COMPUTER NETWORKS

(21CS52)
Module-1
Introduction to Networks
and
Physical Layer
 INTRODUCTION TO
NETWORKS
1. NETWORK HARDWARE
 There is no generally accepted taxonomy into which all computer networks fit, but two
dimensions stand out as important:
 Transmission technology
 Scale

 Transmission technology
 Broadly speaking, there are two types of transmission technology that are in widespread
use:
 Broadcast links
 Point-to-point links

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Point-to-point links
 Point-to-point links connect individual pairs of machines.
 To go from the source to the destination on a network made up of point-to-point links, short
messages, called packets in certain contexts, may have to first visit one or more intermediate
machines.
 Often multiple routes, of different lengths, are possible, so finding good ones is important in
point-to-point networks.
 Point-to-point transmission with exactly one sender and exactly one receiver is sometimes
called unicasting.

Broadcast links
 Broadcast systems usually also allow the possibility of addressing a packet to all destinations
by using a special code in the address field.
 When a packet with this code is transmitted, it is received and processed by every machine
on the network.
 This mode of operation is called broadcasting.
 Some broadcast systems also support transmission to a subset of the machines, which
known as multicasting.

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 Scale
 An alternative criterion for classifying networks is by scale. Distance is important as a
classification metric because different technologies are used at different scales.
 In Figure we classify multiple processor systems by their rough physical size.

Figure: Classification of interconnected processors by scale

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1.1 Personal Area Networks (PAN)
 PANs (Personal Area Networks) let devices communicate over the range of a person.
 A common example is a wireless network that connects a computer with its peripherals.
 Without using wireless, this connection must be done with cables.
 So many new users have a hard time finding the right cables and plugging them.
 some companies got together to design a short-range wireless network called Bluetooth to
connect these components without wires.
 The idea is that if your devices have Bluetooth, then you need
no cables.
 You just put them down, turn them on, and they work
together.
 Bluetooth networks use the master-slave paradigm of Figure.
 The system unit (the PC) is normally the master, talking to the
mouse, keyboard, etc., as slaves.
 The master tells the slaves what addresses to use, when they
can broadcast, how long they can transmit, what frequencies
Figure: Bluetooth PAN configuration.
they can use, and so on.

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 Bluetooth can be used in other settings, too.
 It is often used to connect a headset to a mobile phone without cords and it can allow your
digital music player to connect to your car merely being brought within range.
 A completely different kind of PAN is formed when an embedded medical device such as a
pacemaker, insulin pump, or hearing aid talks to a user-operated remote control.
 PANs can also be built with other technologies that communicate over short ranges, such as
RFID on smartcards and library books.

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1.2 Local Area Network (LAN)
 A LAN is a privately owned network that operates within and nearby a single building like a
home, office or factory.
 LANs are widely used to connect personal computers and consumer electronics to let them
share resources (e.g., printers) and exchange information.
 When LANs are used by companies, they are called enterprise networks.
 Wireless LANs are very popular these days, especially in homes, older office buildings,
cafeterias, and other places where it is too much trouble to install cables.
 In these systems, every computer has a radio modem and an antenna that it uses to
communicate with other computers.
 In most cases, each computer talks to a device in the ceiling as shown in Figure (a).
 This device, called an AP (Access Point), wireless router, or base station, relays packets
between the wireless computers and also between them and the Internet.
 There is a standard for wireless LANs called IEEE 802.11, popularly known as WiFi, which has
become very widespread.
 It runs at speeds anywhere from 11 to hundreds of Mbps.

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 Wired LANs use a range of different transmission technologies.
 Most of them use copper wires, but some use optical fiber.
 Typically, wired LANs run at speeds of 100 Mbps to 1 Gbps, and make very few errors.
 Newer LANs can operate at up to 10 Gbps.
 Compared to wireless networks, wired LANs exceed them in all dimensions of performance.
 The topology of many wired LANs is built from point-to-point links.
 IEEE 802.3, popularly called Ethernet, is, by far, the most common type of wired LAN.
 Figure (b) shows a sample topology of switched Ethernet.
 Each computer speaks the Ethernet protocol and connects to a box called a switch with a
point-to-point link.
 A switch has multiple ports, each of which can connect to one computer.

Figure: Wireless and wired LANs. (a) 802.11. (b) Switched Ethernet.
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1.3 Metropolitan Area Network
 A MAN (Metropolitan Area Network) covers a city.
 Examples: cable television networks available in many cities.

Figure: A metropolitan area network based on cable TV

 Recent developments in highspeed wireless Internet access have resulted in another MAN,
which has been standardized as IEEE 802.16 and is popularly known as WiMAX.
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 1.4 Wide Area Network
 A WAN (Wide Area Network) spans a large geographical area, often a country or continent.
 Example: a company with branch offices in different cities.

Figure 1-10. WAN that connects three branch offices in Australia.

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 We will follow traditional usage and call these machines hosts.
 The rest of the network that connects these hosts is then called the communication subnet,
or just subnet for short.
 The job of the subnet is to carry messages from host to host, just as the telephone system
carries words (really just sounds) from speaker to listener.
 In most WANs, the subnet consists of two distinct components:
 transmission lines and
 switching elements.

 Transmission lines
• Move bits between machines.
• They can be made of copper wire, optical fiber, or even radio links.
• Most companies do not have transmission lines lying about, so instead they lease the
lines from a telecommunications company.

 Switching elements or just switches

• are specialized computers that connect two or more transmission lines.

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VPN (Virtual Private Network)
 A company might connect its offices to the Internet.
 This allows connections to be made between the offices as virtual links that use the underlying
capacity of the Internet.
 Compared to the dedicated arrangement, a VPN has the usual advantage of virtualization,
which is that it provides flexible reuse of a resource.
 A VPN also has the usual disadvantage of virtualization, which is a lack of control over the
underlying resources.

Figure: WAN using a virtual private network

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Internet Service Provider
 The subnet operator is known as a
network service provider and the offices
are its customers.
 The subnet operator will connect to other
customers too, as long as they can pay
and it can provide service.
 Since it would be a disappointing network
service if the customers could only send
packets to each other, the subnet
operator will also connect to other
networks that are part of the Internet. Figure: WAN using an ISP network.
 Such a subnet operator is called an ISP
(Internet Service Provider) and the subnet
is an ISP network.
 Its customers who connect to the ISP
receive Internet service.

