Quality of service improvement in 802.11e: Conditioned Enhanced Distributed Coordination Function (CEDCF)

In this paper, we address the issue of providing service guarantees with EDCF in the 802.11 wireless local area networking standard. We suggest a new scheme to adjust the size of the contention window (CW). In fact, with the EDCF (enhanced distributed coordination function) mechanism introduced in the upcoming IEEE 802.11e standard, the CW is reset upon a successful transmission, assuming that the contention level has dropped. This leads to new collisions and retransmissions because contention level is more likely to change slowly. On the other hand, keeping high sizes of CW is not always the perfect solution because delays increase with high backoff values. To optimize this tradeoff between fast and slow CW decrease, we propose an intermediate CW decrease function that is conditioned by the channel state: if it is congested, the size of the CW should be adjusted according to the adaptive approach AEDCF (adaptive enhanced distributed coordination function). In the other case, when contentions on the medium are negligible, the fast decrease scheme of EDCF is adopted. To conclude about the channel state, each station compares the calculated collision rate to an optimized threshold that depends on the traffic category. The performances of the proposed scheme, namely conditioned enhanced distributed coordination function (CEDCF), are extensively investigated by simulations using the network simulator NS-2. Results obtained indicate that our approach, CEDCF, outperforms both EDCF and AEDCF