Follow us on TwitterTechnical Information


CONTENTS OF
TELEPHONES UK

Home
A-Z Index
Acknowledgements
Bells & Tone Callers
Connection Boxes
Dial Labels
Telephone Exchanges
Feedback
History
Kiosks & Payphones
Latest Additions
Line Jacks
Links
LST & House Phones
Miscellaneous
Museums
Network Serv. Codes
Nostalgia/Documents
Old Dialling Codes
Shared Service
Sounds
Speaking Clock
TAMs & Cordless
Technical Information
Telephones Pre-1960
Telephones 1960-80
Telephones Post-1980
Telephone Systems
Tools & Testers
Vehicles
Wiring Cabinets
Wiring Information

Please note:

This is a site which looks purely at historical telephones & telecoms through the last 100 years plus.

Telecom Systems UK
View our new web site which supplies, installs & maintains Telephone Systems & Door-Entry Systems

TECHNICAL INFORMATION

How a Telephone Line Works
 
Telephones are essentially very straightforward audio devices with a send path, receive path and a means of generating dialing. Any confusion usually lies with the way the device is interfaced with the line/ exchange or with the many features available on modern equipment.

THE LINE
 
This is a pair of wires running from the apparatus (telephone, answering machine, etc) to the local B.T.
exchange. The network of exchanges throughout the country is known as P.S.T.N. (Public Switched Telephone Network). The line has an impedance of 600 ohms and is terminated at the subscriber's end with a modular jack socket known as a master socket (NTE5). Additional sockets may be connected in parallel with the master. These should be secondary sockets which have no components inside.When a piece of equipment is approved it is given a REN (Ring Equivalence Number). This is a way of defining the current it takes from the ring signal. The REN of all equipments connected to a line must add up to 4 or less. So much for the line, now let's look at some of the voltages and see when we can expect to see them.

   
LINE CONDITIONS  
   
IDLE
In this state no calls are in progress and there is no ring signal present. A nominal
50 volts DC can be measured on the line.
   
RING
Now the exchange has superimposed the ring signal onto the 50v, In its most common format this is an AC waveform 75vRMS 25Hz with a cadence of 0.4 sec
on, 0.2 sec off, 0.4 sec on, 2 sec off.
   

ANSWER
The apparatus takes the call by putting a loop across
the line; this stops the ringing at the exchange and causes the 50 volts DC to fall to approximately 10 volts DC. Communication may now take place.

 
Other tones are generated at the exchange from time to time, e.g. dial tone, ringback tone. The parameters of these vary with line loss and type of exchange. However, they are not critical to the operation of most equipment.
   
THE APPARATUS
 
Let's now take a look at some of the signals generated at the customer's end of the line. The most obvious of these is dialling and there are two types:
   
DUAL TONE MULTI-FREQUENCY (DTMF)  
   

This feature is commonly referred to as Tone Dialling. Each key on the keypad selects a tone from a group of high frequencies and one from a group of low frequencies. The two tones are combined and sent to line (either for a preset time or for as long as the button is held down).

   
PULSE DIALLING  
   
Whenever a key is pressed the loop formed when the apparatus went "off hook" is pulsed, 5 times for key 5, 10 times for key 0. In this country the pulsing or loop disconnection is done at 10 pulses per second. This is quite slow compared with tone dialling.
   
RECALL  
   
This function is used on PABXs to transfer an incoming call to another extension. There are two types of recall, Timed Break and Earth Loop. Timed   break   recall   causes   the   loop   to disconnected for approximately 75ms. With earth loop, the recall switch connects line (B) to earth (pin 4 of the modular socket).
   

[Home]  [Telephones]  [Systems]  [TAMs]  [Kiosks]  [Labels]  [Misc.]  [Links]  [A-Z]