BSS Traffic Channels

NOKIA BSS Traffic Channels Training Document

Document Number/Issue

Copyright Nokia Telecommunications Oy Training Document

1 (62)

NTC CTXX 0408/3.0 en

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No. of pages 62/TT

Edited by/Translator 10-Oct-97 Miia Sjgren

Author 10-Oct-97 Helena Itkonen

Approved by 10-Oct-97 Stefan Flick

Previous issue (2.0) approved 03-Jun-97

TABLE OF CONTENTS1. The Traffic Channels in the BSS .................................................................8 2. A Interface....................................................................................................12 3. Transcoder, TCSM2E..................................................................................15 3.1. Inband signalling between TCSM2E and TRX in the BTS....................18

4. Ater Interface...............................................................................................23 5. Base Station Controller, BSC.....................................................................28 6. The Abis Interface.......................................................................................33 7. TRU and D Channel....................................................................................36 8. The Base Transceiver Station, BTS............................................................41 9. Air Interface.................................................................................................46

10. Traffic Channels in Different Interfaces..................................................50 11. Exercises......................................................................................................55

Ater Interface

1.

THE TRAFFIC CHANNELS IN THE BSSThe traffic channels are described in the down link direction (from MSC to MS) in different interfaces and network elements.

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A

B

C

D

E

F

G

H

I

MSCET TCSM2E ET ET

BSC BTSET TRU ET

MS

ET

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Fig. 1.1 The traffic channels in the BSS network. The following interfaces and network elements are handled: A) B) C) D) A interface Transcoder Ater interface Base Station Controller

Ater Interface

E) F) G) H) I)

Abis interface Transmission Unit Base Transceiver Station Air interface Mobile Station

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2.

A INTERFACEThe A interface is based on the CCITT recommendation G.703 (electrically) and G.704 (frame structure). The traffic channels baud rate in the A interface is 64 kbit/s and they are located in the time slots 1 - 15 and 17 - 31. TS 16 in A-interface is normally used for the CCS7 signalling and its baud rate is 64 kbit/s.

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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

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Fig. 2.1

The A interface.

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3.

TRANSCODER, TCSM2EThe speech signal is divided into 20 ms samples (160 2Mbit/s frames). Each sample is taken from the MSC and passed into the Transcoder where the Digital Signal Processor (DSP) performs the RPE-LTP coding of the sample (Regular Pulse Excitation - Long Term Prediction). The resulting coded sample is known as the vocoded block, which contains 260 bits for full rate traffic channels or 112 bits for half rate. Each vocoded block is inserted into a TRAU frame, which contains a vocoded block plus synchronization bits and control bits giving a total of 320 bits (16 kbit/s) for full rate or 160 bits (8 kbit/s) for half rate. The

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frame is reassembled in the DSP in the Transceiver unit of the BTS. Each traffic time slot has its own DSP for transcoding.

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2 M b it/s fr a m e s f r o m th e T r a n s c o d e r to th e B S C ( S M 2 M ) 2 0 m s s a m p le , 1 6 0 x 2 M b it/s F r a m e s

D S P fo r T S 1

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

2 M b it/ s F r a m e , 1 2 5 u s 31 30 29 28 2 1 0

2 M b it/s F r a m e , 1 2 5 u s 31 30 29 28 6 4 k b it/s T im e s lo t, 8 b its B 1 a n d B 2 u s e d fo r th e T C H B1 B2 1 1 1 1 1 1 2 1 0

T R A U F ra m eB1 B2 B3 B4 B5 B6 B7 B8 TS TS TS TS TS TS TS TS TS TS TS TS TS 0 1 2 3 4 5 6 7 8 9 10 11 12 0 0 1 C 1 D 1 D 1 D 1 D 1 0 0 0 0 0 0 C1 C2 8 D1 8 23 D 31 28 53 0 0 0 0 0 0 0 0 C7 C 15 D7 D 15 D 30

D S P fo r T S 3 1B1 B2 1

6 4 k b it/s T im e s lo t, 8 b its B 1 a n d B 2 u s e d fo r th e T C H 1 1 1 1 1

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Fig. 3.1

TRAU frame structure

3.1.

Inband signalling between TCSM2E and TRX in the BTSThe bit rate after transcoding is 16 kbit/s for full rate traffic channels, which includes 13 kbit/s of transcoded speech/data and 3 kbit/s of control data. For half rate the bit rate is 8 kbit/s, which includes 5.6 kbit/s of speech/data and 2.4

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kbit/s of control data. This control data is used for inband signalling between the DSP in the Transcoder and the DSP in the Transceiver Unit of the BTS. The BTS controls the transcoder with 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

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- transferring DTX (discontinuous transmission) information.

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TCSM2E64 kb DSP1 16 kb 16 kb

BSC16 kb

BTSTRXTRU

2MDSP90

2 Mbit/s

ET

GSW

ET

2 Mbit/s

DSP13 kb

33,8 kb

MS

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Fig. 3.2

Inband signalling between the TCSM2E and the TRX.

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4.

