Telephony (Basic Information and Terminology)

Executive Summary

Telephone Switchboard Plug (Diagram)

(almost) All telephony signals are derived from the operator's switchboard plug

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 rome 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)
  • comments:
    1. The labels "Tip", "Ring" and "Sleeve" survive today in the (non-wireless and non-VOIP) telephone industry
    2. I have worked in the telephone industry for over 40-years and can tell you that people still throw around these terms but have no idea where they came from (so now YOU know where they originated)
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 (pressing key 5 yields 5 pulses).
  • North American average installed life: 40 years
  • A single cross-bar swithNumber 5 Cross Bar (for local switching) Number 4 Cross Bar (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)
    • a.k.a. 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
    • An all-electronic telephone switch manufactured by Northern Electric (Nortel)
    • North American average installed life: ?? years
      • many installed in 1980 are still running in 2017 due to in-place upgrades
      • probably will be the last generation of telephone switch since the whole industry is moving from circuit switching to packet switching
  • 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
  • pronounced: cee oh
  • Central Office (a.k.a. telephone exchange, telephone switch)
  • usually serves between 10,000 and 100,000 lines
  • pronounced: cee dee oh
  • Community Dial Office
  • a rural telephone exchange
  • usually serves between 100 and 2,000 lines
  • pronounced: voip
  • 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 (idle)
  • 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 (idle
  • 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 of phone) - mandatory
    • Bell (yellow wire of phone)  - connect to Ring Wire
  • Tip Customer
    • Tip Wire (green)
    • Ring Wire (red)
    • Ground (black of phone) - mandatory
    • Bell (yellow wire of phone) - 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 rings by energizing one wire (tip it you are the tip customer or the ring if you are the ring customer) but the return path (of the BELL) is made through ground (a.k.a. physical 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)
  • Also called Fast Busy
  • -48 Volts
  • 0 Volts

Step by Step (SxS) "Local Call" Overview

Telco wiring frames

SxS call overview
dialing four digits
Short Dialing Demo
The caller (3124) has dialed "3123"

Step by Step (SxS) "Local Call" Details

Example: Short-Dialing "7437970" (digits "74" are not required)

Historical overview from the 1940s:

  1. The following example comes from the Sherwood exchange in Kitchener Ontario Canada where the "SH" letters of Sherwood represent "74".
  2. This was a 40,000 line step-by-step (SxS) exchange implemented as 742, 743, 744 and 745.
  3. The "74" prefix was redundant meaning that a local call could be completed by only dialing the last 5 digits
  4. If the local subscriber began dialing "74", the first selectors were wired in such a way as to absorb them (the switches stepped vertically but then fell back down)
  5. I suppose it goes without saying that whenever someone dialed "0" the circuit actually pulsed 10 times.

Historical overview from the 1960s:

  1. Then in the 1960s a 30,000 line cross-bar (5xB) was added to a floor above and implemented as 576, 578, and 579.
  2. The SxS office was modified so that SxS subscribers could directly connect to the 5xB exchange without needing to dial "1" (it was a local call, not long distance)
  3. A SxS subscriber wishing to call a 5xB subscriber would begin by dialing "5" (which was absorbed if it was the first digit) then "7" to make a connection to the crossbar exchange above. At this point, no one could use short dialing to call 745 numbers so short dialing was discouraged although it continued to work in a limited fashion.

Top Diagram (just below):
  1. Customer Goes off-hook to operate 1 of 200 line relays (two banks of 10 up and 10 across)
  2. A line-finder steps up then across to "find the line" (2/200 in this diagram)
  3. The back of each line-finder is hardwired to a first-choice selector which provides dial tone
  4. The customer dials the first digit (#3 in this case) which caused the selector to step up 3 levels then automatically step across 1 to 10 terminals looking for an idle circuit (sleeve=-48v). It was designed to stop on the first idle circuit of ten.
  200 Line Relays      20 Line Finders (hardwired to)
                       20 First Choice Selectors

