Questions and Answers
What is typically found in the payload of an 802.11 frame?
What is the purpose of the sequence number field in an 802.11 frame?
Which field in the 802.11 frame is used to verify data integrity?
What information is conveyed by the type and subtype fields in an 802.11 frame?
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What does the WEP field in the 802.11 frame indicate?
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The payload in an 802.11 frame typically consists of an IP datagram or an ______ packet.
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The sequence number field in the 802.11 frame is used to distinguish between a newly transmitted frame and its ______.
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The duration value in the frame’s duration field includes the time to transmit its data frame and the time to transmit an ______.
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The type and subtype fields in an 802.11 frame are used to distinguish between various frame types, such as ______, CTS, and ACK.
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The WEP field in the 802.11 frame indicates whether encryption is being ______ or not.
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Study Notes
Wireless LAN Overview
- Wireless LAN (WLAN) is a flexible data communication system, serving as an extension or alternative to wired networks.
- Utilizes radio frequency (RF) technology for data transmission, enhancing user mobility and reducing wired connectivity needs.
Benefits of Wireless LAN
- Installation is quick and straightforward, providing flexibility in setup.
- Offers reduced total cost of ownership compared to wired alternatives.
- Enhances productivity and convenience by enabling mobility.
- Easily scalable to accommodate growing network demands.
Disadvantages of Wireless LAN
- Higher costs for hardware: wireless network cards are significantly more expensive than wired ones.
- Access points, essential for wireless networking, cost more than traditional hubs and cabling.
- Susceptible to environmental factors like weather and obstacles (buildings, trees) that can hinder signal quality.
- Limited bandwidth capacity, leading to slower data transfer rates and potential packet loss affecting performance.
Hidden Terminal Problem
- Occurs when a station is unaware of ongoing transmissions, potentially causing communication collisions.
- Station H1 transmitting while Station H2 attempts to send to the access point (AP) leads to a scenario where both transmissions collide.
Collision Avoidance Techniques (CSMA/CA)
- IEEE 802.11 utilizes a Request to Send (RTS) and a Clear to Send (CTS) control frame to reserve access to the channel and prevent collisions.
- Stations select random backoff values to minimize simultaneous transmissions when the channel is busy.
802.11 MAC Protocol Functionality
- CSMA/CA mechanism allows devices to sense channel status and manage transmission timing, enhancing communication efficiency.
- Upon sensing an idle channel for the Duration of Inter-Frame Space (DIFS), the device may transmit; otherwise, it waits and counts down its backoff timer.
- Acknowledgment from the receiving station verifies that message delivery was successful; if not received, the device increases the backoff value.
802.11 Frame Structure
- The frame consists of various fields, including source and destination MAC addresses, duration, sequence number, payload, and CRC for error checking.
- Four address fields accommodate extended networking needs compared to Ethernet frames, facilitating communication through access points and routers.
Address Field Purpose
- The additional address fields are crucial for internal routing between wireless stations and transitions to wired networks, ensuring proper delivery of data packets.
Payload and CRC Information
- The payload typically holds IP datagrams or ARP packets, with a standard size of fewer than 1,500 bytes. The maximum length can reach 2,312 bytes.
- Each frame incorporates a 32-bit cyclic redundancy check (CRC) for error detection during transmission.
Other Frame Control Subfields
- Type and subtype fields categorize frame purposes (association, RTS, CTS, ACK, etc.).
- The "to" and "from" fields clarify the roles of address fields based on transmission context (ad hoc vs. infrastructure).
- The WEP field indicates the use of encryption for enhanced data security.
Wireless Standards Overview
- 802.11b operates in the 2.4-5 GHz range, reaching speeds of up to 11 Mbps, employing Direct Sequence Spread Spectrum (DSSS).
- 802.11a operates in the 5-6 GHz range, achieving up to 54 Mbps.
- 802.11g also works in the 2.4-5 GHz range, with speeds up to 54 Mbps.
- 802.11n utilizes multiple antennas, supporting up to 200 Mbps in the same frequency range.
- All standards incorporate CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) for efficient multiple access.
802.11 LAN Architecture
- Wireless hosts connect to base stations known as Access Points (APs).
- Basic Service Set (BSS) or "cell" comprises wireless hosts and an AP, defining communication boundaries.
- BSS can operate in both infrastructure mode (with an AP) and ad-hoc mode (host-to-host communication).
802.11 Channels and Association
- 802.11b utilizes a spectrum of 2.4GHz-2.485GHz, divided into 11 channels.
- AP administrators choose specific frequencies, acknowledging potential interference from neighboring channels.
- Wireless hosts must associate with an AP via scanning and detecting beacon frames containing SSID and MAC addresses.
Scanning Methods
- Passive Scanning involves listening for periodic beacon frames from APs.
- Active Scanning requires broadcasting Probe Request frames to receive responses from APs.
Wi-Fi Jungle Concept
- A "Wi-Fi jungle" refers to an area where a wireless station can receive strong signals from multiple APs.
- The wireless station associates with one AP for internet access by selecting from available APs based on beacon frames.
AP Selection Criteria
- The 802.11 standard doesn’t dictate the AP selection algorithm, which is determined by wireless host firmware.
- Selection often prioritizes APs with the strongest signal but may overlook less crowded APs that provide better performance.
802.11 MAC Protocol
- The MAC protocol is based on CSMA/CA, ensuring only idle channels are used for transmission to avoid collisions.
- Unlike Ethernet, 802.11 uses acknowledgment for data integrity, prioritizing collision avoidance due to environmental dynamics and hardware limitations.
Collision Avoidance in 802.11
- The protocol avoids collision detection due to hardware constraints and issues like hidden terminal problems.
- Multiple access is managed effectively through CSMA, preventing two nodes from transmitting simultaneously while maintaining throughput.
802.11 Frame Structure
- The payload typically carries IP datagrams or ARP packets, allowing a maximum payload of 2,312 bytes but usually under 1,500 bytes.
- Frames include a 32-bit cyclic redundancy check (CRC) for error detection.
Frame Control Fields
- The type and subtype fields distinguish between frame types such as data, ACK, RTS, and CTS.
- Addressing fields manage frame meaning depending on whether it originates from an AP or a host.
- WEP field indicates whether encryption is applied to the transmitted data, ensuring security.
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Description
This quiz explores the early concepts of networking, particularly focusing on signal transmission issues like sensing collisions, weak received signals, and fading. Learn about strategies such as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) to effectively manage communication. Test your understanding of hidden terminals and the challenges faced in network communications.
