ocessing engines perform other tasks associated with distributing video. When cable subscribers watch programs from televisions connected to broadband, the authentication software built into a set-top box automatically sends messages to the cable provider's networks verifying that this is a legitimate customer with a subscrip- tion for particular services such as video-on-demand (VOD). Authentication is more complex when subscribers attempt to view content from other devices such as a tablet computer. The authentication software needs to deter- mine if this device belongs to a legitimate subscriber. Authenticating a set-top box associated with a particular subscriber is simpler because it involves only a single set- top box, at a fixed location. ELECTRONIC COMMERCE Social networking, video, and search sites are key generators of e-commerce revenue. Additional drivers are dating sites, massively multiplayer online games, online gam- bling, pornography, and virtual worlds such as Second Life. Advertising adds substan- tial revenue at most of these sites. According to market analysis company eMarketer, Inc., Internet advertising revenues in the United States grew 30 percent from June 2010 to June 2011. Google earned the largest amount of ad revenue. Total online advertising revenue is expected to increase as more people use tablet computers, smartphones, and Internet-connected televisions to browse, shop, and view content over the Internet. Use of these devices is in early phases in most countries. However, as such use proliferates, advertising revenues will continue to grow, under- writing the costs for content, which will lead to lower prices or possibly no fee at all on the Internet. The demographic information collected about people's interests by tracking which articles they read and which sites they visit enables marketers and advertisers to tar- get ads to narrowly defined groups. They can target women older than 55 who are interested in women's health issues, shopping, or travel. Or, they can focus on 18- to