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P2P's
I reading the e-mails on various wavelength plans, and types of laser, I'd like to offer my opinion which was discussed to some degree at the Raleigh, NC meeting. Many points have been raised, pointing out the Pros and Cons of each variation I supporting a single wave length 1310nm Tx and 1310nm Rx. Note this is different from what Vipul was describing in his e-mail relating to single wave length 1310. I believe he was describing a 1310+ and 1310- for Tx/Rx. Where Tx & Rx never overlap. Technology exists to differentiate a reflection from a signal being transmitted at the other end of a link. For example it can be done with link design and signal/reflections being maintained below 6db (As described in Vipul's Portland presentation). Its also possible to differentiate between reflections and a signal being transmitted over a 10KM link via the LOS (threshold) indicator. FP could be used at either end of the link, the limiting factor will of course be the PMD in an ONU where it may be located outdoors. Since we have spec'd the ONU to operate from -40 to +85C the total range is 125 degrees, as described in Vipul e-mail. It is certainly not necessary to double the number if we are not defining a 1310+ / 1310- P2P link. Even with a 125C Temp the delta wave length range is conservatively only 62.5 nm. (Using 0.5nm/C specificed in Vipul's e-mail, although I think the wave length drift is closer to 0.4nm/C) We voted on case temp rating versus ambient for the PMD, the FP laser and IC junction temp's will be at least 10-20C higher within 10-20 seconds of power up. In operation, their will be a theta C-A Case to Ambient of 5-30C/ Watt, the smaller the heat spreading area the greater the delta between Case and Ambient temperature. We also should consider the theta J-C Junction to Case rise. If it's an IC mounted in a package with a thermal heat slug, the theta C-A is very low about one degree C/W, however most of the components that go into a module are ether bare die mounted to an internal surface, or are in plastic packages where theta J-C is large. This just means that the active devices operate at higher Temps then either the Case or Ambient Temperature. I believe the 0.4nm/C I mentioned is not linear, it decreases as temperature increases. In short the max wave length drift the FP would be subjected to depends on the worst case delta nm/C, at 0.5nm/C a FP centered at 1310nm would drift +/- 31nm or 1279nm to 1341nm. At 0.4 nm/C it would only drift +/- 25nm or 1285nm to 1335nm. These are worst case numbers, sample measurments are better. I hope this resolves the wavelength drift over temperature issue for FP's but I welcome additional data or feedback. Another issue raised was the "benefit" of having a single PMD for P2P and P2MP. Someone said the same thing I said on this issue just the other day which was "Lets stop trying to design a Swiss army knife." We will probably need a different PMD for each application. The P2MP PMD's have different needs and requirements, for one, the laser driver needs a control to shut it down. P2P links do not have these requirements. But what does make since is the fact that a true 1310/1310nm Tx/Rx single fiber solution only requires a single PMD design for both end's of the link. This has several advantages, only one PMD to keep in inventory, and fewer mistakes during installation, since there is only one PMD to chose from and only one fiber to connect to it. Finally, a 1310/1310nm link is much easier to add a 1550 video overlay, then a 1310/1490nm link. One other point on reflections, the carriers I've talked with say their existing networks have 20 dB ODN-ORL, thus a 10KM run is no problem. They also say that new installations will have 30 dB ODN-ORL, which makes hitting a reach of 20KM much easier. Of course MPN is the actual limitation for 20KM links, and agreeing on a smaller K value is key to getting there. In short for P2P the least expensive solution available today is FP's, using single mode, single wave length, single fiber
networks.
