24 March 2008

Stealth upgrading

Did I mention that the 2006 attempt to upgrade my broadband speed from 1Mb to 2Mb failed? The line was dropping too often so Silent Fred, the BT engineer who doesn't tell you anything, reset it back to 1MB.

Easter Sunday, with no warning, my connection went up to 2Mb again and this time it's holding steady without a single dropout. I can only suppose there's been an upgrade somewhere along the line.

It took me a long hunt on the Internet to find a webpage that explained with numbers how to check your own connection, so let me pass this on:

The two parameters that matter are Signal to Noise ratio (S/N, Margin) and Drop off (Attenuation). Both are measured in decibels (Db).
S/N is the difference in strength between the signal and the background hiss. It needs to be >10Db. Attenuation is the difference between the signal strength when it leaves the Exchange and when it reaches your modem, and the higher the speed the lower the limit for this. 1Mb requires lower than 60Db, 2Mb lower than 45Db.

For the copper wire telephone lines we have in the UK, this equates to ~6km maximum for 1Mb and ~3.5km for 2Mb.

My Attenuation is a steady 40Mb so 2Mb is the most I can get. My S/N is 24Db at 1Mb but 17Db at 2Mb, still OK though it can fall to 10Db on a bad day. I suspect that it is S/N discrimination that has improved for me.

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