Optical switching/Optical route casting All-Optical Network

Li Guangcheng, vice president of Beacon Communications Technology Co., Ltd.

The role of optical switching/optical routing in all optical networks

With the progress of society, the social demand of the new data business, such as broadband video, multimedia service, ip-based real-time/quasi real time service, which can enrich and improve people's communication effect and quality is increasing. Because of the more bandwidth resources occupied by the new business, the high-speed broadband integrated service network has become the trend of communication network in this century. The fiber has huge bandwidth. The transmission loss is lower in the 200nm range near the 1.55μm wavelength. By the formula F = c/λ, where f is the frequency, λ is the wavelength, C = 3x108m/s is the speed of light, it can be learned that the corresponding bandwidth of 200nm is about 25THz (1thz=1012hz). Around the 1.3μm wavelength, there is also about 25THz of available bandwidth. In this way, an optical fiber can provide a theoretical transmission bandwidth of about 50THz. However, at present, the maximum transmission rate of the serial signal is 40Gbps, even if it is transmitted on the fiber by this rate, only 1 per thousand of the optical fiber capacity is utilized. In many network technology realization Scheme, the network scheme based on electronic technology is limited to the device work upper limit rate 40G, it is difficult to complete the high-speed broadband integrated service transmission and the Exchange processing, the network also may appear the bandwidth "bottleneck". Only optical fiber based all-optical network scheme can provide high-speed, high-capacity transmission and processing capacity, breaking the "bottleneck" of information transmission, can adapt to the bandwidth demand of high-speed broadband service for a long time. All-Optical Network (all-optical Communication network) refers to the optical information flow in the network transmission and exchange is always in the form of light, without the need for optical/electrical, electrical/optical transformation. In other words, the information from the source node to the destination node in the transmission process is always in the optical domain, wavelength becomes the most basic building blocks of all optical network. Because all the signal transmission in All-optical network is carried out in the optical domain, all optical Network has the transparency of the signal, it realizes the route selection through the wavelength selection device. With its good transparency, wavelength routing characteristics, compatibility and scalability, all optical networks become the first choice for the next generation of high-speed (ultra-high speed) broadband networks.

All-optical networks have the following advantages: 1 provide huge bandwidth. 2 with wireless or copper ratio, high processing speed and low error rate. 3 The All-optical network with optical Path Exchange has protocol transparency, that is, the signal form is unrestricted. The flexibility of network applications is facilitated by the use of different rates and protocols. 4 All Optical network uses a lot of passive optical devices, eliminating the large amount of light/electricity/light conversion work and equipment, improve the overall network exchange speed, reduce costs and improve reliability.

In an ideal all-optical network, all functions of signal exchange, route selection, transmission and recovery are all in the form of light. At present, all optical network is not the whole network optical, but refers to the optical information flow in the transmission and exchange process in the form of light, with the circuit method to achieve the control part. From the current situation and development trend of optical electronic components, it is not realistic and unnecessary to try to realize the whole network's all-optical. All optical network mainly consists of core network, metropolitan area Network and access network three layers, the basic structure of the three are similar, by DWDM system, optical amplifier, OADM (Optical Division multiplexer) and OXC (optical crossover equipment) and other equipment. All optical networks have 3 basic types, such as Star Network, bus network and tree-shaped network. The related technologies of all-optical network mainly include optical switching/optical routing (Pangkung Switching), optical cross connection, all-optical relay and optical interpolation multiplexing.

Optical switching/Optical routing is a key optical node technology in all optical networks, which mainly completes the optical signal exchange and route selection between any optical fiber ports in optical nodes, and the most important work is wavelength conversion. Because it is essentially the wavelength of light processing, it is more precisely that optical switching/optical routing should be called wavelength switching/wavelength routing. The advantages of all optical networks, such as bandwidth advantage, transparent transmission and lower interface cost, are embodied by this technology. From the functional division, optical switching/optical routing, OXC, oadm are sequential containment. That is, Oadm is a special case of OXC, and Oxc is a special case of optical switching/optical routing. As OXC and optical switching/optical routing are still developing, the naming of optical switching/optical routing is rather confusing. Some companies put the existing oadm, OXC are called Optical Exchange series (optical switching), and some also called optical router (optical Router). So the current optical switching/optical routing is mostly oxc and even oadm temporarily.

