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Unit II:

GSM services

GSM services are a standard collection of applications and features available to mobile phone

subscribers all over the world. The GSM standards are defined by the 3GPP collaboration andimplemented in hardware and software by equipment manufacturers and mobile phone operators.

The common standard makes it possible to use the same phones with different companies' services,

or even roam into different countries. GSM is the world's most dominant mobile phone standard.The design of the service is moderately complex because it must be able to locate a moving phoneanywhere in the world, and accommodate the relatively small battery capacity, limited input/output

capabilities, and weak radio transmitters on mobile devices.

Accessing a GSM network

In order to gain access to GSM services, a user needs three things:

A billing relationship with a mobile phone operator. This is usually either where services are

paid for in advance of them being consumed (prepaid), or where bills are issued and settled

after the service has been consumed (postpaid).

A mobile phone that is GSM compliant and operates at the same frequency as the operator.

Most phone companies sell phones from third-party manufacturers.

A Subscriber Identity Module (SIM) card, which is activated by the operator once the billing

relationship is established. After activation the card is then programmed with the subscriber's

Mobile Subscriber Integrated Services Digital Network Number (MSISDN) (the telephone

number). Personal information such as contact numbers of friends and family can also bestored on the SIM by the subscriber.

After subscribers sign up, information about their identity (telephone number) and what services

they are allowed to access are stored in a "SIM record" in the Home Location Register (HLR).

Once the SIM card is loaded into the phone and the phone is powered on, it will search for thenearest mobile phone mast (also called a Base Transceiver Station/BTS) with the strongest signal inthe operator's frequency band. If a mast can be successfully contacted, then there is said to be

coverage in the area. The phone then identifies itself to the network through the control channel.Once this is successfully completed, the phone is said to be attached to the network.

The key feature of a mobile phone is the ability to receive and make calls in any area where

coverage is available. This is generally called roaming from a customer perspective, but also called

visiting when describing the underlying technical process. Each geographic area has a databasecalled the Visitor Location Register (VLR), which contains details of all the mobiles currently in

that area. Whenever a phone attaches, or visits, a new area, the Visitor Location Register must

contact the Home Location Register to obtain the details for that phone. The current cellularlocation of the phone (i.e., which BTS it is at) is entered into the VLR record and will be used

during a process called paging when the GSM network wishes to locate the mobile phone.

Every SIM card contains a secret key, called the Ki, which is used to provide authentication andencryption services. This is useful to prevent theft of service, and also to prevent "over the air"

snooping of a user's activity. The network does this by utilising the Authentication Center and is

accomplished without transmitting the key directly.

Every GSM phone contains a unique identifier (different from the phone number), called theInternational Mobile Equipment Identity (IMEI). This can be found by dialing *#06#. When a

phone contacts the network, its IMEI may be checked against the Equipment Identity Register to

locate stolen phones and facilitate monitoring.

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Voice calls

Outgoing

Once a mobile phone has successfully attached to a GSM network as described above, calls may bemade from the phone to any other phone on the global Public Switched Telephone Network.

The user dials the telephone number, presses the send or talk key, and the mobile phone sends a call

setup request message to the mobile phone network via the nearest mobile phone base transceiverstation (BTS).

The call setup request message is handled next by the Mobile Switching Center, which checks thesubscriber's record held in the Visitor Location Register to see if the outgoing call is allowed. If so,

the MSC then routes the call in the same way that a telephone exchange does in a fixed network.

If the subscriber is on a prepaid tariff (sometimes known as Pay As You Go (PAYG) or Pay & Go),

then an additional check is made to see if the subscriber has enough credit to proceed. If not, thecall is rejected. If the call is allowed to continue, then it is continually monitored and the

appropriate amount is decremented from the subscriber's account. When the credit reaches zero, the

call is cut off by the network. The systems that monitor and provide the prepaid services are not partof the GSM standard services, but instead an example of intelligent network services that a mobile

phone operator may decide to implement in addition to the standard GSM ones.

Incoming calls

Gateway MSC contact

When someone places a call to a mobile phone, they dial the telephone number (also called a

MSISDN) associated with the phone user and the call is routed to the mobile phone operator's

Gateway Mobile Switching Centre. The Gateway MSC, as the name suggests, acts as the "entrance"

from exterior portions of the Public Switched Telephone Network onto the provider's network.As noted above, the phone is free to roam anywhere in the operator's network or on the networks of

roaming partners, including in other countries. So the first job of the Gateway MSC is to determinethe current location of the mobile phone in order to connect the call. It does this by consulting the

Home Location Register (HLR), which, as described above, knows which Visitor Location Register

(VLR) the phone is associated with, if any.

Routing the call

When the HLR receives this query message, it determines whether the call should be routed toanother number (called a divert), or if it is to be routed directly to the mobile.

If the owner of the phone has previously requested that all incoming calls be diverted to

another number, known as the Call Forward Unconditional (CFU) Number, then this number

is stored in the Home Location Register. If that is the case, then the CFU number is returned

to the Gateway MSC for immediate routing to that destination.

If the mobile phone is not currently associated with a Visited Location Register (because the

phone has been turned off) then the Home Location Register returns a number known as the

Call Forward Not Reachable (CFNRc) number to the Gateway MSC, and the call isforwarded there. Many operators may set this value automatically to the phone's voice mail

number, so that callers may leave a message. The mobile phone may sometimes override the

default setting. Finally, if the Home Location Register knows that the phone is roaming in a particular

Visited Location Register area, then it will request a temporary number (called an MSRN)

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from that VLR. This number is relayed back to the Gateway MSC, and then used to route the

call to the MSC where the called phone is roaming.

Locating and ringing the phone

When the call arrives at the Visiting MSC, the MSRN is used to determine which of the phones inthis area is being called, that is the MSRN maps back to the SIM of the original phone number

dialled. The MSC pages all the mobile phone masts in the area in order to inform the phone that

there is an incoming call for it. If the subscriber answers, a speech path is created through theVisiting MSC and Gateway MSC back to the network of the person making the call, and a normal

telephone call follows.

It is also possible that the phone call is not answered. If the subscriber is busy on another call (and

call waiting is not being used) the Visited MSC routes the call to a pre-determined Call ForwardBusy (CFB) number. Similarly, if the subscriber does not answer the call after a period of time

(typically 30 seconds) then the Visited MSC routes the call to a pre-determined Call Forward NoReply (CFNRy) number. Once again, the operator may decide to set this value by default to the

voice mail of the mobile so that callers can leave a message.

