COMPILATION OF RF OPTIMIZATION PROBLEM CASE AND ANALYSIS (CELLULAR 2G NETWORKS) 3rd Edition start on September 24, 2010
compiled by Hery Supriyanto
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SDCCH drop reasons 1)High intreference of freq.,like co-channel 2)SDCCH time slot faulty 3)Poor TRX DL quality 4)Hardware fault like antenna or duplexer malfunction 5)May be feeder cable and connectors are faulty. 6)Site taking calls from a very far distance.
Dropped Call(TCH Drop-SDCCH Drop) 1. Radio Link Time-Out
Every time a SACCH message can not be decoded the radio link time-out counter is decreased by 1. If the message can be decoded the counter is incremented by 2. However, the value can not exceed the initial value. The initial value is set by the parameter RLINKT for radio link time-out in the mobile station and by RLINKUP for timeout in the BSC. If the mobile moves out of coverage and no measurement reports are received in the BSC, there will will be a radio radio link link time-o time-out ut and and the mess message age Chann Channel el Relea Release se (caus (cause: e: abnormal release, unspecified) is sent to the mobilestation and the SACCH is deactivated in the BTS. A Clear Request message is sent to the MSC. To be sure that the mobile has stopped transmitting, the BSC now waits RLINKT SACC SACCH H peri period ods s befo before re the the time timesl slot ot is rele releas ased ed and and a new new call call can can be established on the channel. 2. Layer 2 Time-Out If the BTS never get an acknowledge on a Layer 2 message after the time T200XN200, the BTS will send Error Indication (cause: T200 expired) to the BSC, BSC, which which will will send send Chann Channel el Re Relea lease se (caus (cause: e: abno abnorm rmal al releas release, e, timer timer expired) to the mobilestation and a Clear Request to the MSC. The SACCH is deactivated and the BSC waits RLINKT SACCH periods before the timeslot is released and a new call can use the channel. This is only valid if the call is in steady state, i.e. not during handover or assignment.
SDCCH drop reasons 1)High intreference of freq.,like co-channel 2)SDCCH time slot faulty 3)Poor TRX DL quality 4)Hardware fault like antenna or duplexer malfunction 5)May be feeder cable and connectors are faulty. 6)Site taking calls from a very far distance.
Dropped Call(TCH Drop-SDCCH Drop) 1. Radio Link Time-Out
Every time a SACCH message can not be decoded the radio link time-out counter is decreased by 1. If the message can be decoded the counter is incremented by 2. However, the value can not exceed the initial value. The initial value is set by the parameter RLINKT for radio link time-out in the mobile station and by RLINKUP for timeout in the BSC. If the mobile moves out of coverage and no measurement reports are received in the BSC, there will will be a radio radio link link time-o time-out ut and and the mess message age Chann Channel el Relea Release se (caus (cause: e: abnormal release, unspecified) is sent to the mobilestation and the SACCH is deactivated in the BTS. A Clear Request message is sent to the MSC. To be sure that the mobile has stopped transmitting, the BSC now waits RLINKT SACC SACCH H peri period ods s befo before re the the time timesl slot ot is rele releas ased ed and and a new new call call can can be established on the channel. 2. Layer 2 Time-Out If the BTS never get an acknowledge on a Layer 2 message after the time T200XN200, the BTS will send Error Indication (cause: T200 expired) to the BSC, BSC, which which will will send send Chann Channel el Re Relea lease se (caus (cause: e: abno abnorm rmal al releas release, e, timer timer expired) to the mobilestation and a Clear Request to the MSC. The SACCH is deactivated and the BSC waits RLINKT SACCH periods before the timeslot is released and a new call can use the channel. This is only valid if the call is in steady state, i.e. not during handover or assignment.
3. Release Indication When the BTS received a layer 2 DISC frame from the mobile it replies with a Layer 2 UA frame to the mobile station and a Release Indication to the BSC. The system does only react on Release Indication if it is received during a normal disconnection situation. If such a message is received unexpectedly this will usually cause radio link time-out or timer T200 expiration as the mobil mobilest estati ation on stops stops the the trans transmit mittin ting g of measu measure remen mentt repo reports rts.. It is also also possible that the release will be normal depending on when the Release Indication is received. 4. MSC Time-Out Normal Release: If the MSC never received a response on a message (e.g. Identity Request) and there is no radio link time-out or layer 2 time-out, the MSC will send a Clear Command to the BSC. The time-out is depending on the message. When receiving Clear Command, the BSC will send a Channel Release (cause: normal release) and then deactivates the SACCH. Reject (only SDCCH): If the MSC never receives a response on the first message after Establish Indication, the MSC will send a reject message. If the connection was a Location Update it will be a Location Update Reject (cause: network failure) and if the connection was a mobile originating call (CM Service Request) a CM Service Reject (cause: network failure) will be sent. The MSC will then send a Clear Command to the BSC and the call is cleared by Channel Release (cause: normal release). 5. Assignment to TCH Before sending an Assignment Command from the BSC at TCH assignment, the following two criterion have to be fulfilled: a. There must be a TCH channel available, i.e. no congestion b. The locating algorithm must have received at least one valid measurement report.
If either of the criterion is not fulfilled, Assignment Command will not be sent and a Channel Release (cause: abnormal release, unspecified) will be sent to the mobilestation and a Clear Request to the MSC.
TCH Drop reason (1) The classification of TCH Drop Reasons are arranged in the order of priority: 1.Excessive Timing Advance 2.Low Signal Strength 3.Bad Quality 4.Sudden Loss of Connection 5.Other Reasons Excessive Timing Advance The TCH Drop counters due to Excessive Timing Advance will pegged when the during the time of disconnection, the last Timing Advance value recorded was higher than the TALIM Parameter. This drop reason is commonly apparent to isolated or island sites with a wide coverage area. Action: Check if the cell parameter TALIM is < "63" Solution: Set TALIM to a value close to 63. Tilt antenna/reduce antenna height/output power, etc. for co-channel cells. TCH Drop Reasons (2) Low Signal Strength on Down or Uplink or Both Links The drops counters due to Low Signal Strength will be pegged when the Signal Strength during the last Measurement Report before the call dropped is below the LOWSSDL and/or LOWSSUL Thresholds. LOWSSDL and LOWSSUL are BSC Exchange Property parameters which is used only for statistics purposes and does not affect the behavior of calls. If both UL and DL Signal Strength are below the thresholds, only Drop due to Low SS BL will pegged. Normally a call is dropped at the border of large rural cell with insufficient coverage. Bad tunnel coverage cause many dropped calls as well as so called coverage holes. Bad indoor coverage will result in dropped calls. Building shadowing could be another reason. Action: Check coverage plots. Check output power. Check power balance and link budget. Check if Omni site. Check antenna configuration & type. Check antenna installation. Perform drive tests & site survey.
Check TRX/TS with high CONERRCNT. Solution: Add a repeater to increase coverage in for example a tunnel. Change to a better antenna (with higher gain) for the base station. Add a new base station if there are large coverage holes. Block/Deblock TRX
TCH Drop Reasons (3) Poor Quality on Down or Uplink or Both Links The drops counters due to Bad Quality will be pegged when the Signal Strength during the last Measurement Report before the call dropped is above the BADQDL and/or BADQUL Thresholds. BADQDL and BADQUL (expressed in DTQU) are BSC Exchange Property parameters which is used only for statistics purposes and does not affect thebehavior of calls. If both UL and DL Quality are above the thresholds, only Drop due to BAD Quality BL will pegged. Problem on Bad Quality is usually associated with Co-channel Interference on BCCH or TCH. Faulty MAIO assignment can cause frequency collisions on cosited cells especially on 1x1 Reuse. External interference is also one possible cause of problem on quality. Action: Check C/I and C/A plots. Check Frequency Plan (Co-BCCH or Co-BSIC Problem). Check MAIO, HOP, HSN parameters. Check FHOP if correctly configured (BB or SY). Check for External Interference. Perform drive tests. Solution: Change BCCH frequency. Change BSIC. Change MAIO, HOP, HSN. Change FHOP. Record RIR or on-site Frequency Scanning to identify source of interference. Use available radio features. TCH Drop Reasons (4) Sudden Loss of Connection Drops due to Sudden Loss are drops that have not been registered as low signal strength, excessive timing advance, bad quality or hardware (other) reasons, and the locating procedure indicates missing measurement results from the MS. There are some common scenarios that could lead to Sudden Loss of connections such as very sudden and severe drops in signal strength, such as when subscribers enter into buildings, elevators, parking garages, etc., very
sudden and severe occurrence of interference, MS runs out of battery during conversation, Handover Lost, BTS HW faults, Synchronization or A-bis link fault (transmission faults), and MS Faults. Action: Check BTS Error Logs, Alarms and Fault Codes. Check CONERRCNT per TRX and TS. Check Transmission Link (A-bis). Check for DIP Slips. Check LAPD Congestion. Correlate Handover Lost to Drops due to Sudden Loss Solution: Fix Hardware Faults and Alarms. Reset TRX with high CONERRCNT. Ensure that Synchronization and A-bis Link are stable. Change RBLT with high DIP Slips. Change CONFACT or increase Transmission Capacity Investigate HO Lost Problem TCH Drop Reasons (5) TCH Drops due to Other Reasons TCH drops due to Other Reasons are computed by subtracting the sum of drops due to Excessive TA, Low SS, Bad Quality and Sudden Loss from the Total TCH Drop Counts. Drops due to Other Reasons are generally associated withhardware problems, transmission link problems on A-bis, Ater or Ainterfaces, and sometimes Handover Lost. Action: Check BTS Error Logs. Check Alarms and Fault Codes. Check CONERRCNT per TRX and TS. Check Transmission Link (A-bis). Check for DIP Slips. Correlate Handover Lost to Drops due to Other Reasons Solution: Fix Hardware Faults and Alarms. Reset TRX with high CONERRCNT. Ensure that Synchronization and A-bis Link are stable. Change RBLT with high DIP Slips. Investigate HO Lost Problem Problem reason of drop in SDCCH Low Signal Strength on Down or Uplink The reason for poor coverage could be too few sites, wrong output power, shadowing, no indoor coverage or network equipment failure. Action : Check coverage plots.Check output power. Perform drive tests. Check BTS error log Solution : Add new sites. Increase output power. Repair faulty equipment.
Poor Quality on Down or Uplink Action : Check C/I and C/A plots. Check frequency plan. Perform drive tests. Solution : Change frequency. Use available radio features. Too High Timing Advance Action : Check if the cell parameter TALIM is < style="font-weight: bold;">Solution: Set TALIM to a value close to 63. Tilt antenna/reduce antenna height/output power, etc. for cochannel cells. Mobile Error Some old mobiles may cause dropped calls if certain radio network features are used. Another reason is that the MS is damaged and not working properly. Action : Check MS fleet. Solution : Inform operator.
