Optical Burst Switching

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Optical burst switching is a promising solution for all optical WDM networks. It combines the benefits of optical packet switching and wavelength routing while taking into account the limitations of all current optical technology. In
OBS the user data is collected at the edge of the network, sorted based on destinationaddress, and grouped into variable sized bursts. Prior to transmitting a burst, a control packet is created and immediately send toward the destination in order to setup a buffer less optical path for its corresponding burst. After an offset delay time, the data burst itself is transmitted without waiting for positive acknowledgement from the destination node. The OBS framework has been widely studied in the past few years because it achieves high traffic throughput and high resource utilization.In this paper we present in a systematic way the main objectives of OBS design parameters and the solutions that have been proposed.

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 light paths from one node to another for different clients/ services e.g. SDH (Synchronous Digital Hierarchy) circuits.
At present optical burst switching is an area that is attracting a lot of attention and is a potential method by which future optical networks may use the available optical resources more effectively. However, several issues still need to be addressed before optical burst switching can enter service in a real optical network. In particular, the technological demands and restrictions of electronic and optical components have to be considered with regard to an application in optical burst switched networks as well as assessment of the architectural and economic aspects of implementing optical burst switching.
Optical Burst Switching operates at the sub-wavelength level and is designed to better improve the utilisation of wavelenghts 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). These packets can also be aggregated according to quality of service (QoS). Therefore at the OBS edge router, different queues represent the various destinations of class of service. 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 separate 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.
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 or the order of milliseconds but require tens of milliseconds to set up the switch and perform switching. Therefore, OBS utilising this type of switching cannot rely on the one way signalling concept as defined by Just-In-Time (JIT) and Just-Enough-Time (JET).
The initial phase of introducing optical burst switching would be: 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.

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