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Ethernet

• Context

• History

• Bus-based (coax)

• Star-coupled (10BaseT)

• Switched

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Ethernet context

Layer 2 of protocol hierarchy

Arbitration to determine sharing of physical medium

Addressing within local area (not at network layer)

Error detection (but not correction – what do you think happens to any message received with an error?

Identification of type of data being carried at network layer

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Ethernet history

Invented 1973-76 at Xerox PARC - Robert Metcalfe credited with the invention.

Metcalfe’s task was to figure out how to share a high-speed printer among a number of microcomputer workstations.

Metcalfe and Boggs: Ethernet: Distributed Packet-Switching For Local Computer Networks. 1976

Metcalfe persuaded Xerox, Digital and Intel to promote Ethernet as a standard. Original Ethernet slightly different from current IEEE 802.3 standard.

One of the most successful technologies in world of telecom.

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Ethernet history

T T T

NIC NIC NIC

Bus-based, with terminators at each end to absorb an electronic signal

Each station puts its message onto the bus (the “ether”)

Message propagates in both directions

Eventually reaches the station for which it is intended.

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Bus-based

Ethernets

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Ethernets

24 bits specify vendor (OUI)

24 bits allocated by vendor

Every NIC has a unique-in-the-Universe 48-bit MAC (Media Access Control) address.

IEEE 802-controlled MAC addresses:

OUI: Organizationally Unique Identifier

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Ethernets

24 bits specify vendor (OUI)

24 bits allocated by vendor

Every NIC has a unique-in-the-Universe 48-bit MAC (Media Access Control) address.

IEEE 802-controlled MAC addresses:

OUI: Organizationally Unique Identifier

http://www.iana.org/assignments/ethernet-numbers

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EthernetsEthernet Frame

http://www.wildpackets.com/compendium/EN/EN-FrFSn.html

See Figure 3-12 for comparison

SAP: Service Access Point

SNAP: Subnetwork Access Protocol

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EthernetsInternet Assigned Numbers Authority (IANA)

http://www.wildpackets.com/compendium/EN/EN-FrFSn.html

See Figure 3-12 for comparison

SAP: Service Access Point

SNAP: Subnetwork Access Protocol (3 bytes of OUI; 2 bytes of EtherType)

http://www.networksorcery.com/enp/protocol/802/ethertypes.htm

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Bus-based Ethernets

http://www.erg.abdn.ac.uk/users/gorry/course/lan-pages/csma-cd.html

The cable propagates the signal in both directions, so that the signal (eventually) reaches all four of the computers. Termination resistors at the ends of the cable absorb the frame energy, preventing reflection of the signal back along the cable.

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Bus-based Ethernets

What's an obvious problem?

http://www.erg.abdn.ac.uk/users/gorry/course/lan-pages/csma-cd.html

All NICs receive the frame. Only the red NIC recognizes the destination address and forwards the contents of the frame to the computer's network layer.

The other computers ignore the frame.

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Bus-b

ased

Ethernets

Stallings Data and Computer Communication 6th ed.

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Bus-based Ethernets

Works extremely well as long as there is minimal contention.

Degrades with increased contention

Of the 10 Mbps, get about 4 Mbps throughput maximum!

Extremely difficult to troubleshoot (why?)

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Bus-based Ethernets

www.delmar.edu/Courses/ ITNW2313/throuput.jpg

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Think of the bus as shrinking into a piece of hardware and the T connectors becoming RJ45 jacks.

Now think of the connections to each computer getting fairly long, and being Cat 5 twisted pair.

Hub-based Ethernet: 10BaseT

(10Mbps, Baseband, Twisted pair)

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Do the NICs know the difference??

No, they are still running CSMA/CD

Are there still collisions?

What is the role of the hub?

10BaseT

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HUB

Station

Station

Station

Station

Station

Notice the Star topology - "Star-coupled" Ethernet

10BaseT

Notice the limit on stations.

Send on one pair of the twisted pair; receive on another.

http://www.ertyu.org/~steven_nikkel/ethernetcables.html

10BaseT

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Note the mistake in the label on the arrow from HHUB to the left IHUB - it should be Fa

Fa means "a frame from station A

10BaseT

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CP means a "Collision Presence" signal. Notice they are issued by the hubs! But they get to every station.

10BaseT

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10BaseT

A, B and C all began to send when there was silence on the medium. Now they are all aware of the collision, and will back off a random number of ms before trying again.

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10BaseT

A, B and C all began to send when there was silence on the medium. Now they are all aware of the collision, and will back off a random number of ms before trying again.

How many collision domains??

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Switched Ethernet

Collision domains?

Collision domains?

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Switched Ethernet

Transparent to stations/NICs

Scales easily

Eliminates collisions - can reach 10Mbps

http://www.networking.ibm.com/mse/mse0c01.html#Switched Ethernet LANs

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Switched Ethernet

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Switched Ethernet

Once there are no collisions, a station can simultaneously send and receive -> Full Duplex (FDX)

http://www.networking.ibm.com/mse/mse0c01.html#Switched Ethernet LANs

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Switched Ethernet