BSS Interface(A,A-ter&A-bis) with time slots detail

8/9/2019 BSS Interface(A,A-ter&A-bis) with time slots detail

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BSS Interface


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Within the BSS, the user- and signalling data is transported over aseries 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 is transmitted to the mobile station via the air interface

Um


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Let's consider the PCM30 configuration as an example for the framestructure of data transmission between the MSC and the mobile station,to understand the dataflow at the A interface, 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 are mapped onto an A-ter channel

consisting of 4 subchannels of 16 kbps each In total, the 128 channels of 4 A-links are reduced to the 32 channels of

one A-ter link, which are numbered consecutively from 0 to 31 The SS7 signalling, which in our example is to be found in timeslot No

16, is transmitted from A to A-ter transparently, i.e. unchanged


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The frame structure consisting of 32 channels is also found at the A-bis interface Channel 0 is used for synchronization, the remaining 31 channels transmit warning

information for operation and maintenance of the BTS, known as O&M alarms, as well assignalling and voice data

Finally, the information from A-bis is transmitted to the air interface Um via the TRXs, theradio transceivers of the BTS Two A-bis channels of 4 subchannels each correspond exactly to the eight timeslots of a

TDMA frame, which carries the data to the mobile station. A TDMA frame, which we willdiscuss in more detail later in the course, portions the stream of physical channels ortimeslots on a particular carrier frequency into periods

Its timeslots are numbered consecutively from 0 to 7, and can be assigned to one TRX.


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The A-interface transmits user and signalling data between the MSC and the transcoder It's the second completely standardized interface in GSM after the air interface. As an open

interface 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 traffic channels Timeslot number 0 takes over synchronization tasks, and timeslot number 16 contains

signalling information in the No 7 signalling system format, or SS7. Thus the air interfacehas an overall bit rate of 2048 kbps.

The PCM24 configuration, which is generally used in the USA, uses 24 traffic channels In both configurations, each frame has clearly defined channels for signalling andsynchronisation information.


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4 traffic channels of the A interface are bundled into four A-ter channelsof 16 kbps each, which are subsequently transmittted to the BSC in a 64kbps physical A-ter timeslot

Conversely, signals coming from the BSC are transcoded from 16 to 64kbps, which is the bit rate 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 voice data


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Now let's turn to a procedure which takes the original speech, andgenerates the speech description parameters in the TC

During the first phase of GSM, which lasted until 1995, a speech codec inthe MS and in the transcoder was specified as the Full-Rate Codec

The basic characteristics of speech, that is the volume, the basefrequency, and the tone, are extracted in 20 ms segments from the 64kbps signal so that descriptive parameters in 16 kbps signals aregenerated

The prediction algorithms, that is to say the calculability of speech, makethe data less sensitive to the interference a signal meets on its way fromand to the mobile station at the air interface


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

disadvantages: Since the mobile station must send for the whole duration of the call, transmitting power is used

even in silence intervals, i.e. when the subscriber is only listening. This wastes the mobile station'sbattery power

Other subscribers using the same frequency in distant cells could be disturbed more thannecessary

Therefore it is logical to switch off the sender whenever the subscriber is not actively

transmitting 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 linkis less than 40%.

Discontinuous Transmission (DTX) is a remedy to this problem.


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In DTX, a function known as voice activity detection switches off thesender of a mobile station whenever there is no data to be transmitted

During speech pauses, a "stopgap" in the receiver, which in the uplink isthe corresponding transcoder element in the TC, must simulate afunctioning channel for the user

In GSM this is called "comfort noise". It is the background noiseanalysed 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


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

In the PCM30 configuration, the data at this interface is transmitted via cable orvia microwave transmission at a bit rate of 2 Mbit/s

A cable connection is more resistent to interference, but a network operatormust lease it from a fixed network operator

The microwave links can be operated independently, and are easily configuredby the network operator, but they are more sensitive to interference

4 types of information can be transmitted over the A-bis interface: user information

synchronisation data signalling information and data for the operation and maintenance of the BTS, known as O&M alarms


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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. In PCM30, timeslot 0 of the A-bis interface is used for

synchronization The remaining 31 timeslots of the PCM30 configuration carry data from and to thetransceivers of the BTS, as well as signalling information and O&M alarms

In the uplink, 4 traffic channels of 16 kbps each are sub-multiplexed and transmitted fromthe BTS to the BSC in a physical A-bis time slot

The same happens in the downlink, only in the opposite 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 variable

signaling and traffic time slots, according to the current traffic situation


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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, by mapping 8 PCM30 sub-timeslots onto one TDMA frame consisting of timeslots 0

to 7, the entire TDMA frame of the TRX would theoretically be available for the transmissionof payload data

But then there wouldn't be enough space left for the necessary signalling traffic from and tothe mobile stations

