An Efficient Metropolitan WDM Ring Architecture for a Slotted Transmission Technique

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 19, OCTOBER 1, 2008 3307

An Efficient Metropolitan WDM Ring Architecturefor a Slotted Transmission Technique

Peristera A. Baziana and Ioannis E. Pountourakis, Member, IEEE

Abstract—In this paper, we present a wavelength division mul-tiplexing multiring metropolitan area network architecture usinga separate ring as control channel and a finite number of accessnodes. Each access node is equipped with a fixed tuned transmitterand a fixed tuned receiver to exchange control information overthe control wavelength. Also, each access node has a tunable trans-mitter to efficiently exploit all data wavelengths for data transmis-sion. The set of data wavelengths is divided into wavelength bands.This allows a flexible node design with a number of parallel tunablereceivers per node, each operating in a specific wavelength bandand providing limited tuning time. On the one hand, our networkstrategy confronts more efficiently the scalability and maintenanceproblems comparatively with most of the access ring protocols. Onthe other hand, the proposed access algorithm avoids both the datawavelengths and the receiver collisions, improving even more thenetwork utilization. An analytic model is developed for the per-formance measures evaluation. Also, we develop another analysisapproach using discrete event simulation model based on self-sim-ilar statistics. Analysis is accomplished studying various numbersof access nodes, data wavelengths, and buffer size.

Index Terms—Collision avoidance, dropping probability, tun-able transceivers, wavelength division multiplexing (WDM).

I. INTRODUCTION

T HE rapid growth of Internet popularity and the expandof network services that use not only the traditional

voice, but also real-time traffic, have led to the even increasingbandwidth demands in metropolitan area networks (MANs).In modern networks, MANs usually operate as backbone net-works and interconnect access networks of diverse traffic, likeEthernet packets, Internet bursts, and frame relay traffic.

Optical fibers are arisen as the dominant technology for high-speed MANs. Wavelength division multiplexing (WDM) tech-nique manages to properly utilize the enormous fiber data ratethat reaches to Tb/s, by dividing the total bandwidth into many(nowadays more than 100) wavelengths that operate in lowerrates [1]–[3]. Ring networks are prevalent for MANs and WDMaccess (WDMA) rings are expected to be the next generation de-ployment [4]. Several advantages are related with rings such asthe simplicity of: routing policy, resources control and manage-ment, protection from network failures [5].

In literature, many network architectures are proposed to ef-ficiently access the WDM wavelengths of ring MANs. Most of

Manuscript received December 27, 2007; revised May 16, 2008. Current ver-sion published December 19, 2008.

The authors are with the School of Electrical and Computer Engineering,National Technical University of Athens, 157 73 Athens, Greece, (e-mail:[email protected], [email protected]).

Digital Object Identifier 10.1109/JLT.2008.928930

them introduce a restriction about the number of access nodesaround the ring. Particularly, most of them assume that is

an integer multiple of the number of WDM wavelengths ,i.e., . This restriction is faced into two network archi-tectures, in which the access node interface includes: 1) eithera tunable transmitter and a fixed tuned receiver (TT-FR) andfor each node corresponds one wavelength for data receptionavoiding receiver collisions [6]–[13], 2) or a fixed tuned trans-mitter and a tunable receiver (FT-TR) and for each node corre-sponds one wavelength for data transmission avoiding channelcollisions [14]–[17].

Although this restriction aims to improve the performancemeasures, it introduces serious scalability problems. This is be-cause it prohibits the network enlargement by adding new accessnodes, since it requires the fairness control, the entire networkreconfiguration and strongly depends on the tunable range ofthe tunable transceivers, increasing the cost. Also, it makes thenetwork maintenance very complicated in case of possible ac-cess node deactivation. Performance improvement is achievedusing a dedicated wavelength for control communication withextra transceivers [7], [11]–[13]. In [14], the scalability issuesare studied comparing two protocols: 1) the pro-tocol in which each access node uses a number of fixed tunedreceivers equal to the number of wavelengths, 2) the FT-TR pro-tocol where in each access node, the array of fixed tuned re-ceivers has been substituted by a tunable one. The results showthat the first protocol avoids receiver collisions but it introducesscalability issues, while the second one improves scalability butit suffers from receiver conflicts.

