All-Optical Label Swapping

26 Optics & Photonics News � March 2002

DANIEL J. BLUMENTHAL

All-optical label swapping (AOLS), an approach to transparently route IP packets all-optically, has potential application to the future Internet. AOLSuses optical encapsulation to route packetsindependent of their bit-rate, format orlength. Photonic technologies like rapidlytunable all-optical wavelength convertersplay a key role in realizing AOLS. This article also covers semiconductor opticalamplifier and nonlinear optical-fiber-basedwavelength converter implementations.Results of an AOLS systems demonstrationwith 40-Gbps packets is shown.

for the Future Internet for the Future Internet

All-Optical Label Swapping

1047-6938/02/03/0026/8-$0015.00 © Optical Society of America

W ithin today’s Internet, data istransported between powerfulelectronic Internet protocol

(IP) routers using optical-fiber transmis-sion and wavelength-division-multiplex-ing (WDM) systems. Fiber-transmissionsystems today typically carry 32 wave-lengths modulated at 2.5 Gbps to 10 Gbps(1 Gbps = 109 bits/sec.) per wavelength. Atan IP router, multiple WDM fibers are ter-minated and signals are converted fromoptical to electronic at the input and elec-tronic to optical at the output. Today’srouters need to handle almost 0.5 Tbit/s(0.5 x 1012 bits/sec.) of data in order toredirect incoming Internet packets fromfully loaded WDM fibers.

Things become very interesting whenwe consider that the capacity of opticalfibers continues to double every 8-12months. Today’s state-of-the-art singlefiber capacity exceeds 10 Tbps. Compar-ing this increase with that of electronicprocessor speeds which doubles every 18months (Moore’s Law) and comes at theexpense of increased chip power dissipa-tion, we start to see that there is a potentialmismatch in bandwidth handling capabil-ity between fiber-transmission systemsand electronic routers. The story is morecomplex when we consider that futurerouters will terminate potentially hun-dreds or thousands of optical wavelengthsand the increase in bit-rate per wavelengthwill head out to 40 Gbps and potentiallybeyond to 160 Gbps. Additionally, elec-tronic memory access speeds only increaseat the rate of approximately 5% per year,an important data point since memoryplays a key role in how packets arebuffered and directed through the router.It is not difficult to see that the process ofmoving a massive number of packets persecond (100 million packets/sec. and be-yond the 1 billion packets/sec. mark)through the multiple layers of electronicsin a router can lead to router congestionand exceed the performance of electronicsand the ability to efficiently handle the dis-sipated power. Cost is also an importantissue: The cost of performing conversionbetween optics and electronics can con-sume more than half the cost of a router.

In this article we review all-optical labelswapping (AOLS), a technique intended tosolve the potential mismatch betweenfiber capacity and router packet-forward-ing capacity. AOLS imparts the functional-ity to direct packets through an optical

network without the need to pass thesepackets through electronics whenever arouting decision is necessary.1-6 Inherentto this approach is the ability to routepackets independently of bit-rate, packetor coding format and packet length. Forthis reason, AOLS is not limited to IPpackets, but can handle ATM (AQ:1) cells,bursts, data-file transfer and other datastructures. We review research results ob-tained at the University of California atSanta Barbara in collaboration with Pro-fessors John Bowers’ and Larry Coldren’sgroups at UCSB under the support of theDARPA NGI program, Cisco systems,Spirent Technologies, the DARPA-spon-sored MOST Center and a State of Califor-nia CORE grant sponsored by New Focus.An AOLS network is illustrated in Fig.1(a). IP packets enter the network throughan “ingress” node and are encapsulatedwith an optical label and then retransmit-ted on a new wavelength. The optical labeland new wavelength are determined byreading the packet’s IP header (called “IPlayer 3” information) and using informa-tion stored in a pre-established locallookup table. Once inside the network,only the optical label is used to make rout-ing decisions and the wavelength is used todynamically redirect (forward) packets.Nodes inside the network have an in-creased functionality over their ingressnode counterparts. At these internalnodes, labels are read and optically erased,then a new label is attached to the packetand the optically labeled packet is convert-ed to a new wavelength using all-optical

ALL-OPTICAL LABEL SWAPPING

March 2002 � Optics & Photonics News 27

Figure 1. (a) All-optical label switched (AOLS)network. (b) Bit serial and subcarrier multiplexedoptical label coding techniques.

