U-Interface

The `U interface' is a popular interface for relatively high speed data communication. The U interface is used for both ISDN and baseband (IDSL) modems.

ISDN has more or less the following structure;

  +-----------+   +-----+
  | Telephone +---+ A/B +---+ S-Bus
  +-----------+   +-----+   |
                          +-+--+  U-Interface  +----+   +--------------+
                          | NT +---------------+ LT +---+ Tel exchange |
                          +-+--+               +----+   +--------------+
  +-----------+             |
  | ISDN card +-------------+ S-Bus
  +-----------+

A/B  A/B Adaptor
NT   Network Terminator (sync slave)
LT   Line Terminator (sync master)

The S bus consists of two 64 kbps signals (data or voice) + 16 kbps signalling. This means that the U bus has to transport at least 2 * 64 kbps + 16 kbps = 144 kbps.
The NT is in fact a S bus to U interface converter. The LT is an interface between the U bus and the local office.

                NT                             LT
           +----------+    U-Interface    |
  S-Bus ---+ S <--> U +-------------------+ U <--> Exchange
           +----------+                   |

If you replace the S interface with a V35 or X21 interface, you get a 144 kbps modem;

           +------------+                 +------------+
  X21 -----+ X21 <--> U +-----------------+ U <--> V35 +----- V35
           +------------+                 +------------+

Besides the 144 kbps user data, the NT and LT also communicate with each other via a 16 kbps house keeping channel. This means that the total amount of data on the U interface is 144 + 16 = 160 kbps.
By feeding groups of 2 bits each to a 2 bit DA converter, which translates the patterns 00, 01, 10 and 11 to four different voltage levels, the symbol rate is reduced to 80 kbaud. This conversion is called `2B1Q';

 00  -3
 01  -1
 10  +3
 11  +1

    -->  <-- 12.5 us
 +3   +--+           +--+                 +-----+
      |  |           |  |                 |     |
 +1   |  +--+     +--+  |        +--+     |     |  +--
      |     |     |     |        |  |     |     |  |
 -1 --+     |     |     |  +-----+  +--+  |     +--+
            |     |     |  |           |  |
 -3         +-----+     +--+           +--+

   |01|10|11|00|00|11|10|00|01|01|11|01|00|10|10|01|11|

The rise and fall times of the pulses are limited, so in fact the pulses are trapezoid rather than square shaped.
The signal is fed to the transmission line by means of a transformer. The signal strength is about 13.5 dB. The transmission line impedance is 93 ... 150 Ω
A fork circuit separates the transmitted from the received signal. An echo canceller removes echoes of the transmitted signal from the received signal.