Computer Networking and Data Communications

Introduction

• Networks are familiar concept because radio, television, postal, railway networks have been with us for decades.

• An additional form of network, the computer network has also been around more then 25 years.

• Computer networks have less noticeable than other forms of networks because we do not see the network itself when we walk outside. Rather we see the results of using this network when we make travel reservations and perform online transactions.

What is a network?

• A network is a group of various components connected together in such a way that, it is possible to distribute or collect information over the entire group.

• In other words, a network is a link or path used to exchange of data between two or more machines.

Computer networks in addition to exchange of information, can carry out another important function – sharing of resources.

wired networking wireless networking

Data CommunicationDigital communications is the physical transfer of data over a point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optical fibers, wireless communication channels.

Types of Communication systems:-

• Analog communication

• Digital communication

ANALOG DIGITAL

Signalinformation is translated into electric pulses of varying amplitude

translation of information is into binary format (zero or one)

Waves Denoted by sine waves Denoted by square waves

ExampleHuman voice in air, analog electronic devices.

Computers, CDs, DVDs, and other digital electronic devices.

Data transmissions

Subjected to deterioration by noise during transmission and write/read cycle.

Can be noise-immune without deterioration during transmission and write/read cycle.

Response to NoiseMore likely to get affected reducing accuracy

Less affected since noise response are analog in nature

FlexibilityAnalog hardware is not flexible. Digital hardware is flexible in

implementation.

UsesCan be used in analog devices only. Best suited for audio and video transmission.

Best suited for Computing and digital electronics.

Applications Thermometer, Oscilloscope PCs, PDAs

Cost Low cost and portable Cost is high and not easily portable

Errors Error rate is high Low error rate

A Communication Model

Data Communication Model

Communication Technology Applications

voice mail Twitter

e-mailinstant

messagingchat rooms

newsgroups telephony videoconferencing

facebook watsapp global positioning

system (GPS)

Transmission Media

• The transmission medium is the physical path by which a message travels from sender to receiver.

• Computers and telecommunication devices use signals to represent data.

• These signals are transmitted from a device to another in the form of electromagnetic energy.

• Examples of Electromagnetic energy include power, radio waves, infrared light, visible light, ultraviolet light, and X and gamma rays.

Classes of transmission media

Guided media, which are those that provide a

conduit from one device to another.

Unguided media (or wireless communication)

transport electromagnetic waves without using a

physical conductor. Instead, signals are broadcast

through air (or, in a few cases, water), and thus are

available to anyone who has a device capable of

receiving them.

Guided Media

There are three categories of guided media:

1. Twisted-pair cable

2. Coaxial cable

3. Fiber-optic cable

Twisted-pair cable

• Twisted pair consists of two conductors (normally copper), each with its own plastic insulation, twisted together.

• Twisted-pair cable comes in two forms: unshielded and shielded

• The twisting helps to reduce the interference (noise) and crosstalk.

UTP and STP

Unshielded Twisted-pair (UTP) cable

• Any medium can transmit only a fixed range of frequencies

• UTP cable is the most common type of telecommunication medium in use today.

• The range is suitable for transmitting both data and video.

• Advantages of UTP are its cost and ease of use. UTP is cheap, flexible, and easy to install.

The Electronic Industries Association (EIA) has developed standards to grade UTP.

1. Category 1. The basic twisted-pair cabling used in telephone systems. This level of quality is fine for voice but inadequate for data transmission.

2. Category 2. This category is suitable for voice and data transmission of up to 2Mbps.

3. Category 3. This category is suitable for data transmission of up to 10 Mbps. It is now the standard cable for most telephone systems.

4. Category 4. This category is suitable for data transmission of up to 20 Mbps.

5. Category 5. This category is suitable for data transmission of up to 100 Mbps.

UTP connectors

The most common UTP connector is RJ45 (RJ stands for

Registered Jack).

Applications

• Twisted-pair cables are used in telephones lines to provide voice and data channels.

• The DSL(Digital subscriber line) lines that are used by the telephone companies to provide high data rate connections also use the high-bandwidth capability of unshielded twisted-pair cables.

• Local area networks, such as 10Base-T and 100Base-T, also used UTP cables.

Shielded Twisted (STP) Cable

• STP cable has a metal foil or braided-mesh covering that enhances each pair of insulated conductors.

• The metal casing prevents the penetration of electromagnetic noise.

