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)

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?

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

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

Ethernet/IEEE – 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

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

IEEE 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

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

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?

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

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

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

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

Ethernet/IEEE 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

Original Ethernet Diagram

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

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

Ethernet/IEEE802.3 Utilization

Ethernet/IEEE 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

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

Ethernet/IEEE 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

Ethernet/IEEE Addressing You can look up the vendor of the adaptor for your machine

Ethernet Frame Format Preamble SFD Destination Address Source Address Type Data 46–1500 bytes FCS CRC - 32 Ethernet Preamble SFD Destination Address Source Address Length LLC Data 46– 496bytes FCS CRC - 32 IEEE ………