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1.5 Internetworks
 A collection of interconnected networks is called an internetwork or internet.
 The Internet uses ISP networks to connect enterprise networks, home networks, and many
other networks.
 The general name for a machine that makes a connection between two or more
networks and provides the necessary translation, both in terms of hardware and
software, is a gateway.

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2. NETWORK SOFTWARE
2.1 Protocol Hierarchies
 To reduce their design complexity, most networks are organized as a stack of layers or levels,
each one built upon the one below it.
 The number of layers, the name of each layer, the contents of each layer, and the function of
each layer differ from network to network.
 The purpose of each layer is to offer certain services to the higher layers while shielding those
layers from the details of how the offered services are actually implemented.
 In a sense, each layer is a kind of virtual machine, offering certain services to the layer above
it.
 When layer n on one machine carries on a conversation with layer n on another machine, the
rules and conventions used in this conversation are collectively known as the layer n protocol.
 Basically, a protocol is an agreement between the communicating parties on how
communication is to proceed.

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 A five-layer network is illustrated in Figure.
 The entities comprising the corresponding layers on different machines are called peers.
 The peers may be software processes, hardware devices, or even human beings.

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 In reality, no data are directly transferred from layer n on one machine to layer n on another
machine.
 Instead, each layer passes data and control information to the layer immediately below it,
until the lowest layer is reached.
 Below layer 1 is the physical medium through which actual communication occurs.
 In Figure, virtual communication is shown by dotted lines and physical communication by solid
lines.
 Between each pair of adjacent layers is an interface.
 The interface defines which primitive operations and services the lower layer makes available
to the upper one.
 A set of layers and protocols is called a network architecture.
 A list of the protocols used by a certain system, one protocol per layer, is called a protocol
stack.

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 Figure: The philosopher-translator-secretary architecture.

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 Technical example: how to provide communication to the top layer of the five-layer network in
Figure.

Figure: Example information flow supporting virtual communication in layer 5.

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2.2 Design Issues for the Layers

Some of the key design issues that occur in computer networks will come up in layer after layer.

1. Reliability
 The bits of a packet traveling through the network, there is a chance that some of these bits
will be received damaged (inverted) due to
 fluke electrical noise,
 random wireless signals,
 hardware flaws,
 software bugs and so on

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How is it possible that we find and fix these errors?
 Error detection
 Mechanism for finding errors in received information uses codes for error detection.
 Information that is incorrectly received can then be retransmitted until it is received
correctly.
 More powerful codes allow for error correction, where the correct message is recovered
from the possibly incorrect bits that were originally received.
 Both of these mechanisms work by adding redundant information.
 They are used at low layers, to protect packets sent over individual links, and high layers,
to check that the right contents were received.

 Finding a working path through a network

 Often there are multiple paths between a source and destination, and in a large network,
there may be some links or routers that are broken.
 Suppose that the network is down in Germany. Packets sent from London to Rome via
Germany will not get through, but we could instead send packets from London to Rome
via Paris.
 The network should automatically make this decision.
 This topic is called routing.
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2. Evolution of the network

 Over time, networks grow larger and new designs emerge that need to be connected to the
existing network.
 the key structuring mechanism used to support change by dividing the overall problem and
hiding implementation details: protocol layering.
 Since there are many computers on the network, every layer needs a mechanism for
identifying the senders and receivers that are involved in a particular message.
 This mechanism is called addressing or naming, in the low and high layers, respectively.
 An aspect of growth is that different network technologies often have different limitations.
 When networks get large, new problems arise.
 Designs that continue to work well when the network gets large are said to be scalable.

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3. Resource allocation

 Networks provide a service to hosts from their underlying resources, such as the capacity of
transmission lines.
 To do this well, they need mechanisms that divide their resources so that one host does not
interfere with another too much.

 Statistical multiplexing: Many designs share network bandwidth dynamically, meaning

sharing based on the statistics of demand.
 Flow control: An allocation problem that occurs at every level is how to keep a fast sender
from swamping a slow receiver with data. Feedback from the receiver to the sender is often
used.
 Congestion: Sometimes the problem is that the network is oversubscribed because too many
computers want to send too much traffic, and the network cannot deliver it all. This
overloading of the network is called Congestion.
 Quality of service: For uses such as carrying live video, the timeliness of delivery matters a
great deal. Most networks must provide service to applications that want this real-time
delivery at the same time that they provide service to applications that want high
throughput.
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4. To secure the Network

 By defending it against different kinds of threats.

 Mechanisms that provide confidentiality defend against this threat, and they are used in
multiple layers.
 Mechanisms for authentication prevent someone from impersonating someone else.
 Other mechanisms for integrity prevent surreptitious changes to messages, such as altering
‘‘debit my account $10’’ to ‘‘debit my account $1000.’’

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 2.3 Connection-oriented Versus Connectionless service
 Layers can offer two different types of service to the layers above them:
 Connection-oriented
 Connectionless service

 Connection-oriented service
 This is modeled after the telephone system.
 To talk to someone, you pick up the phone, dial the number, talk, and then hang up.
 Similarly, to use a connection-oriented network service, the service user first
establishes a connection, uses the connection, and then releases the connection.
 The essential aspect of a connection is that it acts like a tube:
 the sender pushes objects (bits) in at one end, and
 the receiver takes them out at the other end.
 In most cases the order is preserved so that the bits arrive in the order they were sent.

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 In some cases when a connection is established, the sender, receiver, and subnet conduct a
negotiation about the parameters to be used, such as
 maximum message size,
 quality of service required, and
 other issues
 Typically, one side makes a proposal and the other side can accept it, reject it, or make a
counterproposal.
 A circuit is another name for a connection with associated resources, such as a fixed
bandwidth.
 This dates from the telephone network in which a circuit was a path over copper wire that
carried a phone conversation.

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 Connectionless Service

 This is modeled after the postal system.

