Formats of HDLC Frames

Created: Feb 24, 2021 02:46:07Latest reply: Jan 16, 2022 07:02:11 733 14 13 0 0

Member interfaces of an IP-Trunk interface must be encapsulated with HDLC. That is why it is important to know about the HDLC format.

In HDLC, data and control packets are transmitted in the standard format of a frame.

A complete frame of HDLC consists of the Flag field, Address field, Information field, and Frame check sequence (FCS) field. Figure 1 shows the format of a complete HDLC frame.

Figure 1 Format of an HDLC frame

Table 1 describes the meaning of those fields.

Table 1 Description about the fields of the HDLC frame
Field	Length (Byte)	Description
Flag	1	Indicates the Flag field whose value is fixed 01111110, in bytes. The Flag field identifies the beginning and ending of a frame. Only one Flag field is required between two frames. Two sequential Flag fields are considered as a null frame that is discarded on the receiving end, but the action of discarding the null frame is not regarded as the FCS error. Generally, channels are still in the active state when frames are not transmitted. In such state, the transmitter sends flag fields continuously and the receiver detects every received Flag field. Once the receiver finds that a Flag field is not followed by another Flag field, a new round of frame transmission begins.
Address	1	Indicates the Address field whose content depends on the adopted operation mode. A unique address is allocated to a secondary node and a combination node respectively. The Address field in a command frame is filled with the peer address, but the Address field in a response frame is filled with the local address. One address can be allocated to more than one node. This type of address is called group address. The frame with a group address can be received by all the nodes that belong to this group. However, a node or a combination node sends the response using its own address rather than the group address. The address 255.255.255.255 is a broadcast address. The frames carrying this address will be sent to all nodes on the link. As stipulated, the address 0.0.0.0 is not allocated to any node, and is used only for detection.
Control	1	Indicates the Control field that is used to form all types of commands and responses to monitor and control the link.The primary node or combination node of the transmitter uses the Control field to notify the destination secondary node or combination node to perform the specified operation. Conversely, the secondary node uses this Control field as response to the command and to report the finished operations or the changes of status.
Protocol	2	Indicates the Protocol field.The Protocol field identifies the type of protocols encapsulated in the data in the Information field.
Information	N	Indicates the Information field.The Information field can be arbitrary binary bit string whose length is not limited. The upper limit of the Information field is decided by the FSC field or the buffer capacity of a communication node, currently, the bits ranging from 1000 to 2000 is used frequently. The lower limit can be 0, which means that no Information field exists. However, a monitor frame cannot contain the Information field.
FCS	2	Indicates the FCS field.The FCS field uses 16-bit CRC to check the content of a frame between two Flag fields.