Network aspects
Ensuring the transmission of voice or data of a given quality over the radio link is only part of the function of a cellular mobile network. A GSM mobile can seamlessly roam nationally and internationally, which requires that registration, authentication, call routing and location updating functions exist and are standardized in GSM networks. In addition, the fact that the geographical area covered by the network is divided into cells necessitates the implementation of a handover mechanism. These functions are performed by the Network Subsystem, mainly using the Mobile Application Part (MAP) built on top of the Signalling System No. 7 protocol.
Mobiles can in fact only roam seamlessly if they are multi-band units. Most international phones have two bands, one for the Americas at 1900Mhz, and one for Europe at 900Mhz. Others such as the Ericsson R380 show below, cover the 1800Mhz band as well. This lets the phone roam on Asian and African networks.
The mobile switch communicates with the telephone network using Signaling System Seven, an internationally agreed upon standard. IS-136 and IS-95 also uses SS7. But it uses a standard called IS-41 when communicating between the Home Location Register and the Visitor Location register. (Source for this IS-41 information is http://www.mobilein.com/mobile_basics.htm)
.pdf file on SS7 and mobile networking -- Good reading!
The signalling protocol in GSM is structured into three general layers [1], [19], depending on the interface, as shown in Figure 3. Layer 1 is the physical layer, which uses the channel structures discussed above over the air interface. Layer 2 is the data link layer. Across the Um interface, the data link layer is a modified version of the LAPD protocol used in ISDN (external link), called LAPDm. Across the A interface, the Message Transfer Part layer 2 of Signalling System Number 7 is used. Layer 3 of the GSM signalling protocol is itself divided into 3 sublayers.
* Radio Resources Management
* Controls the setup, maintenance, and termination of radio and fixed channels, including handovers.
* Mobility Management
* Manages the location updating and registration procedures, as well as security and authentication.
* Connection Management
* Handles general call control, similar to CCITT Recommendation Q.931, and manages Supplementary Services and the Short Message Service.
Signalling between the different entities in the fixed part of the network, such as between the HLR and VLR, is accomplished throught the Mobile Application Part (MAP). MAP is built on top of the Transaction Capabilities Application Part (external link) (TCAP, the top layer of Signalling System Number 7. The specification of the MAP is quite complex, and at over 500 pages, it is one of the longest documents in the GSM recommendations [16].
Figure 3. Signalling protocol structure in GSM
I've not written on layers and feel they are beyond the scope of this site.