Network Media
 


Network media is the actual path over which an electrical signal travels as it moves from one component to another. This chapter describes the common types of network media, including twisted-pair cable, coaxial cable, fiber-optic cable, and wireless.

Twisted-Pair Cable

Twisted-pair cable is a type of cabling that is used for telephone communications and most modern Ethernet networks. A pair of wires forms a circuit that can transmit data. The pairs are twisted to provide protection against crosstalk, the noise generated by adjacent pairs. When electrical current flows through a wire, it creates a small, circular magnetic field around the wire. When two wires in an electrical circuit are placed close together, their magnetic fields are the exact opposite of each other. Thus, the two magnetic fields cancel each other out. They also cancel out any outside magnetic fields. Twisting the wires can enhance this cancellation effect. Using cancellation together with twisting the wires, cable designers can effectively provide self-shielding for wire pairs within the network media.

Two basic types of twisted-pair cable exist: unshielded twisted pair (UTP) and shielded twisted pair (STP). The following sections discuss UTP and STP cable in more detail.

UTP Cable

UTP cable is a medium that is composed of pairs of wires. UTP cable is used in a variety of networks. Each of the eight individual copper wires in UTP cable \is covered by an insulating material. In addition, the wires in each pair are twisted around each other.

Unshielded Twisted-Pair Cable

UTP cable relies solely on the cancellation effect produced by the twisted wire pairs to limit signal degradation caused by electromagnetic interference (EMI) and radio frequency interference (RFI). To further reduce crosstalk between the pairs in UTP cable, the number of twists in the wire pairs varies. UTP cable must follow precise specifications governing how many twists or braids are permitted per meter (3.28 feet) of cable.

UTP cable often is installed using a Registered Jack 45 (RJ-45) connector. The RJ-45 is an eight-wire connector used commonly to connect computers onto a local-area network (LAN), especially Ethernets.

RJ-45 Connectors

When used as a networking medium, UTP cable has four pairs of either 22- or 24-gauge copper wire. UTP used as a networking medium has an impedance of 100 ohms; this differentiates it from other types of twisted-pair wiring such as that used for telephone wiring, which has impedance of 600 ohms.

UTP cable offers many advantages. Because UTP has an external diameter of approximately 0.43 cm (0.17 inches), its small size can be advantageous during installation. Because 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 installing a network in an older building. UTP cable 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. And because UTP can be used with most of the major networking architectures, it continues to grow in popularity.

Disadvantages also are involved in using twisted-pair cabling, however. 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.

Although UTP was once considered to be slower at transmitting data than other types of cable, this is no longer true. In fact, UTP is considered the fastest copper-based medium today. The following summarizes the features of UTP cable:

  • Speed and throughput—10 to 1000 Mbps

  • Average cost per node—Least expensive

  • Media and connector size—Small

  • Maximum cable length—100 m (short)

Commonly used types of UTP cabling are as follows:

  • Category 1—Used for telephone communications. Not suitable for transmitting data.

  • Category 2—Capable of transmitting data at speeds up to 4 megabits per second (Mbps).

  • Category 3—Used in 10BASE-T networks. Can transmit data at speeds up to 10 Mbps.

  • Category 4—Used in Token Ring networks. Can transmit data at speeds up to 16 Mbps.

  • Category 5—Can transmit data at speeds up to 100 Mbps.

  • Category 5e —Used in networks running at speeds up to 1000 Mbps (1 gigabit per second [Gbps]).

  • Category 6—Typically, Category 6 cable consists of four pairs of 24 American Wire Gauge (AWG) copper wires. Category 6 cable is currently the fastest standard for UTP.

Shielded Twisted-Pair Cable

Shielded twisted-pair (STP) cable combines the techniques of shielding, cancellation, and wire twisting. Each pair of wires is wrapped in a metallic foil . The four pairs of wires then are wrapped in an overall metallic braid or foil, usually 150-ohm cable. As specified for use in Ethernet network installations, STP reduces electrical noise both within the cable (pair-to-pair coupling, or crosstalk) and from outside the cable (EMI and RFI). STP usually is installed with STP data connector, which is created especially for the STP cable. However, STP cabling also can use the same RJ connectors that UTP uses.

