network coding in next-generation passive optical … · network coding (nc) is an example of such...

IEEE Communications Magazine • September 201138 0163-6804/11/$25.00 © 2011 IEEE

This work was supportedin part by Orange/FranceTelecom.

INTRODUCTION

During the last decade, the major cost and tech-nology barriers to fiber deployment in accessnetworks were gradually removed, hence pavingthe way for the passive optical network (PON)to become a centerpiece of future access net-works. Current PON deployment is dominatedby two major standards: International Telecom-munication Union — Telecommunication Stan-dardization Sector (ITU-T) G.984 Gigabit PON(GPON) and IEEE 802.3ah Ethernet PON(EPON). The benefits of PONs include loweroperational expenditures and transparency todata rate and signal format. This has encouragedcarriers to deploy PONs that can easily beupgraded as new technologies mature and newstandards evolve [1, 2].

Among the several next-generation PON(NG-PON) requirements are the provisioning ofhigher bandwidth per subscriber, an increasedsplitting ratio, and an extended maximum reachcompared to current EPON and GPON archi-tectures. NG-PONs may offer additional func-tionalities such as protection, support topologiesother than conventional tree structures, andenable the consolidation of access, backhaul, andmetro network infrastructures [3]. In addition,substantial research activity is currently focusedon the convergence of optical and wireless accessarchitectures into bimodal fiber-wireless (FiWi)access networks [4], a key feature of NG-PONs.

An important goal of FiWi research is to

combine the most promising technologies pro-posed for wireless and optical access. Networkcoding (NC) is an example of such technologies.Consisting of bit- or packet-level coding opera-tions, NC has been shown to improve through-put, simplify routing, and provide robustnessagainst transmission errors and failures in vari-ous packet networks [5]. In a recent study, sig-nificant throughput gains were demonstratedexperimentally in NC-enabled WiFi-based meshnetworks [6].

In this article, we focus on the integration ofNC within NG-PONs. The aim is to illustrate theNG-PON architectures where NC yields poten-tial performance gains. Our illustrations andsimulations demonstrate significant potentialperformance improvements while clarifyingsome underlying topological constraints of NC invarious NG-PON scenarios.

NC is only starting to be investigated in thecontext of PONs [7, 8]. Figure 1 illustrates thepotential of NC to improve throughput in cur-rent PONs. In this illustrative scenario, twopackets are exchanged between two optical net-work units (ONUs). Due to the PON’s direction-al coupler, ONUs may communicate onlythrough the intermediary of the optical line ter-minal (OLT). In conventional PONs, such anexchange is usually performed in four separatepacket transmissions, with the OLT receivingand then broadcasting each packet individually(Fig. 1a). With NC, the OLT may code thereceived packets into a single packet using a sim-ple bitwise exclusive-OR (XOR) operation,denoted ⊕ (Fig. 1b). Upon receiving the codedpacket, the ONUs decode the packets destinedto them using a copy of their previously trans-mitted packets. NC hence achieves the packetexchange in only three packet transmissions,using 50 percent less downstream bandwidththan conventional PONs. Although the work of[7, 8] emphasized the throughput gains of NC inPONs, there has been little investigation of theeffects of NC on other performance metrics(e.g., packet delay) and in diverse traffic configu-rations. We study the scenario of Fig. 1 in moredetail and provide compelling simulation resultsdelineating the impact of NC on PON perfor-mance.

The remainder of this article is organized as

ABSTRACT

As the emerging access architecture, NG-PONs feature enhanced PON configurations,metro-access integration, and bimodal fiber-wireless (FiWi) networks. The moving landscapeof NG-PONs provides opportunities for applyingnovel and promising technologies such as net-work coding (NC). In this work, we introducethe basic principles of NC and discuss theirapplicability to NG-PONs, with a focus on layer2 design. Our example illustrations and simula-tions demonstrate significant potential perfor-mance improvements in various NG-PONscenarios while clarifying some underlying topo-logical constraints of NC.

TOPICS IN OPTICAL COMMUNICATIONS

Kerim Fouli, MIT

Martin Maier, Optical Zeitgeist Laboratory, INRS

Muriel Médard, MIT

Network Coding in Next-GenerationPassive Optical Networks

FOULI LAYOUT 8/23/11 9:42 AM Page 38

IEEE Communications Magazine • September 2011 39

follows. We first introduce NG-PONs. We nextdefine interflow and intraflow NC, and discusstheir applicability to NG-PONs. We then demon-strate the potential of NC in different NG-PONsettings through illustrative examples. Finally, weconclude the article.

NG-PONSAs illustrated in Fig. 1, the practical implemen-tation of NC in any network requires preciseknowledge of its architecture. In this section, westart by presenting PONs and their performanceenhancements; then we describe their next-gen-eration extensions into wireless and metropoli-tan settings.

