Lesson 5 - The Transport Layer

   Introduction

Thus far in this unit, we have seen that each layer in the OSI model describes a destination in increasingly finer detail. The Physical Layer simply puts the signal onto a transmission medium, with no concept of where it goes. The Data Link Layer addresses a data frame to a hardware device on the other end of a physical link. The Network Layer addresses a data packet to the logical description of a computer that may be located several links away from the source.

However, computers run multiple applications simultaneously. Thus, once the data arrives at the correct computer, it must also be delivered to the correct software process within that computer. If you suspect this requires yet another layer of addressing information, you are correct. The Transport Layer (Layer 4) addresses data to a particular process running on a destination computer.

   Objectives

At the end of this lesson you will be able to:

1.	Describe how applications transmit information to other applications
2.	Describe how "ports" are used to transmit information to the correct applications
3.	Name common Transport Layer protocols

Key Point
The Transport Layer carries messages between processes. Transport Layer addresses are process addresses called "port numbers."

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