  A finite length analysis is introduced for irregular repetition slotted ALOHA
(IRSA) that enables to accurately estimate its performance in the
moderate-to-high packet loss probability regime, i.e., in the so-called
waterfall region. The analysis is tailored to the collision channel model,
which enables mapping the description of the successive interference
cancellation process onto the iterative erasure decoding of low-density
parity-check codes. The analysis provides accurate estimates of the packet loss
probability of IRSA in the waterfall region as demonstrated by Monte Carlo
simulations.
