Department of Computer Science and Engineering: 2. What Is Network Software?

DEPARTMENT OF COMPUTER SCIENCE AND ENGINEERING
2 Marks
1. list the requirements to building a network
 Scalable Connectivity
 Cost-Effective Resource Sharing
 Support for Common Services
 Manageability
2. What is network software?

A general phrase for software that is designed to help set up, manage, and/or monitor
computer networks. Networking software applications are available to manage and monitor
networks of all sizes, from the smallest home networks to the largest enterprise networks
3. Write the parameters used to measure network performance (May 2016)

 Bandwidth and Latency
 Delay×Bandwidth Product
 High-Speed Networks
 Application Performance Needs
4. Define API
Application Programming Interface (Sockets)
The place to start when implementing a network application is the interface exported by
the network. Since most network protocols are implemented in software, and nearly all computer
systems implement their network protocols as part of the operating system, when we refer to the
interface “exported by the network,” we are generally referring to the interface that the OS
provides to its networking subsystem. This interface is often called the network application
programming interface (API)
5. What are the three criteria necessary for an effective and efficient network?
The most important criteria are
 Performance
 Reliability
 Security.
Performance of the network depends on number of users, type of transmission medium, and the
capabilities of the connected h/w and the efficiency of the s/w. Reliability is measured by
frequency of failure, the time it takes a link to recover from the failure and the network’s
robustness in a catastrophe. Security issues include protecting data from unauthorized access and
viruses.
6. Group the OSI layers by function?

The seven layers of the OSI model belonging to three subgroups. Physical, data link and
network layers are the network support layers; they deal with the physical aspects of moving
data from one device to another. Session, presentation and application layers are the user
support layers; they allow interoperability among unrelated software systems. The transport
layer ensures end-to-end reliable data transmission.
7. What are the features provided by layering? (May 2013)
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PREPARED BY: Mr. D. Ramkumar - A.P-CSE
Two features:
 It decomposes the problem of building a network into more manageable components.
 It provides a more modular design.
8. Why are protocols needed?

In networks, communication occurs between the entities in different systems. Two entities
cannot just send bit streams to each other and expect to be understood. For communication, the
entities must agree on a protocol. A protocol is a set of rules that govern data communication.
9. What are the two interfaces provided by protocols?

 Service interface
 Peer interface
Service interface- defines the operations that local objects can perform on the protocol.
Peer interface- defines the form and meaning of messages exchanged between protocol peers to
implement the communication service.
10. Mention the different physical media?

 Twisted pair (the wire that your phone connects to)
 Coaxial cable (the wire that your TV connects to)
 Optical fiber (the medium most commonly used for high-bandwidth, long-distance
links)
 Space (the stuff that radio waves, microwaves and infra red beams propagate through)
11. Explain the two types of duplex?

 Full duplex-two bit streams can be simultaneously transmitted over the links at the
same time, one going in each direction.
 Half duplex-it supports data flowing in only one direction at a time.
12. What is spread spectrum and explain the two types of spread spectrum?
Spread spectrum is to spread the signal over a wider frequency band than normal in such
a way as to minimize the impact of interference from other devices.
 Frequency Hopping
 Direct sequence
13. What are the different encoding techniques?

 NRZ
 NRZI
 Manchester
 4B/5B
14. What are the responsibilities of data link layer?

Specific responsibilities of data link layer include the following.
a) Framing b) Physical addressing c) Flow control d) Error control e) Access control.
15. What are the ways to address the framing problem?

 Byte-Oriented Protocols(PPP)
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 Bit-Oriented Protocols(HDLC)
 Clock-Based Framing(SONET)
16. Mention the types of errors and define the terms? (MAY 2012)
There are 2 types of errors
 Single-bit error.
 Burst-bit error.
Single bit error: The term single bit error means that only one bit of a given data unit (such as
byte character/data unit or packet) is changed from 1 to 0 or from 0 to 1.
Burst error: Means that 2 or more bits in the data unit have changed from 1 to 0 from 0 to 1.
17. List out the available detection methods.

There are 4 types of redundancy checks are used in data communication.
 Vertical redundancy checks (VRC).
 Longitudinal redundancy checks (LRC).
 Cyclic redundancy checks (CRC).
 Checksum.
18. Write short notes on VRC.

The most common and least expensive mechanism for error detection is the vertical redundancy
check (VRC) often called a parity check. In this technique a redundant bit called a parity bit, is
appended to every data unit so, that the total number of 0’s in the unit (including the parity bit)
becomes even.
19. Write short notes on LRC.

In longitudinal redundancy check (LRC), a block of bits is divided into rows and a redundant
row of bits is added to the whole block.
20. Write short notes on CRC.

The third and most powerful of the redundancy checking techniques is the cyclic redundancy
checks (CRC) CRC is based on binary division. Here a sequence of redundant bits, called the
CRC remainder is appended to the end of data unit.
21. Write short notes on CRC checker.

A CRC checker functions exactly like a generator. After receiving the data appended with the
CRC it does the same modulo-2 division. If the remainder is all 0’s the CRC is dropped and the
data accepted. Otherwise, the received stream of bits is discarded and the dates are resent.
22. Define checksum.

The error detection method used by the higher layer protocol is called checksum. Checksum is
based on the concept of redundancy.
23. What are the steps followed in checksum generator?

The sender follows these steps a) the units are divided into k sections each of n bits. b) All
sections are added together using 2’s complement to get the sum. c) The sum is complemented
and become the checksum. d) The checksum is sent with the data.
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24. Write short notes on error correction? (NOV 2011)

It is the mechanism to correct the errors and it can be handled in 2 ways.
 When an error is discovered, the receiver can have the sender retransmit
the entire data unit.
 A receiver can use an error correcting coder, which automatically corrects certain
errors.
25. What is the purpose of hamming code?

A hamming code can be designed to correct burst errors of certain lengths. So the simple strategy
used by the hamming code to correct single bit errors must be redesigned to be applicable for
multiple bit correction.
26. What is redundancy?

It is the error detecting mechanism, which means a shorter group of bits or extra bits may be
appended at the destination of each unit.
27. Define flow control? (NOV 2011)(May 2015) (May 2016)

Flow control refers to a set of procedures used to restrict the amount of data. The sender can send
before waiting for acknowledgment.
28. Mention the categories of flow control?

There are 2 methods have been developed to control flow of data across communication links.
a) Stop and wait - send one from at a time.
b) Sliding window - send several frames at a time.
29. What is a buffer?

Each receiving device has a block of memory called a buffer, reserved for storing incoming data
until they are processed.
30. What is the difference between a passive and an active hub?

