This document describes the roles and procedures of the Border Gateway Control Function (BGCF), Media Gateway Control Function (MGCF), and IP Multimedia Subsystem Media Gateway (IMS-MGW) in interworking between IP Multimedia Subsystem (IMS) networks and circuit switched networks like the Public Switched Telephone Network (PSTN). The BGCF determines routing between IMS networks. The MGCF controls media channels and performs protocol conversion between SIP and ISUP/TCAP. The IMS-MGW terminates media channels and streams and supports media functions under MGCF control. The document then outlines call setup and release procedures between an IMS network and PSTN involving these components.

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2. BGCF:
• Determines the next hop for SIP routing;
• For PSTN terminations, select the network in which the interworking with the PSTN/CS
Domain is to occur. If the interworking is in another network, then the BGCF will forward the
SIP signalling to the BGCF of that network;
• For PSTN terminations, select the MGCF in the network in which the interworking with
PSTN/CS Domain is to occur and forward the SIP signalling to that MGCF. This may not apply if
the interworking is a different network;
• Generation of CDRs
The MGCF:
• Controls the parts of the call state that pertain to connection control for media channels in an
IMS-MGW;
• Communicates with CSCF, BGCF, and circuit switched network entities;
• Determines the next hop depending on the routing number for incoming calls from legacy
networks;
• Performs protocol conversion between ISUP/TCAP/SIP-I and the IM subsystem call control
protocols;
• Out of band information received in MGCF may be forwarded to CSCF/IMS-MGW.
A IMS-MGW may terminate bearer channels from a switched circuit network and media streams
from a packet network (e.g., RTP streams in an IP network). The IMS-MGW may support media
conversion, bearer control and payload processing (e.g. codec, echo canceller, conference
bridge), it:
• Interacts with the MGCF for resource control;
• Owns and handles resources such as echo cancellers etc;
• May need to have Codecs;
• The IMS-MGW will be provisioned with the necessary resources for supporting UMTS/GSM
transport media.
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3. The MGCF in the IM CN subsystem is a SIP endpoint that initiates requests on behalf of
the PSTN and Media Gateway. The subsequent nodes consider the signalling as if it came
from a S-CSCF.
Procedures:
1. The PSTN establishes a bearer path to the MGW, and signals to the MGCF with a IAM
message, giving the trunk identity and destination information.
2-3. The MGCF initiates a H.248 command, to seize the trunk and an IP port.
4. The MGCF initiates a SIP INVITE request addressed to a tel URI or, if directed by
operator's local policy, to a SIP URI (using an E.164 address in the user portion and the
setting user=phone), includes an initial SDP in the INVITE request, and forwards the
request to a configured I-CSCF, as per the proper S-S procedure.
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4. 15. The media stream capabilities of the destination are returned along the signalling
path, per the S-S procedures;
16-17. MGCF initiates a H.248 command to modify the connection parameters and
instruct the MGW to reserve the resources needed for the session.
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6. 25. Terminating end point responds to the Response Confirmation. If Optional SDP is
contained in the Response Confirmation, the Confirmation Acknowledge will also
contain an SDP response;
26-27. MGW reserves the resources needed for the session.
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7. 28. When the resource reservation is completed, MGCF sends the successful Resource
Reservation message to the terminating endpoint, per the S-S procedures.
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8. 32-35. Terminating end point responds to the successful media resource reservation;
36-39. The destination endpoint may optionally perform alerting. If so, it signals this to
the originating party by a provisional response indicating Ringing. This message is sent to
MGCF per the S-S procedure;
40. If alerting is being performed, the MGCF forwards an ACM message to PSTN.
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9. 32-35. Terminating end point responds to the successful media resource reservation;
36-39. The destination endpoint may optionally perform alerting. If so, it signals this to
the originating party by a provisional response indicating Ringing. This message is sent to
MGCF per the S-S procedure;
40. If alerting is being performed, the MGCF forwards an ACM message to PSTN.
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10. 41-44. When the destination party answers, the terminating and S-S procedures result in
a SIP 200-OK final response being sent to MGCF;
45. MGCF forwards an ANM message to the PSTN.
46-47. MGCF initiates a H.248 command to alter the connection at MGW to make it bi-
directional.
