2276 IEEE Transactions on Consumer Electronics, Vol. 56, No. 4, November 2010 Soft Handover Mechanism Based on RTP Parallel Transmission for Mobile IPTV Services Qi Qi, Yufei Cao, Tonghong Li, Xiaomin Zhu, Jingyu Wang

Abstract — Mobile Protocol Television (IPTV) technologies including Wideband Code Division Multiple provides users with multimedia content through wireless Access (WCDMA), Wireless Local Area Network (WLAN), access technologies whenever they want and wherever they and World Interoperability for Microwave Access (WiMAX) are. However, vertical handover in heterogeneous networks etc. When users move among these networks, the continuity results in long interruption time and large rate, of the services should be guaranteed with neither disruption which poses a challenge for seamless mobile IPTV service. nor perceptible degradation. Thus, the mobility management is The existing solutions that make use of the heterogeneous the key point to provide seamless mobile IPTV service. access networks to transmit multimedia content have two Moreover, as the wireless networks have limited resources drawbacks: high bandwidth consumption of the wireless compared to wired networks, overall bandwidth consumption networks and lots of packets loss when a user resides in the needs to be minimized. Although the vertical handover has overlapped area of the boundary. To solve these issues, we been a research subject for several years, it still poses a propose a new soft handover mechanism, based on the IPTV challenge for mobile IPTV service as it typically causes long server and the consumer electronic devices. The proposed handover delay, high packet loss rate and wireless bandwidth Parallel Soft Handover (PSH) divides the Real-time Transport waste. Protocol (RTP) packets according to the packet loss rates, and exploits different networks efficiently by using parallel Fig.1 presents a scenario in which the mobile IPTV service transmission. Moreover, the selective retransmission for RTP is provided in heterogeneous networks. A user moves across packets is introduced to reduce the packet loss. Simulation several networks and several areas covered by different results demonstrate that the PSH can reduce the packet loss wireless access technologies when the IPTV program is rate, as well as obtain a more efficient use of the wireless playing. Before handover takes place, the link to the WCDMA bandwidth1. network (old path) and the link to the WiMax network (new path) are both available. Also, when the user enters the house Index Terms — mobile IPTV, handover, parallel transmission, that is covered by WLAN, the mobile device can explore the heterogeneous networks. paths of the WCDMA and WLAN in parallel to receive the multimedia content. I. INTRODUCTION The rapid growth of wireless access technologies and the increasing number of mobile consumer electronic devices have led to a considerable increase of the users’ demand for mobile networking services. The mobile Internet Protocol Television (IPTV) extends many IPTV services to mobile users, which enables them to receive multimedia traffic such as TV signals, video, audio, text, and graphics, through IP- based networks, whenever they want and wherever they are [1]. In the overlapped heterogeneous networks, the mobile devices can receive the multimedia content by multiple access

1 This work was jointly supported by: (1) National Natural Science Foundation of China (No. 60902051); (2) the Fundamental Research Funds for the Central Universities (BUPT2009RC0505). Qi Qi is with State Key Laboratory of Networking and Switching Technology, Beijing University of Posts and Telecommunications; Beijing, Fig. 1. Vertical handover of IPTV service P.R. China, 100876 (e-mail: [email protected]). Tonghong Li is with the department of computer science, Technical University of Madrid, Madrid, Spain, 28660 (e-mail: [email protected]). Due to the increase of CPU performance and memory Yufei Cao is with EBUPT Information Technology Co., Ltd.; Beijing, P.R. capacity, it is possible to have the capability of media China, 100083 (e-mail: caoyufei@ ebupt.com). processing in mobile devices. In this paper, we propose a Xiaomin Zhu is with State Key Laboratory of Networking and Switching Technology, Beijing University of Posts and Telecommunications; Beijing, terminal driven handover method for mobile IPTV service to P.R. China, 100876 (e-mail: [email protected]). utilize the two network bandwidth and avoid packets loss Jingyu Wang is with State Key Laboratory of Networking and Switching when a user moves in heterogeneous networks. This proposed Technology, Beijing University of Posts and Telecommunications; Beijing, P.R. China, 100876 (e-mail: [email protected]). Parallel Soft Handover (PSH) is based on RTP parallel Contributed Paper transmission, which splits a RTP packet into two parts and Manuscript received 10/12/10 Current version published 12/23/10 Electronic version published 12/30/10. 