BSSPAR-Idle Mode Operation

BSSPAR

Idle Mode Operation Training Document BSSPAR CTXX 03

6-90250 v 1.0

© Nokia Oyj

1 (27)

BSSPAR CTXX 03

The information in this document is subject to change without notice and describes only the product defined in the introduction of this documentation. This document is intended for the use of Nokia Networks' customers only for the purposes of the agreement under which the document is submitted, and no part of it may be reproduced or transmitted in any form or means without the prior written permission of Nokia Networks. The document has been prepared to be used by professional and properly trained personnel, and the customer assumes full responsibility when using it. Nokia Networks welcomes customer comments as part of the process of continuous development and improvement of the documentation. The information or statements given in this document concerning the suitability, capacity, or performance of the mentioned hardware or software products cannot be considered binding but shall be defined in the agreement made between Nokia Networks and the customer. However, Nokia Networks has made all reasonable efforts to ensure that the instructions contained in the document are adequate and free of material errors and omissions. Nokia Networks will, if necessary, explain issues which may not be covered by the document. Nokia Networks' liability for any errors in the document is limited to the documentary correction of errors. Nokia Networks WILL NOT BE RESPONSIBLE IN ANY EVENT FOR ERRORS IN THIS DOCUMENT OR FOR ANY DAMAGES, INCIDENTAL OR CONSEQUENTIAL (INCLUDING MONETARY LOSSES), that might arise from the use of this document or the information in it. This document and the product it describes are considered protected by copyright according to the applicable laws. NOKIA logo is a registered trademark of Nokia Corporation. Other product names mentioned in this document may be trademarks of their respective companies, and they are mentioned for identification purposes only. Copyright © Nokia Oyj 2003. All rights reserved.

6-90250 v 1.0

© Nokia Networks Oy

2 (27)

Idle Mode Operation

Contents

1 Module Objectives................................................................................4 2 Idle Mode Control.................................................................................5 3 PLMN Selection....................................................................................8 3.1 PLMN Access Parameters...................................................................9 3.2 Cell and Location Area Parameters...................................................10 3.3 Measurement Reporting ...................................................................12 4 Cell Selection & Reselection.............................................................14 4.1 C1 Criterion 15 4.2 C2 Criterion 16 4.3 C31 & C32 Criteria............................................................................19 5 Location Updates...............................................................................20 5.1 IMSI Attach & Detach........................................................................22 6 Key Learning Points...........................................................................24 7 Review Questions ..............................................................................26

6-90250 v 1.0

© Nokia Oyj

3 (27)

BSSPAR CTXX 03

1

Module Objectives At the end of the module, the participant will be able to:

6-90250 v 1.0



List the functions of the MS during the idle mode



Explain the parameters used for PLMN and cell selection



State the purpose of location updates and the associated parameters


4 (27)

Idle Mode Operation

2

Idle Mode Control Because of the limited radio spectrum that is available, each MS cannot be allocated a dedicated traffic channel (TCH) at all times. Traffic channels are only allocated to the user when the need arises. This leads to the distinction between two states that the mobile can take, namely Idle Mode and Dedicated Mode. In the idle mode, an MS does not have a dedicated traffic channel (TCH). It listens to and transmits on the broadcast channels. All requests for transmission resources are made while in idle mode. Once the resources are allocated for full duplex point-to-point communication, the mobile moves from idle mode to dedicated mode.

Idle Mode Tasks

When?

• PLMN selection • W hen the MS is switched O N • C ell selection & re-selection • W hen there is no dedicated connection • Location updates Why? • To camp on the best suitable cell Why to camp on a specific cell? • F or MS to receive system info from the NW on DL • F or MS to be able to initiate a call whenever needed • F or the NW to be able to locate the MS when there is a MT call/S MS Figure 1. Idle mode operation

During the idle mode, the MS has to perform a number of functions: (a) During Power On it has to check if it can access a PLMN and cell. (b) It has to stay in contact with the BTS by decoding the BSIC and listening to System Information messages. (c) Listening to BTS transmission for paging messages to see if it being called. (d) Monitor the status of radio links from various BTS and choose the most appropriate BTS. (e) Listen for cell broadcast short messages.

