Multicast Instant Channel Change in IPTV Systems
    Damodar Banodkar1, K.K. Ramakrishnan2, Shivkumar Kalyanaraman1, Alexa...
requirements are relatively independent of the population of                                with the multicast scheme make...
lower channel switching latency, and is independent of the                                   the capacity from a CO to the...
The Unicast ICC method indeed reduces the channel                                     B. Our Proposed Approach: Multicast ...
C. The first frame from the secondary ICC channel change                                 buffering. Subsequently, when the...
frames, but lower variance in sizes, unlike P- and B-frames                                scheme is lower and shows less ...
As a result, the display latency deteriorates sharply (we                              entire trace period of one hour. Ou...
for the multicast scheme is primarily to fill the playout buffer
as the multicast of the full-quality stream is transmitte...
This synthetic workload simulation shows that even                                       Figure 20 shows a similar result ...
time, the transient bandwidth requirement from such unicast-                               [9] Microsoft Corporation, Micr...
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  1. 1. Multicast Instant Channel Change in IPTV Systems Damodar Banodkar1, K.K. Ramakrishnan2, Shivkumar Kalyanaraman1, Alexandre Gerber2, Oliver Spatscheck2 1 Rensselaer Polytechnic Institute (RPI), 2AT&T Labs Research Abstract— IPTV delivers television content over an IP bandwidth and video server I/O capacity, to meet the quality infrastructure with the potential to enrich the viewing of experience goals of clients. experience of users by integrating data applications with To understand this, consider how IPTV is delivered. For the video delivery. From an engineering perspective, IPTV places purposes of this paper, we only address the use of IPTV for both significant steady state and transient demands on the delivery of linear broadcast TV. IPTV typically uses IP network bandwidth. Typical IPTV streaming techniques incur multicast to deliver the content, using a distinct multicast delays to fill the play-out buffer. But, when viewers switch or group for each TV channel. There are typically several surf channels, it is important to minimize this user-perceived hundred channels. The consumer’s set-top box “tunes” to a latency. Traditional Instant Channel Change (ICC) particular TV “channel” by joining the multicast group for that techniques reduce this latency by having a separate unicast channel. There is a channel switching latency due to the time assist channel for every user changing channels. Instead, we taken to join the multicast group and (more significantly) a propose a multicast-based approach using a secondary time to wait for an I-frame and then fill up the play-out buffer "channel change stream" in association with the multicast of for the new channel (to prevent underflow and the resulting the regular quality stream for the channel requested. During jerky/stalled playout). The total channel switching latency can channel change events, the user does a multicast join to this be several seconds, causing unacceptable quality of experience new stream and experiences smaller display latency. In the to users who may want to surf or browse channels. In contrast, background, the play-out buffer of the new full-quality with traditional over-the-air broadcasting, or cable-based multicast stream is filled. Then, the transition to the new analog systems that do not utilize a packetized video channel is complete. We show that this approach has several distribution, the set top box (STB) receives all the channels performance benefits including lower bandwidth consumption simultaneously. When the user changes channels, the STB even during flash crowds of channel changes, lower display immediately “tunes” to the new channel and begins displaying latency (50% lower), and lower variability of network & it on the screen. Hence the channel change time in such a server load. The tradeoff is a lower quality video during the system is minimal. But such systems are dramatically play-out buffering period of a few seconds. Our results are wasteful in the use of network resources. based upon both synthetic channel change arrival patterns as To minimize this delay, techniques called “Instant Channel well as traces collected from an operational IPTV Change” (ICC) have been proposed. Changing channels on environment. IPTV systems, while minimizing the perceived delay to the Keywords- Display Latency, Channel switching Latency, user, goes by a variety of names in the industry (e.g., Bandwidth, IPTV. rapid/fast/instant channel change). In this paper we will refer to this feature generically, and independent of platform or I. INTRODUCTION provider, as “Instant Channel Change” (ICC). One approach With the rapid growth of broadband deployment to homes, first sends a full-quality unicast stream of the new channel’s IP-based distribution of real-time television (IPTV) [1] is now data at a higher bit rate [9]. The higher bit rate allows the play- a reality. The term IPTV refers to an array of video services out buffer to be filled faster and the unicast also masks the (including “linear-TV”, i.e., real-time programming, and multicast join latency. But, each such unicast stream adds a video-on-demand) where the video is compressed, and transient demand to the steady-state multicast traffic. Flash transmitted to subscribers over an IP packet data network. crowds of channel changes place significant demands on A recent white paper [2] projects that consumer IP traffic access links and video server resources. These unicast channel (both Internet and non-Internet IP) will grow annually at 57%, change mechanisms are built on the expectation that ICC driven and dominated by video traffic. Therefore engineering events are mostly uncorrelated, and the impulse load on the IP networks to support the long-term growth of video traffic is system does not grow to unmanageable levels. However, when very important. Since a HD-quality channel can consume 6-15 they are correlated (e.g., during prime-time viewing where Mbps [3], and there may be hundreds of channels for a large subscribers switch at the completion of a program), there is a number of clients, IPTV places significant bandwidth burst load that imposes a significant I/O demand on video demands on the infrastructure. There are both steady state and servers sending the unicast stream and bandwidth on the links transient traffic demands. One of the sources of transient in the network. This paper is focused on these situations where bandwidth demand is from clients switching channels. The the channel change events are correlated and occur close to transient traffic demand can be significant in terms of network each other from a number of STBs in a large-scale IPTV deployment. We seek an approach where the bandwidth Authorized licensed use limited to: UNIVERSITE LOUIS PASTEUR. Downloaded on November 5, 2008 at 15:32 from IEEE Xplore. Restrictions apply.
