In the January 2009 issue of CT, ARRIS Chief Strategy Officer Tom Cloonan provided a thoughtful and detailed critique of Motorola’s DOCSIS over Internet protocol TV (IPTV) bypass architecture (DIBA). In it, he raised many excellent points.

His conclusion, however, was worded in a way that could be misconstrued as a dismissal of cable modem termination system (CMTS) bypass techniques in general, rather than DIBA in particular. In addition, his interpretation of some of Michael Cookish’s original statements deserves additional discussion.

What it is

To begin, "CMTS bypass" simply refers to the technique of allowing high-volume video traffic to bypass the relatively expensive and also throughput-limited DOCSIS CMTS core. However, the CMTS core is still central to the system. DIBA (or any other CMTS bypass architecture) in no way eliminates the need for a DOCSIS core. Every CMTS bypass architecture, of which there are several competing versions, employs DOCSIS cable modems and the DOCSIS CMTS core as the cornerstones that are key to successful implementation.

"With the best interests of the industry in mind, each CMTS bypass solution makes tradeoffs on many axes."

The goal of CMTS bypass is to avoid wasting expensive resources, namely CMTS cores. A CMTS core has three major functions: to process downstream packet flows; to manage and control cable modems; and to coordinate and process upstream packet flows. In addition, a CMTS core has a limited throughput capacity.

CMTS bypass addresses the fact that video traffic is extremely high-volume compared to data and voice, especially when one factors in projected demand for unicast video, and yet requires very little packet processing. It takes advantage of key characteristics of video. Unlike typical Internet traffic, video is highly asymmetric, comes from a trusted source, is scrambled using robust conditional access systems (CASs), and only needs the simplest of quality of service (QoS) algorithms — namely, top priority. It therefore requires minimal packet processing.

In a traditional CMTS configuration, many CMTS cores are needed to process the sheer volume of TV traffic, but are otherwise underutilized. By diverting video traffic from the CMTS core, CMTS bypass drastically reduces the number of CMTS cores required to support a commercial service. A nice corollary is that with fewer cores, fewer upstream channels are needed as well.


It is true that the DOCSIS 3.0 architecture and manufacturing efficiencies will bring down the cost of DOCSIS cores, but the fact remains that in terms of transport expense, the overriding dilemma can be reduced to the fact that on a per-channel basis:

Equation 1

Traditional DOCSIS IPTV cost = DOCSIS core cost + quadrature amplitude modulation (QAM) cost, while

Equation 2

Traditional MPEG-2-TS cost = QAM cost

CMTS bypass endeavors to bring the comparative cost of cable IPTV in line with the cost of traditional MPEG-2-TS video, reducing Equation 1 to:

Equation 3

CMTS bypass IPTV cost = (1/x) DOCSIS core cost + QAM cost

where x is a number greater than 1, typically 12-24, and possibly as high as 192. In addition, manufacturing efficiencies confer similar cost savings on other modular CMTS (M-CMTS) components, with the net effect that the CMTS core remains a significant portion of a traditional CMTS system cost.

In the same vein, questions arose whether DIBA is merely a shell game for functionality and cost, and whether edge QAM modulators could support the additional processing required by DIBA (and by extension, other CMTS bypass implementations). Given the number of edge QAM vendors with deployed or announced CMTS bypass solutions (at least four), the latter concern is a moot point. In terms of functionality migration, the beauty of CMTS bypass is that most of the packet processing functionality is not needed for video traffic, and the remainder that is passed to the edge QAM modulator increases edge QAM costs by significantly less than the cost of a CMTS core.


Mr. Cloonan also raises some very important concerns about operational impacts: whether DIBA precludes the use of channel bonding for statistical multiplexing benefits, whether the complexity of managing and maintaining DIBA infrastructure would outweigh the cost savings, and what the impact would be on existing DOCSIS (data and voice) provisioning and configuration. He could also have asked how CMTS bypass would affect existing data and voice service quality and how to address the thorny question of in-home networking.

In this arena, the answers depend heavily on individual vendors’ design and implementation decisions. CMTS bypass solutions have a broad range of differences, and the commonality is simply that the video packets bypass the CMTS core.

Here are few differences:

  • Some CMTS bypass edge QAM modulators support channel bonding, and others don’t.

  • In some CMTS bypass solutions, the CMTS core works cooperatively with the edge QAM modulators, and in others the system works without the CMTS core’s knowledge.

  • Some solutions operate as standalone video solutions, in parallel with existing services, while others borrow resources (cores or cable modems or both) from an existing data/voice service.

  • Some solutions require DOCSIS 3.0 components, and others work with DOCSIS 1.1/2.0 cable modems as well.

  • Some provide resource management (for example, switched multicast), others use fixed channel assignments, and still others depend on unicast delivery. Similarly, some support both unicast and multicast video, while others support only one or the other.

  • Some solutions provide TV service management, and others leave that as an exercise for the operator or a third-party vendor.

  • Some assume that the cable modems require customized software changes; others use off-the-shelf modems.

  • Some CMTS bypass solutions require that a dedicated, video-only cable modem be deployed per set-top box, eliminating the need for in-home networking and avoiding the possibility that data traffic might inadvertently affect video quality; other solutions assume that a single cable modem delivers data, voice, and video to a central location in the home, simplifying the modem-provisioning process.


With the best interests of the industry in mind, each CMTS bypass solution makes tradeoffs on many axes, including baseline cost, incremental cost, functionality and operational simplicity. At the root of each decision is the motivation to make IPTV over cable more accessible and affordable than it currently is because the demand for cable IPTV is already strong and will grow stronger with time.

I thank Mr. Cloonan for his astute observations that prompted this addition to the discourse surrounding practical delivery of IPTV over DOCSIS and hope that it has addressed — and possibly dispelled — some of the concerns he raised about the advantages of CMTS bypass.

Rei Brockett is vice president of marketing and product management at GoBackTV.

The Daily


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