5.1 ISDN Architecture
5.1.1 ISDN versus asynchronous

Prior to the 1960s, the world’s communication network was relying mainly on analog facilities. Communication networks have been evolving for the past 40 years toward digital transmission. In 1968, the ITU-T convened a meeting to discuss the integration of transmission and switching. Part of the discussions revolved around Integrated Services Digital Network (ISDN).

ISDN developers envisioned that this new technology would provide a digital pipeline, offering integrated access to the broadest range of services. These services were to include voice, networking, packet switching, telemetry, and cable television.  

As its name implies, ISDN uses digital technology. As its name implies, ISDN uses digital technology. As shown in Figure , ISDN replaces the traditional basic analog telephone service equipment and wiring scheme. ISDN uses new, high-speed digital equipment that provides a host of new services.

The major force behind ISDN growth, especially in the United States, is the capability of ISDN to cost-effectively provide reliable service for the World Wide Web and access to other services that require high-speed.

The ISDN call uses a unique set of protocols to communicate from the local terminal equipment to the ISDN switch in the central office for call setup and teardown. These protocols also allow the service provider to offer many other services in addition to basic telephone.

After the call is set up and the connection made, the process is the same as a traditional call. When the call is to be disconnected, once again the unique ISDN protocols are used to communicate between the local switch and the terminal equipment.

ISDN delivers a faster alternative to the following:

  • Analog dialup modems — ISDN provides two to four times the bits-per-second transmission rate.

  • Other switched services — Fast call setup makes a connection in less than one second (compared to 30 to 45 seconds for other switched services).

  • Expensive leased lines — ISDN provides faster connectivity than analog and, for a lower cost than leased lines.

The primary difference between the basic or analog telephone call process and the ISDN call process is the use of digital signaling and data transmission from end to end. ISDN is an access technology that provides a digital local loop. Therefore, ISDN transmissions are digital from end to end. With asynchronous connections, the local loop is analog and requires pulse code modulation (PCM), thus introducing delay in the transmission.