Network problem

[Tom Fogal]

once again ive forgotten to change the 'To:' to the list... *sigh*

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To: Chris Brenton <cbrenton at chrisbrenton.org>
Subject: Re: Network problem 
In-Reply-To: Your message of "Thu, 28 Aug 2003 20:56:37 EDT."
             <3F4EA4C5.6030306 at chrisbrenton.org> 
Date: Thu, 28 Aug 2003 21:33:27 -0400
From: Tom Fogal <tfogal at io.iol.unh.edu>

> Stephen Ingham wrote:
> > 
> > A good repeater hub will automatically turn off a port when 32 consecutive
> > collisions are detected.
> 
> Humm. A collision is defined as a system following the Ethernet CSMA/CD 
> rules that detects a different bit pattern on the receive pair Vs. what 
> it is currently sending out on the transmit pair. Since a hub is little 
> more than a line amplifier, it does not "transmit" per CSMA/CD, and thus 
> has no way of detecting collisions. If a hub had this type capability, 
> we really would not need switching as it would be trivial to retime the 
> circuit as well. My guess is you are thinking of a switch or perhaps a 
> dual speed hub (which is still a form of a bridge).

A collision is actually the detection of an incoming frame while a device is
transmitting. It is not necessarily a different pattern; 100mbs devices
constantly send out all '1's, and this could of course easily happen within
a frame.
A 'repeater' is the term used in the standard, and to be compliant they must
implement CSMA/CD. A hub is generally taken to mean a multi-port repeater but
the term is left undefined.
Not to say his hub definitely performed to spec, but it should.

> > A switch or bridge will also stop all collisions and other errors from
> > propagating over the entire network.
> 
> Collisions yes, as it resets the transmission timing. Errors, maybe. It 
> depends on the switch. Some filter out runts, bad CRC's, etc. Some do 
> not. If the switch runs in cut through mode rather than store and 
> forward, chances are its not error checking. Most modern switches 
> support store and forward, but I doubt the proverbial "everyone" does.

The timing of transmissions is supposed to be set when a device starts 
receiving the preamble of an incoming frame (this is why the preamble exists).
Not to say it must sync clocks via the preamble, its just what the preamble was
intended to do.

'Runts', or fragments of a packet should be discarded by a switch.
I forget about bad CRCs, but I believe a switch should drop the packet without
forwarding.
If a switch is forwarding traffic bit-for-bit without checking for the frames
validity, it is not implementing a compliant ethernet MAC.

In short, a device implementing a compliant ethernet MAC should localize
errors between each link it has.

> and Neal wrote:
> 
> >  My question is this:  Is this normal behavior to have the whole network 
> > go down due to a mis wired cable. I can understand the hub not working 
> > but to cause the whole thing to crash seams bizarre to me

apparently people have seen this, but it seems very awkward to me. only 4 of 
the 8 wires are used in networks <=100mpbs, it would seem to me a device
manufacturer would hook up the two 'idle' pairs to particularly strong 
resistors, if anything at all - not the main circuit of their networking
device or anywhere it could do damage. of course, i dont design circuits, 
either...

the worst ive seen with a bad cable is devices not establishing a link.

> Seen this as well. It usually only effects all systems located off of 
> the same switch and/or router port. Effectively what you did is change 
> your ground reference by 5 volts which tends to bum out electrical 
> circuits. Lucky you are not swapping a lot of NIC cards. ;-)

hrm.. perhaps they use those unused pairs as ground..

anyway.. this is a long response for a dying thread =)

- -tom

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