Delivering quality real-time video over IP
Jul 1, 2008 12:00 PM, By Stephane Billat
Can an IP router replace a video router?
With telco entering in the quadruple play of video, Internet, phone and cell phone, it seems that video over IP is part of a video future. The requirements to deliver acceptable video and audio signals via IP are different from traditional voice and data delivery methods, bringing new challenges. IP is not new to broadcast facilities, as many files get transferred between locations, servers and workstations. However, most of that transfer is not in real time.
As manufacturers develop solutions to support the new IPTV deployment, traditional broadcasters are looking at using some of the IP technology to improve their real-time workflow. One question comes to mind: Can broadcasters replace a traditional video router with a standard IP router?
Can any broadcast signal fit into IP?
Table 1. Number of video streams in standard Ethernet connections. The boxes shaded in blue indicate which signals are practically useable over the given network/bandwidth.
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In theory, any type of data stream can be encapsulated into IP. In most cases, it is a good idea to limit the maximum size of the IP packet to 1500 bytes. The overhead for the encapsulating is small. It's about 40 bytes for 1500 bytes total, which is less than 3 percent. Today's networks commonly run at 1Gb/s (950Mb/s useable), even though there are still a lot of 100Mb/s networks inside existing facilities. The future will bring a 10Gb/s plus backbone and beyond, but this will not be widely spread outside of the large telco companies for some time. Broadcasters can encapsulate hundreds of video streams of H.264 or MPEG-2 SD distribution rate content on a 1GigE connection. There is no defined standard for encapsulating uncompressed SDI, HD or 3Gb/s video in IP packets. So at this time, carrying uncompressed HD-SDI or 3Gb/s around the plant over IP is possible, but not practical. (See Table 1.)
Delivering content between facilities
IP networks are different from traditional video networks. (See Figure 1.) IP networks are switched networks, which — with inconsistent signal paths — make it challenging to troubleshoot. The classic example is VOD, which by definition is only active while the user orders and watches the movie. It is therefore difficult to “follow the wire” from point to point.
Figure 1. This block diagram shows an example of Evertz IPTV monitoring topology, using an MVP display wall to show the results of data supplied by a VistaLINK SNMP interface.
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Delivering packets from the source to the destination is a well-known and controlled process (ATM, SONET). Timing and packet order has always been a challenge but is not critical in the case of e-mail and data. Most IP routers have a buffer built in but not much compared with high-speed video routers.
When delay happens in the network, it can result in underflow and overflow of the buffer at the switch. When data gets lost, freezeframe occurs, or there is tiling on the video output.
The media delivery index (MDI), which measures network jitter and drop of IP packets, is a composite number proposed by the Internet Engineering Task Force (IETF) and adopted by multiple test equipment vendors. It offers a simple but accurate way of measuring the network delivery quality in IP probes.
Using video over IP in the studio
As IP becomes mainstream, broadcasters are wondering if they could use the momentum of the IP networking to improve their real-time workflow.
The attraction comes from the fact that an IP router is perceived to be low-cost, can handle all kinds of data in real time or not real time and seems to be independent of the codec bit rate. The concept is that if you can equip yourself with an IP router, you should be able to encapsulate any signal available now and in the future.
So the question that broadcasters are asking to the industry is: Can I use an IP router for my core video router?
I/O ports
In a video router, the I/O count is key. A typical video router has a large number of input ports and output ports. Each port can carry one video signal SD, HD, 3Gb/s or compressed ASI in the case of an advanced video router. The port count of a video router can now go to 1024 inputs and 1024 outputs in large facilities.
In an IP router, each port is bidirectional and can support multiple video signals. That is a great advantage to the IP router, as multiple SD video sources can flow in both directions, whereas only one video source can travel on a video router I/O. Typical IP routers have a core up to 48 ports, a low number in the traditional video world. Multiple 48-port line cards can be combined in one chassis. The larger one-chassis design in the IP world is 480 ports. Note as most routers today are limited to GigE ports running at 950Mb/s, it is therefore impossible to switch real-time uncompressed HD-SDI and 3Gb/s because these won't “fit” in a GigE. 10GigE ports are becoming available, but they don't yet have the density of video routers.
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