T3 E3 Testing

T3/E3 Datacom HDLC Protocol Analysis

HDLC Protocol Analysis for T3 (DS3) / E3 Analyzers



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

Display Features
  • Displays Summary, Detail, Hex-dump, and Statistics Views
  • Detail View
    • Displays decodes of a user-selected frame from the summary view
    • Provides options to display or hide the required protocol layers
    • Contents of this view can also be copied to clipboard
  • Summary View displays timeslot, sub channel, frame#, device#, LAPD, LAPF specific information and IP information like source/destination IP address, TCP source port, TCP destination port, and so on in a tabular format
  • Any protocol field can be added to the summary view, filtering, and search features providing users more flexibility to monitor required protocol fields.
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
Export Options
  • Exports Summary View information to a comma delimited file for subsequent import into a database or spreadsheet.
  • Capability to export detailed decode information to an ASCII file
  • Captured frames can later be used for traffic simulation using HDLC Transmit/Receive/Playback application
  • Recorded trace files can be played back using HDLC playback option.
Remote Monitoring Remote monitoring capability using GL's Network Surveillance System

Supported Protocols

Following is a description of the protocols supported by GL's HDLC analyzer:

Supported Protocols Structure for HDLC Protocol Analysis

Supported Protocols Structure for HDLC Protocol Analysis

Supported Protocols Specification Used
LAPD CCITT Q.920/921
IP RFC 791
DNS RFC 1035
DHCP RFC 1533, 2131
SNMP RFC 1157,1155,1902,3416,2863,2578,3418,2011,2012 etc
LAPB, X.25 ITU-T Recommendation X.25
Cisco HDLC -

Summary, Detail and Hexdump Views

Protocol Summary, Detailed, and Hexdump Views in HDLC Protocol Analysis

Protocol Summary, Detailed, and Hexdump Views in HDLC Protocol Analysis

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

Port Selection for real-time capture

Port Selection for real-time capture

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

Offline Filtering and Search Criteria for HDLC Protocol Analysis

Offline Filtering and Search Criteria for HDLC Protocol Analysis

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.

Statistics View

Calculated Statistics View in HDLC Protocol Analysis

Calculated Statistics View in HDLC Protocol Analysis

Statistics is an important feature available in HDLC analyzer and can be obtained for all frames both in real-time as well as offline mode. Numerous statis