A channel reuse strategy with adaptive channel allocation for all-optical WDM networks

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Optical Switching and Networking

Optical Switching and Networking 10 (2013) 246–257

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A channel reuse strategy with adaptive channel allocationfor all-optical WDM networks

P.A. Baziana n, I.E. Pountourakis

School of Electrical and Computer Engineering, Department of Communications Electronic & Information Engineering, National Technical

University of Athens, 157 73 Zografou, Athens, Greece

a r t i c l e i n f o

Article history:

Received 4 May 2012

Received in revised form

22 February 2013

Accepted 25 February 2013Available online 14 March 2013

Keywords:

Wavelength division multiplexing (WDM)

Multi-channel control architecture (MCA)

Performance improvement

Propagation delay latency

Receiver collisions

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ail address: [email protected] (P.A. Ba

a b s t r a c t

The fundamental problems of WDM networks are: (1) high rate of control packet loss and

(2) high propagation delay for each (re)transmission. In this paper, we minimize the

station randomness to access the control architecture introducing a collisions-free access

scheme. We propose a synchronous protocol according which at the end of the

propagation delay each station applies a distributed algorithm for packet transmission

following the data channel collisions and the receiver collisions avoidance algorithms.

We introduce two data transmission stages. The time difference between them is one

packet transmission time. At the end of the first stage all data channels are free and can

be reused by the remaining data packets during the second stage. The proposed protocol

ensures a totally collisions-free performance. The main advantage is that the data

channels reuse strategy applied during the second stage provides enhanced transmission

probability to the rejected packets during the first stage. This allows the data packets to

try retransmission in the same cycle without requiring control packets re-coordination

that increases propagation delay. Thus, we achieve large number of data packets

transmission, even more than the data channels number, providing throughput improve-

ment and delay reduction, comparing with other studies.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

In literature, diverse access protocols suitable forWavelength Division Multiplexing (WDM) networks ofpassive star topology have been proposed. Some of theseWDM Access (WDMA) protocols are based on the randomaccess method. It is obvious that the contention nature ofsuch protocols essentially restricts the performanceachieved, while they set an upper bound on the maximumbandwidth utilization. The most important parametersthat reduce the network performance are: (1) the control

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ziana).

channels collisions, (2) the data channels collisions, and(3) the destination conflicts.

Recent studies in literature propose the passive startopology as the most preferable one for WDM networksespecially at local area scale, while they introduce effectiveWDMA protocols to improve the performance measures.Thus, an access strategy that optimizes the performance byeliminating the message delay in a WDM network of passivestar topology is studied in [1]. Also, some modern schedul-ing algorithms based on the use of clustering techniquesthat aim to provide performance enhancement in passivestar WDM networks are presented in [2]. Moreover, aquality of service (QoS) prediction framework to accommo-date different applications with various QoS requirements inWDM networks of passive star topology is proposed in [3].

A determinant parameter that plays key role in theperformance evaluation is the round trip propagation

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delay latency as it is compared with the data packettransmission time. Although its effect is critical enough,only few studies consider its influence to the networkefficiency. For example, in [4] the propagation delay ismeasured between the stations and the hub and thisvalue is considered for the transmission schedule in apassive star network.

In addition, for WDMA protocols with no pre-transmission coordination the effect of propagation delaylatency is not assumed prior to the transmission phase, sincethere is no control information exchange [5]. On the contrary,for WDMA protocols with pre-transmission coordinationschemes the propagation delay latency critically affects thedata packets transmission scheduling at the control coordi-nation phase. Thus, a collision-free pre-transmission coordi-nation protocol is presented in [6] to predict the transmissionrequests and to eliminate the scheduling delay time in WDMpassive star architectures. Also, in [7] the effect of thepropagation delay on the performance of two reservationbased access protocols is studied and the average packetdelay is estimated.

Some WDMA protocols of passive star topology basedon pre-transmission coordination use a single separatecommon shared channel to exchange control information,while the remainder channels are used as data channels[8–11]. Since the maximum control information proces-sing rate is limited by the electronic interface of thestation, a fundamental problem rises: this of the disabilityof a station to receive and process all control packets thatare transmitted over the single control WDM channel.This fact causes the electronic processing bottleneck [12].Also, this major obstacle gives rise to the need to developefficient network architectures and to adopt effectiveaccess techniques to overcome this problem.

