Data Communication

Chapters : 01-06, 08, 10, 12

References :

Chapter 01: Introduction

Data: Data refers to the raw facts that are collected.

Information: Information refers to processed data that enables us to take decision.

Data Communication: Data communications are the exchange of data between two devices by some form of transmission medium such as a wire cable.

The effectiveness of data communication:

• Delivery : Deliver data to the correct destination.

• Accuracy : Deliver the data accurately.

• Timeliness : Deliver data in timely manner. Audio and Video data has to be delivered in a timely manner without any delay; such a data delivery is called real time transmission of data.

• Jitter : It is the variation in the packer arrival time. Uneven jitter may affect the timeliness of data being transmitted.

The components of data communication: There are five components.

• Message: the information or data to be communicated

• Sender: the device that sends the data message.

• Receiver: the device that receives the message.

• Transmission medium: the physical path by which a message travels from sender to receiver.

• Protocol: A protocol is a set of rules that govern data communication. It is an agreed upon set or rules used by the sender and receiver to communicate data. Without protocol, they may be connected but not communicating.

Data Representation

Data flow

Data flow is the movement of data through a system comprised of software, hardware or a combination of both.

Network

A network is the interconnection of a set of devices capable of communication.

• Host

• Connecting-device: connects the network to other networks, router

• Switch: connects devices together, modem

A network criteria:

Type of Connection

Physical Topology

The term physical topology refers to the way in which a network is laid out physically.

Consideration When Choosing a Topology: Money. Length of cable needed, Future growth, Cable type

Network Types

Chapter 02: Network Models

Protocol Layering: A layer protocol architecture provides a conceptual framework for diving the complex task of exchanging information between remote hosts into simpler task.

Principle of Protocol Layering:

• First Principle : The first principle dictates that if we want bidirectional communication, we need to make each layer so that it is able to perform two opposite task, once in each direction.

• Second Principle: The second principle that we need to follow in protocol layering is that the two objects under each layer at both sides should be identical.

Internet Protocol Suite/ TCP/IP : The internet protocol suite, commonly knowns as TCP/IP, is a framework for organizing the set of communication protocols used in the internet and similar computer net

Layered Architecture

OSI : Open System Interconnection

Description of Layers in OSI Model

• Physical Layer
  • The physical layer provides a standardized interface to physical transmission media.
  • On the sender side → it receives data from Data Link layer and encodes it to signals to be transmitted.
    On the receiver side → it receives the signal from the transmission medium decodes it back into data and sends it to Data Link Layer.

• Data Link Layer

• The Data Link layer adds reliability to the physical layer by providing error detection and correction mechanism.

• On the sender side → It receives the data from Network Layer and divides the stream of bits into fixed size manageable units called as Frames and sends it to the physical layer.

  On the receiver side → It receives data from Physical Layer and regroups them intro frames and sends them to Network layer. This process is called Framing.

• Physical Addressing : The data link layer appends the physical address in the header of the frame before sending it to physical layer. Physical layer contains sender and receiver.

• Flow control: It makes sure that the sender sends the data at a speed at which receiver can receive it.

• Error control: The data link layer imposes error control mechanism to identify lost or damaged frames, duplicate frames and the retransmit them.

• Main responsibility: hop to hop transmission of frames.

• Network Layer
  • The network layer makes sure that the data is delivered to the receiver despite multiple intermediate devices.
  • Sending side → it accepts data from transport layer divides in into packets, adds addressing information in the header and passes it to the data link layer.
    Receiving side → It receives the frames from data link layer, converts them into packets, verifies the physical addresses and send the packets to the transport layer.
  • The network layer is responsible for source to destination of delivery of data.
  • Logical addressing: The network layer uses logical address commonly known as IP address to recognize devices on the network.
    • An IP address is a universally unique address which enables the network layer to identify devices outside the sender’s network.
  • Routing: The network layer divides data into units called packets of equal size and bears a sequence number for rearranging on the receiving end.
    • Every packets is independent of the other and may travel using different routes to reach the receiver hence may arrive out of at the receiver.
    • The process of finding the best path is called as Routing, using routing algorithms.
  • It doesn’t perform any flow control or error control.
  • Main responsibility: Transmission of packets from source to destination.

• Transport Layer
  • The transport layer takes care of process to process delivery of data and makes sure that it is intact and in order.
  • Sending side → the transport layer receives data from the session layer, divides it into unit called segments and sends it to the network layer.
    Receiving side → It receives packets from network layer, converts and arranges proper sequence of segments and sends it to the session layer.
  • To ensure process to process delivery it makes sure of port address to identify the data from the sending and receiving process.
    • A port address is the name or label given to a process. It is a 16 bit address. HTTP use port address 80.
  • Error control and flow control

• Session Layer
  • The session layer establishes a session between the communicating devices called dialog and synchronizes their interaction.
  • Sending side → Accept data from presentation layer adds checkpoint to it called syn bits and passes the data to the transport layer.
    Receiving side → It receives data from transport layer removes the checkpoints inserted previously and passes the data to the presentation layer.
  • The checkpoint or synchronization points is a way of informing the status of the data transfer.

• Presentation Layer
  • The presentation layer performs translation, encryption and compression of data.
  • Sending side → It receives data from the application layer adds header which contains information related to encryption and compression and sends it to the session layer.
    Receiving side → It receives data from session layer decompresses and decrypts the data as required and translates it back as per the encoding scheme used at the receiver.
  • Translation: The sending and receiving device may run on different platforms. Hence, a translation service may be required.
  • Compression: It ensures faster data transfer. The data compressed at sender has to be decompressed at the receiving end.
  • Encryption: It is the process of transforming the original message to change its meaning before sending it. The revers process called decryption has be performed at the receiving end to recover the original message.

• Application Layer
  • Main responsibility: provide access to network resources
  • The application layer enables the user to communicate its data to the receiver by providing certain services.
  • Once the data is ready, the application layer initiates communication with the corresponding application on another device. This involves establishing a connection, if necessary, and sending the data packets over the network.
  • Once the data packets are processed, the application layer delivers the data to the appropriate user application running on the receiving device. This enables the application to interpret and utilize the received data for further processing or display to the user.
  • Ensures effective communication with another application program on a network.
  • Facilitating communication between clients and server.
  • User Authentication and authorization
  • Error handling and recovery

Multiplexing and Demultiplexing

• Multiplexing at the source and demultiplexing at the destination

• Multiplexing → A protocol at a layer can encapsulate a packet from several next-higher-layer protocols

• Demultiplexing → A protocol can decapsulate and deliver to a several next-higher layer protocols

Chapter 03: Introduction to Physical Layer

Analog Signal

• They have infinite value in a range

Characteristics of an analog signal (Sine wave is analog) : 3 characteristics

Peak Amplitude

• The peak amplitude of a signal is the absolute value of the highest intensity

• The amplitude of a signal is proportional to the energy carried by the signal

Frequency and Period

• Frequency $(f)$ refers to the number of cycles completed by the wave in one second.
  Frequency is the rate of change w.r.t time.
  If signal doesn’t change at all, $f$ is $0$. If it changes instantaneously, $f$ is $\infin$
  Frequency is independent of the medium

• Period $(T)$ refers to the time taken by the wave to complete one cycle

$f = \frac{1}{T}$

Phase

• Phase describes the position of the waveform w.r.t time.

• Phase indicates the forward and backward shift of the waveform from the axis.

• It is measured in degrees or radian

Example: A sine wave is offset $\frac{1}{6}$ cycle w.r.t time. What is phase degrees in degrees and radian.

$\frac{1}{6} \times 360 = 60 \degree$
$60\degree = 60 \times \frac{2\pi}{360} = \frac{\pi}{3} \text{ rad}$

Wavelength

Time Domain and Frequency Domain

• The time-domain plot shows the changes in signal amplitude w.r.t time. Time-amplitude relation.

• The frequency-domain plot shows the signal frequency and peak amplitude.

Composite Signal

• A composite signal is a combination of two or more simple sine waves with different frequency, phase and amplitude

• A periodic composite signal can be decomposed into a series of signals with discrete frequencies.

• A non-periodic signal when decomposed gives a combination of sine waves with continuous frequencies.

Digital Signal

• They have limited number of defined values

• Information in digital signal can be explained in the form of voltage levels

• If a signal has $L$ levels, each level need $log_2L$ bits.

Bit Length or Interval

• The bit length is the distance one bit occupies on the transmission medium

$\text{Bit length } = \text{propagation speed} \times \text{bit duration}$

Bit Rate

• It is the the number of bits transmitted in one second

• Expressed as bits per second (bps)

Example: An analog signal carries 4 bits in each signal unit. If 1000 signal units are send per second. Find the baud rate and bit rate.

Baud rate : 1000 bauds

Bit rate = 4 * 1000 = 400 bps

Baud Rate

• It is the rate of signal speed

• The number of signal units per second that are required to represent these bits

Channel

• A channel is the medium through which the signal carrying information will be passed

Types of Channels

• Transmission of Digital Signal

Bandwidth

• Bandwidth can be defined as the portion of the electromagnetic spectrum occupied by the signal.

• It may be defined as the frequency range over which a signal transmitted.

Bandwidth of an analog signal:

• Bandwidth of an analog signal is expressed in term of its frequencies.

• It is calculated by the difference between the maximum and minimum frequency.

Bandwidth = 90 - 30 = 60 Hz

Bandwidth of an digital signal

• It is defined as the maximum bit rate of the signal to be transmitted

• it is measured in bits per second

Bandwidth of a Channel

• In terms of analog signal, bandwidth of the channel is the range of frequencies that the channel can carry.

