The Tl Carrier System
`
`By K. E. FULTZ and D. B. PENICK
`
`(Manuscript received May 11, 1965)
`
`Tl carrier pmvides 24 voice channels by time division multiple:~:ing
`and pulse code modulation (PC M ). Each voice channel is sampled
`8000 times a second and ea.ch sample is coded into a 7-digit binm·y
`word. P1·ovision for signaling and . synch1·onization mises the p'ttlse
`repetition rate on the Tepeatered line to 1.544 x 106 pulse positions
`pe1· second. The bipola1· pu.lse train out of the tenninals is tmnsmitted
`over pulp, pape1· or plastic insulated paired cables by the use of 1'e(cid:173)
`genemtive repeaters. For 22-gauge cable pai1·s, 1·epea.ters m·e no1'1nally
`located at 6000-foot intervals.
`The system has been designed fo r low cost and is being widely ap(cid:173)
`plied on many bi.tnks inte1·connecting switching units within metTo-
`1Jolitan aTeas. Westem E lect?-ic Company manufactu1·e of Tl began
`in 1962 and about 100,000 channels m·e now in service th1·oughout the
`Bell System.
`
`I. I NTRODUCTION
`
`The rapid expansion in the telephone network that has occurred
`since 1950 has st imulated a thorough investigation of methods for
`reducing t he cost of additional trunk facilities. T he desire to improve
`the quality of telephone service has given additional emphasis to
`studies of improved trunking arrangements. One way to obtain addi(cid:173)
`tional trunks for growth is to increase the utilization of existing con(cid:173)
`ductors by using them to transmit more than one voice signal. For
`such an arrangement to be economical, t he savings from t he more
`efficient use of t he transmission line must more than offset the cost of
`the terminal equipment required to multiplex a number of voice
`channels. On trunks between cities, carrier systems (systems t rans(cid:173)
`mitting a numbe1· of voice channels) have been economical for many
`years. The lower terminal co "ts achieved in t he Tl carrier system
`1405
`
`ARRIS883IPRI0001326
`
`

`
`TRANS(cid:173)
`MI TTING
`
`24
`CHANNEL f---~
`UNITS
`
`RECEIV(cid:173)
`ING
`
`I
`I
`CROSS(cid:173)
`CONNECTION
`
`TRUNK
`
`r TO 24
`CIRCU ITS l
`
`.... g
`
`..
`
`I
`I
`I
`
`I
`I
`
`POWER
`THROUGH
`
`POWER
`LOOPED
`
`POWER
`THROUGH
`
`I
`I
`CROSS(cid:173)
`CONNECTION
`
`----- -- ----- ------ ----- - -- - - -SPAN LINE ------- ---- - - ----- --- - - --- + ---TE RMINAL ____ ,
`~----- --TERMINAL OFFICE- -- - ----""'1
`
`REPEATER
`
`~ T
`/
`I
`j
`Acdss
`' I
`JACKS
`\
`I
`~ ~
`-'-~ t
`I
`~
`I
`( ____ _ _ _ _ _ ______ ___ ______ ____ _____ __ )
`I
`- - S ECTION - --t----SECTION -- - - t - - - SECTION ---t-----REPEATiR SECTION ------l 0 1 CHANNEL BANK r--
`I
`~
`~·"
`
`REPEATER
`
`REPEATER
`
`LJ -----ti---n----6---
`
`-r-
`
`RECEIV(cid:173)
`ING
`
`24
`f - ----1 CHANNEL
`UNITS
`
`1
`
`TO 24
`TRUNK
`C IRCUITS
`
`POWE R
`THROUGH
`
`POWER
`POWER
`THROUGH
`LOOPED
`Fig. 1 - Typical T l carrier system.
`
`IO
`
`)( )( I 0 I -f
`
`TRANS(cid:173)
`MI TTING
`
`I
`I
`CROSS-
`CONNECTION
`
`J
`
`ARRIS883IPRI0001327
`
`

