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The architecture of the VPN makes use of software-defined intelligence residing in strategic points of the network. AT&T’s SDN, for example, consists of an ACP connected to the PBX through dedicated or switched lines. The ACPs connect with the carrier’s NCP, where the customer’s 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 carrier’s 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 carrier’s Class 4 switch, which distributes the voice channels to the appropriate service.

DATA NETWORKING OVER THE VPN

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.

Low-Speed Data Services

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:

  AT&T SDN Electronic Mail. This worldwide public messaging service offers secure transport and feature-rich functionality that supports access from a variety of computer platforms. The service provides worldwide electronic mail delivery to many systems, including X.400 gateways and the Internet.
  AT&T SDN Shared Folder. This service is an electronic bulletin board service within SDN Electronic Mail that automatically downloads new information to each user’s mailbox whenever the user queries the system for new mail.
  AT&T SDN MailFAX. This service lets users send electronic mail documents from a computer to a receiver’s fax machine. Service features include automatic retry, fax broadcast, and customized logos and signatures.
  AT&T SDN Enhanced FAX. The enhanced fax service provides secure store-and-forward delivery of facsimile transmissions worldwide.
  AT&T SDN Electronic Data Interchange. This EDI service provides message transport, storage, and tracking for the electronic exchange of business documents such as purchase orders or invoices in standard data formats.
  AT&T SDN Telex. This service connects users to the worldwide telex network and the extensive community of Telex subscribers in numerous industries, including international commerce, banking, and shipping.
  AT&T SDN Information Services. These information services give users fast and cost-effective access to a broad spectrum of online news services, interactive research databases, bulletin boards, and research-on-demand services.

High-Speed Data Services

VPNs also are capable of supporting such bandwidth-intensive applications as LAN interconnection, image transfers, and videoconferencing. These services are offered under AT&T’s SDDN, MCI’s VPDS, and Sprint’s VPN Premiere.

AT&T’s SDDN, for example, offers high-speed data networking in conjunction with SDN’s 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&T’s 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:

  Have high-speed or high-volume data transmission requirements.
  Have a time window for completion (e.g., applications performed during the night, morning, or other specified time periods).
  Benefit from bandwidth-on-demand and usage-based pricing (e.g., applications active for a limited duration, used infrequently, or required for unscheduled events).
  Have restoration requirements (e.g., critical applications that must remain operational in the event of a network failure). Such applications are currently protected through a dial backup capability or spare bandwidth and alternate routing in a T1 multiplexer network.
  Have multiple endpoint destinations (e.g., applications requiring serial or nonsimultaneous communications between an originating point and several endpoints).
  Benefit from networking flexibility (e.g., applications with traffic patterns that demonstrate daily or seasonal variations or that change drastically as the network grows). SDDN eliminates the time and expense required to install additional private lines.


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