The document describes the sequence of events that occur during IMS registration and session establishment in 3G/4G networks. 1. A UE first attaches to the GPRS network and establishes a PDP context to get an IP address. It then performs CSCF discovery and IMS registration by exchanging SIP REGISTER messages with the P-CSCF, I-CSCF and S-CSCF. 2. To establish a session, the UE sends a SIP INVITE message which is routed through CSCFs and may involve application servers based on the user's profile. The message is sent to the destination UE to initiate session negotiation. 3. Sessions involving the PST

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2. This slide shows the high-level sequence of events that take place prior to the IMS
registration
1. The UE attaches to the GPRS level
2. The UE establishes a PDP context. This may be a general PDP context or a dedicated
PDP context for IMS signalling. As a result, the UE gets an IPv6 /64 prefix. This allows
the UE to create an IPv6 address.
3. The UE performs the CSCF discovery procedure. As a result, the UE gets the IP
address of the P-CSCF allocated to the UE
4. The UE performs the IMS registration procedures
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3. 1. The UE sends a SIP REGISTER message. This message includes a Public User ID, the Private User ID, and
the home network SIP URI.
2. The P-CSCF finds, with the help of DNS, the home network entry point (I-CSCF). The P-CSCF forwards
the REGISTER request to that I-CSCF.
3. The I-CSCF queries the HSS to find out if there is an already allocated S-CSCF to this user.
4. The HSS returns either:
• The address of the S-CSCF allocated to this user.
• A set of capabilities that will help the I-CSCF to choose an appropriate S-CSCF for this user.
5. The I-CSCF may choose a new S-CSCF or use an already allocated one. In both cases, it forwards the
REGISTER request to that S-CSCF.
6. The S-CSCF informs the HSS that this S-CSCF is taking care of the user.
7. The HSS returns one or more authentication vectors.
8. The S-CSCF challenges the UE.
9. The I-CSCF forwards the SIP response.
10. The P-CSCF forwards the SIP response to the UE.
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4. The registration procedure continues:
11. The UE calculates the credentials, includes them into the REGISTER request, and
sends it to the P-CSCF.
12. The P-CSCF finds, with the help of DNS, the home network entry point (I-CSCF). The
P-CSCF forwards the REGISTER request to that I-CSCF.
13. The I-CSCF queries the HSS to find out if there is an already allocated S-CSCF to this
user.
14. The HSS returns the address of the S-CSCF allocated to this user.
15. The I-CSCF forwards the REGISTER request to that S-CSCF.
16. The S-CSCF informs the HSS that this S-CSCF is taking care of the user.
17. The S-CSCF returns the user profile containing the filter criteria
18. The S-CSCF evaluates the filter criteria, and may contact, if needed, one ore more
application server. In this example, the filter criteria indicates that an application
server has to be informed about the user's registration. The S-CSCF creates a new
REGISTER (3rd party REGISTER)requests and sends to an application server.
19. The AS acknowledges the reception of the REGISTER request
20. The S-CSCF informs the UE about the successful registration.
21. The I-CSCF forwards the response.
22. The P-CSCF forwards the response
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5. The Session Establishment procedures:
1. The UE sends an INVITE request through the originating leg (P-CSCF, optional I-CSCF
and S-CSCF). The INVITE request includes a Request-URI with the SIP URI of
destination subscriber.
2. The INVITE request contains a Route header that helps the P-CSCF to route the
INVITE request to the I-CSCF.
3. The I-CSCF, with the help of the Route header, forwards the INVITE request to the S-
CSCF.
4. The S-CSCF evaluates the filter criteria for subscriber A. In this example, subscriber A
does not have any originating services, so no AS is contacted. Then the S-CSCF
inspects the Request-URI to find the destination network. With the help of DNS, the
S-CSCF finds the entry point (I-CSCF) of Home B. The S-CSCF forwards the INVITE
request to that I-CSCF.
5. The I-CSCF queries the HSS to find out which S-CSCF is serving the destination
subscriber B.
6. The HSS returns the address of the S-CSCF serving subscriber B
7. The I-CSCF forwards the INVITE request to that S-CSCF
8. The S-CSCF evaluates the service criteria for subscriber B. In this example, there are
no AS involved. The S-CSCF, inspecting the Contact information (at registration),
forwards the INVITE request to the P-CSCF.
9. The P-CSCF forwards the INVITE request to the UA
10-16. UE B accepts the session. The 200 (OK) response is forwarded backwards to UE A.
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6. Session initiation involving PSTN interworking:
1. The UE sends an INVITE request. The Request-URI (destination) contains a TEL URL.
2. The INVITE request contains a Route header that helps the P-CSCF to route the
INVITE request to the I-CSCF.
3. The I-CSCF, with the help of the Route header, forwards the INVITE request to the S-
CSCF.