Nevertheless, the critical problem that many WDMAprotocols with pre-transmission coordination face is thatof low bandwidth utilization and limited performancemeasures achieved. Thus, although they aim to reducepacket loss exploiting the control information and addres-sing appropriate collisions-free access schemes for thedata channels, they usually restrict the throughputobtained at much lower rate than that of the fiber’s.This is because of two combined reasons: (1) The controlpacket transmissions compete to gain access over thecontrol channel (s) and they face control channel colli-sions [13,14]. In this way, the throughput obtained islimited by the percentage of the collided control packets.(2) In addition, a number of data channels remain unusedduring a cycle period in order to avoid data channels andreceiver collisions, although there may exist availabledata channels for transmission [10,13–20]]. In this way,they manage low exploitation of the data channels band-width and they essentially restrict the system perfor-mance. Thus, a fundamental objective rises: the need toadopt efficient WDMA protocols to access the control anddata channels, effectively facing the control and datachannels collisions and the destination conflicts, whileoptimally exploiting the fiber bandwidth.

In this study, we adopt the Multi-channel ControlArchitecture (MCA) [13,14] which employs a number ofcontrol channels for control information exchange.

Especially, the MCA utilizes a number v of control chan-nels for the control packets transmission, while theremaining channels are used for the data packets trans-mission. In other words, if a station attempts transmis-sion, it selects randomly one of the v control channelsfrom the MCA for the control packet transmission. In thisway, significantly less control information processingoverhead is provided as compared with the single controlchannel case, while the electronic processing bottleneckproblem is effectively faced. Additionally, we adopt a ‘‘telland wait’’ algorithm to access the MCA and the datachannels, given that the round trip propagation delay islonger than the data packet transmission time. This factensures that the round trip propagation delay can beexploited as acknowledgment time period after the end ofwhich we may schedule the data packet transmissionswhich are totally free from both control and data channelscollisions and destination conflicts [13].

Especially, the proposed access algorithm totallyavoids the control packet collisions at the pre-transmission phase by appropriately assigning a specificcontrol mini-slot to each station for transmission. In thisway, there is no control packet loss and the controlinformation of all stations can be processed in a de-centralized way to address the data channels access,avoiding both the data channels and the receivercollisions.

Additionally, in opposition to other studies like[13–20] where a number of data channels remain unusedduring a cycle period in order to avoid data channels andreceiver collisions although there may exist available datachannels for transmission, the proposed access schemeintroduces a data channel reuse strategy during a cycleperiod and efficiently utilizes all the data channels fortransmission without leaving any of them unused whiletaking under consideration the data channels and receivercollisions avoidance. In this way, it gives the opportunityto many data packets (more than the number of datachannels) to gain access over the data channels in twocollisions-free transmission stages. In other words, theproposed access scheme avoids the access randomness onthe control and data channels that other WDMA protocolslike [13–20] introduce and addresses an efficient datachannel reuse access strategy maximizing the probabilityof a successful data packet transmission. Moreover in thisway, the proposed protocol allows the data packets to tryretransmission in the same cycle period without requiringthe control packets re-coordination, like in other proto-cols for example [13–20], that would provide increases oftotal delay experienced. Thus, the proposed access proto-col exploiting the data channel reuse strategy in conjunc-tion with the MCA and the propagation delay latencyconsideration, achieves performance improvement andreaches not only significant throughput enhancementbut also essential delay reduction.

In opposition to the studies [13,14], in this paper weassume that the number of control channels in the MCA isless than the number of data channels. This fact provides aless costly implementation, since the required number offixed tuned receivers per station is less Moreover, this factenhances the benefits provided by the data channel reuse

P.A. Baziana, I.E. Pountourakis / Optical Switching and Networking 10 (2013) 246–257248

strategy since it assumes a small part of the total fiberwavelengths as control channels in the MCA and itconsiders the remaining channels as data channels essen-tially increasing the data packet transmission probability.

The main novelty of this study is the introduction of twotransmission stages per cycle, in conjunction with thesuitable transmission algorithms that determine thecollisions-free data packets transmission. In other words,the proposed protocol schedules the collisions-free datapackets transmission during the first transmission stageaccording to the proposed access algorithms that totallyavoid both the data channels and the receiver collisions,while it exploits the exchanged control information toaddress the additional packets transmission during thesecond transmission stage of the same cycle period basedon extra access algorithms that introduce the data channelsreuse and ensures the receiver conflicts avoidance, withoutrequiring the control information re-coordination. This factconsists the innovation of our study.

