PDF of this tutorialBy using high-power 1550 transmission and taking the node size down to twelve homes, the HFC architecture merges quite nicely with a copper-based fiber in the loop (FITL) system to arrive at a powerful, full-service architecture. As Figure 5 shows, an HFC–based video overlay can be added to an FITL network conveniently. The 1550 externally modulated transmitter and a high-power optical amplifier are located at the central office (CO), distributing the broadband signal to as many as 16 remote terminals (RTs) or 32,000 subscribers. Out at the remote terminal, another optical amplifier boosts the signal, feeding very low-power video receivers. This receiver can stand alone, be contained in a tap housing, or take the form of a plug-in card in the optical network unit (ONU). Because the video operates at 1550 nm and the voice/data at 1310 nm, a single fiber can be used. With shared housings, power, fiber, and installation labor, the video overlay can be economically added to the FITL system. Crucial to making this architecture work, both technically and commercially, are high-power 1550 optics and low-cost, low-power video receivers.
Figure 5. HFC–Based Video Overlay
Figure 6 shows how the relative costs of 1310 and 1550 technology have progressed over time. This plot shows the cost of light measured in dollars per milliwatt versus the total output power in 1995, when HFC networks began to deploy 1550. These costs may seem high to operators with baseband digital networks, but these transmitters can carry up to 110 6-MHz analog subcarriers with carrier-to-noise ratios (CNRs) in the 50s and second- and third-order distortions in the high 60s. As shown, 1550 technology was quite expensive and hence was limited to those applications requiring high output power, typically in the transport network.
Figure 6. The Costs of 1310 and 1550 Technology
