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Телекоммуникационные системы и компьютерные сети
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3. Andrews J.G., Ghosh R. Muhamed Fundamentals of WiMAX Understanding Broadband Wireless Networking. - Prentice Hall. 2007. - Р. 448.
4. Сюваткин В.С. и др. Wimax - технология беспроводной связи: основы теории, стандарты, применение / Под ред. В.В. Крылова. - СПб.: БХВ-Пе-тербург, 2005. - 368 с.
5. Связь с подвижными объектами в диапазоне СВЧ / Под ред. У.К. Джейкса; пер. с англ., под ред. М.С. Ярлыкова и М.В. Чернякова. - М.: Связь, 1979. - 520 с.
ARRAYED WAVEGUIDE GRATING ALONG WITH SOME POSSIBLE SOLUTIONS FOR THE PROBLEMS
© Urinov E.M.*, Abdukadirov B.A.*
Fergana branch of Tashkent University of Information Technologies
The AWGs are used to multiplex channels of several wavelengths onto a single optical fiber at the transmission end and are also used as de multiplexers to retrieve individual channels of different wavelengths at the receiving end of an optical communication network.
Keywords: multiplex, demultiplexers, wavelengths, optical (de)multiple-xers, switching fabric, transmission, spectrum.
Arrayed waveguide gratings (AWG) are commonly used as optical (de)multiplexers in wavelength division multiplexed (WDM) systems. These devices are capable of multiplexing a large number of wavelengths into a single optical fiber, thereby increasing the transmission capacity of optical networks considerably.
The AWG is used as it gives better performance than a normal space switch interconnection network, as far as insertion losses are concerned. The devices are based on a fundamental principle of optics that light waves of different wavelengths interfere linearly with each other. This means that, if each channel in an optical communication network makes use of light of a slightly different wavelength, then the light from a large number of these channels can be carried by a single optical fiber with negligible crosstalk between the channels. The AWGs are used to multiplex channels of several wavelengths onto a single optical fiber at the transmission end and are also used as demultiplexers to retrieve individual channels of different wavelengths at the receiving end of an optical communication network.
* Assistent-teacher.
* Assistent-teacher.
1550.0 1550.5 1551.0 1551.5 1552.0 Wavelength Я(nm)
Fig. 1
Commercially available 40 channel devices have a channel spacing of 100 GHz and show an insertion loss of less than 7.5 dB. The connection of Transmission Spectrum with wavelength can be obviously seen in Figure 1. The AWG central wavelength changes with temperature. Active temperature stabilization by a heater or Peltier cooler is often used, but it requires continuous power consumption of several watts and temperature control electronics. This can be avoided with an athermal design, with substantially reduced temperature sensitivity.
Packet Switching
Since its introduction in the early 1970s, packet switching has received widespread acceptance. Public networks have been constructed in most developed countries and many developing countries. The internetwork ITU-T X.75 protocol provides for interlinking of national networks at an international level. The ITU-T X.25 Recommendation is the original standard for packet-switching architecture. Packet switching has several advantages over conventional circuit-switched networks. Figure 2 highlights the difference between the two. The circuit-switched network maintains a fixed band width between the transmitter and receiver for the duration of a call. Also, the circuit switched network is bit stream transparent, meaning it is not concerned with the data content or error-checking process. This is not the case for packet switching, where band width is allocated dynamically on an «as required» basis. Data is transmitted in packets, each containing a header that contains the destination of the packet and a tail, or footer, for error checking information. Packets from different sources can coexist on the same customer-to-network physical link without interference. The simultaneous call and variable band width facilities improve the efficiency of the overall network. The buffering in the system which allows terminals operating at different bit rates to interwork with each other is a significant advantage of packet switching. The obvious disadvantage is the extra dimension of complexity with respect to the switches and network-to-customer protocol. Furthermore, in certain circumstances, packet switching has several advantages over other methods of data communication:
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1. Packet switching might be more economical than using private lines if the amount of traffic between terminals does not warrant a dedicated circuit.
2. Packet switching might be more economical than dialed data when the data communication sessions are shorter than a telephone call minimum chargeable time unit.
