Transmission Media

The transmission of bits from one machine to another is the responsibility of physical layer. Physical layer converts the bits stored in the system into signal compatible to the media. There are various types of media eache having their own advantages/disadvantages in terms of bandwidth, delay, cost, ease of installation and maintenance. This chapter discusses various media used for transmission. In general the media can be broadly classified as:

1. Magnetic Media :- Tapes, Floppy, CD, DVD etc2. Guided media :- Copper wire, fiber3. Unguided media :- terrestrial wireless, satellite and laser

Guided Media:

a. Twisted Pair Cables (UTP and STP), (b) Coaxial Cable (c) Power Line (d) Fiber Optics

Unguided Media:

(a) Radio Transmission (b) Microwave (Terrestrial and Satellite) transmission (c) Infrared transmission ( d) Light Transmission

1. Magnetic media

An industry standard Ultrium tape has a capacity of 800 Giga bytes, a box of size 60cm x 60cm x 60 cm can hold 1000 such tapes, so the total capacity is 800 terabytes (800 Giga x 1024) or 6400 terrabits. If this box can be transported to its destination by rail/rod/air in 24 hours then the bandwidth 6400 terabits /(24*60*60)sec or 70Gbps. If destination is at 1 hour by road then the effective bandwidth reaches 1700 Gbps

2. Guided MediaAlthough magnetic media has good bandwidth and very fine for offline communication, for online communication such as in computer communication we need guided or unguided media. Twisting is done because the parallel would constitute a fine antenna. Because of the twists in the wire the wave created from different wires get cancelled out, so radiation is less effective. Signal carried is usually the difference between the two voltage levels, as the noise effects both the wire equally, so the effect cancel out and the difference in voltage remain unchanged.Uses:

Most common usage is in telephone line. Twisted pair can carry both analog and digital signals. Adequate performance and low cost.

2.1 Twisted pair cable

a. Unshielded Twisted Pair cable (UTP) and Shielded Twisted Pair Cable (STP)Unshielded twisted pair find more usage than shielded Twisted pair. Shielded twisted pair was developed by IBM for communication across IBM system

Category

Bandwidth Data bit Rate

Application Type of signal transmission

CAT-1 Very Low < 10Kbps Telephone AnalogCAT-2 <2 MHz 2 Mbps Telephone/computer Analog/DigitalCAT-3 16 MHz 10 Mbps Computer communication DigitalCAT-5 100 MHz, 1 Gbps

Ethernet LAN

More twists per meter which result in less cross talk and better quality signal over long distance, more suitable for high speed computer communication

Digital

CAT-6 500MHz and 10 GBps

Digital

CAT-7 500MHz and 10 GBps

Shileding on individual pair and also around all pairs, this reduces susceptibility to external interference and crosstalk

Digital

Shielded Twisted pair Cable (STP) ‘Shielded’ with a foil jacket to cancel any external

interference Commonly used for large-scale enterprises for high-end

applications as well as exterior cabling that may be exposed to environmental elements

General Applications of Twisted pair Cables:

Telephone Lines to carry voice and data Local Loops DSL and ADSL LAN 10 base-T, 100 base T ISDN

Type of Connectors used

RJ 11 (Male-Female) : For telephone systems RJ 45 (Male-Female) : For computer Communication

b. Coaxial CableThis kind of media overcomes the disadvantages of the twisted pair cables. They are used for long distance communication and offer better bandwidth. It has 80X more transmission capacity than twisted pair cables. Usage:

(i) Commonly used to deliver TV signals (its higher bandwidth makes it more suitable for video applications)

(ii) To connect computers in a network. Properties:(i) Stable transmission of data(ii) Have anti-jamming capabilities and can

effectively protect signals from being interfered. (iii) Cost is slightly higher than twisted pair but still

considered more economical than fiber

Two types of coaxial cable:

(a) 50 ohm coaxial cable :

(i) for digital communication. (ii) Most commonly used to transmit video signals(iii)Often connects video signals between different

components like DVDs, VCRs, or receivers commonly known as A/V cables

(b) 75 ohm coaxial cable

(i) Used for analog transmission and cable television.(ii) Primarily utilized to transmit a data signal in a 2-

way communication system(iii) Most commonly used for computer ethernet

backbones, AM/FM radio receivers, GPS antenna, police scanners, and cell phone systems

Construction

Conclusion:

It gives good bandwidth and excellent noise immunity, bandwidth depends on the cable quality and length

Earlier were used within telephone systems for long distance but are now replaced by Fiber, nowadays used for cable TV and MAN

c. Fiber Optics

Optical fiber is the flexible transparent media made of silica glass or plastic. It consist of two elements core and the cladding having different reflex index. In fiber optics communication presence of the pulse of light indicates a 1 bit and its absence indicate a 0 bit. They are used for long haul transmission in network backbones, high speed LAN and high speed FttP (fiber to the home).

