The Future of Fiber-Optic Computer Networks

Fiber-Optic Computer Networks

Paul E. Green, IBM T.J. Watson Research Center

iber-optic communication is only 25 years old, but it would be difficult to exaggerate the impact it has already had on all branches of information transmission, from spans of a few meters to intercontinental distances. This strange technology, involving -of all things -the transmission of messages as pulses of light along an almost invisible thread of glass, is effectively taking over the role of guided transmission by copper and, to a modest extent, the role of unguided free-space radio and infrared transmission. If the medium itself is not strange enough, reflect on the form taken by the transmitter - the semiconductor laser diode. This device is usually no larger than a grain of sand and transmits milliwatts of infrared light - enough to send information at gigabits per second over long distances with an extremely low received bit-error rate. Yet, looking a little closer at how this supposedly revolutionary technology has been used to date, you come away with a sense that vast new unexploited opportunities await. The bandwidth is 10 orders of magnitude greater than that of phone lines (25,000 gigahertz versus 3 kilohertz), and the raw bit-error rate on the link is 10 orders of magnitude lower. Design points of are in place today (1 GHZ= io9 HZ). When you combine these numbers with the great potential for cost reductions, Fiber-optic networks it becomes clear that fiber is much more promising than anything that network architects have ever had to play with before and that a great deal more can be done have had a profound with photonic communication than is being done today. impact on information The classic example of cost reduction of photonic technology is the short- wavelength laser diode used in compact disc players. Who would have thought transmission over their during experiments with the first large, bulky lasers with their cryogenic cooling and all the other complications that lasers would eventually cost $5 apiece and be first quarter-century. more plentiful than phonograph needles? Orders of magnitude of You sometimes get the impression that the only real network applications that have been found for this remarkable medium are those that substitute fiber for further improvement copper within the framework of some existing architecture. The performance is are waiting to be improved and the cost goes down, but the system is basically what it was before fiber optics came along. accessed. The physical-level topologies, the layer structure, the protocols within the

78 0018-916219110900-0078%01.00 0 1991 IEEE COMPUTER layers, and the network-control functions in use today are all directly de- Glossary rived from the heritage of either voice- grade telephone lines or local-area ANSI American National Standards Institute coaxial cable. For example, today’s DLC ATM Asynchronous time multiplexing frame sizes are usually dictated by con- BlSDN Broadband Integrated Services Digital Network straints on frame buffers that were im- CATV Coaxial cable community antenna television posed long ago by telephone-line bit CD Collision detect rates and bit-error rates. CDMA Code-division multiple access CSMA Carrier-sense multiaccess A number of aggressive activities in DLC Data link control various research laboratories concern DNA Digital Network Architecture (DEC) what I will arbitrarily call here third- DQDB Dual queue, dual bus generation lightwave networks. The ERP Error-recovery protocols purpose of this article (condensed from FC Flow control an upcoming book’) is to speculate about FDDI Fiber data distribution interface where these activities are taking us. HDTV High-definition television These networks are also sometimes Hippi High-performance parallel interface called all-optical networks because the LAN Local area network MAN Metropolitan-area network entire path between end nodes is pas- os1 Open Systems Interconnection sive and optical. There are no conver- PKT Packetizing/depacketizing sions back and forth between electron- RlSC Reduced instruction-set computer ics and photonics, except at the ends of SNA System Network Architecture (IBM) a connection. SONet Synchronous optical network Many groups are contributing to the TCP/IP Transport connection protocol/lnternet protocol current state of such networks.2 The TDM Time-division multiplex active programs that have contributed TDMA Time-division multiple access the most are those at British Telecom WAN Wide area network Research Laboratories (Martlesham WDMA Wavelength-division multiple access Heath in England), Bell Laboratories (Crawford Hill, N.J.), Bell Communi- cation Research (Morristown and Red Bank, N.J.), NTT Laboratories (Yoko- (2) Fiber used in traditional architec- proaches?” How, for example, can you suka, Japan), Heinrich Herz Institute tures. The outstanding example of this use 10 orders of magnitude improve- (Berlin), and IBM Research (Haw- generation is the point-to-point inter- ment in error rate and bandwidth? thorne, N.Y., and Zurich, Switzerland). city trunk that is improved by upgrad- Recently, the US Defense Dept.’s ing single-copper or microwave-radio Advanced Research Projects Agency connections to fiber connections. It al- Applications (DARPA) announced3 that it would lows you to use higher and higher speed sponsor one or more “precompetitive, electronic time-division multiplexing Of course, the answer to this question generic, industrial” consortia to speed and demultiplexing to try to keep up depends on the problem you are trying development of all-optical networks. with the ever-increasing traffic demands; to solve with the network. As we ap- it does so by exploiting the speed of proach the end of the century, an entire laser diodes and other photonic com- new set of applications is emerging that Three generations ponents and the low fiber attenuation appears certain to consume very large and dispersion. bandwidth, probably up to 1 gigabit per Three generations of physical-level Other examples of the second gener- second per node. In some cases, this technology can be defined: ation include such LANs and MANs as 1-Gbps figure means a sustained rather the FDDI ring and the 802.6 DQDB than a burst bit rate. (1) Fiber not used. Some time in the These architectures substitute fi- These applications are quite varie- early 1980s, fiber-optic technology ber for copper, but make only minor gated and depend on whether the phys- reached the point where it could easily changes in topology and protocol-layer ical topology is simply a point-to-point be exploited for computer networks. content. The electronic front end of link, a point-to-multipoint structure, or Examples of LANs and MANs that de- each node must still be fast enough to a full network (any-to-any addressabil- veloped before this period include Eth- handle the bits from all (or most) of the ity available). ernets and 802.5 token rings, 22-gauge N nodes in the entire network, as had The most prominent link application copper local-exchange connections, and been the case with first-generation net- is the long-haul common-carrier trunk. CATV connections. First-generation works. As installed today, such trunks can han- WANs would include such telephone (3) Fiber used for its unique proper- dle more than 1 Gbps of voice and data company-based packet networks as ties. Here, you might ask, “How can the traffic. The challenge we face concerns ARPAnet, IBM’s SNA, Digital Equip- nonclassical properties of fibers be em- how to upgrade long-haul links for the ment’s DNA, OSI, and TCP/IP from ployed to meet the needs of emerging day when each user will want access to the American ARPAnet community. applications by taking entirely new ap- 1 Gbps of capacity.

