Welcome to the real world

Communications News, Feb, 1998 by John Piercy

There is an old engineering axiom that says "Cheap, fast, good -- pick two," referring to the trade-offs of new technology design. Never has this saying been more true than in the design of networks for transporting video images.

Broadcasters and video production houses are under increased pressure to find cost-effective methods for transmitting high-quality digitized video to locations outside their existing plants. Large national broadcasters want to reduce costs by centralizing video clip archives, and large production houses want to add remote editing stations and apply more resources toward completing specific features.

With new techniques, wide area video transport services can be much faster, better, and cheaper than has been possible.

UNCOMPRESSED

According to current standards, 270 Mbps of bandwidth is needed to embed audio and ancillary data for digitized, broadcast-quality video. Local area networks can provide this quality with video switching routers and point-to-point coaxial cable or fiber lines. Coax allows full speed transmission up to 300 meters, typically restricting its application to in-studio type settings. Fiber can be used over distances up to 60 kilometers, making it an effective option in building-to-building and metropolitan-area networks.

Using fiber optic cable with central-office-based video switchers allows for local networking of production houses and television studios. This type of network is used extensively as local access networks in metropolitan areas, where local fiber plants exist with telephone and water utilities. The video quality is uncompressed and therefore very good.

The network allows for real-time video transmission, meaning that the video sent from the source can be transmitted directly on air or viewed at the receiving end on a monitor. Because of the expense of using dedicated fiber over long hauls, the dedicated fiber service does not extend well to the wide area.

COMPRESSED

Recent advances in integrated circuit technology have opened the door to sophisticated file compression algorithms -- such as the Motion Picture Expert Group's MPEG2 -- that reduce the size of video files. The MPEG2 algorithm is capable of compressing a standard 270 Mbps digital video stream down to rates in the range of 10-50 Mbps with no visible signal degradation, even after multiple tandem passes.

The MPEG2 standard defines a "professional profile" that uses spatial and temporal compression to achieve video compression. Spatial compression transforms small blocks of screen pixels from time-based to frequency-based signals and then filters out the high-frequency content (the eye is less sensitive to high-frequency components). Temporal compression is based on the understanding that most video sequences contain duplicate information. With this approach, only different information is sent for a number of frames after each full frame of information.

The new MPEG2 compression standard not only solves the video quality problem, but also permits more video streams to use the same bandwidth as would a single stream using older compression techniques. It can also reduce bandwidth to a rate that complies with telephone company standards, typically 45 Mbps DS3 service in North America and 34 Mbps E3 service in Europe.

PROTOCOL

The asynchronous transfer mode (ATM) backbone, currently being deployed by carriers around the world, is another key component that enables effective networking of video. With ATM, one source can connect to multiple end destinations using the switching capability of the ATM service.

The ATM architecture allows on-demand, high-bandwidth services, lowering the cost of transmitting video over the wide area. ATM addresses cost issues through the use of switched virtual circuits. Normally, dedicated services such as DS3s have costs that are fixed. ATM costs -- based on usage -- allow for a user to "pay-as-you-play," opening the market to less frequent users. Future multicast options will allow the ATM service to be used in a pseudo-broadcast manner for network-to-affiliate type video feeds.

One result of merging MPEG and ATM is additional savings made possible by the bursty nature of the MPEG output. The bandwidth requirements of this type of output vary based on the amount of motion in a video clip. With proper feedback from the network, it is possible for real-time video to use a less-expensive variable bit rate (VBR) service versus a constant bit rate (CBR) service.

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