home › Programs › Cwdm carrier frequency value. What technologies can operators use to enhance the capabilities of existing optical networks? Line quality assessment

Cwdm carrier frequency value. What technologies can operators use to enhance the capabilities of existing optical networks? Line quality assessment

Questions often arise, what is the difference between CWDM (Coarse Wavelength Division Multiplexing) and DWDM (Dense Wavelength Division Multiplexing) technologies, except for a different number of channels. The technologies are similar in the principles of organization of communication channels, input-output channels, but they have a completely different degree of technological precision, which largely affects the parameters of the line and the cost of solutions.

Number of wavelengths and CWDM and DWDM channels

Technology spectral compaction CWDM implies the use of 18 wavelengths 1), while with accurate WDM DWDM can be used from 40 wavelengths.

CWDM and DWDM Frequency Grid

Channels in CWDM technology are separated by wavelengths, in DWDM - by frequency 2) . The wavelength is calculated a second time from the ratio of the speed of light in vacuum to the frequency. For CWDM, a wavelength grid with a step of 20 nm is used, for standard DWDM systems, frequency grids of 100 GHz and 50 GHz, for high-density DWDM, grids of 25 and 12.5 GHz are used.

Wavelengths and frequencies of CWDM and DWDM

The CWDM technology uses wavelengths from 1270 - 1610 nm. Taking into account the tolerances and the bandwidth of the filters, the range expands to 1262.5 - 1617.5, which is 355 nm. we get 18 wavelengths.

For DWDM with a 100 GHz mesh, the carriers range from 191.5 (1565.50 nm) THz to 196.1 THz (1528.77 nm), i.e. 4.6 THz or 36.73 nm wide band. A total of 46 wavelengths for 23 duplex channels.

For DWDM with a 50 GHz grid, the signal frequencies are in the range of 192 THz (1561.42 nm) - 196 THz (1529.55 nm), which is 4 THz (31.87 nm). There are 80 wavelengths here.

CWDM and DWDM amplification capability

WDM systems based on CWDM technology do not imply amplification of a multicomponent signal. This is due to the lack of optical amplifiers operating in such a wide spectrum.

DWDM technology, on the contrary, implies signal amplification. The multi-component signal can be amplified with standard erbium amplifiers (EDFA).

Range of CWDM and DWDM

CWDM systems are designed to operate on relatively short lines, about 50-80 kilometers.

DWDM systems allow data to be transmitted over distances much greater than 100 kilometers. In addition, depending on the type of signal modulation, DWDM channels can operate without regeneration at a distance of more than 1000 kilometers.

Notes

1) At the beginning of 2015, manufacturers of optical modules, including SKEO, introduced CWDM SFP modules with a wavelength of 1625 nm. This wavelength is not specified by ITU G.694.2, but has been used in practice.

2) Frequency grids for CWDM are described in the ITU G.694.2 standard, for DWDM - in the G.694.1 standard (revision 2).

Number of wavelengths and CWDM and DWDM channels

The CWDM WDM technology involves the use of 18 wavelengths 1), while with accurate DWDM WDM, up to 40 wavelengths can be used.

CWDM and DWDM Frequency Grid

Wavelengths and frequencies of CWDM and DWDM

For DWDM with a 100 GHz mesh, the carriers range from 191.5 (1565.50 nm) THz to 196.1 THz (1528.77 nm), i.e. 4.6 THz or 36.73 nm wide band. A total of 46 wavelengths for 23 duplex channels.

For DWDM with a 50 GHz grid, the signal frequencies are in the range of 192 THz (1561.42 nm) - 196 THz (1529.55 nm), which is 4 THz (31.87 nm). There are 80 wavelengths here.

CWDM and DWDM amplification capability

WDM systems based on CWDM technology do not imply amplification of a multicomponent signal. This is due to the lack of optical amplifiers operating in such a wide spectrum.

DWDM technology, on the contrary, implies signal amplification. The multi-component signal can be amplified with standard erbium amplifiers (EDFA).

Range of CWDM and DWDM

CWDM systems are designed to operate on relatively short lines, about 50-80 kilometers.

Notes

2) Frequency grids for CWDM are described in the ITU G.694.2 standard, for DWDM - in the G.694.1 standard (revision 2).

The technology packed wavelength division multiplexing (Dense Wave Division Multiplexing, DWDM) is designed to create a new generation of optical backbones running at multi-terabit speeds and. Information in fiber-optic communication lines passed at the same time a large number of light waves. DWDM networks operate on the principle of channel switching, each light wave is a single spectral channel and is essential information.

Opportunities of DWDM

The number of channels in a single fiber - 64 light beams in the 1550 nm window transparency. Each light wave transmits information at 40 Gb / s. hardware development is also underway with data rates at speeds of up to 100 Gbit/s and Cisco, are already in progress to develop such technology.

In DWDM technology has predecessor - wavelength division multiplexing technology (Wave Division Multiplexing, WDM), which utilizes four spectral channel transmission windows 1310 nm and 1550 nm, with a carrier spacing of 800-400 GHz. Multiplexing DWDM is called "densified" due to the fact that it uses a considerably smaller distance between the wavelengths than the WDM.

frequency plans

At present, two of the frequency plan (ie a set of frequencies that are separated from each other by a constant value) are defined recommendation G.692 Sector ITU-T:

The frequency plan pitch (spacing between adjacent frequency channels) of 100 GHz (0.8 nm = YES), whereby the data transmission wave 41 is applied in the range of 1528.77 (196.1 THz) to 1560.61 nm (192.1 THz);
Frequency plan in increments of 50 GHz (YES = 0.4 nm), allowing you to transfer in the same range of 81 wavelengths.
Some companies also produced equipment, the highly-called wavelength division multiplexing equipment (High-Dense WDM, HDWDM), capable of operating with a frequency up to 25 GHz increments.

The main problem in the construction of super-dense DWDM systems is that with decreasing frequency step there is an overlapping of the spectra of adjacent channels and there is blurring of the light beam. That leads to an increase in the number of errors and the inability to transmit information on the system

Frequency plans of DWDM

In the following channel plans are currently being used for various types of DWDM systems, CWDM, HDWDM, WDM.

