1. GSM History

8/14/2019 1. GSM History

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1. GSM History

During the early 1980s, analog cellular telephone systems were experiencing rapidgrowth in Europe, particularly in Scandinavia and the United Kingdom, but also inFrance and Germany. Each country developed its own system, which was

incompatible with everyone else's in equipment and operation. This was anundesirable situation, because not only was the mobile equipment limited tooperation within national boundaries, which in a unified Europe were increasinglyunimportant, but there was also a very limited market for each type of equipment,so economies of scale and the subsequent savings could not be realized.

The Europeans realized this early on, and in 1982 the Conference of European Postsand Telegraphs (CEPT) formed a study group called the Groupe Spcial Mobile (GSM)to study and develop a pan-European public land mobile system. The proposedsystem had to meet certain criteria:

Good subjective speech quality

Low terminal and service cost Support for international roaming Ability to support handheld terminals Support for range of new services and facilities Spectral efficiency ISDN compatibility

Pan-European means European-wide. ISDN throughput at 64Kbs was neverenvisioned, indeed, the highest rate a normal GSM network can achieve is 9.6kbs.

Europe saw cellular service introduced in 1981, when the Nordic Mobile TelephoneSystem or NMT450 began operating in Denmark, Sweden, Finland, and Norway inthe 450 MHz range. It was the first multinational cellular system. In 1985 GreatBritain started using the Total Access Communications System or TACS at 900 MHz.Later, the West German C-Netz, the French Radiocom 2000, and the ItalianRTMI/RTMS helped make up Europe's nine analog incompatible radio telephonesystems. Plans were afoot during the early 1980s, however, to create a singleEuropean wide digital mobile service with advanced features and easy roaming.While North American groups concentrated on building out their robust butincreasingly fraud plagued and featureless analog network, Europe planned for adigital future. Link to my mobile telephone history series

In 1989, GSM responsibility was transferred to the European TelecommunicationStandards Institute (ETSI), and phase I of the GSM specifications were published in1990. Commercial service was started in mid-1991, and by 1993 there were 36 GSM

networks in 22 countries [6]. Although standardized in Europe, GSM is not only aEuropean standard. Over 200 GSM networks (including DCS1800 and PCS1900) areoperational in 110 countries around the world. In the beginning of 1994, there were1.3 million subscribers worldwide [18], which had grown to more than 55 million byOctober 1997. With North America making a delayed entry into the GSM field with aderivative of GSM called PCS1900, GSM systems exist on every continent, and theacronym GSM now aptly stands for Global System for Mobile communications.

According to the GSM Association as of 2002, here are the current GSM statistics:
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No. of Countries/Areas with GSM System (October 2001) - 172 GSM Total Subscribers - 590.3 million (to end of September 2001) World Subscriber Growth - 800.4 million (to end of July 2001) SMS messages sent per month - 23 Billion (to end of September 2001) SMS forecast to end December 2001 - 30 Billion per month GSM accounts for 70.7% of the World's digital market and 64.6% of the

World's wireless market

http://www.gsmworld.com/membership/mem_stats.html (external link, now dead.)

The developers of GSM chose an unproven (at the time) digital system, as opposedto the then-standard analog cellular systems like AMPS in the United States andTACS in the United Kingdom. They had faith that advancements in compressionalgorithms and digital signal processors would allow the fulfillment of the originalcriteria and the continual improvement of the system in terms of quality and cost.The over 8000 pages of GSM recommendations try to allow flexibility andcompetitive innovation among suppliers, but provide enough standardization toguarantee proper interworking between the components of the system. This is done

by providing functional and interface descriptions for each of the functional entitiesdefined in the system.

The United States suffered no variety of incompatible systems as in the differentcountries of Europe. Roaming from one city or state to another wasn't difficult . Yourmobile usually worked as long as there was coverage. Little desire existed to designan all digital system when the present one was working well and proving popular. Toillustrate that point, the American cellular phone industry grew from less than204,000 subscribers in 1985 to 1,600,000 in 1988. And with each analog basedphone sold, chances dimmed for an all digital future. To keep those phones working(and producing money for the carriers) any technological system advance wouldhave to accommodate them.

