GSM Baics

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INTRODUCTION TO GSM
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INTRODUCTION TO GSM

• INTRODUCTION
• The Global System for Mobile Communications (GSM)
  is a set of recommendations and specifications
  for a digital cellular telephone network (known
  as a Public Land Mobile Network, or PLMN).
• These recommendations ensure the compatibility
  of equipment from different GSM manufacturers,
  and interconnectivity between different
  administrations, including operation across
  international boundaries.
• GSM networks are digital and can cater for high
  system capacities.
• They are consistent with the world-wide
  digitization of the telephone network, and are an
  extension of the Integrated Services Digital
  Network (ISDN), using a digital radio interface
  between the cellular network and the mobile
  subscriber equipment.

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INTRODUCTION TO GSM

• CELLULAR TELEPHONY
• A cellular telephone system links mobile
  subscribers into the public telephone system or
  to another cellular subscriber.
• Information between the mobile unit and the
  cellular network uses radio communication. Hence
  the subscriber is able to move around and become
  fully mobile.
• The service area in which mobile communication is
  to be provided is divided into regions called
  cells.
• Each cell has the equipment to transmit and
  receive calls from any subscriber located within
  the borders of its radio coverage area.

Radio
Cell
Mobile subscriber
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INTRODUCTION TO GSM

• GSM FREQUENCIES
• GSM systems use radio frequencies between 890-915
  MHz for receive and between 935-960 MHz for
  transmit.
• RF carriers are spaced every 200 kHz, allowing a
  total of 124 carriers for use.
• An RF carrier is a pair of radio frequencies, one
  used in each direction.
• Transmit and receive frequencies are always
  separated by 45 MHz.

UPLINK FREQUENCIES
DOWNLINK FREQUENCIES
890
960
935
915
UPLINK AND DOWNLINK FREQUENCY SEPARATED BY 45MHZ
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INTRODUCTION TO GSM

• Extended GSM (EGSM)
• EGSM has 10MHz of bandwidth on both transmit and
  receive.
• Receive bandwidth is from 880 MHz to 890 MHz.
• Transmit bandwidth is from 925 MHz to 935 MHz.
• Total RF carriers in EGSM is 50.

UPLINK FREQUENCIES
DOWNLINK FREQUENCIES
880
925
890
915
935
960
UPLINK AND DOWNLINK FREQUENCY SEPARATED BY 45MHZ
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INTRODUCTION TO GSM

• DCS1800 FREQUENCIES
• DCS1800 systems use radio frequencies between
  1710-1785 MHz for receive and between 1805-1880
  MHz for transmit.
• RF carriers are spaced every 200 kHz, allowing a
  total of 373 carriers.
• There is a 100 kHz guard band between 1710.0 MHz
  and 1710.1 MHz and between 1784.9 MHz and 1785.0
  MHz for receive, and between 1805.0 MHz and
  1805.1 MHz and between 1879.9 MHz and 1880.0 MHz
  for transmit.
• Transmit and receive frequencies are always
  separated by 95 MHz.

UPLINK FREQUENCIES
DOWNLINK FREQUENCIES
1710 MHz
1880 MHz
1805 MHz
1785 MHz
UPLINK AND DOWNLINK FREQUENCY SEPARATED BY 95MHZ
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FEATURES OF GSM
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FEATURES OF GSM

• INCREASED CAPACITY
• The GSM system provides a greater subscriber
  capacity than analogue systems.
• GSM allows 25 kHz per user, that is, eight
  conversations per 200 kHz channel pair (a pair
  comprising one transmit channel and one receive
  channel).
• Digital channel coding and the modulation used
  makes the signal resistant to interference from
  cells where the same frequencies are re-used
  (co-channel interference) a Carrier to
  Interference Ratio (C/I) level of 12 dB is
  achieved, as opposed to the 18 dB typical with
  analogue cellular.
• This allows increased geographic reuse by
  permitting a reduction in the number of cells in
  the reuse pattern.

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FEATURES OF GSM

• AUDIO QUALITY
• Digital transmission of speech and high
  performance digital signal processors provide
  good quality speech transmission.
• Since GSM is a digital technology, the signals
  passed over a digital air interface can be
  protected against errors by using better error
  detection and correction techniques.
• In regions of interference or noise-limited
  operation the speech quality is noticeably better
  than analogue.
• USE OF STANDARDISED OPEN INTERFACES
• Standard interfaces such as C7 and X25 are used
  throughout the system. Hence different
  manufacturers can be selected for different parts
  of the PLMN.
• There is a high flexibilty in where the Network
  components are situated.

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FEATURES OF GSM

• IMPROVED SECURITY AND CONFIDENTIALITY
• GSM offers high speech and data confidentiality.
• Subscriber authentication can be performed by the
  system to check if a subscriber is a valid
  subscriber or not.
• The GSM system provides for high degree of
  confidentiality for the subscriber. Calls are
  encoded and ciphered when sent over air.
• The mobile equipment can be identified
  independently from the mobile subscriber. The
  mobile has a identity number hard coded into it
  when it is manufactured. This number is stored in
  a standard database and whenever a call is made
  the equipment can be checked to see if it has
  been reported stolen.

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FEATURES OF GSM

• CLEANER HANDOVERS
• GSM uses Mobile assisted handover techique.
• The mobile itself carries out the signal strength
  and quality measurement of its server and signal
  strength measurement of its neighbors.
• This data is passed on the Network which then
  uses sophisticated algorithms to determine the
  need of handover.
• SUBSCRIBER IDENTIFICATION
• In a GSM system the mobile station and the
  subscriber are identified separately.
• The subscriber is identified by means of a smart
  card known as a SIM.
• This enables the subscriber to use different
  mobile equipment while retaining the same
  subscriber number.

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FEATURES OF GSM

• ENHANCED RANGE OF SERVICES
• Speech services for normal telephony.
• Short Message Service for point ot point
  transmission of text message.
• Cell broadcast for transmission of text message
  from the cell to all MS in its coverage area.
  Message like traffic information or advertising
  can be transmitted.
• Fax and data services are provided. Data rates
  available are 2.4 Kb/s, 4.8 Kb/s and 9.6 Kb/s.
• Supplementary services like number identification
  , call barring, call forwarding, charging display
  etc can be provided.

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FEATURES OF GSM

• FREQUENCY REUSE
• There are total 124 carriers in GSM ( additional
  50 carriers are available if EGSM band is used).
• Each carrier has 8 timeslots and if 7 can be used
  for traffic then a maximum of 868 ( 124 X 7 )
  calls can be made. This is not enough and hence
  frequencies have to be reused.
• The same RF carrier can be used for many
  conversations in several different cells at the
  same time.