• Materials and manufacturing requirements make STP more expensive than UTP but less susceptible to noise.

Coaxial Cable (or coax)

• Coaxial cable carries signals of higher frequency ranges than twisted-pair cable.

• Coaxial Cable standards:

RG-8, RG-9, RG-11 are used in thick Ethernet

RG-58 Used in thin Ethernet

RG-59 Used for TV cables.

BNC connectors

•To connect coaxial cable to devices, it is necessary to use coaxial connectors. The most common type of connector is the Bayone-Neill-Concelman, or BNC, connectors. There are threetypes: the BNC connector, the BNC T connector, the BNC terminator.Applications include cable TV networks, and some traditional Ethernet LANs like 10Base-2, or 10-Base5.

Optical FiberAn optical fiber is a glass fiber. It carries pulses of light that represent data. Some advantages of optical fibers over metal wires are very low transmission loss, Very high capacity, low noise, small size. Optical fibers can simultaneously carry multiple wavelengths of light, which greatly increases the rate that data can be sent, and helps enable data rates of up to trillions of bits per second. Optic fibers can be used for long runs of cable carrying very high data rates, and are used for undersea cables to interconnect continents.

• Optical fiber is made of glass or plastic and transmits signals in the form of light.

• Light, a form of electromagnetic energy, travels at 300,000 Kilometers/second ( 186,000 miles/second), in a vacuum.

Unguided Media

• Unguided media, or wireless communication, transport electromagnetic waves without using a physical conductor. Instead the signals are broadcast though air or water, and thus are available to anyone who has a device capable of receiving them.

• The section of the electromagnetic spectrum defined as radio communication is divided into eight ranges, called bands,each regulated by government authorities.

Propagation of Radio Waves

• Radio technology considers the earth as surrounded by two layers of atmosphere: the troposphere and the ionosphere.

• The troposphere is the portion of the atmosphere extending outward approximately 30 miles from the earth's surface.

• The troposphere contains what we generally think of as air, Clouds, wind, temperature variations, and weather in general occur in the troposphere.

• The ionosphere is the layer of the atmosphere above the troposphere but below space.

• Broadcast Radio – Distribute signals through the air over long

distance – Uses an antenna – Typically for stationary locations – Can be short range

• Cellular Radio– A form of broadcast radio used for mobile

communication – High frequency radio waves to transmit voice

or data – Utilizes frequency-reuse

• Microwaves – Radio waves providing high speed transmission – They are point-to-point (can’t be obstructed) – Used for satellite communication

• Infrared (IR) – Wireless transmission media that sends signals

using infrared light- waves - Such as?

Physical Transmission Media

100 Mbps is how many bits per sec?

Which is bigger:

10,000 Mbps, 0.01Tbps or 10Gbps?

Wireless channel capacity:

Networks

• Collection of computers and devices connected together

• Used to transfer information or files, share resources, etc.

• What is the largest network?

• Characterized based on their geographical coverage, speed, capacities

• Networks are categorized based on the following characteristics:

– Network coverage: LAN, MAN, WAN

– Network topologies: how the computers are connected together

– Network technologies

– Network architecture

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• The generic term node or host refers to any device on a network

• Data transfer rate:- The speed with which data is moved from one place on a network to another

• Data transfer rate is a key issue in computer networks

LAN vs WANLAN - Local Area Network a group of

computers connected within a building

or a campus (Example of LAN may

consist of computers located on a

single floor or a building or it might link

all the computers in a small company.

WAN - A network consisting

of computers of LAN's

connected across a distance

WAN can cover small to large

distances, using different

topologies such as telephone

lines, fiber optic cabling,

satellite transmissions and

microwave transmissions.

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Wide-area network (WAN) A network that connects two or more local-area networks over a potentially large geographic distance

Often one particular node on a LAN is set up to serve as a gateway to handle all communication going between that LAN and other networks

Communication between networks is called internetworking

The Internet, as we know it today, is essentially the ultimate wide-area network, spanning the entire globe

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So, who owns the Internet???

Well, nobody does. No single person or company owns the Internet or even controls it entirely. As a wide-area network, it is made up of many smaller networks. These smaller networks are often owned and managed by a person or organization. The Internet, then, is really defined by how connections can be made between these networks.