Frequency plans DWDM

Optical fiber amplifiers

The practical success of DWDM technology in many ways defined the appearance of a fiber-optic amplifiers. Optical devices directly amplify light signals in the 1550 nm band, eliminating the need of intermediate conversion to electrical form, as do the regenerators used in the SDH network. The disadvantage of systems of electric signal regeneration is that they have to take a certain type of coding, which makes them quite expensive. Optical amplifiers, "transparent" transmission information, allow to increase the line speed without the need to upgrade the amplifier units. Length of the section between the optical amplifiers can reach 150 km or more, which provides economical DWDM backbones generated in which multiplex section length is today 600-3000 km with use of 1 to 7, the intermediate optical amplifiers.

Recommendation ITU-T G.692 defined three types of amplifying sections, ie sections between two adjacent multiplexers, DWDM:

L (Long)- plot consists of a maximum of 8 spans of fiber-optic communication lines and 7 of optical amplifiers, the maximum distance between the amps - up to 80 km with a maximum total length of the section of 640 km;
V (very long)- plot consists of a maximum of 5 spans of fiber-optic communication lines and 4 optical amplifiers, the maximum distance between the amps - up to 120 km with a maximum total length of 600 km section;
U (Ultra long)- plot without repeaters up to 160 km

Restrictions on the amount of coasting and long associated with the degradation of the optical signal in the optical amplification. Although optical amplifier restores the signal strength, it does not fully compensate for the effect of chromatic dispersion (i.e. propagation of different wavelengths at different rates, due to which the signal at the receiving end is "smeared" fibers) and other nonlinear effects. Therefore, to build a more extensive highways need to be installed between the reinforcing portions DWDM multiplexers performing signal regeneration by converting it into electrical form and back. To reduce non-linear effects in the DWDM signal limitation also applies power systems.

Typical topologies

Ultralong two-point connection on the basis of terminal multiplexers, DWDM

DWDM circuit with input-output in the intermediate nodes

ring topology

The ring topology provides survivability of DWDM network through redundant paths. traffic protection methods used in DWDM, similar to the methods in SDH. To some the connection was secured, two paths are established between its endpoints: main and reserve. Multiplexer endpoint compares the two signals and selects the best signal quality.

Ring DWDM multiplexers

The mesh topology

With the development of DWDM networks are increasingly mesh topology is used, which provides the best performance in terms of flexibility, performance, and resiliency than other topologies. However, to implement a mesh topology, you must have optical cross connects (Optical Cross-Connector, PL), which not only add waves to the overall transit signal and outputting them out, as do the multiplexer input-output, but also support arbitrary switching between optical signals transmitted waves of different lengths.

Mesh DWDM

optical multiplexers

Passive muliplexers used in DWDM networks (without power supply and active conversion) and active multiplexers, demultipleskory.

Passive multiplexers	Active multiplexers
The number of light waves output low	The number of light waves is limited to the applicable frequency plan and a set of light waves
It allows you to display and input signal is a light wave without changing the overall spectrum of the light beam	It does not introduce additional attenuation because it produces a complete demultiplexing of all channels and converting into electrical form
Introduces additional attenuation	It has a high cost
It has a budget cost

Optical cross connections

In networks with mesh topology is necessary to provide the flexibility to change the route of the wave of connections between network subscribers. Such capabilities provide optical cross-connects, to guide the any of the waves at any output port from each input port signal (of course, provided that no other signal of this port does not use the wave must perform another broadcast wavelength).

There are optical cross-connects two types:

Optoelectronic cross connectors with intermediate conversion to electrical form;
all-optical cross-connects, or photonic switches.

Micro Electro Mechanical System, MEMS

Factors to be considered in the construction of DWDM systems

Chromatic dispersion

Chromatic dispersion- as a result of its influence, as it propagates through the fiber, the pulses constituting the optical signal become wider. When transmitting signals over long distances pulses can be superimposed on the adjacent, making it difficult for accurate recovery. With increasing speed of the transmission optical fiber length and chromatic dispersion effect increases. To reduce the effect of chromatic dispersion on the transmitted signals, dispersion compensators are applied.

Polarization Mode Dispersion

PMD occurs in an optical fiber due to the difference in the propagation velocities of the two mutually perpendicular polarization mode components, which leads to distortion of the transmitted pulses. The reason for this phenomenon is the heterogeneity of the geometric shape of the optical fiber. Effect of polarization mode dispersion on the transmitted optical signals with increasing rate with increasing number of channels and sealing system with increasing fiber length.

Stimulated backscatter Mandelstam - Brillouin, the essence of this phenomenon is to create an optical signal of periodic domains with varying refractive index - a kind of a virtual diffraction grating, passing through which signals propagate like the acoustic wave. Reflected this virtual grid signals are added and amplified to form a reverse optical signal with the Doppler frequency down. This phenomenon leads to an increase in the noise level and prevents the spread of the optical signal, since a large part of its power is dissipated in the reverse direction. Often mistakenly called this phenomenon reflected acoustic wave.

Phase modulation at high power levels of the laser signal modulation of its own phase of the signal can occur. This modulation extends the range and broadens or compresses the signal in time, depending on the sign of the chromatic dispersion. In dense WDM systems, self-modulation signal with an expanded spectrum signals may be superimposed on the adjacent channels. Phase modulation signal is increased with increasing power, increasing the transmission rate and with a negative chromatic dispersion. Influence of phase modulation is reduced at zero or a small positive chromatic dispersion

cross-phase modulation, the phenomenon resulting signal modulates the phase of one channel signals from adjacent channels. Factors affecting the cross-phase modulation, coincide with the factors influencing the phase modulation. In addition, the cross-phase modulation effect depends on the number of channels in the system.

four wave mixing, is shown at the threshold power level laser, in which case the non-linear characteristics of the fiber leads to the interaction of three waves and the fourth wave of the new appearance, which may coincide with the frequency of another channel. Such overlay frequency increases the noise level and signal reception difficult

Insertion EDFA amplifier noise, the reason for this phenomenon - the power of the amplified spontaneous emission that occurs due to the design features edfa amplifiers. In the process of passing through the amplifier to the useful component of the optical signal is added to the noise, thereby reducing the ratio of "signal / noise" as a result of the signal can be received in error. This phenomenon limits the amount of in-line amplifiers.