`
`T l CA RRIEH
`
`1-!07
`
`have made carrier systems economically attractive for the longer
`trunks between local offices within a city. In a large number of situa(cid:173)
`tions the T1 carrier system will prove-in over voice frequency circuits
`for distances longer than 10 to 12 miles. Satisfactory performance is
`achieved over lengths up to 50 miles, and the performance over
`longer lengths is being evaluated.
`A major contributor to the low terminal costs in Tl is the economy
`with which the signaling information required to control the switching
`equipment can be transmitted in a digital system. In most carrier
`systems the digital signaling information is converted into analog
`tones for transmission. In a digital system the signaling information
`can be added directly to the coded speech samples with the saving in
`digital-to-analog conversion of the signaling information. Additional
`economies are achieved by an instantaneous compander shared by a
`number of channels rather than individual channel syllabic com(cid:173)
`panders as used in some carrier systems.
`The Tl carrier system now being manufactUl'ed by the Western
`Electric Company is a refinement of the experimental P CM system
`described in the J anuary, 1962, issue of this JoUl'naJ.l-5 The basic
`system plan and the fundamental circuit approaches remain un(cid:173)
`changed.
`It is convenient to consider a PCM system as being composed of two
`parts -
`a PCM terminal and a digital transmission line. For regular
`telephone trunks, the PCM terminal for the T1 system is the D1 chan(cid:173)
`nel bank. The D1 channel bank combines 24 voice channels in a time
`division multiplex and encodes them in a scale of 127 quantized
`amplitude levels (63 steps positive and 63 steps negative from zero)
`into a single pulse tr·ain. In the receiving direction, it reconstructs the
`analog speech signals from the incoming pulse stream. Other terminal
`anangements are being provided which prepare wideband data sig(cid:173)
`nals for transmission over T1 repeatered lines. These terminals are
`discussed in a companion paper.6
`The T1 repeatered line consists of cable pairs equipped with re(cid:173)
`generative repeaters at appropriate spacings. At the end offices, and
`at intermediate offices along the route, each repeatered line passes
`through an office repeater which provides a regenerator for the incom(cid:173)
`ing signal, powering circuits for the line repeaters, access jacks for
`patching, monitoring jacks, and cross-connection points for route
`flexibility. A block schematic of a typical T1 carrier system is shown
`in Fig. 1.
`
`ARRIS883IPRI0001328
`
`

`
`1408 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1965
`
`CL OCK SIGNAL
`
`___ J1JlJlJUU1IlflJ_~_
`
`D1 __ J
`D2 ___ j]
`
`I
`I
`I
`D6 ---
`
`DIGIT TIM ING SIGNALS
`n
`L . -__ __ _ _ ]L_______JL___J __ _
`n
`n L--- ---~---
`n
`.______,,..____n_ ___ _____ll___ll__ __ _
`n
`n
`
`CH 13 ---
`
`CHANNEL TIM ING SIGNALS
`
`_____ ___.n_
`
`CH 2 _______ ___JnL ___ _
`
`I
`
`C )24 - - - - - - -- - - - - - - - - ---__ IL__ __ _
`
`TYPICAL PAM SIGNALS AT ENCODER INPUT
`-l ENCODE If-
`
`~ENCODE 12fc-
`
`_,ENCODE 2f-
`
`GRI
`
`- ..jENCODE 13~­
`GR2-----~~========i_-
`
`- - ..jENCODE 24L.. __
`
`r- r---
`
`TYP ICAL CO MPOSITE SIGNAL AT CO N VERTER INPUT
`
`TYPICAL LIN E SIGNAL
`
`---~---~---
`
`Fig. 2 - Dl bank pulse trains used in multiplexing and encoding.
`
`ARRIS883IPRI0001329
`
`

