Very low SNR and very low line attenuation, what's up with my ADSL?

How can be SNR be so bad if the line attenuation is so low?

I live a couple hundred meters from the local telecommunications cabin.

Notes:

The local telecommunications cabin is not necessarily the location of the DSLAM (where the ADSL line terminates). The DSLAM could well be further away as they are often located in the nearest telephone exchange not the nearest street cabinet.
If this is the case then the calculations I make later for Attenuation need to be modified (the Attenuation should be higher).

SNR is 4.8dB

Your SNR is low. Ideally, this should be 12dB or higher.

The low SNR can be caused by many things, not limited to:

The configuration and line quality of the copper wire pair between the DSLAM and your premises.
Electrical interference from outside sources
- Underground phone lines that have to run under very large buildings and motorways can be affected
- Older phone lines that run along telegraph poles may also be affected.
The configuration and quality of the copper wiring within your premises
- Use a central splitter for optimal performance.
Weather conditions:
- Hot weather can cause a long line to expand.
- Rain on a connection somewhere could cause dampness and deteriorate the signal.

Line attenuation is 9.5 dB

Attenuation also depends on the quality and gauge of cable on your line, but a rough guide is 13.5dB - 14dB of loss per km.

Your attenuation should be around 3dB for the distance according to the above guidelines.

ADSL2 adds 3-4dB
4 Mbps add another ~1dB
That makes a total of ~8dB

In your case (a couple hundred meters) your line attenuation of 9.5dB is about right, assuming you have ADSL2 (remember cable runs may be longer that the direct distance to the DSLAM).

I have normal ADSL, no ADSL2/+

If you don't have ADSL2 then 9.5db is too high. It should be ~4-5dB

Line Speed is 5/6 Mbps downstream

Speedtest average is 5/6 Mbps downstream, against the ADSL limit that is 8 Mbps, and often the line has latency and packet loss peaks.

If you are only a couple of hundred meters from the DSLAM you should be getting around 7Mbps:

A line syncing at 8128 kbps could expect to see a maximum throughput speed of around 7.1Mb (approx 13% for overheads).

Latency and packet loss can increase at times of peak internet usage. Your DSLAM may not have enough peak capacity (oversubscribed).

Conclusion

Given your low SNR, low downstream speed, high latency and packet loss I recommend you call your ISP, open a ticket with them, and ask them to test your line.

What is Attenuation?

Line attenuation is in relation to the "loop loss" on your line. The lower this figure the better, and the better chance you have of getting the faster speeds.

Attenuation is a term used to describe the reduction of the ADSL signal strength that occurs on the copper pair over distance and is measured in dB decibels. The further you are from the exchange, the higher your attenuation figure will be as the signal loss increases.
local loop attenuation loop loss

Attenuation is logarithmic and each 3dB of attenuation halves the strength of the signal power received, therefore a line with 30dB of attenuation only receives 1/1,000th of the power, whilst a 60dB line would only receive 1/1,000,000th.

Attenuation also depends on the quality and gauge of cable on your line, but a rough guide is 13.5dB - 14dB of loss per km.

True line attenuation - or Insertion Loss - can be measured at the DSLAM at the exchange via diagnostic tests and this figure should remain fairly static. Our routers can give us an indication of how much the signal is attenuated as an average against all the frequencies that it uses.

Because the router measures against the frequencies available, some users may notice a very slight increase in attenuation if say moving from a fixed rate 512 kb connection up to 2Mb. Higher frequencies such as those used to transmit faster speeds are more likely to be attenuated (higher frequencies = higher attenuation). ADSL 2+ has an increased frequency spectrum, therefore an increased attenuation of around 3-4dB is not unusual. As a very rough guide a speed increase of 4Mb is said to increase the attenuation by 1dB.

It is also important to note that different routers may load the frequency bins in slightly different ways, and on top of that some routers report the average across the frequencies actually in use, whilst some may report the average across the frequencies available - which is why sometimes using a different router may report a slightly different attenuation figures.

To confuse matters further, there are a few routers which record the insertion loss measured at 300 kHz as being the downstream attenuation figure. Obviously those routers are unlikely to show attenuation changes as the speed range increases. The reason why, is that insertion loss or attenuation measured at 300kHz can be used as a very good guide to calculate the loop length and line capacity.

What is SNR?

SNR is Signal to Noise Ratio - in other words a measurement in decibels of the Signal strength to the level of Noise on the line.

The higher your SNR is, the better, as there is less background Noise.

SNR fluctuates on all lines throughout the course of the day by various amounts. Signal strength is usually best during the early part of the day. During the evenings "Noise" often occurs due to more people being at home and turning on electrical equipment etc. Electromagnetic Noise can be introduced on your line from various sources around your home and neighbouring area. The list of sources for noise is practically endless, but common culprits could be TVs, PCs, lighting, heating, pumps etc. There's also something called "cross talk" which is a small amount interference picked up on your line from other users ADSL connections between the home and the exchange.

SNR can vary on an hour to hour basis, not only is it affected by the speed of your connection but there are other things may reduce your SNR. Hot weather can cause a long line to expand. Wet weather on a connection somewhere could cause dampness and deteriorate the signal. Even street lighting and flashing Xmas tree lights have been known to affect SNR.

Source How to interpret your ADSL Line Stats