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　　NetworkWorkingGroupJuhaHeinanen
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:1483TelecomFinland



　　July1993
　　MultiprotocolEncaps

ulationoverATMAdaptation

Layer5

　　StatusofthisMemo
　　ThisRFCspecifiesanIABstanda
community,andrequestsdiscussiona

rdstrackprotocolfortheInternet
ndsuggestionsforimprovements.

　　Pleaserefertothecurrentedit
forthestandardizationstateandst

ionoftheIABOfficialProtocolStandards
atusofthisprotocol.

　　Distributionofthismemoisunlimited.
　　Abstract
　　Thismemodescribes
interconnecttrafficove

twoencapsulationsmethodsf
rATMAAL5.Thefirstmethod

orcarryingnetwork
allows

　　multiplexingofmultipleprotoco
thesecondmethodassumesthateach

lsoverasingleATMvirtualcircuitwhereas
protocoliscarriedover

　　aseparateATMvirtualcircuit.
　　1.Introduction
　　AsynchronousTransfe
forbothlocalandwide

rMode(ATM)basednetworks
areaapplications.Thismemo

areofincreasinginterest


　　describestwodiffer

entmethodsforcarryingcon

nectionlessnetwork

　　interconnecttraffic
ATMnetwork.Thefirstme

,routedandbridgedProtoco
thodallowsmultiplexingof

lDataUnits(PDUs),overan


　　multipleprotocolsoverasingle

ATMvirtualcircuit.Theprotocol

　　ofacarriedPDUisidentifiedb

yprefixingthePDUbyanIEEE802.2

　　LogicalLinkControl
Encapsulationandasubs

(LLC)header.Thismethodis
etofithasbeenearlier

inthefollowingcalledLLC


　　definedforSMDS[1]
multiplexingimplicitly

.Thesecondmethoddoeshigh
byATMVirtualCircuits(VCs

er-layerprotocol
).Itisinthe

　　followingcalledVCBasedMultiplexing.
　　ATMisacellbasedtransfermod
informationtobesegmentedandreas

ethatrequiresvariablelengthuser
sembledto/fromshort,fixed

　　lengthcells.Thisme
(SAR)methodforbridged

modoesn''tspecifyanewSeg
androutedPDUs.Instead,th

mentationAndReassembly
e

　　PDUsarecarriedin
(CPCS)PDUofATMAdapta
describeshowroutedand

thePayloadfieldofCommon
tionLayertype5(AAL5)[2]
bridgedPDUsare

PartConvergenceSublayer
.Notethatthismemoonly


　　carrieddirectlyovertheCPCSo
ConvergenceSublayer(SSCS)ofAAL5
AAL5July1993RelayServiceSpecific
inI.36x.1[3],isusedovertheCPC
carriedusingtheNLPIDmultiplexing

fAAL5,i.e.,whentheServiceSpecific
isempty.IfFrameRFC1483Multiprotocolover
ConvergenceSublayer(FR-SSCS),asdefined
SofAAL5,thenroutedandbridgedPDUsare
methoddescribedinRFC

　　1294[4].AppendixA
FR-SSCS-PDUaswellash
accordingtoRFC1294.

(whichisforinformationon
owIPandCLNPPDUsareenca


ly)showstheformatofthe
psulatedoverFR-SSCS


　　2.SelectionoftheMultiplexing
Multiplexingwillbedominantinenv
numbersofATMVCsisfastandecono
prevailin

MethodItisenvisionedthatVCBased
ironmentswheredynamiccreationoflarge
mical.Theseconditionsarelikelytofirst


　　privateATMnetworks
whenitisnotpractical

.LLCEncapsulation,ontheo
foronereasonoranothert

therhand,maybedesirable
ohave

　　aseparateVCforea
ATMnetworkonlysupport
dependsheavilyonthen

chcarriedprotocol.Thisis
s(semi)PermanentVirtualC
umberof

thecase,forexample,ifthe
ircuits(PVCs)orifcharging


　　simultaneousVCs.WhentwoATMst

ationswishtoexchangeconnectionlessnetwork

　　interconnecttraffic
manualconfiguration(in

,selectionofthemultiplex
caseofPVCs)orbyB-ISDN

ingmethodisdoneeitherby


　　signallingprocedures(incaseo
signallingarestillunderstudyin

fSwitchedVCs).ThedetailsofB-ISDN
CCITT[5].Itcan,however,

　　beassumedthatB-ISDNsignallin
informationelement,whichwillallo

gmessagesincludeaLowlayercompatibility
wnegotiationof

　　AAL5andthecarried

(encapsulation)protocol.



　　3.AAL5FrameFormat
　　Nomatterwhichmult
shallbeencapsulatedwi
AAL5CPCS-PDUisgivenb

iplexingmethodisselected,
thinthePayloadfieldofAA
elow:

routedandbridgedPDUs
L5CPCS-PDU.Theformatofthe


　　Heinanen[Page2]


　　RFC1483MultiprotocoloverAAL5J

uly1993

　　AAL5CPCS-PDUFormat
　　


　　ThePayloadfieldcontainsuser
fieldpadstheCPCS-PDUtofitexact
octetcellpayloadcreatedbytheSA
rightjustifiedinthecell.TheCPCS
totransparentlytransferCPCSuser
underthemultiprotocolATMencapsul
toanyvalue.TheCPI(CommonPartIn

informationupto2^16-1octets.ThePAD
lyintotheATMcellssuchthatthelast48
RsublayerwillhavetheCPCS-PDUTrailer
-UU(User-to-Userindication)fieldisused
touserinformation.Thefieldhasnofunction
ationdescribedinthismemoandcanbeset
dicator)fieldalingstheCPCS-PDUtrailerto

　　


　　ThePayloadfieldco
fieldpadstheCPCS-PDU
octetcellpayloadcreat
rightjustifiedinthec
totransparentlytransfe
underthemultiprotocol
toanyvalue.

ntainsuserinformationupt
tofitexactlyintotheATM
edbytheSARsublayerwill
ell.TheCPCS-UU(User-to-Us
rCPCSusertouserinformat
ATMencapsulationdescribed


o2^16-1octets.ThePAD
cellssuchthatthelast48
havetheCPCS-PDUTrailer
erindication)fieldisused
ion.Thefieldhasnofunction
inthismemoandcanbeset

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