introduction to ubr. introduction . ubr stants for unspecified bit rate. ubr allows service...

INTRODUCTION TO UBRINTRODUCTION TO UBR

INTRODUCTIONINTRODUCTION

. UBR STANTS FOR UNSPECIFIED BIT RATE.

UBR allows Service Providers to deliver a national broadband service with Unbundled bitstream Service (UBS).It provide a service to deliver traffic

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INTRODUCTION TOINTRODUCTION TO GSM GSM

CELLULAR TELEPHONY

PUT ONE IMAGE WITH ITS WORKING SINGNAL LIKE BELOW SHOWN

3

Radio

Mobile subscriber

Cell

INTRODUCTION TO GSMINTRODUCTION TO GSM

GSM FREQUENCIES PUT HERE FREQUENCIES ON WHICH IT WORKGSM FREQUENCIES PUT HERE FREQUENCIES ON WHICH IT WORK

• 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

4

890 960935915

UPLINK FREQUENCIES DOWNLINK FREQUENCIES

UPLINK AND DOWNLINK FREQUENCY SEPARATED BY 45MHZ



890 915 935 960880 925


Extended GSM (EGSM)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.

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1710 MHz 1880 MHz1805 MHz1785 MHz



DCS1800 FREQUENCIESDCS1800 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.

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FEATURES OF GSMFEATURES OF GSM

INCREASED CAPACITYINCREASED 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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AUDIO QUALITYAUDIO 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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IMPROVED SECURITY AND CONFIDENTIALITYIMPROVED 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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CLEANER HANDOVERSCLEANER 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 IDENTIFICATIONSUBSCRIBER 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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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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FREQUENCY REUSEFREQUENCY 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.

6

43

772

• 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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55

12

1


NETWORK COMPONENTSNETWORK COMPONENTS

MSC

EC

PSTN

AUC HLREIR VLRNMC

OMC-S

OMC-R

IWF

XCDR

BTS BTS

BSC


UM

UM

ABIS

ABC

D

F

H

MMobile obile SSwitching witching CCentre (MSC)entre (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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MMobile obile SSwitching witching CCentre (MSC) – Lucent MSCentre (MSC) – Lucent MSC


Mobile StationMobile Station (MS) (MS)

• The Mobile Station consists of the Mobile Equipment (ME) and the Subscriber Identity Module (SIM).

Mobile EquipmentMobile 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.

CLASS POWER OUTPUT

1 20W2 8W3 5W4 2W5 0.8W


SIMSIM

• 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.


SIMSIM

• 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.

G S M

FULL SIZE SIM CARD MINI SIM CARD


98 XXX 12345

CCNDCSN

CC NDC SN

= Country code= National Destination Code= Subscriber Number 22

Mobile Station International Subscribers Dialling NumberMobile Station International Subscribers Dialling Number ( MSISDN ) :( 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.


MCC MNC MSIN

404 XX 12345..10

MCCMNCMSIN

= Mobile Country Code ( 3 Digits )= Mobile Network Code ( 2 Digits )= Mobile Subscriber Identity Number

International Mobile Subscribers Identity ( IMSI ) :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.


Temporary Mobile Subscribers Identity ( TMSI ) :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.


Equipment Identity Register ( EIR )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 MSC’s in the Network and can also be accessed by an MSC in a different PLMN.

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Equipment Identity Register ( EIR )Equipment Identity Register ( EIR )

White List

All Valid assigned ID’s

Range 1Range 2

Range n

Black List

Service denied

MS IMEI 1MS IMEI 2

MS IMEI n

Grey List

Service allowedbut noted

MS IMEI 1MS IMEI 2

MS IMEI n

EIR


TAC FAC SNR

6 2 6 1

TACFACSNRSP

SP

= Type Approval Code= Final Assembly Code= Serial Number= Spare

International Mobile Equipment Identity ( IMEI ) :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.


HOME LOCATION REGISTER( HLR )HOME LOCATION REGISTER( HLR )

• The HLR contains the master database of all subscribers in the PLMN.

• This data is remotely accessed by the MSC´´s 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


VISITOR LOCATION REGISTER ( VLR )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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• 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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Authentication Centre ( AUC Authentication Centre ( AUC ))


AUTHENTICATION PROCESSAUTHENTICATION PROCESS

HLR AUC

TRIPLESRAND, Kc , SRES

VLR

RAND Kc SRES

RAND

SRES

SRES

SRES = SRESBTS

A5 , HYPERFRAME NUM

Kc

AIR INTERFACE ENCRYPTION

MS

A3 , A8 , A5 , Ki

SRES = A3 (RAND , Ki )

Kc = A8 (RAND , Ki )

Ki, A3, A8A3 ( RAND, Ki ) = SRES

A8 ( RAND, Ki ) = KcTriples

Generated


Base Station Sub-System ( BSS ) :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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Transcoder( XCDR )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 digitised 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).


TRANSCODER(XCDR) - SiemensTRANSCODER(XCDR) - Siemens


TRANSCODING

30 Timeslots1 traffic channel / TS

64 Kbps / TS4 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

0 1 2 3116

Transcoded information from four calls


BBase Station Controller (Base Station Controller (BSCSC))

• 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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BBase Station Controller (Base Station Controller (BSCSC) – Siemens BSC) – Siemens BSC


Base Transceiver StationBase Transceiver Station (BTS (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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Base Transceiver StationBase Transceiver Station (BTS (BTS) )


MSC BSC BTS12

BTS1

BTS2

BTS4

BTS3BTS11

BTS13 BTS14

BTS5

BTS6

BTS7

BTS8

BTS9

BTS11

Open ended Daisy Chain

Daisy Chain with a fork. Fork has a return loop back to the chain

Star

Daisy Chain with a fork. Fork has a return loop back to the chain

BTS ConnectivityBTS Connectivity


• 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 analyse 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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Operation And Maintenance Centre For Radio (Operation And Maintenance Centre For Radio (OMC-ROMC-R))


Operation And Maintenance Centre For Radio (Operation And Maintenance Centre For Radio (OMC-ROMC-R))


• 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 Cell’s using the same BCCH frequencies

BBase Station Identity Codease Station Identity Code

BCC00 BCCNCC

7 6 5 4 3 2 1 0


• The MS is identified by it’s classmark which the mobile sends during it’s 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.

MS CMS Classlass M Markark


MOBILE MAXIMUM RANGE

RANGE=

TIMING ADVANCE = DELAY OF BITS (0-63)

BIT PERIOD= 577/156.25 = 3.693sec =3.693 * 10e-6 sec

VELOCITY= 3 * 10e5 Km/sec

RANGE= 34.9 Km

TIMIMG ADVANCE * BIT PERIOD* VELOCITY

2

MULTIPLE ACCESS TECHNIQUESMULTIPLE 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 )

TERRESTERIAL INTERFACETERRESTERIAL 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 )

•

INTERFACE NAMESINTERFACE 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

2 Mbits/s Trunk 30- channel PCM

This interface carries the traffic from the PSTN to the MSC, between MSC’s, from the MSC to the BSC’s and from the BSC’s to the BTS’s.

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 ConfigurationTypical Configuration

TS 0 TS 1-15 TS 16 TS 17 - 31

TS 0 - Frame allignment/ Error checking/ Signalling/ AlarmsTS 1-15 , 17-31 - TrafficTS 16 - Siganlling

BSS CONNECTIONSBSS CONNECTIONS

MSC

XCDR

BSC

BTS BTSBTS

OMC

CBC

MTL(C7 )

OML (X.25)

RSL ( LAPD)

CBL

Cell Global Identity ( CGI ) :Cell Global Identity ( CGI ) :

MCCMNCLACCI

MCC MNC LAC CI

LAI

CGI

= Mobile Country Code= Mobile Network Code= Location Area Identity= Cell Identity

Physical channelPhysical channel - Each timeslot on a carrier is referred to as a physical channel. Per carrier there are 8 physical channels.Logical channelLogical 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

Downlink

Uplink

CHANNELSCHANNELS

CHANNEL CONCEPTCHANNEL CONCEPT

GSM Traffic ChannelsGSM Traffic Channels

Traffic Channels

TCH/FFull rate 22.8kbits/s

TCH/HHalf rate 11.4 kbits/s


GSM Control ChannelsGSM Control Channels

BCH ( Broadcast channels )Downlink only

Control Channels

DCCH(Dedicated Channels)Downlink & Uplink

CCH(Common Control Chan)Downlink & Uplink

Synch.

