The Big Bandwidth Misconception

Over on Bram Cohen’s blog (of BitTorrent fame), Mark Cuban made a comment that reminded me of a pretty common misconception about the ISP business. Mark said, in reference to Bram and attempting to make money from BitTorrent:

Unfortunately for you,ISPs crack down on heavy bandwidth users, particularly uploaders and enforce their TOS.

By definition, seeders create upstream bandwidth. The ISPs dont want to see more upstream usage Bram, i know its a tough concept for you, but in the mind of the ISP, upstream use = bad. MOre upstream b/w use = more bad. Which in turn pushes them not to increase the bandwidth available to end users, but to evaluate where the upstream use is coming from and look at shutting it off and throttling it. Call me crazy, but that equates to a challenge for the BT universe.

This couldn’t be further from the truth. The upstream bandwidth is not a concern for most ISPs, especially your standard Cable or DSL provider. Broadband ISPs have to order bandwidth synchronously, in various speed increments, because all high capacity circuits come in synchronous form. This means if you order a Gigabit Ethernet connection to an upstream provider, you’re actually buying 2 Gigabits per second worth of bandwidth, one from your provider and another towards your provider. Broadband ISPs sell bandwidth asynchoronously (the A in ADSL), which means they’re selling you something like 6 megabits down and 784k or less upstream. Obviously, it’s well known that ISPs oversubscribe their upstream links which is why peak times can see serious difficulties achieving the full downstream. However, no matter how you do the math of an ISP which serves mostly bandwidth consumers, this still leaves large amounts of bandwidth on the upstream on the ISPs egress connections. In fact, most Broadband ISPs don’t provision large WANs to haul traffic back to central egress points, because it makes more sense to dump the traffic off at a peering point in the market, assuming you’re in a large metro where you can cheap bandwidth. Because ISPs providing broadband to homes and businesses also have such a one way usage pattern, it means that they cannot negotiate peering arrangements with other ISPs for an even trade of bandwidth, meaning they’re always going to be paying for that bandwidth.

In the business, this is generally called Tier 2 bandwidth. Broadband ISPs will sell large hicap circuits to businesses largely interested in serving content much cheaper than you can buy Tier 1 bandwidth from a major internet provider (something like UUNet, etc). This also becomes very evident if you attempt to buy bandwidth from a Tier 1 provider and a Tier 2 or Tier 3 provider, because it becomes very difficult to load balance your traffic over to the Tier 2 providers because of the way BGP routing works. There are methods for overcoming this (AS padding), but it still never works out quite the way you want.

So, in summary, ISPs which are selling bandwidth to users which are using BitTorrent don’t give a rats ass about the upstream. They’ve got loads to spare. What they’re concerned with is the users who are using large amounts of upstream bandwidth are generally also heavy consumers of downstream, and those people are causing oversubscription issues.


2 Comments on “The Big Bandwidth Misconception”

  1. Tom Kirkham says:

    So, if thats the case, why do ISPs limit the upstream traffic so severly compared to the downstream traffic? 512k upstream to 5-6mb down seems like large ratio for ISPs that don’t give a rats ass and could provide more competitive services for businesses and consumers.

  2. Clint Sharp says:

    It’s mainly a technology issue with the data link layer. There’s generally only X amount of spectrum (whether that’s on a wire or over wireless) that can be allocated to data. The consumer demand is largely for downstream bandwidth, so thusly the technologies are designed to use 80% of the available spectrum for data transfer to downstream traffic. If the consumers truly wanted symetrical transfer rates, the ISPs would offer it, but mainly it’s a problem of a smaller market segment (businesses largely) wanting it, so thusly they have to pay higher rates for less utilized technologies like T1 and SDSL.


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