PASSIVE OPTICAL NETWORKSConventional PONs — Typically, current PONsuse a tree topology connecting the OLT at itsroot with the ONUs at its leaves, as depicted inFig. 2 [9]. A passive optical coupler is located atthe remote node. Individual fiber trunks connectthe coupler to the OLT and to the ONUs. PONsuse full-duplex transmission, where the down-stream and upstream traffic is carried on sepa-rate wavelengths, λ1 and λ2, respectively.

In the downstream direction, traffic is broad-cast to all the ONUs through the coupler. In theupstream direction, the transmissions from indi-vidual ONUs are merged at the coupler. Toavoid collisions, an adequate medium accesscontrol (MAC) protocol is necessary. Central-ized polling, where the OLT allocates time win-dows to ONUs based on their queue occupancyreports, is used as the upstream channel sharingmethod for PONs. This type of shared mediumaccess method enables dynamic bandwidth allo-cation (DBA) on the upstream channel andimproves its utilization by means of dynamictime-division multiplexing (TDM), particularlyunder the bursty traffic conditions of access net-works [9]. Figure 2 illustrates the collision-freemultiplexing of the ONU transmissions over theupstream channel.

High-speed TDM PONs — Higher speeds areneeded to support emerging bandwidth-hungryapplications (e.g., high-definition television andvideo on demand) and provide sufficient capaci-ty as backhauls of next-generation IEEE 802.11nwireless LANs with a MAC throughput of 100Mb/s or higher per device [3]. The recent stan-dardization of IEEE 802.3av 10G-EPON pro-vides a 10 Gb/s downstream channel over analternate waveband, and allows upstream trans-missions to switch between 1 and 10 Gb/s forbackward compatibility. Similar standardizationefforts are underway for high-speed GPON [2].

Multichannel WDM PONs — In addition toboosting the available bandwidth, the benefits ofwavelength-division multiplexing (WDM, i.e., theuse of multiple channels upstream and/or down-stream) are manifold [9].

Different forms of WDM PONs have beenactively studied as components of NG-PON. In awavelength-routing WDM PON, each ONU isassigned a dedicated pair of wavelength channelsfor upstream and downstream transmission,

requiring replacing the coupler in installed TDMPONs with a wavelength demultiplexer. Accord-ing to [2], a more practical approach towardsWDM PONs is to leave the existing power-split-ting PON infrastructure in place and to selectwavelengths at each ONU by using a bandpassfilter. To ensure the backward compatibility oflegacy TDM PON infrastructure, conventionalTDM ONUs may be equipped with wavelength-blocking filters which let only the legacy TDMwavelength pass. Legacy TDM MAC protocolscan be extended to support a wide range of pos-sible WDM ONU structures by exploiting thereserved bits of their control messages [10].

Long-Reach PONs — Long-reach PONsincrease the range and splitting ratio of conven-tional TDM and WDM PONs significantly.Importantly, long-reach PONs typically have amultistage topology and allow for the integrationof optical access and metro networks. Thisbroadened PON functionality offers major costsavings by reducing the number of required opti-cal-electrical-optical (OEO) conversions, at theexpense of optical amplifiers required to com-pensate for propagation and splitting losses [11].

METRO-ACCESS INTEGRATIONTo provide backward compatibility with legacyinfrastructures, current TDM PONs are expect-ed to evolve toward NG-PONs in a pay-as-you-grow manner. The combination of long-reach

Figure 1. Network coding in a passive optical network.

Coupler

Packet-2 (ONU-2→ONU-1)

ONU-2 ONU-1

OLTPacket-1

Packet-2

3

(b)

2 1

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Figure 2. Time-division multiplexing PON [9].

Cabinet

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Home

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1 23 2

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IEEE Communications Magazine • September 201140

PONs with the high-speed and WDM perfor-mance enhancements will shift the role of PONsfrom a traditional access network into an opti-cally integrated metro-access environment. NG-PONs are hence required to take upcarrier-grade functionalities such as resiliencewhile exploiting legacy topologies [1, 12, 13].

FIWIWireless and optical networks are complemen-tary. While optical networks provide reliablecommunication and huge bandwidth, their bulkydeployment contrasts sharply with the ubiquityand low installation costs of wireless networks.On the other hand, the bandwidth constraintsand various impairments of wireless channels donot compare favorably with the bandwidth andreliability of the optical domain. Hybrid FiWiaccess networks aim at seamlessly combining theadvantages of both media [4].

To date, various FiWi architectures havebeen proposed that combine different wirelessLAN (WLAN) and wireless mesh network(WMN) technologies with optical access-metroinfrastructures, particularly PONs. A recenttestbed connecting independently running EPONand WLAN-based WMN networks ([14]) demon-strates the benefits of jointly designing layer 2mechanisms, an important aspect of the nascentFiWi research.

