20 GSM Handovers

Suchit Purohit

Learning Objectives

  • Recap of previous modules
  • Basic functions of Network Sub System
  • Entities that form NSS namely MSC,GMSC,HLR and VLR
  • Functions of MSC and GMSC
  • Information stored in HLR and VLR and how they facilitate localization and roaming
  • Functions and entities of Operation Subsystem namely Operation and maintenance control(OMC), Authentication Center(AUC), Equipment Identity register(EIR)

Introduction

 

In the previous module we have started our discussion about GSM telecommunication standard. We had discussed about GSM specifications as in frequencies used, data rate used, modulation and multiple access techniques used, modes of data transfer and so on. We had also studied that GSM is a hierarchical organization with three layers namely Radio Sub-system, Network Sub-system and Operation Sub-system. In the previous module we had discussed the entities of Radio Sub-System, their interfaces and functions. In this module we will study other two layers Network Sub-system and Operation Sub-system

 

Network Subsystem:

 

NSS forms the heart of GSM system. It acts as interface between wireless and fixed network i.e. public networks like PSTN,ISDN,PSPDN through. It consists of  switching center and  databases to perform various functions like worldwide user localization, management of roaming different BSSs, maintenance of user accounts and call charges and roaming between different providers of different countries. The entities of GSM network belonging to NSS are:

  • Mobile Services Switching Center (MSC)
  • Gateway mobile services switching center(GMSC)
  • Visitor location register(VLR)
  • Home location register (HLR)

The basic architecture of NSS and the interfaces between components of NSS with OSS and RSS are shown in Figure 1. The connections and components of NSS are summarized as

  • Each MSC in NSS manages many BSC in RSS
  • Each MSC has a HLR and VLR
  • Each MSC can connect to another MSC, GMSC and IWF
  • HLR connects to AUC in OSS
  • GMSC are connected to PSTN, ISDN or PSPDN

Figure 1: GSM architecture

 

Figure 2: Hierarchical organization of GSM

 

MSC (Mobile switching Centre)

 

MSC is the backbone of Network Subsystem. GSM architecture in NSS is arranged in an hierarchical fashion. MSC is one administrative area. Each MSC controls many Location areas. Each Location area controls many BSC’s and each BSC controls number of cells(Fig. 2.It consists of high performance ISDN switches used to connect to various BSC and also to another MSC. MSC is connected to fixed network like PSTN via GMSC which abbreviates for Gateway Mobile Switching center. Basic functions of MSC are:

  • Signal Processing
  • Manage BSC in a geographical area
  • Establish and terminate  connections between various  mobile  stations via BSC.  The mobile stations connected may fall into its own area or to another MSC’s area.
  • Establish and terminate connection between MSC and fixed network via GMSC
  • Connect to PDN(Public data networks) via x.25
  • Monitoring of calls mode to and from an Mobile Station
  • Manage connection setup, connection release and handover of connections to other MSC via SS7 Standard Signaling System SS7 (SS7 is for control signaling for digital networks)
  • Manage supplementary services like call charging, multi way calling and call forwarding

GMSC (Gateway Mobile Switching Centre)

 

GMSC is a special switching center belonging to NSS which handle connection to fixed network likes ISDN, PSTN, PSPDM and PLMN .Special IWF are used by GMSC to connect to public data networks like x.25(Figure 3).

 

Figure 3: GMSC connections

 

HLR (HomeLocation Register)

 

HLR is an important database which contains information about the subscribers belonging to area covered by that MSC. When a user buys a subscription in the form of SIM card, all the information related to the subscriber is stored in HLR of network operator. For every subscriber there is only one HLR record worldwide. IMSI number which uniquely identifies a GSM subscriber is the primary key to that record.HLR is implemented as a distributed database but there is only one HLR per GSM network. Each user has only one HLR record worldwide which is updated on real time basis. HLR connect to AUC in OSS for authentication of subscriber.HLR stores static as well as dynamic information stored for the subscriber.

 

Static Information stored in HLR

  1. Mobile Station International Subscriber Directory  Number(MSISDN):It is the phone number of GSM user known to the external world. It is the number to which the subscriber can be called.The number is not associated with device but with the subscriber. The MSISDN follows ITU-T standard E.164 for addresses as it is also used in fixed networks. The format of MSISDN number is:
    • Country code (CC) – 1 to 3 decimal digits long (for example 91 for India,49 Germany)
    • National  Destination  Code(NDC)  –  2  to  3  decimal  digits  long.It  is  address  of  network provider (for example In India 94 for BSNL and 98 for all others)
    • Subscriber Number (SN) – Max. 10 digits long. In India it is 8 digits