ChannelsRACH

Random Access Channel

CBCHCell Broadcast

Channel

SDCCHStandalone dedicated

control channel

ACCHAssociated

Control Channels

SACCHSlow associated Control Channel

FACCHFast Associated

Control Channel

PCH/AGCH

Paging/Access grant

FCCHFrequency

Correction channel

SCHSynchronisation

channel

BCCHBroadcast

control channel


BCH ChannelsBCH ChannelsBCCH( Broadcast Control Channel )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 )SCH( Synchronisation Channel )• Downlink only• Carries information for frame synchronisation. Contains TDMA frame

number and BSIC.FCCH( Frequency Correction Channel )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.


CCCH ChannelsCCCH ChannelsRACH( Random Access Channel )RACH( Random Access Channel )

• Uplink only• Used by the MS to access the Network.

AGCH( Access Grant Channel )AGCH( Access Grant Channel )• Downlink only• Used by the network to assign a signalling channel upon

successfull decoding of access bursts.

PCH( Paging Channel )PCH( Paging Channel )• Downlink only.• Used by the Network to contact the MS.


DCCH ChannelsDCCH ChannelsSDCCH( Standalone Dedicated Control Channel )SDCCH( Standalone Dedicated Control Channel )

• Uplink and Downlink• Used for call setup, location update and SMS.

SACCH( Slow Associated Control Channel )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 )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.


NORMAL BURSTNORMAL BURST

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

57 bits 57 bits26 bits 33

FRAME1(4.615ms)FRAME2

Training sequence

Data DataTailBits

TailBits

FlagBit

FlagBit

GuardPeriod

GuardPeriod

0.546ms0.577ms

Carries traffic channel and control channels BCCH, PCH, AGCH, SDCCH, SACCH and FACCH.


DataData - 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 periodGuard 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 bitsFlag bits - This bit is used to indicate if the 57 bits data block is used as FACCH.Training SequenceTraining 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.

NORMAL BURSTNORMAL BURST


FREQUENCY CORRECTION BURSTFREQUENCY CORRECTION BURST

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

142 bits 33

FRAME1(4.615ms) FRAME2

Fixed DataTailBits

TailBits

GuardPeriod

GuardPeriod

0.546ms0.577ms

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


SYNCHRONISATION BURSTSYNCHRONISATION BURST

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

39 bits 33


SynchronisationSequence

TailBits

TailBits

GuardPeriod

GuardPeriod

0.546ms0.577ms

64 bits 39 bits

EncryptedBits

EncryptedBits

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


DUMMY BURSTDUMMY BURST

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

57 bits 57 bits26 bits 33


Training sequence

Data DataTailBits

TailBits

FlagBit

FlagBit

GuardPeriod

GuardPeriod

0.546ms0.577ms

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


AACCESS BURSTCCESS BURST

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

41 bits 68.25 bits8


TailBits

TailBits

GuardPeriod

0.577ms

36 bits

SynchronisationSequence

EncryptedBits

3

• 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.


NEED FOR TIMESLOT OFFSETNEED FOR TIMESLOT OFFSET

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

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

BSS Downlink

MS Uplink


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

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

• 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.

BSS Downlink

MS Uplink

5

0

3 timeslotoffset

CHANNEL CONCEPTCHANNEL CONCEPTNEED FOR TIMESLOT OFFSETNEED FOR TIMESLOT OFFSET

T15

T5

T9

T10

T11

S12

T13

T14

T6

T7

T8

T0

T1

T2

T3

T4

T16

T17

T18

T19

T20

T21

T22

T23

T24

I25

00 11 22 33 44 55 66 77 00 11 22 33 44 55 66 77 00 11 22 33 44 55 66 77

120 msec

4.615 msec26 26 FRAME MULTIFRAME STRUCTUREFRAME MULTIFRAME STRUCTURE

• 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.


BCCH

CCCH

BCCH

CCCH

BCCH

CCCH

BCCH

CCCH

CCCH

CCCH

CCCH

CCCH

CCCH

CCCH

CCCH

CCCH

0

10

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30

40

50 50

40

30

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10

0

IDLECCCH BLOCKBCCH BLOCKSCH BLOCKFCCH BLOCK

BCCH/CCCH NON-COMBINED MULTIFRAMEBCCH/CCCH NON-COMBINED MULTIFRAME

RACH BLOCK

DownlinkDownlink UplinkUplink

0

10

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50 50

40

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0

IDLECCCH BLOCKBCCH BLOCKSCH BLOCKFCCH BLOCK

BCCH/CCCH COMBINED MULTIFRAMEBCCH/CCCH COMBINED MULTIFRAME

51

101 101

51

BCCH

CCCH

BCCH

CCCH

CCCH

BCCH

CCCH

CCCH

CCCH

SACCH

SACCH

CCCH

CCCH

CCCH

CCCHSDCCH

SDCCH

SDCCH

SDCCH

BCCH

CCCH

BCCH

CCCH

CCCH

BCCH

CCCH

CCCH

CCCH

SACCH

SACCH

CCCH

CCCH

CCCH

CCCHSDCCH

SDCCH

SDCCH

SDCCH

BCCHCCCHSDCCH

CCCHSACCH

CCCHSACCH

CCCHCCCHCCCHSDCCH

CCCHCCCHCCCHSDCCH

CCCHCCCHCCCHSDCCH

BCCHCCCHSDCCH

CCCHSACCH

CCCHSACCH

CCCHCCCHCCCHSDCCH

CCCHCCCHCCCHSDCCH

CCCHCCCHCCCHSDCCHRACH BLOCKSDCCH/4 SACCH/4


0

10

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40

50 50

40

30

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0

IDLE

DCCH/8 MULTIFRAMEDCCH/8 MULTIFRAME

51

101 101

51

SDCCH/8 SACCH/C8

BCCH

CCCH

A2

A3

BCCH

CCCH

A0

A1

CCCHCCCHCCCHD7

CCCHCCCHCCCHD6

CCCHCCCHCCCHD5

CCCHCCCHCCCHD4

CCCHCCCHCCCHD3

CCCHCCCHCCCHD2

CCCHCCCHCCCHD1

CCCHCCCHCCCHD0

BCCH

CCCH

A6

A7

BCCH

CCCH

A4

A5

CCCHCCCHCCCHD7

CCCHCCCHCCCHD6

CCCHCCCHCCCHD5

CCCHCCCHCCCHD4

CCCHCCCHCCCHD3

CCCHCCCHCCCHD2

CCCHCCCHCCCHD1

CCCHCCCHCCCHD0

BCCHA0

CCCHCCCHCCCHD7

CCCHCCCHCCCHD6

CCCHCCCHCCCHD5

CCCHCCCHCCCHD4

CCCHCCCHCCCHD3

CCCHCCCHCCCHD2

CCCHCCCHCCCHD1

CCCHCCCHCCCHD0

BCCH

CCCH

A6

A7

CCCHA5

BCCHA4

CCCHCCCHCCCHD7

CCCHCCCHCCCHD6

CCCHCCCHCCCHD5

CCCHCCCHCCCHD4

CCCHCCCHCCCHD3

CCCHCCCHCCCHD2

CCCHCCCHCCCHD1

CCCHCCCHCCCHD0

BCCH

CCCH

A2

A3

CCCHA1


00 11

00 11 22 20452045 20462046 20472047

1 Hyperframe = 2048 superframes = 2,715,648 TDMA frames3h 28min 53s 760ms

1 Superframe = 1326 TDMAframes = 51(26 fr) 0r 26(51 fr) multiframes

11 2 33 494847 50

00 11 2424 2525

00 11 22 2323 2424 2525 00 484811 22 4949 5050

22 33 44 55 66 77

6.12s

0

235.38ms120ms

Control 51 - Frame MultiframeTraffic 26 - Frame Multiframe

4.615ms

TDMA Frame

HYPERFRAME AND SUPERFRAME STRUCTUREHYPERFRAME AND SUPERFRAME STRUCTURE


SPEECHCODING

CHANNELCODING

INTERLEAVING

BURSTASSEMBLING

CIPHERING

MODULATION DEMODULATION

DECIPHERING

BURSTDISASSEMBLING

DEINTERLEAVING

CHANNELDECODING

SPEECHDECODING

Transmission

CODING, INTERLEAVING CIPHERINGCODING, INTERLEAVING CIPHERING

• 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).

