We’ve discussed Long Term Evolution (LTE) in depth in terms of technological architecture, use, features, speed, etc., but we need to remember its predecessor. New technologies, LTE included, would not be the speedy giant it is today without learning from those technologies who lived before it. GSM is one of them. To successfully understand our future, we must understand our past.
So, what is GSM? GSM (Global System for Mobile Communications, originally Groupe Spécial Mobile), is a standard set developed by the European Telecommunications Standards Institute (ETSI) to describe protocols for second generation (2G) digital cellular networks used by wireless phones.
Originally, the GSM standard was developed to replace the first generation (1G) analog cellular networks, and was described a digital, circuit switched network optimized for full duplex voice telephony. Over time, GSM’s capabilities expanded to include data communications, first by circuit switched transport, then packet data transport via GPRS (General Packet Radio Services) and EDGE (Enhanced Data rates for GSM Evolution or EGPRS).
Further improvements to GSM were made when the 3GPP developed third generation (3G) UMTS standards followed by fourth generation (4G) LTE Advanced standards. Now that we’ve covered what GSM is and where it came from, how exactly does it work? Let’s discuss call setup in GSM.
How does the GSM network know where the subscribers are?
The radio network consists of a large number of BTSs. Each BTS is given an identity. These BTSs are grouped according to location area, also given an identity. Each MSC/VLR (Mobile Services Switching Center/Visitor Location Register) serves the BTSs in an number of location areas. The GSM phones reports to the network (VLR) when it moves from a BTS in oneLocation Area to a BTS in another location area.
How does the the GSM network know where the subscribers are? The VLR always knows in which location area the GSM subscriber is located in at any given moment. Accordingly, the HLR always knows which MSC/VLR the GSM subscriber is at as well. Then, the GSM subscriber’s telephone number tells the network which HLR the actual GSM subscriber belongs to.
How a Call Gets to a GSM Mobile Phone
The call is routed through the telephone network to the closest MSC to the called GSM subscriber. Next, Gateway MSC checks with HLR, asking “Where is the GSM subscriber?” After that, the call is established to the actual MSC/VLR (Visiting MSC) either directly or through the fixed or international telephone network. Finally, the request for a mobile call is transmitted over all BTSs in the actual location area of the called GSM subscriber. Mobile recognizes its own identity, and the call begins.
Call from a GSM Phone to a GSM Phone
How does it work for GSM, calling from mobile device to mobile device? Well, aia the radio path and the base station network a call request for a GSM subscriber is sent from a mobile phone to MSC/VLR ). The MSC/VLR then collects authentication data from HLR (if such datahas not been collected earlier). Typically, a bulk of such data is transferred, thus avoiding repeating authentication requests to HLR. Next, the MSC/VLR requests HLR of the actual location of GSM subscriber, and the call is establised as described earlier.
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Roaming and Accounting
The GSM Association has defined standard format on call/billing data which shall be transferred between the operators. Previously, all operators transferred data to all, and as you can imagine, this situation was not ideal. In fact, it was chaos. Today, all operators utilize data clearning houses. [1] Key attributes include:
- Largest actors: MACH, Roameo, EDS
- Single point of contact for data float between operators
- Does not change the responsibilities between the operators (Acts as an extended arm of the operator)
Data Clearing House services:
- Can convert data formats (local format to TAP-format, TAP2 to TAP3…)
- Checks invoices and call records
- Fraud control
[1] Call Setup to/from a GSM Subscriber, telenor, Hans Myhre, http://www.scribd.com/doc/22645860/GSM-Call-Setup