0098 3063/10/$20.00 © 2010 IEEE Q. Qi et al.: Soft Handover Mechanism Based on RTP Parallel Transmission for Mobile IPTV Services 2277 transmits them in parallel via the old and new paths during the [8] present a Session Initiation Protocol (SIP) based mobility vertical handover. Comparing with the existing approaches, management mechanism by making use of the Session Border PSH makes the following three contributions to mobile IPTV Controllers (SBC). The SBC duplicates the RTP packets and services: (i) PSH splits a large payload Real-time Transport sends them to the new and the old interface in separate. Protocol (RTP) packet into two smaller sub-packets and In the state-of-the-art soft handover mechanism for mobile retransmits some of the lost sub-packets so as to decrease the transmission, duplicate packets are packet loss rate. This is very useful when a user resides in the transmitted via the two paths before the old path breaks [7] overlapped area of the boundary between two access [8], so the packet loss rate is reduced comparing with the networks, where the wireless signals of two links are quite “make before break” handover [2]. But the duplicate poor. (ii) PSH utilizes the resources more multimedia content transmission by the soft handover may efficiently by sending different media data via the new and the degrade the network performance. This is particularly a old paths simultaneously. (iii) All modifications are restricted challenge when the user stays for a long time in the to the mobile devices and IPTV servers, and the proposed overlapped area of two networks as the IPTV multimedia technique is fully interoperable with existing IPTV service content always contains large RTP packets. In order to infrastructure, so it can be easily deployed. improve the network performance, two paths are exploited to The remainder of this paper is organized as follows. Section transmit the splitting media flows under load balance [3]. II surveys related work. Section III proposes the RTP parallel When the Frame Error Rates (FERs) of the two links become transmission algorithm in the PSH. Section IV introduces the higher, however, the feasibility of splitting transmission simulation model and investigates the performance of PSH. between the old and the new path is undermined greatly. Finally, conclusions are described in Section VI. Accordingly, we propose a new soft handover mechanism which enhances the RTP processing capability in the mobile II. RELATED WORK device to reduce the packet loss rate and make better use of For the mobile IPTV services, Jeon et al. [2] propose a two networks at the same time. mobile multicasting model based on Proxy Mobile IPv6 (PMIPv6). The new fast handover scheme realizes the “make III. SOFT HANDOVER MECHANISM FOR MOBILE IPTV before break” handover procedure by predicting users’ A. The Architecture and Enhanced Functionalities direction of movement and transferring the context message before the handover execution. Ciubotaru et al. [3] propose a We assume that when a mobile IPTV device performs a Smooth Adaptive Soft Handover Algorithm (SASHA) for vertical handover, it can dynamically configure its IP address mobile IPTV services. The algorithm splits the transmission of from the new wireless network in advance. media flows based on load balancing between two different networks according to their (QoS) metrics. Some methods have been proposed to improve the streaming media system for supporting mobile users. Lee et al. [4] address a movement-aware vertical (MAV) handover algorithm between WLAN and WiMAX for seamless ubiquitous access. The MAV algorithm adjusts the dwell time adaptively and predicts the residual time in the cell of target Base Station (BS). Kim et al. [5] propose an efficient video streaming method, which dynamically adjusts the video transmission rate based on the channel bandwidth and Fig. 2. Protocol architecture for PSH minimizes the error propagation during handover. Pyun et al. [6] proposes a new cross-layer streaming video system for Fig. 2 shows our enhanced functionalities above the vertical handoff. The TCP Friendly Rate Control (TFRC) standard RTP and the Real-time Control Protocol (RTCP) server uses explicit vertical handoff messages for faster rate stack in IPTV server and mobile device. In the IPTV server, adaptation to a new target network, and the TFRC client RTP packet splitter and Link status analyzer are two new determines the switching time between different networks components for PSH mechanism. The RTP packet splitter according to the handoff metrics. divides each RTP packet into two parts according to the On the other hand, researches on the optimization of performance of both networks