BHARTI ENTERPRISES

1145133/8038-11/GTBKIET, Malout

MID-TERM REPORT

Of

Six Months Industrial Training

Held at

AIRTEL

(Gurgaon)

Submitted in the partial fulfillment of the requirement

for the 8th semester curriculum degree of

BACHELOR OF TECHNOLOGY

In

ELECTRONICS & COMMUNICATION ENGINEERING

of

PUNJAB TECHNICAL UNIVERSITY, JALANDHAR

Under the Guidance of :-

Submitted By :-

Mr. Anubhav Raj

Shailesh Pratap Singh

Inst. Roll No. 8038/11

Uni. Regd. No. 1145133

Submitted To :-

Mr. Parminderjeet Singh Bhullar

H.O.D. (E.C.E.)

DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERING

GURU TEG BAHADUR KHALSA INSTITUTE OF ENGINEERING & TECHNOLOGY

CHHAPIANWALI (MALOUT)

AcknowledgementWith deep sense of gratitude, I Would like to take this opportunity to thank my Training Co-ordinator Mr. Anubhav Raj (Training Guide), Airtel who gave his best support & co-ordination during my summer training. His involvement & unstinted support always gives his the confidence to do my work. Without His guidance this project report would not have seen the light of the day. I am also very thankful to Mr. Parminderjeet Singh Bhullar (H.O.D.) Electronics & Communication Engineering, G.T.B.K.I.E.T., Chhapianwali (Malout) for their kind co-opration to complete this report. I would like to thank the people who took their time to help me to complete this project. I would like thanking my friends who were of immense help to me.

Last but not the least, I would like to thank my parents who were a source of support throughout the making of the report.

Thanks

Shailesh Pratap Singh

TABLE OF CONTENTS

Sr. No.

Description

Page No.

1.

Bharti Airtel

1-8

2.

GSM

9

3.

History of GSM

10

4.

Growth of GSM

11-12

5.

GSM Architecture

13-14

6.

Base Station and Network Subsystem

15-16

7.

Burst

17-18

8.

Speech Transmission

19

9.

User Data Transmission

20

10.

GPRS

21

11.

Introduction to Cell Phone Working

22-26

12.

Cell Phone & CBS

27

13.

Cellular Access Technologies

28

14.

FDMA

29

15.

TDMA

30

16.

TDMA/GSM

31-32

17.

Bibliography

331. Bharti Enterprises

History

Bharti Tele-Ventures Limited was incorporated on July 7, 1995 for promoting investments in telecommunications services. Its subsidiaries operate telecom services across India. Bharti Tele-Ventures is India's leading private sector provider of telecommunications services based on a strong customer base consisting of approximately 11.84 million total customers which constitute, approximately 10.98 million mobile and approximately 857,000 fixed line customers, as of March 31, 2005.

The company was initially manufacturing telecom instruments but gradually moved on the other areas of the telecom. Group has moved rapidly to be a Pan Indian integrated telecom service provider as far as the cellular sector is concerned. It is the leader in Northand has entered the western sector.

As a group, it stamps its presence in cellular, fixed line, national long distance, V-sat and broadband including datacom and cyber solutions. Large investments are envisaged in these projects during the next three years. For example, an investment of US $ 650 million is earmarked for submarine cable project along with SingTel, which will have eight-tera bits/sec capacity.

Telecom giant Bharti Airtel is the flagship company of Bharti Enterprises. The Bharti Group, has a diverse business portfolio and has created global brands in the telecommunication sector. Bharti has recently forayed into retail business as Bharti Retail Pvt. Ltd. under a MoU with Wal-Mart for the cash & carry business. It has successfully launched an international venture with EL Rothschild Group to export fresh agri products exclusively to markets in Europe and USA and has launched Bharti AXA Life Insurance Company Ltd under a joint venture with AXA, world leader in financial protection and wealth management.

