ethernet: distributed packet switching for local computer networks - robert m. metcalfe

8/6/2019 Ethernet: Distributed Packet Switching for Local Computer Networks - Robert M. Metcalfe

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C o m p u t e rSys tems

G. Bel l , S . Ful ler andD. S iewiorek , Edi torsEthernet: DistributedPacket Switching for

Local ComputerNetworksRobert M. Metcalfe and David R. BoggsXerox Palo Alto Research Center

Etherne t i s a branching broad cas t communica t ionsystem for ca r ry ing d ig i t a l da ta packe t s amon g loca l lydis t r ibuted computing s tat ions . The packet t ransportmechanism provided by Ethernet has been used to bui ldsys tems w hich can be viewed as ei ther local computernetworks or loosely coupled mult iprocessors . An Ethe r-net ' s shared communicat ion faci l i ty, i ts Ether, is a pas-sive broadcast medium with no central control . Coordi-na t ion of access to the Eth er for packe t broadcas t s i sdis t r ibuted among the contending t ransmit t ing s tat ionsusing control led s tat is t ical arbi t rat ion. Switching ofpackets to their des t inat ions on the Ether is dis t r ibutedamong the receiving s tat ions us ing packet addressrecogni t ion. Design principles and implementat ion ar edescribed, based on experience with an operat ing Ether-net of 100 nodes along a ki lome ter of coaxial cable . Amodel for e s t imat ing performance under heavy loadsand a packe t pro tocol for e r ror cont ro l l ed communica-t ion are included for completeness .

Key W o r d s a n d Phrases: computer ne tworks , packe tswitching, mult iprocess ing, dis t r ibuted con trol , dis-t r ibuted comput ing , broadcas t communica t ion , s t a t is t i -ca l a rb i t ra t ion

CR Categories : 3.81, 4.32, 6.35

Copyright 1976, Association for Computing Machinery, Inc.General permission to republish, but not for profit, all or partof this material is granted provided that ACM's copyright noticeis give n and that reference is made to the publication, to its dateof issue, and t o the fact th at reprinting privileges were grantedby permission of the Association for Computing M achinery.Author's present addresses: R.M. M etcalfe, Transaction Tech-nology, Inc., 10880 Wilshire Boulevard, Los A ngeles, CA 94304; D.Boggs, Xerox Palo Alto Research Center, 3333 Coyote Hill Road,Palo Alto, CA 94304.395

1. BackgroundO n e c a n c h a r a c t e r i z e d i s t r i b u t e d c o m p u t i n g a

spec t rum of ac t iv i t ie s va ry in g in the i r degree of dect r a l i z a t i o n , w i t h o n e e x t r e m e b e i n g r e m o t e c o m p un e t w o r k i n g a n d t h e o t h e r e x t r e m e b e i n g m u l t i p ro c ei n g . R e m o t e c o m p u t e r n e t w o r k i n g i s t h e l o o s e in t e r cnec t ion o f previous ly i so lated , wide ly separa ted , ra the r l a rge comput ing sys tems . Mul t iproces s ing i s c o n s t r u c t i o n o f p r e v i o u s ly m o n o l i t h i c a n d s e ri a l c oput ing sys tems f rom increas ingly numerous and smapieces com put in g in pa ra ll e l . Nea r the middle of spec t rum i s loca l ne tworking , the in te rconnec t ionc o m p u t e r s t o g a i n t h e r e so u r c e s h a r in g o f c o m p une twork ing and the pa ra l l e li sm of mul t iproces sing .

The separa t ion be tween compute rs and the as soa t e d b i t r a t e o f t he i r c o m m u n i c a t i o n c a n b e u s e d t o v i d e t h e d i s t r i b u t e d c o m p u t i n g s p e c t r u m i n t o b r oac t iv i t ie s . The p rod uc t of s epara t ion and b i t ra t e , nabo ut 1 g igabi t -mete r pe r s econd (1 Gbmp s) , i s and i c a t io n o f t h e l im i t o f c u r r e n t c o m m u n i c a t i o n t enolog y and can be expec ted to inc rease wi th time:Activity Separation Bit rateRemote networks > 10 km < .1 MbpsLocal networks 10-.1 km .1-10 MbpsMultiprocessors < .1 km > 10 Mbps1.1 Remote Computer N e t w o r k i n g

C o m p u t e r n e t w o r k i n g e v o l v e d f r o m t e l e c o m m u n it ions terminal-computer c o m m u n i c a t i o n , w h e r e t h e j e c t w a s t o c o n n e c t r e m o t e t e r m i n a l s t o a c e n t r a l c oput ing fac i l i ty . As the need for computer-compuin te rconnec t ion grew, compute rs themse lves were ut o p r o v i d e c o m m u n i c a t i o n [2 , 4 , 29 ]. C o m m u n i c a tusing com pute rs as pac ke t swi tches [15-21 , 26] ac o m m u n i c a t i o n s among c o m p u t e r s f o r r e s o u r c e s h a r[ I0 , 3 2 ] w e r e b o t h a d v a n c e d b y t h e d e v e l o p m e n t o f A r p a C o m p u t e r N e t w o r k .

T h e A l o h a N e t w o r k a t t h e U n i v e r s i ty o f H a w a i i wo r i g i n a l l y d e v e l o p e d t o a p p l y p a c k e t r a d i o t e c h n i qf o r c o m m u n i c a t i o n b e t w e e n a c e n t r a l c o m p u t e r a n d t e rmina l s s ca t t e red amo ng the H awai ian I s l ands [1 ,M a n y o f t h e te r m i n a l s a r e n o w m i n i c o m p u t e r s c om u n i c a t i n g a m o n g t h e m s e l v e s u s i n g t h e A l o h a Nw o r k ' s M e n e h u n e a s a p a c k e t s w i t c h . T h e M e n e h ua n d a n A r p a n e t I m p a r e n o w c o n n e c t e d , p r o v i d i n g tm i n a l s o n th e A l o h a N e t w o r k a c c e ss t o c o m p u t iresources on the U .S . main land.

J u s t a s c o m p u t e r n e t w o r k s h a v e g r o w n a c r o s s c ot i n e n t s a n d o c e a n s t o i n t e r c o n n e c t m a j o r c o m p u t ifac i l i t i e s a round the wor ld , they a re now growing docor r id ors and be tween bui ld ings to in te rcon nec t micom pu ters in off ices and labo rato ries [3, 12, 13, 14, 31.2 Mul t iproces s ing

M u l t i p r o ce s s i n g f i rs t t o o k t h e f o r m o f c o n n e c t i n gI /O cont ro l l e r to a l a rge cent ra l com pute r ; raM's Asp iCommuaications July 1976of Volume 19the ACM Number 7


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classic examp le [29]. Nex t, multiple central pro cesso rswere connected to a com mon memory to p rov ide morepower for compute -bound appl icat ions [33]. For certainof these appl icat ions, more exot ic mult iprocessor archi-tectures such as Illiae IV were introduced [5].More recent ly minicomputers have been connectedin mult iprocessor configurat ions for economy, rel ia-bility, and increased system mo dula rity [24, 36]. Thetrend has been toward decentral izat ion for rel iabi l i ty;loosely coupled mult iprocessor systems dep end less onshared cen t ra l memory and more on thin wires for in-terprocess communicat ion with increased componentisolation [18, 26]. With the c ontinu ed thinning of in-terprocessor communicat ion for rel iabi l i ty and the de-velopment of dis t r ibutable appl icat ions, mu lt iprocessingis gradual ly approac hing a local form of dis t ributedcomputing.1.3 Local Computer Networking

Ethernet shares many object ives with other localnetworks such as Mitre 's M itrix, Bel l Telephone L abora-tory 's Spider, and U.C. Irvine's Distributed ComputingSystem (DC S) [12, 13, 14, 35]. Prototy pes of al l fourlocal networking schemes operate at bi t rates betweenone and three megabits per second. Mitrix and Spiderhave a central minicomputer for switching and band-width al locat ion, while DCS and Ethernet use dis t r ib-uted control . Spider and D CS use a r ing com municat ionpath, Mitrix uses off-the-shelf CAT V techn ology toimplement two one-way busses, and our experimentalEthernet uses a branching two-way passive bus. Differ-ences among these systems are due to differences amongtheir intended appl icat ions, differences among the costconstraints und er which t rade-offs were made, anddifferences of opinion among researchers .

