BSNL Nokia BSS Implementation

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Fundamentals of the Nokia BSSImplementation

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Fundamentals of the Nokia BSS Implementation

Contents

1 Transcoding and the TCSM2E.............................................................. 5

1.1 TCSM2E functions................................................................................... 51.1.1 Transmission between MSC and BSC .................................................... 51.2 Transcoding............................................................................................. 71.2.1 TRAU frame............................................................................................. 91.2.2 Submultiplexing ..................................................................................... 121.3 Timeslot allocation................................................................................. 121.4 TCSM2E units and configurations ......................................................... 141.5 TRAU operating modes......................................................................... 151.5.1 Discontinuous transmission................................................................... 161.5.2 Fixed and adaptive gain adjustment...................................................... 181.5.3 Acoustic echo cancellation .................................................................... 181.6 Operation of TCSM2E ........................................................................... 19

1.6.1 Supervision by the BSC......................................................................... 191.6.2 Alarms ................................................................................................... 191.6.3 Reliability ............................................................................................... 211.6.4 State administration and reconfiguration............................................... 211.7 BSC functions........................................................................................ 231.7.1 Management of radio channels ............................................................. 241.7.1.1 Channel types........................................................................................ 251.8 Management of frequency hopping....................................................... 271.8.1 Handover management ......................................................................... 281.8.2 Signalling management ......................................................................... 311.8.3 Management of terrestrial channels ...................................................... 341.8.4 Interfaces............................................................................................... 351.8.4.1 A Interface to the MSC .......................................................................... 351.8.4.2 Abis interface (BSC-BTS)...................................................................... 351.8.4.3 Q3.......................................................................................................... 351.8.5 Operation and maintenance.................................................................. 361.8.6 Measurements and observations .......................................................... 371.8.7 Support of call control functions ............................................................ 381.8.8 Ciphering management ......................................................................... 381.9 Cellular network functions ..................................................................... 391.9.1 Data services......................................................................................... 391.9.2 Support for multiple speech codecs ...................................................... 391.9.3 Dual band GSM/DCS ............................................................................ 40

1.9.4 Intelligent underlay overlay.................................................................... 401.9.5 Directed retry ......................................................................................... 421.10 Architecture of DX 200 BSC.................................................................. 421.10.1 General design ...................................................................................... 421.10.2 Redundancy principles .......................................................................... 451.10.3 Reliability ............................................................................................... 45

2 Understanding the Nokia BTS............................................................ 462.1 Introduction............................................................................................ 462.2 Problems and solutions for the air interface .......................................... 46

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Contents

2.2.1 Multipath propagation.............................................................................482.2.2 Flat fading ..............................................................................................48

2.2.2.2 Channel coding ......................................................................................512.2.2.1 Frequency hopping ................................................................................50

2.2.2.3 Interleaving.............................................................................................52 2.2.2.4 Antenna receiver diversity......................................................................532.2.3 Selective fading......................................................................................542.2.4 Modulation..............................................................................................54 2.2.5 Shadowing .............................................................................................562.2.6 Propagation delay ..................................................................................572.2.6.1 Frequency correction and synchronisation.............................................592.2.6.2 Transmission of BCCH information........................................................602.2.7 Ciphering................................................................................................60 2.2.8 Signalling................................................................................................60 2.2.9 Measurements .......................................................................................622.3 Implementation.......................................................................................63 2.3.1.1 Transmission unit ...................................................................................642.4 Control functions ....................................................................................642.4.1.1 Operation and maintenance...................................................................642.4.1.2 External alarms and controls..................................................................642.4.1.3 Master clock ...........................................................................................642.4.1.4 Frequency hopping control.....................................................................652.5 Transceiver (TRX)..................................................................................652.5.1 Combiner/coupler (antenna filter)...........................................................66

3 The Traffic Channels in the BSS.........................................................67

4 A Interface.............................................................................................69

5 Ater Interface ........................................................................................71

6 Base Station Controller, BSC..............................................................73

7 The Abis Interface ................................................................................75

8 The Base Transceiver Station, BTS....................................................77

9 Air Interface ..........................................................................................79

10 Traffic Channels in Different Interfaces.............................................80

11 Signalling in BSS..................................................................................82

12 Signalling Layers in BSS.....................................................................8312.1 Signalling Model for GSM.......................................................................8312.1.1 The Physical Layer.................................................................................8512.1.2 The Link Layer .......................................................................................8512.1.3 The Network Layer.................................................................................85

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12.1.4 The Applications.................................................................................... 85

13 Signalling Protocols............................................................................ 8713.1 CCS7..................................................................................................... 8713.1.1 Message Transfer Part, MTP ................................................................ 8913.1.1 Signalling Connection Control Part, SCCP............................................ 9313.2 Link Access Protocol on D-channel, LapD ............................................ 9613.2.1 LapD Frame Format .............................................................................. 9613.2.2 LapDm................................................................................................. 100



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the 2 Mbit/s links between the MSC and the BSC. Up to four E1 interfaces

can be submultiplexed into one 2 Mbit/s link.

MSC

Transcoder, TCSM2E

30 TCH / 64 kbit/s

CCS7 orX.25 / 64 kbit/s

BS

A interface Ater interface

2 Mbit/s2 Mbit/s

DSP90

DSP1

90 TCH / 16 kbit/s

3 x CCS7 or X.25 / 64 kbit/s3 x

2 Mbit/s

2 Mbit/s

2 Mbit/s

Figure 2. Transcoder TCSM2E and A-/A-ter interfaces



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Transcoding and the TCSM2E

1.2 Transcoding

The Transcoder converts 160 A-law PCM samples of an 8 bit speech channel

(20 ms) into a vocoder block of 260 bits in the downlink direction and vice

versa in the uplink direction. Regular Pulse Excited-Long Term Prediction,

RPE-LTP, vocoder is used in the Transcoder. For each speech channel there is

a Digital Signal Processor, DSP, providing the transcoding / de-transcoding.

