Adapting the EPON MAC Protocol to a Metropolitan Burst Switching Network

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1 Adapting the EPON MAC Protocol to a Metropolitan Burst Switching Network Jelena Pesic, Ahmed Triki and Annie Gravey Alcatel-Lucent Bell Labs, Nozay, France ( jelena.pesic@alcatel-lucent.com) Institut Mines Télécom, Télécom Bretagne, Brest, France ( firstname.lastname@telecom-bretagne.eu) Abstract This paper compares the respective efficiencies of the control and the management planes in performing resource allocation for TWIN (Time-domain Wavelength Interleaved Networking) optical networks used to aggregate and distribute traffic within a metropolitan area. While the Management Plane driven MAC (MP-MAC) protocol is based on an quasi-static configuration derived from optimization model, the Control Plane driven MAC (CP-MAC) protocol is based on adapting Passive Optical Network (PON) upstream traffic control, originally designed to access network, to the metropolitan network. The performance levels delivered by both approaches are compared by simulating a TWIN network applied to a Multi-hEad subwavelength switching (MEET) architecture that ensures an all optical aggregation between the regional metropolitan and the core networks. The simulation assesses the QoS delivered to three different classes of service, for a packet level traffic trace obtained from an operational network. I. INTRODUCTION Future networks will have to support very high bitrate interfaces and to deal with increasing and time-varying traffic demands. Optical sub-wavelength switching is a good candidate for meeting that challenge, as it brings flexibility to the optical layer as well as reduces the energy consumption [1]. Optical sub-wavelength switching consists in dynamically sharing a given wavelength between several source-destination pairs in the optical domain. This requires switching of optical bursts at the intermediate nodes of the network (i.e., Optical Burst Switching, OBS). Time-domain Wavelength Interleaved Networking (TWIN) [2] is a promising OBS solution. This solution has been designed by a group of researchers in Bell Labs in 2003 and has been shown to provide lossless OBS with simple and optically transparent intermediate nodes within both, mesh and ring topologies. It consists in assigning a unique wavelength to each node (destination) in order to receive traffic from the other nodes (sources). Each source node requires a tunable transceiver, whereas each destination node only requires a fixed receiver. Burst collision in intermediate nodes is avoided thanks to a MAC layer that schedules the bursts emission times. The schedule computation (also called resource allocation process) takes into account the resource blocking issue. Resource blocking occurs when a source should transmit towards multiple destinations during overlapping time intervals. Author contribution was developed during her affiliation to Télécom Bretagne /15/$31.00 c 2015 IEEE This paper identifies two types of approaches to perform resource allocation in the TWIN network. For both approaches, schedule computation is centralized in a single point (which makes backup as mandatory for redundancy purposes). The first approach consists in attributing the resource allocation process to the management plane that periodically computes a fixed schedule pattern, afterwards applied by sources as long as another one is not uploaded. We refer to this approach as Management Plane driven MAC protocol (MP-MAC protocol). In MP-MAC, the schedule computation is done off-line and it is based on an algorithm, called static algorithm, proposed in [3]. It is based on formulating resource allocation as an Integer Linear Programming (ILP) problem yielding an optimized scheduler pattern. Note that in the current metropolitan network, the allocation of resources is usually done by the management plane and the resource allocation configuration is kept unchanged for a long period of time. A second approach consists in attributing the resource allocation process to the control plane that dynamically schedules bursts transmissions one by one. Consequently, the MAC layer computes transmission schedules on-line and sends one grant per burst to each source while taking into consideration actual demands. In following, we refer to this approach as Control Plane driven MAC protocol (CP-MAC protocol). The schedule computation adopted in CP-MAC is similar to a dynamically controlled PON, except that the allocation algorithm has also to take care of blocking issue at the source. In fact, as illustrated in Fig. 1, the virtual topology of TWIN can be viewed as a juxtaposition of optical multipoint-topoint trees. Each tree has a unique colour associated to a unique destination. The TWIN tree structure implies that the avoidance of burst collision at the destination side leads to avoid collision at all intermediate nodes between the source and the destination. Therefore, each TWIN tree is similar to a Passive Optical Network (PON) [4] tree. Each destination situated at the root of a tree could thus be seen as Optical Line Terminal (OLT) whereas the source nodes situated at the leaves of the tree could be seen as Optical Network Units (ONUs). A PON s DBA-like (Dynamic Bandwidth Allocation) algorithm (here referred also as PON-like algorithm) could be used to manage the bursts transmission within a given TWIN tree. However, the multipoint-to-point trees of TWIN are not mutually independent because each source node has to

