TWIN as a Future-Proof Optical Transport Technology for Next Generation Metro Networks

Transcription

1 as a Future-Proof Optical Transport Technology for Next Generation Metro Networks Ahmed Triki, Ion Popescu, Annie Gravey, Xiaoyuan Cao, Takehiro Tsuritani, Philippe Gravey Institut Mines Télécom, Télécom Bretagne, Brest, France, ( firstname.lastname@telecom-bretagne.eu) KDDI R&D Laboratories, Inc., Saitama, Japan, ( io-popescu, xi-cao, UMR CNRS 674 IRISA, France, UMR CNRS 6285 Lab-STICC, France Abstract The continuous growth of the traffic at a rapid pace raises challenges related to the efficient use of resources and saving of energy in the transport network. The new traffic pattern is characterized by faster growth of metro traffic compared with core, due in part to video content replication. In this paper, we compare the resource requirements of three different transport technologies (optical packet switching, opaque circuit switching and transparent circuit switching) within two architecture scenarios (the current hub-and-spoke architecture and the next generation distributed architecture). The comparison study takes into consideration the predicted traffic growth from 214 to 219 and the potential traffic decentralization due to content replication. For the optical packet switching, we focus on Time-domain Wavelength Interleaved Network () solution which achieves passive switching at the intermediate nodes thanks to the precomputed optical packet emission schedule. Results show that outperforms the other transport technologies in terms of number of transmitters and receivers with a high ability to cope with scalability issues. Index Terms Network architecture, optical packet switching, metropolitan network, traffic profile. I. INTRODUCTION Traffic in all segments of the transport network has been constantly growing in the past years [1] as a result of increasing number of connected user terminals (smartphones, tablets, connected TVs) and of the race toward higher definition contents. This growth of fixed, mobile and video traffic is expected to continue [2], and it leads to significant increase in bandwidth demand. The conventional approach in opaque transport networks consists in aggregating/grooming traffic in the time domain at every node, in order to optimize bandwidth usage at the expense of costly traffic grooming electronics and high power consumption. This method offers the maximum flexibility and control. On the other hand, transparent (i.e., optical) aggregation uses Wavelength Division Multiplexing (WDM) and Reconfigurable Optical Add Drop Multiplexer (ROADM). In this case, many O E O (Optical/Electrical/Optical) conversions are avoided at intermediate nodes, but it induces a non-optimum channel use. The use of Optical Circuit Switching (OCS) method is considered by many references not to be flexible enough for packet oriented traffic [3]. Many approaches try to improve the utilization of resources in OCS by exploiting the sub-wavelength granularity with reduced electronic processing. These methods are referred to as Optical Packet Switching (OPS)/Optical Burst Switching (OBS) [3], [4] deping on the optical packet duration. The time domain sub-wavelength switching requires drastic technical modifications: shifting to burst-mode receivers, ultrafast tunable lasers and new switching paradigms. Many declinations and variants, derived from the original OPS/OBS concepts, have been proposed in the literature. The Sub-Lambda Photonically Switched Networks (SLPSN) [5] is a recently proposed term by International Telecommunication Union - Telecommunication (ITU-T) for the OPS/OBS concept. The new transport networks should support very high bitrate interfaces, dynamic bandwidth provisioning such that they meet customers demands and use the existing fibers. In this context, a SLPSN paradigm may be more appropriate than the currently deployed OCS, in order to bring the required flexibility in the transport layer [6]. Among approaches which have been proposed for OPS, the Time-domain Wavelength Interleaved Network () [7] is considered as a rather attractive solution, as it aims combining passive optical transparency and efficient use of wavelength resources. Previous studies show that is a promising OPS solution for the metropolitan area since it is able to achieve high channel utilization (more than 8%) [8], [9] and low design cost [1] [12]. In this paper we aim to compare the required network resources for, opaque and transparent OCS transport technologies within both the current hub-and-spoke architecture and the next generation distributed architecture. The yearly traffic growth of the metropolitan traffic from 214 to 219 is considered. Moreover, we propose some assumptions to emulate the new distribution of traffic due to the decentralized tr of the next generation network. The comparison takes into account the number of trans-

