gsm architecture lect 5,6,7

7/27/2019 GSM Architecture Lect 5,6,7

http://slidepdf.com/reader/full/gsm-architecture-lect-567 1/50

GSM Network Overview



GSM Services Tele Services:

Telephony (13 kbps) and emergency calls Fax

Short message service Bearer Services:

Data 300 – 9600 kbps

Supplementary Services: Call Barring

Call Forwarding Call waiting, hold and transfer Multiparty Line Identity



Frequency Spectrum GSM 900

890 – 915 MHz uplink (MS -> BS) or Reverse Channel.

935 – 960 MHz downlink (BS -> MS) or Forward

Channel. E-GSM 10 MHz

880 – 915 MHz uplink (MS -> BS) or Reverse Channel.

925 – 960 MHz downlink (BS -> MS) or ForwardChannel.

PCN/DCS/GSM-1800

1710 – 1785 MHz uplink

1805 – 1880 MHz downlink



ARFCN



UL and DL Frequencies



Speech Multiframe Structure



GSM Air Interface Specification



Network Overview Access Network

Mobile Station Base Transceiver Station Base Station Controller

Base Station Subsystem (BTS + BSC) Core Network Switching

Mobile Switching Centre Gateway MSC

Core Network Mobility And Service Control Visitor Location Register Home Location Register Authentication Centre Equipment Identity Register

Maintenance And Management Control Network Management Centre Operation and Management Centre



Network Architecture



Architecture and Interfaces



Network Architecture The Access Network

Consist of Mobile and BSS.

BSS primary role to control the radio link with theMS.

BSS Consists: BTS define cell and handles the radio-link protocols.

BSC and BTS communicate Abis interface.

BSC manages the radio resources such as radio-channelsetup and handovers.

BSC hardware may be located same site as BTS orelsewhere.



Contd. The Core Network Switching

MSC similar normal switching node but additionalfunctions are: Location updating Authentication Registration Handovers Call Routing

GMSC provides interface to external networks. Responsible for fetching location information. GMSC support MSC if network require more

capacity.



Contd. The Core Network Mobility and Service

Control

HLR contains the service subscription.

Stores the current location of user.

VLR contains subset of the information.

VLR and MSC are physically integrated. Authentication centre stores the

authentication algorithm.



Public Land Mobile Network PSTN is collection of MSC areas.

MSC interfaces between fixed networksand PLMN.

PLMN’s may be interconnected throughthe ISDN/PSTN and PDNs.



PLMN



Mobile Station

Terminal Equipment.

Terminal Adaptor – Signal adoption.

Mobile Equipment – Contains transceiver signalprocessing, speech transcoder.

Subscriber Identity Module – Provides user identity,authentication, system and user data.



SIM Card

International Mobile Subscriber Identity (IMSI) number. Mobile Service ISDN (MSISDN) number used to identify

the subscriber to the system.

Secret Key (Ki). Algorithms

A3 to calculate the response to the network challenge. A5 the actual encryption. A8 generates the encryption key used for

authentication. Four Digit PIN number which is stored on the card. Location area identifier (LAI). Accumulated call charges and Remaining call allocation.



Base Transceiver Station

BTS provides: Radio Transceiver Signal Processing Ciphering

Single transceiver support eight radio channel. BTS can handle between 1 and 16 radio carriers. Carrying between 7 and around 115 simultaneously

conversation.



Key Components of BTS



Functions of BTS

Each BTS transmits BCCH channel on one TRX withconstant output.

Encodes, and corrects, multiplexes, interleaves,

modulates and feeds the RF signal to antenna. Ciphering and De-ciphering. Time and frequency synchronizes signals transmitted

from the BTS. Random access detection. Timing advance. Uplink radio channel measurement. Frequency hopping.



Base Station Controller

Monitors and controls several base stationbetween 10 to 100.

Concerned frequency administration andexchange function.

Assign and releases frequencies and timeslots.

Handover operation and reallocates

frequencies to BTS’s. Interface between BSC and MSC is

standardized SS7 interface.



MSC and GMSC

Similar to exchange in a fixed network. Difference MSC has to cope with the mobility of the users. MSC functions are:

Management of handover. Billing Subscribers. Echo cancellation. Paging. Location registration.



Variable Location Register

Database of mobile users.

Data contained in HLR and VLR are same.

Data stored in VLR: The IMSI, MSISDN, MSRN, TMSI.

Location area (LA) of MS.

ID of the MSC.

Authentication key, query and response obtainedfrom the AuC.



IMSI(International MobileSubscriber Identity)

Each SIM uniquely employs IMSI used forsubscriber identification.

