Flow-level QoS guarantees in IEEE 802.11e-EDCA based WLANs

Wireless LANs provide a broadband and mobile access to Internet, allowing multimedia mobile services to become a reality by using an alternative network to actual cellular systems. Simultaneously, the Internet user behavior is evolving to include VoIP and P2P services to the already traditional Web browsing, file transfer or e-mail. However, in this scenario, WLANs show several performance limitations, such as the difficult simultaneous coexistence of TCP and UDP flows, as it was not designed to support these heterogeneous traffic conditions. This dissertation deals with the study of mechanisms to provide QoS guarantees at flow level in Hotspot WLANs, mitigating and solving the different impairments without losing the basic characteristic of this technology: a distributed random access MAC protocol. To solve these limitations, an novel admission and rate control scheme is proposed, based on the IEEE 802.11eEDCA standard. This entity, placed at the Access Point takes control on the Hotspot state and manages the distribution of the transmission resources among the active nodes. However, due the non-linear behavior of the EDCA random access MAC protocol, the estimation of the available set of resources (e.g., the available bandwidth) requires complex computations, specially when different non-saturated traffic profiles are considered. To provide a framework where the MAC behavior can be evaluated, a mathematical model of the DCF/EDCA protocol is developed, being able to capture the non-linear and reciprocal impact between network and traffic flows. Moreover, the model also captures the dynamics of all QoS traffic differentiation parameters, such as: CWmin, CWmax, AIFS, TXOP and different ACK policies. The model is validated and used to understand how a WLAN Hotspot behaves with heterogeneous traffic flows and how EDCA is able to mitigate the performance impairments that appear. The performance of both TCP and UDP traffic flows is analyzed in detail, providing also different models to capture the observed dynamics. These models can be used along with the DCF/EDCA model to evaluate the expected performance of TCP-like or/and UDP-like services, such as Web Browsing or VoIP calls. For the VoIP service, a capacity analysis is provided, focusing on the causes which motivate the lower maximum number of VoIP calls that can be simultaneously active, such as the existent downlink / uplink unfairness and the multi-rate anomaly. Solutions are provided for each case. Finally, the admission control is presented and several polices are evaluated. Adaptive policies are based on adjusting dynamically the EDCA parameters, for which a novel parameter tuning algorithm is used. The algorithm searches for the optimal EDCA parameter configuration in order to maximize both the number of rigid flows and the best-effort throughput. Results show how the use of the proposed admission control improves significantly the overall Hotspot performance, being a feasible solution for future distributed random access multimedia wireless networks.