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Even in comparison to newer LAN internetworking technologies such as Frame Relay, T-carrier holds it own. For instance, a large bank in Missouri recently tripled the size of its private T-carrier network to 30 nodes. They have plans to increase the network by another 20 nodes by 1997. The primary application is the transport of data from ATM machines and remote bank branches to IBM mainframes located at the central site in St. Louis. The T-spans are leased from a combination of IXCs [8-11].
The most significant recent and emerging developments of digital carrier relate to equipment and applications. Many manufacturers of CSUs/DSUs have incorporated intelligence into their products, in effect turning them into commodity-level MUXs. MUX manufacturers have developed adapter cards which allow the transmission of SNA, ISDN, Frame Relay, and ATM over T-carrier. Direct connection of Ethernet and Token Ring LANs is sometimes supported, with the MUX effectively acting as a bridge or router. Some manufacturers have also released direct SONET fiber optic interfaces at speeds up to 155 Mbps. Contemporary MUXs are addressable devices that can be managed and controlled remotely. Additionally, several manufacturers of traditional MUXs have incorporated inverse multiplexing capabilities. Inverse multiplexing will spread a high-bandwidth transmission (a videoconference or file transfer) across multiple DS-0 channels or across multiple DS-1 circuits.
T-carrier certainly offers cost and performance advantages when compared to individual trunking. Additionally, it is attractive due to its ability to integrate voice, data, video, and other forms of information. Additionally, competition and deregulation have caused T-carrier costs to drop dramatically during the past ten years, although they recently have begun to increase. Yet, in a private, leased-line network application, T-carrier suffers from the same flexibility and vulnerability issues that affect alternative leased-line technologies. As a result, virtual private networks have replaced T-carrier networks in many large, voice-intensive user applications. Additionally, ISDN competes effectively with T-carrier in PBX-CO trunking applications. In data-intensive applications, T-carrier is losing ground to emerging broadband network technologies such as Frame Relay, SMDS, and ATM. Ultimately, ATM is likely to replace T-carrier, altogether.
In the meantime, however, T-carrier continues to enjoy a strong following, with T1 MUXs being the hottest-selling equipment for enterprise-wide networking. According to International Data Corporation, 1995 mux sales were $1.2 billion worldwide, an increase of 20% over the previous year. IDC forecast sales of $1.5 billion for 1996 [8-11].
Digital 800 service is a very new offering, having been announced and trialed in 1994 by AT&T; the first general release offerings were those of AT&T and MCI in January, 1995. The service currently is offered nationwide in the United States by AT&T (800 Multimedia) and MCI (MCI 800 Digital Service) at a cost of less than 30 cents per minute. In the case of AT&T, different rates apply to voice calls (typically 6 to 10 cents per minute under contract) and data (22 cents a minute for peak calling and 19 cents for off-peak) calls [8-12]. It is intended only for medium-to-large customers subscribing to high-volume 800 service offerings. Sprint Corporation announced its Toll-Free Switched Digital Service in December 1995.
Digital 800 uses the same numbering conventions used for voice calls, based on the North American Numbering Plan (NANP), which is governed by the ITU standard dialing scheme. Therefore, carrier interconnection is provided, a problem which plagues Switched 56/64 Kbps and Frame Relay data service offerings employing proprietary numbering schemes. Additionally, the 800 numbers are portable. In fact, the same, and even existing, 800 number can be used for both analog and digital voice and data.
The carrier network distinguishes the nature of the call and routes it to the call destination via the appropriate facilities. Digital calls are routed exclusively over digital facilities. Although capabilities differ by carrier, routing considerations can include type of call (digital data versus analog voice versus videoconference), originating NPA, and time-of-day. Separate and distinct and management reports are available for each class of calling activity [8-13].
Bandwidth-on-demand, within limits, is provided as an inherent part of Digital 800. Digital bandwidth is provided in 64 Kbps increments (Nx64), up to full T1 at 1.544 Mbps. Depending on the nature of the call (voice versus data versus videoconference) the appropriate amount of bandwidth is supported.
Termination of Digital 800 requires a digital local loop, which can be in the form of ISDN, T1, or Switched 56/64 Kbps service. Termination equipment varies according to the nature of the terminating loop, although an Inverse MUX is required in order to associate multiple 64 Kbps channels where required. The carriers have suggested that the Inverse MUX requirement is temporary, to be eliminated as soon as multirate ISDN is available.
Access to the service can be in the form of an analog dialup connection, Switched 56/64 Kbps service, or ISDN. Where multiple channels are required for a high-bandwidth transmission, an inverse MUX currently is required at the originating end of the connection.
Digital 800 issues abound, as the LECs are not fully equipped at this point to handle the potential demand for the service. Therefore, users should not assume that Digital 800 termination is possible in every location. The features differ significantly between AT&T and MCI, especially with respect to sophisticated routing capabilities of analog voice versus digital data calls. Additionally, the carriers have not yet established (nor are they likely to establish) network-to-network interfaces; in other words, all incoming call centers must use the same Digital 800 carrier.
Applications for Digital 800 primarily are business-to-business. The envisioned applications include catalog ordering, software uploads/downloads, online information services, publishing, graphics editing, videoconferencing, multimedia communications, video augmentation, and image augmentation. While the carriers have a lot of work to do in order to make Digital 800 ready for mass consumption, it is expected to be a mainstream business service offering by 1998 [8-13] and [8-14].
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