Reminder about the reality of DLM on Openreach FTTC
With BT Wholesale ADSL and ADSL2+ services having ran for some years most people have grown to understand how the system manages their connection, but with the FTTC services there has been a lot less information made public, in addition to a lack of control for the broadband provider.
The Openreach Fibre to the Cabinet products use a DLM system that has three main modes, the only control a retail provider has is to pick which of these three profiles your new FTTC connection will run on, or swap you between the various modes.
- Standard – best overall balance between speed and stability for general internet users.
- Stable – prioritise stability over speed for IPTV videoconferencing, home workers and businesses transferring data and IPTV.
- Speed – prioritise speed over stability for online gamers.
In all three modes the system will tune the connection, setting fast or interleaved mode for error correction and once in interleave mode vary the depth of interleaving, the deeper the interleave the more tolerant the line is to errors at the expense of increasing latency and some throughput speed.
Where the most myths occur is the speed with which the DLM system will react to issues and the pattern is something like the following:
- New connections are left wide open so should run at the upper limit that the modem and DSLAM can negotiate.
- On the first day if severe instability (levels governed by which of the three modes the line is running in) then DLM will intervene.
- Normally the DLM waits for a day after provisioning before making any DLM choices, and then only if the modem was trained for at least 15 minutes the previous day so that some data is actually available.
- Aside from adjusting interleaving depths, the DLM can set a maximum and minimum sync rate (where the minimum is usually half the maximum). This is often referred to as banding and the purpose of the minimum is that to stop a line syncing at a stupidly low speed due to a noise burst e.g. thunderstorm i.e. the modem will keep negotiating until the minimum sync can be obtained.
- The upshot is that usually within 48 hours most lines have had their DLM conditions set.
- VERY IMPORTANT: While the DLM is set at an early stage, changes in line conditions will continue to affect the line and potentially cause DLM changes in the future for as long as the FTTC service is active.