Plain old telephone systems (POTS) telephone line consits of one
wire pair which carries full duplex audio and the operating current for the telephone. The
telephone connected to line is powered from current limited 48V power source, so phones
on-hook, should measure around 48 volts DC. Practically the opearating voltages of
telephone systems can vary from 24V to 60V depending on the application, although 48V
nominal voltage is the most commonly used. Telephone applications often require and use
positive grounding in the central office, where the positive conductor of the 48V power
supply is connected to earth ground. The telecommunication industry began the positive
ground convention in the 1940s and many telecomm companies still employ the traditionally
positive grounded system.
This means that generally when telephone is on-hook, one telephone
line wire is quite near to the ground potential and other one carries -48V. When telephone
is put off-hook the voltage beween wires going to telephone drops down to the 3 to 9 volt
range and typically acurrent of 20-60 mA will flow through the telephone. The remaining
voltage drop occurs over the copper wire path and in the telephone central electronics.
Typical telephone line bandwidth is around 200-3200 Hz, which was
what phone companies decided years ago be sufficient for speech intelligibility while
allowing them to multiplex many calls over coax and twisted pair. The low end is rolled
off early to stay away from the mains frequency (50 or 60 Hz) interference. The high end
cut off is caused by the telephone transmission system (nowadays the audio is digitized at
8 kHz). The typical signal to noise ratio of a telephone line is approximately 45 dB or
somewhat less. The average signal levels on telephone line is -9 dBm average speech.
The voice on a tip/ring pair is full duplex balanced audio which
requires a two wire to four wire hybrid circuit or transformer to convert it into separate
transmit and receive audio paths. The hybrid circuit makes it possible to transmit two
channels of information in opposite directions on a single pair of wires. Historically
hybirid circuits have used one or two transformers. Bulky and expensive hybrid
transformers have been replaced in most telephones by ICs which perform the same function.
In a telephone, the biggest contributor to poor audio quality is the handset microphone
(it has be be cheap and withstand very hard use).
Telephone standards world is fragmented. Typically each country has
its own standards because of both the historical roots of the phone service and the desire
to protect the local phone market from outside competition.
Basically the telephone systems work in the same way in different
countries, but there are are some differences which can mean that a devices designed for
one country does not meet the regulations of other country and work poorly or not at all.
The differences in local technical standards range from minor to severe and affect many of
the signaling conditions on local loops.
The biggest differences are different wiring practices and
connectors, different line impedances, different nominal loop currents, different
signaling tones and different electrical safety regulations. Unfortunately nowadays many
countries are harmonizing many standards across their boundaries so it is nowadays
possible to designe devices which work well and meet the regulations in more than one
country at the time. For example in Europe the European Commission adopted CTR 21 standard
covers nonvoice equipments (for example FAX and MODEM) in more than 20 countries.
It would be a really good thing if telephone tones were standardised
throughout the world. Unfrotunately they are not and will propably never be fully
standardized to be same all around world. Telephone ringivers have differences between
countries. The ring signal is much the same, worldwide. It is around 80V at a frequency
between 16 2/3 Hz and 50Hz (20-25 Hz quite common). But its timings are wildly different,
as are the return tones it generates. Also the "dial tone", "busy
tone" etc. are different in different countries.
Course notes,
terrestrial WAN - Internet plumbing is made up of LANs, terrestrial (wired) WANs and
radio-WANs that are all interconnected with routers. This note addresses the terrestrial
WANs. This document tells also about the telephone company version of data communications.
Nonvoice
Calls and Line Testing Tutorial - This tutorial examines the history of line testing
and the various challenges brought about by recent technological change, all within a
service provider's business context
Telephony
Billing Tutorial - In telephony, billing involves gathering data for customer use and
the provision of features, calculating costs, and invoicing for payment
Wireless Local
Loop (WLL) Tutorial - This technique to replace telephone wire using radio. WLL can
offer the developing world the advantages of a PSTN network, where it would normally be
too costly to install a fixed-wire infrastructure. WLL proposes to replace the fixed-line
element of a standard PSTN inrastructure with a wireless link. Thus, only the final mile
would be a fixed link from a radio transceiver to the user terminal.
