IMS-based IPTV

    The Internet Protocol Multimedia Subsystem (IMS),  in the standard NGN architecture, provides a converged system architecture for fixed and mobile Softswitch networks. It enables the separation of the control layer and the data layer by setting an independent Home Subscriber Server (HSS) database, and helps operators fulfill unified management of both subscriber and business data. More importantly, the separation of the service layer and the control layer in the IMS architecture enables operators to develop more new services.

    According to recent researches of relevant standards, it is becoming a new trend to use the IMS architecture for interactive IPTV and other stream media services. The Call Session Control Function (CSCF) of the IMS can be used for the subscriber authentication and service trigger of IPTV services, and the HSS enables good management of IPTV subscriber data. The growing maturity of the IMS-based Public Switched Telephone Network/Integrated Service Digital Network Emulation Subsystem (PES) has created a new PSTN/ISDN service provisioning model based on the IMS core network.

    Correspondingly, a similar architecture based on the IMS core network can be applied as a solution to IPTV service provisioning[1-4].

1 IPTV Architecture
Generally speaking, the IPTV system architecture is logically made up of a content operation platform, service operation platform, Content Delivery Network (CDN) service network, bearer network, and home network, as shown in Figure 1.

1.1 Content Operation Platform
The content operation platform has a content operator platform and application services from  third-party Service Providers (SPs).

    The content operator platform is mainly responsible for the establishment and management of the Audio and Video (AV) program broadcasting system and monitoring system. Its functions cover service logic processing, service data processing, and organization, production, approval, storage, broadcasting and content security guarantee of AV programs.

    The third-party SPs offer online games, information services, e-commerce, interactive programs and other application services. These services are released on the content provider platform and enjoyed by subscribed users.

1.2 Service Operation Platform
The service operation platform is a multifunctional platform for an operator to manage IPTV subscribers, Content Providers (CPs), SPs, and more.

    It has such functions as service management, SP/CP management, subscriber management, content management and content billing. In addition, it enables the interconnection with external Operation Support System/Business Support System (OSS/BSS).

    This service operation platform is mainly made up of a service processing system, Content Management System (CMS), OSS, content distribution system, Electronic Program Guide (EPG) system, Set-Top Box (STB) version management system, billing service system, interface system and various service function modules.

1.3 CDN Service Network
The CDN service network pushes service content (AV program content) to the network edge to provide services to a subscriber within his nearest possible reach. Therefore, it effectively improves service quality and reduces transport pressure in the backbone network. In addition, the CDN service network solves the problem of video network scalability by flexible deployment of video servers at the network edge.

    The core technology supporting the CDN service network is content service distribution mechanism.

    The network features are as follows:
    (1) The CDN service network has a complete content distribution function. The main distribution technologies used include Push, Pull, and mirror distribution. These technologies support both unified distribution and play of media in various formats.

    (2) It supports two-leveled scheduling mechanism, which includes operator-oriented program content scheduling mechanism and reversible equivalent scheduling mechanism between CDN network nodes.

    (3) The network has a unique live broadcasting relay and transfer function. If a Metropolitan Area Network (MAN) with weak multicasting capability cannot implement live broadcasting services by multicasting in the entire network, the CDN service network may be used for relay and transfer of the live broadcasting services.

    (4) It supports high-performance computing. The server in the CDN is a typical software server with a multithreaded architecture. Therefore, the multithreaded technology is applied in the memory forecast scheduling, multi-disk storage of a program, and stable output of multi-concurrent video streams.

    (5) It uses a unified video-pump to support various stream media formats. In this way, it solves the problems of concurrent distribution of stream media in various formats and of overall load balance.

    (6) In order to support massive concurrent distribution and all sorts of complicated transportation networks, the CDN service nework uses a stream architecture to fulfill video information transmission.

    (7) Its multi-disk storage technology makes full use of the hard disk resource and Input and Output (IO) performance of the system. This not only improves the concurrent distribution capability of the system, but also balances the hard disk load and accordingly extends the average operational life span of the hard disk effectively.

    (8) It supports live broadcasting relay. When a network is unable to support multicasting in the entire network, the live broadcasting program can be sent to the CDN, and finally relayed to the edge node via the level-one and level-two nodes. The edge node then provides the end users with the live broadcasting program through either unicasting or multicasting according to the actual network situation.

    (9) It supports caching and pre dissu technology and QoS control.

    (10) Its open architecture supports Direct Attached Storage (DAS), Network Attached Storage (NAS), Storage Area Network (SAN) and other sharing storage devices.

