CSMA (Cont) In traditional LANs, propagation time is much less than frame transmission time Remark: this may not be true for 1 Gbps and 10gbps ethernet All stations know that a transmission has started almost immediately by carrier sense Details of cSma Stations first listen for clear medium (carrier sense) -If medium is idle, transmit the frame -If two or more stations start at about the same instant there will be a collision To account for this a station waits for an acK If no ack after a reasonable time then retransmit What should a station do if the medium is found busy? Three different approaches: nonpersistent CSMA, 1-persistent CSMA, and p-persistent CSMA 99 CSMA (Cont.) • In traditional LANs, propagation time is much less than frame transmission time — Remark: this may not be true for 1Gbps and 10Gbps Ethernet • All stations know that a transmission has started almost immediately by “carrier sense” • Details of CSMA — Stations first listen for clear medium (carrier sense) — If medium is idle, transmit the frame — If two or more stations start at about the same instant, there will be a collision. • To account for this, a station waits for an ACK • If no ACK after a reasonable time, then retransmit • What should a station do if the medium is found busy? — Three different approaches: nonpersistent CSMA, 1-persistent CSMA, and p-persistent CSMA