The customer also causes current to be drawn from the exchange line feed voltage when the phone goes off-hook at the beginning of a new call. It is this current flow that is detected in the local exchange and results in the application of dial tone and the connection of a subsystem to detect the customer’s dialled digits. Similarly, when the phone goes on-hook at the end of the call, the lack of current flow signals to the local exchange indicates that the call has ended. Again transients are generated in these processes, but they are clearly smaller than those caused during ring-trip.
1.9 Dialing
Dialing is the process by which the party initiating a POTS connection attempts to gen-erate the signals (to be sent to the local exchange) that identify the party at the other end of the desired connection. These signals may be in the form of dial pulses (sequential on-hook or off-on-hook of the terminal equipment causing current pulses) or tones. The local exchange indicates that it is ready to receive these signals by sending a “dial tone” signal to the terminal equipment. Upon receipt of the appropriate pulses or tones, the local exchange equipment interprets the signals to appropriately set up a connection. In older telephone sets, a mechanical technique known as pulse dialing (or loop disconnect dialing) is often used, whereas newer telephones tend to use tone dialing using dual-tone multi-frequency (DTMF) signalling. One of the key features of DSL is that it can be deployed as an overlay service; i.e., it can “peacefully” co-exist on lines with legacy services. In many administra-tions, replacement of telephony terminal equipment is at the user’s discretion and expense.
This at least partially explains why many older telephone sets still exist in networks used for DSL, and hence why pulse dialing is relevant to DSL.
1.9.1 Pulse (Loop Disconnect) Dialing
Once a dial tone has been received by the calling party, a rotary dial such as that shown in Figure 1.12 can provide called party address information to the central office in the
FIGURE 1.12 Rotary dial telephone.
Time Off-hook
On-hook
Seizure
Connect Dialling ‘4’ Inter-digital
Time Dialling ‘2’
Inter-digital Time
Break Interval 60 ms
Make Interval 40 ms
Minimum 200 ms
FIGURE 1.13
Dial pulse sequences with a pulse rate of 10 PPS for the digits“4,20.”
form of dial pulses, i.e., short duration breaks in the loop current caused by the opening and closing of the loop. A conventional telephone set will have ten equally spaced finger-holes, as shown inFigure 1.12.The mechanical operation of pulse dialing is described in [Fike 1983]. The nominal pulse rate is ten pulses per second (i.e., 10 Hz), but a typical rotary dial instrument can vary from eight to twelve PPS. The make/break (off-hook/on-hook) ratio applied to the loop is nominally 40/60; i.e., the loop is closed 40 percent of the time and opened 60 percent. A sample dial pulse pattern is shown in Figure 1.13. Detection of the dial pulses by modern local exchange equipment is primarily based on time-domain signal processing (knowledge of the pulse rate, make/break ratio); however, an additional consideration is the distortion introduced by the copper pair.
1.9.1.1 High Voltage Transients Due to Pulse Dialing
Voltage spikes are produced each time the dial pulsing contacts break the circuit, i.e., inter-rupt the flow of loop current. These spikes can have a large amplitude, in part due to the inductive DC feed used in many local exchanges.
Conventional telephones have an inherent protection against these voltage spikes, which can cause the bell of the ringing circuit in the terminal equipment (see Section 1.6) to sound.
The consequent “ringing” is usually very soft, and hence is referred to as “tinkle” (or “bell tap”). In order to prevent this effect, an “anti-tinkle” circuit such as that shown inFigure 1.14 is used. Basically when the dial is rotated, the ringing circuit is shunted with a resistor R in order to prevent bell tinkle. The ringing capacitor C in Figure 1.1419 serves as a spark quencher to suppress arcing at the dial pulsing contacts. Additionally, the speech circuit is
19The primary function of this capacitor is to prevent direct current from passing through the ringing circuit.
July 22, 2005 10:44 CRC-AU1913 AU1913˙Book
Overview of the POTS Environment—Signals and Circuits 25
S1
S4
S2
S3
S5
S6
Speech Circuit
Bell R = 340 Ohms
Switchhook in on-hook position C
Rotary Dial S5 and S6 are mechanically coupled to the dial.
They close and remain closed when the dial is rotated.
They open when the dial returns to its real position.
FIGURE 1.14 Anti-tinkle circuit.
also shorted by switches 1 and 2 to prevent loud clicks in the receiver during pulse dialing.
In terms of DSL, the previously described transient voltages can have significant energy in the frequency band used for DSL transmission and can thus cause significant interference with DSL performance if not adequately filtered out [ETSI TR 102 139]. This is one of the functions of the splitter.
1.9.2 Tone Dialing
Most newer telephones use tone dialing, not least because it is much faster than pulse dialing. Tone dialing involves a dial pad with push-buttons and almost always uses DTMF signalling. The most common form of DTMF dialing uses a dial pad similar to that shown in Figure 1.15. Each button (and hence each touch-tone digit) is represented by a unique combi-nation of two single-frequency tones. The frequencies are arranged in a matrix, as shown in Figure 1.15. As the button is pressed for a specific digit, the appropriate combination of tones is generated, corresponding to the horizontal and vertical position of the push-button. (For example, pushing “1” would generate tones at 697 Hz and 1209 Hz.) At the central office, the received tones are filtered and detected in order to determine the required digit. For Eu-ropean networks, DTMF equipment and transmission are specified by [ETSI ES 201 235-1],
1 2 3
4 5 6
7 8 9
0 #
FIGURE 1.15 DTMF dial pad.
whereas for North America both [ANSI T1.401-2000] and [ANSI/EIA/TIA-470-A-1987]
contain DTMF specifications. DTMF signalling takes place at a frequency range well below those used in DSL and hence does not typically cause interference with DSL transmission.