AddingVoicetoanOfficeComputerNetworkD.C.Swinehart,L.C.Stewart,andS.M.OrnsteinCSL-83-8August1983[P83-00009]Abstract:ThispaperdescribesthearchitectureandinitialimplementationofanexperimentaltelephonesystemdevelopedbytheComputerScienceLaboratoryattheXeroxPaloAltoResearchCenter(PARCCSL).Aspeciallydesignedprocessor(Etherphone)connectstoatelephoneinstrumentandtransmitsdigitizedvoice,signalling,andsupervisoryinformationindiscretepacketsovertheEthernetlocalareanetwork.Whenusedbyitself,anEtherphoneprovidesthefunctionsofaconventionaltelephone,butitcomesintoitsownwhencombinedwiththecapabilitiesofanearbyofficeworkstation,avoicefileservice,andothersharedservicessuchasdatabases.Mostoftheworksofarhasgoneintothebasicprovisionsforvoiceswitchingandtransmission.Todaythesystemsupportsordinarytelephonecallsandsimplevoicemessageservices.Wewillexpandthesefunctionsasweexploretheintegrationofvoicewithourexperimentalofficesystems.AversionofthispaperwillappearintheproceedingsoftheIEEEGlobeCom'83Conference,November,1983.CRCategoriesandSubjectDescriptors:C.2.4,H.4.3,H.1.2,C.2.2AdditionalKeywordsandPhrases:Voice,recordedvoice,telephones,Ethernet,Cedar,distributedsystemsXEROXXeroxCorporationPaloAltoResearchCenter3333CoyoteHillRoadPaloAlto,California94304p ^k#,w:$q X 2w @Q"&(1*r Se SeSe s H^t#H^H^SC! *,0 ;V=?6 F*!(i. 57O9>An CU%+2 ;B@D A -!'l+ 25 = Eg ?d $r'-257;= =Xs $ +./479<@ ;$  [iR#i +-03W8;? 8OJ  ["$(H*N,/X368 n? *' 248:>n@ G P D &m,.4e7t ?BG M "&+036k:$ BWEk K 5 & Ie&-/ # +-M24J8 ?BC G1 ~t"k */17 ? Ft D 7 $-*-k3j6#;=dB B v rk#Q%'+#,.4m:>?B(FI @  b! ).,.Z1C2\7: AC >_  %(X-/# 6T;PAlD  <* +0 (W+H. 6+7;=AcEG 9 L$8j!#)(,b.05 ;? F 7  5 Yj!n '-4.C 7A >A,B; 3QD 6[ g%,3w <{? G/ 1C "&P -/ 67:>eD . t ii!$V +-0 75A@EBG/ +,L"H! # ),0e6 <?`D:G/ ){ 1 !&?(*.J 5;>0 FH 'F  ,j | #j ){ 41 $q&(.16:%<~?AEE Yj '"+/ 7a:>QDYG % u9 $y&)f/1Z6:<@MFt h$! ( 1348<?C   %p).1N5,9 BCD   i"Q&<'*g.157\:3<,?D FI S /?   $d K"R&6'),S.0q4I69>1?AG  $F&( /46:f<CMG  Qu/ ')+/16& <?/Cx #P',X1 {TVm$4AddingVoicetoanOfficeComputerNetworkassistancetothosecallingfromoutsidetheEtherphonesystem.2.3VoiceasdataLeavingarecordedmessagecanbeaneffectivesubstituteforthosecallsthatgounanswered.Telephoneansweringmachinesmakethispossible,buttheirimperativenaturedemandsawell-constructed,composedmessageratherthananinformalcommunication.Fewofusareabletoproducesuchacompositioninrealtime.Thesemachinesalienatecallers,whooftenhangupwithoutleavinganyusefulmessage.Ourapproachtothisproblemistoshifttheresponsibilityforproducingavoicemessagetothecaller.Thesystemcanallowthesendertorecord,audition,andmodifyamessagebeforesendingit--ordecidingnotto.Themessagemightcontaintext,recordedvoice,oracombination.Ifexperiencewithelectronictextmessagesisanyguide,userswillcometopreferrecordedmessagestotelephonecallsinsomesituations.Somerecentstand-alonevoicemessagesystemssupporttheseideas[Maxemchuk].Voiceinmessagesanddocumentswillfindapplicationfarbeyondsimpletelephony.Weintendtoexplorevoiceannotationofordinaryofficedocuments,spokenreminders,musicalprompts,andothercombinations.Insummary,wefeelthatourcomputingandcommunicationsenvironmentcanbebroughttobearonvoiceintwoways.Onewayistoimproveourcontroloverotherwiseordinaryvoicecommunications;theotheristoincorporatethevoicemediumintoourintegratedenvironmentonanequalfootingwithtext,graphics,anddata.Whilethefacilitieswehavebuiltorproposedarenotentirelynew,webelievethatthefullintegrationofvoiceintoourcomputer-basedofficesystemswillmakeexistingfunctionsmoreeffectiveandnewapplicationspossible.XEROXPARC,CSL-83-8,AUGUST,1983w g/xg/"|%&(|+1{ _/ !n:"$c +| [E ) { W[16"$7&j,1 25*9 <^?oA U& ?"(#*x- 478[>> R; !% 0E357:1=k?ZDHe P R}z#~(-037 9>"BEv N LJm !#v&)7 14-:<,?E]G/ Jz<"$n)%.167=/AFH G sx+G{mGUG_"'+0467 @B E / #'()-e/38>@5F Cx - y $(-27;p> A8L/!$' .05:6 @CG ? i!% ,i16 8 =;CE < 8Q%( 2 ;=?DFt 6 nYQ} %(,/5;W> 3 Rj n n%(+S 1 9;=AFt 1 { $'*b-/|57:Y?`BD / e + !