A line coding scheme in which data is modulated using a single carrier frequency for transmission over a phone line. The transmission is considered «carrierless» because the carrier is suppressed before transmission and is reconstructed at the receiver. Carrierless amplitude and phase modulation (CAP) is algorithmically similar to the quadrature amplitude modulation (QAM) line coding scheme, which encodes bits as discrete phase and amplitude changes but has different transmission characteristics.
Some competitive local exchange carriers (CLECs) deploy Asymmetric Digital Subscriber Line (ADSL) using CAP as the encoding method, but results of some independent tests suggest that CAP-encoded ADSL lines might cause spectral interference with proximate T1 lines and Integrated Services Digital Network (ISDN) circuits, resulting in bit errors that can reduce throughput. However, these tests might be misleading because of the limited number of ADSL circuits currently deployed by CLECs. Check with your carrier before signing up for ADSL services to get the latest information about this issue.
Such interference is not a problem with Symmetric Digital Subscriber Line (SDSL) technologies, which use the 2B1Q encoding scheme. Furthermore, ADSL deployed by incumbent local exchange carriers (ILECs) uses discrete multitone (DMT) technology, which doesn't produce the same degree of spectral interference as CAP. Competitive local exchange carriers generally do not use DMT for ADSL because they must deal with the copper local loop, which effectively supports DMT only about half the time but can support CAP about 85 percent of the time. Furthermore, DMT has been adopted as the standard for ADSL by both the American National Standards Institute (ANSI) and the International Telecommunication Union (ITU).
CAP is an acronym that stands for Carrierless Amplitude and Phase Modulation.