ISUP Tapping

Overview

The ISDN User Part or ISUP is part of the Signalling System No. 7 (SS7) which is used to set up telephone calls in the public switched telephone network (PSTN).

When a telephone call is set up from one subscriber to another, several telephone exchanges could be involved, possibly across international boundaries. To allow a call to be set up correctly, where ISUP is supported, a switch will signal call-related information like called party number to the next switch in the network using ISUP messages.

The telephone exchanges may be connected via E1 or T1 trunks which transport the speech from the calls. These trunks are divided into 64 kbit/s timeslots, and one timeslot can carry exactly one call. Regardless of what facilities are used to interconnect switches, each circuit between two switches is uniquely identified by a circuit identification code (CIC) that is included in the ISUP messages. The exchange uses this information along with the received signalling information (especially the called party number) to determine which inbound and outbound circuits should be connected together to provide an end to end speech path.

In addition to call related information, ISUP is also used to exchange status information for, and permit management of, the available circuits. In the case of no outbound circuit being available on a particular exchange, a release message is sent back to the preceding switches in the chain.

Operation

One can perform tapping on isup calls by using sangoma tapping solution.

ISUP Tapping application will start E1 ports in tapping mode.
Sample_isup_tapping solution mainly monitors messages received in ISUP D-channel and take appropriate action as the diagram given below:

Open

• Once UP/START event is received this application mainly starts tapping the call i.e. application will open a file as specified and start reading the specific channel on which event is received and write contents present on channel in to the file as opened earlier until the channel is in ANSWER i.e UP state as shown below:

Open

• Once call is received this application mainly starting tapping the call i.e. start recording file.

• For each call two files will get recorded one on each [span : channel] i.e. one on which call is being received and another on which call is transmitted.

• One can stop the application on any point of time either by using ctrl + c or can select option to exit as present on the screen i.e. pressing 1 and then enter.

• One can hear the recorded files using audacity tool.

  NOTE: Please refer to sangoma-tdm-sdk-faq in order to get how we can play the recorded file w.r.t ISUP Tapping.

Build and Run sample ISUP Tapping application

   For WINDOWS

1. Change directory to applications\sample_isup_tapping\Win_VSS2010\.

2. Open sample_isup_tapping 2010 Visual Studio Solution File.

3. Run: Rebuild Project

   1. Note do not try to run ISUP Tapping application from Visual Studio.

   2. It needs a console.

   3. Exe file is located: Debug\sample_isup_tapping.exe.

4. Open cmd shell in Administrator mode.

5. Run: applications\sample_isup_tapping\Win_VS2010\Debug\sample_isup_tapping.exe

   NOTE: Before building ISUP Tapping application make sure that ftmod_sangoma_isup_tap module is being properly build.

   For LINUX

1. Change directory to applications/sample_isup_tapping/.

2. Run following commands:

   1. ./configure

   2. make

   3. ./run_sample_isup_tapping

NOTE: For much information w.r.t. isup application and its configuration please go through the README file as present in applications\sample_isup_tapping\README or the README file as attached with this blog.

ISUP README: