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 To help identify the problem each station should know the identity of its upstream neighbor station. The process for obtaining this identity is known as neighbor notification. It is based on address-recognized and frame-copied bits of the frame status field that are transmitted as 0s. If a station recognizes the destination address of the passing frame as its own, and if it copies the frame, it sets the bit A equal to 1 and sets C equal to 1. In the event of a broadcast frame, the A and C bits are all set to 0. This means a broadcast frame’s destination address will be recognized by all stations on the ring. Therefore, the first station downstream will set bit A equal to 1. Stations further downstream will therefore not reset the A and C bits to 0. As this process continues in a daisy-chain fashion, every station learns the identity of its upstream neighbor. A monitor begins neighbor notification by broadcasting the active monitor present (AMP) media access control frame. On receiving this frame, a station immediately downstream takes the following actions: 1.  It resets its timer, standby monitor (TSM), based on the active monitor current value obtained from the frame control field. 2.  The downstream station copies the broadcast active monitor present media access control frame and stores the upstream station’s identity in an upstream neighbor’s address at the proper memory location. 3.  If the frame is copied the station sets the A and C values to 1. 4.  It broadcasts a similar monitor present media access control frame at a convenient time. This process continues until each station has the opportunity to receive an A - 0, C = 0 similar monitor present frame, copy its upstream neighbor’s address, and broadcast a similar monitor present frame itself. The active monitor present frame must pass each station on a regular basis. In addition to the timer, active monitor (TAM) in the active monitor, each standby station has a TSM that is reset each time an AMP media access control frame passes. If the TSM expires, the standby monitor issues a claim-transmission frame; this initiates the recovery process. Token Maintenance. The monitor station plays an active role in maintaining the integrity and stability of the network. It periodically issues an AMP status frame to inform other nodes that it is monitoring the ring. The monitor uses a watchdog valid frame timer with values initialized to a value greater than the time required for a token or frame to circulate around the ring. The timer is reset after every token or frame. The monitor issues a token only at the expiration of the valid frame timer. In order to detect a persistently circulating data frame, the monitor sets the monitor bit M to 1 the first time it sees a data frame. If it detects a data frame with the monitor bit already set to 1, it concludes that the transmitting station failed to remove the frame. The monitor removes the frame and issues a new token. The monitor follows the same algorithm to detect a priority failure and recovery mechanism that prohibits the circulation of a constantly nonzero priority-level token in the ring. If the monitor detects the presence of another monitor in the ring, it enters into standby monitoring mode. This, i turn, triggers the recovery mechanism. Token-Ring Physical Layer Specification The IEEE 802.5 standard specifies a baseband, shielded twisted-pair cable attachment to the trunk cable of the Token Ring (see Exhibit 1-1-27). The communications medium consists of a set of trunk coupling units interconnected sequentially by the trunk cable links. Each trunk coupling unit is connected to the trunk cable/medium interface connector, where all transmitted and received signal specifications are met. Two balanced, 150-ohm shielded twisted-pair cables connect the station to the trunk cable medium. The original standard defines a system with alternative data rates of 1M or 4M bps with a maximum capacity of 250 physical media components (PMCs) per ring, except for 144 in the case of UTP with 4M bps. Each active retiming concentrator (ARC) port counts as one PMC. In cases with two ARCs per station, the number of stations that can be supported per ring are reduced, possibly to only 125 or 72 stations, respectively. Exhibit 1-1-27.  IEEE 802.5 Connection System Hardware Separation In order to accommodate demands for higher bit rates, the 1993 IEEE draft standard 802.5q increases the top bit rate to 16M bps. Physical Layer. In the IEEE draft standard 802.5q, the station’s physical layer (PHY) is divided to form the physical signaling components (PSC) and the physical medium components (PMC) sublayers. The PSC couples the PMC to the MAC. It specifies the data symbol timing, encoding and decoding, and reliability. The physical layer encodes and transmits four symbols passed down to it by the media access control layer. These symbols are binary 0, binary 1, nondata J, and nondata K. Differential Manchester encoding techniques are used to transmit the symbols. The nondata symbols J and K depart from the standard rules. A J symbol has the same polarity as the preceding symbol, whereas K has a polarity opposite to that of the preceding symbol. The nondata symbols must be transmitted as a pair (i.e., J and K) to avoid an accumulating dc component caused by the transmission of only one nondata symbol. The physical layer recovers the symbol timing information encoded in the transmission between levels of the received signal. The timing information is required for internal use and for the transmission of symbols on the ring. During regular operation one station is designated as the active monitor, which acts as the timing source. All other stations on the ring are frequency and phase locked to the active station. A latency buffer is provided by the active monitor to ensure minimum latency and compensate for phase jitter. A ring must maintain a latency equal to 24 bits (i.e., the number of bits in the token sequence) for the token to continuously circulate around the ring when all stations are in repeat mode. Because ring latency varies from one segment to another and no prior knowledge is available, a delay of at least 24 bits should be provided by the active monitor. If the ring latency is not constant, bits either will be dropped as the latency of the ring decreases or will be added as the latency increases. To maintain a constant ring latency, an elastic 6-bit buffer is added to the fixed 24-bit buffer. The result is a 30-bit buffer that is initialized to 27 bits. If the received signal at the monitor station is slightly faster than the clock of the monitor station, the buffer expands to avoid dropping bits. On the other hand, if the received signal is slow, the buffer contracts to avoid adding bits to the repeated bit stream. 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.