4/11/40 page 1 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL Ethernet

4/11/40 page 2 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL Ethernet and IEEE Developed during the mid-1970 抯 at Xerox Palo Alto Research Developed during the mid-1970 抯 at Xerox Palo Alto Research Center (Bob Metcalfe) Center (Bob Metcalfe) Later refinement by DEC, Intel and Xerox (DIX standard) Later refinement by DEC, Intel and Xerox (DIX standard) Became IEEE in 1985 Became IEEE in 1985 Different frame format between Ethernet and IEEE Different frame format between Ethernet and IEEE 802.3

4/11/40 page 3 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL Data link layer Physical layer Ethenet architecture 0.5 ?Coax tap BNC connector transceiver AUI cable NIC station interface data encapsulation link management encoding and decoding transmission and receipt

4/11/40 page 4 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL Multi segments network hub 10Base2 - Thin Ethernet 10Base5 - Thick Ethernet 10BaseT-Twisted pair router server hub bridge

4/11/40 page 5 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL IEEE specification Various standard defined for IEEE802.3 with a concise notation Various standard defined for IEEE802.3 with a concise notation 10Base5 -- thickwire coaxial 10Base5 -- thickwire coaxial 10Base2 -- thinwire coaxial or cheapernet 10Base2 -- thinwire coaxial or cheapernet 10BaseT -- twisted pair 10BaseT -- twisted pair 10BaseF -- fiber optics 10BaseF -- fiber optics 10Broad36 -- broadband 10Broad36 -- broadband Fast Ethernet Fast Ethernet 100BaseTX, 100BaseT4, 100BaseF and 100 VG-AnyLAN 100BaseTX, 100BaseT4, 100BaseF and 100 VG-AnyLAN data rate in Mbps signaling baseband or broadband maximum segment length in hundreds of meters 10 Base 5

4/11/40 page 6 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL 10Base5 tap : cable does not to be cut tap : cable does not to be cut transceiver : send/receive, collison transceiver : send/receive, collison detection, electronics isolation detection, electronics isolation AUI : Attachment Unit Interface AUI : Attachment Unit Interface Use for backbone networks Use for backbone networks 0.5?Coax 0.5?Coax vampire tap BNC connector transceiver AUI cable NIC maximum segment length=500m maximum number of stations per segment=100 minimum distance between two stations = 2.5 m maximum network distance between two stations = 2.8km

4/11/40 page 7 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL 10Base ?Coax BNC T-connector NIC BNC connector BNC connector No drop cable No drop cable use for office LAN use for office LAN maximum segment length=185m maximum number of stations per segment=30 minimum distance between two stations = 0.5 m maximum network distance between two stations = 925 m

4/11/40 page 8 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL 10BaseT NIC A hub functions as a repeater A hub functions as a repeater use for office LAN use for office LAN hub maximum segment length = 100m

4/11/40 page 9 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL 10BaseF 10BaseF specification enable long distance connections with 10BaseF specification enable long distance connections with the use of optical fiber. Three standards are: the use of optical fiber. Three standards are: 10BaseFP - A passive-star topology, up to 1 km link 10BaseFP - A passive-star topology, up to 1 km link 10BaseFL - An asynchronous point-to-point link 10BaseFL - An asynchronous point-to-point link (a new FOIRL), up to 2 km link (a new FOIRL), up to 2 km link 10BaseFB - A synchonous point-to-point link, up to 2 km link 10BaseFB - A synchonous point-to-point link, up to 2 km link with 15 cascade repeaters with 15 cascade repeaters

4/11/40 page 10 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL Carrier Sense Multiple Access Station listens to the medium before transmitting (listen before talking ;LBT) Station listens to the medium before transmitting (listen before talking ;LBT) Non persistent CSMA Non persistent CSMA if medium is idle, transmit if medium is idle, transmit if medium busy, waits a random periods an then resense medium if medium busy, waits a random periods an then resense medium 1-persistent CSMA 1-persistent CSMA if medium is idle, transmit if medium is idle, transmit if medium busy, continue to listen until the channel is sensed idle; then transmit immediately if medium busy, continue to listen until the channel is sensed idle; then transmit immediately

4/11/40 page 11 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL Media Access Control technique Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Carrier Sense Multiple Access with Collision Detection (CSMA/CD) listen before talking listen before talking Station senses to the medium before transmitting : CS Station senses to the medium before transmitting : CS Topology supports multiple access (listenin) : MA Topology supports multiple access (listenin) : MA Manchester encoding ensures a transition every bit Manchester encoding ensures a transition every bit 0 : high-to-low 0 : high-to-low 1 : low-to-high 1 : low-to-high 0 V (high) -2.2 V (low) 100 ns 0 mA -90 mA