Shielded Twisted-Pair Cable

Although STP prevents interference better than UTP, it is more expensive and difficult to install. In addition, the metallic shielding must be grounded at both ends. If it is improperly grounded, the shield acts like an antenna and picks up unwanted signals. Because of its cost and difficulty with termination, STP is rarely used in Ethernet networks. STP is primarily used in Europe.

The following summarizes the features of STP cable:

  • Speed and throughput—10 to 100 Mbps

  • Average cost per node—Moderately expensive

  • Media and connector size—Medium to large

  • Maximum cable length—100 m (short)

When comparing UTP and STP, keep the following points in mind:

  • The speed of both types of cable is usually satisfactory for local-area distances.

  • These are the least-expensive media for data communication. UTP is less expensive than STP.

  • Because most buildings are already wired with UTP, many transmission standards are adapted to use it, to avoid costly rewiring with an alternative cable type.

Coaxial Cable

Coaxial cable consists of a hollow outer cylindrical conductor that surrounds a single inner wire made of two conducting elements. One of these elements, located in the center of the cable, is a copper conductor. Surrounding the copper conductor is a layer of flexible insulation. Over this insulating material is a woven copper braid or metallic foil that acts both as the second wire in the circuit and as a shield for the inner conductor. This second layer, or shield, can help reduce the amount of outside interference. Covering this shield is the cable jacket. 

Coaxial Cable

Coaxial cable supports 10 to 100 Mbps and is relatively inexpensive, although it is more costly than UTP on a per-unit length. However, coaxial cable can be cheaper for a physical bus topology because less cable will be needed. Coaxial cable can be cabled over longer distances than twisted-pair cable. For example, Ethernet can run approximately 100 meters (328 feet) using twisted-pair cabling. Using coaxial cable increases this distance to 500m (1640.4 feet).

For LANs, coaxial cable offers several advantages. It can be run with fewer boosts from repeaters for longer distances between network nodes than either STP or UTP cable. Repeaters regenerate the signals in a network so that they can cover greater distances. Coaxial cable is less expensive than fiber-optic cable, and the technology is well known; it has been used for many years for all types of data communication.

When working with cable, you need to consider its size. As the thickness, or diameter, of the cable increases, so does the difficulty in working with it. Many times cable must be pulled through existing conduits and troughs that are limited in size. Coaxial cable comes in a variety of sizes. The largest diameter (1 centimeter [cm]) was specified for use as Ethernet backbone cable because historically it had greater transmission length and noise-rejection characteristics. This type of coaxial cable is frequently referred to as Thicknet. As its nickname suggests, Thicknet cable can be too rigid to install easily in some situations because of its thickness. The general rule is that the more difficult the network medium is to install, the more expensive it is to install. Coaxial cable is more expensive to install than twisted-pair cable. Thicknet cable is almost never used except for special-purpose installations.

A connection device known as a vampire tap was used to connect network devices to Thicknet. The vampire tap then was connected to the computers via a more flexible cable called the attachment unit interface (AUI). Although this 15-pin cable was still thick and tricky to terminate, it was much easier to work with than Thicknet.

In the past, coaxial cable with an outside diameter of only 0.35 cm (sometimes referred to as Thinnet) was used in Ethernet networks. Thinnet was especially useful for cable installations that required the cable to make many twists and turns. Because it was easier to install, it was also cheaper to install. Thus, it was sometimes referred to as Cheapernet. However, because the outer copper or metallic braid in coaxial cable comprises half the electrical circuit, special care had to be taken to ensure that it was properly grounded. Grounding was done by ensuring that a solid electrical connection existed at both ends of the cable. Frequently, however, installers failed to properly ground the cable. As a result, poor shield connection was one of the biggest sources of connection problems in the installation of coaxial cable. Connection problems resulted in electrical noise, which interfered with signal transmittal on the networking medium. For this reason, despite its small diameter, Thinnet no longer is commonly used in Ethernet networks.

The most common connectors used with Thinnet are BNC, short for British Naval Connector or Bayonet Neill Concelman, connectors. The basic BNC connector is a male type mounted at each end of a cable. This connector has a center pin connected to the center cable conductor and a metal tube connected to the outer cable shield. A rotating ring outside the tube locks the cable to any female connector. BNC T-connectors are female devices for connecting two cables to a network interface card (NIC). A BNC barrel connector facilitates connecting two cables together.