• The radio carriers available are allocated
  according to a regular pattern which repeats over
  the whole coverage area.
• The pattern to be used depends on traffic
  requirement and spectrum availability.
• Some typical repeat patterns are 4/12, 7/21 etc.

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3
1
4
7
5
6
2
1
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NETWORK COMPONENTS
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NETWORK COMPONENTS
D
H
B
F
C
A
UM
ABIS
UM
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NETWORK COMPONENTS

• Mobile Switching Centre (MSC)
• The Mobile services Switching Centre (MSC)
  co-ordinates the setting up of calls to and from
  GSM users.
• It is the telephone switching office for MS
  originated or terminated traffic and provides the
  appropriate bearer services, teleservices and
  supplementary services.
• It controls a number of Base Station Sites (BSSs)
  within a specified geographical coverage area and
  gives the radio subsystem access to the
  subscriber and equipment databases.
• The MSC carries out several different functions
  depending on its position in the network.
• When the MSC provides the interface between PSTN
  and the BSS in the GSM network it is called the
  Gateway MSC.
• Some important functions carried out by MSC are
  Call processing including control of data/voice
  call setup, inter BSS inter MSC handovers,
  control of mobility management, Operation
  maintenance support including database
  management, traffic metering and man machine
  interface managing the interface between GSM
  PSTN N/W.

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NETWORK COMPONENTS
Mobile Switching Centre (MSC) Lucent MSC
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NETWORK COMPONENTS

• Mobile Station (MS)
• The Mobile Station consists of the Mobile
  Equipment (ME) and the Subscriber Identity Module
  (SIM).
• Mobile Equipment
• The Mobile Equipment is the hardware used by the
  subscriber to access the network.
• The mobile equipment can be Vehicle mounted, with
  the antenna physically mounted on the outside of
  the vehicle or portable mobile unit, which can be
  handheld.
• Mobiles are classified into five classes
  according to their power rating.

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NETWORK COMPONENTS

• SIM
• The SIM is a removable card that plugs into the
  ME.
• It identifies the mobile subscriber and provides
  information about the service that the subscriber
  should receive.
• The SIM contains several pieces of information
• International Mobile Subscribers Identity ( IMSI
  ) - This number identifies the mobile subscriber.
  It is only transmitted over the air during
  initialising.
• Temporary Mobile Subscriber Identity ( TMSI ) -
  This number also identifies the subscriber. It
  can be alternatively used by the system. It is
  periodically changed by the system to protect the
  subscriber from being identified by someone
  attempting to monitor the radio interface.
• Location Area Identity ( LAI ) - Identifies the
  current location of the subscriber.
• Subscribers Authentication Key ( Ki ) - This is
  used to authenticate the SIM card.
• Mobile Station International Standard Data Number
  ( MSISDN ) - This is the telephone number of the
  mobile.

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NETWORK COMPONENTS

• SIM
• Most of the data contained within the SIM is
  protected against reading (eg Ki ) or alterations
  after the SIM is issued.
• Some of the parameters ( eg. LAI ) will be
  continously updated to reflect the current
  location of the subscriber.
• The SIM card can be protected by use of Personal
  Identity Number ( PIN ) password.
• The SIM is capable of storing additional
  information such as accumulated call charges.

FULL SIZE SIM CARD
MINI SIM CARD
G S M
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NETWORK COMPONENTS

• Mobile Station International Subscribers Dialling
  Number ( MSISDN )
• Human identity used to call a MS
• The Mobile Subscriber ISDN (MSISDN) number is the
  telephone number of the MS.
• This is the number a calling party dials to reach
  the subscriber.
• It is used by the land network to route calls
  toward the MSC.

CC
NDC
SN
98 XXX 12345
CC NDC SN
Country code National Destination Code
Subscriber Number
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NETWORK COMPONENTS

• International Mobile Subscribers Identity ( IMSI
  )
• Network Identity Unique to a MS
• The International Mobile Subscriber Identity
  (IMSI) is the primary identity of the subscriber
  within the mobile network and is permanently
  assigned to that subscriber.
• The IMSI can be maximum of 15 digits.

MCC
MNC
MSIN
404 XX 12345..10
MCC MNC MSIN
Mobile Country Code ( 3 Digits ) Mobile
Network Code ( 2 Digits ) Mobile Subscriber
Identity Number
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NETWORK COMPONENTS

• Temporary Mobile Subscribers Identity ( TMSI )
• The GSM system can also assign a Temporary Mobile
  Subscriber Identity (TMSI).
• After the subscriber's IMSI has been initialized
  on the system, the TMSI can be used for sending
  messages backwards and forwards across the
  network to identify the subscriber.
• The system automatically changes the TMSI at
  regular intervals, thus protecting the subscriber
  from being identified by someone attempting to
  monitor the radio channels.
• The TMSI is a local number and is always
  allocated by the VLR.
• The TMSI is maximum of 4 octets.

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NETWORK COMPONENTS

• Equipment Identity Register ( EIR )
• The Equipment Identity Register (EIR) contains a
  centralized database for validating the
  international mobile station equipment identity,
  the IMEI.
• The database contains three lists
• The white list contains the number series of
  equipment identities that have been allocated in
  the different participating countries. This list
  does not contain individual numbers but but a
  range of numbers by identifying the beginning and
  end of the series.
• The grey list contains IMEIs of equipment to be
  monitored and observed for location and correct
  function.
• The black list contains IMEIs of MSs which have
  been reported stolen or are to be denied service.
• The EIR database is remotely accessed by the
  MSCs in the Network and can also be accessed by
  an MSC in a different PLMN.
• .

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NETWORK COMPONENTS
Equipment Identity Register ( EIR )
EIR
White List All Valid assigned IDs Range
1 Range 2 Range n

Black List Service denied MS IMEI 1 MS IMEI
2 MS IMEI n
Grey List Service allowed but noted MS IMEI
1 MS IMEI 2 MS IMEI n
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NETWORK COMPONENTS

• International Mobile Equipment Identity ( IMEI )
  
• IMEI is a serial number unique to each mobile
• Each MS is identified by an International Mobile
  station Equipment Identity (IMEI) number which is
  permanently stored in the Mobile Equipment.
• On request, the MS sends this number over the
  signalling channel to the MSC.
• The IMEI can be used to identify MSs that are
  reported stolen or operating incorrectly.

