116
`
`FUNDAMENTAL WIDE AFIEA NETWORKING CONCEPTS
`
`.3 Universal
`jOCl'I
`
`]C|Ck
`
`RJ4-15
`
`Programmed
`
`Figure 1.69 Programmed arrangement jack
`
`W455
`
`Table 1.29 Telephone ordering options
`
`Decision
`A
`
`B
`
`0
`
`Description
`
`
`
`
`
`OD‘-iiG301«F5-(.0i'\.'1|—‘-
`
`Tetephone set controls line
`Data set controls line
`
`No aural monitoring
`Aural monitoring provided
`
`Touchtone dialing
`Rotary dialing
`Switchhook indicator
`Mode indicator
`
`Vfhen the telephone set is optioned for telephone set controls the line, calls are
`originated or answereti with the telephone h_\‘
`lifting the l’Ik1l1I’.l:$Ct' off-hook. To
`enable control of the line to he passed to a moclem or data set an ‘cxclusitari key‘ is
`reqttirecl.
`Tlie exclusion lte_\' telt-phone permits calls; to be manually answered and then
`translierretl to the tnodeni using the exclusion key. The exclusion key telephone is
`wired for either ‘telephone set controls line‘ or ‘data set controls line’. Data set
`control
`is normally selt-acted if you have an autornatit‘ call or automatic answer
`moclem since this permits calls to be originated or answered without taking the
`telephone handset oft‘-hook. To use the telephone for voitte communications the
`handset tnust be 1‘-.tisr:d and the exclusion lte_\' placed in an up\\':irtl location.
`The telephone set control of the line option is
`tisctl with manual answer or
`manual originate modems or automatic answer or m'igi:'1att: l’I“1U(_lt'_‘n"1.‘-‘- that will be
`operated manutilly. To connect the modem to the line the telephone tmtst be off—
`hook and the exclusion key placed in an upward position. To use telephone for
`mice communications the telephone must he off-hook while the exclusion lie)‘ is
`placed in the downward position.
`\\"hen the (lat set controls the line option is selcctetl. calls can he E1l.lt0lTlE1l'.lC8ll_\'
`ori,:__tii1atcd or answerccl by the tlata equipment without lifting the telephone ltandset.
`Aural mt.}nit0t'lng enables the telephone set to monitor call prr)g1'ess tones as well
`as voice answer hack messages without requiring the user to switch from data to
`voice.
`
`

`
`1.15 THE DATA LINK LAYER
`
`117
`
`You can select option B3 if aural monitorin;._g is not required, while option B4
`should be selected if it
`is required. Option C5 should be selected if touchtone
`dialing is to be used. while option C6 should be specified for rotary dial telephones.
`Under option D7, the exclusion key will he h_\'passet'l, resulting in the lifting of the
`telephone handset causing the closure of the switehhook contact in the telephone.
`ln comparison, option D8 results in the ercclusion key contacts being wired in
`series with the switehhook contacts, indicating to the user whether he or she is in a
`Voice or L‘l'dta tnode.
`
`Ordering the business line
`
`Orderin;_r a business line t.o transmit (late: o\'er the switched telephone network
`currently requires you to pr0\'i(l€
`the telephone eompan_\‘ with four items of
`information. First, you must supply the telephone LT(ll‘|'1]')::lI‘l_\‘ with the Federal
`Ctnnmunications Cotninission (FCC) registration nuniher of the device to be
`ctmnected to the switched telephone network. This 1+-character number can be
`obtained from the \-‘entlor who must First register their device for operation on the
`switchetl network prior to making it amilahle. for use on that network.
`Next, you must provitle the ringer equivalence number of the data set to be
`connected to the switched network, This is a three—eharaetei' number, such as
`('}.4.'—\, and represents a unitless quotient formed in 'r1CL‘(')l'ClClI1C€ with certain circuit
`parameters. Finally, you must provide the jack numbers and ai'r-angement to be
`used as well as the telepho|‘1e options if you intend to use a handset.
`
`1.15 THE DATA LINK LAYER
`
`In the ['50 model, the data link l:i_\'(_‘T is responsible for the estahli.~;hment, control,
`and termination of connections among network devices.
`'l‘o ateeornplish these.