SPEECH CODSPEECH CODIINGNGCODINGCODING

• 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.

SPEECH CODSPEECH CODIINGNG

CODINGCODING

CHANNEL CODINGCHANNEL 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.

CODINGCODING

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 = 1/2k=11 bit input

n=22 bit input

CODINGCODING

CHANNEL CODING FOR GSM SPEECH CHANNELSCHANNEL 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 (50+3+132). 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.

CODINGCODING

Speech Channel Coding

Class 1a50 bits

Class 1b132 bits

Class 1a50 bits

Class 1b132 bits

378 bits

Class 278 bits

3 4

Paritycheck

Tailbits

260 bits

456 bits

Convolution coding

CODINGCODING

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.

184 bits

184 bits

456 bits

Firecode

Tailbits

Convolution coding

40 bits 4

Paritybits

CODINGCODING

CHANNEL CODING FOR DATA CHANNELSCHANNEL 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

240 bits

488 bits

Tailbits

Convolution coding

4

456 bits

Punctuate

CODINGCODING

INTERLEAVININTERLEAVINGG

• 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.

INTERLEAVINGINTERLEAVING

GSM SPEECH CHANNEL INTERLEAVINGGSM 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.


4

GSM SPEECH CHANNEL INTERLEAVING ( Diagram )GSM SPEECH CHANNEL INTERLEAVING ( Diagram )

1 2 3 5 6

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

4456 bits

5456 bits

6456 bits

Full rate encoded speech blocks from a single conversation

Bursts

TDMAFrames Frame 1 Frame 2 Frame 3 Frame 4


CONTROL CHANNEL INTERLEAVINGCONTROL 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 (4*114 = 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.


DATA INTERLEAVINGDATA 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.


CIPHERINGCIPHERING

• 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.

MODULATIONMODULATION

• Modulation is done using 0.3 GMSK

MODULATIONMODULATION

SIGNALLINGSIGNALLING

Other Networks

• 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.

SIGNALLING SYSTEMSIGNALLING SYSTEMWHAT IS SIGNALLING ?WHAT IS SIGNALLING ?

GENERAL INTRODUCTIONGENERAL INTRODUCTION

• Today’s 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

SIGNALLING SYSTEM C7SIGNALLING SYSTEM C7

GENERAL INTRODUCTIONGENERAL 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.

SIGNALING IN TELECOMMUNICATION NETWORKSIGNALING IN TELECOMMUNICATION NETWORK

DIGITAL SUBSCRIBER SIG.

SUBSCRIBER LINE SIG.

ACCESS SIG

SIGNALLING

TRUNK SIGNALLING

CHANNEL ASSOCIATED SIG.

COMMON CHANNEL SIG.

Access SignalingAccess 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.

Access SignalingAccess 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 user’s site, the amount of signaling information and the number of variations

Trunk SignalingTrunk 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)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)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.

CHANNEL ASSOCIATED SIGNALING (CAS)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.

COMMON CHANNEL SIGNALING (CCS)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 80’s 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.

OSI REFERENCE MODELOSI 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.

PHYSICAL

LINK

NETWORK

TRANSPORT

SESSION

PRESENTATION

OSI MODEL REFERENCE DIAGRAMOSI MODEL REFERENCE DIAGRAM

APPLICATION

PHYSICAL

LINK

NETWORK

TRANSPORT

SESSION

PRESENTATION

APPLICATION

COMMUNICATION PROCESSCOMMUNICATION 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.

• 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 layer’s protocol.

DESCRIPTION OF LAYERSDESCRIPTION OF LAYERS

Application LayerApplication Layer

• This layer provides services for support of the user’s 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.

Presentation LayerPresentation 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 LayerSession 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.

Transport LayerTransport 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.

Network LayerNetwork 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 LayerData 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.

Physical LayerPhysical 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.

SIGNALING SYSTEM NO. 7 INTRODUCTIONSIGNALING 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.

• 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, ISDN, IN, etc.).

• Even though the standardization of SS7 is now the responsibility of ITU-T, for traditional and historical reasons, the system is often called “CCITT No. 7 signaling system”.

• The signaling system used in GSM follows the CCITT recommendations.

• The modular layer structure allows flexible usage of the specifications.

USER PARTSUSER PARTS• The User Parts (UPs) contain functions dealing with the processing of

signal information before and after it is transmitted through the signaling network.

• The MTP provides the means of reliable transport and delivery of UP information across the SS7 network.

• It also has the ability to react to system and network failures that affect the information from the UPs and take necessary action to ensure that the information is safely conveyed.

• The User does not mean the subscriber involved in the call, but the user of the MTP.

• The MTP is a common transport system developed to serve one or more User Parts in the same node.

• Every Signaling Point(SP) consists of MTP & a number of its users. • Only UPs of the same type can communicate with each other.• To forward signaling messages between UPs, located in different

nodes, the MTP is used.

.

USERS OF SIGNALING SYSTEM CCITT NO 7

MAP CAP BSSAP ISUP TUP

TCAP

MTP

SCCP

CCITT SS NO. 7 PROTOCOLS IN GSM

MTP user partsMTP user parts

ISUP (ISDN User Part)ISUP (ISDN User Part)• It provides control-functions and signaling, needed in an ISDN, to

deal with ISDN subscriber calls and related functions.

TUP (Telephony User Part)TUP (Telephony User Part)• It provides all necessary functions and signaling for dealing with a

telephony user. • TUP is being replaced by ISUP in telecommunication networks.

DUP (Digital User Part)DUP (Digital User Part)• This UP is used for purposes such as file transfer and related

signaling.

SCCPSCCP

• The MTP was designed for the real-time applications of telephony.

• The connectionless nature of the MTP provides a low-overhead facility suiting the requirements of telephony.

• Regarding GSM, other applications such as network management need services such as expanded addressing capability and reliable message transfer.

• The SCCP was developed to meet these requirements.

• The SCCP also sends its messages through the MTP.

• The SCCP provides functions for completely new services, for example, non-circuit-related signaling.

• Some functions, not directly related to users, but necessary for network control, are used.

• The main reason is that they are necessary for serving applications in higher layers and for maintenance purposes.

SCCPSCCP

• These functions use SCCP services:

Transaction Capabilities (TC)Transaction Capabilities (TC)

– First introduced in 1984, TC provides the mechanisms for transaction-oriented applications and functions.

Operation and Maintenance Application Part (OMAP)Operation and Maintenance Application Part (OMAP)

– Specifies network management functions and messages related to operation and maintenance.