Business telephone systems very often take use of technologies like
PABX, multiline telephones and digital telephones.
PABX (Private Automatic Branch eXchange) is a telephone exchange
operated within an organisation, used for switching calls between internal lines and
between internal and PSTN lines. A PABX can route calls without manual intervention, based
entirely on the number dialed. In some old companies the might still be PMBX (Private
Manual Branch eXchange), which involves company employed operators manually switching each
call using a manual switchboard.
Alexander Graham Bell (1847-1922) is most famous for his invention
of the telephone. As a teenager of 18, Bell had been experimenting with the idea of
transmitting speech. In 1874, while working on a multiple telegraph he developed the basic
ideas for the telephone. He and his assistant Thomas Watson finally proved successful on
March 10, 1876, when the first telephone message was transmitted: "Watson, come here;
I want you.". This led eventually to the establishment of the Bell Telephone Company,
still in existence today, which introduced the telephone to the world.
Antique Telephone Collectors
Association - The Antique Telephone Collectors Association, or ATCA, is the largest
telephone collectors organization in the world.
Tribute to the
Telephone - history, schematics, trivia, photos, links and other telephony related
pages
Telephone
Schematics for Western Electric Phones - schematics, drawings, tables, photos, and
other information pertaining to telephones produced by Western Electric for the Bell
System prior to 1984
How
Cordless Telephones Work - A cordless telephone is basically a combination telephone
and radio transmitter/receiver. This document tells how they work.
Telephone
operating parameters - table that tells battery voltages, operating current,
resistance of phone, resistance of loop, loop loss, distortion, and ringing signal
Computer can be used to perform lots of fuctions in modern telephony
systems. This link collectains mostly information on linking computers and traditional
telephone systems. There is a separate link section for
Internet telephony.
CTI
converges on a single TDM bus - computer-telephony resource boards use the CT Bus in
new designs and upgrades, transition to PCI bus made it to agree on one
time-division-multiplex bus
Echo
Cancellation for VoIP - echo cancellation is necessary technology in long distance
telephony, especially in computer based telephony
Managing
Telephony Services with the S.100 API - In the right hands, the S.100 API provides a
versatile interface between hardware components and any computer-telephony application.
This toolset provides a unified method for constructing scaleable, reliable telephony
applications with mixed-vendor equipment.
Telephone
moves to broadband - Voice-over-broadband (VoB) represents the next incremental step
in the evolution of the global voice/data network from a circuit- to packet-switching
architecture.
Voice-Data
Consolidation Tutorial - information on the transmission of both voice and data over a
single packetized communications network
Accessory circuits
Circuit
eliminates PC echoes - long-distance-telephone services available via the Internet
often require the PC user to wear headphones of a headset to prevent echo caused by the
microphone's picking up the loudspeaker outputs, this circuit eliminates the echo while
using the existing PC microphone and speakers
Technical characteristics of tones for the telephone service are
listed in ITU-T Recommendation Q.35 (1988).
Unfortunately that document is not freely available (can be ordered from ITU-T if you are willing to pay), so the information
available in the following documents might be useful if you are looking for free
information on telephone line signaling.
Touch-tone dialing, also know as DTMF dialling, is a method of
sending signals from telephone customer's premises to central offices and beyond. The idea
of touch-tone dialing was first introduced in 1964. Today, most of the telephones in the
in developed countries use touch-tone dialling. The advantage of touch tone signaling is
that the signaling is voice band signal and the dialing can be done faster than with pulse
dialing. The fact that DTMF signaling energy is in the voice frequency band, makes it
possible to transmit signaling information (12 distinct signals) to any point in the
telephone network to which voice can be transmitted. This makes it possible to use DTMF
signals for remote control functions in additions to normal dialling. DTMF tones are used
for controlling many modern automated telephone answering services.
Caller ID system is a feature of digital telephone network, which
allows the telephone number of the caller to be transferred to the called person before
he/she answers the phone. This feature can be useful for example for storing numbers of
the people who have tried to call you when you were not home. In completely digital
systems (like ISDN and GSM) send the caller ID information as part of normal completely
digitla signaling. After digitialization of PSTN network (all parts except last mile are
digital) this caller ID functionality is become available in PSTN network also.