An active hub contains a repeater that regenerates the received bit patterns before sending them
out. A passive hub provides a simple physical connection between the attached devices.
31. For n devices in a network, what is the number of cable links required for a mesh and
ring topology?
 Mesh topology – n (n-1)/2
 Ring topology – n
32. List the Channelization Protocols

 Frequency Division Multiple Access (FDMA)
-The total bandwidth is divided into channels.
 Time Division Multiple Access (TDMA)
-The band is divided into one channel that is time shared
 Code Division Multiple Access (CDMA)
-One channel carries all transmission simultaneously
33. What are the two types of line configuration? (NOV 2010)
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 Point-to-point & Multipoint
34. What do you meant by error control? (NOV 2010)(May 2015)

Error control is used for detecting and retransmitting damaged or lost frames and to prevent
duplication of frames. This is achieved through a trailer added at the end of the frame.
35. Define Error detection (NOV 2011)

Data can be corrupted during transmission. For reliable communication, errors must be detected
and corrected
Types of error:
 Single bit error
 Burst error
The three error detecting techniques are:
 Parity check
 Check sum algorithm
 Cyclic Redundancy Check
36. What is the use of Two dimensional parity in error detection? (NOV 2012)
 It is based on simple parity.
 It performs calculation for each bit position across each byte in the frame.
 This adds extra parity byte for entire frame, in addition to a parity bit for each byte.
37. What are the issues(Services) in data link layer? (NOV 2012) (May 2016) (Nov 2016)
a) Services Provided to the Network Layer
b) Framing
c) Error Control
d) Flow Control
38. Define network and computer network

A network is any collection of independent computers that communicate with one
another over a shared network medium. A computer network is a collection of two or more
connected computers. When these computers are joined in a network, people can share files and
peripherals such as modems, printers, tape backup drives, or CD-ROM drives.
39. List the components of data communication

 Message
 Sender
 Receiver
 Medium
 Protocol
40. Define bit stuffing (MAY 2011)

Bit stuffing is the insertion of one or more bits into a transmission unit as a way to provide
signaling information to a receiver. The receiver knows how to detect and remove or disregard
the stuffed bits.
41. What are the major duties of network layer? (MAY 2012)
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 Logical addressing - If a packet passes the n/w boundary, we need another addressing
system for source and destination called logical address.
 Routing – The devices which connects various networks called routers are responsible
for delivering packets to final destination.
42. What are the functions of application layer? (MAY 2011)

 FTAM (file transfer, access, mgmt) - Allows user to access files in a remote host.
 Mail services - Provides email forwarding and storage.
 Directory services - Provides database sources to access information about various
sources and objects.
43. Define a layer. (Nov/Dec 2013)

The OSI (Open System Interconnection) Model breaks the various aspects of a computer
network into seven distinct layers. Each successive layer envelops the layer beneath it, hiding its
details from the levels above.
44. What do you mean by framing? (Nov/Dec 2013) (Nov/Dec 2014)

Frames are the small data units created by data link layer and the process of creating frames by
the data link layer is known as framing
45. What is protocol? What are its key elements? (NOV/DEC 2007) (May 2016)
Set of rules that govern the data communication is protocol. The key elements are
i) Syntax ii) Semantics iii) Timing
46. Define (or) mechanism of stop and wait protocol (Nov 2016)
The idea of stop-and-wait is straightforward: After transmitting one frame, the sender
waits for an acknowledgment before transmitting the next frame. If the acknowledgment does
not arrive after a certain period of time, the sender times out and retransmits the original frame.
47. Define sliding window algorithm

The sender can transmit several frames before needing an acknowledgement. Frames can
be sent one right after another meaning that the link can carry several frames at once and its
capacity can be used efficiently. The receiver acknowledges only some of the frames, using a
single ACK to confirm the receipt of multiple data frames
48. Define character stuffing

The problem with the sentinel approach is that the ETX character might appear in the
data portion of the frame. BISYNC overcomes this problem by “escaping” the ETX character by
preceding it with a DLE (data-link-escape) character whenever it appears in the body of a frame;
the DLE character is also escaped (by preceding it with an extra DLE) in the frame body. This
approach is called character stuffing.
49. List the 7 OSI layers
 Physical Layer.
 Data link Layer.
 Network Layer.
 Transport Layer.
 Session Layer.
 Presentation Layer.
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 Application Layer
50. Define hamming distance (Nov/Dec 2014)

Hamming distance = the number of bit positions in which two code-words differ.
Eg. How to calculate ?(Exclusive OR=XOR):
10001001
10110001
------------
00111000
=> The number of 1’s give the number of different bits
16 MARKS
1. Discuss the applications, advantages and disadvantages of networks

Applications of Computer Network:-
1. Business Applications
a. Database resource
b. Communication Medium
c. Electronic commerce
d. Home Applications
a. Internet Access
b. Personal Communication
c. Entertainment
d. Electronic Commerce
2. Mobile Computers
a. Wireless networks
Advantages of Network
 Speed. Sharing and transferring files within Networks are very rapid. Thus saving time,
while maintaining the integrity of the file.
 Cost. Individually licensed copies of many popular software programs can be costly.
Networkable versions are available at considerable savings. Shared programs, on a
network allows for easier upgrading of the program on one single file server, instead of
upgrading individual workstations.
 Security. Sensitive files and programs on a network are passwords protected or
designated as "copy inhibit," so that you do not have to worry about illegal copying of
programs.
 Centralized Software Management. Software can be loaded on one computer (the file
server) eliminating that need to spend time and energy installing updates and tracking
files on independent computers throughout the building.
 Resource Sharing. Resources such as, printers, fax machines and modems can be
shared.
 Electronic Mail. E-mail aids in personal and professional communication.
 Flexible Access. Access their files from computers throughout the firm.
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 Workgroup Computing. Workgroup software (such as Microsoft BackOffice) allows
many users to work on a document or project concurrently.
Disadvantages of Network
 Server faults stop applications being available

 Network faults can cause loss of data.
 Network fault could lead to loss of resources
 User work dependent upon network
 Could become inefficient
 Could degrade in performance
 Resources could be located too far from users
2. Discuss the categories (types) and connections of networks
Types of Network
Computer network design is dividing into three basic types such as

 LAN (local area network),
 MAN (Metropolitan area network)
 WAN (wide area network)
LAN (Local area networks)

Generally called LANs, are privately-owned networks within a single building or campus of
up to a few kilometers in size. They are widely used to connect personal computers and
workstations in company offices and factories to share resources (e.g., printers) and exchange
information.
LAN configuration consists of:

 A file server
 A workstation
 Cables
MAN (Metropolitan area network)
A metropolitan area network (MAN) is a large computer network that usually

spans a city or a large campus. A MAN usually interconnects a number of local area networks
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(LANs) using a high-capacity backbone technology, such as fiber-optical links, and provides up-
link services to wide area networks and the Internet
WAN (Wide Area Network) A WAN spans a large geographic area, such as a state, province
or country. WANs often connect multiple smaller networks, such as local area networks (LANs)
or metro area networks (MANs). The world's most popular WAN is the Internet.
TYPE OF CONNECTION:
There are two types are,
1. Point to point
2. Multi point
1. Point To Point:
It provides a dedicated link between two devices of the channel. The entire capacity of
the channel is reserved for transmission between those two devices.
2. Multipoint:
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More than two devices can share a link by using this type of connection. It also called as
multidrop. The capacity channel is shared either temporary or spatially. It simultaneously use, it
is spatially shared. If it takes turns, it is time shared line configuration
3. Discuss about topology and its types

