Telecommunications Services [compatibility Mode]-1

Telecommunications Services

Course Overview • • • • • • •

Capabilities and features of a "Plain Old Telephone Service" (POTS) and modern PSTN services. Basic components and features of the ISDN. Applications and features of public X25, Frame Relay and ATM services. i Accessing the Internet inc. ADSL and coax cable modems. Introduction to the Internet inc. role of ISPs and NSPs; WAP based services. services Introduction to GSM mobile radio networks and services. Introduction to Third Generation (3G) mobile services.

Plain Ordinary Telephone Service (POTS) •

We probably take for granted t d th thatt we can pick i k up a telephone, dial a number and in seconds be connected to a friend or colleague who i th is thousands d of f miles il away. Every country in the world is connected to the worldwide automatically switched network t k and d virtually i t ll every community connected to a PSTN can be called without the intervention of a t l h telephone operator. t

POTS (Contd.) •

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You can even dial a number and be automatically connected to someone aboard a ship. The minimum service provided by a PSTN is often known as a "Plain Old Telephone Service: (POTS) The basic "plain Old Telephone“ service( POTS) is provided by all PTOs. A POTS service allows a caller ll to t dial di l a national ti l or international number and gives 'confidence' (status feedback) signals such as dial tone, ringing tone and engaged d tone. t

POTS (Contd.) •

The sequence of events and possible error conditions occurring when someone makes a telephone call are listed below – Caller dials a number – System selects a route – Connection established – Called party answers – Call terminated – Error conditions

Public Switched Telephone Network (PSTN) •

Modern PSTN are capable of providing more advanced services than a POTS. There is a table intended as an indication of the range g of these extra telephones services offered by PTOs

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Many services have detailed or substantially differing implementations in d ff different countries and d by b different d ff PT PTOS, sometimes as a result l of f regulation. There is considerable overlap between services, and they may have alternative names, perhaps proprietary. Some services may not be available to customers with older telephone equipment. Often services are limited geographically.

PSTN (Contd.) •

How services are charged for varies widely and rapidly and is not discussed in detaill here. Some services may be controlled ll by an authorization code. Some services include: • Per Call Service • Diversion Di i and dA Answering i • PABX emulation • Special rate Services • Information I f ti services i • Other services

PSTN (Contd.) Diversion and answering Per call service

Traditional Service Caller display Call return Ring back when free Call waiting Conference call Charge advise Call barring Hotline

Special rate services

Divert all calls

Emergency services

Divert when busy

Freephone

Divert on no reply

Local call

Voice mail

National rate

Operator answer

Response call Premium rate

PABX CENTREX Automatic Call Distribution Hunting group Call pick-up Group Call transfer Short code dialling

PSTN (Contd.) Information services Directory enquiries Time Announcement Broadcast services

Other services Payphone Call direct Voice response Alarm call Telemessage

PSTN (Contd.)

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In CENTREX, a public Telecommunications Operator (PTO) provides PABX services to customers by segmenting the software in PSTN exchanges so that a CENTREX customer has a "partition“ exchanges, "partition apparently independent of the public exchanges. Every “extension" has an individual path to the CENTREX exchange, and such functions as internal numbering, PABX operators.

ISDN (Integrated Service Digital Network) •

The ISDN allows access to several services. It can be used to carry data from a  terminal to a remote server. ISDN can also carry voice and video communications.  All these services are available using a standard copper pair connection from the  local exchange. They are  – Voice  Voice – Graphics – Video – Fax

X.25, Frame Relay and ATM Services •

Most business use a Local Area Network (LAN) in their day to day activities, allowing terminals to access and store files and access database located on a server. To connect outside the LAN, a WAN service is required.

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Most PTOs offer leased lines which p provide a fixed bandwidth, synchronous y connection between points. Typical bit rates include 64Kbit/s and 2 Mbit/s. Some operators also offer an "nX64Kbit/s" service where n is from one to six, typically.

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Normally a line connects to the customer's equipment (CPE) using a physical interface as specified by ITU-T recommendations G.703.

