 digital service is SS7; it is the network-to- network interface. However, D channel signaling has been implemented in various proprietary ways and one carrier's SS7 system might not be compatible with another. The trouble with SS7 interoperability can be traced to a rather sluggish standards- setting process. In 1984, the ITU-T published the first ISDN specifications. Unfortunately, it was incomplete as it related to signaling over the D channel. Without a comprehensive standard, the switch manufacturers were left to their own devices to fill the holes in the D-channel protocol. AT&T and Northern Telecom, the two dominant CO manufacturers in North America, turned out proprietary interpretations. Since switching systems could not interpret each other's D-channel protocols, inter-network signaling had to be passed in-band. Even today, it is very difficult to arrange for 64 Kbps clear channel endpoint-to-endpoint, using LEC access. Resolving this haphazard arrangement was the goal of National ISDN. Video Communications: The Whole Picture National ISDN In 1991, the Corporation for Open Systems (COS) and the North American ISDN User's Forum (NIUF) developed an initiative to standardize the implementation of ISDN within the United States. In November 1992, Bellcore, the COS, and the NIUF introduced National ISDN-1 (NI-1), a standard that aims to provide a consistent interface among LECs, IXCs, and equipment manufacturers. NI-1 was introduced at a week-long event called the Transcontinental ISDN project, or TRIP '92. The NI-1 standard, which precisely defines ISDN Basic Rate Interface, has been widely implemented. National ISDN-2 got underway in late 1993. In the NI-2 standard, Bellcore strengthened NI-1, and thereby provided for service uniformity and facilitating operations and maintenance. NI-2 also defines various PRI features, including circuit- switched voice and data call control, packet-switched data call control, switched T-1, fractional T-1 service, and D-channel backup. NI-2 did not address Non-Facility Asso