Unit6 - Subjective Questions

A data communications system consists of five fundamental components:

1. Message: The information (data) to be communicated. Popular forms of information include text, numbers, pictures, audio, and video.
2. Sender: The device that sends the data message. It can be a computer, workstation, telephone handset, video camera, and so on.
3. Receiver: The device that receives the message. It can be a computer, workstation, telephone handset, television, and so on.
4. Transmission Medium: The physical path by which a message travels from sender to receiver. Some examples include twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves.
5. Protocol: A set of rules that govern data communications. It represents an agreement between the communicating devices. Without a protocol, two devices may be connected but not communicating.

Guided Transmission Media:
Guided media, also known as wired or bounded media, provide a physical conduit from one device to another. Signals propagating along these media are directed and confined by the physical limits of the medium.

• Examples: Twisted-pair cable, coaxial cable, and fiber-optic cable.

Unguided Transmission Media:
Unguided media, also known as wireless or unbounded media, transport electromagnetic waves without using a physical conductor. Signals are broadcast through free space and are thus available to anyone who has a device capable of receiving them.

• Examples: Radio waves, microwaves, and infrared waves.

Key Differences:

• Physical Path: Guided media use a physical path; unguided media use air/vacuum.
• Directionality: Guided media are highly directional; unguided media range from omnidirectional to highly directional.
• Installation: Guided media usually require physical installation of cables; unguided media require antennas and towers.

Construction:
A fiber-optic cable is made of glass or plastic and transmits signals in the form of light. It consists of three main concentric layers:

• Core: The central tube of very thin glass or plastic that carries the light.
• Cladding: A surrounding layer of glass or plastic with a lower refractive index than the core.
• Jacket: An outer protective cover made of plastic or other materials to protect the cable from moisture, abrasion, and crushing.

Working Principle (Total Internal Reflection):
Fiber optics work on the principle of Total Internal Reflection. When light traveling in a denser medium (core) hits the boundary of a less dense medium (cladding) at an angle greater than the critical angle, the light is completely reflected back into the core.

Mathematically, according to Snell's law, if (where is the critical angle), the light ray bounces down the length of the fiber without escaping, allowing for high-speed data transmission over long distances with minimal signal loss.

Transmission media are not perfect. The imperfection causes signal impairment. The three primary causes are:

1. Attenuation:
   It means a loss of energy. When a signal travels through a medium, it loses some of its energy in overcoming the resistance of the medium. To compensate for this loss, amplifiers are used to amplify the signal. Attenuation is often measured in decibels (dB), calculated as: where is input power and is output power.

2. Distortion:
   Distortion means that the signal changes its form or shape. Distortion can occur 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. This difference in delay creates a phase shift, altering the shape of the composite signal.

3. Noise:
   Noise is any unwanted signal that gets mixed with the original signal. Types of noise include:

   • Thermal noise: Random motion of electrons in a wire.
   • Induced noise: From motors and appliances.
   • Crosstalk: Effect of one wire on another.
   • Impulse noise: A spike (a signal with high energy in a very short time) that comes from power lines, lightning, etc.

Shannon's Channel Capacity Theorem:
Claude Shannon introduced a formula to determine the theoretical highest data rate for a noisy channel. The capacity depends on the bandwidth of the channel and the signal-to-noise ratio (SNR).

The formula is given by:

Where:

• = Channel capacity in bits per second (bps)
• = Bandwidth of the channel in Hertz (Hz)
• = Signal-to-Noise Ratio (unitless power ratio)

Calculation:
Given:

• Bandwidth () = 3 kHz = 3000 Hz
• Signal-to-Noise Ratio (SNR) = 3162

Using the formula:

Since :

Therefore, the theoretical channel capacity is approximately 34.86 kbps.

Twisted Pair Cable:
A twisted pair consists of two conductors (normally copper), each with its own plastic insulation, twisted together. One wire carries the signal, and the other is used as a ground reference. The twisting helps to reduce crosstalk and electromagnetic interference (EMI) from external sources.

Difference between UTP and STP:

• Unshielded Twisted Pair (UTP):

  • Construction: Consists only of the twisted pairs of insulated copper wire covered by a simple plastic jacket.
  • Interference: More susceptible to EMI and crosstalk.
  • Cost & Installation: Cheaper, lighter, more flexible, and easier to install.
  • Usage: Commonly used in telephone systems and typical Ethernet networks (e.g., Cat 5e, Cat 6).

