Telephony Basic Information and Terms

  1. Switchboard Plug
  2. Switching Information
  3. Carrier Information
  4. Telephone Information
  5. SxS "Local Call" Overview
  6. SxS "Local Call" Details
  7. SxS "SD" Numbers
  8. Thomas A. Watson
  9. Links
    • Did Alexander Graham Bell plagiarize the solution for a telephone transmitter from Elisha Gray? Perhaps. It seems that a patent clerk showed Bell a few facts that allowed Bell to improve his transmitter. Also, Bell's father-in-law was a lawyer who was willing to spend 20-years wearing down the other side in court.

Telephone Switchboard Plug (Diagram)

cord-board plug

 Switching Information

Name Description
Plug Board
  • totally manual
  • when a subscriber phone went off-hook, a small lamp (light bulb) associated with the subscriber's jack was lit. In some sites, an associated buzzer was activated because these plug boards were sometimes installed in the front rom of the house where the operator lived but wasn't always present.
  • the operator plugged into your jack to find out what you wanted
  • you instructed the operator to connect to so-and-so. She used a pair of cords to connect your jack to the destination jack  (if she was bored, she sometimes listened in)
  • when you were finished, you went back on hook and she pulled out your plug when she noticed that the light was out (both parties could not make a new call until the operator cleared the previous call)
Panel Board
  • tip and ring wipers move up and down a contact panel on brass rods
  • a mechanical marvel you've got to see to believe
  • an early Step by Step system with no "line finder"
  • when a customer went off-hook, a relay latch dropped to provide them dial-tone etc.
  • when they went back on-hook, they could not make another call until a telephone office attendant manually restored the latch
  • 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, but they did support digi-pulse phones.
  • North American average installed life: 40 years
  • A single cross-bar swithNumber 5 Cross Bar (for local switching; Number 4 Cross Bar was for toll-tandem use)
  • a telephone switch employing a huge matrix of connected crossbar switches (matrices of contacts)
  • typically, each crossbar switch was composed of twenty vertical paths "in" and ten horizontal paths "out". If you wanted a more orthogonal matrix, you could stack (electrically speaking) a second one on top of the first making it 20h x 20v.
  • To close a set of contacts, one of ten select magnets operated one of five horizontal bars either up or down (the center position was neutral) which moved a metal enabler spring into position. This action was followed by the operation of one of twenty vertical hold magnets which forced the metal spring to close three, or more, contacts. Now the select magnet is released (to set up the next connection) while the hold magnet maintains the existing connection until the customer disconnects.
  • the crossbar switches were controlled by an electro-mechanical computer known as a marker (named because they "marked out a path" through the central office)
  • most of these systems supported both touch-tone and dial phones
  • North American average installed life: 20 years
  • SP1 (S-P-One)
    • Stored Program 1
    • a cross bar telephone switch manufactured by Northern Electric (Nortel) with a computerized front-end rather than an electro-mechanical front end (anyone remember markers?). These machines employed a mini-crossbar technology to complete the analog circuit.
    • North American average installed life: 10 years
  • 1ESS
    • a.k.a. ESS 1, No. 1 ESS
    • ESS = Electronic Switching System
    • developed by AT&T
    • electronic switching but analog circuits
  • DMS-100
  • DMS-200
    • A toll (long distance) version of DMS-100
  • DMS-10
    • A small community version of DMS-100
  • DMS-1
    • A 256-1-256 line concentrator
  • 5ESS
    • a.k.a. ESS 5, No. 5 ESS
    • ESS = Electronic Switching System
    • full digital switch
    • run by the UNIX operating system
  • Private Branch Exchange - a small telephone switch associated with a business
  • SL1 (S-L-One) is the PBX version of SP1
  • Central Office (a.k.a. telephone exchange, telephone switch)
  • usually serves between 10,000 and 100,000 lines
  • Community Dial Office
  • a rural telephone exchange
  • usually serves between 100 and 2,000 lines
  • Voice over Internet Protocol
  • An packet based communications technology that rides on the internet
  • Note that most analog technologies employ circuit switching while VoIP is based upon packet switching
  • In the 1960's the Americans (ARPA + DARPA) funded research which resulted in a self-healing packet network which could survive natural disasters or a nuclear attack. ARPAnet merged with NSFnet to become the Internet. Since circuit switching was based upon common control solutions (telephone switches), problems would occur whenever the switch became incapacitated. In a properly designed packet network the intelligence is moved into the network's routers. When the network detects an internal problem, it just routes the packets around the obstacle.

Carrier Information (only a few common technologies are listed)

Name Description
  • European version of T1
  • supports 32 channels each capable of passing 64 kb/s
  • although not usually thought of as carrier technology, this may change with VoIP
  • Asynchronous Transfer Mode
  • LANE (LAN Emulation) is an ATM technology designed to make LANs (local area networks) more dependable
  • This technology was designed to make up for the short comings of LANs. For some reason, network equipment manufacturers would rather sell you MPLS (multi protocol label switching)
  • although not usually thought of as carrier technology, this may change with VoIP
  • Local Area Network
  • Ethernet, Token Ring, etc.
  • works OK sending email and web pages but not a serious contender until technologies like ATM are employed
    • for  example, when packet delays cause VoIP problems on an Ethernet, installing an additional second path or increasing the speed of the existing path is not the answer; you can replace hubs with switches and even add MLPS based solutions but these are just tweaks to a technology that was designed to live with packet delays