In the downlink direction, 20ms samples are taken from the incoming 2

Mbit/s signal from the MSC (20 ms = 160 x 2 Mbit/s frames). 160 x 8 bit

timeslots are then brought into a Digital Signal Processor, DSP, where the

RPE-LTP vocoding is done (160 x 8 bits => 260 bit vocoder block). The

Transcoder creates a TRAU frame that is then packed into the traffic channel

sub-timeslot in the 2 Mbit/s frame in the outgoing direction, in the Aterinterface. The TRAU frame is used between two DSPs, one in the Transcoder

and another one in the TRX of the BTS. One TRAU frame consists of 260 bits

speech / data information together with 60 bits that are used for creating the

TRAU frame structure (inband signalling between the two DSPs).

Transcoder software, i.e. TRAU SW, is capable of handling both full rate (FR

and enhanced full rate speech coding and FR data) and half rate (HR speech

coding and HR data) traffic channels. It is possible to configure the TCSM2

equipment with O&M commands to be one of three types: full rate, half rate

or dual rate (DR). DR refers to the ability of the TRAU to change between FR

and HR traffic channels in real time according to the control information

received from the base station. (In this chapter, FR refers to either FR codingor enhanced full rate (EFR) coding. HR refers to HR coding).

Full rate transcodingmeans that the subchannel handled by the TRAUis a 16 kbit/s channel, containing 16 kbit/s FR or EFR traffic.

Half rate transcodingmeans that the subchannel handled by the TRAUis an 8 kbit/s channel, containing 8 kbit/s HR traffic.

In dual rate transcoding, the subchannel handled by the TRAU is a16 kbit/s subchannel, containing either 16 kbit/s FR or EFR traffic or

8 kbit/s HR traffic.

There are always two HR traffic channels or one FR traffic channel in one

16 kbit/s subchannel between the BSC (Base Station Controller) and the BTS(Base Transceiver Station), i.e. in the Abis interface. In case of DR mode,

however, the TRAU handles only one HR traffic channel or one FR traffic

channel in one 16 kbit/s subchannel in the Ater interface. This is shown in

Figure 3. (Please note that this picture is only an example and that it does not

show all possible configurations).

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Figure 3. The location of the TRAU in the BSS and the use of FR andHR traffic channels

The EFR compression method used is ACELP (Algebraic Code Excited

Linear Prediction). The HR speech compression method is also a CELP-type

(Code Excited Linear Prediction) VSELP-coding (Vector Sum Excited Linear

Prediction), which uses the analysis by synthesis method with fixed code

books and 10th order LPC for short term prediction and adaptive code bookwith fractional lags for long-term prediction, resulting in a bit rate of

5.6 kbit/s.



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160 159 158157 156 157 154 7 6 5 4 3 2 1 160 159 158157 156 157 1547 6 5 4 3 2 1

031 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

B1 B2 B3 B4 B5 B6 B7 B8

2 Mbit/s Frame, 125 us

20 ms sample, 160 x 2 Mbit/s Frames

64 kbit/s Timeslot, 8 bits

DSP for TS 1160 x Timeslot 1 from the 160 x 2 Mbit/s Frames

2 Mbit/s frames from the MSC to Transcoder

160 159 158157 156 157 154 7 6 5 4 3 2 1 160 159 158157 156 157 1547 6 5 4 3 2 1

031 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

2 Mbit/s Frame, 125 us

20 ms sample, 160 2 Mbit/s Frames

64 kbit/s Timeslot where only first two bits are used for traffic channel

160 x 2 bits from Timeslot

2 Mbit/s frames from the Transcoder to the BSC (SM2M)

DSP for TS 31

B1 B2 1 1 1 1 1 1

Figure 4. Transcoding

1.2.1 TRAU frame

The Transcoder creates the TRAU frame , which is used between the DSP in

the Transcoder and the DSP in the TRX of the BTS for carrying the vocoder

traffic channel blocks as well as inband signalling when the call is connected.

The inband signalling thus gives the transcoder the necessary information

about the type of speech coding algorithm, information as to the nature of the

call (i.e. speech or data) information regarding the use of DTX transmission

etc.. The TRAU frame is packed into the 2 Mbit/s frame. That is, the inband

signalling as well as the 260 bit vocoder block are transmitted within the 16kbit/s subchannel. 260 bits carry the vocoder block (13 kbit/s) and 60 bits (3

kbit/s) are used for TRAU frame alignment word (TS0 and 1), Frame type,

(C1-C4), Channel type (C5), Time Alignment info (C6-C11), Frame indicator

(C12-C16), DTX ON/OFF (C17), Spare Bits (C18-C21) and Time Alignment

(T1-T4).




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0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 01 C1 C2

Bit number

1 2 3 4 5 6 7 8

C3 C4 C5 C6 C7

C8

1D8

1

D23

1

D38

1D53

1

D681

D83

1

D98

1D113

C9 C10 C11 C12 C13 C14 C15D1 D7

D112

D114 D115 D116 D117 D118 D119 D120

D106

Octet no.

1

23

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

2728

29

30

31

32

33

34

35

36

37

38

3940

D37

D39 D40 D41 D42 D43 D44 D45

D46 D52

D2 D3 D5 D6

D9 D10 D11 D12 D13 D14 D15

D4

D16 D17 D18 D19 D20 D21

D24 D25 D26 D27 D28 D29 D30D31 D32 D33 D34 D35 D36

D22

D54

D47 D48 D50 D51D49D55 D56 D57 D58 D59 D60

D61 D62 D63 D64 D65 D66 D67

D69D70

D71 D72D73 D74 D75

D76 D77 D78 D79 D80 D81 D82

D84

D91D99

D90D97D105

D85

D92

D100D107

D86 D87 D88 D89

D93

D101D108

D94 D95 D96D104

D111D103D102

D109 D110

1

D1281

1

1

1

1

1

1

1

1

D143

D158

D173

D188

D203

D218

D233

D248

C18

D121 D122 D123 D124 D125 D126 D127

D129 D130 D131 D132 D133 D134 D135

D256 D257 D258 D259 D260 C16 C17C19 C20 C21 T1 T2 T3 T4

D136 D137 D138 D139 D140 D141 D142D144 D145 D146 D147 D148 D149 D150D151 D152 D153 D154 D155 D156 D157

D159 D160 D161 D162 D163 D164 D165

D166 D167 D168 D169 D170 D171 D172D174 D175 D176 D177 D178 D179 D180D181 D182 D183 D184 D185 D186 D187D188 D189 D190 D191 D192 D193 D194D195 D196 D197 D198 D199 D200 D201

