1. 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)

2. CSMA/CD How long should a station listen while transmitting to ensure no collision? Shared channel AB - Suppose A begins to transmit - Suppose B wants to transmit before hearing A’s transmission ☼☼ collision - 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 * 10 8 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/CD 1-persistent  Different speeds  Different media 10BASE5Thick Coax500 meters 10BASE2Thin Coax185 meters 10BASETTwisted pairs100 meters 10BASEFFiber 1 & 2 Km 100BASETtwisted pairs100 meters 1000BaseTtwisted pairs100 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 host Transceiver – Ethernet Media Access Unit (MAU) - IEEE Transceiver/AUI Cable 50 meters maximum Attachment Unit Interface (AUI) 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. 10BASE5 500 meters Repeater Transceiver/AUI Cable 50 meters maximum A B 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.75 µs one- way or 1.5 µs roundtrip Transceiver adds 1.0 µs one-way or 2.0 µs roundtrip (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. 10BASE5 B A 500 meters R R R R 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

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. Ethernet Wiring Topologies Star wired hubs Shared Channel n Mbps capacity ABFGCED n Mbps 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,......, 2 j -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 XXXXXX XXXXXX (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 Frame Format Preamble SFD Destination Address Source Address Type Data 46–1500 bytes FCS CRC - 32 Ethernet 7 1 6 6 2 4 Preamble SFD Destination Address Source Address Length LLC Data 46– 496bytes FCS CRC - 32 IEEE 802.3 7 1 6 6 2 4 10101010 ……….. 10101011