Computer Communications 33 (2010) S110–S121

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Computer Communications

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Metropolitan optical packet bus-based networks: Packet bursting and emulationof TDM services

Viêt Hùng Nguyen *, Tülin AtmacaTELECOM & Management SudParis – 9, Rue Charles Fourier, 91011 Evry, France

a r t i c l e i n f o

Article history:Available online 24 April 2010

Keywords:Circuit emulation serviceQuality of serviceModified packet burstingOptical packetPerformance evaluation

0140-3664/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.comcom.2010.04.023

* Corresponding author. Tel.: +33 664182600.E-mail addresses: viet-hung.nguyen@itron.com, vi

Nguyen), tulin.atmaca@it-sudparis.eu (T. Atmaca).

a b s t r a c t

The volume of network traffic, notably of data traffic, grows continuously and tremendously duringrecent years with the appearance of new value-added services such as video on demand, interactive videoconferencing, peer to peer data sharing, etc. Faced with this challenge, traditional circuit-switched net-works become inappropriate and costly. On the other side, optical packet-switched networks, which sup-port best data traffic thanks to their high flexibility, capacity, and cost-efficiency, appear the mostappropriate candidate. To optimise the transmission efficiency of packets in optical networks, a new tech-nology, Modified Packet Bursting (MPB), has been studied in the framework of a metropolitan opticalpacket bus-based network. Also, in order not to degrade the quality of classical circuit-switched TimeDivision Multiplexing (TDM) services, which still stay the major revenues of telecom operators today, Cir-cuit Emulation Service (CES) on packet-switched networks has been investigated by main standard orga-nizations. This paper presents the performance analysis of the above two technologies applied to ametropolitan optical packet bus-based network. It specially demonstrates the improvement of CES per-formance thanks to MPB features.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Metropolitan Area Networks (MANs) are principally Time Divi-sion Multiplexing (TDM), circuit-switched networks, includingSONET/SDH rings. They were primarily designed to transport voicetraffic, offering high reliability, survivability and superior quality tovoice communications. However the traffic pattern is excessivelychanging with the advent of high bandwidth applications such asmusic/video sharing, interactive video conferencing, video on de-mand, etc., the voice traffic becomes minor compared to multime-dia and data traffic. MANs are faced with a real challenge: mainlytransporting sporadic video and data traffic, while still being ableto offer high quality of service for the minor voice traffic as it rep-resents most revenues of service providers. Circuit-switched net-works were not able to handle data traffic efficiently and cannotscale cost-effectively to accommodate the rapid growth in datatraffic. Therefore modern MANs are moving away from traditionalcircuit-switched networking technologies towards high-speedpacket-switched networking technologies, supporting best datatraffic, offering high flexibility, cost-effective and ease of installa-tion solution.

ll rights reserved.

et_hung77@yahoo.com (V.H.

In order to cope with this new trend, an experimental opticalring architecture (DBORN [1]) has been designed as a flexible andcost-efficient solution for short-term metropolitan networks. Thisarchitecture uses mature Ethernet technology at the electronic le-vel and passive technology at the optical level. It is more efficientthan circuit switching or wavelength switching thanks to theEthernet-based packet-switching granularity. It also offers bettercost perspective than currently proposed Resilient Packet Ring(RPR [2]) solution, thanks to optical passive technology used at ringnodes.

This paper presents two novel technologies applied to the abovementioned network. The first one is Modified Packet Bursting(MPB) [11,12,14] technology that aims to improve the optical Med-ia Access Control (MAC) protocol based on Optical Carrier SenseMultiple Access with Collision Avoidance (Optical CSMA/CA).MPB allows reducing optical overhead volume used for transmit-ting electronic packets, thus improving network resource utilisa-tion efficiency and, as a result, network performance. It extendsthe principle of packet bursting (PB) technology introduced byGigabit Ethernet network [3].

The second technology presented in this paper is Circuit Emu-lation Service (CES) which allows the transport of TDM service(e.g., voice and video) over a packet-switched network. As statedabove, an important challenge for MAN service providers whiledeveloping packet-switched networks is the ability to convergeboth TDM and data services on the same network infrastructure,

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providing a cost-effective and high service-integration solution. Atechnology such as CES should provide the same reliability andquality of transporting TDM service as that provided by existingcircuit-switched networks. This work analyses the feasibilityand performance impact when deploying CES on DBORN. It ex-tends our previous works in [4] while providing deeper perfor-mance analysis. Concretely we specially investigate theperformance analysis of the interaction between MPB and CEStechnologies, and their influence on the global networkperformance.

The rest of this paper is organised as follows. Section 2 de-scribes the optical network architecture and its main features.Section 3 focuses on MPB technology. It specifically introducesa novel concept, the bursting timer, which is associated withMPB mechanism, remarkably enhancing the MPB performance.Some definitions on performance parameters used to evaluateMPB are also given in this section. Section 4 presents CES tech-nology with the considered CES model, CES performance param-eters and QoS requirements. Sections 5 and 6 respectivelyprovide simulation results on MPB and CES performance. Finally,Section 7 finishes with some conclusions and discussions on fu-ture work.

Fig. 2. Optical Ethernet Frame (OEF) structure.

2. Optical network architecture and main features

2.1. Optical bus-based architecture

The network considered in this work is a metropolitan opticalEthernet ‘‘ring” network, the so-called DBORN for Dual Bus Opti-cal Ring Network. We briefly describe this network architecturein this section. For more details the lectors are invited to referto [1]. DBORN logically consists of two unidirectional buses spec-trally disjoint: a transmission (upstream) bus that provides ashared transmission medium for carrying traffic from several ringnodes to a Hub node; and a reception (downstream) bus carryingtraffic from the Hub node to all ring nodes. Thus, a ring node al-ways ‘‘writes” to the Hub node employing the transmission busand ‘‘listens” for the Hub node using the reception bus. The trafficemitted on the transmission bus by a node is first received by theHub node, then is either switched to the reception bus to reachits destination node, or is routed to other MAN or backbone net-works (see Fig. 1).

