Optical communications networks are typically arranged into either tree or ring architectures. Both architectures are well known and are generally employed in different circumstances. Ring architectures are typically employed in long haul or trunk routes where circuit protection is important. Because the network is arranged into a ring, a break in the ring fibre can be overcome by re-routing traffic around the ring in the opposite direction. This arrangement is more expensive as it requires additional fibre and switching complexity, but provides the protection required where large numbers of calls would otherwise be affected. Tree architectures are also well known and tend to be used in access networks where protection is less critical and reduced deployment costs are more important.
Currently the high cost of ring topology metro access networks is hampering deployment in this network space. However tree architectures, while cheaper to deploy, offer no protection and are therefore unsuitable for metro access networks. Prior art ring topologies in the metro access space typically utilise dielectric filters connected to two headends by sections of optical fibre. Each wavelength or channel on the connecting fibres can be tapped at each filter which may be connected to a large customer such as a bank for example, or to a local access tree network for example. Filters tend to be expensive and also relatively high loss thereby reducing the maximum diameter of the ring in a passive network.