21-06-2011, 12:22 PM
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INTRODUCTION
With in today's Internet data is transported using wavelength division multiplexed (WDM) optical fiber transmission system that carry 32-80 wavelengths modulated at 2.5gb/s and 10gb/s per wavelength. Today’s largest routers and electronic switching systems need to handle close to 1tb/s to redirect incoming data from deployed WDM links. Mean while next generation commercial systems will be capable of single fiber transmission supporting hundreds of wavelength at 10Gb/s and world experiments have demonstrated 10Tb/shutdown transmission.
The ability to direct packets through the network when single fiber transmission capacities approach this magnitude may require electronics to run at rates that outstrip Moor’s law. The bandwidth mismatch between fiber transmission systems and electronics router will becomes more complex when we consider that future routers and switches will potentially terminate hundreds of wavelength, and increase in bit rate per wavelength will head out of beyond 40gb/s to 160gb/s. even with significance advances in electronic processor speed, electronics memory access time only improve at the rate of approximately 5% per year, an important data point since memory plays a key role in how packets are buffered and directed through a router. Additionally opto-electronic interfaces dominate the power dissipations, footprint and cost of these systems, and do not scale well as the port count and bit rate increase. Hence it is not difficult to see that the process of moving a massive number of packets through the multiple layers of electronics in a router can lead to congestion and exceed the performance of electronics and the ability to efficiently handle the dissipated power.
By using optical packets to perform “connectionless” communication in the optical network, following the principles of IP packet routing, a demand for optical packet generation and transmission, and optical packet switching naturally comes into scene. Moreover, such photonic networks will require very fast packet switching functions throughout, with minimum amount of buffers in optical nodes. The absence of electronic processing allows unlimited bit rate with any data format. Under such context, the switching functionality is performed within the optical layer, without access to higher electronic layers in the network.
Optical fiber is the most appropriate medium to provide the necessary bandwidth to attend the increasing demand of end users with higher data rates in access networks. Optical packet switching technology offers great potential to provide wider flexibility for bandwidth efficiency, scalability and finer granularity. But, optical packet switching still remains quite unexplored in optical access networks. In order to realize a practical implementation, simple and low cost switching nodes are required, operating with low loss, easy control and good throughput performance. Thus, the principle of self-routing of packets having header and payload architecture is now extended to the optical layer. Various techniques can be used to address header recognition whether in time domain, code domain, frequency domain, or wavelength domain. In all cases, it is just the header, not the payload that is processed in a network node. This means that the optical network is truly rendered transparent to information content, data rate or format carried in the optical signal; and the optical nodes are then immensely simplified, because only straightforward switching and routing is performed.
In this article we review the state of art in optical packet switching and more specifically the role optical signal processing plays in performing key functions. It describe how all-optical wavelength converters can be implemented as optical signal processors for packet switching, in terms of their processing functions, wavelength agile steering capabilities, and signal regeneration capabilities. Examples of how wavelength converters based processors can be used to implement asynchronous packet switching functions are reviewed. Two classes of wavelength converters will be touched on monolithically integrated semiconductor optical amplifiers (SOA) based and nonlinear fiber based.
CHAPTER 2
SWITCHING
Switching provide mechanisms to interconnect inputs to outputs. It is necessary for the efficient utilize the network resources. Three types of switching networks are
(i) Circuit Switching
(ii) Packet switching and Burst Switching
(iii) Cell switching
Fig. 2.1 Two types of switching networks
In circuit switching a dedicated path is established for communication. (E.g.: telephone networks). In circuit switching, there is inefficient use of resources.
In packet switching, the messages to be transmitted are broken to small Packets. It involves the Packetization and transfer of information after source coding. Characteristics of Packet switching are:
(a) Efficient use of line.
(b) More sources can use the line.
© For limited number of sources, the jitter (bursts from more than one sources come at the same instant) induced degradation will be tolerable.
In OBS (Optical Burst Switching) control information is sent separately in a reserved optical channel and in advance of the data (or packets, now called bursts). By doing so, these control signals can be processed electronically, and allow the timely setup of an optical light path to transport the soon-to-arrive data, thereby eliminating the need for either large optical buffers.
Cell Switching is subset of packet switching. It has fixed packet size (e.g. ATM cells). It uses virtual circuits and routing decisions during virtual circuit setup.