Slotted optical switching with pipelined two-way reservations

3616 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 24, NO. 10, OCTOBER 2006

Slotted Optical Switching With PipelinedTwo-Way Reservations

John D. Angelopoulos, Member, IEEE, Konstantinos Kanonakis, Helen C. Leligou, Charalambos Linardakis,Ioannis E. Pountourakis, Member, IEEE, and Alexandros Stavdas, Member, IEEE

Abstract—Optical burst switching is a core architecture de-signed to reconcile the available optical technology with the in-creasing burstiness of traffic. However, disappointing performancein terms of high packet loss and/or low system utilization discour-aged broader experimental implementations. A method to avoidthese losses by first sending over the control channel a short scoutpacket that simulates the events that the actual burst will expe-rience is proposed in this paper. Once the scout message detectsa drop at any intermediate node, it returns back to the sourceto avert the payload emission and repeat the process. The waythe control works results in essential service quality features, i.e.,no loss of bursts, no out-of-order emissions, increased efficiency,much reduced delay variation, and graceful throttling of the loadrespecting the contracted rates.

Index Terms—Access arbitration, medium access control(MAC) protocol, optical burst switching (OBS), reservation based,wavelength division multiplexing (WDM).

I. INTRODUCTION

D EVELOPMENTS in optics have undoubtedly affectedmodern networks more than any other technology. How-

ever, the impact has been uneven; optical technology revo-lutionized the transmission plant but produced a much moremodest success record in the switching plant. While providingvery fast, cheap, and interference-free signal transport, opticsoffer only expensive, unwieldy, and bulky storage solutions,rudimentary processing capabilities, and a hard dilemma be-tween slow or very expensive switching fabrics. Given theimportance of packet switching, a consequence of IP networkexplosion, the effort to exploit the best features of optics whilesidestepping the disadvantages led to the emergence of theoptical burst switching (OBS) paradigm [1], [2]. The principleof operation is simple: The source node sends a burst controlheader ahead of each data burst in an out-of-band channel toprepare all nodes along the data path for the following burst,which therefore need not be buffered on the way to its destina-tion. Its strength lies in achieving dynamic bandwidth handlingof the payload fully in the optical domain while keeping thecontrol processing in the electrical domain. Its weakness is therather heavy burst loss [3], even at low utilization levels due

Manuscript received February 6, 2006; revised June 16, 2006.J. D. Angelopoulos, K. Kanonakis, H. C. Leligou, C. Linardakis,

and I. E. Pountourakis are with the Electrical Engineering Depart-ment, National Technical University of Athens, (NTUA) 15773 Zo-grafou, Greece (e-mail: [email protected]; [email protected];[email protected]; [email protected]; [email protected]).

A. Stavdas is with the Department of Telecommunications Science andTechnology, University of Peloponnese, 22100 Tripolis, Greece (e-mail:[email protected]).

Digital Object Identifier 10.1109/JLT.2006.881847

to its ambitious on-the-fly switching. The problems arise fromconflicts due to the temporal properties of data traffic, whichOBS can only poorly alter (little buffering) or prevent (poorcontention control), leaving as the only possible alternativethe gross overprovisioning of the system. As a result, OBScould not satisfy early expectations, raising skepticism aboutits viability.

Many alternatives regarding the node control algorithms,such as “tell-and-go” [4], “just-in-time” (JIT) [5], [6], “just-enough-time” [2], and several others, have been investigatedand compared [7]. Fast circuit switching techniques havealso been adjusted to the OBS philosophy employing eitherone-way or two-way reservation techniques [8]. The impactof different traffic aggregation methods has also been studied[9]. Slotted operation in OBS systems has also been pursued asa promising direction and shown to outperform the pure OBSsystems [10]–[12].

