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Enhanced CSMA

Enhanced CSMA

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Enhanced CSMA

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  1. Enhanced CSMA • Additional improvement • Use CSMA access • Listen while transmitting • Stop immediately if collision sensed • Called collision detection • Reducestime lost due to collisions • Carrier Sense Multiple Access/Collision Detection protocols (CSMA/CD)

  2. Shared channel A B CSMA/CD How long should a station listen while transmitting to ensure no collision? collision ☼☼ - Suppose A begins to transmit - Suppose B wants to transmit before hearing A’s transmission - How long before A hears collision?

  3. Collision Detection • Let p = maximum propagation delay p = time for signal to travel full length of channel • To ensure detection of a collision, transmitter must listen 2 x propagation delay • Thus, the time we need listen depends on size of network • If we set a limit to size of network, we could define a maximum time to listen • On wired networks we consider propagation delay as 2 * 108 meters per second

  4. Collision Detection • Suppose time to transmit frame is longer than 2 * propagation delay • What can we say about the channel if we have not heard a collision after 2 * propagation delay? • By that time, station will have seized the channel • What if we required a minimum frame size? • What if the time to transmit a frame of minimum size was ≥2 * propagation delay? • Under this condition, if transmission completes without collision, we are assured none will occur

  5. Ethernet/IEEE 802.3 • 1975 – Original Ethernet • Based on a paper by Robert Metcalfe • Based on Aloha concepts • 2.94 Mbps (3 Mbps) • Maximum size of 1 Km • Used CSMA/CD protocol • Used coaxial cable as media • First version developed in collaboration with • Xerox • Intel • Digital Equipment Corporation

  6. Ethernet/IEEE 802.3 • 1980 Ethernet Version 1.0 • 10 Mbps • 500 meter segment • Used 50 ohm coaxial cable • CSMA/CD – 1-persistent • 1982 Ethernet Version 2.0 • Some electrical revisions • 1985 IEEE 802.3 • Defined a family of networks • Minor differences with Ethernet • Some additional electrical functions • Frame format variations

  7. IEEE 802.3 • Family of Networks • CSMA/CD1-persistent • Different speeds • Different media • 10BASE5 Thick Coax 500 meters • 10BASE2 Thin Coax 185 meters • 10BASET Twisted pairs 100 meters • 10BASEF Fiber 1 & 2 Km • 100BASET twisted pairs 100 meters • 1000BaseT twisted pairs 100 meters • Others

  8. 10BASE5 • 500 meters • 10 Mbps • Maximum 100 stations on a segment • Minimum 2.5 meters apart • Manchester encoding • ± .85 volts • High to low = 1 bit • Low to high = 0 bit

  9. 10BASE5 500 meters Transceiver – Ethernet Media Access Unit (MAU) - IEEE Transceiver/AUI Cable 50 meters maximum Attachment Unit Interface(AUI) host How long must we listen to assure no collision? How large a minimum frame do we need to detect any collision before end of transmission?

  10. A B 10BASE5 500 meters Transceiver/AUI Cable 50 meters maximum Repeater 500 meters How long must we listen to assure no collision?

  11. Calculating Propagation Delay • 100 meters of cable adds .5 µs to delay or 1 µs roundtrip • Repeater adds approximately .75µs one-way or 1.5 µs roundtrip • Transceiver adds 1.0 µsone-way or 2.0 µsroundtrip (if not connected to host) • On a 10 Mbps Ethernet, 1 µs is equivalent to transmitting 10 bits • We can calculate delay in time units or in bits

  12. Ethernet/IEEE 802.3 • Use CSMA/CD • Detect collisions while transmitting frame • Requires a minimum size frame • Transmission rate (speed) • Frame size • Network size All related

  13. 500 meters R R R R 500 meters 500 meters 500 meters 500 meters 10BASE5 A 5 * 500 segments = 2500 meters = 12.5 µs 8 * 50 Transceiver cables = 400 meters = 2 µs 4 repeaters * .75 µs = 3 µs 8 transceivers *1 µs = 8 µs Total one way delay = 25.5 µs Total round trip = 51 µs B

  14. Ethernet/IEEE 802.3 • Minimum frame size = 512 bits = 64 bytes • Vulnerable time or contention slot = 51.2 µs • After 51.2 µs, station has seized channel • What happens if a station sends unlimited size frame? • Maximum size frame also specified at 1518 bytes • Maximum size frame enforced by transceiver

  15. Original Ethernet Diagram

  16. Shared Channel n Mbps capacity n Mbps n Mbps A B C D E F G Ethernet Wiring Topologies Star wired hubs All stations share n Mbps A shared hub is actually a repeater

  17. Efficiency of CSMA/CD • Let p = propagation delay (one way) • A = probability a station acquires channel during a contention slot • Tx = time to transmit an average size frame • It can be shown that • Optimal utilization when

  18. Ethernet/IEEE802.3 Utilization

  19. Ethernet/IEEE 802.3 • Advantages • Most widely used network architecture • New stations can be installed on live network • Low delay at low load • Simple access mechanism • Disadvantages • Limited distances • Non deterministic • High collisions rate at high loads • No provisions for priority traffic

  20. Collisions • Minimum frame defined as 512 bits • At 10 Mbps contention slot = 51.2 µs • After a collision each station waits 0 or 1 contention slots • After a second collision each station waits 0, 1, 2, or 3 contention slots • After j collisions, each station waits 0, 1, 2, . . . . . ., 2j-1 contention slots until j = 10 then continue until 16 total tries • This is called binary exponential backoff

  21. Ethernet/IEEE 802.3 Addressing • Each station on a multiple access LAN must be uniquely identified • For Ethernet, addresses are assigned to network interfaces by vendors • Each address is 48 bits XXXXXXXXXXXX (X = 4 bits)vendors address • There is a special broadcast address FFFFFF FFFFFF (all 1s) All stations on shared channel • There are provisions for subgroups (multicast) • This is called the Media Access Control (MAC) address

  22. Ethernet/IEEE 802.3 Addressing • You can look up the vendor of the adaptor for your machine http://standards.ieee.org/regauth/oui/index.shtml

  23. Ethernet IEEE 802.3 Preamble Preamble 7 7 1 1 SFD SFD Destination Address Destination Address 6 6 Source Address Source Address 6 6 Type Length 2 2 Data 46–1500 bytes LLC Data 46– 496bytes FCS CRC - 32 FCS CRC - 32 4 4 Ethernet Frame Format 10101010 ……….. 10101011