| Thread Links | Date Links | ||||
|---|---|---|---|---|---|
| Thread Prev | Thread Next | Thread Index | Date Prev | Date Next | Date Index |
|
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 US is -A
fiber. So the likelihood of finding fiber in the access network that is
not good to 1625 nm is minuscule. How much should we base specification of
the latest product on fear for fiber performance that may not exist in the
field, and almost certainly not where we are going (access)?
In addition, please see
the note from Corning below (they have agreed to let me post it with their
name), which establishes a latest date for the introduction of G.562B
fibers.
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 Corning:
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.
The oldest spec sheet we have quoting performance at 1625nm is December 2000. Chris Kalivoda Applications Engineer 828-901-5573 CORNING CABLE SYSTEMS HICKORY, NC Jim Farmer, K4BSE
From: Frank Effenberger [mailto:feffenberger@xxxxxxxxxx] Sent: Thursday, October 16, 2008 9:31 AM To: STDS-802-3-10GEPON@xxxxxxxxxxxxxxxxx Subject: Re: [8023-10GEPON] Upstream Wavelength Selection 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,
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
|