6 Weeks Industrial Training In Telecom In Chandigarh
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Concept of TDMA Frames AndChannels In GSM
Concept of TDMA Frames and Channels
GSM combines FDM and TDM: bandwidth is subdivided into channels of 200khz, shared by up to eight stations, assigning slots for transmission on
demand.
f
t
c
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GSM uses paired radio channels
0 124 0 124
890MHz 915MHz 935MHz 960MHz
UPLINK
DOWNLINK
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GSM delays uplink TDMA frames
T1 T2 T3 T5 T6 T7T4 T8
R T
R T
R1 R2 R3 R5 R6 R7R4 R8
Uplink TDMA Frame
F1 + 45MHz
Downlink TDMA F1MHz
The start of the uplink TDMA is delayed of
three time slotsTDMA frame (4.615 ms)
Fixed transmit Delay of three time-slots
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1 2 3 4 5 6 7 8
higher GSM frame structures
935-960 MHz124 channels (200 kHz)downlink
890-915 MHz124 channels (200 kHz)uplink
frequ
ency
time
GSM TDMA frame
GSM time-slot (normal burst)
4.615 ms
546.5 µs577 µs
guardspace
guardspacetail user data TrainingS S user data tail
3 bits 57 bits 26 bits 57 bits1 1 3
GSM - TDMA/FDMAwww.arcadianlearning.com
LOGICAL CHANNELS
TRAFFIC SIGNALLING
FULL RATEBm 22.8 Kb/S
HALF RATELm 11.4 Kb/S
BROADCAST COMMON CONTROL DEDICATED CONTROL
FCCH SCH BCCH
PCHRACH
AGCH
SDCCH SACCH FACCH
FCCH -- FREQUENCY CORRECTION CHANNELSCH -- SYNCHRONISATION CHANNELBCCH -- BROADCAST CONTROL CHANNELPCH -- PAGING CHANNELRACH -- RANDOM ACCESS CHANNELAGCH -- ACCESS GRANTED CHANNELSDCCH -- STAND ALONE DEDICATED CONTROL CHANNELSACCH -- SLOW ASSOCIATED CONTROL CHANNELFACCH -- FAST ASSOCIATED CONTROL CHANNEL
DOWN LINK ONLY
UPLINK ONLYBOTH UP & DOWNLINKS
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Broadcast Channel - BCH
Broadcast control channel (BCCH) is a base to mobile channel which provides general information about the network, the cell in which the mobile is currently located and the adjacent cells
Frequency correction channel (FCCH) is a base to mobile channel which provides information for carrier synchronization
Synchronization channel (SCH) is a base to mobile channel which carries information for frame synchronization and identification of the base station transceiver
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Common Control Channel - CCH Paging channel (PCH) is a base to
mobile channel used to alert a mobile to a call originating from the network
Random access channel (RACH) is a mobile to base channel used to request for dedicated resources
Access grant channel (AGCH) is a base to mobile which is used to assign dedicated resources (SDCCH or TCH)
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Dedicated Control Channel - DCCH Stand-alone dedicated control
channel (SDCCH) is a bi-directional channel allocated to a specific mobile for exchange of location update information and call set up information
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Dedicated Control Channel - DCCH Slow associated control channel (SACCH) is a bi-
directional channel used for exchanging control information between base and a mobile during the progress of a call set up procedure. The SACCH is associated with a particular traffic channel or stand alone dedicated control channel
Fast associated control channel (FACCH) is a bi-directional channel which is used for exchange of time critical information between mobile and base station during the progress of a call. The FACCH transmits control information by stealing capacity from the associated TCH
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TAIL BIT
ENCRYPTION BIT
GUARD PERIOD
TRAINING BITS MIXED BITS
SYNCHRONISATION BITSFIXED BITS
FLAG BITS
3 57 1 26 1 57 3 8.25NORMAL BURST - NB
3 142 3 8.25FREQUENCYCORRECTION BURST - FB
3 3 8.25 39 64 39SYNCHRONISATION BURST - SB
