Network Switching SubSystem (NSS)

Mobile Switching Center (MSC):
Heart of the network.

Manages communication between GSM and other networks.

Call setup function and basic switching.

Call routing.

Billing information and collection.

Mobility management
- Registration
- Location Updating
- Inter BSS and inter MSC call handoff

MSC does gateway function while its customer roams to other network by using HLR/VLR.


Home Location Registers (HLR):

Permanent database about mobile subscribers in a large service area (generally one per GSM network operator).

Database contains IMSI, MSISDN, prepaid/postpaid, roaming restrictions, supplementary services.


Visitor Location Registers (VLR):

Temporary database which updates whenever new MS enters its area, by HLR database.

Controls those mobiles roaming in its area.

Reduces number of queries to HLR.

Database contains IMSI, TMSI, MSISDN, MSRN, Location Area, authentication key.


Authentication Center (AUC):

Protects against intruders in air interface.

Maintains authentication keys and algorithms and provides security triplets ( RAND, SRES, Kc).

Generally associated with HLR.


Equipment Identity Register (EIR) :

Database that is used to track handsets using the IMEI (International Mobile Equipment Identity).

Made up of three sub-classes: The White List, The Black List and the Gray List.

Only one EIR per PLMN.