photonic packet switches: architectural and design issues yatindra nath singh electrical engineering...

Photonic Packet Switches: Architectural and design issues

Yatindra Nath Singh

Electrical Engineering Dept./ACES

IIT Kanpur-208016

Email: ynsingh@ieee.org

http://home.iitk.ac.in/~ynsingh

copyright 2001, YNS Photonic Packet Switching 2

Outline of the talk

- What is packet switch?

- Functionalities of packet switch

- Why optical packet switching?

- Basic switching elements and buffering

- Various architectures

- What we have been doing?

- Conclusion

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NxN switch

1

N

1

N

Input ports

Output

ports

What is a packet switch?

- Packet arrivals at inputs

- Header analysis and routing to designated output port

- Header replacement

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Functions in a packet switch

- Routing

- communication between neighbors

- resulting in provisioning of network connectivity information

- ultimately gives routing tables

- Forwarding

- analysis of header

- comparison of destination info with routing table

- decision regarding destination output port.

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- Switching

- directing each packet to proper output (as defined by forwarding process)

- needs switching and interconnect elements

-Buffering

- resolving contention by storing packets

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Problems with optical circuit switching

- BW granularity is poor.

- IP Router A not connected to IP Router B. Both connected to IP Router C.

- Packet from A to B goes via C.

- AC and CB light path may share some physical link.

- If traffic between AB high, Light path should be adjusted.

- Average traffic per physical link should be minimized.

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Why optical packet swtiching?

• clock skew

• cheaper electronic interfaces

• bit rate, modulation and format need not be standard

• Need to be agreed between two edge routers only.

• leads to payload transparency.

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Basic elements needed for the switching

- SOA

- Electro-optic switch (based on 2x2 coupler)

- Spatial Light modulator

- Tunable wavelength converters alongwith wavelength filter/ AWGM

- Fixed wavelength converter with tunable filter

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Basic generic packet switch

Synchroni-zing block

HeaderReplace-

ment block

SwitchingAnd

Bufferingblock

Switch controller

1

N

1

N

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- Routing and forwarding - difficult to implement using optical technology at the moment

- Switching and buffering - can be implemented optically as well as electronically

For photonic packet switch

- hybrid approach preferred

Routing and forwarding - using electronics

switching and buffering - optically

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Format for packets

For implementing certain optical header regeneration techniques

- packet format may be different. This structure need to be standardized over the network.

- for payload only duration need to be standardized.

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For buffering

- No equivalent of RAM in optical domain.

- Bits can be stored in bistable laser diodes or flip-flops

made using optical logic gates. (Technology is not

matured for implementing large optical RAMs.)

- Optical fiber delay lines

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Effective Refractive index in fiber ~ 1.5

Speed of light in fiber ~ 2x108 m/s

For packet with 1024 bytes = 8192 bits ~ 9000 bits (overhead bits, synchronization bits etc.), transmission rate 1Gb/s,

duration of transmission = 9x10-6 secs (slot period)

Fiber length to delay the packet by one slot = 1.8 km

T

b

n

cl

l - length of loop,

c - Speed of light,

n - R.I. of fiber,

b - number of bits in packet,

T - transmission rate

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Various types of buffers

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Other devices available

- WDM (multiplexers, demultiplexers),

- Couplers,

- filters,

- Add drop multiplexers

using these and other elements

switch architectures can be build.

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Wavelength routed all-optical packet switch

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Frontiernet architecture : another wavelength routed switch

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Broadcast and select type of switch

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Fiber loop memory based switch

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Architectural issues

- Processing time for a header

- Switch throughput – limited by number of headers that can be processed by controller.

- Payload should be sufficiently large in size.

- Ideally header processing time should be less than packet duration.

- Header replacement technique

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- Guard band duration.

- too small, tight tolerances on fiber delay lines and switch control implementation.

- too large, channel utilization low.

- Control points

- more separation of control points

- control synchronization required.

- Architectures with control points at inputs or output only – simpler to implement.

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- Noise accumulation

- ASE (amplified spontaneous emission noise) of Optical

amplifiers

- Gain crosstalk of optical amplifier

- Gain in loop if maintained equal to loss of loop, minimization

of noise accumulation.

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What we have been done so far?

(using analysis and simulations – in fiber loop buffer memory

switch)

- degradation due to ASE noise with number of recirculations

- control algorithm for the switch (with and without priority

mechanisms

- queuing performance analysis (exact and approaximate

methods) – appeared in IEEE comm. Letters, May 2001.

Detailed paper submitted in IEEE/OSA Journal of Lightwave

technology.

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- gain control techniques for EDFA to optimize switch performance

- gain in the loop should be maintained equal to loss for optimized operation test

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Recent results on this submitted to Electronics Letters.

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Summary and future scope

- Optical switching can be a viable technique for switching in backbone

- Investigation on queuing performance for multiple priority traffic/ multicast traffic need to be done.

- Investigations in high speed optical memories needed.

- control function of routing and forwarding using optical processing (need investigation)

- Switching architecture using free space optics