摘要 : This paper reviews the telephone loop plant characteristics, current DSL (digital subscriber line) technologies, recent efforts in video coding standards, and the interrelationship between DSL technologies and visual communication... 展开 This paper reviews the telephone loop plant characteristics, current DSL (digital subscriber line) technologies, recent efforts in video coding standards, and the interrelationship between DSL technologies and visual communications over subscriber lines. In overview of the loop plant characteristics we examine its physical makeup and transmission properties, where for the latter we discuss frequency and time responses of wire-pair lines and the impairments of echo, crosstalk, impulse noise, and radio frequency interference. We trace the historical development of various DSL technologies and comment on possible future evolution. Transmission technologies used in the ISDN basic-access DSL, the high bit-rate DSL, and the asymmetric DSL are portrayed. And the issue of spectrum compatibility among different transmission systems is explained. Several important video coding standards are briefly described, including ITU-T's H.261 and ISO's JPEG and MPEG series, which are either completed or emerging. The synergistic relationship between these standards and the DSL technologies is elucidated. As a result, DSL technologies provide the potential of delivering certain broadband services well in advance of direct fiber access for telephone subscribers. 收起