A term used to describe any data/voice connection with a bandwidth of less than 2.048Mb/s. This figure is arrived at by calculating ther maximum data throughput of a PCM cable when using in a System X exchange.

- A System X exchange samples the analogue signal 8000 times a second
- Each sample is digitised into 8 bits
- Each PCM carries 32 channels of information, so:

8 x 8000 x 32 = 2048000 b/s or 2.048Mb/s

This is obviously a UK only definition, as the US telephone network uses a different exchange architecture