Figure 19 Dual Tone Multi Frequency

Dual Tone Multi-Frequency

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When you pick up your telephone handset and press the digits (as shown in Figure 1-9), the tone that passes from your phone to the central office switch to which you are connected tells the switch what number you want to call.

ISDN uses another method of signaling known as out-of-band . With this method, the signaling is transported on a channel separate from the voice. The channel on which the voice is carried is called a bearer (or B channel) and is 64 kbps. The channel on which the signal is carried is called a data channel (D channel) and is 16 kbps. Figure 1-10 shows a Basic Rate Interface (BRI) that consists of two B channels and one D channel.

Figure 1-10. Basic Rate Interface

Figure 1-10. Basic Rate Interface

Out-of-band signaling offers many benefits, including the following:

• Signaling is multiplexed (consolidated) into a common channel.

• Glare is reduced (glare occurs when two people on the same circuit seize opposite ends of that circuit at the same time).

• A lower post dialing delay.

• Additional features, such as higher bandwidth, are realized.

• Because setup messages are not subject to the same line noise as DTMF tones, call completion is greatly increased.

In-band signaling suffers from a few problems, the largest of which is the possibility for lost tones . This occurs when signaling is carried across the voice path and it is a common reason why you can sometimes experience problems remotely accessing your voice mail.

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