video stream is introduced into the inverse multiplexer, the inverse mux splits that up into 24 64Kbps channels, and each of these 24 channels occupies a separate channel on an existing T-1/E-1 facility or PRI ISDN. (PRI ISDN is discussed in Chapter 2, "Traditional Transmission Media.") The channels are carried across the network separately. At the destination point, a complementary inverse mux reaggregates, resynchronizes, and reproduces that high-bandwidth signal SO that it can be projected on the destination video monitor. Inverse multiplexing therefore allows you to experience a bit of elastic band- width. You can allocate existing capacity to a high-bandwidth application without having to subscribe to a separate link just for that purpose. WDM, DWDM, and CWDM WDM was specifically developed for use with fiber optics. In the past, we could use only a fraction of the available bandwidth of a fiber-optic system. This was mainly because we had to convert the optical pulses into electrical signals to regenerate Multiplexing 1.5Mbps 1.5Mbps Inverse T-1/E-1 T-1/E-1 Inverse Multiplexer Digital Multiplexer N-ISDN Dialup N-ISDN Switched Services Switched 56/64Kbps 56/64Kbps Figure 1.18 Inverse multiplexing them as they moved through the fiber network. As optical signals travel through fiber, the impurities in the fiber absorb the signal's strength. Because of this, some- where before the original signal becomes unintelligible, the optical signal must be regenerated (i.e., given new strength) SO that it will retain its information until it reaches its destination or at least until it is amplified again. And because repeaters were originally electronic, data rates were limited to about 2.5Gbps. In 1994, something very important happened: Optical amplifiers called erbium-doped fiber amplifiers (EDFAs) were introduced. Erbium is a chemical that's injected into the fiber. As a light pulse passes through the erbium, the light is amplified and contin- ues on its merry way, without having to be stopped and processed as an elec