ISDN Tapping

Overview

Integrated Services for Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network. It was first defined in 1988 in the CCITT red book. Prior to ISDN, the telephone system was viewed as a way to transport voice, with some special services available for data. The key feature of ISDN is that it integrates speech and data on the same lines, adding features that were not available in the classic telephone system. There are several kinds of access interfaces to ISDN defined as Basic Rate Interface (BRI), Primary Rate Interface (PRI), NarrowbandISDN (N-ISDN), and Broadband ISDN (B-ISDN).

 ISDN is a circuit-switched telephone network system, which also provides access to packet switched networks, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in potentially better voice quality than an analog phone can provide. It offers circuit-switched connections (for either voice or data), and packet-switched connections (for data), in increments of 64 kilobit/s. A major market application for ISDN in some countries is Internet access, where ISDN typically provides a maximum of 128 kbit/s in both upstream and downstream directions. Channel bonding can achieve a greater data rate; typically the ISDN B-channels of three or four BRIs (six to eight 64 kbit/s channels) are bonded.

ISDN Channels

A CHANNEL is the basic unit of ISDN service. The ISDN uses two bearer channels and one delta channel:

• Bearer channels (B channels)

• Delta (or "Demand") channels (D channels)

• H Channels

B Channel

A B channel is a 64-Kbps unit of clear digital bandwidth in which the primary data or voice communication is carried.

D Channel

A D channel is a signalling channel in which the control and signalling information is carried.The D channel can also carry packet-switched data using the X.25 protocol.

The D channel is not a clear channel. It operates according to a well-defined pair of layered protocols:

• Q.921 (LAPD) at the Data Link Layer (Layer 2)

• Q.931 at the upper layers (Layers 3 and above)

The bit rate of the D channel of a basic rate interface is 16 kbit/s, whereas it amounts to 64 kbit/s on a primary rate interface.

H Channel

An H channel is a high-speed channel comprising multiple aggregated low-speed channels to accommodate bandwidth-intensive applications such as file transfer, videoconferencing, and high-quality audio. An H channel is formed of multiple bearer B channels bonded together in a primary rate access (PRA) or primary rate interface (PRI) frame in support of applications with bandwidth requirements that exceed the B channel rate of 64 kbit/s. The channels, once bonded, remain so end-to-end, from transmitter to receiver, through the ISDN network. 

Operation

One can perform tapping by using sangoma sdk i.e. sangoma tapping solution.

ISDN Tapping application will start T1/E1 ports in tapping mode.
Sample_isdn_tapping solution mainly monitors events on ISDN channel i.e. B-channel/D-channel as explained above and take appropriate action as the diagram given below:

Open

• Once UP/START event is received this application mainly start 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

• 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 ISDN Tapping.

Build and Run sample ISDN Tapping application

   For WINDOWS

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

2. Open sample_isdn_tapping 2010 Visual Studio Solution File.

3. Run: Rebuild Project

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

   2. It needs a console.

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

4. Open cmd shell in Administrator mode.

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

   For LINUX

1. Change directory to applications/sample_isdn_tapping/.

2. Run following commands:

   1. ./configure

   2. make

   3. ./run_sample_tapping

NOTE: For much information w.r.t. isdn application and its configuration please go through the README file as present in applications\sample_isdn_tapping\README.