Early multicast collision detection in CSMA/CA networks

Reliable multicast is especially difficult to achieve in IEEE 802.11 CSMA/CA networks when a large number of unicast stations are contending with the multicast transmissions. Motivated by capacity problems in a sports infotainment product the authors developed early multicast collision detection (EMCD) specifically tailored to accommodate a large number of legacy terminals in multiple overlapping cells. The algorithm is based on the fact that in the absence of hidden terminals two stations must start their transmissions simultaneously to cause a collision. A station using EMCD introduces an early pause in the multicast transmission, performs a clear channel assessment (CCA), and if another sender is detected, schedules a retransmission. We have evaluated our method using simulations. Results show that EMCD increases the multicast reliability even at relatively moderate network loads.