chapter 10: optical burst switching
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Connection-Oriented Networks - Harry Perros 1
Chapter 10:Optical burst switching
TOPICS– Optical packet switching– Optical burst switching
• Connection setup schemes
• Reservation/release of resources
• Scheduling
– The Jumpstart project
Connection-Oriented Networks - Harry Perros 2
Optical burst switching (OBS)
• It has not been standardized yet• It is regarded as a viable solution for
transmitting bursts over an optical network• A connection is setup uniquely for the
transmission of a single burst• OBS was preceded by an earlier scheme:
optical packet switching (OPS)
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Optical Packet Switching (OPS)
• A WDM optical packet network consists of optical packet switches interconnected by WDM fiber links.
• Optical packet switches operate in a slotted manner.
• An optical packet are fixed-sized in time, but the actual transmission rate may vary, i.e., the packet size may vary
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WDM optical packet switches
• A WDM optical packet switch consists of the following four parts:– input interfaces– the switching fabric– output interfaces, and – the control unit.
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Main operation in a switch: – The header and the payload are separated.
– Header is processed electronically.
– Payload remains as an optical signal throughout the switch.
– Payload and header are re-combined at the output interface.
payload hdr
Wavelength iinput port j
Opticalpacket
hdr CPU
Optical switch
payload
payload hdr
Re-combinedWavelength ioutput port j
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Output port contention
Assuming a non-blocking switching matrix, more than one optical packet may arrive at the same output port at the same time.
Output ports
payload
payload
payload
.
.
.
Optical SwitchInput ports
.
.
.. . .
.
.
.
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Contention resolution
• Output port contention commonly arises in packet switches, and it is known as external blocking.
• It is resolved by buffering all the contending packets, except one which is permitted to go out.
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Techniques for resolving contention in an optical switching
• optical buffering,
• exploiting the wavelength domain, and
• using deflection routing.
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Optical buffering - The Achilles' heel of OPS!
• Optical buffering currently can only be implemented using fiber delay lines (FDL).
• An FDL can delay an optical packet for a specified amount of time, which is related to the length of the delay line.
• FDLs are not commercially viable.
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• A buffer for N packets with a FIFO discipline can be implemented using N optical delay lines of different lengths.
• FDL i delays an optical packet for i timeslots.
• Assuming C wavelengths, FDL i may be able to store: C*i optical packets .
• Limited by the length of the delay lines, this type of optical buffer is usually small, and it does not scale up.
FDLs
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Exploiting the wavelength domain
• External blocking may be minimized by exploiting the WDM feature on a fiber link that connects two optical switches.
• Two optical packets destined to go out of the same output port at the same time can be sent out on two different wavelengths. This requires converters.
• This method may have some potential since the number of wavelengths that can be coupled together onto a single fiber continues to increase.
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Deflection routing
• When there is a conflict between two optical packets, one will be routed to the correct output port, and the other will be routed to any other available output port.
• A deflected optical packet may follow a longer path to its destination. In view of this:– The end-to-end delay for an optical packet may be
unacceptably high. – Optical packets may have to be re-ordered at the
destination
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Optical packet switch architectures
• Based on the switching fabric used, they have been classified in the following three classes:– space switch fabrics, – broadcast-and-select switch fabrics, and– wavelength routing switch fabrics.
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A space switch fabric architecture
Packet encoder Space switch Packet buffer
1
1
W
1
N
0
d
d
1
N
0
d
0
d
N
1
W
1
N
0
d
0
d
1
N
0
d
0
d
. . .. . .
1
0
0*T
N
d*T0
d
0*T
d*T0
d
. . .
. . .
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Packet encoder
• The switch is slotted, with N input/output ports, and W wavelengths per port
• The incoming signal in input port i is demultiplexed into the W wavelengths.
• Each wavelength carries a packet for that slot, and it is converted to another wavelength to avoid collisions at the destination output port.
11
W
N
1
W
. . .. . .
. . .
Tunable wavelength converter
De-mux
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Space switch
splitter
Optical gate
• The space switch fabric switches an optical packet to any of the N output optical buffers.
• A splitter distributes the same packet to N different output fibers, one per output port. The signal on each of these output fibers is split again d+1 times, one per FDL at the destination output buffer
1
N
0
d
d
1
N
0
d
0
d
1
N
0
d
0
d
1
N
0
d
0
d
0
. . .
. . .
InputPort 1
InputPort N
OutputPort 1
OutputPort N
. . .. . .
0
0
d
d
. . .. . .
. . .. . .
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Packet buffer
• A packet arrives at its destination port and it joins one of the FDLs
• FDL i delays an optical packet for a fixed delay equal to i slots (T), with FDL 0 providing zero delay,
1
0*T
N
d*T0
d
0*T
d*T0
d
. . .
i
coupler
FDL i
. . .. . .