2 Fig. 1: Architecture overview of the general metropolitan network. mitters, receivers and wavelengths for each technology and the ability to deal with the scalability challenge due to the yearly traffic growth. To the best of our knowledge, the comparison of network design cost in and OCS technologies for the current and the next generation architectures in the context of future traffic evolution was not addressed before. The remainder of the paper is organized as follows. Section II describes the architecture and traffic trs of the current and next generation metropolitan network. In Section III the characteristics of are detailed. Section IV illustrates the case study of the addressed architecture scenarios. Section V is devoted to the discussion of the detailed numerical results. Finally, we conclude the paper in the last section. II. ARCHITECTURES AND TECHNOLOGIES FOR METROPOLITAN NETWORKS In general, telecommunication networks are used to connect large groups of users spread over a geographical area. In order to ensure an efficient connectivity, the current operator networks are designed in a hierarchical way deping on the covered area and the traffic aggregation process. A node in a given level aggregates the traffic coming from the immediate lower level, yielding to higher stages of traffic aggregation. As shown in Fig. 1, three levels of hierarchy: access, metro and core are basically defined. A. Current hub-and-spoke architecture In the metropolitan network, the nodes can be classified into two types: Edge Nodes (ENs) and the Concentration Node (CN) [8]. Fig. 1 illustrates a typical architecture for metropolitan networks. ENs aggregate the fixed and/or mobile traffic coming from the access network nodes (e.g., OLTs). The traffic could come from either a middle-sized town in a low density area or from a given district in a high density area. All the aggregated traffic in the ENs is transmitted to a CN, which is responsible for ensuring connection between the metro and the core network. Thus, it is the first aggregation node in the core network. The CN aggregating all the traffic in each metropolitan network is connected to the core Central Office (cco). It is also called Point of Presence (PoP) as it is the Fig. 2: Architecture overview of the next generation metropolitan network. location of the IP edge at least for fixed traffic. The IP edge for mobile traffic may be located even higher in the core network. The above-described architecture requires a huge buffering capacity and computing resources in the CN to deal with all the traffic flows. B. Next generation distributed architecture The next generation metropolitan network will have to present considerable architecture modifications in order to support fifth generation (5G) technologies. In 5G networks, the control and the user planes (C/Uplane) are distributed over the edge nodes via the Network Function Virtualization (NFV). Thus, we assume that the global C/U-plane is composed of multiple Virtual edge Central Offices (VeCO) at the backhaul level and a cco at the core level (Fig. 2) [13]. A VeCO can thus be considered as a next generation PoP (NG- PoP) as the IP edge is moved closer to users. The split of C/U-plane can hopefully address capacity (i.e., data rate) challenges, reduce the cost, and improve the user s Quality of Experience (QoE), as some services can be delivered directly from the NG-PoP, for both fixed and mobile users. In particular, this architecture will facilitate Mobile Edge Computing. Thus, the deployment of VeCO will reduce the aggregated traffic toward the core network, and even in the metro portion as some services (e.g., video content distribution) can be performed from the NG-PoP. C. Traffic profile evolution According to a study done by Cisco [2], the global IP traffic has increased by a factor of five over the last five years, and over the next five years will increase by a factor of three. The annual growth rate between 214 and 219 will be nearly 23%. Global mobile data is expected to grow by a factor of ten to 219. The same trs are confirmed by the recent Ericsson report [14]. Traffic in the metropolitan network will be 66% of the total IP traffic by 219. Metropolitan traffic will grow twice as fast as core traffic from 214 to 219. The trs in the metropolitan traffic grow result from an increasingly significant role of video caching and of

3 Fig. 3: architecture with four edge nodes and three passive nodes. server distribution over the edge nodes (i.e., the deployment of Content Delivery Networks, CDN), which will bypass the core links and deliver traffic directly to metro. With these assumptions, it is expected that the CDN traffic will consist 62% of the total Internet traffic by 219, which will represent 8% of the metropolitan traffic. III. DESCRIPTION is a sub-wavelength switching technique that transmits optical slots (i.e., optical packets) of data on WDM channels without resorting to electronic processing in transit nodes, the electronic processing functions being pushed to its edge source and destination nodes (Fig. 3). A set of fixed-wavelengths is exclusively attributed to each destination node in order to enable it to receive data sent by the other nodes. The switching in the network is based only on the optical packet s wavelength, and thus, is purely passive. Therefore, the logical topology of can be viewed as a set of optical multipoint-to-point trees overlaid on top of a physical mesh/ring network, one for each destination. Fig. 3 illustrates two trees, with nodes D1 and D2 being destinations that receive traffic from nodes S1 and S2, by using a