Network Topologies
Topology refers to the way a network is laid out either physically or logically. Two or
more devices connect to a link; two or more links form a topology. It is the geographical
representation of the relationship of all the links and linking devices to each other.
1. Mesh
2. Star
3. Tree
4. Bus
5. Ring
6. Hybrid
1. Mesh Topology:
Here every device has a dedicated point to point link to every other device. A fully connected
mesh can have n(n-1)/2 physical channels to link n devices. It must have n-1 IO ports.
Advantages:
1. They use dedicated links so each link can only carry its own data load. So traffic
problem can be avoided.
2. It is robust. If any one link get damaged it cannot affect others
3. It gives privacy and security
4. Fault identification and fault isolation are easy.
Disadvantages:
1. The amount of cabling and the number IO ports required are very large. Since every
device is connected to each other devices through dedicated links.
2. The sheer bulk of wiring is larger then the available space
3. Hardware required to connect each device is highly expensive.
Example:
A mesh network has 8 devices. Calculate total number of cable links and IO ports needed.
Solution:
Number of devices = 8
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Number of links = n (n-1)/2
= 8(8-1)/2
= 28
Number of port/device = n-1
= 8-1 = 7
2. STAR TOPOLOGY:
Here each device has a dedicated link to the central ‘hub’. There is no direct traffic
between devices. The transmission are occurred only through the central controller namely hub.
Advantages:
1. Less expensive then mesh since each device is connected only to the hub.
2. Installation and configuration are easy.
3. Less cabling is need then mesh.
4. Robustness.
5. Easy to fault identification & isolation.
Disadvantages:
1. Even it requires less cabling then mesh when compared with other topologies it still
large.
3. TREE TOPOLOGY:
It is a variation of star. Instead of all devices connected to a central hub here most of the
devices are connected to a secondary hub that in turn connected with central hub. The central
hub is an active hub. An active hub contains a repeater, which regenerate the received bit pattern
before sending.
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The secondary hub may be active or passive. A passive hub means it just precedes a physical
connection only.
Advantages:
1. Can connect more than star.
2. The distance can be increased.
3. Can isolate and prioritize communication between different computers.
4. BUS TOPOLOGY:
A bus topology is multipoint. Here one long cable is act as a backbone to link all the
devices are connected to the backbone by drop lines and taps. A drop line is the connection
between the devices and the cable. A tap is the splice into the main cable or puncture the
sheathing.
Advantages:
1. Ease of installation.
2. Less cabling.
Disadvantages:
1. Difficult reconfiguration and fault isolation.
2. Difficult to add new devices.
3. Signal reflection at top can degradation in quality
4. If any fault in backbone can stops all transmission
5. Ring topology
Each node is connected to exactly two other nodes, forming a ring. Can be visualized as a
circular configuration. Requires at least three nodes
Advantages:
1. Easy to install.
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2. Easy to reconfigure.
3. Fault identification is easy.
Disadvantages:
1. Unidirectional traffic.
2. Break in a single ring can break entire network.
6. Hybrid topology
A combination of any two or more network topologies.
4. Explain the list of requirements to building a network
 Scalable Connectivity
 Cost-Effective Resource Sharing
 Support for Common Services
 Manageability
1. Scalable Connectivity
Networks (of which the Internet is the prime example) are designed to grow in a way that
allows them the potential to connect all the computers in the world. A system that is designed to
support growth to an arbitrarily large size is said to scale.
Links, Nodes, and Clouds

A network can consist of two or more computers directly connected by some physical
medium, such as a coaxial cable or an optical fiber. We call such a physical medium a link, and
we often refer to the computers it connects as nodes.
The cloud distinguishes between the nodes on the inside that implement the network (they are
commonly called switches, and their primary function is to store and forward packets) and the
nodes on the outside of the cloud that use the network (they are commonly called hosts, and they
support users and run application programs).
2. Cost-Effective Resource Sharing
Given a collection of nodes indirectly connected by a nesting of networks, it is possible

for any pair of hosts to send messages to each other across a sequence of links and nodes. Of
course, we want to do more than support just one pair of communicating hosts—we want to
provide all pairs of hosts with the ability to exchange messages.
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Multiplexing can be explained by analogy to a timesharing computer system, where a single

physical processor is shared (multiplexed) among multiple jobs, each of which believes it has its
own private processor. Similarly, data being sent by multiple users can be multiplexed over the
physical links that make up a network. There are several different methods for multiplexing
multiple flows onto one physical link. One common method is synchronous time- division
multiplexing (STDM).
3. Support for Common Services

The next requirement of a computer network is that the application programs running on
the hosts connected to the network must be able to communicate in a meaningful way. From the
application developer’s perspective, the network needs to make his or her life easier we use a
cloud to abstractly represent connectivity among a set of computers, we now think of a channel
as connecting one process to another.
Diagram shows a pair of application-level processes communicating over a logical

channel that is, in turn, implemented on top of a cloud that connects a set of hosts. We can think
of the channel as being like a pipe connecting two applications, so that a sending application can
put data in one end and expect that data to be delivered by the network to the application at the
other end of the pipe
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4. Manageability
Managing a network includes making changes as the network grows to carry more traffic
or reach more users, and troubleshooting the network when things go wrong or performance isn’t
as desired. This requirement is partly related to the issue of scalability discussed above—as the
Internet has scaled up to support billions of users and at least hundreds of millions of hosts, the
challenges of keeping the whole thing running correctly and correctly configuring new devices
as they are added have become increasingly problematic.
5. Discuss layering and protocols in detail (or) OSI layers (or) Network architecture
(Nov 2015) (Nov 2016)
Network Architecture
Networks do not remain fixed at single point in time, but it must evolve to accommodate
changes based on the technologies on which they are based and demands made by application
programmer.
Network architecture guides the design and implementation of network. Two commonly
used architecture are
 OSI Architecture
 Internet or TCP/IP architecture
Layering and Protocols

When the system gets complex, the system designer introduces another level of
abstraction. It defines unifying model with important aspects of the system, encapsulated this
model in interface objects and hide it from users
In network, abstraction leads to layering. Layering provides two nice features.
 It decomposes the problem of building a network into more manageable components.
Rather than implementing a monolithic piece of software that does everything
implement several layers, each of which solves one part of the problem.
 It provides more modular design. To add some new service, it is enough to modify
the functionality at one layer, reusing the functions provided at all the other layers.
Protocols
A protocol is a set of rules that governs data communication. It defines what is
communicated, how it is communicated, and when it is communicated. The key elements of a
protocol are syntax, semantics and timing.
Each protocol defines two different interfaces.
 Service interface - to the other objects on the same computer that want to use its
communication services. This service interface defines the operations that local
objects can perform on the protocol.
 Peer interface - to its counterpart (peer) on another machine. It also defines the
form and meaning of messages exchanged between protocol peers to implement
the communication service.
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Fig: Service and peer interfaces.
Except at the hardware level, peer to peer communication is indirect.
Encapsulation
Control information must be added with the data to instruct the peer how to handle with
the received message. It will be added into the header or trailer.
Header - Small data structure from few bytes to few kilobytes attached to the front of
message.
Trailer – Information will be added at the end of the message
Payload or message body – Data send by the program
In this case data is encapsulated with new message created by protocol at each level.
Multiplexing and De-Multiplexing

The fundamental idea of packet switching is to multiplex multiple flows of data over a
single physical link. This can be achieved by adding identifier to the header message. It is known
as demultiplexing or demux key. It gives the address to which it has to communicate.
The messages are demultiplexed at the destination side. In some cases same demux key is
used on both sides and in some cases different keys are used.
OSI Architecture (NOV 2012) (MAY 2012)

ISO defines a common way to connect computer by the architecture called Open System
Interconnection (OSI) architecture.
Network functionality is divided into seven layers.
 Physical Layer.
 Data link Layer.
 Network Layer.
 Transport Layer.
 Session Layer.
 Presentation Layer.
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 Application Layer
Organization of the layers

The 7 layers can be grouped into 3 subgroups
1. Network Support Layers

Layers 1,2,3 - Physical, Data link and Network are the network support layers.
They deal with the physical aspects of moving data from one device to another such as
electrical specifications, physical addressing, transport timing and reliability.
2. Transport Layer
Layer4, transport layer, ensures end-to-end reliable data transmission on a single link.
3. User Support Layers

Layers 5,6,7 – Session, presentation and application are the user support layers.
They allow interoperability among unrelated software systems
Functions of the Layers

