1

Advancesin

Optical Switching

Mohammad Ilyas, PhDCollege of Engineering & Computer Science

Florida Atlantic UniversityBoca Raton, Florida USA

ilyas@fau.edu

2

Outline

• Introduction• Switching techniques• Challenges in optical switching• Future directions• Summary

3

Introduction

• All communication networks have links and switching nodes

• Not all nodes are connected to all other nodes

• Switching nodes route information from links to links

• Switching techniques have changed significantly as the link capacities have increased

4

DDSS

A typical communication networkA typical communication network

Introduction (continued)

5

Introduction (continued)

• Switching nodes switch information from incoming links to outgoing links

• Routing, flow control, and congestion control are some of the important aspects

• Traditional wide area networks are primarily for one type of service

6

Introduction (continued)

• Resource - primarily transmission capacity and buffering capacity

• Supply and demand concept• Resource sharing - pre-

allocations versus dynamic allocationsDynamicDynamic

Pre-allocationPre-allocationTimeTime

7

Functional diagram of a switch Functional diagram of a switch

Switching matrixSwitching matrix

InputInput OutputOutput

InfoInfo

headerheader

RoutingRoutingalgorithmalgorithm

New headerNew header

InfoInfo

Introduction (continued)

8

Switching Techniques

• Two broad categories of switching: space division switch and time division switch

• A space division switch has multiple inputs and multiple outputs

• A time division switch has a single multiplexed input and the time slots are interchanged to switch the information

9

Switching Techniques (continued)

Space switchSpace switch

InputInput OutputOutput

10

Switching Techniques (continued)

Time division switchTime division switch

Logic for timeLogic for timeslot interchangeslot interchange

InputInputOutputOutput

11

Switching Techniques (continued)

• Performance aspects– Connectivity - set of pairs that can be

simultaneously connected through a switch

– Delay - the amount of time the switch takes to route the incoming information to an appropriate output

– Throughput - the amount of information that can be successfully handled by the switch / unit time

12

Switching Techniques (continued)

• Switch architectures– Crossbar switch (also called matrix

type or single-stage space division switch)

– Multi-stage space division switch– bus type– ring type– Optical switches

13

Switching Techniques (continued)

Crossbar switchCrossbar switch

InputInput

OutputOutput

14

Switching Techniques (continued)

• Switch architectures (continued)– Multistage switches have a variety of

configurations– These switches are modular and are

usually made up of 2 x 2 switching modules and may have input or output buffers

InputInput OutputOutput

15

Switching Techniques (continued)

Omega Omega

InputInput OutputOutput

000000001001

010010

011011

100100101101

110110111111

000000001001

010010

011011

100100101101

110110111111

Tag = 011Tag = 0111 = down1 = down

For output 110, the routing tag is 011For output 110, the routing tag is 011

Tag = 01Tag = 011 = down1 = down

Tag = 0Tag = 00 = up0 = up

16

Switching Techniques (continued)

• Impact of input buffers– If we add buffers, then less packets are

discarded and the throughput increases

– At the same time, the addition of buffers adds to the queueing delay and the overall switching delay increases

– Larger the number of buffers, better is the throughput

– May cause HOL blocking

17

Switching Techniques (continued)

InputInputBuffersBuffers

••••••

SwitchSwitch OutputsOutputsInputsInputs

18

Optical Switching

• Optical switching mostly needs optical-to-electronic and electronic-to-optical conversion

• This slows down the process and increases delay

• Pure optical switching has its own challenges:– Can we buffer in optical domain– Utilization– Error recovery– others

19

Optical Switching (continued)

• Switching information in optical domain can reduce the delay

• Optical switching requires capability to direct the light beam to a desired output based on the address

• Storage of information is not possible in optical domain

• Real time processing (possibly optical computing) is needed

20

Optical Switching (continued)

• Once the address of a packet has been identified, it can be used to direct the trailing information properly

• the delay (if needed) is introduced by looping the information (in optical domain) for some time

• The technology is being developed for an eventual goal of building large optical switches

21

Optical Switching (continued)

• Demand on communication network is increasing and is expected to continue

• Need for pushing more and more information through networks

• Many approaches:– WDM optical switching– Lamda switching– Optical burst switching– Several variations are possible

22

Optical Switching (continued)

• The term “burst switching“ means taking several packets together and transmitting them together and no buffering

• In case of conflict for an output port, one of the incoming bursts would be dropped.

• At a switching speed of 1 μs, one could switch bursts of 10 μs length (typically containing many packets)

• Possible use of optical buffers• Optical burst switching allows betters

sharing of bandwidth

23

Optical Switching (continued)

• Optical burst switching:– Increasing data rate versus utilization– A packet of 10,000 bits on an optical link,

occupies only 1 micro second of transmission– A link of 1 Kilometer introduces 5

microseconds of propagation delay– If information is transmitted one packet at a

time, it results in very poor transmission efficiency

– This cannot be ignored in designing optical switching

24

Optical Switching (continued)

• Optical burst switching (continued):– Increasing data rate versus utilization– A packet of 10,000 bits on an optical link,

occupies only 1 micro second of transmission– A link of 1 Kilometer introduces 5

microseconds of propagation delay– If information is transmitted one packet at a

time, it results in very poor transmission efficiency

– This cannot be ignored in designing optical switching

25

Future directions

• Switching in optical domain is almost a necessity

• Innovative approaches such as optical burst switching are needed to better share the resources

• New materials may help with buffering in optical domain

• Optical computing needed for reading packets on the go

26

Conclusions

• Optical networks are rapidly emerging• Several challenging research problems• Utilization versus throughput and delay• Some wastage is unavoidable

27