Re: [EFM] Half-duplex deferral for MAC-PHY rate matching andcomp atibility with existing silicon
Vlad, Steve, and All,
I would like to just make a pointer to the presentations by George
Ginis recently that multiple coordinated lines may enable (say
2 or 4 "bonded" lines) 100 Mbps at ranges well beyond a km, so
there may very well be data suggesting that 100 Mbps is an
excellent EFM rate for which to have duplex interfaces
(of any sort that is appropriate). Basically, a lot more than
200-300 ft is possible, and replacement of the last segment
of copper by fiber is still extremely expensive. Some data
has been presented to support that and would contradict what
is suggested below.
At 07:23 PM 12/11/2001 -0500, Vladimir Oksman wrote:
> I would support Steven's idea. For a full duplex 100 Mb/s customer
> fiber will probably work better. I would like to notice that the expected
> aggregate capacity of a VDSL link with loop length zero is close to 100
> Mb/s. The first hundreds feet will already reduce it significantly.
> If we really consider a business case with a distance of 200-300
> ft, I would look for a fiber solution.
> > Arthur,
> > Our experience has shown that the half duplex method is the safest way
> and works with most MACs. The CRS can always be used to control the flow
> of frames from the MAC to the PHY, even at all the different rates
> proposed for EFM copper objectives.
> > The only drawback is the "limitation" of 100Mbps. This can be overcome
> with most MACs using a flow control message (802.3x). This method is not
> watertight and not all MACs process this in a timely fashion so buffers
> are needed in the PHY. The case of an EFM PHY working above 100Mbps will
> most likely only happen in very short range loop aggregation scenarios
> when there are multiple lines going to a business customer. I expect
> fiber to be used in these cases so creating a new MAC seems unnecessary.
> > I propose the following engineering tradeoff:
> > 1. Use of the half duplex method for most modes of operation. This
> covers all rates up to 100Mbps aggregate.
> > 2. Usage of flow control, combined with the appropriate buffers, in the
> rare cases of rates above 100Mbps.
> > A third option is to create a jump in supported service rates from
> 100Mbps half duplex to 100Mbps full duplex.
> > Steven
> > -----Original Message-----
> > From: Rich Seifert [mailto:rich@xxxxxxxxxxxxxxx]
> > Sent: Monday, December 10, 2001 7:25 PM
> > To: Behrooz Rezvani; 'Arthur Marris'; 'Shimon Muller';
> > 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
John M. Cioffi
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