Regards, Tony Anderson
I've attached some of the e-mail treads from Jack and Vipul on this topic below. In a message dated 2/14/02 3:54:43 PM Pacific Standard Time, Jack.Jewell@xxxxxxxxxxxxx writes: <BLOCKQUOTE TYPE=CITE style="BORDER-LEFT: #0000ff 2px solid; MARGIN-LEFT: 5px; MARGIN-RIGHT: 0px; PADDING-LEFT: 5px">Subj: RE: [EFM-P2P] P2P link design considerations Date: 2/14/02 3:54:43 PM Pacific Standard Time From: Jack.Jewell@xxxxxxxxxxxxx (Jack Jewell) Sender: owner-stds-802-3-efm-p2p@majordomo.ieee.org To: Vipul_Bhatt@xxxxxxxx ('Vipul_Bhatt@xxxxxxxx') CC: stds-802-3-efm-p2p@xxxxxxxxxxxxxxxxxx (P2P reflector) Vipul,
Thanks for this effort and investigation. One correction: the
3rd
option of a dual-wavelength solution with both in the 1310nm band EXCLUDES Fabry-Perot lasers because the FP wavelength drift with temperature is so large that just one channel uses up the entire 1260-1360nm band (for the extended
temperature range sought). The dual-1310 solution is the one best suited for VCSELs, and is one I suggested in March, but the current status of 1310nm VCSELs makes it a tough argument (despite large recent progress). It can also be met with DFB lasers, but cost issues exist just as they do at the 1490nm end of the dual-wavelength (1310/1490) P2P approach. The dual-1310nm solution is somewhat similar to the CWDM solution of 10GigE. To summarize, the dual-1310 option will be attractive if: 1) people accept that directly-modulated DFBs can be cost effective, OR 2) that 1310nm VCSELs will arrive in time, OR 3) if a "DFBs today; VCSELs tomorrow" attitude prevails. I agree with the concensus that EFM products will have a long product lifetime and the long-term optimization should weigh heavily
on
the choice of PMD. As I see it so far, either of the 2-wavelength approaches would fill that bill. Jack
-----Original Message-----
From: Vipul Bhatt [mailto:Vipul_Bhatt@xxxxxxxx] Sent: Thursday, February 14, 2002 3:21 PM To: P2P reflector Subject: [EFM-P2P] P2P link design considerations << File: EFM P2P Options Comparison.ppt >> Dear colleagues,
Yesterday, I met with about 20 members of the 802.3ae (10G
Ethernet), all PMD experts. They were meeting in Santa Rosa to advance their 10G PMD work. I thank Jonathan Thatcher, the chair of 802.3ae, for arranging it. I gladly grabbed that opportunity because it was a chance to gain useful advice from the Green Berets of the PMD Planet! This message summarizes the discussions that took place. The presentation I gave to trigger the discussions is attached. Motivation: While PMD proposals that address our extended
temperature objective and P2MP objective seem to be enjoying good consensus, the P2P objective is drawing more than one suggestions on how to specify the right PMD. Yes, it's important to meet our March deadline, but it's even more important to do it with well-considered proposals. The fact that many competent members are unable to converge on a single P2P proposal indicates the need for more technical and economic analysis of the problem. Considering the deadline, we need to work faster. So I decided to go. I began by presenting the table with two options: 1-wavelength P2P
link at 1310 nm, or a 2-wavelength P2P link. In the 2-wavelength case, I made the initial assumption that the wavelength plan would be 1310/1490, because that would allow us to have a PMD at the ONU end that can be the same for both P2P and P2MP. The leveraging of scale economies seemed like a good idea to me. The table is my fabrication; any factual errors are mine. The choice of 1490 nm implied the use of either a DFB laser or a VCSEL. First off, the discussion led to the realization that there may be a
third good option - a two wavelength PMD that uses both wavelengths in the vicinity of 1310 nm. This will permit the use of Fabry Perot lasers, the most cost-effective and proven lasers available today. To reduce the risk of worst case wavelengths being too far from the zero-dispersion point, some helpful assumptions about temperatures at the two ends of the link can be made. For example, when one transmitter is at +85C, the chances are slim that the transmitter at the other end is at -40C. Thus, when counting wavelength drift with temperature, we can partially overlap the temperature ranges of the two ends. (This idea is promising, but before you can make a votable proposal out of it for March, you will need to crunch some link budget spreadsheet numbers. If you want to work on it, let me know how I can help.) With that issue aside, we moved on to discuss the two options as presented. The group agreed that longwave VCSELs will not be ready for prime
time soon (prime time meant high volume, low cost, high yields, proven robustness over temperature, wide acceptance among customers). Only one member said they will be, by December 2002. More (8) agreed that by December 2003, they may be. And yet, when asked to choose between one-wavelength and
two-wavelength options, majority (8 to 6) chose the two-wavelength option. Huh? I asked, how do you explain that? The answer was
