bss technical details

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dB+4+1-1

-30

-70

-6

10 8 10 10 8 10(147 bits) sec sec sec sec sec sec7056/13 (542.8)

30 1 2 3 7654

TB 3

BN0 –BN2

DATA57

SF1

Training26

SF1

DATA57

TB 3

BN0 –BN2

4.615 ms

148 bits

156.25 bits= 0.577 ms

20 ms

time

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Course Outline:

I. Review1. Logical Channels2. 51& 26-frame multiframe

II. GSM Technical Details1. Bursts in Air Interface

a. Normal Bursts (NB)b. Frequency Correction Burst (FB)c. Synchronization Bursts (SB)d. Access Bursts (AB)e. Dummy Bursts (DB)

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2. Timing Advance a. Why 63 is the maximum?b. Its calculation

3. Speech Processing Flowa. Digitizing & Source Codingb. Channel Codingc. Interleaving

- Bit- Burst

d. Bit Rate Calculation

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LOGICAL CHANNELS

On every physical channel, a number oflogical channels are mapped. Each logical channel is used for a specific purpose.

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LOGICAL CHANNELS

TRAFFIC CHANNELS (TCH)Full Rate ChannelHalf Rate Channel

CONTROL CHANNELS (with horrible abbreviations!)Broadcast Channels (BCH)

Frequency Correction Channel (FCCH)Synchronization Channel (SCH)Broadcast Control Channel (BCCH)

Common Control Channels (CCCH)Paging Channel (PCH)Random Access Channel (RACH)Access Grant Channel (AGCH)

Dedicated Control Channels (DCCH)Stand alone Dedicated Control Channel (SDCCH)Slow Associated Control Channel (SACCH)Fast Associated Control Channel (FACCH)

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BROADCAST CHANNELSall downlink!

FCCH

SCH TDMA#…BSIC...

BCCH

Hey. I’m aGSM xmitter!

GSM?

GSM!!!

LA…neigbors…cell info…max power...

Ok…ok

Hey! Don’t shoutat me, lower your

power...

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COMMON CONTROL CHANNELS

PCH downlink only

Hello! You have a call.

RACH uplink only

Hello! I have to setup a call.

I need SDCCH.

AGCH downlink only

Ok. Use SDCCH.

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DEDICATED CONTROL CHANNELSuplink and downlink

SDCCH

SACCHtiming advanceMS power

FACCH

handover

On SDCCH:-call set up signaling-location updating-periodic registration-IMSI attach/detach-SMS-facsimileetc…..

On SACCH-mobile transmits signalstrength on ncell quality

Don’t shout at me.I can’t hear you little butt.

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TCH = Traffic ChannelTCH = Traffic Channel

Full rate - Used for speech at 13 kbits/s or sending data at 9.6 kbits/s

Half rate - Used for speech at 6.5 kbits/s or sending data at 4.8 kbits/s

Enhanced Full rate - Used for speech at 13 kbits/s or sending data at 9.6 kbits/s but with almost Land line quality

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TS 0 TS 1 TS 2 TS 3 TS 4

200 kHz4.615 ms

0.577 ms

FCCH,SCH,BCCHPCH,RACH,AGCH

SDCCH,SACCH,CBCH

TCH,SACCH,FACCH

TCH,SACCH,FACCH

TCH,SACCH,FACCH

TCH,SACCH,FACCH

TCH,SACCH,FACCH

TCH,SACCH,FACCH

1 TDMA FRAME

TS 5 TS 6 TS 7

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Downlink ….

Uplink ……

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

5432107654321076543210765 76 ….

3 TIMESLOTS3 * 0.577ms = 1.73ms

3 TIMESLOTS3 * 0.577ms = 1.73ms

3 TIMESLOTS3 * 0.577ms = 1.73ms

TCH UP-DOWNLINK OFFSETTCH UP-DOWNLINK OFFSET

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dB

+4+1-1

-30

-70

-6

10 8 10 10 8 10(147 bits)

sec sec sec sec sec sec7056/13 (542.8)

Different Bursts in the AIR Interface

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0 1 2 3 7654

TB 3

BN0 –BN2

DATA57

SF1

Training26

SF1

DATA57

TB 3

BN0 –BN2

Normal BurstNormal Burst

4.615 ms

148 bits

156.25 bits= 0.577 ms

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00 1 2 3 7654

Synchronization BurstSynchronization Burst

4.615 ms

148 bits

156.25 bits= 0.577 ms

TB 3

Encrypted

Bits39

ExtendedTraining seq.

64TB 3

Encrypted

Bits39

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00 1 2 3 7654

Frequency Correction BurstFrequency Correction Burst

4.615 ms

148 bits

156.25 bits= 0.577 ms

TB 3

Fixed all zeros142

TB 3

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00 1 2 3 7654

Access BurstAccess Burst

4.615 ms

148 bits

156.25 bits= 0.577 ms

TB 8

BN0 –BN2

Syncronization

Sequence 48BW8-BW48

EncryptedMessage

36

TB 3

BN85- BN87

Additional guardTime

60

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0 1 2 3 7654

Mixed bits58

BN87-BN144

TB 3

BN0 –BN2

Mixed bits58

BN3 –BN60

Training26

BN61 – BN86

TB 3

BN145–BN147

Dummy BurstDummy Burst

4.615 ms

148 bits

156.25 bits= 0.577 ms

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TA 1

TA 2

RACH

RACH

Timing Advance Calculation

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Where did the value 550 m to one TA come from?

