The diagram below illustrates, in a simplified form, the principle of rectangular interleaving. This is applied to most control channels.
The diagram shows a sequence of ‘control blocks’ after the encoding process. Each block contains 456 bits, these blocks are then divided into four blocks each containing 114 bits. Each block will only contain bits for even or odd bit positions. The GSM burst will be produced using these blocks of control.
Each burst will be transmitted in the designated timeslot of four consecutive TDMA frames, providing the interleaving depth of four.
The control information is not diagonally interleaved as are speech and data. This is because only a limited amount of control information is sent every multiframe. If the control information was diagonally interleaved, the receiver would not be capable of decoding a control message until at least two ultiframes were received. This would be too long a delay.