$p .27:>aC -K  ?x #P',X1TVm$wAddingVoicetoanOfficeComputerNetwork53.SystemArchitecture3.1PARCEnvironmentFormostofficeapplications,adistributednetworkofpersonalworkstations,interconnectedbyhigh-bandwidthlocal-areanetworksorbyspecializedPABXlines,issupplantingthetraditionalcentralcomputinginstallation.AtPARC,boththestandardEthernet,operatingat10.0Mbps,andanearlierprototypeEthernet,operatingat3.0Mbps,areingeneraluse[Metcalfe,Crane].Thesenetworkslinkpersonalworkstationstoeachotherandtosharedresourcessuchasfileservers,mailservers,andhigh-qualitylaserprinters[Birrell82,Mitchell].Communicationsoutsidethelocalareaareprovidedprimarilybyleasedlines--mostoperatingat9.6or56Kbps,withsomeoperatingatupto1.5Mbps[Boggs,NS].Severaltypesofpersonalworkstationsareinuse,includingtheXerox1100,1108,and1132(Dorado)processors[Thacker,Lampson].Thesesameprocessorsareusedasthecontrollersformostservers.Everyworkstationhasahigh-resolutionbitmapdisplay(1000by800pixels),akeyboard,andamouse[Smith].Inourlaboratory,mostusersworkwithintheCedarprogrammingenvironment.Cedarisanexperimentalintegratedsystemthatisusedforsoftwaredevelopmentandcomputer-aideddesign,aswellasforday-to-dayoperationofofficeapplications,suchasdocumentpreparation,graphics,andelectronicmail[Brotz,Smith].OurapproachtoapplicationsresearchistobuildprototypeapplicationsbasedontheCedarenvironment,thentousetheminourdailywork.Thisapproachencouragesustocarrythedevelopmentofourimplementationsfarenoughthatperformanceandreliabilityproblemsdonotimpairourabilitytoevaluatetheirusefulness.Oncetheyareintroduced,theseapplicationssupplybothideasandusefulcomponentsforfurtherdevelopments.Mostofourapplicationsarestructuredsothatuserscaneitheroperatethemdirectly,orwriteprogramsthatcombinetheirfunctionswiththoseofotherfacilities.3.2BasicArchitecturaldecisionsThegoalsofimprovedtelephonefunctionalityandintegratedvoiceaddtwonewrequirementstotheexistingcapabilitiesinCedar:rapidandversatilecontrolovervoicetransmissionandswitching,andahighperformancevoicestoragecapability.Wesurveyedseveralpossiblearchitecturesforthesystem.SubscriberlineaccesstoavoicestoragesystemanduseofourexistingCentrexservicethroughDTMFsignallingwasrejectedduetothelowbandwidthandtheinflexiblecontroloverswitching.UseofacommercialPABXwasattractive,butthenecessaryswitchingcapabilitieswerenotcommerciallyavailable.TheuseofourowncomputerstocontroltheswitchingoperationsofastandardPABXwasrejectedprimarilyduetothenontechnicaldifficultiesofarrangingthenecessarycooperativeeffort.Althoughreluctanttoundertakethenecessarydevelopmentwork,weconcludedthatthemosteffectivewaytosatisfyourneedswastoconstructourowntransmissionandswitchingsystem.WeelectedtousetheEthernettotransmitvoiceaswellascontrolinformation.WechoseEthernetbecauseofitsXEROXPARC,CSL-83-8,AUGUST,1983xg/!$&('+ 0KwHDg/z ^  | [ )y { W=;S  (&-/5 > G U  3 !# */J24e ;>V E R a{V"%/*17B8<@gC=EK PQsr %<').}1 7<@F Nk}NkNk M!1$,{&Nk&Nk*1 46r9>MAF L6 e !` }'L6(L6 27:b=A'C J D$&n(*,037=?iACRE G{GG$!Q )7+-)/58R?AEHe A0F >  d$2(+w/ 8 AzEG <  ! $6&e+ 36 @jEF :  _ * (+4,3: :@CO 8^ u 6)=B /%')-4> <+@dBEw 3 Gal! $e(.+<1. 849;? A^ 1 D" *-} 39;>RBEV / ] }"$ ,C/ 7s;?1BEv -W  "N%'*A 14D :<?C E +#_q#%+j.47;i="@ xE+#| 'G ) 9{ #j1M! ), 3569Q@ A>D PRu"'c,1 7:?BDvF d  # *@-./ 7[;>\ DG/ /  "h * 03n57c9<CJD  Pj  !%(-368P: B  -A ! !$Z* 3#7:9@CF2 ]X5 "''*j-V 5 7=BtD ) 6b#!@#&', 4~7:@EGx #P',X1 TVm$!6AddingVoicetoanOfficeComputerNetworkpervasivenessinourenvironmentandinordertodemonstratetheeffectivenessofEthernetforvoice.ThepotentialoftheEthernetforpacketvoicetransmissionhasbeendescribedelsewhere[Shoch].Inthefollowingsection,wepresentonlyaveryconservativeanalysis.XEROXPARC,CSL-83-8,AUGUST,1983w g/xg/"|%&(|+1{ _/ i !h#&(y 0F2 : G X $'2*04m8 >| U ){ Q,(!#$% , 56<@~B Nb| $(+- 4 W@D J AY"Q&+/1 9=?D HZ n F%o^$v"$y' // 7s;= E C ; #(S)+/h35 <@#BuD A :p ;n "9#}'(*/1d39Q>*A ? ga( %~),W01390=n@5 =S ;~ % -2f6:? BE 8 !")%w , 3 8<AE@ 6 Q  $*.]033 9m< Ea 4 W }e!%S -n14` :?*CG 2L r #&*e/k47 :7 B@F 0[ $ +-24`5~8='?Y E - "&&e)- 24:>ABD + Xb )zlQ!$A ,n/ 6:C;?Ak 'F %!a#.( 02|56W;@\BF> %* W$&*-<1i3!7<A "|  ){ $"b&# .2/17ZA? R5="$ )Z,l.u12+6I{;|<?D  "$(.4 <AF P[ &_)x-1 8 ?HC4 R+"(* 49<AF |  &(- 24t7 ? E  va!