JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. X, MONTH 2005 1

Design and Performance Analysis of SchedulingAlgorithms for WDM-PON under

SUCCESS-HPON ArchitectureKyeong Soo Kim,Member, IEEE,David Gutierrez,Student Member, IEEE,Fu-Tai An, Member, IEEE,

and Leonid G. Kazovsky,Fellow, IEEE, Fellow, OSA

Abstract— We report the results of our design and performanceanalysis of two new algorithms for efficient and fair schedul-ing of variable-length frames in a wavelength division multi-plexing (WDM)-passive optical network (PON) under StanfordUniversity aCCESS-Hybrid PON (SUCCESS-HPON) architec-ture. The WDM-PON under the SUCCESS-HPON architecturehas unique features that have direct impacts on the design ofscheduling algorithms: First, an optical line terminal (OLT) usestunable transmitters and receivers that are shared by all theoptical network units (ONUs) served by the OLT to reduce thenumber of expensive dense WDM (DWDM) transceivers. Second,also for cost reduction, ONUs have no local DWDM light sourcesbut use optical modulators to modulate optical continuous wave(CW) bursts provided by the OLT for upstream transmissions.Therefore, the tunable transmitters at the OLT are used forboth upstream and downstream transmissions. To provide ef-ficient bidirectional communications between the OLT and theONUs and guarantee fairness between upstream and downstreamtraffic, we have designed two scheduling algorithms – batchingearliest departure first (BEDF) and sequential scheduling withschedule-time framing (S3F). The BEDF is based on the batchscheduling mode where frames arriving at the OLT during abatch period are stored in virtual output queues (VOQs) andscheduled at the end of the batch period. It improves transmissionefficiency by selecting the frame with the earliest departuretime from a batch of multiple frames, which optimizes theusage of tunable transmitters in scheduling. Considering the highcomplexity of the optimization process in BEDF, we have alsode-signed the S3F based on the sequential scheduling mode as in theoriginal sequential scheduling algorithm proposed earlier. In S3Fwe use VOQs to provide memory space protection among trafficflows and a granting scheme together with schedule-time framingfor both upstream and downstream traffic to reduce framingand guard band overhead. Through extensive simulations undervarious configurations of the tunable transmitters and receivers,we have demonstrated that both the BEDF and S3F substantiallyimprove the throughput and delay performances over the orig-inal sequential scheduling algorithm, while guaranteeingbetterfairness between upstream and downstream traffic.

Index Terms— Access, media access control (MAC) protocols,passive optical network (PON), scheduling, wavelength divisionmultiplexing (WDM)

This work was supported in part by the Stanford Networking ResearchCenter and STMicroelectronics.

K. S. Kim is with the Advanced System Technology, STMicroelectronics,Stanford, CA 94305, USA (e-mail: kks@stanford.edu).

D. Gutierrez and L. G. Kazovsky are with the Photonics and NetworkingResearch Laboratory, Stanford University, Stanford, CA 94305, USA (e-mail:{degm,kazovsky}@stanford.edu).

F-T. An is with the Marvell Technology Group Ltd. (e-mail:ftan@stanfordalumni.org).

This paper was presented in part at GLOBECOM 2004, Dallas, TX,November, 2004.

I. I NTRODUCTION

Efficient and fair scheduling of variable-length messagesunder the constraints of shared resources is critical for the suc-cess of advanced, next-generation wavelength-routed opticalnetworks where tunable transmitters and receivers are sharedby many users in order to reduce the high cost of wavelengthdivision multiplexing (WDM) optical components.

The scheduling problem we study in this paper is fora WDM-passive optical network (PON) underStanfordUniversity aCCESS-Hybrid PON (SUCCESS-HPON) archi-tecture, which was proposed for next-generation hybridWDM/time division multiplexing (TDM) optical access net-works [1].1 The SUCCESS-HPON architecture is based on atopology consisting of a collector ring and several distributionstars connecting a central office (CO) and optical networkingunits (ONUs). By clever use of coarse WDM (CWDM) anddense WDM (DWDM) technologies, it guarantees the coex-istence of current-generation TDM-PON and next-generationWDM-PON systems on the same network. The semi-passiveconfiguration of remote nodes (RNs) together with the hybridtopology also enables supporting both business and residentialusers on the same access infrastructure by providing protectionand restoration capability, a frequently missing feature intraditional PON systems.

In designing the SUCCESS-HPON architecture, we mainlyfocused on providing economical migration paths from thecurrent-generation TDM-PONs to future WDM-based opticalaccess networks. This has been achieved by sharing somehigh-performance but costly components and resources inSUCCESS WDM-PON2: First, an optical line terminal (OLT)uses tunable transmitters and receivers that are shared byall the optical network units (ONUs) served by the OLT toreduce the number of expensive DWDM transceivers. Second,also for cost reduction, ONUs have no local DWDM lightsources but use optical modulators to modulate optical con-tinuous wave (CW) bursts provided by the OLT for upstreamtransmissions. Therefore, the tunable transmitters at theOLTare used for both upstream and downstream transmissions.The sharing of tunable transmitters and receivers at the OLTand the use of tunable transmitters for both upstream and

1We have changed the name of the architecture fromSUCCESStoSUCCESS-HPONto distinguish it from other architectures under the sameresearch initiative ofSUCCESSat PNRL, Stanford.

2In this paper we use the termSUCCESS WDM-PONto denote the WDM-PON under the SUCCESS-HPON architecture.

2 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. X, MONTH 2005

downstream transmissions, however, pose a great challengeindesigning scheduling algorithms: A scheduling algorithm forthe SUCCESS WDM-PON has to keep track of the status of allshared resources (i.e., tunable transmitters, tunable receiversand wavelengths assigned to ONUs) and arrange them properlyin both time and wavelength domains to avoid any conflictsamong them for both upstream and downstream transmissions.