Fig. 1. Metropolitan all-op

The spectral separation allows the use of a simple passive struc-ture for the optical part of ring nodes. This means that optical pack-ets travel along the ring without any opto-electronic conversion atintermediate nodes. Indeed, at the optical level, ring nodes use anoptical splitter to separate an incoming signal into two identicalsignals: the main transit signal and its copy used for the control.A Fibre Delay Line (FDL) creates a fixed delay on the transit pathbetween the control and the add/drop function. Thus, neither ac-tive optical device nor opto-electronic conversion is employed tohandle transit optical packets. Once the optical packet is transmit-ted on the ring, it cannot be dropped before arriving to the hub.

Before continuing to describe some features of the network, wenotice that in this work we only interested in the analysis andimprovement of the performance of ring nodes on the upstreambus, because the later is a multiple access sharing resource (multi-point-to-point communication) that raises many problems of re-source access control, sharing fairness, etc. These problems donot come up for the communication on the downstream bus sinceit simply is point-to-multipoint communication.

2.2. Optical Ethernet frame format

To have a simple and flexible architecture, Ethernet is used inDBORN as the convergence layer for the data plane. The structureof an optical Ethernet frame (OEF) is shown in Fig. 2. The PDUEthernet frame size is kept unchanged (from 64 bytes to 1518 by-tes). The existing standard extensions (IEEE 802.1Q/802.1p/802.3ad) are still applicable. As DBORN uses burst mode trans-ceiver (BMT) at the hub node for the packet-by-packet detectionon the upstream bus, an optical preamble field of 16 bytes is re-quired for optical frame. Finally, the optical frame includes an in-ter-packet gap (IPG) of 25 ns (i.e., approximately 8 bytes at

tical ring architecture.

Fig. 3. Schema of CSMA/CA in DBORN.

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2.5 Gbs) that allows an asynchronous insertion of frames on themedia.

2.3. Media access control protocol

The main issue in terms of logical performance when usingEthernet is the collision-free packet insertion on a multiple accesswriting bus (i.e., the upstream bus). In order to avoid collision withtransit packets during local packets transmission, a media accesscontrol (MAC) protocol is required at ring nodes. DBORN hasadopted an optical Carrier Sense Multiple Access with CollisionAvoidance (CSMA/CA) based on void detection to ensure packetinsertion on the upstream bus. Fig. 3 shows the schema of opticalCSMA/CA used in DBORN.

As stated earlier, incoming transit packets pass transparentlythrough the local FDL of ring nodes. The local FDL size isslightly larger than the Maximum Transmission Unit (MTU) ofthe used communication protocol, to provide the MAC logic unitwith sufficient time to listen and to measure the media occu-pancy. It also creates a sliding detection window for the MAClogic unit. Once a free state (i.e., a void) of the media is detectedthanks to a signal detection photodiode, the MAC unit measuresthe size of the progressing void. The ring node will begin insert-ing a local packet to fill a void only if the void is large enough.Otherwise, local packet is buffered in the electronic memory ofthe ring node until a large enough void is detected. Notice thatwith this mechanism, a detected void size is not necessarilylimited by the size of the FDL, since the sliding observation win-dow allows a progressing void measurement. Thus, a ring nodecan observe a void whose size is larger than the FDL size(�MTU).

2.4. Traffic control mechanism

In a bus-based topology, fairness issues for bandwidth accessare likely to raise between ring nodes sharing a common transmis-sion channel (wavelength). Indeed, considering the upstream busof DBORN, ring nodes that start the bus (i.e., upstream nodes)can grab all available bandwidth, and, as a result, block the trans-mission of ring nodes close to the hub node (i.e., downstreamnodes). This leads to performance degradation of downstreamnodes. Therefore, in order to face the unfairness issues on the up-stream bus, a traffic control mechanism called TCARD (Traffic Con-trol Architecture using Remote Descriptors) [5] has been designedin the framework of DBORN.

TCARD is a preventive mechanism that statistically guaranteesthe access to the resource for downstream nodes. With TCARD,mean bandwidth is preserved by a ring node according to trafficrequirements of downstream nodes (e.g., based on Service LevelAgreements – SLA). This is completed by the generation of anti-to-kens (ATOK), whose size is around the Ethernet MTU (�1500 by-tes), that forbid the node emission, and hence preserving voidsfor downstream nodes. Indeed, a flow of ATOK is generated withconstant rate at each ring node. Each time an ATOK is generated,the ring node’s transmission is forbidden until a void of size equalto ATOK size has been reserved by the node. Since the ATOK size isaround the Ethernet MTU, this reserved void is sufficient for theinsertion of any packet size at downstream nodes. In this mecha-nism, the ATOK rate is easily computed from the average requiredbandwidth of all downstream nodes. For example, consider an up-stream bus that is shared by three ring nodes, each node requires100 Mbs. Then the ATOK rate at the first ring node is equal to200 Mbs/(1500 � 8 bits) � 16,667 ATOK/s, and that at the secondring node should be 100 Mbs/(1500 � 8 bits) � 8333 ATOK/s, andso on. Obviously the last ring node does not need to reserve band-width for any node, therefore the ATOK rate at this node is zero.

More details about the DBORN architecture and the implemen-tation of TCARD mechanism can be found in [1,5]. Note thatdepending on the volume of client traffic, TCARD mechanism canbe combined (enabled) or not (disabled) with the MAC protocolin order to ensure the functioning of the network.

3. Modified packet bursting technology

This section discusses the new technology aiming to improvethe performance of the optical CSMA/CA protocol previouslydescribed.

3.1. Packet bursting in Gigabit Ethernet

The recent development of the Gigabit Ethernet standard [3]introduced a new technique, called Packet Bursting (PB), in theMAC layer. In Gigabit Ethernet, the ‘‘carrier extension” was intro-duced in order to guarantee the interoperability of Gigabit Ether-net with existing 802.3 Ethernet networks. The slot time wasincreased from 64 bytes to 512 bytes. This is required for the col-lision detection in the half-duplex CSMA/CD (CSMA with CollisionDetection) transmission mode. Thus, if a packet size is smallerthan 512 bytes, the extension field is filled with extension sym-bols (padding) to bring the minimum length of the transmission

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frame up to 512 bytes. This technique highly decreases the proto-col efficiency for transmission of small size packets (e.g., for apacket of 64 bytes, the padding is up to 448 bytes).