Types of Networks

Local-area networks connected across a distance to create a wide-

area network

Network Topologies

• Configuration or physical arrangement in which devices are connected together

• BUS networks: Single central cable connected a number of devices – Easy and cheap– Popular for LANs

• RING networks: a number of computers are connected on a closed loop – Covers large distances– Primarily used for LANs and WANs

• STAR networks: connecting all devices to a central unit – All computers are connected to a central device called hub– All data must pass through the hub

Network Topologies

personal

computer

personal

computer

personal

computer

personal

computer

personal

computer

host

computer

printer

file server

personal computer

personal computer

personal computer

personal computer

Network Architecture• Refers to how the computer or devices are designed in a

network • Basic types:

– Peer-2-Peer• Each computer (peer) has equal responsibilities,

capacities, sharing hardware, data with the other computers on the peer-to-peer network

• Good for small businesses and home networks • Simple and inexpensive

– Client/Server• All clients must request service from the server• The server is also called a host• Different servers perform different tasks: File server, web

server etc.

Computer networks have opened up an entire frontier in the world of computing called the client/server model

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• File server A computer that stores and manages files for multiple users on a network

• Web server A computer dedicated to responding to requests (from the browser client) for web pages

P2P vs Client-Server

Peers make a portion of their resources, such

as processing power, disk storage or network

bandwidth, directly available to other network

participants, without the need for central

coordination by servers or stable hosts

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Internet Connections

• Internet backbone A set of high-speed networks that carry Internet traffic

These networks are provided by companies such as AT&T and IBM

• Internet service provider (ISP) A company that provides other companies or individuals with access to the Internet

15-45

Internet Connections

• There are various technologies available that you can use to connect a home computer to the Internet

– A phone modem converts computer data into an analog audio signal for transfer over a telephone line, and then a modem at the destination converts it back again into data

– A digital subscriber line (DSL) uses regular copper phone lines to transfer digital data to and from the phone company’s central office

– A cable modem uses the same line that your cable TV signals come in on to transfer the data back and forth

15-46

Internet Connections

• Broadband A connection in which transfer speeds

are faster than 128 bits per second

– DSL connections and cable modems are broadband

connections

– The speed for downloads (getting data from the Internet

to your home computer) may not be the same as uploads

(sending data from your home computer to the Internet)

2.47

Network Models

2.48

LAYERED TASKS

We use the concept of layers in our daily life. As an

example, let us consider two friends who

communicate through postal mail. The process of

sending a letter to a friend would be complex if there

were no services available from the post office.

Sender, Receiver, and Carrier

Hierarchy

Topics discussed in this section:

Figure: Tasks involved in sending a letter

THE OSI MODEL

Established in 1947, the International Standards

Organization (ISO) is a multinational body dedicated

to worldwide agreement on international standards.

An ISO standard that covers all aspects of network

communications is the Open Systems

Interconnection (OSI) model. It was first introduced in

the late 1970s.

ISO is the organization.

OSI is the model.

Note

Communication ArchitectureStrategy for connecting host computers and othercommunicating equipment.

Defines necessary elements for data communicationbetween devices.

A communication architecture, therefore, defines astandard for the communicating hosts.

A programmer formats data in a manner defined by thecommunication architecture and passes it on to thecommunication software.

Separating communication functions adds flexibility, forexample, we do not need to modify the entire host softwareto include more communication devices.

OSI Model

Layer ArchitectureLayer architecture simplifies the network design.

It is easy to debug network applications in a layeredarchitecture network.

The network management is easier due to the layeredarchitecture.

Network layers follow a set of rules, called protocol.

The protocol defines the format of the data beingexchanged, and the control and timing for the handshakebetween layers.

OSI Model

Open Systems Interconnection (OSI) Model

International standard organization (ISO) established acommittee in 1977 to develop an architecture for computercommunication.

Open Systems Interconnection (OSI) reference model is theresult of this effort.

In 1984, the Open Systems Interconnection (OSI) referencemodel was approved as an international standard forcommunications architecture.

Term “open” denotes the ability to connect any twosystems which conform to the reference model andassociated standards.

OSI Model

OSI Reference ModelThe OSI model is now considered the primary Architecturalmodel for inter-computer communications.

The OSI model describes how information or data makes itsway from application programmes (such as spreadsheets)through a network medium (such as wire) to anotherapplication programme located on another network.

The OSI reference model divides the problem of movinginformation between computers over a network mediuminto SEVEN smaller and more manageable problems .