DWDM technology

Dense wavelength division multiplexing (DWDM) is modern technology transmission of a large number of optical channels over a single fiber, which underlies the new generation network technologies. Currently, the telecommunications industry is undergoing unprecedented changes associated with the transition from voice-based systems to data transmission systems, which is a consequence of the rapid development of Internet technologies and various network applications. With the large-scale deployment of data transmission networks, the network architecture itself is being modified. That is why fundamental changes in the principles of design, control and management of networks are required. The new generation of network technologies is based on multi-wavelength optical networks based on dense wave multiplexing DWDM (dense wavelength-division multiplexing).

Technology Description

The most important parameter in dense wave multiplexing technology is undoubtedly the distance between adjacent channels. Standardization of the spatial arrangement of channels is needed, if only because on its basis it will be possible to start testing for mutual compatibility of equipment from different manufacturers. The telecommunications standardization sector of the International Telecommunication Union ITU-T approved the DWDM frequency plan with a distance between adjacent channels of 100 GHz (nm), (Table 1). At the same time, a great debate continues around the adoption of a frequency plan with an even smaller 50 GHz (nm) channel spacing. Without an understanding of the limitations and benefits of each frequency plan, carriers and organizations planning to expand their network capacity can face significant difficulties and unnecessary investment.

100 GHz grid.

The table on the right shows the 100 GHz frequency plan grids with varying degrees of channel sparsity. All grids except one 500/400 have equidistant channels. Uniform channel distribution optimizes the performance of wave converters, tunable lasers, and other all-optical network devices, and makes it easier to scale up.

The implementation of a particular frequency plan grid largely depends on three main factors:

the type of optical amplifiers used (silicon or fluorine-zirconate);
transmission rates per channel - 2.4 Gb / s (STM-16) or 10 Gb / s (STM-64);
influence of non-linear effects.

Moreover, all these factors are strongly interconnected.

Standard silicon fiber EDFAs have one drawback - a large gain variation in the region below 1540 nm, which leads to lower signal-to-noise ratio and gain non-linearity in this region. Both very low and very high values of the gain are equally undesirable. As the bandwidth increases, the minimum signal-to-noise ratio allowed by the standard increases - so for the STM-64 channel it is 4-7 dB higher than for the STM-16. Thus, the non-linearity of the gain of silicon EDFA limits the area size more strongly for STM-64 multiplex channels (1540-1560 nm) than for STM-16 channels and lower capacitance (where you can use almost the entire gain area of silicon EDFA, despite the non-linearity) .

50 GHz grid.

A denser yet non-standardized frequency grid plan with an interval of 50 GHz allows more efficient use of the 1540-1560 nm zone, in which standard silicon EDFAs operate. Along with this advantage, this network has its drawbacks.

In- first, with a decrease in the interchannel intervals, the influence of the four-wave mixing effect increases, which begins to limit maximum length inter-regeneration line (lines based only on optical amplifiers).

In- second, the small inter-channel distance of 0.4 nm may limit the ability to multiplex STM-64 channels. As can be seen from the figure, multiplexing of STM-64 channels with an interval of 50 GHz is not allowed, since then overlapping of the spectra of adjacent channels occurs. Only if there is a lower bit rate per channel (STM-4 and below) does no overlap occur.

AT- third, at 50 GHz, the requirements for tunable lasers, multiplexers, and other components become more stringent, which reduces the number of potential equipment manufacturers, and also leads to an increase in its cost.

Multiplexers DWDM

DWDM multiplexers (unlike more traditional WDM) have two distinctive features:

the use of only one transparency window 1550 nm, within the C-band 1530-1560 nm and L-band 1570-1600 nm;
small distance between multiplex channels, 0.8 or 0.4 nm.

In addition, since DWDM multiplexers are designed to work with a large number of channels up to 32 or more, along with DWDM devices in which all channels are multiplexed (demultiplexed) simultaneously, new devices are also allowed that have no analogues in WDM systems and operate in the add mode. or output of one or more channels to/from the main multiplex stream represented by a large number of other channels. Since the output ports/poles of the demultiplexer are assigned to certain wavelengths, such a device is said to be passively routed along the wavelengths. Due to the small distances between channels and the need to work with a large number of channels simultaneously, the manufacture of DWDM multiplexers requires much greater precision compared to WDM multiplexers (usually using transparency windows of 1310 nm, 1550 nm, or additionally the wavelength region around 1650 nm). It is also important to ensure high near (directivity) and far (isolation) crosstalk performance at the poles of a DWDM device. All this leads to a higher cost of DWDM devices compared to WDM.

Figure "a" shows a typical diagram of a DWDM multiplexer with a mirror reflective element. Let's consider its operation in the demultiplexing mode. The incoming multiplex signal enters the input port. Then this signal passes through the waveguide-plate and is distributed over a plurality of waveguides, representing the diffractive structure AWG (arrayed waveguide grating). As before, the signal in each of the waveguides remains multiplexed, and each channel remains represented in all waveguides. Further, the signals are reflected from the mirror surface and, as a result, the light fluxes are again collected in the waveguide-plate, where they are focused and interfered - spatially separated interference intensity maxima corresponding to different channels are formed. The waveguide-plate geometry, in particular the location of the output poles, and the lengths of the waveguides of the AWG structure are calculated so that the interference maxima coincide with the output poles. Multiplexing occurs in the opposite way.

Another way to build a multiplexer is based not on one but on a pair of waveguides-plates (Fig. b). The principle of operation of such a device is similar to the previous case, except that here an additional plate is used for focusing and interference.

DWDM multiplexers, being passive devices, introduce a large attenuation into the signal. For example, the loss for a device (Fig. 1a) operating in the demultiplexing mode is 4-8 dB, with long-range crosstalk

Transponders and transceivers

Two types of devices can be used to transmit data at wavelength from the DWDM mesh - transceivers and DWDM transponders. DWDM transceivers come in a variety of form factors and can be used in passive DWDM solutions.

