The growth -of existing and new services will create a large increase of traffic flow in telecommunication network in the coming years .The demand for bandwidth in a communication network increase continuously .The world wide web alone Ëœfor example require a yearly 8 fold increase in bandwidth per user . All optical networks may be the only solution to cope with such increasing bandwidth .In these network transmitter signals remain in optical format on the way from source to destination
The wave length division multiplexing WDM technology offers a practical way to exploit the bandwidth of fiber optics by partitioning the optical bandwidth into separate channels .Photonic packet switching offers high speed ,data rate transparency ,and configurability which are some of the important characteristics needed in future networks supporting different forms of data
2. TYPES OF SWITCHING
There are three main types of switching:
1. Circuit switching
2. Packet switching
3. Cell switching
Circuit Switching :
This type of switching is the one used in the field of conventional telephony . In an end-to-end dedicated link is set up before communication starts. This link is solely used for this communication and after that it is broken .The steps involved in the process of circuit switching are
1. Circuit setup
2. The actual communication
3. Clearing of the circuit
Here data is sent as packets .Due to this fact, there is an efficient utilization of the communication line. The earlier problem of waiting does not occur here as more than one source can use the line at the same time.
All data to be transmitted is first assembled into one or more messages units called packets, by the source DTE.These packets include both source&destination DTE to its local packets switching exchange (EXE).On receipt of each packets, the exchange first stores the packet and then inspects the destination address it contains . Each PSE containing routing directory specifying the outgoing links to be used for each networkaddress.The PSE forwards the packet on the appropriate link at the maximum available bit rate.
This is a subset of packet switching. It deals with packets of fixed size, for eg. The ATM cells. It buffers in all the switches follow the FIFO discipline, the packets are delivered in order in the case of cell switching, where as in packet switching the packets may not arrive in order. Cells are 53 bytes long, of which 5 bytes are header and 48 bytes are pay load.
3. PHOTONIC SWITCHING
In photonic switching the signals are kept in optical form, while they are being routed from input to the output. It does not convert optical signals into electrical form for switching purpose. Though the lack of interaction among photons would appear to make photonic switching unrealizable, but fortunately optical non-linearity provides bi-stability, which enables optical switching and promises optical computation also similar to digital computers. In fact optical bi-stability enables switching at energies comparable to electronics. Recently, optically controlled photonic switching devices have been developed, which have the potential for switching rates in tetra hertz range.
Photonic switching has three main advantages:
1. No need for optical to electronic conversions.
2. The ability to route high data rate optical signals.
3. Possibility of three dimensional inter-connections.
Though electronic devices need less power, are more controllable and faster than optical logic devices, they rapidly become slowly in operation by electrical connections between them as a result of cross talk, dispersion, and unwanted capacitance. The photonics would be desirable for long wide band inter-connections.
The main disadvantages of photonic switching are:
1. Large physical size of devices.
2. Absence of all optical clock regenerators to remove the loss and cross talk introduced in cross points
3. The polarization sensitivity of devices
3.1 Functions in a switch :
1. Routing: providing networks connectivity information through routing tables.
2. Forwarding: defining the output for each incoming packet based on routing table.
3. Switching: directing each packet to proper output defined by forwarding process.
4. Buffering: resolving contention by storing packets.
3.2 Photonic packet switches:
A WDM optical packet switch consists of four parts; input interface, switching fabric, output interface and control unit .The input interface is mainly used for packet delineation and alignment, packet header information extraction and packet header removal. The switch fabric is the core of the switch and is used to switch packets optically. The output interface is used to regenerate the optical signals and insert the packet header. The control unit controls the switch using the information in the packet headers. Because of synchronization requirements, optical packet switches are typically designed for fixed size packets.
When a packet arrives at a WDM optical packet switch, it is first processed by the input interface. The header and payload of the packet are separated and the header is converted into the electrical domain and processed by the control unit electronically. The payload remains an optical signal throughout the switch. After the payload passes through the switching fabric, it is recombined with the header, which is converted back into the optical domain at the output interface.
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