WAN Protocols : cHDLC

Cisco's HDLC

1. It is a ISO standard protocol.
2. Bit Oriented Data Link Layer protocol.
3. Encapsultes data on Synchronous serial data links using Frame Characters and Checksums.
4. Commonly used in Leased Lines.
5. No Authentication.
6. Cisco's default encapsulation.

Cisco HDLC Frame Format:



Flag:

The 8 bit pattern "01111110" which signals the beginning and end of an HDLC frame. If a piece of data within the frame to be transmitted contains a series of 5 or more 1's, the transmitting station must insert a 0 to distinguish this set of 1's in the data from the flags at the beginning and end of the frame. This technique of inserting bits is called bit-stuffing. These bits are detected and removed upon receipt. If a pattern of five 1's is followed by a 1 and then a 0, it marks the end of the frame. If a pattern of five ones is followed by two more 1's, it is a signal from the transmitting station to abort.

Address:

The address field always contains the address of a secondary station. When the primary station transmits, it is the receiving secondary station's address, if it is a secondary responding, it is it's own address. This field is only populated for Unbalanced connections, it is otherwise empty for point-to-point (Balanced) links.

Control:

The format of the control field varies with the data it contains. There are three categories of HDLC frames:
Information (I-frame) - Carries data
Supervisory (S-frame) - Carries commands and responses
Unnumbered (U-frame) - Carries additional command sequences

Proprietary:

Specifies the proprietary informations. For Eg: Cisco.
Each Vendor's HDLC has a proprietary data field to support Multiprotocol Environments.

FCS:

A frame check sequence (FCS) refers to the extra checksum characters added to a frame in a communication protocol for error detection and correction.

The sending host computes a checksum on the entire frame and sends this along. The receiving host computes the checksum on the frame using the same algorithm, and compares it to the received FCS. This way it can detect whether any data was lost or altered in transit. It may then discard the data, and request retransmission of the faulty frame. A cyclic redundancy check is often used to compute the FCS.

Source: Internet