ISP Overselling Speed on PON

I have a 910 / 910 Synchronous fibre connection I.e. FTTP, this uses a PON (Passive Optical Network).
Fibre circuit provider is City Fibre.
My ISP is Vodafone with their UltraHub and two boosters.
The ONT is a Calix GigaPoint 801Gv2 Model No: 100-04923 10
This is a 1.244Gbs upstream and 2.488 GBs downstream, there could be up to 64 other customers from various ISPs on this PON, the ONT makes it a GPON.
The GPON circuit is capped at 940 Mbps due to the Ethernet encapsulation, the PPPoE used by City Fibre further reduces the user real world throughput to 880 - 900 Mbps also reduces the MTU (Maximum Transmission Unit) 1492 bytes which will impact user data.
Given all the above it seems impossible for Vodafone to supply a synchronous 910/910 fibre circuit.
Other things to keep in mind is upstream, only has a 125 micro second window to do all its transmission, however downstream is broadcasted to all ONTs on that particular PON circuit, you receive everybody’s downstream packets and everyone receives your downstream packets.
My question is , are the ISP services covered by the sales of goods act in that they should be merchantable quality ? , anyone any ideas about this over selling of broadband speed.
The ThinkBroadBand speed test confirms I have an asymmetrical FTTP Circuit I.e downstream is twice the rate as the upstream.

Check out your ONT (fibre to copper converter) this will point to the type of FTTP Circuit you have.

In reply to a post by ultraproblems:

Given all the above it seems impossible for Vodafone to supply a synchronous 910/910 fibre circuit.

Not impossible, this is down to the sharing ratio. Shame with CityFibre as you'd expect this with an Openreach FTTP service. CityFibre at least have announced they have plans to upgrade all their GPONs to XGSPON.

Not really any different to DOCSIS cable either!

Isn't it all still UPTO speeds due to contention?

In reply to a post by broadband66:

Isn't it all still UPTO speeds due to contention?

The fundamental difference between "home broadband" and a "leased line" (aka circuit) is that home broadband is priced because it is shared technology.

With XGSPON the capacity both up and down is so much more, the impact of the sharing/contention ratio is much less obvious.

Don't isps just sell XGSPON to more users?

In reply to a post by broadband66:

Don't isps just sell XGSPON to more users?

Its not the ISP it is the network provider that decides what type of network to build and operate. Openreach, CityFibre, and the other alternate network providers do this, and you get what is built to your home. Vodafone, Plusnet and others are ISPs that then make use of the network in your area.

Sometimes known as "last mile" access.

My contract is with Vodafone, they should check with, in this case City Fibre to see what is available, the contract between Vodafone and City Fibre should reflect available capacity in the PON and also the equipment in use.

In reply to a post by jchamier:

In reply to a post by broadband66:

Isn't it all still UPTO speeds due to contention?

The fundamental difference between "home broadband" and a "leased line" (aka circuit) is that home broadband is priced because it is shared technology.

With XGSPON the capacity both up and down is so much more, the impact of the sharing/contention ratio is much less obvious.

Not necessarily, XGSPON can be used over a greater distance, and can be used with higher splitter ratios up to 1/256, so potentially many more customers can be on a single XGSPON service than a GPON service , I’m not claiming that any UK provider splits fibres to that extent but the XGSPON technology can deliver it , so it not really true to say that the extra bandwidth makes congestion less likely in every case , a GPON splitter with a few customers on it is less likely to ‘congested’ than a XGSPON splitter that has dozens more customers …

Edited by Iniltous (Sat 14-Jun-25 20:13:54)

In the real world PON contention just isn't a thing. Also it's symmetric, not synchronous. Synchronous services are high end products that are all clocked to the same source.

Hi I agree with your comment, policing it at present is a bit of a nightmare, the ISPs are very shy at revealing the configuration and usage of the PON circuits the fibre circuit providers also have this problem.
What else do purchase that you don’t know what you are getting.

In reply to a post by ultraproblems:

Hi I agree with your comment, policing it at present is a bit of a nightmare, the ISPs are very shy at revealing the configuration and usage of the PON circuits the fibre circuit providers also have this problem.
What else do purchase that you don’t know what you are getting.