Subscriber Behavior Poorly educated subscribers could use their handsets incorrectly by not raising antennas, choosing illadvised locations to attempt calls, etc. Action : Check customer complaints and their MS. Battery Flaw When a subscriber runs out of battery during a conversation, the call will be registered as dropped call due to low signal strength or others. Action : Check if MS power regulation is used. Check if DTX uplink is used. Congestion on TCH The SDCCH is dropped when congestion on TCH. Action : Check TCH congestion Solution : Increase capacity on TCH or using features like Assignment to another cell, Cell Load Sharing, HCS, Dynamic Half-Rate Allocation and FR-HR Mode Adaptation etc
TCH ASSIGNMENT SUCCESS RATE Probable Reason: 1) No dominant serving cell: The serving cell cannot cope with the TCH traffic. 2) Severe congestion on TCH: Failing TCH allocation for assignment or handover due to congestion 3) Low signal strength for call access: The signal strength might be higher on the BCCH than on the TCH. 4) Interference: Disturbance on SDCCH or target TCH 5) Faulty transceiver: Faulty equipment
The following procedure should be performed for TCH Assignment analysis: For TCH assignment success rate, the first thing, check the TCH Time Congestion. If there is congestion on TCH, it is recommend doing the dimensioning and adding TRU based on carried TCH traffic demand. If there is no congestion on TCH, check the output power of the BTS. If the output power is low, increase the output power. If the output power is ok, check the faulty BTS by extracting BTS error log. If hardware fault found, swap or repair HW. Perform drivetests to check the coverage and received RxLEV. If no dominant cell or similar signal strengths of a few cells found during drivetests, it is recommended to add BTS. If there is no problem on the dominant cell, check the interference whether co-channel or adjacent channel. Check the disturbance whether it is on SDCCH or target TCH. If disturbance found, improve the frequency plan. Mostly, the problems of low TCH assignment are TCH availability and interference.
Probable Reasons of SDCCH Congestion ---Low Availability Action : Check SDCCH Availability. Check if the channels are manual, control or automatic blocked.
---Increasing Traffic Demand The high traffic could be related to an occasional event or due to a long term growth. Action : Check if short term traffic growth. Make trend comparisons. Check if combined SDCCH is used. Check SDCCH dimensioning. ---Bad use of Adaptive configuration of Logical Channels By using the Adaptive configuration of logical channels feature, the basic SDCCH configuration in a cell will be under-dimensioned. If this feature is not used correctly, it will cause SDCCH congestion. Action : Check if ACSTATE is on. Check parameters related to Adaptive configuration of logical channels ---Long Mean Holding Time If the mean holding time is long, this generates a higher traffic load. Action : Check SDCCH Mean Holding Time ---Too Frequent Periodic Registration Action : Check Random Access Distribution. Check the timer T3212 in the BSC and the parameters ---BTDM and GTDM in the MSC Solution : Decrease the periodic registration.
---Location Area Border Cell If the cell is situated on a misplaced Location Area border, this means that unnecessary many normal LUs are performed. Action : Check site position and location area border. Check Location Update Performance. Check parameter CRH etc. ---Extensive SMS Usage Extensive SMS usage increases the SDCCH traffic and could cause congestion if badly dimensioned SDCCH channels. Action : Check SMS activity. ---Cell Broadcast Used Action : Check if Cell Broadcast is active. .If active, check if it is used by the operator. ---IMSI Attach/Detach in Use. An introduction of IMSI attach/detach will increase the traffic on SDCCH. However, the benefits are that the paging success rate will increase. The recommendation is to use Attach/Detach. ---Cell Software File Congestion Action : Check SAE setting. High Ratio of Random Accesses Action : Check Random Access performance
Probable Reasons of Bad Handover Performance
---Neighboring Cell Relation Action:Add neighbor cell relation.
---Missed measurement frequencies in BA-list Action:Check measurement frequencies list. ---Permitted Network Color Code problem Action:Check NCC Permitted ---HW faults.
Action : Check BTS error log.
---Blocking on Target Cell Action:Remove Blocking on Tager Cell ---Congestion A high congestion might lead to dragged calls (handover performed at a not intended location) and a lot of unsuccessful handovers. Action : Check TCH congestion. ---Timer Expire After MS is Lost The MS never answers the base station. Action : Check coverage. Check interference. ---Link Connection or HW Failure Action : Check BTS error log. Perform site visit. Perform link performance measurements. ---Bad Antenna Installation Action : Perform site survey and check antenna installation. Check antenna cabling.
---Many Neighbors Defined Many defined measurement frequencies defined (>16) will decrease the accuracy of the mobile measurements to locate the best six servers. Many measurement frequencies mean few samples per frequency and problem for mobiles to decode the BSIC. Action : Check number of definitions. ---Delayed Handover Decision A delayed handover decision can be due to congestion in the target cell. Action : Check handover parameters.
---Wrong Locating Parameter Setting Action : Check locating parameters. ---Bad Radio Coverage Action : Check coverage plots. ---High Interference, Co-Channel or Adjacent The potential handover candidate is disturbed by interference. Outgoing handover due to bad uplink quality may indicate interference from cochannel another MS. On the border, the quality may be rather bad and the signal strength low. Bad downlink quality may indicate interference from another co-channel basestation. Action : Check interference. Check if many handovers are performed due to downlink or uplink bad quality. ---Receiver Antenna Problem or RBS HW problems (in candidate cell)
Action : Check antenna installation. Check RBS HW and Error log of the target cell
---Poor Inter-MSC/BSC Handover Performance For outer or external cell, wrong definitions in either MSC or BSC may be reason for the problem. Action : Check inter-MSC/BSC handover performance. ---Incorrect Down Tilt Action : Perform site survey and check antenna installation. Solution : Correct antenna tilting.
Ping Pong Handover
As the cellular network growing, it requires addition of new sites to expand capacity and or coverage services. New mobile operators were expecting to grow their network in exponential order to gain market share against mature competitors. Without any intensive network performance maintenance, common problem such ping pong handover would be degrade user experience like bad voice quality ( SQI speech quality index, MOS mean opinion score ) or even dropped call. Ping Pong handover is shown from the successful handover back to old cell within pre-defined time of total handover, e.g. less than 10 seconds. Since not all BSS vendors provide such performance counters it might be identified by simple metric that shall be expressed as total successfulhandover over number of call or connection, e.g. more than 200% indicates ping pong handover. The actual ping pong handover is easily identified through drive test with sense of optimization engineer without require any calculation. Coverage holes might lead to Ping-Ponghandover especially for slow moving mobiles. It may be cause by shadowing by high building. Several possible equal signals from two or more base stations might lead to Ping-Ponghandover as well which is so called not optimized cell dominance. The following action items shall be used for optimization solutions:
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Check the parameter setting such as hysteresis, offset, priority layer in dual-band case, etc. If poor parameters setting found, then correct the related parameters. Compare to default parameters design. Check the output power BSPWR and BSPWRT which normally put on maximum value or compare to design. Check path balance on transceiver link, e.g. loss in uplink and downlink chain. Defective RF modules might be the reason of imbalanced link. Check the site location such as possible coverage holes or no dominant cells. Perform drive test on that particular area to check the signal strength, if the average of signal strength of the neighbors are the same, no dominant cell is found. For Long term action if no dominant cell shall be adding new site, while short term action shall be uptilt the most dominant cell as the serving cell for that area and reduce coverage for others.
Drive Testing
The Purpose of Drive Testing Drive testing is principally applied in both the planning and optimisation stage of network development. However, there are other purposes for which drive testing can be used: •To provide path loss data for initial site survey work •To verify the propagation prediction during the initial planning of the network. •To verify the network system parameters, as defined in the EG8: GSM/DCS System-Specific Parameters. •To provide the initial test parameters used in Benchmarking (as defined in the “Analysis” section of the Network Performance and Monitoring Guideline). •To verify the performance of the network after changes have been made e.g. When a new TRX is added; the removal or addition of a new site; any power Adjustments or changes to the antenna; any changes in clutter or traffic habits such as the addition of new roads etc.
•To measure any interference problems such as coverage from neighboring Countries. •To locate any RF issues relating to traffic problems such as dropped or blocked calls. •To locate any poor coverage areas. •To monitor the network against a slow degradation over time, as well as Monitoring the network after sudden environmental conditions, such as gales or electrical storms. •To monitor the performance of a competitor’s network.
When to Drive Test Drive testing can take place during the day or at night and is dependant upon the Operator’s requirements and subscriber habits. Drive testing during the day will mimic the conditions as seen by subscribers, but may clog up the network if call analysis is being performed. Drive testing during the night will allow a greater area to be surveyed due to the reduction in vehicular congestion. It will also allow for certain test signals to be transmitted and tested, particularly when setting up a new site, without interrupting normal operation. However, night-time testing does not mimic the conditions experienced by subscribers. For planning purposes, drive testing is typically performed at night and for maintenance purposes, drive testing is performed during the day.
Where to Drive Test Some areas of a network will have greater performance problems than others. Drive testing should not be uniform throughout the whole network, but should be weighted towards areas where there are significant RF problems. There may be other areas of the network that require temporary coverage during a certain time of the year e.g. an exhibition centre or a sports stadium. These areas should be examined and planned in greater detail. It is important that a drive test is documented. This is specified by the Operator and can either take the form of creating a new item of documentation or filling in an existing document. All documentation will be passed to Analysts and Engineers, who will need accurate records of any test work carried out.
----Route Plans The area to be drive tested is ascertained before leaving the office. There are three levels of drive testing depending on the purpose of the test: Primary Route: This includes all major roads, highways and throughfares and should be given priority to all other roads when conducting a coverage test, unless a new site is put into service for a specific objective. Secondary Route: This includes all streets, by-streets and compounds, where accessible, such as a University Campus. Secondary routes are used in areas where problems have been located during a primary route test and further investigation is needed. Miscellaneous Routes: This includes in-building and non-access routes to vehicles such as shopping malls, golf courses, airports, hotels, conference centres etc. A route is prepared by photocopying a map and highlighting the route to be driven. For primary routes, a map of scale no less than 1:20,000 should be used, and a map of scale 1:10,000 is recommended for secondary routes. It is recommended that the route is marked in a contiguous circuit, taking account of one-way streets at this stage. A drive test should be planned in both directions, where possible, and at the same speed. This minimises any errors and checks the point of handovers and cell dimensioning. For new sites that are being tested, it is recommended that the transceiver is forced to camp onto the cell (forbidding any handovers) in order to ascertain the full coverage of the cell. The test should be re-driven with any forced handovers removed.
Layer 1 Messages Other Layer 1 criteria that is useful for field measurements include: C1 criteria • ARFCN of Serving Cell - (TCH in dedicated mode, BCCH in idle mode)) Time Slot (TS) •
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Layer 3 Messages All Layer 3 messages should be collected where possible. Layer 3 Messages are used by Analysts to determine more accurately the cause of a problem within the network. Some field test equipment can perform basic analysis of particular Layer 3 messages
during data collection. This enables certain conditions such as call classification or handovers to be flagged to the survey technician.
Call Classification In principle there are five call classifications, some of which can be subdivided further. Good Calls: These are calls that are successfully placed on the network and maintained for the required duration. Dropped Calls: These are calls that are successfully placed on to the network but are terminated without authorisation. Using Layer 3 Messages, these calls can be sub-divided into: End User Hang-up • System Hang-up • Other • Blocked Calls: These are calls that cannot be placed on to the network. Again, using Layer 3 messages, these can be sub-divided as follows: System Busy • End User Engaged • No Service • Other • Roamed Calls: These are calls that are successfully placed on another network. Roamed calls may also be good calls or dropped calls. Noisy Calls: These are calls which have been successfully completed for the duration of the call but which experienced a number of noise bursts that a subscriber may find intolerable. The threshold for determining the level of poor audio is programmed during the set-up of the test. In GSM, this particular classification is very difficult to determine with great accuracy. It should be noted that it is not enough to monitor just the RxLEV and the RxQUAL.
