IPTV Bearer Network Solution

Currently, telecom operators face a continuous decline in Average Revenue per User (ARPU). Moreover, relevant investigations show that the terminal and content are major factors to restrict the further development of telecom data services. Therefore, in order to improve ARPU, the operators have to solve two problems: content and terminal equipment. Feature-rich content may not only attract new subscribers, but also make old subscribers spend more time in using the network. With regard to terminal equipment, physical equipment and easy use of the terminal are two problems to be solved for the further reduction in the cost of technology and the terminal. Fortunately, the emergence of Internet Protocol Television IPTV services not only solves these two problems, but also conforms to the trend of triple-play network.

    IPTV uses the broadband Internet infrastructure, IP transfer technology, and TV or PC as the medium terminal to provide subscribers with basic, digital media services. These services include TV programs, Video on Demand (VoD) and value-added services.

    As shown in Figure 1, the bearer network of IPTV network is located between the video source and Customer Premise Equipment (CPE). It consists of Metropolitan Area Network (MAN) and broadband access network. It is responsible for transporting video stream from the video source to the CPE.

1 Characteristics of IPTV Services
As real-time stream media services, IPTV services have the following characteristics when compared with other Internet services:

    (1) Characteristics of Traffic Flow
    All traffic flow, no matter what is the broadcast or VoD, is sent from the video server to the customer. That is to say, the traffic is concentrated and only downlink traffic.

    With regard to broadcast services, data stream from the same channel destined for customers is same at any time. The bandwidth required doubles up to down. Therefore, the bandwidth demand is predictable, and is multiple of the number of channels available.

    As for VoD services, data stream destined for different customers at the same time is different, while the bandwidth required is same up to down. For each bearer link, the bandwidth prediction is more difficult than that of broadcast services. The bandwidth demand multiplies along with the increase of customers.

    (2) Characteristics of Authentication
    Compared with other Internet services, IPTV services need secondary authentication. That is to say, they require access authentication, as well as service authentication.

    According to the flow and performance features of IPTV broadcast, multicast technology is especially suitable for the bearer network of IPTV network. Usually the bandwidth occupied by IPTV broadcast on each multicast link is related with the number of channels. However, VoD can only adopt unicast for transmission. With the assistance of the Video Delivery Network (VDN), VoD may use the 80/20 Rule to deliver 20% of hot TV shows to the bearer network edge nearby the subscribers. This reduces the bandwidth pressure on the bearer network and improves QoS performance. Generally, the bandwidth occupied by VoD service traffic has connection with the number of concurrent subscribers. On condition that MPEG-4 coding is adopted and each channel needs 2 M bandwidth, 200 M bandwidth is necessary for 100 broadcast channels of each link, and 10 G bandwidth necessary for 5 000 subscribers to watch VoD simultaneously.

    For IPTV multicast applications, the following factors shall be taken
into account.

    (1) Selection of Multicast Replication Point
    The multicast replication point refers to the network node responsible for replicating video stream for users. It can be either the Broadband Access Server (BAS)/Access Router (AR) or L2 equipment such as the Local Area Network (LAN) switch and Asymmetric Digital Subscriber Line (ADSL). At the multicast replication point, the network device replicates the multicast stream according to the request of Internet Group Management Protocol (IGMP) at the port. The equipment serving as the multicast replication point is required to have good functionality and processing capability. The nearer the multicast replication points approach to the users, the less bandwidth the network needs, and while the management becomes more complex.

    When selected as the multicast replication point, the BAS/AR implements the user-oriented replication without requiring the access network to support multicast. Although simple in management, the BAS/AR poses higher bandwidth requirement for the access network. Moreover, the BAS/AR is more demanding on its port density and processing capability. Therefore, BAS/AR is recommended for trial deployment only.

    If L2 equipment nearest to the user (such as ADSL or LAN switch) is selected as the multicast replication point, the bandwidth of the access network will be saved at the most. However, the access network must support multicast, and the terminal equipment is required to support IGMP Snooping. Centralized management by unified network management technology is the solution to complex management. This solution is a distributed replication, so it has a relatively low requirement for processing capability at the replication point. Hence, it is applicable for large-scale service deployment.

