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Dear Frank,
in lines with what You said about RFoG, there is a nice
post in the Lightwave blog at: http://www.pennwellblogs.com/lw/2008/10/wavelength-debate-makes-rfog-standard.html.
If Victor is following the reflector at this moment, he can probably shed some
more light on this topic I believe.
Regards
Marek From: Frank Effenberger [mailto:feffenberger@xxxxxxxxxx] Sent: quinta-feira, 23 de Outubro de 2008 15:47 To: STDS-802-3-10GEPON@xxxxxxxxxxxxxxxxx Subject: Re: [8023-10GEPON] Upstream Wavelength Selection Dear Jim,
You know, the technical
business is a funny thing. When you talk to the applications engineers,
you get one story. When you negotiate the contract, you get quite a
different one. What I’ve heard from my friends in On a different matter,
I have heard that the SCTE should be sending us a liaison regarding their work
on a new sort of PON system called RFOG (radio frequency over glass). This
system is completing the downstream-only video overlay scheme that we started
with the 1550 to 1560nm wavelength by adding an analog upstream transport.
They are looking at using the 1590 or 1610 bands, and they will need 20nm of
width (most likely) because these upstream lasers are at the ONTs. If we
want to coexist with this system (admittedly a new request), then we should
think twice about using up the entire spectrum. As it is, if we stick to
1577, then this system could use 1610 with a nice 20nm guard band. If we
spread out to 1600nm, then where does that leave them?
If I can elevate the
discussion for a moment, the evolution of PON has been marked with the tendency
to spend our spectrum like water. When it came time to add new features
and upgrades, we were forced to go back and revise our systems to narrow the
windows, add blocking filter specifications that should have been in there in
the first place, jump through multi-rate TDMA hoops, and so on. This is
not a good path. The 1577nm option is a small step towards being more
conservative in our allocation of spectrum. But I’ll admit that this sort
of argument in not strictly in our scope – it is more a general concept
position… just take it for what it’s worth. Sincerely, Frank
E. From: Jim
Farmer [mailto:Jim.Farmer@xxxxxxxxxxxxx] Sorry to have let this
lie for a few days - had to go off and earn a little
money. Thanks for the
replies. It seems Pete has established that existing fiber and
passives should work up to 1625 nm. We have gotten similar information
from a large vendor of optical fiber and components. He tells me that
the oldest long distance fiber he knows of being deployed in the
In addition, please see
the note from We are content to
let PR(X)30 operate at 1577 nm, and if the market decides that is all that
is needed, so be it. With due respect, it may be that vendors who are
working on 1577 nm product are doing so because of the perceived initial market
for a 10 G product, which may be a slightly specialized market. But we
remain concerned about two things: The cost of meeting the narrow bandwidth,
both in terms of money and power, and the filter used to separate the downstream
RF overlay from the lower 1577 nm wavelength. This latter would not be an
issue except it is a filtering function that must be done at the ONT, where we
worry about every penny of cost. I've been trying to get solid numbers
from vendors, but I, too, am having trouble finding
consensus. The narrow
bandwidth (+/- 3 nm) issue is an OLT issue, and so is not quite of as much
concern as is anything at the ONT, but we still worry about it. I can
accommodate the bandwidth by measuring the wavelength of each laser and
determining the temperature at which the cooler must operate to keep its
wavelength centered in the window. This works, but does cost money and
power for the cooler and control circuit, as well as an extra step in
production. While traditional telecom markets may not be concerned,
we feel that emerging deployers, such as municipalities and cable companies, may
not tolerate increased costs. We don't see how a product with a cooler
will be competitive with something that does not have to be cooled, all else
being equal. Regarding the issue of
filtering the 1560 nm maximum video wavelength from the 1574 nm minimum data
wavelength, I, too, have not been able to get a quantitative answer.
