gsm band allocations

8/7/2019 GSM band allocations

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GSM band allocationsThere is a total of fourteen different recognised GSM frequency bands. These are defined in 3GPPTS 45.005.

BAND UPLINK

(MHZ)

DOWNLINK

(MHZ)

COMMENTS

380 380.2 - 389.8 390.2 - 399.8410 410.2 - 419.8 420.2 - 429.8

450 450.4 - 457.6 460.4 - 467.6

480 478.8 - 486.0 488.8 - 496.0

710 698.0 - 716.0 728.0 - 746.0

750 747.0 - 762.0 777.0 - 792.0

810 806.0 - 821.0 851.0 - 866.0

850 824.0 - 849.0 869.0 - 894.0

900 890.0 - 915.0 935.0 - 960.0 P-GSM, i.e. Primary or standard GSMallocation

900 880.0 - 915.0 925.0 - 960.0 E-GSM, i.e. Extended GSM allocation

900 876.0 - 915 921.0 - 960.0 R-GSM, i.e. Railway GSM allocation

900 870.4 - 876.0 915.4 - 921.0 T-GSM

1800 1710.0 - 1785.0 1805.0 - 1880.0

1900 1850.0 - 1910.0 1930.0 - 1990.0

GSM frequency band usageThe usage of the different frequency bands varies around the globe although there is a large

degree of standardisation. The GSM frequencies available depend upon the regulatoryrequirements for the particular country and the ITU (International Telecommunications Union)

region in which the country is located.

As a rough guide Europe tends to use the GSM 900 and 1800 bands as standard. These bands arealso generally used in the Middle East, Africa, Asia and Oceania.For North America the USA uses both 850 and 1900 MHz bands, the actual band used is

determined by the regulatory authorities and is dependent upon the area. For Canada the 1900MHz band is the primary one used, particularly for urban areas with 850 MHz used as a backup in

rural areas.For Central and South America, the GSM 850 and 1900 MHz frequency bands are the most widely

used although there are some areas where other frequencies are used.

GSM multiband phonesIn order that cell phone users are able to take advantage of the roaming facilities offered by GSM,

it is necessary that the cellphones are able to cover the bands of the countries which are visited.Today most phones support operation on multiple bands and are known as multi-band phones.

Typically most standard phones are dual-band phones. For Europe, Middle east, Asia and Oceaniathese would operate on GSM 900 and 1800 bands and for North America, etc dual band phones

would operate on GSM 850 and 1900 frequency bands.To provide better roaming coverage, tri-band and quad-band phones are also available. European

triband phones typically cover the GSM 900, 1800 and 1900 bands giving good coverage in Europeas well as moderate coverage in North America. Similarly North America tri-band phones use the

900, 1800 and 1900 GSM frequencies. Quad band phones are also available covering the 850,900, 1800 and 1900 MHz GSM frequency bands, i.e. the four major bands and thereby allowing

global use.

GSM Power classNot all mobiles have the same maximum power output level. In order that the base station knowsthe maximum power level number that it can send to the mobile, it is necessary for the base


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station to know the maximum power it can transmit. This is achieved by allocating a GSM powerclass number to a mobile. This GSM power class number indicates to the base station themaximum power it can transmit and hence the maximum power level number the base station caninstruct it to use.Again the GSM power classes vary according to the band in use.

GSM

POWERCLASS

NUMBER

GSM 900 GSM 1800 GSM 1900

Power level

number

Maximum

power output

Power level

number

Maximum

power output

Power level

number

Maximum

power output

1 PL0 30 dBm / 1W PL0 30 dBm / 1W

2 PL2 39dBm / 8W PL3 24 dBm/ 250mW

PL3 24 dBm / 250mW

3 PL3 37dBm / 5W PL29 36 dBm / 4W PL30 33 dBm / 2W

4 PL4 33dBm / 2W

5 PL5 29 dBm / 800

mW

GSM power amplifier design considerationsOne of the main considerations for the RF power amplifier design in any mobile phone is itsefficiency. The RF power amplifier is one of the major current consumption areas. Accordingly, to

ensure long battery life it should be as efficient as possible.It is also worth remembering that as mobiles may only transmit for one eighth of the time, i.e. for

their allocated slot which is one of eight, the average power is an eighth of the maximum.Further pages from this tutorial

GSM system overviewThe GSM system was designed as a second generation (2G) cellular phone technology. One of thebasic aims was to provide a system that would enable greater capacity to be achieved than the

previous first generation analogue systems. GSM achieved this by using a digital TDMA (timedivision multiple access approach). By adopting this technique more users could be accommodated

within the available bandwidth. In addition to this, ciphering of the digitally encoded speech wasadopted to retain privacy. Using the earlier analogue cellular technologies it was possible for

anyone with a scanner receiver to listen to calls and a number of famous personalities had been"eavesdropped" with embarrassing consequences.

GSM servicesSpeech or voice calls are obviously the primary function for the GSM cellular system. To achievethis the speech is digitally encoded and later decoded using a vocoder. A variety of vocoders are

available for use, being aimed at different scenarios.In addition to the voice services, GSM cellular technology supports a variety of other data services.

Although their performance is nowhere near the level of those provided by 3G, they arenevertheless still important and useful. A variety of data services are supported with user data

rates up to 9.6 kbps. Services including Group 3 facsimile, videotext and teletex can be supported.One service that has grown enormously is the short message service. Developed as part of the

GSM specification, it has also been incorporated into other cellular technologies. It can be thoughtof as being similar to the paging service but is far more comprehensive allowing bi-directional

messaging, store and forward delivery, and it also allows alphanumeric messages of a reasonablelength. This service has become particularly popular, initially with the young as it provided a

simple, low fixed cost.


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GSM basicsThe GSM cellular technology had a number of design aims when the development started:

It should offer good subjective speech quality

It should have a low phone or terminal cost

Terminals should be able to be handheld The system should support international roaming

It should offer good spectral efficiency

The system should offer ISDN compatibility

The resulting GSM cellular technology that was developed provided for all of these. The overall

system definition for GSM describes not only the air interface but also the network orinfrastructure technology. By adopting this approach it is possible to define the operation of the

whole network to enable international roaming as well as enabling network elements from differentmanufacturers to operate alongside each other, although this last feature is not completely true,

especially with older items.GSM cellular technology uses 200 kHz RF channels. These are time division multiplexed to enable

up to eight users to access each carrier. In this way it is a TDMA / FDMA system.

The base transceiver stations (BTS) are organised into small groups, controlled by a base stationcontroller (BSC) which is typically co-located with one of the BTSs. The BSC with its associatedBTSs is termed the base station subsystem (BSS).

Further into the core network is the main switching area. This is known as the mobile switchingcentre (MSC). Associated with it is the location registers, namely the home location register (HLR)

and the visitor location register (VLR) which track the location of mobiles and enable calls to berouted to them. Additionally there is the Authentication Centre (AuC), and the Equipment Identify

Register (EIR) that are used in authenticating the mobile before it is allowed onto the network andfor billing. The operation of these are explained in the following pages.

Last but not least is the mobile itself. Often termed the ME or mobile equipment, this is the itemthat the end user sees. One important feature that was first implemented on GSM was the use of a

Subscriber Identity Module. This card carried with it the users identity and other information toallow the user to upgrade a phone very easily, while retaining the same identity on the network. It

was also used to store other information such as "phone book" and other items. This item alonehas allowed people to change phones very easily, and this has fuelled the phone manufacturing

industry and enabled new phones with additional features to be launched. This has allowed mobileoperators to increase their average revenue per user (ARPU) by ensuring that users are able to

access any new features that may be launched on the network requiring more sophisticatedphones.

GSM system overviewThe table below summarises the main points of the GSM system specification, showing some of thehighlight features of technical interest.

SPECIFICATION SUMMARY FOR GSM CELLULAR SYSTEM

Multiple access technology FDMA / TDMA

Duplex technique FDD

Uplink frequency band 933 -960 MHz

(basic 900 MHz band only)

Downlink frequency band 890 - 915 MHz

(basic 900 MHz band only)

Channel spacing 200 kHz

Modulation GMSK

Speech coding Various - original was RPE-LTP/13

Speech channels per RF channel 8

Channel data rate 270.833 kbps

Frame duration 4.615 ms


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GSM summaryThe GSM system is the most successful cellular telecommunications system to date. Withsubscriber numbers running into billions and still increasing, it has been proved to have met itsrequirements. Further pages of this GSM tutorial or overview detail many of the GSM basics fromthe air interface, frame and slot structures to the logical and physical channels as well as detailsabout the GSM network.

