Re(2): [EFM] Network timing?
Hi Bob,
No, I wasn't thinking of 8kHz presentation, just bandying about 8kHz as
possible means of conveyance. Sorry for any X.25 flashbacks :-)
I was thinking of Stratum 1 traceability, and of course any presentation is
possible once you have the information.
The application I had in mind is E1/T1 private line emulation over Ethernet
packet transport. As we know things in telecom (and less so datacom) have a
tendency to be demanded/exist for quite a long time after they are hip/state of
the art/commodity. I am sure there are other applications for traceable network
timing.
Matt
Bob Barrett (27/09/01 2:14):
>
>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@fujitsu.com.au Phone: (613) 9845 4313
>>
>