Packet bursting mechanism was introduced to counteract thedisadvantage of ‘‘carrier extension” used in Gigabit Ethernet tech-nology. It allows a given transmission node to keep control of themedia without relinquishing it between two consecutive frames.When a node has several packets to be transmitted, the first packetis transmitted with carrier extension if necessary. Subsequentpackets are transmitted back to back with the minimum Inter-packet gap (IPG), until a burst timer (of 1500 bytes) expires. Moredetails on packet bursting process and performances can be foundin [3].

3.2. MPB basic principle

The modified packet bursting (MPB) mechanism presented in thispaper is based on the same principle as native packet bursting (i.e.,Gigabit Ethernet packet bursting), but with further improvements.The minimum time slot in DBORN is kept to 64 bytes, thereforethere is no consideration of the carrier extension introduced inGigabit Ethernet. However, the issue of protocol efficiency stillarises for small size packets, due to the proportion of optical pre-amble and IPG. (From here we use the terms optical overhead asthe reference to both optical preamble and IPG.) For instance, thetransmission at 2.5 Gbs of a electronic packet of 64 bytes needsan optical overhead of 24 bytes. Hence the protocol efficiency is46/70 = 72%, which is poorly efficient.

As stated above, ring nodes and hub node in DBORN uses BMT(burst mode transceiver) for an asynchronous packet transmissionand reception. The BMT at ring nodes (transmission side) requiresthe IPG field in the optical frame for an asynchronous packet inser-tion on the media. The BMT at the hub node (reception side) re-quires an optical preamble field (PRE) for a packet-by-packetdetection. However, considering optical frames transmitted bythe same BMT of a ring node, once the IPG has been preservedfor the burst mode transmitter and the burst mode receiver hasbeen synchronised thanks to the optical preamble, there is no morea need to repeat those fields for consecutive frames. This observa-tion led us to consider the MPB mechanism, which consists in min-imising the use of optical overheads for transmission of electronicpackets (therefore maximising protocol efficiency).

Fig. 4 illustrates the principle of MPB. If one node has severalpackets to transmit, the first packet is emitted with an optical over-head. Other packets are transmitted back to back without opticaloverhead and without any gap. This process creates on the trans-mission media a sequence of concatenated electronic frames ledby one optical overhead. We call that an Optical Ethernet Concat-enated Frame (OECF).

An important modification on MPB, compared to the nativepacket bursting in Gigabit Ethernet, is that no limit on OECF lengthis imposed. OECF length can be dynamically sized according to the

Fig. 4. Sequence of frames w

distribution of free bandwidth (voids) on the media. It is limitedonly by the volume of traffic to be transmitted, and by the avail-ability of free bandwidth. Concretely, MPB allows one node, whichis transmitting on the detected progressing void, to keep control ofthe media to emit its electronic packets, until the end of the cur-rent void occurs or there are no more electronic packets to trans-mit. Thus the length of the OECF formed by those electronicpackets is not limited to 1500 bytes as in native packet bursting.However, note that even if the length of an OECF can be in theoryinfinite, the size of each electronic packet composing this OECF isalways limited to the MTU of the used transmission protocol (i.e.,�1500 bytes in Ethernet case).

3.3. Bursting timer principle

The protocol efficiency is defined as the proportion of opticaloverhead used to transport electronic packets. Thus the OECFlength is an important factor that impacts the protocol efficiencyof MPB. Indeed, as the optical overhead length is fixed (e.g., 24 by-tes at transmission rate of 2.5 Gbs) for each optical frame transmis-sion, the increase of OECF length leads to the increase of MPBprotocol efficiency. The basic concept of MPB improves the proto-col efficiency of a node only if there exist, at a given moment, atleast two electronic packets waiting for transmission at the node.But what happens if the node has only one packet in the electronicbuffer when it detects a new void on the media? It is clear that thetransmission efficiency cannot be improved if the electronic packetis transmitted alone with an optical overhead.

This observation led us to propose a new mechanism, calledbursting timer (BT), which further enhances the protocol efficiencyof the network by rendering more ‘‘active” the MPB operation.Bursting timer mechanism tries to impose (if possible) a minimumlength for each OECF. As a result, it tries to impose an inferior limitfor MPB protocol efficiency. More specifically, BT unit forbids MPBunit at each ring node from emitting OECF until a bursting timerexpires. During this time, many electronic packets are (probably)accumulated in the local electronic buffer waiting for transmission.Once the bursting timer expires, MPB unit can build up a big OECFthanks to gathered electronic packets, hence increases its protocolefficiency. However, introducing BT mechanism in MPB processcan lead to an increase of access delay for electronic packets (i.e.,additional waiting time during bursting timer period). Thus a com-promise between the protocol efficiency and the access delayshould be considered in the parameterisation of BT. Also, BT mustbe designed in such a way that it should not introduce excessivedelay for high priority traffic, notably for delay-sensitive trafficsuch as voice.

3.4. MPB and bursting timer processes

The BT and MPB processes implemented at an access node arerespectively described in Figs. 5 and 6. Each time the MAC unit

ith and without MPB.

Fig. 5. Bursting timer process.

Fig. 6. MPB process.

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detects a void on the media, it informs BT and MPB units. BT unitfirst looks at its local electronic buffer, allowing OECF transmission(i.e., BT is off) if the current buffer length is sufficient to build a bigOECF (i.e., buffer length must at least equal to a threshold definedby BT size parameter). In order not to introduce excessive delay forelectronic packets, notably for high priority (e.g., voice) traffic, BTunit also permits OECF transmission when the first packet on thetop of local buffer has high priority, or has waiting time exceedinga duration threshold given by BT time parameter. BT unit naturallydoes not block OECF transmission if an OECF is currently underconstruction. In all others cases, OECF transmission of the node isblocked (i.e., BT is on).

Informed by MAC unit about the detection of a void, MPB unitfirst verifies whether it has permission of transmission given byBT unit. In case where BT is off, MPB unit looks at its local elec-tronic buffer for the first electronic packet. If an OECF is under con-struction, the electronic packet is padded at the end of the currentOECF if its size matches the detected void size. In case where noOECF is under construction, if the total size of the electronic packetplus an optical overhead (24 bytes at 2.5 Gbs) matches the voidsize, the optical overhead is added to the electronic packet and anew optical frame is transmitted within the void. As we can notice,MPB process does not impose any limit on the length of OECF. Butwith the introduction of BT mechanism, the minimum length ofOECF could be increased thanks to a number of electronic packetsaccumulated in local buffer during the on-time of BT, hence theMPB protocol efficiency could be increased.