&i+.13:< ?fE /9$&R*N-/2 9w; BG/x #P',X1 "TVm$8AddingVoicetoanOfficeComputerNetworkbandwidthavailableinanetworkdedicatedtovoicetransmissionis5Mbps,yielding28full-duplex"trunks"(55one-wayvoicestreams.)IfweestimateaTASIadvantageof1.6forthissizetrunkgroup[ATT],adedicated10MbpsEthernetcansupportabout45conversations.Ifatmost20%oftelephonesareinuseatonce,suchanetworkcouldsupportinexcessof225subscribers.Thisargumentrequiresthatcontroltrafficandothernon-voicetrafficisnegligiblecomparedtovoicetraffic.Thusitseemsclearthatasimplesingle-cableEthernetvoice-transmissionsystemcansupportasizeableinstallation.ThenumberofsubscriberscouldbefurtherincreasedbylinkinganumberofEthernetcableswithpacketgatewaysorconventionalcircuitswitches.Installationofamultiple-cablesystemwouldrequireattentiontousagepatternsandothertraffic-engineeringconsiderations.4.3SecuritySecuritycanbeaproblemwithEthernetoranyotherbroadcastmedium,sincenophysicalprotectionagainsteavesdroppingispossible.WeimposesecurityonourtransmissionsbyencryptingallvoiceandcontroltrafficusingtheDataEncryptionStandard[DES].Theavailabilityofsingle-chipDESdevicesoperatingat900Kbpsmakesthispossible.Wehandlethedistributionofkeysusingamethodsimilartothetrusted-authentication-serverapproachadvocatedbyNeedhamandSchroeder[Needham].4.4NewFunctionsThebroadcastnatureoftheEthernetadmitssomenewcommunicationpossibilities.ConferencecallsamongseveralEtherphonescanbeachievedwithouttheneedforspecialconference-bridgehardware:multicasttechniquespermitvoicepacketsfromagivenEtherphonetobereceivedandcombinedbyeachoftheothers[Forgie].Intheextreme,voicefromameetingorconferencecanbetransmittedonceandreceivedbyanynumberoflisteners,withoutconsuminganymorebandwidththanasingleconversationconsumes.4.5EtherphoneVoiceProtocolsThevoiceprotocolsfortheEtherphonearebasedontheprecedinganalysis.Wetransmit80008-bitsamplespersecond,usingtheindustry-standardmu-255encoding[Henning].TheEtherphoneactuallysupportstworelatedprotocols,oneforinteractivevoice(telephonecalls),theothertoplayandrecordstoredvoice.Theinteractive-voiceprotocolwasdesignedtomeetadelaybudgetof40millisecondsend-to-end.Thisdelayconsistsofapacketizationdelayof20milliseconds,hardwarelatencyof5millisecondsforencryptionandEthernettransmission,softwaredelaysofupto5milliseconds,andwhatwecallananti-jitterdelayof10milliseconds.Tomeetthisbudget,theprotocolspecifiesthetransmissionoffiftypacketspersecond,eachcontaining160voicesamplesand36bytesofaddressingandcontroloverhead.Anti-jitterdelayisintroducedbybufferingatthereceivingstationtoallowforvariationsinthearrivaltimesofpackets.Onemightsaytheprotocoloperateswithaboutone-halfpacketofbuffering.Thisdelaybudgetdoesnotallowtimeforretransmissionsintheeventoflostpackets.ThishasprovenXEROXPARC,CSL-83-8,AUGUST,1983w g/xg/"|%&(|+1{ _/ %'+ 35.6:@lB} \!y#%3*+/67:  @B X SJB#(*.03Y :>DB V] mz2"&&4' -4K59 T)_" *A0;@gC&He Q: x~! ),/3:%?E  =  %+-4H6= | 9& )N{ 5Jrj#H'+%- 7 ?@ F 33 P%*-a03,7C  0 `#-&'+< 24F6$;>%DtF^ .  5!%)()/0 7:U?k GR R  "+',8.%0j2K3 < >BDGu  {~q !%M'-/n4:q< DF 8K "&"+I-/3n5# ;>CE  @ !( ),R2U68<> E[G/ P+E R"%;*0C3t7k<A`C/ L d!$;-/157m: ?IBkDx #P',X1 TVm$ AddingVoicetoanOfficeComputerNetwork9acceptable,becausethenativepacketlossrateofwell-designedEthernetcomponentshasbeenshowntobelessthanonepacketintwomillion[Hupp].Recordingandplaybackofstoredvoicehasslightlydifferentcharacteristics.Transmissiondelayisnotparticularlyimportant,providedthatthestartupdelayfromrequesttoplaybackisreasonablyshort.Thestored-voiceprotocolcallsfor100millisecondsofbuffering.Theadditionalbufferingisdesirablebecausethesharedvoicefileservermaynotbeabletoschedulethetransmissiontimesofpacketsasaccuratelyasadedicatedvoiceterminal.Theadditionalbufferingalsopermitsretransmission,althoughwehavenotfoundtheneedtoimplementit.XEROXPARC,CSL-83-8,AUGUST,1983xg/!$&('+ 0KwHDg/{ _/  g{ #x&6' 06& =@>CG \ rDM! Zn7"%(=-2; DgH# X| c  i =# %n* -1"57=q> E VH -V"] *C, 25 <B8C T)$V ~#&,(8+L-25Q =NABG Q o'[:#% ,z25d:dC O Z<G$,x #P',X1TVm$ 10AddingVoicetoanOfficeComputerNetwork5.MajorSystemComponents5.1HardwarechoicesPerhapsthemostsignificantaspectoftheEtherphonesystemarchitecturaldesignwasthechoiceofvoiceterminalequipment.GiventheexistingPARCenvironment,voiceterminalsweretheonlynewequipmentrequired.Thealternativesweexploredweretoaddspecialhardwareandsoftwaretoeachworkstation,ortodesignastandardizedEthernettelephoneperipheral--theEtherphone--thatwoulddoallofthenecessaryvoiceconversionandtransmission.WechosetheEtherphoneapproachforseveralreasons.Wewantedtomakevoicecapabilitiesavailabletousersoverourfullrangeofworkstations.Furthermore,wedidnotwanttoredesignthevoicehardwareforeachtypeofworkstation,norwereweconfidentthatallofthemcouldhandlethetransmissionandcontroloftelephoneconversationswithsufficientreliabilityandwithoutsubstantiallossofperformanceintheirotherduties.Anadditionalhardware-relateddecisionwasmotivatedbyeconomics.WewereabletolimitthesizeandspeedoftheEtherphone,andthusitscost,byrestrictingitsactivitiestothosethatcouldnotbereadilyperformedbywritingsoftwarefortheexistingworkstationsorservers.EtherphoneusersconversewitheachotherusingonlyEthernetcommunications.Tomakecallsoutsidethesystem,however,accesstothepublictelephonenetworkisneeded.Thepreferredmethod,transplantedfromtheconventionalPABXenvironment,wouldinvolveconnectingagroupofcentral-officetrunklinestoaserverthatwouldcompletetheconnectionsovertheEthernet.Forourprototypesystem,wechoseinsteadtoprovideeachEtherphonewithajackforconnectiontoanexistingsubscriberline.Tocompletetelephonecallsinvolvingnon-Etherphoneusers,theEtherphoneconnectsthetelephoneinstrumenttothesubscriberline.ThischoiceallowstherelativelysmallcommunityofEtherphoneuserstoretaintheirshortextensionnumbers,avoidsthecostofdevelopingatrunkserver,andpermitsdirectuseofthesubscriberlineasabackupifthehardwareorsoftwarefails.5.2SystemOrganizationInadditiontotheEtherphones,wehavedesignedandimplementedatelephonecontrolserverandavoicefileserver,bothofwhichareCedar-basedapplicationsprograms.Figure1showsthestructureoftheresultingEtherphonesystem.TheEtherphoneisresponsibleforvoicedigitization,fortheanaloginterfacetotheswitchedtelephonenetwork,andforimplementingthevoice-transmissionprotocols.Itisdesignedtobeavoiceperipheral,withoutlocalintelligenceorsystemcontrolresponsibility.Instead,itreportsuseractionstothetelephonecontrolserver,whichreturnsdetailedcommandsthatcontrolitsfunctions.Thetelephonecontrolserveristhesystemcontroller.ItsresponsibilitiesareanalogoustothecontrolresponsibilitiesofaconventionalPABX:monitoringthestateofthesystem,keepingtrackofthestateofeachEtherphone,andsettingupallconnections.Itmustalsocoordinatetheoperationofthesoftwareinusers'workstations.Thisworkstationsoftwareprovidesimproveduserinterfacesforthetelephoneandothermoreexperimentalapplications.XEROXPARC,CSL-83-8,AUGUST,1983w g/xg/"%'R(,71uz ^  | [ )~{ W=@ 0"R$&f -2] :@>A6CG U : }UU!h&+ {3U4QU7U}DF^ P N "(/ 57Du9P:P{@PAvP}BPBPEa Nk  mD "$ ,/e3!{5qNk6XNk<BD L7b+ '-/|36H8;?wAJ J ccb#&.)/15 8*9 ADGg G.0 #M% -057\= Ft E   $'R) 1269 Ce ]%)'.&0 7 9<?AYDF A0  # %(F*` 02 8:v>3A E G >}A=!h#&+ 24 < b!<$(t,R/5z@<BF :8  )!$Z(/4x5;'>D+ 8^  ## +a/e4 :;?A= 6* qQs$O& .1$3x9i<>D 3 "{ &d)*-0 78:}?3@h3{F3F3 1 >R"A ,03; :@C8 / |}/Y/{[//a!t%),/ 25<> F -X Oo"x&)+- 459=}@-XA~-XE +# ay l!k"%++4,2p| 'G ) { #k* "&+. 7u87#k8#k>HB{F#kG}#k !7u !7 !7u{"!7!71 > (( /6:;?BG  K q _   !"$ +.094w9;=C8 \ b+5}0{1234p:0;=?B d X y$,13:7:?I@C8 0 '*/b1< u{]d!&; ,.8:i@BD MT #Z *,0<24y9^>B!CF^   >! )U*. 0 79@AD ^ uX ^@ ^ { ^w ^ !'P,25 <>9@F *  Y x #P',X1 TVm$AddingVoicetoanOfficeComputerNetwork11Thevoicefileserverusesageneral-purposecomputerwithattacheddiskstorageforvoice.Itperformsstandardfileserverfunctions,butisspecializedforthereal-timeneedsoftelephony.Finally,existingstandardfileserversanddata-baseservicesareusedforstorageoftelephonedirectoryinformationandforstorageofusers'callfiltersandotherinformation.Thefollowingsectionsamplifythedescriptionsofthesemajorsystemcomponents.5.3EtherphoneTheEtherphoneisadual-processorcomputerwithinterfacesfortheEthernet,DESencryption,RS-232serialdevices,andvoice.Themainprocessorisresponsiblefornetworkcommunicationsandcontrolwhiletheslaveprocessorhandlesvoicesignal-processingfunctions.Ananalogboard,adigitalboard,andapowersupplyoccupyaconvection-cooledcabinetmeasuring12"x13"x5".Thispackageisdesignedtositunderauser'sdesk,whilethetelephoneset,microphone,andspeakeroccupypositionsofconvenience.ThisarrangementisshowninFigure2.Figure3isablockdiagramoftheEtherphone'sinternalarchitecture.Theanalogboardiscenteredaroundan8by8analogcrossbarswitchthatinterconnectsvariousvoicesourcesandsinks:telephoneset,telephoneline,analog-to-digitalconverters,DTMFdecoder,microphoneandspeaker,andlinelevelauxiliaryinputsandoutputs.Theanalog-to-digitalconversionfunctionsareaccomplishedbycoder-decoder(CODEC)devices.Thetelephonesetandtelephonelineinterfacesareconnectedbyrelayssothatapowerfailureorsystemcrashwillrestorestandardtelephoneservice.Thedigitalboardcontainsthetwoprocessors,memorysystems,timinglogic,anddigitalinterfacesfortheEthernet,RS-232devices,DESencryption,analog-to-digitalconversion,andcontroloftheanaloghardware.Awatchdogtimerisprovidedforautomaticrecoveryfromanintermittentfault.ThemainprocessorisanIntel8088with8KbytesofEPROMand56KofRAM.DESencryptionisaccomplishedbya900Kbpssinglechipdevicewithdirectmemoryaccess(DMA).TheEthernetcontrollerisaninternalXeroxLSIpartusingtheprotocolsofthe3MbpsexperimentalEthernet,butoperatingat1.5Mbps.TheRS-232interfacepermitsthetheconnectionofalocaldisplayandkeyboardasanoption.ProgramswrittenintheClanguagesupplyallofitsfunctionsexceptforasmallnumberoflow-leveldevicedrivers,whicharewritteninanassemblylanguage.Thesoftwareincludesaround-robin-scheduledmultitaskoperatingsystem,andanetworkpackagethatimplementsboththevoiceprotocolsandprotocolsforconnectingalocalcontrolprogramtothetelephonecontrolserver.Thislocalcontrolprogramcommunicatesuseractions(suchasliftingtheswitchhookorkeyingina"5"onthetelephone)totheserverwithoutinterpretation.CommandsfromthecontrolserverinstructtheEtherphone'slocalcontrolprogramtoestablishEthernetvoiceconnections,configurethecrossbarswitch,generateprogrammedtonesequences,andmanagethetelephonelineinterface.Theslaveprocessorisalsoan8088.Ithasaccessto8KofprivateEPROM,2KofRAM,andsharedaccessto48Kofmainmemory.Theslaveprocessorexecutesasmall,carefullycoded,assembly-languageprogram.Itsfunctionsincludesilencedetection,thecombinationofincomingpacketstoachieveXEROXPARC,CSL-83-8,AUGUST,1983x\g/!S$m%'R*/wG~g/{ _,uj_,_,{_,|_,&,/47BWC \ S @ '%)c+15t7- Zy4"%3+#0 285P7i;=C X v-k #7')-n VZ"# )+r.27m | R~ ) { N f\ #* -U 368>B\ Lnu!Ln"Ln{%xLn&?Ln+-C 46<{ F J:uJ:J:{~J:EJ:#'27 8;?DEV H="Rz +07W9;%=>A1DM E NtlX $&-"/ 7|:*?C C   $$l&*|,0j1247=(>A2 Ah ?3$9o$J&W')+/5 9aC : # (-/5^8UB 8l   &/,2 4;h=@jF 6b Z h#.$P(v,.269'=C8 4- 1H"k )f.4B8<]? Ce / hU$4!u (2 9<A3BE* -L"%(+1 4t6e = +[ r_"%(L+-3=58:?fB )' >hj"%*6-17,;h@C & @%"&)/!03b48 @G $%f*W,/ 678<$@C "   UW="^&(*=,16+8U9k<BC !K!#&,/5 :Y;j y#P(q+ 2g57;4AC  l  #\%M'.3~8%;? D  n .$N& ./4]67Z:<> EoG/ O  #L&)-1695 AD v7 (/17;Aj   $N' x6"g#&9*9+.!/4\:V<>IBE) } 6` ,#).049= I43# ), 46=SBDx #P',X1 TVm$12AddingVoicetoanOfficeComputerNetworkconference-callbridging,echosuppression,gaincontrol,CODECcontrol,andlow-levelbuffermanagementinsharedmemory.TheoverallperformanceoftheEtherphoneissufficienttosupporttwosimultaneousEthernetconversations.Forexample,twoconversationsareneededinordertoforwardanarrivingoutsidecalltoanattendant'sEtherphonewhenthetelephonesetisalreadybeingusedforanEthernetcall.Inadifferentconfiguration,theEtherphonewillsupportafour-partyconferencecall.BecauseeachEtherphonehasitsowncrystal-controlledclock,synchronizationoftwoEtherphonesparticipatinginaconversationpresentsanintriguingproblem.ApairofEtherphonesmayhaveclocksdifferingbyasmuchasonepartin10,000.Inthesteadystate,thisfrequencyerrorcausesthequantityofbufferedvoiceatthereceivingEtherphonetoslowlyincreaseordecrease.Sincesilencedetectionisusedtoreducetransmissionbandwidth,thecorrectbufferdepthisreestablishedduringeachsilentinterval.Communicationswiththevoicefileserveravoidthisproblembyuseofsoftwarefeedback--thefileserverisdrivenbytheEtherphoneclock.5.4TelephoneControlServerThetelephonecontrolserverperformsthefunctionsnormallyassociatedwiththecommoncontrolfacilitiesinaconventionalPABXortelephoneswitchingoffice.However,becauseitdoesnothavetoswitchortransmitanyoftheactualvoicetraffic,ithasnospecialhardwareneedsandrequiresonlymodestcomputingpowertosupportitsactivities.Thetelephonecontrolserversuppliestwoclassesoffunctions:Etherphoneintelligenceandcallprocessing.ThecontrolservercontainstheEtherphoneintelligencesoftwarethatinterpretsuseractionsatanEtherphone.MostEtherphoneshaveonlythestandardDTMFkeypad,butasmalldisplayandkeyboardcanbeaddedtoprovideamoreeffectiveuserinterfacewhenaworkstationisnotavailable.Ineithercase,controlserversoftwareinterpretsandrespondstotheuser'sactions.Theseresponsesincludeservicerequestssuchassettingupandtakingdownconnections,andsendingcontrolinstructionsbacktotheEtherphones.Thecontrolserveralsoperformsthetelephone-controlfunctionsusuallyreferredtoascallprocessing.ItprovidescontrolandbookkeepingforconnectionsincollaborationwiththeEtherphoneuserinterfaceprogramsfortheparticipatingparties.Thecontrolserverprovidesaccesstoanumberofdatabases,includingdirectoriesestablishingtherelationshipsamongEtherphonenetworkaddresses,thenamesofindividualsandservices(suchasthevoicefileserver),telephoneextensions,officelocations,andnearbyworkstations