An 802.11 station sends a 300 octet frame (header + data + CRC) to a second stat

Question

An 802.11 station sends a 300 octet frame (header + data + CRC) to a second station in a BSS. (a) How much time would it take (in octet times) before another station could send a frame? (b) What would be the value of the duration field in the Data frame? (c) How much time would it take (in octet times) before another station could send a frame if the RTS-CTS frames were used to capture the link before the station transmitted the 300 octet frame? (d) What would be the value of the duration field in the CTS frame?

Explanation / Answer

Wireless LAN is a half-duplex (simplex) communication medium i.e. the same frequency is used to transmit as to receive. Spread Spectrum was invented in 1942 and started to be used in ISM bands in 1986. More information requires a wider frequency spectrum (bandwidth), e.g. CB Radio uses at 3KHz, FM Radio uses 175KHz and television uses 4.5MHz. In addition, the more complex the transmission and modulation/compression; the more vulnerable to noise is the signal. In very broad terms, 'Wireless' covers a multitude of technologies. Relating to data networking these include: Infrared - 1 to 10Mbps, local coverage Narrowband - 9.6 to 19.6Kbps, local coverage Spread Spectrum - 1 to 50Mbps, local coverage Personal Communications Service (PCS) - 9.6 to 192Kbps, metropolitan coverage 2.5G, 3G, 3.5G GSM - 10 to 384Kbps, wide area coverage Cellular Digital Packet Data (CDPD) - 19.6 to 56Kbps, wide area coverage Free Space Optics - Laser running from 10Mbps to 1Gbps, metropolitan coverage Microwave - 192Kbps to 54Mbps, wide area coverage Satellite - 19Kbps to 1Mbps, wide area coverage In the late 1980s Wireless LANs (WLAN) manufacturers used proprietary Direct Sequence Spread Spectrum (DSSS) technology over 900MHz radio with data throughput of 860Kbps. This could run at quite large distances, however the problem with this is that there were no open standards and many countries did not allow the 900MHz radio frequency to be used. 900MHz DSSS used one channel at 860Kbps, two channels at 344Kbps or three channels at 215Kbps each. There are a number of different Wireless LAN technologies in addition to the 802.11 Wireless LAN. These include: Wireless LAN Interoperability Forum (WLIF) - Proxim and RangeLAN2 Home RF - Frequency Hopping Technology Home RF 2.0 - disbanded in 2003 HiperLAN - European 5GHz WLAN being superceded by 802.11a Bluetooth - PAN on 2.4GHz using Fast Frequency Hopping giving up to 1Mbps over short distances. With the move to 2.4GHz in 1990 the data throughput jumped to 1Mbps and 2Mbps albeit at shorter distances. 1992 saw the beginning of the drafting of the 802.11 standard for wireless LAN technologies. This culminated in 1997 with the 2.4GHz standard which used the following technologies at the physical layer: DSSS - for 2Mbps and 11Mbps Frequency Hopping Spread Spectrum (FHSS) - for 1Mbps and 2Mbps Infra Red This standard had 1Mbps as a standard data rate and 2Mbps as a Turbo mode. In September 1999 the 802.11a and 802.11b standards were developed and ratified, followed by 802.11g in June 2003. Frequency Bands The frequency bands used for WLAN belong to the unlicensed frequency bands. These are: 900MHz - 902MHz to 928MHz - Industrial, Scientific and Medical (ISM) 2.4GHz - 2.4GHz to 2.483GHz (or 2.495GHz in Japan), prone to interference fromcordless phones, microwave ovens, Bluetooth and wireless video 5GHz - 5.150GHz to 5.350GHz and 5.725GHz to 5.825GHz, prone to interference from HiperLAN, Maritime and Satellite frequencies. Unlicensed National Information Infrastructure (UNII) Some countries use frequencies that are slightly different from those listed.

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