6-90250 v 1.0

© Nokia Oyj

5 (27)

BSSPAR CTXX 03

Parameter

Value

locationAreaId • mcc (Mobile Country Code) • mnc (Mobile Network Code) • lac (L ocation Area Code)

0 … 999 0 … 99 0 … 65535

bsIdentityCode • ncc (Network Colour Code) • bcc (BTS Colour Code)

0… 7 0… 7

cell-ID

0 … 65535

trainingSequenceCode

0… 7

Cell Global Identity

MCC + M NC + L AC + CI

Figure 2. ID’s and ID codes

When the mobile station is in idle mode, it needs some information about the GSM network in order to know the right frequencies and find the right cells. This information pertains to Radio Resource Management and to Mobility Management and includes:

6-90250 v 1.0

•

The access parameters to the PLMN

•

Location Area Identification (LAI), which is composed of the Mobile Network Code (MNC), the Mobile Country Code (MCC) and Location Area Code (LAC). LAI = MCC + MNC + LAC

•

Cell Identification Code

•

Base Station Identity Code (BSIC) including Network Colour Code (NCC) and Base Station Colour Code (BCC).


6 (27)

Idle Mode Operation

• BSIC is a combination of NCC and BCC •Reported in Measurement Results to BSC •Can be listed in Hex or Decimal

4 32 0 0 0 0 1 1 1 1

NCC (0...7) 2 16 0 0 1 1 0 0 1 1

U 8 0 1 0 1 0 1 0 1

4 4 x x x x x x x x

BCC (0...7) 2 2 x x x x x x x x

U U x x x x x x x x

Range 0-7 8 - 15 16 - 23 24 - 31 32 - 39 40 - 47 48 - 55 56 - 63

Figure 3. Base Station Identity Code (BSIC)

Once the base station is identified within the network, the initial frequency must be set for each TRX. The basic idea in the GSM system is that the mobile is always within the cell offering the best coverage and that the system knows where the MS is located. In idle mode, mobility is ensured by setting the following criteria and timers:

6-90250 v 1.0

•

Cell selection criteria based on C1 or C2 comparison

•

Location Update

•

IMSI attach/detach

© Nokia Oyj

7 (27)

BSSPAR CTXX 03

3

PLMN Selection When a mobile station is switched on, it tries to locate a GSM network. If the MS is in the home country, it naturally tries to find the home network. If there is coverage, the MS camps on the home network. If there is no coverage, the MS attempts to find other networks of competitive operators and this is called national roaming. Usually this is not possible because different operators are in competition with each other. Then the only possibility of finding a network in the home country is to find the home network. When the MS is abroad, international roaming is usually possible. The Mobile can select any operator offering GSM service in the foreign country, with which the operator of the home network has a roaming agreement. The issue is how the MS selects the network in a foreign country. There are 3 possibilities: 1. The home operator can make a list of preferred operators in different countries. This list of preferred networks is not used. 2. The MS just selects the network with the best field strength level found where the mobile is switched on. The Mobile camps on the network selected and stays on it as long as service (coverage) is available. Usually, the selection is made based on the field strength level only. 3. Home operator can give a list of forbidden networks. The PLMN selection criteria mentioned above are chosen by the operator and they cannot be affected with the parameters which will be discussed next. An operator can control PLMN selection in a number of ways: a) Specify a class of MS that is allowed to access a particular cell. b) Specify the minimum received level signal strength required by the MS. c) Bar some cells from user access except for emergency calls.

6-90250 v 1.0


8 (27)

Idle Mode Operation

• Two Modes • Automatic - utilises a list of P LMNs in priority order. Highest priority (available and allowable) P LMN is selected • Manual - MS indicates to user which P LMNs are available • Three Types of Lists • P referred • F ound • F orbidden Parameter

Value

plmnpermitted

0 ... 7

NOTEThis parameter is confusing. It’s not used network for selection. It’s used A FTER a network has been selected so that measurements from only one PL M N are reported