Our investigation is carried out as follows: The net-work model and the assumptions are described in Section2. In Section 3, the performance evaluation is studiedwhile the benefits of the data channel reuse strategy areexamined, for diverse numbers of control channels, datachannels and nodes. Finally, the concluding remarks areoutlined in Section 4.

2. Network model

We assume a passive star network, as Fig. 1 shows.The system uses vþN (voN) wavelengths lc1,..lcv,ld1,..ldN

to serve a finite number M of stations. The multi-channelsystem at wavelengths lc1,..lcv forms the MCA and operatesas the control multi-channel system, while the remaining N

channels at wavelengths ld1,..ldN constitute the data multi-channel system. The proposed MCA network model isdescribed as [CC]v

�TT�[FR]v�[TR]. This means that there

are v control channels and each station has a tunabletransmitter that can be tuned to a wavelength in the set

Fig. 1. Passive star multi-wa

lc1,..lcv,ld1,..ldN. The outcoming traffic of a station is con-nected to an input of the passive star coupler. Each stationuses v fixed tuned receivers one for each control channeland one tunable receiver to any of data channels ld1,..ldN.The incoming traffic to a station is splitted into vþ1portions by an 1� (vþ1) WDMA splitter, as Fig. 1 shows.

The transmission time of a fixed size control packet isused as time unit (control mini-slot) and the data packettransmission time normalized in time units is L (dataslot). The control packet includes information about: (1)the source address, (2) the destination address, (3) thepacket priority and (4) the data wavelength lk thatbelongs to the set of ld1,..ldN and has been chosen forthe data packet transmission. The normalized round trippropagation time between any station to the star couplerhub and to any other station is equal to R data slots (R� L

time units) for all stations.Both control and data channels use the same time

reference which we call cycle. We define as cycle the timeinterval that includes:

a)

vele

the control packets transmission phase in order toprovide collisions-free control packets transmission, plus

b)
the normalized round trip propagation time R� L, plus c) the data packets transmission phase in order to obtain
the transmission possibility of two contiguous datapackets over each data channel.

The cycle time duration is presented in Fig. 2.Especially, we assume that the control packets trans-

mission phase lasts for K ¼ divðM=vÞþ1 time units. In thisway, we divide the control packets transmission phase ineach MCA channel into K time units, each of which iscalled control mini-slot. Thus, we define at least M timeunits in the whole MCA area, as Fig. 2 shows. For eachstation a specific control mini-slot (one of the M controlmini-slots) is assigned in order to transmit its controlpacket. In this way, the control packets transmissions aretotally collisions-free. After the end of the control packets

ngth architecture.

Fig. 2. Cycle structure.


transmission phase, the outcome of the control informa-tion is known to all stations R� L time units later(acknowledgment time). Then, each station runs in ade-centralized way the transmission decision algorithm(that is described in the Transmission Mode that follows)in order to transmit during the data packet transmissionphase, during either the first or the second transmissionstage. So, the cycle duration is defined as:

C ¼ KþðRþ2Þ � L time units ð1Þ

where:

K ¼ divM

v

� �þ1 time units ð2Þ

We assume a common clock to all stations. Time axis isdivided into contiguous cycles of equal length and sta-tions are synchronized for transmission on the controland data channels during a cycle. At any point in timeeach station is able to transmit at a given wavelength lT

and simultaneously to receive at a wavelength lR. Finally,for the tunable transceivers, we assume negligible tuningtimes and very large tunable ranges.

We assume that each station is equipped with atransmitter buffer with capacity of one data packet. Ifthe buffer is empty the station is said to be free, otherwiseit is backlogged. Packets are generated independently ateach station following a geometric distribution, i.e. apacket is generated at each cycle with probability p.A backlogged station retransmits the unsuccessfullytransmitted packet following a geometric distributionwith probability p1 and defers the transmission by onecycle with probability (1�p1). If a station is backloggedand generates a new packet, the packet is lost.

Free stations whose transmission is canceled due todata channels or receiver collisions during a cycle aregetting backlogged on the next cycle. A backlogged station

is getting free at the next cycle if it manages to gain accessover a data channel (without being rejected due to datachannels collisions) and is not aborted due to receivercollisions.

In the followings, the transmission and receptionmode are presented. Also, in order to obtain a morerealistic performance study, the receiver collisions effectis considered.

