Optical burst switching with burst access mode

passive optical networks

C. Y. Li,' P. K. A. Wai,' and Victor 0. K. Li2'Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hong

Kong, China, {enli, enwai} @polyu.edu.hk2Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong,

China, vli@eee.hku.hk

Abstract In this paper, we investigate the integration of passiveoptical networks (PONs) with optical burst switched (OBS)networks. Owing to the decomposition of the operations betweenthe PONs and OBS nodes, serious problems have been observed.One of them is data burst assembly problem. In some situations,burst assembly delay exceeding tens seconds may be required if thegeneral purpose PON access schemes are used. To solve thisproblem, we propose to have the optical network units (ONUs)take over the burst assembly function from the OBS nodes. Largereduction of the burst assembly delay is obtained. Four ONU databurst scheduling schemes have also been investigated. We observethat a simple scheduling scheme is itself adequate in mostsituations.

Index Terms-Optical Burst switching, passive optical network,burst assembly

I. INTRODUCTION

Optical burst switching (OBS) has been gaining popularity inrecent years because it can be implemented with currenttechnology [1], [2]. For better bandwidth utilization, the trafficbetween OBS nodes are data bursts which are groups ofmultiple packets. OBS networks use one-way reservation fordata transmission [3]-[5]. When a data burst is generated at anode, a control packet is sent immediately to the destination ofthe data burst. Following a pre-determined path, the controlpacket reserves the resources for the data burst at the nodes onthe path. No acknowledgment for the data burst is sent back.After an offset time, the node sends out the data burst using thesame routing path of the control packet. If the reservation by thecontrol packet is successful, the data burst will pass through allnodes on the path without any processing andoptical-to-electrical (O/E) conversion. No optical buffers aretherefore required. Compared to other proposed optical packetswitching methods, OBS is more likely to be implemented soon.Unlike the connection-oriented wavelength-routed opticalnetworks, the large propagation delay between nodes is nolonger a major system performance concern in OBS.

The one way reservation in OBS is designed to minimize the

This research is supported by a grant from The Hong Kong PolytechnicUniversity (Project Number 1-BBZB).

requirement of optical hardware. It also reduces the complexityof signaling between nodes and shortens the waiting time of thedata bursts at the source nodes. The main rationale for the use ofone way reservation scheme in OBS is that the average databurst transmission time is much smaller than the propagationdelay between nodes. Another rationale is that the processingtime of the control packets at intermediate nodes can also besignificant when compared to the transmission time of the databursts. In either cases, feedback acknowledgment schemes willbe inefficient. Because of these reasons, the burst assemblyfunction in OBS has been considered separately from thechannel reservation [3]-[5]. In OBS networks, a packet arrivingat a node will be stored in an electronic buffer until the numberof packets with the same destination reaches a threshold valueor the first packet in the batch exceeds the storage time limit.Then a data burst is built and is handled with the OBS one wayreservation.A passive optical network (PON) is an important component

of broadband networks but its operation is separated from themain network [6]. For example, the user packets are transmittedin optical format on the Ethernet PON but have to be convertedback to electrical format in the main networks for furtherprocessing [7]. When the traditional PONs are used as theaccess networks for an OBS node, multiple O/E and E/Oconversions and packet buffering are required. This increasesthe system implementation cost and also causes unnecessarydelay to the packets and data bursts. Owing to thedecomposition of the operations between the PONs and OBSnode, the problems caused can be much more serious thansimply the duplication of hardware and additional bufferingdelay. One of them is the data burst assembly problem. Toaccommodate the special requirement of quality of service, onlythe packets from the same ONU may be grouped into a databurst. Using a common general purpose PON dynamicbandwidth assignment (DBA) scheme [7], the time required forbuilding a data burst in the OBS node can be up to hundreds ofseconds. To solve this problem, some OBS operations such asthe burst assembly may be pushed to the PONs. The PON DBAshould also be adjusted to satisfy the requirement of OBS.

