What limits the Line Rate on FTTC connections (80mbps product)

[g67575]

I notice that there are Attainable Rate stats, as well as the current 'Line Rate' / 'Data Rate' on most VDSL2 modems.

Presumably, the Line Rate is set by Openreach's DLM, based on the errors (errored seconds) and resyncs recorded for the line, over the previous 1-3 weeks.

Also, is the 'Attainable Rate' / 'Max Rate' just an estimate calculated by the router, based on the line quality stats (e.g Noise Margin and Attenuation etc)? Or, is this what the DLM system will actually sync the line at, if all conditions are good enough (low error count with few / no resyncs)?

 

[Rroff]

I'm well out of touch with that side of it but when I had FTTC first installed the engineer had a tool which showed max attainable rate of something like 125 down, 108 up. BT fiddled about with what the 80mbit product would actually sync at at max as well it used to be 1-2mbit higher.

 

[VersionMonkey]

Also, is the 'Attainable Rate' / 'Max Rate' just an estimate calculated by the router, based on the line quality stats (e.g Noise Margin and Attenuation etc)?

Yes.

The actual sync rate is managed by DLM, which takes into account errors and resyncs, as you say - pushing up the target SnR until the line reaches the stability Openreach want for the product.

 

[g67575]

There's a small amount of interleaving / 'Impulse Noise Protection' on my VDSL2 line, does anyone know if this is something that generally needs to be removed by the DLM system first, before other improvements, like sync speed increases, are made?

 

[Dg834man]

Your on Fastpath, interleave depth 1 means its not interleaved. if the line was interleaved path mode would say interleaved and the interleave depth would be 32, 64, 1024 ect

 

[Scramz]

I notice that there are Attainable Rate stats, as well as the current 'Line Rate' / 'Data Rate' on most VDSL2 modems.

Presumably, the Line Rate is set by Openreach's DLM, based on the errors (errored seconds) and resyncs recorded for the line, over the previous 1-3 weeks.

Also, is the 'Attainable Rate' / 'Max Rate' just an estimate calculated by the router, based on the line quality stats (e.g Noise Margin and Attenuation etc)? Or, is this what the DLM system will actually sync the line at, if all conditions are good enough (low error count with few / no resyncs)?

The Ethernet into the house from the exchange uses 2 or 4 cores instead of 8, this limits the line to 100mbps, 80 down 20 up.

Then distance comes into play.

If you open the master socket you will see the Ethernet cores being used

 

[VersionMonkey]

The Ethernet into the house from the exchange uses 2 or 4 cores instead of 8, this limits the line to 100mbps, 80 down 20 up.

Then distance comes into play.

If you open the master socket you will see the Ethernet cores being used

Er... VDSL uses analogue phone lines, not ethernet. An analogue phone line works over 2 wires, and while you may see 4 or even possibly more wires in a dropwire that is only so you can have more than one phone line into a property.

The speed limitations of VDSL in general are because of the need to overlay high-frequency signals onto cabling designed to carry voice frequencies - it's the significant degradation of those signals over the rubbish single twisted pair of a phone line that means that few phone lines achieve even the 80/20 that Openreach chose as the maximum for that product - VDSL2 can, theoretically do quite a bit more than that (on very short lines).

 

[Scramz]

Er... VDSL uses analogue phone lines, not ethernet. An analogue phone line works over 2 wires, and while you may see 4 or even possibly more wires in a dropwire that is only so you can have more than one phone line into a property.

The speed limitations of VDSL in general are because of the need to overlay high-frequency signals onto cabling designed to carry voice frequencies - it's the significant degradation of those signals over the rubbish single twisted pair of a phone line that means that few phone lines achieve even the 80/20 that Openreach chose as the maximum for that product - VDSL2 can, theoretically do quite a bit more than that (on very short lines).

The master socket in my last 2 houses have been wired with Ethernet out the house to the box, but only using a couple of the wires. They even had Ethernet written on the cable. And my last house got 79/19.

I can only assume they used the Ethernet as a substitute cable. A bit like when people use Ethernet for power to doorbells, only using 2 wires out of the bunch. Which would make sense as the phone cable is basically ethernet but less cores. Which would also probably be partly the reason for the 100mbps limit. Most new homes phone wiring is now done with ethernet cable, just using less cores in the cable, leaving some for redundancy.

My current house has FTTP, no DSL so I cant check the master socket, don’t have one, however my cabling to the phone socket outlets are wired with ethernet but only using 2 cores.

Edit: I have found an old photo from my last house I took at the socket. There were 2x 8 core ethernets, one to the rest of the house and one going out the house to the cabenit. However only a couple of cores were used from each. I did find it odd at the time but then the penny dropped, the spare wires were knotted up behind the master socket.

