Introduction to functions and entities of IMS networks

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2.2 IMS entity and Function Description

This section describes IMS entities and key functions. These entities can be roughly divided into six categories: session control and routing entity family (cscfs), Database entity (HSS, SLF), and interconnected entity
(Bgcf, mgcf, im-MGW, SGW), service-related entities (application server, mrfc, mrfp), supporting entities (thig, seg, PDF), and billing-related entities
Body. It is important to understand that the IMS standard does not detail the internal functions of network entities. For example, there are three internal functions of the HSS: IMS, necessary functions required by the CS domain, and PS
Required functions of the domain. The 3GPP standard does not describe how IMS functions interact with PS functions. Instead, it describes the functions supported by interfaces and interfaces between entities (for example, how cscf obtains
User data ). Section 2.3 describes these different interfaces. In addition, the function body in the GPRS network will be described in the end part of this section.

2.2.1 proxy cscf

The proxy call session control feature entity (P-CSCF) is the first connection point during user access to IMS. All the sip signaling messages from the UE and sent to the UE are sent through the p-
Cscf. Like its name, P-CSCF is like [RFC
The proxy defined in 3261] works the same way. This means that the P-CSCF checks the request message, forwards it to the selected destination, and processes and forwards the response message at the same time. In addition, the P-CSCF can also
For example, [RFC
The user proxy (UA) defined in 3261] works the same way. The role of this UA is used to initiate the release session when an exception occurs (for example, the loss of the user bearer channel is detected according to the local service-based policy.
See section 3.9 ). It is also used in the process of processing registration described in section 5.12.6 to establish an independent SIP transaction (transaction ). One or
Many p-cscf. The features provided by the P-CSCF are described in [3GPP ts 23.228, TS 24.229:

· Based on the domain name placed by UE in the request message
The query cscf (I-CSCF) that the Register message forwards to the domain ). Section 5.5 gives a more detailed description of what the P-CSCF should do before it forwards messages.
Or let cscf know that the Register message was not received on a security association ).

· Forward the SIP request message and Response Message to the service cscf (S-CSCF. Chapter 6th details what the P-CSCF should do before forwarding a request and Response Message except register to the S-CSCF (for example, checking that the user identity used in the message is correct ).

· Forward the SIP request message and Response Message to the UE. Chapter 6th details what the P-CSCF should do before forwarding a request and Response Message except register to the UE (for example, compressing the message ).

· Detect emergency call establishment requests. In IMS version 5, the P-CSCF returns a sip error message, 380, indicating that ue should try CS
CN. This work continues to be defined in version 6, and the P-CSCF has changed the original approach. The P-CSCF selects a S-CSCF to handle this urgent call. This choice is necessary
Because in the case of roaming, the original allocated S-CSCF is located in the local network. The S-CSCF located in the local network cannot route requests to the correct emergency processing.
Heart. (We should find an emergency processing center close to the geographical location, that is, the Access Point Network .)

· Send billing information to the billing collection entity (CCF ).

· Provides integrity protection for SIP messages and maintains secure associations between UE and P-CSCF. Integrity protection is provided through IPSec ESP. Version 6 also provides confidentiality protection. Section 3.6 describes the design of IMS security. Chapter 4 discusses security-related protocols.

· Compress and decompress the SIP messages between the UE. The P-CSCF supports Compression Based on Three RFC standards: [RFC 3320], [RFC 3485], and [RFC 3486]. Section 3.16, Section 6.4, and chapter 19 describe SIP Message compression in more detail [3GPP ts 24.229].

· Subscribe to registration events to the user's Registry (S-CSCF. This is used to download the public user identity for implicit registration, and to obtain notifications when the network initiates a logout. Section 5.12.6 describes a registration event package. Section 3.14 shows how implicit registration works. 5.14.3 introduces more information about cancellation initiated by the network.

· Execute a media control policy. The P-CSCF is able to check the content of SDP and whether it contains media and encoding that is not allowed by the user. If the suggested SDP does not comply with the carrier's policy, the P-CSCF rejects the request and sends the SIP error message 488 to the UE. The carrier may want to restrict users by using this feature due to bandwidth restrictions.

· Maintain the session timer. Version 5 does not provide a means for stateful proxies to know the session status. Version 6 fixes this vulnerability by introducing session timers. This allows the P-CSCF to detect and release resources that are exhausted by dead sessions.

· Interaction with policy control entities (PDF. The responsibility of PDF is to implement a service-based local policy (sblp ). In version 5, PDF is a logical part of P-CSCF. In version 6, PDF becomes an entity independent from the P-CSCF.