• Shielded Twisted Pair (STP):

  • Construction: Has a metal foil or braided mesh covering that encases each pair of insulated conductors, overall covered by a plastic jacket.
  • Interference: Offers better protection against EMI and crosstalk due to the shielding.
  • Cost & Installation: More expensive, heavier, thicker, and harder to install.
  • Usage: Used in environments with high electrical noise, like industrial settings.

1. Bandwidth:
   In networking, bandwidth is the potential measurement of a link, in bits per second (bps). It represents the maximum amount of data that can pass from one point to another in a unit of time. It is the theoretical capacity of a channel.

2. Throughput:
   Throughput is a measure of how fast we can actually send data through a network. While bandwidth is a potential measurement of a link, throughput is an actual measurement of how fast data is successfully transmitted. Throughput is always less than or equal to bandwidth due to overhead, retransmissions, and network congestion.

3. Latency (Delay):
   Latency defines how long it takes for an entire message to completely arrive at the destination from the time the first bit is sent out from the source. It consists of four components:

   • Propagation Time: Time taken for a bit to travel through the medium.
   • Transmission Time: Time taken to push all bits of a packet onto the link.
   • Queuing Time: Time the packet spends waiting in router queues.
   • Processing Delay: Time taken by routers to process the packet header.

1. Radio Waves:

• Frequency Range: 3 kHz to 1 GHz.
• Propagation: Omnidirectional (travel in all directions from the source).
• Penetration: Can penetrate solid walls, making them ideal for indoor/outdoor communication (e.g., AM/FM radio, TV).
• Interference: Susceptible to interference from other signals using the same frequency.

2. Microwaves:

• Frequency Range: 1 GHz to 300 GHz.
• Propagation: Unidirectional. They travel in a straight line, requiring line-of-sight between transmitting and receiving antennas.
• Penetration: Cannot penetrate solid objects well.
• Application: Used for point-to-point communication, satellite communication, cellular networks, and wireless LANs.

3. Infrared Waves:

• Frequency Range: 300 GHz to 400 THz.
• Propagation: Highly directional.
• Penetration: Cannot penetrate walls or opaque objects, which prevents interference between adjacent rooms.
• Application: Used for short-range communication in closed areas, like TV remote controls and wireless keyboards.

Coaxial Cable:
Coaxial cable (or coax) carries signals of higher frequency ranges than twisted-pair cable. It consists of a central inner conductor (solid copper), surrounded by an insulating layer (dielectric). Over this, there is a tubular conducting shield (metallic braid or foil) which serves as the outer conductor and shield against EMI, all wrapped in a protective outer plastic sheath.

Advantages:

• Higher bandwidth and data transmission rates compared to standard twisted pair.
• Better shielding provides higher resistance to electromagnetic interference (EMI) and crosstalk.
• Can support longer cable runs between network devices than twisted-pair cable.

Applications:

• Cable TV Networks: Used widely for distributing television signals.
• Traditional Ethernet LANs: Used in older Ethernet standards like 10Base-2 (Thinnet) and 10Base-5 (Thicknet).
• Broadband Internet: Used by cable internet service providers to deliver data to homes.

The IEEE 802.11 standard defines the architecture for Wireless Local Area Networks (WLANs). It consists of several components that define how wireless stations interact.

1. Basic Service Set (BSS):
The BSS is the fundamental building block of the 802.11 architecture. A BSS is a group of stations that coordinate their access to the medium under a single coordination function. There are two types:

• Independent BSS (IBSS): Also known as an ad-hoc network. Stations communicate directly with each other without an Access Point (AP).
• Infrastructure BSS: Includes an Access Point (AP). All communication between stations goes through the AP.

2. Extended Service Set (ESS):
An ESS is a set of two or more infrastructure BSSs interconnected by a distribution system (usually a wired LAN like Ethernet). This allows for a larger wireless network where stations in different BSSs can communicate with each other. It also enables roaming, where a mobile station can move from the coverage area of one AP (in one BSS) to another AP without losing network connection.

Here is a comparison of the three legacy IEEE 802.11 wireless standards:

1. IEEE 802.11a:

• Frequency Band: 5 GHz.
• Maximum Data Rate: 54 Mbps.
• Modulation Technique: OFDM (Orthogonal Frequency Division Multiplexing).
• Pros: Less interference since fewer devices use the 5 GHz band; high data rate.
• Cons: Shorter range and poor obstacle penetration compared to 2.4 GHz.

