MOST vs. Ethernet Friend or Foe?

This paper discusses the payload efficiency of MOST and Ethernet and the derived influence on the network architecture of vehicles with high bandwidth needs. Furthermore, ideas for coexistence of these competing technologies are described.

The MOST technology is nowadays dominating the upper class infotainment systems due to its support of high bandwidth data. Fostered by the integration of consumer devices and the world-wide success of the Internet Protocol (IP), the research for the usage of IP as the common network layer in an automotive environment has already started. The results presented in this paper have been prepared within the publicly funded project SEIS[1]. In combination with IP, Ethernet is the most commonly used physical layer. Actually, the usage of a cost efficient and automotive-qualified Ethernet solution is already scheduled for implementation in series production[2]. So the competition between MOST and Ethernet is already ongoing.

This paper will focus on a specific part of this competition. The payload efficiency of MOST and certain transport protocols of Ethernet AVB[3] are compared, since Ethernet AVB defines provisions to achieve Quality of Service (QoS) within an Ethernet network. However, simply looking at the payload efficiency is not sufficient, since MOST is a bus system, while today’s Ethernet is a switched network that leads to a multiplication of the system-wide available bandwidth by the number of point-to-point links in the system. Hence, network utilization will also be discussed in this paper as well.  

Increasing bandwidth requirements in automotive applications

Automotive infotainment networks are becoming more open to non-automotive devices like mobile phones and are supporting IP/Web based applications. High-definition video and camera based applications create higher data rates that already need to be handled today. The bandwidth requirements of certain applications in comparison to the bandwidth offered by networks, is shown in figure 1. The continuously increasing bandwidth requirement is a clearly visible trend.

One of the core requirements for a network is that it will deliver application data reliably and that it will provide reasonable response times for inter-node communication. In order to support QoS in asynchronous Ethernet networks, AVB extends the standard with three additional sub-standards. IEEE 802.1Qav[4] uses methods described in IEEE 802.1Q to separate timing critical and non-timing critical traffic into different traffic classes. Output port buffers are separated into different queues, each allocated to a specific class. This ensures a separation of low priority traffic from high priority traffic.

Moreover, all output ports have a credit-based shaping mechanism to prevent burst cycles being used during communication. IEEE 802.1Qat[5] defines a protocol for signal reservation requests and to reserve resources for media streams. This is actually implemented by allocating buffers within switches. - IEEE 802.1AS[6] is responsible for the precise time synchronization of the network nodes to a reference time. IEEE 802.1AS synchronizes distributed local clocks, referred to as slave clocks, with a reference that has an accuracy of better than one microsecond. Additionally, transport protocols like IEEE P1722[7] and IEEE P1733[8] are used for the actual transfer of the media streams.

Payload efficiency (PE) is defined here as ratio between the payload P and the effectively sent data D.

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The available data rate of a network for media streams is defined as B.

For MOST150, the effectively sent data is the content itself without additional headers. Therefore PMOST150 is generally equal to DMOST150. MOST150 has an actual line speed of just 147.5 Mbit/s at a sampling rate of 48 kHz. MOST frames are sent with administrative data including the control channel, which is not related to the streaming data. Hence, the maximum available data rate for streaming data is reduced to BMOST150 = 142.9 Mbit/s.

In comparison to MOST, AVB provides similar QoS when the above introduced sub-standards are supported by the devices of the AVB network. Their bandwidth consumption is considered similarly to the MOST administra-tive data. IEEE 802.1Qat utilizes reservation messages that lead to a negligible background load. IEEE 802.1Qav does not utilize any messages. The time synchronization frames of IEEE 802.1AS lead to a background load of about 0.1 Mbit/s. On top of the AVB protocols, communication for setting up and controlling media streams is required. 

Therefore, a general background load of 1.5 Mbit/s is assumed, as this is also offered by the MOST control channel. Hence, the actually available streaming bandwidth of a 100 Mbit/s AVB network is reduced to BAVB = 98.4 Mbit/s. Note that the Ethernet AVB standard limits the bandwidth of QoS supported data to 75 percent of the totally available bandwidth, since 25 percent are reserved for conventional best effort traffic.

As the preferred AVB transport protocol for media streams, P1722 has been examined. The P1722 frame structure is depicted in figure 2 .

The sent data DAVB for media transport over P1722 is constructed by Ethernet header (preamble + layer 2) + P1722 header + CIP header (Common Isochronous Packet) + payload + CRC. DAVB differs depending on the type of the payload.