Dual tone multifrequency raveling

Dial pulse or loop disconnect raveling

Outgoing raveling

Each telephone in the network is assigned a unique number and each country has been assigned a unique number, therefore to obtain a distant telephone it is necessary to pass this number (address) to the telephone exchange, which will then route the request through the network to the distant telephone. There are two basic types of address raveling:

1. Dial pulse or loop disconnect.

2. Multifrequency.

Dial pulse or loop disconnect raveling takes its name from the rotary dial method of raveling information to the exchange. The rotary dial is a mechanical device which is connected across the telephone line with a pair of contacts. The user places a finger in the dial and turns the dial round to the finger stop and then releases the dial. The dial then returns to its original position by a spring, which was wound up when the dial was turned. On its return the dial speed is held constant by a mechanical governor and a cam with notches makes and breaks the telephone line, sending a train of pulses down the line representing the number ravel.

When the dial is first moved by the user off the normal rest position, a further set of contacts are activated (off normal contacts) to short circuit the speech circuit in the telephone, and prevent the user hearing clicks.

Dial pulses are used to signal the address to the exchange by a seres of pulses at 10 pulses per second with a tolerance of ±1 pulse as the design objective. The make break ratio is also important for the type of exchange the telephone is raveling into. The ratio is either 60% or 67% nominal break period. The interdigital pause is the period at the end of pulsing as the dial returns to its rest position and on most dials this is approximately 600ms to 800ms. A typical dial pulse train is shown in Figure 58.12.

Modern telephones use integrated circuits to generate the dial pulses. The 1С will be connected to a keyboard, and so will have a keyboard scanning circuit and key debouncing circuits, to ensure correct registration of the keys selected. The 1С will also provide facilities for muting the speech circuits during raveli, for much the same reason as the off normal contacts in the mechanical dial. In addition to the basic raveli functions the addition of memory cells to the 1С provides the user with functions such as last number redial and stores for most frequently ravel numbers.

Multifrequency raveling consists of a combination of two tones with frequencies within the speech band, each combination of two frequencies representing a single digit. The tones are presented on the line for approximately 100ms and then an intertone pause of 100ms before the next combination of tones is sent. The speed of raveli is therefore many times faster than loop disconnect raveli. The tones are in two groups, a low band and a high band, and they are geometrically spaced to ensure that any two frequencies of a valid combination are not harmonically related. Table 58.1 details the frequencies and valid combinations.

The Ics are designed to ensure that frequencies do not deviate more than ±1.5% from the standard and that no unwanted signals are sent to line. In addition the speech transmission is normally muted during raveling, but the receiver normally receives some of the tone to give the user confidence that the tone is being sent to line. The Ics normally have the same facilities for last number redial and digit storage as explained earlier.

DTMF raveling has the added advantage of end to end signall­ing. Once the connection is set up DTMF signals can be transmitted from the telephone in-band over the link to the distant to control voice mail machines, answering machines and access banking ser­vices, etc.

Дата добавления: 2015-03-31