A prototype fiber/coaxial design

Twenty-three companies teamed up to demonstrate a fully interoperable ATM-switched full-service network in California early this month, employing the fiber/coaxial “star/bus” design pioneered by the cable industry.

The demonstration, put together by Cable Television Laboratories for the industry’s Western Show convention in Anaheim, CA, served as confirmation that the many streams of advanced protocols now emerging in telecommunications, computing and digital television can be put together for provision of a vast array of media and telephony services over such networks. And it also provided a useful snapshot as to where vendors are going as they push capabilities ever further in the interim between first iterations and wide-scale rollout of their products.

CABLENET DELIVERS INTERACTIVE SERVICES

The demonstration involved all the services we would expect: delivery of distance learning, voice service, teleconferencing, data services, video on demand, community local area network, commercial telecommunications (T-1), access to the Internet, digital ad insertion and automated network control. There were also unique new applications, such as Hewlett-Packard’s color printer system, which captures and prints images shown on the television screen, and Kodak’s digital picture exchange, an extension of the company’s Photo CD system. But the real story was not the applications and services but the underlying technology that made them possible.

All of these applications were running simultaneously, supported by two asynchronous transfer mode (ATM) switches that were interconnected over an OC-48 fiber ring, which is the 2.4-gigabit/second SONET (Synchronous Optical Network) protocol used for regional trunking applications. The demonstration also used fiber links outside the convention area, supplied by Teleport Communications Group, for importation of distant signals.

DEMO BASED ON CABLELABS MODEL

The setup mimicked the type of ATM and fiber node array that CableLabs is recommending for cable systems, starting with regional interconnection and extending down to the individual customer level. The regional ATM switch, a Northern Telecom Magellan Gateway, operated network-to-network and network-to-user interfaces at DS-3 (45 Mbits/sec.) and OC-3 (155 Mbits/sec.) speeds. The Magellan can service up to 24 ports, each running at DS-3 speed, or a smaller number of ports in various combinations of DS-3 and OC-3 speeds.

ATM as the backbone. The role of such a switch is to steer traffic from high-speed inputs and outputs across a broad range of networks and operating territories. Beyond this core interface, the fiber ring extending over a large geographical region would serve several additional ATM nodes, which might be set up along the lines of the second ATM switch at the demonstration, an NEC NEAX Model 10 ATM Service Node. (For a primer on ATM technology, see Mediascape on p. 14.)

The second-level ATM switch serves as the traffic manager for the local serving area, such as a cable system’s franchise territory. It provides the interface among the various cable network components, including servers, ad-insertion systems, head-ends and local telecommunications switches. Eventually, the cable industry envisions another layer of ATM routers that would be dispersed further into the field to accommodate higher levels of user contention for switched services.

Officials at the demonstration noted that many ATM applications envisioned for media distribution, as opposed to the initial commercial applications in network computing and telecommunications, remain to be woven into the set of protocols. For example, the International Standards Organization’s task force recently selected a 188-byte packetized transport layer for MPEG-2 video compression with the intention of achieving an easy mapping of such transmissions into ATM cells, but the details of how this interface will be structured have yet to be worked out.

Backward compatibility. Another detail that needs to be worked out is support for existing (analog) video transmission over the same network. ATM has been formulated to work with SONET protocols but not with RF (radio frequency) transport protocols, such as are used in cable TV. This means that specific allocations of frequency levels within any given RF channel must be matched to each of the bytes within the 53-byte ATM cell, and it probably means some of the ATM functions built into the 5-byte header will be removed to achieve greater bandwidth efficiency and to eliminate unnecessary processing from ATM-compatible premises terminals.

But these are details that vendors appear ready to focus on, according to Richard Green, president of CableLabs. “I’m very encouraged that further work on ATM will maximize its efficiency for applications such as we have in mind,” he said. The key point, he added, is that no major technological innovations are required to make these accommodations.

Meanwhile, as the demonstration proved, ATM is more than adequate to meet the requirements of early market tests such as are scheduled by Time Warner in Orlando, FL, and by US West and Cox Cable in separate trials in Omaha, NE, next year. Here, the ATM units served to support video-on-demand techniques offered by IBM, as well as the Grass Valley Group’s interactive distance learning system, the American Memory program (using Mosaic in conjunction with accessing multimedia material from the Library of Congress) and broadband access to the Internet (the Consortium for School Networking’s Multimedia Internet Applications).

A MIX-AND-MATCH APPROACH TO SERVICES

A number of other applications were also on the network but bypassed the ATM switches, showing how network operators can mix and match various approaches to providing services by dividing service categories by frequency (frequency multiplexing) and routing them through whatever network components they choose. These applications included two uses of Ethernet over cable: one by Zenith Corp. for work-at-home applications and one by Unisys demonstrating “multimedia on demand.” There was also another video-on-demand system in addition to IBM’s operated by Hewlett-Packard, which connected to HP’s server located elsewhere in the convention exhibit hall.