  2. 2. requirements are relatively independent of the population of with the multicast scheme makes provisioning decisions much users switching to a channel or the arrival process of the simpler. The tradeoff in our proposed multicast scheme is the channel change events. use of a lower quality (just I-frames) video displayed during We propose a multicast-based approach using a secondary the play-out buffering period (between point B and point C in "channel change stream" associated with each channel. This Figure 1). We believe that this is a tradeoff worth making to secondary stream is a lower bit rate stream carrying only I- deliver the quick response to clients (they don’t have to see a frames and associated audio. During a channel change event, blank/black image till point C in Figure 1) and the benefit of the client does a multicast join to this secondary stream and reduced bandwidth requirements on the infrastructure. results in a low “channel display latency” (between A and B The rest of the paper is organized as follows: Section II in Figure 1). In the background, the play-out buffer of the new overviews related unicast instant channel change (ICC) full-quality multicast stream is filled. Once the playout point is schemes in IPTV systems. We then provide an overview of the reached, the full-quality video is displayed and the transition is network infrastructure. Section IV outlines the unicast ICC complete (at point C in Figure 1). The time interval from when scheme and develops our Multicast ICC scheme. Section V the user requests the channel change to this completion point provides performance analysis including trace-driven NS-2 C is the “channel switching latency”. simulations to validate the bandwidth requirements and channel change latencies for the two ICC schemes. Finally, Channel Change First frame Displayed Stream starts playing out of section VI concludes the paper. on TV screen the data buffered from Issued primary multicast stream II. RELATED WORK time There have been different approaches to reduce the delay associated with each part of the channel change process. Cho Display Channel switching Latency High Picture et al. [4] propose an approach that is based on the idea that the Latency quality video adjacent channels are going to be watched more frequently by viewers. Their scheme maintains a table that keeps track of adjacent channel requests, and adds currently inactive adjacent channels to the table and joins them while a join is in progress for the new channel requested. One vendor’s approach [8] A B C improves the above scheme by adding a statistical selection of Figure 1: Display Latency vs. Channel Switching Latency channels to join instead of choosing the adjacent channels. These approaches address the issue of channel switching The secondary channel change stream carrying I-frames is latency by issuing joins to different multicast groups, but do likely to use about 50% of the bandwidth compared to the full- not address playout buffering latencies. An analytical study quality video (we present an example later in this paper). The [8]shows that the user channel change requests are not a drawback therefore is the 50% additional capacity required steady flow and tend to double at commercial breaks during the channel switching period on the access network to disrupting the steady state of the system. the home. However, as we describe below, there is likely to be Another commercial approach [9] is to create a unicast sufficient capacity on the access link to the home to carry this stream sent at an accelerated rate so that the play-out buffer channel change stream. Our proposed scheme also requires threshold is reached quickly. When the user requests a channel additional I/O capacity from the video server (up to 50% for change, the STB waits for the unicast stream to fill up to a each channel that has at least one channel change in progress). higher threshold in the playout buffer, and then begins However, unlike the unicast ICC which requires a full-rate playout. After the buffer is filled to the higher threshold, a join stream at an accelerated rate for every single user (of which request is issued to the multicast stream. By the time buffer there may be thousands if there is a correlated channel change drains to the normal playout threshold, the multicast join will event), the multicast approach uses a lower rate I-frame stream complete and the multicast stream continues to deliver the and is relatively independent of the user population requesting video to the client. The unicast session starts the content a channel change. Moreover, we will show through traces that staggered in time, so that the handoff to the new multicast the maximum simultaneous number of channels experiencing session is seamless. As discussed earlier, such unicast streams a channel change event at any time in an operational add a transient demand to the steady-state multicast traffic, environment is small compared to the total number of active proportional to the number of users concurrently initiating a channels. channel change event. Flash crowds of channel changes On the other hand, our multicast approach has several potentially place excessive demands on access links and video performance benefits including lower bandwidth consumption server resources. during a burst (flash crowds) of channel changes (the obvious To summarize some key issues with the existing advantage of using multicast), lower display latency (50% proposals: 1) the schemes may not scale well when there are lower), and lower variability of network & server load. For a many channel change requests over short periods (e.g., during low intensity of channel change events, the reduction in the commercial breaks in popular TV events/shows) and 2) when display latency with the multicast approach may not be users surf between non-contiguous channels. Our multicast significant. But, even with a modest rate of channel changes, ICC scheme addresses both scalability and stability of the the need to use separate unicast assist sessions leads to system even during flash crowds while meeting the user consistently higher bandwidth requirements than our multicast expectation for rapid channel change. Our scheme achieves scheme. The predictable load on the links and video servers Authorized licensed use limited to: UNIVERSITE LOUIS PASTEUR. Downloaded on November 5, 2008 at 15:32 from IEEE Xplore. Restrictions apply.
