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Optical Burst Switching (OBS)

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Optical Internet

• IP runs over an all-optical WDM layer– OXCs interconnected by fiber links– IP routers attached to OXCs

• Increasing discrepancy between optical transmission and electronic switching speed– Want “through” traffic to be switched in the

optical domain to eliminate the electronic bottleneck at the IP layer

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Optical Circuit Switching

• Lightpaths are set up between source and destination nodes– No optical buffer needed at the intermediate nodes

– Bit rate and protocol transparency

• Setting up a connection takes a few hundreds of ms Not suitable for short lived connections

• Bandwidth allocated by one wavelength at a time, however, most applications only need sub- bandwidth

• No statistical multiplexing Inefficient bandwidth utilization when carrying bursty traffic

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Optical Packet Switching

• High bandwidth utilization due to statistical multiplexing

• Need to buffer packets at intermediate nodes• Not feasible in the near future

– Current optical switches (OXCs) too slow for packet switching

– No practical optical buffer

– Immaturity of high-speed optical logic

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The Challenge

• How to efficiently support bursty traffic with high resource utilization as in packet switching while requiring no buffer at the WDM layer as in circuit switching?

• Answer: Optical Burst Switching (OBS)

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OBS

• Burst assembly/disassembly at the edge of an OBS network– Multiple IP packets aggregated into a burst at

the ingress node– Data bursts disassembled at the egress node– Packets/bursts buffered at the edge during burst

assembly/disassembly

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OBS

• Separation of data and control signals in the core– For each data burst, a control packet containing the

header information (including burst length) is transmitted on a dedicated control channel

• A control packet is processed electronically at each intermediate OBS node to configure the OXCs

– An offset time between a control packet and the corresponding data burst

• The offset time is large enough so that the data burst can be switched all-optically without being delayed at the intermediate nodes

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Advantages of OBS

• No optical buffer or fiber delay lines (FDLs) is necessary at the intermediate nodes

• Burst-level granularity leads to a statistical multiplexing gain absent in optical circuit switching

• A lower control overhead per bit than in optical packet switching

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OBS Building Blocks

• Burst assembly: assembly of client layer data into bursts

• Burst reservation protocols: end-to-end burst transmission scheme

• Burst scheduling: assignment of resources (wavelengths) at individual nodes

• Contention resolution: reaction in case of burst scheduling conflict

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Burst Assembly

• Aggregating packets from various sources into bursts at the edge of an OBS network– Packets to the same OBS egress node are

processed in one burst assembly unit– Usually, one designated assembly queue for

each traffic class– Create control packet and adjust the offset time

for each burst

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Burst Assembly Algorithms

• Timer-based scheme:– A timer starts at the beginning of each assembly cycle– After a fixed time T, all the packets that arrived in this

period are assembled into a burst.

• Effect of time out value T– T too large: the packet delay at the edge will be too long. – T too small: too many small bursts will be generated

resulting in a higher control overhead.

• Disadvantage: might result in undesirable burst lengths.

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Burst Assembly Algorithms

• Burstlength-based scheme:– Set a threshold on the minimum burst length. – A burst is assembled when a new packet arrives

making the total length of current buffered packets exceed the threshold.

• Disadvantage: no guarantee on the assembly delay

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Burst Assembly Algorithms

• Mixed timer/threshold-based assembly algorithm: – A burst is assembled when either the burst

length exceeds the desirable threshold or the timer expires

– Address the deficiency of both timer-based and burstlength-based schemes

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Burst Assembly Algorithms

• After a burst is generated, it’s buffered in the queue for an offset time before being transmitted

• During the offset period, packets may continue to arrive– Can’t include the packets in the same burst

– Leaving the packets for the next burst will increase the average delay.

• Use burst length prediction to minimize the extra delay

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Burst Length Prediction

• Let the control packet carry a burst length of l+f(t)– l : the exact burst length when the control packet is sent– f(t) : the predicted extra burst length as a result of

additional packet arrivals during the offset time t.

• Assume the total length of packets actually arrive during the offset time is l(t) – If l(t) < f(t), part of the bandwidth reserved will be

wasted– If l(t) > f(t), the extra packets are delayed to the

next burst

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A Burst Reservation Protocol: Just-Enough-Time (JET)

• Basic ideas– Each control packet carries the offset time and

burst length– The offset time is chosen so that no optical

buffering or delay is required at the intermediate nodes

– Delayed reservation: the reservation starts at the expected arrival time of the burst

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JET

• Control packet is followed by a burst after a base offset time (h): time to process the control packet at hop

h, 1 h H– No fiber delay lines (FDLs) necessary at the

intermediate nodes to delay the burst– At each intermediate node, T is reduced by (h)

H

h

hT1

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JET

• Use Delayed Reservation (DR) to Achieve efficient bandwidth utilization– Bandwidth on the output link at node i is reserved from the

burst arrival time ts to the burst departure time ts + l (l = burst length)

– ts = ta + T(i), where is the offset time remaining after i hops and ta is the time at which the processing of the control packet finishes

• The burst is dropped if the requested bandwidth is not available– Can use FDLs at an intermediate node to resolve contention

i

h

hTiT1

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JET