Seamless Handover for Mobile TV over DVB-H Applications

Lukasz KondradTampere University of

TechnologyTampere,Finland

lukasz.kondrad@tut.fi

Imed BouaziziNokia Research Center

Tampere,Finlandimed.bouazizi@nokia.com

Moncef GabboujTampere University of

TechnologyTampere,Finland

moncef.gabbouj@tut.fi

ABSTRACTIn this paper, we present an approach for achieving applica-tion driven seamless handover across different wireless accessnetworks. The target is thereby to eliminate any content losswhile minimizing the interruption duration that is perceivedby the user during the handover procedure. Ultimately, theuser experience will greatly benefit from the usage of themost suitable network access technology. The focus is set onMobile TV applications, where an ongoing streaming sessionexperiences an application-controlled handover from DVB-H to an alternative unicast access (e.g. Wi-Fi or 3G) or viceversa. A handover decision algorithm is also presented, aim-ing at enhancing the overall system stability by adjusting tothe dynamic conditions of the broadcast channel.

Categories and Subject DescriptorsC.2.1 [Network Architecture and Design]: Wireless com-munication

General TermsAlgorithms, Measurement, Design

KeywordsStreaming, Handover, DVB-H, Wi-Fi

1. INTRODUCTIONThe development and deployment of wireless access tech-

nologies have experienced enormous growth throughout thelast decade. Recent developments in wireless access net-works have enabled bandwidths close to or exceeding thebandwidths that users get at home through wired network-ing technologies.

As a result, a hybrid environment of wireless connectivitysolutions became available, where some provide general pur-pose data connectivity and others are tailored for specificmultimedia applications. Due to uncoordinated network

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planning, overlaps in network coverage, though through dif-ferent technologies, became very common. This has pavedthe way for a new generation of smart receivers that areable to select the most appropriate access technology amongthose available.

As a consequence, the notion of ubiquitous services be-comes extremely appealing, enabling a service to be ac-cessible through any available access technology, and en-abling receivers to choose the most suitable one, based onthe QoS, connection costs, and channel quality. However,several challenges have to be circumvented first. One of themain challenges is how to ensure seamless and interruption-free consumption of the service when a vertical handover isperformed. Yet another question is how to manage serviceinter-operability issues between different networks.

The integration of different wireless technologies has beenextensively studied by several research and standardizationefforts. For example, the 3rd Generation Partnership Project(3GPP) has extensive studies on interworking of UMTS andWLAN networks [1]. Recently, the IEEE has launched a newgroup (IEEE 802.21), which is developing standards to en-able handover and inter-operability between heterogeneousnetworks including both IEEE 802 and non-IEEE 802 net-works. In [10], the author presents a comparative analysisof alternative approaches that have been developed by stan-dardization bodies for handover between PSTN and Inter-net sessions among multiple wireless technologies includingUMTS, Wi-Fi, WiMAX, Mobile-Fi and WPANs. In thatwork Wright describes Quality-of-Service (QoS) aspects forseamless handover for different application types such asvoice, video telephony, gaming, audio/video streaming, andfile downloading.

Since each service has different QoS requirements, theyshould be considered separately. One of the most demand-ing services is Mobile TV. Salkintzis et al. in [7] describechallenges and give performance evaluation of seamless con-tinuity of real-time video across UMTS and Wi-Fi networks.They formulate a number of practical interworking scenar-ios, where UMTS subscribers with ongoing real-time videosessions handover to Wi-Fi network. Work on vertical han-dover focuses mainly on lower layer approaches. For ex-ample, Yang et al. in [11] propose and evaluate a novel softhandover scheme between DVB-H and UMTS networks thatis based on an optimum radio resource allocation algorithm.In [6], the authors address the integration of DVB-H andUMTS at the radio access level, present an interworking ar-chitecture, and discuss necessary modifications to UMTS.All above mentioned vertical handover methods strive to

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minimize the data loss during handover while maintainingthe session from the unchanged source.

In this paper, we propose an approach for enhancing ser-vice continuity during vertical handover procedures. We fo-cus on Mobile TV applications, where an ongoing streamingsession experiences an application-controlled handover frombroadcast/multicast (DVB-H or UMTS) to unicast access(e.g. Wi-Fi or UMTS) or vice versa. In contrast to the pre-viously presented handover algorithms, the approach in thispaper does not make any assumptions on the source of thestreaming session being unchanged. Finally, we also presenta handover decision algorithm, aiming at enhancing the sta-bility of the system by adapting the handover decisions tothe dynamic conditions of the broadcast channel.