1. Physical Layer
The physical layer coordinates the functions required to transmit a bit stream over a
physical medium.
The physical layer is concerned with the following:
 Physical characteristics of interfaces and media - The physical layer defines the
characteristics of the interface between the devices and the transmission medium.
 Representation of bits - To transmit the stream of bits, it must be encoded to signals.
The physical layer defines the type of encoding.
 Data Rate or Transmission rate - The number of bits sent each second – is also defined
by the physical layer.
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 Synchronization of bits - The sender and receiver must be synchronized at the bit level.
Their clocks must be synchronized.
 Line Configuration - In a point-to-point configuration, two devices are connected
together through a dedicated link. In a multipoint configuration, a link is shared between
several devices.
 Physical Topology - The physical topology defines how devices are connected to make a
network. Devices can be connected using a mesh, bus, star or ring topology.
 Transmission Mode - The physical layer also defines the direction of transmission
between two devices: simplex, half-duplex or full-duplex.
2. Data Link Layer

It is responsible for transmitting frames from one node to next node.
The other responsibilities of this layer are
 Framing - Divides the stream of bits received into data units called frames.
 Physical addressing – If frames are to be distributed to different systems on the n/w ,
data link layer adds a header to the frame to define the sender and receiver.
 Flow control- If the rate at which the data are absorbed by the receiver is less than the
rate produced in the sender ,the Data link layer imposes a flow ctrl mechanism.
 Error control- Used for detecting and retransmitting damaged or lost frames and to
prevent duplication of frames. This is achieved through a trailer added at the end of the
frame.
 Access control -Used to determine which device has control over the link at any given
time.
3. NETWORK LAYER
This layer is responsible for the delivery of packets from source to destination.
It is mainly required, when it is necessary to send information from one network to
another.
 Logical addressing - If a packet passes the n/w boundary, we need another addressing
system for source and destination called logical address.
 Routing – The devices which connects various networks called routers are responsible
for delivering packets to final destination.
4. TRANSPORT LAYER
 It is responsible for Process to Process delivery.
 It also ensures whether the message arrives in order or not.

 Port addressing - The header in this must therefore include a address called port address.
This layer gets the entire message to the correct process on that computer.
 Segmentation and reassembly - The message is divided into segments and each
segment is assigned a sequence number. These numbers are arranged correctly on the
arrival side by this layer.
 Connection control - This can either be connectionless or connection-oriented. The
connectionless treats each segment as a individual packet and delivers to the destination.
The connection-oriented makes connection on the destination side before the delivery.
After the delivery the termination will be terminated.
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 Flow and error control - Similar to data link layer, but process to process take place.
5. SESSION LAYER
This layer establishes, manages and terminates connections between applications.
 Dialog control - This session allows two systems to enter into a dialog either in half
duplex or full duplex.
 Synchronization-This allows to add checkpoints into a stream of data.
6. PRESENTATION LAYER
It is concerned with the syntax and semantics of information exchanged between two
systems.
 Translation – Different computers use different encoding system, this layer is
responsible for interoperability between these different encoding methods. It will change
the message into some common format.
 Encryption and decryption-It means that sender transforms the original information to
another form and sends the resulting message over the n/w. and vice versa.
 Compression and expansion-Compression reduces the number of bits contained in the
information particularly in text, audio and video.
7. APPLICATION LAYER
This layer enables the user to access the n/w. This allows the user to log on to remote user.
 FTAM (file transfer, access, mgmt) - Allows user to access files in a remote host.
 Mail services - Provides email forwarding and storage.
 Directory services - Provides database sources to access information about various
sources and objects.
6. Explain Internet architecture in detail (May 2015)
INTERNET ARCHITECTURE (TCP/IP ARCHITECTURE)

TCP/IP model is an implementation of OSI reference model. It has four layers. They are
 Network Interface Layer.
 Internet Layer.
 Transport (also known as Host-to-Host or Transmission) Layer
 Application Layer (known earlier as the Process Layer)
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1) The Host to Network Layer:
Below the internet layer is great void. The TCP/IP reference model does not really say such
about what happen here, except to point out that the host has connect to the network using some
protocol so it can transmit IP packets over it. This protocol is not specified and varies from host
to host and network to network.
2) Internet layer:
Packet switching network depends upon a connectionless internetwork layer. This layer is known
as internet layer, is the linchpin that holds the whole design together. Its job is to allow hosts to
insert packets into any network and have them to deliver independently to the destination. They
may appear in a different order than they were sent in each case it is job of higher layers to
rearrange them in order to deliver them to proper destination.
The internet layer specifies an official packet format and protocol known as internet protocol.
The job of internet layer is to transport IP packets to appropriate destination. Packet routing is
very essential task in order to avoid congestion. For these reason it is say that TCP/IP internet
layer perform same function as that of OSI network layer.
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3) Transport layer:
In the TCP/IP model, the layer above the internet layer is known as transport layer. It is
developed to permit entities on the source and destination hosts to carry on a conversation. It
specifies 2 end-to-end protocols
i) TCP (Transmission Control Protocol)
ii) UDP (User Datagram Protocol)
TCP
It is a reliable connection-oriented protocol that permits a byte stream originating on one
machine to be transported without error on any machine in the internet. It divides the incoming
byte stream into discrete message and passes each one onto the internet layer. At the destination,
the receiving TCP process collects the received message into the output stream. TCP deals with
flow control to make sure a fast sender cannot swamp a slow receiver with more message than it
can handle.
UDP
It is an unreliable, connectionless protocol for applications that do not want TCP’s sequencing on
flow control and wish to offer their own. It is also used for client-server type request-reply
queries and applications in which prompt delivery is more important than accurate delivery such
as transmitting speech or video.
4) Application Layer:
In TCP/IP model, session or presentation layer are not present. Application layer is present on
the top of the Transport layer. It includes all the higher-level protocols which are virtual terminal
(TELNET), file transfer (FTP) and electronic mail (SMTP).
The virtual terminal protocol permits a user on one machine to log into a distant machine and
work there. The file transfer protocol offers a way to move data efficiently from one machine to
another. Electronic mail was used for file transfer purpose but later a specialized protocol was
developed for it.
The Application Layer defines following protocols
i) File Transfer Protocol (FTP)

It was designed to permit reliable transfer of files over different platforms. At the transport layer
to ensure reliability, FTP uses TCP. FTP offers simple commands and makes the differences in
storage methods across networks transparent to the user. The FTP client is able to interact with
any FTP server; therefore the FTP server must also be able to interact with any FTP client. FTP
does not offer a user interface, but it does offer an application program interface for file transfer.
The client part of the protocol is called as FTP and the server part of the protocol is known as
FTPd. The suffix "d" means Daemon this is a legacy from Unix computing where a daemon is a
piece of software running on a server that offers a service.
ii) Hyper Text Transfer Protocol

HTTP permits applications such as browsers to upload and download web pages. It makes use of
TCP at the transport layer again to check reliability. HTTP is a connectionless protocol that
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sends a request, receives a response and then disconnects the connection. HTTP delivers HTML
documents plus all of the other components supported within HTML such as JavaScript, Visual
script and applets.
iii) Simple Mail Transfer Protocol

By using TCP, SMTP sends email to other computers that support the TCP/IP protocol suite.
SMTP provides extension to the local mail services that existed in the early years of LANs. It
supervises the email sending from the local mail host to a remote mail host. It is not reliable for
accepting mail from local users or distributing received mail to recipients this is the
responsibility of the local mail system.
SMTP makes use of TCP to establish a connection to the remote mail host, the mail is sent, any
waiting mail is requested and then the connection is disconnected. It can also return a forwarding
address if the intended recipient no longer receives email at that destination. To enable mail to be
delivered across differing systems, a mail gateway is used.
iv) Simple Network Management Protocol

For the transport of network management information, SNMP is used as standardized protocol.
Managed network devices can be cross examined by a computer running to return details about
their status and level of activity. Observing software can also trigger alarms if certain
performance criteria drop below acceptable restrictions. At the transport layer SNMP protocol
uses UDP. The use of UDP results in decreasing network traffic overheads.
7. Discuss network software

