Introduction To Computer Network

Chanderprabhu Jain College of Higher Studies & School of Law
Plot No. OCF, Sector A-8, Narela, New Delhi – 110040
(Affiliated to Guru Gobind Singh Indraprastha University and Approved by Govt of NCT of Delhi & Bar Council of India)
Semester: THIRD Semester
Name of the Subject:
COMPUTER NETWORKS
Semester: THIRD Semester
Name of the Subject:
COMPUTER NETWORKS
INTRODUCTION TO
COMPUTER NETWORK

Networking
• Computer network A collection of computing
devices that are connected in various ways in order
to communicate and share resources
Usually, the connections between computers in a
network are made using physical wires or cables
However, some connections are wireless, using
radio waves or infrared signals

15-3
Networking
• The generic term node or host refers to
any device on a network
• Data transfer rate The speed with which
data is moved from one place on a
network to another
• Data transfer rate is a key issue in
computer networks

15-4
Networking
• Computer networks have opened up an
entire frontier in the world of computing
called the client/server model

15-5
Networking
• File server A computer that stores and
manages files for multiple users on a
network
• Web server A computer dedicated to
responding to requests (from the browser
client) for web pages

15-6
Types of Networks
• Local-area network (LAN) A network
that connects a relatively small number of
machines in a relatively close
geographical area

15-7
Types of Networks
• Various configurations, called topologies, have been used to administer LANs
– Ring topology A configuration that connects all nodes in a closed loop on which messages travel in
one direction
– Star topology A configuration that centers around one node to which all others are connected and
through which all messages are sent
– Bus topology All nodes are connected to a single communication line that carries messages in
both directions

Types of Networks
Various network topologies
15-10

15-9
Types of Networks
• Wide-area network (WAN) A network that
connects two or more local-area networks over a
potentially large geographic distance
Often one particular node on a LAN is set up to serve
as a gateway to handle all communication going
between that LAN and other networks
Communication between networks is called
internetworking
The Internet, as we know it today, is essentially the
ultimate wide-area network, spanning the entire globe

15-10
Types of Networks
• Metropolitan-area network (MAN) The
communication infrastructures that have
been developed in and around large cities

15-11
Internet Connections
• Internet backbone A set of high-speed
networks that carry Internet traffic
These networks are provided by
companies such as AT&T, GTE, and IBM
• Internet service provider (ISP) A
company that provides other companies
or individuals with access to the Internet

15-12
• There are various technologies available that you can
use to connect a home computer to the Internet
– A phone modem converts computer data into an analog
audio signal for transfer over a telephone line, and then a
modem at the destination converts it back again into data
– A digital subscriber line (DSL) uses regular copper phone
lines to transfer digital data to and from the phone company’s
central office
– A cable modem uses the same line that your cable TV
signals come in on to transfer the data back and forth

15-13
• Broadband A connection in which transfer
speeds are faster than 128 bits per second
– DSL connections and cable modems are broadband
connections
– The speed for downloads (getting data from the
Internet to your home computer) may not be the
same as uploads (sending data from your home
computer to the Internet)

Packet Switching
• To improve the efficiency of transferring information over
a shared communication line, messages are divided into
fixed-sized, numbered packets
• Network devices called routers are used to direct
packets between networks
Figure 15.4
Messages
sent by
packet
switching
15-18

15-15
Open Systems
• Proprietary system A system that uses technologies kept
private by a particular commercial vendor
One system couldn’t communicate with another, leading to the
need for
• Interoperability The ability of software and hardware on
multiple machines and from multiple commercial vendors to
communicate
Leading to
• Open systems Systems based on a common model of
network architecture and a suite of protocols used in its
implementation

15-16
Open Systems
• The International
Organization for
Standardization (ISO)
established the Open
Systems
Interconnection (OSI)
Reference Model
• Each layer deals with a
particular aspect of
network communication
Figure 15.5 The layers of the OSI Reference Model

15-17
Network Protocols
• Network protocols are layered such that
each one relies on the protocols that
underlie it
• Sometimes referred to as a protocol
stack
Figure 15.6 Layering of key network protocols

