Syllabus:

Introduction to networks: Network hardware, Network software, Reference models,

Physical Layer: Guided transmission media, Wireless transmission
Textbook 1: Ch.1.2 to 1.4, Ch.2.2 to 2.3

Textbooks:
1. Computer-Networks- Andrew S. Tanenbaum and David J. Wetherall, Pearson
Education, 5th Edition.
2. Computer Networking A Top-Down Approach -James F. Kurose and Keith W.
RossPearson Education 7th Edition.
Reference Books:
1. Behrouz A Forouzan, Data and Communications and Networking, 5th Edition,
McGraw Hill, Indian Edition
2. Larry L Peterson and Brusce S Davie, Computer Networks, 5th Edition, ELSEVIER
Computer Networks (21CS52) Module 1: Introduction, Physical Layer

Network Hardware
 There are two types of transmission technology that are in widespread use: broadcast
links and point-to-point links.
 Point-to-point links connect individual pairs of machines.
 To go from the source to the destination packets, may have to first visit one or more
intermediate machines.
 Point-to-point transmission with exactly one sender and exactly one receiver is called
unicasting.
 In contrast, on a broadcast network, the communication channel is shared by all the
machines on the network; packets sent by any machine are received by all the others.
 Upon receiving a packet, a machine checks the address field. If the packet is intended
for the receiving machine, that machine processes the packet; otherwise, it is just
ignored.
 Broadcast systems usually 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.
 Some broadcast systems also support transmission to a subset of the machines, which
known as multicasting.
An alternative criterion for classifying networks is by scale.

Classification of interconnected processors by scale

Personal Area Networks

 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.
 Almost every computer has an attached monitor, keyboard, mouse, and printer.
Without being wireless, this connection must be done with cables.

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer
 A short-range wireless network uses Bluetooth to connect these components without
wires.
 Bluetooth networks use the master-slave paradigm. 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 they can use, and so on.

Bluetooth PAN configuration

 Bluetooth can be used to connect a headset to a mobile phone without cords and it can
allow a digital music player to connect to the car merely being brought within range.
 PANs can also be built with other technologies that communicate over short ranges,
such as RFID on smartcards and library books.

Local Area Network

 A LAN (Local Area Network) 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 useful in 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.
 Each computer talks to a device, called an AP (Access Point), wireless router, or
base station, relays packets between the wireless computers and also between them
and the Internet.
 The standard for wireless LANs is IEEE 802.11, popularly known as WiFi, runs at
speeds from 11 to hundreds of Mbps.
 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.

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer

Wireless and Wired LANs

 The topology of many wired LANs is built from point-to-point links. IEEE 802.3,
called Ethernet, is the standard for wired LAN.
 Each computer 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.
 The job of the switch is to relay packets between computers, using the address in each
packet.
 To build larger LANs, switches can be plugged into each other using their ports. It is
also possible to divide one large physical LAN into two smaller logical LANs.
 A Virtual local area network (VLAN) is a virtualized connection that connects
multiple devices and network nodes from different LANs into one logical network.
 For example, the engineering and finance departments of a company might have
computers on the same physical LAN. In this design each port is tagged with a
‘‘color,’’ say green for engineering and red for finance.
 Both wireless and wired broadcast networks can be divided into static and dynamic
designs, depending on how the channel is allocated.
 A typical static allocation would be to divide time into discrete intervals and use a
round-robin algorithm, allowing each machine to broadcast only when its time slot
comes up.
 Dynamic allocation methods for a common channel are either centralized or
decentralized. In the centralized channel allocation method, there is a single entity, the
base station determines who goes next.
 In the decentralized channel allocation method, there is no central entity; each
machine must decide for itself whether to transmit.
 A home network is more likely to have different properties than other networks. First,
the networked devices have to be very easy to install.
 Second, the network and devices have to be foolproof in operation. Third, low price
is essential for success.
 Fourth, it must be possible to start out with one or two devices and expand the reach
of the network gradually. Fifth, security and reliability will be very important.