Thinnet and BNC Connector

The following summarizes the features of coaxial cables:

  • Speed and throughput—10 to 100 Mbps

  • Average cost per node—Inexpensive

  • Media and connector size—Medium

  • Maximum cable length—500 m (medium)

 


 

Primary Cable Types

The vast majority of networks today are connected by some sort of wire or cabling, which act as the network transmission medium carrying signals between computers. There is a variety of cable that can meet the varying needs and sizes of networks, from small to large.

Cabling can be confusing. Belden, a leading cable manufacturer, publishes a catalog that lists more than 2,200 types of cabling. Fortunately, only three major groups of cabling connect the majority of networks:

  • Coaxial

    Twisted-pair

    • Unshielded twisted-pair

    • Shielded twisted-pair

  • Fiber-optic

The next part of this lesson will describe the features and components of these three major cable types. Understanding their differences will help you determine when to use each type of cabling.

Coaxial

At one time, coaxial cable was the most widely used network cabling. There were a couple of reasons for coaxial's wide usage. Coaxial was relatively inexpensive, and it was light, flexible, and easy to work with. It was so popular that it became a safe, easily supported installation.

In its simplest form, coaxial consists of a core made of solid copper surrounded by insulation, a braided metal shielding, and an outer cover. One layer of foil insulation and one layer of braided metal shielding is referred to as dual shielded. However, quad shielding is available for environments that are subject to higher interference. Quad shielding consists of two layers of foil insulation and two layers of braided metal shielding.

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Figure 2.1: Coaxial cable showing various layers

Shielding refers to the woven or stranded metal mesh (or other material) that surrounds some types of cabling. Shielding protects transmitted data by absorbing stray electronic signals, called noise, so that they do not get onto the cable and distort the data.

The core of a coaxial cable carries the electronic signals which make up the data. This core wire can be either solid or stranded. If the core is solid, it is usually copper.

The core is surrounded by a dielectric insulating layer which separates it from the wire mesh. The braided wire mesh acts as a ground and protects the core from electrical noise and crosstalk. Crosstalk is signal overflow from an adjacent wire.

The conducting core and the wire mesh must always be separated from each other. If they touch, the cable will experience a short, and noise or stray signals on the mesh will flow onto the copper wire. This will destroy the data.

The entire cable is surrounded by a non-conducting outer shield, usually made of rubber, Teflon, or plastic.

Coaxial cable is more resistant to interference and attenuation than twisted-pair cabling. Attenuation is the loss of signal strength which begins to occur as the signal travels further along a copper cable.

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Figure 2.2: Attenuation causes signals to deteriorate

The stranded, protective sleeve can absorb stray electronic signals so they do not affect data being sent over the inner copper cable. For this reason, coaxial is a good choice for longer distances and for reliably supporting higher data rates with less sophisticated equipment.

Types of Coaxial Cable

There are two types of coaxial cable:

  • Thin (thinnet)

  • Thick (thicknet)

What type you select depends on the needs of your particular network.

Thinnet

Thinnet is a flexible coaxial cable about .25 inch thick. Because this type of coaxial is flexible and easy to work with, it can be used in almost any type of network installation. Networks that use thinnet have the cable connected directly to a computer's network adapter card.

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Figure 2.3: Close-up view of thinnet cable showing where it connects to a computer

Thinnet coaxial cable can carry a signal up to approximately 185 meters (about 607 feet) before the signal starts to suffer from attenuation.

Cable manufacturers have agreed upon certain designations for different types of cable. Thinnet is included in a group referred to as the RG-58 family and has a 50-ohm impedance. Impedance is the resistance, measured in ohms, to alternating current flowing in a wire. The main difference in the RG-58 family is the center core of copper. It can either be a stranded wire or solid copper core.

