Controller Area Network or CAN protocol
is a method of communication between various electronic devices like
engine management systems, active suspension, ABS, gear control,
lighting control, air conditioning, airbags, central locking etc
embedded in an automobile. An idea initiated by Robert Bosch GmbH in
1983 to improve the quality of automobiles thereby making them more
reliable, safe and fuel efficient. With the developments taking
place in the electronics and semiconductor industry the mechanical
systems in an automobile were being replaced by more robust electronics
system which had an improved performance. New technologies, products and
inventions with added or improved functions started to shape a complete
new era for the automobile industry which promised more robust vehicles
with use of electronics. The increasing number of electronic devices
used communication signals with more complex interrelations between
them. Thereby making the life difficult for automobile engineers when
they designed systems wherein one electronic device needs to communicate
with others to operate. Realizing the problem of communication between
different electronic modules Robert Bosch came up with this new protocol
called CAN which was first released in 1986. CAN
provide a mechanism which is incorporated in the hardware and the
software by which different electronic modules can communicate with each
other using a common cable.
Need for CAN
A
vehicle contains a network of electronic devices to share
information/data with each other. For example A spark ignition engine
requires a spark to initiate the combustion chamber at the correct time
so it communicates with engine control unit that adjusts the exact
timing for ignition to provide better power and fuel efficiency..
Another example is of a transmission control unit that changes the ratio
of gear automatically with the changing speed. It uses information from
engine control unit and various sensors in the system. Every electronic device has an ECU/MCU (electronic/microcontroller
control unit) with its own set of rules to share/transfer information.
For two or more devices to interact they should have the necessary
hardware and software which allows them to communicate with each other.
Before CAN was introduced in the automobiles, each electronic device was
connected to other device using the wires (point to point wiring) which
worked fine, when the functions in the system were limited. The figure below is the pictorial view of the point to point wiring connection.
One
of the major problems for automotive engineers was linking the ECUs of
the different devices so that real time information can be exchanged.
CAN protocol was designed to address this problem. It laid down the
rules through which the various electronic devices can exchange
information with each other over a common serial bus. This in turn
reduced the wiring connections to a great extend thereby reducing the
bulkiness and complexity of the system. The image below shows how the
different devices are connected using the CAN protocol.
Also
the standard technology of time, asynchronous transmitter/receiver did
not support multi domain communications. Domain is a group of electronic
devices that have almost similar requirements to work in the system.
For example CD/DVD PLAYER, GPS system, monitors and displays etc. form a
single domain. Similarly air conditioning and climate control,
dashboards, wipers, lights doors etc. form another domain. Hence the
electronic devices implanted in a vehicle can be classified under
different domain. CAN facilitates multi-domain communication for the
engineers.
The table below shows the different domains
What is CAN protocol and how is it implemented?
CAN
protocol can be defined as the set of rules for transmitting and
receiving messages in a network of electronic devices. It means that it
defines how data is transferred from one device to another in a network.
It was designed specifically looking into the needs of the automobile
industry. However CAN's robust architecture and advantages has forced many industries like Railway, Aircrafts, medical etc to adopt CAN protocol in their systems.
Every electronic device (also known as the node)
which needs to communicate using the CAN protocol is connected with
each other via a common serial bus to transmit and receive messages.
For data exchange to happen among the nodes they must have the necessary hardware and the software embedded inside them.
As
shown in the above figure a typical CAN network consists of various
nodes. Every node has a Host controller (ECU/MCU) which is responsible
for the functioning of the respective node. In addition to the host
controller every node has a CAN controller and CAN transceiver.
CAN controller convert the messages of the nodes in accordance with the
CAN protocols to be transmitted via CAN transceiver over the serial bus
and vice versa. CAN controller is a chip which can either be added
separately or embedded inside the host controller of the node.
CAN
does not follow the master-slave architecture which means every nodes
has the access to read and write data on the CAN bus. When the node is
ready to send data, it checks availability of the bus and writes a CAN
frame onto the network. A frame is defined structure, carrying
meaningful sequence of bit or bytes of data within the network. CAN
transmitted frame does have address neither of transmitting node or the
receiving node. CAN is a message based protocol. A message can be
defined as a packet of data which carries information. A CAN message is
made up of 10 bytes of data. The data is organized in a specific
structure called frame and the information carried in every byte is
defined in the CAN protocol. Protocols are generally of two types:
address based and message based. In an address based protocol the data
packets contain the address of the destination device for which the
message is intended. In a message based protocol every message is
identified by a predefined unique ID rather than the destination
addresses. All nodes on CAN receive the CAN frame and depending on ID on
the node CAN decides whether to accept it or not. If multiple nodes
send the message at the same time than the node with highest priority
(lowest arbitration ID) gets the bus access. Lower priority nodes wait
till the bus is available.
