IP video conferencing service quality technology based on H.323 protocol
In recent years, the H.323-based IP video conferencing system has been greatly developed, and the conditions for public operations have been implemented, and this condition is realized, and the quality of service is key. This article gives the IP video conferencing service quality technology from two levels: network level and business level.
Keywords: h.323; IP video conference; QoS
1 Introduction
In 2001, major operators in China turned their attention to the construction of IP video conferencing systems. Due to the immature of the H.323 protocol itself, this will bring a certain difficulty to the public operations of the IP video conference. Public operation of IP video conferencing must address interoperability such as user management, business management, billing management, and video exchange. Since Internet is an unconnected network, only one bearer service - try to transfer (Best Effort) business. That is, the network does not guarantee the bandwidth required to the application data stream, nor does it guarantee the quality indicators such as the transfer of data streams. For non-real-time services such as data services, try to deliver real-time communication applications such as audio or video, and networks must support end-to-end hosting services with a certain QoS. How to improve real-time performance, ensure that communication QoS is a key technical requirement for IP video systems, and it is also a technical difficulty. In an IP video conference, QoS strategies can be divided into two levels: network level and business level. This paper analyzes the QoS policy of the IP bearer network for video conferencing and the QoS implementation of the H.323 protocol itself, and proposes the QoS implementation technology of IP video conferencing.
2 Make sure the video system QoS method
INTERNET ensures that IP video system QoS has two methods.
2.1 Overweight Method (Overengineering)
That is, sufficient bandwidth is reserved at the time of network planning, so that acceptable QoS can be obtained at any time. This method is very simple, does not require resource reservation agreements and accept control functions, but requires deploying enough routers and high-speed links to ensure that there is enough margin even if the network resources are also available. It can be used for inexpensive network resources, and the maximum traffic of the network can be predicted.
2.2 Integrated Services Internet Method
Defined by the IETF Integrated Services (IntServ) Working Group. It needs to define a call acceptance control function resource reservation protocol, such as RSVP. With RSVP messages, endpoint applications can presence the network resources (such as bandwidth, buffer size, etc.) that must be retained in data transfer, and also determine the transmission scheduling strategy of each router along the way, thereby, QoS can be used for each data stream Control sequentially.
3 Supplementary QoS in network design
3.1 Network Structure
The metropolity IP network is typically composed of core layers, a coinciding layer, and an access layer, and each node of the coinciding layer is connected to the core layer through a high speed link. In the Member IP network, consider the number of routing hops on the router connection to ensure that the network arbitrary two nodes of communication is up to 4 hops, configuring high-performance routers, according to the experience value, in the case of light transmission, one The data packet has been delayed by a hop of 10ms, which is not determined by the length of the line and the performance of the router (for the router of the 7,500), the data packet is approximately 50 ms in the backbone network. From this perspective, the delay problem is not the main problem that affects the quality of IP video services.
3.2 Routing Oscillation Problems
The route oscillation is divided into two aspects.
One is a route change caused by the change in link state. If the routing of IS-IS or OSPF is discovered, the problem is not detected by the Hello package, and it is not necessary to detect several times. Under normal circumstances, from the link interrupt to the new route selected take a few seconds to dozens of seconds, such problems will greatly affect the quality of real-time multimedia services on the backbone network, which mainly uses MPLS's FRR capabilities. protect. Another routing oscillation problem is mainly caused by the unscrupulous network design. For a large number of similar values or a large number of ROUTE RELOOKUP or routing status update oscillation, the main solution to prevent problems is to require all traffic when designing the network. Both direction and circuitry require monitors to be clearly checked.
3.3 Treatment of oscillation
Oscillation is a problem that is very difficult to solve, due to the problem of router principles (relative to the switch), there is always some time possible to be relatively busy, this may make the delay in the single router increase to 100ms or more, this will The quality of the multimedia conference system has declined. This is sometimes not necessarily caused by excessive traffic on the line, it may happen in a bandwidth of 20% to 30%. Such a problem is mainly due to the problem of the Buffer setting of the router. The improved solution is to optimize the Buffer setup of the router to the conference system, but it is possible to degrade the efficiency of traditional IP services. The best situation is to serve the two networks, there is a problem with decision.
3.4 Network congestion
In addition to oscillation, network congestion has a significant impact on IP video conferencing business. Therefore, when we design the network, we should prevent the generation of network congestion. When deploying congestion management, use the following steps:
(1) Determination of congestion in the WAN. (2) Determine the target based on the types of communication, the network topology and design of the network. When determining what kind of results needed, consider whether the target is located in several standards: • A fair bandwidth allocation method can be established for all the communication types determined; • Communication sent from the IP video service, It can specify strict priority. This may hurt to simultaneously support, but not the interests of an emergency communication business; • You can customize bandwidth allocation to share network resources between all applications of services. Each application has a specific and has determined a good bandwidth requirement. (3) Configure the interface with the selected queue strategy and observe the result.