2.1. Transmission mode

At the beginning of each cycle if a station has a datapacket to transmit to another station, it first choosesrandomly a data channel for packet transmission. Then,it transmits a control packet over the control mini-slotthat corresponds to itself, in order to inform the otherstations. As previously described, the control packets aretransmitted without collisions and their information willbe known to all stations R� L time units after the end ofthe control packets transmission phase. At that time, thestation is aware of the data channel claims for transmis-sion of all stations, their data packets priority anddestination.

Also at that time, all stations attempting transmissionrun contiguously the following access algorithms in orderto decide the transmission on either the first or thesecond transmission stage. The following Algorithms 1and 2 concern the collisions-free transmission over thefirst transmission stage, while the Algorithms 3 and 4concern the collisions-free transmission over the secondtransmission stage. Also, the proposed Algorithms 2, 3and 4 constitute the data channel reuse strategy.

Algorithm 1. Is executed by all stations that have trans-mitted a control packet at the beginning of the cycle(Condition 1).


If two or more stations have selected the same datachannel (of the first transmission stage) for transmission,only one of them gains access on the selected datachannel, according to a defined arbitration rule (such aspriority, etc.), in order to avoid data channel collisions.This algorithm is referred as data channel collision avoid-ance (DCCA) algorithm. In addition, if two or morestations have gained access to different data channelsbut they have selected the same destination, only one ofthem is finally allowed to transmit over the selected datachannel, according to the priority criteria, to avoid recei-ver collisions. This algorithm is called receiver collisionsavoidance (RCA) algorithm. The data packet transmissionwill be performed during the first transmission stage.

Algorithm 2. Data Channel reuse: Is executed by thestations that have not gained access to the data channelsin Algorithm 1 to avoid data channels collisions, and theyhave different destinations from the data packets finallytransmitted in Algorithm 1 (Condition 2).

After running the access Algorithm 1, some datachannels of the first transmission stage may remainunused. Thus, for each of the stations which satisfies thecondition 2, an unused data channel of the first transmis-sion stage is assigned for transmission (we may considerthe correspondence of the station number to the firstunused data channel number). Additionally in order toavoid receiver conflicts, the RCA algorithm is performed.Thus, it is examined if two or more of these stations haveselected the same destination. In this case only one ofthem gains the right to transmit over the assigned datachannel, according to the priority criteria. The datachannel assignment (DCA) algorithm is completed eitherif all unused data channels are assigned, or if all stationsof Algorithm 2 are finished. The data packet transmissionwill be performed during the first transmission stage.

Algorithm 3. Data Channel reuse: Is executed by thestations that have not gained access to the data channelsin Algorithm 1 to avoid receiver collisions (Condition 3).

The data packet transmission will be performed duringthe second transmission stage. Especially, for each of thestations which satisfies the condition 3, a data channel ofthe second transmission stage of the cycle is assigned.Moreover in order to avoid receiver conflicts, the RCAalgorithm is introduced. In other words, it is examined iftwo or more of these stations have selected the samedestination. In this case only one of them gains the rightto transmit over the assigned data channel, according tothe priority criteria.

Algorithm 4. Data Channel reuse: Is executed by thestations that have not gained access to the data channelsin Algorithm 2 to avoid receiver collisions and they havedifferent destinations from the data packets finally trans-mitted in Algorithm 3 (Condition 4).

The data packet transmission will be performed duringthe second transmission stage, since some data channelsmay remain unused after running the Algorithm 3.Especially, for each of the stations which satisfies the

condition 4, an unused data channel of the secondtransmission stage of the cycle is assigned. Moreover inorder to avoid receiver conflicts, the RCA algorithm isintroduced. So, it is examined if two or more of thesestations have selected the same destination. In this caseonly one of them gains the right to transmit over theassigned data channel, according to the priority criteria.

The transmission mode flow chart that explains thefour transmission algorithms is given in Fig. 3. Moreoverin order to be clearly introduced, in the following Figs. 4and 5 and we provide the simulation diagrams of theactions performed by a station to run the DCCA algorithmwith priority criteria and the RCA algorithm with prioritycriteria respectively. In Figs. 4 and 5, we define as S thevariable that represents the number of stations thatexecute each algorithm according to the correspondingcondition (1, 2, 3, or 4).

