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My book says that because most ISPs support asymmetrical bandwidth usage and so support greater downstream traffic than upstream, that P2Ps somehow put stress on the ISP due to this? I don't understand how.

This is quoted from the book "Computer Networking: A Top-down Approach": "P2P video-streaming and file distribution applications put stress on ISPs by shifting upstream traffic from servers to residential ISPs."

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Suppose the typical client-server connection, for example watching a YouTube video or browsing the web. When you watch a video or do web browsing the downstream is highly used, but the upstream is almost not used, mainly to send periodical short bursts of data.

When you have a P2P application open 24 hours you are downloading and also continually uploading files, probably using all your upload bandwith. ISPs doesn't expect that behavior and have designed their systems keeping in mind that the typical upstream will be short bursts of data and long silences.

ADSL is the typical assymetrical home solution, it uses POTS cable to transport data, and all the ADSL connections on a zone are gathered on a DSLAM at the ISP zone premises.

The power of the upstream signal is weaker on the DSLAM as there are more distance from the home to the DSLAM, meanwhile the power of the downstream signal on the DSLAM is very high for everybody.

Imagine all the cables coming from the houses and getting on the DSLAM, as they are together for some length, then happens crosstalk (interference from one cable to the other), the high power signals will interfere the low power signals.

Per design, the ISP supposes that the user will use the upstream to send short burts of data then crosstalk seems to be not important, but as more users use upstream to upload files causing a constant stream of data, then crosstalk increases causing loss of data and retransmisions. The DSLAM becomes a bottleneck for the upstream traffic and finally the user gets the impression that the network is slower.

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