Polymorphic Optical Networks

Polymorphic Optical Networks: A Solution for Service Differentiation at the Optical Layer J.C. González1,2, I. Tafur Monroy1, I. de Miguel2, A.K.M. Koonen1, J.C. Aguado2, J. Blas2 1

COBRA Institute, Eindhoven University of Technology, Eindhoven, The Netherlands, E-mail: [email protected] 2

Dpt. of Signal Theory, Communications and Telematic Engineering, University of Valladolid, Valladolid, Spain, E-mail: [email protected]

This paper proposes a novel polymorphic framework for optical networking as an efficient solution for service differentiation at the optical layer. We show that by simultaneously supporting several optical switching paradigms over a single physical network, efficient and flexible optical networks can be built, and moreover, a seamless evolution path from optical circuit-switched (OCS) towards optical packet-switched (OPS) networks is provided. We present the polymorphic paradigm and the motivation for this new concept. We also introduce two novel polymorphic architectures, one based on optical circuit switching paradigms and a second one based on optical label-switched networks with the capacity of express lightpath provisioning. The support for class of service differentiation and a possible seamless evolution toward optical packet-switched networks is briefly presented. 1. Introduction Future optical transport networks are expected to deal not only with an increased volume of data-centric traffic but also with an increase of diversity of services accompanied by a bursty/dynamic variation of traffic patterns. Optical packet switching [1-3] has been identified as a flexible and promising solution for this trend in optical networking. However, mainly due to the lack of optical random access memory (RAM), complex buffering schemes and complex synchronization required, realization of optical packet switched networks is expected to be delayed for some years. It is therefore foreseen that the evolution from current static circuit-based optical networks toward efficient packet switched networks will take place in stages. To cope with this evolution several optical switching paradigms have been proposed. A widely studied solution is the introduction of semi-static and dynamic wavelengthrouted optical networks (WRONs). In semi-static WRONs, a limited set of lightpaths is established between pairs of nodes, which allows embedding a virtual topology in the physical topology [4]. One benefit of this architecture is that the virtual topology can be reconfigured, for instance, to adapt to traffic changes or to react to network failures, but it has the drawback that some traffic may require conversion to the electronic domain at intermediate nodes. In dynamic WRONs [5], lightpaths between any two network nodes are established and released on demand on real

time. In this scenario, traffic is always transmitted from source to destination without electronic conversion at intermediate nodes. Although WRONs are relatively easy to build and manage, they do not fully solve the problem of efficient support of traffic services and optimization of optical network resources. Another proposed alternative is the use of Labeled Optical Burst-Switched Networks (LOBS) [6]. These networks try to reconcile the need for serving packetized traffic and the present technological limitation of optics such as RAM, buffering and synchronization. Wavelength-Routed Optical Burst-Switched Networks (WR-OBS) are another promising architecture for the transmission of packets with guaranteed maximum end-to-end delays [7, 8]. It is a hybrid architecture between dynamic WRONs and OBS networks, and it is based on the acknowledged establishment of dynamic lightpaths for the transmission of bursts. One common feature of the alternatives proposed in the literature is that none of them can transport efficiently different traffic types and services while making efficient use of optical network resources and bandwidth. As an alternative solution to this problem, we propose the utilization of polymorphic optical networks (PMON). The framework of polymorphic optical networks has its origin in the concept of polymorphic control introduced by Qiao et al. [9]. These authors propose a network architecture sliced into several virtual optical networks (VON), each one designed to support a different class of service. For each VON, a dedicated set of resources is allocated and a different kind of control is employed, thereby introducing the concept of polymorphic control. In contrast with their work, we propose the PMON as an integrated architecture that combines several switching paradigms allowing, as long as it is possible, resource reutilization between all the supported paradigms. Moreover, a PMON is based on a unified control plane. In this way, the cost efficiency of such a network is improved. A PMON is able to support concurrently different optical switching paradigms over the same physical network. Each paradigm is selected to best serve a certain traffic type. It is assumed that the network is composed of an access and a core layer. In the access layer, the process of traffic monitoring and classification, as well as the selection of the switching paradigm is performed by edge routers. Furthermore, each core node of the polymorphic optical network is able to support the selected optical paradigms. In this way, not only does the network optimally support different traffic types and services, but it also uses its resources effectively by means of semi-static or dynamic resource reservation for each paradigm, thereby adapting to traffic demand. 2. Polymorphic architectures We propose two novel polymorphic networking architectures, Optical CircuitSwitched Polymorphic Networks (OCSPN) and Labeled Optical Burst-Switched Polymorphic Networks (LOBSPN). We envision these two architectures as an evolutionary path toward all-optical packet switched networks for future data centric networks (Fig. 1).

