Chapter 26: Signaling System No. 7

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A communication system has to handle two types of data—user data and signaling data. The signaling data is used for setting up a call, disconnecting the call, and other things such as billing. The signaling used in the PSTN is not very efficient, so Signaling System No. 7 has been developed. This signaling system uses a separate data network for carrying signaling information. In this chapter, we will discuss the architecture of the Signaling System No. 7.

26.1 DRAWBACKS OF PSTN SIGNALING

In the PSTN, the following signaling information is transmitted from the subscriber to the switch and between switches:

• When a subscriber wants to make a telephone call, the off-hook information is sent to the switch.

• When the subscriber dials a number, the dialed digits are sent to the switch.

• The switch analyzes the dialed digits to route the call to the destination.

• The switch uses the signaling information to allocate trunks.

• The switch monitors the on-hook condition and generates billing information.

• Tones such as a ringing tone and a busy signal are sent to subscribers.

Note 

The signaling information can be divided into two types: (a) signaling between the subscriber terminal and the switch; and (b) signaling between switches.

As we discussed in the chapter on PSTN, in-band signaling and channel associated signaling are used for transferring the signaling information between the subscriber and the switch and between the switches. This type of signaling has the following drawbacks:

• The same communication link is used for both user data and signaling, so the communication resources are not used efficiently.

• Call setup and disconnection are very slow.

• The tones used for signaling will interfere with human speech because the tones are in the voice frequency band.

In the PSTN, signaling information is exchanged to set up and disconnect a call, to exchange billing information, and to exchange network management information.

Due to these drawbacks, a new standard by ITU-T has been developed, and it is called Signaling System No. 7. This system is used extensively in cellular mobile communications, Integrated Services Digital Network (ISDN), and PSTN in many developed countries.

The signaling used in the PSTN is inefficient because the same communication channel is used for both signaling and data transfer.

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