estinations such as the Internet, long-distance providers, or the cable MSO's VoIP equipment. CMTSs are similar in function to Digital Subscriber Line Access Multiplexers (DSLAMs) that are used in traditional telephone company networks in that both devices translate between modems at customers' premises and equipment in backbone networks. They both also aggregate traffic from multiple subscribers into a single stream for transmission in the backbone. (See the section "Digital Subscriber Line Access Multiplexers," earlier in this chapter, for a discussion on DSLAMs.) Supporting More Video via Set-Top Boxes New modulation technologies enable digital cable to increase capacity two to three times from the former 10 to 12 channels of standard-definition video and two to four channels of HDTV video on each 6MHz channel. Standard-definition TV has lower resolution than high- definition television. Moreover, digital cable results in improved resolution because there is less interference from noise, which creates snow and shadows that appear on the television screen. Consumers can opt for set-top boxes with hard drives capable of storing content and playing it back at their convenience. Digital cable is the basis for VOD, which allows subscribers to order movies for an extra fee. VOD www.TechnicalPDF.com movies are available for a limited time period of a day or two. Nearly 100 percent of all cable TV subscribers in the United States are located in areas where they can receive digital cable. Pay-TV via cable and satellite are available to 82 percent of households of the population in the United States. Cable Modem Standards Transition to Higher Speeds CableLabs, the research and development arm of the North and South American cable TV industry, sets cable TV modem standards. CableLab's standards are intended to provide a technology "road map" for cable MSOs to move toward implementing higher-capacity IP networks used for Internet access. These standards are known as the Data Over Cable System Interface Specifications (DOC