Global System for Mobile Communications (GSM) Notes

Global System for Mobile Communications (GSM)


Services and Architecture

If your work involves (or is likely to involve) some form of wireless public communications, you are likely to encounter the GSM standards. Initially developed to support a standardized approach to digital cellular communications in Europe, the “Global System for Mobile Communications” (GSM) protocols are rapidly being adopted to the next generation of wireless telecommunications systems, Personal Communication Systems (PCS) and global Low Earth Orbit (LEO) satellite communication systems. In the US, its main competition appears to be the cellular TDMA systems based on the IS-54 standards. Since the GSM systems consist of a wide range of components, standards, and protocols.

The GSM and its companion standard DCS1800 (for the UK, where the 900 MHz frequencies are not available for GSM) have been developed over the last decade to allow cellular communications systems to move beyond the limitations posed by the older analog systems. Analog system capacities are being stressed with more users that can be effectively supported by the available frequency allocations. Compatibility between types of systems had been limited, if non-existent. By using digital encoding techniques, more users can share the same frequencies than had been available in the analog systems. As compared to the digital cellular systems in the US (CDMA [IS-95] and TDMA [IS-54]), the GSM market has had impressive success. Estimates of the numbers of telephones run from 7.5 million GSM phones to .5 million IS54 phones to .3 million for IS95.

GSM has gained in acceptance from its initial beginnings in Europe to other parts of the world including Australia, New Zealand, countries in the Middle East and the far east. Beyond its use in cellular frequencies (900 MHz for GSM, 1800 MHz for DCS1800), portions of the GSM signaling protocols are finding their way into the newly developing PCS and LEO Satellite communications systems. While the frequencies and link characteristics of these systems differ from the standard GSM air interface, all of these systems must deal with users roaming from one cell (or satellite beam) to another, and bridge services to public communication networks including the Public Switched Telephone Network (PSTN), and public data networks (PDN).

Digital Transmission Enables Delivery of a Rich Set of Services

As a digital network that has many parallels to the Integrated Services Digital Network (ISDN), GSM offers a rich set of services that include voice, circuit switched data, packet data, and fax, all of which are afforded a level of privacy not available through the analog cellular networks. To remain consistent with existing cellular and PSTN systems, GSM also supports a range of supplementary services, such as call barring, call forwarding, call waiting, and advice of charge. The ability to provide these services introduces a new level of complexity. With all information being transferred over the air interface at 13 kbps transcoding schemes and format translation services must be provided by the GSM network components. Voice information is digitized using the Regular Pulse Excitation-Long Term Prediction algorithm that removes enough redundancy from the voice signal to transmit it over the 13 kbps channel; this is translated to PCM and ADPCM by the GSM switching network for transmission over the PSTN. Mobility also presents a unique set of challenges; users may roam into areas supported by other carriers. Algorithms and protocols have been designed to locate users and handle charging while users are visiting areas away from home. Data formats and control signals are transferred between the switching systems and mobile subscriber equipment.

An additional benefit of digital transmission is security. Where the analog systems are able to provide extremely limited protection against eavesdropping and false call origination, GSM has features to address each of these. Users can be authenticated on the basis of information contained in their SIM. The radio path is also encrypted to provide additional confidentiality.

Several Components Must Communicate Within the System

The GSM architecture includes several subsystems:

  • the Mobile Station (MS) — These digital telephones include vehicle, portable and hand-held terminals. A device called the Subscriber Identity Module (SIM) that is basically a smart-card provides custom information about users such as the services they’ve subscribed to and their identification in the network
  • the Base Station Sub-System (BSS) — The BSS is the collection of devices that support the switching networks radio interface. Major components of the BSS include the Base Transceiver Station (BTS) that consists of the radio modems and antenna equipment, and the Base Station Controller (BSC) that manages the radio activities of several BTS and connects to a single NSS. In OSI terms, the BTS provides the physical interface to the MS where the BSC is responsible for the link layer services to the MS. Logically the transcoding equipment is in the BTS, however, an additional component, the Transcoder/Rate Adapter Unit (TRAU) can also provide signal transcoding services.
  • the Network and Switching Sub-System (NSS) — The NSS provides the switching between the GSM subsystem and external networks along with the databases used for additional subscriber and mobility management. Major components in the NSS include the Mobile Services Switching Center (MSC), Home and Visiting Location Registers (HLR, VLR). The HLR and VLR databases are interconnected through the telecomm standard Signaling System 7 (SS7) control network.
  • the Operation Sub-System (OSS) — The OSS provides the support functions responsible for the management of network maintenance and services. Components of the OSS are responsible for network operation and maintenance, mobile equipment management, and subscription management and charging.

Layered Protocols Include Both New (Air Interface) and Old (SS7) Components

As one might suspect, this collection of components and services requires the use of several protocols to control calls, transfer information, and provide overall system management. From the perspective of the MS, there are four layers for communication:

  • the RF interface to the BTS
  • the radio resource management (RR) layer to the BSC
  • mobility management (MM)
  • communications management (CM) to the MSC VLR

Additional protocols are used to provide control services that are managed between the system switching and management components.


