GSM INTRODUCTION

Netwing

INTRODUCTIONINTRODUCTION

• The global system for mobile communications (GSM) is a set of recommendations and specifications for a digital cellular telephone network (known as a Public Land Mobile Network, or PLMN). These recommendations ensure the compatibility of equipment from different GSM manufacturers, and interconnectivity between different administrations, including operations across international boundaries.

THE GSM NETWORKTHE GSM NETWORK

• The GSM network is comprised of the following components:

• Network Elements• The GSM network incorporates a number of network elements

to support mobile equipment. They are listed and described in the GSM network elements section of this chapter.

• GSM subsystems• In addition, the network includes subsystems that are not

formally recognized as network elements but are necessary for network operation. These are described in the GSM subsystems (non-network elements) section of this chapter.

• Standardized Interfaces• GSM specifies standards for interfaces between network

elements, which ensure the connectivity of GSM equipment from different manufacturers. These are listed in the Standardized interfaces section of this chapter.

THE GSM NETWORK - CONTINUEDTHE GSM NETWORK - CONTINUED

• Network Protocols

• For most of the network communications on these interfaces, internationally recognized communications protocols have been used

• These are identified in the Network protocols section of this chapter.

• GSM Frequencies• The frequency allocations for GSM 900, Extended GSM

and Digital Communications Systems are identified in the GSM frequencies section of this chapter.

DIGITAL NETWORKSDIGITAL NETWORKS

• GSM networks are digital and can cater for high system capacities. They are consistent with the world wide digitization of the telephone network, and are an extension of the Integrated Services Digital Network (ISDN), using a digital radio interface between the cellular network and the mobile

subscriber equipment.

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 200kHz. 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 the cells where the same frequencies are re-used (co-channel interference); a Carrier to Interference Ratio (C/I) level of 9 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. Since this number is directly controlled by the amount of interference, the radio transmission design can deliver acceptable performance.

CGI : CELL GLOBAL IDENTITYCGI : CELL GLOBAL IDENTITY

MCC MNC LAC CI

LAI

CGI

MCC = Mobile Country CodeMNC = Mobile Network CodeLAC = Location Area CodeCI = Cell Identity

MSISDNMSISDN

CC NDC SN

98 XXX 12345

CC = Country CodeNDC = National Destination CodeSN = Subscriber Number

MSISDNMSISDN

• 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 towards the MSC.

IMSI IMSI • IMSI (International Mobile Subscriber Identity) Network

Identity Unique To A Sim.

MCC MNC MSIN

404 XX 12345..10

SIM = Subscriber Identity ModuleMCC = Mobile Country CodeMNC = Mobile Network CodeMSIN = Mobile Subscriber Identity Number

IMEIIMEI

• IMEI : Serial number unique to each mobile

TAC FAC SNR SP

6 2 6 1

IMEI = International Mobile Equipment IdentityTAC = Type Approval CodeFAC = Final Assembly CodeSNR = Serial NumberSP = Spare

SUBSCRIBER IDENTIFICATIONSUBSCRIBER IDENTIFICATION• International Mobile Subscriber Identity (IMSI)• Just the IMEI identifies the mobile equipment, other numbers

are used to identify the mobile subscriber. Different subscriber identities are used in different phases of call setup. The International Mobile Subscriber Identity (IMSI) is the primary identity of the subscriber within the mobile network and is permanently assigned to that subscriber.

• Temporary Mobile Subscriber 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 backward and forward 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 transmitted with the Local numbers and is always transmitted with the Location Area Identification (LAI) to avoid ambiguities.

SUBSCRIBER IDENTIFICATION SUBSCRIBER IDENTIFICATION MODULE (SIM)MODULE (SIM)

• By making a distinction between the subscriber identity and the mobile equipment identity, a GSM PLMN can route calls and perform billing based on the identity of the subscriber rather than the mobile equipment being used. This can be done using a removable Subscriber Information Module (SIM). A ”smart card” is one possible implementation of a SIM module.

• IMSI. This is transmitted at initialization of the mobile equipment.

• TMSI This is updated periodically by the PLMN• MSISDN This is made up of a country code, a national code

and a subscriber number.• Location Area Identity (LAI) This identified the current

location of the subscriber.• Subscriber Authentication Key (KI) This is used to

authenticate the SIM.

EQUIPMENT IDENTITY NUMBEREQUIPMENT IDENTITY NUMBER

• International Mobile station Equipment Identity (IMEI)

• 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 MS,s that are reported stolen or operating incorrectly.

• Equipment Identity Register ( EIR )• A listing of the allowed IMEI is maintained by the

PLMN’s in the Equipment Identity Register (EIR) to validate the mobile equipment.

Frequency BandsFrequency Bands

Uplink 890 – 915 MHz 25 MHz

Downlink 935 – 960 MHz 25 MHz

100 KHz 200 KHz 100 KHz

1 43 1242 …………….

A 200 KHz carrier spacing has been chosen. Excluding 2x100 KHz edges of

the band, this gives 124 possible carriers for the uplink and downlink. The

use of carrier 1 and 124 are optional for operators.

GSMGSM Network Architecture Network Architecture

BTS

BTS

BTS

BTS

BTS

BSC

BSC

TRAU

MSC

HLRAUC

VLR

EIR

PSTN

SMSC

MSMS – Mobile Station – Mobile Station

• Mobile station provides user access to GSM network for voice and data

• All GSM mobiles comply to GSM standards• Subscriber data is read from a SIM card that plugs

into ME

SIM ME

MS

MS (cont..)MS (cont..)

• Each MS has a unique number called as IMEI number, which is stored in EIR for authentication purposes

• Mobile camps on to the GSM network through the BTS serving the cell

• Mobile also scans neighboring cells and reports signal strengths• Mobile transmits and receives voice at 13 kb/s over the air

interface

Mobile Station Output PowerMobile Station Output Power

• CLASS 1 20 watts Vehicle and Portable• CLASS 2 8 watts Portable and Vehicle• CLASS 3 5 watts Hand-Held• CLASS 4 2 watts Hand-Held (GSM)• CLASS 5 0.8 watts Hand-Held (DCS

1800)• Output power determines:

– Accessibility in areas of coverage– Talk Time and Standby time

Mobile Station IdentitiesMobile Station Identities

MSISDN : Mobile Station ISDN Number

It is the human identity used to call a Mobile Station

CC SNNDC MSISDN

98 250 00134

• CC – Country Code• NDC – National Destination Code• SN – Serial Number

IMSI (International Mobile IMSI (International Mobile Subscriber Identity)Subscriber Identity)

• MCC – Mobile Country Code• MNC – Mobile Network Code• MSIN – Mobile Subscriber Identity Number

MCC MSINMNC IMSI

3 2 or 3

Not more than 15

NMSI

IMEI (International Mobile IMEI (International Mobile Equipment Identity)Equipment Identity)

TAC SPFAC IMEISNR

6 162 15• TAC – Type Approval Code• FAC – Factory Assembly Code• SNR – Serial Number• SP – Spare digit (usually used to

specify software version)

SIM ( Subscriber Identity SIM ( Subscriber Identity Module)Module)

• Removable module inserted when the subscriber wants to use the ME

• Two sizes: credit card size and stamp size• SIM features and contents are personalized by the Service

Activator• ROM – 6kb to 16 kb• RAM – 128 bytes to 256 bytes• EEPROM – 3kb to 8 kb

Space to insert SIM photo

Contents of SIMContents of SIM• Serial Number• IMSI, Subscriber Key Ki, Ciphering Key Kc• Algorithms for authentication and

ciphering• Network Code• PIN, PUK• Charging Information• Abbreviated Dialling• Supplementary Features (e.g. Call

barring)

SIM SecuritySIM Security• Two level protection• When mobile is turned on, it will ask

for user to enter PIN (Personal Id Number)