Airtel comes to you from Bharti Airtel Limited, Indias largest integrated and the first private telecom services provider with a footprint in all the 23 telecom circles. Bharti Airtel since its inception has been at the forefront of technology and has steered the course of the telecom sector in the country with its world class products and services. The businesses at Bharti Airtel have been structured into three individual strategic business units (SBUs) - Mobile Services, Airtel Telemedia Services & Enterprise Services. The mobile business provides mobile & fixed wireless services using GSM technology across 23 telecom circles while the Airtel Telemedia Services business offers broadband & telephone services in 95 cities and has recently launched India's best Direct-to-Home (DTH) service, Airtel digital TV. The Enterprise services provide end-to-end telecom solutions to corporate customers and national & international long distance services to carriers. All these services are provided under the Airtel brand.

Businesses

Bharti Tele-Ventures current businesses include

Mobile services

Fixed-line

National and international long distance services

VSAT, Internet services and network solution

Airtel Digital

Airtel Broadband

& Many More

Present Strategy and Vision

Bharti Tele-Ventures' strategic objective is: to capitalise on the growth opportunities that the Company believes are available in the Indian telecommunications market and consolidate its position to be the leading integrated telecommunications services provider in key markets in India, with a focus on providing mobile services. The Company has developed the following strategies to achieve its strategic objective:

Focus on maximizing revenues and margins;

Capture maximum telecommunications revenue potential with minimum geographical coverage;

Offer multiple telecommunications services to provide customers with a "one-stop shop" solution; Position itself to tap data transmission opportunities and offer advanced mobile data services;

Focus on satisfying and retaining customers by ensuring high level of customer satisfaction;

Leverage strengths of its strategic and financial partners; and

Emphasize on human resource development to achieve operational efficiencies.

Competitive Strengths

Bharti Tele-Ventures believes that the following elements will contribute to the Company'ssuccess as an integrated telecommunication services provider in India and will provide the Company with a solid foundation to execute its business strategy:

Nationwide Footprint - As of March 31, 2005, 100% of India's total mobile subscribers resided in the Company's twenty three mobile circles. These 23 circles collectively /accounted for approximately 100% of India's land mass;

Focus on telecommunications to enable the Company to better anticipate industry trends and capitalise on new telecommunications-related business opportunities;

The strong brand name recognition and a reputation for offering high quality service to its customers;

Quality management team with vision and proven execution skills; and

The Company's strong relationships with international strategic and financial investors such as SingTel, Warburg Pincus, International Finance Corporation, Asian Infrastructure Fund Group and New York Life Insurance.

The key demographics of Bharti Tele-Ventures' mobile circles are set forth below:

Bharti Tele-Ventures has also come out with a unified brand strategy for all telecom services, with Airtel as the flagship brand. The unified brand strategy will make life simple for customers, providing them with more choice, a larger distribution network, best value plans and unique bundling offers.Under the Airtel brand, the company plans to offer a suite of world-class telecom services and solutions ranging from wireless to fixed-line, from data and broadband to long distance and enterprise services. Bhartis unified brand strategy is pursuant to the coming in of the unified license regime and the companys high growth strategy. The unified brand will result in cost-efficiencies, economies of scale and quality advantages for Bharti while also making Airtel Indias first truly integrated telecom services brand.Consequent to the unified brand strategy, Touchtel and IndiaOne brands will now be Airtel

Under the new brand architecture, Bhartis telecom services are being clubbed under four

Heads:

Airtel Mobile Services - Mobile Services

Airtel Telephone & Broadband Services

Airtel Long Distance Services

Long distance service (ILD & NLD)

Airtel Enterprise Services

Single window end-to-end telecom solutions for large corporates

An integrated brand also brings in the benefits of a larger distribution network, best value plans, and unique bundling offers.

Enhanced Convenience:

Being countrys first truly integrated telecom services brand, Airtel will be a one-stop provider of telecom services, thereby bringing in added convenience to its customers. More than 9 million customers will now have a uniform, special service experience at nearly 600 Airtel Connects in the country spread, across its length and breadth. There will also be added convenience on bill payment, query redressal, access to drop boxes and other best in class common quality standards and processes.