  3. 3. lower channel switching latency, and is independent of the the capacity from a CO to the DSLAM may be around 1 Gbps. increase in the number of channel changes issued by users. The DSL link itself may also be a bottleneck, which the service provider avoids by provisioning and arranging with the III. NETWORK ARCHITECTURE FOR IPTV SERVICE subscriber so that the total demand (e.g., number of TVs Figure 2 shows the network architecture and system viewing distinct channels in the home) does not exceed the components for IPTV systems. The distribution network DSL link capacity. The link from the CO to the DSLAM (a consists of video servers for each metropolitan area connected DSLAM may support 100-200 homes) may also be a through the metro network of IP routers and optical networks bottleneck in certain situations. Finally, the number of parallel to the access network. The access network typically includes a streams carried out of a D-Server may be limited by its I/O tree-like distribution network (whether it is a DSL or cable capacity. We will evaluate the impact of high utilization due environment) with copper (existing environments) or fiber to channel change events on these potential bottleneck links (newer environments) connectivity to the home. In an example and their effects on user quality of experience (QoE) as nationwide distribution environment, content is received from evaluated by the channel display and switching latencies. the point where it is acquired over an IP backbone to the IPTV head-end for the metropolitan area at a Video Hub Office IV. INSTANT CHANNEL CHANGE (ICC) IN IPTV SYSTEMS (VHO). From this point, the video is distributed to subscriber A. Current Approaches for ICC homes in the metro area. The architecture comprises: ! Content sources & D-Server: Video content is We refer to the existing approaches for fast channel received from content providers either live or from change (ICC) as “Unicast Instant Channel Change” or storage (for Video on Demand). The content is buffered at “Unicast ICC” and focus on one baseline approach. As shown Distribution Servers (D-Server) in the Video hub office in Figure 3, when a client connects to the D-server, the D- (VHO). A separate D-server could be used for every server begins unicasting data to the client, starting from an I- channel; all D-servers share the link to the VHO, and on frame in its buffer. The D-Server bursts the data at a higher average a single D-server’s I/O capacity is of the order of rate than the nominal video bitrate rate. ~100s of Mbps. ! Metro Network: The metro network connects the VHO to a number of central offices (CO). The metro network is usually an optical network with significant capacity. Downstream of the CO are several Digital Subscriber Line Access Multiplexers (DSLAMs). SHO S DServer 1 Gbps RG S 10-100 Gbps DServer Central 30Mbps Router Office DSLAM 2Mbps S Figure 3: Unicast ICC scheme: uses accelerated unicast streams to ~100s of Mbps fill the playout buffer, thus reducing the wait at the STB. DServer I/O B/w RG Metro Network S Given this higher unicast rate, the set-top box (STB) buffer fills up faster than the nominal rate at which the Video Hub Office Access Network multicast stream is transmitted. A multicast “join” is issued by the client after sufficient data is buffered in the playout buffer Figure 2: IPTV network Architecture & Components of the STB so that the buffer does not under-run by the time ! Customer access link: Delivery of IPTV over the the multicast join completes and the subsequent multicast “last mile” may be provided over existing loop plant to stream is received. The unicast stream is stopped once the homes using higher-speed DSL technologies such as playout buffer is filled to the desired level. Once the multicast VDSL2. Service providers may use a combination of join is successful, the client can start displaying video received Fiber-to-the Node (FTTN) and DSL technologies or from the multicast stream. implement direct Fiber-to-the-Home (FTTH) access. Figure 5 shows the timeline of the sequence events and ! Customer premises equipment (CPE): CPE buffer dynamics with the Unicast ICC scheme. When a includes the broadband network termination (B-NT) and a channel change is issued, a unicast join is issued to the D- Residential gateway (RG). The RG is typically an IP Server (1). At point 2, the client starts receiving packets from router and serves as the demarcation point between the the unicast stream, and buffers the packets up to the play-out service provider and the home network. point. At point 3, the full quality video is displayed on the ! IPTV Client: The IPTV client (e.g., Set top Box screen starting with the first I-frame. At point 4, the client (STB)) terminates IPTV traffic at the costumer premises. issues a join to the multicast group corresponding to the channel. At point 5, the client starts receiving packets from the Link capacities vary significantly in the architecture. multicast stream. Any gaps between the two streams are While capacity between the Internet routers may be 10 Gbps, reconciled by having the D-Server retransmit packets corresponding to the gap. Authorized licensed use limited to: UNIVERSITE LOUIS PASTEUR. Downloaded on November 5, 2008 at 15:32 from IEEE Xplore. Restrictions apply.