Figure 4. PLMN Selection

3.1

PLMN Access Parameters A number of parameters are needed to control which MS is allowed to access a PLMN, BTS or Cell. The parameter notAllowedAccessClasses (ACC)(BTS)(0..9, 11..15) tells which mobile user classes can not use a particular cell. This allows the operator to divide the subscriber database into different Access Control Classes and hence gives the operator some control over the existing load and access to priority users in congested cells. The plmn-permitted (PLMN)(BTS)(0..7) parameter is broadcast on the BCCH and is used by the mobile to report measurements of that PLMN. This parameter is used after the network selection has been done. The BSIC (Base Station Identity Code) is broadcast on the SCH, so when the mobile presynchronises, it knows if the BTS belongs to the right PLMN or not (BSIC is screened by plmn-permitted (PLMN)(BTS)). The parameter plmn-permitted (PLMN)(BTS)(0..7), doesn’t affect the PLMN selection, it is only used for measurements reporting.

6-90250 v 1.0

© Nokia Oyj

9 (27)

BSSPAR CTXX 03

3.2

Cell and Location Area Parameters AN MS needs information about cell identities. First, there is identity of the each cell (cell-ID) and in addition to this cell-ID more IDs, which are used for location information. Parameter locationAreaId (LAC)(BTS) includes the following: 

Mobile Network Code, mnc (0..99),



Mobile Country Code, mcc (0..999) and

 Location Area Code, lac (0..65535) which describes each location area as shown in Figure 5.

LocationAreaCode MCC =Countrye.gFinland MNC =Operator LAC 1=Helsinki

LAC 2

LAC 3

LAC 4

BSC

BTS BTS 1

2

BTS n

Figure 5. Description of a location area.

There is other information used for Radio Channel Management purposes. Some information is needed in order to separate co-channels used in different Base Stations in the vicinity as shown in Figure 6. Parameter baseStationIdentityCode (BCC)(BTS) includes 

Network Colour Code, ncc (0..7) and



Base Station Colour Code, bcc (0..7)

It is possible to set these parameters into the Background Database, to minimise downtime.

6-90250 v 1.0


10 (27)

Idle Mode Operation

B a s eS ta tio n Co lo u rCo d e

f1

b cc=2

f2 b cc=1

f1 f3 f1 b cc=3

Figure 6. Base Station Colour Code.

Parameter initialFrequency bCC HAllocation-ID bCC HAllocationL ist idleStateBCCHAllocation

Value 1..124 & 975..1023, 0; 512..885 GSM900;1800 (FREQ TRX)

1 ... 255 1 ... 124 in GSM (max. freq 32. for all bands) 0 (BCC H list is taken from the adjacent cell) 1 ... 255 (number of the BC CH list used)

measurementBCCHAllocationADJ (BCC H frequency list taken from adj. cell) IDL E (active MS uses same list as M S in IDL E mode)

Figure 7. Frequencies

6-90250 v 1.0

© Nokia Oyj

11 (27)

BSSPAR CTXX 03

3.3

Measurement Reporting After the MS accesses one network, it reports the measurements of the network to the BTS on which it is camped. However, there are also some other requirements to access one cell. Having coverage might not be enough to access some particular cells. The parameter RxLevAccessMin (RXP)(BTS)(-110..-47 dBm) describes the minimum value of received field strength required by the MS to get any service from the network in that cell in idle mode. However, there are still some cases, even if there is good field strength where, the operator may want to make some tests to keep a cell out of use. For this kind of purposes the cell can be changed to barred state by using the parameter cellBarred (BAR)(BTS)(Yes/No) parameter. An example of using cell barring for test measurements is given in Figure 8. Cell Barred GPS-satelite

Existing Layer •Barred, No New Microcell Layer •Barred, Yes

NMS/X 5.24

•Easy to test new Microcells with NMS/X 5.24 with Nokia 2110 Mobile Figure 8. Use of Cell Barring for test measurements.

NOTE: All adjacent cells have to be also barred. A Mobile can not use any cell for call establishment, which is in barred state. An exception to a barred cell could be implemented using parameter emergencyCallRestricted (EC)(BTS)(Yes/No). This parameter tells if the mobile has right to use the network for emergency calls even if it has no right to use the network for normal calls. Only for MS classes 11 to 15. The network also broadcasts (on the BCCH) some parameters related partly to network planning to mobiles. When the mobile is moving in Idle mode it has to know which is the best cell offering service in each area.