IBM’s video on demand. IBM’s video on demand employed the company’s Ultimedia Server 6000 in conjunction with TRW’s Ramcube 26G, which is a large, solid-state memory subsystem for high-speed, high-volume data-handling applications. Together, the units showed how a video-on-demand service could support a combination of high-volume use of current box office hits with much lower levels of demand for less-popular movies, with the net effect of lowering the overall storage and access capabilities required for the primary server.

As explained by Tom Gritzmacher, program manager for TRW’s Space and Electronics Group, the solid state Ramcube can serve as a temporary storehouse for the movies that are most in demand, accommodating up to 1,000 simultaneous viewings of a single film. The configuration of films in this unit can be changed whenever the network operator wants to by simply downloading selected films from the IBM server into the solid-state unit.

Next round of improvements. The demonstration also highlighted some improvements in bandwidth efficiency that are on the horizon. Even though the MPEG-2 protocols have established a set of bit rates for various types of applications that is likely to be the universal standard for a long time to come, continuing advances in modulation techniques promise considerable improvements in spectrum efficiency — the overall bit count per unit of bandwidth frequency (Hertz) — as well as gains in robustness for any given level of bitstream per channel.

For example, Northern Telecom (NT) showed an application of a technology known as discrete multitone (DMT), which can deliver up to seven bits per Hertz. At five bits per Hertz (as demonstrated in Anaheim) it can support transmission of two-way services over much longer extensions of coaxial cable than once seemed possible.

DMT divides a channel within a given transport medium into a multitude of discrete subchannels and then determines which subchannels are most appropriate on any given network segment for transmission of the telephone or other signal. For example, when DMT technology is used over standard telephone copper wire, the approximately 1.1-MHz capacity of the line is divided into 256 individual carriers, allowing the system to determine the points within the link spectrum that are best suited to a high level of bit loading, a medium level or none.

Telephone service over coax. Earlier this year, the T1E1.4 subworking group of the Exchange Carriers Standards Association chose DMT as the preferred modulation scheme for the telcos’ ADSL (asymmetrical digital subscriber line) technology, owing to the superior robustness and high bit rate of the system. NT claims that the system will support transmission over 12,000-foot runs of standard copper wire at 7.7 megabits/sec., which is enough for four movie channels, two digital telephone circuits, an analog voice circuit and a data channel. Bellcore estimates that about 50–55 percent of all households in the U.S. fall within this line reach.

The technology should provide equal benefits for transmission over coax. Said Stefan Sherman, senior market development manager for residential broadband at NT, “We believe DMT will permit cable companies to operate telephone service over coaxial serving areas of 2,000 homes or more.” This would expand the cable industry’s opportunity in telephony by a considerable margin, since most discussions about “cablephone” have assumed fiber penetration to the 500-home level, which is still a rarity among cable systems.

Sherman said the cablephone application has been under development for only three months at NT and, as yet, has not won a go-ahead for final product development. “We have to run tests over cable systems and talk with the industry more about it,” he said, acknowledging that NT is a latecomer to the burgeoning cable hardware market.

However, given the success of DMT in improving the capabilities of twisted-pair phone lines, the technology does look promising for telephone service or other interactive services over longer coaxial runs, where “noise” from outside signals and online amplifiers is a serious problem.

Sherman noted that NT was not operating the system at the Western show at maximum bit/Hertz rates. “The principle is the same,” he said. “We could run at 7 bits/Hertz; it’s just a matter of how far you want to go versus how much you want to carry. In cable, bandwidth isn’t at such a premium as it is over copper, so you can opt for greater range.”

Seven bits and what do ya’ get? By the time digital communications become a reality, seven bits/Hertz could well be a standard. This would mean that the digital capacity of any transmission medium could be some 30–40 percent above what people had anticipated. At the Western show, ComStream demonstrated another modulation technique known as 64 QAM (quadrature amplitude modulation), which operates at five bits/Hertz. However, according to a company engineer, ComStream (as well as several other companies) is working on a refinement called 256 QAM, which will offer 7 or more bits/Hertz, possibly before 64 QAM ever gets off the ground.

This would appear to put QAM techniques, now under development by a wide range of manufacturers, on course to catch up with another new modulation scheme — Zenith’s 16 VSB (vestigial sideband) technique, which also operates at about 7 bits/Hertz. Zenith specifications call for 23 movie channels per six MHz cable channel or as many as nine live TV programs. The commune also demonstrated its modulation scheme that supports transport of two HDTV channels over a single 6-MHz cable channel, effectively doubling the HDTV carrying capacity within the traditional 6-MHz cable-channelization scheme.

Engineers anticipate that by the end of next year all of these advanced modulation systems will be on the market. The freeing up of bandwidth for more channels over star/bus networks means not only more carrying capacity for point-to-multipoint “broadcast” services, but also more capacity for allocation of dedicated point-to-point channels such as were demonstrated at the CableNet exhibit.

Fred Dawson