Telephony Basic Information and Terms
1. Switching Information 2. 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 telephone transmitter from Elisha Gray? It seems so. Switching Information
Name Description
Operator • totally manual Plug Board • when you went off-hook, a light bulb associated with your jack was lit. Sometimes there would also be an associated buzzer because, in the very early days (or in rural locations), these plug boards were sometimes installed in the front of a house where the operator lived and the operator wasn't always sitting at the plug 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 was bored, she usually listened in on your call)
• when you were finished, you went back on hook and she pulled out your plug when she noticed that the light was out
Panel • tip and ring wipers move up and down a contact panel on brass
1 rods
• you've got to see it in order to believe it
Drop • an early Step by Step system with no "line finder" Relay • 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
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, but they 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 connected crossbar switches (which are also 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.
2 • 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
• http://en.wikipedia.org/wiki/Crossbar
Computeri • SP1 (S-P-One) zed Circuit o Stored Program 1 Switching, Analog o a cross bar telephone switch manufactured by Northern Circuits 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.
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
Computeri • DMS-100 zed Circuit o An all-electronic telephone switch manufactured by Switching, Northern Electric (Nortel) Digital Circuits o North American average installed life: ?? years (many installed in 1980 are still running in 2009 because there were 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
3 • 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 associated with 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 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)
4 Na Description me
L1 • analog carrier used between switching centers N1 N2 • 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 per voice 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 51.840 Mb/s
• http://en.wikipedia.org/wiki/Synchronous_optical_networking
ATM • although not usually thought of as carrier technology, this may change with VoIP
• A synchronous T ransfer M ode
• 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 would rather sell you MPLS (multi protocol label switching) LAN • although not usually thought of as carrier technology, this may change with VoIP
• Ethernet, Token Ring, etc.
• http://en.wikipedia.org/wiki/Local_area_network
5 • works OK sending email and web pages but not a serious contender until technologies like ATM are employed
o 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 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 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 Sleeve • 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"
6 • ground (0 volts) signifies "line busy" Wall • Green (tip #1) Wires • Red (ring #1)
• Yellow (tip #2 or spare)
• Black (ring #2 or ground) Single • Tip Wire (green) Party Phone • Ring Wire (red) Wiring • 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 Two • Most phones come pre-wired for single party operation. Party Internally, the BELL will usually be wired across the TIP and Phone RING terminals. When converting a phone to party line use Wiring (very rare this side of y2k), this internal BELL wiring must be moved 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)
7 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 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 • An open circuit - no DC electricity is flowing. (but an AC Hook connection exists for ringing the BELL) • Typical Ring: -48 volts (battery)
• Typical Tip: 0 volts (ground) Off • A closed circuit - DC electricity is flowing. Hook • Typical Ring: -30 volts to -35 volts
• Typical Tip: -15 volts to -20 volts
• Lowest Current: 15 mA
• Typical Current: 40 mA
Pulse • when you use a rotary dial the signal the number "9", the DC Signalin path of the telephone is interrupted 9 times. When you dial "0" g 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. Tone • Also called DTMF (Dual Tone Multi Frequency) signalling. There Signalin are 3 tones associated with the vertical key columns and 4 g tones associated with the horizontal key rows. Therefore,
8 pressing any key will result in the transmission of two tones.
• http://en.wikipedia.org/wiki/Dual-tone_multi-frequency Ringing • usually 85 Volts AC at 20 Hz Signal • 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) Sleeve • a 3-wire circuit is made to behave like a 6-wire circuit Signalli ng • 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 Busy • 60 IPM (Impulses Per Minute) Signal • Also called Busy Tone All • 120 IPM (Impulses Per Minute) Trunks Busy Tone Battery • -48 Volts Ground • 0 Volts
9 Telephone Switchboard Plug (Diagram)
Step by Step (SxS) "Local Call" Overview
• telco wiring frames o MFD (main distribution frame)
. primarily a two wire (tip + ring) frame allowing connections between the vertical side (VMDF), where the underground cables appear, and the horizontal side (HMDF), where the OE (office equipment) blocks 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.
o IDF (intermediate distribution frame)
. primarily a three wire (tip, ring, sleeve) frame allowing connections between the vertical side (VIDF), where the OE
10 (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: • The line relay provides battery (- 48 Volts) and ground to energize the telephone phone • When the calling phone goes off hook, the line relay operates which requests a line finder (there are only 20 line finders for every 200 lines)
• once a line finder connects to the line, the calling circuit is cut through to the first selector. 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 1st selector). This ground on the sleeve is also used to block incoming calls to the calling phone (green line on this diagram)
• the customer pulse-dials the first digit which causes the 1st selector to step up that many levels (1= one pulse while 0= ten pulses). The selector will then rotate horizontally looking for an available trunk to the next selector bay (battery=available, ground=busy). Phone power now comes from the A-relay of the 2nd selector.
11 • the customer pulse-dials the second digit which causes the 2nd selector to step up that many levels. The selector will then rotate horizontally looking for an available trunk to the connector bay (battery=available, ground=busy)
• the connector processes the last two dialled digits (steps up 'X' digits then steps horizontally 'Y' digits).
• 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)
• 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... 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 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
o 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).
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 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.
12 o 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 just 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 wasn't 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: 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
13 002 + + + + 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) |
|
+------+
14 | | 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 (Community Dial Office)
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: o 10 line relays (one per subscriber)
o 10 cut-off relays (one per subscriber)
15 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 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:
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 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) • each switch
16 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)
o 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.
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). 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) • 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
o The bottom terminal was never used
17 o The top terminal was permanently grounded to prevent the switch from vertically over-stepping
o These contacts appear to be heavily oxidized which would never be allowed in a "production" telephone exchange (clean copper contacts should be 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 Function Number
SD- Selector 30200 SD- Local Rotary Connector 30215
SD- 2-Ring Combination Connector 30220 SD- 1-Ring Combination Connector
18 30228 SD- Incoming Pulse Correcting Repeater 30537
SD- Digit Absorbing Selector 30976 SD- Digit Absorbing Selector 32183
SD- Toll Rotary Connector 31114 SD- Toll Intermediate Selector 31179
SD- Toll Connector - 10 Party Terminal per 31300 Station SD- Test Distributor Control Circuit 31401
SD- Test Trunk Circuit 90018 SD- Interrupter Circuit & Transfer Key 31501
SD- Reverting Call Selector 31647 SD- Toll Transmission Selector 31522
SD- Local Connector - 10 Party Terminal 31526 per Station SD- Line Finder 31530
SD- Subscriber's Line Circuit 32133 Line Finder Multiple Diagram SD- Outgoing Repeater 31779
SD- Coin Box Trunk 31592 SD- Test Distributor 32007
SD- Recording Completing Trunk-3 Wire
19 61605 SD- Recording Completing Trunk-2 Wire 32136 Flat Rate
SD- Recording Completing Trunk-2 Wire 62426 SD- Recording Completing Trunk-Coin 31123 Control (CDO)
SD- Recording Completing Trunk-Coin 62428 Control (Toll) SD- Recording Completing Trunk-Coin 62496 Control
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 about telecommunications by Arthur C. Clarke • stromcarlson.com - telephony document library Includes:
o Nortel's Telephony 101 (PDF)
20 • Science Friday Archives: The Telephone Gambit (broadcast Friday, February 20th, 2009 on NPR)
o Did Alexander Graham Bell plagiarize the solution for a telephone transmitter from Elisha Gray? It seems so.
21