Telephony
Systems.
Each
telephony line from the PSTN consists normally of a single pair of wires, such
a line would be called direct exchange lines or DEL. In the USA this is referred
to as a CO line which stands for central office. These lines are however
remarkably flexible and can be used to carry voice, fax, modem or even mixed
signals which combine all 3, for example DSL.
A
telephone works like all systems in blocks or stages…
1st)
The user lifts the handset / cradle and this releases a switch called the hook switch
– this name has historical origins when the earpiece was suspended from a
hook and the term hang up has it’s roots from the same period, when you
would physically lift the earpiece backup on to the hook switch to end the
call.
When
you lift the handset you permit the telephone circuitry to provide a DC path
to the exchange where a 50V DC supply is connected indirectly to each
subscriber line. When the DC path is completed current will flow and a loop is
formed (local loop – last mile in USA). If the current flow is above 20ma
the exchange will recognise this as a request for service. (historically 20ma
and above was needed to operate a physical relay at the exchange this was
coupled to a coil in series with the local loop – in modern systems this
role has been taken over by microprocessors).
2nd)
On recognising this request for the service the telephone exchange switches
the subscriber line into the system and for the purpose of human ears dial
tone is provided.
3rd)
The subscriber now tells the exchange the route destination (number) of the
party which is desired… (historically this would have been by speaking to
the operator) and
later
by means of loop disconnect (LD) “pulse dialling” which is still
recognised today. Although most modern analogue systems use DTMF, dual tone
multi-frequency.
Loop
Disconnect (LD) temporarily disconnects the local loop in pulses, the pulses
are too short for the line relay to disconnect (typically each pulse is 100ms,
67ms break, 33ms make and at least 240ms of a pause between digits).
LD
dialling was patented in 1891 by Almon Strowger, a funeral director in Kansas City
USA. DTMF dialling is recognised by the modern systems such as Bell &
Howells system X. DTMF has the
advantage of being much faster and more accurate than LD dialling – speed is
largely dictated by the person or equipment making the call, whereas pulse
dialled digits take around 1-2 seconds per digit. DTMF has allowed “star services" and
has led to a whole new generation of products which will respond to DTMF tones
remotely such as answering machines, PABX conference systems, LCR programming
etc…
4th)
The subscriber will now here a tone signal which relates to the condition of
the called number, example – ringing, engaged, unobtainable, fault etc…
What
is going on is that a distant exchange is attempting to apply a ringing signal
to the subscriber line of the called party. This signal is actually an AC
voltage of around 90V at 20hz, it can be relatively high in current as it may
need to drive a bell. When the bell is ringing the actual telephone circuitry
is normally isolated by using the hook switch. The bell itself is powered
through a capacitor so that the bell does not provide a DC path which would
connect the local loop. Bells are now being replaced by electronic ringing
circuits because they are much cheaper and provide a variety of definable
tones and pitches.
What
must be noted is that it is a feature of most fixed PSTN telephone systems
that the call path is maintained until the
calling party hangs up. Even if the recipient hangs up correctly the
path may still be open. Nowadays some but not all telecommunications providers
will disconnect the path if the recipient taps the cradle a couple of times or
if the recipient is using a cell phone (when it may prove impossible to keep
the path open). o
<--
Back to contents.