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Questions and Answers
What maximum data rate can the IEEE 802.11n standard achieve with 4 spatial streams?
What is the main access method used by IEEE 802.11 standards for multiple access?
Which standard is known as WiFi?
What frequency range does Bluetooth operate within?
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Which IEEE standard is designed for low data rate and very long battery life applications?
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What technology does Bluetooth use for transmitting data?
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Which standard is targeted for wireless metropolitan area networks?
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What is the bandwidth of each Bluetooth channel?
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What is the primary purpose of a Wireless Local Area Network (WLAN)?
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Which of the following is NOT a design goal for WLANs?
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What benefit of WLANs increases both working efficiency and productivity?
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What is a significant challenge associated with WLANs as compared to wired networks?
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Which characteristic is a primary goal for WLAN products regarding user convenience?
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Which area is particularly advantageous for WLAN installations due to the difficulty of wiring?
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What is NOT a factor that contributes to long-term cost savings in WLAN deployment?
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What should WLAN security ensure to protect user data?
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What is the purpose of the RTS (Request to Send) in the RTS-CTS exchange scenario?
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In the addressing scheme, what is the role of Address 3?
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What is the consequence of a collision during the RTS(A) phase?
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Which address is used exclusively in ad-hoc mode according to the addressing scheme?
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What does an ACK packet signify in the context of the RTS-CTS framework?
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What is the primary function of DCF in the MAC layer?
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Which mechanism does DCF use to provide contention-free access?
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What problem does the hidden station scenario illustrate in a WLAN?
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What is a key challenge in collision detection for WLANs compared to LANs?
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What does the term 'Carrier' imply in the CSMA protocol?
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How does a node participate in the CSMA protocol when it detects the medium is in use?
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What functionality does PCF provide that DCF does not?
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What happens to the complete packet in WLAN when a collision occurs?
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What is the role of the Access Point (AP) in a wireless LAN?
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Which statement correctly describes the Distribution System (DS)?
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What are the advantages of using radio frequency for wireless transmission?
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What are the two basic access mechanisms defined for IEEE 802.11?
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Which of the following is NOT a feature provided by Access Points?
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What is the primary function of the MAC sublayer in the data link layer?
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What does the term Extended Service Set (ESS) refer to?
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Which frequency band is typically used for wireless transmission in the USA?
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Study Notes
WLAN Overview
- WLANs utilize radio waves for wireless communication.
- The last user connection is wireless, enabling network access within a building, campus, or specific rooms.
- The core network typically uses cables for higher stability and reliability.
WLAN Goals
- Wireless access flexibility in offices, homes, and manufacturing settings.
- Coverage typically limited to buildings, campuses, or individual rooms.
- Aiming to replace wired networks in offices, enhance connection flexibility (especially for portable devices), and enable ad-hoc communications for meetings.
WLAN Design Goals
- Global functionality of WLAN products for seamless operation.
- Low power consumption for battery-powered devices (power-saving modes and management functions).
- No special permissions or licenses required (license-free band operation).
- Robust transmission technology to ensure reliable data transfer.
- User-friendly design for ease of use and simple management.
- Protection of wired network investments (support the same data types and services).
- Strong security features: data privacy, user profile protection, and low radiation safety.
WLAN Benefits
- Mobility: Increases working efficiency and productivity, extends online periods.
- Installation in challenging areas: Inside buildings, road crossings, and other difficult-to-wire locations.
- Enhanced reliability: More resilient against disaster scenarios (earthquakes, fires).
- Reduced installation time: Faster deployment than wired networks, convenient for users, particularly in challenging installations.
- Long-term cost savings: Generally cheaper than wired networks, easier maintenance, reduced cabling costs, improved efficiency, and accuracy.
WLAN Known Problems
- Wireless link characteristics: Error-prone communication media, higher bit error rate (BER) compared to wired networks.
- IEEE 802.11n standard: Up to 300 Mbit/s data rate (2 spatial streams), 600 Mbit/s (4 spatial streams).
- Carrier-sense multiple access with collision avoidance (CSMA/CA) used for multiple access.
WLAN Standards
- 2.4 GHz and 5 GHz frequency bands are used for wireless LANs.
- 802.11, 802.11b, 802.11g, HiSWANa, 802.11a, HiperLAN2 are common standards with varying data rates.
- HomeRF 2.0, Bluetooth, and HomeRF 1.0 are other technologies with different data rates.
- 802.11e (Quality of Service), 802.11f (Inter Access Point Protocol), 802.11h (Transmit Power Control & Dynamic Frequency Selection), and 802.11i (Security) are additional standards.
Wireless Network Technology Options
- Wireless Personal Area Network (WPAN): IEEE 802.15.1, known as Bluetooth.
- Low-rate WPAN (LR-WPAN): IEEE 802.15.4, known as ZigBee.
- Wireless Local Area Network (WLAN): IEEE 802.11, known as WiFi.
- Wireless Metropolitan Area Network (WMAN): IEEE 802.16, known as WiMAX.
WPAN
- Wireless personal area network (WPAN) utilizes Bluetooth technology for short-range radio communication.
- Short range, low power consumption, and low cost define Bluetooth.
- Bluetooth operates in the 2.4 GHz band, a license-free industrial, scientific, and medical (ISM) frequency range.
- Bluetooth uses frequency-hopping spread spectrum (FHSS) for data transmission, dividing data into packets transmitted on one of 79 distinct Bluetooth channels (each channel has a 1 MHz bandwidth).