Today the GSM cell or mobile phone system is the most popular in the world. GSM handsets arewidely available at good prices and the networks are robust and reliable. The GSM system is alsofeature-rich with applications such as SMS text messaging, international roaming, SIM cards andthe like. It is also being enhanced with technologies including GPRS and EDGE. To achieve thislevel of success has taken many years and is the result of both technical development andinternational cooperation. The GSM history can be seen to be a story of cooperation across Europe,and one that nobody thought would lead to the success that GSM is today.The first cell phone systems that were developed were analogue systems. Typically they usedfrequency-modulated carriers for the voice channels and data was carried on a separate sharedcontrol channel. When compared to the systems employed today these systems werecomparatively straightforward and as a result a vast number of systems appeared. Two of themajor systems that were in existence were the AMPS (Advanced Mobile Phone System) that wasused in the USA and many other countries and TACS (Total Access Communications System) that

was used in the UK as well as many other countries around the world.Another system that was employed, and was in fact the first system to be commercially deployedwas the Nordic Mobile Telephone system (NMT). This was developed by a consortium of companiesin Scandinavia and proved that international cooperation was possible.The success of these systems proved to be their downfall. The use of all the systems installedaround the globe increased dramatically and the effects of the limited frequency allocations weresoon noticed. To overcome these a number of actions were taken. A system known as E-TACS orExtended-TACS was introduced giving the TACS system further channels. In the USA anothersystem known as Narrowband AMPS (NAMPS) was developed.

New approachesNeither of these approaches proved to be the long-term solution as cellular technology needed to

be more efficient. With the experience gained from the NMT system, showing that it was possibleto develop a system across national boundaries, and with the political situation in Europe lendingitself to international cooperation it was decided to develop a new Pan-European System.Furthermore it was realized that economies of scale would bring significant benefits. This was thebeginnings of the GSM system.To achieve the basic definition of a new system a meeting was held in 1982 under the auspices ofthe Conference of European Posts and Telegraphs (CEPT). They formed a study group called theGroupe Special Mobile ( GSM ) to study and develop a pan-European public land mobile system.Several basic criteria that the new cellular technology would have to meet were set down for thenew GSM system to meet. These included: good subjective speech quality, low terminal andservice cost, support for international roaming, ability to support handheld terminals, support forrange of new services and facilities, spectral efficiency, and finally ISDN compatibility.With the levels of under-capacity being projected for the analogue systems, this gave a real senseof urgency to the GSM development. Although decisions about the exact nature of the cellular

technology were not taken at an early stage, all parties involved had been working toward a digitalsystem. This decision was finally made in February 1987. This gave a variety of advantages.Greater levels of spectral efficiency could be gained, and in addition to this the use of digitalcircuitry would allow for higher levels of integration in the circuitry. This in turn would result incheaper handsets with more features. Nevertheless significant hurdles still needed to beovercome. For example, many of the methods for encoding the speech within a sufficiently narrowbandwidth needed to be developed, and this posed a significant risk to the project. Neverthelessthe GSM system had been started.

GSM launch datesWork continued and a launch date for the new GSM system of 1991 was set for an initial launch ofa service using the new cellular technology with limited coverage and capability to be followed by acomplete roll out of the service in major European cities by 1993 and linking of the areas by 1995.


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Meanwhile technical development was taking place. Initial trials had shown that time divisionmultiple access techniques offered the best performance with the technology that would beavailable. This approach had the support of the major manufacturing companies which wouldensure that with them on board sufficient equipment both in terms of handsets, base stations andthe network infrastructure for GSM would be available.Further impetus was given to the GSM project when in 1989 the responsibility was passed to the

newly formed European Telecommunications Standards Institute (ETSI). Under the auspices ofETSI the specification took place. It provided functional and interface descriptions for each of thefunctional entities defined in the system. The aim was to provide sufficient guidance formanufacturers that equipment from different manufacturers would be interoperable, while notstopping innovation. The result of the specification work was a set of documents extending tomore than 6000 pages. Nevertheless the resultant phone system provided a robust, feature-richsystem. The first roaming agreement was signed between Telecom Finland and Vodafone in theUK. Thus the vision of a pan-European network was fast becoming a reality. However this tookplace before any networks went live.The aim to launch GSM by 1991 proved to be a target that was too tough to meet. Terminalsstarted to become available in mid 1992 and the real launch took place in the latter part of thatyear. With such a new service many were sceptical as the analogue systems were still inwidespread use. Nevertheless by the end of 1993 GSM had attracted over a million subscribersand there were 25 roaming agreements in place. The growth continued and the next million

subscribers were soon attracted.

Global GSM usageOriginally GSM had been planned as a European system. However the first indication that thesuccess of GSM was spreading further a field occurred when the Australian network provider,Telstra signed the GSM Memorandum of Understanding.

FrequenciesOriginally it had been intended that GSM would operate on frequencies in the 900 MHz cellularband. In September 1993, the British operator Mercury One-to-One launched a network. Termed

DCS 1800 it operated at frequencies in a new 1800 MHz band. By adopting new frequencies newoperators and further competition was introduced into the market apart from allowing additionalspectrum to be used and further increasing the overall capacity. This trend was followed in manycountries, and soon the term DCS 1800 was dropped in favour of calling it GSM as it was purelythe same cellular technology but operating on a different frequency band. In view of the higherfrequency used the distances the signals travelled was slightly shorter but this was compensatedfor by additional base stations.In the USA as well a portion of spectrum at 1900 MHz was allocated for cellular usage in 1994. Thelicensing body, the FCC, did not legislate which technology should be used, and accordingly thisenabled GSM to gain a foothold in the US market. This system was known as PCS 1900 (PersonalCommunication System).

GSM successWith GSM being used in many countries outside Europe this reflected the true nature of the namewhich had been changed from Groupe Special Mobile to Global System for Mobile communications.The number of subscribers grew rapidly and by the beginning of 2004 the total number of GSMsubscribers reached 1 billion. Attaining this figure was celebrated at the Cannes 3GSM conferenceheld that year. Figures continued to rise, reaching and then well exceeding the 3 billion mark. Inthis way the history of GSM has shown it to be a great success.The GSM technical specifications define the different elements within the GSM networkarchitecture. It defines the different elements and the ways in which they interact to enable theoverall network operation to be maintained.The GSM network architecture is now well established and with the other later cellular systemsnow established and other new ones being deployed, the basic GSM network architecture has beenupdated to interface to the network elements required by these systems. Despite thedevelopments of the newer systems, the basic GSM network architecture has been maintained,and the elements described below perform the same functions as they did when the original GSMsystem was launched in the early 1990s.


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GSM network architecture elementsThe GSM network architecture as defined in the GSM specifications can be grouped into four mainareas:

Mobile station (MS)

Base-station subsystem (BSS)

Network and Switching Subsystem (NSS)

Operation and Support Subsystem (OSS)

Simplified GSM Network Architecture

Mobile stationMobile stations (MS), mobile equipment (ME) or as they are most widely known, cell or mobilephones are the section of a GSM cellular network that the user sees and operates. In recent years

their size has fallen dramatically while the level of functionality has greatly increased. A furtheradvantage is that the time between charges has significantly increased.

There are a number of elements to the cell phone, although the two main elements are the mainhardware and the SIM.

The hardware itself contains the main elements of the mobile phone including the display, case,battery, and the electronics used to generate the signal, and process the data receiver and to be

transmitted. It also contains a number known as the International Mobile Equipment Identity(IMEI). This is installed in the phone at manufacture and "cannot" be changed. It is accessed by

the network during registration to check whether the equipment has been reported as stolen.The SIM or Subscriber Identity Module contains the information that provides the identity of the

user to the network. It contains are variety of information including a number known as theInternational Mobile Subscriber Identity (IMSI).

Base Station Subsystem (BSS)The Base Station Subsystem (BSS) section of the GSM network architecture that is fundamentallyassociated with communicating with the mobiles on the network. It consists of two elements:


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Base Transceiver Station (BTS): The BTS used in a GSM network comprises the radio

transmitter receivers, and their associated antennas that transmit and receive to directlycommunicate with the mobiles. The BTS is the defining element for each cell. The BTScommunicates with the mobiles and the interface between the two is known as the Uminterface with its associated protocols.

Base Station Controller (BSC): The BSC forms the next stage back into the GSM

network. It controls a group of BTSs, and is often co-located with one of the BTSs in itsgroup. It manages the radio resources and controls items such as handover within the

group of BTSs, allocates channels and the like. It communicates with the BTSs over whatis termed the Abis interface.

