BT's PSTN switch off question (FTTC Users)

Hey all,

Now that BT will be switching off their PSTN system in the very near future does this mean a potential increase to bandwidth/sync speed?

What I mean by this is that there must've been a % of phone line allocated for the PSTN part of the phone line for voice calls (let's just say for arguments sake, this was 10%), and the remainder (90%) was used for digital internet traffic. When the PSTN switches off, this will mean a 100% data phone line. Will we see a slight increase in sync speed in lieu of that 10% PSTN switch off?

TIA

In reply to a post by Bobby_Valentino:

Hey all,

Now that BT will be switching off their PSTN system in the very near future does this mean a potential increase to bandwidth/sync speed?

What I mean by this is that there must've been a % of phone line allocated for the PSTN part of the phone line for voice calls (let's just say for arguments sake, this was 10%), and the remainder (90%) was used for digital internet traffic. When the PSTN switches off, this will mean a 100% data phone line. Will we see a slight increase in sync speed in lieu of that 10% PSTN switch off?

No, those who are migrated from FTTC (with a analogue telephone service) to SOGEA (with digital voice service) typically see not uplift in speed.

To put it simply VDSL2 broadband uses a defined range of frequency bands higher than the PSTN voice range. This will not change when PSTN is removed so there will be no change to actual sync speeds or bandwidth.

If there's no change in bandwidth/sync speed, what happens to the capacity of the PSTN voice traffic when it's removed? That vacant 10% just gets removed and sits there unused?

Can't that PSTN section not be used to increase speed/sync somehow? (I know we're talking very small amounts here ... but hey every little helps).

Edited by Bobby_Valentino (Wed 18-Sep-24 23:03:14)

Post

Edited by MHC (Thu 19-Sep-24 00:28:25)

The SOGEA frequency profile is the same as the FTTC frequency profile thats why you see no uplift. At this point this is not going to change as there are lots of other considerations including a 5 year trial.

I wonder why they kept the profile the same. They could've repurposed the dropped PSTN frequency spectrum and added it to the upload/download frequency spectrum, rather than letting it go unused.

They knew the switch off was happening, so trails/R & D etc... could've been done along ago, ready for repurposing the dropped PSTN frequency to increase the upload/download.

Edited by Bobby_Valentino (Wed 18-Sep-24 23:19:49)

It just isn't worth the effort.

The baseband PSTN audio plus guard bands before the first upstream VDSL block of bands is 0.025875 MHz of the 17.664 MHz utilised by the 17a profile, so it would add a fraction of a percent to the upstream bandwidth, even taking into account that there are usually more bits per tone for lower frequencies. There would be no change to the downstream bandwidth.

In reply to a post by tdw42:

It just isn't worth the effort.

The baseband PSTN audio plus guard bands before the first upstream VDSL block of bands is 0.025875 MHz of the 17.664 MHz utilised by the 17a profile, so it would add a fraction of a percent to the upstream bandwidth, even taking into account that there are usually more bits per tone for lower frequencies. There would be no change to the downstream bandwidth.

That's assuming using the Profile 17a right? What about a shift to the Profile 35b which is designed to make full use of the line without any presence of PSTN.

In reply to a post by Bobby_Valentino:

In reply to a post by tdw42:

It just isn't worth the effort.

The baseband PSTN audio plus guard bands before the first upstream VDSL block of bands is 0.025875 MHz of the 17.664 MHz utilised by the 17a profile, so it would add a fraction of a percent to the upstream bandwidth, even taking into account that there are usually more bits per tone for lower frequencies. There would be no change to the downstream bandwidth.

That's assuming using the Profile 17a right? What about a shift to the Profile 35b which is designed to make full use of the line without any presence of PSTN.

Why would a business want to spend the time and money to achieve a minimal benefit when they are trying to get rid of the technology anyway by migrating to fibre-optic?

Because it’s a waste of effort and money, the focus is FTTP. Nobody is going to trial something that in the best conditions on a short line delivers about what a decent 4G service manages. We did Gfast, it was accepted as a misstep, we’re finished trying to be innovative over copper.

In reply to a post by Bobby_Valentino:

What about a shift to the Profile 35b which is designed to make full use of the line without any presence of PSTN.

Ain't going to happen, legacy product just waiting to die.

Yeah true. I thought fibre was still some time away and copper was still being focused/developed on but seeing as there's been significant progress to fibre deployment around the country it makes sense to focus on the next shiny new product.

Also bearing in mind that Openreach's competition is all full-fibre, bar VM. Which is also doing a lot of conversion to it.

In reply to a post by Bobby_Valentino:

What about a shift to the Profile 35b which is designed to make full use of the line without any presence of PSTN.

I think it's not possible because of interference from exchange-based ADSL, so couldn't be considered until all ADSL/LLU is gone. By which point we'll all have FTTP anyway.

(The narrowband voice frequencies - 0 to 4kHz - are irrelevant)

Remember that Openreach didn't even deploy VSDL vectoring except in a tiny proportion of cabinets, so I think that gives you a clue.

There's no benefit for Openreach spending money on VDSL card upgrades or software licences. For people with no FTTP yet, they have no choice but to stick with Openreach anyway. For people who do have FTTP available, it's much better if they migrate to FTTP.

Dialup at best was 40-50kbit/s using the 300-3.4khz range. No one would notice the difference.

I assume the switch from FTTC to SOGEA can be done in the background and isnt something an engineer is required for?

In reply to a post by bobble_bob:

I assume the switch from FTTC to SOGEA can be done in the background and isnt something an engineer is required for?

Yes, on the day of the migration you move your telephone from the socket on the wall to the socket on your ISP provided router. Dial tone may or may not remain on the wall socket.

Ah right so if you dont have a telephone no change to current setup really

Edited by bobble_bob (Thu 19-Sep-24 18:46:07)

None at all