Newsgroups: comp.dcom.lans.ethernet
Subject: Re: Repeaters
From: hilbig_t@qis.dofasco.ca
Date: 6 Apr 92 13:07:36 GMT
Organization: Dofasco Inc, Hamilton, Ont. Can.
Nntp-Posting-User: hilbig_t

In article <1992Apr1.151835.8309@picker.com>, SYSNET@engvax.picker.com (Leonard A. Visconti) writes:
> We are beginning to use 10baseT hubs to extend portions of our network
> and I am concerned about the number of these devices which can be 
> daisy-chained. As I understand it, these devices are basically repeaters 
> which do nothing more than accept a signal, clean and amplify it, and 
> send it on its way. 
> 

Basically, that's what it does.  Repeaters are also responsible for
regenerating a packet's preamble to its full length, and passing on
a collision reported on one side to the other side.  The later is done by
"jamming" (sending 62 bits of alternating 1's and 0's) to the non-colliding
side.
                                                                       
                                                                        
> I've often heard of a 'three repeater rule' which says nodes cannot reliably
> communicate through three ( or is it more than three?) repeaters. It would
> seem that this would be based more on timing delay and less on the number of
> devices and that delay would vary somewhat from vendor to vendor. 

The original DIX (DEC, Intel Xerox) Ethernet standard allowed for a maximum of
2 repeaters between any two non-bridged nodes.  I suspect this was to allow for 
large "start of packet propogation delays" (timing delay) of earlier repeaters. 
IEEE 802.3  bumped this up to 4 repeaters max; I don't know where you heard of
the 3 repeater rule.
 
There is more than just fixed timing delay to consider, however.  The
interpacket gap of 96 bit times (9.6us) between two packets may shrink both
pass through repeaters.   This is caused by path variablity (preamble
re-generating, packet propogation variablity, MAU transmit&receive start-up
variability, clock-skew).  In the worst case scenario, the first packet will be
delayed more than normal and the second packet will be delayed less than
normal.  Thus, the gap between packets is decreased.  The gap is allowed to 
shrink by 49 bit times to a minimum of 4.7 usec.  

	        Table 13-3 of 802.3J (10BaseF)
	     Segment Variability Values in Bit Times

	Segment Type		Xmitting	Mid
				End		Segment

	Coax			16		11
	Link except 10BaseFB	10.5		8
	10BaseFB		"not supported"	2
	10BaseFP		11		8


 As an example using coax segments:
         Xmitting end (with repeater)  variability = 16
     3 x Mid sedment (w/repeaters)  variablity     = 33
						    ---
		Total Path Variability Value       = 49

	(Note: the end segment doesn't count towards PVV since
	delays are fixed)

Note the effect of replacing the three mid segments with 10BaseFB (the new
synchronous standard for fibre).  You could have up to 16 10BaseFB repeaters in
this case.  Even more if you replace your remaining coax end with 10BaseT.   

> 
> 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!  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). 

> If this is the case, I should be able to
> calculate, based on cable and repeater propagation to come up with answers
> to how many repeaters I can use in a given segment.

Exactly; but don't forget your Path Variability Value (PVV) calculation as
well.  In fact, 802.3J section 13.4 suggests that the round trip collision
delay and interpacket gap shrinkage are the only two limiting factors.


> 
> Does all this make sense? Does anyone have any information on this? 

Good questions.  I really which I knew of a book that summarizes all of the
802.3 specifications (instead of just throwing things like the 4 repeater rule
at you).  You could always buy the ANSI/IEEE and ISO/IEC standards; but they
seem to be written by
engineers-turned-Pascal-nuts-who-want-to-get-back-at-all-other-engineers.  Life
is a flow-chart to these guys. 

> Thank you for you help.
You're welcome
> 

> Leonard A. Visconti 	Picker International 
> visconti@picker.com	(216)473-4801