Resources allocation and routing in WDM Metropolitan Networks

Transcription

1 Resources allocation and routing in WDM politan Networks T. Almeida 1 and M. Pousa 1 1 Portugal Telecom Inovação SA, Rua Engº Pinto Bastos, Aveiro, Portugal. Abstract 1 politan telecommunications networks is a recent network concept imported from the data communication world, which is gaining considerable importance in present/near future networks. The emerging of optical transport networks (OTN) which uses Wavelength division multiplexing (WDM), associated to the concept of all-optical networking is shaping the evolution scenarios for the politan networks. In spite of the similarities between existing WDM systems for long-distance and metro networks the cost functions are not the same for the two network segments. In metropolitan networks, cost is a major factor, which in spite of the technological advantages that all-optical networking can bring, is preventing its faster penetration. In this paper we present a possible solution to introduce WDM in this segment, based on the usage of less expensive asymmetric optical nodes, in which the handling of optical channels directed from the long distance network to the metropolitan and distribution networks is achieved by the analysis of the specificity of each case, the required functionality implemented having them into account and in most of the cases realised by one or two optical components, which reduces the cost. This approach requires a more detailed planning phase and has some implications in terms of resources allocation and wavelength routing in particular if end-to-end optical channel routing. These will be analysed in the proposed solution for the metropolitan network. I. INTRODUCTION politan telecommunications networks is a recent network concept. It corresponds to a network segment, which lays between the transport, long-distance network, and the access network. Its geographical coverage is not well defined in terms of kilometric distances, varying each country socioeconomic and demographic reality, depending essentially on the user density distribution. These networks have hybrid functionality, as they have most of transport network characteristics, except that they can provide direct connections to the end-users. These users are typically business end-users or new telecommunication services operators (as a result of market liberalisation), both types requiring high capacity connections. The authors wish to express their acknowledgment to EURESCOM, that partially financed this work, and to PTIN partners in the BOBAN project. Traffic aggregation and traffic routing both within the metro network and outgoing/incoming traffic, (either directed to long-distance network or to access network), are two of the most important functions in these networks. The introduction of optical transport networks (OTN) based on Wavelength division multiplexing technology (WDM) in the long-distance networks and its progressive spreading bringing it closer to the end-user, is influencing the evolution scenarios for the politan networks. The introduction of WDM technology in the metro environment is being driven by a crescent capacity and flexibility demand due to the previewed growth of telecommunication services usage both in terms of number of users and number of different new services. Due to the similarities between long distance and metropolitan networks in terms of functionality and due to the starting of the metro equipment business, the solutions for deployment of WDM in the metro environment are identical to long distance solutions. However, the business case is not the same for long distance and metro networks. In this last network segments the reduction in the share factor raise the importance of the cost which in spite of the technological advantages that all-optical networking can bring, is preventing its faster penetration. Due to this fact, simpler optical nodes in terms of functionality, based on low cost components, could be used in the metro and distribution segments of the network. The basis of optical WDM nodes is the usage of wavelength selective optical components. Wider ITU grid spacing utilisation, as one approaches the distribution network, poses less requirements on the optical spectral characteristics of components. However this has implications on wavelength routing strategies from the long distance towards the end-user, that must be taken into consideration. II. NETWORK ARCHITECTURES One of the most used physical network topology in the core and metro networks is the ring topology due to its inherent network resilience. politan optical ring topologies, with WDM transmission are assumed in this paper. The Optical Node functionality depends on its interface role, i.e., whether it interfaces mainly to the long-distance network, or instead it interfaces mainly to distribution (last-mile) network. Resource allocation based on space division and WDM is proposed for the metropolitan network. This configures a stack of WDM rings in which interfacing to the core to the distribution networks use dedicated rings, Fig 1.