While many researchers have studied the issue of schedulingmessages in both time and wavelength domains in networkarchitectures based on tunable transmitters and/or receivers(e.g., [2]–[5]), only a few schemes have been proposed to sup-port variable-length message transmissions without segmenta-tion and reassembly processes. In [4], we studied schedulingalgorithms for unslotted carrier sense multiple access withcollision avoidance (CSMA/CA) with backoff media accesscontrol (MAC) protocol to address the issues of fairness andbandwidth efficiency in multiple-access WDM ring networks.In [5], the authors studied distributed algorithms for schedulingvariable-length messages in a single-hop multichannel locallightwave network with a focus on reducing tuning overhead.To the best of our knowledge, however, scheduling algorithmsfor a network where tunable transmitters are used for bothupstream and downstream transmissions as in the SUCCESSWDM-PON, have not been investigated by other researchers.

In [1] we proposed a sequential scheduling algorithm forthe SUCCESS WDM-PON, which emulates a virtual globalfirst-in-first-out (FIFO) queueing for all incoming frames.Inthis algorithm incoming frames are scheduled sequentiallyin the order of arrival at the OLT. This original sequentialscheduling algorithm is simple to implement, but suffers frompoor transmission efficiency and fairness guarantee betweenupstream and downstream traffic.

To address the limitations of the original sequential schedul-ing algorithm, we propose in this paper two new schedulingalgorithms – batching earliest departure first (BEDF) andsequential scheduling with schedule-time framing (S3F). Thekey idea in the design of BEDF is to provide room foroptimization and priority queueing by scheduling over morethan one frame: In BEDF, frames arriving at the OLT duringa batch period are stored in virtual output queues (VOQs)and scheduled at the end of the batch period, which allowsin scheduling to select the best frame according to a givenoptimal scheduling policy from the batch of multiple framesin the VOQs. We choose the EDF as an optimal schedulingpolicy to minimize the unused time of the tunable transmitters.The throughput versus scheduling delay tradeoff is a majordesign issue in BEDF.

In S3F, considering the high complexity of the BEDFoptimization process, we adopt the sequential scheduling modeas in the original sequential scheduling algorithm, but useVOQs to provide memory space protection among traffic flowsas in BEDF and a granting scheme together with schedule-timeframing for both upstream and downstream traffic to reduceoverhead due to framing and guard bands.

The rest of the paper is organized as follows. In SectionII we provide a high-level overview of the SUCCESS-HPONarchitecture and review the MAC protocol, frame formats andoriginal sequential scheduling algorithm for the WDM-PON

under the SUCCESS-HPON architecture. In Section III wedescribe the BEDF and S3F scheduling algorithms based onthe system model and procedures used in the description ofthe original sequential scheduling algorithm in Section II. InSection IV, we provide the results of the performance analysisof the designed scheduling algorithms through simulations.Section V summarizes our work in this paper and discussesfuture directions for further studies.

II. WDM-PON UNDER SUCCESS-HPONARCHITECTURE

A. Overall Architecture

A high-level overview of the SUCCESS-HPON, includingTDM-PONs and WDM-PONs as its subsystems with wave-length allocations, is shown in Fig. 1. A single-fiber collectorring with stars attached to it formulates the basic topology.The collector ring strings up RNs, which are the centers ofthe stars. The ONUs attached to the RN on the west side ofthe ring talk and listen to the transceivers on the west side ofthe OLT, and likewise for the ONUs attached to the RN onthe east side of the ring. Logically there is a point-to-pointconnection between each RN and the OLT. No wavelength isreused on the collector ring. When there is a fiber cut, theaffected RNs will switch to the transceivers on the other sideof the OLT for continuous operations as soon as they sense asignal loss.

The RN for TDM-PON has a pair of CWDM band splittersto add and drop wavelengths for upstream and downstreamtransmissions, respectively. On the other hand, the RN forWDM-PON has one CWDM band splitter, adding and drop-ping a group of DWDM wavelengths within a CWDM grid,and a DWDM MUX/DEMUX device,i.e., arrayed waveguidegrating (AWG), per PON. Each ONU has its own dedicatedwavelength for both upstream and downstream transmissionson a DWDM grid to communicate with the OLT. Since theinsertion loss of a typical AWG is roughly 6 dB regardless ofthe number of ports, AWGs with more than eight ports willlikely be employed to enjoy better power budget compared topassive splitters.

Fig. 2 shows block diagrams of the portion of the OLT andthe ONU for the SUCCESS WDM-PON. Tunable components,such as fast tunable lasers and tunable filters are employed forDWDM channels. Because the average load of the network isusually lower than the peak load [6], we can expect statisticalmultiplexing gain by sharing tunable components at the OLT,which also reduces the total system cost by minimizing thetransceiver count for a given number of ONUs and userdemand on bandwidth. Downstream optical signals from thetunable transmitters in DWDM channels enter both ends ofthe ring through passive splitters and circulators. Upstreamoptical signals from the ring pass the same devices but inreverse order and are separated from the downstream signalsby the circulators. The scheduler controls the operation ofboth tunable transmitters and tunable receivers based on thescheduling algorithms that will be described in Section III.

Note that the tunable transmitters at the OLT are usedfor both downstream frames and CW optical bursts to be

KIM et al.: SCHEDULING ALGORITHMS FOR WDM-PON UNDER SUCCESS-HPON 3

RN

RN

RN

RN

λ*1, λ2

λ1

λ2

λ21λ22 λ23

λ*1

λ*3, λ4, …

λ1, λ2

λ3, λ4, …

λ4

λ*3

λ3

λ41

λ42

λ43

TDM-PON ONU

RN TDM-PON RN

WDM-PON ONU

RN WDM-PON RN

CentralOffice

Fig. 1. Overview of SUCCESS-HPON.

Scheduler

DownstreamTraffic Queues

TunableTransmitter 1

. . .. . .

TunableTransmitter M

TunableReceiver 1

. . .

TunableReceiver N

UpstreamGrant Queues

Circulator

M:2 Passive Splitter

. . .