4 IP video service itself QoS implementation
How to improve real-time, to ensure that QoS of communication is a key technical requirement for IP video conferencing. At this point, the H.323-based video conferencing system uses IETF's real-time protocols and network technologies. First, the voice signal uses a real-time delivery protocol RTP package transmission. However, RTP itself does not provide any mechanism to ensure QoS, to ensure that the real-time communication of communication requires an IP network itself has the enhancement capacity in this regard.
4. 1 RTP function and design ideas
The RTP protocol provides end-to-end delivery services for audio, video and other real-time data, which can transmit information necessary to recover real-time signals to the receiving endpoint, and provide QoS monitoring to the transmission and reception of both sides and network operators to reduce network bandwidth. Demand. RTP can greatly reduce your bandwidth. RTP can also make the video conference tolerance to a small amount of packet loss to avoid the delay caused by the data package. RTP actually includes two protocols.
(1) RTP itself: used to transmit real-time data. It features a payload type indication, data packet serial number, data transmission timestamp, and data source identity.
(2) RTCP: The quality parameters used to transmit real-time signal delivery provide QoS monitoring mechanism; at the same time, the information of the participants in conference communication can also be transmitted, and "relax" conferences that do not have explicit member control and call. Communication provides control mechanisms.
H.323 protocol monitors QoS using RTCP SR and RR packets.
SR: Mainly used in multi-RTP streams such as synchronization between audio and video, and closely related to H.225.0. The field synchronization-related field is RTP timestamp and NTP timestamp.
RR: User Monitor QoS indicators, including long-time indicators and short indicators. If the packet loss rate is higher than the set value, the media rate should be reduced. If the received report interval exceeds the set value, the network should be determined whether the network is severely blocked according to the loss rate field in the RR package, such as, should reduce the media rate. If the arrival of three consecutive reception reports increases, the transmitting end should take action.
Messages in H.245 also measured round-trip delay: "round-trip time extension" and "round-trip time delay", the message does not contain time parameters, requesting the sender to go back and trip according to the difference between the transmission and reception time of the two messages, The time delay is propagated, and the reception end queuing time delay and the sum of time delay. The RTCP is calculated according to the SR and RR messages, and the QoS directly reflects the QoS transmitted by the network. Therefore, the two monitored different physical quantities, and there is no conflict between each other.
4. 2 certificate QoS specific means
In order to maintain a certain quality of service, when the QoS index is monitored, the H.323 terminal takes a certain measure. In fact, these measures are not to maintain the original QoS, and the quality of various media is sequentially reducing the quality of various media in a certain order, so that the user can provide acceptable services to the user under a given bandwidth and load conditions. First, consider reducing the quality of the video signal, and then it is data, audio, and control signals. Take measures can be divided into two categories: short response and long response. The former aims to solve short-term problems with increasing inclusion of the package short-term loss and delay; the latter is used for the increasingly serious network of network congestion.
(1) Dynamic adjustment image bandwidth
When people are sensitive to the image and speech, when the image code stream is delayed, jitter, the decoded image is generated as a bit error, the frame is lost; when the speech code flow is delayed, the jitter is decoded, and the sound is decomposed. It is more tolerant from the human feel. In order to improve QoS, the RTP / RTCP report can be used to obtain information about the network status, such as packet loss rate, package shake, delay, and dynamically adjust the image bandwidth based on this information. When the network is not good, the encoder can be notified, reduce the image bandwidth, give priority to the sound bandwidth; when the network is good, the encoder is notified, and the image bandwidth is improved.
(2) Lip synchronization
Receiver: For the synchronization of the receiver voice and the image, the terminal receives the voice, the image data is placed in the speech buffer and image buffer, respectively, and the phonetic bag decoding is taken from the voice buffer, and if the speech package is stamped and image Essential, decoding the corresponding image package. The advantage of this is to consider the sensitivity of speech.
Send party: Timestamp. The sender should play up the timestamp of the packet, on the one hand, when the packet (RTP package) is picked, on the other hand, the data control package (RTCP package) is stamp.
5 Conclusion
According to current signs, the H.323-based IP video conferencing system will become a great value-added business of broadband IP network. And its ultimate goal is the public operation, so that thousands of households enjoy video services. However, the public operation of the IP video conferencing system involves many problems, service quality is the key to implementing IP video conferencing, so in the system design of IP video, it is necessary to unify the quality of service. Since IP video conferencing is an emerging technology, it is still in development, and many technologies have to be further studied and discussed. Technology area
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