Before going into a detai led descript ion of Ethernet ,we offer the fol lowing overview (see Figure 1).

2. System SummaryEtherne t is a system for local comm unicat ion amo ng

computing s tat ions. Our experimental Ethernet usestapped coaxial cables to carry variable length digi taldata packets among, for example, personal minicom-puters, printing facilities, large file storage devices,magnetic tape bac kup stations, larger central comp uters,and longer-haul communicat ion equipment .

The shared communicat ion faci l i ty , a branchingEther, is passive. A stat ion 's Ethernet interface con-nects bit-serially through an interface cable to a trans-ceiver which in turn taps into the passing E ther. Apacket is broadcast onto the Ether, is heard by al l s ta-t ions, and is copied from the Ether by dest inat ionswhich select i t according to the packet 's leading ad dressbi ts . This is broadcast packet switching and should bedist inguished from store-and-forward packet switching,in which rout ing is performed by intermediate process-

ing elements . To handle the demands of growth, anEthernet can be extended using packet repeaters forsignal regeneration, pac ket filters for traffic localization,and p acket gateways for internetwork address extension.

Control is completely dis t r ibuted among stat ions,with packet t ransmissions coordinated through stat is t i -cal arbi trat ion. Transmissions ini t iated b y a s tat ion de-fer to any which may already be in progress . Oncestarted, if interference with other packets is detected, atransmission is aborted and rescheduled by i ts sourcestation. Afte r a certain perio d of interference-free trans-mission, a packet is heard by all stations and will run tocomplet ion without interference. Ethernet control lersin col l iding s tat ions each generate random retransmis-sion intervals to avoid repeated col l is ions. The mean ofa packet 's retransmission intervals is adjusted as a func-t ion of col l is ion his tory to keep Ether ut i l izat ion nearthe opt imum with changing network load.

Even when transmit ted without source-detected in-terference, a packet may st i l l not reach i ts dest inat ionwithout error; thus, packets are del ivered only with highprobability. Stat ions requiring a residual error ratelower than that p rov ided by the bare Ethernet packett ransport mechanism must fol low mutual ly agreed uponpacket protocols .

3. Design PrinciplesOur object is to design a communicat ion system

which can grow smoothly to accommodate severalbui ldings ful l of personal computers and the faci l i t iesneeded for their support .

Like the computing s tat ions to be connected, thecommunicat ion system must be inexpensive. We chooseto dis t r ibute control o f the com municat ion s faci l i tyamong the communicat ing computers to e l iminate therel iabi l i ty problems of an act ive central control ler , toavoid creating a bottleneck in a system rich in parallel-ism, and to reduce the fixed costs which make small sys-tems uneconomical .

Etherne t design s tarted with the basic idea of packetcol l is ion and retransmission developed in the AlohaNet wor k [1 ] . We expe cted that, l ike the Aloha Netw ork,Ethernets would carry bursty t raffic so that conven-t ional synchronous t ime-divis ion mult iplexing (STDM)wo uld be inefficient [1, 2, 21, 26]. We saw p rom ise in theAloha approach to d i s t ribu ted cont ro l o f rad io channelmult iplexing and hoped that i t could be appl ied effec-t ively with media sui ted to local computer communica-t ion. With several innovat ions of our own, the promiseis realized.

Ethernet is named for the his torical luminiferousether through which electromagnet ic radiat ions wereonce al leged to propagate. Like an Aloha radio t rans-mit ter , an Ethernet t ransmit ter broadcasts completely-addressed t ransmit ter-synchronous bi t sequences cal ledpackets onto the Ether and hop es that they are heard b y

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Fig. 1. A two -segmen t Ethernet.TERMINATOR

TRANS- INTERFACETAP ~ CABLE

ETIIER

ENT#l

I l c lON NT TE R STATIONR OF LA LC EE RI 0N NT TE R STATIONR 0F LA LC EE R

STATION

CONTROLLERw I INTERFACE

ETH ER SEG MEN T # 2mm

the intended receivers . The Ether is a logical ly pass ivemed ium for the pro paga t io n of d ig i t al s igna l s and canb e c o n s t r u c t ed u s i n g . a n y n u m b e r o f m e d ia i n c l ud i n gcoaxial cables , twis ted pairs , and opt ical f ibers .3 .1 Topology

W e c a n n o t a f f o r d t h e r e d u n d a n t c o n n e c t i o n s a n dd y n a m i c r o u t i n g o f s t o r e - a n d - f o r w a r d p a c k e t s w i t c hi n gto as sure re l i ab le communica t ion , so we choose toachieve re l i ab i l i ty through s impl ic i ty . We choose tomake the shared communica t ion fac i l i ty pas s ive so tha tthe fa i lure of an ac t ive e lement wi ll t end to a f fec t thecomm unica t io ns of only a single s t a tion . The l ay out andchanging need s of of f ice and l abo ra to ry bui ld ings l eadsus to p ick a ne twork topology wi th the potent i a l forconvenient inc rementa l ex tent ion and reconf igura t ionwi th minimal s e rv ice d i s rupt ion .

T h e t o p o l o g y o f t h e E t h e r n e t is t h a t o f a n u n r o o t e dtree. It is a tree so tha t the Ether can br anch a t the en-t rance to a bui ld ing ' s cor r idor , ye t avoid mul t ipa th in-t e r f e r e n c e . T h e r e m u s t b e o n l y o n e p a t h t h r o u g h t h eE t h e r b e t w e e n a n y s o u r c e a n d d e s t i n a t i o n ; i f m o r e t h a none pa th were to ex i s t , a t ransmis s ion would in te r fe rewi th i t s e l f , repea tedly a r r iv ing a t i t s in tended des t ina -t ion havin g t rave l l ed by pa ths of d i f fe rent length . TheEther i s unrooted b e c a u se i t ca n b e e x t e n d e d f r o m a n y o fi t s poin t s in an y d i rec t ion . An y s t a t ion wishing to jo in397

an Etherne t t aps in to the Ether a t the neares t convenientpoin t .

L o o k i n g a t t h e r e l a t io n s h i p o f i n t e r c o n n e c t i o n a n dcont ro l , we see tha t E th erne t i s the dua l o f a s ta r ne t -w o r k . R a t h e r t h a n distributed i n t e r c o n n e c t i o n t h r o u g hmany separa te l inks and central cont ro l in a swi tchingnode , a s in a s t a r ne twork , the Ether ne t has central inter-c o n n e c t i o n t h r o u g h t h e E t h e r a n d distributed c o n t r o lamong i t s s t a t ions .

Unl ike an Aloha Network , which i s a s t a r ne tworkw i t h a n o u t g o i n g b r o a d c a s t c h a n n e l a n d a n i n c o m i n gmul t i -acces s channe l , an Etherne t suppor t s many- to-many communica t ion wi th a s ingle broadcas t mul t i -access channel .3.2 Control

Shar ing o f the Ether i s cont ro l l ed in such a w ay tha ti t i s not o nly possib le but probab le tha t two or mo re s t a -t ions wi l l a t t empt to t ransmi t a packe t a t roughly thesame t ime . Packe t s which over lap in t ime on the Etherare said to collide; they in te r fe re so as to be unreco gniza -b le by a rece iver . A s t a t ion recovers f rom a de tec tedc o l li s io n b y a b a n d o n i n g t h e a t t e m p t a n d r e t r a n s m i t ti n gt h e p a c k e t a f t e r s o m e d y n a m i c a l l y c h o s e n r a n d o m t i m eper iod . Arbi t ra t io n o f conf l i c t ing t ransmis s ion de man dsis both dis t r ibuted and s tat is t ical .

When the Ether i s l a rge ly unused , a s t a t ion t ransmi t si t s packe t s a t wi ll , t he packe t s a re rece ived wi th out e r ror ,and al l is wel l . As more s tat ions begin to t ransmit , thera te o f packe t in te r fe rence inc reases. E th erne t co nt ro l l e rsin each s t a t ion a re bui l t to ad jus t the mean re t ransmis s ionin te rva l in pro por t ion to the f requency of co l li sons ;s h a r in g o f t h e E t h e r a m o n g c o m p e t i n g s t a t io n - s t a t io nt ransmis s ions i s the reby kep t near the opt im um [20 , 21].

A degree of coop era t io n amo ng the s t a t ions i s re -qui red to share the Ether equi t ab ly . In demanding ap-p l i ca t ions ce r t a in s t a t ions might use fu l ly t ake t rans -mis s ion pr ior i ty through some sys temat ic v io la t ion ofequi ty ru les . A s t a t ion could usurp the Ether by not ad-jus t ing i t s re transmis s ion in te rva l wi th inc reas ing t ra f f i cor by sending v ery l a rge packe t s. Both prac t i ces a re nowprohib i t ed by low- level sof tware in each s t a t ion .3.3 Addressing

Each packe t has a source and des t ina t ion , both ofwhich a re ident i f i ed in the packe t ' s header . A packe tp laced on the Ether eventua l ly propaga tes to a l l s t a -t io n s . A n y s t a t i o n c a n c o p y a p a c k e t f r o m t h e E t h e r i n t oi t s loca l memory, but normal ly only an ac t ive des t ina -t ion s t a t ion matching i t s addres s in the packe t ' s headerwi ll do so as the packe t pas ses . By conven t ion , a ze rodes t ina t ion addres s i s a wi ldcard and matches a l l ad-dres ses ; a pack e t wi th a d es t ina t ion of ze ro i s ca l l ed abroadcast packet.3.4 Reliability

An E thern e t i s probabi l i st i c . Packe t s m ay be los t dueto in te r fe rence wi th o ther packe t s , impulse noi se on th eCommunications July 1976of Volume 19eth ACM Number 7


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Ether, an inactive receiver at a packet's intended desti-nation, or purposeful discard. Protocols used to com-municate through an Etherne t must assume that packetswill be received correctly at intended destinations onlywith high probability.

An Ethernet gives its best efforts to transmit packetssuccessfully, but it is the responsibility o f processes in thesource and destination stations to take the precautionsnecessary to assure reliable communication of the qualitythey themselves desire [18, 21]. Recognizing the costli-ness and dangers of promising "error-free" communi-cation, we refrain from guaranteeing reliable deliveryof any single packet to get both economy of transmis-sion and high reliability averaged over many packets[21 ]. Removing the responsibility for reliable communi-cation from the packet transport mechanism allows us totailor reliability to the application and to place error re-covery where it will do the most good. This policy be-comes more important as Ethernets are interconnectedin a hierarchy of networks through which packets musttravel farther a nd suffer greater risks.3.5 M e c h a n i s m s

A station connects to the Ether with a ta p and atransceiver. A tap is a device for physically connecting tothe Ether while disturbing its transmission characteris-tics as little as possible. The design of the transceivermust be an exercise in paranoia. Precautions must betaken to insure that likely failures in the transceiver orstation do not result in pollution of the Ether. In par-ticular, removing power from the transceiver shouldcause it to disconnect from the Ether.

Five mechanisms are provided in our experimentalEthernet for reducing the probability and cost of losinga packet. These are (1) carrier detection, (2) interferencedetection, (3) packet error detection, (4) truncatedpacket filtering, and (5) collision consensus enforcement.

3.5.1 Carrier detect ion. As a packer's bits are placedon the Ether by a station, they are phase encoded (likebits on a magnetic tape), which guarantees that there isat least one transition on the Ether during each bit time.The passing of a packet on the Ether can therefore be de-tected by listening for its transitions. To use a radioanalogy, we speak of the presence of carrier as a packetpasses a transceiver. Because a station can sense the car-rier of a passing packet, it can delay sending one of itsown until the detected packet passes safely. The AlohaNetwork does not have carrier detection and conse-quently suffers a substantially higher collision rate.Without carrier detection, efficient use of the Etherwould decrease with increasing packet length. I n Section6 below, we show that with carrier detection, Etherefficiency increases with increasing packet length.

With carrier detection we are able to implementdeference: no station will start transmitting while hearingcarrier. With deference comes acquisition: once a packettransmission has been in progress for an Ether end-to-

end propagation time, all stations are hearing carrierand are deferring; the Ether has been acquired and thetransmission will complete without an interfering colli-sion.

With carrier detection, collisions should occur onlywhen two or more stations find the Ether silent and be-gin transmitting simultaneously: within an Ether end-to-end propagation time. This will almost always happenimmediately after a packet transmission during whichtwo or more stations were deferring. Because stations donot now randomize after deferring, when the trans-mission terminates, the waiting stations pile on together,collide, randomize, and retransmit.

3.5.2 Interference detect ion. Each transceiver has aninterference detector. Interference is indicated when thetransceiver notices a difference between the value of thebit it is receiving from the Ether and the value of the bitit is attempting to transmit.

Interference detection has three advantages. First, astation detecting a collision knows that its packet hasbeen damaged. The packet can be scheduled for re-transmission immediately, avoiding a long acknowledg-ment timeout. Second, interference periods on the Etherare limited to a m aximum of one round trip time. Collid-ing packets in the Aloha Network run to completion,but the truncated packets resulting from Ethernet colli-sions waste only a small fraction of a packet time on theEther. Third, the frequency of detected interference isused to estimate Ether traffic for adjusting retrans-mission intervals and optimizing channel efficiency.

3.5.3 Packet error detect ion. As a packet is placedon the Ether, a checksum is computed and appended.As the packet is read from the Ether, the checksum isrecomputed. Packets which do not carry a consistentchecksum are discarded. In this way transmission errors,impulse noise errors, and errors due to undetected inter-ference are caught at a paeket's destination.

