RE: Clock Tolerance and WAN PHY
Boaz,
There are two major miss-conceptions about POS. The first is that it
directly connects to SONET transmission systems. It does not. POS
interfaces to customer facing, reduced function SONET facilities. The
primary function that causes the most problem with SONET systems, and
requires a change in service provisioning methodologies has to do with
protection switching. Standard SONET has it, POS interfaces invariably do
not. The second is that IETF creates globally standards. Because of the
way that the IETF standard was written, many of the POS interfaces that
work well with SONET do not work well with SDH because of overhead byte
usage incompatibility.
I was involved with the development of POS several years ago. Almost all
of us within the transmission industry thought that we could make a router
part of the transmission network. We were wrong. A router is data
equipment, not transmission equipment. The level within the OSI stack that
routers operate at is two layers above SONET. There is almost a total
disconnect between the operation of the router, as a router, and SONET
transmission systems. The attempt to make a router be part of the
transmission system is perhaps one of the reasons that POS interfaces are
so expensive, per megabit, compared to GbE.
One of the reasons that I have been pushing 10GbE WAN PHY to not be full
SONET compliant is to prevent network implementers from falling into the
same trap, that an Ethernet data switch could be an active part of the
transmission network. Other reasons are that by reducing the management
overhead bytes to a minimum, a set that is common between SONET, SDH, and
SDHJ could be provided, making 10GbE fully interoperable, as a passive
managed element to all of the transmission systems in the world. Another
reason for the Ethernet WAN PHY is reduced cost. The less complex
management overhead, compared to POS, gives Ethernet an additional
opportunity to come in a greatly reduced cost, compared to POS.
I hope that this helps you understand why 10GbE WAN PHY will be used in
place of POS. In addition, I wonder how many people have recognized that
the ability to use the Ethernet tag and queuing functionality is similar to
what MPLS is advertised to be going to do. This is a customer
implementation issue, but I would speculate that a lot of customer are
looking very hard at this as well.
Thank you,
Roy Bynum
At 10:33 AM 1/23/01 +0200, Boaz Shahar wrote:
>Tom,
>Do you see any justification in using this rather then POS?
>I Mean:
>If you compare the system {XGXS,PCS,WIS,PMA,PMD} + ELTE to a regular POS
>system, you haev:
>
>1.In the POS system you do not need the ELTE; You can directly connect to
>the SONET network
>2.All the world is talking POS today
>3.There are components with I/F dedicated to POS which are different then
>XGMII
>
>The only advantage (I see) in WAN-PHY is that the encapsulation is more
>efficient in terms of BW consumption from the SONET network.
>
>Regards,
>Boaz
>
>
> > -----Original Message-----
> > From: Tom Alexander [mailto:Tom_Alexander@pmc-sierra.com]
> > Sent: Monday, January 22, 2001 10:03 PM
> > To: 'James Colin'; Luigi.Ronchetti@netit.alcatel.it;
> > tripathi@vidyaweb.com
> > Cc: stds-802-3-hssg@ieee.org
> > Subject: RE: Clock Tolerance and WAN PHY
> >
> >
> >
> > James,
> >
> > There is no intent or support for directly interfacing the
> > WAN PHY to standard
> > SONET gear, especially in outside plant applications. Off
> > hand, I can think of
> > the following obstacles, even if you did match the clocks:
> >
> > - The optics are completely different
> > - Most of the overhead bytes are not supported (for instance, it
> > would not be possible to provision the ring)
> > - Much of the defects and alarm reporting is missing
> >
> > While it is certainly possible for someone to put back the
> > missing overhead
> > and defects and also use SONET optics rather than Ethernet
> > optics, all this
> > is totally outside the scope of the 802.3ae standard.
> >
> > Best regards,
> >
> > - Tom
> >
> > -----Original Message-----
> > From: James Colin [mailto:james_colin_j@yahoo.com]
> > Sent: Sunday, January 21, 2001 12:54 AM
> > To: Luigi.Ronchetti@netit.alcatel.it; tripathi@vidyaweb.com
> > Cc: stds-802-3-hssg@ieee.org
> > Subject: Clock Tolerance and WAN PHY
> >
> >
> > Luigi,
> > I think that the motto in the WAN PHY standard is the
> > introduction of a new framing scheme (As opposed to
> > POS), rather than being gluelessly connectable to the
> > SONET network. The WAN PHY is supposed to be connected
> > to a SONET LTE (ELTE) that is doing clock drift and
> > jitter adjustments.
> >
> > Even if the WAN PHY Clock requirements were identical
> > to those of SONET, I'm not sure if the ELTE is still
> > needed or the WAN PHY can be directly interface to the
> > SONET ring. Can anybody comment on that?
> >
> > James
> >
> > --- Luigi.Ronchetti@netit.alcatel.it wrote:
> > > Hi Devendra and all,
> > >
> > > I think that is not enough to reduce the clock
> > > tolerance to 50ppm.
> > >
> > > As far as I know, ITU-T is going to approve
> > > (February 2001) a new
> > > recommendation (G.709) that defines OTN (Optical
> > > Transport Network).
> > > Future optical backbones over long distances will
> > > likely to be realized
> > > using G.709 and this will happen before 10 GbE final
> > > approval.
> > >
> > > In G.709, among the others, a CBR10G client signal
> > > is defined as "a
> > > constant bit rate signal of 9953280 kbit/s +/-20
> > > ppm" (for example an
> > > OC-192/STM-64 signal and then, in principle, also a
> > > 10 GbE WAN signal).
> > >
> > > So, in my opinion, at least for a 10 GbE WAN signal,
> > > the clock
> > > tolerance should be 20ppm.
> > >
> > > Best regards,
> > > Luigi
> > > __
> > > \/ Luigi Ronchetti
> > > A L C A T E L via Trento, 30 - 20059 Vimercate (MI)
> > > Italy
> > > TND R&D phone: +39-039-686.4793 (Alcanet
> > > 2-210-(3)4793)
> > > fax: +39-039-686.3590 (Alcanet
> > > 2-210-(3)3590)
> > >
> > > mailto:luigi.ronchetti@netit.alcatel.it
> > >
> > > > -----Original Message-----
> > > > From: tripathi@vidyaweb.com
> > > [mailto:tripathi@vidyaweb.com]
> > > > Sent: Tuesday, January 09, 2001 10:50 PM
> > > > To: stds-802-3-hssg@ieee.org
> > > > Cc: tripathi@vidyaweb.com
> > > > Subject: Clock tolerance
> > > >
> > > >
> > > >
> > > > Hi,
> > > >
> > > > Right now we are specifying the clock tolerance of
> > > 100 ppm. Currently
> > > > in-expensive
> > > > oscillators are available with tolerance value
> > > less than 50
> > > > ppm. Just like
> > > > we are moving
> > > > voltage levels, it is time we revise the tolerance
> > > value too.
> > > > The elastic
> > > > buffer
> > > > requirements get simplified by this assumption. I
> > > propose
> > > > that we reduce it
> > > > to 50 ppm.
> > > >
> > > > Regards,
> > > > Devendra Tripathi
> > > > VidyaWeb, Inc
> > > >
> > >
> >
> >
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