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186 lines
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<TITLE>The Answer Guy 42: Ethernet Switches vs. Hubs</TITLE>
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<H4>"The Linux Gazette...<I>making Linux just a little more fun!</I>"</H4>
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<H1><A NAME="answer">
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<font color="#B03060">The Answer Guy</font>
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<BR>
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<H4>By James T. Dennis,
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<a href="mailto:linux-questions-only@ssc.com">linux-questions-only@ssc.com</a><BR>
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LinuxCare,
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<A HREF="http://www.linuxcare.com/">http://www.linuxcare.com/</A>
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<H3 align="left"><img src="../../gx/dennis/qbubble.gif"
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height="50" width="60" alt="(?) " border="0"
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>Ethernet Switches vs. Hubs</H3>
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<p><strong>From Louan Handke on Sat, 01 May 1999
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</strong></p>
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<!-- ::
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Ethernet Switches vs. Hubs
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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:: -->
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<P><STRONG><IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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>
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What is the difference between switch hub and unswitched hubs
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</STRONG></P>
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<BLOCKQUOTE><IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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>
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The traditional ethernet hub (concentrator, repeater,
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etc) is a relatively simple device which just amplifies
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the signals on any of its ports out to all of the other
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ports. A "switch" or "intelligent" hub is more of a
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multiport bridge. It "learns" which MAC addresses
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(ethernet hardware assignments) are on each of it's ports
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and only "repeats" (rebroadcasts) data frames to the
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appropriate port.
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</BLOCKQUOTE>
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<BLOCKQUOTE>
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In a traditional hub only one system on a given network
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segment can be "talking" at any given time. The whole
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network segment is virtually a single wire. Any time
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two or more systems attempt to send packets at close to
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the same time there is a "collision." This is called
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CSMA/CD --- carrier sense (listen for quiet), multiple
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access (any card and "speak up"), with collision detection.
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</BLOCKQUOTE>
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<BLOCKQUOTE>
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Whenever a collision occurs the cards involved send a
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short jamming signal, and then they perform a psuedo
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random "backoff" delay before attempting to re-broadcast.
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Since it is incredibly unlikely that two cards will choose
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the same amount of backoff delay one of them will usually
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"win" and get to send first. That's fine with only a
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couple of cards in contention. However, as utilization
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approaches 20% or more, the number of collisions skyrockets
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and the overall average throughput drags to a crawl.
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</BLOCKQUOTE>
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<BLOCKQUOTE>
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The traditional answer was to segment the systems ---
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putting servers in close proximity to their clients
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(work groups), put routers between segments, and
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put lots of interfaces in your workgroup servers
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(four to eight ethernet interfaces was not unusual
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for big netware servers).
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</BLOCKQUOTE>
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<BLOCKQUOTE>
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Etherswitches are used to alleviate some of these
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problems. On a 24 port etherswitch its theoretically
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possible for 12 pairs of systems to be concurrently
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exchanging data frames. This allows for much
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larger segments (called VLANs --- virtual local
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area networks).
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</BLOCKQUOTE>
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<BLOCKQUOTE>
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On the downside, etherswitches are typically much more
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expensive than their more passive cousins. They have to
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contain processors, memory, and firmware. In addition
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their processors have to be pretty quick (usually
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quick RISC chips with a mess of ASICs I guess). Also
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there are degenerate cases. If all of your servers are
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located on one or two legs of an etherswitch then it
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won't help much. All of the clients will be waiting for
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that one (or those couple of) port(s) to be clear ---
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a classic bottleneck.
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</BLOCKQUOTE>
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<BLOCKQUOTE>
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Again the solution is to have lots of smaller servers
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--- segment the network, and replicate the data and
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services so that they clients tend to use local
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copies of everythings. Hierarchies scale!
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</BLOCKQUOTE>
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<BLOCKQUOTE>
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(Not to say that etherswitches don't have their
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place --- its just to say that their deployment should
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be based on an understanding of the situation and the
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benefits vs. the costs of the technology. Most vendors
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have little interest in your needs --- they want to
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sell you the shiny expensive toy).
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</BLOCKQUOTE>
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<!-- sig -->
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<!-- end 6 -->
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<!--startcut ======================================================= -->
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<P> <hr> <P>
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<H5 align="center"><a href="http://www.linuxgazette.com/copying.html"
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>Copyright ©</a> 1999, James T. Dennis
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<BR>Published in <I>The Linux Gazette</I> Issue 42 June 1999</H5>
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<H6 ALIGN="center">HTML transformation by
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<A HREF="mailto:star@starshine.org">Heather Stern</a> of
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Starshine Techinical Services,
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<A HREF="http://www.starshine.org/">http://www.starshine.org/</A>
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</H6>
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