Troubleshooting No Data Collected Occasionally, the equipment fails to trigger the collection device to save the data to file. Check • all cables Ensure the Processing Unit is powered • Re-start the laptop computer • Re-start the equipment •
Re-drive the test.
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No Positional Information Collected If data is collected using GPS only, it may be possible that satellite reception was lost during a drive through a tunnel etc. It is important that back-up equipment is used, such as a Dead-Reckoning device, since a GPS receiver will re-transmit the last known position until it receives an update. If the vehicle moves without GPS cover, the data will be inaccurate and cannot be analysed. Check the • GPS antenna cable to the receiver Drive to an open area and ensure that the GPS system is working correctly • If required, install a back-up positional device to safeguard against lost GPS • Coverage Holes If there are patches of poor coverage in unexpected areas, it may indicate the fringes of a coverage hole. It is important to re-drive this particular area. Complete a route plan using secondary roads as far as possible • Make notes of any buildings / obstructions that may cause shadowing • Take note of pedestrian / vehicular habits in the area •
Dropped Calls Dropped calls can be caused by either RF environments or incorrect system parameters. The following data should be checked to ensure that it has been collected properly. Layer 3 Messages • Neighbour Cell List (BA Table) • RxLEV (Server • & Neighbour) RxQUAL (Server • & Neighbour) Finally, ensure that the automatic setting for the call length is not shorter than that for the timer monitoring for unauthorised call drop-outs. The setting should be a minimum of 30 seconds.
Handover Problems Handover problems are generally caused by inaccurate settings of the handover boundary. This can cause ping-ponging, where the server will keep changing, and congestion at the switch. Check the following. The transceiver antenna is fitted correctly • Collection of Layer 3 Messages • Collection of Neighbour Cell List (BA Table) •
Collection of Scanning Information • Collection of Cell Identities • Collection of T.Adv for the Serving Cell • Also, ensure that the collection of data from the new serving cell immediately after the handover has occurred (particularly RxLEV and RxQUAL) is not timed to occur prior to the-synchronisation of the transceiver itself. If a particular serving cell can be isolated as a potential cause of handover problems, slowly drive around the cell in a radius of around 500m - 1km, checking when handovers occur. Blocked Calls / System Busy If calls are repeatedly classified as blocked, it is recommended that the drive test is temporarily halted in order to try and locate the cause. Check that the number called is fully functional • Check that there is adequate coverage from the expected serving BTS • Check the equipment transceiver is functioning correctly by using an ordinary • mobile to call the office If all appears functional, try to place calls through an alternative BTS. If this • succeeds, inform the office immediately and re-suspend the drive test.
Timers and counters for Radio Resource Management
Timers on the Mobile Station Side T3122: This timer is used during random access, after the receipt of an IMMEDIATE ASSIGN REJECT message.Its value is given by the network in the IMMEDIATE ASSIGN REJECT message. T3124: This timer is used in the seizure procedure during a hand-over, when the two cells are not synchronized.Its purpose is to detect the lack of answer from the network to the special signal. Its value is set to 675 ms if the channel type of the channel allocated in the HANDOVER COMMAND is an SDCCH (+ SACCH); otherwise its value is set to 320 ms. T3126:This timer is started either after sending the maximum allowed number of CHANNEL REQUEST messages during an immediate assignment procedure. Or on receipt of an IMMEDIATE ASSIGNMENT REJECT message, whichever occurs first. It is stopped at receipt of an IMMEDIATE ASSIGNMENT message, or an IMMEDIATE ASSIGNMENT EXTENDED message. At its expiry, the immediate assignment procedure is aborted. The minimum value of this timer is equal to the time taken by T+2S slots of the mobile station's RACH. S and T. The maximum value of this timer is 5 seconds.
T3128:This timer is started when the mobile station starts the uplink investigation procedure and the uplink is busy.It is stopped at receipt of the first UPLINK FREE message. At its expiry, the uplink investigation procedure is aborted. The value of this timer is set to 1 second. T3130:This timer is started after sending the first UPLINK ACCESS message during a VGCS uplink access procedure.It is stopped at receipt of a VGCS ACCESS GRANT message.At its expiry, the uplink access procedure is aborted.The value of this timer is set to 5 seconds. T3110:This timer is used to delay the channel deactivation after the receipt of a (full) CHANNEL RELEASE. Its purpose is to let some time for disconnection of the main signalling link. Its value is set to such that the DISC frame is sent twice in case of no answer from thenetwork. (It should be chosen to obtain a good probability of normal termination (i.e. no time out of T3109) of the channel release procedure.) T3134:This timer is used in the seizure procedure during an RR network commanded cell change order procedure. Its purpose is to detect the lack of answer from the network or the lack of availability of the target cell. Its value is set to 5 seconds. T3142:The timer is used during packet access on CCCH, after the receipt of an IMMEDIATE ASSIGNMENT REJECT message. Its value is given by the network in the IMMEDIATE ASSIGNMENT REJECT message. T3146:This timer is started either after sending the maximum allowed number of CHANNEL REQUEST messages during a packet access procedure. Or on receipt of an IMMEDIATE ASSIGNMENT REJECT message during a packet access procedure, whichever occurs first. It is stopped at receipt of an IMMEDIATE ASSIGNMENT message, or an IMMEDIATE ASSIGNMENT EXTENDED message. At its expiry, the packet access procedure is aborted. The minimum value of this timer is equal to the time taken by T+2S slots of the mobile station's RACH. S and T are defined in section 3.3.1.2. The maximum value of this timer is 5 seconds. T3164:This timer is used during packet access using CCCH. It is started at the receipt of an IMMEDIATE ASSIGNMENT message. It is stopped at the transmission of a RLC/MAC block on the assigned temporary block flow, see GSM 04.60. At expire, the mobilestation returns to the packet idle mode. The value of the timer is 5 seconds. T3190:The timer is used during packet downlink assignment on CCCH. It is started at the receipt of an IMMEDIATE ASSIGNMENT message or of an PDCH ASSIGNMENT COMMAND message when in dedicated mode.It is stopped at the receipt of a RLC/MAC block on the assigned temporary block flow, see GSM 04.60. At expiry, the mobile station returns to the packet idle mode. The value of the timer is 5 seconds.
Timers on the network side T3101:This timer is started when a channel is allocated with an IMMEDIATE
ASSIGNMENT message. It is stopped when the MS has correctly seized the channels. Its value is network dependent. NOTE: It could be higher than the maximum time for a L2 establishment attempt. T3103:This timer is started by the sending of a HANDOVER message and is normally stopped when the MS has correctly seized the new channel. Its purpose is to keep the old channels sufficiently long for the MS to be able to return to the old channels, and to release the channels if the MS is lost. Its value isnetwork dependent. NOTE: It could be higher than the maximum transmission time of the HANDOVER COMMAND, plus the value of T3124, plus the maximum duration of an attempt to establish a data link in multiframe mode.) T3105:This timer is used for the repetition of the PHYSICAL INFORMATION message during the hand-over procedure. Its value is network dependent. NOTE: This timer may be set to such a low value that the message is in fact continuously transmitted. T3107:This timer is started by the sending of an ASSIGNMENT COMMAND message and is normally stopped when the MS has correctly seized the new channels. Its purpose is to keep the old channel sufficiently long for the MS to be able to return to the old channels, and to release the channels if the MS is lost. Its value isnetwork dependent. NOTE: It could be higher than the maximum transmission time of the ASSIGNMENT COMMAND message plus twice the maximum duration of an attempt to establish a data link multiframe mode. T3109:This timer is started when a lower layer failure is detected by the network, when it is not engaged in a RF procedure. It is also used in the channel release procedure. Its purpose is to release the channels in case of loss of communication. Its value is network dependent. NOTE: Its value should be large enough to ensure that the MS detects a radio link failure. T3111:This timer is used to delay the channel deactivation after disconnection of the main signalling link. Its purpose is to let some time for possible repetition of the disconnection. Its value is equal to the value of T3110. T3113:This timer is started when the network has sent a PAGING REQUEST message and is stopped when the network has received the PAGING RESPONSE message. Its value is network dependent. NOTE: The value could allow for repetitions of the Channel Request message and the requirements associated with T3101. T3115:This timer is used for the repetition of the VGCS UPLINK GRANT message during the uplink access procedure. Its value is network dependent. NOTE: This timer may be set to such a low value that the message is in fact continuously transmitted. T3117:This timer is started by the sending of a PDCH ASSIGNMENT COMMAND message and is normally stopped when the MS has correctly accessed the target TBF. Its purpose is to keep the old channel sufficiently
long for the MS to be able to return to the old channels, and to release the channels if the MS is lost. Its value isnetwork dependent. NOTE: It could be higher than the maximum transmission time of the PDCH ASSIGNMENT COMMAND message plus T3132 plus the maximum duration of an attempt to establish a data link in multiframe mode. T3119:This timer is started by the sending of a RR-CELL CHANGE ORDER message and is normally stopped when the MS has correctly accessed the new cell. Its purpose is to keep the old channels sufficiently long for the MS to be able to return to the old channels, and to release the channels if the MS is lost. Its value isnetwork dependent.NOTE: It could be higher than the maximum transmission time of the RR_CELL CHANGE ORDER, plus T3134, plus the maximum duration of an attempt to establish a data link in multiframe mode. T3141:This timer is started when a temporary block flow is allocated with an IMMEDIATE ASSIGNMENT message during a packet access procedure. It is stopped when the mobile station has correctly seized the temporary block flow. Its value is network dependent.
GSM Mobile Terminated Call
The PSTN subscriber dials the MS’s telephone number (MSISDN), the MSISDN is analyzed in the PSTN, which identifies that this is a call to a mobile network subscriber. A connection is established to the MS’s home GMSC. The PSTN sends an Initial Address message (IAM) to the GMSC. The GMSC analyzes the MSISDN to find out which HLR, the MS is registered in, and queries the HLR for information about how to route the call to the serving MSC/VLR. The HLR looks up the MSISDN and determines the IMSI and the SS7 address for the MSC/VLR that is servicing the MS. The HLR also checks if theservice, “call forwarding to C-number” is activated, if so, the call is rerouted by the GMSC to that number.
The HLR then contacts the servicing MSC/VLR and asks it to assign a MSRN to the call. [MSRN - Mobile Station Routing Number].The MSC/VLR returns an MSRN via HLR to the GMSC. The GMSC sends an Initial Addressing message (IAM) to the servicing MSC/VLR and uses the MSRN to route the call to the MSC/VLR. Once the servicing MSC/VLR receives the call, the MSRN can be released and may be made available for reassignment. The MSC/VLR then orders all of its BSCs and BTSs to page the MS. Since the MSC/VLR does not know exactly which BSC and BTS the MS is monitoring, the page will be sent out across the entireLocation Area(LA). When the MS detects the paging message to the BTS’s in the desired LA. The BTS’s transmit the message over the air interface using PCH. To page the MS, the network uses an IMSI or TMSI valid only in the current MSC/VLR service area. When the MS detects the paging message, it sends a request on RACH for a SDCCH. The BSC provides a SDCCH, using AGCH. SDCCH is used for the call set-up procedures. Over SDCCH all signaling preceding a call takes place. This includes: Marking the MS as “active” in the VLR. Authentication procedure (Start ciphering, Equipment identification). The MSC/VLR instructs the BSC/TRC to allocate an idle TCH. The BTS and MS are told to tune to the TCH. The mobile phone rings. If the subscriber answers, the connection is established.
