Monitoring MPEG in an IP Network
The cost and ease-of-use advantages of moving video using IP have been welcomed in cable, IPTV and satellite applications around the world. For the most part, IP remains a rarity in most of today's television studios. However, these same benefits will eventually make the approach more prevalent in the broadcast market. Signal monitoring throughout the video delivery chain is essential to ensuring the viewer's quality of experience (QoE), and for this reason, broadcasters will benefit from having a thorough understanding of what is required to effectively deploy IP and accurately monitor IP content.
IP's Usefulness in Broadcast
While a broadcast facility may never have a fully deployed IP network backbone in the studio, IP links are set to replace ASI interconnects between equipment in many applications.
Encoders, multiplexers and other equipment located at the broadcaster's headend are already IP-capable devices, likely linked by ASI in most environments. Moving forward, IP networks with high-end switches or routers at the center will feed multiple pieces of equipment within the broadcast operation and become increasingly common as organizations begin recognizing the benefits of this technology. Benefits include relatively low-cost, facilitated transmission of signals to multiple destinations (multicasting) and ease of signal monitoring.
Another likely location for the imminent incursion of IP into the broadcaster's world is the connection between the studio and the transmitter. A transmitter is typically linked to the studio by microwave; however, some broadcasters are already considering replacing this link via an IP connection. Leasing fiber eliminates both the expense and uncertainty of microwave systems, which are subject to weather and other interference.
How Video Over IP Works
Digital video is packetized data — ones and zeros moving 188 bytes at a time in a transport stream. IP transmits data from one point to another, and because digital video is essentially data, it can also be arranged in Ethernet frames and transmitted. However, IP does pose fundamental problems as a video transfer scheme, most of them stemming from the nature of IP. IP was developed some 30 years ago as a way to move data quickly and efficiently from Point A to Point B. In traditional data transfers, timing and sequence do not matter very much. For example, when sending or viewing a Web page or e-mail, the order in which the data components arrive is unimportant as long as the content appears correctly once loading is complete. Should data be lost or corrupted in transit, the content can easily be retransmitted and loaded without the end user ever knowing the difference.
Transmitting live video is entirely different. The frames containing the data — typically MPEG — must arrive synchronized, on time and in sequence if the footage is going to appear as intended to the viewer. In a video application, retransmission of lost or corrupt data is nearly impossible because the appropriate moment for display has passed.
When MPEG-compressed video travels on an IP network, the content is typically arranged in groups of seven data packets, each wrapped in an Ethernet frame. These frames each contain source and destination addresses so that the data is routed appropriately within the network. As the frame arrives at the receiver-decoder (or other device), the MPEG packets are extracted and treated the same as they would have had they arrived by ASI.
In an IP network that has been properly designed to carry video, the switches and routers are configured to prioritize video data. If the integrity of the video is to be maintained at a high level, then IP infrastructure must also be maintained and a monitoring solution put in place that addresses the problems unique to IP infrastructure — specifically jitter and dropped, lost or out-of-order data packets.
How Video Over IP is Monitored
In the cable and satellite industries where video delivery over IP is common, effective monitoring techniques have been tried and proven. Typically, a monitoring device performs multiple tests continuously on all inputs. Because almost all of these tests are based on the timing within the data transmission, the monitor's most fundamental job is to keep track of packet arrival times with a high degree of accuracy. Within the MPEG signal, the monitoring device assesses the timing to ensure that it meets a predetermined standard, such as ETSI TR 101 290 in Europe. The timing standard — designed to ensure QoE for the viewer — codifies acceptable arrival timing for the component parts of the stream and the Program Clock Reference (PCR), which contributes to timing accuracy. The component parts are audio and video data, as well as the tables that enable the consumer's television to perform decoding, display the image on the screen, and properly represent auxiliary items like a program guide and subtitles.
Effective monitoring of an IP-based system requires timestamping every Ethernet frame so that the rate and sequence can be tested. The first test that must be performed is for jitter, which is a measure of the cadence of the packets in the line. The packets should arrive at regular intervals, without bursts or prolonged gaps. To some extent, the buffer in the receiver-decoder can compensate for these issues, but if they become extreme, the buffer is overwhelmed and viewer experience suffers.
The second test is for dropped or out-of-order packets. These can be hard to recognize because the IP stream typically lacks both indicators of packet order and a means of notifying the network that a packet has been lost. To get around this, a monitoring device can penetrate the packet, scrutinize the MPEG packets within and use continuity counters to determine whether all the packets are present. This is the same kind of MPEG test that is conducted for an ASI or other traditional broadcast signal. With IP transport, the Ethernet frame adds another layer of complexity to the address with monitoring.
Monitoring devices typically incorporate slots for one or two cards that can perform either IP or ASI monitoring, depending on the needs of the system. In a traditional broadcast station, a monitor with four ASI inputs might be implemented to cover all the necessary streams at the headend or studio. One of IP's advantages over ASI is that a single IP input can simultaneously monitor all the traffic on the network — hundreds of IP multicasts — rendering multiple monitor inputs unnecessary and ultimately saving the broadcaster money. In fact, the number of IP transport streams is limited only by overall network capability.
Once the Ethernet frame is removed from the MPEG layer, the monitoring process is the same as for transport streams carried over any other physical medium. The monitor assesses the timing of elements such as PAT and PMT tables to ensure their rates meet the predetermined standard (ie: ETSI TR 101 290). The time-checks also reveal gaps between packets that contain video and audio. Beyond that, the monitor scrutinizes the accuracy of the PCR.
Because timing really is everything for optimal video delivery, the accuracy of the monitoring device is also important; even a small degree of inaccuracy distorts the information gained from the monitoring process. Some monitors unintentionally introduce delay and inaccuracy because they rely on an off-the-shelf network interface card to input the video streams and disregard the specialized requirements of delicate IP-based transport streams. Because these cards must subsequently pass the data through the operating system's software IP stack, much of the timing information's granularity is inevitably lost to processing delays.
A more effective technique is for a proprietary network interface card to timestamp the Ethernet frames at the time of input — without injecting processing delays. This can be accomplished by using specialized hardware on the input card to separate Ethernet frames carrying transport stream packets from those containing general IP traffic. The general traffic data can be passed through the operating system's normal IP stack while the frames containing transport stream packets are timestamped and passed directly to the analysis software, bypassing the IP stack.
The Ethernet frame timestamping is done using a highly accurate clock reference (such as an oven-controlled crystal oscilator), and then the frame timestamps can be inferred and transferred to the transport packets inside the frames by referencing the physical link speed on the Ethernet interface. This methodology yields more accurate data for subsequent reference and analysis by the specialized software. In fact, timestamping techniques like this have been proven effective and are common with single stream ASI inputs. In an IP network, where there may be hundreds of IP multicasts, it is less common but even more useful and necessary.
Conclusion
As digital television becomes the global standard, delivering video from source to home becomes an increasingly complex process that relies on multiple transport techniques. Each of these techniques — and each combination of them — is accompanied by a potential for error that can diminish the quality of the video signal being delivered. If signal quality deteriorates or is interrupted, viewers may change channels, switch providers, or even turn off the television set altogether. IP-based signal transmission schemes are not yet as familiar to broadcast engineers as their more traditional ASI and RF counterparts. However, as the use of IP-based signals increases, their effective and accurate monitoring becomes commensurately important to the viewer's quality of experience.
By Seth Vermulm, Broadcast Engineering
