Hacker Perspective: Telephony (Basic Information and Terminology)

Executive Summary
  • Telephony (analog voice communication over wire) came in two fundamental categories: Circuit Switching and Packet Switching
  • Circuit Switching includes everything from an operator patch panel (also called a plug board) to a step-by-step switch, to a crossbar switch, to DMS or 1ESS switches.
    caveat: everything just listed was analog except the last two (DMS or 1ESS) which were digital.
  • Packet Switching includes just about everything that is delivered by the internet protocol. This began in earnest with computer applications like Skype but now also includes all so-called smart-phone communications which began with products like the RIM BlackBerry
  • This webpage is primarily dedicated to Circuit Switching

Telephone Switchboard Plug (Diagram)

(almost) all telephony signal names 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 room of the house where the operator lived
  • 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)
Panel Board
  • tip and ring wipers (sliding contacts) move up and down on brass rods while connecting to a contact panel
  • 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
  • https://en.wikipedia.org/wiki/Stepping_switch
  • 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 electromechanical 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
  • https://en.wikipedia.org/wiki/Crossbar
  • 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 electromechanical 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)
    • https://en.wikipedia.org/wiki/Digital_Multiplex_System
    • 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
  • https://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 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.
  • https://en.wikipedia.org/wiki/Local_area_network
  • 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 signaling converts the button you pressed into the appropriate number of DC interruptions.
  • Also called DTMF (Dual Tone Multi Frequency) signaling. 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.
  • https://en.wikipedia.org/wiki/Dual-tone_multi-frequency
  • 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

  • MFD ("main distribution frame" pronounced "em dee eff")
    • primarily a two wire (tip + ring) frame allowing connections between the vertical side (VMDF), where the underground cable terminations appear, and the horizontal side (HMDF), where the OE (office equipment) terminations appear.
    • the horizontal side of the MDF is permanently wired to the vertical side of the IDF using over head cabling.
    • each line on the VMDF also contains electrical protectors known as "heat coils" and "carbons". The heat coils work like a very-slow-blow fuse protecting the telephone switch from too much current while the carbons protected against lightning. Modern protectors employ gas cartridges in the place of carbons (unlike carbons, gas cartridges can recover)
  • IDF ("intermediate distribution frame" pronounced: "eye dee eff")
    • usually only associated with SxS
    • primarily a three wire (tip, ring, sleeve) frame allowing connections between the vertical side (VIDF), where the OE (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 line relays (OE) in the line finder bays
    • the horizontal side of the IDF is permanently wired to telephone connector switch banks.
SxS call overview
dialing four digits
Short Dialing Demo
The caller (3124) has dialed "3123"
  • Diagram: subscriber "3124" calls subscriber "3123"
  • Every subscriber is connected to a dedicated pair of relays called "the OE" (the office equipment)
    • the line relay
      • two windings of approximately 200 ohms each; one is connected to battery (-48 Volts) while the other is connected to ground (0 Volts)
      • The line relay powers the telephone when it is off-hook.
      • The circuit must not be too long (resistive) since the line relay must activate at a lower value of 14.8 mA.
        • Ohm's Law tells us that a telephone line with a dead short inside the central office will not pass anymore than 120 mA (48/400)
        • The DC resistance of a telephone is ~ 200 ohms (IIRC) so this sets a typical upper current limit of 80 mA (48/600). In practice you will never see current levels this high because the resistance of the copper wire between telephone and OE will reduce the current further. 
    •  the cutoff relay
      • when active, this relay opens the line relay circuit blocking dial-tone activation
  • When the calling phone goes off hook, the completed circuit energizes the the line relay which then requests the next-available line finder (there are usually only 20 line finders for every 200 lines which means that only 10% of the customers will be able to make a simultaneous out-going call)
  • once a line finder connects to the line, the calling circuit is cut through to the first selector (line-finders are hardwired to first selectors). This causes a ground to be put back on the sleeve of the calling line which operates the cut-off (CO) relay, which causes the line relay to release (telephone phone power now comes from the A-relay of the first selector). The ground on the sleeve is also used to block incoming calls to the calling phone (green line on the diagram to the right). It is the first selector which provides dial-tone back to the subscriber as a signal to "begin dialing"
  • the customer pulse-dials 3 pulses (at a speed of 10 pulses per second) the first digit which causes the 1st selector to step up 3 levels.
    • The selector will then automatically step horizontally searching for an available path to the next selector bay (sleeve test: -48v=idle while 0v=busy)
    • There are only ten circuits available on each horizontal so if the selector reaches step 11 (you have stepped too far) a small circuit is closed which will return the "all trunks busy" signal (120 IPM a.k.a. fast-busy)
    • Assuming an idle circuit was found, the circuit is cut-through to the A-relay of the 2nd selector.
  • the customer pulse-dials 1 pulse (at 10 pulses per second) which causes the 2nd selector to step up 1 level. The selector will then step horizontally looking for an available trunk to the connector bay (sleeve test: -48v=idle while 0v=busy)
  • the connector dials 2 then 3 (the connector steps up '2' levels then steps horizontally '3' positions).
  • the connector sleeve circuit is now used to operate the called subscriber's cut-off relay (thus disabling the associated Line Relay so the called customer can't draw dial tone when his phone goes off-hook)
  • 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 call was answered (this feature is used to collect coins or signal toll circuits to start timing the call)