Spoiler

 

[g67575]

Indeed, the main limitation on my line (and many others) is the copper cable distance to the FTTC cabinet, which in my case is approx. 753 meters (based on a downstream line attenuation of 10.4dB, worked out with an online calculator).

My line at present seems to be a bit unusual, in that the bit rate for the transmission (upstream) frequency bands seems higher than some of the other receive (downstream) frequency bands. I suppose this could be a result of manually increasing the SNR margin on my line, here is how it looks:

Edit - Nope, thinking about it, the high bit rate on lower transmission (upstream) frequencies is probably a result of disabling UPBO (Upstream Power Back Off) on my modem.

I guess the main thing you can see from this graph, is that on a longer line like mine, most of the reception (downstream) throughput comes from lower VDSL2 frequencies from 0.13-4.4mhz (D1 Band), some from 5.2-8.8 mhz (D2 Band) and very little from the highest frequencies, between 13.2-17.6mhz (D3 Band), although the throughput is a bit higher on higher frequencies with a lower SNR Margin.

More info and UK VDSL frequencies here:
https://1.bp.blogspot.com/-48T2a0H_...6BHOvo9BACLcBGAsYHQ/s1600/VDSL-freq-bands.jpg

In fact, the bitrate (and throughput) of VDSL2 frequencies seems to correspond very closely to the SNR (Signal-to-noise ratio, measured in decibels, shown in a separate graph), the higher the decibels, the higher the bitrate.

 

[VersionMonkey]

The master socket in my last 2 houses have been wired with Ethernet out the house to the box, but only using a couple of the wires. They even had Ethernet written on the cable. And my last house got 79/19.

I can only assume they used the Ethernet as a substitute cable. A bit like when people use Ethernet for power to doorbells, only using 2 wires out of the bunch. Which would make sense as the phone cable is basically ethernet but less cores. Which would also probably be partly the reason for the 100mbps limit. Most new homes phone wiring is now done with ethernet cable, just using less cores in the cable, leaving some for redundancy.

My current house has FTTP, no DSL so I cant check the master socket, don’t have one, however my cabling to the phone socket outlets are wired with ethernet but only using 2 cores.

Edit: I have found an old photo from my last house I took at the socket. There were 2x 8 core ethernets, one to the rest of the house and one going out the house to the cabenit. However only a couple of cores were used from each. I did find it odd at the time but then the penny dropped, the spare wires were knotted up behind the master socket.

You are right that Ethernet cable might have been used as a substitute, but that doesn't make phone cable "basically ethernet". The overwhelming majority of phone lines in the UK are not fed by Ethernet cable. An analogue phone line is just a pair of cables twisted together and it just happens that Ethernet cable exceeds the minimal requirements for a phone line making it a possible substitute for the dropwire. The use of twisted pair cables for phone lines predates the invention of Ethernet by many decades, never mind the use of twisted pair cable for Ethernet connections.

Using ethernet cable for the cable from the cabinet to a property might, due to the higher build standards of the cable, reduce noise and cross-talk compared to modern phone cables. It might not even cost more, given the volume of ethernet cable that is manufactured.

However, none of that means the combined 100mb/s limit on Openreach's FTTC product has anything at all to do with Ethernet networking standards. Not even partly.

Ethernet networking and VDSL broadband are entirely seperate technologies. You've jumped to completely the wrong conclusion by adding together 80 and 20 and seeing ethernet written on the cable.

 

[Scramz]

You are right that Ethernet cable might have been used as a substitute, but that doesn't make phone cable "basically ethernet". The overwhelming majority of phone lines in the UK are not fed by Ethernet cable. An analogue phone line is just a pair of cables twisted together and it just happens that Ethernet cable exceeds the minimal requirements for a phone line making it a possible substitute for the dropwire. The use of twisted pair cables for phone lines predates the invention of Ethernet by many decades, never mind the use of twisted pair cable for Ethernet connections.

Using ethernet cable for the cable from the cabinet to a property might, due to the higher build standards of the cable, reduce noise and cross-talk compared to modern phone cables. It might not even cost more, given the volume of ethernet cable that is manufactured.

However, none of that means the combined 100mb/s limit on Openreach's FTTC product has anything at all to do with Ethernet networking standards. Not even partly.

Ethernet networking and VDSL broadband are entirely seperate technologies. You've jumped to completely the wrong conclusion by adding together 80 and 20 and seeing ethernet written on the cable.

Phone cable is 2 core twist, pull one 2 core twist out of an ethernet cable and you now have a 2 core phone cable.