64-QAM Upstream

It is both "possible" and "practical" to deploy 64-QAM today, Ron Hranac, Cisco Systems technical leader and senior technology editor for Communications Technology, said during a recent CT webinar sponsored by Trilithic. "(64 QAM) is not something that is confined to a laboratory environment."

He pointed, of course, to Midcontinent Communications, which has launched upstream 64-QAM in six nodes. Given this success story, the question becomes: "How on earth can I get 64-QAM to work in my cable system?" Hranac said. Critical pieces His rule of thumb? "You can expect to be about 80 percent successful with doing things like 64-QAM in the upstream if you meet or exceed relevant government technical regulations (Part 76 of the FCC rules in the United States) for the entire cable network (including the headend, distribution network or outside plant, and subscriber drops)."

"The next piece that is critical is to ensure your cable network is DOCSIS compliant," Hranac said, pointing in particular to the assumed RF channel transmission characteristics outlined in the DOCSIS Radio Frequency Interface Specification. For the upstream, this covers parameters like carrier-to-interference, amplitude ripple and micro-reflections.

Additional challenges could include accommodating the lower dynamic data range of 64-QAM modem transmission. Switching from QPSK to 16-QAM loses 3 dB of headroom, meaning that a cable modem could have trouble at a higher modulation if it already is "maxed out" at QPSK. "You need to go into the outside plant and find those modems and figure out why they are transmitting at a maximum output level," Hranac said, noting that trouble often lies at the subscriber drop. Preventive maintenance As for the success story, Midcontinent implemented a preventive maintenance initiative (PMI) program four years ago, which helped ready the network for 64-QAM. Currently on an 18-month cycle, the program includes testing, finding and fixing faults with the optical nodes and power supplies, sweeping each amplifier, and end-of-line digital testing. In addition, the network operations center (NOC) monitors every return path 24 hours a day.

"If (an) alarm is critical and close to affecting service, we will roll a tech out," said David Haigh, Midcontinent’s senior RF engineer.

Midcontinent is preparing to deploy 64-QAM over an additional two nodes. "The long-term plan is to have this across our entire footprint," Jon Pederson, Midcontinent’s vice president of technology, told Communications Technology recently.

"What you will find as Midcontinent found … is that as soon as you fill up the upstream pipe or open up the data throughput on there, the upstream no longer becomes the bottleneck; the downstream does. By getting ready for it with upstream 64-QAM and having a well-maintained plant, that is going to prepare you for the next version of DOCSIS, which is right around the corner," Hranac said.

To view the on-demand version of the 64-QAM webcast, go to CT‘s Web site and click on it in the "CT Events" box, or simply click here. – Monta Hernon