[ntpwg] On preamble and trailer timestamping and 100-Mb Ethernet

[David L. Mills]

Anthony,

Maybe so, but page 127 of the 2002 1588 document specifies the SOF as 
the on-time point. Curiously, this IEEE document specifically expects 
the IETF (!) to amend this assumption. To me the course is clear, boot 
the issue to the TICTOC group.

Dave

anthony.flavin at bt.com wrote:

> Dave,
>
> A pragmatic solution may be to exclude 10BASE-T from hardware
> timestamping. I really don't believe that it is the first choice for
> anything anymore, and certainly not for the type of applications that
> require IEEE 1588 or the like.
>
> Regards,
>
>
> Tony Flavin
>
> -----Original Message-----
> From: ntpwg-bounces+anthony.flavin=bt.com at lists.ntp.org
> [mailto:ntpwg-bounces+anthony.flavin=bt.com at lists.ntp.org] On Behalf Of
> David L. Mills
> Sent: 28 February 2008 02:56
> To: ntpwg at lists.ntp.isc.org
> Subject: [ntpwg] On preamble and trailer timestamping and 100-Mb
> Ethernet
>
>
> Guys,
>
> You may recall that my advice was to leave the media timestamp positions
>
> out of the NTP specification and put it in a separate document specific
> to each network architecture as in IEEE 1588. But, as in the recent
> essay posted, if the general feeling is to the contrary and that a
> specific position is necessary, the best choice would be as in 1588
> following the preamble and SOF octet. Well, a closer examination
> comparing 10BASE-T and 100BASE-TX Ethernet shows the issues are a bit
> more complicated.
>
> Since 10-Mb Ethernets were evolved from half-duplex CSMA/CD technology,
> each frame begins with a preamble to wind up a phase-lock loop and
> demodulate the Manchester code. As the number of preamble bits to lock
> varies somewhat, the natural choice for the on-time mark is the SOF
> octet. So far as I know, this is still the case even if the segment is
> full-duplex.
>
> On the other hand, 100-Mb Ethernets are full-duplex, so the data bits
> flow all the time and no preamble is necessary. Date octets are split in
>
> two 4-bit nibbles and each nibble encoded in a 5-bit channel symbol.
> Channel symbols occur in pairs, a designated pair for IDLE, another pair
>
> JK for the beginning of the frame and another pair TR for the end of the
>
> frame. The frame data itself, which is encoded first in NRZI and then
> MLT-3 and then scambled (!!), includes a conventional Ethernet preamble,
>
> SOF, data and checksum. The NIC chip doesn't care about the encapsulated
>
> Ethernet frame and, in particular doesn't know if it contains a
> preamble, SOF or anything else.
>
> According to this reasoning, a 10BASE-T 1588 chip doesn't know when the
> preamble begins and a 100BASE-TX 1588 chip doesn't know where the SOF is
>
> or even if it exists. It isn't clear from the chip spec sheet I have
> whether the encapsulated preamble is by convention always included or
> not, the chip has provisions to count the number of octets in the
> original frame and smash a 64-bit 1588 timestamp right in the data
> stream. Heroic, that.
>
> I can mumble on about the T1 extended superframe, which in fact is a
> neat candidate for NTP. I once proposed to MCI they could encode NTP
> timestamps in the frame overhead. SONET and FDDI and ADSL and ISDN and
> WiFi would be fun.
>
> Life is very complicated.
>
> Dave
> _______________________________________________
> ntpwg mailing list
> ntpwg at lists.ntp.org https://lists.ntp.org/mailman/listinfo/ntpwg