15-18
TCP/IP
• TCP stands for Transmission Control Protocol
TCP software breaks messages into packets,
hands them off to the IP software for delivery,
and then orders and reassembles the packets
at their destination
• IP stands for Internet Protocol
IP software deals with the routing of packets
through the maze of interconnected networks
to their final destination

15-19
TCP/IP (cont.)
• UDP stands for User Datagram Protocol
– It is an alternative to TCP
– The main difference is that TCP is highly
reliable, at the cost of decreased
performance, while UDP is less reliable, but
generally faster

Multiplexing

Frequency Division Multiplexing
• FDM
• Useful bandwidth of medium exceeds required
bandwidth of channel
• Each signal is modulated to a different carrier
frequency
• Carrier frequencies separated so signals do not
overlap (guard bands)
• e.g. broadcast radio
• Channel allocated even if no dataChanderprabhu Jain College of Higher Studies & School of Law

Frequency Division Multiplexing
Diagram

FDM System

Wavelength Division
Multiplexing
• Multiple beams of light at different frequency
• Carried by optical fiber
• A form of FDM
• Each color of light (wavelength) carries separate data channel
• 1997 Bell Labs
– 100 beams
– Each at 10 Gbps
– Giving 1 terabit per second (Tbps)
• Commercial systems of 160 channels of 10 Gbps now available
• Lab systems (Alcatel) 256 channels at 39.8 Gbps each
– 10.1 Tbps
– Over 100km

WDM Operation
• Same general architecture as other FDM
• Number of sources generating laser beams at different frequencies
• Multiplexer consolidates sources for transmission over single fiber
• Optical amplifiers amplify all wavelengths
– Typically tens of km apart
• Demux separates channels at the destination
• Mostly 1550nm wavelength range
• Was 200MHz per channel
• Now 50GHz

Synchronous Time Division
Multiplexing
• Data rate of medium exceeds data rate of digital signal to
be transmitted
• Multiple digital signals interleaved in time
• May be at bit level of blocks
• Time slots preassigned to sources and fixed
• Time slots allocated even if no data
• Time slots do not have to be evenly distributed amongst
sources

Time Division Multiplexing

TDM System

TDM Link Control
• No headers and trailers
• Data link control protocols not needed
• Flow control
– Data rate of multiplexed line is fixed
– If one channel receiver can not receive data, the others must carry
on
– The corresponding source must be quenched
– This leaves empty slots
• Error control
– Errors are detected and handled by individual channel systems

Data Link Control on TDM

Framing
• No flag or SYNC characters bracketing TDM
frames
• Must provide synchronizing mechanism
• Added digit framing
– One control bit added to each TDM frame
• Looks like another channel - “control channel”
– Identifiable bit pattern used on control channel
– e.g. alternating 01010101…unlikely on a data channel
– Can compare incoming bit patterns on each channel
with sync pattern

Pulse Stuffing
• Problem - Synchronizing data sources
• Clocks in different sources drifting
• Data rates from different sources not related by
simple rational number
• Solution - Pulse Stuffing
– Outgoing data rate (excluding framing bits) higher than
sum of incoming rates
– Stuff extra dummy bits or pulses into each incoming signal
until it matches local clock
– Stuffed pulses inserted at fixed locations in frame and
removed at demultiplexerChanderprabhu Jain College of Higher Studies & School of Law

Statistical TDM Frame Formats

Performance
• Output data rate less than aggregate input
rates
• May cause problems during peak periods
– Buffer inputs
– Keep buffer size to minimum to reduce delay

35
Flow control
refers to a set of procedures used to restrict the amount of data that
the sender can send before waiting for acknowledgment.
Error control
in the data link layer is based on automatic repeat request, which
is the retransmission of data

36
Line discipline
• It determines which device can send and when it
can send.
• It oversees the establishment of links and the right
of a particular device to transmit at a given time.

