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 ADVANCED SWITCH FUNCTION Switches were originally designed to receive a frame at the input port, check its validity, and send it out on the destination port. This is known as store and forward switching (see Exhibit 3-6-3). Bridges and routers both operate this way. Exhibit 3-6-3.  Switch Operation The validity of a LAN frame is checked using a Cyclic Redundancy Check (CRC). The sending device performs a mathematical operation on the bytes in the frame as it is being transmitted, and the result of this calculation is appended to the frame. As the input port receives the frame, the mathematical operation is repeated and the calculated result compared to the appended value. The frame is correct if they compare, and corrupted if they do not. Any frame in error is discarded by the switch, so as not to “bother” the output LAN or device with error traffic. In a quality LAN installation, only one frame in billions has an error. Since the first thing in the frame is the destination address, once these six bytes have been received, and while the rest of the frame flows into the switch, the processor could perform the table lookup to determine the output port. If the output port is available (not currently in use), the frame could be transferred to the output port as it is being received, and immediately transmitted. This is known as cut-through switching. One of the measures of switch quality is their latency. Switchlatency is the time from the first bit arriving at the input port to the first bit being transmitted by the output port. The latency of cut-through switches is as low as 30 microseconds, while that of store-and-forward switches is about 1,300 microseconds. Since many applications (client/server, Web, intranet) require many transmissions to accomplish a single task, latency is cumulative for each task. Thus, cut through is a superior design. Unfortunately, not all of us have LANs with very low error rates, whether because of faulty adapters, wiring, or whatever. While these problems could be permanent, many are transient. To address this situation, and maintain the benefits, adaptive cut-through was invented. These switches operate in cut-through mode until some error threshold is reached, and then switch to store and forward operation. If the number of frames in error falls below another threshold, the switch automatically changes back to cut-through operation Benefit. Adaptive cut-through provides the very low frame latency today’s applications demand, while accommodating LANs with higher error rates. SWITCHING MODES So far, the description of LAN switches has not differentiated between LAN protocols. Readers with a knowledge of protocols know that Ethernet uses a technique called transparent switching (see Exhibit 3-6-4). That is, the forwarding decision is made by the switch, and the end station need not know a switch is even present (hence transparent). Transparent switches operate by doing table lookup against address tables. A consequence of this technique is that there may be one and only one path between end points. Switches either rely on the network designer to manually ensure this, or they support spanning tree, the technique used by transparent bridges to automatically accomplish it. Exhibit 3-6-4.  Token-Ring LAN Switching Token Ring users know that their bridges use another technique, called source route bridging. Source route bridging, among other things, allows active parallel paths between devices, increasing redundancy and network availability. A source route is established by a Token Ring station when it determines the destination station is not on its same ring. The result of this process is that the originating station inserts a routing information field into each frame it sends, and the destination station sets a bit so that the bridges follow the same route in reverse with the answer. Should you install a switch into an environment that contains Token Ring bridges that will remain, the switch must recognize and act on the routing information field in the frames. Even if the existing bridges are to be removed, if the parallelism of Token Ring is to continue, the switches must be able to correctly interpret the route discovery frames, construct routing information fields, and act on them. Token Ring switches that allow this capability are available. If an environment will no longer require source routing, use Token Ring switches that allow operation in transparent mode. This will ease the administrative burden, but at the cost of lost redundancy. If route (switch) redundancy is important, be sure to choose switches that support source routing. Benefit. Both types of Token Ring switches are available today, and many allow setup options to choose how you want them to operate. FULL DUPLEX In shared media LANs, every LAN adapter must have the ability to both listen and talk at the same time. In Ethernet this is used for collision detection and in Token Ring for the ability to repeat received frames around the ring. If there is only one station on the port, rather than a LAN segment, this capability is not needed. Realizing this, some switches allow the device to operate in full duplex mode. Full duplex support allows the device on a dedicated port to receive a request from one station while answering a previous request from another station. In addition to capability in the switch, full duplex requires support by the device adapter and its software driver (see Exhibit 3-6-5). Exhibit 3-6-5.  Full Duplex Benefit. Full duplex operation effectively doubles the transmission speed while reducing the application latency. SWITCH FORM FACTORS LAN switches have evolved from the background, may be produced by the same companies, and often share designers with LAN wiring hubs. So it should be no surprise that LAN switches are available in the same form factors: table top, rack mount, stackable, or as a module in a frame-based system. This means that many different products are available, and a given user’s choice depends on the existing environment. The top of the line modular systems provide flexibility in the number of ports supported, ability to connect to higher speed LAN technologies, and advanced management. The low end device may have a small, fixed number of ports. The other designs have a variety of capabilities for expansion, speed, media, or LAN protocol conversion, or additional functions. Benefit. LAN switches are available today as low cost work group switches that work well at the edge of the network, as backbone switches with high capacity and management tools, or for anywhere in between. 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.