`
`'l'l CARRIER
`
`1409
`
`ll. Dl CHANNEL BANKS
`
`:u Gr01tp and Channel Circuits
`
`Most of the transmission functions in a D1 channel bank are per(cid:173)
`formed in a block of circuits shared by a number of voice channels.
`These group circuits may be di\'ided into two sections - tmnsmitting
`a nd receiving. T he t ran mitting group equipment samples the incom(cid:173)
`for each channel, multiplexes the sample in time
`ing voice signal
`division, compre e and encodes the samples, combines the encoded
`sample with signaling in formation, and prepares the pulse train for
`transmi sion over the line. Fig. 2 shows the more important pulse
`trains involved in this process. The receiving group equipment accepts
`the incoming pul e stream, separates the signaling information from
`t he coded samples, decodes and expands t he speech samples, demulti(cid:173)
`plexes them , and reconstructs the voice signal. Thus, the group equip(cid:173)
`ment provides 24 voice channels plus 24 signaling channels in each di(cid:173)
`rection. Each signaling channel has a t heoretical capacity of 8 kilobits/
`second. In some situa tions - revertive pu lsing and foreign exchange
`lines- additional ·ignaling capabi lity i obtained by using the least
`::;igni ficant* speech digit when ::;pecch would not usually be present.
`The cha nnel units shown in F ig. 1 a re used to match the voice
`a nd signaling paths provided by tho group equipment to the require(cid:173)
`ments of t he individual switching circuits to which each channel is
`connected.
`A block schematic of the group circuits is shown in Fig. 3. Consider
`fi r t the tran mitting direction shO\m in the uppet· half of the sche(cid:173)
`matic. The tran mission circuits in heavy lines come in at the lefl
`side from 24 plug-in channel units not shown. Six channels connect
`to each of four transmitting gate and filter plug-in units. Each gnte and
`filter uni t contnin
`ix low-pac;;s filters and six sampling gates. The
`four gate and filter units a rc arranged in two pairs, a pair for each of
`two 12-cha nnel group . The ampling t imes of t he two groups are in(cid:173)
`terlea ved so that group 1 cha nnels are sampled at odd-numbered sam(cid:173)
`pling times a nd group 2 channels at even-numbered times. Thus the
`channel appear in the PA1\I (pulse amplitude modulated) pulse train
`in the order: 1, 13, 2, 14, · · · 11 , 23, 12, 24, 1, 13, 2, 14, · · ·.
`The common output of each group of twelve gates connects to its
`own compressor, which reduce a wide range of input amplitudes to a
`
`• The seventh digit of a seven-digit binar~· code is the least significant since it
`offeets the coded amplitude by onl~· I part in 128. T he first digit affects the am(cid:173)
`plitude by 64 parts in 128, the second by 32 parts, e tc.
`
`ARRIS883IPRI0001330
`
`

`
`SIG { I
`
`TRANS. f---
`
`TIMING -,_ e '
`(l; TRANS. i CHANNEL
`iU F ILTERS
`
`GATES&
`
`1-6
`
`COUNTERS
`1- 6 ~
`T -
`
`'
`
`PCM
`
`COMP.
`GROUP
`I
`CHANS.
`
`~
`TRANS.
`CONV.
`DUAL ~
`.!.
`ENCODER
`1-12 ~ ~ COMMON
`SIGNALING
`j----
`
`EN-
`CODER
`DRIVE
`
`'--
`l__
`
`I
`
`f--!-
`TPAMI
`I
`I ~ TRANS. 1--.-J
`! GATES.!.
`l ~ FILTERS
`7-12
`~ COON""
`1-- l
`
`:
`TRANS.
`;
`J--!- CHANNEL
`
`7-12
`
`~
`+-
`
`TRANS.
`
`COUNTERS
`
`GATES.!.
`
`f'
`
`_ 12 1
`
`TRANS-
`MISSION
`
`S IG.
`T
`IMING
`
`RANS-
`T
`
`Ml SSION
`
`.!.;:
`3
`.~ !
`(~
`TRANS. ": CHANNEL
`I ~ FILTERS ~ 13-16
`13- 16
`t====-J
`1 PAM I
`w
`
`I
`
`y DIGIT f
`
`ll
`I
`I -----
`.
`r- MASTER
`GENERATOR -
`
`CLOCK.!.
`FRAMING
`GEN-
`ERA TOR
`
`a: w
`
`~}!
`
`f*-~
`0 ....
`
`I ~
`TRAN S. 1--;-
`GATES.!.
`'
`' '
`l~
`FILTERS
`19-24 1--L
`
`L ,_
`
`COMP.
`GROUP.
`2
`CHANS.
`13-24
`
`1-1
`
`TRANS.
`CHANNEL
`COUNT ERS
`19-24
`
`"1.
`
`ALARM
`CONTROL
`CIRCUIT
`
`Ill
`
`.... z :::>
`
`...J w
`2
`2
`
`~ u
`
`<t
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`0 ....
`
`s
`IG.
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`{ 19
`24 !
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`NAL-
`SIG
`IN G
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`I
`
`TIM t'
`
`s
`IG.
`TIM lNG
`
`,
`6'
`
`.]4! {-1
`I (4 REC.
`I-ll ILTERS
`1-e
`·
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`l PAM4
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`REC. 1-o
`i
`GATES.!.
`F ILTERS
`7-12
`
`f-;-
`:
`I
`I
`1
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`(
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`: GATES 11.
`I F
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`NS-
`TRA
`ION
`MISS
`
`SIG. r
`
`: _!2!
`
`TIMIN G
`
`13
`
`TRANS(cid:173)
`MISSION
`
`14
`
`SIG.
`TIMING
`
`16 !
`(~
`1-;-
`REC.
`!
`GATES.!.
`I
`'
`I
`FILTERS
`I
`I ~
`j--L.
`13-16
`1 PAM 4
`I
`1-;-
`REC.
`GATES &
`I
`REC.
`' I
`FILTERS
`~~
`CHAN NEL
`19-24 ~
`COUNTERS
`{
`19
`19-24
`24!
`
`1
`
`REC.
`CHANNEL
`COUNTERS
`1- 6
`
`,__
`
`I
`
`REC.
`CHANNEL
`COUNTERS
`7- 12
`
`~
`
`-c=
`
`+-.!
`
`l
`DIGIT
`GENERATOR 1-
`-
`
`f.. FRAMING
`DETECTOR
`I
`
`---
`
`REC.
`CHANNEL
`COUNTERS
`13-16
`
`""""F=
`I
`
`f---'
`
`J I
`l
`~ REC.
`DECODER ~ CONV.
`~
`AND
`COMMON
`PCM
`SIGNALING
`
`PAM
`
`EX PANDOR
`
`l<'ig. 3- Dl bank group circuits.
`1<110
`
`ARRIS883IPRI0001331
`
`

