Fast growing IPTV services have become a major concern in China information industry. However, the IPTV service model has not been perfected yet after long-time trial made in a couple of Chinese cities. Selecting optimum technologies for IPTV is difficult to solve in the short term. So far most trial IPTV services in China are designed to provide a small number of local viewers with television and recorded programs. Accordingly, the technology used for the available IPTV services can hardly become a paradigm for future IPTV technology development. Therefore, with the ultimate goal of constructing a countrywide IPTV service network that can meet the requirements of commercial operation, it is necessary to conduct research into the design of countrywide IPTV network architecture, improvement of the bearer network, selection of video encoding technology, terminal structure, building of the Content Distribution Network (CDN), establishment of the open Digital Rights Management (DRM) platform, and construction of Media Asset Management (MAM) system.
Different from traditional video communication services and broadcast TV services, IPTV services are the development trend of new IP-based video services. IPTV system will unavoidably encounter many new problems in selecting optimum technologies. So far most trial IPTV systems in China are targeted to provide a small number of local viewers with television and recorded programs. The systems based on traditional and mature Video on Demand (VOD) technologies will hardly meet the requirements of large numbers of users in a long time. Therefore, in order to construct a countrywide IPTV service network that can ultimately meet the requirements of commercial operation, it is necessary to conduct research into IPTV technologies step by step.
1 Overall IPTV Network Architecture
The present IPTV system adopts single-layered plane network architecture. Each trial area has established an independent IPTV system that consists of the Content Distribution Network (CDN), operation support system and terminal system at the service layer, the bearer network equipment and access network equipment at the network layer, as well as devices at the content layer as for content management and edition controlled by Content Providers (CP) and for digital rights protection. This networking model is the result of common impacts of many factors including the policy and market. The biggest problem is that the independent trial IPTV networks can hardly form a countrywide system with shared resources and unified management and authentication. Moreover, as only one vendor supplies the systems and devices in one trial area, devices of different vendors cannot interwork or replace each other. Therefore, the present IPTV network architecture merely meets the service requirements in the short term. If IPTV services cover the whole country and expand from the existing TV broadcast and VOD to other fields, the construction and operation cost of IPTV systems will be very high.
The defining of industrial IPTV standards and other work on IPTV are focusing on IPTV architecture design. Although the final solution has not yet been achieved, many good ideas have surfaced:
(1) For a commercial operation countrywide, the overall IPTV system must be a network consisting of the national level and the local level. The present IPTV trial system has only one CP ? Shanghai Media Group (SMG). But with service expansion, many local TV stations and other CPs will access the system. The access and storage of the contents they provide should be completed locally so as to lower the cost. However, unified distribution and management of these content resources over the national network is necessary for more users enjoying the contents. The practical design of IPTV network architecture must take into full consideration the different requirements of the national and local levels. For example, interfaces of the operation support system, content platform and service platform, and the front end of the media asset management system are distributed on the national or local service platforms according to the service deployment, while the resource distribution and management system and the billing system can only be deployed on the national service platform.
(2) The design for interfaces between the content platform and the service platform must consider two aspects. First, the interfaces should be standardized, and compatible with all the content formats provided by influential CPs in China while allowing access to other content sources. Secondly, considering the content sources of different locations, the interfaces should be divided into local interfaces and national interfaces. Accordingly, the related digital rights protection, media asset management and operation support systems should also be divided into the national and local parts.
(3) The division of functional modules such as the digital rights protection, media asset management, storage and support operation subsystems has to consider the functional completeness of each subsystem as well as the connection and standardization between subsystems. So far all subsystems of the trial IPTV system are bound together, so it is necessary to take their functional division into consideration as a whole. Functions that are frequently used by users such as terminal management, service and program selection, and billing should be performed locally as much as possible, while other functions related to the operation of the entire network such as resource storage, resource dispatch directory, and settlement should be implemented at the national center.
(4) An important project in the future is to construct a complete and open media asset management system. The Electronic Program Guide (EPG) concept has just been put forward in the IPTV system, but it is far from meeting future service requirements. For the commercial deployment of an IPTV system, the content classification, management and display may involve multiple platforms that are distributed around the country. The contents distributed everywhere can hardly be covered by using the EPG that only identifies the single-level directory with simple labels and keywords. Therefore, it is necessary to establish an information management system with an intelligent search function, which will gradually support fuzzy inquiry based on pictures, voice or other information. Moreover, the media asset management system and the storage management system will become a whole unit with dynamic connection, so that the media asset management system can directly affect the distribution of content storage rather than provide simple inquiry and use of contents as the EPG does.