2.2. Reception mode

After the data packet transmission, the destinationwaits R L time units and then it adjusts its tunablereceiver to the data channel specified from the abovealgorithms for the data packet reception.

3. Performance evaluation

In this Section we present the numerical solutions ofthe proposed data channel reuse protocol. The numericalresults are provided by a specific network simulator thatwas developed to simulate the proposed protocol perfor-mance, using the C programming language. The devel-oped simulator implements an extensive discrete-eventsimulation model and simulates the actions performed byall stations during a number Nc of successive cycles, asFigs. 3–5 illustrate. The number Nc of successive cyclesthat the simulation runs is chosen in order the derivedresults to be defined under steady state conditions. In oursimulation, the simulator runs for Nc¼30.000 successivecycles. It is noticeable that the results could be alsoextracted for Nc¼15.000 successive cycles, while in thiscase the performance measures values appear on average7% deviation from the previous case. Also in our simulatorfor each case study, the number of the simulated packetsduring a cycle depends on the number of stations as wellas on the birth probability p and retransmission prob-ability p1 definition. On average and assuming thatp1¼0.3, the following numerical results show that duringeach cycle more than 80% of the stations either transmitor retransmit and contribute at the performance mea-sures evaluation at high new packets generation condi-tions (p¼0.9). It is obvious that the exact time eachsimulation runs does not consist an objective issue sinceit strictly depends on the programming environment aswell as the computer motherboard performance. Theresults take into account the receiver collisions phenom-enon, for more realistic study. In the presented numericalevaluations, we assume that the value of the data packetlength is L¼10 time units.

In the following Figs. 6 and 7, the dependence of theproposed protocol performance on the variation of the

Fig. 3. Protocol flow chart.


number v of the control channels is studied. Especially,Fig. 6 presents the throughput per data channel Sd,rc

curves versus the birth probability p, for M¼50, N¼30,R¼5 and v¼5,10,15 control channels. As it is observed,for a given value of R and for fixed values of M, N and p,the Sd,rc rises as v increases. This is due to the fact that thecycle duration C decreases with the increase of v, as eq.(1) denotes. In other words as v increases, the timereference (cycle) in which the successful data packetstransmissions are counted decreases. Since Sd,rc is areverse proportional of C, it is evident that Sd,rc increaseswith the increase of v. This can be noticed for example forp¼0.8 and M¼50, N¼30 and R¼5, where the Sd,rc is: 0.16for v¼5, 0.168 for v¼10, and almost 0.18 for v¼15.

As an immediate result, as v rises, the performanceimproves: the system reaches higher Sd,rc values andlower delay Drc values, since the cycle duration decreases.The explanation is based on the fact that bandwidthutilization is enhanced. This is validated in Fig. 7 thatpresents the Drc curves versus the Sd,rc, for M¼50, N¼30,R¼5, and v¼5,10,15 control channels. For example, forSd,rc¼0.14 its: Drc¼99 for v¼5, Drc¼90 for v¼10, andDrc¼86 for v¼15.

In the opposite, the variation of the number of datachannels N causes the reverse behavior. It is obvious that

as N increases the probability of a data packet successfultransmission over the data multi-channel systemincreases too. This fact causes the Sd,rc decrease, as N

increases. Also, for this reason, the probability of a datapacket rejection at destination due to the receiver colli-sions increases. This is the reason why Sd,rc decreases, as N

increases. This behavior is noticed in Fig. 8 that presentsthe throughput per data channel Sd,rc curves versus thebirth probability p, for M¼50, v¼10, R¼5 andN¼30,40,50 data channels. The above results areobserved for example, for p¼0.9 where Sd,rc is: 0.17 forN¼30, 0.13 for N¼40, and 0.1 for N¼50.

The above remarks are confirmed in Fig. 9 that depictsthe Drc curves versus the Sd,rc, for M¼50, v¼10, R¼5,N¼30,40,50 data channels. Also, it is worth mentioningthat as N increases the number of backlogged stationsincreases too, due to the rise of receiver collisions. Thisfact causes the significant increase of the delay Drc withthe N increment, as Fig. 9 shows.

The performance as a function of the node populationvariation is studied in Fig. 10 that presents the Sd,rc curvesversus the birth probability p, for v¼10, N¼30, R¼5, andM¼50,75,100 stations. It is observed that as M increases,Sd,rc increases too. This is because, as M increases for fixedN, v, R and p, the offered load to the system increases.