dedicated wavelength. In this configuration, the intermediate nodes are core passive nodes, connecting edge nodes (e.g., S1 and D1). Furthermore, by tuning its transmitter to the appropriate wavelength, a source can s an optical packet to any destination. In [15] is shown that a centralized control plane, where a single control entity manages resource allocation, is more adapted to a metropolitan area and achieves better performance than a distributed control plane. A node knows when to s and receive slots thanks to a schedule. The schedule length is from a few dozen to a few hundred of slots. The schedule is followed by nodes repeatedly until a new schedule is recomputed. The recomputation could be done every few milliseconds, seconds, minutes, or hours, as needed, deping on the traffic variation or network configuration changes. Several mechanisms are proposed in the literature to compute schedules. A heuristic algorithm is proposed in [16]. It simultaneously assesses the routing, scheduling and virtualization in in order to overcome the computation Fig. 4: Studied network topology. complexity of the control plane. [1] proposes an ILP formulation enabling the computation of schedule patterns and the minimization of the number of used transmitters/receivers. IV. CASE STUDY SCENARIO The aim of the present study is to assess the required resources in terms of wavelengths (Ws), transmitters (Txs) and receivers (Rxs) for the current and the next generation network architectures. For both architectures, we apply three different transport technologies: opaque, transparent and. We use the network topology from Fig. 4 which contains 17 nodes: node 1 corresponding to the cco and others 16 nodes corresponding to ENs. The cco ensures the exchange of data between the metropolitan and core network. The EN aggregates the local metro/access traffic and ensures the connection of the local area with the rest of the network. The role of the cco and the ENs deps on the architecture as it is aforementioned. For the three used technologies, connections are established according to the shortest path between nodes using Dijkstra as algorithm of resolution and the distance between nodes (expressed in kilometers) as weight. Based on our previous study [8], we assume that for all technologies the Tx/Rx utilization should not exceed 8% of the channel capacity (C=1 Gbit/s). The number of Txs and Rxs in are computed based on the Eq. 1 and 2, respectively. The number of Ws is the same as the number of Rxs. In what follows, f i,j is the load of traffic from i to j that in some cases could be equal to, and function maps a real number to the smallest following integer. N N N ;j i Tx = Tx i = f i,j ; (1) Rx = i=1 i=1 N N N i=1;i j Rx j = f i,j ; (2) In the opaque case the number of Txs and Rxs are respectively computed based on Eq. 3 and Eq. 4, where fs,d i is the load of the traffic belonging to the flow froms todand passing throughi(i.e.,i V s,d ). The number of

4 TABLE I: The traffic profile evolution from 214 to 219. Video traffic [%] Web/data/file sharing/others [%] Total throughput [ExaByte/month] Billion of devices Number of devices per user [unit] Throughput per device [Kbit/s] Throughput in the metro-network [Gbit/s] required Ws in the network corresponds to the number of Ws on the most loaded link. N N i V Tx = Tx i = s,d,i d fi s,d ; (3) i=1 i=1 N N j V Rx = Rx j = s,d,j s fj s,d ; (4) The number of Txs and Rxs in the transparent case are computed based on the number of optical circuits as shown in Eq. 5 and Eq. 6 respectively. The computation of the number of Ws is based on Alg. 1 that takes into account the possibility to reuse Ws for flows having disjoint paths. N N N Tx = Tx i = f i,j ; (5) Rx = i=1 N Rx j = i=1,j i N N i=1,i j f i,j ; (6) In the present study, we do not reserve extra resources for protection since protection is out of the scope of the paper. The traffic model is derived from the Cisco VNI trs and analysis of the traffic in the interval from 214 to 219 [2]. In fact, taking into account the expected global traffic evolution and the number of devices per user, we estimate the amount of traffic in a metropolitan network containing two million users. The magnitude of the video traffic in this network is also based on the same Cisco VNI report which estimates that the IP video traffic will represent 8% of the total traffic in 219, compared to 67% in 214. The evolution of the average traffic throughput within our synthetic metropolitan network is shown in the Table I. Accordingly, the total traffic throughput considered in this model scenario varies from 49.9 Gbit/s in 214 to Gbit/s in 219. The distribution of the traffic among the different connections deps on the architecture. As a first assumption, we suppose that in each architecture, the 16 ENs behave in the same way and s/receive the same amount of traffic. For the hub-and-spoke architecture scenario, the traffic is concentrated at the cco. The downstream traffic, from the cco to the EN, represents 8% of the total traffic. Initialization: G(V, E) // network topology F = {f i,j : i V,j V} // list of flows load W = {} // list of used wavelengths W e = {}, e E // list of wavelengths per edge Wavelength assignment: forall the f i,j F do P i,j (V,E ) // path