This paper is organized as follows. In Section 2 motiva-tion for the new approach is given. Section 3 examines thedesign and setup of our test-bed that is used for verifyingthe seamless handover algorithm. In section 4, we describein details the proposed re-synchronization methods. Section5 presents and discusses a robust handover decision algo-rithm. The paper is concluded in section 6 with an outlookon future work.

2. STREAMING SERVICE CONTINUITYUSING SEAMLESS HANDOVERS

In this paper, we propose an application-driven handoverapproach, which allows for seamless handover between dif-ferent delivery methods(e.g. multicast and unicast delivery),independently from the underlying access networks.

3GPP has specified open standards for the delivery ofmultimedia services to mobile terminals. 3GPP MultimediaBroadcast Multicast Service (MBMS) [3] is a multicastingservice that targets mobile devices. 3GPP also standard-izes a Packet-switched Streaming Service (PSS) [2]. 3GPPPSS reuses the IETF RTSP[9] and SDP[5] standards andcan be understood as a unicast access to streaming services.Both 3GPP MBMS and PSS can be offered via GSM andUMTS cellular networks. Another mobile multimedia ser-vice is the Digital Video Broadcasting - Handheld (DVB-H)[4]. DVB-H is a broadcast system targeting mobile deviceswith limited power supply.

The seamless switch between two different delivery meth-ods, i.e. between two different streaming sessions, comesalong with some challenges. First, an accurate re-synchronizationpoint between the multimedia sessions needs to be deter-mined. This problem is described in detail in Section 4,where we also propose a method for accurate calculationof the re-synchronization point. However, even in case ofaccurate re-synchronization, viewers may still experience aninterruption time during playout of the media. The interrup-tion is caused by the delay between the initiation of tune-into the new session and resumption of the decoding and ren-dering process. For example, if Real-Time Streaming Pro-tocol (RTSP)[9] is used to establish unicast session, severalrequest/response messages need to be exchanged, which maylast for a significant period of time. Additional delay may beintroduced by incurred buffering time from the new mediasource.

Using the testbed described in Section 3, we measured in-terruption time and unicast session setup time. Interruptiontime is the time gap between the last received packet fromthe old session and the first received packet from the new

session (upon triggering of the switch procedure). This timeis typically equal to the unicast session setup time. Assum-ing the first received packet from the unicast session is thesubsequent packet to be played out, interruption time repre-sents the required buffer duration for a seamless handover.The measured interruption and setup time results from sev-eral consecutive handover events from DVB-H to Wi-Fi arepresented in Table 1.

Table 1:Interruption time [s] 2.09 2.00 1.09 0.77 1.96 1.39

Unicast setup time [s] 1.20 1.75 1.08 0.67 0.56 1.39

3. TESTBED SETUPIn order to investigate handover approaches for broadcast

services, a test-bed including a DVB-H and a Wi-Fi link hasbeen built. The architecture is presented in Figure 1. Thetransmission side consists of: a unicast streaming server,which can be accessed through a Wi-Fi access point, a mul-ticast streaming server which delivers content to the DVB-HIP encapsulator, and a DVB-H transmitter. The multicastand the unicast servers have stored identical content. Themulticast server shares its timeline information with the uni-cast server for synchronization purposes. The unicast serverfurther supports time shifting.

Figure 1: Test-bed architecture

A combined DVB-H and unicast streaming client receiverwas developed and implements different handover decisionalgorithms.

4. RE-SYNCHRONIZATIONIn this section, we propose two methods for re-synchronizing

a unicast streaming session to multicast/broadcast stream-ing session. Our focus is set on the switch between DVB-H and Wi-Fi but the proposed re-synchronization methodsare independent of the access channel and rely solely onthe implementation and deployment of the source (multi-cast/unicast) servers.

Various options are provided for the accurate indication ofa re-synchronization point in a streamed media content. There-synchronization point allows the play-out of a streamedmedia content from or at a desired point. To allow a user toswitch between an MBMS service and a PSS service, 3GPPdefined a method for re-synchronization of a PSS unicastsession to an MBMS multicast session. According to [3], aclient may request a specific start time of the PSS session

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by indicating a Coordinated Universal Time (UTC) clocktime in the ”Range” header field of the RTSP PLAY re-quest. The UTC clock time represents the requested stream-ing start point according to the timeline of the broadcastserver. Since the clock of the multicast server and that ofthe client are not necessarily synchronized, which may resultin the client receiving a media range different from what itoriginally requested, the specification proposes to calculatethe actual time by using the Network Time Protocol (NTP)timestamp of the last received Real Time Control Protocol(RTCP) Sender Report (SR).

