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Equipment dwdm technology development trends. Russian DWDM and CWDM equipment. Operating principle of wavelength division multiplexing systems

The basic principle of WDM technology (Wavelength-division multiplexing, frequency division of channels) is the ability to transmit multiple signals at different carrier wavelengths in one optical fiber. In Russian telecom, transmission systems created using WDM technology are called “compression systems”.

On this moment There are three types of WDM systems:
1. CWDM (Coarse Wavelength-division multiplexing - coarse frequency division of channels) - systems with optical carrier spacing of 20 nm (2500 GHz). The operating range is 1261-1611 nm, in which up to 18 simplex channels can be implemented. ITU standard G.694.2.
2. DWDM (Dense Wavelength-division multiplexing - dense frequency division of channels) - systems with optical carrier spacing of 0.8 nm (100 GHz). There are two operating ranges - 1525-1565 nm and 1570-1610 nm, in which up to 44 simplex channels can be implemented. ITU standard G.694.1.
3. HDWDM (High Dense Wavelength-division multiplexing) - systems with optical carrier spacing of 0.4 nm (50 GHz) or less. It is possible to implement up to 80 simplex channels.

This article (review) focuses on the problem of monitoring in DWDM compaction systems, in more detail about various types WDM systems can be found at the link - link.

DWDM wavelength division multiplexing systems can use one of two ranges of carrier wavelengths: C-band - 1525-1565 nm (conventional band or C-band can also be found) and L-band - 1570-1610 nm (long wavelength band or L-band).

The division into two ranges is justified by the use of different optical amplifiers with different operating gain ranges. The gain bandwidth for a traditional amplifier configuration is approximately 30 nm, 1530-1560 nm, which is the C-band. For amplification in the long wavelength range (L-band), the configuration of the erbium amplifier is changed by lengthening the erbium fiber, which leads to a shift in the amplification range to wavelengths of 1560-1600 nm.

At the moment, C-band DWDM equipment has received great recognition in Russian telecoms. This is due to the abundance of various equipment that supports this range. It should be noted that equipment manufacturers include both venerable domestic companies and leading global brands, as well as numerous faceless Asian manufacturers.

The main issue in any part of the compaction system (regardless of type) is the power level in the optical channel. First, you need to understand what a DWDM sealing system typically consists of.

DWDM system components:
1) Transponder
2) Multiplexer/demultiplexer
3) Optical amplifier
4) Chromatic dispersion compensator

The transponder performs 3R regeneration (“reshaping, “re-amplifying”, “retiming” - restoration of the shape, power and signal synchronization) of the incoming client optical signal. The transponder can also convert client traffic from one transmission protocol (often Ethernet) to another, more noise-resistant (for example, OTN using FEC) and transmit the signal to the linear port.

In more simple systems an OEO converter can act as a transponder, which performs 2R regeneration (“reshaping”, “re-amplifying”) and transmits the client signal to the linear port without changing the transmission protocol.

The client port is often made in the form of a slot for optical transceivers, into which a module is inserted for communication with client equipment. The line port in the transponder can be made in the form of a slot for an optical transceiver or in the form of a simple optical adapter. The design of the linear port depends on the design and purpose of the system as a whole. In an OEO converter, the line port is always designed as a slot for an optical transceiver.
In many systems, the intermediate link, the transponder, is eliminated in order to reduce system cost or due to functional redundancy in a particular task.

Optical multiplexers are designed to combine (mix) individual WDM channels into a group signal for their simultaneous transmission over one optical fiber. Optical demultiplexers are designed to separate the received baseband signal at the receiving end. IN modern systems compaction, multiplexing and demultiplexing functions are performed by one device - a multiplexer/demultiplexer (MUX/DEMUX).

A multiplexer/demultiplexer can be divided into a multiplexing unit and a demultiplexing unit.
An optical amplifier based on impurity optical fiber doped with erbium (Erbium Doped Fiber Amplifier-EDFA) increases the power of the group optical signal included in it (without prior demultiplexing) without optoelectronic conversion. The EDFA amplifier consists of two active elements: an Er3+ doped active fiber and a suitable pump.