TAC
FAC
SNR
SP
6 2 6 1
TAC FAC SNR SP
Type Approval Code Final Assembly Code
Serial Number Spare
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NETWORK COMPONENTS

• HOME LOCATION REGISTER( HLR )
• The HLR contains the master database of all
  subscribers in the PLMN.
• This data is remotely accessed by the MSCs and
  VLRs in the network. The data can also be
  accessed by an MSC or a VLR in a different PLMN
  to allow inter-system and inter-country roaming.
• A PLMN may contain more than one HLR, in which
  case each HLR contains a portion of the total
  subscriber database. There is only one database
  record per subscriber.
• The subscribers data may be accessed by the IMSI
  or the MSISDN.
• The parameters stored in HLR are
• Subscribers ID (IMSI and MSISDN )
• Current subscriber VLR.
• Supplementary services subscribed to.
• Supplementary services information (eg. Current
  forwarding address ).
• Authentication key and AUC functionality.
• TMSI and MSRN

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NETWORK COMPONENTS

• VISITOR LOCATION REGISTER ( VLR )
• The Visited Location Register (VLR) is a local
  subscriber database, holding details on those
  subscribers who enter the area of the network
  that it covers.
• The details are held in the VLR until the
  subscriber moves into the area serviced by
  another VLR.
• The data includes most of the information stored
  at the HLR, as well as more precise location and
  status information.
• The additional data stored in VLR are
• Mobile status ( Busy / Free / No answer etc. )
• Location Area Identity ( LAI )
• Temporary Mobile Subscribers Identity ( TMSI )
• Mobile Station Roaming Number ( MSRN )
• The VLR provides the system elements local to the
  subscriber, with basic information on that
  subscriber, thus removing the need to access the
  HLR every time subscriber information is
  required.

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NETWORK COMPONENTS
Authentication Centre ( AUC )

• The AUC is a processor system that perform
  authentication function.
• It is normally co-located with the HLR.
• The authentication process usually takes place
  each time the subscriber initialises on the
  system.
• Each subscriber is assigned an authentication key
  (Ki) which is stored in the SIM and at the AUC.
• A random number of 128 bits is generated by the
  AUC sent to the MS.
• The authentication algorithm A3 uses this random
  number and authentication key Ki to produce a
  signed response SRES( Signed Response ).
• At the same time the AUC uses the random number
  and Authentication algoritm A3 along with the Ki
  key to produce a SRES.
• If the SRES produced by AUC matches the one
  produced by MS is the same, the subscriber is
  permitted to use the network.

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NETWORK COMPONENTS
AUTHENTICATION PROCESS
HLR
AUC
MS
Ki, A3, A8 A3 ( RAND, Ki ) SRES A8 ( RAND, Ki )
Kc Triples Generated
A3 , A8 , A5 , Ki
RAND
TRIPLES RAND, Kc , SRES
VLR
SRES A3 (RAND , Ki )
SRES
RAND Kc SRES
SRES
SRES SRES
BTS
AIR INTERFACE ENCRYPTION
A5 , HYPERFRAME NUM
Kc A8 (RAND , Ki )
Kc
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NETWORK COMPONENTS

• Base Station Sub-System ( BSS )
• The BSS is the fixed end of the radio interface
  that provides control and radio coverage
  functions for one or more cells and their
  associated MSs.
• It is the interface between the MS and the MSC.
• The BSS comprises one or more Base Transceiver
  Stations (BTSs), each containing the radio
  components that communicate with MSs in a given
  area, and a Base Site Controller (BSC) which
  supports call processing functions and the
  interfaces to the MSC.
• Digital radio techniques are used for the radio
  communications link, known as the Air Interface,
  between the BSS and the MS.
• The BSS consists of three basic Network Elements
  (NEs).
• Transcoder (XCDR) or Remote transcoder (RXCDR) .
• Base Station Controller (BSC).
• Base Transceiver Stations (BTSs) assigned to the
  BSC. .

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NETWORK COMPONENTS

• Transcoder( XCDR )
• The speech transcoder is the interface between
  the 64 kbit/s PCM channel in the land network and
  the 13 kbit/s vocoder (actually 22.8 kbit/s after
  channel coding) channel used on the Air
  Interface.
• This reduces the amount of information carried on
  the Air Interface and hence, its bandwidth.
• If the 64 kbits/s PCM is transmitted on the air
  interface without occupation, it would occupy an
  excessive amount of radio bandwidth. This would
  use the available radio spectrum inefficiently.
• The required bandwidth is therefore reduced by
  processing the 64 kbits/s PCM data so that the
  amount of information required to transmit
  digitized voice falls to 13kb/s.
• The XCDR can multiplex 4 traffic channels into a
  single 64 kbit/s timeslot. Thus a E1/T1 serial
  link can carry 4 times as many channels.
• This can reduce the number of E1/T1 leased lines
  required to connect remotely located equipment.
• When the transcoder is between the MSC and the
  BSC it is called a remote transcoder (RXCDR).

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NETWORK COMPONENTS
TRANSCODER(XCDR) - Siemens
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NETWORK COMPONENTS
TRANSCODING
30 Timeslots 1 traffic channel / TS 64 Kbps /
TS 4 E1 lines 30 X 4 120 Timeslots
Each Timeslot 16 X 4 64 Kb/s 30 timeslots
30 x 4 120 traffic channels
MSC
XCDR
BSC
Transcoded information from four calls
0
1
2
31
16
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NETWORK COMPONENTS

• Base Station Controller (BSC)
• The BSC network element provides the control for
  the BSS.
• It controls and manages the associated BTSs, and
  interfaces with the Operations and Maintenance
  Centre (OMC).
• The purpose of the BSC is to perform a variety of
  functions. The following comprise the functions
  provided by the BSC
• Controls the BTS components.-
• Performs Call Processing.
• Performs Operations and Maintenance (O M).
• Provides the O M link (OML) between the BSS and
  the OMC.
• Provides the A Interface between the BSS and the
  MSC.
• Manages the radio channels.
• Transfers signalling information to and from MSs.
  

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NETWORK COMPONENTS
Base Station Controller (BSC) Siemens BSC
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NETWORK COMPONENTS

• Base Transceiver Station (BTS)
• The BTS network element consists of the hardware
  components, such as radios, interface modules and
  antenna systems that provide the Air Interface
  between the BSS and the MSs.
• The BTS provides radio channels (RF carriers) for
  a specific RF coverage area.
• The radio channel is the communication link
  between the MSs within an RF coverage area and
  the BSS.
• The BTS also has a limited amount of control
  functionality which reduces the amount of traffic
  between the BTS and BSC.

