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Analysis of SK Telecom's Mobile IPTV Live Streaming (Full HD, HD and SD over LTE-A)
September 24, 2013 | By Netmanias (tech@netmanias.com)
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SUMMARY
This report will provide a technical overview of the streaming method used in SKT's Btv Mobile service. Out of over 70 live TV channels offered by SKT's Btv Mobile service, we selected the five most popular channels available, KBS1, KBS2, EBS, TvN and MLB.

SKT, the Korean No. 1 telecom operator, launched Btv Mobile service (in SD/HD) offering a variety of live broadcasting and VoD contents for mobile devices in around July 2012. And a year later in July 2013, boosted by the commercialization of LTE-A, it began a "Full HD" streaming service with drastically enhanced video quality. The following is the press release by SKT about its new full HD service.     

 

 [June 26, 2013, SK Telecom press release] 
With the commercialization of LTE-Advanced, SK Telecom plans to promote ultra-high quality, large volume and multi-channel services including ultra-high quality broadcasting/video contents, multi-networking services, etc... SK Telecom’s ‘Btv mobile,’ an IPTV service with 550,000 paid subscribers, will begin providing full HD (1080p resolution) video streaming service, for the first time in the world, from early July. Full HD video streaming requires a speed of 4 Mbps or above, which is well supported by the LTE-Advanced network. 
※ Required speed (bit rate) for each level of video quality - SD: 1 Mbps, HD: 2 Mbps and Full HD: 4~8 Mbps
 

 

This report will provide a technical overview of the streaming method used in SKT's Btv Mobile service. 

Out of over 70 live TV channels offered by SKT's Btv Mobile service, we selected the five most popular channels available, KBS1, KBS2, EBS, TvN and MLB (MBC and SBS were not available through the service at the time our tests were carried out), and analyzed their streaming as follows: 

 

The Btv Mobile service offered the following 3 video quality levels (full HD, HD and SD):

 

 

So, users can manually choose a quality level they want, SD (low quality), HD (high quality) or full HD (ultra-high quality) when watch a video. Though in 3 different levels, the quality of the video delivered was very satisfying in all levels. 

 

The figure below is a screenshot of MLB played back in full HD. 
 

SKT's Btv Mobile service supporting 3 quality levels (Full HD, HD, SD)

 

1. Analysis of Streaming in SKT's Btv Mobile Service based on our Actual Measurements: Summary

 

We performed an actual measurement of video streaming used in the 3 quality levels (Full HD, HD and SD) offered by SKT’s Btv Mobile service. The test environment in the measurement and the analysis result are as follows:

1.1 Test environment in the actual measurement

  • Internet service: SKT LTE-A service
  • Video service: SKT Btv Mobile service/live broadcasting/KBS1, KBS2, EBS, TvN and MLB channels
  • Device: Samsung Galaxy S4 LTE-A phone 

1.2 Analysis result summary 

 

1) Encoding rates 

* Our actual measurement found little difference between the bit rates marked in the profiles of the playlist (manifest) file and the actual encoding rates measured (For instance, a chunk file with a profile of BANDWIDTH=4096000 in the playlist had an encoding rate of around 4 Mbps when downloaded). 

 
2) SKT Btv Mobile streaming architecture (e.g. KBS2)
 
 

3) Video delivery protocol

  • SKT Btv Mobile service provides 3 quality modes (full HD, HD and SD). Their encoding rates were about 4 Mbps, 2 Mbps and 1 Mbps, respectively and the codec for all three was H.264.
  • HTTP Live Streaming (HLS), an Apple’s Adaptive Bit-Rate (ABR) streaming protocol, was used for all the three modes.
  • The chunk duration was 4 seconds in all three modes.
  • Users can manually switch the video quality into a full HD, SD or HD mode while watching.
  • No bit rate switching among full HD, SD or HD observed.
  • None of the 5 channels allowed bit rate switching while streaming in full HD and HD modes, and therefore a video was played only at the quality level initially selected (No Adaptive Streaming).
  • In case of KBS1, KBS2, EBS and TvN, 3 profiles (1 Mbps, 500 Kbps and 64 Kbps) are provided in an SD mode, allowing automatic bit rate switching depending on the network conditions while watching (Adaptive Streaming)
  • In case of MLB, no bit rate switching was supported while watching in an SD mode.
  • No differentiated charging for different quality levels.

  
2. Analysis of Playlist Files
  
In full HD and HD modes, none of the channels (KBS1, KBS2, EBS, TvN and MLB) supported adaptive streaming. However, in an SD mode, all the 4 channels, except MLB, supported adaptive streaming.

 

2.1 Full HD
The figure below shows how a playlist and a chunklist for a Full HD video provided by KBS2 channel are requested and obtained.
As seen in the playlist, only one profile (BANDWIDTH=4096000) is available for a full HD video, and thus no bit rate switching was expected. 
In the chunklist, the duration was set as 4 seconds for all the chunks (#EXT-X-TARGETDURATION: 4). Upon receiving a request for a chunklist from a device, CDN server sent the device a chunklist including 4 chunks currently available (96.ts ~ 99.ts in the captured example below).  
After obtaining the chunklist, the device made a request for more chunks every four (4) seconds and downloaded them. 
 

 

 
2.2 HD
For HD videos, the procedure for requesting and obtaining the lists was the same as in full HD mode.

 

2.3 SD
In case of SD videos, KBS1, KBS2 and EBS TvN have 3 profiles, allowing adaptive streaming, whereas MLB has only one profile, allowing no adaptive streaming. 