 Each message (letter) carries the full destination address, and each one is routed through the
intermediate nodes inside the system independent of all the subsequent messages.
 There are different names for messages in different contexts;
 Packet: A packet is a message at the network layer.
 Store-and-forward switching: When the intermediate nodes receive a message in full
before sending it on to the next node.
 Cut-through switching: The alternative, in which the onward transmission of a message
at a node starts before it is completely received by the node.
 Normally, when two messages are sent to the same destination, the first one sent will be the
first one to arrive. However, it is possible that the first one sent can be delayed so that the
second one arrives first.

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 Figure: Six different types of service.

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2.4 Service Primitives
 A service is formally specified by a set of primitives (operations) available to user processes
to access the service.
 These primitives tell the service to perform some action or report on an action taken by a
peer entity.
 If the protocol stack is located in the operating system, as it often is, the primitives are
normally system calls.
 These calls cause a trap to kernel mode, which then turns control of the machine over to the
operating system to send the necessary packets.
 The set of primitives available depends on the nature of the service being provided.
 The primitives for connection-oriented service are different from those of connectionless
service.

Figure: Six service primitives that provide a simple connection-oriented service.

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A simple client-server interaction using acknowledged datagrams:

 First, the server executes LISTEN to indicate that it is prepared to accept incoming
connections.
 A common way to implement LISTEN is to make it a blocking system call.
 After executing the primitive, the server process is blocked until a request for
connection appears.
 Next, the client process executes CONNECT to establish a connection with the server.

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 The CONNECT call needs to specify who to connect to, so it might have a parameter giving the
server’s address.
 The operating system then typically sends a packet to the peer asking it to connect. …(1)
 When the packet arrives at the server, the operating system sees that the packet is requesting
a connection.
 It checks to see if there is a listener, and if so it unblocks the listener.
 The server process can then establish the connection with the ACCEPT call.
 This sends a response back to the client process to accept the connection. …(2)
 Then the client executes SEND to transmit its request. …(3)
 Followed by the execution of RECEIVE to get the reply.
 The arrival of the request packet at the server machine unblocks the server so it can handle
the request.
 After it has done the work, the server uses SEND to return the answer to the client. …(4)
 The arrival of this packet unblocks the client, which can now inspect the answer.
 When the client is done, it executes DISCONNECT to terminate the connection. …(5)
 When the server gets the packet, it also issues a DISCONNECT of its own, acknowledging the
client and releasing the connection. …(6)

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2.5 The Relationship of Services to protocols
 Service
 Is a set of primitives (operations) that a layer provides to the layer above it.
 The service defines what operations the layer is prepared to perform on behalf of its
users, but it says nothing at all about how these operations are implemented.
 A service relates to an interface between two layers, with the lower layer being the
service provider and the upper layer being the service user.

 Protocol
 Is a set of rules governing the format and meaning of the packets, or messages that are
exchanged by the peer entities within a layer.
 Entities use protocols to implement their service definitions.
 They are free to change their protocols at will, provided they do not change the service
visible to their users.
 In this way, the service and the protocol are completely decoupled. This is a key concept
that any network designer should understand well.

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 Services relate to the interfaces between layers, as illustrated in Figure.
 In contrast, Protocols relate to the packets sent between peer entities on different machines.

Figure: The relationship between a service and a protocol.

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3. REFERENCE MODELS
 We will discuss two important network architectures:
 OSI reference model
 TCP/IP reference model

 Although the protocols associated with the OSI model are not used any more, the model itself
is actually quite general and still valid, and the features discussed at each layer are still very
important.

 The TCP/IP model has the opposite properties: the model itself is not of much use but the
protocols are widely used.

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3.1 The OSI reference model
 The OSI model (minus the physical medium) is shown in Figure.
 This model is based on a proposal developed by the International Standards Organization
(ISO) as a first step toward international standardization of the protocols used in the various
layers (Day and Zimmermann, 1983).
 It was revised in 1995 (Day, 1995).
 The model is called the ISO OSI (Open Systems Interconnection) Reference Model because
it deals with connecting open systems—that is, systems that are open for communication
with other systems.

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 Figure: The OSI reference model
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 The OSI model has seven layers.
 The principles that were applied to arrive at the seven layers can be briefly summarized as
follows:
 A layer should be created where a different abstraction is needed.
 Each layer should perform a well-defined function.
 The function of each layer should be chosen with an eye toward defining internationally
standardized protocols.
 The layer boundaries should be chosen to minimize the information flow across
the interfaces.
 The number of layers should be large enough that distinct functions need not be
thrown together in the same layer out of necessity and small enough that the
architecture does not become unwieldy.

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The Physical Layer

 The physical layer is concerned with transmitting raw bits over a communication channel.
 The design issues have to do with making sure that when one side sends a 1 bit it is received
by the other side as a 1 bit, not as a 0 bit.
 Typical questions here are
 what electrical signals should be used to represent a 1 and a 0,
 how many nanoseconds a bit lasts,
 whether transmission may proceed simultaneously in both directions,
 how the initial connection is established,
 how it is torn down when both sides are finished,
 how many pins the network connector has, and
 what each pin is used for
 These design issues largely deal with mechanical, electrical, and timing interfaces, as well as
the physical transmission medium, which lies below the physical layer.

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The Data Link Layer

 The main task of the data link layer is to transform a raw transmission facility into a line
that appears free of undetected transmission errors.
 It does so by masking the real errors so the network layer does not see them.
 It accomplishes this task by having the sender break up the input data into data frames and
transmit the frames sequentially.
 If the service is reliable, the receiver confirms correct receipt of each frame by sending back
an acknowledgement frame.
 To keep a fast transmitter from drowning a slow receiver in data, some traffic
regulation mechanism may be needed to let the transmitter know when the
receiver can accept more data.
 To control access to the shared channel a special sublayer of the data link layer, the
medium access control sublayer is available.

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The Network Layer

 The network layer controls the operation of the subnet.

 A key design issue is determining how packets are routed from source to destination.
 Routes can be based on static tables that are ‘‘wired into’’ the network and rarely changed,
or more often they can be updated automatically to avoid failed components.
 They can also be determined at the start of each conversation, for example, a terminal
session, such as a login to a remote machine.
 Finally, they can be highly dynamic, being determined anew for each packet to reflect the
current network load.
 If too many packets are present in the subnet at the same time, they will get in one
another’s way, forming bottlenecks.
 Handling congestion is also a responsibility of the network layer.
 The quality of service provided (delay, transit time, jitter, etc.) is also a network layer issue.

