2007 IEEE International Conference on Signal Processing and Communications (ICSPC 2007), 24-27 November 2007, Dubai, United Arab Emirates

DDSPON: A DISTRIBUTED DYNAMIC SCHEDULING FOR EPON

Marilet De Andrade, Lluis Gutierrez, Sebastia Sallent

Universitat Politecnica de Catalunya, Spain[marilet, lluis.gutierrez, sallent]@entel.ucp.edu

ABSTRACT

Ethernet Passive Optical Networks (EPON) are a cost-effective solution for the bottle-neck problem in theaccess network. An open issue for this technology is thebandwidth allocation over the upstream channel in orderto provide differentiated services, fairness and efficiency.Several algorithms for dynamic bandwidth allocation inEPON have been proposed, but very few of them addressthe possibilities for an on-the-fly (immediate) distributedscheduling method. In this paper we propose and simulatea distributed scheduling method where the user devicesparticipate in the bandwidth allocation process. Theresults show a significant improvement in terms of delayand queue size for high network loads when compared toone of the most popular schemes.

Index Terms- Ethernet Passive Optical Networks(EPON), dynamic bandwidth allocation, distributedscheduling, access network.

1. INTRODUCTION

Passive Optical Networks (PONs) are the most attractivecandidates that are considered as the access networksolution to the current and future traffic demands. Apassive optical network (PON) is a subscriber accessnetwork technology that provides high bandwidthcapacity over fiber. Ethernet-PON is a point to multipointsubscriber access technology that transports Ethernetframes. The Ethernet protocol is highly deployed in localarea networks (LANs) and also it is becoming anemerging technology for metropolitan and wide areanetworks. Ethernet is an attractive protocol choice for theaccess networks because of its technological simplicityand customer familiarity.

An EPON is a point to multipoint (P2MP) networkwith a tree topology in most of the cases. The terminalequipment connected at the trunk of the tree is referred toas the Optical Line Terminal (OLT) and typically residesat the service provider's facility. The OLT is connected toa passive optical splitter using an optical trunk fiber,which fans out at the splitter to multiple optical dropfibers to which Optical Network Units (ONUs) areconnected. According to standard IEEE 802.3ah, anEPON supports a nominal bit rate of 100OMb/s, sharedamongst the ONUs, which can be at a maximum distanceof 20 km. There are two wavelengths - one for thedownstream and one for the upstream direction.

In an EPON, all downstream (from the OLT to theONU) Ethernet frames transmitted by the OLT, reach allONUs. ONUs will discard frames that are not addressedto them. In the upstream direction (from the ONU to theOLT) the signal transmitted from the ONU is receivedonly by the OLT. Since in the upstream direction thechannel is shared in time (TDM) between the ONUs, itmust be controlled to avoid collisions. The OLT arbitratesthe upstream transmissions from the ONUs by grantingtransmission windows, which can have variable lengths.An ONU is only allowed to transmit during the timeslotallocated to it. OLT informs the transmission window tothe ONU by means of a Gate message. In order to informthe OLT about its bandwidth requirements, ONUs useReport messages that are also transmitted (along with thedata) in the transmission window.

An open issue at the standard IEEE 802.3ah (EPONstandard) is the Dynamic Bandwidth Allocation (DBA)algorithm. DBA schemes have been considered as a wayof handling bandwidth and QoS requirements in EPON.Most of the works are based in a centralized algorithmthat runs at the OLT and allocates transmission windowsto every active ONU. One of the most referred wasdesigned by Kramer et al.[1] where the OLT distributesthe bandwidth on-the-fly according to each ONU'srequest and without considering the effect of other ONUson that distribution. Ma et al. [2] provides a uniformdistribution of slots over cycle according to the class ofservice It is able to guarantee delay and bandwidth but itis not very efficient and requires a complex calculation.Another approach suggested by Assi et al. relies in thecalculation of a fair bandwidth distribution taking intoaccount all the ONUs' requirements in the network [3]. Inthis case, there is a need to wait after each cycle oftransmission in order to get all the information from therest of ONUs, which may lead to more delays.

In this paper we propose a distributed schedulingmechanism in order to allocate bandwidth fairly andefficiently in an EPON. The control of the channel iscentralized because it is done by the OLT, but thescheduling process is distributed among the active ONUsover the PON.

2. BANDWIDTH DISTRIBUTION ANDSCHEDULING IN EPON

As mentioned in the previous section, most of theproposed algorithms for bandwidth allocation arecentralized.

1-4244-1236-6/07/$25.00 © 2007 IEEE 840

be sent to the ONUs from the OLT. With this vector, eachONU calculates the instantaneous transmission windowsize for itself. The information needed by the ONU canbe sent through the Gate message in the PAD/Reserveheader fields available. We call this method DynamicDistributed Scheduler for EPON (DDSPON). Each ONUwill send to the OLT one extra parameter (its weight) inthe Report message so that the OLT can update theweight vector. In this paper we will study a case of onequeue per ONU, but it can be easily extended to severalqueues per ONU.