• 3 tries for PIN, after that PIN locked• To unblock PIN, there is PUK (Pin

Unblock Key)• 10 attempts of PUK allowed• After that SIM is blocked

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

• BTS has a set of Transceivers (TRXs) to communicate with mobiles in its area

• One BTS covers one or more than one cell• The capacity of a cell depends on number of transceivers in

the cell• BTS is connected to the BSC through Abis Interface which is

2Mbps• BTS transmits and receives voice at 13kbps over air interface

to the mobiles.• BTS commands mobiles to set Tx. Power, timing advance and

Handovers

BTSBTS

BSC – Base Station BSC – Base Station ControllerController

• Several BTSs are connected to the BSCSeveral BTSs are connected to the BSC• BSC Manages channel allocation, handovers and release of BSC Manages channel allocation, handovers and release of

channels at connected BTSschannels at connected BTSs• BSC connects to the BTS via the Abis interface and to the MSC on BSC connects to the BTS via the Abis interface and to the MSC on

A interfaceA interface• BSC has the entire database of cell parameters associated with BSC has the entire database of cell parameters associated with

the BTSs.the BTSs.• No mobile data is stored in the BSCNo mobile data is stored in the BSC• Less connections for MSC as intelligence is made common to all Less connections for MSC as intelligence is made common to all

BTSs by the BSCBTSs by the BSC

• Several BTSs are connected to the BSCSeveral BTSs are connected to the BSC• BSC Manages channel allocation, handovers and release of BSC Manages channel allocation, handovers and release of

channels at connected BTSschannels at connected BTSs• BSC connects to the BTS via the Abis interface and to the MSC on BSC connects to the BTS via the Abis interface and to the MSC on

A interfaceA interface• BSC has the entire database of cell parameters associated with BSC has the entire database of cell parameters associated with

the BTSs.the BTSs.• No mobile data is stored in the BSCNo mobile data is stored in the BSC• Less connections for MSC as intelligence is made common to all Less connections for MSC as intelligence is made common to all

BTSs by the BSCBTSs by the BSC

BSCBSC

TRAU – Transcoder Rate TRAU – Transcoder Rate Adaptation UnitAdaptation Unit

BTS

BSC PSTN

13 kbps 16 kbps 16 kbps 64 kbps

MSC and TRAU

TRAU (cont..)TRAU (cont..)• The MSC is based on ISDN

switching. The Fixed Network is also ISDN based.

• ISDN has speech rate of 64 kbps. Mobile communicates at 13 kbps.

• TRAU converts the data rates between 13kbps GSM rate to 64kbps Standard ISDN rate

• TRAU can be collocated with the BTS, BSC or MSC or it can be a separate unit.

Location of TranscoderLocation of Transcoder

• Collocated with MSC, BSC, BTS• Separate Unit

MSCTransco

derBSC

MSC – Mobile Switching CentreMSC – Mobile Switching Centre

BSC

BSC

BSC

BTSs PSTN

HLR

VLR

MSC (cont..)MSC (cont..)

• Exchange where calls are established, maintained and released• Database for all subscribers and their associated features.• Communicates with the BSCs on the A interface and with PSTN on

fixed line.• MSC is weighted on the number of subscribers it can support. E.g.

an MSC of 1 lac subscribers means one MSC is enough till subscriber base increases upto 1 lac, beyond which another MSC is required.

Multiple MSCsMultiple MSCs• When there is more capacity, there are more than one MSCs.• All MSCs have to communicate with one another and to the

outside world.• Very complicated to connect each MSC to each other and each

MSC to PSTN• So there is a concept of GMSC (Gateway MSC)

BSC

BSC

MSC

MSC

GMSC PSTN

HLR – Home Location HLR – Home Location RegisterRegister

• MSC has all subscriber database stored in HLR• HLR has all permanent subscriber database• HLR has a database which describes the subscriber’s profile i.e.

basic features and supplementary services• MSC communicates with the HLR to get data for subscribers on

call

VLR – Visiting Location RegisterVLR – Visiting Location Register

• A subscription when activated is registered in VLR• VLR has all the subscriber numbers which are active.• VLR has a temporary database of all active subscribers (on/off,

location information)

MSCVLRVLR

HLR

VLR (cont..)VLR (cont..)

MSC MSCVLRVLR

HLR

VLR

• MSC communicates with HLR for subscribers coming from different MSCs. If the subscriber is found valid, then it registers the subscriber in the VLR

AUC – Authentication CentreAUC – Authentication Centre• Authentication is a process by which a SIM is verified• Secret data and the verification process algorithm are stored in

AUC• AUC is the element which carries out the verification of the SIM• AUC is associated with the HLR

MS MSC HLR AUC

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

• EIR is the Mobile Equipment Database which has a series of IMEIs• MSC asks the Mobile to send its IMEI• MSC then checks the validity of IMEI with the EIR• All IMEIs are stored in EIR with relevant classifications

EIR

MSC

Classification of IMEIsClassification of IMEIsWhite list: This contains the IMEI of type approved mobiles

Black List: List of IMEIs which should be barred because either they are stolen or are not functioning properly

Grey list: List of IMEIs which are to be evaluated before they are put in black list

Billing Centre (BC)Billing Centre (BC)• BC Generates the billing statement

for each subscriber• BC may be directly connected to the

MSC or through a mediation device• MSC sends CDRs (Call Detail

Records) to the BC• According to the template of pulse

rates and units set, BC creates a bill according to the destination called and the call duration

Billing Centre (BC) (cont..)Billing Centre (BC) (cont..)

CDRs

Templates for unit costs

OMC – Operations and OMC – Operations and Maintenance CentreMaintenance Centre

• Also called the NOC (Network Operations centre)

• It is the central monitoring and remote maintenance centre for all network elements

• OMC has links to BSCs and MSCs

OMC – Operations and OMC – Operations and Maintenance CentreMaintenance Centre

• Also called the NOC (Network Operations centre)

• It is the central monitoring and remote maintenance centre for all network elements

• OMC has links to BSCs and MSCs

OMCOMC

OMC System

BSC

BSC

BSC

BTSs

BTSs

BTSs

OMC Terminals

GSM ChannelsGSM Channels

GSM ChannelsGSM Channels

• Physical Channel– One time slot on one carrier is called

physical channel.• Logical Channel

– Information carried by physical channels is called logical Channels.

– Logical channels are mapped on physical channels.

Logical ChannelsLogical Channels• Traffic channels: Used for speech and data

– Full Rate(TCH/F) – Half Rate(TCH/H)

• Control channels: Used for signaling .i.e. setting up a radio connection, call or controlling an MS during conversation– BCH(Broadcast channels)– CCCH(common control channels)– DCCH(dedicated control channels)

Traffic Channels(TCH)Traffic Channels(TCH)

TCH/F(full Rate)

TCH/H(half Rate)

Traffic Channels(TCH)

Control Channels(CCH)Control Channels(CCH)

CCH(Control Channel)

BCH CCCH DCCH

CCH RACH CBCH SDCCH ACCHSynch.Chanels

SACCHFACCHPCH/

AGCHFCCHSCH

BCH(Broadcast ChannelsBCH(Broadcast Channels))• BCCH(Broadcast Control Channels)

– Downlink Only.– Broadcast information of the serving

cell (System Information).– Transmitted on timeslot zero of BCCH

carrier.– Reads only by idle mobile at least once

every 30 secs.

BCH(Broadcast Channels) BCH(Broadcast Channels) cont’dcont’d

• SCH(Synchronisation Channels)– Downlink Only– Carries information for frame

synchronisation.– Contains frame number and BSIC(Base

Station Identity Code).

BCH(Broadcast Channels) BCH(Broadcast Channels) cont’dcont’d

• FCCH(Frequency Correction Channels)– Downlink Only.– Enable MS to synchronies to the

frequency.

CCCH(Common Control CCCH(Common Control Channel)Channel)

• RACH(Random Access Channel)– Uplink only.– Used by the MS when making its first

access to the Network.– The reason for access could be initiation

of a call or a page response.

CCCH(Common Control CCCH(Common Control Channel) cont’dChannel) cont’d

• AGCH(Assess Grant Channel)– Downlink only.– Used for acknowledgement of the

access attempt sent on RACH.– Used by the network to assign a

signaling channel upon successful decoding of access bursts.

CCCH(Common Control CCCH(Common Control Channel) cont’dChannel) cont’d

• PCH(Paging Channel)– Downlink only.– The network will page the MS ,if there is

a incoming call or a short Message.– It contains the MS identity number, the

IMSI or TMSI.