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The Transport Layer

 The basic function of the transport layer is

 to accept data from above it,
 split it up into smaller units if need be,
 pass these to the network layer, and
 ensure that the pieces all arrive correctly at the other end.
 Furthermore, all this must be done efficiently and in a way that isolates the upper layers from
the inevitable changes in the hardware technology over the course of time.
 The transport layer also determines what type of service to provide to the session layer, and,
ultimately, to the users of the network.
 The type of service is determined when the connection is established.
 The transport layer is a true end-to-end layer; it carries data all the way from the source to
the destination.

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The Session Layer

 The session layer allows users on different machines to establish sessions between them.
 Sessions offer various services, including
 dialog control - keeping track of whose turn it is to transmit),
 token management - preventing two parties from attempting the same critical operation
simultaneously, and
 synchronization - checkpointing long transmissions to allow them to pick up from where
they left off in the event of a crash and subsequent recovery.

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The Presentation Layer

 The presentation layer is concerned with the syntax and semantics of the information
transmitted.
 In order to make it possible for computers with different internal data representations to
communicate, the data structures to be exchanged can be defined in an abstract way, along
with a standard encoding to be used ‘‘on the wire.’’
 The presentation layer manages these abstract data structures and allows higher-level data
structures (e.g., banking records) to be defined and exchanged.

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The Application Layer

 The application layer contains a variety of protocols that are commonly needed by users.
 One widely used application protocol is HTTP (HyperText Transfer Protocol), which is the
basis for the World Wide Web.
 When a browser wants a Web page, it sends the name of the page it wants to the server
hosting the page using HTTP.
 The server then sends the page back.
 Other application protocols are used for file transfer, electronic mail, and network news.

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3.2 TCP/IP Reference Model
 TCP/IP Reference Model was first described by Cerf and Kahn (1974), and later refined and
defined as a standard in the Internet community (Braden, 1989).
 The design philosophy behind the model is discussed by Clark (1988).
 The ability to connect multiple networks in a seamless way was one of the major design
goals.

Figure: The TCP/IP reference model.

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The Link Layer

 A packet-switching network based on a connectionless layer that runs across different

networks.
 The lowest layer in the model, the link layer describes what links such as serial lines and
classic Ethernet must do to meet the needs of this connectionless internet layer.
 It is not really a layer at all, in the normal sense of the term, but rather an interface between
hosts and transmission links.

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The Internet Layer

 The internet layer is the linchpin that holds the whole architecture together.
 It is shown in Figure as corresponding roughly to the OSI network layer.
 Its job is to permit hosts to inject packets into any network and have them travel
independently to the destination (potentially on a different network).
 They may even arrive in a completely different order than they were sent, in which case it is
the job of higher layers to rearrange them, if in-order delivery is desired.
 Note that ‘‘internet’’ is used here in a generic sense, even though this layer is present in
the Internet.
 The internet layer defines an official packet format and protocol called IP (Internet
Protocol), plus a companion protocol called ICMP (Internet Control Message Protocol) that
helps it function.
 The job of the internet layer is to deliver IP packets where they are supposed to go.
 Packet routing is clearly a major issue here, as is congestion (though IP has not proven
effective at avoiding congestion).

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The Transport Layer

 It is designed to allow peer entities on the source and destination hosts to carry on a
conversation, just as in the OSI transport layer.
 Two end-to-end transport protocols have been defined here.

 TCP (Transmission Control Protocol)

 Is a reliable connection-oriented protocol that allows a byte stream originating on one
machine to be delivered without error on any other machine in the internet.
 It segments the incoming byte stream into discrete messages and passes each one on to
the internet layer.
 At the destination, the receiving TCP process reassembles the received messages into
the output stream.
 TCP also handles flow control to make sure a fast sender cannot swamp a slow receiver
with more messages than it can handle.

COMPUTER NETWORKS (21CS52) MODULE-1

 UDP (User Datagram Protocol)
 Is an unreliable, connectionless protocol for applications that do not want TCP’s
sequencing or flow control and wish to provide their own.
 It is also widely used for one-shot, client-server-type request-reply queries and
applications in which prompt delivery is more important than accurate delivery, such as
transmitting speech or video.
 The relation of IP, TCP, and UDP is shown in Figure.
 Since the model was developed, IP has been implemented on many other networks.

Figure: The TCP/IP model with some protocols we will study.

COMPUTER NETWORKS (21CS52) MODULE-1

The Application Layer

 The TCP/IP model does not have session or presentation layers.

 Experience with the OSI model has proven this view correct: these layers are of little use to
most applications.
 On top of the transport layer is the application layer.
 It contains all the higher-level protocols.
 The early ones included virtual terminal (TELNET), file transfer (FTP), and electronic mail
(SMTP).
 Many other protocols have been added to these over the years.
 Some important ones that we will study, shown in previous Figure.
 Domain Name System (DNS), for mapping host names onto their network addresses,
 Hyper Text Transfer Protocol (HTTP), the protocol for fetching pages on the World Wide
Web, and
 Real Time Protocol (RTP), the protocol for delivering real-time media such as voice or
movies.

COMPUTER NETWORKS (21CS52) MODULE-1

3.3 Comparison of the OSI and TCP/IP Reference Models
OSI Reference Models TCP/IP Reference Models
OSI represents Open System Interconnection. TCP/IP model represents the Transmission
Control Protocol / Internet Protocol.
OSI is a generic, protocol independent standard. TCP/IP model depends on standard protocols
It is acting as an interaction gateway between about which the computer network has created.
the network and the final-user. It is a connection protocol that assigns the
network of hosts over the internet.

The OSI model was developed first, and then The protocols were created first and then built
protocols were created to fit the network the TCP/IP model.
architecture’s needs.

It provides quality services. It does not provide quality services.

The OSI model represents defines It does not mention the services, interfaces, and
administration, interfaces and conventions. It protocols.
describes clearly which layer provides services.