`tasks the. data link layer assumes responsibilit_\' for the How of user data as well as
`for detecting and providing a mechanism for re-cot-'er_\-‘ from errors and other
`abnormal conditions, such as at station failing to receive a response during a
`predefined time int:-:r\-‘al.
`In this section, we will first examine the l<e_\' element that defines the data link
`la_\'er—its protocol. In this exarnination. we will differentiate between terminal
`protocols and data link protocols to eliminate this l'et‘minolog_\' as -.1 potential area of
`ttoiilusioti. Next, we will
`focus our attention upon several specific wide area
`networking protocols, starting with simple :1s_\-'nt:hronous line-by-line protocols.
`Protocols examined in the second portion of this section include an as}-'nehronous
`teletype protocol, several popular asynchronous file transfer protocols used to
`transfer data to and from personal computers,
`ll3‘.\I's eharacter—oriented binary
`synchronous communication (commonly referred to as BBC or hisyne), Digital
`Equipment C'.orpoi':Ition‘s Digital
`l)ata Comrnunieations Nlcssage Protocol
`(DDCt\'lI’}. and the. bit—orien1ed Higher Lei-'el Data Link Control (I-IDLC) as
`well as its IBRI near—equi\'alent,
`.‘5y11ttl1roi"ioLis Data Link Control (SDLC).
`
`*
`
`_
`"I
`
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`
`113
`
`FUNDAMENTAL WIDE AREA NETWORKING CONCEPTS
`
`Terminal and data link protocols
`
`Two types of protocols should be considered in 21 data (‘.OITIt11lIl1i(.221ti('JnS
`enrii-onment: terminal protocols and data link protocols.
`The data link protocol defines the control characteristics of the netvrorlt and is a
`set oi‘ conventions that are followecl which govern the transmission of data and
`control
`information. A terminal or it personal computer can have a predefined
`control character or set oi" control characters which are unique to the terminal and
`are not interpreted by the line protocol. This internal protocol can include such
`control characters as the bell,
`line Feed and carriage return for conventional
`teletype terminals, blink and cursor positioning characters for a display terminal
`and form control characters for a line printer.
`For experinienting with members of
`the ll-3L\-"l PC series and compatible
`computers, you can execute the one line BASIC" program PI{I1\7'l‘ Ci-IREMX)
`“l_)E.\IO”, substituting different ASCII values For the value of X to see the effect
`of different PC terrninal control ci1aI'acters. As an example, using the value F for X.
`the IBM PC will beep prior to displaying the Inessagxe I_)EL\-"IO, sinee ASCWI 7 is
`interpreted by the PC as a request to beep the speaker. Using the Value 9 for
`will
`cause the message DEMO to be printed commencing in position 9, since ASCII 9
`is 21 tab character which causes the cursor to more on the screen 8 character
`
`positions to the right. :'\nother example of a terminal control character is ASCII
`ll, which is the home character. Using the value ll for X will cause the message
`DER-"IO to be printed in the upper left—hand corner of the screen since the cursor is
`first placed at that location by the home character.
`Altliotigli poll and select is normall_\‘ thought of as a type of line discipline or
`control, it is also a data link protocol. in general, tht-. data linlt protocol enables the
`exchange of information according to an order or setjtience by establishing a series
`of rules for the interpretation of control siegnals which will govern the exchange of
`inioI'matitm. The control signals govern the execution of a number of tasks which
`are essential
`in controlling the exchaiige of information via a communications
`faciiit}-'. Some of these inforrnation control tasks are listed in 'l'al.1le 1.30.
`
`Table 1.30
`
`information control tasks
`
`Connection establishment
`Connection verification
`Connection disengagement
`
`Transmission sequences
`Data sequence
`Error control procedures
`
`Connection establishment and verification
`
`.'—\lthot1gh all of the tasks listed in 'l'able 1.30 are important, not all are required for
`the transmission of data, since the series of taslts required is a function of the total
`data communications en\'ironn'1ent. As an example, a single terminal or personal
`computer connected directly to a mainframe or another terminal device by a leased
`line may not require the establishment and \-'erii'ieation of the connection. Several
`
`
`
`

`
`1.15 THE DATA LINK LAYER
`
`119
`
`ll
`
`,
`
`'
`‘’
`
`l
`
`-
`
`,
`
`.