If the subscriber does not respond to the paging request, either due to being out of coverage, or their

battery has gone flat/removed, then the Visited MSC routes the call to a pre-determined CallForward Not Reachable (CFNRc) number. Once again, the operator may decide to set this value by

default to the voice mail of the mobile so that callers can leave a message.

A roaming user may want to avoid these forwarding services in the visited network as excessive

roaming charges may apply.

Voice charges

In the United States and Canada, callers pay the cost of connecting to the Gateway MSC of the

subscriber's phone company, regardless of the actual location of the phone. As mobile numbers aregiven standard geographic numbers according to the North American Numbering Plan, callers pay

the same to reach fixed phones and mobile phones in a given geographic area. Mobile subscribers

pay for the connection time (typically using in-plan or prepaid minutes) for both incoming andoutgoing calls. For outgoing calls, any long distance charges are billed as if they originate at the

GMSC, even though it is the visiting MSC that completes the connection to the PSTN. Plans thatinclude nationwide long distance and/or nationwide roaming at no additional charge over "local"

outgoing calls are popular.

Mobile networks in Europe, Asia (except Hong Kong, Macau (Macao) and Singapore), Australia &

Argentina only charge their subscribers for outgoing calls. Incoming calls are free to the mobile

subscriber with the exception of receiving a call while the subscriber is roaming as described below.However, callers typically pay a higher rate when calling mobile phones. Special prefixes are usedto designate mobile numbers so that callers are aware they are calling a mobile phone and therefore

will be charged a higher rate.

From the caller's point of view, it does not matter where the mobile subscriber is, as the technical

process of connecting the call is the same. If a subscriber is roaming on a different company'snetwork, the subscriber, instead of the caller, may pay a surcharge for the connection time.

International roaming calls are often quite expensive, and as a result some companies require

subscribers to grant explicit permission to receive calls while roaming to certain countries.

When a subscriber is roaming internationally and a call is forwarded to his or her voice mail, such

as when his or her phone is off, busy, or not answered, he or she may actually be charged for twosimultaneous international phone callsthe first to get from the GMSC to the VMSC and thesecond to get from the VMSC to the Call Forward Busy or Call Forward No Reply number

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(typically the voice mailbox) in the subscriber's country. However, some networks' GMSCs connect

unanswered calls directly, keeping the voice signal entirely within the home country and thusavoiding the double charge.

Data transmission

The GSM standard also provides separate facilities for transmitting digital data. This allows a

mobile phone to act like any other computer on the Internet, sending and receiving data via theInternet Protocol.

The mobile may also be connected to a desktop computer, laptop, or PDA, for use as a network

interface (just like a modem or Ethernet card, but using one of the GSM data protocols describedbelow instead of a PSTN-compatible audio channel or an Ethernet link to transmit data). Some

GSM phones can also be controlled by a standardised Hayes AT command set through a serial cable

or a wireless link (using IRDA or Bluetooth). The AT commands can control anything from ringtones to data compression algorithms.

In addition to general Internet access, other special services may be provided by the mobile phone

operator, such as SMS.

Circuit-switched data protocols

A circuit-switched data connection reserves a certain amount of bandwidth between two points forthe life of a connection, just as a traditional phone call allocates an audio channel of a certain

quality between two phones for the duration of the call.

Two circuit-switched data protocols are defined in the GSM standard: Circuit Switched Data (CSD)and High-Speed Circuit-Switched Data (HSCSD). These types of connections are typically charged

on a per-second basis, regardless of the amount of data sent over the link. This is because a certain

amount of bandwidth is dedicated to the connection regardless of whether or not it is needed.

Circuit-switched connections do have the advantage of providing a constant, guaranteed quality ofservice, which is useful for real-time applications like video conferencing.

General Packet Radio Service (GPRS)

The General Packet Radio Service (GPRS) is a packet-switched data transmission protocol, whichwas incorporated into the GSM standard in 1997. It is backwards-compatible with systems that use

pre-1997 versions of the standard. GPRS does this by sending packets to the local mobile phone

mast (BTS) on channels not being used by circuit-switched voice calls or data connections. MultipleGPRS users can share a single unused channel because each of them uses it only for occasional

short bursts.

The advantage of packet-switched connections is that bandwidth is only used when there is actually

data to transmit. This type of connection is thus generally billed by the kilobyte instead of by the

second, and is usually a cheaper alternative for applications that only need to send and receive datasporadically, like instant messaging.

GPRS is usually described as a 2.5G technology; see the main article for more information.

Short Message Service (SMS)

Short Message Service (more commonly known as text messaging) has become the most used data

application on mobile phones, with 74% of all mobile phone users worldwide already as active

users of SMS, or 2.4 billion people by the end of 2007.

SMS text messages may be sent by mobile phone users to other mobile users or external services

that accept SMS. The messages are usually sent from mobile devices via the Short Message ServiceCentre using the MAP protocol.

The SMSC is a central routing hubs for Short Messages. Many mobile service operators use their

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SMSCs as gateways to external systems, including the Internet, incoming SMS news feeds, and

other mobile operators (often using the de facto SMPP standard for SMS exchange).

The SMS standard is also used outside of the GSM system; see the main article for details.

Supplementary Services

Call forwarding.

Barring of Outgoing Calls.

Barring of Incoming Calls.

Advice of Charge (AoC). This service lets users find out how much a call will cost or has

cost, without having to phone their mobile network operator's customer service department

for information. This makes users satisfied by helping to prevent bill shock. It also reduces

customer service departments' workload. Few mobile network operators offer Advice ofCharge service. MTN Irancell offers an Advice of Charge service to prepaid subscribers, but

it is implemented differently than the GSM standard recommends.

Call Hold.

Call Waiting.

Multiparty service.

Calling Line Identification presentation/restriction.

Closed User Group (CUG).

Explicit Call Transfer (ECT). This service allows a user who has two calls to connect these

two calls together and release its connections to both other parties

)

Architechture and Working Of GSM Networks

We have already read about how GSM technology had taken over mobile communicationtechnologies and grown over to 214 countries around the world. Now it is important to learn how

GSM network works and what is the architecture of GSM networks?. GSM network is consist of

thee major systems, it can also be considered architecture of GSM networks. These three systemsare SS, which is known to be The Switching System, BSS, it is The Base Station and the other

important system is, The operation and support System for GSM networks.

The Switching system is very operative system in which many crucial operations are conducted, SSsystems holds five databases with in it which performs different functions. If we talk about major

tasks of SS system it performs call processing and subscriber related functions. These five databasesfrom SS systems are HLR, MSC, VLR, AUC and EIR. Lets study each database in detail and learnwhat functions this little systems performs.

