BT Reveal Tech Details for Expanded Long Reach VDSL Broadband Trial - ISPreview UK
Openreach (BT) has today published new technical details for their on-going trials of fixed line “Long Reach VDSL” (FTTC) broadband technology, which could be used to deliver the future 10Mbps Universal Service Obligation and possibly even to expand the availability of “superfast” (24Mbps+) speeds.

We’ll begin with a quick recap. The existing ‘up to’ 40-80Mbps capable Fibre-to-the-Cabinet (FTTC) service works by running a fibre optic cable from the telephone exchange to your local street cabinet. After that Openreach uses VDSL2 over the remaining copper wire, which runs from the cabinet and into your home.

Ideally you need to live less than 300-400 metres (copper line length) from your street cabinet to get the best VDSL2 speeds, with performance falling away to around just 10-15Mbps at the extremes of 2000 metres (2km). But many premises can exist a lot further away than this and so LR-VDSL was proposed as one possible way of meeting the Government’s 10Mbps USO pledge.

The new approach is said to work in a narrower frequency band and at a higher power than existing VDSL2 connections, with lab tests showing that a 2km long copper line delivering 9Mbps via normal VDSL2 could be pushed to 24Mbps+ with LR-VDSL (here). A technical Proof of Concept Trial with up to 200 lines has already taken place in the rural village of Isfield (East Sussex).


By comparison the new SIN522 (PDF) document gives us our first real look at how the technology is setup and also confirms that Openreach will this month launch an expanded trial “across a number of exchange areas” (we’ve asked for a list of the locations and have sadly been refused by BT).

At this point it’s worth noting that the first concept trial in Isfield needed to ensure that no exchange based ADSL existed in order to prevent crossover network interference. The new trials also warn about the “potential degradation to ADSL services from application of LR-VDSL” and have recommended that ISPs cancel any SMPF (Shared LLU Line) based services in order to take advantage of the trial (there aren’t many of these left in the UK, so it’s not likely to be a big issue).

CPs will not be able order SMPF on any of the Trial Cabinets but may order MPF subject to an acknowledgement that any ADSL service they want to offer will be impacted by LR-VDSL and that, in the circumstances, we strongly recommend they use GEA FTTC for broadband services,” said BT’s document.

We also note some other requirements, such as the need to enable G.INP and also Vectoring to help cancel out cross-talk interference (so far Openreach has only made minimal use of this on VDSL, partly because future upgrades are regarded as a better investment). On top of that G.INP has had somewhat of a rocky roll-out due to conflicts with ECI kit (here).

LR-VDSL is also being aimed at lines with a D-side length in excess of 1.25km (0.5mm diameter copper) and Seamless Rate Adaptation (SRA) may even be applied to some of the LR-VDSL lines as part of Openreach’s trial activity.

2.2 LR-VDSL Line Selection

In order for a line to be selected for uplifting to LR-VDSL, the following criteria must be met:

1) The D-side insertion loss (measured at 300kHz) of the line shall be greater than 12.5dB and less than 40 dB (both measured at 300kHz). Lines with a loss lower than this will not benefit from LR-VDSL and will remain as a 17MHz vectored VDSL2 line.
2) The CP shall have confirmed to Openreach Product Line that they are prepared to participate in the trial activity.
3) The CP provided CPE shall support Profile 8b operation and vectoring as defined in G.993.2 and G.993.5.
The LR-VDSL trial will only use a single profile and related lines will be configured for a default downstream target margin of 6dB, although some lines may be migrated to operate with lower target margins to “determine what impact this has on [broadband] speed and stability“. Interestingly LR-VDSL will use the VDSL2 band plan 998 (B8-4), which defines a maximum frequency of 12MHz (i.e. less than the 17.664MHz used by normal VDSL2 / FTTC lines).

The current VDSL2 system also has a maximum aggregate transmit power of 14.5dBm in both the upstream and downstream directions, while LR-VDSL will need to push this higher and aims to use 20.5dBm in the downstream direction and 14.5dBm in the upstream direction (i.e. the main focus here is on boosting download speeds by using less frequency and more power).

Apparently it’s expected that customers uplifted to LR-VDSL shall receive a maximum service rate of 40Mbps downstream and 10Mbps upstream, although many will obviously receive less than this.

All in all there’s plenty of useful information to chew on and we’ll finish by posting the full channel, line and spectrum configuration profiles for LR-VDSL. Anybody not familiar with this bit can switch away to read something else, but it’s useful to have confirmation of how the technology will actually work.