With ISDN service, signaling that must be conveyed between the user station and the local switch is carried on a separate digital channel called the D channel.

与必须表达在用户驻地和本地开关之间的ISDN服务，发信号继续称D渠道的一条分开的数字通道。

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Under the Federal Communications Commission's requirement, cable operators must either switch to all-digital systems, or carry both the digital and analog signals of local stations in the first three years after the February 2009 conversion.

WSJ: Cable Must Air Broadcast Signals