Network Switching Subsystem (NSS)The GSM network subsystem contains a variety of different elements, and is often termed the corenetwork. It provides the main control and interfacing for the whole mobile network. The major

elements within the core network include:

Mobile Switching services Centre (MSC): The main element within the core networkarea of the overall GSM network architecture is the Mobile switching Services Centre(MSC). The MSC acts like a normal switching node within a PSTN or ISDN, but alsoprovides additional functionality to enable the requirements of a mobile user to besupported. These include registration, authentication, call location, inter-MSC handoversand call routing to a mobile subscriber. It also provides an interface to the PSTN so thatcalls can be routed from the mobile network to a phone connected to a landline. Interfacesto other MSCs are provided to enable calls to be made to mobiles on different networks.

Home Location Register (HLR):This database contains all the administrative

information about each subscriber along with their last known location. In this way, theGSM network is able to route calls to the relevant base station for the MS. When a userswitches on their phone, the phone registers with the network and from this it is possibleto determine which BTS it communicates with so that incoming calls can be routedappropriately. 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 oneHLR per network, although it may be distributed across various sub-centres to foroperational reasons.

Visitor Location Register (VLR): This contains selected information from the HLR that

enables the selected services for the individual subscriber to be provided. The VLR can beimplemented as a separate entity, but it is commonly realised as an integral part of theMSC, rather than a separate entity. In this way access is made faster and moreconvenient.

Equipment Identity Register (EIR): The EIR is the entity that decides whether a given

mobile equipment may be allowed onto the network. Each mobile equipment has a numberknown as the International Mobile Equipment Identity. This number, as mentioned above,

is installed in the equipment and is checked by the network during registration. Dependentupon the information held in the EIR, the mobile may be allocated one of three states -

allowed onto the network, barred access, or monitored in case its problems. 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.

Gateway Mobile Switching Centre (GMSC): The GMSC is the point to which a ME

terminating call is initially routed, without any knowledge of the MS's location. The GMSCis thus in charge of obtaining the MSRN (Mobile Station Roaming Number) from the HLRbased on the MSISDN (Mobile Station ISDN number, the "directory number" of a MS) androuting the call to the correct visited MSC. The "MSC" part of the term GMSC is misleading,since the gateway operation does not require any linking to an MSC.

SMS Gateway (SMS-G): The SMS-G or SMS gateway is the term that is used to

collectively describe the two Short Message Services Gateways defined in the GSMstandards. The two gateways handle messages directed in different directions. The SMS-

GMSC (Short Message Service Gateway Mobile Switching Centre) is for short messagesbeing sent to an ME. The SMS-IWMSC (Short Message Service Inter-Working MobileSwitching Centre) is used for short messages originated with a mobile on that network.


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The SMS-GMSC role is similar to that of the GMSC, whereas the SMS-IWMSC provides afixed access point to the Short Message Service Centre.

Operation and Support Subsystem (OSS)The OSS or operation support subsystem is an element within the overall GSM networkarchitecture that is connected to components of the NSS and the BSC. It is used to control andmonitor the overall GSM network and it is also used to control the traffic load of the BSS. It mustbe noted that as the number of BS increases with the scaling of the subscriber population some ofthe maintenance tasks are transferred to the BTS, allowing savings in the cost of ownership of thesystem.The network structure is defined within the GSM standards. Additionally each interface betweenthe different elements of the GSM network is also defined. This facilitates the informationinterchanges can take place. It also enables to a large degree that network elements fromdifferent manufacturers can be used. However as many of these interfaces were not fully defineduntil after many networks had been deployed, the level of standardisation may not be quite ashigh as many people might like.

1. Um interface The "air" or radio interface standard that is used for exchanges between amobile (ME) and a base station (BTS / BSC). For signalling, a modified version of the ISDNLAPD, known as LAPDm is used.

2. Abis interface This is a BSS internal interface linking the BSC and a BTS, and it has notbeen totally standardised. The Abis interface allows control of the radio equipment andradio frequency allocation in the BTS.

3. A interface The A interface is used to provide communication between the BSS and theMSC. The interface carries information to enable the channels, timeslots and the like to beallocated to the mobile equipments being serviced by the BSSs. The messaging requiredwithin the network to enable handover etc to be undertaken is carried over the interface.

4. B interface The B interface exists between the MSC and the VLR . It uses a protocolknown as the MAP/B protocol. As most VLRs are collocated with an MSC, this makes theinterface purely an "internal" interface. The interface is used whenever the MSC needsaccess to data regarding a MS located in its area.

5. C interface The C interface is located between the HLR and a GMSC or a SMS-G. When acall originates from outside the network, i.e. from the PSTN or another mobile network itahs to pass through the gateway so that routing information required to complete the callmay be gained. The protocol used for communication is MAP/C, the letter "C" indicatingthat the protocol is used for the "C" interface. In addition to this, the MSC may optionallyforward billing information to the HLR after the call is completed and cleared down.

6. D interface The D interface is situated between the VLR and HLR. It uses the MAP/Dprotocol to exchange the data related to the location of the ME and to the management ofthe subscriber.

7. E interface The E interface provides communication between two MSCs. The E interfaceexchanges data related to handover between the anchor and relay MSCs using the MAP/Eprotocol.

8. F interface The F interface is used between an MSC and EIR. It uses the MAP/F protocol.The communications along this interface are used to confirm the status of the IMEI of theME gaining access to the network.

9. G interface The G interface interconnects two VLRs of different MSCs and uses theMAP/G protocol to transfer subscriber information, during e.g. a location update procedure.

10.H interface The H interface exists between the MSC the SMS-G. It transfers shortmessages and uses the MAP/H protocol.

11.I interface The I interface can be found between the MSC and the ME. Messagesexchanged over the I interface are relayed transparently through the BSS.

Although the interfaces for the GSM cellular system may not be as rigorouly defined as manymight like, they do at least provide a large element of the definition required, enabling the

functionality of GSM network entities to be defined sufficiently.One of the key elements of the development of the GSM, Global System for Mobi

Communications was the development of the GSM air interface. There were many requirements


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that were placed on the system, and many of these had a direct impact on the air interface.Elements including the modulation, GSM slot structure, burst structure and the like were alldevised to provide the optimum performance.During the development of the GSM standard very careful attention was paid to aspects includingthe modulation format, the way in which the system is time division multiplexed, all had aconsiderable impact on the performance of the system as a whole. For example, the modulation

format for the GSM air interface had a direct impact on battery life and the time division formatadopted enabled the cellphone handset costs to be considerably reduced as detailed later.

GSM signal and GMSK modulation characteristicsThe core of any radio based system is the format of the radio signal itself. The carrier is modulatedusing a form of phase sift keying known as Gaussian Minimum Shift Keying (GMSK). GMSK wasused for the GSM system for a variety of reasons:

It is resilient to noise when compared to many other forms of modulation.

Radiation outside the accepted bandwidth is lower than other forms of phase shift keying.

It has a constant power level which allows higher efficiency RF power amplifiers to be usedin the handset, thereby reducing current consumption and conserving battery life.

Note on GMSK:

GMSK, Gaussian Minimum Shift Keying is a form of phase modulation that is used in a number of portable radio andwireless applications. It has advantages in terms of spectral efficiency as well as having an almost constant amplitudewhich allows for the use of more efficient transmitter power amplifiers, thereby saving on current consumption, acritical issue for battery power equipment.Click on the link for aGMSK tutorial

The nominal bandwidth for the GSM signal using GMSK is 200 kHz, i.e. the channel bandwidth andspacing is 200 kHz. As GMSK modulation has been used, the unwanted or spurious emissionsoutside the nominal bandwidth are sufficiently low to enable adjacent channels to be used from thesame base station. Typically each base station will be allocated a number of carriers to enable it toachieve the required capacity.The data transported by the carrier serves up to eight different users under the basic system bysplitting the carrier into eight time slots. The basic carrier is able to support a data throughput ofapproximately 270 kbps, but as some of this supports the management overhead, the data rateallotted to each time slot is only 24.8 kbps. In addition to this error correction is required toovercome the problems of interference, fading and general data errors that may occur. This meansthat the available data rate for transporting the digitally encoded speech is 13 kbps for the basicvocoders.

