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Unshielded Twisted-Pair, or UTP, is a form of cabling that uses pairs of wires that are twisted together to reduce crosstalk. Refer to twisted-pair to understand the background for twisted-pair usage.

UTP is just a set of twisted-wire pairs enclosed in a plastic jacket. There is no foil or braid shielding to reduce electromagnetic interference (EMI) or signal bleeding.

UTP is mostly used in data and voice communications cabling. There are several categories of UTP, and the category determines the use of the cable. These categories were introduced by the Electrical Industries Association (EIA).

The categories are:

Category    Rate      Use

 CAT1,2     4Mbps     Originally used for voice 
                      communications, especially
                      old phone lines. 

 CAT3      10Mbps     Suitable for most computer networks.
                      Most newer phone systems now use
                      CAT3 cabling.

 CAT4      20Mbps     Not as common, allows a higher
                      data rate. Most people opt for the
                      benefits of CAT5 cable instead
                      of CAT4.

 CAT5     100Mbps+    Supports Fast Ethernet systems, but
                      requires more expensive 100Mbps
                      equipment to take advantage of the
                      higher data rate. The wiring is of a
                      higher quality, including better
                      insulation, more twists per linear
                      foot and better outer jackets.
                      Most businesses now install CAT5
                      when running cable due to the
                      expandability. It is cheaper to run
                      CAT5 now than install CAT3 and
                      replace it later with CAT5, but
                      running CAT5 requires a more 
                      stringent installation.

CAT3, 4 and 5 UTP cable use copper conductors. Because copper attenuates signals over distances, the length of one cable is limited to approximately 100 meters. UTP only supports two nodes or devices per cable, one at each end. Ethernet is basically equipment connected by patch cords.

The wires in UTP are grouped in pairs. Most data cables (CAT 3, 4, 5) have four wires or 8 wires. THe 4-wire cable is called two-pair, and the 8 conductor version four-pair. Networking using UTP requires at least a two-pair cable.

The connectors for Ethernet 4-pair UTP are called RJ-45 modular phone connectors. They look very similar to the phone connectors on many telephones, especially in the United States. The two-pair UTP cables can use RJ-11 connectors, which are smaller versions of the RJ-45.

A few additions:
An "e" stuck on the end of the category denotes "enhanced", therefore category 5e cable is enhanced category 5 cable. This simply means that the physical and electrical characteristics surpass those of the numbered category. An "enhanced" category or a larger number improves performance due to better handling of crosstalk, attenuation, impedance, and more.

ANSI/TIA/EIA-568-A is the main standard for UTP. This standard specifies up to category 5e. Any company who claims its product is category 6 compliant is referring to a draft of a standard - and possibly not even the draft of the ANSI standard.

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