The transmission channel between the MS and the BTS is the one component that is unique to GSM cellular networks, modified to operate on different frequencies in the case of PCS and replaced in its entirety in the case of satellite communications systems. The interface between the MS and the BTS consists of a frequency-hopped TDMA channel that is divided in several subchannels, some of which are used for the transmission of user information, the remainder of which are used by the assorted control protocols. To increase battery life and to decrease interference between stations operating in adjacent cell-sites, both the MS and the BTS transmitters automatically adapt their transmission power. Several channels are used in the air interface:

  • FCCH – the frequency correction channel – provides frequency synchronization information in a burst
  • SCH – Synchronization Channel – shortly following the FCCH burst (8 bits later), provides a reference to all slots on a given frequency
  • PAGCH – Paging and Access Grant Channel used for the transmission of paging information requesting the setup of a call to a MS.
  • RACH – Random Access Channel – an inbound channel used by the MS to request connections from the ground network. Since this is used for the first access attempt by users of the network, a random access scheme is used to aid in avoiding collisions.
  • CBCH – Cell Broadcast Channel – used for infrequent transmission of broadcasts by the ground network.
  • BCCH – Broadcast Control Channel – provides access status information to the MS. The information provided on this channel is used by the MS to determine whether or not to request a transition to a new cell
  • FACCH – Fast Associated Control Channel for the control of handovers
  • TCH/F – Traffic Channel, Full Rate for speech at 13 kbps or data at 12, 6, or 3.6 kbps
  • TCH/H – Traffic Channel, Half Rate for speech at 7 kbps, or data at 6 or 3.6 kbps

Slow frequency hopping is employed on the traffic channels that are centered at 200 kHz intervals between 890 and 915 MHz and 935 and 960 MHz. Through slow frequency hopping, frequency diversity is obtained thereby improving the overall signal quality by not dwelling on noisy channels. Each transmission burst is completed prior to switching frequencies.

The radio resource (RR) protocols are responsible for the allocation and reallocation of traffic channels between the MS and the BTS. These services include controlling the initial access to the system, paging for Mobile terminated calls, handover of calls between cell sites, power control, and call termination. The RR protocols provide the procedures for the use, allocation, reallocation, and release of the GSM channels.

Mobility Management

One of the major features used in all classes of GSM networks (cellular, PCS and Satellite) is the ability to support roaming users. Through the control signaling network, the MSCs interact to locate and connect to users throughout the network. “Location Registers” are included in the MSC databases to assist in the role of determining how, and whether connections are to be made to roaming users. Each user of a GSM MS is assigned a Home Location Register (HLR) that is used to contain the user’s location and subscribed services. A separate register, the Visitor Location Register (VLR) is used to track the location of a user. As the users roam out of the area covered by the HLR, the MS notifies a new VLR of its whereabouts. The VLR in turn uses the control network (that happens to be based on SS7) to signal the HLR of the MS’s new location. Through this information, mobile terminated (MT) calls can be routed to the user by the location information contained in the user’s HLR.

Communication Management Features Support Call Control and Supplementary Services

The communication management layer provides three primary classes of services, call control, supplementary services, and the short message service. Call control services are responsible for routing the calls, determining who is responsible for the call charges, and the organization that is to receive payment. Supplementary services include call forwarding, barring, and passwords for security. Finally, the communication management layer includes services to handle short message services, that are more efficiently handled through packet oriented transfers than the traditional circuit switched connections supported by the mainstream GSM system.

Source Encoding and Interwork Functions Support Interoperability With Public Networks

With a data transfer rate that is unique to GSM and too limited to send the typical 8 kHz sampling of the 3 kHz spectrum, functions are included in the system to transcode voice signals into narrowband digital information. In addition, data services that support interoperability with standard modems are provided so that mobile users can communicate with users connected to the public networks. Voice services are compressed for transmission over the air interface and converted to either PCM or ADPCM formats for transmission over the synchronous network. Circuit switched data and fax information is transmitted over the GSM network in a digital form and converted to the appropriate modulation schemes for communication with existing systems. Packet data is supported through use of the GSM circuits providing access to terrestrial hub equipment.

As the growth of wireless communications continues, the use and range of services addressed by GSM are likely to grow. Interoperability between PCS, cellular, and satellite systems can be more readily accomplished. Many of the LEO satellite systems are proposing the development of dual GSM/LEO terminals that can provide users with ubiquitous coverage. With the current activity in distribution of the PCS frequencies for major US markets, GSM will become a major force in the US telecomm market. By providing a standard for wireless communication that has been accepted in many areas of the world, users benefit from the ability to be reached regardless of their location.

The IMEI number

The GSM IMEI (International Mobile Equipment Identity) numbering system is a 15 digit unqiue code that is used to identify the GSM/DCS/PCS phone to a GSM/DCS/PCS network.

When a phone is switched on, its unqiue IMEI number is transmitted and checked against a database of blacklisted or greylisted phones in the network’s EIR (Equipment ID Register). This EIR determines whether the phone can log onto the network to initiate and receive calls.

You can display your phone’s IMEI number by typing *#06# on the keypad. This code works on most phones.

If the EIR and IMEI numbers match, the networks can do a number of things. They can for example greylist or blacklist a phone:

  • Greylisting will allow the phone to be used, but it can be tracked to see who has it (via the SIM info).
  • Blacklisting bars the phone from being used on any network where there is an EIR match.

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