GSM Network Interfaces 
- a summary or tutorial of the different interfaces used to provide communication between various elements in a GSM cell phone network
The network structure is defined within the GSM standards. Additionally each interface between the different elements of the GSM network is also defined. This facilitates the information interchanges can take place. It also enables to a large degree that network elements from different manufacturers can be used. However as many of these interfaces were not fully defined until after many networks had been deployed, the level of standardisation may not be quite as high as many people might like.
- Um interface The "air" or radio interface standard that is used for exchanges between a mobile (ME) and a base station (BTS / BSC). For signalling, a modified version of the ISDN LAPD, known as LAPDm is used.
- Abis interface This is a BSS internal interface linking the BSC and a BTS, and it has not been totally standardised. The Abis interface allows control of the radio equipment and radio frequency allocation in the BTS.
- A interface The A interface is used to provide communication between the BSS and the MSC. The interface carries information to enable the channels, timeslots and the like to be allocated to the mobile equipments being serviced by the BSSs. The messaging required within the network to enable handover etc to be undertaken is carried over the interface.
- B interface The B interface exists between the MSC and the VLR . It uses a protocol known as the MAP/B protocol. As most VLRs are collocated with an MSC, this makes the interface purely an "internal" interface. The interface is used whenever the MSC needs access to data regarding a MS located in its area.
- C interface The C interface is located between the HLR and a GMSC or a SMS-G. When a call originates from outside the network, i.e. from the PSTN or another mobile network it ahs to pass through the gateway so that routing information required to complete the call may be gained. The protocol used for communication is MAP/C, the letter "C" indicating that the protocol is used for the "C" interface. In addition to this, the MSC may optionally forward billing information to the HLR after the call is completed and cleared down.
- D interface The D interface is situated between the VLR and HLR. It uses the MAP/D protocol to exchange the data related to the location of the ME and to the management of the subscriber.
- E interface The E interface provides communication between two MSCs. The E interface exchanges data related to handover between the anchor and relay MSCs using the MAP/E protocol.
- F interface The F interface is used between an MSC and EIR. It uses the MAP/F protocol. The communications along this interface are used to confirm the status of the IMEI of the ME gaining access to the network.
- G interface The G interface interconnects two VLRs of different MSCs and uses the MAP/G protocol to transfer subscriber information, during e.g. a location update procedure.
- H interface The H interface exists between the MSC the SMS-G. It transfers short messages and uses the MAP/H protocol.
- I interface The I interface can be found between the MSC and the ME. Messages exchanged over the I interface are relayed transparently through the BSS.
Although the interfaces for the GSM cellular system may not be as rigorouly defined as many might like, they do at least provide a large element of the definition required, enabling the functionality of GSM network entities to be defined sufficiently.
By Ian Poole
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