3 6 41 36 68.25ACCESSBURST - AB
DEFINITION OF TIME SLOT - 156.25 BITS 15/26ms = 0.577ms
0 1 2 3 4 5 6 2043 2044 2045 2046 2047
0 1 2 3 4 48 49 50
0 1 2 24 25
0 1 2 3 24 25
0 1 2 3 4 48 49 50
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0
1 HYPER FRAME = 2048 SUPERFRAMES = 2 715 648 TDMA FRAMES ( 3 H 28 MIN 53 S 760 MS )
1 SUPER FRAME = 1326 TDMA FRAMES ( 6.12 S ) LEFT (OR) RIGHT
1 MULTI FRAME = 51 TDMA FRAMES (235 .4 ms )
1 SUPER FRAME = 26 MULTI FRAMES
1 SUPER FRAME = 51 MULTI FRAMES
1 MULTIFRAME = 26 TDMA FRAMES ( 120 ms )
TDMA FRAME NO.0 1
0 1
HIERARCHY OF FRAMES
1 2 3 4 155 156
1 TIME SLOT = 156.25 BITS ( 0.577 ms)
(4.615ms)
(4.615 ms)
1 bit =36.9 micro sec
TRAFFIC CHANNELS
SIGNALLING CHANNELS
GSM Frame
0 1 2 3 4 5 6 7
3 57 1 26 1 57 3 8.25
0 1 2 12 24 25
Full rate channel is idle in 25SACCH is
transmitted in frame 120 to 11 and 13 to 24
Are used for traffic data Frame duration =
120ms
Frame duration = 60/13ms
Frame duration = 15/26ms
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114 bits are available for data transmission.
The training sequence of 26 bits in the middle of the burst is used by the receiver to synchronize and compensate for time dispersion produced by multipath propagation.
1 stealing bit for each information block (used for FACCH)
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LOGICAL CHANNELS
TRAFFIC SIGNALLING
FULL RATEBm 22.8 Kb/S
HALF RATELm 11.4 Kb/S
BROADCAST COMMON CONTROL DEDICATED CONTROL
FCCH SCH BCCH
PCHRACH
AGCH
SDCCH SACCH FACCH
FCCH -- FREQUENCY CORRECTION CHANNELSCH -- SYNCHRONISATION CHANNELBCCH -- BROADCAST CONTROL CHANNELPCH -- PAGING CHANNELRACH -- RANDOM ACCESS CHANNELAGCH -- ACCESS GRANTED CHANNELSDCCH -- STAND ALONE DEDICATED CONTROL CHANNELSACCH -- SLOW ASSOCIATED CONTROL CHANNELFACCH -- FAST ASSOCIATED CONTROL CHANNEL
DOWN LINK ONLY
UPLINK ONLYBOTH UP & DOWNLINKS
Mobile looks for BCCH after switching on
RACH send channel request
AGCH receive SDCCH
SDCCH authenticate
SDCCH switch to cipher mode
SDCCH request for location updating
SDCCH authenticate response
SDCCH cipher mode acknowledge
SDCCH allocate TMSI
SDCCH acknowledge new TMSI
SDCCH switch idle update mode
Location update from the mobile
Mobile looks for BCCH after switching on
RACH send channel request
AGCH receive SDCCH
SDCCH do the authentication and TMSI allocation
SDCCH require traffic channel assignment
SDCCH send call establishment request
SDCCH send the setup message and desired number
FACCH switch to traffic channel and send ack (steal bits)
FACCH receive alert signal ringing sound
FACCH acknowledge connect message and use TCH
TCH conversation continues
FACCH receive connect message
Call establishment from a mobile
Mobile looks for BCCH after switching on
Receive signaling channel SDCCH on AGCH
Receive alert signal and generate ringing on FACCH
Receive authentication request on SDCCH
Generate Channel Request on RACH
Answer paging message on SDCCH
Authenticate on SDCCH
Receive setup message on SDCCH
FACCH acknowledge connect message and switch to TCH
Receive connect message on FACCH
Receive traffic channel assignment on SDCCH
Mobile receives paging message on PCH
FACCH switch to traffic channel and send ack (steal bits)
Call establishment to a mobile
GSM speech coding
AIRINTERFACE
MOBILE
BASE TRANSCEIVER STATION
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Transmit Path
BS Side
8 bit A-Law to
13 bit Uniform RPE/LTP speech Encoder To Channel Coder 13Kbps
8 K sps
MS Side
LPF A/D RPE/LTP speech Encoder To Channel Coder 13Kbps
8 K sps,
Sampling Rate - 8KEncoding - 13 bit Encoding (104 Kbps)RPE/LTP - Regular Pulse Excitation/Long Term PredictionRPE/LTP converts the 104 Kbps stream to 13 Kbps
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GSM Speech Coding
GSM is a digital system, so speech which is inherently analog, has to be digitized.