    If a convergence switch serves as the multicast replication point, it has features between those of the two technologies mentioned above. Therefore, it is an interim solution.

    (2) Selection of Multicast Control Point
    The multicast control point refers to the equipment that is responsible for watching authority control of TV channels according to viewers´ channel authority. In order to reduce the complexity of network, it is usually integrated with the multicast replication point.

    (3) Multicast Management
    Originally, the Internet was not built as a telecom-class, service bearer network. In addition, operational problems such as authentication and billing were not considered when multicast technology was first introduced. Therefore, it is necessary to improve multicast technology. Multicast management involves multicast user management, multicast source management and multicast distribution management. Multicast user management authenticates services requested by users to implement authority control over the multicast stream received by the users and collects billing data. Multicast source management distinguishes legal and illegal multicast source, and then makes priority control over and bandwidth restriction to the legal source. Multicast distribution management is to control the scope of multicast distribution via the static multicast.

    Static multicast refers to static setup of a multicast distribution tree. All multicast streams are pushed to the distributed point regardless of being received by the users or not. Dynamic multicast uses multicast routing protocols such as Protocol-Independent Multicast-Sparse Mode (PIM SM) and Protocol-Independent Multicast-Dense Mode (PIM DM), and the Internet Group Management Protocol (IGMP) request to dynamically set up a multicast distribution tree. Static multicast is applicable for IPTV because it may reduce time delay while enabling multicast distribution management.

    (4) QoS of Multicast
    To meet the QoS requirement of IPTV, two methods are usually adopted. One is the configuration of the static multicast path. The other is the construction of the dedicated network. At present, all IPTV bearer networks overseas adopt dedicated networks to guarantee the QoS because the QoS standards for multicast are not yet mature enough to ensure end-to-end QoS of the current network.

2 ZTE´s IPTV Bearer Network Solutions
In terms of networking, the IPTV bearer network can be either a logical network or a dedicated physical network.
Based on Multiple Protocol Label Switching (MPLS)/Virtual Private Network (VPN), the logical network forms a solid Metropolitan Area Network (MAN) with multiple planes. Each plane bears different services. Meanwhile, the logical network uses DS-TE, static configuration and other technologies to build the video path. It offers QoS guarantee based on the DiffServ model. However, the logical network requires numerous alterations to the existing network. Currently, it is difficult to implement it because the multicast QoS technology is not mature enough yet.
The physical dedicated network is an independent network. With reasonable traffic flow planning and bandwidth resource allocation, it adopts broad bandwidth and DiffServ model to guarantee the QoS with little modification to the existing network. The physical dedicated network features easy deployment and good security. It can be divided into network-class, equipment-class and link-class physical dedicated networks. Therefore, according to multicast, QoS and processing capability provided by the existing equipment, multiple interim solutions are available.

    Presently, the physical dedicated network is recommended as an interim solution for building the MAN part of IPTV bearer network, and the IPTV services may share the improved access network with other services. When building a dedicated network, the processing redundancy of port, slot and equipment, as well as the support of MPLS/VPN, shall be considered in order to facilitate the evolution to the logical bearer network. Then it may bear 3G and Next Generation Network (NGN) services in addition to IPTV services in the future.

    Figure 2 illustrates ZTE´s IPTV bearer network solution. Multicast routing protocols such as PIM-SM and PIM-DM are used to set up a multicast distribution tree. IGMP static joins are configured to enable static multicast. Multicast Source Discovery Protocol (MSDP) and Multi-protocol Border Gateway Protocol (MBGP) are used to implement cross-domain multicast.

    IGMP Snooping and Proxy are used to implement multicast distribution control. The billing server collects billing data at the multicast replication points such as the LAN switch or ADSL to accomplish the billing task. The edge equipment adopts multicast Access Control List (ACL) to filter multicast sources, Committed Access Rate (CAR) to restrict the speed of multicast stream and MARKING to mark multicast priority. In this way, it fulfills multicast source management.

