Exercises The Data Link Layer

50 Communication Networks Copyright © 2005 PragSoft • The sliding window protocol is a widely-used flow control protocol which relies on the transmitter keeping track of transmitted but unacknowledged frames, and the receiver keeping track of frames that are expected to be received. • The BSC or BISYNC protocol is a widely-used synchronous, character- oriented protocol devised by IBM. • The HDLC is a widely-used bit-oriented protocol with a number of variations: LAP, LAP-B, LAP-D, and LLC. HDLC offers three modes of operation NRM, ARM, and ABM. It transmits information in frames, of which there are three types: information frames, supervisory frames, and unnumbered frames .

3.7. Exercises

3.17 Explain the difference between synchronization at the physical layer and synchronization at the data link layer. 3.18 Describe what is meant by bit stuffing. Rewrite the following bit sequence after bit stuffing it: 0111110011101111111111010111. 3.19 Explain how the polling technique works. Describe an application for which polling may be appropriate. 3.20 Describe the role of frame sequence numbers and how they are used for acknowledgment purposes. 3.21 Assuming the generator polynomial x 5 + x 3 + x , what is the CRC code for the following bit sequence: 0110111001111011110111111010111? 3.22 Describe the role of T1 and T2 timers and how they can be used to handle a lost frame situation using the Selective Reject and G0-Back-N methods. 3.23 What problems are likely to occur in absence of a flow control scheme? 3.24 Using a diagram, illustrate how the sliding window protocol with a maximum transmitter window size of 5 and receiver window size of 2 operates for the following scenario: 1. Transmitter sends frames 1 and 2 www.pragsoft.com Chapter 3: The Data Link Layer 51 2. Transmitter receives ACK for frame 1 3. Transmitter sends frame 3 4. Transmitter receives ACK for frames 2 and 3 3.25 Describe the role of information, supervisory, and unnumbered frames in HDLC. Use a sequence diagram to illustrate the use of SREJ command in a data transfer situation. 52 Communication Networks Copyright © 2005 PragSoft

4. The Network Layer

This chapter describes the network layer of the OSI model. The network layer handles the routing of data packets across the network, and defines the interface between a host and a network node. We will first discuss the use of network primitives for defining network services. Then we will look at two switching methods and their use for routing. This is followed by a description of data packets and their handling by the network layer. We will then turn our attention to the problem of interconnecting two networks, and discuss the protocol sublayering provided for this purpose. Finally, we will look at four widely-accepted standards for networking as well as internetworking. After completing this chapter you should be able to: • Understand the nature of network services and use network primitives to describe network service scenarios. • Describe how circuit switching works and appreciate its strengths and weaknesses. • Describe how packet switching works and distinguish between the virtual circuit and datagram methods and their packet formats. • Understand the three basic routing algorithms flooding, static routing, and dynamic routing and their characteristics. • Appreciate the importance of congestion control. • Appreciate the need for internetworking and the sublayers provided to support it. • Have a basic knowledge of network layer standards X.25, X.75, IP, and ISO 8473.