Step by Step (SxS) "Local Call" Details

  • SxS call detail: when a subscriber goes off hook...
    • 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 associated sleeve wire in the line finder top terminal bank
      • operates a "group relay" to...
        • ground the associated sleeve commutator in the line finder level switch bank (see picture below)
        • starts the first available line finder stepping vertically
    • a "line finder" switch starts to step vertically looking for the grounded commutator. Once it reaches the correct level, it rotates horizontally (stepping) looking for the subscriber line with battery on the sleeve (where it stops).
    • 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 audible indicator telling the customer to start dialing. Operation of the A-relay in the selector now causes a ground to be placed upon the calling party'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's connector bay.
    • Most line finders were set up to service 200 lines but there were only 17-20 line finder switches which meant that only 10% of the customers in a line finder group could make outgoing calls at the same time.
  • when the subscriber dials the first digit of the telephone number, the first choice selector will rise vertically an equivalent number of levels to the digit being dialed. The first choice selector will then step horizontally looking for an available 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 be presented with 120 IPM (a.k.a. all trunks busy tone)
  • In a medium sized office (~40k lines) the first choice selector bay was usually wired to a fourth choice bay. This saved equipment and also allowed for something called short-dialing. Instead of dialing 7437970 you only needed to dial 37907. This means that if someone did dial all the digits, that a digit absorbing first selector was required to "eat the first 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 would happen if a 4 was dialed as the second digit). Short dialing was not usually available in densely populated areas.
  • The fourth choice connector took care of the fourth digit of a 7 digit dialed number
  • The fifth choice connector took care of the fifth digit of a 7 digit dialed number
  • The connector took care of the last two digits (digit 6 was vertical stepping while digit 7 was horizontal stepping). At this time a DC connection exists between the A-relay of the connector and the calling party's telephone line. Ring signal (86 Volts AC) is now applied to the called party's line as well as an audible ringing tone onto the calling party'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 phone which trips the ring and cuts through the called party to the calling party by way of capacitors. At this same time the tip and ring of the calling 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 a toll timer"

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; processes the
| final 2 dialed digits; here we go up 7 then across 10) | 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)
  • The square area (middle top) contains 20 rows of rectangular relay covers. Each row contains:
    • 10 line relays (one per subscriber)
    • 10 cut-off relays (one per subscriber)
    • 1 group relay
  • This bay can support a maximum of 200 (20 x 10) subscribers.
  • With 20 line finders for every 200 lines, only 10% of the subscribers will be to originate 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 the dialed subscriber's line.
  • The first connector was a special switch reserved for:
    • allowing the operator to break into an "in-progress" call
    • 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 these will be wired to the connectors while others will connect to trunk circuits for carrying traffic 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. ROTS switches were sometimes used to access trunks (toll and non-toll) to remote locations.

