GSM (chapter 1)

GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS (GSM)

In 1982, the Nordic PTT sent a proposal to Conférence Européenne des Postes et Télécommunications (CEPT) to specify a common European telecommunication service at 900 MHz. A Global System for Mobile Communications (GSM) standardization group was established to formulate the specifications for this pan-European mobile cellular radio system. During 1982 through 1985, discussions centered around whether to build an analog or a digital system. Then in 1985, GSM decided to develop a digital system. In 1986, companies participated in a field test in Paris to determine whether a narrowband or broadband solution would be employed. By May 1987, the narrowband Time Division Multiple Access (TDMA) solution was chosen. Concurrently, operators in 13 countries (two operators in the United Kingdom) signed the Memorandum of Understanding (MoU) which committed them to fulfilling GSM specifications and delivering a GSM system by July 1, 1991. This opened a large new market. The next step in the GSM evolution was the specification of Personal Communication Network (PCN) for the 1800 MHz frequency range. This was named the Digital Cellular System (DCS) 1800. The Personal Communication Services (PCS) 1900 for the 1900 MHz frequency range was also established.

THE DIFFERENT GSM-BASED NETWORKS

Different frequency bands are used for GSM 900/1800 and GSM 1900 (Figure 1). In some countries, an operator applies for the available frequencies. In other countries (e.g. United States), an operator purchases available frequency bands at auctions.

Figure 1 Frequency Band for different GSM

NETWORK HARDWARE

Every cellular system has hardware that is specific to it and each piece of hardware has a specific function. The Ericsson GSMbased systems comply to the GSM standard while varying from

it for the purpose of overall system improvement. The system solutions integrate existing hardware and new technology to provide a “total” solution to the mobile telephony market. The major systems in the network are:

· Operation and Support System

· Switching System

· Base Station System

The system is normally configured as depicted in Figure 2.

Figure 2 GSM Based System Model

OPERATION AND SUPPORT SYSTEM (OSS)

For GSM system administration, the OSS supports the network operator by providing:

· Cellular network administration

· Network operation and support

SWITCHING SYSTEM (SS)

Figure 3 shows the main components of the switching system. The following is a brief description of each of these components.

Figure 3 Switching System

· Mobile Services Switching Center (MSC)

The MSC is responsible for set-up, routing, and supervision of calls to and from mobile subscribers. Other functions are also implemented in the MSC, such as authentication. The MSC is built on an AXE-10 platform.

· Visitor Location Register (VLR)

In the GSM based solution, the VLR is integrated with the MSC. This is referred to as the MSC/VLR. The VLR contains non-permanent information about the mobile subscribers visiting the MSC/VLR service area, e.g. which location area the MS is in currently.

· Gateway MSC (GMSC)

The GMSC supports the function for routing incoming calls to the MSC where the mobile subscriber is currently registered. It is normally integrated in the same nodes as MSC/VLR.

· Home Location Register (HLR)

In GSM, each operator has a database containing information about all subscribers belonging to the specific Public Land Mobile Network (PLMN). This database can be implemented in one or more HLRs. Two examples of information stored in the database are the location (MSC/VLR service area) of the subscribers and services requested. The HLR is built on an AXE-10 platform.

· Authentication Center (AUC)

For security reasons, speech, data, and signaling are ciphered, and the subscription is authenticated at access. The AUC provides authentication and encryption parameters required for subscriber verification and to ensure call confidentiality.

· Equipment Identity Register (EIR)

In GSM there is a distinction between subscription and mobile equipment. As mentioned above, the AUC checks the subscription at access. The EIR checks the mobile equipment to prevent a stolen or non-type-approved MS from being used.

· Interworking Location Register (ILR)

Around the world there are market demands for roaming capabilities with GSM. The ILR is the node that forwards roaming information between cellular networks using different operating standards. This currently exists only in the GSM 1900 network.

· Short Message Service Gateway MSC (SMS-GMSC)

A Short Message Service Gateway MSC (SMS-GMSC) is capable of receiving a short message from a Service Center (SC), interrogating an HLR for routing information and message waiting data, and delivering the short message to the MSC of the recipient MS. In GSM system, the SMS-GMSC functionality is normally integrated in the MSC/VLR node.

· Short Message Service Interworking MSC (SMSIWMSC)

A Short Message Service InterWorking MSC (SMSIWMSC) is capable of receiving a mobile originated short message from the MSC or an ALERT message from the HLR and submitting the message to the recipient Service Center (SC). The SMS-IWMSC functionality is normally

integrated in the MSC/VLR node.

· Data Transmission Interface (DTI)

DTI - consisting of both hardware and software - provides an interface to various networks for data communication. Through DTI, users can alternate between speech and data during the same call. Its main functions include a modem and fax adapter pool and the ability to perform rate adaptation. It was earlier implemented as the GSM InterWorking Unit (GIWU).

BASE STATION SYSTEM (BSS)

The Base Station System (BSS) is comprised of two major components. They are:

· Base Station Controller (BSC)

· Base Transceiver Station (BTS)

Figure 4 Base Station System

The Base Station Controller (BSC) is the central point of the BSS. The BSC can manage the entire radio network and performs the following functions:

· Handling of the mobile station connection and handover

· Radio network management

· Transcoding and rate adaptation

· Traffic concentration

· Transmission management of the BTSs

· Remote control of the BTSs