The AVnu Alliance makes Ethernet a deterministic network technology for conformance and interoperability certification of IEEE-compliant devices, ensuring network devices in various applications (eg professional A/V, automotive, industrial control and consumption). Interoperability of electronics). Together with other standards bodies and alliances, the AVnu organization opens up a viable path for deterministic networks based on 802.1 Audio Video Bridging (AVB)/Time Sensitive Networking (TSN) standards, making it easy for designers and engineers to apply in specific products. These standards.
Since its inception, Hengrun Technology has been focusing on the research and application of automotive network technology, including CAN, CANFD, LIN, FlexRay, and MOST. For automotive Ethernet technology, Hengrun Technology has already joined the AutoSAR organization and the OPEN Alliance. In order to facilitate the timely and comprehensive understanding of AVB technology, in October 2016, Hengrun Technology joined the AVnu Alliance. With its accumulated years of traditional in-vehicle network technology, a strong new bus R&D team and experienced foreign partners, Hengrun Technology is committed to providing customers with the most advanced and professional automotive Ethernet and AVB technology solutions.
summary
As consumer demand for automotive connectivity and entertainment comforts is exploding, it is highly desirable for automakers to build a standard, easy-to-deploy A/V network system in the vehicle. The IEEE 802.1 Audio/Video Bridging (AVB) Working Group and the IEEE 1722 Layer 2 Transport Protocol Working Group have developed a range of network-enhanced features for time-sensitive data to facilitate high-reliability audio and video applications. The AVnu Alliance adopted these standards and gave recommendations for in-vehicle applications. At the same time, the internal certification test team (CertificationTestSubgroup, CDS) has also developed the corresponding certification test specifications. This article outlines these new technologies and their advantages in automotive applications.
Introduction
In the past decade, consumer demand has driven a significant increase in car audio and video capabilities. Features that once only appeared in luxury cars, such as DVD playback, reversing cameras and navigation, are now standard on many mainstream cars. More and more resources and options make the RearSeat Entertainment (RSE) more and more complicated. Faced with such a changing demand, how to build a universal automotive network architecture becomes an urgent problem.
When almost all OEMs around the world believe that the vehicle communication bandwidth is low and the CAN network is used to build the in-vehicle network, the application of in-vehicle multimedia brings great challenges (such as bandwidth, QoS, scalability, cost, economies of scale, openness, etc.). What is a multimedia application solution that can meet both technical and cost requirements? There is still much debate.
Previously, due to the high uncertainty, vehicle multimedia transmission avoided the use of packet-switched networks. Today, the IEEE 802.1 Audio Video Bridging (AVB) Working Group and the IEEE 1722 AVTP Working Group provide a standards-based implementation for low-latency, high-reliability transmission requirements in automobiles. The AVB protocol can be applied to a variety of physical layers. This article will focus on AVB applications over Ethernet. The simplification of the in-vehicle cable and the reliable hard-wire solution make the wired Ethernet-based AVB protocol work well in the vehicle environment.
The main market goal of the AVnu Alliance is to successfully apply AVB to audio and video streams in the car. This white paper outlines the features and benefits of using AVB technology in an in-vehicle network.
First, Ethernet AVB overview
AVB is an enhancement of the Ethernet Open Standards Suite. Quality of Service (QoS) is provided for processing audiovisual data within the network through network time synchronization and time-sensitive stream related transport protocols. In an in-vehicle environment, it can meet the more demanding time-sensitive network requirements, making it possible to simultaneously handle infotainment, body control, assisted driving, and even safety-related functions in a single network.
In order to do this, AVB uses many important concepts. "Priority": Indicates the time sensitivity of part of the data stream, which is different from the general best-effort data. “Reservationâ€: Pre-set a fixed-size determined bandwidth in the network to handle high-priority traffic. Synchronous data packets are transmitted through "network time" along with strict delay requirements so that audio and video can be played simultaneously. AVB technology also includes standard traffic shaping and forwarding rules. In addition, AVB technology can be used for standardized methods of discovery, enumeration, and control, enabling the system to be quickly and easily built and configured.
The technical overview at the end of this article presents more details about the independent parts of the standard and related technologies.