HLR- Home Location Register:

HLR is database, which holds very important information of subscribers. It is mostly known for

storing and managing information of subscribers. It contains subscriber service profile, status of

activities, information about locations and permanent data of all sorts. When new connections arepurchased, these subscribers are registered in HLR of mobile phone companies.

MSC- Mobile Services Switching Center:

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MSC is also important part of SS, it handles technical end of telephony.It is build to perform

switching functionality of the entire system. Its most important task is to control the calls to andfrom other telephones, which means it controls calls from same networks and calls from other

networks. Toll ticketing, common channel signaling, network interfacing etc are other tasks which

MSC is responsible for.

VLR- Visitor Location Register:

VLR performs very dynamic tasks; it is database which stores temporary data regarding subscriberswhich is needed by Mobile Services Switching Center-MSC VLR is directly connected to MSC,

when subscribe moves to different MSC location, Visitor location register VLR integrates to MSC

of current location and requests the data about subscriber or Mobile station ( MS ) from the HomeLocation Register HLR. When subscriber makes a call the Visitor location register-VLR will have

required information for making call already and it will not required to connect to Home RegisterLocation - HRL again.

AUC- Authentication Center:

AUC is small unit which handles the security end of the system. Its major task is to authenticate andencrypt those parameters which verify users identification and hence enables the confidentiality of

each call made by subscriber. Authentication center AUC makes sure mobile operators are safefrom different frauds most likely to happen when hackers are looking for even smallest loop wholes

in systems.

EIR Equipment Identity Register:

EIR is another important database which holds crucial information regarding mobile equipments.

EIR helps in restricting for calls been stolen, mal functioning of any MS, or unauthorized access.AUC Authentication center and EIR- Equipment Identity registers are either Stand-alone nodes or

some times work together as combined AUC/EIR nodes for optimum performance.

The Base Station System (BSS)

The base station system have very important role in mobile communication. BSS are basically out

door units which consist of iron rods and are usually of high length. BSS are responsible forconnecting subscribers (MS) to mobile networks.All the communication is made in Radio

transmission.The Base station System is further divided in two systems. These two systems, they

are BSC, and BTS. Lets study these two systems in detail.

BTS The Base Transceiver Station:

Subscriber, MS (Mobile Station) or mobile phone connects to mobile network through BTS; ithandles communication using radio transmission with mobile station. As name suggests, Base

transceiver Station is the radio equipment which receive and transmit voice data at the same time.BSC control group of BTSs.

BSC The Base Station Controller:

The Base Station normally controls many cells, it registers subscribers, responsible for MShandovers etc. It creates physical link between subscriber (MS) and BTS , then manage and controls

functions of it. It performs the function of high quality switch by handover over the MS to next

BSC when MS goes out of the current range of BTS, it helps in connecting to next in range BTS tokeep the connection alive within the network. It also performs functions like cell configuration data,

control radio frequency in BTS. Data moves to MSC-Mobile switching center after BSC doneprocessing it. MSC is switching center which acts as bridge between different mobile networks.

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The radio interface in GSM

The radio interface in GSM provides the means by which a mobile station communicates with thebase station of the network. Figure 1. provides a simplified block diagram of the GSM radio link.

We will begin by examining the modulation scheme and the carrier frequencies used in GSM. Then,

we will discuss the construction of TDMA bursts or packets and the way in which these may bedemodulated in the presence of intersymbol interference (ISI) caused by the radio channel and the

modulation process itself.

Following these we will discuss the different channels that are available in GSM and the mapping

of the radio resources to each of these channels. Then we will turn our attention to the coding,interleaving and ciphering processes that occur on the GSM radio interface specifically for the

speech information, user data and signalling information separately since they are significantlydifferent.

SpeechEncoder

ChannelEncoder Interleaver

CipheringBurst

Assembler

RF Rx

Equalize/

DemodDechiphering

Deinterleaving

ChannelDecoder

Recovered

User Data

SpeechDecoder

RecoveredSpeech

Channel

RF Tx

GMSKModulator

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SM Radio Carriers

GSM uses a combined TDMA/FDMA multiple-access scheme. The available spectrum ispartitioned into a number of bands, each 200 KHz wide. Each of these bands may be occupied by aGMSK modulated RF carrier supporting a number of TDMA time slots. The RF carriers are paired

to allow a simultaneous data flow in both directions; i.e. full-duplex. The GSM900 frequency bandsare 890 MHz to 915 MHz for the and 935 MHz to 960 MHz for the downlink.

GSM Power Classes

The specifications define five classes of MS for GSM900 based on their output power capabilitiesas given in Table 1. The classical handheld is the Class 4 and a classical vehicular unit is Class 2.Each MS has the ability to reduce its output power from its maximum power in steps of 2 dB to a

minimum of 3.2 mW in response to commands from BTS. This facility is used to implementpower control whereby an MSs transmitted power is adjusted to ensure that it is sufficient to

provide a staisfactory up-link quality. This process is used to conserve MS battery power and also

reduce interference. Furthermore, the BTS output power may also be adjusted by upto 15 steps to

allow power control on the downlink.

PowerClass

MaximumPower(Watts)

1 202 83 54 25 0.8

GSM Bursts

Each GSM RF carrier supports 8 time slots per frame and the data are transmitted in the formof bursts that are designed to fit within these slots. Each TDMA frame is 4.615 ms in length and

each TDMA slot is 577 microseconds in length. The GSM specifications define 5 different typesof bursts

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In normal burst (NB), the training sequence is used to sound the radio channel and produce an

estimate of the impulse response at the receiver for equalization. The training sequences used ineach time slot are tabulated in Table 2. The tail bits in normal burst are also always set to 0 to

ensure that the Viterbi decoder begins and ends in a known state. Furthemore, the last bit of thefirst 58 information bits and the first bit of the last 58 information bits are the stealing bits.

Signaling ChannelsTraffic Channels (TCH)

These are the main types of signaling Channels:Broadcast Channels (BCH) - Transmitted by theBTS to the MS. This channel carries system parameters needed to identify the network, synchronize

time and frequency with the network, and gain access to the network.

Common Control Channels (CCH) - Used for signaling between the BTS and the MS and torequest and grant access to the network.

Standalone Dedicated Control Channels (SDCCH) - Used for call setup.

Associated Control Channels (ACCH) - Used for signaling associated with calls and call-setup.

An ACCH is always allocated in conjunction with a TCH or a SDCCH.

*keep in mind, these are only categories of logical channels, they are not logical channels themselves.

