Research on Standard Interface Model Between IPTV Terminal and Platform

Abstract:With the rapid development of the IPTV industry worldwide in recent years, IPTV services typically represent triple-play services. However, a lack of standards for the interfaces between the terminal and the platform has become the bottleneck in IPTV development. In order to fulfill IPTV standardization, it is necessary to conduct studies on the relationship between the IPTV system and the three networks (the broadcast network, communications network and Internet). The horizontal interface model between the IPTV terminal and the three networks is a solution to architecture of the standard interface model at the current phase of service integration. A vertical protocol interface model based on the horizontal interface architecture is proposed to validate the development trend of IPTV protocol interface standardization towards the all-IP system. Based on the standard protocol interfaces, introduction of middleware effectively lowers the threshold of IPTV application development.

      Triple-play services fully embody the service characteristics of IPTV. Terminals of triple-play services present users with broadcast, telecommunications and Internet services in one. It brings great challenge to the development of IPTV service since these 3 systems are independent following no common standards. Without any standard interface models between the IPTV terminal and platform, different IPTV systems are under isolated conditions, which leads to increased operation cost and risks of IPTV and hinders healthy development of the IPTV industry.

      Service convergence needs the support of technical convergence, while technical convergence requires conditional separation of and cuts in available technologies for the three systems. Based on convergence of broadcast, communications and Internet services, this paper proposes a horizontal interface model for service capabilities, and then introduces a standardized vertical protocol interface model. The two models are a solution to the problems existing in the current development of IPTV services.

1 Standardized Interface Model between IPTV  Terminal and Platform
The development of IPTV services follows an inherent process: combination of multiple services? integration?  convergence. Combination of multiple services means simple piling of service functions that are shown by one terminal. From the perspectives of either user or technology, these services are independent of each other. Service integration means that the independent services are unified on the terminal. In users? eyes, services are unified rather than independent from each other. However, from the point of view of technology, the services are still independent. Service convergence is the final stage when multiple services are unified both technologically and in users? eyes. The standardized interface model is proposed for solving possible problems during the process of IPTV service convergence.

1.1 Horizontal Service Capability Interface Model
IPTV services will ultimately fulfill the convergence of services on broadcast networks, telecommunication networks and Internet, with the goal of offering seamless converged services. At present, three technical systems are still under independent conditions although the IPTV services are provided to subscribers through a unified portal. Therefore, IPTV services are now under the integration stage.

      According to the present development stage of IPTV, the horizontal service capability interface model, as shown in Figure 1, creates an architecture which allows services to be converged on the terminal and send service requests to different service platforms.

      In Figure 1, multiple service function modules are integrated on the IPTV terminal. Therefore, in users? eyes, the terminal offers unified services. However, service capabilities are still implemented by different service platforms. With the development of IPTV services, different service platforms tend to be integrated into a unified multi-service platform.

1.2 Vertical Standardized Protocol Interface Model
The IPTV terminal must be intelligent to meet the requirements of the horizontal service capability interface model. In order to fulfill services on the three networks, the terminal needs to support the related protocols conditionally, for which the vertical standardized protocol interface model is proposed.

      According to comparison results of related test data and solutions and the trend of convergence of the three networks, it is concluded that Internet technology is the foundation of the vertical interface architecture. Improvements of Internet protocols are in progress for supporting more services. There have been the standardized Internet protocols that support broadcast services to be fulfilled on IP networks. The development trends of communication services are video communications based on the Session Initiation Protocol (SIP), an Internet protocol. Moreover, Internet services completely follow the Internet protocols from the beginning. Figure 2 shows the vertical standardized protocol interface model.

      The bottom layer is the data link layer (i.e. Layer 2), above which is Transmission Control Protocol/Internet Protocol (TCP/IP). Above TCP/IP are diversified Internet application protocols, which fulfill the interfaces of broadcast, telecommunication and Internet services. According to the model, the protocols for all the interfaces are based on Internet protocols.

