16. GSM Network Elements and Operation

| University of Dar es Salaam | Department of Electronic and Telecommunications Engineering | | University of Dar es Salaam | Department of Electronic and Telecommunications Engineering |

_______________________________________________________________________________ _______________________________________________________________________________

GSM History and Organisation

Date Task Completion Date Task Completion

GSM Network Elementsand Operation

1979 Frequency band reserved for 1991

Europe wide cellular system.

1982 “Groupe Special Mobile” 1992created within CEPT.

1986GSM has full-time team. 1992

DCS1800 phase 1 recommendationfrozen.

“GSM” renamed “Global System forMobile Communications”.

GSM launched for commercial purposes.

1988 ETSI takes over GSM 1993 Phase 2 GSM technical specifications

Committee. First Tender frozen. DCS 1800 launched in the UK ininvitations made for GSM September.

equipment. 1994 GSM commercial operations coverage

1990 Phase 1 GSM world-wide exceeded GSM committee

Recommendations frozen to expectations.

enable development of the 1996 Introduction of microcellular techniques infirst GSM systems. GSM900/DCS1800 networks.

------------------------------------------------------------------------------------------------------------------------

| TE 412 Introduction to Wireless Communications | Christine Mwase | 10/03/2014 |

------------------------------------------------------------------------------------------------------------------------


| University of Dar es Salaam | Department of Electronic and Telecommunications Engineering |

_______________________________________________________________________________

GSM Features and Services

• Compatibility

- GSM has been specified and developed by many European

countries working in co-operation with each other. The result is a

cellular system which has been implemented throughout Europe

and many parts of the world.

• Noise Robust

- In order to combat the problems caused by noise, GSM uses

digital technology instead of analogue.

- This leads to better frequency re-use patterns and more

capacity.

------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________


• Flexibility and Increased Capacity

- GSM enhancement: half rate speech channel, where mobilesuse new speech algorithms requiring half as much data to besent over the air interface. By implementing half rate, one carriersupports 16 users, effectively doubling network capacity.

- International roaming

- Multi-band operation

- GSM equipment is fully controlled by its software. Network re-configurations can be made quickly and easily with a minimum ofmanual intervention required. Although the highly softwaredependent nature of GSM makes it very complex, it alsoprovides for a high degree of flexibility.

------------------------------------------------------------------------------------------------------------------------


| University of Dar es Salaam | Department of Electronic and Telecommunications Engineering |_______________________________________________________________________________


• Use of Standardised Open Interfaces- E.g. C7, X.25

• Improved Security and Confidentiality- GSM offers encryption, ME authentication, Subscriber

authentication, Frequency hopping.

• Flexible Handover Processes- With analogue systems, handovers are frequently a problem

area and the subscriber is often aware that a handover hasoccurred.

------------------------------------------------------------------------------------------------------------------------




• ISDN Compatibility

- The GSM network has been designed to operate with the ISDN

system and provides features which are compatible with it. GSM

can provide a maximum data rate of 9.6 kbit/s while ISDN

provides much higher data rates than this (standard rate 64

kbit/s, primary rate 2.048 Mbit/s).

• Enhanced Range of Services

- Determined by level of service provided by network provider,

level of service purchased by subscriber, and the capabilities of

the subscriber’s mobile equipment.

------------------------------------------------------------------------------------------------------------------------


| University of Dar es Salaam | Department of Electronic and Telecommunications Engineering | | University of Dar es Salaam | Department of Electronic and Telecommunications Engineering |

_______________________________________________________________________________ _______________________________________________________________________________

GSM Features and Services GSM Features and Services

• Enhanced Range of Services • Enhanced Range of Services

- Speech services - Supplementary services

• Telephony • Number identification

• Emergency calls • Call barring

• Short message service point to point • Call forwarding

• Short message cell broadcast • Call completion

• Advanced message handling service • Charging

• Dual personal and business numbers • Multi-party

- Data services

• Raw data

• Fax

------------------------------------------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------------------------------------------

| TE 412 Introduction to Wireless Communications | Christine Mwase | 10/03/2014 | | TE 412 Introduction to Wireless Communications | Christine Mwase | 10/03/2014 |


Data Rates Provided in Basic GSM


Variants of GSM

• GSM supports both voice and data service.

