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Unformatted text preview: 4-1 Chapter 4: Data Link Layer Answers to End-of-Chapter Questions 1.What does the data link layer do? The data link layer controls the way messages are sent on the physical media. The data link layer handles three functions: media access control, message delineation, and error control. The data link layer accepts messages from the network layer and controls the hardware that actually transmits them. The data link layer is responsible for getting a message from one computer to another without errors. The data link layer also accepts streams of bits from the physical layer and organizes them into coherent messages that it passes to the network layer. 2.What is media access control and why is it important? Media access control handles when the message gets sent. Media access control becomes important when several computers share the same communication circuit, such as a point-to-point configuration with a half duplex line that requires computers to take turns, or a multipoint configuration in which several computers share the same circuit. Here, it is critical to ensure that no two computers attempt to transmit data at the same time -- or if they do, there must be a way to recover from the problem. Media access control is critical in local area networks. 3.Under what conditions is media access control unimportant? With point-to-point full duplex configurations, media access control is unnecessary because there are only two computers on the circuit and full duplex permits either computer to transmit at anytime. There is no media access control. 4.Compare and contrast roll call polling, hub polling (or token passing), and contention. With roll call polling, the front end processor works consecutively through a list of clients, first polling terminal 1, then terminal 2, and so on, until all are polled. Roll call polling can be modified to select clients in priority so that some get polled more often than others. For example, one could increase the priority of terminal 1 by using a polling sequence such as 1, 2, 3, 1, 4, 5, 1, 6, 7, 1, 8, 9. Hub polling is often used in LAN multipoint configurations (i.e., token ring) that do not have a central host computer. One computer starts the poll and passes it to the next computer on the multipoint circuit, which sends its message and passes the poll to the next. That computer then passes the poll to the next, and so on, until it reaches the first computer, which restarts the process again. Contention is the opposite of controlled access. Computers wait until the circuit is free (i.e., no other computers are transmitting), and then transmit whenever they have data to send. Contention is commonly used in Ethernet local area networks. Data Link Layer 4-2 5.Which is better, hub polling or contention? Explain....
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This note was uploaded on 09/19/2011 for the course IS 589 taught by Professor Peacock during the Spring '11 term at DeVry Chicago.
- Spring '11