Newsgroups: comp.dcom.lans.ethernet
Subject: Re: Repeaters
From: hilbig_t@qis.dofasco.ca
Date: 16 Apr 92 14:14:03 GMT
Organization: Dofasco Inc, Hamilton, Ont. Can.
Nntp-Posting-User: hilbig_t

In article <jidapq0@sgi.sgi.com>, rpw3@rigden.wpd.sgi.com (Rob Warnock) writes:
> Minor corrections:

> This is incorrect. A legal "jam" is 32 bits minimum, 48 bits maximum.
> You are probably thinking of "preamble", which is 62 bits of "1,0,1,0,..."
> followed by 2 bits of "1,1".

That wasn't minor; I mis-read 9.5.6.2 of 802.3 spec.  It said that the jam 
will be sent with the further constraint that the 62 bits of 1's and 0's are
also sent.  Yes, this is the preamble (why it just doesn't say so is beyond
me).  Section 4.2.8 (Oh no! It's the pseudo-pascal chapter) confirms this
in the BitTransmitter process; the only after the preable and SFD is sent does
it care if there is a collision.  If there is a collision, the process 
continues to transmit another 32 bits (jamSize) -- I can't find any reference
to the 32 to 48 bit range you mentioned; but I am reading the 802.3 specs and
not the Ethernet 2.0 spec.

> 
> +---------------
> | > Is the minimum packet size is based on the fact that the sending station 
> | > must still be transmitting a packet when the farthest node is receiving that
> | > packet (for collision detection)?
> | 
> | Yup!
> +---------------
> Close, but not quite correct.

I agreed too quickly.  
 
> 
> +---------------
> | Yup!  Doubling the end-to-end delay gives the Round Trip Propogation Delay. 
> | This number must be less than 1 slot time (512 bit times;
> | the minimum packet size (64 bits including addresses, excluding preamble). 
> +---------------
> 
> Actually, the maximum calculated round-trip time in the Ethernet 2.0 spec
> based on the maximal permitted network configuration (max stations, max length
> segments, max repeaters, max fiber, etc.) was 464 bits. This plus a maximum
> length "jam" (48 bits) is what *defines* the slot time (464+48 -> 512).
> [See Ethernet 2.0 spec, section 6.3.2.3]
> -Rob

Opps, that should have been 64 bytes, not bits.

We are reading different spec's (you Ethernet 2.0, me 802.3 w/supplements).
Admittedly, this is one area I need to study further.  However, I don't see
why you add the jam bits to arrive at the slot time.  This doesn't make sense 
to me as the slot time is defined to make sure the "near station" is still 
transmitting when the far station's jam signal arrives.  If it has already
finished transmitting (ie missed the collision), then it won't jam will it.

This is an interesting discussion; especially when we see reponses from
the 802.3 Chair.  I hope we can continue our discussions even though we
have probably scared the hell out the person who first asked the repeater
rule question.  At least the IEEE should see some extra orders for the 
spec's. 

Right now I am trying to figure out why the round trip delay of a right
end segment is more than a mid segment (802.3j/D10 table 13-2).  Once I've 
given up trying to understand the spec, I'll probably pose the question
under something like "Understanding 802.3".  

Tom
-- 

Thomas Hilbig,
Network & Hardware Engineer,		Address:    1330 Burlington St East,
Information and Automation Systems,		    Hamilton, Ontario
Dofasco Inc.                            	    Canada   L8N 3J5
hilbig_t@qis.dofasco.ca                 Telephone:  (416) 544-3761