T3/E3 HDLC Protocol Analysis
HDLC Protocol Analysis for T3 (DS3) / E3 Analyzers
Overview
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 HDLC protocol analyzer provides the capability to capture, and analyze HDLC data on a T3/E3 lines. There may be a single data channel per
T3/E3 line. After setting the T3/E3 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 analyzer supports HDLC Tx Rx Test application that further helps in transmitting and
capturing pre-defined HDLC frames.
HDLC Analyzer Main Features
- Displays Summary, detail, hex-dump, statistics, and call trace views.
- Summary view displays device#, frame#, LAPD, LAPF specific information and IP information like source/destination IP address, TCP source port, TCP
destination port etc.
- Detailed view displays decodes of user-selected frames.
- Statistics view displays statistics based on frame count, byte count, frames/sec, bytes/sec etc for the entire captured data
- Hex dump view displays the frame information in HEX and ASCII format.
- Supports filtering and search features based on C/R, SAPI, TEI, CTL, P/F, N(S), N(R) and FUNC
- Exports detailed and summary information to a comma delimited file for subsequent import into a database or spreadsheet.
- Status bar displaying information regarding running percent utilization, Number of frames captured, CRC errors and Frame errors etc.
- Streams can be captured on the selected ports.
- Real-time capture filter can be set for filtering frames based on frame length.
- Multiple streams of HDLC traffic on various T3/E3 ports can be simultaneously decoded with single GUI instance.
- Trace files for offline analysis can be loaded in different GUI instances simultaneously
Supported Protocols
Following is a description of the protocols supported by GL's HDLC analyzer:
Supported Protocols Structure for HDLC Protocol Analysis
- LAPD: Decodes Layer 2 as Link Access Protocol as defined in the ITU Q.921
- LAPF: Decodes Layer 2 as Link Access Procedure/Protocol (LAPF) as defined in the ITU Q.922, an enhanced LAPD (Q.921) with congestion
control capabilities, is for Frame Mode Services in the Frame Relay network.
- LAPD+IP: Decodes Layer 2 as Link Access Protocol as defined in the ITU Q.921 & Layer 3 as Internet Protocol (IP)
- LAPX+IP: Dumps Layer 2 Link Access Protocol data & decodes Layer 3 as Internet Protocol (IP)
Summary, Detail and Hexdump Views
The HDLC analyzer application is invoked from the main menu of GL's T3/E3 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.
Protocol Summary, Detailed, and Hexdump Views in HDLC Protocol Analysis
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).
Port Selection for real-time capture
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.
Offline Filtering and Search Criteria for HDLC Protocol Analysis
Statistics View
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 statistics
can be obtained to study the performance and trend in the HDLC network and it is based on protocol fields and different parameters e.g. Use Type
(Key/Total/Field), Statistic type (Frame count, Byte count, Frames/Sec) and patterns like Range List, Wild card etc.
Calculated Statistics View in HDLC Protocol Analysis
Enhanced Trace Saving Options
Users can control the captured trace files by saving the trace using different conventions such as trace files with user-defined prefixes, trace file with
date-time prefixes, and slider control to indicate the total number of files, file size, frame count, or time limit. This feature also allows the captured frames to
be saved into a trace file based on the filtering criteria set using display filter feature
Protocol Trace Saving Options for HDLC Protocol Analysis
HDLC Test Application
Transmit and Receive HDLC frames Test Application
USB T3/E3 HDLC Test Optional Application is designed to transmit and receive HDLC frames over unstructured T3/E3. It generates frames in memory
and fills them with 64 bit sequential integer numbers. The number is either truncated if the frame length is less than 8 bytes or the same number bytes are
concatenated to make up the complete frame
At the end of each frame a Frame Check Sequence (FCS) is generated to verify the data integrity on the receive side. The FCS (also knows and
cyclic redundancy check CRC) is calculated using a polynomial.
16 bit FCS is generated using polynomial 1+x^5+x^12+x^16
32 bit FCS is defined in RFC 1662 and is using polynomial 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
Three types of frame length sequences are supported:
- Fixed length
- Variable length with framed data length is incremented from the minimum value to the maximum value by one and after maximum length is reached the minimum length is transmitted
- Random length in the minimum – maximum range.
User can specify the number of flags between frames to vary the data utilization.
The receiving part receives the frames and verifies if the received frames conform to the specified pattern data sequence and frame length. The following
errors are counted:
- Frame errors – HDLC zero bit insertion/deletion error that are constituted by non-integral number of octets between flags
- Length errors – frame length is different from expected
- CRC errors – FCS errors
- Sequence errors – sequential 64 bit integer numbers places in the frame are out of sequence.
Higher level protocol headers can be specified by user and prepended to frames.
Multiple instances of the application can run simultaneously for different ports. Each instance can be:
- Receive and transmit
- Receive only
- Transmit only
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