IEEE 802.4-1990 pdf free.Information processing systems—Local area networks Token-passing bus access method and physical layer specifications.
IEEE 802.4 The Essence of the Token Access Method:
(1) A token controls the right of access to the physical medium; the station which holds (possesses)the token has momentary control over the medium.(2) The token is passed by stations residing on the medium. As the token is passed from station to station a logical ring is formed.
(3)Steady state operation consists of a data transfer phase and a token transfer phase.
(4)Ring maintenance functions within the stations provide for ring initialization, lost token recovery, new station addition to the logical ring, andgeneral housekeeping of the logical ring. The ring maintenance functionsare replicated among all the token-using stations on the network.
Shared media generally can be categorized into two major types. These typesare broadcast and sequential. This standard deals exclusively with the broadcasttype.On a broadcast medium, every station may receive all signals transmitted.Media of the broadcast type are usually configured as a physical bus.
In Fig 1-2, note that the token medium access method is always sequential in alogical sense.That is, during normal, steady state operation, the right to accessthe medium passes from station to station. Furthermore, note that the physicalconnectivity has little impact on the order of the logical ring and that stationscan respond to a query from the token holder even without being part of the logi-cal ring.(For example, stations H and F can receive frames and could respondbut cannot initiate a transmission since they will never be sent the token.) The MAC sublayer provides sequential access to the shared bus medium bypassing control of the medium from station to station in a logically circularfashion. The MAC sublayer determines when the station has the right to accessthe shared medium by recognizing and accepting the token from the predecessorstation，and it determines when the token shall be passed to the successorstation.IEEE 802.4 pdf download.