18624159 Network Switching Subsystem

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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 assumeresponsibility for the complete set of control

and database functions required to set up callconnections using one or more of thesefeatures: encryption, authentication, androaming.


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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 orindirectly 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 different

network elements in the NSS


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Home Location Register (HLR)

Every PLMN requires access to HLR as apermanent store of data.

The HLR can best be regarded as alarge database with access times thatmust be kept as short as possible.

The faster the response from thedatabase, the faster the call can beconnected.


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

parameters are maintained, such as the

parameter Ki, which is part of securityhandling.

Ki is known only to the HLR and the SIM

T

o reduce the load on the HLR, the VLR wasintroduced to support the HLR by handlingmany of the subscriber-related queries. (e.g.,localization and approval of features).


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The home location register (HLR) and visitorlocation register (VLR), together with the

MSC, provide the call routing and (possiblyinternational) roaming capabilities of GSM.

So HLR contains all the administrativeinformation of each subscriber registered inthe corresponding GSM network, along withthe 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 andupdate requests from other network nodes.

For this reason it acts as a databasemanagement system.

Each subscriber record contains a substantialamount of parameters.


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HLR Functions contd.., Communication with GMSCs: during call

set-up to an MS, the GMSC requests MS

location information from the HLR, whichthen provides this in the form of routinginformation.

Also, if the subscriber is detached the HLRwill inform the GMSC that there is no need toperform further routing of the call.


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HLR Functions contd.., Communication with MSCs: when

setting up calls to an MS, it is necessaryfor the HLR to contact the MSC servingthe 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 theMS from the HLR of the subscriber.

The HLR provides a copy of the subscriptiondetails, updates its MS location informationand instructs the old VLR to delete theinformation 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 protecteddatabase that stores a copy of the secret keystored in each subscriber's SIM card, which isused 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 staticfunctions, the VLR provides dynamicsubscriber data management.

In one word VLR maintains information aboutsubscribers 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 toanother, data are passed between the VLR ofthe 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 therelated 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 subscribers location. Typically, but not necessarily, a VLR is linked

with a single MSC.

So VLR contains selected administrative

information from HLR, necessary for callcontrol and provisioning of subscribedservices, for each mobile currently located inthe 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 arewithin a particular MSC service area.

Thus, there is one VLR for each MSC servicearea.

This means that the MSC does not have tocontact the HLR every time the subscriberuses a service or changes its status.


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VLR Functions contd.., The following occurs when MSs move into a

new service area:

1. The VLR checks its database to determinewhether or not it has a record for the MS(based on the subscribers IMSI)

2. When the VLR finds no record for the MS,it sends a request to the subscribers HLR fora copy of the MSs subscription


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VLR Functions contd.., 3. The HLR passes the information to

the VLR and updates its locationinformation for the subscriber. The HLRinstructs the old VLR to delete theinformation it has on the MS.

4. The VLR stores its subscriptioninformation 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 subscriptiondetails, including the following information:

Identity numbers for the subscriber

Supplementary service information (e.g.

whether the subscriber has call forwarding onbusy 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 handlethe 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 isresponsible, and the functionality to performand control inter-MSC handover.


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

MSs and from MSs. The primary functions of

an MSC include the following: Switching and call routing:A MSC

controls call set-up, supervision and releaseand may interact with other nodes to

successfully establish a call.

This includes routing of calls from MSs toother networks such as a PSTN.


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

charging mobile calls and information aboutthe particular charge rates to apply to a callat any given time or for a given destination.

During a call it records this information andstores it after the call, e.g. for output to abilling center.

Service provisioning: supplementaryservices are provided and managed by aMSC.


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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 toan MS, when the HLR requests some routinginformation from the MSC.

Communication with the VLR: associated

with each MSC is a VLR, with which itcommunicates for subscription information,especially during call set-up and release.


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MSC Functions contd.., Communication with other MSCs: it may be

necessary for two MSCs to communicate with eachother during call set-up or handovers between cells

belonging to different MSCs. Control of connected BSCs: as the BSS acts as

the interface between the MSs and the NSS, the MSChas the function of controlling the primary BSS nodei.e the BSC.

An MSC may communicate with its BSCs during, forexample, call set-up and handovers between twoBSCs.


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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 thePLMN where calls to mobile subscribers enterthe GSM network.

Network operators may opt to equip all oftheir MSCs with gateway functionality or onlya few.


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Gateway MSC contd.., The GMSC requests call routing information

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

The same is true of a call from an MS toanother MS.

Any MSC in the mobile network can functionas a gateway by integration of the

appropriate software and definition of HLRinterrogation 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 eachMS equipment. Each IMEI is unique.

During any MS access (except in the case ofIMSI-detach), the MSC/VLR may verify the

IMEI.

When necessary, the EIR is requested by theMSC/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 thenetwork.

Each GSM phone has a unique identifier, itsIMEI, which cannot be altered withoutdestroying the phone.

Like the HLR or the VLR, the EIR basically

consists of a database, which maintains threelists: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 orunknown.


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Equipment Identification Procedure The equipment identification procedure uses

the identity of the equipment itself (IMEI) toensure that the MS terminal equipment isvalid.

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

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 withindividual operators. GSM specifications recommend

identification for each attempted call set-up.


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18624159 Network Switching Subsystem