Assignment- Communication Channels and their Features and more

     Communication Channels and their Features

In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel.

A channel is used to convey an information signal, for example a digital bit stream, from one or several senders (or transmitters) to one or several receivers.

 A channel has a certain capacity for transmitting information, often measured by its bandwidth in Hz or its data rate in bits per second

Electrical communications channels are either wireline or wireless channels. Wireline channels physically connect transmitter to receiver with a "wire" which could be:

·         Two-Wire Open Line Cable

·         Twisted pair

·         Coaxial cable

·         Optic fiber

Consequently, wireline channels are more private and much less prone to interference. Simple wireline channels connect a single transmitter to a single receiver: a point-to-point connection as with the telephone. Listening in on a conversation requires that the wire be tapped and the voltage measured.

Some wireline channels operate in broadcast modes: one or more transmitter is connected to several receivers. One simple example of this situation is cable television. Computer networks can be found that operate in point-to-point or in broadcast modes.

Wireless channels are much more public, with a transmitter's antenna radiating a signal that can be received by any antenna sufficiently close enough. In contrast to wireline channels where the receiver takes in only the transmitter's signal, the receiver's antenna will react to electromagnetic radiation coming from any source. This feature has two faces: The smiley face says that a receiver can take in transmissions from any source, letting receiver electronics select wanted signals and disregarding others, thereby allowing portable transmission and reception, while the frowny face says that interference and noise are much more prevalent than in wireline situations. A noisier channel subject to interference compromises the flexibility of wireless communication.

Wireless communication channels are:

·         Microwave Communication

·         Radio

·         Satellite

Features of Communication Channels:

A channel can be modeled physically by trying to calculate the physical processes which modify the transmitted signal. For example in wireless communications the channel can be modeled by calculating the reflection off every object in the environment. A sequence of random numbers might also be added in to simulate external interference and/or electronic noise in the receiver.

1.     Features of Two-Wire Open Line:

•      Insulated Wire and open to free space

•      Signal is applied to one wire and other one is for ground reference

•      it is used for connecting modem to computer

Drawback:

•      It is highly effected by Electromagnetic Radiations

2.     Features of Coaxial Cable

Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing a geometric axis. Coaxial cable was invented by English engineer and mathematician Oliver Heaviside, who patented the design in 1880. Coaxial cable differs from other shielded cable used for carrying lower-frequency signals, such as audio signals, in that the dimensions of the cable are controlled to give a precise, constant conductor spacing, which is needed for it to function efficiently as a radio frequency transmission line.

Coaxial cable is used as a transmission line for radio frequency signals. Its applications include feed lines connecting radio transmitters and receivers with their antennas, computer network (Internet) connections, and distributing cable television signals. One advantage of coax over other types of radio transmission line is that in an ideal coaxial cable the electromagnetic field carrying the signal exists only in the space between the inner and outer conductors. This allows coaxial cable runs to be installed next to metal objects such as gutters without the power losses that occur in other types of transmission lines. Coaxial cable also provides protection of the signal from external electromagnetic interference.

 

 

Types of Coaxial Cable:

Common types of coaxial cable include RG-6, RG-8, RG-58, and RG-59. RG-6 is one of the most common, found in household and business applications such as cable television connections. RG-59 is considered to be the predecessor to RG-6. RG-8 cable is used mainly for radio transmissions such as CB radio while RG-58 is found in Ethernet network applications.

Considerations:

Coaxial cable is resistant to the effect of attenuation (signal loss over long distances) up to a certain length. The average coaxial cable can run 100 meters before attenuation begins to become noticeable.

While coaxial cable is resistant to environmental conditions, running the cable through cable tubes and lines underground is a good way to protect the cables from excessive moisture and the risk of being cut if digging occurs.

The insulator surrounding the inner conductor may be solid plastic, a foam plastic, or air with spacers supporting the inner wire. The properties of dielectric control some electrical properties of the cable. A common choice is a solid polyethylene (PE) insulator, used in lower-loss cables. Solid Teflon (PTFE) is also used as an insulator. Some coaxial lines use air (or some other gas) and have spacers to keep the inner conductor from touching the shield.

