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Telephony Basic Information and

Terms1. Switching Information2. Carrier Information

3.  Telephone Information

4. Switchboard Plug

5. SxS "Local Call" Overview

6. SxS "Local Call" Details

7. SxS "SD" Numbers

8. Links 

o Did Alexander Graham Bell plagiarize the solution for a telephonetransmitter from Elisha Gray? It seems so.

 Switching Information

Name Description

 

OperatorPlug Board

• totally manual 

• when you went off-hook, a light bulb associated with your jackwas lit. Sometimes there would also be an associated buzzerbecause, in the very early days (or in rural locations), these plugboards were sometimes installed in the front of a house wherethe operator lived and the operator wasn't always sitting at theplug board.

• the operator plugged into your jack to find out what you wanted

• you instructed the operator to connect to so-and-so (if she wasbored, she usually listened in on your call)

• when you were finished, you went back on hook and she pulledout your plug when she noticed that the light was out

Panel • tip and ring wipers move up and down a contact panel on brass

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rods

• you've got to see it in order to believe it

Drop

Relay

• an early Step by Step system with no "line finder"

• when a customer went off-hook, a relay latch dropped to providethem dial-tone etc.

• when they went back on-hook, they could not make another calluntil a telephone office attendant manually restored the latch

SxS • Step by Step

• a telephone switch associated with the first "dial" systems

• Click SxS Details for more information

•

most of these systems never supported touch-tone phones, butthey did support digi-pulse phones.

• North American average installed life: 40 years

• http://en.wikipedia.org/wiki/Stepping_switch

5XB • Number 5 Cross Bar (for local switching; Number 4 was for toll-tandem use)

• a telephone switch employing a huge matrix of connectedcrossbar switches (which are also matrices of contacts)

• typically, eachcrossbar switchwas composed of twenty verticalpaths "in" andten horizontalpaths "out". If you wanted a more orthogonal matrix, you could stack(electrically speaking) a second one on top of the first making it20h x 20v.

•  To close a set of contacts, one of ten select magnets operated

one of five horizontal bars either up or down (the centerposition was neutral) which moved a metal enabler spring intoposition. This action was followed by the operation of one of twenty vertical hold magnets which forced the metal spring toclose three, or more, contacts. Now the select magnet is released(to set up the next connection) while the hold magnet maintainsthe existing connection until the customer disconnects.

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• the crossbar switches were controlled by an electro-mechanicalcomputer known as a marker (named because they "marked outa path " through the central office)

• most of these systems supported both touch-tone and dial

phones• North American average installed life: 20 years

• http://en.wikipedia.org/wiki/Crossbar

ComputerizedCircuitSwitching,AnalogCircuits

• SP1 (S-P-One)

o Stored Program 1

o a cross bar telephone switch manufactured by NorthernElectric (Nortel) with a computerized front-end rather thanan electro-mechanical front end (anyone remember

markers?). These machines employed a mini-crossbartechnology to complete the analog circuit.

o North American average installed life: 10 years

• 1ESS 

o a.k.a. ESS 1, No. 1 ESS

o ESS = Electronic Switching System

o developed by AT&T

o electronic switching but analog circuits

ComputerizedCircuitSwitching,DigitalCircuits

• DMS-100 

o An all-electronic telephone switch manufactured byNorthern Electric (Nortel)

o North American average installed life: ?? years (manyinstalled in 1980 are still running in 2009 because therewere many in-place upgrades)

o http://en.wikipedia.org/wiki/Digital_Multiplex_System

• DMS-200 

o A toll (long distance) version of DMS-100

• DMS-10 

o A small community version of DMS-100

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• DMS-1 

o A 256-1-256 line concentrator

• 5ESS 

o a.k.a. ESS 5, No. 5 ESS

o ESS = Electronic Switching System

o full digital switch

o run by the UNIX operating system

PBX • Private Branch Exchange - a small telephone switch associatedwith a business

