Shulou(Shulou.com)11/24 Report--

It can be said that the use of light for information transmission has a long history. In ancient times, the "beacon fire opera princes" have made people experience the convenience of transmitting information through light. However, this original optical communication mode is relatively backward and its reliability is not strong. The development of social information transmission has promoted the birth of modern optical communication.

The beginning of modern optical communication in 1880, Bell invented the first optical telephone system with a distance of 213 meters. This marks the opening of modern optical communication. So how is it possible to make a phone call?

1. Using sunlight or arc lamp as the light source, the light beam is focused on the vibrator of the microphone after passing through the lens.

two。 As people speak, the light reflected by the vibrator will change in intensity, thus carrying the voice information on the light wave.

3. The receiver focuses the light signal on the silicon photocell through a paraboloid and converts the current change of the photocell into a sound signal.

4. The sound signal is sent to the receiver and the sound from the sender can be heard.

Because the optical telephone does not have a reliable, high-intensity light source, nor a stable, low-loss transmission medium, the transmission loss of light in the atmosphere is too large, long-distance communication is difficult, and even unable to communicate when there is a shield.

In order to solve the problems faced by optical telephone, scientists make unremitting experimental research.

In 1966, British Chinese Charles Kao put forward the theory of optical fiber transmission, but at that time the optical fiber loss was as high as 3000 dB / km.

In 1970, the research and development of quartz fiber and semiconductor laser technology reduced the fiber loss to 20 dB / km, and the laser intensity was high and the reliability was strong.

In 1976, with the continuous development of optical fiber technology, the loss has been reduced to 0.47 dB / km, which means that the loss of transmission media has been solved.

After solving the problems of optical fiber and laser, PDH technology promotes the optical transmission network to enter a stage of rapid development.

PDH as early as 1972, CCITT (International Telecommunication Consulting body), the predecessor of ITU-T (International Telecommunication Union Telecommunication Standards Branch), put forward the first batch of PDH (Plesiochronous Digital Hierarchy) suggestions.

PDH is the transmission technology originally adopted in the transmission network, but PDH does not have worldwide electrical and optical interface standards, so it is impossible to directly separate or insert low-speed signals into high-speed signals, which leads to the tedious and inefficient processing flow of separating and inserting low-speed services in high-speed signals.

In the transmission of business, PDH is like a bulk train transported by railway. the goods are cluttered in the carriage. if you want to unload a specific cargo at a certain station, you need to unload all the goods first, find the goods you need, and then load the remaining goods and the goods that need to be newly loaded at the station onto the train and transport them away. In this way, every time you separate or insert the goods, you have to turn the cabinets upside down!

In SDH1988, the International Telecommunications Organization adopted the first SDH (Synchronous Digital Hierarchy, synchronous Digital hierarchy) recommendations. After 1990, SDH has become the basic transmission mode of optical fiber communication.

SDH defines a standard service data structure, which solves the problem that there is no standard interface in PDH, so that the position of low-speed SDH signal in high-speed SDH signal is regular and easy to query, and the low-speed signal can be separated / inserted directly in high-speed signal.

In the transport business, SDH is like a container train, all kinds of goods are labeled and loaded into containers. Then a number of small cases are packed into large cases, one level at a time, and each time a label that can identify the goods is affixed before packing. In this way, through the label query, we can accurately know which box a package of goods is in which container, and we can quickly find the target goods on the train and take them out directly. There is no need to rummage around any more, the efficiency has been greatly improved.

At the beginning of the MSTP21 century, with the rapid development of IP services such as short message, MMS, e-commerce, real-time video and so on, MSTP (Multi-Service Transport Platform, multi-service transmission platform) based on SDH was born in 2002.

PDH and SDH are mainly voice transmission services, but MSTP can transmit images, video and other larger capacity services. Because the implementation of MSTP is based on "SDH + Ethernet + ATM", the full name of ATM is Asynchronous Transfer Mode, that is, asynchronous transmission mode, which supports the transmission of voice, data, image, video and so on.

MSTP delivers business in the same way as SDH, by packing and labeling the goods and putting them into standard-sized containers for transmission. It's just that MSTP has more Ethernet and ATM interfaces, so more types of goods can be transmitted.

WDM as early as 1990s, some researchers put forward the concept of WDM (Wavelength Division Multiplexing, wavelength Division Multiplexing). It was not until the late 1990s or even the 21st century that WDM was widely built and used. WDM solves the previous problem of wasting PDH, SDH, and MSTP resources.

In the era of PDH / SDH / MSTP, an optical fiber has only one wavelength, time division multiplexing between different services, services occupy a fixed time period, if there is no service in this time period, it will be wasted. It is equivalent to a highway with only one lane, which is then leased to a freight company over a period of time, regardless of whether the freight company carries goods or not. Then when the freight company does not transport goods, it results in a waste of resources.