3.5.4 Truncated packet f i l tering . Interference de-tection and deference cause most collisions to result intruncated pack ets of only a few bits; colliding stationsdetect interference and abort transmission within anEther round trip time. To reduce the processing loadthat the rejection of such obviously damaged packetswould place on listening station software, truncatedpackets are filtered out in hardware.

3.5.5 Coll is ion consensus enforcement. When a sta-tion determines that its transmission is experiencing in-terference, it momentari ly jams the Ether to insure thatall other participants in the collision will detect inter-ference and, because of deference, will be forced to abort.Without this collision consensus enforcement mechanism,it is possible that the transmitting station which wouldotherwise be the last to detect a collision might not doso as the other interfering transmissions successivelyabort and stop interfering. Although the packet maylook good to th at last transmitter, different path lengths

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between the colliding transmitters and the intended re-ceiver will cause the packet to arrive damaged.

4. Implementat ionOur choices of 1 kilometer, 3 megabits per second,and 256 stations for the parameters of an experimentalEthernet were based on characteristics of the locally

distributed computer communication environment andour assessments of what would be marginally achiev-able; they were certainly not hard restrictions essentialto the Ethernet concept.We expect that a reasonable maximum network sizewould be on the order of 1 kilometer of cable. We usedthis working number to choose among Ethers of varyingsignal attenuation and to design transceivers with ap-propriate power and sensitivity.

The dominant station on our experimental Ethernetis a minicomputer for which 3 megabits per second is aconvenient data transfer rate. By keeping the peak ratewell below that of the computer's pa th to main memory,we reduce the need for expensive special-purpose packetbuffering in our Ethernet interfaces. By keeping the peakrate as high as is convenient, we provide for larger num-bers of stations and more ambitious multiprocessingcommunications applications.

To expedite low-level packet handling among 256stations, we allocate the first 8-bit byte o f the packet tobe the destina tion address field and the second byte to bethe source address field (see Figure 2). 256 is a numbersmall enough to allow each station to get an adequateshare of the available bandwidth and approaches thelimit of what we can achieve with current techniques fortapping cables. 256 is only a convenient number for thelowest level of protocol; higher levels can accomodateextended address spaces with additional fields inside thepacket and software to interpret them.Our experimental Ethernet implementation has fourmajor parts: the Ether, transceivers, interfaces, and con-trollers (see Figure 1).4.1 Ether

We chose to implement our experimental Ether usinglow-loss coaxial cable with off-the-shelf CATV taps andconnectors. It is possible to mix Ethers on a singleEthernet; we use a smaller-diameter coax for convenientconnection within station dusters and a larger-diametercoax for low-loss runs between clusters. The cost ofcoaxial cable Ether is insignificant relative to the cost o fthe distributed computing systems supported byEthernet.4.2 TransceiversOur experimental transceivers can drive a kilometerof coaxial cable Ether tapped by 256 stations trans-mitting at 3 megabits per second. The transceivers canendure (i.e. work after) sustained direct shorting, im-399

proper termination of the Ether, and simultaneousdrive by all 256 stations; they can tolerate (i.e. workduring) ground differentials and everyday electricalnoise, from typewriters or electric drills, encounteredwhen stations are separated by as much as a kilometer.An Ethernet transceiver attaches directly to theEther which passes by in the ceiling or under the floor.It is powered and controlled through five twisted pairsin an interface cable carrying transmit data, receivedata, interference detect, and power supply voltages.When unpowered, the transceiver disconnects itselfelectrically from the Ether. Here is where our fight forreliability is won or lost; a broken transceiver can, butshould not, bring down an entire Ethernet. A watchdogtimer circuit in each transceiver attempts to preventpollution of the Ether by shutting down the output stageif it acts suspiciously. For transceiver simplicity we usethe Ether's base frequency band, but an Ethernet couldbe built to use any suitably sized band of a frequency di-vision multiplexed Ether.

Even though our experimental transceivers are verysimple and can tolerate only limited signal attenuation,they have proven quite adequate and reliable. A moresophisticated transceiver design might permit passivebranching of the Ether and wider station separation.4.3 Interface

An Ethernet interface serializes and deserializes theparallel data used by its station. There are a number ofdifferent stations on our Ethernet; an interface must bebuilt for each kind.

Each interface is equipped with the hardware neces-sary to compute a 16-bit cyclic redundancy checksum(CRC) on serial dat a as it is transmitted and received.This checksum protects only against errors in the Etherand specifically not against errors in the parallel por-tions of the interface hardware or station. Higher-levelsoftware checksums are recommended for applicationsin which a higher degree of reliability is required.

A tr ansmitting interface uses a packet buffer addressand word count to serialize and phase encode a variablenumber of 16-bit words which are taken from the sta-tion's memory and passed to the transceiver, precededby a start bit (called SYNC in Figure 2) and followed bythe CRC. A receiving interface uses the appearance ofcarrier to detect the start of a packet and uses the SYNCbit to acquire bit phase. As long as carrier stays on, theinterface decodes and deserializes the incoming bitstream depositing 16-bit words in a packet buffer in thestation's main memory. When carrier goes away, theinterface checks that an integral number of 16-bit wordshas been received and that the CRC is correct. The lastword received is assumed to be the CR C and is not copiedinto the packet buffer.

These interfaces ordinarily include hardware foraccepting only those packets with appropriate addressesin their headers. Hardware address filtering helps a sta-tion avoid burdensome software packet processing whenCommunications July 1976of Volume 19the ACM Number 7


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Fig. 2. Ethernet packet layout.ACCESSIBLETO SOFI'WARE

I i -Y DEST SOURCEADDRESS D A T AI I

8 BITS 8 BITS ~ 4OOOBI'PS

I CIIECKSUM [I f

16 BITS

the Ether is very busy carrying traffic intended for otherstations.4.4 Control ler

An Ethernet controller is the station-specific low-level firmware or software for getting packets onto andout of the Ether. When a source-detected collision oc-curs, it is the source controller's responsibility to gene-rate a new random retransmission interval based on theupdated collision count. We have studied a number of al-gorithms for controlling retransmission rates in stationsto maintain Ether efficiency [20, 22]. The most practicalof these algorithms estimate traffic load using recentcollision history.Retransmission intervals are multiples of a slot, themaximum time between starting a transmission and de-tecting a collision, one end-to-end round trip delay. AnEthernet controller begins transmission of each newpacket with a mean retransmission interval of one slot.Each time a transmission attempt ends in collision, thecontroller delays for an interval of random length with amean twice that of the previous interval, defers to anypassing packet, and then attempts retransmission. Thisheuristic approximates an algorithm we have calledBinary Exponent ial Backoff (see Figure 3) [22].

When the network is unloaded and collisions arerare, the mean seldom departs from one and retrans-missions are prompt. As the traffic load increases, morecollisions are experienced, a backlog of packets buildsup in the stations, retransmission intervals increase, andretransmission traffic backs off to sustain channelefficiency.