We adopt the re-synchronization method described abovefor the DVB-H to Wi-Fi handover scenario. Additionally, toincrease the accuracy of the re-synchronization procedure,two possible improvements are introduced.

First, we propose to calculate the UTC clock time basedon information from the last received RTCP SR packet aswell as from the timestamp (TS) of the last received mediapacket. According to [8], RTCP SR, contains the 64 bitsNTP Timestamp field and the 32 bits RTP Timestamp field.The NTP Timestamp indicates the wallclock time while theRTP Timestamp field corresponds to the same time as theNTP Timestamp, but using the media clock rate and offset.Based on that timestamp, the receiver may calculate theexact NTP value of the media packet according to equations1 and 2.

∆ =RTPtssr −RTPtsl

CR(1)

NTPtsl = NTPtssr + ∆ (2)

where

∆ : shift between the last received SR NTP TS and thelast received RTP media packet NTP TSRTPtssr : RTP TS from the last received SRRTPtsl: RTP TS from the last received RTP media packetCR: Clock rate of the mediaNTPtsl: NTP TS from the last received RTP media packetNTPtssr: NTP TS from the last received SR

The re-synchronization method presented above assumesthat both the multicast server and the unicast server havestored the same content. In addition, it has been assumedthat the multicast server, is able to communicate with theunicast server, in order to share necessary information aboutthe session (e.g. the exact start time of the multicast ses-sion).

A second approach for re-synchronization is proposed toaddress the case where the unicast server and the multicastserver receive the streams from the same source (e.g. viamulticast). The unicast server stores the incoming streamsso that it is able to navigate freely within the stored streambased on the RTP packet identifiers (e.g. SSRC, RTP Times-tamp and/or Sequence Number of the packet).

At the receiver side and during a handover procedure,the receiver extracts the Timestamp and SSRC values fromthe last received RTP media packet upon deciding to switchfrom multicast to unicast delivery.

The Real Time Streaming Protocol (RTSP) specification[9], which is commonly used for controlling unicast stream-ing sessions, allows to use optional tags to send the newrange request to the streaming server. The unicast server

is then able to seek through the stored content indexed bythe RTP packet identifiers to locate the requested startingpoint. The advantage of the latter approach is due to there-usage of the same buffer at the receiver side across thedifferent delivery channels. Media packets are not dupli-cated un-necessarily during the handover procedure due tothe accurate re-synchronization. Moreover, this approachis suitable for both stored content and live event streamingwhile the first one is strictly dedicated for pre-stored con-tent.

5. HANDOVER DECISION ALGORITHMOne of the key issues that may result as a by-product

of the handover procedure is the so called ping-pong effect.It occurs as a consequence of varying network conditions,which may subsequently result in successive and frequenthandover decisions. This instability in the handover decisionalgorithm behavior will typically result in a severe degrada-tion of the watching experience that is attributable to thefrequent play-out interruptions.

In order to cope with this problem, a more sophisticatedhandover decision algorithm is presented in the sequel. Itkeeps track of the recent handover decision history, includ-ing information about time and direction of the handover,reception quality and duration. With an aim to avoid theping-pong effect, decisions to handover to the broadcast ac-cess should be limited in frequency. This is achieved byrequiring a minimal observation time (THh) after which thebroadcast channel is judged to be in good state. Duringthe required observation time, the measured packet loss rateshould not exceed a given threshold PLRg.

A key feature of the algorithm is the exponential increasein the THh threshold required for handing over to the broad-cast channel. THh is doubled if the dwell time in the broad-cast reception is shorter than a given stability threshold(THst). THh is reset to its initial value if the broadcastchannel shows sufficiently good reception quality for a longertime period THreset. This feature significantly boosts thestability of the handover decision algorithm and eliminatesany ping-pong effect.

The algorithm is described in the sequel.

