A Brief Introduction to WDM and TDM
We would like to keep this introductory chapter free from technical detail as much as possible. However, it is necessary to introduce two terms before proceeding further: (a) wave division multiplexing (WDM), and (b) time division multiplexing (TDM). Later chapters will embellish on this introduction. Refer to Figure 13 during this discussion.
Figure 13 WDM and TDM.
WDM is based on a well-known concept called frequency division multiplexing or FDM. With this technology, the bandwidth of a channel (its frequency domain) is divided into multiple channels, and each channel occupies a part of the larger frequency spectrum. In WDM networks, each channel is called a wavelength. This name is used because each channel operates at a different frequency and at a different optical wavelength (and the higher the frequency, the shorter the signal's wavelength). A common shorthand notation for wavelength is the Greek symbol lambda, shown as λ.
The wavelengths on the fiber are separated by unused spectrum. This practice keeps the wavelengths separated from each other and helps prevent their interfering with each other. This idea is called channel spacing, or simply spacing. It is similar to the idea of guardbands used in electrical systems. In Figure 13, the small gaps between each channel represent the spacing.
Time division multiplexing (TDM) provides a user the full channel capacity but divides the channel usage into time slots. Each user is given a slot and the slots are rotated among the users. A pure TDM system cyclically scans the input signals (incoming traffic) from the multiple incoming data sources (communications links, for example). Bits, bytes, or blocks of data are separated and interleaved together into slots on a single high-speed communications line.
Combining WDM and TDM
Most optical networks (or, for that matter, most networks in general) use a combination of WDM and TDM by time-division multiplexing fixed slots onto a specific wavelength, as shown in Figure 14. This concept is quite valuable because it allows multiple users to share one WDM wavelength's capacity. With some exceptions, the capacity of one wavelength exceeds an individual user's traffic capacity needs.
Figure 14 Combining WDM and TDM.
These introductory definitions should be sufficient for us to use them in this chapter. In later chapters, TDM and WDM are examined in considerable detail.