`
`'1'1 CARRIER
`
`1411
`
`smaller range of output amplitudes in a n almost logarithmic rela(cid:173)
`tionship. An input range of approximately 60 decibels (1000-to-1 ampli(cid:173)
`tude ratio) is reduced to an output range of 63-to-1 amplitude ratio in
`a modified loga rithmic-to-linear conversion, so that the output varia(cid:173)
`tion in volts amplitude is approximately proportional to the input vari(cid:173)
`ation in decibels over most of the range. The two compressor outputs are
`connected to t he dual encoder. It · two summing amplifiers and com(cid:173)
`parison networks, under the logic control of the encoder dl'ive unit,
`encode the two pulse trains alterna tely into a single strea m of PCM
`pulses.
`This unipolar PCM signal, occupying seven of the eight pulse po(cid:173)
`sitions assigned to each channel sample, is one of three signals fed
`into the transmitting converter and common signaling unit. A second
`signal is processed by the common signaling portion of this unit, which
`accepts a signaling pulse from the scanning gate of each of the 24
`channel units in turn , reshapes each one, and times it to interleave with
`t he P CM pulses in the unipola r train. A third signal entering t his unit
`is a framing signal from the framing generator, occupying a single
`pulse position per frame. Thcr<e three signals, added together in t he
`converter, form a combined pul c t rain of 193 pulse positions per
`frame. This number, multiplied by the frame repetition rate of 8000
`per second, yields the basic pulse repetition rate of 1,544,000 per second.
`As a final step in the converter processing before the pulse train is
`sent out over the repeatered line, each pulse is regenerated and alter(cid:173)
`nate pulses, when t hey appear, are inverted to form a bipola r signal.
`This signal, t hen, is transmitted to the line by way of the alarm con(cid:173)
`trol unit located on the receiving shelf.
`Timing for the signal processing circuits is derived from a crystal(cid:173)
`controlled oscillator, a part of t he master clock. The oscillator out(cid:173)
`put, shaped into square-topped pulses, each occupying about one half
`of its allotted time interval, drives a digit genera tor which is basically
`a ring counter composed of blocking oscillators. Each of eight stages
`sends out one of eight successive digits on a lead per digit for use as
`required in encoding and other timing functions. A second lead per
`digit is also provided for digit pulses of opposite polarity. A ninth stage
`provides a ninth pulse at the end of each frame for fram ing control in
`conjunction with the fra ming generator included in the master clock
`unit.
`Digit pulses, in turn, drive a set of channel counters which provide
`timing for both voice sampling gates and signaling scanning gates.
`As in the case of the digit generator, t he counter stages are blocking
`
`ARRIS883IPRI0001332
`
`

