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GSM Notes

Quick overview of GSM Networks

The GSM system is comprised of a a chain of technologies that are linked to encode receive and route your data from your handset or mobile device to an end destination. The flow of the system is as follows starting from your GSM capable handset or MS Mobile Station.

Overview of BTS System and its components

  • MS (Mobile Station):
SIM (Subscriber Identity Module) holds an IMSI (International Mobile Subscriber Identity) number.
ME (Mobile Equipment/terminal)
  • BSS (Base Station Subsystem):
BTS (Base Transceiver Station)
BSC (Base Station Controller)
  • NSS (Network Switching Subsystem):
MSC (Mobile Switching Center) performs the switching functions and connection to other networks.
GMSC (Gateway Mobile Switching Center) gateway that connects the cellular network and the PSTN.
HLR (Home Location Register) The HLR stores information of the subscribers (such as services) in the coverage area of a given MSC.

The MSC (Mobile Switching Center) connects to:

|----- SGSN (Serving GPRS Support Node), which routes data to and from users’ MEs.

--- GGSN (Gateway GPRS Support Node), which links mobile networks to external networks such as the internet.

|----- GMSC (Gateway Mobile Switching Center), which links the mobile network to other phone networks and handles roaming.

--- PSTN (Public Switched Telephone Network) or regular ol’ phone lines.
--- ISDN (Integrated Services Digital Network) such as new digital phone lines.

|----- SMSC (Short Message Service Center), which handles SMS.

GPRS - General Packet Radio Service.
SMS - Short Message Service.

Typical Call Flow

The ME is linked to a BTS via EM pulses. Once the ME's identity is verified via its International Mobile Subscriber Identity (IMSI) code and a channel is allocated, the transfer of data can begin via voice or data packets. Both are managed by the BSC which connects back to the NSS system which then controls and allows access to the rest of the networks whether to a PSTN system for voice calls or to a TCP/IP gateway for data or VoIP calls.

What does this mean for OpenBTS?

In OpenBTS Asterisk replaces what the MSC (Mobile Switching Center) would do in a traditional GSM network/BTS system. Subscriber Registry acts as HLR (Home Location Register). Smqueue handles all SMS functionality. Transceiver handles interfacing with the actual radio/ethernet.

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