In contrast to optical circuit switching by wavelength routing,where lightpath set up and tear down are too slow to becomeefficient for the predominantly bursty data traffic, OBS canachieve multiplexing gain albeit still with inevitable low systemutilization due to lack of significant buffering. With burstytraffic, the role of buffers is quite critical to reach reasonablesystem utilization. Some relief to this problem is providedby wavelength division multiplexing (WDM), which offers achoice of alternative output channels (despite the fact that theinputs have equally more channels), and this is why OBS ismore useful in the context of WDM [1], [3]. Further similar im-provement can be offered by the limited storage offered by fiberdelay lines (FDLs). Unfortunately, both wavelength convertersand FDLs come at a significant cost. Even more important,the few channels and limited FDLs can only be effective atvery small timescales, which is quite inadequate for the burstsencountered in real networks today. In comparison, the storageused in IP routers today to resolve output port contentionamong packets can buffer 512 MB/port or more, altering thetemporal properties of flows in timescales of seconds. This iswhy this switching approach was named “store-and-forward.”Apart from temporarily storing colliding packets that wouldotherwise be dropped, buffers contribute at the same time tohigh utilization by storing a backlog that will keep outputlines busy between arriving bursts. The resulting delay must,however, be kept below the limits that services can tolerate.

Once the needed size of buffering is prohibitive for all-opticalsolutions, the only alternative left is the acceptance of a lowutilization on the premise that wholesale all-optical handling oftraffic can become cost effective well below the near capacity

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ANGELOPOULOS et al.: SLOTTED OPTICAL SWITCHING WITH PIPELINED TWO-WAY RESERVATIONS 3617

utilization levels of electrically switched networks. To makethis a reality, new improvements are needed not only to lowerthe cost of optics but also to further raise system utilization,bringing the overall system approach to the break-even point.Working in the latter direction, this paper addresses the burstloss problem without introducing out-of-order retransmissionsand the large jitter they bring, neither delegating retransmis-sions to the higher layers (useless for interactive services). Thisis achieved through a novel control method that allows not onlypreprogramming the switch but also reporting back the fate ofthe burst before it is launched, ascertaining a successful passageto the destination or a repetition of the attempt.

This paper is organized as follows: In Section II, the ratio-nale for this novel approach is introduced, while the systemarchitecture is presented in Section III. In Section IV, thenode scheduling mechanism is elaborated, while the systemperformance is evaluated in Section V. Finally, conclusions aredrawn in Section VI.

II. RATIONALE AND CONCEPT OF OPERATION

In the OBS situation, despite the help from WDM channelsand FDLs, the collision situation is quite reminiscent of Alohaprotocols. However, in this case, the burst may cross three to sixswitches, reducing its chances of success to the joint probabilityof avoiding collisions in all nodes. It is then natural to seekthe same improvement of performance that slotting brings onthe basis of the same reasoning that justified slotted Aloha,i.e., avoidance of the quite large waste and likelihood of partialcollisions. Yet, as it will become clear with the description ofthe proposed scheme, the main motivation for the choice ofa slotted system is not just the avoidance of partial collisionsbut mainly the need to keep an easy and detailed accounting oftraffic and exploit this to reach a closed loop reservation controlthat will be used to obviate burst loss and the need for retrans-missions with all the concomitant load control problems [7].Naturally, the creation of slots out of variable packets comes atsome efficiency penalty since some slots will inevitably remainpartly filled, although in the proposed scheme, fill levels arequite high because the slot aggregation runs in parallel with thereservations. The slotting of the system is similar to the onedescribed in [11], except that no frames are considered, andeach slot is accounted for on its own.

This paper, which was carried out in the framework ofarchitectural studies of the EU project Next generation Opticalnetwork for Broadband in Europe (NOBEL), addresses the lossproblem at the optical layer (i.e., independent of higher layerprotocol actions) without any out-of-order deliveries. This iscrucial for service quality, since, in interactive services, trans-mission control protocol (TCP) is not employed, while real-time protocol (RTP) time stamping cannot reorder traffic withlarge delay variations. Even when TCP is used, the loss of largebursts can upset its operation particularly since not all OBSburst losses should be interpreted as congestion (most are justaccidental collisions), but TCP is designed to use loss as a mea-sure of congestion (which was almost always true in electricallybuffered routers). The rationale of our approach is to furtherexploit the cheap electrical buffering at the network periphery

