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End Office Switch Call-Flow Versus IP Phone Call

To simplify a TDM or end office switch call-flow and an IP call-flow, this section looks at ways you can call your next-door neighbor using both the PSTN and the Internet. Figure 7-10 shows a basic call-flow in the PSTN today. Compare this to an IP phone call-flow and notice the similarities of necessary call setup.


Figure 7-10 Calling My Neighbor with Today's PSTN

In this example, Bob calls his neighbor Judy. They are both subscribers on the local end office switch, and therefore, no SS7 is needed. The following steps occur:

  1. Bob picks up his handset (off hook).
  2. The local end office switch gives Bob a dial tone.
  3. Bob dials Judy's seven-digit phone number.
  4. The end office switch collects and analyzes the seven-digit number to determine the destination of the phone call. The end office switch knows that someone from Bob's house is placing the call because of the specific port that it dedicated to Bob.
  5. The switch analyzes the seven-digit called number to determine whether the number is a local number that the switch can serve.
  6. The switch determines Judy's specific subscriber line.
  7. The end office switch then signals Judy's circuit by ringing Judy's phone.
  8. A voice path back to Bob is cut through so that Bob can hear the ring-back tone the end office switch is sending. The ring-back tone is sent to Bob so that he knows Judy's phone is ringing. (The ringing of Judy's phone and the ring-back tone that Bob hears need not be synchronized.)
  9. Judy picks up her phone (off hook).
  10. The end office switch cuts through the voice path from Bob to Judy. This is a 64 Kbps, full-duplex DS-0 (Digital Service, Level 0) in the end office switching fabric to enable voice transmission.

Figure 7-11 demonstrates the call-flow necessary to complete an Internet phone call using a PC application.


Figure 7-11 Calling with an Internet-Phone Application

Both Bob and Judy need to be on the Internet or have some other IP network between their homes to talk to each other. Assuming this IP network exists or that both neighbors have a connection to the Internet, you can then follow this possible call-flow:

  1. Judy launches her Internet phone (I-phone) application, which is H.323-compatible.
  2. Bob already has his I-phone application launched.
  3. Judy knows that Bob's Internet "name," or Domain Name System (DNS) entry, is bob@nextdoorneighbor.com, so she puts that into the "who to call" section in her I-phone application and presses Return.
  4. The I-phone application converts Bob.nextdoorneighbor.com to a DNS host name and goes to a DNS server that is statically configured in Judy's machine to resolve the DNS name and get an actual IP address.
  5. The DNS machine passes back Bob's IP address.
  6. Judy's I-phone application takes Bob's IP address and sends an H.225 message to Bob.
  7. The H.225 message signals Bob's PC to begin ringing.
  8. Bob clicks on the Accept button, which tells his I-phone application to send back an H.225 connect message.
  9. Judy's I-phone application then begins H.245 negotiation with Bob's PC.
  10. H.245 negotiation finishes and logical channels are opened. Bob and Judy can now speak to one another through a packet-based network.

The example does not show all the steps and omits some details that a service provider needs to deploy a VoIP network. Because IP is a ubiquitous protocol, as mentioned in Chapter 6, when a call is packetized, it could be destined to your next-door neighbor or to a relative in Norway.

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