Unsolicited Grant Service (UGS) Turns Mountain Cablevision into a Voice Lighthouse

Signal to Noise: Useful Information and Analysis

By Jim Barthold

Unsolicited Grant Service (UGS) Turns Mountain Cablevision into a Voice Lighthouse

Cable telephony looks great on a white board; sounds stunning at a shareholder meeting; is a fine foundation for clever TV advertising - and is a bear to do right. The earliest cable telephony systems relied heavily on what the phone companies had been doing for the past century by using some version of time division multiplexing (TDM). In the meantime, CableLabs, as is its charter, continued to look for ways to make cable's offering both better and unique to its broadband pipe via the DOCSIS effort.

Even so, the first cable attempts to deliver reliable voice service using IP were akin to setting sail into the Atlantic Ocean without the benefit of a lighthouse to point out onshore shoals. In other words, a lot of early efforts quickly crashed and sank.

"In the early days, we found that even though we used all the normal quality of service (QoS) settings, it really didn't work; particularly Bit Torrent just clobbered the telephone traffic," said Bruce Marshall, technical director of Hamilton, Ontario-based Mountain Cablevision.

That was because Mountain, like most other cable operators, was trying to put the voice traffic into the same pipe as its high-speed data. The pipe was big enough, but the traffic turned out to be like a gnarly mix of oil and water rather than a smooth flowing stream of fresh packets.

A separate data flow

The answer, said Marshall, was "the conceptual idea of building up a separate data flow outside of the Internet bandwidth and prioritizing that 128 kb channel back to the CMTS; strip it out and send it off to the telephony equipment."

Called unsolicited grant service (UGS), the concept was developed by Rob McCann and Dale Turner of ClearCable Networks, a contractor that was incubated by Mountain to do voice services for small-medium companies.

The UGS groundwork had been laid in the DOCSIS specifications, said McCann, ClearCable's president. "It's not that we created it or invented it; at the time that we wanted to do this, none of the vendors had implemented anything in a fashion that was demonstrable."

DOCSIS 1.1, he said, identifies five different types of service flows. The most popular is best effort, used with most high-speed data offerings. The newest one - at least when ClearCable started doing its thing - is UGS. There is a big difference between the two that, in the end, makes a difference in how a voice signal is handled.

Getting permission

"In the best-effort traffic flow, every time a device or a cable modem wants to send something on the Internet, it needs to first ask the CMTS for permission. (With) unsolicited grant service, the cable modem asks once and the CMTS just grants it in perpetuity, presenting the opportunity to transmit on the cable system until the cable modem says it doesn't need it anymore," McCann said.

It's a simple, clever concept that makes all the sense in the world from the perspective of a cable operator doing voice; that's why it was included in DOCSIS. The thing is, when ClearCable started to work with UGS two years ago, "we went to all the vendors and asked how we set this up, what we did to make it work, and nobody really knew. We had to do a lot of actual field testing to get to the point where we had it working successfully."

The key words there are a lot of field testing.

"We tried to create ... a configuration we thought would be good for voice, and we threw it out there with some friendly customers and, sure enough, they broke it," he said.

So ClearCable went back to the vendors, tweaked it and put the gear into the field again.

"We broke it again," he said with a laugh.

A Canadian first

Finally, he said, they got it right, making Mountain the first operator in Canada to implement it and maybe the first in North America to get it up and working.

"It is a reliable approach to ensuring high priority for real-time voice traffic," McCann said. "At the end of the day, DOCSIS cable plant is only TDM anyway; every cable modem gets its opportunity to transmit over the cable plant with TDM."

UGS just takes it a step further and makes it a "much more reliable way to ensure that cable modems get their amount of time that they're supposed to to send real-time packets," he said. "The only real innovation on our side was the ability to actually field trial this stuff and work out all the problems with getting unsolicited grants to work appropriately."

In its own way, then, Mountain Cablevision, by solving its own cable telephony quandary, built a lighthouse to guide other operators and vendors through the dangerous shoals.

- Jim Barthold