Output Power

Tacitus · Dec 20, 2009, 13:58:21

I've just been checking my Sisters' line stats:

noise margin upstream: 25 db
output power downstream: 5 db
attenuation upstream: 22 db

noise margin downstream: 7 db
output power upstream: 12 db
attenuation downstream: 41 db

Speeds: 5760 kbps / 448 kbps

This looks fine except for the downstream output power which seems rather low. Mine is certainly higher than that.

Has anyone any idea what constitutes 'normal' output power? Given the time of the year I don't want it to be a sign of something failing at the exchange.

Glenn · Dec 20, 2009, 14:05:46

These are mine Tac

Down Up

Current Noise Margin:        13.0 dB        8.0 dB
Current Attenuation:          59.0 dB         31.5 dB
Current Output Power:       17.9 dBm       11.9 dBm

Ray · Dec 20, 2009, 14:22:28

These are mine: -

Output Power (Up/Down) [dBm]:   11.5 / 19.5
Line Attenuation (Up/Down) [dB]:   16.0 / 27.0
SN Margin (Up/Down) [dB]:   13.0 / 9.0

Tacitus · Dec 20, 2009, 14:25:57

Thanks folks

On the basis of those figures, downstream output power of 5dB looks pretty low, which makes me wonder if something is failing at the exchange. Probably best to have a word with support.

Rik · Dec 20, 2009, 15:25:53

Mine are:

Current Connection:
   Current Noise Margin:       8.6 dB       6.2 dB
   Current Attenuation:       62.0 dB       31.8 dB
   Current Output Power:       18.7 dBm       -31.0 dBm

Which makes me think routers may not always report the figure accurately. Have a wrd with support to check, Tac.

Tacitus · Dec 20, 2009, 15:41:38

Quote from: Rik on Dec 20, 2009, 15:25:53
...Which makes me think routers may not always report the figure accurately. Have a wrd with support to check,

I wouldn't dispute that Rik, but I think it has been higher, or possibly I simply haven't noticed - after all it's Sis' line not mine.

It was a firmware upgrade which drew it to my attention when I checked the figures after rebooting. Like an idiot I didn't scrutinise them thoroughly prior to the upgrade......

I'll call support in the morning.

dujas · Dec 20, 2009, 15:42:33

They can lower output power to reduce crosstalk, but I thought they only did that for low attenuation lines, i.e. those syncing at the highest speed possible.

davej99 · Dec 20, 2009, 16:09:58

Transmission powers relate to a nominal 1 milliWatt output.

0dBm is 1mW
3dbm is 2mW
7dbm is 5mW
9dbm is 8mW
10dBm is 10mW
13dBm is 20mW
17dBm is 50mw
19dBm is 80mW

I think the power is automatically minimised to prevent cross talk and yet maintain SNR (not margin), especially on good lines.
A low power ought to indicate a clean or short line. I guess it will track attentuation

Tacitus · Dec 20, 2009, 17:34:38

Quote from: davej99 on Dec 20, 2009, 16:09:58
I think the power is automatically minimised to prevent cross talk and yet maintain SNR (not margin), especially on good lines.
A low power ought to indicate a clean or short line. I guess it will track attentuation

That makes sense and would explain why the power is much higher on my own poor line. TBH it's not something I paid much attention to in the past and, if I hadn't upgraded the firmware on Sis' router I doubt I'd have noticed.

For some reason I get the impression it was higher in the past, so it might be that OR have reduced it to minimise crosstalk. There was a short period where the sync speed dropped from around 55/5600 to around 4300. It's now back up to 5760 albeit on an SNR of 4dB, so it could be the two are connected.

What I didn't want is for it to be a symptom of something failing at the exchange end, since BT will go into hibernation over the holiday....

Incidentally for those who slate AR7 based routers, I've seen this one hang on to a signal at 1/2dB for a fair while.

Rik · Dec 20, 2009, 17:36:15

Netgear, Tac? They were notorious for mis-reporting the noise margin, mine used to hold sync at -2db, which is technically impossible.

Tacitus · Dec 20, 2009, 19:37:00

Quote from: Rik on Dec 20, 2009, 17:36:15
Netgear, Tac? They were notorious for mis-reporting the noise margin, mine used to hold sync at -2db, which is technically impossible.

No it's not a Netgear, but I take your point although I've not seen this one go below 1/2dB. It never got to zero, at least not that I've seen.

Tacitus · Dec 21, 2009, 09:39:24

To conclude this thread, I checked with support who couldn't see a problem at their end. Surprisingly the router reported figures seem pretty accurate when I checked. It's still connected at 5760 with SNR of 8dB.

The connection, currently on Max, is pretty stable having been up for 117 hours so all 'seems' to be well. The exchange is on WBC so an upgrade is possible, although support suggest there would be little if any speed improvement. Given a probable drop in SNR which may result in some instability, I'm inclined to leave things as they are, since the added bandwidth allowances are of little benefit in this case.

Rik · Dec 21, 2009, 09:40:42

If it ain't broke...

Tacitus · Dec 21, 2009, 10:08:02

Quote from: Rik on Dec 21, 2009, 09:40:42
If it ain't broke...

True Rik, but what happens next year? Will Max connections stay as they are, or will they move to ADSL2, with attendant higher frequencies and, be limited to 8meg? Theoretically the end user should see similar speeds, but as with all BT products I'd take that with a large helping of salt....

Rik · Dec 21, 2009, 10:12:03

Ultimately, everyone will be moved to an MSAN, but with the option to retain ADSL1 (Max) modulation. In theory, this should present an indentical service to the user.

Tacitus · Dec 21, 2009, 10:31:36

Quote from: Rik on Dec 21, 2009, 10:12:03
Ultimately, everyone will be moved to an MSAN, but with the option to retain ADSL1 (Max) modulation. In theory, this should present an identical service to the user.

That should produce some interesting product differentiation.

Rik · Dec 21, 2009, 10:32:44

It certainly has that potential, Tac.