There are certainly many contributing factors (including a good architecture), but one
thing that has made the Internet such a runaway success is the fact that so much of its
functionality is provided by software running in general-purpose computers.
The significance of this is that new functionality can be added readily with “just a small
matter of programming.” As a result, new applications and services—electronic commerce,
videoconferencing, and IP telephony, to name a few—have been showing up at an incredible
pace.
Application Programming Interface (Sockets)
The place to start when implementing a network application is the interface exported by
the network. Since most network protocols are implemented in software (especially those high in
the protocol stack), and nearly all computer systems implement their network protocols as part of
the operating system, when we refer to the interface “exported by the network,” we are generally
referring to the interface that the OS provides to its networking subsystem. This interface is often
called the network application programming interface (API)
Although each operating system is free to define its own network API (and most have),
over time certain of these APIs have become widely supported; that is, they have been ported to
operating systems other than their native system. This is what has happened with the socket
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interface originally provided by the Berkeley distribution of Unix, which is now supported in
virtually all popular operating systems, and is the foundation of language-specific interfaces,
such as the Java socket library. The advantages of industry-wide support for a single API are that
applications can be easily ported from one OS to another and developers can easily write
applications for multiple operating systems.
The main abstraction of the socket interface, not surprisingly, is the socket. A good way
to think of a socket is as the point where a local application process attaches to the network. The
interface defines operations for creating a socket, attaching the socket to the network, sending/
receiving messages through the socket, and closing the socket. To simplify the discussion, we
will limit ourselves to showing how sockets are used with TCP.
The first step is to create a socket, which is done with the following operation:
int socket(int domain, int type, int protocol)
The reason that this operation takes three arguments is that the socket interface was designed to
be general enough to support any underlying protocol suite. Specifically, the domain argument
specifies the protocol family that is going to be used: PF INET denotes the Internet family, PF
UNIX denotes the Unix pipe facility, and PF PACKET denotes direct access to the network
interface (i.e., it bypasses the TCP/IP protocol stack).
The type argument indicates the semantics of the communication. SOCK STREAM is
used to denote a byte stream. SOCK DGRAM is an alternative that denotes a message-oriented
service, such as that provided by UDP. The protocol argument identifies the specific protocol
that is going to be used. In our case, this argument is UNSPEC because the combination of PF
INET and SOCK STREAM implies TCP. Finally, the return value from socket is a handle for
the newly created socket—that is, an identifier by which we can refer to the socket in the future.
It is given as an argument to subsequent operations on this socket.
The next step depends on whether you are a client or a server. On a server machine, the
application process performs a passive open—the server says that it is prepared to accept
connections, but it does not actually establish a connection. The server does this by invoking the
following three operations:
int bind(int socket, struct sockaddr *address, int addr len)

int listen(int socket, int backlog)
int accept(int socket, struct sockaddr *address, int *addr len)
The bind operation, as its name suggests, binds the newly created socket to the specified address.
This is the network address of the local participant—the server
The listen operation then defines how many connections can be pending on the specified
socket. Finally, the accept operation carries out the passive open.
On the client machine, the application process performs an active open; that is, it says
who it wants to communicate with by invoking the following single operation:
int connect(int socket, struct sockaddr *address, int addr len)
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Once a connection is established, the application processes invoke the following two operations
to send and receive data:
int send(int socket, char *message, int msg len, int flags)
int recv(int socket, char *buffer, int buf len, int flags)
The first operation sends the given message over the specified socket, while the second operation
receives a message from the specified socket into the given buffer. Both operations take a set of
flags that control certain details of the operation
(Example application – Refer Lab Program)
8. Discuss in detail about the network performance measures (Nov 2016)

Like any computer system, however, computer networks are also expected to
perform well. This is because the effectiveness of computations distributed over the network
often depends directly on the efficiency with which the network delivers the computation’s data.
While the old programming adage “first get it right and then make it fast” is valid in many
settings, in networking it is usually necessary to “design for performance.” It is therefore
important to understand the various factors that impact network performance.
 Bandwidth and Latency

 Delay×Bandwidth Product
 High-Speed Networks
 Application Performance Needs
Bandwidth and Latency

Network performance is measured in two fundamental ways: bandwidth (also called
throughput) and latency (also called delay).
The bandwidth of a network is given by the number of bits that can be transmitted over
the network in a certain period of time.
For example, a network might have a bandwidth of 10 million bits/second (Mbps),
meaning that it is able to deliver 10 million bits every second. It is sometimes useful to think of
bandwidth in terms of how long it takes to transmit each bit of data. Ona10-Mbps network, for
example, it takes 0.1 microsecond (μs) to transmit each bit.
The second performance metric, latency, corresponds to how long it takes a message to
travel from one end of a network to the other. (As with bandwidth, we could be focused on the
latency of a single link or an end-to-end channel.) Latency is measured strictly in terms of time.
Latency = Propagation+Transmit+Queue
Propagation = Distance/Speed Of Light
Transmit = Size/Bandwidth
Delay×Bandwidth Product
It is also useful to talk about the product of these two metrics, often called the delay
×bandwidth product. Intuitively, if we think of a channel between a pair of processes as a hollow
pipe (see Figure 1.18), where the latency corresponds to the length of the pipe and the bandwidth
gives the diameter of the pipe, then the delay×bandwidth product gives the volume of the pipe
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the maximum number of bits that could be in transit through the pipe at any given instant. Said
another way, if latency (measured in time) corresponds to the length of the pipe, then given the
width of each bit (also measured in time) you can calculate how many bits fit in the pipe.
The delay×bandwidth product is important to know when constructing high-performance
networks because it corresponds to how many bits the sender must transmit before the first bit
arrives at the receiver
High-Speed Networks
The bandwidths available on today’s networks are increasing at a dramatic rate, and there
is eternal optimism that network bandwidth will continue to improve. This causes network
designers to start thinking about what happens in the limit or, stated another way, what is the
impact on network design of having infinite bandwidth available.
Although high-speed networks bring a dramatic change in the bandwidth available to
applications, in many respects their impact on how we think about networking comes in what
does not change as bandwidth increases: the speed of light.
The effective end-to-end throughput that can be achieved over a network is given by the
simple relationship
Throughput = TransferSize/TransferTime
where Transfer Time includes not only the elements of one-way Latency identified earlier in this
section, but also any additional time spent requesting or setting up the transfer. Generally, we
represent this relationship as
TransferTime = RTT+1/Bandwidth × TransferSize
We use RTT in this calculation to account for a request message being sent across the network
and the data being sent back.
Application Performance Needs

The discussion in this section has taken a network-centric view of performance; that is,
we have talked in terms of what a given link or channel will support. The unstated assumption
has been that application programs have simple needs—they want as much bandwidth as the
network can provide. This is certainly true of the aforementioned digital library program that is
retrieving a 25-MB image; the more bandwidth that is available, the faster the program will be
able to return the image to the user.
9. Explain framing concepts in detail (or) Discuss link layer services (or) Issues in
data link layer (or) Discuss HDLC & PPP (Nov/Dec 2014) (May 2016)
Framing (MAY 2013)
To transmit frames over the node it is necessary to mention start and end of each frame. There are
three techniques to solve this frame
 Byte-Oriented Protocols (BISYNC, PPP, DDCMP)
 Bit-Oriented Protocols (HDLC)
 Clock-Based Framing (SONET)
Byte Oriented protocols

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In this, view each frame as a collection of bytes (characters) rather than a collection of bits.
Such a byte-oriented approach is exemplified by the BISYNC (Binary Synchronous
Communication) protocol and the DDCMP (Digital Data Communication Message Protocol)
Sentinel Approach
The BISYNC protocol illustrates the sentinel approach to framing; its frame format is
Fig: BISYNC Frame format
 The beginning of a frame is denoted by sending a special SYN (synchronization)

character.
 The data portion of the frame is then contained between special sentinel characters:
STX (start of text) and ETX (end of text).
 The SOH (start of header) field serves much the same purpose as the STX field.
 The frame format also includes a field labeled CRC (cyclic redundancy check) that is
used to detect transmission errors.
The problem with the sentinel approach is that the ETX character might appear in the data
portion of the frame. BISYNC overcomes this problem by “escaping” the ETX character by
preceding it with a DLE (data-link-escape) character whenever it appears in the body of a frame;
the DLE character is also escaped (by preceding it with an extra DLE) in the frame body. This
approach is called character stuffing.
Point-to-Point Protocol (PPP)
The more recent Point-to-Point Protocol (PPP). The format of PPP frame is
Fig: PPP Frame Format
 The Flag field has 01111110 as starting sequence.