D204 D205 D206 D207 D208 D209 D210

D211 D212 D213 D214 D215 D216 D217

D219 D220 D221 D222 D223 D224 D225

D226 D227 D228 D229 D230 D231 D232D234 D235 D236 D237 D238 D239 D240D241 D242 D243 D244 D245 D246 D247D249 D250 D251 D252 D253 D254 D255

Table 1. TRAU frame structure

The TRAU frame is disassembled in the TRX of the BTS. This is done by the

DSP , which then performs block coding, convolutional coding, interleaving

and ciphering for the 260 bits. The DSP also formats the burst. The burst is



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then sent to the Carrier Unit/Tx part of the TRX, which performs GMSK

modulation and up-conversion to the RF in the downlink direction. The RF

signal is then brought to the antenna and finally transmitted into the air. In the

Mobile Station, MS, all decoding is carried out and digital speech is converted

into analogue format. In the uplink direction, the MS does the transcoding, block coding, convolutional coding, interleaving, ciphering and formats the

burst. This is then sent to the BTS through the air. In the BTS, the DSP

creates the TRAU frame and sends the vocoded block to the Transcoder

where the de-transcoding is done.

The Transcoder is responsible for vocoded block timing, adjusting the phase

of the blocks in the downlink direction, to achieve a minimum delay.

DSP1

DSP90

TCSM2E

BSC

BTSMS

DSP

TRX(x)

DSP

A-ter Abis Air A-Interface

64 kbit/s 16 kbit/s

Figure 5. Traffic channels between TCSM2E and MS




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1.2.2 Submultiplexing

The submultiplexer is integrated in the Transcoder, TCSM2E. Thesubmultiplexing function provides the possibility to reduce the number of 2

Mbit/s links on Ater-interface. This gives a much more efficient use of the 2

Mbit/s PCM line. Normally, the Transcoder is located at the MSC site even

though it is controlled by the BSC. The TCSM2E can submultiplex up to four

transcoded full rate traffic channel 2 Mbit/s links into one 2 Mbit/s link. An

example of a full rate timeslot is shown in table 2. In the case of half rate

traffic channels seven 2 Mbit/s links can be submultiplexed into one Ater

interface link.

1.3 Timeslot allocation

The routing of signals between the BSC side and MSC side trunk interfaces is

controlled by the chosen timeslot allocation. Different allocations can be

programmed and loaded into the TRCO, the master unit within the TCSM2.

The selected allocation type must also be supported by the BSC. Different

TCSM2Es of the same BSS may use different allocations. Though timeslot

allocations will be described in a later section, a typical full rate allocation is

shown here:

Table 2. Timeslot allocation for 16 kbit/s bit rate channels (typically fullrate or enhanced full rate) on the Ater 2 Mbit/s interface withthe TCSM2E



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BIT-> 1 2 3 4 5 6 7 8

TS-> 0 LINK MANAGEMENT

1 LAPD TCH.1 TCH.2 TCH.3 A-PCM 1

2 TCH.4 TCH.5 TCH.6 TCH.7







9 x TCH.1 TCH.2 TCH.3 A-PCM 2


























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Note:

It is recommended to add CCS7 links starting from TSL 31, (i.e.

31,30,29) to maximise on the number of traffic channels.

1.4 TCSM2E units and configurations

The TCSM2E contains 4 different types of plug-in units: TRCO, TR16,

ET2Es as well as a power supply. Basic functions are as follows:

TRCO: master unit, internal supervision of the other plug-in units,synchronisation, operation and maintenance

TR16: contain the DSPs (16 per card) and therefore perform the speech

coding and other transcoding functions

ET2E: provide the external 2Mbit/s interface to the BSC and MSC

ET2EET2E

TRCOTR16

TR16

TR16

TR16

TR16

TR16

ET

ET

TCSM2E BSC

ET2E

ET2ETRCOTR16

TR16

TR16

TR16

TR16

TR16

to MSC

Ater

A

Figure 6. TCSM2E units and configurations



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The actual plug-in unit configuration depends on the desired traffic channel

capacity. Additional capacity can be added by adding additional TCSM2E

units. The total capacity of the Transcoder rack is 8 transcoder units or a

maximum TCH rack capacity of 8 X 120 TCH or 960 TCH. However, each

TCSM2E unit functions independently and is dependant almost solely on theexternal connection to the BSC and the MSC.

Figure 7. Transcoder rack

1.5 TRAU operating modes

TRAU software is capable of handling both full rate (FR) and half rate (HR)

speed traffic channels, i.e. 16 kbit/s and 8 kbit/s TRAU frames. It is possible

to configure the TCSM2E equipment to be one of three types: full rate, half

rate or dual rate (DR). DR refers to the ability of the TRAU to change

between FR coding, Enhanced Full Rate (EFR) coding traffic channels or HR

coding traffic channels in real time, according to the control information

received from the Base Station (BTS).




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The Transcoder performs the speech activity detection in the downlink

direction. If no speech is detected in the TC, SID-frames (Silence Descriptor),

which include the characteristics of the background noise, are sent to the MS

where it is able to generate comfort noise. In the downlink direction, DTX is

used to reduce the interference in the Air interface.

If no speech has been detected at the Mobile Station, a similar kind of

function takes place. The parameters of the background noise are sent to the

Transcoder , which is then able to generate comfort noise.

In the uplink direction, DTX is used to save the batteries of the MS and to

reduce interference between the TDMA frame timeslots in the Air interface.




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1.5.2 Fixed and adaptive gain adjustment

By means of O&M messages, it is possible to choose a fixed level adjustmentin the TRAU software in the uplink and downlink directions between +6dB

and -6dB, in 1 dB intervals.

In TRAU software, it is also possible to choose an adaptive gain control in the

downlink direction. In order to have an overall solution to the problems

caused by a speech volume that is too low in GSM phones and a variable

volume level in the PSTN, an adaptive gain has been implemented in the

downlink direction to the TRAU. This guarantees sufficient volume level in

the Mobile Station. The gain is attenuated in steps of 1 dB. In adaptive gain

control, it is also possible to select the minimum and maximum values within

which the gain can vary.