NETWORK CODING AND ITSAPPLICABILITY TO NG-PONS

NCNC stems from the observation that the functionof nodes in a communications network is notrestricted to routing, switching, and forwarding.In NC, nodes may perform operations on dataunits (e.g., bits, packets), generally using linearalgebraic approaches, in order to improve net-work performance [5].

In the following, a flow is defined as a streamof data units with the same source and destina-tion. Linear coding denotes the linear combina-tion of individual symbols, defined over finitefields or vectors thereof, such that their extrac-tion at the decoding node is possible throughsolving linear equations. Although nonlinear cod-ing schemes are mentioned in the literature [5],the discussion in this article is restricted to thesimpler and more practical linear coding. All theexamples provided henceforth are thus based onlinear coding schemes. The coding of two packetsor flows denotes the linear combination of theirconsecutive symbols using the same coefficients.Hence, any coded symbol, packet, or flow can beexpressed as a linear combination Σci xi, where xidenotes a native (i.e., uncoded) symbol, packet,or flow, and ci represents the coding coefficients.In the case of a binary field, symbols consist ofsingle bits, and both coding and decoding areperformed through XOR operations [6].

INTERFLOW NCIn interflow NC, coding applies to packets fromdifferent flows. We distinguish two forms ofinterflow NC and discuss their applicability toNG-PONs next.

Reverse Carpooling — The PON example ofFig. 1 is a particular case of inter-flow NC wherethe receiver nodes (ONU-1 and ONU-2) usecopies of their own previously transmitted pack-ets to decode received packets. The concept hasbeen explored in the context of wireless commu-nications, where it is denoted reverse carpooling[15], piggybacking, or pairwise XOR coding.Reverse carpooling requires the uplink from theinformation-exchanging nodes towards a com-mon intermediate node to be unicast while thedownlink from the intermediate node back tothe transmitting nodes must be broadcast. NCcan hence exploit the underlying broadcast archi-tecture to convert unicast transmissions intomore efficient broadcast transmissions, as depict-ed in Fig. 1.

In wireless networks, [6] uses reverse carpool-ing to increase throughput by exploiting thebroadcast nature of wireless mesh networks.Each node is required to:• Store overheard packets that are not des-

tined to it for a limited period of time, aprocedure termed opportunistic listening

• Periodically send control packets calledreception reports to inform its neighboringnodes of its stored packets

This enables nodes to code opportunistically: Ateach transmission, nodes combine the maximumnumber of packets that can be decoded at theirnext hop.

The example of Fig. 1 shows that the condi-tions for reverse carpooling are satisfied in con-ventional PONs, owing primarily to the use ofthe directional coupler. Furthermore, since onlyone intermediate node exists (i.e., the OLT),NC may be applied in a centralized manner.This removes the requirement for signaling(i.e., reception reports) and facilitates the inte-gration of NC within PONs. Favorable condi-tions for reverse carpooling are pervasive inNG-PONs we consider since many of them usecoupler-based tree architectures, includingsplitter-based WDM EPONs [10], LR-PONs[11], and integrated access-metro networkarchitectures [1, 12]. However, reverse carpool-ing is not possible when connections with theintermediate node are reduced to point-to-point links such as in wavelength-routing WDMPONs. Similarly, when the medium is fullybroadcast, interflow NC is not feasible for lackof intermediate nodes. In NG-PONs, this mayoccur when the nodes are connected through areflective or star coupler. In the next section,we illustrate quantitatively the remarkableadvantages of reverse carpooling in convention-al EPONs.

Multipath Interflow NC — In multipath inter-flow NC, a receiver uses different linear combi-nations of the coded packets from differentpaths in order to successfully perform decoding.Multipath interflow NC is particularly relevantfor multicasting, when flows are transmittedfrom multiple sources to multiple destinationsacross a shared network infrastructure wherecapacity bottlenecks arise.

Unlike reverse carpooling, multipath inter-flow NC requires multiple paths from the sourceto the destination. This renders it infeasible in

NC stems from the

observation that the

function of nodes in

a communications

network is not

restricted to routing,

switching, and

forwarding. In NC,

nodes may perform

operations on data

units, generally using

linear algebraic

approaches, in order

to improve network

performance.



tree networks such as PONs, LR-PONs, andaccess-metro networks dominated by tree topolo-gies (e.g., [1]). Nevertheless, more diversifiedaccess-metro architectures and FiWi networksprovide interesting possibilities, as shown in theexamples later.

INTRAFLOW NCRather than coding packets of different flows,intraflow coding implies the coding of consecu-tive packets from the same flow and has beenproposed in particular to improve reliabilitymechanisms in wireless networks [16]. As analternative to acknowledgment-based repetition,a source node generates random linear combina-tions of the next N packets in the flow until Nlinearly independent ones are successfullyreceived, enabling the destination node todecode the N native packets. Such batch codingdoes not require acknowledgment for each pack-et, but rather the entire batch, thus signaling thesource to end the coded transmissions for thatbatch.

