Civil Engineering Reference
In-Depth Information
are all connected as nodes in a system network. A system designer has to combine a
number of modules and make them perform in concert.
In this way, we have three concise layers:
• System layer
• Glue layer
• Module layer
The system is also seen as one big application, broken down into sub-applications
that reside in respective nodes and are coordinated by the glue layer. The glue layer
can then be seen as an API built upon a specific serial communication protocol.
The qualities of the glue layer are therefore highly dependent on the qualities of the
chosen communication protocol.
This concept has many advantages:
• The glue layer makes a clean and simple interface between interacting modules
as it is only control and data messages that are transmitted and received accord-
ing to need.
• Module designers can concentrate on the performance of the module and need
not know much about the system.
• The system designer has only to see to it that each module receives and transmits
messages as needed to perform in concert with the other nodes and does not need
to know much about each ECU.
• The performance of each module can be easily checked individually as a stream
of messages can be used to simulate the rest of the system. If the module re-
sponds to commands and is able to transmit messages according the specifica-
tion, it is OK.
• Modules and systems can be developed in parallel, saving time and money.
This concept does not fit the OSI model. The OSI model is based on a concept
where modules just need to exchange information and do not require any form of
coordination between communication sessions.
CAN is very well suited as the basis for a glue layer:
• It conveys 11-93 bits at a time from one node to all other connected nodes in a
safe, predictable way.
• Any bit rate between 10 kbps and 1 Mbps is supported.
• It can be used for any scheduled and/or unscheduled transmissions.
• Time scheduled, sequence scheduled and unscheduled messages can be transmit-
ted simultaneously with a guaranteed latency time.
4.5.3
Overview
CK is a glue layer based on CAN. It contains a set of rules—all in all 18—that
separate the module layer and system layer as much as possible. Only three of these
rules are mandatory (Table
4.21
).