According to a fixed, producer-, and configuration-specific pattern, the signalling informationis carried in specific A-bis timeslots of 64 kbps each, or in 16 kbps sub-timeslots, to at least1 TRX per cell, where it uses timeslot 0 to be transmitted over the air interface


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Special timeslots carry the O&M alarm traffic between the OMC and the BTS over theBSC

The information is, of course, not transmitted over the air interface. As we could see atthe A-ter interface, each 16 kbps of a traffic channel consist of 13 kbps of payload and 3of inband signalling 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

PCM30 configuration may transport user information, signalling information, and O&Minformation from and to up to 15 transceivers

In the PCM24 configuration, 24 channels achieve an overall bit rate of 1536 kbps at theA-bis interface. Up to 10 transceivers can be assigned to a connection


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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 individual network

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 is done 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 is transmitted over the A-bis interface at

16, 32 or 64 kbps, depending on the producer O&M alarms 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 are transmitted to the BSC, which is also the O&M master for the entireBSS, over the A-bis interface 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 the alarms from the BTS or TC to it


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Let's consider the transmission of speech and user data, which istransmitted at a data rate of 64 kbps over the A interface, at 16 kbps

over the A-ter interface - after being turned into transcoded speech orrate adapted data - and also at 16 kbps per subchannel over the A-bisinterface

SMS messages are transmitted via signalling channels The number of physical timeslots that's available for the transmission of

signalling information over the air interface depends on theconfiguration, and is up to the manufacturer or to the operator.


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Within mobile radio networks, data is transmitted over PCMlines at a bit rate of 2 Mbit/s

Air transmission is used between the mobile station and theBTS, and the information transmitted over the air interfacemust be adjusted to the PCM lines so it can pass through therest of the network

The air interface, or Um, is the weakest part of a radio link. InGSM, a lot is done to ensure high quality, security, andreliability


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At the air interface, the frequencies are arranged in pairs Each uplink frequency has a downlink 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 the base station to the mobile The arrangement in pairs is what actually enables simultaneous communication. The

difference between the frequency pair is fixed and is called "duplex frequency In GSM 900, the duplex frequency is 45 MHz. Accordingly, the uplink frequency range 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 between

1710 and 1785 MHz, the downlink frequency range between 1805 and 1880 MHz In GSM 1900, the duplex frequency is 80 MHz. The uplink frequency lies between 1850

and 1910 MHz, and the downlink frequency between 1930 and 1990 MHz


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The BTS elements which send and receive radio signals in the downlink anduplink channels, are known as transmitter & receivers, or transceivers (TRX) forshort

In GSM networks, the transmission over the air interface is digital. Digitaltransmission in GSM is based on a combination of the FDMA- and the TDMAmethods, which already have been introduced

In Frequency Division Multiple Access - or FDMA - different frequency channelsare assigned to each 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


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Time Division Multiple Access, or TDMA, is a method whereseveral subscribers share one frequency - each subscriberis assigned its own time unit, which is known as a timeslot

In analog mobile systems, on the other hand, a frequency isoccupied by one subscriber for the duration of the call

In TDMA systems, each mobile station sends and receives

information only on the timeslot it has been assigned These timeslots are either used to transmit voice data, orinformation on signalling and synchronization


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To send digital information over the air interface, the analog radio signals mustbe interpreted as bit signals

This process - the transmission of digital information to the air interface - is

called modulation Modulation takes advantage of the physical characteristics of analog signals,

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 Gaussian Minimum Shift

Keying, or GMSK


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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 are referred 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 mobile station and the BTS consists of a continuous stream of

time slots, organized in TDMA frames. 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 The logical channels differ according to the function they fulfil in data transmission


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To organize the radio transmission, various frame types consisting of numbered timeslots are specifiedin GSM. The numbered timeslots are continuously numbered off by the mobile station

A simple TDMA frame consists of eight physical channels, or timeslots. A timeslot is 0.557 ms long Thus a simple TDMA frame is 4.62 ms long The length of a timeslot is also referred to 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 slots 0 to 7 in a basic TRX configuration, contain their information organised

in 26 TDMA periods of 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, which makes 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 superframes and is 3hours, 28 minutes, 53 seconds, and 760 ms long TDMA frames, multiframes, superframes and the hyperframe can be considered as counters to

organize user and signalling information within the TRX, and to support cyphering at the air interface


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The information which is physically transmitted over the air interface Um via thephysical channels must be converted into a 16 kbps signal within a 2 Mbit/sFrame, which connects the BTS and the BSC as the A-bis interface

It is very important that all mobile stations within a cell send their digitalinformation at the right moment, in order to avoid collisions at the timeslots of theair interface, which would destroy the transmitted information

Therefore, each mobile station sends its digital voice data at regular periodicintervals, using a different timeslot to the other mobile stations within the samecell

The medium for this transmission process is the timeslots, or physical channels.The content of such a channel is also known as a burst

Bursts consist of different data blocks containing payload- as well as securityinformation, to guarantee high data reliability and transmission quality.