• Standard GSM data rates range from 300 to 9,600 bps.

• Both asynchronous and synchronous data

communications are supported.

• GSM 900- Rx (uplink) 890 - 915 MHz

- Tx (downlink) 935 - 960 MHz

- 124 ARFCNs

• EGSM 900- Rx (uplink) 880 - 915 MHz- Tx (downlink) 925 - 960 MHz

– 174 ARFCNs

• GSM 1800 (DCS1800)- Rx (uplink) 1710 - 1785 MHz


- 374 ARFCNs

• PCS 1900- Rx (uplink) 1850 - 1910 MHz


- 299 ARFCNs

ARFCN: Absolute Radio Frequency Channels- Bandwidth = 200 kHz

------------------------------------------------------------------------------------------------------------------------


- 8 TDMA timeslots------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________

Variants of GSM - summaryVariant Uplink Downlink Total Duplex- Channels

(MHz) (MHz) Bandwidth frequency

GSM-400 451-458 and 461-468 and Twice 14 MHz 10 MHz Twice 72479-486 489-496

GSM-900 890-915 935-960 Twice 25 MHz 45 MHz Twice 124(primary

band)

Extended 880-915 925-960 Twice 35 MHz 45 MHz Twice 174GSM-900

GSM-R 876-880 921-925 Twice 4 MHz 45 MHz Twice 19

DCS-1800 1,710-1,785 1,805-1,880 Twice 75 MHz 95 MHz Twice 373

PCS-1900 1,850-1,910 1,930-1,990 Twice 60 MHz 80 MHz Twice 300

------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________

NetworkComponents

------------------------------------------------------------------------------------------------------------------------



GSM Network Components

• The previous diagram showed a simplified GSM network.Each network component was illustrated only once, however,many of the components will occur several times throughout anetwork.

• The principle component groups of a GSM network are:

- The Mobile Station (MS)

- The Base Station System (BSS)

- The Network Switching System

- The Operations and Maintenance System

------------------------------------------------------------------------------------------------------------------------



The Mobile Station (MS)

Consists of two parts

• The Mobile Equipment (ME)

• The Subscriber Identity Module (SIM)

------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________

The Mobile Equipment (ME)

• hardware used by subscriber to access the network.

• has unique IMEI (International Mobile Equipment Identity)

number permanently stored in it.

- enables the network operator to identify mobile equipment

which may be causing problems on the system.

• three main types of ME:

- vehicle mounted

- portable mobile unit

- handportable unit

• identified by a classmark.

------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________

The Subscriber Identity Module (SIM)

• electronic ‘smart card’ that plugs into the ME.

• has unique IMSI (International Mobile Subscriber Identity)number.

- calls can be routed and bills provided based on the subscriber.

• contains several pieces of information:

- IMSI

- Temporary Mobile Subscriber Identity (TMSI)

- Location Area Identity (LAI)

- Subscriber Authentication Key (Ki)

- Mobile Station International Services Digital Network (MSISDN)

• also executes the authentication algorithm.

------------------------------------------------------------------------------------------------------------------------



The Base Station System (BSS)

• provides the radio interconnection fromthe mobile station to the land-basedswitching equipment.

• communicates with the MSC via 2Mbit/s links.

• consists of:- the Base Transceiver Station (BTS)

- the Base Station Controller (BSC)

- the Transcoder (XCDR)

------------------------------------------------------------------------------------------------------------------------




• the Base Transceiver Station (BTS)

- contains RF hardware

- limited control functionality

- supports one or more cells

• the Base Station Controller (BSC)

