Signalling and MAC Protocols

The 200 Tbit/s WDMA/TDMA "Switchless" optical network, which has been proposed as a set of interconnected, amplified PONs on a national scale to take advantage of the ultimate limits of WDM channel density, is in essence a single switch covering a whole country. Packets are buffered electrically at the edge terminals. Such a high capacity in a single-hop, geographically distributed switch places enormous demands on signalling and MAC protocols, particularly since connectionless cells or packets must be supported as well as connections. Out-of-band signalling in dedicated signalling wavelength channels would require every terminal to possess at least 12 receivers. Fortunately in-band meta-signalling using all of the available wavelength channels, in combination with temporal manipulation of blocks of wavelength channels, can provide connectivity that is unconstrained by signalling limitations, with just a single transceiver at each terminal. A MAC protocol capable of scheduling cells/packets in the 200 Tbit/s "Switchless" optical network has also been proposed.

Constraint-Based Routing

Constraint-based routing, as enabled by MPLS, is crucial for providing explicit routes across a network for traffic engineering, for fast re-routing and for providing QoS over guaranteed bandwidth paths. When applied to the provision of optical paths and wavelength channels across a WDM network, the number of constraints needing to be considered can be large, due to various analogue transmission impairments, and their values can have very complex analogue behaviour, both as a function of path length and of the numbers of wavelength channels in the various links of the path, including non-linear behaviour. Compromises will be needed to limit the complexity of routing and control protocols in relation to some of the constraints, at the expense of increased network costs. Other key issues are i) whether routing algorithms should be run centrally within a domain, using a centralised database, or distributed throughout the nodes, and ii) whether routes should be pre-computed. Both could allow more sophisticated routing algorithms to be used, which raises the question of the optimum relationship between the route server(s) and the network planning tool.

Path Searching in Multi-Stage Cross-Connects (Optical, SONET, SDH etc.)

Path-searching algorithms such as Clos's and Paull's are needed to establish paths through multi-stage switch fabrics, e.g. in optical cross-connects, for both new connections and protection or restoration paths after network and switch failures. Paull's rearrangement algorithms can result in large numbers of existing connections having to be moved. The major challenges are to find a) algorithmic improvements that minimise rearrangements, thus minimising numbers of connections disrupted and their disruption times, and b) structural improvements in the methods of handling protection/restoration of connections that remove the need for rearrangements.