Introduction to Media Access Control Protocols

1
Introduction to Media Access Control Protocols

• Yerang Hur and Jiaxiang Zhou
• System Design Research Lab.
• Dept. of Computer and Information Science

2
Some Examples of MAC Layer Protocols for Control
Systems

• CSMA/CD Carrier Sense Multiple Access with
  Collision Detection
• Ethernet
• Token bus
• ControlNet
• CAN bitwise priority
• DeviceNet (Rockwell), SDS (Honeywell)
• TDMA Time Division Multiple Access
• TTP

3
Controller Area Network (CAN)

• protocol mainly used in the automotive industry
• used for transmitting short messages
• 1M bps
• a message with a unique identifier
• MAC CSMA with arbitration on message priority
• simultaneous transmission arbitrated based on the
  message priority

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CAN Message Frame
Frames do not contain addresses.
arbitration field
control field
data field
CRC
ACK
EOF
SOF
1bit
11 bits
0-8 bytes
16 bits
6 bits
2bits
7bits
Identifier the identifier with the smallest
value has the highest priority.
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CAN Bus Arbitration

• Bit stream is synchronized on the start bit.
• A sender waits till the bus is free and then
  starts to send the identifier bit by bit.
• A sender listens to the network and if a sender
  observes a bit different from the one itself
  sends, it stops transmission and yields to other
  nodes.

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TDMA
1 2 3 4 5 6 7 8
frame
slot