Handbook of Local Area Networks, 1998 Edition:Advanced LAN Issues and Solutions 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 Auto-Negotiation All Gigabit Ethernet signaling systems use some form of Auto-Negotiation to configure both ends of the link for proper operation at start-up. 1000BASE-T uses the same Auto-Negotiation technology pioneered by Fast Ethernet. This technology, described in clause 28 of IEEE802.3u, supports operation over copper links with RJ45 connectors. It ensures that stations at both ends of the link are compatible and, if so, configures them for optimal operation. In addition to ensuring proper connection and operation and startup, 1000BASE-T Auto-Negotiation can support multi-speed (100/1000M bps) capable devices, much like the 10/100 devices available for Fast-Ethernet. 1000BASE-X uses a different form of Auto-Negotiation, developed by the 802.3z task force, to support full-duplex continuous signaling using 8B10B coding over 1.25GHz links. As with the copperlink version of Auto-Negotiation, it provides a means for stations at two ends of a link to exchange information describing their abilities and to then use this information to configure themselves so they can operate with one another in an optimal configuration. The auto-negotiation information is exchanged via special frames as part of the start-up process. Once Auto-Negotiation is complete, the link is opened for communication. The basic information exchanged as part of the 1000BASE-X Auto-Negotiation process is contained in a “base page” and includes: support of full-duplex operation, support of half-duplex information, support of asymmetric and/or symmetric PAUSE control (needed for flow control), provision of remote fault codes to report Auto-Negotiation problems, acknowledgment that the base page has been received, and notification of additional or next pages to be sent. The Next pages are used to exchange optional or vendor-specific information as part of the Auto-Negotiation process. Half-Duplex/Full-Duplex Gigabit Ethernet supports two modes of operation: the traditional half-duplex mode which uses carrier sense multiple access with collision detect (CSMA/CD), and a full-duplex mode similar to that provided by Fast Ethernet. The two modes operate and behave quite differently. Half-duplex operation requires modifications to the Media Access Control (MAC) layer which change the treatment of small packets and can limit performance. At operating speeds above 100M bps, the MAC appends a sequence of carrier extension bits to short frames to ensure that the length each frame event equals or exceeds the slot time. This means that the minimum frame size for half-duplex operation at 1000M bps is 4096 bit times or 512 bytes. A minimum frame size of 512 bytes ensures half-duplex operation with a collision domain of 200—the same as that provided by 100BASE-T. (Failure to extend the frames would limit the collision domain to approximately 20 meters.) Extension bits are not data and are automatically stripped from frames at receipt. A second change to the MAC enables sending burst of frames. When operating in half-duplex mode at speeds greater than 100M bps, the CSMA/CD MAC may optionally transmit additional frames (up to a total of 65,536 bits) without relinquishing control of the transmission medium. This improves the throughput during half-duplex operation—assuming there are multiple packets in a queue waiting to be sent and that upper layer protocols permit packet bursting. Full-duplex operation provides for simultaneous data flow in both directions by turning off CSMA/CD as per Clause 31 of IEEE802.3. Turning off CSMA/CD allows the links to operate at full bandwidth, maximizing performance. In 10BASE-T and 100BASE-T, full-duplex operation is only possible on dedicated or switched links, but 1000BASE-T also supports shared media full-duplex operation via Full Duplex repeaters, which are described later in this chapter. CABLING AND TOPOLOGY RULES Gigabit Ethernet signaling systems are designed to work with a variety of fiber and copper links. Fiber links are dependent on the interaction between the optics and the fiber; the distances shown in Exhibit 2-5-3 are minimum distances. The TW-style copper users should be aware that the electrical requirements for 1000BASE-CX support are more stringent than the IEC-807-3 specifications. Exhibit 2-5-3.  Gigabit Ethernet Link Distances Gigabit Ethernet link distances assume ISO/IEC 11801 compliant fiber. Distances will vary depending on the modal bandwidth of the installed fiber. Additional details on the impact of modal bandwidth on fiber link distances are provided in Clause 38 of the standard. Supporting 1000M-bps operation over Category 5 links raises concerns about both return loss and crosstalk, performance areas that have previously been unspecified in the US EIA/TIA-568-A cabling standard. A new addendum to that standard specifies how to measure return loss and crosstalk for both component and field testing. By the time 1000BASE-T products reach the market, commercial cable testing devices should be capable of evaluating these cable characteristics. Cabling experts have indicated that cabling plants that are compliant with the current version of EIA/TIA-568-A should have no problem meeting appropriate return loss and crosstalk performance levels. Exhibit 2-5-4 provides collision domain diameter information for 1000M-bps half-duplex operation. Maximum collision domain diameters for half-duplex operation assume 25 meters of TW-style cable and one fiber link of 195 meters. Exhibit 2-5-4.  Maximum Collision Domain Diameters for Half-Duplex Operationa 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.