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NETWORK COMPONENTS
Base Transceiver Station (BTS)
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NETWORK COMPONENTS
BTS Connectivity
Open ended Daisy Chain
MSC
BSC
BTS12
BTS13
BTS14
Star
BTS5
BTS11
Daisy Chain with a fork. Fork has a return loop
back to the chain
BTS1
BTS6
BTS4
Daisy Chain with a fork. Fork has a return loop
back to the chain
BTS2
BTS7
BTS9
BTS8
BTS3
BTS11
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NETWORK COMPONENTS
Operation And Maintenance Centre For Radio (OMC-R)

• The OMC controls and monitors the Network
  elements within a region.
• The OMC also monitors the quality of service
  being provided by the Network.
• The following are the main functions performed by
  the OMC-R
• The OMC allows network devices to be manually
  removed for or restored to service. The status of
  network devices can be checked from the OMC and
  tests and diagnostics invoked.
• The alarms generated by the Network elements are
  reported and logged at the OMC. The OMC-R
  Engineer can monitor and analyze these alarms and
  take appropriate action like informing the
  maintenance personal.
• The OMC keeps on collecting and accumulating
  traffic statistics from the network elements for
  analysis.
• Software loads can be downloaded to network
  elements or uploaded to the OMC.

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NETWORK COMPONENTS
Operation And Maintenance Centre For Radio (OMC-R)
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NETWORK COMPONENTS
Base Station Identity Code

• BSIC allows a mobile station to distinguish
  between neighboring base stations.
• It is made up of 8 bits.
• NCC National Colour Code( Differs from
  operator to operator )
• BCC Base Station Colour Code, identifies the
  base station to help distinguish between Cells
  using the same BCCH frequencies

7 6 5 4
3 2 1 0
BCC
0
0
BCC
NCC
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NETWORK COMPONENTS
MS Class Mark

• The MS is identified by its classmark which the
  mobile sends during its initial message.
• The classmark contains the following information
• Revision level - Identifies the phase of the GSM
  specifications the mobiles complies with.
• RF Power Capabilities - The maximum power the
  mobile can transmit. This information is held in
  the MS Power Class Number.
• Ciphering Algorithm - Indicates the ciphering
  algorithm implemented in the mobile. There is
  only one algorithm (A5 ) in GSM phase 1, however
  GSM phase 2 specifies different algorithms (A5/0
  to A5/7 )
• Frequency Capability - Indicates the frequency
  bands the MS can receive and transmit on.
• Short Message Capability- Indicates whether the
  MS is able to receive short messages or not.

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MOBILE MAXIMUM RANGE RANGE TIMING ADVANCE
DELAY OF BITS (0-63) BIT PERIOD 577/156.25
3.693?sec 3.693 10e-6 sec VELOCITY 3 10e5
Km/sec RANGE 34.9 Km
TIMIMG ADVANCE BIT PERIOD VELOCITY
2
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• MULTIPLE ACCESS TECHNIQUES
• In order for several links to be in progress
  simultaneously in the same geographical area
  without mutual interference , multiple access
  techniques are deployed.
• The commonly used multiple access techniques are
• Frequency Division Multiple Access (FDMA )
• Time Division Multiple Access (TDMA )
• Code Division Multiple Access (CDMA )

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• TERRESTERIAL INTERFACE
• The terrestrial interfaces comprises all the
  connections between the GSM system entities
  ,apart from the Um or air interface.
• The terrestrial interfaces transport the traffic
  across the system and allows the passage of
  thousands of data messages to make the system
  function.
• The standard interfaces used are
• 2 Mb/s
• Signalling System (C7 or SS7
• Packet Switched Data
• A bis using the LAPD protocol (Link Access
  Procedure D )

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INTERFACE NAMES Each interface specified in GSM
has a name associated with it.
NAME INTERFACE Um MS ----- BTS Abis BTS
----- BSC A MSC ------ BSC B MSC ------
VLR C MSC ------ HLR D VLR ----- HLR E MSC
------ MSC F MSC ------ EIR G VLR ------
VLR H HLR ------ AUC
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2 Mbits/s Trunk 30- channel PCM This interface
carries the traffic from the PSTN to the MSC,
between MSCs, from the MSC to the BSCs and from
the BSCs to the BTSs. It represents the
physical layer in the OSI model. Each 2 Mb/s link
provides 30 traffic channels available to carry
speech ,data or control information. Typical
Configuration
TS 17 - 31
TS 0
TS 1-15
TS 16
TS 0 - Frame allignment/ Error checking/
Signalling/ Alarms TS 1-15 , 17-31 - Traffic TS
16 - Siganlling
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BSS CONNECTIONS
MSC
MTL (C7 )
XCDR
OMC
OML (X.25)
BSC
CBC
CBL
RSL ( LAPD)

BTS
BTS
BTS
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Cell Global Identity ( CGI )
LAI
MCC
MNC
LAC
CI
CGI
MCC MNC LAC CI
Mobile Country Code Mobile Network Code
Location Area Identity Cell Identity
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CHANNEL CONCEPT
CHANNELS
Downlink
Uplink

• Physical channel - Each timeslot on a carrier is
  referred to as a physical channel. Per carrier
  there are 8 physical channels.
• Logical channel - Variety of information is
  transmitted between the MS and BTS. There are
  different logical channels depending on the
  information sent. The logical channels are of two
  types
• Traffic channel
• Control channel

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CHANNEL CONCEPT
GSM Traffic Channels
Traffic Channels
TCH/F Full rate 22.8kbits/s
TCH/H Half rate 11.4 kbits/s
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CHANNEL CONCEPT
GSM Control Channels
Control Channels
BCH ( Broadcast channels ) Downlink only
DCCH(Dedicated Channels) Downlink Uplink
CCCH(Common Control Chan) Downlink Uplink
BCCH Broadcast control channel

Synch. Channels
RACH Random Access Channel
CBCH Cell Broadcast Channel
SDCCH Standalone dedicated control channel
ACCH Associated Control Channels
SCH Synchronisation channel
SACCH Slow associated Control Channel
FACCH Fast Associated Control Channel
PCH/ AGCH Paging/Access grant
FCCH Frequency Correction channel
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CHANNEL CONCEPT

• BCH Channels
• BCCH( Broadcast Control Channel )
• Downlink only
• Broadcasts general information of the serving
  cell called System Information
• BCCH is transmitted on timeslot zero of BCCH
  carrier
• Read only by idle mobile at least once every 30
  secs.
• SCH( Synchronisation Channel )
• Downlink only
• Carries information for frame synchronisation.
  Contains TDMA frame number and BSIC.
• FCCH( Frequency Correction Channel )
• Downlink only.
• Enables MS to synchronise to the frequency.
• Also helps mobiles of the ncells to locate TS 0
  of BCCH carrier.