On my TT contract it states what product I have, and what is the minimum acceptable speed, does your contract state that?

Splitters are passive devices hence the name PON, fibre especially single mode 9/125 has very little skew, it’s principle down to launch power, I don’t know if the splitters have any monitoring like some of the old sugar cubes.

Synchronous systems recover the clock to synchronise local system either dedicated space in headers or combined with the data, as opposed to the old asynchronous system which had start indicator and hope for the best, it usually worked I.e. UART chips etc.

Your upstream data is controlled by the BWMap and TDM (Time Domain Multiplex) the time window is 125 microseconds how many of these you get in a second must be down to the number of customers.

In reply to a post by Iniltous:

a GPON splitter with a few customers on it is less likely to ‘congested’ than a XGSPON splitter that has dozens more customers …

Thanks, as always the complexity is in the detail.

I'm a bit confused as to the difference between an optical gpon spliter and a xgs-pon splitter?

There isn’t really a difference between a splitter on a GPON system and a splitter on an XGSPON system, in fact GPON/XGSPON can coexist on the same fibre/splitter network…but XGSPON has the option to go upto a 256 to 1 split , and has an operating range ( distance ) of around 100km, compared to GPON max split of 128 (typically 32/1 is used ) and around 60km….this in essence allows a single fibre to reach further from the headend and have more users hanging of it , OR use 1/32 split so around a quarter of a GPON theoretical capacity, AFAIK , XGSPON operators don’t publish their split ratio but it’s possible they have many more users per PON as well as the PON covering a much larger geographic area, so less headend kit , and although the faster 10g speeds and symmetrical service are undoubtedly beneficial to customers , it also makes it cheaper for the network provider, more customers per ‘PON’ , ( possibly 64 , 128 users or whatever ) , so although the total bandwidth 10Gb is 4 times that of a GPON you may be ‘sharing’ with many more users.

XGSPON somewhat counterintuitively while offering longer range is also more power efficient so more customers and cheaper to run , XGSPON benefits the network operator as much as the customers benefiting from faster symmetrical speeds.

Edited by Iniltous (Sat 14-Jun-25 22:55:39)

In reply to a post by ultraproblems:

Hi I agree with your comment, policing it at present is a bit of a nightmare, the ISPs are very shy at revealing the configuration and usage of the PON circuits the fibre circuit providers also have this problem.
What else do purchase that you don’t know what you are getting.

Every service tells you the guaranteed rate when you sign up. There is no mis-selling occurring.

If you have requirements around contention and are put off by a shared access network then uncontended products are available.

Going back to your original post, if you are only getting 500Mbps up on a 1Gbps symmetric service then you'd be better served by posting a new thread asking for help with a fault than getting into the weeds on PON contention.

Edited by jpm (Sat 14-Jun-25 22:54:24)

I was tempted to write a diatribe, but to be fair been sitting here with the metaphorical popcorn reading all the replies to get the OP on the page so to speak has been far more entertaining.

This thread is like a collective “expectation reset” for the OP. I did wonder if the OP was some random AI / bot but they appear to be genuinely human.

Anyway I’ve polished off the remains of the vermentino (Cotes du Brian, 2020 from Domaine Anne Gros & Jean-Paul Tollot - very good btw) and look forward to the next update…

Crack on crew 😂 🍿

In reply to a post by Iniltous:

There isn’t really a difference between a splitter on a GPON system and a splitter on an XGSPON system, in fact GPON/XGSPON can coexist on the same fibre/splitter network…but XGSPON has the option to go upto a 256 to 1 split , and has an operating range ( distance ) of around 100km, compared to GPON max split of 128 (typically 32/1 is used ) and around 60km…speeds.

For Openreach there isn't a xgs or 25g pon or gpon splitter. As you correctly said, XGSpon can do 256 way compared to 128 for gpon. You can of course add a splitter further back up the path, so that you can split it into 2 or 4 (64 or 128) but of course you reduce the length that light will travel.