Glossary of TermsSemiconductor optical amplifier(SOA): A semiconductor optical waveguide with gain.

Cross-phase modulation (XPM):A nonlinear optical effect where the intensity modulation of one wavelengthmodulates the index of refraction in amaterial where another wavelength ispropagating and therefore modulates its phase.

Wavelength-division multiplexing(WDM): The transmission of multiple independent data streams on a singlefiber using a different optical frequencyfor each stream.

Internet protocol (IP):The addressingscheme for packets on the Internet.

BibliographyIEEE Journal of Lightwave Technology,Special Issue on Photonic Packet SwitchingArchitectures,Techniques and Technologies,Dec. (1998).

IEEE Journal on Selected Areas in Communications, Special Issue on OpticalNetworks, Sept. 1998.

Optical Networks:A Practical Perspective,Ramaswami and Sivarajan, Second Edition,Morgan Kaufmann Publishers, 2002.

(a)

(b)

wavelength conversion. Throughout thisprocess, the contents (e.g., the IP packetheader and payload) are not passedthrough electronics and are kept intactuntil the packet exits the optical networkthrough the “egress” node where the opti-cal label is removed and the original pack-et is handed back to the electronic routinghardware.

A module that performs the label-swapping function is shown schematicallyin Fig. 2. As packets enter the module, asmall percentage of light is redirected, us-ing an optical tap, to a lower label-process-ing circuit where it is converted to an elec-tronic signal and the new label and newwavelength are computed. While the newlabel and new wavelength computationare being performed, the labeled packetpasses through a fixed fiber delay-line thatmatches the electronic-processing delay.The electronic circuit then sets up the newlabel and wavelength in the upper photon-ic processing circuit. The packet arrives atthe first stage of the photonic processingcircuit and the incoming label is opticallyerased. The packet is then converted to anew wavelength and a new optical label isreattached all-optically. Today we use elec-tronics to process labels even though thepacket is switched all-optically. This ap-proach allows us to use more powerfultable lookup and routing decisions than ifcurrent all-optical logic technology wereused. As long as the label bit-rate is operat-ed at a relatively fast rate and compatiblewith today’s state-of-the-art electronics(for example 2.5 Gbps or 10 Gbps), andthe number of bits in a label are held to aminimum, the routing decision can becomputed relatively fast and not decreasethe optical packet throughput. The labelbit-rate is independent of the payload bit-rate and is chosen to be compatible with

burst-mode electronics. For example both40 Gbit/s and 10 Gbit/s packets can berouted using 2.5 Gbit/sec headers. In thefuture, optical logic and memory elementsmay prove feasible to replace the fixed de-lay line and electronic-processing circuit.In electronic routers, digital memory isused to hold packets for arbitrary amountsof time while a routing decision is made.However, it is difficult to get all-opticalbuffer technology that can hold packetswith arbitrary delays to work with the ar-bitrary length inherent to IP packets, an is-sue that adds to the complexity of AOLStoday (AQ:2).

Switching technology to redirect pack-ets at the output must be capable of doingso on the order of a nanosecond (10-9

sec.). Wavelength conversion using fasttunable lasers is a scalable switching tech-nique that can meet the demands of opti-cal-packet switching applications. Figure 3illustrates how a rapidly tunable laser isused as a local source for a wavelengthconverter. When combined with a wave-length-demultiplexing device, such as awavelength router,6 packets can be spatial-ly directed by selecting the correct wave-length at the tunable laser. Switching be-tween any pair of 36 ITU channels in lessthan 5 ns has been reported7 and is scala-ble to a larger number of wavelengths.

