MOST - 1707

Overview
Application
The MOST bus (Media Oriented Systems Transport) was specially developed for the networking of multimedia applications in motor vehicles (infotainment bus). Alongside the classical entertainment functions such as radio receivers and CD players, infotainment systems also provide video functions (DVD and TV), navigation functions, and access to mobile communication and information. The MOST bus supports the logical networking of up to 64 devices and provides a fixed and reserved transmission bandwidth. MOST defines the protocol, the hardware, the software, and the system layers. MOST is jointly developed and standardized by automobile manufacturers and suppliers within the MOST
Cooperation [7]. With a data rate of more than 10 Mbit/s, the MOST bus is a Class D bus system (see Table 1).
For data transfer, the MOST bus supports the following transmission channels:
– Control channel to transport control commands,
– Multimedia channel (synchronous channel) for the transmission of audio and video data,
– Package data channel (asynchronous channel), for example to transmit configuration data for a navigation system
and to update software in control units.

 

Requirements
The transmission of multimedia data – both audio and video data – requires a high data rate and also synchronization of the data transfer between source and sink as well as between a number of sinks.

 

Transfer system
Physical layerPhysical layer
The MOST standard specifies both optical  and electrical technologies of the physical layer (transmission layer). The optical transmission layer is widespread and currently uses fiber-optic cables (polymer optical fibers, POF) made of polymethyl methacrylate as its transfer medium. These have a 1 mm core diameter and are used in combination with LEDs and silicon photo diodes as receivers (see Optical fibers/waveguides).

The outstanding feature of MOST 50 is its suitability for the electrical transmission of data. This enables data transfer across unshielded, twisted copper cables (UTP, unshielded twisted pairs). Whereas MOST 25 technology has continued to develop in Europe for a many years and has become established in the Korean market, the Japanese market in particular prefers MOST 50, the second generation of the multimedia standard. The identification number, for exam -
ple in the case of MOST 25, stands for a transfer rate of approx. 25 Mbit/s. The exact data rate depends on the sampling
rate that the system uses. With a sampling rate of 44.1 kHz, the MOST frame (data frame) is transferred 44,100 times
per second; a frame length of 512 bits results in a data rate of 22.58 Mbit/s. For MOST 50, the same sampling rate
results in double the data rate, as the frame is 1,024 bits long. Higher data rates of 150 MBit/s (MOST 150) are currently
also available.

 

Special features of MOST 150
In addition to the higher bandwidth of 150 Mbit/s, MOST 150 contains an isochronous transport mechanism to transfer
compressed data of HD videos efficiently. MPEG transport streams (MPEG, Moving Picture Experts Group) are transported directly here. With a corresponding MPEG4-based video codec, it is possible to transfer resolutions of up to 1,080 p (1,080 screen lines), as delivered by BluRay players for example. Alongside this, MOST 150 provides an Ethernet channel for the efficient transmission of IP package data (IP = internet protocol). In contrast to the MAMAC protocol (MOST Asynchronous Medium Access Control) used in the case of MOST 25, the Ethernet channel is able to transfer Ethernet frames. The Ethernet channel transfers unmodified Ethernet data blocks, which means that software stacks and applications from the fields of consumer electronics and IT can be integrated seamlessly in vehicles with much shorter innovation cycles. TCP/IP stacks or protocols that use TCP/IP (TCP = Transmission Control Protocol) can thus communicate via MOST 150 without changes.

The MOST Network Interface Controller (NIC) is a hardware controller that is responsible for control of the physical layer
and implements important transfer mechanisms.

 

Protocol
Data transferData transfer
Data transfer on the MOST bus is organized in data frames that are created by the timing master with a fixed data rate
and passed on by the devices in the ring.

 

Data frames
The timing master usually creates data frames with a clock rate of 44.1 kHz, more rarely also 48 kHz. The size of the data
frames thus determines the bus speed of a MOST system. In the case of MOST 25, the size of a data frame is 512 bits (Figure 14). The synchronous and asynchronous areas of MOST 25 jointly use 60 bytes of the data frame. The division between the synchronous channels and the asynchronous channel is determined by the value of the boundary descriptors with a resolution of 4 bytes. The synchronous area must have at least 24 bytes (six stereo channels). This
means that between 24 and 60 bytes are permitted for the synchronous area and between zero and 36 bytes are permitted for the asynchronous area. The preamble is used for synchronization; the parity bit is used to recognize bit errors.

 

Transmission of control messages
The control channel is used to signal device statuses and for the messages required for system administration. So that
the control channel does not occupy too much bandwidth per frame, it has been distributed to 16 frames grouped to form a block. Each frame transports two bytes of the channel (Figure 14). In order to ensure recognition of the block start, the preamble of the first frame of a block bears a special bit pattern. In the case of MOST 25, the control channel has a gross bandwidth of 705.6 kbit/s.