GSM slot structure and multiple access schemeGSM uses a combination of both TDMA and FDMA techniques. The FDMA element involves thedivision by frequency of the (maximum) 25 MHz bandwidth into 124 carrier frequencies spaced200 kHz apart as already described.The carriers are then divided in time, using a TDMA scheme. This enables the different users of thesingle radio frequency channel to be allocated different times slots. They are then able to use thesame RF channel without mutual interference. The slot is then the time that is allocated to theparticular user, and the GSM burst is the transmission that is made in this time.Each GSM slot, and hence each GSM burst lasts for 0.577 mS (15/26 mS). Eight of these burstperiods are grouped into what is known as a TDMA frame. This lasts for approximately 4.615 ms(i.e.120/26 ms) and it forms the basic unit for the definition of logical channels. One physicalchannel is one burst period allocated in each TDMA frame.There are different types of frame that are transmitted to carry different data, and also the framesare organised into what are termed multiframes and superframes to provide overallsynchronisation.

GSM slot structure
http://www.radio-electronics.com/info/rf-technology-design/pm-phase-modulation/what-is-gmsk-gaussian-minimum-shift-keying-tutorial.phphttp://www.radio-electronics.com/info/rf-technology-design/pm-phase-modulation/what-is-gmsk-gaussian-minimum-shift-keying-tutorial.phphttp://www.radio-electronics.com/info/rf-technology-design/pm-phase-modulation/what-is-gmsk-gaussian-minimum-shift-keying-tutorial.php


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These GSM slot is the smallest individual time period that is available to each mobile. It has adefined format because a variety of different types of data are required to be transmitted.Although there are shortened transmission bursts, the slots is normally used for transmitting 148bits of information. This data can be used for carrying voice data, control and synchronisationdata.

GSM slots showing offset between transmit and receive

It can be seen from the GSM slot structure that the timing of the slots in the uplink and thedownlink are not simultaneous, and there is a time offset between the transmit and receive. This

offset in the GSM slot timing is deliberate and it means that a mobile that which is allocated thesame slot in both directions does not transmit and receive at the same time. This considerably

reduces the need for expensive filters to isolate the transmitter from the receiver. It also providesa space saving.

GSM burstThe GSM burst, or transmission can fulfil a variety of functions. Some GSM bursts are used forcarrying data while others are used for control information. As a result of this a number of

different types of GSM burst are defined.

Normal burst uplink and downlink

Synchronisation burst downlink

Frequency correction burst downlink

Random Access (Shortened Burst) uplink

GSM normal burstThis GSM burst is used for the standard communications between the basestation and the mobile,and typically transfers the digitised voice data.The structure of the normal GSM burst is exactly defined and follows a common format. It containsdata that provides a number of different functions:

1. 3 tail bits: These tail bits at the start of the GSM burst give time for the transmitter toramp up its power

2. 57 data bits: This block of data is used to carry information, and most often containsthe digitised voice data although on occasions it may be replaced with signallinginformation in the form of the Fast Associated Control CHannel (FACCH). The type of datais indicated by the flag that follows the data field

3. 1 bit flag: This bit within the GSM burst indicates the type of data in the previous field.

4. 26 bits training sequence: This training sequence is used as a timing reference and forequalisation. There is a total of eight different bit sequences that may be used, each 26bits long. The same sequence is used in each GSM slot, but nearby base stations using the


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same radio frequency channels will use different ones, and this enables the mobile todifferentiate between the various cells using the same frequency.

5. 1 bit flag Again this flag indicates the type of data in the data field.6. 57 data bits Again, this block of data within the GSM burst is used for carrying data.7. 3 tail bits These final bits within the GSM burst are used to enable the transmitter power

to ramp down. They are often called final tail bits, or just tail bits.

8. 8.25 bits guard time At the end of the GSM burst there is a guard period. This isintroduced to prevent transmitted bursts from different mobiles overlapping. As a result oftheir differing distances from the base station.

GSM Normal Burst

GSM synchronisation burstThe purpose of this form of GSM burst is to provide synchronisation for the mobiles on thenetwork.

1. 3 tail bits: Again, these tail bits at the start of the GSM burst give time for thetransmitter to ramp up its power

2. 39 bits of information: 3. 64 bits of a Long Training Sequence: 4. 39 bits Information: 5. 3 tail bits Again these are to enable the transmitter power to ramp down.6. 8.25 bits guard time: to act as a guard interval.

GSM Synchronisation Burst

GSM frequency correction burstWith the information in the burst all set to zeros, the burst essentially consists of a constantfrequency carrier with no phase alteration.

1. 3 tail bits: Again, these tail bits at the start of the GSM burst give time for thetransmitter to ramp up its power.

2. 142 bits all set to zero: 3. 3 tail bits Again these are to enable the transmitter power to ramp down.4. 8.25 bits guard time: to act as a guard interval.

GSM Frequency Correction Burst


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GSM random access burstThis form of GSM burst used when accessing the network and it is shortened in terms of the datacarried, having a much longer guard period. This GSM burst structure is used to ensure that it fitsin the time slot regardless of any severe timing problems that may exist. Once the mobile hasaccessed the network and timing has been aligned, then there is no requirement for the longguard period.

1. 7 tail bits: The increased number of tail bits is included to provide additional marginwhen accessing the network.

2. 41 training bits: 3. 36 data bits: 4. 3 tail bits Again these are to enable the transmitter power to ramp down.5. 69.25 bits guard time: The additional guard time, filling the remaining time of the GSM

burst provides for large timing differences.

GSM Random Access Burst

GSM discontinuous transmission (DTx)A further power saving and interference reducing facility is the discontinuous transmission (DTx)capability that is incorporated within the specification. It is particularly useful because there are

long pauses in speech, for example when the person using the mobile is listening, and duringthese periods there is no need to transmit a signal. In fact it is found that a person speaks for less

than 40% of the time during normal telephone conversations. The most important element of DTxis the Voice Activity Detector. It must correctly distinguish between voice and noise inputs, a task

that is not trivial. If a voice signal is misinterpreted as noise, the transmitter is turned off an effectknown as clipping results and this is particularly annoying to the person listening to the speech.

However if noise is misinterpreted as a voice signal too often, the efficiency of DTX is dramaticallydecreased.

It is also necessary for the system to add background or comfort noise when the transmitter isturned off because complete silence can be very disconcerting for the listener. Accordingly this is

added as appropriate. The noise is controlled by the SID (silence indication descriptor).The GSM system has a defined GSM frame structure to enable the orderly passage of information.

The GSM frame structure establishes schedules for the predetermined use of timeslots.By establishing these schedules by the use of a frame structure, both the mobile and the base

station are able to communicate not only the voice data, but also signalling information withoutthe various types of data becoming intermixed and both ends of the transmission knowing exactly

what types of information are being transmitted.The GSM frame structure provides the basis for the various physical channels used within GSM,

and accordingly it is at the heart of the overall system.

Basic GSM frame structureThe basic element in the GSM frame structure is the frame itself. This comprises the eight slots,

each used for different users within the TDMA system. As mentioned in another page of thetutorial, the slots for transmission and reception for a given mobile are offset in time so that the

mobile does not transmit and receive at the same time.


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GSM frame consisting of eight slotsThe basic GSM frame defines the structure upon which all the timing and structure of the GSM

messaging and signalling is based. The fundamental unit of time is called a burst period and itlasts for approximately 0.577 ms (15/26 ms). Eight of these burst periods are grouped into what is

known as a TDMA frame. This lasts for approximately 4.615 ms (i.e.120/26 ms) and it forms thebasic unit for the definition of logical channels. One physical channel is one burst period allocated

in each TDMA frame.In simplified terms the base station transmits two types of channel, namely traffic and control.

Accordingly the channel structure is organised into two different types of frame, one for the trafficon the main traffic carrier frequency, and the other for the control on the beacon frequency.

GSM multiframeThe GSM frames are grouped together to form multiframes and in this way it is possible toestablish a time schedule for their operation and the network can be synchronised.

There are several GSM multiframe structures:

Traffic multiframe: The Traffic Channel frames are organised into multiframes

consisting of 26 bursts and taking 120 ms. In a traffic multiframe, 24 bursts are used fortraffic. These are numbered 0 to 11 and 13 to 24. One of the remaining bursts is thenused to accommodate the SACCH, the remaining frame remaining free. The actual positionused alternates between position 12 and 25.