X25, Frame Relay and ATM Services (Contd.) •

G.703 is an ITU-T recommendation dealing with the physical interface for copper pair i and d coax cable bl lines li carrying i synchronous h 64 kbit/s kbit/ signals i l and d plesiochronous (PDH) signals at (for example) 1.54 Mbit/s, 2 Mbit/s, and 8 Mbit/s.

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Leased lines not be cost effective, especially if the link is only used intermittently and/or several different sites have to be interconnected. Also, the bandwidth of leased line does not approach the bandwidth of a modern LAN and so may y act as a "bottleneck",, to avoid congestion g 8 Mbit/s or 34 mbit/s lines may be available, but are very expensive.

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Most PTOs offer some form of X25 access to a public data network. The use of such service by business is widespread. widespread

X25, Frame Relay and ATM Services (Contd.) •

As X25 is widely used, a service provider will provide many access points - this means that a business can easily l interconnect may sites. A common application for an X25 data service is to connect many geographically separated terminals to a central server e.g. EPOS

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Electronic Point of Sale - Electronic cash machines connected to a Wide Area Network can upload information to a central database, allowing stock assessment,, price p adjustment j and bank card verification.

X.25, Frame Relay and ATM Services (Contd.)

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Features of X25 – X25 suite of protocols provide flow control, error checking and ARQ. – X25 data services are available not only within national networks b t also but l allow ll connection ti of f terminals t i l in i different diff t countries. t i – Permanent Virtual Circuits (PVC) and Switched Virtual Circuits (SVC) are available. – For these reasons, reasons X25 is used for secure and reliable data transfer. – However, the data transfer rates possible within X25 are limited. When large g f files have to be transferred f or if f interconnection between LANs is required another type of WAN service must be considered.

X.25, Frame Relay and ATM Services (Contd.)

X.25, Frame Relay and ATM Services (Contd.) •

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Automatic Repeat reQuest (ARQ)- A method of error correction in which blocks l of data found to contain errors at the receiving station are automatically requested for repeat transmission from the transmitting station. A virtual circuit provided by a network appears to the end user as if it were a connection over dedicated physical lines. However, a virtual circuit does not necessarily have particular physical or logical network resources dedicated to it. For example, in packet switching, a virtual circuit will share link resources dynamically with many other circuits. If a particular connection has no data to send, no network resources are used by that connection. connection

X.25, Frame Relay and ATM Services (Contd.) •

Frame-relay technology is widely used by network operators that provide l long-distance di t d data t communications i ti service i to t companies. i It iis d designed i df for ordinary data applications and transmits data frames with variable length.

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Frame F m relay y leaves data checking g and acknowledgment gm procedures p to the network users and the protocols in use are much simpler and can support a much higher data rate.

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Frame relay consists of an efficient data transmission technique used to send digital information. It is a message forwarding "relay race" like system in which data packets, packets called frames, frames are passed from one or many startstart points to one or many destinations via a series of intermediate node points.

X.25, Frame Relay and ATM Services (Contd.) •

A frame relay service carries each packet inside a frame that is error checked by the network, any frames that contain errors are simply l discarded. Frame relay leaves it to the communications protocol operating in a terminal to request data retransmission. This allows faster data transfer because unlike X25, X25 packets/frames are not acknowledged. Frame relay only operates efficiently when links have good error performance. If there're more than a few errors, retransmission requests cause performance to deteriorate rapidly.

X.25, Frame Relay and ATM Services (Contd.) •

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A data frame is used to transport data over a synchronous link and consists of a sequence of a certain number of bytes. A frame structure is the set of rules which determines, for example, how many bytes are in the frame, which bytes are available for data and which bytes are reserved for the operation of the link (the overhead bytes). bytes) The first byte or bytes in a frame identify the beginning of the frame; this is the Frame Alignment Word. Other bytes may be reserved for error checking the contents of the frame. The nodes within a Time Division Multiplex network identify individual received bytes by the order in which they are received.