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The mid-1970s’ capability provided by *.+- multimode fiber driven by lasers oper- A *.e* ating in the near-infrared (0.85 micron 1,000 wavelength) was superseded by E soon Y. that of single-mode fiber driven at 1.3-p m 8 100 wavelength, where pulse smearing due (3 to dispersion is small and attenuation is (L, only 0.5 decibels per kilometer. c 10 Systems at 1.5 F,where this attenua- z tion figure is only 0.2 dB per kilometer, ‘t soon followed. The introduction of co- a 1.o herent detection provided still more cE capability, because taking into account 0.8 @m 1.3 pm 1.5 pn Coherent 3 , mm fiber sm fiber sm fiber detection the frequency and phase of the light 0.1 I I I I I I I r) allows lower detectability limits to be 1976 1978 1980 1982 1984 1986 1988 1990 achieved than can be achieved with much Year cruder direct detection. In direct detection, the receiver regis- Figure 1. Progress in fiber-optic links, as measured by product of bit-rate times ters only the amount of energy observed required inter-repeater distance. and ignores the sinusoidal character of the light. Recently, preoccupation with coherent detection has been challenged by the realization that error-rate per- To get a little more topologically IBM Enterprise Connection at 0.2 Gbps,7 formance that is almost as good can be complex, point-to-multipoint structures and the current work in ANSI study achieved much more cheaply and sim- are expected to provide “fiber to the group X3T9.3 to define a 1-Gbps serial ply by suitable use of direct detection home” or “fiber to the offi~e,”~carrying standard (fiber channel). The two latter working with photonic receiver pream- the multiple bit streams of HDTV, efforts are aimed at spanning MAN dis- plifiers. Such amplifiers have become digital audio, and other emerging ap- tances (tens of kilometers). truly commercially available only with- plications. To satisfy all these emerging applica- in the past year. Direct detection is very As for networks, the gigabit applications, it will be necessary to deal with simple, because only the presence or tions are appearing first in LAN and limits on electronic speeds that appear absence of light is detected; phase, fre- MAN environments. A network can be to be effectively permanent. At quency, and polarization are ignored. considered a topological structure, usu- 1-gigabit-per-node sustained demand, Until very recently, the single long- ally of more than one hierarchical level, it is difficult to imagine that second- haul link has dominated thinking about that allows any node to reach any other generation approaches will ever be suf- lightwave communication. But now, an node on a peer-to-peer basis. ficient to build a 1,000-node network. interesting change is taking place in the There is already an urgent require- The 1,000-Gbps electronics that would research agendas of the participating ment6 in university and other research be required to do this simply does not corporations. The emphasis is shifting environments for networks that will seem to be a realistic expectation. from the single long-haul link to multi- provide supercomputer support of hun- points and networks. This is as true of dreds or thousands of high-performance component research as it is of system workstations with live-motion color The fiber links era research. graphics. Since long-haul transport, where Medical imaging is another impor- For most of the 25-year history of bandwidth and distance are the impera- tant application. The image-quality re- fiber-optic communication, the commu- tives, is in relatively good shape, atten- quirements are so exacting that the nication community has been preoccu- tion is turning to providing connectivity medical community is not sure it can pied with sending the highest possible rather than bandwidth, first over LAN trust image-compression techniques. bit rate over a single link for the longest distances and thenover MAN distances. Therefore, the images are usually trans- unrepeatered distance.8This has simply This shift is apparent in the research mitted in uncompressed form, and scan- been a reflection of the priorities of the literature and in the advanced proto- ning one or two of these per second can world’s common carriers and their re- typing efforts of various leading-edge generate bit rates approaching 1 Gbps. search laboratories. Not only has it been companies and carriers.* Another pressing LANIMAN appli- more in their interest to continue to cation is the interconnection of com- upgrade the installed long-haul plant, puter mainframes to each other and to but this has also been much easier to Fiber paths in networks their storage peripherals within one cen- achieve - both technically and eco- ter (the “glass house”) and also the nomically - than upgrading millions of Figure 2 is an attempt to capture the interconnection of glass houses. This individual copper subscriber loops. essential differences in the three net- requirement has led to the introduction Figure 1 shows the remarkable prog- work generations. In the first genera- of such second-generation systems as ress in bit-rate-distance product that tion, all links are copper, and conver- the ANSI 0.8-Gbps Hippi standard, the was made over the past 15 years alone.’ sions between waveforms and bits take