5. Growth

the Ether and extending its signal cover, there is a trade-off between using sophisticated transceivers and usingrepeaters. With increased power and sensitivity, trans-ceivers become more expensive and less reliable. Theintroduction of repeaters into an Ethernet makes thecentrally interconnecting Ether active. The failure of atransceiver will sever t he communications of its owner;the failure of a repeater partitions the Ether severingmany communications.5.2 T raff ic Cover

One can expand an Ethernet just so far by addingEther an d packet repeaters. At some point the Ether willbe so busy tha t additiona l stations will jus t divide morefinely the already inadequate bandwidth. The trafficcover can be extended with an unbuffered traffic-filteringrepeater or packet f i l ter , which passes packets from oneEther segment to another only if the destination stationis located on the new segment. A packet filter also ex-tends the signal cover.5.3 Address Cover

One can expand an Ethernet just so far by addingEther, repeaters, and traffic filters. At some point therewill be too many stations to be addressed with the Ether-net's 8-bit addresses. The address cover can be extendedwith packet ga teways and the software addressing con-ventions they implement [7]. Addresses can be expandedin two directions: down into the station by adding fieldsto identify destination ports or processes within a sta-tion, and up into the internetwork by adding fields toidentify destination stations on r emote networks.A gateway also extends the traffic and signal covers.There can be only one repeater or packet filter con-necting two Ether segments; a packet repeated onto asegment by multiple repeaters would interfere with itself.However, there is no limit to the number of gatewaysconnecting two segments; a gateway only repeats packetsaddressed to itself as an intermediary. Failure of thesingle repeater connecting two segments partitions thenetwork; failure of a gateway need not part ition the netit there are paths through other gateways between thesegments.

5.1 Signal CoverOne can expand an Ethernet just so far by adding

transceivers and Ether. At some point, the transceiversand Ether will be unable to carry the required signals.The signal cover can be extended with a simple un-buffered packet repeater . In our experimental Ethernet,where because of transceiver simplicity the Ether cannotbe branched passively, a simple repeater may join anynumber of Ether segments to enrich the topology whileextending the signal cover.

We operate an experimental two-segment packet re-peater, but hope to avoid relying on them. In branching

6 . Perfo rm a nceWe present here a simple set o f formulas with which

to characterize the performance expected of an Ethernetwhen it is heavily loaded. More elaborate analyses andseveral detailed simulations have been done, but thefollowing simple model has proven very useful inunderstanding the Ethernet's distributed contentionscheme, even when it is loaded beyond expectations[1, 20, 21, 22, 23, 27].

We develop a simple model of the perfo rmance of aloaded Ethernet by examining alternating Ether timeperiods. The first, called a transmission interval, is that

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dur ing which the Ether has been acqui red for a succes s -fu l packe t t ransmis s ion . T he second, ca l l ed a contentioninterval, i s tha t co mp osed of the re t ransmis s ion s lo ts ofSec t ion 4 .4 , dur ing which s t a t ions a t t empt to acqui recont ro l of the Ether . Because the mode l ' s E thern e t s a reloaded and because s t a t ions de fe r to pas s ing packe t s be -fore s t a r t ing t ransmis s ion , the s lo t s a re synchronizedby the t a i l o f the preceding acqui s i tion in te rva l . A s lo twi ll be empty whe n no s t a t ion cho oses to a t t e mpt t rans -miss ion in i t and i t wi l l con tain a col l is ion i f mo re th anone s t a t ion a t t empts to t ransmi t . When a s lo t conta insonly one a t t empted t ransmis s ion , then the Ether hasbeen acqui red for the dura t ion of a packe t , the conten-t ion interval ends , and a t ransmiss ion interval begins .

Le t P be the numb er o f b i t s in an Etherne t packet.Let C be the peak capacity in b i ts pe r s econd, ca r r i ed onthe Ether . Le t T be the time in seconds o f a s lot , thenumber of s econds i t t akes to de tec t a co l l i s ion a f t e rs t a r t ing a t ransmis s ion . Le t us as sume tha t the re a re Qs ta t ions cont inuous ly queued to t ransmi t a packe t ;e i the r the acqui r ing s t a t ion has a new packe t immedi -ately after a successful acquis i t ion or another s tat ioncomes ready. Note tha t Q a l so happens to g ive the to ta loffered load on the ne tw ork which for th i s ana lys i s i s al -ways 1 or grea te r . We as sume th a t a queue d s t a t ion a t -t empts to t ransmi t in the cur rent s lo t wi th probabi l i ty1 /Q , or de lays with probab i l i ty 1 - ( l /Q ) ; th i s i s kno wn

to be the opt imum s ta t i s t i ca l dec i s ion ru le , approxi -mated in E therne t s t a t ions by means of our load-es t i -mat ing re t ransmis s ion cont ro l a lgor i thms [20 , 21 ].6.1 Acquis it ion Probabil i ty

W e n o w c o m p u t e A , the probabi l i ty tha t exac t ly ones ta t ion a t t empts a t ransmis s ion in a s lo t and the re foreacquires the Ether . A i s Q . ( 1 / Q ) . ( ( 1 - ( l / Q ) ) * *(Q - 1) ) ; the re are Q ways in which one s t a t ion canc h o o s e t o t r a n s m i t ( w it h p r o b a b i l i t y ( l / Q ) ) w h i le Q - 1s ta t ions choose to wa i t (wi th probabi l i ty 1 - - ( l / Q ) ) .Simplifying,A = ( 1 - ( l / Q ) ) (Q -l).6 .2 Wai t ing Time

W e n o w c o m p u t e IV , t h e m e a n n u m b e r o f s lo t s o fwaiting in a content ion in te rva l be fore a succes s fu l ac -qui s i t ion of the Ether by a s t a t ion ' s t ransmis s ion . Theproba bi l i ty o f wa i ting no t ime a t a l l is jus t A , the prob-abi l i ty tha t one and only one s t a t ion chooses to t rans -mi t in the f i r s t s lo t fo l lowing a t ransmis s ion . The prob-abi l i ty of wa i t ing 1 s lo t i s A , (1 - A) ; the probabi l i ty ofwai t ing i s lots is A ,( ( l -- A)**i). T h e m e a n o f t hi s g e o-met r i c d i s t r ibut ion i sw = ( l - A ) / A .

Fig. 3. Collision control algorith m.Z E R O L O A D IE S T IM A T E F O RN E W A C K E T

. A s [L O A D E S T IM A T E Y E SO V E R F L O W E D ?

N O

G E N E R A T E IR A N D O M N U M B E R

I I N C R E A S EL O A DE S T I M A T E

C O L L I S I O

I I A = = IW E I G H T E D R A N D O MN U M B E R

I E R R O R I

6.3 Ef f i c i encyW e n o w c o m p u t e E, t h a t f r a c t i o n o f t i m e t h e E t h e r

i s ca r ry ing goo d packe t s , the efficiency. T h e E t h e r ' s t i m ei s d iv ided be tween t ransmis s ion in te rva l s and conten-t ion in te rva l s . A pack e t t ransmis s ion t akes P / C seconds .The mean t ime to acqui s i t ion i s W , T . T h e r e f o r e , b y o u rs imple mode l ,E = ( P / C ) / ( ( P / C ) q - ( W , T ) ) .

Table I present s representa t ive pe r formance f igures( i. e. E) for ou r exper ime nta l E therne t wi th the indica tedpacke t s i zes and n um ber o f continuously queued s tat ions .The e f f i c i ency f igures g iven do n ot accou nt for inevi t ab ler e d u c t i o n s d u e t o h e a d e r s a n d c o n t r o l p a c k e t s n o r f o rlosses due to imprec i se cont r o l o f the re t ransmis s ionp a r a m e t e r 1 /Q; the former i s s t ra ight forwardly pro to-col -dependent and the l a t t e r requi res ana lys i s beyondthe scope o f th is paper . Aga in , we feel tha t a l l o f theE t h e r n e t s i n t h e t a b l e a r e o v e r l o a d e d ; n o r m a l l y l o a d e dEtherne t s wi ll usua l ly have a Q mu ch l es s than 1 andexhib i t behav ior not cov ered by th is mode l .

