A local area network with ordered bus access by low level token passing

In local area networks with token-controlled access to the physical medium, the access rights are passed from one station (or node) to another by transferring a token. There is only one token owner at a time which controls the packet generation onto the physical medium. Token-controlled systems are deterministic, in contrast with systems controlled by carrier sense multiple access with collision detection (CSMA/CD), and therefore worst case access times or transaction times can be defined, which is an important characteristic in distributed process control systems. In this paper a token-passing scheme is described which may be added below a commercially available data link controller with CSMA/CD control. The performance of the two access protocols is compared.