Broadcasters have embraced the Internet in many ways, such as providing content and streaming media via their Web sites, and transferring video files to connect their enterprise information systems together. The last step in this move to the new technology – using IP networks to distribute live, broadcast-quality video streams – is already happening. Path 1 has participated in numerous domestic and international video-over-IP trials in the past 12 months. And the company expects to launch service offerings in early 2002.
CNN multicast trial
Recently, CNN orchestrated a 90-day field trial to test the viability of long-haul, video-over-IP multicasting. Path 1 Network Technologies participated in the trial, along with Leitch Technology, BellSouth, CoreExpress, Cisco Systems, Pixelmetrix and TANDBERG Television. Each participant provided a technology or service integral to building a video-exchange service using IP multicast.
Trial configuration
There were four sites involved in the trial: Atlanta’s CNN Center, the Los Angeles and Washington, DC, CNN News Bureaus, and the St. Louis CoreExpress facilities.
DS-3 or 100 Mbits/s Ethernet/IP links connected these sites to a CoreExpress core network as well as to Sprint’s ISP network. MPEG-2 4:2:2 video streams with data rates ranging from 8 to 31 Mbits/s (CBR) were sent between these sites. Path 1 provided the video-over-IP gateways that performed the conversion from SDI MPEG-2 transport streams to IP streams and back. Video streams were continuously monitored for quality.
The long-haul network supported various forms of QoS methods and mechanisms that had to be effectively managed to ensure acceptable broadcast-quality transmissions. These methods and mechanisms included over-provisioning, priority queuing and dynamic assignment.
Over-provisioning simply allocated enough pipe and switch-processing bandwidth to accommodate all possible application scenarios, in addition to existing data traffic. Priority queuing used existing priority standards, such as Type-of-Service (TOS) in DiffServ, to rank queued data packets. Dynamic assignment used dynamic routing assignment stndards such as MPLS and RSVP to allcate bandwidth, on demand, through all legs of the route. The network bandwidth-management solution, requiring both priority and dynamic-assignment QoS approaches, was critical in making the video-over-IP test successful for CNN.
The Path 1 Cx1000 IP gateways sat at each end of the network cloud and served as the principle mechanism for pumping video streams into the network and receiving them. On the source end, the gateways accepted SDI video input, converted it to IP packets and dispatched the packets into the network cloud. At the far end, another Cx1000 reassembled the SDI data stream with precise synchronization and passed it to the broadcaster’s receiving equipment. In addition, they had to be rapidly configurable to suit the needs of the transport stream and network.
Implementation of QoS and/or MPLS in the trial network guaranteed the performance of the network to a high degree in the presence of other traffic. However, video data packets still experienced some degree of latency, jitter, packet loss and packet reorder while traveling through the multi-hop network environment. Receiving equipment, such as MPEG-2 decoders and video servers, proved highly intolerant to errors. So the gateway at the receive end had to correct these data-channel characteristics before reconstructing the digital video signal. Specific adaptation by the Cx1000 included corrections for jitter, packet loss and recovery.
Thus, the gateways played a critical role in ensuring that the video sent over the IP networks was received and reconstructed according to the stringent requirements of the broadcasters. On Sept. 11, distribution of broadcast video over IP became a reality when a test system was pressed into service by CNN to deliver news coverage from Washington, DC, to Atlanta. The field test proved that IP networks could be used as a reliable, high-quality method for exchanging live and taped news material among CNN-affiliated stations.
Henry Sariowan, Ph.D, is vice president of systems, and Douglas Palmer, Ph.D, is chief scientist and founder of Path 1 Network Technologies.