    Every user accesses the Internet or IPTV network via a Virtual Local Area Network (VLAN) or Permanent Virtual Circuit (PVC). Point-to-Point over Ethernet (PPPoE) and Dynamic Host Control Protocol (DHCP) are employed to complete access authentication of the user, and 802.1p tells the priority of the user. All IPTV users may share one multicast VLAN. The edge equipment adopts CAR to restrict the speed of multicast stream, classifies and marks the stream. Moreover, it makes mutual mapping of 802.1p and the MPLS Experimental (EXP), or that of 802.1p and Differentiated Services Code Point (DSCP), over BAS/AR to fulfill remarking.

    The server mirror is necessary for backup of the service-layer equipment. MPLS VPN is used for VPN isolation. As for the bearer network, all devices connected to the service equipment have to support Virtual Router Redundancy Protocol (VRRP).  The bearer network must adopt the dual-star networking mode. In order to reduce recovery time of the failed network, the bearer network should support such functions as MPLS fast rerouting, fast routing convergence and fast Spanning Tree Protocol (STP) convergence.

3 Case Study: Atlas India´sIPTV Bearer Network  Project
    (1) Project Background
    Atlas India, a wholly owned subsidiary of Atlas Group, is responsible for exploiting telecom services in India. It chiefly provides residential areas and business users with video and Internet services. These services include
high-quality TV program, video/audio on demand, email, videoconference and other high-speed Internet access services. Atlas India adopted ZTE´s ZXR10 data series to build a brand-new MPLS/IP network that delivers triple-play services including IPTV services.

    (2) Topology of Atlas´ IPTV Network
    The IPTV project of Atlas was divided into two phases. In the first phase, 50 000 lines were deployed to cover four big cities. In the second phase, the project added 500 000 lines to the network, and thirty-six cities were covered. The original four major cities were upgraded into the core nodes in the second phase. Each core node adopts two ZXR10 T160G sets that mutually back up their data. The entire core network has ring structure, and uses Link Aggregation Control Protocol (LACP) technology to bind two 10 GE ports between the core nodes. The bandwidth of the entire backbone network reaches 20 G. Additionally, thirty-two ZXR10 T64Gs are used at the convergence layer, and they are distributed in thirty-two cities respectively. The ZXR10 T64G in each city is connected to the two core nodes via two 10 GE ports, and the two ZXR10 T160G sets at the core layer employ VRRP technology to ensure reliability of the entire network. Downlink, the ZXR10 T64G uses GE to connect to
ZTE´s Digiual Subscriber Line Access Multiplexer (DSALAM) equipment—ZXDSL 9210. To ensure good performance of VoD services, the entire network employs distributed video push technology. In certain important cities, video servers are deployed to connect to the ZXR10 T64G. Moreover, the Network Operation Center (NOC) is connected with the video source network and the operation management system. Figure 3 shows the entire network topology.

    (3) Network Features

• The entire physical network adopts dual-star and ring structure to ensure reliability of the network.
• The core and convergence layer of the MAN adopts MPLS network, fully making use of the strengths of MPLS technology in QoS, security and service compatibility to meet the development demands for IPTV services.
• The Hierarchical Virtual Private LAN Service (H-VPLS) technology is used to ensure network scalability and reliability, which guarantees the launch of video, VoIP and High-Speed Internet Access (HSIA) services.
• The entire network adopts the most advanced 10 GE networking technology to meet the requirement of IPTV on wide bandwidth, laying a solid foundation for large-scaled deployment of IPTV services.
• The entire network adopts the multicast technology to ensure the development of IPTV services. The MSDP and PIM technology is used at the core and convergence layer.  ZXDSL9210/9806 at the access layer uses IGMP Snooping to enable multicast distribution control.
  

4 Conclusion
With abundant functions and outstanding performance, ZXR10 data series supplied by ZTE can satisfy the requirements of IPTV bearer network. Moreover, it takes into consideration the bearer network requirements of NGN and 3G services. It serves the goal of building an integrated service bearer network, and is an effective guarantee for operators´ network evolution toward the triple play.

Manuscript: 2005-05-29