FPO
`
`SLIM FILMS
`
`T
`
`oday’s network architects have already be-
`gun to build the Optical Internet using tech-
`nologies that can switch some or all of the
`light signals in one incoming optical fiber to
`one or several outgoing fibers. The new-gen-
`eration Optical Internet will serve as a high-
`speed mail delivery vehicle that will bring
`units of data called packets to a point nearby
`a recipient. There the time-consuming process of sorting out
`which packet goes where—the Internet equivalent of the local
`post office—will fall to electronic routers from companies
`such as Cisco Systems and Juniper Networks. Over time, even
`this task of switching individual packets may be taken over
`by routers that use photons, not electrons, for processing.
`The IP packet is the Internet’s basic unit of currency. In
`today’s networks, every e-mail message gets chopped into
`thousands of packets, which are switched over different path-
`ways and reassembled at a final destination using the Internet
`Protocol (IP). The routers, together with other networking
`equipment, convert data from lightwaves to electronic signals
`in order to read the packets and send them along to their fi-
`nal destination: a mail server, where they are reassembled
`into a coherent message before a synthesized voice proclaims,
`“You’ve got mail!” The key is that the routers all along the
`way can easily read the address of each IP packet.
`The lightwave network that does the heavy hauling may
`serve as only one stage in the evolution toward a truly all-
`optical network, however. When networks routinely shuttle
`
`OPTICAL MULTIPLEXER, a component of an optical packet router,
`sends four incoming wavelengths to two output ports.
`
`terabits (trillions of bits) a second, the conversion step into
`electronics may prove too costly both in time and money. In
`coming years, a router will routinely have to break down a
`data stream carrying 40 gigabits (billions of bits) a second
`over a single wavelength into 16 parallel electronic data
`streams, each transmitting 2.5 gigabits a second within the
`router. Moving a massive number of packets every second
`through multiple layers of electronics in a router can lead to
`congestion and degrade network performance.
`
`Light to Light
`
`In response to these problems, network engineers are con-
`
`templating a solution that will use lightwaves to process
`lightwaves and optical switches that can redirect packets at
`blindingly fast speeds. The technology for photonic routing
`switches is still in its infancy. Creation of a photonic packet-
`switched network will depend on overcoming multiple tech-
`nological hurdles equivalent to those that had to be ad-
`dressed by electronics engineers from the mid-1950s to the
`mid-1970s, going from individual capacitors and resistors sol-
`dered onto a circuit board to monolithic integrated circuits.
`Nevertheless, the quest has begun. The Defense Ad-
`vanced Research Projects Agency (DARPA) has funded pro-
`grams in optical packet switching at the University of Cali-
`fornia at Santa Barbara, Telcordia Technologies in Morris-
`town, N.J., Princeton University and Stanford University.
`Alcatel in France, the Technical University of Denmark and
`
`
`
`9696
`
`
`
`Scientific American January 2001Scientific American January 2001
`
`
`
`Routing Packets with LightRouting Packets with Light
`
`Copyright 2000 Scientific American, Inc.
`
`Ex. 1025
`CISCO SYSTEMS, INC. / Page 1 of 4
`
`

`

`The ultimate all-optical
`network will require
`dramatic advances in
`technologies that use
`one lightwave to
`imprint information
`on another
`
`by Daniel J. Blumenthal
`
`ROUTING
`
`PACKETS
`WITHLIGHT
`
`the University of Strathclyde in Scotland have also launched
`research efforts.
`A potential communications technology for photonic
`packet switching in future networks is known as All-Optical
`Label Swapping (AOLS). In AOLS, individual IP packets or
`groups of packets get tagged with an optical label. The IP
`packet is like an envelope with an address on the front
`(called the header) and contents inside (called the payload).
`AOLS attaches a label to the packet by placing it, say, in
`front of the header. The use of labels resembles packing into
`the same mailbag all letters going through the same set of
`major cities on the way to their final destination. Postal
`workers read only the label at each intermediate routing
`point, not each individual envelope.
`In a communications network, an optical router will look
`only at the smaller label and determine which output fiber or
`wavelength to send the packet to, instead of reading the
`header inside the packet. Current photonic technologies do
`not make this task easy. Optical components that perform the
`function of the integrated-circuit elements that read and
`process a label exist mostly as laboratory demonstrations that
`have not made their way into the commercial mainstream.
`Take something as simple as the buffers used for tempo-
`rary storage of the bits in a packet. In designing an electronic
`router, an engineer would hold the packet in a buffer that is
`similar to the dynamic random-access memory (DRAM) used
`in computers. Photons cannot be stored easily, however. So de-
`signers must then figure out ways to buffer the packet by com-
`
`plex optical circuits that do not hold the photons still but
`rather corral the pulses of light into a holding area similar to
`an automobile traffic circle. Other approaches use techniques
`that match how long it takes the pulses in a packet to move
`through a switch. They do so by sending the pulses through a
`predetermined length of fiber. The time it takes to traverse this
`“detour” fiber equals the time needed by the switch to strip off
`the packet label and read the routing information it contains.
`Optical buffers on laboratory benchtops are currently
`hundreds of times larger than the submicron-size dimensions
`of their electronic buffer counterparts. It is not known if the
`optical version of the dense conglomeration of transistors
`found in the Pentium chip will ever come to be a reality.
`Besides buffers, logic circuits that can process labels
`based on light controlling light have also been shown in the
`lab but are a long way from being engineered into a full-
`scale router. The first examples of photonic routers will
`therefore still rely partly on electronics. A packet that enters
`a first-generation AOLS router will have a small amount of
`its optical energy diverted down a separate pathway, where
`it gets converted by a photodetector into an electronic copy
`of the packet and label. The label goes to circuitry that reads
`its contents and computes the next pathway for the packet
`along the network. The electronic processor generates a new
`label, attaches it to the original packet, and sends a control
`signal to the switch to specify a particular wavelength or
`fiber along which the packet should travel [see box on next
`page]. Despite the need to make an optoelectronic conver-
`
`www.sciam.com
`
`Scientific American January 2001 97
`
`Copyright 2000 Scientific American, Inc.
`
`Ex. 1025
`CISCO SYSTEMS, INC. / Page 2 of 4
`
`

