02_bss Traffic Channels

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BSSTraffic Channels

BSS S11



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BSS Traffic ChannelBSS Traffic Channel

• This module describes the flow of in circuit switched and a packetswitched traffic through following network elements and interfaces:

MSC

SGSN

ET

ET

ET

ET

ET ET

A IF Ater IF AbisIF Air IF

Gb IF

ET

ET

ET

ET

TCSM2E

MS

BTS

TRU

BSC

ET

ET

ET

Network Elements•TCSM: Transcoder Submultiplexer•BSC: Base Station Controller

•BS: Base Station•MS: Mobile Station

(MSC: Mobile Switching Centre)(SGSN: Serving GPRS Support Nodes)

Interfaces•A-Interface•Ater-Interface

•Abis-Interface•Air-Interface•Gb-Interface

TRAU frame



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A interfaceA interfaceTS 1 2 3 4 5 6 7 801 TCH2 TCH3 TCH4 TCH

5 TCH

6 TCH7 TCH8 TCH9 TCH

10 TCH11 TCH12 TCH13 TCH14 TCH15161718 TCH19 TCH20 TCH21 TCH22 TCH23 TCH24 TCH

25 TCH26 TCH27 TCH28 TCH29 TCH30 TCH31 TCH

TCHCCS7 TCH

A interface

The A interface is based on the ITU-T Recommendation G.703 (electrically) and G.704 (framestructure). The traffic channels’transmission rate in the A interface is 64kbit/s and they are

located in the time slots 1 - 15 and 17 - 31.

TSL 16 in the A interface is normally used for the CCS7 signalling and its transmission rate is 64kbit/s.



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TranscodingTranscodingandandsubmultiplexingsubmultiplexing

7 6 5 4 3 2 1 160 159 158 157 156 157 154

31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

x x b1 b2 x x x x

31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

2 Mbit/s framesfrom theMSC to the transcoder

2 Mbit/s frames, 125 us

7 6 5 4 3 2 1 160 159 158 157 156 157 154

B1 B2 B3 B4 B5 B6 B7 B8

7 6 5 4 3 2 1 160 159 158 157 156 157 154 7 6 5 4 3 2 1 160 159 158 157 156 157 154

20ms sample, 160 x 2 Mbit/s frames

64kbit/s timeslot, 8 bits

160 x timeslot from the160 x 2 Mbit/s frames

DSP forTS 1

DSP forTS 31

160 x 2 bitsfromtimeslot

2 Mbit/s frames, 125 us

20 ms sample, 160 2 Mbit/s frames Mbit/s framesfrom the transcoder totheBSC (SM2M)

TranscoderTCSM2

The speech signal is divided into 20 ms samples (160 2Mbit/s frames). Each sample is taken fromthe MSC and passed into thetranscoder where the Digital Signal Processor (DSP) performs the

RPE-LTP coding of the sample (Regular Pulse Excitation - Long Term Prediction). The resultingcoded sample is known as the vocodedblock, which contains 260 bits for full rate trafficchannels or 112 bits for half rate. Each vocodedblock is inserted into a TRAU frame, whichcontains a vocodedblock plus synchronisation bits and control bits giving a total of 320 bits (16kbit/s) for full rate or 160 bits (8kbit/s) for half rate. The frame is reassembled in the DSP in thetransceiver unit of the BTS. Each traffic timeslot has its own DSP for transcoding.



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InbandInbandSignalling between TCSM2 and BTSSignalling between TCSM2 and BTS

TCSM2E BSC BTS

64 kb 16 kb 16 kb

33,8 kb

InbandSignallingwithin the TRAU Frame

16 kb

DSP1

DSP90

2Mbit/s TRU DSPET

GSWMS

TRX

13 kb

ET2

Mbit/s2

Mbit/s

Inbandsignalling between TCSM2E and TRX in the BTS

The bit rate after transcodingis 16kbit/s for full rate traffic channels, which includes 13kbit/s

of transcodedspeech/data and 3kbit/s of control data. For half rate the bit rate is 8kbit/s,which includes 5.6kbit/s of speech/data and 2.4kbit/s of control data. This control data is usedfor inbandsignalling between the DSP in theTranscoder and the DSP in the Transceiver Unit of the BTS.

The BTS controls thetranscoderwith this signalling information for the following purposes:

- shifting between speech and data,- shifting between half and full rate radio channels,- controlling rate adaptation functions for data calls,- timing down link frames for speech frames, and- transferring DTX (discontinuous transmission) information.



