Previous | Table of Contents | Next |
Until this time, cable operators had no method equivalent to methods used by the telcos to monitor their network. Cable companies usually relied on subscribers to call in when there was a problem either no cable signal at all (an outage), snowy images (low levels or carrier/noise problems), or multiple pictures (crossmod, microreflections, or other distortion problems).
With analog TV signals, a wide variety of problems could exist, yet the picture was still visible, thus there was no real need to put in elaborate monitoring systems. With the advent of new, digital-based services, however, there is no graceful fading of the services they either work or they dont. Digital modulation is more resistant to network problems, but it eventually will reach a point where it wont work.
To ensure delivery of quality services, it becomes necessary to manage both the HFC network and the services that run on top of it.
To illustrate the relationship between the HFC network and the services that run on top of it, a few examples are given of the services delivered over HFC and how they might correlate to problems with the network.
As mentioned previously, there are many types of problems that can occur in the HFC network and still allow viewing of analog video. These problems could include signal levels that decrease as a function of frequency or distortion products that superimpose several channels onto one. These problems are well known and characterized, and most HFC networks are optimized to minimize their effects.
These are the new services that will take advantage of the broadband capabilities of the HFC network. What is unknown is how the HFC impairments will affect the digital services.
Recent studies have looked at several different HFC networks to test their ability to support digital services. The tests (done by Cable Television Laboratories, Inc.) used a RF T1 modem to determine the performance factors from five different cable plants. Measurements were compared against ITU-TSS G.821 performance objectives.
What these studies concluded was that the return path is a hostile environment. One cable plant defect can make the entire reverse spectrum unusable.
The field testing emphasized the critical importance of conducting a comprehensive and thorough plant hardening effort and maintaining precise gain alignment. The most troublesome conditions were caused by impulse noise, usually observed with cold temperatures, high wind conditions, increased precipitation (including thunderstorms), and combinations of all of these weather variables.
Under these conditions, the integrity of the transmission path (both downstream and the return path) was affected, resulting in increased error seconds, severely error seconds, and poor availability. This does not rule out the ability of the HFC network to deliver the services; it merely indicates that a well-characterized HFC network, along with a comprehensive network management system, is required to offer the new services that meet subscriber expectations.
One method for managing communications networks such as HFC is the TMN model.
Briefly, the TMN model breaks the operation process into five distinct layers:
Each of these layers is connected, and each affects the ability to offer services to subscribers. For each layer, the management systems must incorporate the following five functions:
Communications between the layers are handled by a standard protocol CMIP.
The TMN model offers a structured approach to meet the new requirements of HFC management discussed in the previous section. Specifically for the HFC network, the following items should be addressed:
Previous | Table of Contents | Next |