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TA 1

TA 2

RACH

RACH

Timing Advance Calculation

t1

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Given:

Bit rate at the Air Interface = 270.8333 kbpsSpeed of Light = 3 x 108 m/s

@ 270.8333 kbps 1 bit = 3.6923 microseconds

Distance = (Velocity) (Time)

Dtotal = (3 x 108 m/s) (3.6923 microseconds)Dtotal = 1107.69 meters

1 Timing Advance = Dtotal / 2 = 553.846 meters

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What is the maximum range of an ordinary GSM BTS?

0 1 2 3 7654

Access BurstAccess Burst4.615 ms

148 bits

156.25 bits= 0.577 ms

TB 8

BN0 –BN2

Syncronization

Sequence 48BW8-BW48

EncryptedMessage

36

TB 3

BN85- BN87

Additional guardTime

608.25 bits

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Guard period = 68.25 bits

Time = (68.25 bits) (3.6928 microseconds / bit) = 252.0336 microseconds

Dist. total = (time) (velocity of propagation) = (252.0336 microseconds) (3 x 108 m/s) = 75610.08 meters

Distance = Dist. total / 2

= 37.8 Kilometers

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Speech Processing Flow

Digitizing & Source Coding

Channel Coding

Interleaving

TDMA Burst

GMSK Modulation

20 ms

time

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Digitizing & Source Coding

Digitizing & Source Coding

20 ms

time

78 182

Side Inf. Residual Data

260 bits

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Where did the 13 Kbps bit rate come from?..

For every 20 ms sample of voice, it is converted into a stream of 260 bits of information.

Hence,

260 bits = 20 ms

Bit Rate = 260 bits / 20 ms = 13 Kbps

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Speech Processing Flow

Digitizing & Source Coding

Channel Coding

Interleaving

TDMA Burst

GMSK Modulation

20 ms

time

13 Kbps

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Channel Coding

13 Kbps78 182

Side Inf. Residual Data

456 bits260 bits

Channel Coding

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Channel Coding

78 182

182 bits Parity Tail

1/2

Conv. Encoded bits78

(class 2)

Channel encoder

3 bits 4 bits

Side Inf. Residual Data

20msec Sample

+ +

+

378

456 bits

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Full rate channel codingFull rate channel coding

Class 1a, 50bits

Class 1b, 132 bitsClass 2, 78

bits

50 132 783 4

78R=1/2k = 5 Convolutional Code

378

Tail bits

ParityCheck

456 bits

260 bits

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Coding for control channelsCoding for control channels

184

184 40 4

Convolutional Code, r=1/2 k=5456

Tail bits

ParityCheck

456 bits

184

Fire-code

From Layer 2

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Summary coding for a GSM systemSummary coding for a GSM system

Cycliccode &tail

260 267

Firecode & tail

184 228

Firecode & tail

184 228

ConvolutionalCode

267 456

Convolutionalcode

228 456

Reordering and partioning Out:

8 sub-blocks

Block diagonalinterleavingout: pairs tosub-blocks

Convolutional

228 456

Blockrectangularinterleavingout: pairs ofsub-blocks

Intra-burstinterleaving

Encryption Unit

Informationbits

Informationand parity bits

Codedbits

Interleavedbits

Speech TCH

Speech frame260 bits

ACCH, BCCH, PCH,AGCH, SDCCH

Message 184 bits

RACH, SCH

Message bits

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Bit Rate after Channel coding:

Bit Rate = 456 bits / 20 ms

= 22.8 Kbps

input = 260 bitsOutput = 456 bits

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Speech Processing Flow

Digitizing & Source Coding

Channel Coding

Interleaving

TDMA Burst

GMSK Modulation

20 ms

time

13 Kbps

22.8 Kbps

22.8 Kbps

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InterleavingInterleaving

1 2 3 4 5 6 7 8

D2

D1

D3

D4

D5

D6

D7

D8

D1

D2

D3

D4

D5

D6

D7

D8

456 bit 456 bit

channelencoding

channelencoding

260 260

speechcoder

speechcoder

20 ms 20 msspeech

5

interleaving

stream of time slots (only one time sent over one frame)

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Basic Interleaving ProcessBasic Interleaving Process

7 6 5 4 3 2 18

1 2 3

Speech Blocks

Training Sequence (“Midamble”)

57bits

57bits

57bits

57bits

57bits

57bits

57bits

57bits

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Details of interleaving processDetails of interleaving process

0 1 2 3 4 5 6 7 8 …. …. 452 453 454 455

0

8

16

.

.

.

.

448

1

9

17

.

.

.

.

449

2

10

18

.

.

.

.

450

3

11

19

.

.

.

.

451

4

12

20

.

.

.

.

452

5

13

21

.

.

.

.

453

6

14

22

.