COMPUTER NETWORKS (21CS52) MODULE-1

 OSI Reference Models TCP/IP Reference Models
The protocols of the OSI model are better The TCP/IP model protocols are not hidden, and
unseen and can be returned with another we cannot fit a new protocol stack in it.
appropriate protocol quickly.
It is difficult as distinguished to TCP/IP. It is simpler than OSI.
It provides both connection and connectionless
It provides connectionless transmission in the
oriented transmission in the network layer; network layer and supports connecting and
however, only connection-oriented transmission
connectionless-oriented transmission in the
in the transport layer. transport layer.
It uses a vertical approach. It uses a horizontal approach.
The smallest size of the OSI header is 5 bytes.
The smallest size of the TCP/IP header is 20
bytes.
Protocols are unknown in the OSI model and In TCP/IP, returning protocol is not difficult.
are returned while the technology modifies.

COMPUTER NETWORKS (21CS52) MODULE-1

 OSI Reference Models TCP/IP Reference Models

COMPUTER NETWORKS (21CS52) MODULE-1

3.4 Critique of the OSI Model and Protocols

 Neither the OSI model and its protocols nor the TCP/IP model and its protocols are perfect.
 Quite a bit of criticism can be, and has been, directed at both of them.
 They can be summarized as:
 Bad timing
 Bad technology
 Bad implementations
 Bad politics

COMPUTER NETWORKS (21CS52) MODULE-1

Bad Timing :

 In the OSI model, it is very essential and important to write standards in between trough i.e.,
apocalypse of two elephants.
 Time of standards is very critical as sometimes standards are written too early even before
research is completed. Due to this, OSI model was not properly understood.
 The timing was considered bad because this model was finished and completed after huge
and significant amount of research time.
 Due to this, the standards are ignored by these companies.
 When the OSI came around, this model was perfectly released regarding research, but at that
time TCP/IP model was already receiving huge amounts of investments from companies and
manufacturers did not feel like investing in OSI model.
 So, there were no initial offerings for using OSI technique.
 While every company waited for any of other companies to firstly use this model technique,
but unfortunately none of company went first to use this model.
 This is first reason why OSI never happen.

COMPUTER NETWORKS (21CS52) MODULE-1

 Figure: The apocalypse of the two elephants

COMPUTER NETWORKS (21CS52) MODULE-1

Bad Technology:

 OSI models were never taken into consideration because of competition TCP/IP protocols
that were already used widely.
 This is due to second reason that OSI model and its protocols are flawed that means both of
them have fundamental weakness or imperfection or defect in character or performance or
design, etc.
 The idea behind choosing all of seven layers of OSI model was based more on political issues
rather than technical. Layers are more political than technical.
 OSI model, along with all of its associated service definitions and protocols, is highly
complex.
 On the other hand, other two layers i.e. Data link layer and network layer both of them are
overfull.
 Documentation is also highly complex due to which it gets very difficult to implement and is
not even very efficient in operation or function.
 Error and flow control are also duplicated i.e., reappear again and again in multiple layers or
each layer.
 On the other hand, most serious and bad criticism is that this model is also dominated by
communications mentality.
COMPUTER NETWORKS (21CS52) MODULE-1
Bad Implementations:

 The OSI model is extraordinarily and much more complex due to which initial
implementations were very slow, huge, and unwidely.
 This is the third reason due to which OSI became synonymous with poor quality in early days.
 It turned out to not be essential and necessary for all of seven layers to be designed together
to simply make things work out.
 On the other hand, implementations of TCP/IP were more reliable than OSI due to which
people started using TCP/IP very quickly which led to large community of users.
 In simple words, we can say that complexity leads to very poor or bad implementation.
 It is highly complex to be effectively and properly implemented.

COMPUTER NETWORKS (21CS52) MODULE-1

Bad Politics:

 OSI model was not associated with UNIX.

 This was fourth reason because TCP/IP was largely and closely associated with Unix, which
helps TCP/IP to get popular in academia whereas OSI did not have this association at that
time.
 On the other hand, OSI was associated with European telecommunications, European
community, and government of USA.
 This model was also considered to be technically inferior to TCP/IP.
 So, all people on ground reacted very badly to all of these things and supported much use of
TCP/IP.
 Even after all these bad conditions, OSI model is still general standard reference for almost all
of networking documentation.
 There are many organizations that are highly interested in OSI model.
 All of networking that is referring to numbered layers like layer 3 switching generally refers to
OSI model.
 Even, an effort has also been made simply to update it resulting in revised model that was
published in 1994.

COMPUTER NETWORKS (21CS52) MODULE-1

3.5 A Critique of the TCP/IP Reference Model
 First, the model does not clearly distinguish the concepts of services, interfaces, and
protocols.
 Good software engineering practice requires differentiating between the specification and
the implementation, something that OSI does very carefully, but TCP/IP does not.
 Consequently, the TCP/IP model is not much of a guide for designing new networks using
new technologies.

 Second, the TCP/IP model is not at all general and is poorly suited to describing any protocol
stack other than TCP/IP.
 Trying to use the TCP/IP model to describe Bluetooth, for example, is completely impossible.

 Third, the link layer is not really a layer at all in the normal sense of the term as used in the
context of layered protocols.
 It is an interface (between the network and data link layers).
 The distinction between an interface and a layer is crucial, and one should not be sloppy
about it.

COMPUTER NETWORKS (21CS52) MODULE-1

 Fourth, the TCP/IP model does not distinguish between the physical and data link layers.
 These are completely different.
 The physical layer has to do with the transmission characteristics of copper wire, fiber optics,
and wireless communication.
 The data link layer’s job is to delimit the start and end of frames and get them from one side
to the other with the desired degree of reliability.
 A proper model should include both as separate layers. The TCP/IP model does not do this.

 Finally, although the IP and TCP protocols were carefully thought out and well implemented,
many of the other protocols were ad hoc, generally produced by a couple of graduate
students hacking away until they got tired.
 The protocol implementations were then distributed free, which resulted in their becoming
widely used, deeply entrenched, and thus hard to replace.
 Some of them are a bit of an embarrassment now.
 The virtual terminal protocol, TELNET, for example, was designed for a ten-character-per-
second mechanical Teletype terminal.
 It knows nothing of graphical user interfaces and mice.

