Unit 2: Physical layer - Subjective Questions

Analog signal: An analog signal is a continuous waveform whose amplitude can take any value within a given range. Voice signals transmitted through a traditional telephone system are an example.

Digital signal: A digital signal has discrete amplitude levels and is commonly represented using binary values, such as 0 and 1. Data exchanged between computers is an example.

Major differences:

• Nature: Analog signals are continuous, whereas digital signals are discrete.
• Values: Analog signals have infinitely many possible values; digital signals use a finite number of levels.
• Noise immunity: Digital signals are generally more resistant to noise because they can be regenerated.
• Transmission quality: Analog quality gradually deteriorates with distance, while digital signals can be restored using repeaters.
• Examples: Radio broadcasts are commonly analog, while Ethernet communication is digital.

A periodic analog signal repeats its pattern after a fixed interval.

• Amplitude: The amplitude is the maximum strength or height of the signal. It is usually measured in volts.
• Period: The period, represented by , is the time required to complete one cycle. It is measured in seconds.
• Frequency: Frequency, represented by , is the number of cycles completed per second. It is measured in hertz.
• Relationship: Frequency and period are related by

• Phase: Phase specifies the position of the waveform relative to time zero. It is measured in degrees or radians.

For example, a signal with a period of seconds has a frequency of Hz.

A composite signal is formed by combining two or more sine waves having different frequencies, amplitudes, or phases. Most practical signals, including voice and data signals, are composite rather than simple sine waves.

• The frequency spectrum is the collection of all frequency components present in a signal.
• The bandwidth of a composite signal is the difference between its highest and lowest significant frequencies.

If the highest frequency is and the lowest frequency is , then

For example, if a signal contains frequencies from Hz to Hz, its bandwidth is

A greater bandwidth allows a signal to carry more rapidly changing information.

Data rate is the number of data bits transmitted per second and is measured in bits per second. Bandwidth is the range of frequencies supported by a channel and is measured in hertz.

Nyquist formula

For a noiseless channel, the maximum bit rate is

where:

• is the maximum bit rate in bits per second,
• is the channel bandwidth in hertz,
• is the number of discrete signal levels.

The factor represents the maximum symbol rate of a noiseless band-limited channel.

Shannon capacity formula

For a noisy channel, the theoretical maximum capacity is

where is channel capacity, is bandwidth, and is the signal-to-noise power ratio.

Significance

• Nyquist shows how bandwidth and signal levels affect the bit rate in a noiseless channel.
• Shannon establishes an absolute upper limit for a noisy channel.
• Increasing the number of signal levels raises the Nyquist rate but may make levels harder to distinguish in the presence of noise.
• Reliable practical transmission must satisfy both limitations.

Bit rate is the number of bits transmitted in one second. Its unit is bits per second.

Baud rate is the number of signal elements or symbols transmitted in one second. Its unit is baud.

If each symbol represents bits, then

For signal levels, the number of bits represented by each symbol is

Given :

Therefore,

Thus, the system has a baud rate of baud and a bit rate of bps.

Transmission impairments are effects that cause the received signal to differ from the transmitted signal.

• Attenuation: It is the loss of signal strength as the signal travels through a medium. Amplifiers or repeaters may be used to compensate for it.
• Distortion: It occurs when different frequency components of a composite signal experience different delays or attenuation. As a result, the shape of the received signal changes.
• Noise: Noise is unwanted energy added to the signal during transmission. It may produce errors in the received data.

Common types of noise include:

• Thermal noise: Random motion of electrons in a conductor.
• Induced noise: Electromagnetic interference from motors, power lines, or appliances.
• Crosstalk: Interference from an adjacent communication channel.
• Impulse noise: Sudden high-energy disturbances caused by lightning or switching systems.

Attenuation or gain is commonly expressed in decibels, abbreviated as dB. For power values, it is calculated using

where is the input power and is the output power.

Given:

• mW
• mW

Therefore,

The negative sign indicates a loss of signal power. Thus, the channel introduces an attenuation of 10 dB, or equivalently a gain of dB.

The signal-to-noise ratio, or SNR, compares signal power with noise power:

In decibels,

A higher SNR indicates a cleaner signal and generally permits a higher reliable data rate.

Given dB:

Using Shannon's formula with Hz:

Since :

Therefore, the theoretical channel capacity is approximately 29.9 kbps.

A twisted-pair cable consists of two insulated copper conductors twisted around each other. Twisting reduces electromagnetic interference and crosstalk.

Types

• Unshielded Twisted Pair: It has no additional metallic shielding. It is inexpensive and widely used in local area networks.
• Shielded Twisted Pair: It includes a metallic shield for better protection from interference.