PHYSICAL

LINK

NETWORK

TRANSPORT

SESSION

PRESENTATION

APPLICATION

SIGNALLING DATA LINK

SIGNALLING LINK

SIGNALLING NETWORK

SCCP

TCAPUSER PARTS

ASE

OSI ModelOSI Model CCITT SS NO 7 ModelCCITT SS NO 7 Model

MT

P

NS

P

CALL FLOWCALL FLOW

Mobile originated callMobile originated callMS

Channel Request (RACH)

BSS MSC

SDCCH SeizureImmediate Assignment [ Reject ] (AGCH)

CM Service Request + Connection Request < CMSREQ >

Connection [ Confirmed / Refused ]

Link Establishment

Authentication Request

Authentication Response

DT1 <CICMD>Ciphering Mode CommandCiphering Mode Complete

DT1 <CICMP>

Identity Request

Identity Response

Setup

Call Proceeding

Connection Management

Assignment Request

Assignment Request [ Failed ]

Assignment Command

Assignment [ Complete / Failure ]


TCH Seizure

SDCCH

TCH

MS BSS MSC

Paging

SDCCH Seizure

Link Establishment

Paging Request (PCH)UDT < PAGIN >

Paging

Channel Request (RACH)

Immediate Assignment [ Reject ] (AGCH)

Paging Response + Connection Request < PAGRES >

Connection [ Confirmed / Refused ]

Authentication Request

Authentication Response

SDCCH

Ciphering Mode Command

Ciphering Mode Complete

DT1 <CICMD>

DT1 <CICMP>

Identity Request

Identity Response

Setup

Call Confirmed

Connection Management

Assignment Request

Assignment Request [ Failed ]

Assignment Command


TCH TCH Seizure


Mobile terminated callMobile terminated call

POWER CONTROLPOWER CONTROL

RF POWER CONTROLRF POWER CONTROL

• RF power control is employed to minimise the transmit power required by MS or BS while maintaining the quality of the radio links.

• By minimising the transmit power levels, interference to co-channel users is reduced.

• Power control is implemented in the MS as well as the BSS.

• Power control on the Uplink also helps to increase the battery life.

• The RF power level employed by the MS is indicated by means of the 5 bit TXPWR field sent either in the layer 1 header of each downlink SACCH message block, or in a dedicated signalling block.

• The MS confirms the power level that it is currently employing by setting the MS_TXPWR_CONF field in the uplink SACCH L1 header to its current power setting. The value of this field is the power setting actually used by the mobile for the last burst of the previous SACCH period.

• The MS employs the most recently commanded RF power level appropriate to the channel for all transmitted bursts on either a TCH (including handover access burst), FACCH,SACCH or SDCCH.

• When accessing a cell on the RACH (random access) and before receiving the first power command during a communication on a DCCH or TCH (after an IMMEDIATE ASSIGNMENT), the MS uses either the power level defined by the MS_TXPWR_MAX_CCH parameter broadcast on the BCCH of the cell, or the maximum TXPWR of the MS as defined by its power class, whichever is the lower.

POWER CONTROL IN THE MSPOWER CONTROL IN THE MS

POWER CONTROL MSPOWER CONTROL MS

• The range over which a MS is capable of varying its RF output power is from its maximum output down to 20mW, in steps of nominally 2dB.

• 0 - 43dBm…….15 - 13dBm.

8 7 6 5 4 3 2 1

Octet 1

Octet 2

Ordered MS Power LevelSpare

Ordered Timing AdvanceSpare

1111111 indicates this field does not have any TA value

TIMING OF POWER CHANGE BY MSTIMING OF POWER CHANGE BY MS

• Upon receipt of a command on the SACCH to change its RF power level (TXPWR field) the MS changes to the new level at a rate of one nominal 2dB power step every 60ms (13 TDMA frames), i.e. a full range change of 15 steps should take about 900ms .

• The change commences at the first TDMA frame belonging to the next reporting period . The MS changes the power one nominal 2 dB step at a time, at a rate of one step every 60 ms following the initial change, irrespective of whether actual transmission takes place or not.

• In case of channel change the commanded power level is applied on the new channel immediately.

BSS POWER CONTROLBSS POWER CONTROL

• Power control at BSS is optional.• The range over which the BS is capable of reducing its RF output

power from its maximum level is nominally 30dB, in 15 steps of nominally 2dB.

RADIO LINK FAILURE

• The criterion for determining Radio Link Failure in the MS is based on the success rate of decoding messages on the downlink SACCH.

• The radio link failure criterion is based on the radio link counter S.

• If the MS is unable to decode a SACCH message, S is decreased by 1.

• If a SACCH message is decoded successfully, S is increased by 2.

• If S reaches 0 a radio link failure is assumed & the MS aborts the conn.

• The RADIO_LINK_TIMEOUT parameter is transmitted by each BS in the BCCH data.

4Decoded

3Not Decoded

2

1

0

SACCH Blocks

RADIO LINK FAILURERADIO LINK FAILURE

• The MS continues transmitting as normal on the uplink until S reaches 0.

• The algorithm will start after the assignment of a dedicated channel and S is initialized to RADIO_LINK_TIMEOUT.

• The aim of determining radio link failure in the MS is to ensure that calls with unacceptable voice/data quality, which cannot be improved either by RF power control or handover, are either re-established or released in a defined manner.

• In general the parameters that control the forced release should be set such that the forced release will not normally occur until the call has degraded to a quality below that at which the majority of subscribers would have manually released. This ensures that, for example, a call on the edge of a radio coverage area, although of bad quality, can usually be completed if the subscriber wishes.

CELL SELECTION AND RE-SELECTIONCELL SELECTION AND RE-SELECTION

• In Idle mode (i.e. not engaged in communicating with a BS), an MS will do the cell selection and re-selection procedures .

• The procedures ensure that the MS is camped on a cell from which it can reliably decode downlink data and with which it has a high probability of communications on the uplink. The choice of cell is determined by the path loss criterion. Once the MS is camped on a cell, access to the network is allowed.

• An MS is said to be camped on a cell when it has determined that the cell is suitable and stays tuned to a BCCH + CCCH of that cell. While camped on a cell, an MS may receive paging messages or under certain conditions make random access attempts on a RACH of that cell, and read BCCH data from that cell.

• The MS will not use the discontinuous reception (DRX) mode of operation (i.e. powering itself down when it is not expecting paging messages from the network) while performing the selection and reselection algorithm. However use of powering down is permitted at all other times in idle mode.

CELL SELECTION AND RE-SELECTIONCELL SELECTION AND RE-SELECTION

• For the purposes of cell selection and reselection, the MS is required to maintain an average of received signal strengths for all monitored frequencies. These quantities termed the "receive level averages” is the averages of the received signal strengths measured in dBm.

• The cell selection and reselection procedures make use of the "BCCH Allocation" (BA) list. There are in two BA lists which may or may not be identical, depending on choices made by the PLMN operator.

• (i) BA (BCCH) - This is the BA sent in System Information Messages on the BCCH. It is the list of BCCH carriers in use by a given PLMN in a given geographical area. It is used by the MS in cell selection and reselection.

• (ii) BA (SACCH) - This is the BA sent in System Information Messages on the SACCH and indicates to the MS which BCCH carriers are to be monitored for handover purposes.

• When the MS goes on to a TCH or SDCCH, it starts monitoring BCCH carriers in BA (BCCH) until it gets its first BA (SACCH) message.

CELL SELECTION - NO BCCH DATA AVAILABLECELL SELECTION - NO BCCH DATA AVAILABLE• The MS searches all 124 RF channels in the GSM system, takes readings of

RSS on each RF channel, and calculate the received level average for each.

• The averaging is based on at least five measurement samples per RF carrier spread over 3 to 5 secs.

• The MS tunes to the carrier with the highest average RSS & determines whether or not this carrier is a BCCH carrier.

• If it is a BCCH carrier, the MS attempts to synchronise to this carrier and read the BCCH data. The MS camps on the cell provided it can successfully decode the BCCH data and this data indicates that it is part of the selected PLMN, that the cell is not barred (CELL_BAR_ACCESS = 0) & that the parameter C1 is greater than 0.