The feasibility of a slotted WDM ring network with theTT-FR node interface is experimentally tested in HORNET[18]. The first HORNET version uses a carrier sense mul-tiple access with collision avoidance (CSMA/CA) scheme withsub-carrier multiplexing (SCM) to face the collisions. The SCMtechnical restrictions are faced in the latest HORNET versionthat employs a separate control channel for collision avoidance.Several transmission queues per node are assumed, one for eachwavelength. A specific queue selection algorithm is applied toresolve contentions by selecting the proper transmission queue.Although this algorithm manages high network efficiency, itprovides access unfairness [17]. Thus, extra control informa-tion is exchanged to enhance fairness, providing (except fromprocessing overhead) insufficient quality of service (QoS) forreal time applications [18].

Despite the variety of multiple WDMA protocols, recentresults have shown that the FT-TR and TT-FR node inter-faces achieve similar theoretical performance [19]. Thus, theuse of a node interface with a fast tunable transmitter and a

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3308 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 19, OCTOBER 1, 2008

Fig. 1. Network architecture; in inset form, the access node logical architecture.

tunable receiver (TT-TR) has arisen, to exploit the availablebandwidth and to improve the scalability. The transceiverstunability benefits to significantly reduce the blocking eventsare examined in [20]. Latest studies propose a token accesspolicy for WDMA ring network architectures with a TT-TRnode interface [21]–[23]. Although these studies make a profitin tunability area, they suffer from very high delays. Also, someaccess strategies for the TT-TR node interface are given in [24],where each node uses many delay lines to exchange controlinformation suffering from very high total delays.

In this study, we propose a slotted WDMA protocol to accessthe multiple wavelengths of a ring MAN. The proposed archi-tecture occupies a separate control wavelength to exchange con-trol information prior to data transmission. An efficient accesscontrol algorithm is applied to avoid both the data wavelengthsand the receiver collisions. Also, we adopt a consideration toimprove the fairness among the access nodes. The proposedprotocol takes advantage of the transceivers tunability benefits.Each access node has a tunable transmitter. Since the tuningtime of tunable receivers is a key parameter for the system per-formance, the set of data wavelengths is divided into wavebands.This allows a flexible node design with a number of paralleltunable receivers per node, each operating in a specific wave-band and providing limited tuning time. Our protocol is appliedto ring MANs of any population, since the number of accessnodes is independent from the number of wavelengths. This isbecause each access node may transmit and receive on any datawavelength. In this way, the restriction is removed.Thus, high scalability at network enlargement and maintenanceat failure is managed.

In this paper, the maximum throughput per access node is an-alytically provided. The results are validated by simulation, as-suming self-similar traffic sources. In order to extensively eval-uate the performance measures, we provide via simulation theaverage queuing delay, the average total delay from the sourceto the destination, and the average packet dropping probability.The analysis is accomplished using various numbers of accessnodes, data wavelengths, and buffer size.

Our study is carried out as follows. The network model andthe assumptions are described in Section II. Section III studies

the theoretical analysis. The performance evaluation is given inSection IV. Some conclusions are outlined in Section V.

II. NETWORK MODEL AND ASSUMPTIONS

We assume a slotted uni-directional single-fiber multichannelring network using wavelengths to interconnect a finitenumber of access nodes on a wide area scale, as Fig. 1 shows.The W wavelengths are used for the data packetstransmission and are called data channels, while the wavelength

