资料来源 : Free On-Line Dictionary of Computing

DS1
     
         A {DS level} and {framing specification} for
        synchronous digital streams, over circuits in the North
        American {digital transmission hierarchy}, at the {T1}
        transmission rate of 1,544,000 bits per second ({baud}).
     
        DS1 is commonly used to multiplex 24 {DS0} channels.  Each DS0
        channel, originally a digitised voice-grade telephone signal,
        carries 8000 bytes per second (64,000 bits per second).  A DS1
        frame includes one byte from each of the 24 DS0 channels and
        adds one {framing bit}, making a total of 193 bits per frame
        at 8000 frames per second.  The result is 193*8000 = 1,544,000
        bits per second.
     
        In the original standard, the successive framing bits
        continuously repeated the 12-bit sequence 110111001000, and
        such a 12-frame unit is called a super-frame.  In voice
        telephony, errors are acceptable (early standards allowed as
        much as one frame in six to be missing entirely), so the least
        significant bit in two of the 24 streams was used for
        signaling between network equipments.  This is called
        {robbed-bit signaling}.
     
        To promote error-free transmission, an alternative called the
        extended super-frame (ESF) of 24 frames was developed.  In
        this standard, six of the 24 framing bits provide a six bit
        {cyclic redundancy check} (CRC-6), and six provide the actual
        framing.  The other 12 form a virtual circuit of 4000 bits per
        second for use by the transmission equipment, for {call
        progress signals} such as busy, idle and ringing.  DS1 signals
        using ESF equipment are nearly error-free, because the CRC
        detects errors and allows automatic re-routing of connections.
     
        Compare {T-carrier systems}.
     
        [Kenneth Sherman, "Data Communications : a user's guide",
        third edition (1990), Reston/Prentice-Hall/Simon & Schuster].
     
        (1996-03-30)