Depending on the type, EDFA can provide an output power of +16 to +26 dBm.
There are several types of amplifiers, the use of which is determined by the specific task:
Input optical power amplifiers (boosters) - installed at the beginning of the route
Optical preamplifiers - installed at the end of the route in front of the optical receivers
Linear optical amplifiers - installed at intermediate amplification nodes to maintain the required optical power

Optical amplifiers are widely used on long data transmission lines with DWDM wavelength division multiplexing systems.

The chromatic dispersion compensator (Dispersion Compensation Module) is designed to correct the shape of optical signals transmitted in optical fiber, which, in turn, are distorted under the influence of chromatic dispersion.

Chromatic dispersion is a physical phenomenon in optical fiber in which light signals with different wavelengths travel the same distance in different periods of time, resulting in broadening of the transmitted optical pulse. Thus, chromatic dispersion is one of the main factors limiting the length of the relay section of the route. Standard fiber has a chromatic dispersion value of about 17 ps/nm.

To increase the length of the relay section, chromatic dispersion compensators are installed on the transmission line. Installation of compensators often requires a transmission line with a speed of 10 Gbit/s or more.

There are two main types of DCM:

1. Chromatic dispersion compensating fiber - DCF (Dispersion Compensation Fiber). The main component of these passive devices is a fiber with a negative chromatic dispersion value in the wavelength range 1525-1565 nm.

2. Chromatic dispersion compensator based on a Bragg grating - DCM FBG (Dispersion Compensation Module Fiber Bragg Grating). Passive optical device, consisting of a chirped fiber and an optical circulator. Due to its structure, chirped fiber creates conditionally negative chromatic dispersion of incoming signals in the wavelength range 1525-1600 nm. The optical circulator in the device acts as a filtering device that directs signals to the appropriate pins.

Thus, the standard circuit consists of only two types of active components - a transponder and an amplifier, with which you can monitor the current power level of transmitted signals. The transponders implement the function of monitoring the status of linear ports, either based on the built-in DDMI function in optical transceivers, or with the organization of their own monitoring. Using this function allows the operator to receive up-to-date information about the status of a specific communication channel.

Due to the fact that optical amplifiers are amplifiers with feedback, they always have the function of monitoring the input group signal (the total optical power of all incoming signals) and the outgoing group signal. But this monitoring is inconvenient in the case of monitoring specific communication channels and can be used as evaluative (presence or absence of light). Thus, the only tool for controlling optical power in a data transmission channel is a transponder.

And since compaction systems consist not only of active, but also passive elements, organizing full monitoring in compaction systems is a very non-trivial and in-demand task.

Options for organizing monitoring in WDM compaction systems will be discussed in the next article.

At the moment, there are three types of WDM systems:
1. CWDM (Coarse Wavelength-division multiplexing - coarse frequency division of channels) - systems with optical carrier spacing of 20 nm (2500 GHz). The operating range is 1261-1611 nm, in which up to 18 simplex channels can be implemented. ITU standard G.694.2.
2. DWDM (Dense Wavelength-division multiplexing - dense frequency division of channels) - systems with optical carrier spacing of 0.8 nm (100 GHz). There are two operating ranges - 1525-1565 nm and 1570-1610 nm, in which up to 44 simplex channels can be implemented. ITU standard G.694.1.
3. HDWDM (High Dense Wavelength-division multiplexing) - systems with optical carrier spacing of 0.4 nm (50 GHz) or less. It is possible to implement up to 80 simplex channels.

This article (review) pays attention to the problem of monitoring in DWDM compaction systems; more details about the various types of WDM systems can be found at the link - link.

The main issue in any part of the compaction system (regardless of type) is the power level in the optical channel. First, you need to understand what a DWDM sealing system typically consists of.

