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 x16 + x12 + x5 + 1 (FCS 16 bits) or x0 + x1 + x2 + x4 + x5 + x7 + x8 + x10 + x11 + x12 + x16 + x22 + x23 + x26 + x32 (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.
HDLC Analyzer Main Features
|Supported Protocols||LAPF, LAPD, LAPD+IP, LAPX+IP, X.25, Cisco HDLC Protocols|
|Filtering and Search||Advanced filtering and search based on any user selected protocol fields|
|Capturing Streams||Streams can be captured on the selected ports|
|Remote Monitoring||Remote monitoring capability using GL's Network Surveillance System|
Following is a description of the protocols supported by GL's HDLC analyzer:
|Supported Protocols||Specification Used|
|DHCP||RFC 1533, 2131|
|SNMP||RFC 1157,1155,1902,3416,2863,2578,3418,2011,2012 etc|
|LAPB, X.25||ITU-T Recommendation X.25|
Summary, Detail and Hexdump Views
The HDLC analyzer application is invoked from the main menu of GL's T3/E3/Datacom Analyzer for real-time analyzer. The analyzer displays summary, detail and hex dump view in different panes. The summary pane displays Frame Number, Time, Length, Error, C/R, SAPI, TEI, CTL, P/F, N(S), N(R), FUNC etc. User can select a frame in summary view to analyze and decode each frame in the detail view. The selected frame is analyzed and decoded according to HDLC frame specifications. The Hex dump view displays the frame information in HEX and ASCII.
Real-time and Offline Analysis
Users can capture and analyze multiple HDLC links in real-time and record all or filtered traffic into a trace file. The recorded trace file can then be analyzed offline and exported to ASCII file, or printed. Real-time capturing requires user to specify Tx/Rx ports FCS (16 & 32 bits).
Filtering and Search
Users can record all or filtered traffic into a trace file. Filtering and search capability adds as another powerful feature to the HDLC Analyzer. This feature can isolate required frames from all frames in real-time as well as off-line. Users can specify custom values for frame length to filter frames during real-time capture. The frames can also be filtered after completion of capture according to CTL, C/R, Modifier Function, N(R), N(S), P, P/ F, SAPI, Supervisory function and TEI. Similarly, search capability helps user to search for a particular frame based on specific search criteria.