IEEE 802.11 DCF is the dominant protocol used in
existing WLANs. However, the efficiency of DCF progressively
degrades with the increase of contending clients in the network
as well as the wireless link rate. To address this issue, in this
paper, we present a distributed random media access protocol,
named CHAIN, which significantly improves uplink performance
of WLANs. CHAIN mainly uses overhearing to coordinate clients
in a network, and thus introduces little control overhead. The
key in CHAIN is a novel piggyback transmission opportunity.
In CHAIN, clients maintain a precedence relation among one
another, and a client can immediately transmit a new packet
after it overhears a successful transmission of its predecessor,
without going through the regular contending process. When
the network load is low, CHAIN behaves similar to DCF; But
when the network becomes congested, clients automatically start
chains of transmissions to improve efficiency. CHAIN is derived
from DCF and co-exists friendly with it. Moreover, it possesses
all the advantages of the 802.11 DCF standard – simplicity,
robustness, and scalability. We analytically prove the correctness
and fairness of CHAIN. Our extensive simulations on J-SIM verify
our analytical results, and demonstrate significant performance
gain of CHAIN over DCF.