As a system that meets the requirements is not yet available today, it will be a step-by-step process to develop the IPTV network architecture. The current network design must consider this evolution trend and leave space for future development while meeting the need of existing services.
2 Media Delivery Network
In the IPTV network, contents are transported via the media delivery system to a single user or a group of users as required. In the current commercial trial, the CDN technology is employed to store, dispatch, forward and play multimedia contents. The media delivery system can reduce excess consumption of servers and bandwidth and improve QoS. However, the existing CDN was developed for streaming media services based on the PC. It provides low service efficiency and needs complex redirection mechanisms when many users select the same content simultaneously. The media delivery system is more suitable for VOD services although it has a limited capability of supporting live TV, time-shifted TV and value-added services. Although some improvements in the CDN have been put forward, it can hardly meet content requirements yet due to its inherent structural disadvantages. It not only cannot bear all the IPTV functions, but also may cause a bottleneck in traffic.
To solve the problems of the CDN, much research has been made on reconstructing the CDN into a
P2P-based media delivery network. It is necessary to undertake the reconstruction in service convergence, content distribution and streaming media playing. First, CP is regarded as a node of the entire P2P network to provide programs for the entire service platform, and all contents delivered can be selectively converged at the central service node for related content processing. Second, in order to reduce the transport pressure at the core network, the P2P-based content distribution mechanism can be used to distribute audio and video content slices from the central service node to edge service nodes that are close to the users. Moreover, different content distribution policies are adopted for different content. Third, during the process of delivering streaming media services to users via the edge service nodes, the P2P-based streaming media playing mode can also be used. In this mode, the edge service node consists of many P2P streaming media servers that can operate simultaneously to provide streaming media services, so that different streaming media servers can have a well-balanced load growth to lower the risk of system congestion.
So far the research has involved the standardization of hybrid media delivery architecture using the P2P mechanism. Since no large-scale commercial application is available in China now, it will be quite some time before the P2P-based media delivery system is developed and perfected. It is a temporary solution to employ the existing CDN system for the current IPTV services, and to gradually upgrade the whole media delivery network from top to down when the services expand on a certain scale.
3 Digital Rights Management
In the industry chain of IPTV services, the Digital Rights Management (DRM) technology plays a crucially important role in ensuring the profits of CPs. It involves digital identification, security and encryption, as well as electronic transaction technologies. In digital video rights protection, IPTV services are distinctly different from digital TV services. The rights protection of IPTV services should follow its characteristic of two-way communication.
There are multiple solutions for service operators to fulfill DRM. For example, a service operator can build a full set of DRM systems by itself, or cooperates with CPs to build a DRM system, or give up establishing a DRM system. Each of the 3 solutions has its advantages and disadvantages. Moreover, when deploying services in different areas, the same operator may have different requirements for building the DRM systems because users in different areas grow differently.
In the initial stage of service deployment, a service operator needs to take the DRM solutions specified by the CPs. Therefore, the IPTV system must be able to integrate multiple DRM systems provided by different CPs easily, and be open enough to meet the CPs？ differing requirements for the DRM system. The necessary work to be considered by the operator is to build a full set of the DRM system by itself, which can provide related protection mechanisms for a great number of non-mainstream CPs and expand the scope of content service. Since the IPTV DRM system has to receive contents from different sources, it is necessary to take into account the standardization of the DRM system.
There have been no complete solutions to standardization of IPTV DRM for operators yet. Besides, the patent fee charged by the Open Mobile Alliance (OMA) for its DRM standardization solution is very high. Therefore, establishing an open and standard DRM system requires great and dedicated efforts in the long term. It is predicted that multiple DRM solutions will coexist for quite some time.
4 Video Coding
Since IPTV services consume a great amount of network bandwidth, the suitable video coding technology is the key to implementing IPTV services.