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Enquiry / Acknowledgment
ENQ/ACK

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• ENQ/ACK
It is used in systems where there is no question
of the wrong receiver getting the transmission
 i.e. when there is a dedicated link between two
devices so that the only device capable of
receiving data is the intended one.
The initiator sends ENQ
The receiver sends ACK The receiver sends NAK No response
Repeat three times
Disconnect
Repeat three timesDisconnect
and start again at
another time
Send data till EOT
Disconnect

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ENQ/ACK

40
Poll / SelectPoll / Select
• It works with topologies where one device is designated as a
primary stationprimary station and the other devices are secondary stationssecondary stations
and all are using a single transmission line.
• All data communication must be made through the primary device
• If the primary wants to receiveprimary wants to receive data, it asks the secondaries if
they have anything to send; this function is called polling.polling.
• If the primary wants to sendprimary wants to send data, it tells the target secondary to
get ready to receive; this function is called selectingselecting

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Multipoint Discipline

42
AddressesAddresses
• We need addressing for multipoint transmission.
• Each secondary device has an address that
differentiates it from the others.
• If the transmission comes from the primary
device, The address indicates the recipient of
the data.
• If the transmission comes from a secondary
device, the address indicates the originator of
the data.

43
PollPoll
• It is used by the primary device to solicit
transmissions from the secondary devices.
• There are two possibilities for terminating the exchange
 The secondary sends all its data and sends (EOT) frame
 The primary “Time’s up”

44
SelectSelect
• It is used whenever the primary device has
something to send.
• Any frame on the link is available to every
device.
• When a device recognizes its own address,
it opens the frame and reads the data.

45
 It coordinates the amount of data that can be sent before
  receiving acknowledgment.
 It provides the receiver’s acknowledgment of frames
   received corrupted.
Flow controlFlow control

46
Stop and Wait

47
Sliding Window
Sender Sliding
Window
Receiver Sliding Window

48
Sliding Window Example

49
Error Control

50
Normal operation
In StopandWait ARQ, numbering frames prevents the
retaining of duplicate frames.

51
StopandWait ARQ, lost frame

52
StopandWait ARQ, lost ACK frame

53
Damaged Frame

54
Damaged Frame

55
Lost Frame

56
Lost ACK
Figure 1021

57
Selective Reject
Figure 1022

58
Data Link Protocols
• Is a set of specifications used to implement
the data link layer
• Data link protocols differ by message
delineation, frame length, and frame field
structure.
• Another fundamental difference is between
asynchronous and synchronous
transmission data link protocols.

59
AAsynchronous Protocols
• In aasynchronous transmission (sometimes called start-stop transmission),
each character is sent independently.
• The transmission sequence begins with
– a start bit
– next the character is sent
– then the parity bit
– and finally a stop bit are sent.
• The start bit is usually a 0 and the stopstop bit a 1.
• Between transmissions (called “idle time”), a series of stop bits are
sent.
• When a new character is sent, the start bit is used by the receiver for
synchronization.

60
Asynchronous Protocols
• Protocols that belong to asynchronous protocols
• XMODEM
• YMODEM
• ZMODEM
• BLAST
• Kermit

61
Synchronous Protocols
• In synchronous transmission
– data is sent in a large block called a frame
• Synchronous transmission is used on both
– point-to-point
– multipoint circuits
• In multipoint circuits, addressing information needs to be
included in the frame.
• Synchronous packets sometimes begin and end with a
series of synchronization (SYN) characters that are used to
help the receiver recognize incoming data.

62
Synchronous Protocols
• Synchronous transmission protocols can be:
– character-oriented:
• Also known as byte-oriented protocols
• Interpret a transmission frame as a succession of
characters
– bit-oriented:
• Interpret a transmission frame as a succession of individual
bits
• Control information in a bit-oriented protocol can be one
or multiple bits depending on the information embodied in
the pattern

63
Bit-oriented protocols

64
HDLCHDLC :: High-level Data Link ControlHigh-level Data Link Control
It is a bit-oriented data link protocol
Designed to support both half duplex and full duplex
communication over point-to-point and multipoint links.
It implements the ARQ mechanisms.
The HDLC protocol embeds information in a data frame
that allows devices to control data flow and correct errors

