Last Updated on January 30, 2026

Businesses are always searching for technology to improve their communications and a reliable high-performance telecommunications system is Integrated Services Digital Network Primary Rate Interface Transmission System 1 (ISDN PRI T1). This technology provides many benefits to companies of any size such as delivering fast data transfer and superior call quality running on Time Division Multiplexing (TDM). What exactly is TDM and how does it work?

This article will describe Time Division Multiplexing and how it works on an ISDN PRI T1 environment.

What is Time Division Multiplexing?

Time Division Multiplexing is a transmission technique that enables multiple inputs to be transferred into a single output path. TDM is accomplished by synchronizing switches at the source and receiving end of the transmission line and ensures the signal occupies the channel at a specified slot in time. Each transmitter is assigned a turn to operate and send a signal.

What is ISDN PRI T1?

ISDN is a set of communications standards for digital transmission of voice, data, and video over a Public Switched Telephone Network (PSTN). Learn more about what is an ISDN. ISDN was designed to create an end-to-end digital communication which uses two types of communication lines known as Basic Rate Interface (BRI) and PRI. BRI consists of two Bearer (B) channels and one Data (D) channel whereas a PRI consists of 23 B channels and a single D channel. Learn more about what is PRI. These channels are designed to run on a T1. A T1 is a hardware specification designed to transmit multiple TDM channels over a single line.

What is a Codec?

A Codec is a Coder / Decoder. An audio codec is a device or software that converts sound into digital and back. An audio software codec is a program that compresses and decompresses audio data while maintaining sound quality. A hardware audio codec is a device that receives audio and converts it into digital data and back into sound. The digital data support a format called Pulse Code Modulation (PCM) which means changing or modifying into a series of digital pulses.

How do Encoders create 8-bit samples?

The encoding process contains three steps Sample, Quantize, and Encode (See Encoding Process image below). At step one, a microphone records an audio sample. The sample is measured during the Quantize step two. The measurement is then converted in the bit sequencer to an eight-bit code that represents that measurement in the third step. There are 8,000 per second samples, which is the minimal sampling rate for telephone speech or Nyquist rate, and an eight-bit byte is produced for each. This creates a signal known as Digital Signal 0 (DS0) which is 64 kilobits per channel.

How does TDM work on PRI ISDN T1?

TDM provides equal time access to each channel to the shared transmission line (See Interleaving Process image below). The image represents the interleaving process of TDM. Each color represents a specific channel and interleaves 24 channels into a single line. The signal contains 24 independent time slots representing each channel. This means there are 24 channels of eight-bits equating to 192-bits with a single one-bit frame totaling 193-bits per frame or per rotation. There are 8,000 rotations per second of 193-bits which totals 1.544 million bits per frame or per rotation. This is known as a Digital Signal 1 (DS1) in a T-type system.

How to Record Calls on ISDN PRI T1 TDM.

There are several reasons a business would need call recording software. New company policies may be in effect, government rules or regulations may change, or the business may be focusing on improving the performance of their customer service or sales team. For businesses running on ISDN PRI T1 TDM there is a call recording solution offered by Versadial designed to quickly and easily record phone calls on this environment.

Versadial offers an effective method known as Call Recording Scenario 3 – Business Phone System, Passive Recording From The Trunk Lines. Analog and TDM lines are physically tapped to recording boards in this call recording scenario. The tapping takes place between the PBX or phone system and the trunk lines. All calls are recorded via the passive method meaning the recording device does not participate in the recording process. The participants in the call may not be aware of the existence of the call recorder. In Active call recording, participants “invite” the call recorder into to conversation and control what is recorded in the conversation. Learn more about Recording Calls on T1 / E1 / ISDN PRI Lines.

Contact Us to learn more about call recording solutions for your company.

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Last Updated on January 30, 2026