Paradigm OPS, Hybrid OPS/OCS

OPS

LOBSPN

OBS, LOBS

OCSPN

WR-OBS

OCS

WRONs Nowadays

Time

Figure 1: Evolution of optical networks.

2.1 The Optical Circuit-Switched Polymorphic Network (OCSPN) The OCSPN architecture is entirely based on relatively mature optical circuit switching. It combines semi-static WRONs, dynamic WRONs and WR-OBS networks. The semi-static WRON is chosen as an efficient solution for supporting bursty traffic such as Internet traffic, due to its adaptability to traffic pattern changes. A virtual topology is embedded in the physical topology by establishing a set of lightpaths. When data requiring this scheme arrive at the network, they are automatically sent to the destination node through the set of pre-established lightpaths. Moreover, if the traffic demand associated to this scheme changes, the virtual topology can be reconfigured in order to adapt efficiently to the new scenario. For services requesting dedicated lightpaths or virtual private optical network establishment on demand, the dynamic wavelength routing scheme is used. In this case, when a request soliciting one of these services arrives at the network, a process for establishing dynamically a lightpath between a pair of network nodes is initiated. The WR-OBS paradigm is used for services requiring bounded delays such as realtime video and video on demand, which require an end-to-end delay lower than 100 ms and 500 ms respectively, due to the capability of this architecture to provide such guarantees [8]. It must be remarked that all the paradigms employed in an OCSPN are based on circuit switching. This feature simplifies the architecture of the core nodes and facilitates resource sharing between all the paradigms supported, which is the key point for the efficiency of this architecture. 2.2 The Labeled Optical Burst-Switched Polymorphic Network (LOBSPN) The LOBSPN architecture is based on optical burst switching, combining the LOBS and the WR-OBS paradigms. The LOBS paradigm is used for providing connectionless services such as best and excellent effort traffic. To transport these traffic types across the network, optical

label swapping is performed in the intermediate core routers, providing fast data switching. For instance, the label can be orthogonally modulated to the payload (and in the same wavelength) as proposed in the IST-STOLAS project [10]. The WR-OBS paradigm is selected for services requiring bounded delay and for express optical circuit provisioning, thereby, providing connection oriented services. Some attractive features of LOBSPN are a simplified routing and label processing in high-capacity WDM networks while providing efficient delivery of services with service differentiation, and dynamic adaptation to bursty traffic and traffic pattern changes. 3. Network and Node architectures As we have previously discussed, a PMON supports different optical switching paradigms at the optical layer. Each switching paradigm has a different operation principle, so that in order to be simultaneously supported by the same physical network, a unified network architecture has to be defined. 3.1 General Network Architecture of a PMON A PMON consists of two network layers, namely an access layer and a core layer (Fig. 2). In the access layer, traffic aggregation and classification is performed by edge routers, i.e, for all the incoming traffic, the class of service required is identified to determine which of the supported switching paradigms best serves the particular service. This layer is also the interface between the different transmission sources and the all-optical core layer. The core layer consists of several core routers connected in a mesh topology. These core routers perform switching and routing functions, and also functionalities related to protection and restoration issues. Moreover, the core layer transports data transparently in the optical domain (without opto-electronic conversion).

Access layer

edge router

core router

Core layer

Figure 2: PMON architecture.

3.2 Edge Router Architecture for a PMON The block-diagram of an edge router is similar for the two polymorphic architectures presented in this paper, but differs in some of the functionalities offered by each block, and therefore in the way each block is implemented (Fig. 3).

Control Unit

Class 1 buffer Routing Unit Traffic Monitoring

Label Generation (Only in LOBSPN) Class K buffer

1 N 1 N

Direct Transmission Lines (Only in OCSPN) Burst Formation Buffers

Figure 3: Block diagram of an edge router.

