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COMPUTER NETWORKS

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Title: COMPUTER NETWORKS


1
COMPUTER NETWORKS
  • UNIT -1

2
DATA COMMUNICATIONS
The term telecommunication means communication at
a distance. The word data refers to information
presented in whatever form is agreed upon by the
parties creating and using the data. Data
communications are the exchange of data between
two devices via some form of transmission medium
such as a wire cable.
  • Components of a data communications system
  • Data Flow

3
5 Basic Components of Communication
4
Simplified Communications Model - Diagram
5
Simplified Data Communications Model
6
Transmission Media Speed
  • BandwidthThe amount of data which can be
    transmitted on a medium over a fixed amount of
    time (second). It is measured on Bits per Second
    or Baud
  • Bits per Second (bps) A measure of transmission
    speed. The number of bits (0 0r 1) which can be
    transmitted in a second
  • Baud Rate Is a measure of how fast a change of
    state occurs (i.e. a change from 0 to 1)

7
Data Representation
  • Text
  • Email, articles, etc
  • Coding (Unicode, ASCII)
  • Numbers
  • Direct conversion
  • Images
  • Pixels, resolution
  • Audio
  • Continuous, signal conversion
  • Video
  • Movie, continuous/discrete

8
Data can be of two types analog and digital.
Analog data take on continuous values on some
interval. Typical examples of analog data are
voice and video. The data that are collected
from the real world with the help of transducers
are continuous-valued or analog in nature.
Digital data take on discrete values. Text or
character strings can be considered as examples
of digital data. Characters are represented by
suitable codes, e.g. ASCII code, where each
character is represented by a 7-bit code.
9
Data flow
  • Communication between two devices can be simplex,
    half-duplex, or full-duplex (as shown in next
    slide).
  • Simplex
  • The communication is unidirectional, as on a
    one-way street. An example of simplex is
    Television, or Radio.
  • Half-duplex
  • Each station can both transmit and receive, but
    not at the same time. It's like a one-lane bridge
    where two way traffic must give way in order to
    cross. Only one end transmits at a time, the
    other end receives.
  • Full-duplex
  • Both stations can transmit and receive
    simultaneously. It's like a one-lane bridge where
    two way traffic must give way in order to cross.

10
Transmission Direction
- simplex One direction only
11
Half Duplex Transmission
  • half duplex Both directions but only one
    direction at a time

12
Full Duplex Transmission
  • full duplex send and receive both directions at
    once

13
Data Flow
Figure Data flow (simplex, half-duplex, and
full-duplex)
14
Signaling It is an act of sending signal over
communication medium. Transmission
Communication of data by propagation and
processing is known as transmission. Analog
Signal The sound waves that your mouth produces
when you speak are analogue - the waves vary in a
smooth way. These waves can be converted into an
electrical signal by a microphone. This
electrical signal is also analogue
15
Packets
  • Transmissions are broken up into smaller units or
    data transmissions called packets

Example A data file is divided into packets. It
does not matter what the transmission is. It
could be Word document, a PowerPoint or an MP3.
This file has now been broken into four packets
PACKET
PACKET
PACKET
PACKET
16
DATA COMMUNICATIONS
  • What do we need?
  • Hardware
  • Software
  • Four fundamental characteristics
  • Delivery correct destination
  • Accuracy correct data
  • Timeliness fast enough
  • Jitter uneven delay

17
Centralized Data Processing
  • A Centralized Data Processing Centre may consists
    of
  • Centralized computers,
  • Centralized processing,
  • Centralized data,
  • Centralized control,
  • Centralized support staff
  • The Basic advantages
  • Economies of scale (equipment and personnel)
  • Lack of duplication
  • Ease in enforcing standards, security

18
Centralized Data Processing
19
Centralized Data Processing
20
Example of Distributed Data Processing
Facility is a WLAN that supports both data
traffic and VoIP. The WLAN connects to the
outside world via a satellite link that connects
to the Internet, to Carnivals private wide area
network (WAN), and to the public switched
telephone network (PSTN) in the US.
Carnival Valor Wireless LAN
21
Distributed Data Processing
22
Distributed Data Processing
  • Computers are dispersed throughout organization
    with some means of interconnection among them
  • Allows greater flexibility in meeting individual
    needs
  • More redundancy
  • More autonomy

23
Why is DDP Increasing?
  • Means and Motive
  • Dramatically reduction in hardware costs
  • Dramatically increased distributed processing
    capabilities (Hardware capabilities)
  • Dramatically increased need for new applications
    and shorter development times
  • Ability to share data across multiple servers

24
DDP Pros Cons
  • There are no one-size-fits-all solutions
  • Key issues
  • How does it affect end-users?
  • How does it affect management?
  • How does it affect productivity?
  • How does it affect bottom-line?