2 Fig. 1: Overlaid trees for burst transfer in TWIN transmit bursts to more than one destination. In other words, a PON-like algorithm natively deals with the issue of burst collision at the destination side, but does not address resource blocking at source side. To overcome this issue, it is proved in [5] that a CP-MAC protocol can be applied in a TWIN context as long as the network architecture has a central point. In a central-point based network architecture, all optical burst flows pass through a central point (called root ). Multi-hEad sub-wavelength switching (MEET) [6] architecture is a new vision of the metropolitan area that could be adapted to a central point based architecture and that we will consider as a scenario of our simulation study. The current paper is organized as follows: Section II briefly describes MEET architecture. Section III describes the MP- MAC protocol and explains its characteristics when applied to a central point based architecture (such as MEET). Section IV describes the CP-MAC protocol and shows how the central point assumption adapts the EPON DBA to TWIN scheduling process. Section V compares the two approaches in terms of packets end-to-end delay and jitter. The study also discusses whether CP-MAC can meet the QoS requirements for a metrobackhaul network. Performance evaluation is carried out using a simulation platform driven by a packet-level traffic trace captured on a metropolitan network. II. MEET ARCHITECTURE The MEET architecture shown in Fig. 2 has been originally proposed by Orange (French operator) as a next generation metro-backhaul network. Current operator networks are designed in an hierarchical way: connections between each metropolitan network and the core network are realised by large electrical routers called Concentration Nodes (CNs). A CN is designed to deal with a large number of flows, and thus requires a huge buffering capacity and a large amount of computing resources. The MEET architecture, as explained in [6] is intended to substitute active electrical CNs by passive optical switches providing all-optical aggregation. The MEET architecture thus optically extends the Metropolitan Area Network (MAN) in order to reach some key core nodes (e.g. internet access nodes and regional nodes). Compared with currently rolled out architectures, MEET avoids several electrical multiplexing stages by replacing them with an alloptical aggregation. In the particular case where a CN is replaced by a single optical switch, the MEET architecture presents a central point as all flows, those within the metro area and those running Fig. 2: Central point based MEET architecture between the metro area and the core network area, have to pass through a single optical switch. III. MANAGEMENT PLANE DRIVEN MAC PROTOCOL The MP-MAC protocol is based on static algorithm for the resource allocation. It considers a slotted allocation of resources. One time slot carries a single burst and represents the resource allocation granularity. The distribution of slots between the different source-destination couples forms the resource allocation pattern that performs during a fixed period called data cycle. Nodes repeatedly follow the slot pattern until a new configuration is uploaded by the management entity. As the optimal solution of the resource allocation problem in TWIN network is complex, resolving this problem cannot be done in real time and the time required to resolve it depends mainly on the number of source-destination pairs in the network. We assume that the computation of the new allocation patterns is done off-line by taking into account the history or the information collected by the management plane about the traffic load variation of each flow. A. Resource Allocation Optimization Problem The allocation mechanism is formulated as an optimization problem. It focuses on maximizing the utilization of the wavelengths by taking into account collision constraints at destination side, slot blocking constraints at source side and a dimensioning constraint that ensures that each sourcedestination demand is satisfied. In the problem formulation [3], s is the number of sources, n is the number of slots per data cycle and X j is a binary vector indicating the pattern related to the reception of bursts at the destination j. The size of X j is equal to s.n. Each index m of the vector X j is written as m = s.(p 1) + i where, 1 i s and 1 p n. Each element of the vector X (j,m) indicates whether slot p is attributed to source i or not. The purpose of this optimization problem is to find the vector X j for each destination j. d n max s j=1 p=1 i=1 X j,s(p 1)+i (1)