The change to leverage opportunities:

With a vision to be "a globally admired company that delights customers", the integrated brand Airtel, will bring in the virtues of trust, innovation and scale to the entire spectrum of telecom services being offered by Bharti. Airtel will also bring simplicity in identity, build greater product and brand differentiation, offer enhanced value for every segment and be a single window provider of customized telecom service and solutions, thereby delivering simple but special brand experience at all touch points - every single time.

This will help the company emerge as a leader, which offers the largest menu of relevantworld-class telecom products and services for all segments across the length and breadth of India.With a view to target 5 million expats, NRIs and ethnic Indians across the globe, the Bharti group has its prepaid cards sold overseas in the local currency of a country. Distribution of AirTel prepaid cards in Singapore, Malaysia and across the Gulf is already being done. Off-the-shelf distribution of prepaid cards is done across the US, UK, Canada, Thailand and Indonesia. Under the proposed distribution model, international travelers can buy AirTel prepaid cards in their home country and activate them on landing in India.

2. GSM Introduction

Global System for Mobile Communications

The name says it all. GSM is a system for connecting mobile communications, such as mobile phones, on a worldwide basis.

3. The History of GSM

The history of GSM stretches back as far as 1982, when Groupe Spciale Mobile, (which originally gave the standard its acronym) was formed within the European Conference of Postal and Telecommunications Administrations (CEPT). The Memorandum of Understanding (MoU) Group was set up in September 1987 and subsequently evolved into the GSM (MoU) Association, a non-profit organization with its headquarters in Dublin. Starting as a GSM network operators club, the GSM Association now also roofs equipment manufacturers, suppliers and operators for satellite and 3G systems, as well as regulators and administrative bodies.

The GSM Association represents over 460 members from 150 countries, with 200 networks currently in operation worldwide. It claims to represent 66% of the digital cellular market, a total of more than 360 million subscribers.

The GSM standard was set when the major players in the European telecommunications industry got together and discussed a new digital mobile communications standard, which would suit everybody. The standard evolved with a lot of work by different groups of engineers, combining their ideas. These committees comprised of experts from manufacturers, future network providers and some permanent expert staff assigned from European Telecommunication Standard Institute (ETSI).

The GSM standard, originally aimed at the 900 MHz band in Europe, has proved so robust that it has been adopted for the PCN service at 1800 MHz, initially in Europe and later for one of the 1900 MHz PCS services in North America. New bands have been identified where analog networks are being phased out: in the 450/480 MHz range in Europe and Asia, and at 850 MHz in America.

4. Growth of GSM

At the start of GSM in 1992, it was a system intended to ensure that Europeans could roam their continent and use their handset wherever they chose to travel. A system designed by Europeans for deployment in Europe.

Four years later the GSM network had over one million subscribers. The worldwide trend towards the deregulation and liberalization of telecommunications meant that a host of new players were entering the mobile market. The GSM specifications were feature-rich and offered a range of services that would enable new operators to differentiate themselves from their rivals. And at the heart of the trend towards GSM ascendancy was its capacity for global roaming. In short, the world was ready for GSM. By the middle of the nineties GSM had expanded beyond Europe and Australia, establishing a presence in areas as diverse as India, Africa, Asia and the Arab world and it is not stopping there.

Global Cellular Market Trends

As you can see on the diagram below, GSM is the largest cellular system, with some market research organizations predicting users of 1 billion by the year 2005.

Benefits and Features of GSM

Users benefit from the following capabilities:

Superior speech quality (equal to or better than the analog cellular technologies)

Low terminal, operational and service costs

High level of security (confidentiality and fraud prevention)

International roaming (under one subscriber directory number)

Support of low-power hand-portable terminals

Long talk time and standby time due to constant envelope modulation

5. GSM Architecture

The functional architecture of GSM can be broadly divided into:

Mobile station (MS)

Base station (BS)

Network subsystem

Mobile Station

The mobile station can be divided into two parts. The first part is the mobile equipment, which in network terms is anonymous. The subscriber information, which includes a unique identifier called the International Mobile Subscriber Identity (IMSI) is stored in the Subscriber Identity Module (SIM). The SIM is a smart card with a processor and memory chip permanently installed in a plastic card, which is either the size of a credit card, or in a smaller version called the plugin SIM (see next page).