• If the cell is part of the selected PLMN but is barred or C1 is less than zero, the MS uses the BCCH Allocation obtained from this cell and subsequently only searches these BCCH carriers. Otherwise the MS tune to the next highest carrier and so on.

CELL SELECTION - NO BCCH DATA AVAILABLECELL SELECTION - NO BCCH DATA AVAILABLE• CELL_BAR_ACCESS may be employed to bar a cell that is only intended

to handle handover traffic etc. For example of this could be an umbrella cell which encompasses a number of microcells.

• If at least the 30 strongest RF channels have been tried, but no suitable cell has been found, provided the RF channels which have been searched include at least one BCCH carrier, the available PLMN's shall be presented to the user, otherwise more RF channels shall be searched until at least one BCCH carrier is found.

• 30 RF channels are specified to give a high probability of finding all suitable PLMN's, without making the process take too long.

CELL SELECTION - BCCH INFORMATION AVAIL.CELL SELECTION - BCCH INFORMATION AVAIL.• The MS stores the BCCH carriers in use by the PLMN selected when it was

last active in the GSM network. A MS may also store BCCH carriers for more than one PLMN which it has selected previously (e.g. at national borders or when more than one PLMN serves a country).

• If an MS includes a BCCH carrier storage option it searches only for BCCH carriers in the list.

• If an MS decodes BCCH data from a cell of the selected PLMN but is unable to camp on that cell, the BA of that cell is examined. Any BCCH carriers in the BA which are not in the MS's list of BCCH carriers to be searched is added to the list.

• If no suitable cell has been found after all the BCCH carriers in the list have been searched, the MS acts as if there were no stored BCCH carrier information. Since information concerning a number of channels is already known to the MS, it may assign high priority to measurements on those of the 30 strongest carriers from which it has not previously made attempts to obtain BCCH information, and omit repeated measurements on the known ones.

PATH LOSS CRITEREON( C1) PATH LOSS CRITEREON( C1)

• This parameter is used to ensure that the MS is camped on the cell with which it has the highest probability of successful communication on uplink and downlink.

• The path loss criterion parameter C1 used for cell selection and reselection is defined by:

C1 = (A - Max(B,0))

where A = Received Level Average - RXLEV_ACCESS_MIN

B = MS_TXPWR_MAX_CCH - P

RXLEV_ACCESS_MIN =Minimum received level at the MS required for access to the system.

MS_TXPWR_MAX_CCH = Maximum TXPWR level an MS may use when accessing the system.

P = Maximum RF output power of the MS.

• All values are expressed in dBm.

PATH LOSS CRITEREON( C1) PATH LOSS CRITEREON( C1)

A = + Good Downlink

- Poor Downlink

B = - Good Downlink

+ Poor Downlink

Monitoring of Received Level and BCCH dataMonitoring of Received Level and BCCH data

• In Idle Mode an MS continues to monitor all BCCH carriers as indicated by the BCCH Allocation .

• A running average of received level in the preceding 5 to 60 seconds is be maintained for each carrier in the BCCH Allocation.

• For the serving cell receive level measurement samples is taken at least for each paging block of the MS and the receive level average is determined using samples collected over a period of 5 s or five consecutive paging blocks of that MS, whichever is the greater period.

Monitoring of Received Level and BCCH data Monitoring of Received Level and BCCH data

• At least 5 received level measurement samples are required per receive level average value. New sets of receive level average values is calculated as often as possible.

• The same number of measurement samples is taken for all non serving cell BCCH carriers, and the samples allocated to each carrier is as far as possible uniformly distributed over each evaluation period.

• The list of the 6 strongest carriers is updated at least every minute and may be updated more frequently.

• In order to minimise power consumption, MSs that employ DRX (i.e. power down when paging blocks are not due) monitor the signal strengths of non-serving cell BCCH carriers during the frames of the Paging Block that they are required to listen to. Received level measurement samples can thus be taken on several non-serving BCCH carriers and on the serving carrier during each Paging Block.

• The MS includes the BCCH carrier of the current serving cell (i.e. the cell the MS is camped on) in this measurement routine.

Monitoring of Received Level and BCCH dataMonitoring of Received Level and BCCH data

• The MS has to decode the full BCCH data of the serving cell at least every 30 seconds.

• The MS attempts to decode the BCCH data block that contains the parameters affecting cell reselection for each of the 6 strongest non-serving cell BCCH carriers at least every 5 minutes.

• When the MS recognizes that a new BCCH carrier has become one of the 6 strongest, the BCCH data shall be decoded for the new carrier within 30 seconds.

• The MS attempts to check the BSIC for each of the 6 strongest non serving cell BCCH carriers at least every 30 seconds, to confirm that it is monitoring the same cell.

• If a change of BSIC is detected then the carrier is treated as a new carrier and the BCCH data redetermined.

• When requested by the user, the MS monitors the 30 strongest GSM carrier to determine, within 15 seconds, which PLMN's are available. This monitoring is done so as to minimise interruptions to the monitoring of the PCH.

CALL RE-ESTABLISHMENTCALL RE-ESTABLISHMENT

• In the event of a radio link failure, call re-establishment may be attempted if it is enabled in the database.

• The received level measurement samples taken on surrounding cells and on the serving cell BCCH carrier in the last 5 seconds is averaged, and the carrier with the highest average received level which is part of a permitted PLMN is taken.

• A BCCH data block containing the parameters affecting cell selection is read on this carrier.

• If the parameter C1 is greater than zero, it is part of the selected PLMN, the cell is not barred, and call re-establishment is allowed, call re-establishment is attempted on this cell.

• If the above conditions are not met, the carrier with the next highest average received level is taken, and the MS repeats the above procedure.

• If the cells with the 6 strongest average received level values are tried but cannot be used, the call re-establishment attempt is abandoned.

bbs_ag_blk_ress_ag_blk_res

• To ensure that some of the blocks are always left clear for access grant messages the parameter bs_ag_blk_res is used to input the number of blocks to be reserved for this purpose.

• The reserved blocks is not be used for paging whatever the demand.

• If more than one timeslot exists within a cell, this parameter will reserve the indicated number of blocks on each timeslot.

• This parameter is broadcast on the BCCH.

• This parameter is used to calculate the number of paging groups available.

COMBINED CCCH BLOCKS AGCH BLOCKS PCH BLOCKS

No 9 0 9No 9 1 8No 9 2 7No 9 3 6No 9 4 5No 9 5 4No 9 6 3No 9 7 2Yes 3 0 3Yes 3 1 2Yes 3 2 1

Bs_pa_mfrmsBs_pa_mfrms• Used to indicate the number of 51 frame multiframes between

transmission of paging messages to MS of the same group.

• Is transmitted on BCCH.

• Used by the MS to calculate its paging group.

BCCH

AGCH

1

2

3

4

5

6

7

8

BCCH

AGCH

1

2

3

4

5

6

7

8

BCCH

AGCH

9

10

11

12

13

14

15

16

BCCH

AGCH

17

18

19

20

21

22

23

24

BCCH

AGCH

25

26

27

28

29

30

31

32

Value

0 = 2 multiframes

1 = 3 multiframes

2 = 4 multiframes

3 = 5 multiframes

4 = 6 multiframes

5 = 7 multiframes

6 = 8 multiframes

7 = 9 multiframes

PAGINGPAGING

Example

cch_conf = 0

bs_ag_blk_res = 1

bs_pa_mfrms = 2

If cch_conf = 1

minimum = 2

If cch_conf = 6

Maximum = 81 * 4

Min time between pages = 2 * 235.5 = 471ms

Max time between pages = 9 * 235.5 =2.1195 sec

max_retranmax_retran

• An MS requests resources from the network by transmitting an ``access burst´´ containing the channel request message.

• For a single request, channel request will be repeated upto M + 1 times where M = max_retran.

max_retrans M1 12 23 44 7

tx_integertx_integer

• To reduce the chances of collision the wait period is randomised for each MS.