65
HDLCHDLC :: High-level Data Link ControlHigh-level Data Link Control
• In 1979, the ISO made HDLC the standard as a Bit-
oriented control protocol
• The HDLC provides a transparent transmission (‫فاف‬‫ف‬ ‫)ش‬
service at the data link layer of the OSI
• The users of the HDLC service provides PDUs which
are encapsulated to form data link layer frames. These
frames are separated by HDLC "flags" and are modified
by "zero bit insertion" to guarantee transparency

66
 Each piece of data is encapsulated in an HDLC frame by
adding a trailer and a header.
 The headerThe header contains an HDLC address and an HDLC control
field.
 The trailerThe trailer is found at the end of the frame, and contains a
(CRC) which detects any errors which may occur during
transmission.
 The frames are separated by HDLC flag sequences which are
transmitted between each frame and whenever there is no data
to be transmitted.
HDLC : High-level Data Link Control

67
HDLC frame typesHDLC frame types

68
HDLCHDLC Frame FieldsFrame Fields
Flag fieldFlag field
– is 8 bits of a fixed pattern (0111 1110).
– There is one flag at the beginning and one at the end frame.
– The ending flag of one Frame can be used as the beginning flag of the
next frame.
– To guarantee that the flag does not appear anywhere else in the frame
– HDLC uses a process called Bit Stuffing.
– Every time a sender wants to transmit a bit sequence having more than
6 consecutive 1’s, it inserts 1 redundant 0 after the 5th 1
Exceptions:
• When the bit sequence is really a flag.
• when transmission is being aborted.
• when the channel is being put into idle.

Switching Techniques
In large networks there might be multiple paths linking sender
and receiver. Information may be switched as it travels through
various communication channels. There are three typical
switching techniques available for digital traffic.
•    Circuit Switching
•    Message Switching
•    Packet Switching

Circuit Switching
• Circuit switching is a technique that directly connects the
sender and the receiver in an unbroken path.
• Telephone switching equipment, for example, establishes a
path that connects the caller's telephone to the receiver's
telephone by making a physical connection.
• With this type of switching technique, once a connection is
established, a dedicated path exists between both ends
until the connection is terminated.
• Routing decisions must be made when the circuit is first
established, but there are no decisions made after that time.

Circuit Switching
• Circuit switching in a network operates almost the same
way as the telephone system works.
• A complete end-to-end path must exist before
communication can take place.
• The computer initiating the data transfer must ask for a
connection to the destination.
• Once the connection has been initiated and completed to
the destination device, the destination device must
acknowledge that it is ready and willing to carry on a
transfer.

Circuit switching
Advantages:
• The communication channel (once established) is dedicated.
Disadvantages:
•  Possible long wait to establish a connection, (10 seconds,
   more on long distance or international calls.) during which
   no data can be transmitted.
•  More expensive than any other switching techniques,
   because a dedicated path is required for each connection.
•  Inefficient use of the communication channel, because the
   channel is not used when the connected systems are not
   using it.

Message Switching
• With message switching there is no need to establish a
dedicated path between two stations.
• When a station sends a message, the destination
address is appended to the message.
• The message is then transmitted through the network,
in its entirety, from node to node.
• Each node receives the entire message, stores it in its
entirety on disk, and then transmits the message to the
next node.
• This type of network is called a store-and-forward
network.

Message Switching
A messageswitching node is typically a generalpurpose computer. The
device needs sufficient secondarystorage capacity to store the incoming
messages, which could be long. A time delay is introduced using this type
of scheme due to store andforward time, plus the time required to find
the next node in the transmission path.

Message Switching
Advantages:
•   Channel efficiency can be greater compared to circuit
    switched systems, because more devices are sharing the
    channel.
•  Traffic congestion can be reduced, because messages may be
   temporarily stored in route.
•  Message priorities can be established due to storeandforward
   technique.
•  Message broadcasting can be achieved with the use of
   broadcast address appended in the message.

Message Switching
Disadvantages
•   Message switching is not compatible with interactive
    applications.
•   Storeandforward devices are expensive, because they
    must have large disks to hold potentially long messages.

Packet Switching
•  Packet switching can be seen as a solution that tries to combine the

   advantages of message and circuit switching and to minimize the
   disadvantages of both.
•  There are two methods of packet switching: Datagram
   and virtual circuit.