3 Subject to, j, j [1..d] i [1..s] p, p [1..n]: X j,s(p 1)+i {0, 1} (2) s X j,s(p 1)+i 1 (3) i=1 X j,s(p 1)+i + X j,s(p 1)+i 1 (4) (if slots p and p are overlapping at source i) n X j,s(p 1)+i R i,j (5) p=1 R i,j, expressed in number of slots, is the traffic demand for the source/destination pair (i, j). The constraints in (3) avoid collisions at destination side, the constraints in (4) avoid blocking at source side and the constraints in (5) ensure that traffic demands are satisfied. The constraints in (4) require the knowledge of the overlapping slots p and p at each source. Therefore, we assume that all data cycles start at the same time for all the destinations and we consider that the propagation delays δ i,j and δ i,j between the two source-destination pairs (i, j) and (i, j ) respectively, are equal to: δ i,j = k. d + α i,j. s (6) δ i,j = k. d + α i,j. s (7) Where d is the data cycle duration, s is the slot duration, k N, k N, α i,j R and 0 α i,j < n. DC is the time offset, within the data cycle, between these two propagation times at the source i, and is expressed in number of slots (8): DC = α i,j α i,j (8) DC is an integer if slots are aligned at the source side. We assume that α i,j > α i,j, so that the relationship between the two overlapping slots p and p intended respectively to destinations j and j is expressed by the equation (9). p = (p + DC )mod(n + 1) (9) Here, xmody gives the remainder of division of x by y. DC is real, if slots are not aligned at the source side. In this case, slot p is overlapping with two slots p and p, intended to the destination j. If we assume that α i,j > α i,j, p and p are given by the equations (10) and (11). p = (p + DC )mod(n + 1) + 1 (10) p = (p + DC )mod(n + 1) + 1 (11) Where DC and DC are the nearest integer larger and smaller than DC, respectively. B. Synchronization in a Central Point Based Architecture The synchronization issue is a crucial problem in TWIN. Usually, the resource allocation pattern is calculated according to reference time which is common for all destinations [2]. Obviously, if T is the burst reception time at a destination j, the burst emission time at the source i (T i,j ), depends on the propagation time between them (δ i,j ) as it is expressed in the equation (12). T i,j = T δ i,j (12) Due to the difference between propagation delays, slots related to different destinations may be unaligned at the source side, which leads to a waste of the bandwidth [3]. In a central-point based architecture, the root is a common intermediate node for all TWIN trees as all paths pass through this node. So, avoiding collisions at the root yields avoidance of collision at all intermediate and destination nodes. Hence, the root can be considered as a virtual destination for all the flows when computing resources and the management entity can compute the resource allocation pattern by using the propagation time between the source i and the root (δ i,root ). According to this assumption, if T is the burst reception time at the root, the burst emission time at the source i (T i ) is given by the equation (13). T i = T δ i,root (13) T i applies for each destination j, which yields slot alignment at the source (i) even when the distances between the sourcedestination pairs are not a multiple of slots. Consequently, the central point based architecture can achieve an efficient bandwidth utilisation, due to slot alignment. Note that this architecture, on the other hand, may increase propagation delays since all paths have to pass through the root. IV. CONTROL PLANE DRIVEN MAC PROTOCOL A. Connectivity The present CP-MAC protocol is dedicated to a central point based architecture. We assume that the Control Entity (CE) is co-located with the root and communicates with TWIN edge nodes using a dedicated bidirectional control channel that is carried by a specific wavelength on the same fiber as the data channels. The control channel is used for ranging operations, for auto-discovery process, to transmit report messages from edge nodes to the CE (upstream) and to transmit grant messages from CE to edge nodes (downstream). Note that the CE could also be collocated with an other edge node that is not necessary the root but in that case it must be connected to the root by one fiber for each direction to enable it to receive and send on the control channel. B. Report/Grant Signalling We propose to use a polling scheme inspired by a classical EPON (Ethernet PON) DBA [7]. Unlike the EPON, however, reports and grants do not use in-band signaling but a dedicated control channel. Sources report their current demand on the upstream control channel while the CE issues grants on the downstream control channel. It is important to note that reports and grants are not strictly coupled and that each report does not systematically give rise to one and only one grant. Reports are emitted periodically by all sources. Grants are issued according to an independent process to active sources with the grant duration computed using the latest reports received by the CE. Reports are emitted in a static TDMA (Time Division Multiple Access) cycle. Each source in turn generates a burst including a fixed size report for each wavelength. The interval between successive reports is s((w 1)b/r + g ) where s is the number of sources, w the number of wavelengths, b the number of bytes in each source-wavelength report, r is the