Unlike transceivers, transponders allow you to convert the radiation wavelength of the terminal device to the DWDM wavelength for transmission to the multiplexer. The inputs of the optical multiplexer receive optical signals, the parameters of which comply with the standards defined by the G.692 recommendations. The transponder may have a different number of optical inputs and outputs. But if an optical signal can be applied to any input of the transponder, the parameters of which are determined by rec. G.957, then its output signals must correspond in parameters to rec. G.692. In this case, if m optical signals are compressed, then at the output of the transponder, the wavelength of each channel must correspond to only one of them in accordance with the grid of the ITU frequency plan.

Application of optical amplifiers

The development of optical amplification technology based on EDFA has greatly changed the methodology for designing fiber optic communication systems. Traditional fiber optic systems use repeaters-regenerators that increase the signal power (Fig. 3a). When the length between remote nodes begins to exceed the maximum allowable span length between neighboring nodes in terms of signal attenuation, additional regenerators are installed at intermediate points, which receive weak signal, amplify it in the process of optoelectronic conversion, restore the duty cycle, fronts and temporal characteristics of the pulse repetition, and after conversion to the optical form, the correct amplified signal is transmitted further, in the same form as it was at the output of the previous regenerator. Although such regeneration systems work well, they are very expensive and, once installed, cannot increase line capacity.

Based on EDFA, line power losses are overcome by optical amplification, (Fig. 3b). Unlike regenerators, this "transparent" amplification is not tied to the signal bit rate, which allows you to transfer information at higher speeds and increase throughput until other limiting factors such as chromatic dispersion and polarization modal dispersion enter into force. . EDFAs are also capable of amplifying a multi-channel WDM signal, adding another dimension to the bandwidth.

Although the optical signal generated by the original laser transmitter has a well-defined polarization, all other nodes along the path of the optical signal, including the optical receiver, should show a weak dependence of their parameters on the polarization direction. In this sense, EDFA optical amplifiers, characterized by a weak polarization dependence of the gain, have a tangible advantage over semiconductor amplifiers.

Unlike regenerators, optical amplifiers introduce additional noise that must be taken into account. Therefore, along with the gain, one of the important parameters of EDFA is the noise figure.

Application of ROADM devices

The use of a configurable optical input/output multiplexer (ROADM) enables flexible deployment and remote configuration of spectral channels. At any node in the ROADM network, it is possible to switch the state of the spectral channel to I/O and end-to-end without interrupting existing services. When working with a tunable laser, ROADM provides flexible control of spectral channels. ROADM allows you to build networks with multiple rings or mixed networks: based on selective switching of spectral channels (WSS) technology.

Building DWDM networks

City DWDM networks, as a rule, are built using a ring architecture, which allows the use of protection mechanisms at the DWDM level with a recovery speed of no more than 50 ms. It is possible to build a network infrastructure on equipment from several vendors with an additional level of distribution based on Metro DWDM equipment. This level is introduced to organize the exchange of traffic between networks with equipment from different companies.

In DWDM technology, the minimum signal resolution is the optical channel, or wavelength. The use of whole wavelengths with a channel capacity of 2.5 or 10 Gbit/s for traffic exchange between subnets is justified for building large transport networks. But multiplexer transponders allow you to organize traffic exchange between subnets at the level of STM-4/STM-1/GE signals. The distribution level can also be built on the basis of SDH technology. But DWDM has a big advantage related to the transparency of control channels and overhead channels (eg overhead). When packing SDH/ATM/IP signals into an optical channel, the structure and contents of the packets do not change. DWDM systems only monitor individual bytes to verify the correctness of the signals. Therefore, the connection of subnets over a DWDM infrastructure at a single wavelength can be considered as a connection with a pair of optical cables.

When using equipment from different manufacturers, two data transmission subnets of one manufacturer are connected through a DWDM network of another manufacturer. A control system that is physically connected to one subnet can also control the operation of another subnet. If SDH equipment were used at the distribution level, this would not be possible. Thus, on the basis of DWDM networks, networks of different manufacturers can be combined to transmit heterogeneous traffic.

SFP (WDM, CWDM, DWDM) - WHAT IS IT? WHAT IS IT FOR?

Wavelength division multiplexing (WDM) technologies.

Spectral multiplexing is based on the optical channel multiplexing method. Principle this method is that each information stream is transmitted over one optical fiber at a different wavelength (at a different carrier frequency), separated from each other at a distance of 20 nm.

With the help of special devices - optical multiplexers - the streams are combined into one optical signal, which is injected into the optical fiber. On the receiving side, the reverse operation is performed - demultiplexing, carried out using optical demultiplexers. This opens up truly inexhaustible possibilities for both increasing the line capacity and building complex topological solutions using a single fiber.

When choosing the number of channels, pay attention to the type of single-mode fiber used!
For example, in G.652B fibers (fiber with a water peak at a wavelength of 1383 nm) at short wavelengths, there are large radiation losses, in connection with this, the allowable transmission distance is reduced and the number of spectral channels will be less than required.

In Coarse WDM systems, in accordance with ITU G.694.2 recommendation, no more than 18 carriers with a 20 nm step should be used: 1270, 1290, 1310 ... 1570, 1590, 1610, i.e. if the total required bandwidth does not exceed 340 nm. It should be taken into account that at the edges of such a wide range, the attenuation is quite large, especially in the region of short waves. The so-called Zero Water Peak Fiber (ZWPF, Zero Water Peak Fiber; LWPF, Low Water Peak Fiber) allowed to increase the number of channels to 18, the parameters of which are determined by the ITU-T G.652.C/D recommendation. in fibers of this type the absorption peak at a wavelength of 1383 nm is eliminated and the attenuation at this wavelength is about 0.31 dB/km.

G.653 fiber proved unsuitable for the rapidly developing new WDM technology due to zero dispersion at 1550 nm, resulting in a sharp increase in signal distortion from four-wave mixing in these systems. G.655 optical fiber turned out to be the most suitable for dense and high-density WDM (DWDM and HDWDM), and the recently standardized G.656 optical fiber for sparse WDM
The creation of fibers without a “water peak” made it possible to use all waves in the range from 1260 to 1625 nm in communication systems, i.e. where the quartz optical fiber has the greatest transparency.

