Data Frame

Typesofdataframes

Atthesendingend,thedatalinklayerencapsulatesthedatatransmittedfromthenetworklayerintoframes,andthensendsthemtothelink;atthereceivingend,thedatalinkThelayertakesoutthedatainthereceivedframeanddeliversittothenetworklayer.Differentdatalinklayerprotocolscorrespondtodifferentframes,sotherearemanykindsofframes,suchasPPPframes,MACframes,etc.,andtheirspecificformatsarealsodifferent.

Exampleofadataframe

ThefollowingtakestheformatofaMACframeasanexample:

TheframeheaderofaMACframeincludesthreefields.Thefirsttwofieldsarerespectivelya6-bytedestinationaddressfieldandasourceaddressfield.ThedestinationaddressfieldcontainsdestinationMACaddressinformation,andthesourceaddressfieldcontainssourceMACaddressinformation.Thethirdfieldisa2-bytetypefield.Theinformationcontainedinitisusedtoindicatewhatprotocolisusedbytheupperlayer,sothatthereceivingendcanhandoverthedatapartofthereceivedMACframetotheprotocoloftheupperlayer.Forexample,whenthevalueofthetypefieldis0x0800,itmeansthattheupperlayerisusinganIPdatagram;ifthevalueofthetypefieldis0x8137,itmeansthattheframeissentbyNovellIPX.

ThedatapartoftheMACframehasonlyonefield,thelengthofwhichisbetween46and1500bytes,andtheinformationcontainedisthedatapasseddownbythenetworklayer.

TheframetailoftheMACframealsohasonlyonefield,whichis4byteslong,andtheinformationcontainedistheframechecksequenceFCS(usingCRCcyclicredundancycheckcode).

RelatedDifferences

PacketisthedataunitinTCP/IPprotocolcommunicationandtransmission,generallyalsocalled"datapacket".

TheTCP/IPprotocolworksonthethirdlayer(networklayer)andthefourthlayer(transportlayer)oftheOSImodel,andtheframeworksonthesecondlayer(datalinklayer).Thecontentoftheupperlayeristransmittedbythecontentofthelowerlayer,sointhelocalareanetwork,the"packet"iscontainedinthe"frame".Theso-calleddataframeistheprotocoldataunitofthedatalinklayer,whichincludesthreeparts:frameheader,datapart,andframetail.Amongthem,theframeheaderandframetailcontainsomenecessarycontrolinformation,suchassynchronizationinformation,addressinformation,errorcontrolinformation,etc.;thedatapartcontainsdatapasseddownfromthenetworklayer,suchasIPdatapackets.

Frameformat

Frameformatreferstotheframeformatspecifiedbydifferentprotocols.Itusuallyconsistsoftwoparts:"frameheader+datainformation".

FrameformatsmainlyincludeEthernetframeformat,dataframeformat,802.3frameformat,andMACframeformat.