HDLC Frame Formats

The standard frame of the HDLC protocol handles both data and control messages. It has the following format:

The HDLC frame structure consists of:

1. Flag - The value of the flag is always (0x7E). In order to ensure that the bit pattern of the frame delimiter flag does not appear in the data field of the frame (and therefore cause frame misalignment), a technique known as Bit Stuffing is used by both the transmitter and the receiver.

2. Address field - The first byte of the frame after the header flag is known as the Address Field. HDLC is used on multipoint lines and it can support as many as 256 terminal control units or secondary stations per line. The address field defines the address of the secondary station which is sending the frame or the destination of the frame sent by the primary station.

3. Control Field - The field following the Address Field is called the Control Field and serves to identify the type of the frame. In addition, it includes sequence numbers, control features and error tracking according to the frame type. Every frame holds a one bit field called the Poll/Final bit. In the NRM (Normal Response Mode) mode of HDLC this bit signals which side is 'talking' and provides control over who will speak next and when. When a primary station has finished transmitting a series of frames, it sets the Poll bit, thus giving control to the secondary station. At this time the secondary station may reply to the primary station. When the secondary station finishes transmitting its frames, its sets the Final bit and control returns to the primary station.

4. Modes of operation - HDLC has 3 modes of operation according to the strength of the master/slave relationship. This is determined by a unique frame type specifier. The three modes of operation are:

• Normal Response Mode (NRM) - The primary station initiates the session and full polling is used for all frame transmissions.
• Asynchronous Response Mode (ARM) - This mode is similar to NRM and is signified by the SARM(E) frame. The difference, however, is that secondary stations can transmit freely without waiting for a poll.
• Asynchronous Balanced Mode (ABM) - This mode is totally balanced (i.e. no master/slave relationship) and is signified by the SABM(E) frame. Each station can initialize, supervise, recover from errors and send frames at any time.

5. FCS (Frame Check Sequence) - The Frame Check Sequence (FCS) enables a high level of physical error control by allowing the integrity of the transmitted frame data to be checked. The sequence is first calculated by the transmitter using an algorithm based on the values of all the bits in the frame. The receiver then performs the same calculation on the received frame and compares its value to the CRC.