Upstream TrafficTo the Network

N:2 Passive Splitter

(a)

Burst-ModeReceiver

Modulator

MAC

DownstreamTraffic

Upstream TrafficQueue

Circulator

1:2 Passive Splitter

(b)

Fig. 2. Block diagrams of (a) the portion of OLT and (b) the ONUforSUCCESS WDM-PON.

modulated by the ONUs for their upstream frames. With thisconfiguration only half-duplex communications are possible atthe physical layer between the OLT and each ONU using avariation of the time compression multiplexing (TCM) scheme[7]. Compared to a similar architecture with a two-fiber ring,two sets of light sources and two sets of MUX/DEMUXfor full-duplex communications [8], our design significantlylowers deployment cost. As a tradeoff, however, we need acareful design of a scheduling algorithm to provide efficientbidirectional communications at the MAC layer.

As discussed before, the ONU has no local light sourceand uses an optical modulator to modulate optical CW burstsreceived from the OLT for its upstream transmission. A semi-conductor optical amplifier (SOA) can be used as a modulatorfor this purpose [9]. The ONU MAC protocol block notonly controls the switching between upstream and downstreamtransmissions but also coordinates with the scheduler at theOLT through a polling mechanism.

For implementation details, especially at the physical layer

8-BitFlags

Ethernet Frame

Grant (CW)

or

8-BitPreamble

16-BitDelimiter

Overhead

16-BitReport Ethernet Frame …

ForDownstream

ForUpstream

8-BitPreamble

16-BitDelimiter

Overhead

16-BitGrant

8-BitFlags

8-BitPreamble

16-BitDelimiter

Overhead

Ethernet FrameEthernet Frame …

Order of Transmission

Fig. 3. Frames formats for SUCCESS WDM-PON MAC protocol.

of the SUCCESS WDM-PON, readers are referred to [1].

B. MAC Protocol and Frame Formats for SUCCESS WDM-PON

Like APON and EPON systems [10], the SUCCESS WDM-PON OLT polls to check the amount of upstream traffic storedat the ONUs and sends grants – but in the form of optical CWbursts in this case – to allow the ONUs to transmit upstreamtraffic. Since there is neither a separate control channel nora control message embedding scheme using escape sequencesas in [11], the MAC protocol has to rely on in-band signalingand uses the frame formats shown in Fig. 3, where the reportand grant fields are defined for the polling process.3 Note thatthe 1-bit ‘ID’ field in [1] for downstream frames has beenextended to 8-bit flags for future extensions: Now the ‘FrameType’ field of the flag is used to indicate whether this frame isfor normal data traffic or not. Usage of the fields in the 8-bitflags is summarized in Table I.

Each ONU reports the amount of traffic waiting in itsupstream traffic queue in octets through the report field in anupstream frame when the ‘Force Report’ field of a receiveddownstream frame is set4, and the OLT uses the grant field

3In this paper, we assume that Ethernet frames are carried in the payloadpart of SUCCESS WDM-PON frames. Note that, however, any other protocolframe or packet. e.g., IP packets, can be encapsulated and carried in aSUCCESS WDM-PON frame because it does not depend on any specificlayer 2 or 3 protocols unlike APON or EPON.

4In this paper, we assume this field is always set.

4 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. X, MONTH 2005

TABLE I

8-BIT FLAGS IN DOWNSTREAMSUCCESS WDM-PON FRAME

Bit Field Values0 - Normal Data

0-3 Frame Type 1 - Grant2-15 - Unused0 - No action required4 Force Report1 - ONU should report in the corresponding

upstream frame5 Unused -6 Unused -7 Unused -

to indicate the actual size of each grant (also in octets). Notethat, as shown in Fig. 3, the length of the whole CW burstcorresponds to that of all upstream Ethernet frames (i.e., thesize of grant) plus the report field and the overhead.

We use two control parameters to govern the polling processconsisting of reporting and granting operations as follows:

• ONU TIMEOUT : The OLT maintains one timer perONU and resets it whenever a grant frame is sent down-stream to an ONU. It clears the timer when the corre-sponding upstream frame with a nonzero report field is re-ceived. If the timer expires after theONU TIMEOUTperiod, which means either there was no upstream trafficwhen the ONU received a grant frame or the reportmessage was lost during the transmission to the OLT,the OLT sends a new grant to poll that ONU again andresets the timer. This parameter keeps the polling processgoing on even in the case of the loss of polling messagesand bounds the maximum polling cycle. It also affects theaverage packet delay of upstream traffic when the systemis under light load.

• MAX GRANT : This parameter limits the maximumsize of a grant (i.e., the payload part of the CW burst)for ONU upstream traffic.

C. Original Sequential Scheduling Algorithm

Here we describe how the scheduling of transmission and/orreception of a SUCCESS WDM-PON frame is done underthe original sequential scheduling algorithm proposed in [1].This will be the basic building block of the new schedulingalgorithms in Section III. For this purpose, we considera SUCCESS WDM-PON system withW ONUs (thereforeW wavelengths),M tunable transmitters, andN tunablereceivers. Because the tunable transmitters are used for bothupstream and downstream traffic but tunable receivers are onlyfor upstream traffic, we usually need more transmitters thanreceivers,i.e., W ≥ M ≥ N . We include in the algorithmdescription the guard band ofG ns between consecutiveSUCCESS WDM-PON frames that accounts for the effectsof unstable local ONU clock frequencies and tuning time oftunable transmitters and receivers at the OLT.

We define the following arrays of global status variablesused in the algorithm description:

• CAT: Array of Channel Available Times. CAT[i]=t,wherei = 1, 2, ..., W , means that the wavelengthλi willbe available for transmission after timet.

• TAT: Array of Transmitter Available Times. TAT[i]=t,wherei = 1, 2, ..., M , means that theith tunable trans-mitter will be available for transmission after timet.

• RAT: Array of Receiver Available Times. RAT[i]=t,wherei = 1, 2, ..., N , means that theith tunable receiverwill be available for reception after timet.

• RTT: Array of round trip times (RTTs) between the OLTand the ONUs. RTT[i] denotes the RTT between the OLTand theith ONU.