In a PON system there are N ONUs, and each ONU ihas a predefined (nominal) weight Di. The nominalweight is used to define ONU's transmission window size(in bytes):

WI = IN Wj=l

(1)

Figure 1. Illustration of the Interleaved Polling mechanism

Among the centralized approaches, the work ofKramer et al. [1] is considered as a reference with the so-called IPACT (Interleaved Polling with Adaptive CycleTime) algorithm. This algorithm is also taken as referencein our proposal, and we describe it here briefly. By meansof Gate and Report messages, the OLT fills a table withthe transmission window size requirements of every ONU(among other parameters). IPACT uses an interleavedpolling approach where the next ONU is polled before thetransmission of the previous one finishes.

The main idea is to keep the channel occupied asmuch as possible in order to obtain an efficient use of it.As the downstream channel is independent from theupstream channel, the OLT can inform an ONU the start-time of its timeslot before the previous transmission ends.When OLT receives a Report message from any ONU, itimmediately computes the grant time based on predefinedmaximum bandwidth. If the requested timeslot is biggerthan its maximum timeslot permitted (according to themaximum bandwidth), the OLT grants the maximumtimeslot for that ONU, otherwise it grants the requestedtimeslot. The maximum cycle time is the maximum timerequired for all ONUs to transmit their respectivemaximum predefined timeslot. So, in IPACT the cycle isvariable.

In our scheme the interleaved polling mechanism isalso applied. However, in our case, the size of thetransmission window for the ONUs is calculated by eachrespective ONU. Extra information about the weightvector ((I) for the ONUs is included in the Gatemessages, as can be seen in figure 1. Our approach willbe explained with more detail in the next section.

where Wvmx is the maximum transmission window sizethat corresponds to the maximum cycle time. The cycle isthe period during which all active ONUs have transmittedtheir traffic, when a new cycle starts again. So, the OLTguarantees the instantaneous window size Ii WMA,x) if:

J= (2)

The algorithm for DDSPON is explained as follows:1. The OLT receives a Report message (from ONU i)

that contains two values: Ri(n) (requested window size forcycle n) and P1(n) (the weight for cycle n): The OLT thenupdates its vector of weights for the cycle n:

ONU | ONU2 ONU3 ...... ONUN(T I(n) I 12(n) 1 (3(n) ...... | N(n)

and proceeds to send a gate message to ONU igranting Ri(n) and including the previous vector in themessage.

If the EPON system has M QoS classes, the gatemessages would include a (NxM) matrix:

(5)

being N the number of ONUs andM the number of QoSclasses. In this description we consider M=1.

2. When the ONU i receives the Gate message, ittransmits the data in the queue up to the granted windowsize (in bytes). Then the ONU takes new weight vector,sets its own weight to the nominal one Di, and calculatesthe new maximum window size that such ONU can takeon cycle n+±:

3. REMOTE DISTRIBUTED SCHEDULER

The bandwidth scheduling algorithm we propose isperformed mainly by each active ONU in the EPONnetwork. An extra information (the weight vector) must

1=l (DIj (n)(6)

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((DII(n) (Dlm(n)"(D(n) =

(D,l (n) (D. (n))

The ONU also calculates the required transmissionwindow size:

(7)

where Qi is the queue size in ONU i at that moment.Finally the ONU sends to the OLT (inside the Reportmessage) two values: Ri(n+±), and the new weight forthe next cycle:

Ri (n + 1) oj (n)

WA,MX(8)

where the addition of weights in (8) is the same usedfor (6).

The process is performed in the same way by everyONU. At the initial operation point, the weight of theONU i is set to be nominal one: 'i.

It should be noticed that the scheduling is executedon-the-fly. In this case, there is no need to wait until allthe reports arrive to the OLT in order execute thescheduling algorithm.

Another interesting remark is that the ONU willallocate the right size of bytes that can fit inside themaximum transmission window size after calculating (7).There is no need to inform the OLT about thresholds inthe ONU's queues. Here the ONU is the one whoschedules dynamically the size of its transmissionwindow fixing it to the real number of bytes in eachEthernet frame, which cannot be fragmented.

4. SIMULATION RESULTS

We have studied the DSSPON scheme by means ofevent-driven simulations in C++. We propose the analysisoftwo scenarios. The first scenario of the system is a treetopology with 16 ONUs, each of them separatedrandomly from the OLT over the interval from 10km to20Km. That leads to a round-trip time that ranges from50j.s to 100pts. We chose a maximum cycle size of 2ms.All the sources are set with a rate of 62,5Mbps (includingprotocols overhead), and the channel capacity is 1Gbps.The nominal weight for every ONU is set to 1/16. Thetraffic is self-similar as defined in [1]. The ONU's bufferis long enough to avoid packet drops.

In the simulations we compare the performance ofthe proposed algorithm with IPACT algorithm. In figure2 we present the results regarding the average packetdelay and the average queue size using self-similartraffic.