and sends them via the old and handover control for seamless session handover are currently the new paths respectively. The Link status analyzer collects in progress. Banerjee et al. [7] firstly introduce a SIP-based the network performance parameters from the RTCP Receiver architecture to support soft handover for IP-centric wireless Report (RR) messages, and calculates the average packet loss networks. At the BS of the old network, the packet replicator rate. In the mobile devices, RTP packet combiner and RTP duplicates a RTP packet and sends it to the new interface of retransmission processer are two new components. The RTP Mobile Host (MH) with a different IP address. Salsano et al. packet combiner recovers the original RTP packet by 2278 IEEE Transactions on Consumer Electronics, Vol. 56, No. 4, November 2010 combining the sub-packets obtained from the old and the new combining the two sub-packets. To handle the scenario that the paths. The RTP retransmission processer decides whether the connection of one network breaks suddenly, the “network break lost packets should be retransmitted from the IPTV server, and disposal” is added in the RTP receiver, which is triggered by the it takes charge of packet retransmission. The details of the loss of consecutive packets. Two arrays M_0 and M_1 are used processing flows and algorithm are presented in Section III.B. to cache the sequence number (sn) of the lost packet. Also, as two sub-packets may not arrive at the mobile device B. The Algorithm of RTP Parallel Transmission simultaneously (the interval may be some hundreds of Our PSH algorithm consists of new path probe, RTP packet milliseconds), the first received one is cached in a map with its parallel transmission and lost packet retransmission. sn as a key. When the other part of the original packet arrives, We add two new bits (‘D’ and ‘N’) in the extension header they are combined. The RTT-timer is set in accordance with the of RTP for packet division and reconstruction. ‘X’ bit is the average value of the Round Trip Time (RTT) via the two paths, header extension marker. When the ‘X’ bit is set to 1, ‘D’ bit which is obtained from experiments. If the other sub-packet and ‘N’ bit have the following meanings: cannot arrive before the RTT-timer expires, packet (i) Dummy data (D) bit. When ‘D’ is set to 1, it means that retransmission is initiated. the payload in this RTP packet is used to probe the link The existing RTP retransmission method is proposed for real- performance. time applications with relaxed delay bounds [10]. Different (ii) Sub-packet number (N) bit. When ‘N’ is set to 0 (or 1), from this method, in the PSH the RTP retransmission processor it means that this packet is a sub-packet and it is the first part calculates whether the lost packets can arrive before being (or the second part) of the original packet. processed by the RTP application. Then the IPTV server The RTP packet division for parallel transmission is based retransmits the lost packet according to the NACK feedback on the estimation of the packet loss rates (PLRs) in the old and message. If the retransmitted sub-packet cannot arrive before the the new path. The PLR during the period time T (T is RTCP retransmit-timer expires, the other received sub-packet is sent message transmission interval) can be obtained from the directly to the RTP application. “fraction lost” (FL) field of the RTCP RR message sent by the Let PSpayload be the payload size of the packet received from RTP receiver. As users may wander around the border area for RTP layer. Let sn_l, sn_p, and sn_m be the sequence number of a period of time before the handover execution, we use the the last packet sent to the RTP application, current played average PLR of m RTCP intervals to represent the link status packet and the missing packet, respectively. Let the old path (or [9], in order to avoid the dithering of network performance. the new path) be path 0 (or path 1). And assume that Bi (i=0,1) is Then, the average value of PLR is denoted as: the bandwidth of path i. Assume the RTP packet played rate in m PLR1() m FL k (1) the application is O . Then the PSH algorithms at IPTV server ii ¦ k 1 and mobile device are shown in Algorithm 1 and Algorithm 2, Here, i denotes the wireless path, and FLi(k) denotes the respectively. PLR of path i during the kth RTCP period. At first, the performance of the new path is probed by the Algorithm 1: The algorithm at the IPTV server dummy packets. The Link status analyzer calculates the initial 1: if the received packet is from RTP layer PLR of the new path by using (1). When the PLR of the old 1.2: Link status analyzer calculates the RTP division as follows: path is greater than that of the new path, the packet parallel PS B(1 PLR ) paylosd 11 (2) transmission is triggered. div1 B1122(1 PLR ) B (1 PLR ) 1.2: RTP packet splitter divides the received packet into two

