How ISDN works :

ISDN uses a digital infrastructure to transport information. Unlike analog telephone lines that transmit signals as continuous electrical waves, ISDN digitizes data by converting it to 0s and 1s, resulting in faster transmission and better signal quality.

ISDN offers two types of channels :

Bearer Channel : It is used for the transmission of user data, such as voice or computer data. Channel B has a transmission capacity of up to 64 kbps (kilobits per second) per channel. In some cases, multiple B-channels can be aggregated to increase bandwidth.

Data Channel : It is used for connection control and signaling. The D channel carries the signaling information needed to establish, maintain, and terminate calls.

Digital telephony :
ISDN allows voice to be transmitted in digital form, resulting in clearer and more stable audio quality compared to analog phone lines.
Digital telephony via ISDN supports advanced features such as call forwarding, call waiting, direct dialing and caller ID.
Users can also have multiple phone numbers on a single ISDN line, each associated with a different Multiple Subscriber Number (ISDN MSN).

Internet access :
ISDN has been widely used to provide a connection to the Internet to individuals and businesses.
With an ISDN Baseline (BRI), users can achieve download speeds of up to 128 kbps and upload speeds of up to 64 kbps.
The higher connection speeds were an advantage over traditional analog modems, which allowed for faster access to websites and an improved online experience.

Fax :
ISDN supports the transmission of faxes at faster speeds and with better quality than analog telephone lines.
Users can send and receive faxes reliably and efficiently using ISDN's digital infrastructure.
The improved quality of data transmission ensures that faxed documents are received with fewer errors and distortions.

Video conferencing :
ISDN has also been used for video conferencing, allowing users to hold remote meetings with colleagues, clients, or other stakeholders.
The bandwidth available on ISDN lines allowed for the transmission of real-time video streams with acceptable quality, although limited compared to newer video conferencing technologies.

Data Services :
In addition to voice and video, ISDN enabled the transmission of computer data, making it a popular choice for businesses needing reliable and fast connectivity.
ISDN data services were used for connecting local area networks (LANs) and wide area networks (WANs), as well as for remote access to computer systems.

Central Office (CO) :
The Central Office is the central node of the ISDN network. This is where the ISDN lines of subscribers are connected to the network. The CO manages the establishment and maintenance of ISDN connections.

Terminal Equipment (TE) :
Terminal Equipment represents the terminal equipment used by subscribers to connect to the ISDN network. These can be ISDN phones, fax machines, data terminals, user interface adapters (UIAs), and more.

Network Termination (NT) :
Network Termination is the point at which the subscriber's equipment physically connects to the ISDN network. This can be an NT1 (for BRI baseline connections) or an NT2 (for PRI trunk connections).

User Interface (UI) :
The User Interface is the interface between the subscriber equipment (CT) and the ISDN network. For Baseline Connections (BRIs), the user interface is typically provided by an NT1. For mainline connections (PRIs), the user interface can be an NT1 or terminal equipment (for example, a PBX).

Signaling protocols :
ISDN uses signaling protocols to establish, maintain, and terminate connections. The main signaling protocols used in ISDN are DSS1 (Digital Subscriber Signaling System No. 1) for baseline connections and Q.931 for trunk connections.

Bearer Channel :
Channel B is used to transport user data, such as voice, computer data, etc. Each B-channel has a transmission capacity of up to 64 kbps. For Baseline Connections (BRI), there are two B channels available. For mainline connections (PRIs), there can be multiple B-channels.

Data Channel :
Channel D is used for connection control and signaling. It carries the signaling information needed to establish, maintain, and terminate ISDN calls.

Types of ISDN lines :
There are two main types of ISDN lines : the Basic Rate Interface (BRI) and the Primary Rate Interface (PRI). BRI is typically used for residential and small business installations, while PRI is used for larger businesses and grids.

- Better sound quality for phone calls.
- Faster data transmission.
- Support for multiple services on a single line.
- Direct dialing and caller ID capability.

- Relatively high cost compared to analog services.
- Limited deployment in some regions.
- ISDN technology has become obsolete with the advent of more advanced technologies such as ADSL, cable, and fiber optics.

Despite its advantages at the time, ISDN has largely been replaced by more modern technologies that offer higher speeds and better efficiency, such as ADSL, fibre optics and mobile broadband networks.