Elementary Switching

by 910

Signals are sent over the telephone network to control its operation and indicate its status.  Signaling is essential to the internal coordination of transmission and switching facilities.  It also allows the user to submit requests to the network and allows the network to provide the user interpretable responses.

At the beginning of time, human beings employed at the local telco central office watched for flashing lamps on their consoles to learn that someone wanted to make a call.  The flashing was initiated by my Great Aunt Muriel turning a crank on her phone.  The operator plugged her headset into Muriel's jack and determined through verbal interaction the person or number Muriel wanted.  If the lamp at the receiving party's jack was unlit, the operator rang the party's phone and connected Muriel's jack to the receiving party's.  If the receiving party's lamp was lit, the operator informed Muriel that the line was in use.

If the receiving party was served by another exchange, the operator called an operator at the distant exchange through an interoffice trunk, and told her the number of the receiving party.  If the receiving party's lamp was unlit, the distant operator rang the receiver's phone and completed the connection.

More recently, the request for service is made by simply lifting the handset, closing a 48-volt Direct Current (DC) circuit.  The flow of current is interpreted by the switch at the central office as a request for service.  This current carries two concurrent sine waves, one 350 Hz and one 440 Hz, which produce a reassuring sound in the user's earpiece, often called "dial tone."  The flow of DC continues as long as the phone is off-hook, and the switching facility uses this information in supervising the line, specifically, in determining whether the line is still in use.

The number of the party to be called is conveyed to the switch by the caller with either tones or pulses.  The early telephone was equipped with a spring-loaded rotating disk, which had numbered "finger holes."  After the caller spun the disk until blocked by a stationary "finger stop," the disk would unwind to its original position at a fixed speed.  During its return the disk would interrupt the DC flow as many times as the number dialed (except ten times for 0).  If the number dialed was 4, as the disk rewound, the DC circuit would be broken four times for about 6/100 of a second, and restored in between each break for 4/100 of a second.  Each pulse cycle took about 1/10 of a second.

Newer, non-rotary phones, capable of pulse dialing, interrupt the current similarly, using an electronic control circuit.  A very nimble finger can accomplish the same thing with the hang-up button.  More modern phones emit a concurrent pair of sine waves to communicate numbers to the central office.

On a standard dial pad, each button on the top row (1, 2, and 3) emits 697 Hz; second row, 770 Hz; third row, 852 Hz; and fourth row (*, 0, and #) 941 Hz.  Each button in the first column (1, 4, 7, and *) emits 1209 Hz; second column, 1336 Hz; and third column (3, 6, 9, and #) 1477 Hz.  These tone pairs are interpreted by the switching facility as the number pressed on the dial pad.  Although ancient switches cannot interpret tones, new (all) switches can interpret pulses.

The central office provides callers with an aural representation of the receiving party's phone in the act of ringing with a simultaneous pair of tones called "ring-back."  They are 440 Hz and 480 Hz, and bleep for two of each six seconds while the distant phone is ringing.

The famous "line-busy" signal is comprised of simultaneous 480 Hz and 620 Hz tones, bleeping one half of each second until the caller hangs up.

The "trunk-busy" (also called "reorder") signal is issued when switching or transmission facilities are unable to handle the call.  It is identical to the line-busy signal but bleeps at twice the rate.

When all goes well, the receiving party's telephone is sent a ringing signal, not audible at the earpiece, but usually inciting a loud bell, chirping sounds, or flashing lights, often invoking considerable excitement.  This is accomplished with a 20 Hz signal of about 75 volts, issued for two of each six seconds until the ringing phone is picked up or the caller interrupts the flow of DC in her phone by hanging up.

A call to a party served by a central office other than one's own requires the use of one or more interoffice trunks.  Older long distance lines used a 2600 Hz tone to indicate that a trunk is available.  When the switch began using the trunk, the caller's central office ceased its issuance of the tone.  The distant office was alerted to an incoming request for service by this change.

Mote recently, interoffice signaling has been moved from the voice transmission circuit to a separate, dedicated circuit.  A single data circuit can control thousands of voice circuits, conveying telephone number, trunk availability, and other information.

"Line-busy" signals are no longer sent from the distant office.  A data signal is sent via the signal circuit, initiating the generation of the audible signal at the caller's office.  Previously, sending an audio signal from the distant office required the use of a voice circuit, which is now left free for other users' conversation.

The caller's telephone number is also conveyed through the separate circuit.  The distant office knows the caller's number, and the receiving party may also get it.  It is sent to the receiving party's equipment as a short burst of digital data, encrypted by phase-shift keying.  The receiver's equipment must decrypt the signal, and display or otherwise act on it.  Depending on the number, the call may be automatically rejected, preventing the phone from ringing, or it may be forwarded to another location.