Fair Queuing - Properties

Properties

Fair queuing is used in routers, switches, and statistical multiplexers that forward packets from a buffer. The buffer works as a queuing system, where the data packets are stored temporarily until they are transmitted. The buffer space is divided into many queues, each of which is used to hold the packets of one flow, defined for instance by source and destination IP addresses.

With a link data-rate of R, at any given time the N active data flows (the ones with non-empty queues) are serviced each with an average data rate of R / N. In a short time interval the data rate may be fluctuating around this value since the packets are delivered sequentially.

Fair queuing achieves max-min fairness, i.e., its first priority is to maximize the minimum data rate that any of the active data flows experience, the second priority is to maximize the second minimum data rate, etc. This results in lower throughput (lower system spectrum efficiency in wireless networks) than maximum throughput scheduling, but scheduling starvation of expensive flows is avoided.

In contrast to round-robin scheduling, fair queuing takes into account data packet sizes to ensure each flow is given equal opportunity to transmit an equal amount of data. A weighted version of fair queuing is called weighted fair queuing (sometimes referred to as fair queuing). Weighting is achieved by multiplying the packet size considered by the fair queuing algorithm with the inverse of a weight for the associated queue. Fair queuing is a special case of weighted fair queuing with equal weights for all queues.