ATER INTERFACEThe A-ter interface is based on the CCITT recommendation G.703 (electrically) and G.704 (frame structure). The traffic channels and signalling channels coming from three different PCMs from the MSC are reallocated in the transcoder.

Ater Interface

C

MSCET TCSM2E ET ET

BSC BTSET TRU

MSET

Ater Interface

Fig 4.1

Ater interface.

Ater Interface

Ater Interface

Fig. 4.2

The Ater interface frame structure.

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5.

BASE STATION CONTROLLER, BSC

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D

MSCET TCSM2E ET ET

BSC BTSET TRU ET

MS

ET

Ater Interface

Fig. 5.1

The Base Station Controller in GSM/DCS 1800 network.

The traffic channels in the Ater interface coming from the transcoder are connected to the BSC Exchange Terminal, ET, and from there to the Group Switch, GSW. The GSW connects a traffic channel to the correct time slot which is then sent to the ET and further on to the BTS. The GSW is controlled by the Marker and Cellular Management Unit (MCMU).

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D

BSC

GSW

ET

BTSTRU

TCSM2EET

MS

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Fig. 5.2

The traffic channels in the BSC.

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6.

THE ABIS INTERFACEThe Abis is the interface between the BSC and the BTS. It is a 2 Mbit/s interface which can carry up to 96 channels. The capacity of the Abis depends on the type of signalling used between the BSC and the BTS. When using 64 kbit/s signalling, the 2 Mbit/s link capacity is 80 traffic channels. Using 16 kbit/s signalling, the 2 Mbit/s link capacity is 96 traffic channels.

Ater Interface

E

MSCET TCSM2E ET

BSC BTSET TRU

MSET

ET

ET

Ater Interface

Fig. 6.1 Fig. 6.2

The Abis interface. The traffic and signalling channels in the Abis interface.

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The Transmission Unit, TRU, provides the A-bis interface to the BSC and also the D channel to the TRXs. The traffic channels and signalling channels are reallocated in the TRU and connected to the correct unit (BCFU or TRX).

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TRUAA bis D1 Bus

Tch 1,2 Sig 25 (1,2) Tch 5,6 Sig 26 (1,2)

BBM TRX 1

HW Address 1 Logical Adds 1

BBM TRX 3


Branching TablesDir1 Dir2 Dir3 DBus

Tch 9,10 Sig 27 (1,2)

BBM TRX 5


Sig 31 (1,2)

BCFU

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Fig 7.1

TRU and the D channel.

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Ater Interface

Fig. 7.2

The D channel.

Ater Interface

Ater Interface

G

ET ET ET ET ET ETTRU

BBM

TX RX

AFE

Base Transceiver Station

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Fig. 8.1

The Base Transceiver Station.

The baseband part of the transceiver unit, TRX, is responsible for: (in down link direction) 1. the block coding 1. the convolutional coding 1. the interleaving

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1. the encryption 1. the TDMA formatting. The Transmitter part, TX, of the TRX is responsible for: (in down link direction) 1. GMSK modulation 1. Up conversion 1. power amplification.

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The signal from the TX part is connected to the Antenna Filter Unit, AFE. In the up link direction, the signal from the received antenna is connected first to AFE and then to the RX part of the TRX.

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The Air interface is located between the BTS and the MS.The traffic channels in the Air interface are allocated onto a TDMA frame. The TDMA frame consists of 8 time slots. Generally, all time slots are used for traffic channels. Time slot 0 and sometimes also time slot 1 can be used for the signalling between the BTS (BSC, MSC) and the MS.

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H

ET ET ET ET TRU

ET

ET

Ater Interface

Fig. 9.2

The air interface.

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Fig. 9.3

The TDMA frame.

Ater Interface

Ater Interface

2

BSC M S CA5 4

T C S M 2 E

BTSE T

Ater

G S W

E T Abis

T R U

TRX

A F E

Air

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Ater Interface

1

A Interface

2

Ater Interface

3

Abis Interface

4

D-Bus

5

TDMA

0 1 2 3 4 5 6 7

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3.1. Label the frame structures used within the BSS interfaces.

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1

2

3

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4

5 0 1 2 3 4 5 6 7

6

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3.2

Specify the bitrate used in the A-interface

______________________________________________________________

3.3

List the five main functions of the transcoder

_______________________________________________________________ _______________________________________________________________

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_______________________________________________________________ _______________________________________________________________ _______________________________________________________________

3.4

Name the element which is responsible for connecting the Ater- and AbisInterface traffic channels.

_______________________________________________________________

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3.5

Name the element within the BTS which terminates the 2 Mbit/s connection

_______________________________________________________________

3.6. Name the element within the BTS which is responsible for the TDMA frame

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_______________________________________________________________

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DATE 21-MAR-96 13-FEB-97 03-Jun-97

ISSUE 1.0 1.1 2.0

AUTHOR P.Lepist H.Itkonen S.Flick

SUMMARY OF CHANGES first release added half rate, and exercises approval

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09-Oct-97

2.2

S.Flick

page 10&11 failure removed

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NTC CTXX 0651/1.0 en

BSSSOM course program

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(Document number)

(Name of attached document)

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Documents

BSS Traffic Channels