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GSM Basics, Version 2.2 T.O.P. BusinessInteractive GmbH Page 1 of 33

4 BSS Interfaces




4 BSS Interfaces .......................................................................1 4.1 The BSS Interfaces (1/3) ....................................................3 4.1 The BSS Interfaces (2/3) ....................................................4 4.1 The BSS Interfaces (3/3) ....................................................5 4.2 The A Interface....................................................................6 4.3 The A-ter Interface..............................................................7 4.3.1 Fullrate vs. Enhanced Fullrate Speech Codec (1/2) .....8 4.3.1 Fullrate vs. Enhanced Fullrate Speech Codec (2/2) .....9 4.3.2 Discontinuous Transmission (1/2)...............................10 4.3.2 Discontinuous Transmission (2/2)...............................11 4.4 The A-bis Interface (1/4)...................................................12 4.4 The A-bis Interface (2/4)...................................................13 4.4 The A-bis Interface (3/4)...................................................14 4.4 The A-bis Interface (4/4)...................................................15 4.5 The Terrestrial Interfaces – Summary (1/2)....................16 4.5 The Terrestrial Interfaces – Summary (2/2)....................17 4.6 The Air Interface Um (1/2) ................................................18 4.6 The Air Interface Um (2/2) ................................................19 4.6.1 Basic Principles of Transmission (1/5) .......................20 4.6.1 Basic Principles of Transmission (2/5) .......................21 4.6.1 Basic Principles of Transmission (3/5) .......................22 4.6.1 Basic Principles of Transmission (4/5) .......................23 4.6.1 Basic Principles of Transmission (5/5) .......................24 4.6.2 The Physical Channels .................................................25 4.6.3 The Logical Channels (1/6)...........................................26 4.6.3 The Logical Channels (2/6)...........................................27 4.6.3 The Logical Channels (3/6)...........................................28

4.6.3 The Logical Channels (4/6)...........................................29 4.6.3 The Logical Channels (5/6)...........................................30 4.6.3 The Logical Channels (6/6)...........................................31 4.7 Channel Coding (1/2) .......................................................32 4.7 Channel Coding (2/2) .......................................................33




4.1 The BSS Interfaces (1/3)

Within the BSS, the user- and signalling data is transported over a series of interfaces. The A

interface connects the Mobile Services Switching Center (MSC) with the Transcoder TC.The A-ter interface connects the Transcoder with the Base Station Controller (BSC). The A-bis interface connects the BSC with the Base Transceiver Station (BTS). Finally, the data istransmitted to the mobile station via the air interface Um.





Let's consider the PCM30 configuration as an example for the frame structure of data

transmission between the MSC and the mobile station, to understand the dataflow at the Ainterface, the A-ter, A-bis and Um interfaces.We see that the 4 A-links are mapped onto one A-ter link. 4 A-channels of 64 kbps each aremapped onto an A-ter channel consisting of 4 subchannels of 16 kbps each. In total, the 128channels of 4 A-links are reduced to the 32 channels of one A-ter link, which are numberedconsecutively from 0 to 31. The SS7 signalling, which in our example is to be found intimeslot No 16, is transmitted from A to A-ter transparently, i.e. unchanged.





The frame structure consisting of 32 channels is also found at the A-bis interface. Channel 0is used for synchronization, the remaining 31 channels transmit warning information for operation and maintenance of the BTS, known as O&M alarms, as well as signalling andvoice data. Finally, the information from A-bis is transmitted to the air interface Um via theTRXs, the radio transceivers of the BTS. Two A-bis channels of 4 subchannels eachcorrespond exactly to the eight timeslots of a TDMA frame, which carries the data to themobile station. A TDMA frame, which we will discuss in more detail later in the course,portions the stream of physical channels or timeslots on a particular carrier frequency intoperiods.Its timeslots are numbered consecutively from 0 to 7, and can be assigned to one TRX.