• SL1 (S-L-One) is the PBX version of SP1

CO • Central Office (a.k.a. telephone exchange, telephone switch)

• usually serves between 10,000 and 100,000 lines

CDO • Community Dial Office

• a rural telephone exchange

• usually serves between 100 and 2,000 lines

VoIP • Voice over Internet Protocol 

• An packet based communications technology that rides on theinternet

• Note that most analog technologies employ circuit switchingwhile VoIP is based upon packet switching

• In the 1960's the Americans (ARPA + DARPA) funded researchwhich resulted in a self-healing packet network which couldsurvive natural disasters or a nuclear attack. ARPAnet mergedwith NSFnet to become the Internet. Since circuit switching wasbased upon common control solutions (telephone switches),problems would occur whenever the switch became

incapacitated. In a properly designed packet network theintelligence is moved into the network's routers. When thenetwork detects an internal problem, it just routes the packetsaround the obstacle.

Carrier Information (only a few commontechnologies are listed)

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Name

Description

L1N1

N2

• analog carrier used between switching centers

• N carrier employed vacuum tubes

• http://en.wikipedia.org/wiki/L-carrier

 T1 • supports 24 channels - each one capable of passing 64 kb/s (8 kb/s pervoice channel)

• popular in North America

• 24 * 64000 * (193 / 192) = 1.544 Mb/s

• http://en.wikipedia.org/wiki/T-carrier

E1•

European version of T1

• supports 32 channels each capable of passing 64 kb/s

OC1 • fiber based

• SONET (Synchronous Optical NETwork) lines capable of sending 51.840Mb/s

• http://en.wikipedia.org/wiki/Synchronous_optical_networking

ATM • although not usually thought of as carrier technology, this may changewith VoIP

• Asynchronous Transfer Mode

• LANE (LAN Emulation) is an ATM technology designed to make LANs(local area networks) more dependable

• http://en.wikipedia.org/wiki/Asynchronous_Transfer_Mode

•  This technology was designed to make up for the short comings of LANs. For some reason, network equipment manufacturers wouldrather sell you MPLS (multi protocol label switching)

LAN•

although not usually thought of as carrier technology, this may changewith VoIP

• Local Area Network

• Ethernet, Token Ring, etc.

• http://en.wikipedia.org/wiki/Local_area_network

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• works OK sending email and web pages but not a serious contenderuntil technologies like ATM are employed

o for example, when packet delays cause VoIP problems on anEthernet, installing an additional second path or increasing the

speed of the existing path is not the answer; you can replacehubs with switches and even add MLPS based solutions butthese are just tweaks to a technology that was designed to livewith packet delays

Telephone Information

Name Description

 Tip • Name of an analog telephone wire. Usually measures ground

when on-hook.• Gets its name from the tip conductor of a manual operator's

three conductor plug (tip of the plug)

• Usually a GREEN wire in the home of a single party customer

• click: plug diagram

Ring • Name of an analog telephone wire. Usually measures -48 Voltswhen on-hook

•

Gets its name from the ring conductor of a manual operator'sthree conductor plug (a small ring of metal)

• Usually a RED wire in the home of a single party customer

• Also receives the 80 Volt AC (20 Hz) ringing signal on a singleparty line

• click: plug diagram

Sleeve • Name of an analog telephone wire used for supervision in non-electronic systems

• Gets its name from the sleeve (third) conductor of a manualoperator's three conductor plug (a long metal sleeve)

• click: plug diagram

• battery (-48 volts) signifies "line idle"

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• ground (0 volts) signifies "line busy"

WallWires

• Green (tip #1)

• Red (ring #1)

•  Yellow (tip #2 or spare)

• Black (ring #2 or ground)

SinglePartyPhoneWiring

•  Tip Wire (green)

• Ring Wire (red)

• Ground (black) - not required but should be connected forsafety

• Bell Wire (yellow) - not used

• Note: only two wires, tip + ring, actually connect back to thetelephone exchange

 TwoPartyPhoneWiring

• Most phones come pre-wired for single party operation.Internally, the BELL will usually be wired across the TIP andRING terminals. When converting a phone to party line use(very rare this side of y2k), this internal BELL wiring must bemoved to the BLACK and YELLOW terminals.

o When a RING party is alerted to an incoming call, ringing

signal will be applied across the RING wire and GROUND.o When a TIP party is alerted to an incoming call, ringing

signal will be applied across the TIP wire and GROUND.