80 COMPUTER place at every node along the path between end users. In the second generation, all links are fiber, which improves the bit rate and bit-error rate but leaves propagation latency unchanged, as diagrammed in the figure by showing the links to be V fatter but the same length as before. 10-First generation4 1 3 Conversion at each node still exists. First generation In third-generation networks, there are electrical-to-optical conversions only 4 at the ends of a physical connection. Second generation4 le3 Electronic bottlenecks stay out of the Second generation picture except at the ends. I The feasibility of all-optical commu- 4 nication at 1 Gbps per user is already Third generation being proven at LAN distances and to I some extent for MANs, though it is only Figure 2. Schematic representation of the three generations for a toy network emerging slowly for WANs. This pattern consisting of four nodes. matches the likely future availability of the dark fiber that will be required to build a third-generation network with a clear optical path connecting all Nnodes. can cities, are eager to provide dark The jargon term dark fiber refers to fiber over tens of kilometers’ distance.“’ fiber onto which the user can directly Thus, before dark fiber is available na- attach his or her own optical termina- tionally or internationally,we may see a tions without having to go through some- repetition of the court battles of the one else’s electronic equipment with its 1970s over what constitutes a “trans- restrictions on protocol, framing, bit rate, parent, highly usable” common-carrier I 3 and so forth. offering. In fact, some litigation has al- It is estimated that typically one-third ready begun.” A more promising possi- 1 I t I of all the fiber that has been installed by bility is the growth of interest in provid- WDMA 7 the regional Bell operating companies, ing dark fiber on the part of the alternate V AT&T, and other interexchange carri- access carriers or the cable TV industry. ers, and by the alternate-access carriers The limits on distance in third- 4 is dark.’” This is not because third- generation networks do not appear to generation networks were planned, but be technical. In particular, attenuation is the result of the usual practice of is not the problem that it used to be. installing plenty of overcapacity for fu- Fiber attenuation is about 0.25 dB/km I I ture growth. at 1.55 p wavelength, and purely pho- TDMA T When you realize that each fiber has tonic amplifiers that have recently be- V a potential carrying capacity of 25,000 come commercially available have 20- Gbps, equal to the volume of all US 30 dB of gain across several thousand telephone calls at the peak busy hour gigahertz of bandwidth, while introduc- (in the US, on Mother’s Day), and that ing only a modest amount of noise. even those that are “lit” are using only Therefore, optical-to-electronic conver- one-tenth of 1 percent of this potential, sions involving traditional repeaters can t I I it is hard not to wax enth,usiastic about be entirely avoided with technology CDMA t the possibilities. available today. Providing dark fiber is widely consid- In long, high-speed links, chromatic Figure 3. The three classes of address- ered by the user community to be the dispersion arises; the propagation ve- ing schemes: wavelength (frequency) responsibility of the common carriers, locity is slightly different across the sig- division, time division, and code divi- but the issue of whether they will readi- nal bandwidth if the bit rate is too high. sion (spread spectrum). ly do so is controversial. Instead of mar- For third-generation LANs and MANs, keting capacity, the carriers traditional- this is unlikely to be the problem that it ly prefer to market service. This involves traditionally has been with long-haul interposing their own standardized TDM telephone company links. This is be- resources, whose main function is to cause any single wavelength carries only Forms of addressing support voice, not computer communi- the bit rate of one connection, not the cation. aggregated bit rate of all connections as As Figure 3 shows, you can address Today, only the alternate access ven- with second-generation links and net- messages from one node to another ac- dors, serving mainly the larger Ameri- works. cording to wavelength or frequency

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- I \\ ' Star -\-U%coupler

Figure 4. The three most popular forms of third-generation lightwave-network topology: (a) star connection, (b) reentrant bus, and (c) tree.

