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One of the problems with Comcast’s new 250 GB bandwidth cap is that, as Om points out on GigaOM, it’s metered without a meter. Comcast doesn’t provide you with a central tally of all your data use. The company instead suggests its customers install bandwidth metering software on their machines and then add up the numbers. Its FAQ reads: “Customers using multiple PCs should just be aware that they will need to measure and combine their total monthly usage in order to identify the data usage for their entire account.” Got multiple home machines consuming data every day? Better bust out that spreadsheet — and get ready for some wild guesstimates. After all, you can’t just install a bandwidth metering application on your Slingbox.

The Slingbox is only one example of why the absence of a central bandwidth meter for your account is not inconvenient, but a central flaw in Comcast’s cap. More and more devices are bringing video to the living room, in turn consuming huge amounts of bandwidth. Most of them are not computers, but home entertainment devices with simplified interfaces that don’t burden their users with complicated stats and settings. That makes for a good user experience — unless you’re a Comcast customer that’s already using a lot of data and the box in your living room is busting your bandwidth-capped behind.

Here are five innocent-looking devices that could get you in trouble with Comcast:

The Slingbox: This shouldn’t clock too much bandwidth — unless you’re, say, a sports junkie stuck in a cubicle, trying watch as many NFL games as possible while your boss is out of the office. The bandwidth consumption of your Slingbox largely depends on your ISP’s upstream capability. Many users access their video with 400-500 kbps, which comes to about a Gigabyte for 5 or 6 hours, but the recently unvieled Slingbox HD has no problems eating up 2 Mbps, or about 900 Megabyte per hour. Americans watch about 8 hours of TV a day, so you could theoretically rack up to 212 gigs per month if you put the HD Slingbox to full use.

The XBox 360: Microsoft’s gaming console features HD downloads straight to the living room. Movies are around 5 GB a pop; TV shows, 2 GB. That means that getting addicted to Battlestar Galactica and downloading all four available seasons for a multiweekend marathon eats up about half of your monthly Comcast bandwidth.

Your good old TiVo: Broadband-connected TiVo machines download their EPG data through your Comcast line, but can also be used for Amazon Unbox downloads. The typical Unbox movie is about 2-3 gigs. Then there free are podcasts, which could add up to a few gigs per week. Oh, and did I mention you can access Last.fm or Real Rhapsody through your TiVo as well? Five hours of music per day eat up about 9 gigs per month.

The Netflix Roku: This little box streams Netflix movies straight to your TV, at up to 2.2 Mbps. A Roku box gives you “unlimited access” to a fairly limited catalog of 12,000 titles, so you probably won’t spend all your quality tube time with this device. Still, just watching a single movie per day with your Roku uses up more than 40 GB per month.

Vudu’s set-top box: This one may not be around for much longer, but it’s still worth mentioning because it uses P2P distribution to deliver content, which means more bits on your bill. The company’s web site isn’t too forthcoming about the bandwidth use of its product, and in fact hardly mentions the uploading activity at all, but users of the official Vudu forum report that the box utilizes between 200 and 300 Kbps upload capacity for serving content to others. This most likely won’t happen all day long, but one streamed movie and 5 hours of P2P content serving per day still could cause up to 60 GB of traffic per month.

To be sure, most of these devices aren’t using up those 250 GB on their own. But they’re consuming enough bandwidth to get you in trouble, and even get your contract with Comcast terminated, especially if you’re already backing up lots of data to Mozy while surfing YouTube and listening to Internet radio all day.