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Q1. Explain all design issues for several layers in Computer. What is connection oriented

and connectionless service?

Answer:

The various key design issues are present in several layers in computer networks. The important design

issues are:

1. Addressing: Mechanism for identifying senders and receivers, on the network need some formof addressing. There are multiple processesrunning on one machine. Some means is needed for a

process on one machine to specify with whom it wants to communicate.

2. Error Control: There may be erroneous transmission due to several problems duringcommunication. These are due to problem in communication circuits, physical medium, due to

thermal noise and interference. Many error detecting and error correcting codes are known, but

both ends of the connection must agree on which one being used. In addition, the receiver must

have some mechanism of telling the sender which messages have been received correctly and which

has not.

3. Flow control: If there is a fast sender at one end sending data to a slow receiver, then there must beflow control mechanism to control the loss of data by slow receivers. There are several mechanisms

used for flow control such as increasing buffer size at receivers, slow down the fast sender, and so

on. Some process will not be in position to accept arbitrarily long messages. Then, there must be

some mechanism to disassembling, transmitting and then reassembling messages.

4. Multiplexing / de-multiplexing: If the data has to be transmitted on transmission media separately,it is inconvenient or expensive to setup separate connection for each pair of

communicating processes. So, multiplexing is needed in the physical layer at sender end and de-

multiplexing is need at the receiver end.

5. Routing: When data has to be transmitted from source to destination, there may be multiple pathsbetween them. An optimized (shortest) route must be chosen. This decision is made on the basis of

several routing algorithms, which chooses optimized route to the destination.

Connection Oriented and Connectionless Services:

Layers can offer two types of services namely connection oriented service and connectionless service.

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Connection oriented service:

The service user first establishes a connection, uses the connection and then releases the connection.

Once the connection is established between source and destination, the path is fixed. The data

transmission takes place through this path established. The order of the messages sent will be same at

the receiver end. Services are reliable and there is no loss of data. Most of the time, reliable service

provides acknowledgement is an overhead and adds delay.

Connectionless Services:

In this type of services, no connection is established between source and destination. Here there is no

fixed path.

Therefore, the messages must carry full destination address and each one of these messages are sent

independent of each other. Messages sent will not be delivered at the destination in the same order.

Thus, grouping and ordering is required at the receiver end, and the services are not reliable.

There is no acknowledgement confirmation from the receiver. Unreliable connectionless service is often

called datagram service, which does not return an acknowledgement to the sender. In some cases,

establishing a connection to send one short messages is needed. But reliability is required, and

then acknowledgement datagram service can be used for these applications.

Another service is the request-reply service. In this type of service, the sender transmits a single

datagram containing a request from the client side. Then at the other end, server reply will contain

the answer. Request reply is commonly used to implement communication in the client-server model.

Connection-Oriented and Connectionless Services

Two distinct techniques are used in data communications to transfer data. Each has its own advantages

and disadvantages. They are the connection-oriented method and the connectionless method:

Connection-oriented Requires a session connection (analogous to a phone call) be establishedbefore any data can be sent. This method is often called a "reliable" network service. It can

guarantee that data will arrive in the same order. Connection-oriented services set up virtual links

between end systems through a network, as shown in Figure 1. Note that the packet on the left is

assigned the virtual circuit number 01. As it moves through the network, routers quickly send it

through virtual circuit 01.

Connectionless Does not require a session connection between sender and receiver. The sendersimply starts sending packets (called datagrams) to the destination. This service does not have the

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reliability of the connection-oriented method, but it is useful for periodic burst transfers. Neither

system must maintain state information for the systems that they send transmission to or receive

transmission from. A connectionless network provides minimal services.

Connection-oriented methods may be implemented in the data link layers of the protocol stack and/or

in the transport layers of the protocol stack, depending on the physical connections in place and the

services required by the systems that are communicating. TCP (Transmission Control Protocol) is a

connection-oriented transport protocol, while UDP (User Datagram Protocol) is a connectionless

network protocol. Both operate over IP.

The physical, data link, and network layer protocols have been used to implement guaranteed datadelivery. For example, X.25 packet-switching networks perform extensive error checking and packet

acknowledgment because the services were originally implemented on poor-quality telephone

connections. Today, networks are more reliable. It is generally believed that the underlying network

should do what it does best, which is deliver data bits as quickly as possible. Therefore, connection-

oriented services are now primarily handled in the transport layer by end systems, not the network. This

allows lower-layer networks to be optimized for speed.

LANs operate as connectionless systems. A computer attached to a network can start transmitting

frames as soon as it has access to the network. It does not need to set up a connection with the

destination system ahead of time. However, a transport-level protocol such as TCP may set up a

connection-oriented session when necessary.

The Internet is one big connectionless packet network in which all packet deliveries are handled by IP.

However, TCP adds connection-oriented services on top of IP. TCP provides all the upper-level

connection-oriented session requirements to ensure that data is delivered properly. MPLS is a relatively

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new connection-oriented networking scheme for IP networks that sets up fast label-switched paths

across routed or layer 2 networks.

A WAN service that uses the connection-oriented model is frame relay. The service provider sets up

PVCs (permanent virtual circuits) through the network as required or requested by the customer. ATM is

another networking technology that uses the connection-oriented virtual circuit approach.

Q2. Discuss OSI Reference model.

