The classic definition of a CDN, or content delivery network, is an aggregation of servers dishing out live streams, on-demand content and other files to end users on behalf of various clients. In broadcasting, typical clients range from small radio and television stations to larger broadcast networks and media conglomerates.
The CDN’s role in the broadcast space isn’t so much changing but evolving. The now-simple idea of distributing content to a desktop grows more complex by the year. The explosion of mobile streaming and the ever-expanding OTT universe is evidence alone.
Simply put, there are more formats to learn, varying bit rates to accommodate and more devices in the consumer space. But it’s clear that broadcasters have as much to gain by diversifying their streaming blueprint as they have to lose by standing pat. The good news is that CDNs have the tools, the knowledge and the relationships to help broadcasters evolve their platforms.
Table 1. Shown here are several wrappers and the end devices that use them. H.264 video and AAC audio are the most common codecs in use for Web delivery. A CDN can deliver these underlying codecs as several different on-demand container types or live delivery protocols without changing the H.264- and AAC-encoded content. This process is often called packetizing or remuxing.
How effectively a broadcaster communicates its needs to a CDN will go a long way toward establishing and growing a successful streaming strategy. Multiformat streaming and bandwidth management are two key engineering aspects to understand for enhancing quality and reach, along with software tools to effectively realize the associated business opportunities.
Bandwidth management
There is an inherent value in understanding bandwidth management, and the techniques and technologies involved, when streaming in multiple formats to reach many devices.
The MPEG-4 AVC/H.264 video codec is the primary choice in streaming environments today. It is the most pervasive mainly because it excels at representing the video picture at an efficient bit rate. Some have claimed that similar codecs, such as WebM’s VP8, offer higher efficiency, as well as real-time processing and licensing advantages. Most agree that H.264 offers an edge in picture quality.
However, many devices have difficulty supporting less well-known codecs because either the required decoding software is not widely supported or decoding the video demands a faster CPU or specific graphic processor on the device. In broadcast-quality streaming, H.264 offers a comfortable middle ground between high efficiency and not taxing devices at the decoding stage.
The most appropriate bit rate used for a specific frame size is dependent on the efficiency of the codec, as well as the amount of motion, color and detail of the picture. We often measure NetFlix, Hulu, Vudu and other professional OTT delivery platforms to determine the appropriate high-end bit rate for the content in question.
With NetFlix, we’re used to seeing encoded bit rates of 3.5MB to 5MB for 1080p downloads; roughly 2.5MB to 3.5MB for 720p, and 1.5MB/s to 2.5MB/s for SD video. These bit rates are sufficient to reproduce the high motion and detail of a Hollywood blockbuster. A lower bit rate can be used if the content is of low motion, as in a talking head or presentation slides, without a noticeable degradation in image quality. In multiformat streaming, bit rates will also be determined by the efficiency of the video codec used within corresponding container files, or wrappers. An H.264 codec in a multiformat streaming scenario will have at least several different wrappers around it. Table 1 shows several wrappers and corresponding end devices.
Multi-bit-rate delivery
A growing number of OTT and mobile devices now offer bandwidth detection to determine the most appropriate bit rate stream for the current network conditions. Those devices and players can dynamically adjust the stream as the network performance changes.
Broadcasters can ensure a high-quality user experience across multiple network conditions by taking advantage of this functionality. This remains true whether the user is connected with a set-top-box on a high-capacity fiber network, or with a mobile phone over a congested cellular network. The proliferation of these devices makes multi-bit-rate delivery a growing trend for broadcasters and CDNs. This capability to monitor network conditions and dynamically adapt is integrated today within Roku OTT devices, iOS mobile devices and many digital set-top boxes. For example, the HLS protocol is used for content delivery to the iPhone and iPad, and supports multi-bit-rate delivery. Any iOS device capable of playing back an HLS stream can simultaneously activate bandwidth detection.
A CDN will generate playlists that note which files and live streams are available at what bit rate. The high-bit-rate stream might be 2.5MB/s, the medium 1.5MB/s, and the low 700kb/s. The device accesses this playlist and determines which stream is most appropriate to play based on network conditions. (Flash Player and Microsoft Silverlight also support multi-bit-rate delivery.)