The method employed by current telephone systems for multiplexing voice lines over high speed trunks and is pulse coded modulation (PCM). The output stream from PCM is 64 kbps, too high a rate to be feasible over a radio link.
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GSM Frame
0 1 2 3 4 5 6 7
3 57 1 26 1 57 3 8.25
0 1 2 12 24 25
Full rate channel is idle in 25SACCH is
transmitted in frame 120 to 11 and 13 to 24
Are used for traffic data Frame duration =
120ms
Frame duration = 60/13ms
Frame duration = 15/26ms
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GSM Speech Coding
Speech is divided into 20 millisecond samples, each of which is encoded as 260 bits, giving a total bit rate of 13 kbps.
Regular pulse excited -- linear predictive coder (RPE--LPC) with a long term predictor loop is the speech coding algorithm.
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The 260 bits are divided into three classes: Class Ia 50 bits - most sensitive to bit errors. Class Ib 132 bits - moderately sensitive to bit errors. Class II 78 bits - least sensitive to bit errors.
Class Ia bits have a 3 bit cyclic redundancy code added for error detection = 50+3 bits.
132 class Ib bits with 4 bit tail sequence = 132 + 4 = 136. Class Ia + class Ib = 53+136=189, input into a 1/2 rate
convolution encoder of constraint length 4. Each input bit is encoded as two output bits, based on a combination of the previous 4 input bits. The convolution encoder thus outputs 378 bits, to which are added the 78 remaining class II bits.
Thus every 20 ms speech sample is encoded as 456 bits, giving a bit rate of 22.8 kbps.
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To further protect against the burst errors common to the radio interface, each sample is interleaved. The 456 bits output by the convolution encoder are divided into 8 blocks of 57 bits, and these blocks are transmitted in eight consecutive time-slot bursts. Since each time-slot burst can carry two 57 bit blocks, each burst carries traffic from two different speech samples.
3 57 bits 26 1 1 57 bits 3
3 57 bits 26 1 1 57 bits 3
3 57 bits 26 1 1 57 bits 3
3 57 bits 26 1 1 57 bits 3
3 57 bits 26 1 1 57 bits 3
3 57 bits 26 1 1 57 bits 3
3 57 bits 26 1 1 57 bits 3
3 57 bits 26 1 1 57 bits 3
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GSM Protocol Suite www.arcadianlearning.com
BTS
Radio interface
HLR
MSCVLR
BSC
RR
MM + CM
SS
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Link Layer LAPDm is used between MS and BTS LAPD is used between BTS-BSC MTP2 is used between
BSC-MSC/VLR/HLR
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Network Layer To distinguish between CC, SS, MM and RR
protocol discriminator (PD) is used as network address.CC call control management MS-MSC.SS supplementary services management MS-
MSC/HLR.MM mobility management(location management,
security management) MS-MSC/VLR.RR radio resource management MS-BSC.
Messages pertaining to different transaction are distinguished by a transaction identifier (TI).
