RE: [802.3ae_Serial] another question on pattertn defined in the current draft:
Juergen and Piers,
The decision to not send AIS upon loopback was made for the following reasons:
1. During loopback, it is suggested that as much of the WIS functionality
be tested (i.e., involved in the loopback path) as possible. Therefore,
the WIS transmit process is expected to be completely occupied in processing
the data being looped back to the PCS, and will not have any facility
to send any type of SONET signal to the PMA. If a valid SONET AIS is
to be sent to the PMA during loopback, then it will be necessary for the
implementer to create an entirely separate subset of SONET framing
functionality, active ONLY during loopback, for the sole purpose of
transmitting AIS to the PMA. This was deemed by the WIS group as being
an unnecessary and excessive burden on the implementer.
The alternative that has been specified in the WIS clause requires no such
extra hardware; the 00-FF pattern can be sent to the PMA by simply forcing
the PMA service interface to a fixed binary value.
2. The constant pattern, originally 00-00 but now 00-FF, does not correspond
to any SONET framing characters. Therefore, the far-end WIS will lose frame
synchronization almost immediately when the local WIS has been placed into
loopback. The effect will be similar to that of a failure of the WIS
(loss of framing). This is precisely the logical effect upon the far-end
WIS as a consequence of placing the local WIS into loopback; effectively,
the transmit path of the local WIS has been disabled or shut off by putting
it into loopback, and shutting off the WIS transmit path would have most
likely raised an LOF alarm at the far-end under normal circumstances.
3. Loopback is an administratively configured mode; i.e., a WIS cannot enter
loopback in any automatic manner during normal operation. Therefore, some
management entity must be aware that the local WIS is in loopback. This
management entity is also hence in a position to ensure that the far-end
WIS does not generate and propagate any false alarms or defects during
loopback. If it is desired that false alarm conditions are not raised,
the system designer is perfectly capable of implementing whatever suppression
measures are required.
4. The WIS can talk only to another WIS. This has been frequently stated,
frequently emphasized, and is the underlying assumption behind the WIS
clause. In fact, most of the first page of Clause 50 spends a great deal
of verbiage explaining that the WIS is not SONET/SDH compliant and is not
supposed to talk directly to SONET transport equipment. The WIS, as
specified, permits only a Path Relay function within the context of the
transport network; interfacing to SONET/SDH STEs or LTEs is outside the
declared scope. Therefore, I do not understand why the pattern output
during WIS loopback - which is certainly an abnormal and administratively
configured condition - should be permitted to have any effect upon the
transport network.
5. From the point of view of the remainder of the network, placing a
local WIS into loopback is logically equivalent to disconnecting it or
powering it down. Why, then, should placing the WIS into loopback be
treated any differently from disconnecting it (in terms of sending AIS
or not)? The effect upon the transport network is the same.
I hope the above answers your question as to why the WIS group selected
a constant pattern during loopback as opposed to AIS. Loopback is already
not all that simple, from a hardware implementation point of view, in a
10 Gb/s WIS device - I would hope that we would not make it even worse.
Best regards,
- Tom Alexander
WIS Scribe
-----Original Message-----
From: DAWE,PIERS (A-England,ex1) [mailto:piers_dawe@agilent.com]
Sent: Wednesday, June 27, 2001 9:20 AM
To: 'Rahn, Juergen (Juergen)'; stds-802-3-hssg-serialpmd@ieee.org
Subject: RE: [802.3ae_Serial] another question on pattertn defined in
the current draft:
Juergen,
Draft 3.0 had said "In this mode, ... the WIS shall transmit a continuous
stream of all-zero data words to the PMA sublayer, and shall ignore all data
presented to it by the PMA sublayer." This meant zeroes on the line: zeroes
where the header would be and zeroes where the payload would be. I thought
that this could lead to optical power glitches and/or chattering optics and
even worse problems than you describe, so I suggested changing it to any
balanced pattern. It turns out that the pattern chosen by the group is
being proposed for the square wave test pattern for WAN PHY.
That's the history: I'll let the SONET experts debate the merit of AIS,
which I presume is balanced, or nearly so.
Piers
> -----Original Message-----
> From: Rahn, Juergen (Juergen) [mailto:krahn@lucent.com]
> Sent: 27 June 2001 10:28
> To: stds-802-3-hssg-serialpmd@ieee.org
> Subject: AW: [802.3ae_Serial] another question on pattertn defined in
> the current draft:
>
>
> Hi,
> Can somebody help me to understand why the pattern in case of
> Loop back is set to 00-FF?
> My question is was there a strong reason for this that I do
> not know. What I know is that there are some
> ugly effects possible with such pattern when the interface is
> connected to a Sonet or other type transport network.
> I understand that at the last meeting some work has been done
> on this loop back facilities. Now I read in
> 50.3.9 Loopback:
> ..............................................................
> ..............
> ...........................the WIS shall transmit a constant pattern
> to the PMA sublayer, and shall ignore all data presented to
> it by the PMA sublayer. The pattern output to
> the PMA transmit path at this time shall consist of a
> sequence of 8 logic zero bits and 8 logic one bits, form-ing
> the 16-bit word 00-FF hexadecimal. No SONET overhead or fixed
> stuff shall be output to the PMA at this time.
>
>
> While agreeing that the data incoming to the PMA sublayer
> should not be
> sent further such constant pattern is likely to generate some ugly
> additional
> effects . So maybe this would lead to undefined and miss
> leading alarms
> or( or even protection actions) in a possible transport network. This
> transport
> network that can either be a plane Sonet transport network, but also a
> traditional
> WDM network with Sonet non-intrusive monitoring or transport
> via an OTN. In all those
> cases the consequence on such patterns are not defined and
> the alarms that will
> be generated are likely to be miss interpreted. For instance
> it can be
> interpreted that a wrong signal is connected. In case a proper AIS is
> inserted however the transport network will react in the standardized
> way and no alarms (or even protection switches as worst case
> scenario)
> will be activated (which should not be done when a client
> equipment is in test mode).
> So can somebody help me with the reason for this pattern ( In
> contrast to a normal AIS
> that would not generate such effects). If there is no
> particular overriding reason for this
> I would strongly suggest to take the AIS signal instead.
> Regards Juergen Rahn
>
>