Bounded dan Unbounded Media

Data Transmission

Data transmission is the process of conveying data between two points by way of a communication medium.  A wide variety of media are available, but they fall into two classes:  bounded and unbounded.

Bounded media confine the data to specific physical pathways.  Common examples of bounded media are wire and optical fiber cables.  Cable TV uses bounded media.

Unbounded media transmit the data-carrying signal through space, independent of a cable.  Broadcast radio and television are examples of unbounded media.

Bounded Media

By far the most common media employed for data transmission are defined as bounded -- the data signal is confined in a specific transmission pathway.  When practical, cable represents a low-cost and reliable means of transmitting data between computing devices.

Practicality is a relative thing.  Certainly cables are likely to be the logical choice within a building or even a building complex.  It may not be possible, however, to run a cable between two buildings on different sides of a public road, and it is certainly a major undertaking when the buildings are located on different continents.  Such conditions may call for use of unbounded media.

You should be alert to several characteristics when examining cables:

1. Resistance to electromagnetic interference (EMI).
2. Bandwidth, the range of frequencies that the cable can accommodate.  LANs generally carry data rates of 1 to 100 megabits per second and require moderately high bandwidth.
3. Attenuation characteristics. Attenuation describes how cables reduce the strength of a signal with distance.  Resistance is one factor that contributes to signal attenuation.
4. Cost.

NOTE: EMI (ElectroMagnetic Interference) can be a major headache for LAN technicians.  Many electrical devices generate magnetic fields that produce unwanted electrical currents in data cables.  The noise that results from these currents can degrade data signals, sometimes stopping communication altogether due to excessive error rates.  Electrical motors and fluorescent lights are common sources of EMI, and it can be a genuine challenge to cable a network in environments such as factories that contain many electrical devices.

Cable Types

Cables fall into two broad categories -- electrical conductors and fiber optic -- with various types of cables available in each category.  Prior to an examination of fiber optic cables, this section examines two types of electrical cables: coaxial and twisted pair.

NOTE: Electrical cable types are frequently referred to as "copper"  because that metal is the most frequently used conductor.  You may hear fiber optic cables called simply "fiber" or "glass".

Coaxial Cable

This type of cable is called coaxial (or coax for short) because two conductors share a COmmon AXis.  A typical coaxial cable has the following components:

• Center conductor.  This conductor usually consists of a fairly heavy, solid yet flexible wire; stranded wires can also be used.  Solid conductors are preferred for permanent wiring, but stranded conductors make the cable more flexible and easier to connect to equipment.
• Insulation layer.  Also called a dielectric layer, this layer provides electrical insulation and keeps the inner and outer conductors in precise coaxial relationship.
• Outer conductor or shield.  This layer shields the inner conductor from outside electrical interference.  The shield can consist of braided wires, metal foil, or a combination of both.  Because of this shield, coax is highly resistant to electromagnetic interference (EMI).
• Jacket or sheath.  A durable PVC plastic or Teflon jacket coats the cable to prevent damage.

Coax has many desirable characteristics.  It is highly resistant to EMI and can support high bandwidths.  Some types of coax have heavy shields and center conductors to enhance these characteristics and to extend the distances that signals can be transmitted reliably.

A wide variety of coax cable is available.  You must use cable that exactly matches the requirements of a particular type of network.  Coax cables vary in a measurement known as the impedance (measured in a unit called the ohm), which is an indication of the cable's resistance to current flow.  The specifications of a given cabling standard indicate the required impedance of the cable.

Here are some common examples of coaxial cables used in LANs, along with their impedances, and the LAN standards with which they are associated:

• RG-8 and RG-11 are 50 ohm coax cables required for thickwire Ethernet.  (10Base5 - ThinkNet)
• RG-58 is a smaller 50 ohm coax cable required for use with thinwire Ethernet.  (10Base2 - ThinNet)
• RG-59 is a 75 ohm coax cable most familiar when used to wire cable TV (CATV)  and is also used to cable broadband Ethernet (10Broad36).
• RG-62 is a 93 ohm cable used for ARCnet.  It is also commonly employed to wire terminals in an IBM SNA (minicomputers & mainframes) network.

Some advantages of coaxial cable are as follows:

• Highly insensitive to EMI
• Supports high bandwidths
• Heavier types of coax are sturdy and can withstand harsh environments
• Represents a mature technology that is well understood and consistently applied among vendors

Coax also has some disadvantages including the following:

• Although fairly insensitive to RF, coax remains vulnerable to EMI in harsh conditions such as factories.
• Coax is among the most expensive types of wire cables.
• Coax can be bulky.

Twisted Pair

This image shows how two conductors are twisted together to form the cable type known as twisted pair (TP/UTP).  Cables can be constructed of multiple twisted pairs of cables contained in a common jacket.

The twists in the conductor pairs are an important part of the electrical characteristics of UTP cable.  Twists reduce the cable's sensitivity to outside EMI and the degree to which the cables radiate radio frequency signals (RF).  Remember that the frequencies at which LANs operate fall into the range of radio signals.  If UTP cable is left insufficiently twisted when terminated (end connectors put on), the cable can function as an antenna and radiate significant amounts of radio signals (NEXT) that can interfere with local broadcast reception equipment.

