Optical Burst Switching
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Joined: Dec 2008
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 .
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
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.
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Joined: Feb 2012
03-03-2012, 10:06 AM
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Joined: Apr 2012
26-06-2012, 11:19 AM
Optical burst switching
Optical burst switching.pptx (Size: 1.92 MB / Downloads: 42)
Optical burst switching
Optical burst switching (OBS)
Combines merits of optical circuit switching (OCS) & optical packet switching (OPS) while avoiding respective shortcomings
Switching granularity at burst level allows for statistical multiplexing at lower control overhead than OPS
Only control packets carried on one or more control wavelength channels undergo OEO conversion at each intermediate node
Data bursts are transmitted on separate set of data wavelength channels that are all-optically switched at intermediate nodes
OBS combines transparency of OCS with statistical multiplexing gain of OPS.
collect traffic originating from upper layers (e.g., IP),
sort it based on destination addresses, and
aggregate it into variable-size bursts by using burst assembly algorithms
Burst assembly algorithms have significant impact on performance of OBS networks & have to take the following parameters into account
Used by OBS user to determine when to assemble new burst
Minimum & maximum burst size
Determine length of assembled burst
Timer & burst size parameters must be set carefully
Long bursts hold network resources for long time periods => higher burst loss
Short bursts cause increased number of control packets => higher control overhead
Padding may be used to assemble minimum-size burst when timer expires
Burst assembly helps reduce degree of self-similarity of higher-layer traffic & make it less bursty => decreased queueing delay & smaller packet loss .
Used to set up connection for assembled burst between given pair of source & destination edge OBS users
OBS networks may deploy one of two types of signaling
Distributed signaling with one-way reservation or
Centralized signaling with end-to-end reservation
Most of proposed OBS network architectures use distributed signaling with one-way reservation
Source OBS user sends control packet on separate out-of-band control channel to ingress OBS node prior to transmitting corresponding burst after certain offset
Out-of-band control channel may be dedicated signaling wavelength channel or separate control network
Control packet contains information about burst (e.g., size)
Control packet is OEO converted & processed in electronic domain at each intermediate OBS node .
One-way vs. two-way reservation
In one-way reservation, burst is sent out after prespecified delay, called offset, without waiting for acknowledgment (ACK) that connection has been established
In conventional two-way reservation, source OBS user would wait for ACK before sending any data
Benefit of one-way reservation
Significantly decreases connection set-up time to one-way end-to-end propagation delay plus time required to process control packet & configure optical switch fabric at intermediate OBS nodes
Shortcoming of one-way reservation
Nonzero burst loss probability since control packets may not be successful in setting up connections due to congestion on data wavelength channels
Retransmission of lost bursts left to higher-layer protocols.
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