Wireless Networks Overview

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Questions and Answers

What is the primary characteristic of BPSK 3/4 in terms of data presentation?

It presents all new data without any repetition.

It presents one-fourth new data and repeats three-fourths.

It presents three-fourths new data and repeats one-fourth. (correct)

It presents half the new data and repeats half.

Which of the following represents a correct relationship among signal-to-noise ratio, data rate, and bandwidth?

Increasing bandwidth decreases data rate.

A lower data rate reduces the required signal-to-noise ratio.

Higher signal-to-noise ratio allows for a higher data rate. (correct)

Bandwidth is irrelevant in determining signal-to-noise ratio.

In a multiple-input multiple-output (MIMO) system described as T×R, what do T and R represent?

T represents the total number of channels, R represents the maximum frequency.

T represents transmission speed, R represents data loss.

T represents time slots, R represents error correction codes.

T represents the number of transmitters, R represents the number of receivers. (correct)

Which modulation technique is a combination of Amplitude Shift Keying (ASK) and Phase Shift Keying (PSK)?

QAM Signup and view all the answers

What is the significance of channel aggregation in 802.11n?

It enables operation on either a single 20-MHz or a 40-MHz channel. Signup and view all the answers

What is the range of frequencies typically referred to as the 2.4-GHz band in wireless LAN communication?

2.400 to 2.4835 GHz Signup and view all the answers

Which property of RF signals is NOT one of the primary signal characteristics mentioned?

Wavelength Signup and view all the answers

What does a higher amplitude in an RF signal typically indicate?

Stronger signal strength Signup and view all the answers

What is the reference power level commonly used in wireless networks to measure signal strength?

1 mW Signup and view all the answers

Which formula is used to compare power levels between transmitters?

$dB = 10 imes ext{log}_{10}(P_2 / P_1)$ Signup and view all the answers

What is the main function of an isotropic antenna in RF communication?

To provide a reference point for gain measurements Signup and view all the answers

What does EIRP stand for in the context of wireless communication?

Effective isotropic radiated power Signup and view all the answers

What does RSSI stand for in relation to signal measurement?

Received signal strength indicator Signup and view all the answers

What is the effect of doubling the channel width to 40 MHz on throughput?

Throughput is doubled. Signup and view all the answers

Which feature of a 3×3:2 MIMO device supports two unique spatial streams?

It has three transmitters and three receivers. Signup and view all the answers

What does transmit beamforming (T×BF) accomplish?

It optimizes the signal phase for a specific receiver. Signup and view all the answers

Which modulation technique does 802.11ac use to enhance throughput?

256-QAM Signup and view all the answers

How does 802.11ac achieve better channel aggregation?

By bonding 40-MHz channels into wider ones. Signup and view all the answers

Which type of MIMO technology does 802.11ac use to send multiple frames to multiple devices at once?

Multi-user MIMO (MU-MIMO) Signup and view all the answers

What does the Null Data Packet (NDP) function in 802.11ac?

It is used to enable explicit transmit beamforming. Signup and view all the answers

Which of the following features is NOT associated with improving the reliability of 802.11n signals?

Intersymbol interference (ISI) Signup and view all the answers

What is the definition of gain in the context of an antenna?

The measure of how effectively an antenna can focus RF energy in a specific direction Signup and view all the answers

What type of antenna has the largest beamwidth?

Omnidirectional antennas like dipole and monopole Signup and view all the answers

What occurs when there is a mismatch in polarization between antennas?

Lower efficiency and potential signal loss Signup and view all the answers

Which statement is true regarding the gain and beamwidth of antennas?

As beamwidth narrows, gain increases for directional antennas. Signup and view all the answers

Is 802.11 a full duplex communication standard?

No, 802.11 only supports half duplex communication. Signup and view all the answers

What is the primary function of a wireless access point (AP) in a Basic Service Set (BSS)?

To operate in infrastructure mode and manage wireless communication Signup and view all the answers

What does the term Basic Service Area (BSA) refer to in the context of BSS?

The area where the wireless signal from the AP is usable Signup and view all the answers

Which statement accurately describes the role of a distribution system (DS) in a wireless BSS?

It serves as the upstream wired Ethernet for managing data traffic Signup and view all the answers

What is a key requirement for an access point (AP) to support multiple SSIDs?

Being connected to the switch by a trunk link carrying multiple VLANs Signup and view all the answers

In what scenario would a workgroup bridge be most appropriately used?