and introduce a meticulous reservation method that operateson a slot-by-slot basis. Compared with other reservation-basedsolutions, the novelty of this paper is that two-way reservationsare pipelined, i.e., the next one is launched without waitingfor the outcome of the previous ones since all scouts refer tosame size payload slots over the same source–destination pairs.The concept can be described as “probe-and-go” because beforeemitting a data burst (which in our system has become a fixed-size slot), the outcome of future contentions in all nodes alongthe path is first found out by the burst header, which will becalled scout to underline its enhanced role. The scout travelsin the control channel carrying information not only to preparethe optical path for the data slot as typical OBS headers butalso to be informed of the outcome of scheduling by each node.The scout, in contrast to the burst header, will return back withthe outcome of the reservation attempt. To make this possible,an offset slightly longer than the round trip time is used. Inthe event of a negative outcome, no data slot will be emittedand another scout will be dispatched. Otherwise, the first slotin the queue will be sent. Control is distributed with eachnode executing the scheduling and accepting or not acceptingreservations on the basis of local information.

The intolerable effects of heavy OBS losses on the servicequality of real-time applications and the disturbance on TCPcongestion control mechanisms have prompted the emergenceof OBS variants, which seek reliable delivery of bursts bynegative acknowledgements and retransmissions in the opti-cal layer, e.g., [6]. The problem with this approach is poorload control since retransmissions cause further deteriorationof performance by aggravating congestion, cause out-of-orderdeliveries, and violate contracted rates inviting policing actions.The proposed scheme should be distinguished from such meth-ods since the failing scouts are not accompanied by payloadbursts, thus acting as load throttling while retransmissionsare gracefully embedded in the operation. Above all, no out-of-order bursts are delivered, something that is detrimental tomost services, while reordering such bursts is not realistic incore networks of terabits per second rates.

Probe-and-go should also be distinguished from the asyn-chronous transfer mode (ATM)-inspired “tell-and-wait” [4],which is quite similar in philosophy: A control packet reservesbandwidth for a burst that will only be sent if the reservationis successful. However, this method cannot reserve the channelfor only a fraction of the round trip time, lacking the relevantcontrol mechanisms that probe-and-go uses and thus becomingonly meaningful for bursts longer than the round trip time.Wavelength-routed OBS [8] also employs the same strategy,avoiding transit loss by two-way reservations, but again only forvery large bursts (several tens of milliseconds) and for the samereason. The nearest paradigm offering the flexibility of shortand pipelined reservations is the medium access control (MAC)protocol used in passive optical networks [13], metropolitanrings [14], and cable modems [15], which provided the in-spiration for this scheme. In all these systems, which are ofmedium size, the standardization bodies adopted reservation-based MAC protocols to avoid the disruption of service qualityfrom the heavy losses of Aloha-based protocols, which is anexpediency even more valid in the core.


Fig. 1. (a) System architecture. (b) Node architecture.

III. SYSTEM ARCHITECTURE

The architecture on which the two-way slot reservationsapply is similar to any typical OBS environment consistingof periphery nodes (edge nodes) possessing electronic buffersand all-optical core nodes, where only the control channel isconverted and processed electronically, as shown in Fig. 1(a).The periphery nodes, where fixed-size slots are formed byaggregating (and occasionally splitting) variable length packetsfrom slower links (e.g., gigabit Ethernet or slower), maintaindifferent queues per destination. Source routing is employed,and knowledge about each route (found by a topology discoveryprotocol operating in the background) also includes the roundtrip distance measured in slots, which is obtained by rangingafter every route update. The core nodes are interconnected viaW data wavelengths and one (λ0) for control.

The maximum size of such a network is determined by theround trip scout travel delay, which places a lower bound tothe data delay, interfering with the needs of delay demandingservices. Therefore, like other two-way reservation methods, itcannot be applied to distances more than about 3000 km (30-msround trip).

