The future development of IPTV is becoming the focus of the industry. According to the statistics of market research company MRG, global IPTV users will increase from 2 million in 2004 to 20 million in 2010. The compound growth rate of the global IPTV market from 2005 to 2010 is expected to be 102%. In China, the IPTV industry is still in the experimental stage, and China Telecom, China Netcom, and China Railcom are actively carrying out IPTV trials.
At present, many equipment manufacturers have provided IPTV system platforms and equipment. It is recognized in the industry that IPTV services include two basic services: VOD on-demand and TV live broadcast. The development of domestic IPTV is in full swing, and there is an urgent need to standardize the testing of the IPTV system. This article intends to briefly discuss the performance testing of these two basic services of the IPTV system. This article only considers the end-to-end performance test, the observation point of the test is on the user side, and does not consider the hierarchical cache performance on the server side.
Due to the difference in network structure between VOD on-demand network and TV live broadcast network, VOD distribution needs to rely on CDN network, while TV distribution needs to rely on multicast network without CDN network for caching. Based on this, the performance test of these two services The focus is also different.
1. Introduction to VOD Service Performance Test
VOD programs are usually delivered in an on-demand manner. On-demand users can freely control the progress of the program and terminate the playback of the program, but each on-demand user needs to occupy a certain network bandwidth, so the number of concurrent users is limited by the network bandwidth and server performance.
The test indicators of VOD service performance mainly include the maximum number of concurrent streams, bandwidth fluctuations, average bandwidth used, packet loss rate and average response time.
Maximum number of concurrent streams: The maximum number of online users that the streaming media system can support at the same time, and it is required that each user can watch the complete video program normally. Before the number of users is gradually increased to the maximum value, the system will not cause users who are already online to be unable to obtain services because of newly added users. Although some video systems can provide a huge number of concurrent streams, the number of failures is large. Since most users use TCP, the number of concurrent streams does not decrease after retransmission. However, for a single stream that has failed, the user has already failed. Unable to continue to receive video services. Therefore, the maximum number of concurrent streams should be the effective maximum number of users that can be supported by the video system and can watch programs online at the same time.
Bandwidth fluctuation: During the playback of a video file, the bandwidth of the media stream will change due to changes in the picture. For example, for classroom teaching programs, the continuous picture does not change much before and after, so the program code rate is relatively constant, and the bandwidth fluctuation is relatively small; for sports programs, the continuous picture changes greatly before and after, so the program code rate The change is correspondingly greater, and the bandwidth fluctuation is also greater. The fluctuation of the program code rate is a severe test for the throughput of the video server. If too many users order programs with a higher code rate, it is likely that the number of users that the server can support will decrease, or the video server will Reduce the output bandwidth of the program stream.
Average bandwidth used: The average bandwidth used by all users during the user watching the program. The larger the value, the stronger the bandwidth capability that the streaming video system can provide.
Packet loss rate: When users receive video-on-demand services, they need to obtain video files from the server, and whether packet loss occurs during video file transmission. The value of the packet loss rate is generally small, but the impact on the user is very important, especially the compression ratio currently used is relatively high, even if a few video packets are lost, it will affect the user's viewing or even disconnect the connection. The upper limit of the end-to-end packet loss rate of VOD should not exceed 0.1%.
Average response time: The overall distribution of the time from the time a user sends an on-demand request to the time a video service is received. Of course, the average response time is as short as possible. Generally speaking, the end-to-end average delay of video on demand should not exceed 2s (tentative).
2. Introduction to TV service performance test
TV programs are usually transmitted in multicast mode. Multicast is a network technology that allows one or more senders (multicast sources) to send a single data packet to multiple receivers at the same time. The multicast source sends data packets to a specific multicast group, and only addresses belonging to the multicast group can receive the data packets. In IPTV, there is often only one multicast source. Even if the number of users doubles, the backbone bandwidth does not need to increase, because no matter how many destination addresses there are, only a single video is transmitted on any backbone link in the entire network. Streaming is the so-called "send once, broadcast within the group". Multicast improves the efficiency of data transmission and reduces the possibility of congestion in the backbone network. For the TV system, the number of users it can support is only limited by the ability to access the network, and has nothing to do with the push capability of the front-end push system.
