Optical Switching
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computer science crazy
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23-09-2008, 01:16 AM


Explosive information demand in the internet world is creating enormous needs for capacity expansion in next generation telecommunication networks. It is expected that the data- oriented network traffic will double every year.

Optical networks are widely regarded as the ultimate solution to the bandwidth needs of future communication systems. Optical fiber links deployed between nodes are capable to carry terabits of information but the electronic switching at the nodes limit the bandwidth of a network. Optical switches at the nodes will overcome this limitation. With their improved efficiency and lower costs, Optical switches provide the key to both manage the new capacity Dense Wavelength Division Multiplexing (DWDM) links as well as gain a competitive advantage for provision of new band width hungry services. However, in an optically switched network the challenge lies in overcoming signal impairment and network related parameters. Let us discuss the present status, advantages and challenges and future trends in optical switches.

A fiber consists of a glass core and a surrounding layer called the cladding. The core and cladding have carefully chosen indices of refraction to ensure that the photos propagating in the core are always reflected at the interface of the cladding. The only way the light can enter and escape is through the ends of the fiber. A transmitter either alight emitting diode or a laser sends electronic data that have been converted to photons over the fiber at a wavelength of between 1,200 and 1,600 nanometers.

Today fibers are pure enough that a light signal can travel for about 80 kilometers without the need for amplification. But at some point the signal still needs to be boosted. Electronics for amplitude signal were replaced by stretches of fiber infused with ions of the rare-earth erbium. When these erbium-doped fibers were zapped by a pump laser, the excited ions could revive a fading signal. They restore a signal without any optical to electronic conversion and can do so for very high speed signals sending tens of gigabits a second. Most importantly they can boost the power of many wavelengths simultaneously.

Now to increase information rate, as many wavelengths as possible are jammed down a fiber, with a wavelength carrying as much data as possible. The technology that does this has a name-dense wavelength division multiplexing (DWDM ) - that is a paragon of technospeak.Switches are needed to route the digital flow to its ultimate destination. The enormous bit conduits will flounder if the light streams are routed using conventional electronic switches, which require a multi-terabit signal to be converted into hundreds of lower-speed electronic signals. Finally, switched signals would have to be reconverted to photons and reaggregated into light channels that are then sent out through a designated output fiber.
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computer science crazy
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Posts: 3,048
Joined: Dec 2008
21-02-2009, 10:54 PM

Optical switching enables routing of optical data signals without the need for conversion to electrical signals and, therefore, is independent of data rate and data protocol.Optical Burst Switching (OBS) is an attempt at a new synthesis of optical and electronic technologies that seeks to exploit the tremendous bandwidth of optical technology, while using electronics for management and control.

In an OBS network the incoming IP traffic is first assembled into bigger entities called bursts. Bursts, being substantially bigger than IP packets are easier to switch with relatively small overhead. When a burst is ready, reservation request is sent to the core network. Transmission and switching resources for each burst are reserved according to the one-pass reservation scheme, i.e. data is sent shortly after the reservation request without receiving an acknowledgement of successful reservation.

The reservation request (control packet) is sent on a dedicated wavelength some offset time prior to the transmission of the data burst. This basic offset has to be large enough to electronically process the control packet and set up the switching matrix for the data burst in all nodes. When a data burst arrives in a node the switching matrix has been already set up, i.e. the burst is kept in the optical domain. The reservation request is analysed in each core node, the routing decision is made, and sent to the next node. When the burst reaches its destination node it is disassembled, and the resulting IP packets are sent to their respective destinations.

The benefit of OBS over circuit switching is that there is no need to dedicate a wavelength for each end-to-end connection. OBS is more viable than optical packet switching because the burst data does not need to be buffered or processed at the cross-connect. Advantages

* Greater transport channel capacity

* No O-E-O conversion

* Cost effective


* Burst dropped in case of contention

* Lack of effective technology
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