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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. Ethernet Properties.

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Ethernet Properties

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ethernet properties
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

CS 428 Computer Networks

ethernet properties2
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

CS 428 Computer Networks

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

CS 428 Computer Networks

csma cd4

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

CS 428 Computer Networks

binary exponential backoff
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

CS 428 Computer Networks

ethernet hardware address
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

CS 428 Computer Networks

ethernet addresses
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?

CS 428 Computer Networks

ethernet address
48-bit address is one of 3 types


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

CS 428 Computer Networks

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

Ethernet Frame

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internet services
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
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

CS 428 Computer Networks

conceptual layering
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

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iso 7 layer reference model
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

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physical layer
Physical Layer

Level 1

Physical interconnection to the network

Includes electrical characteristics of voltage, current

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data link layer
Data Link Layer

Level 2

Handles transmission errors

Frame checksum

Transmission is unreliable

Specifies exchange of acknowledgements between machines to know when frametransferred successfully

Does not mean content is right!!

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network layer
Network Layer

Level 3

Defines basics unit of transfer across network

Includes concepts of destination addressing, routing

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transport layer
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

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session layer
Session Layer

Level 5

Remote terminal access

Terminal to host connection

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presentation layer
Presentation Layer

Level 6

Provides network functions that many applications use

Text compression, graphic images to bit stream formats etc.

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application layer
Application Layer

Level 7

Application programs that use the network

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

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tcp ip internet layering model
TCP/IP Internet Layering Model

Looked at as 4 layers plus a 5th hardware layer




Network Interface


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application layer22
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

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transport layer23
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

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internet layer
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

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network interface layer
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

CS 428 Computer Networks

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 500m each

Ethernet Bridges

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repeater example
Repeater Example

| 500Meters |

Floor 1

Floor 2

Floor 3


Computer (Host)

CS 428 Computer Networks

ethernet bridges28
Superior because they do not replicate



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

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Almost arbitrary number of bridges can be connected

Why? What is different?


Hide details of the interconnection

Acts like single huge Ethernet

CS 428 Computer Networks


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

CS 428 Computer Networks

adaptive bridges
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!)

CS 428 Computer Networks


Make sure you read all of chapter 2 & 3

CS 428 Computer Networks