Hardware Reference
In-Depth Information
Figure 17.9. An AT&T 258A/EIA 568B standard-compliant 8P8C (RJ-45) connector.
Note
You also might encounter the similar EIA 568A standard. It reverses the position of the orange
and green pairs listed previously.
Crossover UTP Cables
Crossover cables, which change the wiring at one end of the cable, connect two (and only two)
computers when no hub or switch is available or connect a hub or switch without an uplink port to
another hub or switch. The pinout for a crossover cable is shown in
Table 17.5
. This pinout is for one
end of the cable only; the other end of the cable should correspond to the standard EIA 568B pinout,
as shown previously in
Table 17.4
.
Table 17.5. 8P8C (RJ-45) Connector Wire Pairing and Placement for EIA 568A Standard
Note
Most standard cables have both ends wired using the EIA 568B standard; however, it is also
possible to wire them with both ends using the EIA 568A standard. As long as both ends are
wired the same in a straight-through configuration, the cable will work. Crossover cables
should have one end in an EIA 568B configuration, while the other end is in an EIA 568A
configuration, thus crossing pairs 2 and 3. It should be noted that other wiring schemes exist for
connecting UTP cables with 8P8C (RJ-45) connectors. The ones listed in this chapter are the
most common.
Most newer switches are designed so as to automatically detect whether a crossover connection is
required and configure the connection appropriately. This feature is called Auto-MDIX (automatic
medium-dependent interface crossover) and essentially negates the need for having dedicated
“uplink” ports or using crossover cables when connecting devices.
Constructing the Cable
Making your own network cables requires a few tools that aren't found in a typical toolbox. Those
items that you might not already have you can typically purchase for a single price from many
network-products vendors. You need the following tools and supplies to build your own Ethernet
cables:
• UTP cable (Category 5e or better)