is used for the control information exchange and is calledcontrol channel. The data channels are divided into wave-bands of four channels each [32]. Each access node is equippedwith a number of parallel tunable receivers, each operating ina specific data waveband and providing limited tuning time.Thus, the number of tunable receivers per node depends on thenumber and the maximum accepted tunable receiver tuningtime. Also, each access node includes one fast tunable trans-mitter that can be tuned over the data channels. The tuningtime of the tunable transmitter and tunable receiver is denotedby and , respectively. Since the technology up to nowdoes not provide us with reliable tunable receiver time response,extra guard band time interval is assumed. Each node is fur-ther equipped with one fixed tuned transmitter and one fixedtuned receiver that are always tuned at the control channel .Thus, the proposed node interface is referred as FT-FR-TT-TR.Each access node has optical add-drop capabilities to access thering. Also, it is connected to one or more access networks. Asingle queue with buffer size of data packets is assumed ateach node. Since the majority of the local area networks (LANs)are Ethernet based, the access networks generate data packetswith fixed size equal to the Ethernet maximum transfer unitsize, i.e., bits.

A set of parallel delay lines is assumed at each accessnode, one for each data channel. This feasibility is discussedin [24] where the delay lines are very long and the total delayis too high. Unlike [24], our study requires only a slight delayper node giving an inexpensive and efficient deployment. Theaccess node logical architecture is depicted in Fig. 1.

All channels are synchronized. The time axis is divided intofixed slots with duration equal to the data packet transmission

BAZIANA AND POUNTOURAKIS: EFFICIENT METROPOLITAN WDM RING ARCHITECTURE FOR A SLOTTED TRANSMISSION TECHNIQUE 3309

Fig. 2. Structure of multidata slot. At time instant � , the multidata slot arrives at the access node; the data slots insert the delay lines for time interval � , whilethe control packet is received, processed, re-generated, and the tunable transceivers are tuned. At time instant � � � , the access node: 1) starts receiving from �

data channel, 2) starts transmitting over � data channel, and 3) starts transmitting the new control packet and the data packets start exiting the delay lines. At timeinstant � � � � � , the multidata slot has departed from the access node.

time . A set of parallel slots over the data channels consista multidata slot, as Fig. 2 shows. At the beginning of eachmultidata slot, the control channel carries a control packetthat consists of: 1) address mini-packets, 2) a number

of status mini-packets. The control packetstructure is depicted in Fig. 2. The control packet transmissiontime is . Each of the address mini-packets corresponds toa data channel, while its size is bits, [25].The address mini-packet carries the desti-nation address (in binary mode) of the data packet that hasbeen transmitted on the data channel of the multidata slot.If the address mini-packet is empty, the data channel isavailable for transmission. On the other hand, each of thestatus mini-packets carries the id (in binary mode) of the datachannel that may be destroyed in case of a network fault. Thenumber defines the maximum number of data channels thatmay be concurrently destroyed. Each status mini-packet size is

bits, [25]. When a multidata slot arrives atan access node, at time instant let’s say , the node experiencesthe following procedure as Fig. 2 shows.

A. Delay of Data Packets and Control Management

time instant the arriving data packets insert to thedelay lines for time interval . The time interval equals tothe time required for the control management and consists ofthe following.

1) Reception of control packet: At time instant , the arrivingcontrol packet starts being received by the fixed tuned re-ceiver. This lasts for time interval .

2) Processing of control packet: At time instant , theaccess node has received the control packet and has erasedit from the control channel. Since it is in optical form, theaccess node converts it to electronic form (O/E conversion)and locally processes it. Thus, the access node is informedby the control packet about: a) the available and active datachannels for transmission to avoid data channels collisions;

b) the destination address of the data packets transmittedover the data channels. Based on this information the ac-cess node runs the next transmission and reception accessalgorithms.Transmission Access Algorithm: If the access node has adata packet to transmit, it examines: i) if there are availableand active data channels; ii) if none of the transmitted datapackets has the same destination with its own data packet.If the above conditions are satisfied, the node chooses ran-domly one of the available and active data channels, let’ssay data channel for the transmission. Otherwise, thetransmission is cancelled. Thus, both data channels and re-ceiver collisions are avoided.Reception Access Algorithm:We suppose that the node in-spects a data packet destined to it and transmitted over the

data channel. After the above accessalgorithms, the access node locally re-generates in elec-tronic form a new control packet. Thus, it re-generates the

