Networks of all three speeds can use cable

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Twisted Pair Cable

Networks of all three speeds can use cable. Twisted-pair cable is popular because it’s inexpensive, yet it can carry signals over long distances. The twists reduce noise in the cable in two ways: by reducing the size of the magnetic field that emanates from the wires and by canceling any noise the wires pick up via magnetic coupling.

Cable Categories

Networks of all three speeds can use cable that meets the Category 5e specification defined in EIA/TIA-568-B. A Category 5e cable contains four unshielded twisted pairs (UTPs) of wires. Each pair consists of two insulated conductors that spiral, or twist, around each other, with about one to three twists per inch (Network article 2-5). Varying the number of twists per inch from pair to pair helps to reduce noise in the wires. The wire diameter is 24 or 22 AWG. An outer layer of insulation surrounds the pairs. The conductors may be solid or stranded. A stranded conductor, which consists of many small-diameter wires, can withstand repeated flexing and is a good choice for patch cords and other uses where you’re likely to move or reroute the cable frequently. A solid conductor, which is a single, larger-diameter wire, has better electrical performance but may break after repeated flexing. Solid conductors are a good choice inside walls and in other locations where the wiring doesn’t move once installed. In the EIA/TIA-568-B standard, Category 5e cable replaces the Category 5 cable recommended in earlier editions of the standard. Although Category 5 cable is acceptable for all three Ethernet speeds, Category 5e cable has stricter manufacturing and testing specifications, and the standard recommends it for new installations. In 2002, an addendum to the standard defined a new cable type, Category 6 cable, which has improved performance over Category 5e. Category 6 cable is suitable for all twisted-pair Ethernet networks with the exception of some networks in industrial environments, as explained later in this networking tutorial. For 10-Mb/s Ethernet, you can use lower-quality Category 3 cable, but Category 5 or greater doesn’t cost much more and won’t need changing if you later upgrade to a faster speed. A 10-Mb/s or Fast Ethernet cable segment uses two of the four pairs in a cable. One pair carries data in each direction. In a 10-Mb/s segment, phone wiring can use the other two pairs, though there is some risk of interference and the wires will need to be moved if you upgrade to a higher speed. So it’s best to use separate cables for phone lines and networking if possible. Gigabit Ethernet uses all four wire pairs in a complex signaling protocol that enables all eight wires to transmit and receive at the same time.

All three speeds can use twisted-pair cable segments of up to 100 meters. The usual connectors for twisted-pair Ethernet are RJ-45 plugs and jacks. A cable segment’s performance is no greater than the lowest rating of the cable, its connectors, and the jacks the connectors plug into. To gain the benefit of a cable’s rated performance, the cable should use connectors with the same or better rating, and the connectors should plug into jacks with the same or better rating. For example, Category 5e cable should use connectors and jacks rated as Category 5e or Category 6. Ethernet transceivers are designed to work with cables that have a characteristic impedance of 100 ohms. A cable’s characteristic impedance is the input impedance of an infinite, open line. The value varies with the wire diameter, the spacing of the wires in relation to each other in the cable, and the insulation type. A line’s physical length has no effect on its characteristic impedance. There are techniques for calculating or measuring characteristic impedance, but you shouldn’t have to resort to these. Cable manufacturers specify the characteristic impedance of cables suitable for networking. Category 3 through Category 6 cable all have 100-ohm characteristic impedance. Using cable with a different characteristic impedance can degrade signal quality and result in data errors. The wires in the twisted-pair cables are color-coded (Table 2-2), with a color (blue, orange, green, or brown) assigned to each twisted pair. One wire in the pair is predominantly white with colored stripes or splashes and the other wire in the pair is predominantly colored, with optional white stripes or splashes. For example, if one wire in a pair is blue, the other wire in the pair is predominantly white with blue stripes. The electrical interface for twisted-pair cable uses differential signaling, which requires two wires to carry a signal. The voltage on one wire is the negative, or complement, of the voltage on the other wire. The receiver detects the difference between the voltages, and any noise that is common to both wires cancels out. A line that uses differential signaling is called a balanced line.

Shielded Twisted-pair Cable

Unshielded cables aren’t the only choices available for twisted-pair cables. Ethernet networks can also use shielded cable. A shield can reduce noise due to capacitive, electromagnetic, and high-frequency magnetic coupling. The TIA-EIA-568 standard specifies requirements for two shielded-cable types suitable for Ethernet networks: 100-ohm screened twisted-pair (ScTP) cable and 150-ohm shielded twisted-pair (STP) cable. In ScTP cable, the shielding consists of a layer of plastic and metal tape surrounding the pairs and a conductive drain wire or braid contacting the metal side of the tape. In STP cable, each pair has its own shield as well. STP cable requires 100Ω:150Ω transformers for impedance matching between the 150-ohm cable and the 100-ohm network interface. Connectors for shielded cable must also be shielded, with a continuous shield from the cable to the connector. The shield should be grounded at one end only. Shielded cable is more expensive and many networks don’t need it. For 10-Mb/s networks, the IEEE 802.3 standard doesn’t forbid shielded cable but says that unshielded twisted pairs meet most networks’ requirements. A Fast Ethernet cable segment may use unshielded or shielded twisted-pair cable. For Gigabit Ethernet, the standard just says that the use of shielded cable is outside the scope of the document.