rfoxwor1@tampabay.rr.com wrote:
> ---- Eben King <eben01@verizon.net> wrote:
>> From Wayne Pollock:
>>
>> ---------- Forwarded message ----------
>
>> Finally, while it is possible to violate standards by
>> using longer cables, the 100 meter limit per segment
>> still applies. It takes time for electricity to
>> propagate through a wire, and Ethernet devices will
>> only wait so long before assuming the segment isn't
>> in use and begin transmission of their own. Remember
>> you're using a shared media with Ethernet, so a too
>> long segment will result in intermittent dropped
>> packets. (Of course using full duplex, switched
>> Ethernet may mitigate that and allow longer segments.)
>>
>> -Wayne
>
> To add to this a bit, ethernet is officially known as CSMA/CD.
> Carrier Sense with Multiple Access/Collision Detect. This
> just means that every station has to "fight" for access to a
> shared bus (Multiple Access) and has to incorporate a
> mechanism to tell when more than one station is trying
> to access the backbone buss (Collision Detect).
>
> The protocol was designed so that the first signal in every
> ethernet packet is the Preamble. This is an alternating
> binary 0 and 1 bit stream, intended for all other receivers
> on the bus to tell that the buss is occupied, with no
> regard to possible data loss.
>
> Each local LAN is a "collision domain" (crossing a repeater
> or router means your signal leaves the CD, or, collision domain).
> The two factors that set an outside bound on any CD's size are
> the propagation delay for a packet to cross the max. length
> of the CD (time) and the attenuation of the wiring (signal level
> becoming too weak to trigger another receiver).
>
> The preamble length originally 56 bits was designed so that
> the max size of the CD would enable any station to hear
> a preamble from the furthrest other station, then hold off
> its own sending, which would otherwise cause a collision. The
> preamble is simply a binary 10101010 repeated, to let
> the other transceivers sync up to it, then ended with a
> 10101011 (end delimiter), then followed by the dest. MAC
> address. That's where they typical NIC starts paying
> attention to the data content of the frame.
>
> Only a specialized packet analyzer such as a Sniffer (TM)
> or a Wandel & Goltermann, or a Shomiti, would decode
> fragments of a preamble that might be present if you have
> some broken hardware on the LAN. These show us as
> 12 to 30 (or so) bytes of hex AA AA or 55 55, which are
> composed of 01010101 or 10101010. This can really
> slow down your net throughput and most packet analyzers
> won't even reveal the presence of such junk. This is all
> strictly PHY (Layer 1 stuff) or, just meaningless voltage
> on the wire, with no inherent value in the "data". The
> newer gear is less susceptible to this problem, but bad
> wiring can lead to it, not to mention the crosstalk issue.
>
> When a transceiver in a NIC becomes deaf, it will not
> hear the preamble, so may start sending "blind" while
> thinking the buss is clear, This guarantees a re-transmission
> by the other station and is known as a Late Collision.
>
> The Binary Exponential Backoff was developed so that
> two stations, both trying to send, would not backoff identically
> then re-send again at the same time. The specified limit of
> no more than 1024 stations on any single LAN is a representation
> of fairness given to any one station that could theoretically be
> forced to back off the max. possible (8x) (due to collisions)
> and yet still be afforded a chance to eventually transmit.
>
> Collisions are a normal part of ethernet and are expected, to
> a reasonable value that does not unduly affect throughput.
> If your app needs a more guaranteed delivery time, you
> might need a deterministic format. Token Ring has zero
> collisions and a max. size (length/time) for a token rotation,
> but I doubt anyone specs TR nowadays as ethernet works
> well enough. Also the reassembly feature of TCP is in
> itself, as well as the MAC level checksumming, a
> guarantee against data loss.
>
> - Bob Foxworth (a former Wayne Pollock student)
> (who does packet analysis for fun, if not profit...)
CSMA/CD is just for half-duplex communication, which is basically dead.
I haven't seen a hub in a really long time. If I did find one in my
environment it would be ripped out immediately :)
Full Duplex Ethernet, collisions are impossible since tx and rx are on
different wires, and each segment is connected directly to a switch.
Therefore, CSMA/CD is not used on Full Duplex Ethernet networks.
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