2. IEEE 802.11b:

• Frequency Band: 2.4 GHz.
• Maximum Data Rate: 11 Mbps.
• Modulation Technique: DSSS (Direct Sequence Spread Spectrum).
• Pros: Good range, better obstacle penetration, lower cost initially.
• Cons: Slowest data rate among the three; highly susceptible to interference from other 2.4 GHz devices (microwaves, cordless phones).

3. IEEE 802.11g:

• Frequency Band: 2.4 GHz.
• Maximum Data Rate: 54 Mbps.
• Modulation Technique: OFDM (for higher speeds) and DSSS (for backward compatibility with 802.11b).
• Pros: High speed comparable to 802.11a, excellent range, backward compatible with 802.11b.
• Cons: Operates in the crowded 2.4 GHz band, so it is still susceptible to interference.

IEEE 802.11n (also known as Wi-Fi 4) introduced several significant enhancements over its predecessors (a/b/g) to increase throughput and range:

1. MIMO Technology (Multiple-Input Multiple-Output): 802.11n uses multiple antennas at both the transmitter and receiver. This allows the transmission of multiple data streams simultaneously (spatial multiplexing), vastly increasing data rates and signal reliability.
2. Dual-Band Operation: It can operate on both the 2.4 GHz and 5 GHz frequency bands, providing flexibility and backward compatibility with 802.11b/g and 802.11a devices.
3. Channel Bonding: 802.11n introduced the ability to bond two adjacent 20 MHz channels together to create a 40 MHz channel, effectively doubling the data transmission rate.
4. Frame Aggregation: To reduce MAC layer overhead, 802.11n aggregates multiple MAC frames into a single larger frame before transmission, which improves throughput efficiency.
5. Higher Data Rates: By combining MIMO, channel bonding, and frame aggregation, 802.11n can theoretically achieve data rates up to 600 Mbps.

Bluetooth architecture defines how devices connect and form networks. The fundamental topologies are Piconet and Scatternet.

1. Piconet:
A piconet is the basic unit of a Bluetooth network. It can have up to eight active nodes (1 primary/master and up to 7 secondaries/slaves).

• Master/Primary Node: Controls the communication in the piconet. It determines the frequency-hopping sequence and timing.
• Slave/Secondary Nodes: Follow the master's synchronization and communicate only with the master, not directly with other slaves.
• Parked Nodes: A piconet can have up to 255 parked nodes. These nodes are synchronized with the master but cannot send or receive data until they are transitioned to an active state.

2. Scatternet:
A scatternet is formed when multiple piconets are combined.

• A slave in one piconet can act as a master in another piconet.
• A node can also act as a slave in two different piconets simultaneously.
• This overlapping node receives data from the master in the first piconet and can forward it to devices in the second piconet, thus expanding the network's reach beyond the limited range of a single piconet.

The Bluetooth protocol stack does not strictly follow the OSI or TCP/IP models but maps to their lower layers. The core layers include:

1. Radio Layer: Equivalent to the Physical Layer. It deals with radio transmission and modulation. Bluetooth operates in the 2.4 GHz ISM band and uses Frequency Hopping Spread Spectrum (FHSS) with 79 channels to avoid interference.
2. Baseband Layer: Equivalent to the MAC sublayer. It handles the physical links (SCO for synchronous/voice and ACL for asynchronous/data), establishes piconets, handles frequency hopping, and performs error correction and encryption.
3. L2CAP (Logical Link Control and Adaptation Protocol): Roughly equivalent to the LLC sublayer in IEEE 802 LANs. It provides multiplexing for higher-layer protocols, segmentation and reassembly of packets, and Quality of Service (QoS) management.
4. Host Controller Interface (HCI): Provides a command interface to the baseband controller and link manager, and access to hardware status and control registers.
5. Higher Layers: Include protocols like RFCOMM (serial cable emulation), SDP (Service Discovery Protocol for finding available services), and various application profiles (like A2DP for audio).

The Hidden Station (or Hidden Terminal) Problem:
This problem occurs in wireless networks when two stations are out of range of each other but both are within range of the same Access Point (AP) or a central receiving station.

Scenario:

• Let Station A and Station C be out of each other's radio range.
• Let Station B be in the middle, within range of both A and C.
• Station A starts transmitting data to Station B.
• Since Station C cannot hear Station A's transmission (because A is "hidden" from C), C assumes the medium is idle.
• Station C also starts transmitting data to Station B.
• The signals from A and C collide at Station B, causing data corruption.

