BRI - Frame format between NT and TE
Each frame has 16 bits for each B channel and 4 bits for D channel, thus yielding a data rate of 64kbps and 16kbps for B and D channel respectively
Interleaving scheme used is
Channel: B1 D B2 D B1 D B2 D
No: of bits: 8 1 8 1 8 1 8 1