That aside, I dobut many would 64 or 128 user splits on gpon. City fibre is going from gpon to xgs pon, so i suspect the contention will stay the same

In reply to a post by Taras:

In reply to a post by Iniltous:

There isn’t really a difference between a splitter on a GPON system and a splitter on an XGSPON system, in fact GPON/XGSPON can coexist on the same fibre/splitter network…but XGSPON has the option to go upto a 256 to 1 split , and has an operating range ( distance ) of around 100km, compared to GPON max split of 128 (typically 32/1 is used ) and around 60km…speeds.

For Openreach there isn't a xgs or 25g pon or gpon splitter. As you correctly said, XGSpon can do 256 way compared to 128 for gpon. You can of course add a splitter further back up the path, so that you can split it into 2 or 4 (64 or 128) but of course you reduce the length that light will travel.

That aside, I dobut many would 64 or 128 user splits on gpon. City fibre is going from gpon to xgs pon, so i suspect the contention will stay the same

A 1/32 splitter is basically 5 x 1 to 2 optical splits contained within in a single device, built as 1 input to 2 outputs , 2 to 4 outputs , 4 to 8 , 8 to 16 , 16 inputs to 32 outputs , each internal split is a 3bB loss so 15dB overall loss for the device, to get to more than 32 outputs obviously needs more splits , a 1/4 split then 4 x 1/32 splitters hanging on each of those 4 output fibres gives potentially 128 customers , this is possible on GPON , but Openreach don’t do it .

When it comes to XGPON and XGSPON , a 1 to 8 split with 8 x1/32 splits gives 256 users , Alt Nets may do this on their XGPON and XGSPON systems because it is one of the major design benefits of these systems, if it can be done , it would be daft not to use at least some of this benefit when designing a network, but I’ve no knowledge what split ratio any particular Alt Nets uses, perhaps not 256 per PON , but 64 or 128 seems likely.

Edited by Iniltous (Sat 14-Jun-25 23:37:53)

My thoughts, and I am unclear which part you specifically unhappy with.

All consumer broadband products and almost all business broadband products use shared infrastructure. This is not unusual.
Over selling I suppose can also be interpreted in different ways, so e.g. if you sell a contended product but in practice there is no congestion, I dont consider that overselling personally, I only consider it overselling if you cant hit the "up to" speeds due to visible congestion.
CityFibre allow both IPoE and PPPoE, the PPPoE will be Vodafone's choice.
1492 MTU limit will barely register on potential throughput, % wise it is 0.6%.
Vodafone speeds are up to, and I think even on a gigabit ONT, 910 throughput is possible.

In terms of visible congestion, definitely possible on GPON, but seems rare in this country, mainly due to low take up levels, immature rollout. CityFibre especially have poor take up figures, and remember most customers wont be on the faster tier products.

Vodafone have some problems on their network, they getting a lot of CityFibre sign ups, but are showing up localised issues to themselves.

I dont see how this would be an issue with sales of good act, everything you have described is not a problem in the service you have purchased.

In reply to a post by Taras:

That aside, I dobut many would 64 or 128 user splits on gpon. City fibre is going from gpon to xgs pon, so i suspect the contention will stay the same

Just GYI CityFibre increased the split ratio from 1:32 to 1:64 on XGSPON.

In reply to a post by ultraproblems:

Your upstream data is controlled by the BWMap and TDM (Time Domain Multiplex) the time window is 125 microseconds how many of these you get in a second must be down to the number of customers.

It's mostly down to demand. Network operators can configure to an extent how much of the capacity is provided on demand and how much is unsolicited slots. Unsolicited slots mean lower latency at the expense of capacity so it's a mix of both with demand by a long way the biggest factor.

In reply to a post by ultraproblems:

Other things to keep in mind is upstream, only has a 125 micro second window to do all its transmission

No. That's the minimum length of a burst not the maximum and ONTs do not have to be provided them one at a time.

You aren't sharing with 64 others, their GPON is a 1:32 split. 8 bytes per 1500 isn't going to take 5% off, maths.

Stop reading technical specifications, it's not wise to quote stuff you don't understand and often makes it harder to get things fixed as you end up confusing the issue. Speak with your provider, see if there's a fault, if not you potentially have a torrent fiend or someone fixated with backups nearby using capacity. Vodafone should get you to the minimum they sold you.