COMPUTER NETWORKS (21CS52) MODULE-1

 PHYSICAL LAYER
1. Guided Transmission Media
 The purpose of the physical layer is to transport bits from one machine to another.
 Various physical media can be used for the actual transmission.
 Each one has its own niche in terms of bandwidth, delay, cost, and ease of installation and
maintenance.
 Media are roughly grouped into
 Guided media
 copper wire
 fiber optics
 Unguided media
 terrestrial wireless
 satellite
 lasers through the air

COMPUTER NETWORKS (21CS52) MODULE-1

1.1 Magnetic Media
 One of the most common ways to transport data from one computer to another is to write
them onto magnetic tape or removable media (e.g., recordable DVDs), physically transport
the tape or disks to the destination machine, and read them back in again.
 Although this method is not as sophisticated as using a geosynchronous communication
satellite, it is often more cost effective, especially for applications in which high bandwidth or
cost per bit transported is the key factor.

COMPUTER NETWORKS (21CS52) MODULE-1

1.2 Twisted Pairs
 A twisted pair consists of two insulated copper wires, typically about 1 mm thick.
 The wires are twisted together in a helical form, just like a DNA molecule.
 Twisting is done because two parallel wires constitute a fine antenna.
 When the wires are twisted, the waves from different twists cancel out, so the wire radiates
less effectively.
 A signal is usually carried as the difference in voltage between the two wires in the pair.
 This provides better immunity to external noise because the noise tends to affect both wires
the same, leaving the differential unchanged.
 The most common application of the twisted pair is the telephone system.
 Both telephone calls and ADSL Internet access run over these lines.
 Twisted pairs can be used for transmitting either analog or digital information.
 The bandwidth depends on the thickness of the wire and the distance traveled.
 Due to their adequate performance and low cost, twisted pairs are widely used.

COMPUTER NETWORKS (21CS52) MODULE-1

  Twisted-pair cabling comes in several varieties.
 The garden variety deployed in many office buildings is called Category 5 cabling, or Cat 5.
 A category 5 twisted pair consists of two insulated wires gently twisted together.
 Four such pairs are typically grouped in a plastic sheath to protect the wires and keep them
together.

Figure: Category 5 UTP cable with four twisted pairs.

 Cat 5 replaced earlier Category 3 cables

 New wiring is more likely to be Category 6 or even Category 7.
 Some cables in Category 6 and above are rated for signals of 500 MHz and can support the
10-Gbps links that will soon be deployed.
 Through Category 6, these wiring types are referred to as UTP (Unshielded Twisted Pair) as
they consist simply of wires and insulators.
 In contrast to these, Category 7 cables have shielding on the individual twisted pairs, as
well as around the entire cable.
COMPUTER NETWORKS (21CS52) MODULE-1
1.3 Coaxial Cable
 It has better shielding and greater bandwidth than unshielded twisted pairs, so it can span
longer distances at higher speeds.
 Two kinds of coaxial cable are widely used.
 One kind, 50-ohm cable, is commonly used when it is intended for digital transmission from
the start.
 The other kind, 75-ohm cable, is commonly used for analog transmission and cable
television.
 This distinction is based on historical, rather than technical, factors.
 A coaxial cable consists of a stiff copper wire as the core, surrounded by an insulating
material.
 The insulator is encased by a cylindrical conductor, often as a closely woven braided mesh.
 The outer conductor is covered in a protective plastic sheath.

COMPUTER NETWORKS (21CS52) Figure: A cutaway view of a coaxial cable MODULE-1

 Advantages:
 High Bandwidth
 Better noise Immunity
 Easy to install and expand
 Inexpensive

 Disadvantages:
 Single cable failure can disrupt the entire network

 Applications:
 Radio frequency signals are sent over coaxial wire.
 It can be used for cable television signal distribution, digital audio (S/PDIF), computer
network connections (like Ethernet), and feedlines that connect radio transmitters and
receivers to their antennas.

COMPUTER NETWORKS (21CS52) MODULE-1

1.4 Power Lines
 Power lines (electrical power lines) deliver electrical power to houses, and electrical wiring
within houses distributes the power to electrical outlets.
 The use of power lines for data communication is an old idea.
 Power lines have been used by electricity companies for low-rate communication such as
remote metering for many years, as well in the home to control devices (e.g., the X10
standard).
 In recent years there has been renewed interest in high-rate communication over these lines,
both inside the home as a LAN and outside the home for broadband Internet access.
 Simply plug a TV and a receiver into the wall, which you must do anyway because they need
power, and they can send and receive movies over the electrical wiring.
 This configuration is shown in Figure.
 There is no other plug or radio.
 The data signal is superimposed on the low-frequency power signal (on the active or ‘‘hot’’
wire) as both signals use the wiring at the same time.

COMPUTER NETWORKS (21CS52) MODULE-1

 Figure: A network that uses household electrical wiring

 Electrical signals are sent at 50–60 Hz and the wiring attenuates the much higher frequency
(MHz) signals needed for high-rate data communication.
 The electrical properties of the wiring vary from one house to the next and change as
appliances are turned on and off, which causes data signals to bounce around the wiring.
 Transient currents when appliances switch on and off create electrical noise over a wide range
of frequencies.
 And without the careful twisting of twisted pairs, electrical wiring acts as a fine antenna,
picking up external signals and radiating signals of its own.
 Despite these difficulties, it is practical to send at least 100 Mbps over typical household
electrical wiring by using communication schemes.
COMPUTER NETWORKS (21CS52) MODULE-1
1.5 Fiber Optics
 Fiber optics are used for long-haul transmission in network backbones, highspeed LANs
and high-speed Internet access such as FttH (Fiber to the Home).
 An optical transmission system has three key components:
 the light source
 the transmission medium
 the detector
 Conventionally, a pulse of light indicates a 1 bit and the absence of light indicates a 0 bit.
 The transmission medium is an ultra-thin fiber of glass.
 The detector generates an electrical pulse when light falls on it.
 By attaching a light source to one end of an optical fiber and a detector to the other.
 We have a unidirectional data transmission system that
 accepts an electrical signal
 converts and transmits it by light pulses
 reconverts the output to an electrical signal at the receiving end

COMPUTER NETWORKS (21CS52) MODULE-1

 When a light ray passes from one medium to another—for example, from fused silica to air—
the ray is refracted (bent) at the silica/air boundary, as shown in Figure (a).
 Here we see a light ray incident on the boundary at an angle α1 emerging at an angle β1.
 The amount of refraction depends on the properties of the two media (in particular, their
indices of refraction).
 For angles of incidence above a certain critical value, the light is refracted back into the silica;
none of it escapes into the air.
 Thus, a light ray incident at or above the critical angle is trapped inside the fiber, as shown in
Figure (b), and can propagate for many kilometers with virtually no loss.
 Each ray is said to have a different mode, so a fiber having this property is called a multimode
fiber.