4 report channel rate in byte/s and g is the inter-burst guard time. This interval contributes to packet latency and could be reduced if necessary by creating multiple report channels, each dedicated to a group of sources. Grants are broadcast by the CE using continuous mode transmission. Each grant identifies source and wavelength, and also specifies a start time and duration of the emission. Grants are generated independently for each wavelength and scheduled in a FIFO queue. C. Scheduling The ranging operations enable the CE to measure Round Trip Times (RT T c,i ) between itself and every edge node. The CE also sets slave clocks in each edge node to its own time minus a one way propagation time. This information is sufficient for the CE to compute schedules on every data channel that would avoid collisions. Since there is a single path from the root to each destination node, the same schedules also avoid collisions at every destination. The couple CE and root fulfills the role of OLT in EPON. The following recursive relations define a schedule that avoids collisions on data and control channels. They ignore the requirement that the source tuner should be available at the start of a slot and therefore would result in capacity loss due to resource blocking, as previously discussed. For the sake of simplicity, equations presented below and implemented in the simulation testbed are provided for a specific wavelength corresponding to a destination node. This could imply that each EPON tree is separately handled but, these EPON trees are actually related by the information of occupancy of the transmitter. The scheduling algorithm works in parallel for each wavelength but takes into account this transmitter occupancy constraint. Let T G (n) be the instant the CE decides to emit the n th grant, according to the CE clock. We assume the processing to determine the grant takes negligible time. The n th grant is sent to some source i and allocates a time slot of duration s (n) starting at time T S (n, i) according to the source i local clock. These start times and grant times are calculated as follows: T S (n, i) = T G (n) + RT T RT T c,i (14) T G (n + 1) = T G (n) + s (n) + guard (15) where guard is a guard time and RT T an offset that compensates for differences in round trip times. The guard time between successive grants is necessary to account for laser tuning delay and residual ranging imprecision. The offset must be greater than max j (RT T c,j ) plus the maximum delay that can occur before the grant message is sent on the grant channel (equal to (w 1) grant emission times). This ensures the grant arrives at destination before time T S (n, i) when the source is required to begin transmission. To avoid transmitter blocking, the recursive relations must be modified to allow the CE to delay start times and grant times, as necessary. Suppose source i is chosen for grant n on wavelength w but its transmitter is already reserved for another wavelength or, successively, for several wavelengths. The starting time must be delayed until the transmitter is free. The fact that propagation times between the source and the CE are identical for all wavelengths implies that the end of the transmitter busy period is determined by the last reservation to be performed. It is impossible to transmit on any other wavelength before this time. Let free i denote the end of the last reservation by any wavelength for the transmitter of source i, measured using its local clock. The grant assignment algorithm for a specific wavelength is given as follows: At time T G (n), compute T S (n, i) by equation (14). If T S (n, i) > free i, send the grant with start time T S (n, i) and duration s (n) and reiterate at time T G (n+ 1) given by equation (15). If not, send the grant to i with start time free i and update free i = free i + s (n) + guard. Reiterate at grant time T G (n + 1) = free i RT T + RT T c,i. The above algorithm avoids collisions. Its effectiveness depends on how the source that should receive the next grant, at each grant time is chosen, and on how the grant durations s (n) are determined. Various possibilities exist. One possible DBA is proposed in the next section. D. Dynamic Bandwidth Allocation As for EPON, the above scheduling algorithm can be completed with a range of possible DBA algorithms [8]. We propose one such algorithm in this section that we believe is well adapted to the transport network considered in this paper. Each source maintains several queues of packets for each destination wavelength, that correspond to several potential classes of services. At each report epoch, it reports the total amount of bytes received in all classes since the last report was sent. When a source i uses a grant at the designated starting time T S (n, i), it proceeds as follows: the burst is first used by packets from the highest priority queue, in their order of arrival. If there is remaining time, this is used for packets from lower priority queue until the total grant has been used. A maximum grant size may be introduced to prevent any overloaded source from hogging a wavelength to itself. We address the impact of this limited service option [7] in the following. V. RESULTS AND DISCUSSION The goal of the present study is to determine the respective efficiencies of management plane versus control plane resource allocation procedures when TWIN is applied to the MEET architecture. This includes discussing the applicability of EPONlike upstream traffic control procedures to OBS central point based metro-backhaul networks. The comparison study is simulation based, using OM- NET++. Simulations are fed by packet level traffic traces. The original trace has been collected from a metro-backhaul network consisting in 10 traffic nodes over one minute, at a peak hour (21:00), and is composed of eight million packets. The original data trace has been processed to create other