• Ring Customer

o  Tip Wire (green)

o Ring Wire (red)

o Ground (black) - mandatory

o Bell (yellow) - connect to Ring Wire

•  Tip Customer

o  Tip Wire (green)

o Ring Wire (red)

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o Ground (black) - mandatory

o Bell (yellow) - connect to Tip Wire

o If the touch-tone keypad does not work, reverse the tip

and ring wires to the phone (some electronic phonesmay never work in party-line situations)

• Note: only two wires, tip + ring, actually connect back to thetelephone exchange. The BELL return path is made throughground (a.k.a. earth)

 

OnHook

• An open circuit - no DC electricity is flowing. (but an ACconnection exists for ringing the BELL)

•

 Typical Ring: -48 volts (battery)

•  Typical Tip: 0 volts (ground)

Off Hook

• A closed circuit - DC electricity is flowing.

•  Typical Ring: -30 volts to -35 volts

•  Typical Tip: -15 volts to -20 volts

• Lowest Current: 15 mA

•

 Typical Current: 40 mA

PulseSignaling

• when you use a rotary dial the signal the number "9", the DCpath of the telephone is interrupted 9 times. When you dial "0"it is interrupted 10 times.

• contact time: 40% make, 60% break

• contact speed: 10 pulses per second

• minimum IDT (inter digit time): 600 mS

• Digi-pulse signalling converts the button you pressed into theappropriate number of DC interruptions.

 ToneSignaling

• Also called DTMF (Dual Tone Multi Frequency) signalling. Thereare 3 tones associated with the vertical key columns and 4tones associated with the horizontal key rows. Therefore,

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pressing any key will result in the transmission of two tones.

• http://en.wikipedia.org/wiki/Dual-tone_multi-frequency

Ringing

Signal

• usually 85 Volts AC at 20 Hz

• usually 2 seconds of ringing followed by 4 seconds of silence(this means that a phone will ring 10 times per minute)

• interrupted by going "Off Hook" (which establishes a DC pathfor the -48 Volts)

SleeveSignalling

• a 3-wire circuit is made to behave like a 6-wire circuit

• high current is placed upon the sleeve (200 ohms via -48Volts)

•

trunk dialing occurs over the tip + ring (like operating a testconnector)

• the sleeve state is switched to low current (1200 ohms via -48Volts)

• the trunk is now held up while the tip + ring are extendedthrough to the customer's line for testing

BusySignal

• 60 IPM (Impulses Per Minute)

• Also called Busy Tone

All TrunksBusy

 Tone

• 120 IPM (Impulses Per Minute)

Battery • -48 Volts

Ground • 0 Volts

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Telephone Switchboard Plug (Diagram)

Step by Step (SxS) "Local Call" Overview

• telco wiring frameso MFD (main distribution frame)

primarily a two wire (tip + ring) frame allowing connectionsbetween the vertical side (VMDF), where the undergroundcables appear, and the horizontal side (HMDF), where theOE (office equipment) blocks appear.

the horizontal side of the MDF is permanently wired to thevertical side of the IDF using over head cabling.

each line on the VMDF also contains electrical protectorsknown as "heat coils" and "carbons". The heat coils worklike a very-slow-blow fuse protecting the telephone switch

from too much current while the carbons protected againstlightning. Modern protectors employ gas cartridges in theplace of carbons.

o IDF (intermediate distribution frame)

primarily a three wire (tip, ring, sleeve) frame allowingconnections between the vertical side (VIDF), where the OE

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(office equipment) blocks appear, and the horizontal side(HIDF) where the telephone number blocks appear.

the vertical side of the IDF is permanently wired to linerelays (OE) in the line finder bays

the horizontal side of the IDF is permanently wired totelephone connector switch banks.