6-90250 v 1.0


12 (27)

Idle Mode Operation

CellReselectHysteresis (HYS)(BTS)(0..14 dB) is a parameter that the mobile uses as a margin in the comparison of the field strength levels of the adjacent cells in different Location Areas in Idle mode. This margin prevents ping-pong location updates, which will result in a waste of SDCCH capacity. The other parameter which is actually directly related to frequency planning is msTxPwrMaxCCH (TXP)(BTS)(13..43 dBm) which tells the mobile the maximum transmitting power when accessing the system.

6-90250 v 1.0

© Nokia Oyj

13 (27)

BSSPAR CTXX 03

4

Cell Selection & Reselection One basic idea in the GSM system is that the Mobile Station is always within the cell offering the best coverage. In Dedicated mode this is handled by handovers, but in Idle mode the concept of handover does not exist because the MS does not have a dedicated channel. Mobile has to find the best cell in each area. There is a process for this purpose called Cell Selection, based on C1 or C2 comparison. The idea is that the Mobile compares field strength levels coming from different BTSs and selects the best BTS. The mobile uses the parameter CellReselectHysteresis (HYS)(BTS)(0..14 dB) parameter between cells that belong to different Location Areas in order to avoid the "Ping-Pong" phenomenon. This means that before the mobile changes to a different cell in Idle mode, between different location areas, the field strength level of the new cell has to be at least the value of CellReselectHysteresis (HYS)(BTS) better than the value of the serving cell. There is no margin between the cells that belong to the same Location Area. The equation for the cell selection is presented next.

Two methods for selecting a: cell • Normal cell selection (a)- M S has no prior knowledge of which R F channels are BC C H • S tored list cell selection - optional (b)- MS uses list of BC C H carriers used by P LMN ➾ If no suitable cell is found using method (b)then(a) is tried. What constitutes a 'suitable' cell: • C ell is in the selected P LM N • C ell is not barred • It is not in a forbidden location area for national roaming • C 1 >0 (range -99..0..+99) • If there is no normal priority cell then low priority cell Figure 9. Cell Selection in Idle mode

6-90250 v 1.0


14 (27)

Idle Mode Operation

4.1

C1 Criterion The C1 criterion has been designed to maximise transmission quality. It takes into account the following  L = Average Level of the signal received by the mobile station on the beacon frequency.  p1 = RxLevelAccessMin (RXP)(BTS)(-110 … -47 dBm) discussed earlier. 

M = Maximum RF power of the MS based on the MS specifications.



p2= MxTxPwrMaxCCH(TXP)(BTS)(13 … 43 dBm) discusses earlier

C1 is defined as follows C1 = (A – Max (B, 0)) Where A = L – p1 And B= p2 – M When looking for cells (neighbouring or during Power On), only cells of positive C1 computed from the broadcast parameters p1 and p2 values are taken into account. The cell with the best C1 is chosen. Thus the C1 criteria is used to determine the coverage limit of each cell and the boundary between adjacent cells. A practical example of C1 radio criteria is shown in Figure 10 • R adio C riteria C 1 = (A - Max(B,0)) • A = R eceived Level Average - p1 • B = p2 - M aximum R F O utput P ower of the Mobile S tation • p1 =rxLevelAccessMin (Min. received level at the M S required for access to the system) (R XP BTS..-105 -110 … -47dBm) • p2 =msTxPowerMaxC CH (Max. Tx power level an M S may use when accessing the system) (TXP BTS G S M33..43/2 900 5.. 33; G S M1800 0..30/2 30dBm) Parameter

Value

rxL evelAccessMin

-110 ... -47 dBm 'RXP' (BTS)

msTxPowerM axCC H

0 … 30dBmGSM 1800 (5 … 43 dBmGSM 900) 'T XP' (BTS)

Figure 10. Cell (Re-) Selection in Idle mode Using C1 criterion

6-90250 v 1.0

© Nokia Oyj

15 (27)

BSSPAR CTXX 03

MS Moving LA2

LA1 A = 4 dB

1

B = 6 dB

A

C = 8 dB 1 •

2 •

A B C

Parameter

BC

2

Value

cellReselectHysteresis

0 … 14 dB

'HY S' (BTS) 2dB steps def MML 4dB

Figure 11. Cell selection based on C1 in practice.

