Newsletter sign-up

View all newsletters

Sign up for our technology specific newsletters.

Enterprise Java

Email Address:

The evolution of cable television to interactive communications service provider

By Jim Connors, JavaWorld.com, 10/01/96

Page 2 of 7

Figure 1.1 - traditional cable television architecture

The smaller distribution or feeder cables branch out from the trunks and are responsible for serving local neighborhoods. To avoid excessive attenuation and noise, feeder cable is also severely limited in length, since usually a maximum of two amplifiers, called line extenders, are allowed per feeder. The statement that 95% of households can be reached by cable services means that those homes are sufficiently close enough to a feeder cable.

Feeder cables are tapped at periodic locations to furnish the familiar coaxial drop cables that enter directly into the customer's premises. Drop cables too are limited in length to about 150 feet. Terminal equipment (consumer electronics) is connected to the drop cable inside the home. Among the more common devices are televisions, VCRs, set-top boxes, converters, descramblers, splitters, and cable modems.

Cable television advantages and disadvantages

Compared to the telephone industry, cable television systems do have, and have had for 50 years, a truly high-bandwidth delivery system to the home. The simple reason for this fact is that television is, even by today's standards, a high-bandwidth application. During the 1940's, when broadcast standards were being set by the NTSC (National Television Systems Committee), technology and compromise dictated that each television channel be assigned a 6-MHz portion of the electromagnetic spectrum. Since that time, much has improved to increase utilization of the spectrum, but the standards still remain the same -- it would be very difficult to force all consumers to replace their NTSC compliant equipment for devices based upon newer albeit better standards.

Figure 1.2 graphically represents the television spectrum allocation. The 6-MHz-wide channels were prudently assigned to avoid inter channel interference. The remaining unallocated terrestrial spectrum is dedicated to a host of other communication services.

Figure 1.2 - television spectrum frequency allocation

Cable television, however does not utilize the terrestrial spectrum, but rather uses coaxial cable to broadcast its signals. Coaxial cable has the ability not only to "emulate" over-the-air spectrum, but is designed such that its sealed environment does not interfere with other signals. Consequently, it is possible for cable operators to safely re-use previously allocated spectrum, or to deploy multiple cables, each of which contains its own separate impervious spectra.

Regrettably, the original cable systems architecture was never really envisioned to be a general purpose two-way communications medium. Its primary goal was simply to deliver high bandwidth video signals to residences. In order to accommodate upstream communications, the existing cable plant must be upgraded.

Recent cable system developments

As is often the case in industry, the chief motivation for change revolves not necessarily around technology but around business. In cable television's case, the late 1980's marked the time that it made business sense to begin replacing the coaxial cable distribution plant with fiber optics. Since signals transmitted by optical fiber can be carried for significantly longer distances, fewer amplifiers are needed. This results in fewer points of failure, lower maintenance costs, and better signal quality.

 

Improve efficiency with Visual Studio(r) 2010 Beta 2. - Microsoft

About Us | Advertise | Contact Us | Terms of Service/Privacy

Copyright, 2006-2008 Network World, Inc. All rights reserved.