Advantages

• Low cost and easy installation
• Flexible and lightweight
• Commonly available
• Suitable for telephone and Ethernet networks

Disadvantages

• Higher attenuation than coaxial cable or optical fiber
• Limited distance and bandwidth
• Susceptible to interference, especially when unshielded

Applications

• Telephone subscriber lines
• Ethernet LANs
• Digital subscriber line connections
• Internal building communication wiring

A coaxial cable contains conductors arranged around a common axis.

Structure

• A central copper conductor carries the signal.
• A dielectric insulator surrounds the central conductor.
• A metallic foil or braided shield protects the signal from electromagnetic interference.
• An outer plastic jacket provides mechanical protection.

The outer metallic shield also acts as a return path and reduces signal leakage. Because of its shielding, coaxial cable supports greater bandwidth and longer distances than conventional twisted-pair cable.

Advantages

• Better resistance to noise and crosstalk
• Greater bandwidth than basic twisted pair
• Strong and durable construction
• Can carry analog and digital signals

Applications

• Cable television distribution
• Broadband Internet access
• Closed-circuit television systems
• Radio-frequency connections
• Older Ethernet installations

An optical fiber carries data as pulses of light.

Construction

• Core: The central glass or plastic region through which light travels.
• Cladding: A surrounding layer with a lower refractive index.
• Buffer and jacket: Protective layers that prevent physical damage.

Working principle

Optical fiber operates using total internal reflection. When light reaches the core-cladding boundary at a suitable angle, it is reflected back into the core instead of escaping.

Single-mode fiber

• Has a very small core.
• Carries one main propagation mode.
• Provides high bandwidth and low dispersion.
• Suitable for long-distance communication.
• Commonly uses laser transmitters.

Multimode fiber

• Has a larger core.
• Carries multiple light paths or modes.
• Has more modal dispersion.
• Suitable for shorter distances.
• Commonly uses LEDs or lower-cost laser sources.

The three major guided media can be compared as follows:

• Material: Twisted pair and coaxial cable use copper conductors, whereas optical fiber uses glass or plastic.
• Signal type: Copper cables carry electrical signals; optical fiber carries light signals.
• Bandwidth: Twisted pair generally offers the lowest bandwidth, coaxial cable offers moderate bandwidth, and fiber offers the highest bandwidth.
• Distance: Fiber supports longer transmission distances with lower attenuation.
• Noise immunity: Twisted pair is most vulnerable to electromagnetic interference. Coaxial shielding improves immunity, while fiber is immune to electromagnetic interference.
• Security: Fiber is difficult to tap and does not radiate electromagnetic energy, making it more secure.
• Cost: Twisted pair is usually cheapest. Coaxial cable has moderate cost, while fiber equipment and installation may be more expensive.
• Applications: Twisted pair is used in telephone and Ethernet networks, coaxial cable in cable television, and fiber in backbones and long-distance high-speed networks.

Radio waves are electromagnetic waves used for wireless communication. They are generally omnidirectional, meaning that they can spread in many directions from the transmitting antenna.

Characteristics

• Radio waves can travel through walls and other obstacles.
• Lower-frequency radio waves can cover large geographical areas.
• Their propagation may occur through ground waves, sky waves, or line-of-sight paths.
• Omnidirectional antennas can support mobile receivers.

Advantages

• Supports mobility and broadcast communication
• Does not always require precise antenna alignment
• Can cover large areas
• Useful where cable installation is difficult

Limitations

• Susceptible to interference and noise
• Signals may be intercepted, creating security risks
• Frequency bands are regulated
• Shared spectrum can cause congestion

Applications

• AM and FM broadcasting
• Television broadcasting
• Mobile communication
• Wi-Fi and Bluetooth
• Maritime and aviation communication

Terrestrial microwave communication uses high-frequency electromagnetic waves transmitted between directional antennas.

Main features

• Microwave signals are usually unidirectional.
• Parabolic dish antennas concentrate energy into narrow beams.
• Directional transmission reduces interference and supports point-to-point links.
• Microwaves provide relatively high bandwidth.

Line-of-sight requirement

Microwaves generally travel in straight lines and do not bend significantly around large obstacles. Therefore, transmitting and receiving antennas must have a clear line of sight. Antennas are often installed on towers or tall buildings.

Need for repeaters

The curvature of the Earth and physical obstacles limit the distance of a direct microwave link. Repeaters are placed between endpoints to receive, regenerate or amplify, and retransmit the signal.

Applications

• Cellular network backhaul
• Point-to-point telephone links
• Television distribution
• Communication between buildings
• Long-distance data transmission

Satellite communication

A communication satellite receives an uplink signal from an Earth station, processes or amplifies it, and sends it back through a downlink. A satellite acts as a microwave repeater in space.