    (11) Its scalability expands the network smoothly into a leveled-and-distributed stream media service network. This secures QoS for hundred-to-ten-thousand-level stream services.

1.4 Bearer Network
The bearer network is mainly made up of the access router, Broadband Remote Access Server (BRAS), Digital Subscriber Line Access Multiplexer (DSLAM) and Modulator-Demodulator (MODEM). All these devices share the responsibility of transmitting IPTV service stream from the video service network to subscriber terminals.

    For IPTV services on demand, the broadband access network only provides subscriber access control, data transport channels, and secured QoS. The service control is implemented through service management and terminals.

    For live broadcasting services, the network may implement authorization control, statistics and QoS guarantee of live broadcasting services along with its transportation functions.

1.5 Home Network
To cope with the tendency of the increasing abundance in home information services, the IPTV system may offer a powerful network solution for home services and unified access. As an important part of the home network, various series of
low-end-to-high-end-oriented STBs have been commercially ready to meet the demands of different IPTV subscribers.

2 IMS Architecture

2.1  NGN IMS Network Architecture
Figure 2 illustrates the NGN IMS architecture (session control and bearer). Main logic entities in this architecture are introduced as follows:

    (1) P-CSCF
    The Proxy-Call Session Control Function (P-CSCF) fulfills proxy of subscriber IMS services, security of signaling, authorization of the bearer, and billing policy download function.

    When User Equipment (UE) is getting an IMS service, the P-CSCF is the first connection node. The UE obtains the P-CSCF address through a “Local CSCF detection” process.

    The P-CSCF functions as a proxy server, processing and transferring requests and services being received. It mainly completes the subscriber-access-related functions such as signaling compression and security.

    Additionally, the introduction of the P-CSCF enables the separation of service access and service control, and makes the control of home services possible.

    (2) I-CSCF
    The Interrogating Call Session Control Function (I-CSCF) mainly fulfills distribution of the Serving-Call Session Control Function (S-CSCF) in the home network (upon subscriber’s registration). In addition, it implements the routing of IMS incoming calls.

    Telecommunication operators want to hide internal logic of their networks in some cases. Therefore, a device called I-CSCF will be inserted to the interface of the network with internal logic to be hided. The I-CSCF may make transfer of incoming and outgoing Session Initialization Protocol (SIP) signaling to hide internal routing information of SIP signaling.

    (3) S-CSCF
    The S-CSCF mainly fulfills subscriber authentication, registration, service authorization, service trigger, service routing and billing. The S-CSCF maintains information about session states according to demands of the network operator. Within the same network, different S-CSCFs may function differently. As the main service node for subscribers, the S-CSCF is distributed upon subscriber’s registration. The P-CSCF and the S-CSCF cooperate to fulfill the home control of IMS services.

    (4) BGCF
    The Breakout Gateway Control Function (BGCF) is used to select a network connected with the interface point of the PSTN (or circuit domain). When the BGCF find itself in a network connected with the interface point, it will choose a Media Gateway Control Function (MGCF) which is responsible for interacting with the PSTN (or circuit domain). If the interface point is connected to another network, the BGCF will transfer session signaling to the BGCF of that network.

    The BGCF may use the local routing configuration to select a network connected to the PSTN. Moreover, it is possible for it to use the information exchanged with other protocols for the selection.

    (5) MGCF
    The MGCF is the interconnection point between the IMS control plane and the traditional PSTN/CS network. It controls the IMS-Media Gateway (MGW) or Trunking Media Gateway Function (T-MGF) to fulfill interworking between media planes (via H.248). Moreover, the MGCF interworks with the I/S-CSCF at the IMS side, while completes protocol translation from SIP to Bearer Independent Call Control/ ISDN User Part (BICC/ISUP) at the PSTN/CS side.

    (6) HSS
    The HSS stores IMS subscription data, service attribute data, location information and authentication information.

    It also offers traditional Home Location Register (HLR) functions (such as subscription data of Circuit-Switched (CS) domain and Packet-Switched (PS) domain) and all sorts of interfaces (such as Diameter and MAP).

    (7) SLF
    The Subscription Locator Function (SLF) locates the HSS by the SIP Unified Resource Identifier (URI). All entities visiting the HSS have to call for SLF. (No call for SLF is needed in a single HSS environment.)

    (8) AS
    The Application Server (AS) mainly performs control and execution of service logic.