  4. 4. The Unicast ICC method indeed reduces the channel B. Our Proposed Approach: Multicast ICC change time. However, it is designed with the expectation that We propose a multicast-based Instant Channel Change the number of concurrent ICC requests is small. When there (Multicast ICC) mechanism that reduces the channel switching are a number of concurrent ICC requests (e.g., during prime latency, especially under high loads of channel change events. time TV viewing periods, when requests from multiple users The approach makes the system scale better as the user are correlated) this approach can impose a substantial load on population grows. The bandwidth demand is more predictable. the network. We have observed in practice. Moreover, the D- servers with limited I/O capacity also have to support this 1) Motivation: large number of unicast streams simultaneously, potentially The network topology from the D-Server to the DSLAM requiring the service provider to deploy additional servers to is shared by all the users. Unicasting the same stream for a meet the peak demand. given channel is wasteful use of network resources. When a user “channel surfs” or changes channels rapidly, she intends to catch a glimpse of the channel that is being played and then decides whether to continue to watch the channel being displayed currently or to move on to another channel. We believe it is sufficient for the user to briefly (for 1-2 seconds) see a lower quality, less-than-full-motion video with good quality sound, and that it is better than displaying a blank screen for the corresponding period (as in some satellite TV systems) when a channel change occurs. In the underlying network, there are bandwidth constraints on the links from the DSLAM to the CO; hence it is desirable to limit the number of concurrent streams delivered to a particular DSLAM. Similar limits also exist for the number of parallel streams delivered out of the D-Server. Multicast is a natural fit to address these performance Figure 4: Characteristics of concurrent channel change events bottlenecks. As we noted earlier, examining the number of (measured over 1 second intervals) from a 7-day trace. channels receiving concurrent channel change events, from an We examined the trace of channel change events over a 7 operational IPTV system. We find that a relatively small day time period in a portion of the metro network that proportion (less than 25%) of the total number of channels supported over 5000 users in an operational IPTV system. receive change requests when measured in 1 second bins, Figure 4 shows the cumulative distribution of the number of where the request was initiated a few seconds earlier, as ICC requests and the number of unique channels that were shown in Figure 4. delivered by the ICC measured over 1 second intervals (the request may have been initiated a couple of seconds earlier). 2) Multicast ICC The peak number of distinct channels experiencing channel We introduce a secondary lower-bandwidth channel- change, measured in the 1 second bins saw a peak of 175, change stream corresponding to each channel at the D-server. which was less than 25% of the number of distinct channels This stream will consist of I-frames only Therefore, each observed over the trace period. On the other hand, the channel will now add another IP multicast group called the maximum number of ICC events measured in the 1 second secondary ICC multicast group that transmits only the I- intervals saw a peak of 326 requests. Thus, even in this small frames for each channel. The secondary ICC channel change sample set, we see that the unicast ICC has to support the stream is offset by a short time interval (approximately 2.6 additional 326 unicast (faster than full bit rate) streams. The seconds in our experiments) so that the secondary stream is multicast ICC has to support the additional 175 I-frame (at delayed in time relative to the primary high quality multicast about 50% bit rate) channel change streams. This is more than stream. This enables the client to display the (less than full a factor of two improvement compared to unicast ICC. motion) video of the new channel that the user switches to, while allowing the play-out buffer to be filled with the Additional data buffered primary multicast stream. Figure 6 shows the mechanism of 1. A unicast 5. Starts buffering the Multicast Instant Channel change process: stream “start” the Multicast A. The channel change request for a particular channel signal is issued Stream to the D-Server results in a multicast join request being issued by the STB client. A join is issued for both the primary multicast STB STB Network STB Network STB stream as well as the secondary ICC multicast group for Network Join Delay the channel change stream obtained by extracting the I- Delay 4. Join is issued for the frames from the primary stream. With IP multicast, the 1. A channel multicast channel change is join progresses as far up the distribution tree as Stream and “Stop” for signaled by 2. The first packet the multicast stream is necessary, until it hits an “on-tree” node. the user from the issued B. The D-server transmits both the secondary channel accelerated change stream as well as the primary multicast channel Unicast stream if there is an outstanding channel change event for that Figure 5: Timeline of events in Unicast ICC. channel. Authorized licensed use limited to: UNIVERSITE LOUIS PASTEUR. Downloaded on November 5, 2008 at 15:32 from IEEE Xplore. Restrictions apply.