address mini-packets that it has received, but it modi-fies only the fields of: a) address mini-packet writing thedestination address of its own data packet; b) addressmini-packet marking it empty according to the destinationstripping. A slot reuse is assumed which restricts the imme-diate reuse of a data slot that has just been marked emptyforcing a node to release it to the next node [14]. This pro-vides fairness without extra processing overhead and delaycost. Also, the access node locally re-generates in elec-tronic form the status mini-packets that it has received. Ifthe access node has inspected that a data channel markedas active in the received status mini-packets is faulty,it modifies the first empty status mini-packet, writing theid of the faulty data channel. Then, the access node con-verts the new control packet (i.e., the new address mini-packets and the status mini-packets) to optical form (E/Oconversion). The control process lasts for time intervalas Fig. 2 shows.


3) Tuning of tunable transceivers: At time instant, the access node starts tuning its tunable transmitter to

the data channel to transmit and its tunable receiver tothe data channel to receive. This lasts for time interval

.4) Guard band: A guard band time interval is assumed

before the multidata slot starts departing, that takes intoaccount the electronic components time to response.The data packets delay at the delay lines equals to thetime interval for control management

.

B. Departure of Multidata Slot

At time instant , the multidata slot starts departing fromthe access node. As Fig. 2 shows this time instant start:

• Transmission and reception of the data packets: The accessnode starts: a) the data packet transmission over the datachannel with its tunable transmitter, b) the data packet re-ception from the data channel with its tunable receiver.This lasts for time interval .

• Transmission of control packet: The access node startstransmitting with its fixed tuned transmitter the new con-trol packet. This lasts for time interval .

• Exit from delay lines: The data packets (except that of thedata channel) start exiting from the delay lines.

III. ANALYSIS

The propagation delay around the ring is

(1)

where is the ring length and the light group velocity inthe fiber. Since the propagation of a data slot is delayed for timeinterval at each access node, the time duration for a com-plete rotation of a data slot around the ring is

(2)

We assume that all the access nodes are equally spaced (phys-ically) around the ring. Let’s consider that an access node trans-mits a data packet over a data slot to its adjacent node. In thiscase, the time it takes for the data slot, once leaving thesource access node to be received is

(3)

Generally, if the source node transmits a data packet over adata slot to the destination node and there are access nodes

interposed among them, the time it takes forthe data slot once leaving the source node to be received is

(4)

We assume that each access node transmits to all other nodeswith equal probability . Thus, the average time

it takes for a data slot carrying a data packet, once leaving thesource node to be received by any destination node is

(5)

Since the first factor of (5) is the sum of an arithmetic pro-gression and using (2) and (3), we get

(6)

A data slot upon the arrival at its destination node cannot beimmediately reused but is released empty to the adjacent node.The average time it takes for a data slot, once leaving thesource node to be made reusable for transmission is

(7)

From (2), (3), (6), and (7), we get

(8)

We define the maximum throughput per data slot and per datachannel as the maximum rate of data packets successfultransmissions by a data slot over a data channel during a com-plete rotation around the ring, i.e.,

(9)

From (2), (8), and (9), we get

(10)

The transmission rate of each channel in Gb/s is denoted as. We define the maximum throughput per data channel

(11)

We denote as the total data transmission rate over all datachannels on the ring, i.e., . We define the max-imum throughput over all data channels on the ring in Gb/s

(12)

It is considered that all access nodes transmit at the same rate.Thus, we define the maximum throughput per access node overall data channels on the ring in Gb/s as

(13)

From (10), (11), (12), and (13), we get

(14)

It is remarkable that the is independent from the ringlength and only depends on the number of access nodesand the total data rate . This is because the affects onlyto the propagation delay in the fiber, as (1) shows. Also,is a reverse proportional of . This is because as increases


Fig. 3. Simulation diagram for access node actions by a multidata slot arrival.

the offered load to the ring increases too. Thus, the probabilityof inspecting an empty data slot decreases, causing thedecrease. The result of (14) is extended in case that the accessnodes are not equally spaced around the ring. In this case, thetime it takes for a data slot carrying a data packet to be receivedby the source’s adjacent node is on average (not exactly) equalto . Also if the access nodes do not transmit at the same rate,the result of (14) remains. This is explained by the facts that eachaccess node transmits to all other nodes with equal probability

and that the slot reuse scheme forces the destinationnode to release the empty data slot to the downstream node.Thus, the number of empty data slots that arrives at an accessnode is on average identical for all nodes, providing identicaltransmission probability.

