lecture2: Framing and flow control

Lecture2:Framing and Flow Control
Data Link Control
The two main functions of the data link layer are data link control and media access control. The first, data link control, deals with the design and procedures for communication between two adjacent nodes: node-to-node communication. We discuss this functionality in this part. The second function of the data link layer is media access control, or how to share the link. We will discuss this functionality later.
Data link control functions include framing, flow and error control, and software implemented protocols that provide smooth and reliable transmission of frames between nodes. We first discuss framing, or how to organize the bits that are carried by the physical layer. We then discuss flow and error control.
To implement data link control, we need protocols. Each protocol is a set of rules that need to be implemented in software and run by the two nodes involved in data exchange at the data link layer. We will discuss five protocols: two for noiseless (ideal) channels and three for noisy (real) channels. Those in the first category provide a foundation for understanding the protocols in the second category.

FRAMING
Data transmission in the physical layer means moving bits in the form of a signal from the source to the destination. The physical layer provides bit synchronization to ensure that the sender and receiver use the same bit durations and timing.
The data link layer, on the other hand, needs to pack bits into frames, so that each frame is distinguishable from another. Our postal system practices a type of framing. The simple act of inserting a letter into an envelope separates one piece of information from another; the envelope serves as the delimiter. In addition, each envelope defines the sender and receiver addresses since the postal system is a many-to-many carrier facility.
Framing in the data link layer separates a message from one source to a destination, or from other messages to other destinations, by adding a sender address and a destination address. The destination address defines where the packet is to go; the sender address helps the recipient acknowledge the receipt.