An optical transmission system has three key components: the light source, transmission medium and the detector. Figure shows the fiber optics transmission system.

At the source of the transmitter is the input signal which is converted to the appropriate signal by the code converter which in turn is converted to light. The light wave travels through the fiber media. On the other end of the fiber is the interface for the detector, the detector generates the light source into an electrical pulse. The electrical pulse then after amplification/shaper becomes as the usable output.

Such type of communication where a light source is attached at one end and the detector at the other end make the communication as unidirectional.

Critical Angle is the angle of incidence in denser medium for which angle of refraction in rarer medium is 90 degrees.

Total internal reflection – if the angle of incidence in denser medium is increased beyond critical angle then ray of light is reflected back completely into the silica and none would escape the silica into the air. Thus a light ray incident at or above the critical angle is trapped inside the fiber and can propaget many kilometers with virtually no loss.

Construction and Types:

Core: Glass or plastic with a higher index of reflection than cladding, it carries the signalCladding: Glass or plastic with a lower index of reflection than coreBuffer: Protects the fiber from damage and moistureJacket: Holds one or more fibers in a cableTypes of fiber optics cable:

Multimode fiber : Figure ( c) on the right shows only one ray trapped in the fiber, but since any light ray incident on the boundary above the critical angle will be reflected internally, many different rays will be bouncing around at different angles. Each ray is said to have a different mode, so a fiber having this property is called multimode fiber.

Inexpensive

easy to couple light into Fiber 

result in higher signal distortion

lower TX rate

Single mode fiber :If the diameter of the fiber is reduced to a few wavelength of light the fiber acts like a wave guide and the light can propagate only in a straight line, without bouncing, yielding a single mode fiber. Single mode fiber is more expensive but widely used for long distance and can transmit data at 100 Gbps for 100 Km without amplification.

Advantage:

Minimum dispersion: all rays take same path, same time to travel down the cable. A pulse can be reproduced at the receiver very accurately

Less attenuation, can run over longer distance without repeaters Larger bandwidth and higher information rate 

Disadvantages:

Difficult to couple light in and out of the tiny core Highly directive light source (laser) is required Interfacing modules are more expensive

Comparison of Guided Media(Ref CISCO web material)

edia Type

Maximum Segment Length

Speed Cost Advantages Disadvantages

UTP 100 m 10 Mbps to 1000 Mbps

Least expensive

Easy to install; widely available and widely used

Susceptible to interference; can cover only a limited distance

STP 100 m 10 Mbps to 100 Mbps

More expensive than UTP

Reduced crosstalk; more resistant to EMI than Thinnet or UTP

Difficult to work with; can cover only a limited distance

Coaxial 500 m (Thicknet)

185 m (Thinnet)

10 Mbps to 100 Mbps

Relatively inexpensive, but more costly than UTP

Less susceptible to EMI interference than other types of copper media

Difficult to work with (Thicknet); limited bandwidth; limited application (Thinnet); damage to cable can bring down entire network

Fiber-Optic

10 km and farther (single-mode)

2 km and farther (multimode)

100 Mbps to 100 Gbps (single mode)

100 Mbps to 9.92 Gbps (multimode)

Expensive Cannot be tapped, so security is better; can be used over great distances; is not susceptible to EMI; has a higher data rate than coaxial and twisted-pair cable

Difficult to terminate

Review Exercises

1. What is the maximum cable length for STP?a. 100 feetb. 150 feetc. 100 metersd. 1000 meters

2. Which connector does UTP use?a. STPb. BNCc. RJ-45d. RJ-69

3. What is an advantage that coaxial cable has over STP or UTP?a. It is capable of achieving 10 Mbps to 100 Mbps.b. It is inexpensive.c. It can run for a longer distance unboosted.d. None of the above.

4. A ______ fiber-optic cable transmits multiple streams of LED-generated light.a. multimodeb. multichannelc. multiphased. None of the above

5. Wireless communication uses which of the following to transmit data between devices on a LAN?

a. Radio frequenciesb. LED-generated lightc. Fiber opticsd. None of the above

6. What is one advantage of using fiber-optic cable in networks?a. It is inexpensive.b. It is easy to install.c. It is an industry standard and is available at any electronics store.d. It is capable of higher data rates than either coaxial or twisted-pair cable.