IV. PERFORMANCE EVALUATION

In order to verify the accuracy of the theoretical analysis, wedeveloped the whole code of a specific network simulator basedon C programming to simulate the proposed system perfor-mance. Our simulator implements an extensive discrete-eventsimulation model that describes in a discrete way the rotationof the multidata slots around the ring. The simulator reachesconfidence level providing predictions fornot only the , like the theoretical study. The simulatorprovides additional estimations for the average throughput, theaverage queuing delay at the buffer, the average total delay (theaverage sum of the queuing delay and the propagation delayfrom the source to the destination node), and the average packetdropping probability per node.

The parameter that plays a key role to the simulator estima-tion is the model used to simulate the access networks traffic.Many studies [8], [26], [27] have modeled the voice traffic, andby extension the data traffic in LANs, with Poisson processesthat have well-known theoretical models and can evaluate theperformance measures via mathematical analysis. Nevertheless,

TABLE INETWORK PARAMETERS

latest studies prove that the multitype traffic and especially theInternet traffic in LANs and MANs are characterized by highvariability and burstiness and can be better modeled by self-sim-ilar processes [28]–[31]. In our simulator, we simulate the trafficreceived by a node from the attached access networks consid-ering self-similar traffic. The aggregated ON-OFF sources ofPareto distribution are used to generate the self-similar traffic[31]. The high burstiness is achieved using the valueto the Hurst parameter, like in [31]. The number of the Paretodistributed ON-OFF sources in each access network is obtainedusing the log(variance)-log(aggregation level) of the generatedload.

In our simulator, the access networks traffic is generated priorto the ring network simulation and the data packets are queuedaccording to their generation time. In ring simulation, the fol-lowing C modules with their properties are assumed:

• NODE: id, buffer;


Fig. 4. Average node throughput versus average offered load per access node for � � 64, 120, 150,� � ��, � � ��.

• DATA_PACKET: generation time, destination address,transmission channel;

• ADDRESS_MINI_PACKET: id, destination address;• STATUS_MINI_PACKET: status, max number of faulty

data channels;• BUFFER: id, size, load, dropped packets, data packets

stack;• DATA_CHANNEL: id, rate, length, propagation delay,

status, data slot size, number of multidata slots, structureof multidata slots;

• CONTROL_CHANNEL: rate, length, propagation delay,size of control packet, size of address mini-packet, size ofstatus mini-packet, structure of control packets.

The main module is the node and is associated with adedicated buffer. The buffer module is represented by aFirst-In-First-Out (FIFO) stack of data packets waiting to betransmitted. The buffer module keeps the number of droppeddata packets for the average packet dropping probability cal-culation. Also, it keeps its current load, and by extensionthe number of transmitted data packets, for the average nodethroughput estimation. The data packet module keeps thegeneration time for the average queuing and the total delaycalculation. The address mini-packet module has an identity tocorrespond to a data channel and carries a destination address(or NULL). Similar, the status mini-packet module has anspecification for the maximum number of data channels thatmay be concurrently faulty and is characterized by its status(ACTIVE or FAULTY).The data channel module is describedby a structure of multidata slots that rotates around the ring,over which the nodes transmit and receive. Each module isassociated with a number of algorithms in the simulator. Thesequence of algorithms that each access node performs by amultidata slot arrival is depicted in Fig. 3.