25
Benefits of DDP
  • Responsiveness
  • Availability
  • Correspondence to Organizational Patterns
  • Resource Sharing
  • Incremental Growth
  • Increased User Involvement Control
  • End-user Productivity
  • Distance location independence
  • Privacy and security
  • Vendor independence
  • Flexibility

26
Drawbacks of DDP
  • More difficulty test failure diagnosis
  • More components and dependence on communication
    means more points of failure
  • Incompatibility of components
  • Incompatibility of data
  • More complex management control
  • Difficulty controlling information resources
  • Suboptimal procurement
  • Duplication of effort

27
Reasons for DDP
  • Need for new applications
  • On large centralized systems, development can
    take years
  • On small distributed systems, development can be
    component-based and very fast
  • Need for short response time
  • Centralized systems result in contention among
    users and processes
  • Distributed systems provide dedicated resources

28
What is a Network?
  • A network is a number of computers and peripheral
    devices connected together so as to be able to
    communicate (i.e. transfer data)
  • Each device in a network is called a node.
  • Terminals are data entry points which can also
    display.

29
Classification Based on Transmission Technology
  • Computer networks can be broadly categorized into
    two types based on transmission technologies
  • Broadcast networks
  • Point-to-point networks
  • Broadcast Network have a single communication
    channel that is shared by all the machines on the
    network.

30
Example of a broadcast network based on satellite
communication
  • This system generally also allows possibility of
    addressing the packet to all destinations (all
    nodes on the network). When such a packet is
    transmitted and received by all the machines on
    the network. This mode of operation is known as
    Broadcast Mode.

31
Point-to-point Communication
  • The end devices that wish to communicate are
    called stations. The switching devices are called
    nodes. Some Nodes connect to other nodes and some
    to attached stations.
  • There may exist multiple paths between a
    source-destination pair for better network
    reliability.
  • The switching nodes are not concerned with the
    contents of data. Their purpose is to provide a
    switching facility that will move data from node
    to node until they reach the destination.

32
Types of connections point-to-point and
multipoint
PMP is usually used for establishing private
enterprise connectivity to offices in remote
locations,
33
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34
NETWORKS categorized by size
LAN a network that connects computers in a
limited geographical area. MAN a backbone
that connects LANs in a metropolitan area such
as a city and handles the bulk of
communications activity across that region.
WAN covers a large geographical area such as
a city or country. Communication channels
include telephone lines, Microwave, satellites,
etc.
35
An isolated LAN connecting 12 computers to a hub
in a closet
  • In a building, office, or school
  • Share hardware,
  • software and data
  • Client-server, hub
  • LAN normally uses
  • only one type of
  • transmission medium
  • Bus, ring and star
  • LAN typically used transmission technology
    consisting of single cable to which all machines
    are connected.
  • more reliable as compared to MAN and WAN.
  • Knowing this bound makes it possible to use
    certain kinds of design that would not otherwise
    be possible.
  • Simplifies network management.

36
WANs a switched WAN and a point-to-point WAN
WAN provides long-distance transmission of data,
voice, image and information over large
geographical areas that may comprise a country,
continent or even the whole world.
37
A heterogeneous network made of four WANs and two
LANs
A WAN that is wholly owned and used by a single
company is often referred to as enterprise
network.
38
Metropolitan Area Network
  • Interconnection of networks in a city into a
    single larger network (which may then also offer
    efficient connection to a wide area network).
  • It is also used to mean the interconnection of
    several local area networks by bridging them with
    backbone lines.
  • For example, a company can use a MAN to connect
    the LANs in all its offices in a city.

39
Topologies
  • 3 different types of LANS are
  • Ring
  • Bus
  • Star

40
Ring
Uses an empty data packet called a token and a
special protocol called token ring. Packets
travel around the ring in a clockwise direction.
Clients require an empty token to transmit
data. Advantages - no collisions because all
data travels in same direction. Disadvantages -
fails if an individual node in the network fails
41
BUS TOPOLOGY
A bus is a form of Ethernet. Nodes linked by a
cable known as the bus. Bus transmits in both
directions and uses CSMA/CD protocol
Advantages - Easy to set up and maintain failure of one node does not affect network Disadvantages Higher rate of data collision than with a bus network fails if there is any damage to the bus
42
Star Topology
All data is sent from one client to another
through the server. Advantages - If one client
fails no other clients are affected. Disadvantage
s - If central file server fails the network
fails.
43
Mesh Topology
  • Example telephone regional offices
  • Advantages
  • no traffic problems
  • Robust. No link failure no
  • effect on others.
  • Privacy security
  • Easy to detect the abnormal situation.
  • Disadvantages
  • Amount of cables, i/o ports
  • Efficiency and effectiveness
  • Space
  • Cost

1
2
4
5
3
A fully connected mesh topology (five devices)
44
Why a layered model?
  • Easier to teach communication process.
  • Speeds development, changes in one layer does not
    affect how the other levels works.
  • Standardization across manufactures.
  • Allows different hardware and software to work
    together.
  • Reduces complexity

45
The OSI Model
  • OSI Open Systems Interconnection".
  • OSI model was first introduced in 1984 by the
    International Organization for Standardization
    (ISO).
  • Outlines WHAT needs to be done to send data from
    one computer to another.
  • Not HOW it should be done.
  • Protocols stacks handle how data is prepared for
    transmittal (to be transmitted)
  • In the OSI model, The specification needed
  • are contained in 7 different layers that interact
    with each other.