`
`E
`
`_l
`
`-
`ii
`l
`
`I
`' i
`l
`l
`
`Il
`
`II I
`
`I
`
`_.
`
`I
`
`de\-"ices connected to a mainframe coniputer on a multidrop or multipoint line
`would, however, require the verification ol‘ the ideiitification of each terminal
`device on the line to insure that data transmitted from the computer would be
`received by the proper device. Similarly, when a device's session is completed, this
`
`fact must he recognized so that the mainframe eomputei-’s resources can be made
`available to other users. Thus, connection disengagernent on devices other than
`those connected on a point-to-point leased line permits a port on the front-end
`processor to become available to service other users.
`
`Transmission sequence
`
`Ptnother important task is the transmission sequence which is used to establish the
`precedence and order of transmission, to include both data and control inform-
`ation. As an example, this task defines the rules for when devices on a multipoint
`
`circuit may transmit and receive information. In addition to the transmission of
`information folloxving a sequence,
`the data itself may be sequenced. Data
`sequencing is normally e.mployed in synchroiintis transmission and in asynchro-
`
`nous file transfer operations where a long block is broken into smaller blocks for
`transmission, with the size of
`the blocks being a
`function of the personal
`computer’s or terminal's buffer area and the error control procedure employed. B}-'
`dividing a block into smaller blocks for transmission.
`the amount of data that
`must he retransmitted,
`in the event that an error in transmission is detected,
`is
`1'edL1ced.
`Although man_\-' error-checking techniques are more efficient when short bloclts
`of information are transmitted,
`the efficiency of transmission correspondingl_\-'
`decreases since an acknt.m'leclgement (negative or positive) is returned to the device
`transmitting after each block has been received and checked. For communications
`between remote joh entry terminals and computers, blocks of up to several
`thousand characters are t_\-'pit::1ll_\-‘ used. Block lengths from 80 to 1024- characters
`are,
`l'10\\-'{3\-‘I31’.
`the most common sizes. Altliougli some protocols specify block
`length, most protocols permit the user to set the size of the block, while other
`protocols automatically \-'ar_\= the block size based upon the error rate experienced
`by the transmission progress.
`
`Error control
`
`The simplest method of error control does not actually ensure errors are corrected.
`This method of error control is known as echoplex and results in each character
`transmitted to a receiving device being sent back or echoed from the receiver to the
`transmitting device, hence, the term ‘echoplex’.
`the transmitting device
`Under
`the echoplex method of error correction,
`examines the echoed data. If the echoed data differs from the transmitted data
`an error is assumed to have occurred and the data must then be retransmitted.
`
`Since a transmission error can occur in either direction. it is possible for a character
`corrupted to another character durinig,
`transmission in one direction to be
`
`:
`
`

`
`120
`
`FUNDAMENTAL WIDE AREA NETWORKING CONCEPTS
`
`eorruptetl hack into its original hit form when echoed to the tr:-msmittingz tl(_'\'iL‘(':. in
`ntltlition, a L‘.i'11lI‘11CI€I'
`reeeivetl eorreetl_\‘ only he eorrttptetl tll.Il'iI‘t_s'_{ its echo. resulting
`in the I":-Ilse itnpressism that an L'I‘t't)r oL‘cL:ri‘e(l.
`Ee.hop|ex “"218 a popular method for detecting tralnsmission errors that was us-eel
`\\'itl)
`teletype terminals. This method of error Lleteetiran was also used in such
`message switching .s_\'s.te1‘ns as TV.-VX and is eurrently used with mz1n_\‘
`types of
`ttsynehronous trnrisniission. including pt-i'so11a| computers. Coneerning the latter.
`it PC Colnnttlliieating in L-1|‘! 215:_\'nCl1i'ormt1.s tiull-(luplcx I'nt)L'le to zmothttr lull-dttplcx
`computer will have each ehm':.ieter t1':1l'I:-ir‘I‘Il’ftL'Ll eehoetl hnel<.
`.‘a'inee the detection of
`erroneous t:l1ui'nt:tei's.
