Introduction to GSM

What is GSM?

Global System for Mobile Communication (GSM) is a set of ETSI standards specifying the infrastructure for a digital cellular service. The standard is used in approx. 85 countries in the world including such locations as Europe, Japan and Australia¹.

Home Location Register (HLR)

A Home Location Register (HLR) is a database that contains semi-permanent mobile subscriber information for a wireless carriers' entire subscriber base. HLR subscriber information includes the International Mobile Subscriber Identity (IMSI), service subscription information, location information (the identity of the currently serving Visitor Location Register (VLR) to enable the routing of mobile-terminated calls), service restrictions and supplementary services information.

The HLR handles SS7 transactions with both Mobile Switching Centers (MSCs) and VLR nodes, which either request information from the HLR or update the information contained within the HLR. The HLR also initiates transactions with VLRs to complete incoming calls and to update subscriber data.

Traditional wireless network design is based on the utilization of a single Home Location Register (HLR) for each wireless network, but growth considerations are prompting carriers to consider multiple HLR topologies.

Visitor Location Register (VLR)

A Visitor Location Register (VLR) is a database which contains temporary information concerning the mobile subscribers that are currently located in a given MSC serving area, but whose Home Location Register (HLR) is elsewhere.

When a mobile subscriber roams away from his home location and into a remote location, SS7 messages are used to obtain information about the subscriber from the HLR, and to create a temporary record for the subscriber in the VLR. There is usually one VLR per MSC.

International Mobile Subscriber Identity (IMSI) Number

The IMSI is a unique non-dialable number allocated to each mobile subscriber in the GSM system that identifies the subscriber and his or her subscription within the GSM network. The IMSI resides in the Subscriber Identity Module (SIM), which is transportable across Mobile Station Equipment (MSE). The IMSI is made up of three parts (1) the mobile country code (MCC) consisting of three digits, (2) the Mobile Network Code (MNC) consisting of two digits, and (3) the Mobile Subscriber Identity Number (MSIN) with up to 10 digits.

Mobile Subscriber ISDN (MSISDN) Number

The MSISDN is the dialable number that callers use to reach a mobile subscriber. Some phones can support multiple MSISDNs - for example, a U.S.-based MSISDN and a Canadian-based MSISDN. Callers dialing either number will reach the subscriber.

Mobile Station Equipment (MSE) Subscription Services

GSM carriers typically order Mobile Station Equipment (MSE) (or GSM phones) from their suppliers (Nokia, Motorola, Sony, etc.) in large quantities (e.g. 1000 Units). After receiving an order, the equipment supplier will program the ordered MSE SIMs with a range of IMSI numbers.

Example: ABC Communications Inc. orders 1000 MSE Units with the following range of IMSIs.

Once the range of IMSI numbers has been determined, the HLR can be populated with the new IMSI records that will be configured and activated at a future date by authorized sales or service subscription representatives. The fact that the HLR can be populated with ranges or blocks of IMSI numbers creates efficiencies in the storage and retrieval of routing information.

The wireless carrier distributes the Mobile Station Equipment to Sales Outlets that sell GSM subscription services. When a new subscriber orders a GSM phone at one of the outlets, the service representative will create a Service Order (SO) to enter the new subscriber's service subscription information, including the MSISDN number. The key to the Service Order is the IMSI that is programmed in the SIM. The SO is sent to the HLR, where the IMSI record is created. It can either be set to an active state immediately, allowing the new subscriber to send and receive telephone calls or it can be activated at a future date.

Note that the MSISDN numbers are assigned one at a time as each new customer subscribes. The MSISDN numbers are therefore provisioned individually, rather than in blocks, which complicates the MSISDN based routing of messages. The ramifications of MSISDN provisioning are discussed further in the example found at the end of this document.

GSM Call Routing

Mobile Subscriber Roaming

When a mobile subscriber roams into a new location area (new VLR), the VLR automatically determines that it must update the HLR with the new location information, which it does using an SS7 Location Update Request Message. The Location Update Message is routed to the HLR through the SS7 network, based on the global title translation of the IMSI that is stored within the SCCP Called Party Address portion of the message. The HLR responds with a message that informs the VLR whether the subscriber should be provided service in the new location.