The slotted operation of the system requires that bursts/slotsneed to enter in alignment from all ports. This is accomplishedby synchronizer units like the ones described in [11] at the entryports of each node, as shown in Fig. 1(b). The synchronizersturn the propagation distance toward every other node of thesystem into an integer number of slots. It is worth notingthat still the system remains bit asynchronous, not requiringglobal clock synchronization in the sense of synchronous digitalhierarchy (SDH). Burst-mode transceivers are used, and eachreceiver is bit/byte aligned by means of a synchronizationpattern at the beginning of the burst (slot in our case) as inclassical OBS. The adjustable delay (with maximum one slot)of the synchronizer operates simultaneously on all λs, and theircost is much lower than that of FDL arrays, which are oftenembedded in OBS switches, because they are much simplerwithout the costly fabric to switch the bursts in and out of thearray of FDLs. Yet their side effect of avoiding partial collisionscan amortize their cost by increased system utilization alone.

In the typical core node of Fig. 1(b), incoming links are firstdirected to the optical synchronizer, and then the WDM chan-nels are separated on different fibers that are directed to the op-tical crossbars for space switching and wavelength conversionas necessary to further improve contention performance underthe guidance of the node controller. The FDLs are includedin the bottom of the figure to stress the point that there is notechnical reason to prevent their use for improved contentionresolution capabilities, but the achieved performance obviatestheir need, avoiding their cost. The switching schedules havealready been prepared by the time the relevant scouts havepassed through the node as is elaborated in the next section. Theoutputs of the crossbars lead to either the pool of delay linesfor slots that are programmed to be delayed or directly to thepassive optical multiplexers at the output that will combine allthe data channels as well as the control channel (which carriesthe control information from the node controller) and exit ateach output port.

As mentioned earlier, in contrast to other OBS systems,data slots are preceded by scouts in place of burst headers,which depart with the arrival of the first byte of the slot (sinceslots have a fixed size), giving ample time to fully fill theslot during their travel. The scouts are situated inside controlslots of the same size also synchronized to data slots. Theformat of the scout is shown in Fig. 1(a). Each scout carries thenecessary information for one data slot, including the sourceand destination address (local OBS address which are 10 bitswide covering a domain of 1024 nodes), the wavelength onwhich the data slot is to arrive (10 bits), and the offset Td(12 bits) indicating how many slots later it is to arrive. In addi-tion, it carries two flags: one with the outcome of the reservation(Ack or Nack), and one indicating whether the scout is travelingin the forward direction (making reservations) or the reverse(i.e., returning with a positive or negative reservation). Finally,it carries a number N (8 bits), which counts traversed nodes.This number is not restarted when a scout receives a Nack andhas to be resent, but continues counting until it reaches its desti-nation successfully. At the departure of the scout, the parameterTd is initialized to the known round trip time in slots for the


Fig. 2. Scheduling log.

specific destination plus the scout processing time in all transitnodes. Finally, each scout contains an 8-bit error correction(EC) field allowing correction of 1 bit for robustness. With thefour spare bits, it reaches an overall size of 64 bits. For atransmission rate of 2.5 Gb/s in the control wavelength and100-µs slots, each control slot has a size of 250 000 bits andcan host about 3900 scouts, which is more than any nodecan process. Therefore, congestion in the control channelis extremely unlikely. Since a lost scout is treated like anegative scout after Td slots, the impact of scout contention onperformance is negligible.

IV. DESCRIPTION OF THE CONTROL AND SCHEDULING

An incoming scout informs the core node switch that Tdslots later a data slot will enter from the same port and inthe indicated wavelength claiming the also indicated output,implying a request to reserve the targeted future slot in thesame wavelength if possible (when conversion is supported,the output wavelength is to be decided by the scheduler). Asin other OBS systems, the nodes in this system are given anearly warning by the scouts to program future switching actionsmany slots later in time while simultaneously executing currentswitching actions that were planned some slots earlier in time.When a new scout claims an output, any contentions from otherscouts entering from other input ports are resolved within thepresent slot time, considering also any previous claims for thesame slots that are already marked in the scheduling log, whichis shown schematically in Fig. 2.

The algorithm is very simple, and its logic diagram is shownin Fig. 3. To decide the order of writing the entries in thescheduling log, scouts entering during each slot time and foreach Td are sorted per decreasing N . Scouts with differentTd do not contend for the same slot and can be processedin parallel. Among scouts with the same Td, the one with ahigher N wins the contention since dropping it would produce

worse damage as it has traveled more nodes (including previousunsuccessful attempts). Among scouts with same N , a randomchoice is exercized.

To execute the scheduling, the switch controller goes tothe time on the schedule Td positions later and checks if theport has this λ free for that time slot. If it does, it writesthe future switching action (i.e., input port and wavelength tooutput port and wavelength) and releases the scout. If it isoccupied, it checks whether another wavelength is free andprograms the relevant converter. If no position was found, thecontroller prepares a return scout going in the opposite directionback to the input link from where the scout entered, fills inall the relevant fields, sets the negative reservation bit, andreleases the returning scout. The above are illustrated in theexample of scheduling log shown in Fig. 2, where the darksquares indicate occupied positions (containing all switchinginformation), while empty positions are white. The size ofthe scheduling log equals the maximum possible Td in thesystem augmented by the total FDL capacity (if used), and itis implemented in a RAM block.

Contention is fought among the scouts with same Td andthose that are already logged in the relevant position of thescheduling log. But the latter have already won the contention,having crossed this switch earlier. In most cases, this happenedbecause they have a longer journey. Consequently, a scout thathas crossed or has to cross many nodes will almost never bedropped in the contention against a scout that has only a coupleof nodes to go. In contrast, in a typical OBS system, a header(hence, a burst) has equal probability to be dropped in thelast contention point in favor of another with only one hop.Favoring the longer destinations is one of the best features ofthis scheme as will be explained in the performance evaluationsection, since it reduces the chances of dropping at the last leg,wasting the already used—in vain—resources.

The scouts that found no empty scheduling position and wereswitched around return home without completing their trip tothe other end carrying a negative reservation, prompting thedispatch of a new scout by the source edge node. No dataslot will be sent, avoiding losses of data, and this is the mostimportant difference with other OBS systems. The winningscouts, on the other hand, continue their journey toward theirdestination trying to plan a secure complete trip for a data slotof the relevant queue. The end result is that the scouts executea preview of what the future holds if their data slots were to besent, probing the traffic conditions along the path and acceptingonly successful transit patterns. In summary, Td slots after thearrival of a payload slot, and hence the launch of a scout, either apositive scout returns causing departure of a data slot, or a neg-ative scout has already arrived, causing the issue of a new scout.

The round trip time of control slots is larger than that of dataslots by the extra time needed in each node for processing. Dueto the simplicity of the node scheduling algorithm amenableto fast hardware implementations, one slot processing delay inevery node is enough. So for each reservation, at least the roundtrip time of control slots is required, and each departing scoutis marked with a Td of the round trip time to destination of thecontrol slot. Each node reduces this number by one to subtractthe one extra slot processing needed by the control compared


Fig. 3. Scheduling logic.

with the data slot. Thus, the scout appears in all nodes ahead ofthe data by Td slots (the value it carries in his relevant field),allowing easy scheduling, even if this Td is not kept the sameas the scout travels.

It is worth noting that, in principle, sources do not needto keep track of Td values to decide when to send the data.On the next slot after the arrival of a positive scout, simplythe first data slot from the queue marked in the address fieldof the returning scout is sent (it is reminded that queues arearranged per destination). The return of any negative scout, onthe other hand, will result in a scout retransmission as shownin Fig. 4. It is clear that scouts do not belong to specific dataslots. As already mentioned, the first payload slot in a queuedeparts with the first returning positive scout irrespective ofwhether the specific scout was launched with the arrival of thenext slot in line. An example of this is illustrated in Fig. 4,where data slot 1 departs using the scout sent when slot 2 had

arrived since its “own” scout (sc 1) failed and the one sent in itsreplacement (sc 4) came later and was used by slot 3. This effectprovides an important improvement on delay performance, asexplained later. Thus, the first-in–first-out (FIFO) principle isnever violated in the same queue (no out-of-order slots are eversent), while at the same time as many scouts as queued payloadslots are always in transit.