2 CORE METRO DISTRIBUTION C/M interface IC Inner Rcore 1 Rcore k Rcore K Rmetro 1 Rmetro 2 Rmetro i Rmetro I Access Node Inner Fig.1 The metropolitan rings will in this case act as feeder rings for the distribution network, laying partially on the core and on the access network. It will be based on a high-count number of fibre rings, with WDM in each fibre. The easiest way to comply with distinct traffic routing function of this interface ring is to dedicate WDM rings to each network segment, built on individual fibre rings to interface specific functionality WDM rings in the adjacent network segment rings, i.e., in the ring a/some specific WDM ring(s) will be dedicated to interface core network, and some other to interface distribution network (in the figure also rings are represented for distribution, but could other topologies as Pons). There is also need for inner ring traffic routing in the metro ring, and also for this functionality there will be dedicated rings. This space (fibre) division multiplexing will allow for in each optical interface node Core/, or /Distribution to have a dedicated subsystem for routing within each network segment of a WDM specific ring, and to implement the interface by passing traffic between these subsystems within the Optical Node. In order to implement the different ITU grid spacing within the different network segments, the optical network node subsystems will be built on different optical component technology, depending on the direction and sense of traffic flow handled by them, being that the most demanding in terms of optical component performance are those handling traffic directed to the hierarchically superior network segments, i.e., from distribution to the core network. ID Rdist. the core to the access, and vice-versa. However this is an expensive solution, and still not for near future implementation due to technology immaturity, unless it is based on O/E/O transponders. Another possible mechanism is the use of a pre-defined set of rules for wavelength allocation in the several network segments, associated to network planning and network configuration. One of the possibilities is to use space division multiplexing in the feeder ring, as referred in the previous section, reserving different fibre rings according to traffic flowing direction (to the access segment, to the core segment, inner ring traffic) therefore allowing the use of similar set of wavelengths for interfacing with upper and lower rings, and also permitting the wavelength routing from/to core to/from access. In the same fibre rings traffic should refer to the same network segment and should be routed for maximum wavelength reuse within the fibre ring. Distribution rings would use for inner ring routing only a subset of available wavelengths. If different grid spacing anchored to the same reference are used, being the grid narrower in the core and going wider towards the access, e.g. (50GHz, 100GHz and 200GHz), for the direction from the core towards the access, and providing that these grids were compatible, it would be possible to route sets of wavelengths grouped, for the same part of the network, but for different same level rings. Inside a particular ring, routing and therefore wavelength allocation have to respect this level ring grid spacing. In the inverse direction, at the interconnection of rings, high-performing components namely in terms of crosstalk rejection should be required in order to narrow the channel spectrum. This would also allow the upgrade of network domains and reuse of components on domains of the same network segment. Direct wavelength routing from core to access network is also possible in this scheme, Fig. 2. CORE METRO C/M interface Inner ring WL transposition to free WL channel III. ROUTING STRATEGIES Independent wavelength assignment on the several domains on the optical network (core, metropolitan and distribution) can be based on several mechanisms. The more evident is the use of wavelength conversion functionality, allowing reusing of the same wavelength allocation grids and components for all the segments of the network. This allied to allocation reconfigurability within rings also permits direct wavelength routing from DISTRIBUTION Fig.2 Access Node Inner ring WL transposition to free WL channel