Certain subscriber parameters are stored on the SIM card, together with personal data used by the subscriber, such as personal phone directory numbers. The SIM card identifies the subscriber to the network. Since only the SIM can personalize a phone, it is possible to travel abroad, taking only the SIM card, rent a mobile phone at the destination, and then use the phone just as if it were a personal mobile phone at home. To protect the SIM card from improper use, a security feature is built-in. Before the mobile phone can be used, a four-digit personal identification number (PIN) must be entered. The PIN is stored on the card.

6. Base Station and Network Subsystem

With the SIM card, each mobile phone is given a unique identity. As soon as the mobile phone is switched on, it registers with the network and is authenticated. This way the network can always find the mobile phone. A large amount of data is transferred to and from the following:

Visitor Location Register (VLR)

Contains the relevant data of all mobiles currently located in a serving Mobile Services Switching Center (MSC), but not belonging to the area. Home Location Register (HLR)

Stores the identity and user data of all the subscribers belonging to the area.

Authentication Center (AC)

Provides the HLR with different sets of parameters to complete the authentication

of a mobile station.

Equipment Identity Register (EIR)

An option that the network operator can use to enforce security. With this feature the network can identify whether the mobile phone is stolen or due to some defect in the hardware, may not be used in the network. A subscriber wishing to make an outgoing phone call, dials the requested number, sending a signal to the nearest base station. The base station sends the data to the MSC, whose job is to switch calls between the mobile users and between mobile and fixed network users. In this case the data is transferred over the Public. Switched Telephone Network (PSTN) to a fixed telephone.

Channel Utilization

A key feature of GSM is channel utilization. Each radio frequency (RF) channel of 200 kHz uses TDMA (Time Division Multiple Access) to provide 8 time slots. FDMA (Frequency Division Multiple Access) is used to provide other 200 kHz wide RF channels. On each RF channel, a stream of bits is transmitted, carrying information to/from various mobile stations. This bit stream is divided into units called time frames and then each frame is split into eight time slots. When you make a phone call, the information is compressed into one of these time slots, which is then transported to the person you are having a conversation with, and then decompressed. It all happens incredibly quickly, at a rate of 4.615 ms for a whole frame. This means that eight people can be having a conversation at the same time, using the same carrier frequency.

One time frame consists of eight time slots. Data is compressed and sent, for example,

in time slot 3, as shown in the figure below.

7. Burst

A mobile station may only transmit data during the time slot assigned to it. Apart from this, it must not emit any power. Therefore, it must increase the transmitted power very quickly (within less than 30 s) from zero to nominal. And once it has transmitted data, it must abruptly decrease the power again. This radio pulse is called a burst.

Below you can see a diagram of the Power/Time Template. The radio is only allowed

to transmit RF power contained within the Power/Time Template shown.

There are five types of burst defined in GSM. These carry data or process speech or provide other network functions within the time slot.

Normal Burst

With the normal burst the actual information about the conversation is carried. 116 of the 148 bits are the real data, the other bits are basically for synchronization. The normal burst is shown in the figure below.

Frequency Correction Burst

Since frequency timing is critical in the system, the BS has to allow an MS to synchronize with the master frequency of the system. To do this, the BS transmits a

pure sine wave signal for one time slot. It is 148 bits long and made up of zeros to

provide a reference carrier.

Synchronization Burst

This burst helps the MS to synchronize its timing with the network, using a longer

training sequence.

Random Access Burst

The MS transmits this kind of burst randomly and only when the MS is trying to

gain initial access to the system.

Dummy Burst

Basically, this burst is transmitted by the BS on unused time slots. In order to increase the system capacity, GSM utilizes TDMA in combination with FDMA. This means that n different carrier frequencies are used within a radio cell to establish communication. Consequently, the combined use of TDMA and FDMA increases the number of possible concurrent calls within one radio cell by a factor of n. Example of the TDMA being used in combination with FDMA.

