Example of Sigtran Usage

Figure 8-18 provides an example of how IP and SS7 networks can interwork
using Sigtran. The IP-to-SS7 connectivity diagram shows how a SIP
device could be connected to an MG and an MGC such that it can communicate
with a standard telephone in the PSTN. The IP-to-SS7 protocol interworking
diagram shows how the protocol interworking can take place via a
signaling gateway (SG). The net effect is that the nodes, such as a PSTN
switch, in the SS7 network can communicate with the SIP terminal via the
SG and MGC and MG without realizing that the SIP terminal is not a standard
telephone connected to a standard SS7-enabled switch.
Of course, the MGC must be able to translate SIP messages to ISUP
messages and vice versa. Although these two protocols are different, the
messages of SIP and those of ISUP do serve similar functions, and it is possible
to map from one protocol to the other. For example, the ISUP Initial
Address Message (IAM) maps quite well to the SIP INVITE. The SIP 183 (Session Progress) response, an extension to the original SIP specification,
maps to the ISUP Address Complete message (ACM). The SIP 200 (OK)
response maps to the ISUP Answer (ANS) message.