Collision avoidance is very important in contentionbased
medium access control protocols for multi-hop ad
hoc networks due to the adverse effects of hidden terminals.
Four-way sender-initiated schemes are the most popular
collision-avoidance schemes to date. Although there
has been considerable work on the performance evaluation
of these schemes, most analytical work is confined to singlehop
ad hoc networks or networks with very few hidden terminals.
In this paper, we use a simple analytical model to
derive the saturation throughput of collision avoidance protocols
in multi-hop ad hoc networks with nodes randomly
placed according to a two-dimensional Poisson distribution,
which to our knowledge has not been investigated sufficiently
before. We show that the sender-initiated collisionavoidance
scheme achieves much higher throughput than
the idealized carrier sense multiple access scheme with an
ideal separate channel for acknowledgments. More importantly,
we show that the collision avoidance scheme can
accommodate much fewer competing nodes within a region
in a network infested with hidden terminals than in
a fully-connected network, if reasonable throughput is to
be maintained. This shows that the scalability problem
of contention-based collision-avoidance protocols looms
much earlier than people might expect. Simulation experiments
of the popular IEEE 802.11 MAC protocol validate
the predictions made in the analysis.