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DESCRIPTION
Digital computer communicationTRANSCRIPT
Data and Communication network (MCA SEMIII)
Prepared By : Sangita Oswal( Lecturer VESIT) Brought to you by :Md. Azaz 1
Data communication network
Data -Information
Communication Sharing Information
Communicate (Share information)
1. Local (face to face)
2. Remote (separated by geographical distance)
Computer network
Definition: large no. of separate but interconnected computers (nodes) is call a computer
network
Components of data communication:
Message :It is the information or data to be communicated . It consist of text,sound or
video
Sender : The sender is the device that sends the data message.
Receiver :The receiver is the device that receive the message.
Medium ; The physical path by which the message travel from sender to receiver .it
could be a twisted pair, co-axial cable fiber optics etc
Protocol: is a set of rules that governs data communication. Without protocol two device
may be connected but not communicating.
Data and Communication network (MCA SEMIII)
Prepared By : Sangita Oswal( Lecturer VESIT) Brought to you by :Md. Azaz 2
Modes of communication Communication between two device can be simplex ,half-duplex or full –duplex
a. Simplex :communication is unidirectional as on a one way street . only one of the
two device on a link can transmit the other can only receive e.g k/b , monitor
b. Half duplex :each station can both transmit and receive but not at same time .if
one device is sending other as to receive e.g. wakie- takie
c. Full-Duplex :both station can transmit and receive simultaneously .It is like a two
way street traffic flowing in both direction at the same time .e.g telephone line .
Topologies
It is a geometric representation of the relation of all the links and linking
device(nodes) to one another .
1.MESH : every device as a dedicated point –to – point link to every other device
If n is no. of nodes Links = n(n-1)/2.
Data and Communication network (MCA SEMIII)
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Advantage :
1. use of dedicated links eliminate traffic problem as link is not shared .
2. Robust : if one link becomes unusable ,it does not affect the entire system
3. privacy or security : due to dedicated link only intended receiver sees it other user
cannot access to message
4. Fault identification and isolation easy
Disadvantage :
1. amount of cabling and no. of I/O port required
2. installation and reconnection is difficult
STAR : each device as a dedicate point to point link only to a central controller
called HUB.
Advantage :
1. Less cabling needed compare to mesh
2. adding and removing any node is easy
3. roubtness : If one link fails only that is affected
Data and Communication network (MCA SEMIII)
Prepared By : Sangita Oswal( Lecturer VESIT) Brought to you by :Md. Azaz 4
4. fault identification and isolation easy
BUS : one long cable act as a backbone and all nodes are connected to it by a drop
line and
tap.
Advantage :
1. easy installation
2. less cabling than mesh and star
Disadvantage :
1. it is difficult to add new device b’cos signal reflection at tap cause degradation in
quality .(it is controlled by limiting the no. of device and spacing between them)
2. fault or break stops transmission ,even if two device on the same side of problem
RING :each device as a dedicated point to point connection only with two device on
either side of it.
A signal is passed along the ring in one direction from device to device until it reach
destination .
Advantage:
1. easy to install and reconfigure ,each device is linked only to its immediate
neighbors
Disadvantage :
1. unidirectional traffic can be disadvantage ,it could be solved by using dual ring.
Analog data /Digital data :
Analog data is produce by human beings when we speak .This analog data
is captured in microphone converted to analog signal and send over longer distance in
case of telephone system.
Digital data is the data in computer represented by 0’s and 1’s .It is converted into digital
signal and transferred between computer memory , cpu etc.
Modulation (analog to analog(radio system)) : Analog-to-analog conversion is the
representation of analog information by an analog signal. One may ask why we need to
modulate an analog signal; it is already analog. Modulation is needed if the medium is
bandpass in nature or if only a bandpass channel is available to us.
Data and Communication network (MCA SEMIII)
Prepared By : Sangita Oswal( Lecturer VESIT) Brought to you by :Md. Azaz 5
Amplitude Modulation (AM)
Carrier signal is modulated so that the amplitude varies with the changing amplitude of
the modulating signal ,frequency and phase remains constant .
B.W=2*B.W of modulating signal.
Carrier frequency in range of 530 to 1700khz
Frequency modulation(FM)
Frequency of carrier is modulated according to changing volatsge level of modulating
signal , amplitude and phase remains constant.
B.W=10*B.W of modulating signal
Data and Communication network (MCA SEMIII)
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Carrier frequency anywhere in range 88 to 108 mhz.
Phase modulation Phase of carrier is modulated according to the changing volatsge level of modulating
signal,amplitude and frequency remains the same .
Definition
Carrier signal : in analog transmission the sending device produce a high frequency
signal that act as a basic for information signal called carrier signal
Modulation : digital information modulate the carrier signal by modifying one or more
of its charactertics (A,F,P).this kind of modification is called modulation and information
signal is called modulating signal..
Sampling (Analog to digital)
PAM: (pulse amplitude modulation) this techniques takes analog signal samples it and
generate a series a pulse based on result of sampling(measuring ampltitude of signal at
equal interval).
In PAM, the original signal is sampled at equal interval using technique of sample and
hold i.e. at a moment the signal level is read and held briefly
PAM is not useful b’cos it translate the original waveform to series of pulse but these
pulses are still amplitude.
It is a foundation for PCM
PCM (pulse code modulation) ; PCM modifies the pulse created by PAM to create a
complete digital signal.To do so it first quantize the PAM pulses which is a method of
assigning integral values in a specific range to sampled instance.
Then assign sign and magnitude to quantized sample. Each value is translated into 7 bit
binary equivalent and 8th
bit indicate sign. The binary digit are then transformed to a
digital signal using line coding techniques .
Data and Communication network (MCA SEMIII)
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Transmission Impairment:
Signal travel through transmission media which are not perfect. The imperfection
cause impairment in the signal this means signal at beginning and end the medium are not
the same.
Attenuation: means loss of energy. when a signal travel through a medium it
loses some of its energy so that it can overcome the resistance of the medium ,that is why
a wire carrying electrical signal gets warm (hot).To compensate for this loss amplifiers
are used to amplify the signal .
Distortion: means that the signal changes its form or shape. Distortion occurs in a
composite signal made of different frequencies.
Each signal component has its own propagation speed through a medium and therefore its
own delay in arriving at the final
destination.
Data and Communication network (MCA SEMIII)
Prepared By : Sangita Oswal( Lecturer VESIT) Brought to you by :Md. Azaz 8
Noise:
Several types of noise such as thermal noise, induced noise crosstalk and impulse noise
may corrupt the signal.
Thermal noise: is a random motion of electron in a wire which create an extra signal not
originally send with transmitter
Induced noise: comes from source like motor or application
Crosstalk is the effect of one wire on the other
Impulse noise is a spike (a signal with high energy in a very short period of time) that
comes from power lines , lightening
Multiplexing: Whenever the bandwidth of a medium linking two devices is greater than the bandwidth
needs of the devices, the link can be shared. Multiplexing is the set of techniques that
allows the simultaneous transmission of multiple signals across a single data link. As data
and telecommunications use increases, so does traffic.
Data and Communication network (MCA SEMIII)
Prepared By : Sangita Oswal( Lecturer VESIT) Brought to you by :Md. Azaz 9
FDM
FDM is an analog multiplexing technique that combines analog signals which can be
applied when B/W of a link is greater than combined B/W of signal to be transmitted.
Multiplexing Process: Each telephone generate a signal, inside multiplexer these similar
signal are modulated onto different carrier frequency (f1,f2,f3).
The resulting modulated signal is then combined into one composite signal then is send
over a media link that has enough b/w to accommodate it.
Demultiplexing process: it uses a series of filter to decompose the multiplexed signal into
its constitute signal
The signal are the passed to demodulator that desperate them from their carrier and pass
to waiting receiver.
Data and Communication network (MCA SEMIII)
Prepared By : Sangita Oswal( Lecturer VESIT) Brought to you by :Md. Azaz 10
WDM
WDM is an analog multiplexing technique to combine optical signals.
Optical signal as a high data rate. WDM is conceptually similar to FDM only the fact that
it combines optical signal and use fiber optics cable for transmission
TDM is a digital process that allows several connection to share the high B/W of a link
instead of sharing a portion of B/W as in FDM, time is shared.
Each Connection occupies a portion of time in link .link is sectioned on time
rather than frequency.
Data and Communication network (MCA SEMIII)
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Switching
Data and Communication network (MCA SEMIII)
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Circuit switching 1.Seeks a physical path all the way from sender to receiver ,this is called circuit
switching
2. when a call passes a physical connection is established between lines on which
call came in and one of the o/p line
3. It needs an end to end path before any data can be sent. e,g telephone
Message switching: 1. No physical path is established in advance between sender and receiver
2. Instead when sender has a block of data to be sent ,it is stored in first
switching office and than forwarded later ,one hop at a time
3. Each block received is inspected for error and then retransmitted called store
and forward network e.g telegraph
Packet switching: 1. With message switching there is no limit at all on block size which means router
have disk to buffer long blocks
2. Packet switching network places a tight upper bound on block size allowing
packet to be buffered in router main memory instead of on disk
3. By making sure that no user can monopolize any transmission line very long it is
suited for interactive traffic
4. Also the first packet of a multi-packet can be forwarded before second one has
fully arrived reducing delay and increasing throughput.
Compare circuit and packet switching
Timings Diagram:
Data and Communication network (MCA SEMIII)
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A comparison of circuit switched and packet-switchednetworks.
Reference Model
OSI Model (open system interconnection) The principle that were applied to arrive at the seven layer are
1. layer created where a different abstraction needed
2. each layer should perform well defined function
3. layer boundary should be chosen to minimize the information flow across
interface
4. no. of layer should be large enough to distinct function need ,not throw together in
same layer
Data and Communication network (MCA SEMIII)
Prepared By : Sangita Oswal( Lecturer VESIT) Brought to you by :Md. Azaz 14
Physical Layer: is responsible for sending bits from one computer to another
(not concern with meaning of it)
Function of physical layer:-
1. Define transmission rate i.e. the no. of bit transmitted per sec
2. It deals with physical topologies, transmission mode and multiplexing
3. It also deals with connection type multipoint or point to point connection.
Data link layer: It is responsible for node to node delivery of data. It accept
packet from the n/w layer and forms frame and give it to physical layer.
Function includes :
1. Framing – it divides the stream of bit received from n/w layer into manageable
data unit called frame .
2. Flow control – by buffering the extra bit
3. Error control – is achieved by adding trailer at end of frame
DLL divided into LLC and MAC sub layer
LLC (logic link control) establish and maintain links between devices
MAC (Medium access control) control the way multiple device share the channel.
Network layer: main function is to delivery packet from source to destination
across multiple links (if only one link no need of network layer)
Its function includes:
1. Translate logical network address into physical address.
2. Quality of service by deciding priority of message and route a message will take if
there are several ways
3. It is concern with circuit, message and packet switching
Data and Communication network (MCA SEMIII)
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4. Transport layer – It is responsible for source to destination delivery of the
entire message, it ensure that the wide message arrive intact and in order .
Functions are:
1. divide each message into packet at source and reassemble them at destination
2. Header includes SAP (service access point) to deliver a specific process at source
to a specific process at destination.
Session layer: The layer establishes maintain and synchronize the interaction between the
communicating system
Its function includes:
1. allow two system to enter into a dialog
2. To add check point i.e. synchronization point into stream of data.
Presentation layer: unlike lower layer which are mostly concern with moving
bits around the presentation layer is concern with syntax and semantics of the
information transmitted
Function includes
1. it translate data between the format the network requires and computer excepts
2. Encryption at transmitter and decryption at receiver.
3. data compression to reduce B/W of data to be transmitted
Application layer – contains variety of protocol that are commonly needed by
user like HTTP, SMTP, FTP, DNS etc
TCP/IP Reference model: TCP/IP reference model is used in
internet.
Data and Communication network (MCA SEMIII)
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1. Transport Layer: layer above the internet layer in TCP/IP is called
transport layer . It is design to allow peer entities on the source and destination to
carry conversation just like OSI Transport layer. Two protocol define here are
TCP(Transmission control protocol) and UDP (user datagram
protocol)
TCP is reliable connection oriented protocol that allow byte stream
originating on one machine to delivered without error on another machine .it
fragments the incoming byte stream into discrete message and pass to internet
layer
UDP is unreliable and connectionless protocol
2. Internet Layer :this layer is the base which hold whole architecture
together .its job is to permit host to inject packet into network and have them
travel independently to the destination .they may even arrive in different order
than they were sent ,in that case it is job of higher layer to rearrange in order .
It defines a protocol IP its job is to delivery IP packets .TCP/IP Internet
layer is same as network Layer in OSI
APPLICATION LAYER: TCP/IP Application Layer includes OSI
Presentation, session and application layer . this include all the processes that involve
user interaction and also presentation of data.
In TCP/IP the term socket and port is used to describe path over which application
communicate. protocol used are SMTP,HTTP,FTP etc…
HOST TO NETWORK LAYER: Data link layer and physical layer of OSI
are used rather than defining its own.
Data and Communication network (MCA SEMIII)
Prepared By : Sangita Oswal( Lecturer VESIT) Brought to you by :Md. Azaz 17
PHYSICAL LAYER
Purpose of the physical layer is to transport raw bit of data from one machine to
another.
Various media can be used for transmission
Guided media / unguided media
Guided media
Twisted pair –
1. Two insulated copper wire ,typically about 1mm thick are used
2. Twisted pair can run several km without amplification ,but for longer distance
amplification is needed it is generally used in telephone .
3. It can be used for transmitting either analog or digital data.
4. Bandwidth depend upon thickness of wire and distance traveled
5. Classified as category 3 and category5
6. category3 : two insulated copper wire gently twisted together ,4 such pair are
grouped together in a plastic sheath to protect wire and keep them
Data and Communication network (MCA SEMIII)
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together.
7. category5: more twist per centimeter, it have less crosstalk and better quality of
signal over longer distance
Co-axial cable:
1. Better shielding than twisted pair ,so can span longer distance at higher speed
2. The co-axial cable consist of stiff copper wire as the core surrounded by
insulating material .the insulator is enchased by a cylindrical conductor ,which in
turn is covered by plastic sheath.
3. The construction and shielding of a co-axial cable give a good combination of
high B/W and excellent noise immunity.
4. use in MAN and cable television
Fiber optics: the fiber optics cable is made of glass or plastic and transmit signal
in form of light.
Data and Communication network (MCA SEMIII)
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Light travel in straight line as long as it is moving through a single uniform substance
If ray of light traveling through one substance suddenly enter another (more or less
dense) the ray changes direction.
If angel of incidence is less than the critical angel the ray refracts and moves closer to
the surface.
If equal ray bends along surface
If greater the ray reflects make turn and travel again in denser substance
Construction of fiber optics
Propagation modes
Data and Communication network (MCA SEMIII)
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UnGuided Media
Data signal flow through air
Not guided or bounded to a channel
Classified as per wave propagation
Electromagnetic Spectrum
When electron move electromagnetic wave is created
Wave travel through space
No. of oscillation /sec=frequency (f hz)
Distance between two consecutive maxima or minima is wave length
Speed is usually speed of light
C=3*108 m.sec
Data and Communication network (MCA SEMIII)
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Radio Microwave infrared and visible light
Portion of spectrum is used for transmitting information by modulating (A,F,P)
of wave .
Ultra Violet Ray X-Ray Gamma Ray:
Are better as they have frequency
Hard to produce and modulate
Not travel well through building
Dangerous to living things
Radio Transmission:
Easy to Generate
Travel long distance
Penetrate building easily
Omni directional: travel in all direction from source
Properties of radio wave are frequency dependent:
At low frequency it pass through obstacles but power falls sharply with distance
At high frequency radio wave travel in straight line and bounce off obstacles.
There are 3 types of RF frequency propagation
Ground Wave :
Fallows the curvature nature of the earth
Carrier frequency up to 2 MHZ.
Line of sight :
Transmit exactly in line of sight
Receiver station in view of transmitter
Ionospheric propogation :
Bounces off the earth Ionosphere layer in upper atmosphere
Data and Communication network (MCA SEMIII)
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Frequency 30-85 MHz
Because it depend on earth ionosphere it change with weather and time of day
(a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth.
(b) In the HF band, they bounce off the ionosphere
Microwave Transmission:
Above 100 MHz the wave travel in straight line and narrowly focused
Concentrating all energy in small beam give higher signal to noise ratio
Transmitter and receiver aligned to each other
Multiple Transmitters lined up to communicate with multiple receivers w/o
interfacing and following minimum spacing rule
But for longer distance repeater needed
Even though the beam may be well focused there is still divergence in space
Some wave are refracted and they take a longer to arrive at destination then direct
wave
Delayed wave is out –o f- phase with direct wave called multi path fading
Microwave is in expensive
Putting up two towers and putting antennas on each one.
Data and Communication network (MCA SEMIII)
Prepared By : Sangita Oswal( Lecturer VESIT) Brought to you by :Md. Azaz 23
DATA LINK LAYER
The data link layer functions include
1. providing a well-defined service interface to network layer
2. delaying with transmission error
3. regulating the flow of data so that slow receiver are not swapped by fast sender
Data link layer encapsulate data into frames
Relationship between packets and frames.
Service provided to network layer:
1. Unacknowledged connectionless service.
2. Acknowledged connectionless service
3. Acknowledged connection-oriented service.
1. Unacknowledged connectionless service: consist of having the source
machine send independent frame to destination machine without having destination to
acknowledge them.
No logical connection is established before or released afterward.
If frame is lost no attempt is made to detect and recover it.
This service is appropriate when error rate is low and recovery is left to higher-level.
2. Acknowledged connectionless service: still no logical connection is used
but each frame sent is individually acknowledged
Sender knows whether the frame has arrived correctly
If not, it can sent again
This service is used for unreliable channels like wireless.
3. Acknowledged connection-oriented service. it is most sophisticated
service that data link layer can provided to network layer .
With this service the source and destination machine establish a connection brfore
any data is to be transferred
Data and Communication network (MCA SEMIII)
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Each frame is numbered and guarantees that it is received exactly once and in right
order at the receiver.
Framing To provide service to network layer the data link layer must use service
provided by physical layer .To break bit stream into frames can be achieved by
inserting time gap between frame .but it is risky to count on timing to mark start and
end of each frame so other methods are devised.
They are:
1. Character count
2. Byte stuffing
3. Bit stuffing
4. Physical layer coding violations.
1. Character count: A field in the header is used to specify the number of
character in the frame.
When the data link layer at the destination sees the character count it knows how
many character fallows and hence the end of frame.
The trouble with the algorithm is that count can be corrupted in transmission. for eg
character count 5 of second frame get corrupted to 7,the destination will go out of
synchronization and will be unable to locate start of next frame.
i.e it is impossible for the receiver to tell the transmitter from where to restart..
Byte Stuffing To get around the problem of resynchronization Byte Stuffing is used
Data and Communication network (MCA SEMIII)
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Bit Stuffing Each frame start and end with a particular bit pattern 0111110
Sender encounter 5 consequtive one ‘s it stuff a 0 bit into outgoing bit stream
Receiver sees five consequative one ‘s followed by a 0 it automatically destuff the 0
bit.
Bit stuffing
(a) The original data.
(b) The data as they appear on the line.
(c) The data as they are stored in receiver’s memory after destuffing.
Elementary DLL Protocol Unrestricted Simplex:
This protocol is very simple
Data transmission is one direction
Both Transmitter and receiver are always ready
Processing time is ignored
Infinite buffer space is available at both ends
Data and Communication network (MCA SEMIII)
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No damage or loss of frame
Ultimately the channel is assumed to be totally error free
It is called unrealistic protocol i.e no data loss ,no data retransmission no ack
facility
Simplex stop and wait
The main unrealistic assumption that the transmitter and receiver are
equally fast is removed and result is stop and waits protocol.
After having passed a packet to network layer the receiver send a dummy frame
back to sender which in effect give sender permission to transmit next frame .
The senders do not need to inspect the frame (ACK) because it is positive only.
Protocol in which the sender sends one frame and then wait for an ack before
proceeding in called stop and wait.
Simplex protocol for noisy channel:
This protocol removes one more restriction i.e a communication channel
can never be ideal it can make errors frame may be damaged or lost
The protocol should have some schema the receiver should ack about the received
frame.
If possible go for retransmission or may try to correct frame at receiver end.
To his protocol sender will send a frame but receiver will ack only when the
frame received is correct
Damaged frame will be directly discarded by the receiver
For the damage frame sender goes time out and retransmit the frame
This schema will fail if Ack will get corrupted i.e transmitter will transmit same
frame again
At receiver end it may cause problem of duplication of same frame.
To deal with problem of duplication a sender has to duplication a sender has to
put sequence no. in header of each frame to be send. Then the receiver has to ack
seq no of each incoming frame to see that whether it is new frame or duplicate
frame and if duplicate discard it and send the ACK and if it is new accept now.
This schema is called PAR (Positive acknowledgement with retransmission)
Piggy Backing Concept :
The limitation of data link protocol is they are all simplex type but in most of
application we need full duplex system.
One way to achieve this is you should use separate channel one in each direction.
Forward direction
Reverse direction
But in most of cases capacity of reverse channel is wasted as user is paying for cost of
two circuit but actually using one circuit.
Piggy Backing: when a frame is accepted by a receiver instead of immediately
sending ACK ,receiver wait for some time for some outgoing frame and then attach
the ACK to the outgoing frame going from receiver to transmitted this technique is
called piggy backing.
Data and Communication network (MCA SEMIII)
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Advantage :
Better use of available bandwidth
No separate frame has to be transmitted only for ACK
Disadvantage :
It produces time delay in ACK process .
Due to max delay the sender may go time out and transmit the same frame.
Sliding Window concept :The essence of sliding window protocol is that at any
instant of time, the sender maintain a set of sequence numbers corresponding to frame
it is permitted to send called sending window.
The receiver maintains a receiving window corresponding to set of frame it is
permitted to accept.
Whenever a new packet arrive it is given the next highest sequence number and upper
edge of window is advance by one.
Any frame falling outside the window is discarded by receiver .when frame whose
sequence number is equal to lower edge of window is received it is passed to network
layer an acknowledgement is generate and window rotate by one.
A sliding window of size 1, with a 3-bit sequence number.
(a) Initially.
(b) After the first frame has been sent.
(c) After the first frame has been received.
(d) After the first acknowledgement has been received.
One bit sliding window protocol:
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Window size is 1 .It still uses stop and wait concept.
It uses only 1 Bit i.e 0 or 1 as a sequence number (1 bit sliding window)
It uses 2 procedures
Next frame to send
Frame accepted
Under normal circumstances one of the two nodes may proceed and transmit
the frame.
When this is received by the receiver data link layer it is pass to network
layer and receiver slid up.
In an event when both start off simultaneously duplicate frame arrive more
even though there is no transmission error.
Two scenarios for protocol 4. (a) Normal case. (b) Abnormal case. The notation is (seq,
ack, packet number). An asterisk indicates where a network layer accepts a packet.
Pipelining concept: To make use of RTDT(Round Trip Delay Time) transmitter can
keep ready some no. of frame to transmit online before receiving acknowledgment of last
frame send. This concept is called is called pipelining . i.e some no. of frame are ready to
transmit always
It does not fallow stop and wait.
Go back N : In Go back N receiver simpy discard all the subsequent frame followed by
the damage frame .center will go time out due to damage frame and it will transmit all
frame i.e damage plus discarded .
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Selective Repeat: bad frame that is received is discarded but good frame received
after is buffered. When the sender goes time out only the oldest un acknowledged frame
is retransmitted. Selective repeat s often combined with having the receiver send a
negative acknowledgment when it detect error.
NACK simulate retransmission before the corresponding timer expires and improve the
performance
HDLC: HDLC is a protocol designed to support both half duplex and full duplex communication
over point –to –point or multipoint link
Transmission Modes:
Normal Response Mode:Is unbalanced we have on primary station and multiple
secondary station. primary can send and secondary can only respond.
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Asynchronous balanced mode:
Both primary and secondary can send and receive data.
Frame format :
Flag field: is an 8 bit sequence with a pattern 01111110 that define beginning and end of
frame.
Address field: contain the address of destination of the frame .An address field can be 1
byte or several byte long.
Control Field: is used for flow control and error control. It can be 1 or 2 byte segment an
its interpretation is different for different frame type.
Information field: user data
FCS checksum: HDLC error detection
Frame types:HDLC have 3 types of frame I Frame,S-Frame and U-Frame
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I Frame:
The bits in control frame are interpreted as
The first bit is 0 this means the frame is I frame
The next 3 bit defines sequence number N(S) of frame in travel
The next bit is called P/F bit, a single bit with dual purpose. It has meaning only it is set
(1) and means poll or final
Poll primary to secondary
Final secondary to primary
The next 3bit called N(R) correspond to ACK when piggy backing is used
S Frame: S frame is used to transfer control information
The first 2 bit of control field is 10 means S-Frame
The second 2 bit for code which defines4 types of S-frame
a. Receiver Ready
b. Receiver Not Ready
10 Reject (Go back N Negative Ack)
11 Selective Reject (selective Repeat Nack)
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The fifth bit is P/F
The next 3 bit is N(R) correspond to Acknowledgement
U-Frame U frame is used for system management not for user data
It is used to exchange session management and control information between connected
devices
U frame code is divided into 2 section 2 bit prefix before P/F and 3 bit suffix after P/F.
Together 5 bit create 32 different types of frame
Code
00 001 SNRM 11 100 SABM
11 011 SNRME 11 110 SABME
Error Detection and Correction:
Types of Error
Single Bit error
Burst Error In a single-bit error, only 1 bit in the data unit has changed.
It occur mainly in parallel transmission
A burst error means that 2 or more bits in the data unit have changed.
It occurs mainly in serial transmission Length of burst is first corrupted bit to last
corrupted bit.
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Error Detection
To detect or correct errors, we need to send extra (redundant) bits with
data.
Detecting Methods
Parity check
CRC
Checksum
Parity Check Simple parity check: Redundant bit – parity bit is added to every data unit so that total number of 1’s
in unit(including parity )become even(or odd).
Example 1110111 1101111 1110010 1100100
11101110 11011110 11100100 11001001
Simple parity check can detect all single bit error .It can detect brust error only if total
number of error in each data unit is odd
Eg 11101110 single bit 11111110 Reject
11101110 Burst (odd) 10100010 Reject
Burst(even) 10100110 Accept
Two dimension parity check 1100111 1011101 0111001 0101001
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11001111 10111011 01110010 01010011 01010101
DATA AND PARITY BIT
CRC – Binary division: Instead of adding bit to achieve desire parity a sequence of redundant bit called
CRC is appended to end of data unit so that the resulting data unit is exactly divisible by
a second predetermined binary number.
At destination the incoming data unit is divided by the same number. if at this step there
is no remainder the data unit is accepted
The redundancy bit is obtained by dividing the data unit by a predetermined
divisor the remainder in CRC.
Receiver side Sender side
Data+ CRC Data + n-1bit
Divisor Divisor(Nbit)
Remainder CRC
Remainder zero-accepted
Non zero reject
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AT receiver
Checksum Sender site:
1. The message is divided into 16-bit words.
2. The value of the checksum word is set to 0.
3. All words including the checksum are
added using one’s complement addition.4. The sum is complemented and becomes the
checksum.
5. The checksum is sent with the data.
Receiver site:
1. The message (including checksum) is
divided into 16-bit words.2. All words are added using one’s
complement addition.3. The sum is complemented and becomes the
new checksum.4. If the value of checksum is 0, the message
is accepted; otherwise, it is rejected.
Example :
10101001 00111001
At sender :
10101001
00111001
11100010 sum
00011101 checksum
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10101001 00111001 00011101
At receiver :
10101001
00111001
00011101
11111111 sum
00000000 complement of sum
Hamming code: Hamming code can be applied to data unit of any length
7 bit ASCII code requires 4 redundant bit to be added to the data unit
The bits are placed in the poition 1,2,4 and 8(poition in 11 bit sequence) bits are
referred as r1,r2,r4 and r8
D D D R8 D D D R4 D R2 R1
Calculating R values
R1: takes care of 1 3 5 7 9 11
R2 : takes care of 2 3 6 7 10 11
R4: takes care of 4 5 6 7
R8 : takes care of 8 9 10 11
We calculate even parity for various bit combination
Example :
1001101
1 0 0 R8 1 1 0 R4 1 R2 R1
Calculate R1
1 3 5 7 9 11
R1 1 0 1 0 1
R1=1
R2: r2 3 6 7 10 11
R2 =0
R4 : r4 5 6 7
R4=0
R8: r8 9 10 11
R8=1
Hamming code: 10011100101
Suppose 7th
bit is corrupted from 1 to 0
10010100101
Recalculate r1 r2 r4 and r8
1 0 0 1 0 1 0 0 1 0 1
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R1 :1
R2: 1
R4: 1
R8:0
Arrange r8r4r2r1
0111 7 in binary
So 7th
bit is corrupted
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MEDIUM ACCESS SUB-LAYER
When only a single channel is available determining who should go next is
an issue .
The protocol used to determine who goes next on a multi-access channel
belong to a sub layer of the data link layer called MAC (Medium Access sub
layer)
The Channel Allocation Problem
• Static Channel Allocation in LANs and MANs
When there is only a small and constant number of users, each of which
has a heavy load of traffic
FDM is a simple and efficient allocation mechanism.
However when the number of sender is large and continuously varying or
traffic is bursty, FDM as some problem.
If spectrum is cut into N region and fewer than N region is using, the
spectrum is wasted.
The basic problem is that if some user who are assigned the BW are not
using it no body else can use it either.
Computer traffic may be bursts 1000:1.
Dynamic Channel Allocation in LANs and MANs
1. Station Model.: the model consist of N independent station
2. Single Channel Assumption: A single channel is available for all
communication .All station can transmit and receive for it.
3. Collision Assumption: if two frames are transmitted simultaneously
they overlap in time and resulting signal is grabled.this event is called
collision.
4. (a) Continuous Time. Frame transmission can begin at any instant of
time.
(b) Slotted Time: frame transmission can begin at start of slot/
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5. (a) Carrier Sense. Station senses the channel before using it, if busy no
station attempt to use it.
(b) No Carrier Sense: station cannot sense the channel before using it.
only later it can come to know whether transmission was successful.
Multiple Access Protocols
• ALOHA
• Carrier Sense Multiple Access Protocols
• Collision-Free Protocols
Pure ALOHA : the basic idea is simple let the user transmit whenever they
have data to be sent.
There will be collision and colliding frame will be damaged
However due to the feedback property of broad casting a sender can find out
whether the frame was destroyed or not.
If the frame was destroyed the sender waits for random period of time and
sent again.
The waiting time should be random or the same frame will collide over and
over again.
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System in which user share a common channel in a way that can lead to
conflict are called contention system.
Whenever two frames try to occupy the channel at the same time, there will
be a collision and both will be garbled.
If the first bit of a new frame overlap with last bit of frame almost finished,
both frame will be totally destroyed.
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Slotted aloha : Time is divided into discrete interval each interval
corresponds to a frame .This approach require user to agree on slotted
boundaries
Station is not permitted to send the data at any instant of time instead it has
to wait for the beginning of the next slot.
Continuous pure aloha is turned into discrete one.
CSMA Carrier Sense Multiple Access Protocol Protocol in which stations listen for carrier and act accordingly are called
carrier sense protocol
1-persistent CSMA : when a station has data to send it first listen to the
channel to see if anyone else is transmitting at that moment .if channel is
busy the station wait until it becomes idle.
When the station detect and idle channel it transmit a frame .
If collision occurs the station wait for random amount of time and start all
over again.
The station transmit with a probability of 1 when it find the channel ideal so
called 1 persistence CSMA
The propagation delay has an important effect on the performance of
protocol.
Non persistent CSMA: before sending a station senses the channel .if no
one else is sending the station begin doing so itself.
If the channel is already in use the station does not continuously sense it
.instead, it wait for random period of time and then repeat the algorithm
P-persistent CSMA: It applies to slotted channels .
When a station becomes ready to send it sense the channel .if ideal it
transmit with the probability p.
With a probability q=1-p it defers until next slot .
The process is repeated until either the frame has been transmitted or another
station begins transmitting.
If channel is busy it wait until next slot and applies the above steps.
CSMA with collision detection: another improvement is that station to
abort their transmission as soon as they detect a collision.
If two stations sense the channel to be idle and begin transmission
simultaneously they detect collision almost immediately. So rather than
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finishing transmission they should abort transmission immediately. Quickly
terminating damage frame saves time and bandwidth .this protocol is called
CSMA/CD.
After a station detects a collision it aborts its transmission wait for random
period of time and then tries again. The model consists of alternate
contention and transmission period with idle period occurring when all
station are quiet.
Collision-Free Protocols Collision in the contention period adversely effect the system performance
.protocol devised to solve the problem of collision is called collision free
protocol they are
A bit Map protocol
Binary countdown
A bit map protocol: Contention period consist of N slot .If station 1 has a frame to send it
transmit 1 bit during 1 slot. similarly station 2 as a opportunity to transmit a
1 during t slot only if it has frame queued up.
After all N slot have passes by each station has a complete knowledge of
which station wish to transmit. transmission begins in numerical order.
Since everyone agrees on who goes next there will never be collision.
After the last ready station has transmitted its frame another N bit contention
period begins.
If a station becomes ready after its bit slot has passes by, it has to wait until
the bit map comes around again.
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Protocols like this in which the desire to transmit is broadcast before actual
transmission is called reservation protocol
Binary countdown: the basic problem of 1 bit map is the overhead of 1
bit per station so it does not scale in network of thousand of computers.
In binary countdown station waiting to use the channel broadcast its address
as a binary bit string.
All address are assumed to be of same length.
The bit in each address position from different station is Boolean ORed
together.
It has the property that higher numbered station have a higher priority than
lower numbered station which can either be good or bad depending on the
context.
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Channelization : channelization is a multiple access method in
which the available bandwidth of a link is shared in time frequency
or code between different station
FDMA : Frequency division multiple access the bandwidth is shared by
all station .each station uses its allocated band to send its data
Each band is reserved for a specific station
The band belongs to the sation all the time.
In FDMA bandwidth is divided into channels
TDMA : Time division multiple access the entire bandwidth is just one
channel .The station share the capacity of channel in time .
Each station is allocated a time slot durning which it can send data.
In TDMA the bandwidth is just one channel that is timeshared.
CDMA (Code division multiple access) :
In CDMA one channel carries all transmission simultaneously
CDMA is based on coding theory .Each station is assigned a code which
is sequence of number called chips.
To generate sequence we use Walsh Table a two dimensional table with
a equal number of rows and columns.
Each row is a sequence of chip.
W1=[+1]
W2= +1 +1
+1 -1
W4= +1 +1 +1 +1
+1 -1 +1 -1
+1 +1 -1 -1
+1 -1 -1 +1
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Satellite Networks:
A Satellite network is a combination of nodes that provides
communication from one point on earth to another.
Orbits : An artifical satellite needs to have an orbit the path in which I
travels around earth.
The orbit can be equatorial ,inclined or polar
The period of a satellite is the time required for a satellite to make a
complete trip around earth .
Footprint : The signal a satellite is normally aimed at a specific area
called footprint .
