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OPTICAL NETWORK ARCHITECTURE
Abstract
One of the major issues in the networking industry today is tremendous demand for more and more bandwidth. However, with the development of optical networks and the use of Dense Wavelength Division Multiplexing (DWDM) technology, a new and probably, a very crucial milestone is being reached in network evolution. The existing SONET/SDH network architecture is best suited for voice traffic rather than today’s high-speed data traffic. To upgrade the system to handle this kind of traffic is very expensive and hence the need for the development of an intelligent all-optical network. Such a network will bring intelligence and scalability to the optical domain by combining the intelligence and functional capability of SONET/SDH, the tremendous bandwidth of DWDM and innovative networking software to spawn a variety of optical transport, switching and management related products. This paper deals with optical network architecture and explains virtual topology along with optical layer and higher layer interface.
1.Introduction
Just like every other layer defined in networking, a layer architecture has to be defined for the optical layer. A multi-wavelength mesh-connected optical network is used to define the architecture of the optic layer. A lightpath is defined as the path between two nodes and is equivalent to a wavelength on each page link on that path. Two aspects of the network topology have been described : physical topology and virtual topology. The physical topology has WDM cross-connect nodes interconnected by pairs of point-to-point fiber links in an arbitrary mesh topology as shown in the following figure.
Fig. A WDM network consisting of crossconnect nodes interconnected by pairs of point-to-point fiber optic links(i.e physcial topology)
2. Virtual Topology
As shown in the below figure, the virtual topology of a network is the set of all lightpaths. It is a logical topology and the direction of the arrows actually show the direction of the lightpaths.
Fig. The virtual topology of the WDM network of previous figure
An optical layer, also known sometimes as Layer 1, is the layer between the physical layer and the datalink layer or other higher layers for that matter. By using this additional layer in say an ATM network, we would be removing the need of having to convert to optical signal to electric signals and to cells before switching them through ATM switches, if we have wavelength switches in the network. ITU-T SG 15 has defined the optical layer itself as consisting of layers. The three sublayers of the optical layer are:
(i) Optical Channel(OCh) layer: This corresponds to light paths
(ii) Optical Multiplex Section(OMSn) layer: This corresponds to links
(iii) Optical Amplifier Section(OASn) layer : This corresponds to page link segments between optical amplifiers.
3. Optical Layer and Higher Layer Interface
Just like every other layer-layer communication, optical layer communicates with the higher layers, both above and below in the protocol stack by means of Service Data Units (SDUs). Besides these guarantees, SDUs also have to be defined to allow for proper exchange between higher layers and the optical layer (in both directions).There are certain services that this new layer must provide to the higher layers.
1 Addressing :
It is obvious that there must be some mechanism for the higher layers to ask for lightpaths from particular nodes in the network. This is done by having an addressing scheme to describe the nodes in the network.
(i) Multicast capability:
This is an optional capability depending on whether multicasting is a
feature enabled in the network.
(iii) Light tree:
A light tree is a point-to-multipoint version of a lightpath. Optical multicasting capability at routing nodes has been suggested to increase logical connectivity and thereby further reduce the hop distances that have to be traversed. Optical multicasting is better than electronic multicasting because it is easier to split an optical signal into many identical optical signals rather than copying a packet in an electronic buffer. Using optical splitters does this function of "splitting" an optical signal. An n-way optical splitter is a passive device that does the above defined "splitting" in such a way that at least one output signal has a power less than or equal to 1/n th times the input power. Optical amplifiers would be needed in the network. The suggested approach is to have a so-called splitter bank. This splitter bank will do the optical splitting and also the optical signal amplification. An interesting point is that this splitter bank could have more features such as wavelength conversion and signal regeneration for "multicast" as well as "unicast" signals in the network. Now this splitter bank is then used to construct a multicast-capable wavelength-routing switch(MWRS). The basic components of this MWRS are optical switches, splitter bank, multiplexors, demultiplexors. Information coming in through a fiber page link is first demultiplexed into separate signals (different wavelengths) and then switched by an optical switch. At this point depending on whether the signal is unicast or multicast, they are sent through different paths. The multicast signals are sent to the splitter bank and the amplified multiple identical signals are then switched by another optical switch. Finally all the signals that are to be sent out on one fiber page link are multiplexed together before being sent out.Mathematic formulation of the light-tree-based virtual topology design problem isthe next step. An optimization problem having any one of the following objective functions is possible:
(a)Minimization of the network-wide average packet hop distance.
(b)Minimization of the total number of opto-electronic components.
(iv) Uni or Bi-directional lightpaths:
When all wavelengths travel in the same direction within a fiber, those wavelengths are called unidirectional wavelengths(or lightpaths). The implication here is that another parallel fiber has to be there that supports the opposite direction lightpath When we have the whole channel split in such a way that for each lightpath in the forward direction there is another lightpath in the opposite direction within the same fiber, such lightpaths are known as bi-directional lightpaths. It is obvious that the transmission bandwidth is reduced. Which one of these lightpaths is chosen depends on the type of traffic. Since failure recovery is a very important aspect of any network, network control has been proposed as a decentralized function though for the early versions a centralized function is also acceptable. Network management criteria, interfacing between network control and network management has all been properlydefined.
4.Conclusion
In this paper we have discussed optical network was described. Virtual topology and Optical layer have been described in much detail. Issues such as Network Control and Network Management ware also discussed. Research work is also being done to try and achieve the difficult goal of a high-speed all-optical network. New concepts such as All-optical switching are coming up. 1 Tbps systems are expected in the market by early 2002/2003. Network providers will start leasing out wavelengths (or "lambdas") instead of leasing lines. Cost will be an important issue in widespread deployment of optical systems. A lot of implementation issues, the setting up of standards need to be addressed for an all-optical network to come out at a reasonable cost. How long or for that matter whether we will ever achieve an all-optical network is a moot question