Protocol Stack

802.11 Protocol Stack 802.11 standard specifies three transmission techniques allowed in the physical layer. The infrared method uses much the same technology as television remote controls do. The other two use short range ratios using techniques called FHSS and DSSS. Both of these use a part of the spectrum that does not require licensing the 2.4 GHz ISM bands. Radio-controlled garage two openers also use this process of the spectrum. Cordless telephones and microwave ovens also use this band. All of these techniques operate at 1 or 2 Mbps and a flow enough power that does not conflict too much. In 1999 two new techniques were introduced. OFDM and HR DSSS. They operate at up to 54 Mbps and 11 Mbps. 802.11 Physical Layer The infrared option uses diffused ie., not line of sight transmission at 0.85 or 95 microns. Two speeds are permitted 1 Mbps and 2 Mbps. At 1 Mbps, an encoding scheme is used in which a group of 4 bits is encoded as a 16-bits code word containing fifteen, 0’s and single one using what is called gray code. A 2 Mbps the encoding takes 2 bits and protons - a 4-bit code word also with only a single 1, that is one of 0001, 0010, 0100 or 1000. Infrared signal cannot penetrate walls, 50 cells in different rooms are wall isolated from each other. Due to low bandwidth land the fact that sunlight swamps infrared signals, that is not a popular option. Frequency Hopping Spread Spectrum (FHSS) Use 79 channels each 1 MHz size; starting at the low end of the 2.4- GHz ISM band. A pseudorandom number generator is used to produce the sequence of frequencies hopped to. As long as all stations use the same required to the pseudorandom number generator and stay synchronized in time, they will hop to the same frequencies simultaneously. The amount of time spent at each frequency the full time is an adjustable parameter, but must be less than 400 MHz FHSS Randomization provides a fair way to allocate spectrum in the non-regularized ISM bands. In the FHSS FSK of1 Mbands, system allows (two level FSK or 1 or 2 bits for FSK) Direct Frequency Spread Spectrum (DSS) Direct Frequency Spread Spectrum is also restricted 1 or 2 Mbps. The scheme used has some similarities to the CDMA System but differs in other ways. Each bit is transmitted as 11 chips using what is called a Borker sequence. It uses phase shift modulation at M baud, transmitting 1 bit per band when operating at 1 Mbps and 2 bit per band when operating at 2 Mbps. Chip code for 0:11011 0 0 1 Chip code for 1: 00011 1 0 Although this scheme looks like similar to CDMA, here is modulation technique i.e., this specification is PSK at 1 M bauds. The system allows 1 or 2 bits/band (BPSR or QPSK) which results in a data rate of 1 or 2 Mbps. The first of the high speed wireless LAN, 802.11a uses OFDM (Orthogonal Frequency Division Multiplexing) to deliver up to 54 Mbps in the wider 5 GHz ISM band. As the form FDM suggests, different frequencies are used 52 of them 48 for data and 4 for synchronization. Not unlike ADSL, since transmissions are present on multiple frequencies at the same time, this technique is considered a form of spreads spectrum, but different from both CDMA an FHSS. Dividing the band into sub bands diminishes the effects of interference. Modulation and Data Rates: OFDM uses FSK and QAM for modulation. The common data rates are 18 Mbps (PSK) and 54 Mbps (QAM). Note: Asymmetrical digital subscriber line is an asymmetric communication technology designed for residential uses: it is not suitable for business.