Data Link Control PDF

DATA LINK CONTROL Data Link Layer Design Issues Services Provided to the Network Layer Framing Error Control Flow Control 11-1 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 f...

DATA LINK CONTROL Data Link Layer Design Issues Services Provided to the Network Layer Framing Error Control Flow Control 11-1 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. Figure 11.1 A frame in a character-oriented protocol Figure 11.2 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. Figure 11.3 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. Figure 11.4 Bit stuffing and unstuffing 11-2 FLOW 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. Figure 11.5 Taxonomy of protocols discussed in this chapter 11-4 NOISELESS 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 Figure 11.6 The design of the simplest protocol with no flow or error control Figure 11.7 Flow diagram for Example 11.1 Stop-and-Wait Protocol Sender sends one frame, stops until it gets confirmation from receiver. Figure 11.8 Design of Stop-and-Wait Protocol Figure 11.9 Flow diagram for Example 11.2 11-5 NOISY 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 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. Note In Stop-and-Wait ARQ, we use sequence numbers to number the frames. The sequence numbers are based on modulo-2 arithmetic. Note In Stop-and-Wait ARQ, the acknowledgment number always announces in modulo-2 arithmetic the sequence number of the next frame expected. Figure 11.10 Design of the Stop-and-Wait ARQ Protocol Figure 11.11 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.  Example 11.4 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. Figure 11.12 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. Figure 11.13 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. Figure 11.14 Design of Go-Back-N ARQ Figure 11.15 Window size for 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. Figure 11.16 Flow diagram for Example 11.6 Figure 11.17 Flow diagram for Example 11.7 Note Stop-and-Wait ARQ is a special case of Go-Back-N ARQ in which the size of the send window is 1. Figure 11.18 Send window for Selective Repeat ARQ Figure 11.19 Receive window for Selective Repeat ARQ Figure 11.20 Design of Selective Repeat ARQ Figure 11.21 Selective Repeat ARQ, window size Note In Selective Repeat ARQ, the size of the sender and receiver window must be at most one-half of 2m. Figure 11.22 Delivery of data in Selective Repeat ARQ Figure 11.23 Flow diagram for Example 11.8 Figure 11.24 Design of piggybacking in Go-Back-N ARQ