The above categories can be divided into the following logical channels:

Broadcast Channels (BCH)

Broadcast Control Channel (BCCH)

Frequency Correction Channel (FCCH)

Synchronization Channel (SCH)Cell Broadcast Channel (CBCH)

1 TDMA Frame=8 Time Slots

1 Time Slot=156.25 bits

Access Burst

Synchronization Burst

Frequency Correction Burst

Normal Burst

Tail Bits

Tail Bits

Tail Bits

Tail Bits Guard Period

Tail Bits

Tail Bits

Tail Bits

Information InformationTraining

Fixed Bits

Information InformationTraining

Training Information Tail Bits

3 58 26 58 3 8.25

3

3

3

3

3

8.25

8.25

68.25

142

39 3964

8 41 36

Guard Period

Guard Period

Guard Period

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Common Control Channels (CCCH)Paging Channel (PCH)Random Access Channel (RACH)

Access Grant Channel (AGCH)

Dedicated Control Channel (DCCH)Standalone Dedicated Control Channel (SDCCH)Fast Associated Control Channel (FACCH)

Slow Associated Control Channel (SACCH)

Broadcast Channels (BCH)

Broadcast Control Channel (BCCH) - This channel contains system parameters needed toidentify the network and gain access. These paramters include the Location Area Code (LAC), the

Mobile Network Code (MNC), the frequencies of neighboring cells, and access parameters.

Frequency Correction Channel (FCCH) - This channel is used by the MS as a frequencyreference. This channel contains frequency correction bursts.

Synchronization Channel (SCH) - This channel is used by the MS to learn the Base StationInformation Code (BSIC) as well as the TDMA frame number (FN). This lets the MS know whatTDMA frame they are on within the hyperframe.

Cell Broadcast Channel (CBCH) - This channel is not truly its own type of logical channel. TheCBCH is forpoint-to-omnipointmessages. It is used to broadcast specific information to network

subscribers; such as weather, traffic, sports, stocks, etc. Messages can be of any nature dependingon what service is provided. Messages are normally public service type messages or

announcements. The CBCH isnt allocated a slot for itself, it is assigned to an SDCCH. It onlyoccurs on the downlink. The CBCH usually occupies the second subslot of the SDCCH. The mobile

will not acknowledge any of the messages.

Common Control Channels (CCCH)Paging Channel (PCH) - This channel is used to inform the MS that it has incoming traffic. Thetraffic could be a voice call, SMS, or some other form of traffic.

Random Access Channel (RACH) This channel is used by a MS to request an initial dedicatedchannel from the BTS. This would be the first transmission made by a MS to access the network

and request radio resources. The MS sends anAccess Burston this channel in order to requestaccess.

Access Grant Channel (AGCH) - - This channel is used by a BTS to notify the MS of theassignement of an initial SDCCH for initial signaling.

Dedicated Control Channels (DCCH)Standalone Dedicated Control Channel (SDCCH) - This channel is used for signaling and callsetup between the MS and the BTS.

Fast Associated Control Channel (FACCH) - This channel is used for control requirementssuch as handoffs. There is no TS and frame allocation dedicated to a FAACH. The FAACH is a

burst-stealing channel, it steals a Timeslot from a Traffic Channel (TCH).

Slow Associated Control Channel (SACCH) - This channel is a continuous stream channel thatis used for control and supervisory signals associated with the traffic channels.

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The layered model of the GSM architecture integrates and links the peer-to-peer communications

between two different systems. The underlying layers satisfy the services of the upper-layerprotocols. Notifications are passed from layer to layer to ensure that the information has been

properly formatted, transmitted, and received.

MS Protocols:

The signaling protocol in GSM is structured into three general layers, depending on the interface.

Layer 1: The physical layer, which uses the channel structures over the air interface.

Layer 2: The data-link layer. Across the Um interface, the data-link layer is a modified

version of the Link access protocol for the D channel (LAP-D) protocol used in ISDN,called Link access protocol on the Dm channel (LAP-Dm). Across the A interface, the

Message Transfer Part (MTP), Layer 2 of SS7 is used. Layer 3: The third layer of the GSM signaling protocol is divided into three sublayers:

Radio Resource management (RR)

Mobility Management (MM) and

Connection Management (CM).

The MS to BTS Protocols:

The RR layer oversees the establishment of a link, both radio and fixed, between the MS and theMSC. The main functional components involved are the MS, the BSS, and the MSC. The RR layer

is concerned with the management of an RR-session, which is the time that a mobile is in dedicated

mode, as well as the configuration of radio channels, including the allocation of dedicated channels.

The MM layer is built on top of the RR layer and handles the functions that arise from the mobilityof the subscriber, as well as the authentication and security aspects. Location management is

concerned with the procedures that enable the system to know the current location of a powered-on

MS so that incoming call routing can be completed.

The CM layer is responsible for CC, supplementary service management, and Short MessageService (SMS) management. Each of these may be considered as a separate sublayer within the CM

layer. Other functions of the CC sublayer include call establishment, selection of the type of service(including alternating between services during a call), and call release.

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BSC Protocols:

After the information is passed from the BTS to the BSC, a different set of interfaces is used. The

Abis interface is used between the BTS and BSC. At this level, the radio resources at the lowerportion of Layer 3 are changed from the RR to the Base Transceiver Station Management (BTSM).

The BTS management layer is a relay function at the BTS to the BSC.

The RR protocols are responsible for the allocation and reallocation of traffic channels between the

MS and the BTS. These services include controlling the initial access to the system, paging for MTcalls, the handover of calls between cell sites, power control, and call termination. The RR

protocols provide the procedures for the use, allocation, reallocation, and release of the GSMchannels. The BSC still has some radio resource management in place for the frequency

coordination, frequency allocation, and the management of the overall network layer for the Layer 2

interfaces.

From the BSC, the relay is using SS7 protocols so the MTP 1-3 is used as the underlyingarchitecture, and the BSS mobile application part or the direct application part is used to

communicate from the BSC to the MSC.

MSC Protocols:

At the MSC, the information is mapped across the A interface to the MTP Layers 1 through 3 from

the BSC. Here the equivalent set of radio resources is called the BSS MAP. The BSS MAP/DTAP

and the MM and CM are at the upper layers of Layer 3 protocols. This completes the relay process.Through the control-signaling network, the MSCs interact to locate and connect to users throughout

the network. Location registers are included in the MSC databases to assist in the role ofdetermining how and whether connections are to be made to roaming users.

Each user of a GSM MS is assigned a HLR that is used to contain the user's location and subscribed

services. A separate register, the VLR, is used to track the location of a user. As the users roam outof the area covered by the HLR, the MS notifies a new VLR of its whereabouts. The VLR in turnuses the control network (which happens to be based on SS7) to signal the HLR of the MS's new

location. Through this information, MT calls can be routed to the user by the location information

contained in the user's HLR.

