GSM Core Network: Network Switching Subsystem

So after finishing Base station subsytem, today we are going to cover one of the most integral parts of GSM core network: Network Switching Subsystem

So what are we waiting for? Lets start:)

NSS is responsible for call connection, authorization and supervision of all the calls. This system is also liable for handling short messages and packet data, maintaining database of its own users as well as visitors, providing authentications and encryptions. It is a gateway to PSTN, other mobile networks and data networks including the Internet. In other words, this system allows mobile devices to communicate with each other and with landlines (PSTN). In layman’s terms, BSC transfers the call to MSC which is a part of NSS where call switching and mobility management functions are being taken care of. This system allows mobile devices to communicate with each other and with landlines (PSTN)

Lets discuss all the components of this system in details. Grab a coffee as it can get little boring L What to do, that’s the topic we are discussing, cant put much entertainment masala.

Anyways, back to studies:

Mobile Switching Centre

This is the main component of NSS as the BSCs coordinate with it. This component controls the entire network. It interacts with other databases and PSTN. The MSC sets up and releases the end-to-end connection, handles mobility and hand-over requirements during the call and takes care of charging and real time pre-paid account monitoring. The responsibilities of this entity included checking if a customer has a valid account or not, what profile he is in, what kind of services he had been activated for etc. In short, it provides the needed user registration and authentication information.

Each MSC manages dozens of cell sites and their base stations. Large systems may have two or more MSCs.

Home location register (HLR)

A MSC queries several databases before permitting a call. One of them is HLR which has been explained below:

The home location register (HLR) is a central database that contains details of each mobile phone subscriber that is authorized to use the GSM network. The HLRs store details of every SIM card issued by an operator. Each SIM has a unique identity no called an IMSI which is the most important key or the primary key to each HLR record.

Another important data coupled with the SIM is the MSISDN, which is basically nothing but the phone no of the subscribers which s/he uses for making calls (yes, we have a technical term for every damn thing!). The MSISDN is also a primary key to the HLR record. The HLR data is stored for as long as a subscriber remains with the operator.

Other keys of this database apart from MSISDN and IMSI are: Current TMSI (Temporary IMSI), IMEI (International Mobile Equipment Identity), Name and address of the subscriber, Current service subscription profile, Current location (MSC/VRL address), Authentication and encryption keys, Mobile Country Code (MCC) and MNC (Mobile Network Code).

The main function of the HLR is to manage the mobility of subscribers by means of updating their position in administrative areas. It is also responsible for sending the subscriber data to a VLR or SGSN when a subscriber first roams there. A subscriber’s data is removed from the previous VLR when a subscriber has moved away from it. One of major roles of HLR is also that it plays the role of a broker between the MSC and the subscriber's VLR in order to allow incoming calls or text messages to be delivered.

Authentication centre (AuC)

As the name suggests, the authentication center (AUC) provides authentication and encryption parameters that validates a user's identity and ensures the discretion of each call.

When a mobile makes a call, AUC authenticate the SIM card that attempts to connect to the GSM core network.  Once this authentication is done and is successful, the HLR is allowed to manage the SIM. During the process, an encryption key is also generated which is later used to encrypt all wireless communications be it voice, SMS or any other mean, between a mobile and the GSM network.

If the authentication fails, then no communication is possible from that SIM. The AUC actually protect the operators from different types of fraud. Also, GSM has standard encryption and authentication algorithm which are used for a call

Now lets have a look at how this whole process of security takes place. There is a secret key which is actually shared between the AuC and the SIM and is known as K_i. This K_i is burned into the SIM during manufacture and is also replicated onto the AuC. This K_i is combined with the IMSI to produce an encryption key called K_c which the MSC sends to the base station controller (BSC) so that all communications can be encrypted and decrypted.

Visitor location register (VLR)

The visitor location register (VLR) is a database attached to an MSC to contain information about the subscribers who have roamed into the premises of the MSC (Mobile Switching Center) which it serves. Each base station in the network is served by only one VLR and so, its just not possible for a subscriber to be present in more than one VLR at a time.

The data stored in the VLR has either been received from the HLR or from the MS. Whenever a MSC detects a new MS in its network, it creates a new record in the VLR for the same. It also updates the HLR of the mobile subscriber with the new location of that MS.

Data which is stored in a VLR is  Information of currently attached mobile stations, their IMSI/TMSI numbers, their MSISDN, location Area Identity i.e. where the MS is currently located), authentication and encryption information, List of base stations that belong to this MSC/VLR (by their Base Station Identity Code), List of location areas that belong to this MSC/VLR (by their LAI or Location Area Identity code)

Other entities

There are many other entities which are connected to NSS. They are:

Equipment Identity Register

The equipment identity register (EIR) is a database that contains information about the identity of mobile equipment that prevents calls from stolen, unauthorized, or defective mobile stations. It function can be implemented with AUC or can be made separate. The EIR keeps a list of mobile phones (identified by their IMEI) which are to be banned from the network or monitored

Billing centre (BC)

The billing centre is basically for postpaid subscribers. It is responsible for processing the toll tickets generated by the VLRs and HLRs and generating a bill for each subscriber. It is also responsible for generating billing data of roaming subscriber.

Short message service centre (SMSC)

SMSC supports the sending and receiving of text messages.

Gateway MSC

The Gateway MSC (GMSC) is an MSC that connects the PLMN (Public Land Mobile Network) to a PSTN/ISDN.

Multimedia messaging service centre (MMSC)

A MMSC supports the sending and receiving of multimedia messages

Voicemail system (VMS)

A VMC system records and stores voicemails.