A slip occurs when a transmitter (tx) emits frames at a different rate to the rate the receiver(rx) accepts the frames. When the transmitter's rate is greater than the receiver's, the receiver deals with the situation by discarding one frame every so often.

Conversely, when the transmitter's rate is lower than the receiver's the receiver duplicates a frame every so often.

On E1, a frame is 32 octets of data, one octet per timeslot.

On voice circuits, a slip results in an audible 'pop'.

On signalling circuits it results in corrupted packets. In MTP-2, a slip will (almost) always result in one corrupted packet.

Slip also occurs when there is Tx Rx Mismatch