4.2 The A Interface

The A-interface transmits user and signalling data between the MSC and the transcoder. It'sthe second completely standardized interface in GSM after the air interface. As an openinterface it is not tied to a specific producer.The A-interface is an ISDN-S2M interface that has been adjusted to GSM with a data rate of 64 kbps per timeslot. In the PCM30 configuration, the A interface contains 30 trafficchannels. Timeslot number 0 takes over synchronization tasks, and timeslot number 16contains signalling information in the No 7 signalling system format, or SS7. Thus the air interface has an overall bit rate of 2048 kbps.The PCM24 configuration, which is generally used in the USA, uses 24 traffic channels. Inboth configurations, each frame has clearly defined channels for signalling andsynchronisation information.




4.3 The A-ter Interface

4 traffic channels of the A interface are bundled into four A-ter channels of 16 kbps each,

which are subsequently transmittted to the BSC in a 64 kbps physical A-ter timeslot.Conversely, signals coming from the BSC are transcoded from 16 to 64 kbps, which is the bitrate typically used in fixed networks. Signalling channels are not transcoded. At the A-ter interface, 120 speech channels of 16 kbps each form a 2 Mbit/s multiplex connection. Four times as many A links as A-ter links are necessary to transmit the same amount of voicedata.




4.3.1 Fullrate vs. Enhanced Fullrate Speech Codec (1/2)

Now let's turn to a procedure which takes the original speech, and generates the speech

description parameters in the TC.During the first phase of GSM, which lasted until 1995, a speech codec in the MS and in thetranscoder was specified as the Full-Rate Codec. The basic characteristics of speech, that isthe volume, the base frequency, and the tone, are extracted in 20 ms segments from the 64kbps signal so that descriptive parameters in 16 kbps signals are generated. The predictionalgorithms, that is to say the calculability of speech, make the data less sensitive to theinterference a signal meets on its way from and to the mobile station at the air interface. Let'slisten to a couple of audio samples.




4.3.1 Fullrate vs. Enhanced Fullrate Speech Codec (2/2)

With the Fullrate Speech Codec, and with low interference, speech comes out sufficiently

true to the original.With increasing interference at the air interface, this quality ensuring measure soon hasproblems.The specifications of the second phase - which started in 1995 - brought higher developed,and more powerful speech encryption procedures. In cases of low interference, theEnhanced Full-Rate Codec stands out for its better speech quality.With high interference at the air interface, its advantages become even more evident. Thevoice quality has improved a great deal.




4.3.2 Discontinuous Transmission (1/2)

In GSM, all voice signals are transmitted the same way and in a continuous data stream. Thechannel is occupied even during silence intervals. This has two fundamental disadvantages:

1. Since the mobile station must send for the whole duration of the call, transmittingpower is used even in silence intervals, i.e. when the subscriber is only listening. Thiswastes the mobile station's battery power.

2. Other subscribers using the same frequency in distant cells could be disturbed morethan necessary.

Therefore it is logical to switch off the sender whenever the subscriber is not activelytransmitting information. Considering the pauses in the dialogue, and also the pausesbetween and within the sentences, we will find that the average occupation of the radio link is

less than 40%.Discontinuous Transmission (DTX) is a remedy to this problem. Let's go into more detail onDTX by listening to some audio samples.




4.3.2 Discontinuous Transmission (2/2)

In DTX, a function known as voice activity detection switches off the sender of a mobile

station whenever there is no data to be transmitted.During speech pauses, a "stopgap" in the receiver, which in the uplink is the correspondingtranscoder element in the TC, must simulate a functioning channel for the user. In GSM thisis called "comfort noise". It is the background noise analysed before the MS is switched off,re-generated by the TC. The comfort noise is even updated during a speech pause, by themobile station transmitting relevant information to the TC.The following audio samples will prove the facts.The first sample is a live recording of a GSM call with a strong background noise and withoutvoice activity detection.Now we will listen to the same call, but with the voice activity detection activated.This is not very pleasant to listen to, is it?

It will be totally different in the final version of this call, where comfort noise is activatedduring the speech pauses.