Ex – If the MSISDN Number is +491791234567

CC (1-3 digits) NDC(2-3digits) SN(10 digits max)
+49 179 1234567

+ 49 is CC for Germany where + indicates International dialing, 179 is NDC and 1234567 is SN

 

In India, SN is 8 digits long. The format of SN is:

  • Operator Code (2 digits )
  • Level number (1 digit)
  • Subscriber number (5 digits)

Ex – If the MSISDN Number in India is 91 98 45062050, 91 is CC, 98 is NDC and 45062050 is SN

CC                   NDC                      SN
91                   98 45062050
Op-                          Level                                   Subscriber No

code                        Number
45                             0                                           62050
  • 45 is operator code which is for Airtel
  • 0 is level number
  • 62050 is Subscriber ID

(ii) International Mobile Subscriber Identity (IMSI) Number: Every subscriber in GSM network is assigned an identifier called IMSI number which is unique worldwide.It is store in SIM card and it is secured. It is the 15 digit number with format as follows

MCC (3 digits) MNC(2 digits) MSIN(10 digits )
Mobile Country Code, 404 in India Mobile Netwok Code, 10 for Airtel in Delhi Mobile subscriber Identification Number

 

3 digit Country Code(MCC)eg. 404 forIndia

2 digit Mobile Network code (MNC) – It identifies operator with in the countryeg. Airtel in Delhi has 10 as MNC

10 digit Mobile subscriber Identification Number (MSIN)this is unique number subscriber within

home network

 

(i) SubscriberServices: The HLR  store  details  of  permission to  subscribe  like  call  forwarding, Roaming etc.

 

Difference between IMSI number and MSISDN number

MDISDN IMSI
Known to outer world Burned on the SIM
Used for dialing Used for identifying the subscriber by the network

operator
Used to route the call Used to connect to the network by authenticating

Dynamic information stored in HLR

This information is updated on real time basis

  • Current location area of MS: When the subscriber is moving from one area to another area, its location changes hence the location area in which it is residing also changes. The information about its current location is updated in the HLR so that the call for it can be routed there.
  • Current  VLR  and  MSC  status:  The  current  MSC  and  the  corresponding  VLR  in  which  the subscriber currently is.
  • MSRN(Mobile Station Roaming Number): A temporary address that hides the identity and location of subscriber when it is roaming to other MSC. It is a temporary location dependent ISDN number assign to the subscriber when it is roaming in other network. It has same structure as MSISDN but contains current visitor Country Code(VCC), Visitor National Destination Code (VNDC), Identification of current MSC together with subscriber number. It is assigned by locally responsible VLR. With the help of MSRN, HLR finds the subscriber when there is a mobile terminated call so that the call can be routed there.

Visitor Location Register (VLR)

 

It is a dynamic database which contain temporary information of some of the important information store in HLR. A perfect analogy to differentiate HLR and VLR is that HLR can be thought of as storage whereas VLR is a cache. It consists of borrowed information from HLR required for call control and provisioning of services. VLR is implemented as integral part of MSC so that geographical area corresponding to that MSC correspond to that control by VLR. When subscriber enter new MSC area VLR associated with this MSC requests information about this new subscriber from corresponding HLR in its Home networks.

 

Example – A subscriber of GSM network in Ahmedabad is roaming in Jaipur. HLR data will remain home network i.e. in Ahmedabad. VLR data will be copied to roaming network i.e Delhi. Hence there will be sufficient information about subscriber to assure the subscribed services to the user without needing to refer to HLR. Thus visiting network provides the services while billing will be done by home network.

 

Figure 4: HLR and VLR during roaming

 

Difference between HLR and VLR

 

There is only one HLR per mobile network operator and one VLR / MSC HLR contains information about all subscriber in the network while VLR contains information about subscriber roaming in VLR area Information in VLR is more dynamic than HLR (though it contains some part of information of HLR). When user moves from one location area to other VLR is updated. When user moves from on VLR and new VLR information is updated in HLRVLR and HLR are used conjunctively to inform ation loc alizat ion of mobile device and facilitate roaming.

 

Localization

 

When there is any service requested for a mobile phone, like a mobile terminated call, it is necessary to localize the mobile phone. This is done via Localization. Localization is a process by which a mobile phone is identified, traced, authenticated and services provided to it when the mobile phone is either in its home networks or at a visiting location. This is because the service provider will provide the services only when it is identified and the access to the services to it has been verified.The localization process consists of two phases. First the location area of the mobile station is found then mobile station is searched in that location area using paging method.

 

Location Update

 

Through location update process, the presence andlocation information of the devices is kept up-to- date with HLR and VLR. Presence of the mobile station means whether it is available for communication or not. A mobile station might be unavailable due to many reasons like it is powered off, out of network etc.

