WDM systems have been installed across the globe (a Nortel Networks statistic). Modulation on a fiber in a traditional system is unipolar amplitude shift keying (ASK), which is another name for amplitude modulation (AM). In the optics world, intensity modulation (IM) describes ASK/AM. Traditional systems ASK modulate a light beam at, for example, 1,310 nm. In a simple modulation environment like IM, the clock speed determines the digital throughput of the system; for example, if the clock is 2.4 GHz, then the bit rate is 2.4 Gbps. Because the optical domain is large (the infrared spectrum where telecom- munications optics operate has a bandwidth of 1014 Hz), you should theo- retically be able to derive between 1014 bps and 1010 bps; this is between 0.5x105 (50,000) and 0.5x106 (500,000) times more achievable on a system with a single ASK-modulated signal operating at some fixed frequency.36 Hence, if you were to employ IM-based systems operating at practically spaced spectral points, you should be able to obtain additional throughput. In effect, you want to use frequency division multiplexing (FDM) Chapter 1 Figure 1-19 Typical regional architecture. AR = Access Ring CR = Collector Ring (SONET AM + DCS) (MSR) MSR = Meshed Super Ring (Collapsed SONET Ring/P-T-P DWDM) DN = Data Network (IP Routers MPLS Switches ATM/FR) techniques in conjunction with the digital ASK-based modulation employed at each of the frequencies. This methodology is WDM (or DWDM when 16 or more frequencies are used). WDM technology started with systems that carried 2, then 4, then 8, then 16, then 32, and SO on light beams. Systems Advances and Opportunities in Next-Generation SONET with more than 160 channels are now available. For example, some systems could support 64 channels unprotected or 32 channels protected. A 16- channel system in essence provides a virtual 16-fiber cable with each fre- quency channel serving as a unique STM-16/OC-48 carrier. Advances in aggregate bandwidth have been seen in recent years, from 400 Gbps on high-end sy