Metropolitan Area Network

 A MAN (Metropolitan Area Network) covers a city. The best-known examples of
MANs are the cable television networks available in many cities.
 These systems grew from earlier community antenna systems used in areas with poor
over-the-air television reception.

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer
 At first, these were locally designed, ad hoc systems. Later business-oriented
companies, got contracts from local governments to wire up entire cities.
 The next step was television programming and even entire channels designed for
cable only. But from their inception until the late 1990s, they were intended for
television reception only.
 When the Internet began attracting a mass audience, the cable TV network operators
began to realize that with some changes to the system, they could provide two-way
Internet service in unused parts of the spectrum.
 At that point, the cable TV system began to morph from simply a way to distribute
television to a metropolitan area network.

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.

Wide Area Network

 A WAN (Wide Area Network) spans a large geographical area, often a country or
continent. For example, a company with branch offices in different cities.
 Each of these offices contains hosts intended for running user (i.e., application)
programs. The rest of the network that connects these hosts is then called the
communication subnet.
 The job of the subnet is to carry messages from host to host. The subnet consists of
two distinct components: transmission lines and switching elements.

WAN using a virtual private network

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer
 Transmission lines move bits between machines. Switching elements are specialized
computers that connect two or more transmission lines.
 Most commonly used switching computer is a router.
 Differences between LAN and WAN:
1. Usually in a WAN, the hosts and subnet are owned and operated by different
people.
2. WANs will in fact be internetworks, or composite networks, the routers will
usually connect different kinds of networking technology.
3. In LAN individual computers are connected to the subnet, in case of WAN
entire LANs are connected to the subnet.
 A company might connect its offices to the Internet This allows connections to be
made between the offices as virtual links, called a VPN (Virtual Private Network).
 A VPN has the usual advantage of virtualization, which is that it provides flexible
reuse of a resource (Internet connectivity).
 A VPN also has the usual disadvantage of virtualization, which is a lack of control
over the underlying resources.
 The subnet may be run by a different company, the subnet operator is known as a
network service provider and the offices are its customers.
 Such a subnet operator is called an ISP (Internet Service Provider) and the subnet is
an ISP network.

WAN using an ISP network

 There may be many paths in the network that connect two routers. How each router
makes the decision as to where to send a packet next is called the forwarding
algorithm.
 How the network makes the decision as to which path to use is called the routing
algorithm.
 Other kinds of WANs make heavy use of wireless technologies. In satellite systems,
each computer on the ground has an antenna through which it can send data to and
receive data from to a satellite in orbit.
 The cellular telephone network is another example of a WAN that uses wireless
technology.

Internetwork
 Many networks exist in the world, often with different hardware and software. People
connected to one network often want to communicate with people attached to a
different one.

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer
 The fulfillment of this desire requires that different, and frequently incompatible,
networks be connected.
 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.
 Subnet refers to the collection of routers and communication lines owned by the
network operator in a wide area network.
 A network is formed by the combination of a subnet and its hosts. A network is a
collection of computers interconnected by a single technology.
 An internet is formed when distinct networks are interconnected. Connecting a LAN
and a WAN or connecting two LANs is the usual way to form an internetwork.
 Two or more different networks can be connected with a gateway. Gateway provides
the necessary translation, both in terms of hardware and software.

Network Software
Protocol Hierarchies
 To reduce the design complexity, most networks are organized as a stack of layers or
levels, each one built upon the one below it.
 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.
 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.
 A protocol is an agreement between the communicating parties on how
communication is to proceed.
 The entities comprising the corresponding layers on different machines are called
peers.
 The peers may be software processes, hardware devices, or even human beings.
 Each layer passes data and control information to the layer immediately below it, until
the lowest layer is reached.

