Transmission Media: Guided and Unguided

• Module 2: Transmission Media

Types of Transmission Media

• Guided media: Electromagnetic waves are guided along a solid medium like copper twisted pair, copper coaxial cable, and optical fiber.
• Unguided media: Wireless transmission occurs through the atmosphere, outer space, or water.

Guided Media: Twisted Pair

• Least expensive.
• Most widely used.

Transmission Medium and Physical Layer

• Transmission medium connects the physical layers of the sender and receiver.
• Can use cable or air.
• Guided media (wired) includes twisted pair, coaxial cable, and fiber optic cable.
• Unguided media (wireless) includes radio wave, micro wave, and infrared.

Guided Media: Twisted Pair - Physical Description

• A twisted pair consists of two insulated copper wires arranged in a regular spiral pattern.
• A wire pair acts as a single communication link.
• Often bundled into cables.
• Usually installed in buildings during construction.
• It is an advantage if the entire network doesn't go down if one part is damaged.
• Twisting reduces cross-talk interference.
• It is cost-effective and less expensive.
• It is easy to install and connect.
• It's easy to implement and terminate.
• Twisted pair is flexible to use.
• Can be used for both analog and digital transmissions.
• Multiple pairs are bundled together by wrapping them in a tough protective sheath.
• Over long distances, cables may contain hundreds of pairs.

Twisted Pair - Characteristics

• Twisting reduces cross-talk interference between adjacent pairs in a cable.
• Neighboring pairs in a bundle typically have different twist lengths to reduce cross talk.
• The twist length typically varies from 5 to 15 cm.
• The wires in a pair have a thickness of 0.4 to 0.9 mm.

Twisted Pair - Applications

• Used for transmission for both analog and digital signals.
• It is the most commonly used medium in the telephone network.
• Used for communications within buildings.
• In telephone systems, residential telephones are connected to a local telephone exchange by twisted pair wire; these are referred to as subscriber loops.
• Used for digital signaling.
• Supports voice traffic.
• Bandwidth: 1 MHz.
• Communication data rate: 64 kbps.
• Commonly used within a building for local area networks, supporting personal computers.
• For long distance communication, twisted pairs can be used at data rates of 4 Mbps or more.
• If one part of the network is damaged, the entire network doesn't go down.

Twisted Pair - Transmission Characteristics

• Twisted pair can be used for both analog and digital transmission.
• For analog signals, amplifiers are used at every 5 to 6 km.
• For digital signals, repeaters are required every 2 to 3 km.

Twisted Pair - Impairments and Solutions

• Attenuation for twisted pair is a strong function of frequency.
• Susceptible to interference and noise including impulse noise.
• Shielding the core with metallic braid or sheathing reduces interference.
• Twisting the wire reduces low-frequency interference and using different twist lengths in adjacent pairs reduces cross talk.

Twisted Pair - Signaling and Types

• For point-to-point analog signaling, a bandwidth of up to about 1 MHz is possible, accommodating a number of voice channels.
• Unshielded Twisted Pair (UTP): Ordinary telephone wire; subjected to external electromagnetic interference; can transfer data over long distances without repeaters; but very bulky.
• Shielded Twisted Pair (STP): Pairs of copper wires are twisted, and can shield each pair together.
• UTP is classified into Category 3 and Category 5.
• Category 3: 100-ohm twisted pair, 16 MHz frequency
• Category 5: The twist length is 0.6 to 0.85 cm, 100 MHz frequency, much more tightly twisted; the tighter twisting of Category 5 is more expensive but provides much better performance than Category 3.
• Wires that are not shielded but simply bundled together in a protective sheath are called UTP.
• UTP has a maximum length of 100 meters.

Coaxial Cable

• Also uses copper wires, but is better shielded than twisted pair cable.
• Stiff copper wire is covered by insulator, outer conductor (shield), and plastic cover.
• Physical description: Widely used for cable TV connections and LANs.
• Consists of two conductors.
• A hollow outer cylindrical conductor surrounds a single inner wire conductor.
• The outer conductor is covered with a jacket as shield, this outer conductor is metal braid.
• Inner conductor is solid metal.
• Has insulating material separating conductors
• Inner conductor is held in place by insulator rings.
• Single cable has diameter from 1 to 2.5cm.
• Can be used over longer distances and supports more stations on a shared line.
• Has high bandwidth.

Coaxial Cable - Characteristics

• It carries signals of higher frequency, rather than twisted pair cable.
• Coaxial cables are categorized by radio government (RG) ratings.

Coaxial Cable - Types

• RG-59: 75 impedance, used for Cable TV.
• RG-58: 50 impedance, used for Thin Ethernet.
• RG-11: 50 impedance, used for Thick Ethernet.

Coaxial Cable - Connectors

• To connect, coaxial cable needs connectors.
• A popular connector is the BNC connector.

Coaxial Cable - BNC Connectors

• BNC connector
• BNC-T connector
• BNC terminator

Coaxial Cable - Advantages

• Excellent noise immunity.
• Can travel longer distances at higher speed.
• Performs best over short distances.
• Less cost.
• Can transmit both analog and digital.
• Easy to wire and expand due to flexibility.
• Supports high bandwidth signal transmission.

Coaxial Cable - Disadvantages

• Signal loss over long distance.
• Signal leak at points of egress.
• Speed fluctuations under heavy usage.
• Bulky.
• More expensive for long distances as compared to twisted pair.
• Security is a great concern.
• Not compactible with TP.
• Less flexible.