When scheduling each SUCCESS WDM-PON frame, wefirst select the earliest available transmitter and receiver. As-suming that theith transmitter and thejth receiver are theearliest available transmitter and receiver respectively, we canobtain the transmission timet of a SUCCESS WDM-PONframe destined for thekth ONU as follows:

t =

max(RAT [j] + G − RTT [k]− GOH ,TAT [i] + G, CAT [k] + G)

if the frame is a grant for upstream traffic,max(TAT [i] + G, CAT [k] + G)

if the frame is for downstream traffic,(1)

where GOH is a transmission delay for the grant overheadconsisting of the overhead, 8-bit flags and grant fields of theSUCCESS WDM-PON grant frame at a line rateR bit/s. Ifthe frame is a grant frame for upstream traffic, the receptionof the corresponding upstream frame from the ONU shouldbe scheduled att + GOH + RTT [k].

After scheduling the frame transmission and/or reception,the related status variables should be updated as follows:

TAT [i] = t + l/RCAT [k] = t + l/R

, (2)

and if the frame is a grant frame for upstream traffic,

RAT [j] = t + l/R + RTT [k], (3)

wherel is the length of the whole frame in bits.Fig. 4 illustrates the timing relations among tunable trans-

mitters and receivers, and frames over channels through anexample: At t1, a report for upstream traffic from ONU4arrives at the OLT. First, the scheduler at the OLT checksthe transmitter availability and finds that TX3 is availablenow. Then, it checks the receiver availability and finds thatRX1 will be available att0 + GOH + RTT1 + lcw1. Then,it also checks the channel availability and finds thatλ4 isavailable now. Finally, based on all these information, thescheduler schedules the transmission of a grant frame att0 + RTT1 + lcw1 + G − RTT4 through TX3 on λ4 and thereception of a corresponding upstream frame from ONU4 att0 + GOH + RTT1 + lcw1 + G. Pseudocode for the wholeprocedure is given in Fig. 5.

III. D ESIGN OFNEW BATCH AND SEQUENTIAL

SCHEDULING ALGORITHMS FORSUCCESS WDM-PON

In this section we describe two new scheduling algorithms– the BEDF and the S3F – designed in order to improve thefollowing performance measures over the original sequentialscheduling algorithm: 1) Fairness guarantee between upstream

KIM et al.: SCHEDULING ALGORITHMS FOR WDM-PON UNDER SUCCESS-HPON 5

RX1

RX2

TX1

TX2

TX3 t1

RTT4

λ1

λ2 λ4

G

λ4λ1

New transmissionscheduled!

lcw1

RTT1

λ2

λ1

Time

Grant Overhead(= OH + Flags + Grant)

CW1

CW2

CW3

lcw3

lcw2

t0

Fig. 4. An example of the original sequential scheduling att1 for a systemwith W = 4, M = 3 andN = 2.

begink←− destination(frame);l←− length(frame);tnow ←− current time;for i = 1 to W do CAT [i]←− max(tnow, CAT [i]);for i = 1 to M do TAT [i]←− max(tnow, TAT [i]);for i = 1 to N do RAT [i]←− max(tnow, RAT [i]);selecti s.t. TAT [i] ≤ TAT [m] ∀m = 1, . . . , M ;if the frame is a grant for upstream trafficthen

selectj s.t. RAT [j] ≤ RAT [n] ∀n = 1, . . . , N ;t←− max(RAT [j] + G−RTT [k]−GOH , TAT [i] + G, CAT [k] + G);schedule reception at timet + GOH + RTT [k] withthe jth receiver via the wavelengthλk;RAT [j]←− t + l/R + RTT [k];

elset←− max(TAT [i] + G, CAT [k] + G);

endTAT [i]←− t + l/R;CAT [k]←− t + l/R;schedule transmission at timet with the ith transmitter viathe wavelengthλk;

end

Fig. 5: Pseudocode for the original sequential schedulingalgorithm.

and downstream traffic flows for a given ONU and 2) overallthroughput. Here we use a simple but intuitive definition of’fairness’: On the assumption that all received traffic flowsare legitimate, the scheduler assigns bandwidth so that theresulting throughput of a traffic flow should be in propor-tion to its incoming rate. By ‘traffic flow’ we mean theaggregated traffic between the OLT and each ONU in eachdirection (upstream or downstream); thus, the scheduler atthe OLT deals with a total of 2W separate traffic flows. Inthe original sequential scheduling algorithm, a downstreamEthernet frame is encapsulated in a SUCCESS WDM-PONframe immediately after its arrival and put into a globalFIFO queue that is shared by all upstream and downstreamtraffic. As the simulation results in [1] show, the lack ofprotection for memory space among traffic flows leads intopoor fairness between upstream and downstream traffic. Also,because there is no room for optimization in scheduling, themaximum achievable throughput is much lower than the total

transmission capacity. To address the issue of memory spaceprotection among traffic flows, we base both the schedulingalgorithms on VOQing with one VOQ per traffic flow eitherupstream or downstream for an ONU.

A. Batching Earliest Departure First (BEDF) Scheduling

The idea of batch scheduling, where a batch of arrivedmessages during a certain period forms a task set to whicha scheduling algorithm is applied, has been already studiedin [12], but in a slightly different context where the mainconcern is the reduction of the frequency and complexity ofthe scheduling algorithm at the cost of deferring considerationof new tasks. On the other hand, the major concern in ourdesign of the BEDF is to provide room for optimization inscheduling by forming a task set consisting of multiple framesby batching process. Rather than sequentially scheduling eachframe in the order of arrival, by forming a batch of arrivedframes and searching for a frame with an optimal valueaccording to a given scheduling policy, we can optimizethe scheduling performance. Because transmission efficiencyunder the constraint of sharing limited resources is one ofthe major design goals, we select the EDF as an optimalscheduling policy to minimize the time when transmitters andchannels are wasted.

Building upon the basic sequential scheduling algorithm de-scription in Section II, we can describe the BEDF schedulingalgorithm as follows: At the end of each batch period,

Step 1 Choose the earliest available transmitter and receiver(i.e., whose TAT and RAT are minimum).

Step 2 Given the earliest available transmitter and receiver,calculate a possible transmission time using Eq. 1 forthe first unscheduled frame in each VOQ that is notmarked as ‘Unschedulable’.

