RE: [EFM] Half-duplex - definition of a repeater
Bob and Roy,
Prior to the development of the 1GbE standards, the definition of a repeater
was explicitly Half Duplex. Repeater time/state and functionality is defined
for 10Mbps and 100Mbps for all copper and fiber media EXCEPT 10Broad36. In
the early days of 10Mbps when the delay budget was not so stringent, a
particular issue had to do with the number of preamble bits "eaten" by each
repeater (as well as delay introduced)which led to limits on number of
repeater hops supported by the standard. One cannot have frames arriving at
a far end PHY with no preamble bits left, now can we?
At the time of IEEE 802.3 1000BaseX development, we "looked into" the
definition of the "Full duplex" repeater, and in fact at least two hit the
market. However, it turned out that unidirectional IEEE 802.3 PAUSE frame
and a suitable auto-negotiation protocol was all that was needed to
implement the Full Duplex repeater, so it ended up out of scope of the
1000BaseX definitions. (Note: While PAUSE is usually observed by 1Gb/s MACs
even when they do not generate PAUSE, I do not believe that implementation
of PAUSE on 100Mbps MAC's is universal, so depending on it can be dicey!)
Doug Ruby
-----Original Message-----
From: owner-stds-802-3-efm@majordomo.ieee.org
[mailto:owner-stds-802-3-efm@majordomo.ieee.org]On Behalf Of Roy Bynum
Sent: Tuesday, December 11, 2001 9:39 AM
To: bob.barrett@xxxxxxxxxxxxxxxxxx
Cc: stds-802-3-efm@ieee.org; Rich Seifert; Behrooz Rezvani; 'Arthur
Marris'; 'Shimon Muller'; Geoff Thompson
Subject: RE: [EFM] Half-duplex - definition of a repeater
Bob,
I had an e-mail discussion about this with Geoff Thompson several months
ago. His comment that there was nothing in the current standard that
defined a full duplex "repeater". He did say that for 100Mb, a FDDI
repeater could be used because of the encoding scheme that was used for
100BaseX. As he is the co-author of much of the "repeater" clauses in the
standard, he would be best qualified to answer that question.
Thank you,
Roy Bynum
At 10:52 AM 12/11/2001 +0000, Bob Barrett wrote:
>General question:
>
>Is the 802.3 definition of a repeater explicitly limited to half duplex
>only?
>
>I am only asking this here for the sake of expediency. I could go through
>the CDROM but that would take me a day and I still wouldn't be sure of the
>negative answer.
>
>I guess there is a supplementary question that members of this group may
>ask, which is 'why would anybody want a full duplex repeater?'. The answer
>being 'as a full duplex media converter, with management'. If there isn't a
>definition for this within 802.3 then would anybody be interested in
>supporting a proposal to create a standard definition for one?
>
>Please respond to this directly rather than busy the reflector.
>
>Thanks
>
>Bob
>
> > -----Original Message-----
> > From: owner-stds-802-3-efm@majordomo.ieee.org
> > [mailto:owner-stds-802-3-efm@majordomo.ieee.org]On Behalf Of Rich
> > Seifert
> > Sent: 10 December 2001 17:25
> > To: Behrooz Rezvani; 'Arthur Marris'; 'Shimon Muller';
> > stds-802-3-efm@ieee.org
> > Subject: RE: [EFM] Half-duplex deferral for MAC-PHY rate matching and
> > comp atibility with existing silicon
> >
> >
> >
> > At 7:17 AM -0800 12/10/01, Behrooz Rezvani wrote:
> > >Arthur,
> > >
> > >As I mentioned in our conference call we need to support data
> > rate greater
> > >than 100 Mbps in total. As I have been listening to you, Rich
> > and Shimon, I
> > >gather that there is a way to operate the MAC such that we can exceed
the
> > >half duplex rate.
> >
> > Operating at data rates in excess of 50 Mb/s (full duplex) would be
> > problematic with the system being proposed so far. Since it is
> > predicated on a single, 100 Mb/s MAC operating in half-duplex mode,
> > the combined transmit+receive rate cannot exceed 100 Mb/s; this is
> > the equivalent of a 50 Mb/s symmetrical full-duplex PHY.
> >
> > In order to operate at greater data rates, one would need to use a
> > Gigabit MAC operating in half-duplex mode. Such MACs are relatively
> > rare; indeed, even if they exist, the use of half-duplex GbE is more
> > theoretical than practical--there are no GbE repeaters in commercial
> > use. It is not even clear that such MACs work properly in half-duplex
> > mode.
> >
> > In addition, it is not possible to aggregate multiple 100 Mb/s MACs
> > when operating in half-duplex mode. The current Link Aggregation
> > standard restricts aggregation to full-duplex links only.
> >
> > >Note that VDSL PHY is full duplex system, and it can transmit and
receive
> > >independently.
> > >
> >
> > I suspect that EFM will want to operate over a variety of PHY types
> > and speeds. Rather than trying to cobble all of these systems to some
> > pre-existing MAC chips (which I agree may provide some short-term
> > benefit), perhaps it would be wiser in the long run to define a
> > full-duplex MAC with a variable (quasi-static) data rate.
> > --
> >
> > --
> > Rich Seifert Networks and Communications Consulting
> > rich@xxxxxxxxxxxxxxx 21885 Bear Creek Way
> > (408) 395-5700 Los Gatos, CA 95033
> > (408) 395-1966 FAX