Transmission of multimedia data
The synchronous channels are used for realtime communication of audio and video data, whereby the data interchange is controlled via the corresponding control commands on the control channel. A synchronous channel can be assigned a
certain bandwidth, which occurs with a resolution of one byte of a data frame. A stereo audio channel with a resolution
of 16 bits, for example, requires four bytes. In the case of MOST 25, depending on the value of the boundary descriptor, a maximum of 60 bytes are available for synchronous channels; this corresponds to 15 stereo audio channels.

 

Transmission of package data
Data are transferred packet-wise on the asynchronous channel. It is therefore suitable for the transmission of information that has no fixed data rate but requires high data rates at short notice. Examples are the transmission of track information of an MP3 player or a software update.

In the case of MOST 25, the asynchronous channel has a gross bandwidth of up to 12.7 MBit/s and it currently supports two modes: a slower 48-byte mode in which 48 bytes are available in each package for the net data transmission as well as a 1,014-byte mode that is more complex to implement. In order to ensure reliable transmission and flow control for the typically large asynchronous channel data volumes, an additional transport protocol (Data Link Protocol) is usually deployed; this is implemented in a driver layer at a higher level. This is either the MOST High Protocol (MHP), which has been specially developed for the MOST, or the common TCP/IP protocol that is placed on a corresponding adaptation layer called the MOST Asynchronous Medium Access Control (MAMAC).

 

Topology
MOST is organized in a ring structure (Figure 15). This is a point-to-multipoint data flow system (i. e. the streaming data
have one source and a number of sinks) and all the devices therefore share a joint system frequency that they acquire from the data flow. The devices are thus in phase and can transfer all data synchronously. This makes mechanisms for signal buffering and signal processing superfluous. A certain device acts as “timing master” and generates the data frames used for data transfer and to which the other devices synchronize.

Addressing
The devices are addressed on the MOST bus via a 16-bit address. Various types of addressing are available: Logical and
physical addressing as well as group addressing for simultaneous addressing of a defined group of control units.

 

Administration functions

The MOST standard defines the management mechanisms (network and connection masters) necessary for the operation of a MOST system. These mechanisms are described below.

Network master

The network master is implemented by a marked device in a MOST system and is responsible for the configuration of the
system. In current systems, the network master is usually implemented using the head unit (i. e. control panel) of the infotainment system. This device is often also the timing master at the same time. The other devices of the MOST system are referred to as network slaves in this context.

 

Connection master
The connection master manages the synchronous connections that exist in a MOST system at any particular point in time.

 

MOST application layer
For the transfer of control commands, status information, and events, the MOST standard defines a corresponding proto-
col on the application level. This protocol enables triggering of a certain function of an application interface (i. e. of an FBlock = function block) that is provided by any device within the MOST system.
The protocol for MOST control messages includes the following elements of a control message:
– The address of a device in the MOST system (DeviceID)
– An identifier for an FBlock (FBlockID) implemented by this device and its instance in the MOST system (InstID)
– The identifier for the function called up within the FBlock (FunctionID)
– The type of an operation (OpType).

 

Function block
A function block (FBlock) defines the interface of a certain application or of a system service. The sinks and sources for
multimedia data are each assigned to an FBlock that provides the correspondingfunctions for their administration. An FBlock can therefore have a number of sources and sinks enumerated via a source and sink number. An FBlock has functions that deliver information regarding the number and type of sources and sinks that it provides (SyncDataInfo, SourceInfo and SinkInfo). Over and above this, each FBlock with a source has an “Allocate” function that ituses to request a synchronous channel and connect the source to that channel. Accordingly, an FBlock with a sink has
a “Connect” function to connect the sink to a certain synchronous channel and a “DisConnect” function to break off this connection. An FBlock is addressed by means of an 8-bit FBlockID, which specifies the type of the FBlock, and an additional 8-bit InstID.

 

Function classes
To standardize the way in which functions are defined, the MOST standard specifies a series of function classes for prop-
erties. These determine which properties the function has and which operations are permissible.

 

Applications
As well as defining the lower layers necessary for data transfer, the MOST standard defines the interfaces for typical applications from the area of vehicle infotainment systems, e. g. a CD changer, amplifier or radio tuner. The FBlocks defined by the MOST Cooperation are summarized in a function catalog.

Standardization
The MOST standard is maintained by the MOST Cooperation, which also publishes the corresponding specifications. The
specifications are available on the homepage of the MOST Cooperation [7]. The MOST Corporation was founded in 1998 by BMW, Daimler, Becker Radio, and OASIS Silicon Systems with the aim of standardizing MOST technology.