  Conventional optical networks are based on SONET rings, but since rings are
known to use bandwidth inefficiently, there has been much research into shared
mesh protection, which promises significant bandwidth savings. Unfortunately,
most shared mesh protection schemes cannot guarantee that failed traffic will
be restored within the 50 ms timeframe that SONET standards specify. A notable
exception is the p-cycle scheme of Grover and Stamatelakis. We argue, however,
that p-cycles have certain limitations, e.g., there is no easy way to adapt
p-cycles to a path-based protection scheme, and p-cycles seem more suited to
static traffic than to dynamic traffic. In this paper we show that the key to
fast restoration times is not a ring-like topology per se, but rather the
ability to pre-cross-connect protection paths. This leads to the concept of a
pre-cross-connected trail or PXT, which is a structure that is more flexible
than rings and that adapts readily to both path-based and link-based schemes
and to both static and dynamic traffic. The PXT protection scheme achieves fast
restoration speeds, and our simulations, which have been carefully chosen using
ideas from experimental design theory, show that the bandwidth efficiency of
the PXT protection scheme is comparable to that of conventional shared mesh
protection schemes.
