SONET, DWDM, and CWDM – CompTIA Network+ N10-006 – 1.4

Carrier networks are all about large bandwidths over large distances. In this video, you’ll learn about SONET and WDM networks.

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If you’re going to be working on a carrier network or working with a carrier in their network, then you’re going to be involved with a SONET network. SONET stands for Synchronous Optical Networking. And it’s a way of multiplexing many different kinds of signals on to the same optical media. What’s interesting about these circuits is that they are all synchronized to the same atomic clock, which means that it minimizes the amount of buffering that’s going back and forth making for a very efficient way of communication.

SONET is standardized through the American National Standards Institute. It is an ANSI standard. And you’ll commonly see SONET used in the United States and Canada. There’s a very similar standard called SDH that stands for Synchronous Digital Hierarchy. And it’s a standard coming from the International Telecommunications Union.

You’ll see SDH used everywhere except the United States and Canada. These two standards are so similar to each other that they could practically be the same thing, but there’s some minor differences between those two standards. But when we refer to SONET, we are generally also referring to SDH at the same time.

The speeds of a SONET network are calculated and described as Synchronous Transport Signals or STS. You might also see this referred to as optical carrier numbers as well. The speeds for an SDH network are described as Synchronous Transport Modules or STM. And to confuse things even further, it’s a different numbering mechanism for STS than you might use for STM.

If we just look at SONET, something like OC-1, that first value of OC-1 is 51.84 megabits. This is useful, because as we use other terms like OC-3, we’re multiplying that base bandwidth of 51.84 by 3 to get 155.52. So you can take any number like OC-48, and that is 48 times an OC-1. So it may be easier to calculate if you happen to remember that an OC-1 is 51.84 megabits.

STM works a little bit differently. The STM-1 is 155.52 megabits, which is also the same thing as OC-3. But as long as you know that STM-1 is 155.52 megabits, you can then multiply up to determine what an STM-4 or an STM-16 is by multiplying by four or by 16 to get the bandwidth value. Both of these very similar of course in nature, but as you can see the numbering mechanism is very different between the two.

Another way of multiplexing traffic across a carrier network is by using WDM. It stands for Wavelength-Division Multiplexing where we’re sending multiple carriers over a single fiber. But in this particular case, we’re separating out those carriers by different wavelengths. And of course, in an optical world, those different wavelengths are effectively a different color, or you can certainly think of it in that fashion.

CWDM stands for Coarse Wavelength-Division Multiplexing. This is also a standard called 10BASE-LX4. It uses four separate carriers. And each carrier is 3.125 gigabits per second in bandwidth. So you’ve got four different wavelengths that are going over the Coarse Wavelength-Division Multiplexing.

You can also see a type of multiplexing called DWDM or Dense Wavelength-Division Multiplexing. And here, when we say dense, we really mean very dense. This is taking in many different optical carriers into a single fiber. So there can be 160 signals going across that same fiber, which would make you have a total of 1.6 terabits per second of bandwidth. If you’re a carrier and you’re trying to maximize the amount of traffic that you can put through a single pair of fibers, you’re probably going to want to look at one of these flavors of WDM.