Base Station Subsystem (BSS) in System Architecture of Global System (GSM)

In post Introduction to System Architecture of Global System for Mobile communications (GSM), already introduced about component of GSM network and detail Mobile Station. Mobile Station comprises all user equipment and software needed for communication with a mobile network by Air Interface (Um) to Base transceiver station.
Base transceiver station is a piece of equipment that facilitates wireless communication between MS and BSC.
The Base Station Subsystem BSS consists of Base Transceiver Stations (BTSs) and the Base Station Controllers (BSCs).

GSM Topology

But some document (BSS) is composed of three parts, the base transceiver station (BTS), the base station controller (BSC), which controls the BTSs, and the transcoder (TC).


Base Station Subsystem and Transcoder

1- Base Transceiver Station (BTS)

The BTS establishes and maintains the connection to the MSs within its cell. The interface between the MS and the BTS is the air interface, called the Um-interface in the GSM context. The BTS hosts, at a minimum, the antennas and the Radio Frequency (RF) hardware of a BS, as well as the software for multiple access. Several – or, rarely, one – BTSs are connected to one BSC. They are either colocated, or connected via landline, directional microwave radio links, or similar connections.

The BTS performs the following functions under the control of the BSC

• Transmit and receive functions
• Antenna diversity
• Frequency hopping
• Radio channel measurements
• Radio frequency testing
• Antenna Hopping

The BTS houses the radio transceivers (TRXs) that define a cell and handle the radio link with the mobile station. As was seen, each TRX can handle up to eight full-rate users simultaneously.If more than eight full-rate users request resources within the TRX then they will receive a busy tone, or a network busy message may be displayed on the mobile device. It is possible to increase the number of simultaneous users in a cell by increasing the number of TRXs, hence the number of frequencies used. When a mobile device moves from one cell to another the BTS may change. Within the GSM system a mobile device is connected to only one BTS at a given time. The first TRX in a cell can actually only handle a maximum of seven (possibly less) simultaneous users since one channel on the downlink is used for broadcasting general system information through what is known as the broadcast and control channel (BCCH). The BTS is also responsible for encrypting the radio link to the mobile device based on security information it receives from the core network. It connects to Base Station Controller (BSC) via Abis Interface.

Also see Why does wireless communication system need multiple access techniques?

BTS Hardware

2- Base Station Controller (BSC)

The BSC manages the radio resources for one or more BTSs and radio parameters. It handles the radio channel setup, frequency hopping and handover procedures when a user moves from one cell to another. When a handover occurs, the BSC may change; it is a design consideration that this will not change with the same regularity as a BTS change. A BSC communicates with the BTS through time division multiplex (TDM) channels over what is referred to as the Abis interface, generally implemented using E1 or T1 lines. From a transmission point of view, the BSC also performs a concentration function if more radio traffic channels than terrestrial channels are connected to the MSC. A single BSC can control a large number of BTS. The exact number is a function of the BSC equipment and the configurations used.

The BSC provides:

• Resource management
• Radio measurement processing
• Channel management
• Operations and maintenance functions within the BSS
• Communication with the OMC-R
• Switching between the Air Interface channels (and their associated Abis channels), and the A Interface channels

3- Transcoder (TC)

The Transcoder is the key component for the transmission function, which provides efficient use of the terrestrial links between the equipment of the BSS. In addition to the Transcoder, Submultiplexers are also used for transmission functions.

The Transcoder provides:

• Conversion between A-law and Radio Test Equipment-Long Term Prediction encoded traffic (speech) 
• Conversion between A-law and Algebraic Code Excited Linear Prediction encoded traffic (speech) 
• Rate adaptation (data) 
• O&M control of the transmission function.

The Transcoder is normally located next to the MSC. The Submultiplexer performs submultiplexing on the Ater Interface, between the MSC and the BSC. When submultiplexing is used, a  Submultiplexer is located at each end of the link.

Here are Content for GSM Mobile Communication System

• Introduction to System Architecture of Global System for Mobile communications (GSM)
• Base Station Subsystem(BSS) or in post Base Station Subsystem (BSS) in System Architecture of Global System (GSM)
• Network switching subsystem (NSS)
• Operation Support System (OSS)