PCC (Policy Control and Charging)
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PCC (Policy Control and Charging)
VoLTE services rely on the connection of the LTE (and existing 3G/2G) infrastructure to the IMS. This connection is made at the point of the PDN-GW via the IP based SGi interface. The IMS components will also have connections to the PCRF for policy and charging control, and in addition the IMS will have access to subscriber-related information from the HSS. A point to note here is that, although the connection between the user and the IMS is via the user plane of the network, the IMS will only terminate signalling information, no user data is passed to the IMS. The signalling relationship between the user and the IMS is based on SIP (RFC 3261). The SIP messages are passed via the user plane using a default bearer established when the mobile registers with the LTE core network.
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PCC (Policy Control and Charging)
There are several steps required for the VoLTE client device to find and register for VoLTE services. First, the UE must establish a radio connection to the E-UTRAN. This involves a random access process and the establishment of and RRC (Radio Resource Control), which configures a radio bearer suitable for conveying signalling-type messages over the air. Once the radio connection is established the UE will perform EPS registration using NAS (Non Access Stratum) protocols. During the registration process the default bearer will be established and an IP address allocated to the UE. The EPS may perform security measures during registration. Now that the default bearer is in place, the IMS client service on the UE will register for services on the IMS. The IMS may then perform additional authentication. At this point the sequence could end and the RRC connection could be deactivated. However, as far as the IMS services and the UE client is concerned, the IP connection is still active. When the client device initiates a call, the RRC connection must be re-established and the VoIP session can be established via SIP signalling between the UE client and IMS servers. Before the media can flow from calling to called party, a bearer must be created for both users. Once the call has ended, SIP signalling will be used to terminate the session, then the RRC connection can be de-activated.
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PCC (Policy Control and Charging)
Before the UE can communicate with the IMS it must first establish the radio connection, register with the EPC and set up the necessary bearers. 1.
The mobile performs random access and establishes a signalling radio connection to the eNB and MME.
2.
The mobile sends an attach request message to the eNB. This message will contain the UE preferences in respect of voice capabilities, e.g. a) Voice domain preferences – CS or IMS-based PS b) UE’s usage settings – voice/data centric c) SRVCC to GERAN/UTRAN capability
3.
The eNB forwards the message to the selected MME using an S1AP control message.
4.
The MME begins the procedures for authenticating the UE.
5.
The HSS receives an Update Location Request message from the MME, to update the records for the IMSI provided by the UE.
6.
An Update Location Ack is sent to the MME. This message will contain the default subscription context including ‘ims_apn’ and QoS parameters used for SIP signalling a) b) c) d) e) f)
PDN – IPv4v6 APN – ims QCI – 5 ARP – 9 APN AMBR for UL and DL VPLMN allowed (for roaming)
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PCC (Policy Control and Charging)
7.
The MME will validate the subscription data and QoS values; there may some restrictions for roaming subscribers. The APN ims, is generally the preferred APN for PS voice subscribers. The MME will select a PDN-GW based on the APN, and the SGW may be selected based on the mobile location. Once the user plane nodes have been determined, the MME will send a Create Session Request message to the selected SGW to begin building the default bearer.
8 & 9. Dynamic PCC rules are used for VoLTE subscribers. The PDN-GW will use the CAN (Connectivity Access Network) session establishment procedure to retrieve the default PCC rules for the UE. The PCRF communication will contain, for example, the subscriber identity, the IP address (v4 or v6), the IP-CAN type and access network type. If the PCRF requires additional information it may query the SPR (Subscriber Profile Repository). The PCRF may provide (to the PDN-GW) PCC rules, bearer authorisation, and QoS parameters, including maximum bit rates, QCI and ARP.
10.
A new entry is created in the PDN-GW EPS bearer context table for the user plane routing, PDN-GW–SGW. The PDN-GW will send a Create Session Response message to the SGW, including an identity for the default bearer, EPS bearer QoS, IP address (UE) and P-CSCF. The P-CSCF address is sent in the PCO (Protocol Configuration Options) inside the response message.
11.
Any bearer modifications are checked by the MME and the aggregated maximum bit rate is calculated. If a combined attach (PS/CS) is required for CSFB the IMSI attached will be sent to an MSC via the SGs interface. An Attach Accept message is forwarded to the UE via the eNB; this message will include the TAI list. The Accept message is inside the Initial Context Set-up Request message sent from the MME to the eNB.
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PCC (Policy Control and Charging)
12.