- controls one or more BTSs

- switches traffic and signalling to/from the BTSs and the MSC

- connects terrestrial circuits and channels on the air interface

- controls handovers performed by BTS’s under its control

------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________

BSS Functionality Control

Terrestrial Channel Management

Channel Allocation BSC

Radio Channel Management BSC

Channel Configuration Management BSC

Handover Control BSC

Frequency Hopping BSC/BTS

Traffic Channel Management BSC/BTS

Control Channel Management BSC/BTS

Encryption BSC/BTS

Paging BSC/BTS

Power Control BSC/BTS

Channel Coding/Decoding BTS

Timing Advance BTS

Idle Channel Observation BTS

Measurement Reporting BTS

------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________

BSS

Configurations

--------------------------------



XCDR

TCH =

Traffic CHannel

------------------------------------------------------------------------------------------------------------------------




• the Transcoder (XCDR)

- converts the speech or data output from the MSC (64 kbit/sPCM), into the form specified by GSM specifications fortransmission over the air interface, i.e. between the BSS and MS(64 kbit/s to 16 kbit/s and vice versa)

- the content of the 16 kbit/s data depends on the coding algorithmused. There are two speech coding algorithms

• Full Rate speech algorithm

• Enhanced Full Rate speech algorithm

and selecting which one to use depends on the capabilities ofthe mobile equipment and the network configuration.

------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________

The Network Switching System

Consists of:

Mobile Services Switching Centre (MSC)

Home Location Register (HLR)

Visitor Location Register (VLR)

Equipment Identity Register (EIR)

Authentication Centre (AUC)

InterWorking Function (IWF)

Echo Canceller (EC)------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________

The Mobile Services Switching Centre (MSC)

• The MSC is included in the GSM system for call-switching.

• When the MSC provides the interface between the PSTNand the BSSs in the GSM network it will be known as aGateway MSC.

• One MSC is capable of supporting a regional capital withapproximately one million inhabitants.

• Functions carried out by the MSC:

- Call Processing- Operations and Maintenance Support

- Internetwork Interworking

- Billing------------------------------------------------------------------------------------------------------------------------



The Home Location Register (HLR)

• The HLR database contains the master database of allsubscribers to a GSM Public Land Mobile Network (PLMN).

• Parameters stored in the HLR:- Subscriber ID (IMSI and MSISDN)

- Current subscriber VLR (current location)

- Supplementary services subscribed to

- Supplementary service information (e.g. currentforwarding number)

- Subscriber status (registered/deregistered)

- Authentication key and AUC functionality- Mobile Subscriber Roaming Number

------------------------------------------------------------------------------------------------------------------------



The Visitor Location Register (VLR)

• The VLR provides a local database for the subscriberswherever they are physically located within a PLMN, thismay or may not the “home” system.

• Contains some duplicate data as well as more precise datarelevant to the subscriber remaining within the VLRcoverage.

• Additional data stored in the VLR:

- Mobile Status- Location Area Identity- Temporary Mobile Subscriber Identity (TMSI)

- Mobile Station Roaming Number

------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________

The Equipment Identity Register (EIR)

• Concerned solely with MS equipment and not the subscriberwho is using it to make or receive a call, the EIR contains acentralized database for validating the IMEI.

Call Processing Functions (EIR)

IMEI checked against WHITE LIST

If NOT found, checked against ‘Grey/Black’ List

IMEI checked against BLACK/GREY LIST

If found, returns a ‘Black’ or ‘Grey’ List indicator as appropriate------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________


• A GSM network entity which provides the functionality forverifying the identity of an MS when requested by the system.

• The AUC is a processor system, it performs the“authentication” function.

• The AUC is normally co-located with the HLR as it will berequired to continuously access and update, as necessary,the system subscriber records. The AUC/HLR centre can beco-located with the MSC or located remote from the MSC.

------------------------------------------------------------------------------------------------------------------------



AuthenticationProcess

------------------------------------------------------------------------------------------------------------------------




Authentication Process

1. HLR/AUC produce authentication parameters - RAND / SRES / Kc - called“triples”. Triples are sent to VLR where the MS is registered and stored there.

• RAND = RANDom number

• SRES = Signed RESponse

• Kc = Ciphering key

2. VLR sends “Authenticate” message to MSC. MSC sends “AuthenticationRequest” message to MS via BSS unencrypted. Message contains RAND.

3. MS uses A3 & A8 algorithm, Ki parameter on SIM and RAND to calculateSRES and Kc. MS sends “Authentication Response” to VLR with SRES.