Control multiframe: the Control Channel multiframe that comprises 51 bursts andoccupies 235.4 ms. This always occurs on the beacon frequency in time slot zero and itmay also occur within slots 2, 4 and 6 of the beacon frequency as well. This multiframe issubdivided into logical channels which are time-scheduled. These logical channels andfunctions include the following:

o Frequency correction burst

o Synchronisation burst

o Broadcast channel (BCH)

o Paging and Access Grant Channel (PACCH)

o Stand Alone Dedicated Control Channel (SDCCH)

GSM SuperframeMultiframes are then constructed into superframes taking 6.12 seconds. These consist of 51 trafficmultiframes or 26 control multiframes. As the traffic multiframes are 26 bursts long and thecontrol multiframes are 51 bursts long, the different number of traffic and control multiframeswithin the superframe, brings them back into line again taking exactly the same interval.

GSM Hyperframe

Above this 2048 superframes (i.e. 2 to the power 11) are grouped to form one hyperframe whichrepeats every 3 hours 28 minutes 53.76 seconds. It is the largest time interval within the GSMframe structure.


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Within the GSM hyperframe there is a counter and every time slot has a unique sequential numbercomprising the frame number and time slot number. This is used to maintain synchronisation ofthe different scheduled operations with the GSM frame structure. These include functions such as:

Frequency hopping: Frequency hopping is a feature that is optional within the GSMsystem. It can help reduce interference and fading issues, but for it to work, the

transmitter and receiver must be synchronised so they hop to the same frequencies at thesame time.

Encryption: The encryption process is synchronised over the GSM hyperframe period

where a counter is used and the encryption process will repeat with each hyperframe.However, it is unlikely that the cellphone conversation will be over 3 hours and accordingly

it is unlikely that security will be compromised as a result.

GSM Frame Structure Summary

GSM frame structure summaryBy structuring the GSM signalling into frames, multiframes, superframes and hyperframes, thetiming and organisation is set into an orderly format that enables both the GSM mobile and basestation to communicate in a reliable and efficient manner. The GSM frame structure forms thebasis onto which the other forms of frame and hence the various GSM channels are built.GSM uses a variety ofchannels in which the data is carried. In GSM, these channels are separatedinto physical channels and logical channels. The Physical channels are determined by the timeslot,whereas the logical channels are determined by the information carried within the physicalchannel. It can be further summarised by saying that several recurring timeslots on a carrierconstitute a physical channel. These are then used by different logical channels to transferinformation. These channels may either be used for user data (payload) or signalling to enable thesystem to operate correctly.

Common and dedicated channelsThe channels may also be divided into common and dedicated channels. The forward commonchannels are used for paging to inform a mobile of an incoming call, responding to channel


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requests, and broadcasting bulletin board information. The return common channel is a randomaccess channel used by the mobile to request channel resources before timing information isconveyed by the BSS.The dedicated channels are of two main types: those used for signalling, and those used for traffic.The signalling channels are used for maintenance of the call and for enabling call set up, providingfacilities such as handover when the call is in progress, and finally terminating the call. The traffic

channels handle the actual payload.The following logical channels are defined in GSM:TCHf- Full rate traffic channel.TCH h - Half rate traffic channel.BCCH - Broadcast Network information, e.g. for describing the current control channel structure.The BCCH is a point-to-multipoint channel (BSS-to-MS).SCH - Synchronisation of the MSs.FCHMS - frequency correction.AGCH - Acknowledge channel requests from MS and allocate a SDCCH.PCHMS - terminating call announcement.RACHMS - access requests, response to call announcement, location update, etc.FACCHt - For time critical signalling over the TCH (e.g. for handover signalling). Traffic burst isstolen for a full signalling burst.SACCHt - TCH in-band signalling, e.g. for link monitoring.

SDCCH - For signalling exchanges, e.g. during call setup, registration / location updates.FACCHs - FACCH for the SDCCH. The SDCCH burst is stolen for a full signalling burst. Function notclear in the present version of GSM (could be used for e.g. handover of an eight-rate channel, i.e.using a "SDCCH-like" channel for other purposes than signalling).SACCHs - SDCCH in-band signalling, e.g. for link monitoring.Audio codecs or vocoders are universally used within the GSM system. They reduce the bit rate ofspeech that has been converted from its analogue for into a digital format to enable it to be carriedwithin the available bandwidth for the channel. Without the use of a speech codec, the digitisedspeech would occupy a much wider bandwidth then would be available. Accordingly GSM codecsare a particularly important element in the overall system.A variety of different forms of audio codec or vocoder are available for general use, and the GSMsystem supports a number of specific audio codecs. These include the RPE-LPC, half rate, and AMRcodecs. The performance of each voice codec is different and they may be used under differentconditions, although the AMR codec is now the most widely used. Also the newer AMR wideband(AMR-WB) codec is being introduced into many areas, including GSMVoice codec technology has advanced by considerable degrees in recent years as a result of theincreasing processing power available. This has meant that the voice codecs used in the GSMsystem have large improvements since the first GSM phones were introduced.

Vocoder / codec basicsVocoders or speech codecs are used within many areas of voice communications. Obviously thefocus here is on GSM audio codecs or vocoders, but the same principles apply to any form ofcodec.If speech were digitised in a linear fashion it would require a high data rate that would occupy avery wide bandwidth. As bandwidth is normally limited in any communications system, it is

necessary to compress the data to send it through the available channel. Once through thechannel it can then be expanded to regenerate the audio in a fashion that is as close to the originalas possible.To meet the requirements of the codec system, the speech must be captured at a high enoughsample rate and resolution to allow clear reproduction of the original sound. It must then becompressed in such a way as to maintain the fidelity of the audio over a limited bit rate, error-prone wireless transmission channel.Audio codecs or vocoders can use a variety of techniques, but many modern audio codecs use atechnique known as linear prediction. In many ways this can be likened to a mathematicalmodelling of the human vocal tract. To achieve this the spectral envelope of the signal is estimatedusing a filter technique. Even where signals with many non-harmonically related signals are used itis possible for voice codecs to give very large levels of compression.A variety of different codec methodologies are used for GSM codecs:

CELP: The CELP or Code Excited Linear Prediction codec is a vocoder algorithm that was

originally proposed in 1985 and gave a significant improvement over other voice codecs of


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the day. The basic principle of the CELP codec has been developed and used as the basisof other voice codecs including ACELP, RCELP, VSELP, etc. As such the CELP codecmethodology is now the most widely used speech coding algorithm. Accordingly CELP isnow used as a generic term for a particular class of vocoders or speech codecs and not aparticular codec.

The main principle behind the CELP codec is that is uses a principle known as "Analysis bySynthesis". In this process, the encoding is performed by perceptually optimising thedecoded signal in a closed loop system. One way in which this could be achieved is tocompare a variety of generated bit streams and choose the one that produces the bestsounding signal.

ACELP codec: The ACELP or Algebraic Code Excited Linear Prediction codec. The ACELPcodec or vocoder algorithm is a development of the CELP model. However the ACELP codeccodebooks have a specific algebraic structure as indicated by the name.

VSELP codec: The VSELP or Vector Sum Excitation Linear Prediction codec. One of themajor drawbacks of the VSELP codec is its limited ability to code non-speech sounds. This

means that it performs poorly in the presence of noise. As a result this voice codec is notnow as widely used, other newer speech codecs being preferred and offering far superior

performance.

GSM audio codecs / vocodersA variety of GSM audio codecs / vocoders are supported. These have been introduced at different

times, and have different levels of performance.. Although some of the early audio codecs are notas widely used these days, they are still described here as they form part of the GSM system.

CODEC NAME BIT RATE

(KBPS)

COMPRESSION TECHNOLOGY

Full rate 13 RTE-LPC

EFR 12.2 ACELP

Half rate 5.6 VSELP

AMR 12.2 - 4.75 ACELP

AMR-WB 23.85 - 6.60 ACELP

GSM Full Rate / RPE-LPC codecThe RPE-LPC or Regular Pulse Excited - Linear Predictive Coder. This form of voice codec was thefirst speech codec used with GSM and it chosen after tests were undertaken to compare it withother codec schemes of the day. The speech codec is based upon the regular pulse excitation LPCwith long term prediction. The basic scheme is related to two previous speech codecs, namely:RELP, Residual Excited Linear Prediction and to the MPE-LPC, Multi Pulse Excited LPC. Theadvantages of RELP are the relatively low complexity resulting from the use of baseband coding,

but its performance is limited by the tonal noise produced by the system. The MPE-LPC is morecomplex but provides a better level of performance. The RPE-LPC codec provided a compromisebetween the two, balancing performance and complexity for the technology of the time.Despite the work that was undertaken to provide the optimum performance, as technologydeveloped further, the RPE-LPC codec was viewed as offering a poor level of voice quality. As otherfull rate audio codecs became available, these were incorporated into the system.