I don’t get the minimum, if I use the thinkbroadband’ s measurement I get upstream around 330 Mbps and 630 Mbps downstream , sometimes this is has low as 200Mbps. This infers the PON to the ONT is a GPON, hence the difference between upstream and downstream.

Has for Tech Manuals I’ve been reading them for the last 50 years or so, along with RFCs, it was part and parcel of of the job has the network tech evolved from thick wire and taps and the AUI cables to GIGBit fibre Ethernet.

The only way to make a judgment is to be informed,

It may not even be the PON congestion but more than likely Vodafone themselves. We’ve seen this before and it’s super rare for the PON to be congested.

Have you asked any of your near neighbours on the CF network if they are experiencing similar?

In reply to a post by ultraproblems:

This infers the PON to the ONT is a GPON

Does it?

In reply to a post by ultraproblems:

I don’t get the minimum, if I use the thinkbroadband’ s measurement I get upstream around 330 Mbps and 630 Mbps downstream , sometimes this is has low as 200Mbps. This infers the PON to the ONT is a GPON, hence the difference between upstream and downstream.

Has for Tech Manuals I’ve been reading them for the last 50 years or so, along with RFCs, it was part and parcel of of the job has the network tech evolved from thick wire and taps and the AUI cables to GIGBit fibre Ethernet.

The only way to make a judgment is to be informed,

Quote just to ensure flow easier to follow.

That your ONT is a GPON ONT tells you that you've GPON. This in no way accounts for the difference between upstream and downstream performance else it'd be that way for everyone and is not.

Do you have a Broadband Quality Monitor set up? How's that looking? Please share a snapshot as congestion should be quite evident.

Tried other speed testers? Getting the speedtest.net app to eliminate browser weirdness a good call.

https://speed.cloudflare.com/ the packet loss, bufferbloat, jitter and overall speeds would be super userful. All 3 latency and jitter numbers alongside the packet loss. Ta.

As others have said CityFibre GPON is a 1:32 split, the specs may go to 1:64 but they don’t go beyond 1:32. It will be very unlikely there are actually 32 on a given PON due to the way the physical network is.

And CityFibre don’t use PPPoE, they provide a semi transparent layer 2 Ethernet link to your ISP. Your ISP can pick to use PPPoE or IPoE. There are at least two major ISPs using CityFibre (and Openreach FTTP) who use IPoE with DHCP so no extra overhead with those ISPs. It is Vodafone that have chosen to use PPPoE.

The speeds ISPs including Vodafone advertise are covered by very strict rules from Ofcom. The 910/910 is the agreed number and the ISPs have to provide evidence of the speeds to meet the speed code of conduct.

If you are not getting 910/910 then their is either a problem with your own network or their is an issue that needs raising with Vodafone. Vodafone should have told you what their guaranteed speed is for your service. I’ve seen lots of results of people getting the above speeds.

Hi,

In reply to a post by Ad_G:

As others have said CityFibre GPON is a 1:32 split, the specs may go to 1:64 but they don’t go beyond 1:32. It will be very unlikely there are actually 32 on a given PON due to the way the physical network is.

And CityFibre don’t use PPPoE, they provide a semi transparent layer 2 Ethernet link to your ISP. Your ISP can pick to use PPPoE or IPoE. There are at least two major ISPs using CityFibre (and Openreach FTTP) who use IPoE with DHCP so no extra overhead with those ISPs. It is Vodafone that have chosen to use PPPoE.

The speeds ISPs including Vodafone advertise are covered by very strict rules from Ofcom. The 910/910 is the agreed number and the ISPs have to provide evidence of the speeds to meet the speed code of conduct.

If you are not getting 910/910 then their is either a problem with your own network or their is an issue that needs raising with Vodafone. Vodafone should have told you what their guaranteed speed is for your service. I’ve seen lots of results of people getting the above speeds.

Note that Cityfibre support an ethernet packet size of at least 1900 bytes from ISP to customer. Some ISPs will use that to allow their customers to have a 1500byte MTU, even with the PPPoE overhead. Of course, the PPPoE header is still an overhead which reduces throughput at the IP level and above.