Re: [EFM] Network timing?
Bob,
Real-time Fax Over Packets (FoIP) (T.38 ?) requires strict timing.
/Ed.
----- Original Message -----
From: Bob Barrett <bob.barrett@xxxxxxxxxxxxxxx>
To: Beanland Matthew <Matthew.Beanland@xxxxxxxxxxxxxx>;
<stds-802-3-efm@ieee.org>
Sent: Wednesday, September 26, 2001 9:14 AM
Subject: 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
> >
>
>