Network Switching Subsystem

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The Network Switching Subsystem

The NSS plays the central part in every mobile network.

BSS provides the radio access for the MS Network elements within the NSS

assume responsibility for the complete set of control and database functions required to set up call connections using one or more of these features: encryption, authentication, and roaming.

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NSS contd.., To satisfy those tasks, the NSS consists of the following:

MSC (mobile switching center)HLR (home location register)authentication center (AuC)VLR (visitor location register)EIR (equipment identity register)

The subsystems are interconnected directly or indirectly via the worldwide SS7 network.

The network topology of the NSS is more flexible than the hierarchical structure of the BSS.

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Overview of the interfaces between the differentnetwork elements in the NSS

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Home Location Register (HLR)

Every PLMN requires access to HLR as a permanent store of data.

The HLR can best be regarded as a large database with access times that must be kept as short as possible.

The faster the response from the database, the faster the call can be connected.

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HLR contd.., Within the HLR, subscriber-specific

parameters are maintained, such as the parameter Ki, which is part of security handling.

Ki is known only to the HLR and the SIM To reduce the load on the HLR, the VLR

was introduced to support the HLR by handling many of the subscriber-related queries. (e.g., localization and approval of features).

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The home location register (HLR) and visitor location register (VLR), together with the MSC, provide the call routing and (possibly international) roaming capabilities of GSM.

So HLR contains all the administrative information of each subscriber registered in the corresponding GSM network, along with the current location of the mobile.

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HLR Functions Subscription database

management: as a database, the HLR must be able to process data quickly in response to data retrieval and update requests from other network nodes.

For this reason it acts as a database management system.

Each subscriber record contains a substantial amount of parameters.

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HLR Functions contd.., Communication with GMSC’s: during

call set-up to an MS, the GMSC requests MS location information from the HLR, which then provides this in the form of routing information.

Also, if the subscriber is detached the HLR will inform the GMSC that there is no need to perform further routing of the call.

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HLR Functions contd..,

Communication with MSC’s: when setting up calls to an MS, it is necessary for the HLR to contact the MSC serving the MS for routing information.

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HLR Functions contd.., Communication with VLR: when an MS

moves into a new MSC service area the VLR for that area requests information about the MS from the HLR of the subscriber.

The HLR provides a copy of the subscription details, updates its MS location information and instructs the old VLR to delete the information it has about that MS.

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Authentication Center (AuC)

The AuC is always implemented as an integral part of the HLR.

The authentication center is a protected database that stores a copy of the secret key stored in each subscriber's SIM card, which is used for authentication and ciphering of the radio channel.

Major function assigned to the AuC is to calculate and provide the authentication-triplets. (SRES, RAND, Kc).

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Visitor Location Register (VLR) The VLR, like the HLR, is a database, but

its function differs from that of the HLR. While the HLR is responsible for more

static functions, the VLR provides dynamic subscriber data management.

In one word VLR maintains information about subscribers currently physically in that region.

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VLR contd.., Consider the example of a roaming

subscriber. As the subscriber moves from one location

to another, data are passed between the VLR of the location the subscriber is leaving (“old” VLR) to the VLR of the location being entered (“new” VLR).

In this scenario, the old VLR hands over the related data to the new VLR.

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VLR-HLR interaction

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VLR contd.., VLR is assigned a limited geographical

area, while HLR deals with tasks that are independent of subscriber’s location.

Typically, but not necessarily, a VLR is linked with a single MSC.

So VLR contains selected administrative information from HLR, necessary for call control and provisioning of subscribed services, for each mobile currently located in the geographical area controlled by VLR.

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VLR Functions The role of a VLR in a GSM network is to

act as a temporary storage location for subscription information for MSs which are within a particular MSC service area.

Thus, there is one VLR for each MSC service area.

This means that the MSC does not have to contact the HLR every time the subscriber uses a service or changes its status.

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VLR Functions contd.., The following occurs when MS’s move

into a new service area: 1. The VLR checks its database to

determine whether or not it has a record for the MS (based on the subscriber’s IMSI)

2. When the VLR finds no record for the MS, it sends a request to the subscriber’s HLR for a copy of the MS’s subscription

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VLR Functions contd..,

3. The HLR passes the information to the VLR and updates its location information for the subscriber. The HLR instructs the old VLR to delete the information it has on the MS.

