SS7 Core Network Architecture


The GSM Core network (CN) (or Network Switching Subsystem NSS) is the component of the GSM system that carries out call switching and mobility management for mobile phones. The CN consists of the following components

The Mobile-services Switching Center (MSC): constitutes the interface between the radio system and the fixed network. It performs all the needed functions to handle the circuit switched services to and from the mobile stations (3GPP, 2015, p26). The MSC usually consists of two systems: the MSC server, responsible for the signaling, and the media gateway (MGW) handling the user traffic.

The Home Subscriber Server (HSS) is the master database for a given user. It is the entity containing the subscription-related information to support the network entities actually handling calls/sessions (3GPP, 2015, p22). The HSS holds: user identification,numbering and addressing information, User security information, User location information, and user profile information. The HSS function is performed by two network components: The Home Location Register (HLR) and the Authentication Center (AuC)

The Home Location Register (HLR) is a central database that contains details of each mobile phone subscriber that is authorized to use the GSM core network. Each user can only be associated to a single HLR.

The authentication center (AuC) is associated with a given HLR and stores an identity key for each subscriber registered with the associated HLR. The key is used to generate security data that are subsequently used for mutual authentication between the subscriber and the network, protect the integrity and the confidentiality of the communications (3GPP, 2015, p23).

The Visitor Location Register (VLR) is a database of the subscribers who have roamed into the jurisdiction of the MSC which it serves. When a Mobile Station (MS) enters a new location area it starts a registration procedure. An MSC in charge of that area notices this registration and transfers to a Visitor Location Register the identity of the location area where the mobile station (MS) is situated. If this MS is not yet registered in the VLR, the VLR and the HLR exchange information to allow the proper handling of CS calls involving the MS. In practice, for performance reasons, most vendors integrate the VLR directly to the V-MSC (3GPP, 2015, p25).

The Short Message Service Gateway (SMSC) acts as an interface between a Short Message Service Centre and the mobile network, to allow short messages to be delivered to mobile stations from the Service Centre (3GPP, 2015, p26).

The Signaling Transfer Point (STP) is responsible for the transfer of SS7 messages between other SS7 nodes, acting somewhat like a router in an IP network (Dryburgh, Hewet, 2005).