(There is a margin only between cells that belong to different location areas.)

4.2

C2 Criterion The criterion C1 is used in Phase 1. C2 is used in GSM phase 2 with more features, for use with two-layer microcell / macrocell architecture. C2 uses more parameters: • MS will calculate the C 1 and C 2 for the serving cell, every 5 s • MS will calculate the C 1 and C 2 for the six best neighbour cells, every 5 s

Cell re-selection is needed if • P ath Loss criterion C 1 < 0 for cell camped on ,for more than 5 seconds. • There is DL signalling failure • The cell camped on has become barred. • There is a better cell in terms of C 2 criterion • A random access attempt is still unsuccessful maxNumberRetransmission after repetitions.

6-90250 v 1.0


16 (27)

Idle Mode Operation

Figure 12. Cell Re-selection in Idle Mode

Using C2 Re-Selection Criterion (PH 2 MS) The parameter cellReselectParamInd (PI)(BTS)(Yes/No) becomes activated if C2 parameters are sent to the Mobile (activates C2), and the parameter cellBarQualify (QUA)(BTS)(Yes/No) controls whether the cell barring can be overridden.

C1,C2

microcell

C1=C2

macrocell

C2
time

BCCH

fast moving mobile BCCH

slow moving mobile

Figure 13. Cell Re-selection with C2

The rest of the C2 parameters are related to microcellular planning. Parameter penaltyTime (PET)(BTS)(20..640 s) describes the time delay before the final comparison is made between two cells. Parameter temporaryOffset (TEO) (BTS)(0..70 dB) describes how much field strength could have been dropped during this penalty time, and parameter cellReselectOffset (REO)(BTS)(0..126 dB) describes an offset to cell reselection. C2 cell reselection is calculated by the following equation:

6-90250 v 1.0

© Nokia Oyj

17 (27)

BSSPAR CTXX 03

C2 =

C 1 + cellReselectOffset - temporaryOffset*H(penaltyTime-T) <= penaltyTime < 640 C 1 - cellReselectOffset … … … … … … … … … … … … … … … <=…penaltyTime .. = 640 H(x) =

1 when T < = penaltyTime 0 when T > penaltyTime

Parameter

Value

cellReselectParamInd cellReselectOffset penaltyTime temporaryOffset

Y /N 0 ... 126 dB 20 ... 640 sec 0 ... 70 dB

Figure 14. Cell Re-selection with C2 Equation & Parameters

The network consists of two cellular layers: GSM macro layer and microcellular layer. In order to prevent unnecessary camping between layers, C2 will be introduced. The idea is: the micro cell, having good DL signal strength and therefore very attractive, has to belong to one of the best cells of the neighbour list for the time set as penaltyTime (PET)(BTS), say 20 sec, in order to allow the MS to camp on that micro cell. The parameter temporaryOffset (TEO)(BTS) has been set to be 30 dB and cellReselectOffset (REO)(BTS) has been set to 20 dB. Let’s assume that C1 of both serving cell and the neighbour cell have been measured as 32. Therefore two alternative cases are possible: M icrocell 900

C 1 "A"=30

GSM900 GSM1800GSM900micro 20 0

cellReselectOffset 0

C 1 "B"=25 penaltyTime C 1 "C "=5 C 1 "D"=50 temporaryOffset

0

20

40

0

30

30

C2 = C 1 + cellReselectOffset - temporaryOffset*H(penaltyTime-T) C 2 "A"=30 + 0 serving cell H(x) = 0 C 2 "B"=25 +20-30 * H (20-T) M acrocell 1800

C 2 "C "=5 + 0 - 0 * H(0-T ) C 2 "D"=50+0-30*H (40-T)

M acrocell 900

Time T: between 0 and 19 seconds Time T: between 20 and 39 seconds Time T: greater than 40 seconds C 2 "A"=30