Step 3 Select the frame with the minimum transmission time(i.e., the earliest departure time) and if the transmissiontime is within the boundary of the next batch period,schedule its transmission; otherwise, cancel its schedul-ing and mark the corresponding VOQ as ‘Unschedu-lable’. If the scheduled frame is a grant for upstreamtraffic, schedule the reception of the corresponding up-stream frame from the ONU afterGOH + RTT fromits transmission time.

Step 4 Update the status variables using Eq. 2 for the transmit-ter, the channel and if needed, the receiver.

Step 5 Repeat the whole procedures from the steps 1 through4 until there is no unscheduled frame or all VOQs aremarked as ‘Unschedulable’.

Note that in contrast to the batch scheduling schemeproposed in [12], once the scheduled transmission time ex-ceeds the boundary of the next batch period, we cancelthe scheduling of that frame, mark the corresponding VOQas ‘Unschedulable’, and exclude the frames in that VOQin further scheduling during the current batch period. Thisprevents the frames arriving in the current batch period fromconsuming the resources available in the next batch periodand therefore provides some protection for network resourcesbetween batches of frames. The interleaving of scheduling and

6 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. X, MONTH 2005

Time

Scheduling results of the2nd batch + remnants

from the 1st batch

Scheduling results ofthe 1st batch

1st batch period 2nd batch period 3rd batch beriod

Snapshots of VOQsat the beginning of batch periods

Fig. 6. A scheduling example showing the interleaving of scheduling andtransmission phases in BEDF.

transmission phases in BEDF is illustrated through an examplein Fig. 6.

B. Sequential Scheduling with Schedule-time Framing (S3F)

The major downside of BEDF compared to the originalsequential scheduling algorithm, is the higher computationalcomplexity due to the optimization process in scheduling tosearch for the frame with the earliest departure time. Forexample, in the worst case where all VOQs have framesto schedule, the BEDF needs roughly2W times as manycalculations as the original sequential scheduling algorithm toschedule one frame.

Here we propose S3F, an improved sequential schedulingalgorithm. S3F is based on the sequential scheduling mode,but unlike the original sequential scheduling algorithm, thescheduling is done at the end of each frame transmission(except in the case when a frame arrives at an empty VOQ,where the scheduling is done immediately after its arrival). Italso uses grants for downstream traffic as well as upstreamtraffic to provide better fairness guarantee and schedule-timeframing of downstream Ethernet frames in the VOQs to over-come the low transmission efficiency of the original schedulingalgorithm. Due to the memory space protection among trafficflows through VOQing, the S3F can provide better fairnessguarantee than the original sequential scheduling algorithm.

For the purpose of granting downstream traffic, we maintaina downstream transmission counter per downstream VOQ.When granting upstream traffic based on a received requestfrom an ONU, we also grant downstream traffic as well basedon the VOQ status at the time of the arrival of the reportmessage. Granting downstream traffic is done by setting thesaid grant counter to the minimum of the queue length ofthe VOQ andMAX GRANT . When scheduling downstreamtransmission, the grant counter value controls the numberof Ethernet frames to be scheduled and transmitted in oneSUCCESS frame through the procedure shown in Fig. 7.

Note that the procedure in Fig. 7 allows at least one Ethernetframe transmission to be scheduled, irrespective of the valueof the downstream transmission counter (dsTxCtr[i]). Thisallows the OLT to transmit downstream traffic for a particularONU even when there is no granting for the ONU: In thecase where there is no request for upstream traffic from that

beginif VOQ[i] is not emptythen

numBits←− 0;pos←− 0;ptr←− &ethFrame(V OQ[i], pos);repeat

dsTxCtr[i]←− dsTxCtr[i]− length(∗ptr);numBits←− numBits + length(∗ptr);pos←− pos + 1;ptr ←− &ethFrame(V OQ[i], pos);if ptr is NULL then

// no more frames to scheduleexit the loop;

enduntil dsTxCtr[i] < length(∗ptr);schedule the transmission of a SUCCESS frame whosepayload length isnumBits;// using the sequential scheduling

algorithm in Fig. 5storepos for the scheduled transmission later;

endend

Fig. 7: Pseudocode for the scheduling of downstream dataframe transmission for a given channeli in S3F. Note thatpos denotes the relative position of an Ethernet frame fromthe head of the VOQ (e.g., pos = 0 means it is the head-of-line (HOL) frame.).

ONU and therefore no granting, it is still possible to transmitdownstream frames, but one at a time.

The benefit of granting and schedule-time framing of down-stream traffic is three-fold. First, by encapsulating multipleEthernet frames in one SUCCESS WDM-PON frame as inupstream transmission, we can reduce the overhead due tothe SUCCESS WDM-PON framing and the guard bands.Second, we can also reduce the waste of tunable transmittersand channels and therefore minimize scheduling delays bypreventing spread of smaller frames over multiple transmittersand channels. This is well illustrated in the examples in Fig.8. Here we assume that for both the framing schemes, thereare three Ethernet frames att1 in the VOQ for channel 1,four Ethernet frames att2 for channel 2 and one Ethernetframe att3 for channel 3.t

′

idenotes the resulting scheduled

transmission time of the first frame for channeli, so thecorresponding scheduling delay is given byt

′

i− ti. The effectsof the inefficient use of transmitters in the arrival-time framing,where each incoming Ethernet frame is encapsulated in aSUCCESS WDM-PON frame at the moment of its arrival,become clear when we compare its scheduling delay of theframe for channel 3 in (b) to that of the schedule-time framingin (a). Third, by integrated and intelligent granting of bothupstream and downstream traffic, we could better control thewhole traffic flows for guarantee of fairness and support ofquality of service (QoS) in the future.

