PDF of this tutorialIt has become generally accepted that the closer to the customers the virtually unlimited bandwidth offered by fiber is deployed, the more service capability and flexibility it offers. The long-range prospect of 1,000 wavelengths per fiber promised by Lucent's new ALLWAVE fiber gives even more impetus to increase fiber deployment.
With the ever-deeper migration of fiber, the key architectural issue becomes how to use the last-mile media most effectively, whether coax or copper-based. Gigahertz-based coax pipes, with 1,000 times the bandwidth of copper, can effectively be used to offer a wide range of interactive video, data, and voice services.
As an example of how these networks can evolve, let us look at a typical HFC network being developed today. Fiber optics has been utilized in cable TV networks since 1991. Initially, 1310-nm–based optical transmitters and fiber nodes were dropped into the middle of long cascades of radio frequency (RF) coax amplifiers, hence the term hybrid fiber/coax. Figure 1 depicts this type of analog television broadcast-based network with upstream capabilities used for set-top controls and network management. The HFC transport network is typically limited to the use of 1550-nm analog transmitters to extend the reach of the cable plant. Usually, each optical node serves 500 to 2,000 homes. The key network drivers are low cost and good performance of the analog video signal in terms of noise and distortion. The user receives up to 78 channels of video and optional control of premium channels and pay-per-view via an analog set-top.
Figure 1. An Analog Television Broadcast-Based Network with Upstream Capabilities
What if, however, there were no video signals in these networks? One of the key advantages of the HFC architecture is the ability to carry multiple types of information in multiple formats shared by a scalable number of users. If the video is eliminated and the pipes are used entirely for data through a spectrally efficient modulation scheme such as 256 QAM, which yields 7 bits per hertz, the result is a 5-gigabit pipe. Taking the coax up to a gigahertz would yield 7 gigabits of data capacity. As analog video usage declines and this bandwidth is reused for other purposes, an enormous potential for new services, in any medium or combination, becomes available. The flexibility to use bandwidth for different services in multiple formats is an essential strength of HFC networks.
The reason that much of the bandwidth today is utilized for analog video highlights a fundamental difference between HFC and copper-based networks. Almost 300 million televisions, many of which are cable-ready, are already in use in this country alone. In fact, more households in this country have televisions than telephones. HFC gives sufficient bandwidth to broadcast services to these appliances, yielding a low-cost video distribution. Limitations of copper bandwidth, notwithstanding significant advances in digital subscriber line (DSL) technology, force a switched approach, which, though technically feasible, results in an untenable in-home cost for interface devices, at least in terms of mass deployment. To expand the service set offered over HFC, certain network changes are needed, both in access and transport.
