Overview of GSM Cellular Network and Operations

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Overview of GSM Cellular Network and Operations
Ganesh Srinivasan NTLGSPTN
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Network and switching subsystem

• NSS is the main component of the public mobile
  network GSM
• switching, mobility management, interconnection
  to other networks, system control
• Components
• Mobile Services Switching Center (MSC)controls
  all connections via a separated network to/from a
  mobile terminal within the domain of the MSC -
  several BSC can belong to a MSC
• Databases (important scalability, high capacity,
  low delay)
• Home Location Register (HLR)central master
  database containing user data, permanent and
  semi-permanent data of all subscribers assigned
  to the HLR (one provider can have several HLRs)
• Visitor Location Register (VLR)local database
  for a subset of user data, including data about
  all user currently in the domain of the VLR

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Operation subsystem

• The OSS (Operation Subsystem) enables centralized
  operation, management, and maintenance of all GSM
  subsystems
• Components
• Authentication Center (AUC)
• generates user specific authentication parameters
  on request of a VLR
• authentication parameters used for authentication
  of mobile terminals and encryption of user data
  on the air interface within the GSM system
• Equipment Identity Register (EIR)
• registers GSM mobile stations and user rights
• stolen or malfunctioning mobile stations can be
  locked and sometimes even localized
• Operation and Maintenance Center (OMC)
• different control capabilities for the radio
  subsystem and the network subsystem

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Mobile Handset
TEMPORARY DATA
PERMANENT DATA - Temporary Subscriber Identity
Permanent Subscriber
Identity - Current Location
Key/Algorithm for
Authentication. - Ciphering Data
Provides access to the GSM n/w Consists
of Mobile equipment (ME) Subscriber Identity
Module (SIM)
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The GSM Radio Interface
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The GSM Network Architecture

• Time division multiple access-TDMA
• 124 radio carriers, inter carrier spacing 200khz.
• 890 to 915mhz mobile to base - UPLINK
• 935 to 960mhz base to mobile - DOWNLINK
• 8 channels/carrier

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GSM uses paired radio channels
UPLINK
DOWNLINK
890MHz
915MHz
935MHz
960MHz
0
124
0
124
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Access Mechanism

• FDMA, TDMA, CDMA

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Frequency multiplex

• Separation of the whole spectrum into smaller
  frequency bands
• A channel gets a certain band of the
• spectrum for the whole time
• Advantages
• no dynamic coordination necessary
• works also for analog signals
• Disadvantages
• waste of bandwidth if the traffic is
  distributed unevenly
• inflexible
• guard spaces

k2
k3
k4
k5
k6
k1
c
f
t
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Time multiplex

• A channel gets the whole spectrum for a certain
  amount of time
• Advantages
• only one carrier in themedium at any time
• throughput high even for many users
• Disadvantages
• precise synchronization necessary

k2
k3
k4
k5
k6
k1
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Time and Frequency Multiplex

• Combination of both methods
• A channel gets a certain frequency band for a
  certain amount of time

k2
k3
k4
k5
k6
k1
c
f
t
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Time and Frequency Multiplex

• Example GSM
• Advantages
• Better protection against tapping
• Protection against frequency selective
  interference
• Higher data rates compared tocode multiplex
• But precise coordinationrequired

k2
k3
k4
k5
k6
k1
c
f
t
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• GSM combines FDM and TDM bandwidth is subdivided
  into channels of 200khz, shared by up to eight
  stations, assigning slots for transmission on
  demand.

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GSM uses paired radio channels
UPLINK
DOWNLINK
890MHz
915MHz
935MHz
960MHz
0
124
0
124
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Code Multiplex
k2
k3
k4
k5
k6
k1

• Each channel has a unique code
• All channels use the same spectrum at the same
  time
• Advantages
• Bandwidth efficient
• No coordination and synchronization necessary
• Good protection against interference and tapping
• Disadvantages
• Lower user data rates
• More complex signal regeneration
• Implemented using spread spectrum technology

c
f
t
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Various Access Method
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Cells
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Capacity Spectrum Utilization Solution

• The need
• Optimum spectrum usage
• More capacity
• High quality of service
• Low cost

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Representation of Cells
Ideal cells
Fictitious cells
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Cell size and capacity

• Cell size determines number of cells available to
  cover geographic area and (with frequency reuse)
  the total capacity available to all users
• Capacity within cell limited by available
  bandwidth and operational requirements
• Each network operator has to size cells to handle
  expected traffic demand

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Cell structure

• Implements space division multiplex base station
  covers a certain transmission area (cell)
• Mobile stations communicate only via the base
  station
• Advantages of cell structures
• higher capacity, higher number of users
• less transmission power needed
• more robust, decentralized
• base station deals with interference,
  transmission area etc. locally
• Problems
• fixed network needed for the base stations
• handover (changing from one cell to another)
  necessary
• interference with other cells
• Cell sizes from some 100 m in cities to, e.g., 35
  km on the country side (GSM) - even less for
  higher frequencies

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Capacity of a Cellular System