Therefore, the main test indicators for TV service performance include the maximum number of channels pushed, average channel bandwidth, average channel switching time, average channel departure delay, average channel joining delay, channel overlap time, and channel interval time.
The maximum number of channels pushed: The maximum number of channels that the streaming media system can provide, and the user can traverse all channels in a short period of time.
Channel average bandwidth: the bandwidth occupied by each channel program. The bandwidth occupied by standard definition programs (SDTV) and high definition programs (HDTV) is different, and the bandwidth occupied by HDTV programs is much higher than that of SDTV programs.
Average channel switching time: The time from sending an IGMPleave message to receiving the first multicast packet of the requested channel. Since the channel switching of TV programs on the IP network can only be achieved through IGMP, in contrast, the channel switching of traditional TV sets is much faster. Therefore, according to personal subjective feelings, usually the IPTV channel switching time should not exceed 2s (tentative), otherwise the user cannot bear the waiting time for channel switching. At present, the slow switching time of IPTV channels is also an urgent problem to be solved.
Average channel leaving delay: the time from sending an IGMPleave message to receiving the last multicast packet of the channel. Generally, the shorter the channel departure delay, the better.
Average channel joining delay: the time from sending an IGMPjoin message to receiving the first multicast packet of the channel. Generally, the shorter the channel joining delay, the better.
Channel overlap time: If the system channel switching adopts the method of stopping the multicast packet of the original channel after receiving the multicast packet of the requested channel, the channel overlap time refers to the first multicast packet received and the reception of the requested channel The time interval between the last multicast packet of the original channel.
Channel interval time: If the system channel switching adopts the method of first stopping the multicast packet of the original channel and then receiving the multicast packet of the requested channel, the channel interval time refers to the last multicast packet received from the original channel and the request received The time interval between the first multicast packet of the channel.
3. Other considerations
In addition to the performance test indicators discussed above, there are some practical factors that need to be considered and tested.
The stability of the system. Since most of the tasks of the IPTV video system are to provide entertainment programs, users pay more attention to the integrity and continuity of programs when watching entertainment programs. Therefore, stability is a very critical point for the video system. Users are full of emotions when watching favorite video programs. Any interruption in the stream will greatly affect the user's appreciation effect, such as a black screen when the user is watching a TV series. The stability of the IPTV video system includes: stable maximum number of concurrent streams/number of programs, stable program bandwidth, and stability of long-term work.
Network impairment factors. Various damages in the network have a great impact on the quality of video programs. The occasional damage in the network directly affects the user’s psychological acceptance of the video program. The most direct impact is that users’ subjective evaluation of the video image quality is reduced, for example When the bit error rate of the network line reaches 10E-6, the VOD program may have obvious mosaics, discoloration of the picture, and picture lag. In this case, the user is likely to terminate the video on demand. Therefore, we should carefully study the damage to the quality of video programs caused by various impairments (including delay, jitter, packet loss, disorder, and line errors) in the network. When formulating relevant technical indicators for IPTV services, the impact of network impairments on service quality and QoS should be fully considered.
4. concluding remarks
In recent years, with the rapid development of broadband construction, video streaming media services have become one of the most important applications of the Internet. The IPTV system is the most important entry point for providing video services. At present, major operators have begun to gradually provide users with To provide IPTV services, the IPTV trial network has been trial-run in many cities in China. Testing the service performance of the IPTV system is a key step that operators need to carry out before launching IPTV services. At present, IPTV technology is in the early stage of development. Since IPTV involves a lot of content, related technologies and services are still in the research process. However, with the continuous development of technology, the testing technology of IPTV services will continue to mature and improve, which will inevitably promote the development of domestic IPTV services.
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How far(long) the transmitter cover?
The transmission range depends on many factors. The true distance is based on the antenna installing height , antenna gain, using environment like building and other obstructions , sensitivity of the receiver, antenna of the receiver . Installing antenna more high and using in the countryside , the distance will much more far.
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