IV. PERFORMANCEANALYSIS

We have developed a simulation model for the performanceevaluation of the designed scheduling algorithms usingOb-jective Modular Network Testbed in C++(OMNeT++) [13].The simulation model is for a WDM-PON system under the

KIM et al.: SCHEDULING ALGORITHMS FOR WDM-PON UNDER SUCCESS-HPON 7

TX

Time

H1

EF

EF

EF

1 2 3

H2

EF

EF

EF

t1

EF

Gt2’t2

Gt1’Hi

EFEthernetFrame

SUCCESS WDM-PONFrame Header for Channel i

t3

H3

EF

t3’

(a)TX

Time

1 2 3t1

t2H1

EF

H1

EF

H1

EF

G

H2

EF

G

t2’

Gt1’

H2

EF

H2

EF

H2

EF

H3

EF

t3

t3’

G

G

(b)

Fig. 8. Effects of the framing on the scheduling delays wheret′

i− ti is the scheduling delay of the first frame for channeli: (a) For the schedule-time

framing and (b) for the arrival-time framing.

TABLE II

DEFAULT PARAMETER VALUES FORSIMULATIONS

Parameter Value Description

R 10 Gbps Line rate for upstream anddownstream transmissions

W 16 Number of ONUs and Wave-lengths

G 50 ns Guard band between adjacentSUCCESS WDM-PON frames(including tuning overhead oftunable transmitters and re-ceivers)

Q 10 Mbytes Size of OLT VOQs and ONUupstream traffic queue

ONU TIMEOUT 1 ms Expiration time of ONU timer

SUCCESS-HPON architecture with 16 ONUs. The ONUs aredivided into four groups with 4 ONUs per group and placedfrom the OLT 5 km, 10 km, 15 km, and 20 km, respectively.The line rateR for both upstream and downstream transmis-sions is set to 10 Gbps. TheONU TIMEOUT and the guardbandG are set to 1 ms and 50 ns respectively.

As for traffic modeling, we choose a simple Poisson processfor IP packet generation because the major purpose of the sim-ulations in this paper is to compare the relative performancesof the designed scheduling algorithms rather than to investigatethe actual performances under realistic conditions. The packetsize distribution is configured to match that of a measurementtrace from one of MCI’s backbone OC-3 links [14], and thedestination distribution for downstream packets at the OLTfollows a uniform distribution. The generated IP packets areencapsulated in Ethernet frames before their arrival at theOLTand ONUs. The size of VOQs at the OLT and the upstreamtraffic queue at the ONU is set to 10 megabytes. The defaultparameter values for the simulations are summarized in TableII.

For each scheduling algorithm, we ran simulations forseveral different configurations of tunable transmitters and

TABLE III

TUNABLE TRANSMITTER AND RECEIVER CONFIGURATIONS FOR

SIMULATIONS

Number of Number of Total Offered LoadTransmitters (M ) Receivers (N ) [Gbps]

4 2 1, 2, ..., 404 4 1, 2, ..., 408 4 1, 2, ..., 808 8 1, 2, ..., 80

receivers. Due to space limitation, however, we show thesimulation results for the chosen subsets of configurationssummarized in Table III in this paper. The total offered loadis the sum of the arrival rates for downstream and upstreamtraffic, where we fix the ratio of the former to the latterto 2 considering that there is more downstream traffic thanupstream traffic in access networks. The maximum load foreach configuration is set to the total transmitter capacity(= M × R), which slightly overloads the system.

We first investigate the effects of important control pa-rameters on the performances of the scheduling algorithms– the batch period for the BEDF and the maximum grantsize (MAX GRANT ) for the S3F – to determine optimalparameter values. Then, based on the optimal values of thosecontrol parameters, we compare the performances of the twoscheduling algorithms.

A. Effects of Batch Period on BEDF Performance

To investigate the effects of the batch period on the BEDFperformance, we ran simulations for three different batch pe-riods and show the throughput and delay results in Figs. 9 and10, respectively. In the simulations we setMAX GRANT to120 percent of the average amount of ONU incoming trafficduring a given batch period when the system load is maximum.For example, withM = 8, the maximum system load is80 Gbps and the average ONU incoming rate is 1.67 Gbps.For a batch period of 10 ms, because the average amount of

8 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. X, MONTH 2005

the ONU incoming traffic during this period is 16.7 Mbits,MAX GRANT is set to 20 Mbits. In this way we canminimize the effects ofMAX GRANT in our investigationof the effects of the batch period on the BEDF performances.

Note that MAX GRANT should be large enough tohandle the longest possible Ethernet frame. Otherwise thatlong Ethernet frame in an ONU upstream queue would blockthe whole upstream traffic from the ONU. Likewise, the batchperiod should be long enough to handle the maximum sizerequests. This implies that there is a limit in the minimumlength of the batch period.

From the simulation results we found that the batch periodof 1 ms provides the best overall performances of the threeperiods considered. Unlike our intuitive expectation, theeffectsof longer batch period on the actual transmission performancesare not always positive: As the batch period increases, thenegative effect of increasing delay becomes dominating overthe effect of better optimization in scheduling with a biggertask set consisting of more frames. In general, however,the performance differences are not significant especiallyinthroughput. We also observed that the number of receivers,given the number of transmitters, has minor impacts on theBEDF performances.

B. Effects of Maximum Grant Size on S3F Performance

In Figs. 11 and 12 we show the throughput and delay perfor-mances of S3F with four different values ofMAX GRANT .

In Fig. 11 we can see that the total throughput approachesthe total transmitter capacity in most of the cases exceptwhen the maximum grant size is less than 5 Mbits and thenumber of receivers is half the number of transmitters. Thishigher transmission efficiency has been achieved because aswe expected, the schedule-time framing combined with thegranting scheme efficiently reduces the framing and the guardband overhead in downstream transmission by encapsulatingmultiple Ethernet frames in one SUCCESS WDM-PON frame.Note that the upstream traffic is no longer penalized as in[1] by the downstream traffic even when the system is highlyoverloaded, which results from the memory space protectionby VOQing as well as the change in downstream framescheduling. From the results, we also observed that whenthe maximum grant size is less than 5 Mbits and there arefewer receivers than transmitters, the downstream throughputactually decreases after reaching its maximum as the systemload further increases.

