23-01-2013, 12:07 PM
Data Link Layer
Data Link.ppt (Size: 2.3 MB / Downloads: 35)
Data Link Layer Design Issues
Services Provided to the Network Layer
Framing
Error Control
Flow Control
Functions of the Data Link Layer
Provide service interface to the network layer
Dealing with transmission errors
Regulating data flow
Slow receivers not swamped by fast senders
How data is moved.
The DLL is responsible for taking the packets of information that it receives from the Network Layer and putting them into frames for transmission.
Each frame holds the payload plus a header and a trailer (overhead).
It is the frames that are transmitted over the physical layer.
Unacknowledged Connectionless service
Losses are taken care of at higher layers
Used on reliable medium like coax cables or optical fiber, where the error rate is low.
Appropriate for voice, where delay is worse than bad data.
Acknowledged Connectionless service
Useful on unreliable medium like wireless.
Acknowledgements add delays.
Adding ack in the DLL rather than in the NL is just an optimization and not a requirement. Leaving it for the NL is inefficient as a large message (packet) has to be resent in that case in contrast to small frames here.
On reliable channels, like fiber, the overhead associated with the ack is not justified.
What about Bit Stuffing?
We could have problems with two machines communicating where one uses 8-bit characters and one uses 16-bit characters.
We stuff bits instead of bytes.
Most DLL protocols use a combination of character count with another method for extra safety. This increases the chances of catching an error.