The Justice Department in the United States could be conducting an antitrust investigation into whether cable companies are acting to limit competition from online video, the Wall Street Journal reports, citing unnamed sources. Any such investigation could go to the heart of the tension between the lucrative legacy business of bundling programming packages with network delivery and the challenges posed by and to other online services delivered over the top of those networks.

The report said that officials from the Justice Department had questioned cable companies, including Comcast and Time Warner Cable, about concerns such as setting limits on the amount of data subscribers could download per month.

Comcast limits subscribers to 300 gigabytes a month, over which they pay extra. The company claims caps are required to prevent heavy users from overwhelming its network. New entrants like Netflix have expressed concerns that cable companies could favour their own content over competitors that could bypass their bundled packages of programming.

An example of this could be the Comcast Xfinity app on the Microsoft Xbox, which does not count against data caps in the same way as videos viewed from Netflix.

Comcast gave a commitment when it acquired NBC Universal that it would not unreasonably discriminate against other companies delivering programming over its network, or treat its own services differently.

For its part, Comcast says it is not privileging its own service because Xfinity is delivered over its own network rather than being served from the public internet. It seems that Comcast uses markers in the packets associated with Xfinity data, although it claims it does not use this to prioritise such traffic.

The distinction between operator services and those delivered by third parties over the top of their networks may be difficult to draw when they are delivered over the same infrastructure using the same internet protocols. The difference is more one of business model rather than technology and that may invite investigation in terms of fair competition.