DCN Notes

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.



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.



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



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.



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



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 .



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.



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.



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.



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.



Switching



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:



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



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



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.



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.



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



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.



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



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



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



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 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



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



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



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.



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:



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 .



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.



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



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)



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.



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



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



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



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



R1 :1

R2: 1

R4: 1

R8:0

Arrange r8r4r2r1

0111 7 in binary

So 7th

bit is corrupted



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/



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.



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.



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



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.



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.



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



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



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.



Network Layer:

Store-and-Forward Packet Switching:



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.



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.



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.



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.



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.



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



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.



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



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.



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 .



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.



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



(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.



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



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



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



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

mailto:[email protected]



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.



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.



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.



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



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.



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.



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



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



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



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.



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



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



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.



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



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



Asymmetric-key cryptography

Keys used in cryptography

Comparison between two categories of cryptography



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



Documents

DCN Notes