Routing and wavelength assignment: a problem for interconnected ring networks in .NET framework

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941 Routing and wavelength assignment: a problem for interconnected ring networks
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The number of wavelengths in a WDM system is nite, and hence considered a scarce resource Further, to establish a lightpath (all-optical connection) on a particular wavelength from a source node to a destination, the primary requirement at the ingress and egress network elements is that the wavelength concerned must be available throughout (across multiple ring spans) this path This requirement is called the wavelength continuity constraint This leads to a complex problem, that of assigning wavelengths in the ring for a given traf c matrix The problem intensi es if we consider the traf c pattern to be temporal (dynamic) The problem becomes even more complicated if we take into consideration the available set of xed wavelength lasers (within the set of transponders) at a given node A lot of literature (eg [4]) is devoted to the wavelength assignment problem For interconnected ring networks, the problem is almost unsolvable in real-time (NP-complete) If we consider a path from an ingress node in one ring to an egress node in another ring, then we have to assume that the wavelength should be freely available across the whole path, and further, that the available wavelength has matching transponders (lasers) at the ingress node This complication is a premier design problem in interconnected ring networks A real-time solution for dynamic traf c is almost impossible to nd, but several heuristics that yield some sort of approximation are available [5]
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942 Mesh optical networks
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A mesh can be differentiated from a ring as a network graph where there is at least one pair of nodes that has a degree of connectivity greater than two Note here that the degree of connectivity means the number of ingress and egress bers into a node, and for a ring-based network element, this is two Mesh networks are generally dif cult to manage, and current optical mesh level protection is not mature In fact, most optical mesh backbones use a protection mechanism that is primarily borrowed from the ring topology (shared path or line switched) Typically, backbone mesh graphs have irregular shapes The motivation for network planners to build mesh backbones is based on the fact that a mesh represents direct source destination connectivity At the optical layer, this means more capacity and easier management for routing However, the protection aspect is in stark contrast Further, as compared to ring networks, mesh networks have a better wavelength reuse ratio (ability to use the same wavelength in graphically disjoint segments) Architecturally, the primary mesh element is an optical cross connect, or OXC The OXC has the ability to switch wavelengths from one ingress ber to another It may, in the process, also incur some degree of wavelength conversion through optoelectronic means However, to save costs, all-optical OXCs are preferred The OXC switching fabric is generally based on MEMS technology Mesh node design has not yet matured for metropolitan and community backbones We expect maturity of this technology in the next 2 3 years and deployment beyond that
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Backbone Optical Network Design for Community Networks
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943 Ring network element design considerations
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It is estimated that 98 % of community networks currently deploy ring optical backbones, and this high percentage will remain valid for at least another ten years unless there is suf cient technological innovation to provoke change Ring network elements are a primary design concern for network planners because of the sheer diversity in the alternatives available It then becomes important to choose the right architectural element, taking into account the plausible alternatives and the requirements the network has, as well as cost All optical ring elements are variants of the basic optical add drop multiplexer The OADM has been a standard system with its own management complex Ring network management is done through an optical supervisory channel, or OSC The OSC is an out-of-band control signal that runs through the complete ring It is demultiplexed at each node through the ring and processed using some modi cation of an element management system (using, say, a network processor) The OSC is out-of-band with the data This means that the data wavelengths are spectrally different from the OSC This leads to reliability one of the key features desired in optical networks As a rule of the thumb, it is considered that in-band systems are more scalable than out-of band ones, but when it comes to reliability, out-of-band systems perform far better Based on this initial description of ring elements, we will now consider three manifestations of ring OADMs: xed add drop multiplexers (FOADMs), recon gurable optical add drop multiplexers (ROADMs) and, nally, dynamic optical add drop multiplexers (DOADMs) [6]
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944 Fixed OADMs (FOADMs)
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Figure 97 shows the conceptual realization of the FOADM The basic principle of this network con guration is to allow the drop and add of a known, xed set of wavelengths at a node site In this case, a dropped wavelength is assigned a particular drop port and this mapping is always preserved
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945 Recon gurable OADMs (ROADMs)
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Figure 98 shows the conceptual realization of the ROADM This is a popular metro core network element In the ROADM implementation, unlike in FOADM, any channel from the complete band of WDM channels can be dropped or added This means that every channel that ows through the system can either be dropped or allowed to continue (or both in case of drop and continue) Further, if a channel is dropped then, due to wavelength reuse, the same wavelength can be used to add another signal into the network at the add side However, in ROADM systems, each port is mapped to a particular channel This means that if we have a transmitter at, say, port # 23, and the wavelength emitted by this transmitter corresponds to port # 22, then the transmitter cannot be used in this system The mapping of wavelength to ports, thus, is always preserved and static
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946 Dynamic OADMs (DOADMs)
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Figure 99 shows the conceptual realization of a DOADM The DOADM is the ultimate in terms of exibility, as it allows any channel (or group of channels) to be dropped, and further,
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