There are two different commonly used caller ID signalling systems
for sending caller ID informationg using normal telephone line (PSTN). Bellcore system
(which is used in USA) send the numer in data packet which is sent at 1200 bps speed using
FSK-modem. Eropean system send the same information using standard DTMF (Dual Tone Multi
Frequency) signalling.
Ensuring Voice
Quality with Adequate Tail Length - Designers who worked on the POTS never dreamed of
the challenges facing those working on next generation telecommunications equipment.
Today's engineers live in the digital world where equipment designs are required to
codify, compress, cancel echo, control jitter and loss, packetize, switch, route, and
bill, and do it all more quickly and efficiently than their competitors. It is easy to
lose sight of the need to provide a quality voice connection when the principal design
goal is to maximize the volume of data the device can handle. Given these concerns,
worrying about the length of the circuit tail delay in an echo-cancellation algorithm
seems downright old fashioned and low on the design requirements list. But failure to
appreciate tail delay will adversely affect voice quality.
The following telephone circuits are mainly ment to give ideas for
your own desigs. Many of the circuits are quite propably do not meet your local telephone
network terminal equipment specs (they vary from country to country) and are quite
propably poorly designed (this applies as well to many circuits in books and hobby
magazines). Connecting non-approved equipment to public telephone network is illegal in
many countries. If you connect non-approved circuit which do not meet the specs to the
public telephone network the telephone do not work properly with them and they also cause
safety hazards.
For information what is allowed to be connecte to telephone line the
reader form USA should check FCC part 68 regulations and European reader should check NET4
(ETS 300 001) regulations. Good information on telephone equipment regulations can be
found from Compliance Engineering Magazine website.
Telephone
Headgear - This circuit is very useful for hands-free operation of EPABX and pager
communication. This circuit can be used in parallel with existing electronic telephone
set.
Wiring
Inside Phones - Since wiring inside most fones is pretty standard, this description
should do the trick. This applies to all WE phones and ITT phones that use the standard
dial/ringer/network block/handset configuration.
Hands-Free
Telephone - Tis is a circuit to interface telephone handset connector to stereo
heaphone and small microphone. This circuits to connect to 4-pin telephone handset
connector.
Isolated Telephone
Interface - This circuit allows you to record audio from a telephone line into a tape
recorder or computer soundcard. Most of the parts for this circuit can be scrounged from
an old modem, with some work, it is possible to rewire the modem circuitry and use the old
modem case.
Simple Phone
Tap - will record any conversation on any phone on same line, the circuit seems to be
a bad design because it does not provide proper isolation and has too low DC impedance
(can keep you line off-hook all the time)
Telephone
Record Control - allows you to connect any tape recorder that has a mic and remote
input to a phone line and automatically record both sides of a conversation when ever the
phone is in use
Telephone
Recorder - lets you record your phone conversations automatically
Telephone
recorder - This circuit automatically records all incoming and outgoing phone
conversations with an ordinary cassette or tape recorder. The circuit below distills the
audio signal from the phone line and automatically starts/stops the casette recorder.
Telephone
Tap - wire your telephone conversion to amplifier or tape recorder
1
line status indicator - circuit gives you a visual indication when a line is in
operation, line polarity insensitive and operates from two AA batteries for one year, pdf
file
2
line telephone line status indicator - circuit gives you a visual indication when a
line is in operation, line polarity insensitive and operates from two AA batteries for one
year, pdf file
4
line telephone line status indicator - circuit gives you a visual indication when a
line is in operation, line polarity insensitive and operates from two AA batteries for one
year, pdf file
Audio
Visual Ringer - This is an extra telephone ringer circuit. All that needs to be done
is to connect the given circuit in parallel with the existing telephone lines using twin
flexible wires. This circuit does not require any external power source for its operation.
Free-line
indicator stops interruptions - A simple circuit lights an LED, which indicates
whether the line is free (Figure 1a). Batteries are unnecessary; the phone line powers the
circuit, and an accumulator saves energy for an "in use" indication.