Layers, protocols, and interfaces

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer

 Below layer 1 is the physical medium through which actual communication occurs.
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.
 In addition to minimizing the amount of information that must be passed between
layers, clear-cut interfaces also make it simpler to replace one layer with a completely
different protocol or implementation.
 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.
 A message, M, is produced by an application process running in layer 5 and given to
layer 4 for transmission.
 Layer 4 puts a header in front of the message to identify the message and passes the
result to layer 3.
 The header includes control information, such as addresses, to allow layer 4 on the
destination machine to deliver the message.
 Layer 3 decides which of the outgoing lines to use and passes the packets to layer 2.
Layer 2 adds to each piece not only a header but also a trailer, and gives the resulting
unit to layer 1 for physical transmission.
 At the receiving machine the message moves upward, from layer to layer, with
headers being stripped off as it progresses.

Example information flow supporting virtual communication in layer 5

Design issues for the Layers

 Reliability is the design issue of making a network that operates correctly even
though it is made up of a collection of components that are themselves unreliable.

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer
 One 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.
 Both of these mechanisms work by adding redundant information.
 Another reliability issue is 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.
 In that case, the network should automatically make a decision to route the packets
called routing.
 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.
 Not all communication channels preserve the order of messages sent on them, leading
to solutions that number messages.
 Solution to differences in the maximum size of a message that the networks can
transmit leads to mechanisms for disassembling, transmitting, and then
reassembling messages.
 Designs that continue to work well when the network gets large are said to be
scalable.
 A third design issue is resource allocation. Many designs share network bandwidth
dynamically called statistical multiplexing.
 An allocation problem that occurs is how to keep a fast sender from swamping a slow
receiver with data. Feedback from the receiver to the sender is often used called as
flow control.
 Sometimes the network is oversubscribed because too many computers want to send
too much traffic. This overloading of the network is called congestion.
 To carry live video, the timeliness of delivery matters a great deal. Applications wants
real-time delivery at the same time they want high throughput.
 Quality of service is the name given to mechanisms that reconcile these competing
demands.
 The last major design issue is to secure the network by defending it against different
kinds of threats.
 One of the threats is that of eavesdropping on communications. Mechanisms that
provide confidentiality defend against this threat.
 Mechanisms for authentication prevent someone from impersonating someone else.
Other mechanisms for integrity prevent modification to messages.

Connection-oriented versus connectionless service

 Layers can offer two different types of service to the layers above them: connection-
oriented and connectionless.
 To use a connection-oriented network service, the service user first establishes a
connection, uses the connection, and then releases the connection.
 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.
 In a connectionless service each message is routed through the intermediate nodes
inside the system independent of all the subsequent messages.
 When the intermediate nodes receive a message in full before sending it on to the next
node, this is called store-and-forward switching.

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer
 The alternative, in which the onward transmission of a message at a node starts before
it is completely received by the node, is called cut-through switching.
 A reliable service is implemented by having the receiver acknowledge the receipt of
each message, so the sender is sure that it arrived.
 Reliable connection-oriented service has two minor variations: message sequences
and byte streams.
 For some applications, the transit delays introduced by acknowledgements are
unacceptable. One such application is digitized voice traffic for voice over IP.
 Not all applications require connections. The spammer probably does not want to go
to the trouble of setting up and later tearing down a connection to just send one item.
 Unreliable connectionless service is often called datagram service. The
acknowledged datagram service can be provided for the convenience of not having
to establish a connection to send one message, but reliability is essential.
 Still another service is the request-reply service. In this service the sender transmits a
single datagram containing a request; the reply contains the answer.

Six different types of service

 Unreliable communication are used, as the delays inherent in providing a reliable

service may be unacceptable. And reliable services are built on top of an unreliable
datagram service.

Service Primitives
 A service is formally specified by a set of primitives (operations) available to user
processes to access the service.
 If the protocol stack is located in the operating system, 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.

Six service primitives that provide a simple connection-oriented service

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer

 These primitives might be used for a request-reply interaction in a client-server

environment.
 First, the server executes LISTEN to indicate that it is prepared to accept incoming
connections.
 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.
The client process is suspended until there is a response.
 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 (2) back to the client process to accept the connection.
 The arrival of this response then releases the client. At this point the client and server
are both running and they have a connection established.

