The GSM Network

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GSM (Global System for Mobile communications: originally from Groupe Spécial Mobile) is the most popular standard for mobile phones in the world. Its promoter, the GSM Association, estimates that 80% of the global mobile market uses the standard. GSM is used by over 3 billion people across more than 212 countries and territories. Its ubiquity makes international roaming very common between mobile phone operators, enabling subscribers to use their phones in many parts of the world. GSM differs from its predecessors in that both signaling and speech channels are digital, and thus is considered a second generation (2G) mobile phone system. This has also meant that data communication was easy to build into the system. [Wikipedia]

Registering a handset in the GSM network works this way– The handset listens for a SID (System Identity) being broadcasted by the closest BTS (Base Transceiver Station) a.k.a. the tower. Once it receives a SID, the handset transmits its IMEI (International Mobile Equipment Identity) number and MIN (Mobile Identity Number). It goes through the BSC (Base Station Controller) which determines whether the transmission is voice or data. The BTS and BSC combined is called the BTS (Base Station Subsystem). (The TRAU (Transcoding Rate Adaption Unit) which isn’t usually identified, also exists to provide repeater functionality to maintain signal strength.)

It then goes to the MSC (Mobile Switching Centre) which authenticates, validates, and locates the subscriber. It checks the HLR (Home Location Register) which is the database of the customer’s SIM services (i.e. SMS, Voicemail, Long distance, Call display, etc.). Once it confirms the subscriber services, it goes back to the handset the same way, but all these happen so fast that it becomes seamless to the subscriber.

Another component in the network is the GMSC (Gateway Mobile Switching Centre) which routes calls targeted to another carrier’s wireless network or the PSTN (Public Switch Telephone Network). There is also a VLR (Visitor Location Register) which is used for load balancing as it stores subsets of information from the HLR when the subscriber is not in his home area.

When roaming, there also is the EIR (Equipment Identity Register) which is a database of all blacklisted equipment (though this one is rarely used these days already). There is also the AuC (Authentication Centre) which stores the private key for SIM authentication. Both of these are connected to the MSC.

For data services, a.k.a. GPRS (General Packet Radio Service), the SGSN (Serving GPRS Support Node) acts as the MSC. It then goes to the GGSN (Gateway GPRS Support Node), then depending on the APN (Access Point Name), it will go out to the Internet in varied ways.

There are three APNs commonly used., which is used for plain text browsing, accesses the Internet via a WAP Gateway., which is used for HTML browsing, accesses the Internet via a NAT Server. Thirdly,, which is used for multimedia messaging, accesses the Internet through a WAP Gateway, then a MMSC (Multimedia Service Centre) or SMTP server (for email).

These are all the network components in the GSM network which are nice to know especially if you work in the telecommunications industry.

Photo Credit: Atomic Taco


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