Adam,
Your point on asymmetrical technologies is excellent. But you may not be aware that residential optical fiber is also asymmetrical. For example, GPON, the latest ITU specified PON standard, and the most widely deployed, calls for a 2.4 Gbps downstream and a 1.25 Gbps upstream optical line rate.
-mel
Ah, I did miss that, you’re right. We don’t have very much GPON up where I am.
-Adam
Why would they mandate such a thing? That seems like purely an operator decision.
Mike
There are also vendor issues involved. I am glad that Mel mentioned 'optical line' rate. Which becomes a theoretical thing. If the line cards aren't set up with buffering properly, then line rate won't be seen. And I think the line cards can also be easily over-subscribed. Oh, and due to the two or three step fan-out of 8/16/32, upstream becomes even more limited.
So, if you have FTTH with 1::1 house::port, then you are cooking with fire. Else, it is the luck of the draw in terms of how conservative the ISP is provisioning a GPON infrastructure. Which, I suppose, depends if it is 1G or 10G GPON.
I’m not mistaken, it also depends on the optics in the splitter, given that GPON is bidirectional single strand fiber.
-mel via cell
GPON is TDM (Time Division Multiplexing). The downstream is essentially OC-48 (2.4Gbps). The OLT sets the clock and each ONT has a specific timeslot for uploading. Some vendors can adjust the timeslot reservations to ‘guarantee’ specific upload speeds to specific ONTs
Why would they mandate such a thing? That seems like purely an operator decision.
It wasn't an arbitrary decision. The downstream has a single "talker",
the OLT, so it can use 100% of the "airtime" for itself to talk to
anyone on the port.
The upstream on the other hand has 1-32 or even more talkers that all
have to be syncrozied to talk at specific times even including guard
bands to account for slight differences is time keeping, not only
that, not every ONT is at the same fiber distance so an ONT that is
closer must wait for a signal from an ONT that is farther to "pass"
it.
All of this creates some inefficiencies in the upstream. Now, it has
gotten better with better technology of course but GPON is already
12-19ish years old.
I did believe that it is about the cost of SFP on the CPE/ONT side: 5$ against 7$ makes a big difference if you multiply by 1000000.
By the way, there are many deployments of 10G symmetric PON. It was promoted for "Enterprise clients".
CPE cost hurts in this case.
But some CPE could be 10GE and another 1GE upstream (10G downstream) on the same tree.
Ed/
We are rolling out XGS-PON everywhere which is 10G symmetric. Just because the PON runs at 10G, doesn’t mean you need to provision all of your customers at 10G.
We have a range of residential packages from 150Mbps up to 1Gbps symmetric. The ONT is the same in all situations. There is no SFP cost, due to it being a copper port. If we were to offer residential packages beyond 1G, a CPE swap would be required, but there is little demand for that… yet…
The future is bright for PON with NG-PON2, and 50G PON on their way.
Regards,
Dave
Yes, XG-PON.
Most FTTH operator stories I've heard of are still running regular GPON, thought.
Seems XG-PON has a high barrier-to-entry for el-cheapo home consumers.
Mark.
Indeed - XG-PON does not mean you have to deliver 10Gbps to customers. It just makes it easier to offer higher bandwidth that is symmetrical, at what-should-be a lower cost than Active-E, for more customers at the same time.
Mark.
ONT always has SFP for PON. It is inside (built-in) – this way is cheaper. OK. In this case, it is not SFP because it is not “pluggable”.
1G and 10G optics have a big cost difference for ONT.
You would be surprised. The equipment isn't that expensive in
the grand scheme of things.
- Jared
Especially when you consider that XGSPON and GPON and coexist.
K
Fair point, it's not part of our scope at $day_job.
Most of the greenfields I'm seeing in my region are standard GPON, and I'm not hearing of existing deployments being upgraded to XG-PON. But then again, perhaps I don't have my hand on the pulse as much as I think I should do :-)...
Mark.
We've seen proposals from Huawei, for example, where OLT shelves can support both GPON and XG-PON line cards.
Just not seeing our market going in that direction yet.
Mark.
I’ve been installing PON equipment for 2+ years where all the ports can be fitted with optics (SFPs) that support both GPON and XGS-PON simultaneously on the same fibre.
Aled
This isn't just Huawei. I know at least Adtran can do GPON+XGS-PON in the same chassis, and I'm pretty sure I remember Nokia telling me the same. I'd imagine Calix can, too, if the other two big names in North America have it.
I know at least Adtran even has a combo card where the same card can handle optics with all the filters built in to do XGS-PON+GPON on the same fiber without external muxes and only consuming one port. The pricing is even what I'd call not just reasonable but "compelling" for immediate deployment even if you don't plan to use the 10G function since it would of course drastically extend the useful lifespan of that card plus give you in-service upgrades to XGS-PON overlay if you equipped it with suitable optics from the get-go (which are also not overly expensive).
Given that the parts clearly exist for combo cards with combo optics, I'd imagine all of the major players have it in their portfolio at this point.
The XGS-PON ONTs are still about double the price of the GPON ONTs last I checked.
Less vanity over there?
Adtran offers the same functionality. As the wavelengths are different, both GPON and XGSPON can coexist on the same fiber plant with a single OLT transceiver supporting both. We are using Adtran gear for this and it's working fine.