The evolution of Optical network technology

Absrtact: This paper describes the colorful situation of optical network world, expounds the core technology of each series of optical Network, discusses the adaptation of optical network products to IP service, and puts forward the fusion node and intelligent Optical Network technology.

Key words: Synchronous Digital Series SDH, digital cross connection equipment based on SDH s-dxc, dense wavelength division multiplexing DWDM, Optical Division multiplexer OADM, Optical crossover connector oxc, wavelength router wavelength Router, wavelength conversion wavelength Conversion, erbium-doped fiber amplifiers, EDFA, Raman amplifiers, optical signal-to-noise ratio (OSNR), optical transmission network OTN, digital encapsulation Digital wrapper, multiprotocol switching MPLS, multiprotocol wavelength switching mplms, and SDH based link Access Protocol laps, Simplifies data link protocols SDL, continuous cascade contiguous concatenation, virtual cascade virtual concatenation, time-division multiplexing ETDM, optical-time multiplexing OTDM, optical soliton O-soliton

One, the colorful network world of light

As the infrastructure of Telecommunication network, optical transmission network has made great progress. From the optical fiber line, the transmission is the light signal, but from the tail fiber into the specific equipment or systems, some systems need to be photoelectric conversion processing, and some are directly for the light signal processing. For large systems, the whole optical processing is difficult to achieve (at least the control or management of the signal is not possible), so the current optical transmission system can be divided into "signal processing-oriented" system and the "Light Signal processing" system.

At present, the relatively mature optical transmission technology is mainly "to signal processing" of SDH and S-DXC system, representing the electrical time Multiplexing ETDM format, as well as the "light signal processing-oriented" dense wavelength division multiplexing system, representing the DWDM format. Driven by the network business, SDH continues to high-bit rate, while DWDM continues to evolve to hyper-dense wavelength numbers.

Now the telecommunications business mainly by SDH and S-DXC to carry, and DWDM is mainly used to build point-to-point (point-to-point) transparent transmission channel, this way there are delays in scheduling, inefficient shortcomings. If the "light signal processing-oriented" oadm, OXC or wavelength routers (wavelength Router, referred to as WR) and other technologies mature, a new "optical transmission network" OTN will gradually build up, by then, most of the telecommunications service will be delivered to the OTN to carry. Based on the future, we are urgent to the optical transmission family renamed Optical Network family.

 recently proposed a classification method, which is to divide optical network products into one wavelength solution SWS (single wavelength Solution), multi wavelength solution MWS (multiple wavelength Solution) and free wavelength solution FWS (freedom wavelength Solution). SWS's representative or SDH and S-DXC,MWS is based on the point-to-point network of DWDM, and FWS is the gradual commercialization of the "light signal processing-oriented" OADM, OXC and WR.

It can not be neglected that the SWS series of optical time multiplexing OTDM technology, optical soliton O-soliton technology and even optical Code Division multiplexing OCDM technology, but also hope to become members of the optical network family, but these technologies have not been out of the laboratory. Experts predict that the 4x40 GB/s systems implemented using OTDM will be first commercially available in ETDM's 160 GB/s.

Two, one wavelength solution (SWS)

1. The evolution of SDH technology

At present, the world-wide SDH 2.5gb/s and the following systems are relatively mature, 10gb/s system is in the commercial scale, in the ascendant, 40gb/s system dawning, has formally entered the ITU-T recommendations, the 160GB/S system does not seem to "Arabian Nights."

(1) The Breakthrough of Microelectronics technology

Most SDH manufacturers are committed to microelectronics technology research and practical, branch mapping, low-order multiplexing, overhead processing technology is more mature, but the cross connection, Gao Jie and other technologies to face the test, the corresponding ASIC launch relatively slow, and this is the focus of product competition. At present, the business of SDH 10gb/s ADM at the VC-4 level of the crossover ability up to 768x768, the next step will break 1024x1024. For the S-DXC system, the crossover matrix can be extended smoothly through Clos technology, and the powerful and flexible cross connection function can be realized by using the power management system control. For the 40GB/S system, the development of indium phosphide INP material and high electron activity transistor hemt will make it out of the laboratory, and then let the ETDM technology flourish again, and let the "electronic bottleneck" temporarily become "alarmist".

(2) Osnr problem of optical signal-to-noise ratio

For the SDH system above 2.5GB/S rate, the most popular method is to use forward error-correcting FEC, which can still obtain better BER performance index when the signal-noise ratio Osnr is lower in the receiving end. The new g.707 proposed to use SDH segment overhead Soh P1, Q1 in BCH-3 code to increase the FEC option, the application of high-speed SDH system is expected to obtain 2dB ber performance improvement. In the hope of further improvement, the Out-of-band FEC may be used, which is derived from the submarine cable system. Now many companies have developed SDH systems using Out-of-band FEC, and some even claimed that in the 10GB/S system can improve the 8dB. But so far the ITU-T has not yet determined what error-correcting codes apply to the Out-of-band FEC of land-based systems, each of which is based on ASIC and system design in its own way, and in interoperability there is a 40gbit/s system that can also consider using Raman technology to improve OSNR, i.e. The Raman pump with l450nm wavelength is added to the input end of EDFA, and the optical signal on the upstream section fiber near the input of EDFA is amplified, and 23dB Raman Peak gain is expected to be obtained in 1550nm window.

