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The architecture of the VPN makes use of software-defined intelligence residing in strategic points of the network. AT&Ts SDN, for example, consists of an ACP connected to the PBX through dedicated or switched lines. The ACPs connect with the carriers NCP, where the customers seven-digit on-net number is converted to the appropriate code for routing through the virtual network.
Instead of charging for multiple local access lines to support different usage-based services, the carriers allow users to consolidate multiple services over a single T1 access line. A user who needs only 384K bps for a data application, for example, fills the unused portion of the access pipe with 18 channels of voice traffic to justify the cost of the access line. At the carriers cross-connect system, the dedicated 384K-bps channel and 18 switched channels are split out from the incoming DS1 signal. The 384K-bps DS0 bundle is then routed to its destination, whereas the voice channels are handed off to the carriers Class 4 switch, which distributes the voice channels to the appropriate service.
Although obtaining economical voice traffic has traditionally been the primary motivation behind the move to VPN service, a variety of low-speed and high-speed VPN data services are available as well.
AT&T has been especially aggressive in offering its SDN customers the means to access a wide array of AT&T EasyLink messaging services. The offering, AT&T SDN EasyLink Solutions, enables customers to use their SDN networks to connect directly to electronic messaging features from AT&T EasyLink Services, including electronic mail, shared folders, text-to-fax (MailFAX), electronic data interchange, Telex, and a variety of information services.
SDN EasyLink Solutions includes the following services:
VPNs also are capable of supporting such bandwidth-intensive applications as LAN interconnection, image transfers, and videoconferencing. These services are offered under AT&Ts SDDN, MCIs VPDS, and Sprints VPN Premiere.
AT&Ts SDDN, for example, offers high-speed data networking in conjunction with SDNs advanced call-handling capabilities. SDDN shares the network capabilities of ACCUNET SDS for reliable transport of data at rates of 56K bps and higher. (Low-speed data is transported over SDN using dial-up modems or PBX data connections.) SDN supports low-speed dial-up modem connections and higher-speed connections through a PBX, T1 multiplexer, or D4 channel bank. AT&Ts SDDN offering supports 56K- and 64K-bps service, 64K-bps clear channel, and 384K- and 1.536M-bps connections utilizing the ISDN PRI. These high transmission speeds are achieved by stacking contiguous 64K-bps clear channels. Users take full advantage of virtual networking by combining and routing their voice and data traffic in a single T1 access line to the SDN/SDDN network.
Users access SDDN with DDS lines for data transmission at rates of up to 56K bps using dial-up modems or DSUs with an optional auto-dial or re-dial capability; alternatively, access is obtained through AT&T ACCUNET T1.5 lines. Customer premises equipment (e.g., intelligent multiplexers and PBXs) interprets ISDN PRI messages for call setup, detection of facility failures, and reinitiation of call setup in response to abnormal call disconnects. Real-time restoration is achieved within seconds of a service disruption so that critical data applications remain operational; SDDN also supports SDN network management capabilities such as call screening, flexible routing, periodic traffic reports, and customer-initiated testing. SDDN is well-suited for applications that:
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