Module 5. Principles of communication

Lesson 12

COMMUNICATION CHANNELS

12.1 Introduction

This lesson will introduce the need and concept of communication systems. Students will learn about Modulation and Encoding; Analog Versus Digital Communications; Electromagnetic Spectrum (Frequency Spectrum) – Frequency bands and their application areas; Communication Channels and characteristics of transmission media. These topics will be useful for students to develop an understanding about the basics of communication process and intricacies involved in transmitting data.

A signal by itself does not carry any information. The signal is modified so that it contains identifiable changes that are recognizable to the sender and receiver as representing the information intended. In computer, data is stored in form of 0s and 1s i.e. in digital form. This digital data must be converted to either into digital signal or analog signal for further transmission to destination. During transmission, four type of conversion of data may occur depending on the nature of signals as shown in the following table 12.1.

Table 12.1 Conversion schemes

Source	Converted to Signal	Type of Conversion	Purpose/ usage
Digital data	Digital	Digital to digital conversion or encoding digital data into a digital signal	For example communication between computer and printer
Analog data	Digital	Analog to digital conversion or digitizing an analog signal	To digitize human voice for sending over long distances.
Digital data	Analog	Digital to analog conversion or modulating a  digital signal	Transmission of data from one computer to another through public telephone network which carry analog signals.
Analog data	Analog	Analog to analog conversion or modulating an  analog signal	Transmission of voice through radio system

12.2 Modulation and Encoding

Encoding is a process of transforming source information into signals. Source data may be in either digital or analog form is encoded into a digital signal using an encoding technique to optimize the usage of transmission media. Some important encoding schemes used for digital data to digital signal conversion are Non return to zero-level, Non return to zero-Invert, Manchester, Differential Manchester, Bipolar 8-zero substitution (B8ZS), and High density bipolar 3 (HDB3).

Modulation is a process of encoding source data onto analog signal of a continuous constant high-frequency periodic waveform called as carrier signal. Modulation technique involves operation of varying one or more properties of carrier signal, like amplitude, frequency, and phase with respect to a modulating signal (which contains information to be transmitted). Typically a high-frequency sinusoid waveform or square wave pulse is used as carrier signal. A device that performs modulation is known as a modulator. Modulation is of two types:

12.2.1 Continuous wave modulation (Digital/ analog to analog conversion)

When carrier wave is continuous in nature then modulation process is known as continuous or analog communication, .e.g.,

· Amplitude modulation: In amplitude modulation process amplitude of carrier wave is varied in accordance to the instantaneous value of modulating wave or message signal.

· Frequency modulation: In frequency modulation process, frequency of carrier signal is varied accordance to the instantaneous value of modulating signal or message signal.

· Phase modulation: In this process phase of carrier signal is varied in accordance to the instanttaneous vlaue of modulating signal.

12.2.2 Pulse modulation (analog to digital conversion)

When carrier wave is a digital pulse train, the modulation process is known as pulse modulation. This is called analog to digital conversion or digitizing an analog signal. Examples of pulse modulations are:

Pulse code modulation (PCM): A PCM is a method to represent analog signal in digital form, in which magnitude of analog signal is sampled regularly at uniform intervals, with each sample being quantized to the nearest value within a range of digital steps.

12.3 Analog verses Digital Communication

I. The set up required for digital communication systems are simple and cheaper compared to analog communication systems because of the advancement made in the IC technologies.

II. In digital communication data is binary in nature; therefore a large amount of noise interference may be tolerated as compare to analog communication.

III. Since channel coding is used in digital communication that makes the error detection and correction easier at the receiver side compare to analog communication.

IV. In digital communication transmission is digital and channel encoding is used, therefore the noise does not accumulate from repeater to repeater in long distance communications.

V. Privacy can be maintained in digital communication using data encryption technique which allow only permitted receiver to detect the transmitted data.

VI. In digital communication, speech, video and other data may be merged and transmitted over a common channel using multiplexing.

