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Breaking Down a UTP Ethernet Link

The term Ethernet link refers to any physical cable between two Ethernet nodes. To learn
about how a UTP Ethernet link works, it helps to break down the physical link into those
basic pieces, as shown in Figure 2-6: the cable itself, the connectors on the ends of the
cable, and the matching ports on the devices into which the connectors will be inserted.
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Figure 2-6 Basic Components of an Ethernet Link
First, think about the UTP cable itself. The cable holds some copper wires, grouped as
twisted pairs. The 10BASE-T and 100BASE-T standards require two pairs of wires, while the
1000BASE-T standard requires four pairs. Each wire has a color-coded plastic coating, with
the wires in a pair having a color scheme. For example, for the blue wire pair, one wire’s
coating is all blue, while the other wire’s coating is blue-and-white striped.

Many Ethernet UTP cables use an RJ-45 connector on both ends. The RJ-45 connector
has eight physical locations into which the eight wires in the cable can be inserted, called
pin positions, or simply pins. These pins create a place where the ends of the copper
wires can touch the electronics inside the nodes at the end of the physical link so that
electricity can flow.

NOTE If available, find a nearby Ethernet UTP cable and examine the connectors closely.
Look for the pin positions and the colors of the wires in the connector.

To complete the physical link, the nodes each need an RJ-45 Ethernet port that matches
the RJ-45 connectors on the cable so that the connectors on the ends of the cable can connect
to each node. PCs often include this RJ-45 Ethernet port as part of a network interface
card (NIC), which can be an expansion card on the PC or can be built in to the system itself.
Switches typically have many RJ-45 ports because switches give user devices a place to connect
to the Ethernet LAN.

Figure 2-7 shows photos of the cables, connectors, and ports.
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Figure 2-7 RJ-45 Connectors and Ports (Ethernet NIC © Mark Jansen, LAN Cable ©
Mikko Pitkänen)

NOTE The RJ-45 connector is slightly wider, but otherwise similar, to the RJ-11 connectors
commonly used for telephone cables in homes in North America.

The figure shows a connector on the left and ports on the right. The left shows the eight pin
positions in the end of the RJ-45 connector. The upper right shows an Ethernet NIC that is
not yet installed in a computer. The lower-right part of the figure shows the side of a Cisco
2960 switch, with multiple RJ-45 ports, allowing multiple devices to easily connect to the
Ethernet network.

Finally, while RJ-45 connectors with UTP cabling can be common, Cisco LAN switches
often support other types of connectors as well. When you buy one of the many models
of Cisco switches, you need to think about the mix and numbers of each type of physical
ports you want on the switch.

To give its customers flexibility as to the type of Ethernet links, even after the customer has
bought the switch, Cisco switches include some physical ports whose port hardware (the
transceiver) can be changed later, after you purchase the switch.

For example, Figure 2-8 shows a photo of a Cisco switch with one of the swappable transceivers.
In this case, the figure shows an enhanced small form-factor pluggable (SFP+) transceiver,
which runs at 10 Gbps, just outside two SFP+ slots on a Cisco 3560CX switch. The
SFP+ itself is the silver colored part below the switch, with a black cable connected to it.
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Figure 2-8 10Gbps SFP+ with Cable Sitting Just Outside a Catalyst 3560CX Switch

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