A generic monitoring architecture for assuring the QoS in Mobile TV platforms

Transcription

1 A generic monitoring architecture for assuring the QoS in Mobile TV platforms Antonio Cuadra-Sanchez Telefonica R&D Madrid, Spain Clara Casas-Caballero Telefonica Spain Madrid, Spain Abstract Nowadays main service providers are launching the initial mobile TV commercial services, which are offered from wireless broadband networks. This article shows a common architecture for monitoring the QoS (Quality of Service) of mobile TV platforms. This solution, which is based on the analysis of quality figures through network probes, will allow service providers to offer their audiovisual services in a competitive manner. The proposal includes the development of an monitoring architecture founded on a common open architecture that gathers the data from the different network segments in a convergent way. This aspect means that the mobile TV services are permanently end-to-end supervised from both the access and the core network. In this specific scenario, the UMTS/3G underlying transport network has been considered to carry the mobile TV service. On the other hand, the core network is constituted by the content delivery platform and the central services centre (CSC), which orchestrates the services offered to end users, such as multicast TV, Video on Demand (VoD) or Pay-per-view (PPV). This paper includes the results obtained from this monitoring platform, applied to on of the major mobile Telco companies, explicitly for the VoD service based on videostreaming sessions. I. INTRODUCTION The IPTV content delivery platforms on mobile networks allow users to access real-time services by subscribing to a multimedia service: broadcast TV, video on demand, pay-perview, etc. These contents are delivered to end users devices (i.e. Set-top-box) from the mobile TV platform through a broadband network, such as UMTS or CDMA2000. The process is coordinated by a dialogue between the application servers and decoders for users to request a specific action or content control (join, play, pause, etc.). This proposal sets out an IPTV services monitoring system offered on convergent content delivery platforms supported on mobile (3G) networks. For that purpose, the traffic sent from servers to applications and content decoders and terminals is permanently monitored through a series of sensors deployed across the network. These probes guarantee the retrieval of real multimedia traffic received by end users in order to monitor the quality of service in a network-independent way. This application makes it possible to determine the behaviour of IPTV services based on the analysis of standard control and transport protocols for multimedia streams in IP networks. The methodology covers the processing and analysis of traffic in order to monitor the most critical services, such as videostreaming contents offered on fixed and mobile networks. These features include QoS and traffic metrics, as well as troubleshooting tools, such as a protocol analyzer. The main feature of the system is the reconstruction of detailed records of the session of mobile TV services, which contain the most relevant information of a comprehensive dialogue for requested service. The most significant parameters of the traced session are stored, such as the content, the traffic downloaded, the delay, the jitter and the packet loss rate. This information allows supervising the main problems in mobile TV, such as frozen images or pixeled effect. The proposed monitoring solution for VoD (video on demand) services is based on the analysis of the videostreaming control protocol (RTSP [1]) and media transport protocols (RTP [2]). The information is obtained from probes and is used to reconstruct detail records (XDR [3]) in order to build quality indicators to quantify the quality of video services. The following sections introduce the protocols used to offer videostreaming services, as well as the most significant IPTV products. Subsequent session cover the main functionalities for supervising the QoS of such services, from the generation of detailed records to the description of specific measures of quality for videostreaming services. Finally, the results are displayed on a real mobile TV platform. II. MOBILE IPTV PLATFORMS Mobile IPTV (Television over IP) can be defined as an audiovisual content delivery offered through a wireless broadband network over Internet Protocol. Although mobile TV can be freely broadcasted, this article refers only to the platforms owned by IPTV service providers, which have a proper infrastructure isolated from the rest of the networks /10/$ IEEE 1227