X.25, Frame Relay and ATM Services (Contd.) • Frame relay supports data rates up to 50 Mbps. Frame-relay technology is usually used to provide semi semi-permanent permanent connections for LAN interconnections. • The network operator sets up a virtual connection between endpoints and frames with circuit identifiers are routed through the network • The network capacity is shared between users and the cost for longdistance connections is much lower than cost of leased-line connections. Frame relay is a technology for data transmission and it does not support isochronous transmission, transmission such as voice or video, video which requires low and constant delay. • Although SVC operation with frame Relay access to a data network is possible, poss ble, many service serv ce providers prov ders offer just PVC V serv service ce w with th each PVC V pre-configured for a certain average level of throughput. Both national and international connections are possible.

Internet •

Please open your texts to ADSL for discussion

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DMT - Discrete Multi Tone. A form of QAM where many different carrier frequencies are used, each modulated in phase and amplitude.

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A system using DMT can dynamically change the frequencies used to avoid any interfering signals that may be affecting the line.

Internet (Contd.) • •

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Several types of organisations are involved in providing the Internet's resources POP - A point i t of f presence (POP) acts t as th the iinterface t f b between t customers t and d th the internet service Provider(ISP) and houses routers, WWW servers and local domain name servers. Residential and business customers gain access to the internet service using one of several different methods; – An analogue modem connected to the PSTN – An ISDN connection – An ADSL or SDSL broadband connection – Larger business customers may be connected using a leased line. The ISP combines customers' internet traffic into a single steam of IP packets. before passing them to the internet backbone

Internet (Contd.) •

ISP - An internet service provider (ISP) allows customers access to the I t Internet t th through h a POP POP. An A ISP maintains i t i one or severall POPs. POP A national ti l or international ISP will use its own WAN(ISP backbone) to interconnect its POPs and to connect to a NAP. The links for the ISP's WAN may be leased from a Long Distance Carrier such as a PTO

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Smaller ISPs may gain access to the Internet backbone using a leased line terminated at a REP REP - Regional Exchange Point - A mechanism for Internet Service Providers (ISPs) to interconnect with at least one Network Access Point (NAP).

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Broadband WANs have been built in Europe and Asia to serve the rapid growth of f internet i t t traffic. t ffi Th These s WANs are connected t d to t th the USA b by hi high h capacity it submarine fibre-optic cables using SDH and WDM to carry telephony, video and data in addition to internet traffic.

Internet (Contd.) ( ) •

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Network Access Point - Provides a mechanism for Network Service Providers (NSP) and d Internet I t t Service S i Providers P id (ISP) to t interconnect i t t their th i Wid Wide A Area Networks (WANs). ISPs and NSPs must have bilateral agreements with at least another ISP/NSP to attach to a NAP. At the time of writing, NAPs include: Sprint New York NAP, AmeriTech Chicago NAP, PacBell in San Francisco and Los A Angeles l and d MCI C Communications i ti f facilities iliti iin W Washington hi t (MAE (MAE-East), E t) San S J Jose (MAE-West), Dallas (MAE-Central), France (MAE-Paris) and Germany (MAEFrankfurt). Some NAPs are run by not-for profit organisations including the London Internet Exchange (LINK). Usually, a REP will not provide inter-NAP connectivity but does allow two or more ISPs to exchange traffic.

Internet (Contd.) ISPs offer several services • ACCESS INTERFERING (PSTN and ISDN) – Many small business and residential users access the internet using the PSTN. The ISP has racks of modems which answer PSTN calls. – The customers IP traffic is combined with other customer's customer s traffic before being routed over the ISPs connection to the Internet backbone. • ROUTER and SERVER MANAGEMENT – The ISP maintains routers, switches and local Domain Name (DNS) servers which hi h direct di t a customer's t ' IP traffic t ffi th through h ISP ISPs L Locall network t k tto th the internet backbone. As well as a local DNS server, the ISP customers’ mails are stored in its Email server so that each message can be retrieved by its intended recipient. Web site servers store the HTML files that make up a W b Sit Web Site. Th The servers s s and d routers t s att a POP are interconnected i t t d by b a LAN