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CHANNEL CONCEPT

• CCCH Channels
• RACH( Random Access Channel )
• Uplink only
• Used by the MS to access the Network.
• AGCH( Access Grant Channel )
• Downlink only
• Used by the network to assign a signalling
  channel upon successfull decoding of access
  bursts.
• PCH( Paging Channel )
• Downlink only.
• Used by the Network to contact the MS.

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CHANNEL CONCEPT

• DCCH Channels
• SDCCH( Standalone Dedicated Control Channel )
• Uplink and Downlink
• Used for call setup, location update and SMS.
• SACCH( Slow Associated Control Channel )
• Used on Uplink and Downlink only in dedicated
  mode.
• Uplink SACCH messages - Measurement reports.
• Downlink SACCH messages - control info.
• FACCH( Fast Associated Control Channel )
• Uplink and Downlink.
• Associated with TCH only.
• Is used to send fast messages like handover
  messages.
• Works by stealing traffic bursts.

60
CHANNEL CONCEPT
NORMAL BURST
FRAME1(4.615ms)
FRAME2
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
0.577ms
0.546ms
57 bits
57 bits
26 bits
3
3
Training sequence
Tail Bits
Tail Bits
Flag Bit
Flag Bit
Guard Period
Guard Period
Data
Data
Carries traffic channel and control channels
BCCH, PCH, AGCH, SDCCH, SACCH and FACCH.
61
CHANNEL CONCEPT
NORMAL BURST
Data - Two blocks of 57 bits each. Carries
speech, data or control info. Tail bits - Used to
indicate the start and end of each burst. Three
bits always 000. Guard period - 8.25 bits long.
The receiver can only receive and decode if the
burst is received within the timeslot designated
for it.Since the MS are moving. Exact
synchronization of burst is not possible
practically. Hence 8.25bits corresponding to
about 30us is available as guard period for a
small margin of error. Flag bits - This bit is
used to indicate if the 57 bits data block is
used as FACCH. Training Sequence - This is a set
sequence of bits known by both the transmitter
and the receiver( BCC of BSIC). When a burst of
information is received the equaliser searches
for the training sequence code. The receiver
measures and then mimics the distortion which the
signal has been subjected to. The receiver then
compares the received data with the distorted
possible transmitted sequence and chooses the
most likely one.
62
CHANNEL CONCEPT
FREQUENCY CORRECTION BURST
FRAME1(4.615ms)
FRAME2
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
0.577ms
0.546ms
142 bits
3
3
Tail Bits
Tail Bits
Guard Period
Guard Period
Fixed Data

• Carries FCCH channel.
• Made up of 142 consecutive zeros.
• Enables MS to correct its local oscillator
  locking it to that of the BTS.

63
CHANNEL CONCEPT
SYNCHRONISATION BURST
FRAME1(4.615ms)
FRAME2
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
0.577ms
0.546ms
39 bits
3
3
64 bits
39 bits
Synchronisation Sequence
Encrypted Bits
Tail Bits
Tail Bits
Guard Period
Guard Period
Encrypted Bits

• Carries SCH channel.
• Enables MS to synchronise its timings with the
  BTS.
• Contains BSIC and TDMA Frame number.

64
CHANNEL CONCEPT
DUMMY BURST
FRAME1(4.615ms)
FRAME2
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
0.577ms
0.546ms
57 bits
57 bits
26 bits
3
3
Training sequence
Tail Bits
Tail Bits
Flag Bit
Flag Bit
Guard Period
Guard Period
Data
Data

• Transmitted on the unused timeslots of the BCCH
  carrier in the downlink.

65
CHANNEL CONCEPT
ACCESS BURST
FRAME1(4.615ms)
FRAME2
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
0.577ms
41 bits
68.25 bits
8
36 bits
3
Synchronisation Sequence
Tail Bits
Tail Bits
Guard Period
Encrypted Bits

• Carries RACH.
• Has a bigger guard period since it is used during
  initial access and the MS does not know how far
  it is actually from the BTS.

66
CHANNEL CONCEPT
NEED FOR TIMESLOT OFFSET
BSS Downlink
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
MS Uplink
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7

• If Uplink and Downlink are aligned exactly, then
  MS will have to transmit and receive at the same
  time. To overcome this problem a offset of 3
  timeslots is provided between downlink and uplink

67
CHANNEL CONCEPT
NEED FOR TIMESLOT OFFSET
BSS Downlink
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
0
MS Uplink
5
6
7
0
1
2
3
4
5
6
7
0
1
2
3
4
5
3 timeslot offset

• As seen the MS does not have to transmit and
  receive at the same time. This simplifies the MS
  design which can now use only one synthesizer.

68
CHANNEL CONCEPT
26 FRAME MULTIFRAME STRUCTURE
4.615 msec
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
T 15
T 5
T 9
T 10
T 11
S 12
T 13
T 14
T 6
T 7
T 8
T 0
T 1
T 2
T 3
T 4
T 16
T 17
T 18
T 19
T 20
T 21
T 22
T 23
T 24
I 25
120 msec

• MS on dedicated mode on a TCH uses a 26-frame
  multiframe structure.
• Frame 0-11 and 13-24 used to carry traffic.
• Frame 12 used as SACCH to carry control
  information from and to MS to BTS.
• Frame 25 is idle and is used by mobile to decode
  the BSIC of neighbor cells.

69
BCCH/CCCH NON-COMBINED MULTIFRAME
Downlink
Uplink
50
50
IDLE
CCCH BLOCK
BCCH BLOCK
SCH BLOCK
FCCH BLOCK
40
40
RACH BLOCK
30
30
20
20
10
10
0
0
70
BCCH/CCCH COMBINED MULTIFRAME
Downlink
Uplink
50
50
101
101
IDLE
CCCH BLOCK
BCCH BLOCK
SCH BLOCK
FCCH BLOCK
40
40
RACH BLOCK
SDCCH/4
SACCH/4
30
30
20
20
10
10
0
0
51
51
71
DCCH/8 MULTIFRAME
Downlink
Uplink
50
50
101
101
IDLE
SDCCH/8
SACCH/C8
40
40
30
30
20
20
10
10
0
0
51
51
72
CHANNEL CONCEPT
HYPERFRAME AND SUPERFRAME STRUCTURE
3h 28min 53s 760ms
1 Hyperframe 2048 superframes 2,715,648 TDMA
frames
0
1
2
2045
2046
2047
1 Superframe 1326 TDMAframes 51(26 fr) 0r
26(51 fr) multiframes
6.12s
1
2
3
49
48
47
50
0
0
1
24
25
235.38ms
120ms
0
1
2
23
24
25
0
48
1
2
49
50
Control 51 - Frame Multiframe
Traffic 26 - Frame Multiframe
4.615ms
TDMA Frame
0
1
2
3
4
5
6
7
73
CODING, INTERLEAVING CIPHERING
SPEECH CODING
SPEECH DECODING
CHANNEL CODING
CHANNEL DECODING
INTERLEAVING
DEINTERLEAVING
BURST ASSEMBLING
BURST DISASSEMBLING
CIPHERING
DECIPHERING
Transmission
MODULATION
DEMODULATION
74
CODING
SPEECH CODING