It is worth mentioning that, for robustness reasons, timers areemployed redundantly to detect lost scouts in the rare case oftransmission errors that could not be corrected by the EC field,ascertaining that the lack of a returning scout after Td slots willcause the launch of a replacement and making sure that no slotsare queued for which no scout is on the way. Another significantdetail is that the negative scouts do not cancel any successfulscheduling in previous nodes on their return path. This wasdecided for implementation simplicity since the straightforwardtiming interpretation of Td on which the whole concept is based


Fig. 4. Indicative examples of scout operation.

is not meaningful on the return path, necessitating complextiming calculations. In addition, the performance benefit isinsignificant since, by that time, only very short paths wouldhave the opportunity to reuse the released slot, reducing theusefulness of such a measure.

V. PERFORMANCE EVALUATION

The probe-and-go scheme was evaluated by computer sim-ulation on a 4 × 4 torus [2] as well as on the NationalScience Foundation (NSF) topology [16] and compared to the“classic” JIT OBS with and without retransmissions. All nodesincluded a periphery interface generating input traffic destinedwith equal probability to each one of the other nodes. Theroutes used were determined by the shortest path algorithm,while for equal cost routes, one was randomly chosen and usedfor all communication between that pair of nodes during thesimulation run. There were W wavelengths at 10 Gb/s for dataand one control wavelength in every link of the network, whereW was 4 in the NSF topology while 4, 16, and 64 in thetorus. Enough converters for full wavelength conversion wereassumed in all nodes and no FDLs.

In the experiments carried out in the fully symmetric torus[2] and shown in Fig. 5, the generators did not directly produceslots, but IP packets from self-similar sources, which the frameaggregation unit, simulated in each periphery node, turned intopayload slots. The fill level inside the slots was found to be al-most 100% due to the big difference between the round trip timeand the slot size for slots of 100 µs (125 kB) or even 200 µs.The load is indicated in the figure as the average loading

over all network links. The length of links between nodesaccommodated 11 slots, which for a 100-µs slot corresponds to1.1 ms, i.e., a distance of about 220 km (assuming propagationspeed of 200 km/ms) and a maximum network dimension of880 km in the 16-node torus (four hops). Note that the sameresults are valid both for loss and delay for a twice as largenetwork with a twice as large slot, i.e., 200 µs. The one-way JITOBS was also simulated for comparison with Poisson arrivalsand exponentially distributed burst length equal on average tothe slot size.

Fig. 5(a) shows the burst/slot loss probability versus the aver-age loading of the system for two values of the electrical periph-ery buffers (45 and 60 slots/port and λ, i.e., 5.625 and 7.5 MB)as well as two values of W (4 and 64). The observed loss isorders of magnitude lower than for the simple OBS, and infact, so low that only at very high loads is it detectable bysimulation. Rather small electrical periphery buffers were used(ranging from 22.5 MB for 4 λs to 480 MB/port for 64 λs) toproduce measurable losses. Loss can easily be further reducedby using buffer values comparable to those (512 MB/port to1 GB/port) encountered in a typical IP core router given that allbuffering is collected at the periphery only. It is worth notingthat in very high loads, larger buffers result in higher loss.This counter-intuitive effect is explained by the fact that in abacklogged situation, the offered load, and hence congestion innode scheduling, is determined by the buffer size rather than theoriginal rate of traffic generation. What we see is the effect ofearlier congestion collapse with large buffers, which, however,is of no consequence since it occurs in unrealistic operatingconditions.


Fig. 5. (a) Burst loss probability comparison for Torus at 4 and 64 λs. (b) Average queuing delay versus offered load in Torus for 4, 16, and 64 λs.

The improved loss probability comes at the penalty of longerdelay. Fig. 5(b) shows the average queuing delay over allsource–destination pairs in the torus topology expressed in slotsversus the total offered load, again expressed as the averageloading over all network links. The results for 4, 16, and 64 λsare provided. The improvement from 16 to 64 wavelengths isnot as high as typically observed in one-way OBS, indicatingthe possibility to achieve good performance with much fewerexpensive wavelength converters. At low loads, the queuingdelay remains essentially very close to the average round triptime since each slot departs with the return of the, almostalways positive, scout. When the load rises further, delay beginsto rise slowly, offering predictability up to saturation (emptyroutes become hard to find), and eventually collapsing, unableto accommodate the offered load.