The architecture of the edge router consists of the following blocks: Traffic monitoring block, it continuously monitors the incoming traffic (data or requests for sending data) in order to determine the destination edge router and the class of service (CoS) required. This information will be used by the control unit to select the switching paradigm required and to properly forward data to the burst formation buffers block. Burst formation buffers, this block is used for building the bursts that will be transported by the network. In the OCSPN architecture, the block consists of several electronic buffers (one buffer per destination edge router and class of service) were burst aggregation is performed. Bursts are then transmitted using the WR-OBS paradigm. In this architecture, there is also a set of direct transmission lines, which bypass the buffers and forward data directly to the routing unit (see below) for its transmission through an already established lightpath. These lines are used by data associated to the semi-static and dynamic WRON paradigms, as they do not require buffering for burst aggregation. When the LOBSPN architecture is used, there are no direct transmission lines, but only buffers for burst aggregation. Bursts are transmitted by using either the WR-OBS or the LOBS paradigm, depending on the service required. Label generation block, this block is only used in the LOBSPN. Each burst to be transmitted is marked with a label. A promising technique for this aim is the use of orthogonal angle and intensity modulation schemes as proposed in [10]. Label information can be either modulated in FSK (frequency shift keying) or DPSK (differential phase shift keying) format, while the burst payload is in intensity modulation format. The label generated by this block is used for identifying the

paradigm to be used in core routers (LOBS or WR-OBS) and the class of service required. Besides, the label is also used for switching and routing (performing label swapping) in the case of LOBS-based traffic. Routing unit, it sends the data to the core network over a suitable wavelength and optical fiber. Control unit, it is in charge of communicating with other edge routers and/or with the network control node in order to request network resources and to check resource availability. Besides, it also controls the internal operation of the edge router by coordinating the operation between all the blocks. 3.3 Core Router Architecture for a PMON The core layer consists of multiple core routers interconnected by optical links in a general mesh topology. Each core router is capable of switching data streams (lightpaths in the OCSPN, and bursts or lightpaths in the LOBSPN) from a given input port to a given output port. Basically, four different functional blocks are defined in a core router of a PMON (Fig. 4). Again, the block diagram is valid for both architectures (OCSPN and LOBSPN), although the inner structure of the blocks is different.

Control Unit

1

IIB

OSB

OIB

N

N

WDM Channels 1

1

IIB

WDM Channels OIB

N

1 N

Figure 4: Block diagram of a core router.

Input interface block (IIB), this block demultiplexes the WDM spectrum into individual channels or wavebands. It may also include tunable wavelength converters in order to increase switching flexibility. In the case of the LOBSPN, this block also differentiates traffic types by processing the label imposed onto the transmitted payload data and may perform label swapping. Optical switch block (OSB), it switches an incoming channel to a proper outgoing port. Output interface block (OIB), it multiplexes individual WDM channels to an optical fiber. Control unit, it is in charge of communicating with other core/edge routers and with the network control node (if a centralized control scenario is employed) in order to request network resources and to check resource availability. It also controls the internal operation of the core router by coordinating the operation

between all the blocks, configuring the switch fabric and performing tasks such as detection and resolution of contention problems. 4. Dynamic resource allocation A PMON is able to support service differentiation at the optical layer by supporting simultaneously different optical switching paradigms. For each optical scheme, a certain amount of resources (wavelengths, fibers, transmitters, receivers, etc) are either statically or dynamically assigned. When the static reservation is used, a fix amount of network resources are reserved for each paradigm defined in the PMON. This approach is consistent with the definition of polymorphic control by Qiao et al. [9]. However, as next generation networks are characterized by a dynamic behavior and variable traffic patterns, we envision the dynamic resource allocation as the most effective mechanism because it allows a seamless adaptation to traffic patterns changes. In an OCSPN, all the paradigms employed are based on circuit switching, thereby facilitating dynamic resource sharing between all the paradigms supported. The set of resources assigned to the virtual topology embedded in the physical network is not static but it changes dynamically in response to traffic changes. If each edge router is equipped with T transmitters and T receivers, a subset of these transceivers (say TSSWRON) is employed for setting the virtual topology. The remaining transceivers are used for establishing dynamic connections following either the dynamic WRON or the WR-OBS paradigms. In order to optimize network performance, methods for traffic monitoring and analysis must be applied as well as protocols for network reconfiguration. If the traffic pattern suffers a significant change, the virtual topology can be reconfigured, and moreover the set of resources assigned to it can also be modified. For example, if the demand of dedicated lightpaths increases, the set of transceivers reserved for establishing the virtual topology can be dynamically decreased so that more resources become available for the dynamic WRON and WR-OBS paradigms. This dynamic assignation also applies to other network resources, namely, fibers and wavelengths. In the case of the LOBSPN, all paradigms are highly dynamic (in contrast with semistatic WRONs in the OCSPN). In this way, resources are dynamically allocated for each paradigm on demand by means of the signaling information received from the network. 5. Conclusion Although a number of optical switching paradigms have been proposed, none of them alone is able to provide service differentiation while making efficient use of the network resources. Moreover, a seamless evolution from current optical networks towards next generation networks is required. For instance, the introduction of provisioning of packetized services while still providing legacy services and making use of legacy equipment is a desirable scenario for telecom operators. Our proposal aims to be a solution for these issues. A polymorphic optical network is conceived as a network supporting simultaneously different optical switching paradigms over the same physical layer. This property permits to offer service differentiation at the optical layer by means of employing the switching paradigm most appropriate for each service and by using a reservation scheme that allocates dynamically

