Real-Time Communication Support in IEEE 802.11-Based Wireless Mesh Networks

Background

The IEEE 802.11 standard defines four MAC sub-layer functions to control medium access (IEEE Standard 802.11-2012, 2012):

• 1.

  Distributed Coordination Function (DCF) is the basic medium access mechanism based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA);

• 2.

  Point Coordination Function (PCF) is a polling scheme where stations are interrogated in order to determine which have the right to transmit;

• 3.

  Hybrid Coordination Function (HCF) introduces the concept of QoS and Transmission Opportunities by means of Enhanced Distributed Channel Access (EDCA) and HCF Controlled Access (HCCA) mechanisms;

• 4.

  Mesh Coordination Function (MCF) defines an optional medium control mechanism for IEEE 802.11s mesh networks called MCF Controlled Channel Access (MCCA).

However, regarding real-time communication support, these functions present some limitations. DCF only provides best effort services, where packets are simply discarded when queue is full. This leads to an undesirable behavior when considering real-time communication, since important messages may be discarded. In addition, there is no traffic differentiation mechanism to separate traffic according to its priorities (Ni, 2005).

PCF is not able to handle multiple QoS requirements of different traffic types, because it defines only a round-robin scheduling algorithm. In addition, the access point contends for the medium with the same priority of other stations in order to transmit the beacon frame, causing delays and decreasing network performance (Ni, 2005; Mangold, Choi, Hiertz, Klein, & Walke, 2003).