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Internet Protocols

by: Melyssa Aufderhar

Internet Protocols CSC 343

Melyssa Aufderhar
GPA 3.56

Errin Fulp

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Errin Fulp
Class Notes
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This 3 page Class Notes was uploaded by Melyssa Aufderhar on Wednesday October 28, 2015. The Class Notes belongs to CSC 343 at Wake Forest University taught by Errin Fulp in Fall. Since its upload, it has received 16 views. For similar materials see /class/230722/csc-343-wake-forest-university in ComputerScienence at Wake Forest University.


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Date Created: 10/28/15
Manchester encoding opposing definitions resolved by Roger Forster Manchester miterlira is iiltlcly used to provide lotc Ilitl lam i39lifoi rrlatimz SIMIIJlitllIt OtlSl Via a single connection limit are two opposing 39til39ltl int I 39ll J quot Qllll llfll Vfl l encoding where tlic rising edge in tic tciilrc oft1c lam lilf italarcs citer l logic quotI or allci39imtiiely a logic 0 This paper resellcs tie WitterIces am 139 lCIIlllltis llt interim0r Qiic riiutiorl anchester encoding is named alter the University ot lVlanchester where the rst recorded use of it occurred in the late 1940 s It is a digital phase modulation encoding method which has the lollowing advantages over NRZ non return to zero signals such as those used in conventional 39ll39l IIMOS logic These are 39 A separate clock is not required 0 39l39here are no long strings ol logic quotll or logic 039 levels 0 There is no lX component and so a signal encoded by it can be AC coupled 39I39he clock can be extracted from the data at the receiving data terminal equipment DTIE by using a digital phase locked loop l LI This makes the method suitable for use on single core systems such as optical bre and coaxial cable In current engineering systems it is most commonly used on Ethernet and token ring local area networks LANs It does have two minor disadvantages i 39l he modulation bandwidth is twice that FNRZ 39 It needs complex decoding circuitry literature search For Manchester encoding produces two opposing lactions as to how it should be generated Faction l centred on authors such as Stallingsl and alsalll states that the encoding scheme is based on the modulo 2 addition of the data and clock signals using an exclusive4ilk gate This method causes the output data to change state on the ll Ii e WWI twi data am Ll Ll Ll lil W Ll L output ll39l l quotlquotll 7 l dl lll l ll l cloak fLWFL H mm output ll39 quotquot39l Iquotllquot positive edge of the clock Faction 2 with authors such as 39lanenbaunrl and Sandersonquot states that the scheme is based on the modulo 2 addition ofthe data and clock but where the output data changes on the negative edge of the clock In each case there is always a change of the output data state to Facilitate easy bit synchronisa tion 0F the data by providing a level change in the middle of the data bit For Faction 1 this means that for a logic 1 this change is in a positive direction and For a logic 0 in a negative direction see Fig la For Faction 2 it is vice versa the change in the centre of the bit is in a positive direction for a logic 0 and in a negative Fig 1 ENGINEERING SCIENCE AND lilUlATINJURNAl Manchester encoding a faction 1 definition b faction 2 definition 278 direction for a logic l see Fig 1b DECI IM BER 2000 Investigation These opposing definitions can cause a problem to the conscientious student who reads around the subject as they will nd this conflicting information This is because it is obvious that Manchester eiicoded data stream generated by a DTH using the first method will not be received successfully by a Tl 1 that decodes using the rules of the method as the received data will be inverted A modification to Manchester second encoding called diiicrential Manchester encoding over comes any data inversion caused by the transmission path but requires a more complex encoding and decoding process than conventional Manchester encoding This problem of the two opposing definitions was brought to the attention of39 the author in late 190 by a student comparing the textbook recommended for their course unit with the author s lecture notes Upon investigation it was noted that Stallings stated that his method although the opposite of Tanenbaum s was in accordance with the llelF 802 and other recognised standards From communications with Stallings and Tanenbaum it was determined that the originator of the method was a postgraduate student at the University of Manchester during the early 1950 s but the definitive reference to Manchester encoding was not identified at that time literature search using the internet and online databases yielded references that were split between the two methods339 The lEliE 8024 speci cation section 171 quite clearly states that a high todow transition represents a logic 0 and a loxwto liigli transition represents a logic 1 A single reference was found that pointed towards what might have been the original papcr7 search of listings of UK theses For l his taken at Manchester University in the early 1950 s also failed to identify conclusively the originator The author of the original reference Was finally identi ed as a result of a letter published in EU Review8 requesting help from the Ilili membership Numerous replies identi ed Dr G li Thomas as the original author in fact the first eply coincidentally was from Dr Thomas himself almost two weeks before the author received his own copy of the magazine Historical background Thomas graduated at Manchester University in 1948 He then became a postgraduate student and finally a member of stat there working under Professor i I ENGINEERING SClliNClu ANI ilU lYl lONJOUlLNAl 27 Williams on early computers in particular the Manchester Mark l computer Williams introduced Thomas to phase modulation for digital encoding which Thomas believes that Williams had used during World Xar ii at the Tcleconununication Research Establishment 39l39lUd Thomas described the phase modulation technique as used for vritinp to and reading from the high speed magnetic backing store of the Fig 2 F C Williams Manchester Mark 1 computer coflrtesy in a number of papersiil his Un39VerS39ty 0f MSc dissertation and nally Manchester H II his l hl thesis A druiu provided the lnain store for the Manchester Mark I computer F 3 which used a CRT store as the 39g t Dr G E random acctss incmol y RAM Thomas device courtesy l lence although 39l39homas did Dr Thomas not invent the system his RFC 110 original references and the rst published practical implementai tion as far as the author can determine The scheme adoph ed for the Manchester Mark l for a the drum edge titi computer WM negative going in middle of the bit A phase modulation system was chosen in preference to NR7 or R7 return to zero schemes due to the balanced nature of its write currents NRZ and R2 would have given unbalanced write currents in the Manchester Mark l computer a logic 1 was represented by a negativegoing pulse and a logic 0 by a positive going pulse 391 hoinas and other members of the Manchester Mark I team have been unable to shed any light on why that scheme was used as opposed to the inverse method used by the IliLlLl 802 standards A history ofquot the Manchester Mark 1 computer can be found at the Web site of Manchester University s Department of Ioinputer Sciencelz Cunningham has written about Professor Williams and Thomas has published an article on the drum The work at Manchester was subject to numerous patents at the time including ili707637 and Gli707634 which cover the phase modulation scheme These patents and others can be Viewed at the Patent Office Web site httpwwwpatentgovukdbservicesindexlitinl using the esplcenet system under l atcnt Services Discussion The author s initial thoughts as to why the two methods should be in existence focused on the concept oi the mark and space de nitions used on telegraph and early data coinumnication systems versus that of DECEMBER 200i Fig 4 Definitive definition of Manchester encoding clock output conventional logic markspace logic data 0 m VLl th lTll l Ll l lll l 3 ll If Ll l V l l 39 quotfl rpm 1af lewlmll ll sv 5V L 0v 6V conventional logic levels In markspace terminology a mark 7a logic l jhas traditionally been represented by a negative voltage and a space by a positive voltage this is the representation for instance in the EIA R8232l7 or the l39l UT V24V28 speci cations This would give rise to the opposite method of generation when considering conventional logic wl 39 a logic l is a positive voltage and a logic 0 is 0 volts Web sites on Manchester encoding such as that of Optimized hitgiiit el iIigl5l and a Philips Semii conductors application note do not show the actual voltage levels which was the author s rst clue to the conundrum if we look at the BER 8023 or WITH 8024 standards then it is clear that the author s initial thoughts are probably correct however only members ot the original lliHE 802 committee would be able to shed tiirther light on tlle topic F Conclusion From the literature search it is most likely that the prilnary dill erence between the authors in the two Factions concerns Conventional logic versus that of markspace logic A logic 1 is always represented by a transition in the middle ot the data bit from the voltage level representing a logic 1 to the voltage level representing a logic 0 Hence the two wavetorins shown in liig 4 represent the definitive status ol Manchester encoding Acknowledgment The author wishes to thank Dr Thomas and the many other correspondents who replied to his letter in L915 Review3i References STALLlNGS W 39l ata and computer nelworlts39 Prentice lall 199 6th etln 2 lllS li Data communication computer networks and open systems Addison Wesley 6 4th edn IiNGlNliliRlNG SCIENCE ANI lilUCATIONJOURNAI 3 TANIENBAUM A Ioinputer networks Prentice Hall 1996 3rd edn 1 SANDERSON IE lincoding39 See httpwwwcsisuisu eduNpeteese l65totesericotlinglitinl liebruary W98 KUROSI l3 and ROSS K X Lithernet See httpgaiacsumassetlukuroseethernetethernetihtln l 6 lntroduetion to the l bilips httptvstviitudelllJilcasesaudiohtml Delll University Ul audio protocol See oli39lechnology BR K RR R A quotthe programming strategy used with the Manchester University Mark 1 computerquot ll39U 1515 1056 103 suppl pp l 517157 FORSTER IL Made in Manchesterquot 1139 March 2000 p42 THOMAS L F Magnetic storage lst Cambridge Computer ionti 194 WILLIAMS ll 3 139 Ill2 Universal highrspeed digital computers a magnetic sto e39 I rur Hill W52 9 Pt ll p94 Tl IOMAS i electronic digital coinputer Manchester University 1954 llistory oi the Department oF Computer Seience See httpwwwcsananaeukVisitorsubvebhistoryphp3 3 IUNNINGI IAM Mi E L Williamsquot ling Sri lmy j April 1094 3 2 pp Si 4 4 THOMAS 1 LL lkecolleetions ot39 Magnetic drum Mark l l lh g l See http I quotprismquot mmimtwitmtM I l y RelIt ll C 39 quotI he tlesign and construction of an 3 storage for the drumshtml Signal encoding in lithclaret8023 See httpwww optinii7edcoinIMI UNIlFN l erslitm Optimized Engineering Corporation 1999 Manehester signal eneoding See httpwww optiniizetlatoni XMl liNlliNrSiglinlitm Optim ized Engineering Corporation 990 AN070 Verilog implementation of a ent oderdecoder in Philips Il lls l hilips Semicour dnctors application note l 7 Ln C Manchester 3 1 llili 2000 The author is a Senior lecturer in the acuity otTeehnology Southampton institute liast l ark 39l erraee Southampton 8014 0YN UK le is an llili Member DECEMBER 2000


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