Multiple access protocols cont. #

CSMA cont. #

CSMA/CD is used by ethernet.

CD = collision detection, CA = collision avoidance

It is not easy to detect collision (because of signal strength) in wireless LAN.

CSMA/CD algorithm #

NIC = network interface card

Binary backoff:

After the \( m \) th collision, we choose \( K \) randomly in the range of \( \{0, 1, 2, \ldots, 2^m - 1\} \) . We then wait \( 512K \) bit times to retransmit.

So, if

• \( m = 0 \) , there is no collision, immediate transmission
• \( m = 1 \) , there has been 1 collision, \( K \) will be either 0 or 1, and we we wait \( 512K = 0 \) or \( 512K = 512 \) bit times until retransmission.
• and so on…

Note: A bit time is the speed that the NIC takes to send 1 bit. Read more.

“Taking turns” MAC protocols #

Cable access network #

Summary of MAC protocols #

LAN #

LAN = local area network

ARP: address resolution protocol #

Addressing #

• A will know B’s IP address via DNS.
• A will know R’s address via DHCP.
• A will know R’s MAC address via ARP.

Ethernet #

Switches #