The technique of encoding multiple codecs for a particular stream is also more prevalent. For example, there is a Baseline and a Main level of H.264, the latter of which offers better video compression. This means the consumer with a device supporting H.264 Main will receive better picture quality at the same bit rate. A broadcaster looking to maximize video quality for every device being reached will want to direct its CDN to deliver H.264 Baseline to older mobiles and legacy iOS devices, while delivering Main to Roku and modern iOS handhelds.
Key frames
Multi-bit-rate delivery is more about delivery than compression. However, in H.264 applications, there are additional benefits on the encoding side — notably the ability to unlock advanced features of the H.264 codec for added efficiency. This includes the effective use of key frames.
Key frames, or i-frames, are complete image frames on which neither previous nor subsequent frames are based. Frame 1 might be an i-frame, and frames 2 through 5, known as p-frames, are based off that initial i-frame. This means that the only data the video codec needs to recreate frames 2 through 5 is the data that is different from Frame 1.
Some advanced encoders, like Haivision’s line of KulaByte live encoders, make use of b-frames. These can be used preceding or following i-frames as reference. This means when Frame 5 is the i-frame, Frames 1-4 can serve as b-frames, based off the following Frame 5.
Less data is needed to accurately recreate the video when the video codec only has to use data to store the difference between two images. The result is lower bandwidth costs and less network congestion.
Multiformat delivery
The term multi-platform delivery is oft-heard in the broadcast industry, but multiformat delivery is a more appropriate description in the streaming universe. This is chiefly because a CDN is supporting multiple streaming formats to ensure it is delivering high-quality video and audio to every device the broadcast client is targeting.
Simply put, there is no “one-size-fits-all.” That makes the server technology in the CDN architecture extremely important. Wowza Media particularly excels in supporting all major streaming protocols, and with minor changes can quickly adapt to new devices and software. Wowza has grown from strictly RTMP-to-Flash delivery to quickly supporting most emerging OTT and mobile-device protocols.
Adobe of course developed RTMP and remains a reliable, if expensive, service for targeting a wide range of platforms. RealNetworks, which originated RTSP streaming to Real Media Players, has grown to support more protocols, but it is an expensive option compared to Wowza. Meanwhile, emerging companies like EvoStream are building a niche for themselves in specific markets; they would have some catch-up work to do to compete in all fields. The point is that CDNs rely on these technologies to enable multiformat delivery and support desktops, phones, tablets, OTT devices and smart TVs — and all of these devices play an integral role in modern viewing habits.
Although the OTT and smart-TV markets show some overlap effects, tablets have become an especially important streaming target as consumer sales escalate. Tablet users expect to see full-resolution HD quality filling the screens, and as Wi-Fi-enabled devices, tablets can support higher bit rates than their cellular network counterparts. This makes the tablet a viable target for high-bandwidth encoded content to maximize quality.
Regardless, the most important aspect for broadcasters to communicate to the CDN is the list of devices they require the CDN to support. This will help the CDN identify format needs, and make recommendations on how live streams and on-demand files are prepared. It also helps the CDN tailor server configurations to reach all targeted devices.
On the cutting edge
Content delivery networks do more than simply deliver content. Many CDNs are equipped to help broadcasters increase reach, expand viewer bases and generate revenue through supporting mobile apps, offering audience and business analytics, and/or enabling dynamic ad insertion to work sponsors into the mix.
This is an emerging trend in streaming for broadcast that will surely make its mark on the television side moving forward. With the proliferation of OTT devices, there is a growing demand for on-demand viewing in comparison to live program viewing. This makes pre-, mid- and post-rolls a potential new revenue stream — and also opens the door wider for subscription-based streaming.
Business analytics are also taking off in the streaming world, offering richer data that note audience numbers, geographic locations and average dwell times. These services are important to help CDNs and broadcast clients understand the appropriate time to scale services at the infrastructure, from adding new live server nodes to increasing storage and for on-demand files.
The exploration and support for emerging, high-efficiency codecs is also a trademark of a reliable CDN. At press time, the International Telecommunications Union announced first-stage approval for HEVC H.265, the next-generation video codec following the MPEG-4 AVC/H.264 standard. Early reports tout efficiency gains of 30 percent to 50 percent over the previous standard.
The role of the content delivery network has certainly evolved, from supporting more formats, bit rates and devices today to helping broadcasters develop a future roadmap through audience analytics and measurement to generate revenue now and into the future.
—Andrew Jones is director of sales engineering at StreamGuys.
