Data Link Control

Definition of Terms

• Data link control deals with the design and procedures for communication between two adjacent nodes: node-to-node communication.
• The two main functions of the data link layer are data link control and media access control.
• Data link control functions include framing, flow and error control, and software-implemented protocols that provide smooth and reliable transmission of frames between nodes.
• Flow control is the regulation of the sender’s data rate so that the receiver buffer does not become overwhelmed.
• Error control is both error detection and error correction.  
• Frames can be of fixed or variable size. In fixed-size framing, there is no need for defining the boundaries of frames; in variable-size framing, we need a delimiter (flag) to define the boundary of two frames.
• Variable-size framing uses two categories of protocols: byte-oriented (or character-oriented) and bit-oriented. In a byte-oriented protocol, the data section of a frame is a sequence of bytes; in a bit-oriented protocol, the data section of a frame is a sequence of bits.
• In byte-oriented (or character-oriented) protocols, we use byte stuffing; a special byte added to the data section of the frame when there is a character with the same pattern as the flag.
• Byte stuffing is the process of adding 1 extra byte whenever there is a flag or escape character in the text.
• In bit-oriented protocols, we use bit stuffing; an extra 0 is added to the data section of the frame when there is a sequence of bits with the same pattern as the flag.
• Bit stuffing is the process of adding one extra 0 whenever five consecutive 1s follow a 0 in the data, so that the receiver does not mistake the pattern 0111110 for a flag.
• Flow control refers to a set of procedures used to restrict the amount of data that the sender can send before waiting for acknowledgment. Error control refers to methods of error detection and correction.
• For the noiseless channel, we discussed two protocols: the Simplest Protocol and the Stop-and-Wait Protocol. The first protocol has neither flow nor error control; the second has no error control. 
• In the Simplest Protocol, the sender sends its frames one after another with no regards to the receiver. 
• In the Stop-and-Wait Protocol, the sender sends one frame, stops until it receives confirmation from the receiver, and then sends the next frame.
• For the noisy channel, we discussed three protocols: Stop-and-Wait ARQ, Go-Back- N, and Selective Repeat ARQ. 
• The Stop-and-Wait ARQ Protocol, adds a simple error control mechanism to the Stop-and-Wait Protocol.
• Error correction in Stop-and-Wait ARQ is done by keeping a copy of the sent frame and retransmitting of the frame when the timer expires.
• In Stop-and-Wait ARQ, we use sequence numbers to number the frames. The sequence numbers are based on modulo-2 arithmetic.
• In Stop-and-Wait ARQ, the acknowledgment number always announces in modulo-2 arithmetic the sequence number of the next frame expected.
• Stop-and-Wait ARQ is a special case of Go-Back-N ARQ in which the size of the send window is 1.
• In the Go-Back-N ARQ Protocol, we can send several frames before receiving acknowledgments, improving the efficiency of transmission.
• In Go-Back-N ARQ, multiple frames can be in transit at the same time. If there is an error, retransmission begins with the last unacknowledged frame even if subsequent frames have arrived correctly. Duplicate frames are discarded.
• In the Go-Back-N Protocol, the sequence numbers are modulo 2^m, where m is the size of the sequence number field in bits. 
• The send window is an abstract concept defining an imaginary box of size 2^m − 1 with three variables: S_f, S_n, and S_size. The send window can slide one or more slots when a valid acknowledgment arrives.
• In Go-Back-N ARQ, the size of the send window must be less than 2^m; the size of the receiver window is always 1.
• In the Selective Repeat ARQ protocol we avoid unnecessary transmission by sending only frames that are corrupted.
• In Selective Repeat ARQ, multiple frames can be in transit at the same time. If there is an error, only the unacknowledged frame is retransmitted.
• In Selective Repeat ARQ, the size of the sender and receiver window must be at most one-half of 2^m.
• Both Go-Back-N and Selective-Repeat Protocols use a sliding window. In Go-Back- N ARQ, if m is the number of bits for the sequence number, then the size of the send window must be less than 2m; the size of the receiver window is always 1. In Selective Repeat ARQ, the size of the sender and receiver window must be at most one-half of 2m.
• A technique called piggybacking is used to improve the efficiency of the bidirectional protocols. When a frame is carrying data from A to B, it can also carry control information about frames from B; when a frame is carrying data from B to A, it can also carry control information about frames from A.
• The bandwidth-delay product is a measure of the number of bits a system can have in transit. 
• High-level Data Link Control (HDLC) is a bit-oriented protocol for communication over point-to-point and multipoint links. However, the most common protocols for point-to-point access is the Point-to-Point Protocol (PPP), which is a byte-oriented protocol. 
• HDLC is a protocol that implements ARQ mechanisms. It supports communication over point-to-point or multipoint links.
• PPP is a byte-oriented protocol using byte stuffing with the escape byte 01111101.
• HDLC stations communicate in normal response mode (NRM) or asynchronous balanced mode (ABM).
• HDLC protocol defines three types of frames: the information frame (I-frame), the supervisory frame (S-frame), and the unnumbered frame (U-frame).
• HDLC handle data transparency by adding a 0 whenever there are five consecutive 1s following a 0. This is called bit stuffing. 

Two Main Functions of the Data Link layer

• Data link control - deals with the design and procedures for communication between two adjacent nodes: node-to-node communication.
• Media access - control deals with procedures for sharing the link.

Two categories of protocols in Variable-size framing

• Byte-oriented protocol - data to be carried are 8-bit characters from a coding system. Character-oriented protocols were popular when only text was exchanged by the data link layers.
• Bit-oriented protocol -  the data section of a frame is a sequence of bits. Bit-oriented protocols are more popular today because we need to send text, graphic, audio, and video which can be better represented by a bit pattern than a sequence of characters.

Taxonomy of protocols discussed in this chapter

Note: You can proceed to take the multiple choice exam regarding this topic. Data Link Control - Set 1 MCQs

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credit: Behrouz A. Forouzan©2013 www.FroydWess.com

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