Although the overall throughput and the fairness betweenupstream and downstream traffic have been greatly improvedby the granting with schedule-time framing, one may wonderwhether there is any downside in the delay performance dueto the effects of large transmission delay of possibly very longSUCCESS WDM-PON frames. From the delay performancesshown in Fig. 12, we can verify that this is not the case: Infact, the total average packet delay is maintained well below5 ms until the total offered load exceeds around 95 percentof the total transmitter capacity, again except for those caseswhere the maximum grant size is less than 5 Mbits and thenumber of receivers is half the number of transmitters. Unlike

traditional TDM-PONs, because there can exist multiple chan-nels simultaneously between the OLT and the ONUs in theSUCCESS WDM-PON, giant frames using one channel hardlyblock other frames. The effects of the number of simultaneouschannels between the OLT and the ONUs, which is directlyrelated with the number of transmitters and receivers, becomeclear when we compare the results in Fig. 12 where the averagepacket delay for the larger number of transmitters and receiversin (a) is less than that for the smaller number of transmittersand receivers in (b).

Note that the effects of the maximum grant size on thedownstream packet delay are opposite to those on the upstreampacket delay: As the maximum grant size increases, thedownstream packet delay decreases while the upstream packetdelay increases. In our simulations where the downstreamtraffic dominates over the upstream traffic, the opposite effectson the upstream traffic are negligible in the total averagepacket delay. But with different traffic conditions, one mayhave to take into account these opposite effects.

Also note that the initial dip in uptream packet delay underlight system load: Under the current granting scheme describedin this paper, there is no further grant frame generated for anONU when the ONU reports to the OLT no frame waiting inthe upstream traffic queue. Therefore the regular polling cycleof granting and reporting pauses until the ONU timer at theOLT expires, when the polling cycle is restarted by sendinga new polling message to the ONU. This whole procedurerelated with the ONU timer expiration results in the increasein upstream packet delay. This can be controlled by adjustingthe ONU TIMEOUT value or a new granting scheme thatgenerates a certain minimum size grant even when the ONUreports no frame in the upstream traffic queue.

In general the effects of the maximum grant size aresalient with less number of receivers for the given numberof transmitters, which is different from the results for theoriginal sequential scheduling algorithm where the numberof receivers, given the number of transmitters, has marginalimpacts on the overall performances: This is because, whenthere is less number of receivers, the polling period for theupstream traffic becomes longer and the maximum grant sizeis a more limiting factor in this case than in the case with alonger polling period.

Considering throughput and delay performances altogether,we can conclude that the maximum grant size of 5 Mbitsprovides the best overall performance for all the configurationsconsidered in the simulations.

C. Performance Comparison between BEDF and S3FScheduling Algorithms

We compare the performances of the BEDF and the S3Fwith the optimal parameter values – 1-ms batch period forthe BEDF and 5-MbitMAX GRANT for the S3F – throughsimulations and show the results in Figs. 13 and 14. The resultsfor the original sequential scheduling algorithm in [1] arealsoshown in the figures for the purpose of comparison.

From the results we can see that both the BEDF and S3Fgreatly improves the scheduling performances over the original

KIM et al.: SCHEDULING ALGORITHMS FOR WDM-PON UNDER SUCCESS-HPON 9

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Fig. 9. Throughput of BEDF scheduling algorithm for total, downstream and upstream traffic: (a) ForM = 8; (b) for M = 4.

sequential scheduling algorithm. The maximum achievabletotal throughput now approaches the total transmitter capacityand the upstream traffic is no longer penalized by the down-stream traffic as the system load increases. The performanceimprovement of the new scheduling algorithms becomes alsoclear in average packet delay. Both the BEDF and S3F main-tain the total average packet delay below 2 ms until the systemload exceeds 87.5 percent of the total transmitter capacityforall the configurations in consideration.

The comparison study shows that, of the two schedulingalgorithms, the S3F provides better overall performances thanthe BEDF in terms of both throughput and average packetdelay. Considering the lower complexity of the sequential

scheduling mode and the potential for better control of QoSand fairness through integrated granting of both upstreamand downstream traffic flows, we can conclude that the S3Fis a better choice for a scheduling algorithm in practicalimplementations.

V. CONCLUSIONS

We have presented the results of the design and performanceanalysis of the two new scheduling algorithms – BEDF andS3F – providing efficient and fair bidirectional communica-tions between the OLT and the ONUs in WDM-PON underthe SUCCESS-HPON architecture. The major design goal isto overcome the low transmission efficiency and the poor

10 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. X, MONTH 2005

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Fig. 10. Average packet delay of BEDF scheduling algorithm for total, downstream and upstream traffic: (a) ForM = 8; (b) for M = 4.

fairness guarantee between upstream and downstream trafficflows of the original sequential scheduling algorithm proposedin [1]. To achieve this goal, we adopt batch scheduling modein BEDF to do optimization in scheduling with a batch offrames. In S3F we maintain sequential scheduling mode as inthe original sequential scheduling algorithm but use grants fordownstream traffic, in addition to upstream traffic, togetherwith schedule-time framing to reduce the overhead due toframing and guard bands. We base both scheduling algorithmson VOQing to separate and protect memory spaces amongtraffic flows.

Through simulations we found that in BEDF, the effectsof the batch period on the throughput are not significant; on

the other hand, the average packet delay is strongly dependentupon the size of the batch period. The simulation results alsoshowed that the number of receivers, given the number oftransmitters, has negligible effects on the performances.Of thethree batch periods considered, 1-ms batch period providesthebest overall performances.

In S3F we investigated the effects of the maximum grantsize, (i.e., MAX GRANT ). The simulation results showedthat asMAX GRANT increases, the throughput also in-creases; as for delay, the downstream delay decreases but theupstream delay increases. In most of the cases we can see thatthe total throughput approaches the total transmitter capacityexcept when the maximum grant size is less than 5 Mbits and

KIM et al.: SCHEDULING ALGORITHMS FOR WDM-PON UNDER SUCCESS-HPON 11

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Fig. 11. Throughput of S3F scheduling algorithm for total, downstream and upstream traffic: (a) ForM = 8; (b) for M = 4.

the number of receivers is half the number of transmitters. Inthe case of unlimited granting, we observed minor decreasein downstream throughput only when the system is highlyoverloaded but the overall performance is as good as in thebest case. Considering the throughput and delay performancesaltogether, we found that the maximum grant size of 5 Mbitsis the best of the four values considered.

