Intro to GSM TechnologyIntro to GSM Technology

Pre-era of GSM Technology.Early Technological Ideas.Approach to GSM Networking.Criteria and Features.Cell Concept and Frequency Techniques.Network Architecture and Call procedures.Security features.

Pre-era of GSM TechnologyPre-era of GSM TechnologyIt's difficult to recall the bad old days. Just imagine a tank that will tackle the terrorists of Pakistan but stops dead when you drive it over into LoC. This pretty much sums up the mobile phone scene in the early 1980s.As business was becoming increasingly international, the cutting edge of the communications industry focused on exclusively local cellular solutions. And none was remotely compatible with any of the others enabling you to call the office if you were in your own home, but not if you were with a client in another country.It was clear there would be an escalating demand for a technology that facilitated flexible and reliable mobile communications.The problem was capacity. Or the lack of it. It would soon become obvious that by the early 1990s the disparate analogue networks would collapse under the pressure of demand.

Eulogy of Cell PhonesEulogy of Cell PhonesFull-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! Range - A walkie-talkie can transmit about 1 mile (1.6 km) using a 0.25-watt transmitter. A CB radio, because it has much higher power, can transmit about 5 miles (8 km) using a 5-watt transmitter. Cell phones operate within cells, and they can switch cells as they move around. Cells give cell phones incredible range. Someone using a cell phone can drive hundreds of miles and maintain a conversation the entire time because of the cellular approach.

Walkie-talkie modeWalkie-talkie mode

In 1983, the analog cell-phone standard called AMPS (Advanced Mobile Phone System) was approved by the FCC and first used in Chicago. AMPS uses a range of frequencies between 824 megahertz (MHz) and 894 MHz for analog cell phones. In order to encourage competition and keep prices low, the U. S. government required the presence of two carriers in every market, known as A and B carriers. One of the carriers was normally the local-exchange carrier (LEC), a fancy way of saying the local phone company.

AMPS SetsAMPS Sets

Approach towards GSM NetworkingApproach towards GSM NetworkingOne of the most interesting things about a cell phone is that it is actually a radio an extremely sophisticated radio, but a radio nonetheless.

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.

Criteria of GSM TechnologyCriteria of GSM Technology

Good subjective speech quality Low terminal and service cost Support for international roaming Ability to support handheld terminals Support for range of new services and facilities Spectral efficiency ISDN compatibility

Structure of the NetworkStructure of the Network

One of the most attractive features of GSM is that it is a very secure network. All communications, both speech and data, are encrypted to prevent eavesdropping. In fact, in the early stages of its development it was found that the encryption algorithm was too powerful for certain technology export regulators. This could have had serious implications for the global spread of GSM by limiting the number of countries to which it could be sold. Fortunately, the MoU reacted promptly to this threat. Alternative algorithms were developed that enabled the free dissemination of the technology worldwide

Features of a typical GSM NetworkFeatures of a typical GSM Network

Suppliers Ability to demonstrate commitment to security and reduction of risks for customersReduced number of individual operator inspectionsCertification from the world’s leading wireless industry representative bodyOpportunity to receive a world-class security review of operationsProvides a uniform approach to security audits

Security Accreditation Security Accreditation SchemeScheme

Network Operators

•No longer need to invest, in financial or resource terms, in conducting individual audits•Scheme is conducted by highly qualified individuals at no cost to the operator•The scheme sets a rigorous security standard demonstrating supplier commitment to the highest levels of securityOffers peace of mind that suppliers have implemented appropriate security measures.

Network OperatorsNetwork Operators

Concept of cellsConcept of cells

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 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 Kms).

Cells are normally thought of as hexagons on a big

Hexagonal cells.

1

6

5

4

7 3

2

1

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

Cell Planning in GSMCell Planning in GSM

Mobile StructureMobile Structure

MicroprocessorMicroprocessor

Flash Memory card on Ckt BoardFlash Memory card on Ckt Board

FDMA & TDMA TechniquesFDMA & TDMA Techniques

Architecture of the GSM networkArchitecture of the GSM network

Call ProcessCall Process

Þ                 When you first power up the phone, it listens for an SID 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. Þ                 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. Þ                 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. Þ                 The MTSO gets the call, and it tries to find you. It looks in its database to see which cell you are in. Þ                 The MTSO picks a frequency pair that your phone will use in that cell to take the call. Þ                 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! Þ                  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. As you travel, the signal is passed from cell to cell.

Call ProcedureCall Procedure

GSM Phone signal AnalysisGSM Phone signal Analysis

how the transmitter power is reduced once the call has been set up and the conversation begins. how the transmitter power is reduced once the call has been set up and the conversation begins. Note also the 'gaps' during the silence in the conversation, where the transmitted signal returns to a Note also the 'gaps' during the silence in the conversation, where the transmitted signal returns to a 'holding' signal -- which looks superficially similar to the signal during ringing. 'holding' signal -- which looks superficially similar to the signal during ringing.

This represents a section of the voice transmission, the timescale of the whole plot being This represents a section of the voice transmission, the timescale of the whole plot being a little over 1 seconda little over 1 second

GSM Signal analysis GSM Signal analysis

The pulses come at 4.62 millisecond intervals (approx. 217Hz frequency), each The pulses come at 4.62 millisecond intervals (approx. 217Hz frequency), each lasting 0.57 milliseconds. This gives a mark:space ratio of 1:7, allowing up to eight lasting 0.57 milliseconds. This gives a mark:space ratio of 1:7, allowing up to eight calls to be time-multiplexed (TDMA) onto the same carrier frequency. Every 26th calls to be time-multiplexed (TDMA) onto the same carrier frequency. Every 26th pulse is omitted, causing an 8.3Hz periodicity in the signal.pulse is omitted, causing an 8.3Hz periodicity in the signal.

