Protocols and Hardware

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

By the Way

Not every networked computer is on a LAN. The network-access software might provide support for something other than a standard network adapter and a LAN cable. One of the most common alternatives is a modem connection to a remote network, such as the connection you establish when you dial in to an Internet service provider (ISP). Modem protocol standards such as Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP) provide network access for the TCP/IP protocol stack through a modem connection. You'll learn more about these protocols in Hour 8, "Dial-Up TCP/IP."

The good news is that the Network Access layer is almost totally invisible to the everyday user. The network adapter driver, coupled with key low-level components of the operating system and protocol software, manages most of the tasks relegated to the Network Access layer, and a few short configuration steps are usually all that is required of a user. These steps are becoming simpler with the improved plug-and-play features of desktop operating systems.

As you read through this hour, remember that the logical, IP-style addressing discussed in Hours 1, 2, 4, and 5 exists entirely in the software. The protocol system requires additional services to deliver the data across a specific LAN system and up through the network adapter of a destination computer. These services are the purview of the Network Access layer.

By the Way

It is worth mentioning that the diversity, complexity, and invisibility of the Network Access layer has caused some authors to exclude it from discussions of TCP/IP completely, asserting instead that the stack rests on LAN drivers below the Internet layer. This viewpoint has some merit, but the Network Access layer really is part of TCP/IP, and no discussion of the network-communication process is complete without it.