Chapter 10: Optical burst switching

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


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)


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


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.


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


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

.

.

.. . .

.

.

.


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.


Techniques for resolving contention in an optical switching

• optical buffering,

• exploiting the wavelength domain, and

• using deflection routing.


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.


• 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


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.


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


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.


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

. . .

. . .


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


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

. . .. . .

. . .. . .


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

. . .. . .


• 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


• 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

…

…

…

…

…


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.


A

B

End-device

End-device

SETUP

Burst

SETUP

Burst

offset

time


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


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



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.


time

Control packet

Burst


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


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


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


A B

timeoffset

Delayed setup, timed burst

Controlpacket

Burst

Processing time of control packetTime during which resources were allocated


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


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


Controlpacket

Burst

offset time

Immediate setup, timed release

Burst Arrival

No other bursts are acceptedduring this time


prior the end of the current burst

offset

Controlpacket


SETUP

Burst

offset time

Delayed setup, timed release: void filling

Burst Arrival

A new bursts is acceptedduring this time if it fits


prior the end of the current burst

offset

SETUP


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


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


• 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.


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.


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.


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.


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


On-the-fly connection setup

Time

Burst

A

SETUP

SETUP

SETUP

SETUPACK

RELEASE

RELEASE

RELEASE

CONNECT

B


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.


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


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)


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


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


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.


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.


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


The routing architecture

OXC

OXC

OXC

OXC

OXC

JITPAC

JITPAC

JITPAC

JITPACJITPAC

Data plane

Control plane


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.


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.


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.

Chapter 10: Optical burst switching

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