• A3 = authentication algorithm that produces SRES using RAND and Ki

• A8 = ciphering key generating algorithm that produces Kc using RAND and Ki

• Ki = individual subscriber authentication key

4. VLR compares its SRES with the received SRES. If they match, authenticationsuccessful. Else, “Authentication Reject” sent to MS and HLR notified.

------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________


Ciphering Process

1. If authentication is successful, VLR sends “Start Ciphering” messageto request that the MSC begin ciphering procedures. Messagecontains information indicating whether ciphering is required.

2. If ciphering is to be used, MSC sends “Cipher Mode Commend”message to BSS. Message contains Kc. Kc is passed to the BTS.

3. MS calculates Kc from RAND, A8 and Ki on the SIM.

BSS responds with “Cipher Mode Complete” message. Using Kc, A5and the GSM hyperframe number, encryption between the MS and theBSS can now occur over the air interface.

------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________

The Interworking Function (IWF)

• Provides the function to enable the GSM to interface with thevarious forms of public and private data networks currentlyavailable.

• Basic features:

- Data rate adaption

- Protocol conversion

• Also incorporates a “modem bank” which may be used when,for example, the GSM Data Terminal Equipment (DTE)exchanges data with a land DTE connected via an analoguemodem.

------------------------------------------------------------------------------------------------------------------------



Echo Canceller (EC)

------------------------------------------------------------------------------------------------------------------------



Echo Canceller (EC)

• Used on the PSTN side of the MSC for all voice circuits.

• Echo control is required at the switch because the inherentGSM delay can cause an unacceptable echo condition, evenon short distance PSTN circuit connections.- The total round trip delay introduced by the GSM system (the

cumulative delay) is approximately 180 ms.

• As a standard telephone connection is 2-wire, a hybrid 2-wireto 4-wire transformer is required at the land party’s localswitch. The transformer causes the echo.

• The standard EC will provide cancellation of up to 68 ms onthe “tail circuit” (tail circuit = connection between output of ECand land telephone).

------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________

The Operations and Maintenance Centre

• Provides the capability to manage the GSM network

remotely.

• Consists of:

- Network Management Centre (NMC)

- Operations and Maintenance Centre (OMC)

------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________

The Network Management Centre (NMC)

------------------------------------------------------------------------------------------------------------------------



The Network Management Centre (NMC)

• Provides global network management

• Single NMC per network

• Used by network managers and planners

• 24 hour supervision

• Functionality:

- Monitors nodes on the network

- Monitors GSM network element statistics

- Monitors OMC regions and provides information to OMC staff

- Passes on statistical information from one OMC region toanother to improve problem solving strategies

- Enables long term planning for the entire network------------------------------------------------------------------------------------------------------------------------



The Operations and Maintenance Centre (OMC)

• Multiple OMCs per network

• Regionalized network management

• Provides central point from which to control and monitor theother network entities as well as monitor the quality of servicebeing provided by the network.

• Employed in daily operations

• Used by network operators

• Provides database for long term network engineering andplanning tools

------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________

The Operationsand MaintenanceCentre (OMC)FunctionalArchitecture

------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________

The Network in Reality

• GSM network is more

complicated than the

previous diagrams have

shown.

• The diagram on the right

shows how multiple BSS

and Network Switching

System components will be

connected within a network.

------------------------------------------------------------------------------------------------------------------------


| University of Dar es Salaam | Department of Electronic and Telecommunications Engineering_______________________________________________________________________________


• A city can have approximately the following number ofnetwork components:

| | University of Dar es Salaam | Department of Electronic and Telecommunications Engineering |_______________________________________________________________________________


• A network can have approximately the following number ofnetwork components:

Network Component Quantity Network Component Quantity

Operations & Management Centre (Base 1 Operations & Management Centre (Base 6

Station Equipment) - OMC (R) Station Equipment) - OMC (R)

Operations & Management Centre (Switching) - 1 Operations & Management Centre (Switching) - 6

OMC (S) OMC (S)

Mobile Services Switching Centre - MSC / VLR 1 - 2 Mobile Services Switching Centre - MSC / VLR 6

Base Station Controller - BSC 5 - 15 Base Station Controller - BSC 40+

Base Station Transceiver - BTS 200 - 400------------------------------------------------------------------------------------------------------------------------


Base Station Transceiver - BTS 1200+------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________

GSM Basic Call Sequence - Mobile to Land Sequence

1. Subscriber pressing “send” initiates a “Channel Request”

message from MS to BSS. This is followed by assignment

of a dedicated control channel by BSS and establishment of

signalling link between MS and BSS.

