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GSM stands for Global System for Mobile Communications
Network architecture - 23.002, Section 5, Fig 1b
GSM Protocol layers - 24.007, Section 5.2, Diagrams 5.1/5.6 gives inside view GSM/GPRS Protocol layers for MS.
24.007, Diagram A.1 (Annexure) shows L3 (CC/MM/RR) messages for MO (voice) Call.
48.052, Section 7, diagram 7.3 shows GSM signaling protocol stack layers for Abis and A interfaces.
Below is chronological order of 3GPP technologies:
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R96 and before
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GSM
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R97
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GPRS
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R99
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EGPRS, UMTS (3G)
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R8
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LTE (4G)
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R15
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5G NR (5G)
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Phyical layer:
FDD, FDMA+TDMA phy organization
8 slots is one frame.
51 and 26 multi-frame structures
Few slots follow 51, few follow 26 (based on configuration).
Superframe, Hyperframe
Phy configurations maps channels to slots
45.001, Section 5.3 shows Phy configurations for both GSM and GPRS channels.
Numbering for UL slots is delayed by 3 slots
one slot, one burst, about 577us (microseconds)
Different types of bursts
Based on Phy channel, burst is used
Broadcast + Common channels, Dedicated signaling channels, dedicated traffic channels
Different bands, 200kHz separation between carriers
GSM burst uses GMSK modulation.
GMSK is basically QAM technique.
In QAM, two amplitude modulated quadrature components are added to control the phase of the resulting wave.
For smoother phase changes, instead of 0’s and 1’s, Gaussian pulse is used.
BT for Gaussian pulse is 0.3 for GSM.
Procedures:
NAS components
AS components
Steps: Random access, Dedicated signaling channel, Dedicated traffic channel (if applicable)
LAPDm frames
BTS manages LAPDm and GSM Phy, rest of the decisions from BSC
RR ends at BSC (part of RR information may be given to MSC)
RR plays critical role for Phy configurations
MO call, MT call (Paging), SMS (Signaling), SS (Signaling)
Paging and MS (battery) power consumption, paging groups
Cells and Location areas
Location update (responsibility of MS)
48.052:3,
"Transceiver (TRX): in the GSM PLMN is the functional entity which supports the 8 basic radio
channels of the same TDMA-frame.
Base Control Function (BCF): functional entity which handles common control functions
within a BTS, e.g. frequency hopping sequences etc. At a multi BTS site, one of the BCFs
can also be chosen to perform functions common to the site (e.g. external alarms, power
supply, time base)."
We may consider TRX as DSP functionality per Carrier.
RSL, OML, and L2ML is available in 48.052, section 7.
RSL is used for MS and BTS communication.
OML is NMS (52.021).
L2ML seems to be for LAPD management.
BTS takes care of L1/L2 of Air interface and BSC acts as its Master.
From GSM frames/TCH, Voice is to be collected and sent to network (and vice versa).
This function is part of CCU (Channel Codec Unit), located at BTS.
The channel could be half rate or full rate.
Refer 45.001, Section 4 for Net bit rates for TCH (e.g. full rate speech TCH has net bit rate of 13 kbps).
Further conversion to 64kbps is function of TRAU (Transcoder/Rate Adaptor Unit).
TRAU is formally part of BSC, but usually located at MSC side (on A interface).
More precise information is in 48.060; check section 4 and figure 4.1 for Top level view.
GSM speech frame is of 20ms duration.
6 speech frames will go in one 26-multiframe (120ms).
Due to interleaving, one speech frame is sent over 8 bursts for full rate TCH.
Full rate TCH gives us bit rate of 13 kbps.
Top view of FR speech coding
There are two aspects of Timing advance - Initial (UL synchronisation) and Updates in connected mode.
44.004, Section 6.1, 7.1.1
45.010, Section 5.4, 5.6.1, 6.5.1
Both UL and DL Power control is with BSS.
Idea is to maintain both BER (RxQual) and measured power levels (RxLev) within certain (likely configurable) thresholds.
For traffic channels, measurements are performed periodically, and power levels are adjusted.
DL measurements are done by MS and UL measurements are done by BTS.
MS informs DL measurements to BTS via RR MEASUREMENT REPORT message (sent over SACCH) [44.018:3.4.1.2].
UL power adjustments are informed to MS via DL SACCH block (L1) field [44.004:7.1.1]
One SACCH period is of 480 ms.
One SACCH burst in each 26-multiframe, one SACCH block requires four bursts , so 120 x 4 = 480ms (almost half second).
More information about averaging for measurement, range of values, power control steps, logic etc. can be found in 45.008:
Measurment calculations - Section 8
Range of values - Section 8.1.4 RxLev, 8.2.4 RxQual
Algorithm - Annexure A
GSM speech codecs [26.103:6.1, 48.058:9.3.6, 44.018:10.5.2.6]:
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GSM Full Rate (13.0 kbps)
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GSM FR
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GSM Half Rate (5.6 kbps)
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GSM HR
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GSM Enhanced Full Rate (12.2 kbps, uses FR TCH)
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GSM EFR
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Full Rate Adaptive Multi-Rate
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FR AMR
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Half Rate Adaptive Multi-Rate
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HR AMR
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8PSK Full Rate Adaptive Multi-Rate WideBand
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OFR AMR-WB
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8PSK Half Rate Adaptive Multi-Rate WideBand
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OHR AMR-WB
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Full Rate Adaptive Multi-Rate WideBand
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FR AMR WB
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8PSK Half Rate Adaptive Multi-Rate
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OHR AMR
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DTX stands for Discontinuous Transmission.
This will not only save MS power but also reduce (RF) inteference.
