ATM + MPEG2 = efficient multimedia

Demand for rich media services such as Internet access, video on demand, digital television and voice over IP grows more clamorous every day. So, too, does the need for high-performance distribution technology.

To meet this demand, service providers are turning to ATM technology - a flexible, scalable way of moving high-speed voice, video and data across networks. ATM's sophisticated bandwidth utilization capabilities enable providers to efficiently transport large, complex video packets without taxing a network.

ATM, however, cannot efficiently distribute these services alone. Combined with MPEG-2 compression technology, providers can effectively transport rich media over ATM, while ensuring integrity and quality of service (QoS). Together, ATM and MPEG-2 provide competitive local exchange carriers and last-mile service providers - including cable television (CATV), telecommunications, fiber to the curb and wireless companies - with the technology they need to distribute high-quality rich media services.

The majority of traffic ported over the ATM infrastructure is voice and data. Video will soon be as prominent and will drive the need for more high-capacity ATM networks. The basis of ATM technology is a high-efficiency, low-latency switching and multiplexing mechanism ideally suited to an environment in which there are specific bandwidth limitations.

ATM allocates bandwidth on demand by constructing virtual channels and virtual paths between source and destination points on the ATM network boundaries. These channels are not dedicated physical connections, but are permanent virtual connections or switched virtual connections that are deconstructed when no longer needed.

The speed and reliability of ATM switched networks can't be matched by other popular WAN technologies, which are ill-equipped to transport high-performance data. However, even in an ATM environment, the nuances and peculiarities of digital video make it impractical to transport real-time video in its native uncompressed format over ATM. Using MPEG-2's sophisticated compression techniques, providers can alleviate technical roadblocks when managing and ensuring the integrity of large, super-fast video streams over ATM.

To efficiently transport video in an ATM environment without jeopardizing qual-ity, the Moving Pictures Expert Group (MPEG) developed a series of hardware and software standards designed to reduce the storage requirements of digital video. The group created MPEG-2, a compression scheme to efficiently distribute high-quality, full-motion video data.

Full-motion, digitized and uncompressed video requires a data rate transfer of roughly 270M bit/sec. With virtually imperceptible quality degradation, MPEG-2 can reduce this to 16M bit/sec for contribution-quality video - used during the editing process - and to 4M bit/sec or 5M bit/sec for distribution-quality video as received by end users. Providers can ensure video quality by applying algorithmic compression techniques such as chroma subsampling, which uses color variation undetectable to the human eye. Motion vector estimation randomly samples frames to assess video quality at that given moment, enabling providers to ensure QoS in an ATM environment. Traditional IP routing technology, in contrast, cannot guarantee QoS.

Getting MPEG-2 onto ATM networks and then picking it off in good order takes care. The ATM network edge device must be particularly adept at handling MPEG switching and jitter management to compensate for propagation delays in the network.

Local MPEG-2 video streams are typically transported via an interface known as digital video broadcast asynchronous serial interface. ATM edge devices deconstruct either an MPEG-2 multiple program transport stream (MPTS) or single program transport stream to the program level and ultimately to the packet-identifier (PID) level. At the PID level, streams can be reordered and combined back into another MPTS. This process is referred to as remultiplexing. Each packet of MPEG-2 data is then tagged with a PID, a 13-bit field that identifies the association between a program, transport stream and packet.

This architecture is likely to become the predominant distribution method for rich media services.

Pecorella is product marketing manager and Dugan is manager of corporate marketing at Artel Video Systems. They can be reached at dpecorel@ artel.com and mdugan@artel.com.