.Thecall-processingportionofthecontrolserverhasanadditionalfunction:itsynchronizestheactivitiesofeachEtherphonewiththeworkstationapplicationsthatmanagethatEtherphoneandthatuseittoprovidetheirvoiceconnections.WhenuseractionscomingfromtheEtherphoneandfromtheworkstationsareinconsistent,theserverresolvestheconflicts.Thetelephonecontrolserverincludessomemaintenancefacilitiesnotdirectlyconnectedwithsystemoperation.Forexample,theservercontainssoftwaretodetectthedistresspacketsthatEtherphonesbroadcastwhentheyarepoweredonorwhenunrecoverablefailuresoccur.ThiscaretakersoftwarerespondsbyperformingautomaticdownloadingandremotemaintenanceofEtherphonesoftware.ByXEROXPARC,CSL-83-8,AUGUST,1983w g/xg/"%'R(,71u{ _/b= &*7/6;>E` \  Z !y )=* 1.3 8o;_ C X d 'G).0@45:<B+G V]   z O")C+- 159C;=CuFwHe T) \ "<$)+ 1 8 Q> P!9$L/!3=>A O  "]$] *02I516 ? B!Ew MN+{ 0$&)-W03:=zAD KW d & &(g,239=BH# I" gT 0 #&A+/35V =BuE FZ  >#_%*-06/8B:<B.H D K-|x $X| @ ){ 4=g*%'j-k3= 9<?*D : f,m  'L-x18=A>ADG 8G-sN $)/*-=/d4:+>.AFu 6e " (+2O7;=@CeG 40 d  -!$o 1g | ( /s57 >*A;EG / 1 ( a#l%+K0578<8@C - $5'D-02 9; = CE +^ "' !@#)+:-1M61:"@:E  )*Yb"% -05: BElG/ & $f O"-m3L7=>@B " _I i" *+ 479y @C X& 1"%).*389;:@BlE $  a (- 5: A C}G  N S#&G+!1s 8j<; BJD  &"$&+/13 :M@5A  o 8j "L$ ,\ 479; CvF(  K 9 e $),[ rSL#' / 4[6;B E    b%+-14g9q>A C%a!#' 05:>D L Q (#+/ 79 A}Gsx #P',X1 TVm$QAddingVoicetoanOfficeComputerNetwork13consultinganotherdatabase,thecontrolservercansupportthesimultaneousoperationofdifferentmodelEtherphonesorEtherphoneswithdifferentsoftwareandcapabilities.Forsystemcontrol,wewereabletotakeadvantageoftherecentdevelopmentofremoteprocedurecallprotocols[Nelson,Birrell].RemoteProcedureCall(RPC)isameansfortransformingthemessage-passingsemanticsofpacket-switchingnetworksintotheprocedure-callformatfamiliartoprogrammers.RPClargelyrelievesapplicationsprogrammersfromanyworriesaboutpacketformats,addressing,reliablecommunication,andsecurity.Thismakesitpossibletodeferdecisionsaboutthepartitioningofadistributedsystemuntilverylateinthedesign.OurtaskwasreducedtoproducinganRPCimplementationfortheEtherphonecontrolprocessor,thenspecifyingtheproceduralinterfacesbetweendifferentsystemcomponents.5.5VoiceFileServerThevoicefileserverusesthestored-voiceprotocoltorecordandplaybackdigitizedvoicesegments.ItrunsonaXeroxDoradoequippedwitha300Mbytedisk.At8000bytespersecond,theresultingvoice-storagecapacityisover8hours.Capacitycanbeexpandedasusagewarrants.Thediskisorganizedsothatstorageisallocatedinonesecondunits.Thispermitsdiskactivityonbehalfofasinglerecordorplaybackoperationtobelimitedtoonecontiguousdisktransferpersecond.Nevertheless,usersoftwarecanspecifytheorderanddurationofvoice-segmentaccessatagrainofonemillisecond.Thisfacilitymakesitpossibletoexperimentwithvoiceediting.Theveryhighnetwork-communicationloadspresentedbymultiplevoice-protocolconnectionspresentaspecialchallenge.Thepresentimplementationiscapableofhandlingabouteightsimultaneoustransfers.Allvoiceisstoredinencryptedform.TheassociatedkeysarestoredbytheEtherphonecontrolserverinadirectoryalongwithinformationgrantingappropriateaccesstoeachvoicesegment.Sinceeachvoicesegmentisstoredonlyonce,regardlessofthenumberofusersgrantedaccess,thefileserverdirectoryalsokeepstrackofthenumberofoutstandingreferencestoeachsegment.Voicestorageisreclaimedautomaticallywhennoreferencesremain.5.6WorkstationapplicationsMostusersoftheEtherphonesystemwillhavebothanEtherphoneandapersonalworkstation.Althoughtherecanbeconflictingrequestsforserviceoriginatingfromtheworkstationandtheuser'sEtherphone,voice-relatedapplicationssoftwareontheworkstationgenerallytakespriorityindecisionmaking.Asanexample,consideranarrivingcall.TheEtherphoneisalwayspreparedtoannouncethecallwitharingingtone,butworkstationsoftwareisfirstgiventheopportunitytorejectthecall,allowtheEtherphonetoringnormally,orchoosesomeotherwaytoannouncethecall.OurgoalsincludebothproducingusefulworkstationapplicationsandmakingtheEtherphonefunctionsavailabletootherprogrammerswhowouldliketoproducevoice-relatedapplications.Forthisreason,theworkstationsoftwareincludesabasicpackageofsupportfacilities,thenanotherlayerofspecificapplicationprogramsthatoperateasclientsofthebasicpackage.XEROXPARC,CSL-83-8,AUGUST,1983x\g/!S$m%'R*/wG~g/{ _/ ofD",&$(-/ 8 >!?EV \  $z), 'Z66R!