But intuitively, the narrower you make the transition region, the more costly
the filter, and the more loss it will have. We had one discussion with a
company that buys filters and packages them, and at first they thought this was
a non-issue based on their experience with CWDM filters. But then we
pointed out that we need a lot more rejection at the reject band (probably
on the order of 50 dB), and we have to do it at ONT prices. This made them
worry. Right now, with current-generation product, we have a transition
region from 1500 to 1550 nm - 50 nm transition - and we are talking
about reducing it to 20 nm or so. I hope someone with filter design
experience can give us guidance. We continue to seek
same. In answer to Marek's
question, while we normally talk about 1550 nm for broadcast use, the actual
band over which transmitters are available is 1550 - 1560 nm, corresponding to
the actual amplification band of commonly-available EDFAs. It may be
possible to restrict the actual wavelengths used to 1550 - 1555 nm. (The
transmitters use a cooled DFB laser with good stability, so the occupied band is
relatively narrow, but transmitters are produced at various wavelengths.)
I have already floated this idea by the RFoG reflector, and as usual, initial
reaction was mixed but generally unfavorable. But I think we could
ease the filtering requirements some by doing this. Now if I just knew
where the cost curve flattens out, so I'd know if I was tilting at windmills or
addressing a real concern... Regarding the bending
issue, I think the primary issue occurs in or near the end customer, and this is
likely to be new fiber anyway. We have bend-insensitive fiber available
now from all vendors. This thread seems to
have bifurcated last week due to time zone differences. I think I have all
the responses below, in the correct temporal order. Thanks, jim From The G.652B
standardization added the PMD requirement in October 2000. Prior to the
above date, there was not a standardized method to deterimine PMD.
Different manufacturers tested differently. So, fibers made earlier
may have met the standard but they were not measured. G.652B fibers can
be integrated with later fibers such as G.652D; however, system capabilities are
limited to that of the legacy fiber. Jim Farmer,
K4BSE
From: Dear Pete,
Thank you for your
additional information. I had missed that 2006 amendment on the splitters.
On the status of real
PON deployments, it is difficult to get really solid information. Since I
work for an equipment vendor company, I don’t have direct access to what the
operator’s measure on their fibers. All I have to go on is what they tell
me. What they tell me is mixed. Many companies are deploying
PONs which are guaranteed to work out to 1600nm. Other companies only go
out to 1580nm (at least, that is what their specifications / measurements go
to). Yet others are interested in using OTDR equipment, and so the long
wavelengths need to be reserved for that. Actually, I’m glad that
we are having this discussion – hopefully the interested parties who have access
to the data will come forward, and we can hear the first-hand story of what is
really out there. Based on that information, we can make a better informed
decision. Sincerely, -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- From: Marek Hajduczenia
[marek_haj@xxxxxxx] Dear all,
Speaking as a member of
this group, I believe Frank has one very strong point which we cannot forget
about - at the beginning of the project we set forth to provide 10G-EPON specs
which will be backward compatible with the existing 1G-EPON deployments. If we
are to live up to that promise, I would expect nothing less than a situation in
which I can replace ONUs and OLTs on both sides of the ODN and run
10G-EPON at the magnificent data rate of 10.3125 Gbps. As a carrier, I
would not want to go out and requalify my ODN. I would not want to go and change
my splitters. As a matter of fact, I would not want to touch anything apart from
ONUs and OLTs. That said, consider how
much fiber is deployed and which does not meet more stringent G.652-B
specifications. Some of carriers who enter the fibre deployment phase only right
now will not have any problem meeting 1625 nm marker (newer fiber and PLC
splitters), other will have tremendous constraints moving past 1580 nm, for
whatever technical reason (ODTR, additional filters, poor fiber or splitters
etc. etc.), but mainly due to the fast that their fiber is simply older and will
not qualify (probably) for more stringent G.652-B specifications. There is a
parallel discussion on the same topic going on at FSAN and I believe (Frank,
correct me if I am mistaken here) this is exactly what the conclusions were. We
do not have however the possibility of defining optional wavelengths for the
same PMD, as some are proposing for FSAN NG-PON. As far as I understood IEEE
approach to defining specifications, we try to boil the available options down
to the set of common features. That means that we need to guarantee support for
a more narrow wavelength range in this case i.e. stick with 1580 nm limit upper,
at least for certain PMDs. Our project was born
with a set of constraints we have to live with, when considering the target
deployment scenarios. Additionally, we have to also look at what is happening on
