FTTP Backhaul

I watched this video the other day which is a tour of a Full Fibre Ltd (Fibre Heroes) cabinet which, if you are like me and love to know how things work, is an interesting watch.

In the video the engineer said that there are 16 line cards and each has 16 ports (256 ports in total), so with it being XGS-PON and they are serving up to 64 customers per port, the cabinet has a capacity of up to 16,384 premises. I was interested to find how this is backhauled to a larger exchange so I started searching for the OLT hardware.

I found the Nokia ISAM FANT-H, a Network Termination Card that has has 3.2 Tb/s switching matrix which it seems connects to all line cards in the frame. The specifications say it supports 200 Gb/s connection to each line card slot which makes sense as 16 ports of 10 Gbps would be 160 Gbps, therefore 200 Gbps would be more than enough.

What has confused me is it says it "supports 2x (100 Gigabit Ethernet or 40 Gigabit Ethernet or 4x10 Gigabit Ethernet) on the front" and later on in the data sheet says "2 QSFP cages each supporting 100 Gb/s link 40 Gb/s link or 4x10 Gb/s link". Is this the maximum capacity it has for backhaul, just 2 x 100 Gbps, so 200 Gbps? It doesn't seem to mention anything else regarding higher capacity links which is surprising given the 2.5 Tbps potential capacity connected. Even with some contention, it seems a low figure as people's bandwidth demands increase.

It also says "supports additional 16x10 Gb/s or 4x40/100 Gb/s connections via NT Input/Output card but I presume this is for the line cards, with the larger capacities being business customers instead of PONs?

Do these really only have a couple of hundred Gbps for backhaul? While that might be enough now, can that be scaled up in the future? I think Nokia's line cards can be upgraded to 25G-PON on some ports.

Yes this is totally normal. These sorts of multi-function access nodes, which may have OLT functionality, aren’t necessarily 1:1 / non-blocking on their uplink ports compared to the sum of their aggregated access ports.

Just as you’ll find a similar on Ethernet switches in many cases, the uplink ports may well have less aggregated capacity than the aggregated sum of the access ports.

In any event backhaul / upstream network capacity is a finite thing (read relative expensive commodity) and definitely won’t be on a 1:1 ratio with access. That’s the nature of the beast and how telecoms and other data networks have been architected for decades.

Edited by Pheasant (Sat 25-Jan-25 15:22:31)

Think of it another way. Which ISP is going to waste a hugely expensive 400G port on their core router, for a single OLT?

Whilst in theory there might be 16,384 premises on it, in practice PONs are built up to cover a service area, and there's no way that every PON will have 64 subscribers (i.e. 100% take-up).

Then there is statistical multiplexing. Even if they sell 10Gbps customer connections, they will burst for tiny amounts of time, and almost never overlap with each other. And the vast majority of customers will barely go any faster than 10Mbps while they stream Netflix.

And if in future it ever becomes a problem, they'll swap out the OLT - which probably needs doing after 5 years anyway, and by which time 400G ports may become affordable.

It was probably over ten years ago now, but there's a presentation from Gigaclear on YouTube somewhere where the person giving the talk showed 20Mb/s usage on average across an entire deployment.

The key there being the average, nobody is providing the capacity on a broadband service for an entire customer base to run a speedtest at the same time and get throughput to match the service tier they've signed up to. The vast majority of residential services get used heavily for a few minutes after they're installed as people run speed tests and then sit idle for the rest of their life, perhaps bursting a bit if a game update downloads, which if your console is left in standby tend to happen overnight anyway so you'd have no idea if you were seeing congestion.

According to Ofcom's market report for 2023, average broadband usage was 482 GB per month, which is about 1.5Mbps averaged 24x7.

That "20Mbps average" figure from 10 years ago doesn't sound right. It might have been peak or busy-hour I guess. Even then that seems unlikely, given that most of us were still on ADSL, and relatively new services like Netflix were tuned for that.

It was the entire network averaging at 20Mb/s, presumably a group of villages representing a few hundred customers

Edit: Found it https://youtu.be/rvjIhu7JiXk?si=H6Q_zc6h6JJY8gZ0&t=985

Edited by jpm (Sun 26-Jan-25 15:43:45)

In reply to a post by jpm:

It was the entire network averaging at 20Mb/s

Aha, that makes more sense!