University combines fiber and WDM for switched video network
David Howley Artel Video Systems
The University of North Carolina met both its video networking and fiber-enhancement requirements seamlessly with a single stroke.
Over the past two years, the University of North Carolina (UNC) at Chapel Hill has systematically implemented a campus-wide, 270-Mbit/sec switched video network, making it the first American university to do so. Recently, UNC also became the first educational user of dense wavelength-division multiplexing (DWDM) for campus-based video and data applications.
In 1983, UNC was offered a 2-channel drop from a new microwave system that carried educational video programming throughout the state. After considering the alternatives, UNC chose a two-way RF transmission system as the most cost-effective method of transmitting video and audio from the closest microwave terminal to the UNC studio. The RF system simply takes the video and AM-modulates a carrier, while the audio is applied to an FM modulator (see Fig. 1). All the modulation techniques and frequencies are identical to those of a cable-TV system. In fact, commercial-grade cable-TV equipment is often used for such an application because it is readily available and reasonably priced.
If the microwave site and the studio had remained the only two sites to be connected, the rest of this story might never have happened. But once word got around that a new video feed had been connected to Chapel Hill from other educational facilities in the state, the clamor for connectivity started.
"It was a good news/bad news scenario," according to Tyler Miller Johnson, a telecommunications systems analyst at Chapel Hill. "The faculty were getting the video connectivity they wanted, but the system that supported it was never really designed for this purpose."
Johnson characterizes the existing RF system as being overbuilt repeatedly without a great deal of attention to its limitations. An RF system is essentially the same as those used for ITFS, cable-TV systems, and TV translators. While RF systems are adequate for video distribution, they fall short when used for contribution-quality systems. Contribution-quality video demands that the material can be edited, changed, or simply viewed and subsequently retransmitted without adding perceptible distortion. Linear systems are simply not well suited for this purpose. "The system worked," says Johnson, "but we had some serious quality and reliability problems; we knew that a better system had to be built."
Figure 2 is a simplified depiction of the switched linear system. The RF demodulators at the UNC studio received signals from various sources such as the state network or one of the classrooms. In all cases, video and audio signals received at the studio were fully demodulated to baseband, processed as needed, and switched to their final destinations. Individual switching and processing equipment was required for both audio and video, and the opportunity for adding noise and distortion was high.
Setting the stage for change
There are many people, especially on college campuses like UNC, who understand computers, Internet protocol (IP) networking, and the movement of compressed video on computer-based networks. There are, however, few who really understand video the way someone with a broadcast or cable-TV background can. Johnson had the dual task of understanding the university`s IP networking infrastructure needs while working toward building a new overlay system for high-quality video.
"It took a long time to prove that video is essentially a layered service," Johnson says. His key point with the UNC administration was that different levels of video quality were needed to support different applications. Although IP video clearly had its place in the scheme of things, it was not the only answer. Desktop video could be provided via such compression-based schemes as H.323, while interactive video classrooms, distance education, and video for broadcast had to be of the highest possible quality, with no compression. Other levels in between could be used for additional distribution tasks such as campus cable and dormitory service.
Johnson settled on switched digital video for the campus contribution network, and chose Artel Video Systems Inc. (Marlborough, MA) as his primary vendor.
Standards-based approach
UNC has a fixed annual technology budget, so getting the most functionality out of each dollar spent on a new piece of capital equipment is a must. In the computer environment, the university uses mainstream, standards-based technology that will work in a multivendor environment, to avoid being locked into a proprietary technology. The university used a similar standards-based approach to build its video network as well as optimize its optical network, which was beginning to suffer from a shortage of available fiber strands.
On the video side of the equation, UNC chose DigiLink 1220 video encoders and decoders in combination with MegaLink 1290X digital repeaters. The DigiLink 1220 encodes NTSC video and related audio programming up to a 270-Mbit/sec rate. The MegaLink acts as a repeater and an electrical interface between the fiber and the digital video routing switch. Combining these two transports in a network allows the use of SMPTE 259M standards-based serial digital video routing switches, like Artel`s Utah 300, at the hub.
NTSC video and analog audio are digitally encoded at 270 Mbits/sec at the North Carolina state network drop point and at one of the classrooms (see Fig. 3). The 270-Mbit/sec signal is transmitted over fiber to the studio, where it is converted by the digital repeater to an electrical 270-Mbit/sec stream without any demodulation. The signal is then re-clocked, switched by the video routing switch, and transmitted via another repeater to decoders in classrooms and to the local TV transmitter. Conversion between NTSC and digital occurs only at the end points, thus eliminating any added distortion as well as the need for complex baseband switching and video processing. In addition to working in the serial digital domain, the digital video routing switch may be used to support existing analog video and audio and can be later combined with a larger switch from the same product line to support high-definition television (HDTV) at 1.5 Gbits/sec.