This separation into smaller more manageable functions isknown as layering.

OSI Model

OSI Reference Model: 7 Layers

OSI Model

OSI: A Layered Network Model

The process of breaking up the functions or tasks of networkinginto layers reduces complexity.

Each layer provides a service to the layer above it in the protocolspecification.

Each layer communicates with the same layer’s software orhardware on other computers.

The lower 4 layers (transport, network, data link and physical —Layers 4, 3, 2, and 1) are concerned with the flow of data from endto end through the network.

The upper four layers of the OSI model (application, presentationand session—Layers 7, 6 and 5) are orientated more towardservices to the applications.

Data is Encapsulated with the necessary protocol information as itmoves down the layers before network transit.

OSI Model

The interaction between layers in the OSI model

Physical Layer

Provides physical interface for transmission of information.

Defines rules by which bits are passed from one system toanother on a physical communication medium.

Covers all - mechanical, electrical, functional and procedural- aspects for physical communication.

Such characteristics as voltage levels, timing of voltagechanges, physical data rates, maximum transmissiondistances, physical connectors, and other similar attributesare defined by physical layer specifications.

OSI Model

Physical layer

The physical layer is responsible for movements of

individual bits from one hop (node) to the next.

Note

Data Link LayerData link layer attempts to provide reliable communicationover the physical layer interface.Breaks the outgoing data into frames and reassemble thereceived frames.Create and detect frame boundaries.Handle errors by implementing an acknowledgement andretransmission scheme.Implement flow control.Supports points-to-point as well as broadcastcommunication.Supports simplex, half-duplex or full-duplex communication.

OSI Model

Data link layer

The data link layer is responsible for moving

frames from one hop (node) to the next.

Note

Hop-to-hop delivery

Network Layer

Implements routing of frames (packets) through thenetwork. In other words, The network layer is responsiblefor delivery of a packet between the original source andfinal destination.

Defines the most optimum path the packet should take fromthe source to the destination

Defines logical addressing so that any endpoint can beidentified. Using logical addresses (IP address) instead ofphysical address

Handles congestion in the network.

Facilitates interconnection between heterogeneousnetworks (Internetworking).

The network layer also defines how to fragment a packetinto smaller packets to accommodate different media.

OSI Model

Network layer

The network layer is responsible for the

delivery of individual packets from

the source host to the destination host.

Note

Source-to-destination delivery

Transport LayerPurpose of this layer is to provide a reliable mechanism forthe exchange of data between two processes in differentcomputers.

It is responsible for breaking the entire message intoseveral packets and delivery them to the network layer.

It is responsible for ensuring that the whole message istransmitted.

Ensures that the data units are delivered error free.

Ensures that data units are delivered in sequence.

Ensures that there is no loss or duplication of data units.

Provides connectionless or connection oriented service.

Provides for the connection management.

Multiplex multiple connection over a single channel.

OSI Model

Session LayerSession layer provides mechanism for controlling the dialoguebetween the two end systems. It defines how to start, control andend conversations (called sessions) between applications, inother words, it is designed to control the dialog between users.The synchronization points divides a long message into smallerones and ensure that each section is received and acknowledgedby the receiver.This layer requests for a logical connection to be established onan end-user’s request.Any necessary log-on or password validation is also handled bythis layer.Session layer is also responsible for terminating the connection.This layer provides service which can be full duplex or half duplex.Session layer can also provide check-pointing mechanism suchthat if a failure of some sort occurs between checkpoints, all datacan be retransmitted from the last checkpoint.

OSI Model

Presentation Layer

Presentation layer defines the format in which the data is tobe exchanged between the two communicating entities. Itdeals with the fact that different systems use differentcoding methods.

Also handles data compression and data encryption(cryptography).

OSI Model

Application Layer

Application layer interacts with application programs and isthe highest level of OSI model.

Application layer contains management functions tosupport distributed applications.

Examples of application layer are applications such as filetransfer, electronic mail, remote login etc.

OSI Model

OSI in ActionA message begins at the top applicationlayer and moves down the OSI layers tothe bottom physical layer.

As the message descends, eachsuccessive OSI model layer adds aheader to it.

A header is layer-specific informationthat basically explains what functionsthe layer carried out.

Conversely, at the receiving end,headers are striped from the messageas it travels up the correspondinglayers.