• The transmission of speech is one of the most
  important service of a mobile cellular system.
• The GSM speech codec, which will transform the
  analog signal(voice) into a digital
  representation, has to meet the following
  criterias
• A good speech quality, at least as good as the
  one obtained with previous cellular systems.
• To reduce the redundancy in the sounds of the
  voice. This reduction is essential due to the
  limited capacity of transmission of a radio
  channel.
• The speech codec must not be very complex because
  complexity is equivalent to high costs.
• The final choice for the GSM speech codec is a
  codec named RPE-LTP (Regular Pulse Excitation
  Long-Term Prediction).

75
CODING
SPEECH CODING

• This codec uses the information from previous
  samples (this information does not change very
  quickly) in order to predict the current sample.
• The speech signal is divided into blocks of 20
  ms. These blocks are then passed to the speech
  codec, which has a rate of 13 kbps, in order to
  obtain blocks of 260 bits.

76
CODING
CHANNEL CODING

• Channel coding adds redundancy bits to the
  original information in order to detect and
  correct, if possible, errors ocurred during the
  transmission.
• The channel coding is performed using two codes
  a block code and a convolutional code.
• The block code receives an input block of 240
  bits and adds four zero tail bits at the end of
  the input block. The output of the block code is
  consequently a block of 244 bits.
• A convolutional code adds redundancy bits in
  order to protect the information. A convolutional
  encoder contains memory. This property
  differentiates a convolutional code from a block
  code.
• A convolutional code can be defined by three
  variables n, k and K.
• The value n corresponds to the number of bits at
  the output of the encoder, k to the number of
  bits at the input of the block and K to the
  memory of the encoder.

77
CODING
CHANNEL CODING ( Cont )

• The ratio, R, of the code is defined as R k/n.
• Example - Let's consider a convolutional code
  with the following values k is equal to 1, n to
  2 and K to 5. This convolutional code uses then a
  rate of R 1/2 and a delay of K 5, which means
  that it will add a redundant bit for each input
  bit. The convolutional code uses 5 consecutive
  bits in order to compute the redundancy bit. As
  the convolutional code is a 1/2 rate
  convolutional code, a block of 488 bits is
  generated. These 488 bits are punctured in order
  to produce a block of 456 bits. Thirty two bits,
  obtained as follows, are not transmitted
• C (11 15 j) for j 0, 1, ..., 31
• The block of 456 bits produced by the
  convolutional code is then passed to the
  interleaver

Convolution code R k/n 1/2
n2 2 bit input
k1 1 bit input
78
CODING
CHANNEL CODING FOR GSM SPEECH CHANNELS

• Before applying the channel coding, the 260 bits
  of a GSM speech frame are divided in three
  different classes according to their function and
  importance.
• The most important class is the class 1a
  containing 50 bits.Next important is the class
  1b, which contains 132 bits.The least important
  is the class 2, which contains the remaining 78
  bits.
• The different classes are coded differently.
• First of all, the class 1a bits are block-coded.
  Three parity bits, used for error detection, are
  added to the 50 class 1a bits.The resultant 53
  bits are added to the class 1b bits.
• Four zero bits are added to this block of 185
  bits (503132). A convolutional code, with r
  1/2 and K 5, is then applied, obtaining an
  output block of 378 bits.
• The class 2 bits are added, without any
  protection, to the output block of the
  convolutional coder. An output block of 456 bits
  is finally obtained.

79
CODING
Speech Channel Coding
260 bits
Class 1a 50 bits
Class 1b 132 bits
Class 2 78 bits
Parity check
Tail bits
Class 1a 50 bits
Class 1b 132 bits
3
4
Convolution coding
378 bits
456 bits
80
CODING
CHANNEL CODING FOR CONTROL CHANNELS

• In GSM the signalling information is just
  contained in 184 bits.
• Forty parity bits, obtained using a fire code,
  and four zero bits are added to the 184 bits
  before applying the convolutional code (r 1/2
  and K 5). The output of the convolution code is
  then a block of 456 bits which does not need to
  be punctured.

Parity bits
184 bits
Fire code
Tail bits
184 bits
40 bits
4
Convolution coding
456 bits
81
CODING
CHANNEL CODING FOR DATA CHANNELS

• In data information is contained in 240 bits.
• Four tails bits are added to the 240 bits before
  applying the convolutional code (r 1/2 and K
  5). The output of the convolutional code is then
  a block of 488 bits which when punctuated yields
  456 bits.

240 bits
Tail bits
240 bits
4
Convolution coding
488 bits
Punctuate
456 bits
82
INTERLEAVING
INTERLEAVING

• An interleaving rearranges a group of bits in a
  particular way.
• It is used in combination with FEC codes( Forward
  Error Correction Codes ) in order to improve the
  performance of the error correction mechanisms.
• The interleaving decreases the possibility of
  losing whole bursts during the transmission, by
  dispersing the errors.
• As the errors are less concentrated, it is then
  easier to correct them.

83
INTERLEAVING
GSM SPEECH CHANNEL INTERLEAVING

• A burst in GSM transmits two blocks of 57 data
  bits each.
• Therefore the 456 bits corresponding to the
  output of the channel coder fit into 8 57 data
  bits (8 57 456). The 456 bits are divided
  into eight blocks of 57 bits.
• The first block of 57 bits contains the bit
  numbers (0, 8, 16, .....448), the second one the
  bit numbers (1, 9, 17, .....449), etc.
• The last block of 57 bits will then contain the
  bit numbers (7, 15, .....455).
• The first four blocks of 57 bits are placed in
  the even-numbered bits of four consecutive
  bursts.
• The other four blocks of 57 bits are placed in
  the odd-numbered bits of the next four bursts.
• The interleaving depth of the GSM interleaving
  for speech channels is eight.
• A new data block also starts every four bursts.
  The interleaver for speech channels is called a
  block interleaver.