To connect a group of wired devices to a wireless network Signup and view all the answers

Study Notes

Comparing Wired and Wireless Networks

Wireless networks provide advantages such as mobility, flexibility, and ease of installation.
Wireless networks can be deployed faster and more cost-effectively, especially in areas with difficult terrain or where installing wired infrastructure is challenging.

Wireless Signal Properties

RF signals travel as a combination of electric and magnetic fields that are always at right angles to each other.
The frequency of an RF signal is the number of complete cycles per second (Hz).
Two main frequency ranges used for wireless LAN communication are:
- The 2.4-GHz band, between 2.400 and 2.4835 GHz
- The 5-GHz band, between 5.150 and 5.825 GHz
Signal bandwidth refers to the actual frequency range required for the transmitted signal.
Bandwidth is the difference between the highest and lowest frequencies in a signal, while channel width refers to the frequency range assigned to a specific communication channel.

RF Signal Properties: Amplitude, Phase, and Frequency

Amplitude refers to the signal’s strength or power.
Phase describes the position of a wave relative to a reference point, measured in degrees or radians.
Frequency measures the number of cycles per second (Hz).
Changes in amplitude, phase, and frequency impact the shape of the signal and can affect its transmission and reception.

Signal Power Levels

Power levels are often expressed in decibels (dB), which provide a logarithmic measure of power ratios.
The formula for comparing power levels between two transmitters is:
```
dB = 10 * log10 (P2 / P1)
```
The dBm (dB-milliwatt) unit is used to express power levels in wireless networks, with a reference power level of 1 mW.

Antenna Gain

Antenna gain is the measure of its ability to focus RF energy in a specific direction.
It’s measured in dBi (dBisotropic), comparing the antenna’s performance to an isotropic antenna, which radiates equally in all directions.
Effective isotropic radiated power (EIRP) represents the actual power level radiated from the antenna, considering antenna gain.

Signal Encoding Techniques: Digital Modulation

Digital modulation is used to encode digital data onto analog RF signals.
Signal encoding criteria are influenced by the signal-to-noise ratio (SNR), the desired data rate, and the available bandwidth.
Factors used to compare encoding schemes include spectrum efficiency, power efficiency, and implementation complexity.
The data rate is the number of bits transmitted per second, while the signal rate (baud rate) represents the number of symbols transmitted per second.
Unipolar NRZ (Non-Return to Zero) is a basic encoding scheme where a high voltage level represents a "1" bit, and a low voltage level represents a "0" bit.
Differential coding uses the transition between voltage levels to represent bits, rather than the absolute voltage levels themselves.
Encoding relationship:
```
Rb (Bit rate) = M * Rs (Baud rate)
```
where 'M' bits are encoded in each symbol element.

Modulation Techniques

Basic Modulation Techniques:
- Amplitude-Shift Keying (ASK): Encodes data by varying the amplitude of the carrier signal while keeping the frequency and phase constant.
- Frequency-Shift Keying (FSK): Encodes data by varying the frequency of the carrier signal while keeping the amplitude and phase constant.
- Phase-Shift Keying (PSK): Encodes data by varying the phase of the carrier signal while keeping the amplitude and frequency constant.
Two-level PSK:
- BPSK (Binary Phase-Shift Keying): Uses two distinct phase shifts (0° and 180° ) to represent "0" and "1" bits.
- DPSK (Differential Phase-Shift Keying): Encodes data based on the difference in phase between consecutive bits, eliminating the need for absolute phase reference.
Four-level PSK (QPSK): Encodes data using four different phase shifts (0°, 90°, 180°, 270°) to represent bit combinations.
QAM (Quadrature Amplitude Modulation): Combines both amplitude and phase shifts to encode data, offering higher data rates and more efficient spectrum utilization.

Regulatory Bodies and Standards

Regulatory bodies like the FCC (Federal Communications Commission) and CE (European Conformity) establish rules and regulations to govern wireless device operation and spectrum allocation.
ISM (Industrial, Scientific, and Medical) bands are frequency ranges designated for non-licensed use.
IEEE 802 Working Groups define standards for wired and wireless networking.
802.11 standards govern wireless LANs and specify channel usage, modulation techniques, and data rates.

802.11 Channel Use and Modulation

The 2.4-GHz ISM band uses 14 channels (1–14), each with a 22-MHz bandwidth.
The 5-GHz U-NII (Unlicensed National Information Infrastructure) bands offer various channel configurations with different bandwidths and spacing.