 

The figure below shows how a playlist and a chunklist for an SD video provided by KBS2 channel are requested and obtained.

  • As seen in the playlist, 3 profiles (BANDWIDTH=1000000, 500000 and 64000 (1 Mbps, 500 Kbps and 64 Kbps)) are available for an SD video, and thus bit rate switching was allowed. 
  • In the chunklist, the duration was set as 4 seconds for all the chunks (#EXT-X-TARGETDURATION: 4). Upon receiving a request for a chunklist from a device, CDN server sent the device a chunklist including 4 chunks currently available (47.ts ~ 50.ts in the captured example below).  

 

 
3. Analysis of the Patterns of Chunk Request and Download
  
The figure below illustrates how chunks are requested and downloaded in case of a Full HD mode, as captured by Wireshark.

  • Initial Buffering State:  The device made a back-to-back request for the first 3 or 4 chunks (3 or 4 requests in a row) and filled the receiving buffer with the downloaded chunks immediately. 
  • Steady State: Thereafter, it made a request for a chunk every 4 seconds and had the chunk downloaded. 

* This back-to-back request (for the first 3~4 chunks) is the same method as the typical Internet video delivery method used by OTTs who do not have their own network and therefore QoS supports from the network layer are not available.

 

 Patterns of chunk request and download in full HD mode
(Galaxy S4, SKT LTE-A and SKT Btv Mobile/Live)

 

4. Adaptive Streaming

 

In our test, we intentionally degraded the network quality while watching a full HD video from KBS2 to check whether or not bit rate switching into a lower quality video was caused. As can be seen in the figure below, requests were made only for chunks in a full HD mode throughout the test. And when we purposely downgraded the network quality after around 90 seconds, what we experienced was buffering, not bit rate switching. So, we can see that adaptive streaming was not utilized in full HD video (and not in HD video either).  

 

 
 The figure below provides an illustration of bit rate switching observed while watching an SD video from KBS2. The video was delivered with profile 1 (1 Mbps) first, and then automatically switched to profile 2 (500 Kbps) through adaptive streaming as the network quality was degraded.    

  

5. Analysis of the Chunk File 
  
We captured the chunk files (*.ts) downloaded in each video quality from KBS2 channel, and analyzed them using an analysis tool (MediaInfo) as listed in the following table (Detailed information on chunks in each quality level is in Appendix B attached at the end of this report):   

The figure below provides an illustration of bit rate switching observed while watching an SD video from KBS2. The video was delivered with profile 1 (1 Mbps) first, and then automatically switched to profile 2 (500 Kbps) through adaptive streaming as the network quality was degraded.  
 

 
 Information on chunks measured in each quality level

 

* Our actual measurement found no difference between the bit rates marked in the profiles of the playlist (manifest) file and the actual encoding rates measured (For instance, a chunk file with a profile of BANDWIDTH=4096000 in the playlist had an actual encoding rate of around 4 Mbps when downloaded). 
 
 
Closing 
  
SKT Btv Mobile service provides its contents in 3 different quality levels (full HD, HD and SD). The video quality of full HD video was particularly impressive, making me wonder if there will be a limit on such enhancement. 

 

One thing that concerns me is that, 
For full HD and HD, users have to stick to the quality level they initially select throughout the whole video unless they manually switch it into a different level while watching it. So, buffering was occasionally experienced while watching in full HD mode during our test.
 
As the number of LTE-A users (and their data traffic) and Btv Mobile subscribers is expected to grow, we can easily predict more viewers will be watching a full HD video simultaneously. So, it is doubtful whether or not we will still be able to watch videos in full HD quality (4 Mbps or higher) uninterruptedly without buffering then.
 
SKT's Btv Mobile service is a paid service and you pay a monthly fee of KRW3,000 for the service. As long as you have to pay, you would expect the same satisfying reliable user experience TV viewers have.  

 

Let's say there is a telecom operator with quite a narrow road (i.e. bandwidth) for flow of traffic. For now, the operator may do one of the followings to ensure uninterrupted video delivery:  

  1. Pour millions into keeping the road wider to cover increasing traffic (i.e. providing higher bandwidth with broader frequency and backhole network capacity, and higher SAE-GW capacity, etc.);
  2. Let a device automatically adjust the bit rates of a video by reflecting each user’s road (i.e. bandwidth) conditions (adaptive streaming); or
  3. Transcode video files with an encoding rate of 4Mbps to ones with a lower bit in real time based on the network conditions by using MVO (Mobile Video Optimization) tools, and deliver them to a device (Dynamic Bit-Rate Adaptation: DBRA). 

With option 1, the network will cost too much. Option 3) is used by telecom operators to process and deliver video files provided by OTTs, not themselves, when the operators have no control over encoding and delivery protocols of the video files. Btv Mobile service has been provided by SKT, and video files in SD have already been encoded in 3 different quality levels. Given that, it sounds most reasonable for SKT to go with adaptive streaming as in option 2).
 
Anyway, our prediction is that SKT will adopt adaptive streaming in full HD and HD sooner or later (I wonder why they haven’t yet, though).


 
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Appendix A. Playlist and Chunklist (via Wireshark)
 
Playlist and chunklist for KBS2 HD video

 
 

Appendix B. Chunk File Analysis (via MediaInfo)

 

B1. Information on chunk file for KBS2 full HD video

 
 

B2. Chunk file for KBS2 HD video

 

B3. Chunk file for KBS2 SD video (SD Profile: 1Mbps)


B4. Chunk file for KBS2 LD video (LD Profile: 500Kbps)

 
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