8. Speech Transmission

Three different algorithms are currently in use for speech transmission:

Full Rate Speech (TCH/FS)

The TCH/FS was the first method for speech transfer over a digital GSM channel. The algorithm is called RPE-LTP (Regular Pulse Excited Codec with Long Term Prediction), with a data rate of 13 kbit/s before channel encoding.

Half Rate Speech (TCH/HS)

The TCH/HS codec uses the VSELP (Vector Sum Excited Linear Predictor) algorithm which can be used to multiplex two calls into one time slot, so that more calls can be made on the same number of carriers. It usually saves some power from the battery as well. The bit rate is 5.6 kbit/s.

Enhanced Full Rate Speech (TCH/EFS)

The TCH/EFS codec uses an ACELP (Algebraic Code Excited Linear Prediction) algorithm at 12.2 kbit/s. It is a development of the mid-90s yielding superior voice quality.

Another method is currently being implemented:

Adaptive Multirate (AMR) (TCH/AFS, TCH/AHS)

The AMR codec allows fast switching between full- and half-rate operation, depending on the network operator's priority for either higher capacity or higher quality (especially at bad reception quality). In the full-rate mode, it gives the radio link 4 to 6 dB more sensitivity, whereas in the half-rate mode, there is still wireline quality at low error conditions.

9. User Data Transmission

In addition to speech, mobile phones may also be able to transmit user data, e.g. from a connected PC or Personal Digital Assistant (PDA). The standard rate supported by GSM is 9.6 kbit/s. This rate may be sufficient for fax transfer but seems inadequate in todays high-speed communications world, where e-mails may contain large attachments and the World Wide Web is multimedia-oriented. A higher data rate of 14.4 kbit/s has been introduced lately. This 50% gain is, however, achievable under good radio conditions only, and the data rate perceived by the user may drop below the 9.6 kbit/s rate under less than ideal conditions. So many operators are hesitant to introduce 14.4 kbit/s over a single channel, and are looking for other, improved ways to offer better data communications.

HSCSD

High Speed Circuit Switched Data (HSCSD) bundles several time slots to achieve higher data rates (e.g. three time slots can carry 28.8 kbit/s). This mechanism is called multislot operation. The multislot operation demands of the network only a software update, but novel mobile phones supporting HSCSD are required. The downlink typically carries more data than the uplink, so HSCSD can have asymmetric time slot usage normally two or three slots in the downlink and one in the uplink. The HSCSD is not very widespread yet. Just 20 to 25 networks worldwide deploy the service or plan to install it, starting in 1999. First handsets came on the market in September 2000. The disadvantage of the HSCSD from the users point of view is that the user pays as long as he or she is connected, even if no data are transferred just as with a speech call. Consequently, this makes it rather expensive for Web browsing. Network operators, on the other hand, have to contend with congested cells in some urban areas and are not able to afford to make more channels available to a single user.

10. GPRS

General Packet Radio Service (GPRS) proves to be a solution to this problem. Using the GPRS, one or more channels are offered to all users for data transfer, but the channel is dynamically shared between multiple users. The mobile phone or network assigns a time slot for a block of data only when there is data to be transferred. Channel assignment is very fast and the mechanism is similar to the Ethernet on a Local Area Network, where many users share a physical line. GPRS is a completely new protocol reusing the lowest (physical) GSM layer. Running on a single channel, it does not offer higher data rates, but in conjunction with channel bundling (multislot operation), it can be very powerful, cost- and bandwidth-efficient.

Depending on the channel coding algorithm chosen for a given network and radio channel situation, a data rate of 9 to 22 kbit/s is achievable for each time slot; so in

a data transfer using two slots, the resulting rate may be 18 to 44 kbit/s, at least for

a short period of time.

As GPRS uses radio resources only when data needs to be transmitted, it allows a

user to be virtually always connected, which is an advantage when e.g. browsing

the World Wide Web.

EDGE

Enhanced Data rates for the Global Evolution (EDGE) involves a new modulation

format, 8PSK, that will provide three times the data rate of GSM/GPRS on the

same physical channel. It can be used together with GPRS to result in EGPRS. It

is expected on the market in late 2001.