The RRC Reconfiguration message is sent from the eNB to the UE in which the UE will receive EPS bearer information, QoS, IP address, and P-CSCF address.
13.
RRC Reconfiguration Complete.
14.
The eNB responds the Context Set-up message (step 11).
15.
The UE sends a Direct Transfer message to the eNB, including the Attach Complete (EPS Bearer identity, NAS sequence number [SQN –for signalling]).
16.
The Attach Complete message is sent directly to the MME via an UL transport message (S_MME). It is now possible for the UE to send UL packets to the UE, which will be tunnelled to the SGW and PDN-GW.
17.
A Bearer Modify message is sent from the MME to the SGW.
18.
Modify Bearer Response is sent to the MME. UL and DL packets can now be exchanged; this means the UE can now exchange SIP signalling with the IMS.
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PCC (Policy Control and Charging)
The diagram shows the relationship between the PDN connection and the default bearer. The PDN connection comprises multiple connections (bearers and tunnels) over the air interface, S1 and S5 interfaces and is identified primarily with an IP address. The default bearer is a construct that makes use of the PDN connection. It is a managed link in that there are a set of parameters that determine the manner in which the packets flowing through the bearer will be handled by each of the E-UTRAN and EPS nodes. These parameters are determined when the mobile first registers with the EPS. (See previous page).
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PCC (Policy Control and Charging)
Now that the UE is connected to the EPS by way of the EPS connection, and to the IMS by way of the default bearer, it can now communicate freely with the IMS and IMS components. The first communication will be the registration process, where the SIP client in the UE informs the IMS of its IP address in order that IMS can determine the location of the end user identity. (In SIP the mobile’s location does not necessarily mean its physical location, simply its current IP address; the mobile network will worry about the actual location [physical] of the UE). The other important function is to establish a route to and from the UE and the serving CSCF. 1.
The UE client will generate the REGISTER message, addressed to the P-CSCF. The REGISTER message will contain the identity (SIP) of the registering user, the final destination (S-CSCF) for the message.
2.
The P-CSCF will proxy (forward the message) to the I-CSCF, which will find and select the S-CSCF. There will be some interaction with the HSS.
3.
The S-CSCF will obtain the authentication credentials for the user. The security procedure will be invoked. The S-CSCF will pass the 401 (unauthorised) message back to the UE, essentially rejecting it from the system, but this process allows the security mechanisms to be agreed between the UE and the IMS.
4.
The IPsec security associations can be established.
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PCC (Policy Control and Charging)
5.
The UE and S-CSCF will learn the route between them. There is routing information passed in the SIP headers that allow the route to be learnt.
6.
The S-CSCF will download the subscriber profile information from the HSS and register the default public identity of the user.
7.
The UE will register the supported IMS communication services at the S-CSCF.
8.
The S-CSCF may also register the UE with application servers, depending on subscription information.
9.
The UE and S-CSCF become aware of all public user identities that are assigned and the registration status.
10.
The P-CSCF is made aware of the charging function addresses.
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PCC (Policy Control and Charging)
IMS service subscriptions are based on user identities. Specifically, a user will have one private identity and one or more public identities which are mapped to the private identity. The private identity is referred to as the IMPI (IM Private Identity) and the public identity, or identities, are known as the IM Public Identity (IMPU). This is identical in concept to the IMSI (private) and MSISDN (public), where a user may have several MSISDNs mapped to one IMSI. The IMS profile of a user stored on the HSS is made up of one or more service profiles. There is only one service profile per public identity, although one profile may be mapped to more than one identity. Service profiles contain triggers (known as filter criteria) which are used to launch IMS services. These filters are relatively static; in future releases dynamic filters will be set by the service logic on an AS. The public and private user identities are stored in the IM Subscriber Identity Module (ISIM) and the HSS. Service profiles are downloaded from the HSS to the S-CSCF at, for example, registration. A public user identity can be a SIP URL, for example: sip:user@domain, or a ‘tel’ URL, for example: tel:+123-123-1234567. Where a subscriber has multiple public addresses, the registration of one of those addresses can be used to implicitly register other addresses. For this to work the addresses must be grouped together in a registration set. The private user identity used for IMS services is in the form of an NAI (Network Access Identifier), for example:
[email protected] or
[email protected]. It is possible for a representation of the IMSI to be coded into the NAI.