Enter DWDM
In the fiber domain, the approach that UNC adopted maximized their fiber investment by allowing them to carry many signals over the same fiber. "On-campus is one issue, but we are establishing remote facilities off campus and leasing fiber to get there," says Johnson. DWDM will allow UNC to use the same fiber for multiple services and save money in the process (see Fig. 4).
In the final analysis, a DWDM system has the advantage over electrical multiplexing in that it is a totally transparent method (except for optical loss) of combining all types of formats and protocols on the same strand or strands of fiber. "The move to DWDM will enable us to have multivendor networks while staying in a digital domain, without having to do any special transcoding," Johnson says. "From an operational and support perspective, it is a more cost-effective, long-term solution, and we`ve had zero failures with minimal maintenance since turning it up."
The big picture
UNC`s Academic Technology and Networks (ATN) group is responsible for developing and maintaining the network infrastructure that transports these images, enabling different departments to transport compressed and uncompressed full-motion images around the campus. Its backbone transmits images from different sites to the studios that broadcast them to the end user. Departments send the images in a variety of formats, meaning the backbone must support everything from 28.8-kbit/sec modem and compressed video to full uncompressed digital video.
Demanding applications, such as certain areas of telemedicine, scientific imaging, and distance learning, require the extremely low latency and high quality that only noncompressed video can provide. The backbone must be capable of sending undistorted images from studio to studio. This capability is particularly important when signals leave UNC`s campus and enter the wide area heterogeneous network environment, where analog and compressed signals can degrade after traveling through numerous pathways.
"We have multi-hop transmission paths where every time a signal is sent through a different network, it is degraded. By using an uncompressed transmission for the highest level of video, we can minimize the degradation as video is compressed or repeated for lower-level distribution tiers," says Johnson.
Students throughout the state can now attend classes interactively via videoconferencing. Simultaneously, the university can transmit educational programming to the local cable-TV station, which sends it to households all over the Chapel Hill area, as well as to desktop computers on campus and throughout the world via the Internet.
The new system has given UNC a cost-effective, low-maintenance fiber backbone for video and a host of other applications that would have previously choked the system. The equipment that UNC purchased will also provide the university with an easy migration path from analog to digital and ultimately to HDTV, a challenge all broadcasters face due to the Federal Communication Commission`s mandate to switch to digital over the next six years. The encoders and decoders, for example, have been discussed here only in the context of being able to support NTSC encoded to a digital format. However, the equipment can be reconfigured via a front-panel switch to provide true SMPTE 259M serial digital transport, capable of supporting direct transmission of serial digital video from any source. This capability allows UNC to continue to use its existing base of analog video production equipment, while positioning itself to make a quick conversion to digital. The university thus enjoys the benefits of digital technology without having to re-build its studios before it is required to do so.
"As soon as our studios decide to migrate to digital, we`ll have the infrastructure in place to support them. In the meantime, we can continue to provide high-quality video production while keeping our analog users on the network," Johnson explains.
Expanding to the wide area
Since UNC`s ATN group standardized on this solution for central campus services, a number of the university`s departments have decided to adopt the same switching and transport technology to provide full digital-quality images to their students and faculty. In addition, other universities have expressed interest in UNC`s design. Johnson is hopeful that if these universities adopt a similar SMPTE 259M transport and switching architecture, it can seamlessly extend its infrastructure to other campuses.
UNC is also an Internet2 site, doing research on transporting video streams over high-speed, packet-based IP networks. While its new video and fiber backbone carries production signals, it also serves as a backbone for Internet-based video transport all the way down to individual desktops on campus, using MPEG1, MPEG2, and eventually MPEG4. This dual-service approach allows for the transmission of high-quality video for production sites, while maintaining the cost-effectiveness of compressed video for the campus`s 30,000 end users.
The Southern Universities Research Association (SURA) has selected UNC, along with Georgia Tech, the University of Tennessee at Knoxville, and North Carolina State University, to act as a video working group for the southern higher-education region. This group has two primary purposes: videoconferencing and video-on-demand (VOD) over IP. The videoconferencing initiative is focusing on promoting and developing H.323 teleconferencing for distance education and scientific collaboration. The VOD initiative is designed to support instructional applications and globally scaled digital libraries.
"We have proven that careful network planning, attention to future migration options, and the selection of well-qualified vendors pay major dividends in building a system that will help to maximize the investment in our educators," Johnson says. "It will also help UNC to build an educational system with a broader range of applications and wider reach than ever before. It`s something we can be proud of." u
David Howley is a product manager at Artel Video Systems (Marlborough, MA).