#)&,.004 <>pC  X \F$b+h.35|7;> G/ V]n'."1\4 =B_G T( U  ' 0\36;?D7 Q 2F {#\(,80279_=C$G/ O f%T K#&-(*,147^:?WA G M z  $<) /3 9 &++-*1j47:@?CP Aj ~V{!$&%(-V026,9w?CG :s#"y )h,S1\38 AD 8 DW! *.0157[: AE 6c=` #t(I+-0?CN 4/ 0  !v&2'C+ 149u C;D 1 aB /&r!$' .14E8:=6 D -3%A %+z 27)8Z?AdEGg \ $V>$ *- 34:t@MB% Zu y ),/5j:>@ X Xwc! )-05;O<? GR V] SG !%_),* 2K36T9@ DG/ T) { Y!G #%B'+ 25;?D%F Q  O#l !H'3)j.+479 B& M~T^  #&,. 135:T> G\ KJ !R"%| +-0!2L49[:<>DE I4D\%) 1O7n9@BG > #%(M,061<}>C9D ; tIU!t$J -(13 95D 9 7l #s)0 2\67;t@BD 7d o!YL!Y&(T*,.b2 9u:>_@E|G/ 50   $*~- .38:>~@AE x #P',X1TVm$AddingVoicetoanOfficeComputerNetwork156.StatusandPlansWearecurrentlytestinganinitialsystemusingengineering-prototypeEtherphones.WeareplanningtoinstalladditionalEtherphones,offeringservicetoasizeablesegmentofourlaboratory,inordertotestthevalueoftheenhancedcontrolandmessagecapabilitiesandtoallowCedarprogrammerstodeveloptheirownapplications.Withtheenablingfacilitiesinplace,wearejustbeginningtoexplorethepotentialapplicationsofintegratedvoiceintheofficeenvironment.XEROXPARC,CSL-83-8,AUGUST,1983x\g/!S$m%'R*/wG~g/z ^ { Z%f1b!;%):6 ?AC X   F#o')*/5698 @AEbG V M>!' .!0263:L BDc T_  Q#" (q* -/244;<AC R* ; F!T%* x #P',X1TVm$16AddingVoicetoanOfficeComputerNetwork7.References[ATT]AmericanTelephoneandTelegraphCompany,NotesonTheNetwork,1980.[Birrell82]AndrewD.Birrell,RoyLevin,RogerM.Needham,andMichaelD.Schroeder,"Grapevine:AnExerciseinDistributedComputing,"Commun.ACM,Vol.25,pp.260-274,April1982.[Birrell83]A.D.Birrelletal,"RPCimplementationpaper",SOSP/TOCS1983.[Boggs]DavidR.Boggs,JohnF.Shoch,EdwardA.Taft,andRobertM.Metcalfe,"Pup:AnInternetworkArchitecture,"IEEETrans.Commun.,Vol.Com-28,pp.612-624,April1980.[Brady]P.T.Brady,"EffectsofTransmissionDelayonConversationalBehavioronEcho-FreeTelephoneCircuits,"BellSyst.Tech.J.,Vol.50,pp.115-134,January1971.[Brotz]D.K.Brotz,"LaurelManual,"XeroxPARCReportCSL-81-6,May1981.[Crane]R.C.CraneandE.A.Taft,"PracticalConsiderationsinEthernetLocalNetworkDesign,"Proc.ofthe13thHawaiiInternationalConferenceonSystemsSciences,Honolulu,pp.166-174,January1980.[DES]DataEncryptionStandard,FederalInformationProcessingStandard(FIPS),Publication46,NationalBureauofStandards,U.S.DepartmentofCommerce,January1977.[Forgie]J.W.Forgie,"VoiceConferencinginPacketNetworks,"InternationalConferenceonCommunications,Seattle,June1980.[Gonsalves]T.A.Gonsalves,"Packet-VoiceCommunicationonanEthernetLocalComputerNetwork:anExperimentalStudy,"Proc.SIGCOMM1983SymposiumonCommunicationsArchitecturesandProtocols,pp.178-185,AustinTX,March1983.[Hatch]"ModelsfortheSubjectiveEffectsofLoss,Noise,andTalkerEchoonTelephoneConnections,"BellSystemTechnicalJournal,Volume55,November1976.[Henning]H.H.HenningandJ.W.Pan,"D2ChannelBank:SystemAspects,"BellSystemTechnicalJournal,Volume51,pp.1641-1657,October1972.[Hupp]J.F.ShochandJ.A.Hupp,"MeasuredPerformanceofanEthernetLocalNetwork,"Commun.XEROXPARC,CSL-83-8,AUGUST,1983w g/xg/"%'R(,71uz ^ { [ X{")-/2y8 Vj T5+ i$'-068/ ?, G< R  #W).03:5;> O MQfKW &6+ 3 J H#U &'+C-24:>AT Fm W$]'T,/X48n C AlC $M(c*} 39;B ? !#&++0 = :$(j-36 8^ 6*i81 $r -/y5 8>DNG 3  %',29+;}@E 1zu /F {$/F/F! )r 05: AC -t-  %1&.=3j * (b &r(.9 6R ?\ G &-r # !~ # !Z +. 0G68:NA&G I V<&!)1C3x >@ F  jR"&  ed? "$(2,/o379@ 1@%'A.  );jv"l'+069`>DB M a"  @"pFM&} .028U<>Cx #P',X1TVm$AddingVoicetoanOfficeComputerNetwork17ACM,Vol.23,pp.711-721,December1980.[Johnson]D.H.JohnsonandG.C.O'Leary,"ALocalAccessNetworkforPacketizedDigitalVoiceCommunication,"IEEETrans.Commun.,Vol.Com-29,pp.679-688,May1981.[Lampson]B.W.LampsonandK.A.Pier,"AProcessorforaHigh-PerformancePersonalComputer"Proc.7thSymposiumonComputerArchitecture,SigArch/IEEE,LaBaule,pp.146-160,May1980.[Maxemchuk]N.F.Maxemchuk,"AnExperimentalSpeechStorageandEditingFacility,"BellSyst.Tech.J.,Vol.59,pp.1383-1395,October1980.[Metcalfe]R.M.MetcalfeandD.R.Boggs,"Ethernet,DistributedPacketSwitchingforLocalComputerNetworks,"Commun.ACM,Vol.19,pp.395-404,July1976.[Mitchell]J.G.MitchellandJ.Dion,"AComparisonofTwoNetworkBasedFileServers,"Commun.ACM,Vol.25,pp.233-245,April1982.[Needham]R.M.NeedhamandM.D.Schroeder,"UsingEncryptionforAuthenticationinLargeNetworksofComputers,"Commun.ACM,Vol.21,pp.993-998,December1978.[Nelson]B.J.Nelson,RemoteProcedureCall,XeroxPARCReportCSL-81-9,May1981.[NS]InternetTransportProtocols,XeroxSystemIntegrationStandardXSIS028112.December1981.