The choice of wavelength-convertertechnology tends to be strongly coupled tothe label-coding technique. Two main ap-proaches to optical label coding are bit-se-rial and subcarrier multiplexed,6 as illus-trated in Fig. 1(b). Here we focus on seriallabels to illustrate the process of erasingand rewriting labels using wavelength con-verters. The serial label is placed at thehead of the IP packet following an opticalguard-band. This guard-band is used tofacilitate label removal and reinsertion

without static packet buffering and to ac-commodate finite switching times of opti-cal switching and wavelength conversion.The bit-serial label is encoded on the sameoptical wavelength as the IP packet.

Two types of wavelength converterhave been employed for AOLS, semicon-ductor-optical-amplifier (SOA) based andnonlinear-optical-fiber based converters.A popular converter configuration thatperforms most of the AOLS functions isthe SOA interferometric wavelength con-verter (SOA-IWC). The SOA-IWC usescross-phase modulation (XPM) in an SOAto realize an optically driven modulatorand has been shown to operate to datarates exceeding 40 Gbit/s. Its operation isbased on the well-known principle of con-structive and destructive interference ofoptical waves. A tunable laser, located atone input of the SOA-IWC as shown inthe inset of Fig. 3, is split to two opticalwaveguides. The original modulatedwavelength drives an SOA in one of the in-terferometer arms, thereby modulatingthe local laser signal and transferring itsdata over to the new wavelength. TheSOA-IWC can also be used as the outputstage in a two-stage converter. One exam-ple is shown in Fig. 4, where a XGM con-verter is turned off as the serial label pass-es through and blocks the label bits. TheXGM-SOA is then turned on as the packetbits arrive and converts the packet to anintermediate wavelength (�int). Mean-while, the new serial label is premodulatedonto the tunable laser that has been tunedto the new wavelength (�out). The label istransmitted to the SOA-IWC outputahead of the packet arrival at the secondstage. The SOA-IWC converts the packetto the final wavelength while regeneratingand reinverting the bits. An optical tap,photodetector and burst-mode receiver at

ALL-OPTICAL LABEL SWAPPING

28 Optics & Photonics News � March 2002

Figure 3. AOLS and packet routing using a fast tunable wavelength converterand wav length demultiplexer/router.

Figure 2. AOLS module functionality.

the input are used to detect the label forselection of the new wavelength and label.

For operation at ultrahigh bit-rates(from over 40 Gbps up to 160 Gbps) awavelength converter based on nonlinear-ities in an optical fiber has been demon-strated.8 This converter employs XPM in adispersion-shifted fiber to phase modulatecontinuous wave (cw) light from the localtunable laser using the intensity-modulat-ed signal on the original wavelength asshown in Fig. 5. The resulting phase-mod-ulated signal is converted to intensitymodulation by optically filtering the con-verter output. The fiber XPM converter iscapable of erasing and writing new labels.The high-speed packet bits are coded re-turn to zero (RZ) and the lower-speed se-rial-label bits are coded non-return to zero(NRZ). XPM effectively converts the RZbits and not the NRZ bits. Therefore thelabel is automatically erased. A new NRZlabel is premodulated onto the local laserand passes straight through the converteron the final wavelength.

A systems experiment that demon-strated AOLS8 using a fiber XPM wave-length converter with 40 Gbps packets isshown in Figure 6. The payload was 40Gbit/s RZ and the serial label at 2.5 Gbit/sNRZ. In the lower left hand trace, thepackets with labels at the input to the con-verter are compared to the output wherethe labels are erased. In the lower righthand scope traces are packets beingswitched between two output wave-lengths.