VII. In digital communication data is in binary form means have a higher data rate which require more bandwidth for transmission as compare to analog communication

12.4 Frequency/Electromagnetic Spectrum

Computers and other telecommunication devices use signals to represent data. These signals are transmitted from one device to another in form of electromagnetic energy which may travel through vacuum, through air, or through other transmission media. The electromagnetic energy includes power, voice, radio waves, infrared light, visible light ultraviolet light, and X, gamma, and cosmic rays. Not all of these forms of electromagnetic energy are usable for telecommunication. Electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiations from gamma rays to the longest radio waves.  A complete range of frequencies and their usages in communication is given the following table -12.2:

Table 12.2 Electromagnetic spectrum

Radio band	Frequency Range (Hz)	Transmission Media	Propagation Mode	Applications
Audio Frequency	0K – 3K	Twisted pairs	Surface (air, water)	Telephone, telegraph
Very Low Frequency (VLF)	3K – 30K			Long radio navigation, submarine communication, aeronautical communication
Low Frequency (LF)	30K–300K
Medium Frequency (MF)	300K-3M	Coaxial cable	Line of Sight, Sky wave	AM radios, maritime radio, radio direction finding (RDF), emergency frequencies
High Frequency (HF)	3M-30M		Sky wave	Citizen’s band (CB) radio, business, Civil defence, military communication, international broadcasting, amateur radio
Very High Frequency (VHF)	30M-300M		Line of Sight	VHF television, FM radio, aeronautical, mobile radio
Ultra High Frequency (UHF)	300M-3G	Wave guide		UHF television, mobile telephone, cellular radio, paging, microwave link beyond 1GHz
Super High Frequency (SHF)	3G-30G			Experimental, Terrestrial and satellite microwave communication, radar communication,
Extremely High Frequency (EHF)	30G-300G
Infrared	300G-200T	Optical fiber	Line of sight, Laser beams	Lasers beams, Guided missiles, Range finders, wireless communication between to devices inside a lab, remote control, wireless local area networks (WLANs),
Visible Light	200T-1000T	Optical fiber	Laser beams	Optical fibers
Ultraviolet, X-ray, Gama rays	> 1000T		Laser Beam	Under experiments

12.5 Communication Channels

Communication channel in refers to a transmission medium to transport data from one or several senders (or transmitters) to one or several receivers. A wide variety of communication channels are used in data communication. These channels can be classified into two groups:

· Wire-line: Copper wires (Twisted pairs, coaxial cables) and Optical fibers.

· Wireless: Radio waves, Micro Waves and Infrared waves.

12.5.1 Wire-line media

Wire-line (or wired) media consists of a solid medium generally of copper metal or glass fiber. A signal in form of electromagnetic wave travels through this solid medium from source to destination. A signal traveling along any of these media is directed and contained by the physical limits of the medium. Therefore, a wired media is also called as guided media. Voice-band frequencies are generally transmitted as current over metal cables, such as twisted-pair or co-axial cable. Visible light used for communications, is harnessed using fiber-optic cable. Different types of cables used in wired media are twisted pair, coaxial cable, and fiber optic cable.

12.5.1.1 Copper wires

The primary medium to connect communicating devices in a network is copper cables because of its low resistance to electric current which means signals can travel for a longer distance. Basically two types of copper cables are used in networks – twisted pair and coaxial cable.

a.  Twisted pair

A twisted pair consists of two insulated copper wires of about 1 mm thick. The pair of wires is twisted to each other. This pair of wire is treated as a single unit in a communication link. In the twisted pair cable one wire is used for signal (a voltage level or current level) and another for ground reference. These two wires may be separate and run parallel but are twisted to each other to reduces the effects of external electrical noise by using differential signaling. A number of pairs are bundled together and encased in a protective sheath.

The twisting minimizes the crosstalk interference between adjacent pairs in multi pair cable. Twisted pair cables are the most commonly used transmission media for both analog and digital communication. These cables are used to connect telephone handset with local exchange and local area network (LAN). The bandwidth depends on the quality of wire and the distance traveled. The data rate varies from 64kbps to 100mbps. Twisted pair cable is cost effective and easy to install. Twisted pair cables are available in two varieties unshielded twisted pair (UTP) and shielded twisted pair (STP).

In UTP, twisted pairs of insulated wires are grouped or placed in a flexible plastic jacket. Un-shielding the cable reduces the cost, size, and installation time of the cable and connecters. But it increases the effect of outside the interferences from nearby twisted pair and from the noise generated in the environment. The UTP cables are available in different varieties. The CAT3, CAT5, CAT6 are the most common varieties of UTP cables. The major difference between category -3 and category -5 is the number of twists per unit distance. In category -3 there are 3-4 twists per foot while category -5 is has 3-4 twists per inch.