`i”1()\\'l'_'\'(_‘l', depends upon the \'lSLI2Il ae.euit_\-' ol‘ the operator
`more mo(le1‘n rnethods of L.'I'l'(_)I' detection :-Ind eoI'i‘eeti0n ltzive replttcecl the Llse of
`
`eCht)pl(‘X in zippliczititins where we camntit t'el_\' upon an tJpe1'ato1' to eoi‘1‘eL‘t (:I‘l't.)I'S_
`Thus.
`21
`l&1t‘,(_{e nLImlJt_‘.r of‘ file tI':'it1sfL1t‘ pI'otoeol.~': that group (_‘l1.'<iI'E1(:T.l.‘.’1'S
`into (him
`bloeks and zippentl ii t:heekin;_v meelttniisni m;-.re elevelopecl to uL:tomatieall_\' detect
`and eorI'et.“t trzmsmiss-ion errors.
`
`'l‘o{l:I_\'. the most t‘omI'i'1tml}' employs;-cl method to t:o1't'eL;1' tI‘imsn'1itte.Ll Crt'ot‘s is to
`infiorrn the ti':msi'nittin;_g devitre simpl_\'
`to retratnsmit
`-.1
`l3lU(.‘l{. This procedure
`requires eoorclin:-ttion l')t..'l'\\'L‘.t.'f'l
`the sending klntl
`reeeix-in;_: devices, with the
`rt-eei\'in,u device either eor1tinuousl_\' informing the seiitliitg tievitse of the status
`of each pre\'iou.~sly trainsrnitted hloelq or transmitting ii netgdtive ;1el\'no\\'Ietl,s,rement
`o1‘il_\‘ when 21 block is l‘Cl.‘(‘l\'(.‘('l
`in erroi‘.
`If the protocol used requires ;: re.-;ponse to each hloek Lintl the hloek pre\'ioL1sl_\-'
`T1"dl'l:-3lTlltI.'l'_'Ll eontztinetl no deteetecl errors.
`the rcL‘ei\'eI‘ will
`transmit
`21 positive
`
`If the recei\'er
`the next block.
`trmismit
`and the sender will
`2l(.‘l{l‘I()\\'le(lg€l’I'lI.311l.'
`detects an error. it will tl‘zI1"lSt1‘Iil a I‘I(:‘_E__{éltl\-‘t' ueltr1o\\'letlgement and disc:-trei the block
`containing: an error. The transmitting station will then retransmit the ])I‘(:\'lUL1Sl_\'
`sent block. Depending upon the protocol employetl. 11 number of retransmissitms
`rna_\' he attempted.
`l-lo\x'e\'er_.
`if d deftiult limit is reetelted owing to at bald circuit
`or other problems,
`then the eomptiter or terminal cleviee -.1;-ting; as the rnaster
`station ma._\' termin:-ate the session. and the operattor will have to re-establish the
`connection.
`
`If the p1'ott.}eol supports trzmsmission ol‘ 2'1 negattive aeknt>\\'let|;_{ement only when
`:1 block is reeeitetl in error, atltlitiomtl rules are required to go\'ern trztnsniissitm.
`As an example, the sending Ll(.'\'i(:L’ eotlld transmit .~;e\'em] blocks and. in fact, eouitl
`be transmitting laloek H + 4 prior
`to receiving;
`2! negzttixre aeltnowletlgement
`concerning hloek H. Depending upon the protoeol‘s rules, the transmitting device
`could retrtmsmit bl0(.‘l< J‘? and all hloelts after that block or finish transmitting hloek
`srz —:— 4, then transmit block is and resutne trzinsmission with hloek '1'?-l‘ 5.
`
`Types of protocols
`
`_\'t)\\' that \\'t: h2I\'e exatminetl protocol tiishs. let us Focus 01.11‘ eitterition upon the
`cltmzteteristies. operettion and utili-/.ntion tll‘.\‘€\'t.'I'1ll
`types of protocols that provide 2!
`Dretletined 5l§_.‘,‘l‘I;‘(".]’I‘It3H.1.' for the ortlerl_\' L‘XL'l‘L':‘1l1,.‘1L‘ ot‘ information. To facilitate this
`exatnittzttion we will start with an o\'ei‘\'ie\=\' oi" one of the simplest protocols in use
`amcl Stl'I.ICTuI'e our o\'er\'ie\\' ol" p:'otoe.olt-2 with respect to their C0mplexit_\'.
