Explanation Of GSM Channel Structures BCCH DECODE STATUS BCCH Decode Status
Explanation
Not Synced
The Mobile Station (MS) is neither synchronized to the network with respect to time or frequency. If the MS is out of sync, only the RxLev can be viewed and all other parameters will read ???
No FCB
The Frequency Correction Burst (FCB or FB) which is used for frequency synchronization of the mobile is not detected. The FCB defines the Frequency Control Channel (FCCH) which is assigned to every other Ts. The FCB is 142 bits long, but carries no information (It is equivalent to an unmodulated carrier, shifted in frequency, with the same guard time as the normal burst), it identifies the FCCH and allows the synchronization Channel (SCH) to be found on at Ts 0 of the following 51-multiframe.
FCB Detect
The FCB was detected on the FCCH - now the MS is frequency synced. and the SCH can be found at Ts 0 of the following 51-multiframe.
SCH Decode
SCH decode means that the synchronization channel (SCH) was decoded and the MS now is time synced. with the network. The Synchronization Burst (SB) has a long (64 bits) training sequence. It also carries a 78 bit payload consisting of the TDMA frame number (FN) and the Base Station Identity Code (BSIC) which consists of the Network (PLMN) Color Code (NCC) & Base station Color Code (BCC). RxLevAm, MSTxPwr & C1 will still read ???
BCCH Decode
All information on the Broadcast Control Channel was decoded. This includes the SCH & FCCH, CellID, LAC, MCC, MNC, frequency hopping & more that describes the current control channel structure.
Sync State ?
The Synchronization state can not be determined.
GSM CHANNEL STRUCTURE GSM distinguishes between physical channels and logical channels (the information carried by the physical channels). Several recurring timeslots on a carrier constitute a physical channel (GSM-900 : 124 RF carriers x 8 timeslots = 992 physical channels), which are used by different logical channels to transfer information - both user data and signaling.
Broadcast Control CHannel
| | | Signaling Channel
Frequency Correction CHannel
BTS->MS: MS frequency correction
Synchronization CHannel
BTS->MS: MS synchronization to the BTS
|
BTS->MS: MS terminating call announcement BTS->MS: Acknowledge channel Common Control Access Grant CHannel requests from MS and allocate a CHannel SDCCH BTS->MS: MS access requests, Random Access CHannel | response to call announcement, Paging CHannel
location update, etc.
| | Dedicated Control CHannel
Fast Associated Control CHannel
Time critical signaling: Handovers
Slow Associated Control CHannel
Link monitoring
Standalone Dedicated Control CHannel
For signaling exchanges (during call setup, registration / LU).
Common channels. The forward common channels are used for broadcasting bulletin board information, paging and response to channel requests. The return common channel is a slotted Aloha type random access channel used by the MS to request channel resources before timing information is conveyed by the BSS, and uses a burst with an extended guard period. Dedicated point-to-point channels. The dedicated point-to-point channels are divided into two main groups, the dedicated signaling channels and the traffic channels. The dedicated signaling channels are used to set-up the connection, and the traffic channel of a variety of rates is used to convey the user information once the session is established. Both channel types have in-band signaling: SACCH for e.g. link monitoring, and FACCH for time-critical signaling during e.g. a handover. The FACCH "steals" the entire traffic burst for signaling. POWER CONTROL To minimize co-channel interference and to conserve power, both the mobiles and the Base Transceiver Stations operate at the lowest power level that will maintain an acceptable signal quality. Power levels can be stepped up or down in steps of 2 dB from the peak power for the class down to a minimum of 13 dBm (20 miliwatts) on GSM-900 and 10 dBm (10 miliwatts). Use this table to translate between dBm, PwrLev and Power. Green indicates the range for a 2 W GSM-900 hand portable (Class 4) the bottom 2 rows indicate the range for a 1800/1900 1 W unit. Power level
0
1
2
3
4
900-dBm
43
41
39
37
35
5 33
6
7
8
9
10
11
12
13
14
15
31
29
27
25
23
21
19
17
15
13
900-Watts 20.0 12.6 8.0 5.0 3.2 2.0 1.30 0.80 0.50 0.32 0.20 0.13 0.08 0.05 0.03 0.02 1800-dBm 1800-mW
30
28
26
24
22
20
1000 631 398 251 158 100
18
16
14
12
10
63
40
25
16
10
BIT ERROR RATE The Bit Error Rate (BER) is the percentage of received bits on a digital link that are in error relative to the number of bits received, usually expressed using a logarithmic scale: The RxQual is a 3 bit value, which means that it has a 0 to 7 value. Use the table below to convert the RxQual to a BER percentage: RxQual Percentage of bits failed (BER) TIMING ADVANCE
0 < 0.2
1
2
3
4
5
6
7
0.20.4
0.40.8
0.8-1. 6
1.63.2
3.26.4
6.412.8
>12.8 %
The Timing Advance is used to compensate for the time it takes a RF signal to go at the speed of light between the BTS and MS. The maximum BTS radius of 35 Km is divided into 64 TA steps ( This means 547 meters / TA step - As a simplification 550 meters will be used) .TA multiplied with 550 meters will give the minimum distance to the BTS. The maximum distance will be (TA +1). A TA value will pinpoint a BTS to a circular band 550 meters wide, with an inner radius of (TA x 550) meters. This can be used to triangulate the position of a BTS by having 2 -3 readings at different positions. Timing Advance
0
Distance to < 550 m BTS
1
2
3
4
5
.......
63
550 m1100 m
1100 m1650 m
1650 m2200 m
2200 m2750 m
2750 m3300 m
.......
35 Km
TDMA FRAME STRUCTURE & DURATION The TDMA technique means that the data are interleaved several times and collected in frames. Each TDMA frame has a 22 bit identifier so it can be told apart from the 2.7 million other TDMA frames any given hyperframe. You can also see a graphic representation by Thomas Kochanek here Traffic Channel (TCH)
Consists of:
.
Control Channel (CCH)
Consists of:
TDMA frame
4,6 ms 8 Timeslots
.
4.6 ms 8 Timeslots
Multiframe
120 ms 26 TDMA frames
.
234 ms 51 TDMA frames
Superframe
6 s 120 ms 51 Multiframes
.
6 s 120 ms 26 Multiframes
Hyperframe
3 h 28 m 53 s 760 2048 ms Superframes
© 1998 Janus Christian Krarup
.
3 h 28 m 53 s 760 2048 ms Superframes
