Ethernet MAC(CSMACD) Carrier Sense Multiple Access/Collision Detection Packet? Sense Send Detect Carrier Collision Yes Discard Packet Jam channel attempts <16 b=CalcBackoffO wait(b) attempts++; attempts == 16 16
16 16 Ethernet MAC (CSMA/CD) Packet? Sense Carrier Discard Packet Send Detect Collision Jam channel b=CalcBackoff(); wait(b); attempts++; No Yes attempts < 16 attempts == 16 Carrier Sense Multiple Access/Collision Detection
Ethernet CSMA/CD: Making it word Jam Signal: make sure all other transmitters are aware of collision; 48 bits Exponential Backoff If deterministic delay after collision, collision will occur again in lockstep Why not random delay with fixed mean? >Few senders> needless waiting > Too many senders> too many collisions Goal: adapt retransmission attempts to estimated current load heavy load; random wait will be longer
17 17 Ethernet CSMA/CD: Making it word Jam Signal: make sure all other transmitters are aware of collision; 48 bits; Exponential Backoff: If deterministic delay after collision, collision will occur again in lockstep Why not random delay with fixed mean? »Few senders → needless waiting »Too many senders → too many collisions Goal: adapt retransmission attempts to estimated current load »heavy load: random wait will be longer
Ethernet Backoff Calculation Exponentially increasing random delay Infer senders from of collisions More senders> increase wait time First collision: choose K from [0, 1]; delay is K x 512 bit transmission times After second collision choose K from After ten or more collisions, choose K from {0,1,2,34,…,1023} 18
18 18 Ethernet Backoff Calculation Exponentially increasing random delay »Infer senders from # of collisions »More senders → increase wait time First collision: choose K from {0,1}; delay is K x 512 bit transmission times After second collision: choose K from {0,1,2,3}… After ten or more collisions, choose K from {0,1,2,3,4,…,1023}
Outline Aloha Ethernet mac Collisions Ethernet frames 19
19 19 Outline Aloha Ethernet MAC Collisions Ethernet Frames
Collisions
20 20 Collisions Time A B C