Different implementations of intraflow NChave been examined and a comparison of theirperformances provided in [17], among others.Intra-flow NC may be applied in an end-to-endfashion, similarly to fountain codes, or withencoding at intermediate nodes. The ability tore-encode at intermediate nodes is particularlyimportant for dead spot mitigation and in multi-casting scenarios [16].

Coding may be applied along a sliding win-dow rather than in fixed-size batches. In thisscheme, decoding occurs as soon as the desti-nation receives enough linear combinations forany subset of native packets. Both batch-basedand sliding-window techniques deliver nativepackets to higher layers only after decodingevents (i .e. , the arrival of enough linearlyindependent packets to perform decoding).While one could expect this to have possibleadverse effect on delay-sensitive applications(e.g. , voice, streaming), the overall delayrequired to transfer content over lossy links isreduced [18].

In the context of NG-PONs, end-to-end NCmay be applied between any source and destina-tion to reduce the complexity of feedbacks andto increase reliability against packet losses dueto link losses or congestion. In particular, end-to-end coding mechanisms may be implementedacross the wireless part of a FiWi network toalleviate wireless link losses.

Since they require the existence of multiplepaths from source to destination, general codingmethods cannot be deployed across tree-basedNG-PONs. Nevertheless, they may be employedin metro ring networks and FiWi networks. Inaddition to increasing throughput and reducingdelay in the presence of packet losses, they pro-vide inherent reliability enhancement, as depict-ed in the next section.

OPPORTUNITIES IN NG-PONS

In this section, we use examples or numericalsimulations to illustrate some of the potentialNC applications in NG-PONs.

NC IN PONS

Figure 1 represents the generic framework forapplying reverse carpooling to intra-PON unicasttransmissions. In the following, we simulate astandard IEEE 802.3ah EPON with 16 ONUsand a symmetrical data rate of 1 Gb/s. TheONUs are 20 km from the OLT and maintain 1Mbyte queues. The upstream channel is allocat-ed dynamically among the ONUs through Inter-leaved Polling with Adaptive Cycle Time(IPACT), a benchmark EPON polling protocolthat is based on the interleaving of granted timewindows in order to improve upstream channelutilization and average packet delay [19]. EachONU’s transmission window is limited to 15kbytes per polling cycle. The OLT maintains 16first-in first-out (FIFO) downstream queues, onefor each ONU, each with a capacity of 1 Mbyte.The downstream channel is allocated dynamical-ly among the downstream queues in a round-robin fashion with a maximum transmissionwindow of 15 kbytes/queue.

In our simulations, two types of traffic com-pete for OLT output queue space:• At the ONUs, intra-PON traffic (i.e., traffic

destined to other ONUs) is generated forupstream transmission.

• At the OLT, an external traffic stream ofpackets destined to the ONUs is injected,representing traffic generated outside theEPON.

We assume Poisson traffic where the packet sizeis uniformly distributed over the Ethernet packetsize range (64–1518 bytes). In addition, the des-tination of both intra-PON and external packetsfollows a uniform distribution over all ONUs.After a 5 s warmup period, we simulate thetransmission of 105 packets.

Opportunistic coding is integrated withinlayer 2 as follows. Each intra-PON packet to betransmitted downstream and having sourceONU-i and destination ONU-j is coded with theearliest packet having inverted source and desti-nation (i.e., with source ONU-j and destinationONU-i). If no such packet exists at the time oftransmission, the packet is transmitted uncoded.To determine the effects of NC, we fix the exter-nal traffic rate to 0.5 Gb/s and vary the intra-PON traffic rate from 0.1 Gb/s to 0.9 Gb/s.Figure 3 compares the performance of nativeand NC-enhanced EPON in terms of meanaggregate throughput (Fig. 3a), average OLTdownstream queue size (Fig. b), and mean delay(Fig. 3c). In Fig. 3, native and NC-enhancedEPON are represented through dashed and solidplots, respectively, with 95 percent confidenceintervals.

The aggregate throughput plots of Fig. 3ashow that coding gains appear at the point ofcongestion, when the intra-PON traffic load is0.5 Gb/s. This point corresponds to the inputaggregate traffic level (of both intra-PON andexternal packet streams) reaching the down-stream data rate. As the OLT downstreamqueues grow, more coding opportunities arise,and the coding gain increases almost to 30 per-cent (0.2 Gb/s) for the intra-PON traffic. It isimportant to note that throughput gains are alsoachieved by the uncoded external traffic stream,

Rather than coding

packets of different

flows, intraflow

coding implies the

coding of

consecutive packets

from the same flow

and has been

proposed in

particular to improve

reliability

mechanisms in

wireless networks.

FOULI LAYOUT 8/23/11 9:42 AM 1


reaching 27 percent (0.1 Gb/s) at the highestintra-PON traffic load.

To shed more light on the throughput gains,we turn to the average queue size plots of Fig.3b. Figure 3b represents the average steady-statesize of all OLT downstream queues. On onehand, the queue in native EPON expectedly sat-urates when aggregate downstream traffic ratesexceed the data rate (intra-PON load of 0.6Gb/s), translating into the loss of all excess pack-ets, and the flattening of the throughput curve.However, this is not the case when NC isemployed, as the queue remains two orders ofmagnitude below its saturation level, thus avoid-ing any significant packet losses and allowing thethroughput to continue rising. The downstreamqueues eventually saturate for the NC-enhancedEPON, but at significantly higher loads. Thecapability of NC to drain the downstream queuesat higher rates hence provides a window of oper-ation (0.5–0.8 Gb/s) where the information rateexceeds the data rate without significant losses,and where the congestion limit is pushed beyondthe capacity limit.

Figure 3c shows the mean packet delay forintra-PON and external traffic, defined as theaverage value of the delay experienced by pack-ets from the moment they are queued at theirsource ONU (intra-PON) or OLT (external) tothe moment they arrive at their destinationONU. By definition, opportunistic coding willintroduce no delay penalty. This is apparent forintra-PON traffic at low loads where few codingopportunities exist. As the load increases, pack-ets are coded more often, thus spending lesstime in the queue. Remarkably, this translatesinto a delay reduction of more than one order ofmagnitude as the aggregate traffic rate risesabove the downstream data rate (intra-PONtraffic loads 0.6 Gb/s and 0.7 Gb/s) for bothintra-PON and external traffic. As queuesapproach saturation in the NC-enhanced EPON,packet delays remain below native EPON levels.

NC IN METRO-ACCESS NETWORKSIn contrast to tree-based networks such as PONs,integrated metro-access networks feature moreopportunities for multiple paths between sourcesand destinations where NC may be applied. Forexample, Fig. 4 illustrates the use of multipathinter-flow NC for multicasting within hybridring-star metro networks.

The hybrid ring-star topology was shownto improve the resilience, spatial reuse, andbandwidth efficiency of packet-based opticalmetro rings [13]. Figure 4a illustrates such anarchitecture, where a subset of the ring nodesare attached to a single-hop WDM star net-work built from widely available metropoli-tan dark fiber. Hybrid ring-star architecturesare powerful metro ring candidates becausethey a l low caut ious WDM upgrades andexploit low-cost passive technology and darkfibers. In addition, they may be deployed toal l -opt ica l ly interconnect mult ip leTDM/WDM PONs [12].

Although different star network architectureswere proposed [13], the passive star coupler(PSC) implementation is of particular interesthere due to its wavelength broadcasting nature.

Figure 3. Performance enhancement of an EPON through network coding:External traffic to the ONUs has a constant load (0.5 Gb/s). For an increasingintra-PON traffic load, we plot: a) the mean aggregate throughput; b) the aver-age OLT downstream queue size; c) the mean packet delay. The solid anddashed curves represent the results with and without NC, respectively. The dif-ferent colors in a) and c) show the effects of NC on intra-PON traffic (black)and external traffic (grey).

Offered load (Gb/s) (Intra-PON traffic)

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Using the PSC, star nodes such as node n canuse a single transmission to broadcast packets tothe star nodes.

We assume the multicast requests of Fig. 4a:Each of the sources s1 and s2 multicasts one flowto destinations d1 and d2 simultaneously. Each ofthe flows A and B, originating from s1 and s2,respectively, requires the capacity of a singlewavelength. Assuming shortest path routing (i.e.,minimum hop routing), Fig. 4b depicts a possiblerouting configuration, where each destinationreceives one flow over the ring and forwards itthrough the star subnetwork to the second desti-nation. Note that the routing scheme of Fig. 4brequires two wavelengths on the star network.Furthermore, although other shortest pathsexist, they all require two wavelengths on thestar network.

In the NC solution (Fig. 4c), copies of theflows are routed through node n, where they arecoded and broadcast through the star subnet-work. In this example of interflow coding, eachdestination receives one coded and one nativeflow through different paths and is thus able toperform decoding. The use of interflow NCremoves the requirement for an additional wave-length on the star subnetwork, hence realizing a50 percent throughput gain at the expense ofhigher spatial utilization on the ring.

NC IN FIWI NETWORKSSome of the most promising applications of NCrelate to FiWi networks. The remainder of thisarticle presents two illustrative applications ofintraflow NC in FiWi networks.

FiWi Broadcast Scenario — We illustrate thepotential of intraflow NC to achieve optical-to-wireless broadcast through the example of Fig.5a shows an NG-PON where the two ONUsoperate as access points for the wireless subnet-work. Furthermore, the two wireless nodes r1and r2 are located near access points AP-1 andAP-2 such that r1 is connected to AP-1 whereasr2 may connect to both AP-1 and AP-2, as shownin Fig. 5a.