1.5.3 Acoustic echo cancellation

On the MS side, the voice coming from the earpiece of MS will also be picked

up by the microphone. There will be an acoustic echo travelling by air and

along the body of the MS. The GSM recommendation 3.50 states that a

handset and a hands-free MS should perform acoustic echo cancellation. In

other words, the mobile should have a built-in echo suppressor or cancellor

and no acoustic echo cancellation should be needed on the network side.

However, it seems that some mobiles are not capable to remove the acoustic

echo sufficiently and the subscriber may sometimes hear the mobileoriginated echo.

BTS BSC TCSM MSC

ECU

Downlink

Uplink

BSS

AEC

Echo No echo

Acousticecho

Figure 9. Acoustic echo cancellation



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1.6 Operation of TCSM2E

Operation of the TCSM2E is facilitated by the integrated operation and

maintenance offered by the BSC and the OMC. All TCSM2E functionsrelated to operation and maintenance can be carried out

from the BSC or the OMC by a remote session;

by using a local terminal.

The O&M link of the TCSM2E towards the BSC uses a 16 kbit/s LAPD

channel.

The configuration data including the internal settings of the TCSM2E as well

as the hardware configuration data are stored in the non-volatile memory of

the TRCO. The hardware layout data of the rack as well as the PCM

configurations are stored in the BSC. This means that, from a practical point

of view, the TCSM2E is seen almost as a unit within the BSC.

1.6.1 Supervision by the BSC

The BSC is responsible for supervising that the state administration program

in the TCSM2E works as intended. This is achieved by way of comparing the

TCSM2E state information as supposed by the BSC to the state reported by

the TCSM2E. Internally, the master unit of the TCSM2E, the TRCO,

supervises the other unit.

1.6.2 Alarms

Alarms are transferred over the O&M link to the BSC. Current alarms can,

however, be viewed on a local MMI terminal. The BSC assigns to each alarm

an identity, time stamp, alarm class, text string and physical location. The

BSC holds information on the physical location of each TCSM2E equipment.

It converts the logical location information of the alarms received from the

TCSM2E into a physical address.

Alarms which indicate that traffic channels, trunks or other portions of the

traffic capacity are lost, lead to blocking of the respective elements by the

BSC. The TCSM2E is not itself aware of the blocking measures taken by theBSC. More common alarms received by the Transcoder would be either

transmission related or related to individual channel failures.




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WELCOME TO THE TCSM LOCAL TERMINAL DIALOGUE

TCSM_034:LUC> ZAI

/*** ALARMS CURRENTLY ON ***/

2910 FRAMING ERROR

AF 00 01 00 11

2915 FAULT RATE MONITORING

AF 00 01 00 00

/*** COMMAND EXECUTED ***/

DX 200 BSC1-KUTOJA 1998-06-26 10:49:08

ALARMS CURRENTLY ON

BSC1-KUTOJA OMU TRANSM 1998-06-26 10:34:44.11

*** ALARM TCSM-34 1C087-01 PXRECE *RECOV* (0040)2915 FAULT RATE MONITORIN

TCSM 1d 00

BSC1-KUTOJA OMU TRANSM 1998-06-2610:34:07.11

* ALARM TCSM-34 1C087-01 PXRECE

(0039) 2910 FRAMING ERROR

TCSM 1d 11

END OF ALARMS CURRENTLY ON

Figure 10. Alarms viewed locally and in the BSC



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1.6.3 Reliability

The design objectives have been adopted to ensure that the downtime of theTCSM2E is very low. The figures are calculated on the basis of an average

repair time of 4.5 h. The objective for TCSM2E fault intensity is max. 1 fault

every 17000 hours.

To achieve the highest availability level, a minimum of two TCSM2E units

(possibly only partly equipped) per BSC is recommended, even if the traffic

dimensioning requires only one.

The following objectives have been established for the servicing of the

TCSM2E:

Mean active repair time per fault: less than half an hour

Fault localisation accurate to one plug-in unit: 95 %

Fault localisation accurate to one tcsm2e unit: 100%

1.6.4 State administration and reconfiguration

The TCSM2E is a functional unit from the point of view of the BSC, and as a

functional unit it has the following states:

WO-EX: working, executing;

WO-RE: working, restarting; TE-EX: test execution;

BL-EX: blocked, executing;

BL-ID: blocked, idle.

The user can invoke a restart command by MMI from the BSC or locally

from the TCSM2E in a situation where he/she supposes that the TCSM2E

is not working in a normal mode. The BSC also commands a restart when

downloading a new configuration to the TCSM2E.




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Nokia DX 200 BSC

The Base Station Controller, DX 200 BSC, is a modern digital switching

product for GSM/DCS networks. The DX 200 Base Station Controller, BSC,

belongs to DX 200 Switching System Product Family. The DX 200 SwitchingSystem Product Family is based on a Fault Tolerant Computing Platform ,

which constitutes a base for a Switching Platform.

Figure 11. DX200 platform

The main function of the BSC is to control and manage the Base Station

Subsystem (BSS) and the radio channels. Based on Nokia's long experience in

cellular networks, the BSC is designed for efficient use of radio resources,

easy operability and maintainability and comprehensive information about

quality of service. The Nokia BSC is a stable, mature product, which has field

proven high reliability. One major feature of the BSC is also field proven

multivendor functionality.

Together with functionally distributed modular architecture of the DX 200

Switching System Product Family and the latest commercially available

industry standard hardware components, the DX 200 BSC is easilyexpandable and cost-efficient and has high capacity.



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1.7 BSC functions

The BSC manages a variety of tasks ranging from channel administration to

short message service. The tasks are explained in brief below.

Figure 12. Base station controller in the GSM / DCS 1800 network




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1.7.1 Management of radio channels

Figure 13. Configuration and management of the radio resources

Configuration and management of Radio Resources is one of the primary

functions of the BSC. The actual channel configuration, i.e. how many trafficchannels and signalling channels can be used in the BSS, is done in

connection with radio network planning during integration (or

reconfiguration) and initial configuration.

Management of traffic channels (TCH) and stand-alone dedicated control

channels (SDCCH) can be further subdivided into following tasks:

Resource management

Channel allocation



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Link supervision

Channel release

Power control

Management of broadcast control channels (BCCH) and common control

channels (CCCH) be subdivided into the following tasks:

Channel management

Random access

Access grant

Paging

1.7.1.1 Channel types

Physical channels can be defined as one timeslot in a TDMA-frame, whereas

Logical channels define what the actual resource is used for. The following is

a definition of the uses of the different logical channels:

Broadcast channels, BCH

Frequency Correction Channel, FCCH

Carries frequency correction information for MS

Downlink, point-to-multipoint

Synchronisation Control Channel, SCH

Carries frame synchronisation information, e.g. TDMA frame number

and BTS identification, BSIC


Broadcast Control Channel, BCCH

Carries general data about the BTS (cell specific)


Common Control Channels, CCCH

Paging Channel, PCH

Used for paging the specific Mobile Station

Downlink, point-to-point




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The mobile then forwards these measurements to the BTS , which in turn

averages them and forwards them to the BSC for processing. At the same

time, the BTS is measuring for processing by the BSC the following:

Uplink quality Uplink Rx level

MS distance

MS speed

Idle channel quality

Note:

MS speed detection is an optional BSC feature




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1.8.2 Signalling management

Figure 18. BSS signalling

GSM signalling is based on the lower three layers of the Open System

Interconnection model: physical, link and network.

OSI Layer 1 represents the physical layer. It is a digital interface at

2048 kbit/s, based on the ITU-T Recommendation G.703. This

interface is normally used as an A interface between the MSC and the

BSS providing a 64 kbit/s transmission rate on each channel. In the

Abis interfaces signalling links may have either a 16, 32 or 64 kbit/s

rate of transmission. In the air interface this function is fulfilled by the

radio path.




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OSI Layer 2 is based on the MTP of the Signalling System No.7. The

MTP provides a mechanism for reliable transfer of signalling messages

between the MSC and the BSS. At least two signalling links are

normally provided between the BSS and the MSC for both capacity and

reliability reasons. In the Abis, this function is performed by the LAPD.In the air interface, this function is performed by a modified form of

LAPD - LAPDm

OSI Layer 3 of the signalling network includes the SCCP and a part of

the MTP. Together, the MTP and the SCCP functions provide both

connection-less and connection-oriented network services. These

services are used to transfer circuit-related and non-circuit-related

signalling information and other types of information between the MSC

and the BSS.

The user part protocol used between the MSC and BSC is the BSS application

part (BSSAP) , which can be divided into two sublayers:

1. Direct transfer part, DTAP : used for direct communication between the

MSC and MS

2. BSS Management Application Part, (BSSMAP): used for

communication between the MSC and BSC

BSC

ET

GSW

BTS

MSOMU

BCSU

OMUSIG

TRXSIG

FU

BIEET

Figure 19. LAPD signalling

The DX 200 BSC can handle LAPD signalling links with the bit rates of 16

kbit/s, 32 kbit/s and 64 kbit/s. Each site will have one BCF/OMU signalling

link allocated to it as well as one TRX signalling link per TRX.



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1.8.3 Management of terrestrial channels

Figure 20. BSS trafic channels

Though part of the MSC's function is to hunt free circuits toward a BSC, the

BSC's role is to provide and indication of blocking on the channels between

the BSC and the MSC.

In the Abis interface, the allocation of the traffic channels between the BSC

and the BTS is performed according to the allocation of a circuit in the Air

Interface. That is, a circuit in the Air interface corresponds directly to an Abis

circuit.

The BSC also manages both the LAPD signalling links in the Abis interface

as well as the CCS7 links in the A interface.



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1.8.4 Interfaces

Figure 21. BSS Interfaces

1.8.4.1 A Interface to the MSC

The A interface between the MSC and the DX 200 BSC is implemented

according to the GSM standards.

1.8.4.2 Abis interface (BSC-BTS)

The Abis interface telecommunication part between the DX 200 BSC2E and

the BTS is implemented according to the 08.5X Series of GSM

Recommendations. The Abis O&M part is Nokia property supporting

additional features like the Site Test Monitoring unit, alarm consistency,

remote transmission equipment management and BTS database management.

1.8.4.3 Q3

The X.25 protocol is used on the interface between the DX 200 BSC and the

Nokia NMS/2000. Either the PSPDN or the PCM-time-slot-based connection

or LAN connection can be used.




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2 Understanding the Nokia BTS

2.1 Introduction

To say that a BTS only transmits and receives information between the mobile

station and the BSC might perhaps be an over-simplification. The BTS

performs the radio function for the Base Station System (BSS) and is

connected to the Base Station Controller (BSC) via the Abis interface and to

the Mobile Stations (MS) via the Air interface. The BSC is further connected

to the Mobile Switching Centre (MSC) and the Network Management System

(NMS). The BTS provides the Air interface, creates TDMA frames (Time

Division Multiple Access) and the bursts that are used for carrying speech /

data and signalling information between the BTS and MS.

BSC BTS

OMC

MS

To MSC

A-if Abis-if Air-if

Figure 27. Base station sub-system

2.2 Problems and solutions for the air interface

The radio link is the most vulnerable part of the GSM connection. That being

said, there are three major sources of problems in the air interface , which can

lead to loss of data. These are:

Multipath propagation



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Understanding the Nokia BTS

Shadowing

Propagation delay




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Since the frequency in GSM/DCS 1800 is of the order 900/1800 MHz, the

dips will occur at approximately every 17/8.5cms.

If the dip is severe enough. the strength of the received signal may go below

that of the receiver sensitivity, resulting in loss of signal.The speed that you travel through the radiating field is also a factor, since the

faster you travel the less time is spent in each potential dip, and the less

information is lost.

approx 17 cm

Rx sensitivity

Fading dips

Figure 29. Flat fading




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In order to overcome the problem of flat fading the following methods are

employed:

1. Frequency hopping

2. Channel coding

3. Interleaving

4. Antenna/receiver diversity

2.2.2.1 Frequency hopping

The BSC, as was mentioned in the previous section, manages frequency

hopping. That is, the BSC provides the Frequency Hopping parameters.

However, the BTS performs it. The frequency on which the information is

sent is changed for every burst. The main benefit is in the ability to average

out the effect of fading dips as well as the effects of interference, thereby

improving the overall quality of the network.

