Concise Guide to DWDM

Chapter 1- Evolution of Optical Network to DWDM

Fiber optics as a transmission technology came about in the 1980s when it became economically feasible to mass produce fiber optic cable. Advances in laser and semiconductor technologies compounded the growth and within decades fiber optic based transmission systems were the bedrock of long haul high capacity data transmission.

In the nineties and 2000s TDM (time division multiplexing), FDM (frequency division multiplexing) and WDM (wavelength division multiplexing) were the three technologies proposed to resolve the chronic capacity crisis. Demand for bandwidth was growing exponentially and the existing transmission technologies were failing to keep up let alone inspire confidence that they could scale to meet future requirements. The problem was that although SDH, a TDM based technology, was widely deployed and was capable of aggregating client data input to meet 10 GBPS or more of transport, there were inherent restrictions. With TDM each client is assigned a timeslot thus making the transmission shared by all the users. In effect SDH/Sonet takes asynchronous or synchronous electrical signals and multiplexes them onto a single optical bit rate. This however requires optical-to- electrical or optical-to-electrical-to-optical (OEO) signal conversion. Furthermore, the optical technology for transporting the data provided a theoretical available bandwidth that could exceed several Terahertz (10 to the power of 12).  The problem was though that TDM could not be made to work to take advantage of this vast bandwidth because the electrical signaling it required simply cannot work at those frequencies. The same was true for FDM systems it just wasn't possible to use frequency multiplexing, at the electrical level, at these high end frequencies. WDM on the other hand takes multiple optical signals and multiplexes them onto a single fiber and there is no signal conversion. Therefore being purely optical DWM was able to scale to use the full optical bandwidth potential.

The solution however was to use frequency division but at the optical wavelength level by using a technique called wavelength division multiplexing (WDM). The idea being, that as light consists of many  differing wavelengths or colors these could individually be isolated and used as optical carriers to carry data across the same fiber. Theoretically each wavelength could carry a signal of unlimited bandwidth, furthermore these individual wavelengths could them be multiplexed across a single fiber providing potentially limitless bandwidth. For example, if one wavelength carried 10Gbpsps, a standard SDM bandwidth of OC-192/STM-64, then it was already matching SDH’s capacity and it was doing so over a single fiber. Additionally if more channels were utilized, let’s say 16 wavelengths were used, then the line capacity was now 160GBPS, and this was transmitted over one single fiber.

The diagram below shows the comparison of an SDH network with a DWDM system and how much less equipment and lines are required the whole topology has been greatly simplified.

CWDM can make use of any of the frequency bands across the useable spectrum, however DWDM is constraint to bands C and L.

The diagram below shows the grid for DWDM frequency wavelengths with a 100 GHz grid with a 0.8 per channel guard between frequencies. CDWM uses a fixed channel separation of 20nm which means it can support a much lower channel density than DWDM.

At this point, we must make a distinction between WDM and what we will come to know as DWDM. The primary difference being that WDM is generally used to describe the parent technology that used only a few channels of the available spectrum and had larger spacing between channels. In fact, there are now two distinct types of WDM:

CDWM (course division wavelength multiplexing) - CWDM systems typically use fewer than eight active wavelengths per fiber and operate over short distances, (in practice CDWM uses 1 to 16 channels and operates over hundreds of meters)

DWDM (Dense Wavelength Division Multiplexing) - DWDM systems operate with more than eight active wavelengths per fiber. (in practice this is now 40, 80 or 160 channels)

In the following paragraphs, we will briefly discuss the merits of each. However our focus in this book is on DWDM.

CWDM Vs DWDM

Currently, there are two types of WDM in existence today: Coarse WDM (CWDM) and Dense WDM (DWDM). Backwards as it may seem, DWDM came well before CWDM, which appeared only after a booming telecommunications market drove prices to affordable lows. Whereas, CWDM breaks the spectrum into big chunks, DWDM dices it finely. DWDM fits 40/80/160 channels into the same frequency range used for 16 CWDM channels.

CWDM is defined by wavelengths. DWDM is defined in terms of frequencies. DWDM’s tighter wavelength spacing fits more channels onto a single fiber, but cost more to implement and operate.

––––––––

Distinctive CWDM differences

CWDM is often considered to be the