Voip

Table Of Contents

Section Description Page No.

1. Signaling Classifications #

2. The Early Days #

3. Single-Frequency Signaling #

4. Multi-Frequency Signaling #

5. Loop Signaling #

5.1 Loop Start Signaling #

6. E&M Signaling #

Signaling Classifications

The two major signaling classifications are Interoffice signaling, or signals transmitted between central offices, and Subscriber loop signaling, or signals transmitted between the central office and the subscriber. Signaling intelligence transmitted over subscriber loops is further subdivided into three primary categories. They are:

* Information Signals - Information signals are audible tones within the voice frequency range. They communicate to the subscriber or operator, intelligence relative to the status of a call. This includes informing the subscriber that dialing may begin (dial tone), alerting the far-end subscriber of an incoming call (ringing), conveying to the call originator that the far-end subscriber is being called (ring-back tone) or that the subscriber line is in use (busy tone), and indicating to the originator that all switching paths or toll trunks are busy (reorder tone).

* Supervisory Signals - Supervisory signals are generally subscriber requests for service. Activated by the switch-hook of a telephone set, supervisory signals convey to the central office that the subscriber wishes to originate, answer, or terminate a call. The direct current present in the subscriber loop circuit is used to generate supervisory signals. These signals are transmitted either before or after conversation has been established and reflect two signaling conditions: on-hook (circuit open, no current flow), or off-hook (circuit closed, current flowing).

* Control Signals - Control signals are used to direct and complete a call. Dial pulsing, which interrupts the flow of DC loop current, and pushbutton-generated tone signals (DTMF) represent the majority of subscriber loop control signals.

The Early Days

The earliest form of signaling - used to this day - is ringing, or the transmission of 20Hz alternating current to alert the receiving end that information is about to be communicated. The ordinary telephone bell or electronic tone ringer is the audible signaling device with which most people are familiar. In fact, during the pioneer days of telephony when most subscribers resided on party lines, ringing current was applied to either the "tip" lead or the "ring" lead of a circuit, or talk path, to effect selective ringing to the appropriate party. The term "tip and ring" is used to designate the two wires of a standard telephone line and was derived from the early switchboard era. In those days, the tip lead was connected to the tip portion of an operator jack plug, while the ring lead was connected to the brass ring located just behind the tip. A third lead, known as the "sleeve" lead, was wired to the shaft or main body of the plug to affect supervision or control of the manually switched connection.

This ringing signaling method was also used to alert the local exchange operator. To originate a call, the subscriber would simply crank the generator on a magneto telephone, which sent ringing current to the central office switchboard. Instead of an audible device, the ringing signal activated a small latched door known as a "drop". The magneto current operated a relay, which literally dropped a hinged flag over the switchboard jack appearance of the calling subscriber's line. This method of signaling was known as "ring-down" or "magneto" signaling and was also used over trunks between central offices. While hand-cranked magnetos have gone the way of the "EH Holden", ring-down signaling is still used today in an automatic fashion, specifically in conjunction with private-line circuits serving most residential areas.

Single-Frequency Signaling

As subscriber populations expanded, so did local telephone exchanges. The need to communicate over longer distances became obvious, so toll trunks were established between central offices. However, the use of ringing current as a signaling medium over long distances proved to be impractical. Initially, DC methods such as reverse-battery signaling were employed for supervision and control of trunk circuits, but as distance between central offices continued to grow, DC signaling also proved to be unfeasible.

The solution was a new signaling system using a single, high frequency tone. Known as Single Frequency In-Band Signaling, this system employs a 2600Hz tone, which is carried over the speech path and within the frequency range of the transmitted message, usually 300 to 3400Hz. Since the signal is audible, it is present on the line in the idle condition but removed once the circuit is seized. Because of this characteristic, the receiving equipment remains active during conversation in order to respond to signaling addresses, but it is designed to prevent erroneous operation during ordinary speech or due to transmission irregularities.

To improve signaling performance and avoid the problem of speech interference associated with in-band signaling, the Out-of-Band Signaling system was developed. This signaling arrangement uses frequencies just above the voice frequency range, generally between 3400 to 3700Hz. Because it employs higher signaling tones, it is much more reliable and less susceptible to speech anomalies than the In-Band arrangement.

Multifrequency Signaling

The most commonly used form of multifrequency signaling is Dual Tone Multi-frequency, or DTMF. Popularly known as "touch-tone", DTMF is used to transmit numerical address information from a telephone by sending a combination of two voice frequency tones: one from a group of four low frequencies, the other from a group of four high frequencies. The eight tones reside in the center