`
`1412 THE BELL SYSTEM TECHNICAL J OURNAL, SEPTEMBER 1965
`
`oscillators. They are turned on in rotation by one digit pulse and
`t urned off by another. Each counter unit accommodates six stages, so
`that for a completely equipped D l bank, four units are required. The
`circuits are arranged, however, so t hat the two units associated with
`the group 1 channels form a 12-stage ring counter which is self-sus(cid:173)
`taining. The two units fot· the group 2 channels are separately driven
`from the ring, and may be omitted in a partially equipped bank with(cid:173)
`out disturbing the group 1 operation. Some of the functions of the
`group 1 countet·s are not required for group 2, so a separate network
`code, simpler and less expensive, is provided for group 2 only. The
`group 1 counter will also operate in group 2 positions, and therefore is
`conveniently used as a spare.
`The interconnections of t he plug-in units which make up the re(cid:173)
`ceiving portion of the Dl bank are shown in the lower half of Fig. 3.
`The combined pulse train from the distant terminal, transmitted over
`the repeatered line and t hrough the local office repeater, is received by
`the alarm control circuit at t he right side of the schematic. Reduced
`by a pad to a convenient amplitude, it is sent into the receiving con(cid:173)
`verter and common signaling unit. At this point the pulse t rain is re(cid:173)
`converted to unipolar form, regenerated, and impressed simultaneously
`on framing, signaling, and PCM circuits. These circuits time-select
`appropriate pulses from t he combined pulse train for further processing.
`The PCM circuit connects to the decoder, which scans the seven
`pulse positions allocated to each sample and synthesizes from the
`code the compressed sample amplitude for the corresponding PAM
`pulse. T he resulting train of PAM pulses passes through the expandor,
`which restores the original, uncompressed amplitudes and tt·ansmits
`them to t he bank of receiving gates. The gates, operating one at a time
`in rotation, route each PAM pulse through an individual low-pass
`filter to the receiving branch of its associated channel unit.
`The signaling pulse associated with each seven-pulse code at the
`conver ter output is selected by t he common signaling timing, is am(cid:173)
`plified to a suitable pulse amplitude and duration, and is passed to
`the bank of receiving signaling gates in the channel units. The gate
`in the appropriate channel unit transmits the individual pulse in each
`frame to its corresponding amplification and reconstruction circuit ,
`also in the channel unit, and reproduces the signaling state correspond(cid:173)
`ing to t hat which was scanned at the distant terminal for that channel.
`Timing for the receiving circuits is very similar to that for the
`transmitting circuits except that the clock signal, instead of originating
`in a crystal-controlled oscillator, is derived from t he incoming pulse
`
`ARRIS883IPRI0001333
`
`

`
`'1'1 CARR IER
`
`1413
`
`train itself acting on a tuned circuit resonant at the expected bit rate.
`The dissipation of the tuned circuit is low enough so that oscill a(cid:173)
`tion of the slave clock is maintained over moderately long blank pe(cid:173)
`riods in the incoming pulse train. The clock signal, produced in the
`converter as part of the pulse regeneration process, also drives a digit
`generator, a duplicate of the one in the transmitting circuit.
`D igit pulses, as in the transmitting circuit, drive channel counters
`which time both the transmi sion gates and signaling receiving gates
`associated with the individual channels. Also, as in the t ransmitting
`circuit, a framing pulse is produced at the end of each frame as deter(cid:173)
`mined by the state of the channel counters. Thus, the bit rate, digit
`pulse rate, channel rate, and frame rate are identical with those in the
`transmitting circuit. Synchronism, once achieved, is therefore main(cid:173)
`tained indefinitely as long as the incoming pulse train is not inter(cid:173)
`rupted.
`Restoration of phase synchronism, or framing, after an interrup(cid:173)
`tion is accomplished under the control of the framing detector. This
`unit receives the framing signal generated in the receiving timing cir(cid:173)
`cuits and compares it with the corresponding signal in the incoming
`pulse train. The framing signal is a fixed pattern consisting of alternat(cid:173)
`ing ones and zeros in every 193rd pulse position, a pattern seldom du(cid:173)
`plicated for more than two or three frame intervals at a time in any
`other pulse position. When the framing detector comparison indicates
`a number of rapidly occulTing differences between t he received pat(cid:173)
`tern and the local framing signal , a logic circuit starts a hunting ac(cid:173)
`tion by inserting an additional pulse per frame in the local signal, thus
`comparing the local framing signal with each pulse position in turn of
`the incoming signal until the framing position is reached. When the
`two patterns match, t he system is in frame and the hunting action
`ceases.
`As noted earlier, t he function of the channel units is to match t he
`24 sets of voice and signaling paths to the 24 individual trunk circuits
`to which they are connected at each end. A channel unit may provide a
`4-wire terminating set and sign aling conve1·ters for connecting conven(cid:173)
`tional de signaling to the carrier derived signaling channels or may
`connect the voice paths directly to a 4-wire trunk circuit. Instead of
`making numerous cross-connections at intermediate distribution frames
`to interconnect the specific terminating equipment required to imple(cid:173)
`ment a circuit order, a channel unit with the appropriate functions is
`selected and inserted in the carrier bay.
`The use of channel units solely for matching the conditions on the
`
`ARRIS883IPRI0001334
`
`