GSM was an all digital system that started new from the beginning. It did not have toaccommodate older analog mobile telephones or their limitations. American digitalcellular, first called IS-54 and then IS-136, still accepts the earliest analog phones.American cellular networks evolved slowly, dragging a legacy of underperformingequipment with it. Advanced fraud prevention, for example, was designed in later forAMPS, whereas GSM had such measures built in from the start. GSM was arevolutionary system because it was fully digital from the beginning.

2. Services provided by GSM

From the beginning, the planners of GSM wanted ISDN compatibility in terms of the

services offered and the control signalling used. However, radio transmissionlimitations, in terms of bandwidth and cost, do not allow the standard ISDN B-channel bit rate of 64 kbps to be practically achieved.

Isn't this a shame? What many wireless customers need most is a high speed dataconnection and this is what GSM provides least. Only 9.6kbs if everything worksright. It is possible the GSM designers in the early 1980s never envisioned the needfor such bandwidth. It may be true, too, that in most countries the radio spectrumneeded to give every caller a 64kbs channel was never available. The add on


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technology EDGE (external link) promises higher data speed rates in the near tomid-term for GSM. Highest data rates will come in the long term when GSM changesinto a radio service based on wide band code division multiple access, and not TDMA.

Using the ITU-T definitions (external link), telecommunication services can bedivided into bearer services, teleservices, and supplementary services. The most

basic teleservice supported by GSM is telephony. As with all other communications,speech is digitally encoded and transmitted through the GSM network as a digitalstream. There is also an emergency service, where the nearest emergency-serviceprovider is notified by dialing three digits (similar to 911).

Bearer services: Typically data transmission instead of voice. Fax and SMSare examples.

Teleservices: Voice oriented traffic. Supplementary services: Call forwarding, caller ID, call waiting and the like.

A variety of data services is offered. GSM users can send and receive data, at ratesup to 9600 bps, to users on POTS (Plain Old Telephone Service), ISDN, Packet

Switched Public Data Networks, and Circuit Switched Public Data Networks using avariety of access methods and protocols, such as X.25 or X.32. Since GSM is a digitalnetwork, a modem is not required between the user and GSM network, although anaudio modem is required inside the GSM network to interwork with POTS.

GSM is an all digital network but many machines are still analog, as is most of thelocal loop. Thus, we need a modem, even though we are dealing with digital.

A FAX machine's digital signal processor converts an analog image into aninstantaneous digital representation; a series of bits, all 0s and 1s. A modulator thenturns these bits into audio tones representing the digital values. An analog FAXmachine at the other end converts the tones received back into digital bits and then

into an image.

This tedious process was required initially because local loops were and are primarilyanalog. In addition, digital services such as T1, fractional T1, or ISDN, whereavailable, was and is extremely expensive. All digital equipment, such as Group 4Fax machines, are far higher priced than their analog counterparts. The local loopwill remain primarily analog for some time.
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Other data services include Group 3 facsimile, as described in ITU-T recommendationT.30, which is supported by use of an appropriate fax adaptor. A unique feature ofGSM, not found in older analog systems, is the Short Message Service (SMS). SMS is

a bidirectional service for short alphanumeric (up to 160 bytes) messages. Messagesare transported in a store-and-forward fashion. For point-to-point SMS, a messagecan be sent to another subscriber to the service, and an acknowledgement of receiptis provided to the sender. SMS can also be used in a cell-broadcast mode, forsending messages such as traffic updates or news updates. Messages can also bestored in the SIM card for later retrieval [2].

Supplementary services are provided on top of teleservices or bearer services. In thecurrent (Phase I) specifications, they include several forms of call forward (such ascall forwarding when the mobile subscriber is unreachable by the network), and callbarring of outgoing or incoming calls, for example when roaming in another country.Many additional supplementary services will be provided in the Phase 2

specifications, such as caller identification, call waiting, multi-party conversations.

3.Architecture of the GSM network

A GSM network is composed of several functional entities, whose functions andinterfaces are specified. Figure 1 shows the layout of a generic GSM network. TheGSM network can be divided into three broad parts. The Mobile Station is carried bythe subscriber. The Base Station Subsystem controls the radio link with the MobileStation. The Network Subsystem, the main part of which is the Mobile servicesSwitching Center (MSC), performs the switching of calls between the mobile users,and between mobile and fixed network users. The MSC also handles the mobilitymanagement operations. Not shown is the Operations and Maintenance Center,

which oversees the proper operation and setup of the network. The Mobile Stationand the Base Station Subsystem communicate across the Um interface, also knownas the air interface or radio link. The Base Station Subsystem communicates with theMobile services Switching Center across the A interface.

