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WDM vs. OTN, What's Your Choice?

Updated on Mar 8, 2023 by
3.8k

It's so hard to live without the Internet since it has brought great conveniences to our lives. But the dramatic development of the Internet causes great pressure on network transmission capacity. For example, everyday we use Facebook, Instagram, Twitter, YouTube, Linkedln or TikTok on mobile phones and computers. These daily simple activities need to be supported by a large-capacity transmission system. Otherwise, news, videos and other information can not be accurately transmitted to our mobile phones or computers. As one of the network transmission technologies, optical network is undoubtedly one of the most mature and popular. However, with the increasing needs of high-bandwidth, the transmission capacity of optical network can no longer meet the requirements of rapidly developing Internet information data businesses. Then how to deal with the bandwidth issue? In this large-capacity transmission system, technologies that must be mentioned are WDM and OTN (Optical Transport Network). I'll talk about WDM vs. OTN in the following text.

Overview of WDM and OTN Technology

What Is WDM?

It might be better to understand WDM with an example. In a WDM system, optical fiber is an highway and the light waves for transmitting services are trucks. As to different transmission signals such as news, blogs or vedios are like packages to be transported. These packages are directly placed on different trucks. If all these trucks want to enter the optical fiber transmission regardless of the lanes, then crowding in will cause chaotic traffic flow on the entire expressway and affect transmission efficiency. Is there any way to avoid this phenomenon? The answer is WDM. With WDM, different transmission data can be realized simultaneously on the same optical fiber, which is equivalent to dividing lanes for different vehicles on the highway, allowing different vehicles to run on separate lanes at the same time, improving transmission efficiency.

WDM

Meanwhile, 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 on streets, the WDM system can be divided into two types: CWDM (Coarse Wavelength Division Multiplexing) and DWDM (Dense Wavelength Division Multiplexing) . The former has a relatively large lane interval (wavelength interval), generally 20 nm, while the latter has a small interval, generally less than 0.8 nm. You can read this article to know more about WDM technology: WDM Basics: Understanding Wavelength Division Multiplexing Technology

How Does a WDM System Work?

In a WDM system, can different signals to be transmitted on a single fiber at the same time as long as the lanes are divided? No. So how does WDM work? Techniquely, a WDM system generally consists of OTU (Optical Transponder Unit), Mux/Demux unit, monitoring channel, and optical amplifiers unit. Then, how do these parts work together to complete the signal transmission?

How Does a WDM System Work

First, services to be transmitted need to to be sent to WDM's dedicated vehicles (i.e. OTU units). Then these signals would be converted into standard wavelength optical signals which can be recognized by WDM.

Second, the standard wavelength optical signal vehicle with services drives to the checkpoint (i.e. Mux unit), is arranged to enter different lanes through the checkpoint, and drives on the highway one by one at the same time.

Third, the patrol car (i.e. the monitoring channel) supervises the driving condition of the vehicle to ensure normal transmission.

For long-distance transmission, it is necessary for vehicles driving into the unified gas station (i.e. optical amplifier unit) to regenerate and amplify the signals to ensure no damage.

Finally, when signals reach the terminal station, the vehicle will come out of the checkpoint (i.e. Demux unit) and be shunted to the corresponding receiving client terminal exit, where the service is unloaded from the vehicle, converted into a client service signal (i.e. a service signal without wavelength information) through the OTU and delivered to the client.

WDM Technology Disadvantages

From the above brief introduction, it is not difficult to find that the biggest advantage of WDM technology is that it makes good use of optical fiber resources and can provide large-capacity data transmission. But it also has the following obvious disadvantages:

  • If the business "package" on the WDM vehicle makes a mistake during transmission, there is no way to identify it. That is, the WDM system is weak in monitoring and managing the operation and maintenance of the service.

  • If a service is transmitted on a designated channel of the WDM system, the channel cannot be used by other services, which will cause a waste of resources. For example, each vehicle takes a fixed lane on an highway. Although the lane is free, other types of vehicles are not allowed to take this lane.

OTN Technology

With the development of communication networks, digital traffic has increased rapidly. So experts had to continue to develop the potential of WDM and improve its capabilities. A new technology was born - OTN.

OTN is a telecommunications industry standard protocol, defined in various ITU Recommendations, such as G.709 and G.798—that offers an efficient way to transport, switch, and multiplex different services onto high-capacity wavelengths across the optical network. The OTN technology provides a network-wide framework that adds SONET/SDH-like features to WDM equipment. To find out more information about OTN, you can read this article: What Is OTN—Optical Transport Network?

Is OTN Superior to WDM?

As mentioned above, WDM system is similar to highway traffic system, while OTN is an upgraded version of highway traffic system. Is OTN technology superior to WDM? What kinds of advantages are OTN over WDM?

 WDM and OTN system comparison diagram

  WDM and OTN system comparison diagram

OTN Support monitoring, management, operation and maintenance

From the above figure, it can be seen that signals are converted into services with wavelength information before being transmitted into the WDM system. In other words, the WDM system has no supervision mechanism for transmitted services. It only guarantees that the signals can be transmitted to the receiving end.

While in the OTN system, a set of rules is provided for signals, which is also known as the frame structure requirement. Signals entering the OTN system are first packaged according to the OTN frame structure requirements, that is adding information on monitoring, management, operation and maintenance, and then transmitting the converted signals with wavelength in the OTN system.

OTN Support Electrical Crossover Function

By adding the electrical crossover function, OTN system can process client service signals and WDM signals separately.

OTN Support Electrical Crossover Function

We have mentioned that the WDM system can transmit client services but the premise is to convert the client service signals into WDM signals at first. Traditional WDM system directly realizes the convertion through the same single board, corresponding to occupying a light wave bearer for each client service. When there are more and more types of client 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 signals, which will increase the cost of network construction; on the other hand, these services will also occupy more light waves, leading to resource constraints. Therefore, the OTN system introduces the electrical crossover function, which is like adding a cargo dispatch center to the traditional WDM system. The cargo dispatching center packs and dispatches different cargoes (i.e. different services) entering the OTN system into different vehicles (i.e. carried by different light waves).

OTN system

The advantage of the cargo dispatch center is that if the network adds new client service, it only needs to add client-side boards for accessing new services, and borrow the existing board transport service on the line side, saving network construction costs. At the same time, when a truck in a certain lane is free, it can load client service to the truck at any time through the cargo dispatch center, avoiding the waste of resources caused by the truck running empty on the lane.

WDM vs. OTN Difference

Networking Mode

The networking mode of WDM is basically point-to-point, chain, star and ring networking. The networking mode of OTN is usually a ring network, mesh network and other networking modes.

WDM multiplexes optical signals of different wavelengths into one fiber for transmission. There are two modes: DWDM and CWDM. Currently, DWDM is the mainstream. the mainstream WDM systems are 40-wavelength and 80-wavelength systems (which are being converted to more than 96-wavelength and 120-wavelength systems) that carry more wavelengths. OTN is asynchronous mapping of payloads.

Components

WDM mainly contain multiplexer, demultiplexer, amplifier and ROADM. While OTN, composed of optical cross-connector (OXC) and optical add/drop multiplexer (OADM), possesses functions like optical cross-ability and wavelength conversion.

Application Scenarios

WDM is mainly used in inter provincial trunk network and intra provincial backbone network. OTN provides a very ideal solution for large broadband granular services.

Comparing WDM vs. OTN technology, you can find that OTN increases O&M rules by adding the frame structure to improve the monitoring, management, operation and maintenance capability of services. To sum up, OTN is actually an optimization of WDM.

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