2. © 2002 JW Ryder CS 428 Computer Networks 1 Ethernet Properties 10Mbps/100Mbps broadcast bus technology –Bus: all stations share single channel –Broadcast: all transceivers receive every transmission Transceiver passes all packets from bus to host adapter Host adapter chooses some and filters others

3. © 2002 JW Ryder CS 428 Computer Networks 2 Ethernet Properties Best-effort delivery: hardware provides no information to the sender about whether packet was actually delivered Destination machine powered down, packets will be lost TCP/IP protocols accommodate best-effort delivery

4. © 2002 JW Ryder CS 428 Computer Networks 3 CSMA/CD Carrier Sense Multiple Access / Collision Detect When transceiver begins transmission –Not all parts of network reached simultaneously –80% speed of light speed Free for all transmission yields possibility of 2 transceivers sensing idle bus and beginning transmission simultaneously Draw picture

5. © 2002 JW Ryder CS 428 Computer Networks 4 CSMA/CD Each transceiver monitors cable while transmitting in an attempt to detect foreign signals (interference) Monitoring called Collision Detection If collision detected, a host adapter – aborts transmission –waits predictable number of arbitrary length time units –tries again

6. © 2002 JW Ryder CS 428 Computer Networks 5 Binary Exponential Backoff Show BEB example Many stations try to transmit at exactly the same time yields a non-recoverable traffic jam Ethernet capacity –10Mbps means for everyone –Highway system can manage a max amount of traffic

7. © 2002 JW Ryder CS 428 Computer Networks 6 WinIPCfg find and view 48-bit addressing scheme Ethernet manufacturers buy ‘lot’s of addresses –Unique address in the universe Fixed in machine readable form on host interface hardware Ethernet Address belong to hardware devices –Physical addr., Hardware addr. Ethernet Hardware Address

8. © 2002 JW Ryder CS 428 Computer Networks 7 Ethernet Addresses Moving hardware interface to another machine changes the original machine’s physical address Host interface examines packets and decides which ones to send ‘up’ –Destination address used –CPU could perform check but what would happen?

9. © 2002 JW Ryder CS 428 Computer Networks 8 48-bit address is one of 3 types –Unicast –Broadcast (All 1’s) –Multicast (Group) Host interface recognizes more that just its physical address Not all forms of addressing implemented by all host adapter manufacturers –Usually unicast and broadcast, multicast is newest idea Ethernet Address

10. © 2002 JW Ryder CS 428 Computer Networks 9 (Data) Link level connection among machines “Phil Daily Never Took Sarah Prentice Anywhere” -- jwr Variable length, no smaller than 64 octets or larger than 1518 octets –See figure 2.7 on page 30 Preamble extra - used for synchronization Sender computes CRC, receiver regenerates and checks Ethernet Frame

11. © 2002 JW Ryder CS 428 Computer Networks 10 Internet Services Read Chapter 11 3 layers Connectionless Delivery Service –Provides foundation Reliable Transport Service –Applications depend on this Application Service

12. © 2002 JW Ryder CS 428 Computer Networks 11 Need for Multiple Protocols “… protocols allow one to specify or understand communication without knowing the details of a particular vendor’s network hardware.” Same or different? –Ethernet frame format, network access policy, frame error handling –IP addresses, datagram format, unreliable delivery

13. © 2002 JW Ryder CS 428 Computer Networks 12 Conceptual Layering Heterogeneous network transport protocols require a buffering between actual network implementation view of implementation See figure 11.1 on page 179 Protocol software more complex than simple model See figure 11.2 on page 180 See figure 11.3 on page 180

14. © 2002 JW Ryder CS 428 Computer Networks 13 ISO 7 Layer Reference Model Application - 7 Presentation - 6 Session - 5 Transport - 4 Network - 3 Data Link - 2 Physical - 1 See figure 11.4 on page 181

15. © 2002 JW Ryder CS 428 Computer Networks 14 Physical Layer Level 1 Physical interconnection to the network Includes electrical characteristics of voltage, current

16. © 2002 JW Ryder CS 428 Computer Networks 15 Data Link Layer Level 2 Handles transmission errors –Frame checksum –Transmission is unreliable Specifies exchange of acknowledgements between machines to know when frame transferred successfully Does not mean content is right!!