SxS call overview:•  The line relay

provides battery (-48 Volts) andground to energizethe telephonephone

• When the calling

phone goes off hook, the line relayoperates whichrequests a linefinder (there areonly 20 line findersfor every 200lines)

• once a line finderconnects to theline, the calling

circuit is cutthrough to the firstselector. Thiscauses a ground tobe put back on thesleeve of thecalling line whichoperates the cut-off (CO) relay,which causes theline relay torelease (telephone phone power now comes from the A-relay of the 1st

selector). This ground on the sleeve is also used to block incoming calls to thecalling phone (green line on this diagram)

• the customer pulse-dials the first digit which causes the 1st selector to stepup that many levels (1= one pulse while 0= ten pulses). The selector willthen rotate horizontally looking for an available trunk to the next selector bay(battery=available, ground=busy). Phone power now comes from the A-relayof the 2nd selector.

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• the customer pulse-dials the second digit which causes the 2nd selector tostep up that many levels. The selector will then rotate horizontally looking foran available trunk to the connector bay (battery=available, ground=busy)

• the connector processes the last two dialled digits (steps up 'X' digits thensteps horizontally 'Y' digits).

• the connector sleeve circuit is now used to operate the called subscriber'scut-off relay (thus disabling the associated Line Relay so the called customercan't draw dial tone)

• ringing signal (86 Volts AC) is now applied to the called subscriber's line

• when the call is answered, a DC path will interrupt the ringing signal and cut-though the voice path (tip and ring) over coupling capacitors

•  The calling party's tip and ring will reverse polarity to indicate that the callwas answered (this feature is used to collect coins or signal toll circuits tostart timing the call)

Step by Step (SxS) "Local Call" Details

• SxS call detail: when a subscriber goes off hook...o tip and ring line current causes a "line relay" (one associated with

every subscriber's line) to operate which does two things:

places battery (-48 volts via the C.O. relay) on the associatedsleeve wire in the line finder top terminal bank

operates a "group relay" to...

ground the associated sleeve commutator in the linefinder level switch bank (see picture below)

starts the first available line finder stepping vertically

o a "line finder" switch starts to step vertically looking for the groundedcommutator. Once it reaches the correct level, it rotates horizontally(stepping) looking for the subscriber line with battery on the sleeve(where it stops).

o now the subscriber's tip + ring circuits are cut-through to the A-relay of the "first choice" selector switch hardwired to the line finder. This first

choice selector switch also supplies the dial tone which is an audibleindicator telling the customer to start dialing. Operation of the A-relayin the selector now causes a ground to be placed upon the callingparty's sleeve. This causes two things:

operates the CO (cut-off) relay to release the line relay

signifies that the calling line is busy at the calling line'sconnector bay.

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o Most line finders were set up to service 200 lines but there were only17-20 line finder switches which meant that only 10% of the customersin a line finder group could make outgoing calls at the same time.

• when the subscriber dials the first digit of the telephone number, the firstchoice selector will rise vertically an equivalent number of levels to the digit

being dialed. The first choice selector will then step horizontally looking for anavailable trunk (it does this by testing the sleeve wiper looking for battery). If it steps to position eleven (a.k.a. "off the deep end") the subscriber will bepresented with 120 IPM (a.k.a. all trunks busy tone)

• In a medium sized office (~40k lines) the first choice selector bay wasusually wired to a fourth choice bay. This saved equipment and alsoallowed for something called short-dialing. Instead of dialing 7437970 you just needed to dial 37907. This means that if someone did dial allthe digits, that a digit absorbing first selector was required to "eat thefirst two digits only if they were "74" (the switch would step up 7 levels

and then just fall back the normal position; the same thing wouldhappen if a 4 was dialed as the second digit). Short dialing wasn'tusually available in densely populated areas.