3 IV. OPTICAL NODE IMPLEMENTATION OXC and configurable are identical in terms of functionality, as they allow one or more input/output trunk ports (fibre DWDM carrying signal ports) and none, one or more single channel input /output ports, being individual channels routed by means of optical switches (switching matrix) to the desired output port. The choice of an or a OXC for the Optical Node implementation depends much on the type of interconnected rings in terms of number of fibres and used channels in each ring, in the number of channels used for interconnecting and on the possibility of direct wavelength routing from the core to the distribution ring. In most of the cases, with one or a few channels dropped /inserted for interconnection of rings is the more suited solution, as is not expected in general the same traffic density from distribution to the and Core, nor the need of direct wavelength routing for a high number of channels but just a few (one or two). OXC for ring interconnection is recommended in the case of high traffic flowing in the ring interface and the requirement of high flexible assignment of channels and high demand for direct wavelength routing. It is also possible that the co-location of several optical nodes for different distribution rings, interconnecting to the metro ring makes more viable the use of a OXC instead of several s, which also allows the interconnection of the several distribution rings without using the metro ring. In an initial phase, /EADM(EXC)/ implementation is a fast straightforward solution for interconnection of pre-existing rings with nodes. In this solution the traffic can be further manipulated in the electrical domain, as this is not pure optical solution, which may present some advantages as routing to and or aggregating traffic to legacy rings (e.g. SDH rings) with a co-located node. Very simple implementations of Optical Node functionality for optical configurable interconnection are possible. Such is the case of a single channel configurable functionality which can be achieved by a tuneable filter associated to a power splitter per direction in the ring as depicted in Fig. 3. Fj Fi Distribution Control Fj λk->m λk->m λk->m Fig.3 λk->m Fi Distribution Recently, an implementation of Optical Node that directly handles different compatible ITU grid spacing has been reported by Wavesplitter[2], based on an all-fibre interleaver, and thus allows the packing of a high number of optical channels in an optical fibre, by shrinking the grid spacing. Both interleaver and de-interleaver function are reported. This type of component is a very promising one for the implementation of the referred networks, depending on the actual price, as the same technology can be used in all interfacing nodes, independent of interface function (Longdistance/metro or /distribution). As one approaches the end-user, the interface between network segments becomes simpler, especially if one considers the particularities of services provided. In Fig. 4 and Fig. 5, some examples of possible implementations are show for distributive and interactive services over a PON topology. In the case of service sharing provisioning, e.g. CATV distribution, the functionality required is the filtering of the correct wavelength transporting the service previous to power splitter which is either co-located with the optical node or connected to it by a PON feeder cable, Fig.4a). If service discrimination provisioning and/or client priority has to be satisfied, this filtering moves in the PON towards the user, being located in the ONU, Fig.4 b). This case corresponds to virtual PONs superposed on different wavelengths. The number of connected in the same virtual PON corresponds to number of filtering the same wavelength. Several can be connected on the same power splitter output port, but on different virtual PONs. Another possibility is to use AWG to discriminate clients and/or services, Fig.4c), which presents the advantage of eliminating all ONU associated filters, but on the other hand limits flexibility. λ filter = o a) b) Fig.4 BBFilter AWG The case of interactive services provisioning, requires upstream traffic handling. The amount of upstream traffic c)

4 depends much on the type of interactive service, but for most of the services is considerably less than on the downstream direction. In the case of equal distribution of downstream/upstream traffic, in the feeder ring interconnection functionality is required. This is also true for the unequal traffic distribution, but in this case some aggregation of upstream traffic could be performed, eventually for superposed virtual PONs. In Fig.5, three types of WDM PONs and the respective interconnections to Ring, for interactive service provisioning are depicted. λ add/drop = Directional coupler a) b) o Fig.5 OXC/ BB Directional coupler OXC/ AWG c) Distinct traffic routing handling for each network segment achieved by space division multiplexing Wavelength group routing from long-distance to metropolitan networks The following disadvantages can be pointed out: Diversity of optical nodes Tailored optical node implementation, depending on application Asymmetry of optical interface nodes More complexity on network planning phase and on routing strategy definition. Some of the pointed disadvantages, in particular those regarding the diversity of optical node implementation, will likely vanish with the usage of new optical components as the wavelength interleaver, provided that its cost is suitable for metropolitan environment. We therefore think that the proposed architecture is a good solution for low cost, short-term metropolitan network implementation and that it can evolve in the medium long time for a simple network implementation, while as maintaining the same flexibility. VI. REFERENCES [1] M. Pousa et al, DWDM Technologies for the Access Network, Eurescom P917 BOBAN- Building and Operating Broadband Access Network, Deliverable 10, September [2] Bob Shine and Jerry Bautista, Interleavers power the jump in channel count, Fiber Systems Europe, Vol.4, Number 10, pp 53-56, December V. CONCLUSIONS In this work we have proposed a solution for WDM metropolitan networks, in a ring topology, which allows a low cost implementation of such networks, by relaxing the requirements on the performance of optical components used for wavelength routing from long distance to metropolitan and distribution networks. The optical nodes obtained that realise the interfaces between the different segments in the network are asymmetrical as the long distance optical signal requirements have to be met in traffic directed to this network segment. The nodes achieved this way are more economical then the commercial solutions currently proposed which are basically the same as for long-distance. The proposed approach presents the following advantages: Low cost implementation of WDM politan networks, Usage of commercial available components. Less restringing wavelength requirements on politan networks Allows for end-to-end direct wavelength channel provisioning.

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