Chapter 2: Traditional Transmission Media

Chapter 2: Traditional Transmission Media • • • • • • • • •

Introduction Copper Wires Glass Fibers Radio Frequency and Satellites Geosynchronous Satellites Low Earth Orbit Satellites and Arrays of them Microwave Infrared Light From A Laser

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Introduction • Existing types of transmission media including cables and wireless means are described here • At the lowest level, all computer communication involves – encoding data in a form of energy – and sending the energy across a transmission medium – Hardware devices attached to a computer perform the encoding and decoding of data

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About Cables •

Cables are the backbone of a network; all data/info runs through cables either in the form of Radio Frequency (RF) via coaxial or twisted pair or in the form of light via fiber optic.

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Q: How to choose the right cable? A: Based on your network needs consider the following factors: - Type of data to be transferred and security issues - Cost (cabling could take 25% - 40% of total network cost) - Installation and Maintenance (how easy?) - Reliability and speed - Distance affect in: 1- Signal strength and quality (Repeaters may be used!) 2- Possibility of packet collision (Token ring Vs Logical Bus) 3- Possibility of RF noise is likely (long antenna)

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Cost of Cabling •

Q: How to determine your cable cost? A: - Determine number of nodes - Do unit measurement first, and then total all units. Q: What kind of preparations you need to do? A: - Building facilities (built-in, over ceiling, or across the floor) - Testing, Repair and Maintenance Contract. - Fire requirement - Documentation (physical topology, ip-addresses, etc.) - Avoid interference (preventing RF noise) 1- Local Radio stations 2- Other network cables 3- Large motors

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Copper Wires •

Conventional computer network use wires as the primary medium – Copper used almost exclusively because its low resistance

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Network wire is chosen to minimize interference

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Interference arises because wire emit a small amount of electromagnetic energy, which can travel through the air

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Whenever it encounters another wire, an electromagnetic wave generates a small electric current in the wire. When two wires are placed close together and in parallel, a strong signal sent on one wire will generate a similar signal on the other

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Copper Wires (cont.) • Problem of interference is severe – because wires that comprise a network often are placed in parallel with many other wires

• To minimize interference, networks use one of three basic wiring types: – Twisted Pair • Unshielded Twisted Pair (UTP) • Shielded Twisted Pair (STP)

– Coaxial Cable – Fiber Optic (immunized from interference)

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Twisted Pair • The figure below illustrates a twisted-pair cable • Oldest and still most common used, because of: – Adequate performance – Low cost – Easy to install

• Twists change the electrical properties of the wire: – They limit the electromagnetic energy the wire emits: • So they help prevent radiating energy that interferes with other wires

– They make the pair of wires less liable to electromagnetic energy: • They help prevent signals on other wires from interfering with the pair

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Coaxial cable • Coaxial cables provides even more protection from interference than twisted pair • a coaxial cable consists of a single wire surrounded by a metal shield (Figure 4.2 below) that forms a flexible cylinder around the inner wire to provide a barrier for electromagnetic radiation – The barrier isolates the inner wire in two ways: • it protects the wire from incoming/radiating electromagnetic energy

• The cable can be placed parallel to other cables or bent and twisted around corners

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Shielded Twisted Pair (STP) The STP cable consists of a pair of wires surrounded by a metal shield • The additional shielding provided by STP or coaxial cabling is often used when wires from a network pass near equipment that generates strong electric or magnetic fields

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Glass Fibers • Network also use flexible glass fibers to transmit data – known as an optical fiber

• Medium uses light to transport data • The miniature glass fiber is encased in a plastic jacket – which allows the fiber to bend without breaking

• A transmitter at one end of a fiber uses – a light emitting diode (LED) or a laser to send pulses of light

• A receiver at the other end uses – a light sensitive transistor to detect the pulses

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Glass Fibers (cont.) Advantages: • Light neither cause electrical interference in other cables nor liable to electrical interference • Glass fibers can be manufactured to reflect most of the light inward can carry a pulse of light much farther than a copper wire signal • Light can encode more information than electrical signals  can carry more information than a wire & MORE SECURE • unlike electricity, which always requires a pair of wires connected into a complete circuit, Light can travel from one computer to another over a single fiber

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Glass Fibers (cont.) Disadvantages • Installing a fiber requires special equipment – that polishes the ends to allow light to pass through

• If a fiber breaks inside the plastic jacket: – finding the location of the problem is difficult

• Repairing a broken fiber is difficult – special equipment is needed to join two fibers

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Radio Frequency (RF) • In RF transmissions – each participating computer attaches to an antenna – Antenna can both transmit and receive RF

• Physically, the antennas used with RF networks may be large or small, depending on the range desired: – An antenna designed to propagate signals several miles • A metal pole approximately 2 meters long that is mounted vertically on top of a building