In this paper, we consider OBS networks with PON access. InSection II, we first describe the problem of OBS PON

Proceedings of the 2007 15th IEEE Workshop on Local and Metropolitan Area Networks1-4244-11 00-9/07/$25.00 ©2007 IEEE 54

packets

*E E _

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PON1

PON2

PON3

PONh

opticalONU piiT ower splitter

_'0 ~data burstONU

I ONU OLT

ON .k T T,"c ---

Fig. 1 An OBS node with two passive optical networks.

integration in detail. It is demonstrated with simulation that theburst assembly delay can be up to tens of seconds in some

occasions. We therefore propose to have the optical networkunits (ONUs) take over the burst assembly operation from theOBS nodes in Section III. We then introduce four ONU databurst scheduling schemes in Section 111-A to 111-D. InSection IV, we use simulations to evaluate the proposedscheduling schemes. We observe that the simple schedulingscheme is itself adequate in most situations. We conclude inSection V.

II. OBS NODE WITH PASSIVE OPTICAL NETWORKS

Figure 1 shows an OBS node with two passive opticalnetworks (PONs) attached. There are three optical networkunits (ONUs) at each PON in the figure. The number of ONUs,however, can be up to 16 or 32 depending on the standards [6],[7]. Each ONU is connected to the OBS node via a passiveoptical power splitter. Owing to the optical power splitting, thedistance between an ONU and the OBS node is normally lessthan 20 km. There is an optical line terminal (OLT) for eachPON at the OBS node to handle the data transmissions betweenthe ONUs and the OBS node. There is one upstream (ONUs toOBS node) channel and one downstream (OBS node to ONUs)channel that are shared by all ONUs. The transmissions from theOBS node can be received simultaneously by all ONUs butthose from an ONU are received only by the OBS node. Hence,no ONU knows the status of other ONUs without help from theOLT at the OBS node. To prevent contention of transmissionsfrom different ONUs, the OLT must schedule only one ONU totransmit at any time. Also, a guard time of 1 to 5 ptS is requiredbetween the transmissions from different ONUs in order toreduce the signal crosstalk. The role of the OLT of each PON isto balance the packet waiting time at each ONU and thebandwidth utilization of the upstream channel [6], [7].

Figure 2 shows a possible architecture of the OBS node. Weassume that there are h PONs connected to the OBS node whichhas k output links. For convenience of illustration, we omit theinput links and the control packet processing unit in Fig. 2. Wealso treat a wavelength channel as an output link and assume

that full wavelength conversion is available in the node, i.e., twoor more output links in Fig. 2 may connect to the same

destination node. Each OLT in Fig. 2 monitors its associatedONUs and periodically adjusts the upstream channel

Fig. 2 A possible architecture of the OBS node.

transmission bandwidth allocated to the ONUs according tosome dynamic bandwidth assignment (DBA) schemes, e.g.,

interleaved polling with adaptive cycle time (IPACT) [7]. SinceOBS networks can be built with optical switches of largeswitching time [1]-[5], the packets received from OLTs are

grouped into data bursts of multiple packets before being sent totheir destination to reduce the switching overhead atintermediate nodes. Apart from the packet destinations, theburst assembly (packet grouping) can be based on other criteriasuch as the requirements of packet priority, security, and qualityof services (QoS). The maximum number of data bursts q beingin assembly can therefore be larger than the number of nodes Nin the network. In Fig. 2, an h x q electronic switch SWE isshown to connect the h OLTs to the q burst assembly queues

BAs. In practice, all the electronic switch and burst assemblyqueues can be replaced by a common memory pool if both the hand q are small.To minimize the waiting time of the data bursts at the source

nodes, control packets can be sent out at the beginning of theassembly of the data bursts. The data burst length informationcarried in the control packets, however, will not be accurateusing this approach. Overestimating the burst data length isrequired to avoid the unnecessary discards of data bursts atintermediate nodes. The time for burst assembly is alsorestricted because the data bursts must be sent out at the timedictated by the control packets. As all these will lower thenetwork bandwidth utilization, the control packets should besent at the completion of the data burst assembly unless thepacket delivering time is critical or the packet arrival is highlypredictable. In Fig. 2, a completed data burst will pass throughthe q x p multiplexer MUX and then the p x k optical switchSWO to the appropriate output link. In some proposed OBSnetworks, the optical switch is considered as part of the pure

optical routing node, i.e., the core node, and the burst assemblybelongs to an edge node that has electronic packet processingcapability. In this paper, we assume that an OBS node has bothfunctions of the edge node and core node.One can observe from Figs. 1 and 2 that some functions of the