In STP, twisted pairs of wires are covered by a protective metal shield. The shielding reduces the effects of interference signals and provides better performance but increases the cost and are difficult to handle in comparison to UTP cables.

b.  Coaxial cable

Coaxial cable consists of an inner insulated core of stranded or solid wire (generally of copper) surrounded by an outer insulated flexible wire braid. The inner wire carries the signals and outer metal braid works as conductor for ground reference. There are four parts in this cable. First the inner wire which works as inner conductor, second this wire is covered by a hard plastic material for providing insulation to wire. Third, the inner wire with its plastic cover is surrounded by a net of metal wires which works as outer conductor. Fourth, the whole wire is again encased with insulated plastic material. Both the conductors share the same axis therefore it is called coaxial cable.

The shielding of cable provides a better protection from external interferences and crosstalk than twisted pair. The coaxial cable provides a higher bandwidth by carrying high frequency signals of range from 10MHz to 500MHz. The latest wires are able to carry 1GHz frequency signals. The diameter of the cable varies from 0.4 to 1 inch. It can be used for transmitting the analog and digital signals for long distances up to 1Km without any repeaters. It is used for a wide range of applications in data communication such as cable television network, Long distance telephone networks, local area network.  The commonly used coaxial cables are thin coaxial cable and thick coaxial cable. Thin coaxial cables are generally used in thin Ethernet. The diameter of cable is around 0.5 inch. These cables are flexible and easy to handle. Thick Coaxial cables are generally used in thick Ethernet. The diameter of cable is around 1 inch. These cables are very stiff and difficult to handle.

12.5.1.2 Glass fibers

Computer networks also use flexible glass fibers to transmit data. This new transmission media is known as “optical fiber” and uses light to transmit a signal rather than voltages used in copper media. The optical fiber is made up of glass or plastic material. Usually silica glass optical fiber used for high data speed networks where more accuracy is required in data transmission over long distances. Plastic optical fiber is also used as low cost solution for limited distance applications. These cables can be used in LAN, telephone systems, long distance communication, and establishing the backbone of country wide network. The optical fiber consists of the following:

• Inner core: Inner core is made up of very thin strands or fibers of glass or plastic material. The diameter of fiber is generally in the range of 2-125 microns. It must be of good quality material and smooth in size and shape. Usually silica glass is used for better performance.
• Cladding: Cladding means the process of protecting one metal by bonding a second metal to its surface. Each fiber is surrounded by its own cladding which is a glass or plastic material of lower density than inner core.
• Outer Jacket: This is the outer most layer of the optical fiber cable. It contains one or more strands of fibers with cladding. It is made up of different kind of materials to protect the inner core form moisture, abrasion, crushing and other damages.

The optical fiber uses the light to transmit the data. Generally infrared light is used which has a longer wavelength and therefore invisible to humans. A presence of a light pulse can be used to represent a 1 bit and absence to represent a 0 bit. For transmitting the data a transmitter is required to transmit the data. This transmitter uses light emitting diode (LED) or laser diode (LD) to converts the electrical data signals into light pulses and transmits the signals in optical fiber.   At the other end of the cable a receiver is required to receive and sense the light pulses. The receiver uses the light sensitive photodiode or photo transistor to convert the light pulses into electrical voltages. The transmitter is also known a light source and receiver as detector.

12.5.1.3 Advantages of optical fiber

· Higher data rate can be up to 2Gbps or more for longer distances (tens of km).

· Smaller size and lighter weight than coaxial cables.

· Since optical fibers use light therefore these cable are not susceptible to electrical interference, cross talk.

· These cables do cause the electrical interference to other nearby cables.

· The glass fibers are designed in such a way that whole light is reflected inside the core, so a fiber can carry signal for a long distance than the copper wire.

· Lower attenuation.

· The Light can be encoded with more information than electrical voltage therefore an optical fiber can carry more information.

· Light can travel in a single fiber while in electricity it requires a pair of wires to complete a circuit.

12.5.1.4 Disadvantages of optical fibers

· Installation of optical fiber is difficult than coaxial cables. It requires special care at the terminal ends to pass through the light.