GSM EFR - Enhanced Full Rate codecLater another vocoder called the Enhanced Full Rate (EFR) vocoder was added in response to thepoor quality perceived by the users of the original RPE-LPC codec. This new codec gave muchbetter sound quality and was adopted by GSM. Using the ACELP compression technology it gave asignificant improvement in quality over the original LPC-RPE encoder. It became possible as theprocessing power that was available increased in mobile phones as a result of higher levels ofprocessing power combined with their lower current consumption.


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GSM Half Rate codecThe GSM standard allows the splitting of a single full rate voice channel into two sub-channels thatcan maintain separate calls. By doing this, network operators can double the number of voice callsthat can be handled by the network with very little additional investment.

To enable this facility to be used a half rate codec must be used. The half rate codec wasintroduced in the early years of GSM but gave a much inferior voice quality when compared toother speech codecs. However it gave advantages when demand was high and network capacitywas at a premium.The GSM Half Rate codec uses a VSELP codec algorithm. It codes the data around 20 ms frameseach carrying 112 bits to give a data rate of 5.6 kbps. This includes a 100 bps data rate for amode indicator which details whether the system believes the frames contain voice data or not.This allows the speech codec to operate in a manner that provides the optimum quality.The Half Rate codec system was introduced in the 1990s, but in view of the perceived poor quality,it was not widely used.

GSM AMR CodecThe AMR, Adaptive Multi-rate codec is now the most widely used GSM codec. The AMR codec wasadopted by 3GPP in October 1988 and it is used for both GSM and circuit switched UMTS / WCDMAvoice calls.The AMR codec provides a variety of options for one of eight different bit rates as described in thetable below. The bit rates are based on frames that are 20 millisceonds long and contain 160samples. The AMR codec uses a variety of different techniques to provide the data compression.The ACELP codec is used as the basis of the overall speech codec, but other techniques are used inaddition to this. Discontinuous transmission is employed so that when there is no speech activitythe transmission is cut. Additionally Voice Activity Detection (VAD) is used to indicate when thereis only background noise and no speech. Additionally to provide the feedback for the user that theconnection is still present, a Comfort Noise Generator (CNG) is used to provide some backgroundnoise, even when no speech data is being transmitted. This is added locally at the receiver.The use of the AMR codec also requires that optimized link adaptation is used so that the optimum

data rate is selected to meet the requirements of the current radio channel conditions including itssignal to noise ratio and capacity. This is achieved by reducing the source coding and increasingthe channel coding. Although there is a reduction in voice clarity, the network connection is morerobust and the link is maintained without dropout. Improvement levels of between 4 and 6 dB maybe experienced. However network operators are able to prioritise each station for either quality orcapacity.The AMR codec has a total of eight rates: eight are available at full rate (FR), while six areavailable at half rate (HR). This gives a total of fourteen different modes.

MODE BIT RATE(KBPS)

FULL RATE (FR) /HALF RATE (HR)

AMR 12.2 12.2 FR

AMR 10.2 10.2 FR

AMR 7.95 7.95 FR / HR

AMR 7.40 7.40 FR / HR

AMR 6.70 6.70 FR / HR

AMR 5.90 5.90 FR / HR

AMR 5.15 5.15 FR / HR

AMR 4.75 4.75 FR / HR

AMR codec data rates

AMR-WB codecAdaptive Multi-Rate Wideband, AMR-WB codec, also known under its ITU designation of G.722.2, isbased on the earlier popular Adaptive Multi-Rate, AMR codec. AMR-WB also uses an ACELP basisfor its operation, but it has been further developed and AMR-WB provides improved speech quality


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as a result of the wider speech bandwidth that it encodes. AMR-WB has a bandwidth extendingfrom 50 - 7000 Hz which is significantly wider than the 300 - 3400 Hz bandwidths used bystandard telephones. However this comes at the cost of additional processing, but with advancesin IC technology in recent years, this is perfectly acceptable.The AMR-WB codec contains a number of functional areas: it primarily includes a set of fixed ratespeech and channel codec modes. It also includes other codec functions including: a Voice Activity

Detector (VAD); Discontinuous Transmission (DTX) functionality for GSM; and Source ControlledRate (SCR) functionality for UMTS applications. Further functionality includes in-band signaling forcodec mode transmission, and link adaptation for control of the mode selection.The AMR-WB codec has a 16 kHz sampling rate and the coding is performed in blocks of 20 ms.There are two frequency bands that are used: 50-6400 Hz and 6400-7000 Hz. These are codedseparately to reduce the codec complexity. This split also serves to focus the bit allocation into thesubjectively most important frequency range.The lower frequency band uses an ACELP codec algorithm, although a number of additionalfeatures have been included to improve the subjective quality of the audio. Linear predictionanalysis is performed once per 20 ms frame. Also, fixed and adaptive excitation codebooks aresearched every 5 ms for optimal codec parameter values.The higher frequency band adds some of the naturalness and personality features to the voice.The audio is reconstructed using the parameters from the lower band as well as using randomexcitation. As the level of power in this band is less than that of the lower band, the gain is

adjusted relative to the lower band, but based on voicing information. The signal content of thehigher band is reconstructed by using an linear predictive filter which generates information fromthe lower band filter.

BIT

RATE(KBPS)

NOTES

6.60 This is the lowest rate for AMR-WB. It is used for circuit switched connections for GSM and UMTSand is intended to be used only temporarily during severe radio channel conditions or during network

congestion.

8.85 This gives improved quality over the 6.6 kbps rate, but again, its use is only recommended for use in

periods of congestion or when during severe radio channel conditions.

12.65 This is the main bit rate used for circuit switched GSM and UMTS, offering superior performance to

the original AMR codec.

14.25 Higher bit rate used to give cleaner speech and is particularly useful when ambient audio noise levels

are high.


are high.


are high.


are high.

23.05 Not suggested for full rate GSM channels.

23.85 Not suggested for full rate GSM channels, and provides speech quality similar to that of G.722 at 64

kbps.

Not all phones equipped with AMR-WB will be able to access all the data rates - the differentfunctions on the phone may not require all to be active for example. As a result, it is necessary toinform the network about which rates are available and thereby simplify the negotiation betweenthe handset and the network. To achieve this there are three difference AMR-WB configurationsthat are available:

Configuration A: 6.6, 8.85, and 12.65 kbit/s

Configuration B: 6.6, 8.85, 12.65, and 15.85 kbit/s

Configuration C: 6.6, 8.85, 12.65, and 23.85 kbit/s

It can be seen that only the 23.85, 15.85, 12.65, 8.85 and 6.60 kbit/s modes are used. Based onlistening tests, it was considered that these five modes were sufficient for a high quality speech

telephony service. The other data rates were retained and can be used for other purposesincluding multimedia messaging, streaming audio, etc.


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GSM codecs summaryThere has been a considerable improvement in the GSM audio codecs that have been in use.Starting with the original RTE-LPC speech codec and then moving through the Enhanced Full Rate,EFR codec and the GSM half rate codec to the AMR codec which is now the most widely used andprovides a variable rate that can be tailored to the individual conditions. Also the newer AMR-WBcodec wills ee increasing use. Although with newer technologies such as LTE, Long Term Evolution

which uses an all IP based system, codecs are still used to provide data compression and improvedspectral efficiency, the idea of a codec will still be used, although some of the GSM codecs that arein use today will be superseded.One of the key elements of a mobile phone or cellular telecommunications system, is that thesystem is split into many small cells to provide good frequency re-use and coverage. However asthe mobile moves out of one cell to another it must be possible to retain the connection. Theprocess by which this occurs is known as handover or handoff. The term handover is more widelyused within Europe, whereas handoff tends to be use more in North America. Either way, handoverand handoff are the same process.

Requirements for GSM handoverThe process of handover or handoff within any cellular system is of great importance. It is a criticalprocess and if performed incorrectly handover can result in the loss of the call. Dropped calls areparticularly annoying to users and if the number of dropped calls rises, customer dissatisfactionincreases and they are likely to change to another network. Accordingly GSM handover was anarea to which particular attention was paid when developing the standard.

Types of GSM handoverWithin the GSM system there are four types of handover that can be performed for GSM onlysystems:

Intra-BTS handover: This form of GSM handover occurs if it is required to change thefrequency or slot being used by a mobile because of interference, or other reasons. In this

form of GSM handover, the mobile remains attached to the same base station transceiver,but changes the channel or slot.