11. INTRODUCTION ABOUT CELL PHONE WORKING

Millions of people in the United States and around the world use cellular phones. They are such great gadgets -- with a cell phone, you can talk to anyone on the planet from just about anywhere!

These days, cell phones provide an incredible array of functions, and new ones are being added at a breakneck pace. Depending on the cell-phone model, you can:

Store contact information

Make task or to-do lists

Keep track of appointments and set reminders

Use the built-in calculator for simple math

Send or receive e-mail

Get information (news, entertainment, stock quotes) from the Internet

Play simple games

Integrate other devices such as PDAs, MP3 players and GPS receivers

But have you ever wondered how a cell phone works? What makes it different from a regular phone? What do all those confusing terms like PCS, GSM, CDMA and TDMA mean? In this article, we will discuss the technology behind cell phones so that you can see how amazing they really are.

The Cell Approach

One of the most interesting things about a cell phone is that it is actually a radio -- an extremely sophisticated radio, but a radio nonetheless. The telephone was invented by Alexander Graham Bell in 1876, and wireless communication can trace its roots to the invention of the radio by Nikolai Tesla in the 1880s (formally presented in 1894 by a young Italian named Guglielmo Marconi). It was only natural that these two great technologies would eventually be combined!

In the dark ages before cell phones, people who really needed mobile-communications ability installed radio telephones in their cars. In the radio-telephone system, there was one central antenna tower per city, and perhaps 25 channels available on that tower. This central antenna meant that the phone in your car needed a powerful transmitter -- big enough to transmit 40 or 50 miles (about 70 km). It also meant that not many people could use radio telephones -- there just were not enough channels.

The genius of the cellular system is the division of a city into small cells. This allows extensive frequency reuse across a city, so that millions of people can use cell phones simultaneously. In the next section, we'll look closer at these cells.

Cell Engineering

In a typical analog cell-phone system in the United States, the cell-phone carrier receives about 800 frequencies to use across the city. The carrier chops up the city into cells. Each cell is typically sized at about 10 square miles (26 square kilometers). Cells are normally thought of as hexagons on a big hexagonal grid, like this:

Because cell phones and base stations use low-power transmitters, the same frequencies can be reused in non-adjacent cells. The two purple cells can reuse the same frequencies.

Each cell has a base station that consists of a tower and a small building containing the radio equipment (more on base stations later

COOL FACTS

Most newer digital cellular phones have some sort of entertainment programs on them, ranging from simple dice-throwing games to memory and logic puzzles.

Approximately 20 percent of American teens (more girls than boys) own a cellular phone.

Cellular phones are more popular in European and Asian countries than they are in the United States -- more than 90 percent of Europeans or Asians own a cell phone, compared to about 50 percent of Americans.

FREQUENCIESA single cell in an analog system uses one-seventh of the available duplex voice channels. That is, each cell (of the seven on a hexagonal grid) is using one-seventh of the available channels so it has a unique set of frequencies and there are no collisions:

A cell-phone carrier typically gets 832 radio frequencies to use in a city.

Each cell phone uses two frequencies per call -- a duplex channel -- so there are typically 395 voice channels per carrier. (The other 42 frequencies are used for control channels -- more on this on the next page.)

Therefore, each cell has about 56 voice channels available.

In other words, in any cell, 56 people can be talking on their cell phone at one time. With digital transmission methods, the number of available channels increases. For example, a TDMA-based digital system can carry three times as many calls as an analog system, so each cell has about 168 channels available

TRANSMISSIONCell phones have low-power transmitters in them. Many cell phones have two signal strengths: 0.6 watts and 3 watts (for comparison, most CB radios transmit at 4 watts). The base station is also transmitting at low power. Low-power transmitters have two advantages:

The transmissions of a base station and the phones within its cell do not make it very far outside that cell. Therefore, in the figure above, both of the purple cells can reuse the same 56 frequencies. The same frequencies can be reused extensively across the city.

The power consumption of the cell phone, which is normally battery-operated, is relatively low. Low power means small batteries, and this is what has made handheld cellular phones possible.

