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GPON Saturation. A Possibility?


*Please correct me if Im wrong. The following comes from my own research, knowledge and assumptions* 


At the moment Gigabit Passve Optical Networks provide 2.5 Gb/s downstream and up to 1.25 Gb/s upstream capacity per fibre coming from the Optical Line Terminal (CO). This is shared between 16, 32, 64, or 128 users. 


Im assuming TELUS Splits each fibre by 16 since 2500 Mb / 16 = 156 Mb/s (Their top residential download speed currently) And for them to promise speeds of 150Mb/s consistently It means no more than 16 subscribers per single fibre from the OLT. 


BUT 1250 Mb/s  ( the max upstream rate per fibre ) / 16 = 78 Mb/s so for TELUS to truly offer 150 symmetrical service, there would have to be less than 16 subscribers per fibre.  I understand that the chances that everyone in the neighborhood will be uploading at the same time are slim, and that mixing subscribers of different speeds could help minimize this issue, but in the end this all kinda sounds like it could lead to the current issues with over saturation of nodes on Shaw's cable internet service. When you think about it, the whole GPON network is, in essence, just like the cable network consisting of nodes (OLTs) and splitters, less the interference, distance limitations and cost.


Wondering what other people think about this and if my understanding is correct. If we hope to someday have gigabit internet to the home, I guess either TELUS would have to dedicate one OLT node per 2 customers or there would have to be some big advancements in PON technology. It seems to me like direct fibre is a more future proof solution.



You are on the right path. However, each PON (the port on the OLT, there are 12 on each card), supports 240GBs. At every Fibre Distribution Hub the CO fibres, one per PON, are split to 32 subscribers. Each ONT (the terminal at the premise) likewise supports 240GBs. Obviously things don't add up so service must be rationed. At the moment 150M symmetrical can be done but not if all 32 subs want that service. Yes, there could be a theoretical bottle-neck. You could have gig service at home. But right now, in the early days of FTTH, that would not meet the standards of reliability we want to deliver.

By 240GBs do you mean 240 gigabytes per second? And if my cheap Alcatel-Lucent ONT is capable of receiving 240 GB per second that's very impressive considering SFP+ modules max out at 10GB/s and cost hundreds of dollars. I don't know if you can disclose the model of OLT telus is using but id be very curious to look it up.

240Gbit/s would be the total backplane switching capacity of the terminal back in the cabinet where all the different GPON strands for that part of the neighbourhood connect.  


240Gbit/s is a pretty common number to find published on network switching equipment spec sheets of many manufacturers.  Probably they all use the same chipsets for their backplane switching so they all get that same statistic to publish.  It's not an important consideration in this topic, it only indicates that the OLT backplane shouldn't ever be the source of congestion as long as it's configured as a simple passive switch.  Usually any routing is a separate capacity figure and likely a more important number to consider if the OLT is doing anything more than just passing the traffic on down the line to some aggregation point device further up the chain.


So maximum of 32 customers would share the 2.5Gbit downstream and 1.25Gbit upstream.  Yes it can become congested, but in practice there is very low chances of that happening.  Residential users typical traffic is in bursts The old upload to download ratios were 4:1 on the asymmetric services and this is closer to 2:1 download to upload ratio.  Even given the new symmetric service most users traffic will still primarily be in the downstream direction. So it would take some very unusual conditions for it to happen.

1. A significant proportion of the customers would have to be subscribed to Telus fibre to fill up the 32:1 splitter close to full.

2. A significant proportion would need to be subscribed to the top 1 or 2 tiers of service speeds

3. You probably need an event like the recent olympics where you get a critical mass of everyone streaming high bitrates at some future super ultra HD resolution to fill the pipe

4. This event would have to be localized to a single node or small cluster of nodes.  Whatever mass hysteria caused everyone in your node to go do massive downloads at the same exact time couldn't happen on a wider scale or the bottlenecks would be at the upstream level because the backbones themselves do not have anywhere close to enough bandwidth for all nodes in the city at the same time to max their connections.  


At the GPON speeds and split ratio given, I would say the backbone and upstream connections are more limiting than the last mile connection.  The 10 and 40Gbit links they tend to use these days for major peering points are likely the points where you would slow down first.