Page 40 - DCAP406_DCAP_207_Computer Networks

Unit 3: Physical Layer

Unshielded Twisted Pair (UTP) Notes

UTP as depicted in Figure 3.2 is the copper media, inherited from telephony, which is being used
for increasingly higher data rates, and is rapidly becoming the de facto standard for horizontal
wiring. Horizontal wiring specifies the connection between, and including, the outlet and the
termination in the communication closet. The horizontal is limited to a maximum of 90 meters.
This is independent of the media type so that the communication closet is common to all media
and all applications operating over the media. In addition, there is an allowance for 3 meters in
the work area and 6 meters for cross connecting in the closet for a total of 99 meters.
The recommended media and connectors for the horizontal are as follow:
100-ohm unshielded twisted pair - 4 pairs, 8-pin modular connector (ISDN)

150-ohm shielded twisted pair - 2 pairs (IBM connector or RJ45)
50-ohm coax (thin) - IEEE 10BASE2, standard BNC connector
62.5/125 multi mode fiber
A UTP cable contains from 2 to 4200 twisted pairs. The advantages of UTP are the flexibility, low
cost media, and can be used for either voice or data communications. Its greatest disadvantage
is the limited bandwidth, which restricts long distance transmission with low error rates.

Figure 3.2: Unshielded Twisted Pair (UTP)

Shielded Copper or STP

Shielded twisted pair (STP) differs from UTP in that a metallic shield or screen surrounds the
pairs, which may or may not be twisted. As illustrated in Figure 3.3, the pairs can be individually
shielded. A single shield can surround a cable containing multiple pairs or both techniques can
be employed in tandem. The shield itself is made of aluminum, steel, or copper. This is in the
form of a metallic foil or woven meshes and is electrically grounded. Although less effective,
the shield sometimes is in the form of nickel and/or gold plating of the individual conductors.

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