Questions about BT Openreach's implementation of G.INP Retransmission.
| A short while ago I was fortunate enough to speak in person with Ian Lawrence who is Chief Engineer at BT Openreach and their Director of Network Strategy & Engineering. Our chat covered various topics, but because G.INP was ’the’ red-hot topic at the time I asked if he would be good enough to answer some of the more common questions about G.INP/ Retransmission posed by members of the kitz community.
Below are some of those questions asked and Ian's answers.
- Is G.INP being permanently removed from the upstream?
DLM will continue to operate in its usual manner. Retransmission will operate in the downstream on any modem that supports retransmission (which should be all of them) and will operate in the upstream for those modems that support it. In addition for modems that do not support retransmission in the upstream, we will not be automatically applying interleaving in the upstream.
- Will MK2 G.INP Upstream lines be moved onto a G.INP profile when the errored seconds have gone above the DLM threshold if using a Modem that is capable of both G.INP US and DS on the Huawei cabinet ?
What are the configurable options for the DLM and what order of preference does the DLM apply them in?
Initially, low interleaving is enabled by default on all retransmission lines. Once DLM has positively identified that the modem supports retransmission then retransmission is enabled. This happens after a few days. If the low level of retransmission is not good enough to correct all errors then the high level is selected by DLM. If the line remains unstable and cannot be adequately managed by DLM with retransmission profile then Interleaving is applied as normal.
Initially, fast mode is applied by default on all lines. 90% of lines stay in this mode and never move. If the line is experiencing trouble in the upstream then DLM will enable retransmission at the low level. If retransmission is no longer needed (because the noise has gone away for example) then fast mode can again be selected by DLM.
If a line is reset then it will initially move back to interleaved downstream and fast mode upstream. Retransmission will be applied again after a few days.
- We have been seeing all users on Huawei cabs using ECI modems and the BT HomeHub 5A losing anything up to 10Mbps of downstream speed and experiencing large amounts of latency. The affected equipment is BT Openreach/BT Retail supplied that should surely comply with SIN498. Yet these are the ones seeing the most problems due to automatic DLM intervention -regardless if the line needed it or not - by application of Error Correction (RS overheads cause reduction in sync) and Interleaving (causes delay). The increased delay through application of Interleaving of 8ms may be the intention but we are often seeing more like 16ms added due to application to both upstream and downstream. What is happening?
As you note above, to comply with SIN 498, modems and routers must support retransmission in the downstream, but it is not a mandatory requirement in the upstream. Taking this into account and to maintain a high level of service, when we first rolled out retransmission to our Huawei estate we introduced interleaving on all lines where the modem/routers did not support retransmission in the upstream.
We found that in some occasions this introduced increased latency of approx. 8ms, and could also impact some downstream headline rates for some modem/routers.
As soon as we realised that for some modem/routers that don’t support retransmission in the upstream, that the introduction of interleaving in the upstream was causing issues, we set about trialling, and subsequently rolling out a reduced interleaving profile in the upstream. This trial has now completed, and all lines have returned to their normal pre-retransmission interleaved state.
- We have observed that G.INP does not appear to have been enabled on any new Huawei cabinets enabled since the beginning of April. Also we have seen several lines that have had a DLM reset having G.INP removed from the DLM profile. (Despite the fact that the line may previously have had retransmission).
Whilst we were rolling out the retransmission profile, there were occasions where a DLM reset would reset the profile, and the modem/router would in effect go to the back of the queue to have the profile reapplied. This was only temporary and all lines on our Huawei estate have the retransmission profiles applied.
- What is happening with end users who don’t have any form of G.INP on their service? ie once the issue with the GINP MK1 was noticed, no lines got it?
Any new and refreshed lines are being moved over to the Retransmission policy each night and we will continue to trawl our Huawei estate each month to move new adds/DLM resets onto the new profiles ad infinitum.
- Is the current fix (for upstream) perceived as a workaround, with a more complete solution due later? If so, any idea what might change?
As mentioned before this is not a fix but a permanent solution.
- Will end users supplied with BT ECI equipment be offered replacement modems?
Following the removal of the automatic application of interleaving in the upstream for any modems that do not support it, there is no requirement for any infrastructure to be replaced.
- How has the roll out progressed so far and what % of lines is left to do?
We have now completed our rollout to our Huawei estate.
- Are there any ECI cabs which have retransmission ?
Apart from some small trials, retransmission is not yet active on ECI cabinets.
- When will retransmission be rolled out to the ECI cabs?
The issue here is that some of our field test equipment requires new firmware to work with retransmission across our ECI estate. As soon as we are comfortable with the stability of this firmware, we will begin the rollout of retransmission across the ECI DSLAMs.
As soon as we have an update on our plans for the ECI rollout we will let you and the industry know.
- There has been some confusion with regards to a previous statement “ECI equipment (either modems or DSLAMS) doesn’t currently support upstream retransmission”.
Does this mean that the ECI DSLAMs don’t have sufficient hardware resources to fully support G.INP in both directions?
To clarify, the ECI Infrastructure supports retransmission in the downstream direction only. We have completed some small trials, but at present retransmission is not active on ECI cabinets.
- G.INP was reportedly tested alongside SRA. Are there any plans to deploy SRA?
To my knowledge Openreach have not tested SRA.
- What is happening with ECI cabs re vectoring support? Will they be replaced with Huawei cabinets? Will there be a two tiered fibre network? Those on Huawei and the unfortunate on ECI cabinets?
Openreach are continuing to assess the benefits of Vectoring in its network.
I’d like to thank Ian for taking the time to answer our questions in print. These should hopefully help the DSL broadband community further understand BT Openreach’s implementation of Retransmission.
©kitz Aug 2015