Unlike routing in the fixed network, where a terminal is semi-permanently wired to a central office, a GSM user can roam nationally and even internationally. (With, if needed, a properly enabled handset.) The directory number dialed to reach a mobile subscriber is called the Mobile Subscriber ISDN (MSISDN), which is defined by the E.164 numbering plan. This number includes a country code and a National Destination Code which identifies the subscriber's operator. The first few digits of the remaining subscriber number may identify the subscriber's HLR within the home PLMN.
These abbreviations don't seem uniform with all GSM writers. But all words and phrases point to a Mobile Subscriber ISDN or International Mobile Subscriber Identity (IMSI) Number. Whatever you call it, the number is made up of three parts:
a.) An International Mobile Subscriber Identity (IMSI) Number, say, 44510000
b.) The mobile country or network code, MCC, consisting of three digits, say, 310
c.) The national destination code or the mobile network code, MNC. This is a two digit number, say, 68.
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An incoming mobile terminating call is directed to the Gateway MSC (GMSC) function. The GMSC is basically a switch which is able to interrogate the subscriber's HLR to obtain routing information, and thus contains a table linking MSISDNs to their corresponding HLR. A simplification is to have a GSMC handle one specific PLMN. It should be noted that the GMSC function is distinct from the MSC function, but is usually implemented in an MSC.
PLMN: Public land mobile network. In this context a cellular telephone network. PLMN is chiefly a European useage.
The routing information that is returned to the GMSC is the Mobile Station Roaming Number (MSRN), which is also defined by the E.164 numbering plan. MSRNs are related to the geographical numbering plan, and not assigned to subscribers, nor are they visible to subscribers.
The most general routing procedure begins with the GMSC querying the called subscriber's HLR for an MSRN. The HLR typically stores only the SS7 address of the subscriber's current VLR, and does not have the MSRN (see the location updating section). The HLR must therefore query the subscriber's current VLR, which will temporarily allocate an MSRN from its pool for the call. This MSRN is returned to the HLR and back to the GMSC, which can then route the call to the new MSC. At the new MSC, the IMSI corresponding to the MSRN is looked up, and the mobile is paged in its current location area (see Figure 4).
Figure 4. Call routing for a mobile terminating call