Second, the AVB advantage of the automotive market
• Simplified cable, lighter weight and higher reliability
The networked wiring method reduces the wiring harness cost and design complexity in the car. Reducing the number of connectors increases reliability, reduces weight, and increases fuel efficiency.
With the widespread use of CAN networks and basic control information transmission technologies, it is very common to implement multi-channel or network-based control communication within a vehicle. Due to the continuous expansion of audio and video content in infotainment systems, the continued use of point-to-point dedicated connections, such as the use of shielded LVDS cables, is no longer sufficient.
• A robust ecosystem
The developers of AVB chose Ethernet as the underlying transmission path, not only because of voice technology, but also because Ethernet technology is robust, easy to expand, and widely used. The developers decided to create an open, standard technology based on these advantages. Open standards can breed a strong, competitive ecosystem, and many vendors now deploy AVB systems in their products. In addition, AVnuCDS ​​defines the AVB instructions for use and has developed related certification procedures. This will allow OEMs to develop consistency and interoperability requirements that can be implemented by many Tire1s. Vendors can in turn build their solutions, not just rely on a single technology provider.
This approach is in stark contrast to previous in-vehicle solutions. In-vehicle infotainment network technology has always been dominated by MOST, but the lack of openness of MOST technology has hindered its application and development. As early as November 2008, the Hansen report pointed out that “the industry insiders believe that MOST is far from open enough compared to CAN, LIN and FlexRay.†This report continues to predict “if there is no more openness and strong purchases. Capabilities, MOST may give way to its replacements in a few years, such as Ethernet." This prediction is being confirmed, and MOST is being replaced by more open new technologies.
• Interoperability certification
In addition to its open nature, AVB technology is based on IEEE standards and has a robust, rigorous consistency and interoperability (C&I) standard. These C&I standards are defined by their respective Protocol Implementation Conformance Statements (PICS). AVnuC&I extends IEEEPICS to automotive applications.
Given that AVB certification guarantees compatibility between different products, OEMs can work with different suppliers to build a rich, growing ecosystem that can quickly meet quality requirements.
• Predictability and high reliability
AVB core protocols: priority, reservation, traffic shaping and time synchronization allow designers to build predictable and reliable network requirements in automotive environments.
The IEEE 802.1Q Queue and Forwarding Protocol (Qav) is the key to ensuring predictability. It prioritizes scheduling high-priority data while ensuring that time-sensitive information is not interfered with by low-priority data.
Bandwidth reservation refers to the reserved end-to-end available bandwidth for audio and video streams, and ensures that this bandwidth can be used by the stream data until it is explicitly released. At the same time, in order to meet the requirements of fast start, bandwidth reservation can be pre-statically configured. By default in the vehicle application scenario, the system can configure static reserved bandwidth for special data.
The IEEE 802.1Q Stream Reservation Protocol (SRP) is more flexible. It allows the terminal to dynamically reserve or release bandwidth, so that OEMs can configure and reliably transmit audio and video streams for each different optional package or future vehicle without performing a large number of network changes. However, the in-vehicle application scenario does not require support for SRP.
• Many-to-many flexible configuration
Building an in-vehicle network that can handle large amounts of data and control signals is challenging. The network needs to ensure timely delivery of audio and video data from multiple sources to multiple destinations, such as mute programs during incoming call events, and pass information such as warnings and route planning navigation commands. This requires the network to configure reserved bandwidth from the beginning to handle these requirements.
In fact, dynamic bandwidth reservation by SRP is not required in the vehicle application scenario. Meeting the many-to-many communication needs is one of the most fundamental reasons why automakers are moving from point-to-point transmission systems to networked A/V systems.
• Low latency
Many automotive applications have very strict delay requirements. Typical examples are tachographs, driver assistance cameras, Bluetooth microphones, and various sound sources. Equally important are the constraints imposed by external systems, such as car hands-free devices that need to confirm the latency requirements of mobile cellular networks. For these applications, the AVB protocol is able to meet the most stringent latency requirements.
• Precise synchronization
The ultimate goal of the car audio and video network is to present audio and video streams in the terminal, providing a high quality audiovisual environment for vehicle users. In addition, car manufacturers can also build a flexible network that can transmit different types of data, including multiple source nodes and playback nodes.
The key to the above requirements is synchronization.
Technically, synchronization has two basic purposes:
First, a common time reference is provided for the sampled data on the source device and presented on the one or more destination devices in the same cycle.
