This document discusses wavelength-division multiplexing (WDM) and how it can be combined with time-division multiplexing (TDM). It explains that WDM separates different channels by assigning each a unique optical wavelength, similar to how frequency-division multiplexing separates channels by frequency. When combining WDM with TDM, fixed time slots are assigned to each wavelength, allowing multiple users to share the bandwidth of a single wavelength. Most optical networks today use this combination of WDM and TDM technologies to efficiently transmit multiple signals simultaneously over one fiber.

1. Submitted by
MURTADHA ALI NSAIF SHUKUR
(M.Tech (ECE))
Second semester (1ST year)

2. WDM(Wavelength-Division Multiplexing)
 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)

3. Con.WDM
 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 below, the small gaps
between each channel represent the spacing. The relation
between frequency and wavelength is ( c = ƛ*f)
Mean
ƛ c f when increasing (ƛ) performance
decreasing (f) and vice versa .

4. Con.WDM

5. (TDMA) Time Division Multiple Access
 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.

6. TDMA

7. 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 below. 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.

8. Combining WDM and TDM

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