ONUs and OBS node are duplicated. For example, the ONUshave to buffer the packets before transmission to the OBS node.The OBS node also has to buffer the incoming packets to build a

data burst, and waits for the output link to become available.Since electronic memory is required for the packet buffering,

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-01

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-03

packets have to pass multiple O/E and E/O conversions beforethey can be sent out in data bursts. Apart from the duplicatedhardware, this causes additional delay to the packets and theassembly of the data bursts. In some situations, however, wefind that much more serous problems can be caused by sucharchitecture in addition to the packet buffering delay.Owing to the requirements of security and QoS, a data burst

can contain packets only from the same ONU. In such situations,the required delay to burst assembly can be significant. Figure 3shows the required burst assembly delay for building the databursts with the node architecture shown in Fig. 2. For PON with16 ONUs and average burst size of 20 packets, the required timeto complete a data burst starting from the first packet enteringthe queue is plotted as the curve with crosses. We assume thatthe Gated IPACT scheme is used for the upstream channelbandwidth assignment [7]. The upstream (ONU to OLT)channel is 1 Gbps and the channel from each user to the ONU is100 Mbps. The packets arrive at each ONU as Poissondistribution and the average packet size is 1000 bytes. Weassume that the PON is attached to a node in the network ofFig. 4. A new packet will choose an ONU and one of the 13destination nodes at random. For the best performance of theGated IPACT scheme, we assume that the propagation delaybetween ONUs and OLT is zero. The size of polling messagesand the guard time between transmissions from different ONUsare also zero. From Fig. 3, the burst assembly delay can be up totens of seconds. In Gated IPACT, the number of packets perONU transmission grows with the loading. Hence, the burstassembly delay decreases with the loading increase but it is slow.It is because increasing the system loading also extends thepolling cycle. When the loading approaches one, a singletransmission of an ONU may contain all the required packets fora data burst. Hence, the burst assembly delay drops significantlyin such situation but the increased packet waiting time at theONU will largely offset the savings.

III. ONU BURST ASSEMBLY AND SCHEDULING

To solve the problem we described in Section II, we proposeto have the ONUs take over the burst assembly operation fromthe OBS node in such situations. We assume that the ONUs alsocheck the requirement of the incoming packets apart fromstoring them for transmission to the OBS node. If an ONUdetermine that some packets should not be mixed with packetsfrom other ONUs, it starts the burst assembly process for thesepackets instead of sending them out. The accuracy of the packetdetermination will have significant impact on the systemperformance. In this paper, however, we assume prefect packetdetermination. In Fig. 3, the curve with circles represents theburst assembly delay for the proposed approach, i.e., burstassembly at the ONUs. Since the burst assembly is no longerinterfered with other ONUs, we observe a ten times reductioncompared to the approach of burst assembly at OBS node.Upon completion of the assembly of a data burst, the data

burst is ready to be sent to the OBS node without furtherprocessing by the OLT and the burst assembly queues. Since the

-burst assembly at OBS node|Fburst assembly at ONUs4

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cZ 10

2!,

E

m 10

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Fig. 3 The required delay for different burst assembly approaches.

OBS node output links are shared by both transit data bursts(input links for transit data bursts are omitted in Fig. 2) and thenew data bursts, we have investigated four scheduling schemesfor the ONU data burst transmission with different assumptionsof the OBS node.

A. Electronic buffering for output contention resolutionWe assume that an electronic buffer is available at the OBS

node to temporarily store the new data bursts if the requiredoutput links are busy. The ONU data burst scheduling is toavoid the conflict between transmissions from different ONUs.It is similar to that of traditional PONs but the operation is morecomplicated. We assume that the OBS node is notified once adata burst is ready at an ONU. According to the OBS scheme [4],the minimum offset time between a data burst and its controlpacket at the source node can be represented as

Toff = H x Tcp + Tsw, (1)

where H is the number of intermediate nodes, Tcp is the controlpacket processing time at intermediate node, and Tsw is therequired optical switch reconfiguration time at the intermediatenodes. Let the propagation delay between the ONU and OBSnode is much smaller than Tcp and Tsw. If either the requiredoutput link or the PON upstream channel is not available aftertime Toff, the data burst will be scheduled to be sent to the OBSnode using the latest idle period of the PON upstream channel.The data burst then is stored in the electronic buffer until theoutput link is available. To minimize the data burst transmissiondelay, however, the OBS node will send the control packetimmediately if both the required output link and the PONupstream channel are available after time Toff. Hence, the databurst can be sent from the ONU to the destination node withoutadditional delay at the OBS node. Since the scheduledtransmission time of the data bursts is fixed after the controlpackets being sent, the idle period of the PON upstream channelwill be highly fragmented if the Tcp is large. The OBS nodetherefore has to keep a record of the fragmentation condition ofeach PON upstream channel. An idle time window search andrecord update are required whenever the OBS node is to

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schedule an ONU data burst.

B. Advance scheduling for output contention resolutionSimilar to Section III-A, the OBS node will send the control

packet immediate if both the required output link and the PONupstream channel are available after time Toff when the databurst is ready at the ONU. However, no buffer is assumed in theOBS node for output link contention resolution. If either theoutput link or the PON upstream channel is not available aftertime Toff, the data burst will stay in the ONU until a suitable idletime window is found in both the output link and the PONupstream channel. Similar to Section III-A, the idle period of thePON upstream channel may be highly fragmented. The searchof the suitable idle time window for the ONU data burst delayedtransmission, however, is more complicated because the statusof the output link must be considered during the operation.Since the idle period of the output link can also be highlyfragmented if the latest available unused channel with voidfilling (LAUC-VF) type channel reservation schemes are used[5], the required computation and storage for the search of thesuitable idle time window will be much larger than that thescheme proposed in Section 111-A.

C. Simple scheduling for output contention resolutionThis scheme is similar to that of Section III-B but the search

of the suitable idle time window for the ONU data burst delayedtransmission is simplified. In this scheme, the OBS node usesonly a channel available counter tca to record the time when aPON upstream channel has no scheduled transmission. When adata burst is ready at an ONU, the OBS node will send thecontrol packet immediately if the counter tca has a smaller valuethan the current time plus Toff, and the required output link isalso available at that time. Otherwise, the data burst has to stayat the ONU to wait for the output link to become available whenthe time is larger than the counter tca. Then, the data burst isscheduled to be sent at that time and the counter tca is updated tothe end of the transmission of that data burst. Although someidle periods in the PON upstream channel may be wasted, thescheme is simple and requires much smaller computation andstorage.

D. No scheduling for output contention resolutionIn some implementations of the OBS network, the dashed

box in Fig. 2 becomes an edge node and the optical switch is ina pure optical routing node, or core node [5]. No blocking isassumed between the edge node and its connected core nodes.In such implementations, however, the edge node will not havethe detail record of the output link status of the core nodes. Itwill not have large performance improvement even if the OBSnode has considered the status of the optical switch output linksbefore scheduling the ONU data burst transmission. To reduceunnecessary hardware and processing, the OBS node mayconsider only the PON upstream channel utilization whenscheduling the ONU data burst transmission. Hence, thescheduling mechanism is similar to that of Section 111-C butwithout the output link status information.

Fig. 4 The NSFNet (1991) network topology. There are 14 nodes and21bi-directional links. The original map of the network is available fromthe Internet (ftp://ftp.uu.net/inet/maps/nsfnet/) at August 2005.

IV. PERFORMANCE EVALUATION

We use simulations to study the performance of the proposedONU data burst scheduling schemes when the network has onlydata bursts that require all packets from the same ONU. At thesimulations, we use the network topology of NSFNet (1991)shown in Fig. 4. We assume that each PON has 16 ONUs andeach OBS node has 4 PONs attached. In each PON, there is onlyone upstream (ONU to OBS routing node) channel shared by allONUs. Each ONU can transmit data bursts to the upstreamchannel under the scheduling of the OBS node. To preventsignal crosstalk, there is a guard time (Tguard) between the databursts from different ONUs. In the simulations, Tguard has beenset to 0.01 of the average data burst transmission time. The PONdownstream (OBS node to ONUs) channel is shared by allONUs with time division multiplexing. No data burst contentionis therefore assumed at the PON downstream channel. EachONU connects to its users with multiple data links. Thetransmission rate of such data links is assumed to be a factorRONU of that between an ONU and its OBS node. In thesimulations, RONU is set to 0.1.We assume that the packets arrive at ONUs as a Poisson

