Re: [8023-10GEPON] Upstream Wavelength
All,
Cut off wavelength is an interesting property. Actually, fibers always have
more than one mode. However, some modes are bound, while others are
radiating. The theoretical cut off wavelength refers to a perfectly
straight fiber, at the point when the second modes (there are several
degenerate ones) just barely become bound. However, any real fiber is not
exactly straight. The incidental bends that happen in the cable act as mode
strippers, and they cause loss to the higher order modes. What becomes
important is the minimum distance between fiber joints (connectors or
splices), because such joints are likely where significant energy can be
exchanged between the modes. Factoring the shortest repair length together
with the bending environment of the cable allows us to arrive at the "cabled
cut-off wavelength", or Lambda_CC.
As for the characteristics of G.652 fiber, I can see an evolution in the
recommended values:
1988: Lambda_CC should be 1270, with some administrations using 1260nm.
1993: Lambda_CC should be either 1260 or 1270nm.
1997: Lambda_CC should be either 1260 or 1270nm.
2000: Lambda_CC max = 1260nm
2003: Lambda_CC max = 1260nm
2005: Lambda_CC max = 1260nm
As mentioned above, the most important pieces of fiber are those between
joints that are close together. These will be the jumpers and patches of
cable. I should think that all of these will be made of relatively modern
fiber (i.e. post-2000).
So, I don't think that we really have a problem here.
Sincerely,
Frank Effenberger
-----Original Message-----
From: Glen Kramer [mailto:glen.kramer@TEKNOVUS.COM]
Sent: Friday, May 09, 2008 4:02 PM
To: STDS-802-3-10GEPON@LISTSERV.IEEE.ORG
Subject: [8023-10GEPON] Upstream Wavelength
All,
E-mail below expresses my opinion as a TF member, not that of a chair.
1) In July 2007, we discussed the choice between 1260-1280 and 134-1360
bands. We decided to use 1260-1280 because the dispersion was negative in
this region (see
http://www.ieee802.org/3/av/public/2007_07/3av_0707_uematsu_2.pdf)
But this discussion never mentioned the issue of proximity to the cut-off
band.
2) I understand that cut-off wavelength is a function of core diameter and
refractive indices of the core and the cladding. It is difficult to lower
the cutoff wavelength without reducing the fiber core diameter or reducing
the difference between refractive indices of core and cladding. Both of
these have undesirable effects.
3) Consider ITU-T recommendation G.957 "Optical interfaces for equipments
and systems relating to the synchronous digital hierarchy". Of interest are
3 table (attached) listing operating parameters of various interfaces. In
most case, the O-band starts with wavelengths slightly higher than 1260,
i.e., 1261-1360 or 1263-1360. The tables also show a footnote that says that
some users may require the shortwave limit of 1270. Further, this document
says:
"The wavelength regions permitting system operation are partially determined
by either the cut-off wavelength values of the fibre or of the fibre cable.
For G.652 and G.653 fibres, these values have been chosen to allow
single-mode operation of the fibre cable at 1270 nm and above, with values
as low as 1260 nm permitted by some Administrations. For G.654 fibre cables,
the cut-off wavelength values have been accepted for single-mode operation
at 1530 nm and above."
So, my questions are:
1) Are we taking unnecessary risk specifying 1260-1280 nm band for our use
for 10G EPON?
2) Is all currently-deployed fiber based on IEC 60793-2 B1.1 and B1.3?
Are there areas where older fiber is deployed? What about the
bend-insensitive fiber that goes into the apartments? Does it have the same
cutoff?
3) What are the disadvantages of using 1340-1360nm? Dispersion is positive
instead of negative. But would dispersion penalty be noticeably higher?
Would we need to recalculate power budgets if we use this region?
4) Is it correct that using 1340-1360 is still friendly to G984.5?
I do not have a very strong opinion about the wavelength we should choose,
but I don't want to make a mistake of overlooking something, and least of
all I want 10GEPON to use a suboptimal wave band, leaving better real estate
to some other use.
Please, comment and let's hear all the pros and cons.
Thank you,
Glen
________________________________________
From: Hajduczenia, Marek (NSN - PT/Portugal - MiniMD)
[mailto:marek.hajduczenia@NSN.COM]
Sent: Thursday, May 08, 2008 1:02 AM
To: STDS-802-3-10GEPON@LISTSERV.IEEE.ORG
Subject: Re: [8023-10GEPON] Upstream Wavelength
Dear Jim,
some thoughts on the comment You are referring to ... I am the one
responsible to its rejection so I believe some more explanation is
justified, especially if we do not want to have this discussion during the
meeting.