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Application Layer protocols BSSMAP between BSC and MSC DTAP messages between MS and MSC. All messages on the A interface bear a
discrimination flag, indicating whether the message is a BSSMAP or a DTAP.
DTAP messages carry DLCI(information on type of link on the radio interface) to distinguish what is related to CC or SMS.
MAP protocol is the one between neighbor MSCs. MAP is also used between MSC and HLR.
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Q.921
Radio Interface
Q.931
Q.921
MAP
TCAP
CCS7 MTP
CCS7 SCCP
Mobile Application Part
Q931 BSSAP
SCCP
CCS7 MTP
A Interface
A-Bis Interface
Um
Base Station System
GSM Functional Architecture and Principal Interfaces
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GSM protocol layers for signaling
CM
MM
RR
MM
LAPDm
radio
LAPDm
radio
LAPD
PCM
RR’ BTSM
CM
LAPD
PCM
RR’BTSM
16/64 kbit/s
Um Abis A
SS7
PCM
SS7
PCM
64 kbit/s /2.048 Mbit/s
MS BTS BSC MSC
BSSAP BSSAP
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Protocols involved in the radio interface Level 1-Physical
TDMA frame Logical channels multiplexing
Level 2-LAPDm(modified from LAPD) No flag No error retransmission mechanism due to real time
constraints Level 3-Radio Interface Layer (RIL3) involves three sub
layers RR: paging, power control, ciphering execution, handover MM: security, location IMSI attach/detach CM: Call Control(CC), Supplementary Services(SS),
Short Message Services(SMS),
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LAPDm on radio interface In LAPDm the use of flags is avoided. LAPDm maximum length is 21 octets of
information. It makes use of “more” bit to distinguish last frame of a message.
No frame check sequence for LAPDm, it uses the error detecting performance of the transmission coding scheme offered by the physical layer
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ADDRESS CONTROL INFORMATION 0-21 OCTETS
SAPI
N(S) N(R)
LAPDm Message structure
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LAPDm on radio interface The acknowledgement for the next expected frame in
the indicator N(R ). On radio interface two independent flows(one for
signaling, and one for SMS) can exist simultaneously. These two flows are distinguished by a link identifier
called the SAPI(service access point identifier). LAPDm SAPI=0 for signaling and SAPI=3 for SMS. SAP1=0 for radio signaling, SAPI=62 for OAM and
SAPI=63 for layer 2 management on the Abis interface.
There is no need of a TEI, because there is no need to distinguish the different mobile stations, which is done by distinguishing the different radio channels.
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Protocols involved in the A-bis interface Level 1-PCM transmission (E1 or T1)
Speech encoded at 16kbit/s and sub multiplexed in 64kbit/s time slots.
Data which rate is adapted and synchronized. Level 2-LAPD protocol, standard HDLC
Radio Signaling Link (RSL)Operation and Maintenance Link (OML).
Level 3-Application ProtocolRadio Subsystem Management (RSM)Operation and Maintenance procedure (OAM)
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Presentation of A-bis Interface Messages exchanges between the BTS and BSC.
Traffic exchangesSignaling exchanges
Physical access between BTS and BSC is PCM digital links of E1(32) or T1(24) TS at 64kbit/s.
Speech:Conveyed in timeslots at 4X16 kbit/s
Data:Conveyed in timeslots of 4X16 kbit/s. The initial user
rate, which may be 300, 1200, … is adjusted to 16 kbit/s
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FLAG ADRESS CONTROL INFORMATION 0 – 260 OCT FCS FLAG
SAPI TEI
N(S) N(R)
LAPD message structure
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LAPD The length is limited to 260 octets of
information. LAPD has the address of the destination
terminal, to identify the TRX, since this is a point to multipoint interface.
Each TRX in a BTS corresponds to one or several signaling links. These links are distinguished by TEI (Terminal Equipment Identities).
SAPI=0, SAPI=3, SAPI=62 for OAM.