As John states, he presents a generic GSM architecture. Lucent, Ericsson, Nokia, andothers feature their own vision in their own diagrams. But they all share the samemain elements and parts from different vendors should all work together. The links
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below show how these vendors picture the GSM architecture. You can remember thedifferent terms much better by looking at all these diagrams.

Lucent GSM architecture/ Ericsson GSM architecture / Nokia GSM architecture /Siemen's GSM architecture

Figure 1. Generalarchitecture of a GSM network

SIM: Subscriber identifymodule.

ME: Mobile equipment.BTS: Base transceiver

station.

BSC: Base station

controller.

HLR: Home location

register.

VLR: Visitor location

register.

MSC: Mobile services

switching center.

EIR: Equipment identity

register.

AuC: Authentication

Center.

UM: Represents the radio link.

Abis: Represents the interface between the base stations and base stationcontrollers.

"A": The interface between the base station subsystem and the networksubsystem.

PSTN and PSPDN: Public switched telephone network and packet

switched public data network.

3.1. Mobile Station

The mobile station (MS) consists of the mobile equipment (the terminal) and a smartcard called the Subscriber Identity Module (SIM). The SIM provides personalmobility, so that the user can have access to subscribed services irrespective of aspecific terminal. By inserting the SIM card into another GSM terminal, the user isable to receive calls at that terminal, make calls from that terminal, and receiveother subscribed services.

The mobile equipment is uniquely identified by the International Mobile EquipmentIdentity (IMEI). The SIM card contains the International Mobile Subscriber Identity(IMSI) used to identify the subscriber to the system, a secret key for authentication,and other information. The IMEI and the IMSI are independent, thereby allowingpersonal mobility. The SIM card may be protected against unauthorized use by apassword or personal identity number.
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GSM phones use SIM cards, or Subscriber information or identity modules. Memorymodules. They're the biggest difference a user sees between a GSM phone orhandset and a conventional cellular telephone. With the SIM card and its memory theGSM handset is a smart phone, doing many things a conventional cellular telephonecannot. Like keeping a built in phone book or allowing different ringtones to bedownloaded and then stored. Conventional cellular telephones either lack the

features GSM phones have built in, or they must rely on resources from the cellularsystem itself to provide them. Let me make another, important point.

With a SIM card your account can be shared from mobile to mobile, at least intheory. Want to try out your neighbor's brand new mobile? You should be able to putyour SIM card into that GSM handset and have it work. The GSM network cares onlythat a valid account exists, not that you are using a different device. You get billed,not the neighbor who loaned you the phone.

This flexibility is completely different than AMPS technology, which enables onedevice per account. No swtiching around. Conventional cellular telephones have theirelectronic serial number burned into a chipset which is permanently attached to the

phone. No way to change out that chipset or trade with another phone. SIM cardtechnology, by comparison, is meant to make sharing phones and other GSM devicesquick and easy.

On the left above: Front of a Pacific Bell GSM phone. In the middle above: Samephone, showing the back. The SIM card is the white plastic square. It fits into the

grey colored holder next to it. On the right above. A new and different idea, a holderfor two SIM cards, allowing one phone to access either of two wireless carriers.Provided you have an account with both. :-) The Sim card is to the left of the body.

3.2 Base Station Subsystem

The Base Station Subsystem is composed of two parts, the Base Transceiver Station(BTS) and the Base Station Controller (BSC). These communicate across the


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standardized Abis interface, allowing (as in the rest of the system) operationbetween components made by different suppliers.

An explanation of the Abis interface is here

The Base Transceiver Station houses the radio tranceivers that

define a cell and handles the radio-link protocols with theMobile Station. In a large urban area, there will potentially bea large number of BTSs deployed, thus the requirements for aBTS are ruggedness, reliability, portability, and minimum cost.

