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Module05_-_deck_1

Module 8: 802.11 Medium Access

Overview

Study of Medium Access Control (MAC) principles in wireless networks highlights the importance of efficient medium access techniques to maintain communications and facilitate the operation of devices in these networks. Key contention mechanisms that ensure fair access to the shared communication medium among multiple users include:

  • Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

  • Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)

Medium Access Control (MAC)

This term refers to a set of rules that determine how devices on a network gain access to the transmission medium to send and receive data in an orderly and efficient manner. The MAC protocols play a critical role in minimizing collisions and ensuring that transmission occurs fairly among all devices in a network.

Important Forms of Contention in Networks:

  • CSMA/CD: Commonly utilized in wired networks like Ethernet. It allows devices to detect collisions when multiple nodes attempt to transmit data simultaneously.

  • CSMA/CA: Employed in wireless networks like 802.11 where the collision detection isn’t feasible due to the nature of radio transmissions. Instead of detecting collisions, CSMA/CA focuses on preventing them through various techniques.

CSMA/CD

Basics

This mechanism is primarily used in 802.3 Ethernet networks. Before a node transmits data, it must first listen to the medium to check for ongoing transmissions.

Collision Management

  • If a device detects no activity, it will proceed to send data.

  • While sending, it continues monitoring for collisions using the listening approach.

  • In the occurrence of a collision, the involved nodes will stop transmitting immediately and wait for a random amount of time (backoff period) before attempting to retransmit. This random wait helps in reducing the likelihood of repeated collisions.

CSMA/CA

Overview

CSMA/CA is specifically designed for 802.11 wireless networking environments, due to the inherent difficulties in detecting collisions while transmitting over the airwaves. Its approach emphasizes collision avoidance rather than detection.

Key Techniques:

  • Carrier Sense: Before transmission, devices check the medium to determine if it is clear.

  • Multiple Access: Multiple radios can access the medium fairly.

  • Collision Avoidance: Mechanisms are in place to ensure that only one radio transmits at a time, which prevents collisions from occurring.

Acknowledgment Mechanism

  • Unicast Acknowledgment: After data frames containing CRC are sent, the receiver acknowledges by sending back an acknowledgment (ACK).

  • Collision Detection: If the sender does not receive an ACK within a prescribed time frame, it infers that a collision has occurred and retransmits the data.

Distributed Coordination Function (DCF)

Definition

This fundamental access method within the 802.11 standard is based on CSMA/CA principles and ensures that only one radio transmits at any given time, which helps in managing potential collisions.

Components of DCF

  • Physical Carrier Sense: This component checks the medium's busy state before any data is sent.

  • Virtual Carrier Sense: Involves using a Duration/ID field embedded in MAC headers to predict traffic conditions.

  • Pseudo-Random Backoff Timer: Implements a randomized backoff period before transmission to ensure fairness among multiple users.

  • Interframe Spaces (IFS): Different types of IFS are utilized to control the pacing of frame transmission, ranging from shortest to longest:

    • Reduced IFS (RIFS)

    • Short IFS (SIFS): Used for high-priority frames like ACKs.

    • PCF IFS (PIFS)

    • DCF IFS (DIFS): Typically used for standard data frames.

    • Arbitration IFS (AIFS)

    • Extended IFS (EIFS)

Hybrid Coordination Function (HCF)

Description

Introduced in the 802.11e standard to enhance quality-of-service (QoS) capabilities, HCF combines elements of DCF with additional mechanisms to prioritize traffic and ensure efficient medium access based on traffic types.

TXOP (Transmit Opportunity)

This feature allows devices to send multiple frames in a single transmission opportunity, which optimizes throughput and reduces latency in the network.

Enhanced Distributed Channel Access (EDCA)

Access Categories

EDCA introduces four access categories ordered from lowest to highest priority:

  • AC_BK (Background): Lowest priority for general data.

  • AC_BE (Best Effort): Default access category for regular traffic.

  • AC_VI (Video): Medium priority tailored for video traffic.

  • AC_VO (Voice): Highest priority for voice transmissions, ensuring minimal delay. Lower backoff values for higher-priority categories help to allocate bandwidth efficiently and meet the demands of real-time applications.

Wi-Fi Multimedia (WMM)

Certification

This concept, endorsed by the Wi-Fi Alliance and rooted in the 802.11e standard, prioritizes network traffic using the four defined access categories to manage bandwidth during high-demand situations.

Airtime Fairness

The WMM standard aims to ensure fairness in the distribution of airtime among users, prioritizing high data rate transmissions by ensuring equitable time allocation rather than just fair access opportunities.

Conclusion

A comprehensive understanding of 802.11 MAC mechanisms is crucial for effective wireless networking. The evolution of these standards has significantly improved QoS metrics and has enhanced the ability to manage contention effectively across multiple connected devices, leading to more reliable and efficient wireless communications.