To achieve this improved performance, probe-and-go re-lies on a combination of effects that improve the schedulingefficiency. At first, slotting avoids partial collisions withoutthe need for burst segmentation and “tail dropping” [17] andinherently includes burst interleaving and void filling [7], [9].In addition, probe-and-go improves throughput by its choiceof scout offset times, which favor longer routes. Thus, burststraveling more nodes are protected more compared with thosewith short journeys. Short distance scouts (often in a secondattempt) visiting a point in the schedule fill in empty gapsleft by earlier (often long distance) scouts, resulting in fewerempty places and improved utilization. Consequently, conges-tion is first suffered by single-hop queues, which can havemany repeated attempts at little delay cost. In contrast, in atypical one-way OBS system, the probabilities of successfullycrossing many nodes diminish exponentially, so shorter routesare favored, leaving longer routes with poor chances of success.

What is of prime importance for real-time services is thedelay quantile that can be guaranteed and not the average delay.Thus, high delay jitter annihilates the usefulness of good aver-age causing degradation perceptible to users. This is why delaycan be purposefully introduced to reduce jitter as, for example,in the case with the play-back buffer. In probe-and-go, the effectof later slots relinquishing their reservations to earlier slotswhenever their “own scouts” failed (described earlier with thehelp of Fig. 4) keeps the FIFO order and allows the sharing ofthe delay among successive slots in an elegant manner, reducingthe delay variation. Improved delay jitter and utilization are two

Fig. 6. Burst loss probability comparison for NSF network at 4 λs.

performance aspects that probe-and-go offers when comparedwith the direct two-way reservation schemes such as WR-OBS[8]. Both stem from its ability to handle smaller chunks of dataand the pipelining of reservations.

Although probe-and-go does not retransmit payloads, it isstill in essence a retransmission scheme (retransmits scouts),and it is fair to be compared with a classical OBS variant,which employs retransmissions at the physical layer. To thisend, comparison is made with the results given in [16], whichinvestigates physical layer retransmission over the NSF net-work with four wavelengths. The same input traffic generatorswere used (Poisson interarrivals and exponential burst lengthwith an average of 100 µs for the OBS and fixed of 100 µs forprobe-and-go), and loading is the input traffic injected into thesystem expressed in Erlangs as in [16].

The results are shown in Fig. 6, where the simple one-wayOBS is provided from our simulator, but the measurements upto the first 30 Erlang are the same as those in [16], confirmingcoincidence of simulation tools and network conditions. Thetwo best curves (giving the lower losses) from the retransmis-sion scheme provided in [16] are then drawn (only availableup to 30 Erlang) for ease of comparison. As commented in theconclusions of [16], “retransmissions generate additional traf-fic load into the network, thereby increasing burst contentionprobability.” In contrast, the next two curves depicting theloss values of probe-and-go with a buffer size of 45 slots (=5.625 MB) and 60 slots (= 7.5 MB) per port and λ showsan improvement of orders of magnitude. It takes a very highoffered load to detect losses by simulation as is normal in net-works with electrical switching. No time-stamping mechanisms


were used in the buffers to purge bursts that exceeded a timelimit as in the retransmissions scheme in [16], keeping theimplementation simple. The vastly improved performance isbecause probe-and-go does not just save lost bursts from re-transmission but vastly improves the effective throughput ofthe system by finding more noncolliding routes by its repeatedattempts without overloading the network.

Before closing, it is worth commenting that during momen-tary or even persisting system overloads, probe-and-go exer-cises an indirect throttling action by probing before sending,which regulates the rates of load entering the system to avoidcollapse due to congestion. This throttling is selectively appliedonly to traffic going through congested paths and not all traffic.In contrast, a typical one-way OBS system enhanced withretransmissions would suffer a chain reaction as retransmittedbursts further increase loading, leading to throughput collapsedue to persistent congestion [7]. Delay to some extent butmainly delay variation would also rise significantly due toretransmissions. If, on the other hand, retransmissions are leftto TCP (only feasible for delay tolerant services), seriousthroughput deterioration by 50% is observed for burst loss aslow as 0.003 by means of the window reduction it causes [18].All such effects are avoided in probe-and-go while increasedeffective capacity is extracted from the network by finding morecontention-free itineraries for bursts.