Localzaton and callng

One fundamental feature of GSM system is the automatic , worldwide localization of users. GSM

perform periodic location updates even if a user does not use the MS.The HLR always containsinformation about the current location , and VLR currently responsible for the MS informs the HLR

about location changes. As soon as an MS moves into the range of a new VLR , the HLR sends alluser data needed to new VLR.

Changing position of services is also called roaming.

To locate an MS and to address the MS following number are required..

Mobile station inter national ISDN number (MSISDN) :---------

The only important number for a user of GSM in is the phone number. Phone no is associated with

SIM ,which is personalized for a user . This no consists of country code (CC) ,the national

destination code(NDC) , and subscriber number (SN) .

International mobile subscriber identity(IMSI) :-------

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GSM uses the IMSI for internal unique identification of a subscriber .it consists of mobile countrycode (MCC) ,the mobile network code(MNC) , and mobile subscriber identification

identity(MSIN).

Temporary mobile subscriber identity(TMSI):

To hide the IMSI ,which would give the exact identity of the user signaling over the air interface

,GSM uses the 4 byte TMSI for local subscriber identification .TMSI is selected by the current VLRand is only valid temporarily and within the location area of VLR.

Mobile station roaming number(MSRN) :---Another temporary address that hides the location of asubscriber is MSRN. MSRN contains the current visitor country code(VCC) ,the visitor national

destination code (VNDC)

The steps involve in calling from a fixed network to a MS (MTC) , and from a MS to fixed

network(MOC) is given below..

MOBILE TERMINATED CALL(MTC)

1: calling a GSM subscriber

2: forwarding call to GMSC

3: signal call setup to HLR

4, 5: request MSRN from VLR

6: forward responsibleMSC to GMSC

7: forward call to

current MSC

8, 9: get current status of MS

10, 11: paging of MS

12, 13: MS answers

14, 15: security checks

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16, 17: set up connection

MOBILE OREGINATED CALL (MOC)

1, 2: connection request

3, 4: security check

5-8: check resources (free circuit)

9-10: set up call

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HandoverIn cellular telecommunications, the term handover or handoff refers to the process of transferring anongoing call or data session from one channel connected to the core network to another. In satellite

communications it is the process of transferring satellite control responsibility from one earth

station to another without loss or interruption of service.

Purpose

In telecommunications there may be different reasons why a handover might be conducted:

when the phone is moving away from the area covered by one cell and entering the area

covered by another cell the call is transferred to the second cell in order to avoid calltermination when the phone gets outside the range of the first cell;

when the capacity for connecting new calls of a given cell is used up and an existing or new

call from a phone, which is located in an area overlapped by another cell, is transferred tothat cell in order to free-up some capacity in the first cell for other users, who can only be

connected to that cell;

in non-CDMA networks when the channel used by the phone becomes interfered by another

phone using the same channel in a different cell, the call is transferred to a different channelin the same cell or to a different channel in another cell in order to avoid the interference;

again in non-CDMA networks when the user behaviour changes, e.g. when a fast-travelling

user, connected to a large, umbrella-type of cell, stops then the call may be transferred to a

smaller macro cell or even to a micro cell in order to free capacity on the umbrella cell forother fast-traveling users and to reduce the potential interference to other cells or users (this

works in reverse too, when a user is detected to be moving faster than a certain threshold,the call can be transferred to a larger umbrella-type of cell in order to minimize the

frequency of the handovers due to this movement);

in CDMA networks a (see further down) may be induced in order to reduce the interference

to a smaller neighboring cell due to the "near-far" effect even when the phone still has anexcellent connection to its current cell;

etc.

The most basic form of handover is when a phone call in progress is redirected from its current cell(called source) and its used channel in that cell to a new cell (called target) and a new channel. Interrestrial networks the source and the target cells may be served from two different cell sites or

from one and the same cell site (in the latter case the two cells are usually referred to as two sectors

on that cell site). Such a handover, in which the source and the target are different cells (even if theyare on the same cell site) is called inter-cell handover. The purpose of inter-cell handover is to

maintain the call as the subscriber is moving out of the area covered by the source cell and enteringthe area of the target cell.

A special case is possible, in which the source and the target are one and the same cell and only theused channel is changed during the handover. Such a handover, in which the cell is not changed, is

called intra-cell handover. The purpose of intra-cell handover is to change one channel, which maybe interfered or fading with a new clearer or less fading channel.

Types of handover

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In addition to the above classification of inter-cell and intra-cell classification of handovers, they

also can be divided into hard and soft handovers:

A hard handover is one in which the channel in the source cell is released and only then the

channel in the target cell is engaged. Thus the connection to the source is broken before or'as' the connection to the target is madefor this reason such handovers are also known as

break-before-make. Hard handovers are intended to be instantaneous in order to minimize

the disruption to the call. A hard handover is perceived by network engineers as an event

during the call. It requires the least processing by the network providing service. When themobile is between base stations, then the mobile can switch with any of the base stations, sothe base stations bounce the link with the mobile back and forth. This is called ping-

ponging.

A soft handover is one in which the channel in the source cell is retained and used for a

while in parallel with the channel in the target cell. In this case the connection to the target is

established before the connection to the source is broken, hence this handover is called

make-before-break. The interval, during which the two connections are used in parallel, maybe brief or substantial. For this reason the soft handover is perceived by network engineers

as a state of the call, rather than a brief event. Soft handovers may involve using connectionsto more than two cells: connections to three, four or more cells can be maintained by one

phone at the same time. When a call is in a state of soft handover, the signal of the best of all

used channels can be used for the call at a given moment or all the signals can be combinedto produce a clearer copy of the signal. The latter is more advantageous, and when such

combining is performed both in the downlink (forward link) and the (reverse link) thehandover is termed as softer. Softer handovers are possible when the cells involved in the

handovers have a single cell site.

GSM Security

Overview of CryptographyThis section provides a brief overview of cryptography, with an emphasis on the features that

appear in the GSM system.

Symmetric Algorithms

Symmetric algorithms are algorithms in which the encryption and decryption use the same key. For

example, if the plaintext is denoted by the variable P, the ciphertext by C, the encryption with key xby the function Ex( ), and the decryption with key x by Dx( ), then the symmetric algorithms are

functionally described as follows:

C=Ex(P)P=Dx(C)P=Dx(Ex(P))

For a good encryption algorithm, the security of the data rests with the security of the key, which

introduces the problem of key management for symmetric algorithms. The most widely-knownexample of a symmetric algorithm is the Data Encryption Standard (DES). Symmetric encryption

algorithms may be further divided into block ciphers and stream ciphers.