• Frequency Re-Use Distance
• The K factor or the cluster size
• Cellular coverage or Signal to interference ratio
• Sectoring

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The K factor and Frequency Re-Use Distance
7
2
6
K i2 ij j2 K 22 21 12 K 4 2 1
K 7
1
5
3
j
R
7
2
6
i
1
D
5
3
4
D ??3K R D 4.58R
Frequency re-use distance is based on the cluster
size K The cluster size is specified in terms of
the offset of the center of a cluster from the
center of the adjacent cluster
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The Frequency Re-Use for K 4
K i2 ij j2 K 22 20 02 K 4 0 0
K 4
D ??3K R D 3.46R
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The Cell Structure for K 7
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Cell Structure for K 4
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Cell Structure for K 12
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Increasing cellular system capacity

• Cell sectoring
• Directional antennas subdivide cell into 3 or 6
  sectors
• Might also increase cell capacity by factor of 3
  or 6

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Increasing cellular system capacity

• Cell splitting
• Decrease transmission power in base and mobile
• Results in more and smaller cells
• Reuse frequencies in non-contiguous cell groups
• Example ½ cell radius leads 4 fold capacity
  increase

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Tri-Sector antenna for a cell
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Cell Distribution in a Network
Rural
Highway
Town
Suburb
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Optimum use of frequency spectrum

• Operator bandwidth of 7.2MHz (36 freq of 200 kHz)
• TDMA 8 traffic channels per carrier
• K factor 12
• What are the number of traffic channels available
  within its area for these three cases
• Without cell splitting
• With 72 cells
• With 246 cells

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Re-use of the frequency
One Cell 288 traffic channels
8 X 36 288
72 Cell 1728 traffic channels
8 X (72/12 X 36) 1728
246 Cell 5904 traffic channels
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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.

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GSM uses paired radio channels
UPLINK
DOWNLINK
890MHz
915MHz
935MHz
960MHz
0
124
0
124
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GSM delays uplink TDMA frames
The start of the uplink TDMA is delayed of three
time slots
TDMA frame (4.615 ms)
Downlink TDMA F1MHz
Uplink TDMA Frame F1 45MHz
Fixed transmit Delay of three time-slots
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GSM - TDMA/FDMA
935-960 MHz 124 channels (200 kHz) downlink
frequency
890-915 MHz 124 channels (200 kHz) uplink
time
GSM TDMA frame
GSM time-slot (normal burst)
guard space
guard space
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LOGICAL CHANNELS
TRAFFIC
SIGNALLING
FULL RATE Bm 22.8 Kb/S
HALF RATE Lm 11.4 Kb/S
BROADCAST
COMMON CONTROL
DEDICATED CONTROL
FCCH
SCH
BCCH
RACH
AGCH
PCH
FCCH -- FREQUENCY CORRECTION CHANNEL SCH --
SYNCHRONISATION CHANNEL BCCH -- BROADCAST
CONTROL CHANNEL PCH -- PAGING CHANNEL RACH
-- RANDOM ACCESS CHANNEL AGCH -- ACCESS
GRANTED CHANNEL SDCCH -- STAND ALONE DEDICATED
CONTROL CHANNEL SACCH -- SLOW ASSOCIATED CONTROL
CHANNEL FACCH -- FAST ASSOCIATED CONTROL CHANNEL
SDCCH
SACCH
FACCH
DOWN LINK ONLY
BOTH UP DOWNLINKS
UPLINK ONLY
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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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DEFINITION OF TIME SLOT - 156.25 BITS 15/26ms
0.577ms
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HIERARCHY OF FRAMES
1 HYPER FRAME 2048 SUPERFRAMES 2 715 648 TDMA
FRAMES ( 3 H 28 MIN 53 S 760 MS )
0 1 2 3 4
5 6
2043 2044 2045 2046
2047
1 SUPER FRAME 1326 TDMA FRAMES ( 6.12 S )
LEFT (OR) RIGHT
TRAFFIC CHANNELS
1 SUPER FRAME 51 MULTI FRAMES
SIGNALLING CHANNELS
1 SUPER FRAME 26 MULTI FRAMES
0 1 2
24 25
1 MULTIFRAME 26 TDMA FRAMES ( 120 ms )
0 1 2 3
24 25
1 MULTI FRAME 51 TDMA FRAMES (235 .4 ms )
TDMA FRAME NO.
(4.615ms)
0
1
1 TIME SLOT 156.25 BITS ( 0.577
ms)
(4.615 ms)
0
1
1 2 3 4 155 156
1 bit 36.9 micro sec
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GSM Frame
Full rate channel is idle in 25
SACCH is transmitted in frame 12
0 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 RATE Bm 22.8 Kb/S
HALF RATE Lm 11.4 Kb/S
BROADCAST
COMMON CONTROL
DEDICATED CONTROL
FCCH
SCH
BCCH
RACH
AGCH
PCH
FCCH -- FREQUENCY CORRECTION CHANNEL SCH --
SYNCHRONISATION CHANNEL BCCH -- BROADCAST
CONTROL CHANNEL PCH -- PAGING CHANNEL RACH
-- RANDOM ACCESS CHANNEL AGCH -- ACCESS
GRANTED CHANNEL SDCCH -- STAND ALONE DEDICATED
CONTROL CHANNEL SACCH -- SLOW ASSOCIATED CONTROL
CHANNEL FACCH -- FAST ASSOCIATED CONTROL CHANNEL
SDCCH
SACCH
FACCH
DOWN LINK ONLY
BOTH UP DOWNLINKS
UPLINK ONLY
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Location update from the mobile
Mobile looks for BCCH after switching on
RACH send channel request
AGCH receive SDCCH
SDCCH request for location updating
SDCCH authenticate
SDCCH authenticate response
SDCCH switch to cipher mode
SDCCH cipher mode acknowledge
SDCCH allocate TMSI
SDCCH acknowledge new TMSI
SDCCH switch idle update mode
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Call establishment from a mobile
Mobile looks for BCCH after switching on
RACH send channel request
AGCH receive SDCCH
SDCCH send call establishment request
SDCCH do the authentication and TMSI allocation
SDCCH send the setup message and desired number
SDCCH require traffic channel assignment
FACCH switch to traffic channel and send ack
(steal bits)
FACCH receive alert signal ringing sound
FACCH receive connect message
FACCH acknowledge connect message and use TCH
TCH conversation continues
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Call establishment to a mobile
Mobile looks for BCCH after switching on
Mobile receives paging message on PCH
Generate Channel Request on RACH
Receive signaling channel SDCCH on AGCH
Answer paging message on SDCCH
Receive authentication request on SDCCH
Authenticate on SDCCH
Receive setup message on SDCCH
Receive traffic channel assignment on SDCCH
FACCH switch to traffic channel and send ack
(steal bits)
Receive alert signal and generate ringing on FACCH
Receive connect message on FACCH
FACCH acknowledge connect message and switch to
TCH
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GSM speech coding
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Transmit Path
BS Side
8 bit A-Law to 13 bit Uniform
RPE/LTP speech Encoder
8 K sps
To Channel Coder 13Kbps
MS Side
RPE/LTP speech Encoder
8 K sps,
LPF
A/D

