Cable television operators have a long history of providing service to institutional customers such as schools, businesses and government users. In the past 18 months, the combined factors of increased competition and demanding franchise renewals have made the provision of distance learning service mandatory for many operators, and for some, an opportunity to generate new revenues.

This article reviews the history of fiber optic distance learning system design. The focus is on the current state-of-the art, which is a dedicated fiber optic network that can deliver a comprehensive set of interactive video and data services. Also discussed are some real-life experiences related to bidding and building multimedia distance learning networks.

The early days-FM rings

In the second half of the 1980s, FM fiber optics became a commercially viable solution for headend interconnects and other applications such as distance learning. These first networks were either directly owned and operated by the school district or supplied by a telephone or alternate access company. In both cases, the parties involved, including the design consultants, had no desire to deal with a coax/amplifier-based system and wholeheartedly embraced a fiber optic solution.

The first systems were designed in a "ring" topology with a maximum capacity of 16 simultaneous channels on the ring. Each location was capable of transmitting one channel and receiving three. This channel configuration stemmed from the conventional wisdom that the maximum number of remote sites an instructor could manage during one session was three, therefore, three receive channels.

In general, these systems met the needs of the first users, but the limitations of the basic design approach made it difficult and expensive to add new sites, increase channel capacity and troubleshoot problems. It was a good solution for its time, but it never provided the functionality and cost to allow for the wide-scale deployment of distance learning networks.

AM fiber optics

In 1990 the cable TV industry embraced AM fiber optic solutions for trunking applications. Acceptance of AM technology in the world of distance learning did not occur for several more years and can be attributed to two factors. Distance learning projects often have a 12- to 36-month cycle, beginning with a consulting engineer defining a bid specification. Many projects in the early '90s were built based upon bid specifications that were written prior to the introduction of AM transmission electronics.

The second factor relates to the type of people who were doing the design of distance learning systems- they were not cable television engineers! It took a number of years, more time than you would reasonably expect, for the majority of the engineering community outside of the cable industry to understand the advantages of AM systems.

One of the notable exceptions was Ameritech. In 1991, the company built a large distance learning network in Indianapolis that connected 90 schools for interactive multichannel video and 10 MB data using AM electronics in a star architecture. It is still today one of the largest interactive distance learning networks in the country.

AM/FM hybrid designs-TEEN network

By 1993, AM fiber optics was becoming more commonly understood, and in that year, some of the first "AM/FM hybrid" networks were designed and built. The TEEN (Technology Excellence in Education Network) was one of those systems and was an extremely interesting project because of how the network was designed, and because of the people involved.

The TEEN Network represents the efforts of a group of dedicated educators and administrators to provide a modern curriculum to students at five rural schools in Kansas (Figure 1). Working with engineering consultant Tele-Systems, the group first considered the classic FM ring approach but determined it was too limiting in terms of channel capacity and site expansion.

The conclusion was a design approach that used both AM and FM transmission electronics in a star architecture. For the inbound portion of the network, the remote sites were configured the same as in a classic FM design. Each site used an individual FM modulator and FM transmitter to deliver one channel to the next site, and eventually to the system headend, which was located at Marion.

At Marion, all of the signals were received and FM demodulated to baseband. The signal package was then AM modulated and fed to AM transmitters for delivery outbound. Cost considerations dictated a minimal fiber count and the need to share fibers between sites. To address the situation, the outbound portion of the system was designed with cascaded AM links. The output of high-power DFB transmitters delivers the entire channel package to each site. AM receivers detect the signal package at each site, and the receiver output is simply fed to a cable-ready TV set for channel selection and viewing.

The AM/FM hybrid approach provided the best advantages of both transmission techniques. The robust nature of FM inbound transmission allowed for the received signals at the headend to be of broadcast quality, even though most of the sites were located many miles away. The high quality of the inbound signals at this point allows them to be either retransmitted outbound to the TEEN locations or to another network. In either case, the system is designed so that the roundtrip signal performance meets or exceeds what the students are watching in their own home.

The AM/FM hybrid approach provided the best advantages of both transmission techniques. The robust nature of FM inbound transmission allowed for the received signals at the headend to be of broadcast quality, even though most of the sites were located many miles away. The high quality of the inbound signals at this point allows them to be either retransmitted outbound to the TEEN locations or to another network. In either case, the system is designed so that the roundtrip signal performance meets or exceeds what the students are watching in their own home.

Using AM electronics reduced the total cost of the network while providing expanded channel capacity. The TEEN outbound transmitters were built for 450 MHz of usable bandwidth, which will meet the needs of the users for many years to come. It also made the classroom relatively simple to install, because the end user interface device is a standard cable-ready TV with a handheld remote control.

In the years that have passed, the TEEN users have consistently expanded the service capability of the system. Today, the network includes Ethernet data service, as well as a codec for long distance conferencing with other networks. The next challenge for the TEEN group is Internet access, which it hopes to have up and running sometime in 1996.

AM/FM hybrid design - today

The basic concepts of the AM/FM hybrid design approach are still being used today. The most significant differences between the TEEN design and the design work done today have to be considered refinements, more than real changes.

Today's outbound design (Figure 2) looks just like a cable TV trunking application in that optical splitting is used to allow one DFB transmitter to serve multiple sites.


The inbound portion uses the same design approach but more cost-effective FM transmission equipment.

The TEEN design used frequency agile FM modulators and demodulators with separate transmitters and receivers. This allowed the network to be easily configured for a full 16 inbound channels. Today, it is generally recognized that most schools will never need to transmit more than four simultaneous video channels, and the majority of sites will only require one channel. This has led to the use of lower cost fixed frequency modulation and demodulation equipment, that in the case of the single channel requirement, is all integrated into one small package (Figure 3).