2. “Request for Service” is passed to MSC which relays it to

VLR. VLR carries out authentication process if MS has

previously registered on this VLR - if not, VLR obtains

authentication parameters from HLR.

3. Authentication takes place. If successful, call setupcontinues. If ciphering is to be used it is initiated at this time.

------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________


4. Message “Set-Up” is sent by MS to MSC accompanied bycall information (type of call, number being called, etc.).Message is forwarded from MSC to VLR.

5. MSC may initiate MS IMEI check (is the mobile stolen?Etc). [This check may occur later in message sequence].

6. In response to “Set-Up”, VLR sends message “CompleteCall” to MSC, which notifies MS with “Call Proceeding”.

7. MSC then assigns traffic channel to BSS. BSS assigns anair interface traffic channel. MS responds to BSS (whichresponds in turn to MSC) with “Assignment Complete”.

------------------------------------------------------------------------------------------------------------------------




8. “Initial and Final Address Message (IFAM)” is sent to PSTN.Ring tone is applied at MS in response to “Alerting”, whichMSC sends to MS when PSTN responds with “AddressComplete Message (ACM)”.

9. When answered, the message “Connect” is forwarded toMS by MSC, stopping the ring tone. MSC then connectsGSM traffic channel to PSTN circuit, completing end to endtraffic connection.

10. Conversation takes place for the duration of the call.

------------------------------------------------------------------------------------------------------------------------



Paging

• A process of broadcasting a message to alert a specific mobile

to take some action, e.g. if there is an incoming call to receive.

• If the system does not know the precise cell in which a mobile is

located it must perform paging in a number of cells.

• An extreme approach would be to undertake paging throughout

the entire coverage area of a cellular system whenever a mobileis to be alerted; however, in anything but the smallest system

this would be wasteful of valuable signalling capacity,

particularly over the air interface.

• The problem is addressed by the use of location areas and

location updating.

------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________

Location Updating

• Used to reduce the area over which paging must be undertaken

in a cellular system.

• The cellular coverage area is divided up into location areas. All

cells broadcast the Identity of their Location Area (LAI).

• Each time a MS moves into a new location area it informs the

network through a location update; this allows the network to

perform paging over smaller area than would otherwise be used.

• Extreme case: each cell could be a location area, system would

know precisely where a mobile was, at the expense of a high

level of location update signalling.

• Compromise: location areas are defined as a group of cells.------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________

Location Area

Public Land Mobile Network (PLMN)

MSC / VLR Area

Location Area

Cell

------------------------------------------------------------------------------------------------------------------------



GSM Network Areas

- Public Land Mobile Network

• the area served by one operator, e.g. Vodacom / Celtel etc.

- MSC / VLR Area

• a group of Location Areas served by one MSC / VLR, i.e. all

the cells connected to that MSC / VLR

- Location Area

• a Group of Cells within one MSC/VLR Area identified by its

Location Area Identity (LAI)

- Cell

• identified by its Cell Global Identity number (CGI) corresponds

to the radio coverage area of one base transceiver station

------------------------------------------------------------------------------------------------------------------------



Location Update

• MS detects that it has entered a new location area by comparing

last known LAI (on SIM) with the info. broadcast by the local cell.

• MS gains access to a radio channel and requests a location update.

- If the serving MSC/VLR is unchanged the network can immediatelyauthenticate the MS and note the change of LA.

• If the MS has moved MSC/VLR, the MSC/VLR addresses a

message to the HLR.

