21-09-2008, 10:40 AM
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 current all optical technology In OBS the user data is collected at the edge of the network, sorted based on destination address,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 .
Introduction:
Optical communication has been used for a long time and it very much popular with the invention of wavelength-division multiplexing(WDM) Current WDM works over point-to-point links,where optical-to-electrical-to-optical(OEO) conversion is required over each step The elimination of OEO conversion in all optical networks(AON) allows for unprecedented transmission rates AON's can further be categorized as wavelength-routed networks(WRNs).,optical burst switched networks(OBSNs),or optical packet switched networks(OPSNs).Now we discuss here about optical burst switching(OBS)
In optical burst switching(OBS) data is transported in variable sized units called bursts Due to the great variability in the duration of bursts the OBS network can be viewed as lying between OPSNs and WRNS That is, when all burst durations are very short,equal to the duration of an optical packet,OBSN can be seen as resembling an OPSN On the other hand,when all the burst durations are extremely long the OBSN may seem resembling a WRN In OBS there is strong separation between the data and control planes,which allows for greater network manageability and flexibility In addition its dynamic nature leads to high network adaptability and scalability,which makes it quite suitable for transmission of bursty traffic .
In general,the OBS network consists of interconnected core nodes that transport data from various edge users The users consist of an electronic router and an OBS interface, while the core OBS nodes require an optical switching matrix,a switch control; unit and routing and signaling processors OBS has received considerable attention in the past few years and various solutions have been proposed and analyzed in an attempt to improve it's performance Here we describe the various OBS architectures by grouping the material logically per OBS design parameter
Burst aggregation:
OBS collects upper layer traffic and sort it based on destination addresses and aggregate it into variable size bursts The exact algorithm for creating the bursts can greatly impact the overall network operation because it allows the network designers to control the burst characteristics and therefore shape the burst arrival traffic The burst assembly algorithm has to consider a preset timer and maximum and minimum burst lengths The burst aggregation algorithm may use bit-padding ,the differentiation of class traffic , create classes of service by varying the preset timers and maximum/minimum burst sizes
One of the most interesting benefit of burst aggregation is it shapes the traffic by reducing the degree of self-similarity,making it less bursty in comparison to the flow of the original higher-layer packets Traffic is considered bursty if busy periods with a large of arrivals are followed by long idle periods The term self-similar traffic refers to an arrival process that exhibits burstiness when viewed at varying time scales:milliseconds,seconds,minutes,hours even days and weeks Self-similar traffic is characterized by longer queuing delays therefore degrades network performance Therefore reducing self-similarity is a desirable feature of the burst assembly process and concluded that traffic is less self-similar after the assembly.