Low-rate WPAN (LR-WPAN)
- Designed for low data rates, extended battery life (months or years), and low-complexity applications.
- Differences lie in the physical and link layers, with distinct media and access control mechanisms.
IEEE 802.11 Terminology
- Basic Service Set (BSS): Group of stations sharing the same radio frequency.
- Access Point (AP): Station integrated into the WLAN and the distribution system.
- Station (STA): Terminal with wireless medium access and radio contact to the access point.
- Portal: Bridge to other (wired) networks.
- Distribution System (DS): Interconnection network forming a logical network.
- Extended Service Set (EES): Based on multiple BSSs.
Distribution System (DS)
- Interconnects wireless cells (multiple BSSs to form an ESS).
- Enables multiple mobile stations to access fixed resources.
- Interconnects 802.11 technology.
Access Points (AP)
- Bridges wireless LAN traffic to the wired LAN.
- Acts as a repeater for wireless nodes, extending the maximum distance between nodes.
- Allows stations to associate with it.
- Supports Distributed Coordination Function (DCF) and Point Coordination Function (PCF).
- Provides management features: Joining/Associating with BSS, time synchronization (beaconing), and power management.
- All traffic flows through APs.
- Supports roaming.
BSS, ESS, and DS
- BSS (Basic Service Set): A group of stations connected to the same access point, sharing the same wireless channel.
- ESS (Extended Service Set): Multiple BSSs interconnected to form a larger network, allowing roaming between APs.
- DS (Distribution System): The infrastructure that connects and manages multiple BSSs in an ESS.
Physical Layer
- Wireless transmission via infrared (IR) or radio frequency (RF).
- Radio frequency: Typically using the license-free 2.4 GHz band.
- Advantages: Experience from wireless WAN (microwave links) and mobile phones, coverage of larger areas, higher transmission rates (~11-54 Mbit/s).
- Disadvantages: Limited license-free frequency bands, shielding challenges, interference from other senders or electrical devices.
Media Access Control (MAC)
- The MAC sublayer and logical link control (LLC) sublayer together form the data link layer.
- Provides flow control and multiplexing for the transmission medium, enabling multiple network nodes to communicate.
- Two basic access mechanisms defined for IEEE 802.11:
- CSMA/CA (mandatory), known as distributed coordination function (DCF).
- Point Coordination Function (PCF), a contention-free polling method for time-bounded service.
DCF and PCF
- DCF provides asynchronous service.
- PCF offers both asynchronous and time-bounded service, but requires the access point to control medium access and avoid contention.
- DCF is also known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol:
- An asynchronous access method based on contention for shared channels.
- A contention-free access mechanism is provided through RTS/CTS (Request to Send/Clear to Send) exchange.
- PCF is used for time-bounded data transfer with the access point polling terminals based on a list.
Carrier Sense Multiple Access (CSMA)
- Carrier refers to a shared transmission medium.
- Sense implies nodes can listen and detect activity on the medium.
- Multiple Access indicates all nodes have the right to access the shared medium.
- CSMA means a node can detect activity on the transmission medium (regardless of the medium type).
- If a node detects ongoing activity, it waits (backoff) for a random time before checking again.
Hidden Station Problem
- When a node (A) is communicating with another node (B), a third node (C) might be hidden from A's perspective but within range of B.
- Node C might start transmitting to B without being aware of the ongoing communication between A and B, leading to collisions.
Collisions in WLAN
- Occur when two or more nodes transmit simultaneously.
- A key difference between WLAN and LAN protocol design is the difficulty in detecting all collisions in WLANs due to:
- Weak received signals (fading) making it difficult to sense collisions while transmitting.
- With receiver and transmitter antennas close together, a station cannot see any signal except its own.
- As a result, the complete packet is sent before an incorrect checksum reveals a collision.
- Receiver and transmitter are not typically active at the same time.
- Collisions cannot be sensed in all cases due to hidden terminals and fading.
CSMA/CA
- In LANs, protocols use collision detection (CD) mechanisms to detect collisions and take corrective actions.
RTS-CTS Exchange Scenario
- A node (A) sending data to another node (B) via an access point (AP) can use a Request to Send (RTS) and Clear to Send (CTS) exchange to avoid collisions.
- Node A sends an RTS message to the AP, reserving the channel for communication.
- Node B also sends an RTS message, potentially resulting in a collision if sent at the same time.
- Node A's RTS takes priority, and the AP sends a CTS to A, confirming the channel reservation.
- Node A sends the data to B, and B sends an ACK (acknowledgment) to A.
Addressing
- Frame control fields include:
- MAC addresses of the source and destination devices.
- Router interface address to which the wireless access point is attached.
- Address used only in ad-hoc mode.
- Sequence number.
- Duration field for the frame.
- Payload: Data to be transmitted.
- Cyclic Redundancy Check (CRC): Error detection code.
Addressing (continued)
- Internet (Internet Protocol)
- H1 (Wireless host 1)
- R1 (Router 1)
- AP (Wireless Access Point)
- The destination MAC address indicates the receiving device.
- The frame includes the router's MAC address, the access point's MAC address, the source device's MAC address, and the destination device's MAC address.
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Description
This quiz focuses on the fundamental aspects of Wireless Local Area Networks (WLANs), their functionality, flexibility, and design goals. Participants will learn about the advantages of wireless communication and the essential considerations for WLAN implementation in various environments such as offices, homes, and manufacturing settings.
