ITU G.992.5 vs ITU G.992.1
Kapil Kapre | Monday, March 22, 2010

Surprisingly, in the fifteen or so years I've been on the intertubes (remember FidoNet?!), I haven't ever been a subscriber of a DSL based ISP until recently. The progression of ISP tech has been roughly - Dial UP, LAN, Cable, LAN, Cable, MOTOwi4, Cable, DSL. Because of this I was fairly excited about getting the DSL connection. The first thing I did was to try out all the different DMT modulation settings and find the most reliable one and the one with the highest throughput. ITU G.992.5 satisfied both of those criteria.

As I was reading up on G.992.1 it seemed like it should win over G.992.5 on reliability as it can sustain a higher line rate over correspondingly high line attenuation. Ofcource, this kind of petty evidence isn't enough so I sampled the DSL noise margin for both modulations over the course of a day each amounting to roughly two thousand samples. Here is the graph for 992.1 followed by 992.5.

GMT NM Graph
Arithmetic Mean : ~12.823
Standard Deviation: ~0.068

ADSL2+ NM Graph
Arithmetic Mean : ~23.45
Standard Deviation: ~1.0733

We now have some empirical evidence to guide us. Looking at the graphs, 992.1 does seem much more stable as compared to 992.5. However, as a general rule, a higher noise margin is always preferable to a more stable one (Yeah, I just made that up right now). Now, on the actual values of the noise margin, this is what the wikipedia hive-mind has to say.

Noise Margin Range Effect
6dB or below is bad and will experience no sync or intermittent sync problems
7db - 10dB is fair but does not leave much room for variancies in conditions
11dB - 20dB is good with no synch problems
20dB - 28dB is excellent
29dB or above is outstanding

Now, ofcource there are tons of other factors that are possibly affecting my connection, namely line attenuation between my modem and the DSLAM, DSL Occupancy, and a bunch of other stuff I have no clue about. However, given that I have fewer connection issues with ADSL2+, I'm going to go with that for now. What a terribly unscientific way to make a decision ! :)