The type of frequency hopping , which can be implemented in the BTS

depends on the type and configuration of BTS and can be either synthesised

(RF hopping) or Base-band hopping.

0 1 2 3 4 5 6 7

TDMA frame n

7FU1

CU 1

FU3

FU3

CU1

CU2

CU3

FHU

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

0 1 2 3 4 5 6 77

CU 20 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 77

CU 30 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

BTS

TDMA frame n+1 TDMA frame n+2

Figure 30. Frequency hopping in the BTS



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2.2.2.3 Interleaving

Interleaving means spreading of the coded speech into many bursts. As the

transmission of a 20ms block of speech is spread over 8 separate bursts, the

system will be able to recover the data even if up to one burst is lost.

2, 10 ... 4501, 9, 17, 25 ... 449

3, 11 ... 451 4, 12 ... 452

5, 13 ... 453 6, 14 ... 454

7, 15 ... 455 8, 16 ... 456

Figure 32. First level of interleaving



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2.2.2.4 Antenna receiver diversity

In this case, there are two antennas used for receiving the signal. This helps in

such a way that if a fading dip occurs at the position of one antenna, the other

antenna will still be able to receive the signal. Since the distance between two

antennas is a few metres, it can only be implemented at the base transceiver

station. Theoretically, the optimum value for this distance is 20 (where is

the wavelength of the carrier). Figure 33 shows examples of frequency

hopping and antenna receiver diversity.

Received signalf1

f2

f3

f4

Rx Rx

DSPUBTS

Frequency Hopping

Antenna Receiver Diversity

Figure 33. Frequency hopping and antenna receiver diversity




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2.2.3 Selective fading

Selective fading is like flat fading caused by reflections, but unlike flat fading,the reflected signal comes from objects that are some distance away from the

receiver. This distance is of the order 1-5 kms. This sort of problem is

particularly noticeable in areas of mountainous terrain or areas with large

expanses of water, or worse both.

Since the bit rate in GSM/DCS 1800 is 270 Kbits / sec, this means that the

time between bits is 3.7uS. This time corresponds to 1.1 Km. Now if signals

are reflected from objects , which cause the reflected path to be of a length of

this, or greater, then the individual bits arriving at the receiver will be

coincident, and result in difficulty interpreting them. This problem is also

referred to as ISI, Inter Symbol Interference.

When user information is transmitted by either the base transceiver station or

mobile station, the information contained in the burst is not all user data.

There are certain bits , which are called training sequence.

T B

3

E n c ry p te d b i ts

5 8

T r a i n . s e q .

2 6

E n c ry p te d b i ts

5 8

T B

3

G P

8 . 2 5

TB: Tail bitsGP: guard periodNormal burst (NB)

(1 bit duration 48/13 3.69 us)

Figure 34. Normal burst

These bits are known to both mobile station and base transceiver station. By

analysing the effect of the air on these training bits, the air interface is

modelled as a filter. Using this mathematical model of the air, the transmitted

bits are estimated based on the received bits. The algorithm used for this

purpose is called Viterbi equalisation.

2.2.4 Modulation

GSM is a digital mobile standard. However, radio frequencies are analogue.

So, the question is how do we transmit digital information in an analogue

signal.

This looks difficult, but if you think about the values that must be transmitted

(0 and 1), an easy solution can be found. Suppose that the frequency varies

between two values, one representing 0 and the other representing 1. By

altering the value of a certain characteristic of frequency at every specified



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interval (the bit duration), we can translate an analogue signal into a bit stream

in the frequency domain. This technique is called modulation. The

characteristics that can be varied are the frequency itself, the amplitude or the

relative phase shift. Figure 35 illustrates this.

Frequency Modulation Amplitude Modulation

Figure 35. Modulation examples

The modulation technique used in GSM is the Gaussian Minimum Shift

Keying (GMSK). This is a phase modulation. In order to understand what it

means, let us take a simple example.

In GSM transmission in the air, the bit rate is approximately 270 kilo bits per

second, which means that the duration of one bit is 3.69 s, i.e. the value ofthe bit requires 3.69 s of transmission time. If we measure the phase

variation of the carrier wave every 3.69 s, we can determine the value of the

bit. If the phase shift is +90, the bit isset to1. If the variation is 0, the valueof the bit is set to 0.




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BTS

Figure 37. Shadowing effect

Shadowing is generally a problem in the uplink direction, because a base

transceiver station transmits information at a much higher power compared to

that from the mobile station. The solution adopted to overcome this problem

is known as adaptive power control. Based on quality and strength of the

received signal, the base station informs the mobile station to increase or

decrease the power as required. This information is sent to the mobile station

on the SACCH, the slow associated control channel.

2.2.6 Propagation delay

Information is sent in burst from the mobile station to the Base Transceiver

Station (BTS). These bursts have to arrive at the base transceiver station such

that they have to map exactly onto their allocated timeslots. But if the distance

between the mobile station and the base station is several kilometres, then the

time taken by the signal (burst) to travel from the mobile station to base

station (and vice versa) is not instantaneous. There is finite time delay. This

means that if the mobile station or base station transmits the burst only when

the timeslot appears, then when the burst arrives at the other end, it smears

onto the region of the next timeslot, corrupting data from both sources.




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2.2.6.2 Transmission of BCCH information

A mobile station needs a certain amount of information in order have access

to the network. The BTS transmits information from the BSC regarding the

local network environment. Each cell/sector would have one BCCH TRX ,

which would transmit information to all mobiles camping under that particular

cell:

Cell identity

Location area

Neighbour cell descriptions

Cell selection parameters

Control channel description

PLMNs permitted

RACH control parameters

2.2.7 Ciphering

The air interface can be ciphered to provide additional security Ciphering and

deciphering of the Air interface traffic and signalling are supported as defined

in the GSM recommendations. The BTS supports several ciphering algorithms

optional as defined in the GSM recommendations.

A5/0 no ciphering

A5/1 A5/2

The ciphering key is calculated in the HLR, forwarded to the VLR and BSC

and then to the BTS , which performs the actual ciphering.

2.2.8 Signalling

In order for the system to function correctly, measurements, fault reports as

well as telecom signalling must be transferred between the BSC, BTS and

MS.LAPD signalling is used between the BSC and BTS. There are two types of

signalling used in the Abis interface: OMUSIG/ BCFSIG and TRXSIG.