2 The topology of a generic mobile IPTV platform [4], implemented by a Telco operator, consists of several network elements in terms of the network distribution: Content delivery centers, which send the IP multimedia content to end-users. For multicast services center, it is called as TV head-end. Sometimes these elements are distributed over the access network in order not to overflow the core network, such as videostreaming. CSC (Central Services Centre) that orchestrates all the operation, admission and maintenance tasks, from billing and authentication, to parental control. LSC (Local Services Centers) that interconnects the core network with the access network There are several access technologies depending on the wireless or wireline network. The main fixed access technologies are ADSL, VDSL and FTTH. On the other hand, inside the scope of this article, for mobile Telco networks, 3G (UMTS/CDMA2000) is the most common technology, and is commonly referred as mobile TV. In this way mobile TV users access real-time and deferred time services depending on the content: Broadcast TV, VoD (Video on Demand), PPV (pay-per-view) and a bundle of supplementary services. The mobile TV platform delivers the multimedia content to the customer's 3G device (mobile terminal). Mobile TV provides two basic multimedia services: Broadcast TV and Video on Demand (VoD). Internet Group Management Protocol (IGMP) version 2 [5] is the main control protocol for multicasting, meanwhile Real Time Streaming Protocol (RTSP[1]) is the most common protocol used for VoD. Regarding the transport protocols, Real-time Transport Protocol (RTP[2]) is used to convey the multimedia flows, although high quality services require also MPEG-2 [6] or 4 [7]. The following sections introduce the services object of this article, that are related to the videostreaming multimedia flows. A. Video-streaming Services These services allow users to access multimedia contents (live TV, video on demand, etc.) by paying a variable amount depending on the event: news, entertainment, music, etc. The IPTV platform sends the multimedia flow (audio and / or video) to terminal equipments. This process is ruled by a dialogue between both elements (TV content server and terminal equipment), which is used to request a specific action or content control (pause, fast forward, play, etc.). For this purpose RTSP[1] (Real-Time Streaming Protocol) is used as a standard to control multimedia flows in IP networks. It has been specified in RFC 2326 and this is an application layer protocol that is used to control the delivery of data that require sending information in real time. The protocol messages include the described methods, Play, Pause and Record among others, through which it controls the delivery of multimedia packets. Although RTSP does not require a specific transport mechanism for carrying multimedia traffic, in video streaming services RTP [2] (Real-Time Transport Protocol, standardized by the IETF in RFC 3550) is commonly used. The delivery of frames is controlled by RTP Control Protocol RTCP [2], which provides a feedback of the source of and participants of a session. This protocol is based on the regular transmission of control packets to all members of a session, using the same mechanisms for the distribution of data to RTP (consecutive and odd port, flow identifiers, etc). The main function of RTCP is to get the quality of the information received from the edge ends. Some of the main statistics compiled, both in reception and in transmission, are the number of sent and lost packets, delays, etc. The application that sends these data may use this information to alter the transmission, for example to set another compression ratio or to improve quality. There are five types of RTCP packets: Sender Report, Receiver Report, Source Description Message, Bye Message and Application- Specific Message. The most significant attributes in RTCP messages are the Counters of packet and bytes sent, and the accumulated rate of lost packets, jitter and inter-arrival delay since the last report submitted. Figure 1. Access to videostreaming contents with RTSP. The previous figure shows the procedure used to get a video on demand content, where the user requests a session that can change through various control commands (pause, fast forward, play, etc.). As it is implemented through RTSP, the request procedures are sent to the Content Server (Content delivery server) with this protocol. These requests are acknowledged (responses) through the same protocol, whose sequence number (Cseq) must match the one included in the previous method. III. MONITORING ENVIRONMENT This section describes the management environment inside the mobile IPTV platform. On the one hand, the architecture of the proposal is described, while on the other hand a subsection shows how to implement this solution on a convergent IPTV network [8]. All this work is mainly focused on the data gathered from probes that are deployed on the IPTV network. 1228

3 A. Architecture of the solution Probes systems are becoming a fundamental tool for network operators and service providers to monitor and manage the quality of service [9]. Probes can be placed at any point in the network, so they provide a greater flexibility than the systems based on network elements or other data sources. The traffic collected through probes, due to the detail with which they are elaborated, provide a real vision of the behavior of the network and of the perception that users have of the services. Therefore, this architecture will allow to guarantee the service and to satisfy customer necessities throughout the processed information. In addition, these systems of massive traffic capture constitute the main input for the SQM/SLM (Service Quality Management/ Service Level Management) that are beginning to establish in main telecommunications networks. Flexibility and multiprotocol capabilities are two basic characteristics