Answer:

Open Systems Interconnection (OSI ) is a standard reference model for communication

between two end users in a network. The model is used in developing products and

understanding networks. Also see the notes below the figure.

OSI divides telecommunication into seven layers. The layers are in two groups. The upper four layers are

used whenever a message passes from or to a user. The lower three layers are used when any message

passes through the host computer. Messages intended for this computer pass to the upper layers.

Messages destined for some other host are not passed up to the upper layers but are forwarded to

another host. The seven layers are:

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Layer 7: The application layer ...This is the layer at which communication partners are identified, quality

of service is identified, user authentication and privacy are considered, and any constraints on data

syntax are identified. (This layer is notthe application itself, although some applications may perform

application layer functions.)

Layer 6: The presentation layer ...This is a layer, usually part of an operating system, that converts

incoming and outgoing data from one presentation format to another (for example, from a text stream

into a popup window with the newly arrived text). Sometimes called the syntax layer.

Layer 5: The session layer ...This layer sets up, coordinates, and terminates conversations, exchanges,

and dialogs between the applications at each end. It deals with session and connection coordination.

Layer 4: The transport layer ...This layer manages the end-to-end control (for example, determining

whether all packets have arrived) and error-checking. It ensures complete data transfer.

Layer 3: The network layer ...This layer handles the routing of the data (sending it in the right direction

to the right destination on outgoing transmissions and receiving incoming transmissions at the packet

level). The network layer does routing and forwarding.

Layer 2: The data-link layer ...This layer provides synchronization for the physical level and does bit-

stuffing for strings of 1's in excess of 5. It furnishes transmission protocol knowledge and management.

Layer 1: The physical layer ...This layer conveys the bit stream through the network at the electrical and

mechanical level. It provides the hardware means of sending and receiving data on a carrier.

OSI Reference Model

The International Organization for Standardization (ISO) developed the OSI reference model in the early

1980s. OSI is now the de facto standard for developing protocols that enable computers to

communicate. Although not every protocol follows this model, most new protocols use this layered

approach. In addition, when starting to learn about networking, most instructors will begin with this

model to simplify understanding.

The OSI reference model breaks up the problem of intermachine communication into seven layers. Each

layer is concerned only with talking to its corresponding layer on the other machine (see Figure 1). This

means that Layer 5 has to worry only about talking to Layer 5 on the receiving machine, and not what

the actual physical medium might be.

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Figure 1. OSI Reference Model

In addition, each layer of the OSI reference model provides services to the layer above it (Layer 5 to

Layer 6, Layer 6 to Layer 7, and so on) and requests certain services from the layer directly below it (5 to

4, 4 to 3, and so on).

This layered approach enables each layer to handle small pieces of information, make any necessary

changes to the data, and add the necessary functions for that layer before passing the data along to the

next layer. Data becomes less human-like and more computer-like the further down the OSI reference

model it traverses, until it becomes 1s and 0s (electrical impulses) at the physical layer. Figure 1 shows

the OSI reference model.

The focus of this chapter is to discuss the seven layers (application, presentation, session, transport,

network, data link, and physical). Understanding these layers allows you to understand how IP routing

works and how IP is transported across various media residing at Layer 1.

The Internet Protocol suite (see Figure 1) maps to the corresponding OSI layers. From the IP Suite figure,

you can see how applications (FTP or email) run atop protocols such as TCP before they are transmitted

across some Layer 1 transport mechanism.

The Application Layer

Most users are familiar with the application layer. Some well-known applications include the following:

E-mail Web browsing Word processing

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The Presentation Layer

The presentation layer ensures that information sent by the application layer of one system is readable

by the application layer of another system. If necessary, the presentation layer translates between

multiple data formats by using a common data representation format.

The presentation layer concerns itself not only with the format and representation of actual user data,

but also with data structures used by programs. Therefore, in addition to actual data format

transformation (if necessary), the presentation layer negotiates data transfer syntax for the application

layer.

Common examples include

Encryption Compression ASCII EBCDIC

The Session Layer

As its name implies, the session layer establishes, manages, and terminates sessions between

applications. Sessions consist of dialogue between two or more presentation entities (recall that the

session layer provides its services to the presentation layer).

The session layer synchronizes dialogue between presentation layer entities and manages their data

exchange. In addition to basic regulation of conversations (sessions), the session layer offers provisions

for data expedition and exception reporting of session-layer, presentation-layer, and application-layer

problems.

The Transport Layer

The transport layer is responsible for ensuring reliable data transport on an internetwork. This is

accomplished through flow control, error checking (checksum), end-to-end acknowledgments,

retransmissions, and data sequencing.

Some transport layers, such as Transmission Control Protocol (TCP), have mechanisms for handling

congestion. TCP adjusts its retransmission timer, for example, when congestion or packet loss occurs

within a network. TCP slows down the amount of traffic it sends when congestion is present. Congestion

is determined through the lack of acknowledgments received from the destination node.

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The Network Layer

The network layer provides for the logical addressing which enables two disparate systems on different

logical networks to determine a possible path to communicate. The network layer is the layer in which

routing protocols reside.

On the Internet today, IP addressing is by far the most common addressing scheme in use. Routing

protocols such as Enhanced Interior Gateway Routing Protocol (Enhanced IGRP, or EIGRP), Open

Shortest Path First (OSPF), Border Gateway Protocol (BGP), Intermediary System to Intermediary System

(IS-IS), and many others are used to determine the optimal routes between two logical subnetworks

(subnets).

