Telco Networks – SDH/SONET and DWDM


 Wednesday November 2, 2016  

Recently, I have been required to look into options for connecting data centres between states. There are two main technologies that our provider can offer us; SDH and DWDM.

I’m not in the telco space, so I’m vaguely aware that they both exist, but I really don’t know much about either. If you’re in the campus or data centre networking space, then like me you probably have to get a connection from site-A to site-B over ethernet, and probably know little more of the underlying technology than that.

This blog will have a brief look at the two technologies, and how they can help us get our frames and packets from site-A to site-B.

Optical Networking

Both SDH and DWDM are telco products that use optical fibre. Most of us won’t see the underlying technology that makes it work. Your provider will quote you a price, and give you an ethernet hand-off at either site. You’re then free to do what you do best; the switching and routing.

Underneath the hood though, the provider has thousands of fibre runs across the country. Each of these fibre strands can carry multiple conversations or bit-streams, often traditional voice as well as digital data.

Both of these technologies are delivered to you in a point-to-point fashion (or site-to-site if you prefer). This is different to IP based networks such as frame relay or MPLS, which operate at a higher layer, which have a more cloud-like approach. When you use these links, you are paying for a very low-level circuit, not an IP based WAN.


SONET (Synchronous Optical Network) and SDH (Synchronous Digital Hierarchy) are essentially the same technology. The main difference is that SONET is deployed in North America, and SDH is deployed in the rest of the world.

SDH goes back to the early 1990’s, and was originally used for transmitting multiple phone calls (encoded in PCM) over a single strand of fibre. Because of these origins, it is protocol independant, meaning that it can carry ATM, ethernet and others.

There are various speeds supported, and offerings often start at relatively low values, such as 10Mbps. Providers may be able to offer fractions of a ‘circuit’ to a customer, so bandwidths available will vary.


DWDM (Dense Wavelength Division Multiplexing) is a newer technology, and works at an even lower layer than SDH. DWDM is essentially a method of increasing the bandwidth available over existing fibre. This makes it cost-effective, as DWDM can be used instead of laying more fibre strands.

This goal is achieved by using different wavelengths (or colours of visible light) to create multiple ‘virtual’ fibre strands over a single physical fibre strand. And you thought virtualization was only in the server rack…

Originally, DWDM was meant to provide a cost-efffective means of addressing long distance fibre runs. A run of 1500km (932mi) will run at 10G. Longer runs of up to 4500km (2796mi) are also possible. To put this in perspective, Australia is 4042km (2512mi) wide, and the USA is 4343km (2680mi) wide. That means it is possible to have a single DWDM run across either country!

Metro deployments have made good use of DWDM too. Due to the dense multiplexing of wavelengths, it has been possible to provide more bandwidth over existing fibre, which addresses the fibre exhaustion issue in some areas.


Both of these services can provided dedicated and secure bandwidth to the customer. DWDM is the most cost-effective for long-haul networks, but has the down side of being offered at higher speeds only (it’s not uncommon for providers to start their offerings at a minimum of 1G).

In the end, for those of us in the enterprise networking space, it comes down to three main questions that your provider has to answer:

  1. What does it cost?
  2. What are the SLA’s?
  3. How will it be handed off?

To get some more information on these technologies, have a look at these websites:

Telecom Transmission Made Simple

The Relation and Difference between SONET/SDH and DWDM