17. © 2002 JW Ryder CS 428 Computer Networks 16 Network Layer Level 3 Defines basics unit of transfer across network Includes concepts of destination addressing, routing

18. © 2002 JW Ryder CS 428 Computer Networks 17 Transport Layer Level 4 End to End reliability Destination host communicates with the source host Even though lower level layers provide reliable checks at each transfer, end to end layer double checks

19. © 2002 JW Ryder CS 428 Computer Networks 18 Session Layer Level 5 Remote terminal access Terminal to host connection

20. © 2002 JW Ryder CS 428 Computer Networks 19 Presentation Layer Level 6 Provides network functions that many applications use Text compression, graphic images to bit stream formats etc.

21. © 2002 JW Ryder CS 428 Computer Networks 20 Application Layer Level 7 Application programs that use the network Electronic mail, File transfer programs, finger, daytime, echo

22. © 2002 JW Ryder CS 428 Computer Networks 21 TCP/IP Internet Layering Model Looked at as 4 layers plus a 5th hardware layer Application Transport Internet Network Interface Hardware

23. © 2002 JW Ryder CS 428 Computer Networks 22 Application Layer Highest layer Application programs that access services available across TCP/IP internet Interacts with one of the transport layer protocols –Sequence of individual messages –Continuous stream of bytes

24. © 2002 JW Ryder CS 428 Computer Networks 23 Transport Layer Messages or Streams pass between AL and TL Provides communication from one application program to another –Called End-to-End May regulate flow of information May provide reliable transport –Arranges for receiving side to send back acknowledgements –Can retransmit packets with errors Must be able to service many applications

25. © 2002 JW Ryder CS 428 Computer Networks 24 Internet Layer Transport Protocol Packets pass between TL and IP Handles communication from one machine to another Encapsulates packet into IP datagram IP Security implemented here Handles incoming datagrams

26. © 2002 JW Ryder CS 428 Computer Networks 25 Network Interface Layer IP Datagrams pass between IP and NL Responsible for accepting datagrams and transmitting them over a specific network –Device driver when attaching to LAN

27. © 2002 JW Ryder CS 428 Computer Networks 26 One way of extending an Ethernet network. Max cable length = 500 meters Repeater is a hardware device that relays electronic signals from one cable to another At most, 2 repeaters can be placed between any 2 machines Total length - 3 segments at 500m each Ethernet Bridges

28. © 2002 JW Ryder CS 428 Computer Networks 27 Repeater Example Floor 1 Floor 2 Floor 3 Repeater Computer (Host) | 500 Meters |

29. © 2002 JW Ryder CS 428 Computer Networks 28 Superior because they do not replicate –Noise –Errors –Malformed frames Frame must be completely valid to retransmit on another wire Follow CSMA/CD rules –Collisions, Propagation delays remain isolated on 1 wire Ethernet Bridges

30. © 2002 JW Ryder CS 428 Computer Networks 29 Almost arbitrary number of bridges can be connected Why? What is different? Bridges Hide details of the interconnection Acts like single huge Ethernet

31. © 2002 JW Ryder CS 428 Computer Networks 30 Bridges Adaptive, Learning bridges 1 computer, 2 Ethernet interfaces Software keeps 2 address lists Frame arrives from E 1, adds 48-bit source addr. to list associated with E 1 Learns topology of each wire Check destination addr. If on frame from whence it came, disregard

32. © 2002 JW Ryder CS 428 Computer Networks 31 Adaptive Bridges Helps improve performance by isolating traffic If there exists a natural dichotomy between computers which communicate, then there is major gain From TCP/IP pov, bridged Ethernets simply another form of physical network connection Modern bridges use distributed spanning tree algorithms to figure how to forward frames - Why? (Cycles & packets in 2 directions simultaneously!)

33. © 2002 JW Ryder CS 428 Computer Networks 32 Reading Make sure you read all of chapter 2 & 3