Beyond Speed: A Platform for New Services

ISDN was more than just a faster alternative to dial-up modems; it was a comprehensive platform for integrated digital services. Its digital nature enabled a wide array of applications that were previously impossible over a single analog phone line. These services are typically categorized into two main groups: Bearer Services, which define the raw data transport, and Teleservices, which are complete end-to-end applications for the user.

Bearer Services: The Digital Pipes

Bearer services are the fundamental transport capabilities offered by the ISDN network. They provide a clear, transparent digital pipe between two points, allowing the user to send any type of data they wish.

• Unrestricted 64 kbps Digital Service: This is the core ISDN bearer service. It provides a synchronous, clear-channel $64 \text{ kbit/s}$ connection. It's "unrestricted" because the network doesn't care what the bits represent (it could be voice, data from a computer, fax images, or anything else).
• Speech Service (3.1 kHz Audio): While a B-channel could carry voice using the unrestricted service, this specific bearer service signaled to the network that the content was voice. This allowed the network to potentially use processing techniques like echo cancellation or digital-to-analog conversion if connecting to the traditional phone network.
• Multiple-Rate Service (N x 64 kbps): For applications needing more than 64 kbps, this service allowed users to aggregate multiple B channels into a single, higher-speed connection. For instance, an early video conferencing system could combine two B channels for a $128 \text{ kbit/s}$ connection, or use six for a $384 \text{ kbit/s}$ H0 channel, providing better video quality.

Teleservices: End-to-End Applications

Teleservices are complete applications that utilize the underlying bearer services and also define the functionality of the terminal equipment. They represent what the end-user actually experiences.

• Telephony (7 kHz): While standard telephony uses a 3.1 kHz audio bandwidth, ISDN enabled high-fidelity voice calls using a 7 kHz bandwidth, providing radio-broadcast quality audio for applications like professional sound studios or teleconferencing.
• Group 4 Telefax: A high-speed, high-resolution digital fax standard. Unlike older analog faxes, which could take a minute or more per page, Group 4 fax over a 64 kbps B-channel could transmit a page in a matter of seconds with superior quality.
• Videotex: A pre-World Wide Web service that allowed users to access text-based and simple graphical information from remote databases, like train schedules, stock quotes, or electronic phone books. ISDN made this service much faster and more responsive than its dial-up predecessors.

Supplementary Services: The "Smart" Features

One of the biggest advantages of ISDN was its powerful set of supplementary services, enabled by the separate D-channel for signaling. These are the features we now take for granted in modern phone systems.