  Outage probabilities in wireless networks depend on various factors: the node
distribution, the MAC scheme, and the models for path loss, fading and
transmission success. In prior work on outage characterization for networks
with randomly placed nodes, most of the emphasis was put on networks whose
nodes are Poisson distributed and where ALOHA is used as the MAC protocol. In
this paper we provide a general framework for the analysis of outage
probabilities in the high-reliability regime. The outage probability
characterization is based on two parameters: the intrinsic spatial contention
$\gamma$ of the network, introduced in [1], and the coordination level achieved
by the MAC as measured by the interference scaling exponent $\kappa$ introduced
in this paper. We study outage probabilities under the signal-to-interference
ratio (SIR) model, Rayleigh fading, and power-law path loss, and explain how
the two parameters depend on the network model. The main result is that the
outage probability approaches $\gamma\eta^{\kappa}$ as the density of
interferers $\eta$ goes to zero, and that $\kappa$ assumes values in the range
$1\leq \kappa\leq \alpha/2$ for all practical MAC protocols, where $\alpha$ is
the path loss exponent. This asymptotic expression is valid for all
motion-invariant point processes. We suggest a novel and complete taxonomy of
MAC protocols based mainly on the value of $\kappa$. Finally, our findings
suggest a conjecture that tightly bounds the outage probability for all
interferer densities.