(WDMA), timeslot (TDMA),orwave- Limit of nanoseconds tunability with lasers shape (spread spectrum or CDMA). Limit of doped-fiber amplifier bandwidth Of these, WDMA seems to be the current favorite. Limit of detectability and fiber bandwidth One reason for this is that TDMA t requires that there be many slots per 104 bit time. CDMA goes even further by subdividing each bit time into hundreds or thousands of "chips" (waveform sam- -z 1,000- Finesse = 1.000 ples) per bit time. In fact, you can show v) that for about 10.'" bit-error rate, U CDMA requires more than 40 chips : 100 per bit per node in the network. L All-optical TDMA and CDMA en- n(I) 5 coders and decoders are feasible 2 10 enough, but when you thinkabout 1,000- node networks at 1 Gbps each, the dispersion in the fiber, plus the requirement to synchronize to within one slot 1 I \ I I I I I* time (for TDMA) or one chip time (for 1 10 100 1 10 100 1,000 CDMA), renders these two options Mbps Bit rate per node Gbps relatively less attractive than WDMA, where nothing has to run any faster Figure 5. Capacity limits imposed by various technology choices. than the bit rate.

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m The space- be required - at each trans- Phase Tunable mitter, at each receiver, or division Active control mirror both. This requires either alternative region region region tunable lasers or tunable filters. The most convenient way to modulate digital in- There is, of course, an- formation onto the laser light other way large numbers of output is simply to turn the nodes can be connected so laser on and off (“on-off key- that the electronics in each ing”) with the two levels of node runs no faster than that the bit stream. node’s own bit rate. That way Lasers that will tune over is to use a central space- the entire 200-nanometer division switch.’* wavelength range are clear- Progress on large purely I waveguide ly desirable, since they can photonic (that is, third- Active provide transmitter tunabil- generation) switches does not ity or receiver tunability - appear to be very promising the latter by using the tun- so far.13Therefore, most cur- able laser as the local oscilla- rent gigabit-per-port switch Figure 6. Experimental tunable laser diode (Nippon Elec- tor in a coherent receiver proposals involve the second- tric Co.). structure that resembles a ra- generation approach of con- dio receiver. verting from photons to elec- An even more important trons, doing the switching advantage of tunable lasers electronically with a high degree of par- important than any link attenuation that is that the speed of retuning the emitted allelism (to minimize the amount of gi- occurs due to network physical size. wavelength of such a device can be as gabit technology internal to the switch), Figure 5 shows the limit in number of short as a few nanoseconds, since it is and converting back to photons again. nodes N and bit rate per node, assuming limited by device capacitance and carri- Such switches might be developed a star topology and a lo-’’ bit-error er-transport times involving very tiny rapidly and successfully, but the N-log- rate. The rightmost diagonal line indi- dimensions of less than a tenth of a N internal connectivity complexity, plus cates thelimit of 25,000-GHz bandwidth. millimeter. Unfortunately, the tunable the problem of call-queuing at a shared Interestingly, this limit on product of laser diode, although the object of much controller, could be important inhibi- number of nodes and modulation band- research worldwide, still remains a lab- tors. In addition, if the electronic speed width per node happens to be very close oratory device. bottleneck is to be avoided, the topolo- to that obtained by asking how many The currently favored design, the gy of the network is forced to be a single photons per bit will give a lo-l’bit-error “three-section’’ laser diode (see Figure star with the switch at the center. rate.I4 (Commercial-grade photodetec- 6), is capable of tuning in nanoseconds, tors and 1-milliwatt laser diodes are as- but only over about 9 nm (1,100 GHz); sumed.) That is, first you take the num- hence, the third diagonal line from the Overall network ber of photons per bit generated at the right in Figure 5,which gives the capac- transmitter (divide the number of pho- ity limits for a network where nanosec- throughput capacity tons per second in 1 mW of power by the ond tuning times are a requirement. reciprocal of the bit rate) and split this Such a network would use packet switch- Figure 4 shows -in its simplest form across N nodes. At this stage, you can ing at gigabit rates, with the ability to - what a WDMA all-optical network see how large N must get before the re-address in a few hundred bit times looks like. It can be a star connection to number of photons per bit becomes less appropriate to packet switching. a central passive optical star coupler, a than that required to develop a lO-’’bit- The three-section laser diode shown reentrant bus in which energy is taken error rate. The answer is the same as in Figure 6 operates by carefully con- from or added to a common bus by that given by the fiber bandwidth. trolling the currents injected into the means of taps, or it can be a tree. The next diagonal line in Figure 5 three sections. The current into the left- Work in the early 1980s on third- shows the limit imposed by the finite most section controls the intensity of generation lightwave networking tend- bandwidth of today’s commercially avail- the light emitted, and the other two ed to emphasize the bus geometry. How- able photonic amplifiers. The third line currents control the way two reso- ever, investigators soon realized that, shows the tuning limit of today’s tun- nances interact to define the radiated since decibel loss accumulates linearly able laser diodes. optical frequency. with the number of nodes along the bus The first of these resonances is a sim- but only logarithmically with a star, the ple cavity resonance between two mir- latter would be preferable. Even the Technologies rors, the leftmost facet of the device and logarithmic loss is severe enough to con- an effective mirror at the right formed stitute the principal limit on bit rate and If you plan to use wavelength (fre- by the tunable mirror region. The sec- number of nodes N in the network. For quency) as the address parameter, it ond resonance is due to the fact that the LANs or MANS. these losses are more follows that some sort of tunability will bottom of the tunable mirror region is