Block Ciphers

As the name suggests, block ciphers encrypt or decrypt data in blocks or groups of bits. DES uses a56-bit key and processes data in 64- bit blocks, producing 64-bits of encrypted data for 64-bits of

input, and vice-versa. Block algorithms are further characterized by their mode of operation, such as

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electronic code book (ECB), cipher block chaining (CBC) and cipher feedback (CFB). CBC and

CFB are examples of modes of operation where the encryption of successive blocks is dependent onthe output of one or more previous encryptions. These modes are desirable because they break up

the one-to-one correspondence between ciphertext blocks and plaintext blocks (as in ECB mode).

Block ciphers may even be implemented as a component of a stream cipher.

Stream Ciphers

Stream ciphers operate on a bit-by-bit basis, producing a single encrypted bit for a single plaintextbit. Stream ciphers are commonly implemented as the exclusive-or (XOR) of the data stream withthe keystream. The security of a stream cipher is determined by the properties of the keystream. A

completely random keystream would effectively implement an unbreakable one-time padencryption, and a deterministic keystream with a short period would provide very little security.

Linear Feedback Shift Registers (LFSRs) are a key component of many stream ciphers. LFSRs are

implemented as a shift register where the vacant bit created by the shifting is a function of the

previous states. With the correct choice of feedback taps, LFSRs can function as pseudo-randomnumber generators. The statistical properties of LFSRs, such as the autocorrelation function and

power spectral density, make them useful for other applications such as pseudo-noise (PN) sequence

generators in direct sequence spread spectrum communications, and for distance measurement insystems such as the Global Positioning System (GPS). LFSRs have the additional advantage of

being easily implemented in hardware.

The maximal length sequence (or m-sequence) is equal to 2n-1 where n is the degree of the shiftregister. An example of a maximal length LFSR is shown below in Figure 3. This LFSR will

generate the periodic m-sequence consisting of the following states (1111, 0111, 1011, 0101, 1010,

1101, 0110, 0011, 1001, 0100, 0010, 0001, 1000, 1100, 1110).

In order to form an m-sequence, the feedback taps of an LFSR must correspond to a primitive

polynomial modulo 2 of degree n. A number of stream cipher designs consist of multiple LFSRswith various interconnections and clocking schemes. The GSM A5 algorithm, used to encrypt voice

and signaling data in GSM is a stream cipher based on three clock-controlled LFSRs.

Public Key Algorithms

Public key algorithms are characterized by two keys, a public and private key, which perform

complementary functions. Public and private keys exist in pairs and ideally have the property thatthe private key may not be deduced from the public key, which allows the public key to be openly

distributed. Data encrypted with a given public key may only be decrypted with the corresponding

private key, and vice versa. This is functionally expressed as follows:

C=Epub(P), P=Dpriv(C)

C=Epriv(P), P=Dpub(C)

Public key cryptography simplifies the problem of key management in that two parties may

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exchange encrypted data without having exchanged any sensitive key information. Digital

Signatures also make use of public key cryptography, and commonly consist of the output of a one-way hash function for a message (discussed in Section 3.3) with a private key. This enables security

features such as authentication and non- repudiation. The most common example of a public key

algorithm is RSA, named after its inventors Rivest, Shamir, and Adleman. The security features ofGSM, however, do not make use of any type of public key cryptography.

One-Way Hash FunctionsGenerally, one-way hash functions produce a fixed-length output given an arbitrary input. Secureone-way hash functions are designed such that it is computationally unfeasible to determine the

input given the hash value, or to determine two unique inputs that hash to the same value. Examplesof one-way hash functions include MD5 developed by Ron Rivest, which produces a 128-bit hash

value, and the Secure Hash Algorithm (SHA) developed by the National Institutes of Standards and

Technology (NIST), which produces a 160-bit output.

A typical application of a one-way hash function is to compute a "message digest" which enablesthe receiver to verify the authenticity of the data by duplicating the computation and comparing the

results. A hash function output encrypted with a public key algorithm forms the basis for digital

signatures, such as NIST's Digital Signature Algorithm (DSA).

A key-dependent one-way hash function requires a key to compute and verify the hash value. Thisis useful for authentication purposes, where a sender and receiver may use a key-dependent hash

function in a challenge-response scheme. A key-dependent one-way hash function may beimplemented by simply appending the key to the message and computing the hash value. Another

approach is to use a block cipher in cipher feedback (CFB) mode, with the output being the last

encrypted block (recall that in CFB mode a given block's output is dependent on the output ofprevious blocks). The A3 and A8 algorithms of GSM are key- dependent one-way hash functions.

The GSM A3 and A8 algorithms are similar in functionality and are commonly implemented as asingle algorithm called COMP128.

Authentication

The GSM network authenticates the identity of the subscriber through the use of a challenge-response mechanism. A 128-bit random number (RAND) is sent to the MS. The MS computes the

32-bit signed response (SRES) based on the encryption of the random number (RAND) with the

authentication algorithm (A3) using the individual subscriber authentication key (Ki). Upon

receiving the signed response (SRES) from the subscriber, the GSM network repeats the calculationto verify the identity of the subscriber. Note that the individual subscriber authentication key (Ki) isnever transmitted over the radio channel. It is present in the subscriber's SIM, as well as the AUC,

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HLR, and VLR databases as previously described. If the received SRES agrees with the calculated

value, the MS has been successfully authenticated and may continue. If the values do not match, theconnection is terminated and an authentication failure indicated to the MS. Figure 5 shown below

illustrates the authentication mechanism.

GSM Authentication Mechanism

The calculation of the signed response is processed within the SIM. This provides enhancedsecurity, because the confidential subscriber information such as the IMSI or the individual

subscriber authentication key (Ki) is never released from the SIM during the authentication process.

High-Speed Circuit-Switched Data

High-speed circuit-switched data (HSCSD), is an enhancement to Circuit Switched Data (CSD), theoriginal data transmission mechanism of the GSM mobile phone system, four times faster than

GSM, with data rates up to 38.4 kbit/s.

Channel allocation is done in circuit-switched mode, as with CSD. Higher speeds are achieved as aresult of superior coding methods, and the ability to use multiple time slots to increase data

throughput.

packet-switched general packet radio service (GPRS), which typically has lower pricing (based on

amount of data transferred rather than the duration of the connection), has become more commonthan HSCSD.

