In the cable industry's unending quest for unlicensed wireless spectrum, the NCTA has run up against the US Department of Transportation's own spectrum quest.

The National Cable & Telecommunications Association (NCTA) filed last-minute comments yesterday on a proposal by the DOT to mandate Dedicated Short Range Communications technology in vehicles for vehicle-to-vehicle communications. The DOT believes DSRC could radically improve road safety by allowing cars to communicate with each other using Basic Safety Messages (BSM) that share information about a vehicle's speed, direction, momentum and more. However, the NCTA has concerns about the spectrum being used to support these V2V communications; namely, how much real estate in the 5.9GHz frequency band any mandated V2V technology could section off from other applications.

According to the NCTA, "the 5.9 GHz band is widely recognized as the single best hope to address the current Wi-Fi spectrum deficit." As such, the NCTA doesn't want the DOT to make rules on how the spectrum is used until studies have been completed on how best to manage spectrum-sharing in the frequency band to meet all needs.

Currently, the Federal Communications Commission (FCC) is studying two proposed spectrum-sharing approaches. The first, called "detect and avoid" or "detect and vacate," would prevent unlicensed devices from using the entire DSRC spectrum band if any DSRC signals were detected. The second, called "re-channelization," would break up the spectrum band into two blocks, maintaining one for safety-related communications that would then be unavailable for unlicensed devices.

The NCTA sees potential problems with both strategies. The cable trade association notes that the "some have advocated before the FCC that the detect-and-vacate approach to coexistence would render the entire 5.9 GHz band unusable for Wi-Fi," and could cost the industry between $10 and $20 billion per year in lost use. The NCTA adds that the re-channelization approach would cause less financial harm, but would still limit other WiFi uses of the 5.9GHz band.

In its comments, the NCTA urges several actions by the DOT and its subsidiary, the National Highway Traffic Safety Administration (NHTSA). The NCTA recommends that the NHTSA do a better job in conducting its cost-benefit analysis by taking into account lost WiFi opportunities. And it recommends that the NHTSA not mandate a specific technology for achieving safety goals given the costs involved. The NCTA also notes that the NHTSA should leave spectrum policy issues to the FCC, which has the jurisdiction to govern on them.

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If there's one thing the contretemps between the NCTA and the DOT highlights, it's how debate over spectrum use will continue to extend into new areas as more and more devices need wireless frequency access to connect and communicate. Considering all of the smart-city applications that have yet to be implemented, it's a guarantee that spectrum fights will only grow more contentious moving forward.

Meanwhile, the cable industry isn't going to back off in fighting for access to more unlicensed spectrum given that it relies on that connectivity for wireless services. Aside from MVNO agreements with Verizon Communications Inc. (NYSE: VZ), and the possibility that Comcast Corp. (Nasdaq: CMCSA, CMCSK) could acquire spectrum in the 600MHz auction, WiFi is pretty much all that cable's got. (See New Comcast Wireless Details Drop This Week .)

— Mari Silbey, Senior Editor, Cable/Video, Light Reading