Quality-of-service State Information-Based Solutions ◾ 289
a contention-based scheme and a centralized polling scheme, the proposed scheme improves the
channel utilization by avoiding collisions (in the contention-based scheme) and the polling over-
head (in the polling scheme).
In [25,26], two similar stateful proactive protocols (called Coop-MAC and Relay-Enabled
Medium Access (rDCF)) based on the IEEE 802.11 DCF are proposed to mitigate the throughput
bottleneck caused by low-data-rate nodes. A high-rate node is allowed to help a low-rate node
through 2-hop transmission. With joint routing and cooperation, a cross-layer approach is intro-
duced in [6]. Clusters of nodes near each transmitter form virtual multiple-input single-