• After the first channel request is sent the next is repeated after a random wait period in the set

(S, S+1,….., S+T-1)

• Wait period from this set is chosen randomly from this set.

TX INTEGER RACH SLOTS

S FOR NON-COMB CCCH

S FOR COMB CCCH

3, 8, 14 55 414, 9, 16 76 52

5, 10, 20 109 586, 11, 25 163 867, 12, 32 127 115

AVAILABLE PAGING BLOCKS ON 1 CCCH_GROUPAVAILABLE PAGING BLOCKS ON 1 CCCH_GROUP

Maximum AGCH reservation for non-combined multiframe = 7

Available paging blocks = 2

Maximum AGCH reservation for combined multiframe = 1


Minimum AGCH reservation for non-combined multiframe = 0


Minimum AGCH reservation for combined multiframe = 0


No of paging blocks will have a range of 2 - 9

CALCULATION OF CCCH AND PAGING GROUP NOCALCULATION OF CCCH AND PAGING GROUP NO

CCCH_GROUP = [ ( IMSI mod 1000) mod (BS_CC_CHANS * N ) ] div N

Paging group no = [ ( IMSI mod 1000) mod (BS_CC_CHANS * N ) ] mod N

HANDOVERHANDOVER

• The GSM handover process uses a mobile assisted technique for accurate and fast handovers, in order to:

– Maintain the user connection link quality.

– Manage traffic distribution

• The overall handover process is implemented in the MS,BSS & MSC.

• Measurement of radio subsystem downlink performance and signal strengths received from surrounding cells, is made in the MS.

• These measurements are sent to the BSS for assessment.

• The BSS measures the uplink performance for the MS being served and also assesses the signal strength of interference on its idle traffic channels.

• Initial assessment of the measurements in conjunction with defined thresholds and handover strategy may be performed in the BSS. Assessment requiring measurement results from other BSS or other information resident in the MSC, may be perform. in the MSC.

HANDOVERHANDOVER

HANDOVERHANDOVER

• The MS assists the handover decision process by performing certain measurements.

• When the MS is engaged in a speech conversation, a portion of the TDMA frame is idle while the rest of the frame is used for uplink (BTS receive) and downlink (BTS transmit) timeslots.

• During the idle time period of the frame, the MS changes radio channel frequency and monitors and measures the signal level of the six best neighbor cells.

• Measurements which feed the handover decision algorithm are made at both ends of the radio link.

HANDOVERHANDOVER (Cont (Cont))

HANDOVERHANDOVER

• At the MS end, measurements are continuously signalled, via the associated control channel, to the BSS where the decision for handover is ultimately made.

• MS measurements include:

–Serving cell downlink quality (bit error rate (BER) estimate).

–Serving cell downlink received signal level, and six best neighbor cells downlink received signal level.

• The MS also decodes the Base Station ID Code (BSIC) from the six best neighbor cells, and reports the BSICs and the measurement information to the BSS.

MS ENDMS END

HANDOVERHANDOVER

• The BTS measures the uplink link quality, received signal level, and MS to BTS site distance.

• The MS RF transmit output power budget is also considered in the handover decision.

• If the MS can be served by a neighbor cell at a lower power, the handover is recommended.

• From a system perspective, handover may be considered due to loading or congestion conditions. In this case, the MSC or BSC tries to balance channel usage among cells.

BTS ENDBTS END

HANDOVERHANDOVER

• During the conversation, the MS only transmits and receives for one eighth of the time, that is during one timeslot in each frame.

• During its idle time (the remaining seven timeslots), the MS switches to the BCCH of the surrounding cells and measures its signal strength.

• The signal strength measurements of the surrounding cells, and the signal strength and quality measurements of the serving cell, are reported back to the serving cell via the SACCH once in every SACCH multiframe.

• This information is evaluated by the BSS for use in deciding when the MS should be handed over to another traffic channel.

• This reporting is the basis for MS assisted handovers.

MS IDLE TIMEMS IDLE TIME REPORTINGREPORTING

HANDOVERHANDOVER

MEASUREMENT IN ACTIVE MODEMEASUREMENT IN ACTIVE MODE

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

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

Frame 24 Frame 25 Idle Frame Frame 0

Frame 24 Frame 25 Idle Frame Frame 0

1 2 3 1 2 1 2

1. MS receives and measures signal strength on serving cell(TS2).2. MS transmits3. MS measures signsl strength for at least one neighbor cell.4. MS reads BSIC on SCH for one of the 6 strongest neighbor.

4

Downlink

Uplink

HANDOVERHANDOVER

• Maximum 32 averaging of RSS takes place.

• Practically a cell neighbors can be equipped for a cell.

• If high numbers of neighbors are equipped, then the accuracy of RSS is decreased as should have 8 to 10 neighbors.

T15

T5

T9

T10

T11

S12

T13

T14

T6

T7

T8

T0

T1

T2

T3

T4

T16

T17

T18

T19

T20

T21

T22

T23

T24

I25

T15

T5

T9

T10

T11

S12

T13

T14

T6

T7

T8

T0

T1

T2

T3

T4

T16

T17

T18

T19

T20

T21

T22

T23

T24

I25

T15

T5

T9

T10

T11

S12

T13

T14

T6

T7

T8

T0

T1

T2

T3

T4

T16

T17

T18

T19

T20

T21

T22

T23

T24

I25

T15

T5

T9

T10

T11

S12

T13

T14

T6

T7

T8

T0

T1

T2

T3

T4

T16

T17

T18

T19

T20

T21

T22

T23

T24

I25

NUMBER OF NEIGHBORSNUMBER OF NEIGHBORS

HANDOVERHANDOVER

NUMBER OF NEIGHBORSNUMBER OF NEIGHBORS

• In one SACCH multiframe there are 104 TDMA frames.

• Out of this 104 frames 4 frames are idle and are used to decode the BSIC.

• Remaining 100 TDMA frames are used to measure RSS( Received Signal Strength) of the neighbor.

• If 25 neigbors are equipped, then in one SACCH multiframe each neigbor is measured 100/25 = 4 times and averaged out. This produces a less accurate value.

• If 10 neigbors are equipped, then in one SACCH multiframe each neigbor is measured 100/10 = 10 times and averaged out. This produces a more accurate value.

HANDOVERHANDOVER

• GSM causes its own time interference.

• The MS has a omni-directional antenna. Much of the MS power goes to the server but a lot is interfering with surrounding cells using the same channel.

• The TDMA frames of adjacent cell are not aligned since they are not synchronised. Hence the uplink in the surrounding cell suffers from interference.

INTERFERENCE ON IDLE CHANNELINTERFERENCE ON IDLE CHANNEL

Channel 10Cell 1

1 2 3 4 5

1 2 3 4 5

UPLINK CELL1

UPLINK CELL2

Channel 10Cell 2

HANDOVERHANDOVER

• The BSS keeps on measuring the interference on the idle timeslots.

• Ambient noise is measured and recorded 104 times in one SACCH multiframe.

• These measurements are averaged out to produce one figure.

• The BSS then distributes the idle timeslots into band 0 to band 5.

• Since the BSS knows the interference level on idle timeslots, it uses this data to allocate the best channel first and the worst last.

INTERFERENCE ON IDLE CHANNELINTERFERENCE ON IDLE CHANNEL

0 1 2 3 4 5 6 7

Inteference on idle channel measured on Idle Timeslot by BSS

HANDOVERHANDOVER

The following measurements is be continuously processed in the BSS :

i) Measurements reported by MS on SACCH

- Down link RXLEV

- Down link RXQUAL

- Down link neighbor cell RXLEV

ii) Measurements performed in BSS

- Uplink RXLEV

- Uplink RXQUAL

- MS-BS distance

- Interference level in unallocated time slots

Every SACCH multiframe (480 ms) a new processed value for each of the measurements is calculated..