OSI Model

TCP/IP MODEL

TCP/IP Model

OSI & TCP/IP Models

TCP/IP Model

TCP/IP Model

TCP/IP Model

Application LayerApplication programs using the network

Transport Layer (TCP/UDP)

Management of end-to-end message transmission,

error detection and error correction

Network Layer (IP)

Handling of datagrams : routing and congestion

Data Link LayerManagement of cost effective and reliable data delivery,

access to physical networks

Physical Layer

Physical Media

Digital-to-digital conversion

The process for converting digital data into digital signal is said to be Line Coding. Digital data is found in digital format,which is binary bits. It is represented (stored) internally as series of 1s and 0s.

Digital signals which represents digital data, represented as discrete signals. There are three types of line coding schemes available:-

• UNI-POLAR ENCODING

Unipolar encoding schemes uses single voltage level to represent data. In this case, to represent binary 1 high voltage is transmitted and to represent 0 no voltage is transmitted. It is also called Unipolar-Non-return-to-zero, because there‟s no rest condition i.e. it either represents 1 or 0.

• POLAR ENCODING

Polar encoding schemes multiple voltage levels are used to represent binary values. Polar encodings are available in four types:

-POLAR-NRZ (NON-RETURN TO ZERO)

It uses two different voltage levels to represent binary values, generally positive voltage represents 1 and negative value represents 0. It is also NRZ because there‟s no rest condition. NRZ scheme has two variants: NRZ-L and NRZ-I.

NRZ-L changes voltage level at when a different bit is encountered whereas NRZ-I changes voltage when a 1 is encountered.

-RZ (RETURN TO ZERO)

Problem with NRZ was the receiver cannot conclude when a bit ended and when the next bit is started, in case when sender and receiver‟s clock are not synchronized.

RZ uses three voltage levels, positive voltage to represent 1, negative voltage to represent 0 and zero voltage for none. Signals change during bits not between bits.

• MANCHESTER

This encoding scheme is a combination of RZ and NRZ-L. Bit time is divided into two halves. It transitions at the middle of the bit and changes phase when a different bit is encountered.

• DIFFERENTIAL MANCHESTER

This encoding scheme is a combination of RZ and NRZ-I. It also transitions at the middle of the bit but changes phase only when 1 is encountered.

• BIPOLAR ENCODING

Bipolar encoding uses three voltage levels, positive, negative and zero. Zero voltage represents binary 0 and bit 1 is represented by altering positive and negative voltages.

Analog-to-digital conversion

• Microphones creates analog voice and camera creates analog videos, which here in our case is treated is analog data. To transmit this analog data over digital signals we need an analog to digital conversion.

• Analog data is wave form continuous stream of data whereas digital data is discrete. To convert analog wave into digital data we use Pulse Code Modulation.

• Pulse Code Modulation is one of the most commonly used method to convert analog data into digital form. It involves three steps: Sampling, Quantization and Encoding.

• SAMPLING

The analog signal is sampled every T interval. Most important factor in sampling is the rate on which analog signal is sampled. According to Nyquist Theorem, the sampling rate must be at least two times of the highest frequency of the signal.

• QUANTIZATION

Sampling yields discrete form of continuous analog signal. Every discrete pattern shows the amplitude of the analog signal at that instance. The quantization is done between the maximum amplitude value and the minimum amplitude value.

• ENCODING

In encoding, each approximated value is then converted into binary format.

Transmission Modes How data is to be transferred between to computer is decided by the transmission mode they are using. Binary data i.e. 1s and 0s can be sent in two different modes: Parallel and Serial.

• PARALLEL TRANSMISSION

The binary bits are organized in to groups of fixed length. Both sender and receiver are connected in parallel with the equal number of data lines. Both computer distinguish between high order and low order data lines. The sender sends all the bits at once on all lines. Because data lines are equal to the number of bits in a group or data frame, a complete group of bits (data frame) is sent in one go.

Advantage of Parallel transmission is speed and disadvantage is the cost of wires, as it is equal to the number of bits needs to send parallelly.

• SERIAL TRANSMISSION

In serial transmission, bits are sent one after another in a queue manner. Serial transmission requires only one communication channel as oppose parallel transmission where communication lines depends upon bit word length.

Serial transmission can be either asynchronous or synchronous.

