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<font size="-1"><font face="Helvetica, Arial, sans-serif">CSMA/CD
Ethernet was a slotted Aloha system with CD. Slotting increased the
throughput from Aloha's 18% to something like 38%, and CD raised it
into the 90s. You can't really do CD without a wire. A station
transmitting wireless data can't really see anyone elses transmitter
while his is active because the strength of the local signal is so
great that the receiver has to be shut off during transmission to avoid
saturation. <br>
<br>
Limits on the network size are driven by the slotting system, as the
slot size has to capture the maximum propagation delay from one network
edge to another. Aloha wasn't slotted, so it could be arbitrarily
large; Ethernet was limited to 2.5 km from edge to edge (three 500 m
segments and two bridges, IIRC.) Coax Ethernet used Manchester Code and
was able to detect collisions by looking for a voltage drop. Twisted
pair CSMA/CD Ethernet detected collisions in the hub and using a code
violation to signal the transmitters. Modern "Ethernet" is full duplex
with no collisions.</font></font> <br>
<br>
On 6/3/2010 10:22 AM, Noel Chiappa wrote:
<blockquote cite="mid:20100603172259.4C05F6BE5D8@mercury.lcs.mit.edu"
type="cite">
<pre wrap=""> > From: John Day <a class="moz-txt-link-rfc2396E" href="mailto:jeanjour@comcast.net"><jeanjour@comcast.net></a>
> Alohanet and Ethernet make the same "effort." The difference is the
> theoretical limit of the media. Ethernet is higher because the aether
> has been replaced by coax. Not because Ethernet made more effort than
> Alohanet.
Not sure if this is what you're referring to with your reference to "aether
... replaced by coax", but... Ethernet's channel access algorithm (CSMA-CD) is
slightly different from Aloha's (which was, IIRC, CSMA). (To get the -CD to
work semi-reliably they had to limit the network's physical size, and increase
the minimum packet size, in ways that weren't feasible with the aether.) I
seem to recall that adding the -CD upped the theoretical throughput (as a
%-age of channel bit rate) considerably.
Noel
</pre>
</blockquote>
<br>
<pre class="moz-signature" cols="72">--
Richard Bennett
Research Fellow
Information Technology and Innovation Foundation
Washington, DC</pre>
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