DCCH(Dedicated Control DCCH(Dedicated Control Channel)Channel)

• SDCCH (Stand-alone Dedicated Control Channel)– Uplink and Downlink.– Used for call setup, authentication,

ciphering location update and SMS.

DCCH(Dedicated Control DCCH(Dedicated Control Channel) cont’dChannel) cont’d

• SACCH(Slow Associated Control Channel)– Downlink and Uplink.– Used to transfer signal while MS have

ongoing conversation on traffic or while SDCCH is being used.

– On the forward link, the SACCH is used to send slow but regularly changing control information to each mobile on that ARFCN, such as power control instructions and specific timing advance instructions

• SACCH(Slow Associated Control Channel) cont’d– The reverse SACCH carries information

about the received signal strength and quality of the TCH, as well as BCH measurement results from neighboring cells.

DCCH(Dedicated Control DCCH(Dedicated Control Channel) cont’dChannel) cont’d

• FACCH(Fast Associated Control Channel)– Downlink and uplink.– Associate with TCH only.– It is used to send fast message like hand

over message.– Work by stealing traffic bursts.

Mapping on Physical Mapping on Physical ChannelsChannels

• The Logical channels are mapped on the physical channels.

• The TDMA frames are grouped together into multi-frame.– 26 TDMA multi-frame for Traffic.– 51 TDMA multi-frame for control signal.

Channel CombinationChannel Combination• Combined

– All the controlling signals are in the time slot 0 of the Multi-frame.

• Non Combined– Dedicated controlling signals are in time

slot 1 of the Multi-frame.

CombinedCombined• Cell with single carrier.

– Timeslot 0 :BCCH+CCCH+SDCCH.– Timeslot 1-7 :TCH/FACCH+SACCH.

Non CombinedNon Combined• Cell with Two carrier

– Timeslot 0 (of carrier 1) BCCH+CCCH.– Timeslot 1 (of carrier1) SDCCH+SACCH.– Timeslot 2-7 & 0-7(of both carriers)

TCH/FACCH+SACCH.

•SYSTEM INFORMATION

SYSTEM INFORMATION 1SYSTEM INFORMATION 1• When frequency hopping is used in cell MS needs to

know which frequency band to use and what frequency within the band it should use in hopping algorithm.

• Cell channel descriptionCell Allocation Number(CANO)-Informs the

band number of the frequency channels used. 00-Band 0(current GSM band)

Cell Allocation ARFCN(CA ARFCN):- ARFCN’s used for hopping.It is coded in a bitmap of 124 bits.

SYTEM INFORMATION 1SYTEM INFORMATION 1

124 123 122 121

024 023 022 021 020 019 018 017

016 015 014 013 012 011 010 009

008 007 006 005 004 003 002 001

SYSTEM INFORMATION 1SYSTEM INFORMATION 1• RACH Control Parameters

Access Control Class(ACC) :-Bitmap with 16 bits. All MS spread out on class 0 –9 . Priority groups use class 11-15. A bit set to 1 barred access for that class. Bit 10 is used to tell the MS if emergency call is allowed or not.

0 – All MS can make emergency call.1 - MS with class 11-15 only can make emergency calls.

Cell barred for access(CB):-0- Yes1- No

SYSTEM INFORMATION 1SYSTEM INFORMATION 1• RACH Control Parameters

Re-establishment allowed(RE):-

0- Yes1- No

Max_retransmissions(MAXRET):-Number of times the MS attempts to access the Network [1,2,4 or 7].

Tx-integer(TX):- Number of slots to spread access retransmissions when a MS attempts to access the system.

Emergency call allowed:- Yes/No.

SYSTEM INFORMATION 2SYSTEM INFORMATION 2• System Information Type 2 message

consists of the Double BA list which defines the BCCH frequencies used in the neighboring cells.

• The Double BA list provides the MS with different frequencies on which to measure, depending on whether the MS is in idle or active mode.

• In active mode, the MS should measure on a reduced number of frequencies in order to improve the accuracy of measurements.

SYSTEM INFORMATION 2SYSTEM INFORMATION 2• In Idle mode,the MS should measure on

larger number of frequencies, so that the time required for the MS to access the network after power on is reduced.

• The MS is also informed which PLMN’s it may use.

• As well as System Information Type 2,it is also possible to have System Information Type 2 Bis and System information Type 2 Ater, depending on the size of the BA List.

• System Information Type 2 Bis/Ter are optional.

SYSTEM INFORMATION 2SYSTEM INFORMATION 2• Neighbor Cell Description:-

BA Indicator(BA IND):- Allows to differentiate measurement results related to different list of BCCH frequencies sent to MS.

BCCH Allocation number(BANO):- Band 0 is used.

• PLMN Permitted(NCCPERM):-This the PLMN color codes permitted and tells the MS which network color codes(NCC) on the BCCH carriers it is allowed to monitor when it is in this cell.

.

SYSTEM INFORMATION 2SYSTEM INFORMATION 2• RACH Control Parameters

Access Control Class(ACC) :-Bitmap with 16 bits. All MS spread out on class 0 –9 . Priority groups use class 11-15. A bit set to 1 barred access for that class. Bit 10 is used to tell the MS if emergency call is allowed or not.

0 – All MS can make emergency call.1 - MS with class 11-15 only can

make emergency calls. Cell barred for access(CB):-

0- Yes1- No

SYSTEM INFORMATION 2SYSTEM INFORMATION 2Re-establishment allowed(RE):-

0- Yes1- No

Max_retransmissions(MAXRET):-Number of times the MS attempts to access the Network [1,2,4 or 7].

Tx-integer(TX):- Number of slots to spread access retransmissions when a MS attempts to access the system.Emergency call allowed:- Yes/No.

SYSTEM INFORMATION 2SYSTEM INFORMATION 2 BCCH ARFCN Number(BAIND):- ARFCN’s

used for in a Bitmap of 124 bits

124 123 122 121

024 023 022 021 020 019 018 017

016 015 014 013 012 011 010 009

008 007 006 005 004 003 002 001

SYSTEM INFORMATION 3SYSTEM INFORMATION 3• The System Information Type 3 contains

information on the identity of the current LA and cell identity, because a change means that the MS must update the network.

• System Information 3 also as Control Channel Description parameters used to calculate the Paging group.

• When the MS is in idle mode it decides which cells to lock to. Information needed by the MS for cell selection is also broadcast in the Type 3 information.

SYSTEM INFORMATION 3SYSTEM INFORMATION 3

8 7 6 5 4 3 2 1

1 1 1 1

LAC

LOCATION AREA IDENTITTY(LAI)

MCC DIG 1MCC DIG 2MCC DIG 1MNC DIG 1MNC DIG 2

CICI

CELL IDENTITY

LAC

SYSTEM INFORMATION 3SYSTEM INFORMATION 3• Control Channel Description

Attach / Detach(ATT):-0 = Allowed

1 = Not Allowedbs_agblk:-Number of block reserved for

AGCH [0-7] Ba_pmfrms:-Number of 51 frame multi-frames between transmission of paging messages to MS of the same group

T3212:- Periodic location update timer . [1-255 deci hours].

SYSTEM INFORMATION 3SYSTEM INFORMATION 3

cch_conf Physical channels combined No. of CCH0 1 timeslot(0) No 91 1 timeslot(0) Yes 32 2 timeslot(0,2) No 184 3 timeslot(0,2,4) No 276 4 timeslot(0,2,4,6) No 36

SYSTEM INFORMATION 3SYSTEM INFORMATION 3• Cell options

DTX:-Whether Discontinuous Transmission used or not.

PWRC:-Power control on the downlink. 0 = Not used.

1 = Used.Radio link

timeout(RLINKT):-Radio link time-out is the time before an MS disconnects due to failure in decoding SACCH message. Sets the timer T100 in the MS.

SYSTEM INFORMATION 3SYSTEM INFORMATION 3• Cell Selection Parameters

Rxlev_access_min:- Minimum received signal level at the MS for which it is permitted to access the system.

0-63 = -100 dBm to –47 dBm. Mx_txpwr_cch:- Maximum power the MS will use when accessing the system.

Cell_reselect_hysteresis:- Used for cell reselection.

