ISDN Services

Overview of bearer services, teleservices, and supplementary services in the Integrated Services Digital Network.

What is ISDN? A Digital Revolution

represented a major leap forward from the traditional analog telephone network. Its core mission was to provide a fully digital connection from one end-user to another (end-to-end digital connectivity). This eliminated the need for analog modems and enabled higher quality, more versatile communication services over the existing copper telephone lines.

ISDN Basic Services: The Two Foundational Layers

ISDN telecommunication services are fundamentally divided into two main groups: Bearer Services, which handle the raw transport of data, and Teleservices, which are complete, user-facing applications.

1. Bearer Services

These are the fundamental transport services. They are responsible for reliably moving bits of information across the network between access points (). The network itself doesn't care what the bits represent (it's just a transport pipe). Bearer services define parameters like data rate and transmission mode.

2. Teleservices

Teleservices are complete, end-to-end applications that utilize the underlying bearer services. They define not only the network transport but also the functions of the end-user terminals. A teleservice ensures that both users have compatible equipment to understand and use the transmitted information. A telephone call is a classic example of a teleservice.

A Closer Look at Bearer Services

ISDN offers several circuit-mode bearer services tailored for different needs:

Unrestricted 64 kbit/s Digital Information: A transparent, all-purpose digital channel for any type of data, offering a guaranteed data rate of $64 \text{ kbit/s}$ . It is the fundamental building block, known as a 'B-channel'.
3.1 kHz Audio: A service specifically for transmitting signals within the 3.1 kHz audio band, such as data from traditional modems or Group 1-3 fax machines. The network may use techniques like echo cancellation on this channel, but it guarantees integrity for binary information that fits within this band.
Speech: This service is optimized specifically for human speech. The network is permitted to use signal processing techniques like compression to enhance quality, meaning it does not guarantee bit-for-bit transparency for data.
N × 64 kbit/s: A "multi-rate" service that allows a user to combine multiple B-channels ( $N$ of them) to create a single, higher-bandwidth connection. This is useful for applications like video conferencing that require more than $64 \text{ kbit/s}$ .

Common Teleservices

Building upon the bearer services, ISDN defines several standard user applications:

Telephony (3.1 kHz & 7 kHz): The standard voice call service. ISDN also standardized a high-quality 7 kHz audio service, offering superior voice clarity compared to traditional analog phones.
Telefax Group 4: A high-speed, high-quality digital facsimile service that operates directly over the $64 \text{kbit/s}$ ISDN channels.
Videotex: An early online service for accessing information from remote databases, a precursor to the World Wide Web.
File Transfer (e.g., FTAM): Services defining end-to-end compatibility for transferring computer files.

Supplementary Services: Enhancing the Call Experience

Supplementary Services are features that modify or enhance the basic call services. These are the features that made digital telephony powerful and user-friendly, many of which we take for granted today.

Number Identification Services

CLIP (Calling Line Identification Presentation): Simply known as "Caller ID". It displays the phone number of the person calling you.
CLIR (Calling Line Identification Restriction): The opposite of CLIP; it allows the caller to hide their number from being displayed.
COLP (Connected Line Identification Presentation): Displays the number that actually answered the call. This is useful if your call was forwarded, as it confirms which number you've actually connected to.

Call Management and Forwarding Services

CFU/CFB/CFNR (Call Forwarding): Allows you to redirect incoming calls to another number under different conditions: Unconditionally (all calls), when you are Busy, or on No Reply.
CW (Call Waiting): Notifies you of a new incoming call while you are already on another call, giving you the option to answer it.
HOLD (Call Hold): Allows you to temporarily put an active call on hold.

Addressing Services for Businesses

DDI (Direct Dialling In): A crucial feature for businesses with a private telephone exchange (PABX). It allows external callers to dial an extension number directly, bypassing the receptionist.
MSN (Multiple Subscriber Number): Allows a single ISDN line to have multiple phone numbers. This is useful for a home office, where one number could be for voice calls and another for a fax machine, all on the same physical line.