Sistema ss7

C H A P T E R

1

Overview
Everything in the telecommunications network is based on signaling—call setup, connection, teardown, and billing. The two forms of signaling used by the network are:
• •

Channel Associated Signaling (CAS) Common Channel Signaling (CCS)

Signaling System Number Seven (SS7) is a form of common channel signaling, that provides intelligence to the network, andallows quicker call setup and teardown—saving time and money.

PSTN Signaling
In order to route telephone traffic through the Public Switched Telephone Network (PSTN), it is necessary to communicate with the switches that make up the PSTN. Signaling is a means for transferring network-related information between switching nodes, and also between the end office switches and their subscribers. (SeeFigure 1-1.) Signaling is used to do the following:
• • • • •

Request service from the central office switch (via going off-hook). Provide central office switch with the information necessary to route a telephone call (via DTMF addressing digits in a specific format). Alert destination address of incoming call (ringing). Provide status information and call supervision for billing. Managenetwork lines/trunks (set up and teardown calls).
End-to-End Signaling

Figure 1-1

DTMF

End Office

MF or CCS

End Office

Ringing
44902

Cisco SS7 Fundamentals 78-11278-01

1-1

Chapter 1 Channel Associated Signaling (CAS)

Overview

Channel Associated Signaling (CAS)
When used for in-band signaling:
• • • •

Call setup information (off-hook, dialtone, address digits,ringback, busy) is transmitted in the same band of frequencies as used by the voice signal. Voice (talk) path is cut over only when the call setup is complete, using the same path that the call setup signals used. SF (single-frequency) signaling uses tones to represent on-hook or payphone deposits. MF (multi-frequency) signaling is used for switch-to-switch call setup

The principal advantage ofCAS is that it is inexpensive to implement and can be used on any transmission medium. However, CAS has the following disadvantages:
• • •

Fraud— “phone freaks” can build boxes to play call setup and teardown tones. Interference is possible between signaling tones used by the network and frequencies of human speech patterns. Speed—call setup and teardown is slower, less efficient use ofresources.

Common Channel Signaling (CCS)
CCS employs a separate, dedicated path for signaling. (See Figure 1-2.) Voice trunks are used only when a connection is established, not before. Call setup time is quicker because resources are more efficiently used. CCS is the technology that makes ISDN and SS7 possible.
Figure 1-2 Common Channel Signaling

Switch A

Voice Trunks

Switch B
44903Signaling Link

ISDN-PRI
Integrated Services Digital Network— Primary Rate Interface (ISDN-PRI) divides digital transport services into bearer channels (B-channels) for voice and data transmission and data channels (D-channels) for signaling data. (See Figure 1-3.) In North America T1-PRI employs 24 channels (23B+1D at 64 Kbps per PCM channel) with an aggregate bandwidth of 1.536 Mbps. InEurope E1-PRI employs 32 channels (30B+2D at 64 Kbps per PCM channel) with an aggregate bandwidth of 2.048 Mbps. ISDN-PRI offers the following advantages:
• •

Data links running at either 56 or 64 Kbps are much quicker than outpulsing MF address digits. Signaling is possible at any time during the call, rather than only during call setup.

Cisco SS7 Fundamentals

1-2

78-11278-01

Chapter1

Overview Common Channel Signaling (CCS)

• • •

Voice trunks are used more efficiently—others can use them during call setup. Allows better control over fraud. Supports enhanced services.

The principal disadvantage of ISDN-PRI is its use of Associated Signaling mode, which only works with directly trunked switches.
Figure 1-3 ISDN Bearer Vs. Data Channels

64 Kbps Signaling Data...