GSM Mobile Originating Call Flow Mobile User calling a Land Line Subscriber. 1. MS after dialing a number & pressing SEND key, sends Channel
Request(Chan_Req) message
on
RACH
to
ask
for
a
signalingchannel (Radio Resources). [RACH - Random Access channel] 2. The BSC allocates a Traffic Channel(TCH) using AGCH. TCH allocation
assigns a specific Frequency & a Timeslot on that frequency. [AGCH Access Grant Channel] 3. The MS sends a call setup request through SDCCH, to the MSC/VLR.
[SDCCH
- slow
dedicated
control
channel].
Over
SDCCH,
all
signaling takes place. This includes: marking the MS status as active in the VLR
4. Then comes Authentication Procedure which includes Ciphering (The
channel is ciphered so as to protect the call), Equipment Identification, etc. 5. The MSC/VLR instructs the BSC to allocate an Idle TCH (this message
contains the dialed digits and other information needed for call establishment). The BTS and MS are told to tune to the TCH. 6. The MSC allocates a voice circuit on one the digital trunks between the
MSC and the BSS. 7. MSC informs the BSS about the allocated voice circuit. The call is also
switched from signaling to voice. 8. The BSS notifies the Mobile about the changeover to voice mode. 9. The MSC routes the call and sends the call towards the called
subscriber. 10. The PSTN indicates to the MSC that it has received all the digits and the
called subscriber is being rung. 11. The
MSC
informs
the
mobile
that
the
called
subscriber
is
beingalerted via a ring. 12. The called subscriber answers the call.
GSM IDENTITY NUMBERS(IMSI,TMSI,CGI,MSRN,IMEI) GSM identities
The GSM network is complex and consists of the Switching System (SS) and the Base Station System (BSS). The switching system, which consists of HLR, MSC, VLR, AUC and EIR, interfaces both the Base Station System and also other networks like PSTN/ISDN, data networks or other PLMNs. In order to switch a call to a mobile subscriber, the right entities need to be involved. It is therefore important to address them correctly. The numbers used to identify the identities in a GSM/PLMNnetwork is described in this chapter. See also Figure 56. Numbering plans are used to identify different networks. For a telephone number in the PSTN/ISDN network, numbering plans E.164 is used. Mobile Station ISDN Number (MSISDN)
The MSISDN is a number which uniquely identifies a mobile telephone subscription in the public switched telephone network numbering plan.
According to the CCITT recommendations, the mobile telephone number or catalogue number to be dialled is composed in the following way: MSISDN = CC + NDC + SN CC = Country Code NDC = National Destination Code SN = Subscriber Number A National Destination Code is allocated to each GSM PLMN. In some countries, more than one NDC may be required for each GSM PLMN. The international MSISDN number may be of variable length. The maximum length shall be 15 digits, prefixes not included. Each subscription is connected to one Home Location Register (HLR). The length of the MSISDN depends on the structure and numbering plan of each operator, as an application of CCITT recommendation E.164. The following is an example of dialling a GSM subscriber.
International Mobile Subscriber Identity (IMSI)
The IMSI is the information which uniquely identifies a subscriber in a GSM/PLMN. For a correct identification over the radio path and through the GSM PLMN network, a specific identity is allocated to each subscriber. This identity is called the International Mobile Subscriber Identity (IMSI) and is used for all signalling in the PLMN. It will be stored in the Subscriber Identity Module (SIM), as well as in the Home Location Register (HLR) and in the serving Visitor Location Register (VLR). The IMSI consists of three different parts:
IMSI = MCC + MNC + MSIN MCC = Mobile Country Code (3 digits)
MNC = Mobile Network Code (2 digits) MSIN = Mobile Subscriber Identification Number (max 10 digits) According to the GSM recommendations, the IMSI will have a length of maximum 15 digits. All network–related subscriber information is connected to the IMSI. See also Figure 56. Mobile Station Roaming Number (MSRN)
HLR knows in what MSC/VLR Service Area the subscriber is located. In order to provide a temporary number to be used for routing, the HLR requests the current MSC/VLR to allocate and return a Mobile Station Roaming Number (MSRN) for the called subscriber, see Figure 56. At reception of the MSRN, HLR sends it to the GMSC, which can now route the call to the MSC/VLR exchange where the called subscriber is currently registered. The interrogation call routing function (request for an MSRN) is part of the Mobile Application Part (MAP). All data exchanged between the GMSC - HLR - MSC/VLR for the purpose of interrogation is sent over the No. 7 signalling network. The Mobile Station Roaming Number (MSRN), according to the GSM recommendations, consists of three parts: MSRN = CC + NDC + SN CC = Country Code NDC = National Destination Code SN = Subscriber Number Note: In this case, SN is the address to the serving MSC.
Temporary Mobile Subscriber Identity (TMSI)
The TMSI is a temporary number used instead of the IMSI to identify an MS. It raises the subscriber’s confidentiality and is known within the serving
MSC/VLR-area and changed at certain events or time intervals. The structure of the TMSI may be chosen by each administration but should have a maximum length of four octets (8 digits). International Mobile station Equipment Identity (IMEI)
The IMEI is used for equipment identification. An IMEI uniquely identifies a mobile station as a piece or assembly of equipment. (See IMEI, chapter 5.) IMEI = TAC + FAC + SNR + sp TAC = Type Approval Code (6 digits), determined by a central GSM body FAC = Final Assembly Code (2 digits), identifies the manufacturer SNR = Serial Number (6 digits), an individual serial number of six digits uniquely identifying all equipment within each TAC and FAC sp = spare for future use (1 digit) According to the GSM specification, IMEI has the length of 15 digits. Location Area Identity (LAI)
LAI is used for location updating of mobile subscribers. LAI = MCC + MNC + LAC MCC = Mobile Country Code (3 digits), identifies the country. It follows the same numbering plan as MCC in IMSI. MNC = Mobile Network Code (2 digits), identifies the GSM/PLMN in that country and follows the same numbering plan as the MNC in IMSI. LAC = Location Area Code, identifies a location area within a GSM PLMN network. The maximum length of LAC is 16 bits, enabling 65 536 different location areas to be defined in one GSM PLMN. Cell Global Identity (CGI)
CGI is used for cell identification within the GSM network. This is done by adding a Cell Identity (CI) to the location area identity. CGI = MCC + MNC + LAC + CI CI = Cell Identity, identifies a cell within a location area, maximum 16 bits Base Station Identity Code (BSIC)
BSIC allows a mobile station to distinguish between different neighboring base stations.
BSIC = NCC + BCC NCC = Network Colour Code (3 bits), identifies the GSM PLMN. Note that it does not uniquely identify the operator. NCC is primarily used to distinguish between operators on each side of border. BCC = Base Station Colour Code (3 bits), identifies the Base Station to help distinguish between BTS using the same BCCH frequencies Location Number (LN)
Location Number is a number related to a certain geographical area, as specified by the network operator by ”tying” the location numbers to cells, location areas, or MSC/VLR service areas. The Location Number is used to implement features like Regional /Local subscription and Geographical differentiated charging.
Dropped Call due to Sudden Drop On circuit switch service, when a call is abnormally disconnected, a Clear Message with cause code Call Control be treated as normal Disconnection is sent to the MSC – named Clear Request Message. Refer to Ericsson system, the following Urgency condition is checked at that time and the relevant counter is incremented as a consequence: 1. 2. 3. 4.
Excessive TA Low Signal Strength Bad Quality Sudden Drop
As named, stright forward meaning for the dropped call is described for the first three items. However, Sudden Drop is quite not easy to understand. Sudden Loss are drops that have not been registered as bad quality, signal strength, timing advance. The term Sudden Loss is used because if the network cannot establish a connection with the lost MS after a pre-defined period, the sudden loss counter is incremented if the last reported measurement from the MS does not fulfill any of the reasons mentioned. A connection is marked as Sudden loss if none of the three types of urgency states (that is excessive TA, low signal strength or bad quality) are indicated and the locating procedure indicates missing measurement results from the MS. Drops due to ‘Other’ reasons are generally associated with hardware problems and disturbances, number of drops due to ‘Other’ reasons is obtained by subtracting the drops with known reasons from the total number of drops.
Main contributors in sudden and other TCH drop: •
Very sudden and severe drop in signal strength, such as when subscribers enter into buildings, elevators, parking garages, etc.
•
Very sudden and severe occurrence of interference or bad quality. MS running out of battery during conversation. • • • •
Handover Lost. BTS HW faults. Synchronization or Abis link fault (transmission faults). MS Faults.
SMS (SHORT MESSAGES SERVICES)*
*mostly of
term
SMS is a service that allows subscribers to send short messages (up to 160 characters) to other mobile subscribers. Rather than having to set up a call on a traffic channel, SMS uses spare capacity on the Standalone Dedicated Control Channel (SDCCH). SMS is classified as a GSM Teleservice and three SMS teleservices (TS21-23) have been defined: · TS 21 – Mobile terminated point-to-point messaging. A mobile can terminate an SMS message either from another MS or from the fixed network. · TS 22 – Mobile originated point-to-point messaging. A mobile can send a message either to another MS or into the fixed network (as an Email for example). · TS23 – SMS Cell Broadcast (SMSCB). A more recent variation of SMS is SMSCB. SMSCB messages are generally broadcast only in a specific network region. An MS cannot initiate such a message and does not acknowledge receipt of one. Only MSs in idle mode can receive SMSCB messages. These messages differ from standard SMS messages in that they are only 92 characters long. However, procedures exist to concatenate up to 15 SMSCB messages using a special reassembly mechanism. In order to implement SMS, a network operator must establish a SMS Service Centre which receives and processes SMS messages in a store-and-forward mode. Messages can be initiated in the fixed or mobile network and delivered to either the fixed or mobile network.
Short Messaging Entities Short messaging entity (SME) is an entity which may receive or send short messages. The SME may be located in the fixed network, a mobile station, or another service centre. Short Message Service Centre Short message service centre (SMSC) is responsible for the relaying and store-and-forwarding of a short message between an SME and mobile station. SMS Gateway Mobile Switching Centre The SMS–gateway mobile switching centre (SMS–GMSC) is an MSC capable of receiving a short message from an SMSC, interrogating a home location register (HLR) for routing information, and delivering the short message to the visited MSC of the recipient mobile station. The SMS interworking MSC (SMS–IWMSC) is an MSC capable of receiving a short message from the mobile network and submitting it to the appropriate SMSC. The SMS– GMSC/SMS–IWMSC are typically integrated with the SMSC. Home Location Register The HLR is a database used for permanent storage and management of subscriptions and service profiles. Upon interrogation by the SMSC, the HLR provides the routing information for the indicated subscriber. The HLR also informs the SMSC, which has previously initiated unsuccessful short message delivery attempts to a specific mobile station, that the mobile station is now recognized by the mobile network to be accessible. Mobile Switching Centre The MSC performs the switching functions of the system and controls calls to and from other telephone and data systems. Visitor Location Register The visitor location register (VLR) is a database that contains temporary information about subscribers. This information is needed by the MSC to service visiting subscribers. The Base Station System All radio-related functions are performed in the base-station system (BSS). The BSS consists of base-station controllers (BSCs) and the base-transceiver stations (BTSs), and its primary responsibility is to transmit voice and data traffic between the mobile stations.