•  The fourth choice connector took care of the fourth digit of a 7 digitdialed number

•  The fifth choice connector took care of the fifth digit of a 7 digit dialednumber

•  The connector took care of the last two digits (digit 6 was verticalstepping while digit 7 was horizontal stepping). At this time a DC

connection exists between the A-relay of the connector and the callingparty's telephone line. Ring signal (86 Volts AC) is now applied to thecalled party's line as well as an audible ringing tone onto the callingparty's line as a form of feed back. When the called party is answered,a DC path exists between the connector and the called party's phonewhich trips the ring and cuts through the called party to the callingparty by way of capacitors. At this same time the tip and ring of thecalling party is reversed to signify that the call has been answered. This is used to do thing like "collect coins" from a payphone or "start atoll timer"

Example: Dialing "7437970" (digits "74" are not required or used)

200 20 Line Finder First Choice Bay 

(terminal bank view)

Line Relays Switches  (terminal bank

view)

001 +------- 1/200 -----+ Switch -------------------->  Switch 

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002 + + + + 0 -> Operator

trunks (0)

003 +------- 3/200 -----+<-+ + 9 -> outbound

trunks (9xx)

... + + + + 8 -> outbound

trunks (8xx)

... + + + + 7 -> vacant

level (digit absorb for 74x)

... + + + + 6 -> outbound

trunks (6xx)

... + + + + 5 -> to fourth 

choice bay (745)

... + + + + 4 -> to fourth 

choice bay (744)

198 + + + + 3 -> to fourth 

choice bay (743) ----->+

199 + + + + 2 -> to fourth 

choice bay (742) |

200 +----- 200/200 -----+ + + 1 -> Long

Distance trunks (1+) |

 |

 

+----------------------------------------------------------------------------

-------------------+

|

| Fourth Choice Bay (743xxxx) Fifth Choice Bay 

(7437xxx)

  | (terminal bank view) (terminal bank

view)

|

+-----> Switch +-----> Switch 

+ 0 -> to fifth choice bay (7430) | + 0 -> to fifth

choice bay (74370)

+ 9 -> to fifth choice bay (7439) | + 9 -> to fifth

choice bay (74379) ---->+

+ 8 -> to fifth choice bay (7438) | + 8 -> to fifth

choice bay (74378) |

+ 7 -> to fifth choice bay (7437) ----->+ + 7 -> to fifth

choice bay (74377) |

+ 6 -> to fifth choice bay (7436) + 6 -> to fifth

choice bay (74376) |

+ 5 -> to fifth choice bay (7435) + 5 -> to fifth

choice bay (74375) |

+ 4 -> to fifth choice bay (7434) + 4 -> to fifth

choice bay (74374) |

+ 3 -> to fifth choice bay (7433) + 3 -> to fifth

choice bay (74373) |+ 2 -> to fifth choice bay (7432) + 2 -> to fifth

choice bay (74372) |

+ 1 -> to fifth choice bay (7431) + 1 -> to fifth

choice bay (74371) |

 

|

 

+----------------------------------------------------------------------------

-------------------+

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|

| 74379xx Connector Bay 

| (terminal bank view) 

|

+-----> Switch 

+ 0 -> to HIDF vertical 0 supports subscribers "7437991 to

7437990"

+ 9 -> to HIDF vertical 9 supports subscribers "7437981 to

7437980"

+ 8 -> to HIDF vertical 8 supports subscribers "7437971 to

7437970"

+ 7 -> to HIDF vertical 7, horizontal 10 (zero) is subscriber

"7437970"

+ 6 -> to HIDF vertical 6 supports subscribers "7437961 to

7437960"

+ 5 -> to HIDF vertical 5 supports subscribers "7437951 to

7437950"