  5. 5. C. The first frame from the secondary ICC channel change buffering. Subsequently, when the playout buffer reaches the stream is displayed on the screen by the client, without playout threshold, it can start playing the video. In contrast, an buffering, along with the audio, resulting in what we I-frame only stream can play out an I-frame once it is received believe is an acceptable viewing experience, even in its entirety. If the buffer then runs out, the current I-frame is though it is not necessarily full-motion. displayed as a freeze frame until the next I-frame arrives. D. In the mean-time the higher quality, primary multicast stream fills the buffer. V. SIMULATION RESULTS E. Once the buffer is filled up to the playout point, the We built an NS-2 [10] simulation of the metro/access client can start displaying the high quality picture from network and the VHO servers. Our objective is to evaluate the the playout buffer. Because of the time-offset between unicast and multicast instant channel change (ICC) schemes in the secondary ICC channel change stream and the terms of the following metrics: bandwidth consumption at the primary stream, when the buffer meets its threshold, the D-servers and the bottleneck link, the display latency and the client (STB) can seamlessly switch to the buffered channel switching latency. For the scenarios we considered, (primary) input without a user-perceived time-shift of the link between the central office (CO) and the DSLAM (see the video. Figure 2) and the D-server I/O were the bottlenecks. We 3. The first frame present results based on both traces collected from an from the I-Frame operational IPTV environment and synthetic channel change stream is arrival patterns. displayed on the 1. “join” screen Our trace-driven simulation uses data from channel change logs in an operational IPTV system during the prime time Multicast 2. a) I-Frame viewing period (8pm-9pm) of a given day. First, we used a Replicator stream flow TV 4. Buffer the primary Client multicast stream up trace of the channel change requests initiated by users served 2. b) Primary Multicast to the playout point by a single CO for a time period of 150 sec to simulate the of the buffer. stream flow effects on the link between the CO and DSLAM. Second, we 5. Issue a leave to selected the most popular channel from the actual trace and the I-Frame simulated the channel change requests handled by a particular stream and D-Server for a period of 1600 sec. Third, we investigate the start playing out of the buffer behavior of unicast and multicast ICC under low loads, with respect to the channel change events. Fourth, we show a stress Figure 6: Steps of the Multicast ICC Scheme test based upon a synthetic workload that has a higher intensity of channel change events. The video for our trace-driven simulations was from a Advantages & Tradeoffs with Multicast ICC library of traces of encoded video [6][7]. These traces were We observe that our proposed multicast ICC approach generated by encoding ~60 minute long videos. The video requires approximately 50% additional capacity for each codec used for the streams was H.264/MPEG-4 AVC channel that has a channel change event to carry the secondary (Advanced Video Coding), from the ITU-T Video Coding channel change stream. This requirement is both on the D- Experts Group (VCEG) and the ISO/IEC Moving Picture server I/O and the network links. However, the capacity Experts Group (MPEG), a.k.a. the Joint Video Team (JVT). requirement is relatively independent of, and does NOT grow with, the user population requesting a channel change. Moreover, we showed from traces of channel change events that the maximum simultaneous number of channels experiencing a channel change event at any time on an operational environment is small. In comparison to the unicast ICC, with our multicast ICC scheme, both the D-server and network require lower capacity during periods of high channel change rates. The channel change processing is more scalable, the D-server I/O capacity requirement are more predictable even with bursts of channel change requests. Unlike the unicast ICC approach, our proposed multicast ICC scheme can more easily deal with correlated channel change events from users. The tradeoff in our proposed multicast scheme is a lower quality (less than full motion) video that is displayed during Figure 7: Comparison of I-frame (secondary, multicast) the period that the playout buffer has to be filled (~2-3 stream vs. Full-rate stream (with I, P, B frames). I-frame seconds). We believe that the STB can play out I-frames as stream has a lower mean (~1Mbps) and lower variance. they come without any buffering to prevent under-runs. On the other hand, the full quality MPEG-4 video stream is much Some sample statistics of the video frame sizes (mean, max, more susceptible to buffer under-runs as players have min) in bytes are: I-frames (16011, 36027, 1162) bytes, P- difficulty in adjusting to gaps in the stream. The player will frames (6587, 32754, 60) bytes, B-frames: (2133, 19013, 16) have to wait for the next I-frame and only then re-start bytes. The frame statistics indicate a larger per-frame size of I- Authorized licensed use limited to: UNIVERSITE LOUIS PASTEUR. Downloaded on November 5, 2008 at 15:32 from IEEE Xplore. Restrictions apply.
