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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:
R96 and before GSM (2G)
R97 GPRS
R99 EGPRS, UMTS (3G)
R8 LTE (4G)
R15 5G NR (5G)
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 (we did *not* cover this)
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.
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
Power control - 45.008:Annexure A, 8.1.4 RxLev, 8.2.4 RxQual
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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