Despite the accelerating rollout of Docsis 3.0, cable still faces a big bandwidth problem. Only the big problem isn't with the downstream bandwidth, it's with the upstream.

As a few speakers noted during Light Reading's Cable Next-Gen Broadband Strategies conference in Denver last week, the upstream limitations of the Docsis specs are becoming increasingly apparent. Even though Docsis 3.0 attempts to ease this problem through the miracle of channel-bonding, it still doesn't provide the needed relief.

That's because Docsis 3.0, unlike, say, Ethernet PON (EPON), doesn't offer a symmetrical boost in bandwidth. By bonding four RF channels with the wideband spec, cable operators can offer up to 160 Mbit/s of shared downstream bandwidth to broadband users. But, using the same number of bonded channels, cable operators can only deliver up to 120 Mbit/s of shared upstream bandwidth.

Rouzbeh Yassini, founder and CEO of YAS Broadband Ventures , warned last week's conference attendees that this shortfall will haunt cable operators as they try to compete against Verizon Communications Inc. (NYSE: VZ)'s superior FiOS network and other telco fiber-to-the-premises (FTTP) initiatives. He urged the industry to stretch beyond Docsis 3.0 and develop a stronger two-way platform for data, voice, and video services.

"My upstream is more crowded than my downstream," said Yassini, known as the father of the cable modem. "We have major upstream issues to resolve." He predicted that cable's upstream needs "will dictate the next topology of networks."

The other problem with Docsis 3.0 is that upstream channel-bonding seems pretty hard to do. While one relatively small vendor, Casa Systems Inc. , has earned full Cable Television Laboratories Inc. (CableLabs) approval for its 3.0 cable modem termination systems (CMTSs), the three CMTS leaders – Arris Group Inc. (Nasdaq: ARRS), Cisco Systems Inc. (Nasdaq: CSCO), and Motorola Inc. (NYSE: MOT) – have not, because they haven't mastered upstream channel-bonding yet. At last week's conference, representatives of all three companies declined to say when they will lick the problem.

Several conference speakers suggested that the Radio Frequency over Glass (RFoG) standard being drafted by the Society of Cable Telecommunications Engineers (SCTE) should help out by enabling MSOs to string fiber all the way to the home or business without changing any headend, customer, or back-office equipment, thereby increasing their potential capacity. RFoG might also generate some bandwidth gains by eliminating much of the current noise in the upstream spectrum.

But, as a new Heavy Reading report makes clear, those capacity gains will still be minimal. The report, "Next-Gen Cable Networks: Opportunities for Fiber-Based Technologies," stresses that even with the deployment of RFoG, the amount of bandwidth that could be delivered to subscribers will still be constrained by the upper limits of cable's hybrid fiber/coax network spectrum and Docsis-based protocols. In other words, even with fresh fiber links, the upstream payoff will still not be enough.

So what's an MSO to do? As the report notes, cable engineers increasingly believe that some kind of PON technology may be the answer. That's why most of the growing number of RFoG equipment vendors are promoting RFoG as a stepping stone to EPON, Gigabit PON (GPON), or the still emerging 10-Gbit/s versions of those two PON flavors. In the latest example, Arris unveiled a new RFoG optical network unit (ONU) earlier this week that can also work with EPON.

In his question-and-answer session with Cable Digital News editor Jeff Baumgartner, Yassini urged cable operators to embrace whatever next-gen broadband technology makes sense, whether it's called GPON, EPON, Cable PON, or Docsis 4.0. "Don't get hung up with the name or the type of technology," he said.

Amen, brother.

— Alan Breznick, Senior Analyst, Heavy Reading

For more information about Heavy Reading's "Next-Gen Cable Networks: Opportunities for Fiber-Based Technologies," or to request a free executive summary of this report, please contact: