CSMA

CSMA is a Multi-Access Protocol

Carrier Sense Multiple Access (CSMA) is like human conversation:

• Carrier sense:
  • Listen before speaking and don’t interrupt
• Collision detection:
  • Detect simultaneous talking and shut up
• Random access:
  • Wait for a random period of time before trying to talk again
  • $p$-persistent CSMA - if channel is idle, transmit the frame with probability $p$

But collisions can still occur

• Wasted transmissions

Collision Detection

The “CD” part of CSMA/CD

• Collision happens when two packets overlap at receiver

Note

Ethernet does collision detection, but not WiFi

Worst-case scenario of collision:

• Two farthest nodes in the LAN
• Transmission time should be greater than or equal to 2 * propagation time for CSMA/CD to work
• Formula for minimum packet size (P) or maximum length (L):
  • $2L/c=2d=P/B$, where $c$ is the speed of propagation

• When receiver detects collision
  • Stop sending
  • Send jamming signal to ensure collision detection by all nodes in the link

CSMA for WiFi

802.11 does Collision Avoidance, but cannot do Collision Detection because of these two problems:

1. Cannot detect all collisions
   • Hidden terminal problem
     • Happens because 802.11 uses physical carrier sensing, which is susceptible to the hidden terminal problem
     • Virtual Carrier Sensing can help with that
   • Signal strength fades over distance
   • (see WiFi Broadcast Limitations for more info)
2. Cannot listen while sending
   • Strength of received signal is much smaller
   • Expensive to build hardware that detects collisions