4.4 The A-bis Interface (1/4)

The A-bis interface connects the Base Transceiver Station (BTS) with the Base Station

Controller (BSC). In the PCM30 configuration, the data at this interface is transmitted viacable or via microwave transmission at a bit rate of 2 Mbit/s. A cable connection is moreresistant to interference, but a network operator must lease it from a fixed network operator.The microwave links can be operated independently and are easily configured by thenetwork operator, but they are more sensitive to interference. Four types of information canbe transmitted over the A-bis interface: user information, synchronization data, signalinginformation, and data for the operation and maintenance of the BTS, known as O&M alarms.





In the basic configuration, the channels of the A-bis interface are directly connected to thetimeslots of the radio transmission at the air interface. The physical data rate is 64 kbps. InPCM30, timeslot 0 of the A-bis interface is used for synchronization. The remaining 31timeslots of the PCM30 configuration carry data from and to the transceivers of the BTS, aswell as signalling information and O&M alarms.In the uplink, 4 traffic channels of 16 kbps each are sub-multiplexed and transmitted from theBTS to the BSC in a physical A-bis time slot. The same happens in the downlink, only in theopposite direction, i.e. from the BSC to the transceivers of the BTS.Today's BSC - BTS connection can also be configured as a dynamic link with variablesignaling and traffic time slots, according to the current traffic situation.





Two PCM30 channels can be assigned to one TRX. These channels consist of 4 sub-timeslots each. Each PCM30-subtimeslot corresponds to a timeslot in the TRX. Thus, bymapping 8 PCM30 sub-timeslots onto one TDMA frame consisting of timeslots 0 to 7, theentire TDMA frame of the TRX would theoretically be available for the transmission of payload data. But then there wouldn't be enough space left for the necessary signalling trafficfrom and to the mobile stations. According to a fixed, producer-, and configuration-specificpattern, the signalling information is carried in specific A-bis timeslots of 64 kbps each, or in16 kbps sub-timeslots, to at least 1 TRX per cell, where it uses timeslot 0 to be transmittedover the air interface.





Special timeslots carry the O&M alarm traffic between the OMC and the BTS over the BSC.The information is, of course, not transmitted over the air interface. As we could see at the A-ter interface, each 16 kbps of a traffic channel consist of 13 kbps of payload and 3 of inbandsignalling between the BTS and the transcoder.Only the 13 kbps of payload data may be transmitted over the air interface.Depending on the producer, and on the configuration, each A-bis connection in the PCM30configuration may transport user information, signalling information, and O&M informationfrom and to up to 15 transceivers.In the PCM24 configuration, 24 channels achieve an overall bit rate of 1536 kbps at the A-bisinterface. Up to 10 transceivers can be assigned to a connection.




4.5 The Terrestrial Interfaces – Summary (1/2)

Let's summarize what we have learned about the three terrestrial interfaces A, A-ter and A-bis:Each of these three interfaces transmits information for the synchronization of the individualnetwork elements point-to-point, at a data rate of 64 kbps, and using timeslot 0.The transcoder merely forwards the SS7 signalling between the MSC and the BSC. This isdone transparently, at a bit rate of 64 kbps, both over the A and over the A-ter interface, for example in timeslot 16. The TRX-related signalling between the BSC and the BTS istransmitted over the A-bis interface at 16, 32 or 64 kbps, depending on the producer. O&Malarms from the transcoder are transmitted to the BSC over the A-ter interface at 16 kbps, or as inband signals through a normal traffic channel. O&M alarms from the BTS aretransmitted to the BSC, which is also the O&M master for the entire BSS, over the A-bisinterface at 16 or at 64 kbps. If the BSC is unable to correct the errors that caused the

alarms, or if it detects an error within itself, it informs the OMC directly, or forwards thealarms from the BTS or TC to it.




4.5 The Terrestrial Interfaces – Summary (2/2)

Let's consider the transmission of speech and user data, which is transmitted at a data rateof 64 kbps over the A interface, at 16 kbps over the A-ter interface - after being turned into

transcoded speech or rate adapted data - and also at 16 kbps per subchannel over the A-bisinterface. SMS messages are transmitted via signalling channels. The number of physicaltimeslots that's available for the transmission of signalling information over the air interfacedepends on the configuration, and is up to the manufacturer or to the operator.