sub-packets: copy the header of the packet and the first div1 bytes of its payload into a sub-packet and set X=1, N=0, D=0; copy the header of the packet and the remaining bytes into a new sub-packet and set X=1, N=1, D=0. Transmit two sub-packets via the two paths; 2: if the received packet is RTCP 2.2: if the received RTCP is NACK message Get the sub-packet from the cache with the sn obtained from NACK message and retransmit it Fig. 3. RTP parallel transmission 2.2: if FL(i)=100% // start the process of “network break disposal” The main idea of the RTP parallel transmission is shown in Set PLRi =1; Fig. 3. The IPTV server divides the RTP packet into two parts Update div1 according to (2); and transmits them via the new and the old paths else simultaneously. The mobile device receives the packets from Link status analyzer calculates PLR1 and PLR2 two paths respectively and recoveries the original packet by according to (1) from RR messages ; Q. Qi et al.: Soft Handover Mechanism Based on RTP Parallel Transmission for Mobile IPTV Services 2279

if k = 5 // k is the number of received RRs discrete-event simulators simulate the user’s mobility Set k=0; //begin a new calculating period behaviors. The wireless links between the IPTV server and the Algorithm 2: The algorithm at the mobile device mobile device via the access networks are simulated as several 1: Receive packets and put them into the cache queues which carry some background traffic and communicate 2: while cache is not empty do with each other using specified packet loss rate. Get packet from cache with sn = sn_l+1; if two sub-packets of the same packet are received TABLE I Copy the fixed part of the head and the payload of the two SIMULATION PARAMETERS sub-packets; Set X=0; Send it to the RTP application; Parameter Value Parameter Value else Start the RTT-timer for the not received sub-packet; FER_(N1 center) 0.05 FER _ (N1 border) 0.08 3: if RTT-timer t expires FER _(N2 center) 0.05 FER _ (N2 border) 0.08 if two sub-packets of the same packet are received FER _(N3 center) 0.03 PSpayload 1000 Bytes Form the RTP packet and send it to the RTP application; else if // only one sub-packet of the original packet is received Request RTP retransmission; Put the sn into M_(‘N’+1)%2; 4: if three continuous sequence numbers exist in M_i // the path i breaks suddenly Set FL(i)=1; Clear the M_i; 5: if (sn_msn_p)>= RTT* O // the packet can be retransmitted Sends a NACK request and start retransmit-timer; 6: if retransmit-timer rt expires if only receive one sub-packet Set X=0; Remove extended header D and N; else Form the RTP packet; Fig. 5. Simulation results of PSH algorithm Give the packet to the RTP application. The simulation parameters are shown in Table I. The IV. PERFORMANCE EVALUATION simulation results in 8 minutes are shown in Fig. 5. From 2 minute to 4 minute, the user enters the border area of Network In this section, we firstly introduce the simulation models, 1 and Network 2, where the FERs of the two networks are and then evaluate the performance of PSH by comparing it higher than the FER in the center of Network 1. By using PSH with other existing schemes. algorithm, the IPTV server transmits the RTP packets via the A. Simulation Model two links from different access networks, but the PLR is still higher than that in the first two minutes. Then, the user moves to the center of Network 2, the PLR decreases and equals to that in the first 2 minutes. From 6 minute to 8 minute, the user enters the Network 3 that is covered by Network 2. In this case, the mobile device can explore the paths of the two networks in parallel to receive the multimedia streaming content. As the FER of Network 3 is smaller than that of Network 2, the PLR is lower than that in the last 2 minutes. In the PSH, the RTP parallel transmission algorithm adds packet processing function to the RTP layer. However, our Fig. 4. Mobility pattern for simulation test result shows that the parsing time for an RTP packet is approximately in the level of microseconds. As the RTT is To evaluate our new handover algorithm for mobile IPTV more than 100 milliseconds, the cost of packet processing in service, we developed a Java-based test bed consisting of an PSH can be ignored. IPTV server, a mobile device and three access networks. In B. Analysis of Packet Loss Rate the IPTV server and the mobile device, the RTP module implements the standard RTP stack and our proposed PSH Assume p0 and p1 are the FERs of the old and the new mechanism, based on the open source code RTP stack. We paths, respectively. The PLRs for Multicast based Handover assume that the user’s moving trail in Network 1 (N1), (MBH) [2], Soft Handover (SH) [8], SASHA [3] and PSH are evaluated along with p in Fig. 6. Before performing