84
INTERLEAVING
GSM SPEECH CHANNEL INTERLEAVING ( Diagram )
Full rate encoded speech blocks from a single
conversation
4
1
2
3
5
6
4 456 bits
5 456 bits
6 456 bits
Bursts
TDMA Frames
Frame 1
Frame 2
Frame 3
Frame 4
85
INTERLEAVING
CONTROL CHANNEL INTERLEAVING

• A burst in GSM transmits two blocks of 57 data
  bits each.
• Therefore the 456 bits corresponding to the
  output of the channel coder fit into four bursts
  (4114 456).
• The 456 bits are divided into eight blocks of 57
  bits. The first block of 57 bits contains the bit
  numbers (0, 8, 16, .....448), the second one the
  bit numbers (1, 9, 17, .....449), etc. The last
  block of 57 bits will then contain the bit
  numbers (7, 15, .....455).
• The first four blocks of 57 bits are placed in
  the even-numbered bits of four bursts.
• The other four blocks of 57 bits are placed in
  the odd-numbered bits of the same four bursts.
• Therefore the interleaving depth of the GSM
  interleaving for control channels is four and a
  new data block starts every four bursts.
• The interleaver for control channels is called a
  block rectangular interleaver.

86
INTERLEAVING
DATA INTERLEAVING

• A particular interleaving scheme, with an
  interleaving depth equal to 22, is applied to the
  block of 456 bits obtained after the channel
  coding.
• The block is divided into 16 blocks of 24 bits
  each, 2 blocks of 18 bits each, 2 blocks of 12
  bits each and 2 blocks of 6 bits each.
• It is spread over 22 bursts in the following way
  
• the first and the twenty-second bursts carry one
  block of 6 bits each
• the second and the twenty-first bursts carry one
  block of 12 bits each
• the third and the twentieth bursts carry one
  block of 18 bits each
• from the fourth to the nineteenth burst, a block
  of 24 bits is placed in each burst
• A burst will then carry information from five or
  six consecutive data blocks. The data blocks are
  said to be interleaved diagonally.
• A new data block starts every four bursts.

87
MODULATION
CIPHERING

• Ciphering is used to protect signaling and user
  data.
• A ciphering key is computed using the algorithm
  A8 stored on the SIM card, the subscriber key and
  a random number delivered by the network (this
  random number is the same as the one used for the
  authentication procedure).
• A 114 bit sequence is produced using the
  ciphering key, an algorithm called A5 and the
  burst numbers.
• This bit sequence is then XORed with the two 57
  bit blocks of data included in a normal burst.
• In order to decipher correctly, the receiver has
  to use the same algorithm A5 for the deciphering
  procedure.
• MODULATION
• Modulation is done using 0.3 GMSK

88
Other Networks
SIGNALLING
89
SIGNALLING SYSTEM
WHAT IS SIGNALLING ?

• The term signaling is used in many contexts.
• In technical systems, it very often refers to the
  control of different procedures.
• With reference to telephony, signaling means the
  transfer of information and the instructions
  relevant to control and monitor telephony
  connections.

90
SIGNALLING SYSTEM C7

• GENERAL INTRODUCTION
• Todays global telecom networks are included in
  very complex technical systems.
• Naturally, a system of this type requires
  extensive signaling, both internally in different
  nodes (for example, exchanges) and externally
  between different types of network nodes.
• During this training we will focus on external
  signaling.
• Thus, the term signaling in the following slides
  always refers to external signaling traffic.
• The main purpose of using signaling in modern
  telecom networks where different network nodes
  must cooperate and communicate with each other
  is to enable transfer of control information
  between nodes in connection with
• Traffic control procedures as set-up,
  supervision, and release of telecommunication
  connections and services

91

• GENERAL INTRODUCTION
• Database communication, for example, database
  queries concerning specific services, roaming in
  cellular networks, etc.
  
• Network management procedures, for example,
  blocking or deblocking trunks.
• Traditionally, external signaling has been
  divided into two basic types
• Access signaling (for example, Subscriber Loop
  Signaling) This means signaling between a
  subscriber terminal (telephone) and the local
  exchange.
• Trunk signaling (that is, Inter-Exchange
  Signaling) This is used for signaling between
  exchanges.

92
SIGNALING IN TELECOMMUNICATION NETWORK

SIGNALLING
ACCESS SIG
TRUNK SIGNALLING
SUBSCRIBER LINE SIG.
CHANNEL ASSOCIATED SIG.
DIGITAL SUBSCRIBER SIG.
COMMON CHANNEL SIG.
93

• Access Signaling
• There are many types of access signaling, for
  example, PSTN analogue subscriber line signaling,
  ISDN Digital Subscriber Signaling System (DSS1),
  and signaling between the MS and the network in
  the GSM system.
• Signaling on the analogue subscriber line between
  a telephony subscriber and the Local Exchange
  (LE) is performed by means of on/off hook
  signals, dialed digits, information tones (dial
  tone, busy tone, etc.), recorded announcements,
  and ringing signals.
• The dialed digits can be sent in two different
  ways as decadic pulses (used for old-type
  rotary-dial telephones), or as a combination of
  two tones (used for modern pushbutton
  telephones). The latter system is known as the
  Dual Tone Multi Frequency (DTMF).
• The information tones (dial tone, ringing tone,
  busy tone, etc.) are audio signals used to keep
  the calling party (the A-subscriber) informed
  about what is going on in the network during the
  set-up of a call.

94

• Access Signaling
• Digital Subscriber Signaling System No. 1 (DSS1)
  is the standard access signaling system used in
  ISDN. It is also called a D-channel signaling
  system
• D-channel signaling is defined for digital access
  lines only.
• The signaling protocols are based on the OSI
  (Open System Interconnection) reference model,
  layers 1 to 3.
• Consequently, the signaling messages are
  transferred as data packets between the user
  terminal and the local exchange.
• Due to the much more complex service environment
  at the ISDN users site, the amount of signaling
  information and the number of variations

95

• Trunk Signaling
• The Inter-exchange Signaling information is
  usually transported on one of the time slots in a
  PCM link, either in association with the speech
  channel or independently.
• There are two commonly used methods for Inter
  Exchange Signaling.
• Channel Associated Signaling (CAS)
• In CAS, the speech channel (in-band), or a
  channel closely associated with a speech channel
  (out-band), is used for signaling.
• Common Channel Signaling (CCS)
• In this case a dedicated channel, completely
  separate from the speech channel, is used for
  signaling. Due to the high capacity, one
  signaling channel in CCS can serve a large number
  of speech channels.
• In a GSM network, CCITT Signaling System No. 7 is
  used.
• Signaling System No. 7 is a Common Channel
  Signaling system.