MAIN EQUIPMENT

Multiplexers/demultiplexers (MUX/DEMUX); allow summing and separating optical signals.

allow you to select and add a signal to the fiber at certain carrier frequencies.

Depending on the task, the configuration of the multiplexer / demultiplexer (Mux / Demux) is determined by the following characteristics:

Dual fiber multiplexer (2 fibers)
Single fiber multiplexer(1 fiber (single fiber) or bidirectional)
4 or 8 channel multiplexer(8 or 16 wavelengths) working on one fiber
8 or 16 channels, dual fiber
multiplexer with two "common"(COMMON) conclusions to implement a "ring" topology
For “Point-to-Point” or “Ring” topologies, a “pairwise” (Tx-Rx ports) set of multiplexers is required - Mux / Demux Type I, Mux / Demux Type II
Connectors - FC,SC,LC,ST,FA,SA

Delivery of Multiplexers is possible in the following versions:
Rackmount 19" 1RU
In a plastic case(for wall or sleeve mounting)
By connector type– LC, SC, etc.

SFP (Small Form Factor Pluggable) transceivers (SFP,SFP+, X2, XFP) – form and receive optical signals (of certain wavelengths) in the CWDM system; convert the signal from electrical to optical and vice versa. SFP module combines a transmitter (transmitter) and a receiver (receiver) at once. Therefore, it supports simultaneous transmission and reception of data over two links within a single channel. Since the days of radio, such devices have been called transceivers. That is why SFP modules are called transceivers.

Each SFP transceiver operates on two fibers and, unlike standard two-fiber 1000Base LX transceivers, operates on two different wavelengths - broadband receiver works with one wavelength and the transmitter with another.
To form a data channel in the SFP system, transceivers are completed in pairs.

Transceivers also differ in signal strength (mileage), i.e. they work at different distances.

For a stronger compression of the optical signal, "color" SFP modules are used that operate in a certain wavelength range (CWDM). Such SFP transceivers are designed to generate optical signals of the "main carrier" from 1270 to 1610nm (20nm step).

SFP modules are available that operate on both single and dual fibers with a throughput of 1.25, 2.5 and 4.25Gbps. These modules can be installed directly into almost any manufacturer's switching equipment, enabling seamless integration of CWDM into existing infrastructure. The same module can serve as a Gigabit Ethernet, Fiber Channel or SDH interface, which greatly adds to the flexibility of the solution.

It is also possible to install CWDM SFP modules in the media converter chassis. The use of a chassis is the most flexible solution, completely eliminating hardware incompatibility issues. Using the chassis, you get standard 1000BASE-T Gigabit Ethernet ports at the output, which eliminates expensive switches with SFP ports.

Special attention should be paid to the compression of 10 Gb / s channels. Three years ago, there were no transceivers operating at speeds of 10 Gbit / s and supporting the wavelengths of the frequency grid of sparse wavelength division multiplexing systems, at present, such modules have appeared, however, their use imposes significant restrictions on the capabilities of the system, compared with channel multiplexing 1 .25 Gbps and 2.5 Gbps.

Currently, there are no 10 Gb/s lasers operating in the 1350-1450 nm wavelength range, so the maximum number of multiplexed 10 Gb/s channels cannot exceed 12 when using two G.652D fibers. In addition, when using 10 Gbit / s channels, it must be taken into account that the maximum optical budget of such modules is currently no more than 28 dBm, which corresponds to a range of approximately 80 kilometers via single-mode fiber. In cases where it is necessary to condense and transmit more than 12 10 Gbit / s channels, incl. for distances greater than 80 kilometers, DWDM equipment is used.

OADM modules - input/output multiplexers; allow you to select and add a signal to the fiber on certain carriers.

Basic properties:
Single channel input/output
Passive optics
Low insertion loss for backhaul links
Dedicated wavelength to the end user

Fundamentally, OADM modules are single-channel and dual-channel. Their difference lies in the ability to receive and receive an optical signal from one or two multiplexers and is physically due to the presence of one or two transceiver units. Accordingly, a single-channel OADM module has one transceiver unit and is able to work with only one multiplexer in one direction. The two-channel OADM module has two transceiver units and is able to work "in two directions" with two multiplexers / demultiplesors.

The transceiver unit of a single-channel OADM module has four interfaces:

Com port - receives a signal from the multiplexer
Express port - passes the signal to other elements of the system
Add port - adds a channel to the line at a certain wavelength,
Drop port - extracts a channel at a certain wavelength from the line.

Such devices have no restrictions on protocols or bandwidth.
Accordingly, a two-channel OADM module has two additional Add and Drop ports.
In the case of using a two-fiber system, Com2 and Express2 ports are also added.
A single-channel OADM module is paired with one SFP transceiver, a dual-channel OADM module is paired with two

Terminal transit module OADM ( drop/pass module) diverts one link from the backbone and directs it to the local port. The remaining channels are passed directly to other network nodes.

The OADM Single Channel Multiplexing Module (drop/add module) has two local interfaces. The first one takes one channel out of the backbone and directs it to the local port, the second adds this channel back to the backbone in the opposite direction. Such a module is necessary when building a "ring" topology network.

Delivery of OADM modules is possible in the following versions:
Rack 19” 1RU
In a plastic case (for wall or sleeve mounting)
Connectors - LC, SC, etc.

The main WDM systems are:

- WDM (Wavelength Division Multiplexing)

- CWDM (Coarse Wavelength Division Multiplexing)

So what is WDM?

Technology for adding optical signals with different wavelengths transmitted simultaneously on one fiber 2 or more signals separated at the far end by wavelengths. The most typical (2-channel WDM) combines 1310 nm and 1550 nm wavelengths in a single fiber.

