Just like Ethernet before, IEEE 802.11 is now transcending the borders of its usage from the office environment toward real-time communication on the factory floor. However, similar to Ethernet, the availability of synchronized clocks to coordinate and control communication and distributed real-time services is not a built-in feature in WLAN. Over the years, this has led to the design and use of a wide variety of customized protocols with varying complexity and precision, both for wired and wireless networks, in accordance with the increasingly demanding requirements from real-time applications. This survey looks into the details of synchronization over IEEE 802.11 with a particular focus on the infrastructure mode which is most relevant for industrial use cases. It highlights the different parameters which affect the performance of clock synchronization over WLAN and compares the performance of existing synchronization methods to analyze their shortcomings. Finally, it identifies new trends and directions for future research as well as features for wireless clock synchronization which will be required by the applications in the near future.

Clock Synchronization Over IEEE 802.11—A Survey of Methodologies and Protocols

The introduction of the precision time protocol has brought forth the possibility to have a standardised synchronization mechanism in networks, independent from the actual communication technology. However, it can be observed, for example in the annexes of the standard, that many implementations focus only on Ethernet based communication. The logical next step is to investigate how this protocol will fare when used for synchronizing clocks in a distributed manner over IEEE 802.11 based devices. The availability of features like roaming, the broadcast nature of the wireless medium and different hardware platform architectures require an investigation on how clock synchronization can be realized in wireless environments. This paper proposes an approach to import the precision time protocol to IEEE 802.11. Furthermore, standard nodes are enhanced with software timestamping, leading to a synchronization accuracy of a few microseconds.

Towards high accuracy in IEEE 802.11 based clock synchronization using PTP

The combination of broadcast and broadband (hybrid) technologies for delivering TV related media contents is already a reality. It has been motivated by the large amount and diversity of media contents, together with the ubiquity and multiple connectivity capabilities of modern consumption devices. The use of connected TVs and companion devices (e.g., tablets, smartphones etc.) is gaining momentum. It enables personalized and enriched TV media experiences, by also exploiting social communication opportunities. Likewise, the media consumption paradigm is worldwide evolving from passive and isolated consumer experiences toward interactive and group shared experiences between remote consumers. Nevertheless, despite the specification of standards, such as hybrid broadcast broadband TV, and the efforts from operators and content providers in the last years, the adoption of hybrid TV media services in Europe is still not as high as expected. This paper presents the concept and some examples of hybrid TV media services , emphasizing the importance of including a combination of media synchronization solutions, known as hybrid sync , to guarantee a satisfactory level of quality of experience. Additionally, it includes the summary and discussion of the results of a research study focused on more than 1000 Spanish users’ habits, preferences, and expectations regarding four representative hybrid TV media services. Many valuable insights and conclusions have been derived. For instance, the current low market adoption of hybrid TV media services, despite their advantages and the high interest of consumers, and that key technological challenges still need to be overcome. The obtained results and impressions foresee the impact (or potential) of such services in the upcoming TV related media consumption landscape. Therefore, devising proper standard-compliant technological solutions (paying special attention to hybrid sync) and equipment should be continued producing appropriate contents, deploying proper hybrid TV media services and applications. As well, it should be convenient to undertake marketing and commercial efforts to boost their deployment. The contributions of this paper can be very valuable to the interested agents to be aware of the remaining challenges, envisage the opportunities, and drive their efforts to maximize the market adoption of such services.

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Hybrid Broadcast/Broadband TV Services and Media Synchronization: Demands, Preferences and Expectations of Spanish Consumers

Nowadays, media content can be delivered via diverse broadband and broadcast technologies. Although these different technologies have somehow become rivals, their coordinated usage and convergence, by leveraging of their strengths and complementary characteristics, can bring many benefits to both operators and customers. For example, broadcast TV content can be augmented by on-demand broadband media content to provide enriched and personalized services, such as multi-view TV, audio language selection, and inclusion of real-time web feeds. A piece of evidence is the recent Hybrid Broadcast Broadband TV (HbbTV) standard, which aims at harmonizing the delivery and consumption of (hybrid) broadcast and broadband TV content. A key challenge in these emerging scenarios is the synchronization between the involved media streams, which can be originated by the same or different sources, and delivered via the same or different technologies. To enable synchronized (hybrid) media delivery services, some mechanisms providing timelines at the source side are necessary to accurately time align the involved media streams at the receiver-side. This paper provides a comprehensive review of how clock references (timing) and timestamps (time) are conveyed and interpreted when using the most widespread delivery technologies, such as DVB, RTP/RTCP and MPEG standards (e.g., MPEG-2, MPEG-4, MPEG-DASH, and MMT). It is particularly focused on the format, resolution, frequency, and the position within the bitstream of the fields conveying timing information, as well as on the involved components and packetization aspects. Finally, it provides a survey of proofs of concepts making use of these synchronization related mechanisms. This complete and thorough source of information can be very useful for scholars and practitioners interested in media services with synchronization demands.

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Understanding Timelines Within MPEG Standards

A wireless media distribution system is provided comprising an access point (6) for broadcasting media and a plurality of stations (2) for reception and playback of media. Each station is configured for receiving and decoding a timestamp in a beacon frame transmitted repeatedly from the access point. This is used to control the output signal of a station physical layer clock (12) which is then used as a clock source for an application layer time synchronisation protocol. This application layer time synchronisation protocol can then be used in the station to control an operating system clock (8) for regulating playback of media.

Method and apparatus for time synchronisation in wireless networks

When a single source of multimedia contents is distributed to multiple reproduction devices, the audio and video contents require synchronous play for multi-channel stereo sound and lip-synchronization. This paper investigates capability of IEEE 802.11n wireless LANs for providing synchronized services of real-time multimedia traffic. We implement IEEE 1588 Precision Time Protocol in wireless LAN devices to characterize delay jitter and clock synchronization accuracy. The results indicate a strong potential to provide a high fidelity real-time multi-channel multimedia networking service within 430-microsecond synchronization accuracy at an approximately 80 Mbps streaming rate.

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Wireless network synchronization for multichannel multimedia services

When a single source of multimedia contents is distributed to multiple reproduction devices, the audio and video contents require synchronous play for multi-channel stereo sound and lip-synchronization. The multimedia system in vehicle, especially, has researched to move to wireless environments from legacy wired environments. This paper proposes the advanced algorithm for providing synchronized services of real-time multimedia traffic in IEEE 802.11 WLANs [1]. For these, we implement the advanced IEEE 1588 Precision Time Protocol [2] and the environments for simulation. Also, we estimate and analysis performance of the algorithm, then we experiment and analysis after the porting of algorithm in wireless LAN devices (Linksys wrt-350n AP network device) to characterize timing synchronization accuracy.

Jongwon Yoon

Papers

Wireless network synchronization for multichannel multimedia services

Wireless Network Synchronization Algorithm based on IEEE 802.11 WLANs (Wireless Local Area Networks) for Multimedia Services