Credit-Based Wireless Medium Access Control
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Solution Overview
Problem
Wireless communication systems face challenges in managing concurrent transmissions, particularly the 'hidden node' problem, which leads to collisions and degrades network throughput and fairness, especially under high traffic loads.
Innovation Solution
Implementing a credit-based coordination algorithm that divides communication into credit periods for real-time applications and free periods for latency-insensitive applications, using an access point to poll stations with assigned credits and fair queuing algorithms to manage medium access, ensuring quality of service and preventing collisions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If CSMA/CA protocol is used for medium access control, then wireless devices can independently transmit data, but collisions occur between concurrent transmissions from hidden nodes
Solution Approach 1:
The access point acts as an intermediary that coordinates transmissions between wireless devices. It polls stations to grant transmission rights and manages the medium access, preventing hidden node collisions while maintaining independent transmission capability through centralized control
2Reliability
If DCF function with RTS/CTS frames is used, then data frame collisions are prevented, but RTS and CTS frame collisions still occur under high traffic load
Solution Approach 1:
The system performs preliminary actions by assigning credits to stations in advance before the contention-free period begins. This pre-allocation of transmission rights eliminates the need for RTS/CTS handshaking during actual data transmission, preventing both data frame and control frame collisions while maintaining high throughput
3Reliability
If PCF with polling is used to control medium access, then collisions are prevented, but real-time application requirements are not met due to latency
Solution Approach 1:
The contention-free period is segmented into multiple polling rounds, allowing the access point to poll multiple stations within a single CFP. This reduces the waiting time for real-time applications by providing multiple transmission opportunities per contention-free period, thereby reducing latency while maintaining collision-free operation
Solution Approach 2:
The system ensures continuous useful action by structuring the contention-free period to maximize transmission opportunities. Multiple polling rounds allow stations to transmit data continuously without idle waiting time, maintaining both collision prevention and low latency for real-time applications
4Loss of time
If multiple polling rounds are implemented in contention-free period, then real-time application latency is reduced, but system complexity increases
Solution Approach 1:
Stations self-manage their transmission opportunities by monitoring their credit balances and transmitting when polled. This self-service approach reduces access point complexity while enabling multiple polling rounds, as stations autonomously determine when they have data to send based on their credit status rather than requiring complex access point scheduling
Data Source
AI summary
Some representative embodiments are directed to controlling access to a wireless medium using “credit” periods and “free” periods. During a credit period, credits are initially assigned to the stations and each station is repetitively polled as long as the station has credit remaining and data is available to communicate. During the free periods, stations are polled using a start-time fair queuing algorithm. By dividing the access to the wireless medium in this manner, real time applications may use the credit periods to prevent communication latency from deteriorating application performance. Specifically, the repetitive occurrence of the credit periods and the assignment of credits to the stations ensures that each station will communicate at least at a minimum data rate. Additionally, controlling access to the wireless medium in this manner prevents frame collisions and, thereby, increases network throughput.