on G(V,E) according to the shortest path W i,j = f i,j /() // number of wavelengths isattributed = true while (k [1.. W i,j ]) and (isattributed == true) do isattributed = f alse while (w W) and (isattributed == false) do if (w / W e, e E ) then isattributed = true Add w to W e, e E if isattributed == false then for p [k.. W i,j ] do Add w p to W // w p is a new wavelength Add w p to W e ; e E Algorithm 1: Wavelengths dimensioning in the transparent technology. In the next generation network architecture scenario, we assume that 75% of the video traffic will be provided locally by the ENs (i.e., VeCO, CDN), while the remaining 25% of traffic comes from the servers, located in the long-haul area, via the cco node. For the other kind of traffic (e.g., web browsing and file sharing), we suppose that the traffic sent by one EN represent 2% of the total received traffic by the same EN. The traffic sent by the EN toward the cco represents 8% of the total sent traffic by the node. V. NUMERICAL RESULTS 1) Current architecture: Figs. 5a and 5b show that the number of Txs/Rxs in the opaque technology continually increases over the years from 214 to 219. The intermediate node in this case ss/receives its

5 Nb of active transmitters [unit] Nb of active transmitters [unit] (a) Number of transmitters (a) Number of transmitters Nb of active receivers [unit] own traffic flows as well as an important amount of transit flows. Therefore, the accumulated traffic to be sent/received by an opaque node increases faster than those of and the transparent technologies. In, the tunable Tx at the cco is used to s traffic toward several trees. As the downstream traffic from the cco to the ENs is significant and grows from 214 to 219, the number of Txs at the cco rapidly scales as well. However, the number of Rxs slightly increases since the upstream traffic from the EN to the cco is less important than the downstream one. On the other side, the number of Txs/Rxs in the transparent technology is steady. This is due to the fact that the Txs/Rxs are supposed to s/receive a single flow having a volume distinctly lower than their capacity (i.e., 1 Gbit/s). The number of Txs/Rxs used by is smaller than those used by opaque and transparent. But, it is worth to note that the burst-mode Tx/Rx used by is more expensive than continuousmode Tx/Rx needed by the opaque and transparent solutions (the cost of Tx/Rx cannot be estimated since they are not yet on the market). Although the total metropolitan traffic grows more than twice from 214 to 219, the resources used by and transparent technologies remain constant. This stability over the time represents an advantage for both technologies. Contrary to the opaque technology that suffers from scalability issue that obliges to re-plan the network resource every year. Fig. 5c depicts that the opaque technology uses the least number of Ws compared to and the transparent technologies because the traffic is electrically aggregated at each link and Ws are reused. The transparent technology uses a low number of Ws thanks to its capability to reuse Ws for flows having disjoint (b) Number of receivers Fig. 5: Current hub-and-spoke architecture. Nb of active receivers [unit] (b) Number of receivers Fig. 6: Next generation distributed architecture. Nb of wavelengths [unit] Nb of wavelengths [unit] (c) Number of wavelengths (c) Number of wavelengths paths. As one W is attributed to each flow and the traffic per flow does not exceed the W capacity over the years, the number of Ws is steady for the transparent case. In, the number of Ws is higher than that of opaque and transparent, and it is equal to the number of Rxs as each W is exclusively attributed to only one Rx. Despite of the low number of used Ws, the opaque solution cannot be considered as preferable solution since it requires an important number of Txs/Rxs. Moreover, the O E O at each intermediate node induces latency and increased energy consumption. The transparent solution in this scenario presents the best trade-off in terms of number of Txs, Rxs and Ws. 2) Next generation architecture: Figs. 6a and 6b illustrate the number of Txs/Rxs needed by, opaque and the transparent technologies when they are deployed in next generation scenario. The number of Txs/Rxs in is a third of those in opaque solution, thanks to the transparent optical switching at the intermediate nodes. The high gain in the number of Txs/Rxs, compensates the possible cost difference between burst-mode and continues-mode Tx/Rx. The number of Txs/Rxs in the transparent case is much higher than in and opaque cases. In fact, the distributed nature of the next generation architecture leads to the increase in the number of flows for the transparent solution, which increases the number of needed Txs/Rxs as well. For the three transport technologies, the number of Txs/Rxs is steady over the years because the amount of traffic per flow is small with respect to the channel capacity. Fig. 6c shows that the number of Ws in the opaque case is smaller than the number of Ws required by. The number of Ws required by is steady