 The Address and Control fields usually contain default values
 The Protocol field is used for demultiplexing.
 The frame payload size can he negotiated, but it is 1500 bytes by default.
 The PPP frame format is unusual in that several of the field sizes are negotiated rather
than fixed.
 Negotiation is conducted by a protocol called LCP (Link Control Protocol).
 LCP sends control messages encapsulated in PPP frames—such messages are denoted
by an LCP identifier in the PPP Protocol.
Byte-Counting Approach
The number of bytes contained in a frame can he included as a field in the frame header.
DDCMP protocol is used for this approach. The frame format is
Fig: DDCMP frame format
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 COUNT Field specifies how many bytes are contained in the frame’s body.
 Sometime count field will be corrupted during transmission, so the receiver will
accumulate as many bytes as the COUNT field indicates. This is sometimes called a
framing error.
 The receiver will then wait until it sees the next SYN character.
Bit-Oriented Protocols (HDLC)

In this, frames are viewed as collection of bits. High level data link protocol is used. The format
is
Fig: HDLC Frame Format
 HDLC denotes both the beginning and the end of a frame with the distinguished bit
sequence 01111110.
 This sequence might appear anywhere in the body of the frame, it can be avoided by bit
stuffing.
 On the sending side, any time five consecutive 1’s have been transmitted from the body
of the message (i.e., excluding when the sender is trying to transmit the distinguished
01111110 sequence), the sender inserts a 0 before transmitting the next bit.
 On the receiving side, five consecutive 1’s arrived, the receiver makes its decision based
on the next bit it sees (i.e., the bit following the five is).
 If the next bit is a 0, it must have been stuffed, and so the receiver removes it. If the next
bit is a 1, then one of two things is true, either this is the end-of-frame marker or an error
has been introduced into the bit stream.
 By looking at the next bit, the receiver can distinguish between these two cases:
7 If it sees a 0 (i.e., the last eight bits it has looked at are 01111110), then it is the end-of-
frame marker.
8 If it sees a 1 (i.e., the last eight bits it has looked at are 01111111), then there must have
been an error and the whole frame is discarded.
Clock-Based Framing (SONET)

 Synchronous Optical Network Standard is used for long distance transmission of data
over optical network.
 It supports multiplexing of several low speed links into one high speed links.
 An STS-1 frame is used in this method.
 It is arranged as nine rows of 90 bytes each, and the first 3 bytes of each row are
overhead, with the rest being available for data.
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 The first 2 bytes of the frame contain a special bit pattern, and it is these bytes that
enable the receiver to determine where the frame starts.
 The receiver looks for the special bit pattern consistently, once in every 810 bytes, since
each frame is 9 x 90 = 810 bytes long.
 The STS-N frame can he thought of as consisting of N STS-1 frames, where the bytes
from these frames are interleaved; that is, a byte from the first frame is transmitted, then a
byte from the second frame is transmitted, and so on.
 Payload from these STS-1 frames can he linked together to form a larger STS-N payload,
such a link is denoted STS-Nc. One of the bit in overhead is used for this purpose.
10. Explain Error detection techniques or Error detecting codes (NOV 2012)
(MAY 2012) (Nov 2014) (May 2015) (May 2016)
Data can be corrupted during transmission. For reliable communication, errors must be detected
and corrected.
Types of Errors
Single-bit error
The term Single-bit error means that only one bit of a given data unit (such as byte, character, data unit
or packet) is changed from 1 to 0 or from 0 to 1.
Burst Error
The term Burst Error means that two or more bits in the data unit have changed from 1 to 0 or from 0
to 1.
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Error detection
Redundancy
One method is to send every data twice, so that receiver checks every bit of two copies and
detect error.
Drawbacks
 Sends n-redundant bits for n-bit message.
 Many errors are undetected if both the copies are corrupted.
Instead of adding entire data, some bits are appended to each unit.
This is called redundant bit because the bits added will not give any new information. These bits
are called error detecting codes.
The three error detecting techniques are:

 Parity check
 Check sum algorithm
 Cyclic Redundancy Check
1)Parity Check
Simple parity check

Only one redundant bit, called parity bit is added to every data unit so that the total number of
1’s in unit become even (or odd)
Two Dimensional Parity

 It is based on simple parity.
 It performs calculation for each bit position across each byte in the frame.
 This adds extra parity byte for entire frame, in addition to a parity bit for each byte.
Fig: Two-dimensional parity
For example frame containing 6 bytes of data. In this third bit of the parity byte is 1 since there
are an odd number of 1’s is in the third bit across the 6 bytes in the frame.
In this case, 14 bits of redundant information are added with original information.
2)Check sum algorithm
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 In the sender side all the words are added and then transmit the result of sum called
checksum with the data.
 The receiver performs the same calculation on the received data and compares the result
with the received checksum.
Sender Receiver
7 7
5 5
9 9
Sum 21 9
Check Sum 9 Sum 30
Check Sum 0
1010 1
1 1111 0
1
0110
9 1001 15 1111
0 0000
 If any transmitted data, including the checksum itself, is corrupted, then the results will
not match, so the receiver knows that an error occurred.
 Instead of sending the checksum as such, one’s complement of that sum will be send to
the receiver. If the receiver gets the result as zero then it will be the correct one.
 In this, we can represent unsigned number from 0 to 2n using n bits.
 If the number has more than n bits, the extra leftmost bits need to be added to the n
rightmost bits.
 Data can be divided in to 16 bit word and the Checksum is initialized to zero.
3) Cyclic Redundancy Check
 It uses small number of redundant bits to detect errors.

 Divisor is calculated by the polynomial functions under two conditions
a. It should not be divisible by x
b. It should be divisible by x+1
Consider the original message as M(x) – n+1 bits
Divisor C(x) – K bits
Original sent message = M(x) + k-1 bits
Steps
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 Append k-1 zeros with M(x) – P(x)
 Divide P(x) by C(x)
 Subtract the remainder from T(x)
 Subtraction is made by making XOR operation
Eg: 100100 by 1101
11. Discuss link level flow control (or) Reliable transmission (or) Explain the
various flow control mechanisms (MAY 2013) (NOV 2012) (Nov 2015)
Flow control is a technique that a transmitting entity does not conquer a receiving entity with
data. Two fundamental mechanisms are acknowledgement and timeouts.
 After getting each frame the receiver will send ACK to sender.
 If the sender does not receive ACK up to reasonable amount of time then it retransmit the
original frame waiting for reasonable amount of time is called timeout.
The two flow control mechanisms are