For our ca lcula t ions we use a C of 3 megabi t s pe rsecond and a T of 16 mic roseconds . The s lo t dura t ion Tmus t be long enough to a l low a co l l i s ion to be de tec tedor a t l eas t tw ice the Ethe r ' s rou nd t r ip t ime . We l imi t ins o f t w a r e t h e m a x i m u m l e n g th o f o u r p a c k e t s t o b e n e a r4000 b it s to keep the l a t ency of ne twor k acces s dow n andto p e rmi t e f f ic i ent use o f s t a t ion pac ke t buf fe r s torage .

For packets whose s ize is above 4000 bi ts , the eff i -c i ency of our exper imen ta l E thern e t s t ays we ll above 95

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percent . For packe t s wi th a si ze appro ximat in g tha t o f as lo t, E therne t e f fi c iency approa ches l / e , the asymp tot i cefficiency of a s lot ted Alo ha netw ork [27].

7 . Pro to co lT h e r e i s m o r e t o t h e c o n s t r u c t i o n o f a v i a b le p a c k e t

c o m m u n i c a t i o n s y s t e m t h a n s i m p l y p r o v i d i n g t h em e c h a n i s m s f o r p a c k e t t r a n s p o r t . M e t h o d s f o r e r r o rcor rec t ion , f low cont ro l , p roces s naming, s ecur i ty , anda c c o u n t i n g m u s t a l so b e p r o v i d e d t h r o u g h h i g h e r- l ev e lp r o t o c o l s i m p l e m e n t e d o n t o p o f t h e E t h e r c o n t r o l p r o -tocol described in Sect ions 3 and 4 above. [7, 10, 12, 21,28 , 34] . E the r con t ro l inc ludes packe t f raming, e r ror de -t ec t ion , addres s ing and mul t i -acces s cont ro l ; l ike o therl i n e c o n t r o l p r o c e d u r e s , E t h e r n e t i s u s e d t o s u p p o r tn u m e r o u s n e t w o r k a n d m u l t i p r o c e s s o r a r c h i t e c t u r e s[ 3 0 , 3 1 ] .

Her e i s a br i e f desc r ip t ion of one s imple e r ror -con -t r o l l i n g p a c k e t p r o t o c o l . T h e E F T P ( E t h e r n e t F i l eTran s fe r P ro toc ol ) i s of in te res t both becau se i t i s re l a -t i v el y e a s y t o u n d e r s t a n d a n d i m p l e m e n t c o r r e c t ly a n dbecause i t has dut i fu l ly ca r r i ed m any va luable f i le s dur -ing the deve lopm ent of mor e genera l a nd e f f i c ien tp r o t o c o l s .7 .1 . G enera l T erm ino lo g y

In d i scuss ing packe t pro tocol s , we use the fo l lowinggenera l ly use fu l t e rminology. A packe t i s s a id to have asource a n d a des t i na t i on . A f low of da ta i s s a id to hav e as e n d e r a n d a rece i ver , recogniz ing tha t to supp or t a f lowof da ta some pack e t s ( typica l ly acknowled gments ) wi llbe sourced a t the rece iver and des t ined for the s ender .A connec t ion i s s a id to have a l i s tener a n d a n ini t iatorand a service is said to ha ve a s e r v e r a n d a user . I t i s veryusefu l to t rea t these as or thog ona l desc r ip tors of thepar t i c ipant s in a commu nica t ion . O f course , a s e rver i susua l ly a li s t ener and the source of da ta -bear ing packe t sis usual ly the sender.7 . 2 E F T P

The f i r s t 16 b i t s of a l l E therne t packe t s conta in i t sin te r face - in te rpre table des t ina t ion and source s t a t ionaddres ses , a byte each , in tha t order ( s ee F igure 2) . Bysof tware convent ion , the s econd 16 b i t s of a l l E therne tp a c k e t s c o n t a i n t h e p a c k e t t y p e . D i f f e r e n t p r o t o c o l suse d i s jo in t s e ts of packe t types . The E FT P uses 5packe t types : da ta , ack , abor t , end , and endreply . Fol -l o w i n g t h e 1 6 - b i t t y p e w o r d o f a n E F T P p a c k e t a r e 1 6bi t s of s equence number , 16 b i t s of l ength , opt io na l lysome 16-bit da ta words , a nd f ina l ly a 16-bi t sof twarec h e c k s u m w o r d ( se e F i g u r e 4 ) . T h e E t h e r n e t ' s h a r d -w a r e c h e c k s u m i s p r e s e n t o n l y o n t h e E t h e r a n d i s n o tcoun ted a t th i s l eve l of pro tocol .

I t should be obvious tha t l i t t l e ca re has been t akento c ram cer t a in f i e lds in to jus t the r ight nu mb er of b i ts .The emphas i s h e re i s on s impl ic ity and ease of pro-

gramming . Despi t e th i s d i sc la imer , we do fee l tha t i t ismore advi sable to e r r on the s ide of spac ious f i e lds ; t ryas you may, one f i eld or ano ther wi l l a lways turn o ut tobe too small .

T h e s o f t w a r e c h e c k s u m w o r d i s u s e d t o l o w e r t h eprobabi l i ty o f an und e tec ted e r ror . I t s e rves not o nly asa backup for the exper imenta l E therne t ' s s e r i a l ha rd-ware 16-bit cyc l i c redu ndan cy che cksum ( in F igure 2) ,but a l so for pro tec t ion aga ins t fa i lures in pa ra l l e l da tapa ths wi th in s t a t ions which a re not check ed by the CRC.T h e c h e c k s u m u s e d b y t h e E F T P i s a l ' s c o m p l e m e n tadd and cyc le over the en t i re packe t , inc luding headera n d c o n t e n t d a t a . T h e c h e c k s u m c a n b e i g n o r e d a t t h euser ' s pe r il a t e i the r end; the s ender may put a l l l ' s (animposs ib le va lue) in to the check sum w ord to indica tet o t h e r e c e iv e r t h a t n o c h e c k s u m w a s c o m p u t e d .

7.2.1 Data transfer. Th e 16-bi t word s o f a fi le areca r r i ed f rom sending s t a t ion to rece iv ing s t a t ion in da tap a c k e t s c o n se c u t iv e l y n u m b e r e d f r o m 0 . E a c h d a t apacke t i s re t ransmi t t ed pe r iodica l ly by the s ender unt i lan ack packe t wi th a matching sequence number i s re -turned f rom the rece iver . The rece iver ignores a l l dam-aged packe t s , packe t s f rom a s t a t ion o ther than thes e n d e r , a n d p a c k e t s w h o s e s e q u e n c e n u m b e r d o e s n o tmatch e i the r the expec ted one or the one preceding .W h e n a p a c k e t h a s t h e e x p e c t e d s e q u e n ce n u m b e r , t h epacke t i s acked , i t s da ta i s accepted as pa r t of the f i le ,a n d t h e s e q u e n c e n u m b e r i s i n c r e m e n te d . W h e n a p a c k e ta r r ives wi th a s equence number one l es s than tha t ex-pec ted , i t i s acknowledged and d i sca rded; the presump-t ior t i s tha t i t s ack was los t and needs re t ransmis s ion[21].

