HUMPTY DUMPTY told Alice that "When I use a word, it means just what I choose it to mean—neither more nor less." American telecommunications companies seem to have adopted a similar semantic strategy when they use the word "unlimited". AT&T is the latest to join the bandwagon. It has just imposed a cap on unlimited mobile data plans for the heaviest users, soon to be followed by limits, and fees for exceeding them, on its wired broadband network. The deployment of zippy next-generation 4G networks, too, is hampered by outdated caps on usage many telecoms firms have and will put in place.

Verizon Wireless engaged in doublespeak of its own several years ago with its first-generation 3G offering, which the operator plugged as "Unlimited Broadband Access". In a footnote of its terms, however, Verizon explained that although e-mail, web browsing and remote corporate activities were not subject to limits, streaming and downloading any media was forbidden altogether. As a result, any use beyond 5GB per month risked engendering potential early termination of the contract on the assumption that even unlimited legitimate use would not exceed that figure. In 2007 the company reached an agreement with the state of New York to stop using the term "unlimited" and compensate users terminated for exceeding the five-gigabyte limit. It also paid a modest fine.

Since Verizon was slapped down, cable and telecoms firms have become more sparing in their use of universal adjectives. At the same time, they have become cagier about explicitly revealing in what ways they restrict service. Comcast, for instance, appears to have had a secret limit for its cable broadband beyond which customers were cut off. Use more than 250GB a month for two months in one year, and your service could be cut. Comcast was eventually shamed into confessing to this ruse, even though it had never promised customers could download or stream endlessly. Undisclosed caps might prove a problem in the courts or with regulators, but limits on use, so long as they are imposed across the board and do not target particular types of data like videos or music, do not violate network-neutrality rules adopted by the Federal Communications Commission (FCC). Nor do they fall foul of the broader principles that underpin the FCC's decision.

What irritates many users, other than being shut out from the network for no apparent reason, is the network operators' putative justification for imposing such limits in the first place. All manner of networks have peak periods when capacity is stretched. This might occur at particular times of day, or while political or sporting events are on. Using dynamic pricing or peak-usage thresholds to reduce usage is a perfectly sound idea. By contrast, rationing data, which disconnecting a bandwidth-hogging user amounts to, is an inefficient way to manage a scarce resource.

AT&T's latest ploy is more elaborate. The company is targeting the heaviest 5% of subscribers to the unlimited plans it offered from 2007 to 2010. As a result up to a million users of the 20m or so with unlmiited plans, may be affected each month if the size of their online diet is deemed too grasping. Instead of cutting off service completely, however, AT&T has taken a leaf out of its European peers' rulebook and plans merely to throttle connection speeds available to the most egregious digital gluttons. These will probably be knocked onto 2.5G GPRS or EDGE networks (with speeds of roughly 50 kilobytes per second and up to 200 kilobytes per second, respectively) that AT&T will maintain for the foreseeable future. (Data received over Wi-Fi, whether at AT&T's hot spots or on private networks, does not count towards the totals.)

Networks already employ throttling and congestion management in each cell in a network. A network can only push out so much data at any given time and operators expend substantial effort to ensure that no subscriber is entirely cut out by pinching bandwidth from extant connections, slowing them down. One could argue that AT&T is simply taking this a step further by extending it from instantaneous use to longer periods and across its entire network.

These new limits, reasonable though they may be during peak times given current capacity, remain in place as more 4G networks, with much higher speeds and throughput, are rolled out. This happened with Comcast's cable broadband, which remains subject to a 250GB monthly cap even as the network's capacity increased tenfold in some areas, and at least doubled in most others, from 2008 levels. Verizon offers plans up to 10GB per month before extra fees kick in despite continually expanding its 4G network. This is equivalent to just two hours of full-throttle transfers—two or three high-definition films, say—or a few dozen hours of lower-resolution video streaming. Public Knowledge, an advocacy group which styles itself as a defender of digital consumer rights, recently released a report making the point that a blazing 4G network is useless if caps mean that all one can conceivably afford to use it for could be done equally well over existing 3G networks.

As it stands, AT&T's plan means that 5% of users will have their mobile broadband crimped each month, regardless of future improvements to network capacity. Many may choose to pay up to restore speedier connections. Operators are right when they point out that caps and fines are perfectly legitimate ways to keep a network fluid. Their true intentions will, however, be revealed when the 4G floodgates open. If the restrictions are not loosened, they can no longer be justified as a reasonable tax on bandwidth hogs who make online life slow and miserable for more restrained types. They will start looking more and more like a rent-seeking ruse.