Suppose the OLT needs to broadcast a batchof packets {p1, p2, p3} to r1 and r2. Once theOLT broadcasts the packets to both APs, thelatter must transmit them to r1 and r2. In a con-ventional WLAN setting, native packets aretransmitted by each access point in sequence.Each packet is then separately acknowledged byr1 and r2. Typically, r2 selects the AP with thestrongest signal for the transaction. With NC,the APs keep transmitting random linear combi-nations of the native packets without waiting foracknowledgments. Hence, AP-1 and AP-2 trans-mit the linear combinations {p′1, p′2, p′3, …} and{p1′′, p2′′, p3′′, …}, respectively (Fig. 5a). Once r1and r2 receive enough independent linear combi-nations to decode the native packets (i.e., three),they use a single block acknowledgment for thewhole batch.

To illustrate the exchanges between theaccess points and the wireless nodes, the follow-ing simplifying assumptions were made:• Time is slotted, and packet transmission in

both upstream and downstream directionstakes one time slot.

Figure 4. Interflow network coding (NC) in a metro-edge ring: a) trafficrequirements; b) shortest-path solution without NC; c) alternative solutionwith NC. Note that this hybrid ring-star topology is a practical example of thebutterfly configuration widely discussed in theoretical NC works [5].

PSC

BA

n

n‘

(a)

s1 s2

d1 d2

PSC

B

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n‘

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d1 d2



• The MAC protocol avoids interference byimposing the cyclic transmission pattern ofFig. 5b, where the solid and dashed arrowsindicate packet and acknowledgment trans-missions, respectively.

• The wireless channel experiences losses in25 percent of the time slots.

• When NC is not used, APs wait for onetime slot before retransmitting a packet,unless an acknowledgment is received.Figure 5c shows the time-space diagrams rep-

resenting the overall transaction without (upperdiagram) and with (lower diagram) NC. In bothcases, the same 25 percent channel loss patternis assumed, where the shaded squares representtimeslots with channel losses. Without NC(upper diagram), r1 receives the three packets bytime slot 13 and requires 9 packet transmissionsfor the transaction. In contrast, with intraflowNC (lower diagram), r1 is able to decode allthree packets at time slot 10 and uses a total of6 transmissions, thus achieving gains of 23 and33 percent in delay and energy, respectively.

Being connected to both APs simultaneously,r2 achieves better performance. In conventionalWLANs, r2 selects the strongest of the two sig-nals. With no coding (upper diagram), r2 startsignoring broadcasts from AP-1 after timeslot-1.Therefore, r2 receives all three packets by timeslot 14 and requires 11 packet transmissions.Using NC (lower diagram), r2 can receive bothcoded flows simultaneously and use them todecode the native packets. r1 is able to decodeall three packets at time slot 5 and uses 3 trans-missions, thus achieving gains of 64 percent indelay and 72 percent in energy. (Note that delayand energy gains are still significant if r2 ignoresAP-1, reaching 42 and 54 percent, respectively).Moreover, the use of two different paths to r2enhances the reliability of the transfer againstfailures along the wireless paths. Overall, theexample of Fig. 5 shows that intraflow NCenables the network to react more efficiently tothe losses of the wireless medium.

Alternatively, coding may be implementedwithin the optical part of the FiWi network. In

Figure 5. FiWi broadcast scenario: a) the use of intraflow network coding to broadcast a batch of packets from the OLT to two wirelessreceivers; b) assumed transmission pattern; c) time-space diagrams of the transaction without (upper) and with (lower) network coding.

(p‘i=Σβipi) (p“

i=Σαipi)

p“ip‘

i

p‘i p“

i

(b)

Cyclic transmissionpattern

(a)

Wirelessoptical

r2

r1

OLT

r1

p1Code

p2 p3

AP-2AP-1

CodeAP-1

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p3 p2 p1

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r1

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Cycle C1

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p“3



this case, the coded packet streams {p1′, p2′, p3′,…} and {p1′′, p2′′, p3′′, …} are generated by theOLT and halted by an acknowledgment from theAPs once the wireless nodes have received threeindependent linear combinations. Such a config-uration leverages the higher optical bandwidthand processing capabilities. Furthermore, it isrequired when the PON is a point-to-point medi-um (e.g., WDM PON) rather than a broadcastmedium.

FiWi Resilience Scenario — NC may alsopotentially improve the resilience of FiWi net-works to fiber cuts. Figure 6 shows an NG-PONwhere the three ONUs operate as access pointsfor the wireless node r1.

The probability of success per packet trans-mission is shown for each of the links between r1and the three APs.

Suppose a flow {p1, p2, p3 …} is to be trans-mitted from the OLT to r1. For illustration pur-poses, we assume that time is divided equallyamong r1 and the access points, so that eachnode is allowed to transmit one packet per trans-mission cycle. Also, let the flow be initially rout-ed via AP-2 such that r1 receives it at a rate ofone packet per cycle, ignoring acknowledgments.Figure 6a shows a cut in the distribution fiberbetween the OLT and AP-2. Clearly, if r1 picks asingle replacement path via AP-1 or AP-2, theflow may only be delivered at an average rate of0.5 packets per cycle.