  6. 6. frames, but lower variance in sizes, unlike P- and B-frames scheme is lower and shows less variability even with the which display high variance. Moreover, the I-frame stream increase in the number of channel changes (for all channels). averages 1Mbps, compared to 3.2 Mbps for the full-quality stream. The time-dynamics of the I-frame secondary stream Number of Channel changes vs. the full-rate stream are shown in Figure 7. Clearly, the I- frame stream has substantially lower mean and variance compared to the full-rate stream. The accelerated unicast stream would thus require a higher average rate (i.e., potentially even greater than 3-4 times the I-frame rate). Also recall that this accelerated stream is unicast to each receiver compared to the multicast of the I-frame stream in our proposal. Figure 8 shows the time series for the channel change events from the trace of an operational IPTV system collected over a 24-hour period. This period was chosen to include the peak concurrent channel change events observed over the 7- Time (sec) day trace whose statistics were shown in Figure 4. This figure Figure 9: Empirical channel change request distribution for a shows the maximum number of ICC requests per second, single Central Office (CO): 150 second trace. examined over 5 minute intervals. There is considerable variability and burstiness in the channel change requests (almost an order of magnitude). With a unicast ICC mechanism will result in a corresponding burstiness in the traffic emanating from the D-server and flowing over the metro and access networks. Figure 10: Bandwidth consumption (CO-DSLAM link): Unicast ICC requires more bandwidth especially at peak loads. Figure 11 shows the display latencies for this trace-driven simulation. With Unicast ICC, the CO-DSLAM link (with reduced capacity to reflect the limited scale of our simulation) Figure 8: Channel Change events (in requests/sec) examined saturates during peak channel change periods. over 5 minute intervals for an entire day A. Bottleneck link from Central Office (CO) to DSLAM In the actual channel change request trace, there were a few hundred distinct channels being transmitted. However, on an average there were far fewer channels active below a given DSLAM. To deal with NS-2 simulation constraints, we restricted the number of channels at the DSLAM to 10, and scaled down the link capacity from the DSLAM to CO to 200 Mbps. The trace-driven simulation was run for 150 seconds. The empirical distribution of the channel change requests across all channels initiated from all users below the CO is shown in Figure 9. Figure 10 shows the bandwidth consumption of the unicast and multicast ICC schemes as a function of time. Observe that the unicast ICC scheme interestingly has a higher average bandwidth requirement, and has higher volatility (especially Figure 11: Display Latency: Multicast ICC vs. Unicast ICC during the peak periods of channel change (between 50-90 for channel changes under a single Central Office (CO) seconds). The bandwidth requirement for our multicast ICC Authorized licensed use limited to: UNIVERSITE LOUIS PASTEUR. Downloaded on November 5, 2008 at 15:32 from IEEE Xplore. Restrictions apply.
  7. 7. As a result, the display latency deteriorates sharply (we entire trace period of one hour. Our simulation used the assume that network buffering is sufficient to deal with the channel change event trace for the time period from 800 to increased traffic and there is no loss). At a low intensity of 2400 sec (see Figure 13). The channel change rate has a peak channel changes, the Unicast ICC display latency stays at around 1900 sec. constant at around 400msec. On the other hand, the Multicast We set the nominal capacity of the D-server I/O to 50 ICC scheme in general has a lower the display latency that Mbps, based on the constraints of the NS-2 simulation as well varies between 0 to 480 msec (due to variability in the arrival as the consideration that only a portion of the physical D- time for the I-frame, relative to the channel change event). server capacity would be available for supporting one of the Recall the two measures of latency that we examine – the channels (across a line-up of hundreds of channels). “display latency” and “switching latency” (Figure 1). In terms of the channel switching latency (i.e., when the switchover to the multicast video stream occurs), the unicast ICC scheme performs a little better (see Figure 12). This is a tradeoff with our multicast ICC approach (but the average difference is only 0.5 seconds) which we believe is tolerable since the user will be seeing a lower quality video in the interim. The spike in the unicast scheme’s channel switching latency again reflects the degradation that occurs during peak periods of channel change requests. Figure 14: D-server I/O capacity requirements: Unicast ICC again requires much more capacity than Multicast ICC. Figure 14 shows the I/O capacity requirements at the D- server. The Unicast ICC scheme consumes significantly more capacity than Multicast ICC both in the steady state and during peak times. Multicast ICC requirements are also stable even during peak channel change times. Figure 15 shows consistent performance benefits of Multicast ICC over Unicast ICC in terms of display latency. Also, this metric is not affected by aggregate number of channel changes initiated by the total user population. Figure 12: Channel Switching Latency (i.e. final switchover to the multicast video session). B. Bottleneck: D-Server I/O In this scenario the channels change requests for the most popular channel at a D-Server are collected (Figure 13). Figure 15: Display latency (popular channel case): Multicast ICC consistently outperforms Unicast ICC The channel switching latency (recall from Figure 14) is Figure 13: The number of incoming channel change events plotted in Figure 16. The behavior is similar to the prior for the most popular channel at the D-Server results where the unicast scheme performs better, subject to spikes during overloads. As mentioned earlier, since a lower The key bottleneck we examine here is the D-server I/O quality, less-than-full-motion video is played out in the capacity. The popularity of a channel is defined by the largest multicast ICC scheme we think this additional channel number of users changing to the particular channel during the switching latency will be acceptable. The additional latency Authorized licensed use limited to: UNIVERSITE LOUIS PASTEUR. Downloaded on November 5, 2008 at 15:32 from IEEE Xplore. Restrictions apply.
