Physical Layer

Overview

The physical layer is responsible for providing transmission services between nodes, and is a mix of data communications (the last topic) and physical layer architecture. The physical layer architecture is based on the data transmission scheme used and the intended use of the network.

Objectives

• Know the difference between the three classes of physical layer (LAN, MAN and WAN).
• Identify the attributes that distinguish physical layer schemes.
• Know the different framing methods.
• Know the different access control methods.
• Understand the basics of physical layer error control.
• Understand basic performance assessment

Preparation

Study sections 2.3 and 2.4 in the book and the notes.

Terms

Access Control Method
Byte counting
Byte stuffing
Bus topology
Checksum
Clock-based framing
Distributed topology
Contention
Cyclical Redundancy Check
Frame Format
Framing
Frequency Division Multiplexing
Parity
Point-to-point topology
Ring topology
Sentinel framing
Star topology
Time Division Multiplexing
Utilization

K&C

A protocol uses STX and ETX to represent beginning and end of a frame and the DLE character for escape. What would be the result of byte stuffing the message string

"A B STX C ETX D ETX ETX E DLE F DLE ETX G H STX I ETX"

The generating polynomial is 11001 and the message is 1001010. What gets sent?

Using the same generating polynomial, what do you decide if the following message arrives: 1110001100

What is the utilization of a network that transmits continuously and every frame sent consists of 1000 bytes of data and 100 bytes of framing information?

In the network above, suppose that TDM is used and that each of the 100 users gets and uses 5 msec of send time each second. What is the utilization?

ASE

Prove that 2-dimensional parity detects all three bit errors. Proof by contradiction is an appropriate method.

In the network above in 5 and 6, the length of the send time is raised to 12 msec when it is discovered that 25% of the users don't have anything to send. It takes a node 1 msec to notify the network that it doesn't intend to send. What is the utilization?

For each of bus, ring, wireless and star network topologies, describe a method of sharing the use of a single medium between multiple nodes. What is the maximum utilization? What is the worst case utilization?