Computer Networks

Question

Why CRC is added at the end of frame?

Comments

Thanks, Rameesh, 

I can't be reaching a perfect solution...

Have you any point?

---

It is not "compulsory" to put CRC at the end of a frame. It depends on your implementation and implementation depends on where you are going to use it. So if you feel like your case requires CRC in the middle(or somewhere else), you can do it. But that implementation would be expected to work only within your own network as in the standard one CRC is always appended at the end. It is more like a standard rather than a compulsion. Standard doesn't always need to make sense but here, as mentioned by Sukanya, hardware buffering can be one of the many reasons we can think of putting CRC at the end to make it a standard.

---

In fact the placement has very much to do with what CRC is about: polynomial division. If you move the CRC remainder to the front of the payload bitstream, you will invalidate some of the CRC properties, such as burst error detection.

The key to understanding this is the fact, that a CRC is always operating on a stream of bits, not bytes, or a block of payload. Sometimes you can find fault CRC implementations, where bits are transmitted little-endian, but actually the CRC is calculated big-endian (in term of bit ordering within individual bytes).This argument seems a lil bit more significant still. :)

Answer 1

there are two benefit  of CRC at the end of frame

1st at the  receiver side

at the transport layer of receiver side when the data passes through  the TL   ,TL calculate the CRC as the data passes through it and at the end of the frame TL layer calculate whole of the CRC bit and immediately matches it with the CRC bit which is actually attach to the frame if it found that both CRC (one which is calculated and second which actually attach to frame) are same then data is correct otherwise  something is wrong (may be data or CRC) .

and same process can happen at DLL

second at the sender  side

placing the CRC at the end of a frame reduces packet latency and reduces hardware buffering requirements. On the transmit side, hardware can read and transmit bytes of the frame immediately. The transmitter calculates the CRC on the fly as data passes through, then simply appends the CRC the tail of the frame