Note

You can switch IP traffic outside its own subnetwork only if you use an IP router.

Traditional routers route IP packets based on their network layer address.

Key functions of the network layer include the following:

Packet formatting, addressing networks and hosts, address resolution, and routing Creating and maintaining routing tables

The Data Link Layer

The data link layer provides reliable transport across a physical link. The link layer has its own addressing

scheme. This addressing scheme is concerned with physical connectivity and can transport frames based

upon the data link layer address.

Traditional Ethernet switches switch network traffic based upon the data link layer (Layer 2) address.

Switching traffic based on a Layer 2 address is generally known as bridging. In fact, an Ethernet switch is

nothing more than a high-speed bridge with multiple interfaces.

The Physical Layer

The physical layer is concerned with creating 1s and 0s on the physical medium with electricalimpulses/voltage changes. Common physical layer communication specifications include the following:

EIA/TIA-232Electrical Industries Association/Telecommunications Industry Associationspecification used for communicating between computing devices. This interface is often used

for connecting computers to modems, and might use different physical connectors.

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V.35International Telecommunication Union Telecommunication Standardization Sector (ITU-T)signaling mechanism that defines signaling rates from 19.2 Kbps to 1.544 Mbps. This physical

interface is a 34-pin connector and also is known as a Winchester Block.

RS-449Specification used for synchronous wide area communication. The physical connectoruses 37 pins and is capable of significantly longer runs than EIA/TIA-232.

802.3One of the most widely utilized physical mediums is Ethernet. Currently, Ethernet speedsare deployed from 10Mbps to 1000Mbps.

Q3. Describe different types of Data transmission modes?

Answer:

Transmission modes

A given transmission on a communications channel between two machines can occur in several different

ways. The transmission is characterised by:

the direction of the exchanges the transmission mode: the number of bits sent simultaneously synchronisation between the transmitter and receiverSimplex, half-duplex and full-duplex connections

There are 3 different transmission modes characterised according to the direction of the exchanges:

A simplex connection is a connection in which the data flows in only one direction, from thetransmitter to the receiver. This type of connection is useful if the data do not need to flow in both

directions (for example, from your computer to the printer or from the mouse to your computer...).

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A half-duplex connection (sometimes called an alternating connection or semi-duplex) is aconnection in which the data flows in one direction or the other, but not both at the same time. With

this type of connection, each end of the connection transmits in turn. This type of connection makes

it possible to have bidirectional communications using the full capacity of the line.

A full-duplex connection is a connection in which the data flow in both directions simultaneously.Each end of the line can thus transmit and receive at the same time, which means that the

bandwidth is divided in two for each direction of data transmission if the same transmission medium

is used for both directions of transmission.

The transmission of binary data across a link can be accomplished in either parallel or serial mode. In

parallel mode, multiple bits are sent with each clock tick. In serial mode, 1 bit is sent with each clock tick.

While there is one way to send parallel data, there are three subclasses of serial transmission:

asynchronous, synchronous, and isochronous.

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Serial and parallel transmission

The transmission mode refers to the number of elementary units of information (bits) that can be

simultaneously translated by the communications channel. In fact, processors (and therefore computers

in general) never process (in the case of recent processors) a single bit at a time; generally they are able

to process several (most of the time it is 8: one byte), and for this reason the basic connections on a

computer are parallel connections.

Parallel connection

Parallel connection means simultaneous transmission ofN bits. These bits are sent simultaneously

overN different channels (a channel being, for example, a wire, a cable or any other physical medium).

Theparallel connection on PC-type computers generally requires 10 wires.

These channels may be:

N physical lines: in which case each bit is sent on a physical line (which is why parallel cables aremade up of several wires in a ribbon cable)

one physical line divided into several sub-channels by dividing up the bandwidth. In this case, eachbit is sent at a different frequency...

Since the conductive wires are close to each other in the ribbon cable, interference can occur

(particularly at high speeds) and degrade the signal quality...

Serial connection

In a serial connection, the data are sent one bit at a time over the transmission channel. However, since

most processors process data in parallel, the transmitter needs to transform incoming parallel data into

serial data and the receiver needs to do the opposite.

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These operations are performed by a communications controller (normally a UART(Universal

Asynchronous Receiver Transmitter) chip). The communications controller works in the following

manner:

The parallel-serial transformation is performed using a shift register. The shift register, workingtogether with a clock, will shift the register (containing all of the data presented in parallel) by one

position to the left, and then transmit the most significant bit (the leftmost one) and so on:

The serial-parallel transformation is done in almost the same way using a shift register. The shiftregister shifts the register by one position to the left each time a bit is received, and then transmits

the entire register in parallel when it is full:

Synchronous and asynchronous transmission

Given the problems that arise with a parallel-type connection, serial connections are normally used.

However, since a single wire transports the information, the problem is how to synchronise thetransmitter and receiver, in other words, the receiver can not necessarily distinguish the characters (or

more generally the bit sequences) because the bits are sent one after the other. There are two types of

transmission that address this problem:

An asynchronous connection, in which each character is sent at irregular intervals in time(for example a user sending characters entered at the keyboard in real time). So, for

example, imagine that a single bit is transmitted during a long period of silence... the

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receiver will not be able to know if this is 00010000, 10000000 or 00000100...