HANDOVERHANDOVER

HANDOVERHANDOVER

Handover is done on five conditions

– Interference

– RXQUAL

– RXLEV

– Distance or Timing Advance

– Power Budget

Interference - If signal level is high and still there is RXQUAL problem, then the RXQUAL problem is because of interference.

RXQUAL - It is the receive quality. It ranges from 0 to 7 , 0 being the best and 7 the worst

RXLEV - It is the receive level. It varies from -47dBm to -110dBm.

Timing Advance - Ranges from 0 to 63.

Power budget - It is used to save the power of the MS.

HANDOVERHANDOVER CONDITIONS CONDITIONS

HANDOVERHANDOVER

• Handover takes place in the same cell from one timeslot to another timeslot of the same carrier or different carriers( but the same cell).

• Intra-cell handover is triggered only if the cause is interference.

• Intra-cell handover can be enabled or disabled in a cell.

HANDOVER TYPESHANDOVER TYPESIntra-Cell HandoverIntra-Cell Handover

BSC

BTSCall is handed

from timeslot 3 to timeslot 5

0 1 2 3 4 5 6 7

HANDOVERHANDOVER

• Handover takes place between different cell which are controlled by the same BSC.

HANDOVER TYPESIntra-BSC Handover

BSC1

BTS1Call is handed from timeslot 3 of cell1 to timeslot 1 of cell2 . Both the cells are controlled

by the same BSC.

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

HANDOVERHANDOVER

• Handover takes place between different cell which are controlled by the different BSC.

HANDOVER TYPESHANDOVER TYPESInter-BSC HandoverInter-BSC Handover

BSS1


by the different BSC.

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7BSS2

MSC

BTS2

HANDOVERHANDOVER

• Handover takes place between different cell which are controlled by the different BSC and each BSC is controlled by different MSC.

HANDOVER TYPESHANDOVER TYPESInter-MSC HandoverInter-MSC Handover

BSS1


by the different BSC, each BSCbeing controlled by different MSC.

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7BSS2

MSC1

BTS2

MSC2

HANDOVERHANDOVER

LOCATION UPDATELOCATION UPDATE


• MSC should always know the location of the MS so that it can contact it by sending pages whenever required.

• The mobile keeps on informing the MSC about its current location area or whenever it changes from one LA to another.

• This process of informing the MSC is known as location updating.

• The new LA is updated in the VLR.

• LAI = MCC + MNC + LAC

MCC MNC LAC

3 digits 1-2 digits Max 16 bits

MCC = Mobile country code.

MNC = Mobile Network Code.

LAC = Location area code. Identifies a location area within a GSM PLMN network. The maximum length of LAC is 16 bits. Thus 65536 different LA can be defined in one GSM PLMN.

• Normal location update

• Periodic location update

• IMSI attach

Normal Location UpdateNormal Location Update

• Mobile powers on and is idle.

• Reads the LAI broadcast on the BCCH.

• Compares with the last stored LAI and if it is different does a location update.

LOCATION UPDATE TYPESLOCATION UPDATE TYPES


MS MSCBSS

RACH

Imme. Assign

Location update request

Authentication request

Authentication response

DTI<CICMD>

DTI<CICMP>

Cipher mode command

Cipher mode complete

Location update accepted

IMSI ATTACHIMSI ATTACH

• Saves the network from paging a MS which is not active in the system.

• When MS is turned off or SIM is removed the MS sends a detach signal to the Network. It is marked as detached.

• When the MS is powered again it reads the current LAI and if it is same does a location update type IMSI attach.

• Attach/detach flag is broadcast on the BCCH sys info.

PERIODIC LOCATION UPDATEPERIODIC LOCATION UPDATE

• Many times the MS enters non-coverage zone.

• The MS will keep on paging the MS thus wasting precious resources.

• To avoid this the MS has to inform the MSC about its current LAI in a set period of time.

• This time ranges from 0 to 255 decihours.

• Periodic location timer value is broadcast on BCCH sys info messages.

• During conversation user talks alternatively.

• In DTX mode of operation the transmitter are switched on only for frames containing useful information.

• Helps to increase battery life and reduce interference level.

T15

T5

T9

T10

T11

S12

T13

T14

T6

T7

T8

T0

T1

T2

T3

T4

T16

T17

T18

T19

T20

T21

T22

T23

T24

I25

T15

T5

T9

T10

T11

S12

T13

T14

T6

T7

T8

T0

T1

T2

T3

T4

T16

T17

T18

T19

T20

T21

T22

T23

T24

I25

T15

T5

T9

T10

T11

S12

T13

T14

T6

T7

T8

T0

T1

T2

T3

T4

T16

T17

T18

T19

T20

T21

T22

T23

T24

I25

T15

T5

T9

T10

T11

S12

T13

T14

T6

T7

T8

T0

T1

T2

T3

T4

T16

T17

T18

T19

T20

T21

T22

T23

T24

I25

DISCONTINOUS TRANSMISSIONDISCONTINOUS TRANSMISSION

SID

IMPLEMENTATION OF DTXIMPLEMENTATION OF DTX

Voice Activity Detector ( VAD )Voice Activity Detector ( VAD )

• Determines which specific block of 20ms from the speech coder contains speech.

• Removes statinary noise.

• Inserts comfort noise.

• The frames containing this background noise are called SID frames and are sent in blocks of 8 frames within every 104 frame block.

VAD Speech / No speech20 ms speech

block

FREQUENCY HOPPINGFREQUENCY HOPPING

MOTOROLA

aaaa

1

4

7

2 3

5 6

8 9

0* #

63250/0/0/

Information is transmittedby different frequencies atdifferent timeslot

Voice

ModulatedRF signal

f1

f2

f3

f4

f1

INTRODUCTION TO FREQUENCY HOPPINGINTRODUCTION TO FREQUENCY HOPPING

Introduction to Frequency HoppingIntroduction to Frequency Hopping

• Can be used to improve the quality of the network

• Also can be used to increase the capacity of the Network thereby reducing the number of sites required for CAPACITY.

The way it works

• Each burst is transmitted on a different frequency

• Both mobile and base station follow the same hopping sequence

Introduction to Frequency HoppingIntroduction to Frequency Hopping• Fading

– Causes quality deterioration– Is frequency dependent

• FH diversifies the impact of fading and improves quality.• The immunity to fading increases by exploiting its frequency selectivity,

because using different frequencies the probability of being continuously affected by fading is reduced, so the transmission link quality is improved.

• This improvement is much more noticeable for slow moving mobiles.

•

Increased Immunity to fadingIncreased Immunity to fading

• In a cellular urban environment in most cases multipath propagation will be present and, as a consequence of that, important short term variations in the received level are frequent . This is called Rayleigh fading which results in quality degradation because some of the information will be corrupted.

• For a fast moving mobile, the fading situation can be avoided from one burst to another because it also depends on the position of the mobile so the problem is not so serious.

• For a stationary one the reception may be permanently affected resulting in a very bad quality, even a drop call.

• Once the information is received by the mobile or the base station, the only way to cope with the disturbance produced by the fading (errors in the information bits) are the decoding and deinterleaving processes, with an effectiveness limited by the number of errors they have to deal with.


• Interference

• A result of frequency reuse & irregular terrain and sites

• FH diversifies the impact of interference and improves quality

• The situation of permanent interference coming from neighbour cells transmitting the same or adjacent frequencies is avoided using Frequency Hopping because the calls will spend the time moving through different frequencies not equally affected by interfering signals. This effect is called Interference Averaging.


• Considering a non hopping system, the set of calls on the interferer cells which can interfere with the wanted call is fixed for the duration of those calls and some calls will be found with very good quality (no interference problems) whereas some others with very bad quality (permanent interference problems).

• With hopping, that set of interfering calls will be continually changing and the effect is that calls tend to experience an average quality rather than extreme situations of either good or bad quality (all the calls will suffer from a controlled interference but only for short and distant periods of time, not for all the duration of the call).