The adopted network configuration is: The ring length ism. The number of access nodes is . The

number of data channels is and a separate controlchannel is assumed. Since the address mini-packet carries thedestination address, its size is bits, . Also,since the status mini-packet carries the data channel id, itssize is bits, . It is assumed that 4% of thedata channels may be concurrently faulty. Thus, the maximumnumber of faulty data channels is . Thesize of the control packet is bits. Thebuffer size is data packets. Each channel transmissionrate is . The data channels transmission rate is

. The transmission time of a datapacket is s and this of a control packet is

s. The number of multidataslots around the ring is . The networkparameters are summarized in Table I.

Since the technology up to now determines that the tuningtime it takes for a single tunable receiver to be tuned over awide wavelength range (like data channels) is highenough, we divide the data channels into wavebands of fourdata channels each [32]. Thus, each node has paralleltunable receivers, each operating in a specific waveband andproviding limited tuning time. Finally, we assume that: 1) timeinterval s like [33]; 2) guard bandtime interval ns like [34].

In simulation results, a perfect fairness among all accessnodes is achieved. Thus, the values of the average throughput,queuing and total delay, and dropping probability per nodeare very close among all nodes. Numerical results showthat the difference between these values is less than 8%.Thus, the average performance measures give a very closeestimation of the individual nodes performance. Finally, each


Fig. 5. Average queuing delay versus average offered load per access node for � � 64, 120, 150, � � ��, � � ��.

simulation was run for a long time to obtain steady-stateconditions.

Fig. 4 illustrates the average node throughput versus the av-erage offered load per node, for various number of access nodes

. As it is observed for , the proposed FT-FR-TT-TRprotocol is able to serve the entire offered load up to the value3050 Mb/s. This means that in this high load range, the total loadis able to access the ring without being dropped at the buffers.Thus, in this range, the entire load becomes actual throughput.Above this range, the system reaches saturation. This is becausethe increasing load gradually utilizes the entire data slots and theaverage queuing delay is high. Consequently the gradual buffercongestion is noticed which leads to fewer dropping events.As Fig. 4 shows, for load values higher than 3050 Mb/s, theproposed FT-FR-TT-TR protocol achieves the maximum dataslots utilization and reaches on average node throughput value3480 Mb/s.

From the above remarks, it is understood that the proposedFT-FR-TT-TR node interface in conjunction with the data chan-nels and receiver collisions avoidance algorithm provides effi-cient bandwidth utilization, while it faces the scalability issues.This is due to: 1) the possibility to transmit and receive over alldata channels exploiting the empty data slots; 2) the efficientmechanism to resolve collisions based on control information.

Fig. 5 shows the average queuing delay versus the offeredload per node, for various number of access nodes . In thecase of , the proposed FT-FR-TT-TR protocol providesvery low (almost zero) values of average queuing delay for loadup to 2000 Mb/s. This is because in this range there are enoughempty slots over all data channels to serve the incoming traffic.For higher loads, the queuing delay increases since the systemreaches maximum slot utilization. It is remarkable that the pro-posed FT-FR-TT-TR protocol achieves low average queuingdelay for the entire load range. This is because of the efficient

collisions avoidance algorithm which avoids the bandwidth con-sumption from the continuous packet loops and provides lowvalues of the average queuing and total delay, as well as of thedropping events. For example, for the high offered load value6000 Mb/s, the queuing delay reaches only 250 s. Finally, theaverage queuing delay of the proposed FT-FR-TT-TR protocolin the entire offered load range is almost 67 s. This value islow enough for MANs and can serve time-dependent traffic, likevideo and real-time services.

The average packet dropping probability at buffers versus theoffered load per node is shown in Fig. 6, for various numberof access nodes . The dropping events variation is estimatedin conjunction with that of queuing delay. In fact, in case of

the proposed FT-FR-TT-TR protocol provides verylow (almost zero) values of dropping probability for load upto 2000 Mb/s. In this range the queuing delay holds very lowvalues, as previously discussed. Above this range the droppingprobability gradually increases as a result of the gradual increaseof the queuing delay. For example, for and offered load6000 Mb/s the dropping probability reaches 0.18. This is be-cause in this case the proposed FT-FR-TT-TR protocol providesonly 250 s queuing delay. Thus, the buffers are early gettingempty and are capable of storing many incoming data packetsproviding few dropping events.