• RACH Control Parameters.

SYSTEM INFORMATION 4SYSTEM INFORMATION 4• Location Area Identification.• Cell Selection Parameters

Rxlev_access_min:- Minimum received signal level at the MS for which it is permitted to access the system.

0-63 = -100 dBm to –47 dBm. Mx_txpwr_cch:- Maximum power the MS will use when accessing the system.

Cell_reselect_hysteresis:- Used for cell reselection.

SYSTEM INFORMATION 4SYSTEM INFORMATION 4• RACH Control Parameters

max_retransmissions(MAXRET)tx_integer(TX)Cell barred for access(CB).

Re-establishment allowed(RE)Emergency Call Allowed

Access Control Class (ACC)

SYSTEM INFORMATION 4SYSTEM INFORMATION 4• CBCH Description(Optional) :

CHN:- This is the channel number for CBCH. It is controlled internally in BSC.

TSC:- Training Sequence Code. Base Station Color Code(BCC) part of BSIC is used.

CBCHNO:- Absolute RF channel number of CBCH.

MAC:- Mobile Allocation in the cell, describes the frequencies to be used in the hopping sequence if frequency hopping is used.

SYSTEM INFORMATION 4SYSTEM INFORMATION 4Hopping Channel(H):-Informs if CBCH

Channel is hopping or single.ARFCN:- If H=0;MAIO:- If H=1, informs the MS where to

start hopping. Values [0-63].HSN:- If H=1,

informs the MS in what order the hopping should take place. Values[0 –63]. HSN=0 Cyclic Hopping. MA:-Indicates which RF Channels are used

for hopping. ARFCN numbers coded in bitmap.

SYSTEM INFORMATION 5SYSTEM INFORMATION 5• Sent on the SACCH on the downlink to the MS in

dedicated mode.• On SAACH, the MS also receives information

about the BCCH carrier in each neighboring cell. This may differ from those sent in System information type 2.

• It is also possible to have system Information Type 5 Bis and System Information Type 5Ter, depending on the size of the BA list.

SYSTEM INFORMATION 5SYSTEM INFORMATION 5• Neighbor Cell Description:-

BA-IND:-Used by the Network to discriminate measurements results related to different lists of BCCH carriers sent by the MS(Type 2 or 5). Values 0 or 1(different from type 2).

BCCH Allocation number:-00-Band 0(current GSM band).

SYSTEM INFORMATION 5SYSTEM INFORMATION 5BCCH ARFCN:-Neighboring cells ARFCN’s.

Sent as a bitmap.0-Not used1-Used.

124 123 122 121

024 023 022 021 020 019 018 017

016 015 014 013 012 011 010 009

008 007 006 005 004 003 002 001

SYSTEM INFORMATION 6SYSTEM INFORMATION 6• Ms in dedicated mode needs to know if the LA

has changed.If so, it must perform location updating when the call is released.

• MS may change between cells with different Radio link timeout and DTX.

• Cell Identity.• Location Area Identification.• PLMN permitted.

SYSTEM INFORMATION 6SYSTEM INFORMATION 6• Cell options:

DTXPWRC

Radio Link timeout.

SYSTEM INFORMATION 7/8SYSTEM INFORMATION 7/8

• System Information Types 7 and 8 contain Cell Reselect parameters. Their function is to supplement System Information Type 4.

GSM InterfacesGSM Interfaces

• (Um) Air interface - MS to BTS• A bis interface - BTS to BSC• A Interface - BSC to MSC• B Interface - MSC to VLR• C interface - MSC to HLR

MSC

BSC

VLRHLR

AUC

EIR

GMSC

MS

A Interface

A bis Interface

Air Interface

B Interface C Interface

F Interface

D Interface H Interface

To otherNetworks

GSM InterfacesGSM Interfaces• The interfaces between MSC and MS is

called A, Abis and Um interfaces.• On these interfaces only three layers are

defined.They are not corresponding to the OSI (Open System Interconnection) model.

A InterfaceA Interface• A interface between the BSC and the MSC • The A interface provides two distinct

types of information, signalling and traffic, between the MSC and the BSC.

• The speech is transcoded in the TRC and the SS7 (Signalling system) signalling is transparently connected through the TRC or on a separate link to the BSC.

Abis InterfaceAbis Interface• The A-bis interface responsible for transmitting

traffic and signalling information between the BSC and the BTS.

• The transmission protocol used for sending signalling information on the A-bis interface is Link Access Protocol on the D Channel (LAPD)

(Um) Air Interface (Um) Air Interface

• This is the interface between the mobile station and the Base station.

• The Air interface uses the Time Division Multiple Access (TDMA) technique to transmit and receive traffic and signalling information between the BTS and MS.

• The TDMA technique is used to divide each carrier into eight time slots.These time slots are then assigned to specific users,allowing up to eight conversations to be handled Simultaneously by the same carrier.

7 5 6 3 4 1 2 0

1 2 43 5 7 6

Down Link

Up Link 0

Time Slot

• This interface is the radio interface between the mobile station and the network and uses layer Three messages. • On Layer three messages we have the division of message types into CM (communication Management), MM (Mobility Management), and RR (Radio Resource Management).

Connection Management Connection Management (CM)(CM)

There are three entities within CM:• Call Control(CC) – Which handles the

procedures concerning call control. e.g. setup,Change of bearer service.

• Supplementary Service (SS) – Which handles such as call bearing, call waiting , call forwarding etc.

• Short Message Service (SMS) – Enables the MS to handle short message transfer to and from the network.

Mobility Management (MM)Mobility Management (MM)

• Mobility management handles functions for authentication, location updating, identification and others concerning the mobility of the mobile station.

Radio Resource Radio Resource Management (RR)Management (RR)

• It contains the functions concerning the radio link. Here we find the capability to establish,maintain and release the radio connection between the network and the mobile station, which includes the handover procedure.

B InterfaceB Interface• The B interface between the MSC and the VLR

uses the MAP/TCAP protocol. • Most MSCs are associated with a VLR, making the

B interface "internal". • Whenever the MSC needs access to data

regarding a MS located in its area, it interrogates the VLR using the MAP/B protocol over the B interface.

C InterfaceC Interface

• The C interface is between the HLR and a MSC. • Each call originating outside of GSM (i.e., a MS

terminating call from the PSTN) has to go through a Gateway to obtain the routing information required to complete the call, and the MAP/TCAP protocol over the C interface is used for this purpose.

• Also, the MSC may optionally forward billing information to the HLR after call clearing.

D InterfaceD Interface• The D interface is between the VLR and HLR.• It uses the MAP/TCAP protocol to exchange the

data related to the location of the MS and to the management of the subscriber.

E InterfaceE Interface• The E interface interconnects two MSCs. • The E interface exchanges data related to

handover between the anchor and relay MSCs using the -MAP/TCAP+ISUP/TUP protocol.

F InterfaceF Interface• The F interface connects the MSC to the EIR.• It uses the MAP/TCAP protocol to verify the status

of the IMEI that the MSC has retrieved from the MS.

G InterfaceG Interface• The G interface interconnects two VLRs of

different MSCs.• It uses the MAP/G protocol to transfer subscriber

information, during e.g. a location update procedure.

Encoding and Encoding and InterleavingInterleaving

of Information of Information Signal Signal

inin GSM GSM

Topics for discussionTopics for discussion• Speech Encoding• Data Encoding• Interleaving for Voice,Control and

Data signals

Speech EncodingSpeech Encoding• We shall start with a raw voice signal

fed into the microphone, travel through the various stages involving vocoding, channel coding etc till it reaches the final burst format on the Air Interface.

Speech Encoding cktSpeech Encoding ckt

Voice Encoding

Channel coding

interleaving

RF Modulation

Raw Voicesignal

Speech Encoding cktSpeech Encoding ckt• The voice is sampled at the rate of 50

samples per second.• This results in 20 msec blocks of

speech• Each of this 20 msec block is passed

on to the 13Kbps vocoder.• There are 260 information bits from

the output of the vocoder for every 20 msec input i.e.; 13Kbps *20msec = 260 bits.

Voice Encoding cktVoice Encoding ckt

Vocoder I/p20 msec speech

blocks

13Kbps Vocoder Vocoder O/p260 bits

Channel codingChannel coding• Channel Coding is done to protect

the logical channels from transmission errors introduced by the radio path.