The comparison study with the optimal parameter values –1-ms batch period for the BEDF and 5-MbitMAX GRANTfor the S3F – showed that S3F provides better overall perfor-mances than BEDF in terms of both throughput and averagepacket delay, although the differences between the two are notsignificant. Considering the lower complexity of the sequential

scheduling mode and the potential for better control of QoSand fairness through integrated granting of both upstream anddownstream traffic flows, we can conclude that the S3F is abetter choice for practical implementations.

As for the fairness issue, we have mainly focused onthe guarantee of fairness between upstream and downstreamtraffic flows for a given ONU in this paper. We demonstratedthrough simulations that both BEDF and S3F can guaranteegood fairness between upstream and downstream traffic withproper choice of control parameter values except when thesystem is severely overloaded. Later in practice, however,fairness guarantee with QoS support among individual userconnections with a guaranteed minimum bandwidth and a

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Fig. 12. Average packet delay of S3F scheduling algorithm for total, downstream and upstream traffic: (a) ForM = 8; (b) for M = 4.

weight per connection will be important, which is beyond thescope of the current paper. In this regard the extension ofthe results in [15] for the cousin-fair hierarchical schedulingin access networks – mainly in the context of the current-generation TDM-PONs – to the case of the next-generationhybrid WDM/TDM networks with shared tunable transmittersand receivers, including WDM-PONs under the SUCCESS-HPON architecture, could be a solution and an interestingtopic for further research.

ACKNOWLEDGMENT

The authors would like to thank the Associate Editor andanonymous reviewers for their constructive comments. The

authors would also like to thank Mr. Salvatore Rotolo ofSTMicroelectronics for his encouragement and support for thiswork.

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Fig. 13. Comparison of throughput for total, downstream andupstream traffic: (a) ForM = 8; (b) for M = 4.

[4] K. S. Kim and L. G. Kazovsky, “Design and performance evaluationof scheduling algorithms for unslotted CSMA/CA with backoff MACprotocol in multiple-access WDM ring networks,”Information Sciences,vol. 149, no. 1–2, pp. 135–148, Jan. 2003, invited Paper.

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Fig. 14. Comparison of average packet delay for total, downstream and upstream traffic: (a) ForM = 8; (b) for M = 4.

Y. Ye, “Fair queueing with service envelopes (FQSE): A cousin-fairhierarchical scheduler for subscriber access networks,”IEEE J. Select.Areas Commun., vol. 22, no. 8, pp. 1497–1513, Oct. 2004.

Kyeong Soo Kim (S’89-M’97) received the B.S.,M.E., and Ph.D. degrees, all in electronics engi-neering, from Seoul National University, Seoul, Ko-rea, in 1989, 1991, and 1995, respectively. From1996 to 1997, he was engaged in development ofmulti-channel ATM switching systems as a Post-Docresearcher at Washington University in St. Louis,Missouri, where he also taught undergraduate andgraduate courses as an Instructor of WashingtonUniversity and Adjunct professor of University Mis-souri, St. Louis. From 1997 to 2000, he was with

the PON Systems R&D organization of Lucent Technologies as aMemberof Technical Staff and co-developed the first commercial APON-based Fiber-To-The-Home/Business (FTTH/B) system, which won the 1999 Bell LabsPresident’s Silver Award. Since 2001 he has been with STMicroelectronics,working on next-generation access and metro area networks as Researcher-in-Residence at Stanford Networking Research Center. Dr. Kim has served as

KIM et al.: SCHEDULING ALGORITHMS FOR WDM-PON UNDER SUCCESS-HPON 15

a Member of the Technical Program Committee for ICC 2005, STFOC 2005,GLOBECOM 2004, and JCIS 2005, 2003 and 2002. Dr. Kim is a member ofIEEE.

David Gutierrez (S’93) received the B.S. degreein electrical engineering from the Universidad delos Andes, Columbia, in 1998, and the M.S. degreein electrical engineering from Stanford University,Stanford, CA, in 2002. He is currently workingtoward the Ph.D. degree with the Electrical En-gineering Department, Stanford University. He haspreviously worked with such companies as Nortel,Reuters, BASF, and AT&T. At Stanford, he hasworked with the Stanford Learning Laboratory andthe Stanford University Medical Media and Informa-

tion Technologies (SUMMIT) Laboratory. He is a Member of thePhotonicsand Networking Research Laboratory (PNRL), where he is working on accessnetworks. Mr. Gutierrez is also a Fellow of STMicroelectronics, Stanford, CA.

Fu-Tai An (S’98-M’04) received the B.S. degreein electrical engineering from National Taiwan Uni-versity, Taiwan, in 1996, and the M.S. and Ph.D.degrees in electrical engineering from Stanford Uni-versity, Stanford, CA, in 1998 and 2004. Duringthe summer of 1999, he helped to start a company,Excess Bandwidth Company. He was a Memberof Research Staff of the Analog-Front-End Groupfor DSL applications. During the summers of 2000and 2001, he was with Sprint ATL to investigatehigh-performance optical transmission gears. He is

now with Marvell Technology Group Ltd. His research interests includephotonic networking, optical communication system design, wireless andwired communication system design, and mixed-signal circuit design. Dr.An received the IEEE Lasers & Electro-Optics Society (LEOS)JapaneseChapter Student Award at the IEEE OptoElectronics and CommunicationConf. (OECC).

Leonid G. Kazovsky (M’80-SM’83-F’91) is a Pro-fessor of electrical engineering at Stanford Univer-sity, Stanford, CA, since 1990. After joining Stan-ford, he founded the Photonics and Networking Re-search Laboratory (PNRL). Prior to joining Stanford,he was with Bellcore (now Telcordia) conductingresearch on wavelength-division-multiplexing, high-speed and coherent optical fiber communication sys-tems. He has authored or coauthored two books,some 150 journal technical papers, and a similaramount of conference papers. Dr. Kazovsky is a

Fellow of the Optical Society of America (OSA).