It is worth mentioning that in case of , the pro-posed FT-FR-TT-TR protocol provides average packet drop-ping probability almost 0.06 in the entire offered load range.This means that on average 6% of incoming from the access net-works packets are dropped. This average value is hardly accept-able for a metropolitan environment. This is because it causeslarge number of retransmitted packets which are added to theordinary traffic and provide performance deterioration. In orderto improve the protocol performance, the increase of the buffersize is investigated in the following.


Fig. 6. Average packet dropping probability versus average offered load per access node for � � 64, 120, 150,� � ��, � � ��.

Fig. 7. Average total �� delay versus average node throughput for � � 64, 120, 150,� � ��,� � ��.

The overall proposed FT-FR-TT-TR protocol performance isrepresentatively shown in Fig. 7 which depicts the average totaldelay versus the average node throughput, for various number ofaccess nodes . Indeed, the proposed FT-FR-TT-TR protocolreaches saturation at considerable high load values, while it pro-vides acceptable average total delay values.

It is evident that the proposed FT-FR-TT-TR protocol per-formance strongly depends on the following network parame-ters: the buffer size , the number of access nodes , and thenumber of data channels . The relation among them deter-

mines the system efficiency. In other words, the buffer sizeis associated with the dropping events, the number is relatedwith the data channels collisions, while the combination ofand determines the receiver conflicts.

The impact of the number variation is shown in Figs. 4–7that present the above performance measures variation for var-ious number 64, 120, 150 of access nodes and ,

. In Fig. 4, it is noticed that the maximum achievedaverage node throughput is a decreasing function of . This re-sult that conforms to the theoretical analysis of (14) is because


Fig. 8. Average total �� delay versus average node throughput for � � ��,� � 4, 8, 32, � � ��.

as increases, the incoming to the ring traffic is higher. Thus,the maximum node throughput that corresponds to system sat-uration is lower. For example, for , it is 3480 Mb/s,while for is reduced to 3250 Mb/s and forto 3075 Mb/s. Also, for the same node throughput, as in-creases, the maximum number of available data slots for trans-mission is lower. This is the reason why the average queuingdelay, dropping probability, and total delay essentially increase,as Figs. 5–7 show, respectively.

On the other hand, the impact of the number variationis depicted in Fig. 8 that shows the average total delay versusthe average node throughput for various number 4, 8, 32data channels (i.e., 1, 2, and 8 tunable receivers per access noderespectively) and , . Thus, for the same nodethroughput, the average total delay is a decreasing function of

. This is because as increases, the number of empty dataslots for transmission increases too. Thus, the time that a packethas to wait in the buffer to be transmitted is less. This results tolower average queuing and total delay. The immediate result isthat as increases the maximum node throughput increases,too. For example, it is1200 Mb/s for , while it reaches2350 Mb/s for , and 3500 Mb/s for .

Finally, the effect of the buffer size variation is depicted inFig. 9 that presents the average total delay versus the averagenode throughput for various buffer size 100, 300, 500data packets and , . It is shown that for nodethroughput up to 2400 Mb/s, the buffer size increase provides al-most identical values of total delay. This is because in this rangethe system has not reached the maximum data slot utilizationand the entire incoming load is able to access the ring withoutbeing dropped at the buffers. Above this value, the system grad-ually reaches saturation and the average total delay and drop-ping probability increase. Although, in this range, the increase

of buffer size provides reduction of dropping events, it unavoid-ably results to increase of queuing delay since the data slot uti-lization is optimum. In possible system implementation, the en-gineer decision to increase to reduce the average droppingprobability should balance and take into account, apart from thefinancial cost, the consequent average total delay increase.