• This interference averaging means again spreading the raw bit errors (BER caused by the interference) in order to have a random distribution of them instead of bursts of errors, and therefore enhance the effectiveness of decoding and deinterleaving processes to cope with the BER and lead to a better value of FER.

TYPES OF HOPPINGTYPES OF HOPPING

Base Band Hopping (BBH)Base Band Hopping (BBH)

• The radio units transmit always the same frequency

• Number of frequencies for hopping = Number of carriers

• The radio units are always transmitting a fixed frequency and frequency hopping is performed by moving the information for every call among the available radio units in a cell on a per burst basis.

• In reception the call is always processed by the same radio unit (the one where the call started).

• The number of frequencies to hop over is limited by the number of radio units equipped in the cell.

• The BCCH carrier can hop in timeslots 1 to 7 (without power control/DTX).

TYPES OF HOPPINGTYPES OF HOPPING

Synthesiser Frequency Hopping (SFH)Synthesiser Frequency Hopping (SFH)

• The radio units change (retune) the frequency every burst.

• The call always stays in the same radio unit.

• Number of frequencies for hopping > Number of carriers.

• The radio units can hop over a range of different frequencies( 64 in case of Motorola).

• Hybrid combiners are required in the base station (Cavity Combiners can not be used with SFH).

• The BCCH carrier can never hop.

Hopping ParametersHopping Parameters

For frequency hopping operability, GSM defines the following set of parameters:

• Mobile Allocation (MA): Set of frequencies the mobile is allowed to hop over. MA is a subset of all the frequencies allocated by the system operator to the cell (cell allocation) although it can be the same. Eg:- If the operator has frequencies from 1 -32, then he can use 1-15 for BCCH and 17-32 for hopping ( MA).

• Hopping Sequence Number (HSN): Determines the hopping order used in the cell. 64 different HSNs can be assigned, where HSN = 0 provides a cyclic hopping sequence and HSN = 1 to 63 provide various pseudorandom hopping sequences.

• Mobile Allocation Index Offset (MAIO): Determines inside the hopping sequence which frequency the mobile starts to transmit on.

• Frequency Hopping Indicator (FHI): Defines a hopping system made up by an associated set of frequencies (MA) to hop over and a hopping sequence (HSN).

WHAT IS FADING ?WHAT IS FADING ?

• The communication between the base station and mobile station in mobile systems is mostly non-LOS.

• The LOS path between the transmitter and the receiver is affected by terrain and obstructed by buildings and other objects.

• The mobile station is also moving in different directions at different speeds.

• The RF signal from the transmitter is scattered by reflection and diffraction and reaches the receiver through many non-LOS paths.

• This non-LOS path causes long-term and short term fluctuations in the form of log-normal fading and rayleigh and rician fading, which degrades the performance of the RF channel.

WHAT IS FADING ?WHAT IS FADING ?

Sig

nal

Pow

er (

dB

m) Large scale fading component

Small scale fadingcomponent

LONG TERM FADINGLONG TERM FADING

• Terrain configuration & man made environment causes long-term fading.

• Due to various shadowing and terrain effects the signal level measured on a circle around base station shows some random fluctuations around the mean value of received signal strength.

• The long-term fades in signal strength, r, caused by the terrain configuration and man made environments form a log-normal distribution, i.e the mean received signal strength, r, varies log-normally in dB if the signal strength is measured over a distance of at least 40.

• Experimentally it has been determined that the standard deviation, , of the mean received signal strength, r, lies between 8 to 12 dB with the higher generally found in large urban areas.

RAYLEIGH FADINGRAYLEIGH FADING• This phenomenon is due to multipath propagation of the signal.

• The Rayleigh fading is applicable to obstructed propagation paths.

• All the signals are NLOS signals and there is no dominant direct path.

• Signals from all paths have comparable signal strengths.

• The instantaneous received power seen by a moving antenna becomes a random variable depending on the location of the antenna.

RICEAN FADINGRICEAN FADING

• This phenomenon is due to multipath propagation of the signal.

• In this case there is a partially scattered field.

• One dominant signal.

• Others are weaker.

INTERFERENCEINTERFERENCE

WHAT IS INTERFERNCE ?WHAT IS INTERFERNCE ?

• Interference is the sum of all signal contributions that are neither noise not the wanted signal.

EFFECTS OF INTERFERENCEEFFECTS OF INTERFERENCE • Interference is a major limiting factor in the performance of cellular systems.

• It causes degradation of signal quality.

• It introduces bit errors in the received signal.

• Bit errors are partly recoverable by means of channel coding and error correction mechanisms.

• The interference situation is not reciprocal in the uplink and downlink direction.

• Mobile stations and base stations are exposed to different interference situation.

SOURCES OF INTERFERENCESOURCES OF INTERFERENCE • Another mobile in the same cell.

• A call in progress in the neighboring cell.

• Other base stations operating on the same frequency.

• Any non-cellular system which leaks energy into the cellular frequency band.

TYPES OF INTERFERENCETYPES OF INTERFERENCE • There are two types of system generated interference

– Co-channel interference

– Adjacent channel interference

Co-Channel InterferenceCo-Channel Interference

• This type of interference is the due to frequency reuse , i.e. several cells use the same set of frequency.

• These cells are called co-channel cells.

• Co-channel interference cannot be combated by increasing the power of the transmitter. This is because an increase in carrier transmit power increases the interference to neighboring co-channel cells.

• To reduce co-channel interference, co-channel cells must be physically separated by a minimum distance to provide sufficient isolation due to propagation or reduce the footprint of the cell.


• Some factors other then reuse distance that influence co-channel interference are antenna type, directionality, height, site position etc,

• GSM specifies C/I > 9dB.

Carrier f1 Interferer f1

dB

Distance

C

I


• In a cellular system, when the size of each cell is approximately the same, co-channel interference is independent of the transmitted power and becomes a function of cell radius(R) and the distance to the centre of the nearest co-channel cell (D).

C1

C2C3

C1

C2C3

D


• Q = D / R = 3N

• By increasing the ratio of D/R, the spatial seperation between the co-channel cells relative to the coverage distance of a cell is increased. In this way interference is reduced from improved isolation of RF energy from the co-channel cell.

• The parameter Q , called the co-channel reuse ratio, is related to the cluster size.

• A small value of Q provides larger capacity since the cluster size N is small whereas a large value of Q improves the transmission quality.

Adjacent-Channel InterferenceAdjacent-Channel Interference

• Interference resulting from signals which are adjacent in frequency to the desired signal is called adjacent channel interference.

• Adjacent channel interference results from imperfect receiver filters which allow nearby frequencies to leak into the passband.

• Adjacent channel interference can be minimized through careful filtering and channel assignments.

• By keeping the frequency separation between each channel in a given cell as large as possible , the adjacent interference may be reduced considerably.

Adjacent-Channel InterferenceAdjacent-Channel Interference

Carrier f1 Interferer f2dB

A

C

Distance

COUNTERING INTERFERENCE COUNTERING INTERFERENCE

POWER CONTROLPOWER CONTROL

• RF power control is employed to minimise the transmit power required by MS or BS while maintaining the quality of the radio links.

• By minimising the transmit power levels, interference to co-channel users is reduced.

• Power control is implemented in the MS as well as the BSS.

• Power control on the Uplink also helps to increase the battery life.

• Power received by the MS is continously sent in the measurement report.

• Similarly uplink power received from the MS by the BTS is measured by the BTS.

• Complex algorithm evaluate this measurements and take a decision subsequently reducing or increasing the power in the Uplink or the downlink.

COUNTERING INTERFERENCE COUNTERING INTERFERENCE

SECTORIZATIONSECTORIZATION

• For 120 degrees sectored site as compared to an omni site almost 1/3rd interference is received in the uplink.

• The more selective and directional is the antenna, the smaller is the interference.

• Reduction in interference results in higher capacity in both links.

introduction to ubr. introduction . ubr stants for unspecified bit rate. ubr allows service...

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