Slotted AloHA To improve efficiency a modification of ALOHA known as slotted ALOHA, was developed Time is organized into uniform slots whose size equals the frame transmission time -Need a central clock ( or other sync mechanism) Transmission begins only at a slot boundary -Consequence: frames either miss or overlap totally Maximum channel utilization can be improved to about 37%
6 Slotted ALOHA • To improve efficiency, a modification of ALOHA, known as slotted ALOHA, was developed. • Time is organized into uniform slots whose size equals the frame transmission time —Need a central clock (or other sync mechanism) • Transmission begins only at a slot boundary —Consequence: frames either miss or overlap totally • Maximum channel utilization can be improved to about 37%
Slotted AloHA Node 1 packet Nodes 2& 3 packets Retransmission Retransmission 283 3 Time Slot Collision
7 Slotted ALOHA
CSMA Why aloha and slotted aloha are so inefficient? Stations don't check the channel status. They just send out frames without considering whether the channel is free or not, which creates too many collisions In fact it is not difficult for a station to sense the channel status(free or not) CSMA: Carrier Sense Multiple access Stations listen to the channel(carrier sense) Stations knowwhether the channel is free or not Stations transmit only if the channel is free -Collisions become rare Only if two or more stations attempt to transmit at about the same time, collisions could happen 8
8 CSMA • Why ALOHA and slotted ALOHA are so inefficient? — Stations don’t check the channel status. They just send out frames without considering whether the channel is free or not, which creates too many collisions. • In fact, it is not difficult for a station to “sense” the channel status (free or not). • CSMA: Carrier Sense Multiple Access — Stations listen to the channel (carrier sense) — Stations “know” whether the channel is free or not — Stations transmit only if the channel is free — Collisions become rare • Only if two or more stations attempt to transmit at about the same time, collisions could happen
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 9
9 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
Nonpersistent CSMA a station wishing to transmit listens to the medium and obeys the following rules 1. If medium is idle, transmit; otherwise, go to step 2 2. If medium is busy, wait an amount of time drawn from a probability distribution and repeat step 1 The use of random delays reduces probability of collisions Consider two stations become ready to transmit at about the same time while another transmission is in progress If both stations delay the same amount of time before retrying both will attempt to transmit at same time>collision Drawback: Capacity is wasted because medium will generally remain idle following the end of a transmission even if there are one or more stations waiting to transmit. 10
10 Nonpersistent CSMA • A station wishing to transmit listens to the medium and obeys the following rules: 1. If medium is idle, transmit; otherwise, go to step 2 2. If medium is busy, wait an amount of time drawn from a probability distribution and repeat step 1 • The use of random delays reduces probability of collisions — Consider two stations become ready to transmit at about the same time while another transmission is in progress — If both stations delay the same amount of time before retrying, both will attempt to transmit at same time → collision • Drawback: — Capacity is wasted because medium will generally remain idle following the end of a transmission, even if there are one or more stations waiting to transmit