C 2 "A"=30

C 2 "A"=30

C 2 "B"=15

C 2 "B"=45

C 2 "B"=45

C 2 "C "=5

C 2 "C "=5

C 2 "C "=5

C 2 "D"=20

C 2 "D"=20

C 2 "D"=50

Figure 15. Cell Re-selection with C2 – Example

6-90250 v 1.0


18 (27)

Idle Mode Operation

During time 0..19 sec (within the set penaltyTime): C2 =C1+cellReselectOffset-temporaryOffset * H(penaltyTime-T) C2 =32+20 -30*1 C2=22 => C2 < C1, so MS will be kept in macro layer i.e. target cell (micro cell) is NOT attractive. 1.

During time 20..∞ (penaltyTime over):

C2=C1 +cellReselectOffset-temporaryOffset*H(penaltyTime-T) C2=32+20-30*0 C2=52 =>C2 > C1, now target cell is very attractive and the idle mode MS will camp on the microcell. If the C2 > C1 before the penalty time is over, the cell reselection will be done immediately. If the C2 = C1 before the penalty time is over, the cell reselection will not be done until the penalty time is expired. Note that C2 is just meant for idle mode.

4.3

C31 & C32 Criteria There are other criteria C31 and C32, which are used in GPRS. For further information, see the module BSSPAR: GPRS & EGPRS (EDGE).

6-90250 v 1.0

© Nokia Oyj

19 (27)

BSSPAR CTXX 03

5

Location Updates A subscriber keeps moving from one place to another and the network must be able to locate it for incoming calls. The transaction that enables the network to keep track of the subscriber is called a Location Update. The mobile phone constantly receives information sent by the network. This information includes the VLR area in which the mobile is currently located. In order to keep track of its location, the mobile stores the ID of the area in which it is currently registered. Every time the network broadcasts the ID of the area, the mobile compares this information to the area ID stored in its memory. When the two IDs are no longer the same, the mobile sends the network a request, i.e. a registration inquiry to the area it has just entered. The network receives the request and registers the mobile in the new VLR area. Simultaneously, the subscriber’s HLR is informed about the new VLR location and the data concerning the subscriber is cleared from the previous VLR as shown below. Figure 16. Elements involved in location update

MS

BSS

MSC

H L R

VLR

HLR

LOCATION UPDATE REQUEST

REQUEST SUBSCRIBER ID

SEND SUBSCRIBER ID

V L R

M (S o C l SEND SUBSCRIBER INFO d )

REQUEST SUBSCRIBER INFO

AUTHENTICATION

AUTHENTICATION RESPONSE

M o b ALL OK - HLR UPDATE

Figure 17 Location update procedures The VLR area is commonly referred to as a location area. As defined in GSM specifications, it is the smallest area, into which a terminating call towards a mobile subscriber will be paged. Also a location area is the area in which a mobile needs not to update its location with its home location register. Location update is performed in idle mode when the mobile is roaming into a cell having a different location area code (LAC). In connected mode, the mobile will update its location with the network as soon as it becomes idle again, i.e. after call completion. While handover boundaries affect only mobiles that are in “connected mode”, location area boundaries affect all mobiles in the network, including the (many) idle mobiles. Location updating causes signalling and processing load across the entire network hierarchy up to the mobile’s HLR. In case of foreign roaming mobiles

6-90250 v 1.0


20 (27)

Idle Mode Operation

(tourists), this is often even international signalling traffic. Therefore planning of location area boundaries should be considered with some thought, such as to avoid “oscillating” location updates along a heavily frequented road. Furthermore, different MSCs can not use the same LAC, otherwise the BSC will not know to which MSC the mobile belongs. In a location area, there is a trade-off between paging traffic and location updating traffic. This means that concatenating e.g. a large city into a single location area will avoid any location updating traffic, but on the other hand causes a maximum in paging traffic, since every single terminating call within the area is broadcast to every single cell in the area. (Even several times per call attempt, depending on network parameters). This can cause significant traffic loads within the network. signalling traffic

function of user density, cell size, call arrival rate ...