5 Fig. 3: CoS-1 packet delay traffic traces corresponding to different traffic loads [6]. The derived packet arrival series are obtained from the original trace by multiplying the inter-arrival times by different load factors. Hereafter, we refer to a load factor as being the ratio between the average amount of data (per second) intended to the most loaded receiver and the channel capacity (10 Gbps). This is intended to represent realistic traffic profiles with intensities up to 10 Gbps for the most loaded node. Each node in the simulation scenario is equipped with a single 10 Gbps transceiver and presents infinite capacity queues. The largest propagation delay is equal to 1.5 ms (which roughly corresponds to a 300 km distance). We consider two versions of CP-MAC. In CP-MACv1, the maximum transmission time attributed to a given source is not limited, meaning that each node will obtain the grant for the exact amount of traffic that has in its waiting queues while in CP-MACv2, the maximum transmission time is limited to 200 µs per source. Packets are classified into three classes of service: CoS-1, CoS-2 and CoS-3. Burst assembly is performed at the source node according to Priority Queuing policy (PQ). PQ gives to the CoS-1 and CoS-3 the highest and the lowest priority respectively. In this paper, we do not consider packet fragmentation during the assembly process. We compare the performances of the above mentioned MAC protocols in terms of packet delay and packet jitter while taking into consideration specific QoS threshold [6]. The packet delay is the sum of the propagation time between the source-destination couple and the time spent by a packet in the source node queue. The propagation time is fixed for a source-destination pair but the delay varies as packets experience variable waiting times in source nodes. For a given source-destination pair, jitter is defined as the difference between the 1 st percentile and the 99 th percentile of the delay distribution. The results are given below for the average waiting time and the jitter for the most loaded source-destination pair. Similar results are obtained for other pairs. Performance parameters are shown versus offered load. To estimate the accuracy of the obtained mean values of each simulation, we calculate the 95% confidence interval by grouping the obtained data into ten batches and computing their corresponding 1.96 estimated standard deviation from the mean. The size of batch is chosen large enough to ensure their approximate independence (it corresponds to time slots in the MP-MAC case). Fig. 3 shows that CoS-1 packets in all MAC protocols Fig. 4: CoS-1 packet jitter (i.e. MP-MAC and CP-MAC protocols) experience a delay significantly lower than the QoS threshold (3 ms). The delay is almost stable for all protocols and for all load values except for the load factor value of 0.9 where the latency recorded for CP-MACv1 is slightly higher. According to Fig. 4, the jitter in CP-MACv1 exceeds the threshold boundary of 1 ms for a load factor equal to 0.8. This can be explained by the absence of the maximum transmission time limit in the allocation strategy of this protocol which gives the opportunity to some sources to monopolize transmission opportunities during an unpredictable period of time. However, with CP-MACv2, enforcing a maximum transmission time obviously alleviates the competition between sources and reduces the CoS-1 packets waiting time. In the case of MP- MAC, CoS-1 packet jitter performance is almost insensitive to the global traffic load because they have the highest priority and benefit from regular resources periodically attributed. Fig. 5 shows that all MAC protocols deliver almost the same delay values for CoS-2 packets and that this delay is lower than the QoS-2 threshold (5 ms). However, the evolution of CoS-2 packet jitter as depicted in Fig. 6 reveals different behaviors for the three protocols. In the case of MP-MAC protocol, the packet jitter proportionally increases with the traffic load and reaches the QoS threshold at a traffic load equal to 0.9. For load factors between 0.8 and 0.9, the confidence intervals become noticeably larger although the mean values are inferior to the QoS threshold. This proves that the system becomes unstable at these points and the static algorithm is unable to manage the high variations of the traffic at this high load. Packet jitter increases more steeply for CP-MAC protocols, and are larger than the jitter for MP-MAC at low load. For CP- MACv1 the network becomes unstable at a load factor equal to 0.8 with jitter performance larger than the 3 ms threshold. However, for CP-MACv2 with limited service duration, the jitter only slightly increases for a traffic load factors less than 0.8, and stays stable for larger values, thus presenting a better performance than MP-MAC. We also observe that each source gets almost the same transmission period of time with each grant which creates an artificial periodicity of burst emissions. In Fig. 7, showing CoS-3 packets delays, MP-MAC performs better than CP-MAC protocols until a load factor value close to 0.8. Beyond this value, MP-MAC protocol becomes unstable, while CP-MAC protocols remain stable and experience small confidence intervals. In fact, as MP- MAC protocol shares all available slots in the data cycle between sources proportionally to their resource needs, for low