4. The S-CSCF evaluates the filter criteria for subscriber A. In this example, subscriber A
does not have any originating services, so no AS is contacted. Then the S-CSCF
inspects the Request-URI to find the destination network. As the Request-URI
contains a TEL URL, the S-CSCF queries DNS to translate the TEL URL into a SIP URI.
5. The DNS returns a "no match" for this TEL URL.
6. The S-CSCF, as it has no options, routes the call to the BGCF in the home network.
7. The BGCF in the home network selects a suitable gateway operator (another BGCF)
8. The BGCF selects an appropriate MGCF.
9. The MGCF reserves resources (TDM slot, DSP processors, etc.) in the Media
Gateway.
10. The MG confirms the successful reservation of resources.
11. The MGCF initiates BICC/ISUP signalling towards the PSTN.
12. Eventually, the subscriber in the PSTN answers the call.
12-16 The call is completed.
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7. This slide shows an example of a roaming subscriber initiating a session. Only the
originating leg of the session is shown. Note that there is not I-CSCF or HSS involved.
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9. This slide shows an example of a roaming subscriber terminating a session. Only the
terminating leg of the session is shown. Note that there is not I-CSCF involved.
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11. The service profile defines the service handling for a user at the S-CSCF. The profile itself
is a structured data format with the following information elements:
•Application address: The SIP URI the S-CSCF uses to address the application
•Filter criteria: This element specifies the conditions for the invocation of the
application. The condition is formulated as a filter rule, which can be applied to a SIP
request (e.g. request method, presence or absence of header, header contents, etc.).
The filter “language” allows formulating nearly arbitrary complex invocation rules.
•Application priority: The priority of an application determines the sequence of
application execution in case the invocation conditions of multiple applications match a
single SIP request received by the S-CSCF.
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12. This slides shows an example of an application server involvement.
The service running in Application Server 33 is designed so that sessions received during business hours
from the user's mother are diverted to an automatic answering machine, but calls received outside
business hours proceed as usual.
The service running is Application Server 38 is a Call Forwarding Unconditional, but in this example, as the
answering machine service has priority, it is not triggered.
Note that AS 33 is acting as a User Agent (UA) for this particular instance of the service.
1. The S-CSCF receives an INVITE request. The Request-URI contains the destination user,
sip:user1@home1.net
2. At registration time, the S-CSCF downloaded the user profile containing the Filter Criteria.
• The S-CSCF evaluates the user's Filter Criteria against the received INVITE request.
• The Filter Criteria indicates that:
• INVITE request
• Address to sip:user1@home1.net
• Where the originating subscriber is sip:users_mom@home1.net
• ACTION: forward the INVITE to the Application Server 33
• Therefore, the S-CSCF forwards the INVITE request to AS33.
3. AS33 evaluates the current time: It is 11:00. The logic of the service says that between 9 and 17 all
session attempts from the user's mother are diverted to an answering machine. The AS acts as a SIP
User Agent and returns a 200 OK response.
4. The 200 OK response is forwarded to the user's mother
The AS 33 plays some personalized welcome message and starts the recording of the message.
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13. This slides shows an example of an application server involvement.
The service running in Application Server 33 is designed so that sessions received during business hours
from the user's mother are diverted to an automatic answering machine, but calls received outside
business hours proceed as usual.
The service running is Application Server 38 is a Call Forwarding Unconditional.
Note that AS 33 is acting as a proxy server for this particular instance of service.
1. The S-CSCF receives an INVITE request. The Request-URI contains the destination user,
sip:user1@home1.net
2. The S-CSCF evaluates the user's Filter Criteria against the received INVITE request.
• The Filter Criteria indicates that:
• INVITE request
• Address to sip:user1@home1.net
• Where the originating subscriber is sip:users_mom@home1.net
• ACTION: forward the INVITE to the Application Server 33
• Therefore, the S-CSCF forwards the INVITE request to AS33.
3. AS33 evaluates the current time: It is 19:00. The logic of the service says that between 9 and 17 all
session attempts from the user's mother are diverted to an answering machine. Otherwise, the AS is
not interested in the signalling. The AS acts as a SIP proxy and forwards the INVITE request again to
the S-CSCF.
4. The S-CSCF continues the Filter Criteria evaluation. As a result of it, it forwards the INVITE to another
AS, AS 38.
5. AS 38 is a Call Forwarding AS. It re-writes the Request-URI (destination) to another location
(sip:user44@home33.net) where the user may be contacted. Acting as a SIP proxy, the AS 38
forwards the INVITE back to the S-CSCF
6. The S-CSCF finished the evaluation of the Filter Criteria. It routes the INVITE request based on the
destination (Request-URI). It forwards the INVITE to an entry point in home33.net network.
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