RE: [EFM] Half-duplex deferral for MAC-PHY rate matching and compatibility with existing silicon
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.
From: Rich Seifert [mailto:rich@xxxxxxxxxxxxxxx]
Sent: Monday, December 10, 2001 7:25 PM
To: Behrooz Rezvani; 'Arthur Marris'; 'Shimon Muller'; firstname.lastname@example.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:
>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
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
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