GSM protocol architecture:

• Figure shows the protocol architecture of GSM with signaling protocols, interfaces.
• The main interest lies in the Um interface, as the other interfaces occur between entities in a fixed network. 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.
• Synchronization also includes the correction of the individual path delay between an MS and the BTS.
• All MSs within a cell use the same BTS and thus must be synchronized to this BTS. The BTS generates the time-structure of frames, slots etc. A problematic aspect in this context are the different round trip times (RTT). An MS close to the BTS has a very short RTT, whereas an MS 35 km away already exhibits an RTT of around 0.23 ms.
• If the MS far away used the slot structure without correction, large guard spaces would be required, as 0.23 ms are already 40 per cent of the 0.577 ms available for each slot. Therefore, the BTS sends the current RTT to the MS, which then adjusts its access time so that all bursts reach the BTS within their limits. This mechanism reduces the guard space to only 30.5 µs or five per cent (see Figure).
• Adjusting the access is controlled via the variable timing advance, where a burst can be shifted up to 63 bit times earlier, with each bit having a duration of 3.69 µs (which results in the 0.23 ms needed). As the variable timing advance cannot be extended a burst cannot be shifted earlier than 63 bit times. This results in the 35 km maximum distance between an MS and a BTS. It might be possible to receive the signals over longer distances; to avoid collisions at the BTS, access cannot be allowed
• 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. FEC adds redundancy to user data, allowing for the detection and correction of selected errors.
• The power of an FEC scheme depends on the amount of redundancy, coding algorithm and further interleaving of data to minimize the effects of burst errors. The FEC is also the reason why error detection and correction occurs in layer one and not in layer two as in the ISO/OSI reference model. The GSM physical layer tries to correct errors, but it does not deliver erroneous data to the higher layer.
• Different logical channels of GSM use different coding schemes with different correction capabilities.
• Speech channels need additional coding of voice data after analog to digital conversion, to achieve a data rate of 22.8 kbit/s (using the 13 kbit/s from the voice codec plus redundancy, CRC bits, and inter-leaving (Goodman, 1997). 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.
• This mechanism helps to decrease interference as a channel might be silent approximately 60 per cent of the time (under the assumption that only one person speaks at the same time and some extra time is needed to switch between the speakers).
• During periods of silence (e.g., if a user needs time to think before talking), the physical layer generates a comfort noise to fake a connection (complete silence would probably confuse a user), but no actual transmission takes place. The noise is even adapted to the current background noise at the communication partner’s location.
• All this interleaving of data for a channel to minimize interference due to burst errors and the recurrence pattern of a logical channel generates a delay for transmission. The delay is about 60 ms for a TCH/FS and 100 ms for a TCH/F9.6
• Mobile communications (within 100 ms signals in fixed networks easily travel around the globe). These times have to be added to the transmission delay if communicating with an MS instead of a standard fixed station (telephone, computer etc.) and may influence the performance of any higher layer protocols, e.g., for computer data transmission
• Signaling between entities in a GSM network requires higher layers (see Figure). 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 (Goodman, 1997), (Halsall, 1996).
• LAPDm is a lightweight LAPD because it does not need synchronization flags or check summing for error detection. (The GSM physical layer already performs these tasks.) LAPDm offers reliable data transfer over connections, re-sequencing of data frames, and flow control (ETSI, 1993b), (ETSI, 1993c). As there is no buffering between layer one and two, LAPDm has to obey the frame structures, recurrence patterns etc. defined for the Um interface.
• Further services provided by LAPDm include segmentation and reassembly of data and acknowledged/unacknowledged data transfer.
• The network layer in GSM, layer three, comprises several sublayers as Figure shows. The lowest sublayer is the radio resource management (RR). Only a part of this layer, RR’, is implemented in the BTS, the remainder is situated in the BSC.
• The functions of RR’ are supported by the BSC via the BTS management (BTSM). The main tasks of RR are setup, maintenance, and release of radio channels. RR also directly accesses the physical layer for radio information and offers a reliable connection to the next higher layer.
• Mobility management (MM) contains functions for registration, authentication, identification, location updating, and the provision of a temporary mobile subscriber identity (TMSI) that replaces the international mobile subscriber identity (IMSI) and which hides the real identity of an MS user over the air inter-face. While the IMSI identifies a user, the TMSI is valid only in the current location area of a VLR. MM offers a reliable connection to the next higher layer.
• Finally, the call management (CM) layer contains three entities: call control (CC), short message service (SMS), and supplementary service (SS). SMS allows for message transfer using the control channels SDCCH and SACCH (if no signaling data is sent), while SS offers the services. CC provides a point-to-point connection between two terminals and is used by higher layers for call establishment, call clearing and change of call parameters. This layer also provides functions to send in-band tones, called dual tone multiple frequency (DTMF), over the GSM network.
• These tones are used, e.g., for the remote control of answering machines or the entry of PINs in electronic banking and are, also used for dialing in traditional analog telephone systems. These tones cannot be sent directly over the voice codec of a GSM MS, as the codec would distort the tones. They are transferred as signals and then converted into tones in the fixed network part of the GSM system.
• Additional protocols are used at the Abis and A interfaces (the internal inter-faces of a GSM system not presented here). Data transmission at the physical layer typically uses pulse code modulation (PCM) systems. While PCM systems offer transparent 64 kbit/s channels, GSM also allows for the submultiplexing of four 16 kbit/s channels into a single 64 kbit/s channel (16 kbit/s are enough for user data from an MS). The physical layer at the A interface typically includes leased lines with 2.048 Mbit/s capacity. LAPD is used for layer two at Abis, BTSM for BTS management.
• Signaling system No. 7 (SS7) is used for signaling between an MSC and a BSC. This protocol also transfers all management information between MSCs, HLR, VLRs, AuC, EIR, and OMC. An MSC can also control a BSS via a BSS appli-cation part (BSSAP).