-ASYNCHRONOUS SERIAL TRANSMISSION

It is named so because there’s no importance of timing. Data-bits have specific pattern and helps receiver recognize when the actual data bits start and where it ends. For example, a 0 is prefixed on every data byte and one or more 1s added at the end.

Two continuous data-frames (bytes) may have gap between them.

-SYNCHRONOUS SERIAL TRANSMISSION

It is up to the receiver to recognize and separate bits into bytes. The advantage of synchronous transmission is speed and it has no overhead of extra header and footer bits as in asynchronous transmission.

Sine Wave

• Peak Amplitude (A)– maximum strength of signal

– volts

• Frequency (f)– Rate of change of signal

– Hertz (Hz) or cycles per second

– (T)= 1/f

• Phase ()– Relative position in time, from 0-2*pi

• General Sine wave

)2sin()( ftAts

Varying Sine Waves

Digital-to-Analog Conversion

When data from one computer is sent to another via some analog carrier, it is first converted into analog signals. Analog signals are modified to reflect digital data, i.e. binary data. An analog is characterized by its amplitude, frequency and phase. There are three kinds of digital-to-analog conversions possible:

AMPLITUDE SHIFT KEYING

In this conversion technique, the amplitude of analog carrier signal is modified to reflect binary data.

When binary data represents digit 1, the amplitude is held otherwise it is set to 0. Both frequency and phase remain same as in the original carrier signal.

• FREQUENCY SHIFT KEYING

In this conversion technique, the frequency of the analog carrier signal is modified to reflect binary data.

This technique uses two frequencies, f1 and f2. One of them, for example f1, is chosen to represent binary digit 1 and the other one is used to represent binary digit 0. Both amplitude and phase of the carrier wave are kept intact.

• PHASE SHIFT KEYING

In this conversion scheme, the phase of the original carrier signal is altered to reflect the binary data.

Amplitude Modulation (AM)

1

0

1

0

Amp. 1 Amp. 2

1 = Amp. 1

0 = Amp. 2

A B

Characteristics of Amplitude Modulation

• Amplitude of the analog signal is modulated

• One amplitude represents a 0

• Another amplitude represents a 1

• Frequency remains unchanged in both cases

• Signals that are modulated at one end are demodulated at the other end

Usage

• Amplitude is susceptible to interference

– This technique in not normally used in modems

• A variation of this technique is used in AM radio transmission

– Analog-to-analog modulation takes place

Frequency Modulation (FM)

1

0

1

0

Freq. 1 Freq. 2

1 = Frequency F1

0 = Frequency F2

Characteristics of Frequency Modulation

• Frequency is modulated

• Frequency f1

– Represents 1

• Frequency f2

– Represents 0

• The amplitude remains unaltered in both cases

Usage• Variations in frequency are easy to detect

– They are less susceptible to interference

• FM and variations of this technique are used in modems

• Easy to implement full duplex transmission under FM

• A variation of the FM technique described here is used in FM radio transmission

Modulation in Modern Day Modems

• Modern day modems may not use the FM technique for modulation

• They may be using a technique known as Phase Shift Modulation (or Phase Shift Keying)

Phase of an Analog Signal

Y

Strength

X

Time Frame

0 90 180 270 360

Phase Modulation Technique

10

90 Degrees

phase shift0 Degree

phase shift

•This is also known as phase shift keying.

Characteristics of Phase Shift Modulation

• Phase is modulated

• Phase shift of 0 represents a 0

• Phase shift of 90 degrees represents a 1

• Both amplitude and frequency remain unaltered is both cases

• Also known as Phase Shift Keying, it is used in a number of modern modems as well

Fourier Analysis

• We model the behavior of variation of voltage or current

with mathematical functions

• Fourier series is used to expand any periodic function with period T

• f=1/T – fundamental frequency.

• an, bn – are the sine and cosine amplitudes of the n’th harmonic.

• c – is a constant.

Fourier Analysis

• Function reconstructed with

Multiplexing• In telecommunications and computer networks, multiplexing is

a method by which multiple analog message signals or digital data streams are combined into one signal over a shared medium. The aim is to share an expensive resource.

• The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium (e.g. a cable), air (radio frequency), and light (optical fiber).

• The multiplexing divides the capacity of the high-level communication channel into several low-level logical channels, one for each message signal or data stream to be transferred.

• A reverse process, known as de-multiplexing, can extract the original channels on the receiver side.