Packet Switching
•   In both packet switching methods, a message is broken into
     small parts, called packets.
•   Each packet is tagged with appropriate source and destination
    addresses.
•   Since packets have a strictly defined maximum length, they
    can be stored in main memory instead of disk, therefore access
    delay and cost are minimized.
•   Also the transmission speeds, between nodes, are optimized.
•   With current technology, packets are generally accepted onto
    the network on a firstcome, firstserved basis. If the network
    becomes overloaded, packets are delayed or discarded
    (``dropped'').

Packet size
• The size of the packet can vary from 180 bits, the size for the
Datakit® virtual circuit switch designed by Bell Labs for
communications and business applications; to 1,024 or
2,048 bits for the 1PSS® switch, also designed by Bell Labs
for public data networking; to 53 bytes for ATM switching,
such as Lucent Technologies' packet switches.

Packet switching
• In packet switching, the analog signal from your phone is
converted into a digital data stream. That series of digital bits is
then divided into relatively tiny clusters of bits, called packets.
Each packet has at its beginning the digital address -- a long
number -- to which it is being sent. The system blasts out all those
tiny packets, as fast as it can, and they travel across the nation's
digital backbone systems to their destination: the telephone, or
rather the telephone system, of the person you're calling.
• They do not necessarily travel together; they do not travel
sequentially. They don't even all travel via the same route. But
eventually they arrive at the right point -- that digital address
added to the front of each string of digital data -- and at their
destination are reassembled into the correct order, then converted
to analog form, so your friend can understand what you're saying.

Packet Switching: Datagram
•  Datagram packet switching is similar to message switching in
   that each packet is a selfcontained unit with complete
   addressing information attached.
•  This fact allows packets to take a variety of possible paths
    through the network.
•  So the packets, each with the same destination address, do not
   follow the same route, and they may arrive out of sequence at
   the exit point node (or the destination).
• Reordering is done at the destination point based on the
  sequence number of the packets.
• It is possible for a packet to be destroyed if one of the nodes on
  its way is crashed momentarily. Thus all its queued packets may
  be lost.

Packet Switching:Virtual Circuit
•   In the virtual circuit approach, a preplanned route is established
    before any data packets are sent.
•  A logical connection is established when
      a sender send a "call  request packet" to the receiver and
      the receiver send back an acknowledge packet "call accepted
         packet" to the sender if the  receiver agrees on conversational
         parameters.
•  The conversational parameters can be maximum packet sizes,
   path to be taken, and other variables necessary to establish and
   maintain the conversation.
• Virtual circuits imply acknowledgements, flow control, and error
  control, so virtual circuits are reliable.
• That is, they have the capability to inform upperprotocol layers
  if a transmission problem occurs.

Packet Switching:Virtual Circuit
•   In virtual circuit, the route between stations does not mean that
    this is a dedicated path, as in circuit switching.
•  A packet is still buffered at each node and queued for output over
   a line.
•  The difference between virtual circuit and datagram approaches:
  With virtual circuit, the node does not need to make a routing
     decision for each packet.
  It is made only once for all packets using that virtual circuit.

Packet Switching: Virtual Circuit
VC's offer guarantees that
 the packets sent arrive in the order sent
 with no duplicates or omissions
 with no errors (with high probability)
regardless of how they are implemented internally.

Advantages of packet switching
Advantages:
•  Packet switching is cost effective, because switching
   devices do not need massive amount of secondary
   storage.
•  Packet switching offers improved delay characteristics,
   because there are no long messages in the queue
   (maximum packet size is fixed).
•  Packet can be rerouted if there is any problem, such as,

   busy or  disabled links.
•  The advantage of packet switching is that many
   network users can share the same channel at the same
   time. Packet switching can maximize link efficiency by
   making optimal use of link bandwidth. Chanderprabhu Jain College of Higher Studies & School of Law

Disadvantages of packet switching
Disadvantages:
•   Protocols for packet switching are typically more complex.
•   It can add some initial costs in implementation.
•   If packet is lost, sender needs to retransmit the data.
•   Another disadvantage is that packetswitched systems still
    can’t deliver the same quality as dedicated circuits in
    applications requiring very little delay like voice
    conversations or moving images.


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