3.5. MPB performance parameters

We focused on the main performance parameters below: packetloss rate, average protocol efficiency, average access delay, net-work effective load and useful load.

Packet Loss Rate (PLR) at a ring node is the number of lost elec-tronic packets divided by the total number of electronic packets re-ceived by the node during the simulation time.

Average protocol efficiency (APE) for optical frames (OEF/OECFs)is the average proportion of client data that they contain. The APEis expected to be highest possible. If we call n the number of OEF/OECF emitted by an access node, and mi the number of electronicpackets in each OEFi/OECFi, then we have the formula for comput-ing the APE for an access node below:

APE ¼ 1n

Xn

i¼1

Pmij¼1EthernetPDUj

Overheadi þPmi

j¼1EthernetPDUjð1Þ

Average access delay (AAD) is the average waiting time from the mo-ment when a electronic packet is inserted in the electronic localbuffer of a node, until it is transmitted successfully on the media.AAD is expected to be lowest possible.

Effective load (EL) is the number of bits transmitted through thenetwork in a time unit divided by the bit rate of the network. TheEL includes client electronic packets as well as optical overheadused to transport these packets. Useful load (UL) is the size in bitsof all client electronic packets transmitted through the networkin a time unit divided by the bit rate of the network. The UL isnot aware of optical overhead transmitted in the network. EL is ex-pected to be lowest possible, but UL is expected to be highestpossible.

4. Circuit emulation service technology

4.1. Introduction

Traditionally, voice has been carried over Time Division Multi-plexed (TDM) based networks. TDM based networks such as

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SONET/SDH offer high reliability and survivability for connectionsand predictable delays for voice samples, thus providing a superiorquality. Nevertheless, the widespread development and deploy-ment of packet-switched networks due to data traffic growth lednetwork provider to a new challenge: transport of TDM (not onlyvoice but also video) traffic and data traffic over the same pack-et-switched network architecture. This convergence of TDM anddata traffic in an existent packet-switched architecture could saveconsiderable equipment and installation cost. The problem to re-solve for network providers is to find a technology that providesthe same reliability and quality of transporting TDM traffic as inexisting TDM based networks.

Circuit Emulation Service (the so-called CES) is a technologyallowing the transport of TDM service such as PDH (E1/T1/E3/T3)as well as SONET/SDH circuit over a packet-switched network. Cir-cuit emulation originally comes from Asynchronous Transfer Mode(ATM) world [6]. The idea has been taken up in the packet-switched world by a number of organisms, including the InternetEngineering Task Force (IETF), the Metro Ethernet Forum (MEF)and the Multi-Protocol Label Switching (MPLS) forum. The Pseu-do-Wire Emulation Edge to Edge (PWE3) working group in the IETF[7] is setting the main CES standards. This group is chartered to de-velop methods to carry Layer-1 and Layer-2 services across a pack-et-switched network (principally IP or MPLS). Hence the group islooking at TDM circuit emulation, and also carriage of Layer-2 ser-vice such as ATM, Frame Relay and Ethernet across a packet-switched network. The Metro Ethernet Forum [8] is looking to ex-tend the work of the PWE3 group to make it applicable to a metro-politan Ethernet context. Similarly, the Multi-Protocol LabelSwitching, Frame Relay and ATM alliance (MFA forum) [9] is alsolooking at the same standards from the point of view of an MPLSnetwork.

DBORN that was designed as a metropolitan packet-switchednetwork should naturally support the transport of TDM traffic.Our previous work in [4] has described the studied model of CESon DBORN network, and also provided the first CES performanceanalysis. We briefly resume our previous work on model, qualityof service (QoS) requirements and performance parameters ofCES in the following subsections.

4.2. CES model and QoS requirements

4.2.1. Reference model of CESThe reference model for CES on DBORN [4] is based on the glo-

bal model for circuit emulation described in PWE3 draft [7]. Fig. 7presents the general model of CES on DBORN. We have two TDMcustomers’ edges (CE) communicating via DBORN. One CE is con-nected to a ring node (ingress CE), the other CE is connected tothe hub node (egress CE). TDM service generated by ingress CE istransported/emulated by DBORN to egress CE. The emulatedTDM service between two CEs is managed by two inter-workingfunctions (IWF) implemented at appropriate nodes.

Fig. 7. CES refere

CES has two principle modes of functioning. In the first one,called ‘‘unstructured” emulation mode, the entire TDM servicebandwidth is emulated transparently. The frame structure ofTDM service is ignored. The ingress bit stream is encapsulated intoan emulated TDM flow (called also CES flow) and is reproduced atthe egress side. The second mode, called ‘‘structured” emulation,requires the knowledge of TDM frame structure being emulated.Individual TDM frames are visible and are byte aligned in orderto preserve the frame structure. ‘‘Structured” mode allows frame-by-frame treatment, permitting overhead stripping, flow multi-plexing/demultiplexing. In the reference model of CES, the NativeService Processor (NSP) block performs some necessary operations(in TDM domain) on native TDM service such as overhead treat-ment or flow multiplexing/demultiplexing, terminating the nativeTDM service coming/going from/to CE.

The main functions of an Inter-Working Function are to encap-sulate TDM service in transport packets (i.e., Ethernet packets inour case), to perform TDM service synchronisation, sequencing,signalling, and also to monitor performance parameters of emu-lated TDM service. Each TDM emulated service requires a pair ofIWF installed respectively at the ingress and egress provider edges(PE). The aim of our study is to evaluate the logical performance ofCES on DBORN. Hence, we ignore some operations and functionalblocks in the CES reference model, which are outside the scopeof this work, such as the synchronisation and signalling aspect ofthe IWF block, and also the functioning of the NSP block.