function of user mobility

Paging

LocUp optimum number of cells in Loc. area

# of cells in Loc. area

Figure 18. Trade-off between location update and paging traffic

Location updates are carried out every time a Mobile changes its location area under one MSC, or between two different MSCs. When the location area changes between two MSCs, the HLR is updated. There are two types of location updates: automatic (random) location update and periodic location update. An automatic location update occurs when the Mobile is switched on if IMSIAttachDetach (ATT)(BTS) parameter is used. Periodic location updates carried out by the Mobile Station and is used to check that the location information in MSC/VLR is correct, because by error in the MSC/VLR, the location information of Mobile Station can disappear. Periodic location update is controlled by the timerPeriodicUpdateMS (PER)(BTS) (0.0..25.5 hours) parameter.

6-90250 v 1.0

© Nokia Oyj

21 (27)

BSSPAR CTXX 03

• MS => MS C / VLR • Mobile S tation switched O N • IMS I Attach / Detach • S ame Location Area => No Location Update • Different Location Area => Location Update • C hange of the Location Area • Location Area under the same MS C / VLR • Location Area under another MS C / VLR => HLR will be updated • S ervice is rejected (MS unknown in VLR ) • Time-P eriodic LU (MS -> MS C /VLR ) Parameter

Value

timerPeriodicUpdateMS allowIMSIAttachDetach

0.0 ... 25.5 hrs 'PER' (BTS) see note in dictionary! Yes/No 'ATT' (BTS)

Figure 19. Location Updates

5.1

IMSI Attach & Detach The IMSIAttachDetach (ATT)(BTS)(Yes/No) parameter is used to decrease signalling load. The Mobile Station sends a message to the MSC notifying that it is switching on or off. When the MSC knows that the Mobile Station is switched off it does not try to page it, and useless paging is avoided.

6-90250 v 1.0


22 (27)

Idle Mode Operation

S earch all the R F channels, take samples during 3-5 s and calculate averages. And put them in ascending order with respect to signal level. Then tune to the strongest R F channel.

S earch for the frequency correction burst in that carrier in order to verify if it is a BC C H carrier

T une to the next highest R F channel which is not tried before

No

Is it a BC C H carrier? Y es

T ry to synchronize to the carrier and read the BC C H data

No

Is it a correct P LM N ? Y es

Y es

Is the cell barred? No Is C 1>0

No

Y es C amp on the cell

Figure 20. Normal Cell Selection

6-90250 v 1.0

© Nokia Oyj

23 (27)

BSSPAR CTXX 03

6

Key Learning Points During the idle mode, the MS has to perform a number of functions: 1. cell.

During Power On it has to check if it can access a PLMN and

2. It has to stay in contact with the BTS by decoding the BSIC and listening to System Information messages. 3.

It has to listen to BTS transmission for paging messages.

4. It has to monitor the status of radio links from various BTS and choose the most appropriate BTS using C1 and C2 criteria. 5.

It has to listen for cell broadcast short messages.

The parameters used by operators to control PLMN and cell selection and cell reselection are:  notAllowedAccessClasses (ACC)(BTS)(0..9, 11..15) allows an operator to define the MS access classes that are not allowed to access a cell.  plmn-permitted (PLMN)(BTS)(0..7) which is broadcast on the BCCH and is used by the mobile to report measurements of that PLMN  rxLevAccessMin (RXP)(BTS)(-110..-47 dBm) which describes the minimum value of received field strength required by the MS to get any service from the network in that cell, in idle mode.  cellBarred (BAR)(BTS)(Yes/No) parameter to keep a cell out of use for testing purposes. An exception to a barred cell could be implemented using parameter emergencyCallRestricted (EC)(BTS)(Yes/No).  cellReselectHysteresis (HYS)(BTS)(0..14 dB) is used as a margin in the comparison of the field strength levels of the adjacent cells in different Location Areas in idle mode. This margin prevents ping-pong location updates, which will result in a waste of SDCCH capacity.  msTxPwrMaxCCH (TXP)(BTS)(13..43 dBm) tells the mobile the maximum transmitting power when accessing the system.  cellReselectParamInd (PI)(BTS)(Yes/No) becomes activated if C2 parameters are sent to the Mobile.  cellBarQualify (QUA)(BTS)(Yes/No) controls if the cell barring can be overridden or not.  penaltyTime (PET)(BTS)(20..640 s) describes the time delay before the final comparison is made between two cells.  temporaryOffset (TEO)(BTS)(0..70 dB) describes how much field strength could have been dropped during this penalty time.