`
`

`
`1.15 THE DATA LINK LAYER _
`
`—
`
`121
`
`Teletype DFOIOCOIS
`
`'I‘e1et_\.vpe and ii»_1et_\-pe compatible terminals support i'elati\-‘ely simple protocols
`used for conveying information. In general, a teletype protocol is a line-b}'—linc
`protocol that requires no acknowledggeinerit of line rec.eipt. Thus, the l<c_\-' elements
`of this protocol define how characters are displayed and when a line is terminated
`and the next line is to be displa_\-'ecl. Some additional elements included in line by
`line tele.t_vpc protocols actually are part of the terminal protocol. since they define
`how the terminal should respond to specific control characters.
`
`Teletype Model 33
`
`One commonly used teletype protocol is the Teletype“ .\-Iodel 33 data terminal.
`This terminal trainsmits and receives data asynchronotisly on a line b_\_-' line basis
`using a modified ASCII code in which lo“-'er~case character received by a :\'[odel
`33 are actually printed as their upper-case equivalent, a term known as ‘fold—over'
`printing, Although the ASCII code defines the operation of 32 control characters,
`only 11 control characters can be used for communications control purposes, Prior
`to examining the use of communications control characters in the telct_\-'pe
`protocol, let us first review the operational Function and typical use of each control
`character. These characters were previously listed in Table 1.11 with the two—
`character designator CC folloxx-'ing their meaning and will be reviewed in the order
`of their appearance in the referenced table.
`
`Communications control characters
`
`NUL
`
`The null (_\'-UL) character is a non-printable time dela_\-' or filler character. This
`character is primarily used for communicating with printing dex-"ices that require a
`defined period of time after each carriage return in which to reposition the
`printhead to the IJI’.-’.gi!'1I‘1iI‘1g of the next line. In the early days of PC communications
`many mainframe computers would he p1'ogi‘arnmed to prompt users to ‘Enter the
`number of nulls’; this is a mechariisrn to permit electromechanical terminal devices
`that require a delay to return the print head to the first position on the next line
`without obtaining garbled output.
`
`SOH
`
`The start of heading (S011) is a communications control character used in se\-'eral
`character-oriented protocols to define the beginning of a message heading data
`block. In synchronous transmission on a multipoint or multidrop line structure.
`the SOH is followed by an address which is checked by all devices on the common
`line to ascertain if they are the recipient of the data. In asynchronous transmission,
`the SOH character can be used to signal
`the beginning of a filename during
`multiple file transfers, permitting the transfer to occur without treating each file
`
`&
`‘
`
`\
`
`E
`
`-
`
`I
`I
`
`:-
`.
`ti
`';
`I
`
`_'
`
`-|-
`
`_|
`ll;
`
`~_
`
`.,
`
`
`
`H:
`ll
`
`
`
`

`
`122
`
`FUNDAMENTAL WlDE AREA NETWORKING CONCEPTS
`
`ti'miste1' as a separate communications session. Since asynchronous communica-
`tions t_vpicall_V in\-‘ohre point-to-point comrnunications, no address is required after
`the SO]-l character; ho\\-ever, both devices must have the same communications
`software program that permits multiple rile ti'ansle.i's in this manner.
`
`8 TX
`
`The start of text (STX) character signilies the end of heading data and the
`beginning of the actual
`information contained within the block. This commu-
`nications control character is used in the bis_\'nchronous protocol
`that will be
`examined later in this section.
`
`ETX
`
`the
`The end of text {ETX} character is used to inform the receiver that all
`information within the block has heen transmitted and normally terminates a block
`of data started with an STX or SOH. This character is also used to denote the
`
`beginning of the block check characters appended to a transmission hlock as an
`error detection t‘necl1anism. This communications control character is primarily
`
`used in the bisynchronous protocol and its receipt requires a status acknowl-
`eclgement, such as an .-'—\CI( or N.-'-\l(.
`
`EOT
`
`The end of transmission (l7.()'T) character defines the end ol‘ transmission of all
`data associated with a message transmitted to a device. If transmission occurs on a
`multidrop circuit the HUT also informs other devices on the line to check later
`transmissions for the occurrence of messages that could be addressed to them. The
`HOT is also used as a response to a poll \-when the polled station has nothing to
`sencl and as an ahort signal when the sender cannot continue transmission. In
`the X3-'I()DE.\"i protocol the l:I()'l.'
`is used to indicate the end of a file transfer
`operation.
`
`ENG
`
`'|'hc enquiry (Ti-NQ) communications control character is used in the bis}-‘nchro—
`nous protocol to request a response or status from the other station on a point to
`point line or to a specil'lcall_\' addressed station on a tnultidrop line. In response to
`the ENQ character, the receiving station tnay respond with the number of the last
`lziloclt of data it successl'ull_\' received. in a multidrop environment, the mairifrarne
`computer would poll each device on the line by addressing the ENQ to one
`particular station at a time. Each station would respond to the poll positixrely or
`negati\'el_\'. depending upon whether or not they had information to send to the
`rnainframe computer at that point in time.
`
`

`
`1.15 THE DATA LINK LAYER
`
`ACK and DLE
`
`123
`
`The acknovrledgenient (ACK) character is used to verify that a block of data was
`received correctly. After the receiver computes its own ‘internal' ehecksum or
`cyclic code and compares it to the one appended to the transmitted block, it will
`transmit
`the ACK character if the two ehecksums match.
`ln the Xl\*IODEi\-"l
`protocol the ACK character is used to inform the transmitter that the next block of
`data can be transmitted. ln the bis_\-nchronous protocol the data link escape (DLE)
`character is normally used in conjunction with the 0 and 1 characters in place of
`the .-'5\CK character. Alternating DIJEU and DLE1 as positive acknowledgement to
`each correctly received block of data eliminates the potential of a lost or garbled
`acknox-vledgemcnt resulting in the loss of data. In some literature, DLEU and
`DLEl are referred to as ACKU and ACKI.
`
`NAK
`
`is
`(NAK) communications control character
`The negative acknowledgement
`transmitted b_v a recei\-'in,:__I device to request the transmitting device to rt-transmit
`the previously sent data block. 'l‘l'1is character is transmitted when the receiver's
`internally generated checksum or c_\'c.liC code does not match the one transmitted,
`indicating that a transmission error has occurred. In the XN1ODl.CV'l protocol this
`character is used to inform the transmitting device that the receiver is read)-' to
`commence a File transfer operation as well as to inform the transmitter of any
`blocks of data received in error. In the bis)-'nchronous protocol. the ‘.\7.»"-\K is also
`used as a station-not—ready reply to an ENQ line bid or a station selection.
`
`SYN
`
`The synchronous idle (SYN) character is employed in the bis_vnehronous protocol
`to establish and then rnaintain line synchronization between the transmitter and
`receiver during periods when no data is transmitted on the line. \-Vhen a series of
`SYN characters is interrupted, this indicates to the receiver that a block of data is
`being transmitted.
`
`E TB
`
`The end of transmission block (RTE) character is used in the bisvnchronous
`protocol in place of an ETX character when data is transmitted in multiple blocks.
`This character then indicates the end of a particular block of transmitted data that
`commenced with an SOH or STX character. A block check character (BCC) is
`sent after an BT13. The receipt of an ETB is ’f(illo\=\'e(l by an acknowledgement by
`
`the receiving device, such as an ACK or .=\J.'—\K.
`
`Information flow
`
`Figure 1.70 illustrates in a time chart format the possible How of information
`between a teletype compatible terminal and a computer system employing a basic
`
`'-
`
`I
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`
`_
`I
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`
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`
`.
`
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`124
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`FUNDAMENTAL WIDE AREA NETWORKING CONCEPTS
`
`Terminal
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`Compufey
`
`Figure 1.70 Basic teletype protocol
`
`telct}-‘pt: protocol. In this pr(_)t(>t.‘0l the terminal opoi'2itoi' might tirst transmit the
`ENQ chnracster, which is formed lay pressing tht: shift and E keys SiFT‘lLIlt3nC(}|.1Sl}-'.
`If the call originated over the PS"l"N,
`the ELVQ (‘.l1{ll';1t_‘!.'€1‘,
`in effect.
`tells the
`computer to respond with its st-.1tL1s. Since the computer is beginning its st-:r\'it:ing
`of 21 new connection request, it normally responds with a log-on message. This log-
`on niess-age can contain one or more lines of data.
`l.?{.} is pl‘f3l'l.‘{(.‘(] with 21
`The first
`line of the log-on message shown in Pi,_c,rL1re
`(ta:-ria,_r__r,e return (CR) line feed (LIV) sequence, \\-'hi¢:h positions the pi'intht:ad to the
`first column on at maw line prior to printing the data in the rccci\'r:d log-on inessztge
`line. The !og—on inessage line, as well as all
`followiniz, lines transmitted by the
`computer", will have Li CR LF suffix, in eftlrtrt, preparing the terminal for the next
`line of data. Upon receipt of the log—on mt:s:sag'e the tormimil operator keys in his
`or her lo,t.:—or1 code, which is E!'ElI‘I5-iYT|l‘l'.‘El.2(l to the computer as data followed by the
`CR l.l*' sulilix \\'hit:l1 [e1'I'nil1:itL:s the line enti'_\-'.
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`1.15 THE DATA LINK LAYER
`
`Variations
`
`125
`
`There are numerous variations to the p]'t:\-'i0I.lSl_\' discussed teletypewriter protocol,
`of which space permits mentioning only two.
`Some eornputers will not recognize an l_C.\‘-Q character on an asvnclironous
`.—\HCIl port. Those computers are normally programmed to respond to a sequence
`of two or more cat":-iage returns. Thus,
`the sequence ENQ LI" CR would be
`replaced by CR CR or CR CR (IR.
`\\-"ith the growth in popularity and use of personal computers as terminals, it was
`found that the time delay transmitted b_v computers in the form of null cliaracters
`to separate multiple lines of output from one another by time was not necessary.
`(_)i'i,;imill_v, the transmission of one line was separated through the use of NLII.
`characters by several character intervals from the next line. This separation was
`required to provide the electromechanical printer used on teletype terminals with a
`sulifieient amount of time to reposition its printhead from the end of one line to the
`l)i_'gil‘1I‘Ii1'i;;_ of the next line prior to reeeivin_r_{ the tirst character to be printed on the
`next line. Since a cursor on a video display can be repositioned almost instantly.
`the growth in the use of personal computers and video display terminals resulted in
`the rcrnoval oi‘ time delays between computer transmitted lines.
`'l‘odav, some computer software tlesignetl to service asynchronous terminals, as
`well as personal computers. will prompt
`the terminal operator with a message
`similar to: “I§N'l.‘lCR .\'l.TLl_.S (U T0 5)—“. This message provides the terminal
`operator with the abilitv to inform the computer whether he or she is using an
`eleetrtamt-chanical terminal. If (I is entered, the computer assumes the terminal has
`a Cl{"l" tlisplav and does not separate multiple lines transmitted from the computer
`by anything more than the sranclard CR L17 sequence. If a number greater than
`zero is entered, the computer separates multiple lines by the. use of the indicated
`number of null characters. The .\'L'L character, also called a PAD character, is
`considered to be a blank character which is discarded by the receiver. Thus,
`transmitting one. or more -.\'L'L characters between lines only serves to Phovide
`time for the. terminal's printh -ad to be repositioned to column 1 and has no effect
`upon the received data.
`lf you are accessing a computer system that assumes all
`users have CRT terminals or personal computers, more than lil<el_v the null
`message will not be displayed.
`friueh systems assume all users do not require a
`timing delay between transmitted lines and do not insert XUL characters between
`lines.
`
`Error control
`
`\\’h-at happens it" a line hit occurs during the transmission of data when a teletvpe
`protocol is used? L..'-nfortunatelv, the only error detection mechanisms emplo_ved by
`teletvpe terminals and computer ports that supports this protocol are parity
`t‘l‘Il.’t.'l<in}_{ and echople.\'.
`it may simply substitute and
`ll‘ parity checking is supported by the terminal,
`display a special error character received with a parity error. This places the.
`responsibility for error detection and correction upon the terminal operator, who
`must first visually observe the error and then request the computer to retransmit
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`126
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`FUNDAMENTAL WIDE AREA NETWORKING CONCEPTS
`
`.‘5imilarl_v. echoplex requires the operator to
`the line containing the parity error.
`visuall_\' note that the echoed character differs front the character key just pressed.
`As previousl_v discussed in Section 1.11, the response of a computer to a parity
`error can range from no action to the generation of a special symbol to denote the
`occurrence of a parity error. In fact, most asynchronous line by line protocols do
`not check for parity errors. These protocols use echoplex, which as previously
`explained can result in a false indication of a transmission error or the appearance
`that all is well even though a transmitted character was received in error.
`
`X.-MODEM protocol
`
`\7\-"ard Christensen
`The X.\-"IOIJlii_\"l protocol which was originally developed b_\_-‘
`has been implemented into many asynchronous personal computer communica-
`tions software programs and is supported by a large number of bulletin boards.
`Figure l.?1 illustrates in a time chart format the use of the X3-'IODEJ\-"I protocol
`for a file transfer consisting of two blocks of data. As illustrated, under the
`X.\"lODEi\'[ protocol the receiving device transmits a negative acknowledgement
`(.\7AK) character to signal the transmitter that it is ready to receive data.
`The .‘{t\-"lODl:I.\-'l protocol
`is a ‘receiver-driven’ protocol
`in that the receiver
`transmits a cliaraeter as a signal for the transmitter to start its data transfer opera-
`tion. L-inder the .‘{1\-"IOl)I*IM protocol the receiver has a 10s timeout. It transmits a
`N.'—\}{ each time it times out; hence, if the software on the personal computer that is
`to transmit a File
`is not set up to do so a period of Ills can transpire until
`transmission actually starts. In response to the NAK the transmitter sends a start
`of header (HOE) communications control character followed by two characters
`that represent the block number and the one’s complement of the block number.
`The block number used in the X.\'IODE.\-I protocol starts at 0'1, increments by
`'l_. and wraps from a maximum value of OFFI-I to UUH and not to 011-1. The one’s
`complement is obtained b_v sttbtraeting the block number from 255. Next a '128~
`character data block is transmitted which in turn is followed by the checksum
`eha 'acter. As previously discussed in Section 1.11, the cheeksum is computed by
`first adding the ASCII values of each of the characters in the l28—eharacter block
`and dividing the sum b_v 255. Next. the quotient is discarded and the remainder is
`retained as the I’.Tl‘t{.‘¢..‘l(SLII'|'l.
`
`If the data blocks are damaged during transmission, the receiver can detect the
`ocettrrence of an error in one of three \va_\'s. If the start of header is darnaged, it will
`be detected b_v the receiver and the data block will be neg-ati\‘el_v acknowledged. lf
`either the block count or the one's complement field are damaged, they will not be
`the one's complement of each other.
`l7inall_\',
`the receiver will compute its own
`cheeltsum and compare it to the transmitted checksum. If the ehecksums do not
`match this is also an indicator that the transmitted block was received in error.
`
`Each of the preceding situations results in the block being considered to have
`been received in error. Then the receiving station will transmit a t\'AI( character
`which serves as a request to the. transmitting station to retransmit the previously
`transmitted block. As illustrated in Fi_quI'e 1.71, a line hit occurring during" the
`transmission of the second block resulted in the receiver transmitting a .\lAl{ and
`
`

`
`1.15 THE DATQ LINK LAYER
`
`Rece,,,,ng
`computer
`
`127
`
`Transmitting
`C0"”P1-"9"
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`Time
`
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`Figure 1.71 XMODEM protocol file transfer operation
`
`the transmitting device resending the second block. Suppose more line hits occur
`\\'l1icl1 affects the retransmission of the second block.
`[.'-nder the X_\-lODE.\-"I
`protocol the retransrnissions process will be repeated until the hloek is correctly
`
`receix-‘edoruntiltenretransmissionattemptsoccur.If,owingtoathunderstormor
`
`other disturhance. line noise is a problem, after [0 attempts to retransmit a hlock
`the file transfer process will he aborted. This will require :1 manual operator
`intervention to restart
`the file transfer at
`the beginning and is one of many
`tleficieticitrs of the Xl\-'lOI)l€3-"I
`protocol. Other deficiencies of the X.\'lODE'i\-"I
`protocol
`include its
`relatix-'el_\-' small block size,
`its half—duple-2:
`transmission
`scheme, and its use of the checksum that provides a less reliable error detection
`cap-al)ilit_\-' in comparison to the ri