Handbook of Local Area Networks, 1998 Edition:LAN Basics Click Here! Search the site:   ITLibrary ITKnowledge EXPERT SEARCH Programming Languages Databases Security Web Services Network Services Middleware Components Operating Systems User Interfaces Groupware & Collaboration Content Management Productivity Applications Hardware Fun & Games EarthWeb sites Crossnodes Datamation Developer.com DICE EarthWeb.com EarthWeb Direct ERP Hub Gamelan GoCertify.com HTMLGoodies Intranet Journal IT Knowledge IT Library JavaGoodies JARS JavaScripts.com open source IT RoadCoders Y2K Info Previous Table of Contents Next 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: •  Synchronous. Service class 6. •  Asynchronous urgent. Service class 4. •  Asynchronous urgent. Service class 2. •  Asynchronous time available. Service class 0. 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 class’s 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 shift—keyed), uses a bidirectional bus with an active head-end repeater. Exhibit 1-1-24.  Physical Hardware Partitioning for Token-Bus LAN The physical layer also performs management services. IEEE 802.4 recommends that the physical layer be responsible for: •  Resetting the physical layer entity, determining the network topology, and determining its own role in that LAN. •  Determining the available and current operating modes of the physical layer entity and selecting the appropriate operating modes. The modes include such features as: —  Transmit and receive channel assignment. —  Transmitted power-level assignment (per drop cable). —  Transmitter output enable and disable (per drop cable). —  Receive signal source. —  Signaling mode selection and reporting. —  Received signal-level reporting. 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. Exhibit 1-1-25.  Comparison of Various MAP and OSI Layers 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. Previous Table of Contents Next Use of this site is subject certain Terms & Conditions. Copyright (c) 1996-1999 EarthWeb, Inc.. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of EarthWeb is prohibited. Please read our privacy policy for details.