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PCC (Policy Control and Charging)
In this example of identity relationships, the user has two different service profiles identified with differing public user identities. Each of the service profiles may result in different handling of the communication services due to the way in which the services are registered with the relevant S-CSCF and AS. Regardless of how many public user services and profiles there are they will be mapped back to a single private identity which is used in the initial registration of the user. Where the user has multiple devices it is possible to have an public identity that is linked or registered to several private identities.
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PCC (Policy Control and Charging)
LTE and UMTS PS are simply IP-based packet networks that efficiently route packets from and to the UE. Moving packets can not be classed as a ‘service’; the IMS, however, can be considered to provide a function for creating and managing actual services, such as voice, video, IM, etc. The name given to this process within IMS is ‘service provisioning’. The data concerning the IMS subscriber and services resides in the HSS. This information will be accessed by the S-CSCF (SIP server) when the IMS user registers for the service. The S-CSCF can then determine how to manage the service based on this subscription information. This may require that the request for session (coming from the IMS subscriber) will be sent to the AS for additional parsing. An example of an AS might be the Telephony AS (or TAS), which understands how to execute services related specifically to telephone services, for example call forwarding, conferencing, etc.
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PCC (Policy Control and Charging)
In this example of service provisioning the S-CSCF receives a request for a session to be established (e.g. INVITE). In order that the S-CSCF understands how to proceed, the request will be evaluated by the server, comparing the session request with subscriber-related information, which may be held at the S-CSCF or downloaded from the HSS. At this point the S-CSCF will also consider the filter criteria (see the following diagram). The filter criteria help the S-CSCF determine the next logical step in executing the service. The filter criteria can be any element or information present or not present in the incoming request; for example, the SIP message type or session description information can be used. The filter criteria can therefore be used to determine which AS the request should be forwarded to in order to execute the service. The AS that the request is forwarded to, may also interrogate the HSS for additional subscriber information. The IMS may have several ASs, each one specialising on a particular service, i.e. voice, SMS, IM, video streaming, therefore the filter criteria and the logic used to process it must setup correctly to ensure the service request is routed to the correct AS.
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PCC (Policy Control and Charging)
In this voice call session establishment sequence the originating user presents the SIP INVITE to the originating network’s IMS. In this case the P-CSCF will determine the route to the S-CSCF and forward the message. The S-CSCF will route the message to an AS determined by the filter criteria; the AS may modify the INVITE according to the requirements for the session. This may include modifying the session description and providing routing information to the intended network. The S-CSCF will route the INVITE message toward the destination user via the I-CSCF. The I-CSCF will determine which S-CSCF should handle the session request. it may do this by interrogating the HSS. The IMS components will then begin to forward the INVITE toward the user within that network, applying filter criteria if necessary. This initial routing and forwarding of the session request is important, not just to deliver the message to the destination party, but also to establish the actual route the subsequent message will follow. Each of the SIP servers (CSCFs) will add or remove routing labels from the SIP header information to ensure accurate routing. The destination system will indicate that an attempt to contact the user is being made (100 Trying). In this example, when the destination party responds to the initial INVITE, it sends a progress message 183, which starts the media negotiation process in which both ends determine which media codecs should be used for the session. The subsequent messages, PRACK from the originating party and 200 OK from the destination, also form part of this media negotiation.
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PCC (Policy Control and Charging)
The UPDATE and 200 OK messages indicate that the required resources have been reserved for the communication and the ringing (alerting) process can begin. When the user answers the call a final 200 OK message is sent, and acknowledges with ACK. At this point both the originating and destination parties will know the source and destination IP addresses and have resources (dedicated EPS bearers) established, so the flow of media can begin.
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PCC (Policy Control and Charging)
When a SIP session is being set up, it will generally require a bearer to be established from each party in the call to the PDN-GW. During session signalling the P-CSCF will inform the PCRF that a bearer is required along with the QoS parameters relevant for the session. The PCRF will then initiate the dedicated bearer toward the user. Note that the bearer being created is the path that will be taken by the media (voice) once the session is been fully set up. All the SIP-based signalling is using the default bearer established by the EPC during the UEs registration to the EPC.
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PCC (Policy Control and Charging)
When either user indicates that the session is to be released, either by hitting the endcall button or closing an application, the SIP BYE message will be sent to the S-CSCF. The BYE message will pass through all the IMS signalling nodes allowing them to terminate any billing -elated activities (STR/STA to the PCRF) and to indicate to the transport plane that dedicated bearer should be terminated (RAR/RAA to the PDNGW).
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