• A device that performs the multiplexing is called a multiplexer (MUX), and a device that performs the reverse process is called a demultiplexer (DEMUX).

There are three major types of multiplexing:-

• Frequency division multiplexing (FDM)– E.g. AM/FM Radio

• Time division multiplexing (TDM)– Used in Digital carriers

• Wavelength division multiplexing (WDM)– Used in optical carriers (colors carry signals)

Frequency-division multiplexing

• Frequency-division multiplexing (FDM) is an analog multiplexing technique that combines analog signals.

• FDM is applied when the bandwidth of a link (in hertz) is greater than the combined bandwidths of the signals to be transmitted.

• In FDM, signals generated by each sending device modulate different carrier frequencies. These modulated signals are then combined into a single composite signal that can be transported by the link.

• Carrier frequencies are separated by sufficient bandwidth to accommodate the modulated signal. These bandwidth ranges are the channels through which the various signals travel.

• Channels can be separated by strips of unused bandwidth i.e guard bands, to prevent signals from overlapping. In addition, carrier frequencies must not interfere with the original data frequencies.

Wavelength-division multiplexing

• Light has different wavelength (colors).

• Wavelength-division multiplexing (WDM) is an analog multiplexing technique to combine optical signals.

• WDM is designed to use the high data rate capability of fiber-optic cable. The optical fiber data rate is higher than the data rate of metallic transmission cable. Using a fiber-optic cable for one single line wastes the available bandwidth. Multiplexing allows us to combine several lines into one.

• WDM is conceptually the same as FDM, except that the multiplexing and demultiplexing involve optical signals transmitted through fiber-optic channels.

• The idea is the same: We are combining different signals of different frequencies. The difference is that the frequencies are very high.

Time Division Multiplexing

• TDM is applied primarily on digital signals but can be applied on analog signals as well.

• In TDM the shared channel is divided among its user by means of time slot. Each user can transmit data within the provided time slot only. Digital signals are divided in frames, equivalent to time slot i.e. frame of an optimal size which can be transmitted in given time slot.

• TDM works in synchronized mode. Both ends, i.e. Multiplexer and De-multiplexer are timely synchronized and both switch to next channel simultaneously.

• When at one side channel A is transmitting its frame, on the other end De-multiplexer providing media to channel A. As soon as its channel A’s time slot expires this side switches to channel B. On the other end De-multiplexer behaves in a synchronized manner and provides media to channel B.

• Synchronous time-division multiplexing-In synchronous time-division multiplexing, the term synchronous means that the multiplexer allocates exactly the same time slot to each device at all times, whether or not a device has anything to transmit.

• Asynchronous time-division multiplexing-

- Synchronous TDM does not guarantee that the full capacity of a link is used. Because the time slots are

pre-assigned and fixed, whenever a connected device is not transmitting, the corresponding slot is empty.

- Asynchronous time-division multiplexing, or statistical time-division multiplexing, is designed to avoid this type of waste.

Space-division multiplexing• In wired communication, space-division multiplexing

simply implies different point-to-point wires for different channels. Examples include an analogue stereo audio cable, with one pair of wires for the left channel and another for the right channel, and a multi-pair telephone cable. Wired space-division multiplexing is typically not considered as multiplexing.

• In wireless communication, space-division multiplexing is achieved by multiple antenna elements. If we remember our work with directional antennas, we can actually reuse both time and frequency, by transmitting our information along parallel channels.

Code Division Multiplexing • Multiple data signals can be transmitted over a single

frequency by using Code Division Multiplexing.

• FDM divides the frequency in smaller channels but CDM allows its users to full bandwidth and transmit signals all the time using a unique Code.

• Each station is assigned with a unique code, called chip. Signals travels with these codes independently travelling inside the whole bandwidth. The receiver in this case, knows in advance chip code signal it has to receive signals.

• Code Division Multiplex techniques are used as an channel access scheme, namely Code Division Multiple Access (CDMA), e.g. for mobile phone service and in wireless networks. Another important application of CDMA is the Global Positioning System (GPS).

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Switched Communications Networks

• Long distance transmission between stations (called “end devices”) is typically done over a network of switching nodes.

• Switching nodes do not concern with content of data. Their purpose is to provide a switching facility that will move the data from node to node until they reach their destination (the end device).

• A collection of nodes and connections forms a communications network.

• In a switched communications network, data entering the network from a station are routed to the destination by being switched from node to node.