Apart from the fact that the full allocated bandwidth of the connection is available to the HSCSD

user, HSCSD also has an advantage in GSM systems in terms of lower average radio interface

latency than GPRS. This is because the user of an HSCSD connection does not have to wait for

permission from the network to send a packet.HSCSD is also an option in enhanced data rates for GSM evolution (EDGE) and universal mobile

telephone system (UMTS) systems where packet data transmission rates are much higher. In theUMTS system, the advantages of HSCSD over packet data are even lower since the UMTS radio

interface has been specifically designed to support high bandwidth, low latency packet connections.

This means that the primary reason to use HSCSD in this environment would be access to legacydial up systems

GPRS is a data network that overlays a second-generation GSM network. This data overlay network

provides packet data transport at rates from 9.6 to 171 kbps. Additionally, multiple users can share

the same air-interface resources simultaneously.

Following is the GPRS Architecture diagram:

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GPRS attempts to reuse the existing GSM network elements as much as possible, but to effectively

build a packet-based mobile cellular network, some new network elements, interfaces, and

protocols for handling packet traffic are required.

Therefore, GPRS requires modifications to numerous GSM network elements as summarizedbelow:

GSM Network Element Modification or Upgrade Required for GPRS.

Mobile Station (MS)

New Mobile Station is required to access GPRS services.

These new terminals will be backward compatible with GSM

for voice calls.

BTSA software upgrade is required in the existing base

transceiver site.

BSC

The base station controller (BSC) requires a software upgradeand the installation of new hardware called the packet control

unit (PCU). The PCU directs the data traffic to the GPRS

network and can be a separate hardware element associatedwith the BSC.

GPRS Support Nodes (GSNs)

The deployment of GPRS requires the installation of new

core network elements called the serving GPRS support node

(SGSN) and gateway GPRS support node (GGSN).

Databases (HLR, VLR, etc.)

All the databases involved in the network will require

software upgrades to handle the new call models andfunctions introduced by GPRS.

GPRS Mobile Stations:

New Mobile Station are required to use GPRS services because existing GSM phones do not handlethe enhanced air interface or packet data. A variety of MS can exist, including a high-speed version

of current phones to support high-speed data access, a new PDA device with an embedded GSM

phone, and PC cards for laptop computers. These mobile stations are backward compatible formaking voice calls using GSM.

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GPRS Base Station Subsystem:

Each BSC requires the installation of one or more Packet Control Units (PCUs) and a softwareupgrade. The PCU provides a physical and logical data interface to the base station subsystem

(BSS) for packet data traffic. The BTS can also require a software upgrade but typically does notrequire hardware enhancements.

When either voice or data traffic is originated at the subscriber mobile, it is transported over the airinterface to the BTS, and from the BTS to the BSC in the same way as a standard GSM call.

However, at the output of the BSC, the traffic is separated; voice is sent to the mobile switchingcenter (MSC) per standard GSM, and data is sent to a new device called the SGSN via the PCU

over a Frame Relay interface.

GPRS Support Nodes:

Following two new components, called GPRS support nodes (GSNs), are added:

Gateway GPRS support node (GGSN):

The Gateway GPRS Support Node acts as an interface and a router to external networks. The

GGSN contains routing information for GPRS mobiles, which is used to tunnel packets through theIP based internal backbone to the correct Serving GPRS Support Node. The GGSN also collects

charging information connected to the use of the external data networks and can act as a packetfilter for incoming traffic.

Serving GPRS support node (SGSN):

The Serving GPRS Support Node is responsible for authentication of GPRS mobiles, registration of

mobiles in the network, mobility management, and collecting information for charging for the useof the air interface.

Internal Backbone:

The internal backbone is an IP based network used to carry packets between different GSNs.Tunneling is used between SGSNs and GGSNs, so the internal backbone does not need any

information about domains outside the GPRS network. Signaling from a GSN to a MSC, HLR or

EIR is done using SS7.

Routing Area:

GPRS introduces the concept of a routing area. This is much the same as a Location Area in GSM,

except that it will generally contain fewer cells. Because routing areas are smaller than LocationAreas, less radio resources are used when a paging message is broadcast.

GPRS Interfaces

Different network components of the GPRS are connected together by well defined interfaces.Some new interfaces to GSM have been added in GPRS to support packet switched data mainly

between GGSNs, SGSNs and other network components. The following interfaces have been

defined:

Um interface between MS and BTS is very similar to GSM and defines the modulation type,

error correction/detection technique, power control information etc.

A interface between BTS and BSC defines the channel allocation, power measurementinformation etc.

Gb interface connects BSCs to SGSN.

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Gn interface is used between GSNs of same PLMN to exchange user profile when the user

moves from one SGSN to another.

All GSNs are connected over a GPRS backbone network over IP. Within this backbone the GSNs

encapsulate and transmit PDN packets by using GPRS Tunneling Protocol (GTP). This backbone

network is of two types:

Intra-PLMN backbone connects the GSNs of same PLMN and are therefore private IP based

networks of the service provider. Inter-PLMN backbone connects the GSNs of different PLMNs if there is a roaming

agreement is between them. The gateways between these PLMNS are called border

gateways and perform security functions apart from regular functions to protect the private

information against unauthorized usage.

GPRS channels

Like other cellular systems, GPRS uses a variety of physical and logical channels to carry the data

payload as well as the signalling required to control the calls.

GPRS physical channel

GPRS builds on the basic GSM structure. GPRS uses the same modulation and frame structure that

is employed by GSM, and in this way it is an evolution of the GSM standard. Slots can be assigned

dynamically by the BSC to GPRS calls dependent upon the demand, the remaining ones being usedfor GSM traffic.

There is a new data channel that is used for GPRS and it is called the Packet Data Channel (PDCH).

The overall slot structure for this channel is the same as that used within GSM, having the same

power profile, and timing advance attributes to overcome the different signal travel times to thebase station dependent upon the distance the mobile is from the base station. This enables the burst

to fit in seamlessly with the existing GSM structure.

Each burst of information for GPRS is 0.577 mS in length and is the same as that used in GSM. Italso carries two blocks of 57 bits of information, giving a total of 114 bits per burst. It therefore

requires four bursts to carry each 20 mS block of data, i.e. 456 bits of encoded data.

The BSC assigns PDCHs to particular time slots, and there will be times when the PDCH is

inactive, allowing the mobile to check for other base stations and monitor their signal strengths toenable the network to judge when handover is required. The GPRS slot may also be used by the

base station to judge the time delay using a logical channel known as the Packet Timing Advance

Control Channel (PTCCT).

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GPRS channel allocation

Although GPRS uses only one physical channel (PDCH) for the sending of data, it employs several

logical channels that are mapped into this to enable the GPRS data and facilities to be managed. Asthe data in GPRS is handled as packet data, rather than circuit switched data the way in which this is

organised is very different to that on a standard GSM link. Packets of data are assigned a spacewithin the system according to the current needs, and routed accordingly.