Figure: (a) Three examples of a light ray from inside a silica fiber impinging on the air/silica
boundary at different angles. (b) Light trapped by total internal reflection.
COMPUTER NETWORKS (21CS52) MODULE-1
Transmission of Light Through Fiber

 The attenuation of light through glass depends on the wavelength of the light.
 It is defined as the ratio of input to output signal power.
 For the kind of glass used in fibers, the attenuation is shown in Figure in units of decibels per
linear kilometer of fiber.

Figure: Attenuation of light through fiber in the infrared region.

COMPUTER NETWORKS (21CS52) MODULE-1

Fiber Cables
 Fiber optic cables are similar to coax, except without the braid.
 Figure (a) shows a single fiber viewed from the side.
 At the center is the glass core through which the light propagates.
 In multimode fibers, the core is typically 50 microns in diameter, about the thickness of a
human hair.
 In single-mode fibers, the core is 8 to 10 microns.
 The core is surrounded by a glass cladding with a lower index of refraction than the core, to
keep all the light in the core.
 Next comes a thin plastic jacket to protect the cladding.
 Fibers are typically grouped in bundles, protected by an outer sheath.
 Figure (b) shows a sheath with three fibers.

Figure: (a) Side view of a single fiber. (b) End view of a sheath with three fibers.
COMPUTER NETWORKS (21CS52) MODULE-1
 Fibers can be connected in three different ways.
 They can terminate in connectors and be plugged into fiber sockets.
 They can be spliced mechanically.
 Two pieces of fiber can be fused (melted) to form a solid connection.
 For all three kinds of splices, reflections can occur at the point of the splice, and the reflected
energy can interfere with the signal.
 Two kinds of light sources are typically used to do the signaling.
 LEDs (Light Emitting Diodes)
 Semiconductor lasers.
 They have different properties, as shown in Figure.

Figure 2-9. A comparison of semiconductor diodes and LEDs as light sources

COMPUTER NETWORKS (21CS52) MODULE-1

1.6 Comparison of Fiber Optics and Copper Wire
Basis Fiber Optic Cable Copper Wire
It carries data in the form of electric
Data Carrier It carries data in the form of light. signals.

Bandwidth It offers higher bandwidth. It offers lower bandwidth.

Structure It is thin, lighter in weight, and smaller It is heavier and thicker.

in size.

It can be laid in different environments It cannot be laid in a different

Environment because it is more resistant to environment because it is more prone
corrosive materials. to corrosive materials.

Attenuation Attenuation is very low. Attenuation is high.

As this data travel in the form of light, As in this data travel in the form of
Interface they are not affected by the electrical electric signals, they are affected by the
and magnetic interfaces. electrical and magnetic interfaces.
COMPUTER NETWORKS (21CS52) MODULE-1
 Basis Fiber Optic Cable Copper Wire
They provide security against the
wiretappers, because there is no They do not provide security against
Security leakage of light and are difficult to the wiretappers, because there is
tap. leakage of signals, and are easy to tap.

Cross-talk problem There is no such kind of problem. These are prevalent this problem.
In this charge carriers are photons, In this charge carriers are electrons,
Effect on charge they do not carry any charge, so they they carry a negative charge, so they
carriers do not get affected. get affected when they move in a wire.
Break-ability They are easily breakable. They cannot be easily broken.
Installation Cost Installation Cost is high. Installation Cost is less.
Bandwidth Size It is a bandwidth size 60Tps. It is a bandwidth size 10Gbps.
Fiber Optic width around 4lbs/1000 Copper wire width around
Width ft. 39lbs/1000ft.

COMPUTER NETWORKS (21CS52) MODULE-1

2. Wireless Transmission

2.1 The Electromagnetic Spectrum

 When electrons move, they create electromagnetic waves that can propagate through space
(even in a vacuum).
 These waves were predicted by the British physicist James Clerk Maxwell in 1865 and first
observed by the German physicist Heinrich Hertz in 1887.
 The number of oscillations per second of a wave is called its frequency, f, and is measured in
Hz (in honor of Heinrich Hertz).
 The distance between two consecutive maxima (or minima) is called the wavelength, which
is universally designated by the Greek letter λ (lambda).
 When an antenna of the appropriate size is attached to an electrical circuit, the
electromagnetic waves can be broadcast efficiently and received by a receiver some distance
away.
 All wireless communication is based on this principle.

COMPUTER NETWORKS (21CS52) MODULE-1

 In a vacuum, all electromagnetic waves travel at the same speed, no matter what their
frequency.
 This speed, usually called the speed of light, c, is approximately 3 × 108 m/sec, or about 1
foot (30 cm) per nanosecond.
 The fundamental relation between f, λ, and c (in a vacuum) is
λf = c where c is a constant

 The electromagnetic spectrum is shown in Figure.

 The bands listed at the bottom of Figure are the official ITU (International
Telecommunication Union) names and are based on the wavelengths.

COMPUTER NETWORKS (21CS52) MODULE-1

 Figure: The electromagnetic spectrum and its uses for communication.

COMPUTER NETWORKS (21CS52) MODULE-1

Forms of Spread Spectrum with a wider band

 Frequency hopping spread spectrum

• In frequency hopping spread spectrum, the transmitter hops from frequency to
frequency hundreds of times per second.
• It is popular for military communication because it makes transmissions hard to detect
and next to impossible to jam.
• It also offers good resistance to multipath fading and narrowband interference because
the receiver will not be stuck on an impaired frequency for long enough to shut down
communication.
• This robustness makes it useful for crowded parts of the spectrum, such as the ISM
bands we will describe shortly.
• This technique is used commercially, for example, in Bluetooth and older versions of
802.11.