Fig. 4. Simulation diagram for a station actions to run the DCCA algorithm.


Also, it is noticeable that for small populations (M¼50)the system has not reached saturation yet and it is able toserve all the incoming traffic. This is noticed, for example,for M¼50 where the Sd,rc curve is almost linear for allvalues of p. On the contrary, for large population systemsthe protocol reaches congestion and provides themaximum value of Sd,rc. This is observed, for example,for M¼100 where the Sd,rc curve is not linear and reachesthe maximum value for p¼0.8, while it almost keeps it forhigher values of p.

Finally, the propagation delay latency R effect on thesystem performance is studied in Figs. 11 and 12. Especially,Fig. 11 shows the Sd,rc curves versus the birth probability p,for M¼50, v¼10, N¼30, R¼0,5,10. It is observed that Sd,rc isa decreasing function of R. This is understood, since thecycle duration C is a proportional function of R as eq. (1)denotes. So, it is evident that Sd,rc decreases with the R

increment. For example, for p¼0.9 where Sd,rc is: 0.47 forR¼0, 0.17 for R¼50, and 0.1 for R¼10.

Also as an immediate result, for the same value of Sd,rc

the delay Drc increases as R rises. This is validated inFig. 12 that presents the delay Drc curves versus the

throughput per data channel Sd,rc, for M¼50, v¼10,N¼30, R¼0,5,10. For example, for Sd,rc¼0.1 it is: Drc¼160for R¼10, Drc¼80 for R¼5, and Drc¼27 for R¼0.The latest two figures representatively depict the signifi-cant effect of R on the system performance and prove thefaulty performance estimation of other studies that ignoreits influence.

3.1. Protocol performance comparison

In this sub-section, we study the advantages of theproposed access protocol and especially we investigatethe benefits provided by the proposed data channel reusestrategy. In order to obtain a quantitative base for theestimation of the proposed protocol performance, wechoose to compare its performance measures with thoseof the relative access scheme which: adopts the samecollisions-free control packet coordination scheme but itassumes only the Algorithm 1 for data packet transmis-sion during the first transmission stage. This means that itdoes not exploit the data channel reuse strategy andsimply uses a single data slot per cycle for data packet

Fig. 5. Simulation diagram for a station actions to run the RCA algorithm.


transmission. In this way, the comparison representa-tively describes the performance enhancement obtainedby the proposed channel reuse strategy, as it is shown infigures.

It is evident that the time reference definition, i.e. thecycle duration, in the two compared protocols is different.Especially, the cycle duration Cs of the protocol withoutthe data channel reuse strategy is smaller than that of theproposed one, i.e

Cs ¼ KþðRþ1Þ � L time units ð3Þ

Since CaCs the performance comparison should notconsider the measures obtained per cycle, but it assumesa specific network configuration: Thus, the number ofstations is assumed to be M¼50. The number of WDMdata channels is the variable N, while the number of WDMcontrol channels in the MCA is the variable v. Each of thechannels is assumed to operate at the transmission rate ofRw¼1 Gbit/s. Since the vast majority of the Local AreaNeworks (LANs) are Ethernet based, we assume that thedata packets are fixed-sized, with size equal to theEthernet Maximum Transfer Unit (MTU) frame size, i.e.

Z¼12000 bits (1500 Bytes). Thus, the data packet trans-mission time is T¼Z/Rw¼12 ms. Since the control packetcarries: the source address, the destination address, thepriority and the selected data channel number, its size is(2� x)þ1þy, because: 2x�1oMr 2x and 2y�1oNr 2y.The transmission time of each control packet istr¼((2� x)þ1þy)/Rw. Assuming that the propagationdelay latency parameter R is 5 times bigger than the datapacket transmission time (R¼5 data slots), the propaga-tion delay time is Prop¼5� T ¼60 ms. Finally, the cycleduration is: (a) for the proposed protocol with the datachannel reuse strategy is given by (1), and (b) for theprotocol without the data channel reuse strategy is givenby (3).

The advantages provided by the proposed data channelreuse strategy are studied in the following Figs. 13 and 14where the performance comparison between the twoprotocols is investigated. Especially, Fig. 13 presents thedelay Drc curves versus the throughput per data channelSd,rc, for M¼50, N¼30, R¼5, and for diverse MCA chan-nels v¼1,5,10,15, for the proposed protocol with the datachannel reuse strategy and the relative protocol without

Fig. 8. Throughput per data channel Sd,rc versus birth probability p, for

M¼50, v¼10, R¼5, and N¼30,40,50.