for monitoring and management through probes the new generation networks (NGN, UMTS, IMS, etc.) and the new services as VoIP, mobiletv, quadruple play offer, etc. The development of these technologies has been focused on a criterion of scalability based on an open architecture that allows the monitoring of new emergent technologies. In addition it is independent of the services that supervises, and provides the customer point of view as far as quality and availability of all offered services. The architecture of the probe section is a distributed model where the data is extracted by the probes located at specific points on the network depending on their topology. This information is received and preprocessed in different Remote Systems [10], which are physically close to the probes, thus allowing the Remote Systems to draw up specific traffic and quality measurements. In addition, the frames captured during a configurable amount of time are also stored in these Remote Sites so that they can be later accessed for the study of any reported abnormality. Finally, the preelaborated measurements are sent to the Central System for processing (information grouping and correlation, if necessary) and consolidated in a Database left at the disposal of the corresponding operators. B. Deployment of the solution on a mobile IPTV platform As already introduced, the QoS monitoring system is fed from probes located at the network interfaces. The probes can be classified depending on the location. Access probes are deployed over the different access networks, and its functionality is common to other services apart from IPTV, such as VOIP, multimedia messaging, etc. Therefore this paper does not include specific results for these datasources. More information can be found in [11]. The following points describe these access probes: Iu-PS Interface (mobile): This interface communicates the RNC with the SGSNs [11]. Gn Interface (mobile): This is used both to send signaling between network elements (SGSN and GGSN) and to transmit packet traffic [11]. Gi Interface (mobile): Interconnection with external data network, in this scenario with the IPTV platform. IPTV services are transmitted through this interface [12]. Platform probes are placed inside the proper IPTV architecture. There are basically two types of passive platform probes: OAM (Operation, Administration and Operation) probes, for monitoring transactional services such as PPV and VoD purchases, webmail, parental control, authentication, etc. TV / Video probes, that allow to monitor the control plain of multimedia services by analysing the delivered protocols, such as VoD (RTSP) or multicast and PPV (IGMPv2). In addition, depending on the requirements, the multimedia flow (RTP) is also monitored [13]. The following figure shows where those probes are placed. Figure 2. Deployment of probes at mobile IPTV platforms IV. ASSURING THE QOS IN MOBILE IPTV: VIDEOSTREAMING SCOPE The monitoring system described in the previous section is used to guarantee the quality of the IPTV services. This chapter describes the functionalities of this OSS, mainly focused on the mobile IPTV QoS measurements. The specific functionality for the access networks can be found in [3], [10] and [11]. The monitoring tasks are currently focused on videostreaming applications, since those services are the first convergent ones at this stage of the development of the solution. This QoS monitoring functionalities rely on the reconstruction of Service-level Detailed Records (SDRs). This concept is derived from the records generated in other communication environments, such as CDRs for traffic 1229

4 associated with voice calls (Call Detailed Records, SS7 ISUP protocol). In order to rebuild each service into a SDR it is necessary to correlate all the IPTV frames associated with one session [12]. Each record contains the most relevant information of the complete dialogue of just one procedure requested between an origin (server) and a destination (user). These procedures are implemented for each Application level protocol. In this videostreaming scenario two protocols are involved: RTSP for the control plane, and RTP (indeed RTCP) for the transport plane. The most important parameters of the messages associated with an IPTV session are stored inside the SDRs. The main fields stored for each session are described as follows: RTP packets and octets: Total number of RTP packets and octets in the IPTV session. Throughput: Mean throughput of the multimedia session (Bytes/s). Ratio Packet Lost: Ratio of the RTP packet lost in the IPTV session. Total Packet Lost: Total number of RTP packet lost. Mean Max & Min Delay: Measurements of the inter- RTP frames delay. Mean Max & Min Jitter: Measurements of the IPTV flow delay variation. Service Time: Total time of visualization of the content. Establishment Time: Time between initial control procedure and first content served. Mean response time: Measurement of the mean time between a request an its response. Packet Loss of multimedia traffic: is the main cause of pixeled images. Throughput: when this parameter drops below a concrete threshold, the video looks frozen and the sound is lost. Jitter: it has little impact on such services, since it affects mainly in real-time interactive applications. The