EE 122 Lecture 6 Ion Stoica istoica cs berkeley edu September 13 2001 this talk is based in part on the on line slides of J Kurose K Rose High Level View Goal share a communication medium among multiple hosts connected to it Problem arbitrate between connected hosts Solution goals High resource utilization Avoid starvation Simplicity non decentralized algorithms istoica cs berkeley edu 2 Medium Access Protocols Channel partitioning Divide channel into smaller pieces time slots frequency Allocate piece to node for exclusive use Random access Allow collisions recover from collisions Taking turns Tightly coordinate shared access to avoid collisions istoica cs berkeley edu 3 Random Access protocols When node has packet to send transmit at full channel data rate R no a priori coordination among nodes Two or more transmitting nodes collision Random access MAC protocol specifies how to detect collisions how to recover from collisions Examples of random access MAC protocols slotted ALOHA CSMA and CSMA CD istoica cs berkeley edu 4 Slotted Aloha Time is divided into equal size slots packet transmission time Node with new arriving pkt transmit at beginning of next slot If collision retransmit pkt in future slots with probability p until successful Success S Collision C Empty E slots istoica cs berkeley edu 5 Slotted Aloha Efficiency What is the maximum fraction of successful transmissions Suppose N stations have packets to send each transmits in slot with probability p prob successful transmission S is by a single node S p 1 p N 1 by any of N nodes S Prob only one transmits N p 1 p N 1 1 e 0 37 istoica cs berkeley edu 6 CSMA Carrier Sense Multiple Access CS Carrier Sense means that each node can distinguish between an idle and a busy link Sender operations If channel sensed idle transmit entire packet If channel sensed busy defer transmission Persistent CSMA retry immediately with probability p when channel becomes idle Non persistent CSMA retry after a random time interval istoica cs berkeley edu 7 CSMA collisions Collisions can occur spatial layout of nodes along ethernet propagation delay means two nodes may not hear each other s transmission Collision entire packet transmission time wasted Note role of distance and propagation delay in determining collision prob istoica cs berkeley edu 8 CSMA CD Collision Detection Collisions detected within short time Colliding transmissions aborted reducing channel wastage Easy in wired LANs measure signal strengths compare transmitted received signals Difficult in wireless LANs receiver shut off while transmitting istoica cs berkeley edu 9 CSMA CD collision detection istoica cs berkeley edu 10 Ethernet Dominant LAN technology CSMA CD protocol Cheap 20 for 100Mbs istoica cs berkeley edu 11 Ethernet Frame Structure Sending adapter encapsulates IP datagram Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates istoica cs berkeley edu 12 Ethernet Frame Structure more Addresses 6 bytes frame is received by all adapters on a LAN and dropped if address does not match Type 2 bytes indicates the higher layer protocol E g IP Novell IPX AppleTalk CRC 4 bytes checked at receiver if error is detected the frame is simply dropped Data payload maximum 1500 bytes minimum 46 bytes istoica cs berkeley edu 13 Ethernet s CSMA CD Sense channel if idle If detect another transmission Abort send jam signal Delay and try again Else Send frame Receiver accepts Frames addresses to its own address Frames addressed to the broadcast address broadcast Frames addressed to a multicast address if it was instructed to listen to that address All frames promiscuous mode istoica cs berkeley edu 14 Ethernet s CSMA CD more Jam signal make sure all other transmitters are aware of collision 48 bits Exponential back off Goal adapt retransmission attempts to estimated current load Heavy load random wait will be longer First collision choose K from 0 1 delay is K x 512 bit transmission times After second collision choose K from 0 1 2 3 After ten or more collisions choose K from 0 1 2 3 4 1023 istoica cs berkeley edu 15 Minimum Packet Size Why put a minimum packet size Give a host enough time to detect collisions In Ethernet minimum packet size 64 bytes two 6 byte addresses 2 byte type 4 byte CRC and 46 bytes of data If host has less than 46 bytes to send the adaptor pads adds bytes to make it 46 bytes What is the relationship between minimum packet size and the length of the LAN istoica cs berkeley edu 16 Minimum Packet Size more Time t Host 1 tarts to send frame Time t d Host 2 tarts to send a frame ust before it hears from ost 1 s frame Time t 2 d Host 1 ears Host 2 s frame etects collision Host 1 Host 2 propagation delay d Host 1 Host 2 propagation delay d Host 1 Host 2 propagation delay d LAN length min frame size light speed 2 bandwidth 8 64b 2 5 108mps 2 107 bps 6400m approx istoica cs berkeley edu 17 Ethernet Technologies 10Base2 10 10Mbps 2 under 200 meters max cable length Thin coaxial cable in a bus topology Repeaters used to connect up to multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only istoica cs berkeley edu 18 10BaseT and 100BaseT 10 100 Mbps rate latter called fast ethernet T stands for Twisted Pair Hub to which nodes are connected by twisted pair thus star topology CSMA CD implemented at hub istoica cs berkeley edu 19 10BaseT and 100BaseT more Max distance from node to Hub is 100 meters Hub can disconnect jabbering adapter Hub can gather monitoring information statistics for display to LAN administrators istoica cs berkeley edu 20 Gbit Ethernet Use standard Ethernet frame format Allows for point to point links and shared broadcast channels In shared mode CSMA CD is used short distances between nodes to be efficient Uses hubs called here Buffered Distributors Full Duplex at 1 Gbps for point to point links istoica cs berkeley edu 21 Interconnecting LANs Why not just one big LAN Limited amount of supportable traffic on single LAN all stations must share bandwidth Limited length Large collision domain can collide with many stations istoica cs berkeley edu 22
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