Network Switching Subsystem (NSS)

Network Switching Subsystem (NSS)

In GSM Network,there are three different subsystem.

1. Network Switching Subsystem (NSS).

2. Base Station Subsystem (BSS).

3. Network Management Subsystem (NMS).

Network Switching Subsystem (NSS)

 Network Switching Subsystem (NSS) - Contains the elements

·     Mobile Services Switching Center (MSC)

·     Home Location Register (HLR)

·     Visitor Location Register (VLR)

·     Authentication Center (AC)

·     Equipment Identity Register (EIR)

The Authentication Center (AC) and Equipment Identity Register (EIR) are used to provide security. The subscriber and the mobile station have to be identified and authorized before accessing the network. These functions will be discussed later.

Function of NSS:

1.      Call control. A mobile terminated call requires HLR inquiry to locate the called subscriber.

2.      Mobility Management.

3.      The HLR always knows in which MSC/VLR area a particular subscriber is located.

4.      An MSC/VLR knows in which Location Area a subscriber is located. This is enabled by a Location Update of which there are three types: Power On, Generic and Periodic

5.      Mobility Management also helps in maintaining ongoing calls for a moving subscriber by a procedure known as Handover. There are four types of Handovers: Intra Cell, Inter Cell-Intra BSC, Inter Cell - Inter BSC and Inter MSC

6.      Subscriber Data handling. A subscriber’s data is located in three places: the HLR, VLR and SIM card

7.      Security Issues. Subscriber verification is performed in the VLR by an authentication process. Speech encryption is carried out between BTS and Mobile Station.

8.      Various types of numbers are used in the GSM network for different functions. The most important ones are: IMSI, MSISDN, MSRN, LAI, LAC, CGI, TMSI and HON.

9.      Charging. The MSC is responsible for collecting charging information. It is sent to the Billing Center which creates bills for the subscriber.

    10.  Signalling towards Base Station Subsystem and other networks.

MOBILE SWITCHING  CENTER  (MSC) 

MSC performs the switching functions for all mobile stations located in the geographic covered by its assigned BSSs.

Function  of the MSC include :-

(1) Call handling that copes with the mobile nature of  subscribers

      (example : paging )

(2) Management of required logical radio link channel during  calls.

(3) Management of  MSC-BSS signaling protocol

(4) Handling location registration and ensure inter working between mobile station 

      and VLR. 

(5) Control of  inter -BSS handovers .

(6) Acting as gateway MSC to interrogate the HLR.

(7) Exchange of signaling with other system entities.

(8) Other normal functions of  the local exchange switch in the fixed network     

      (example :-charging )

Home Location Register (HLR)

The home location register (HLR )contains the identities of mobile subscribers their service parameters and their location information.

The location information is stored as mobile station roaming number (MSRN) which is a directory number that network can

use to route calls to MSC where the mobile subscriber is located at the time of call.

In summary HLR contains :-

(1) Identify of mobile subscribers.

(2) ISDN directory number of mobile station.

(3) Subscription information on teleservices and bearer service.

(4) Service  restrictions  (if any)

(5) Supplementary service.

(6) Location information for call routing.

Visitor Location Register (VLR)

Visitor Location Register contains the subscriber parameters and location information for all mobile subscribers currently located in

the geographical area (i.e. cells) controlled by the VLR .The VLR allocates the MSRN(when required) a Temporary Mobile Subscriber Identity (TMSI) for secret identification of mobile subscriber on the radio link.

In summary ,the VLR contains :-

(1) Identity  of mobile subscriber

(2) Any temporary mobile subscriber identity.

(3) ISDN directory number of mobile.

(4) Location area where the mobile station is registered.

(5) Copy of subscriber data from HLR.

Equipment Identity Register (EIR)

This database is accessed during the equipment validation procedure when a mobile accesses the system it contains the identities of mobile station equipment’s(called International Mobile Station Equipment Identity or IMEI) which may be valid, suspect, or known to fraudulent.

This contains:

(1) Valid list-list of valid MS equipment identities.

(2) Suspect list-list of mobiles under observation.

(3)Fraudulent list-list of mobiles for which service is barred.

Authentication Center (AUC)

The Authentication Center :-

(1) Contains subscriber authentication data called authentication keys (Ki).

(2) Generates security related parameters  needed to service using Ki.

(3)Generate unique data pattern called a cipher key (Kc) needed for encrypting speech and data.