• The coding schemes depend on the type of the logical channels, hence the coding can differ from speech, control and data .

Channel Coding for speechChannel Coding for speech

Class class 1b class 21a

50 3 132 4 tailBits parity bits

Convolutional coder½ coder, k=5

456 bits=378 bits from Convolution coder + 78 class 2 bits

260 bits

Channel coding for SpeechChannel coding for Speech• The 260 bits of speech info from the

vocoder is broken down into three parts.• Class 1a- 50 bits , these represent the

filter coefficients of the speech and are the most important for proper detection of the speech at the receiver and hence are given maximum protection. 3 additional parity bits are derived from the class 1a bits for cyclic redundancy check (CRC).

Channel coding for Speech Channel coding for Speech cont’dcont’d

• Class 1b - 132 bits are not parity checked but are fed into the convolutional coder along with 4 tail bits which are used to set the registers in the receiver to a known state for decoding purpose.

• Class 2- 78 bits, these are not so important and are not protected but are combined with the output of the convolution coder.

Control Channel CodingControl Channel Coding

184 bits Control data

184 40 4 tailFire coded parity bits

½ Convolutional Coder456 bits output

Control Channel CodingControl Channel Coding• The control information is received in blocks

of 184 bits.• These bits are first protected with a cyclic

code called as Fire code, which is useful in correction and detection of burst errors.

• 40 Parity bits are added, along with 4 tail bits.• These 228 bits are given to the CC whose

output is again 456 bits at a bitrate of 22.8Kbps.

• The control channels include the RACH, PCH, AGCH etc.

Data Channel CodingData Channel Coding

240 bits 4 tail Data bits

½ Convolutional CoderOutput= 488 bits

After PuncturingOutput=456 bits

Data Channel CodingData Channel Coding• The data bits are received in blocks of 240

bits. These are directly convolution coded after adding 4 tail bits.

• The output of the CC is now 488 bits, which actually increases the bitrate to 24.4 Kbps.

• To keep the bitrate constant on the air interface we need to puncture the output of the CC. Hence, we have a final bitrate of 22.8 Kbps again .

Channel Coding cont’dChannel Coding cont’d• The above explanation was given

keeping in view a full rate Traffic, Control, or Data channel.

• For Half rate or Lesser rates the same principle of channel coding holds good, with slight differences in the encoding process.

Interleaving Interleaving • Having encoded the logical channel

information, the next step is to build its bit stream into bursts that can be transmitted within the TDMA frame structure. This is the stage where the interleaving process is carried out.

• Interleaving spreads the content of one information block across several TDMA timeslots or bursts.

Interleaving cont’dInterleaving cont’d• The following interleaving depths are used :• Speech – 8 blocks• Control – 4 blocks• Data – 22 blocks• The interleaving process for a speech block

is shown wherein which a 456 bit speech block is divided into 8 blocks of 57 bits each and each of these odd and even 57 bit blocks are interleaved diagonally on to alternate bursts on the TDMA frame.

Speech InterleavingSpeech Interleaving8* 57 bits each = 456 bits

Of Speech block N

57Even

Of N-1

57EvenOf N

Speech blockN-1

57odd

Of N-1

57odd

Of N

The speech is spread over 8 such normal burstsEach normal burst consists of two blocks of 57 bit speech

from different 20msec blocks (say N, N-1) along with26 bit training sequence T and 2 flag F plus 6 start stop bits .

T+FT+FT+F

456 bit speech data

Control Data InterleavingControl Data Interleaving

114 114 114 114

456 bits control data

The control data is spread over 4 blocks using rectangular interleaving instead of diagonal interleaving as in speech the receiver will have to wait for at least

2 multiframes before being able to decode the controlmessage

TDMABurst blocks

Data InterleavingData Interleaving

114 114 114 114

Burst 1 Burst 22Burst 2 Burst 3 Burst 4 Burst 19

First 6bits

First 6bits

Last 6bits

Last 6bits

456 bit data block

Data Interleaving cont’dData Interleaving cont’d• Here the data block of 456 bits is divided

into 4 blocks of 114 bits each.• The first 6 bits from each of the 114 bit

blocks is inserted in to each frame, the second 6 bits from each of the 114 bits into the next frame and so on spreading each 114 block over 19 TDMA bursts while the entire 456 bits is spread over 22 TDMA bursts.

• Thus the data interleaving is said to have a depth of 22 bursts.

Data Interleaving cont’dData Interleaving cont’d• The reason why data is spread over such along

period of time is that if data burst is corrupted or lost, only a small part of it is lost which can be reproduced at the receiver.

• This wide interleaving depth does produce a time delay during transmission but that is acceptable since it does not affect the data signal quality at the receiver, unlike speech where delay could result in bad quality of signal to the subscriber.

• *Note – The interleaving used in data is diagonal interleaving.

Before Deinterleaving3 successive bursts corrupted

After DeinterleavingThe corrupted bursts are spread over a length equal to the

interleaving depth so that the effect of the errors isminimized.

Interleaving AdvantageInterleaving Advantage

Air Interface BitrateAir Interface Bitrate• The information which is now coded and

interleaved at 22.8 Kbps now has to be transmitted over the Air interface to the BTS.

• The information burst is not sent directly , but is sent in ciphered form within a burst envelope. This ciphering is done using ciphering keys and algorithms known both by the mobile and the BSS.

Air Interface Bitrate cont’dAir Interface Bitrate cont’d• The Kc is the ciphering key and A5

algorithm are applied to the information(speech or data) which increases the bitrate to a final rate of 33.8 Kbps from/to each mobile.

• If we assume all 8 timeslots of the cell to be occupied then the bitrate of the Air interface comes to 33.8 * 8= 270.4 Kbps/channel.

Air Interface Bitrate cont’dAir Interface Bitrate cont’d

A5 Algorithm

Kc Information Block 22.8 Kbps

Sent on Air interface

Ciphered information burst33.8 Kbps

Air Interface Bitrate cont’dAir Interface Bitrate cont’d

1 2 3 4 5 6 7 8

Mobile Tx’s at

33.8 Kbps

Cell rx’s 8*33.8 KBps = 270.4 KbpsPer TDMA frame

Cell coverage area

TDMA Fn TDMA Fn+1

Decoding and Deinterleaving Decoding and Deinterleaving at the Receiverat the Receiver

• At the receiver the reverse process of Deinterleaving and decoding have to take place respectively, so as to recover the information from the signal.

• After Deinterleaving the signal will be decoded which is the reverse process of the Convolutional coding, using Viterbi decoders.

• The decoder can recover lost or corrupted data up to 4 successive bits, because the memory depth of the CC is 4(for k=5).

ChannelizationChannelization

• Frequency band has several application segments

• Certain blocks of the Band are reserved for certain applications by regulating authorities

• Technologies have decided their frequency bands

• E.g. AMPS/DAMPS: 824-894 MHz

Channelization methodsChannelization methods

Channelization can be done primarily by three methods:– FDMA (Frequency Division Multiple

Access)– TDMA (Time Division Multiple Access)– CDMA (Code Division Multiple Access)

FDMA FDMA

Frequency

Time

Power

• E.g. AMPS band is divided into 30 KHz channels (1666 Freq. channels)

• Television Channels (Star, Zee, Sony,..)