the market of optical components. I agree with Frank's comment from the last
recirculation here - I have been also tracking progress in several
companies developing optical subassemblies and for now I have only seen products
focusing on 1574 - 1580 nm band, while 1580 - 1600 nm band seems to be
pushed much further in the future. Additionally, companies seem to be unwilling
to invest into development of components the volume of which may be much more
limited. In short, most people I have spoken to so far seem to bet more on
PR(X)30 type devices than 10 and 20 classes. That seems to follow the trend from
the past and if we believe it happens, than application will indeed drive
adequate volume to get the cost down at the target wavelength, despite the fact
that it might be more narrow than standard CWDM band of 20 nm. If we are to look
at the problem at hand through the prism of backward compatibility, then indeed
PR(X)30 type PMDs are in a better position to be deployed, mainly because of the
quantity of compatible ODN. The main problem I see here is that we are trying to
predict what market does at times when I think we all learnt that market is not
predictable. An interesting point to
note in favour of PR(X)30 PMDs: if any of the current GPON favouring carriers
were to switch to EPON technology, PR(X)30 would be exactly the PMD they would
need to use. Much as it seems unlikely, I did receive querries from a few large
carriers on such a possibility. In such a case, PR(X)10 and 20 PMDs could not be
used for obvious reasons. Additionally, when considering harmonization with
FSAN/ITU-T NG-PON systems, PR(X)30 PMDs are yet again the ones which will
receive the main focus, since they are considered for reuse for XG-PON systems.
Should that move forward, this will represent a clear signal to the
manufacturers to focus on these devices. In the long run, the cost of a more
narrow band PR(X)30 OLT PMD may be comparable if not lower than uncooled DML
based PR(X)10/20 PMDs. Jim and Alan are right here - the bigger the volume and
more companies in the segment, the better the prices. Concerning filter cost:
I have seen already presentations on this topic done at our group and at FSAN
and conclusions are contradictory. Some say a decrease to 15/14 nm isolation
between video and data channel should have minimum impact on the filter
cost, others say it is going to hurt us bad. If I recall right, we had only one
presentation on this topic at our group. I am not an expert in this area
myself so I rely on others for data and information. Yet in the view of
conflicting information, I find myself wondering where we really are.
Perhaps people workign with filter design could contribute with more down to the
ground relative cost comparison for 20, 15, 10 nm channel separation ?
Just a mental note -
aren't RF Video systems supposed to operate at 1550 nm ? Who would push them to
1560 nm ? Regards Marek From: Pete Anslow
[mailto:pja@xxxxxxxxxx] Frank, While not wishing to
enter the debate on what the wavelength choice for 10G EPON should be, your
statement “The G.671 standard for
couplers specifies the two traditional windows: 1260~1360 and 1480~1580.
So, if we continue to specify that our ODN is composed of fibers recommended in
G.652 and couplers recommended in G.671 then we really shouldn't go beyond
1580nm.“ is not really
correct. G.671 Amendment 1
(03/2006) available here: http://www.itu.int/rec/T-REC-G.671-200603-I!Amd1/en adds an “Optical
branching component (wavelength non-selective) for PONs” which is specified from
1260 to 1360 nm and 1450 to 1600
nm. For the loss of G.652
type fibre, some information was presented in: http://www.ieee802.org/3/av/public/2007_03/3av_0703_anslow_1.pdf see slides 5 to
11. The red curve on slide
6 is derived from measurements of 1549 fibres at 1550 and 1625 nm together with
full spectrum measurements of uncabled fibres. The only room for
uncertainty between these results and your requirements is whether the bends
seen in a PON environment produce a significantly different curve from those
seen in the transport environment. Regards, Pete
Anslow
Nortel Networks UK Limited, External +44 1279 402540 ESN 742
2540 Fax +44 1279 402543
From: Dear Group,
Let's not forget that
it is not just splitters and the raw fiber we need to worry about at 1600nm, but
also the effects of bends. And that is something that is more field
operations dependent. This is the biggest uncertainty that has made my
operator contacts concerned about using wavelengths longer than 1580nm. I
will point out that the data I’m getting from network operators is very sporadic
and somewhat contradictory. But, if there is doubt, then I think the right
way to go is to assume the worst case. There is another thing
to consider: We may consider proprietary specifications all we want, but
in the end we should be more standards driven. If I look at the G.652-A
fiber recommendation, all it tells me is that the loss (both basic and with
bends) is specified at 1550nm at the longest wavelength. Only G.652-B
specifies the losses at 1625nm. The G.671 standard for couplers specifies
the two traditional windows: 1260~1360 and 1480~1580. So, if we continue
to specify that our ODN is composed of fibers recommended in G.652 and couplers
recommended in G.671 then we really shouldn't go beyond 1580nm.
