3.1.8 STP cable combines the techniques of cancellation, shielded, and twisted wires. Each pair of wires is wrapped in metallic foil. The two pairs of wires are wrapped in an overall metallic braid or foil. It is usually 150-ohm cable. As specified for use in Token Ring network installations, STP reduces electrical noise within the cable such as pair to pair coupling and crosstalk. STP also reduces electronic noise from outside the cable such as electromagnetic interference (EMI) and radio frequency interference (RFI). STP cable shares many of the advantages and disadvantages of UTP cable. STP provides more protection from all types of external interference. However, STP is more expensive and difficult to install than UTP.
A new hybrid of UTP is Screened UTP (ScTP), which is also known as foil screened twisted pair (FTP). ScTP is essentially UTP wrapped in a metallic foil shield, or screen. ScTP, like UTP, is also 100-ohm cable. Many cable installers and manufacturers may use the term STP to describe ScTP cabling. It is important to understand that most references made to STP today actually refer to four-pair shielded cabling. It is highly unlikely that true STP cable will be used during a cable installation job.
The metallic shielding materials in STP and ScTP need to be grounded at both ends. If improperly grounded or if there are any discontinuities in the entire length of the shielding material, STP and ScTP can become susceptible to major noise problems. They are susceptible because they allow the shield to act like an antenna that picks up unwanted signals. However, this effect works both ways. Not only does the shield prevent incoming electromagnetic waves from causing noise on data wires, but it also minimizes the outgoing radiated electromagnetic waves. These waves could cause noise in other devices. STP and ScTP cable cannot be run as far as other networking media, such as coaxial cable or optical fiber, without the signal being repeated. More insulation and shielding combine to considerably increase the size, weight, and cost of the cable. The shielding materials make terminations more difficult and susceptible to poor workmanship. However, STP and ScTP still have a role, especially in Europe or installations where there is extensive EMI and RFI near the cabling.
The following page discusses UTP cable.
3.1.9 UTP is a four-pair wire medium used in a variety of networks. Each of the eight copper wires in the UTP cable is covered by insulating material. In addition, each pair of wires is twisted around each other. This type of cable relies on the cancellation effect produced by the twisted wire pairs to limit signal degradation caused by EMI and RFI. To further reduce crosstalk between the pairs in UTP cable, the number of twists in the wire pairs varies. Like STP cable, UTP cable must follow precise specifications as to how many twists or braids are permitted per foot of cable.
TIA/EIA-568-B.2 contains specifications that govern cable performance. It involves the connection of two cables, one for voice and one for data, to each outlet. The cable for voice must be four-pair UTP. Category 5 is the cable most frequently recommended and implemented in installations. However, analyst predictions and independent polls indicate that Category 6 cable will supersede Category 5 cable in network installations. The fact that Category 6 link and channel requirements are backward compatible to Category 5e makes it very easy for customers to choose Category 6 and supersede Category 5e in their networks. Applications that work over Category 5e will work over Category 6.
UTP cable has many advantages. It is easy to install and is less expensive than other types of networking media. In fact, UTP costs less per meter than any other type of LAN cabling. However, the real advantage is the size. Since it has such a small external diameter, UTP does not fill up wiring ducts as rapidly as other types of cable. This can be an extremely important factor to consider, particularly when a network is installed in an older building. When UTP cable is installed with an RJ-45 connector, potential sources of network noise are greatly reduced and a good solid connection is almost guaranteed.
There are some disadvantages of twisted-pair cabling. UTP cable is more prone to electrical noise and interference than other types of networking media, and the distance between signal boosts is shorter for UTP than it is for coaxial and fiber optic cables.
Twisted pair cabling was once considered slower at transmitting data than other types of cable. This is no longer true. In fact, today, twisted pair is considered the fastest copper-based media.
For communication to occur the signal that is transmitted by the source needs to be understood by the destination. This is true from both a software and physical perspective. The transmitted signal needs to be properly received by the circuit connection designed to receive signals. The transmit pin of the source needs to ultimately connect to the receiving pin of the destination. The following are the types of cable connections used between internetwork devices.
In Figure , a LAN switch is connected to a computer. The cable that connects from the switch port to the computer NIC port is called a straight-through cable.
In Figure , two switches are connected together. The cable that connects from one switch port to another switch port is called a crossover cable.
In Figure , the cable that connects the RJ-45 adapter on the com port of the computer to the console port of the router or switch is called a rollover cable.
The cables are defined by the type of connections, or pinouts, from one end to the other end of the cable. See Figures , , and . A technician can compare both ends of the same cable by placing them next to each other, provided the cable has not yet been placed in a wall. The technician observes the colors of the two RJ-45 connections by placing both ends with the clip placed into the hand and the top of both ends of the cable pointing away from the technician. A straight-through cable should have both ends with identical color patterns. While comparing the ends of a cross-over cable, the color of pins #1 and #2 will appear on the other end at pins #3 and #6, and vice-versa. This occurs because the transmit and receive pins are in different locations. On a rollover cable, the color combination from left to right on one end should be exactly opposite to the color combination on the other end.
In the first Lab Activity, a simple communication system is designed, built, and tested.
In the next Lab Activity, students will use a cable tester to determine if a straight-through or crossover cable is good or bad.
This page concludes this lesson. The next lesson will discuss optical media. The first page will describe the electromagnetic spectrum.
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