EthernetEthernet

• The most popular LAN technology

• Open Standard

• Open and wide market

Ethernet

• 1970: Xerox in PARC, Robert M. Metcalfe, David Boggs, Charles Thacker, Butler Lampson

• Experimental Ethernet, 3Mbps

• 1979: Formal specifications DIX (DEC-Intel-Xerox). 10Mbps

• 1985: “IEEE 802.3 CSMA/CD” is published

• It has included new technologies periodically

Operation

• Stations connected to a SHARED MEDIUM

• No main controller

• Serial transmission to the medium, Manchester

• CSMA/CD

• Colissions

SharedMedium

BUS

Application

Presentation

Session

Transport

Network

Datalink

Physical

Medium

Physical

Medium

MACLLC

OSI IEEE 802

UpperLayers

802

Sublayers of IEEE 802• Logical Link Control

– 802.2– SAPs to the upper layer– Assembly/Reassembly of frames– Addresses and fields for error detection

Medium Access Control– 802.3, 802.4, 802.5, 802.11, . . . . . .– Arbitrar el acceso al medio físico– Varias MACs para una sola LLC

Transmission in the BUS

Colission Colission

Ethernet 10 MbpsEthernet 10 Mbps

Medium Access Control (MAC) EthernetMedium Access Control (MAC) Ethernet

10Base5Thick coaxial

10Base5Thick coaxial

10Base2Thin Coaxial

10Base2Thin Coaxial

10BaseTTwisted Pair

10BaseTTwisted Pair

10BaseFFiber Optic

10BaseFFiber Optic

10Broad36Coaxial

10Broad36Coaxial

10Base5

10Base5

10Base5

10Base2

10Base2

10BaseT

Switches

• Filter frames– With MAC destination address– damaged (CRC)– incomplet

• Self learning of addresses/port– Based on source MAC address

• Switching made by hardware• Store-and-forward y cut-through

HUB

SWITCH

CSMA/CDCSMA/CD

Station readyfor transmission

Station readyfor transmission

Transmitlistening

Transmitlistening

End oftransmission

End oftransmission

Sense thechannel

Sense thechannel

Stop transmissionand

Start jam signal

Stop transmissionand

Start jam signal

Wait a randomtime (BEB)

Wait a randomtime (BEB)

Busy

Free

Collision

MTU

• Maximum Transfer Unit

• Big frames are not good for multiplexing– Big delays in “conversations”– More collisions

Importance of 2tp

tp

~ tp

~ 2tp

LLminmin(t) = 2t(t) = 2tppLLminmin(t) = 2t(t) = 2tpp