6 over the five years. In fact, in the next generation architecture, contents, especially video, become closer to the access network (i.e., localized at the ENs). As a result, the traffic exchanged within the metropolitan area decreases compared to the corresponding traffic in the current architecture. As the number of flows increases in the next generation architecture, the number of Ws required by the transparent technology increases as well, and becomes more under-used. This makes the transparent technology less appropriate to the traffic of the next generation network. VI. CONCLUSION The tr of the next generation metropolitan network shifts toward a more distributed architecture. It might replace the hub-and-spoke architecture which has shaped the metropolitan area for decades. In the present paper, we have studied the ability of the existing transport technologies to carry the next generation traffic pattern. We have also evaluated the potential of sub-wavelength switching solution, with the focus on Time-domain Wavelength Interleaved Network (), as a new transport solution for the metropolitan area. Therefore, we have applied three different transport technologies (i.e., opaque, transparent and ) on a metropolitan network designed according to the current and the next generation architectures. We aimed to assess, for both architecture scenarios, the requirements of each technology in terms of number of Txs, Rxs and Ws. The traffic pattern for both scenarios was derived from the traffic forecast study done by the Cisco VNI for the period between 214 and 219. Our results show that the transparent and opaque technologies could operate on the current architecture with acceptable amount of resources, while requires the smallest number of Txs and Rxs, but the largest number of Ws. The requirements on resources in the next generation network are significantly higher for the transparent technology, which makes it less adapted to this evolution. On the other hand, the opaque technology and require a somewhat stable amount of resource when applied to both architectures throughout the next five years. However, the energy consumption of the opaque technology is expected to be much higher than that of, thus making this latter technology a promising transport solution for the next generation metro network. In the future, we shall evaluate the gain in terms of energy consumption presented by and compare the technology performance to the one presented by other Sub-Lambda Photonically Switched Networks technologies. ACKNOWLEDGMENT This work was partly supported by the French National Research Agency in the framework of the N- GREEN project (ANR-15-CE25-9-2). REFERENCES [1] Alcatel-Lucent Bell Labs, Metro network traffic growth: An architecture impact study, Alcatel-Lucent Bell Labs Strategic White Paper, 213. [2] Cisco, VNI, The zettabyte era: Trs and analysis, White Paper, 215. [3] C. Qiao and M. Yoo, Optical burst switching (OBS) a new paradigm for an optical Internet, Journal of High Speed Networks, vol. 8, no. 1, pp , [4] M. J. Mahony, D. Simeonidou, D. K. Hunter, and A. Tzanakaki, The application of optical packet switching in future communication networks, Communications Magazine, IEEE, vol. 39, no. 3, pp , 21. [5] ITU-T, Terms and definitions for sub-lambda photonically switched networks, International Telecommunication Union- Telecommunication, Geneva [6] 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 Software, Telecommunications and Computer Networks (SoftCOM), 212 2th International Conference on, pp. 1 5, IEEE, 212. [7] 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. S3 S36, 23. [8] 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 Optical Network Design and Modeling (ONDM), th International Conference on, pp , 214. [9] A. Gravey, B. Uscumlic, Y. Pointurier, I. Popescu, P. Gravey, and L. Alahdab, Efficient resource allocation in time-domain wavelength interleaved networks, in Optical Network Design and Modeling (ONDM), th International Conference on, pp , IEEE, 214. [1] A. Triki, A. Gravey, and P. Gravey, CAPEX and OPEX saving in SDN-compliant sub-wavelength switching solution, in Photonics in Switching (PS), 215 International Conference on, pp. 1 3, IEEE, 215. [11] I. Popescu, B. Uscumlic, Y. Pointurier, A. Gravey, P. Gravey, and M. Morvan, Cost of protection in time-domain wavelength interleaved networks, in Networks and Optical Communications-(NOC), th European Conference on, pp , IEEE, 214. [12] I. Popescu, B. Uscumlic, Y. Pointurier, A. Gravey, P. Gravey, and M. Morvan, A cost comparison of survivable subwavelength switching optical metro networks, in Teletraffic Congress (ITC), th International, pp. 1 9, IEEE, 214. [13] P. Agyapong, M. Iwamura, D. Staehle, W. Kiess, and A. Benjebbour, Design considerations for a 5G network architecture, Communications Magazine, IEEE, vol. 52, no. 11, pp , 214. [14] Ericsson, Ericsson mobility report, november, Report, 215. [15] A. Triki, P. Gavignet, B. Arzur, E. Le Rouzic, and A. Gravey, Bandwidth allocation schemes for a lossless optical burst switching, in Optical Network Design and Modeling (ONDM), th International Conference on, pp , IEEE, 213. [16] I. Popescu, B. Uscumlic, A. Triki, Y. Pointurier, A. Gravey, and P. Gravey, Scalable routing, scheduling and virtualization for optical burst switching networks, in Networks and Optical Communications-(NOC), 215 2th European Conference on, pp. 1 6, IEEE, 215.