GSM Coding ChannelsGSM Coding ChannelsThe overall data rate for the radio channel is 270kb/s. This is split into B full rate or 16 half rate traffic channels, plus the signaling channels. The coding is complex in order to have the maximum chance to detect and correct the errors encountered in a typical propagation path. The output of the speech coder is encrypted, coded and interleaved in a sophisticated way to allow Forward Error Correction to be applied. The data is then sent as bursts in time slots of 577µs, each containing 116 encrypted bits. There are 8 or 16 of these time slots per TDMA frame, and the receive and transmit time slots are staggered so that the mobile station is not receiving at the same instant as it is transmitting, thus simplifying the filtering requirements. With this scheme, there can be at least one spare slot between transmit and receive, leaving time for the synthesizer to change frequency (whether or not hopping is employed).

The receiver also monitors adjacent cell for one time slot each frame to determine their signal strength to optimize a possible handover.

GSM Channel GSM Channel Allocation.Allocation.

  Base station transmit

Handset transmit

Peak handset power

Used in Karnataka

primarily by

GSM 900 935-960MHz 890-915MHz 2 watts Spice & Airtel

GSM 1800 (PCN)

1805-1880MHz 1710-1785MHz 1 watt BSNL & Hutch

GSM Traffic & Signaling GSM Traffic & Signaling ChannelsChannels

Traffic channels are defined for speech, and for data at the rates Traffic channels are defined for speech, and for data at the rates of 9.6kb/s, 4.8kpbs and 2.4kbps.of 9.6kb/s, 4.8kpbs and 2.4kbps.These may be full or half rate (except for 9.6kb/s which must be These may be full or half rate (except for 9.6kb/s which must be 'full rate').'full rate').In addition a number of logical signaling channels are specified. In addition a number of logical signaling channels are specified. These operate at different rates, and have individual functions:These operate at different rates, and have individual functions:•Slow associated control channels - for control and supervisory Slow associated control channels - for control and supervisory signals associated with the traffic channels.signals associated with the traffic channels. •Fast associated control channels - which steal time slots from the Fast associated control channels - which steal time slots from the traffic allocation and are used for control requirements such as traffic allocation and are used for control requirements such as handover.handover. •Dedicated control channels - which are multiplexed on to a Dedicated control channels - which are multiplexed on to a standard channel and are used for registration, location updating, standard channel and are used for registration, location updating, authentication and call set-up.authentication and call set-up. •Broadcast control channels (down link only) - which provide the Broadcast control channels (down link only) - which provide the mobile stations with information such as base station identity.mobile stations with information such as base station identity. •Paging channel (down link only) - which informs the mobile that Paging channel (down link only) - which informs the mobile that the network requires to signal it.the network requires to signal it. •Random-access channel (up link only) - which is used by the Random-access channel (up link only) - which is used by the mobile station to request access to the network.mobile station to request access to the network. •Access grant channel (clown link only) - which replies to a Access grant channel (clown link only) - which replies to a random access arid assigns a dedicated control channel for random access arid assigns a dedicated control channel for subsequent signaling.subsequent signaling.                   Cell Broadcast Channel (down link only) - transmits the cell Cell Broadcast Channel (down link only) - transmits the cell broadcast information.broadcast information.

FeaturesFeaturesCall waiting & Call holdCall waiting & Call holdIf you are using the phone, Call Waiting will alert you to a second caller. You need If you are using the phone, Call Waiting will alert you to a second caller. You need never miss an incoming call.never miss an incoming call.Use Call Hold to put the person you are talking to on hold so you can call another Use Call Hold to put the person you are talking to on hold so you can call another party, and then switch between the two calls.party, and then switch between the two calls.Call forwardingCall forwardingUse Call forwarding to divert incoming calls to another number.Use Call forwarding to divert incoming calls to another number.Calling line identity (CLI)Calling line identity (CLI)CLI displays the number (name if stored on your phone) of the incoming call.CLI displays the number (name if stored on your phone) of the incoming call.SMS (short messaging service)SMS (short messaging service)SMS (text messaging) allow you to send & receive text messages on your phone. SMS (text messaging) allow you to send & receive text messages on your phone. Services available from many of the world's GSM networks today - in addition to Services available from many of the world's GSM networks today - in addition to simple user generated text message services - include news, sport, financial, simple user generated text message services - include news, sport, financial, language and location based services, as well as many early examples of mobile language and location based services, as well as many early examples of mobile commerce such as stocks and share prices, mobile banking facilities and leisure commerce such as stocks and share prices, mobile banking facilities and leisure booking services. (More information about SMS can be found in GSM Technology.) booking services. (More information about SMS can be found in GSM Technology.) Data ServicesData ServicesUsing your GSM phone to receive and send data is the essential building block Using your GSM phone to receive and send data is the essential building block leading to widespread mobile Internet access and mobile data transfer. GSM leading to widespread mobile Internet access and mobile data transfer. GSM currently has a data transfer rate of 9.6k. New developments that will push up data currently has a data transfer rate of 9.6k. New developments that will push up data transfer rates for GSM users are HSCSD (high speed circuit switched data) and GPRS transfer rates for GSM users are HSCSD (high speed circuit switched data) and GPRS (general packet radio service) are now available(general packet radio service) are now available..

TowersTowers

Multiple Providers Base stationMultiple Providers Base station

Grounding EquipmentGrounding Equipment

Multiple ProvidersMultiple Providers

Weird AntennasWeird Antennas