3/6 10:53pm Well, after a week of calls to Verizon tech support, pondering my options, and one small technical adjustment, our DSL situation seems to be finally resolved for the better - it works. We've had a stable and silent connection for three days now.

Yes, it was a hardware problem. A rather simple problem, and one that the phone company is supposed to handle. But no, Verizon tech support has very little ability to diagnose problems. They're courteous and friendly, and do try to be helpful, just don't recognize obvious problems when described to them.

I was finally pointed in the right direction by an eBay seller, who commented that an audible DSL modem is most likely a "backline connection" problem: a corroded connection somewhere between the house and the phone company exchange. So I checked the only connection accessible to me, the one on the corner of the house, and sure enough, the wire leads were green and gray. I scraped them coppery clean with the screwdriver blade, tightened the connection, and all symptoms - scratchy line, dropped DSL connections, and hiss on the line - have disappeared.

Now that I have both Comcast and Verizon internet service working, here is how I perceive the difference between the two:

Transfer speed.
The rated download/upload speeds of Comcast and Verizon are 400/40 and 200/45 kilobytes (KB) per second. Supposedly upload speeds are roughly equal, and Comcast is twice as fast at downloads.

On downloads, both providers deliver on their promise. Comcast has just recently increased the download speed to 550 KB/sec, and it is really neat to watch a large download stream in at a megabyte every two seconds. In practice, however, the experience is not very common. Many (if not most) web servers do not deliver data anywhere near the connection speed - and often at no more than 65 KB/sec. This lower limit is well below the capacity of either provider.

On uploads, however, Comcast averaged for me between 11 and 13 KB/sec, much worse than the expected 40 KB/sec. Verizon delivers as advertised, and I measured 47 KB/sec when uploading large files. This may be due to the difference in the two implementations (the cable that Comcast uses to deliver the internet connection is shared by all users on the block. Each house, on the other hand, has its own non-shared wire all the way to the telephone exchange, so the individual rate limits combine only when they are merged onto the phone company's own internet connection).

Name servers.
For most web sites, the Verizon name servers look up the address faster than Comcast. This results in the subtle impression that pages load faster over DSL than cable. (The Verizon name servers honor lookups from any host on the internet, so one can use them even without having Verizon internet service; Comcast restricts access to Comcast customers only.)

Ping times (round-trip latency of a small message between PC and server), on the other hand, is lower over cable than on DSL. I measured 36.6 milliseconds to norfolknet.com using Verizon, and 26.0 with Comcast. In practive, the absolute difference may be too small to be noticeable.

E-mail.
This subtle difference only affects Linux/BSD users who want to use the provider's mail server to relay messages. Comcast is very straightforward to use, it authenticates based on the IP address of the sender - all Comcast customers are automatically permitted use of the anonymous mail relay, which is very convenient. Verizon's mail server uses the AUTH protocol, which can require a custom mail client configuration. This difference should not be visible from within most desktop e-mail readers, however.

IP address and visibility.
Whenever a home PC needs an IP address, Comcast tries to assign the same internet address as before. (The mapping is based on the MAC address of the ethernet adapter connected to the cable modem.) Verizon assigns the next unsed address, which is most often a different one than the last time. Each method has its own subtle advantages and disadvantages. With a near-permanent address one can trivially make files available to friends. However, surfing behavior patterns can be more easily tracked with a permanent IP addresses (but then again, are probably tracked with cookies). A variety of addresses help maintain anonymity while browsing, but are not as suited to having a permanent presence on the web (though dynamic dns can be a solution).

The Verizon routers block traceroute probes in both directions. This protects the provider's network and the home computers behind it, but it also prevents certain diagnostic tools from working from a home workstation.

Home