Ethernet Cabling
Twisted-Pair Cables
The networks in most homes and schools are wired with twisted-pair copper cable. This type of cable is inexpensive compared to other types of cabling, and it is readily available. The Ethernet patch cables that you can purchase through the internet or at a retail store are an example of copper twisted-pair cable.
Twisted-pair cables consist of one or more pairs of insulated copper wires that are twisted together and housed in a protective jacket. Like all copper cables, twisted-pair uses pulses of electricity to transmit data.
Data transmission over copper cable is sensitive to electromagnetic interference (EMI), which can reduce the data throughput rate that a cable can provide. Common items in a home that can create EMI include microwave ovens and fluorescent light fixtures.
Another source of interference, known as crosstalk, occurs when cables are bundled together for long lengths. The electrical impulses from one cable can cross over to an adjacent cable. This occurs most frequently when cables are improperly installed and terminated. When data transmission is corrupted due to interference such as crosstalk, the data must be retransmitted. This can degrade the data carrying capacity of the medium.
The figure illustrates how data transmission is affected by interference.
Types of Twisted-Pair Cables
There are two commonly installed types of twisted-pair cable:
UTP Cable
UTP cable is inexpensive, offers a high bandwidth, and is easy to install. This type of cable is used to connect workstations, hosts and network devices. It can come with many different numbers of pairs inside the jacket, but the most common number of pairs is four. Each pair is identified by a specific color code.
STP Cable
There are electrical environments in which EMI and RFI are so strong that shielding is a requirement to make communication possible, such as in a factory. In this instance, it may be necessary to use a cable that contains shielding, such as shielded twisted-pair (STP). Unfortunately, STP cables are very expensive, not as flexible, and have additional requirements because of the shielding that make them difficult to work with.
Many different categories of UTP cables have been developed over time, as shown in the table. Each category of cable was developed to support a specific technology and most are no longer encountered in homes or offices. The cable types which are still commonly found include Categories 3, 5, 5e, and 6.
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All categories of data grade UTP cable are traditionally terminated into an?RJ-45?connector. There are still some applications that require the smaller?RJ-11?connector, such as analog phones and some fax machines. In the figure below, an example of an RJ-11 connector is on the left. The RJ-45 connector is on the right.
RJ-11 and RJ-45 Connectors
Twisted-Pair Wiring Schemes
Have you ever looked closely at the plastic RJ-45 connector at the end of an Ethernet patch cable? Did you ever wonder why each of the wires terminating in the connector has a specific color or pattern? The color coding of the wire pairs in an UTP cable is determined by the type of standard that is used to make the cable. Different standards have different purposes and are closely governed by the standards organizations.
For typical Ethernet installations, there are two standards that are widely implemented. The TIA/EIA organization defines two different patterns, or wiring schemes, called T568A and T568B, as shown in the figure. Each wiring scheme defines the pinout, or order of wire connections, on the end of the cable.
On a network installation, one of the two wiring schemes (T568A or T568B) should be chosen and followed. It is important that the same wiring scheme is used for every termination in that project.
Twisted-Pair Transmit and Receive Pairs
Ethernet NICs and the ports on networking devices are designed to send data over UTP cables. Specific pins on the connector are associated with a transmit function and a receive function. The interfaces on each device are designed to transmit and receive data on designated wires within the cable.
When two devices are directly connected using an UTP Ethernet cable, it is important that the transmit function and the receive function on each end of the cable are reversed. One device sends data on a specific set of wires and the device on the other end of the cable listens for the data on the same wires.
Two devices that use different wires for transmit and receive are known as unlike devices. They require a straight-through cable to exchange data. Straight-through cables have the same color patterns on both ends of the cable.
Devices that are directly connected and use the same pins for transmit and receive, are known as like devices. They require the use of a crossover cable in order to reverse the transmit function and receive function so that the devices can exchange data.