4.2.2. Logical model of CES implemented at a ring nodeThe logical model of a DBORN access node supporting CES is de-

scribed in Fig. 8. Each access node (CES ingress side) is composed ofan electronic part and an optical part. The incoming TDM service(treated as ‘‘structured” or ‘‘unstructured”) is mapped into Ether-net packets thanks to the Inter-Working Function (IWF) block. Astatic segmentation mechanism, which fragments TDM frames intosmaller segments according to a predefined threshold, is appliedon large TDM frames in order to fit them to Ethernet packet. Ether-net packets transporting data service and Ethernet packets trans-porting TDM service are aggregated into local electronic buffers.Here all packets are classified, according to their destinations andclasses of service (CoS), into three separated buffers correspondingto three CoS. A scheduler, taking into account the CoS priority, dis-tributes all packets from local buffer to temporary sending elec-tronic buffers. Optical Ethernet frames (OEF) are built by addingan optical preamble (Pr) to each electronic packet, and then aresent on appropriate wavelength. In case where MPB mechanismis enabled, MPB unit builds optical Ethernet concatenated frames(OECF) and send them over the ring.

At the egress side (the HUB node), the same architecture withsome modifications concerning IWF block is used. A jitter buffer(or playout buffer) is introduced in IWF block in order to accom-modate the expected TDM frame jitter. The main function of theegress IWF at the hub node is to measure the CES flow performance(e.g., delay, jitter, and loss) based on delivered Ethernet packets

nce model.

Fig. 8. Logical DBORN access node architecture.

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transporting TDM service. The other aspects of egress IWF, such asjitter buffer dimensioning and reconstruction of native TDMframes, are not considered in this work.

4.2.3. CES Ethernet packet formatWe adopted the CES packet format proposed in the PWE3 draft

[7] (Fig. 9). A CES control word is added to each TDM payload. Themain functions of the CES control word are to differentiate the net-work outage and the emulated service outage, to signal problemsdetected at the IWF egress to the IWF ingress, to save bandwidthby not transferring invalid data, and to perform packet sequencingif RTP header is not used. An RTP header may be added to theresulting packet for the synchronisation and sequencing. Thenew resulting packet is encapsulated in the CES Ethernet packetby adding Ethernet and multiplexing header. All details about thestructure of CES control word and RTP header are described in [7].

4.2.4. Segmentation mechanism for TDM framesAs we explained above, in order to perform CES on DBORN, TDM

frames are encapsulated into Ethernet packets. A TDM framewould ideally be relayed across the emulated TDM service as a sin-gle unit. However, when the combined size of TDM frame and itsassociated header exceeds the MTU size of DBORN, a segmentationand re-assembly process should be performed in order to deliverTDM service over DBORN.

We have proposed two segmentation mechanisms. The firstone, called dynamic segmentation, fragments a TDM frame intosmaller segments according to void size detected on the media(wavelength) by the MAC unit. This approach promises a gooduse of wavelength bandwidth, but technically it is complex toimplement. The second one, called static segmentation, segmentsthe TDM packet according to a pre-defined threshold. This tech-nique is simple to implement, and it provides resulting TDM seg-ments with predictable size. Thus current TDM monitoring

Eth, Mux Header TDM Payload

12 octets

RTP Header

4 octets

CES Control Word

Fig. 9. CES Ethernet packet format.

methods could be reused, simplifying the management of CES. Inthe framework of this study, we used the static segmentationmethod to evaluate the performance of CES on DBORN.

Segmentation threshold is a parameter that we have to deter-mine during this work. In [7] authors have recommended somerules to determine the segmentation threshold. First, the segmen-tation threshold should be either an integer multiple or an integerdivisor of the TDM payload size. For example, for all unstructuredSONET/SDH services, the segmentation threshold could be an inte-ger multiple of STS-1 or STM-0 frame of 810 bytes. Second, forunstructured E1 and DS1 services, the segmentation threshold forE1 could be 256 bytes (i.e., multiplexing of eight native E1 frames),and for DS1 could be 193 bytes (i.e., multiplexing of eight nativeDS1 frames).

4.2.5. QoS definitionTo be able to support circuit emulation in DBORN, we have de-

fined three CoS for electronic packets as given in Table 1. Ethernetpackets transporting TDM service require very high quality of ser-vice, therefore they are given the highest priority. The media class(data traffic with guarantee of QoS) can be considered as pseudoreal-time traffic (e.g., video streaming). The CoS3 or Best-Effort(BE) class is sporadic Internet traffic, which has no guarantee ofQoS.

4.3. CES performance parameters

We focused on three main parameters for CES performanceevaluation: CES Ethernet Frame Loss (FL), CES Ethernet end-to-end Frame Delay (FD) and CES Ethernet Frame Jitter (FJ).

The CES Ethernet Frame Loss is defined as the ratio of lostEthernet frames carrying TDM service among total sent Ethernetframes carrying TDM service. The CES Ethernet end-to-end FrameDelay (FD) is the maximum delay measured for a percentile P(superior to 95%) of successfully delivered Ethernet frames carry-ing TDM service over a measured interval T. The CES EthernetFrame Jitter (FJ) is derived from the FD measured over the samemeasurement interval T and percentile P. FJ is obtained by sub-traction of the lowest frame delay from FD. FJ is typically usedto size the Jitter buffer at the egress side. These parameters mustmeet the MEN requirements for CES given in [8]. Concretely, FL

Table 1QoS definition.

CoS type Service QoS

Priority PLR Delay Jitter

CoS1 TDM High 10�9 Strictly limited Strictly limitedCoS2 Data with QoS guarantee Media 10�6 Limited LimitedCoS3 Best-Effort (BE) Low No guarantee No guarantee No guarantee

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and FD shall be kept to a minimum, and FJ shall not exceed10 ms.

Fig. 10. PLR vs. ring node rank.

Fig. 11. APE vs. ring node rank.

5. Simulation parameters and results on MPB performance

We use discrete-event network simulator tool to simulate thestudied network with eight nodes sharing one wavelength operat-ing at rate R = 2.5 Gbs (we only deal with one wavelength in oursimulations). The size of electronic buffer at each node can be setto a large value since electronic memory cost today is negligible.However, as shown in our previous work [13,14], the mean num-ber of client packets in the electronic buffer merely is of some tenspackets for the network load less than 0.60. Thus in order to be ableto study the network performance under higher loads, we choosefor our simulations the buffer size of 250 Kbytes, which is equiva-lent to some 170 Ethernet packets of maximum length(1500 bytes).

The incoming best-effort traffic is modelled as an aggregation ofmany Interrupted Poisson Process (IPP) sources configured withburstiness of 10 (i.e., peak source rate is ten times mean sourcerate) and mean burst length of 3 packets. We assume that IPPsources generate packets whose the length distribution is closeto the ‘‘real-life” Internet packet length distribution [10]. Thismeans that there are three types of packet in the best-effort traffic:short length packet of 50 bytes (10% total volume), medium lengthpacket of 500 bytes (40% total volume) and long length packet of1500 bytes (50% total volume). Finally, we assume that all nodesshare the same rate of packet arrivals and the same BT parameters(i.e., uniform traffic and identical configuration at all ring nodes).The bandwidth reservation TCARD mechanism is not used in thisexperimentation.

Before analysing the simulation results, it is worth noting thatall mean values in our simulation results are computed with anaccuracy of no more than a few percents at 95% confidence levelusing Batch Means method. In the following paragraphs we willcompare the network performance when MPB is used (MPB case)with the case where only classical MAC protocol is used (MAC case).We set average arrival rate at each access node to around 250 Mbs,which gives the average offered ring load about 80%. For the choiceof BT parameters, we first notice that the protocol efficiency for thetransmission at 2.5 Gbs of an optical frame whose size equal to theMTU size of DBORN is high (1500/1524 � 98.43%). This value canbe considered as good protocol efficiency. Therefore we suggestthat the BT size value should be superior or equal to the DBORNMTU size in order to effectively improve the MPB protocol effi-ciency. Note that if we set the BT size to zero we obtain the basicMPB mechanism without bursting timer. The BT time actually isthe transmission duration at the used wavelength bit rate (e.g.,2.5 Gbs) of the number of bits defined by BT size.

We first look at the packet loss event in this simulation. Fig. 10plots the PLR measured at each ring node, comparing the MAC caseto the MPB case with different values of BT size. We observe thatthe MAC case evokes highest PLR at several downstream nodes, fol-lowed by the MPB case with BT size equal to zero, to MTU, to 4times MTU and so on. The very high PLR (almost 100% at the 8thnode) measured in MAC case shows the inefficiency of the classical

MAC protocol using CSMA/CA in sharing the wavelength band-width, notably at heavy ring load. This evolution of PLR at ringnodes in MAC case is readily explained by the positional and band-width fragmentation properties of the studied network. Indeed, foroptical CSMA/CA protocol, it was pointed out in [13,14] that underheavy load the upstream nodes may monopolise all the bandwidth(or fragment it into small unusable voids) and cause head-of-the-line (HOL) blocking at downstream nodes. On the contrary, withMPB mechanism, the PLR strongly decreases even at the mostdownstream node when using relatively big values of BT_size(e.g., no loss any more when BT size exceeds 10 times MTU).

To understand how MPB reduces the PLR, we now investigatethe average protocol efficiency and the average OECF length ob-tained at each node (Figs. 11 and 12) with the same simulationparameters in the preceding example. For the uniform workloadconsidered, all nodes have the same (relatively small) mean ofprotocol efficiency in MAC case since each OECF only contains

Fig. 12. Average OECF length vs. Ring node rank. Fig. 13. AAD vs. ring node rank.

Fig. 14. EL & UL for different configurations.

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one client payload. In case with MPB, the average protocol effi-ciency at each node clearly increases with the growth of OECFlength. (Exceptionally for the case of BT size equal to zero we ob-serve that the OECF length increases at some downstream nodesand suddenly drops at the last ring node. This is simply due tothe fact that almost 100% of electronic packets are dropped atthe last ring node (see Fig. 10), which leads to an inexact computa-tion of OECF length in this case.) The bigger the value of BT_size(and hence BT_time), the higher the probability of having a bignumber of client payloads accumulated in the buffer of the nodeduring the ‘‘gathering period” of MPB, therefore the longer theaverage length (accordingly, the higher the average protocol effi-ciency) of OECF. The gain in terms of transmission efficiency ofMPB versus OU-CSMA/CA is likely to vary from some 10–20%depending on the value of BT_size.

Figs. 11 and 12 also show that owing to the unfairness prop-erty of the ring that reduces the accessible bandwidth (i.e., lim-ited void length) of downstream nodes, some downstreamnodes cannot build as long length OECF as upstream nodes canbuild (e.g., OECF length slightly decreases at some downstreamnodes), even if they may have the same quantity of payloads intheir buffers. In other words, a downstream node must uninten-tionally repartition the total volume of payloads in its buffer intoseveral smaller OECFs instead of one big OECF due to limited voidlength.

Now we may explain the drop of PLR when MPB is employed.Recall that the OCB length at a given node depends not only onthe number of client payloads accumulated on the buffer, but alsoon the availability of accessible bandwidth for the node. It is easyfor the first few upstream nodes to form big OCB since they caneasily access to the transmission bandwidth. But it is generally dif-ficult for other downstream nodes to find large void to transmit bigpackets in MAC case. In the above results, we can notice that withMPB, downstream nodes are able to build very long length OECFwith big values of BT_size. This means that MPB is able to provideconsiderably larger accessible bandwidth for downstream nodes incomparison with the MAC case (thus, explaining why the PLR atdownstream nodes strongly drops in case with MPB). A reasonfor this behaviour is that big value of BT_size (hence BT_time) atan upstream node implies long ‘‘idle” (‘‘gathering”) period of thisnode before each OECF transmission. This amounts to saying thatdownstream nodes benefit from free bandwidth left by upstreamnodes during their ‘‘idle” periods. Clearly, the bandwidth releasedfor downstream nodes grows with the increase of BT_size at up-stream nodes.

