The Network Access Layer

What You'll Learn r:

• Physical addresses

• Ethernet frames

• LAN technologies

At the base of the TCP/IP protocol stack is the Network Access layer, the collection of services and specifications that provide and manage access to the network hardware. In this hour you'll learn about the duties of the Network Access layer and how the Network Access layer relates to the OSI model. This hour also looks at some common physical network technologies you'll find in the Network Access layer.

At the completion of this hour, you'll be able to

• Explain the Network Access layer

• Discuss how TCP/IP's Network Access layer relates to the OSI networking model

• Explain a network architecture

• List the contents of an ethernet frame

• Identify the methods that ethernet, token ring, and FDDI use for controlling access to the transmission medium

The Network Access layer is the most mysterious and least uniform of TCP/IP's layers. The Network Access layer manages all the services and functions necessary to prepare the data for the physical network. These responsibilities include

• Interfacing with the computer's network adapter.

• Coordinating the data transmission with the conventions of the appropriate access method. You'll learn more about access methods later in this hour.

• Formatting the data into a unit called a frame and converting that frame into the stream of electric or analog pulses that passes across the transmission medium.

• Checking for errors in incoming frames.

• Adding error-checking information to outgoing frames so that the receiving computer can check the frame for errors.

• Acknowledging receipt of data frames and resending frames if acknowledgment is not received.

Of course, any formatting tasks performed on an outgoing frame must occur in reverse when the frame reaches its destination and is received by the computer to which it is addressed.

The Network Access layer defines the procedures for interfacing with the network hardware and accessing the transmission medium. Below the surface of TCP/IP's Network Access layer, you'll find an intricate interplay of hardware, software, and transmission-medium specifications. Unfortunately, at least for the purposes of a concise description, there are many different types of physical networks that all have their own conventions, and any one of these physical networks can form the basis for the Network Access layer. You'll learn about these physical network types later in this hour. A few examples include

• Ethernet

• Token ring

• FDDI

• PPP (Point-to-Point Protocol, through a modem)

• Wireless networks