A simple client-server interaction using acknowledged datagrams

 The next step is for the server to execute RECEIVE to prepare to accept the first
request. The RECEIVE call blocks the server.
 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.
 If the client has additional requests, it can make them now. 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)

The Relationship of services to protocols

 A 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, but it says
nothing 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.
 A protocol, in contrast, 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.

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer
 Entities use protocols to implement their service definitions.

The relationship between a service and a protocol

Reference Models
The OSI Reference Model
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 in 1983. It was revised in 1995. The model is called the ISO OSI (Open Systems
Interconnection) Reference Model because it deals with connecting systems that are open for
communication with other systems.
The OSI model has seven layers. The principles that were applied to arrive at the seven layers
can be briefly summarized as follows:
1. A layer should be created where a different abstraction is needed.
2. Each layer should perform a well-defined function.
3. The function of each layer should be chosen with defining internationally
standardized protocols.
4. The layer boundaries should be chosen to minimize the information flow across the
interfaces.
5. The number of layers should be large enough that distinct functions need not be
thrown together in the same layer and small enough that the architecture does not
become unwieldy.

The OSI reference model

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer

The Physical Layer

 The physical layer is concerned with transmitting raw bits over a communication
channel.
 When one side sends a 1 bit it must be 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.
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.
 Broadcast networks have an additional issue in the data link layer: how to control
access to the shared channel. A special sublayer of the data link layer, the medium
access control sublayer, deals with this problem.
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.
 If too many packets are present in the subnet at the same time, leads to 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.
The Transport Layer
 The basic function of the transport layer is to accept data from above it, split it up into
smaller units, pass these to the network layer, and ensure that the pieces all arrive
correctly at the other end.
 The most popular type of transport connection is an error-free point-to-point channel.
 Other possible kinds of transport service are, such as the transporting of isolated
messages with no guarantee about the order of delivery, and the broadcasting of
messages to multiple destinations.
 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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Computer Networks (21CS52) Module 1: Introduction, Physical Layer
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.

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 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.
 Other application protocols are used for file transfer, electronic mail, and network
news.

The TCP/IP Reference Model

The TCP/IP reference model

The Link Layer
 The network is 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, but rather an interface between hosts and transmission links.

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer

The Internet Layer

 The internet layer 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.
 The internet layer defines the packet format and protocol called IP (Internet Protocol),
plus another 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.
The Transport Layer
 The transport layer is designed to allow peer entities on the source and destination
hosts to carry on a conversation.
 Two end-to-end transport protocols have been defined here.
 The first one, 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.
 The second protocol in this layer, UDP (User Datagram Protocol), is an unreliable,
connectionless protocol for applications.
 It is 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 Application Layer
 The TCP/IP model does not have session or presentation layers. Instead, applications
simply include any session and presentation functions that they require.
 On top of the transport layer is the application layer. It contains all the higher-level
protocols.
 This includes virtual terminal (TELNET), file transfer (FTP), electronic mail (SMTP),
the Domain Name System (DNS), for mapping host names onto their network
addresses, HTTP, and RTP, the protocol for delivering real-time media such as voice
or movies.

The TCP/IP model with some protocols

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer

A comparison of the OSI and TCP/IP reference models

 The OSI and TCP/IP reference models have much in common. Both are based on the
concept of a stack of independent protocols. Also, the functionality of the layers is
roughly similar.
 Despite these fundamental similarities, the two models also have many differences.
 Three concepts are central to the OSI model:
1. Services
2. Interfaces
3. Protocols

