Insights into the Launch of MOST150 in the New Mercedes-Benz S-Class
Since the very beginning of the MOST Cooperation in 1998, Daimler has actively contributed and developed MOST Technology. This was characterized by an initial double technology leap consisting of
a) digital transmission (to improve sound quality) and
b) optical transmission (to overcome EMC issues).
By 2005, every new Mercedes-Benz car was equipped with MOST at a bit rate of 25 Mbit/s. During this time frame it became clear that new requirements and challenges would show up on the horizon, and Daimler, together with the MOST Cooperation, started towards the 3rd generation of MOST with 150 Mbit/s [1].
During the development of several innovations on the one hand, it became clear on the other hand that it is important to exploit the bandwidth of the existing Plastic Optic Fiber (POF), thus allowing for an easy migration path from MOST25 to MOST150.
1. Requirements of the Infotainment in the New S-Class
In the new S-Class infotainment system, many new features have been required. Fig. 1 gives an overview of the supported infotainment use cases:
- 12.3” central display
- HDD navigation system with 3D city models and live traffic
- Entertainment for front passenger with split-view technology.
- Rear seat entertainment with 2 x 10.2” displays and independent access to all entertainment sources of the car
- Playback of A/V entertainment sources
- Integration of cell phones and CE devices
- SMS/E-Mail
- Web browser for all passengers in the car
- Wi-Fi Hotspot
- Remote Control App
- Mercedes-Benz Online Apps
Fig.1: Infotainment in the new S-Class
As already mentioned in 2011 [2], when Daimler announced the adoption of MOST150 in a series project, one of the challenges consisted of the totally changed bus access structure: Whereas in traditional infotainment systems the Head Unit solely provided the user interface for the driver and optionally the co-driver, the rear seat entertainment system (RSE) was no longer a stand-alone system but connected and integrated to the MOST bus.
The infotainment system architecture consists of the Head Unit, amplifier, (market-specific) tuner and the rear-seat entertainment (Fig. 2). Each component needs access (Table 1) and supports
- Audio/Video Playback
- Control
- Cabin Playback
Fig.2: Infotainment System Architecture
The mechanisms for connection handling are the basis for a multi-seat management that is able to handle a large number of sources and connections in parallel, as well as the dynamic sequences that may occur, for example when audio sources are changed because of a traffic announcement.
Table 1: Multi Seats vs. Entertainment Sources
Beyond this, the new S-Class provides access to internet services and web browsing for all seats. Therefore, IP traffic has to be transmitted efficiently over the MOST150 network, especially against the background of the increasing data rates of mobile networks (e.g., LTE).
Finally, the bandwidth requirements for existing features have increased to a great extent. Reasons for this are, among others, the use of metadata (including cover art) for music tracks, the data services of digital broadcast radio (e.g., the weather information), accessing video files on storage media and access to HTML-based information content provided by one device to the other devices in the system. All of these features have to be provided in a multi-seat context.
To summarize, the following requirements were the basis for the design of this new generation of infotainment systems:
- Independent access to audio and video sources from all seats.
- Access to internet services and increased connectivity to customer devices.
- Increased available bandwidth for new features.
2. Realization of the Infotainment System in the New S-Class
2.1 Physical Layer – Smooth Transition to MOST150
The MOST150 Physical Layer, with a bandwidth of 150 Mbit/s, allows continued use of POF and LEDs as light sources. Thus, the established optical wire harness and assembly processes can be reused. This makes the transition to higher bandwidth networking a smooth evolution. The MOST150 optical physical layer specification introduces SMD technology for the FOTs, which allows for a state-of-the-art reflow manufacturing process.
Increased interoperability of MOST150 components is achieved through the specification of eye diagrams, providing a measurement specification with defined test setups and a worst-case data pattern. The specification limits are testable and can be checked during compliance verification.
Another important aspect for easy migrations was the adoption of proven MOST25 technologies, i.e. an INIC architecture and the NetServices, to abstract from the physical interface. Through this, the system development became very stable from the beginning.
2.2 Network Management Improvements
With the MOST Specification 3.0 [1], further mechanisms for failure detection and diagnosis have been added to improve the stability of the system, such as detection of a sudden signal off caused by a device, or an extended ring break diagnosis. With the sudden signal off detection, the system can determine the spontaneous failure of a certain device. Previously, this could only be detected through a special test setup and could not be diagnosed in a production system.
The Electrical Control Line (ECL) is used currently for wakeup.
2.3 Application Layer Drives Distributed Entertainment
Regarding the application layer functions, the Function Catalog for the S-Class consists of more than 1000 pages using more than 30 FBlocks and over 70 FBlock instances.
The scenario description consists of more than 700 message sequence charts (MSCs). Example features include a distributed entertainment system (all sources on all seats including metadata), distributed telephony system (2 phones, address book, messaging) and software updates via MOST High Protocol.