Coaxial Cable - Use Cases

• Single coaxial cable is less flexible and has a diameter of 2.5 cm.
• Used for long distance communication (185 m to 500m).
• High bandwidth, less electromagnetic interference, thicker than twisted pair.
• Coaxial cable is a versatile transmission media, used in wide variety of applications
• The most important are television distribution, long distance telephone transmission, short range computer system links, local area networks.

Optical Fiber

• Data transmission in the form of light signals.
• Physical Description:
• A thin (2 to 125 μm), flexible medium capable of guiding an optical ray.
• Various glasses and plastics can be used to make optical fibers.
• Low loss when using ultra pure fused silica
• Plastic fibers are more economical, less costly and can be used for short haul links with moderately high loss

Optical Fiber - Disadvantages

• Expensive for production and installation
• Fragility.
• High precision requirements.
• An optical fiber cable has a cylindrical shape and consists of 3 concentric circles, namely the core, cladding, and jacket.
• The core is the inner most section, that consists of one or more thin strands of fibers, that are made of either glass or plastic, and can be from 8 to 50 μm in diameter and transmits the light signal.

Optical Fiber - Construction and Characteristics

• Each fiber is surrounded by its own cladding or glass/plastic coating coating that has optical properties that are different from that of the properties of the core, and the core has a diameter of 125 μm.

• The interface between the core and the cladding acts as reflector as to better transmit the signal forward.

• The outermost layer surrounding the core and the cladding, is composed of another layer, or material that acts as a jacket, that help protect the underlying material that protects against, abrasions, crushing, or other environmental layers.

Optical Fibre Cable Considerations

• Made up of silica glass or plastic, depending on whether the fiber is carrying the core
• Cladding is the Refractive Index.

Characteristics of Optical Fiber Cable

• High bandwidth for voice, video and data
• Carries data over long distances
• Not susceptible to EMI, but its installation is difficult.
• Advantages: High bandwidth, immune to electromagnetic interference.
• EMI resistant for electromagnetic isolation.
• Performs well over long distances,
• Small size, low weight and no electrical noise.
• Can travel long distances without signal weakening
• Disadvantages:
• More expensive.
• Harder to install
• Fragile.
• Light waves are unidirectional, so two frequencies are required for full duplex transmission.
• Distinguishing Characteristics versus Twisted and Coaxial Pairs: Greater capacity with data rates of hundreds of Gbps demonstrated, smaller size/lighter weight, considerably thinner, lower attenuation, and electromagnetic isolation.
• The system is not vulnerable to interference, impulse noise, or crosstalk.
• Provides Greater repeater spacing.

Optical Fibre Cable Considerations

• Five basic "application" categories:
• Long haul trunks: long haul routes average about 1500km and offer high capacity
• Metropolitan trunks: average circuits have an average length of 12km
• Rural exchange trunk: have circuit lengths ranging from 40 to 160km, and link towns and villages.

Optical Fibers - Transmission Characteristics

• At the source end optical transmitter converts electrical signals to optical
• Optical fiber transmits a single encoded beam of light by means of total internal reflection.
• At the destination, optical receiver converts light back to electrical signals.
• Light from a source enters the cylindrical glass or plastic core.
• Rays at shallow angles are reflected and propagated along the fiber; other rays are absorbed by the surrounding material.

Optical Fibers - Step Index Multimode

• Graded Index Optical Fiber:
• In this case, the core itself is made up of a material and with use with a varying density
• The density is the highest at the code and gradually decreases towards the edge.
• The higher refractive "inden" at the center, make light rays moving down here, on average.

Optical Fibers - Single versus Multi Mode

• The number of light waves that can be accommodated is less, so it can transmit the signals more slowly, as opposed to the cladding.
• In this case, light of the core curves helically because of the gradiation and redices its travel distance diameter of roughly 100 nm.
• Single Mode versus Multi Mode, Multi Mode is used and smaller distances along distance over lower speed reduction
• By reducing the radius of the cord of the order of the wavelength of only a single angle mode can parse to provide a signal
• This distortion in Multi Mode cannot occur on a segment length of 2 km and with a banwidth of 900 Mbps.

Unguided Media: Terrestrial Microwave

• Physical description
• The most common type of microwave antenna is the parabolic dish.
• A typical size is about 3 m in diameter.
• The antenna is fixed rigidly and focuses a narrow beam to achieve line of sight transmission to a receiving antenna.
• Microwave antennas are usually located at substantial heights above ground level to extend the range between antennas and be able to transmit over intervening obstacles.
• The primary use is for providing long haul telecommunication services.
• An alternative to coaxial or optical fiber.
• Requires fewer amplifiers or repeaters.
• Requires line of sight transmission.
• Used for both voice and television transmission.
• Used for short point-to-point links between buildings.
• Can be used for closed circuit TV.
• Can be used in cellular systems.

Microwave Transmission Characteristics

• Common frequencies used for transmission are in the range 1 to 40 GHz.
• The higher the frequency used, the higher the potential bandwidth and higher the data rate.
• The loss can be expressed as attenuation: L = 10 log((4πd)/λ)^2 dB, where 'd' is distance and 'λ' is wavelength.
• Repeaters or amplifiers may be used - 10 to 100km is typical.
• Attenuation increased with rainfall.
• The most common bands for long-haul telecommunications are the 4 GHz to 6 GHz bands.
• The 12 GHz band is used as a component of cable TV systems.
• Higher frequency microwave is being used for short point to point links between buildings.
• Higher microwave frequencies are less use than lower ones for most application due to increased attenuation.
• Another source of impairment is interference.