GSM Architecture

Introduction to mobile networks

There are two types of mobile networks that can be formed: infrastructure networks and ad hoc network or infrastructure independent networks. Both types can be designed for voice/data services.

The infrastructure application is aimed at office areas or to provide a hotspot. It can be installed instead of a wired system and can provide considerable cost savings, especially when used in established offices. A backbone wired network may be still required and is connected to a server. The wireless network is then split up into a number of cells, each serviced by a base station or access point (AP), which acts as a controller for the cell. Each access point may have a range dependent upon the environment and the location of the access point.

The other type of network that may be used is termed as an ad hoc network. These are formed when a number of computers and various devices like phones, printers….etc are brought together to establish their own network in a small domain. They may be needed when several people come together and need to share data, or if they need to access a printer without the need for having to use wire connections. In such a situation, the users only communicate with each other and not with a larger wired network. As a result there is no access point as it is in infrastructure network. Special algorithms within the protocols are used to enable one of the devices to take over the role of the master to control the network with the others acting as slaves.

GSM System

The GSM system is the most widely used mobile telecommunication system in the world today. The GSM originally stood for groupe special mobile, but afterwards it was changed to global system for mobile communications. Since it was first deployed in 1991, the use of GSM has grown steadly, and it is now the most widely spread out cell phone system in the world. The GSM reached the one billion subscriber point in February 2004 and continues to grow in popularity. Wireless technology growth for mobile telephony and among all positions of GSM is as follows:

·         Firstly, there was the analog mobile phone and limited area courage of 1G. This type of system offered limited mobility and voice communication within a group only. No data communication was possible.

·         Secondly, the analog cellular phone with wide area coverage came. This system offered complete mobility within the service area and connectivity to PSTN and other networks, however, still without data communication facility.

·         Thirdly, the digital cellular phone systems (GSM) of 2G came. This system offers complete mobility within the service area and connectivity to PSTN and other networks. They provided data communication facility also along with other advanced features.

·         Lastly, came the digital cellular communication systems (3G). This system offers complete mobility within the service area and connectivity to PSTN and Internet/intranet. They have a very high data rate communication facilities and other advanced features.

The GSM system was designed as a second-generation (2G) cellular telecommunication system. It is a fully digitized technology for better speech quality. One of the basic aims was to provide a system that would enable a greater capacity to be achieved than the previous first-generation analog systems. The GSM achieved this by using a digital TDMA along with FDMA. By adopting this technique, more users can be accommodated within the available bandwidth. In addition to this, ciphering of the digitally encoded speech was adopted to retain privacy. Because of the encryption of user information, GSM calls cannot be captured by unauthorized persons.

Components of GSM System

The architecture of the GSM system with its hardware can broadly be grouped into three main areas: the mobile station, the base station subsystem, and the network subsystem. Each area performs its own functions and when used together, they enable the fully operational capability of the system to be realized.

Mobile Station

The mobile station consists of two units – mobile handset with battery and subscriber identity module (SIM).

The first element is mobile handset, which is a piece of hardware that the customer purchases from the equipment manufacturer or their dealers. This hardware piece contains all the components needed for the implementation of the protocols to interface with the user and the air-interface to the BSS. The components include speaker, microphone, keypad, and the radio modem. Therefore, the ME is an expensive piece of hardware. To encourage more users to subscribe to the wireless services, a number of service providers in the early days of the cellular industry, and even today, subsidize the price of the MEs. Each handset has a unique identity number known as the international mobile equipment identity (IMEI). This is installed in the phone at the time of manufacturing and cannot be changed. It is accessed by the network during registration to check whether the equipment has been reported as stolen.

The second element of the MS in the GSM is the subscriber identity module (SIM) that is a smart card issued at the subscription time identifying the specifications of a user such as address and type of service. The calls in the GSM are directed to the SIM rather than the terminal. Short messages are also stored in the SIM card. Using SIM cards was not possibly with the analog cellular systems, and the existing North American digital cellular standards have not implemented this option. Although implementing a SIM is a fairly simple concept, it has an important impact on the way that a user transacts with the service provider. A SIM card carries every user’s personal information which enables a number of useful applications. The SIM is a removable module that fits in the mobile handset. Each SIM has unique IMSI International Mobile Subscriber Identity (IMSI). It has a built-in microcomputer and memory in it. It contains a ROM of 6 to 16 KB, RAM of 128 to 256 KB (least phones have phone memories in Mbytes and even one GB and can be additionally provided) and EEPROM of three to eight KB. Memory is the limitation for accessing large web pages on mobile phones.

