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The original application for X.25 packet switching was that of interactive time-sharing, which involves long connect times and low data volume. While such applications are still supported effectively by X.25, contemporary applications include online interactive processing (reservations systems), messaging (email), batch file transfer (data backup), information service access (America Online, CompuServe, and Prodigy), and Internet access.
Packet-switching offers the advantage of being a highly mature, if limited, network technology. Therefore, it is relatively inexpensive to deploy and is highly cost-effective in support of applications that require many-to-many connectivity and which involve relatively low volumes of data transport. Additionally, it is virtually ubiquitous, having been deployed in every corner of the globe. However, it is limited in terms of speed and latency. As a result, many applications and service providers are moving toward newer network technologies. For example, the airline reservations systems are rapidly moving to Frame Relay, at least for the domestic (U.S.) networks. Internet Service Providers prefer ISDN or Frame Relay. The Internet backbone network largely has shifted to Frame Relay or ATM, operating at T1 or T3 speeds at a minimum; much of the backbone is being upgraded to fiber optic facilities operating at speeds of 155 Mbps. The future of packet switching is assured because of its low cost and high availability. Additionally, low-intensity users will continue for many years to access the Internet on a dialup basis through X.25 packet switches.
X.25 is growing at record rates in Latin America, Central Europe, and other developing regions. In such areas, the poor quality of the networks make X.25s error correction capabilities a must for data communications. Demand also is growing in Western Europe and North America, where Internet access is X.25-based, at least at the edges of the networks [8-21]. The postal administration in France recently built a X.25 LAN internetwork that eventually will extend to most of its 7,000 post offices. Bank branches in Belgium and other countries of Western Europe also are expanding their X.25 networks due to the low cost and high degree of flexibility [8-22].
Integrated Services Digital Network (ISDN)
ISDN (Integrated Services Digital Network) was first explored as a concept from 1968 to 1971 by a CCITT study group. A more focused conceptual study took place during the 1981 to 1984 CCITT study period. The first set of published standards recommendations appeared in 1984 in the form of a CCITT Red Book, which provided the basic framework for the concept, network architecture, UNI (User Network Interface) protocols and common channel signaling protocols. As a result of the 1985 to 1988 study period, a Blue Book was published, which provided description of supplementary services, rate adaptation, ISDN Frame Relay, and the initial set of B-ISDN (Broadband ISDN) recommendations. (The color of the books has no significance, other than the fact that a different color is chosen for each study period.)
ISDN is a suite of services based on a set of technologies, including transmission, switching, and signaling and control. ISDN is a set of international standards recommendations which will allow the provisioning of a wide range of services which are intended to be available on a ubiquitous basis. Additionally, the ISDN network is accessible through a standard set of interfacesone for low-bandwidth applications and another for high-bandwidth.
The specific characteristics of ISDN include its entirely digital natureCPE, transmission facilities and switching systems all are fully digital. Three channel types are identified, including B-channels (Bearer channels) that carry the information, D-channels (Data channels) for signaling and control, and H-channels (High-speed channels) for channel aggregation in order to accommodate bandwidth-intensive applications. The User Network Interface (UNI) protocols include Basic Rate Interface (BRI) for low-speed termination, and Primary Rate Interface (PRI) for high-speed access. Common Channel Signaling System #7 (SS7) is a fundamental requirement of ISDN.
Announced to the world with great fanfare, ISDN quickly captured the imagination of carriers, manufacturers and user organizations worldwide. ISDN offers the compelling advantages of improved bandwidth, flexibility, error performance, reliability, availability, and interconnection to a wide range of services. Unfortunately, it has stalled. For the past 25 years, ISDN has progressed at such a slow pace that it has become legendary. Among the many reasons for its slow development are slow standards development, lack of adherence to standards, lack of availability, regulatory hurdles, circuit and equipment costs, and poor marketing.
Standards development at the ITU-T is infamously slow. Standards traditionally were released every four years in monsoon fashion and with total droughts in the interim. Over time, the various committees were afforded the privilege of developing and releasing certain standards recommendations on an intermediate basisstandards now come in sprinkles and showers.
Standards from the ITU actually are in the form of standards recommendations. Individual member nations are free to implement ISDN options as they see fit, or to deviate from the standards, as long as international interconnectivity is accomplished at some reasonable level. The most notable international difference is that of the basic ISDN hierarchy. The North American version follows the T1 hierarchy, with PRI including 24 B channels; the European (ITU) version is based on E-1, providing 30 B channels. While this difference is understandable in the context of maintaining backward compatibility with existing networks, it also perpetuates issues of basic protocol incompatibility.
Systems manufacturers of CO exchanges and PBXs have a strong interest in maintaining the proprietary nature of their systems architectures. Therefore, they have implemented ISDN in distinctly different ways. ISDN compatibility became ISDN compliance a decidedly lower level.
Additionally, carriers have implemented nonstandard versions of ISDN. Pacific Bell, for instance, offers ISDN at a rate of 56 Kbps per channel, rather than the standard 64 Kbps. This limitation is due to the fact that SS7 is not fully deployed in Pacific Bells carrier network; as a result, in-band signaling and control consumes 8 Kbps of channel bandwidth. Pacific Bell has announced its intention to rectify this situation in 1997, although the recently announced Pacific Bell/Southwestern Bell merger may affect that decision.
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