2.4EthernetProtocolOverview
2.4.1EthernetFrameFormat
Preamble SFD Destination Source Len Data
7 1 6 6 2 46−1500 4
FCS
Number of bytes
Legend: SFD=Start Frame Delimeter; FCS=Frame Check Sequence (CRC)
Architecture
Ethernetprovidesanunreliable,connection-lessservicetoanetworkingapplication.Abriefoverviewof
theEthernetprotocolisgiveninthefollowingsubsections.Forin-depthinformationontheCarrierSense
MultipleAccesswithCollisionDetection(CSMA/CD)AccessMethod,whichistheEthernet’smultiple
accessprotocol,seetheIEEE802.3standarddocument.
AlltheEthernettechnologiesusethesameframestructure.TheformatofanEthernetframeisshownin
Figure4anddescribedinTable3.TheEthernetpacket,whichisthecollectionofbytesrepresentingthe
dataportionofasingleEthernetframeonthewire,isshownoutlinedinbold.TheEthernetframesareof
variablelengths,withnoframesmallerthan64bytesorlargerthanRXMAXLENbytes(header,data,and
CRC).
Figure4.EthernetFrameFormat
Table3.EthernetFrameDescription
FieldBytesDescription
Preamble7Preamble.These7byteshaveafixedvalueof55handservetowakeupthereceiving
EMACportsandtosynchronizetheirclockstothatofthesender’sclock.
SFD1StartofFrameDelimiter.Thisfieldwithavalueof5Dhimmediatelyfollowsthepreamble
patternandindicatesthestartofimportantdata.
Destination6Destinationaddress.ThisfieldcontainstheEthernetMACaddressoftheEMACportfor
whichtheframeisintended.Itmaybeanindividualormulticast(includingbroadcast)
address.WhenthedestinationEMACportreceivesanEthernetframewithadestination
addressthatdoesnotmatchanyofitsMACphysicaladdresses,andnopromiscuous,
multicastorbroadcastchannelisenabled,itdiscardstheframe.
Source6Sourceaddress.ThisfieldcontainstheMACaddressoftheEthernetportthattransmitsthe
frametotheLocalAreaNetwork.
Len2Length/Typefield.ThelengthfieldindicatesthenumberofEMACclientdatabytes
containedinthesubsequentdatafieldoftheframe.Thisfieldcanalsobeusedtoidentify
thetypeofdatatheframeiscarrying.
Data46toDatafield.Thisfieldcarriesthedatagramcontainingtheupperlayerprotocolframe,thatis,
(RXMAXLEN-18)IPlayerdatagram.Themaximumtransferunit(MTU)ofEthernetis(RXMAXLEN-18)
bytes.Thismeansthatiftheupperlayerprotocoldatagramexceeds(RXMAXLEN-18)
bytes,thenthehosthastofragmentthedatagramandsenditinmultipleEthernetpackets.
Theminimumsizeofthedatafieldis46bytes.Thismeansthatiftheupperlayerdatagram
islessthen46bytes,thedatafieldhastobeextendedto46bytesbyappendingextrabits
afterthedatafield,butpriortocalculatingandappendingtheFCS.
FCS4FrameCheckSequence.Acyclicredundancycheck(CRC)isusedbythetransmitand
receivealgorithmstogenerateaCRCvaluefortheFCSfield.Theframechecksequence
coversthe60to(RXMAXLEN-4)bytesofthepacketdata.Notethatthis4-bytefieldmay
ormaynotbeincludedaspartofthepacketdata,dependingonhowtheEMACis
configured.
SPRUEQ6–December2007EthernetMediaAccessController(EMAC)/ManagementDataInput/Output(MDIO)19
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