Csma
- 1. Topic : Networking and LAN Sub topic : Describe CSMA . 1
- 2. Objectives On completion of this period , you would be able to know about • CSMA ( Carrier Sense Multiple Access) • Types of CSMA – Persistent CSMA – Non Persistent CSMA – CSMA / CD • Comparison 2
- 3. CSMA (Carrier Sense Multiple Access) • CSMA: Its concept is simply listen before talk (LBT) • If channel is sensed idle : transmit entire frame • If channel is sensed busy : defer transmission • We could achieve better throughput if we could listen to the channel before transmitting a packet • This way, we would stop avoidable collisions • To do this, we need CSMA protocols 3
- 4. Assumptions with CSMA Networks • Constant length packets • No errors, except those caused by collisions • No capture effect • Each host can sense the transmissions of all other hosts • The propagation delay is small compared to the transmission time 4
- 5. CSMA (cont’d) • There are several types of CSMA protocols – 1-Persistent CSMA – Non-Persistent CSMA – P-Persistent CSMA 5
- 6. 1-Persistent CSMA • Sense the channel – If busy, keep listening to the channel and transmit immediately when the channel becomes idle – If idle, transmit a packet immediately • If collision occurs – Wait a random amount of time and start over again This protocol is called 1-persistent because the host transmits with a probability of 1 whenever it finds the channel idle 6
- 7. 1-Persistent CSMA (cont’d) • No longer propagation delay • Even if prop. delay is zero, there will be collisions Example - If stations B and C become ready in the middle of A’s transmission, B and C will wait until the end of A’s transmission and then both will begin transmitted simultaneously, resulting in a collision - If B and C were not so greedy, there would be fewer collisions 7
- 8. Non-Persistent CSMA • Sense the channel – If busy, wait a random amount of time and sense the channel again – If idle, transmit a packet immediately • If collision occurs – wait a random amount of time and start all over again Merits • Better channel utilization • Longer delays • Reduces chances of collision • Reduces efficiency 8
- 9. Tradeoff between 1- and Non Persistent CSMA • If only B becomes ready in the middle of A’s transmission – 1-Persistent: B succeeds as soon as A ends – Non-Persistent B may have to wait • If B and C become ready in the middle of A’s transmission – 1-Persistent B and C collide – Non-Persistent B and C probably do not collide 9
- 10. Fig .1 Fig .2 10
- 11. P-Persistent CSMA • Applicable to slotted channels • When a station becomes ready to send, it senses the channel – if it is idle, station transmits with a probability of p – it defers until next slot with a probability of q = 1-p • If the slot is also idle – either station transmits or it defers with probabilities of p & q • This is repeated until either the frame has been transmitted or another station begun transmitting 11
- 12. CSMA efficiency • Decreases with t (prop) / t (trans) • Decreases with increasing distance between nodes • T (trans) = R / L where R is the link speed in bits/sec and L is the frame length in bits • So, efficiency decreases with increasing link speed R 12
- 13. CSMA / CD : Network components Carrier Sense Multiple Access / Collision Detection • Medium (coax, tp, fiber) • Transceivers • AUI cable (drop cable; station to coax) • NIC (network interface card -MAC protocol logic) • Repeaters (needed to extend coax) • Test equipment • Hub (multipart repeater) : for star configuration • Bridges (to connect to other LANs) 13
- 14. CSMA / CD : topologies, media , etc. • media – coaxial cable – twisted pair – fiber (less common) • topologies – bus - original design; widespread for many years – star with hub in middle; now becoming common • data rates – 10 Mbps – 100 Mbps – Gbps on the way 14
- 15. CSMA / CD Sending nodes are able to detect collisions while transmitting • If medium is idle , transmits • If busy, listens for idle then transmits • While transmitting node continues to listen to see if a collision has occurred • If collision detected stops transmission, generates a jamming signal on the bus to indicate all nodes about 15 collision
- 16. CSMA/CD Protocol Fig .3 16
- 17. CSMA/CD Protocol • All hosts transmit & receive on one channel • Packets are of variable size • When a host has a packet to transmit 1. Carrier Sense: Check that the line is idle before transmitting 2. Collision Detection: Detect collision as soon as possible If a collision is detected, stop transmitting; wait a random time, sense the channel again 17
- 18. Summary In this class, you have learnt about • Types of CSMA – Persistent CSMA – Non Persistent CSMA • CSMA / CD 18
- 19. Quiz 1. ____________ helps Ethernet to recover from simultaneous transmissions causing errors (a) CSMA /CD (b) CSM (c) CD (d) Collisions 19
- 20. Quiz 2. Which one is applicable to slotted channel ? (a) 1- persistent CSMA (b) P- Persistent CSMA (c) Non – Persistent CSMA (d) All of the above 20
- 21. Frequently Asked Questions 1. Describe Persistent and Non Persistent CSMA 2. Compare Persistent and Non Persistent CSMA 3. Describe CSMA / CD 21