4. The VLR stores its subscription information for the MS,

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VLR Functions contd.., For the duration when the MS is within

one MSC service area, then the VLR contains a complete copy of the necessary subscription details, including the following information:

Identity numbers for the subscriber Supplementary service information (e.g.

whether the subscriber has call forwarding on busy activated or not)

Current LA of MS

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NSS Hierarchy

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Data in HLR and VLR

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The Mobile-Services Switching Center

MSC is just an ordinary Integrated Services Digital Network (ISDN) exchange with some

modifications specifically required to handle the mobile application.

The modifications of exchanges required for the provision of mobile service affect.

In particular, the assignment of user channels toward the BSS, for which the MSC is responsible, and the functionality to perform and control inter-MSC handover.

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MSC Functions It is the node, which controls calls both to

MS’s and from MS’s. The primary functions of an MSC include the following:

Switching and call routing: A MSC controls call set-up, supervision and release and may interact with other nodes to successfully establish a call.

This includes routing of calls from MS’s to other networks such as a PSTN.

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MSC Functions contd.., Charging: an MSC contains functions

for charging mobile calls and information about the particular charge rates to apply to a call at any given time or for a given destination.

During a call it records this information and stores it after the call, e.g. for output to a billing center.

Service provisioning: supplementary services are provided and managed by a MSC.

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MSC Functions contd.., Communication with HLR: the primary

occasion on which an MSC and HLR communicate is during the set-up of a call to an MS, when the HLR requests some routing information from the MSC.

Communication with the VLR: associated with each MSC is a VLR, with which it communicates for subscription information, especially during call set-up and release.

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MSC Functions contd.., Communication with other MSC’s: it may

be necessary for two MSC’s to communicate with each other during call set-up or handovers between cells belonging to different MSC’s.

Control of connected BSC’s: as the BSS acts as the interface between the MS’s and the NSS, the MSC has the function of controlling the primary BSS node i.e the BSC.

An MSC may communicate with its BSC’s during, for example, call set-up and handovers between two BSC’s.

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Gateway MSC An MSC with an interface to other

networks is called a gateway MSC. The Gateway MSC (GMSC) is the point in

the PLMN where calls to mobile subscribers enter the GSM network.

Network operators may opt to equip all of their MSCs with gateway functionality or only a few.

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Gateway MSC contd.., The GMSC requests call routing

information from the HLR that provides information about which MSC/VLR to route the call to.

The same is true of a call from an MS to another MS.

Any MSC in the mobile network can function as a gateway by integration of the appropriate software and definition of HLR interrogation information.

In effect it then becomes a GMSC.

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Equipment Identity Register The Equipment Identity Register (EIR) is a

database that stores the International Mobile station Equipment Identity (IMEI) for each MS equipment. Each IMEI is unique.

During any MS access (except in the case of IMSI-detach), the MSC/VLR may verify the IMEI.

When necessary, the EIR is requested by the MSC/VLR to check the IMEI.

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EIR contd.., EIR was introduced to identify, track,

and bar stolen equipment from being used in the network.

Each GSM phone has a unique identifier, its IMEI, which cannot be altered without destroying the phone.

Like the HLR or the VLR, the EIR basically consists of a database, which maintains three lists:White list, black list and gray list.

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EIR contd.., Equipments can be classified under

different lists as: white list (permitted for use) gray list (should be tracked for evaluation) black list (barred)

The network rejects any access attempt if the equipment is classified as “black-listed” or “unknown”.

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Equipment Identification Procedure

The equipment identification procedure uses the identity of the equipment itself (IMEI) to ensure that the MS terminal equipment is valid.

1. The MSC/VLR requests the IMEI from the MS.

2. MS sends IMEI to MSC. 3. MSC/VLR sends IMEI to EIR. 4. On reception of IMEI, the EIR

examines three lists: white list, black list, grey list

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Equipment Identification Procedure contd..,

5. The result is sent to MSC/VLR, which then decides whether or not to allow network access for the terminal equipment.

The decision to identify equipment remains with individual operators. GSM specifications recommend identification for each attempted call set-up.

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