In Figure 2(a) IPACT experiences an abrupt stepwhen the network load is bigger than 0.8 arriving to anaverage delay of 0.6 seconds, where the maximum delayarrives to 0.85 seconds. In the case of DDSPON, thebehavior is very smooth and shows lower delays forhigher network loads compared with IPACT.

1000 -

100-

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0 0,2 0,4 0,6 0,8 1

Load

2(a) Average packet delay and maximum packet delay

1 ,OOE+07 -

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D 1,OOE+05-I.-PACT

@ 1,OOE+04 -DDPON

m 1,OOE+03

> 1,OOE+02

1,OOE+010 0,2 0,4 0,6 0,8 1

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2(b)Average queue size

Figure 2. Delay and average queue size as function of thenetwork load

In Figure 2(b) DDSPON also presents a smoothcurve related with the average queue size. With IPACTalgorithm, the system would require a bigger resources inorder to avoid packet dropping, compared to DDSPON.In this scenario we can see the main contribution of ourdistributed approach that shows an improvedperformance at high loads.

We introduce a second scenario that will allow us toanalyze the transient response. In this scenario we takeagain a tree topology with only three ONUs: ONU 0,ONU 1 and ONU 2. Each ONU has a CBR sourcegenerating frames of 700 bytes, and their rates changeover time according to figure 3(a). Each ONU has anominal weight of 1/3.

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600 N 0

GNU =

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200

100

0 1 2 3 4

Time (seconds)

3(a) Source traffic rates for ONUs 0, 1 and 2

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R, (n + 1) = AIIIN(W, (n + 1), Qi)

3(b) Cycle size distribution over time for DDSPON and IPACT

Figure 3. Scenario for studying the time behaviour of thealgorithms: DDSPON and IPACT

In figure 3(b) we show the changes of the total cyclesize over the simulation time. The maximum cycle sizethat corresponds to 2ms is 250000 bytes including theguard times between ONUs' transmissions. In IPACT, as

the algorithm only grants as a maximum 250000/3(83333) to each ONU, when the traffic of any of theONUs is low, then the total cycle decreases. And as theONU's traffic fluctuates also the cycle size varies in thesame way. And even when, for example, between 2 and 3seconds there are only two sources generating traffic,IPACT only allows the maximum size to each respectiveONU, giving a total of 166666 bytes. In the case ofDDSPON, the distributed exchange of the changingweights creates a dynamic and fair distribution of thebandwidth for each cycle. As can be seen the cycle sizeremains almost constant.

Figure 4. Average packet delay per ONU (ONUs 1 and 2).A comparison over the proposed simple test scenario

cycle is 2 ms. and the queue delay adds some delay equalor less than half of the cycle. ONU 1 experiences similardelays for both cases.

5. CONCLUSIONS

In this paper a novel distributed dynamic bandwidthallocation algorithm, called Distributed DynamicScheduling for EPON (DDSPON), is presented forEthernet-based Passive Optical Networks. Using a simplealgorithm, the ONU (user device) is able toproportionally schedule its transmission window sizebased on its current queue requirements and therequirements of the rest of the ONUs.

The simulation results confirm that this algorithm isa good alternative to centralized schemes. Especially itoutperforms in terms of delay and queue size at highnetwork load. Also it has been shown that it fairlydistributes the bandwidth when the traffic rates are

changing. However, the proposed scheme requiressending extra information inside the control messages(Report and Gate).As future work we plan to study the performance of theproposed mechanism defining several QoS classes.DDSPON can be easily extended to the WDM/TDMPON, and we keep this issue as the next step of our

research.

6. ACKNOWLEDGEMENTS

This research was supported by the Spanish Ministry ofEducation and Science, and FEDER within theframework of the projects TS12005-06092 and TS12006-12507-C03-03.

7. REFERENCES

[1] G. Kramer, B. Mukherjee and G. Pesavento, "IPACT a

dynamic protocol for an Ethernet PON (EPON)", WEEECommunications Magazine, vol. 40, issue. 2, pp. 74-80, Feb2002.

[2] M. Ma, Y. Zhu and T. Cheng, "A Bandwidth GuaranteedPolling MAC Protocol for Ethernet Passive Optical Networks",EEE INFOCOM 2003, vol. 1. pp. 22-3 1, Mar 2003.

[3] C. Assi, Y. Ye, S. Dixit, and M. Ali, "Dynamic BandwidthAllocation for Quality-of-Service Over Ethernet PONs", IEEEJournal on Selected Areas in Communications, vol. 21, issue9,pp. 1467-1477, Nov 2003.

In figure 4 it is shown the average packet delay perONU. The results for ONU 0 are not represented as theyare very small: 1.8ms. with IPACT and 2.6ms. withDDSPON. The ONU 2 that is under big congestion oftraffic experiences less average with DDSPON than withIPACT. The difference in this case is 126.6ms. As for thebigger delay in ONU 0 for the case of DDSPON, this isthe maximum delay when all the bandwidth is occupiedby the rest of the network devices. So this is the delayexpected by ONU 0 in normal conditions because the

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Time (microseconds)

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0 400 -EMIPACT300-

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ONU 1 ONU 2