Simple Switching Network

Switching Techniques

Switching is process to forward packets coming in from one port to a port leading towards the destination.In large networks there might be multiple paths linking sender and receiver. Information may be switched as it travels through various communication channels. There are three typical switching techniques available for digital traffic.

• Circuit Switching • Message Switching • Packet Switching

Circuit Switching

• Circuit switching is a technique that directly connects the sender and the receiver in an unbroken path.

• Telephone switching equipment, for example, establishes a path that connects the caller's telephone to the receiver's telephone by making a physical connection.

• With this type of switching technique, once a connection is established, a dedicated path exists between both ends until the connection is terminated.

• Routing decisions must be made when the circuit is first established, but there are no decisions made after that time.

• Circuit switching in a network operates almost the same way as the telephone system works.

• A complete end-to-end path must exist before communication can take place.

• The computer initiating the data transfer must ask for a connection to the destination.

• Once the connection has been initiated and completed to the destination device, the destination device must acknowledge that it is ready and willing to carry on a transfer.

Advantages:• The communication channel (once established) is dedicated.

Disadvantages: • Possible long wait to establish a connection, (10 seconds,

more on long- distance or international calls.) during which no data can be transmitted.

• More expensive than any other switching techniques, because a dedicated path is required for each connection.

• Inefficient use of the communication channel, because the channel is not used when the connected systems are not using it.

Message Switching

• With message switching there is no need to establish a dedicated path between two stations.

• When a station sends a message, the destination address is appended to the message.

• The message is then transmitted through the network, in its entirety, from node to node.

• Each node receives the entire message, stores it in its entirety on disk, and then transmits the message to the next node.

• This type of network is called a store-and-forward network.

• A switch working on message switching, first receives the whole message and buffers it until there are resources available to transfer it to the next hop. If the next hop is not having enough resource to accommodate large size message, the message is stored and switch waits.

This technique was considered substitute to circuit switching. As in circuit switching the whole path is blocked for two entities only. Message switching is replaced by packet switching. Message switching has some drawbacks:

• Every switch in transit path needs enough storage to accommodate entire message.

• Because of store-and-forward technique and waits included until resources available, message switching is very slow.

• Message switching was not a solution for streaming media and real-time applications.

Packet Switching

• Packet switching can be seen as a solution that tries to combine the

advantages of message and circuit switching and to minimize the disadvantages of both.

• There are two methods of packet switching: Datagram and virtual circuit.

• In both packet switching methods, a message is broken into small parts, called packets.

• Each packet is tagged with appropriate source and destination addresses.

• Since packets have a strictly defined maximum length, they can be stored in main memory instead of disk, therefore access delay and cost are minimized.

• Also the transmission speeds, between nodes, are optimized.• With current technology, packets are generally accepted onto

the network on a first-come, first-served basis. If the network becomes overloaded, packets are delayed or discarded

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Packet Switching Technique

• A station breaks long message into packets

• Packets are sent out to the network sequentially, one at a time

• How will the network handle this stream of packets as it attempts to route them through the network and deliver them to the intended destination?

– Two approaches

• Datagram approach

• Virtual circuit approach

Datagram

• Each packet is treated independently, with no reference to packets that have gone before.

– Each node chooses the next node on a packet’s path.

• Packets can take any possible route.

• Packets may arrive at the receiver out of order.

• Packets may go missing.

• It is up to the receiver to re-order packets and recover from missing packets.

• Example: Internet

Virtual Circuit

• In virtual circuit, a preplanned route is established before any packets are sent, then all packets follow the same route.

• Each packet contains a virtual circuit identifier instead of destination address, and each node on the preestablished route knows where to forward such packets.

– The node need not make a routing decision for each packet.

• A route between stations is set up prior to data transfer.

• All the data packets then follow the same route.

• But there is no dedicated resources reserved for the virtual circuit! Packets need to be stored-and-forwarded.

Virtual Circuits v Datagram• Virtual circuits

– Network can provide sequencing (packets arrive at the same order) and error control (retransmission between two nodes).

– Packets are forwarded more quickly

• Based on the virtual circuit identifier

• No routing decisions to make

– Less reliable

• If a node fails, all virtual circuits that pass through that node fail.

• Datagram

– No call setup phase

• Good for bursty data, such as Web applications

– More flexible

• If a node fails, packets may find an alternate route