· Fiber glass is fragile. Therefore it cannot be bent like copper wire otherwise it will break.

· Identifying the location of breaks in fiber is difficult.

· Repair of broken fiber is difficult. It needs special instruments to join the two fibers so that the light can pass through the joint.

· Optical fiber cable and associated equipments are more expensive than copper wires.

· It requires high quality manufacturing standards because even small impurity or imperfection in core glass can defeat the purpose. 

12.5.2 Wireless media

In wireless media the signals are transmitted in the form of electromagnetic waves through air (it may be water also in some cases). A special device, antenna is used to receive and transmit signals in wireless communication. For transmission antenna radiates the electromagnetic waves into medium. At the other end antenna receives the electromagnetic waves from the surrounding medium. Signals in the medium can be transmitted in two ways broadcasting (omni-directional) and directional. In broadcasting, electromagnetic wave (signals) spreads out in all directions and can be received by more than one antenna. In directional, the electromagnetic wave is directed in particular direction through antenna. The receiving device must be aligned in that direction to receive the signals. In general, the higher, the frequency of a signal, the more it is possible to focus it into a directional beam. Three general ranges of frequencies are common for wireless transmission and are identified as radio waves, microwaves and infrared waves. Radio frequencies can travel through air or space, but require specific transmitting and receiving mechanisms. The mode of propagation of electromagnetic waves in the atmosphere and in free space may be subdivided into following categories:

12.5.2.1 Surface

The radio waves travel through the lowest portion of the atmosphere, just touching the surface. The distance travel depends on the power in the signal greater the power greater the distance. Surface propagation can also take place in seawater. VLF and LF waves are propagated as surface waves.

12.5.2.2 Tropospheic propagation

The signal can be directed in a straight line from antenna to antenna (line of sight). Another way is that waves can be broadcasted at an angle so that the waves get back on the earth after reflecting from troposphere. Troposphere is the portion of atmosphere extending outward approximately up to 30 miles from earth surface.  MF signals are propagated in the troposphere since these frequencies are absorbed by the ionosphere. Absorption increases during day time. Therefore, most MF transmission relies on light-of-sight antennas. Atmospheric noise, man-made noise, and thermal noise from electronic components at the receiver are main disturbances for signal transmission of MF.

12.5.2.3 Ionospheric propagation or sky-wave propagation

Sky-wave propagation results from transmitted signals being reflected from the ionosphere, which consists of several layers of charged particles ranging in altitude from 30–250 miles above the surface of the earth. This type of transmission allows for greater distances to be covered with lower power output.

12.5.2.4 Line-of-sight (LOS) propagation

In LOS propagation very high frequency (VHF) signals are transmitted in straight line directly from antenna to antenna. Antennas must be directional, facing each other and separated from each other so the LOS should not be affected by curvature of the earth.  This propagation is being used for important services like TV, FM radio, aircraft navigation etc.

12.5.2.5 Space propagation

This utilizes satellite relays in place of atmospheric refraction. A broadcast signal is received by an orbiting satellite, which rebroadcasts the signal to the receiver back on the earth. It acts like a super high antenna and increase the distance remarkably between sender and receiver.  Basically it is LOS communication used to transmit signals of SHF and EHF for covering longer distances because SHF and EHF signals can cross without absorbance in the ionosphere layers. 

12.6 Characteristics of Communication Channels

The following are the main characteristics of communication channels:

12.6.1 Bandwidth

The bandwidth refers to the total capacity of a communication channel to transmit data. In analogous communication, it is the difference between the highest and lowest frequencies capable of being carried over a channel. The greater the bandwidth, the more signals that can be carried over a given frequency range. For example voice grade lines transmit frequencies from 300 Hz to 3400 Hz. Thus the bandwidth is 3400 Hz - 300 Hz = 3100 Hz or 3.1 KHz.

In digital communication and networking, bandwidth refers to data rate – the amount of data that can be transferred over a communication medium in a given period. Data rate is measured in bits per second (bps) and can vary considerably from one type of channel to another. For example LAN has data rates ranging from 4 Mbps (megabits per second) to 1000Mbps (or 1Gbps). Now days it is up to 10Gbps also on optical media. The bandwidth of dialup connections using modem ranges from 33.6Kbps to 56Kbps or 1Mbps.