Solution:
IEEE 802.11 solves this using the RTS/CTS (Request to Send / Clear to Send) mechanism. Station A sends an RTS to B. B broadcasts a CTS. Since C hears the CTS from B, it knows the medium is reserved and defers its transmission.

Purpose:
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is the MAC protocol used in IEEE 802.11 wireless networks. Unlike wired networks which use CSMA/CD (Collision Detection), wireless networks cannot detect collisions effectively because listening while transmitting is difficult (the transmitted signal overwhelms the received signal). Therefore, they must try to avoid collisions.

How it works:

1. Listen Before Talk: A station with a frame to transmit listens to the channel. If the channel is idle for a time period called DIFS (Distributed Inter-Frame Space), it proceeds.
2. Backoff Strategy: If the channel is busy, the station waits until it becomes idle, waits for a DIFS, and then selects a random backoff time slot within a contention window. It counts down this timer while the channel is idle; if the channel becomes busy, the timer pauses.
3. Transmission & ACK: When the timer reaches zero, the station transmits the frame. It then waits for an Acknowledgement (ACK) from the receiver. If no ACK is received (implying a collision or error), the station doubles its contention window and repeats the backoff process.

The Exposed Station Problem:
This problem occurs when a station refrains from transmitting because it incorrectly senses the medium as busy, leading to an underutilization of the network bandwidth.

Scenario:

• Assume four stations: A, B, C, and D arranged in a line.
• B and C are within each other's range.
• A is within B's range but not C's. D is within C's range but not B's.
• Suppose B is transmitting data to A.
• Station C wishes to transmit data to Station D.
• C listens to the medium and hears B's transmission. Following the carrier sense protocol, C concludes the medium is busy and defers its transmission to D.
• The Issue: B's transmission is aimed at A, which is out of C's range. C's transmission to D would not interfere with B's transmission to A at all. Thus, C is unnecessarily "exposed" to B's transmission and wastes time deferring.

While RTS/CTS helps with the hidden station problem, it does not perfectly solve the exposed station problem, which remains a challenge in wireless ad-hoc networks.

In a Bluetooth piconet, the Baseband layer can establish two types of physical links between the master and a slave:

1. Synchronous Connection-Oriented (SCO) link:

   • Purpose: Primarily used for time-sensitive data, such as real-time voice or audio.
   • Mechanism: The master allocates specific, regular slots for SCO links. It guarantees bandwidth and latency.
   • Error Handling: SCO links do not retransmit corrupted packets because real-time voice cannot tolerate the delay of retransmissions. Error correction (like FEC) is used instead.

2. Asynchronous Connectionless Link (ACL):

   • Purpose: Primarily used for general data transmission where integrity is more important than timing.
   • Mechanism: Uses the slots not reserved by SCO links. The master decides which slave can transmit based on a polling mechanism.
   • Error Handling: Data integrity is crucial, so ACL uses ARQ (Automatic Repeat reQuest). If a packet is lost or corrupted, it is retransmitted. It supports variable data rates and packet sizes.

Signal-to-Noise Ratio (SNR):
SNR is a critical metric in data communications that compares the level of a desired signal to the level of background noise. It is defined as the ratio of average signal power to average noise power.

A high SNR indicates that the signal is much stronger than the noise, which is ideal for accurate data transmission. A low SNR indicates a noisy channel where the signal might be corrupted.

SNR in Decibels ():
Because SNR can vary over a very wide range, it is often expressed in decibels, a logarithmic scale. The formula is:

Calculation:
Given SNR = 1000.

Since :

Nyquist Bit Rate:
Harry Nyquist formulated a theorem to determine the maximum data rate of a theoretical noiseless channel. The Nyquist bit rate depends on the bandwidth of the channel and the number of signal levels used to represent data.

The formula is:

Where:

• = Maximum data rate in bits per second (bps)
• = Bandwidth of the channel in Hertz (Hz)
• = Number of signal levels used to represent data

Explanation:

• According to Nyquist, a signal can be sampled or changed at most times per second without causing Inter-Symbol Interference (ISI).
• If a signal has levels, each signal element can carry bits.
• Therefore, by increasing the number of signal levels (), one can increase the bit rate without increasing the bandwidth, though this requires the receiver to be more sophisticated to distinguish between the smaller voltage differences in the levels.