The BTS or Base Transceiver Station is also called an RBS orRemote Base station. Whatever the name, this is the radiogear that passes all calls coming in and going out of a cell site.

The base station is under direction of a base station controllerso traffic gets sent there first. The base station controller,described below, gathers the calls from many base stations

and passes them on to a mobile telephone switch. From thatswitch come and go the calls from the regular telephonenetwork.

Some base stations are quite small, the one pictured here is a large outdoor unit.The large number of base stations and their attendant controllers, are a bigdifference between GSM and IS-136.

Want to read more about a base station? Download this product brochure fromSiemens. It's about 228K in .pdf

The Base Station Controller

The Base Station Controller manages the radio resources for oneor more BTSs. It handles radio-channel setup, frequency hopping,and handovers, as described below. The BSC is the connectionbetween the mobile station and the Mobile service SwitchingCenter (MSC).

Another difference between conventional cellular and GSM is thebase station controller. It's an intermediate step between thebase station transceiver and the mobile switch. GSM designersthought this a better approach for high density cellular networks.

As one anonymous writer penned, "If every base station talkeddirectly to the MSC, traffic would become too congested. Toensure quality communications via traffic management, the

wireless infrastructure network uses Base Station Controllers as a way to segmentthe network and control congestion. The result is that MSCs route their circuits toBSCs which in turn are responsible for connectivity and routing of calls for 50 to 100wireless base stations."
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Want to read more about a base station controller? Download

this product brochure from Siemens. It's about 363K in .pdf Twopage .pdf file on the network subsystem by Nokia. It's a glossyproduct brochure but it does mention all the important elements.(363k in .pdf)

Many GSM descriptions picture equipment called a TRAU, whichstands for Transcoding Rate and Adaptation Unit. Of course. Alsoknown as a TransCoding Unit or TCU, the TRAU is a compressorand converter. It first compresses traffic coming from the mobilesthrough the base station controllers. That's quite an achievementbecause voice and data have already been compressed by thevoice coders in the handset. Anyway, it crunches that data downeven further. It then puts the traffic into a format the MobileSwitch can understand. This is the transcoding part of its name,where code in one format is converted to another. The TRAU is notrequired but apparently it saves quite a bit of money to install one.Here's how Nortel Networks sells their unit:

"Reduce transmission resources and realize up to 75%transmission cost savings with the TCU."

"The TransCoding Unit (TCU), inserted between the BSC and MSC, enables speechcompression and data rate adaptation within the radio cellular network. The TCU isdesigned to reduce transmission costs by minimizing transmission resources betweenthe BSC and MSC. This is achieved by reducing the number of PCM links going to theBSC, since four traffic channels (data or speech) can be handled by one PCM timeslot. Additionally, the modular architecture of the TCU supports all three GSMvocoders (Full Rate, Enhanced Full Rate, and Half Rate) in the same cabinet,providing you with a complete range of deployment options."

(PCM? To read more about that click here.)

Voice coders or vocoders are built into the handsets a cellular carrier distributes.They're the circuitry that turns speech into digital. The carrier specifies which ratethey want traffic compressed, either a great deal or just a little. The cellular systemis designed this way, with handset vocoders working in league with the equipment ofthe base station subsystem.

3.3 Network Subsystem

The Mobile Switch

The central component of the Network Subsystem is the Mobile services SwitchingCenter (MSC). It acts like a normal switching node of the PSTN or ISDN, andadditionally provides all the functionality needed to handle a mobile subscriber, suchas registration, authentication, location updating, handovers, and call routing to aroaming subscriber. These services are provided in conjunction with severalfunctional entities, which together form the Network Subsystem. The MSC providesthe connection to the fixed networks (such as the PSTN or ISDN). Signalling between
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functional entities in the Network Subsystem uses SignallingSystem Number 7 (SS7), used for trunk signalling in ISDNand widely used in current public networks.

.pdf file on SS7 and mobile networking -- Good reading!

Mobile switches go by many names: mobile switch (MS), mobile switching center(MSC), or mobile telecommunications switching office (MTSO). They all do the samething, however, and that is to process mobile telephone calls. This switch can be anormal landline switch like a 5ESS, a Nokia, an Alcatel, or an Ericsson AXE(Automatic Exchange Electric) or a dedicated switch, built just to handle mobile calls.Each mobile switch manages dozens to scores of cell sites. In GSM the mobile switchhandles cell sites by first directing the base station controllers. Large systems mayhave two or more MSCs. It's easy understand what a switch does. What is harder tounderstand is the role the switch has to do with other network resources.

