InnoWave Telecommunications

Ccitt No 5 Signaling 85 Iscb

Switchblock TC4 TC4 Switchblock

Tone(s) On

Tones Off

Time

Tone(s) On

Tones Off

Time

Figure 4.3-1 Supervision Signals of CCITT No. 5. (a): signal on send channel at ISC-A. (b): signal on receive channel at ISC-A.

Signaling Frequencies. Two in-band signaling frequencies are used: /, = 2400 Hz, and/2 is 2600 Hz. There are three signals:/,,/2, and a composite signal that contains /, and/2.

Signaling Circuitry in Trunk Circuits. The functions and circuitry for supervision signaling in the TC4 circuits of analog CCITT No.5 trunks are similar to those discussed in 4.2.2. The tone detectors (TD) have to detect the presence of the signaling frequencies /, and/ or f2 on the receive (R) channel of the trunk. Since the signaling frequencies are ifi-band, a TD that detects their presence has to block them from entering the switchblock of the exchange. In addition, the tone detectors include circuitry that prevents the simulation of signaling tones by the speech (or other voiceband communications) of the subscribers.

Because of its in-band supervision signals, CCITT No.5 signaling is vulnerable to "blue box" fraud.

4.3.2 Supervision Signals and Call Handling Procedures

Table 4.3-1 lists the supervision signals, their acknowledgments, and the signaling frequencies. A particular physical signal can have several logical meanings, depending on its direction (forward or backward), and the state of the call. The supervision signals for a typical call on a trunk seized by ISC-A in Fig. 4.3-1 are outlined below.

Table 4.3-1 CCITTNo.5 supervision signals and their acknowledgments.

Signal	Direction	Frequency
Seizure	fwd	/.
Proceed-to-send	bkwd	h
Answer	bkwd

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