Environmental Engineering Reference
In-Depth Information
been common for multiple access collision detection (MA/CD) protocols in the
past. Time triggered protocols (TTP) have been proposed for many years and are
now beginning to enter the automotive arena as a new protocol derived from a TTP/
C basis known as FlexRay.
To understand automotive communications, it is important to understand the
basics of networked communications. In 1983 the Open Systems Interconnection
(OSI) committee of the International Standards Organization (ISO) developed a
layered model to describe how two different computer systems may share files with
each other over a common network. This OSI model became the industry standard,
7-layered network functionality, open architecture network used universally. Open
architecture means that once defined the standard is open to the world to use and
build systems that meet the interface definition without need for patents or licen-
sing. Closed architectures are proprietary, their message format and protocol not
accessible, and expansions and enhancements generally costly.
In the OSI 7-layer model shown in Figure 4.48, the network architecture is
clearly defined and published.
In Figure 4.48, an application running on the computer at Node A is sending a
packet of data to a remote computer running at Node B over a communications
channel. The communications channel may be cable, twisted pair or wireless. The
arrows indicate the progress of the packet of data, for example, an engineering
drawing, as it moves from application layer 7 on computer A to applications layer 7
on computer B. This process describes very closely how the procedure of email, file
transfer, Internet web browsers etc. work. It also describes the background of in-
vehicle network protocols to be addressed later. For this illustration, the data packet
Computer: Node A
Computer: Node B
7 Application
7 Application
6 Presentation
6 Presentation
5 Session
5 Session
4 Transport
4 Transport
3 Network
3 Network
2 Data link
2 Data link
1 Physical
1 Physical
Communications channel
Figure 4.48 OSI 7-layer network model