Two page .pdf file on the network subsystem by Nokia. It's a glossy product

brochure but it does mention all the important elements. (363k in .pdf)

Home Location Register and the Visitor/ed Location Register

The Home Location Register (HLR) and Visitor Location Register (VLR), together withthe MSC, provide the call-routing and roaming capabilities of GSM. The HLR containsall the administrative information of each subscriber registered in the correspondingGSM network, along with the current location of the mobile. The location of themobile is typically in the form of the signalling address of the VLR associated with themobile station. The actual routing procedure will be described later. There is logicallyone HLR per GSM network, although it may be implemented as a distributeddatabase.

The Visitor Location Register (VLR) contains selected administrative information fromthe HLR, necessary for call control and provision of the subscribed services, for eachmobile currently located in the geographical area controlled by the VLR. Althougheach functional entity can be implemented as an independent unit, all manufacturersof switching equipment to date implement the VLR together with the MSC, so thatthe geographical area controlled by the MSC corresponds to that controlled by theVLR, thus simplifying the signalling required. Note that the MSC contains noinformation about particular mobile stations --- this information is stored in thelocation registers.

The Home Location Register and the Visitor or Visited Location Register worktogether -- they permit both local operation and roaming outside the local service

area. You couldn't use your mobile in San Francisco and then Los Angeles withoutthese two electronic directories sharing information. Most often these these twodirectories are located in the same place, often on the same computer.

The HLR and VLR are big databases maintained on computers called servers, oftenUNIX workstations. Companies like Tandem, now part ofCompaq, make the servers,which they call HLRs when used for cellular. These servers maintain more than thehome location register, but that's what they call the machine. Many mobile switches
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Interfaces are standardized methods for passing information back and forth. Thetransmission media isn't important. Whether copper or fiber optic cable or microwaveradio, an interface insists that signals go back and forth in the same way, in thesame format. With this approach different equipment from any manufacturer willwork together. See my page on standards.

Let's consider the the A-bis interface as an example. Tektronix says the A-bis "is aFrench term meaning 'the second A Interface.' " Good grief! In most cases the actualspan or physical connection is made on a T1 line or in Europe its equivalent, theE1.But regardless of the material used, the transmission media, it is the signalingprotocol that is most important.

Although the interface is unlabeled, the mobile switch communicates with thetelephone network using Signaling System Seven, an internationally agreed uponstandard. More specifically, it uses ISUP over SS7. As the Performance Technologiespeople tersely put in in their tutorial on SS7, "ISUP defines the protocol andprocedures used to set-up, manage, and release trunk circuits that carry voice anddata calls over the public switched telephone network (PSTN). ISUP is used for both

ISDN and non-ISDN calls."

Using SS7 throughout is a big difference between conventional cellular and GSM. IS-136 and IS-95 also uses SS7 but to communicate between the HLR and VLR it uses astandard called IS-41.

What about the mysterious UM? That's the radio link between a mobile and a basestation. Um are the actual radio frequencies that calls are put on. Possibly the lettersstand for User Mobile. R.C. Levine clears up this matter nicely,

"Interface names (A, Abis, B, C, etc.) were arbitrarily assigned in alphabetical order.The Um label is taken from the customer-network U interface label used in ISDN.Although mnemonics have been proposed for these letters, they are after-the-fact."

.pdf file on SS7 and mobile networking -- Good reading!

SIM: Subscriber identifymodule.

ME: Mobile equipment.BTS: Base transceiver

station.
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BSC: Base station

controller.

HLR: Home location

register.

VLR: Visitor location

register.

MSC: Mobile services

switching center.

EIR: Equipment identity

register.

AuC: Authentication

Center.

UM: Represents the radio link.

Abis: Represents the interface between the base stations and base stationcontrollers.

"A": The interface between the base station subsystem and the networksubsystem.

PSTN and PSPDN: Public switched telephone network and packet

switched public data network.

Figure 1. General architecture of a GSM network
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1. GSM History

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