DTX is mandatory feature for both MS and BSS.
Configuration:
UL DTX configuration is indicated to MS via System Information (3 and 6) [44.018:10.5.2.3,10.5.2.3a].
BTS receives DL/UL DTX configuration via Abis (RSL) interface during Channel Activation [48.058:9.3.6].
Procedure:
Transmitter side requires Voice Activity Detector.
SID frame detection at receiver side:
46.031 (FR Specch), Section 6.1.1,
"The ternary SID flag is the output of a SID frame detector,
which compares bit by bit the relevant bits of the received
traffic frame (the SID field) with the SID code word defined in GSM 06.12"
A good traffic frame (BFI=0) with SID flag=2 is considered as valid SID frame.
Similarly, A good traffic frame (BFI=0) with SID flag=0 is considered as good Speech frame.
A traffic frame with SID flag=1 is considered as an invalid SID frame.
Specification references:
46.031/41/81 - DTX for FR/HR/EFR Speech
46.012/22/62 - Comfort noise for FR/HR/EFR Speech
46.032/42/82 - Voice Activity Detector for FR/HR/EFR Speech
AMR stands for "Adaptive Multi-Rate".
26.071, Section 4,
"The AMR speech coder consists of the multi-rate speech coder, a source controlled rate scheme including a voice
activity detector and a comfort noise generation system, and an error concealment mechansim to combat the effects of
transmission errors and lost packets.
The multi-rate speech coder is a single integrated speech codec with eight source rates from 4.75 kbps to 12.2 kbps,
and a low rate background noise encoding mode. The speech coder is capable of switching its bit-rate every 20 ms
speech frame upon command."
Refer 26.071:Figure 1 for TX and RX side block diagram.
26.071, Section 5, Table 1,
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AMR_12.20
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12,20 kbps (GSM EFR)
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AMR_10.20
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10,20 kbps
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AMR_7.95
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7,95 kbps
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AMR_7.40
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7,40 kbps (IS-641)
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AMR_6.70
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6,70 kbps (PDC-EFR)
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AMR_5.90
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5,90 kbps
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AMR_5.15
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5,15 kbps
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AMR_4.75
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4,75 kbps
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AMR_SID
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1,80 kbps
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Highlighted is one of the preferred combinations [28.062:Table 7.11.3.1.3-2].
The basic idea is to change codec rate based on measured Carrier-to-inteference (C/I) ratio [45.009:3.3].
Change in codec rate is done via inband signaling, as shown below [45.009:3.2].
| | BTS
| | MS
| | | | | | | | | BTS does UL measurements and decides UL codec rate.
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| | | | |  | | | | | CMC
Codec Mode Command
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| | | | | | MS applies modified codec rate.
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| | | | |  | | | | | CMI
Codec Mode Indication
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| | | | | | MS does DL measurements and requests change in DL codec rate.
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| | | | | | | | | | CMR
Codec Mode Request
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| | | | | | BTS (or far end of the network, say MGW) decides and applies different/same codec rate.
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| | | | | | | | | | CMI
Codec Mode Indication
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Configuration for AMR includes,
Codec set (Codec combination) and
Thresholds/Hysteresis (w.r.t C/I ratio) as each Codec (change) [45.009:3.4].
Above flaour of AMR is known as AMR or AMR NB (Narrowband).
AMR WB (Wideband) provides codec rates from 6.6 kbps till 23.85 kbps; from 26.171, Section 5, table 1,
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AMR-WB_23.85
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23.85 kbps
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AMR-WB_23.05
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23.05 kbps
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AMR-WB_19.85
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19.85 kbps
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AMR-WB_18.25
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18.25 kbps
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AMR-WB_15.85
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15.85 kbps
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AMR-WB_14.25
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14.25 kbps
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AMR-WB_12.65
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12.65 kbps
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AMR-WB_8.85
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8.85 kbps
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AMR-WB_6.60
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6.60 kbps
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AMR-WB_SID
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1.75 kbps
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Highlighted is one of the allowed combinations [26.103:Table 5.7-1].
From TR 26.901 v4, Section 6.1,
"The current GSM speech codecs (FR, HR, EFR and AMR) operate for narrowband speech (audio bandwidth limited to
3.4 kHz). While these codecs achieve good performance for narrowband speech, the introduction of a wideband speech
service (audio bandwidth extended to 7 kHz) would provide improved voice quality and increased naturalness of voice,
especially in handsfree situations."
More details about AMR:
AMR (NB) Frame Structure - 26.101
AMR WB Frame Structure - 26.201
RTP payload format for AMR and AMR-WB - RFC 3267
Location services (LCS) were introduced in R99 for GSM (and UMTS).
Rel 99 3.2 Location services,
"The location relies on three key functions: the measurement of the radio signals,
performed by the LMU (Location Measurement Unit); the dialogue between the network
and the external LCS client, performed by the GMLC (Gateway MLC);
and the calculation of the position, performed by the SMLC (Serving Mobile Location Centre),
also coordinating the overall process."
22.017 - LCS Stage 1
23.271 - LCS Stage 2
43.059 - GERAN Functional description for LCS Stage 2
Figure 1: Functional LCS Architecture in GERAN
Figure 2: General sequence for LCS operation
Figure 4: Generic Model for LCS Signalling to an SMLC
44.071 - L3 interface between LMU and SMLC
44.031 - RRLP protocol between MS and SMLC
48.071 - SMLC-BSS (Lb) interface, L3 specification
References:
GSM/GPRS Rel 4 Specifications
GSM by Mouly and Pautet
GSM Networks by Gunnar Heine
GPRS RAN refresh notes (includes EGPRS)
Back to GSM Index
Copyright © Samir Amberkar 2024
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