distribution and the packet size is exponentially distributed witha mean of 0.5 time unit. Hence, the offered load AONU to anONU is equal to half of the average packet arrival rate. Weassume that all ONUs have the same offered traffic. Since anONU user side has data transmission rate RONU times theupstream/downstream channel, the offered traffic at a node isAOBS = NPONNONUAONU/RONU, where NPON is the number ofPONs per node and NOUN is the number of ONUs per PON. Thenormalized loading is the ratio of a node offered traffic to thenumber of wavelength channels (W) in an OBS node output link.In the simulations, W is set to 4. The AONU varies from 0 to0.625 when we change the normalized loading from 0 to 1(NPON= 4, NONU= 16, and RONU= 0.1). The average data bursttransmission time is set to one time unit. The number of packetsper data burst is a Poisson random number of mean Nburst = 20.When a new packet arrives at an ONU, it randomly chooses a

destination from the rest of the nodes in the network. Once thefirst packet of a destination is received in the simulation, the

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0.2F

0.1

"0 0.2 0.4 0.6 0.8 1loading

Fig. 5 The throughput of OBS with different ONU data burst schedulingschemes on the NSFNet topology (shown in Fig. 4}) network. Themaximum extra data burst delay at the source node is set to 10 time units.

ONU generates a Poisson random number of packets per burstfor that destination. A data burst of that destination is generatedwhen sufficient number of packets with that destination havebeen received. Note that each data burst has its own randomnumber of packets per burst. We assume that the propagationdelay from each ONU to its OBS node is a random number thathas a mean of 0.1 time units. When a new data burst is ready atan ONU of a node, if the required output link and the upstreamchannel are available after time Toff, a control packet is sentimmediately to reserve the required wavelength channels on thepath. The data burst is then transmitted after an offset time Toffaccording to the value calculated with Eq. (1). If two or more

wavelength channels are available at an intermediate node, thechannel with latest available time is chosen, i.e., LAUC-VF [5].If there is no output channel available at one of intermediatenodes, both the control packet and data burst are discarded atthat node. In the simulations, the control packet processing timeis set to one average data burst transmission time (Tcp = 1.0).Because we are only interested in the performance of the ONUdata burst scheduling schemes, the switching overhead at allnodes is simply assumed to be negligible (Tsw = 0). We alsoassume that data bursts are routed to their destinations usingminimum hop routing. The propagation delay of a link is set to100 time units. All simulations are run sufficiently long suchthat the 95% confidence intervals are less than 1% of thecorresponding averages.

Figure 5 is the throughput of the OBS with different ONUdata burst scheduling schemes on the NSFNet topology (shownin Fig. 4) network. The curves with pluses and circles are thosewith the advance and simple ONU data burst schedulingschemes, respectively. We plot the performance of electronicbuffering as the solid curve in Fig. 5. The dashed curve in Fig. 5is the throughput performance of ONU data burst schedulingwithout buffering or scheduling for the output link contentionresolution. The maximum extra data burst delay at the source

node is set to 10 time units. From Fig. 5, we observe that all

-electronic buffering7 advance scheduling

simple scheduling

6 ---no scheduling

>5

-0

23x

2 /

) 0.2 0.4 0.6loading

0.8 1

Fig. 6 The extra burst delay of OBS with different ONU data burstscheduling schemes on the NSFNet topology network. The maximum extradata burst delay at the source node is set to 10 time units. The notations are

similar to that in Fig. 5.

proposed schemes have similar performance when the loading isbelow 0.4. It is because most data burst discards in LAUC-VFoccur at the last intermediate nodes of the path unless the Tcp issmall [5]. From loading 0.4 to 0.8, the advance and simple ONUdata burst scheduling schemes have similar throughputperformance that is slightly better than that of the schemewithout scheduling for output contention resolution, but notquite as good as that of the electronic buffering. When theloading increases further, the throughput of the simple ONUdata burst scheme drops, and it is finally smaller than that of thescheme without scheduling for output contention resolution.We have investigated the utilization of the extra delay of the

data bursts in solving the output link contention and PONupstream channel transmission. From Fig. 6, we observe that themajor part of the extra burst delay is due to the PON upstreamchannel reservation. Comparing the delay curves of electronicbuffering and the scheme of no scheduling for output contentresolution, we observe that the required delay for a data burst toavoid output contention at the source node is very small if thedata burst has been stored in the electronic buffer. Because allnew arrival data bursts can only access the PON upstreamchannel sequentially, the delay of a data burst at the upstreamtransmission accumulates to the subsequent data bursts.Therefore, larger delay is required for the two ONU data burstscheduling schemes to solve the output link contention even ifthe system loading is not high. From Fig. 6, the simplescheduling scheme needs more delay if the loading is high. Ittherefore has larger throughput drop when the delay approachesthe limit.From simulation results including Figs. 5 and 6, we observe