TDMATDMA

• E.g. AMPS has 3 timeslots on each 30 KHz channel

Frequency

Time

Power

CDMACDMA

• Frequency channel is divided into code channels

• E.g. in IS-95 CDMA, 1.228 MHz channel is divided into 64 Code Channels

• Each user has a particular code• Codes are orthogonal to each other,

do not interfere with each other

Duplex Access MethodsDuplex Access Methods

F1 F2 Frequency

Amplitude

Time

Tx Rx

• Frequency Division Duplex (FDD)• Transmit on one frequency and

receive on another frequency

Time Division DuplexTime Division Duplex

• Time division duplex• Tx and Rx is on the same frequency

but on different times

F1 Frequency

Amplitude

Time

Tx

Rx

GSM Air InterfaceGSM Air Interface• Separate Bands for Uplink and Downlink

– Downlink: 935-960Mhz (EGSM: 925-960MHz)– Uplink: 890-915 MHz (EGSM: 880-915 MHz)

• TDMA and TDMA Multiplex– 124 Frequency Channels (ARFCN) for

GSM900– 1 to 124 fro current band– 975 to 1023 for E-GSM – 200kHz Channels– 8 Mobiles share ARFCN by TDMA

GSM Air interface (1800)GSM Air interface (1800)– 1800: Downlink: 1805-1880 MHz– 1800: Uplink: 1710-1785 MHx

• 374 ARFCNs• Separation of 95 MHz• ARFCNs are numbered from 512 to

885 inclusive

The GSM BurstThe GSM Burst

3 357 261 571 8.25

Tail Bits

Data

Control Bit

Midamble

Control Bit

Data

Tail Bits

Guard Period

Speech CoderSpeech Coder

• RPE/LTP coder (Regular Pulse excitation/Long term Prediction)

• Converts 64 kbps speech to 13 kbps

• At the end we get 13kbps speech i.e. 260 bits in 20 ms

20 ms blocks

Speech Coder

Bits Ordered

50 very important bits

132 important bits

78 other bits

Error CorrectionError CorrectionType 1a 50 3(CRC)Type 1b 132 Type II 78

Reordering

25 66366 25 4 Type II 78

Type 1a

Type 1b Type 1b

Type 1a

Tail

Half rate convolutional code

378 Type II 78

456 bits from 20 ms of speech

Diagonal InterleavingDiagonal Interleaving456 bits from 20ms of speech 456 bits from 20ms of speech

57 57575757575757 57 57575757575757

57 57 57 5757 5757 5757 5757 5757 5757 57

• Traffic channel (TCH) bursts carry two 57 bit blocks (114)

• Each 120 ms of speech = 456*6 = 2736 bits2736/114 = 24 bursts i.3. 24 frames

Multiframe has 26 frames in 120ms.There are 2 spare frames .. 1 SACCH, 1 Idle

Convolutional Coding and Convolutional Coding and InterleavingInterleaving

• Bits to be Tx ed: HELLO• Convolutionally encoded:

HHEELLLLOO• Interleaved: EE HH LL LL OO

• Bits Rx ed: EE HH LL LL OO• De-Interleaved: HHEELLLLOO• Viterbi Decoded: HELLO

Speech Coding ProcessSpeech Coding Process

20 ms

Speech Coder

260 bits 13 kbps

50 1a 132 1b 78 II

Channel Coder

456 bits 22.8 kbps

Transceiver (BTS)

Transcoder Handler

260 bits

456 bits

16 kbps

TRAU frame

260 + 60 = 320 bits

Abis

13 kbps

TRAU frameTRAU frame• 260 bits info + 60 TRAU bits = 320

bits/20ms = TRAU frame• 60 bits contain frame Information

data which indicates speech, data, O&M, full rate/half rate

• 60 bits = 35 synchronization + 21 control + 4 timing

Midamble or Training BitsMidamble or Training Bits

3 357 261 571 8.25

Tail Bits

Data

Control Bit

Midamble

Control Bit

Data

Tail Bits

Guard Period

• 8 midamble patterns (Colour codes) of 26 bits (BSIC)

• RACH and SCH have longer 41 and 64 bit Midambles

• Equalizer estimates channel impulse response from midamble

• Mathematically construct inverse filter• Uses inverse to decode bits

Downlink and UplinkDownlink and Uplink

• Uplink lags downlink by 3 timeslots• Uplink and downlink use same

timeslot number• Uplink and downlink use same

channel number (ARFCN)• Uplink and downlink use different

bands (45 MHz apart for GSM 900)

Measurements made by MS Measurements made by MS and BTSand BTS

• RxQual0 < 0.2% 1 0.2 – 0.4 %3 0.4 – 0.8 % 4 0.8 – 0.16 %5 1.6 – 3.2 % 6 3.2 – 6.4 %7 6.4 – 12.8 %

Uplink RXLEV (-48 to -110 dbm)Uplink RXQUAL (0-7)

Uplink RXLEV (-48 to -110 dbm)Uplink RXQUAL (0-7)

Mobile Power ControlMobile Power ControlPath Loss

Power Command

• Mobile is commanded to change its Transmit Power

• Change in Power is proportionate to the Path Loss

• Change in Power is done in steps of 2 dbs

Timing AdvanceTiming Advance

• TDMA approach requires signals to arrive at BTS at the correct time

• A mobile at 30 km will be late by 100micro seconds

• Timing advance is in the range of 0-62• One unit is 550m• So maximum cell size is 63*0.55 =

~35 kms

Concepts of Channels in GSMConcepts of Channels in GSM• A company vehicle is used for several

purposes in a day• Similarly in GSM, the timeslots are used for

different purposes at different times

Frames and MultiframesFrames and Multiframes

0 654321 7

3 Data 1Midamble1 Data 3 8.25 bits

156.25 bits 576.92 micro sec

4.615 ms

Time

Slot

Frame

0 50 0 25

Control Channel Multiframe

Traffic Channel Multiframe

GSM OperationsGSM Operations• Location Update• Mobile Originated

Call• Mobile Terminated

Call• Handover• Security

Procedures• Cell Barring

• DTX• Cell Broadcast• Short Message

Service• Emergency calls• Supplementary

Services• Roaming

Mobile Turn OnMobile Turn On• Mobile Searches for Broadcast

Channels (BCH)• Synchronizes Frequency and Timing• Decodes BCH sub-channels (BCCH)• Checks if Network Allowed by SIM• Location Update• Authentication

Location AreaLocation Area

Location Area 1Location Area 1

LocationArea 2

LocationArea 2

BTS

BTSBTS

BTS

BTS

BTS

BTS

BTS

BSC

BSC

BSCMSC

Location Area IdentityLocation Area Identity

• Location area is the area covered by one or more BTSs where a mobile can move freely without updating the system

• One Location area can be covered by one or more BSCs, but ony one MSC.

MCC LACMNC

Importance of Location AreaImportance of Location Area• Reduce Paging load• Resource Planning

Smaller Location Areas – Location update increases

Larger Location Areas – Paging load increases

What is Location Update?What is Location Update?• MSC should know the location of the

Mobile for paging• Mobile is continuously changing

location area• Mobile when changes Location Area

informs the MSC about its new LA• Process of informing MSC about new

Location area is Location Update

Types of Location UpdatesTypes of Location Updates1. Normal Location

Update2. IMSI Attach3. Periodic Location

Update

Hi,I am in Location area

xxx

IMSI AttachIMSI Attach• Mobile turns off and sends an IMSI

Detach to MSC• Mobile turns on again and compares

LAI• If same, sends an IMSI attach to

MSC

Is the received LAI same as

before

If same,Sends IMSI

attach

Normal Location UpdateNormal Location Update• Mobile Turns on Power• Reads the new LAI• If different, does a Location Update

Is the received LAI same as

before

If different,does

Location Update

Periodic Location UpdatePeriodic Location Update

• The periodic location Update time is set from OMC/MSC

• After the periodic location update timer expires, the mobile has to do a location update

What happens at Location What happens at Location Update?Update?

• Mobile changes location area• Reads the new Location Area from

BCCH• Sends a RACH (request for channel)• Gets a SDCCH after AGCH• Sends its IMSI and new and old LAI in

a Location Update request to MSC on SDCCH

What happens at location What happens at location update cont..update cont..

….. . .• MSC starts Authentication• If successful, Updates the new

Location area for the Mobile in the VLR

• Sends a confirmation to the Mobile• Mobile leaves SDCCH, and comes to

idle mode

Mobile Originated CallMobile Originated CallChannel RequestImmediate AssignService Request

Call ProceedingSet Up

CipheringAuthentication

AlertingAssignment

Connection

Mobile Terminated CallMobile Terminated CallPaging

Channel RequestImmediate Assign

Set UpCiphering

AuthenticationPaging Response

AssignmentCall Confirmed

AlertingConnection

Security FeaturesSecurity Features• Authentication

– Process to verify Authenticity of SIM

– Mobile is asked to perform an operation using identity unique to SIM• Ciphering

– Process of coding speech for secrecy

– The speech bits are EXORed with bit stream unique to MS

Security Features (TMSI Security Features (TMSI Reallocation)Reallocation)

GSM Infrastructure Mobile

Location Update

TMSI Allocation

Call Setup

TMSI Reallocation

TMSI- Temporary Mobile Subscriber Identity

Security Features Security Features (Identity Check)(Identity Check)

EIR

Sends IMEI

Identity Check

White listed /Grey Listed/ Black Listed mobiles

HandoverHandover

Cell 1 Cell 2

Handover is a GSM feature by which the control/communication of a Mobile is transferred from one cell to another if certain criteria’s are

met. It is a network initiated process.

