O2 Broadband Connection

Some stats and a few theories.

Since I had my O2 broadband activated I’ve been carefully monitoring the bandwidth and S/N margins. S/N margins are definitely better during the day and then go down to around 6.5dB at night. The modem/router box will re-train to a lower speed if conditions deteriorate but does not appear to re-train back up to a higher speed if they improve.

ADSL2+ uses frequencies up to 2.2MHz to transmit the signal down the phone lines. This part of the radio spectrum is subject to the effects of the ionosphere which means that radio signals will travel longer distances after dark due to reflections from the E-layer and F-layer. During the daytime, the signals are absorbed by the D-layer and very little reflection takes place. If you want a practical demonstration of this, tune around on your medium wave (AM) radio during the day and note what station you can here. Repeat the exercise after dark and you will find you can hear stations from a lot further away. Signals will also be stronger and subject to fading and distortion effects.

So how does this all affect ADSL? Well in my case, overhead telephone lines will behave like a radio aerial and these signals will be passed to the modem and appear as an increase in the noise floor. Although the lines present a balanced connection and this should be rejected, this balance will not be perfect and external signals will still be present as common-mode interference and cause a deterioration in the ADSL signal.

Room for improvement? What I need to investigate is if it is possible to improve this common-mode rejection, by trying different filters and faceplate splitters.

29-20-08 11:42 Before reboot.
Note that the downstream S/N Margin is 10.5dB so there should be room for improvement in the downstream bandwidth, 7789.

Uptime: 1 day, 14:33:38

DSL Type: G.992.3 annex A

Bandwidth (Up/Down) [kbps/kbps]: 1,282 / 7,789

Data Transferred (Sent/Received) [kB/kB]: 0.00 / 0.00

Output Power (Up/Down) [dBm]: 12.0 / 19.0

Line Attenuation (Up/Down) [dB]: 24.5 / 40.5

SN Margin (Up/Down) [dB]: 7.5 / 10.5

Vendor ID (Local/Remote): TMMB / BDCM

Loss of Framing (Local/Remote): 0 / 0

Loss of Signal (Local/Remote): 0 / 0

Loss of Power (Local/Remote): 0 / 0

Loss of Link (Remote): 0

Error Seconds (Local/Remote): 116 / 0

FEC Errors (Up/Down): 729 / 990,518

CRC Errors (Up/Down): 729 / 20

HEC Errors (Up/Down): 1,821 / 9

29-10-08 11:48. After reboot.

Downstream bandwidth has increased to 9177

Uptime: 0 days, 0:00:17

DSL Type: G.992.5 annex A

Bandwidth (Up/Down) [kbps/kbps]: 1,274 / 9,177

Data Transferred (Sent/Received) [kB/kB]: 0.00 / 0.00

Output Power (Up/Down) [dBm]: 12.0 / 19.0

Line Attenuation (Up/Down) [dB]: 24.5 / 43.5

SN Margin (Up/Down) [dB]: 7.0 / 7.0

Vendor ID (Local/Remote): TMMB / BDCM

Loss of Framing (Local/Remote): 0 / 0

Loss of Signal (Local/Remote): 0 / 0

Loss of Power (Local/Remote): 0 / 0

Loss of Link (Remote): 0

Error Seconds (Local/Remote): 0 / 0

FEC Errors (Up/Down): 0 / 0

CRC Errors (Up/Down): 0 / 0

HEC Errors (Up/Down): 1,826 / 0

So now we wait until after dark and repeat the stats check, looking for signs of a re-train/reboot.

Leave a Reply

Your email address will not be published. Required fields are marked *