Wi-Fi. Управление доступом к разделяемой среде | Курс "Компьютерные сети"

Wi-Fi Access Control in Shared Medium

Introduction to Wi-Fi Technology

• The lecture is presented by Andrey Sazykhin, focusing on Wi-Fi as a leading technology for wireless data transmission in computer networks.

• Discussion begins on the physical layer of Wi-Fi, which has six different variants outlined in IEEE 802.11 standards.

Channel Layer and Collision Management

• Transitioning to the channel layer of Wi-Fi, which operates uniformly regardless of the physical layer standard used.

• Emphasis on the need for mechanisms to prevent simultaneous data transmissions from multiple computers, which can lead to collisions and data corruption.

Challenges in Wireless Communication

• Introduction of hidden and exposed station problems:

• Hidden Station Problem: Computer A can transmit to B but C cannot detect A's transmission due to range limitations, leading to potential collisions when both A and C attempt to send data simultaneously.

• Explanation of how computers check carrier frequency before transmitting; if they find it free, they proceed with sending data.

Exposed Station Problem

• Exposed Station Problem: Computer C may refrain from transmitting to D because it detects that B is transmitting even though D is out of B's range.

Error Handling in Wireless Networks

• In Wi-Fi, acknowledgment (ACK) is used at the channel level; after sending a frame, sender waits for an ACK from the receiver before proceeding with further transmissions.

• If no ACK is received within a timeout period due to collision or other issues, the sender retransmits the frame.

Collision Detection Mechanisms

• Comparison between collision detection methods in wired (CSMA/CD - Carrier Sense Multiple Access with Collision Detection) versus wireless networks:

• Wired networks can detect collisions during transmission; however, this method isn't feasible for Wi-Fi due to signal strength differences and potential hidden stations.

Costly Collisions in Wi-Fi

• In Wi-Fi, collisions are detected through lack of acknowledgment rather than direct detection during transmission. This makes collision management more expensive time-wise since full frames must be sent before detecting issues.

Conclusion on Access Methods

How Does the CSMA/CA Method Work in Wi-Fi?

Overview of CSMA/CA

• The CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) method is similar to Ethernet's approach. Before transmitting data, Wi-Fi computers listen to the carrier frequency to check if any data is currently being transmitted.

• If a transmission is detected, all other computers wait until the frame transmission completes. After this, they must observe an Interframe Space (IFS), which can be either normal or short depending on the type of message being sent.

Interframe Spaces and Acknowledgments

• In Wi-Fi, after the IFS period ends, a competition period begins where computers attempt to transmit data. If a collision occurs, it is quickly detected.

• To avoid collisions—which are costly in Wi-Fi—computers enter a silence period instead of competing directly. Each computer randomly selects a waiting time known as "slots."

Slot Waiting and Transmission Initiation

• A waiting slot is defined as a time interval during which a computer waits before attempting to transmit. The length of these slots varies across different physical layer standards in Wi-Fi.

• The computer that chooses the fewest waiting slots initiates its data transmission first. CSMA/CA effectively manages shared medium access but does not fully resolve hidden node and exposed node issues.

What Is MAC Protocol and How Does It Address Hidden Node Issues?

Introduction to MAC Protocol

• The MAC (Media Access Control) protocol addresses hidden node problems by requiring computers to send a control message called "Request to Send" (RTS) before transmitting actual data.

• This RTS includes information about the size of the data intended for transmission. Upon receiving this request, if ready, the recipient sends back a "Clear to Send" (CTS).

Handling Hidden Nodes

• When Computer A wants to send 1,500 bytes to Computer B, it sends an RTS. Computer B responds with CTS; however, Computer C—out of range from A—also receives this CTS.

• Knowing that another computer will transmit soon prevents Computer C from sending its own messages simultaneously.

Managing Exposed Nodes

• In scenarios where Computers B and C both send RTS messages simultaneously but are out of each other's range, they may mistakenly believe that they can transmit at once due to lack of awareness about each other's transmissions.

Conclusion: Importance of Collision Avoidance in Wireless Networks

Key Takeaways on Collision Management

• Effective methods for accessing shared media are crucial for avoiding collisions in wireless environments where only one device should transmit at any given moment.