Figures 6a and 6b depict two alternative solu-tions where both lossy links are used simultane-ously as backup paths. In Fig. 6a, each nativepacket pi (i = 1, 2, 3, …) is transmitted fromboth AP-1 and AP-3. In Fig. 6b, however, ran-dom linear combinations pi′ and pi′′ are transmit-ted from AP-1 and AP-3, respectively. Assumingthat losses across the two links are independent,Figs. 6a and 6b show the likelihoods of the pack-et pairs received by r1 at each cycle. Without NC(Fig. 6a), the average packet rate attained is 0.75

packets/cycle. NC, however, enables r1 to receivethe flow at its full rate of 1 packet/cycle (Fig.6b).

CONCLUSIONWe have shown that very simple approaches tonetwork coding yield considerable gains inthroughput and delay in PONs. Moreover, theimplementation of NC in NG-PONs holdspromise for enhanced throughput, delay, andreliability, in adverse conditions with high packetlosses. A study of the relationship between thetopology of access-metro networks and the effec-tiveness of various NC techniques may yield fur-ther performance gains.

The deployment of NC across the wireless-optical boundary may reap particular advantagesof the complementarity of the two media, whereoptical networks provide the processing capabili-ty and bandwidth, whereas wireless networksprovide mobility and cost-effective coverage ofgeographical areas.

REFERENCES[1] L. G. Kazovsky et al., “Next-Generation Optical Access

Networks,” IEEE/OSA J. Lightwave Tech., vol. 25, no.11, Nov. 2007, pp. 3428–42.

[2] F. Effenberger et al., “An Introduction to PON Tech-nologies,” IEEE Commun. Mag., vol. 45, no. 3, Mar.2007, pp. S17–S25.

[3] R. Lin, “Next Generation PON in Emerging Networks,”Proc. OFC/NFOEC, San Diego, CA, Feb. 2008, pp. 1–3.

[4] N. Ghazisaidi and M. Maier, “Fiber-Wireless (FiWi) Net-works: Challenges and Opportunities,” IEEE Network,vol. 25, no. 1, Jan./Feb. 2011, pp. 36–42.

[5] T. Ho and D. Lun, Network Coding: An Introduction,Cambridge University Press, Apr. 2008.

[6] S. Katti et al., “XORs in the Air: Practical Wireless Net-work Coding,” IEEE/ACM Trans. Net., vol. 16, June2008, no. 3, pp. 497–510.

[7] M. Belzner and H. Haunstein, “Network Coding in Pas-sive Optical Networks,” Proc. ECOC, Vienna, Austria,Sept. 2009, pp. 1–2.

[8] K. Miller et al., “Network Coding in Passive Optical Net-works,” Proc. IEEE Int’l. Symp. Network Coding, Toron-to, Ontario, Canada, June 2010, pp. 1–6.

Figure 6. FiWi survivability scenario: To react to a fiber cut between the OLT and AP-2, the OLT uses thetwo remaining paths to reach r1 a) without; b) with network coding. For each case, the likelihood of thepacket pairs being received within one wireless transmission cycle is depicted.

(-,-)

r1

p“ip‘i

50% 50%100%

Packet pairreceived

1 packets/cycleLikelihood

25%

wirelessoptical

AP-1 AP-2 AP-3

p1,p2,p3...

OLT

r1

pipi

50% 50%100%

wirelessoptical

AP-1 AP-2 AP-3

p1,p2,p3...

OLT(a) (b)

(p’i,-) (-,p”i ) (p’i,p”i )(-,-)

Packet pairreceived

0.75 packets/cycleLikelihood

25%

(pi,-) (-,pi) (pi,pi)

The use of inter-flow

NC removes the

requirement for an

additional

wavelength on the

star subnetwork,

hence realizing a

50 percent

throughput gain,

at the expense of

higher spatial

utilization on

the ring.



[9] T. Koonen, “Fiber to the Home/Fiber to the Premises:What, Where, and When?” Proc. IEEE, vol. 94, no. 5,May 2006, pp. 911–34.

[10] M. P. McGarry, M. Reisslein, and M. Maier, “WDM Eth-ernet Passive Optical Networks,” IEEE Commun. Mag.,vol. 44, no. 2, Feb. 2006, pp. S15–S22.

[11] D. Shea and J. Mitchell, “Long-Reach Optical AccessTechnologies,” IEEE Network, vol. 21, no. 5, Sept./Oct.2007, pp. 5–11.

[12] M. Maier, M. Herzog, and M. Reisslein, “STARGATE:The Next Evolutionary Step Toward Unleashing thePotential of WDM EPONs,” IEEE Commun. Mag., vol.45, no. 5, May 2007, pp. 50–56.