Let us now analyse the average access delay curves in Fig. 13.We observe that the MAC case provides lowest access delay at up-stream nodes, but highest access delay at downstream nodes (ex-cept at the last node the computation of access delay iserroneous due to very high (99%) electronic packets loss rate asshown in Fig. 10). On the contrary the MPB cases give higher accessdelay for electronic packets at upstream nodes but offer very lowaccess delay (notably when BT size is big) compared to MAC caseat downstream nodes. The explanation for this behaviour is verysimilar to that given above. Indeed, the behaviour of the curvefor MAC case is due to the positional priority. In MPB case withBT size equal to zero, thanks to the reduction of optical overheadvolume by including many electronic packets in one OECF, ringnodes can: on one hand reduce the waiting time for other elec-tronic packets in the local buffer; on the other hand leave more freebandwidth for other downstream nodes. This leads to a slight de-crease of AAD at downstream nodes. When BT size becomes higher,because of the time constraint introduced by MPB at upstreamnodes, the AAD of electronic packets at upstream nodes increases.Higher BT time value obviously produces higher AAD at upstreamnodes. However, during the period where upstream nodes are for-bidden from transmission by MPB mechanism, downstream nodesbenefit from free bandwidth left by upstream nodes to transmittheir traffic and, as a result, they reduce access delay for their elec-tronic packets. Thus MPB balances access delay at ring nodes onthe upstream bus. Furthermore, as BT mechanism is easily cus-tomisable with its BT size and BT time parameters, we can com-pletely imagine a scenario where all access nodes have the same

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AAD, promising an AAD fairness protocol for a shared mediaarchitecture.

In Fig. 14, we measure the effective bandwidth and useful band-width (or effective load and useful load respectively) that the net-work uses to transport client traffic in the same experiment above.Owing to very high PLR, the MAC case provides useful load lowerthan the one offered by client traffic (i.e., 63.53% against 80%),and it needs some 5% added bandwidth for optical overheads. Onthe contrary, MPB with big values of BT_size is able to transport100% offered traffic (i.e., useful load is equal to offered load of80%) with some 0.1% added bandwidth merely for optical over-head. Note that the gain in terms of optical overhead may changewith the change of wavelength bit rate as the IPG will change, andwith the change in client packet length distribution. Our results in[14] indicated that MPB may reduce the wasted volume of band-width carrying optical overheads up to �30% (compared to MACcase) when there are 100% short length client packets offered tothe network.

6. Simulation parameters and results on CES performance

We use the same simulation model as in the study of MPB per-formance, except some modifications below. Each ring node is nowequipped with three CoS buffers as shown in Fig. 8. At each accessnode, the size of the CoS3 buffer which stocks sporadic BE datatraffic is set to 250 Kbytes according to above stated reasons. TheCoS2 buffer is used to stock QoS-guarantee data traffic. In realitythe volume of data traffic requiring QoS is usually smaller thanthe volume of best-effort data traffic, thus we suppose that theCoS2 traffic requires smaller buffer than the best-effort data traffic.Then we set CoS2 buffer size to 100 Kbytes. We also set the CoS1buffer capacity to 100 Kbytes since it serves only constant TDMtraffic that requires generally small buffer.

To model the packet arrival process at each access node, weused three types of traffic source. The Constant Bit Rate (CBR)source is used to model TDM traffic. The CoS2 traffic is modelledby an exponential source for the reason of simplicity. It generatespackets with size of 250 bytes. The same source type and parame-ters studied in Section 5 are used to model the sporadic BE datatraffic.

The offered traffic is also repartitioned uniformly. This time, theTCARD mechanism is enabled at each access ring node, statisticallyallocating for downstream nodes a bandwidth equal to their meanbit rate.

Fig. 15. PLR vs. ring node rank.

6.1. Preliminary performance analysis

In [4] we have proved that a segmentation threshold of 810 by-tes, which is the STS-1 frame size, is appropriate to emulateunstructured SONET/SDH circuits. Other unstructured TDM ser-vices whose frame size is under 810 bytes (e.g., E1/T1/E3/T3 PDHservice) could be emulated entirely (without frame segmentation)in DBORN. With a simple asynchronous reservation mechanism(i.e., TCARD), DBORN can support CES with satisfied performance(i.e., no loss for Ethernet packets transporting TDM service, low ac-cess delay and FJ lower than 10 ms). However, we have observedthat CES performance at downstream nodes may not satisfy QoSrequirements when the volume of BE traffic inserted on the ringby upstream nodes is high (e.g., exceeding 70% of the total offeredtraffic). The reason is that BE traffic inserted by upstream nodesdisturbs and consumes free bandwidth for premium service(TDM service) at downstream nodes. Moreover, with classicalMAC protocol and simple reservation mechanism used in DBORN,free bandwidth left by upstream nodes for the emission of down-stream nodes can be fragmented into small voids or unusable

voids, which probably are not sufficient for packet insertion atdownstream nodes. By consequent it degrades the performanceof CES and other services, notably at heavy ring load. The work pre-sented in this paper aims to improve CES performance on DBORNby introducing MPB mechanism in the MAC layer, improving band-width utilisation efficiency. Therefore this could counteract thenegative impact of sporadic BE traffic and of inefficient bandwidthfragmentation on the CES performance.

6.2. CES in combination with MPB

6.2.1. At high volume of BE trafficIn [4], we have shown that the important volume of BE traffic

inserted on the ring by upstream nodes can degrade CES perfor-mance at downstream nodes. We focus in this work on CES perfor-mance analysis when the volume of BE traffic is higher than 70% ofthe offered traffic. For TDM traffic parameters, each access node isfed with six unstructured E-1 TDM flows, each flow corresponds toframe of size 32 bytes generated each 125 ms. In order to fit E-1frames to Ethernet packets, we group eight native E-1 frames(8 � 32 = 256 bytes) in one Ethernet packet, according to PWE3draft [7] recommendation. The volume distribution of the offeredtraffic at each access node is set as following: BE traffic volume in-creases to 77%, the rest is 5% of TDM traffic and 18% of CoS2 traffic.We set the average offered load of the upstream bus to 80%. In thefollowing paragraphs we will analyse the performance of CES inthree cases: MAC case, BT size equal to zero and BT size equal to3 times MTU that gives �14.4 ls to BT time at 2.5 Gbs.

For this experimentation, we only observe loss for BE traffic(Fig. 15). There is no loss for CoS2 traffic and TDM traffic, meaningthe FL for TDM traffic is zero, satisfying one of the MAN require-ments on CES performance [8]. We notice that even if offered ringload is acceptable (80% according to the precedent work [1]), thehigh volume of BE traffic evokes loss at the last node in MAC case(highest PLR) and in MPB case with BT size equal to zero. However,the MPB case with BT size equal to 3 times MTU provides no loss atall nodes, proving its efficiency in terms of bandwidth utilisation.Lectors can refer to Section 5 for more explanations about thebehaviour of PLR curves in function of BT size value.