Internet (Contd.) •

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DOMAIN NAME REGISTRATION – A customer subscribing to an ISP ISP'ss Email service is allocated a unique identifier. identifier The identifier(URL) may be based on the ISP's domain name for example, of he ISP "compuserve“ may have an Email address such as [email protected] where jsmith is the "local part "of the address and compuserve.com is the ISPs registered domain name. – Alternatively the ISP will, On behalf of the customer, register a new domain name with the appropriate DNS registrar. EMAIL SERVICES – Email messages are transported over the internet using the SMTP protocol which provides an end-to-end delivery system. If the intended recipient is not available the SMTP server will try later. Eventually, if a message cannot be delivered, a notification may be returned to the sender. – An ISP maintains an Email server which acts as a mailbox for remotes users. Messages are stored on the server until a customer "logs on "and accepts delivery of the message. Security measures, such as passwords, are designed to ensure that only authorised recipients can access messages.

Internet (Contd.) •

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WEB SITE HOSTING and DESIGN – Most ISPs provide Web site hosting services; the ISP ISP'ss web site servers store the HTMl Files belonging to different sites. A particular site is identified by a uniform resource Locator (URL). A Web site can also be located on a server at the customer's premises with a dedicated link provided by a leased line or an SDSL connection ) to the POP WEB EB SERVICES E E – THE WAP protocol are designed to provide Internet based services over GSM mobile phone networks. WAP GATEWAYS – The h WAP gateway retrieves requested d information f from f a server, processes it and d passes it via the mobile network to the client. Ideally, the data stored on the server should be in WML format to suit the presentation capabilities of a mobile phone. However, if necessary, the gateway may be able to convert form HTML to WML. Some gateways can communicate over different data bearer services such as SMS,GPRS and 3G (UMTS) – The gateway may also pass billing information to the mobile networks' billing system

Internet (Contd.) •

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WAP SERVICES – WAP allows ll a network t k operator t to t supply l various i text, t t graphics hi and d menu based b d interactive services to mobile phone users. services include message notification and call management. – To access these services, a mobile phone must be loaded with WAP browser software and use a WAP enabled connection. connection WAP CONNECTIONS – The existing SMS service can be employed to provide a connection between the mobile phone and the WAP service gate way, an interactive service requires a fast response prevent user frustration. f time to p – Network operators are beginning to provide bearer services which offer a fast, more responsive service. These include HSCSD and GPRS. The faster bearer service offered by 3G mobile networks is ideal for WAP . Special WAP enabled phone with capability of employing these enhanced transfer services are now available.

Internet (Contd.) WAP PROTOCOL • Several mobile phone manufacturers and service providers have agreed upon a set of common standards for the delivery of Internet based services to mobile phones. The WAP protocol include WML which defines the format of the information stored on a server ready for display o a mobile phone screen • Other WAP protocol are responsible for transferring data between the mobile p phone and the WAP gateway g y over different types yp of WAP connection

GSM Mobile Radio Networks •

GSM - Global System for Mobile communications (originally Groupe Speciale l Mobile), l ) the European E body responsible l for specifying digitall mobile radio systems. Also used for systems meeting those specifications.

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The GSM specifications are now managed by 3GPP, the 3rd Generation Partnership Project.

GSM Mobile Radio Networks (Contd.) ANALOGUE SYSTEMS • The first generation public mobile systems used analogue techniques and frequencies in the 450 MHZ and 900 MHZ bands. A number of different and incompatible arrangements for signalling and frequency channels were used. used • These analogue mobile services, aimed at car users, were very popular and, therefore, users in some areas experienced poor GoS (grades of service)) at p peak time due to lack of channels. Digital g systems y make use of frequency spectrum and so DAMPS, GSM and PCS have been designed and implemented in many countries-to common standards.

GSM Mobile Radio Networks (Contd.) •

DAMPS - Digital Advanced Mobile Phone System. AMPS is the North American analogue l cellular ll l phone h system; t digital di it l AMPS is i a TDMA (time (ti division di i i multiple lti l access) development of AMPS, sharing many characteristics with GSM but with fundamental differences also.

