Previous Table of Contents Next


Distortion.
With any active components, distortion is unavoidable. In the case of HFC network, the main cause of distortion comes from multiple carriers (i.e., frequencies) in amplifiers.
If the combined signal strength is too strong, the amplifier will go into compression and generate distortion products. These will cause the existing carrier levels to be distorted by multiples and combinations of other carriers.
Carrier Levels.
As previously stated, if the carrier levels are too high, they will cause the amplifiers to generate distortion products. If they are too low, then the carrier-to-noise ratio will be too low (i.e., not enough signal strength at the receiving end).
The frequency response of the amplifiers can also cause variation of the carrier levels, thus the lower frequencies may be fine and the higher frequencies will have a low (bad) carrier-to-noise ratio.
Corroded Connectors.
HFC networks are constructed either above ground (on telephone poles) or underground. In both environments, the connectors (between the cable and the amplifiers) will corrode or degrade with time, either from water seeping into the connection or from other types of wear — everything from animals chewing on the connectors to varying tension on the cable.
This type of destruction can either let other signals in (known as “ingress”) or distort the signals passing through it, most noticeably the 60 Hz power supply used to power the amplifiers. This distortion causes 60 Hz and other multiples (usually 120 Hz) distortion across all frequencies.
Ingress.
Although the cable network is a closed network, there are still external signals that can get into, and interfere with, upstream traffic. Ham radio has components at 7, 14, and 21 MHz; shortwave radio has components at 6,10, 12 and 15 MHz; arc welders produce broadband noise that can last for long time periods (minutes); lightning produces broadband noise that can last for several seconds. Home wiring (as when consumers decide to wire up their entertainment system) is responsible for common causes of noise imposed on the return path.
Noise Funneling.
The return path also experiences a phenomenon referred to as noise funneling, where the collective noise of all the return path amplifiers collects, or funnels, back into the fiber node. This phenomenon can be difficult to troubleshoot, because it will be difficult to determine which amplifier is causing the significant noise problem.

“Digital” Requires Monitoring

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 don’t. Digital modulation is more resistant to network problems, but it eventually will reach a point where it won’t 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.

SERVICES RUNNING ON THE HFC NETWORK

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.

Analog Video Service

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.

Digital Services

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.

IDEAS FOR MANAGING THE HFC NETWORK

One method for managing communications networks such as HFC is the TMN model.

Telecommunications Management Network (TMN) Model

Briefly, the TMN model breaks the operation process into five distinct layers:

  Business management.
  Service management.
  Network management.
  Element management.
  Network elements.

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:

  Fault management.
  Configuration management.
  Performance management.
  Security management.
  Accounting management.

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:

  Monitor status of HFC network (fault management).
  Monitor status of head end equipment (fault management).
  Manage head end equipment (configuration, performance, security, and accounting management functions).
  Monitor QOS into the head end.


Previous Table of Contents Next

Copyright © CRC Press LLC