2.2.2 policy decision function entity (PDF)

The responsibility of PDF is to make policy decisions based on the session information and media-related information obtained from the P-CSCF. It works like a policy decision point defined in the sblp protocol. The following features of policy decision points in sblp need to be supported:

· Stores session and media information (such as IP addresses, port numbers, and bandwidth ).

· Generate an authorization token to identify the PDF and session.

· When a channel authorization request is received from ggsn, the policy determines whether to authorize the request based on the stored session and media information.

· When a session is modified, the authorization is updated based on the session information and media-related information.

· The ability to revoke authorization decisions at any time.

· Capabilities that enable authorized bearer channels to be used (such as PDP context ).

· Maintain the ability to authorize but prevent authorized bearer channels from being used.

· Notify p-cscf when the bearer channel (such as PDP context) is lost or changed. The modification instruction is issued only when the bearer channel increases from 0 kbit/s or drops to 0 kbit/s.

· Forward the IMS billing identifier to the ggsn and the GPRS billing identifier to the p-cscf.

2.2.3 query point cscf (I-CSCF)

I-CSCF is the entry point of a network, and all the connections to users in the network will pass through the I-cscf of the network. A single operating network may have multiple I-cscf instances. I-CSCF provides the following features:

· Contact the HSS and get the name of the S-CSCF that provides services to a user.

· Assign a s-cscf that meets the requirements based on the capabilities required to be supported from the HSS. One S-cscf is assigned only when the current user has not assigned a S-CSCF. This process will be described in more detail in section 3.8.

· Forward the SIP request or response message to s-cscf.

· Send billing information to CCF.

· Provides the hide function. A I-CSCF can contain a functional entity called a network topology to hide a thig. Thig can be used to hide the configurations, capabilities, and topology of networks outside the operating network.

2.2.4 service cscf (S-CSCF)

The S-CSCF is located in the local network and is the brain of IMS. It provides registration service and session control for UE. When the UE joins a session, the S-CSCF maintains the session state and
The Platform deals with billing entities to support services required by carriers. In an operating network, there may be multiple S-CSCF, and each S-CSCF may also support different capabilities and functionality. More
Body, S-CSCF to complete the following features:

· Process registration requests like the registration center defined in [RFC 3261. The S-CSCF knows the IP address of UE and the P-cscf used to access IMS.

· Use IMS authentication and key agreement (aka) plan to authenticate users. The IMS aka authentication plan provides two-way authentication for UE and the local network.

· When a user registers or processes a request sent to an unregistered user, the user information and service-related information of the user are downloaded from the HSS.

· Route the communication to the mobile side to the P-CSCF, route the communication initiated by the mobile side to the I-cscf, The out gateway control function entity (bgcf) or the application server ().

· Implement session control. The S-CSCF can work like the proxy server and user proxy (UA) defined in [RFC 3261.

· Interaction with the service platform. When receiving a request or response message, determine whether to route to an as for further processing.

· Translate a telephone number in the form of e.164 into a sip URI using the domain name resolution server (DNS) in the format described in [Draft-ietf-enum-rfc2916bis. Because the sip signaling routing in IMS only uses the SIP Uri, this translation is required.

· Regulatory Registration timer. Users can be logged out as needed.

· When the carrier supports IMS emergency calls, it can select an Emergency Handling Center. This is a feature of version 6.

·
Execute a media control policy. The S-CSCF is able to check SDP content and whether it contains media and encoding that are not permitted by the user. If the SDP recommended in the message does not comply with the carrier's policy, then s-
Cscf rejects the request and sends the SIP error message 488 to the UE. Section 3.11 shows how a media control policy is included in the user description (
Profile.

· Maintain session timers. Version 5 does not provide a means for stateful proxies to know the session status. Version 6 fixes this vulnerability by introducing session timers. This allows the S-CSCF to detect and release resources that are exhausted by dead sessions.

· Sends billing information to CCF to support the offline billing function. Sends billing information to the online billing system (OCs) to support the online billing function.

2.2.5 local subscriber server (HSS)

HSS is the primary storage device for all the subscriber information and service-related information in IMS. The main data stored in the HSS includes the User Identifier, registration information, access parameters, and service trigger information [3GPP ts 23.002].