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Icssness, and therefore the cost goes up with the finesse. Several tricks can be exploited to make relatively inexpensive devices with fi- nesses in the thousands, as would be required for networks with thousands of nodes. A nuniber of researchers are also seeking ways to speed up tuning by electrically changing the index of the cavity material rather than the physical length of the cavity. In the past year, purely photonic amplifiers having 20-30 dB of gain across niany thousands of gigahertz of bandwidth have become commercially available. ’I hebe use several meters of spe- cidlly doped fiber that is spliced into the link at the appropriate point. When the electrons in the dopant at- oms are pumped to high energy levels by sume 100 mW of light from a nearby small but powerful laser diode, any inci- Figure 7. Optical portion of a plug-in cad tor an IHM YS/2 or RS-6000 comput- dent signal within the bandwidth of the er, corrstitutrrig one node in an all-optical network using wavelerrgth addressing. device will be strongly amplified. Com- plete comiiiercially available amplifiers that include the fiber, the pump-laser, arid suitable couplers and isolators oc- ehpy only two or three cubic inches. I here die a iiuiiibcli of techniques for 32 Such riodez cdii be interconnected locking the frequency of tunable devic- ovtr 25 kilonieter distdnces Edch node es to reference standards or to the wave- Ldn Oprrdte dt bit rdtes up to 300 Kbps length of the incoming signal (as the In receivers usirig such devices the circuitry in Figure 7 does). 1ighllt‘~elVtdflorri the Lelltrdl atdr LUU Several of the approaches to third- plrl (dl1dlhubLdlr)iillgddl~tht bit StIedn15 geiieration network technology being dl dll tht dlffclent wdvelr‘ilgths) is til addressed worldwide are so promising tacd III the tdbry-krot device su ds to that cxperiniental 1,000-nodeMANS (50 pdrs oiily one node s trdnsrnibsion, arid kim in diameter) at I sustained gigabit thtii ispd~sedtu datariddid photodetec- per second per node will undoubtedly 101 tullowtd by low iloist: di~ip~iti~~be built within the next three years. stages drid d thirshold dt\lLe 1 ht horiLuiltal lines in bigure 5 show the cdpdcity ut nttwoiks usirig tabry- Protocol layers before yerot tUIidblt tiltera I ht: Ii1iiiIiiig pd- iarneter is not tuiiing range, but the and after interchannel crosstalk thdt occurs when you place the chdiinels too close togeth- Figure 8 presents a view of the pro- er in frequency Scverd different val- tocol stack - the suite of architectural ucs of theinaxiiriurii N (nuiiiber ut nodes) layers - for second- and third- ait show11 fur the firiesbe, or “U,” of generation lightwave networks. The these filters binesbe vdlues such ds 100- left two diagrams represent first- and 200 are typical of such comniercial de- second-generation nodes, and the right vices ds that of Figure 7 two represent projected third- Finesse is defined ds the ratio of the generation possibilities. The second spdeing in opticdl frequency of the re- and fourth diagrams indicate the tradi- petitive fesolldnce peaks i eldtive to their tional protocol layers: width b~oiriIhrs defiiiitluri 11 IS dedi lhdI Ihc dilloullt ut ddJdLtll1 Lhd1111el physical, crussldlk thdt ledks through tu produce multiaccess control (MAC), bit errors will be smaller when the fi- data link, nesse is higher The finesse goes up with network, mirror parallelism, smoothness, and loss- transport,

