HDLC frame format and frame type
HDLC frames include the following types:
Information frame (I frame)—Responsible for transmitting useful data or information.
Supervision frame (S frame)—Responsible for error control and flow control.
Unnumbered frame (U frame)—Responsible for the link establishment, teardown, and so on.
An HDLC frame is composed of the following fields:
Flag—The flag field, 01111110, marks the beginning and end of an HDLC frame. Each frame begins with a 7E and ends with a 7E. The 7E between two neighboring frames functions both as the end of the frame in the front and as the beginning of the following frame.
Address—The address field is 8 bits; it identifies the source or destination where the frame is sent or received.
Control—The control field is 8 bits and identifies the control type and defines the frame type (control or data).
Information—The information field can be an arbitrary binary bit set. The minimum length can be zero and the maximum length is decided by the FCS field or the buffer size of the communicating node. The maximum length is between 1000 and 2000 bits.
Checksum—The checksum field can use a 16-bit CRC to check the content of a frame.