Tuesday, August 19 2008, 2:04 AM

I'm really tired of needing to apologize to Comcast. They're evil. I know this. But I'm really getting annoyed with having what I perceive to be their problems actually be problems on my end.

I've been having a lot of problems with random cable modem disconnects due to lack of signal over the past few weeks. I know this because I'm a dork and wrote a script that plugs into my Cacti installation and logs the signal levels displayed in my cable modem's admin interface. I did this years ago because - you guessed it - Comcast is evil and their stuff cut out on me enough for me to want to be able to monitor it. There has been a steady and constant trend of ever-dropping signal levels that has made itself apparent since the beginning of the year. I've had the same general layout in the apartment in terms of devices and number signal paths, so I could only assume that the problem was upstream on Comcast's side. I was so confident that this was yet another Comcast screw-up that I ordered DSL.

Well, it wasn't.

I had a service appointment scheduled with Comcast on Friday. The guy did some tests outside where my line plugs in, and he said he noticed a lot of signal "reflection", and said that my splitters and/or wiring were probably bad. He looked around, and also noticed that I was running my cable through the Monster way-too-much-money power strip, and called that a no-no as well. He replaced three splitters with two nicer ones, and ran custom length cables to each one of my devices. The result?

As you can see, the signal levels improved dramatically. The downstream power, which is what I receive from Comcast, is up by at least 10dBmV (depending on the time of day). 10dB corresponds to a ten-fold signal power increase. Signal-to-noise ratio is also better than it was, but not by as striking of an amount.

What's are the lessons here? Use good splitters. Use good cable. Don't assume Comcast is at fault because they're evil.

EDIT: My dad has corrected me. The 10dB increase probably only corresponds to around a 3.1 fold increase in power. I think I read the wikipedia chart wrong.

comcast frustrations

Wednesday, June 11 2008, 12:51 AM

Even though they're an over-priced pseudo-monopoly with a track record of shitty customer service and only somewhat better service uptime, I owe Comcast an apology. My internet was down for most of the past weekend, and for most of Monday as well. I figured it was due to the storms that rolled through on the night of the outage (Saturday), but after a couple of days with no service, I started getting mad. I was cursing their name and anything related to them. I was particularly unhappy when I found out that a coworker that lives in my apartment complex had no interruptions in service. After I heard that, I started thinking of ways that my setup would be sabotaging the process.

And then it hit me. As part of my process to convert my firewall machine into a Xen instance, I altered the physical networking layout so my cable modem would plug directly into my "Core Switch", an old Cisco 2924XL. I gave the cable modem service its own VLAN, which would be accessable via my firewall instance running on a Xen machine. What I failed to consider is that managed switches tend to have features that allow for communication with other switches in order to facilitate ease of management and network health. This communication is typically broadcasted to any device that is listening on regular intervals.

These broadcasts are what caused my issue. In a normal residential cable modem service (with Comcast at least), the cable modem latches on to the first network device it hears traffic from, and assumes that it will be the one it deals with when connecting to the internet. By having my cable modem plugged directly into the switch, it was receiving the switch's broadcast messages before my firewall instance had a chance to make itself heard. Because of this, my firewall's attempts to connect to the internet fell on deaf electronic ears.

This was remedied easily enough by disabling spanning tree protocol on the VLAN that my cable modem connects to, and disabling Cisco Discovery Protocol broadcasts on the port it connects to. I don't like disabling spanning tree, because quite frankly, network loops suck. The chance that somehow make a loop in that VLAN is pretty damn low though, so there's not much to worry about.

Let this be a lesson to those with way too much time on their hands, like myself.

cisco comcast frustrations linux networking stp vlan xen

Saturday, May 10 2003, 4:02 PM

Well, after a week of around 80% cable modem down time, I was ready to bite the head off of anyone who uttered the word Comcast. They had a area wide outage over the past weekend according ot their tech support line, but two days later, tech support didn't seem to know about it. The cable modem kept going up and down, being down much more than up, and we called back ready to bitch. They scheduled a repair guy to come out today. I didn't think our line had anything to do with it, but I was wrong. Turns out our we were losing at least 90% of the signal in the 100 or so feet of cable between the pole and the modem. The repair man replaced the cable from the pole to the house, and the signal was a ton better. I'm going to wait a while to make sure it's still good, and if it is, I might have to repent a bit on the hateful statements I've made about them. :)

Nothing much else is going on... my car hit 10,000 miles on friday... I really need to move to Lansing!

comcast

Sunday, April 13 2003, 9:58 PM

Grrr... The cable modem has been down for a solid two days now. It would seem that it isn't Comcast's fault though, since there was an Ameritech guy working on the pole friday, and the cable modem went down shortly after that. Still, it's a pain in my ass.

comcast frustrations

Monday, January 13 2003, 9:24 PM

Yah... my cable modem is down again... Comcast can suck the fat one!

comcast frustrations

Sunday, December 08 2002, 10:24 PM

Grr... Stupid Comcast... they hike our prices $15 a month, and then we get an entire day with no cable modem service.

comcast frustrations

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