Friday, August 05, 2005

802.11 Notes continues...

Bit Error Rates in Wireless medium are the main drawback of them, comparatively BERs in Wireless medium are 10 times more than Wired LAN cables. The main reason for BERs are Atmospheric Noise, Physical Obstructions along the path, Multipath Propagation and interference.
Most of the WLANS operate on Spread Spectrum Modulation. which operates over a wide amount of bandwidth. Interference is also caused by multipath fading of the WLAN signals, which results in random phase and amplitude fluctuations in the received signal.

Inward interference comes from devices transmitting in the frequency spectrum used by the WLAN. A number of techniques the operate either on the physical or MAC layer like, Alternative modulation techniques, antenna diversity and feedback equalization in the physical layer, Automatic Repeat Requests (ARQ), Forward Error Control (FEC) in the MAC layer are used to mitigate this inteference.

Outward interference occurs when the WLAN signals disrupts the operation of adjacent WLANs or radio devices.

Further problems are Hidden and Exposed terminal cases.

Physical layers:

IR: operates near to visible rays wavelength 850nm, produced by semiconductor laser diodes or LEDs, since their electrical to optical conversion behaviour is linear.
Can be produced using one of three ways: Diffused transmission in omni direction, reflection by ceiling or focused transmission. IRs are demodulated at the receivers using their amplitude, not their frequency or phase, reduces the receivers complexity. IR rediatons are immune to electromagnetic noise. IR systems share a part of spectrum that is also used by the Sun, hence can work practically in indoor application, same the case with Florescent lights. The IEEE 802.11 physical layer specfication uses Pulse Position Modulation (PPM) to transmit data using IR radiation. PPM varies the position of a pulse in order to transmit different binary symbols. IR transmission works at 1 or 2 Mbps.

RF: RF is robust to florescent lights and outdoor operations, can penetrate non-metalic objects. RF equipment is subject to increased cochannel interferece, atmospheric and man-made noise, high-current circuits.
RF Bands: 902 MHz, 2.4 Ghz and 5.8 Ghz
Adv Factor: The higher the RF band, lesser would be the interfernce.
Disadv Factor: The higer the band, lesser would be the transmission range.
Spread Spectrum: RF uses spread spectrum technology. The idea is to spread the transmitted information over a wider bandwidth in order to make interception and jamming more difficult. In a spread spectrum system, the input data is fed into a channel encoder, which uses a carrier to produce a narrowband analog signal centered around a certain frequency. This signal is then spread in frequency by a modulator, which uses a sequence of pseudorandom numbers. At the receiving end, the same sequence is used to demodulate the spread signal and recover the original narrowband analog signal. The recoverd original narrowband analog signal is fed into a channel decoder to recover the initial digital data. The hopping sequence is defined by the 'seed' of the random number generator.
Spread spectrum has been proven very effective in combating fading. Since a spread spectrum is very wide in frequency, fading only affects a small part of it.

FHSS: Hops over frequency to frequency. Time spend on one frequency is called a chip. The receiver executes the same hopping sequence while remaining in synchronization with the transmitter and thus receives the transmitted data.

DSSS: Each bit in the original spectrum is represented by a number of bits in the spread signal. This can be done by binary multiplication (XOR) of the data bits with a higher rate pseudorandom bit sequence., known as the chipping code. The resulting stream has a rate equal to that of the chipping code and is fed into a modulator, which converts it to analog form in order to be transmitted.

Bit Rates: IEEE 802.11 covers FHSS, DSSS and Infrared Technologies. IEEE 802.11b works at 5.5-11 Mbps and uses DSSS for spreading the spectrum. IEEE 802.11a (HiperLAN2) operates at 5Ghz, and acheives data rates around 54 Mbps. It uses OFDM as opposed to spread spectrum used in Bluetooth, 802.11 and 802.11b. Bluetooth hops around 1600 hops per second which is extremely high as compared to IEEE 802.11 (2.5 hops per second). Thus, due to overhead of switching between the frequencies could cause some delays, and affect the throughtput in that way, that's why actual output data rate of bluetooth is around 30 to 400 kbps, whereas that of 802.11 is 1-2 Mbps. Obviously, 802.11b has a higher data rate than 802.11, 5.5-11 Mbps.

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