the Network 2 (N2) and Network 3 (N3) is shown in Fig. 4. The 0 2280 IEEE Transactions on Consumer Electronics, Vol. 56, No. 4, November 2010 handover, the air-link condition of the old path begins to PLR for PSH is smaller than that for SASHA, and the decay gradually, which causes p0 to increase. The IPTV server advantage becomes evidence when the packets size increases. with SH method has the smallest PLR as it transmits the In SASHA, when the packet size increases, the packets duplicate packets via the two paths. When p0 is less than 0.01, transmitted via the new or the old path have higher probability most of the RTP packets are transmitted successfully via the of being lost. But PSH transmits small sub-packets via two old path, and hence MBH has a smaller PLR than PSH and paths with fewer frames than SASHA, which helps to reduce SASHA. When p0 is larger than 0.05 (p1), the PLR for MBH the probability of packet loss. In addition, the selective RTP becomes larger than that for SASHA. This is because MBH packet retransmission leads to lower PLR as well. only transmits RTP flows via the old path until the handover performs; but SASHA exploits the two paths to transmit the C.Analysis of Utilized Bandwidth Ratio media flows. On the other hand, the PLR for PSH is always The IPTV service usually transmits large multimedia data, less than that for the SASHA, and approaches to that for SH. the traditional soft handover schemes may lead to The reason is that the packet division in PSH reduces the unacceptable resource waste. To measure the used network number of frames contained in a transmitted packet, and thus resource, we define the Utilized Bandwidth Ratio (UBR) as the loss probability due to one of the frame error is decreased. the ratio of the occupied bandwidth to the maximum available Moreover, the RTP packet retransmission in PSH can reduce bandwidth. The occupied bandwidth is the used bandwidth for the PLR significantly. transmitting packets from IPTV server to mobile devices.

Fig. 6. Packet loss rate vs. p0 (PSpayload =500, p1=0.05) Fig. 8. Utilized Bandwidth Ratio vs. packets size

Fig. 8 depicts the UBRs of the heterogeneous networks for MBH [2], SH [8], SASHA [3] and PSH along with various packet sizes. With the increase of the size of RTP packet, the UBR increases. Fig. 8 shows that the UBR for PSH is much smaller than that for SH. This is because SH transmits duplicate multimedia content, whereas PSH splits the RTP packet into two parts for transmission. On the other hand, we can see that the UBR for PSH approximately equals to that for SASHA, so the packet retransmission has little impact on the UBR. Importantly, when several users in a vehicle perform handover at the same time, SH may cause the BSs located in the border area to become heavily loaded suddenly. In case of

PSpayload =1000, the handover events cause the UBR to be as p p Fig. 7. Packet loss rate vs. packets size ( 0= 1=0.05) high as 0.4 for the SH, which leads to the reduction of the available bandwidth for the new coming sessions. Therefore, Fig. 7 shows the comparing results for MBH [8], SASHA PSH is very useful in such a scenario because it can reduce the [3] and PSH as the RTP packet size increases. The PLRs for UBR significantly. all the four schemes increase as the RTP packet size increases. Fig. 9 presents the comparison results for MBH [2], SH[8], Although SASHA transmits multimedia content via the two SASHA [3] and PSH with the increase of p0. We can see that paths, it has the same PLR as MBH, because the FERs of the PSH has a bit higher UBR than SH and SASHA. The sub- two paths are the same. The PLR for PSH is a little higher figure inside shows that the UBR for PSH grows slowly as p0 than that for SH, because the duplicate flow transmission can becomes higher. Because the increase of the FER of the old reduce the number of lost RTP packets to some extent. The Q. Qi et al.: Soft Handover Mechanism Based on RTP Parallel Transmission for Mobile IPTV Services 2281 path results in more and more lost packets that must be [7] N. Banerjee, A. Acharya, and K. D. Sajal, “Seamless SIP-based mobility retransmitted, the UBR for PSH increases. As the scale of this for multimedia applications”, IEEE Network, vol. 20, no. 2, pp. 6-13, Mar. 2006. metric is 0.01, the bandwidth for packet retransmission in [8] S. Salsano, A. Polidoro, and C. Mingrardi, “SIP-based moblity PSH can be ignored. management in next generation networks”, IEEE Wireless Commun., vol. 15, no. 2, pp. 92-99, Apr. 2008. [9] L. H. Xu, and Sh. H. Ai, “A new feedback control strategy of video transmission based on RTP”, 1ST IEEE Conference on Industrial Electronics and Applications (ICIEA), Singapore, pp:1-4, May 2006. [10] J. Rey, D. Leon, A. Miyazaki, V. Varsa, and R. Hakenberg, “RTP retransmission payload format”, RFC4588, Jul. 2006.