+ 4 -> to HIDF vertical 4 supports subscribers "7437941 to

7437940"

+ 3 -> to HIDF vertical 3 supports subscribers "7437931 to

7437930"+ 2 -> to HIDF vertical 2 supports subscribers "7437921 to

7437920"

+ 1 -> to HIDF vertical 1 supports subscribers "7437911 to

7437910"

A Small CDO(CommunityDial Office)

Line-Finderbay (2 rows of 

10 switches;top middle-left)

•  Thesquarearea(middletop)contains20 rows of 

rectangular relay covers. Each row contains:o 10 line relays (one per subscriber)

o 10 cut-off relays (one per subscriber)

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o 1 group relay

•  This bay can support a maximum of 200 (20x10) subscribers.

• With 200 lines and but only 20 line finders, only 10% of the subscribers will be tooriginate calls at the same time (sometimes more switches would be added to

support businesses)

Connector Bay (2 rows of 11 switches; bottom middle-left)

•  These switches processed the last 2 digits of the dialed number to connect to thedialed subscriber's line.

•  The first connector was a special switch reserved for:

o allowing the operator to break into an "in-progress" call 

o line testing by maintenance personnel 

• Since each row has 11 switches, this picture most likely represents two separate

connector shelves, each one supporting 100 terminating subscribers

Selector Bay (6 rows of 10 switches; far right)

•  The six rows of switches on the far right appear to be a selector bay. Some of thesewill be wired to the connectors while others will connect to trunk circuits for carryingtraffic to other telephone exchanges.

ROTS Bay (3 switches in extreme left)

• It is difficult for me to identify this equipment but it appears to be 3 ROTS (rotary

outgoing trunk selector) switches just bellow the curved piece of paper. ROTSswitches were sometimes used to access trunks (toll and non-toll) to remotelocations.

Line FinderSwitches (acloser view)

• eachswitch

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contains 3 terminals banks x 10 levels x 10 rows x 2 connections for a total of 600wires

• the lower bank contains tip + ring wires for customers 1-100

• the middle bank contains tip + ring wires for customers 101-200

•

the upper bank contains sleeve circuits for all 200 customers

• all 600 wires are daisy chained horizontally from bank to bank in the back of the bay(not visible here) 

o the bank wiring between each switch is staggered in order to reduce theamount of "find" time for each OE. This means that "up one, in one" for switch9 is terminal 91. However, terminal 91 is "up two, in one" on switch 8 and "upthree in one" for switch 7.

o OEs 91-90 (and 191-190) will start switch 9 as their first choice line finder. If that switch is busy, the start lead is transferred to switch 8 and so on.

• the commutator (level seeking) wiper can be seen mounted to the middle of each

shaft

• wiper wires have a cloth covering to allow increased flexibility

• two wire-spring relays can be seen below switches D & E (see the can covers). These

are part of the bay alarm system.

• three wire-wrap blocks can be seen below switches I & N (see the can covers). Thesewires connect the bay alarm system to an aisle alarm.

• a black block below switch F contains indicator lamps and Alarm Cut Off (ACO)switches.

Line Finder Bank (up close)• each switch contains 3 terminals banks x 10

levels x 10 rows x 2 connections for a total of 600 wires

• the lower bank contains tip + ring wires forcustomers 1-100

• the middle bank contains tip + ring wires forcustomers 101-200

• the upper bank contains sleeve circuits for all 

200 customers (lower wiper: 1-100, upperwiper: 101-200)

• directly under the paper switch tag (with thenumber "1" on it) you'll notice eleven coppercommutator contacts 

o  The bottom terminal was never used

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o  The top terminal was permanently grounded to prevent the switch fromvertically over-stepping

o  These contacts appear to be heavily oxidized which would never be allowed ina "production" telephone exchange (clean copper contacts should beyellowish-orange rather than brown)

