Data Link Control PDF

DATA LINK CONTROL Data Link Layer Design Issues Services Provided to the Network Layer Framing Error Control Flow Control FRAMING The data link layer needs to pack bits into frames, so that each frame is distinguishable from another. Our postal system practices a type of fra...

DATA LINK CONTROL Data Link Layer Design Issues Services Provided to the Network Layer Framing Error Control Flow Control FRAMING The data link layer 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. Topics discussed in this section: Fixed-Size Framing Variable-Size Framing Types of Framing Fixed Size Framing Variable Size Framing--- Character oriented protocols and Bit oriented Protocols. A frame in a character-oriented protocol Byte stuffing and unstuffing Note Byte stuffing is the process of adding 1 extra byte whenever there is a flag or escape character in the text. A frame in a bit-oriented protocol Note 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. Bit stuffing and unstuffing W AND ERROR CONTROL The most important responsibilities of the data link layer are flow control and error control. Collectively, these functions are known as data link control. Topics discussed in this section: Flow Control Error Control Note Flow control refers to a set of procedures used to restrict the amount of data that the sender can send before waiting for acknowledgment. Note Error control in the data link layer is based on automatic repeat request, which is the retransmission of data. Taxonomy of protocols discussed in this chapter ISELESS CHANNELS Let us first assume we have an ideal channel in which no frames are lost, duplicated, or corrupted. We introduce two protocols for this type of channel. Topics discussed in this section: Simplest Protocol Stop-and-Wait Protocol Simplest Protocol No flow and Error Control The design of the simplest protocol with no flow or error control Flow diagram for Simplest Stop-and-Wait Protocol Sender sends one frame, stops until it gets confirmation from receiver. Design of Stop-and-Wait Protocol Flow diagram for Stop and Wait OISY CHANNELS Although the Stop-and-Wait Protocol gives us an idea of how to add flow control to its predecessor, noiseless channels are nonexistent. We discuss three protocols in this section that use error control. Topics discussed in this section: Stop-and-Wait Automatic Repeat Request Go-Back-N Automatic Repeat Request Selective Repeat Automatic Repeat Request Note In Stop-and-Wait ARQ, we use sequence numbers to number the frames. The sequence numbers can be 1,0,1,0,1… Note In Stop-and-Wait ARQ, the acknowledgment number is 0 if sequence number is 1 and acknowledgment number is 1 if sequence number is 0 Design of the Stop-and-Wait ARQ Protocol Flow diagram for Example 11.3 Disadvantage Stop-and-Wait ARQ Protocol Inefficient---if channel is thick and long Thick means high bandwidth Long means roundtrip delay Product of both is bandwidth delay. Bandwidth delay is number of bits we can send while waiting for news from receiver.  Assume that, in a Stop-and-Wait ARQ system, the bandwidth of the line is 1 Mbps, and 1 bit takes 20 ms to make a round trip. What is the bandwidth-delay product? If the system data frames are 1000 bits in length, what is the utilization percentage of the link? Solution The bandwidth-delay product is 11.28 Pipelining Task begins before end of first task. Stop-and-Wait ARQ does not use pipelining but other two techniques do. This improves efficiency. Go-Back-N Protocol This sends multiple frames before receiving acknowledgment from receiver. Note In the Go-Back-N Protocol, the sequence numbers are modulo 2m, where m is the size of the sequence number field in bits. Sliding Window Defines the range of sequence numbers that is concern of sender and receiver. The range which is concern of sender is called sender sliding window. The range which is concern of receiver is called receiver sliding window. Send window for Go-Back-N ARQ Note The send window is an abstract concept defining an imaginary box of size 2m − 1 with three variables: Sf, Sn, and Ssize. Note The send window can slide one or more slots when a valid acknowledgment arrives. Receive window for Go-Back-N ARQ Note The receive window is an abstract concept defining an imaginary box of size 1 with one single variable Rn. The window slides when a correct frame has arrived; sliding occurs one slot at a time. Design of Go-Back-N ARQ Note In Go-Back-N ARQ, the size of the send window must be less than 2m; the size of the receiver window is always 1. Flow diagram Flow diagram Note Stop-and-Wait ARQ is a special case of Go-Back-N ARQ in which the size of the send window is 1. Send window for Selective Repeat ARQ Receive window for Selective Repeat ARQ Design of Selective Repeat ARQ Note In Selective Repeat ARQ, the size of the sender and receiver window must be at most one-half of 2m. Delivery of data in Selective Repeat ARQ Flow diagram Design of piggybacking in Go-Back-N ARQ