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F-T1: buy only what you need.


In the year since its introduction, AT&T's fractional T1 offering has created a mini-revolution in network planning and design.

Previously, AT&T customers with high-volume communications traffic had to choose between a full T1 link or a 56-kb/s Dataphone Digital Service (DDS) link.

Even though the price of T1 service has declined steadily since the mid-1980s, there still are few companies which can use enough of the 1.544-Mb/s bandwidth to justify the expense of a full T1 link.

AT&T's Accunet Spectrum of Digital Services (ASDS) solved this dilemma last June by offering portions of the T1 bandwidth at a fraction of the cost of a full T1 link.

Since then, US Sprint, MCI, and other carriers have introduced fractional T1 service, and a number of firms now offer multiplexers, DSU/CSUs and LAN bridges for fractional T1 uses.

Fractional T1 installations are expected to grow by 60% annually over the next five years, fueled by the economics of the new offerings and the growth of such applications as CAD/CAM, X-ray and other image transmissions, video-conferencing and LAN-to-LAN communications.

Fractional T1 also provides a strong incentive for analog users to switch to digital networks.

AT&T's 64-kb/s ASDS offering upgrades analog private-line services to digital transmission quality at a savings of up to 17% on intercity links.

To illustrate relative service pricing, AT&T notes that, for a 500-mile circuit, for every $1 spent on ASDS at 64 kb/s a customer would spend $1.17 ona voice-grade private line.

That customer would spend about $4.13 for 56-kb/s DOS service, AT&T says.

AT&T acknowledges that one of its objectives with ASDS was to migrate its leased-analog-line customers onto its digital backbone.

High-capacity digital networks are more economical to build and maintain than analog facilities.

At the same time, AT&T feels its voice-grade private-line customers will find the upgrade very attractive, since monthly bills for intercity service will be lower and the conversion will not affect existing applications, network management systems or promises equipment.

Less clear is AT&T's objective in offering the higher-bit-rate ASDS services, unless it was to counter the threat of bypass by alternate carriers.

These intermediate bit rates range from 128 to 768 kb/s in 128-kb/s increments.

Cost Of Service

Monthly charges for the 64-kb/s ASDS channel range from a fixed cost of $72 and $2.84 per mile for circuits 50 miles or less, to $248 fixed plus 32 cents per mile for circuits more than 100 miles.

There is also a monthly $16.40 connection charge at each end of the circuit.

Pricing for intermediate bit rates follows the same pricing structure, but offers 5-10% discounts off single-channel pricing.

Initially, ASDS was available in cities.

However, new locations are being added, with a target of 175 by mid-1990, and the conversion of all locations where voice-grade private-line service is available to ASDS by mid-1991.

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

Last December, AT&T expanded its ASDS capabilities by adding a multipoint service, transmission at 9.6 kb/s, a circuit-diversity option for better service availability, and the ability to combine lower-speed circuits in an AT&T office to use a more efficient, high-speed backbone circuit.

AT&T says that current users of its multipoint analog voice-grade private-line service will be converted automatically to the lower-priced ASDS multipoint circuits as their locations are converted to the new digital capabilities.

The subrate data multiplexing option uses equipment in AT&T serving offices to combine up to five 9.6-kb/s access line to be carried in one ASDS circuit across the network, and to disaggregate the circuit into its components at the far end.

Fractional T1 Pioneers

AT&T was not the first to offer fractional 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.

Soon after, another carrier, Lightnet, packaged eight DSO circuits on its fiber-optic network as a bundle under the name "T1 Third," pricing it at approximately 45% of the T1 tariff for the same distance.

Lightnet is now part of an 11,000-mile network built or acquired over the past five years by Williams Telecommunications Group of Tulsa, Okla., at a cost of nearly $1 billion.

WilTel claims it is the fourth-largest digital fiber-optic network in the country, behind the systems operated by AT&T, MCI and US Sprint, and serving customers from Portland to Miami and from Los Angeles to Boston.

Its fractional T1 service is a multiple-DSO, point-to-point private-line service provided with DS1 local access.

Recently, WilTel announced clear channel capability (CCC) as a no-cost option with its fractional T1 (F-T1) service.

This capability allows customers to utilize the full 64 kb/s for user information. Without CCC, 8 kb/s is needed to keep the network running smoothly, leaving an effective data throughput of 56 kb/s.

To provide the clear channel service, WilTel employs B8ZS (Binary 8 Zero Substitution) line coding, which maintains the "ones" density requirement in the transmitted bit stream.

AT&T also offers 64-kb/s clear channel capability using B8ZS as the preferred coding, but also supports the Alternate Channel Alternate Mark Inversion (ACAMI) line coding.

ACAMI is not as efficient as B8ZS and wastes bandwidth since two DS0s are used to send one 64 kb/s of user data.

AT&T says it should only be used when the application requires clear channel and B8ZS is not available from the local exchange carrier.

LECs Slow To Move

While local carriers have been slow to offer fractional T1 local loops, AT&T expects that the 60% annual growth in fractional T1 interexchange links will drive the market to total end-to-end fractional T1 service.

In the meantime, a number of firms are starting to fill the breach.

One of them, Metropolitan Fiber Systems Inc., has emerged as a leading provider of local access fiber-optic networks, with service in Chicago, Philadelphia, Baltimore, Boston, Minneapolis, San Francisco, Los Angeles and Houston.

A Pittsburgh network is under construction, and additional networks for New York, Dallas and other major cities are planned for later this year.

Its MFS F-T1 service allows customers to connect to interexchange fractional T1 links at either 56 kb/s or 64 kb/s clear channel.

The Oakbrook Terrace, Ill., firm has also upgraded its MetroHub 1 DS1 Hub product to provide 56 or 64 kb/s clear channel end links.

The MetroHub 1 allows carriers to distribute any fraction of a DS1 service directly to the customer premises.

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

This equipment allows the carrier to switch a user's DS0 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) DS0s into a T1 pipe.

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

Mux Compatibility

Multiplexers provide fractional T1 users with additional savings and efficiencies, provided they are compatible with the public network.

Besides providing virtually any data rate, these multiplexers enable users to create and route logical channels through the network, and expand the number of channels on demand.

They also allow users to configure and add DS0s quickly as they are ordered from the carrier, and to pass network management functions through the public network.

With this capability, the reach of proprietary network management can extend to every remote location, even though the DS0s are being routed to those destinations via the public DACS network.

Compatibility with the public network, though, is a significant multiplexer requirement.

Some multiplexers may not interoperate with the public network, but only act as gateways to it.

Such products require additional hardware to work with new fractional T1 services, adding considerable costs and significantly reducing network flexibility and options for future growth.

For a T1 multiplexer to transmit data over fractional T1 services, data must be formatted as 24 DS0s that are D4-channelized and ESF-formatted.

The AT&T Paradyne Comsphere Acculink family of T1 multiplexers allows users to mix proprietary and network-compatible channels freely on a single T1 link.

Multiple logical channel groups can be multiplexed onto a single link, travel on the same access T1 facility, and then be individually switched inside the public network to a destination.

Supervisory Data Links provide network management capabilities to each node.

Last January, General DataComm, of Middlebury, Conn., announced that its intelligent networking multiplexer, the Megamux Transport Management System (TMS), had met AT&T compatibility standards for ASDS and Accunet T1.5 Service.

Megamux TMS employs a Unix-based operating system to provide sophisticated networking and management capabilities.

Through the OACS-compatibility, the system permits flexible bandwidth allocation and extends network management and control to multiple remote locations.

The Avanti ONX T1 network processors and ONC 200, 100 and 10 network concentrators support fractional T1 services at all levels of compatibility.

The Newport, R.1., firm recently introduced an entry level multiplexer for fractional T1 applications, the ONC 100 VS, which can bandle up to 30 subrate data circuits, six high-speed data circuits or six voice circuits.

Timeplex Inc. of Woodcliff Lake, N.J., was another early supporter of fractiona T1 service through enhancements to its LINK and microLINK systems.

In March, another pioneer, Telematics International, introduced networking software for its Digital Wideband Exchange (DX) product line of time division multiplexers capable of supporting 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.

In September, the Fort Lauderdale, Fla., firm added low-end access multiplexers, the DX-10 and DX-20, to serve as economical feeder units to the higher-capacity DX-50 and DX-500 systems, providing unified management and control of the entire network.

Infotron Systems of Cherry Hill, N.J., offers one of the broadest arrays of networking products for fractional T1 applications.

Its Infostream 2000N, designed specifically for fractional T1 use, also complies with the G.732 international standard.

Last February, CASE/Datatel announced a new T1 multiplexer designed specifically for fractional T1 applications.

Designated the DCP9401 T1 Network Access Miltiplexer, the DAC-compatible unit can collect up to 60 data inputs, 24 voice inputs, or a mix of the two, and combine them for transmission over a full or fractional T1 facility.

Options include built-in subrate data multiplexer cards and T1 ESF Channel Service Units.

The PCSI CS4100 voice/data multiplexer takes advantage of DSO availability by putting up to four "network-quality" voice channels plus a synchronous data channel on the link.

The San Diego firm, a Pacific Telesis company, uses adaptive transform coding to digitize and compress the four analog channels simultaneously.

The encoded voice channels are then combined with the data for transmission via a CSU/DSU.