BENEFITS
· Low cost:
As CAN serial bus uses two wires, it offers good price/performance
ratio. Also, driven by high volume production of low cost protocol
devices, they are relatively cheap.
· Reliable: Because of excellent error
detection and error handling mechanisms used by CAN, it offers high
reliability transmission. It is also highly immune to Electromagnetic
Interference
· Flexibility: CAN Nodes can be easily connected / disconnected. Also, the number of nodes is not limited by the protocol
· Good Speed: CAN supports data rate of 1 MBit/s @ 40m bus length.
· Multi-master communication: Any node can access the bus
· Fault Confinement: Faulty nodes do not disturb the communication.
· Broadcast capability: Messages can be sent to one /many/all nodes.
· Standardized:
ISO has standardized CAN via ISO-DIS 11898 high speed applications) and
ISO-DIS 11519-2 (low speed applications). CAN protocol is also
standardized by industry organizations such as SAE-Society of Automotive
Engineers.
CAN Architecture
CAN
uses the existing OSI reference model to transfer data among nodes
connected in a network. The OSI reference model defines a set of seven
layers through which the data passes during communication between
devices connected in a network. The 7-layered structure of the OSI model
is a very robust approach widely adopted in many communication
protocols. The figure below gives the clear picture of OSI model.
Each layer has its specific function that supports the layer above and below as described under-
· Application layer
It
serves as a window for users and application processes to access
network services. The common functions of the layers are resource
sharing, remote file access, network management, electronic messages and
so on.
· Presentation layer
The
most important function of this layer is defining data formats such as
ASCII text, EBCDIC text BINARY, BCD and JPEG. It acts as a translator
for data into a format used by the application layer at the receiving
end of the station.
· Session layer
It
allows to establishing, communicating and terminating sessions between
processes running on two different devices performing security, name
recognition and logging.
· Transport layer
The
transport layer ensures that messages are delivered error-free, in
sequence, and without loss or duplication. It relieves the higher layer
from any concern with the transfer of data between them and their peers.
· Network layer
It
provides end to end logical addressing system so that a packet of data
can be routed across several layers and establishes, connects and
terminates network connections.
· Data link layer
It
packages raw data into frames transferred from physical layer. This
layer is responsible for transferring frames from one device to another
without errors. After sending the frame it waits for the acknowledgement
from receiving device. Data link layer has two sub layers:
{C1. MAC (Medium Access Control) layer: It performs frame coding, error detection, signaling, serialization and de-serialization.
LLC (Logical Link Control) layer: The LLC sub layer provides multiplexing mechanisms that make it possible for several network protocols (IP, Decnet and Appletalk)
to coexist within a multipoint network and to be transported over the
same network medium. It performs the function of multiplexing protocols
transmitted by MAC layer while transmitting and decoding when receiving
and providing node-to-node flow and error control.
· Physical layer
The
physical layer transmits bit from one device to another and regulates
the transmission of bit streams. It defines the specific voltage and the
type of cable to be used for transmission protocols. It provides the hardware means of sending and receiving data on a carrier defining cables, cards and physical aspects.
CAN
protocol uses lower two layers of OSI i.e. physical layer and data link
layer. The remaining five layers that are communication layers are left
out by BOSCH CAN specification for system designers to optimize and
adapt according to their needs.
Function of each part is shown with the figure below.
The concept of the CAN protocol can be understood using the figure above. Every node has a Host controller also known as micro-controller which is a
small and low-cost computer .Host controller implements application
layer of OSI model. Micro-controller gathers information from other
electronic control units like braking, steering, power windows etc. to
communicate with other nodes and transfers it to CAN controller. CAN
controller incorporate logical link control and MAC medium access
control of data link layer. LLC allows filtering of messages by using
unique ID on each message then MAC sub layer frames the message. Once,
framing is done it is followed by arbitration, error detection and
acknowledgement that all comes under MAC sub layer of data link. The
frame is transferred to CAN trans-receiver, for encoding and decoding.