MCC(3 decimal places)

MNC(2 decimal places)

MSIN(10 decimal places)



MSISDN(Mobile SubscriberISDN Number)

Country Code (CC)-Upto 3 decimalplaces

National Destination Code (NDC)-2to3decimal places

Subscriber Number (SN)-Maximum 10

decimal places.



Mobile Station RoamingNumber(MSRN)

MSRN is a temporary mobile dependentISDN number,assigned by locally

responsible VLR to each MS. CC- of the visited network

NDC-of the visited network

SN-in current mobile network



TMSI(Temporary MobileSubscriber Identity)

Assigned by VLR which has onlysignificance in the location of current

subscriber.It is stored in Network side.TMSI allows a subscriber to be identified

uniquely in a network and can consists

up to 4*6 bits



Home Location Register

Mobile subscriber information IMSI. Call Divert settings. Subscription basis. Authentication Key The subscribers permitted supplementary services.

Temporary data is also stored The current location area. The mobile station roaming number. The address of the current visitor location register.



Equipment Identity Register

Responsible for tracking mobile terminals.

Each mobile phone equipped with an unambiguousidentification code (IMEI).

EIR internally consists of three databases: The WHITE LIST contains all certified mobile equipment that

may be used in a network operator’s PLMN.

The BLACK LIST contains all mobile that shall be barredbecause f theft or technical problems.

The GREY LIST for mobiles with faulty software, that aredropping an excessive number of calls or seem to have oddusage.



International Mobile Equipment Identifier

IMEI is a unique 15 digit code.

The IMEI can be displayed on most phones by dialingthe code *#06#.

Usually printed on the compliance plate under thebattery.

IMEI code is divided into four section e.g. 490520 30123456 0:

TAC Type Approval Country 490520 FAC Final Assembly Code 30

SNR Serial Number 123456

SP Spare 0 usually this digit is zero.



Burst Structure of TDMA Slot

GSM have five different Burst structure

Normal Burst

Synchronization Burst

Frequency Correction Burst

Access Burst

Dummy Burst



Normal Burst

Almost every kind of data transmission. Normal burst contain 114 bit of useful data. Tail bits at the start and the end of data are

used as guard time. Stealing flag used to indicate special signaling

data than the normal burst traffic e.g. inHandover.



Contd.

Uses of Training sequence: Mobile synchronize their receivers with the base station. Used to compensate for multipath fading by the help of

equalizer. Equalizer detects the impulse response of the transmission

channel. Equalizer can compensate for timing delays up to 16 usec.

Training sequence is placed in the middle of data. Because channel condition change very rapidly, so

we can protect our data easily. Guard period protects messages from overlapping

into adjacent time slots.



Synchronization Burst

Tail and Guard bit are used for same purposeas Normal burst.

Coded data contains information such asTDMA frame number.

Training sequence contains information aboutthe base station number.

Synchronization burst appears withsynchronization channel SCH.



Frequency Correction Burst

Enables the mobile to synchronize its frequency with the masterfrequency.

Base station transmits fixed sequence of zeroes within time slot. GMSK modulator to produce sine wave of fixed frequency,

which is exactly 67.7 kHz. This constant transmission frequency allows an MS to lock on to

the BCCH frequency. Frequency correction burst appears on frequency correction

channel. Synhronization channel always follows the frequency correction

channel.



Random Access Burst

When first switched on, mobile needs to orientate

itself within the network. Mobile firstly synchronizes itself in frequency. Then synchronizes itself in time using the

synchronization burst. Burst is only 88 bits and there are 68.25 guard bits at

the end of burst. If mobile is far away 37 Km radius, then random

access burst will not interfere or overlap withadjacent slots.



Dummy Burst

Dummy burst are sent out from the base station on

the base channel. In order to fill idle time slots on this channel. So base channel power density broadcast higher than

that all other frequency. This enables mobile station to detect the base

channel, when first switched on. The dummy burst is coded with pseudorandom bit

sequence to prevent accidental confusion withfrequency correction burst.



Logical Channels

Logical channels are really just structures of data or signaling information.

Which are in certain forms and be presented

in certain order onto the physical channel. Ten general logic channels. That can be mapped onto physical channel in

seven combination. Two types:

Traffic channels, used for speech or data. Control channels, used for managing the network.



Logical Channels



Traffic Channels

Full rate speech traffic channel (TCH/F) Carries raw speech at 13 kbps.

Which is encoded and operates at a grossrate of 22.8 kbps.

Half rate speech traffic channel (TCH/H) Can be used to double the capacity of the

system.

By compressing speech by factor of twoand operates at gross rate of 11.4 kbps.



Control/Signaling Channels

Broadcast Channels Transmitted only by the base station, Downlink

only.

