1 Multiple Access: Ethernet Section 2.6

2 Point-to-Point vs. Broadcast Media Point-to-point –PPP for dial-up access –Point-to-point link between Ethernet switch and host Broadcast (shared wire or medium) –Traditional Ethernet – wireless LAN mmmmmmmmmm mmmmmmmmmm mmmmmmmmmm mmmmmmmmmm mmmmmmmmmm mmmmmmmmmm mmmmmmmmmm m

3 Multiple Access Protocol Single shared broadcast channel –Avoid having multiple nodes speaking at once –Otherwise, collisions lead to garbled data Multiple access protocol –Distributed algorithm for sharing the channel –Algorithm determines which node can transmit Classes of techniques –Channel partitioning: divide channel into pieces –Taking turns: passing a token for the right to transmit –Random access: allow collisions, and then recover

4 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 –CSMA/CD (Ethernet: IEEE 802.3) –CSMA/CA (Wireless: IEEE )

5 Key Ideas of Random Access Carrier sense –Listen before speaking, and don’t interrupt Checking if someone else is already sending data … and waiting till the other node is done Collision detection –If someone else starts talking at the same time, stop Realizing when two nodes are transmitting at once …by detecting that the data on the wire is garbled Randomness –Don’t start talking again right away Waiting for a random time before trying again

6 CSMA/CD (Collision Detection) Carrier Sense Multiple Access, CSMA: listen before transmit –If channel sensed idle: transmit entire frame –If channel sensed busy, defer transmission CD, Collision Detection : –Collisions detected within short time –Colliding transmissions aborted, reducing wastage –Easy in wired LANs: measure signal strengths, compare transmitted, received signals –Difficult in wireless LANs: receiver shut off while transmitting Human analogy: the polite conversationalist

7 CSMA/CD Collision Detection

8 Ethernet Dominant wired LAN technology First widely used LAN technology Simpler, cheaper than token LANs and ATM Kept up with speed race: 10 Mbps – 10 Gbps Metcalfe’s Ethernet sketch

9 Ethernet Topologies Bus Topology: Shared All nodes connected to a wire Star Topology: All nodes connected to a central repeater

10 Ethernet Connectivity 10Base5 – ThickNet < 500m Controller Vampire Tap Transceiver Bus Topology Detects when the line is idle Drives the signal when the host is transmitting Receives incoming signals Controller: Implements all the logic that makes up the Ethernet

11 Ethernet Connectivity 10Base2 – ThinNet < 200m Controller BNC T-Junction Transceiver Bus Topology

12 Ethernet Connectivity 10BaseT < 100m twisted pair hub Controller Star Topology

13 Ethernet MAC Protocol, CSMA/CD Carrier sense: wait for link to be idle –Channel idle: start transmitting must wait 9.6 µ s between back-to-back frames –Channel busy: wait until idle and transmit immediately Collision detection: listen while transmitting –No collision: transmission is complete –Collision: abort transmission, and send a 32-bit “jamming sequence”

14 Ethernet MAC Protocol, CSMA/CD(cont.) Random access: exponential back-off –After collision, wait a random time before trying again –After m th collision, choose K randomly from {0,..., 2 m -1} –… and wait for K*512 bits transmission time before trying again K*51.2 µ s on 10Mbps Ethernet

15 Delay Times After Collision Delay and try again –1st time: 0 or 51.2 µ s –2nd time: 0, 51.2, or µ s –3rd time: 0, 51.2, 102.4, or µ s –After n collisions, wait time: k x 51.2 µ s, for randomly selected k=0 … 2 n - 1 –give up after several tries (up to 16) –exponential backoff

16 Collisions AB How can we ensure that A knows about the collision?

17 Limitations on Ethernet Length Latency depends on physical length of link –Time to propagate a packet from one end to the other Suppose A sends a packet at time t –And B sees an idle line at a time just before t+d –… so B happily starts transmitting a packet B detects a collision, and sends jamming signal –But A doesn’t see collision till t+2d latency d A B

18 Limitations on Ethernet Length A needs to wait for time 2d to detect collision –So, A should keep transmitting during this period –… and keep an eye out for a possible collision Imposes restrictions on Ethernet –Maximum length of the wire: 2500 meters –Minimum length of the packet: 512 bits (64 bytes) –2d is bounded to 51.2  s –At 10Mbps, and during 51.2  s, 512bit are transmitted (64B) –Packet length  64B latency d A B

19 Ethernet Frame Structure Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble: 8 bytes 7 bytes with pattern followed by one byte with pattern Used to synchronize receiver, sender clock rates (Manchester encoding)

20 Ethernet Frame Structure (con. I) Address: 6 bytes –unique, 48-bit unicast address assigned to each adapter (2 48 > 281 Billion); human readable, burned in ROM (example: 8:0:2b:e4:b1:2 ( ) –broadcast: all 1s –multicast: first bit is 1 used to send messages to some subset of the hosts on an Ethernet (e.g. all file servers) adaptor programmed to accept some set of multicast addresses

21 Ethernet Frame Structure (con. II) Adaptor receives all frames and accepts: –Frames addressed to its address; –Frames addressed to the broadcast address; –Frames addressed to a multicast address, if it has been instructed to listen to that address; –All frames, if it has been placed in promiscuous mode. Type: 2B; indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk) CRC: 4B; checked at receiver, if error then frame is discarded

22 Ethernet Frame Structure (con. III) Frame Data: Up to 1500 data bytes – Minimally 46 bytes of data –Minimal frame = 46 (data) + 14 (header) + 4 (CRC) = 64 bytes The reason for a minimum frame size is that the frame must be long enough to detect a collision 14-byte header

23 Unreliable, Connectionless Service Connectionless –No handshaking between sending and receiving adapter. Unreliable –Receiving adapter doesn’t send ACKs –Packets passed to network layer can have gaps –Gaps will be filled if application is using TCP –Otherwise, the application will see the gaps

24 Hubs: Physical-Layer Repeaters Hubs are physical-layer repeaters –Bits coming from one link go out all other links –At the same rate, with no frame buffering –No CSMA/CD at hub: adapters detect collisions twisted pair hub

25 Interconnecting with Hubs Backbone hub interconnects LAN segments All packets seen everywhere, forming one large collision domain Can’t interconnect Ethernets of different speeds hub

26 Switch Link layer device –Stores and forwards Ethernet frames –Examines frame header and selectively forwards frame based on MAC destination address –When frame is to be forwarded on segment, uses CSMA/CD to access segment Transparent –Hosts are unaware of presence of switches Plug-and-play, self-learning –Switches do not need to be configured

27 Switch: Traffic Isolation Switch breaks subnet into LAN segments Switch filters packets –Same-LAN-segment frames not usually forwarded onto other LAN segments –Segments become separate collision domains hub switch collision domain

28 Benefits of Ethernet Easy to administer and maintain Inexpensive Increasingly higher speed Moved from shared media to switches –Change everything except the frame format –A good general lesson for evolving the Internet

29 Conclusions IP runs on a variety of link layer technologies –Point-to-point links vs. shared media –Wide varieties within each class Link layer performs key services –Encoding, framing, and error detection –Optionally error correction and flow control Shared media introduce interesting challenges –Decentralized control over resource sharing –Partitioned channel, taking turns, and random access –Ethernet as a wildly popular example Next time: Token Ring –Reading: Section 2.7