Categories of satellite:
Based on location of orbit satellites can be divided into three categories
GEO LEO MEO
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GEO (Geosynchronous earth orbit): Line of sight propagation requires
that sending and receiving antennas to be locked onto each other all the
time . Therefore satellite must move at same speed as the earth so that it
remains fixed above at a certain spot such satellite are called
geosynchronous.
To provide global transmission minimum 3 satellites are required
equidistant from each other.
MEO (Medium earth orbit): satellite is positioned between the two van
Allen belts. It takes 6 hours to circle earth.
GPS(Global Positioning system ):is an example of MEO .
GPS uses 24 satellites in six orbits. the orbits and the location of satellite
in each orbit are designed in a way that at any time four satellite are
visible from any point on earth.
It is used in land and sea navigation to provided time and location for
vehicles .It is not used for communication.
It is based on the principle of triangulation. On a plane if we know our
distance from 3 points we known exactly where we are.
LEO(low earth orbit): satellites have polar orbits .An LEO is used for
cellular type of access .the altitude is between 500 to 2000 K.M with
rotation period of 90 or 120 min. the footprint normally has a diameter
of 8000 K.M.
A LEO system is made of a constellation of satellites that work together
as a network each satellite acts as a switch..Satellite that is close to each
other are connected through inter-satellite links (ISL).A mobile system
communicating with satellite through a user mobile link (UML). A
satellite can also communicate with earth station through a gateway
link(GWL).
Iridium System Global star and teledesic are example of LEO.
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Network Layer:
Store-and-Forward Packet Switching:
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A host with a packet to send transmits it to the nearest router
either on its LAN or over point-to-point link to the carrier.
The packet is stored until it has fully arrived so that checksum
can be verified.
Then it is forwarded to next router along the path until it
reaches the destination
Implementation of Connectionless Service:
In connectionless service packets are injected into the subnet
individually and routed independently of each other. NO
advance setup is needed.
Packets are called datagram’s and subnet is called datagram
subnet.
Every router has an internal table telling it where to send
packets for each possible destination.
Each table entry is a pair consisting of a destination and
outgoing line to use for that destination.
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0
.
Packets 1,2, and 3 are forwarded to C according to A’s table.
Something different happen with packet 4 .
When it arrived at A it was send to B even though the
destination is F as traffic jam could occur along ACE path and
update its routing table
The algorithm that manages the table and make routing
decision are called routing algorithm.
Implementation of Connection-Oriented Service:
Host 1 has established connection 1 with host2 it is
remembered has first entry in each of the routing table.
The first line of A’s table says that if a packet bearing
connection identifier 1 comes in from H1 it is to sent to C and
given same connection identifier.
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If H3 also want to establish a connection to H2.it chooses
identifier 1 this lead to second entry in table.
We have a conflict here because A can easily identify H1 or H3
but C cannot. For this reason a different connection identifier
is taken for outgoing traffic for second connection.
Routing Algorithms
The main function of the network layer is routing packets from
source to destination machine
The routing algorithm is that part of the network layer
responsible for deciding which output line an incoming packet
should be transmitted on.
Routing algorithm are classified has no adaptive and adaptive.
Non adaptive algorithm: do not base their routing decision on
measurement or estimate of current traffic and topology.
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Instead the route is computed in advance offline and
downloaded to the routers when network is booted.
Adaptive Algorithm: chance the routing decision to reflect
changes in the topology and traffic
• Shortest Path Routing
• Flooding
• Distance Vector Routing
• Link State Routing
• Hierarchical Routing
• Broadcast Routing
• Multicast Routing
The Optimality Principle: It states that if router J is on the
optimal path from router I to router K than optimal path from
J to K also falls along same route.
The set of optimal routes from all source to a given destination
form a tree rooted at destination. Such a tree is call sink tree.
Sink tree does not have loops so packet will be delivered within
finite and bounded numbers of HOP.
(a) A subnet. (b) A sink tree for router B.
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Shortest Path Routing
Flooding
Another static algorithm is flooding in which every incoming
packet is sent out on every outgoing line except the one is
arrived on. Flooding generally generate vast numbers of
duplicate packets. Flooding always select shortest an best path
b’cos it choose every possible path.
A variation of flooding that is slightly more practical is
selective flooding , in this algorithm the router do not send
every incoming packet out on every line only on those line that
are going approximately in the right direction.
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Distance Vector Routing
(a) A subnet. (b) Input from A, I, H, K, and the new
routing table for J.
Fiq a shows a subnet table shows a delay vector for the IMP J
in ms. This algorithm is use in Arpanet
Each IMP periodically exchange a routing info to each of its
neighbor IMPS
Each IMP maintain a routing table containing one entry for
each other IMP in the subnet
This entry contain 2 part
a. The preferred outgoing line to go that destination
b. The time required to reach at that destination.
J has measured its current delay to its neighour A,I,H and K as
8,10,12 and 6 ms respectively
Now calculate the route from j to g .
J to G VIA A =(18+8)=26 VIA I =(10+31)=41
VIA H =(12+6)=18 VIA k=(6+31)=37
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Hierarchical Routing:
If the size of network increases size of router also increases.
For this more time is require to scan the tables and more B.W
is required to send the status information.
At certain limit the network may grow up to point where it is
no longer possible for router to manage it, so routing will be
done hierarchically
as in telephone network.
When hierarchical routing is use the router are divided into
region with each knowing all the details of its own region but
nothing about the internal structure of other region.
For huge network where 2 level hierarchies is not sufficient we
may bunch the region into cluster, cluster to zone and zone to
groups so on.
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Link State Routing
Each router must do the following:
1. Discover its neighbors, learn their network address.
2. Measure the delay or cost to each of its neighbors.
3. Construct a packet telling all it has just learned.
4. Send this packet to all other routers.
5. Compute the shortest path to every other router.
Learning about the Neighbors: When a router is booted its
first task is to learn who the neighbors are. it accomplishes this
goal by sending a special hello packet on each point to point
link and router at other end is expected to send back a reply
telling who it is. The names must be globally unique.
(a) Nine routers and a LAN. (b) A graph model of (a).
Measure the delay or cost to each of its neighbors : The link
state routing algorithm requires each router to known or at
least have a reasonable estimate of the delay to each of its
neighbors. The direct way is to send the echo packet that the
other side is required to send back immediately. By measuring
round trip delay time and dividing it by 2,the sending router
can get a reasonable estimate of the delay. An issue is whether
to take the load into account when measuring the delay. Round
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trip timer must start when the ECHO packet is queued or
when it reaches front of queue. to avoid oscillation in path it
may be wise to distribute the load over multiple lines with some
fraction going over each line.
Building Link State Packets:
Once information is collected the next step is for each router to
build a packet containing all the data. The packet starts with
identification of sender followed by sequence number, age and
list of neighbor. The packet could be build periodically or after significant event occur.
Distributing the Link State Packets
As the packet are distributed and installed the router getting
the first one will change their routed. Different router may use
different version of topology and can lead to inconsistencies.
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The idea is to use flooding to distribute the link state packet.
When a new packet comes in it is checked against the list of
packet if new forwarded on all lines else discarded. The age
field is decremented by each router to make sure no packet can
get lost and live for indefinite period of time.
