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2002 JWRyder CS 428 ComputerNetworks 1

Ethernet Properties

10Mbps/100Mbps broadcast bus

technologyBus: all stations share single

channel

Broadcast: all transceivers receive

every transmission

Transceiver passes all packets

from bus to host adapter

Host adapter chooses some andfilters others

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Ethernet Properties

Best-effort delivery: hardware

provides no information to thesender about whether packet

was actually delivered

Destination machine powereddown, packets will be lost

TCP/IP protocols accommodate

best-effort delivery

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CSMA/CD

Carrier Sense Multiple Access /Collision Detect

When transceiver begins

transmissionNot 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

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CSMA/CD

Each transceiver monitors cable

while transmitting in an attempt

to detect foreign signals

(interference) Monitoring called Collision

Detection

If collision detected, a hostadapter

aborts transmission

waits predictable number of

arbitrary length time units

tries again

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Binary Exponential

Backoff

Show BEB example

Many stations try to transmit atexactly the same time yields a

non-recoverable traffic jam

Ethernet capacity10Mbps means for everyone

Highway system can manage a

max amount of traffic

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WinIPCfg find and view

48-bit addressing scheme

Ethernet manufacturers buy

lots of addresses

Unique address in the universe

Fixed in machine readable form

on host interface hardware

Ethernet Address belong tohardware devices

Physical addr.,Hardware addr.

Ethernet Hardware

Address

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Ethernet Addresses

Moving hardware interface to

another machine changes theoriginal machines physical

address

Host interface examines packetsand decides which ones to send

up

Destination address used

CPU could perform check but

what would happen?

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48-bit address is one of 3 types

Unicast

Broadcast (All 1s)

Multicast (Group)

Host interface recognizes more

that just its physical address

Not all forms of addressing

implemented by all host adapter

manufacturersUsually unicast and broadcast,

multicast is newest idea

Ethernet Address

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(Data) Link level connection

among machines Phil Daily Never Took Sarah

Prentice Anywhere -- jwr

Variable length, no smaller than64 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

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Internet Services

Read Chapter 11

3 layers

Connectionless Delivery

Service

Provides foundation

Reliable Transport Service

Applications depend on this

Application Service

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Need for Multiple

Protocols

protocols allow one to

specify or understand

communication without

knowing the details of a

particular vendors network

hardware.

Same or different?Ethernet frame format, network

access policy, frame error

handling

IP addresses, datagram format,

unreliable delivery

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Conceptual Layering

Heterogeneous network

transport protocols require a

buffering between actual

network implementation viewof implementation


Protocol software morecomplex than simple model



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ISO 7 Layer Reference

Model

Application - 7

Presentation - 6

Session - 5

Transport - 4

Network - 3

Data Link - 2

Physical - 1


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Physical Layer

Level 1

Physical interconnection to thenetwork

Includes electrical

characteristics of voltage,current

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Data Link Layer

Level 2

Handles transmission errors

Frame checksum

Transmission is unreliable

Specifies exchange ofacknowledgements between

machines to know when frame

transferred successfully

Does not mean content is right!!

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Network Layer

Level 3

Defines basics unit of transferacross network

Includes concepts of destination

addressing, routing

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Transport Layer

Level 4

End to End reliability

Destination host communicates

with the source host

Even though lower level layersprovide reliable checks at each

transfer, end to end layer double

checks

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Session Layer

Level 5

Remote terminal access

Terminal to host connection

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Presentation Layer

Level 6

Provides network functions thatmany applications use

Text compression, graphic

images to bit stream formatsetc.

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Application Layer

Level 7

Application programs that usethe network

Electronic mail, File transfer

programs, finger, daytime, echo

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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

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Application Layer

Highest layer

Application programs thataccess services available across

TCP/IP internet

Interacts with one of thetransport layer protocols

Sequence of individual messages

Continuous stream of bytes

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Transport Layer Messages or Streams pass

between AL and TL Provides communication from

one application program to

anotherCalled 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

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Internet Layer

Transport Protocol Packets pass

between TL and IP Handles communication from

one machine to another

Encapsulates packet into IPdatagram

IP Security implemented here

Handles incoming datagrams

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Network Interface Layer

IP Datagrams pass between IP

and NL Responsible for accepting

datagrams and transmitting

them over a specific networkDevice driver when attaching to

LAN

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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 at500m each

Ethernet Bridges

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Repeater Example

Floor 1

Floor 2

Floor 3

Repeater

Computer (Host)

| 500Meters |

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Almost arbitrary number ofbridges can be connected

Why? What is different?

Bridges

Hide details of the interconnection

Acts like single huge Ethernet

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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

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Reading

Make sure you read all of

chapter 2 & 3

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