Introduction to Media Access
Control Protocols
Yerang Hur and Jiaxiang Zhou
System Design Research Lab.
Dept. of Computer and Information
Science
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
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
CAN Message Frame
Frames do not contain addresses.
SOF arbitration control data field CRC ACK EOF
field field
1bit 11 bits 6 bits 0-8 bytes 16 bits 2bits 7bits
Identifier: the identifier with the smallest value has the highest priority.
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.
TDMA
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
frame
slot