46
What Each Layer Does
2
47
  • Gives end-user applications access to network
    resources
  • Where is it on my computer?
  • Workstation or Server Service in MS Windows

3
48
Application Layer
OSI Model
  • Application layer interacts with application
    programs and is the highest level of OSI model.
  • Application layer contains management functions
    to support distributed applications.
  • Examples of application layer are applications
    such as file transfer, electronic mail, remote
    login etc.

49
Presentation Layer
OSI Model
  • Presentation layer defines the format in which
    the data is to be exchanged between the two
    communicating entities.
  • Also handles data compression and data encryption
    (cryptography).

50
Session Layer
  • Allows applications to maintain an ongoing
    session
  • Where is it on my computer?
  • Workstation and Server Service (MS)
  • Windows Client for NetWare (NetWare)

3
51
Session Layer
OSI Model
  • Session layer provides mechanism for controlling
    the dialogue between the two end systems. It
    defines how to start, control and end
    conversations (called sessions) between
    applications.
  • This layer requests for a logical connection to
    be established on an end-users request.
  • Any necessary log-on or password validation is
    also handled by this layer.
  • Session layer is also responsible for terminating
    the connection.
  • This layer provides services like dialogue
    discipline which can be full duplex or half
    duplex.
  • Session layer can also provide check-pointing
    mechanism such that if a failure of some sort
    occurs between checkpoints, all data can be
    retransmitted from the last checkpoint.

52
Basic Functions for the Session Layer Protocols
  • Managing multiple sessions
  • A computer can establish multiple sessions with
    several other computers
  • e.g., session 1 exchanging information over the
    World Wide Web with www.yahoo.com
  • session 2 exchanging information over the World
    Wide Web with www.google.com
  • session 3 exchanging information over the World
    Wide Web with www.espn.com
  • Two computers can also establish multiple
    sessions,
  • e.g., function 1 exchanging information over the
    World Wide Web
  • function 2 exchanging information over the FTP
  • function 3 exchanging information over the email

53
Transport Layer
  • Provides reliable data delivery
  • Its the TCP in TCP/IP
  • Receives info from upper layers and segments it
    into packets
  • Can provide error detection and correction

3
54
Transport Layer
OSI Model
  • Purpose of this layer is to provide a reliable
    mechanism for the exchange of data between two
    processes in different computers.
  • Ensures that the data units are delivered error
    free.
  • Ensures that data units are delivered in
    sequence.
  • Ensures that there is no loss or duplication of
    data units.
  • Provides connectionless or connection oriented
    service.
  • Provides for the connection management.
  • Multiplex multiple connection over a single
    channel.

55
Network Layer
  • Provides network-wide addressing and a mechanism
    to move packets between networks (routing)
  • Responsibilities
  • Network addressing
  • Routing
  • Example
  • IP from TCP/IP

3
56
The Network Layer
csie.ndhu.edu.tw
cs.berkeley.edu
routing path selection
different network translation
congestion control
network accounting
57
The Network Layer
Network Layer Design Issues
Services Provided to the Transport Layer
58
Network Layer
OSI Model
  • Implements routing of frames (packets) through
    the network.
  • Defines the most optimum path the packet should
    take from the source to the destination
  • Defines logical addressing so that any endpoint
    can be identified.
  • Handles congestion in the network.
  • Facilitates interconnection between heterogeneous
    networks (Internetworking).
  • The network layer also defines how to fragment a
    packet into smaller packets to accommodate
    different media.

59
5. The Network Layer
5.1 Network Layer Design Issues
5.1.2 Internal Organization of the Network Layer
60
Data Link Layer
OSI Model
  • Data link layer attempts to provide reliable
    communication over the physical layer interface.
  • Breaks the outgoing data into frames and
    reassemble the received frames.
  • Create and detect frame boundaries.
  • Handle errors by implementing an acknowledgement
    and retransmission scheme.
  • Implement flow control.
  • Supports points-to-point as well as broadcast
    communication.
  • Supports simplex, half-duplex or full-duplex
    communication.

61
Physical Layer
  • Determines the specs for all physical components
  • Cabling
  • Interconnect methods (topology / devices)
  • Data encoding (bits to waves)
  • Electrical properties
  • Examples
  • Ethernet (IEEE 802.3)
  • Token Ring (IEEE 802.5)
  • Wireless (IEEE 802.11b)

3
62
Physical Layer
OSI Model
  • Provides physical interface for transmission of
    information.
  • Defines rules by which bits are passed from one
    system to another on a physical communication
    medium.
  • Covers all - mechanical, electrical, functional
    and procedural - aspects for physical
    communication.
  • Such characteristics as voltage levels, timing of
    voltage changes, physical data rates, maximum
    transmission distances, physical connectors, and
    other similar attributes are defined by physical
    layer specifications.
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