SA HDLC and SSM Protocol Analysis

Timeslot 0 of every basic frame is reserved for basic frame alignment and contains either a Frame Alignment Signal (FAS) or a Non-Frame Alignment Signal (NFAS). FAS and NFAS occur in timeslot0 of consecutive basic frames. Bits four to eight of the NFAS (i.e., Sa4 - Sa8) are additional spare bits generally called as Signaling Associated (Sa) bits.

The HDLC frames can be transmitted on timeslot 0 Sa bits (bits 4-8 of the non-frame alignment signal) and can operate at 4,8,12,16 or 20 kbit/s depending on which Sa bit is selected.

GL's Sa Bits HDLC Analyzer performs Sa Bits HDLC protocol analysis on E1 timeslot 0 during real-time and off-line. Users can capture stream of Sa Bit HDLC frames on the selected even or odd frames of the E1 multiframe on timeslot 0. Captured information can be saved to disk for later off-line analysis. Both real-time and off-line analysis presents summary and detailed views of Sa Bit HDLC decode information. Captured frames can later be used for traffic simulation using the Sa Bits HDLC transmit application.

GL Communications support the following types of Sa Bit HDLC analyzers:

• Real-time Sa Bit HDLC Analyzer (Pre-requisites: GL's field proven E1 or T1 internal cards or Laptop E1 or T1 external units, required licenses and Windows Operating System)
• Offline Sa Bit HDLC Analyzer (Pre-requisites: Hardware Dongle, and Windows Operating System)

In addition, the following Sa Bit HDLC applications are supported that further help in transmitting and capturing pre-defined Sa Bit HDLC frames

• Sa Bit HDLC Playback
• Automated Sa Bit HDLC Testing using Client Server
• SSM Analyzer

Sa Bits HDLC Analyzer application is invoked from the main menu of GL's E1 Analyzer for real-time analyzer. The offline Sa Bits HDLC analyzer is invoked from the installation directory of the offline Sa Bits HDLC Analyzer. The analyzer displays, Summary, Detail, and Hex dump View in different panes. Summary View displays Device No, Time Slots, Sub Channel, Frame Number, Time, Frame Length, Frame Errors, C/R, CTL, P/F, SAPI N (S), N(R), etc. for all frames. User can select a frame in Summary View to analyze and decode each frame in the Detail View. Hex dump view displays the captured data for the selected frame in hex format.

User can capture and analyze Sa Bit frames using real time feature and can record and filter traffic into a trace file. The recorded trace file can be used for offline analysis or exported to a comma-delimited file, or ASCII file. Real-time capturing requires user to specify even or odd frames on a timeslot, signaling links, and bit inversion. Recorded trace file can be played back on E1 using Sa Bits HDLC file playback option.

Users can record all or filtered traffic into a trace file. Filtering and Search capabilities is an important feature of Sa Bits HDLC analyzer s/w. These features isolate certain specific frames from all frames in real-time as well as offline. 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 C/R, CTL, P/F,SAPI, N (S), N(R), and more. The criteria for filtering can be set by user and a separate GUI is provided for that. Similarly, Search capability helps user to search for a particular frame based on specific search criteria.

Statistics is an important feature available in Sa Bits 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 Sa Bits network and it is based on protocol fields and different parameters.

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 Configuration window provides a consolidated interface for all the important settings required in the analyzer. This includes various options such as protocol selection, startup options, stream/interface selection, filter/search criteria and so on. Any protocol field can be added to the summary view, filtering, and search features from this GUI providing the users more flexibility to monitor required protocol fields. All the configuration settings done in any of these options can be saved to a file, loaded from a configuration file, or user may just revert to the default values using the default option.

The SA Bits HDLC Playback provides HDLC and/or codeword transmission using SA bits on E1 line(s). Files captured by the SA Bits HDLC Analyzer can be replayed using this application on one or multiple E1 lines. Code words can be specified via user interface.

The SA HDLC playback main screen allows the user to selecting card (line), SA bits and make a choice either to transmit HDLC frame transmission or Codewords transmission by clicking 'Add' in the main screen.
There are various options with which HDLC transmission could be done such as:

• Continuous play (after end of last frame, the first frame is transmitted) with manual intervention to terminate.
• Limited option to limit the number of frames to user defined value.
• Invert bits specification conversion.
• Option to specify 'Flags between frames'.

After specifying various parameters, the selected SA bits would be displayed.

Starting and stopping transmission

After Code words and/or HDLC Information has been specified, the application is started. Transmission stops automatically only when HDLC frames are transmitted (no code words) and no continuous play has been selected. Playback stops when the file end or frame limit has been reached. In this case flags are transmitted at the end.

If the transmission is interrupted by clicking on the 'Stop' button, it is not guaranteed that flags will be transmitted. When mix of Code words and HDLC frames is transmitted the Codeword transmission does not stop until the 'Stop' button is clicked.

GL’s Synchronization Status Message (SSM) analyzer operates in accordance with G.704 standard and analyzes 4 bit long SSM messages received over E1 San bits in time slot 0. The SSM Analyzer is licensed along with SA bits HDLC transmit and analysis modules using the same part number.