[13] M. Herzog and M. Maier, “RINGOSTAR: An Evolution-ary Performance-Enhancing WDM Upgrade of IEEE802.17 Resilient Packet Ring,” IEEE Commun. Mag., vol.44, no. 2, Feb. 2006, pp. 8–14.

[14] P. Chowdhury et al., “Hybrid Wireless-Optical Broad-band Access Network (WOBAN): Prototype Develop-ment and Research Challenges,” IEEE Network, vol. 23,no. 3, May/June 2009, pp. 41–48.

[15] M. Effros, T. Ho, and S. Kim, “A Tiling Approach toNetwork Code Design for Wireless Networks,” Proc.Info. Theory Wksp., Punta del Este, Uruguay, Mar.2006, pp. 62–66.

[16] C. Fragouli et al., “Wireless Network Coding: Opportu-nities & Challenges,” Proc. IEEE MILCOM, Orlando, FL,Oct. 2007, pp. 1–8.

[17] D. S. Lun, M. Médard, and R. Koetter, “Network Codingfor Efficient Wireless Unicast,” Proc. Int’l. Zurich Seminaron Commun., Switzerland, Feb. 2006, pp. 74–77.

[18] A. Eryilmaz, A. Ozdaglar, and M. Médard, “On DelayPerformance Gains from Network Coding,” Proc. CISS,Princeton, NJ, USA, Mar. 2006, pp. 864–70.

[19] G. Kramer, B. Mukherjee, and G. Pesavento, “IPACT: ADynamic Protocol for an Ethernet PON (EPON),” IEEECommun. Mag., vol. 40, no. 2, Feb. 2002, pp. 74–80.

BIOGRAPHIESKERIM FOULI ([email protected]) is a postdoctoral fellow at MIT.He received his B.Sc. degree in electrical engineering atBilkent University, Ankara, Turkey, his M.Sc. degree in opti-cal communications at Laval University, Quebec City, Cana-da, and his Ph.D. in optical networking at INRS, Montreal,Canada. He was a research engineer with AccessPhotonic

Networks, Quebec City, from 2001 to 2005. His researchinterests are in the area of access and metropolitan net-work architectures with a focus on fiber-wireless integra-tion.

MARTIN MAIER [SM] ([email protected]) is an associate pro-fessor at the Institut National de la Recherche Scientifique(INRS), Montreal, Canada. He was educated at the Techni-cal University of Berlin, Germany, and received M.Sc. andPh.D. degrees (both with distinctions) in 1998 and 2003,respectively. In the summer of 2003, he was a postdoc fel-low at the Massachusetts Institute of Technology (MIT),Cambridge. He was a visiting professor at Stanford Univer-sity, California, October 2006 through March 2007. He is aco-recipient of the 2009 IEEE Communications Society BestTutorial Paper Award and Best Paper Award presented atThe International Society of Optical Engineers (SPIE) Pho-tonics East 2000-Terabit Optical Networking Conference.He is the founder and creative director of the Optical Zeit-geist Laboratory (www.zeitgeistlab.ca). His research activi-ties aim at rethinking the role of optical networks andexploring novel applications of optical networking conceptsand technologies across multidisciplinary domains, with aparticular focus on communications, energy, and transportfor emerging smart grid applications and bimodal fiber-wireless (FiWi) networks for broadband access. He is theauthor of the book Optical Switching Networks (Cam-bridge University Press, 2008), which was translated intoJapanese in 2009. He served on the Technical ProgramCommittees of IEEE INFOCOM, IEEE GLOBECOM, and IEEEICC, and is an Editorial Board member of IEEE Communica-tions Surveys and Tutorials as well as Elsevier ComputerCommunications.

MURIEL MÉDARD [F] ([email protected]) is a professor inEECS at MIT. She received five degrees from MIT. She hasbeen associate or guest editor for numerous journals andTPC chair or member for numerous conferences. Shereceived the 2009 Communication Society and InformationTheory Society Joint Paper, the 2009 William R. BennettPrize in the Field of Communications Networking, and the2002 IEEE Leon K. Kirchmayer Prize Paper Award. Shereceived an NSF Career Award in 2001, the 2004 MITHarold E. Edgerton Faculty Achievement Award, and wasnamed a Gilbreth Lecturer by the National Academy ofEngineering in 2007.

Very simple

approaches to

network coding yield

considerable gains in

throughput and

delay in PONs.

Moreover, the

implementation of

NC in NG-PONs

holds promise for

enhanced

throughput, delay,

and reliability, in

adverse conditions

with high packet

losses.

NEW PUBLICATION COMING SOON IN FEBRUARY

2012

Editor-in-Chief: Dong In Kim Sponsored by the IEEE Communications Society, IEEE Signal Processing Society and IEEE Vehicular Technology Society

IEEE Wireless Communications Letters

Bi-monthly journal publishing shorter, high-quality manuscripts on advances in the state-of-the-art of wireless communications

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