Inter-BTS Intra BSC handover: This for of GSM handover or GSM handoff occurs when

the mobile moves out of the coverage area of one BTS but into another controlled by thesame BSC. In this instance the BSC is able to perform the handover and it assigns a new

channel and slot to the mobile, before releasing the old BTS from communicating with themobile.

Inter-BSC handover: When the mobile moves out of the range of cells controlled by

one BSC, a more involved form of handover has to be performed, handing over not onlyfrom one BTS to another but one BSC to another. For this the handover is controlled by

the MSC.

Inter-MSC handover: This form of handover occurs when changing between networks.

The two MSCs involved negotiate to control the handover.

GSM handover processAlthough there are several forms of GSM handover as detailed above, as far as the mobile isconcerned, they are effectively seen as very similar. There are a number of stages involved inundertaking a GSM handover from one cell or base station to another.In GSM which uses TDMA techniques the transmitter only transmits for one slot in eight, andsimilarly the receiver only receives for one slot in eight. As a result the RF section of the mobilecould be idle for 6 slots out of the total eight. This is not the case because during the slots in whichit is not communicating with the BTS, it scans the other radio channels looking for beaconfrequencies that may be stronger or more suitable. In addition to this, when the mobilecommunicates with a particular BTS, one of the responses it makes is to send out a list of the

radio channels of the beacon frequencies of neighbouring BTSs via the Broadcast Channel (BCCH).


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The mobile scans these and reports back the quality of the link to the BTS. In this way the mobileassists in the handover decision and as a result this form of GSM handover is known as MobileAssisted Hand Over (MAHO).The network knows the quality of the link between the mobile and the BTS as well as the strengthof local BTSs as reported back by the mobile. It also knows the availability of channels in thenearby cells. As a result it has all the information it needs to be able to make a decision about

whether it needs to hand the mobile over from one BTS to another.If the network decides that it is necessary for the mobile to hand over, it assigns a new channeland time slot to the mobile. It informs the BTS and the mobile of the change. The mobile thenretunes during the period it is not transmitting or receiving, i.e. in an idle period.A key element of the GSM handover is timing and synchronisation. There are a number of possiblescenarios that may occur dependent upon the level of synchronisation.

Old and new BTSs synchronised: In this case the mobile is given details of the new

physical channel in the neighbouring cell and handed directly over. The mobile mayoptionally transmit four access bursts. These are shorter than the standard bursts andthereby any effects of poor synchronisation do not cause overlap with other bursts.However in this instance where synchronisation is already good, these bursts are only usedto provide a fine adjustment.

Time offset between synchronised old and new BTS: In some instances there maybe a time offset between the old and new BTS. In this case, the time offset is provided so

that the mobile can make the adjustment. The GSM handover then takes place as astandard synchronised handover.

Non-synchronised handover: When a non-synchronised cell handover takes place, the

mobile transmits 64 access bursts on the new channel. This enables the base station todetermine and adjust the timing for the mobile so that it can suitably access the new BTS.

This enables the mobile to re-establish the connection through the new BTS with thecorrect timing.

Inter-system handoverWith the evolution of standards and the migration of GSM to other 2G technologies including to 3GUMTS / WCDMA as well as HSPA and then LTE, there is the need to handover from one technologyto another. Often the 2G GSM coverage will be better then the others and GSM is often used asthe fallback. When handovers of this nature are required, it is considerably more complicated thana straightforward only GSM handover because they require two technically very different systemsto handle the handover.These handovers may be called intersystem handovers or inter-RAT handovers as the handoveroccurs between different radio access technologies.The most common form of intersystem handover is between GSM and UMTS / WCDMA. Here thereare two different types:

UMTS / WCDMA to GSM handover: There are two further divisions of this category of

handover:

o Blind handover: This form of handover occurs when the base station hands offthe mobile by passing it the details of the new cell to the mobile without linking toit and setting the timing, etc of the mobile for the new cell. In this mode, the

network selects what it believes to be the optimum GSM based station. The mobilefirst locates the broadcast channel of the new cell, gains timing synchronisation

and then carries out non-synchronised intercell handover.o Compressed mode handover: using this form of handover the mobile uses the

gaps I transmission that occur to analyse the reception of local GSM base stationsusing the neighbour list to select suitable candidate base stations. Having selecteda suitable base station the handover takes place, again without any timesynchronisation having occurred.

Handover from GSM to UMTS / WCDMA: This form of handover is supported within

GSM and a "neighbour list" was established to enable this occur easily. As the GSM / 2G

network is normally more extensive than the 3G network, this type of handover does notnormally occur when the mobile leaves a coverage area and must quickly find a new basestation to maintain contact. The handover from GSM to UMTS occurs to provide an


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improvement in performance and can normally take place only when the conditions areright. The neighbour list will inform the mobile when this may happen.

SummaryGSM handover is one of the major elements in performance that users will notice. As a result it isnormally one of the Key Performance Indicators (KPIs) used by operators to monitor performance.Poor handover or handoff performance will normally result in dropped calls, and users find thisparticularly annoying. Accordingly network operators develop and maintain their networks toensure that an acceptable performance is achieved. In this way they can reduce what is called"churn" where users change from one network to another.

ATMThe Asynchronous Transfer Mode (ATM) was developed to enable a single data networking

standard to be used for both synchronous channel networking and packet-based networking.Asynchrnonous transfer mode also supports multiple levels of quality of service for packet traffic.In this way, asynchronous transfer mode can be thought of as supporting both circuit-switchednetworks and packet-switched networks by mapping both bitstreams and packet-streams. Itachieves this by sending data in a series or stream of fixed length cells, each of which has its ownidentifier. These data cells are typically sent on demand within a synchronous time-slot pattern ina synchronous bit-stream. Although this may not appear to be asynchronous, the asynchronouselement of the "Asynchronous Transfer Mode", comes from the fact that the sending of the cellsthemselves is asynchronous and not from the synchronous low-level bitstream that carries them.One of the original aims of Asynchronous Transfer Mode was that it should provide a basis forBroadband Integrated Services Digital Network (B-ISDN) to replace existing PSTN (Private ). Asa result of this the standards for Asynchronous Transfer Mode standards include not only thedefinitions for the Physical transmission techniques (Layer 1), but also layers 2 and 3.In addition to this, the development of Asysnchronous Transfer Mode was focussed heavily on the

requirements for telecommunications providers rather than local data networking requirements,and as a result it is more suited to large area telecommunications applications rather than smallerlocal area data network solutions, or general computer networking.While Asynchronous Transfer Mode is widely used for many applications, it is generally only usedfor transport of IP traffic. It has not become the single standard for providing a single integratedtechnology for LANs, public networks, and user services.

Basic asynchronous transfer mode systemThere are two basic elements to an ATM system. Any system can be made up a number of each ofthese elements:

ATM switch This accepts the incoming cells or information "packets" from another ATMentity which may be either another switch or an end point. It reads and updates the cellheader information and switches the information cell towards its destination

ATM end point This element contains the ATM network interface adaptor to enable

data entering or leaving the ATM network to interface to the external world. Examples ofthese end points include workstations, LAN switches, video codecs and many more items.

ATM networks can be configured in many ways. The overall network will comprise a set of ATMswitches interconnected by point-to-point ATM links or interfaces. Within the network there are

two types of interface and these are both supported by the switches. The first is UNI and this isused to connect ATM end systems (such as hosts and routers) to an ATM switch. The second type

of interface is known as NNI. This connects two ATM switches.


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ATM operationIn ATM the information is formatted into fixed length cells consisting of 48 bytes (each 8 bits long)of payload data. In addition to this there is a cell header that consists of 5 bytes, giving a total celllength of 53 bytes. This format has been chosen because time critical data such as voice packets isnot affected by very long packets being sent. The data carried in the header comprises payloadinformation as well as what are termed virtual-circuit identifiers and header error check data.

ATM is what is termed connection orientated. This has the advantage that the user can define therequirements that are needed to support the calls, and in turn this allows the network to allocatedthe required resources. By adopting this approach, several calls can be multiplexed efficiently andensuring that the required resources can be allocated.There are two types of connection that are specified for asynchronous transfer mode:

Virtual Channel Connections - this is the basic connection unit or entity. It carries a

single stream of data cells from the originator to the end user.

Virtual Path Connections- this is formed from a collection of virtual channelconnections. A virtual path is an end to end connection created across an ATM

(asynchronous transfer mode) network. For a virtual path connection, the network routesall cells from the virtual path across the network in the same way without regard for the

individual virtual circuit connection. This results in faster transfer.

