Browse Prior Art Database

Priority Preemption in a Token Ring Communication System

IP.com Disclosure Number: IPCOM000044564D
Original Publication Date: 1984-Dec-01
Included in the Prior Art Database: 2005-Feb-06
Document File: 2 page(s) / 100K

Publishing Venue

IBM

Related People

Andrews, DW: AUTHOR [+5]

Abstract

In token ring local area networks, it is desirable to provide several priority levels of the "token." It is also desirable to allow stations having high priority messages to non-disruptively preempt the passage of a lower priority token by using a reservation technique. However, such a priority scheme needs to be fair; that is, all messages at a given priority level must be provided access before any message at a lower priority level is provided accesses, and at any given priority level the free token must pass from station to station in a round robin manner. The priority and reservation technique described provides both preemption and fairness. In the closed ring structure contemplated, a token has the format illustrated in Fig.

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Priority Preemption in a Token Ring Communication System

In token ring local area networks, it is desirable to provide several priority levels of the "token." It is also desirable to allow stations having high priority messages to non-disruptively preempt the passage of a lower priority token by using a reservation technique. However, such a priority scheme needs to be fair; that is, all messages at a given priority level must be provided access before any message at a lower priority level is provided accesses, and at any given priority level the free token must pass from station to station in a round robin manner. The priority and reservation technique described provides both preemption and fairness. In the closed ring structure contemplated, a token has the format illustrated in Fig. 1 in which the common token is bounded by a start and end delimiter and includes a control field which has three priority bits "P"; a token bit "T" that when set to "0" indicates a token and when set to "1" a frame; a monitor bit M that is used by a monitor station to purge exhausted messages from the ring during certain device malfunctions; and three reservation bits R that are used by devices to cause the priority level of the token to change. In operation, a station that has acquired a free token at the lowest priority level marks the token busy and sends its message followed by an end delimiter. Another station on the ring at a higher priority level and needing service changes the reservation bits "R" to correspond to its priority level. Thus, upon return to the transmitting station the reservation bits "R" will indicate the highest priority required on the network. This station will now issue a free token at the received reservation level...