`

`Optical Router
`
`1 DEMULTIPLEXER
`
`2 OPTICAL SPLITTER
`
`3 LABEL ERASER
`
`4 LABEL WRITER
`
`5 WAVELENGTH
` CONVERTER
`
`INCOMING FIBER
`
`WAVELENGTHS
`
`PACKET
`
`PACKET
`
`LABEL
`
`ALL-OPTICAL LABEL SWAPPING is an emerging tech-
`nology for photonic switching of individual packets of
`data. A packet enters the switch, gets stripped off and
`read electronically (A) or with lightwave processing ele-
`ments (B). The processors decide on which wavelength
`and fiber a packet should exit the switch. A new label
`gets placed on the packet, which then gets switched to
`a new wavelength and an output fiber.
`
`PHOTODETECTOR
`
`SEMICONDUCTOR-
`OPTICAL
`AMPLIFIER
`
`CONTROL ELECTRONICS
`
`SEMICONDUCTOR-
`OPTICAL
`AMPLIFIER
`
`CONTROL PHOTONICS
`
`A
`
`B
`
`LASER
`
`COMBINER
`
`LAURIE GRACE
`
`sion for the tiny label, perhaps 20 bits
`in size, the optical switches can forgo
`electronic processing of the much larger
`packet, which may range from 40 to lit-
`erally thousands of bytes. The big pack-
`et moves through the switch at the
`higher-speed optical bit rate. Making
`the conversion for the entire packet
`might be prohibitive in cost as well as
`consuming power and space for elec-
`tronic equipment.
`Once the label has been processed,
`the packet finally arrives at the physical
`elements that switch it from the input to
`the output fiber. Optical switching ele-
`ments pose another major engineering
`challenge, because the switching com-
`ponents must be able to send a packet
`to a new fiber or wavelength in a nano-
`second or less. Several technologies have
`emerged that are capable of switching
`packets at the desired speeds. One ex-
`ample is the semiconductor-optical am-
`
`plifier, a device that uses stimulated
`emission of light as the basis for switch-
`ing, the same process that drives a laser.
`To switch a signal on an incoming
`fiber to any of many outgoing ones, the
`amplifier forms an optical bridge be-
`tween the desired input and output
`fibers. When a data-carrying optical sig-
`nal reaches the bridge, an electric cur-
`rent in a control signal injects electrons
`and “holes” (areas where electrons are
`absent) into the amplifier. Light entering
`the amplifier causes the electrons and
`holes to combine with one another, giv-
`ing off more photons that are exact
`copies of the optical signal trying to
`cross the bridge. Once the signal reaches
`a certain level of power, it moves from
`one side of the bridge to the other.
`When the control current is shut off,
`light at the amplifier input is absorbed
`and does not make it through to the
`output fiber. Although the control signal
`
`that opens the gateway is electronic, the
`stream of photons carrying packets
`races through the switch without get-
`ting converted to an electronic signal.
`The technology remains in early de-
`velopment. Investigators have devised
`fast switches with up to eight incoming
`and outgoing fibers each, although for
`marketable products they will need to
`create switches with hundreds or thou-
`sands of input and output ports.
`Semiconductor-optical amplifiers may
`also perform a vital role in realizing a
`technology that can transfer a stream of
`bits directly from one lightwave to an-
`other. This wavelength converter can
`switch packets to multiple output fibers
`by combining the technology with a de-
`vice called a wavelength multiplexer
`that, much like a prism, can separate
`light into its individual colors. The
`wavelength converter will determine
`how to route packets to avoid switching
`
`98
`
`Scientific American January 2001
`
`Routing Packets with Light
`
`Copyright 2000 Scientific American, Inc.
`
`Ex. 1025
`CISCO SYSTEMS, INC. / Page 3 of 4
`
`