S/No

Frequency Band

Channel Spacing

Countries

NMT 450

450 MHz

12.5KHz

Nordic Countries, Luxemborg,  Belgium, Spain, Austria l i S i i

NMT 900

900 MHz

12.5KHz

Some of the above and switzerland

AMPS

900 MHz

30 KHz

USA

TACS & ETACS

900 MHz

25KHz

UK and Ireland and Nigeria

GSM Mobile Radio Networks (Contd.) GSM SERVICES • H High gh Quality Qual ty Phone hone – The prime requirement if for a high quality, fully integrated telephone service, just like the PSTN. Generally, users will not tolerate any noticeable degradation in voice quality although a slightly longer time to connect may be b acceptable. bl G GSM M and d PCS PC systems provide d just about b all ll the PSTN advanced services we have previously discussed, though charging patterns may be very different from those for PSTN services. • FAX – The GSM specification does provide for a facsimile service using a laptop PC or hand held terminal although at al lower bit rate(9,6Kbit/s) than is p possible with a PSTN fax service. Some mobile network operators provide a service where a mobile user can send a text message using the SMS to a particular fax machine.

GSM Mobile Radio Networks (Contd.) •

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Message Service – GSM and PCS provide a class of service by means of the control channel between base stations and mobiles. The fundamental service is the short message service. In SMS, a message input from the kea pad of a mobile, is delivered to another mobile phone. there it may be read when the owner whises,stored,deleted or replied to. to IF the recipient is switched off or out of range range, the message is stored and forwarded the next time he/she appears. Data – Two classes of data service are available. Transparent bearer services use FEC b the but h application l has h to be b able bl to tolerate l higher h h residual d l error ratios and d an end-to-end delay of tenths of a second. Non-transperent bearer services use, in addition to FEC an ARQ protocol to achieve much improved BERs, but the ARQ mechanism may result in varying delays of up to several seconds in block transfer, l d leading to b buffer ff problems bl and d perhaps h call ll termination, unless l applications l are designed specially.

GSM Mobile Radio Networks (Contd.) ENHANCED MOBILE DATA SERVICE • Enhanced mobile data service are being deployed which offer faster connection to the internet and other internet related services •

GPRS DATA SERVICE – The GPRS service is an alternative to HSCSD and is based on packet switching rather than circuit switching techniques. Each user can begin transferring data in less than a second. Up to eight TDMA timeslots can be shared by all GPRSGPRS enabled phones in a cell area with data transfer rates of up to 115Kbit/s. However, the GPRS services available typically use three timeslots or less available to each user depends on the traffic demands of other users and may reduce even father. GPRS allows new services such as MMS and WAP access to be offered

GSM Mobile Radio Networks (Contd.) •

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CONVENTIONAL GSM DATA SERVICES – As with speech calls, a standard GSM data service reserves a single TDMA timeslot for each call connection. This data service uses a high degree of FEC with associated overheads so the transfer rate is only 9.6kbit/s. 9 6kbit/s The only other service, SMS is limited to a maximum number of text 160 characters per message. Newer mobile data service offer data transfer rated near to those p provided by y a fixed connection such as ISDN

GSM Mobile Radio Networks (Contd.) •

SMS Short Message Service - in GSM, the service which allows the transfer of messages up to 160 characters long l between mobiles l and between mobiles and other services - the Internet, for example. Developments of SMS to provide longer messages and other information services have been implemented by different operators. operators

HSCSD DATA SERVICE • Some network operator have an enhanced service known as HSCSD. HSCSD This service allocates two or more TDMA timeslots per user to increase the data transfer rate.

GSM Mobile Radio Networks (Contd.) •

PCS - Personal Communications System - a development of GSM which uses the 1800 MHz band (1900 ( MHz in North America). ) PCS networks are designed for hand-portables rather than car-mounted units. PCS operators often offer innovative charging arrangements, for example, free calls off off-peak peak, pre pre-purchased purchased calls and/or tariffs inclusive of some call charges. Except where specifically noted, "GSM" includes PCS in this course.

GSM Mobile Radio Networks (Contd.)

