Local Area Networks part II

2 Wireless LANs Why wireless LAN –Cost with wired LANs is that of installing the physical wire cable If the layout of the interconnected computers changes, –Then a cost similar to the initial installation coast can be incurred as the wiring plan is changed –The advent of handheld terminals and portable computers

3 Wireless LANs (cont ’ d)

4

5 Wireless media Two type of media –Radio-frequency wave –Infrared optical signal Radio –Be used extensively for many applications Radio and television broadcasting Cellular telephony networks

6 Radio Radio waves can readily propagate through objects such as walls and doors Radio bandwidth is scarce –For a particular applications A specific frequency band must be officially allocated The circuitry associated with radio-based system –is more sophisticated that that used in infrared optical system –Reasonable cost

7 Radio (cont ’ d) Path loss –SNR (signal-to-noise) The radio power of the received signal to the power of the receiver noise signal must not fall below the specified value –Receiver noise Temperature Bandwidth of the received signal

8 Radio (cont ’ d) –Signal power The power of the transmitted signal The distance between the transmitter and receiver –In free space »The power of a radio signal decays inversely with the square of the distance from the source –In an indoor environment »The decay is increased »Because of the presence of objects such as furniture and people »Because of destructive interference of the transmitted signal caused by the reflected signals from the these objects (Path loss)

9 Radio (cont ’ d) –With portable computers The power of the transmitted signal is limited by the power consumption of the radio Adjacent channel interference –The available bandwidth can be divided into a number of sub-bands So that the area of coverage of adjacent sub- bands utilize a different frequency

10 Radio (cont ’ d)

11 Radio (cont ’ d) Multipath –Multipath dispersion (delay spread) The receiver receives multiple signals originating from the same transmitter –each of which has followed a different path between the transmitter and receiver –Intersymbol interference(ISI) The signals relating to a previous bit/symbol to interfere with the signals relating to the next bit/symbol

12 Radio (cont ’ d) –Frequency-selective fading Caused by the variation in path lengths of the different received signals Relative phase shifts between them which, at radio frequencies, can cause the various reflected signals to significantly attenuate the direct path signal and in the limit, to cancel each other out Rayleigh fading

13 Infrared Infrared emitter and detector –It include optical fiber transmission systems and various remote control applications Such as television sets, CD players, VCRs Very much higher than radio frequency waves –Greater than Hz

14 Infrared (cont ’ d) Wavelength – = c / f c is the speed of light f is the signal frequency in Hz Advantage –The lack of regulations relating to its use –A similar wavelength to visible light Be reflected from shiny surfaces It will pass through glass but not through walls or other opaque objects –Be limited to a single room which, in wireless LAN applications, reduces the level of adjacent channel interference

15 Infrared (cont ’ d) When using infrared as the physical medium is the interference caused by background light –The noise power can be high, which leads to a requirement for a high signal power to obtain an acceptable SNR It can lead to a high power demand on a battery source To reduce the level of noise –Optical bandpass filter »Attenuates those infrared signals that are outside of the frequency band of the transmitted signal

16 Infrared (cont ’ d) Topology –Point-to-point mode The emitter is pointed directly at the detector –Much lower power emitters –Less sensitive detectors can be used –Diffused mode (broadcast mode) The output of the infrared source is optically diffused so that the light is spread over a wide angular area

17 Infrared (cont ’ d)

18 Transmission schemes Radio propagation characteristics –Direct sequence spread spectrum –Frequency-hopping spread spectrum –Single-carrier modulation –Multi-subcarrier modulation

19 Direct sequence spread spectrum ISM band –Free radio spectrum available –The frequency bands set aside for general industrial, scientific, and medical applications Heating equipment, microwave ovens, amateur radio operator –In order to coexist with such applications It is essential that the transmission scheme selected has a high level of co-channel interference rejection Spread spectrum –Direct-sequence –Frequency-hopping

20 DSSS (cont ’ d) Pseudorandom binary sequence –The source data to be transmitted is first exclusive-ORed –That is the bits making up the sequence are random but the same sequence is made much larger than the source data rate –Exclusive-ORed signal It occupies a proportionately wider frequency band than the original source data bandwidth

21 DSSS (cont ’ d)

22 Frequency hopping spread spectrum Channel –The allocated frequency band is divided into a number of lower-frequency sub-bands –A transmitter uses each channel for a short period of time before moving/hopping to a different channel –Hopping sequence The pattern of usage of the channel is pseudorandom –Chip period The time spent on each channel –Chipping rate The hopping rate

23 FHSS (cont ’ d) Fast frequency-hopping –When the chipping rate is higher than the data rate Slow frequency-hopping –When the chipping rate is lower than the data rate

24 FHSS (cont ’ d)

25 Single-carrier modulation –Signal located in the center of the allocated band is modulated with the data –It is simply an extension of the modulation schemes for transmitting data over an analog switched telephone network

26 Multi-subcarrier modulation –First to divide the high bit rate binary signal to be transmitted into a number of lower bit rate streams –Each lower bit rate stream is then used to modulate a separate subcarrier – from the allocated frequency band – as with a single-carrier scheme

27 Protocols The protocols standards for LANs –IEEE 802, ISO 8802 IEEE standards –IEEE : CSMA/CD bus –IEEE : Token bus –IEEE : Token ring –IEEE : Wireless

28 Protocols (cont ’ d)

29 MAC sublayer services For CSMA/CD, confirm primitive indicates the successful(or not) transmission of requests For token LAN, confirm primitive indicates the successful(or not) delivery of requests

30 LLC sublayer L_DATA.request –The only user service primitive –Because this is a best-try protocol All data is transferred using the unnumbered information(UI) frame

31 Network layer The primary role –To route the messages associated with the higher protocol layers above it It involves –Creating an NPDU from the parameters associated with the incoming N_UNITDATA.request primitive –Passing this to the LCC sublayer in the user data parameter of an L_DATA.request

32 Network layer