Data Link Layer

Topics: Data transmission, Parity bit, Hamming code Pages: 10 (2855 words) Published: September 13, 2010
Table of Contents

Part 1: General review of data link layer2

a)Explain the working principles of the data link layer.2

b)Is controlled access better than contention for media access control? Discuss.2

c)Why is error detection important in this layer? What is being measured?3

d)Identify three significant noises that can cause errors in data communication digital circuits. Briefly explain.3

Part 2: General review of error correction4

a)Why is cyclical redundancy check (CRC) most popular error checking scheme?4

b)How is Hamming distance used in error correction? Explain.4

c)Briefly explain how parity is used to find the location of single-bit errors in the forward error correction method?5

Part 3: Demonstration of data link protocols5

a)Explain the necessity of data link protocols?5

b)Asynchronous communication is sometimes called start-stop transmission. Discuss with necessary diagram?6

c)Explain the Ethernet protocols categories?6


Part 1: General review of data link layer

Explain the working principles of the data link layer.

The data link layer sits between the physical layer and the network layer. It is responsible for sending and receiving messages to and from other computers. It is responsible for moving a message from one computer to next computer where the message needs to go. The data link layer performs the main functions and is divided into two sub layers. The first sublayer called logical link control (LLC) sublayer and the second sublayer called media access control (MAC) sublayer. The LLC sublayer software at the sending computer is responsible for transmitting the network layer Protocol Data Unit (PDU) with the data link layer. At the receiving computer the MAC sublayer software takes the data link layer PDU from the LLC sublayer and coverts into a stream of bits and also controls when the physical layer actually transmits the bits over the circuit. The data link layer controls the way messages are sent on the physical media. The data link layer performs various functions depending upon the hardware protocol used in the network and both sender and receiver have to agree on the rules and protocols that govern how they will communicate with each other. The data link layer is concerned with physical addressing, network topology, physical link management, error notification, ordered delivery of frames and flow control (Fitzgerald & Dennis 2009).

Is controlled access better than contention for media access control? Discuss.

There are two fundamental approaches to media access control that are control access and contention. In control access the polling process is used in which the mainframe computer (i.e. server) controls the circuit and determines which clients (i.e. computer or terminal) can access media at what time. Polling is like a classroom situation where the instructor (i.e. server) calls students who have raised their hands to gain access to the media. Contention is altogether opposite to controlled access. In this case the computers wait until a circuit is free which means they have to check whether any computers are transmitting and then only they are allowed to transmit whenever they have data to send. But to determine which is better can be considered based on the largest amount of user data to be transmitted through the network. The contention approach has worked better than controlled access approach for small network where there is low usage, but can be problematic in heavy usage networks. In heavy usage networks where many computers want to transmit at the same time the controlled access approach is better because it prevents collisions and delivers better throughput in such networks. But today’s contention approach is better than controlled access because they have been improved to the point where they are able to deliver significantly better throughput than controlled access and...
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