    (9) MRFC
    The Media Resource Function Control (MRFC) maintains and controls the media resources in the Media Resource Function Processor (MRFP) in the IMS domain (via H.248). It interworks with the S-CSCF, and accepts indirect control of the AS. At the initial stage of the IMS establishment, the MRFC can be integrated into the AS.

    (10) MRFP
    The MRFP has the media resources within the IMS domain. Under the control of the MRFC (via H.248), it provides vocoder resources, announcement tone resources and conference bridge resources.

    (11) RACS
    The Resource and Admission Control Subsystem (RACS) mainly fulfills management control, resource reservation, policy control, and Network Address Translation (NAT) traversal and control.

    Gq’ is attributable to a Diameter interface. It interacts with the core network, and cooperates with the Network Attachment Subsystem (NASS) to implement QoS and policy control.

    (12) NASS
    The NASS chiefly fulfills IP address distribution for UE terminals, configuration for network access, and authentication and authorization for network access.

2.2 IMS-based PES Architecture
The MS-based PES architecture is illustrated in Figure 3. 

    The IMS-based PES system is the result of the introduction of an Access Gateway Control Function (AGCF) entity into the IMS architecture. Other entities of the IMS-based PES correspond to those of the IMS architecture, but their functions are improved and expanded. Traditional terminals and/or visiting nodes access the PES via the Media Gateway (MG) and Gateway (GW).

    The interface protocol may be H.248 (the P1 reference point) or SIP (the Gm reference point). H.248 is used for the MG, while SIP for the GW. An Independent PSTN/ISDN can also be connected through the relay media gateway (the Mn reference point).

    The functional entity of the AGCF is specially defined for the PES. It is the first contact point for the MG and GW to visit the PES. Its functions are listed as follows:

• Serves as the MGCF to control all the MGs
• Interacts with the RACS
• Interacts with the NASS to obtain attributes of subscriber lines
• Fulfills transfers between analog signaling and SIP signaling via the P1 reference point
• Acts with other IMS SIP functional entities as proxy for SIP subscribers
• Performs P-CSCF functions for traditional access terminals behind the MG (such as managing the SIP registration process, generating subscriber correlated identification, and producing billing identification)
  

    The AGCF is actually a P-CSCF for other CSCFs. Besides, SIP signaling follows the definition of the Mw reference point.

3 IMS-based IPTV System

3.1 IMS-based IPTV Architecture
The IMS-based IPTV architecture is illustrated in Figure 4.

    Borrowing the concepts of the IMS-based PES, the IPTV system can also utilize the IMS core Network Elements (NE) to fulfill subscriber authentication and authorization, and to obtain addresses of relevant service servers. Moreover, the IMS core NEs can be applied for message transfer between terminals and the IPTV AS. The SIP call model in the IMS can be used for negotiation and transmission of IPTV media addresses.

    The introduction of the Hyper Text Transfer Protocol-Call Server Control Function (H-CSCF) logic functional entity has two purposes. One is that the H-CSCF fulfills the translation of Hyper Text Transfer Protocol (HTTP) into SIP. In this way, all Web-based terminals, besides IPTV STBs, can be used to enjoy services provided by the IMS core network. The other is that more succeeding Web-based terminals (including STB) are expecting the services provided by the IMS network, as well as the internet-based services such as network gaming, information browsing, and the like. These services can be realized through the H-CSCF bypassing.

    The interface Mw between the H-CSCF and the I/S-CSCF uses SIP. The interface H1 between the STB and the H-CSCF utilizes HTTP. H2 is the interface of H-CSCF with the management system and external database. The H-CSCF, a basically upgraded version of EPG system in IPTV, also has P-CSCF functions.

    The CDN in IPTV corresponds to the Media Resource Function (MRF) NE in the IMS. Current STBs mainly support HTTP/Real Time Stream Protocol (RTSP). The principle definition of integration of the IPTV system into the IMS involves authentication, authorization, accounting,  QoS, and the interaction with IMS services. In this way, the solutions to QoS, accounting and more in IMS can directly be used to those in the IPTV system.

3.2 IMS-based IPTV Subscriber  Authentication Process
The configuration for the H-CSCF address and IP address of the version server in a STB is a prerequisite for IPTV service access.

    The subscriber authentication process is as follows:

    (1) The STB starts self-checking at power on.

    (2) The STB requests broadband for network access.

    (3) Network access succeeds. The IP address is obtained, and network connection is established.

    (4) The STB requests the version server for version update.