To remedy this problem, each character is preceded by some information indicating the

start of character transmission (the transmission start information is called a START bit)

and ends by sending end-of-transmission information (called STOP bit, there may even be

several STOP bits).

In a synchronous connection, the transmitter and receiver are paced by the same clock. The receivercontinuously receives (even when no bits are transmitted) the information at the same rate the

transmitter send it. This is why the transmitter and receiver are paced at the same speed. In addition,

supplementary information is inserted to guarantee that there are no errors during transmission.

During synchronous transmission, the bits are sent successively with no separation between eachcharacter, so it is necessary to insert synchronisation elements; this is called character-level

synchronisation.

The main disadvantage of synchronous transmission is recognising the data at the receiver, as there may

be differences between the transmitter and receiver clocks. That is why each data transmission must be

sustained long enough for the receiver to distinguish it. As a result, the transmission speed can not be

very high in a synchronous link.

Q4. Define Switching. What is the difference between circuit switching and Packet Switching?

Answer:

Switching is referred as the controlling or routing of signals in circuits to execute logical or

arithmetic operations or to transmit data between specific points in a network.

Note: Switching may be performed by electronic, optical, or electromechanical devices.

A switch is network consists of a series of interlinked nodes, called switches. Switches are

devices capable of crating temporary connections between two or more devices linked to the

switch. In a switched network, some of these nodes are connected to the end systems

(computers or telephones). Others are used only for routing. Switched networks are divided, as

shown in the figure.

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Packet-switched and circuit-switched networks use two different technologies for sending messages and

data from one point to another.

Each has their advantages and disadvantages depending on what you are trying to do.

Circuit Switching

Circuit switching was designed in 1878 in order to send telephone calls down a dedicated channel. This

channel remained open and in use throughout the whole call and could not be used by any other data or

phone calls.

There are three phases in circuit switching: Establish Transfer Disconnect

The telephone message is sent in one go, it is not broken up. The message arrives in the sameorder that it was originally sent.

In modern circuit-switched networks, electronic signals pass through several switches before aconnection is established.

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During a call, no other network traffic can use those switches. The resources remain dedicated to the circuit during the entire data transfer and the entire

message follows the same path.

Circuit switching can be analogue or digital With the expanded use of the Internet for voice and video, analysts predict a gradual shift away

from circuit-switched networks.

A circuit-switched network is excellent for data that needs a constant link from end-to-end. Forexample real-time video.

Disadvantages:

Inefficient the equipment may be unused for a lot of the call, if no data is being sent, thededicated line still remains open

Takes a relatively long time to set up the circuit During a crisis or disaster, the network may become unstable or unavailable. It was primarily developed for voice traffic rather than data traffic.

It is easier to double the capacity of a packet switched network than a circuit network a circuit

network is heavily dependent on the number of channel available.

It is cheaper to expand a packet switching system. Circuit-switched technologies, which take four times as long to double their performance/cost,

force ISPs to buy that many more boxes to keep up. This is why everyone is looking for ways to

get Internet traffic off the telephone network. The alternative of building up the telephone

network to satisfy the demand growth is economically out of the question.

The battle between circuit and packet technologies has been around a long time, and it isstarting to be like the old story of the tortoise and the hare. In this case, the hare is circuit

switchingfast, reliable and smart. The hare starts out fast and keeps a steady pace, while the

tortoise starts slow but manages to double his speed every 100 meters.

If the race is longer than 2 km, the power of compounding favours the tortoise.Packet Switching

In packet-based networks, the message gets broken into small data packets. These packets are sentout from the computer and they travel around the network seeking out the most efficient route to

travel as circuits become available. This does not necessarily mean that they seek out the shortest

route. Each packet may go a different route from the others.

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Each packet is sent with a header address. This tells it where its final destination is, so it knowswhere to go.

The header address also describes the sequence for reassembly at the destination computer so thatthe packets are put back into the correct order.

One packet also contains details of how many packets should be arriving so that the recipientcomputer knows if one packet has failed to turn up.

If a packet fails to arrive, the recipient computer sends a message back to the computer whichoriginally sent the data, asking for the missing packet to be resent.

Advantages:

Security Bandwidth used to full potential Devices of different speeds can communicate Not affected by line failure (rediverts signal) Availability do not have to wait for a direct connection to become available During a crisis or disaster, when the public telephone network might stop working, e-mails and texts

can still be sent via packet switching

Disadvantages

Under heavy use there can be a delay Data packets can get lost or become corrupted Protocols are needed for a reliable transfer

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Not so good for some types data streams e.g real-time video streams can lose frames due to theway packets arrive out of sequence.

Comparison of switching techniques

Item Circuit Switched Packet Switched

What is send Voice Messages (divided)

Call setup Required Not Required

Dedicated Physical path Yes No

Each packet follows the

same route

Yes No

Packets arrive in order Yes No

Is a switch crash is fatal Yes No

Bandwidth available Yes No

Time of possible congestion At setup time On every packet

Store-and-forward No Yes

Q5. Classify Guided medium (wired).Compare fiber optics and copper wire.

Answer:

Guided Transmission Medium

Guided media, are those that provide a conduit from one device to another, include twisted-pair cable,

coaxial cable, and fiber-optic cable.

Guided Transmission Media uses a "cabling" system that guides the data signals along a specific path.

The data signals are bound by the "cabling" system. Guided Media is also known as Bound Media.