• HLR notes the new location (VLR) and downloads security

parameters to allow authentication to take place. It also passes on

user subscription details to the new VLR and informs the old VLR to

delete its records.------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________

Location Update Message Sequence

• MS to NEW MSC (via BSS) - location updating request


_______________________________________________________________________________

Attach and Detach

• A procedure related to location updating is the IMSI attach and detach.The attach procedure is the complement of the detach procedure.

• New MSC to NEW VLR

• New VLR to HLR

• HLR to OLD VLR

• OLD VLR to HLR

• HLR to NEW VLR

• NEW VLR to HLR

• HLR to NEW VLR

• NEW VLR to NEW MSC

• NEW VLR to MS

- update location area

- update location

- cancel location

- cancel location ACK

- insert subscriber data

- insert subscriber data ACK

- update location ACK

- update location area ACK

- location updating accept

• The IMSI detach / attach procedures mark the MS as detached /attached in the VLR (and optionally in the HLR) on MS power down /power up or SIM removal / insertion.

• IMSI attach: indicates IMSI is active in the network. Attach is invoked ifan IMSI is activated in a MS (power up or SIM insertion) in the coveragearea of the network, or an activated MS enters the coverage area.

• IMSI detach: indicates that the mobile station is unreachable anddisables the location updating function to prevent unnecessarysignalling overhead. Any incoming calls are either rejected or forwardedas specified by the user.

• Attach/detach activation is up to the operator on an individual cell basis.ACK = acknowledge------------------------------------------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------------------------------------------

| TE 412 Introduction to Wireless Communications | Christine Mwase | 10/03/2014 | | TE 412 Introduction to Wireless Communications | Christine Mwase | 10/03/2014 |


Call Routing When Roaming

• Gateway MSC contains a table linking MSISDNs to theircorresponding HLR.

• Gateway MSC can interrogate the subscriber's HLR to obtainrouting information or for simplicity, can have one Gateway MSChandle one PLMN.

• Assuming the former:- Gateway MSC queries the subscriber's HLR for an MSRN.

- HLR queries the subscriber's current VLR for an MSRN.- Subscriber’s current VLR temporarily allocates an MSRN for the call.

- This MSRN is returned to the HLR and back to the Gateway MSC.- Gateway MSC then routes the call to the new MSC.- At the new MSC, the IMSI corresponding to the MSRN is found, and the

mobile is paged in its current location area.

------------------------------------------------------------------------------------------------------------------------



Radio Interface Optimisation

• Transmission Timing

- Synchronization of a TDMA system is critical because burstshave to be transmitted and received within the “real time”timeslots allocated to them.

- The further the MS is from the base station, the longer it takesthe bursts to travel the distance between them.

- The GSM BTS caters for this problem by instructing the MS toadvance its timing (i.e. transmit earlier) to compensate for theincreased propagation delay.

- Timing advance info. is sent to the MS twice every second.

- Max. timing advance ≈ 233µs (caters for max cell radius ≈ 35km)

------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________


• Battery Life

- One of the main factors which restricts reducing the size of a MS

is the battery.

- A battery must be large enough to maintain a telephone call for

an acceptable amount of time without needing to be recharged.

- Four features which enable the life of a GSM MS battery to be

extended:

• Power Control

• Voice Activity Detection (VAD)

• Discontinuous Transmission (DTX)

• Discontinuous Reception (DRX)------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________


• Multipath Fading

- The frequency band used for GSM transmission means that a

“good” location may be only 15 cm from a “bad” location!

- A typical urban profile would cause dispersion of up to 5 µs,

whereas, a hilly terrain would cause dispersion of up to 20 µs.

- GSM offers five techniques that combat multipath fading effects:

• Equalization (must be able to cope with dispersion of up to 17µs

• Diversity

• Frequency hopping

• Interleaving

• Channel coding

------------------------------------------------------------------------------------------------------------------------



GSM Modulation

Transmission of Analogue and Digital Signals

• The main reasons why GSM uses a digital air interface:

- It is “noise robust”, thus enabling the use of tighter frequency re-use patterns and minimizing interference problems;

- It incorporates error correction, thus protecting the traffic that itcarries;

- It offers greatly enhanced privacy to subscribers and security tonetwork providers;

- It is ISDN compatible, uses open standardized interfaces andoffers an enhanced range of services to its subscribers

------------------------------------------------------------------------------------------------------------------------



GSM ModulationModulation Techniques

• There are three methods of modulating a signal:

- Amplitude Modulation (AM):• very simple to implement for analogue signals but it is prone to noise.