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11. 48-51. MGCF acknowledges the SIP final response with a SIP ACK message.
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12. 52. PSTN party hangs up, which generates an ISUP REL message to the MGCF;
53. The MGCF sends a Hangup (Bye message in SIP) to the S-CSCF to notify the terminal
that the far end party has disconnected;
54. The MGCF notes the reception of the REL and acknowledges it with an RLC. This is
consistent with the ISUP protocol;
55. The MGCF requests the MGW to release the vocoder and ISUP trunk using the
H.248/MEGACO Transaction Request (subtract). This also results in disconnecting the
two parties in the H.248 context. The IP network resources that were reserved for the
message receive path to the PSTN for this session are now released. This is initiated
from the MGW. If RSVP was used to allocated resources, then the appropriate release
messages for that protocol would be invoked here;
56. The MGW sends an acknowledgement to the MGCF upon completion of above step;
57-58. The S-CSCF invokes whatever service logic procedures are appropriate for this
ending session;
59. The S-CSCF forwards the Hangup to the P-CSCF. The P-CSCF/PCRF removes the
authorization for resources that had previously been issued for this endpoint for this
session. This step also results in a release indication to the IP-CAN to confirm that the IP
bearers associated with the UE#2 session have been deleted;
60. The P-CSCF forwards the Hangup to the UE.
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13. 61. The terminal responds with an acknowledgement, the SIP OK message (number
200), which is sent back to the P-CSCF. The IP network resources that had been reserved
for the message receive path to the endpoint for this session are released, taking into
account the bearer establishment mode used for the IP-CAN session.
62. The SIP OK message is sent to the S-CSCF.
63. The S-CSCF forwards the message to the MGCF.
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14. The MGCF in the IM CN subsystem is a SIP endpoint that initiates and receives requests
on behalf of the PSTN and Media Gateway (MGW).Other nodes consider the signalling
as if it came from a BGCF (S-CSCF).
Procedures:
1. MGCF receives an INVITE request, containing an initial SDP, through one of the
origination procedures and via one of the inter-serving procedures;
2. MGCF initiates a H.248 interaction to pick an outgoing channel and determine media
capabilities of the MGW;
3. MGCF determines the subset of the media flows proposed by the originating
endpoint that it supports, and responds with an Offer Response message back to the
originator. This response is sent via the S-S procedure;
4. The originating endpoint sends a Response Confirmation. The Response
Confirmation may also contain SDP. This may be the same SDP as in the Offer
Response sent in Step 3 or a subset. The originating UE is free to continue to offer
new media on this operation or on subsequent exchanges using the Update method;
5. MGCF initiates a H.248 interaction to modify the connection established in step #2
and instruct MGW to reserve the resources necessary for the media streams;
6. MGCF responds to the offered media towards the originating party;
7. GW reserved the resources necessary for the media streams;
8. When the originating endpoint has completed its resource reservation, it sends the
successful Resource Reservation message to MGCF, via the S-S procedures;
9. MGCF sends an IAM message to the PSTN;
10. MGCF sends response to the successful resource reservation towards originating
end.
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15. 11. The PSTN establishes the path to the destination. It may optionally alert the
destination user before completing the session. If so, it responds with an ACM
message;
12. If the PSTN is alerting the destination user, MGCF indicates this to the originating
party by a provisional response indicating Ringing. This message is sent via the S-S
procedures;
13. When the destination party answers, the PSTN sends an ANM message to MGCF;
14. MGCF initiates a H.248 interaction to make the connection in the MGW bi-
directional;
15. MGCF sends a SIP 200-OK final response along the signalling path back to the
session originator;
16. The Originating party acknowledges the final response with a SIP ACK message.
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