LAPDm signalling is used between the BTS and MS, which means that

messages destined for a mobile from the network are restructured into LAPD

format in the BTS.

TRXSIG signalling is used for carrying signalling information between MSC

- MS, BSC - MS and BSC - BTS. The TRXSIG is used for:



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Location updates

Call control

Handover

Paging

Up/downlink measurement results/ controls

Radio link management procedures

Dedicated channel management procedures

Common channel management procedures

TRX management procedures

The BCFSIG is used for control of a particular BTS site. Information carried

on the BCFSIG link can be relative to:

Fault management

Fault reporting

BTS recovery

BTS test handling

Configuration management

BSC

ET

GSW

BTS

MSOMU

BCSU

OMUSIG

TRXSIG

FU

BIEET

Figure 42. Abis signalling links




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2.3.1.1 Transmission unit

The task of the transmission unit is to connect the BTS to the Abis interface

and, in doing so, create different types of transmission configuration

possibilities. All of the Nokia BTSs have integrated transmission units.

Certain Talk-family models offer additional integrated radio relay links.

Transmission units are monitored by the operation and maintenance unit by

means of an internal Q1 bus.

2.4 Control functions

Control functions can be split into four individual functions:

1. Operation and maintenance

2. Master clock function

3. Frequency hopping control

4. External alarms and controls

That being said, depending on the type of BTS this could mean from one

integrated unit to up to four individual plug-in units.

2.4.1.1 Operation and maintenance

The O&M processor controls and supervises the operation of all BTS units

alone or in co-operation with other processors. It is the main interface forlocal O&M and controls and supervises the other units as well as delivers all

status information to the BSC by means of the O&M signalling link , which it

manages. It stores SW as well as downloads SW to the other units. It also

downloads the software and configuration information received from the BSC

or the MMI to other processors.

2.4.1.2 External alarms and controls

External alarms and controls are programmable interfaces to other devices in

the BTS , which can be used to monitor environmental conditions at the BTS

site as well as monitor the state of units, which do not have a processor of

their own. An example of external alarm might be an intruder alarm or asmoke detector.

2.4.1.3 Master clock

The master clock generates the accurate 13MHz time base for the BTS. Most

of the time the BTS would operate in hierarchical mode and would tune itself

to the clock signal from the 2Mbit/s link, though in many cases it can also

function in plesiochronous mode.



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The Traffic Channels in the BSS

3 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.

MSC

TCSM2E

BSC

BTSET

ET

ET

ET

ET

ETFXC

A I/F Ater Abis Air I/F

A B C D E F G H I

MS

Fig. 1.1 The traffic channels in the BSS network.




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A Interface

4 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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Ater Interface

5 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.

MSC

TCSM2E

BSC

BTSET

ET

ETET

ET

FXC

Ater

C

MS

Fig 4.1 Ater interface.




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1 TCH0 TCH1 TCH2 TCH3




5 TCH0 TCH1 TCH2 TCH36 TCH4 TCH5 TCH6 TCH7


















24 TCH4 TCH5 TCH6 TCH725 TRX1 OMU1 TRX2 OMU2

26 TRX3 OMU3 TRX4 OMU4





31 XX XX XX XX

TS 1 2 3 4 5 6 7 8

0

Fig. 6.2 The traffic and signalling channels in the Abis interface.



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The baseband part of the transceiver unit, TRX, is responsible for:

(in down link direction)

the block coding

the convolutional coding

the interleaving

the encryption

the TDMA formatting.

The Transmitter part, TX, of the TRX is responsible for:

(in down link direction)

GMSK modulation

Up conversion power amplification.

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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Air Interface

9 Air InterfaceThe 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.

MSC

TCSM2E

BSC

BTSET

ET

ET

ET

ET

ET

TRU

0 1 2 3 4 5 6 7

TDMA frame = 8 time slots

0 1 2 3 4 5 6 70 1 2 3 4 5 6 7

Air Interface

H

MS

Fig. 9.1 The air interface.

Fig. 9.2 The TDMA frame.




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BSSAP BSS Application Part

BSSMAP BSS Management Application Part

DTAP Direct Transfer Application Part

MM Mobility Management

CM Connection ManagementCC Call Control

SS Supplementary Services

SMS Short Message Services

SCCP Signalling Connection Control Part

MTP Message Transfer Part

RR Radio Resources

BTSM BTS Management

LapD Link Access Protocol on the D channel

LapDm Link Access Protocol on the D channel modified



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Messages transferred between the BSC and MSC use the BSS management

application part (BSSMAP). Radio resources (RR) and BTS management

(BTSM) are used to transfer messages between the BSC and the BTS.



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Only the lowest level in the protocol stack has the actual connection to the

physical medium between the network nodes, and all data transfer must go

through the lowest three levels. Network elements which are to communicate

with each other must have the same protocols in each level.

The Message Transfer Part contains the functions corresponding with OSI

layers 1 to 3. It is responsible for delivering signalling messages reliably from

one node to another. The User Parts use the services provided by the MTP.



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Signalling Protocols

Level 3 is the Signalling Network level. Its functions are divided into two

parts:

5. message handling, which includes routing of outgoing and transfer

messages to a neighbouring node, and the distribution of incomingmessages to the respective user part of it's own node; and

6. network management, which provides all necessary procedures for

using the signalling network in an optimised and fault tolerant way.

Control of message routing and reconfigurations is performed in order

to preserve or restore the normal message transfer capability.

As far as the message handling is concerned, Level 3 matches with OSI layer

3. The network management functions go beyond OSI layer 3 functionality.

As mentioned earlier, MTP is just a transmission service for the User Parts.

So it does not normally send its own messages but rather delivers User Part

messages through the network. In order to perform its task of network

management, however, an exception has been made: MTP sends Signalling

Network Testing (SNT) and Signalling Network Management (SNM)

messages to the MTPs of other network nodes without distributing them

further to any User Part.

In order to route and distribute outgoing, transferred or incoming messages

MTP needs two types of addresses: one address for the network node as a

whole and another for MTPs neighbours in the protocol stack. The former is

called Signalling Point Code (SPC) and the latter Service Information

Octet (SIO).

