Handbook of Local Area Networks, 1998 Edition:LAN Interconnectivity 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 Ethernet MAC The Ethernet frame format and MAC functions were originally developed into a coherent link layer LAN protocol during the late 1970s by Digital Equipment Corp., Intel Corp., and Xerox Corp. Ethernet is a contention access protocol (CSMA/CD), services many processor types (including DOS, Macintosh and UNIX), and assigns use of the medium on a first-come, first-served basis. The Ethernet access mechanism is analogous to a telephone party line, in which multiple phones hare a single telephone line. If someone wants to use the line, the phone is taken off-hook, the caller listens to make sure no one else is using the phone, and either hangs up if someone is already on the line or completes the call if the line is free. This is the way workstations access an Ethernet. When a workstation initiates a send process, the network interface card first monitors the LAN broadcast medium to determine if it is in use. Once the medium is determined to be clear for transmission then the frame is transmitted onto the physical medium. All stations on the medium (i.e., the LAN segment) hear and ignore the transmission, except the station whose address matches the destination MAC address and the originating station. The originating station receives its transmitted sequence and compares it to the original transmission. If the transmit and receive data match then the data has been successfully transmitted. If the transmit and receive data do not match then it is likely that a collision has occurred and the transmitting station ceases transmission, generates a jam signal to all other stations, and waits a random amount of time before initiating transmission again. The 64-byte minimum frame size, maximum cable lengths, and maximum propagation delay are critical to the proper operation of the collision detect mechanism. Together these parameters ensure that by the time the last bit of the last byte of information is transmitted, the originating node is guaranteed of detecting a collision between its original transmission and any other workstation transmissions. Another characteristic that is a result of the shared media is that frame destination addresses fall into three categories, referred to as single-cast, multicast, and broadcast. Single-cast addressing uses a specific 6-byte destination address for a dedicated machine. In essence, this type of transmission is a point-to-point communication over a broadcast medium. Multicast addresses look like a standard 6-byte single-cst address but are actually special addresses that can be received by a subset of the stations connected to the LAN segment. Multicast addresses are typically used to support directed software loads. Broadcast addressing is accomplished by setting a specific destination address (i.e., FF-FF-FF-FF-FF-FF) in the Ethernet frame. Then all stations on the network will receive the transmission. Although broadcasting is an efficient mechanism to get information out to a large number of users, it can consume large amounts of bandwidth as the broadcast frame propagates across the entire network. The MAC layer frame formats are currently in use, the Ethernet V.2 format and the Institute of Electrical and Electronics Engineers (IEEE) 802.3 format. The differences between the two formats are illustrated in Exhibit 3-3-5. First, the 2 byte type field in the Ethernet frame is replaced with the 2 byte length field in the 802.3 frame. The two MAC layer frame header formats are the same length, so the frame types may coexist on the same shared medium. From the network layer up, the two frame types are incompatible, due to different information (length versus type fields) being transferred to the network layer. Another significant difference occurs at the LLC link sublayer. The Ethernet frame has no provisions for an LLC component, the 802 MAC layer protocols include provisioning for an LLC component. Because the frame formats are slightly different, some special care may have to be taken in mixing components if interoperability is to be ensured. Exhibit 3-3-5.  The Ethernet and IEEE 802.3 Frame Structures 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.