Two-channel WDM (and three-channel) can be used to quickly and easily add additional (or two additional) wavelengths. It is very easy to install and connect and very inexpensive. In most cases, WDM is the most cost-effective solution for fiber shortages, providing a 2 to 1 or 3 to 1 fiber gain by combining 1310nm, 1550nm and 1490nm wavelengths into a single fiber.

When more channels are needed to expand an existing fiber infrastructure, CWDM provides an efficient solution for short optical spans (up to 80 km). CWDM can easily and quickly add up to 18 additional wavelengths on ITU-standardized frequencies. It is ideal for moderate sized networks with transverse dimensions up to 100 km. Since the wavelength spacing is 20 nm, less expensive lasers can be used, resulting in a very low cost. CWDM systems, although they are multi-channel, do not have any optical amplification mechanisms and range limitations are determined by the channel with maximum attenuation. Moreover, channels from the 1360nm to 1440nm region may experience the most attenuation (1 to 2 dB/km) due to the water peak in this region for some types of optical cable.

Where high capacity or long distance transmission is required, solutions DWDM is the preferred method for increasing fiber capacitance. With its high-precision lasers optimized to operate within the 1550nm window (to reduce loss), DWDM systems are ideal solution for more demanding networks. DWDM systems can use EDFA to amplify all wavelengths in the DWDM window and extend the transmission length up to 500 km.

DWDM systems are typically limited in range to 4-5 gain spans due to Amplified Spontaneous Emissions (ASE) noise in EDFA. Simulation tools are available to determine exactly how many EDFAs can be installed. Over long distances (> 120 km) dispersion can be problematic, requiring the installation of dispersion compensation modules. The DWDM band is limited to wavelengths ranging from 1530 nm to 1565 nm by the EDFA gain range.

Solution types:

1. Point - point.

Adding a point-to-point spectral system to an optical system is a simple and cost-effective solution to fiber shortages.
Systems with such a topology are typical in solving problems of simultaneous transmission of a large number of data streams to increase the number of services provided (video, voice, etc.). In this case, the fibers of an already existing optical transport network are used. In this mode of operation, information is transmitted over channels between two points. For successful data transmission over a distance of up to 50-80 km, multiplexers / demultiplexers are required in those nodes where the information flows will be combined and then separated.

Branch connection

Such an architecture implements the transfer of information from one node to another with intermediate nodes along this path, where it is possible to add and remove individual channels using OADM modules. The maximum number of taps is determined by the number of duplex transmission channels (for example, 4 or and the optical budget of the line. When calculating, it must be remembered that each OADM module introduces attenuation, as a result of which the total length of the path is correspondingly reduced. The optical channel can be extracted at any point in the path.

In this case, OADM modules (two-channel) are installed between two multiplexers / demultiplexers.
In this case, each dual-channel OADM module must be equipped with two SFP transceivers.

Branch point.

The fundamental difference from the first option is the absence of a second multiplexer / demultiplexer. Thus, the exchange of signals takes place between the central communication node and the end equipment at different sections of the line. Such an architecture seems to be promising from an economic point of view, since in fact, it allows you to exclude the aggregation level switch from the network with significant savings in fiber. At the same time, the distance from the OADM module (single-channel) to the location of the final equipment (switch, router, media converter) is limited only by the signal power in the line and the insertion loss from the compression equipment.

Advantages
Optical fiber savings - the WDM system allows you to transmit up to 8 channels on one fiber with a bandwidth of up to 2.5 Gb / s per channel
Power independent - only active equipment needs power
No problems of "falling", reboots, etc.
No need to organize permanent access to the locations of the system elements - there are OADM modules designed for placement in optical boxes
Reducing the level of influence of the "human factor" - the absence of active components that require configuration, management, etc.
Significant reduction in cost of ownership - lower operating costs
Relatively low cost, the possibility of refusing aggregation-level equipment
The maximum operating range is 80 kilometers or more
Independence from client protocols - transmission of up to 18 independent services over two pairs of optical fibers; transparency for all data transfer protocols
Availability various kinds equipment for mounting in various conditions: in a rack, in a sleeve, on a wall.

Surely everyone has heard about the transmission of information over fiber optic networks, and also that this method provides the highest speeds to date. It is the latter that gives a good reason for the development of data transmission technologies over optical fiber. Even today, throughput can reach the order of terabits (1000 gigabits) per second.

If compared with other methods of information transfer, then the order of magnitude of TB / s is simply unattainable. Another plus of such technologies is the reliability of transmission. Fiber optic transmission does not have the disadvantages of electrical or radio signal transmission. There is no interference that can damage the signal, and there is no need to license the use of the radio frequency. However, not many people understand how information is transmitted over fiber in general, and even more so are not familiar with specific implementations of technologies. In this article, we will look at one of them - DWDM technology (dense wavelength-division multiplexing).

First, let's look at how information is generally transmitted over an optical fiber. An optical fiber is a waveguide that carries electromagnetic waves with a wavelength of about a thousand nanometers (10-9 m). This is a region of infrared radiation not visible to the human eye. And the main idea is that with a certain selection of the fiber material and its diameter, a situation arises when for some wavelengths this medium becomes almost transparent, and even when it hits the boundary between the fiber and the environment, most of the energy is reflected back into the fiber. This ensures the passage of radiation through the fiber without much loss, and the main task is to receive this radiation at the other end of the fiber. Of course, such a brief description hides the huge and difficult work of many people. One should not think that such material is easy to create or that this effect is obvious. On the contrary, it should be treated as a big discovery, as it now provides the best way to convey information. You need to understand that the waveguide material is a unique development and the quality of data transmission and the level of interference depend on its properties; The waveguide insulation is designed to minimize the amount of energy escaping to the outside. As for specifically the technology called "multiplexing", it means that you transmit several wavelengths at the same time. They do not interact with each other, and when receiving or transmitting information, interference effects (the superposition of one wave on another) are insignificant, since they are most pronounced at multiple wavelengths. Here we are talking about using close frequencies (the frequency is inversely proportional to the wavelength, so it doesn't matter what to talk about). A device called a "multiplexer" is an apparatus for encoding or decoding information into a waveform and vice versa. After this short introduction, let's move on to a specific description of DWDM technology.

