Optical Burst Switching (OBS) - PowerPoint PPT Presentation

Optical Burst Switching (OBS)

• 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

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

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

• 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)

• 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

• 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

• 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

• Burst assembly: assembly of client layer data into bursts
• Burst reservation protocols: end-to-endburst transmission scheme
• Burst scheduling: assignment of resources (wavelengths) at individual nodes
• Contention resolution: reaction in case of burst scheduling conflict

• 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

• Timer-basedscheme:
• 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.

• Burstlength-basedscheme:
• 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

• Mixed timer/threshold-basedassembly 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

• 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

• 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

• 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

• 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)

• 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