When the call is answered and the MGCF
BGCF BGCF
MGCF
MGCF
MGCF
MGCF
MGCF
MGCF
S-CSCF S-CSCF
Network domain A Network domain B
Figure 5.2 Relationship between the BGCF and MGCF
Establishing a Session in the IMS 121
receives the Answer Message (ANM) from SS7, it sends the response 200 OK to the IMS
originator. Figure 5.3 shows a typical call flow between the two domains.
There are numerous variables for the responses, depending on the values within
the SS7 parameters (if originated in the SS7 domain), or dependent on the SIP header
values (if originated in the SIP domain). Tables 5.2 and 5.3 show the SIP responses to
the various SS7 messages given different parameter values.
If a Confusion (CON) message is received, the MGCF will usually discard the message
without sending any SIP response. There is an option of encapsulating the CON
message into a SIP message body and forwarding it through the IMS to another SS7
entity, but there is no mapping of parameter values to SIP in this case.
When the MGCF receives an Exit Message (EXM), the SS7 EXM is encapsulated
within SIP and sent through the IMS to the destination entity, using the response
Device P-CSCF S-CSCF
SIP/IMS domain SS7/CS domain
MGCF STP EO Phone
INVITE
INVITE
INVITE
IAM
IAM
IAM
100 TRYING
100 TRYING
100 TRYING
SESSION
PROGRESSION
SESSION
PROGRESSION SESSION
PROGRESSION
ACM
ACM
ACM 180 RINGING
180 RINGING
180 RINGING ANM
ANM
ANM
200 OK
200 OK
200 OK
ACK ACK
ACK
CONVERSATION
BYE BYE
BYE
REL
REL
REL
RLC RLC
RLC
200 OK
200 OK
200 OK
Figure 5.
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