Primer on DOCSIS

OLYMPUS DIGITAL CAMERAAnybody who uses a cable modem at home has probably heard the word DOCSIS. This is a set of standards that define how data is transmitted over coaxial cable networks. DOCSIS stands for Data Over Cable Service Interface Specification. It was developed by CableLabs, which is a research and standards organization that the cable companies have created for research and development purposes. CableLabs is to the large cable TV companies what Bell Labs has been for the large telephone companies.

DOCSIS 1.0 was first issued in 1997 as a standard and created the basis for cable modems. It established a data network that started with a CMTS (cable modem terminal system) that talked to cable modems in each home. DOCSIS 1.0 was limited to a single data channel which means that data speeds were limited to a usable 38 Mbps download and 9 Mbps upload for everybody together on a cable node. Because the data was shared with anywhere up to 200 homes, speeds on DOCSIS 1.0 were practically limited to a maximum of about 7 Mbps, although these speeds could be much slower at peak times.

The standard was updated in 1999 to DOCSIS 1.1 which allowed for QoS (Quality of Service) which enabled cable systems to carry voice calls, with priority, on the cable modem data path. There are still a significant number of field deployments using DOCSIS 1.0 and 1.1, particularly in smaller and rural cable systems.

DOCSIS 2.0 came out in 2001 and the major improvement was to increase upload speeds. Version 2.0 also improved the ability to transmit VoIP. The standard still kept the single channel downstream. As cable companies lowered node sizes there were DOCSIS 2.0 systems that supported speeds of up to 15 Mbps download.

The biggest improvement with DOCSIS came with version 3.0 which was released in August 2006. This standard allows for bonding cable channels together to make larger data paths to each node. Cable companies that have deployed DOCSIS 3.0 are offering much faster speeds than in the past. Comcast in the US offers 107 Mbps download in urban markets using the newer modems. In Canada, Shaw Cable and Videotron have used DOCSIS 3.0 to offer products over 200 Mbps download. Virgin Media in Britain announced a speed of 1.5 Gbps download and 150 Mbps upload.

Why don’t US cable companies offer speeds that fast? There is a trade-off in any cable system between the number of channels that are used for programming versus data. While US cable companies have undergone digital conversion to free up channels, they have used most of that new space to add high definition channels to their network rather than dedicate the extra space to data. In the future, cable companies will be able to free up even more space for data by converting their cable channels to IPTV. Today they multicast every channel in the system to every home, but with IPTV they would send only the channels people are watching.

The CEOs of the largest cable companies have often been quoted saying that they are providing the bandwidth that people need. And I am sure that the believe this. But we have a very long way to go to just convert all of the cable systems in the US to DOCSIS 3.0 and increase speeds. I work every day in markets where the speeds are far slower than they are in upgraded urban markets. But it’s good to know that the tools are there for the cable systems to increase speeds, when they finally decide the time is right to do so.

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