• In terms of digital signal, bandwidth of the channel is the maximum bit rate supported by the channel.

• The Channel bandwidth determines the type of signal to be transmitted.

Attenuation : It means a loss of energy. When a signal travels through a medium, it loses some of its energy in overcoming the resistance.

$\text{Signal lost ot gained} = 10 log_{10}{\frac{P_2}{P_1}} \text{ dB}$

Distortion: It means that the signal changes its form or shape. It can occur in a composite signal made of different frequencies. Each signal has its own propagation speed through a medium and therefore its own delay in arriving at the final destination.

SNR : Signal to Noise Ratio

$\text{SNR} = \frac{\text{average signal power}}{\text{average noise power}}$

$\text{SNR}_{dB} = 10 log_{10} \text{SNR}$

The maximum data rate of a channel

Data rate depends on three factors:

• The bandwidth available

• The level of the signal we use

• The quality of the channel (noise level)

The Quality of the channel

Nyquist Bit Rate

$\text{Bit Rate} = 2 \times \text{Bandwidth } \times log_2 L$

Shannon Capacity

$\text{Capacity} = \text{Bandwith } \times log_2 (1 + \text{SNR})$

• SNR is the Signal to Noise Ratio

• Measured in bps

Propagation time : Time required for a bit to travel from the source to the destination.

$\text{Propagation time} = \text{Distance}/ \text{Propagation Speed}$

Transmission Time

$\text{Transmission Time} = \text{Message size} / \text{Bandwidth}$

Chapter 05: Digital Transmission

Line Coding

• It is the process of converting digital data to digital signals.

• $r = \frac{\text{data element}}{\text{signal element}}$

• $\text{Signal rate (S) = } \frac{\text{Data rate (N)}}{r}$

• $\text{Average signal rate, S}_{ave} = c \times N\times \frac{1}{r} \text{ baud}$

Example: A signal is carrying data in which one data element is encoded as one signal element (r = 1). If the bit rate is 100 kbps, what is the average value of the baud rate if c is between 0 and 1?

$S = c \times N \times 1/r = 1/2 \times 100000 \times 1/2 = 50 \text{ kbaud}$

Line coding scheme line

• Unipolar

all the signal levels are on one side of the time axis

NRZ (Non-Return-to-Zero)

• Positive voltage defines bit 1 and the zero voltage defines bit 0;

• Polar

In polar schemes, the voltages are on both sides of the time axis.

NRZ-L (NRZ_Level)

If bit 0 : 1
if bit 1 : 0 (under the reference)

NRZ-I (NRZ Invert)

If bit 0 : No change

if bit 1 : Transition

NRZ-L and NRZ-I both have an average signal rate of N/2 Bd.

Polar RZ (Return to Zero)

1 → Z

0 → Reverse Z

Polar Manchester

0 → + to - transition

1 → - to + transition

Polar Differential Manchester

• Bipolar

Bipolar RZ

0 → Reference line
1 → Z, if again 1 reverse Z, continues …

Bipolar AMI

0 → Draw line on reference

1 → Draw line above, then again below, alternating..

Bipolar Pseudoternary

1 → Draw line on reference

0 → Draw line above, below, alternating.

Skipped.

Chapter 05 : Analog Transmission

Digital to Analog Conversation

It is the process of changing one of the characteristics of an analog signal based on the information in digital data.

$S = N \times \frac{1}{r}$; $r = log_2 L$

$\text{S = signal per second or baud rate}$

$\text{N = data rate/ bit rate/ bits per second }$

$\text{L = number of different signal elements}$

Analog Transmission

A method of sending/transmitting information over long distance by encoding it as an analogue signal.

Carrier Signal

In analog transmission. the sending device produces a high-frequency signal that acts as a base for the information signal. The base signal is known as carrier signal or frequency.

Chapter 06

Frame Size = Number of source * output slot carries + synchronization bit

Frame rate = source bandwidth/output slot carries

Frame duration = 1/frame rate

Data rate = frame rate * frame size

Efficiency =

$SNR_{\text{dB}} = 6.02 \times r + 1.76$

PCM bandwidth : $r \times B_{\text{analog}}$

• Assume that a voice channel occupies a bandwidth of 4 kHz. We need to multiplex 10 voice channels with guard bands of 500 Hz using FDM. Calculate the required bandwidth. Find the maximum effect of a 2-ms burst of noise on data transmitted a 12000 bps.

  10*4*1000 + 9*500
  2*10^-3 *12000

• What is the minimum number of bits in a pseudorandom noise (PN) sequence if we use FHSS with a channel bandwidth of B = 4 KHz and Bss = 128 KHz?

  The number of hops = 100 KHz/4 KHz = 25. So we need log225 = 4.64 ≈ 5 bits