CUMPU I kK

m session, presentation, and application. Application The activation, deactivation, and pa- 0 Session rameter setting for all the layers is pre- sided over by the control point (usually I 0 0 an application), indicated by the shad- 0 0 ed inverted L at the right of each layer 0 I Transport i 0 diagram. Multiaccess 0 The content of each of the first four Multiaccess control cont ro I layers is dictated by the parameters of Physical the network technology, and that of the upper three by the demands of the ap- ERP PC PKT plication user. ERP PC PKT For WANs, the intermediate-node routing function (network layer) is im- First and second generation 7 hiid gerieidtiot portant, but media access is simple, usually a matter of making a leased or dial- Figure 8. A representation of the potential for third-generation networks to re- up connection (voice-grade dial-up, alize several forms of improvement: (1) fewer and simpler protocol layers and ISDN connection, and so forth). Con- their associated control point (layer diagrams at right), and (2) elimination of versely, for LANs and MANS,no rout- redundant instances of “atomic protocol functions” (circles at left). (ERP = er- ing decisions need be made, and the ror-recovery protocols, FC = flow control, and PKT = packetizing/depacketiz- network layer is essentially absent, al- ing) though the multiaccess protocol can become quite sophisticated. With the arrival of third-generation networks, changes occur in the upper protocol levels, reflecting the session and presentation requirements of new ogies and the corresponding protocols through the protocol stack is a steady- applications. Some of the new applica- is to greatly simplify the communica- state process in the sense that it takes tions are opened up by the great perfor- tion layers. similarly simplify the con- place on a continuous basis as informa- mance and ease-of-use advantages of trol-point function, and execute many tion is transmitted. Thus, it is extremely third-generation networks. of the atomic functions only once on time-sensitive. (Note that ingigabit net- The immediate changes, however, are behalf of the application only. The dia- works, even 1 millisecond of delay means occurring in the communication layers gram doesn’t show the network layer, 1 megabit of data delayed). and in the control point. as is appropriate for a LAN or MAN, The network control functions, on Among the atomic protocol functions where there is only one hop between the other hand, are also time-sensitive, are packetizing/depacketizing, rese- any two nodes (see Figure 4). although not so much so. A logical con- quencing, splittingherging, error re- A number of lightweight protocols nection between application-layer cover, flow control, priority-class han- have already been implemented. These entities at the two ends of the connec- dling, and encryption/decryption. are all concerned with presenting, at the tion needs to be set up and taken down Various architectures handle them at top of the transport layer, an interface as fast as possible. Part of the light- various layers. that will have the greatest efficiency. weight protocol effort consists of prun- In the higher layers, such functions This means at least ing not only the steady-state processes are intended to serve the needs of the within the layers but also the intermit- application; in the lower layers, they (1) removing redundant instances of tent network-control functions within cater to the peculiarities of the commu- the atomic functions, the control point (shaded inverted L in nication technology. As indicated in (2) pruning the size and number of Figure 8). Figure 8, for first- and second- the finite-state machine representations generation networks, many of these of each individual protocol, atomic functions were often repeated (3) designing toward an execution Making the several times as the same data stream environment that makes the fewest pos- communication layers traversed the protocol stack on its way sible memory moves and, if possible, no into or out of the node. Three of the calls to the operating system, and invisible atomic functions -error control, flow (4) implementing as much of the pro- control, and packetizingidepacketizing tocol stack as possible in hardware very Work continues not only on making - are illustrated alongside the left side near the physical port, without the sys- the protocol stack and its control more of Figure 8, as done in a typical first- tem bus’s intervening between the hard- efficient, but also on making it serve the generation architecture (SNA). ware and the node’s CPU. idiosyncrasies of the applications and The potential effect on this structure hide from the application any depen- of introducing third-generation technol- Sending the messages up and down dencies on the underlying communica-