To Channel Coder 13Kbps
Sampling Rate - 8K Encoding - 13 bit Encoding
(104 Kbps) RPE/LTP - Regular Pulse
Excitation/Long Term Prediction RPE/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
Full rate channel is idle in 25
SACCH is transmitted in frame 12
0 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 503 bits.
• 132 class Ib bits with 4 bit tail sequence 132
  4 136.
• Class Ia class Ib 53136189, 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.

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GSM Protocol Suite
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HLR
SS
MSC VLR
MM CM
RR
BSC
BTS
Radio interface
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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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GSM Functional Architecture and Principal
Interfaces
Um
Base Station System
BSC
BTS
A-Bis Interface
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GSM protocol layers for signaling
Um
Abis
A
MS
BTS
BSC
MSC
CM
CM
MM
MM
RR BTSM
BSSAP
RR
BSSAP
RR
BTSM
SS7
SS7
LAPDm
LAPDm
LAPD
LAPD
radio
radio
PCM
PCM
PCM
PCM
16/64 kbit/s
64 kbit/s / 2.048 Mbit/s
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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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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 SAPI0 for signaling and SAPI3 for SMS.
• SAP10 for radio signaling, SAPI62 for OAM and
  SAPI63 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 Protocol
• Radio 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 exchanges
• Signaling 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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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).
• SAPI0, SAPI3, SAPI62 for OAM.

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Presentation of the A-ter interface
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TRAU
BSC
LAPD TS1
OAM
Transcoding
Speech TS
MSC
Speech TS
CCS7 TS
CCS7 TS
X.25 TS2
OMC
X.25 TS2
PCM LINK
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), 430
• 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 interface
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Signaling Protocol Model
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Presentation on the A-Interface
BSSMAP - 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 presentation
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NSS
MS
CM
M A P
CM
MM
MM
BTS
BSC
O A M
O A M
R R
BSSAP
BSSAP DTAP/ BSSMAP
T C A P
R R
ISUP/TUP
D T A P
B S S M A P
L A P D
SCCP
L A P D
SCCP
SCCP
LAPDm
LAPDm
MTP3
MTP3
MTP3
MTP2
MTP2
MTP2
MTP1
RADIO
RADIO
PCM
PCM
PCM E1 T1
A Interface
Um Interface
A bis Interface
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BSC
MSC
MS
TRXTEIT1
SCCP RefR1
ChannelC1
Channel ID N1
Link SAPI0
PDRR
DTAP
DLCI SAPI0
PDMM
PDCC
TIa
TIb
DLCI SAPI3
Link SAPI3
TIA
PD protocol discriminator TI Transaction
Identifier for RIL3-CC protocol DLCI Data
Link connection Identifier SAPI
Service Access Point Identifier on
the radio Interface TEI Terminal
Equipment Identifier on the Abis I/F
Channel ID N1
SCCP RefR2
ChannelC2
A Interface
Radio Interface
Abis Interface
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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

87
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

89
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

90
Thank You