Troubleshooter's Guide - Frequent disconnections in MTNL Broadband

When I learnt last month that I am eligible for one of MTNL's 8 mbps plans, I upgraded with lots of excitement. But the last 30 days have been a nightmare.

The connection goes down literally every 4-5 minutes, and comes back up on its own after a few minutes. These casual brief disconnections play havoc with the usability of the internet, especially with large downloads. I could literally do nothing on my 8 mbps broadband (yes, I was getting that speed till the modem, despite MTNL's initial apprehension that I could not get it, being more than 1 km away from the exchange).

I did some research and figured out two things

1) I have a noisy line (literally, it makes a lot of noise)
2) The broadband line drops are related to a low SNR/ noise margin. SNR (Signal to Noise Ratio) compares the level of the signal with the amount of inteference. A higher value is desirable. A lower SNR means a more unstable line. There is also a relation between line speed and SNR. At a lower speed, the SNR level is usually higher. You need to find the optimum balance between speed and SNR.

Both the above values are also related to the distance from the exchange. The closer you are, the better the quality of your line. 

I monitored the SNR/ Noise Margin value for a few days using the excellent Router Stats utility. Then I figured out the danger zone for SNR. Every time the SNR dropped below 6, my broadband would slow down, and when it dropped below 1, the line would disconnect. 

High fluctuation in SNR/ Noise Margin, and frequent drops below 6 dB. Using : Router Stats Lite

So I visited the MTNL office and spoke to one of the officers in charge (go with some time in hand for this!). I requested him to get my line checked - especially the 'joints'. And secondly, I asked him to get the speed reduced/ capped at around 5 mbps. I have not yet gone back to a 4 mbps plan. For this month, I am testing out if I can get a stable connection at this speed or not. I am willing to pay for 8 mbps, even if I get a slight increase in speed.I have the luxury of this option because MTNL's price differential between a 4 mbps and 8 mbps plan is very small. Currently the 4 Mbps plan with 50 GB download limit is Rs. 1250 and the 8 Mbps plan is Rs. 1350.

While I was writing this blog post, the powers-that-be decided to drop my connection speed below 1 Mbps and look at the immediate improvement in SNR values:

When speed dropped, Noise Margin improved immediately

Of course, the ideal scenario would be, a high SNR with a high speed, but as I said earlier, you have to find the sweet spot for your connection, the lowest possible SNR you can live with and the speed you get at that SNR. For this, you need to monitor the connection for a little time and test at different speeds.

This whole episode re-inforced an important lesson for me. Speed is not everything. A stable line is more useable and valuable than a fast line. Without stability, speed is nothing.

Update : 29/03/2015 : My speed is still stuck at less than 1 mbps and the connection is rock solid. It has not dropped even once. I think I am going to be forced to trade off the higher speed, and stay with something below 4 mbps.

These are the results of my test:
  • At 1 mbps, there has been no disconnection since last 24 hours and noise margin is 24-plus
  • At 4 mbps, there are disconnections at least every 2 hours and noise margin is between 0 and 14
  • At 8 mbps, internet is not useable, and disconnects every 5 min. noise margin keeps dropping as I have shown in my graph.
My conclusion is that I would probably be better off with a 2 Mbps connection (sigh) but if MTNL manages to improve my line quality, maybe I can live with 4 mbps.

Edit (May, 2015) : After repeated complaints, and with the assistance of an extremely co-operative manager at my local exchange, the cabling to my house was replaced by MTNL. I replaced the last mile internal wire in my flat at my own cost. Now, at 4 Mbps, I have  a rock solid connection since the past month. Not a single drop!


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