 OSI model makes the distinction between these three concepts explicit. Each layer
performs some services for the layer above it.
 A layer’s interface tells the processes above it how to access it. It specifies what the
parameters are and what results to expect.
 Peer protocols used in a layer depends on the layers services. Each layer can use any
protocols it wants to get the job done.
 The TCP/IP model did not originally clearly distinguish between services, interfaces,
and protocols.
 The protocols in the OSI model are better hidden than in the TCP/IP model and can be
replaced relatively easily as the technology changes.
 The OSI reference model was devised before the corresponding protocols were
invented. This ordering meant that the model was not biased toward one particular set
of protocols.
 With TCP/IP the reverse was true: the protocols came first, and the model was really
just a description of the existing protocols.
 There was no problem with the protocols fitting the model. The only trouble was that
the model did not fit any other protocol stacks.
 An obvious difference between the two models is the number of layers: the OSI
model has seven layers and the TCP/IP model has four.
 The OSI model supports both connectionless and connection-oriented communication
in the network layer, but only connection-oriented communication in the transport
layer.
 The TCP/IP model supports only one mode in the network layer (connectionless) but
both in the transport layer, giving the users a choice.

A critique of the OSI model and protocols

Neither the OSI model and its protocols nor the TCP/IP model and its protocols are perfect.
1. Bad timing
2. Bad technology
3. Bad implementations
4. Bad politics

Bad Timing
 The competing TCP/IP protocols were already in widespread use by research
universities by the time the OSI protocols appeared.
 While the billion-dollar wave of investment had not yet hit, the academic market was
large enough that many vendors had begun cautiously offering TCP/IP products.

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer
 When OSI came around, they did not want to support a second protocol stack until
they were forced to, so there were no initial offerings.
 With every company waiting for every other company to go first, no company went
first and OSI never happened.
Bad Technology
 The second reason that OSI never caught on is that both the model and the protocols
are flawed.
 The choice of seven layers was more political than technical, and two of the layers
(session and presentation) are nearly empty, whereas two other ones (data link and
network) are overfull.
 The OSI model, along with its associated service definitions and protocols, is
extraordinarily complex.
 They are also difficult to implement and inefficient in operation. Some functions, such
as addressing, flow control, and error control, reappear again and again in each layer.
Bad Implementations
 Given the enormous complexity of the model and the protocols, the initial
implementations were huge, unwieldy, and slow.
 In contrast, one of the first implementations of TCP/IP was part of Berkeley UNIX
and was quite good and free.
 People began using it quickly, which led to a large user community, which led to
improvements, which led to an even larger community.
Bad Politics
 On account of the initial implementation, many people, especially in academia,
thought of TCP/IP as part of UNIX, and UNIX in the 1980s was widely used in
academia.
 OSI, on the other hand, was widely thought to be from the European
telecommunication ministries, the European Community, and later the U.S.
Government.

A critique of the TCP/IP reference model

 The TCP/IP model and protocols have their problems too. First, the model does not
clearly distinguish the concepts of services, interfaces, and protocols.
 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.
 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.
 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.

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer
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.
 Media are roughly grouped into guided media, such as copper wire and fiber optics,
and unguided media.

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.
 It is often more cost effective, especially for applications in which high bandwidth or
cost per bit transported is the key factor.

Twisted pair
 One of the oldest and still most common transmission media is twisted pair. 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. Twisted
pairs can be used for transmitting either analog or digital information.
 The bandwidth depends on the thickness of the wire and the distance travelled, but
several megabits/sec can be achieved for a few kilometers in many cases.
 Twisted-pair cabling comes in several varieties:
 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.

Category 5 UTP cable with four twisted pairs

 Different LAN standards may use the twisted pairs differently. For example, 100-
Mbps Ethernet uses two (out of the four) pairs, one pair for each direction.
 To reach higher speeds, 1-Gbps Ethernet uses all four pairs in both directions
simultaneously.
 Category 5 replaced earlier Category 3 cables with a similar cable that uses the same
connector, but has more twists per meter.

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer
 More twists result in less crosstalk and a better-quality signal over longer distances.
 New wiring is more likely to be Category 6 or even Category 7. These categories
have more stringent specifications to handle signals with greater bandwidths.
 Some cables in Category 6 and above are rated for signals of 500 MHz.
 Category 6 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. Shielding reduces the susceptibility to external
interference and crosstalk.

Coaxial cable
 Coaxial cable 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.
 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.

A cutaway view of a coaxial cable

 The construction and shielding of the coaxial cable give it a good combination of high
bandwidth and excellent noise immunity.

