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Get T1's clout at a fraction of the cost.

As fractional T1 offerings become more widely available, companies of all sizes are starting to take advantage of the service's dramatic cost savings and performance benefits that come with high-speed digital links.

Until the advent of F-T1 service, highvolume communications users had to choose between a full T1 link or a 56-kb/s Dataphone Digital Service (DDS) link.

Further, since the 56-kb/s DDS service is expensive compared with analog offerings, smaller companies were deterred from embracing digital networks.

With AT&T's Accunet Spectrum of Digital Services (ASDS), companies can now use a portion of the T1's 1.544-Mb/s bandwidth at a fraction of the cost of a full T1 link. Also, since the price for a 56- or 64-kb/s ASDS channel is slightly less than that for a leased voice-grade line, cost is no longer a barrier to upgrading from analog to digital transmission quality.

AT&T acknowledges that one of its objectives with ASDS is to migrate its leased analog-line customers to its digital backbone. High-capacity digital networks are mor eeconomical to build and maintain than analog facilities. With this in mind, AT&T palns to offer ASDS wherever voice-grade private-line services are available.

Already, ASDS availability has grown from the initial coverage of 24 cities in June, 1989 to a total of 500 locations today.

Besides the 56/64-kb/s channel, ASDS offers service at 9.6, 128, 256, 384, 512 and 768 kb/s. The higher-speed offerings are designed to accommodate the growing number of applications requiring wideband links, such as video conferencing, CAD/CAM and other image transmissions, and LAN-to-WAN links.

Monthly charges for a 56/64-kb/s ASDS channel range from a fixed cost of $73.80 and $2.94 per mile for circuits 50 miles or less, to $254.30 fixed plus 33 cents per mile for circuits more than 100 miles.

Pricing for the higher-speed channels follows the same structure, with escalating discounts of from five to 10 percent.

T1 vs F-T1

Compared with F-T1 service, full T1 links are still cost-effective if the bandwidth is almost fully used or if the links are fairly short.
 Monthly Charges Per
 Mileage Channel
Per Channel Band Fixed Per Mile
 9.6 kbps 1-50 $ 73.80 $ 2.92
 51-100 152.30 1.36
 101plus 254.30 .33
 56/64 kbps 1-50 73.80 2.92
 51-100 152.30 1.35
 101plus 254.30 .33
 128 kbps 1-50 141.45 5.54
 51-100 290.45 2.56
 101plus 480.45 .63
 256 kbps 1-50 283.93 11.06
 51-100 581.43 5.11
 101plus 966.43 1.26
 384 kbps 1-50 425.38 16.60
 51-100 872.38 7.66
 101plus 1451.38 1.87
 512 kbps 1-50 538.13 20.97
 51-100 1103.13 9.67
 101plus 1834.13 2.36
 768 kbps 1-50 807.70 31.44
 51-100 1654.20 14.51
 101plus 2750.20 3.55


AT&T's rates for its Accunet Spectrum of Digital Services includes both fixed and mileagesensitive charges.

For example, with links of between 75 and 250 miles, F-T1 service is only more cost-effective than a full T1 link if you need less than half the T1 bandwidth. As the distance increases, the economies of using F-T1 increase. For 1000-mile links, F-T1 service is more cost-effective for any application requiring less than three-fourths the T1 bandwidth.

These same economics apply to local loops, which is why local carriers have been reluctant to offer F-T1 service. Some observers argue that the 60 percent annual growth in F-T1 links will drive the market to total end-to-end F-T1 service.

Others believe the technology for full T1 transmission is so well developed and relatively inexpensive for the distances involved that F-T1 local loops will never become practical.

For the moment, most F-T1 networks use a full T1 circuit at each end, running from the local carrier's central office to the customer site.

For local access, AT&T accepts F-T1 traffic either from a T1 link, analog leased lines or from 56-kb/s DDS feeds.

Last November, AT&T introduced a new access arrangement known as Accunet Flexible Digital Access Service. AFDAS offers users individual sub-T1 digital access at a savings of up to 40 percent compared with existing analog and DDS lines.

AT&T has further enhanced ASDS by adding Customer Controlled Reconfiguration (CCR) service and a secondary channel option which allows users to monitor 9.6- and 56-kb/s ASDS circuits from their own premises. CCR enables users to route voice or data traffic around faulty equipment or facilities on ASDS circuits, and to isolate the problems through tests.