    (5) The STB version server checks if the STB needs update. If update is necessary, the server sends the latest version to the STB. Otherwise, it notifies the STB that version update is unnecessary.

    (6) The STB version is updated.

    (7) The STB sends a log-in request to the H-CSCF.

    (8) The H-CSCF returns to the log-in page.

    (9) The subscriber enters his account and password, and sends a service authentication request to the H-CSCF. The request reaches the IMS core network through the H-CSCF.

    (10) The IMS core network interacts with the authentication-related functional entities. After a successful authentication, a temporary ID and the authorization message are generated. The information about the authentication and authorization completion is then sent to the IPTV AS.

    (11) The IMS core network sends the authentication result, temporary ID and authorization information back to the H-CSCF.

    Based on the information received, the H-CSCF generates a new page and sends it together with the information mentioned back to the STB.

    (12) The STB renews the EPG, and records the authorization information
and ID.

3.3 IMS-based IPTV VOD Service Process
The Video on Demand (VOD) process refers to a flow of operations for playing a subscribed program. A subscriber selects one of his subscribed VOD programs via the H-CSCF, and then sends a VOD request to the system. It obtains the video stream in the end. The detailed process is as follows:

    (1) The subscriber selects a program subscribed according to his EPG, and sends a call request to the IMS core network via the H-CSCF.

    (2) The IMS core network verifies the subscriber’s information. The Initial Filter Criteria (iFC) triggers the processing on to the IPTV AS.

    (3) Through interaction with the CDN, the IPTV AS makes a selection of the video server and video stream for the service, and then sends the relevant information to the H-CSCF via the core network.

    (4) The IPTV AS sends billing information to the operation support management system.

    (5) The H-CSCF sends the information about the video server and video stream for the service, together with the related authorization information, back to the STB.

    (6) The STB presents the ID and authorization certificate and sends a video access request to the selected video service network.

    (7) The video service network verifies the ID and authorization certificate via the OSS service control system. The service statistical information is simultaneously generated in the OSS management system.

    (8) After the verification is passed, the video service network will allow the video access, and then send a reply to the STB.

    (9) After a successful establishment of video connection, the STB starts video stream transmission from the stream media server in the video service network. For video content with digital rights, the STB has to make a digital rights check before decoding the video content.

    In video stream transmission, a subscriber may make fast forward, rewind, pause and other Video Cassette Recorder (VCR) operations. The operations are fulfilled by the STB and the stream media server.

4 Conclusions
The IMS architecture has been regarded as an NGN convergence architecture. Both its service provisioning modes and QoS solutions are studied and technically supported by specialized standardization organizations.

    The IMS-based IPTV system is advised to be combined with existing IMS services like online services and video conferencing. In addition, it is recommended to be integrated with mobile stream media services to provide IPTV services for mobile terminals.

References
[1] ETSI TS 183 043 V1.1.1. Telecommunications and Internet converged Services and Protocols for Advanced Networking (TISPAN): IMS-based PSTN/ISDN Emulation [S]. 2006.
[2] Jiang Lintao. IPTV—Beginning of Triple Play [J]. ZTE Communications, 2006,12(3):1-4.
[3] ETSI ES 282 007 V1.1.1. Telecommunications and Internet converged Services and Protocols for Advanced Networking (TISPAN): IP Multimedia Subsystem (IMS): Functional architecture [S]. 2006.
[4] ZXBIV IPTV Technical Manual [R]. Shenzhen:ZTE Corporation.
[5] Zhang Xueli. Service Provision for Next Generation Networks [J]. ZTE Communications, 2006,12(5):7-10.
[6] ITU-T FG IPTV-C-0156. Proposal of NGN based IPTV architecture [S]. 2006.

Manuscript received：2006-11-03

[Abstract] A solution is proposed to implement interactive Internet Protocol Television (IPTV) services in the Internet Protocol Multimedia Subsystem (IMS) network. The solution enables the IPTV system to access the IMS core network via the Mw interface provided by the HTTP Call Session Control Function (H-CSCF), fulfilling converged networking of IPTV and IMS services. It allows operators to offer IPTV services in IMS-based converged networks without upgrading and modifying the existing IPTV terminals. Moreover, it enables unified subscriber management and guaranteed Quality of Service (QoS), and supports the convergence with other NGN services. Based on this solution, IMS operators can offer IPTV services or IPTV-based converged services through an IMS architecture; IPTV operators can implement networking of IPTV and IMS by updating and modifying the core equipment, to reduce operational costs.