Valid frames are terminated by a closing flag. If the link layer protocol needs to transmit a higher priority frame before the current frame has been fully sent, it may "Abort" the frame. An "aborted" frame is terminated by an "abort sequence": 0111 1111 instead of the normal "flag sequence". Abort sequences may also be caused by bit errors which occur while the frame is travelling from the transmitter to the receiver. (For instance a flag may suffer a 1-bit inversion of the least significant bit, resulting in a bit sequence identical to an "abort").
A frame which is terminated by an abort is received by the receiver in the normal way, but marked as being "aborted". The frame is then discarded without further processing. This is shown below:
(fig)An HDLC frame terminated by an "abort" is discarded by the HDLC receiver
An abort sequence is often followed by a series of 1's. The sequence of all 1's may be used to fill the gaps between frames (or alternatively a continuous series of flags may be transmitted). The all 1's sequence is known as the "idle" sequence, since the line becomes idle (N.B. represented at the physical layer by 0 Volts). This is useful for applications requiring half duplex operation (also known as Two Way Alternate (TWA) operation).
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