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I tion technology. Both jobs are made or a full-screen high-resolution work- much easier by the new lightwave tech- station with continuous video refresh, nology. the streaming type of service class can The agenda for network architects is Special circuit-switch be provided in which a “packet” might to understand and exploit the revolu- and packet-switch be megabytes in length. tionary properties of optical fibers, par- protocols for third- This should allow each of the many ticularly the 10 orders of magnitude generation networks instances of the higher layers that sit on greater bandwidth and smaller error rate, top of the transport layer to do its own and to do this in the most effective way are an active area error recovery and packetizing inde- possible. With this in mind, 1 next dis- of research. pendently. Since all the instances would cuss each of the communication layers. share the same lower layers through a Obviously, the LAN and MAN phys- common buffer, flow control at the trans- ical layer changes completely for the port layer is still required. third generation. Commands entering It is conceivable that eventually pho- the layer from the control point will tonic technology will become so inex- lead to changing a wavelength, a time- pensive that many physical ports per division slot, or a CDMA waveform - rates of photonic technology (say lo-*’) node would be feasible. In the case of most probably the first of these, if are as good as the uncorrected error WDMA, this means one fiber port per WDMA continues to be the format of rates of existing DLCs and will be dom- node, but many wavelengths at that port. choice. inated by buffer overflow errors any- However, photonics cost levels are WDMA third-generation lightwave way, so why do any error detection currently so high that it will not be easy networks that use circuit switching of- and retransmission at all in third- to provide many physical ports per node fer a high degree of protocol transpar- generation networks? using today’s lightwave components, ency, since different protocol stacks can Many applications - for example, even though the fiber will often have be implemented on different physical- certain graphics applications - do not more than enough bandwidth to serve level connections at different wave- require such heroic error-control in any all the ports in the network. lengths. This protocol transparency ex- event. Thus, it is possible to think of If costs ever drop low enough, the ists during the steady-state interval after relegating all bit-error recovery to the number of physical ports per node could the connection is set up and before it is transport layer or even the higher (ap- grow until it becomes as large as the taken down -just as with voice-grade plication-driven) layers of the stack. If number of logical ports (communica- phone lines. this proves workable, an essentially null tion-based application instances); that The MAC layer changes completely. link layer may be expected in the third is, one protocol stack per application. First-generation LAN protocols such as generation. You can compare this with today’s situ- 802.3 carrier-sense multiaccess with col- For third-generation LANs and ation in which the number of instances lision detect (CSMAKD)and 802.5 (to- MANS that use one physical hop, the of the physical and data-link level (that ken rings) assumed that the propaga- intermediate routing function of the is, the number of physical ports) is one tion time across the network was a small network layer is no more necessary than or two orders of magnitude smaller than fraction of a packet duration. Waiting it was for the first two generations. These the number of instances of the higher for a collision detection slot to expire or are the cases in which nodes can reach layers. a token to arrive imposes an unaccept- each other directly rather than by going Transport-level flow control to pre- able performance hit as the network through intermediate nodes. vent shared-buffer overflow could then size grows larger than about 1kilometer As for packetizingldepacketizing in be done away with, since it might now and the bit rate climbs above a few this layer and the transport layer, the take place entirely to serve individual megabits. very low bit-error rate has some impor- applications. In today’s packet-switching In the second-generation FDDI and tant consequences that recall the old networks, buffers in the lower layers DQDB (802.6), some relief is obtained arguments about the relative virtues of are not dedicated to individual applica- by allowing a limited amount of concur- packet switching and circuit switching, tions but are instead dedicated to indi- rency (more than one packet in progress and whether packet switching always vidual links that are shared across all somewhere on the network at any in- has to mean short packets - for exam- applications. stant of time), but the multiple is not ple, less than a thousand bytes. Low-cost all-optical solutions will also large - somewhere between 1 and 10. With third-generation networks, pack- mean that eventually this form of net- What is needed is for the amount of et size can be matched entirely to the working will be accessible to PCs and concurrency to approach N, the num- application, including a packet length Macintoshes. ber of stations; this is what the all- so long that you might normally consid- optical technology provides. er it a circuit-switched connection. Special circuit-switch and packet- If the application is credit-card check- Network control switch protocols for third-generation ing, the packet length provided by the networks are an active area of research. transport interface to the application Imaginative simplifications should be First-order changes in the link layer instance can be that of the record, per- possible in the control point, shown as and in the packetizing/depacketizing haps only a few hundred bytes. At the the inverted L in Figure 8. Previously, functions of the network layer are like- other extreme, if the application is to network control consisted of the fol- ly. Some argue that the achievable error support an uninterrupted file transfer lowing steps (not necessarily in the or-