Criteria for HandoverCriteria for Handover• Receive Quality (RXQUAL) on uplink

and downlink• Receive Signal Strength (RXLEV) on

uplink and downlink• Distance (Timing Advance)• Interference Level• Power Budget

Handover DecisionHandover Decision• BSC process the measurements reported by

Mobile and the BTS

BTS

BTS

BTS

BTS

BTS

BTS

Mobile has measurements of six neighbors

Handover Decision (cont..)Handover Decision (cont..)

• BSS performs averaging function on these measurements every SACCH frame (480ms)

• Handover Decision algorithm is activated after a set number of SACCH frame periods by comparison against thresholds

Types of HandoversTypes of Handovers

• INTRA-CELL HANDOVERS• INTER-CELL HANDOVERS• INTRA-BSC HANDOVERS• INTER-BSC HANDOVERS• INTER-MSC HANDOVERS

INTRA-CELL HANDOVERINTRA-CELL HANDOVER

C0

C1

Handover between timeslots of same frequency

Handover between different frequencies of the same cell (to reduce interference)

MSC is not aware about this

Inter-cell HandoverInter-cell Handover

Handover between cells of the same BTS

BTSCell 1 Cell 2

Inter-cell Handover (cont..)Inter-cell Handover (cont..)

• MSC is told about HO• BTS -> BSC -> MSC• Why MSC is informed?

– In case of change of LA, MSC may need LAC for paging. As MS is busy, a link already exists. So, MSC can send a tone in case of call waiting, and does not need to page again.

– This is needed also for billing and call tracing

INTRA-BSC HandoverINTRA-BSC Handover

MSC BSC

BTS

BTS

This HO takes place if the cell to which handover is to be done belongs to the same BSC

Inter BSC HandoverInter BSC Handover

MSC

BSC BTS

BTSBSC

The MSC is completely involved in this Handover

Inter MSC HandoverInter MSC Handover

BSC

BSC

MSC

MSC

BTS

BTS

GMSC/PSTN/

Backbone

In this case the handover takes place through the interconnecting element which can be GMSC or PSTN or private Backbone between the MSCs

Cell BarringCell Barring

BTS

Cell Barring is a GSM feature by which certain mobiles could be barred access to certain cells

Cell barring is activated/deactivated at BTS level

Cell barring is done for mobile categories and priorities

Cell BarringCell Barring• Every mobile has an access class• The access class is stored in the SIM

– Classes 0-9 are termed normal calsses– Classes 11-15 are emergency classes

• Every cell has a set parameter which defines which access classes are barred for the particular cell. This parameter is broadcasted on the BCCH

What is DTX?What is DTX?• DTX (Discontinous Transmission)• Each direction of Transmission is only

50% • Transmitter is switched ON for useful

information frames

Need for DTX•To increase battery life•To reduce the average interference levelDTX is done by DTX handlers which have the following functions.

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

• Senses for speech in 20ms blocks • Removes stationary noise• VAD is an energy detector• Compares Energy of filtered speech

threshold• It determines which 20ms blocks

contain speech and it only forwards those frames

Evaluation of Background Evaluation of Background NoiseNoise

• Background noise is always present with speech

• DTX cuts off this noise with speech• Gives an uncomfortable feeling to

the listener• VAD takes care of this by inserting

comfort noise at the receiving end when speech discontinues.

Emergency CallsEmergency Calls• GSM specs define 112 as an

emergency number• ‘112’ is accessible with or without SIM• Without SIM it is sent on the best

channel• Mobile on sensing ‘112’ sets the

establishment cause to emergency call in the RACH

• Routing of this call be done to a desired location defined in the switch

Cell (Re)selectionCell (Re)selection• Cell reselection is done using C1 path

loss criterion.• The purpose is to ensure that the MS

is camped on to the cell with the best transmission quality.

• The MS will camp on to the cell with the highest C1 value if C1 > 0.

The following parameters are used to The following parameters are used to calculate the C1 criterioncalculate the C1 criterion

• The received signal at the MS side.• Rxlev_access_min - broadcast on

the BCCH - The minimum received level at the MS required for access to the network.

• Ms_txpwr_max_cch - the maximum power that an MS may use when initially accessing the network.

• The maximum power of the MS

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

• A = Received level Average - Rxlev_access_min.

• B = MS_txpwr_max_cch - maximum output power of the MS

Cell Reselect HysteresisCell Reselect Hysteresis• Cell reselection on the border of two location areas

result in a location update. When an MS moves on the border of two location areas lots of location updates take place. To avoid these location updates, the reselect hysteresis is introduced.

• A location update is performed only if:– The C1 value of the new location area is higher

than the C1 value in the current location area and

– The received signal strengths have at least a difference of the reselect hysteresis.

Cellular conceptCellular concept

Why to use the Why to use the cellular concept ?cellular concept ?

Solves the problem of Spectral congestion and user capacity by means of frequency reuse.

Offers high capacity in a limited spectrum allocation.

Offers system level approach, using low power transmitters instead of a single, high power transmitter (large cell) to cover larger area.

A portion of the total channels available is allocated to each base station.

Neighboring base stations are assigned different groups channels, in order to minimize interference.

Cell shapeCell shape

1-Omni-directional cell-site (Omni-directional antenna).

2-Rhombus-shaped sectors (Directive antenna).

3-Hexagonal shaped sectors (Directive antenna).

Cell sizeCell sizeLarge cell : (up to 70km in diameter)It exists where :1-Radio waves are unobstructed.2-Transmission power can cover the area.3-low subscriber density.

Small cell : (up to 2km in diameter)It exists where :1-Radio waves are obstructed.2-Low transmission power to decrease

interference.3-High subscriber density.

Types of cellsTypes of cells1-Macro-cells 2-Micro-cells. 3-Pico-cells. 4-Umbrella-

cells.

What is a cluster ?What is a cluster ?A cluster is a

group of cells.No channels are

reused within a cluster.

It is the unit of design.

Cluster sizeCluster sizeDefinition : It is The number of

cells per cluster

N = i^2 + ij + j^2 Where :

i = 0, 1, 2….& j = 0,1,2…. etc. N = 1 , 3 , 4 ,7, 9 , 12 ,……

Types of clustersTypes of clusters1-N=7 omni frequency plan (2-

directional).2-N=7 trapezoidal frequency plan (1-directional).3-N=9 omni frequency plan.4-Tricellular plans a) N=3 tricellular plan (3/9). b) N=4 tricellular plan (4/12).

Channel Channel assignment assignment strategiesstrategies

Considerations : 1) Max. capacity. 2) Min interference. 3) Perfect handover.

Types of assignment strategies : 1) Fixed : Each cell has permanent predetermined set of

voice channels. New calls served by unused channels of this cell. Borrowing strategy if all channels are occupied. High probabiltity that call is Blocked if channels

are occupied.( disadv.)

2) Dynamic :

Channels are not allocated to different cells permanently.

Each new call BTS requests new channel from MSC.

MSC allocate a channel, by using an algorithm that takes into account:

1- Frequency is not already in use. 2- Min. reuse distance to avoid co-

channel interference.

Adv. of dynamic assignment strategy :

1) Increase channel utilization ( Increase trunking efficiency ).

2) Decrease probability of a blocked call.

Frequency reuseFrequency reuseConceptConcept

Reuse clusterReuse cluster

Co-channel Reuse Co-channel Reuse ratio (Q)ratio (Q) : :

R : cell radius.D : reuse distance.

Q = D/R. = sqrt(3N).

