The HDLC protocol is a data link layer used for transmitting data between called nodes. The HDLC protocol embeds information in a data frame that allows devices to control data flow and correct errors. A frame of data is encapsulated by flags. The beginning and end of an HDLC frame are marked by flag characters - 01111110 binary.

At the end of the frame, a Frame Check Sequence (FCS) is used to verify the data integrity. The FCS is a CRC calculated using polynomial x^16 + x^12 + x^5 + 1 (FCS 16 bits) or x^0+x^1+x^2+x^4+x^5+x^7+x^8+x^10+x^11^x^12+x^16+x^22+x^23+x^26+x^32 (FCS 32 bits).

Idle periods in the data stream contain only flags.

GL's T3/E3/Datacom HDLC protocol analyzer provides the capability to capture, and analyze HDLC data on a T3/E3/Datacom lines. There may be a single data channel per T3/E3/Datacom line. After setting the T3/E3/Datacom ports and FCS type and starting the decoding process, the main screen displays the captured HDLC frames. Flags are stripped, and all other data is presented, including FCS bytes.

In addition to HDLC Analyzer, T3/E3/Datacom analyzer supports HDLC Tx Rx Test application that further helps in transmitting and capturing pre-defined HDLC frames.