(3) Dispersion compensation problem

More than GB/s ETDM system to be practical also need to solve the problem of dispersion compensation, g.655 fiber relative g.652 optical fiber needs less dispersion compensation, the use of dispersion compensation fiber DCF is a relatively mature compensation method, but the introduction of the loss of the amplifier additional gain to compensate. For the 40gb/s system, it is not only to compensate the dispersion, but also to compensate the dispersion slope. For this application, the inverse dispersion fiber with negative dispersion and opposite slope has emerged as compared with conventional optical fiber. Fiber Bragg Grating Compensation is a potential application, but it is urgent to solve the problem of temperature stability and wide wavelength range.

The light waves propagating in single-mode fibers actually contain two electromagnetic fields, which are perpendicular to each other's polarization modes. Due to the optical fiber manufacturing process of the fiber core of the oval, asymmetric mechanical thermal stress and external bending or distortion effects, the two polarization modes are spread at different velocities, the group delay is different, resulting in optical signal distortion or pulse broadening, that is, the transmission rate and distance are limited, which is called polarization mode dispersion PMD. For the signal, because the PMD value is very small, the rate of 2.5gb/s light signal, but the effect on the 10gb/s rate and above the light signal. For optical fiber, because of the complex profile design of g.653 and g.655 fiber, the refractive index is very bad and susceptible to external factors, its PMD is slightly worse than g.652 fiber. Generally speaking, because of the randomness and uncertainty generated by PMD, the need for adaptive compensation, there are now manufacturers claim to develop the corresponding PMD compensator.

(4) Network protection problem

A General Protection methods

SDH Classic protection Switching has been widely accepted. Types include two fiber ring/four fiber ring, Tanxiang/bidirectional ring, Channel Loop/multiplexing De, and a variety of combinations of subnet connection protection SNCP. For the general multiplex segment ring NET, protection switching time can be controlled within 50ms, but for thousands of kilometers of long-distance, up and down business nodes more than the ring network, some advanced SDH systems through fast electric switch bridge, fast slot switching Fast-tsi and efficient APS protocol/algorithm processing, etc. The final switching recovery time is guaranteed to be less than 100ms.

For trans-ring business protection, the typical two-node interconnection Dni method in g.842 is worth recommending. However, if the trans-loop business volume is too large, DNI also powerless, then need to DXC to achieve business transfer and protection. The problems needing attention in the DNI include the error connecting the error, the delay Hold-off time and the waiting recovery time WTR can be set by network management.

B. Logical subnet protection

The traditional subnet is to divide the network on the basis of "physical topology", can be called "physical subnet", and logical subnet is based on "logical topology" as the basis for the division of the network, the circuit layer of the business and functional characteristics as the basis for the channel layer and segment layer after the horizontal division of the subnet formed. The SDH logical subnet is composed of several lower grade SDH logical subnets, channels, segment overhead and links. A single channel or segment overhead can comprise the smallest SDH logical subnet. The segmentation of SDH logical subnet follows the following principles:

The business function features of the circuit layer are used as the segmentation basis to ensure the business integrity of the circuit layer;

The network structure of logical subnet should be as simple as possible, and must conform to the basic type of network topology.

Segment cost, High-order channel and low order channel are the basic components of logical subnet segmentation, and it is recommended to use High-order Channel as the basic component to reduce the number of components contained in the logical subnet.

From the perspective of network management and protection, the basic components of the same business function should be allocated to the same logical subnet as far as possible to avoid fragmentation of the network.

The method of using logical subnet can easily realize the protection function, for example, those logical subnets that contain segment overhead can use the mature APS protocol to share the protection of the business, while the logical subnets that do not contain the segment overhead can be protected by the channel, and the subnet connection protection SNCP can be further provided between different logical subnets.

(5) Synchronization timing problem

According to the requirements of some new network operators, SDH network is not only the user of synchronous network, but also the bearer of synchronous network. In SDH network, the timed signal can not be equated with other service signals, because the pointer adjustment mechanism of SDH may cause some degree of jitter (jitter), which will bring the transmission damage, and thus affect the timing precision. So the timing signal needs to be transparent and unimpaired to reach the destination site from the source site.

At present, most SDH manufacturers use SSM mechanism, the software set S1 byte of different states to indicate timing priority and timing availability, to a certain extent, can ensure the optimization and reconfiguration of the timing route, and prevent the timing loop (two site to each other timing) this worst-case scenario.