April 1, 2009

Reality Check: Back to the Basics

By Bruce Marshall

A few years back, I was working for the engineering division of a large Canadian MSO that operated cable systems in both Canada and the United States. One of my duties was performing system technical audits - Canadian BP 23 testing of the Canadian systems and FCC Proof of Performance testing of the U.S. cable systems and yearly technical audits for head office.

The first test I would normally do when I arrived at a system was a very quick system drive-through to measure the amount of signal leakage in the system. Within 15 minutes of measuring the signal leakage, I could accurately predict if the system would pass, possibly pass or fail dismally. This early prediction was all based on how many leaks I found and how bad these leaks were - all within 15 minutes.

If the system was relatively tight, it would almost always pass on all technical measurements. It doesn't take much highlighting to demonstrate that a system with more leaks would have many more technical failure points. If the leakage detector was making as much noise as an arcade game, the rest of the audit was sure to be a disaster.

Protecting our muxes

Quantifying the signal leakage "score" of a cable system is a good way of showing the age of cable plant and the level of craftsmanship practiced by the technical staff. To this day, signal leakage is the first thing on my report card as it's a measurement of the quality of construction and integrity of cable plant.

Ron Hranac's February column "Signal Leakage in an All-Digital Network" doesn't mention this, but once a cable system is filled with quadrature amplitude modulation (QAM) carriers, the real-world chance of cable's signal leakage interfering with a ham radio operator will be very slim; the radiated power spread is very low. However, the opportunity for a ham radio transmission to clobber a QAM carrier (or any other ingress carrier) will be very high.

Now, a single interfering carrier falling into a QAM carrier will cause a 10 channel outage (typical). All signals within the clobbered QAM carrier are now a victim of a single ingress carrier, so rather than an interfering carrier taking out a single analog channel, our efficiency gained by muxing channels at a 10:1 per 6 MHz ratio increases an ingress outage by a factor of 10. Sure, there is forward error correction (FEC) within the transport network, but it can't fix everything, and look at the fun the techs will have looking for these intermittent 10-channel outages. Imagine the look on the customer's face when, for the third time in a week, you have to say: "It's not happening now; I can't find anything. Call us when it goes off again."

I would suggest that our performing and correcting signal leakage to minimize our interfering with somebody else's operation will change to protecting the quality and reliability of our muxes. There is a significant value in ensuring we look after the efficiencies of our 256-QAM/10:1 multiplexing so it remains reliable and stable for our customers.

Very boring, but ...

Ron Hranac is an evangelist when he speaks of quality components, good craftsmanship, connector integrity and attention to detail; I couldn't agree with him more. Ron's articles have covered these areas in great detail, and he has clearly put a lot of thought into researching these basics.

Here are a few things that really do work to minimize ingress: tri-shield drop wire, tri-shield jumpers on traps, tri-shield wiring throughout the home, tri-shield jumpers in the home between customer's devices (especially VCRs), flooding compound (dry) in aerial drops, high-quality compression drop fittings, house splitters with soldered backs and cast F-fittings, and finally wrench tightening of all F-fittings (carefully in the house). Yes, these are all drop-related and very boring stuff, but it's where you get the greatest performance improvement. Just think about how long it would take to install 100 percent tri-shield in every home if you started now, and you get an idea of the time frame and long-term commitment necessary.

In summary, figure out what it will take to have a meaningful leakage patrol system within your operations and standardize on a high level of quality components and craftsmanship to minimize leakage, but more so to minimize the ingress problems.

Bruce Marshall is technical director for Mountain Cablevision.