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AterAter interfaceinterface

1st A-IFtrunk

2nd A-IFtrunk

3rd A-IFtrunk

CCS7

4th A-IF

trunk

1st A-IF

trunk (HR)

2nd A-IFtrunk (HR)3rd A-IFtrunk (HR)

4th A-IFtrunk (HR)5th A-IFtrunk (HR)

6th A-IFtrunk (HR)

7th A-IF

trunk (HR)CCS7 / X.25

1st A-IFtrunk

2nd A-IFtrunk

3rd A-IFtrunk (HR)

4th A-IFtrunk (HR)

5th A-IF trunk

(HS4)

1 2 3 4 5 6 7 8

00

01 0 1 0 2 0 3 0 4 0 5 0 6

02 0 7 0 8 0 9 1 0 1 1 1 2 1 3 1 4

03 1 5 1 7 1 8 1 9 2 0 2 1 2 2 2 304 2 4 2 5 2 6 2 7 2 8 2 9 3 0 3 1

05 0 1 0 2 0 3 0 4 0 5 0 6

06 0 7 0 8 0 9 1 0 1 1 1 2 1 3 1 4

07 1 5 1 7 1 8 1 9 2 0 2 1 2 2 2 3

08 2 4 2 5 2 6 2 7 2 8 2 9 3 0 3 1

09 0 1 0 2 0 3 0 4 0 5 0 6

10 0 7 0 8 0 9 1 0 1 1 1 2 1 3 1 4

11 1 5 1 7 1 8 1 9 2 0 2 1 2 2 2 3

12 2 4 2 5 2 6 2 7 2 8 2 9 3 0 3 1

13 0 1 0 2 0 3 0 4 0 5 0 6

14 0 7 0 8 0 9 1 0 1 1 1 2 1 3 1 4

15 1 5 1 7 1 8 1 9 2 0 2 1 2 2 2 3

16 2 4 2 5 2 6 2 7 2 8 2 9 3 0 3 1

17 0 1 0 2 0 3 0 4 0 5 0 6

18 0 7 0 8 0 9 1 0 1 1 1 2 1 3 1 4

19 1 5 1 7 1 8 1 9 2 0 2 1 2 2 2 3

20 2 4 2 5 2 6 2 7 2 8 2 9 3 0 3 1

21 0 1 0 2 0 3 0 4 0 5 0 6

22 0 7 0 8 0 9 1 0 1 1 1 2 1 3 1 4

23 1 5 1 7 1 8 1 9 2 0 2 1 2 2 2 3

24 2 4 2 5 2 6 2 7 2 8 2 9 3 0 3 1

25 0 1 0 2 0 3 0 4 0 5 0 6

26 0 7 0 8 0 9 1 0 1 1 1 2 1 3 1 4

27 1 5 1 7 1 8 1 9 2 0 2 1 2 2 2 3

28 2 4 2 5 2 6 2 7 2 8 2 9 3 0 3 1

29 - -- - -- - -- - -- - -- - -- - -- - --

30 - -- - -- - -- - -- - -- - -- - -- - --

31 - -- - -- - -- - -- - -- - -- - -- - --

LAPD

1 2 3 4 5 6 7 8

00

01 01 02 03

02 04 05 06 07

03 08 09 10 1104 12 13 14 15

05 16 17 18 19

06 20 21 22 23

07 24 25 26 27

08 28 29 30 31

09 01 02 03

10 04 05 06 07

11 08 09 10 11

12 12 13 14 15

13 16 17 18 19

14 20 21 22 23

15 24 25 26 27

16 28 29 30 31

17 01 02 03

18 04 05 06 07

19 08 09 10 11

20 12 13 14 15

21 16 17 18 19

22 20 21 22 23

23 24 25 26 27

24 28 29 30 31

25 01 02 03

26 04 05 06 07

27 08 09 10 11

28 12 13 14 15

29 16 17 18 19

30 --- --- --- --- --- --- --- ---

31 --- --- --- --- --- --- --- ---

LAPD

1 2 3 4 5 6 7 8

00

01 01 02 03

02 04 05 06 07

03 08 09 10 1104 12 13 14 15

05 16 17 18 19

06 20 21 22 23

07 24 25 26 27

08 28 29 30 31

09 01 02 03

10 04 05 06 07

11 08 09 10 11

12 12 13 14 15

13 16 17 18 19

14 20 21 22 23

15 24 25 26 27

16 28 29 30 31

17 0 1 0 2 0 3 0 4 0 5 0 6

18 0 7 0 8 0 9 1 0 1 1 1 2 1 3 1 4

19 1 5 1 7 1 8 1 9 2 0 2 1 2 2 2 3

20 2 4 2 5 2 6 2 7 2 8 2 9 3 0 3 1

21 0 1 0 2 0 3 0 4 0 5 0 6

22 0 7 0 8 0 9 1 0 1 1 1 2 1 3 1 4

23 1 5 1 7 1 8 1 9 2 0 2 1 2 2 2 3

24 2 4 2 5 2 6 2 7 2 8 2 9 3 0 3 1

25 01

26 02

27 03

28 04

29 05

30 06

31 07

LAPD

Ater interface

The Ater interface is based on the ITU-TRecommendation G.703 (electrically) and G.704 (framestructure). The traffic channels and signalling channels coming from the different PCMsof the

MSC are reallocated in thetranscoder.

The number of thePCMscoming from the MSC depends on the use of the line:

16kbit/s per speech circuit (e.g. Full Rate): max 4PCMs(max 120 TCH)

8kbit/s per speech circuit (e.g. Half Rate): max 7PCMs(max 210 TCH)

64kbit/sperspeech circuit (e.g. HSCSD4): 1PCM (max 30 TCH)



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GbGbinterfaceinterface

ET

TransmissionNetwork

ET

BSC

TCSM

MSC

