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Is a crisis possible in optical networks?

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Is a crisis possible in optical networks?
Figuratively speaking, today practically every phone call, every sms-sent, every video clip uploaded on youtube is at some moment transformed into elementary particles of light (photons) and carried at breakneck speed (over 200 thousand km/s) maybe even along the bottom of the ocean by superfine glass threads. More than two billion kilometers of such threads are involved today, they can wrap around the globe more than 50, 000 times.
Now if we talk about optical networks in more serious terms, we can note the following. Until recently, the potential of standard single-mode fibers exceeded the bandwidth requirements of optical networks to solve the problem of traffic growth. Compared to trying to develop a brand new fiber platform, there have long been far more cost-effective ways to increase capacity. These methods have long kept pace with the global rate of traffic growth and have been implemented, for example, by simply upgrading terminal equipment to make more efficient use of the available bandwidth. However, times are changing and today’s laboratory experiments involving data transmission over standard single-mode fibers are getting closer to the fundamental bandwidth limits of single-mode fibers. According to information theory, this limit for current fibers is estimated to be around 100-200 Tb/s. This fact causes concern that in the future the network capacity cannot be increased systematically and has been called "capacity crisis" in optical networks. Various symposiums are already being held to discuss the situation and try to find solutions. Let us try to explain further the essence of this fact. For a better understanding, let us first consider as a chronology of growth rates of data transmission speeds on the globe, along with the evolution of the speeds of linear interfaces.
Features of Speed Evolution of Communication Systems and Linear Interfaces

