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Monday, February 8, 2010

Transmission Media



On any network, the various entities must communicate through some form of media. Just as humans can communicate through telephone wires or sound waves in the air, computers can communicate through cables, light, and radio waves. Transmission media enable computers to send and receive messages but do not guarantee that the messages will be understood.

Some of the most common network transmission media, such as coaxial cable, shielded twisted-pair cable, and unshielded twisted-pair cable, network fiber-optic cable and wireless communications.

Here are the types of transmission media

1. Twisted-pair cable
2. Coaxial cable
3. Fiber-optic cable
4. Wireless communications

Twisted-Pair Cable

Twisted-pair cable has become the dominant cable type for all new network designs that employ copper cable. Among the several reasons for the popularity of twisted-pair cable, the most significant is its low cost. Twisted-pair cable is inexpensive to install and offers the lowest cost per foot of any cable type.

A basic twisted-pair cable consists of two strands of copper wire twisted together This twisting reduces the sensitivity of the cable to EMI and also reduces the tendency of the cable to radiate radio frequency noise that interferes with nearby cables and electronic components. This is because the radiated signals from the twisted wires tend to cancel each other out. (Antennas, which are purposely designed to radiate radio frequency signals, consist of parallel, not twisted, wires.)

Twisting also controls the tendency of the wires in the pair to cause EMI in each other. Whenever two wires are in close proximity, the signals in each wire tend to produce noise, called crosstalk, in the other. Twisting the wires in the pair reduces crosstalk in much the same way that twisting reduces the tendency of the wires to radiate EMI.

Coaxial cables

Coaxial cables were the first cable types used in LANs. Coaxial cable gets its name because two conductors share a common axis; the cable is most frequently referred to as a coax.

The components of a coaxial cable are as follows:

* A center conductor, although usually solid copper wire, sometimes is made of stranded wire.
* An outer conductor forms a tube surrounding the center conductor. This conductor can consist of braided wires, metallic foil, or both. The outer conductor, frequently called the shield, serves as a ground and also protects the inner conductor from EMI.
* An insulation layer keeps the outer conductor spaced evenly from the inner conductor.
* A plastic encasement (jacket) protects the cable from damage.

Fiber optics cable

In almost every way, fiber-optic cable is the ideal cable for data transmission. Not only does this type of cable accommodate extremely high bandwidths, but it also presents no problems with EMI and supports durable cables and cable runs as long as several kilometers. The two disadvantages of fiber-optic, however, are cost and installation difficulty.

The center conductor of a fiber-optic cable is a fiber that consists of highly refined glass or plastic designed to transmit light signals with little loss. A glass core supports a longer cabling distance, but a plastic core is typically easier to work with. The fiber is coated with a cladding that reflects signals back into the fiber to reduce signal loss.
Fiber-optic network cable consists of two strands separately enclosed in plastic sheaths—one strand sends and the other receives. Two types of cable configurations are available: loose and tight configurations. Loose configurations incorporate a space between the fiber sheath and the outer plastic encasement; this space is filled with a gel or other material. Tight configurations contain strength wires between the conductor and the outer plastic encasement. In both cases, the plastic encasement must supply the strength of the cable, while the gel layer or strength wires protect the delicate fiber from mechanical damage.

Optical fiber cables don’t transmit electrical signals. Instead, the data signals must be converted into light signals. Light sources include lasers and light-emitting diodes (LEDs). LEDs are inexpensive but produce a fairly poor quality of light suitable for less-stringent applications.

A laser is a light source that produces an especially pure light that is monochromatic (one color) and coherent (all waves are parallel). The most commonly used source of laser light in LAN devices is called an injection laser diode (ILD). The purity of laser light makes lasers ideally suited to data transmissions because they can work with long distances and high bandwidths. Lasers, however, are expensive light sources used only when their special characteristics are required.

The end of the cable that receives the light signal must convert the signal back to an electrical form. Several types of solid-state components can perform this service.

One of the significant difficulties of installing fiber-optic cable arises when two cables must be joined. The small cores of the two cables (some are as small as 8.3 microns) must be lined up with extreme precision to prevent excessive signal loss.

Wireless Media The extraordinary convenience of wireless communications has placed an increased emphasis on wireless networks in recent years. Technology is expanding rapidly and will continue to expand into the near future, offering more and better options for wireless networks.

Presently, you can subdivide wireless networking technology into three basic types corresponding to three basic networking scenarios:

* Local area networks (LANs). Occasionally, you will see a fully wireless LAN, but more typically, one or more wireless machines will function as members of a cable-based LAN. A LAN with both wireless and cable-based components is called a hybrid.
* Extended local networks. A wireless connection serves as a backbone between two LANs. For instance, a company with office networks in two nearby but separate buildings could connect those networks using a wireless bridge.
* Mobile computing. A mobile machine connects to the home network using cellular or satellite technology.

The following sections describe these technologies and some of the networking options available with each.

Wireless point-to-point communications are another facet of wireless LAN technology. Point-to-point wireless technology specifically facilitates communications between a pair of devices (rather than attempting to achieve an integrated networking capability). For instance, a point-to-point connection might transfer data between a laptop and a home-based computer or between a computer and a printer. Point-to-point signals can pass through walls, ceilings, and other obstructions. Point-to-point provides data transfer rates of 1.2 to 38.4 Kbps for a range of up to 200 feet indoors (or one third of a mile for line-of-sight broadcasts).

Here are the types and frequencies used in radio and microwaves transmission.
Radio Frequencies

The frequency spectrum operates from 0 Hz (DC) to Gamma Rays (1019 Hz).

Name


Frequency (Hertz)


Examples

Gamma Rays


10^19 +



X-Rays


10^17



Ultra-Violet Light


7.5 x 10^15



Visible Light


4.3 x 10^14



Infrared Light


3 x 10^11



EHF - Extremely High Frequencies


30 GHz (Giga = 10^9)


Radar

SHF - Super High Frequencies


3 GHz


Satellite and Microwaves

UHF - Ultra High Frequencies


300 MHz (Mega = 10^6)


UHF TV (Ch. 14-83)

VHF - Very High Frequencies


30 MHz


FM / TV (Ch2 - 13)

HF - High Frequencies


3 MHz2


Short Wave Radio

MF - Medium Frequencies


300 kHz (kilo = 10^3)


AM Radio

LF - Low Frequencies


30 kHz


Navigation

VLF - Very Low Frequencies


3 kHz


Submarine Communications

VF - Voice Frequencies


300 Hz


Audio

ELF - Extremely Low Frequencies


30 Hz


Power Transmission

Radio Frequencies are in the range of 300 kHz to 10 GHz. We are seeing an emerging technology called wireless LANs. Some use radio frequencies to connect the workstations together, some use infrared technology.
Microwave
Microwave transmission is line of sight transmission. The Transmit station must be in visible contact with the receive station. This sets a limit on the distance between stations depending on the local geography. Typically the line of sight due to the Earth's curvature is only 50 km to the horizon! Repeater stations must be placed so the data signal can hop, skip and jump across the country.

Microwaves operate at high operating frequencies of 3 to 10 GHz. This allows them to carry large quantities of data due to the large bandwidth.

Advantages:

1. They require no right of way acquisition between towers.
2. They can carry high quantities of information due to their high operating frequencies.
3. Low cost land purchase: each tower occupies small area.
4. High frequency/short wavelength signals require small antenna.

Disadvantages:

1. Attenuation by solid objects: birds, rain, snow and fog.
2. Reflected from flat surfaces like water and metal.
3. Diffracted (split) around solid objects
4. Refracted by atmosphere, thus causing beam to be projected away from receiver.

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