GSM Mobile Radio Networks (Contd.) TARRIFS • Technically, GSM phones can be used with any GSM network and operator therefore need to encourage customers to continue service with then rather than repeatedly change to competing operator( a problem known as churn). It was therefore normal to require new customers to sign minimum 12 months contract and to discourage churn by requiring substantial payments for reprogramming SIMs • Now that the market for mobile phones has matured, tariffs have been set up to encourage g use as a replacement p phone p system y for all,, rather than an additional phone system for mobile users. To this end, various options are offered by the competing GSM PTOs • Newer mobile services include higher bandwidth services with mobile videophones integrated with satellite mobile service and "third phones, third generation "Universal Universal Mobile Telecommunications Service

GSM Mobile Radio Networks (Contd.) •

Churn - especially in mobile systems, when a customer repeatedly changes service supplier l (for ( example, l to take advantage of "signing up" offers). Churn is a problem for service suppliers because of repeated administration costs and possible bad debts.

3rd Generation Mobile Radio Networks •

We have seen that GSM systems are well-developed and are fulfilling many demands for mobile l services. so why is another system needed? there are two main answers and this chapter deals with both. – Answer (1) - we need more of the same. Third Generation (3G) mobile systems attempt to address the spiral demand for mobile services. – Answer (2) - we need new, different things. The design of 3G has been structured for new services,, including g services not yet y conceived.

3rd Generation Mobile Radio Networks (C d ) (Contd.) • •

Mobile Explosion Second generatrion,2G,mobile systems have been extremely successful and have exceeded all predictions for traffic growth. Traditionally in telecoms, it has been possible to limit growth by charging high prices or by setting up waiting lists for connection connection, or both both. But in the competitive mobile market these methods are impossible and consequently congestion has been a feature at peak times in urban areas, resulting in lost calls and other difficulties. the resulting pressure has led to demands for vastly increased capacity-bandwidths, switching capacity and interconnection. All of these are addressees by third generation, 3G systems, the dominant example of which is the Universal Mobile Telecommunications system(UMTS). system(UMTS)

3rd Generation Mobile Radio Networks (C d ) (Contd.) •

Another explosion of traffic has been in the use of internet and mobile users demand mobile l access to the internet. The originall 2G of accessing the internet is now seen as limited and other facilities such as GPRS have been added to meet demands for wider-band mobile internet UMTS has been specifically designed with IP traffic as a internet. major consideration ultimately it is foreseen that almost all traffic will eventually be carried by IP with Voice over IP (VoIP) being used for 3G telephony service

3rd Generation Mobile Radio Networks (C d ) (Contd.) •

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GPRS - General Packet Radio Service - A service designed for transporting emaill and WAP (internet) ( ) traffic. The technology l allows ll mobile phone users to remain attached to the network eliminating the need for call set-up each time a customer wants to send or receive data. data A data transfer rate equivalent to an ISDN connection is achievable. However, data rates vary, dependent on the levels of network traffic and the distance of the user from a base station and other factors. GPRS uses packet switching techniques to allow GPRS-enabled mobile phone clients in a cell to share up to eight timeslots. Timeslot resources can be shared dynamically between speech and data services according to traffic load and operator preference.

3rd Generation Mobile Radio Networks (C d ) (Contd.) •

3G SERVICES MESSAGE SERVICE – SMS was initially thought of as a minor part of GSM, but it popularity for exchanging messages has grown very rapidly. The success of SMS drove the development of an enhanced SMS service -Enhanced Enhanced Messaging Service (EMS) which allows sounds and pictures to be transmitted or received. – A service with even m more facilities f was then developed p for f use with 3G systems in particular. This Multimedia Messaging Service(MMS) allows the exchange of audio, pictures and even video.

3rd Generation Mobile Radio Networks (C d ) (Contd.) LOCATION SERVICE – The location of a mobile is used internally by the network for example it is necessary to know a users location to put calls through to him. However, this information could also be used as a component of a service for example emergency call centre could be advised of a users' position as part of handling his emergency. A 3G service provider could use a user's user s location to apply a cheaper 'home' tarriff when the mobile is located at the registered home address and a more expensive tariff when the mobile located at the registered home address and a more expensive tarriff when a user is on the move. th

3rd Generation Mobile Radio Networks (C d ) (Contd.) DATA SERVICES • UMTS comes with a data transfer service with a performance at least as good as GPRS and possibly far faster. such a high speed service could be used, for example, to access a WAP based information service to browse the World Wide Web. Web Data cards are now available that a slot into a laptop computer to give radio access to the internet • It also provides Roaming. • An upgrade to WCDMA systems known as High Speed Downlink Packet Access (HSDPA) an improve data transfer rates in the direction from the network to the mobile device. Up to 2Mbit/s is possible in the downlink direction when this upgrade is implemented.