Finally CAN trans-receiver synchronizes with the CAN bus to send the
message to anther node.
Terms associated with CAN protocol.
BUS VALUES
Binary values in CAN protocol are termed as dominant and recessive bits.
· CAN define the logic “0” as dominant bit.
· CAN define the logic “1” as recessive bit.
In the CAN system dominant bit always overwrites the recessive bit.
Message based communication
A Message is packet of data that carries the information to be exchanged between the nodes. Each
message in CAN has a unique identification number. The identification
number is specified according to the content of the message and stored
in message identifier. This identification number is also unique within
the network so when the transmitting node places the data on the network
for access to all nodes it checks unique ID number to allow the message
to pass through the filter and rest are ignored. This is done to save
the time spent on sorting. With message based protocol other nodes can
be added without re-programming since the units connected to the bus
have no identifying information like node addressing. So there is no
change needed in the software and hardware of any of the units connected
on the bus.
Message framing
Messages
in CAN are sent in a format called frames. A frame is defined
structure, carrying meaningful sequence of bit or bytes of data within
the network. Framing of message is done by MAC sub layer of Data Link
Layer .There are two type of frames standard or extended .These frames
can be differentiated on the basis of identifier fields. A CAN frame
with 11 bit identifier fields called Standard CAN and with 29 bit identifier field is called extended frame.
Standard frame
Various fields in standard CAN are as follows-
{C· SOF
- Start of Frame bit. It indicates start of message and used to
synchronize the nodes on a bus. A dominant bit in the field marks the
start of frame.
· IDENTIFIER
- It serves dual purpose one, to determine which node has access to the
bus and second to identify the type of message.
· RTR
- Remote Transmission Request. It identifies whether it’s a data frame
or a remote frame .RTR is dominant when it is a data frame and recessive
when it is a remote frame.
{C}{· IDE
– Identifier Extension. It is used to specify the frame format.
Dominant bit is for standard frame and recessive for extended frame.
{C}· R0 - Reversed bit. Not used currently and kept for future use.
{C· DLC – Data Length Code. It is 4 bit data length code that contains the number of bytes being transmitted.
{· DATA– Used to store up to 64 data bits of application data to be transmitted.
{C}· CRC–
Cyclic Redundancy Check. The 16-bit (15 bits plus delimiter) cyclic
redundancy check (CRC) contains the checksum of the preceding
application data for error detection.
{· ACK –
Acknowledge (ACK) field. It compromises of the ACK slot and the ACK
delimiter. When the data is received correctly the recessive bit in ACK
slot is overwritten as dominant bit by the receiver.
{C}· EOF– End of Frame (EOF). The 7-bit field marks the end of a CAN frame (message) and disables
Bit - stuffing, indicating a stuffing error when dominant.
{C}· IFS - Inter Frame Space that specifies minimum number of bits separating consecutive messages. It provides the intermission between two frames and consists
of three recessive bits known as intermission bits. This time allows
nodes for internal processing before the start of next frame.
EXTENDED CAN
It is same as 11-bit identifier with some added fields
{C· SRR- Substitute Reverse Request. The
SRR bit is always transmitted as a recessive bit to ensure that, in the
case of arbitration between a Standard Data Frame and an Extended Data
Frame, the Standard Data Frame will always have priority if both
messages have the same base (11 bit) identifier.
{· R1- It is another bit not used currently and kept for future use.
Message frame
There are four different frames which can be used on the bus.
Data frames-
These are most commonly used frame and used when a node transmits
information to any or all other nodes in the system. Data Frames
consist of fields that provide additional information about the message
as defined by the CAN specification. Embedded in the Data Frames are
Arbitration Fields, Control Fields, Data Fields, CRC Fields, a 2-bit
Acknowledge Field and an End of Frame.
· Remote frames
- The purpose of the remote frame is to seek permission for the
transmission of data from another node. This is similar to data frame
without data field and RTR bit is recessive. For example, the
microprocessor controlling the central locking on your car may need to
know the state of the transmission gear selector from the power train
controller.
· Error frames
– If transmitting or receiving node detects an error, it will
immediately abort transmission and send error frame consisting of an
error flag made up of six dominant bits and error flag delimiter made up
of eight recessive bits. The CAN controller ensures that a node cannot
tie up a bus by repeatedly transmitting error frame.
· Overload frame-It
is similar to error frame but used for providing extra delay between
the messages. An Overload frame is generated by a node when it becomes
too busy and is not ready to receive.
