IMS In A 5G Network

An Overview of 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.

fig. IMS in a 5G Network (Click to enlarge)

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 –

fig. IMS Service Access in a 5G Network (Click to enlarge)

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 –

fig. IMS Voice Session in a 5G Network (Click to enlarge)

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

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Understanding how IMS works with 5G Core is essential for mastering Voice over 5G (VoNR) and multimedia services. My book "The 5G Core: Architecture and Functions Explained" provides comprehensive coverage of IMS integration, charging, and policy control in 5G networks.

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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

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