COMPUTER NETWORKS (21CS52) MODULE-1

 Direct sequence spread spectrum
• uses a code sequence to spread the data signal over a wider frequency band. It is widely
used commercially as a spectrally efficient way to let multiple signals share the same
frequency band.
• These signals can be given different codes, a method called CDMA (Code Division
Multiple Access).
• This method is shown in contrast with frequency hopping in Figure.
• It forms the basis of 3G mobile phone networks and is also used in GPS (Global
Positioning System).
• Even without different codes, direct sequence spread spectrum, like frequency hopping
spread spectrum, can tolerate narrowband interference and multipath fading because
only a fraction of the desired signal is lost.
• It is used in this role in older 802.11b wireless LANs.
• For a fascinating and detailed history of spread spectrum communication, see Scholtz
(1982).

COMPUTER NETWORKS (21CS52) MODULE-1

 Figure: Spread spectrum and ultra-wideband (UWB) communication.

COMPUTER NETWORKS (21CS52) MODULE-1

 UWB (UltraWideBand)
• UWB sends a series of rapid pulses, varying their positions to communicate information.
• The rapid transitions lead to a signal that is spread thinly over a very wide frequency
band.
• UWB is defined as signals that have a bandwidth of at least 500 MHz or at least 20% of
the center frequency of their frequency band. UWB is also shown in previous Figure.
• With this much bandwidth, UWB has the potential to communicate at high rates.
• Because it is spread across a wide band of frequencies, it can tolerate a substantial
amount of relatively strong interference from other narrowband signals.
• Just as importantly, since UWB has very little energy at any given frequency when used
for short-range transmission, it does not cause harmful interference to those other
narrowband radio signals.
• It is said to underlay the other signals.

COMPUTER NETWORKS (21CS52) MODULE-1

2.2 Radio Transmission
 Radio frequency (RF) waves are easy to generate, can travel long distances, and can
penetrate buildings easily, so they are widely used for communication, both indoors and
outdoors.
 Radio waves also are omnidirectional, meaning that they travel in all directions from the
source, so the transmitter and receiver do not have to be carefully aligned physically.
 The properties of radio waves are frequency dependent.
 At low frequencies, radio waves pass through obstacles well, but the power falls off sharply
with distance from the source—at least as fast as 1/r2 in air—as the signal energy is spread
more thinly over a larger surface.
 This attenuation is called path loss.
 At high frequencies, radio waves tend to travel in straight lines and bounce off obstacles.
 Path loss still reduces power, though the received signal can depend strongly on reflections
as well.

COMPUTER NETWORKS (21CS52) MODULE-1

 High-frequency radio waves are also absorbed by rain and other obstacles to a larger extent
than are low-frequency ones.
 At all frequencies, radio waves are subject to interference from motors and other electrical
equipment.
 In the VLF, LF, and MF bands, radio waves follow the ground, as illustrated in Figure(a).
 These waves can be detected for perhaps 1000 km at the lower frequencies, less at the
higher ones.
 In the HF and VHF bands, the ground waves tend to be absorbed by the earth.
 However, the waves that reach the ionosphere, a layer of charged particles circling the earth
at a height of 100 to 500 km, are refracted by it and sent back to earth, as shown in
Figure(b).

Figure(a): In the VLF, LF, and MF bands, radio waves follow the curvature of the earth.
Figure(b): In the HF band, they bounce off the ionosphere.
COMPUTER NETWORKS (21CS52) MODULE-1
2.3 Microwave Transmission
 Above 100 MHz, the waves travel in nearly straight lines and can therefore be narrowly
focused.
 Concentrating all the energy into a small beam by means of a parabolic antenna (eg. satellite
TV dish) gives a much higher signal-to-noise ratio, but the transmitting and receiving
antennas must be accurately aligned with each other.
 In addition, this directionality allows multiple transmitters lined up in a row to communicate
with multiple receivers in a row without interference, provided some minimum spacing rules
are observed.
 Microwaves travel in a straight line, so if the towers are too far apart, the earth will get in the
way.
 The higher the towers are, the farther apart they can be.
 The distance between repeaters goes up very roughly with the square root of the tower
height.
 For 100-meter-high towers, repeaters can be 80 km apart.

COMPUTER NETWORKS (21CS52) MODULE-1

 Disadvantages
 Microwaves do not pass through buildings well.
 Some waves may be refracted off low-lying atmospheric layers and may take slightly
longer to arrive than the direct waves.
 The delayed waves may arrive out of phase with the direct wave and thus cancel the
signal.
 This effect is called multipath fading and is often a serious problem.

 Advantages
 No right of way is needed to lay down cables.
 Microwave is also relatively inexpensive.

COMPUTER NETWORKS (21CS52) MODULE-1

2.4 Infrared Transmission
 Unguided infrared waves are widely used for short-range communication.
 The remote controls used for televisions, VCRs, and stereos all use infrared communication.
 They are relatively directional, cheap, and easy to build but have a major drawback: they do
not pass through solid objects.
 On the other hand, the fact that infrared waves do not pass through solid walls well is also a
plus.
 It means that an infrared system in one room of a building will not interfere with a similar
system in adjacent rooms or buildings: you cannot control your neighbor’s television with
your remote control.
 Security of infrared systems against eavesdropping is better than that of radio systems
precisely for this reason.
 Therefore, no government license is needed to operate an infrared system.

COMPUTER NETWORKS (21CS52) MODULE-1

2.5 Light Transmission
 Unguided optical signaling or free-space optics has been in use for centuries.
 Optical signaling using lasers is inherently unidirectional, so each end needs its own laser and
its own photodetector (Figure).

 Advantages:
 This scheme offers very high bandwidth at very low cost and is relatively secure because
it is difficult to tap a narrow laser beam.
 It is also relatively easy to install and, unlike microwave transmission, does not require
an FCC license.

 Disadvantages:
 The laser’s strength, a very narrow beam, is also its weakness here.
 Wind and temperature changes can distort the beam and laser beams.
 Cannot penetrate rain or thick fog.

COMPUTER NETWORKS (21CS52) MODULE-1

Problem:
 heat from the sun during the daytime caused convection currents to rise up from the roof of
the building, as shown in Figure.
 This turbulent air diverted the beam and made it dance around the detector, much like a
shimmering road on a hot day.

Figure: A bidirectional system

with two lasers is pictured here.
COMPUTER NETWORKS (21CS52) MODULE-1
COMPUTER NETWORKS (21CS52) MODULE-1