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Priority Option The token-passing-access method provides a priority option mechanism. This permits higher layers to assign priority (according to service class, rank, or order) to the frames destined for transmission. The priority option procedure allows the media access control sublayer to support eight service classes (four are currently defined) for the logical link control sublayer and higher-layer protocols. The currently defined service classes are:
The priority algorithm incorporates the maximum allowable time that a token can circulate for each access class. If the token hold timer has a remaining positive value, then the station can transmit frames at this access class until either the token hold timer value reaches 0 or this access classs queue becomes empty. When either event occurs, the station begins to service the next lower access class. At the completion of the lowest-level service request, the station performs any required logical ring maintenance and passes the token to its successor. Physical Layer Specification Exhibit 1-1-24 illustrates the partitioning of the physical hardware for token bus networks. The standard specifies three bus media and corresponding physical layer entities for use with the token passing access method. Two of these, phase-continuous frequency shift keying and phase-coherent frequency shift keying, use a nondirectional bus; the third, multilevel duobinary AM/PSK (a form of modulation where the radio frequency carrier is both amplitude-modulated and phase shiftkeyed), uses a bidirectional bus with an active head-end repeater.
The physical layer also performs management services. IEEE 802.4 recommends that the physical layer be responsible for:
MAP Versus IEEE 802.4 Although the Manufacturing Automation Protocol (MAP) was behind the development of the IEEE 802.4 standard, it differs from the standard in several ways. The MAP specification is not confined to the OSI physical and data link layers; it covers all the higher layers as well. The applications software provides file transfer and manufacturing messaging services. The MAP 3.0 specification also includes options for carrier band media and data link options that extend the applicability of MAP into lower levels of factory automation. The relationship among the various MAP versions, OSI, and IEEE 802.4 is shown in Exhibit 1-1-25.
IEEE 802.5 (Token Ring) The Token Ring topology was originally proposed in 1969 and was known as the Newhall ring. It has been popular in Europe and is often used in the US as a medium for connecting IBM networks. A Token Ring is composed of a number of stations serially connected by a medium. Information is transferred along the ring serially, bit by bit, from one node to another. In general, each station regenerates and repeats each bit and serves as the means for attaching one or more devices to the ring. A station gains transmission access to the medium by capturing a token that is passing on the medium. It then puts information on the token which circulates around the ring until it reaches the destination and is copied by the intended station. When the information has been transferred, the sending station removes the data from the ring. The sending station then generates a new token. To prevent a single station from taking over the medium, a token-holding timer limits the length of time a station can occupy the medium before passing the token. The protocol also supports a multiple priority scheme.
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