• Provides very wide geographical coverage
• Supports broadcasting and long-distance links
• Requires line-of-sight communication
• Can experience propagation delay, particularly with high-altitude satellites
• Used for television, navigation, weather monitoring, telephony, and Internet access

Infrared communication

Infrared communication uses electromagnetic waves with frequencies above microwaves and below visible light.

• Commonly supports short-range communication
• Usually uses line-of-sight or reflected indoor propagation
• Cannot penetrate walls effectively
• Experiences limited interference between separate rooms
• Used in remote controls, sensors, and short-range device communication

Comparison: Satellites provide long-distance and wide-area coverage, whereas infrared is mainly intended for short-range indoor communication. Both may require a clear path, but their ranges and applications differ greatly.

Radio waves, microwaves, and infrared waves transmit information without a physical conductor.

• Directionality: Radio waves are often omnidirectional. Microwaves are mainly directional. Infrared is generally directional or confined by reflection within a room.
• Range: Radio can provide local or wide-area coverage. Terrestrial microwave supports point-to-point links, while satellite microwave can cover very large areas. Infrared usually has a short range.
• Obstacle penetration: Radio waves may penetrate walls. Microwaves are more dependent on line of sight. Infrared generally cannot penetrate walls.
• Interference: Radio bands are vulnerable to interference from many sources. Directional microwave links reduce interference but can be affected by obstacles and weather. Infrared communication in one room usually does not interfere with another room.
• Antennas: Radio may use omnidirectional antennas, while microwave commonly uses dish or horn antennas.
• Applications: Radio is used in broadcasting and mobile systems, microwave in cellular backhaul and satellites, and infrared in remote controls and short-range links.

A repeater is a physical-layer device that receives a weakened or distorted signal, regenerates it, and retransmits it. It extends the maximum distance of a network but does not inspect addresses or filter traffic.

A hub is a multiport repeater. Data received on one port is repeated to all other ports. Devices connected through a conventional hub share bandwidth and belong to the same collision domain.

Types of hubs

• Passive hub: Provides a physical connection between devices but does not regenerate or amplify signals.
• Active hub: Regenerates and retransmits incoming signals. It requires electrical power.
• Intelligent hub: Provides active-hub functions together with monitoring, diagnostics, and management capabilities.

Neither a repeater nor a basic hub makes forwarding decisions based on MAC or IP addresses.

Both bridges and switches mainly operate at the data-link layer and forward frames using MAC addresses.

Bridge

• Connects a small number of LAN segments.
• Learns source MAC addresses and stores them in a forwarding table.
• Filters a frame if the destination is on the same segment as the source.
• Forwards the frame when the destination belongs to another segment.

Switch

• Is effectively a high-speed multiport bridge.
• Usually provides one dedicated collision domain per port.
• Supports many simultaneous frame transfers.
• Often provides features such as virtual LANs, port security, and link aggregation.

Address-learning process

1. The device reads the source MAC address of an incoming frame.
2. It associates that address with the incoming port.
3. It looks up the destination MAC address.
4. It forwards the frame to the known destination port or floods it if the destination is unknown.

A switch generally offers more ports, better performance, and greater functionality than a traditional bridge.

A router is primarily a network-layer device that connects different IP networks. It reads destination IP addresses, consults a routing table, selects a suitable path, and forwards packets toward their destination. Routers also separate broadcast domains.

A gateway connects systems that may use different protocols, data formats, or architectures. It can perform protocol conversion and may operate across several layers. The term also commonly refers to the router through which a host reaches external networks.

Difference from a switch

• A basic switch works mainly at the data-link layer, while a router works at the network layer.
• A switch normally forwards frames using MAC addresses; a router forwards packets using IP addresses.
• A switch connects devices within a LAN; a router connects different networks.
• Each switch port creates a separate collision domain, while a router also separates broadcast domains.
• A gateway may translate protocols, whereas ordinary switches and routers generally forward data according to protocols they support.

A suitable network design would use the following devices:

• Switches for laboratories: A switch connects computers within each laboratory. It learns MAC addresses and sends frames to the appropriate port, reducing unnecessary traffic and collisions.
• Routers for departmental LANs: A router connects different IP networks or subnets, selects routes, and prevents ordinary broadcasts from passing between departments.
• Repeater for excessive cable distance: If a cable run exceeds the supported distance, a repeater regenerates the weakened signal before retransmission. Fiber may be a better long-term solution for very long links.
• Wireless access points: An access point connects Wi-Fi devices to the wired LAN and provides managed wireless coverage.
• Gateway or edge router for Internet access: The default gateway forwards traffic to external networks. It may also provide network address translation, firewall rules, and traffic filtering.
• Modem or optical network terminal: This terminates the service provider's access technology and converts signals where required.

The final design should also consider bandwidth, media type, security, coverage, redundancy, and future expansion.