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What is the need of error control at data link layer when Transport layer provides error control ? What is the difference between the two error controls ?
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The data link layer is the protocol layer in a program that handles the moving of data into and out of a physical link in a network. The data link layer is Layer 2 in the Open Systems Interconnection (OSI) architecture model for a set of telecommunication protocols.
The network layer provides communication between hosts of different networks. The transport layer receives the data from the upper layer and converts it into smaller parts known as segments. The network layer divides the data received from the transport layer in the form of packets.
Examples of data link protocols are Ethernet, Point-to-Point Protocol (PPP), HDLC and ADCCP. In the Internet Protocol Suite (TCP/IP), the data link layer functionality is contained within the link layer, the lowest layer of the descriptive model, which is assumed to be independent of physical infrastructure.
The Data-link layer is the second layer from the bottom in the OSI (Open System Interconnection) network architecture model. It is responsible for the node-to-node delivery of data. Its major role is to ensure error-free transmission of information.
Transport layer data could be broken down to many data-link layer frames/packets.
So it is possible that even without any data-link errors the transport layer stream/packet may be corrupt. Edit: This is because a transport layer path is usually composed of many data-link layer hops, for example:
Host1 <----> switch1 <----> switch2 <----> Host2
if a packet was lost between switch1 and switch2 then there would be no errors recorded on the switch2 Host2 link, but the corresponding transport layer stream would be corrupted.
On the other hand - once a data-link error is encountered it's possible to drop/restart the transport-layer transmission, without wasting resources.
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This is because Data link layer deals exclusively with bit-level error correction. It takes a packet the receiving computer already has in its possession and determines if an error occurred in transmission and whether the data is intact or corrupt. However, there need to be additional controls in place to make sure the system knows that all the packets are arriving. This is called end to end error control and is the responsibility of transport layer. Transport layer couldn't care less whether the data in the payload is good or bad. That's Data link's job. Transport only cares if it is getting every packet that it is supposed to, and whether or not there are arriving in the right order. It is the transport layer that detects the absence of packets or the corruption of packets that occurred on the transmission end before they arrived at the Data link layer.
For additional details, refer to
http://books.google.ca/books?id=9c1FpB8qZ8UC&pg=PA216&lpg=PA216&dq=why+error+detection+and+correction+both+in+transport+and+link+layer+?&source=bl&ots=RI7-DU8RO0&sig=0U5Z9AmKkx3m3TA71WfIe1uTeW0&hl=en&sa=X&ei=LbqPUsahOtDEqQHyvIHQCw&ved=0CDUQ6AEwAQ#v=onepage&q=why%20error%20detection%20and%20correction%20both%20in%20transport%20and%20link%20layer%20%3F&f=false
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