3rd Generation Mobile Radio Networks (C d ) (Contd.) •

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Wideband Code Division Multiple Access. The multiple access method used in UMTS systems. The “Wideband” “ is historicall and has had no significance since a 1999 international decision that all UMTS CDMA systems shall use similar bandwidths – though there is now a narrower bandwidth alternative. alternative Code Division Multiple Access provides the means of sharing radio resources between users.

3rd Generation Mobile Radio Networks (C d ) (Contd.) •

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GPS - Global Positioning System - The GPS was built by the United States Defence Department to provide accurate navigationall positioning for its military forces. It consists of a series of satellites which transmit signals to mobile receiving stations. These stations can use the GPS signals to determine their position with a accuracy of a few metres. The GPS is also available for non military use. The stability of the frequency of the GPS signals has to be very high in order to provide the necessary navigational accuracy.

3rd Generation Mobile Radio Networks (C d ) (Contd.) Determining Location • A mobile location can be determined by identifying the cell it is currently using though a cell could be relatively large especially rural area. • A better accuracy can be achieved by measuring the time it takes for a signal from a mobile to reach three different base stations. In this case a mobile's location can be calculated with an accuracy of between 200 to 500 meters • The third and most accurate method used a satellite navigation system such as a GPS to determine a mobile's location. Mobile devices equipped to use such a location service can be located to within 5 to 50 meters

3rd Generation Mobile Radio Networks (C d ) (Contd.) •

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GSM uses TDMA. UMTS uses a wide-band modulation/multiple access technique known as Wideband Code Division Multiple l l access technique known as WCDMA. In a GSM cell mobiles use different frequencies and transmits at different times. times In a UMTS cell all active mobiles in a cell simultaneously transmit at the same frequency. A receiver can tune into a particular signal because each signal is encrypted with a different security code per user. GSM networks use circuit switched techniques although packetswitched functionality has been added later in the form of GPRS. IP mode has been designed into UMTS from the beginning.

3rd Generation Mobile Radio Networks (C d ) (Contd.) •

Another difference which causes considerable technical concerns in UMTS is the compatibility l with existing mobile l systems. 2G systems were not required to interwork with the then-existing mobile systems (except in the USA) which gave them considerable flexibility. A requirement of UMTS systems is that they interoperate with GSM systems at least for voice calls. It has also been a design objective that as much as possible of the existing GSM infrastructure should be usable for UMTS reducing 3G implementation costs and the numbers of necessary extra t b base station t ti sites. it

3rd Generation Mobile Radio Networks (C d ) (Contd.) FREQUENCY SHARING • The pressure on frequency allocations for mobile systems, caused by the phenomenal success of 2G systems and the demand for wideband service, led to the selection of a fundamentally different multiple access system, system Wideband Code Division Multiple Access WCDMA WCDMA,for for 3G • In contrast with GSM where GMSK is used to limit the bandwidth of the modulated signal to roughly 200KHZ, WCDMA deliberately spreads basebands traffic signals g so that the modulated signal g occupy py 5MHz. However, this 25-fold increases in occupied bandwidth saves frequency spectrum because, in general, all cells and users transmit on the same frequency

3rd Generation Mobile Radio Networks (C d ) (Contd.) •

In GSM systems, several carriers can be used in a cell with each carrier using TDMA to carry up to eight channels. l Adjacent cells ll use a different set of carriers. However, UMTS uses a radically different technique for both channel and cell separation. Each transmission is allocated a spreading code which when multiplied with a baseband baseband, spreads it to the full bandwidth. At the receiver the same spreading code is multiplied with the received signal and the baseband signal recovered. Spreading codes are pseudo-random sequences and the process iis, in i essence, encryption/decryption. ti /d ti I Indeed, d d WCDMA originated in long-established military communications.

END OF MODULE Q QUESTIONS!!!