VI. CONCLUSION

By first simulating the travel of a packet along an OBS path,scouts can, upon their return to the sending node, inform thenode controller about the fate of the payload, avoiding theemission of those destined to be dropped. Thus, by means ofpipelined slot-by-slot reservations, the probe-and-go protocolcan avoid the intolerable losses of one-way OBS, keep thedelay variation lower than any other method of retransmittinglost bursts (which is important to interactive services), improveutilization by reversing the success probabilities for burstscrossing more contention points, and obviate out-of-orderbursts. It also sidesteps the problems of typical two-waysignaling-based systems that need to amass bursts comparableto the round trip while probe-and-go pipelines the reservationshandling smaller chunks of data. Reservations act as an inherentclosed-loop back-pressure control of the offered load that pre-vents congestion and uncontrolled loss by selectively adaptingthe ingress traffic to the rates that each path can tolerate.

ACKNOWLEDGMENT

The views expressed herein are those of the authors and donot necessarily represent the position of the whole NOBELconsortium.

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John D. Angelopoulos (S’92–M’95) is a Professorwith the Technological Institute of Piraeus, Aiga-leo, Greece, and participates in research activitieswith the National Technical University of Athens(NTUA), Zografou, Greece, in the areas of high-speed networks. He has been involved in severalResearch and development in Advanced Commu-nications technologies in Europe (RACE), Euro-pean Strategic Program for Research in InformationTechnology (ESPRIT), Advanced CommunicationsTechnologies and Services (ACTS), and Information

Society Technologies Programme (IST) projects with emphasis on broadbandaccess systems.


Konstantinos Kanonakis received the Dipl.-Ing. de-gree from the School of Electrical and Computer En-gineering, National Technical University of Athens(NTUA), Zografou, Greece, in 2004. He is currentlyworking toward the Ph.D. degree in the same univer-sity.

His main research interests are in the area of trafficengineering, architectures, and control protocols foroptical networks and their implementation in hard-ware.

Helen C. Leligou received the Dipl.Ing. and Ph.D.degrees from the National Technical University ofAthens (NTUA), Zografou, Greece, in 1995 and2002, respectively, both in electrical and computerengineering.

Her research interests include high-speed net-works for tree-topology hybrid fiber/coax andgigabit passive optical network systems, wavelength-division-multiplexed metro-ring networks, and opti-cal burst switching.

Charalambos Linardakis received the Ph.D. degreefrom the National Technical University of Athens(NTUA), Zografou, Greece, in 2004.

His research interests include wavelength divi-sion multiplexing (WDM) access networks, pas-sive optical network access networks, convergenceof TCP/IP and asynchronous transfer mode net-works, and broadband telecommunications. He hasdesigned, simulated, and implemented medium ac-cess control protocols for WDM systems in theframework of the Data And Voice Integration overDWDM (DAVID) European project.

Ioannis E. Pountourakis (M’91) is a Professorwith the Computer Science Division, Departmentof Electrical Engineering, National Technical Uni-versity of Athens (NTUA), Zografou, Greece. Hisresearch interests are in the fields of data communi-cation networks, multichannel multiaccess protocols,performance evaluation and stability, broadband net-works, integrated communications, local area net-works, metropolitan area networks, and wide areanetworks.

Alexandros Stavdas (M’97) is an Associate Profes-sor of optical networking with the Department ofTelecommunications Science and Technology, Uni-versity of Peloponnese, Tripolis, Greece.

He is also heading the Optical Networking Groupof the National Technical University of Athens(NTUA), Zografou, Greece. His current interestsinclude physical layer modeling of optical networks,ultrahigh capacity end-to-end optical networks, opti-cal cross connect architectures, optical packet/burstswitching, and wavelength division multiplexingaccess networks.

Documents

Slotted optical switching with pipelined two-way reservations