Since the propagation delay between consecutive ring nodes isfixed, the total end-to-end delay of client traffic is mainly influ-enced by the access delay. Therefore we only investigate the aver-age access delay for TDM traffic in this work. Fig. 16 shows the AADfor TDM traffic versus ring node rank. We observe that in all casesthe AAD is under 300 ls. And the lowest delay is provided by the

Fig. 16. AAD for TDM traffic vs. Ring node rank.

Fig. 17. FJ vs. ring node rank.

Fig. 18. PLR vs. ring node rank.

Fig. 19. AAD vs. ring node rank: load = 0.87.

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MPB case with BT size equal to 3 times MTU, followed by the MPBcase with BT size equal to zero and the MAC case. The same expla-nation as in the precedent section applies also here.

Fig. 17 plots the curves representing the FJ for TDM traffic mea-sured at each ring node. We notice that in MAC case, the FJ at thelast access node exceeds 10 ms, unsatisfying QoS requirements onCES performance. This is because the high volume of BE traffic in-serted on the ring by upstream nodes disturbs the transmission ofTDM traffic at downstream nodes. Nevertheless, the MPB cases de-crease FJ for TDM traffic to highly under 10 ms, thanks to opticaloverhead reduction and better bandwidth utilisation. We can seethat MPB, notably when associated with BT mechanism, can coun-teract negative effect of sporadic BE traffic on the QoS of premiumtraffic (TDM) in a shared media network. Therefore they improvethe availability and reliability of the realising CES on DBORN.

Fig. 20. FJ vs. ring node rank: load = 0.87.

6.2.2. At high offered ring loadWe now focus on CES performance at high offered ring load, in

order to show the impact of MPB on network performance, notablyon the performance of high QoS requirement traffic such as TDM.In this experimentation, the volume distribution of the offered traf-fic at each access node is set to 10% of TDM traffic, 19% of CoS2 traf-fic and 71% of BE traffic. We set the average offered load of theupstream bus to �87%. The same BT parameters are kept.

We observe in this simulation work that MAC case causes loss(for BE traffic only) at several access nodes, while MPB cases onlycauses very small loss at 3rd node with BT size equal to zero andno loss at all with BT size equal to 3 times MTU (Fig. 18). This dem-onstrates that MPB considerably improves the network perfor-mance and capacity.

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Fig. 19 depicts the AAD for TDM traffic versus ring node rank.We notice the same trend of AAD at access nodes as discussed inthe precedent sections. MPB case with BT size equal to 3 timesMTU always provides lowest access delay at downstream accessnodes. Moreover this AAD remains acceptable (lower than 1 ms)even if the offered ring load is very high.

Observing Fig. 20, which plots the FJ versus ring node rank, wenote that at high offered ring load, the FJ in MAC case exceeds10 ms at many nodes, unsatisfying CES requirements. On the con-trary the FJ in MPB cases still remains under 10 ms, again insuringCES performance requirements.

7. Conclusion

In this paper we introduce and analyse two novel technologiesfor a metropolitan all-optical Ethernet bus-based network: Modi-fied Packet Bursting aiming at improving protocol transmissionefficiency, and Circuit Emulation Service aiming at ensuringTDM-like QoS for required services. Performance evaluation showsthat MPB allows to remarkably reduce the wasted volume of band-width carrying optical overheads (from 5% to 30% depending on cli-ent traffic profile). Furthermore, if we choose BT_size value bigenough, MPB is able to offer to the network the capability of beingstable under different network configurations (e.g., high percent-age of short length packets volume, offered ring load up to 0.90,etc.), while ensuring good performance in terms of low access de-lay and low packet loss rate for network customers. It even is com-patible with CES which delivers the quality of service to customers.

We also point out that the CES technology, with the help of MPBmechanisms, could be realised with high reliability and offer supe-rior quality for TDM service in DBORN, even under worse networkconfigurations (e.g., high BE traffic volume, high offered ring load,etc.). For instance, with a good parameterisation of MPB and BT, wecan increase the acceptable offered ring load to superior to 80%,while still satisfying QoS requirements for high priority TDM(voice, video) traffic. In [14], we further proved that although theclassical optical CSMA/CA protocol seems to be enough for the net-work to provide satisfying QoS for TDM service, it might not be en-ough to guarantee QoS for lower priority services (data services).More specifically, by only using the optical CSMA/CA protocol,the network might lose all data services in order to support TDMservice, especially under heavy ring load. Thus a performance-en-hanced mechanism such as MPB is highly recommended to be usedwith CES, since it globally guarantee the transport of all services(both TDM and data) with satisfying QoS, even under unfavourablecontext (e.g., heavy ring load, unbalanced traffic pattern, etc.)

The main purpose of this paper is not to perform an exhaustivestudy of the above technologies, but to demonstrate their perfor-mance, their feasibility and the interoperability between them.Therefore the present simulation works are restricted on somesimple assumptions on traffic profiles: exponential source modelfor CoS2 traffic, IPP source model for BE traffic, uniform traffic

partition on the ring, etc. We expect to present in our future worksmore pragmatic scenarios that could confirm the availability andreliability of these technologies. Furthermore, we also expect to an-swer to some following questions. Can we automatically adaptMPB parameters for any network traffic change? Can MPB be gen-eralised to meshed topology?

Acknowledgments

This work was the result of a fruitful collaboration with Alcatel-Lucent, Bell Labs France at Nozay. We would like to thank all theparticipant of this collaboration. We also specially thank River Pub-lishers for authorising us to re-use some parts of the results in ourprevious publication [12] in their book.

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[12] V.H. Nguyên, T. Atmaca, Circuit emulation service technologies and modifiedpacket bursting in metropolitan optical networks, in: D.D. Kouvatsos (Ed.),Performance Modelling and Analysis of Heterogeneous Networks, vol. 2, RiverPublishers Series in Information Science and Technology, ISBN 978-87-92329-18-9, 2009, pp. 339–370 (Chapter 17).

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