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Figure 2.4: RG-58 coaxial showing stranded wire and the solid copper cores

Cable

Description

RG-58 /U

Solid copper core

RG-58 A/U

Stranded wire core

RG-58 C/U

Military specification of RG-58 A/U

RG-59

Broadband transmission such as cable television

RG-6

Larger in diameter and rated for higher frequencies than RG-59, but used for broadband transmissions as well

RG-62

ArcNet networks

Thicknet

Thicknet is a relatively rigid coaxial cable about 0.5 inch in diameter. It is sometimes referred to as Standard Ethernet because it was the first type of cable used with the popular network architecture Ethernet. The copper core is thicker than a thinnet core.

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Figure 2.5: Thicknet cable has a thicker core than thinnet

The thicker the copper core, the farther the cable can carry signals. This means that thicknet can carry signals farther than thinnet. Thicknet can carry a signal for 500 meters (about 1,640 feet). Therefore, because of thicknet's ability to support data transfer over longer distances, it is sometimes used as a backbone to connect several smaller thinnet-based networks.

A device called a transceiver connects the thinnet coaxial to the larger thicknet coaxial cable.

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Figure 2.6: Thicknet cable transceiver with detail of a vampire tap piercing the core

A transceiver designed for thicknet Ethernet includes a connector known as a vampire tap or a piercing tap to make the actual physical connection to thicknet core. This connector is pierced through the insulating layer and makes direct contact with the conducting core. Connection from the transceiver to the network adapter card is made using a transceiver cable (drop cable) to connect to the attachment unit interface (AUI) port connector on the card. An AUI port connector for thicknet is also known as a Digital Intel Xerox (DIX) connector after the three companies that developed it and its related standards, or as a DB-15 connector.

Thinnet vs. Thicknet

As a general rule, the thicker the cable, the more difficult it is to work with. Thin cable is flexible, easy to install, and relatively inexpensive. Thick cable does not bend easily and is, therefore, harder to install. This is a consideration when an installation calls for pulling cable through tight spaces such as conduits and troughs. Thick cable is more expensive than thin cable, but will carry a signal farther.

Coaxial Connection Hardware

Both thinnet and thicknet use connection components, known as a BNC (British Naval Connector), to make the connections between the cable and the computers. There are several important components in the BNC family, including the following:

  • The BNC cable connector

    The BNC cable connector is either soldered or crimped to the end of a cable.

    Figure 2.7: BNC cable connector

    Figure 2.7: BNC cable connector
  • The BNC T connector

    This connector joins the network interface card in the computer to the network cable.

    Figure 2.8: BNC T connector

    Figure 2.8: BNC T connector
  • The BNC barrel connector

    This connector is used to join two lengths of thinnet cable to make one longer length.

    Figure 2.9: BNC barrel connector

    Figure 2.9: BNC barrel connector
  • The BNC terminator

    A BNC terminator closes each end of the bus cable to absorb stray signals. Without BNC terminators, a bus network will not function.

    Figure 2.10: BNC terminator

    Figure 2.10: BNC terminator

Coaxial Cable Grades and Fire Codes

The type of cable grade that you should use depends on where the cables will be in your office. Coaxial cables come in two grades:

  • Polyvinyl chloride

  • Plenum

Polyvinyl chloride (PVC) is a type of plastic used to construct the insulation and the cable jacket for most types of coaxial cable. PVC coaxial cable is flexible and can be easily routed in the exposed areas of an office. However, when it burns, it gives off poisonous gases.

A plenum is the short space in many buildings between the false ceiling and the floor above; it is used to circulate warm and cold air through the building. Fire codes are very specific on the type of wiring that can be routed through this area, because any smoke or gas in the plenum will eventually become part of the air breathed by everyone in the building.

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Figure 2.11: Plenum grade cabling is required by fire code in the plenum

Plenum cabling refers to coaxial that contains special materials in its insulation and cable jacket. These materials are certified to be fire resistant and produce a minimum amount of smoke. This reduces poisonous chemical fumes. Plenum cable can be used in the plenum area and in vertical runs (for example, in a wall) without conduit. However, plenum cabling is more expensive and less flexible than PVC cable.

Note: Please consult your local fire and electrical codes for specific regulations about running networking cable in your office.

Coaxial Considerations

Consider these coaxial capabilities when making a decision on the type of cabling to use.

Use coaxial cable if you need:

  • A medium that will transmit voice, video, and data.

  • To transmit data longer distances than less expensive cabling can transmit.

  • A familiar technology that offers reasonable data security.