Fig. 9. Delay Drc versus throughput per data channel Sd,rc, for M¼50,

v¼10, R¼5, and N¼30,40,50.


v¼10, N¼30, R¼5, and M¼50,75,100.


M¼50, N¼30, R¼5 and v¼5,10,15.


N¼30, R¼5, and v¼5,10,15.


the data channel reuse strategy. As it is representativelyshown, the proposed protocol provides significant perfor-mance improvement for all values of v as compared withthe protocol without the data channel reuse strategy.Particularly, for all values of v, for the same Sd,rc valuethe proposed protocol with the data channel reuse strat-egy provides essentially lower Drc values. For example forSd,rc¼5 Mbits/s, the Drc experienced: (a) for the proposedprotocol with the data channel reuse strategy is: 1.5 timeunits for v¼5, 1.3 time units for v¼10 and 1.2 time unitsfor v¼15, while (b) for the protocol without the datachannel reuse strategy increases to: 2.1 time units forv¼5, 1.8 time units for v¼10 and 1.6 time units for v¼15.This means that the proposed protocol with the datachannel reuse strategy provides almost 27% delay reduc-tion for all values of v. The performance improvement canbe representatively noticed for the system that employs asingle control channel, as in the most protocols in


N¼30, v¼10, and R¼0,5,10.

Fig. 13. Protocols Performance Comparison—Delay Drc versus through-

put per data channel Sd,rc, for M¼50, N¼30, R¼5, and v¼1,5,10,15.

Fig. 14. Protocols Performance Comparison—Delay Drc versus through-

put per data channel Sd,rc, for M¼50, v¼10, R¼5, and N¼30,40,50.


M¼50, N¼30, v¼10, and R¼0,5,10.


literature. As Fig. 13 depicts, in this case (v¼1) for Sd,rc¼5Mbits/s the proposed protocol that exploits the datachannel reuse strategy provides almost 44% Drc reduction,as compared to the protocol without this strategy.

This behavior is explained by the following fact: In theprotocol without the data channel reuse strategy, if astation does not succeed after running the Algorithm 1 totransmit a data packet due to either the DCCA or the RCAscheme, it cancels its transmission for the next cycle. Thismeans that it has to re-transmit the control packet duringthe next cycle and to wait for additional propagationdelay latency time before re-trying a new data packettransmission. This fact consequently results to very highdelay values. On the contrary, the proposed protocol withthe data channel reuse strategy allows the stations to try adata packet retransmission in the same cycle periodwithout requiring the control packets re-coordination. Inthis way, it provides essential total delay reduction, as

Fig. 13 depicts. This behavior is more noticeable in case ofa single control channel v¼1, as previously discussed.

Moreover, the protocol without the data channel reusestrategy reaches very low Sd,rc values, while it providesvery high delay. This means that for all values of v, itreaches saturation at low offered loads, despite the factthat the cycle duration is smaller than that of theproposed protocol. On the other hand, the proposedprotocol with the data channel reuse strategy is able toserve significantly more incoming traffic, while keepingessentially low delay values. For example, the maximumthroughput obtained is: (a) for the proposed protocol withthe data channel reuse strategy: 23 Mbits/s for v¼5,24 Mbits/s for v¼10 and 24.8 Mbits/s for v¼15, and (b)for the protocol without the data channel reuse strategy:12.5 Mbits/s for v¼5, 13 Mbits/s for v¼10 and13.4 Mbits/s for v¼15. In other words, the proposedprotocol with the data channel reuse strategy providesalmost 85% maximum throughput improvement for all

Table 2Performance Improvement for v¼10, M¼50, R¼5,

N¼30,40,50.

N p Improvement

Sd,rc (%) Drc (%)

30 0.8 100 118

40 70 93.7

50 53 80.6


values of v. Especially, in the case of a single controlchannel v¼1, the maximum throughput improvementprovided by the data channel reuse strategy reaches 114%.