– An antenna designed to permit communication within a building • May be small enough to fit inside a portable computer (e.g., less than 20 centimeters)

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Satellites • RF technology can be combined with satellites – to provide communication across longer distances

• Figure 4.3 illustrates a satellite in orbit • The satellite contains a transponder – that consists of a radio receiver and transmitter

• The transponder – accepts an incoming radio transmission – amplifies it – and transmits the signal back toward the ground at a slightly different angle than it arrived

• A single satellite usually contains multiple transponders – Each transponder uses a different radio frequency (i.e., channel) 16

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Geosynchronous Satellites •

Communication satellites can be grouped into categories according to the height at which they orbit: – The easiest is geosynchronous or geostationary satellites – The name arises because a geosynchronous satellite is placed in an orbit that is exactly synchronized with the rotation of the earth. – Such an orbit is classified as a Geostationary Earth Orbit (GEO) – When viewed from the ground, • satellite appears to remain at exactly the same point in the sky at all times

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Laws of physics determine the exact distance from the earth that a satellite must orbit to remain synchronized with the earth's rotation – The distance is 35,785 kilometers or 22,236 miles

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Geosynchronous Satellites (cont.) • GEO is about one tenth of the distance to the moon – Engineers refer the distance as “high earth orbit”

• There is a limited amount of ``space'' available in the GEO above the equator – because satellites using a given frequency must be separated from one another to avoid interference • The minimum separation depends on the power of the transmitters

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Low Earth Orbit Satellites • Second category of satellites operate in what is called Low Earth Orbit (LEO) – which means that they orbit a few hundred miles above the earth (typically 200 to 400 miles)

• The chief disadvantage of a LEO lies in the rate at which a satellite must travel – Their period of rotation is faster than the rotation of the earth • LEOs do not stay above a single point on the earth's surface • An observer, who stands on the earth looking upward through a telescope, sees LEOs move across the sky

• A single satellite can complete an entire orbit in approximately 1.5 hours 20

Low Earth Orbit Satellites (cont.) From a communication provider's point of view: • having a satellite that does not appear to remain stationary causes problems: – First, the satellite can only be used during the time • that its orbit passes between two ground stations

– Second, maximal utilization requires complex control systems • that continuously move the ground stations so they point directly at the satellite

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Low Earth Orbit Satellite Arrays •

Instead of focusing on one satellite, – the scheme requires a communication company to launch a set of satellites into low earth orbits

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Although a given satellite orbits quickly, – the set of orbits is chosen so that each point on the ground has at least one satellite overhead at any time – sixty-six (66) satellites are required to provide service over the entire surface of the earth

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From the point of view of an observer on earth, – it appears that a satellite emerges from a point on the horizon – flies overhead – and then disappears into a point on the opposite horizon

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The key to the scheme lies in the set of orbits – guarantees at least one satellite is available at any time

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Low Earth Orbit Satellite Arrays (cont.) • In addition to transponders used to communicate with ground stations – an array of satellites in low earth orbit contains radio equipment used to communicate with other satellites in the array

• As they move through their orbits – the satellites communicate with one another and agree to forward data

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Microwave •

Many long-distance telephone companies use microwave (MW) to carry telephone conversations – A few large companies have also installed MW systems as part of the company's network

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MW are merely a higher frequency version of radio waves, but they behave differently – Instead of broadcasting in all directions, • a MW transmission can be aimed in a single direction, preventing others from intercepting

– In addition, MW transmission can carry more information than lower frequency RF transmissions

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MW cannot penetrate metal structures: – transmission works best in a clear path exists between two parties – most MW installations consist of two towers • that are taller than the surrounding buildings and vegetation

– each MW transmitter aimed directly at a MW receiver on the other

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Infrared • Infrared is limited to a small area (e.g., a single room) • Usually requires that the transmitter be pointed toward the receiver • Infrared HW – is inexpensive compared to other mechanisms, – and does not require an antenna

• It is possible to equip a large room with a single infrared connection – that provides network access to all computers – computers can remain in contact with the network while they are moved within the room

• Infrared network are especially convenient for small, portable computers 25

Light From A Laser • •

A beam of light can also be used to carry data through the air A communication link that uses light consists of two sites that each have a transmitter and receiver – equipment is mounted in a fixed position, often on a tower – aligned so the transmitter at one location sends its beam of light directly to the receiver at the other

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The transmitter uses a laser to generate the beam of light – because a coherent laser beam will stay focused over a long distance

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Light from a laser must travel in a straight line and must not be blocked A laser beam cannot penetrate vegetation or weather conditions such as snow and fog: – Thus, laser transmission has limited use 26