 Stop and wait Flow Control
 Sliding Window Flow Control
Stop and Wait Algorithm

 After transmitting one frame, the sender waits for an acknowledgment before transmitting
the next frame.
 If the acknowledgment does not arrive after a certain period of time, the sender times out
and retransmit the original frame.
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a) The ACK is received before the timer expires b) The original frame is lost
c) The ACK is lost d)The timeout fires too soon
Fig: illustrates four different scenarios that result from this basic algorithm. The sending side is
represented on the left, the receiving side is depicted on the right, and time flows from top to
bottom.
 In Fig (a) ACK is received before the timer expires, (b) and (c) show the situation in which
the original frame and the ACK, respectively, are lost, and (d) shows the situation in which
the timeout fires too soon..
 Suppose the sender sends a frame and the receiver acknowledges it, but the acknowledgment
is either lost or delayed in arriving. This situation is in (c) and (d). In both cases, the sender
times out and retransmit the original frame, but the receiver will think that it is the next
frame, since it correctly received and acknowledged the first frame.
 This makes the receiver to receive the duplicate copies. To avoid this two sequence numbers
(0 and 1) must be used alternatively.
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 The main drawback of the stop-and-wait algorithm is that it allows the sender have only one
outstanding frame on the link at a time.
Sliding Window Algorithm

 The sender can transmit several frames before needing an acknowledgement.
 Frames can be sent one right after another meaning that the link can carry several frames at
once and it s capacity can be used efficiently.
 The receiver acknowledges only some of the frames, using a single ACK to confirm the
receipt of multiple data frames
 Sliding Window refers to imaginary boxes at both the sender and the receiver.
 Window can hold frames at either end and provides the upper limit on the number of frames
that can be transmitted before requiring an acknowledgement.
 Frames are numbered modulo-n which means they are numbered from o to n-1
 For eg. If n=8 the frames are numbered 0,1,2,3,4,5,6,7. i.e the size of the window is n -1.
 When the receiver sends ACK it includes the number of the next frame it expects to receive.
 When the sender sees an ACK with the number 5, it knows that all frames up through
number 4 have been received.
There are two methods to retransmit the lost frames

 GO-Back N
 Selective Repeat
Go – Back N Method
Sender Window
 At the beginning of transmission, the sender window contains n-1 frames. As frames are
sent out, the left boundary of the window moves inward, shrinking the size of the window
 If size of window is W if three frames have been transmitted since the last
acknowledgement then the number of frames left in the window is w -3.
 Once an ACK arrives, the window expands to allow in a number of new frames equal to
the number of frames acknowledged by that ACK.
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Receiver Window
 The receive window is an abstract concept defining an imaginary box of size 1 with one
single variable Rn.
 The window slides when a correct frame has arrived, sliding occurs one slot at a time.
When the timer expires, the sender resends all outstanding frames. For example, suppose the sender
has already sent frame 6, but the timer for frame 3 expires. This means that frame 3 has not been
acknowledged; the sender goes back and sends frames 3, 4,5, and 6 again. That is why the
protocol is called Go-Back-N.
Selective Repeat (Nov 2016)
Sender Window
Receiver window
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 The Selective Repeat Protocol allows as many frames as the size of the receive window to
arrive out of order and be kept until there is a set of in-order frames to be delivered to the
network layer.
 Because the sizes of the send window and receive window are the same, all the frames in the
send frame can arrive out of order and be stored until they can be delivered.
 If any frame lost, sender has to retransmit only that lost frames.
Sliding window algorithm - Working
The sliding window algorithm works as follows. First, the sender assigns a sequence
number, denoted SeqNum, to each frame. For now, let’s ignore the fact that SeqNum is
implemented by a finite-size header field and instead assume that it can grow infinitely large.
The sender maintains three variables:

 The send window size, denoted SWS, gives the upper bound on the number of
outstanding (unacknowledged) frames that the sender can transmit;
 LAR denotes the sequence number of the last acknowledgment received; and
 LFS denotes the sequence number of the last frame sent.
The sender also maintains the following invariant:
LFS−LAR ≤ SWS
When an acknowledgment arrives, the sender moves LAR to the right, thereby allowing
the sender to transmit another frame. Also, the sender associates a timer with each frame it
transmits, and it retransmits the frame should the timer expire before an ACK is received. Notice
that the sender has to be willing to buffer up to SWS frames since it must be prepared to
retransmit them until they are acknowledged.
The receiver maintains the following three variables:
 The receive window size, denoted RWS, gives the upper bound on the number of outof-
order frames that the receiver is willing to accept;
 LAF denotes the sequence number of the largest acceptable frame; and
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 LFR denotes the sequence number of the last frame received.
The receiver also maintains the following invariant

LAF−LFR ≤ RWS
When a frame with sequence number SeqNum arrives, the receiver takes the following
action. If SeqNum ≤ LFR or SeqNum > LAF, then the frame is outside the receiver’s window
and it is discarded. If LFR < SeqNum ≤ LAF, then the frame is within the receiver’s window and
it is accepted.
Now the receiver needs to decide whether or not to send an ACK. Let SeqNumToAck
denote the largest sequence number not yet acknowledged, such that all frames with sequence
numbers less than or equal to SeqNumToAck have been received.
The receiver acknowledges the receipt of SeqNumToAck, even if higher numbered
packets have been received. This acknowledgment is said to be cumulative. It then sets LFR =
SeqNumToAck and adjusts LAF = LFR+RWS.
12. The message 11001001 is to be transmitted, using CRC error detection algorithm.
Assuming the CRC polynomial to be x 3+1, determine the message that should be
transmitted. if the second left most bit is corrupted, shoe that it is detected by the
receiver.(8) (NOV 2013)
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13. Discuss Error Correction (The process of correcting bits) in detail (May 2015)
Error Correction can be handled in two ways

1. When an error is discovered, the receiver can have the sender to retransmit the entire
data unit. (Retransmission)
2. Forward error correction (FEC) – sending the codes in advance. Eg. Hamming code
Error correcting codes are more sophisticated than error-detection codes and require more
redundancy bits.
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In single bit error detection only two states are sufficient.
1) error
2) no error
Two states are not enough to detect an error but not to correct it.
Redundancy Bits
 To calculate the number of redundancy bit(r) required to correct a given number of

data bits (m), we must find a relationship between m and r.
 Add m bits of data with r bits. The length of the resulting code is m+r.
Data and Redundancy bits
 If the total number of bits are m+r, then r must be able to indicate at least m+r+1
different states. r bits can indicate 2r different states. Therefore, 2r must be equal to or
greater than m+r+1
2r >=m+r+1
 For example if the value of m is 7 the smallest r value that can satisfy this equation is
4.
Relationship between data and redundancy bits
Number of Data Number of Total bits (m+r)