The Mobile Station The mobile station (MS) is the wireless terminal capable of receiving and originating short messages as well as voice calls. The wireless network signalling infrastructure is based on signalling system 7 (SS7). SMS makes use of the mobile application part (MAP), which defines the methods and mechanisms of communication in wireless networks, and uses the services of the SS7 transaction capabilities application part (TCAP). An SMS service layer makes use of the MAP signalling capabilities and enables the transfer of short messages between the peer entities.
GSM Interfaces
Interface between the MSC and Base Station System (A-interface) The interface between the MSC and its BSS is specified in the 08-series of GSM Technical Specifications. The BSS-MSC interface is used to carry information concerning: · BSS management; · call handling; · mobility management. A-BIS INTERFACE (BSC - BTS) When the BSS consists of a Base Station Controller (BSC) and one or more Base Transceiver Stations (BTS), this interface is used between the BSC and BTS to support the services offered to the GSM users and subscribers. The interface also allows control of the radio equipment and radio frequency allocation in the BTS. This interface is specified in the 08.5x-series of GSM Technical Specifications. B INTERFACE (MSC - VLR) The VLR is the location and management data base for the mobile subscribers roaming in the area controlled by the associated MSC(s). Whenever the MSC needs data related to a given mobile station currently
located in its area, it interrogates the VLR. When a mobile station initiates a location updating procedure with an MSC, the MSC informs its VLR which stores the relevant information. This procedure occurs whenever an MS roams to another location area. Also, when a subscriber activates a specific supplementary service or modifies some data attached to a service, the MSC informs (via the VLR) the HLR which stores these modifications and updates the VLR if required. C INTERFACE (HLR and the MSC) The Gateway MSC must interrogate the HLR of the required subscriber to obtain routing information for a call or a short message directed to that subscriber. D INTERFACE (HLR - VLR) This interface is used to exchange the data related to the location of the mobile station and to the management of the subscriber. The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area. To support this, the location registers have to exchange data. The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station. The HLR sends to the VLR all the data needed to support the service to the mobile subscriber. The HLR then instructs the previous VLR to cancel the location registration of this subscriber. Exchanges of data may occur when the mobile subscriber requires a particular service, when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means. E INTERFACE (MSC – MSC) When a mobile station moves from one MSC area to another during a call, a handover procedure has to be performed in order to continue the communication. For that purpose the MSCs have to exchange data to initiate and then to realize the operation. After the handover operation has been completed, the MSCs will exchange information to transfer A-interface signalling as necessary. When a short message is to be transferred between a Mobile Station and Short Message Service Centre (SC), in either direction, this interface is used to transfer the message between the MSC serving the Mobile Station and the MSC which acts as the interface to the SC.
F INTERFACE (MSC and EIR) This interface is used between MSC and EIR to exchange data, in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station. G INTERFACE (VLR – VLR) When a mobile subscriber moves from a VLR area to another Location Registration procedure will happen. This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR. H INTERFACE (HLR - AuC) When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data, the HLR requests
the data from the AuC. The protocol used to transfer the data over this interface is not standardized. Um INTERFACE (MS- BTS) The interface between the MS and the BSS is specified in the 04- and 05series of GSM Technical Specifications.
TENTANG SATUAN dB The decibel (dB) represents a logrithmic ratio between two quantities and is unitless. If the ratio is refered to a specific quantity this is indicated by a suffix (dBm is referenced against 1 mW and dBV is 1 Volt). Originally the dB was for power ratios, given by dB=10 log (P 1/P2) Power is proportional to voltage squared, hence the ratio of voltages or currents across a constant impedence is given by dB=20 log(V 1/V2) or 20 log(I 1/I2) dBi = The output Gain of the Antenna compared to an isotropic antenna. dBm = The Gain of the Antenna Output signal compared to one milli volt. notice where the "output" is in each sentence. dBi refers to free space radiated power. dBm refers to voltage measure at the antenna terminals. dBi is relative. dBm is an actual numerical value. What is Optimum Value of T200? we like to know the optimum value for T200 on LAPDm. All vendors have different default values (Siemens 145 ms, Nokia 220 ms, Satellite Abis 400 ms etc.) for this timer - but which value is the best to reduce SDCCH drops and to keep the retransmissions at an acceptable level ? Example: SDCCH/8 During a 51er multiframe the SDCCH/8 occupies four consecutive TDMA frames (four bursts are sent). Than the MS / BTS has to wait for the next 51er multiframe (i.e. 235 ms) before the next Layer 2 frame could be sent. 145 ms / 220 ms are shorter th an the 51er multiframe (235 ms) so in case of an missing acknowledgement this is always a T200 expiry. The SDCCH drop will occur if T200 expired N200+1 times. If the T200 is increased (for example to 500 ms) we have two 51er multiframe to get the acknowledgement and the SDCCH drops are reduced. The Qos Stats show a clear & strong corelation between the KPI and the parameter T200. That's amazing !
Regarding the Ack from the BTS, I'm not sure (and I'm tired to look in the 3GPP specs :) ). Take the subchannel "0" from the SDCCH ts. in DL : the BTS sends SDCCH/0 on burst 0, 1, 2, 3 and the SACCH/0 on burst 32, 33, 34, 35 in UL : th MS sends SDCCH/0 on burst 15, 16, 17, 18 and the SACCH/0 on 47, 48, 49, 50 And I ***believe*** that the Lapdm acknowledgments can be sent on either the SACCH or the SDCCH, since both of them are sent over the same LapDm link. I'm not sure at all about this though, but it sounds logical. The value of the SDCCH Drop due to Radio failures (in ALU) is usually around 1% in a fairly good network. The SDCCH Drop due to Radio Failures is a counter that encompasses both the Radio Link Timeout and the "T200*N200+1 times" failures. I am not able to test your changes because I am not working on a live network (i am a gsm trainer, living in a world of theory...) T200 = 220ms for sDCCH SAPI0 = 450ms for SDCCH SAPI3 = 900ms for SACCH associated to SDCCH Looking at those timers, I fear that the LapDm link for SDCCH is different than the LapDm link for the SACCH of the SDCCH... I'll let you look it up th ough :)
Handover failure due to protocol error Answer 1:plz check ur msc version. Answer2:Can you verify the ciphering algo used in both External Cells? You can also confirm this via DT. (In case of ciphering issue, handover from one side to other should be happening. i.e. the side using higher ciphering algo will be able to transfer the call to cell with lower ciphering algo)
Low Signal Strength Analysis Low Signal strength is one of the reason of drop call. It can be indicated by many calls disconnected at low signal strength by subscriber, drop calls due to excessive TA, poor handover performance and poor call setup performance.
Probable Reason Poor BSC
High LOWSSDL & LOWSSUL will Exchange give more drop reason due to SS and this might not show the Property actual drop. It is because drop setting due to SS is more priority than Quality. No dominant Cell might be isolated or cell standalone. Antenna tilt & Too much downtilt sometimes orientation might not cover a larger area and the subscriber might lose the SS. Output Power Low output power might cause smaller border cell.
The following procedure should be performed for low signal strength analysis: 1:Identify the baseline LOWSSUL/LOWSSDL).
requirement
of
design
and
BSC
exchange
property
(setting
for
2:Check the value for LOWSSDL & LOWSSUL. If it is higher than ACCMIN, change the parameter to a reasonable value since the drop reason will be more priority to SS compared to Quality. 3:Check the site position, antenna direction, position etc. This is to ensure the possible location is open to interference (open water environment) or isolated. Good map is needed for this. 4:Check if the site is sectorized or Omni. If it is Omni, set the cell into sectorized cell. 5:Check if the signal strength is uplink or downlink limited. Mostly, It is designed to be downlink limited. 6:Check the coverage cover expected area from the planet. If it is not, check the antenna tilt and orientation. Change the direction or tilt if it is too much downtilt or pointing to a wrong direction. 7:Sometime, low output power might cause low SS. Check output power and if it is low, increase the output power. 8:Check cell whether it has hotspots from drivetests. If found, adding new site is recommend. 9:In order to check power distribution, run Cell Traffic Recording (CTR) to that particular cell. 10:Check if the cell has indoor coverage problem. If yes, add micro site instead.
Call a balanced solution Call a balanced solution 1, change the angle under directional antenna, hanging high, and the corresponding cell parameters such as base station transmit power to change the size of coverage to achieve the purpose of regulating telephone traffic. 2, the temporary increase in telephone traffic, can temporarily increase or decrease the carrier frequency transmission power to change the signal coverage. 3, changing the carrier frequency is the number of residential telephone traffic is one common method of regulation, drawn from telephone traffic small cell carrier frequency to high residential telephone traffic. 4, using OVERLAY / UNDERLAY-level cell structure or addition of micro-cellular base stations, reducing telephone traffic per channel. 5, the verification value to allow access to the minimum level ACCMIN, changes in coverage by cell telephone traffic indirect adjustment.Note that this value adjustment is too large may cause blind spots, too small may result in decreased voice quality. 6, according to the site re-election testing, adjusting cell reselection parameters CRO. 7, adjusting the switching offset and delay parameters, change the switch and the switch to bring the border to achieve Call divert. 8, directed retry is enabled, the load switch.
What is E1 and T1
The PDH (plesiochronous Digital Hierarchy) has 2 primary communication systems as its foundation. These are, T1 system based on 1544kbit/s that is recommended by ANSI & E1 system based on 2048kbit/s that is recommended by ITU-T. Common Characteristics :-
1. 2. 3. 4. 5.
Both are having Same Sampling Frequency i.e. 8kHz. In both (E1 & T1) Number of samples/telephone signal = 8000/sec. In both (E1 & T1) Length of PCM Frame = 1/8000s = 125µs. In both (E1 & T1) Number of Bits in each code word = 8. In both (E1 & T1) Telephone Channel Bit Rate = 8000/s x 8 Bit = 64 kbit/s.
Differing Characteristics :-
1. In E1 Encoding/Decoding is followed by A-Law while in T1 Encoding/Decoding is 2. 3. 4. 5. 6.
followed by µ-Law. In E1 - 13 Number of Segments in Characteristics while in T1 - 15Number of Segments in Characteristics. In E1 - 32 Number of Timeslots / PCM Frame while in T1 - 24 Number of Timeslots / PCM Frame. In E1 - 8 x 32 = 256 number of bits / PCM Frame while in T1 - 8 x 24 + 1* = 193 number of bits / PCM Frame. (* Signifies an additional bit). In E1 - (125µs x 8)/256 = approx 3.9µs is the length of an 8-bit Timeslot while in T1 - (125µs x 8)/193 = approx 5.2µs is the length of an 8-bit Timeslot. In E1 - 8000/s x 256 bits = 2048kbit/s is the Bit Rate of Time-Division Multiplexed Signal while in T1 - 8000/s x 193 bits = 1544kbit/s is the Bit Rate of Time-Division Multiplexed Signal.
TENTANG QUALITY , INTERFERENCE dan AMR (HR/FR – decompression mode) 1. Bad Quality di Level yang bagus bisa jadi ada masalah hardware. 2. idle band interferences untuk cek external interferences , di katakan jelek atau ada interferences terutama bila ada alarm di Band 4, 3. Bila tidak ada IDLE Intereference Alarm, bisa jadi INTERNAL Intereferences, 4. AMR Half Rate, kualitasnya LEIH BAIK dari pada FULL RATE biasa. 1. beware ! that operators are , looking for TRAFFIC mostly ...not just good KPI but low traffic.
