|Optical IP Switching|
Improvements in the optical transmission technology, over the past fifteen years, have substantially facilitated the development and worldwide deployment of the Internet, by reducing the cost of data transport. The exponential growth in Internet traffc however, demands new solutions at the routing layer, as current IP routing technology struggles to deliver the necessary bandwidth at competitive costs.
Hybrid electro-optical architectures, where dynamical optical circuit switching is combined with legacy packet routing, have been introduced in the past few years as promising solutions to reduce costs at the IP layer, and to deliver new revenue-generating services and applications.
Most of the architectures currently proposed however have focused on end-to-end lightpath provisioning, coordinated through a centralized management plane.
We have proposed Optical IP Switching (OIS), a hybrid electro-optical network architecture that combines IP routing and wavelength switching, using a distributed decision-making process. The idea behind Optical IP Switching is that of an IP network that adapts the underlying physical topology to the traffic flows encountered at the IP layer. The decision-making process is completely distributed and is only based on local traffic observation. An OIS node monitors the traffic by sampling IP packets at a certain rate, using mechanisms similar to those adopted by Netflow. The goal is to identify elephant flows, aggregate them on the basis of their upstream and downstream directions, and evaluate the feasibility of switching those flows into a dedicated optical cut-through path. This path is then established between the selected upstream and downstream neighbors. As clearly illustrated in the figure below, the advantage of the cut-through path is that the middle node can switch the flows at the optical layer, without consuming expensive router resources. A path extension process then allows upstream and downstream neighbors to extend the optical path to their own neighbors, availing of the advantages of transparent switching.
The main advantage of this distributed decision mechanism is that every node can au- tonomously evaluate the convenience of switching or routing a flow aggregate, depending on its available electrical and optical resources. This makes OIS especially suitable for inter- domain networking, and more generally for highly heterogeneous network, because it allows every node to make its own traffic analysis and optimization.
above shows the creation of an optical cut-through path that bypasses part of the IP traffic.
The figure below gives a logical overview of the Optical IP Switching architecture.
The upper part of the diagram specifies the IP/OIS functional elements together with their logical interconnections. The OIS functions are closely integrated with the IP layer entities, with read/write access to the routing table and ability to avail of the routing engine to forward signaling messages. Both the OIS and IP protocols can influence the routing mechanism by modifying the entries in the routing table independently from each other.
The advantage brought about by the close integration of IP routing and optical switching is twofold. Firstly, it associates the routing at the IP and optical layers, allowing multi-layer traffic engineering. Secondly, it guarantees full backward compatibility with default IP networks, a crucial characteristic for the practical implementation of any Internet architecture.
The optical interfaces allow the router to create and terminate optical links, over which packets are transmitted and received. Some of these are used to create static default links to neighbors, through which normal IP traffic and control packets are exchanged. The remaining interfaces are used to accommodate the dynamic optical paths.
The optical switch is the element that physically links the node to the external world. On one hand it allows each interface to connect to any incoming or outgoing fiber; on the other hand it allows the transparent switch of an incoming to an outgoing fiber, creating optical bypasses of the IP layer. The switch is directly controlled by the dynamic allocation engine through a dedicated interface. In the figure, we have illustrated a fiber switch to clarify the interconnections of the optical elements. For a real implementation however, wavelength- selective switches (incorporating the WDM multiplexers) could be employed.