Frame Relaynetwork

ET

ET

ET

ET

SGSN

1

2

3

Gbinterface

TheGbinterface is located between the BSC and the SGSN, and it is implemented using FrameRelay (FR). The Frame Relay can be either point-to-point (PCU – SGSN) or there can be a Frame

Relay network located between the BSC and the SGSN. The Frame Relay network will becomprised of third-party off-the-shelf products. The following figure displays examples of theGbinterface transmission solutions:

In the first solution (1) spare capacity of the Ater and the A interfaces is used for theGbinterface. TheGbtimeslots are transparently through connected in the TCSM and in the MSC. The second solution (2) represents a transmission network that provides a point-to-pointconnection between the BSC and the SGSN. In the third solution (3) the Frame Relay network isused.

Gb interface allows many users to be multiplexed over the same physical link using Frame Relay.Bandwidth is allocated to a user upon activity (when data is sent or received) and is reallocated

immediately thereafter. This is in contrast to the A interface, where a single user has theexclusive use of a dedicated physical resource throughout the lifetime of a call irrespective of activity.

A Gbinterface Bearer channel can use 1 to 30 64kbit/s timeslots.



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Base Station Controller (BSC)Base Station Controller (BSC)

ET

GSW

PCU

BCSU

AS7

MSC/TCSM2

SGSN

BTS

MCMU

TCH (Circuit Switched) TCH (Circuit Switched)

Data (Packet Switched)Data (Packet Switched)

Signalling (CCS7)Signalling (LAPD)

ET ET

Speech/ HSCSD: The traffic channels in the Ater interface coming from thetranscoderare connected to the BSCExchange Terminal (ET), and from there to the Group Switch (GSW). The GSW connects thetraffic channel to the correct time slot which is then sent to the ET and further on to the base

station (BS). The GSW is controlled by the Marker and Cellular Management Unit (MCMU).

GPRS Data:In the BSC, the physical connection to theGbinterface is made through the 2Mbit/s links (ET2E)and the Group Switch (GSWB), while the PCU unit implements the BSSGP protocol in theGbinterface. The PCU unit receives and transmits TRAU frames to the BS and Frame Relay packetsto the Serving GPRS Support Node (SGSN).

Signalling:Signalling data from the MSC (CCS7) and from the base stations (LAPD) is handled by the BCSU.



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Abisinterface

17 TCH0 TCH1 TCH2 TCH318 TCH4 TCH5 TCH6 TCH7

19 TCH0 TCH1 TCH2 TCH3





25 TRX1 OMU1 TRX2 OMU2

OMU4

OMU6OMU8

OMU10

OMU12

TS 1 2 3 4 5 6 7 8

0






7 TCH0 TCH1 TCH2 TCH38 TCH4 TCH5 TCH6 TCH79 TCH0 TCH1 TCH2 TCH3






26 TRX3 OMU3 TRX4

27 TRX5 OMU5 TRX628 TRX7 OMU7 TRX8

29 TRX9 OMU9 TRX10

30 TRX11 OMU11 TRX1231 XX XX XX XX

DynamicAbis Pool

Abisinterface

TheAbisis the interface between the BSC and the BTS. It is a 2 Mbit/s interface which can carryup to 96 channels. The capacity of theAbis interface depends on the type of signalling (16kbit/s,

32kbit/s, 64kbit/s) used between the BSC and the BTS. The allocation of the channels within theAbis interface is free (TCH must occupy two successive timeslots). For EDGE configuration it’spossible to define dynamicallyAbispools in 64kbit/s steps.