96

• CHANNEL ASSOCIATED SIGNALING (CAS)
• Channel Associated Signaling (CAS) means that the
  signaling is always sent on the same connection
  (PCM link) as the traffic.
• The signaling is associated with the traffic
  channel.
• In a 2 Mb/s PCM link, 30 time slots are used for
  speech, whereas TS 0 is used for synchronization
  and TS 16 is used for the line signaling.
• All 30 traffic connections share TS 16 in a
  multiframe consisting of 16 consecutive frames.
• On TS 16, each traffic channel has a permanently
  allocated recurring location for line signaling,
  where two traffic channels share TS 16 in one
  frame.

97

• COMMON CHANNEL SIGNALING (CCS)
• In CCS, signaling messages (or data packets) are
  transmitted over time slots in a PCM link
  reserved for the purpose of signaling.
• The system is designed to use a common data
  channel (or signaling link) as the carrier of all
  signals, required by a large number of traffic
  channels.
• In 1968, CCITT specified a Common Channel
  Signaling system called CCS System No. 6, which
  was designed especially for international
  analogue telephony networks.
• However, very few installations of this system
  remain today. It has, as already mentioned, been
  replaced by Signaling System No. 7.
• The first version of SS7 (1980) was designed for
  telephony and data.
• In the 80s the demand for new services
  dramatically increased and the SS7 was therefore
  developed to meet the signaling requirements,
  specified for all these new services.
• Today SS7 is used in many different networks and
  related services typically betn PSTN, ISDN, PLMN
  IN services throughout the world.

98

• OSI REFERENCE MODEL
• The Signaling System No. 7, which is a type of
  packet switched data communication system, is
  structured in a modular and layered way.
• Such a design of SS7 is similar to the Open
  System Interconnection model.
• Open Systems are systems that use standardized
  communication procedures developed from the
  reference model.
• Thus, all such open systems are able to
  communicate with each other.
• The word system can refer to computers,
  exchanges, data networks, etc.

99




OSI MODEL REFERENCE DIAGRAM
APPLICATION
APPLICATION
PRESENTATION
PRESENTATION
SESSION
SESSION
TRANSPORT
TRANSPORT
NETWORK
NETWORK
LINK
LINK
PHYSICAL
PHYSICAL
100

• COMMUNICATION PROCESS
• Each layer has its own specified functions and
  provides specific services for the layers above.
• It is important to define the interfaces between
  different layers and the functions within each
  layer.
• The way a function is realized within a layer is
  not predicted.
• Logically, the communication between functions
  always takes place on the same level according to
  the protocols for that level.
• Only functions on the same level can talk to
  each other.
• In the transmitting system, the protocol for each
  layer adds information to the data received from
  the layer above.
• The addition usually consists of a header and/or
  a trailer.
• In the receiving system, the additions are used,
  for example, to identify bits or data fields
  carrying information for that specific layer
  only.
• These fields are decoded by layer functionality
  and are removed when delivering the message to
  the applications orlayers above.

101

• When the data reaches the application layer on
  the receiving side, it consists of only the data
  that originated in the application layer of the
  sending system.
• Logically, each layer communicates with the
  corresponding layer in the other system.
• This communication is called Peer-to-Peer
  communication and is controlled by the layers
  protocol.
• DESCRIPTION OF LAYERS
• Application Layer
• This layer provides services for support of the
  users application process and for control of all
  communication between applications.
• Examples of layer 7 functions are file transfer,
  message handling, directory services, and
  operation and maintenance.

102

• Presentation Layer
• This layer defines how data is to be represented,
  that is, the syntax.
• The presentation layer transforms the syntax used
  in the application into the common syntax needed
  for the communication between applications.
• Layer 6 contains data compression.
• Session Layer
• This layer establishes connections between
  presentation layers in different systems.
• It also controls the connection, the
  synchronization and the disconnection of the
  dialogue.
• It allows the presentation layer to determine
  checkpoints, from which the retransmission will
  start when the data transmission has been
  interrupted.

103

• Transport Layer
• This layer guarantees that the bearer service has
  the quality required by the application in
  question.
• Examples of functions are error detection and
  correction (end-to-end), and flow control.
• The transport layer optimizes the data
  communication, for example by multiplexing or
  splitting data streams before they reach the
  network.

104

• Network Layer
• The basic network layer service is to provide a
  transparent channel.
• This means that the application requesting a
  channel ignores network problems and the related
  signal exchange because that is the task of the
  lower levels.
• It just requires an open channel, transparent for
  the transmission of data, between transport
  layers in different systems.
• The Network Layer establishes, maintains, and
  releases connections between the nodes in the
  network and handles addressing and routing of
  circuits.
• Data Link Layer
• This layer provides an essentially error-free
  point-to-point circuit between network layers.
• The layer contains resources for error detection,
  error correction, flow control, and
  retransmission.

105

• Physical Layer
• This layer provides mechanical, electrical,
  functional, and procedural resources for
  activating, maintaining, and blocking physical
  circuits for the transmission of bits between
  data link layers.
• The physical layer contains functions for
  converting data into signals compatible with the
  transmission medium.
• For the communication between only two exchanges,
  layers 1 and 2 are sufficient.
• For the communication between all exchanges in
  the network, layer 3 must be added because it
  provides addressing and routing.

106

• SIGNALING SYSTEM NO. 7 INTRODUCTION
• The Signaling System (SS)No. 7 is an elaborate
  set of recommendations defining protocols for the
  internal management of digital networks.
• These recommendations were introduced in 1980 and
  revised in 1984 and 1988 in different-colored
  books (yellow, red, and blue).
• CCITT SS No. 7 is intended primarily for digital
  networks, both national and international, where
  the high transmission rates (64 kbps) can be
  exploited.
• It may also be used on analogue lines especially
  on international trunks (CCITT SS No 6).
• CCS was initially meant for telephony only, but
  has now evolved into non-telephony and
  non-connection related applications (for example,
  location updating of a mobile subscriber).
• A dialogue with a database or between two
  databases is a typical application for CS in GSM.

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• Thus, there is a need for a generic system that
  is able to support a wide variety of applications
  in telecommunication.
• The variety of applications is increasing as new
  types of telephony systems and a wider use of
  databases in the network become necessary (mobile
  telephony networks, I