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Marek,
If the OLT uses a Gen 3 or Gen 4 Transceiver on a PON, and 90% of the ONUs are Gen 1, they could be allocated in such a way as to leave one channel 'lightly utilized'. Since many ONU vendors are trying to make a visible speed test
an actual button on the ONU itself (would you count on BGP-4 peering to
speedtest.net servers to work at 25 Gbps....), then the ONU is aware of the task at hand and can be told to change channel by the OLT to perform the test.
There are technologies which can enable sub millisecond tuning (especially in reception) and the pressure to have them undergo industrialization is happening on the NG-PON2 front.
F.
Sent from my mobile device. Apologies for any typo.
Francois
Such *dynamic* wavelength reallocation with sub-ms times, and in a way that the end application does not become aware of the change. While it is certainly
possible (given the right amount of effort and complexity), it would be putting very tight requirements on tuning mechanism to be used trying to solve the problem that I am not sure exists to begin with. Operators tend to manage capacity in the access network
quire closely – I am not sure I see the value in a solution which assumes that the target 25G channel is free all the time and we can use it as always available. If there are 4 x 25G channels to use, they will be populated, and there is not going to be anything
free to treat as a backup capacity, just in case.
I think we need to focus as the group on the actual requirements we discussed at SG phase – this is what drove the project to begin with.
Marek
I do not see how this solves the serviceability of a 25G NG-EPON whereas if the costs pan out to be OK and if the standard accommodated it, it would be possible
to take advantage of tunability for this purpose as well as the other that I’ve stated before, such as better average throughput by having fewer ONUs participate in the same TDMA scheduler by moving ONUs out of the PON. So for instance, if ONU wants to do
a 25G speed test, it could be moved dynamically onto a ‘Speed Test’ channel, without slowing down the traffic of all other ONUs while the speed test is occurring….
Absolutely. A gen-1 25 Gb/s single-lane EPON should provide 2.5x more bandwidth than 10G EPON at less than 2.5x the cost. That won’t happen if you toss in a tunable
laser (and tunable receiver filter) in the ONU, and put wavelengths on a DWDM grid. Let’s defer that cost and complexity to the 2nd lane.
Ed
Somehow, just because the end target is 100G-EPON, people think that we need to utilize the 100G EPON capacity from day one. And if we don’t have 100G ONUs
at day one, then we have to fill this capacity with 25G ONUs.
This is not what we set to do. The generation 1 is a
single-lane EPON. Yes, starting with tunable optics and utilizing 4 lanes will provide 4x of sustained throughput per ONU. But that would be at more than 4x the cost.
According to my latest market data, the cost of tunable ONU optics today is ~9x that of 10G/10G-EPON ONU optics. Projections out to 2020 show it to drop to
3.5x the cost of 10/10 ONU optics, which is still very high.
Is this not why after completing NG-PON2 standard, SG15 shifted focus to XGS-PON that uses a fixed single wavelength 10/10 optics?
If the same cost ratio between tunable and fixed optics remains for the 25G tunable and 25G fixed, then the cost of 50G ONUs will likely be lower than tunable
25G ONUs. A 50G ONU obviously can burst at 50Gb/s peak rate, but additionally can operate as a 25G ONU on either of the channels, or even can operate as two independent 25G ONUs. So, we may spend time and efforts developing the first generation based on tunable
optics, but then why wouldn’t operators just skip gen 1 and go directly to 50G ONUs with 2 fixed channels?
Glen
Here is what I understand so far:
Per what Glen has presented: The OLT starts with a Gen 1 transceiver, which is stuck at 25 Gbps until it is replaced with a Gen 2 at 50 Gbps. Only the OLT
transceiver is replaced with a Gen 3 transceiver, would it then become possible to add 100 Gbps ONUs on the PON. With a Gen 1 OLT transceiver on the PON, 100 Gbps ONUs would be limited to 25 Gbps.
However, in NG-PON2, the use of an external WM allows for different OLT ports (or different OLT’s) to be the source of the additional instances of 10 Gbps channel
(up to 8 from 8 different line cards or OLT shelves is allowed). Therefore this allows pay as you grow, in service, with no downtime without requirement of retiring out OLT transceivers. Is this a benefit or a pain in the rear end for operators ? Benefits
allow for greater reliability, pay as you grow from cheaper 10 Gbps fixed XFPs/SFP+ with burst mode receivers. Pain in the butt means dealing with the WM and increased footprint.
With regards to the benefits of being able to get a 25 Gbps Tunable Tx / Tunable Rx ONU to roam across channels, here are the benefits:
- Serviceability of the PON port. If an ONU can move to another channel while the one that is down is being serviced, then everybody
is happy.
- Enhanced average throughput on the PON at the expense of peak speed. For instances, with 32 ONUs and 8 lambdas @ 25 Gbps, average
throughput would be 25 Gbps / 4 = 6.25 Gbps rather than 25 Gbps / 32 = 0.78 Gbps, which is 8 times greater. Perhaps this is not required for all applications, but as soon as you have mobile fronthaul, or business services on the PON, this capability becomes
important.
- As soon as the tuning range at the ONU exceeds the number of users on the PON (and you have an equally sized multi-wavelength
Comb laser + Rx Array in the OLT transceiver like what is being done in the OpenOptics MSA), then it becomes possible to turn off burst mode operation both for transmission on the ONU as well as for reception in the OLT. You’re now doing WDM-PON on power
splitters. Each user gets a dedicated channel. This is where we want to go in the end with all of this.
- Achieving greater 4 times the throughput of the maximum line rate of a single channel, is all about packaging the cost structure
of 4 tunable ONUs in one ONU and to have the MAC and ASIC to deal with the four transceivers. It is perfectly possible to imagine the OLT transceiver would support 32 channels and that an ONU would only be able to bond 4 channels. Then the question becomes
which 4 channels to bond in a pool of 32. If the ONU is not tunable, then you are not able to take advantage of assigning different bonded channels to different ONUs.
Francois Menard
CTO & Co-Founder
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