RE: [EFM] Network timing?
Matthew,
There is scope for confusion between network timing, as you put it, at 8Khz
(also know as frame synchronisation) with precision (Stratum level) timing,
in my experience usually presented as 2MHz, 10MHz or 1PPS, and defined in
G.811 and G.812 (from memory) a.k.a. BITS clock. I haven't seen a 8Khz
outside of a box since the old days of X.25.
The 8Khz when required for frame alignment can be recovered from the framing
of T1 / E1 stream, and the stream can be carried by circuit emulation or in
a side-band. ATM, SONET and SDH can transport circuits but I am aware that
the clock recovery (on the circuit emulation) of these systems is usually
pretty poor, resulting in the recovered E1/T1 clock having a lot of jitter.
It's the 'cost optimised' design of the PLLs in the ADMs that creates the
problem. There are ways of overcoming this issue, and conditioning the
circuit back to Stratum 1 level, without recourse to 8kHz. Most of the ATM /
SONET systems have a 2MHz or a 10Mhz reference clock, recovered from the
bearer.
Consequently I can't think of a useful application for 8Khz, unless it is to
Stratum 1 quality, and then it's not one of the standard rates or
presentations. I am sure you have an application in mind other then frame
sync for E1/T1. Care to enlighten me please?
Best regards
Bob
> -----Original Message-----
> From: owner-stds-802-3-efm@majordomo.ieee.org
> [mailto:owner-stds-802-3-efm@majordomo.ieee.org]On Behalf Of Beanland
> Matthew
> Sent: 26 September 2001 07:04
> To: stds-802-3-efm@ieee.org
> Subject: [EFM] Network timing?
>
>
>
> Hi EFMers,
>
> I guess this is a question for the service providers out there.
> Imagining an
> EFM ONU supporting bearer emulation (say, in order to provide
> E1/T1 interfaces
> for connection to a legacy PABX), is there any interest in having the OLT
> propagate network timing (usually 8kHz, traceable back to some
> reference) to
> the ONUs by some method?
>
> Propagation of network timing is allowed for in the xDSL standards.
>
> Should we require propagation of network timing in EFM it could
> be propagated
> by either the Ethernet symbol rate itself or via some coding method. Some
> physical layer schemes (ATM25 comes to mind) use a low spec
> oscillator for the
> line rate and insert special line tokens at 8kHz to allow user
> side equipment
> to recover network timing if required. It would be possible to
> use one of the
> non-data 8B/10B tokens as a timing marker and send at 8kHz,
> alternatively if
> there is an OAM block it could be sent at 8kHz rate.
>
> Best Regards,
>
> Matt
>
> Matt Beanland, Project Manager/Principal Architect
> Telecommunications Research and Development, Fujitsu Australia Ltd
> 5 Lakeside Drive, Burwood East 3151, Victoria, Australia
> e-mail: matthew.beanland@xxxxxxxxxxxxxx Phone: (613) 9845 4313
>