 | Issue: | Asia-Pacific I 2006 |
| Article no.: | 8 | |
| Topic: | Reliable IPTV | |
| Author: | Danny Wilson | |
| Title: | President | |
| Organisation: | Pixelmetrix Corporation, Singapore | |
| PDF size: | 52KB |
About author
Danny Wilson is founder and President of Pixelmetrix Corporation, a manufacturer of preventive moni-toring solutions for digital broadcasters. Previously, Mr Wilson was an executive with Hewlett-Packard’s Communication Measurement Division. In this role, he was responsible for the introduction of the MPEGScope, transport stream tester and the world’s first ATM/B-ISDN Test System, which accelerated the development and deployment of ATM technology worldwide. A native of Edmonton, Canada, Mr Wilson holds a degree in Computer Engineering from the University of Alberta.
Article abstract
Many operating companies and service providers see IPTV as their salvation since it will enable them to compete with the service offerings of the cable and direct to home satellite TV companies. IP’s advantages – efficient use of bandwidth, digital audio, voice and data transmission on the same network – are undeniable. Unfortunately, given the current state of the technology, it is still quite difficult to guarantee the same quality viewing experience with IPTV that conventional services provide; this requires automated, integrated solutions.
Full Article
Transmitting video over IP has serious quality of service challenges that companies ignore at their peril. Monitoring the network for problems has become more complicated as the number of channels has exploded. Noticing there is a problem with the transmission is one issue, while locating the source of the problem is another. As digital video compression technology matures, Internet Protocol (IP) is gaining widespread acceptance as a means of sending digital video along the transmission chain. This has numerous advantages. For example, it enables the most efficient use of expensive bandwidth. In addition, those same IP networks can also carry digital audio, telephony, data and metadata, allowing cable companies and phone companies to engage in a triple play strategy with their existing infrastructure. However, while IP networks have many advantages, there are serious disadvantages as well. There are significant Quality of Service challenges, which, if left unaddressed, will result in high customer churn as users abandon the service. What the customer wants TV viewers expect to be able to turn on the TV, zap through channels and watch without the transmission breaking down. They do not expect a simple thing like changing the channel to take two seconds. They do not expect to watch TV programmes that continuously stutter. Televisions are expected to just work. Something as simple as channel surfing has become a complex network engineering issue. Yet few viewers think about channel surfing, they just sit back scan the channels in search of something interesting. With video-over-IP, however, channel surfers can be in for an unpleasant shock. As a viewer attempts to channel surf, the television blanks out for a few seconds before displaying the video from the next channel. Channel surfing becomes channel wading. The reason that a normal TV viewer can change from one channel to another so quickly is because his television is already receiving all the programmes, either over the air or via the cable infrastructure. All the channels are received; they are just not being shown. On an IP-based network, though, channel surfing can become an issue because only one stream of video is transmitted to the TV at any one time. When the TV viewer wants to change the channel, a signal is sent from the set-top box to a router on the network. The router then has to stop sending the original stream and then resend a new stream based on the requested channel. This may cause a delay between the time the router stops transmitting the old channel and when it starts transmitting the new requested channel. How long this delay is depends on a number of factors, such as network speed, design, and how quickly the router can respond. It can result in a considerably longer response time than most people expect, especially in comparison to the instant nature of the current channel surfing experience. The situation is worsened when the router is subjected to multiple change requests at the same time. In a normal IP network, this might not happen very often. However, on an IP network transmitting a television programme, this might happen quite often. One notable example is the World Cup final. One of the most popular sporting events in the world, it draws millions of viewers. The final itself is broadcast live all around the world. As such, in most markets, you can expect that many people will be tuning in to watch the final of the World Cup at the same time. However, once the match breaks for half-time, people are suddenly able to do other things. During this time, many systems get stretched to their limits. On an IP network, the half-time break during the World Cup could mean that people watching the World Cup might all suddenly switch to, say, American Idol. Worse still, people might successfully change channels, but find themselves unable to change back in time for the match because of overloaded routers. Jitter bugs Another problem with video-over-IP comes from the way the network handles data. In an IP network, data is broken up into small packets and then sent off separately. At its final destination, all the data packets are reassembled again. Because of routing changes, network congestion or timing drift, the packets do not all arrive at the same rate. This problem is known as jitter. The solution to this problem is a jitter buffer. The jitter buffer stores the packets as they come in. These packets are assembled in the buffer ahead of their use so that late packets can be added. Unfortunately, the buffer solution is not flawless. Depending on the rate at which the packets of video data arrive, the buffer can suffer from underflow or overflow. Underflow occurs when the data arrives too slowly. This causes the video to stutter as the packets eventually arrive. Overflow happens when the data arrives too fast for the buffer to handle and the packets are lost. In either case, the end result is the same, an unacceptable consumer experience as the video lurches from one frame to the next. The challenges of building an IP network Building an IP network that supports video is a challenging job. Sending video on a data network puts a tremendous strain on the network so engineers have to find ways to transmit data in a more efficient way. For multiple users, video-over-IP usually uses a technique known as multicasting. In multicasting, instead of the video server sending a separate video stream to each TV viewer, a network router duplicates the data and sends it off to other routers. The other routers, in turn, receive the data, duplicate it and then send them off. In this way, the data is distributed in a tree-like structure that puts minimal strain on the network. However, while this is very efficient, there are technical issues with the scalability of this structure. In addition, real-time video is usually sent using UDP (User Datagram Protocol). UDP is a very simple protocol that does not require any acknowledgement of success or failure of the transmission. As such, UDP is inherently unreliable. If a packet goes missing, the source will never know and the recipient will never be able to retrieve dropped packets. The design of the IP network then is a wildcard that can have tremendous influence on the signal quality of the transmission, and hence the quality of the picture. Keeping an eye on things Given that video delivery over IP is going full speed ahead despite these limitations, companies need a way to monitor their networks to ensure that they are able to deliver a high level of service to their customers. Traditionally, operators are employed to eyeball a bank of monitors to look for problems with the video transmission. In a digital world, however, this is no longer viable. The explosion of channels means that it is not humanly possible to monitor hundreds of channels, which might include programmes with options for soundtracks and subtitles in different languages. How can an operator know if the soundtrack and subtitles on any one channel are the right ones? In addition, given the complexity of today’s networks, determining what causes a problem is a big challenge as well. The source of the problem could lie in any number of places, from the multicast router, to the way the video was compressed, or at the head-end receiving the satellite feed. Given all the potential problems associated with service delivery when using an IP network, it is critical that companies relying on IP have automated and cost-effective solutions that help them to monitor their network so they can deliver on their service level agreements. Ideally, such a system would monitor service and signal integrity throughout the entire chain of transmission across all media. The monitoring systems should be able to do a check and compare at each stage of the delivery chain. Such a system needs to be able to monitor all of the equipment regardless of geography. It should not matter whether the equipment being checked is in the next room or the next city. Information gathered on fault and performance should plug into other information systems such as the billing platform or customer information database so that customer support issues can be dealt with quickly. On an IP network, a problem with the TV transmission isn’t a question of the wrong frequency being used. While IP networks are efficient at transmitting data, transmitting television programmes is much more challenging. Delivering not only high QoS, but exceptional Quality of Experience is where the service companies must pay careful attention. To ensure service and signal integrity, preventive monitoring across the entire transmission chain is not optional. The volume, complexity and rapidity of data flow require automated, integrated solutions. In the era of digital video over IP networks, “golden eyes” alone are not enough.