User identifiers include private user identifiers and public user identifiers. Private user identifiers are allocated by the local network operator for registration and authorization purposes. The public User Identifier is
The user used to initiate a call. The IMS access parameter is used to establish a session, which includes information such as user authentication, roaming authorization, and assigned S-CSCF. The service trigger information is used to support the execution of the SIP service.
Line. As well as requirements for S-CSCF capabilities for a particular user. This information is used by the I-CSCF for selecting the most appropriate S-cscf.

In addition to IMS-related functions, the HSS contains the functional entities required for the PS domain and CS domain, that is, the subset of the Local Location registration server and the authentication center (HLR/AUC. The structure of the HSS is represented in. Communication interfaces between different HSS instances are not standardized.

Figure 7 structure of HSS

HLR functions are required to support entities in the PS domain, such as sgsn and ggsn. This allows the subscriber to use the PS domain service. Similarly, HLR also provides
Body support, such as MSC and MSC server. This allows the subscriber to use services in the CS domain and roam to the GSM/umts cs domain network.

AUC stores a key for each subscriber, which is used to dynamically generate security data for the subscriber. This secure data is used for mutual authentication between the International Mobile Subscriber identifier (imsi) and the network. This secure data is also used to provide encryption and Integrity protection for data in the UE and wireless path between networks.

Depending on the number of subscribers, device capabilities, and network organization, a network may have multiple HSS. There are multiple interfaces between HSS and other network entities.

2.2.6 subscription information positioning function entity (SLF)

When multiple separately addressable hsss are deployed in a network, SLF acts as a solution, enables I-cscf, S-CSCF, and as to find the user subscription information corresponding to the given User Identifier.

2.2.7 Multimedia Resource Controller (mrfc)

Mrfc is used to support services related to the bearer channel, such as meetings, user announcements, or Transcoding of the bearer channel. Mrfc explains the sip signaling received from the S-CSCF and uses media gateway control protocols
Megaco command to control the Multimedia Resource processor (mrfp ). Mrfc can send billing information to CCF and OCS. Chapter 2 shows how mrfc is used in conference services.

2.2.8 Multimedia Resource processor (mrfp)

Mrfp provides user-layer resources that are required and indicated by mrfc. Mrfp provides the following functions:

· Mixed operations on received media data (for example, mixing and image processing in multi-party meetings ).

· Generate media (for example, sending user prompts ).

· Media Processing (such as voice transcoding and media analysis) [3GPP ts 23.228 and TS 23.002].

2.2.9 Application Server ()

Remember that in a hierarchical design, as is not a pure IMS entity. On the contrary, it is a functional part of IMS. However, as is introduced here as the functional entity of IMS. This is because the as entity provides multimedia value-added services for IMS networks.

As is located in the local network or in a third party. The third party indicates a network or a separate. The main functions of AS are as follows:

· Process and affect the SIP sessions received from the S-CSCF.

· Initiate a SIP request.

· Send billing information to CCF and OCS.

The provided services are not limited to SIP-based services. This is because the operator provides subscribers with the ability to access services based on the camel service environment (CSE) and OSA [3GPP
TS 23.228]. Therefore, "as" is a common reference for SIP as, OSA server (OSC), and camel.
The term IP multimedia service exchange feature entity (IM-SSF.

With OSA, carriers can utilize some service capabilities to facilitate service development, such as call control, user interoperability, user status, data session control, terminal capabilities, account management, and
And billing and policy control [3GPP TS
29.198]. An additional benefit of the Osa framework is that it can be used as a secure way to provide a third-party as standardization mechanism for IMS. This is because OSA contains
Certification, authorization, registration and discovery features (S-CSCF does not provide authentication and security functions for third-party secure access to IMS ). Because the support of the OSA service is determined by the carrier
The surface should not allow multiple entities to support the Osa protocol and features. So osa scs are used to end the sip signaling at the S-CSCF. OSA
SCS use the Osa application programming interface (API) to interact with the real OSA application server.

The IM-SSF feature entity is introduced into the IMS architecture to support legacy services developed in the camel service environment (CSE. It carries the camel network features (trigger monitoring points, camel service switching finite state machine, etc.) and interacts with the camel Application Part (CAP) interfaces.

Figure 8 Relationship between different as types

SIP as is a SIP-based server. It undertakes a wide range of multimedia value-added services. SIP as can be used to provide online services, short message services, and conference services. The different functions of the SIP server will be provided as part of the service in more detail in section 8.3 and section 3.12.4.

Shows how different feature entities are connected. From a S-CSCF perspective, SIP as, OSA SCs and IM-SSF have the same interface behavior.