Enable the mobile station to synchronize with thenetwork.

Obtain key information to facilitatecommunication.

Frequency Correction Channel (FCCH) Provides the mobile with the reference frequency and is

mapped onto frequency correction burst.

When mobile first switched on, enables to lock on thebroadcast channel frequency on BCCH and SCH.



Contd.

Synchronization Channel (SCH) Channel is mapped onto the synchronization burst. Carries the Base Station Identity Code (BSIC) plus current Frame Number

(FN) of BTS. Absolute value of the FM is not transmitted, but the relative position of an

FN in the frame hierarchy. FN is sent as a combination of the parameters T1, T2 and T3. BSIC does not uniquely identify single BTS. BSIC consist:

3 bit long network color code (NCC). 3 bit long base station color code (BCC).

Broadcast Control Channel (BCCH) Gives the mobile to identify and gain access to the network e.g.

Location area code. The radio frequency channels. RF Channel number for allocation. The hopping sequence.



GSM Frame Hierarchy



Common Control Channels

Used to carry signaling information for accessmanagement and call setup.

Random Access Channel (RACH) Random access burst is mapped on this channel. This uplink channel is used by the mobile to request

dedicated channel on the network.

The channel operates on slotted ALOHA protocol.

Answered is not received within specific time, mobile

assumes collision has occurred and repeat the request. Analysis shows that if load on the RACH exceeds 35% so

many collision occurs.

System can control make a group is called an ACCESS LIST.



Contd.

Paging Channel (PCH) This downlink channel is used by the base station to call the mobile

when an incoming call has arrived. This channel is mapped to normal burst.

MS responds to paging call with the RACH. Once allocated channel (frequency and time slot) the MS sends

message like “I am MS qqqq you paged me”. BSC does not immediately reserve a channel when paging request

is sent as it does not know which cell within LA contains the MS.

Access Grant Channel (AGCH)

This downlink is used by the base station to inform the mobile onwhich channel it should transmit. Channel also contains the timing advance information and response

to the random access channel message. This channel is mapped to normal burst.



Dedicated Control Channels

Used for general message transfer between the mobile stationand vice versa.

Stand-alone Dedicated Control Channel (SDCCH) Bi-Directional channel used for mobile registration, location update

and authentication. Slow Associated Control Channel (SACCH)

Bi- Directional channel. In the downlink the base station to inform the mobile of the latest

system changes. In the uplink direction the mobile informs the base station about

power measurements made on the neighboring cells. This channel always used in association with an SDCCH or a traffic

Channel. The word slow is used because the data rate is low as SACCH is

sent only once in every 26 frames.



Contd.

Fast Associated Control Channel (FACCH)

Bi- Directional channel is used to replace trafficchannel when some heavy duty signaling, such ashandover is needed.

This channel is used to transmit long signalingmessages.

To replace the traffic channel with this channel thesealing flags on the traffic channel are set.

The word fast is used because a higher data rateis used than SACCH.



Logical Channel Combinations

Seven possible combinations

1) TCH/F+FACCH/F+SACCH/F

2) TCH/H(0,1)+FACCH/H(0,1)+SACCH/H(0,1)3) TCH/H(0)+FACCH/H(0)+SACCH/H(0)+TCH/H(1)

+FACCH/H(1)+SACCH/H(1)

4) FCCH+SCH+CCCH+BCCH

5) FCCH+SCH+CCCH+BCCH+SDCCH/4+SACCH/46) CCCH+BCCH

7) SDCCH/8+SACCH/8



Combinations Based on Cell Utilization

Low Capacity Cell With 1 TRX TN 0: FCCH+SCH+BCCH+CCCH+SDCCH/4 +SACCH/4 TN 1-7: TCH/F+FACCH/F+SACCH/F

Medium Capacity Cell With 4 TRX’s TN 0: FCCH+SCH+BCCH+CCCH TN 2+4: SDCCH/8+SACCH/8 29 TN’s: TCH/F+FACCH/F+SACCH/F

High Capacity Cell With 12 TRX’s

TN 0: FCCH+SCH+BCCH+CCCH TN 2+4+6: BCCH+CCCH 5 TN’s: SDCCH/8+SACCH/8 87 TN’s: TCH/F+FACCH/F+SACCH/F



Athentication Process

From HLR(ki,A3,A5) From VLR(RAND,SRES,KC) VLR send RAND towards MS MS generate KC and SRES KC=A8(RAND,Ki) SRES=A3(RAND,Ki)

Both VLR and MS send SRES toward AUC Where as KC is verified at BTS(Air Interface

Encryption)

gsm architecture lect 5,6,7

Documents