4.6 The Air Interface Um (1/2)

Within mobile radio networks, data is transmitted over PCM lines at a bit rate of 2 Mbit/s. Air transmission is used between the mobile station and the BTS, and the informationtransmitted over the air interface must be adjusted to the PCM lines so it can pass throughthe rest of the network. The air interface, or Um, is the weakest part of a radio link. In GSM, alot is done to ensure high quality, security, and reliability.




4.6 The Air Interface Um (2/2)

At the air interface, the frequencies are arranged in pairs. Each uplink frequency has adownlink frequency permanently assigned to it. The uplink signal goes from the mobile

station to the base station, and the downlink signal goes in the opposite direction - from thebase station to the mobile. The arrangement in pairs is what actually enables simultaneouscommunication. The difference between the frequency pair is fixed and is called "duplexfrequency". In GSM 900, the duplex frequency is 45 MHz. Accordingly, the uplink frequencyrange 890 to 915 MHz, is assigned to a frequency range of 935 to 960 MHz in the downlink.In GSM 1800, the duplex frequency is 95 MHz. The uplink frequency range lies between1710 and 1785 MHz, the downlink frequency range between 1805 and 1880 MHz. In GSM1900, the duplex frequency is 80 MHz. The uplink frequency lies between 1850 and 1910MHz, and the downlink frequency between 1930 and 1990 MHz.




4.6.1 Basic Principles of Transmission (1/5)

The BTS elements which send and receive radio signals in the downlink and uplink channels,are known as transmitter & receivers, or transceivers (TRX) for short. In GSM networks, the

transmission over the air interface is digital. Digital transmission in GSM is based on acombination of the FDMA- and the TDMA methods, which already have been introduced. InFrequency Division Multiple Access - or FDMA - different frequency channels are assigned toeach BTS. Mobile phones in neighbouring cells - or within the same cell - can be usedsimultaneously, but occupy different frequencies. The FDMA method uses different carrier frequencies - 124 in GSM 900, 374 in GSM 1800, and 299 in GSM 1900.





Time Division Multiple Access, or TDMA, is a method where several subscribers share onefrequency - each subscriber is assigned its own time unit, which is known as a timeslot. In

analog mobile systems, on the other hand, a frequency is occupied by one subscriber for theduration of the call. In TDMA systems, each mobile station sends and receives informationonly on the timeslot it has been assigned. These timeslots are either used to transmit voicedata, or information on signalling and synchronization.





To send digital information over the air interface, the analog radio signals must be interpretedas bit signals. This process - the transmission of digital information to the air interface - iscalled modulation. Modulation takes advantage of the physical characteristics of analogsignals, and changes them in a certain way, depending whether the digital value to betransmitted is 1 or 0. Signals can be modulated on the basis of their amplitude, their frequency, or their phase. GSM uses a specific phase modulation known as the GaussianMinimum Shift Keying, or GMSK.





Time Division Multiple Access, or TDMA, splits a radio frequency into consecutive periodsknown as TDMA frames. A TDMA frame, in turn, consists of 8 short time units, which arereferred to as time slots. These time slots represent the physical basis for data transmission.Therefore they are also called physical channels. The radio signal between the mobilestation and the BTS consists of a continuous stream of time slots, organized in TDMAframes. Each connection is always assigned one timeslot.Thus, the physical channels provide the resources used to transmit specific types of information. The types of information and the functions define the logical channels. Thelogical channels differ according to the function they fulfil in data transmission.