thought-provoking: We should define a single-fiber P2P link that is forward looking. Longwave VCSELs and/or low cost DFB-isolator combinations will be available in the near future. Products based on the EFM standard will have a long market life. As for the present need for P2P deployments, the 2-fiber temperature extended LX will do just fine. Therefore, we can afford to wait. ("Two-fiber one-wavelength now, one-fiber two-wavelengths later.") Technical feasibility can be proven at a later stage in the standards timeline, but for Draft 1.0, the 2-wavelength option should be included. I then asked how they ranked the technical risk of option 2,
compared to option 1. Answer: If the risk level of option 1 is 1.0, then the risk level of option 2 is about 0.5. Reason? Mostly, the fact that the reliability and performance of a single-wavelength link depends on sustaining the high return loss of the PMD at the media interface. But a dust particle just a few microns in size can produce an air gap big enough to cause reflections which will make the return loss out of spec. Even if the PMD-media interface is kept within spec, every potential connector joint in the cable plant, including the often-used patch cord, is a place where dust particles can get in and defeat the high return loss assumption. This is not to say that high return loss implementations are not feasible; this is only to point out that Ethernet deployments can happen in huge volumes, and the statistical risk of dust particles causing failure in even a small percentage of links would be unacceptable. This concludes my report. I hope you find it useful in making an
informed decision. Whatever your preference, please remember that we can only vote on proposals - no proposals, no votes. Regards,
Vipul vipul_bhatt@xxxxxxxx
408-857-1973 _______________________________________________________________
Subj: RE: [EFM-P2P] P2P link design considerations
Date: 2/15/02 1:29:32 AM Pacific Standard Time From: Vipul_Bhatt@xxxxxxxx (Vipul Bhatt) Sender: owner-stds-802-3-efm-p2p@majordomo.ieee.org Reply-to: Vipul_Bhatt@xxxxxxxx <mailto:Vipul_Bhatt@xxxxxxxx> To: stds-802-3-efm-p2p@xxxxxxxxxxxxxxxxxx (P2P reflector) Jack,
Let me try a thought on you: if we get aggressive with FP specs, it
may be possible to accommodate FP lasers in the 2-wavelength solution in the 1310 nm band. Here's my argument. We assume that the wavelength drift with temperature for FP lasers
is 0.5 nm/C. The extended temperature range is 125C wide. For two lasers, one at each end of the link, it may be overly conservative to allocate a wavelength span capable of handling 250 C temperature range. This is because when a laser at one end of the link is at its extreme cold, it is highly unlikely that the laser at the other end will be at its extreme hot. We can overlap the two temperature ranges a bit, for a total planning range that is less than the sum of the two. One can also argue that the OLT end PMD will need to operate over a much smaller temperature range, say 70C. After all, switches are normally rated for 0 to 70C, and the OLT end PMD will be in a switch. (I am assuming that if it is housed in a Remote Terminal enclosure outdoors, it will be temperature controlled.) Suppose we work out the details and end up with a total temperature range of 180C. This translates to 90 nm wavelength range. Suppose we give back another 20 nm in manufacturing tolerances for laser wavelength and WDM filter spacing (aggressive, yes). This creates a goal of 110 nm wavelength span. Now we push on the transmitter specs. Dare I say: 2.5 nm rms
spectral width, k factor of 0.4, rise time of 150 psec. (I can half-heartedly defend these values if necessary, but for now, I will keep the message short.) Given all that, it may be possible to span a range of, say 1265 nm
to 1375 nm, while still keeping MPN penalty to a manageable ~1.6 dB. Is my optimism blinding me, or do you think this is a promising line
of thought? Regards,
Vipul vipul_bhatt@xxxxxxxx
408-857-1973 =====================
>
> > Vipul, > > Thanks for this effort and investigation. One > correction: the 3rd option of > a dual-wavelength solution with both in the 1310nm band > EXCLUDES Fabry-Perot > lasers because the FP wavelength drift with temperature > is so large that > just one channel uses up the entire 1260-1360nm band (for > the extended > temperature range sought). The dual-1310 solution is the > one best suited > for VCSELs, and is one I suggested in March, but the > current status of > 1310nm VCSELs makes it a tough argument (despite large > recent progress). It > can also be met with DFB lasers, but cost issues exist > just as they do at > the 1490nm end of the dual-wavelength (1310/1490) P2P > approach. The > dual-1310nm solution is somewhat similar to the CWDM > solution of 10GigE. > > To summarize, the dual-1310 option will be attractive if: > 1) people accept > that directly-modulated DFBs can be cost effective, OR 2) > that 1310nm VCSELs > will arrive in time, OR 3) if a "DFBs today; VCSELs > tomorrow" attitude > prevails. I agree with the concensus that EFM products > will have a long > product lifetime and the long-term optimization should > weigh heavily on the > choice of PMD. As I see it so far, either of the > 2-wavelength approaches > would fill that bill. > > Jack Tony Anderson
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