3.     Features of Twisted Pair Cable

Twisted pair cabling is a type of wiring in which two conductors of a single circuit are twisted together for the purposes of canceling out electromagnetic interference (EMI) from external sources

·         Pairs of wires twisted together

·         It is the most common medium used for communication over a large distance

·         It is used for internet and television connections

·         Extensively being used in LAN Connections

Types of Twisted Pair Cables:

1.      Shielded Twisted Pair Cable

·         Covered with a foil shield to reduce electromagnetic interference

·         Better in performance than UTP cable but more expensive than UTP cable

2.      Unshielded twisted Pair Cable

·         Does Not Include any extra sheilding around the wire pairs

·         Used for ordinary phone lines and local area networks

·         Less Expensive and easy to work

·         Support shorter distance

Twisted Pair Cable Pros and Cons:

Pros:

1.      It is Cheap

2.      It is easy to use

3.      Less effected by noise

Cons

1.      Supports Low Data rate

2.      Used for short Distant Communication

4.     Features of Optical Fiber

An optical fiber cable is a cable containing one or more optical fibers. The optical fiber elements are typically individually coated with plastic layers and contained in a protective tube suitable for the environment where the cable will be deployed.

An optical fiber is a thin (2 to 125µm), flexible medium capable of guiding an optical ray.

Preferable because of:

·         Greater capacity

·         Smaller size and lighter weight

·         Lesser attenuation

·         Greater repeater spacing

·         Electromagnetic isolation

5.     Features of Microwave Communication Media

Microwave transmission refers to the technology of transmitting information or energy by the use of radio waves whose wavelengths are conveniently measured in small numbers of centimeter; these are called microwaves. This part of the radio spectrum ranges across frequencies of roughly 1.0 gigahertz (GHz) to 30 GHz. These correspond to wavelengths from 30 centimeters down to 1.0 cm.

Microwaves are widely used for point-to-point communications because their small wavelength allows conveniently-sized antennas to direct them in narrow beams, which can be pointed directly at the receiving antenna. This allows nearby microwave equipment to use the same frequencies without interfering with each other, as lower frequency radio waves do. Another advantage is that the high frequency of microwaves gives the microwave band a very large information-carrying capacity; the microwave band has a bandwidth 30 times that of all the rest of the radio spectrum below it. A disadvantage is that microwaves are limited to line of sight propagation; they cannot pass around hills or mountains as lower frequency radio waves can.

Microwave radio transmission is commonly used in point-to-point communication systems on the surface of the Earth, in satellite communications, and in deep space radio communications. Other parts of the microwave radio band are used for radars, radio navigation systems, sensor systems, and radio astronomy.

Uses of Microwave Communication

Wireless transmission of information

•      One-way (e.g. television broadcasting) and two-way telecommunication using communications satellite

•      Terrestrial microwave radio broadcasting relay links in telecommunications networks including e.g. backbone or backhaul carriers in cellular networks linking BTS-BSC and BSC-MSC.

Wireless transmission of power

•      Proposed systems e.g. for connecting solar power collecting satellites to terrestrial power grids

6.     Features of Radio Communication

Radio is the transmission of signals through free space by electromagnetic waves with frequencies significantly below visible light, in the radio frequency range, from about 3 kHz to 300 GHz. These waves are called radio waves. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space.

Information, such as sound, is carried by systematically changing (modulating) some property of the radiated waves, such as their amplitude, frequency, phase, or pulse width. When radio waves strike an electrical conductor, the oscillating fields induce an alternating current in the conductor. The information in the waves can be extracted and transformed back into its original form.

How Radio Communication works:

7.     Features of Satellite Communication

In satellite communication, signal transferring between the sender and receiver is done with the help of satellite. In this process, the signal which is basically a beam of modulated microwaves is sent towards the satellite. Then the satellite amplifies the signal and sent it back to the receiver’s antenna present on the earth’s surface. So, all the signal transferring is happening in space. Thus this type of communication is known as space communication.

Two satellites which are commonly used in satellite communication are Active and passive satellites.

•      Passive satellites: 

It is just a plastic balloon having a metal coated over it. This sphere reflects the coming microwave signals coming from one part of the earth to other part. This is also known as passive sphere. Our earth also has a passive satellite i.e. moon.

