KEY IDEAS IN NETWORKING

Wi-Fi

Wi-Fi is the name of the industry consortium that decides implementations of the wireless Ethernet standards IEEE802.11. In this section, we explain the multiple access scheme that Wi-Fi uses.

In a Wi-Fi network, the devices communicate via a special node called the access point (AP). All the devices share the same communication channel (same range of frequencies). The basic issue is how to regulate access to this shared channel in a distributed way.

Wi-Fi uses a scheme called carrier sense multiple access with collision avoidance (CSMA/CA). The principle is (1) listen before you talk, (2) if you collide by speaking at the same time as someone else, wait a random time before you try again. This is essentially a cocktail party protocol.

The devices compute random wait times. The wait time of a device is the number of idle time slots it must wait before transmitting, counted D seconds after then end of the previous transmission. The device must add D seconds to its wait time whenever that wait time is interrupted by a transmission.

If devices compute different random times, they do not collide after their wait times. If the channel is busy at the end of a wait time, the device computes a new random wait time and starts waiting again.

 

In Wi-Fi, the random wait time is zero for a node that just receives a new packet to transmit while it is not waiting to transmit and the channel is idle. If the channel is busy, the node computes a wait time that is uniform in {0, 1, , 15} time slots. If the node tries to transmit a packet and does not receive an ACK, it doubles the set of time slots among which it picks uniformly a wait time.