Broadcast Routing:
sending packet to all destination simultaneously is called
broadcasting . one solution is flooding but it consume too much
bandwidth and generate too many duplicate packet.
The next algorithm is multi destination routing. In this each
packet contains either a list of destination or bit map
indicating the desired destination. When packet arrives the
router checks the entire destination to determine the set of
output lines that will be needed
The next algorithm makes use of sink tree. A spanning tree is a
subset of subnet that included all the router but contain no
loops. The method makes excellent use of bandwidth
generating minimum number of packets to do job.
The last is reverse path forwarding where router does not
known about the spanning tree nor does it have overhead of
destination list .
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Reverse path forwarding. (a) A subnet. (b) a Sink tree. (c) The tree
built by reverse path forwarding.
Multicast Routing : sending message to group is called
multicasting . it requires group management. To do multicast
routing each router computes a spanning tree covering all the
routers. when a process sends a multicast packet to a group the
first router examines its spanning tree and prunes it removes
all lines that do not lead to host that are member of the group.
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Congestion control : when too many packets are present in the
subnet performance degrade .This situation is called
congestion .
When the packet are within the carrying capacity of subnet
they are delivered however then traffic increases the router are
no longer capable to cope up and then packet are lost.at high
traffic almost no packet are delivered.
The Leaky Bucket Algorithm
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(a) A leaky bucket with water. (b) a leaky bucket with packets.
Figure illustrate a bucket with a small hole at bottom .No
matter the rate at which water enters the bucket the outflow is
at a constant rate .Also when bucket is full any additional
water entering it spills over the side and is lost.
The same idea is applied to packet .Host is connected to the
network by an interface containing a leaky bucket that is finite
internal queue. if packet arrive an queue is full, packets are
discarded.
The Token Bucket Algorithm : The leaky bucket enforce a
rigid output pattern at the average rate no matter how burst
the traffic is.It is better to allow the output to speed up
somewhat when large burst arrive so more flexible algorithm is
needed preferably one that never loses data.One such
algorithm is Token Bucket.
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Tunneling
The case when source and destination host are on the same
type of network but there is a different network in between
them. The solution to this problem is tunneling to send an IP
packet to HOST 2 ,Host 1 construct an packet containing IP
address of HOST 2.
When the multiprotocol router gets the frame it removes the IP
packet insert it in the payload field of WAN network layer
packet. when the destinations multiprotocol router gets it
removes the IP packet and sends it to HOST 2 inside Ethernet
frame.
The Wan can be seen as a big tunnel extending from one router
to another .the IP packet just travel from one end of tunnel to
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other end .Only router has to understand IP and WAN
packets. The entire distance is a serial line.
Internetwork Routing
(a) An internetwork. (b) A graph of the internet work.
The internet work of 5 network connected connected by 6
router.
Once the graph has been constructed known routing algorithm
can be applied to a set of multiprotocol router .this gives 2 level
routing algorithm
Exterior gateway protocol : between networks.
Interior gateway protocol : within each network.
Fragmentation: Each network imposes some maximum size on
packets ,when large packet wants to travel through a network
problem arises. one solution is to allow gateway to break up
packets into fragments sending each fragments as a separate
internet packet.
There is trouble in putting back the fragments together. Two
opposing strategies are:
1. Transparent Fragmentation : in this approach when an
oversize packet arrives at a gateway the gateway breaks it
up into fragments .each fragment is address to exit
gateway where it recombines .Te problem is all packets
fallow same route and exit gateway must known when it
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receives all the packets .also overhead require to
repeatedly reassemble and then refragment.
2. Non transparent fragmentation : once a packet has been
fragmented each fragment is treated as though it where
an original packet all fragment are passed to exit gateway
Recombination occur only at destination HOST, an
advantage is that multiple exit gateway can be used and
higher performance can be achieved.
(a) Transparent fragmentation.
(b) Nontransparent fragmentation.
Internet Control Protocols
ARP– The Address Resolution Protocol : every machine has
one or more IP address these cannot be used for sending
packets because the DDL hardware does not understand IP
address it understand Ethernet address (48 bit).it sends and
receive frame using Ethernet address . The question is
mapping and IP address to an Ethernet address
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Host 1 sends a packet to user on host2 for e.g something like
[email protected] .the first step is to find IP address for Host 2.This
look up is performed by DNS lets say it returns the IP address
of HOST 2 .Next is to known the Ethernet address the solution
is HOST 1 to output a broadcast packet onto the Ethernet
asking who owns IP address 192.31.65.5. The broadcast arrive
to every machine HOST 2will respond with Ethernet address
E2. The protocol used for asking this question and getting the
reply is called ARP(address Resolution Protocol).
RARP : Given the Ethernet address what is the corresponding
IP address ? This problem occur in diskless workstation is
booted .Such machine has a binary image of OS .
The solution is RARP .this allows newly booted
workstation to broadcast its Ethernet address and say My 48
bit Ethernet address is ----------- what is the IP address? the
RARP server sees this request looks up Ethernet address and
send back IP address.
Embedding IP address in memory image requires each
workstation to have a separate image.
BOOTP: BOOTP uses UDP message which are forwarded to
every router. It also provide additional information like IP
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address of server holding mirror image,IP address of router
subet mask etc.
BOOTP requires manual configuration of table mapping IP
address to Ethernet address. When a new host is added it
cannot use BOOTP until it is assign IP address and entered
into configuration table by hand.
Dynamic Host Configuration Protocol :There is a special
server to assign IP address to the host asking for it. The server
need not be on the same LAN.A DHCP relay is needed on each
LAN .
To find the IP address a newly booted machine broadcast a
DHCP Discover packet .the DHCP relay finds it and send it as
a unicast packet to DHCP server may be on different network.
the only information relay needs is IP address of DHCP server.
Operation of DHCP.
An issue is how long an IP address should be allocated. If
HOST leaves and does not return the IP address the address is
permanently lost. To prevent this IP address assignment may
be for a fixed period of time by a technique called Leasing. Just
before the lease expires Host has to renew it or else it would no
longer be able to use the IP address.
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In classful addressing, the address space is divided into five
classes: A, B, C, D, and E
Class A: class a is divided into 128 blocks with each block
having a different netid.the first block covers 0.0.0.0 to
0.255.255.255(netid 0).
The second is (netid1)1.0.0.0 to 1.255.255.255.
The last block is 127.0.0.0 to 127.255.255.255(netid 127)
Number of address in each block is 16,777,216 is probably larger than the need of almost all organization .Millions of class
A address are wasted.
Class B: class B is divided into 16,384 blocks with each block
having a different netid. Sixteen are reserved for private use
leaving 16,368 blocks for assignment.
First block is 128.0.0.0 to 128.0.255.255 (netid 128.0)
The last is 191.255.0.0 to 191.255.255.255 (netid 191.255)
Each block in this class contain 65536 address.
Class B is designed for midsize organization but many addresses are wasted.
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Class C: class C is divided into 2,097,152 blocks with each
block having a different netid..
256 are reserved for private use leaving 2,096,896 blocks for
assignment. The first block covers 192.0.0.0 to 192.0.0.255
(netid 192.0.0) Last block covers (223.255.255.0 to 223.255.255.255)
netid 223.255.255
Each blocks contain 256 address
The number of address in class c is smaller than the needs of
most organization.