DWDM system components:
1) Transponder
2) Multiplexer/demultiplexer
3) Optical amplifier
4) Chromatic dispersion compensator

In simpler systems, an OEO converter can act as a transponder, which performs 2R regeneration (“reshaping”, “re-amplifying”) and transmits the client signal to the linear port without changing the transmission protocol.

Optical multiplexers are designed to combine (mix) individual WDM channels into a group signal for their simultaneous transmission over one optical fiber. Optical demultiplexers are designed to separate the received baseband signal at the receiving end. In modern compaction systems, multiplexing and demultiplexing functions are performed by one device - a multiplexer/demultiplexer (MUX/DEMUX).

Optical amplifiers are widely used on long data transmission lines with DWDM wavelength division multiplexing systems.

There are two main types of DCM:

2. Chromatic dispersion compensator based on a Bragg grating - DCM FBG (Dispersion Compensation Module Fiber Bragg Grating). A passive optical device consisting of a chirped fiber and an optical circulator. Due to its structure, chirped fiber creates conditionally negative chromatic dispersion of incoming signals in the wavelength range 1525-1600 nm. The optical circulator in the device acts as a filtering device that directs signals to the appropriate pins.

Due to the fact that optical amplifiers are feedback amplifiers, they always have a function of monitoring the input group signal (the total optical power of all incoming signals) and the outgoing group signal. But this monitoring is inconvenient in the case of monitoring specific communication channels and can be used as evaluative (presence or absence of light). Thus, the only tool for controlling optical power in a data transmission channel is a transponder.

And since compaction systems consist not only of active, but also passive elements, organizing full monitoring in compaction systems is a very non-trivial and in-demand task.

Options for organizing monitoring in WDM compaction systems will be discussed in the next article.

Questions often arise about what is the difference between CWDM (Coarse Wavelength Division Multiplexing) and DWDM (Dense Wavelength Division Multiplexing) technologies, besides the different number of channels. The technologies are similar in the principles of organizing communication channels and input-output channels, but have completely different degrees of technological precision, which significantly affects the parameters of the line and the cost of solutions.

Number of wavelengths and channels CWDM and DWDM

CWDM wavelength division multiplexing technology involves the use of 18 wavelengths 1), while precision wavelength division multiplexing DWDM can use 40 wavelengths or more.

CWDM and DWDM frequency grid

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

CWDM and DWDM wavelengths and frequencies

CWDM technology uses wavelengths from the range 1270 - 1610 nm. Taking into account the tolerances and 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 grid, the carriers are located in the range from 191.5 (1565.50 nm) THz to 196.1 THz (1528.77 nm), i.e. a range of 4.6 THz or 36.73 nm wide. Total 46 wavelengths for 23 duplex channels.

For DWDM with a 50 GHz grid, the signal frequencies are in the range 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

Wavelength division multiplexing systems based on CWDM technology do not involve amplification of a multi-component signal. This is due to the lack of optical amplifiers operating in such a wide spectrum.

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

Operating range CWDM and DWDM

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

DWDM systems allow data transmission 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 found use 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).

Optical fiber has enormous bandwidth. Even twenty years ago, people thought that they would hardly need even a hundredth part of it. However, time passes and the needs for transmitting large volumes of information are growing faster and faster. Technologies such as ATM, IP, SDH (STM-16/64) in the near future may not be able to cope with the “explosive” growth of transmitted information. They were replaced by DWDM technology.

DWDM (Dense Wavelength Division Multiplexing) is a dense wavelength division multiplexing technology. The essence of DWDM technology is that several information channels are transmitted over one optical fiber at different wavelengths, which allows the most efficient use of the fiber's capabilities. This allows you to maximize the throughput of fiber-optic lines without laying new cables or installing new equipment. In addition, working with multiple channels in a fiber is much more convenient than working with different fibers, since a single DWDM multiplexer is required to handle any number of channels.

DWDM systems rely on the ability of optical fiber to simultaneously transmit light of different wavelengths without mutual interference. Each wavelength represents a separate optical channel. Let us first explain the concept of interference.