2.4 MOST150 Bandwidth Partitioning
The total amount of bandwidth is 136 Mbit/s, which is partitioned on MOST as follows:
- Synchronous Channel: 26 Mbit/s
- Isochronous Channel: 65 Mbit/s
Packet Data: 45 Mbit/s
2.4.1 Synchronous Channel (Audio Transmission)
The connection management has to handle up to 20 entertainment audio sources and up to ten information audio sources. These sources can be played on four sinks in parallel. In case of a source change, the connection handling will have to deal with the change of audio settings, the conflict handling and prioritisation, as well as the mixing, fading, and muting of audio sources dynamically and within certain timing constraints.
The quantitative structure is as follows:
- 18 parallel synchronous connections (entire system)
- 15 parallel synchronous connections in the Head Unit:
- 16 bit, 48 kHz audio quality
- 1x 7.1 surround sound
- 10x stereo channels
- 1x 4 mono channel
- 3x mono channels
This leads to up to 26 Mbit/s synchronous bandwidth.
2.4.2 Isochronous Channel (Video Transmission)
To reduce the high number of dedicated video links, which would otherwise have been necessary, the video sources of the Head Unit are transmitted as streaming connections via the MOST150 network. They use the isochronous streaming channels provided by MOST150, which were designed to transmit streaming data that does not have a fixed bit rate like compressed video or audio. Similar to MOST25, bandwidth can be allocated for streaming channels or for packet data.
The quantitative structure is as follows:
- Infotainment system supports playback of 4 HD videos
- All videos are lip-synched on all displays
- 2 videos are streamed from Head Unit to RSE
- 2 isochronous connections for video streaming
- 1 additional isochronous connection for lip synching
- 15 Mbit/s per streaming connection
- Support of DTCP encryption
- Blu-ray ready, bandwidth reserved to expand one video stream to 50 Mbit/s
Correspondingly, this leads to a bandwidth of 65 Mbit/s on the isochronous channel.
2.4.3 MOST Ethernet Protocol (MEP) – Internet Communication
The MEP channel is used to transmit IP traffic between the main units of the system (HU and RUs) to provide access to internet services and the internet itself. The MEP channel has proven to be an easy-to-use and efficient way to transmit IP data.
The prevalence of internet applications in the automotive environment will grow significantly. Consequently, today’s infotainment platforms have to meet these new requirements. MOST150 makes a crucial contribution to this by providing the MOST Ethernet protocol. With this new communication channel, both high-performance IP networks and internet applications can easily be implemented. The MOST Ethernet channel can transport unmodified Ethernet frames according to IEEE 802.3. This permits software stacks and applications from both the consumer and IT domains to be seamlessly migrated into the car. TCP/IP stacks or protocols utilizing TCP/IP can communicate via MOST without any modification. Thus the new generation of MOST provides the automotive-ready physical layer for Ethernet in the car.
The MEP channel is used as follows:
- Transmission of IP based data:
- Shared internet access (Head Unit, RSE, Wi-Fi tethering)
- IP packet routing within the system
- MOST application layer communication:
- Head Unit <–> RSE data exchange
- Internet like data, e.g., HTML pages via HTTP
- Very large data, e.g., address book
Finally there is a total bandwidth of 45 Mbit/s on the packet data channel.
Fig.3 Infotainment System in the current Mercedes-Benz S-Class
3 Conclusion
The development of this latest MOST generation has been accompanied by a cost/benefit analysis that led to several innovations on one hand and to a high level of backward compatibility on the other. Existing applications can easily be reused in the new network by simply modifying their network interfaces. Therefore, it offers a smooth adaption with the majority of features being continued. Daimler is able to reuse and adopt many components from the proven system architectures, as for example, main parts of topologies, network management and application structures. MOST150 is able to cope with the challenges of a modern infotainment system as shown in the Mercedes-Benz S-Class.
Again, as with MOST25 in the past, MOST150 technology will be rolled out onto all other Mercedes-Benz car lines, step by step.
Literature:
[1] MOST Specification Rev. 3.0 E2. MOST Cooperation, 2010.
[2] New Project, New Features, Leonhardi, A., Wachter, S., Bösinger M., Pech, T., Special Issue MOST 2011.
[3] MOST: The Automotive Multimedia Network, Editor: Grzemba A. , Franzis Verlag; Auflage: 1, 1. Mai 2008.
About the author:
Jan Bauer is responsible for the MOST Bandwidth Partitioning and is in charge of Audio- and Videohandling in the Head-Unit development in the Telematic Network team at Daimler AG series development in Sindelfingen. He represents Daimler in the MOST Cooperation Working Group Device Architecture.
Article previously published in Elektronik automotive, MOST Special Edition, May 2014, www.elektroniknet.de
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