The main characteristics of DWDM multiplexers that distinguish them from just WDM multiplexers are:

the use of only one window of transparency 1550 nm, within the amplification region EDFA 1530-1560 nm (EDFA - optical amplification system);
small distances between multiplex channels - 3.2/1.6/0.8 or 0.4 nm.

For reference, let's say that the wavelength of visible light is 400-800 nm. In addition, since the name itself speaks of dense (dense) transmission of channels, the number of channels is greater than in conventional WDM schemes, and reaches several tens. Because of this, there is a need to create devices that are able to add a channel or remove it, in contrast to conventional schemes, when all channels are encoded or decoded at once. With such devices operating on one channel out of many, the concept of passive wavelength routing is associated. It is also clear that working with a large number of channels requires greater accuracy of signal encoding and decoding devices and places higher demands on the quality of the line. Hence the obvious increase in the cost of devices - while reducing the price for the transfer of a unit of information due to the fact that now it can be transferred in a larger volume.

This is how the demultiplexer works with a mirror (scheme in Fig. 1a). The incoming multiplex signal enters the input port. Then this signal passes through the waveguide-plate and is distributed over a plurality of waveguides, which are an AWG (arrayed waveguide grating) diffraction structure. As before, the signal in each of the waveguides remains multiplexed, and each channel remains represented in all waveguides, that is, so far only parallelization has occurred. Further, the signals are reflected from the mirror surface, and as a result, the light fluxes are again collected in the waveguide-plate, where they are focused and interfered. This leads to the formation of an interference pattern with spatially separated maxima, and the geometry of the plate and mirror is usually calculated so that these maxima coincide with the output poles. Multiplexing occurs in the opposite way.

Another way to build a multiplexer is based not on one, but on a pair of waveguides-plates (Fig. 1b). The principle of operation of such a device is similar to the previous case, except that here an additional plate is used for focusing and interference.

DWDM multiplexers, being purely passive devices, introduce a lot of attenuation into the signal. For example, the loss for a device (see Fig. 1a) operating in the demultiplexing mode is 10-12 dB, with far crosstalk less than -20 dB and a half-width of the signal spectrum of 1 nm (according to Oki Electric Industry). Due to high losses, it is often necessary to install an optical amplifier before and/or after the DWDM multiplexer.

The most important parameter in DWM technology is undoubtedly the distance between adjacent channels. Standardization of the spatial arrangement of channels is needed if only because on its basis it will be possible to start testing for mutual compatibility of equipment from different manufacturers. The telecommunications standardization sector of the International Telecommunication Union ITU-T has approved a DWDM frequency plan with a distance between adjacent channels of 100 GHz, which corresponds to a wavelength difference of 0.8 nm. The issue of transmitting information with a difference in wavelengths of 0.4 nm is also being discussed. It would seem that the difference can be made even smaller, thereby achieving a greater throughput, but in this case, purely technological difficulties arise associated with the manufacture of lasers that generate a strictly monochromatic signal (of a constant frequency without interference) and diffraction gratings that separate the maxima in space corresponding to different wavelengths. When using a 100 GHz split, all channels fill the used band evenly, which is convenient when setting up equipment and reconfiguring it. The choice of separation interval is determined by the required bandwidth, the type of laser and the degree of interference on the line. However, it should be taken into account that when operating even in such a narrow range (1530-1560 nm), the influence of non-linear interference at the boundaries of this region is very significant. This explains the fact that with an increase in the number of channels, it is necessary to increase the laser power, but this, in turn, leads to a decrease in the signal-to-noise ratio. As a result, the use of a stiffer seal has not yet been standardized and is under development. Another obvious disadvantage of increasing the density is the reduction in the distance over which the signal can be transmitted without amplification or regeneration (a little more about this will be discussed below).

Note that the nonlinearity problem mentioned above is inherent in silicon-based amplification systems. Now more reliable fluorine-zirconate systems are being developed, which provide greater linearity (in the entire region of 1530–1560 nm) of the gain. With an increase in the working area of the EDFA, it becomes possible to multiplex 40 STM-64 channels with an interval of 100 GHz with a total capacity of 400 GHz per fiber (Fig. 2).

The table shows specifications one of the powerful multiplex systems using the 100/50 GHz frequency plan manufactured by Ciena Corp.

Let us dwell in more detail on the optical amplification system. What is the problem? Initially, the signal is generated by the laser and sent to the fiber. It propagates along the fiber, undergoing changes. The main change to deal with is signal scattering (dispersion). It is associated with nonlinear effects that arise during the passage of a wave packet in a medium and is obviously explained by the resistance of the medium. This raises the problem of transmission over long distances. Large - in the sense of hundreds or even thousands of kilometers. This is 12 orders of magnitude greater than the wavelength, so it is not surprising that even if the nonlinear effects are small, then in total at such a distance they must be taken into account. Plus, the nonlinearity can be in the laser itself. There are two ways to achieve reliable signal transmission. The first is the installation of regenerators that will receive a signal, decode it, generate a new signal that is completely identical to the one that arrived, and send it further. This method is effective, but such devices are quite expensive, and increasing their bandwidth or adding new channels that they must handle is associated with difficulties in reconfiguring the system. The second method is simply optical amplification of the signal, completely analogous to amplifying the sound in a music center. This amplification is based on EDFA technology. The signal is not decoded, but only its amplitude is increased. This allows you to get rid of speed losses in the amplification nodes, and also removes the problem of adding new channels, since the amplifier amplifies everything in a given range.

Based on EDFA, line power losses are overcome by optical amplification (Fig. 3). Unlike regenerators, this "transparent" amplification is not tied to the signal bit rate, which allows you to transfer information at higher speeds and increase throughput until other limiting factors such as chromatic dispersion and polarization mode dispersion come into force. EDFAs are also capable of amplifying a multi-channel WDM signal, adding another dimension to the bandwidth.

Unlike regenerators, optical amplifiers introduce additional noise that must be taken into account. Therefore, along with the gain, one of the important parameters of the EDFA is the noise figure. EDFA technology is cheaper, for this reason it is more often used in real practice.

