A High Efficiency MAC Protocol for WLANs: Providing Fairness in Dense Scenarios

TL;DR

This paper introduces CSMA/ECA, a decentralized MAC protocol that creates collision-free schedules, improving throughput and fairness in dense WLAN scenarios, with extensive evaluation including real hardware tests.

Contribution

It presents the CSMA/ECA protocol and its extensions, along with the first comprehensive evaluation under various realistic conditions and hardware implementation results.

Findings

CSMA/ECA achieves higher throughput than CSMA/CA.

CSMA/ECA provides short-term fairness among contenders.

Experimental hardware tests confirm the protocol's effectiveness.

Abstract

Collisions are a main cause of throughput degradation in WLANs. The current contention mechanism used in IEEE 802.11 networks is called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). It uses a Binary Exponential Backoff (BEB) technique to randomise each contender attempt of transmitting, effectively reducing the collision probability. Nevertheless, CSMA/CA relies on a random backoff that while effective and fully decentralised, in principle is unable to completely eliminate collisions, therefore degrading the network throughput as more contenders attempt to share the channel. To overcome these situations, Carrier Sense Multiple Access with Enhanced Collision Avoidance (CSMA/ECA) is able to create a collision-free schedule in a fully decentralised manner using a deterministic backoff after successful transmissions. Hysteresis and Fair Share are two extensions of CSMA/ECA to support a large number of contenders in a collision-free schedule. CSMA/ECA offers better throughput than CSMA/CA and short-term throughput fairness. This work describes CSMA/ECA and its extensions. Additionally, it provides the first evaluation results of CSMA/ECA with non-saturated traffic, channel errors, and its performance when coexisting with CSMA/CA nodes. Furthermore, it describes the effects of imperfect clocks over CSMA/ECA and present a mechanism to leverage the impact of channel errors and the addition/withdrawal of nodes over collision-free schedules. Finally, experimental results on throughput and lost frames from a CSMA/ECA implementation using commercial hardware and open-source firmware are presented.