ISO and ITU have rolled out a series of international video compression and coding standards, including H.261,
H.262, H.263, H.264, MPEG1, MPEG2 and MPEG4. Present IPTV systems mainly adopt H.264 and MPEG4. Additionally, some influential foreign companies have put forward their own video coding standards, such as ASF, nAVI, AVI, DIVx, QuickTime, Real Audio, Real Video and Real Flash. The Audio Video Coding Standard (AVS) developed by China is a new-generation coding method with proprietary intellectual property rights. The defining of the AVS is in progress. As one of the effective coding methods targeted to replace MPEG2, the AVS enables coding efficiency equal to that of H.264 and Windows Media 9 (VC-1)[1].
When selecting a video coding technology for an IPTV system, the coding compression rate, service requirement, interoperability and usage cost should be taken into account, out of which the usage cost is the most crucial factor. Currently IPTV systems have to pay the patent fees for using the H.264 and MPEG4 video coding standards. As these video coding standardization organizations adopt the policy of charging fees from both vendors and operators, the total operation cost of the whole industry is increased considerably. China provides a relatively preferential policy to its AVS license.
The IPTV trial systems built in different areas in China are concerned more about network and technical performance when selecting video coding technologies. However, in the long term, it is necessary to conduct a comparison analysis of the AVS and other video coding standards as soon as possible and ultimately fulfill a unified and efficient video coding technology for the entire network.
5 Storage System
Digital video files generally occupy much storage space, so it is necessary to set up a cost-effective storage and distribution mechanism. On one hand, this mechanism can optimize the IPTV system bandwidth occupation in the data network, and on the other hand it can improve the stability of system security and response speed at the user end. The storage system consists of 3 parts: storage devices, storage network and storage management system. They are responsible for data storage, improvement of storage capacity and performance, and data management respectively.
The IPTV storage devices can be redundant arrays of disks, optical discs and magnetic tapes. With strengths of high speed, large capacity and good security and reliability, redundant arrays of disks are generally used as the
on-line storage device for streaming media applications. Compared with the hard disk, the optical disk and magnetic tape are apparently slow in read/write and access speeds, but they have distinct advantages in terms of price per capacity unit and capacity scalability. Therefore, they are usually used as the near-line and off-line storage devices for the IPTV system. In practice, the three kinds of storage devices are used in combination to meet the requirements of different scenes.
Direct Attached Storage (DAS), Network Attached Storage (NAS) or Storage Area Network (SAN) can fulfill the storage network. DAS is a simple technology with low investment. It can meet requirements for the mass storage of IPTV contents. However, as DAS provides low security and has difficulty in implementing effective data backup as well as in management and maintenance, it is not the development trend of the storage system. NAS can make use of the established Local Area Networks (LAN), with easy scalability and simple fulfillment. However, but it is unable to meet the requirement for large capacity and real-time data storage and access, which will have great impact on network performance. Hence, NAS is not the preferred IPTV storage solution. SAN is a network that consists of storage devices and system components. Through the optical network, it provides high data access speed and additional redundancy links to the storage system, and supports the highly available cluster system. However, its output bandwidth cannot expand linearly with the growth of the subscriber base and service scale, and the system construction cost is relatively high. Though SAN has lots of technical advantages, it is inevitable for IPTV system to make use of the existing storage system before the SAN cost is considerably reduced.
Since the IPTV system greatly taxes network sources, a distributed storage mode has to be considered in
large-scale deployment. At present, a multi-level storage structure consisting of one central node and multiple edge nodes are usually used to reduce cost. However, this storage structure may lead to an increase in delay and network load in the process of content distribution, so it is an important topic in IPTV to select an optimal storage management mechanism. There are two ways to organize and manage stored contents in present IPTV system. One is the file mode in which the file is used as the minimum storage unit, and video streams are stored in files on the disk. The file mode is disadvantageous in delivering real-time services. The other is the slice mode that can support real-time multimedia services well, which cannot be fulfilled by the file mode. It can support more flexible content exchange and routing policies to greatly improve the network capabilities of load balancing and fast response while reducing bandwidth and storage space usage. With great strengths in content distribution, content exchange, content integration and linkage, system performance and real experience of users, the slice mode has become the development trend of storage technology for IPTV services.
6 User Access Authentication
The present IPTV system may adopt the access authentication modes commonly used by broadband networks, such as authentication by virtual, web, Dynamic Host Control Protocol (DHCP) and IEEE 802.1x.