2 Key Technologies for Vertical Standardized  Protocol Interface Model
The vertical standardized protocol interface model plays a crucial role in fulfilling the interworking of the terminal and the platform of the IPTV system. The key to the interface standardization is the standardization of the protocol process. A unified protocol process enables the platform compatible with different kinds of terminals, which solves the problem of the terminal binding with the platform.

2.1 Streaming Control and Real-time Streaming Media Protocol for Broadcast Services
In the IPTV system, the control interface of basic audio/video services fulfills the transfer of medium data from the server to the user end. Video on-demand services are generally fulfilled by the Content Distribution Network (CDN), that is, video data are duplicated by CDN to the edge server at the network edge, and then transferred to the set-top box by streaming transport technology to fulfill the functionality of playing while downloading finally.

      The content of streaming transport[1] includes the encapsulation format of data packets, transport format of data packets, establishment of streaming transport, and processing of pause/fast-forward/rewind requests of users.

      Correspondingly, the streaming transport technology involves streaming control protocol, file packaging format, and streaming transport protocol. The widely used Real Time Streaming Protocol (RTSP) is adopted here for the standardized interface.

      RTSP[2] is a client/server multimedia program protocol. It controls the transport of streaming media data on IP networks, and meantime enables remote control of the Video Cassette Recorder (VCR) mode for audio and video streams, such as stop, fast-forward, rewind and positioning. RTSP is also an application-layer protocol, together with lower layer protocols such as Real-time Transport Protocol (RTP) and Resource Reservation Protocol (RSVP), to provide the complete set of streaming services based on Internet.

      The interface of RTSP implements streaming control request and respond by using the key control word "Method". Different Methods represent different control requests from clients, as shown in Table 1.

2.2 Video and SIP for Communication Services
SIP[3] aims to help offer advanced cross-Internet telephony services. IP phone (IP phone) is becoming a formal commercial phone model, and SIP is an important NGN series protocol for fulfilling this model[4]. Based on Simple Mail Transfer Protocol (SMTP) and Hypertext Transfer Protocol (HTTP), SIP is used to set up, change and terminate calls between clients of IP networks.

      SIP has two message types: request (from the client to the server) and respond (from the server to the client).

      The SIP request message includes three elements: the request line, header and message body.

      The SIP respond message includes three elements: the status line, header and message body.

      The request line and header define the nature of a call according to service, address and protocol characteristics, while the message body is independent from SIP and may include any contents.

      Table 2 gives the basic methods SIP defines.

      IPTV system uses SIP as the foundation protocol for communication services such as video communications and instance communication. SIP itself keeps expanding to cater to the future development of communication services.

2.3 Application Authentication and HTTP for Internet Services
As a kind of telecom-level services, IPTV services should be manageable and billable, and provide valid subscribers with Quality of Service (QoS). The application authentication interface module is the most important one in the IPTV interface model. Every user needs to use the application authentication interface module to exchange identification information with the IPTV platform, and only valid subscribers are allowed to use IPTV services.

      HTTP is used for the standardized application authentication interface here. HTTP is based on a request/respond mechanism (equivalent to client/server). A client sends one request to the server after setting up a connection with the server. The request format is Uniform Resource Locator (URL), the version number of protocol, and then Multipurpose Internet Mail Extension (MIME) messages that include the request modifier, information about the client, and other possible contents. After receiving the request, the server gives a respond message with the following format: one status line including the version number of protocol, one successful or false code, and then MIME messages that include information about the server, entity messages and other possible contents.

      In the IPTV system, application authentication adopts WWW-Authentication[5], i.e. challenge interaction. First, a user sends identity information and service request to the platform. Then the platform returns a random challenge string. After that, the terminal uses the symmetric key shared with the platform to encrypt the challenge string and sends it to the platform. In the end, the platform verifies the user磗 identity and provides service to the valid user.
This authentication model integrates the shared key, and effectively unifies the authentication and authorization of all the services. It features efficient transmission and security of information.

      A lot of Internet applications such as information browsing and related protocols such as Simple Object Access Protocol (SOAP) and Hypertext Markup Language (HTML) are based on HTTP.

