Note

SS7 is beneficial because it is an out-of-band signaling method and it interconnects to the Intelligent Network (IN). Connection to the IN enables the PSTN to offer Custom Local Area Signaling Services (CLASS) services.

SS7 is a method of sending messages between switches for basic call control and for CLASS. These CLASS services still rely on the end-office switches and the SS7 network. SS7 is also used to connect switches and databases for network-based services (for example, 800-number services and Local Number Portability [LNP]).

Some of the benefits of moving to an SS7 network are as follows:

• Reduced post-dialing delay

There is no need to transmit DTMF tones on each hop of the PSTN. The SS7 network transmits all the digits in an initial setup message that includes the entire calling and called number. When using inband signaling, each MF tone normally takes 50 ms to transmit. This means you have at least a .5-second post-dialing delay per PSTN hop. This number is based on 11-digit dialing (11 MF tones x 50 ms = 550 ms).

• Increased call completion

SS7 is a packet-based, out-of-band signaling protocol, compared to the DTMF or MF in-band signaling types. Single packets containing all the necessary information (phone numbers, services, and so on) are transmitted faster than tones generated one at a time across an in-band network.

• Connection to the IN

This connection provides new applications and services transparently across multiple vendors' switching equipment as well as the capability to create new services and applications more quickly.

To further explain the PSTN, visualize a call from my house to my Grandma's house 10 miles away. This call traverses an end office switch, the SS7 network (signaling only), and a second end office switch. Figure 1-11 displays the call flow from my house to Grandma's.

Figure 1-11. PSTN Call Flow to Grandma's House

Figure 1-11. PSTN Call Flow to Grandma's House

To better explain the diagram in Figure 1-11, let's walk through the flow of the call:

1. I pick up the phone and send an off-hook indication to the end office switch.

2. The switch sends back a dial tone.

3. I dial the digits to call Grandma's house (they are sent in-band through DTMF).

4. The switch interprets the digits and sends an Initial Address Message (IAM, or setup message) to the SS7 network.

5. The SS7 network reads the incoming IAM and sends a new IAM to Grandma's switch.

6. Grandma's switch sends a setup message to Grandma's phone (it rings her phone).

7. An alerting message (alerting is the same as the phone ringing) is sent from Grandma's switch (not from her phone) back to the SS7 network through an Address Complete Message (ACM).

8. The SS7 network reads the incoming ACM and generates an ACM to my switch.

9. I can hear a ringing sound and know that Grandma's phone is ringing. (The ringing is not synchronized; your local switch normally generates the ringing when the ACM is received from the SS7 network.)

10. Grandma picks up her phone, sending an off-hook indication to her switch.

11. Grandma's switch sends an ANswer Message (ANM) that is read by the SS7, and a new ANM is generated to my switch.

12. A connect message is sent to my phone (only if it's an ISDN phone) and a connect acknowledgment is sent back (again, only if it's an ISDN phone). (If it is not an ISDN phone, then on-hook or off-hook representations signal the end office switch.)

13. I can now talk to Grandma until I hang up the phone (on-hook indication).

If Grandma's phone was busy, I could use an IN feature by which I could park on her line and have the PSTN call me back after she got off the phone.

Now that you have a basic understanding of how the PSTN functions, the next section discusses services and applications that are common in the PSTN.

If you want more information on PSTN signaling types, see Chapter 3 and Chapter 4.

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