Cabling is meant in a generic sense in the previous sentences and is not meant to be interpreted as

copper wire cabling only. Cable is the medium through which information usually moves from onenetwork device to another.

Twisted pair cable and coaxial cable use metallic (copper) conductors that accept and transport signals

in the form of electric current. Optical fiber is a glass or plastic cable that accepts and transports signals

in the form of light.

There four basic types of Guided Media:

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1. Open Wire

2. Twisted Pair

3. Coaxial Cable

4. Optical Fiber

OPEN WIRE

Open Wire is traditionally used to describe the electrical wire strung along power poles. There is a single

wire strung between poles. No shielding or protection from noise interference is used. We are going to

extend the traditional definition of Open Wire to include any data signal path without shielding or

protection from noise interference. This can include multiconductor cables or single wires. This media is

susceptible to a large degree of noise and interference and consequently not acceptable for data

transmission except for short distances under 20 ft.

TWISTED-PAIR (TP) CABLE

Twisted pair cable is least expensive and most widely used. The wires in Twisted Pair cabling are twisted

together in pairs. Each pair would consist of a wire used for the +ve data signal and a wire used for the -

ve data signal. Any noise that appears on one wire of the pair would occur on the other wire. Because

the wires are opposite polarities, they are 180 degrees out of phase When the noise appears on both

wires, it cancels or nulls itself out at the receiving end. Twisted Pair cables are most effectively used in

systems that use a balanced line method of transmission : polar line coding (Manchester Encoding) as

opposed to unipolar line coding (TTL logic).

Physical description

Two insulated copper wires arranged in regular spiral pattern. Number of pairs are bundled together in a cable. Twisting decreases the crosstalk interference between adjacent pairs in the cable, by usingdifferent twist length for neighboring pairs.

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A twisted pair consists of two conductors (normally copper), each with its own plastic insulation, twisted

together. One of the wire is used to carry signals to the receiver, and the other is used only a ground

reference.Why the cable is twisted?

In past, two parallel flat wires were used for communication. However, electromagnetic interference

from devices such as a motor can create noise over those wires.

If the two wires are parallel, the wire closest to the source of the noise gets more interference and ends

up with a higher voltage level than the wire farther away, which results in an uneven load and a

damaged signal. If, however, the two wires are twisted around each other at regular intervals, each wire

is closer to the noise source for half the time and farther away for the other half. The degree of

reduction in noise interference is determined specifically by the number of turns per foot. Increasing thenumber of turns per foot reduces the noise interference. To further improve noise rejection, a foil or

wire braid shield is woven around the twisted pairs.

Twisted pair cable supports both analog and digital signals. TP cable can be either unshielded TP (UTP)

cable or shielded TP (STP) cable. Cables with a shield are called Shielded Twisted Pair and commonly

abbreviated STP. Cables without a shield are called Unshielded Twisted Pair or UTP. Shielding means

metallic material added to cabling to reduce susceptibility to noise due to electromagnetic interference

(EMI).

IBM produced a version of TP cable for its use called STP. STP cable has a metal foil that encases each

pair of insulated conductors. Metal casing used in STP improves the quality of cable by preventing the

penetration of noise. It also can eliminate a phenomenon called crosstalk.

Crosstalk is the undesired effect of one circuit (or channel) on another circuit (or channel). It occurs

when one line picks up some of the signal traveling down another line. Crosstalk effect can be

experienced during telephone conversations when one can hear other conversations in the background.

Twisted-pair cabling with additional shielding to reduce crosstalk and other forms of electromagnetic

interference (EMI). It has an impedance of 150 ohms, has a maximum length of 90 meters, and is used

primarily in networking environments with a high amount of EMI due to motors, air conditioners, power

lines, or other noisy electrical components. STP cabling is the default type of cabling for IBM Token Ring

networks. STP is more expensive as compared to UTP.

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UTP is cheap, flexible, and easy to install. UTP is used in many LAN technologies, including Ethernet and

Token Ring.

In computer networking environments that use twisted-pair cabling, one pair of wires is typically used

for transmitting data while another pair receives data. The twists in the cabling reduce the effects of

crosstalk and make the cabling more resistant to electromagnetic interference (EMI), which helps

maintain a high signal-to-noise ratio for reliable network communication. Twisted-pair cabling used in

Ethernet networking is usually unshielded twisted-pair (UTP) cabling, while shielded twisted-pair (STP)

cabling is typically used in Token Ring networks. UTP cabling comes in different grades for different

purposes.

The Electronic Industries Association (EIA) has developed standards to classify UTP cable into seven

categories. Categories are determined by cable quality, with CAT 1 as the lowest and CAT 7 as the

highest.

Category Data Rate Digital/Analog Use

CAT 1 < 100 Kbps Analog Telephone systems

CAT 2 4 Mbps Analog/Digital Voice + Data Transmission

CAT 3 10 Mbps Digital Ethernet 10BaseT LANs

CAT 4 20 Mbps Digital Token based or 10baseT LANs

CAT 5 100 Mbps Digital Ethernet 100BaseT LANs

CAT 6 200 Mbps Digital LANs

CAT 7 600 Mbps Digital LANs

Categories of UTP cable

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Unshielded twisted pair cableThe quality of UTP may vary from telephone-grade wire to extremely high-speed cable. The

cable has four pairs of wires inside the jacket. Each pair is twisted with a different number of twists per

inch to help eliminate interference from adjacent pairs and other electrical devices. The tighter the

twisting, higher the supported transmission rates hence greater the cost per foot.