  8. 8. for the multicast scheme is primarily to fill the playout buffer as the multicast of the full-quality stream is transmitted at the regular playout rate. Figure 18: Display Latency with rare channel change requests Figure 16: Channel Switching Latency: Unicast ICC is better than Multicast ICC, subject to overload spikes. The trace-driven simulation results demonstrate the bandwidth requirements and the latencies involved in the Unicast and Multicast ICC. To examine the behavior of the systems under extreme conditions we subject the mechanism to synthetic channel change workload intensity to show that the results based upon synthetic workloads are consistent with the trace-driven simulations. C. Synthetic Workload: Rare Channel Changes In this scenario, we simulate a channel change process in which the network is almost free of any transient channel Figure 19: Bandwidth Requirement on DSLAM- CO link. changes and hence does not excessively load the network. Our Despite small number of channel changes, the multicast ICC first topology has 150 nodes per DSLAM, with 5 channels has comparable bandwidth costs to unicast ICC. involved in relatively rare channel changes (an average of a Figure 19 shows that despite rare channel changes (less single request every 12 seconds per DSLAM). than half the number of channels at any time), the additional demand on bandwidth from the secondary channel change multicast stream is no more than the unicast ICC bandwidth requirement. This is because the secondary stream operates at a lower rate, while the unicast ICC scheme uses an accelerated rate compared to full-quality bit rate for the video. Scheme Display Latency (sec) CO - DSLAM link bandwidth (Mbps) Peak Avg. Std. 95 % Peak Avg. Std. 95 % Dev C.I. Dev C.I. Unicast 0.8 0.43 0.01 0.42- 100 52.5 1.2 51.46- ICC 0.44 53.35 Multicast 0.7 0.26 0.01 0.25- 95 51.2 0.1 51.1- ICC 0.27 51.2 Figure 17: Rare channel changes: Distribution of Requests Table I: Summary statistics across 10 runs for the rare channel case with different, random seeds. Note in all cases Figure 17 shows the distribution of the channel change the standard deviation is small compared to the mean, and requests: observe that the number of concurrent channel hence the confidence intervals are tight. changes (<= 3 changes) tends to be smaller than the number of channels (5 channels). Figure 18 shows that the display latency of multicast ICC is still better than unicast ICC. Authorized licensed use limited to: UNIVERSITE LOUIS PASTEUR. Downloaded on November 5, 2008 at 15:32 from IEEE Xplore. Restrictions apply.
  9. 9. This synthetic workload simulation shows that even Figure 20 shows a similar result as shown earlier (Figure though the multicast ICC scheme uses a secondary lower 14) – the unicast ICC imposes significant requirements on the quality channel change stream per channel that is multicast, average I/O capacity of the D-server, and also results in (the bit-rate of this channel change stream is lower than the increased variability of the load on the D-server (compared to regular high-quality stream (less than half)), the overall Figure 14). bandwidth requirements for the multicast ICC scheme (both in Figure 21 shows the display latency during peak loads terms of the D-server load and the metro access network from increasing significantly to about 6-7 seconds. The multicast the CO down to the DSLAMs) do not exceed the unicast ICC ICC scheme displays remarkable stability both in terms of scheme even for low channel change workloads. display latency and the bandwidth requirements. Table I summarizes the statistics for the Display latency and the Bandwidth comparison for the two schemes, across 10 Scheme Display Latency (sec) D-Server output B/w different simulation runs. The multicast ICC has comparable (Mbps) bandwidth requirements as the unicast ICC case, even with a Peak Avg. Std. 95 % Peak Avg. Std. 95 % small number of channel changes. Dev C.I. Dev C.I. D. Synthetic Workload: Popular Channel, Stress Test Case Unicast 6.79 0.6 0.04 0.58- 50 23.64 4.3 20.57- The workload in this simulation is a more intense version ICC 0.62 26.71 of the corresponding trace driven simulation for the most popular channel as discussed in subsection B. It helps us to Multicast 1.25 0.26 0.02 0.25- 16 7.76 0.4 7.48- understand the impact of rapid channel changes when users ICC 0.27 8.04 switch to a given popular channel (e.g., switching to a channel at the start of a live event or to an adjacent channel during an Table II: Summary statistics across 10 simulations for the advertisement). We assume that about 25% of the viewers stress test case (different random seeds). Standard deviation is served by a DSLAM tune to this popular channel within a time small compared to the mean, but higher for the Unicast ICC. period of 120 seconds. We first focus on D-Server I/O capacity requirement. This second synthetic workload simulation shows that as the system is stressed, the unicast ICC scheme incurs much higher display latency, and places much higher D-Server I/O capacity demands during times of peak loads compared