Monday, April 22, 2013

Fixing the cable

So a few months ago I switched from ADSL to cable. The installation didn't really go smoothly as everyone seems to have forgotten about an extra splitter in the attic which was left in the path of the modem.. D'OH!
The tech was already running late on our install and was forced to take half-measures. Because the modem was seeing crappy signal levels (and we didn't know about the splitter at the time) I believe he adjusted the "street amplifier" so we get a stronger signal. This still only kicked the modem into the far edge of their recommended levels however, but off he went to the next job saying that'll do.. and it did for most of the winter the modem was hovering around -5dBmV receive and 53dBmV transmit but as temperatures rose so did signal levels decline and on the first modem desync I was looking at -9/58 which is very far from ideal. 60dBmV is the maximum transmit power the modem can do AFAIK, so it took almost 100% capacity for the outgoing signal to get through

I was looking at the modem's customer-accessible network information page throughout the months and eventually coded a quick php script that Cosm, an online data gathering and graphing system can poll.

The Cosm feed for my modem data can be found here (like removed, see below)

Well after the first desync I realized I had to do something. Getting up to that particular attic section was something I really didn't want to do but the alternative was just way too invasive. What I found up there explained the issues perfectly. A 20 year old coaxial tap (amazingly with F connectors, but they were extremely weathered..) in a small puddle of water. I have absolutely no idea how that happened. There was no water anywhere else except around the input of that tap. Murphy? Or maybe water was coming in through the coax but hopefully that's not the case..
The cable that the modem actually got the signal through was also very low quality and mangled in several places. It looked like something was chewing on it.
There was also an ancient non-F splitter that supplied 3 TVs in the back of the house.

I threw out everything I could see up there, installed new F connectors, relocated the cable modem tap so it connects directly to the incoming cable and installed a high quality splitter for the rest of the house.

The effect from the modem's perspective:


Cosm graphs

From the low end of the threshold to the high end in 123514 easy steps. The signal level might be a teensy bit high now because of the adjustment they made initially but if that causes any problems I'll just have to call and get them to send a tech out to adjust the street amp back to the default level.

I also discovered self amalgamating tape. Specifically that I had some in my drawer for years not knowing what it was. (Don't laugh..) In case you're like me and this is the first time you hear of it: it's great stuff. Also called self-vulcanizing tape or self-fusing tape, you wrap it on something (a cable connection for example) like a bandage overlapping layers of itself while stretching it out and it'll fuse together into a watertight insulating rubber sleeve after a while.

I didn't want to take any chances so I used it on all primary connections.


Attic-shot

As a bonus of the rewiring the in-house network is now perfectly symmetrical.. which I've read is also not a bad thing.

UPDATE: Yes, sadly Cosm had turned into Xively or whatever... Since my data upload was based on Cosm's "Pull" feature and the new thing didn't retain that (while also being severely deficient at presenting data at least at launch) I didn't bother updating my script. Oh well... Good luck at their attempt at monetizing the service I guess.

UPDATE #2: Now pushing data to data.sparkfun.com instead and using imp.guru to plot. You can check it out here