Signaling allows the phone to communicate with the network and other phone systems. Off-hook notification, dialtone, ringing, number dialing, and busy signals are all examples of signaling used in analog telephony.
Off Hook
The first hurdle in building a telephone network is knowing when a phone is taken off-hook. Once the phone switch identifies that the phone is off hook, it can provide dial tone and listen for dialed digits. Given the 2-wire connection between the phone company and the installed phone, a method of in-band signaling is required to indicate the off-hook condition. Loop start is the most common method of alerting the CO switch and seizing
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Figure 3-3
(a) On hook (no current flow) and (b) off hook circuit (current flows) completed.
the phone line. When a phone is taken off hook, it closes the off-hook switch which completes the circuit by connecting tip and ring through the phone's circuitry. Once the circuit is completed,
dc current flows from the phone switch through the phone powering the phone's electronics and indicating to the phone switch that the phone is off hook. In response, the phone switch provides dialtone over the circuit. The completing of the circuit forms a contiguous wire loop between the phone switch and the end phone. This is sometimes referred to as the local loop. A logical view of both on-hook and off-hook conditions is presented in Figure 3-3.
Ringing
While on hook, the circuit between the phone switch and the phone is open. When the phone switch needs to connect a call, it has no means of completing the circuit between the switch and the phone. Without the dc current generated by closing the off-hook switch, the phone does not have the power to make any noise. To overcome this, the phone network places the phone's ringing device between the off-hook switch in the phone and the phone switch. As such, the ringer in an analog phone is placed in parallel with the incoming tip and ring lines. To activate the ringer, the phone switch generates a ring tone by sending an ac signal at 20—47 Hz on the circuit. It is usually a 90-V signal at 20 Hz. The ring tone crosses the ringer's leads and activates a bell or, more commonly today, an electronic circuit which generates a ring tone. The ac nature of the signal causes the bell to ring without requiring the hook switch to be closed. The basic phone layout is depicted in Figure 3-4.
Upon answering the phone, the dc circuit is completed and the switch, recognizing this, disables the ring tone. While the switch is ringing the
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Figure 3-4
Simplified phone diagram: the ringer is placed before the switch hook, allowing it to receive the ac ring signal.
call destination, it also generates a ringing tone in band, back to the call initiator.
Dialing
Dialing from an analog phone can be performed in one of two ways: pulse or tone. Pulse is the simpler of the two and consists of transmitting sequences of pulses that indicate the numbers being dialed. The pulses are generated by opening and closing the local loop in short bursts. A digit is represented by the number of on/off bursts made in succession. There is a direct relationship between pulses and dialed digits, so the number 4 requires four pulses. The phone
switch acknowledges a pulse if the circuit is closed, for 40 ms. The period of the pulse waves is 100 ms, so the expected time for the circuit to be open is 60 ms. The circuit-open time between pulses is called the break time, and the time the circuit is closed during the pulsing is called the make time. The time between pulses should be 700 ms. All phone switches should support pulse dialing and most phones provide the option of using pulse dialing. Figure 3-5 displays the line voltage over the local loop when dialing the digit 3. Note the time intervals for each pulse. Tone dialing or touch-tone dialing uses a process called dual tone multifrequency, or DTMF. DTMF breaks up the keyboard into columns and rows. For each row, the keypad generates a unique frequency. Thus, when any of the buttons in that row are pressed, the keypad will generate the same frequency. Similarly, for each column the keypad generates a unique frequency. By pressing a single key, two frequencies are actu-
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Figure 3-5
Pulse dialing—line voltage during the three successive off-hook/on-hook intervals required to dial the number 3.
1209 Hz 1336 Hz 1477 Hz 1633 Hz 697 Hz 1 2 3 A 770 Hz 4 5 6 B 852 Hz 7 8 9 C 941 Hz * 0 # D Figure 3-6
DTMF frequencies: identifies the frequencies associated with each row and column of the keypad.
sent over the line and can identify the digit pressed by using simple logic circuits and its knowledge of the keypad matrix and frequencies. Figure 3-6 represents the keypad and the frequencies associated with each row and column.
For example, pressing the number 4 would generate a 1209-Hz tone and a 770-Hz tone. Whether pulse or tone, the dialing digits are sent in band and interpreted by the phone switch for call routing.
Network Call Progress Tones
The switched network indicates various network conditions by sending network call progress tones to the appropriate phone set. These tones include dialtone, busy signals, congestion (fast-busy), a receiver off-hook notifier, and other special messages. The frequencies and on/off times for these signals are provided in Table 3-1.
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TABLE 3-1 Call Progress Tones
Tone Frequency, Hz On Time, s Off Time, s
Dial tone 350 + 440 continuous n/a
Ring back 440 + 480 2 4
Busy 480 + 620 0.5 0.5
Congestion 480 + 620 0.2 0.3
Off-hook notifier 1400 + 2060 + 0.1 0.1
2450 + 2600
No such number 200 to 400 continuous n/a
Figure 3-7
Initial idle state: both phones are on-hook, both circuits are open.
Basic Call Processing Sequence
In order to review the concepts discussed above, a step-by-step walkthrough of a call setup is provided.
Step 1. The user lifts the handset of the phone. This activates the off-hook switch, completes the dc circuit, and triggers the phone switch, which generates the dialtone of the link. (See Figure 3-8.)
Step 2. The user acknowledges the dialtone and dials the digits for the phone number associated with the destination. The digits are sent in band and received by the phone switch. (See Figure 3-9.)
Step 3. The phone switch interprets the digits, looks up the destination in its routing table, and identifies the outbound path for the call. (See Figure 3-10.)
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Figure 3-8
The left-side user lifts the handset, closing the circuit.
Figure 3-9
Pulse digits are sent from the phone to the CO switch.
Figure 3-10
Page 40 Step 4. The phone switch generates a ring signal on the destination phone by sending the 90-V, 20-Hz ac signal down the output port. The switch also generates the ringing sound back to the call initiator. (See Figure 3-11.)
Step 5. The remote phone rings, and the user picks up the phone, completing the dc circuit. (See Figure 3-12.)
Step 6. The switch senses the completed circuit, disables the ringing and ring back, and connects the call.
This is a simplified description of call processing for a call which is placed in and out of two ports on the same phone switch. Calls which span multiple switches require more complicated call processing. These techniques are discussed later in the chapter.
Figure 3-11
The switch generates a ring signal for the receiving phone.
Figure 3-12
The receiving phone goes off hook, completing the circuit.
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