1 / 23

Enhanced CSMA

Enhanced CSMA. Additional improvement Use CSMA access Listen while transmitting Stop immediately if collision sensed Called collision detection Reduces time lost due to collisions Carrier Sense Multiple Access/Collision Detection protocols ( CSMA/CD ). Shared channel. A. B. CSMA/CD.

randy
Télécharger la présentation

Enhanced CSMA

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related