It is obvious that the variation of the network configurationparameters , , determines the saturation conditions aswell as the overall system performance. Concluding, it is re-markable that although the proposed FT-FR-TT-TR protocolachieves to properly face the scalability issues and efficientlyimprove the bandwidth utilization by avoiding both the datachannels and the receiver collisions and adopting tolerance tonetwork faults, it introduces additional cost. This is because ofthe introduction of multiple devices per access node: the delaylines and the transceivers. The additional cost is due to nowa-days technology constraints about the time response reliabilityof the tunable transceivers. It is evident that the unavoidable costhas to be balanced with the tunability benefits, the collisions res-olution strategy, and the effective bandwidth exploitation.

V. CONCLUSION

This paper presents a slotted protocol to access the mul-tiple WDM wavelengths of a ring network architecture in ametropolitan area environment. The data slot reuse strategywith fairness control is followed. The network uses a separatecontrol wavelength to exchange control information prior tothe data transmission. Based on the control information, anefficient access algorithm is applied to avoid both the datachannels and the receiver collisions. Each access node isequipped a pair of fixed tuned transceivers. Also, each accessnode has one tunable transmitter and a set of tunable receivers(FT-FR-TT-TR). Theoretical analysis is provided to evaluate


Fig. 9. Average total �� delay versus average node throughput for � � ��,� � ��, � � 100, 300, 500.

the maximum node throughput of the proposed FT-FR-TT-TRprotocol. Also, a discrete-event simulation model is developedto estimate the performance under self-similar traffic giving arealistic investigation of the traffic burstiness.

The innovation of this study is the utilization of the proposedFT-FR-TT-TR node interface in conjunction with the data chan-nels and receiver collisions avoidance algorithm, to optimallyexploit the entire fiber bandwidth and to erase the packets losson the ring. Also, the strategy to inspect possible data channelsdeactivation provides tolerance to network faults. In oppositionto other WDMA protocols, the proposed FT-FR-TT-TR pro-tocol achieves to enhance the network scalability and mainte-nance, while it manages excellent performance especially underhigh load conditions. Thus, it achieves high maximum and av-erage node throughput values, while it provides significantlylow values of queuing delay, total delay, and packet droppingprobability. Finally, extensive investigation is given for the im-pact of the network parameters: the buffer size, the number ofaccess nodes, and the number of data channels.

This study could be a useful tool for engineers of WDMmetropolitan ring networks to adopt relevant WDMA protocolsto further improve the network performance by exploiting theuse of FT-FR-TT-TR node interface. Also, the presented sim-ulation study could tease the researchers’ scientific interest toinvestigate the effect of other network parameters, like the vari-able packet size, the number of queues per access node and thedata slot size.

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Peristera A. Baziana received the Diploma degreein electrical and computer engineering from the Uni-versity of Patras, Patras, Greece, in 1998. She is cur-rently working toward the Ph.D. degree in the fieldof architectures and protocols for optical networksat the Communications, Electronic, and InformationEngineering Division, School of Electrical and Com-puter Engineering, National Technical University ofAthens, Athens, Greece.

From 1999 to 2002, she participated in researchprograms for the Greek PTT Organization (O.T.E.) as

a researcher with the University of Patras. Her current research interests includeoptical communications, OBS networks, MAC protocols, and queuing analysis.She is a member of the Technical Chamber of Greece.

Ioannis E. Pountourakis (M’90) is a Professor atthe Communications, Electronic, and InformationEngineering Division, School of Electrical and Com-puter Engineering of National Technical Universityof Athens (N.T.U.A), Athens, Greece. His researchinterests include optical communication networks,network architecture and protocols, performanceevaluation, and stability. He has taught severalundergraduate and graduate courses at NTUA,supervised many doctoral students working in theareas of queuing analysis of contention resolution

mechanisms in local area networks and optical networks, WDM networkdesign, optical networks architectures, analysis of data link layer protocols,evaluation of performance of computer systems, etc., and has been reviewerfor international journals, conferences, and research project proposals. Hehas participated and organized many international conferences. He has alsoparticipated in several RACE projects and in several national research programsdealing with communication networks.

Prof. Pountourakis is member of the Greek society of Computer Science.

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An Efficient Metropolitan WDM Ring Architecture for a Slotted Transmission Technique