The Bt8954 framer has been tailored specifically to meet the needs of voice pair gain systems (also referred to as "cable relief systems" and "digital subscriber line carriers") by providing a direct connection to the DSL modem and the CODEC. It performs data, clock, and format conversions necessary to construct a Pulse Code Multiplexed (PCM) channel from a Symmetrical Digital Subscriber Line (SDSL) or a High-Bit-Rate Digital Subscriber Line (HDSL) channel. The PCM channel consists of transmit and receive data, clock, and frame sync signals configured for 218 voice channels. The PCM channel connects directly to popular PCM codecs. The Digital Subscriber Line (DSL) channel interface consists of serial data and clock connected to a RS8973, Bt8970 or a Bt8960 DSL Transceiver. The Bt8954 supports clear and compressed voice system. When coupled with a Bt8960, the Bt8954 provides PCM4 functions at greater than 5 km reach with no voice compression, allowing V.34 modem operation. At one end, Bt8954 multiplexes payload data from several PCM codecs with the appropriate overhead and signaling bits into one transport frame that is passed on to the bit-pump, for transport over a single twisted pair. At the other end, Bt8954 demultiplexes the DSL bit stream into payload data sent to the PCM codec, and overhead data written into microcomputer-accessible registers. Embedded Operations Channel (EOC) and signaling overhead can be inserted via the Microcomputer Interface (MCI). Control and status registers are accessed via the MCI. One common register group configures the PCM interface formatter, Phase-Locked Loop (PLL), and PCM Loopback (LB). Another group of DSL channel registers configures the elastic store FIFOs, overhead muxes, receive framer, payload mapper, and the DSL loopback. Status registers monitor received overhead, PLL, FIFO, and framer operations, including CRC and FEBE error counts.