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• Optical burst switching is a new technology that it is currently under study. It has not as yet been commercialized.
• Unlike optical packet switching, it does not require optical buffering.
• It can be seen as lying between optical packet switching and wavelength-routing networks.
Optical Burst Switching
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• An OBS network consists of OBS nodes interconnected with WDM fiber in a mesh topology.
• An OBS node is an OXC which has a very low configuration time, due to the fact that connection do not stay up for a long time.
Control Unit
Input WDM fibers
Output WDM fibers
…
Switch fabric
…
…
…
…
…
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Main features of OBS networks
• Each user transmits data in bursts.• For each burst, it first sends a SETUP message to
the network, to announce its intention to transmit.• Transmission of the burst takes place after a delay
known as offset.• The network nodes allocate resources for just this
single burst.
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A
B
End-device
End-device
SETUP
Burst
SETUP
Burst
offset
time
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A B
time
offset
Burst is transmitted without knowing if the connection has been successfully established
Offset = Sum of processing at each OXC
+ 1 configuration delay
On-the fly connection setup
Control packet
Burst
Processing time of control packet
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A B
time
offset
If the offset is not long enough, then the burst may arrive at an OXC before the SETUP request, or before the OXC has a chance to configure its switch!!
Control packet
Burst
Processing time of control packet
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A B
Confirmed connection setup
This is equivalent to circuit-switching.
It incurs a round-trip delay to set up the transmission, and the delivery of the burst is guaranteed.
Processing time of control packet
time
Control packet
Burst
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Reservation of resources in an OXC
• Immediate setup– The switch is configured immediately after the
SETUP request has been processed.
• Delayed setup– The SETUP request provides information that
is used to estimate when to configure the switch for the incoming burst
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Release of resources in an OXC
• Timed burst– The control packet contains information re. the
length of the burst. This permits the OXC to know when to release its resources.
• Explicit release– An OXC releases the resources allocated to a
burst upon receipt of an explicit release message
Connection-Oriented Networks - Harry Perros 27
A B
timeoffset
Immediate setup, timed release
Controlpacket
Burst
Processing time of control packetTime during which resources were allocated
A B
offset
Immediate setup, explicit release
Controlpacket
Burst
Re;easepacket
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A B
timeoffset
Delayed setup, timed burst
Controlpacket
Burst
Processing time of control packetTime during which resources were allocated
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Classification of reservation/release schemes
1. Immediate setup/explicit release
2. Immediate setup/timed release
3. Delayed setup/explicit release
4. Delayed setup/ timed release
Connection-Oriented Networks - Harry Perros 30
Controlpacket
Burst
offset time
Immediate setup, explicit release
Burst Arrival
No other bursts are acceptedduring this time
A new burst will be acceptedif its control packet arrives
after the end of the current burst
Scheduling bursts in an OBS node
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Controlpacket
Burst
offset time
Immediate setup, timed release
Burst Arrival
No other bursts are acceptedduring this time
A new burst will be acceptedif its control packet arrives
prior the end of the current burst
offset
Controlpacket
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SETUP
Burst
offset time
Delayed setup, timed release: void filling
Burst Arrival
A new bursts is acceptedduring this time if it fits
A new burst will be acceptedif its control packet arrives
prior the end of the current burst
offset
SETUP
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Lost bursts
• A burst is blocked when upon arrival at a node, its wavelength is at the output port is not free.
• Solutions:– Burst is dropped– Wavelength conversion– Deflection routing
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Burst assembly
• Each end-device maintains a queue for each destination end-device.
• Packets arriving at the end-device are placed accordingly to the destination queues, from where they are transmitted out in bursts.
• When to transmit a burst:– Timer– Max/min burst size
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• When a timer expires, all packets in the queue are transmitted out in a single burst, as long as:
Min. burst size < burst size
Also,
burst size < max. burst size
• A burst can also be transmitted out if the queue size reaches the max. burst size before the timer expires.
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Priorities
• The end-device can also introduce priorities when transmitting bursts.
• Each destination queue may be further sub-divided to a number of quality-of-service queues. The arriving packets are grouped into these queues, which are served according to a scheduler.
• In addition, different timers and maximum/minimum burst sizes can be used for different queues.
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The Jumpstart architecture
• Jumpstart is a DoD-funded project carried out by NC State University and MCNC, an RTP-based non-profit research organization.
• The objectives of Jumpstart are:– Define an architecture for signaling in OBS
networks and demonstrate proof of concept
– Define a routing architecture for OBS networks and demonstrate proof of concept.