6-90250 v 1.0


24 (27)

Idle Mode Operation

 cellReselectOffset (REO)(BTS)(0..126 dB) describes an offset to cell reselection. Location information is updated by the MS to the network at a regular interval. This is necessary for the paging to be carried out efficiently by the network. Each paging message is sent from every cell in a location area. Location updates are carried out every time a Mobile changes its location area under one MSC, or between two different MSCs. When the location area changes between two MSCs, the HLR is updated. There are two types of location updates: automatic location update and periodic location update. Periodic location updates are carried out by the MS to update the location information in the MSC/VLR. The interval is set by timerPeriodicUpdateMS (PER)(BTS)(0.0..25.5 hours) parameter. The IMSIAttachDetach (ATT)(BTS)(Yes/No) parameter is used to indicate that an MS is switched off. The MSC does not try to page thus avoiding unnecessary paging. An automatic location update occurs when the Mobile is switched on if IMSIAttachDetach (ATT)(BTS) parameter is used.

6-90250 v 1.0

© Nokia Oyj

25 (27)

BSSPAR CTXX 03

7

Review Questions Q1. Which of the following functions are not performed by an MS in idle mode? a)

During Power On it has to check if it can access a PLMN and cell.

b)

It has to stay in contact with the BTS.

c)

Listen to BTS transmission for paging messages.

d) Monitor the status of radio links to various BTS and choose the most appropriate BTS using C1 and C2 criteria. e)

Listen for cell broadcast short messages.

f)

Listen to RACH messages from other mobiles.

The answers to choose for questions Q2-Q5 are given below a) - e): a)

notAllowedAccessClasses (ACC)(BTS)(0..9, 11..15)

b)

plmn-permitted (PLMN)(BTS)(0..7)

c)

rxLevAccessMin (RXP)(BTS)(-110..-47 dBm)

d)

cellBarred (BAR)(BTS)(Yes/No)

e)

cellReselectHysteresis (HYS)(BTS)(0..14 dB)

Q2. Which parameter is used as a margin to prevent ping-pong location updates? Q3. Which parameter is used to keep a cell out of use for testing purposes? Q4. Which parameter describes the minimum received field strength required by an MS to get any service from the network in that cell in idle mode? Q5. Which parameter allows an operator to define MS access classes that are not allowed to access a cell? Q6. Which parameter given below is used to tell the mobile the maximum transmitting power when accessing the system.

6-90250 v 1.0

a)

msTxPwrMaxCCH (TXP)(BTS)(13..43 dBm)

b)

cellReselectParamInd (PI)(BTS)(Yes/No)

c)

cellBarQualify (QUA)(BTS)(Yes/No)

d)

penaltyTime (PET)(BTS)(20..640 s)

e)

temporaryOffset (TEO)(BTS)(0..70 dB).

f)

cellReselectOffset (REO)(BTS)(0..126 dB)


26 (27)

Idle Mode Operation

Q7. When are location updates carried out? a) Every time an MS changes its location area under one MSC. b) Every time an MS changes between two different MSCs. c) On a periodic basis set by timerPeriodicUpdateMS (PER)(BTS)(0.0..25.5 hours) parameter. d) All of the above. Q8. Which of the following is true about IMSIAttachDetach (ATT)(BTS) (Yes/No) parameter? a)

It is used to indicate that a BTS is switched on.

b)

Its purpose is to avoiding unnecessary paging by the BSC.

c) An automatic location update occurs when the Mobile is switched on if IMSIAttachDetach (ATT)(BTS) parameter is used. d) It is used for periodic location updates carried out by the MS to update location information in MSC/VLR. e)

6-90250 v 1.0

© Nokia Oyj

All of the above.

27 (27)

BSSPAR-Idle Mode Operation

Recommend Documents