6 Fig. 5: CoS-2 packet delay Fig. 6: CoS-2 packet jitter load values, sources will get more resources than they really need. However, CP-MAC protocols allocate grants, in the most favorable case, exactly according to what is requested by the sources. During the period of time from the moment when the report is sent to the controller to the moment when the grant is received, additional packets may arrive to the source. Higher priority packets arriving after the report is sent may thus benefit from resources allocated by the CE on the basis of requests corresponding to lower priority packets (this is a wellknown phenomenom in EPON). Since CP-MAC protocols react fast, the CoS-3 packets delays are usually smaller than the 10 ms threshold and confidence intervals remain small even at high load. VI. CONCLUSION TWIN is a promising sub-wavelength switching solution destined to metropolitan-backhaul network. The MAC protocol is central to this technology since it is supposed to efficiently use the optical resources while satisfying the QoS requirements of the flows. In this study, we presented two approaches to design the TWIN MAC layer. The first approach is the MP-MAC protocol for which resource allocation is performed by the management plane. The reconfiguration of the resource allocation pattern could be done several times a day. The second approach (CP-MAC protocols) are based on EPON-like upstream traffic control. It considers the virtual structure of TWIN as juxtaposition of overlapping PON trees in order to use an EPON-like DBA in the TWIN context. Resource allocation is performed dynamically by the control plane that ensures a dynamic reconfiguration of the network based on a real-time knowledge of the traffic variations. The comparison between these two approaches has been done by simulating a MEET metropolitan network based on a central point architecture. The burst assembly process considered in this study is QoS sensitive and Ethernet com- Fig. 7: CoS-3 packet delay pliant. Simulation results show that the CP-MAC protocol achieves better performance than the MP-MAC protocol at high load when a burst duration limitation is enforced (CP- MACv2). Otherwise, the CP-MACv1 protocol delivers a bad packet jitter performance, even for CoS1 packets at low load. CP-MACv2 globally manages well the available resources and follows the traffic variation within the network at the expense of an intensive control message exchange between edge nodes and the controller. The performance delivered by the MP-MAC protocol is very good at low load, and remains satisfying until a load factor value equal to 0.8. ACKNOWLEDGMENT This work was partly supported by the DGCIS, in the frame of the CELTIC-Plus project SASER-SaveNet, as well as the European community s 7 th framework programme FP7/ under grand agreement COMBO project. Authors would like to thank Paulette Gavignet, Esther Le Rouzic and Bernard Arzur from Orange Labs for providing traffic traces and their valuable comments within SASER project. Authors also thank Jim Roberts from IRT-System X for his contribution in designing EPON-like algorithm. REFERENCES [1] E. Bonetto, A. Triki, E. Le Rouzic, B. Arzur, and P. Gavignet, Circuit switching and time-domain optical sub-wavelength switching technologies: Evaluations on the power consumption, in SoftCOM, pp. 1 5, IEEE, [2] I. Widjaja, I. Saniee, R. Giles, and D. Mitra, Light core and intelligent edge for a flexible, thin-layered, and cost-effective optical transport network, Communications Magazine, IEEE, vol. 41, no. 5, pp. S30 S36, [3] A. Triki, P. Gavignet, B. Arzur, E. Le Rouzic, and A. Gravey, Bandwidth allocation schemes for a lossless optical burst switching., in ONDM, pp , [4] A. Banerjee, Y. Park, F. Clarke, H. Song, S. Yang, G. Kramer, K. Kim, and B. Mukherjee, Wavelength-division-multiplexed passive optical network (wdm-pon) technologies for broadband access: a review [invited], Journal of optical networking, vol. 4, no. 11, pp , [5] R.-M. Indre, J. Pesic, and J. Roberts, Popi: A passive optical pod interconnect for high performance data centers, in ONDM, pp , IEEE, [6] A. Triki, R. Aparicio-Pardo, P. Gavignet, B. Arzur, E. Le Rouzic, and A. Gravey, Is it worth adapting sub-wavelength switching control plane to traffic variations?, in ONDM, pp , [7] G. Kramer, B. Mukherjee, and G. Pesavento, Interleaved polling with adaptive cycle time (ipact): a dynamic bandwidth distribution scheme in an optical access network, Photonic Network Communications, vol. 4, no. 1, pp , [8] M. T. Ngo, A. Gravey, and D. Bhadauria, Controlling qos in eponbased fttx access networks, Telecommunication Systems, vol. 48, no. 1-2, pp , 2011.