IMS in a 5G Network

IMS is a service framework to deliver multimedia services over IP Networks. It is predominantly used for delivering interactive services like Voice, IP Centrex Service, Video chat, Video Conferencing, and Instant Messaging-based multimedia services (VoNR, VoLTE, VoWiFi, RCS, etc.).

As IMS is access-independent, it is quite relevant in 5G Networks.

It connects to the 5G SBA for delivering services like Voice over 5G (VoNR or Vo5G) and allows roaming features over 5G/4G networks.

To know more about how traditional IMS (used for 4G/WiMAX network) needs to be evolved for a Cloud-based 5G SA Network, please check my other article by clicking on the link.

Let’s discuss some of the important IMS Nodes –

P-CSCF/I-CSCF/S-CSCF: These IMS nodes are used for Voice session control and call routing. Handling of user registration and service authorization has also been taken care of via CSCFs.

AS/AF: Executes Telephony, IDDs, Supplementary services, and several multimedia services like Video calls, Video conferencing, etc.

MRF: Handles media processing, media streams (like announcements), media stream mixing, etc.

MGCF: Takes care of Signaling interworking between legacy networks (PSTN/PLMN) and IMS network, that is, ISUP to SIP conversions. It also takes care of controlling the resources of MGW.

BGCF: It is used during call routing to PSTN/PLMN networks. From a protocol perspective, it handles SIP as well as SS7.

IMS-GWF: Enables online charging in S-CSCF for IMS services like voice calls, video chats, etc.

HSS: Maintains Subscriber’s user profiles in the home network and helps to manage user identification.

Let’s delve into IMS in a 5G Network –

For any Non-IMS terminating call, IMS detects the B-Party as an Off-net subscriber and forwards the SIP signaling message to MGCF via BGCF. SIP messages get converted to ISUP and reach PLMN via MGW. Later, Bearer gets established between IMS and non-IMS users through the MGW node.

For IMS to IMS On-net calls, the call arrives in the IMS server (comprised of P-CSCF, I-CSCF, and S-CSCF). IMS server interacts with AS incase of any supplementary services initiation. IMS server forwards?SIP signaling messages?to the B-party within the network. Once the call is connected, a Bearer path gets established between both the IMS users.

IMS allows maintaining QoS for Service types and Subscriptions based on its integration with PCF/PCRF. Not only it allows call control within the IMS network, but it also connects calls to legacy/non-IMS networks.

A typical Voice session in a 5G Network is shown below –

IMS nodes communicate in SIP protocol whereas 5G CCS understands HTTP/2 Rest APIs.

CSCFs and AS perform the authentication & service authorization by interacting with 5G CCS. On sufficient balance or available credit limit, call signaling gets established with the B-Party. Once the Voice session gets established, subsequent balance checks & reservations occur using Nchf Update requests/responses. As the call terminates, with a successful Nchf Release request/response, the actual balance update happens over the Customer account.

Here, Nchf is the Network Function Service-based interface with a Converged Charging Function.

IMS nodes must be deployed over the cloud like other 5G Core network functions like AMF, SMF, etc.

Through Service Control over SIP, IMS can manage parallel services. From a Charging perspective, it supports Offline Charging, Online Charging, Flow-based Charging, etc.

Along with standard Services, as mentioned at the beginning of the article, it facilitates IDD (International Direct Dialing), Supplementary services like Call Forwarding, Missed Call Notifications, Call Barring, Speed Dialing, CLIP/CLIR, Multi-party Calling, Call Waiting, Voicemail, etc.

Please check my article for further details about 5G Service-based Architecture and various 5G Network functions.

NOTE: This article was originally published on my Website.

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Glossary: IMS (IP Multimedia Subsystem), SBA (5G Service Based Architecture), SIP (Session Initiation Protocol), SS7 (Signaling System 7), CSCF (Call Session Control Function), P-CSCF (Proxy CSCF), S-CSCF (Serving CSCF), I-CSCF (Interrogating CSCF), ISUP (ISDN User Part), HSS (Home Subscriber Server), MRF (Media Resource Function), MGCF (Media Gateway Control Function), BGCF (Breakout Gateway Control Function), SGW (Serving Gateway), PGW (Packet Gateway), AMF (Access Management Function), SMF (Session Management Function), UPF (User Plane Function), UDM (Unified Data Management), IMS-GWF (IMS Gateway Function), PCF (Policy Control Function), PCRF (Policy & Charging Rules Function), CCS (Convergent Charging Server), ABMF (Account Balance Management Function), RF (Rating Function), CGF (Charging Gateway Function), CHF (CHarging Function), VoLTE (Voice over LTE), VoWiFi (Voice over WiFi), RCS (Rich Communication Services), VoNR (Voice over New Radio), 5G SA (5G Standalone Architecture), CLIP (Calling Line Identification Presentation), CLIR (Calling Line Identification Restriction), IDD (International Direct Dialing), PLMN (Public Land Mobile Network), PSTN (Public Switched Telephone Network)