To promote environmental protection and save resources, WDM came into being.

In the WDM era, an optical fiber can transmit multiple wavelengths, which is equivalent to the highway can provide multiple lanes, the overall bandwidth or service carrying capacity has been improved. However, each lane may still be provided to PDH / SDH / MSTP, that is, each lane is still leased to freight companies over a period of time, but there are more lanes and multiple goods can be transported at the same time.

According to the lane spacing, WDM can be divided into two categories:

CWDM: sparse wavelength division multiplexing. The distance between lanes is larger, usually 20 nm, and less lanes are divided.

DWDM: dense wavelength division multiplexing. The lane spacing is smaller, less than or equal to 0.8 nm, and more lanes are divided. DWDM is the most widely used transmission method in the future.

In the OTN21 century, digital television, remote conference, network live broadcast and other services have sprung up everywhere, these emerging services have higher requirements for the bandwidth and reliability of the transmission network. Compared with WDM technology, OTN (Optical Transport Network, Optical Transport Network) network can provide larger bandwidth and more reliable transmission.

WDM, especially DWDM technology, can easily multiply the transmission bandwidth by dividing multiple channels. However, when WDM continues to expand its bandwidth, it ignores the need to synchronously strengthen its supervision, resulting in the inflexibility of WDM scheduling in transmission.

For example, if a cargo is to be transported from Chengdu to Shenzhen, the pre-allocated lane is the eighth lane (wave 8), then the whole journey from Chengdu to Shenzhen is the eighth lane, even if the eighth lane is blocked for some reason. Unless you pass through the expressway (optical regeneration section), such as Chengdu-Guangzhou and Guangzhou-Shenzhen, you can have a chance to change lanes in Guangzhou, and the price of changing lanes is to build a special road (laying optical fiber) for your behavior this time. However, when SDH encounters this kind of situation, it is fine to build a dispatching center in Guangzhou and change to the desired lane at the dispatching center.

Therefore, the comprehensive consideration of WDM and SDH, learning from each other to offset their weaknesses, trying to give full play to the advantages of both, and then the birth of OTN.

OTN is based on the large-capacity transmission of WDM, draws lessons from the powerful supervision function of SDH (OAM function), realizes the large-capacity transmission of flexible scheduling, and also has the perfect protection mechanism of SDH.

The rise of new services in the PTN/IPRAN21 century, especially the rapid development of IP services, has given birth to two efficient ways to transmit IP data, PTN and IPRAN, which solve the problems faced by MSTP.

The original purpose of MSTP is to solve the problem of carrying IP business in the transmission network, but its improvement is not complete, because MSTP only provides several fixed-size boxes and transports them according to the largest boxes they need. There will be large boxes containing small-size goods, and a fixed-size box will be sent regardless of whether there are goods or not. Equivalent to a freight company contracted a train car, how much goods will be sent, at most a full car, there is no goods on the empty car. Obviously, this has caused two major wastes: large boxes for small-size goods, and transportation of empty boxes.

For example, if you want to express a Qixi Festival gift, a lipstick, a box of chocolates, or a puppet to your girlfriend, they may be packed in the same size box, so the box containing lipstick and chocolate wastes a lot of space.

PTN / IPRAN technology can just solve the waste problem of MSTP. PTN / IPRAN is equivalent to all freight companies sharing all the carriages of the train, sending as many goods as there are, as long as the total volume of goods does not exceed the capacity of the whole train. At the same time, when packing the goods, the boxes of the corresponding size can be customized according to the size of the goods. Then in the transportation, the corresponding size of the packing box will be customized according to the size of lipstick, chocolate and puppet, so that the transport space can be used to the maximum extent.

The main difference between IPRAN and PTN lies in the way goods are labelled. It can be understood that PTN is like the traditional manual way to fill out express orders, while IPRAN can print express orders automatically through the machine.

Summary and prospect of traditional optical transmission technology is mainly these, let's review the history of optical transmission.

We can see clearly that the evolution direction of optical transmission technology is: increasing bandwidth, lower and lower delay, supporting more and more types of services. At present, we are about to enter a new 5G era. Through the introduction of FlexE, FlexO, SR, IPv6, ROADM, high-precision synchronization, network slicing and other new 5G technologies, the transmission network provides ultra-large bandwidth, ultra-low delay and flexible interconnection, thus strongly supporting the realization of the three application scenarios of eMBB, URLLC and mMTC, and bringing us a better tomorrow!

This article comes from the official account of Wechat: ZTE documents (ID:ztedoc)

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