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Telephone technology relating links

General information links

  • How Cordless Telephones Work   
  • How Telephones Work - Although most of us take it completely for granted, the telephone you have in your house is one of the most amazing devices ever created. This article is a good introduction to telephone technology.   
  • How Telephones Work: The Telephone - Surprisingly, a telephone is one of the simplest devices you have in your house. It is so simple because the telephone connection to your house has not changed in nearly a century. If you have an antique phone from the 1920s, you could connect it to the wall jack in your house and it would work fine!   
  • How Wiretapping Works - If people did want to eavesdrop, they could tap into almost any phone line quite easily. This edition of HowStuffWorks explores the practice of wiretapping to see just how simple it is.   
  • International Dialling Codes - country codes and dialling prefixes   
  • Phone Line Basics - Revisited   
  • Phone Lines Demystified From the Engineering staff at Telecom Audio - Phone lines come in many configurations, but they can be summed up as either PBX lines or "outside" lines.   
  • Telecommunication Tidbits - links and information articles   
  • Phone Line Basics - Reprint from an article printed in June 1996, Radio Guide Magazine   
  • Telephone Plug Types by Country   
  • Telephony Billing Tutorial - In telephony, billing involves gathering data for customer use and the provision of features, calculating costs, and invoicing for payment   
  • Tips for Correcting Bad Phone Lines   
  • Transmission lines: Why 600 Ohms? - There are broad definitions, under which every cable becomes a transmission line. Most engineers would narrow the definition considerably. This article introduces you to transmission lines and by 600 ohms impedance has become commonly used.   
  • 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.   
  • World Wide Phone Guide - What you need to hook up your modem just about anywhere!   

PBX systems

  • How a PBX works   
  • How IP PBX works   
  • Is Your PBX Ready for the Junk Heap? - So, what's the deal with VoIP? Should you believe the vendors and embrace it as the newest way for ISPs to increase revenues and for businesses to cut costs, or should you listen to the analysts who say it's still too soon to take the plunge? The answer probably has more to do with your situation than with the technology.   
  • Network Prep and QoS Assurance - Yes, you should look at your network before installing an IP PBX. But the news is probably good. If not, here's a make-ready recipe, and some products to help.   
  • On speaking terms: a network-based PBX lexicon   
  • Pack up your PBX--VoIP is here - IP technology is increasingly becoming the standard for corporate voice communications.   
  • PbxTech - Free Technical Forums and News for PBX Administrators   
  • Voice&Data GoldBook - IP Is Future but Circuit Switch Stays   
  • VoIP--do it right - VoIP can help your company save on telephone costs, leverage its existing network infrastructure, and add communications features that enhance productivity--assuming, of course, that it's done right. If you're planning to take the plunge and swap out your old PBX for a VoIP system, you need to keep your eye on what's critical--and know the pitfalls to avoid.   

How telephone network works

    Fax

    Computer telephony

    Signaling and coding

    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.

      DTMF dialling

      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

      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.

      Misc

      • 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.   
      • E-Series Recommendations Excerpts   
      • How message waiting lights work - description of different message waiting light systems   
      • A Brief Introduction to CCS7 - CCS7 is somewhat analogous to TCP/IP in that it is a protocol that allows networked computers (in this case telephone switches) to talk to each other.   

    Audio Interfaces

    Telephone line audio interfaces allow you to get the sound from the telephone line to some other system or sound from other system to the telephone line. The safest bet in all connections is to have and audio isolation transformer between the telephone equipment and your audio system. Because signal is from telephone has pretty limited signal quality, you don't need the highest quality transformers you can find (for exmaple the line transformer from an old modem should do the job nicely here, it passes telephone frequencies nicely and proves safe isolation level from line).

    Caller ID and DTMF decoding circuits

    Test equipments

    • Digital/Standard Phone Line Tester - make sure that the line is suitable for modem use and your PCMCIA does not damage if you plug it in   
    • 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   
    • Telephone In-Use Indicator, Telephone Ringer and The Surfers Preserver - This Phone Ringer circuit will work with any ordinary phone including older bell ringer types. The circuit rings the phone in a completely realistic manner until someone answers. When the receiver is lifted the user hears the audio of your choice. DC current is passed through the phone to activate the phone?s electronics.   

    Remote controlling using telephone

    Telephone line can be used for remote controlling applications besides normal telephone call. It possible to use DTMF signals for remote control functions in additions to normal dialling. DTMF tones are often used for controlling telephone remote systems. The phone remote system is a device connectedto both telephone line and power line network. It isable to detect the phone ring, to hang up the line,and "converse" with the householder. This dialogueis done by using the DTMF (digital tone multifrequency)generated by the remote phone. The remote system can communicate back to user by sending tones indicating that the user commands get through (not all have this feedback). Most teleremote circuits enables switching 'on. and 'off. of appliances through telephone lines. The general idea on those is that they usually have a set of relays or switched mains outputs that can be controlled through telephone line. The basic operation of such device is that you dial up your phone from anywhere in the world, input your pin number then turn on or off any of the controllable outputs (relays or switched mains output).

    • 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.   

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