Ethernet Properties 10 Mbps 100 Mbps broadcast bus

Ethernet Properties • 10 Mbps/100 Mbps 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 © 2002 JW Ryder CS 428 Computer Networks 1

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 © 2002 JW Ryder CS 428 Computer Networks 2

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 © 2002 JW Ryder CS 428 Computer Networks 3

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 © 2002 JW Ryder CS 428 Computer Networks 4

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

Ethernet Hardware Address • Win. IPCfg 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. © 2002 JW Ryder CS 428 Computer Networks 6

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? © 2002 JW Ryder CS 428 Computer Networks 7

Ethernet Address • 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 © 2002 JW Ryder CS 428 Computer Networks 8

Ethernet Frame • (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 © 2002 JW Ryder CS 428 Computer Networks 9

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

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 © 2002 JW Ryder CS 428 Computer Networks 11

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 © 2002 JW Ryder CS 428 Computer Networks 12

ISO 7 Layer Reference Model • • Application Presentation Session Transport Network Data Link Physical - 7 - 6 - 5 - 4 - 3 - 2 - 1 See figure 11. 4 on page 181 © 2002 JW Ryder CS 428 Computer Networks 13

Physical Layer • Level 1 • Physical interconnection to the network • Includes electrical

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!! © 2002 JW Ryder CS 428 Computer Networks 15

Network Layer • Level 3 • Defines basics unit of transfer across network •

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 © 2002 JW Ryder CS 428 Computer Networks 17

Session Layer • Level 5 • Remote terminal access • Terminal to host connection

Presentation Layer • Level 6 • Provides network functions that many applications use •

Application Layer • Level 7 • Application programs that use the network • Electronic

TCP/IP Internet Layering Model • Looked at as 4 layers plus a 5 th

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 © 2002 JW Ryder CS 428 Computer Networks 22

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 © 2002 JW Ryder CS 428 Computer Networks 23

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 © 2002 JW Ryder CS 428 Computer Networks 24

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 © 2002 JW Ryder CS 428 Computer Networks 25

Ethernet Bridges • 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 500 m each © 2002 JW Ryder CS 428 Computer Networks 26

Repeater Example | 500 Meters | Floor 1 Floor 2 Floor 3 Repeater Computer

Ethernet Bridges • 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 © 2002 JW Ryder CS 428 Computer Networks 28

Bridges • Almost arbitrary number of bridges can be connected • Why? What is

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 © 2002 JW Ryder CS 428 Computer Networks 30

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!) © 2002 JW Ryder CS 428 Computer Networks 31