The PCM frame rate is 8000 frames a second. With DS1, each frame has one framing bit. Thus 8000 framing bits are transmitted per second. With modern processor technology, all of the 8000 framing bits are not needed to keep the system frame-aligned. Only one-quarter of the 8000 framing bits per second are actually necessary for framing and the remainder of the bits, 6000 bits per second, can be used for other purposes such as on-line gross error detection and for a maintenance data link. To make good use of these overhead bits, DS1 frames are taken either 12 or 24 at a time. These groupings are called superframe and extended superframe, respectively. The extended superframe, in particular, provides excellent facilities for on-line error monitoring and troubleshooting.
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