Do you understand network large-capacity transmission technology - WDM/OTN?
383 2023-03-09
We take mobile phones and computers every day, and we can smoothly browse Weibo, watch videos, and chat. These daily simple activities require a large-capacity transportation system to support them. Otherwise, Weibo, video, and WeChat information cannot be accurately delivered to your mobile phone or computer display. In this large-capacity transportation system, a technology that has to be mentioned is WDM/OTN.
What is WDM?
If the optical fiber is compared to a highway, the light waves used to transmit services in the WDM system are compared to trucks, and different transmission services such as Weibo, video, and WhatsApp are equivalent to packages to be transported, and these packages are directly placed on different trucks. If these trucks want to enter the optical fiber transmission regardless of the lanes, then crowding in will cause chaos and disorder in the entire expressway traffic flow and affect the transmission efficiency. With WDM, different transmission services can be transmitted simultaneously on the same optical fiber, which is equivalent to dividing lanes for different vehicles on the expressway, allowing different vehicles to run on different lanes at the same time, improving transmission efficiency.
At the same time, to ensure smooth traffic, it is necessary to distinguish lanes so that different vehicles can go their own way. Similar to the division of large and small lanes in highway traffic, there are two types of lane division in the WDM system: CWDM (Coarse Wavelength Division Multiplexing, sparse wavelength division multiplexing) and DWDM (Dense Wavelength Division Multiplexing, dense wavelength division multiplexing) , the former has a relatively large lane interval (ie wavelength interval), generally 20 nm, while the latter has a small interval, generally less than 0.8 nm.
What does a WDM system consist of?
Does WDM allow different services to be transmitted on one optical fiber at the same time as long as the lanes are divided? Things are not that simple, let‘s take a look at how WDM does it!
In technical terms, a WDM system generally consists of an OTU (Optical Transponder Unit, Optical Transponder Unit), a wavelength combining/demultiplexing unit, a monitoring channel, and an optical amplification unit.
Then, how do the various parts of WDM work together to complete service transmission?
To transmit services in WDM, it is first necessary to send the services to WDM‘s dedicated vehicle (that is, OTU unit), and convert these service signals into standard wavelength optical signals recognized by WDM.
The standard wave-optical signal vehicle carrying the business drives to the checkpoint (that is, the multiplexing unit), arranges to enter different lanes through the checkpoint, and drives on the expressway one by one at the same time.
The driving status of the vehicle needs to be supervised through the patrol car, that is, the monitoring channel, to ensure the normal business transmission.
If the transportation distance is long, it is also necessary for the vehicle to drive into the unified gas station, that is, through the optical amplifier unit, the business signal is regenerated and amplified to ensure that the long-distance business transportation is not damaged.
When the business is transported to the terminal station, the vehicle comes out of the inspection station (that is, the wavelength splitting unit) and is shunted to the corresponding receiving customer terminal exit. The business is unloaded from the vehicle, that is, converted into customer service signals through the OTU unit (that is, no wavelength information business signal), sent to the customer.
From the above brief introduction, it is not difficult to conclude that the biggest advantage of WDM technology is that it makes good use of optical fiber resources and can provide large-capacity data transmission capabilities. But WDM has the following obvious disadvantages:
If the business "package" on the WDM vehicle is wrong during transportation, there is no way to identify it. That is, the WDM system has weak monitoring, management, operation and maintenance capabilities for services.
If a service is transmitted on a designated channel of the WDM system, the channel cannot be used by other services, which will cause waste of resources. For example, on a highway, each type of vehicle has a fixed lane. If the lane is free, other types of vehicles are not allowed to use this lane.
What is OTN?
With the development of communication networks, the amount of data on the data network has increased rapidly, and experts have to continue to develop the potential of WDM and improve the ability of WDM, so a new technology was born - OTN.
As mentioned above, the WDM system is similar to the highway traffic system, while the OTN is an upgraded version of the highway traffic system. Its upgrade functions are mainly manifested in the following two aspects:
1)Add operation and maintenance rules. The specific measure is to increase the frame structure and improve the monitoring, management and operation and maintenance capabilities of the business.
From the above simplified comparison diagram between WDM and OTN systems, it can be seen that in the WDM system, only services without wavelength information entering the WDM system are converted into services with wavelength information, which are transmitted in the WDM system. That is to say, the WDM system has no supervision mechanism for the transmitted services, and only guarantees that the services can be transmitted to the receiving end. In the OTN system, a set of rules for placing services in the OTN system is provided, that is, the so-called frame structure requirements. Services entering the OTN system are first packaged according to the OTN frame structure requirements, that is, information for monitoring, management, operation and maintenance is added, and then converted into services with wavelength information and sent to the OTN system for transmission.
2)The electrical crossover function is added so that the OTN system can process customer service signals and WDM signals separately.
When we talked about "What is the composition of the WDM system?", it was mentioned that the WDM system can transmit customer services. First, the customer service signals need to be converted into WDM signals. When the traditional WDM system processes this function, it directly passes through the same single board. Realization, corresponding to each customer service needs to occupy a light wave bearer. When there are more and more types of customer services on the network, in order for these services to be transmitted in the WDM system, on the one hand, it is necessary to develop new boards to carry these services, which will increase the cost of network construction; on the other hand, these services will also occupy more light waves, causing resource strain. Therefore, the OTN system introduces the electrical crossover function, which is like adding a cargo dispatch center to the traditional WDM transportation system. The cargo dispatching center packs and dispatches different cargoes (that is, different services) entering the OTN transportation system into different vehicles (that is, uses different optical waves to carry them).
The advantage of the cargo dispatch center is that if the network adds new customer services, it only needs to add customer-side boards for accessing new services, and borrow the existing board transportation services on the line side, saving network construction costs. At the same time, when a truck in a certain lane is free, the cargo dispatch center can be used to load customer business on the truck at any time, so as to avoid the waste of resources caused by the truck running empty on the lane.
To sum up, OTN is actually an optimization of WDM, which further improves the operation and maintenance capability and flexible resource scheduling capability of the WDM system.
In short, WDM/OTN technology is serving as a large-capacity transportation system for data networks in today‘s rapidly expanding information data volume, and is constantly delivering these information data "goods" with higher reliability, higher flexible scheduling capabilities, and higher resource utilization.