802.11n and 802.11ac Features

802.11n introduced several features, including:
- MIMO (Multiple-Input, Multiple-Output) technology for enhanced data rates and range by using multiple antennas for both transmission and reception.
- Channel aggregation to combine two 20-MHz channels into a wider 40-MHz channel for increased throughput.
- Spatial multiplexing (SM) to distribute data across multiple parallel data streams.
- Multipath to improve signal reliability by leveraging multiple signal paths to reach the receiver.
- Block acknowledgments to reduce overhead and enhance efficiency.
- Guard intervals to protect against intersymbol interference (ISI).
- Transmit beamforming (T×BF) to focus the transmitted signal toward the intended receiver, improving signal strength and reducing interference.
802.11ac further enhanced performance, including:
- Wider channel aggregation: Utilizing larger channels by combining 20-MHz channels into 40, 80, or even 160 MHz channels.
- Denser modulation: Employing 256-QAM (Quadrature Amplitude Modulation) with 256 different phase and amplitude combinations to encode more data per symbol.
- Improved MAC layer efficiency: Increased data aggregation and reduced overhead for enhanced throughput.
- Explicit T×BF: Simplifying transmit beamforming with a single feedback method.
- Scalable MIMO: Supporting up to eight spatial streams.
- Multi-user MIMO (MU-MIMO): Allowing an access point (AP) to transmit data to multiple devices simultaneously.

Antenna Gain and Beamwidth

Antenna gain is a measure of its effectiveness in focusing RF energy in a specific direction.
Beamwidth refers to the angular range over which an antenna radiates its signal effectively.
Radiation patterns chart the antenna’s signal strength in various directions.
Omnidirectional antennas radiate equally in all directions, having a wide beamwidth and lower gain.
Directional antennas focus signal in a specific direction, resulting in a narrower beamwidth and higher gain.

Antennas: Types and Characteristics

Omnidirectional Antennas:
- Dipole antennas: Simple and common, with a figure-eight radiation pattern.
- Monopole antennas: Similar to a dipole but with one terminal grounded, commonly used in mobile devices.
- Integrated omnidirectional antennas: Compact and integrated into devices for convenience.
Directional Antennas:
- Patch antennas: Used for high-frequency applications, often found in WLAN equipment.
- Yagi antennas: Provide directional gain with multiple elements, widely used in TV reception.
- Parabolic dish antennas: Highly directional with a large reflective surface, used for satellite communication.
Polarization describes the orientation of the electromagnetic field within an RF signal.
Linear polarization is the most common type, where the electric field oscillates in a straight line.
Circular polarization is another type where the electric field rotates in a helical pattern.
Antenna polarization mismatches can lead to signal loss.

Wireless Network Topologies

Unidirectional communication: Data flows in only one direction.
Bidirectional communication: Data can flow in both directions.
Half Duplex: Data can flow only in one direction at a time, requiring switching (like a walkie-talkie).
Full duplex: Data can flow in both directions simultaneously, achieving full-time communication.
Basic Service Set (BSS): The fundamental unit of a wireless network, consisting of one or more stations (STAs) communicating within the coverage area of an AP (Access Point).
APs operate in infrastructure mode and provide central control and data routing for STAs within their coverage.
The Basic Service Area (BSA), also known as a cell, is the physical coverage area of an AP, defined by the usable signal strength.
BSS identifier (BSSID): A unique MAC address assigned to each BSS.
Service set identifier (SSID): A user-defined name for the BSS, used by STAs for network identification.
Association: When an STA connects to a BSS, it is associated with that AP.
Distribution System (DS): A wired infrastructure that connects the APs to the rest of the network, providing backhaul connectivity.
AP as a translational bridge: Bridges between different media (wireless and wired) to enable smooth communication.
Multiple SSIDs on one AP: Multiple BSSs can be created on a single AP using Virtual LANs (VLANs).
Extended Service Set (ESS): A collection of interconnected BSSs, extending the wireless network coverage by allowing STAs to roam between APs.
Independent Basic Service Set (IBSS), Ad Hoc network: A peer-to-peer network where STAs directly communicate with each other without an AP.
Other Wireless Topologies:
- Repeater: Extends the range of a wireless network by retransmitting signals.
- Workgroup Bridge (WGB): Connects two different wired networks using a wireless bridge.
- Outdoor Bridge (Point-to-Point): Connects two fixed locations directly using a wireless link.
- Outdoor Bridge (Point-to-Multipoint): Establishes a wireless link from a central point to multiple remote locations.
- Mesh Network: A self-healing network with interconnected nodes that can communicate with each other, providing redundancy and robustness.

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Description

This quiz explores the comparison between wired and wireless networks, focusing on the benefits of wireless technology. Topics include the properties of wireless signals, frequency ranges, and concepts like bandwidth and channel width.