IEEE 802.11 was created with three air interface options: infrared at 10 meters coverage, and direct sequence spread spectrum or frequency hopping spread spectrum at 100 meters coverage. Today, IEEE 802.11 products typically implement direct sequence spread spectrum.
Currently the 802.11 family has four variations for the radio air interface. The first was 802.11b, also known as Wi-Fi, which operates in the 2.4 GHz unlicensed band at speeds up to 11 Mbps. The second was 802.11a, which operates in the 5.8 GHz unlicensed band at speeds up to 54 Mbps. The third was 802.11g, which operates in the 2.4 GHz band at speeds up to 54 Mbps. The last is 802.11n, which operates at over 200 Mbps in the 2.4 and 5 GHz bands and has backward compatibility with 802.11a/b/g. 802.11n was accepted in draft form in January 2006 with formal ratification expected by 2008.
Wi-Fi Performance: CSMA/CA
Carrier sense multiple access (CSMA) is basically a listen before talking process that is used in a shared LAN environment. However, if two users listen at the same time and hear nothing, then they both will talk, a collision will occur, and data will be lost. In Ethernet this problem is solved by adding a collision detection phase—listen before talking, send, listen for a collision, and stop transmitting if a collision occurs.
In radio environments, collision detection is often not possible, especially if there are hidden nodes. A hidden node is one that is detected by the access point but not by other stations on the wireless LAN. For example, 802.11g has about 300 feet of radio coverage. Consider two stations, each of which is 300 feet from the access point. The access point can hear both stations, but because of the 600 foot distance between the stations they cannot hear each other and thus cannot detect a collision.
Collision avoidance (CA) attempts to solve the problem in one of two ways. The first is to send a jamming signal before transmitting, which alerts all the stations that a transmission is about to begin. Unfortunately, this approach does not work with hidden nodes. The second is to use a request to send/clear to send (RTS/CTS) procedure, found in the 802.11 protocols. In this model, a station wishing to send transmits an RTS message to the destination (the access point in 802.11), and the destination sends a CTS back to the station. Any station that hears the RTS or CTS message must hold off transmitting for a period of time that is specified in the RTS/CTS sequence.
While CSMA/CA attempts to efficiently use the bandwidth, the whole process of listening for a clear channel and sending the RTS/CTS sequence takes time. Since the bandwidth is shared, the number of potential collisions increases as the number of active stations increases. This means that the more active the environment, the lower the average throughput per station. Mixing 802.11b stations with 802.11g stations in a single radio environment results in lower throughput (e.g., 802.11b is slower than 802.11g). This mix-and-match approach has led some manufacturers to implement multiple radio solutions.
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