Telephone Information

Name Description
  • 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
  • Name of an analog telephone wire. Usually measures -48 Volts when on-hook
  • Gets its name from the ring conductor of a manual operator's three 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 single party line
  • click: plug diagram
  • Name of an analog telephone wire used for supervision in non-electronic systems
  • Gets its name from the sleeve (third) conductor of a manual operator's three conductor plug (a long metal sleeve)
  • click: plug diagram
  • battery (-48 volts) signifies "line idle"
  • ground (0 volts) signifies "line busy"
  • Green (tip #1)
  • Red (ring #1)
  • Yellow (tip #2 or spare)
  • Black (ring #2 or ground)
  • Tip Wire (green)
  • Ring Wire (red)
  • Ground (black) - not required but should be connected for safety
  • Bell Wire (yellow) - not used
  • Note: only two wires, tip + ring, actually connect back to the telephone exchange
  • Most phones come pre-wired for single party operation. Internally, the BELL will usually be wired across the TIP and RING terminals. When converting a phone to party line use (very rare this side of y2k), this internal BELL wiring must be moved to the BLACK and YELLOW terminals.
    • When a RING party is alerted to an incoming call, ringing signal will be applied across the RING wire and GROUND.
    • When a TIP party is alerted to an incoming call, ringing signal will be applied across the TIP wire and GROUND.
  • Ring Customer
    • Tip Wire (green)
    • Ring Wire (red)
    • Ground (black) - mandatory
    • Bell (yellow) - connect to Ring Wire
  • Tip Customer
    • Tip Wire (green)
    • Ring Wire (red)
    • Ground (black) - mandatory
    • Bell (yellow) - connect to Tip Wire
    • If the touch-tone keypad does not work, reverse the tip and ring wires to the phone (some electronic phones may never work in party-line situations)
  • Note: only two wires, tip + ring, actually connect back to the telephone exchange. The BELL return path is made through ground (a.k.a. earth)
On Hook
  • An open circuit - no DC electricity is flowing. (but an AC connection 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
  • when you use a rotary dial the signal the number "9", the DC path 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 the appropriate number of DC interruptions.
  • Also called DTMF (Dual Tone Multi Frequency) signalling. There are 3 tones associated with the vertical key columns and 4 tones associated with the horizontal key rows. Therefore, pressing any key will result in the transmission of two tones.
  • 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 path for the -48 Volts)
  • a 3-wire circuit is made to behave like a 6-wire circuit
  • high current is placed upon the sleeve (200 ohms via -48 Volts)
  • trunk dialing occurs over the tip + ring (like operating a test connector)
  • the sleeve state is switched to low current (1200 ohms via -48 Volts)
  • the trunk is now held up while the tip + ring are extended through to the customer's line for testing
  • 60 IPM (Impulses Per Minute)
  • Also called Busy Tone
All Trunks
Busy Signal
  • 120 IPM (Impulses Per Minute)
  • -48 Volts
  • 0 Volts

Step by Step (SxS) "Local Call" Overview

dialing four digitsSxS call overview:

Step by Step (SxS) "Local Call" Details

	Example: Dialing "7437970" (digits "74" are not required or used)
  200                      20                     First Choice Bay
  Line Relays              Line Finder Switches   (terminal bank view)
  001 +----- 1/200 -----+  Switch --------------> Switch
  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+)     |
                   Bank Terminals                                                      |
|    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)   |
|     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 "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"

Small CDO (Community Dial Office)

A small Step-by-step office
A Line-Finder bay (2 rows of 10 switches; top middle-left)

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

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

ROTS Bay (3 switches in extreme left)

Line Finder Switches (a closer view)

Line Finder Switches

Line Finder Bank (up close)

Line-finder contacts

SD Numbers

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

SD Number Function
SD-30200 Selector
SD-30215 Local Rotary Connector
SD-30220 2-Ring Combination Connector
SD-30228 1-Ring Combination Connector
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 per Station
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 Terminal per Station
SD-31530 Line Finder
SD-32133 Subscriber's Line Circuit
Line Finder Multiple Diagram
SD-31779 Outgoing Repeater
SD-31592 Coin Box Trunk
SD-32007 Test Distributor
SD-61605 Recording Completing Trunk-3 Wire
SD-32136 Recording Completing Trunk-2 Wire Flat Rate
SD-62426 Recording Completing Trunk-2 Wire
SD-31123 Recording Completing Trunk-Coin Control (CDO)
SD-62428 Recording Completing Trunk-Coin Control (Toll)
SD-62496 Recording Completing Trunk-Coin Control

Thomas A Watson

The Thomas A. Watson version of the story I am familiar with is a little more complicated (and accurate) than the versions published by newspapers or Hollywood movies.
  1. The Charles Williams Machine Shop was a manufacturer of fine electrical instruments -and- had a side line doing consultant engineering.
  2. Back in those days, no one could afford a full time engineer so they would take their ideas to a consulting engineer, or a company that hosted consulting engineers.
  3. Alexander Graham Bell took his ideas for the telephone to the Charles Williams Company where he was assigned to Thomas Watson.
    off-topic comment about consultants: around this same time, Arthur Conan Doyle was a consulting physician (ophthalmologist) who would not receive any referrals for weeks at a time; he started writing the Sherlock Holmes stories to fill his time and make some money in case his medical consultancy failed.
  4. Many people are surprised to learn that Thomas Edison also employed the Charles William Machine Shop before he built his research facility in Orange, New Jersey.
  5. At the urging of Bell's future father-in-law, Gardiner Greene Hubbard , Watson is convinced to leave Charles Williams and work full time with Bell. Apparently, Hubbard paid to equip the new lab and bank-rolled Watson's fee.
  6. Thomas A. Watson's only son, Thomas J Watson, goes on to create IBM

A few related links:


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Neil Rieck
Kitchener - Waterloo - Cambridge, Ontario, Canada.