4.10 Systematic Important Timers 4.10.1 T3101
I. Definition T3101 is the BSC timer controlling time of immediate assignment process.
II. Format T3101 ranges from 0 to 255s. The recommended value is 3s.
III. Configuration and Influence In an immediate assignment process, the BSC requires BTS to provide SDCCH to set up signaling channel. When the BSC sends a channel activation message, T3101 starts timing. When the BSC receives the setup instruction sent by BTS, T3101 stops timing. When T3101 expires, the system releases corresponding SDCCH resources. Proper configuration of T3101 reduces congestion due to dual assignment SDCCH effectively. The greater the T3101 is, the longer the inefficient time for using signaling resources is. For example, if the extended transmission delay is improperly configured (usually the sum of T and S is over small), the MS fails in responding to the network side, so the MS resends the random access request message. Therefore, the network side will assign SDCCH (the network cannot distinguish the repeated sending access request from the first send). For better use of signaling resources, especially in activating queue function, you must configure T3101 to a smaller value. The minimum interval for sending channel activation message and receiving setup indicator is 600ms. For non-overload BSS, the maximum interval is 1.8s. 4.10.2 T3103
I. Definition In inter- and intra-BSS handover, the BSC determines the time for keeping TCH both in handover-originated cell and target cell. When
the time receives handover completion (intra-BSC) or clearing (inter-BSC) message, T3103 stops.
II. Format T3103 ranges from 0 to 255s. The recommended value is 5s.
III. Configuration and Influence The following paragraph is an example of inter-BSS handover. When T3103 receives the handover command, it is reset and starts timing. When it receives clearing command, it is reset. This means that T3103 reserves two channels when it is timing, one channel for source BSC, and one channel for target BSC. If it is over long, two channels are occupied for a long time and resources might be wasted. According to the tests, if the NSS timer is properly configured, the handover process occurs within 5s. Therefore, the recommended value is 5s. 4.10.3 T3105
I. Definition See the protocol 0408 and 0858. When sending physical information, the network starts T3105. If the timer expires before receiving any correct frames from MS, the network resends physical information and restarts the T3105. The maximum repeated times is Ny1.
II. Format T3105 ranges from 0 to 255, with unit of 10ms.
III. Configuration and Influence The physical information is sent on FACCH. The time for sending four TDMA in a time on FACCH is about 18ms. If the next physical information is just sent 18ms after the first one, probably the first physical information is still being sent. The minimum time for sending physical information continuously and most quickly is 20ms.
IV. Precautions T3105 is related to the timer NY1. If T3105 is small, configure NY1 to a greater value. If a handover trial fails and the T3105 of the target cell expires for Ny times before the original cell receives the HANDOVER FAILURE message, the target BTS sends the CONNECTION FAILURE INDICATION message to the target BSC. The counter of target BSC is renewed though MS might return to the original channel. To avoid this, the T3105 must meet the following foulard: Ny * T3105 > T3124 + delta
Wherein, delta is the time between expiration of T3124 and receiving HANDOVER FAILURE message by original BSC. 4.10.4 T3107
I. Definition T3107 is a BSC timer, restricting the time for executing TCH assignment instruction. It caters for TCH assignment of intracell handover and channel assignment of calling.
II. Format T3107 ranges form 0s to 255s. The recommended values are as follows: 10s when channel resources are enough. 5s when channel resources are limited.
III. Configuration and Influence T3107 starts after the BSC sends the ASS_CMD message to BTS. It stops after the BSC receives the ASS_CMP or ASS_FAIL message sent by BTS. If T3107 expires, the system judges that the MS disconnects to the network, so the occupied resource is released to other MSs. According to the measured statistics result of network, the channel assignment is complete within 2s. If the BSC does not receive ASS_CMP message after 2s, the assignment command fails. If the radio link is bad and some information must be resent, the process might be prolonged to 5s. To avoid premature disconnection, configure T3107 to 10s. In this way, the MS can reuse the original channel when handover or assignment fails. Therefore the call drop due to intracell handover decreases or the system service quality of re-assignment is improved (if the system supports re-assignment function). However, the channel resource might be wasted for several seconds. When the network capacity is limited, you must save the resource as possible. 4.10.5 T3109
I. Definition The BSC restricts the releasing resource of SACCH by T3109.
II. Format T3109 ranges from 3s to 34s. The recommended T3109 is as follows: T3109 = a + RdioLinktimeOut x 0.480s, a = 1s or 2s.
III. Configuration and Influence T3109 measures the time for channel releasing indicator after sending MS clearing instructions. It starts after the BSC sends DEACT_SACCH message to BTS. It stops after the BSC receives the
REL_INC message sent by BTS. When T3109 expires, the BSC sends the CLEAR REQUEST message to MSC.
IV. Precautions The sum of T3111 and T3109 must be greater than RadioLinkTimeOut. If T3109 is over small, the corresponding radio resources are re-allocated before RadioLinkTimeOut is due (radio link is not released). 4.10.6 T3111
I. Definition T3111 is a connection release delay timer, used in deactivation of delayed channel after disconnection of major signaling link. T3111 aims to spare some time for repeated disconnections. When BSC receives the REL_IND message sent by BTS, T3111 starts. For time protection, T3111 stops until expiration and the BSC sends the RF_CHAN_REL message to BTS.
II. Format T3111 ranges from 0s to 5s. The recommended value is 2s.
III. Configuration and Influence After the disconnection of major signaling link, T3111 delays the release of channels. It allows the base station to retransmit the instruction for releasing radio channels to MS within delayed time. After the base station sends a release request massage, the radio resources remain for T3111 time. If the system capacity is small, configure T3111 as short as possible. The minimum value of T3111 is 2s, over five multiples of the time for resending MS the instruction for releasing radio channel resources. A greater T3111 might be of no help, but affects congestion of SDCCH and TCH easily. 4.10.7 Parameter T3212
I. Definition In a GSM network, the causes to location updating are as follows: The MS attach. The MS detects that its location area changes. The network forces MS to update location periodically. The network controls how frequent the MS updates location, and the period for location updating is determined by the parameter T3212.
II. Format T3212 ranges from 0 to 255, with unit of 6 minutes (1/10 hour). If T3212 = 1, it means that T3212 is 6 minutes. If T3212 = 255, it
means that T3212 is 25 hours and 30 minutes. If T3212 = 0, it means that MS is not required for periodical location updating in the cell. The recommended T3212 is 240.
III. Configuration and Influence As an important means, the periodical location updating enables network to connect to MSs closely. Therefore, the short the period is, the overall service performance of the network is. Anyhow frequent periodical location updating brings two negative aspects: The signaling flow of the network increases sharply and the utilization of radio resource declines. When the period is over long, the processing capability of network elements (NE, including MSC, BSC, and BTS) is directly affected. The MS must transmit signals with greater power, so the average standby time is shortened sharply. Therefore, configure T3212 according to resource utilization in various aspects of network. T3212 is configured by equipment room operators. Its value depends on the flow and processing capability of each NE. Configure T3212 as follows: Configure T3212 to a greater value (such as 16 hours, 20 hours, or even 25 hours) in areas with heavy traffic and signaling flow. Configure T3212 to a smaller value (such as 3 hours or 6 hours) in areas with low traffic and signaling flow. Configure T3212 to 0 in areas with traffic overrunning the system capacity. To configure T3212 properly, you must permanently measure the processing capability and flow of each UE in the running network, such as: The processing capability of MSC and BSC A interface, Abis interface, and Um interface The capability of HLR and VLR If any of the previously listed NEs is overloaded, you can consider increasing T3212.
IV. Precautions T3212 cannot be over small. Otherwise, the signaling flow at each interface increases sharply and the MS (especially handset) consumes increasing power. If the T3212 is smaller than 30 minutes (excluding 0), the network will be fiercely impacted. Configuring T3212 of different cells in the same location area to the same value is recommended. In addition, the T3212 must be consistent with related parameters of switching side (smaller than the implicit detach timer at switching side). If the T3212 of different cells in the same location area is the same, in the cell reselection, the MS continues to time according the T3212 of the original cell. If the T3212 of the original and target cell
in the same location area is different, the MS uses the T3212 of the original cell modulo that of the serving cell. According to the actual tests of MS in the network, if the T3212 in the same location area is different, after the MS performs modulo algorithm based on behaviors of some users, the MS might power on normally. However, the MS fails in originating location updating, so the network identifies it as implicit detach. Now the MS powers on normally, but a user has powered off prompt appears when it is called. 4.10.8 T3122
I. Definition T3122 defines the period that the MS must wait for before the second trial calling if the first trial calling fails. It aims to avoid congestion of SDCCH due to repeated trial calling by MS and to relieve system load.
II. Format T3122 ranges from 0s to 255s. The recommended value is 10s.
III. Configuration and Influence The value of T3122 is included in the immediate assignment reject message. After the MS receives the immediate assignment reject message (no channels for signaling, A interface failure, overload of central processing unit, namely, CPU), it can send new trial calling request after T3122. T3122 aims to relieve radio signaling and voice channel resources. T3122 also help avoid systematic overload. When the CPU is overloaded, the system multiplies T3122 by a factor (determined by processorLoadSupconf) to increase T3122 through overload control. In peak load time, you can manage network access by increasing T3122. Namely, you can increase the interval between two continuous trial callings to relieve network load. 4.10.9 T3124
I. Definition T3124 is used in occupation process in asynchronous handover. It is the time for MS to receive the physical information send by network side.
II. Format Configure it to 675ms when the channel type of assigned channel for HANDOVER COMMAND message is SDCCH (+ SACCH). Configure it to 320ms in other situations.
III. Configuration and Influence When the MS sends the HANDOVER ACCESS message on the primary DCCH, T3124 starts. When the MS receives a PHYSICAL INFORMATION message, the MS stops T3124, stops sending access burst, activates the PCH in sending and receiving mode, and connects to the channel if necessary. If the assigned channel is a SDCCH (+ SACCH), you must enable MS to receive a correct PHYSICAL INFORMATION message sent by network side in any block. If T3124 expires (only in asynchronization) or the low layer link fails in the new channel before sending the HANDOVER COMPLETE message, the MS proceeds as follows: 1) Deactivate the new channel 2) Restart the original channel 3) Reconnect to TCH 4) Trigger to setup primary signaling link Then the MS sends the HANDOVER FAILURE message on the primary signaling link and return normal operation before trial handover. The parameters for returning the original channel are those before response to the HANDOVER COMMAND message (such as in encryption mode). 4.10.10 T11
I. Definition T11 is an assignment request queue timer.
II. Format T11 is determined by equipment room operators. It indicates the maximum queuing delay for assignment request.
III. Configuration and Influence When the BSC is sending the ASSIGNMENT REQUEST message, no TCHs are available. The ASSIGNMENT REQUEST message must be put to a queue and the BSC sends the QUEUING INDICATION message to MSC. Meanwhile, T11 starts timing. When the BSC sends the ASSIGNMENT COMPLETE message (TCH is successfully assigned) or the ASSIGNMENT FAILURE message (TCH is not assigned) to MSC, T11 stops timing. If T11 expires, the corresponding ASSIGNMENT REQUEST message is removed from queue and the BSC sends a CLEAR REQUEST message with the cause of no radio resource available to MSC to clear calling. Assignment queuing helps reduce service rejection times due to congestion, so enabling it is recommended in a network. Anyhow, T11 cannot be over great and it must be configured according to customer habits. 4.10.11 T200
I. Definition T200 is important (both the MS and base station have T200) at Um interface in data link layer LAPDm. LAPDm has different channels, such as SDCCH, FACCH, and SACCH, and the transmission rate of different channel is different, so T 200 must be configured with different values. The type of the channels corresponding to T200 is the value of the T200.
