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Networking: A Beginner's Guide
For a complete network connection, data flows from the top layer on one computer,
down through all the lower layers, across the wire, and back up the seven layers on the
other computer. The following sections discuss each layer, making comparisons to real
networking systems as appropriate.
Physical Layer
The bottom layer, layer 1, is called the physical layer. It defines the properties of the
physical medium used to make a network connection. The physical layer specifications
result in a physical medium--a network cable--that can transmit a stream of bits
between nodes on the physical network. The physical connection can be either point-
to-point (between two points) or multipoint (between many points, such as from one
point to many others), and it can consist of either half-duplex (one direction at a time)
or full-duplex (both directions simultaneously) transmissions. Moreover, the bits can
be transmitted either in series or in parallel. (Most networks use a serial stream of bits,
but the OSI model allows for both serial and parallel transmission.) The specification
for the physical layer also defines the cable used, the voltages carried on the cable,
the timing of the electrical signals, the distance that can be run, and so on. A NIC, for
example, is part of the physical layer.
Data-Link Layer
The data-link layer, layer 2, defines standards that assign meaning to the bits carried by
the physical layer. It establishes a reliable protocol through the physical layer so the
network layer (layer 3) can transmit its data. The data-link layer typically includes error
detection and correction to ensure a reliable data stream. The data elements carried by
the data-link layer are called frames. Examples of frame types include X.25 and 802.x
(802.x includes both Ethernet and Token Ring networks).
The data-link layer is usually subdivided into two sublayers, called the logical
link control (LLC) and media access control (MAC) sublayers. If used, the LLC sublayer
performs tasks such as call setup and termination (the OSI model can be applied
to telecommunications networks as well as LANs) and data transfer. The MAC
sublayer handles frame assembly and disassembly, error detection and correction, and
addressing. The two most common MAC protocols are 802.3 Ethernet and 802.5 Token
Ring. Other MAC protocols include 802.12 100Base-VBG, 802.11 Wireless, and 802.7
Broadband.
On most systems, the software drivers for the NIC perform the work done at the
data-link layer.
Network Layer
The network layer, layer 3, is where a lot of action goes on for most networks. The
network layer defines how data packets get from one point to another on a network and
what goes into each packet. The network layer uses different packet protocols, such
as Internet Protocol (IP) and Internet Protocol Exchange (IPX). These packet protocols