results of tuning the main QoS parameters for videostreaming sessions are shown in table 1. Those QoS metrics are obtained from the reconstruction of each session (SDRs). TABLE I. EXAMPLE OF MOBILE TV PARAMETERS FOR DIFFERENT QOS. Parameter Media Good Regular Poor Throughput Video >20Kbps <15Kbps Audio >10Kbps 10-5 <5Kbps Packet Loss Video < 1% 1% - 5% >5% Audio < 2% 2% - 5% >5% Average Jitter Video < E4 >1E4 Audio < E4 >1E4 Apart from the indicators related to the traffic flow, it is also important to monitor the quality of the establishment and management of the video sessions. The following chart shows a use case based on the number of daily sessions from a set of 4 channels offered to customers. A sudden decrease in the number of mobile TV sessions usually involves hidden service faults. This information is also used for planning and dimensioning tasks. Note that some information is not shown for confidential purposes. The quality of service measures are derived from mobile IPTV SDRs, that will identify the main problems associated with these information flows, such as the pixel and the freezing of the images. It is also necessary to identify the media, because their characteristics determine how they affect the parameters on the quality of service. The services discussed in this paper cover the VoD (video on demand), where the contents are transmitted through buffers, and streaming in "real time", usually transmitted through shortsize buffers that are close to the quasi-real time. V. RESULTS The proposal presented in this article has been applied on an IPTV convergent platform. This section shows the results obtained from a commercial IPTV videostreaming service. The QoS data has been generated from the deployed monitoring system that receives all the VoD sessions from the different network probes. The main quality of service parameters and its influence on the customer experience are detailed bellow: Figure 3. Daily evolution of IPTV sessions per channel In addition, the following graph is an example of the quality of service in the establishment control for the different sessions in one of the channels offered. Some of these values are also hidden for confidential reasons. This way the probebased monitoring tool is able to detect connection failures, and its reason, when users try to join the different mobile TV channels or VoD sessions. 1230

5 [6] [6] ISO/IEC Information technology -- Generic coding of moving pictures and associated audio information: Video.. Switzerland, [7] [7] ISO/IEC :2004 Information technology -- Coding of audio-visual objects -- Part 1: Systems. Switzerland, [8] [8] R. Ansorena, A. Cuadra, F. Garces, et al. How to Face Convergence to Guarantee End-to-End Quality of Service. ICT- Mobile-summit 08. Stockholm, June [9] [9] A. Cuadra, "Feeding the Quality of Experience, Contribution to Inside Latin America Nr 5. TeleManagement Forum, November [10] [10] A. Cuadra "Network and service monitoring in next generation networks.", Conference in XIV Telecom I+D. Madrid, November [11] [11] J.M. Hernando, C. Lluch, C. Casas, A. Cuadra et al GPRS: Technology, Services and Business", ISBN Madrid, October [12] [12] A. Cuadra, Perceptual QoS in IPTV platforms, Contribution to XVII Telecom I+D. Valencia, November [13] [13] F. Garcés, A. Cuadra, J. A. del Sol, C. Casas et al. "II QoS Assurance Conference in Telefónica Latinoamérica. Sao Paulo, July Figure 4. Daily evolution of QoS of the establishment sessions for one channel for one channel VI. CONCLUSIONS In this proposal a system to monitor and assure the quality of media services in mobile IPTV platforms has been presented. The QoS data are gathered by probes that sniff end users multimedia traffic from the different network segments (access, core and operations centre) in a passive way. In order to calculate quality indicators, the solution makes a summary of each IPTV service by generating session detailed records (SDRs). In addition, this article shows the results of its implementation on an mobile operator for videostreaming services, since those are the first commercial convergent products. In the same way the paper presents the main proposed QoS indicators for videostreaming services, such as packet loss, throughput and jitter, as well as their effect on real VoD sessions. Finally, the results of this work are currently being applied to a convergent IPTV Telco operator in Spain. Further works will extend the management necessities to monitor the rest of convergent services inside a service provider, mainly focused on IMS and VoIP services. Moreover, innovation activities will be based on the research of the quality perceived by the customers QoE (Quality of Experience) since the developed functionality already contains the detailed data of each service from the user's point of view. REFERENCES [1] [1] H. Schulzrinne, A. Rao, and R. Lanphier. Real Time Streaming Protocol (RTSP), RFC IETF, April [2] [2] H. Schulzrinne, S. Casner et al. "RTP: A Transport Protocol for Real-Time Applications, RFC IETF, June [3] [3] A. Cuadra, "QoS Monitoring in VoIP networks, Conference in XIII Telecom I+D. Madrid, November [4] [4] P. Granado et al Imagenio: The new way of entertainment on demand. Madrid, September [5] [5] W. Fenner "Internet Group Management Protocol Version 2, RFC IETF, November