Mobile Subscriber ISDN Number (MSISDN) Call Routing

When a user dials a GSM mobile subscriber's MSISDN, the PSTN routes the call to the Home MSC based on the dialed telephone number. The MSC must then query the HLR based on the MSISDN, to attain routing information required to route the call to the subscribers' current location.

The MSC stores global title translation tables that are used to determine the HLR associated with the MSISDN. When only one HLR exists, the translation tables are trivial. When more than one HLR is used however, the translations become extremely challenging, with one translation record per subscriber (see the example below). Having determined the appropriate HLR address, the MSC sends a Routing Information Request to it.

When the HLR receives the Routing Information Request, it maps the MSISDN to the IMSI, and ascertains the subscribers' profile including the current VLR at which the subscriber is registered. The HLR then queries the VLR for a Mobile Station Roaming Number (MSRN). The MSRN is essentially an ISDN telephone number at which the mobile subscriber can currently be reached. The MSRN is a temporary number that is valid only for the duration of a single call.

The HLR generates a response message, which includes the MSRN, and sends it back across the SS7 network to the MSC. Finally, the MSC attempts to complete the call using the MSRN provided.

Adding a Second HLR to the GSM Network

As a GSM wireless carrier's subscriber base grows, it will eventually become necessary to add a second HLR to their network. This requirement might be prompted by a service subscription record storage capacity issue, or perhaps an SS7 message processing performance issue. It might possibly be prompted by a need to increase the overall network reliability.

The new HLR can be populated with service subscription records as new subscribers are brought into service or existing service subscription records can be ported from the old HLR to the new HLR to more evenly distribute the SS7 traffic load.

Typically, when new subscribers are brought into service, the second HLR will be populated with blocks of IMSI numbers that are allocated when new MSE equipment is ordered. As the following example shows, this grouping of IMSI numbers within a single HLR simplifies the routing translations that are required within the SS7 network for VLR to HLR Location Update Request transactions. Global Title Translation (GTT) tables will contain single translation records that translate an entire range of IMSIs numbers into an HLR address. Even if some individual records are moved between the HLRs, as shown in the example, the treatment of IMSIs as blocks results in a significant simplification of the Global Translation tables.

Much more complicated SS7 message routing Global Title Translations are required for Routing Information Request transactions between the MSCs distributed over the entire wireless carrier serving area and the two or more HLRs. MSC Routing Information Requests are routed to the appropriate HLR based on the dialed MSISDN and not the IMSI. Unlike the IMSI numbers, the MSISDN numbers can not easily be arranged in groups to reside within a single HLR and therefore, the MSC must contain an MSISDN to HLR address association record for every mobile subscriber homed on each of the MSCs. As the example illustrates, the MSC routing tables quickly grow much more extensive than the STP tables. The network administration becomes increasingly complex and prone to error.

Example: Simple Network with two MSCs and two HLRs

The following example illustrates the issues relating to GSM network routing table administration with multiple HLRs. A simple GSM network is shown, with the various routing tables following:

HLR Datafill

HLR #1 is populated with IMSI Range 310-68-4451000 to 310-68-4451005 and is populated with service subscribers from two different MSCs.

HLR #1

New HLR#2 is populated with IMSI Range 310-68-5568095 to 310-68-5568100 and is populated with new service subscribers from the same two MSCs. One subscriber has been moved from HLR #2 to HLR #1 (IMSI = 310-68-5568099).

HLR #2

STP Datafill

The STPs route SS7 messages to these HLRs based on the IMSI numbers which are usually provisioned in blocks. In this case, the STPs (which have identical GTT tables) are provisioned to route one block of IMSIs to the each HLR. Note that individual records can be moved between HLRs with the addition of another record in the routing table which specifies the individual IMSI. Individual records take precedence over IMSI block entries.

STP #1, #2

MSC Datafill

When a GSM subscriber receives a phone call, the call attempt messages are routed to the subscriber's MSC, based on the dialed numbers (the MSISDN). The MSC is provisioned with routing tables which relate each MSISDN to an HLR. Note that the MSISDN numbers cannot be assigned in convenient blocks like the IMSI numbers.

MSC #1