802.11 How WiFi Works - Wireless Networks | Computer Networks Ep. 7.3 | Kurose & Ross

Summary

This video provides an in-depth look at the evolution and functioning of 802.11 wireless LAN protocols, commonly known as Wi-Fi. It explores key concepts like CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance), the association process for devices, and the RTS/CTS mechanism to manage network congestion. The video also covers mobility between access points, power management strategies for battery-operated devices, and a comparison with Bluetooth networks. Aimed at understanding how Wi-Fi networks operate, the video addresses both technical challenges and solutions for efficient wireless communication.

Takeaways

• 😀 802.11 Wi-Fi protocols, also known as wireless LANs, are designed to address the challenges of wireless communication, including interference, range, and bandwidth limitations.
• 😀 The evolution of Wi-Fi standards includes 802.11b, 802.11g, 802.11ac (Wi-Fi 5), and 802.11ax (Wi-Fi 6), each offering improvements in speed and capacity.
• 😀 Wi-Fi operates over both 2.4 GHz and 5 GHz bands, with different channels that must be carefully selected to avoid interference between neighboring access points.
• 😀 The CSMA/CA algorithm is crucial for wireless communication, ensuring that devices listen before transmitting to avoid collisions, with collision avoidance rather than detection.
• 😀 The RTS/CTS mechanism helps manage channel contention in congested environments by allowing devices to reserve the channel before transmitting data.
• 😀 Security concerns exist in Wi-Fi networks, particularly with active scanning where devices may inadvertently connect to rogue access points, exposing them to man-in-the-middle attacks.
• 😀 802.11 frames include multiple address fields to handle both wireless and wired network addressing, supporting scenarios like ad hoc and infrastructure modes.
• 😀 Mobility in Wi-Fi networks is handled by re-associating devices with new access points as they move, ensuring seamless communication across the network.
• 😀 Power management is important in Wi-Fi, as devices can signal to access points to pause their transmissions, conserving battery life for mobile devices.
• 😀 Bluetooth operates on the same 2.4 GHz spectrum but is designed for short-range, ad-hoc networks, using frequency hopping to minimize interference and including energy-saving modes like 'parked mode'.

Q & A

What is the main difference between the 802.11b and 802.11g Wi-Fi standards?

802.11b was the earliest version widely deployed for consumers and operated only at 2.4 GHz, offering slower speeds. 802.11g, on the other hand, increased available bandwidth and was also limited to the 2.4 GHz band, but provided faster speeds than 802.11b.

Why do some 802.11 protocols operate in the 5 GHz band instead of the 2.4 GHz band?

The 5 GHz band provides higher bandwidth and less interference than the 2.4 GHz band, making it suitable for newer, faster Wi-Fi protocols such as 802.11ac and 802.11ax (Wi-Fi 6).

What is the role of an access point in an 802.11 wireless network?

An access point (AP) serves as the central base station in a wireless LAN (WLAN), allowing devices to connect to the network. The AP handles communication between wireless devices and the wired network.

How does a device associate with an access point in a wireless network?

A device associates with an access point by scanning available channels for beacon frames. It then sends an association request to the AP, which responds with an association response to establish the connection.

What is the difference between passive and active scanning in the association process?

In passive scanning, the device listens for beacon frames from APs across channels, which may cause delays if the AP is on a different channel. In active scanning, the device sends probe requests, and APs respond immediately with probe responses, speeding up the process.

What is the purpose of the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) protocol in Wi-Fi?

CSMA/CA helps avoid data collisions in wireless networks by requiring devices to listen for a clear channel before transmitting. If the channel is busy, devices wait for a random backoff period before retrying, reducing the chance of simultaneous transmissions and collisions.

What is the RTS/CTS (Request to Send / Clear to Send) mechanism and why is it used?

RTS/CTS is used to reduce collisions in congested wireless environments. The sender first sends a small RTS control frame to the access point, which responds with a CTS to indicate which device is allowed to transmit, preventing hidden terminal collisions.

How does the 802.11 protocol handle collisions if the sender does not receive an acknowledgment (ACK)?

If the sender does not receive an acknowledgment (ACK) after transmission, it assumes a collision has occurred and retransmits the frame. This process ensures data is reliably transmitted in the network.

What is the significance of the four address fields in the 802.11 frame header?

The four address fields in the 802.11 frame header allow the inclusion of both MAC addresses relevant to wireless communication and the wired network. They help in routing frames in both infrastructure and ad hoc modes.

How does mobility between access points work in a wireless LAN?

When a device moves from one access point to another within the same network, it undergoes a reassociation process. The switch or access point learns the new location of the device's MAC address, ensuring continuous communication as the device roams.