Second, to achieve synchronization between multiple streams (such as before and after audio).
AVB is synchronized by the IEEE802.1AS Precision Clock Protocol (PTP). It provides a common time reference for all nodes in the network. This benchmark is called "wallclock." The IEEE1722 audio and video transmission protocol proposes the concept of "presentationtime", which is derived from "wallclock" and allows the sending node to define the presentation time of the data packet at the receiving end.
Network nodes maintain their own local clocks, and by exchanging timestamp information, they calculate the difference between themselves and the network time. By reconstructing the original sampling clock, the receiving end not only realizes accurate and low-jitter delivery of content, but also enables the same AVB network to simultaneously accommodate data of different sampling rates and a variety of different types of devices.
•Quick Start
One of the most important requirements for in-vehicle multimedia systems is to provide "earlyaudio" and "earlyvideo". The system must be able to start up quickly and be ready to present audio and video when the vehicle is started. Audio is usually used to play security alarms, and video is used to play back-view camera images. These two scenarios are mandatory by NHTSA (US Highway Safety Administration): available within 2 seconds of vehicle start-up.
To meet these needs, the AVnu Alliance has specifically defined automotive applications to simplify the startup process of automotive Ethernet AVB products. Specifically, fast network time synchronization is achieved by using a pre-configured fixed clock tree, and the delay caused by dynamic AVB stream reservation is eliminated by pre-configuration of traffic.
• Scalable general topology
Unlike the shared network bandwidth of MOST, the AVB network consumes bandwidth only between the source node and the destination node. Even under the same network bandwidth, this approach will allow more data to be transmitted over the AVB network than the MOST network, and it is easier to extend the star topology or tree topology. In addition, the use of multicast services, ie one-to-many transmission, further increases bandwidth usage.
Figure 1 MOST ring topology. All nodes will monitor traffic from other nodes.
Figure 2 shows an example of the AVB architecture. The camera signal at the front of the car is only transmitted to the driver assistance module.
In addition, designers have the flexibility to choose compatible speeds. For example, a given vehicle design can easily mix and match high bandwidth 1000 Mbps links with low bandwidth 100 Mbps links. A large amount of technical input from various organizations guarantees this interoperability, while at the same time bringing the possibility of higher speeds, such as 10Gbps or even 100Gbps. In contrast, the three current MOST rates—MOST25, MOST50, and MOST150—are not compatible. The entire MOST network can only use the same rate, and has low flexibility. For devices with low bandwidth requirements, unnecessary costs are required to meet the communication requirements of high-bandwidth devices.
Third, AVB Ethernet use case
• Lip-synced multimedia playback
Providing true Lip-synced playback of AV content through different multimedia devices in a car environment is a core in-vehicle application for AVB Ethernet.
Whether it's from a DVD player installed in the front center or audio and video content from an external wireless mobile device, it can be played on multiple front and rear seats, car amplifiers, and even connected headphones. All passengers provide a pleasant audiovisual experience.
In the example below, two RSE displays play DVD video while audio is transmitted to the amplifier through another channel. AVB technology guarantees precise synchronization between these three devices.
Figure 3 DVD player with multiple audio and video transmission paths
• Connected vehicle applications
In interconnected vehicles, the availability and dependencies of external data are high. Regardless of the transmission of audio and video streams, online map navigation, incoming external Internet media information in the car, or various remote data and service requests from the car, the network bandwidth requirements are very high.
The ability to transmit the above data as well as internal audio and video data over a single network while simultaneously meeting the different QoS requirements of different data is a powerful advantage of AVB Ethernet.
• Advanced driver assistance system
Many modern Advanced Driver Assistance Systems (ADAS) are available through a flexible, high-bandwidth AVB network.
For example, a set of interconnected cameras can provide a panoramic image of a synchronized 360° vehicle environment that can be transmitted synchronously with more sensor data on the same network, ultimately for driver alerts to improve occupant and pedestrian safety. Sex.
• Cable diagnosis
Automakers are eager to troubleshoot vehicle problems directly through the vehicle diagnostics on the assembly line and at the after-sales service station.
The Ethernet PHY has powerful physical layer diagnostic capabilities, including: automatic detection and compensation switching pairs, cable breaks, and detection and compensation for bandwidth degradation caused by link impedance mismatch. With these established and tested Ethernet diagnostics, assembly and service issues can be discovered and located faster.