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Features– Jumpstart uses the immediate setup with timed
or explicit release. – Both on-the-fly and confirmed connection setup
methods are used.– Uses out-of-band packet-based signaling (ATM
network) – The signaling messages for establishment and
tearing down of a connection are processed by the OBS nodes in hardware to assure fast connections.
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Basic signaling messages
• The following signaling messages have been defined for the basic operation of an OBS network:– SETUP– SETUP ACK– KEEP ALIVE– RELEASE– CONNECT– FAILURE
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On-the-fly connection setup
Time
Burst
A
SETUP
SETUP
SETUP
SETUPACK
RELEASE
RELEASE
RELEASE
CONNECT
B
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KEEP ALIVE messages
• In the case of explicit release, to guard against lost RELEASE messages, the control unit of each OBS node associates the transmission of a burst with a timer.
• The control unit assumes that the transmission of a burst has been completed if the timer expires and it has not received a RELEASE message.
• In view of this, when an end-device transmits a very long burst, it must periodically send to the network KEEP ALIVE messages which are used by each control unit to reset the timer.
Connection-Oriented Networks - Harry Perros 42
SESSION DECLARATION
SESSION ACK
KEEP ALIVE
SESSION RELEASE
Persistent connection setup
Data transfer
Tear down
SESSION DECLARATION
SESSION DECLARATION
SESSION ACK
KEEP ALIVE
KEEP ALIVE
SESSION RELEASE
SESSION RELEASE
SESSION ACK
BA • A persistent connection guarantees that a series follows the same path through the network. The following additional messages are used: – SESSION
DECLARATION,
– DECLARATION ACK
– SESSION RELEASE.
Persistent connections
Connection-Oriented Networks - Harry Perros 43
The signaling message structure
• Signaling messages are structured so that they can be partly processed in hardware and partly in software.
• The information carried in a message is organized in information elements (IEs):– Hardpath IEs (processed in hardware)– Softpath IEs (processed in software)
• IE structure: TLV (type, length, value)
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Common header Hardpath IEs Softpath IEs CRC 32
Protocoltype
Protocolversion
Message type
Messagelength
Softpath IEs offset
IE mask . . .
Hardpath IEs
Number of softpath IEs
. . .
TLVs
Headerflags
flags
Message format
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Addressing
0x1A 0x1B
0x01
0x02
0x0E
0x03
0x0B
0x001F
0x001
0x000F
0x035
Domain 0xA
Domain 0xB
OBS top Domain
• Hierarchical addresses, similar in spirit to the NSAP address format
Connection-Oriented Networks - Harry Perros 46
JITPAC: The Jumpstart signaling processing engine
• The JITPAC processes SETUP/RELEASE messages and controls the optical fiber.
• Hardware-based• Uses ATM/AAL5 frames for signaling• Controls the OXC (2D MEMS) via RPC calls
done over dedicated Ethernet.
Connection-Oriented Networks - Harry Perros 47
Main operation: SETUP message
• JITPAC receives a SETUP message.• Using the destination address it looks up the next
hop (i.e. the output port number).• Instructs the switch fabric to setup the path from
input to output.• Forwards the SETUP message to the JITPAC of
the next hop OXC.
Connection-Oriented Networks - Harry Perros 48
JITPAC
155Mb/sUTP
Ethernet OBS
OXC
ATM Network
Ethernet Network
Ethernet10BaseT
Ethernet10/100
ATM Interface
MPC8260
AlteraEP20K400
FPGA
SD
RA
M D
IMM
M
od
ule
(6
4 M
eg
)
Flash16M
SDRAM4M
60x Bus
Local BusSerial Port 1
Serial Port 2
Connection-Oriented Networks - Harry Perros 49
The routing architecture
OXC
OXC
OXC
OXC
OXC
JITPAC
JITPAC
JITPAC
JITPACJITPAC
Data plane
Control plane
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Features• Different routing architectures for control
messages and data bursts are used.• Signaling messages were not considered, since
they use the same routes as the data bursts.• Each JITPAC maintains two logical forwarding
tables: the control forwarding table, and the burst forwarding table.
• Two separate and independent path computation components were defined for the control forwarding table and the burst forwarding table.
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The routing architecture for control messages
• The control plane was implemented on an electrical packet-switched network.
• The primary routing goal is the computation of shortest paths between JITPAC controllers to enable the efficient exchange of control messages.
• To that effect a link-state protocol such as OSPF or IS-IS can be used to establish paths for control messages.
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Routing architecture for data bursts
• A centralized architecture is used for computing paths for data bursts within a network domain.
• The path computation is the responsibility of the Routing Data Node (RDN), a server attached to one of the OBS nodes. It is responsible for – collecting routing information regarding the data plane,
– computing the burst forwarding tables for each JITPAC controller, and
– downloading the tables to the JITPAC controllers.
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