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Contention-Access Protocol for a Ring Network

IP.com Disclosure Number: IPCOM000042835D
Original Publication Date: 1984-Jun-01
Included in the Prior Art Database: 2005-Feb-04
Document File: 5 page(s) / 55K

Publishing Venue

IBM

Related People

Bederman, S: AUTHOR

Abstract

This article describes a contention-access mechanism for a network ring. The described protocol differs from conventional token-access schemes in that a station, upon detecting that the ring is in an idle state, can immediately begin to transmit a message, rather than having to wait for the receipt of a token pattern. The described protocol also differs from typical buffer-insertion schemes in that the message which a station begins to transmit is subject to overlapping the transmission of messages by one, or more, other stations. Such overlapping (herein called "collisions") can result in one of the messages going around the ring, while the other message(s) are interrupted to clear a path for the successful message.

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Contention-Access Protocol for a Ring Network

This article describes a contention-access mechanism for a network ring. The described protocol differs from conventional token-access schemes in that a station, upon detecting that the ring is in an idle state, can immediately begin to transmit a message, rather than having to wait for the receipt of a token pattern. The described protocol also differs from typical buffer-insertion schemes in that the message which a station begins to transmit is subject to overlapping the transmission of messages by one, or more, other stations. Such overlapping (herein called "collisions") can result in one of the messages going around the ring, while the other message(s) are interrupted to clear a path for the successful message. The interrupted stations will subsequently again try to gain access to the ring and will retransmit their messages. Ring Modulation The description of the invention assumes that the signals on the ring employ differential Manchester code. Fig. 1 illustrates one definition of this code. Note that there is a transition at each bit boundary, and that binary 0's also have transitions at the midpoints between bit boundaries. With this definition, it follows that a "code violation" occurs whenever there is no transition at a bit boundary. Fig. 2 illustrates two particular patterns which can occur over a duration of 4 bit times. These patterns employ code violations. Electrical circuitry can be designed to detect the occurrence of these patterns. In the description which follows, one of the 4-bit patterns is used to place the ring in an idle state. The other pattern is employed, in the variable-length message format, as an ending delimiter, and is used to indicate the end of the portion of the message which is included within the cyclic redundancy check. The idle patterns help establish bit synchronization of the clocks on the ring, and also provide tentative byte synchronization for messages which may start to flow. Message Format Fig. 3 shows the message format which is used in the description. A station will not start to transmit message(s) until it is certain that the ring is in an idle state. It determines this state by noting that an idle pattern, received from the upstream ring segment, has been retransmitted on the downstream segment. Thus, the first message transmitted by a station will be preceded by at least one idle pattern. The first field transmitted by a station is the monitor count/priority field. In the following, this field is assumed to consist of 4 bits. One bit is used for the monitor count. A transmitting station sets this bit to 0 when it originates a message.

When the message flows past one of the stations on the ring which serves as the monitor station, the monitor station sets the MC bit to 1. Three bits of the field encode the priority. This is a binary number which can be used in resolving access contention. The 4-bit monitor count/priority count field...