• the copper commutator wiper (mounted on the shaft) is currently making contactwith the first (unused) commutator terminal. (this switch is sitting in the rest position;the tip, ring + sleeve wipers are not currently switched into the terminal bank)

• look carefully at the top bank wipers near the shaft and you'll see two wiresindicating a path for both top and bottom circuits in that terminal bank. Barely visiblein this picture are reddish-brown insulators between all three pairs of wipers as wellas their respective contacts in the terminal bank

• wiper wires have a cloth covering to allow increased flexibility

• two green wire rings can be seen "dressing up" the wiper wires. These werenecessary to prevent the wiper wires from flopping around which could result in thembecoming snagged by the wipers.

• on either side of the terminal banks you'll notice a vertical brass tube (the right handtube is partially blocked from view by the commutator terminal block). This tubecontains an internal bolt which goes through a hole in the base plate of the switchand is then mated with a nut (not seen in this picture). Removing this nut allowed theswitch to be removed from the bay and taken to a shop area for repair and/oradjustment.

SD Numbers

Documentation going back to 1928 states that the letters "SD" stand for"Schematic Drawing". However, you'll hear telephone technicians also referto "SD" as "Special Drawing" and "Switch Drawing". The following drawingnumbers come from the May 1954 edition of "Training Manual for theStep by Step Dial Switching System" by American Telephone andTelegraph" (AT&T) 

SDNumber

Function

SD-30200

Selector

SD-30215

Local Rotary Connector

SD-30220

2-Ring Combination Connector

SD- 1-Ring Combination Connector

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30228

SD-30537

Incoming Pulse Correcting Repeater

SD-

30976

Digit Absorbing Selector

SD-32183

Digit Absorbing Selector

SD-31114

 Toll Rotary Connector

SD-31179

 Toll Intermediate Selector

SD-31300

 Toll Connector - 10 Party Terminal perStation

SD-31401

 Test Distributor Control Circuit

SD-90018

 Test Trunk Circuit

SD-31501

Interrupter Circuit & Transfer Key

SD-31647

Reverting Call Selector

SD-31522  Toll Transmission Selector

SD-31526

Local Connector - 10 Party Terminalper Station

SD-31530

Line Finder

SD-32133

Subscriber's Line CircuitLine Finder Multiple Diagram

SD-

31779

Outgoing Repeater

SD-31592

Coin Box Trunk

SD-32007

 Test Distributor

SD- Recording Completing Trunk-3 Wire

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61605

SD-32136

Recording Completing Trunk-2 WireFlat Rate

SD-

62426

Recording Completing Trunk-2 Wire

SD-31123

Recording Completing Trunk-CoinControl (CDO)

SD-62428

Recording Completing Trunk-CoinControl (Toll)

SD-62496

Recording Completing Trunk-CoinControl

Links• BELL HOMESTEAD ~ National Historic Site 94 ~ Tutela Heights Rd.

Brantford, Ontario, Canada. tel: 519-756-6220 519-756-6220

• http://www.telephonetribute.com/

• Antique Telephone Collecting Web-Ring

•  TelecomWriting.com: Step by Step

• Privateline.com Telephone History

• Vintage Telephone Equipment Museum - Step by Step CDO

• Strowger History

• Strowger - Manufacturer Representatives

• Almon Strowger @ Wikipedia

•  Telephone exchange @ Wikipedia

• Electrodata Telecom Acronym List

• How The World Was One is a non-fiction book abouttelecommunications by Arthur C. Clarke

• stromcarlson.com - telephony document libraryIncludes:

o Nortel's Telephony 101 (PDF)

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7/27/2019 Telephony Basic Information and Terms

http://slidepdf.com/reader/full/telephony-basic-information-and-terms 21/21

• Science Friday Archives: The Telephone Gambit (broadcast Friday,February 20th, 2009 on NPR)

o Did Alexander Graham Bell plagiarize the solution for a telephonetransmitter from Elisha Gray? It seems so.

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