                       Switch - Switch (dialed 3)
  001 +----- 1/200 -----+  +    + 0 -> Operator trunks (0)
  002 +----- 2/200 -----+<-+    + 9 -> outbound trunks (911)
  003 +----- 3/200 -----+  +    + 8 -> outbound trunks (893,894)
  ... +                 +  +    + 7 -> outbound trunks (576,578,579)
  ... +                 +  +    + 6 -> outbound trunks (632,648,669)
  ... +                 +  +    + 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+)    |
                                       (terminal bank view)         |
|   Fourth Choice Bay (743xxxx)            Fifth Choice Bay (7437xxx)
|   (terminal bank view)                   (terminal bank view)
Switch (dialed 7)                 +-Switch (dialed 9)
+ 0 -> to fifth choice bay (7430) | + 0 -> to connector bay (74370)
+ 9 -> to fifth choice bay (7439) | + 9 -> to connector bay (74379)-+ 
+ 8 -> to fifth choice bay (7438) | + 8 -> to connector bay (74378) |
+ 7 -> to fifth choice bay (7437)-+ + 7 -> to connector bay (74377) |
+ 6 -> to fifth choice bay (7436)   + 6 -> to connector bay (74376) |
+ 5 -> to fifth choice bay (7435)   + 5 -> to connector bay (74375) |
+ 4 -> to fifth choice bay (7434)   + 4 -> to connector bay (74374) |
+ 3 -> to fifth choice bay (7433)   + 3 -> to connector bay (74373) |
+ 2 -> to fifth choice bay (7432)   + 2 -> to connector bay (74372) |
+ 1 -> to fifth choice bay (7431)   + 1 -> to connector bay (74371) |
|   74379xx Connector Bay
|   (terminal bank view; connectors process the final 2 dialed digits
Switch (dialed 7 then 0)
+ 0 this vertical supports 7437901 up to 7437900
+ 9 this vertical supports 7437991 up to 7437990
+ 8 this vertical supports 7437981 up to 7437980
+ 7 this vertical supports 7437971 up to 7437970
+ 6 this vertical supports 7437961 up to 7437960
+ 5 this vertical supports 7437951 up to 7437950
+ 4 this vertical supports 7437941 up to 7437940
+ 3 this vertical supports 7437931 up to 7437930
+ 2 this vertical supports 7437921 up to 7437920
+ 1 this vertical supports 7437911 up to 7437910
Line-finder contacts
A line finder
Line Finder Switches
A line finder bay

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

Elisha Gray


The Patent Controversy

Did Alexander Graham Bell plagiarize Gray's idea for the liquid-based telephone transmitter? It seems so. We know that an alcoholic patent clerk (Zenas Fisk Wilber ) showed Bell a few facts from Gray's filing. But how could this have happened?


Thomas A Watson

The Thomas A. Watson version of the story I am familiar with is somewhat more complicated than the versions published by newspapers, magazines and movies.
  1. The Charles Williams Machine Shop (of Boston) 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.
    observation: During this same time period, Arthur Conan Doyle was a consulting physician (ophthalmologist) who would not get a patient referral for weeks. To keep himself busy, he wrote fictional stories about a consulting detective by the name of Sherlock Holmes
  3. Alexander Graham Bell took his ideas for the telephone to the Charles Williams Company where he was assigned to a consulting engineer by the name of Thomas Watson.
  4. 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.
  5. Thomas A. Watson's only son, Thomas J Watson, becomes a rising star at Computing-Tabulating-Recording Company which he later morphs into IBM
  6. Many people today are surprised to learn that Thomas Edison also employed the Charles Williams Machine Shop before he built his own research facility in Orange, New Jersey. In fact, Edison maintained an office in the same building just so he could get quick access to the human talent.

A few related links:

Examples of synchronicity (and cross-fertilization)

  1. Bell Labs invents the transistor which revolutionizes IBM's computer business (see: IBM 7070) as well as the telephone system hardware (see: 1ESS)
  2. Bell Labs invents the Unix operating system then creates the C programming language in order to produce a better version of Unix. This action is so successful that the C language takes off on its own as an alternative to assembly language making it the first truly portable computer language.
  3. Since Bell Labs is run as a monopoly, they are not allowed to profit from Unix so many universities get it for free, or at a very low price license (under one thousand dollars). They use access to the source code to teach computer engineering and one variation at the University of California at Berkley becomes very popular and is called BSD Unix.
  4. Since universities are not chartered to sell software, BSD Unix is given away when possible or licensed at cost (once again, under one thousand dollars)
  5. DARPA had been working with Universities to develop software for something that would eventually be called the Internet. They focus on a talented C-language programmer at Berkley named Bill Joy to write the TCP (transmission control protocol) code which would be built on top of IP (internet protocol) which would eventually be known as TCP/IP. Not only does this enable the US government to build a command-and-control system able to survive a nuclear attack, it provides the computer industry with the internet. It also enables the telephone industry to move from "circuit switching" to "packet switching" which later is used for VOIP (voice over IP). It should be no surprise that a faster internet is able to support Video over IP. From this point on, TCP/IP is given away for free as part of Unix.
  6. the C programming language morphs into the object-oriented C++ programming language
  7. The breakup of the Bell system under Ronald Regan causes UNIX to come under control of AT&T. Universities are told they will no longer get UNIX or UNIX source code (which they need to teach computer engineering) at bargain-basement prices.
    1. This causes them to focus their collective efforts on alternatives to AT&T Unix like "BSD Unix", GNU, and MINIX.
    2. GNU is basically a total rewrite of AT&T Unix apps to avoid copyright claims. The GNU project did not have the skills to produce an alternate kernel.
    3. Enter Linus Torvalds with an alternate kernel but no apps. Put them together and you end up  with Linux (the most popular operating system in the world which changes the computer industry forever; even IBM joins in with )
    4. Events like these kick off the open source movement
  8. Smart phones from companies like Blackberry, Apple (iPhone) and Google (Android) allow the telephone to morph into a handheld computer


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