Light interference is a redistribution of light intensity as a result of the superposition (superposition) of several coherent light waves. This phenomenon is accompanied by alternating maxima and minima of intensity in space.

In the definition of interference there is an important concept of coherence. Light waves are coherent when their phase difference is constant. If the waves overlap in antiphase, the amplitude of the resulting wave is zero. Otherwise, if the waves overlap in the same phase, then the amplitude of the resulting wave will be greater.

At this stage it is important to understand that if two waves have different frequencies they will no longer be coherent. Accordingly, they should not influence each other. Based on this, it becomes clear that we can simultaneously transmit modulated signals with different wavelengths (frequencies) over the same medium and they will not have any influence on each other. This idea is the basis of DWDM technology. Today, DWDM technology makes it possible to transmit channels over a single fiber with a wavelength difference between adjacent channels of just a fraction of a nanometer. Modern equipment DWDM supports dozens of channels, each with a capacity of 2.5 Gbps.

It would seem that if waves of different frequencies do not overlap each other, then an almost infinite number of channels can be introduced into an optical fiber, because the spectrum of light is huge. In theory this is true, but in practice there are certain problems. Firstly, we previously considered a strictly monochromatic wave (one frequency). Achieving such monochromaticity is very difficult, since the light waves are generated by lasers - electronic components that are subject to phenomena such as thermal noise. When generating a light wave, the laser will unknowingly distort the output signal, resulting in slight variations in frequency. Secondly, a monochromatic wave has a spectral width equal to zero. On the graph it can be represented as one single harmonic. In reality, the spectrum of the light signal is different from zero. These issues are worth keeping in mind when we talk about DWDM systems.

The essence of spectral (optical) multiplexing technology is the ability to organize multiple separate client signals (SDH, Ethernet) over one optical fiber. For each individual client signal, the wavelength must be changed. This transformation performed on a DWDM transponder. The output signal from the transponder will correspond to a specific optical channel with its own wavelength. Then, using a multiplexer, the signals are mixed and transmitted to the optical line. At the final point, the reverse operation occurs - using a demultiplexer, the signals are separated from the group signal, the wavelength is changed to a standard one (on the transponder), and transmitted to the client. Because of this, the optical signal tends to fade. In order to amplify it, amplifiers are used on the optical line.

We looked at the operation of the DWDM system in general terms. Next will be a more detailed description of the components of the DWDM system.

The DWDM transponder is a frequency converter that provides an interface between the terminal access equipment and the DWDM line. Initially, the transponder was intended to convert a client signal (optical, electrical) into an optical signal with a wavelength in the range of 1550 nm (typical for DWDM systems). However, over time, the signal regeneration function appeared in transponders. Signal regeneration quickly passed through three stages of development - 1R, 2R, 3R.

1R – relay. Only the amplitude is restored. This limited the length of early DWDM systems, since essentially the remaining parameters (phase, shape) were not restored and the result was “garbage in, garbage out.”
2R – restoration of signal amplitude and duration. These transponders used a Schmidt trigger to clear the signal. Didn't gain much popularity.
3R – restoration of the signal amplitude, its duration and phase. Fully digital device. Capable of recognizing service bytes of the control level of SONET/SDH networks.

A DWDM (multiplexer-transponder) muxponder is a system that time-multiplexes a low-speed signal into a high-speed carrier.

A DWDM (de)multiplexer is a device that, using various wave separation techniques, combines multiple optical signals to transmit signals over an optical fiber and separates these signals after transmission.

Often you want to add and extract only one channel from a composite signal without changing the entire structure of the signal. For this purpose, input/output multiplexers of OADM (Optical Add/Drop Multiplexer) channels are used, which perform this operation without converting the signals of all channels into electrical form.