1. Your comment is submitted incorrectly, since D1.3 does not include Annex
91A - please check the latest version of the draft
http://www.ieee802.org/3/av/private/draft_1_3/3av_d1_3.pdf which is subject
to commenting per email
http://www.ieee802.org/3/10GEPON_study/email/msg01027.html. Please bear in
mind that editors have dozes of comments to process and we cannot look for
the piece of text which You might refer to - that is interpreting the
comment and we are not allowed to do it. If the commenter does not make sure
I can trace the comment location, I will have to reject it. That is what
happened with Your comments.
2. The second reason for the rejection is technical. We have been discussing
the issue of the 1260 - 1280 nm transmission window for 10G signals in
upstream (versus 1300 - 1320 nm or 1340 - 1360 nm) and it was decided that
we would aim for this particular window, with the TF wide consensus. Various
reasons can be mentioned and I am pretty sure that if You look through the
past presentations, You will be able to trace it back quite easily. All in
all, everyone (almost) seems to be in agreement that such a window
allocation is feasible and will not warrant transmission problems.
As such, it is hard for me to decide on the validity of the claims of Your
expert on this issue. The deployed fibre is supposed to have the cut off
frequency below 1260 nm and the system should operate fine as long as the
fibre type used in the PON is consistent with the spec (IEC 60793-2 B1.1 and
B1.3). If someone wants to get hurt, we cannot prevent it. One can easily
deploy fibre which will overmode in this range but that is already not
standard compliant plant and we cannot provide for all the possible use
cases.
IMHO we do not need to change anything. The restrictions which are placed on
the fibre plant assure that the transmission in the selected window is
feasible.
I will be looking forward to receiving any feedback from You
Best wishes
Marek Hajduczenia (141238)
NOKIA SIEMENS Networks S.A.
System Architect - COO BBA DSLAM R&D
Rua Irmãos Siemens, 1, Ed. 1, Piso 1
Alfragide, 2720-093 Amadora, Portugal
A marek.hajduczenia@nsn.com
m+351.21.416.7472 ++351.21.424.2337
________________________________________
From: ext Jim Farmer [mailto:Jim.Farmer@W7OPTICS.COM]
Sent: quarta-feira, 7 de Maio de 2008 22:00
To: STDS-802-3-10GEPON@LISTSERV.IEEE.ORG
Subject: [8023-10GEPON] Upstream Wavelength
I'm campaigning for a comment submitted barely in time for the May meeting.
The 10 Gb/s upstream wavelength is currently defined as 1260 to 1280 nm.
See for example, Table 91-6 and many following references. This wavelength
is also specified for the for 10/1GBASE-PRX-D3 (Table 91-7). According to
my expert, this wavelength is dangerous because single mode fiber becomes
overmoded at about 1260 nm, resulting in a second propagation mode at a
different velocity. In addition, this wavelength will be overlapped by the
normal 1310 nm 1 Gb/s upstream optical signal. This is acknowledged in the
proposed document:
91A.3.2 Upstream wavelength allocation
The 1.25 GBd upstream transmission uses the 1270 - 1360 nm wavelength band,
as specified in Clause 60. The 10.3125 GBd upstream transmission uses the
1260 - 1280 nm wavelength band, as specified in Clause 92. The two
wavelength bands overlap, thus WDM channel multiplexign {sic} cannot be used
to separate the two data channels.
An OLT supporting both upstream channels must use TDMA techniques to avoid
collisions between transmissions originating from different ONUs.
We presume that the reason for specifying a separate wavelength band is to
afford the opportunity to build a mixed-mode OLT receiver (capable of
receiving both 1 Gb/s and 10 Gb/s) that is optimized for 10 Gb/s. However,
this would imply a WDM at the OLT receiver, whose loss could rob a good bit
of the improvement to be gained. This is acknowledged in Section 91A.5.
Furthermore, this implies tight standards for the ONT 10 Gb/s laser, which
could add cost. Since we have already acknowledged that the OLT will need
to use TDMA to separate the 1 Gb/s from the 10 Gb/s upstream, we propose
that the 1260-1280 nm upstream requirement be dropped, and that upstream
transmitters for both speeds use the same wavelength band.
Figure 91A-5 shows some dual-rate receiver architectures that seem practical
and which can provide optimization for each speed without having to use dual
receivers. Implicit in these receivers is the ability to not only switch
bias, but to also switch low pass filtering - since you have electrical
amplification ahead of the filters, you can even leave both filters
connected to the source and their respective loads all the time, so that the
transient response of the filter is minimized.
This seems a better solution than using dual wavelengths, with the potential
issues of the 1260 - 1280 nm band.
Thanks,
jim
Jim Farmer, K4BSE
Chief Technical Officer,
Wave7 Optics, Inc.
An Enablence Technologies Company
1075 Windward Ridge Parkway
Alpharetta, GA 30005 USA
678-339-1045
678-640-0860 (cell)
jim.farmer@w7optics.com
www.wave7optics.com