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Presentation of the A-ter interface
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BSC
TRAU
MSC
OMC
OAM
Transcoding
LAPD TS1
Speech TS
CCS7 TS
X.25 TS2
Speech TS
CCS7 TS
X.25 TS2
PCMLINK PCM
LINK
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Presentation on the A-ter interface Signaling messages are carried on specific timeslots (TS)
LAPD signaling TS between the BSC and the TCU SS7 TS between the BSC and the MSC, dedicated for
BSSAP messages transportation. X25 TS2 is reserved for OAM.
Speech and data channels (16kbit/s) Ater interface links carry up to:
120 communications(E1), 4*30 92 communications(T1).
The 64 kbit/s speech rate adjustment and the 64 kbit/s data
rate adaptation are performed at the TCU.
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Presentation of the A interfacewww.arcadianlearning.com
Signaling Protocol Model
Presentation on the A-InterfaceBSSMAP - deals with procedures that take place logically between the BSS and MSC, examples:
Trunk Maintenance, Ciphering, Handover, Voice/Data Trunk Assignment
DTAP - deals with procedures that take place logically between the MS and MSC. The BSS does not interpret the DTAP information, it simply repackages it and sends it to the MS over the Um Interface. examples:
Location Update, MS originated and terminated Calls, Short Message Service, User Supplementary Service registration, activation, deactivation and erasure
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Inter MSC presentationwww.arcadianlearning.com
OAM
LAPD
BTS
MTP2
SCCP
MTP3
LAPD
OAM
RR
DTAP
BSSMAP
BSSAP
BSC
MTP1
MTP3
MTP2
SCCP
MTP2
MTP3
SCCP
BSSAPDTAP/
BSSMAP
TCAP
MM
CM MAP
NSS
RR
MM
CM
MS
UmInterface
A bisInterface
AInterface
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SCCP Ref=R2
TRX:TEI=T1
Channel ID = N1
SCCP Ref=R1
DTAP
DLCI: SAPI=3
DLCI: SAPI=0
Channel=C1Link: SAPI=3
Link: SAPI=0PD=CC
TI=a
TI=b
PD=MM
PD=RR
TI=A
MS BSC MSC
Channel=C2 Channel ID = N1
Radio Interface Abis Interface A Interface
PD: protocol discriminatorTI: Transaction Identifier for RIL3-CC protocolDLCI: Data Link connection IdentifierSAPI: Service Access Point Identifier on the radio InterfaceTEI: Terminal Equipment Identifier on the Abis I/F
Bearer Services Telecommunication services to transfer
data between access points Specification of services up to the terminal
interface (OSI layers 1-3) Different data rates for voice and data
(original standard)Data service
○ Synchronous: 2.4, 4.8 or 9.6 kbit/s○ Asynchronous: 300 - 1200 bit/s
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Tele Services Telecommunication services that enable voice communication
via mobile phones. All these basic services have to obey cellular functions, security
measurements etc. Offered services.
Mobile telephonyprimary goal of GSM was to enable mobile telephony offering the traditional bandwidth of 3.1 kHz.
Emergency numbercommon number throughout Europe (112); Mandatory for all service providers; Free of charge; Connection with the highest priority (preemption of other connections possible).
Multinumberingseveral ISDN phone numbers per user possible.
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Performance characteristics of GSM
Communication mobile, wireless communication; support for voice and data
services Total mobility
international access, chip-card enables use of access points of different providers
Worldwide connectivity one number, the network handles localization
High capacity better frequency efficiency, smaller cells, more customers
per cell High transmission quality
high audio quality and reliability for wireless, uninterrupted phone calls at higher speeds (e.g., from cars, trains)
Security functions access control, authentication via chip-card and PIN
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Disadvantages of GSM No full ISDN bandwidth of 64 kbit/s to the
user Reduced concentration while driving Electromagnetic radiation Abuse of private data possible High complexity of the system Several incompatibilities within the GSM
standards
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