Class D is just one block use for multi cast
Class E is just one block. it was designed for use as reserved
address.
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Find the class of each address.
a. 00000001 00001011 00001011 11101111
b. 11000001 10000011 00011011 11111111
c. 14.23.120.8
d. 252.5.15.111
Solution
a. The first bit is 0. This is a class A address.
b. The first 2 bits are 1; the third bit is 0. This is a class C
address.
c. The first byte is 14; the class is A.
d. The first byte is 252; the class is E.
Sub netting : IP address are designed with two levels of
hierarchy (netid and hosted) however often an organization
requires the HOST to assemble into groups .The network need
to be divided into several sub network (subnet)
For e.g. university may want to group its host according to
department .in this case university has one network address but
needs several sub network address. Outside world known only
network address inside the organization we have sub network
address. In sub netting a network is divided into several smaller groups
with each subnet having its own sub network address. when we
divided a network into several subnet we have three level of
hierarchy.
141.14.192.2
Netid Hostid
141.14 192 132
Netid subnetid hostid
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MASK : when a router receive a packet with a destination
address .it need to route the packet. the route outside the
organization route the packet based on network address and
inside the organization route the packet based on sub network
address .
The problem is how can a router find network address or subnetwork address .the solution is 32 bit key called mask .the
router outside the organization use default mask and inside the
organization use a subnet mask.
Default mask : is a 32 bit binary number that gives the network
address when ANDed with an address of block
The network address can be found by applying the default mask
to any address in the block .it retains the netid of block and set
the hostid to 0’s.
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Address 190.240.7.91 find network address
Soln : it is class B default mask is 255.255.0.0
So AND Address and default mask
We get 190.240.0.0
Subnet Mask : the number of 1’s in a subnet mask is more than number of 1’s in the corresponding default mask .i.e we change
the leftmost zero’s in default mask to make subnet mask .
255.255.0.0 11111111 11111111 00000000 00000000
255.255.224.0 11111111 11111111 11100000 00000000
The number of subnet is determined by the extra 1’s if extra one
is n the number of subnet is 2n
If number of subnet is N the no. of extra ones is log2N.
Supernetting : Although class and B address are almost depleted class C
address are still available .However maximum no. of address in
each block is 256 if more address are needed solution is super
netting .
In super netting an organization can combine several class C to
create a larger range of address. if organization needs 1000 class C address four class C blocks can be granted .the organization
can use these address in one super network.
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Transport Layer The transport layer is responsible for process-to-process delivery.
DLL – node to node delivery
Network layer – Host to Host delivery
Transport layer – process to process delivery
Port Number: Transport layer address is called port number to chose among multiple process
that are running on destination HOST .
Port number is a 16 bit integer between 0 to 65,535
The client port number can random(ephemeral port) but server have well known port
number
e.g port number 13 for date and time cannot be random 13
client may use this service on random port number 52000
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IANA Range
Socket address :Process to process delivery need to identify IP address and port number
at each end
Combination of IP address and port number is socket address
UDP
The User Datagram Protocol (UDP) is called a connectionless, unreliable transport
protocol. It does not add anything to the services of IP except to provide process-to-
process communication instead of host-to-host communication.
The field are:
Source port number: This is the port number used by the process running source host.
It is 16 bit long
Destination Port number: This is the port number used by the process running destination
host .It is 16 bit long
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Length : this is 16 bit field that defines the total length of the user datagram header plus
data .
Checksum : This field is use for error detection over entire user datagram (header plus
data)
Application:
UDP is suitable for the process that requires simple request response
communication
UDP is suitable for process with internal flow and error control mechanism
UDP is suitable for transport protocol for multicasting
TCP
Stream delivery
TCP allows sending process to delivery data as a stream of byte and receive it as a stream
of byte
Sending and receiving buffers:
As sending and receiving process are not equally fast TCP needs buffer for storage .At
transport layer TCP groups a number of byte together into a packet called segment .TCP
adds header to each segment and deliver it to the destination IP layer
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TCP HEADER
Source port address and destination port address :
This is a 16 bit field that defines the port number of host that is sending and receiving the
segment.
Sequence number : this is a 32 bit field defines the number assigned to first byte of data
contained in the segment.
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Acknowledgement number : this is 32 bit field that defines the byte number that the
sender of the segment is expecting to receive from the other party. if byte number x has
been successfully received x+1 is the acknowledgment number.
Header length : the header can be 20 or 60 byte therefore value of this field can be 5 or
15.
Control : this field defines 6 different control flags .one or more bits can be set at a time.
Window size: this field define size of window in byte max window size is 65535 bytes.
Checksum : this is 16 bit fiels use for error detection and correction.
Urgent pointer: it is valid only if urgent flag is set is use when segment contain urgent
data
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Connection establishment using three-way handshaking
Steps:
1.The client sends the first segment a SYN segment .The segment include the
source and destination port number .
2.The server produce the second segment a SYN and an ACk segment . this
segment has a dual purpose . First it acknowledge the recipt of first segment using
ACK flag . ACK number is client initializing sequence number plus 1 .
Second it used as the initialization segment for server .It contain intial sequence
number used to number the byte sent from the server to the client.
3. The client send a third segment .This is ACK segment, it acknowledges the
receipt of second segment using the ACK flag .ACK number is the server
initialization sequence number plus 1
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Connection termination using three-way handshaking
Steps :
1. The client TCP sends the first segment a FIN Segment
2. The server TCP sends the second segment an ACK segment to confirm the
receipt of the FIN segment from client
3.The server TCP can continue sending data in the server client direction. When it
does not have any more data to send ,it send a third segment .This segment is a
FIN segment
4.The client TCP sends the fourth segment an ACK segment to confirm the
receipt of the FIN segment from TCP server.
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Note : the second and third step can be combined into 1 step if the server also wants to
close the connection. This segment will contain ACK plus FIN flag and the client
acknowledges the FIN from server side in third segment.
The connection closes in both direction
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Application Layer Cryptography components
The word cryptography means secret writing. The orginal message is called plain
text and after it is transferred it is called as cipher text
Encryption transfer plain to cipher text and decryption transfer cipher to plain text.
A key is a number that the cipher operates on .
To encrypt a message we need an encryption algorithm ,encrypt key and plain text
To decrypt we need decryptalgorithm decryption key and cipher text.
Symmetric-key cryptography
In symmetric-key cryptography, the same key is used by the sender
(for encryption) and the receiver (for decryption).The key is shared
The disadvantage is that each pair of user must have a unique symmetric key
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Asymmetric-key cryptography
Keys used in cryptography
Comparison between two categories of cryptography
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A substitution cipher replaces one symbol with another
Monoaplabetic substitution : a character in the plain text is always changed to the same
character in cipher text regardless of its poition in text.. also called caesar cipher
Use the shift cipher with key = 15 to encrypt the message “HELLO.”
WTAAD.
Polyalphabetic substitution : each occurance of a character c an have a different
substitution .the relationship between plain text and cipher text is one to many.
We may define the key like take the position of character in text ,divided by 10 and let
remainder be the shift value.
An exaple of polyalpabetic is vigenere cipher
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