7 .2 .2 E nd. Wh en a l l the da ta has been transmi t t ed ,an end packe t i s s ent wi th the next consecut ive s equencen u m b e r a n d t h a n t h e s e n d e r w a i t s f o r a m a t c h i n g e n d -reply . Having accepted an end packe t in s equence , theda ta rece iver respond s wi th a matchin g endrep ly andt h e n da l l y s f o r s o m e r e a s o n a b l y l o n g p e r i o d o f t im e ( 1 0seconds ) . Upo n ge t t ing the endreply , the s ending s t a t iont ransmi t s an echo ing endreply and i s f ree to go of f wi ththe as surance tha t the f i le has been t rans fe r red succes s -fu l ly . The da l ly ing rece iver then ge t s the echoed end-reply and i t t oo goes of f a s sured .

Table I. Ethernet Efficiency.Q P = 4096 P = 1024 P = 512 P = 481 1.0000 1.0000 1.0000 1.00002 0.9884 0.9552 0 .9 14 3 0.50003 0.9857 0.9447 0.8951 0,44444 0.9842 0.9396 0.88~ 0.42195 0.9834 0 . 9 3 6 7 0.8810 0.409610 0.9818 0 .9 31 0 0.8709 0.387432 0.98~ 0.9272 0.8642 0.373764 0.9805 0.9263 0.8627 0.3708128 0.9804 0.9259 0.8620 0 ,3 ~ 3256 0.9803 0.9 2~ 0.8616 0. 3~ 6

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Fig. 4. E FF P packet layout.Destination Source

P ack e t T yp e

Sequence Number

Length (in words)

Data (words)

Software Checksum

- - 1 16-bit Word

IDataPacketOnly

The compara t ive ly comp lex end-da l ly s equence i sin tended to make i t p rac t i ca l ly ce r t a in tha t the s enderand receiver of a f i le wil l agree on whether the f i le hasbeen t ransm i t t ed cor rec t ly . I f the end pack e t i s los t, t heda ta s ender s imply re t ransmi t s i t as i t would a ny pac ke tw i th a n o v e r d u e a c k n o w l e d g e m e n t . I f t h e e n d r e p l y f r o mthe dat a receiv er is los t , the data sender wil l t ime o ut inthe same way and re t ransmi t the end packe t which wi l lin turn be ackn owledged by the da l ly ing rece iver . I f theechoed endreply is los t , the dal lying receiver wil l be in-convenienced having to wa i t for i t , bu t when i t hast imed out , the rece iver can never the les s be as sured o fsucces s fu l t rans fe r of the f i le because the end packe t hasbeen rece ived .

At any t ime dur in g a l l o f th i s , e i the r s ide i s f ree todec ide communica t ion has fa i l ed and jus t g ive up , i t i scons idered pol i t e to s end an abor t packe t to end thecom mun ica t ion pr omp t ly in the event of , s ay , a use r -ini t ia ted abort or a f i le sys tem error .

7 . 2. 3 E F T P s h o rt c om i n g s. T h e E F T P h a s b e e n v e r yusefu l , bu t i t s shor tcomings a re many. F i r s t , t he pro to-col provides only for f il e t rans fe r f rom s ta t ion to s t a t ionin a s ingle ne twork and speci f ica lly not f rom proces s toproces s wi th in s t a t ions e i the r on the s ame ne twork orthrough a ga teway. Second, proces s rendezvous i s de -genera te in tha t the re a re no mechani sms for f indingproces ses by name or for convenient handl ing of mul t i -ple users by a s ingle server. Third, there is no real f lowcont ro l . I f da ta a r r ives a t a rece iver unable to accep t i tin to i t s buf fe rs, the da ta can s imply be thrown away wi thcomple te as surance tha t i t w i l l be re t ransmi t t ed eventu-a l ly . There i s no way for a rece iver to quench the f low ofsuch was ted t ransmis s ions or to expedi t e re t ransmis s ion .Fou r th , da ta i s t ransmi t t ed in in tegra l num bers o f 16-bit40 3

w o r d s b e l o n g i n g t o u n n a m e d f il es a n d t h u s t h e E F T P i se i the r t e r r ib ly rest r i c t ive or dem ands some nes ted f i l et rans fe r formats in te rna l to i t s da ta words . And f i f th ,func t ion a l genera l i ty i s los t because the rece iver i s a l sothe l is tener and server.

8 . C o n c l u s i o nOur exper ience wi th an opera t ing Eth erne t l eads us

t o c o n c l u d e t h a t o u r e m p h a s i s o n d i s t r i b u te d c o n t r o lwas well p laced . By keeping the shared c om pon ent s oft h e c o m m u n i c a t i o n s y s t e m t o a m i n i m u m a n d p a s s i v e ,we have achieved a very high level of rel iabi l ity. Ins tal la-t i o n a n d m a i n te n a n c e o f o u r e x p e r i m e n t a l E t h e r n e t h a sbeen m ore than sa t i s fac tory . The f l ex ib i l ity of s t a t ioni n t e r c o n n e c t io n p r o v i d e d b y b r o a d c a s t p a c k e t s w i tc h i n gh a s e n c o u r a g e d t h e d e v e l o p m e n t o f n u m e r o u s c o m p u t e rne tworking and mul t iproces s ing appl i ca t ions .

A c k n o w l e d g m e n t s . Our col l eagues a t the Xero x Pa loAl to Re search Cente r , e spec ia l ly Ta t C . Lam, But le r W.Lampson, John F . Shoch, and Char les P . Thacker , havec o n t r i b u t e d i n m a n y w a y s to t h e e v o l u t i o n o f E t h e r n e tideas and to the con s t ruc t ion of the exper imenta l sys-t em wi th out which such ideas would be jus t so m uchspecula t ion .Received May 1975; revised December 1975References1. Abramson,N. Th e Aloha system. AFIPS Conf. Proc., Vol.37, 1970 FJCC, A FIPS Press, Montvale, N.J., 1970, pp. 281-285 .2. Abramson, N. and K uo, F.F. Computer-Communication Net-works. Prentice-Hall, Englewo od Cliffs, N.J., 1975.3. Ashenhurst, R.L., and Vonderohe, R.H. A hierarchicalnetwork. Datamation 21, 2 (Feb . 1975), 40--44.4. Baran, P. On distributed communications. Rand Corp. MemoRM-3420-PR, Aug. 1964.5. Barnes,G.H., Brown, R.M., K ato, M., K uck, D.J., Slotaick,D.L., and Stokes, R.A. T he Illiac IV Computer. IEEE Trans.Computers C-17, 8 (Aug. 1968), 758-770.6. Binder,R., Abramson, N., K uo, F., Okinaka, A., and Wax,D. A loha packet broadcasting--a retrospect. AFIPS Conf. Proc.,Vol. 44, 1975 NCC, A FIPS Press, Montvale, N.J., 1975.7. Cerf, V.G., and K ahn, R.E. A protocol for packet networkintercommunication. IEE E Trans. Comm. CO M M- 22 , 5 (May1974), 637-648.8. The shrinking world: computer networks and communica-tions. Computer 7, 2 (Fe b. 1974).9. Distributed-fu nction compu ter architectures. Computer 7, 3(March 1974).10. Crocker, S.D., Heafner, J.F., Metcalfe, R.M., and Postel,J.B. Function-oriented protocols for the Arpa computer network.AFIPS Conf. Proc., Vol. 40, 1972 SJCC, AFIPS Press, Montvale,N.J., 1972, pp. 271-279 .11. Crowther, W.R., Heart, F.E., McK enzie,A . A . , McQuillan,J.M., and Walden, D.C. Issues in packet-switchingnetwork de-sign. AFIPS Conf. Proc., Vol. 44, 1975 NCC, AFIPS Press, Mont-vale, N.J., 1975, pp. 161-175.12. Farber, D .J., et al. Th e distributed computing system. Proc.7th Ann . IEEE Computer Soc. International Conf., Feb. 1973,pp. 31-34.13. Farber, D.J., A ring network. Datamation 21, 2 (F eb. 1975),44-46.14. Fraser, A.G. A v irtual channel network. Datamation 21, 2(Feb. 1975), 51-53.Communications July 1976of Volume 19the ACM Number 7


