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 IEEE 802.4 Token Passing Bus Protocol The development of the IEEE 802.4 token passing bus protocol was spurred by General Motors Corp. and the Manufacturing Automation Protocol (MAP) User Group. They felt that IEEE 802.3, which was intended for light industrial and commercial environment applications, failed to meet the timing constraints of real-time heavy industrial applications. IEEE 802.4 was designed to operate well with both broadband and carrier-band signaling. It covers a wide range of topics—electrical signaling methods, frame format, access methods, contention resolution, and the media access control-logical link control layer interface. It also specifies the physical and media layer requirements for single-channel, phase-continuous frequency shift keying bus, single-channel, phase-coherent frequency shift keying bus, and broadband bus. Although the 802.4 standard uses a bus topology, it forms a logical ring during typical operation. Right-of-access to the communication medium is indicated by a token passed along the network from node to node. To become a member of the logical ring, a station must know three addresses: the predecessor’s address, the next station address, and its transmit address. Steady-state operation is composed of two phases: token transfer and data transfer. Stations perform path initialization, token recovery, new station admission, and general housekeeping of the logical path. Physical connectivity has little impact on node sequence within the logical ring. In addition, stations can receive information without being part of the logical ring. In Exhibit 1-1-19, stations A, E, D, B, and C form the logical ring; they are not physically connected in sequence. Stations F and G, which are outside the ring, cannot initiate transmission but can receive all ongoing transmissions. Exhibit 1-1-19.  Token Bus Topology Based on Logical Ring The IEEE 802.4 recommendation defines several functions as resident at the media access control sublayer. These general utilities are the lost token timer, distributed initialization, token holding timer, station address recognition, token preparation and frame encapsulation, frame check sequence generation and checking, valid token recognition, addition of new member to the ring, and station failure error recovery. These functions are assigned to and implemented as asynchronous logical machines (see Exhibit 1-1-20)—an interface machine (IFM), an access control machine (ACM), a receive machine (RxM), a transmit machine (TxM), and an optional regenerative repeater machine (RRM). Exhibit 1-1-20.  IEEE 802.3 Media Access Control Layer Partitions Interface Machine The IFM supports the logical link control to media access control sublayer and station management to media access control sublayer. This machine interprets all media-access-data and incoming-service primitives and generates appropriate responses. It maps the quality of service parameters from the logical link control to the media access control, manages service queuing and the address recognition function on received logical link control frames, and admits only those frames addresses to the station. Access Control Machine The ACM is the central nerve of the media access control sublayer. It cooperates with its peers on other stations in managing the token that controls transmission access. It also initializes and maintains the logical ring, including the admission of new nodes into the ring. Finally, the ACM performs fault or failure detection and error recovery, if needed in the network. Receive Machine The RxM accepts bits from the physical layer, reconstructs frames, and validates them before forwarding them to the IFM and ACM. Assembly and validation of frames are achieved through the detection of start delimiters, end delimiters, frame check sequence (FCS), and validating frame’s structural integrity. The RxM is also responsible for identification and reception of noise burst and bus quiet conditions. Transmit Machine The TxM accepts a data frame from the ACM and delivers it to the physical layer as a properly formatted frame. The TxM builds a media-access control-protocol data unit by adding the required preamble and start delimiters and by appending a frame check sequence and end delimiter to each frame. In the presence of a regenerative repeater machine, the TxM operates somewhat differently. Regenerative Repeater Machine The RRM is available as an option in special repeater stations (e.g., at a broadband or head-end remodulator). In such an operation both the RxM and the TxM cooperate with the RRM to repeat the bit stream coming in from the physical layer back to the physical layer. Logical Link Control-Media Access Control Service The token bus standard uses the same service primitives specified for the logical link control-media access control functions (802.3) MA__DATA.request, MA__DATA.indication, and MA__DATA.confirmation. Frame Format The IEEE 802.4 standard calls for two types of frames for information interchanges. The control information is conveyed by token frames and the data by logical link control frames. Frames may contain a maximum of 8,191 octets, excluding the preamble, start delimiter, and end delimiter. The format for these two generic types is the same as for IEEE 802.3, shown in Exhibit 1-5-14. Preamble As in 802.3, the preamble is affixed to the beginning of every transmitted frame. It can be one or more (integral) octets long and is used by the receiving station’s modem to identify signal level and detection of phase lock. The preamble also serves as a marker between successive frames. It guarantees a minimum end-to-start-delimiter interval of 2 ms independent of the data rate to ensure stations have had ample time to process the previously received frame. 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.