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Base Transceiver Station (BTS)Base Transceiver Station (BTS)

TRU BCF

BTS

BSC

TRXs

BB TX

RX

BB TXRX

Couplingunit

Transmission Unit (TRU):

- The transmission unit provides theAbis interface to the BSC. The traffic channels andsignalling channels are reallocated in the transmission unit andconnected to the correct unit

(BCFU or TRX).Base Controller Function Unit (BCFU):

- Controls the connection between the BSC and the BTS (O/M)

Transceiver (TRX):

- The transceiver unit contains two main parts, the base band part (BB) and the radio part(TX/RX).

- The base band part of the transceiver unit is responsible for:(in the downlink direction)

• block coding

• convolutional coding

• interleaving

• encryption

• TDMA formatting

The transmitter (Tx) part of the TRX is responsible for:

(in down link direction)

• GMSK modulation

• Power amplification



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Air interfaceAir interface

BCCHBCCH SDCCHSDCCH TCHTCH TCHTCH TCHTCH TCHTCH TCHTCH TCHTCH

TDMA frame= 8 time slots

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

Air interface

The Air interface is located between the BTS and the MS. The traffic channels in the Air interfaceare allocated onto a TDMA frame. The TDMA frame consists of 8 time slots. Generally, all time

slots are used for traffic channels. The time slot 0 and sometimes also the time slot 1 can beused for the signalling between the BTS (BSC, MSC) and the MS:

BCCH: Broadcast Control Channel

SDCCH: Standalone Dedicated Control Cannel



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ExerciseExercise

SGSN

__

__

__

__

__

MSC

ET

____ _____ ______ _____

_____

__

__

__

__

_______

MS

____

__

______

__

__

Label the network elements and their interfaces

1

TS

1

2

3

4

5

6

7

8

0 1 TCH 2 TCH 3 TCH 4 TCH 5 TCH 6 TCH 7 TCH 8 TCH 9 TCH

10 TCH 11 TCH 12 TCH 13 TCH 14 TCH 15 TCH 16 CCS7 17 TCH 18 TCH 19 TCH 20 TCH 21 TCH 22 TCH 23

TCH

24 TCH 25 TCH 26 TCH 27 TCH 28 TCH 29 TCH 30 TCH 31 TCH

2

CCS7 CCS7 CCS7

3

TS 1 2 3 4 5 6 7 8 0 1 TCH0 TCH1 TCH2 TCH3 2 TCH4 TCH5 TCH6 TCH7 3 TCH0 TCH1 TCH2 TCH3 4 TCH4 TCH5 TCH6 TCH7 5 TCH0 TCH1 TCH2 TCH3 6 TCH4 TCH5 TCH6 TCH7 7 TCH0 TCH1 TCH2 TCH3 8 TCH4 TCH5 TCH6 TCH7 9 TCH0 TCH1 TCH2 TCH3

10 TCH4 TCH5 TCH6 TCH7 11 TCH0 TCH1 TCH2 TCH3 12 TCH4 TCH5 TCH6 TCH7 13 TCH0 TCH1 TCH2 TCH3 14 TCH4 TCH5 TCH6 TCH7 15 TCH0 TCH1 TCH2 TCH3 16 TCH4 TCH5 TCH6 TCH7 17 TCH0 TCH1 TCH2 TCH3 18 TCH4 TCH5 TCH6 TCH7 19 TCH0 TCH1 TCH2 TCH3 20 TCH4 TCH5 TCH6 TCH7 21 TCH0 TCH1 TCH2 TCH3 22 TCH4 TCH5 TCH6 TCH7 23 TCH0 TCH1 TCH2 TCH3 24 TCH4 TCH5 TCH6 TCH7 25 TRX1 OMU1 TRX2 OMU2 26 TRX3 OMU3 TRX4 OMU4 27 TRX5 OMU5 TRX6 OMU6 28 TRX7 OMU7 TRX8 OMU8 29 TRX9 OMU9 TRX10 OMU10 30 TRX11 OMU11 TRX12 OMU12 31 XX XX XX XX

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02_bss Traffic Channels