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In GSM, there are two types of logical channels: the dedicated channels, and the common channels

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 the principle of "equal shares for all", and are not directed to a

specific target They are used to broadcast 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-broadcast SMS If, on the other hand, we only want to water a specific plant and deliberately leave out the neighbouring

ones, we use a jet of water This metaphor corresponds to the Dedicated Channels These are always directed to a particular addressee Various types of signalling channels, known as the dedicated control channels, facilitate communication

between the mobile 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 individual logical channels, we will now look at how a mobile station logs

on to the network


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After the subscriber has switched on his mobile station and typed in his PIN code the


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After the subscriber has switched on his mobile station and typed in his PIN code, themobile station searches for a network

But how does it log on to the network the subscriber is registered with?

For this purpose, the BTS sends out the Frequency Correction Channel (FCCH) at shortregular intervals, to help the mobile station find a frequency for downlink reception andadjust its frequency oscillator for the uplink transmission

To do so, it picks out the strongest received signal The Synchronization Channel (SCH) then helps the mobile station to synchronize itself to

timeslot 0 sent out by the BTS

This means the mobile station must 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 theinfo stored on the SIM card

After this step, the mobile is able to decide whether it has chosen the proper 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 forsynchronization and network identification, the Broadcast Control Channel (BCCH), whichis also sent by the BTS, supplies the mobile station with additional information about theselected cell, for example for ciphering

For some Value Added Services, for example location-dependent services, additional

information has to be transmitted from the BTS to the mobile The Cell Broadcast Channel CBCH is used for this purpose to transmit geographical

parameters, 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 are the first logical channels belonging to the Common Channels.


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The mobile station has now adjusted its frequency and synchronized its TDMAs, and has picked out thebest cell available But before it can be reached by other subscribers, and before 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 can now be reached by other mobile stations, or initiate a call. For this purpose, Common Control

Channels are required Common Control Channels are "point-to-multipoint" channels, which exist 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 all base stations

within a Location Area, so that the mobile station concerned can react To initiate a call, the mobile station sends out a Random Access Channel (RACH), which carries its

identification and request, for example for registration, to the network

This channel only exists in the uplink. In return, the network sends the Access Grant Channel (AGCH) inthe downlink direction, to assign resources to the mobile station, by granting it a Stand-Alone DedicatedControl Channel, SDCCH. The PCH, RACH and AGCH form the group of the Common Control Channelsbelonging also to the Common Channels.


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A Stand-alone Dedicated Control Channel (SDCCH) has to be assigned to themobile station to exchange the requested signaling with the network, forexample authentication, ciphering or call set-up. Also, it assigns a trafficchannel, and it transmits short messages

The SACCH is always linked with an SDCCH or a traffic channel. It sends

measurement reports to the network, and is used for power control and tohandle the exact temporal alignment of the channels, the so-called TimingAdvance

If the subscriber moves into the service area of another BTS, the handovercommand needed is transmitted over the FACCH. This channel is also used forevery call release. During the call, FACCH data is transported over the TrafficChannel assigned

The Dedicated Control Channels are bidirectional point-to-point channels andbelong to the group of Dedicated Channels


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User speech and data are transmitted over the traffic channels we havealready spoken about. Traffic channels are bidirectional, and also belongto the group of dedicated channels

There are two different channel types supporting different gross bit rates The Traffic Channel Full rate (TCH/F) has a gross bit rate of 22.8 kbps. It

is used for speech encoded by a Full Rate or Enhanced Full Rate codecas well as for user data encapsulating a net bit rate of 9.6 kbps forstandard bearer services, 14.4 kbps per timeslot in the case of HSCSD,

or up to 21.4 kbps with GPRS The Traffic Channel Half rate (TCH/H) supports 11.4 kbps and is only

used for Half Rate codec speech.


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Let us sum up what we just learned about theclassification of logical channels. Commonchannels include FCCH, SCH, BCCH, PCH, RACH,

AGCH and, finally, CBCH. All contain point-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.


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To be able to detect and correct bit errors at the air interface, GSM performschannel 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 three classes, which define if the data is important, required or unimportantfor speech intelligibility. With convolutional coding, the information relevant to speech intelligibility is

doubled with an arithmetical operation. That means a copy of the data is madeso the data can be restored if necessary. This procedure allows to fullycompensate bit error rates of up to 12.5 % in the secured relevant data. Channelcoding increases the bit rate necessary at the air interface from 13 to 22.8 kbps.

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BSS Interface(A,A-ter&A-bis) with time slots detail