The various entities in the GSM network are connected to one another through signalling networks. Signalling is used for example, for subscriber mobility, subscriber registration, call establishment, etc. The connections to the various entities are known as ‘reference points’. Examples include:

• A interface – the connection between MSC and BSC;
• Abis interface – the connection between BSC and BTS;
• D interface – the connection between MSC and HLR;
• Um interface – the radio connection between MS and BTS.

Various signalling protocols are used over the reference points. Some of these protocols for GSM are the following:

• Mobile application part (MAP) – MAP is used for call control, subscriber registration, short message service, etc.; MAP is used over many of the GSM network interfaces;

• Base station system application part (BSSAP)– BSSAP is used over the A interface;

• Direct transfer application part (DTAP)– DTAP is used between MS and MSC; DTAP is carried over the Abis and the A interface. DTAP is specified in GSM TS 04.08 [49];

• ISDN user part (ISUP) – ISUP is the protocol for establishing and releasing circuit switched calls. ISUP is also used in landline Integrated Services Digital Network (ISDN). A circuit is the data channel that is established between two users in the network. Within ISDN, the data channel is generally a 64 Kbps channel.

The circuit is used for the transfer of the encoded speech or other data. ISUP is specified in ITU-T Q.763 [137]. When it comes to call establishment, GSM makes a distinction between signalling and payload. Signalling refers to the exchange of information for call set up; payload refers to the data that is transferred within a call, i.e. voice, video, fax etc.

For a mobile terminated GSM call, the signaling consists of exchange of MAP messages between GMSC, HLR and visited MSC (VMSC). The payload is transferred by the ISUP connection between GMSC and VMSC. It is a continue aim to optimize the payload transfer through the network, as payload transfer has a direct cost aspect associated with it. Some network services are designed to optimize the payload transfer. One example is optimal routing.

BSC is a communicator between the MSC and a number of RBS. One or more torture in the system switching can be controlled by one MSC. BSC control each BTS / RBS to transmitter performance and implementation of handover to other cells in the areas of torture itself. Handover is the ability of MS to get a better signal quality when MS move from one cell to another cell.

Handover BTS / RBS there are two types:

Intra Cell Handover:
Handover, which occurred in the same cell.

Inter Cell Handover:
Handover in a different cell. There are two methods used to connect between the TRC and the BSC in the BSS Ericson, namely:

- BSC / TRC is a combination between the BSC and the TRC on the same Ax. This system is suitable for medium capacity applications and high sample area network and urban and sub-urban. Part of this can handle until 1020 TRX.15 BSC remote can be expensive from a BSC / TRC.

-BSC and TRC standalone.
Standalone-BSC without a transcoder is used for applications low and sub-urban. BSC can be handle to 300 TRX. While the TRC can standalone handled 16 BSC remote.

The main functions of BSC during the call include:

Locating
Evaluating the relationship to the radio MS and if necessary, suggest handover to the other direction. This handover decision based on the results of the measurement of the quality of the signal received by MS.

Handover
The function of locating recommend to the handover, BSC will automatically take over the cell which will be in handover and will start the handover process. Switching is done to provide good quality signal to the MS.

Frequency hopping
Frequency random that is done on a sector with the aim to prevent interference as a result of almost the same frequency of the BTS / RBS other.

Dynamic Power Control in MS and RBS
BSC takes into account the output power to the BTS and MS / RBS based on measurements received from the uplink and Downlink every 480 ms to maintain the quality of a good relationship.

Other functions BSC
• Radio Network Management: Contains cell data and information system for cell in the BSC.

• Radio Base Station Management: To configuration Matching Cell equipment with RBS, the system between RBS and torture must be the same as well as for the election hardwared. RBS Management also handle software maintenance device RBS and RBS.

• just Rate Adaption: transcoding is the conversion process information from PCM code (A / D converter) to the information in the speech GSM Coder. Rate Adaption convert information on the receiving end of the MSC / VLR at a speed of 64 Kbps, which comprises 16 Kbps from 13Kbps to 3Kbps to traffic and signaling information. If the Rate Adaption or without adjustment speed link to the BSC will be a four-speed data capabilities.

• Tranceiver Handling: handle signaling to the radio at RBS.

• BSC manage the functions of all the GSM radio from Ericson GSM system, include:
- Controls Channel radio.
- Movement / MS movement.
- Collecting data configuration cell.
- Collecting statistical data on the number of calls, handover and traffic per cell.

MS-Mobile Station Wireless communication devices that use the air interface which is the SIM (Subscriber Identity Module) that connects customers with the BTS. MS consists of:

ME (Mobile Equipment)
ME consists of the radio functions and user interface and terminal equipment to the other. ME identification have a serial number called IMEI.

SIM (Subcriber Identity Module)
Contains information about users that can be used with ME. Access to that used to form a GSM smart card that can control various functions. There are a number of others in the SIM IMSI, TMSI, Ki, Algorithm A3 and A8, memory phone number, PIN. Communication that occurs from the MS to the BTS (Base Station Transceiver) is a radio communication through electromagnetic waves, and in transmitter MS from the antenna to antenna receiver in the BTS through the air interface.

Consists of 2 channels on the air interface channel on the GSM namely:

• UP Link (MS to the BTS) 890 - 915 MHz.

• Down Link (BTS to MS) 935 - 960 MHz.

Mobile Service Switching Center have important functions in switching communication in the entire MS in the MSC area. Such as setting up, routing, supervising calls to and from MS.

MSC-Mobile Service Switching Center functions:

- Establish a connection and traffic signaling.

- Billing: Recording of the duration of speech.

- Pagging: The process of giving signals to the BTS, which has the frequency with the same frequency signal MSC issued with the purpose to find the Subscriber calling.

- Handover: This process can also occur in the MSC. The process of handover to the MSC called the inter handover MSC.

- Access to the data base to PLMN (VLR, HLR, EIR).

- Implement security functions.

Data Transmision MSC contains Interface (DTI), which is implemented on a GSM interworking function (IWF). DTI to handle data functions such as data conversion rate.

MSC / VLR implemented on AXE as follows:

• MSS (Mobile Switching Subsystem) handle and to set up the call, charging implemented for supporting function and the gateway GMSC on roaming.

• MMS (Mobile Mobility and Radio Subsystem) handle up date location, authentication and chippering access to all the radio. handle requests handover and paging. Save information about the cell and torture.

• MMS creation (Mobile Data Subsystem) Save the data received by the HLR.

• SHA (Short Message Service Subsystem) Contains the Short Message Service (SMS).

Base Transceiver StationEach BTS coverage to a specific area. Here is a geographical BTS:

Cell
An area served by one BTS. MS will differentiate each cell that is by using the BSIC (Base Station Identification Code).

Local Area (LA)
The combination of several cell. One local area is also a region where pagging message emanating Subscriber to find it's correct. In one LA, there are more than one BTS, depending on one or more of the BSC and one still in the MSC / VLR.

PLMN service area (Public Line Mobile Network)
The combination of some of LA, which provides telecommunications service.

GSM service area
Regional network interconnection services, the GSM network.

MSC service area:
A geographic area from the GSM network coverage area is served by one MSC. Each MS can enter this area when it is registered in the VLR (Visitor Location Register) BTS is a radio device that is required to serve one cell. BTS system contain antenna, RF Power Amplifiers and Digital Equipment Signalling.