.

.

.

454

7

15

23

.

.

.

.

455

57 Rows

A B C

57 5757 57 57 57 57570 1 2 3 4 5 6 7

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DetailsDetails of interleaving process of interleaving process

C B CC B C BC ….B B BC …. B

57 5757 57 57 57 5757

A B C0 1 2 3 4 5 6 7

57 57

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TDMA BURSTTDMA BURST

51 2 3 4 5 6 7 8

Single frame

Out of first 20msec Out of second 20msec

26 bit(training)3 57 bit 1 1 57 bit 3 8.25

normalburst

data

data

tailtail Guard

55

156.25 bits

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Speech Processing Flow

Digitizing & Source Coding

Channel Coding

Interleaving

TDMA Burst

GMSK Modulation

20 ms

time

13 Kbps

22.8 Kbps

22.8 Kbps33.8 Kbps270.833 Kbps

Air

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Bit Rate at the Air Interface:

1 burst = 156.25 bits1 burst period = 0.577 ms

Bit Rate = (156.25 bits) / (0.577 ms) = 270.8333 Kbps

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Signalling Layer in BSS

- Physical Layer

- Link Layer

- Network Layer

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Physical Layer

- This Layer is responsible for the the error free transfer of physical data units. It defines the electrical and mechanical characteristics of the transmission path.

- Air Interface ( Radio Path @ 270.833 Kbps)

- Abis, Ater & A Interface (2.048 Mbps CEPT data stream, E1)

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The Link Layer

- The function of this Layer is to delimit frames and then transfer them between two nodes with a guaranteed low level of undetected errors.

- Its tasks include opening, maintaining and closing a connection between two nodes.

- Abis Interface (LAPD)

- Air Interface (LAPDm)

- A Interface (MTP & SCCP)

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The Network Layer

- The network Layer isolates the higher levels from the routing and connection tasks.

- It is responsible for getting the message to its destination through an arbitrary network topology.

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The Applications- This is the actual dialogue between the

network elements which uses the services of the lower layers.

- MS-MSC (DTAP)- DTAP (MM & CM)

- BTS-BSC (RR & BTSM)- BSC-MSC (BSSMAP)

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Message Transfer Part, MTP

Level 1 – Signalling Data Link Level- It defines the physical, electrical and functional

characteristics of a signalling link and the physical interface towards the transmission media.

Level 2 – Signalling Link Level- It defines a message structure, frraming, error

detection and correction and allignment procedures. - This provides reliable transfer of signalling

messages between two neighbors.

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Three different Signalling units used in MTP L2:

1. MSU (Message Signalling Unit)- this carries messages from higher layers

2. LSSU (Line Status Signalling Unit)- it is used in the initial alignment procedure when setting up a link. Once alligned, it is used to inform the far end about failures and congestion on the link.

3. FISU (Fill in Signal Unit)- this message is transmitted in the link if there is no LLSU or MSU to send.

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Level 3 – Signallling network levela. Message handling – which includes

routing of outgoing and transfer messages to a neighboring node, and the distribution of incoming messages to the respective user part of it’s own node.

b. Network Management – which provides all necessary procedures for using the signalling network in an optimized and fault tolerant way.

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Link Access Protocol on D-channel (LAPD)

- this is the GSM layer 2 protocol between the BSC and the BTS.

2 Types of LAPD Signalling Link:1. BCFSIG – used for downloading the BTS software

from the BSC and for configuration and monitoring purposes.

2. TRXSIG – is used for telecom signalling which involves call set-up, measurements, power control, etc..

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LAPD Frame Format

Flag Address Control FCS FlagInformation

8 bits 16 bits 8/16 bits Up to 260 octets 16 bits 8 bits

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Flag- The flag delimits the beginning and end of a frame,

and consists of the following bit pattern:

- 01111110

Address

SAPI C/R 0

TEI 1

6 1 1

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The SAPI, Service Point Identifier is used to identify BCFSIG (=62), TRXSIG (=0) and SMS (=3).

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

TEI, Terminal End Point Identifier identifies a specific connection endpoint. TEI for BCFSIG is 1. Each TRX has a different TEI depending on the Logical ID of the TRX NOT the Physical address.

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ControlThe control field is 16 bits and the contents change

depending on the purpose of the frame which can be information, supervisory or unnumbered.

InformationData contains up to 260 data octets.

FCS (Frame Check Sequence)The FCS defined by ITU-T is used for error detection.

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LAPDm

- This is a modified LAPD protocol. - The FCS is not required in the LAPDm frame as

error detection.- The use of flags to delimit the the start and stop

frames is not necessary due to the ready-made blocks of the physical layer.

Address Control Length Information8 bits 8 bits 8 bits 21 or 23 octets

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Address field

SAPI0 0 C/R EA

SAPI = 0 , Mobility Management, Call Control and Radio Resource messages= 3 , for SMS

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Information

- Fixed length of 23 0ctets.- for SACCH only 21 octets is used because 2 octets will be

used for power control and timing advance- Unused octets are filled with a default pattern (00101011)- Segmentation and reassembly is used for too long

messages. It uses a “MORE” bit in the message header.

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