CDN is a distributed content distribution network built on the IP network, which mainly adopts the "center-edge" storage and service distribution method. Its basic principle is to deploy an edge server in a distributed area or network. First of all, all programs are stored in the central server, and then the relatively concentrated content accessed by users is distributed to the edge server through the backbone network, and the edge router provides user services directly. .
In the VOD business, due to the high quality and real-time requirements of video signals from TV viewers, the status of CDN is particularly important. Not only does it bear the responsibility of improving user response speed, but also can reduce the huge pressure of massive data traffic on the backbone network. .
The distributed VOD architecture based on the HFC network makes full use of the existing CATV network, and can provide massive programs to users quickly and efficiently, providing a reasonable framework for developing multimedia interactive services such as VOD. The core equipment of this framework, IPQAM modulation equipment, integrates the functions of "multiplexing, scrambling, modulation, and frequency conversion". It can re-multiplex the program stream input by DVB / IP_GbE in the specified MPTS, and then perform
QAM modulation and frequency conversion, output to RF. After using IPQAM, the control information and video stream between the set-top box and the edge video server will be transmitted through different channels: the access authentication of the set-top box, EPG information browsing and other processes interact through the two-way backhaul channel; the edge video server receives After the user's request, the audio and video stream is output to the IPQAM device in an appropriate packet form. IPQAM modulates the audio and video stream into an RF signal and transmits it to the set-top box through the HFC network. The set-top box demodulates and decodes the audio and video stream.
After adopting IPQAM technology, on the one hand, it can make full use of the bandwidth resources and transmission characteristics of HFC to provide users with QoS guaranteed video services, which is more suitable for the high downlink bandwidth requirements of high-definition services on the transmission network; Network requirements no longer require the IP network access layer to provide higher bandwidth, allowing broadcast and television operators to implement interactive video services at a lower cost.
Combining CDN and IPQAM
In a large interactive TV platform, the CDN pushes the video content to the edge node according to a suitable strategy, and then sends the IP data packet modulation to the HFC access network through the IPQAM device, which is a typical deployment.
In the network system that combines IPQAM and CDN technology to develop VOD services, it is different from the traditional centralized VOD transmission mode of telecommunications IPTV and radio and television. The main issues involved are:
★ Because of the network characteristics of HFC, the data reachability of end users and IPQAM ports needs to be considered, and related network planning and resource management should be carried out;
★ The combination of CDN and IPQAM technology at the data level, including the requirements for video stream data packet and bit rate control;
★ The combination of CDN and IPQAM technology at the routing level includes global routing management, load balancing, redundancy mechanism, and QAM resource management scheduling.
1. Resource management scheduling
In the HFC-based VOD system, the downlink of the video content ordered by the user is carried by the HFC network, and the IP data packets are modulated and sent to the wired network through the IPQAM device. In view of the shared nature of the HFC network, a particular set-top box can only receive data from a specific set of IPQAM frequency points. Therefore, it is necessary to have clear planning and management of the service area. Generally, such a service area is defined as a Region.
Region is a predefined set of frequency resources, or an IPQAM channel resource planning unit.
In the service area covered by different nodes, the VOD frequency planning unit of the network can be reused in whole or in part. A service area with the same path is a region. To identify different Regions, each Region has a unique ID number, that is, RegionID. RegionID is used to locate STB users in the VOD service. When a STB requests a service, it needs to submit this information to the VOD head-end system. Based on this information, the head-end system can distinguish which region of the STB is sending the request, so as to allocate the corresponding route. For the video server to transmit TS streams.
(1) QAM resource management
The VOD headend system resource management module SRM (Session & ResourceManager) needs to understand the resource usage of each actually deployed IPQAM to complete resource allocation and recovery. The IPQAM device is required to provide an interface for SRM to complete heartbeat maintenance and status query. The interface can be through IPQAM Provided in the form of SNMP, can also be provided through other API methods.
The mapping rules of IPQAM's UDP ports, frequency points, and program numbers should be set uniformly across the network to schedule all resources according to a unified algorithm. The mapping rules can be set through SNMP or API. Because IPQAM and the corresponding frequency and channel resources are limited, the system is required to manage the above resources during the service process.
(2) RegionID management
The set-top box can obtain the RegionID through fixed distribution or automatic network download. The implementation of the fixed allocation method is relatively simple, but the organic set-top box needs to re-bind RegionID when roaming across regions or when the head-end network structure is adjusted; while using the real-time delivery of RegionID through the head-end, the set-top box can be dynamically updated, which is more flexible. The common business requirements of operators.
In the real-time delivery method, the RegionID information is sent to the set-top box through the IPQAM through the HFC channel. The specific sending method can be set and broadcast in real time to the set-top box DVB network parameters, the set-top box is resolved according to the preset rules to obtain the RegionID. The specific parameters for setting RegionID can be TSID or NID.
TSID method: complete the configuration of TSID of each channel in IPQAM according to the frequency band planning and deployment of the system to ensure that the TSID of each stream of IPQAM is unique in the entire network. After the set-top box is turned on, the frequency band that is divided into the on-demand service is scanned and received. To save the TSID.
Set the TSID in all IPQAMs, and the IPQAM can download the TSID information by broadcasting the PAT table in real time. In this way, the set-top box will be able to detect the TSID information of all IPQAMs that can reach the set-top box by scanning. The set-top box may obtain multiple TSIDs belonging to a Region, and the set-top box parses the TSID according to the set rules to obtain the RegionID.
NID method: insert a service entry descriptor in the NIT table of each non-on-demand frequency point, and the set-top box obtains the main frequency information of the on-demand through the descriptor, and then the set-top box obtains the NetworkID information according to the NIT information on the main frequency of the on-demand. RegionID. When the on-demand main frequency information or RegionID information in the descriptor cannot be read, the information stored in the set-top box is used.
2. The combination of data
After routing and resource allocation, the CDN's edge video server will output video stream data to IPQAM. According to the transmission characteristics of IPQAM equipment, it is necessary to consider appropriate data packets and bit rate control methods. (1) Data packet
The video encoding format currently generally selected by operators is MEPG2TS or MPEG4 / H.264ISMA. These two types of formats use different packaging processes:
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