Protocols Continued

MM (Mobility Management) encompasses the functions for registration, authentication, identification, location updating and the provision of TMSI (Temporary Mobile Subscriber Identity) that replaces the IMSI (International Mobile Subscriber Identity) and is needed to obscure the true identity of the MS over the radio air interface. Although the IMSI identifies the user, the TMSI is only valid within the location area of a VLR. MM also offers a reliable connection to the next higher layer.

Finally the CM (call management) layer contains three entities : CC (Call Control), SMS (Short Message Service) and SS (Supplementary Services). SMS allows for short messages transfer using the control channels SDCCH and SACCH, while SS offers the services described in section 2.1.1.3. CC provides a point-to-point connection between two terminals, the higher layers for call management use this, call clearing and change of call parameters. This layer also contains functions to send in-band tones, called DTMF (Dual Tone Multiple Frequency), over the GSM network.

Additional protocols are used at the Abis and the A interfaces. Data transmission at the physical layer is typically done using PCM (Pulse Code Modulation) systems. Although PCM systems offer transparent 64 kbit/s channels, GSM allows for the sub-multiplexing of four 16 kbit/s channels into single 64 kbit/s (while remembering that 16kbit/s are enough for user data from an MS). At the physical layer, the A interface typically includes leased lines with a capacity of 2.048 Mbit/s. LAPD is used for layer two at Abis, BTSM for the BTS management.

For signaling between the MSC and a BSC, the SS7 (Signaling System No, 7) is used. This protocol also transfers all the management information between MSC's, HLR, VLR's and OMC. Additionally, and MSC can control a BSS via the BSSAP (Base Station [Sub] System Application Part).

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