•      Active satellites:

It basically does the work of amplifying the microwave signals coming. In active satellites an antenna system, transmitter, power supply and a receiver is used. These satellites are also called as transponders. The transmitters fitted on the earth generate the microwaves. These rays are received by the transponders attached to the satellite. Then after amplifying, these signals are transmitted back to earth. This sending can be done at the same time or after some delay. These amplified signals are stored in the memory of the satellites, when earth properly faces the satellite. Then the satellite starts sending the signals to earth. Some active satellites also have programming and recording features. Then these recording can be easily played and watched. The first active satellite was launched by Russia in 1957. The signals coming from the satellite when reach the earth, are of very low intensity. Their amplification is done by the receivers themselves. After amplification these become available for further use.

b.    How the bandwidth and channel capacity of a particular channel can be measured?

Bandwidth of a particular channel is dependent on the channel material and cross sectional area of the medium used for communication.

Channel capacity can be found using Shannon capacity formula of Nyquist formula for finding channel capacity

Measuring Channel Capacity by Shannon Capacity Formula:

An application of the channel capacity concept to an additive white Gaussian noise (AWGN) channel with B Hz bandwidth and signal-to-noise ratio S/N is the Shannon–Hartley theorem:

1.      C is measured in bits per second

2.      W is Bandwidth of Channel

3.      SNR is Signal to Noise Ratio

Nyquist Formulation

if the rate of signal transmission is 2B, then a signal with frequencies no greater than B is sufficient to carry the signal rate.

•      Given bandwidth B, highest signal rate is 2B.

Why is there such a limitation?

•      due to intersymbol interference, such as is produced by delay distortion.

Given binary signal (two voltage levels), the maximum data rate supported by B Hz is 2B bps.

•      One signal represents one bit

c.      What is Nyquist criterion of channel capacity?

Signals with more than two levels can be used, i.e., each signal element can represent more than one bit.

E.g., if a signal has 4 different levels, then a signal can be used to represents two bits: 00, 01, 10, 11

With multilevel signalling, the Nyquist formula becomes:

C = 2B log₂M

M is the number of discrete signal levels, B is the given bandwidth, C is the channel capacity in bps.

How large can M be?

The receiver must distinguish one of M possible signal elements.

Noise and other impairments on the transmission line will limit the practical value of M.

Nyquist’s formula indicates that, if all other things are equal, doubling the bandwidth doubles the data rate.

Example:

We assume

F1=0Hz and F2=20 kHz

so
B= F2-F1=20000 Hz

By Nyquist formula:

C = 2*20000* log₂ (16) = 2*20000* log₁₀ (16) / log10 (2) = 160 000 bps

d.    How particular channel capacity is affected in term of data rate in the presence of noise and in particular in presence of extreme noise?

Noise Effects Leading to :

•      Higher the data rate of the signal, the greater the effective BW it requires.

•      The greater the BW of the tx system, the higher is the data rate that can be tx-ed over the system.

•      With the introduction of levels what we are trying to do is to increase the number of information pieces that travel in one signaling element.

•      For a given BW, the data rate can be increased by increasing the number of signal elements. However this places an additional burden on the receiver since now he has to discern many possible amplitude values.

•      The channel capacity is the maximum rate at which data can be transmitted over a gives communication path and with presence of noise the channel capacity is highly affected in term of data rate and the data rate capacity may decrease in channel in presence of noise.

•      With noise the ability of the receiver to recognize many levels becomes low

According to Shannon:

1. Faster data rate shortens each bit, so burst of noise affects more bits

2.      The key parameter is the SNR: Signal-to-Noise Ratio, which is the ratio of the power in a signal to the power contained in the noise

3.      C = B log₂(1+SNR)  in bps - maximum data rate

4.      The wider the bandwidth, the more noise is admitted to the system. Thus, as B increases, SNR decreases.

5.      Lower S/N leads to higher bit error rates thus reducing the effective data rate.

6.      Noise targets multilevel signalling more

Now according to SNR, if our noise is greater in a channel corresponds to low SNR which leads to weak signal.

In the channel considered by the Shannon-Hartley theorem, noise and signal are combined by addition. That is, the receiver measures a signal that is equal to the sum of the signal encoding the desired information and a continuous random variable that represents the noise. This addition creates uncertainty as to the original signal's value. If the receiver has some information about the random process that generates the noise, one can in principle recover the information in the original signal by considering all possible states of the noise process. In the case of the Shannon-Hartley theorem, the noise is assumed to be generated by a Gaussian process with a known variance. Since the variance of a Gaussian process is equivalent to its power, it is conventional to call this variance the noise power.