© 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

© 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

© 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

© 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

© 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

© 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

© 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?

© 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

© 2002 JW Ryder

CS 428 Computer Networks

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• (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

© 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

© 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

© 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

© 2002 JW Ryder

CS 428 Computer Networks

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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

© 2002 JW Ryder

CS 428 Computer Networks

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Physical Layer

• Level 1

• Physical interconnection to the network

• Includes electrical characteristics of voltage, current

© 2002 JW Ryder

CS 428 Computer Networks

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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

16

Network Layer

• Level 3

• Defines basics unit of transfer across network

• Includes concepts of destination addressing, routing

© 2002 JW Ryder

CS 428 Computer Networks

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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

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Session Layer

• Level 5

• Remote terminal access

• Terminal to host connection

© 2002 JW Ryder

CS 428 Computer Networks

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Presentation Layer

• Level 6

• Provides network functions that many applications use

• Text compression, graphic images to bit stream formats etc.

© 2002 JW Ryder

CS 428 Computer Networks

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Application Layer

• Level 7

• Application programs that use the network

• Electronic mail, File transfer programs, finger, daytime, echo

© 2002 JW Ryder

CS 428 Computer Networks

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TCP/IP Internet Layering Model

• Looked at as 4 layers plus a 5th hardware layer

• Application

• Transport

• Internet

• Network Interface

• Hardware

© 2002 JW Ryder

CS 428 Computer Networks

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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

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

© 2002 JW Ryder

CS 428 Computer Networks

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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

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

© 2002 JW Ryder

CS 428 Computer Networks

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• 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

© 2002 JW Ryder

CS 428 Computer Networks

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Repeater Example

Floor 1

Floor 2

Floor 3

RepeaterComputer (Host)

| 500 Meters |

© 2002 JW Ryder

CS 428 Computer Networks

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• 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

© 2002 JW Ryder

CS 428 Computer Networks

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• Almost arbitrary number of bridges can be connected

• Why? What is different?

Bridges

Hide details of the interconnection

Acts like single huge Ethernet

© 2002 JW Ryder

CS 428 Computer Networks

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Bridges

• Adaptive, Learning bridges

• 1 computer, 2 Ethernet interfaces

• Software keeps 2 address lists

• Frame arrives from E1 , adds 48-bit source addr. to list associated with E1

• Learns topology of each wire

• Check destination addr. If on frame from whence it came, disregard

© 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!)

© 2002 JW Ryder

CS 428 Computer Networks

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Reading

• Make sure you read all of chapter 2 & 3