The graph below shows the values of indicators characterizing the capacity of telecommunication systems over the last 500 years. In general, a communication system can be quantified by the value of symbol rate and the amount of information per 1 symbol (number of bits per symbol). The green markers in Fig. 1 show the value of the speed of the communication system in bits/sec at a particular point in time.
Values of indicators describing the capacity of telecommunications systems over the past 500 years.
Fig. 1
Is a crisis possible in optical networks?
These results reflect the historical development of information transmission systems, beginning with early communications systems such as a set of relay towers, which used signals in the form of lighted lights to transmit messages from station to station. Such early communications systems were notoriously slow in transmitting messages, and they usually served to transmit alarms. For the transmission of more complex messages, it was necessary to increase the performance considerably by increasing the information content of each signal.
The crosses in the figure describe the speeds of wireless communication systems, including such prehistoric ones as the Murray optical telegraph and the Schapp Brothers semaphore. As you can see from the figure, there was exponential growth in transmission speeds in that early period, although the average annual growth rate was less than 10%.
In the 19th century electrical signals began to be used to transmit messages, and the telegraph appeared. The growth rate of communication systems increased to about 20% per year. The use of electrical signals made it possible to significantly reduce the cost of operating communication lines through the introduction of electronic amplifiers instead of human-driven relay towers. In the next 50 years transatlantic cables were already constructed and soon the lines of communication covered the entire globe, reaching Adelaide, Australia, in 1872.
Throughout the next century, copper-core communication cables made it possible to increase the capacity of networks, and coaxial cables emerged. Various multiplexing, modulation, etc. schemes began to appear in communication systems. However, the capacity limit for such communication lines was about 1 Gbit/s. In addition, the combination of high attenuation and extremely limited channel bandwidth became a severe constraint for deploying high-capacity systems. A new technology of data transfer was required – FOCL appeared. Further development of networks, I think you can not describe. If you are not familiar with the history of FOCL, but want to know, you can read about it, for example, here.
School history textbooks might have noted that the speeds of commercial line interfaces in optical transport systems have been steadily increasing at about 20% per year since the mid-1980s. As a consequence of these rates, the volume of data transmission over a single fiber in the mid-1990s was already in the order of tens of gigabits, which required the development of new technologies to increase the overall throughput capacity of the networks. The solution to the problem was WDM systems. Today, network traffic is also growing rapidly, at an annual rate of 30 to 90 percent, depending on the type of traffic.
The following interesting points can be noted. During the last decades, the growth of router performance (due to the evolution of microprocessors and other computing devices in accordance with Moore’s law) has coincided with the rate of traffic growth (see Fig. 2). At the same time the growth rate of high-speed optical interfaces was only 20%. Thus we can note the following fact: the growth rate of router performance appeared to be faster compared to the growth rate of their interface speeds by approximately 40-60%.
Over the past decades, the growth of router performance (due to the evolution of microprocessors and other computing devices in accordance with Moore’s Law) has coincided with the rate of increase in traffic.
Fig. 2 – Comparison of speeds of optical systems and speeds of linear interfaces
Is a crisis possible in optical networks?
As can be seen in figure 2, around 2005 the optical line-rate capabilities began to limit the speed growth of router interfaces. The standardization of 100G Ethernet interface and OTN transport module for 100G was not done until 2010, and the standardization of 400G Ethernet interface was not done until 2017. Another consequence, which follows from the disproportionate growth of electrical and optical interfaces, can be highlighted – a future decrease in the level of aggregation of Ethernet streams into a single optical channel. That is, if previously one 100G optical channel included ten 10GE component streams, then with the standardization of the 100G Ethernet interface this level of aggregation will no longer be possible.
Today, the capacity growth rate of WDM transmission systems has slowed from 100% annual growth in the 1990s to only 20% per year. In 2010, commercial networks with WDM systems could support approximately 100 optical channels at 100 Gbps each. This translated into a single fiber capacity of up to 10 Tbps. With an annual traffic growth rate of 40%, we expect commercial systems to support a capacity of about 1 Pbit/s in 2024. This by no means means means that such systems will be 100% utilized by then, which was not the case with the 2010 systems, but there will likely be a commercial need to start installing such systems.
What follows from all the facts presented? Imagine that when you were 6 years old, you could eat one apple and a small bowl of porridge. At 10, we already needed twice as much : two apples plus a portion of porridge doubled in size. At the age of 14 we already eat apples, a bowl of porridge, a bowl of soup plus we need more compote. Our appetite grows exponentially with time, and at 20 we eat like a Sumo wrestler, and at 32, like Robin Bobin Barabek (as in Marshak’s poem). Thus, if in the beginning our appetite was completely traditional and real for an ordinary person, then later multiple rates of growth led us to inconceivable amounts, difficult to realize in practice. The same situation is happening in today’s optical networks.
Continuous improvement in spectral efficiency in existing networks cannot continue indefinitely – there are limitations in the form of fundamental limits to the channel capacity. Limitations are caused both by technological imperfections in transmitters, receiving modules, multiplexers and optical amplifiers (internal noise in optical amplifiers), and by properties of the fiber itself (nonlinear effects). All this leads to various distortions of the signal and, accordingly, to practical limitations of the transmission speed. Recall that the fundamental limitation of the maximum transmission rate in the channel is called the Shannon limit – the physical limitation underlying the "capacity crisis" itself.
Today, the concepts of optical network capacity crisis and the Shannon limit are widely used in the scientific community as justification for the urgent need to develop innovative solutions. However, it should be noted that this term can be perceived and interpreted in different ways. For example, in the oil exploration and production industry, the phrase "capacity crisis" or "capacity crisis" appeared much earlier and implied the approaching depletion of available resources. A common interpretation of this phrase means a limited but still continuous supply of finite resources. In the same context we can refer to the "capacity crisis" as applied to optical networks. That is, in the future, a capacity crisis will mean the continued existence of optical communication networks that support much of modern society, economically, administratively, and socially, but with limitations on the availability of network services.
It can be assumed that the "capacity crisis" may lead to changes in the way providers pay for network services. For example, consumers of services will pay operators for the actual bandwidth used. It can also be assumed that centuries after the creation of networks with higher bandwidth, network resources will finally meet our ultimate desires, and traffic growth rates above 10% will be recorded in history simply as a feature of the late 20th century – first half of the 21st. However, judging by the emergence of more and more information applications, there is insufficient evidence that demand is beginning to saturate. The "build it, and it will come" approach prevails now, i.e. new technologies are created first, and then they become in demand from people. There are other supporters who take the approach of "it is necessary to develop new things as needed. The latter believe that only with this kind of thinking, humanity will develop fully and will not drive itself, ultimately, to a dead end. What approach society, and with it telecommunications networks, should develop is a controversial question, and discussions on this issue are no longer young.
Prepared by Dmitry Kusaykin

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