`

`7 MULTIPLEXER
`
`6 BUFFER
`
`ELECTRONIC
`CONTROL
`SIGNAL
`
`OPTICAL
`CONTROL SIGNAL
`
`OUTPUT
`FIBERS
`
`1 Data on an incoming fiber get
`separated by wavelength (demulti-
`plexed) into different pathways in
`the router.
`
`2 An optical splitter makes a copy
`of the packet and sends it either to
`a control module, which may be ei-
`ther electronic (A) or photonic (B).
`The control module reads only the
`label, discarding the rest of the
`packet.
`
`3 The other copy of the packet goes
`to a label eraser, where the label is
`removed.
`
`4 A label writer then writes a new
`label on receipt of a signal from the
`control modules—denoting the next
`destination on the network where
`the packet is to be sent.
`
`5 Another signal from a control
`module directs the wavelength con-
`verter to send a packet from one
`wavelength to another. (The same
`process occurs for packets being
`channeled on the incoming blue,
`green and red wavelengths.)
`
`6 An optical buffer holds a packet
`until the control module commands
`it to send the packet through a
`multiplexer.
`
`7 A multiplexer directs the packet-
`containing wavelengths to one of
`the output fibers.
`
`conflicts. If two packets in different
`fibers both need to be transferred to the
`red wavelength in a given output fiber,
`the wavelength converter will route one
`of the packets to a green wavelength to
`ensure that they do not conflict.
`
`Interacting Wavelengths
`
`Experimental converters have em-
`
`ployed photonic integrated-circuit
`(PIC) technology, the optical equivalent
`of integrating electronic components on
`microchips. In a converter built with
`PICs, light from a laser moves along
`waveguides, similar to optical fibers. In
`some types of optical processing, a
`wavelength carrying a stream of bits in-
`teracts with another wavelength, im-
`printing information on it. The light-to-
`light transfer occurs when the stream of
`optical bits causes the waveguide to
`change the phase of the recipient light-
`
`wave, which then exits into an output
`fiber. A change in phase might represent
`a digital one and no change a zero.
`To make converters feasible, research-
`ers have devoted massive efforts to the
`development of tunable lasers that can
`be set to different wavelengths onto
`which bits from incoming packets can
`be modulated. Currently laboratory la-
`sers allow an incoming data stream to
`be switched to any of 80 wavelengths,
`and that number will grow to hundreds
`in the future.
`Ultimately even the decisions on how
`to route a packet may by necessity be-
`come all-optical. As packet speeds on a
`given wavelength exceed 160 gigabits a
`second, a typical packet will race through
`the switch in a nanosecond, whereas it
`would take 100 nanoseconds to process
`a label using electronics. In essence, the
`address label will have become larger
`than the envelope (packet) onto which it
`
`is affixed. Increasing the speed at which
`the label moves through the processor—
`to beyond 100 gigabits a second—might
`tax the limits of the electronic control
`circuitry (although new exotic electron-
`ic technologies that could operate at
`speeds above 100 gigabits per second
`are being tested).
`Anticipating this problem, a few labo-
`ratories have fashioned early prototypes
`of ultrahigh-speed optical logic gates
`that can be built from the same technol-
`ogy used to build light-controlled switch-
`es. These devices—developed at Prince-
`ton, the M.I.T. Lincoln Laboratory and
`British Telecom Labs, among others—
`utilize different configurations of semi-
`conductor-optical amplifiers or other op-
`tical materials to implement simple opti-
`cal Boolean logic gates (that is, AND,
`XOR and NOT) that can process light
`signals moving at speeds in excess of 250
`gigabits a second.
`These gates have made extremely sim-
`ple packet-routing decisions in the labo-
`ratory but cannot yet be scaled up to the
`number needed to make complex rout-
`ing decisions required for a commercial
`switch. Still, device integration and new
`optical switching technologies may one
`day make photonic control possible,
`marking the true advent of the all-opti-
`cal router. If such issues can be resolved,
`a packet might travel from New York
`City to Los Angeles through IP routers
`and never pass through electronics.
`
`SA
`
`DANIEL J. BLUMENTHAL is co-
`founder of Calient Networks, head-
`quartered in San Jose, Calif., which
`makes intelligent optical switches based
`on microelectromechanical devices. He
`is on a leave of absence from the Uni-
`versity of California, Santa Barbara,
`where he is associate professor in the
`department of electrical and computer
`engineering and associate director for
`the Center on Multidisciplinary Opti-
`cal Switching Technology (MOST),
`which is funded by the Defense Ad-
`vanced Research Projects Agency.
`
`Further Information
`Photonic Packet Switching Technolo-
`gies, Techniques and Systems. Special
`issue of Journal of Lightwave Technology
`(IEEE/OSA). Vol. 16, No. 12; December
`1998.
`Optical Networking Solutions for
`Next-Generation Internet Networks.
`Marco Listanti and Roberto Sabella in IEEE
`Communications Interactive; September
`2000. Available by subscription at www.
`comsoc.org/~ci/ on the World Wide Web.
`
`www.sciam.com
`
`Scientific American January 2001 99
`
`Copyright 2000 Scientific American, Inc.
`
`Ex. 1025
`CISCO SYSTEMS, INC. / Page 4 of 4
`
`