bss technical details
DESCRIPTION
BSS Technical ParameterTRANSCRIPT
\\afagcaoili
\\afagcaoili
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
\\afagcaoili
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)
\\afagcaoili
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
\\afagcaoili
LOGICAL CHANNELS
On every physical channel, a number oflogical channels are mapped. Each logical channel is used for a specific purpose.
\\afagcaoili
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)
\\afagcaoili
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...
\\afagcaoili
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.
\\afagcaoili
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.
\\afagcaoili
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
\\afagcaoili
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
\\afagcaoili
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
\\afagcaoili
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
\\afagcaoili
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
\\afagcaoili
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
\\afagcaoili
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
\\afagcaoili
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
\\afagcaoili
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
\\afagcaoili
TA 1
TA 2
RACH
RACH
Timing Advance Calculation
\\afagcaoili
Where did the value 550 m to one TA come from?
\\afagcaoili
TA 1
TA 2
RACH
RACH
Timing Advance Calculation
t1
\\afagcaoili
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
\\afagcaoili
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
\\afagcaoili
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
\\afagcaoili
\\afagcaoili
Speech Processing Flow
Digitizing & Source Coding
Channel Coding
Interleaving
TDMA Burst
GMSK Modulation
20 ms
time
\\afagcaoili
Digitizing & Source Coding
Digitizing & Source Coding
20 ms
time
78 182
Side Inf. Residual Data
260 bits
\\afagcaoili
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
\\afagcaoili
Speech Processing Flow
Digitizing & Source Coding
Channel Coding
Interleaving
TDMA Burst
GMSK Modulation
20 ms
time
13 Kbps
\\afagcaoili
Channel Coding
13 Kbps78 182
Side Inf. Residual Data
456 bits260 bits
Channel Coding
\\afagcaoili
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
\\afagcaoili
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
\\afagcaoili
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
\\afagcaoili
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
\\afagcaoili
Bit Rate after Channel coding:
Bit Rate = 456 bits / 20 ms
= 22.8 Kbps
input = 260 bitsOutput = 456 bits
\\afagcaoili
Speech Processing Flow
Digitizing & Source Coding
Channel Coding
Interleaving
TDMA Burst
GMSK Modulation
20 ms
time
13 Kbps
22.8 Kbps
22.8 Kbps
\\afagcaoili
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)
\\afagcaoili
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
\\afagcaoili
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
\\afagcaoili
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
\\afagcaoili
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
\\afagcaoili
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
\\afagcaoili
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
\\afagcaoili
\\afagcaoili
Signalling Layer in BSS
- Physical Layer
- Link Layer
- Network Layer
\\afagcaoili
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)
\\afagcaoili
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)
\\afagcaoili
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.
\\afagcaoili
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)
\\afagcaoili
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.
\\afagcaoili
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.
\\afagcaoili
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.
\\afagcaoili
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..
\\afagcaoili
LAPD Frame Format
Flag Address Control FCS FlagInformation
8 bits 16 bits 8/16 bits Up to 260 octets 16 bits 8 bits
\\afagcaoili
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
\\afagcaoili
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.
\\afagcaoili
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.
\\afagcaoili
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
\\afagcaoili
Address field
SAPI0 0 C/R EA
SAPI = 0 , Mobility Management, Call Control and Radio Resource messages= 3 , for SMS
\\afagcaoili
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.
\\afagcaoili