Arbitration
It is a mechanism which resolves the conflict when two or more nodes try to send the message at the same time. In this technique whenever
the bus is free any unit can transmit a message. If two or more units
starts transmitting at the same time access to the bus is conflicted,
but this problem can be solved by arbitration using identifier. During
arbitration every transmitter compares the value of transmitted bit with
bit value on the bus. If the bit value is same, the node continues to
send the bits. But at any time if transmitted bit value is different
from bus value the dominant bit overwrites the recessive bits. The
arbitration field of the CAN message consists of an 11- or 29-bit
identi?er and a remote transmission (RTR) bit. The identifier having
lowest numerical value has the highest priority. RTR simply
distinguishes between remote frame for which RTR is recessive and data
frame for which RTR is dominant. If both data frame and remote frame
with the same identifier is initiated at the same time data frame will
prevail over remote frame. With the concept of arbitration neither
information nor time is lost.
CAN as a CSMA protocol
CSMA
is a carrier sense, multiple-access protocol in which node verifies the
absence of traffic before transmitting on a shared medium such as
electrical bus. In CSMA each node on a bus waits for a specific
time before sending the message. Once this wait period is over every
node has equal opportunity to send the message. Based on pre-programmed
priority of each message in identifier field i.e. highest priority
identifier wins the bus access. It is implemented on the physical layer
of OSI model. Let us understand CSMA with an example. In a discussion
every person gets an equal opportunity to voice their thoughts however
when a person is talking others keep quiet and listens and waits for
their chance to speak (carrier sense). But if two or more people start
speaking at the same time then they detect the fact and quit speaking
(collision detection).
Error Checking and Fault Confinement
This
is one of the attributes of CAN that makes it robust. CAN protocol has
five methods of error checking, out of which three are at message level
while other two are at bit level. Every frame is simultaneously accepted
or rejected by every node in the network. If a node detects an error it
transmits an error flag to every node and destroys the transmitted
frame and the transmitting node resends the frame
Message level
CRC check
In
this stage a 15-bit cyclic redundancy check value is calculated by
transmitting node and is transmitted in the CRC field. This value is
received by all nodes. Then all the nodes calculate CRC value and
matches the results with the transmitted value. If values differ than an
Error Frame is generated. Since one of the nodes did not receive the
message properly it is resent.
ACK slots
When
transmitting node sends a message, a recessive bit is sent in
acknowledgement slot. After message is received acknowledge slot is
replaced by dominant bit which would acknowledge that at least one node
correctly received the message. If this bit is recessive, then none of
the node has received the message properly.
Form Error
End
of frame, Inter-frame space, Acknowledge Delimiter are fields that are
always recessive, if any node detects dominant bit in one of these
fields than CAN protocol calls it a violation and a Form Frame is
generated and original message is resent after certain period.
Bit level
Stuff error
Bit
stuffing - It is a very common technique used in telecommunication and
data transmission to insert non -informative bits to have same bit
rates or to fill the frames .These extra bits are removed by data link
layer to retrieve the original message. This same technique is used in
bit error.CAN bus is never idle because it uses NRZ method. After five
consecutive bits of the same value, a bit with a complement or opposite
value is stuffed into the bit stream. If six bits of the same value are
detected between SOF and CRC delimiter, error frame is generated. Upon
detection of errors, the transmission is aborted and frame is repeated.
If errors continue, then the station or node may switch itself off to
prevent the bus from being tied up.
Bit error
A
node that is sending the bit always monitors the bus. If the bit sent
by transmitter differs from the bit value on the bus then error frame is
generated. But there is an exception in case of arbitration field or
Acknowledge slot where a recessive bit is sent and a dominant bit is
received. Then no Bit Error is occurs when dominant bit is monitored.
Conclusion
CAN protocol
initially developed for in-vehicle networking of automobiles has
expanded its applications in various other industries. The application
started for luxurious cars is now being used in heavy duty vehicles like
trucks, buses, trains and rail vehicles. The unique feature of CAN that
allows various electronic units to communicate with each other made it
important in healthcare domain. For example intensive care units and
operating rooms where time and communication is of utmost importance.
Entertainment industry also used CAN protocols to improve features in
studios to control lights and door system and to control stage of
theatres, event halls etc. Gambling machines and toys are other examples
in entertainment field. In the field of science the high energy
experiments and astronomical telescope use CAN in embedded network.