The MAC layer is central to this and there are three MAC modes that are used to control the

transmissions. These are named fixed allocation, dynamic allocation, and extended dynamic

allocation.

The fixed allocation mode is required when a mobile requires a data to be sent at a consistent datarate. To achieve this, a set of PDCHs are allocated for a given amount of time. When this mode is

used there is no requirement to monitor for availability, and the mobile can send and receive data

freely. This mode is used for applications such as video conferencing.

When using the dynamic allocation mode, the network allocates time slots as they are required. A

mobile is allowed to transmit in the when it sees an identifier flag known as the Status Flag(USF) that matches its own. The mobile then transmits its data in the allocated slot. This is requiredbecause up to eight mobiles can have potential access to a slot, but obviously only one can transmit

at any given time.

A further form of allocation known as extended dynamic allocation is also available. Use of this

mode allows much higher data rates to be achieved because it enables mobiles to transmit in morethan one slot. When the USF indicates that a mobile can use this mode, it can transmit in the

number allowed, thereby increasing the rate at which it can send data.

Logical channels

There is a variety of channels used within GPRS, and they can be set into groups dependent upon

whether they are for common or dedicated use. Naturally the system does use the GSM control and

broadcast channels for initial set up, but all the GPRS actions are carried out within the GPRSlogical channels carried within the PDCH.

Broadcast channels:

Packet Broadcast Central Channel (PBCCH): This is a downlink only channel that is

used to broadcast information to mobiles and informs them of incoming calls etc. It is very

similar in operation to the BCCH used for GSM. In fact the BCCH is still required in theinitial to provide a time slot number for the PBCCH. In operation the PBCCH broadcastsgeneral information such as power control parameters, access methods and operational

modes, network parameters, etc, required to set up calls.

Common control channels:

Packet Paging Channel (PPCH): This is a downlink only channel and is used to alert the

mobile to an incoming call and to alert it to be ready to receive data. It is used for control

signalling prior to the call set up. Once the call is in progress a dedicated channel referred toas the PACCH takes over.

Packet Access Grant Channel (PAGCH): This is also a downlink channel and it sends

information telling the mobile which traffic channel has been assigned to it. It occurs afterthe PPCH has informed the mobile that there is an incoming call.

Packet Notification Channel (PNCH): This is another downlink only channel that is used

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to alert mobiles that there is broadcast traffic intended for a large number of mobiles. It is

typically used in what is termed point-to-point multicasting.

Packet Random Access Channel (PRACH): This is an channel that enables the mobile to

initiate a burst of data in the . There are two types of PRACH burst, one is an 8 bit standard

burst, and a second one using an 11 bit burst has added data to allow for priority setting.

Both types of burst allow for timing advance setting.

Dedicated control channels:

Packet Associated Control Channel (PACCH): : This channel is present in both anddownlink directions and it is used for control signalling while a call is in progress. It takes

over from the PPCH once the call is set up and it carries information such as channelassignments, power control messages and acknowledgements of received data.

Packet Timing Advance Common Control Channel (PTCCH): This channel, which is

present in both the and downlink directions is used to adjust the timing advance. This isrequired to ensure that messages arrive at the correct time at the base station regardless of

the distance of the mobile from the base station. As timing is critical in a TDMA system and

signals take a small but finite time to travel this aspect is very important if long guard bandsare not to be left.

Dedicated traffic channel:

Packet Data Traffic Channel (PDTCH): This channel is used to send the traffic and it is

present in both the and downlink directions. Up to eight PDTCHs can be allocated to amobile to provide high speed data.

DECT (Digital Enhanced Cordless Telecommunications) technology is widely used for residential,

and business cordless phone communications. Designed for short-range use as an access mechanismto the main networks, DECT technology offers cordless voice, fax, data and multimedia

communications, wireless local area networks and wireless PBX.

With the flexibility offered by cordless phone communications, DECT technology has become the

major standard for this application and DECT is now in use in over 100 countries worldwide.mobility management DECT

DECT technology development

The standard for DECT or Digital Enhanced Telecommunications system was developed bymembers of the European Telecommunications Standards Institute (ETSI). The first release of the

standard was available in 1992 after which much of the work was focussed on inter-working

protocols (DECT / GSM, DECT/ISDN, etc).

As a result of this work, DECT / GSM inter-working has been standardized and the basic GSM

services can be provided over the DECT air interface. This enables DECT terminals to inter-workwith DECT systems which are connected to the GSM infrastructure. All roaming scenarios based on

SIM roaming as described in GSM specifications are applicable.

Along with requirements arising from the growing use of DECT, this work gave rise to a number ofextensions to the basic DECT standard. This led to a second release of the standard at the end of

1995. This included facilities including : emergency call procedures, definition of the WirelessRelay Station (WRS), and an optional direct portable to portable communication feature.

DECT codecs

The basic telephony speech quality offered by DECT is very high compared to many other wirelesssystems. This is the result of the use of the ITU-T Recommendation G.726 codec that is employed.

This is a 32 kbit/s ADPCM speech codec and although it uses 32 kbps, the quality it affords is high

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and there is more than sufficient bandwidth within the system to support it.

TDMA structure

The DECT TDMA structure enables up to 12 simultaneous basic voice connections per transceiver.The system is also able to provide widely varying bandwidths by combining multiple channels into

a single bearer. For data transmission purposes error protected net throughput rates of integral

multiples of 24 kbps can be achieved. However the DECT standard defines a maximum data rate of

552 kbps with full security.

UMTS

UMTS stands for Universal Mobile Telecommunications System. UMTS is one of the emergingmobile phone technologies known as third-generation, or 3G. Third-generation systems are

designed to include such traditional phone tasks as calls, voice mail, and paging, but also new

technology tasks such as Internet access, video, and SMS, or text messaging.

One of the main benefits of UMTS is its speed. Current rates of transfer for broadband informationare 2 Mbits a second. This speed makes possible the kind of streaming video that can support movie

downloads and video conferencing. In a sense, UMTS makes it possible for you to enjoy all of the

functionality of your home computer while you are roaming. By combining wireless and satellitecellular technologies, UMTS takes advantage of all existing options to result in the Holy Grail of

3G presentation: seamless transitions between WiFi and satellite.

UMTS went live as a network for the first time in Japan in 2001. Austria had its own network twoyears later. A handful of other European countries joined the UMTS bandwagon in the next two

years, with South Africa and a few other African countries soon following suit. The U.S. hasemployed UMTS networks in several large cities, and the number is steadily growing.