To organize the radio transmission, various frame types consisting of numbered timeslots are

specified in GSM. The numbered timeslots are continuously numbered off by the mobilestation. A simple TDMA frame consists of eight physical channels, or timeslots. A timeslot is 0.557ms long. Thus a simple TDMA frame is 4.62 ms long. The length of a timeslot is also referredto as the burst period. A burst is the content of a physical channel.Information is transmitted as bursts each TDMA frame period. Traffic channels, i.e. time slots0 to 7 in a basic TRX configuration, contain their information organised in 26 TDMA periodsof time known as a multi-frame. This is 26 x 4.62 ms = 120 ms long. Signaling information,normally provided in time slot 0, is organised in 51 TDMA periods of 4.62 ms each, whichmakes 235 ms altogether. 26 of these "long" 51-multiframes, or 51 of the "short" 26-multiframes form a superframe, which is 6.12 seconds.The largest transmission unit defined is the hyperframe, which contains 2,048 superframesand is 3 hours, 28 minutes, 53 seconds, and 760 ms long. TDMA frames, multiframes,superframes and the hyperframe can be considered as counters to organize user andsignalling information within the TRX, and to support cyphering at the air interface.




4.6.2 The Physical Channels

The information which is physically transmitted over the air interface Um via the physicalchannels must be converted into a 16 kbps signal within a 2 Mbit/s Frame, which connectsthe BTS and the BSC as the A-bis interface. It is very important that all mobile stations withina cell send their digital information at the right moment, in order to avoid collisions at thetimeslots of the air interface, which would destroy the transmitted information. Therefore,each mobile station sends its digital voice data at regular periodic intervals, using a differenttimeslot to the other mobile stations within the same cell. The medium for this transmissionprocess is the timeslots, or physical channels. The content of such a channel is also knownas a burst. Bursts consist of different data blocks containing payload- as well as securityinformation, to guarantee high data reliability and transmission quality.




4.6.3 The Logical Channels (1/6)

In GSM, there are two types of logical channels: the dedicated channels, and the commonchannels. Let's explain the difference between the two with a metaphor from gardening. If we

want to water a whole area, and not a particular plant in it, we use a watering can.This metaphor describes the common channels. These supply their data according to theprinciple of "equal shares for all", and are not directed to a specific target. They are used tobroadcast information area-wide to all the mobile stations within the service area of a BTS.This is general signaling information, for example to log onto the network and cell-broadcastSMS.If, on the other hand, we only want to water a specific plant and deliberately leave out theneighbouring ones, we use a jet of water. This metaphor corresponds to the DedicatedChannels. These are always directed to a particular addressee. Various types of signallingchannels, known as the dedicated control channels, facilitate communication between themobile station and the mobile radio network. And, of course, traffic channels that carry user

speech and data also belong to this category. To understand the tasks of the individuallogical channels, we will now look at how a mobile station logs on to the network.





After the subscriber has switched on his mobile station and typed in his PIN code, the mobilestation searches for a network. But how does it log on to the network the subscriber isregistered with? For this purpose, the BTS sends out the Frequency Correction Channel(FCCH) at short regular intervals, to help the mobile station find a frequency for downlinkreception and adjust its frequency oscillator for the uplink transmission. To do so, it picks outthe strongest received signal. The Synchronization Channel (SCH) then helps the mobilestation to synchronize itself to timeslot 0 sent out by the BTS. This means the mobile stationmust adjust to the rhythm given by the BTS.The SCH contains the TDMA frame number as well as the Base Station Identity Code,containing basic information about the network operator that can be compared with the infostored on the SIM card. After this step, the mobile is able to decide whether it has chosen theproper network. If not, it starts the same procedure again trying with the second strongest

FCCH received.While the mobile station uses the FCCH to adjust its frequency, and the SCH for synchronization and network identification, the Broadcast Control Channel (BCCH), which isalso sent by the BTS, supplies the mobile station with additional information about theselected cell, for example for ciphering. For some Value Added Services, for examplelocation-dependent services, additional information has to be transmitted from the BTS to themobile. The Cell Broadcast Channel CBCH is used for this purpose to transmit geographicalparameters, for example Gauss-Krueger-Coordinates of the BTS, to the mobile. The FCCH,SCH, BCCH and CBCH are Broadcast Channels, and exist only in the downlink. They arethe first logical channels belonging to the Common Channels.