Unshielded Twisted Pair Connector

The standard connector for unshielded twisted pair cabling is an RJ-45 connector. This is a plastic

connector that looks like a large telephone-style connector. A slot allows the RJ-45 to be inserted only

one way. RJ stands for Registered Jack, implying that the connector follows a standard borrowed from

the telephone industry. This standard designates which wire goes with each pin inside the connector.

Table: STP Cabling categories

STP Cabling

Category

Description

IBM Type 1 Token Ring transmissions on AWG #22 wire up to 20 Mbps.

IBM Type 1A Fiber Distributed Data Interface (FDDI), Copper Distributed Data

Interface (CDDI), and Asynchronous Transfer Mode (ATM)

transmission up to 300 Mbps.

IBM Type 2A Hybrid combination of STP data cable and CAT3 voice cable in one

jacket.

IBM Type 6A AWG #26 patch cables.

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Figure:STP cable

COAXIAL CABLE

A form of network cabling used primarily in older Ethernet networks and in electrically noisy industrial

environments. The name coax comes from its two-conductor construction in which the conductors

run concentrically with each other along the axis of the cable. Coaxial cabling has been largely replaced

by twisted-pair cabling for local area network (LAN) installations within buildings, and by fiber-optic

cabling for high-speed network backbones.

Coaxial cable (or coax) carries signals of higher frequency ranges than twisted-pair cable. Instead of

having two wires, coax has a central core conductor of solid or standard wire (usually copper) enclosed

in an insulating sheath, which is, in turn, encased in an outer conductor of metal foil, braid, or a

combination of the two (also usually copper).

Figure : Coaxial cable

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FIBER-OPTIC CABLE

Fiber-optic is a glass cabling media that sends network signals using light. Fiber-optic cabling has higher

bandwidth capacity than copper cabling, and is used mainly for high-speed network Asynchronous

Transfer Mode (ATM) or Fiber Distributed Data Interface (FDDI) backbones, long cable runs, and

connections to high-performance workstations. A fiber-optic cable is made of glass or plastic and

transmits signals in the form of light. Light is a form of electromagnetic energy. It travels at its fastest in

a vacuum: 3,00,000 kilometers/sec. The speed of light depends on the density of the medium through,

which it is traveling (the higher the density, the slower the speed). Light travels in a straight line as long

as it is moving through a single uniform substance. If a ray of light traveling through one substance

suddenly enters another (more or less dense), the ray changes direction. This change is called.

Refraction : The direction in which a light ray is refracted depends on the change in density

encountered. A beam of light moving from a less dense into a denser medium is bent towards vertical

axis.

When light travels into a denser medium, the angle of incidence is greater than the angle of refraction;

and when light travels into a less dense medium, the angle of incidence is less than the angle of

refraction.

Figure : Fiber Optic cable

No Twisted-Pair Cable Coaxial Cable Fiber Optic Cable (FOC)

1. It uses electrical signals for

transmission.

It uses electrical signals for

transmission.

It uses optical form of signal (i.e.

light) for transmission.

2. It uses metallic conductor to

carry the signal.

It uses metallic conductor to

carry the signal.

It uses glass or plastic to carry

the signal.

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3. Noise immunity is low.

Therefore more distortion.

Higher noise immunity than

twisted-pair cable due to

the presence of shielding

conductor.

Highest noise immunity as the

light rays are unaffected by the

electrical noise.

4. Affected due to external

magnetic field.

Less affected due to

external magnetic field.

Not affected by the external

magnetic field.

5. Cheapest Moderately costly Costly

6. Can support low data rates. Moderately high data rates. Very high data rates.

7. Power loss due to conduction

and radiation.

Power loss due to

conduction.

Power loss due to absorption,

scattering, dispersion.

8. Short circuit between two

conductors is possible.

Short circuit between two

conductors is possible.

Short circuit is not possible.

9. Low bandwidth. Moderately high bandwidth. Very high bandwidth.

Table : Comparison of Guided media

Q6. What are different types of satellites?

Answer:

Satellites fall into five principal types:

1. Research Satellites,2. Communication Satellites,3. Weather Satellites,4. Navigational Satellites, and5. Application Satellites.

Communication satellites have some interesting properties that make them attractive for many

applications. In its simplest form, a communication satellite can be thought of as a big microwave

repeater in the sky. It contains several transponders, each of which listens to some portion of the

spectrum, amplifies the incoming signal, and then rebroadcasts it at another frequency to avoid

interference with the incoming signal.

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Communications satellites allow radio, television, and telephone transmissions to be sent live anywhere

in the world. Before satellites, transmissions were difficult or impossible at long distances. The signals,

which travel in straight lines, could not bend around the round Earth to reach a destination far away.

Because satellites are in orbit, the signals can be sent instantaneously into space and then redirected to

another satellite or directly to their destination.

The satellite can have a passive role in communications like bouncing signals from the Earth back to

another location on the Earth; on the other hand, some satellites carry electronic devices

calledtranspondersfor receiving, amplifying, and re-broadcasting signals to the Earth.