Unguided Media

Unguided Media transmit electromagnetic waves without using a solid conductor. Some authors say that air or water is unguided media's media. However it should be noted that Electromagnetic waves do not require any media to propagate and can travel even through vacuum. Wireless Communication would be a better term.

Wireless Communication

Wireless communication uses radio frequencies (RF) or infrared (IR) waves to transmit data between devices on a LAN. For wireless LANs, a key component is the wireless hub, or access point, used for signal distribution.

Figure :  Wireless Network

To receive the signals from the access point, a PC or laptop must install a wireless adapter card (wireless NIC). Wireless signals are electromagnetic waves that can travel through the vacuum of outer space and through a

medium such as air. Therefore, no physical medium is necessary for wireless signals, making them a very versatile way to build a network. Wireless signals use portions of the RF spectrum to transmit voice, video, and data. Wireless frequencies range from 3 kilohertz (kHz) to 300 gigahertz (GHz). The data-transmission rates range from 9 kilobits per second (kbps) to as high as 54 Mbps.

The primary difference between electromagnetic waves is their frequency. Low-frequency electromagnetic waves have a long wavelength (the distance from one peak to the next on the sine wave), while high-frequency electromagnetic waves have a short wavelength.

Some common applications of wireless data communication include the following:

Accessing the Internet using a cellular phone Establishing a home or business Internet connection over satellite Beaming data between two hand-held computing devices Using a wireless keyboard and mouse for the PC

Another common application of wireless data communication is the wireless LAN (WLAN), which is built in accordance with Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards. WLANs typically use radio waves (for example, 902 megahertz [MHz]), microwaves (for example, 2.4 GHz), and IR waves (for example, 820 nanometers [nm]) for communication. Wireless technologies are a crucial part of the today's networking. See Chapter 28, "Wireless LANs," for a more detailed discuss on wireless networking.

Electromagnetic Spectrum

Wireless Data Communication frequencies

1. AM Radio (530Khz - 1600Khz)2. FM Radio (88Mhz - 108hz)3. Terrestrial and Satellite (1Ghz - 100Ghz)4. Infrared (1Thz - 100Thz)Propagation Methods

1. Ground Propagation - radio waves travel through the lowest portion of the atmosphere following the curvature of the planet.2. Sky Propagation - high frequency radio waves radiate upward into the ionosphere where they are reflected back to the earth.3. Line-of-sight Propagation - very high frequency signals are transmitted in straight lines directly from antenna to antenna

Radio Transmission

Although there is no clear cut division between radio waves and microwaves, electromagnetic frequencies between 3Khz and 1Ghz. Behavior of the waves, rather than frequencies, is a better criterion for classification. Radio waves are mostly omni-directional and propagated using an omnidirectional antenna. When an antenna transmits radio waves, they are propagated in all directions. This means that sending and receiving antennas do not need to be aligned. Radio waves are used for multi-casting, in which there is one sender but many receivers such as AM and RM radio, television, maritime radio, cordless phones and paging. Radio waves can penetrate walls. Radio waves travel to long distances. The disadvantage of omnidirectionality is that the signal is susceptible to interference by another signal of the same frequency. The entire band is regulated by authorities and any part of the band requires permission.

Microwave transmission : Terrestrial and satellite communication

Microwaves have frequencies between 1 and 300 GHz. Microwaves are unidirectional. When an antenna transmits microwaves, they can be narrowly focused. This means that the sending and receiving antennas need to be aligned. This means that even if two pairs of antennas transmit microwaves of the same frequency, they will not interfere with each other as long as the pairs are not aligned. Microwave propagation needs to be line-of-sight. The microwave band is relatively wide. Therefore wider sub-bands can be assigned and a high data rate for each sub-band is possible. Though microwaves travel shorter distances relative to radio waves. Microwaves use unidirectional antennas such as a parabolic dish and a horn antenna. They are used in cellular phones, satellite networks and wireless LANs.

Infrared (IR) Transmission

Infrared signals, with frequencies from 300Ghz to 4Thz, can be used for short-range communication. Infrared signals, having high frequencies never pass through walls. This is advantageous when communication systems are separated by physical walls. However we can not use infrared outside a building because the sun's rays contain infrared waves that interfere with the communication.