Erbium-Doped Fiber Amplifiers (EDFA) have revolutionized the telecommunications industry over the past few years. EDFA amplifiers provide direct amplification of optical signals without conversion to electrical signals and vice versa, have a low noise level, and their operating wavelength range almost exactly matches the transparency window of quartz optical fiber. It is thanks to the advent of amplifiers with this combination of qualities that communication lines and networks based on DWDM systems have become economical and attractive.

Attenuators are often installed in the communication line after the optical transmitter, which allow them to reduce their output power to a level corresponding to the capabilities of downstream multiplexers and EDFA amplifiers.

Optical fiber and some components of DWDM systems exhibit chromatic dispersion. The refractive index of the fiber depends on the wavelength of the signal, which leads to a dependence of the speed of signal propagation on the wavelength (material dispersion). Even if the refractive index were independent of wavelength, signals of different wavelengths would still travel with at different speeds due to the intrinsic geometric properties of the fiber (waveguide dispersion). The resulting effect of material and waveguide dispersion is called chromatic dispersion.

Chromatic dispersion causes optical pulses to broaden as they travel along the fiber. If the line is long, this leads to the fact that nearby pulses begin to overlap, worsening the signal. DCD dispersion compensation devices give the signal equal but opposite sign dispersion and restore the original pulse shape.

DWDM systems have many topologies: ring, mesh, linear. Let's consider the most popular ring topology today. The ring topology ensures the survivability of the DWDM network due to redundant paths. In order for any connection to be secure, two paths are established between its endpoints - the main and backup. The endpoint multiplexer compares the two signals and selects the signal best quality(or default signal).

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Recently, modern highways (modern with a capital “C”) have ceased to have enough standard capabilities of compaction systems, both in terms of operating range and the number of simultaneously used channels, and in overall bandwidth systems and expansion options for sealing systems. In Ukraine, DWDM technology has begun to actively enter the network arena, both as a backbone system and as a local densification system.

Not long ago, one of our Ukrainian providers (they asked us not to point fingers, otherwise we would be severely scolded) needed to transfer several dozen “ZhE” over 162 kilometers (over one fiber) with the desire to add several more of the same tens of “ZhE” to this system in the future. . It is clear that you can “grade” in breadth and not be afraid that the lambdas will suddenly end, only with DWDM (well, or a very thick and very black, and also a very long and very multi-core cable). And if we take into account the distance over which a huge number of packets need to be delivered in one hop (without regeneration “in the field”), then choosing DWDM is the only correct and correct decision.

In order to cover such a serious distance in one span, it was decided to design a line that, in addition to standard multiplexers/transceivers/switches, also includes power amplifiers, dispersion compensators and red-blue dividers.

Calculations made when designing the system:

Transceiver sensitivity to dispersion (A-Gear SFP+ DWDM 80LC and A-Gear XFP DWDM 80LC) – 1600 ps/nm;

Path on G.652D fiber, fiber dispersion 17 ps/(nm*km);

The total dispersion indicator on a 162 km track: 17 ps/(nm*km) * 162 km == 2754 ps/nm;

Exceeding the dispersion norm: 2754 ps/nm – 1600 ps/nm == 1154 ps/nm – it was decided to install a dispersion compensator A-Gear DMC-FC120 (completely compensates for the dispersion of 120 km of fiber, total dispersion indicator: -2001 ps/nm at a wavelength of 1545 nm, length fibers in the compensator 12.3 km);

Line loss budget: (162km + 12.3km) * 0.3dBm/km == 52.29dBm;

Optical budget of transceivers (A-Gear SFP+ DWDM 80LC and A-Gear XFP DWDM 80LC) – 26 dBm;

Exceeding the attenuation norm: 52.29 dBm - 26 dBm == 26.29 dBm - it was decided to install the EDFA amplifier A-Gear BA4123 (sensitivity (-10) dBm, maximum output power 23dBm) and A-Gear PA4325 preamplifier (sensitivity (-30)dBm, maximum output power (-5)dBm).