3. Destination information is contained in each packet, so numerous messages can be sent very quickly to many different destinations. The rate depends on how fast the data terminal equipment (DTE) can transmit the packets.
4. Computers at each node allow dynamic data routing. This inherent intelligence in the network picks the best possible route for a packet to take through the network at any particular time. Throughput and efficiency are therefore maximized.
5. The packet network inherent intelligence also allows graceful degradation of the network in the event of a node or path (link) failure. Automatic rerouting of the packets around the failed area causes more congestion in those areas, but the overall system is still operable.
6. Other features of this intelligence are error detection and correction, fault diagnosis, verification of message delivery, message sequence checking, reverse billing (charging), etc.
Fig. 2. Packet switching (b) compared to circuit switching (a)
Output-Buffered Photonic Packet Switch
Another space switching based architecture is an output-buffered photonic packet switch using wavelength-routed packet buffer [TZ99]. Each input is converted to an individual wavelength. Therefore several packets traveling to the same output can be multiplexed together in the space switch. Packets traveling to the same output are given different delays in the buffering part of the switch. The first AWGM delivers packets to right delay lines and the second AWGM directs packets to right outputs, i.e., to the same outputs as the input ports where packets arrived to this block. Inputs and outputs can contain one packet at time.
Wavelength Routing Switch (WRS)
As the name of the switch suggests, WDM is used for switching in wavelength routing switch. The network demonstrator implemented during the project consists of a 44 switches with 4 wavelengths and payload signals of 2.5 Gbit/s. The demonstrator included contention resolution with optical delay lines, payload routing according to the input header content, header updating in the switch, and rewriting with respect to the payload position. Implementing a wavelength routing switch is expensive.
Fig. 3. The function of the 3R regenerator, adopted from [CLJ + 98]
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Broadcast-and-Select Switch
The switch implemented in the test-bed was a 161 < switch with 1 < wavelengths and 1 < memory positions and 10 Gbit/s signals [CJR + 98]. The test-bed results for the broadcast-and-select switch were promising. The experimental validation of the KEOPS concept at the network level was successful [CJR + 98]. The drawback of the architecture is the need for a great number of components [TZ99].
Among various targets of current AWG here are some problems associated with this issue, like: crosstalk, blocking and phase correction. One of the difficulties in large-scale AWGs is crosstalk deterioration caused by phase errors arising from variations in the arrayed waveguide width, thickness, material composition, and stress. Because the influence of such errors increases with the size of the waveguide array, the effect can be severe for densely spaced AWGs. The crosstalk can be reduced by adjusting the phase delays in the individual arrayed waveguides, but this is rather tedious task. The phase can be measured for each waveguide of the array by low coherence interferometry [3]. The phase correction can be achieved by ultra-violet-induced refractive index changes in the glass. All the waveguides are exposed at the same time by using a metal mask with different opening lengths over each waveguide that are proportional to the phase errors to be compensated. A typical AWG has a symmetric intensity distribution across the waveguide array, and as such its chromatic dispersion D is negligible. However in a practical AWG this symmetry is disturbed by phase and amplitude errors that are randomly distributed in the arrayed waveguides. This increases chromatic dispersion. Because the errors increase with decreasing channel separation, the chromatic dispersion increases similarly. One of the problems concern to AWG is blocking in a switching fabric. This issue can be reduced by the implementation of Tunable and Fixed wavelength converters (TWC and FWC opportunities). Recently, AWG devices made of polymeric materials have been gaining a great deal of attention because of their excellent particular features such as easier optical integrating, lower transmission loss and easier control of the refractive index, compared to other AWG devices. Prosperity of AWG is very bride, in the World is going on very great researches on this, and it will be implemented in Uzbekistan very soon, moreover it will vastly enhance the development of Uzbekistan.
References:
1. George N. Rouskas, Lisong Xu. Optical packet switching, E_11_2005_ Springer_Book
2. Rouskas, G N., and Stevenson, D. (2002). JumpStart: A just-in-time signaling architecture for WDM burst-switched networks. IEEE Communication, 40(2): 82-89.
3. Robert G Winch. Telecommunication Transmission System. - 2009.
4. Maurizio Maurizio Casoni. Novel Switch Architectures. - 2008.
