ANSI standard T1.601 describes the  U-reference point for BRI. The local loop will be between NT1 and  LE. The U reference point provides serial, full duplex, point-to-point communication.  Echo cancellation is used to reduce noise.  Echo cancellation involves inserting negative of transmitting voltage in the receiving circuitry.  It uses hybrid, to  do 4-wire to 2-wire conversion.

The scheme that achieves two wire to four wire conversion is depicted below. It uses a hybrid balanced
impedance with echo cancellation to mutiplex and demultiplex the Tx and Rx signals at the Two-Four
wire interface.

The line coding scheme   used for U-interfaces is 2B1Q (2 Binary 1 Quarternary).  The input voltage can be one of 4 levels (0 volt is not a valid level). These levels are called Quarternaries. Each quartenary represents 2 data bits, the first bit of each pair represents the sign, of quarternary symbol., while second bit represents the magnitude.

  The U reference point operates at 160kbps, since 2B1Q represents pair of bits with single signal, the   rate of signaling is 80kbaud. (80,000 signals per second).
The summary is as follows.
* two bits per  baud.
* 80k baud per second.
* transfer rate of 160kbps per second.