Unit 2

Wireless LAN satellite communication wireless application protocol (WAP) Antennas narrow-band and spread-spectrum technologies cellular telephony Propagation frequencies and spectrum and personal communication system

Wireless LAN

Standards de jure standards IEEE IEEE b de facto standards

Strengths Avoids cabling issues – Cost – Installation problems – Delay – Aesthetics of cabling lying about in offices Excellent choice for older buildings. Provides easy access to students in College campuses.

Weakness 1.most WLANs operate at 1 – 2 Mbps – Ethernet – 10 Mbps – Token Ring – 16 Mbps 2.Susceptible to construction materials and objects 3.Controversial issue of health effects on living tissues Higher rates – greater damage Lower rates – little or no damage

Satellite Communication

Strengths Efficient transmission of data High speed upto 3Gbps Impervious to signal obstacles Difficult to intercept data coming from or going to satellite

Weaknesses Highly expensive due to costs involved in launching LEO or MEO satellite launching takes minimum of 200 million dollars GEO satellite launching requires 300 million dollars Average life span of satellites is over 10 years Susceptible to solar activities like – Solar flares – Which limits communication from few minutes to few days

GPS Applications: – Track military vehicles – Aim military weapons – Can detect nuclear explosions By use of onboard nuclear detection hardware/software suite – Civilian use – In ships – Automotive applications

WAP Wireless Application Protocol Attempts to emulate the very successful WWW (internet) model – With the assumption that user are not at the work station, but are using a hand-held device to access the internet

WAP contents and applications are based on familiar WWW content formats Content is transferred using a set of standard communication protocols based on WWW communication protocols. A micro browser in the wireless terminal coordinates the user interface and is comparable to standard web browser.

WAP model defines a set of standard components that enable communication between mobile devices and network servers : – Standard naming model WWW standard URLs are used to – identify WAP content on origin servers – Identify local resources in a device (eg. Call control functions) – Content typing WAP content is given a specific type consistent with WWW typing This allows WAP user agents(hand-held devices) to correctly process the content based on its type

– Standard content formats These are based on WWW technology And include, – Display markup – Calendar information – Electronic business card objects – Images…etc – Standard communication protocols Enable the communication of browser requests from the mobile terminal to the network web server

Example of WAP network

Provides a scalable and extensible environment for application development for mobile communication devices. This is achieved by the use of layered architecture :

Antenna Transmits and receives electrical energy. Function : Radio transmission – Unidirectional – Omni directional Electrical Signals Airborne Waves

Functions of Antenna Antenna is : – a piece of conducting material in the form of a wire, rod or any other shape – a source or radiator of EM waves – a sensor of EM waves – a transducer – a impedance matching device – a coupler between generator/transmission line and space or vice-versa

Antenna Characteristics Size and shape depends of the frequency and direction of the signal 1.Lower the frequency larger the antenna must be Broadcast radio stations – 530KHz – hundred feet high Cellular phones – 900MHz – less than 6 inches 2.Shape varies depending on the direction of the airborne signal Unidirectional Omni directional

3. Relation between wavelength and antenna size. – A perfect antenna is one half the length of wavelength it receives

Narrowband and Spread Spectrum Technology Band ?? Bandwidth ?? Narrowband ?? Spectrum ??

Most conventional radio transmitters operate in Narrowband – i.e. Transmitted signal contains all its power in the very narrow RF bandwidth Drawback: – Low bandwidth  more interference of signal – Sometimes a single interfering signal at or near the tuned frequency of radio makes the desired signal unusable

Spread Spectrum Technology It is a technique by which a signal (e.g. an electrical, electromagnetic, or acoustic signal) generated with a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth. i.e. signal is transmitted on a bandwidth considerably larger than the frequency content of the original informationbandwidthfrequency The bandwidth is expanded using Complex Coding Schemes

Expanded Signal ( Increased Bandwidth ) Original Signal ( Narrow Bandwidth ) Applying Spread Spectrum Technique Recovering back Original Signal

Originally there were two motivations: – to resist enemy efforts to jam the communications (anti-jam, or AJ) – to hide the fact that communication was even taking place

Advantages of using Spread Spectrum Technique: the establishment of secure communication increasing resistance to – natural interference – noise – jamming to prevent detection to limit power flux density

Two main signal spreading techniques are : 1.Direct Sequencing Spread Spectrum 2.Frequency Hopping Spread Spectrum

1.Direct Sequencing Spread Spectrum (DSSS) – It continuously distributes the data signal across a broad portion of the frequency band – This technique modulates a carrier by a digital code (i.e. pseudorandom sequence) with a bit rate which is much higher than the information signal bandwidth pseudorandom

Benefits of using DSSS – Resistance to intended or unintended jammingjamming – Sharing of a single channel among multiple users Sharing – Reduced signal/background-noise level – avoids interceptioninterception – Determination of relative timing between transmitter and receiver

Benefits of using DSSS – Resistance to intended or unintended jammingjamming – Sharing of a single channel among multiple users Sharing – Reduced signal/background-noise level – avoids interceptioninterception – Determination of relative timing between transmitter and receiver

2.Frequency Hopping Spread Spectrum (FHSS) – It moves the radio signal from frequency to frequency in a fraction of second – it is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels – It uses a pseudorandom sequence known to both transmitter and receiver.pseudorandom

Channel Sharing using FHSS

Spread Spectrum technique differs from commercial radio technology, – As it spreads the signal over a wide frequency range within its assigned bands – it is more immune to noise and interference – Allows sharing of communication channels – Privacy and security is given prime importance On the other hand, – The commercial or conventional radios operate over a specific frequency – Thus confining he signal over a limited band of frequency called as Narrowband

Accessing Channels: Cellular Telephony Frequency band allocated for cellular telephony is 806 MHz to 902 MHz Different channel accessing techniques available are: – Frequency Division Multiple Access (FDMA) – Time Division Multiple Access (TDMA) – Code Division Multiple Access (CDMA)

Frequency Division Multiple Access (FDMA)

Time Division Multiple Access (TDMA)

Code Division Multiple Access (CDMA)

Propagation

Factors Affecting Propagation: 1.Reflection – Objects >Wavelength 2.Scattering – Objects<Wavelength 3.Diffraction – Obstruction due to sharp and irregular edges 4.Mobility Effects – Channel varies with user location 5.Attenuation Effects – Signal loss over distance (in clear and unobstructed environment) 6.Movement Effects – Movement of surrounding objects 7.Indoor Propagation – Indoor receiving signal decays much faster 8.Solar Activity

Personal Communication System Current state of cellular telephony Cellular telephony – Utilizes frequency in range 800 MHz – Has both Analog and Digital capabilities PCS – Operates in 1900 MHz range – It is purely digital Need: – To deal with explosive growth of cellular telephony – To avoid overloading in 800 MHz range

Advantages: PCS is developed as a digital tool that provides user with: – Voice communication – Paging service – Fax service – Mobile connection to internet