With CCR, users can also meet time-of-day or event-specific needs by readily reconfiguring their network on a demand or scheduled basis.

Since last summer, users of the 9.6- and 56/64-kb/s ASDS channels have also benefited from a volume discount plan that saves from six to 13 percent off the regular monthly rates. The discount does not apply to the intermediate bit rates of 128 kb/s and beyond, but only to "noncontiguous" ASDS channels; that is, 9.6- or 56/64-kb/s circuits that do not occupy adjacent slots on the T1 facility.

AT&T has also given a bonus to wideband users in the form of a switched 384-kb/s service, which is six times faster than any other switched 384 Service is available initially in 29 cities, with others to be added based on demand.

Getting the most from F-T1

AT&T was not the first to offer F-T1 service. Cable & Wireless's U.S. carrier services division in Vienna, VA began offering customers multiple DSO (64-kb/s) facilities in 1987 under the name Intelliflex. The service now provides from one to 24 channels of 56 or 64 kb/s each.

Similar F-T1 offerings are also available from MCI, US Sprint and members of the National Telecommunications Network, a consortium of carriers operating over 16,000 miles of fiber-optic network.

One of its members, Williams Telecommunications Group of Tulsa, OK was among the first to offer clear channel capability (CCC) as a no-cost option with its F-T1 service. This capability allows users to utilize the full 64 kb/s of the DSO circuit for user information. Without CCC, 8 kb/s is needed to keep the network running smoothly, leaving an effective data throughput of only 56 kb/s.

For all carriers, what makes F-T1 possible is the digital cross-connect system or, in AT&T parlance, the Digital Access and Cross-Connect System (DACS).

What DACS does

This equipment allows the carrier to switch a user's DSO to any available time slot in a DS1 while maintaining the synchronization, framing and control needed for each digital trunk. In this way, a carrier can efficiently fill or "groom" (selectively assign) DSOs into a T1 pipe.

To improve monitoring of network performance, carriers have standardized on the Extended Super Framing (ESF) format, which groups every 24 frames and uses the framing bits as a byte of information for network synchronization, error correction and diagnostics.

F-T1 users can gain additional savings and efficiencies with a variety of multiplexers, channel banks and combination CSU/DSUs.

Channel service units effectively act as a repeater, conditioning, shaping and enhancing the signal while providing testing and signal monitoring capabilities. Data service units mold the information into a D4-compatible signal.

Multiplexers provide users with virtually any data rates over the F-T1 link. They also allow users to configure and add DSOs quickly as they are ordered from the carrier, and to pass network management functions through the public network.

However, some multiplexers may not interoperate with the public network, but only act as gateways to it. Such products require additional hardware to work with the F-T1 services, adding considerable cost and significantly reducing network flexibility and options for future growth.

Many multiplexers are merely downsized versions of standard T1 models. For a T1 multiplexer to transmit data over F-T1 service, data must be formatted as 24 DSOs that are D4-channelized and ESF formatted.

T1 multiplexers and their F-T1 counterparts are typically designed for users of T1 links who are utilizing the F-T1 services to improve network reliability through mesh networking, or to bring outlying corporate sites onto the company network through drop-and-insert techniques.

Network management is an important capability for these users, and the multiplexers generally offer sophisticated capabilities.

For users unable to justify a T1 link, F-T1 services provide a means for replacing analog or DDS facilities with an improved or less expensive alternative.

In some cases, these users may be looking for data rates higher than 64 kb/s to handle their communications traffic or special wideband applications.

Generally, the most cost-effective approach is a combined CSU/DSU, possibly with a small built-in time division multiplexer.

What's new

AT&T Paradyne of Largo, FL has added a F-T1 multiplexer to its Comsphere family for point-to-point and single-ended access to T1 and F-T1 services. Model 5312 accommodates up to 60 channels and can assign aggregate bandwidth as contiguous DSOs to full T1 capacity. Fully DACS-compatible, it provides clear channel operation and also supports DDS and 56-kb/s applications.

Telematics International of Fort Lauderdale, FL offers network software for its DX multiplexer family to support up to seven fractional bundles within a T1 access line. The software retains all of the integrated network management capabilities of the DX product line and supports hybrid D4 and European G.732 networking.