Twisted pair cable used in early networks was most frequently surrounded by a braided shield that served to reduce both EMI sensitivity and radio emissions.  An example of this shielded twisted pair (STP) cable is IBM Type 1, Type 6, and Type 9 cable used in Token Ring installations.  In the past, shielded twisted pair cable (STP) was required for all high-performance networks such as IBM Token Ring.  STP cable, however, is expensive and bulky, and manufacturers of network equipment have devoted extensive research to enabling high-speed networks to work with unshielded twisted pair (UTP).

UTP is the cost leader among network cables.  The 10Base-T, 100Base-TX, and Gigabit Ethernet standards define Ethernet configurations that utilizes UTP.  Work by IBM and other vendors have developed network equipment that can use UTP even for the higher speed 16 megabit per second Token Ring.  In most cases, UTP cable is implemented using telephone-type modular connectors such as the RJ-11 (2 pair) and RJ-45 (4 pair) connectors.  Telephone type modular connectors are inexpensive and easy to install, serving to further reduce the cost of UTP cabling systems.

NOTE: UTP looks much like the cable used to wire voice grade telephones.  In newer telephone installations, it may indeed be possible to use the wiring installed for the voice grade telephone system as data grade cable in a network.  UTP cable comes in a variety of grades, ranging from CAT3 (low quality) to CAT6 (high quality).  When investigating the use of UTP cabling, be sure to determine the cable quality required for your network.

When utilizing UTP cable, it is necessary to ensure that all components in the data network are data grade.  Voice grade components used in voice telephone systems are not of sufficiently high quality for the bandwidth needed for networking.

Shielded twisted pair cable (STP) types are the standard cables specified for IBM's Token Ring networks and for Apple's LocalTalk.

Unshielded twisted pair cables (UTP) can be utilized for some configurations of Token Ring, Ethernet, and ARCnet networks.

Here are some advantages of twisted pair wiring:

• Telephone cable standards are mature and well established.  Materials are plentiful, and a wide variety of cable installers are familiar with the installation requirements.
• It may be possible to use in-place telephone wiring if it is of sufficiently high quality.
• UTP represents the lowest cost cabling.  The cost for STP is higher and is comparable to the cost of coaxial cable.

Some disadvantages of twisted pair are as follows:

• STP can be expensive and difficult to work with.
• Compared to fiber optic cable, all UTP cable is more sensitive to EMI.
• UTP especially may be unsuitable for use in high-EMI environments.
• Cable segment lengths are also more limited with UTP.
• UTP cables are regraded as being less suitable for high-speed transmissions than coax or fiber optic.  Technology advances, however, are pushing upward the data rates possible with UTP.

Fiber Optic

Fiber optic cables utilize light waves to transmit data through a thin glass or plastic fiber.  The structure of a typical fiber optic cable is shown in the graphic.  The parts of the fiber optic cable are as follows:

• The light conductor is a very fine fiber core.  Glass is the most common material, allowing signals to be transmitted for several kilometers without being refreshed.  Plastic is used in some circumstances, but plastic cables allow only short cable runs.
• The cladding is a glass layer that surrounds the optical fiber core.  The optical characteristics of the cladding reflect light back to the core, ensuring that very little of the light signal is lost.
• A sheath or jacket protects the cable from damage. A single sheath can be used to bundle multiple core/cladding fibers into a multi-fiber cable.

The light signals on fiber optic cables are generated either by light emitting diodes (LEDs) or by injection laser diodes (ILDs), which are similar to LEDs but produce laser light.  The purity of laser light is desirable, increasing both data rates and transmission distance.  Light Signals are received by photodiodes, solid state devices that detect variations in light intensity.

The interface devices required to operate with fiber optic cable are more expensive than those required for copper cable.  The higher cost is the result of several factors, including cost of the components and tighter design characteristics because fiber optic cables generally are operated at high data rates.  The cost of fiber optic cable installation, however, is trending downward.

Fiber optic cables have many desirable characteristics.  Because the fibers are small in diameter, a cable of a given size can contain more fibers than copper wire pairs.  Because fiber optic cables use light pulses instead of electrical signals, they offer very high bandwidth.  Bandwiths of 100 megabits (million bits per second) are commonplace, and bandwidths in the Gigabit (billion bits per second) and 10 Gigibit (10 billion bits per second) range are available.

Because the signal in a fiber optic cable consists of light pulses, the signal cannot be affected by electromagnetic interference.  Nor can the cables radiate radio frequency noise.  Optical fibers are, therefore, suitable for use in the noisiest and most sensitive environments.  Because these cables radiate no electromagnetic energy, it is impossible to intercept the data signal with electronic eavesdropping equipment.  Fiber optic transmissions are extremely secure.

Installation of fiber optic cable requires greater skill than is necessary to install most copper cables.  Cables must not be bent too sharply, and connectors must be installed by skilled technicians using special tools.  However, new connector technologies have simplified installation and reduced cost.

Here are some advantages of fiber optic cable:

• Very high bandwidth.
• Immunity to EMI;  fiber optic cables can be used in environments that make copper wire cables unusable.
• No radio frequency (RF) emissions; signals on fiber optic cables cannot interfere with nearby electronic devices and cannot be detected by conventional electronic eavesdropping techniques.

Summary of Cable Characteristics
Cable Type	Cable Cost	Installation Cost	EMI Sensitivity	Data Bandwidth
UTP STP Coax Fiber Optic	Lowest Medium Medium Highest	Lowest Moderate Moderate Highest	Highest Low Low None	Lowest Moderate High Very high

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