Data Link Layer

The goal of the data link layer is to provide reliable, efficient communication between adjacent machines connected by a single communication channel.

Specifically:

1. Group the physical layer bit stream into units called frames. Note that frames are nothing more than ``packets'' or ``messages''. By convention, we'll use the term ``frames'' when discussing DLL packets.
2. Sender checksums the frame and sends checksum together with data. The checksum allows the receiver to determine when a frame has been damaged in transit.
3. Receiver recomputes the checksum and compares it with the received value. If they differ, an error has occurred and the frame is discarded.
4. Perhaps return a positive or negative acknowledgment to the sender. A positive acknowledgment indicate the frame was received without errors, while a negative acknowledgment indicates the opposite.
5. Flow control. Prevent a fast sender from overwhelming a slower receiver. For example, a supercomputer can easily generate data faster than a PC can consume it.
6. In general, provide service to the network layer. The network layer wants to be able to send packets to its neighbors without worrying about the details of getting it there in one piece.

      Framing

The DLL translates the physical layer's raw bit stream into discrete units (messages) called frames. How can the receiver detect frame boundaries? That is, how can the receiver recognize the start and end of a frame?

Length Count:

Make the first field in the frame's header be the length of the frame. That way the receiver knows how big the current frame is and can determine where the next frame ends.

Error Control

Error control is concerned with insuring that all frames are eventually delivered (possibly in order) to a destination. How? Three items are required.

Acknowledgements:

Typically, reliable delivery is achieved using the ``acknowledgments with retransmission'' paradigm, whereby the receiver returns a special acknowledgment (ACK) frame to the sender indicating the correct receipt of a frame.In some systems, the receiver also returns a negative acknowledgment (NACK) for incorrectly-received frames. This is nothing more than a hint to the sender so that it can retransmit a frame right away without waiting for a timer to expire.

Timers:

One problem that simple ACK/NACK schemes fail to address is recovering from a frame that is lost, and as a result, fails to solicit an ACK or NACK. What happens if an ACK or NACK becomes lost?Retransmission timers are used to resend frames that don't produce an ACK. When sending a frame, schedule a timer to expire at some time after the ACK should have been returned. If the timer goes off, retransmit the frame.

Sequence Numbers:

Retransmissions introduce the possibility of duplicate frames. To suppress duplicates, add sequence numbers to each frame, so that a receiver can distinguish between new frames and old copies.

Error Detection and Correction

In data communication, line noise is a fact of life (e.g., signal attenuation, natural phenomenon such as lightning, and the telephone repairman). Moreover, noise usually occurs as bursts rather than independent, single bit errors. For example, a burst of lightning will affect a set of bits for a short time after the lightning strike.
Detecting and correcting errors requires redundancy -- sending additional information along with the data.