Channel partitioning mac protocols TDMA TDMA: time division multiple access access to channel in rounds each station gets fixed length slot (length pkt trans time)in each round unused slots go idle example: 6-station LAN, 1, 3, 4 have pkt, slots 25.6 idle 6-slot rame
Channel Partitioning MAC protocols: TDMA TDMA: time division multiple access • access to channel in "rounds" • each station gets fixed length slot (length = pkt trans time) in each round • unused slots go idle • example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle 1 3 4 1 3 4 6-slot frame
Channel partitioning mac protocols FDMA FDMA: frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example: 6-station LAN, 1, 3, 4 have pkt frequency bands 25.6 idle time ∩00 FDM cable
Channel Partitioning MAC protocols: FDMA FDMA: frequency division multiple access • channel spectrum divided into frequency bands • each station assigned fixed frequency band • unused transmission time in frequency bands go idle • example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle frequency bands FDM cable
Random access protocols When node has packet to send transmit at full channel data rate r no a priori coordination among nodes Two or more transmitting nodes + collision Random access mac protocol specifies how to detect collisions how to recover from collisions(e. g. via delayed retransmissions) Examples of random access mac protocols slotted AlOHa ALOHA CSMA, CSMA/CD, CSMA/CA
Random Access Protocols • When node has packet to send – transmit at full channel data rate R. – no a priori coordination among nodes • Two or more transmitting nodes ➜ “collision”, • Random access MAC protocol specifies: – how to detect collisions – how to recover from collisions (e.g., via delayed retransmissions) • Examples of random access MAC protocols: – slotted ALOHA – ALOHA – CSMA, CSMA/CD, CSMA/CA
Slotted aloha Assumptions: Operation: o all frames same size When node obtains fresh ● time divided into frame, transmits in next equal size slots(time slot to transmit 1 frame) - if no co∥ IS/on:node nodes start to can send new frame transmit only slot in next slot beginning -ifCo∥ lision;node nodes are retransmits frame in synchronized each subsequent slot ●f2 or more nodes with prob. p until transmit in slot,a‖l success nodes detect collisⅰon
Slotted ALOHA Assumptions: • all frames same size • time divided into equal size slots (time to transmit 1 frame) • nodes start to transmit only slot beginning • nodes are synchronized • if 2 or more nodes transmit in slot, all nodes detect collision Operation: • when node obtains fresh frame, transmits in next slot – if no collision: node can send new frame in next slot – if collision: node retransmits frame in each subsequent slot with prob. p until success
Slotted aloha nods[1[[[1 node 2 2 node 3 alot C E C S E C E S Pros Cons single active node can collisions wasting slots continuously transmit idle slots at full rate of channel nodes may be able to highly decentralized detect collision in less than time to transmit only slots in nodes need to be in sync packet clock synchronization sImple
Slotted ALOHA Pros • single active node can continuously transmit at full rate of channel • highly decentralized: only slots in nodes need to be in sync • simple Cons • collisions, wasting slots • idle slots • nodes may be able to detect collision in less than time to transmit packet • clock synchronization