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Re: Proposal for accomodating 10.0000 and 9.58464 line rates


I have only one question.  What happens if or when the 802.3 WG decides to
standardize on extended frame sizes?

Thank you,
Roy Bynum
MCI WorldCom

Steve Haddock wrote:

> Judging by the reflector traffic, it seems that there is quite a bit of
> support for the proposal made by Dan Dove and Kevin Daines for defining a
> MAC/PLS interface that runs at 10.0000 Mb/s but has a flow control mechanism
> that allows the effective payload rate to be matched to 9.58464 Mb/s.  This
> would allow the specification of a 10.0000 Mb/s PHY and a 9.58464 Mb/s PHY
> that interfaced to the MAC using the same 10.0000 Mb/s MAC/PLS interface.
> At the Montreal meeting I suggested that the MAC already had such a flow
> control mechanism, called "deference", which could potentially be used for
> this purpose.  After reviewing the MAC spec, I believe that this is very
> straight-forward.
> For those new to 802.3, the core MAC functionality is specified in PASCAL
> code in clause 4 of the standard.  Within this code there is a process call
> "deference" which generates a variable called "deferring".  When deferring
> is true, the MAC transmitter will not initiate a new frame transmission.  In
> the traditional CSMA/CD half-duplex operation, this variable was asserted in
> response to CarrierSense to prevent the transmitter from initiating a
> transmission when the medium was already busy.  In full-duplex operation the
> variable is used simply to time interFrameSpacing (aka inter packet gaps).
> The CarrierSense input to the deference process is currently not used in
> full duplex operation.  I propose that we define the use of CarrierSense in
> full duplex to be conceptually the same as its use in half duplex:  to
> inhibit the initiation of new transmission.
> In practical terms this means that we either define a new signal "Hold" (as
> suggested by Dan), or use the "CRS" signal, at the MAC/PLS interface that
> drives the carrierSense input to the deference process.  A PHY could then
> use this signal in full duplex mode to control the spacing between
> transmitted frames.  This would allow a PHY with a transmit line rate of
> 9.58464 to connect to a MAC/PLS interface running at 10.0000 Mb/s without
> loss of data.  It would require the PHY to have some nominal buffering to
> prevent overrunning the transmit channel during a maximum size frame (max
> frame of 1522 bytes * 9.58464 / 10.0000 requires a 64 byte FIFO).
> The simplest way to implement this in the MAC PASCAL is to add a single line
> to the deference process.  The deference process is divided into a
> half-duplex loop and a full-duplex loop.  The full-duplex loop, with the new
> line in bold, would become:
>         else cycle {full-duplex loop}
>                 while not transmitting do nothing;  {wait for the start of a
> transmission}
>                 deferring := true;  {inhibit future transmissions}
>                 while transmitting do nothing;  {wait for the end of the
> current transmission}
>                 StartRealTimeDelay;  {time out an interframe gap}
>                 while RealTimeDelay(interFrameSpacing) do nothing;
>                 while carrierSense do nothing;  {wait for deassertion of
> carrier sense}
>                 deferring := false {don't inhibit transmission}
>         end {full-duplex loop}
> Notice that this means the MAC will only sample carrierSense at the end of
> an interframe gap immediately following a transmission.  This is sufficient
> if the goal is to control interframe spacing.  If we want the MAC to defer
> to carrierSense even if it has not recently transmitted, then the PASCAL
> modifications get slightly more complex but are still contained within this
> "full-duplex loop".  The biggest complication in trying to get the MAC to
> defer to carrierSense at any time rather than just following a transmission
> is that we will have to tightly define the latency from when a PHY asserts
> CRS to when the MAC responds.
> There is also a backwards compatibility issue to resolve if we choose to use
> CRS as opposed to defining a new signal.  Currently the MAC/PLS interface
> says CRS is undefined for full-duplex operation.  We would need to word the
> specification such that we accomplish what we desire for 10Gbps operation,
> but without making existing 10/100/1000 Mbps implementations non-compliant
> if they happen to assert CRS in response to receive activity even in full
> duplex mode.
> Steve Haddock
> Extreme Networks