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ISDN Characteristics And Benefits

ISDN is unusual, if not unique, as it is undoubtedly the most carefully planned, well coordinated, and best documented network technology in history. Despite this fact, it will be at least a decade before it is widely available and implemented. The key characteristics of ISDN include its end-to-end digital nature, unusual for a circuit-switched network. Through a small family of interfaces, a wide range of services can be accessed through the LEC, IXC or CAP. Rate adaption and channel aggregation allow bandwidth-intensive applications to be supported on a dynamic basis.

ISDN’s reliance on SS7 (discussed in Chapter 6) offers a number of advantages, including faster call setup and nonintrusive signaling and control. Additionally, SS7 (either with or without ISDN) makes possible a number of interesting CLASS services, including Caller ID, Name ID, call trace, selective call forwarding, and selective call blocking.

ISDN also is interoperable with X.25, Frame Relay, SMDS and ATM. In fact ISDN standards were developed specifically with these services in mind. Frame Relay, in particular, is closely aligned with ISDN link-level protocols. Broadband ISDN (B-ISDN), although many years in the future, is dependent on ATM network technology.

ISDN Characteristics and Drawbacks

ISDN is not without its drawbacks, which include limited availability, standards variations, channel application, and high cost. As discussed earlier, availability is limited in most nations, because ISDN and SS7 software are costly for the carriers to deploy. While ISDN capability can be extended to non-ISDN CO exchanges, that incremental cost is not trivial. As discussed previously, standards vary by carrier and manufacturer, and BRI limits the user to a single voice channel.

Not surprisingly, cost/benefit considerations dictate the success or failure of technologies, regardless of how compelling they appear at first glance. The cost of an ISDN BRI circuit is more than twice that of an analog line—installation costs can range as high as $578.00 per BRI (Southwestern Bell). These costs tend to discourage ISDN to a considerable extent, particularly in voice-intensive environments—although ISDN provides three channels, only one can be used for voice at any given time.

Additionally, many carrier tariffs impose a usage surcharge in the form of a flat rate per minute. The usage charge may apply to local, as well as long distance, calls. Additionally, data calls often carry a higher usage rate than do voice calls. While this additional usage charge tends to discourage ISDN usage, the call connect time may be reduced significantly through ISDN. Pacific Bell [8-31] suggests that a 20MB file can be transmitted from California to Japan via a 90 Kbps ISDN connection in 3.7 minutes at a cost of $6.32. This compares quite favorably with an analog modem transfer at 14.4 Kbps, which would take 23.1 minutes and cost $29.85. Group IV facsimile transmission over ISDN lines also offers significant long distance cost savings when compared to Group III analog transmission.

Hardware Costs are additional in support of ISDN. In a BRI environment, such additional equipment might include Terminal Adapters (TAs), Inverse MUXs, and BRI contention devices. In a PRI application, ISDN software for PBXs and routers is an additional cost. Older systems might require replacement in order to effect ISDN compatibility.

A final cost issue centers on equal access charges, or Subscriber Line Charges (SLCs). In 1995, the FCC decision considered the application of the SLC to ISDN on a per-channel basis, rather than the standard per-circuit approach [8-33]. Although the FCC relented under pressure from carriers and users, regulators may strike a similar posture in the future.

ISDN Standards

As mentioned earlier in this chapter, ISDN standards are voluminous. While still under development in some respects, current ITU-T standards for ISDN include the following:

  I.441/4511: ISDN Primary Rate Interface (PRI)
  I.515: Parameters for ISDN internetworking
  Q.700: Signaling System Number 7 (SS7) specifications
  Q.921: Layer 2 specification for D-channel; Link Access Protocol-D (LAP-D)
  Q.931: Layer 3 User Network Interface (UNI) specifications
  V.110: B channel procedures (Europe) for Terminal Adapters (TAs)
  V.120: B channel procedures (North America) for TAs

In addition to the ITU-T, other organizations are active in the development and promotion of ISDN standards. For example, ANSI (US) and ETSI (Europe) each lobby the ITU for the acceptance of their parochial ISDN variations.

ISDN Applications

The applications for ISDN are broad in range. While ISDN was long described as “a technology in search of an application,” it recently has been opened to applications developers, and aggressively so. There is no killer ap for ISDN, rather there are a number of applications that, in total, promise to ensure the future of ISDN. A host of applications which benefit from the improved quality of digital networking and that are bandwidth-intensive are well served by ISDN. Further, ISDN offers an affordable circuit-switched alternative to DDS, Switched 56, and T-carrier, which simply cannot be cost-justified in many cases.

Personal office internetworking, remote office internetworking, and telecommuting (or Telework) all are facilitated by the increased bandwidth and error performance offered by ISDN BRI. In such applications, file transfers and facsimile transmission are accomplished much more quickly and with much greater clarity; the improved quality of the voice communications are an added benefit [8-34].

ISDN also is used for access to X.25 packet networks, with users benefiting from the faster call setup and teardown time made available by virtue of SS7. As either the B channels or D channel can be used for packet data transfer in a BRI implementation, ISDN offers additional flexibility and bandwidth utilization. Additionally, some manufacturers of Terminal Adapters offer built-in X.25 PADs for end-to-end error correction [8-35].

As a replacement or backup for dedicated digital services, ISDN performs well for data and image networking, whether in a host-to-host, LAN-to-LAN, or remote LAN access application. Intensive users of the Internet and World Wide Web find ISDN bandwidth to be of great advantage, as the speed of call setup and file transfer are much increased compared to dialup analog connections. For instance, a typical WWW page takes 3 minutes to load over an analog line with a 14.4 Kbps modem, but less than 20 seconds at BRI speed of 128 Kbps [8-36]. As discussed in Chapter 4, incoming call centers can take advantage of ISDN to increase productivity, as well as make use of remote agents working from home.


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