Communications satellites are often in geostationary orbit. At the high orbital altitude of 35,800

kilometers, a geostationary satellite orbits the Earth in the same amount of time it takes the Earth to

revolve once. From Earth, therefore, the satellite appears to be stationary, always above the same area

of the Earth. The area to which it can transmit is called a satellite's footprint. For example, many

Canadian communications satellites have a footprint which covers most of Canada.

Communications satellties can also be in highly elliptical orbits. This type of orbit is roughly egg-shaped,

with the Earth near the top of the egg. In a highly elliptical orbit, the satellite's velocity changes

depending on where it is in its orbital path. When the satellite is in the part of its orbit that's close to the

Earth, it moves faster because the Earth's gravitational pull is stronger. This means that a

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communications satellite can be over the region of the Earth that it is communicating with for the long

part of its orbit. It will only be out of contact with that region when it quickly zips close by the Earth.

Weather satellite

Because of weather satellite technology and communications satellite technology, you can find out the

weather anywhere in the world any time of the day. There are television stations that carry weather

information all day long.Meteorologistsuse weather satellites for many things, and they rely on images

from satellites. Here are a few examples of those uses:

Radiation measurements from the earth's surface and atmosphere give information on amountsof heat and energy being released from the Earth and the Earth's atmosphere.

People who fish for a living can find out valuable information about the temperature of the seafrom measurements that satellites make.

Satellites monitor the amount of snow in winter, the movement of ice fields in the Arctic andAntarctic, and the depth of the ocean.

Infrared sensors on satellites examine crop conditions, areas of deforestation and regions ofdrought.

Some satellites have a water vapour sensor that can measure and describe how much watervapour is in different parts of the atmosphere.

Satellites can detect volcanic eruptions and the motion of ash clouds. During the winter, satellites monitor freezing air as it moves south towards Florida and Texas,

allowing weather forecasters to warn growers of upcoming low temperatures.

Satellites receive environmental information from remote data collection platforms on thesurface of the Earth. These include transmitters floating in the water called buoys, gauges of

river levels and conditions, automatic weather stations, stations that measure earthquake and

tidal wave conditions, and ships. This information, sent to the satellite from the ground, is then

relayed from the satellite to a central receiving station back on Earth.

There are two basic types of weather satellites: those ingeostationary orbitand those inpolar orbit.

Orbiting very high above the Earth, at an altitude of 35,800 kilometres (the orbital altitude),

geostationary satellites orbit the Earth in the same amount of time it takes the Earth to revolve once.

From Earth, therefore, the satellite appears to stay still, always above the same area of the Earth. This

orbit allows the satellite to monitor the same region all the time. Geostationary satellites usually

measure in "real time", meaning they transmit photographs to the receiving system on the ground as

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soon as the camera takes the picture. A series of photographs from these satellites can be displayed in

sequence to produce a movie showing cloud movement. This allows forecasters to watch the progress

of large weather systems such as fronts, storms, and hurricanes. Forecasters can also find out the wind

direction and speed by monitoring cloud movement.

The other basic type of weather satellite is polar orbiting. This type of satellite orbits in a path that

closely follows the Earth'smeridian lines, passing over the north and south poles once each revolution.

As the Earth rotates to the east beneath the satellite, each pass of the satellite monitors a narrow area

running from north to south, to the west of the previous pass. These 'strips' can be pieced together to

produce a picture of a larger area. Polar satellites circle at a much lower altitude at about 850 km. This

means that polar satellites can photograph clouds from closer than the high altitude geostationary

satellites. Polar satellites, therefore, provide more detailed information about violent storms and cloud

systems.

Navigation satellite

Satellites for navigation were developed in the late 1950's as a direct result of ships needing to know

exactly where they were at any given time. In the middle of the ocean or out of sight of land, you can't

find out your position accurately just by looking out the window.

The idea of using satellites for navigation began with the launch of Sputnik 1 on October 4, 1957.

Scientists at Johns Hopkins University's Applied Physics Laboratory monitored that satellite. They

noticed that when the transmitted radio frequency was plotted on a graph, a pattern developed. This

pattern was recognizable to scientists, and it is known as the doppler effect. The doppler effect is an

apparent change of radio frequency as something that emits a signal in the form of waves passes by.

Since the satellite was emitting a signal, scientists were able to show that the doppler curve described

the orbit of the satellite.

Today, most navigation systems use time and distance to determine location. Early on, scientists

recognized the principle that, given the velocity and the time required for a radio signal to be

transmitted between two points, the distance between the two points can be computed. The calculation

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must be done precisely, and the clocks in the satellite and in the ground-based receiver must be telling

exactly the same time - they must be synchronized. If they are, the time it takes for a signal to travel can

be measured and then multiplied by the exact speed of light to obtain the distance between the two

positions.

Research satellite

NASA's Voyager 1 spacecraft has entered a new region between our solar system and interstellar space.

Data obtained from Voyager over the last year reveal this new region to be a kind of cosmic purgatory.

In it, the wind of charged particles streaming out from our sun has calmed, our solar system's magnetic

field has piled up, and higher-energy particles from inside our solar system appear to be leaking out into

interstellar space.

"Voyager tells us now that we're in a stagnation region in the outermost layer of the bubble around our

solar system,"said Ed Stone, Voyager project scientist at the California Institute of Technology. "Voyager

is showing that what is outside is pushing back. We shouldn't have long to wait to find out what the

space between stars is really like."

