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Open interfaces of GSM

One of the main purposes behind the GSM specifications is to define several open interfaces, which then limit certain parts of the GSM system. Because of this interface openness, the operator maintaining the network may obtain different parts of the network from different GSM network suppliers. When an interface is open, it also strictly defines what is happening through the interface, and this in tum strictly defines what kind of actions/procedures/functions must be implemented between the interfaces.

The GSM specifications define two truły open interfaces within the GSM network. The first one is between the Mobile Station (MS) and the Base Station (BS). This open-air interface is appropriately named the “air interface”. It is relatively easy to imagine the need for this interface to be open, as mobile phones of all different brands must be able to communicate with GSM networks from all different suppliers.

The second interface is located between the Mobile services Switching Centre, MSC, (which is the switching exchange in GSM) and the Base Station Controller (BSC). This interface is called the “A-interface”. These two network elements will be discussed in greatcr detail in later chaptcrs. The system includes morę than the two defined interfaces, but they are not totally open, as the system specifications had not been completed when the commercial Systems were launched.

When operating analogue mobile networks, experience has shown that centralised intelligence generates excessive load in the system, thus decreasing the capacity. For this reason, the GSM specification. in principle, provides the means to distribute intelligence throughout the network. Referring to the interfaces, the morę complicated the interfaces in use, the morę intelligence is reąuired between the interfaces in order to implement all the fiinctions reąuired. In a GSM network, this decentralised intelligence is implemented by dividing the whole network into three separate subsystems:

• Network Switching Subsystem (NSS)

• Base Station Subsystem (BSS)

• Network Management Subsystem (NMS)

The actual network needed for establishing calls is composed of the NSS and the BSS. The BSS is responsible for radio path control and every cali is connected through the BSS. The NSS takes carc of cali control functions. Calls are always connected by and through the NSS.

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