The explanation comes from the fact that the proposedprotocol which exploits the data channel reuse strategyprovides three additional algorithms for possible trans-mission and supports the effective data channels alloca-tion to maximize the transmission possibility. In otherwords, the protocol without the data channel reusestrategy gives access only to those stations which arenot aborted due to the DCCA and RCA schemes inAlgorithm 1. On the contrary, the proposed protocol withthe data channel reuse strategy gives additional accesspossibilities to those stations whose transmission iscanceled due to the DCCA or RCA schemes in Algorithm1. In this way, the proposed protocol significantlyenhances the data packet transmission probability, pro-viding impressively higher values of Sd,rc and conse-quently lower values of Drc, especially in the case of asingle control channel v¼1.

It is evident that for the same value of v, M, and R, asthe number N rises the proposed protocol with the datachannel reuse strategy provides higher transmissionprobability. This is due to the fact that as N increases,the three additional transmission algorithms that theprotocol supports, essentially contribute to the perfor-mance improvement. This behavior is observed in Fig. 14that presents the delay Drc curves versus Sd,rc, for M¼50,v¼10, R¼5, and N¼30,40,50 data channels for bothprotocols. For example, the proposed protocol with thedata channel reuse strategy provides maximum through-put improvement: 84.6% for N¼30, 41.6% for N¼40 and27.3% for N¼50. Although the cycle duration of theproposed protocol with the data channel reuse strategyis bigger than that of the protocol without the datachannel reuse strategy, the former protocol providesessentially lower Drc values than those of the latest one.This behavior is an immediate result of the significanttransmission probability enhancement and the essentialthroughput improvement that the primer protocol pro-vides. Moreover, this behavior is explained by the factthat the proposed protocol with the data channel reusestrategy allows the transmission of more data packets percycle, decreasing the total delay, as previously described.

Summarizing, the overall performance improvement isstudied in the following Tables 1 and 2 which present theSd,rc and Drc values and the percentage improvementprovided by the proposed protocol with the data channelreuse strategy comparatively to the protocol without the

Table 1Performance Improvement for N¼30, M¼50, R¼5,

v¼5,10,15.

v p Improvement

Sd,rc (%) Drc (%)

1 0.8 167 44

5 100 118

10 84 120

15 77.7 124

data channel reuse strategy, for diverse number of v andN, for M¼50, R¼5 and p¼0.8.

4. Conclusions

In this paper, we study a synchronous transmissionWDMA protocol for passive star topology that manages toessentially improve the bandwidth utilization, as com-pared with other studies. This is achieved by introducingan efficient data channel reuse strategy and applying twostages for transmission in each cycle period. This meansthat the proposed WDMA protocol provides enhanceddata packet transmission probability, since during thesecond transmission stage it permits the successful trans-mission of many of the data packets that are rejectedduring the first stage. The access scheme adopts a pre-transmission coordination scheme that totally avoids thecontrol packets collisions. In this way after the propaga-tion delay time, the control information is available to allstations in a decentralized way in order to schedule datatransmissions without data channels and receivercollisions. In opposition to other studies, the proposedWDMA protocol exploits all the data channels for success-ful transmission and gives the opportunity to a largenumber of data packets (bigger than the number of datachannels) to be successfully transmitted during a cycleperiod. As a result, the proposed WDMA protocol reachessignificant performance enhancement and effectivelyfaces the critical problem that other protocols introduce:that of low bandwidth utilization. This result is validatedby comparative numerical evaluations that prove that theproposed WDMA protocol with the data channel reusestrategy not only provides significant throughputimprovement, but it also achieves essential delay reduc-tion, for various network parameters. As it is representa-tively shown, the throughput improvement achievedreaches even 100% for specific number of MCA channels,while the delay deterioration reaches almost 124%. Parti-cularly for the single control channel case, the proposeddata channel reuse strategy provides 167% throughputimprovement for high load conditions. Thus, for a givennetwork topology and offered load traffic, the determina-tion of the number of MCA channels should take intoaccount the required throughput and delay improvementobtained as well as the relative financial cost.

The proposed WDMA protocol requires perfect timesynchronization among all stations in order to support thecontrol and the data packets transmission. This require-ment implies a more complicated access strategy withprocessing overhead and would appear as a protocol


disadvantage. For this reason, it is an engineer decision tobalance the cost for synchronization with the significantperformance enhancement obtained.

The presented work consists a useful base for thedevelopment and evaluation of new access schemes,suitable for similar WDM network architectures. As afuture work, the study of the number of contiguoustransmission stages per cycle would appear scientificallyinteresting, as a function of the network parameters.

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A channel reuse strategy with adaptive channel allocation for all-optical WDM networks