TPs : period of time measuring continuous, sufficientlygood DVB-H reception (PLRm < PLRs)TPg : period of time measuring continuously good DVB-Hreception (PLRm < PLRg)THst : stability threshold: minimal period of continuousDVB-H reception necessary for declaring the DVB-Hchannel conditions as stableTHh : minimal duration of good DVB-H reception prior tohandover back to DVB-HTHreset : duration of continuous, sufficiently good DVB-Hreception necessary for resetting Th to its default valuePLRm : continuously measured packet loss rate over theDVB-H channelPLRs : packet loss rate threshold for initiating a handoverto Wi-FiPLRg : packet loss rate threshold characterizing a goodreception quality over DVB-H

To evaluate the proposed decision algorithm, it was com-pared against two other decision algorithms. In the firstone the handover decision is made based on the PLRs and

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Algorithm 1 Handover decision algorithm

1: Measure PLRm and update TPs, TPg

2: if PLRm > PLRs then3: Perform the handover to Wi-Fi4: if TPs > THst then5: if TPs > THreset then6: THh = default value7: end if8: Go to 11.9: else

10: THh = 2*THh . exponential increase11: TPs = 0 and TPg = 012: Measure PLRm and upadte TPs, TPg

13: if TPg > THh then14: Perform the handover to DVB-H15: Go to 1.16: else17: Go to 12.18: end if19: end if20: else21: Go to 1.22: end if

PLRg threshold values. In the second one, apart from thePLRs and PLRg threshold values, a condition was addedthat the handover to DVB-H can occur only if TPg is biggerthan a default value of THh.

A packet loss model was extracted from transport stream(TS) error trace files. Following parameters were used to cre-ate the TS error pattern: mode 8k, guard interval 1/4, coderate 2/3, modulation 16QAM, SNR 14 dB, and a Dopplerspectrum pedestrian -indoor at a speed 3 Km/h.

The threshold values for the algorithm were set as follows:THst = 10 seconds, default THh = 2 seconds, THreset = 20seconds, PLRs = 60%, PLRg = 30%. In figure 2, the re-sults of a 180 seconds long simulation are presented. Becauseof the bad channel condition the decision algorithm based onPLRs and PLRg threshold values resulted in 50 handovers.The second algorithm performed better thanks to the TPg

value and yielded 30 handovers. The proposed algorithmresulted in only 5 handovers and prevented the ping-pongeffect, due to the exponential increase in the THh thresholdvalue.

Figure 2: Handovers

6. CONCLUSIONIn this paper, we introduced an approach to achieve seam-

less handover between DVB-H and Wi-Fi networks. Ensur-ing seamless handover the user experience of Mobile TVservices can be significantly improved. We presented anefficient mechanism to reach re-synchronization across thebroadcast and the unicast streaming sessions. Next, we pro-posed an algorithm to increase robustness against un-stablebroadcast channel conditions. Finally, we described the ar-chitecture of the testbed and the softweare implementation.

7. REFERENCES[1] 3GPP TS 23.234. 3GPP system to Wireles Local Area

Network (WLAN) interworking; System description(Release 8).

[2] 3GPP TS 26.234. Transparent end-to-endPacket-switched Streaming Service (PSS); Protocolsand codecs(Release 7).

[3] 3GPP TS 26.346. Multimedia Broadcast/MulticatsService (MBMS); Protocols and codecs (Release 7).

[4] EN 302 304 V1.1.1. Digital Video Broadcasting(DVB): Transmission systems for handheld terminals.

[5] M. Handley and V. Jacobson. SDP: SessionDescription Protocol. RFC 2327 (Proposed Standard),1998. Obsoleted by RFC 4566, updated by RFC 3266.

[6] N.Vulic, S. de Groot, and I. Niemegeers. DVB-H -UMTS Integration at Radio Access Level. In 65thIEEE Vehicular Technology Conference, pages1250–1254, April 2007.

[7] A. Salkintzis, G. Dimitriadis, D. Skyrianoglou,N. Passas, and N. Pavlidou. Seamless continuity ofreal-time video across UMTS and WLAN networks:challenges and performance evaluation. IEEE WirelessCommunications, 12:8–18, June 2005.

[8] H. Schulzrinne, S. Casner, R. Frederick, andV. Jacobson. RTP: A Transport Protocol forReal-Time Applications. RFC 3550 (Standard), July2003.

[9] H. Schulzrinne, A. Rao, and R. Lanphier. Real TimeStreaming Protocol (RTSP). RFC 2326 (ProposedStandard), 1998.

[10] D. J. Wright. Maintaining QoS during handoveramong multiple wireless access technologies. In SixthInternational Conference on the Management ofMobile Business (ICMB 2007), pages 10–18, July2007.

[11] X. Yang and T. J. Owens. Intersystem soft handoverfor converged DVB-H and UMTS networks. IEEEWireless Communications, 57:1887–1898, May 2008.

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