10/10

15. He art, F.E., K ahn, R.E., Omstein, S.M., Crowther, W .R., andWalden, D.C. The interface message processor for the Arpacom puter network, A FIP S Conf. Proc ., Vol. 36, 1970 SJCC,AF IPS Press, Montvale, N.J., 1 970, pp. 551-567.16. Heart, F.E., Ornstein, S.M., Crowther, W.R., and Barker,W.B. A new minicomputer-multiprocessor for the Arp a network.AFIP S Conf. Proc ., Vol. 42, 1972 SJCC, A FIPS Press, Montvale,N.J., 1972, pp. 529-537.17. K ahn, R .R. The organization of comp uter resources into apacket ratio network. AFIPS Conf. Proc., Vol. 44, 1975 NCC ,AF IPS Press, Montvale, N .J., 19 75, pp. 177-186.18. Metcalfe, R.M . Strategies for interprocess com mun icatio n in adistributed comp uting system. Prec. Symp. on Comp uter Com-mun. N etworks and Teletratiic. Polytechnic Press, New Y ork,1972.19. Metcalfe, R.M. Strategies for Operating Systems in Comp uterNetworks, Proc. AC M N ational Conf., August 1972, pp. 278-281.20. Metcalfe, R.M . Steady-state analysis of a slotted an d con-trolled aloha system with blocking. Proc. 6th Hawaii Con f. onSystem Sci. Jan. 1973, pp. 3 75-380.21. Metcalfe, R.M. Packe t commu nication. Harv ard Ph.D. Th.,Project Mac TR-114, Dee. 1973.22. Metealfe, R.M . Distributed algorithms fo r a broadc ast queue.Talk given at Stanford University in Novembe r 1974 and at theUniversity of California a t Berkeley in Febr uar y 1975, pape r inpreparation.23. Mu rthy, P. Analysis of a carder-sen se random-access systemwith rando m pack et length. Aloha System Tech. Rep. B75-17, U.of Hawaii, May 1975.24 . Ornstein, S.M., Crowtber, W. R., Kra ley, M.F., Bressler, R.D .,Michel, A., and Heart, F.E. Pluribus--a reliable multiprocessor.AFIP S Conf. Proc., Vol. 44, 1975 NCC , AFI PS Press, Montvale,N.J., 1970, pp. 551-559.25. Retz, D.L. Operating system design considerations for thepack et switching environment. AF IPS Conf. Proc., Vol. 44, 1975NC C, AF IPS Press, Montvale, N.J ., 1970, pp. 155-160.26. Roberts, L., an d Wessler, B. Com puter network developmentto achieve resource sharing. AFIPS Conf. Proc., Vol. 36, 1970SJCC, A FIP S Press, Montvale, N.J ., 1970, pp. 543-549.27. Roberts, L. Capture effects on Aloha channels. Proc. 6thHawaii C onf. on System Sci., Jan. 1973.28. Rowe, L.A. The distributed comp uting operating system.Tech. Rep. 66, Dep. of Information and Co mputer Sci., U. o fCalifornia, Irvine, June 1975.29. R ustin, R. (Ed.) Com puter Netwo rks (Proc. Coura nt Com -puter S ci. Symp. 3, December 1970), Prentice-Hall, En glewo odCliffs, N.J., 1970.30 . IB M synchronous data link control--general information. IB MSystems Development Div., Pub. Center, Research Triangle Park,N.C., 1974.31. IBM system network architecture--general information.IBM Systems Development Div., Pub . Center, ResearchTriangle Park, N.C., 1975.32. Thom as, R.H . A resource sharing executive for the Arpanet.AFIP S Conf. Proc., Vol. 42, 1973 NCC , A FIPS Press, Montvale,N.J., 1973, pp. 155-163.33. Thornton, J.E. Design of a Computer: the Control Data 6600.Scott Fo resm an and Co., Glenview, I11. 1970.34. Walden, D.C. A system for interprocess comm unication in aresource sharing compu ter network. Comm. ACM, 15, 4 (April1972), 221-230.35. Willard, D.G. Mitrix: A sophisticated digital cable communi-cations system Proc. National Telecommunications Conf., Nov.1973.36. Wulf, W., and Levin, R. C.mm i>--a multi-mini-processor,AFI PS Conf. Proc., Vol. 41, 1972 FJCC , A FIPS Press, Montvale,N.J., 1972.40 4

M a n a g e m e n t H . M o r g a nA p p l i c a t i o n s E d i t o rSynthesis ofDecision RulesCheng-Wen Cheng and Jonas RabinWestern Electric

Decision tables can be used as an effective toolduring an interview to record the logic of processes to beautomated. T he result of such an interview is not astructure of complete decision tables but rather sets ofdecision rules. The purpose of this paper is to provide aprocedure for synthesizing the decision rules and thusprovide an aid in developing a structure of completedecision tables.

Key W ords and Phrases: decis ion rules, decis iontables, logical tables , logical des ign, system des ign,specif ication langua ge

CR Categories: 3 .50 , 4 .33

1. IntroductionO n e o f t h e m o s t i m p o r t a n t s t a g e s i n th e s y s t e m s

d e v e l o p m e n t p r o c e s s i s f a c t f i n d i n g . F a c t f i n d i n g i s a ni n v e st i g at i o n a n d r e c o r d i n g o f c u r r e n t p r o c e d u r e s a n df u t u r e r e q u i r e m e n t s . T h e m o s t e f f ec t iv e m e t h o d o f f a c tf i nd i ng i s i n t e rv i ewi ng .

D e c i s i o n t a b l e s a r e a n e f f e c t i v e t o o l f o r r e c o r d i n gt h e l o g i c d u r i n g t h e i n t e r v i e w p r o c e s s , a s d i s c u s se d b yL o n d o n [ 1] . T h e p r i m a r y o b j e c t iv e o f t h e i n t e r v i e w is

Copyright 1976, Association fo r Compu ting Machinery, Inc.General permission to republish, but not for profit, all or partof this material is granted provided that ACM's copyright noticeis given and that reference is made to the publication, to its dateof issue, and to the fact tha t reprinting privileges were grantedby permission of the Association for Co mputing Machinery.Au tho rs' present addresses: C. W. Cheng, We stern Electric, En -gineering Research Center, P.O. Box 900, Princeton, NJ 08540.J. Rabin , Bell Laboratories, Napierville, IL 08540.Communications July 1976of Volume 19the ACM Number 7

ethernet: distributed packet switching for local computer networks - robert m. metcalfe

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