Phone Busy
Indicator - simple busy indicator, does not meet regulations of many countries because
this circuit draws excessive continuous current from the phone line even when not active
and thi can cause problems with telephone system
Phone-line
privacy circuit - circuit uses three HS20s, bilateral silicon trigger switches, to
form a phone-line privacy circuit with three telephone outputs
Telephone
Line Indicator design - information on the characteristics of telephone lines and
circuit which connects to a telephone line and indicates whether or not the line is in use
(seems to be quite good design)
Telephone privacy
adapter - revents other telephones from hearing your call and disturbing your modem
connection
Chip
recorder customizes phone ringer - By placing a circuit in series with the telephone,
you can customize a phone ringer without modifying the phone. The heart of the ringer is a
chip-recorder IC, which can play as much as 10 seconds of telephone-quality recorded
sound.
Phone
rang indicator light - will detect the ring signal, energize the relay which latches
up, and the LED comes on and stays on till you push a switch
Smart
Phone light - The circuit shown here is used to switch on a lamp when the telephone
rings, if the ambient light is insufficient.
CID.ZIP -
Adapter for getting aller ID information from Finnish telephone network to PC. THis
circuit decodes DTMF format caller ID. This document includes source code (Pascal) for
simple programs to access the circuit in DOS and Windows. The document language is
Finnish.
DTMF Products for you -
DTMF decoder can be powered from a 9V battery or from your parallel printer, schematic and
windows demo program, design by Schmidt Technologies
Generate
CID/CIDCW analog signals - A high-speed µP can generate CID (caller-ID) and CIDCW
(caller-ID-on-call- waiting) analog signals at 1200 baud FSK between the first and second
20-Hz ring of an incoming call
Telephone
Number Display - The given circuit, when connected in parallel to a telephone,
displays the number dialled from the telephone set using the DTMF mode.
Off line
Telephone tester - Here is a circuit of an off-line telephone tester which does not
require any telephone line for testing a telephone instrument. The circuit is so simple
that it can be easily assembled even by a novice having very little knowledge of
electronics.
Off
line Telephone tester - Here is a circuit of an off-line telephone tester which does
not require any telephone line for testing a telephone instrument. The circuit is so
simple that it can be easily assembled even by a novice having very little knowledge of
electronics. This circuit provice the line current, sign signal and audio interface.
Telephone
line monitor - detects if there is another telephone connected to the line, if there
is a short or an open line
Phone
Remote System - a telephone interface that allows the householder to give instructions
to appliances from outside, application note from SGS-Thomson
Remote
control using telephone - Here is a teleremote circuit which enables switching
"on" and "off" of appliances through telephone lines. The circuit
described here can be used to switch up to nine appliances (corresponding to the digits 1
through 9 of DTMF the telephone key-pad). The telephone can be used to switch on or switch
off the appliances also while being used for normal conversation.
Remote
control using telephone - This is a teleremote circuit which enables switching
‘on’ and ‘off’ of appliances through telephone lines. It can be used
to switch appliances from any distance, overcoming the limited range of infrared and radio
remote controls.
Remote
control using telephone - Here is a teleremote circuit which enables switching
'on’ and 'off’ of appliances through telephone lines. The circuit described here
can be used to switch up to nine appliances (corresponding to the digits 1 through 9 of
the telephone key-pad). The DTMF signals on telephone instrument are used as control
signals.
Telephone Line
Relay Switch - Control 4 relays over your phone line. Dial up your phone from anywhere
in the world, input your pin number then turn on or off any of 4 relays. Use the relays to
control any devices you have in your house or office. The pin number is set by 4 on-board
BCD rotary switches. The circuit has telephone line protection circuitry built-in (but not
approved). Standard telephone line impedance termination can be customised by the user for
their own particular country if they want to get better impedance matching.
Misc telephone accessories
9-Line Telephone
Sharer - This circuit is able to handle nine independent telephones (using a single
telephone line pair) located at nine different locations, say, up to a distance of 100m
from each other, for receiving and making outgoing calls, while maintaining conversation
secrecy. This circuit is useful when a single telephone line is to be shared by more
members residing in different rooms/apartments.
Having
secrecy in parallel telephones - Often a need arises for connection of two telephone
instruments in parallel to one line. But it creates quite a few problems in their proper
performance, such as overloading and overhearing of the conversation by an undesired
person. In order to eliminate all such problems and get a clear reception, a simple scheme
is presented here.
