Basics of Wi-Fi Study Notes

• 802.11 − This pertains to wireless LANs and provides 1- or 2-Mbps transmission in the 2.4-GHz band using either frequency-hopping spread spectrum (FHSS) or direct-sequence spread spectrum (DSSS).
• 802.11a − This is an extension to 802.11 that pertains to wireless LANs and goes as fast as 54 Mbps in the 5-GHz band. 802.11a employs the orthogonal frequency division multiplexing (OFDM) encoding scheme as opposed to either FHSS or DSSS.
• 802.11b − The 802.11 high rate WiFi is an extension to 802.11 that pertains to wireless LANs and yields a connection as fast as 11 Mbps transmission (with a fallback to 5.5, 2, and 1 Mbps depending on strength of signal) in the 2.4-GHz band. The 802.11b specification uses only DSSS. Note that 802.11b was actually an amendment to the original 802.11 standard added in 1999 to permit wireless functionality to be analogous to hard-wired Ethernet connections.
• 802.11g − This pertains to wireless LANs and provides 20+ Mbps in the 2.4-GHz band.

Wi-Fi Concepts: There are two general types of Wi-Fi transmission: DCF (Distributed Coordination Function) and PCF (Point Coordination Function). DCF is Ethernet in the air. It employs a very similar packet structure, and many of the same concepts. There are two problems that make wireless different then wired.

• The hidden substation problem.
• High error rate.

These problems demand that a DCF Wi-Fi be a CSMA/CA network (Collision Avoidance) rather than a CSMA/CD network (Collision Detect). The results are the following protocol elements,

• Positive Acknowledgement. Every packet sent is positively acknowledged by the receiver. The next packet is not sent until receiving a positive acknowledgement for the previous packet.
• Channel clearing. A transmission begins with a RTS (Request to Send) and the destination or receiver responds with a CTS (Clear to Send). Then the data packets flow. For the channel is cleared by these two messages.
• Channel reservation: Each packet has a NAV (Network Allocation Vector) containing a number X. The channel is reserved to the correspondents (the sender and receiver of this packet) for an additional X milliseconds after this packet. Once you have the channel, you can hold it with the NAV. The last ACK contains NAV zero, to immediately release the channel.