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 GSM Backhaul:


The worldwide popularity of GSM has driven operators to deploy services in most metropolitan areas and, increasingly, into smaller and more remote areas. Often the terrestrial infrastructure is not sufficient to reach these locations and satellite is the only viable way to extend the service into these regions. The implementation of GSM over satellite is in common use in many regions of the world.


The GSM network technology is normally implemented using fiber, cable or microwave transmission where bandwidth is not normally a concern. Once a fiber is installed, there is little motivation in using one E1s. Satellite transmission however, is very bandwidth sensitive since every kHz of satellite bandwidth must be leased and incurs an additional cost. When implementing GSM links over satellite it is important to minimize the required link bandwidth in order to reduce operating cost.


While GSM is inherently satellite friendly and is easy to implement in a brute force way, there are more elegant approaches which can lead to significant bandwidth, and therefore cost, reductions.





   GSM Backhaul Network



From GSM Backhaul application:


  • The Abis Interface:


The Abis interface is used to connect a BSC and a BTS. Since there are more BTSs in the network than other components, the Abis interface is the most common interface in a GSM network and is often implemented via satellite.




Abis Interface


The Abis interface contains compressed voice and GSM information. The Abis Interfaces format is shown in Figure. A single Abis interface can be used to support up to eight GSM radio access channels. Each radio channel supports eight GSM voice channels. Radio channels can be used either for multiple cells to increase geographic coverage or for multiple frequencies within single cell to increase the traffic handling capacity.




Abis Interface Format


The voice channels on the Abis interface have been compressed by the BSC. Four GSM voice streams are then placed in a 64 Kbps timeslot and two timeslots are used for each 8 channel GSM radio. A single Abis interface can be used for up to eight radio channels. If fewer radios are supported then the unused timeslots are left empty.


There are five timeslots which are always unused, even on a fully loaded Abis interface. In addition, if fewer than eight radios are deployed, there are three timeslots that are unused for each radio that is not deployed. The result is many unused timeslots that should not be transmitted over satellite.


*Africanwaves successfully provided GSM backhaul solution to a petroleum company in the West Desert.




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