The Data Link Layer

1. Both the physical and data link layer are not really about "networks" but rather about communication between simple pairs of "directly" connected machines.

2. In my introduction to the course, I suggested that the data link layer concentrated on the software issues of a machine to machine connection while the physcial layer worried about the hardware. Like all definitions of the roles of the layers, this isn't entirely accurate. Data link layer issues are sometimes address in hardware.

3. Another way to distinguish these two layer is the way they view the data being transmitted.

   • The physical layer simply transmits a stream of bits from a source to a destination.

   • The data link layer transmits a stream of messages or packets (each of which is just a sequence of bits).

4. While a stream of packets of bits is in some sense just a stream of bits, this difference in viewpoint has a very practical implication.

5. While it is not practical to associate control information with each bit in a message stream, it is practical to associate control information with groups of bits viewed as packets. Such control information can support several enhancements to the basic functionality provided by the physical layer:

   • Error Detection and Correction:

     The detection of errors requires that redundant information be included in the data stream that can be used to detect errors. To use this redundant information, the receiving end of the connection must know what collection of bits the redundant information is associated with. Packets provide natural units to associate with error checks.

   • Time Division Multiplexing:

     If a channel is to be shared by taking turns, groups of bits have to be packaged in such a way that some form of "address label" can be associated with the groups so that the receiver will know to which process they should be delivered.

   • Multiaccess networks:

     In some cases the "direct" connection between two machines is not really very direct. For example, in a network based on radio transmission, each message broadcast can he heard by many machines. It is still appropriate to view such a connection as direct because there is no need to perform any routing to find a path through one or more intermediaries from the sender to the receiver. All communications is "direct".

     To make such communications possible, data bits must be grouped into messages and an address field indicating the destination machine must be included. (Obviously, this is similar to the TDM issue).

6. While the notion of messages or packets is quite central to modern networking techniques, the data link layer actually concentrates primarily on three issues in the handling of packets:

   Framing

   Including information in the bit stream to identify the beginning of and end of each packet.

   Error detection and correction

   The data link layer usually includes mechanisms for including error detecting codes and arranging for the retransmission of damaged packets.

   Flow Control

   Arranging for enough communication between the sender and receiver to ensure that the receiver does not get overloaded by a transmitter that is capable of sending message faster than the receiver can process them.