Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

Figure 11.4 SIP-T call flow with ISUP tunneling

SIP-T also uses the INFOmethod to carry midcall signaling information, as
shown in Figure 6.7.
The advantages of implementing SIP-T are obvious—it allows a carrier to
build a PSTN network using a SIP IP telephony core, and provides trans-
parency and full features. The disadvantages are not so obvious but can be
seen on closer examination. For example, the complexity of gateways imple-
menting SIP-T is much greater than a normal gateway, since a SIP-T gateway
must still do all the PSTN-to-SIP mapping of a regular gateway, plus the addi-
tional encoding, decoding, and parsing, of the ISUP attachments. For example,
the INVITEMessage 2 of Figure 11.4 would be:

INVITE sip:[email protected];user=phone SIP/2.0

Via: SIP/2.0/UDP ngw1.carrier.com:5060;branch=z9hG4bK74bf9

Max-Forwards: 70

From: <sip:[email protected];user=phone>;tag=gx3432

To: <sip:[email protected];user=phone>

1 IAM

3 100 Trying

7 ACM

2 INVITE (IAM)

4 100 Trying

6 183 Session Progress

(ACM)

One way RTP Media

8 ANM

9 200 OK (ANM)

11 ACK

13 BYE (REL)

No Speech Path No Media Session No Speech Path

12 REL

17 RLC

3 IAM

One way speech

5 ACM

One way speech

ISUP Messages

are carried as

message bodies

in the SIP

messages

between the

SIP-T Gateways.

Two way speech RTP Media Session Two way speech

15 RLC

16 200 OK (RLC)

10 ANM

14 REL

PSTN Switch SIP-T Gateway SIP-T Gateway PSTN Switch

SIP Telephony 193