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Open Systems Interconnect (OSI) Model
Interacts with software requiring network communications;
identifies partners, resources and synchronization
Layer 5
Session
Establishes, manages, and terminates connections between
computers
Segment/
Datagram
Layer 4
Transport
Provides transparent transfer of data between hosts; end-toend error recovery and flow control
Packet
Layer 3
Network
Provides switching, routing, addressing, error handling,
congestion control, and packet fragmentation and sequencing
Frame
Layer 6
Formats and encrypts data; unifies syntax and semantics
Presentation
Layer 2
Data Link
Encodes/decodes data packets into bits
Logical Link Control: handles error in physical layer, flow control
and frame synchronization
Media Access Control: defines transmission protocol and
management
Bit
Media
Host
Data
Layer 7
Application
Layer 1
Physical
Carries bit stream; defines physical characteristics such as
voltage/light levels and frequencies
Controller Area Network Standard
• Defines Physical Layer (L1)
• Defines Data Link Layer (L2)
• Defines how to Transport (L4) small (8 bytes)
datagrams
• No flow control (L3 and L4)
• No sequencing and fragmentation (L3)
• No Session (L5) or Presentation (L6) specs
• Different Higher Layer Protocols (HLPs) handle
the rest
Common CAN HLPs
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CanKingdom
CANopen
CCP/XCP
DeviceNet
SAE J1939
OSEK
SDS
• These define the “Object”
Layer (layers not defined
by the CAN standard)
CAN Physical Layer – Voltages
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Open collector (wired-OR); NRZ
Dominant bits are logical 0
Recessive bits are logical 1
Provides arbitration free transmission
Automatic Collision Detection
Transmitter B
Transmitter A
Dominant (0)
Recessive (1)
Dominant (0)
Dominant (0)
Dominant (0)
Recessive (1)
Dominant (0)
Recessive (1)
If A transmits recessive (1) and sees dominant (0) from B, A knows
collision occurred and stops transmitting (will retry 6 clock cycles
after end of dominant message)
CAN Physical Layer – Timing
• Each node has its own clock
• Synchronization done by dividing bit time into
four segments
Bit Time
Sync
Propagation
Phase Segment 1
Phase Segment 2
Clock
• Phases 1 & 2 adjusted based on network and node
conditions
• Sample between Phase 1 & 2
CAN Data Link Layer
• Specifies four message types
• Data: contains data for transmission
• Data Request (Remote): requests transmission
of a specific identifier
• Error: transmitted by any node detecting an
error
• Overload: injects a delay between data and/or
remote frames
CAN Data Frames
Field
Bit Length Description
Start-of-Frame
1
Signals start of frame transmission
Identifier
11
Unique id for data (embedded priority)
Remote Trans. Request (RTR)
1
Optional, but must be dominant (0)
ID Extension bit (IDE)
1
Optional, but must be dominant (0)
Reserved bit (r0)
1
Reserved, but must be dominant (0)
Data Length Code (DLC)
4
Number of data bytes (0-8)
Data Field
0-8 bytes
Actual data
CRC
15
Cyclic redundancy check
CRC Delimiter
1
Must be recessive (1)
ACK Slot
1
Transmit recessive, receiver sends dominant
ACK Delimiter
1
Must be recessive (1)
End-of-Frame (EOF)
7
Must be recessive (1)
For Data Request RTR = 1 (recessive) and DLC = 0 (data field empty)
Error Frames
• Active error generated by transmitter
• Passive error generated by receiver
• Error Types
– Bit: Send recessive, read dominant
– Stuff: more than 5 consecutive bits of same
polarity
– CRC: computed and received CRCs not equal
– Form: invalid bits in field
– ACK: no acknowledgement from receiver
Overload Frames
• Two overload conditions
– Internal conditions of receiver – it can’t keep up
– Dominant bit detected during expected
intermission (interframe space)
Byte Data Link Controller (BDLC)
• Physical Layer has three forms
– 2-wire: 10.4 Kbps, UART, NRZ (Chrysler)
– 2-wire: 41.6 Kbps pulse width modulated (Ford)
– 1-wire: 10.4 Kbps variable pulse width (GM)
• High level 4.25-20 V; Low level < 3.5 V
• Buses use weak pull-down, driver pulls it high
• High signals are dominant
• High and low values are bit symbols with specific times
Active
Passive
Logic 0
Logic 1
128 s high
64 s low
64 s high
128 s low
BDLC – Data Link Layer
SOF
Header
Field
Data
Length
CRC EOD
IFR
CRC
EOF
Description
Start-of-Frame (SOF)
200 s high Signals start of frame transmission
Header
1 or 3 bytes
Data
variable
CRC
1 byte
End-of-Data (EOD)
In-Frame Response (IFR)
CRC
End-of-Frame (EOF)
200 s low
Message priority; IFR required/disallowed;
addressing mode; and message type
Actual data
Cyclic Redundancy Check for data
Signals end of data; allows receivers to respond
variable
Receivers may append response here
1 byte
Cyclic Redundancy Check for response
80 s low
Signals end of frame
3 byte headers contain destination and source addresses