Micom Communications Corp. of Simi Valley, Calif., uses Advanced Packetized Voice technology with its VoiceMux 100 time division multiplexer to put four voice channels and a data channel over a 56- or 64-kb/s link.

It can also be configured to support two voice and five data channels or four voice and four data channels.

Republic Telcom Systems of Boulder, Colo., also uses packet technology to optimize bandwidth usage in fractional 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 common substrate digital circuits.

Its patented RLX voice packetizing technology reportedly provides up to 10 high-quality voice conversations per 64-kb/s circuit.

The RNET network control system provides a single point for network management control and reporting via Zenix-based software running on a 386 PC.

In March, OSC Communications Corp. of Plano, Texas, acquired Integrated Telecom Corp., forming a new group, CP Network Systems Division, to supply T1 multiplexers and customer premises equipment.

Its latest product, the CP1000S multiplexer, is intended specifically for fractional T1 applications.

It is fully compatible with the public network at every level, including DDS substrate formatting and ADPCM (Adaptive Differential Pulse Code Modulation) voice compression.

Racal-Milgo of Sunrise, Fla., has also introduced a three-member fractional T1 multiplexing series: the D4-compatible Omnimux 7000-D; the intelligent D4/ESF-compatible Omnimux 7000-I, and the Omnimux 7000-F, a fractional T1 multiplexer.

Channels supported may consist of any combination of 64-kb/s PCM voice, high-speed data and compressed video signals.

For high-speed synchronous applications, the Omnimux 7000 series provides dynamic bandwidth control of DS0 channels in increments of Nx56 kb/s, or Nx64 kb/s with Clear Channel Capability.

Data Service Units

Three California firms have recently unveiled fractional-T1-compatible data service units.

Digital Link of Sunnyvale now offers a dual-channel DSU/CSU that is compatible with LAN bridge, router and gateway products, CAD/CAM workstations and channel extender products.

It provides programmable channelization of incoming DTE data, which can then be routed via a DACS and multiplexed onto a higher-rate transmission line.

Network Equipment Technologies Inc. of Redwood City has introduced an intelligent DSU/CSU that can connect two data interfaces, or one voice and one data interface, onto a fractional T1 access trunk.

The Larse Corp. Split-T DSU allows Nx56- and Nx64-kb/s data from multiple LAN bridges, CAD/CAM Terminals, front-end processors, and other high-speed data terminals, to be bundled into contiguous, alternate, or random DS0 time slots.

The Santa Clara firm offers an optional T1 drop-and-insert input for integrating voice channels from fractional T1 into digital PBXs or channel bank environments.

Codex Corp. also has committed to support fractional T1 capabilities on its networking products.

Codex and its strategic partner, StrataCom Inc. of Campbell, Calif., are developing fractional T1 capabilities for the 6290/6292 line of fast-packet T1 switches, to be available in the third quarter.

Meanwhile, the Canton, Mass., firm has introduced a "multiplexing bridge," the 6310 EtherSpan, which provides fractional T1 services and also works with carrier-based DASC networks.

Two other firms have also introduced LAN bridges with fractional T1 capabilities: Network Application Technology of Cupertino, Calif.; and Newport Systems Solutions of Newport Beach, Calif.

In March, Phoenix Microsystems Inc. introduced a gateway fractional T1 network which allows drop-and-insert of up to 24 DS0 channels.

In addition, the Model 1564 Data-T acts as a controller for other DS0-level peripheral units in the Huntsville, Ala., firm's family of T1 products, including the Model 1565 subrate miltiplexer.

Data-T provides diagnostic error counting on DS1 lines to identify network problems before they affect user applications.

Also, built-in BERT capability for high-speed data ports isolates network problems.

Testing Tools

For testing the complete transmission system, Tekelec of Calabasas, Calif., offers the Chameleon 8000 test system, using 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 fractional T1 service.

Fast recovery from fractional T1 service interruptions is a key feature of the "intelligent" channel bank from Coastcom Inc. of Concord, Calif.

The D/I Mux III can accommodate up to 48 DS0 channels in a dual-channel bank mode.

Remotely configurable via software, the unit provides bi-directional drop-and-insert capabilities.

Furthermore, it may be programmed to automatically reroute high-priority DS0 channels in the event of network failure.

The Bytex Unity digital network switch also allows users to configure or reconfigure a network to use carrier facilities for wide-area connection in a most economical manner.

In addition to network access management, the Southborough, Mass., firm lists trunk-link management and wide-area switching as other principal applications for the product.

It supports fractional T1 services, as well as DDS interconnection at 56 kb/s for lower bandwidth requirements.

Cross-connection capabilities permit the mixing of digital voice as well as data traffic, the company says.
COPYRIGHT 1990 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1990 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Edwards, Morris
Publication:Communications News
Date:Jun 1, 1990
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