Tweaking ADSL2+ performance on Billion BiPAC 5200S ADSL router, via telnet CLI
(same chipset as ZyXel P660R-D1?, Speed-Modem 50B?)

Telnet to the router's IP address to access the CLI.

View relevant line data:

router> show wan adsl chandata
near-end interleaved channel bit rate: 0 kbps
near-end fast channel bit rate: 2062 kbps
far-end interleaved channel bit rate: 0 kbps
far-end fast channel bit rate: 903 kbps

router> show wan adsl linedata near
relative capacity occupation: 100% 
noise margin downstream: 17.5 db
output power upstream: 12.3 dbm
attenuation downstream: 44.0 db


MANUALLY ADJUSTING LINE RATE FOR ADSL2+

The default setting is normally 0 0

Check your existing setting before making a change:
router> wan dmt2 set snrmoffset -1792 -1792

Subtract 512 from both values to reduce SNR margin by 1 dB:
reduces speed, increases stability

Add 512 to both values to reduce SNR margin by 1 dB:
increases speed, reduces stability

Example:
Auto-negotiated SNR margin was 17.5 dB
Reducing margin by 14 dB should increase speed:  -1792 + (14 * 512) = 5376

router> wan dmt2 set snrmoffset 5376 5376
router> wan adsl reset

After re-sync (1-2 mins), view line data again to compare:

router> show wan adsl chandata
near-end interleaved channel bit rate: 0 kbps
near-end fast channel bit rate: 5017 kbps
far-end interleaved channel bit rate: 0 kbps
far-end fast channel bit rate: 911 kbps

router> show wan adsl linedata near
relative capacity occupation: 100% 
noise margin downstream: 5.4 db
output power upstream: 12.3 dbm
attenuation downstream: 44.0 db

Noise margin actually reduced by about 12.1 dB to 5.4 dB;
downstream line rate is now 5017 kbps! (up from 2062 kbps)

If connection fails to resynchronise, retry with a setting that allows for a
larger noise margin (usually it seems at least 3 dB is needed for day/night
atomospheric variation of radio interference in the phone line;  preferably
6 dB, or even more if strong local sources of interference are causing frequent
resyncs.

Use 'attentuation downstream' figure plus the chosen target SNR margin to
estimate theoretical maximum rate for a line in perfect condition:
* http://www.kitz.co.uk/adsl/max_speed_calc.php

eg. 44.0 dB (from data above) + 6 dB (my chosen target SNR) = 50.0 dB;
estimates 5152 kbps as the best achievable rate.

To make changes persistent, the CLI contains a basic text editor for
autoexec.net which contains CLI commands that are run automatically after a full
reboot of the modem:

router> sys edit autoexec.net
EDIT cmd: q(uit) x(save & exit) i(nsert after) d(elete) r(eplace) n(ext)