MOST and Ethernet: Payload efficiency and network considerations

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 worldwide 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 fully 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.

Description of problem/challenge
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, compared to the bandwidth offered by networks, is shown in the figure below. The continuously increasing bandwidth requirement is a clearly visible trend.



Figure 1: Bandwidth requirements and network capabilities over time.

One of the core requirements for a network is that it will deliver application data reliably and will provide reasonable response times between any nodes. 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 time critical and non-time 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 bursty behavior.

IEEE 802.1Qat (5) defines a protocol to signal reservation requests and 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), 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.

PE = P/D. (Eq 1)

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.

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