Pure(Unslotted) ALOHA Unslotted Aloha: simpler, no synchronization Pkt needs transmission >)Send without awaiting for beginning of slot Collision probability increases >)Pkt sent at to collide with other pkts sent in [to-1, to +1 will overlap will overlap with start of with end of i' s frame→←- is frame node i frame t- 1 t+1 0 11
11 11 Pure (Unslotted) ALOHA Unslotted Aloha: simpler, no synchronization Pkt needs transmission: » Send without awaiting for beginning of slot Collision probability increases: »Pkt sent at t0 collide with other pkts sent in [t0 -1, t0+1]
Outline Aloha Ethernet MAc Collisions Ethernet frames 12
12 12 Outline Aloha Ethernet MAC Collisions Ethernet Frames
Ethernet First practical local area network built at Xerox parc in 70s Dominant 'LAN technology: >Cheap >Kept up with speed race: 10, 100, 1000 Mbps TRANSCENVER STATION TAP XI NTERFACE CABLE INTERFACEU COTELER 正MARR THE ETHER A 13
13 13 Ethernet First practical local area network, built at Xerox PARC in 70’s “Dominant” LAN technology: »Cheap »Kept up with speed race: 10, 100, 1000 Mbps Metcalfe’s Ethernet sketch
Ethernet mac- Carrier Sense Basic idea >> Listen to wire before transmission Hidden Exposed 》 Avoid collision with St louis active transmission Why didn't ALOHA Chicago have this? 、CMU 、CMU )In wireless, relevant contention at the Chicago receiver, not sender Hidden terminal Exposed terminal 14
14 14 Ethernet MAC – Carrier Sense Basic idea: »Listen to wire before transmission »Avoid collision with active transmission Why didn’t ALOHA have this? »In wireless, relevant contention at the receiver, not sender – Hidden terminal – Exposed terminal NY CMU Chicago St.Louis Chicago CMU NY Hidden Exposed
Ethernet mac- Collision Detection But: aloha has collision detection also? >)That was very slow and inefficient Basic idea: >Listen while transmitting >If you notice interference> assume collision Why didn't ALOHa have this? >Very difficult for radios to listen and transmit ) Signal strength is reduced by distance for radio Much easier to hear"local, powerful radio station than one in ny You may not notice any“ interference” 15
15 15 Ethernet MAC – Collision Detection But: ALOHA has collision detection also? »That was very slow and inefficient Basic idea: »Listen while transmitting »If you notice interference → assume collision Why didn’t ALOHA have this? »Very difficult for radios to listen and transmit »Signal strength is reduced by distance for radio – Much easier to hear “local, powerful” radio station than one in NY – You may not notice any “interference