Protocols architecture in gsm
The following Section Explains the Protocol architecture Description in GSM.
- The figure 8.12 shows the protocol architecture of GSM with signaling protocols. Considering the wireless interface Um.
- Layer 1: the physical layer handles all radio-specific functions.
- This includes the creation of bursts according to the five different formats, multiplexing of bursts into a TDMA frame, synchronization with the BTS, detection of idle channels, and measurement of the channel quality on the downlink.
- The physical layer at Um uses GMSK for digital modulation and performs encryption/decryption of data, i.e., encryption is not performed end-to-end, but only between MS and BSS over the air interface.
- The main tasks of the physical layer comprise channel coding and error detection/correction, which is directly combined with the coding mechanisms. Channel coding makes extensive use of different forward error correction (FEC) schemes.
- As voice was assumed to be the main service in GSM, the physical layer also contains special functions, such as voice activity detection (VAD), which transmits voice data only when there is a voice signal.
- Layer-2: LAPDm, Signaling between entities in a GSM network requires higher layers.
- For this purpose, the LAPDm protocol has been defined at the Um interface for layer two. LAPDm, as the name already implies, has been derived from link access procedure for the D-channel (LAPD) in ISDN systems, which is a version of HDLC.
- It does not need synchronization flags or check summing for error detection.
- LAPDm offers reliable data transfer over connections, re-sequencing of data frames, and flow control.
- Further services provided by LAPDm include segmentation and reassembly of data and acknowledged/unacknowledged data transfer.
Figure 8.12 Protocol Architecture ofGSM