The mobile station has now adjusted its frequency and synchronized its TDMAs, and haspicked out the best cell available. But before it can be reached by other subscribers, andbefore it can initiate calls, a Location Update and authentication procedure are necessary.Only after that is the mobile station logged on to the network and has radio coverage. It cannow be reached by other mobile stations, or initiate a call. For this purpose, Common ControlChannels are required. Common Control Channels are "point-to-multipoint" channels, whichexist either only in the uplink, or only in the downlink.When a subscriber is called, the Paging Channel (PCH) is broadcast in the downlink by allbase stations within a Location Area, so that the mobile station concerned can react. Toinitiate a call, the mobile station sends out a Random Access Channel (RACH), which carriesits identification and request, for example for registration, to the network. This channel onlyexists in the uplink. In return, the network sends the Access Grant Channel (AGCH) in the

downlink direction, to assign resources to the mobile station, by granting it a Stand-AloneDedicated Control Channel, SDCCH. The PCH, RACH and AGCH form the group of theCommon Control Channels belonging also to the Common Channels.





A Stand-alone Dedicated Control Channel (SDCCH) has to be assigned to the mobile stationto exchange the requested signaling with the network, for example authentication, cipheringor call set-up. Also, it assigns a traffic channel, and it transmits short messages.The SACCH is always linked with an SDCCH or a traffic channel. It sends measurementreports to the network, and is used for power control and to handle the exact temporalalignment of the channels, the so-called Timing Advance.If the subscriber moves into the service area of another BTS, the handover commandneeded is transmitted over the FACCH. This channel is also used for every call release.During the call, FACCH data is transported over the Traffic Channel assigned.The Dedicated Control Channels are bidirectional point-to-point channels and belong to thegroup of Dedicated Channels.





User speech and data are transmitted over the traffic channels we have already spokenabout. Traffic channels are bidirectional, and also belong to the group of dedicated channels.

There are two different channel types supporting different gross bit rates. The TrafficChannel Full rate (TCH/F) has a gross bit rate of 22.8 kbps. It is used for speech encoded bya Full Rate or Enhanced Full Rate codec as well as for user data encapsulating a net bit rateof 9.6 kbps for standard bearer services, 14.4 kbps per timeslot in the case of HSCSD, or upto 21.4 kbps with GPRS. The Traffic Channel Half rate (TCH/H) supports 11.4 kbps and isonly used for Half Rate codec speech.





Let us sum up what we just learned about the classification of logical channels. Commonchannels include FCCH, SCH, BCCH, PCH, RACH, AGCH and, finally, CBCH. All containpoint-to-multipoint signaling information.Dedicated Channels contain point-to-point signalling, such as SDCCH, SACCH and FACCH,or traffic, such as TCH/F and TCH/H.




4.7 Channel Coding (1/2)

To be able to detect and correct bit errors at the air interface, GSM performs channel coding.This procedure is organized in two consecutive processes: block coding and convolutional

coding.In block coding, the parameters describing the speech data are first subdivided into threeclasses, which define if the data is important, required or unimportant for speech intelligibility.With convolutional coding, the information relevant to speech intelligibility is doubled with anarithmetical operation. That means a copy of the data is made so the data can be restored if necessary. This procedure allows to fully compensate bit error rates of up to 12.5 % in thesecured relevant data. Channel coding increases the bit rate necessary at the air interfacefrom 13 to 22.8 kbps.



4.7 Channel Coding (2/2)

The following audio samples will prove the effectiveness of channel coding as a protectionagainst transmission errors.

In the first example, channel coding is deactivated, and the radio transmission isconsiderably altered, due to a bit error rate of 6%.Now we will activate the channel coding. Although the bit error rate is just as high as in thefirst sample, we will realize that the speech quality has clearly improved.Now we will switch on another function in the transcoder element, which does not restore theoriginal speech from the heavily altered speech parameters, but uses predictive algorithms -a sort of estimate - for speech reconstruction. Please note: in this example the bit error rateis also 6%.

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