The cellular approach requires a large number of base stations in a city of any size. A typical large city can have hundreds of towers. But because so many people are using cell phones, costs remain low per user. Each carrier in each city also runs one central office called the Mobile Telephone Switching Office (MTSO). This office handles all of the phone connections to the normal land-based phone system, and controls all of the base stations in the region.

CELL PHONE CODES

All cell phones have special codes associated with them. These codes are used to identify the phone, the phone's owner and the service provider.

Let's say you have a cell phone, you turn it on and someone tries to call you. Here is what happens to the call:

When you first power up the phone, it listens for an SID (see sidebar) on the control channel. The control channel is a special frequency that the phone and base station use to talk to one another about things like call set-up and channel changing. If the phone cannot find any control channels to listen to, it knows it is out of range and displays a "no service" message.

When it receives the SID, the phone compares it to the SID programmed into the phone. If the SIDs match, the phone knows that the cell it is communicating with is part of its home system.

Along with the SID, the phone also transmits a registration request, and the MTSO keeps track of your phone's location in a database -- this way, the MTSO knows which cell you are in when it wants to ring your phone.

The MTSO gets the call, and it tries to find you. It looks in its database to see which cell you are in.

The MTSO picks a frequency pair that your phone will use in that cell to take the call.

The MTSO communicates with your phone over the control channel to tell it which frequencies to use, and once your phone and the tower switch on those frequencies, the call is connected. You are talking by two-way radio to a friend!

As you move toward the edge of your cell, your cell's base station notes that your signal strength is diminishing. Meanwhile, the base station in the cell you are moving toward (which is listening and measuring signal strength on all frequencies, not just its own one-seventh) sees your phone's signal strength increasing. The two base stations coordinate with each other through the MTSO, and at some point, your phone gets a signal on a control channel telling it to change frequencies. This hand off switches your phone to the new cell.

CELL PHONE CODES

Electronic Serial Number (ESN) - a unique 32-bit number programmed into the phone when it is manufactured

Mobile Identification Number (MIN) - a 10-digit number derived from your phone's number

System Identification Code (SID) - a unique 5-digit number that is assigned to each carrier by the FCC

AS YOU TRAVEL, THE SIGNAL IS PASSED FROM CELL TO CELL.

While the ESN is considered a permanent part of the phone, both the MIN and SID codes are programmed into the phone when you purchase a service plan and have the phone activated.

ROAMINGIf the SID on the control channel does not match the SID programmed into your phone, then the phone knows it is roaming. The MTSO of the cell that you are roaming in contacts the MTSO of your home system, which then checks its database to confirm that the SID of the phone you are using is valid. Your home system verifies your phone to the local MTSO, which then tracks your phone as you move through its cells. And the amazing thing is that all of this happens within seconds!

12. CELL PHONES AND CBS

A good way to understand the sophistication of a cell phone is to compare it to a CB radio or a walkie-talkie.

Full-duplex vs. half-duplex - Both walkie-talkies and CB radios are half-duplex devices. That is, two people communicating on a CB radio use the same frequency, so only one person can talk at a time. A cell phone is a full-duplex device. That means that you use one frequency for talking and a second, separate frequency for listening. Both people on the call can talk at once.

Channels - A walkie-talkie typically has one channel, and a CB radio has 40 channels. A typical cell phone can communicate on 1,664 channels or more!

13. CELLULAR ACCESS TECHNOLOGIES

There are three common technologies used by cell-phone networks for transmitting information:

Frequency division multiple access (FDMA)

Time division multiple access (TDMA)

Code division multiple access (CDMA)

Although these technologies sound very intimidating, you can get a good sense of how they work just by breaking down the title of each one.

The first word tells you what the access method is. The second word, division, lets you know that it splits calls based on that access method.

FDMA puts each call on a separate frequency.

TDMA assigns each call a certain portion of time on a designated frequency.

CDMA gives a unique code to each call and spreads it over the available frequencies.

The last part of each name is multiple access. This simply means that more than one user can utilize each cell.

