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Re: [EFM] Half-duplex deferral for MAC-PHY rate matching and compatibility with existing silicon

>What is the case for <100 Mb/s
>Jack Andresen

At 10 Mb/s the issues are the same as for 100 Mb/s operation. A lot 
of legacy 10 Mb/s-only devices *cannot* receive frames while 
transmitting. This goes for all of the LANCE-based systems (and there 
are a LOT of those), as well as some others. If anyone intends to use 
such equipment on an EFM link, then the proposed deferral-based 
scheme will not work.

Most newer equipment that is 10 Mb/s-capable is also 100 
Mb/s-capable. There is little price difference between the two today.

>Rich Seifert wrote:
>>  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
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