From the April, 2015 Issue of Cabling Installation & Maintenance Magazine
Several technologies are available to manage the capacity required to transmit and store IP-video-generated images.
By Patrick McLaughlin
Manufacturers of Internet Protocol (IP)-based video surveillance cameras and other devices have incorporated compression technology that reduces the bit rates at which video signals are transmitted over a network. Without such compression, these video systems could consume significant amounts of a network's throughput capacity and, depending upon the network setup, risk slowing down other network traffic and/or risk losing some of the video footage.
In January 2015 we hosted a web seminar titled "IP Security Systems and Their Impact on Structured Cabling," which included a presentation delivered by Axis Technologies' (www.axis.com) partner-program business development manager Vince Ricco. His presentation directly addressed video surveillance systems and bandwidth consumption and in it he explained how compression technology fits into the overall picture.
Four factors
"The amount of bandwidth required for a video stream can be defined by several key variables," Ricco said. "The first variable affecting the camera's bit-rate output is the video codec being used. The codec determines how the video is packaged and compressed before being sent over the network. The second variable, which probably is the most difficult to assess and draw general recommendations for, is scene complexity. A complex scene with lots of movement, and color and contrast variances, will require more bandwidth, independent of codec choice.
"The third variable is resolution," he continued. "As camera technology and lenses have evolved, we have moved from low-resolution cameras delivering 352x288 pixels, to HDTV and beyond cameras delivering 1920x1080 pixels. Assuming each pixel delivers the same amount of data, the data amount, for each frame, has increased by more than 20 times. As a consequence, not over-specifying camera resolution is important within a project, as it is a driver leading to both direct and indirect costs.
"The fourth and final variable is the frame rate of the camera, as a higher frame rate requires more data to be sent across the network than with a lower frame rate. However, the benefit of the different compression algorithms varies as frame rate varies. The four variables interact and are dependent, affecting each other."
Throughout his presentation Ricco emphasized that it is critical for IP-video system planners to base decision on the intended use of the camera and the video it captures. For example, at one point he displayed two versions of the same surveillance image showing a cardboard box at the receiving area of a building. If a building occupant's needs are satisfied with the knowledge that there is a box at the back door, then lower-resolution imagery will be sufficient. But if the occupant needs to be able to read a shipping label affixed to the box, higher resolution will be necessary. Similarly, factors including indoor or outdoor use, the possible forensic use of the footage, and others will dictate the level of resolution needed and the corresponding compression technology chosen to manage the bandwidth impact.
Different technologies
Several compression technologies exist, and a number of camera manufacturers have published educational documents comparing these options. For example, Avigilon (www.avigilon.com) offers a paper titled "Understanding Compression Technologies for HD and Megapixel Surveillance." In it, the company explains, "There are now a wide variety of compression technologies available on the market, but no clear standard has emerged. At the same time, implementations of a particular technology may vary from one vendor to another. Often, installers think only of file and disk size and how that determines the number of days video is stored—neglecting the fact that video compression can also impact a video surveillance system design. For example, video compression technology impacts the choice of hardware for client workstations, what transmission systems can be used, and the speed, success, and efficiency of investigations."
Avigilon's document also describes two types of compression technologies: frame-by-frame and temporal. "Each technology group incorporates different formats and in turn has its own tradeoffs," the company notes. "Frame-by-frame, or intraframe, compression technologies compress video by applying a compression algorithm to each frame captured by a camera. The end result is a series of individually compressed images. Temporal compression technologies rely both on compressing data within a single frame and on analyzing changes between frames. The result is a stream of video that is compressed over multiple frames rather than a series of individual frames. Typically, a temporal compression technology will attempt to store only incremental changes between frames and store whole frames only on periodic intervals."
Frame-by-frame compression methods include JPEG and JPEG2000, while temporal compression methods include MPEG-4 and H.264. Avigilon's paper provides some level of detail on each.
The paper positions frame-by-frame technology as favorable over temporal: "Video that is compressed using a frame-by-frame compression technology presents a number of benefits over the more-complicated temporal compression technologies," Avigilon says. "The resulting video through frame-by-frame compression is a series of individually compressed frames that do not require information from other frames; they can be compressed and retransmitted out of the camera more quickly to reduce latency. Because each frame acts as an independently accessible frame and is not built up from multiple frames, recorded video can be accessed more quickly."
Avigilon then says that although the technique used in temporal compression "can result in bandwidth efficiencies, it can also lead to the loss of information because the whole frame is not retained."
In the end, Avigilon takes a fairly even-handed approach to explaining the compression types and making recommendations. "Once resolution has been determined, it is important to select a compression technology suited to the application. Resolution, frame rate, activity level and investigative needs will all influence the selection of compression technology. … As resolution increases, the benefits of JPEG2000 and HDSM [Avigilon's own High Definition Stream Management] increase. For scenes on a site that require large multi-megapixel cameras, JPEG2000 compression is required to manage the high-resolution information effectively.
"As resolution increases and frame rate decreases, the benefits of temporal compression technologies like H.264 diminishes. But for smaller scenes that require a higher frame rate and use a camera with 2MP resolution or less, a temporal compression may be the most effective option. For low resolution, high-frame-rate scenes, a temporal compression technology like H.264 can help minimize the required network bandwidth and storage. For scenes of moderate activity with high frame rates, H.264 compression can offer a significant advantage."
Variable or constant
On its website, Axis provides technical guides on a number of topics including compression technologies. In one lesson, the company explains, "With MPEG-4 and H.264, users can allow an encoded video stream to have a variable or a constant bit rate. The optimal selection depends on the application and network infrastructure."
Axis continues, "With variable bit rate, a predefined level of image quality can be maintained regardless of motion or the lack of it in a scene. This means that bandwidth use will increase when there is a lot of activity in a scene and will decrease when there is no motion. This is often desirable in video surveillance applications where there is a need for high quality, particularly if there is motion in a scene. Since the bit rate may vary, even when an average target bit rate is defined, the network infrastructure (available bandwidth) must be able to accommodate high throughputs.
"With limited bandwidth available, the recommended mode is normally constant bit rate (CBR) as this mode generates a constant bit rate that can be predefined by a user. The disadvantage with CBR is that when there is, for instance, increased activity in a scene that results in a bit rate that is higher than the target rate, the restriction to keep the bit rate constant leads to a lower image quality and frame rate."
Far from being a "connect-it-and-forget-it" technology, IP video systems are going to consume some amount of network bandwidth. Professionals who plan and implement these video systems must consider a number of use factors before making decisions that will impact the system's bandwidth consumption.
Patrick McLaughlin is our chief editor.
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