The result was a really working system, stable as the world itself, long-range - not every bird will fly, expandable, and generally the best. A photo of this system is presented below, and even lower we decided to write a short review of the DWDM components that exist today, methods for their inclusion, terminology - we tried to cover everything that is available on DWDM.

The photo shows (from top to bottom): a switch with transceivers, two power amplifiers (booster and preamplifier), a DWDM multiplexer, again a switch with a transceiver and at the very bottom (gray, almost invisible) – a dispersion compensator. This set of equipment is located at point A and point B (they also asked not to name the points, threatening the phone with a thick leather army belt). Having such a relatively small and inexpensive set of equipment, it is easy and simple to shoot 162 kilometers, which was achieved.

On this optimistic note, the introductory part comes to an end, and we begin a methodical analysis of the technology that has become the “main flagship” modern world network engineering.

1. What is DWDM, the differences between DWDM and CWDM.

For those for whom the throughput of CWDM systems is not enough (180 Gbit/s is the extreme maximum), there are two options to satisfy the “traffic appetite”: increase the number of fibers (which is usually associated with diggers, pole climbers and generally the last century) or use a more “advanced” technology seals – DWDM.

DWDM(English: Dense Wavelength Division Multiplexing - dense wavelength multiplexing) is a technology for compressing information flows, in which each primary information flow is transmitted by light beams at different wavelengths, and the optical communication line contains a total group signal formed by a multiplexer from several information flows.

Abstruse. Let's try to figure it out. By analogy with CWDM (for those in the know), DWDM is the same sealing system, physically consisting of devices generating information flow(media converters, routers... well, you know) transceivers (transceivers that create an information flow at different wavelengths of IR radiation invisible to the eye), multiplexers(devices that create/share group light signal) and optical waveguide(fiber optic cable). In addition, DWDM includes a group of components designed to amplify/restore the group light signal, but in order for everything to go consistently, this will be discussed below.

Let’s immediately decide on the words with which we will operate. In this article we will call the channel one way information flow(one side “speaks” the information flow, the other “listens” to this same flow). The channel is located on its only carrier, which has a specifically defined wavelength (or frequency). But, as you know, it is impossible to build a full-fledged connection between a pair of subscribers, one of whom is deaf and the other is mute. Therefore, to create one full-fledged communication line, it is necessary to use two physical channels, and we will call this connection “ full duplex channel».

So, DWDM and CWDM do the same thing - compaction. What's the difference? And the difference is in the frequency grid (or in the wavelengths of the carriers, whichever is more convenient for you) of the carriers of the primary information flows (channels). And in the operating ranges of the group signal itself.

Operating range and frequency (wave) grid. Another obscure word, the meaning of which we will try to understand. What's happened wavelength? Let's imagine a sinusoid. So, wavelength is the distance between two adjacent peaks of a sine wave. Wavelength is usually denoted by the Greek letter λ (lambda). Clearly shown in the figure below:

In the CWDM standard, it is convenient to measure radiation in wavelengths: 1550 nm, 1310 nm, etc. (nanometers – 10 -9 meters!). Convenient, first of all, because the numbers are integers. In standard CWDM systems, the distance between two adjacent carriers (channels) is 1610 – 1590 == 20 nm (also an integer! Well, convenient!).

Now let's look at the same situation from the frequency side, first by understanding what frequency is. Frequency is the number of complete oscillations(peak to peak) electromagnetic wave per second (denoted in Hertz, or Hz). For protozoa For calculations, frequency can be thought of as the speed of light divided by the wavelength. Let's consider the information flow on a 1550nm carrier, its frequency is approximately equal to 300000000/0.00000155 == 193548387096774 Hz, or 193548 GHz (Gigahertz!). and the distance between adjacent carriers will be 300000000/0.00000020 == 1500000000000000 Hz, or 1500000 GHz. It’s completely inconvenient - there are a lot of numbers and it’s unclear.

Today, CWDM systems operate in the 1270nm-1610nm range, representing 18 separate channels (1270nm, 1290nm, 1310nm ... 1590nm, 1610nm). But in DWDM things are a little different.

DWDM systems operate in two bands, cut for CWDM systems, namely: C band (C-Band) and L band (L-Band). RangeC is within from 1528.77nm(channel C61) up to 1577.03nm(channel C01), and rangeL is within from 1577.86nm(channel L100) up to 1622.25nm(channel L48). The numbers are already scary, and if you also take into account the fact that the wave grid is uneven (that is, the distance between two adjacent channels is not always the same - from 0.5 nm to 0.8 nm), then it’s easier to get confused than to figure it out. This is why DWDM systems use the band name and channel numbering in this range (for example, C35 or L91). Everything is clear ordinary DWDM system channels are presented in Figure 1.2, data on frequencies and wavelengths are presented in Table 1.1:

Figure 1.2 – C and L bands of DWDM systems in the general range of CWDM systems.

Table 1.1 is a typical 100 GHz DWDM mesh.

Here we should immediately make several reservations.

Firstly ( and this is important for further understanding! ), range C is conventionally divided into two “color ranges” - blue(1528nm-1543nm) and red(1547nm-1564nm). Why divide - more on this in subsequent articles, now it’s just important to note for yourself that division exists.

Secondly, the L-band is just beginning to be used, and not all manufacturers can afford to make equipment for the L-band (Table 1.1, marked in blue, channels L48-L65 are missing from the table).

Thirdly, the word “ordinary” appears in the caption to the table - which means that there must also be “unusual” grids. And they really are.

As we found out above, it is inconvenient to distinguish DWDM channels by wavelength. But in terms of frequencies - very much so, and if you look closely at Table 1.1, you can see that the difference between two adjacent channels is always 100 GHz. And, if we consider the C band (currently mastered by most manufacturers of DWDM systems), then we can display the total number of channels in it - 61 channels. Let’s immediately make a reservation that, as in CWDM systems, each channel is a one-way information flow, which means that for full data exchange, two of them are needed (30 full-fledged duplex channels in the C band and 26 in the L band, for a total of 56 full-fledged duplex channels).

In addition to the usual 100 GHz grid, they use 200 GHz grid (odd C-band channels). This is due to the fact that a number of DWDM equipment manufacturers are not capable of producing multiplexers for a 100 GHz network, because components for it are quite expensive and should be more High Quality relative to 200 GHz systems. In this compaction scheme there are 31 unidirectional communication channels or 15 full duplex channels.

Very rarely (well, very rarely) DWDM compaction systems with a 50-gigahertz mesh are used. This means that between two adjacent main channels of a conventional 100-GHz mesh there is an additional subchannel. Such channels are called Q and H: Q– subchannels in the rangeL(for example, Q80 – frequency 188050 GHz, wavelength 1594.22 nm), H– subchannels in the rangeC(for example, H23 – frequency 19230 GHz, wavelength 1558.58 nm). In such compaction systems in the C range there are 61 main channels and 61 additional channels, for a total of 122 channels. In the L band there are 53 main and 53 subchannels, for a total of 106 channels. Total power == 122+106 == 228 unidirectional channels, or 114 full duplex communication channels! It's a lot. So many. But it is very, very expensive, and the author has not seen any mention of projects with a fully loaded DWDM system with a 50 GHz grid.

Let's summarize:

- the “light version” of the DWDM system has a 200 GHz grid and is capable of providing 15 full duplex channels in the C band, while leaving room for 15 CWDM channels (1270nm-1510nm, 1590nm, 1610nm);

A standard DWDM system has a 100-GHz grid and is capable of providing 30 full-duplex channels in the C-band and 26 full-duplex channels in the L-band, while also leaving room for 15 CWDM channels (1270nm-1510nm, 1590nm, 1610nm);

The full DWDM system has a 50-GHz grid and is capable of providing 60 full-duplex channels in the C-band and 52 full-duplex channels in the L-band, again leaving room for 15 CWDM channels (1270nm-1510nm, 1590nm, 1610nm);

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