Power Lines
 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.
 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.

A network that uses household electrical wiring

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer
 The difficulty with using household electrical wiring for a network is that it was
designed to distribute power signals.
 Electrical signals are sent at 50–60 Hz and the wiring attenuates the much higher
frequency (MHz) signals needed for high-rate data communication.
 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 that resist impaired
frequencies and bursts of errors.

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, and the detector.
 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, and then reconverts the output to an
electrical signal at the receiving end.
 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.

Three examples of a light ray from inside a silica fiber impinging on the air/silica
boundary at different angles.

 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 and
can propagate for many kilometers with virtually no loss.

Light trapped by total internal reflection

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer
 Since any light ray incident on the boundary above the critical angle will be reflected
internally, many different rays will be bouncing around at different angles called a
multimode fiber.
 However, if the fiber’s diameter is reduced to a few wavelengths of light the fiber acts
like a wave guide and the light can propagate only in a straight line, without
bouncing, yielding a single-mode fiber.

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.

Attenuation of light through fiber in the infrared region

 For the kind of glass used in fibers, the attenuation is shown in above figure in units
of decibels per linear kilometer of fiber.
 The figure shows the near-infrared part of the spectrum, which is what is used in
practice. Visible light has slightly shorter wavelengths, from 0.4 to 0.7 microns.
 Light pulses sent down a fiber spread out in length as they propagate. This spreading
is called chromatic dispersion.
 One way to keep these spread-out pulses from overlapping is to increase the distance
between them, but this can be done only by reducing the signaling rate.
 Fortunately, it has been discovered that making the pulses in a special shape related to
the reciprocal of the hyperbolic cosine causes nearly all the dispersion effects cancel
out, so it is possible to send pulses for thousands of kilometers without appreciable
shape distortion. These pulses are called solitons.
Fiber Cables
 Fiber optic cables are similar to coax, except without the braid. 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.

(a) Side view of a single fiber. (b) End view of a sheath with three fibers

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer
 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. Terrestrial fiber sheaths are normally laid in
the ground within a meter of the surface.
 Fibers can be connected in three different ways:
1. They can terminate in connectors and be plugged into fiber sockets.
2. They can be spliced mechanically.
3. Two pieces of fiber can be fused (melted) to form a solid connection.
 Two kinds of light sources are typically used to do the signaling. These are LEDs
(Light Emitting Diodes) and semiconductor lasers.

A comparison of semiconductor diodes and LEDs as light sources

Comparison of Fiber Optics and Copper Wire

 Fiber has many advantages. It can handle much higher bandwidths than copper. This
alone would require its use in high-end networks.
 Due to the low attenuation, repeaters are needed only about every 50 km on long
lines, versus about every 5 km for copper.
 Nor is it affected by corrosive chemicals in the air, important for harsh factory
environments.
 Fiber is thin, lightweight and have more capacity. Fiber has much lower installation
cost. Fibers do not leak light and are difficult to tap.
 These properties give fiber good security against potential wiretappers.
 On the downside, fiber is a less familiar technology requiring skills not all engineers
have, and fibers can be damaged easily by being bent too much.
 Since optical transmission is inherently unidirectional, two-way communication
requires either two fibers or two frequency bands on one fiber. Finally, fiber interfaces
cost more than electrical interfaces.

Wireless Transmission
The Electromagnetic spectrum
 When electrons move, they create electromagnetic waves that can propagate through
space. The number of oscillations per second of a wave is called its frequency, f, and
is measured in Hz (Hertz).
 The distance between two consecutive maxima (or minima) is called the wavelength,
λ (lambda).
 In a vacuum, all electromagnetic waves travel at the same speed. The fundamental
relation between f, λ, and c (in a vacuum) is λf = c

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer

The electromagnetic spectrum and its uses for communication

 Most transmissions use a relatively narrow frequency band (i.e., Δf/f << 1). They
concentrate their signals in this narrow band to use the spectrum efficiently and obtain
reasonable data rates by transmitting with enough power.
 However, in some cases, a wider band is used, with three variations.
 In frequency hopping spread spectrum, the transmitter hops from frequency to
frequency hundreds of times per second.
 It makes transmissions hard to detect and 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.
 A second form of spread spectrum, 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).
 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.