There are usually 3 implementations of OXC: Fiber crossover, wavelength crossover, and wavelength conversion crossover. Among them, the fiber-optic crossover connection is based on the total capacity of all wavelengths on a fiber, the capacity is large but not flexible; a wavelength crossover connects any wavelength on any fiber to any optical fiber with the same wavelength. For example, wavelength λ 1, lambda 2, λ3, and λ4 from input 1th fiber input, wavelength crossover can be selected from the 4 wavelength to the output port of 1, 2, 3 and 4th fiber up. This wavelength crossover is now known as a passive optical router (passive Router), and its wavelength can be reused through space segmentation. The routing route of wavelength is determined by the internal cross Matrix, and a nxn cross matrix can simultaneously establish N2 route. Several of its other aliases are the Latin router (Latin routers), the waveguide raster router Wgrs (waveguide grating routers), and the wavelength router wrs (wavelength routers) Wavelength conversion crossover connects any wavelength on any fiber to any optical fiber with a different wavelength, with the highest flexibility. The difference between it and wavelength crossover is that wavelength conversion can be done.

Technical principles of optical switching/optical routing

The traditional optical switching has the power of light and electricity, and the switching capacity is limited by the working speed of the electronic device, which limits the bandwidth of the optical communication system. Direct optical switching eliminates the switching process of light/electricity and electricity/light, making full use of the broadband characteristics of optical communication. Therefore, optical switching is considered to be the most potential new generation switching technology in the future broadband Communication network. The exploration of light exchange began in the 70 's, and the mid-80 development was relatively rapid.

Similar to the electric switching technology, the optical switching technology can be divided into circuit switching and packet switching according to the switching mode. The circuit exchange also contains the air separation (SD), Time (TD), Wavelength Division/Frequency Division (WD/FD) and other means, packet switching has ATM optical exchange and so on. Its principle, structure characteristic and research progress status are as follows.

1. Air Separation Light Exchange

The optical exchange is realized by the switch matrix, the switch matrix node can be controlled by mechanical, electric or light, and the physical channel is established according to the requirement, so that any channel of the input port is connected with any channel of the output end, and the exchange of information is completed. All kinds of mechanical, electrical or optical control related devices can constitute the air separation light exchange. The switches which compose the light matrix are lithium niobate directional coupler, MEMS and so on.

2. Time Division Light Exchange

The time Division optical switching system uses the optical device or the photoelectric device as the slot switch, and completes the exchange action through the controlled ordered read-write operation of the optical memory. Because the time Division optical switching system and optical transmission system is very good to form a full optical network, so the research and development of time division optical switching technology is very fast, the exchange rate of almost one times a year, has developed a number of hours of optical switching system. The optical time Switching system of 256Mbps (4-way 64Mbps) color image coding signal was successfully implemented in the middle of 80. It uses the 1x4 lithium niobate directional coupler matrix switch as the selector, the Bistable laser diode as the memory (the switching speed 1Gbps), constitutes a single-stage switching module. The 512Mbps test system was launched in the early 90. The key to realize the optical time division switching system is to develop the high speed optical logic devices, namely the optical reading and writing devices and the memory devices.

3. Wavelength Division/Frequency Division optical switching

Wavelength Division exchange is the signal through different wavelength, select a different network path to achieve, by switching to switch to exchange. WDM networks consist of wavelength multiplexer/multiplexer, wavelength-selective space switch and wavelength switch (wavelength switches).

At present, Wavelength division multiplexing (WDM) system is developed, which is about 10. Recently, a kind of too-grade optical switching system has been developed, it uses the wavelength division multiplexing number 128, the maximum terminal number reaches 2048, the multiplex level is equal to the 1.2Tbps exchange throughput.

4. ATM Optical Switching

ATM optical switching follows the basic principle of ATM switching in electricity domain, using wavelength division multiplexing, electric or optical buffering technology, and selecting the path from the signal element wavelengths. According to the wavelength of the signal element, the signal is selected to the optical buffer memory of the output port, and then the signal to the same output port is stored in the input common optical buffer memory to complete the exchange.