4/11/40 page 12 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL Collision More than two station send frame at the same time More than two station send frame at the same time Station transmits its data while checking the cahannel for collision Station transmits its data while checking the cahannel for collision If average DC voltage level exeeds the CD threshold, collision is detected If average DC voltage level exeeds the CD threshold, collision is detected

4/11/40 page 13 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL Collision detection How to detect How to detect Station send frame and sense the medium Station send frame and sense the medium Collision detected if Collision detected if station sense the exeeded signal strength (coax) station sense the exeeded signal strength (coax) there is signal on more than one inut (UTP) there is signal on more than one inut (UTP) How to do How to do Transmitting nodes send a jamming signal after a collision is detected. Transmitting nodes send a jamming signal after a collision is detected. jam

4/11/40 page 14 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL Ethernet frame format IEEE PA : Preamble s for synchronization SFD : Start of Frame delimiter to start frame DA: Destination Address -- MAC address SA: Source Address -- MAC address LEN: Length -- Number of data bytes Type: identify the higher -level protocol LLC PDU+pad -- minimum 46 bytes, maximum 1500 FCS : Frame Check Sequence -- CRC FCS PA SALEN SFD DA LLC PDU Pad FCS SAType PA DA Data Pad Ethernet calculation of the FCS bytes

4/11/40 page 15 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL Ethernet MAC address I/G =0 Individual address I/G =1 Group address U/L=0 Global administered address U/L=1 Local administered address Unicast : define a single destination Broadcast : FFFFFFFF each station on the network receive and accept frames multicast : a group address defines multiple recepient I/G U/L 46-bit address I/G 15-bit address 1 15 Two addressformat: 48 bits 48 bits 16 bits 16 bits

4/11/40 page 16 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL Interframe gap Obligatory 9.6ms interval between the emitted frame and the new one Obligatory 9.6ms interval between the emitted frame and the new one To enable other stations wishing to transmit to take over at this time To enable other stations wishing to transmit to take over at this time FCS SAType PA DA Data Pad PA Interframe gap > 9.6  s

4/11/40 page 17 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL Frame transmission Assemble frame carrier sense signal ON ? Wait interframe gap time Start trnasmission collision detected? N N Y Y send jam sequence Increment attempt. attempt limited? Discard frame N Y Compute backoff and wait backoff time transmission done? tranmit OK N Y

4/11/40 page 18 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL Frame reception done receiving? Pass frame to next layer Y Y N N Discard frame Y N start receiving Matched DA FCS and frame size OK?

4/11/40 page 19 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL Binary Backoff collision detected? Y send frame Discard frame Y random wait between 0 and delay-1 double delay limited delay<1024 N max delay is limited at 1023*51.2  s=52.4 ms >16 attempts delay= 2 transmission done? tranmit OK N Y N 1 st wait 0 or 1 slot time 2 nd wait 0,1,2 or 3 slot time 3 rd wait 0,1,2,..7 slot time k th wait k slot time after 10th collisions the interval is frozen at 1023 slots after 16th collisions, frame is discarded and report failure back to the upper layer

4/11/40 page 20 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL Binary Backoff collision detected? N Y max_delay= 1 slot time >16 attempts Discard frame Y transmission done? tranmit OK N Y N max_delay is limited at 1024*51.2  s=52.4 ms random wait 0 or max_delay double max_delay limited max_delay<1024

4/11/40 page 21 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL Minimum frame size (1) packet starts at time 0 (2) packet almost at B at t-  (3) B send packet; collision occurs at t (4) jam signal gets back to A at 2t A frame must take more than 2t to send to prevent the situation that the sender incorrect conclude that the frame was successfully sent. This slot times is = 51.2  s corresponds to 512 bit = 64 bytes The minimum frame length is 64 bytes (excluding preamble) This answers why data field must have 46 bytes minimum A A A A B B B B A and B locates at the far end of the cable

4/11/40 page 22 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL Late Collision Late collisions are collisions which occur after the first 64 bytes have been transmitted on to the network Late collisions are collisions which occur after the first 64 bytes have been transmitted on to the network Primary causes: excessive cable lengths and repeaters Primary causes: excessive cable lengths and repeaters

4/11/40 page 23 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL Ethernet Connectors

4/11/40 page 24 Department of Computer Engineering, Kasetsart University Introduction to Computer Communications and Networks CONSYL Do you know? A propagation speed 0.77c on 10Base5 and 0.59c on twisted pair A propagation speed 0.77c on 10Base5 and 0.59c on twisted pair