14. CELLULAR ACCESS TECHNOLOGIES: FDMA

FDMA separates the spectrum into distinct voice channels by splitting it into uniform chunks of bandwidth. To better understand FDMA, think of radio stations: Each station sends its signal at a different frequency within the available band. FDMA is used mainly for analog transmission. While it is certainly capable of carrying digital information, FDMA is not considered to be an efficient method for digital transmission.

In FDMA, each phone uses a different frequency

15. CELLULAR ACCESS TECHNOLOGIES: TDMA

TDMA is the access method used by the Electronics Industry Alliance and the Telecommunications Industry Association for Interim Standard 54 (IS-54) and Interim Standard 136 (IS-136). Using TDMA, a narrow band that is 30 kHz wide and 6.7 milliseconds long is split time-wise into three time slots.

Narrow band means "channels" in the traditional sense. Each conversation gets the radio for one-third of the time. This is possible because voice data that has been converted to digital information is compressed so that it takes up significantly less transmission space. Therefore, TDMA has three times the capacity of an analog system using the same number of channels. TDMA systems operate in either the 800-MHz (IS-54) or 1900-MHz (IS-136) frequency bands.

TDMA splits a frequency into time slots.

16. CELLULAR ACCESS TECHNOLOGIES: TDMA/GSM

TDMA is also used as the access technology for Global System for Mobile communications (GSM). However, GSM implements TDMA in a somewhat different and incompatible way from IS-136. Think of GSM and IS-136 as two different operating systems that work on the same processor, like Windows and Linux both working on an Intel Pentium III. GSM systems use encryption to make phone calls more secure. GSM operates in the 900-MHz and 1800-MHz bands in Europe and Asia, and in the 1900-MHz (sometimes referred to as 1.9-GHz) band in the United States. It is used in digital cellular and PCS-based systems. GSM is also the basis for Integrated Digital Enhanced Network (IDEN), a popular system introduced by Motorola and used by Nextel.

GSM is the international standard in Europe, Australia and much of Asia and Africa. In covered areas, cell-phone users can buy one phone that will work anywhere where the standard is supported. To connect to the specific service providers in these different countries, GSM users simply switch subscriber identification module (SIM) cards. SIM cards are small removable disks that slip in and out of GSM cell phones. They store all the connection data and identification numbers you need to access a particular wireless service provider.

Unfortunately, the 1900-MHz GSM phones used in the United States are not compatible with the international system. If you live in the United States and need to have cell-phone access when you're overseas, the easiest thing to do is to buy a GSM 900MHz/1800MHz cell phone for traveling. You can get these phones from Planet Omni, an online electronics firm based in California. They offer a wide selection of Nokia, Motorola and Ericsson GSM phones. They don't sell international SIM cards, however. You can pick up prepaid SIM cards for a wide range of countries at Telestial.com.

COOL FACTS

The GSM standard for digital cell phones was established in Europe in the mid-1980s -- long before digital cellular phones became commonplace in American culture.

It is now possible to locate a person using a cellular phone down to a range of a few meters, anywhere on the globe.

3G (third-generation wireless) phones may look more like PDAs, with features such as video-conferencing, advanced personal calendar functions and multi-player gaming

CELLULAR ACCESS TECHNOLOGIES: CDMA

CDMA takes an entirely different approach from TDMA. CDMA, after digitizing data, spreads it out over the entire available bandwidth. Multiple calls are overlaid on each other on the channel, with each assigned a unique sequence code. CDMA is a form of spread spectrum, which simply means that data is sent in small pieces over a number of the discrete frequencies available for use at any time in the specified range.

17. BIBLIOGRAPHY The GSM system for mobile communication-Michel Mouly & Marie- Bernadette Pautet.

GSM system Engineering-Asha Mehrotra (Artech House Publisher) haug, T.,Developing GSM standard, pan-European Digital Cellular Radio Conf., Nice, France, 1991.

Mouly, M., and pautet Marie-Bernadette,Current Evolution of the GSM system,

IEEE Personal Communications, October 1995, PP.9-19.

Beddoes, E, W., GSM Network Architecture, GSm Seminar, Budapest, October 1990, Session 2.1.

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