26-06-2012, 11:19 AM
Optical burst switching
Optical burst switching.pptx (Size: 1.92 MB / Downloads: 215)
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.
Burst assembly
OBS users
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
Timer
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 .
Signaling
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.
Optical burst switching.pptx (Size: 1.92 MB / Downloads: 215)
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.
Burst assembly
OBS users
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
Timer
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 .
Signaling
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.