resources (fibers, wavelengths and transceivers) to each paradigm supported. We have introduced and defined two novel polymorphic architectures: the optical circuitswitched polymorphic networks (OCSPN) and the labeled optical switched polymorphic networks (LOBSPN). For a seamless evolution of optical networks towards next generation networks, OCSPN can be introduced as a first step. Note that this architecture can provide support for both IP traffic as well as for non-IP services due to the use of lightpaths. The mature WDM and OXC technologies can be used, provided that the edge routers perform the required traffic aggregation and classification followed by the selection of the proper switching paradigm. As the demand for more IP-centric data increases, optical labeled burst switching may be introduced. LOBSPN offers the possibility to provide express circuit bandwidth for legacy services while offering efficient and fast forwarding of packetized traffic. Moreover, both polymorphic architectures can adopt a unified control plane based on the GMPLS approach. For these reasons, we believe that the two proposed network architectures form a promising seamless evolutionary path toward next generation networks. Acknowledgements This work has been supported by IST-STOLAS (Switching Technologies for Optical Labeled Signals) of the European Commission, and by the Spanish Ministry of Science and Technology (Ministerio de Ciencia y Tecnología) under Grant TIC2002-03859. References [1] S. Yao, B. Mukherjee, S. Dixit, “Advances in Photonic Packet Switching: An Overview”, IEEE Communications Magazine, Vol. 38, No. 2, pp. 84-94, February 2000. [2] S. Tarek, El-Bawab, J. Shin, “Optical Packet Switching in Core Networks: Between Vision and Reality”, IEEE Communications Magazine, Vol. 40, No. 9, pp. 60-65, Sep. 2002. [3] M. O’Mahony, D. Simeonidou, D. Hunter, A. Tzanakaki, “The Application of Optical Packet Switching in Future Communication Networks”, IEEE Communications Magazine, Vol. 39, No. 3, pp. 128-135, March 2001. [4] R. Dutta, G.N. Rouskas, “A Survey of Virtual Topology Design Algorithms for Wavelength Routed Optical Networks”, Optical Networks Magazine, Vol. 1, No. 1, pp. 73- 89, Jan. 2000. [5] H. Zang, J. Jue, B. Mukherjee, ”A Review of Routing and Wavelength Assignment Approaches for Wavelength Routed Optical WDM Networks”, Optical Networks Magazine, Vol. 1, No. 1, pp. 47-60, Jan. 2000. [6] C. Qiao, “Labeled Optical Burst Switching for IP-over-WDM Integration”, IEEE Communications Magazine, Vol. 38, No. 9, pp. 104-114, Sep. 2000. [7] M. Düser, P. Bayvel, “Analysis of a Dynamically Wavelength-Routed Optical Burst Switched Network Architecture”, Journal of Lightwave Technology, Vol. 20, No. 4, pp. 574-586, April 2002. [8] I. de Miguel, E. Kozlovski, P. Bayvel, “Provision of End-to-End Delay Guarantees in WavelengthRouted Optical Burst-Switched Networks”, Next Generation Optical Network Design and Modelling – Proc. of ONDM'02, (A. Bianco, F. Neri, eds.), Kluwer Academic Publishers, 2003, pp. 85-100. [9] C. Qiao, “Polymorphic Control for Cost-Effective Design of Optical Networks”, European Transactions on Telecommunications, Vol. 11, No. 1, pp. 17-26, 2000. [10] T. Koonen, Sulur, I. Tafur, J. Jennen, H. de Waardt, “Orthogonal Optical Labeling of Packets in IP over WDM Networks”. Proc. NOC 2002, pp. 82-89.