Since EDFA, at least in terms of price, looks more attractive, let's break down the main characteristics of this system. This is the saturation power, which characterizes output power amplifier (it can reach and even exceed 4 W); gain, defined as the ratio of the power of the input and output signals; the power of amplified spontaneous emission determines noise level, which creates the amplifier itself. Here it is appropriate to give an example of a music center, where you can trace analogies in all these parameters. The third one (noise level) is especially important, and it is desirable that it be as low as possible. Using an analogy, you could try including music Center without playing any disc, but at the same time turn the volume knob to the maximum. In most cases, you will hear some noise. This noise is created by amplification systems simply because they are powered. Similarly, spontaneous emission occurs in our case, but since the amplifier is designed to emit waves in a certain range, then photons of this particular range will be more likely to be emitted into the line. This will create (in our case) light noise. This imposes a limitation on the maximum line length and the number of optical amplifiers in it. The gain factor is usually chosen to restore the original signal level. On fig. Figure 4 shows the comparative spectra of the output signal in the presence and absence of a signal at the input.

Another parameter that is convenient to use when characterizing an amplifier is the noise factor - this is the ratio of the signal-to-noise parameters at the input and output of the amplifier. In an ideal amplifier, this parameter should be equal to one.

There are three applications for EDFA amplifiers: preamplifiers, line amplifiers, and power amplifiers. The first are installed directly in front of the receiver. This is done to increase the signal-to-noise ratio, which allows the use of simpler receivers and can reduce the cost of the equipment. Linear amplifiers are intended to simply amplify the signal in long lines or in the case of a branching of such lines. Power amplifiers are used to amplify the output directly after the laser. This is due to the fact that the power of the laser is also limited and sometimes it is easier to simply install an optical amplifier than to install a more powerful laser. On fig. 5 schematically shows all three EDFA applications.

In addition to the direct optical amplification described above, an amplifying device using the Raman amplification effect for this purpose and developed at Bell Labs is currently preparing to enter the market. The essence of the effect is that a laser beam of a certain wavelength is sent from the receiving point towards the signal, which shakes the crystal lattice of the waveguide in such a way that it begins to emit photons in a wide frequency spectrum. Thus, the overall level of the useful signal rises, which allows you to slightly increase the maximum distance. Today this distance is 160-180 km, compared to 70-80 km without Raman enhancement. These Lucent Technologies devices will hit the market in early 2001.

What was said above is technology. Now a few words about implementations that already exist and are actively used in practice. First, we note that the use of fiber optic networks is not only the Internet and, perhaps, not so much the Internet. Fiber optic networks can transmit voice and TV channels. Second, let's say that there are several different types networks. We are interested in long-distance backbone networks, as well as localized networks, for example, within one city (the so-called metro solutions). At the same time, for trunk communication channels, where the rule “the thicker the pipe, the better” works perfectly, DWDM technology is an optimal and reasonable solution. Another situation develops in urban networks, in which the requests for traffic transmission are not as large as those of backbone channels. Here, operators use the good old SDH/SONET-based transport operating in the 1310 nm wavelength range. In this case, to solve the problem of insufficient bandwidth, which, by the way, is not very acute for urban networks yet, you can use the new SWDM technology, which is a kind of compromise between SDH / SONET and DWDM (read more about SWDM technology on our CD-ROM ). With this technology, the same fiber optic ring nodes support both 1310nm single-channel data transmission and 1550nm wavelength division multiplexing. Savings are achieved by "inclusion" of an additional wavelength, which requires the module to be added to the appropriate device.

DWDM and traffic

One of important points when using DWDM technology is transmitted traffic. The fact is that most of the equipment that currently exists supports the transmission of only one type of traffic at one wavelength. As a result, a situation often arises when traffic does not completely fill the fiber. Thus, less “dense” traffic is transmitted over a channel with a formal bandwidth equivalent to, for example, STM-16.

Currently, there is equipment that implements the full load of wavelengths. In this case, one wavelength can be "filled" with heterogeneous traffic, say, TDM, ATM, IP. An example is the equipment of the Chromatis family from Lucent Technologies, which can transmit on the same wavelength all types of traffic supported by I / O interfaces. This is achieved with a built-in TDM cross-switch and ATM switch. Moreover, the additional ATM switch is not price-forming. In other words, additional hardware functionality is achieved at virtually the same cost. This allows us to predict that the future belongs to universal devices capable of transmitting any traffic with optimal use of bandwidth.

DWDM tomorrow

Moving smoothly to the development trends of this technology, we will certainly not discover America if we say that DWDM is the most promising optical data transmission technology. This can be attributed to a greater extent to the rapid growth of Internet traffic, whose growth rates are approaching thousands of percent. The main starting points in development will be an increase in the maximum transmission length without optical signal amplification and the implementation of a larger number of channels (wavelengths) in one fiber. Today's systems transmit 40 wavelengths, which corresponds to a 100 GHz frequency grid. Devices with a 50 GHz mesh, supporting up to 80 channels, which corresponds to the transmission of terabit streams over a single fiber, are next to enter the market. And already today you can hear the statements of the laboratories of development companies, such as Lucent Technologies or Nortel Networks, about the imminent creation of 25-GHz systems.

However, despite such a rapid development of engineering and research ideas, market indicators are making their own adjustments. The last year was marked by a serious drop in the optical market, as evidenced by a significant drop in the price of Nortel Networks shares (29% in one day of trading) after the announcement of difficulties with the sale of its products. Other manufacturers found themselves in a similar situation.

At the same time, if some saturation is observed in the western markets, then the eastern ones are just beginning to unfold. The most striking example is the Chinese market, where a dozen national operators are racing to build backbone networks. And if “they” have already practically solved the issues of building backbone networks, then in our country, sadly, there is simply no need for thick channels for transmitting our own traffic. Nevertheless, the exhibition “Departmental and corporate networks Communications” revealed a huge interest of domestic telecom operators in new technologies, including DWDM. And if such monsters as Transtelecom or Rostelecom already have state-wide transport networks, then the current power engineers are just starting to build them. So, despite all the troubles, optics is the future. And DWDM will play a significant role here.

ComputerPress 1 "2001

Popular in category: