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I want to know why error control is used in transport layer although it is used in data link layer?While data link layer has all data came from upper layers in its frames so its error control should be sufficient.

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There are many types of error controls. The link layer error control that you are referring to is probably to do with ensuring that no bits were flipped in transit i.e. safe guarding against data corruption.

The error control in the transport layer usually refers to the guaranteed delivery mechanism with TCP, which attempts to safe guard against frames/packets getting lost entirely.

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  • $\begingroup$ +1 Good observation. Bit flips are only one kind of possible error. Packets may also be dropped or arrive out of order. As for why you might want to run a checksum on incoming packets... well, it's always possible that the information was changed in flight, but not by bit flips... or maybe you've got a bum router/switch somewhere that's not working quite right. $\endgroup$ – Patrick87 Oct 20 '13 at 14:48

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