Racal-Milgo of Sunrise, FL used last fall's TCA show to introduce its Omnimux FT1, a F-T1 multiplexer with integral DSU/CSU. It comes with Nx56- or 64- kb/s DTE ports to accommodate current bandwidth needs and allow for future growth. Omnimux FT1 supports both contiguous and alternate DSO placement. It may be controlled from its front panel, a VT100 supervisory port or from the firm's CMS 400 network management system.

Phoenix Microsystems of Huntsville, AL recently had its F-T1 product family certified by AT&T as compatible with its Accunet services. The products certified include: the Model 1564 multiplexer, which provides a network interface for up to three T1 links; the Model 1565 subrate data multiplexer, which supports up to 20 RS-232 inputs for feeding onto the T1 link via the Model 1564; and the Model 1608 data port, which allows fractional portions of the T1 bit stream to be allocated at 24 data rates (Nx56/N64 kb/s) for high-speed applications.

Digital Link Corp. of Snnyvale, CA employs plug-in modules with its DL100 digital service multiplexer to support up to five data DTE inputs and one DS1 input with DSO add/drop capability. A combination DSU/CSU and multiplexer, the DL100 provides an expandable T1 interface for access to both full T1 and F-T1 services. It accepts Nx56/64-kb/s data streams on any DTE port and multiplexes the data onto the T1 network. The DS1 port will interface to T1 outputs from a PBX, multiplexer, channel bank or other equipment, permitting data from any DTE port or combination of ports to be inserted into unused time slots.

General Data Comm of Middleboro, CT recently enhanced its F-T1 product family with a combination DSU/CSU that supports D4 framing, clear channel signaling using B8ZS and both sequential and alternate framing structures. Network interface rates can be multiples of either 56 or 64 kb/s. Two serial ports on the DataComm 552 permit mapping of two independent data streams into one 1.544-Mb/s aggregate. A third, full T1 "cascade" port allows the linking of up to three DSU/CSUs.

Larse Corp. of Santa Clara, CA also provides format flexibility with its Split-T fractional T1 DSU. The Split-T allows Nx56 and 64 kb/s data from multiple LAN bridges, CAD/CAM terminals, front-end processors and other highspeed data terminals to be bundled into contiguous, alternate or random DSO time slots.

Pacific Communication Sciences of San Diego, CA employs a 9:1 voice compression algorithm to put up to eight voice-grade channels on a 56/64-kb/s digital link. The CS8000 also handles inputs from up to 20 data channels for multiplexing with voice and fax traffic over links to 128 kb/s.

Republic Telecom Systems of Boulder, CO uses packet technology to optimize bandwidth usage in F-T1 applications. By combining low-bit-rate voice, high-speed packet switching and advanced call processing, the firm's RNET private networking system can integrate multiple voice and data channels over digital links. Its latest model, the RLX-4N, can integrate four voice channels and a 32-kb/s data channel over a 64-kb/s circuit. Alternatively, it can handle three voice and four 9.6-kb/s data channels and similar combinations.

Micom Communications Corp. of Simi Valley, CA also uses advanced packetized voice technology with its Voice Mux 100 time division multiplexer to put four voice channels and a data channel over a 56- or 64-kb/s link. The multiplexer can also be configured to support two voice and five data channels, or four voice and four data channels. Micom recently unveiled a remote terminal server to give remote site users the same access to the corporate Ethernet LAN as local users. The Marathon server lets terminals access LANs from remote locations with the added benefit of sending toll-free voice and fax traffic over the same F-T1 network.

Codex Corp. of Mansfield, MA has introduced a digital circuit switch that interfaces with T1, E1 (CEPT 2.048) and F-T1 services, enabling both U.S. and multinational users to create subrate and superrate data channels and to integrate voice traffic into the networks using T1 and CEPT high-speed circuits. Designed for small- to medium-sized networks, the 6250 network multiplexer supports mesh networking of up to 5 nodes.

Tekelec of Calabasas, CA provides the means for testing F-T1 systems with its Chameleon 8000 test system, which uses a modular architecture to test and simulate a variety of configurations. The Chameleon 8000 provides verification and analysis of both digital communications services and terminal equipment, including full-duplex testing of F-T1 services.
COPYRIGHT 1991 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Fractional T1; includes related article on F-T1 service
Author:Edwards, Morris
Publication:Communications News
Date:Apr 1, 1991
Words:2332
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