Components of Data Communication

Data communication refers to the transfer of data or information between a source and a receiver.

Data communications is concerned with:

• Transfer of data
• Method of transfer
• Preservation of data

Functionally, it consists of:

• Message
• Medium
• Sender
• Protocol
• Receiver

Effectiveness of Data Communication System

The effectiveness of a data communication system depends on:

• Delivery
• Accuracy
• Timeliness

Modes of Transmission

Unicast

Information sent from one sender to one receiver.

Uses standard unicast applications like FTP, HTTP, SMTP, and Telnet.

Broadcast

Information sent from one sender to all other connected receivers.

ARP (Address Resolution Protocol) uses broadcast to resolve addresses.

Example: 255.255.255.255

Multicast

Information sent from one or more senders to a particular set of users. Example: video server transmitting TV channels.

Transmission Impairment

• Attenuation
• Propagation delay
• Distortion
• Noise
• Crosstalk
• Jitter

Signal Impairment

Signals travel through transmission media, which are not perfect. The imperfection causes signal impairment. This means that the signal at the beginning of the medium is not the same as the signal at the end of the medium.

What is sent is not what is received. Causes of impairment are:

Attenuation

Reduction in strength of signals, also referred to as Loss.

Signals traveling long distances lose their strength.

Signals lose some of their energy and are converted into heat.

Represented in Decibels, cables measured in ‘decibels per foot.’

More efficient cable = less attenuation per unit distance.

Repeaters are used to overcome attenuation by regenerating signals.

Propagation Delay

Delay from the time signal is transmitted and the time signal is received.

Measured in milliseconds.

Varies from medium to medium.

Noise

Addition of external factors in signals. Noise can disturb data.

Two wires can generate voltage noise which affects data.

Types of noise:

• Thermal noise (signals generated by electrons by random motion)
• Induced noise (generated by motors and appliances)
• Crosstalk (effect of one wire on another)
• Impulse noise (generated by power lines, lightning)

Distortion

Distortion, in acoustics and electronics, is any change in a signal that alters the basic waveform or the relationship between various frequency components; it is usually a degradation of the signal.

Crosstalk & Jitter

Crosstalk: In electronics, crosstalk is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel.

Jitter: Jitter is the variation in periodicity of a signal or periodic event from its target or true frequency. In telecommunications, jitter further refers to the variation in latency of packets carrying voice or video data over a communications channel.

Network Performance

Bandwidth is the maximum amount of data that can travel through a channel.

Throughput is how much data actually does travel through the channel successfully.

This can be limited by many different factors, including latency and the protocol being used.

Bandwidth vs Throughput

Latency is the delay between the message transmitted and the message received.

Latency can be caused by:

• Propagation time
• Transmission time
• Queuing time
• Processing time

Propagation Time

Time required by a bit to travel from source to destination. It is the total distance per unit speed.

Transmission Time

Time required to send a complete message. Measured in message size per unit bandwidth available.

Queuing Time

Time required by an intermediate device to process data. It varies with load on the network. Example: packets queuing.

Latency Formula

Propagation Delay = Distance / Propagation speed

Transmission Delay = Message size / Bandwidth (bps)

Latency = Propagation delay + Transmission delay + Queueing time + Processing time

Analog and Digital Data Transmission

Data communications deal with two types of information: analog and digital.

An analog signal is characterized by a continuous mathematical function—when the input changes from one value to the next, it does so by moving through all possible intermediate values.

In contrast, a digital signal has a fixed set of valid levels, and each change consists of an instantaneous move from one valid level to another.

Analog vs Digital Signals

Analog signal: Varies continuously over continuous time.

Digital signal: Varies in steps over discrete intervals of time.

Unguided Media: Wireless Transmission

Unguided media transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication.

Wireless Transmission

• Radio Waves
• Microwaves
• Infrared

The Electromagnetic Spectrum

When electrons move, they create electromagnetic waves that can propagate through space (even in a vacuum).

These waves were predicted by the British physicist James Clerk Maxwell in 1865 and first observed by the German physicist Heinrich Hertz in 1887.

The number of oscillations per second of a wave is called its frequency, f, and is measured in Hz (in honor of Heinrich Hertz).

The distance between two consecutive maxima (or minima) is called the wavelength, universally designated by the Greek letter λ (lambda).

When an antenna of the appropriate size is attached to an electrical circuit, the electromagnetic waves can be broadcast efficiently and received by a receiver some distance away.

In a vacuum, all electromagnetic waves travel at the same speed, no matter what their frequency. This speed, usually called the speed of light, c, is approximately 3 × 10⁸ m/sec, or about 1 foot (30 cm) per nanosecond.

Frequency and Wavelength

The fundamental relation between f, λ, and c (in a vacuum) is:

$$λf = c$$

Since c is a constant, if we know f, we can find λ, and vice versa. As a rule of thumb, when λ is in meters and f is in MHz, λf ≈ 300.

For example, 100-MHz waves are about 3 meters long, 1000-MHz waves are 0.3 meters long, and 0.1 meter waves have a frequency of 3000 MHz.

Radio Frequency

Radio frequency (RF) waves are easy to generate, can travel long distances, and can penetrate buildings easily, so they are widely used for communication, both indoors and outdoors.

Radio waves are omnidirectional, meaning that they travel in all directions from the source, so the transmitter and receiver do not have to be carefully aligned physically.

Radio Wave Properties

The properties of radio waves are frequency dependent.

• At low frequencies, radio waves pass through obstacles well, but the power falls off sharply with distance from the source.
• At high frequencies, radio waves tend to travel in straight lines and bounce off obstacles. Path loss still reduces power, though the received signal can depend strongly on reflections as well.
• High-frequency radio waves are also absorbed by rain and other obstacles to a larger extent than low-frequency ones.

Microwave Transmission

Above 100 MHz, the waves travel in nearly straight lines and can therefore be narrowly focused. Concentrating all the energy into a small beam by means of a parabolic antenna (like the familiar satellite TV dish) gives a much higher signal-to-noise ratio, but the transmitting and receiving antennas must be accurately aligned with each other.

Microwave communication is widely used for long-distance telephone communication, mobile phones, television distribution, and other purposes. However, a severe shortage of spectrum has developed.

Microwave has several key advantages over fiber, the main one being that no right of way is needed to lay down cables. By buying a small plot of ground every 50 km and putting a microwave tower on it, one can bypass the telephone system entirely.

Infrared Transmission

Unguided infrared waves are widely used for short-range communication, such as the remote controls used for televisions.

Infrared waves are relatively directional, cheap, and easy to build but have a major drawback: they do not pass through solid objects.

For example, you cannot control your neighbor’s television with your remote control because infrared waves do not pass through solid walls.

Electromagnetic Spectrum