Spread spectrum and ultra-wideband (UWB) communication

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer
 A third method of communication with a wider band is UWB (UltraWideBand)
communication.
 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 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.

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.
 Radio waves also are omnidirectional that is, they travel in all directions from the
source, so the transmitter and receiver do not have to be 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.
 High-frequency radio waves are also absorbed by rain and other obstacles to a larger
extent than are low-frequency ones.
 In the VLF, LF, and MF bands, radio waves follow the ground. These waves can be
detected for perhaps 1000 km at the lower frequencies, less at the higher ones.
 Radio waves in these bands pass through buildings easily, which is why portable
radios work indoors.
 The main problem with using these bands for data communication is their low
bandwidth.

In the VLF, LF, and MF bands, radio waves follow the curvature of the earth

 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.

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer

In the HF band, they bounce off the ionosphere

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 gives
a much higher signal-to-noise ratio, but the transmitting and receiving antennas must
be accurately aligned with each other.
 Microwaves travel in a straight line, so if the towers are too far apart, repeaters are
needed periodically. For 100-meter-high towers, repeaters can be 80 km apart.
 Unlike radio waves at lower frequencies, 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.
 Microwave communication is so widely used for long-distance telephone
communication, mobile phones, television distribution, and other purposes, so that a
severe shortage of spectrum has developed.
 It has several key advantages over fiber. The main one is that there is no need to lay
down cables. Microwave is also relatively inexpensive.

The Politics of the Electromagnetic Spectrum

 To prevent total chaos, there are national and international agreements about who gets
to use which frequencies.
 National governments allocate spectrum for AM and FM radio, television, and mobile
phones, as well as for telephone companies, police, maritime, navigation, military,
government, and many other competing users.
 Worldwide, an agency of ITU-R (WRC) tries to coordinate this allocation. However,
countries are not bound by ITU-R’s recommendations.
 When a piece of spectrum has been allocated to some use, the additional issue is
which carrier is allowed to use which frequencies.
 Three algorithms were widely used in the past. The oldest algorithm, often called the
beauty contest, requires each carrier to explain why its proposal serves the public
interest best.
 Algorithm 2, holding a lottery among the interested companies. Algorithm 3, auction
off the bandwidth to the highest bidder.
 A completely different approach to allocating frequencies is to not allocate them at all.
 Instead, let everyone transmit at will, but regulate the power used so that stations have
such a short range that they do not interfere with each other.
 Most governments have set aside some frequency bands, called the ISM (Industrial,
Scientific, Medical) bands for unlicensed usage.

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer
 The location of these bands varies from country to country.

ISM and U-NII bands used in the United States by wireless devices

 The unlicensed bands have been a huge success over the past decade. The ability to
use the spectrum freely has unleashed a huge amount of innovation in wireless LANs
and PANs.
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.
 Thus, infrared system in one room of a building will not interfere with a similar
system in adjacent rooms or buildings.
 Furthermore, 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, in contrast
to radio systems, which must be licensed outside the ISM bands.
 Infrared communication has a limited use on the desktop, for example, to connect
notebook computers and printers with the IrDA (Infrared Data Association) standard.

Light Transmission
 Optical signaling using lasers is inherently unidirectional, so each end needs its own
laser and its own photodetector.
 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.
 It is difficult to focus laser beam 1 mm wide at a target. Wind and temperature
changes can distort the beam and laser beams also cannot penetrate rain or thick fog.
 Unguided optical links need to be engineered with a sufficient margin of error.
 Communicating with visible light is inherently safe and creates a low-speed network
in the immediate vicinity.

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Computer Networks (21CS52) Module 1: Introduction, Physical Layer

A bidirectional system with two lasers. Convection currents can interfere with laser
communication systems

26