| Foreword: [ According to the forecast of the market consulting firm Winter Green Research, the development cycle of the unified communications market is about to enter the stage of healthy growth and rapid application, and video will become an uncompromising dark horse in enterprise-level unified communications deployment. Traditional enterprise video technologies include: MCU, H.323, Frame Relay, etc., and video applications based on IP bearer networks will push more rich functions to users. But to use these features smoothly, users need to pay more attention to some new technology hotspots. |
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A, LPR: stable video stream <br> unified communications applications enable enterprise video and voice, data synchronization carried over IP networks, video transmission in the form of data packets, which inevitably have problems of network packet loss . Packet loss can cause mosaic of video images; local distortion of the image, frequent refresh or flicker of the screen; video and audio are not synchronized; frame rate is reduced and the image is still. In the unified communication environment, since video, voice, and data streams are transmitted synchronously, packet loss can also cause overall audio distortion, intermittent or intermittent noise, and degradation of the quality of content presentation, such as blurred slides or The page turning speed is slower, etc. These issues will greatly affect the user's application experience. If the network packet loss rate is too large or too frequent, it will also delay the video communication excessively, and even cause communication interruption.
Packet loss is caused by internal network congestion or external network priority traffic conflicts. To avoid packet loss, first determine the packet loss rate of the bearer network, the distribution of packet loss over time, and the performance of each terminal and device in the video call.
LPR (Loss Packet Recovery) technology is one of the most effective methods to solve the problem of video transmission loss. The technology is based on a new algorithm developed by Polycom, which aims to protect IP video calls from network packet loss. In fact, LPR is a mechanism for implementing coverage of lost packets using Forward Error Correction (FEC). The sender system adds redundant data to the outgoing data stream so that the receiving system can detect and correct the error without requesting the sender system to retransmit the lost information. This ability to perform error correction without waiting for network transmission makes FEC ideal for real-time communications such as television broadcasting, IP telephony, and IP video conferencing.
The working principle of LPR is to monitor all incoming call incoming packet loss information through the video terminal. Once the packet loss is detected, the terminal will use three methods to guarantee the quality of the call: Lost Packet Recovery (LPR), Dynamic Bandwidth Allocation (DBA), and traditional video error concealment without LPR support. PVEC) technology.
Different from most error concealment or avoidance algorithms only involved in receiving the packet loss information system, LPR will intervene in all levels of the video call system, and temporarily allocate a part of the call bandwidth to a data channel to send FEC. The data is used by the receiving system to function. When a packet loss occurs in the network transmission, the LPR increases or decreases the size of the FEC data channel through the above-mentioned loop processing until the minimum bandwidth that must be allocated to the FEC data channel is found, so that the receiving system can recover all the lost data packets. .
When the size of the FEC data channel exceeds the standard value, the system will repeatedly detect whether the size of the FEC data channel can be reduced, thereby providing more available call bandwidth for audio, video, and content data. Since LPR only occupies bandwidth when packet loss is detected, this feature makes LPR ideal for random or bursty packet loss environments.
For network environments where the packet loss rate is relatively fixed, the Dynamic Bandwidth Allocation (DBA) function can be combined with LPR. DBA is also an algorithm that automatically and dynamically adjusts the video bit rate during a video call to eliminate or avoid packet loss. For example, when a 10% packet loss rate is detected in a 384 kbps video call, the DBA will reduce the video rate by about 10% from 320 kbps to 288 kbps, and then resample the signal to see if there is still a packet loss. phenomenon. If necessary, the DBA can further reduce the video bitrate until no more packet loss occurs in a few seconds.
After reducing the bit rate, if the DBA determines that the packet loss is temporary, the DBA will gradually increase the video bit rate. Conversely, if the DBA continues to detect packet loss, it will not adjust the video bit rate, but will continue the call. Therefore, DBAs are suitable for packet loss environments caused by network overuse or throughput issues.
The Video Error Concealment (PVEC) function can be used in cases where the parties involved in the call do not support LPR. PVEC is an algorithm for IP Video Quality of Service (QOS) that compensates for the effects of packet loss. It estimates the content of the current video frame by using adjacent macroblocks, pre-frames, and back-frames. Unlike LPR to recover lost packets and DBA's packet loss avoidance technology, PVEC's role is to mask the impact of packet loss.
On this basis, LPR enables users to make high-quality video calls over IP networks with high bit error rates (DSL, cable, satellite, high-conflict LAN/WAN, etc.) without having to endure the impact of packet loss. By temporarily adjusting the bit rate of the ongoing call, LPR can protect video calls from short-term network problems. Once the network problem is resolved, LPR will automatically call back the bit rate to the highest level based on the originating call rate or current bandwidth availability. This allows the bandwidth of the network to be fully utilized, realizing the real-time reconstruction of lost data packets and real-time reconstruction, making audio and video data flow smoothly, eliminating waiting, intermittent, noise or information confusion, and reducing dynamic jitter buffering. Zone space and associated delays.
However, using LPR generates FEC channel bandwidth, which is used to reduce the available bandwidth of the video call itself. In the case of a low connection rate, a temporary frame rate reduction or a decrease in resolution may be caused.
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