BIOGRAPHIES Qi Qi was born in 1982, obtained her B.S. degree from Southwest University in 2005. She is currently working toward the Ph.D. degree at the Department of Computer Science and Technology, Beijing University of Posts and Telecommunications. Her research interests include SIP protocol, mobility management, Next Generation Network, Ubiquitous services, and multimedia Fig. 9. Utilized Bandwidth Ratio vs. p0 communication.

V.CONCLUSION Yufei Cao was born in 1974, obtained his PhD degree from Beijing University of Posts and Mobile IPTV has been becoming popular as it promises to Telecommunications in 2008. He joined the EBUPT deliver multimedia contents to users while they are moving. Information Technology Company, China, in 2008, and is Nevertheless, in the heterogeneous access networks, how to currently working as a Research Engineer. His research interests include SIP protocol, communications software, guarantee service continuity is a challenge to mobile IPTV. Next Generation Network, and IP multimedia subsystem. This paper proposes the PSH mechanism for mobile IPTV service in heterogeneous networks to deliver high quality multimedia content as well as improve the performance of the Tonghong Li was born in 1968, obtained his PhD degree from Beijing University of Posts and whole IPTV network. Without the duplicate packet Telecommunications in 1999. He is currently an assistant transmission, the PSH can realize the seamless handover by professor with the department of computer science, using RTP parallel transmission. The evaluation results Technical University of Madrid, Spain. His main research interests include resource management, distributed demonstrate that the PSH can reduce the packet loss rate and system, middleware, wireless networks, and sensor obtain a more efficient use of the scarce wireless bandwidth. networks.

Xiaomin Zhu was born in 1974, obtained his PhD degree REFERENCES from Beijing University of Posts and [1] Soohong Park and Seong-Ho Jeong, “Mobile IPTV Approaches, Telecommunications in 2001. Now he is an associate Challenges, Standards, and QoS Support”, IEEE Internet Comput., vol. professor in Beijing University of Posts and 13, no. 3, pp. 23-31,May. 2009. Telecommunications. His major is Telecommunications [2] Seil Jeon, Namhi Kang, and Younghan Kim, “Mobility Management and Information Systems. His research interests span the Based on Proxy Mobile IPv6 for Multicasting Services in Home area of intelligent networks and next-generation networks Networks”, IEEE Trans. Consum. Electron., vol. 55, no. 3, pp. 1227- with a focus on 3G core network and protocol conversion. He has published 1232, Aug. 2009. over 110 papers, among which there are more than 30 first-authored ones, in [3] B. Ciubotaru, and G. M. Muntean, “SASHA—a quality-qriented different journals and conferences. handover algorithm for multimedia content delivery to mobile users”, IEEE Trans. Broadcasting, vol. 55, no. 2, pp. 437-450, Jun. 2009. Jingyu Wang was born in 1978, obtained his PhD degree [4] W. Lee, E. Kim, J. Kim, I. Lee, and C. Lee, “Movement-Aware Vertical from Beijing University of Posts and Handover of WLAN and Mobile WiMAX for Seamless Ubiquitous Telecommunications in 2008. Now he is an assistant Access”, IEEE Trans. Consum. Electron., vol. 53, no. 4, pp. 1268-1275, professor in Beijing University of Posts and Nov. 2007. Telecommunications, China. His research interests span [5] H. S. Kim, H. M. Nam, J. Y. Jeong, S. H. Kim, and S. J. Ko, broad aspects of performance evaluation for Internet and “Measurement Based Channel-Adaptive Video Streaming for Mobile overlay network, traffic engineering, image/video coding, Devices over Mobile WiMAX”, IEEE Trans. Consum. Electron., vol. multimedia communication over wireless network. 54, no. 1, pp. 171-178, Feb. 2008. [6] J.Y. Pyun, “Context-Aware Streaming Video System for Vertical Handover over Wireless Overlay Network”, IEEE Trans. Consum. Electron., vol. 54, No. 1, pp. 71-79, Feb. 2008.