The service package today

What has changed significantly in the past three years is the set of services most end users expect to be provided by a modern distance learning network, including:

  • Outbound video-550 MHz is a minimum requirement, and many users want 750 MHz. The actual number of channels to be delivered outbound upon system turn-up varies depending on available funds but is rarely less than 40 channels. The bottom line is that users want as many channels and as much usable bandwidth as possible for broadcast-only service, interactive videoconferencing and media retrieval.
  • Fax-Any capability of the network that reduces the end user's existing cost helps pay for the network. Most users expect the network to provide the capability for fax service between sites. This capability is relatively simple and inexpensive to implement, and in fact, was provided as part of the functionality of the TEEN Network.
  • Data-10 MB Ethernet & Internet Access. This is the must-have service. For many educators it is more important than the video service. The 10 MB Ethernet capability can be used for administrative functions, such as attendance, payroll, record keeping, etc. Schools see this as a means to centralize their data processing functions and to reduce MIS expenditures. Internet is almost self-explanatory. Does anyone want their children to attend schools that aren't connected to the Internet? It is the 1990s' version of the encyclopedia.

Commercially available and proven RF modems have greatly simplified the simultaneous transmission of 10 MB Ethernet, Internet and multiple full motion video services. RF bridges connect each remote Local Area Network (LAN) into a multi-site Wide Area Network (WAN). The WAN can approach a 200-mile service radius and be fully symmetrical with full 10 MB throughput. The bridges connect to the WAN via standard cable TV video RF frequencies to allow data carriers to be combined and transmitted with the video channels (Figure 2).


A central headend collects the inbound data carrier from each LAN and uses a frequency translator to convert the data package to a high frequency outbound carrier returned to each site's bridge. The headend equalizer or "pacer" router synchronizes the data flow and network authorization between LANs (Figure 4).

Internet Protocol (IP) access is connected from the local Internet provider or "gateway" and connected to the WAN using an IP router. The IP router allows simultaneous Internet and Ethernet service over the WAN Ethernet bridge/translator network. The IP router directs Internet access to LAN users via their IP address, while other protocols are directed by the Ethernet pacer.

The network management software allocates data bandwidth and access between sources and users.

Any network that provides all of these capabilities has to include system switching or be designed so that system switching can easily be added in the future. Many of the early distance learning networks were complicated to use and discouraged rather than encouraged teachers and students to use the system.

The technology can't interfere with the learning process, and this is best accomplished by giving the teacher very simple tools to set-up and control conferences. Based upon the feedback received, the following are some of the user capabilities that need to be provided by a modern distance learning network:

  • End users can schedule their own conference and receive immediate feedback upon their request.
  • If the conference can't be scheduled as requested, the network provides alternate conference times and dates.
  • The classroom environment provides the teacher and students with a full-motion, continuous view of all participants.
  • The teacher can control and switch the video images being viewed.
  • The teacher has access to multiple views of any one location.
  • The teacher can remotely control source devices, such as VCRs, cameras and microphones.
  • The control functions are all through a handheld remote control, like the one teachers use in their own home.

The classic approach to setting up a video switch is to establish a centralized baseband matrix switch at the system headend. This approach can be very expensive both in terms of the penalty it places on the transmission electronics and the cost of growing the switch in the future. To avoid these problems, the focus has moved to distributed switching solutions using simple programmable demodulators at the school controlled by the EDCOMM network management system.

The advantages of this approach, beyond reduced cost, include the capability to designate any site on the network as a gateway and the ability to grow the switching capability of the network in an incremental manner based upon the needs of each site.

A growing number of cable operators are exploring the possibility of providing distance learning solutions. The common perception is that this growth is resulting from a need to satisfy franchise requirements. This is partly true, but ignores the fact that cable operators see the opportunity to generate new revenues and enhance their public image. It also does not recognize the reaction of many cable operators to imminent competition for video service in their franchise areas.

Distance learning-the business case

This multi-purpose strategy is the case for Comcast in New Jersey and Marcus in Wisconsin. End users are capable and willing to pay for both video and data services, if they are delivered with cost-performance tradeoffs that are of real value. The authors' company has worked very closely with Comcast and Marcus to put together cost-effective solutions that include simultaneous full-motion videoconferencing, 10 MB Ethernet service and full Internet access.

This combined set of services and the guaranteed capability for future expansion to more sites and other systems gave Comcast and Marcus dramatic advantages (and the winning bids) versus the competition.

These systems provide the infrastructure and demonstrate the capability to reach a new customer base with an expanding number of site and service requirements. Most importantly, both systems use current, cost-effective technology in designs suitable for wider scale business plans.

Areas up to 70 square miles are serviced with expansion capability provided for 30, 60 or more fully interactive sites. These sites can include government institutions, medical facilities, colleges and schools to provide multiple market opportunities all supported by one network.

The building of infrastructure and the delivery of service today at a realistic price positions Marcus and Comcast extremely well for future growth. Their competition has been pushed back, their networks have expanded their reach and service capability, and as corporate citizens, they are viewed as providing an innovative, important service to their communities.

It isn't hard to understand that this is a winning strategy for these two operators and others who share the vision.

In conclusion

Distance learning-defined as providing interactive video, audio and data services to schools, businesses and government-is a real business that is of strategic importance to the cable industry.

The existing franchise, when combined with the use of a comprehensive multimedia design solution, provides the cable operator with enormous advantages over the competition.