II. Format Different channels corresponds different values of T200. According to the protocol, when SAPI = 0 and SAPI = 3, the T200 of corresponding data link is dependently implemented, depending on delay of synchronous processing mechanism and process in layer 1 and layer 2. Table 7-1 Value range and default of each type of T200 T200
Minimum Maximum Default
T200_SDCCH_SAPI0
50
100
60; /* = 60 * 5 ms */
T200_FACCH_Full_Rate
40
100
50; /* = 50 * 5 ms */
T200_FACCH_Half_Rate
40
100
50; /* = 50 * 5 ms */
T200_SACCH_TCH SAPI0
120
200
150; /* = 150 * 10 ms */
T200_SACCH_TCH SAPI3
120
200
150; /* = 150 * 10 ms */
T200_SACCH_SDCCH
50
100
60; /* = 60 * 10 ms */
T200_SDCCH_SAPI3
50
100
60; /* = 60 * 5 ms */
III. Configuration and Influence T200 avoids deadlock in sending data in data link layer. The data link layer changes the physical link in which error occurs easily to data link with no errors. At the two ends of the data link communication system, a confirm-to-resend mechanism is used. Namely, receiving a message by the receiver must be confirmed by the sender. If it is unknown that the message is lost, both two ends wait for messages, so the system confronts a deadlock. Therefore, T200 is used by the sender. When T200 expires, the sender judges that the receiver fails in receiving the message, so it resends the message. When the sender needs to confirm whether the receiver has received the message, T200 starts. When the sender receives the response from the receiver, T200 stops. When T200 expires, the resending mechanism starts. If the sender receives no response from the receiver after multiple resendings, it sends ERROR INDICATION (T200 expiration) to layer 3.
IV. Precautions T200 must be properly configured to ensure a predictable behavior at Um interface. The rules for configuring T200 include: The potentially-existing lost frames in radio link must be detected as possible. Necessary retransmission of frames must start at the earliest possible moment. If the response is delayed due to UE failure, the T200 cannot expire before receiving and processing the next frame from the opposite end. If T200 expires and no other frames are sent by preference, the related frames must be resent in the message block. T 200 starts immediately after next PH-READY-TO-SEND.
4.10.12 N200
I. Definition N200 is the resending times after expiration of T200.
II. Format To configure N200, follow rules below: 1) When SAPI = 0 or 3, N200 depends on the state and the channel used. When multiframe operation is set up, it ensures a common time value for layer 2 link failure in all channels. For layer 2 link establishment and release, configure N200 to 5. 2) In timer recovery state, configure N200 as below: 5 (SACCH) 23 (SDCCH) 34 (FACCH of full rate) 29 (FACCH of half rate) 3) When SAPI is unequal to 0 or 3, configure N200 to 5, as shown in Table 1-6.
Table 7-2 Situations of SAPI unequal to 0 or 3 Minimum Valid response Maximum resending delay resending delay delay Tresp Trmin Trmax Note 3 0 SDCCH MS: 11 51 51 BSS: 32 0 FACCH/Full rate 9 26 39 0 FACCH/Half rate 10 34 44 3 SDCCH MS: 11 51 51 Note 1 BSS: 32 3 SACCH(with TCH) 25/129 Note 2 312 416 Note 2 The TDMA frame is the measurement unit of values in this table, equal to 120/26ms (approximately 4.615ms) Note 1: It caters for the process without SAPI 0 transmission. Otherwise, it does not have a upper limit due to the priority of SAPI 0 transmission. SAPI
Channel
Note 2: You can configure it to a greater value only when PCH is unavailable due to SAPI frame transmission if SAPI = 3. Note 3: It caters only for sending monitoring frames that are available and without F equal to 1.
III. Configuration and Influence If the BSC fails in receiving lay 2 response message after multiple resending, it sends the ERROR INDICATION message (T200 expires) to layer 3. The BSC takes statistics of ERROR INDICATION message by corresponding traffic measurement counter. When T200 or N200 is configured to an over small value, call drop occurs probably due to ERROR INDICATION.
Introduction To GSM (Global System for Mobile Communications) Why “cellular”? Radio spectrum is very limited, that’s why we have only 10-25MHz dedicated to wireless communication. Such narrow bandwidth allows 100-400 channels of reasonable quality, which is not rational and commercially not profitable to develop network for such small number of mobile subscribers. Genius idea lead to division of the whole geographical area to relatively small cells, and each cell may reuse the same frequencies by reducing power of transmission. Each cell has its own antenna (base station), and all base stations are interconnected using microwave or cable communication.
History Once upon a time there was analog cellular communication that didn’t support encryption, compression, and ISDN compatibility; in addition each country (company) developed its own system, which was incompatible with everyone else’s in equipment and operation. So, in early 80s Europeans realized that pan-European public mobile system should be developed. The new system had to meet certain criteria: • • • • •
Good subjective speech quality Low terminal and service cost International roaming ISDN compatibility Digital
GSM Network Architecture
Figure 1. Layout of generic GSM network
GSM Network consists of three main parts: • • •
Mobile Station (MS) carried by the subscriber Base Station Subsystem (BSS) controls radio link with mobile station Network & Switching Subsystem (NSS) mobility management and switching of calls between mobile users, and between mobile and fixed network users.
Mobile Station Consists of: Mobile Equipment (ME) such as hand portable and vehicle mounted unit Subscriber Identity Module (SIM), which contains the entire customer related information (identification, secret key for authentication, etc.) • •
Base Station Subsystem Consists of: Base Transceiver Station (BTS) defines a cell and is responsible for radio link protocols with the Mobile Station •
Base Station Controller (BSC) controls multiple BTSs and manages radio channel setup, and handovers. The BSC is the connection between the Mobile Station and Mobile Switching Center . •
Network and Switching Subsystems Consists of: Mobile Switching Center (MSC) is the central component of the NSS. Operates all switching functions for the mobiles within its jurisdiction. Interface between mobile and other (including fixed) network. Its functions: •
• • • • • • • •
Manages the location of mobiles Switches calls Manages Security features Controls handover between BSCs Resource management Interworks with and manages network databases Collects call billing data and sends to billing system Collects traffic statistics for performance monitoring
Network Databases – Home Location Register and Visitor Location Register together with MSC provides the call routing and roaming capabilities of GSM. •
•
•
•
•
Home Location Register (HLR) contains all the subscriber information for the purposes of call control, and location determination. There is logically one HLR per GSM network, although it may be implemented as a distributed database. Visitors Location Register (VLR) is only a temporary storage while the particular subscriber is located in the geographical area controlled by the MSC /VLR. Contains only the necessary information provision of subscribed services. Authentication Center (AuC) is a protected database that stores the security information for each subscriber (a copy of the secret key stored in each SIM). Equipment Identity Register (EIR) is a list of all valid mobile equipment on the network.
Radio Link – Physical Layer As we have mentioned above radio spectrum is very limited resource shared by all users. The method to divide up the bandwidth among as many users as possible, chosen by GSM, is a combination of Time- and Frequency-Division Multiple Access (TDMA /FDMA). FDMA divides frequency bandwidth of the (maximum) 25 MHz into 124 carrier frequencies. Each Base Station (BS) is assigned one or more carrier frequencies. Using a TDMA scheme each carrier frequency is divided in time, which forms logical channels. Time Division Multiple Access (TDMA) - the users take turns (in a round robin), each one periodically getting the entire bandwidth for a little burst of time. Frequency Division Multiple Access (FDMA) - the frequency spectrum is divided among the logical channels, with each user having exclusives possession of some frequency band.
Mobile unit can be in two modes
• •
Idle - listening Dedicated – sending/receiving data
There are two kinds of channels: •
Traffic channels (TCH) carry speech and data traffic.
Figure 2. Organization of bursts, TDMA frames, and multiframes for s peech and data The fundamental unit of time in TDMA scheme is called a burst period and it lasts 15/26 msec. Eight bust periods are grouped in one TDMA frame (120/26 msec), which forms a basic unit of logical channels. One physical channel is one burst period per TDMA frame. Traffic channels are defined as 26-frame multiframe. 26-frame multiframe lasts 120 msec (26 * 120/26). Out of 26 frames, 24 are for traffic, 1 is used for Slow Associated Control Channel (SACCH), and 1 is currently unused. Control channels used by idle mode mobiles to exchange signaling information, required changing to dedicated mode. Mobiles in dedicated mode monitor the surrounding Base Stations for handover and other information. The Control channels include: •
•
•
•
• •
Broadcast Control Channel (BCCH) serves for BS identification, broadcasts, and frequency allocations. Frequency Correction Channel (FCCH) and Synchronization Channel (SCH) – used for synchronization, and physical layer definition (time slots, burst time…) Random Access Channel (RACH) used by mobile to request access to the network. Paging Channel (PCH) used for locating the mobile user Access Grant Channel (AGCH) used to obtain a dedicated channel. (Following the request of RACH)
Speech coding The speech is analog, so in order to be transmitted over digital communication it should be digitized. The method used by GSM is Regular Pulse Excited – Linear Predictive Coder (RPE-LPC) with a Long Term Predictor loop. The main idea behind this smart-looking name is simple. Speech is divided into 20 millisecond samples; current sample may be predicted from previous samples,
that’s due to slow change of voice patterns. Predicted and real information are compared and the difference is saved. Each 20-millisecond sample is encoded using 260 bits (that requires 13 kbps). Testing let to distinguish three classes of bits out of 260, that are classified by they sensitivity to errors. The most sensitive class has CRC and together with moderate sensitivity class is encoded using ½ rate convolutional encoder of length 4 – each input bit is encoded as two bits, based on 4 previous bits. Thus we have 456 bits per 20 milliseconds sample (that requires 22.8 kbps).
Discontinuous transmission The idea is based on the fact that a person speaks less than 40% of time in normal conversation, so turning the transmitter off can save power. In order to distinguish voice and background noise, very accurate Voice Activity Detector should be used. While transmitter is off, the receiving end will hear a total silence, that’s due to digital transmission. To avoid this, comfort noise is generated trying to match the c haracteristics of background noise.
Discontinuous reception While being in idle mode mobile station has to listen only to Paging Channel, that uses almost no power.
Power Control To minimize co-channel interference and to conserve power, both the mobile and BTS operate at the lowest power level that will maintain an acceptable signal quality. Mobile decides that power level is acceptable using bit errors ratio.
Network Aspects
Figure 3. Signaling protocol structure in GSM • •
Layer 1 is the physical layer. Layer 2 is the data link layer.
•
Layer 3 is the GSM signaling protocol.
We have already seen structure used by physical layer , so we won’t expand it any more. Data layer is modified version of some protocol used in ISDN and in Signaling System Number 7. So the only interesting thing that is left for us is Layer 3 - GSM signaling protocol. Layer 3 is itself divided into three sub-layers. • • •
Radio Resource Management Mobility Management Connection Management
Radio Resource Management (RR-Layer) The RR-Layer is concerned with the management of RR-session, which is the time that a mobile is in dedicated mode, as well as the configuration of radio channels. In addition RR-Layer manages power control, discontinues transmission and reception, and handovers.