When the traditional access authentication modes are used for broadband video services, bottlenecks occur, such as narrow uplink bandwidth, low processing capability, difficulty in dealing with concurrent and heavy traffic flow, and poor support of multicast protocols. Furthermore, as there are more and more IPTV services based on
Set-top Boxes (STB), the traditional access authentication modes are inconvenient for users.
The DHCP authentication mode has the problems of inaccurate billing time, long idle time for users and automatic off-line operation. However, it is not restricted by the 2-layer network and can support multicast protocols. Since IPTV services have no need for accurate time-based billing, provide no idle state of users in service provisioning, and can support multicast communication, the DHCP authentication mode is suitable for IPTV services. Currently, an independent IP address pool and targeted policy configuration can be used to separate IPTV services from other Internet services.
In deployment of future IPTV services, a unified authentication mode that integrates multiple access authentication modes has to be considered, and needs further research and demonstration.
7 Media Asset Management
As for IPTV services, video programs are the main data processed, which are greatly different from traditional data like text and pictures. For this reason, the concept of Media Asset Management (MAM) is introduced in IPTV systems. Although the MAM system has been proved to be feasible, it is still necessary to solve some problems such as format specifications for the content index. The MAM technology involves media data storage management, content search, catalogue data management, media asset application and distribution, and more. The EPG technology used in the present IPTV systems pertains to the domain of media asset application and distribution.
EPG enables an easy way for users to access programs quickly with a friendly interface. It also allows users to browse information they are interested in.
As there are more and more IPTV content providers, content classification becomes increasingly complex. Accordingly, it will be very difficult to access IPTV services by means of EPG. Moreover, contents for some services in the future will require search and inquiry in multiple ways rather than strictly by the EPG menu. Therefore, it is necessary to consider introducing the concept of a complete MAM system in the future IPTV system.
8 Streaming Media
Streaming media is a technology that uses the streaming transmission model to play multimedia information such as audio and video data over the IP network. Compared with pure data download, streaming media not only greatly reduces start-up delay but also lowers the requirements of buffer capacity for the system. After coding and compression is completed, digital video streams can be transported and controlled via different network transport protocols such as the frequently used Real-time Transport Protocol (RTP) and Real-time Streaming Protocol (RTSP). The Internet Streaming Media Alliance (ISMA) has defined the related technical specifications for data transport and control. Moreover, the Transport Stream (TS) protocol is also used to specify data transport and control.
With respective advantages and disadvantages, the ISMA and TS streaming formats have different applications on the market. The ISMA streaming format is fitful for the features of IP networks with little overhead, but requires audio and video to be transported separately in two streams and makes it difficult to synchronize them. The TS streaming format allows audio and video streams to be multiplexed into one stream, provides relatively high synchronization accuracy, and can carry abundant program related information in a stream, but at the expense of high overhead. In light of the application of present IPTV systems, it is difficult to integrate the two streaming formats in a short time, although the two formats have no effect on normal system operation. The STB that supports the two streaming formats or special ISMA-TS conversion module can be used to solve this problem.
As the streaming media server used in the IPTV system is responsible for transporting in a streaming format the video files that are coded and compressed in advance or on a real-time basis, it has a high requirement on system performance and reliability. To meet requirements, it is necessary to improve a single server reliability or use server cluster technology. To solve the problem of overall system performance and service availability by means of improving the performance of a single server brings about relatively low cost-performance ratio, while the introduction of server cluster technology can effectively solve the problem. The loosely coupled cluster system with load balance is an important trend for large-scale streaming media services.
9 Bearer Network
To ensure users？ easy access to IPTV services, it is necessary for the operator to provide an appropriate bearer network. Yet the present technologies for IP bearer network have many disadvantages.
IPTV services require a large-capacity and high-speed access system. So far the access solutions available for IPTV services are Asymmetric Digital Subscriber Line (ADSL), Gigabit Ethernet Passive Optical Network (GEPON), Local Area Network (LAN) and broadband wireless access. Affected by the subscriber line quality and Digital Subscriber Line Access Multiplexer (DSLAM), the ADSL technology popularly adopted in China has the problems of low rate, short transmission distance, unguaranteed QoS, and restricted support of multicast. Though the DSL technology has been developing, and the transmission bandwidth has increased, the QoS is not yet acceptable. To solve the QoS problem, operators have tried to use ADSL2+ that can provide higher access bandwidth and longer transmission distance. Moreover, GEPON is receiving increasing attention. By employing an all-optical network structure, GEPON provides high symmetric uplink/downlink bandwidth and allows dynamic allocation of bandwidth for each IPTV user. Besides, it supports the overall QoS guarantee on the access link, ensures wide coverage, and greatly reduces maintenance costs. The GEPON system can meet the need for IPTV service deployment, but the initial equipment and fiber installation cost is very high, which affects its popularization. Therefore, most operators are currently delivering IPTV services by upgrading the ADSL.
