Optical burst switching

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Optical burst switching (OBS) is a switching concept which lies between optical circuit switching and optical packet switching. Firstly, a dynamic optical network is provided by the interconnection of optical cross connects. These optical cross connects (OXC) usually consist switches based on 2D or 3D Micro electro Mechanical mirrorsMEMS which reflect light coming into the switch at an incoming port to a particular outgoing port. The granularity of this type of switching is at a fibre, waveband (a band of wavelengths) or at a wavelength level. The finest granularity offered by an OXC is at a wavelength level. Therefore this type of switching is appropriate for provisioning lightpaths from one node to another for different clients/ services e.g. SDH (Synchronous Digital Hierarchy) circuits.

Optical Burst Switching operates at the sub-wavelength level and is designed to better improve the utilisation of wavelengths by rapid setup and teardown of the wavelength/lightpath for incoming bursts. In OBS, incoming traffic from clients at the edge of the network are aggregated at the ingress of the network according to a particular parameter (commonly destination, type of service (TOS bytes) class of service and quality of service(e.g. profiled Diffserv code points)). Therefore, at the OBS edge router, different queues represent the various destinations or class of services. Therefore based on the assembly/aggregation algorithm, packets are assembled into bursts using either a time based or threshold based aggregation algorithm. In some implementations, Aggregation is based on a Hybrid of Timer and Threshold. From the aggregation of packets, a burst is created and this is the granularity that is handled in OBS.

Also important about OBS is the fact that the required electrical processing is decoupled from the Optical process. Therefore the burst header generated at the edge of the network is sent on a separate control channel which could be a designated out-of-band control wavelength. At each switch the control channel is converted to the electrical domain for the electrical processing of the header information. The header information precedes the burst by a set amount known as an offset time. Therefore giving enough time for the switch resources to be made available prior to the arrival of the burst. Different reservation protocols have been proposed and their efficacy studied and published in numerous research publications. Obviously the signalling and reservation protocols depends of the network architecture, node capability, network topology and level of network connectivity. The reservation process has implications on the performance of OBS due to the buffering requirements at the edge. The one-way signalling paradigm obviously introduces a higher level of blocking in the network as connections are not usually guaranteed prior to burst release. Again numerous proposals have sought to improve these issues.

Optical burst switching has many flavours determined by the current available technologies such as the switching speed of available core optical switches. Most optical cross connects have switching times of the order of milliseconds but require tens of milliseconds to set up the switch and perform switching. New switch architectures and faster switches of the order of micro and nano second switching times can help to reduce the path setup overhead. Similarly, control plane signalling and reservation protocols implemented in hardware can help to speed up processing times by several clock cycles.

The initial phase of introducing optical burst switching would be based on an acknowledged reservation protocol i.e. two-way signalling: after burstification process, based on a forwarding table bursts of a particular destination are mapped to a wavelength. As the burst requests a path across the network, the request is sent on the control channel, at each switch, if it is possible to switch for the wavelength, the path is set up and an acknowledge signal is sent back to the ingress. The burst is then transmitted. Under this concept, the burst is held electronically at the edge and the bandwidth and path is guaranteed prior to transmission. This reduces the amount of bursts dropped. The effects of dropping bursts can be detrimental to a network as each burst is an amalgamation of IP packets which could be carrying keepalive messages between IP routers. If lost, the IP router would be forced to retransmit and reconverge.

Under the GMPLS control plane, forwarding tables are used to map the bursts and the MPLS (Multiprotocol Label Switching) base 'PATH' and 'RESV' signals are used for requesting a path and confirming setup respectively. This is a two way signalling process which can be inefficient in terms of network utilisation. However for increasingly bursty traffic, the conventional OBS is the preferred choice.

Under this conventional OBS, a one way signalling concept as mentioned previously is used. The idea is to hold the burst at the edge for an offset period while the control header traverses across the network setting up the switches, the burst follows immediately without confirmation of burst setup. There is an increased likelihood for bursts to be dropped but contention resolution mechanisms can be used to ensure alternative resources are made available to the burst if the switch is blocked ( being used by another burst for the incoming or outgoing switch port). An example contention resolution solution is deflection routing, where blocked bursts are routed to alternative port until the required port becomes available. This requires optical buffering which is implemented mainly by fibre delay lines.

One way signalling makes more efficient use of the network and the burst probability of blocking can be reduced by increasing the offset time, thereby increasing the likely hood of switch resources being available for burst.