Handover (handoff) is switching of an on-going call to a different channel or cell. There are four types of handovers • • •
•
Switching channels in the same cell. Switching cells under control of the same Base Station Controller ( BSC) Switching cells under the control of different BSCs, but belonging to the same Mobil service Switching Center (MSC) Switching cells under control of different MSCs.
The first two types of handover, called internal because they involve only BSC, and MSC is notified only on completion of the handover. The last two types of handover, called external because they involve MSC.
Handover may be initiated by MSC (traffic balancing) or by mobile unit. The mobile unit always scans Broadcast Control Channel of up to 16 neighboring cells, and forms a list of the six best candidates for possible handover. This information is transmitted to current Base Station at least once per second. BSC and MSC use this information for handover algorithm.
One of the problems while making handover decision is whether the poor signal quality is due to physical interference or mobile having moved to another cell. There are two basic algorithms for making handover decision: •
•
Minimum acceptable performance. If signal degrades beyond some point, then transmission power is increased. If power increase does not lead to improve then handover is performed. Disadvantages: increasing transmission power may cause interference with neighbor cell. Power budget. Uses handover to improve transmission quality in the same or lower power level. This method avoids neighbor cell interference, but is quite complicated.
Mobility Management (MM-Layer) Manages problem that arise from mobility of the subscriber. The ideal situation is when system always knows where the subscriber is located (what cell) in each moment. But this will cause the subscriber to update the system on every move, and this means a lot of obsolete update messages, wasting bandwidth. Another extreme situation is when system never knows subscriber’s position, but this will cause the system to look for the user over the whole geographical area, that means a lot of paging messages on every terminating call. Strategy used by GSM is as following, group of neighbor cells is grouped in one location area and subscriber updates its position when moving from one location area to another. Paging is done only in the current location area. The only question is "what division of cells to location areas is optimal?". There are various algorithm for solving this problem, they are mostly based statistical data.
Figure 4. Registering to Mobile Switching Center ( MSC). As seen form Figure 4, when a subscriber registers to MSC it sends registration message that contains subscriber’s information. MSC updates its VLR and sends a message to subscriber’s HLR. Authentication and security Since the radio medium may be accessed by anyone, authentication is used to prove that the users are who they claim to be. Each subscriber is given a secret key that is recorded in subscriber’s SIM and Authentication Center (AuC), during authentication AuC generates a random number that is sent to mobile. Using the secret key and this random number mobile produces a response using ciphering algorithm A3. The response number should be equal to the one calculated by AuC.
The same initial random number in conjunction with secret key is used to generate the ciphering key using A8 algorithm. This ciphering key together with TDMA frame number is used compute a sequence that is XORed with the sent data.
Connection Management. (CM-Layer)
Figure 5. Paging process.
An incoming mobile termination call is directed to Gateway MSC (GMSC). GMSC is basically a switch, which is able to interrogate the subscribers HLR to obtain routing information. The routing information that is returned to GMS is the Mobile Station Roaming Number (MSRN) . MSRN are related to the geographical numbering plan, and not assigned to subscribers. To obtain subscriber’s MSRN, subscriber’s HLR have to query subscriber’s current VLR.
Tantang BLACKBERRY
If your BlackBerry cannot send or receive email If your BlackBerry cannot send or receive email, follow the steps below in the order given to determine the problem.
Check the signal strength To make sure that you are in a place with a strong enough signal for BlackBerry communication, click the Options icon, and then click Status . For some models, the path is Settings , then Options , and then Status . In the "Signal" field, verify that the value is above -100Bm. Note the "-"; this is a negative number. If the signal is weaker than -100Bm (i.e., the figure you see is bigger [105Bm or 110Bm]), the signal strength in your area is too weak for normal functions. If your signal is stronger than -100Bm (with a smaller number, i.e., -90Bm), continue troubleshooting.
Verify that you are in a coverage area where email transmission is supported
For AT&T The network status indicator for AT&T should be "EDGE" or "3G".
The Bold on the left shows a 3G signal indicator in the upper right corner of its screen; the Curve 8900 on the right shows an "EDGE". Here is a closeup of the screen for a Bold on a 3G network:
Note the uppercase letters on both. If the "G" or "EDGE" is not uppercase, this indicates that the devices are in range of an area with limited coverage. Phone calls and texting will work, but data like web browsing and BES mail delivery will most likely not.
For more, including a breakdown on what an AT&T customer can and cannot do depending on the indicated signal, see AT&T's Check the Wireless Network Availability for the RIM BlackBerry Bold 9000 using Handheld Software Version 4.6. The 3G network is faster; Indianapolis and Bloomington users should see that level of service. Older models that do not support EDGE or 3G will most likely have a "GPRS" indicator; "GPRS" should be capitalized. Following is a table of network status indicators for those models:
For Verizon The network status indicator for Verizon should be "1X" or "1XEV". The "X" or "XEV" should be upper case, and the same size on the screen as the "1":
Following is a table of network status indicators for Verizon:
Regardless of whether you use AT&T, Verizon, or any other carrier, if your network status indicates email is not supported in your area, you must move to a location where email is
supported. If your network status indicates email support, but you still cannot send or receive email, continue troubleshooting.
Check the date and time On most models, click the Options icon, and then click Date/Time . If the date and time are incorrect, fix them, and then test to see if you can send or receive email. If the date and time are correct, or if you have problems after fixing them, continue troubleshooting.
Restart To restart your BlackBerry, see How do I restart my BlackBerry? Once the BlackBerry has restarted, test to see whether you can now send and receive email. If you cannot, continue troubleshooting.
Check network settings To check the network settings on most models: Network or Network Settings. 1. Click the Options icon, and then click
2. In the "Radio" field, verify that the value is set to On. 3. In the "Roaming" field, verify that the correct country is displayed. 4. Verify that the "Status" is active, and then click the trackwheel. A menu will appear. 5. Click Register Now and verify that the registration request is sent. If you are unable to register the handheld or connect to the network, or do not have the correct options displayed in the "Roaming" or "Status" fields, contact your service provider. If these settings are all correct, and you are able to register your BlackBerry, continue troubleshooting.
Disconnect from the BlackBerry network Remove the device battery for about five minutes to ensure a complete disconnect from the BlackBerry network. This causes a hard reset, forcing the network to register the device as a new device on the network. This is a common fix for coverage areas in which you normally have a connection and can send and receive email, but are now having trouble (see the chartsabove to determine coverage areas).
Check to see if redirection is enabled
From the Desktop Manager Software Open Desktop Manager and double-click Redirector Settings . Make sure Redirect incoming messages to your handheld is checked.
From the BlackBerry handheld 1. Open the messages screen and select Options . 2. Choose Email Settings . 3. On the Email Settings screen, choose Folder Redirection . 4. On the Folder Redirection page, select Mailbox- (Name of User) , and choose Expand . You should see a list of folders that includes Deleted Items, Inbox, Junk E-mail, and Sent Items. For mail messages to be received, the Inbox folder must have a checkmark next to it. Sent Items is checked by default and will not affect mail delivery. For more information, see With a BlackBerry, how do I wirelessly reconcile the folders from my mailbox? If redirection is enabled and you are still unable to send or receive email, continue troubleshooting.
Check mail filters and mail redirection For general information on mail filters and mail redirection, see With the BlackBerry Desktop Manager, how do I configure filters to control mail redirection? For more information on wireless mail redirection, see With a BlackBerry, how do I wirelessly reconcile the folders from my mailbox? If your device is still not able to send or receive email, contact the Support Center or your BlackBerry provider. Few Solution for Low TCH Traffic Probable Reasons 1)Site Intended for Special Events Action: Check if the site in located in a expo,stadium etc 2)Rural Site with Low or No Traffic Action: Check site position Solution: If the site is a rural site, no activity is needed. However, the position should be reconsidered as well as the antenna direction. 3)No or too few Handover Neighbours Action: Check neighbour cell definitions Solution: Define new neighbouring cell relations. Make cell relations mutual if only one-way relations are defined. 4)Hardware Fault Action: Check TCH availability Check BTS error log Check if sleeping TS. 5)Wrong Antenna Position
Action: Conduct physical verification. 6)Corrupt Cell Data Definitions Action: Check the cell parameter setting
Handover is an important function, which shows the integrity of the GSM network. If the handover performance is poor, the subscriber will perceive the quality of the network as bad . Probable Reason of Bad Handover Performance Bad locating parameter Bad setting might cause the locating will not rank the best cell settings as a candidate Uplink interference Incoming handover failed as the target cell could not decode the handover burst message from the mobile Missing measurement Prevent any handover to affected neighbor cells frequency in BA-list Extra measurement Can provide inaccuracy of measurement for handover decisions frequency Co-Channel and Co-BSIC Measurement result from neighbors can not be distinguished and MS may perform a handover to the wrong cell. Unnecessary neighbor None or few handover might indicate a unsuitable neighbor relation relation. HW faults Can cause bad neighbor relation. Permitted NCC (network Missing NCC of the neighbors will not allow any han dover. color code) Wrong use of HCS Cause unnecessary handover parameters Congestion High congestion might lead to dragged calls (handover performed at a not intended location) a lot of unsuccessful handover.
The following procedure should be performed for handover analysis: - Check the successful handover per cell - Check for the handover success rate below certain criteria for example X%. If it doesn’t meet the criteria and change the X% to a higher value and check for the next cell. - Check the handover activity from the number of handover performed. If the number is above certain value for example Y and then take into consideration. - Check the site location, whether the site to site distance or co-sited. (Good map is needed here). - Check whether the cell is isolated or not, if already known problem of no dominant serving cell and take other cell. - Check the handover flow whether is balance between incoming and outgoing.
- Check the difference performance of incoming and outgoing handover. This is to ensure the priority for more problem direction. - Focus on the bad direction. - Check the worst relation and pick one cell. - Check the cell whether it is external or internal. If external, start inter-BSC analysis. - If the cell is internal, check the high ratio of lost handover, urgency handover and reversion. - If one of them is yes, check the uplink & downlink interference problem. - Check the frequency plan. There might be problem on co-channel or adjacent channel. - Check the handover related parameters such as relation, BA-list, BSIC, hysteresis, offsets etc. - Check if many Ping-Pong handover. - Check if assignment handover are used. - Check if cell has HW problem. If problem found, swap or repair hardware. - Check if congested target cell. If the target cell is congested and then solve the congestion by adding TRU. - Perform measures to improve HO performance. This is the assumption comes from the above analysis. No or Few Handover Attempts The following procedure should be performed for no or few handover attempts: - Check the neighbor relation with low traffics (less than 10% of average number of handover per relation) and unbalance relations. - Check from the unbalance relation whether the target cell is congested, if congested, solves the congestion problem. - Check the missing measurement frequency (MBCCHNO) in Double BA-list in active list (if yes, add the measurement frequency in the BA-list. - Check unnecessary neighbor cell relations and if found, remove unnecessary relations. - Check NCCPERM, if NCC of the neighbor is missing, add the NCC of the neighbor list into the NCCPERM list. - Check the setting of the cells whether they are set in different layer of HCS. (Layer 1 is the highest priority) - Check if the cell is at the inter BSC border, if so , check the inter BSC HO performance (SIEMENS System).