 

An IMSI detach procedure occurs in HLR when the mobile station is not present. When mobile phone is powered off, explicit IMSI detach occurs and HLR and VLR are updated When a mobile station is out of coverage area, an implicit IMSI detach procedure occurs.

 

IMSI detach indicates mobile phone is not available

  • When connected again, an IMSI attach procedure occurs. During IMSI attach, mobile station gives its IMSI number to the network. This is location update process also updates the HLR and VLR. The location update is undergone under following conditions:
  • During Mobile originating calls
  • When mobile station moves from one location area to another its new location is updated in HLR and VLR
  • Mobile Station updates the location when it moves more than k cells away from the location information of the last update i.e. when it crosses exact k cell boundaries. This is done to avoid frequent updates as it would be an overhead on signaling system.
  • There is also periodic location update. This is specified by GSM operator and range from 6 minutes to 24 hours.

Paging

 

By location update, the HLR and VLR are notified of location of mobile phone. But it gives only location area. To search the mobile phone in a location area, paging is used. Paging is a technique by which the Number of mobile phone is broadcasted in different administrative areas. A location area pages to all BSC’s under it, each BSC pages the number to all BTS’s under its area and each BTS broadcasts it on its downlink. A mobile phone continuously monitors its downlink and when it sees its own number, it response.

 

RoamingChanging VLR with uninterrupted services is called roaming. Roaming is of 2 types

  • Horizontal Roaming
  • Vertical Roaming

 

Horizontal Roaming: Roaming between two networks from same family. Eg. GSM to GSM or GSM to UMTS

Vertical Roaming: Roaming between two networks of different families. Eg. GSM to CDMA or GPRS to Wi-Fi. As soon as mobile phone moves to new VLR, HLR sends all the user related data to new VLR When vertical roaming happens without disruption of services, it is called seamless roaming Illustration of Roaming

Suppose user’s mobile number is +91 9844012345

Suppose this is number of spice network in Bangalore and the user is roaming in Mumbai

CASE I: Fixed landline phone in Mumbai calls the subscriber

  • i It will dial as 09844012345. Since the call is from fixed network, the call will be switched at PSTN network in Mumbai
  • ii Call will be routed to its home network
  • iii The MSC of its home network will refer to HLR and will find that subscriber is within coverage of Vodafone operator in Mumbai. This is known by the help of MSRN provided by the VLR
  • iv Call will be routed to Mumbai MSC at Vodafone
  • v Vodafone MSC at Mumbai will look at VLR to locate Spice subscriber and route the call vi When call is over, charging information is forwarded to Spice network in Bangalore

Figure 5: Roaming Illustration: CASE I

 

CASE II: A call is being made from the mobile phone in roaming state Suppose the spice subscriber makes a call in Mumbai. Here home network will not be referred. Vodafone Mumbai, looking at the VLR information finds the mobile phone in its area and routes the call to it.

 

Figure 6: Roaming Illustration: CASE II

 

CASE III: Both caller and called party are in roaming state in foreign network A and B are 2 subscribers in Bangalore. Both are in Kolkata i.e. in roaming state. A dials B, call will be routed to Bangalore. In Bangalore finding that B is roaming in Kolkata, Call will be routed to Kolkata

 

Figure 7: Roaming Illustration: CASE III

Operationand Maintenance Sub-System

 

It manages the operation and maintenance of the entire GSM network. It is connected to GMSC and MSC in NSS and BSCs in RSS. The components of OSS are

  • OMC- Operation Maintenance Control
  • AUC- Authentication Centre
  • EIR- Equipment Identity Register.

AUC

 

AUC is used for authentication of subscriber. It is used by HLR to authenticate the user. It storessubscriber authentication keys and generate of cipher keys. It calculates the authentication parameters and forwards them to HLR.

 

OMC

 

As the name indicates OMC performs management related tasks like management of status reports billing. It communicates with MSC and BSC.

 

EIR

 

Equipment Identity registers stores IMEI numbers of the devices in the GSM network. EIR maintains 3 types of lists related to devices

  • Black List– MS which have been reported stolen or blocked due to some
  • White list– All MS that are valid and operating
  • Grey List- All MSs that are not functioning properly

Suggested Reading:

  1. “Mobile Communications”, 2nd edition, by Jochen Schiller
  2. Asoke K Talukder, Hasan Ahmed, Roopa R Yavagal, “Mobile Computing: Technology, Applications and Service Creation”, 2nd ed, Tata McGraw Hill, 2010.
  3. Rajkamal, “Mobile Computing” 2nd ed, Oxford Press
  4. Eberspächer, J., Vögel, H. – J., Bettstetter, C. and Hartmann, C. (2008) References, in GSM – Architecture, Protocols and Services, Third Edition, John Wiley & Sons