At present, the cost will be relatively high if the access network adopts the Gigabit Passive Optical Network (GPON) solution to support IPTV and other broadband multimedia services. However, with the growth of subscribers and popularity of IPTV services, GPON will become the preferred access mode for IPTV services in the coming years.
IPTV services also create a new requirement for the transmission quality of Metropolitan Area Networks (MAN). Currently there are two solutions. One is to reconstruct the original IP MAN into a real multi-service bearer network to deliver network-wide multicast services, guarantee network-wide QoS and support high reliability and broad bandwidth. The solution is applicable in cities with good MAN infrastructure. The other is to construct a dedicated network for IPTV services, with the original MAN simply carrying original Internet services and the traffic flow at the access network being separated at the convergence layer. This solution is applicable in cities with complex MANs. The two solutions can be selected according to the actual network conditions, but other economical and temporary solutions can also be adopted.
Multicast is a key technology for IPTV services. At present it has the following problems.
- Both Version 1 and Version 2 of the existing Internet Group Management Protocol (IGMP) provide no related security mechanism for a multicast user entry into or exit from a multicast group and for establishment of multicast sources. Therefore, multicast probably has impacts on existing networks.
- The existing core network devices support the Multi-protocol Border Gateway Protocol (MBGP) and Protocol Independent Multicast (PIM) protocol, but this function has not yet been verified in practical services. Hence there exist deficiencies in supporting multicast.
- The existing MAN convergence devices are chiefly Broadband Remote Access Servers (BRAS), so there are deficiencies in supporting multicast.
- Lots of devices in the access network fail to support a large number of multicast groups and high reproduction efficiency (CPU occupation rate) well, so performance problems may occur in large-scale implementation of multicast technology.
Multicast in IPTV system is a very important issue that cannot be solved in a short time. Hence it is practical to provide a temporary solution to meet the actual service requirements, and then take all into consideration after the service attributes and contents of IPTV are completely specified.
10 Terminals
Terminals for IPTV services currently fall into two types: the Personal Computer (PC) plus software and the STB plus TV set. The former has the advantages of easy deployment and low investment, but its player software restricted to the vendor private file format provides poor interoperability, and anyone unable to use a PC has no opportunity to enjoy this service. Therefore, using a PC as the IPTV terminal system is a solution applicable for low-cost development and promotion of IPTV services, and it will not become the development trend. However, the P2P-based player software is still a highlight of recent development.
The STB-based IPTV terminals that integrate functions of PC and TVs can directly use the TV terminals available at the users？ homes to deploy IPTV services, so they become the focus of recent development. Now the IPTV STB contains 3 types: ASIC-based STB, DSP-based STB and CPU-based STB. The ASIC-based STB features high decoding efficiency and low cost, but has the disadvantages of poor scalability and single function. The DSP-based STB is programmable, so it provides good scalability and can easily add new functions. However, it has the disadvantages of high development cost and poor interoperability. The CPU-based STB provides simple development and good scalability, but the cost is high. Since technologies for video coding, transport streaming format and DRM have not yet been finalized, the IPTV terminals are required to enable certain flexibility and scalability. Accordingly, the DSP-based or CPU-based STB is a preferred choice at present. When the concerned technologies remain stable, the low-cost ASIC-based STB will enjoy a more promising market.
The middleware concept is more and more introduced in the design of IPTV terminals. However, the concept and application scope of middleware itself are still in dispute, so it will take some time to conduct the related research and practical verification.
11 Conclusions
This article merely discusses main technical trends of the IPTV system. Due to limited space, many details about IPTV are not covered here, which are also very important to the development of the IPTV system. The technical development trends discussed in the article are based on the present situation of IPTV service development. We should be quite aware that IPTV itself is a brand-new type of services and that many of its service features and development trends are to be further specified. Accordingly, we also need to adjust and improve our knowledge about IPTV technical development trends.