- Frequency Modulation (FM):• more complicated to implement but provides a better tolerance to noise.

- Phase Modulation (PM) / Phase Shift Keying (PSK):• best tolerance to noise but it is very complex to implement for analogue

signals and is therefore rarely used.

• Digital signals can use any of the methods, but phase modulationprovides the best noise tolerance. Since phase modulation can beimplemented easily for digital signals, this is the method which isused for the GSM air interface.

------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________

GSM ModulationTransmission of Digital Signals

• Phase Shift Keying (PSK)

- High degree of noise tolerance. However, there is a problem.- When the signal changes phase abruptly, high frequency components are

produced, thus a wide bandwidth would be required for transmission.

- GSM has to be efficient with available bandwidth. Therefore it is not thistechnique, but a more efficient development of this technique that is actuallyused by the GSM air interface.

• Gaussian Minimum Shift Keying (GMSK)

- The phase change (represents the change from a digital ‘1’ or a ‘0’) doesnot occur instantaneously. It occurs over a period of time and therefore theaddition of high frequency components to the spectrum is reduced.

- First the digital signal is filtered through a Gaussian filter, causing distortionto the signal - the corners are rounded off. The distorted signal is then usedto phase shift the carrier signal. The phase change therefore is spread out.

------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________

Short Message Service (SMS)

• Globally accepted wireless service that first appeared in 1991 withGSM.

• Enables transmission of alphanumeric messages between mobilesubscribers and external systems e.g. email. Enables transmissionof short messages to and from wireless devices.

• Uses a short message service centre (SMSC), which acts as astore-and-forward system for short messages.

• Compared to other text-message services e.g. alphanumeric paging

- guaranteed delivery of text messages to the destination.

- supports many input mechanisms allowing interconnection withdifferent message sources and destinations.

------------------------------------------------------------------------------------------------------------------------




• An active mobile handset is able to receive or submit a shortmessage at any time, regardless of whether a voice or data call is inprogress (in some implementations, this may depend on the MSC orSMSC capabilities).

• Initial applications:- eliminating alphanumeric pagers by permitting two-way general-purpose

messaging and notification services, primarily for voice mail.

• Variety of services introduced:- e-mail, fax, and paging integration, interactive banking, information

services such as stock quotes, and integration with Internet-basedapplications.

------------------------------------------------------------------------------------------------------------------------



Short Message Service (SMS) - Benefits

To subscribers

• Guaranteed message delivery

• Delivery of notifications and alerts

• Reliable, low-cost communication for concise information• Delivery of messages to multiple subscribers at a time

• Integration with other data and Internet-based applications

To providers• Protection of network resources (such as voice channels), due to

SMS’ sparing use of the control and traffic channels

• Revenue resulting from provision of value-added services such as e-mail, voice mail, fax and Web-based application integration

• Notification mechanisms for newer services

------------------------------------------------------------------------------------------------------------------------



_______________________________________________________________________________

Short Message Service (SMS) - Basic NetworkArchitecture for an SMS Deployment (IS-41)

Source: International Engineering Consortium

------------------------------------------------------------------------------------------------------------------------| TE 412 Introduction to Wireless Communications | Christine Mwase | 10/03/2014 |


_______________________________________________________________________________


• External Short Messaging Entities - devices that may receive orsend short messages. A short message entity (SME) may belocated in the fixed network, a mobile device, or another servicecentre.

• SMSC - a combination of hardware and software responsible for therelaying and storing and forwarding of a short message between anSME and mobile device.

• Signal Transfer Point (STP) - a network element that allows IS-41interconnections over signalling system 7 (SS7) links with multiplenetwork elements.

------------------------------------------------------------------------------------------------------------------------


16. GSM Network Elements and Operation

Documents

Transcript of 16. GSM Network Elements and Operation