• Brief technical overview
For an in-depth look at AVB technology, please see the Technical Resources and CouncilMaterial available on the AVnu website.
IEEE 802.1AVB and IEEE 1722 are the technical foundations that are being promoted by the AVnu Alliance in conjunction with other IEEE standards that describe different network link interconnection or bridging technologies. It is worth noting that the services provided by the AVB standard on different types of network links are not the same, as each link technology has different characteristics.
The following are the basic criteria:
♦ IEEE802.1AS (PTP): “TimingandSynchronizationforTime-SensitiveApplicationsinBridgedLocalAreaNetworks.†automatically selects a device as the master clock, and then sends time information to other nodes by bridging the LAN or IP subnet. The 802.1AS clock is not used as a media clock, but is used as a shared clock reference between nodes to transmit talker media clock information to the listener. In order to estimate the actual media rate of the sender in a network with a large amount of delay jitter, this reference removes the need for fixed packet switching delays and does not require calculation of long-term running averages. IEEE 802.1AS is based on the IEEE 1588-2008 standard.
♦ IEEE802.1Q-2012 (SRP): As a P802.1Qat project development, "VirtualBridgedLocalAreaNetworks-Amendment9: StreamReservationProtocol (SRP)". Allows bridging between LAN and IP subnets, and sets up flow reservation between talker and listener.
♦ IEEE802.1Q-2012 (FQTSS): As a P802.1Qav project development, “VirtualBridgedLocalAreaNetworks-Amendment11: ForwardingandQueuingforTime-SensitiveStreams.†describes a method for token bucket shaping network traffic, making delay and bandwidth reserved traffic available. control.
♦ IEEE802.1BA: “Audio/VideoBridging (AVB) Systemsâ€. It defines how to apply the relevant IEEE802.1 and other standards when building an AVB system.
♦ IEEE1722: “Layer2TransportProtocolforTime-SensitiveApplicationsinBridgedLANs.†defines data encapsulation, presentation time, and interoperability between audio and video end devices, and these devices are standard network services provided by IEEE 802 networks and meet time-sensitive applications. QoS requirements.
When a special message leaves and arrives at the interface or PHY, an accurate timestamp is recorded, and the PTP can measure and compensate for the queue and forward transmission delay. In order to use 802.1AS, the desired stream contains the presentation time and forms a cross-time stamp in conjunction with the network time to reconstruct the sampling clock. For streams that use 802.1AS time and encapsulate presentation time, in essence, the AVB network supports any number of different media sampling rates, and also supports clock sources as destination devices, synchronized to their respective source devices. The AVB network also provides a mechanism for synchronizing between different paths in the network.
The IEEE 802.1Q stream reservation protocol provides a bandwidth reservation mechanism that allows the terminal node to dynamically configure routing. SRP is not required to be implemented in automotive applications. In order to simplify the operation of the terminal node and achieve the fast start requirement, the network is considered to be fixed by default, that is, the reserved bandwidth is pre-configured according to the transmission scenario. But whether you want to pre-configure the flow reservation or use SRP, you need to use FQTSS. FQTSS schedules time-sensitive stream data to ensure timely delivery of network data. Ordinary non-streaming data such as IP-based control or metering messages cannot affect the reserved AVB stream data.
The AVnu Automotive Ethernet AVB Functionality and Interoperability Specification uses the following standards and defines the configuration and option optimization for automotive applications, including:
♦ Fixed pre-configured 802.1AS clock tree with built-in clock redundancy
♦ Preconfigured stream reservation
♦ Start to meet NHTSA requirements
♦ Car audio AVB traffic classification optimization
♦ Optimize AVB performance mechanisms in low-cost ECUs and endpoints
♦ Cable diagnosis
to sum up
The IEEE 802.1AVB working group defines key technologies for vehicle infotainment systems and driver assistance systems to apply Ethernet. The AVnu Alliance provides interoperability and certification testing of automotive applications to verify the implementation of these requirements. In the context of a large and continuous investment by organizations, low-latency, precisely synchronized audio and video transmission technologies will provide a powerful solution for next-generation applications. The AVB protocol is an open standard that allows OEMs and suppliers to use it.
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