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.
>> --
>
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Rich Seifert Networks and Communications Consulting
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