3 Relationship between Standardized Interface Model and Middleware
Traditional digital TV systems generated the concept of Java middleware[6-7] to solve the problem of interconnection and interworking. At present many IPTV terminal and platform vendors have also proposed the concept of IPTV middleware, which is expected to satisfy the interworking of interfaces. Interface interworking is essentially based on a standardized protocol process, while middleware is just a means to fulfill the vertical standardized interface model.

3.1 IPTV Middleware
There are terminal middleware and platform middleware in IPTV system.

      Terminal middleware is a sofuware abstracted layer used to separate terminal service applications from bottom-layer operation system and hardware. It has downward interfaces for different operation systems and hardware, and upward Application Programming Interfaces (API) supporting upper-layer service applications. The APIs are unrelated to the operation system. They define a stable high-layer application environment, masking the difference between bottom-layer hardware and system software. For terminal application software vendors, terminal middleware helps quicken the development of new applications.

      Platform middleware is the universal service between the IPTV platform and applications. Platform middleware is located above platform hardware, operation system and database, and below application programs. Platform middleware has the standardized program interfaces and protocols, enabling application service to be quickly deployed among heterogonous platforms.

      Since IPTV services are based on network, a lot of middleware is related to network protocols. Basic service processes or protocol standards can be encapsulated by using middleware API, which facilitates the development of applications. For instance, there are standardized stream playing control API, service authentication API and browser API. However, for middleware developers, the encapsulation of these APIs must be strictly standardized protocol interfaces, otherwise, it is impossible to fulfill interworking between terminal and platform interfaces through middleware.

3.2 Interface Standardization and Middleware
Middleware aims to facilitate integration and quick deployment of services among heterogeneous platforms. From the viewpoint of application developers, middleware, by providing them with standard API sets or complete software development tool kits, saves them from concerning the hardware platform, operation system and network architecture at the bottom layer. On the other hand, middleware developers must understand protocols encapsulated in middleware, and divide the protocols into multiple logical elements, i.e. various APIs. Figure 3 shows the relationship between middleware and the standardized interface function module.

      For the standardized interface model, middleware specifies the interfaces for application developers. However, only when the content encapsulated in middleware is in accordance with the protocol process defined by the standardized interface model, can middleware solve the problem of standardization between IPTV terminal and platform interfaces and further fulfill interworking and interoperability.

4 Conclusions
The standardization of IPTV is a long-term goal, and the interface model will change with the service convergence going through different phases. The IPTV industry needs to integrate the broadcast, communications and Internet systems. The technical development of all  three systems will finally move towards all-IP systems. The Internet protocols have been improved into the protocols that support streaming media, communications and Internet, and can accordingly be regarded as the foundation of IPTV service convergence. They are the development direction of IPTV standardized interface protocol models. The standardized interface protocol based on encapsulation of middleware APIs provides application developers with a low threshold, and may cater to the changing requirements from users. It is believed that IPTV can fulfill real service convergence and provide triple-play services based on Next Generation Network (NGN) soon.

References
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[5] Fielding R, Gettys J, Mogul J, et al. IETF RFC 2616 Hypertext Transfer Protocol - HTTP/1.1[S]. 1999.
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Manuscript received?2006-03-15

Author Introduce:

  Yan Haining is a chief engineer of the Shanghai Research Institute of China Telecom Corporation Limited. He has long been engaged in the research of communication networks, and took charge of multiple important research projects at China Telecom.

Chen Yan  received his Master磗 degree from Shanghai Jiaotong University. He is a product development engineer at the Shanghai Research Institute of China Telecom Corporation Limited. As the deputy leader of the terminal and platform interface team of the China Communications Standards Association IPTV special working group, he is mainly engaged in IPTV product development and standardization.

Hu Bingsong received his
post-doctoral degree from Shanghai Jiaotong University. He is a senior engineer at the Shanghai Research Institute of China Telecom Corporation Limited. As the deputy leader of the terminal team of the CCSA IPTV special working group, he is mainly engaged in IPTV product management and standardization.