The alternative would
be to require the fiber to conform to G.652-B, and then to require the couplers
to conform to a revised G.671. But that sort of defeats the whole idea of
backward compatibility with existing PONs, which did not have such restrictions.
So that is our problem. I realize that the
window is somewhat narrow, but it seems to be the only safe choice at this
point. Sincerely, Frank
E. From: Jim Farmer
[mailto:Jim.Farmer@xxxxxxxxxxxxx] Before submitting a
formal comment, we wanted to run this by the reflector for comment.
Regarding the upstream wavelength action reflected below, from the September
meeting in Cost and power
dissipation: The use of a tight wavelength band (e.g., 1577 +/-3 nm) at
any wavelength is going to significantly increase the cost of the laser, both
due to the manufacturing tolerance imposed and by the need for heating the
laser. Any DFB laser so far as we know, has a temperature drift of about
0.1 nm/deg. C. If one were to provide for a normal indoor temperature
range, which can cover 60 degrees, then one will have a 6 nm wavelength shift
due to drift. We have found that there is market demand for wider
temperature range OLTs, which can easily have a wavelength drift of 10 nm over
their operating temperature range. Of course, the answer is to
temperature-stabilize the laser, but this requires an added heater/cooler and
control, and the best estimate we can make right now is that it will add 2-4
watts per PON to the OLT power dissipation when operated at room
temperature. Granted, this is small compared with the total power
dissipated by the OLT, but in an age in which we are trying to minimize the
power draw of all equipment, it is going in the wrong direction. And it
will cost money, not to mention that the laser will have to be specified to a
tight wavelength tolerance (or the wavelength tuned by the cooler), adding more
cost. Concerning the
availability of devices, we expect that this application will drive adequate
volume to get the cost down at whatever wavelength we choose. We’ve been
told by one laser manufacturer that the wavelength specified has little bearing
on cost, though of course, tolerance and volume play a big part in determining
the cost. Specification: We
question the statement that fiber and couplers are not fully specified beyond
1580 nm. Some people are using 1610 nm for OTDRs. A check with a
couple of major manufacturers of fiber and couplers indicates that
specifications are controlled out to 1625 nm. See, for example,
http://www.corning.com/WorkArea/showcontent.aspx?id=15535, and
http://www.ofsoptics.com/resources/AllWaveFLEX136web.pdf. As for
couplers, I understand that planar couplers are fine over this wavelength range,
though fused biconic couplers may not be as good. Yet another concern we
have with the 1577 nm selection is the filtering when a 1550 nm video carrier is
used. The video optical carrier can be as high as 1560 nm. For a
1577 nm downstream data carrier, which can go as low as 1574 nm by
specification, the WDM to separate the wavelengths has a 14 nm transition
region. If we use 1590 +/-10 nm for data, the transition region expands
43%, to 20 nm. This can help reduce ONT cost, as well as the cost of the
WDM at the OLT. For these reasons, we
seek reconsideration of the decision to abandon 1590 nm. We would like to
receive comments from the reflector, then we are planning to submit a formal
comment before the deadline. Thanks, Jim
Farmer, K4BSE |