12.6.2 Data transmission speed – bps and baud

Serial data speed is expressed as the number of bits transmitted per second (bps). This is also often referred to as the baud rate, but actually baud and bps not necessarily the same. When data is given in bps, the actual number of bits transmitted per second is specified, whereas, baud rate is the number of signals transmitted per second. 

12.6.3 Baseband and broadband

Baseband is a signaling technique in which the signal is transmitted in its original form and not changed by modulation. Broadband makes use of multiple channels over the same medium by frequency division of the bandwidth.

12.6.4 Narrowband

In this bandwidth, data is transmitted in a range of 300 to 1200bps. They are mainly used for low speed terminals.

12.6.5 Voice band or medium band

This channel is the standard telephone lines wich allows transmission rates from 300 to 2400bauds or upto 9600bauds or more.

12.6.6 Broadband or high speed

Transmission rate varies from 19200bps to billion bps. It is being used for transmission of large volume of data such as TV network satellite communication etc.

12.6.7 Transmission technology

Broadcast networks: Two or more communicating devices on the network share a single communication channel. This is also known as multipoint or multidrop line configuration. A message sent by a device is received by all devices but only the concerned device process the message and other discard the message.

Point-to-point networks: This type of network consists of a dedicated link between communicating devices. This is also known as point-to-point line configuration. The entire capacity of the channel is reserved for transmission between those two communicating devices.

12.6.8 Transmission mode

This term is used to define the direction of signal flow between two linked devices. Different types of transmission mode are described as follow:

Simplex: In this mode, the communication is unidirectional. Only one of the two stations on a link can transmit; the other can only receive. Data flows in one direction only. For example Radio system, TV transmission, Printers, Keyboards, PAGER system etc.

Half duplex: In half duplex mode, each station can both transmit and receive, but not at the same time. Data flows in both directions but at a time only in one direction. For example voice communication (telephone talk), Transmission of data to and from hard disk etc.

Full duplex: In full duplex mode, both stations can transmit and receive simultaneously. Data flows in both directions. For example data transmission, Internet access, Chatting etc.

12.6.9 Digital data transmission

Data transmission across the media can be either in serial or parallel. In parallel mode multiple bits each on separate channel, are sent simultaneously at a time. For example, for 8 bits transmission 8 channels are required. Advantage of parallel channels is high speed but cost is too high because of multiple channels. Therefore it is generally used for short distances. In serial transmission one bit is transmitted at one time over single channel. Thus it reduces the cost but also speed. Serial communication is generally used for long distance communication. Communication within the devices is parallel therefore conversion devices are required at the interfaces between sender and line and between line and receiver.   

Serial transmission may either be asynchronous or synchronous. Asynchronous transmission transmits one character (8 bits) at a time, with each character preceded by a start bit and followed by a stop bit because the timing of signal is unimportant. Sender and receiver are not alert to send and receive the signals with time binding. Asynchronous transmission is inefficient because of additional bit are required to indicate start and stop bit and an ideal time between the transmission of characters. It is normally used for low speed data transmission at rates below 2400 bps. In synchronous transmission a group of characters is sent at a time. The start and end of a character is determined by a timing signal indicated by the sending device. Thus it eliminates the need for that start and stop bit as well as gaps between characters. However the sender and receiver must be in perfect coordination to avoid the loss or gain of data. A unique string of bits called sync bits is used to synchronize the timing of sender and receiver. Synchronous transmission is generally used for high speed data communication.

12.6.10 Transmission error control

A communication channel may be subject to noise which may corrupt the original signals.  This necessitates accuracy controls for data transmission. These controls consist of bits known as parity bits that are similar to check sums added to data by the sender. Parity bits are checked at the receiving end to find whether bits were lost during data transmission. If errors are detected, these may be rectified by using backward or forward error correction methods. In backward error correction method, the sender is requested to retransmit the entire data or a particular part as per requirement. Forward error correction method makes use of knowledge of data stream and mathematical algorithms (for example Cyclic redundancy checks (CRC), Hamming code, Burst error correction (BEC) etc.) to allow the receiver to correct the received data without going back to sender. However this method is more complex but preferred over long distances where retransmission is costly.