Base Station Subsystem (BSS)

This section of the GSM network is fundamentally associated with communication with the mobiles on the network. It consists of two elements, namely the base transceiver station (BTS) and the base station controller (BSC).

Base Transceiver Station (BTS)

The BTS used in a GSM network comprises radio transmitter receivers and their associated antennas that transmit and receive to directly communicate with the mobiles. The BTS is defining element for each cell. One BTS covers one or more cell. The capacity of BTS depends on the transceivers. The BTS communicates with the mobiles and the interface between the two is known as the Um interface with its associated protocols.

The BTS is connected to BSC via an A’bis interface. The transmission rate on A’bis is 2 Mbps. The interface between MS and BTS is called air interface. The transmission rate on the air interface is 13 kbps.

Base Station Controller (BSC)

The BSC controls several BTSs. It manages channel allocation and handover of calls from one BTS to another BTS. The BSC is connected to MSC via the A interface. The transmission rate on the A interface is 2 Mbps. The BSC has the database for all its BTS parameters. It provides path from MS to MSC.

Network Subsystem

The network subsystem contains a variety of different elements and is often termed the core network. It provides the main control and interfacing for the whole mobile network. It includes the elements, such as MSC, HLR, VLR, AUC, and more.

Mobile Switching Centre (MSC)

The MSC is heart of the entire network connecting the fixed line network to the mobile network. It manages all call-related functions and billing information. It is connected to the HLR and VLR for subscriber identification and for routing incoming calls. The MSC capacity is in terms of the number of subscribers. The MSC is connected to BSC at the one end and to the fixed line network at the other end. Call Detail Record (CDR) is generated for each and every call in the MSC.

Home Location Register (HLR)

The entire subscriber’s data is stored in HLR. It has a permanent database of all the registered subscribers. The HLR has a series of numbers for all subscribers. When a user switches on the phone, the phone registers with the network and from there, it is possible to determine which BTS it communicates with so that the incoming calls can be routed appropriately. Even when the phone is not active (but switched on), it re-registers periodically to ensure that the network (HLR) is aware of its latest position. There is one HLR per network, although it may be distributed across various subcentres too for operational reasons.

Visiting Location Register (VLR)

An active subscriber is registered in VLR. It has a temporary database of all the active subscribers used for their call routing. The HLR validates the subscriber before registration. The MSC asks VLR before routing incoming call.

Authentication Centre (AUC)

The AUC is a protected database that contains the secret key also contained in the user’s SIM card. It is used for authentication and for ciphering on the radio channel. Authentication is a process to verify the subscriber SIM. Secret data and verification algorithm are stored in the AUC. The AUC and HLR combine to authenticate the subscribers. Subscriber information can be done on every call, if required.

Equipment Identity Register (EIR)

All subscriber’s mobile handset data is stored in EIR. The EIR is the entity that decides whether a given mobile equipment may be allowed onto the network. Each mobile equipment has a unique IMEI. This number, as mentioned above, is installed in the equipment and is checked by the network during registration. The MSC also asks the mobile to send its IMEI and then checks it with the data available in EIR. The EIR has different classifications for mobile handsets like white list, grey list, and black list. Depending upon the information held in the EIR, the mobile may be allocated one of the three states – allowed onto the network, barred access, or monitored in case of problems. According to the categorization, the MS can make calls or can be stopped from making calls.

Operation and Maintenance Centre (OMC)

All the network elements are connected to the OMC, which monitors the health of all the network elements are carries out any maintenance, if required. The OMC links to BTS are via the parent BSC. The OMC keeps records of all the faults occurred. It can also do traffic analysis and prepares the MIS report for the network.

Resources Used:

1. Principles of Wireless Networks (By: Prashant Krishnamurthy)
2. Wireless Communication (By: Upena Dalal)
3. Mobile Communication (By: Schiller)

Compiled By: Chaudhary Amit V.