Although Voyager 1 is about 11 billion miles (18 billion kilometers) from the sun, it is not yet in

interstellar space. In the latest data, the direction of the magnetic field lines has not changed, indicating

Voyager is still within the heliosphere, the bubble of charged particles the sun blows around itself. The

data do not reveal exactly when Voyager 1 will make it past the edge of the solar atmosphere into

interstellar space, but suggest it will be in a few months to a few years.

The latest findings, described today at the American Geophysical Union's fall meeting in San Francisco,

come from Voyager's Low Energy Charged Particle instrument, Cosmic Ray Subsystem and

Magnetometer.

Scientists previously reported the outward speed of the solar wind had diminished to zero in April 2010,

marking the start of the new region. Mission managers rolled the spacecraft several times this spring

and summer to help scientists discern whether the solar wind was blowing strongly in another direction.

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It was not. Voyager 1 is plying the celestial seas in a region similar to Earth's doldrums, where there is

very little wind.

During this past year, Voyager's magnetometer also detected a doubling in the intensity of the magnetic

field in the stagnation region. Like cars piling up at a clogged freeway off-ramp, the increased intensity

of the magnetic field shows that inward pressure from interstellar space is compacting it.

Voyager has been measuring energetic particles that originate from inside and outside our solar system.

Until mid-2010, the intensity of particles originating from inside our solar system had been holding

steady. But during the past year, the intensity of these energetic particles has been declining, as though

they are leaking out into interstellar space. The particles are now half as abundant as they were during

the previous five years.

At the same time, Voyager has detected a 100-fold increase in the intensity of high-energy electronsfrom elsewhere in the galaxy diffusing into our solar system from outside, which is another indication of

the approaching boundary.

"We've been using the flow of energetic charged particles at Voyager 1 as a kind of wind sock to

estimate the solar wind velocity,"said Rob Decker, a Voyager Low-Energy Charged Particle Instrument

co-investigator at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. "We've found

that the wind speeds are low in this region and gust erratically. For the first time, the wind even blows

back at us. We are evidently traveling in completely new territory. Scientists had suggested previously

that there might be a stagnation layer, but we weren't sure it existed until now."

Launched in 1977, Voyager 1 and 2 are in good health. Voyager 2 is 15 billion km away from the sun.

The Voyager spacecraft were built by NASA's Jet Propulsion Laboratory in Pasadena, Calif., which

continues to operate both. JPL is a division of the California Institute of Technology. The Voyager

missions are a part of the NASA Heliophysics System Observatory, sponsored by the Heliophysics

Division of the Science Mission Directorate in Washington.

Applications of Satellites:

Traditionally

Weather satellites Radio and TV broadcast satellites Military satellites Satellites for navigation and localization (e.g., GPS)

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Telecommunication

Global telephone connections Connections for communication in remote places or underdeveloped areas Satellite systems to extend cellular phone systems (e.g., GSM or AMPS)Broadband Digital Communications

Broadband satellites transmit high-speed data and video directly to consumers and businesses. Markets

for broadband services also include interactive TV, wholesale telecommunications, telephony, and

point-of-sale communications, such as credit card transactions and inventory control.

Direct-Broadcast Services

Direct-broadcast satellites (DBS) transmit signals for direct reception by the general public, such as

satellite television and radio. Satellite signals are sent directly to users through their own receiving

antennas or satellite dishes, in contrast to satellite/cable systems in which signals are received by a

ground station, and re-broadcast to users by cable.

Environmental Monitoring

Environmental monitoring satellites carry highly sensitive imagers and sounders to monitor the Earth's

environment, including the vertical thermal structure of the atmosphere; the movement and formation

of clouds; ocean temperatures; snow levels; glacial movement; and volcanic activity. Large-scale

computers use this data to model the entire earth's atmosphere and create weather forecasts such as

those provided by national weather services in the U.S. and abroad.

These satellites are typically self-contained systems that carry their own communications systems for

distributing the data they gather in the form reports and other products for analyzing the condition of

the environment. Satellites are particularly useful in this case because they can provide continuous

coverage of very large geographic regions.

Fixed-Satellite Services

Satellites providing Fixed-Satellite Services (FSS) transmit radio communications between ground Earth

stations at fixed locations. Satellite-transmitted information is carried in the form of radio-frequency

signals. Any number of satellites may be used to link these stations. Earth stations that are part of fixed-

satellite services networks also use satellite news gathering vehicles to broadcast from media events,

such as sporting events or news conferences. In addition, FSS satellites provide a wide variety of services

including paging networks and point-of-sale support, such as credit card transactions and inventory

control.

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Government

Providing X-band satellite communications services to governments is a new commercial application

with substantial growth potential. SS/L has designed and built two X-band satellites, which will be

available for lease to government users in the United States and Spain, as well as other friendly and

allied nations within the satellites' extensive coverage areas. Government communications use specially

allocated frequency bands and waveforms.

Beyond environmental applications, government sensors gather intelligence in various forms, including

radar, infrared imaging, and optical sensing.

Mobile Satellite Services

Mobile Satellite Services (MSS) use a constellation of satellites that provide communications services to

mobile and portable wireless devices, such as cellular phones and global positioning systems. The

satellite constellation is interconnected with land-based cellular networks or ancillary terrestrial

components that allow for interactive mobile-to-mobile and mobile-to-fixed voice, data, and multimedia

communications worldwide. With repeaters located on orbit, the interference of traditional fixed-

ground terminals can be eliminated.