that the scheduling scheme with electronic buffering (SectionIII-A) always has the best performance. This scheme is alsolikely to be used in most OBS nodes with PONs because not alldata bursts require all packets from the same ONU. Theelectronic buffers and burst assembly queues will probably stillbe required in most of the OBS nodes. The scheduling scheme

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-electronic bufferingadvance schedulingsimple scheduling

---no scheduling

0.6

0.5F

0 0.40)

2O 0.3

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() Iv D:

without output contention consideration (Section III-D) showsan extreme. It should be used if the OBS node has beenseparated into edge and core nodes, or most of the data burstsare discarded at the last intermediate nodes of the path, e.g.,LAUC-VF without small Tcp.

The advance and simple scheduling schemes (Section III-Band III-C) move the data burst buffering function from the OBSnode to the ONUs. While the two schemes reduce the hardwarecomplexity of the OBS node, they also have the throughputperformance similar to that of electronic buffering in a largerange of loading. The two schemes will have similar throughputperformance if large delay is permitted. Since we can have largesavings (over a ten times in Fig. 3) in the burst assembly delayafter pushing the burst assembly to the ONU, this allows us toincrease slightly the allowable delay in the scheduling. Forexample, if the parameters of Fig. 3 are used, the unit time inFig. 6 is equal to 0.16 milliseconds. There will be aninsignificant increase to the total delay even if a hundred timeunits are used for the scheduling delay. In general, the simplescheduling scheme is itself adequate in most situations.

V. CONCLUSION

In this paper, we have investigated the integration of passiveoptical networks into the optical burst switched network. Owingto the requirements such as QoS and security, we observe thatthe burst assembly time can be very large in some situations. Wetherefore propose to have the optical network units (ONUs) takeover the burst assembly function to reduce the burst assemblytime and the required hardware at the OBS nodes. Moreover,four schemes have been investigated to schedule the data bursttransmissions from the ONUs to the OBS nodes. From thesimulation results, we observe that a simple scheduling is itselfadequate in most situations.

REFERENCES

[1] J.Y. Wei, and R.I. McFarland, Jr., "Just-In-Time Signaling for WDMOptical Burst Switching Networks," Journal of Lightwave Technology,Vol. 18, No. 12, pp. 2019-2037, 2000.

[2] Y. Sun, T. Hashiguchi, N. Yoshida, X. Wang, H. Morikawa, and T.Aoyama, "Architecture and design issues of an optical burst switchednetwork testbed," Proceedings ofOECCICOIN 2004, pp. 386-387, 2004.

[3] J.S. Tuner, "Terabit burst switching," Journal of High Speed Networks,Vol. 8, pp. 3-16, 1999.

[4] C. Qiao, and M. Yoo, "Optical burst switching (OBS) - a new paradigmfor an optical Internet," Journal of High Speed Networks, Vol. 8, pp.69-84, 1999.

[5] Yijun Xiong, M. Vandenhoute, and H.C. Cankaya, "Control architecturein optical burst-switched WDM networks," IEEE Journal on SelectedAreas in Communications, Vol. 18, No. 10, pp. 1838-1851, 2000.

[6] T. Koonen, "Fiber to the home/fiber to the premises: what, where, andwhen?" Proceedings of the IEEE, Vol. 94, No. 5, pp. 911-934, 2006.

[7] G. Kramer, Ethernet passive optical networks, McGraw-Hill, New York,2005.

[8] A. Banerjee, et. al., "Wavelength-division-multiplexed passive opticalnetwork (WDM-PON) technologies for broadband access: a review,"Journal of Optical Networking, Vol. 4, No. 11, pp. 727-758, 2005.

[9] G.I. Papadimitriou, C. Papazoglou,and A.S. Pomportsis, "OpticalSwitching: Switch Fabrics, Techniques, and Architectures," Journal ofLightwave Technology, Vol. 21, No. 2, pp. 384-405, 2003.

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