Line Finder Switches (a closer view)

Line Finder Switches
  • 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 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)
    • the bank wiring between each switch is staggered in order to reduce the amount of "find" time for each OE. This means that "up one, in one" for switch 9 is terminal 91. However, terminal 91 is "up two, in one" on switch 8 and "up three in one" for switch 7.
    • 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). These wires 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)

Line-finder contacts
  • 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 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 (lower wiper: 1-100, upper wiper: 101-200)
  • directly under the paper switch tag (with the number "1" on it) you'll notice eleven copper commutator contacts
    • The bottom terminal was never used
    • The top terminal was permanently grounded to prevent the switch from vertically over-stepping
    • These contacts appear to be heavily oxidized which would never be allowed in a "production" telephone exchange (clean copper contacts are yellowish-orange rather than brown)
  • the copper commutator wiper (mounted on the shaft) is currently making contact with 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 wires indicating a path for both top and bottom circuits in that terminal bank. Barely visible in this picture are reddish-brown insulators between all three pairs of wipers as well as 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 were necessary to prevent the wiper wires from flopping around which could result in them becoming snagged by the wipers.
  • on either side of the terminal banks you'll notice a vertical brass tube (the right hand tube is partially blocked from view by the commutator terminal block). This tube contains an internal bolt which goes through a hole in the base plate of the switch and is then mated with a nut (not seen in this picture). Removing this nut allowed the switch to be removed from the bay and taken to a shop area for repair and/or adjustment.

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


  • a successful inventor
  • with the help of investors, creates Graybar Electric (a manufacturer of electrical communications equipment)
  • with the help of investors, the manufacturing arm of Western Union merges with Graybar Electric into Western Electric
    • Western Electric gets out of the wholesaling business in 1926 by selling off that division as Graybar (an electrical equipment wholesaler still in business in 2017 today in North America)
    • Western Electric is broken up in 1984 by AT&T Technologies
  • possibly the first inventor of the telephone (if you do not count Philipp Reis)
    • at the very least, a good case could be made for Gray and Bell being co-creators; people didn't know about synchronicity in those days

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?

  • everyone has heard the story of how "Bell submitted his patent four hours before Gray" but how many know that submitted patents were not time-stamped?
  • on top of this, conflicting patents submitted on the same day always enter into something called an interference investigation but that did not happen in this case
    • or more accurately, an investigation was begun but was later withdrawn
    • clerk Wilber sent interference letters to all concerned parties including Bell's attorneys: and Marcellus Bailey (a war buddy of WIlbur) and Anthony Pollok
  • facts:
    • on Feb-14, 1876 Bell filed a patent for the "harmonic telegraph" while Gray filed a caveat for the "telephone".
      • what is a caveat? In those days it was treated as a 3-month place-holder indicating "more information will follow shortly" along with a "proof of invention" and the actual patent application
    • since these were different inventions, could the clerk have been tricked into showing Alec Bell the caveat submitted by Gray?
      • this is where we learn that someone inserted a lengthy hand-written entry into the margin of Bell's typed patent which appears to be a reworded description of "variable resistance" taken from Gray's caveat
    • Bell races back to Boston then begins work on the telephone. He and Watson produce a working telephone on Mar-10, 1876
    • On top of this, there is the troublesome line-drawing in Bell's notebook showing a man talking down into a cone. This same image can be seen in Gray's document which was filed on Feb-14, 1867 in Washington DC. Bell's notebook was never seen by anyone until 1976 (the 100-year anniversary of the telephone) when the Bell family donated a large trove of documents to the Library of Congress
  • Elisha Gray was reputed to be a very honest man (in fact, he was a Quaker)
  • Alec Bell was reputed to be a very honest man but he was also in love with the daughter of his investor and attorney, Gardiner Green Hubbard. It appears that Hubbard may have engineered the conflict between Bell and Gray by using Bell's emotions for Hubbard's daughter as leverage (we all know how that usually turns out smile)
    • Bell's behavior in the year after the patent does not seem to be the behavior of an inventor. In fact, it seems the behavior of someone with a guilty conscience


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.
    • full time research? sounds to me like the precursor to Bell Labs
  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
Waterloo, Ontario, Canada.