Payload efficiency and network considerations of MOST and Ethernet
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 continuously increasing bandwidth requirement is a clearly visible trend.
Figure : Bandwidth requirements and network capabilities over time. For better resolution, click here.
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.
(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.
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 administrative 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 % of the totally available bandwidth, since 25 % is 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 , P1722 embeds several media stream formats, like e.g. the IEC 61883 based formats, to an IEEE 802 network.
AVB supports two QoS classes. Class A provides a maximum latency of 2 ms and Class B provides a maximum latency of 50 ms over 7 hops. For Class A, P1722 frames are sent each 125 µs, which e.g. allows collecting six stereo audio samples at a sampling rate of 48 kHz. For Class B, P1722 frames are sent every 250 µs.
Figure : IEEEP1722 frame structure. For high resolution, click here.
The streaming use cases defined in the left column of Table 1a) were used for the comparison of MOST and AVB. The Blu-ray disc is specified with a maximum data rate of 53.95 Mbit/s for audio and video and will soon enter the infotainment domain. Due to the maximum support of 50 Mbit/s for a single transport stream of the MOST INIC Transceiver (9), we used a reduced media content for the calculation composed of Dolby Digital Plus audio (1.7 Mbit/s) and video (38.5 Mbit/s). This leads to an actual transport stream Payload PBlu-ray of 41.1 Mbit/s considering the transport stream header.
The payload efficiency PE for the use cases is shown in column PE of the respective network in Table 1a) and was calculated using (Eq1).
Table : Payload efficiency of streaming use cases. To read the table in full resolution click here.
As mentioned above, MOST150 has a payload efficiency of 100 %. When transmitting an audio stream with P1722, the overhead (including CRC) is about 62 bytes. This causes bad payload efficiency when transmitting a small amount of data, as is the case with CD-Audio streams. Since the P1722 payload for a single packet can be up to 1472 bytes long, the payload efficiency also reaches values beyond 90 %, as visible in a). Unlike audio, video packets sent via P1722 have a better balance between payload and header information. Applications needing lots of bandwidth generate better payload efficiency because larger frames need to be used. The substantial overhead of each frame is spread out over more payload bytes.
However, just looking at the payload efficiency is not sufficient. Additionally, the network utilization NU needs to be considered. NU is defined by the ratio of the required to the available bandwidth of the complete network.
(Eq. 2)
Mapping the use cases above to a MOST (10) and Ethernet based vehicle architecture, as shown in Figure 3, leads to network utilizations as summarized in Table 1b. Looking into the infotainment domain, this leads to one exemplary combination of streaming use cases to
(Eq 3)
i.e. actually to NUMOST150_INFO = 63.3 % and NUAVB_INFO = 37.9 %.
Looking into the Advanced Driver Assistance Systems (ADAS) domain, with a Top-View system as a possible application generating a kind of birds’ eye view of the vehicle, this requires at least 4 live video streams. For MOST, this leads to
(Eq 4)
since the available streaming bandwidth is shared. For Ethernet AVB this leads to
(Eq 5)
since each link also multiplies the available streaming bandwidth. Because shows a 5th camera used, e.g. for driver observation, birds’ eye, traffic sign recognition, etc, the NUMOST150_ADAS is more than 100 %, which is not the case for NUAVB_ADAS: that remains at 40.4 %.
Figure : Possible MOST and Ethernet based vehicle architecture. For full resolution, click here.
Conclusion
Since MOST was originally designed for automotive infotainment, it is using the available bandwidth in an optimal way for all kinds of media streaming and is, in this respect, superior to Ethernet and other physical layers. However, MOST is designed as a bus system that shares the available bandwidth, rather than a switched network that multiplies the available bandwidth. Another upcoming topic is the communication between the ADAS and Infotainment domain. With MOST, there is no simple layer 2 bridging, as is the case with Ethernet by simply connecting the ADAS unit and Infotainment Head Unit.
An alternative approach is to implement an efficient MOST / Ethernet Gateway, maintaining the main advantages of Ethernet (higher total network bandwidth, highly scalable, flexible) and MOST (highly efficient media streaming). Due to the IEEE 802.1AS protocol, Ethernet AVB simplifies the implementation of such a MOST / Ethernet Gateway, since it allows maintain the synchronous MOST timing on the Ethernet side. Furthermore, a media source on the Ethernet side can stream synchronously using MOST as reference clock. A demonstrator system, which was built up in line with the SEIS project, was successfully used to validate this MOST / Ethernet AVB Gateway concept.
Figure 1 illustrates that bandwidth requirements keep rising. Both technologies have the capability to address the bandwidth needs of the future using alternative system architecture approaches. The suggestion is to use Ethernet for applications that will need high bandwidth, like the different kinds of video streaming. Furthermore, the transformation to an IP-based common network layer, as targeted by the SEIS project, can be supported by an efficient MOST / Ethernet Gateway realization.
1. SEIS. Sicherheit in eingebetteten IP-basierten Systemen. SEIS – Security in Embedded IP-based Systems. [Online] BMBF – German ministry for Education and Research. https://www.strategiekreis-elektromobilitaet.de/projekte/seis/.
2. Bruckmeier, Dr. Robert. Freescale Technology Forum. [Online] 23. June 2010. https://www.freescale.com/files/ftf_2010/Americas/WBNR_FTF10_AUT_F0558.pdf.
3. IEEE, Inc. Audio Video Bridging Task Group. IEEE Standard 802.1 edition. [Online] October 2010.
4. IEEE, Inc. Forwarding and Queuing Enhancements for Time-Sensitive Streams. IEEE802.1Qav/D7.0 edition. [Online] October 2009.
5. IEEE, Inc. Stream Reservation Protocol. IEEE 802.1Qat/D6.1 edition. [Online] June 2010.
6. IEEE, Inc. Timing and Synchronization for Time-Sensitive Applications in Bridged Local Area Networks. IEEEP802.1AS/D7.5 edition. [Online] October 2010.
7. IEEE, Inc. Layer 2 Transport Protocol Working Group for Time-Sensitve Streams. IEEE 1722/D2.5 edition. [Online] September 2010.
8. IEEE, Inc. Standard for Layer 3 Transport 2 Protocol for Time Sensitive 3 Applications in Local Area Networks. IEEE 1733/D6.0 edition. [Online] October 2009.
9. SMSC. OS8110 Data Sheet. s.l. : SMSC, Oct. 2010.
10. Schöpp, Harald. Elektronik automotive . Special Issue MOST. 2010, Bd. S2, March 2010.
About the Authors:
Josef Nöbauer is team leader at Continental and an expert for the cross divisional usage of automotive network technologies like MOST, Ethernet, FlexRay. He is coordinating Continental’s activities around the introduction of Ethernet as automotive bus system.
Continental Automotive GmbH, Siemensstrasse 12, 93055 Regensburg, Tel: +49-941/790-8684, josef.noebauer@continental-corporation.com
Helge Zinner is currently working towards the Ph.D. degree at the Institute for Communication Networks, University Ilmenau (Germany) and at Continental. He has been with Continental as a development engineer for infotainment systems since 2004.
Continental Automotive GmbH, Siemensstrasse 12, 93055 Regensburg, Tel: +49-941/790-90327, helge.zinner@continental-corporation.com
Article published in Elektronik automotive, Special Edition MOST, April 2011, www.elektroniknet.de
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