Parallel Telephones
with Secrecy - enable the incoming ring to be heard at both the ends and any one time,
only one telephone is connected to the line
Tape Recorder
Switch for Telephone - This kit uses a FET to switch on a tape recorder when the phone
is picked up. It will work with low voltage tape recorders as low as 1.5V. It requires the
REMOTE & MIC jacks of the tape recorder to be available.
Telephone
call meter using calculator & COB - In this circuit, a simple calculator, in
conjunction with a COB (chip-on-board) from an analogue quartz clock, is used to make a
telephone call meter.
Telephone
Headgear - A compact, inexpensive and low component count telecom head- set can be
constructed using two readily available transistors and a few other electronic components.
. Since the circuit draws very little current, it is ideal for parallel operation with
electronic telephone set.
Telephone Line Based
Audio Muting and Light-On Circuit - circuit would automatically light a bulb on
arrival of a telephone ring and simultaneously mute the music system/TV audio for the
duration the telephone handset is off-hook
Telephone
Recorder - lets you record your phone conversations automatically, notice that this
circuit does not provide isolation between telephone line and the recorder
Creating Your
Own Telephone Network - you can easily create your own intercom system using two
telephones, a 9-volt battery (or some other simple power supply) and a 300-ohm resistor
Fax saver
cuts wear, tear, and power - less than $20 to cut wear and tear on your fax machine
while saving power, circuit intercepts the incoming ring signal and powers up the fax
machine in time to receive the fax and then turns off the fax machine after a
predetermined time and awaits the next incoming fax
Telephone
Ringer using 556 dual timers - Using modulated rectangular waves of different time
periods, The circuit presented here produces ringing tones similar to those produced by a
telephone.
Notes on the
Telephone-To-Sound-Card Interface Project - The purpose of this circuit is to allow a
sound card (a SoundBlaster AWE 64 in my case) to interface with the phone line so that
output from the sound card that normally goes to the speakers goes instead to the phone
and can be heard by a remote caller or local phone, and also speech on the phone line will
be sent to the sound card via the sound card's microphone interface.
Return Loss
Calculations - four page note explains how return loss can be determined and contains
a MathCad analysis, an Excel spreadsheet analysis and a QuickBASIC program listing, zipped
file from Midcom Technical Note
library
Transformer Seminar
Notes - companion to the seminar entitled "The Telecommunications Transformer:
low-cost solution to product reliability and continuing regulatory compliance" by
Dave LeVasseur, in pdf format, from Midcom
Phone
Remote System - a telephone interface that allows the householder to give instructions
to appliances from outside, application note from SGS-Thomson
Telephone Ring
Generator Using Small Power Transformer - This ring generator will ring a telephone
once every 10 seconds. The interval between rings can be lengthened or shortened if
needed. The 70 volt/ 30 Hz ring voltage is produced from the 120 volt side of a small 12.6
VAC power transformer.
Telephone Ring
Generator Using Switching Supply - This telephone ring generator below generates the
needed high voltage from a simple switching mode power supply (SMPS) which employs a CMOS
Schmitt Trigger square wave oscillator, 10 mH inductor, high voltage switching transistor
(TIP47 or other high voltage, 1 amp transistor) and a driver transistor (2N3053).
How
to make residential phones regulate their own power - With the advent of broadband
access, manufacturers of small residential-phone systems are delivering products that no
longer depend on the central office to regulate and control the power they use.
The following telephone hacking circuits are here only for
educational purposes. Most of them don't work anymore and connecting them to telephone
network is illegal.
2600 Magazine - link to famous
pheaker magazine web pages
Phone Losers of America -
web site devoted to telephone stuff that phone companies don't want you to know
Phrack -
information about everything you shouldn't know about telephone
Telephone Line
Attacks - The telephone system presents a major threat from audio eavesdropping
attacks. Telephone tapping, a term the general public is well aware of, is the act of
monitoring telephone conversations. Telephone taps are of two general types: Direct and
Inductive.
The
Hacker's Handbook - very early version of the book that was later banned in Great
Britain in 1985
