all of the packets within a session and analyze the conversation. Basic firewalls can only analyze individual
Routers connect networks together. Switches and hubs connect computing devices together. All of these
devices are connected together using some type of transmission media.
Today's networks use twisted-pair, fiber-optic, and wireless connections. Both twisted-pair and fiber-op-
tic media are cables you can touch. However, wireless connections use transceivers to transmit and receive
radio frequency transmissions over the air.
Chapter 7 covers the details of twisted-pair and fiber connections. Chapter 12 covers the details
of wireless networking.
Twisted pair is used for short distances up to 100 meters. Fiber-optic runs can be as long as 2 km for mul-
timode fiber and up to 40 km for single-mode fiber. Wireless networks are primarily used within buildings.
The most common type of transmission media is twisted pair. Twisted-pair cables can be wired as either
a straight-through cable or a crossover cable.
Straight-Through Cable Wires are connected to the same pins on both connectors of a straight-through
cable. A straight-through cable connects computers to networking devices. For example, it would con-
nect a computer to a hub or a computer to a switch.
Figure 2-10 shows the wiring diagram of a straight-through cable. Just as the name implies, the connec-
tions are straight through end to end and each wire is connected on the same pins on both ends. The colors
of the cable are based on the T568B standard.
Crossover Cable Specific wires are crossed on opposite connectors of the crossover cable . A crossover
cable connects similar devices to each other. For example, you would use a crossover cable to connect
any two networking devices together such as the following:
• A switch and a switch
• A switch and a hub
• A switch and a router
• A computer and a computer
Figure 2-11 shows the wiring diagram for a crossover twisted-pair cable. The straight-through cable has
the pairs connected from the same pins on one side to the same pins on the other side. However, the cros-
sover cable crosses over some key wires so that transmit signals on one side go to receive on the other side.
Many modern routers and switches autosense the connection. In other words, if the connection
needs a crossover cable, the wiring is internally changed.
You can easily identify a crossover cable by placing both connectors of the same cable side by side. If the
orange and green pairs are swapped, it's a crossover cable.
Figure 2-10: Straight-through cable