Bits (m) redundancy Bits(r)
1
2 3
2
3 5
3
3 6
4
3 7
5
4 9
6
4 10
7
4 11
Hamming Code
38
R.W. Hamming provides a practical solution for the error correction.
Positioning the Redundancy Bits
For example, a seven-bit ASCII code requires four redundancy bits that can be added to
the end of the data or intersperse with the original data bits. These redundancy bits are placed in
positions 1, 2, 4 and 8. We refer these bits as r1, r2, r3 and r4
Position of redundancy bits in Hamming code
The combination used to calculate each of the four r values for a seven-bit data sequence are as
follows
 The r1 bit is calculated using all bits positions whose binary representation include a
1 in the rightmost position
 r2 is calculated using all bit position with a 1 in the second position and so on
r1: bits 1,3,5,7,9,11
r2: bits 2, 3, 6, 7, 10, 11
r3: bits 4, 5, 6, 7
r4: bits 8, 9, 10, 11
Redundancy bits calculation
39
Calculating the r values
 Place each bit of the original character in its appropriate position in the 11-bit unit.
 Calculate the even parities for the various bit combination.
 The parity value for each combination is the value of the corresponding r bit
For example,
 The value of r1 is calculated to provide even parity for a combination of bits 3,5,7,9
and 11.
 The value of r2 is calculated to provide even parity with bits 3, 6, 7, 10 and 11.
 The value of r3 is calculated to provide even parity with bits 4,5,6 and 7.
 The value of r4 is calculated to provide even parity with bits 8,9,10 and 11.
Error Detection and Correction
Now imagine the received data has 7th bit changed from 1 to 0.
Single-bit error
The receiver takes the transmission and recalculates four new data using the same set of bits
used by the sender plus the relevant parity (r) bit for each set.
Error detection
40
 Then it assembles the new parity values into a binary number in order of r position
(r8,r4,r2,r1).
 This step gives us the binary number 0111(7 in decimal) which is the precise location of
the bit in error.
 Once the bit is identified, the receiver can reverse its value and correct the error.
Hamming Distance
One of the central concepts in coding for error control is the idea of the Hamming
distance.
 The Hamming distance between two words (of the same size) is the number of
differences between the corresponding bits. The Hamming distance between two words x
and y is d(x, y).
 The Hamming distance can be found by applying the XOR operation on the two words
and count the number of 1’s in the result.
 In a set of words, the minimum Hamming distance is the smallest Hamming distance
between all possible pairs. We use dmin to define the minimum Hamming distance in a
coding scheme.
UNIVERSITY QUESTIONS
B.E/B.TECH NOVEMBER/DECEMBER 2014 (2008 regulation)

2 MARKS
41
1. What is meant by framing (Q.NO 44)
2. Define hamming distance (Q.NO 50)
16 MARKS
1. Discuss the issues in the data link layer (16) (Q.NO 9)
2. Explain in detail the error detecting codes (16) (Q.NO 10)
B.E/B.Tech April May 2015
2 MARKS
1. What do you mean by error control?(Q.NO 34)
2. Define flow control (Q.NO 27)
16 MARKS
1. Discuss in detail about Internet Architecture (16) (Q.NO 6)
2. What is the need for error detection? Explain with typical examples. Explain methods
used for error detection and error correction (16) (Q.NO 10 & 13)
B.E/B.Tech Nov-Dec 2015
2 MARKS
1. State the issues of data link layer (Q.NO 37)
2. Define the term protocol (Q.NO 45)
16 MARKS
1. Draw the OSI network architecture and explain the functionalities of every layer in detail (16)
(Q.NO 5)
2. Explain the various flow control mechanisms (16) (Q.NO 11)
B.E/B.Tech April-May 2016

2 MARKS
1. Define flow control. (Q.NO 27)
2. Write the parameters used to measure network performance (Q.NO 3)
16 MARKS
1. Explain any two error detection mechanism in detail (16) (Q.NO 10)
2. Explain in detail about HDLC & PPP (8+8) (Q.NO 9)
B.E/B.Tech Nov-Dec 2016
2 MARKS
1. List the services provided by data link layer (Q.NO 37)
2. Write the mechanism of stop and wait flow control (Q.NO 46)
42
16 MARKS
1. Draw the OSI network architecture and explain the functionalities of every layer in detail (16)
(Q.NO 5)
2. a)Discuss in detail about the network performance measures (8) (Q.NO 8)
b) Explain selective-repeat ARQ flow control method.(8) (Q.NO 11)
UNIT 1 - IMPORTANT QUESTIONS

1. Define a layer.
2. What do you mean by framing?
3. Give the purpose of layering? Or What are the features provided by layering?
4. list the requirements to building a network
5. What is the use of Two dimensional parity in error detection?
6. What are the major duties of network layer?
7. What are the two different types of errors occurred during data transmission?
8. What is flow control? & its types
9. Define error detection and correction
10. What are the functions of application layer?
11. Define bit stuffing
12. What is network software?
13. What do you meant by error control?
14. What are the layers of OSI model?
15. What are the two interfaces provided by protocols?
16. Define LRC, CRC & VRC
17. Define checksum.
18. What is protocol? What are its key elements? Or Why are protocols needed?
19. Define network and computer network
20. List the performance of computer networks
21. Define API or Socket
22. What are the responsibilities (issues) of data link layer?
23. List the techniques used in framing
24. Give the applications, advantages & disadvantages on computer network
25. Define hamming distance
PART- B
1. a) What are the requirements to building a network?
b) Discuss about topologies and its types
2. How layering & protocols can be accessed in network? Or Explain network architecture
Or Discuss OSI model in detail
3. Explain internet architecture Or TCP/IP model in detail
4. a)How connectivity can be made by network through software give example Or Discuss
API
b) How performance can be evaluated in network
5. Explain framing concepts in detail (Or) Explain in detail about the following i)PPP
ii)HDLC iii)SONNET (or) Discuss link layer services (or) issues in data link layer
6. a) Describe how bit stuffing works in HDLC protocol
b) Discuss CRC detection techniques
43
7. Discuss briefly about Link level flow control (or) discuss the principle of stop and wait
flow control algorithm & Sliding window flow control algorithm
8. Explain Error detection techniques in detail or Error correcting codes
9. Suppose we want to transmit the message 11001001 and protect it from errors using the
CRC polynomial x3+1. Use polynomial long division to determine the message that
should be transmitted
44

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DEPARTMENT OF COMPUTER SCIENCE AND ENGINEERING

2. What is network software?

3. Write the parameters used to measure network performance (May 2016)

6. Group the OSI layers by function?

8. Why are protocols needed?

9. What are the two interfaces provided by protocols?

10. Mention the different physical media?

11. Explain the two types of duplex?

13. What are the different encoding techniques?

14. What are the responsibilities of data link layer?

15. What are the ways to address the framing problem?

17. List out the available detection methods.

18. Write short notes on VRC.

19. Write short notes on LRC.

20. Write short notes on CRC.

21. Write short notes on CRC checker.

22. Define checksum.

23. What are the steps followed in checksum generator?

24. Write short notes on error correction? (NOV 2011)

25. What is the purpose of hamming code?

26. What is redundancy?

27. Define flow control? (NOV 2011)(May 2015) (May 2016)

28. Mention the categories of flow control?

29. What is a buffer?

30. What is the difference between a passive and an active hub?

32. List the Channelization Protocols

34. What do you meant by error control? (NOV 2010)(May 2015)

35. Define Error detection (NOV 2011)

38. Define network and computer network

39. List the components of data communication

40. Define bit stuffing (MAY 2011)

42. What are the functions of application layer? (MAY 2011)

43. Define a layer. (Nov/Dec 2013)

44. What do you mean by framing? (Nov/Dec 2013) (Nov/Dec 2014)

47. Define sliding window algorithm

48. Define character stuffing

50. Define hamming distance (Nov/Dec 2014)

1. Discuss the applications, advantages and disadvantages of networks

 Server faults stop applications being available

2. Discuss the categories (types) and connections of networks

Computer network design is dividing into three basic types such as

LAN (Local area networks)

LAN configuration consists of:

MAN (Metropolitan area network)

A metropolitan area network (MAN) is a large computer network that usually

3. Discuss about topology and its types

4. Explain the list of requirements to building a network

Links, Nodes, and Clouds

2. Cost-Effective Resource Sharing

Given a collection of nodes indirectly connected by a nesting of networks, it is possible

Multiplexing can be explained by analogy to a timesharing computer system, where a single

3. Support for Common Services

Diagram shows a pair of application-level processes communicating over a logical

Layering and Protocols

Fig: Service and peer interfaces.

Except at the hardware level, peer to peer communication is indirect.

Multiplexing and De-Multiplexing

OSI Architecture (NOV 2012) (MAY 2012)