Where :N : cluster size

HandoverHandover

Definition : procedure that allows MS to change the cell or time-slot to keep as good link as possible during all the call.

Types of handoverTypes of handover

IntraCell : bet. 2 channels of same cell.

InterCell : bet. 2 channels of 2 different cell & same BTS.

InterBTS (intra BSC) : 2 cells of different BTS Same BSC.

InterBSC : bet. 2 cells of different BSC’s & same MSC.

Measurements Measurements before handoverbefore handover

1- Measurements from MS to BSC : a) Strength of BTS signal. b) Quality of BTS signal. c) Signal strength of 6 neighbor BTS’s.

2-Measurements from BTS to BSC : a) Strength of MS signal. b) Quality of MS signal. c) Distance between serving BTS & MS.

Different causes of Different causes of handoverhandover

Better cell HOEmergency HO

Level QualityPBGT

Traffic causesInterferenceDistance

Different causes of

Handover

Basic handover Basic handover algorithmsalgorithms

a)“Min. acceptable performance” algorithm:

MS power is increased when quality deceases till handover is the only way.

b) “Power budget “ algorithm: Prefer direct handover when quality

deceases without increasing MS power first .

Handover priorityHandover priority1) UL quality cause (or interference).2) DL quality cause (or

interference).3) UL level cause.4) DL level cause.5) Distance cause.6) Better cell cause.

InterferenceInterference

1) Another mobile in the same cell. 2) A call in progress in the

neighboring cell.

3) Other BTS’s operating in the same frequency band.

Sources of Sources of interference include:interference include:

Interference effectsInterference effects : :• In voice channel causes crosstalk

• In control channels it leads missed and blocked calls due to errors in the digital signaling.

Main types of Main types of interference :interference :

1) Co-channel interference.

2) Adjacent channel interference.

1) Co-channel 1) Co-channel interferenceinterference

• Source : Near cell using same frequency. It is a function of reuse distance(D/R).• General rule :

io = No. of co-channel interfering cells.S = Signal power from a desired BS.Ii = interference power caused by the ith

interfering co-channel cell BS.

• Another form : C/I = 10 log {(1/n)(D/R)*m} Where : m = propagation constant

(dep’s on nature of environment)

n = number of co-channel interferers.

Can be minimized by : Choosing minimum reuse distance = (2.5….3)(2R).

2) Adjacent channel 2) Adjacent channel interferenceinterference

• Source : A cell using a frequency adjacent to the one in another cell due to imperfect reciever’s filter.

Can be minimized by :

1-careful filtering2-careful channel assignments3-Directional antenna.

• General rule : ACI= -10 Log[(d1/d2)*m] – Adj ch isolation.

Where : d1: distance between MS & proper

BTs d2: dist. Bet MS & adj BTS causing interference.

Adj ch isolation = Filter isolation = - 26db.

Traffic Traffic engineering engineering

theorytheory

Why do we need to Why do we need to know traffic?know traffic?

The amount of traffic during peak hours allows us to dimension our wireless system for a certain GOS.

GOS : probability of having a call blocked during busy hour (block rate).

Traffic intensity (E)Traffic intensity (E)

Erlang : A unit of traffic intensity measure.

1 Erlang = 1 circuit in use for 1 hour.

T ( in Erlangs) = [No. of calls per hour*average call holding time(sec.)] / [3600]

Typical traffic profileTypical traffic profile

Traffic tablesTraffic tables

Erlang B Table

Blocked calls are not held

Erlang C Table

Blocked calls are held in the queue indefinitely

Poisson Table

Blocked calls are held in the queue for a time = the mean holding time of a call

Erlang – B tableErlang – B tableP(N;T) = [ (T^N)*exp(-T) ] / N!

N GOS 1% GOS 2%

2 0.153 0.223

4 0.869 1.093

10 4.46 5.084

20 12.0 13.182

40 29.0 30.997

TrunkingTrunking

Sharing channel among several users.

Trunking efficiency (nT) : Measures the number of subscribers that each channel in every cell can accommodate.

nT = (traffic in Erlangs / no. of channels)*100.

Trunking efficiency in presence of one operator :

N = 7 , 312 one direction voice channels

No. of channels / cell = 312 / 7 = 44 ch./cell.

From Erlang-B table @GOS 2%,this’s equivalent to 35 Erlangs

nT = 35 / 44 = 79.55.

Trunking efficiency in presence of two operators :

N = 7 , 312 / 2 = 156 one direction voice channel for each operator.

No. of channels / cell = 156 / 7 = 22 ch./cell.

From Erlang-B table @GOS 2%,this’s equivalent to 15 Erlangs.

nT = 15 / 22 = 68.18.

System capacitySystem capacity

S : total duplex channels available for use = k*N

Where: N : cluster size. k : No. of channels / cell.

C : total No. of duplex channels in system;

C = M*k*N.

Where : M : No. of times the cluster is repeated.

Improving system Improving system capacitycapacity

Cell splitting.Sectoring.

Cell splittingCell splitting

Sectoring Sectoring We use directional antennas instead

of being omnidirectional

What does sectoring What does sectoring mean?mean?

We can now assign frequency sets to sectors and decrease the re-use distance to fulfill :

1) More freq reuse. 2) Higher system capacity. 3) Improve S/I ratio ( better signal quality ).How S/I ratio is improved? -e.g. In 120 degree sectoring there’s only 2 interferers instead of 6 incase of

omnidirectional N=7 cluster.

Directional frequency Directional frequency reusereuse

Here we use 7/21 pattern for frequency allocation.

Comparison Comparison between various between various types of clusterstypes of clusters

N = 7 omni frequency N = 7 omni frequency plan :plan :

n = 6 , m = 4.

D / R = 4.583.

1) Co-channel interference ratio :

C / I = 18.6 dB.

2) Adjacent channel interference :

ACI = -26 dB @ d1= d2.

N = 7 trapezoidal N = 7 trapezoidal frequency planfrequency plan

n = 2 , m = 4.

D / R = 6.245.

1) Co-channel interference ratio :

C / I = 28.8.

2) Adjacent channel interference : disappears because the channels are assigned alternatively to the cells.

Trunking efficiency :• 312 one direction voice channels N = 7 312 / 7 = 44.57 ~ 44 ch./cell.

• From Erlang-B table @ GOS = 2%T = 35 E. nT = 35 / 44 = 79.55 %.

N = 9 omni frequency N = 9 omni frequency planplan

n = 4 , m = 4.

D / R = sqrt ( 3 * 9 ) = 5.2.

1) Co-channel interference :

C / I = 22.6 dB.

2) Adjacent channel interference :

ACI = -38 dB @ d2 = 2 (d1).

Trunking efficiency :• 312 one direction voice channels N = 9 312 / 9 = 34.67 ~ 34 ch./cell.• From Erlang-B table @ GOS = 2%T = 25.529 E. nT = 25.529 / 34 = 75.085 %.Conclusion : nT 7 > nT 9 But C/I 7 > C/I 9 ACI 7 > ACI 9

4 / 12 cell pattern4 / 12 cell pattern n = 1 , m = 4. D / R = sqrt (3* 4) =

3.732.

C / I = 22.87 dB.

Trunking efficiency :• No. of channels/cell = 312 / 12 = 26

ch./cell.• From Erlang-B table @

GOS = 2 %. T = 18.4 E/cell.nT = 18.4 / 26=

70.77%.

3 / 9 cell pattern3 / 9 cell pattern n = 1 , m = 4. D / R = sqrt (3* 3) = 3.

C / I = 19.1 dB.

Trunking efficiency :• No. of channels/cell =312 / 9 = 34 ch./cell.• From Erlang-B table @

GOS = 2 %. T = 25.5 E/cell.nT = 25.5 / 24 = 75 %.

120 degree cell 120 degree cell sectoringsectoring

n = 2 , m = 4. D / R = sqrt(3 * 7) = 4.583.

Co-channel interference : C / I = 23.436 + 6dB(due to

isolation) = 29.436 dB.

Trunking efficiency :• No. of channels/cell = 312 /

21 = 14.857.• From Erlang-B @ GOS=2%

T= 8.2003. nT = 8.2003 / 14.857 =56.216%.

References :• Motorola CP02• NOKIA SYSTRA