In summary, we have described all-op-tical label swapping (AOLS) and its poten-tial application to the future Internet.AOLS uses optical encapsulation to trans-parently route packets independent of bit-rate, format or length. The basic functionsof AOLS are reviewed including label era-sure, rewriting and all-optical wavelengthconversion. Two types of wavelength con-verters to implement the AOLS and packetrouting, one semiconductor optical ampli-fier based and the other nonlinear fiber-based were reviewed. This approach hasbeen shown to operate at packet bit-ratesout to 40 Gbps. (Does this mean Gbit/s?)

AcknowledgmentsThe author would like to acknowledge themany individuals at UCSB who con-tributed to the work described here, inparticular Lavanya Rau, Suresh Rangara-jan, Milan Masonovic, Olga Lavrova, Wei

Wang, Xuejin Yan, Yijen Chiu, Hsu-FengChiu, Erik Skogan, and Marcelo Davanco.We also wish to thank our collaborators atTelcordia Technologies in the area of opti-cal label swapping. (AQ:3)

References1. D. J. Blumenthal, Scientific American, January (2001)

(AQ:4)2. D. J. Blumenthal, P. R. Prucnal, and J. R. Sauer, Proc.

IEEE, 82 (11), 1650-67, November (1994).3. B. Bostica,,A. Cappellari, M. Burzio, B.Vercellone, C.

Guillemot,A. Gravey, P. Gravey, F. Masetti, M. Sotom,M. Renaud,“A Novel Packet Switching networkAdopting Transparent Optical Packets,” CSELTTechnical Reports, 24 (6), CSELT, 1049-56, Dec.1996.

4. D. J. Blumenthal, B-. E. Olsson, G. Rossi,T. Dimmick,L. Rau, M. Masanovic, O. Lavrova, R. Doshi, O. Jer-phagnon, J. E. Bowers,V. Kaman, L.A. Coldren and J.Barton,“All-Optical Label Swapping Networks andTechnologies,” IEEE J. Lightwave Technol., Special Is-sue on Optical Networks, Dec. 2000 (Invited Pa-per).

5. Y. M. Lin,W. I.Way, and G. K. Chang, IEEE Photon.Technol. Lett. 12 (8), 1088-90,Aug. 2000.

6. A. Carena, M. D.Vaughn, R. Gaudino, M. Shell and D.J. Blumenthal IEEE J. Lightwave Technol., Special Issueon Photonic Packet Switching, 16 (12) 2135-45,Dec. (1998).

7. O.A. Lavrova, G. Rossi, and D. J. Blumenthal,“Rapidtunable transmitter with large number of ITU chan-

nels accessible in less than 5ns,” Proc. 26th Euro-pean Conference on Optical Communication(ECOC 00), Munich, Germany, Paper 6.3.5, 169-70,September 4-7 (2000).

8. B. E. Olsson, P. Ohlen, L. Rau, G. Rossi, O. Jer-phagnon, R. Doshi, D. S. Hunphries, D. J. Blumenthal,V. Kaman, and J. E. Bowers,“Wavelength routing of40 Gbit/s packets with 2.5 Gbit/s headererasure/rewriting using an all-fiber wavelength con-verter,” Postdeadline Paper, ECOC 2 52-3, Nice,France, September 1999.

Daniel J. Blumenthal is with the Department of Elec-trical and Computer Engineering, University of Cali-fornia, Santa Barbara, California. His e-mail address isdanb@ece.ucsb.edu.

Author queries1. Dan, please spell out ATM the first timeit is used. Thanks.2. Does sentence as edited convey yourmeaning?3. Please re-check acknowledgments andmake sure no names were inadvertentlyleft out: this section was compiled usingthe automatic reference function and inreformatting we want to make sure nonames were inadvertently deleted.4. Please supply page range in Ref. 1. Ourstyle is not to use article title.

ALL-OPTICAL LABEL SWAPPING

March 2002 � Optics & Photonics News 29

Figure 4. AOLS using a two-stage SOA-based wavelength converter.

Figure 5. Nonlinear fiber cross-phase modulation (XPM) converter operating at 40 Gbps.