`
`1414
`
`'l'HE BELL SYSTEM 'l'ECHNICAL J OUUNAL, SEP'l' EMBER 1965
`
`2 -WIRE
`CHANNEL UNIT
`
`TO TRANSMITTING
`COUNTER
`
`,- ------
`
`~~-'-~-:_-!--•
`TO
`,
`TRANS M ITTING
`COMMON
`SIGNALING UNIT
`TO
`r------*"~!--------1-- TRANSMITTING
`- 9SL
`GATE
`
`1
`
`I
`I
`I
`
`I
`I
`I
`I
`I
`L __ ___ __ _
`
`+3SL
`
`TO RECEIVING
`COUNTER
`
`TO
`RECEIVING GATE
`
`TO RECEIVING
`COMMON
`SIGNALING UNIT
`
`TO TRANSMITTING
`COUNTER
`
`I K- TRANSMITTING
`
`TO
`
`COMMON
`SIGNALING UNIT
`
`4 -WIRE
`CHANNEL UNIT
`,- -- ~--------- ± --~
`I
`I
`(_:..:M"-r--------------~• SCANNING
`GATE
`I
`I
`
`TO
`4-WIRE
`TRUNK
`CIRCUIT
`
`- 16
`
`r ~/
`I
`<J
`+7 I
`c~:
`I
`l E I
`I L _ __ _
`
`)(
`)(
`- 9S L
`
`I • TRANS~~TTING
`
`GAT E
`
`I
`I
`I
`I
`
`TO RECEIVING
`GATE
`
`TO RECEIVING
`CO MMON
`SIGNALING UNIT
`
`:/s)~ vlv
`
`_
`
`_
`
`_
`
`TO RECEIV ING
`COUNTER
`
`Fig. 4- Typica.I D l bank channel circui ts.
`
`ARRIS883IPRI0001335
`
`

`
`'PI CAR RIER
`
`141 5
`
`voice frequency inputs to the carrier channels is quite different from
`the fun ction of channel units in most frequency division multiplex
`(FDM) carrier systems. FDM channel units usually include filters
`which are different for each channel in a system. There is no difference
`in the Tl carrier channel units with respect to t heir position in the
`time division cycle. The different types of channel units required to
`meet local circuit necct may be intermixed in a channel bank in any
`order.
`The two major types of channel units a rc the two-wire and th€
`four-wire types as shown in the block schematic of Fig. 4. The two(cid:173)
`wire channel units include a hybrid coil used as a terminating set.
`They a lso include transmitting and receiving access jacks for lineup
`use, a level adjusting pad, and two fixed pads, one of which may be
`strapped out. These elements constitute the transmission circuit and
`are the same for all two-wire units.
`The four-wire unit transmission circuit does not require a hybrid
`coil, but provides an amplifier and an access jack in each direction of
`transmission, as well as a level adjusting pad in t he receiving direc(cid:173)
`tion. The amplifier gains are adjusta ble over a range of about 1.5 db
`each for overcoming office wiring losses and are arranged to pmvide
`the nominal levels of - 16 db and + 7 db respectively, within 0.2
`rib, at the channel unit when t he gain adjustments are turned to mini(cid:173)
`mum.
`The basic signaling functions for all channel units are the same. At
`the transmitting end in each direction, a scanning gate monitors the
`signa ling state presented to it and converts it to a stream of corre(cid:173)
`sponding signaling pulses, off or on, for transmission to t he receiving
`end. There, a selecting gate recognizes the pulses, amplifies each one,
`and operates a reconstruction circuit which produces the signaling
`state con-esponding to that scanned at the transmitting end. The dif(cid:173)
`ferences between channel units lie in the methods required to trans(cid:173)
`late the varying signaling states to pulses and reconstruct them again
`from pulses.
`The most commonly u ed types of trunks in the exchange plant,
`which Tl carrier is designed to provide, are one-way trunks with either
`dial pulse or revertive pulse signaling and reverse battery supervision.
`The dial pulse signaling functions are quite straightforward. Loop
`closures are transmitted in the originating-to-terminating direction
`and battery reversals in the terminating-to-originating direction. In
`both directions, the digit 1 position in the train of eight pulses per
`channel is used to transmit the required information. Since the scan-
`
`ARRIS883IPRI0001336
`
`

`
`1416
`
`'l'HE BELL SYS'l'E M 'l'ECH NICAL J OURNAL, SEP'l'EMBE R 1965
`
`ning gate requirements and relay requirements are quite different for
`the two directions, it is convenient and economical to use different
`designs for the originating and terminating channel units.
`The same design basis applies also for revertive pulse signaling.
`Here the loop closures and loop opens in the originating-to-terminating
`direction represent start and stop signals, respectively. In the terminat(cid:173)
`ing-to-originating direction, it is necessary to transmit both battery
`reversals for supervision, and loop closures for the revertive ground
`pulse during dialing periods. The second signal in this direction re(cid:173)
`quires an additional scanning gate in the terminating unit and an ad(cid:173)
`ditional selecting gate, amplifier, and reconstruction circuit in the orig(cid:173)
`inating unit.
`It also requires another signaling state, provided by "borrowing"
`another digit in addition to the digit 1 normally provided. Since the
`added digit is not needed for signaling during the normal talking
`period, digit 8, the least significant of the 7 PCM digits, is used and is
`returned to the PCM function as soon as the called customer returns
`the normal supervisory signal. One result of this arrangement is that
`operator connections, or others which do not return supervision, will
`have only 6 PCM digits available for transmission. These added
`functions, of course, require two additional channel unit designs, one
`each for originating and terminating units.
`A demand for foreign exchange trunk service over Tl bas in(cid:173)
`spired the design of two more channel units, which are now available.
`They connect the line circuits at the serving office end and customer
`end, respectively. All three available signaling states are used in both
`directions of transmission. In the serving office-to-customer direction,
`a tip ground signal and a ringing signal are transmitted. In the cus(cid:173)
`tomer-to-serving office direction, a loop closure signal and a ring
`ground signal are transmitted.
`The four-wire channel unit is designed for symmetrical two-way
`trunks with identical signaling in the two directions. In either direc(cid:173)
`tion, ground and battery on the M lead at the transmitting end become
`open and ground, respectively, on the E lead at the receiving end.
`Thus, the same design of channel unit is used at both ends of such
`trunks.
`The four-wire channel unit may also be used with existing trunk
`converter circuits to connect to any of a large number of other types of
`trunks for which specific channel units have not been provided.
`It is also feasible to use a four-wire channel unit at one end of a Tl
`carrier circuit and a two-wire unit at the other end to avoid the use
`of a converter, which in some cases would otherwise be required.
`
`ARRIS883IPRI0001337
`
`

`
`Tl CA RRIER
`
`1417
`
`2.2 Bay
`
`The basic cha nnel bank bay is 11 feet, 6 inches high and 23 inches
`wide. It mounts three Dl banks, each associated with one 24-channel
`system, with their associated power supplies. Fig. 5 is a photograph
`of a typical installation showing an unequipped bay and a working
`bay filled with plug-in units. The unequipped bay consists of a sup(cid:173)
`porting fra mework, die-cast metal shelves for t he plug-in units, multi(cid:173)
`pin connectors, and terminal strips. The termina l stri ps and connec(cid:173)
`tors, including special screw connectors for hanger-mounted power
`supply panels, are prewired and fully tested at the factory. A 9-foot
`bay mounting two Dl banks and a 7-foot double bay mounting three
`Dl banks are a lso available.
`The cost of these unequipped bays is comparable to the co t of
`engineering and installing them. Since the engineering and in talla(cid:173)
`tion costs per bay are lower when a number of bays are in talled at a
`time, it is economical to in -tall more bays than a re required imme(cid:173)
`dia tely. At installation, the voice frequency connections arc wired to
`the distributing frame, the 1.544-megabit digita l leads are extended to
`a 1.544-megabit cross-connect field on either the office repeater bay or
`a separate bay, and the - 48 volt power leads are connected to the
`battery supply. When traffic requirements materialize, the more ex(cid:173)
`pensive plug-in units may be inser ted in t he carrier bays. At that time,
`office personnel will cross-connect t he additional carrier-derived voice
`channels to switching trunk circuits and cross-connect the outgoing
`and incoming digital circuits to the appmpriate repeatered line.
`
`2.:.1 Plug-I n Um.ts
`
`The active circuits are of three general clas es: power supply, group
`ti ming and proce sing circuit , and channel units. The power upply
`con i ts of a de- to-de conver ter and regulator . It provides well reg(cid:173)
`ulated voltages of -24 v, + 24 v, - 42 v and +48 v from t he -48-volt
`offi ce battery. In general, the lO\\·er voltages supply t he digital circuits
`and the higher voltages supply the de stabilized ana log circuits. The
`de-to-de conver ter a nd the analog voltage regulators serve a ll th ree
`Dl banks on a bay, but each Dl bank has its individua l regulator for
`the digital circuit voltages.
`The group equi pment pl ug-in units are eight inches high a nd eight
`inches deep. One shelf, mounting fifteen units, i devoted to t rans(cid:173)
`mitting and a second shelf, mounting fourteen units, is used for re(cid:173)
`ceiving. Thus the group equipment for one Dl ba nk consists of 29
`plug-in units moun tecl in a bout 16 inches of vertical bay space.
`
`ARRIS883IPRI0001338
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`

`
`1418 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1965
`
`Fig. 5- Typical Dl bank installation.
`
`ARRIS883IPRI0001339
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`

`
`Tl CAHR IER
`
`1-4- 1!)
`
`The 24 channel units in each Dl bank are mounted in two rows of
`twelve. Each channel unit is a bout 6 inches high, 8 inches deep and
`1% inches wide. A photograph of a set of group and channel plug-in
`units in place for one Dl bank is shown in Fig. 6. Three typical plug-in
`units are shown in Fig. 7.
`While the advantages of compactness are recognized, no major com(cid:173)
`promi es were made in the equipment design for extreme miniaturiza(cid:173)
`instead on high reliability, design for
`tion. Empha is was placed
`
`Fig. 6- Group nnd channel units in place for one Dl bank.
`
`ARRIS883IPRI0001340
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`

`
`1420 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1965
`
`mechanized assembly, mass soldering capability, and accessibility for
`inspection and repair. The book-case arrangement of units, however,
`uses the available volume efficiently.
`One of the early choices in the arrangement of circuits and equip(cid:173)
`ment was the consideration and rejection of the use of larger numbers
`of small, universal circuit packages such as gates, flip-flops, and
`blocking oscillators. The prospect of high production rates for rela(cid:173)
`tively few types of basic modules is economically attractive. Analy(cid:173)
`sis of the circuits showed, however, that each type of circuit block re(cid:173)
`quired so many variations for different points of application t hat the
`economies inherent in high production rate could not be realized. In
`t he CW'rent design, t he circuit packages are made large enough to in(cid:173)
`clude all of the specialized circuit blocks required to perform a larger
`circuit function. A digit generator, for example, uses nine similar
`blocking oscillators which operate in rotation. Several variations in
`these stages, however, would preclude using nine identical blocking os(cid:173)
`cillator packages unless each included all of the variations.
`
`III. REPEA TERED LINE
`
`3.1 Span Complements
`
`The digital transmission line, or repeatered line, extends from ter(cid:173)
`minal to terminal of a system, and consists of two cable pairs equipped
`with repeaters for t he two directions of transmission. The administra(cid:173)
`tive line unit is a span line extending between office repeaters. A span
`line is composed of a number of repeater sections permanently con(cid:173)
`nected in tandem at repeater apparatus cases mounted in manholes
`or on poles along t he span. A span is defined7 as the group of span lines
`which extend between two office repeater points. The repeatered line of
`the typical system of Fig. 1 is composed of two span lines, each of
`which happens to have four repeater sections.
`Span lines are engineered, cable pairs assigned, and repeater mount(cid:173)
`ing arrangements provided in multiples of 25-line complements. In one(cid:173)
`cable installations (both directions of transmission in the same cable
`sheath), each set of apparatus cases along the cable serves a 25-line
`complement. In two-cable operation (the two directions of trans(cid:173)
`mission in separate cable sheat hs), two sets of apparatus cases, one for
`each cable, serve two 25-line complements. A single shop-wired re(cid:173)
`peater bay provides mounting arrangements for office repeaters for one
`end of three 25-line complements.
`
`ARRIS883IPRI0001341
`
`

`
`Tl CARRIER
`
`1421
`
`Fig. 7- T~·pical Dl bank plug-in uni ts.
`
`Messrs. Crater and Cravis7 hnve discussed the factors involved in the
`selection of cable sheaths and the spacing of apparatus cases.
`The functions of the office repeater are to feed power to the re(cid:173)
`peatered line, regenerate the low-l