to the multicast ICC scheme. One effect seen more clearly for the unicast ICC scheme here (going beyond the results seen with the trace driven simulations) is that until bandwidth saturation occurs, the user-experienced display latency is not severely affected. But, after this, the display latency can severely degrade. Further, there is a significant bandwidth demand with the unicast ICC scheme. Table II provides the summary statistics for 10 different simulation runs of the two schemes with the synthetic workload. With the increased rate of channel changes, the Unicast ICC consumes about 3 times the D-server output bandwidth as the Multicast ICC. The peak display latency with unicast ICC is also much higher, just as we observed Figure 20: D-server I/O Capacity Requirement: Popular earlier. The variability for both bandwidth demand and display Channel Stress Test. Unicast ICC is more volatile and latency with Multicast ICC is lower than with Unicast ICC. saturates the link at peak loads VI. SUMMARY AND CONCLUSIONS As IPTV usage grows, viewers will have novel interfaces to interact with TV. One result of this is the continued propensity of viewers to switch or surf channels. With IP multicast, and especially due to the jitter arising from protocols that have to recover from packet loss and the possible delay variation in the IP network need there is the need to have a playout buffer at the receiver (STB) to prevent underflows. This leads to latencies with channel switching and the consequent impact on the user quality of experience. Previous instant channel change (ICC) schemes use unicast assist streams that transmit at an accelerated rate to reduce the channel switching latency. However, they come at the cost of additional bandwidth load on the server I/O subsystems and the network. Our analysis Figure 21: Display Latency (popular channel stress test) shows that even for average channel switching loads in prime Authorized licensed use limited to: UNIVERSITE LOUIS PASTEUR. Downloaded on November 5, 2008 at 15:32 from IEEE Xplore. Restrictions apply.
  10. 10. time, the transient bandwidth requirement from such unicast- [9] Microsoft Corporation, Microsoft TV: IPTV Edition, based channel change schemes can be significant. Bursts in channel switching load (from correlated channel changes [10] NS-2 Simulator, (URL: across users) increase the utilization on the infrastructure further and the queuing effects can lead to longer display latencies. In this paper, we introduced a pure multicast ICC approach that involves the use of a secondary lower quality (I-frame) channel change stream for each channel. Users issue multicast joins to this stream on a channel change event. This aspect of the scheme does not require pre-buffering since I-frames can be displayed without the concern related to buffer under-runs. Concurrently, a join to the full-rate multicast stream is also issued. With a modest number of channel changes, this scheme outperforms the unicast on all the key metrics of display latency and bandwidth requirements. The tradeoff is a lower quality (less than full-motion) video being displayed during the play-out buffering period lasting a small number of seconds (between point B and point C in Figure 1). We believe that this is a tradeoff worth making to deliver the quick response to clients (they don’t have to see a blank/black image till the point that the playout buffer is filled to the threshold (point C in Figure 1). The scheme results in a significant reduction in the bandwidth requirement for the infrastructure. ACKNOWLEDGEMENTS We gratefully acknowledge the help, input and suggestions we received from Amy Reibman and Chris Chase of AT&T and John Woods of RPI. REFERENCES [1] IPTV, Wikipedia, (URL: ) [2] Cisco Systems, “The Exabyte Era,” White paper, (URL: s537/c654/cdccont_0900aecd806a81a7.pdf), 2007. [3] Cisco Systems, “Optimizing Video Transport in Your IP Triple Play Network”, White Paper , 2006 (URL: ) [4] Chunglae Cho, Intak Han, Yongil Jun and Hyeongho Lee, Improvement of Channel Zapping time in IPTV Services using the Adjacent Groups Join Leave Method, International Conference on Advanced Communication Technology, 2004. [5] Donald E. Smith, IPTV Bandwidth Demand: Multicast and Channel Surfing, Proceedings of IEEE INFOCOM 2007, Anchorage, AK, May 2007 [6] Geert Van der Auwera and Prasanth T. David and Martin Reisslein "Traffic Characteristics of H.264/AVC Variable Bit Rate Video" Technical Report, Arizona State University, March 2007 [7] Geert Van der Auwera and Prasanth T. David and Martin Reisslein "Traffic and Quality Characterization of Single- Layer Video Streams Encoded with the H.264/MPEG-4 Advanced Video Coding Standard and Scalable Video Coding Extension" Technical Report Arizona State University, July 2007 [8] Lucent Technologies, Optimization of IPTV Multicast Traffic Transport over Next Generation Metro Networks, Telecommunications Network Strategy and Planning Symposium, 2006 Authorized licensed use limited to: UNIVERSITE LOUIS PASTEUR. Downloaded on November 5, 2008 at 15:32 from IEEE Xplore. Restrictions apply.