GSM Data Services Architecture
GSM (Global System for Mobile Communications) originally designed for voice services evolved to support data through GPRS (General Packet Radio Service) and EDGE (Enhanced Data rates for GSM Evolution). These technologies introduced packet-switched data to the circuit-switched GSM network.
Key Evolutionary Steps
- • CSD (Circuit Switched Data): 9.6 kbps - Original GSM data service
- • HSCSD (High Speed CSD): Up to 57.6 kbps using multiple time slots
- • GPRS: 114 kbps theoretical max - Packet switching introduced
- • EDGE: 384 kbps - 8-PSK modulation added to GPRS
GPRS Network Architecture
New Network Elements
- PCU (Packet Control Unit) - BSC upgrade
- SGSN (Serving GPRS Support Node)
- GGSN (Gateway GPRS Support Node)
- CG (Charging Gateway)
Interface Definitions
- Gb: BSC ↔ SGSN (Frame Relay)
- Gn: SGSN ↔ GGSN (GTP over IP)
- Gi: GGSN ↔ External PDN
- Gr: SGSN ↔ HLR (MAP)
Coding Schemes & Throughput
| Coding Scheme | Modulation | Code Rate | Data Rate/slot | Protection |
|---|---|---|---|---|
| CS-1 | GMSK | 1/2 | 9.05 kbps | Highest |
| CS-2 | GMSK | ≈2/3 | 13.4 kbps | High |
| CS-3 | GMSK | ≈3/4 | 15.6 kbps | Medium |
| CS-4 | GMSK | 1 | 21.4 kbps | None |
| MCS-9 (EDGE) | 8-PSK | 1 | 59.2 kbps | None |
Note: GPRS uses 4+1 coding schemes (CS-1 to CS-4) while EDGE uses 9 Modulation and Coding Schemes (MCS-1 to MCS-9). The air interface allocates resources in blocks of 4 bursts (20ms) across 1-8 time slots.
GPRS Protocol Stack
Application Layer
IP, X.25
IP, X.25
LLC (Logical Link Control)
Error recovery, ciphering, flow control
Error recovery, ciphering, flow control
RLC/MAC (Radio Link/Medium Access)
ARQ, scheduling, multiplexing
ARQ, scheduling, multiplexing
Physical Layer (GSM RF)
GMSK/8-PSK, 200kHz channels, TDMA
GMSK/8-PSK, 200kHz channels, TDMA
Time Slot Structure
Frame Duration: 4.615 ms
Slots per Frame: 8
Slot Duration: 0.577 ms
Bursts per Block: 4
Block Duration: 20 ms
TBDataStealingDataTBGP
Normal Burst Structure
Learning Objectives
- •Understand GPRS/EDGE physical layer
- •Analyze coding scheme trade-offs
- •Calculate theoretical throughput
- •Visualize time-slot allocation
- •Compare CSD vs Packet data