Metastability in Loss Networks with Dynamic Alternative Routing

Consider $N$ stations interconnected with links, each of capacity $K$, forming a complete graph. Calls arrive to each link at rate $\lambda$ and depart at rate $1$. If a call arrives to a link $x y$, connecting stations $x$ and $y$, which is at capacity, then a third station $z$ is chosen uniformly at random and the call is attempted to be routed via $z$: if both links $x z$ and $z y$ have spare capacity, then the call is held simultaneously on these two; otherwise the call is lost. We analyse an approximation of this model. We show rigorously that there are three phases according to the traffic intensity $\alpha := \lambda/K$: for $\alpha \in (0,\alpha_c) \cup (1,\infty)$, the system has mixing time logarithmic in the number of links $n := \binom N2$; for $\alpha \in (\alpha_c,1)$ the system has mixing time exponential in $n$, the number of links. Here $\alpha_c := \tfrac13 (5 \sqrt{10} - 13) \approx 0.937$ is an explicit critical threshold with a simple interpretation. We also consider allowing multiple rerouting attempts. This has little effect on the overall behaviour; it does not remove the metastability phase. Finally, we add trunk reservation: in this, some number $\sigma$ of circuits are reserved; a rerouting attempt is only accepted if at least $\sigma+1$ circuits are available. We show that if $\sigma$ is chosen sufficiently large, depending only on $\alpha$, not $K$ or $n$, then the metastability phase is removed.