Both addresses are included in the signalling message. In cases where the

identification of the receiver of an incoming message is the same as the

network nodes own SPC, the message is distributed to SCCP or one of the

User Parts. This is done by means of the SIO. If it's own SPC is different, then

the message is forwarded to the most suitable of the neighbouring network

nodes.

Since MTP is fundamental to the whole signalling system, it has to be present

in every network element that participates in CCS7 signalling. Such network

nodes are called Signalling Points (SP).

The following diagram shows some examples of CCS7 signalling on the A

interface as recorded by Nethawk, a GSM protocol analyser.




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Conn:1 Card:1 TS:25 Subch:0 2016 19:35:50.59

MSU

- BSN: 126 (7Eh) BIB: 1 FSN: 7 (07h) FIB: 1

- Signalling Network Test & Maintenance regular

- network indicator : national network

- data, length : 14, (0Eh)

SLTM - SIGNALLING LINK TEST MESSAGE

- DPC : 432 (01B0h) OPC : 12600 (3138h)

- SLC : 0 (00h)

- test pattern length : 8 (8h) 14 15 16 17 18 19 1A 1B

Conn :1 Card:1 TS:25 Subch:0 2017 19:35:50.603

MSU

- BSN: 7 (07h) BIB: 1 FSN: 127 (7Fh) FIB: 1

- Signalling Network Test & Maintenance regular


- data, length : 14, (0Eh)

SLTA - SIGNALLING LINK TEST ACKNOWLEDGE

- DPC : 12600 (3138h) OPC : 432 (01B0h)

- SLC : 0 (00h)

- test pattern length : 8 (8h) 14 15 16 17 18 19 1A 1B


MSU

- BSN: 7 (07h) BIB: 1 FSN: 0 (00h) FIB: 1

- SCCP


- data, length : 50, (32h)


MSU

- BSN: 0 (00h) BIB: 1 FSN: 8 (08h) FIB: 1

- SCCP


- data, length : 13, (0Dh)

SLTM - SIGNALLING LINK TES

- DPC : 432 (01B0h) OPC : 1- SLC : 0 (00h)- test pattern length : 8 (8h)

Service Indicator :- indicates what typprovided by the hig

layer messages- eg. Sign NW testin

MTPL2

MTPL3

Figure 13.3 MTP Layer 3 carried by MTP Layer 2

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The SAPI, Service Access Point Identifier is used to identify BCFSIG (= 62),

TRXSIG (= 0) and SMS (= 3)

The address field can be 8 or 16 bits.

6 1 1

SAPI C/R

TEI

0

1

The C/R bit indicates whether the frame is a command or a response frame.

The TEI, Terminal End Point Identifier identifies a specific connection

endpoint. In the case of a BTS, each TRX has a different TEI depending on the

logical ID of the TRX, NOT the physical address. The TEI for BCFSIG = 1.

Control

The control field is 16 bits and the contents change depending on the purpose of

the frame which can be information, supervisory or unnumbered.

The control field of an information frame is shown below.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

N(S) 0 N(R) P/F

N(S) is the number of the frame in a sequence of frames, and N(R) is the

number of the next expected frame.

The P/F (Poll/Final) bit indicates whether a response is required. If the bit is set

(= 1) then the frame requires a response, and the response to this frame must

also have its P?F bit set.

The control field of a supervisory frame is shown below.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

1 0 S Not used P/F N(R)

The supervisory frames are used for flow control. The S bits are used to

indicate:

RR Receiver Ready acknowledgement to indicate that the receiver is

ready for the next frame. If the link is idle, this frame is sent every 10

seconds (timer T203).

RNR Receiver Not Ready.


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INFO

SAPI: 00H TEI: 01H C/R: 1 P/F: 0

N(R): 19H N(S): 52H: 08 2F 01 0E 0D 02

Conn:2 Card:2 TS:3 Subch:1 2243 19:36:43.078RECEIVER READY


N(R): 53H


INFO


N(R): 19H N(S): 53H

03 01 01 0E 02 00 0B 00 09 83 25 02 E0 90 1E

02 E0 88


RECEIVER READY


N(R): 54H

Conn:2 Card:2 TS:3 Subch:1 2247 19:36:43.476UNNUMBERED INFO


08 28 01 0E 1B 33 19 03 2A 29 00 04 0E 0A 08

00 0B 00 12 06 15 18 16 10 00 00 00 00 00 00

00 00 00 00 00 00 00

LapD frame used for

sending higher levelmessages

Figure 13.6 Example of LapD communication taken from Nethawk


Nokia Telecommunications Oy


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13.2.2 LapDm

LapDm is LapD modified for use on the air interface between the BTS and theMS. The modifications are imposed by the nature of the air interface and this is

discussed below.

The frame check sequence is not required in the LapDm frame format as error

detection is already carried out on the air interface by the transmission coding of

the physical channel.

The use of flags to delimit the start and end of frames is not necessary due to the

ready-made blocks of the physical layer. So the frame format becomes:

8 bits 8 bits 8 bits 21 or 23 octets

Address Control Length Information

The address field is as follows, where the SAPI is 0 for Mobility Management,

Call Control and Radio Resource messages, and 3 for SMS.

0 0 SAPI C/R EA

The control field is formatted as illustrated below, where the abbreviations are

the same as defined for LapD earlier.

Information N(R) P/F N(S) 0

Supervisory N(R) P/F S S 0 1

Unnumbered 0 M M P/F M M 1 1

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Each frame is designed to fit into a single physical block which is 23 octets long

on the TCH. The frame is only 21 octets long on the SACCH block as 2 octets

are needed for power control and timing advance.

Since the effective information length may be less than the specified number ofoctets, a length indicator is included in each frame. Unused octets are filled with

the default pattern (00101011).

The number of octets available for messages on the LapDm (21 or 23) are not

sufficient for most signalling needs, and so a segmentation and reassembly

facility is defined. It involves the use of a "more" bit in the message header.

When the value of this bit is 1, it is indicating that this frame is not the last to

contain message information. A 0 indicates the final frame.

Message

1

1

0

1

T

T

T

F T

More bits

T = Tail Bits

F = Fill Bits

Figure 13.7 Segmentation of messages in LapDm

Documents

BSNL Nokia BSS Implementation