86 COMPUTER

m der listed) when some application re- many new ideas for gigabit networking 9. P.S. Henry,R.A.Linke,and A.H. Gnauch, quired a logical connection to another could be tested, third-generation opti- “Introduction to Lightwave Systems,” Chapter 21 of Optical Fiber Telecommu- application somewhere in the network, cal networks among them. The technol- nications 11, S.E. Miller and 1.P Kamin- known only by name: ogy for interconnecting individual LAN ow, eds., Academic Press, San Diego, or MAN “islands” is to use existing Calif, 1988. The topology of the network chang- SONet and ATM long-haul transmis- 10. Special issue on metropolitan-area net- es when a new node or link joins or sion facilities, each having up to 2.49 works, Fiber Optics, K. Kelly, ed., Infor- leaves (this new topology is made known Gbps aggregated capacity. mation Gatekeepers Inc., Boston, Vol. to all nodes). 12, NO. 2, 1990, pp. 1-32. .The location of the named target application is learned by invoking some he state of today’s lightwave 11. FCC Dark Fiber Proc., CC Docket 88- 136, Washington, DC, 1986. sort of directory function. technology is such that LANs The best route to the node contain- T and MANS providing 1-Gbps 12. J.Y. Hui, Switching and Traffic Theory ing the target application is determined. sustained capacity for each of 1,000nodes for Integrated Broadband Networks, Klu- The logical connection (session) to will be practical within three to five wer, New York, 1990. years, certainly for circuit switching and that application is set up and this fact is 13. Special issue on photonicswitching, IEEE confirmed at both ends. probably for packet switching as well. If Comm., J. Baldini, ed., Vol. 25, No. 5, Use of the connection commences. the opportunity is exploited with imag- May 1987. ination, the positive effect on network In principle, in a single one-hop third- architectures and on applications to be 14. P.S. Henry, “High-Capacity Lightwave Local-Area Networks,” IEEE Comm., generation network, where every node supported will be profound. Vol. 27, NO. 10, Oct. 1989, pp. 20-26. sees every other directly, as in Figure 4, The public interest will best be served all this could be collapsed into one ex- by enabling rather than inhibiting the change - an ask-and-go style of usage. diffusion of this exciting new genera- The session request verb is broadcast tion of networking to the widest possi- and, if there is any node anywhere in the ble community of users. W LAN or MAN containing an instance of the named resource, that node’s address is returned in the session response and the connection is thus completed. References

1. P.E. Green, Fiber-optic Comm. Net- Interconnection of works, Prentice Hall, Englewood Cliffs, third-generationLANs N.J., to be published in 1992. and MANs 2. Special issue on dense wavelength divi- Paul E. Green, Jr., is manager of advanced sion for high-capacity and multiple- optical networking at the IBM T.J. Watson access communications systems, IEEEJ. As mentioned previously, it is expect- Research Center, Hawthorne, N.Y., where Selected Areas in Comm., N.K. Cheung, he has been employed since 1969. His re- ed that third-generation networks will K. Nosu, and G. Winzer, eds., Vol. 8, No. search interests include applied communica- proliferate first as LANs, then as MANs, 6, Aug. 1990. tion theory and computer network architec- and later as WANs, partly because this ture. 3. “Precompetitive Consortia for All- is the order in which dark fiber will Green received degrees at the University Optical Network Technology - SOL,” become freely available. The topic of of North Carolina and North Carolina State DARPA Broad Agency Announcement University and was awarded his PhD at the extending third-generation LAN and 91-14, Commerce Business Daily, July Massachusetts Institute of Technology in MAN technology to wide area network- 16, 1991. 1953. He is a member of the National Acad- emy of Engineering and the IEEE Computer ing (for example, nationwide) is begin- 4. R.M. Newman, Z.L. Budrikis, and J.L. ning to draw serious thought. Society, is president-elect of the IEEE Com- Hullett,“TheQPSXMAN,”lEEEComm., munication Society, and has been an IEEE It appears that, for the foreseeable Vol. 26. No. 4, Apr. 1988. pp 20-28. fellow since 1962. He received the IEEE future, the only gigabit wide-area links Aerospace Society’s Pioneer Award in 1981, that will be available are those second- 5. P.W. Shumate,Jr., “Optical Fibers Reach the IEEE Communication Society’s E.H. into Homes,” IEEE Spectrum, Vol. 26, Armstrong Technical Achievement Award generation point-to-point links that use No. 2, Feb. 1989, pp. 43-47. specific TDM call-setup and framing in 1989, and the IEEE Simon Ram0 Medal in 1991. He was named a distinguished engi- conventions - for example, BISDN, 6. Special issue on visualization in scientific neering alumnus of North Carolina State SONet, ATM, and so forth. computing,” Computer, G.M. Nielson, University in 1983. ed., Vol. 22, No. 8, Aug. 1989. Components are being developed that could be used for all-optical gateways 7. J.C. Elliott and M.W. Sachs, “The IBM between networks to do wavelength- Enterprise Systems Connection Archi- swapping or wavelength-sensitive rout- tecture,” IBM J. Research and Develop- ment, to be published in 1992. ing. However, these components are Readers can write to the author at Rm. presently only at the research stage. 8 Optical Fiber Communications-11, S.E. 3D54, IBM T.J. Watson Research Center, The National Research and Educa- Miller and I.P. Kaminow, eds., Academic Hawthorne, NY 10532, or contact him by tion Network is a testbed upon which Press, San Diego, Calif., 1988. electronic mail at [email protected].

September 1991 87

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