[Nutt]G.J.NuttandD.L.Bayer,"PerformanceofCSMA/CDNetworksUnderCombinedVoiceandDataLoads,"IEEETrans.Commun.,Vol.Com-30,pp.6-11,January1982.[Shoch]J.F.Shoch,"CarryingVoiceTrafficThroughanEthernetLocalNetwork--AGeneralOverview,"presentedattheIFITWG6.4Int.WorkshopLocal-AreaComputerNetworks,Zurich,Switzerland,August1980.[Smith]D.C.Smith,E.Harslem,C.Irby,andR.Kimball,"TheStarUserInterface,anOverview,"Proc.ofNationalComputerConference,pp.515-528,June1982.[Thacker]C.P.Thacker,E.M.McCreight,B.W.Lampson,R.F.Sproull,andD.R.Boggs,"ALTO:APersonalComputer,"inComputerStructure:ReadingsandExamples,D.Sieworek,C.G.Bell,andA.Newell,Eds.McGraw-Hill,NewYork1981.XEROXPARC,CSL-83-8,AUGUST,1983x\g/!S$m%'R*/wG~g/{ _/$ \ Z mA^%'+069 @E XJ&)/17:1 U S!>#),%-i9>F Qf) 'e 037@9?"BR N LX  #(-0B5 ; =ADF JR +x H E1 <X" ) 1]5<>C C a!#&8+. A! > %S ')[,~21619>EW <J` :< 8 $)e 0~2 <=AG 5 ;s"k$',3. 3W 1"nu1"1" C{"1"#1"',07P: . ,rC "'y .4h8=C ) '_ ')107;BF %(#&,~.2O7{ # "o#C(*04W: ;=B^ 7D~"( /6b<A ti  "&$&,0E386 <> F d  "F$*(-z  n` "-$&-/-169k;=B;H 3 l! (g.^179?ACF v "&^x #P',X1TVm$18AddingVoicetoanOfficeComputerNetwork[Tobagi]F.A.TobagiandN.Gonzalez-Cowley,"OnCSMA-CDNetworksandVoiceCommunication,"Proc.Infocom,pp.122-1278,1982.XEROXPARC,CSL-83-8,AUGUST,1983w g/xg/"%'R(,71u{ _' \TI.$h'].47<;F Zx #P',X1TVm$UEthernetWork-stationEP:LTIczSubscriber lineControlServerVoiceFile2LargeDisks:LTIczEP:LTIczEPstationWork-Subscriber lineSubscriber lineExisting NetworkServicesGatewayto OtherNetworksGlobeComSystem.silL. Stewart 8/8/83)e8G+>G$-l>G$.8GG)e8G)e>$)?G)e@P$,P>GG(,>GG.>G],sx4X$V$WGVGG(VGGVGGVGG9VGVG9VVG]VGVQGrQGQGZ$$Z$$Z$$Z$$AZ$$Z$$Z$$kZ$$$Z$$Z$$Z$$OZ$$Z$$Z$$yZ$$2Z$$Z$$]Y.k$ZJ$$ZJ$$dZJ$$ZJ$$ZJ$$ZJ$$HZJ$$ZJ$$ZJ$$rZJ$$+ZJ$$ZJ$$ZJ$$VZJ$$ZJ$$2Z$$yZ$$Z$$Z$$OZ$$Z$$Z$$$Z$$kZ$$Z$$Z$$AZ$$Z$$Z$$Y$$dY$$Y$$Y$$Y$$HY$$Y$$Y$$rY$$+Y$$Y$$Y$$VY$$]YQ$YQ$ZG$YG$YG$ZG$ZJG$yYu$^u$A[f$2[f$2[C$^.$[$[$[$9tT9SO-GSrGQG9QGQGxRO-G|RORSSZ$Yu$YQ$Yu$V[$$[$$[$$Z$$VZ$$Z$$ZJ$$VZJ$$ZJ$$Z$$Z$$VZ$$:Y.G$OY.G$[$k[$kZ$[$[$][f$sZ$RZ&$$ORZ&$$O[$$][$$S|$S|$@Y9$R_$KJ$9K& $tKms8Gs8G8Grs=G=G]s=G9=Gl=GG=GG(O=GG=GGs>Gs=$s?$;:!V8G!V8G(8Gr!V=G(=G]!V=G9!=G'O=GG&=GG(&3=GG%=GG!V>G!V=$!V?$.8G1P>G$2>G$38GG.8G.>$/?G.@P$1>GG(2l>GG3>G]!;!:!9(9G.9G9;7IG$6G9*:;*::#R_$#V9$#S|$y#S|$%^|P'P%P(HP%W<$%V$%V$(%V$&XX$$&XX$$',XX$$&X$$&X$$',X$$',W$$&W$$&W$$',W$$&W$$&W$$%VfG$'VfG$'sX$k&WX$k%X5$%Y.$&X$%X$(X5$%O%W_$$'O(%W_$$'X$$%X$$&WxR%:QG%:QG)QG9%:SrG'sO-GJ $#J-$V#\$$(l\<$$(%\<$$'\<$$'\<$$'O\<$$'\<$$&\<$$&z\<$$&3\<$$%\<$$%\<$$%^\<$$%\<$$$\<$$$\<$$$A\<$$#Zk$(H[$$([$$'[$$'s[$$',[$$&[$$&[$$&W[$$&[$$%[$$%[$$%:[$$$[$$$[$$$e[$$$[$$$[$$%[$$%^[$$%[$$%[$$&3[$$&z[$$&[$$'[$$'O[$$'[$$'[$$(%[$$([f$$'[f$$'s[f$$',[f$$&[f$$&[f$$&W[f$$&[f$$%[f$$%[f$$%:[f$$$[f$$$[f$$#Z$(Z$(l[G$(H[fG$$A[fG$$[G$#[G$$[$#\$#]k$(\$#\$$$A\k$$\$$&z\$%^\$)S|$)[C9$+WS$8R_$8Y9$8S|$@8S|$9|R;QG99SrG<O-G0WtS0WT/X$/V$/WG4VGG(4VGG53VGG5VGG0WVG/VG96tVG]/VG6tQGr/QG/QG0Z$$53Z$$4Z$$4Z$$4^Z$$4Z$$3Z$$3Z$$3BZ$$2Z$$2Z$$2lZ$$2%Z$$1Z$$1Z$$1PZ$$1 Z$$0zY.k$5ZJ$$4ZJ$$4ZJ$$4;ZJ$$3ZJ$$3ZJ$$3eZJ$$3ZJ$$2ZJ$$2ZJ$$2IZJ$$2ZJ$$1ZJ$$1sZJ$$1,ZJ$$1PZ$$1Z$$1Z$$2%Z$$2lZ$$2Z$$2Z$$3BZ$$3Z$$3Z$$4Z$$4^Z$$4Z$$4Z$$4Y$$4Y$$4;Y$$3Y$$3Y$$3eY$$3Y$$2Y$$2Y$$2IY$$2Y$$1Y$$1sY$$0zYQ$5YQ$53ZG$5YG$1 YG$0ZG$0ZJG$1Yu$1^u$4^[f$21[f$21[C$2%^.$4[$2%[$2%[$H($8I$ rJQI4N05?$5=$5>G:4=GG(:=GG;P=GG;=GG6t=G5=G9<=G]5=G<8Gr58G58G6A$$;PA$$; A$$:A$$:{A$$:4A$$9A$$9A$$9^A$$9A$$8A$$8A$$8BA$$7A$$7A$$7lA$$7%A$$6@-k$;-AI$$:AI$$:AI$$:WAI$$:AI$$9AI$$9AI$$9;AI$$8AI$$8AI$$8eAI$$8AI$$7AI$$7AI$$7IAI$$7lA$$7A$$7A$$8BA$$8A$$8A$$9A$$9^A$$9A$$9A$$:4A$$:{A$$:A$$; A$$:@$$:@$$:W@$$:@$$9@$$9@$$9;@$$8@$$8@$$8e@$$8@$$7@$$7@$$6@P$;@P$;PAG$;-@G$7%@G$7AG$6AIG$7@t$7Et$:{Bf$27Bf$27BB$8BE-$:4B$8BB$8BB$6t; 6t:6t9?6s606G98G8G:8Gr=G:=G]=G9=G=GG=GG(k=GG=GG>G=$?$;:96G93G3G,sx 2,s  0T EthernetControllerTransceiverEthernetControllerDESEncryptionDMACPUEPROMControllerRS-23256 K bytesshared RAM2K bytesRAM8K bytes8K bytesEPROMCPU8088 Main8088 SlaveA/D, D/AVoice I/OzcITL:AnalogCrossbarSwitchMikeSpeakerDTMF DecoderSubscriber line interfaceTelephone setControlLogicEthernetGlobeComBlock.silL. Stewart 8/8/83J-<PGrLGULGLGVtNVM ]XGsZJGUZGZG:Z :\_s`9G_Gs^Gs^G9 JG9sM LGLGLGUPGUG$RGUR<GR<GsTQsS4 sN PJG9JG99M(MPG$LGULGLG3M JG93N GfGCGCG$DGUGGE D AIG+WAG9(GG/DGU(CG(CG AG9CGCGrDGUGGVEVD(N(E(DGGDGUCGCGAG99D9N 9E +JG9(LG(LG/LGU(PG3PG:LGU3LG3LG7JG9>;Gr;-GU:G:G >GAI)7GfG3CG3CG:DGU3GG9<9; ;