The idea of virtual path connections are also used within the ATM network itself to routetraffic between switches

ATM networks can be configured in many ways. The overall network will comprise a set of ATMswitches interconnected by point-to-point ATM links or interfaces. Within the network there are

two types of interface and these are both supported by the switches. The first is UNI and this isused to connect ATM end systems (such as hosts and routers) to an ATM switch. The second type

of interface is known as NNI. This connects two ATM switches.

E-Carrier, E1The E carrier system has been created by the European Conference of Postal andTelecommunications Administrations (CEPT) as a digital telecommunications carrier scheme forcarrying multiple links. The E-carrier system enables the transmission of several (multiplexed)voice/data channels simultaneously on the same transmission facility. Of the various levels of theE-carrier system, the E1 and E3 levels are the only ones that are used.More specifically E1 has an overall bandwidth of 2048 kbps and provides 32 channels eachsupporting a data rate of 64 kbps. The lines are mainly used to connect between the PABX (PrivateAutomatic Branch eXchange), and the CO (Central Office) or main exchange.The E1 standard defines the physical characteristics of a transmission path, and as such itcorresponds to the physical layer (layer 1) in the OSI model. Technologies such as ATM and otherswhich form layer 2 are able to pass over E1 lines, making E1 one of the fundamental technologies

used within telecommunications.A similar standard to E1, known as T1 has similar characteristics, but it is widely used in NorthAmerica. Often equipment used for these technologies, e.g. test equipment may be used for both,and the abbreviation E1/T1 may be seen.

E1 beginningsThe life of the standards started back in the early 1960s when Bell Laboratories, where thetransistor was invented some years earlier, developed a voice multiplexing system to enable betteruse to be made of the lines that were required, and to provide improved performance of theanalogue techniques that were used. The step of the process converted the signal into a digitalformat having a 64 kbps data stream. The next stage is to assemble twenty four of the datastreams into a framed data stream with an overall data rate of 1.544 Mbps. This structured signal

was called DS1, but it is almost universally referred to as T1.In Europe, the basic scheme was taken by what was then the CCIT and developed to fit theEuropean requirements better. This resulted in the development of the scheme known as E1. This


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the telecommunications traffic so that the volume can be quantified in a standard way andcalculations can be made. Telecommunications network designers make great use of the Erlang tounderstand traffic patterns within a voice network and they use the figures to determine thecapacity that is required in any area of the network.

Who was Erlang?The Erlang is named after a Danish telephone engineer named A.K Erlang (Agner Krarup Erlang).He was born on 1st January 1878 and although he trained as a mathematician, he was the firstperson to investigate traffic and queuing theory in telephone circuits.After receiving his MA, Erlang worked in a number of schools. However, Erlang was a member ofthe Danish Mathematician's Association (TBMI) and it was through this organization that Erlangmet the Chief Engineer of the Copenhagen Telephone Company (CTC) and as a result, he went towork for them from 1908 for almost 20 years.While he was at CTC, Erlang studied the loading on telephone circuits, looking at how many lineswere required to provide an acceptable service without installing too much over-capacity thatwould cost the company money. There was a trade-off between cost and service level.Erlang developed his theories over a number of years, and published several papers. He expressedhis findings in mathematical forms so that they could be used to calculate the required level ofcapacity, and today the same basic equations are in widespread use..In view of his groundbreaking work, the International Consultative Committee on Telephones andTelegraphs (CCITT) honoured him in 1946 by adopting the name "Erlang" for the basic unit oftelephone traffic.Erlang died on 3rd February 1929 after an unsuccessful abdominal operation.

Erlang basicsThe Erlang is the basic unit of telecommunications traffic intensity representing continuous use ofone circuit and it is given the symbol "E". It is effectively call intensity in call minutes per sixtyminutes. In general the period of an hour is used, but it actually a dimensionless unit because thedimensions cancel out (i.e. minutes per minute).The number of Erlangs is easy to deduce in a simple case. If a resource carries one Erlang, thenthis is equivalent to one continuous call over the period of an hour. Alternatively if two calls werein progress for fifty percent of the time, then this would also equal one Erlang (1E). Alternatively ifa radio channel is used for fifty percent of the time carries a traffic level of half an Erlang (0.5E)From this it can be seen that an Erlang, E, may be thought of as a use multiplier where 100% useis 1E, 200% is 2E, 50% use is 0.5E and so forth.Interestingly for many years, AT&T and Bell Canada measured traffic in another unit called CCS,100 call seconds. If figures in CCS are encountered then it is a simple conversion to change CCS toErlangs. Simply divide the figure in CCS by 36 to obtain the figure in Erlangs

Erlang function or Erlang formula and symbolIt is possible to express the way in which the number of Erlangs are required in the format of asimple function or formula.A = x h

Where: = the mean arrival rate of new callsh = the mean call length or holding timeA = the traffic in Erlangs.Using this simple Erlang function or Erlang formula, the traffic can easily be calculated.

Erlang-B and Erlang-CErlang calculations are further broken down as follows:

Erlang B: The Erlang B is used to work out how many lines are required from aknowledge of the traffic figure during the busiest hour. The Erlang B figure assumes that

any blocked calls are cleared immediately. This is the most commonly used figure to beused in any telecommunications capacity calculations.


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Extended Erlang B: The Extended Erlang B is similar to Erlang B, but it can be used to

factor in the number of calls that are blocked and immediately tried again.

Erlang C: The Erlang C model assumes that not all calls may be handled immediatelyand some calls are queued until they can be handled.

These different models are described in further detail below.

Erlang BIt is particularly important to understand the traffic volumes at peak times of the day.

Telecommunications traffic, like many other commodities, varies over the course of the day, andalso the week. It is therefore necessary to understand the telecommunications traffic at the peak

times of the day and to be able to determine the acceptable level of service required. The Erlang Bfigure is designed to handle the peak or busy periods and to determine the level of service

required in these periods.

Erlang CThe Erlang C model is used by call centres to determine how many staff or call stations are

needed, based on the number of calls per hour, the average duration of call and the length of timecalls are left in the queue. The Erlang C figure is somewhat more difficult to determine because

there are more interdependent variables. The Erlang C figure, is nevertheless very important todetermine if a call centre is to be set up, as callers do not like being kept waiting interminably, as

so often happens.

Erlang summaryThe Erlang formulas and the concepts put forward by Erlang are still an essential part of

telecommunications network planning these days. As a result, telecommunications engineersshould have a good understanding of the Erlang and the associated formulae.

despite the widespread use of the Erlang concepts and formulae, it is necessary to remember thatthere are limitations to their use. It is necessary to remember that the Erlang formulas make

assumptions. Erlang B assumes that callers who receive a busy tone will not immediately tryagain. Also Erlang C assumes that callers will not hold on indefinitely. It is also worth remembering

that the Erlang formulas are based on statistics, and that to make these come true an infinitenumber of sources is required. However for most cases a total of ten sources gives an adequate

number of sources to give sufficiently accurate results.The Erlang is a particularly important element of telecommunications theory, and it is a

cornerstone of many areas of telecommunications technology today. However one must be awareof its limitations and apply the findings of any work using Erlangs, the Erlang B and Erlang C

formulas or functions with a certain amount of practical knowledge.

Ethernet IEEE 802.3 tutorialThis Ethernet, IEEE 802.3 tutorial is split into several pages each of which addresses differentaspects of Ethernet, IEEE 802.3 operation and technology:

[1] Ethernet IEEE 802.3 tutorial[2] Ethernet IEEE 802.3 standards[3] Ethernet IEEE 802.3 dataframes structure[4] 100 Mbps Ethernet inc 100 Base-T[5] Gigabit Ethernet 1GE[6] Ethernet

cables[7] Power over Ethernet, 802.3af and 802.3atEthernet, defined under IEEE 802.3, is one of today's most widely used data communications

standards, and it finds its major use in Local Area Network (LAN) applications. With versionsincluding 10Base-T, 100Base-T and now Gigabit Ethernet, it offers a wide variety of choices of

speeds and capability. Ethernet is also cheap and easy to install. Additionally Ethernet, IEEE 802.3offers a considerable degree of flexibility in terms of the network topologies that are allowed.

Furthermore as it is in widespread use in LANs, it has been developed into a robust system thatmeets the needs to wide number of networking requirements.

Ethernet, IEEE 802.3 history

The Ethernet standard was first developed by the Xerox Corporation as an experimental coaxialcable based system in the 1970s. Using a Carrier Sense Multiple Access / Collision Detect
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