Why both Data link layer and Transport layer do Error Checking?

[Shankar Murali]

Dear all,

  I have a very simple question. Basically information converted in to stream of bits (1's and 0's) and transmit, in the receiving end Both Link layer and Transmission Layer runs error checking. Any one please elaborate why is these two checks?

Regards

Shankar

[Dennis Mink]

Tell us where you are seeing this?  paste output

[Shankar Murali]

Sorry for late reply.. Here what i mean is why we need to error check -->  Layer2 check - CRC and Layer4 check checksum. Can we combine these two in to a single check.

Shankar

[Joseph W. Doherty]

A very, very belated response, but only now caught site of this question.

Answer:

Because they can.  (Laugh - just kidding.)

Answer:

Because layers operate somewhat independently of each other, and errors can happen as one layer and not another.

Also with independent error checking, finding an error at one level but not another might help identify the cause of the error condition.

[Shankar Murali]

Thank you joseph.. Instead of these two checks can IETF combine to a single check. CRC+Checksume = Xcheck kind of..

[Joseph W. Doherty]

It might, but that would negate both the independence of each layer being independent for error checking.

BTW, why the concern of the multiple checks?  If you're worried about efficiency, years ago I encoded a CRC-32 check in assembler that was based on a table lookup - i.e. it didn't have to do the polynomial match.