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UPDATE BT Openreach Brief UK FTTC Fibre Broadband ISPs on G.INP Issues

Friday, April 24th, 2015 (4:18 pm) - Score 7,526

Back in March we reported that BTOpenreach’s roll-out of Physical Retransmission ReTX (G.INP – ITU G.998.4) technology, which can improve the performance of their ‘up to’ 80Mbps capable Fibre-to-the-Cabinet (FTTC) lines, had hit a few snags (here) with some customers losing performance, connectivity and or suffering higher latency.

In simple terms, G.INP is an error correction solution that can help to tackle spikes / bursts of Electromagnetic Interference (impulse noise), which once enabled should make some problematic lines more stable and less prone to errors. Openreach completed the rollout of retransmission across their Huawei footprint / street cabinets (approx. 2.5m lines) on the 3rd April 2015, although they’ve yet to begin the roll-out for ECI cabs.

Broadly speaking many end-users can expect to see a benefit from G.INP, with most seeing their sync speeds improve and or lower network latency. But unfortunately this was not the same for everybody and some reported a spike in latency times, which was sometimes accompanied by a loss of speed. A few people even had trouble getting a connection after the upgrade was applied.

Part of the issue appeared to stem from consumers who were using VDSL2 modems and or routers that did not appear to include proper support for G.INP, which also impacted a few of Openreach’s ECI modems. In some cases ISPs have been able to work with customers in order to get the firmware update applied, but a few devices may still have problems and not all providers are as pro-active on this as they could be.

But today we have a few bits of new information, which come via way of a communication that Openreach has been sending out to ISPs. The document itself reveals some useful details and we’ve summarised those below.

Openreach G.INP Briefing to ISPs

* Retransmission can operate in both downstream and upstream channels simultaneously, although ECI equipment (either modems or DSLAMS) doesn’t currently support upstream retransmission.

* Any infrastructure that doesn’t support retransmission in the upstream will default to interleaving, increasing upstream latency by approx. 8ms.

g_inp_interleaving_fallback

* We have not yet started the rollout for the ECI infrastructure as we are investigating a minor issue with our hand held testers.

* Retransmission has so far demonstrated an improved performance over interleaving, resulting in a 6 fold improvement in error performance across the network.

* Retransmission has also had a positive impact on those lines supporting TV, with a greater proportion of lines now performing at less than 1 errored second per hour. There has also been a 15% increase in the number of lines able to receive a TV service.

* For the majority of lines, we have noticed a small (1-2Mbit/s) increase in line speeds, as a result of retransmission enabling the DLM to increase the headline rates. … Note that in some instances the available memory in the modem can limit the maximum headline rate, by a few Mbit/s, however this is quite rare.

* When retransmission is active on a line (retransmitting packets) latency naturally increases for a short period of time – but once retransmission stops the latency reduces.

The final part of the document focuses on what Openreach is doing to resolve the problem. “Openreach has recognised that the automatic application of interleaving in the upstream could be causing issues for some customers. We are investigating if we can deploy a change to remove the automatic application of interleaving in the upstream for those lines that do not need it. This will not impact the normal DLM processes, which may still apply retransmission / interleaving if the line requires it as was the case previously,” said the operator.

Apparently an update on their progress is due by the very end of this month.

UPDATE 27th April 2015

A trial to test the change mentioned almost directly above will not now begin for another two weeks.

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By Mark Jackson
Mark is a professional technology writer, IT consultant and computer engineer from Dorset (England), he also founded ISPreview in 1999 and enjoys analysing the latest telecoms and broadband developments. Find me on Twitter, , Facebook and Linkedin.
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