A local area network (LAN) is a grouping of individual stations or personal computers in a relatively small area such as an office building. The stations are interconnected or linked to facilitate the transfer of data and the sharing of peripherals. A conventional LAN is "hardwired", i.e., the stations are linked by high performance cables. A recent improvement in LANs is the development of a wireless LAN in which stations are linked by radio signals. The wireless LAN is cheaper to install than a conventional wired LAN and is more flexible in the movement and placement of stations.
In general, a specific protocol must be followed in order for a station to gain access to the medium (network). In a particular class of wired LANs, referred to as Ethernet, a random access protocol is employed.
An issue common to both conventional and wireless LANs is collision avoidance. A collision occurs when more than one station transmits at the same time. One widely used method of collision avoidance for conventional LANs is known as CSMA/CD (carrier sense multiple access with collision detection). According to this known method, a station wishing to transmit a message listens until the link or transmission channel is idle before commencing to transmit an information packet. The station continues to listen to the channel during its transmission, and if a collision is detected, terminates its transmission and transmits a jam pattern to notify other stations of the collision.
Data is transferred in frames which are separated by a fixed minimum time referred to as the interframe spacing (IFS). According to the typical CSMA/CD protocol, each station is free to attempt to broadcast a data frame at the end of the IFS period. A collision results when two stations start to transfer data at the same time or within the time needed to detect a carrier signal on the medium. When a collision is detected the stations are pushed into a random backoff period. The random backoff period prevents further attempts to transmit data until after the period expires. As the backoff period is random, the chances of two stations again attempting to access the network at the same time is reduced. The transfer of data, including the backoff algorithm, is handled by a so-called Medium Access Controller chip (MAC) on each station.
The CSMA/CD protocol is the subject of an international (ISO) standard and corresponding IEEE Standard Number 802.3, and is a generally used random access protocol for wired LANs. Consequently, CSMA/CD controller chips are widely available. For example, the Intel 82586 LAN coprocessor chip available from Intel Corporation of Santa Clara, Calif.
Collision detection on a wireless LAN is difficult to implement because a transmitting station cannot generally receive a much lower level signal on the medium while it is transmitting a much higher level signal. U.S. patent application Ser. No. 633,691, Kamerman et al., now U.S. Pat. No. 5,369,639, referred to above, describes a novel station which is adapted to use a standard CSMA/CD controller chip in a wireless LAN environment. In Kamerman et al. each station that wants to transmit immediately after the IFS period is pushed into the random backoff period at the end of the IFS period, even though there has been no collision. This greatly reduces the number of collisions which occur.
An issue not addressed by Kamerman et al. is that of equitable access to the network by the various stations on the LAN. For example, the random backoff period is only applied to those stations not presently broadcasting data. In other words, the broadcasting station need only wait until the end of the IFS period before broadcasting another data frame. For certain network operating systems, such as Novell's Netware v 2.15, client workstations which transmit a data frame must wait until receiving an acknowledgment before transmitting a second frame. Thus, no client workstation can unfairly dominate the network. However, a server frequently has a number of data frames ready for transmission. Therefore, the server is likely to tie up the network for multiple data frame transfers. This reduces data throughput and the efficiency of the system.