One as may only be used to implement one service, and one user may have multiple services. Therefore, one subscriber may have one or more. In addition, one or more as users may participate in one session.
. For example, a carrier may have an as to control the traffic to the user based on the user's choice (redirect all multimedia sessions to a reply phone between five o'clock P.M. and am ), however
Another as is used to modify the content of instant messages based on the UE's capabilities (screen size, pixel size, etc.

2.2.10 outbound gateway controller (bgcf)

Bgcf is responsible for selecting where to exit and entering the CS domain. The result may be an error in the bgcf network or in other networks. If the outbound traffic occurs on the network where bgcf is located
, Bgcf selects an mgcf to process the session later. If the outbound traffic occurs in other networks, bgcf passes the session to a bgcf [3GPP TS
23.228]. The selected rule is not defined. In addition, bgcf can collect statistics and report billing information to CCF. The interoperability between IMs and CS is described in Section 3.13.

2.2.11 media gateway controller (mgcf)

Mgcf is a functional entity used to implement communication between IMS users and CS users. All call signals from Cs are sent to mgcf. Mgcf performs ISUP, BICC, and sip
Protocol conversion and session forwarding to IMS. Similarly, all sessions initiated by the IMS side to the CS user pass through mgcf. Mgcf also controls associated user-layer entities (I .e. IMS-MGW)
. In addition, mgcf can report billing information to CCF. The interoperability between IMs and CS is described in Section 3.13.

IMS Media Gateway (IMS-MGW)

The IMS-MGW provides the user layer link between the CS Network (PSTN, GSM) and IMS. It ends the bearer channel from the CS network and the media stream from the backbone network (
RTP streams and AAL 2/ATM connections in the ATM Backbone Network), which are converted between the two networks to provide transcoding operations. If necessary, user-layer signal processing is also provided. In addition, IMS-MGW can
Provides signal and prompt sound for CS users. IMS-MGW is controlled by mgcf.

2.2.13 signaling gateway (SG)

SG is used to connect different signaling networks, such as sctp/IP-based Signaling Networks and SS7 Signaling Networks. SGW performs signaling transmitted on SS7 and transmits on IP (for example
Sigtran sctp/IP and SS7
Between MTP. SGW does not explain the application layer of the message (such as BICC and ISUP ). Only ISUP is displayed, and BICC is similar.

Figure 9 signaling transformation in SGW

2.2.14 Security Gateway (SEG)

To protect the security of the message stream at the control layer between security domains, the message stream must pass through a Seg when it enters or leaves the security domain. A security domain is managed by a single administration.
(Administrative
Authority), which is consistent with the network boundary of the carrier. The seg is placed on the boundary of the security domain. It is used to enhance the security policy of this security domain to the seg in other security domains.
. There may be multiple Seg in the network to avoid single-point errors or to improve performance. A seg can be set to interact with all other accessible security domains or a subset. Security Domain-related
The concept is described more thoroughly in section 3.6.3.

2.2.15 billing entity

Different billing entities and related interfaces are described separately in section 3.10.

2.2.16 GPRS entity 2.18..1 service GPRS Support Node (sgsn)

Sgsn connects the ran to the packet switching core network. It is also responsible for the control and communication stream processing functions in the PS domain. The control function includes two main aspects: mobile management and session management. Mobile management Processing
The location and status of the UE, and authenticate the subscriber and UE. Session management processes connection license control and changes to existing data connections. The control function also monitors 3G services and resources. Communication stream processing is also managed by sessions.
A part. Sgsn works like a gateway to provide tunnel transmission for user data. In other words, it transmits user traffic between UE and ggsn. As part of this function, sgsn is also true
Guaranteed connections have appropriate QoS protection. In addition, sgsn generates billing information.

2.2.16.2 Gateway GPRS Support Node (ggsn)

Ggsn provides interconnection with external packet exchange networks. The main function of ggsn is to connect ue to an external Packet Exchange Network, where there will be IP-based applications and services. For example
The local data network can be IMS or the Internet. In other words, ggsn routes the IP package containing the SIP Message from the UE to the P-CSCF, and vice versa. In addition, ggsn also helps
IP packets are routed to the destination network (for example, the ggsn routed to the called end ). The provided interconnection service is generally implemented on the Access Point Where the subscriber wants to access the network. In most cases, IMS has its own access
Point. When you activate an access point (IMS) bearer channel (PDP)
Context), ggsn will assign a dynamic IP address to ue. The assigned IP address will be used by UE as the contact address for IMS registration and call initiation. In addition, ggsn will
Maintains and supervises the use of PDP context for IMS media streams and generates billing information.

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