Radio SubSystem (RSS)

As suggested by the name, the radio subsystem is comprised of all the radio specific elements, i.e. the mobile stations (MS) and the base station subsystem (BSS). The connection between the RSS and the NSS (shown in figure x.ii) via the A interface (solid lines) and the connection to the OSS via the O interface (dashed lines). The A interface is generally based on a circuit-switched PCM-30 system (2.048 Mbit/s), carrying up to 30 X 64 kbit/s connections, whereas the O interface uses the Signalling System No. 7 (SS7) based on X.25 carrying system management data to/from the RSS.

Base Station Subsystem (BSS): A GSM network is made up of many BSSs, each one being controlled by a base station controller (BSC). The main function of the BSS is to maintain the radio connections to an MS, however, it does have several other functions such as the coding/decoding of voice, and rate adaptation to/from the wireless network part. As well as a BSC, the BSS contains several BTSs.

Base Transceiver Station (BTS): A BTS contains all the radio equipment (antennas, signal processing, amplifiers) necessary for radio transmission. A BTS can be used to form a radio cell, or if sectored antennas are used, several cells. The BTS is connected to the MS by the Um interface, and the BSC by the Abis interface. The Um interface comprises of all the mechanisms necessary for wireless transmission (TDMA, FDMA). Abis interface consists of 16 or 64 kbit/s connections. The area coverage from a GSM cell can vary from 100m and 35km depending on the expected traffic and the location environment.

Base Station Controller (BSC): Basically, the BSC controls the BTS The functions of the BSC include reserving radio frequencies, handling handovers from one BTS to another and performing the paging of the MS. The BSC also multiplexes the radio channels onto the fixed network connections at the A interface.

	Function	BTS	BSC
	Management of radio channels		X
	Frequency hopping	X	X
	Management of terrestrial channels		X
	Mapping of terrestrial onto radio channels		X
	Channel coding and decoding	X
	Rate adaptation	X
	Encryption/decryption	X	X
	Paging	X	X
	Uplink Signal measurement	X
	Traffic measurement		X
	Authentication		X
	Location registry, location update		X
	Handover management		X

Mobile Station (MS) : The MS is the user equipment which contains the software required for communication with the GSM network. The MS consists of user independent hard/software and the subscriber identity module (SIM), which stores the user specific data. While an MS can be identified via the international mobile equipment identity (IMEI)