IP Multimedia Subsystem Fundamentals: Architecture to Protocols

The IP Multimedia Subsystem (IMS) is a fundamental component of modern telecommunications networks. It enables the delivery of multimedia services over IP networks.

IMS architecture is based on a layered approach, with the core network consisting of the Call Session Control Function (CSCF), which handles call and session control, and the Home Subscriber Server (HSS), which stores subscriber data.

The IMS protocol suite includes protocols such as SIP, Diameter, and MIPv6, which facilitate communication between different network elements. These protocols ensure seamless communication and efficient resource allocation.

The CSCF is responsible for routing requests between different network elements, ensuring that calls and sessions are established and maintained correctly.

Recommended read: ISDN Digital Subscriber Line

The IP Multimedia Subsystem (IMS) architecture is a collection of different functions, linked by standardized interfaces, which grouped form one IMS administrative network. Each function is not a node (hardware box), so an implementer is free to combine two functions in one node, or to split a single function into two or more nodes.

You might enjoy: Node Ip Openshift

Each node can also be present multiple times in a single network, for dimensioning, load balancing or organizational issues. This flexibility allows for a more efficient and scalable architecture.

The IMS architecture is made up of three distinct layers: the service/application layer, control layer, and transport layer. The service/application layer hosts and executes services provided to users, such as 4G/5G mobile networks, Wifi, or any other network infrastructure.

The control layer is responsible for session controlling and management, regulating the traffic between the transport layer and the service/Application layer by authenticating and distributing traffic. This layer is crucial in ensuring seamless and secure communication services.

The transport layer is responsible for supporting the architecture's core network by acting as a gateway linking access layers and IP networks. It ranges from Application servers for supplementary and other services, from SMS, Call-Forwarding or even more sophisticated services such as IP-TV.

Here's a breakdown of the three layers:

The IP Multimedia Subsystem (IMS) relies on several key network components to function. The P-CSCF is the central entry point into the operator network, and all requests need to go through it.

The P-CSCF is located close to the connecting endpoint to increase network efficiency by avoiding useless traffic transport and reducing latency. This is crucial for maintaining a smooth user experience.

The Interrogating-CSCF (I-CSCF) is responsible for querying the Home Subscriber Server (HSS) and routing messages to the correct Serving-CSCF (S-CSCF). If no S-CSCF is indicated by the HSS, the I-CSCF will select one.

The S-CSCF is the worker in the IMS world, communicating with the HSS using Diameter (Cx-interface) to retrieve information about authentication and user profiles.

The HSS is a master user database that supports IMS network entities, containing subscription-related information, performing authentication and authorization, and providing subscriber location and IP information.

Here's a brief overview of the IMS components:

The Call Control function in the IP Multimedia Subsystem (IMS) is a crucial aspect of managing real-time communications. It involves the handling of call/session control and routing within the network.

The Call Session Control Function (CSCF) is a key component of the IMS architecture, responsible for handling call/session control and routing. There are three types of CSCFs: Proxy-CSCF (P-CSCF), Interrogating-CSCF (I-CSCF), and Serving-CSCF (S-CSCF).

Here's a breakdown of the roles of each CSCF:

These CSCFs work together to establish, modify, and terminate sessions, ensuring seamless communication between users.

CSCF Call Control is a crucial aspect of IMS architecture, responsible for handling call/session control and routing within the network. There are three types of CSCFs: P-CSCF, I-CSCF, and S-CSCF, each serving specific roles.

P-CSCF acts as the first point of contact for UE and handles initial session signaling, forwarding requests to the appropriate destination to facilitate session establishment. It may also perform Registrar functions, accepting registration requests from the UE and making information available through the location server.

I-CSCF serves as the entry point within the operator's network for connections directed to a user, assigning a S-CSCF during SIP registration and routing requests/responses accordingly. It retrieves the S-CSCF address from the HSS and forwards SIP requests or responses to the determined S-CSCF.

S-CSCF manages session control services for the UE, maintaining a session state essential for supporting various services as required by the network operator. It may act as a Registrar, accepting registration requests and making its information available through the location server, or behave as a Proxy Server, handling requests internally or forwarding them.

Here's a summary of CSCF roles:

S-CSCF provides endpoints with service event-related information, such as notification of tones/announcements and location of additional media resources, along with billing notifications. It may also route SIP requests/responses based on the type of procedure.

The Cx Interface is a crucial part of the Call Control system, enabling IMS registration and the transfer of subscriber data to the S-CSCF.

It operates on the Diameter protocol, which facilitates communication between the I/S-CSCF and HSS.

The Cx Interface allows the I/S-CSCF to communicate with the HSS to retrieve user profiles, assisting in user registration and authentication.

This process is essential for setting up and managing IMS sessions.

The Cx Interface plays a vital role in the Call Control system, making it possible to download subscriber data from the HSS to the S-CSCF.

Here's a quick rundown of the Cx Interface's key functions:

Application Servers are a crucial part of the IMS network, hosting and executing services that deliver a wide range of services and enrich the user experience with multimedia capabilities.

An Application Server (AS) can be located in the home network or in an external third-party network, and it interfaces with the S-CSCF using SIP.

The AS can operate in SIP proxy mode, SIP UA (user agent) mode, or SIP B2BUA mode, depending on the actual service being executed.

Consider reading: High Power User Equipment

Here are some key functions of an Application Server:

The AS-ILCM and AS-OLCM store transaction state, and may optionally store session state depending on the specific service being executed.

Application Servers are a crucial component of the IMS network, responsible for hosting and executing various services that enhance the user experience with multimedia capabilities. They can operate in different modes, including SIP proxy mode, SIP UA mode, or SIP B2BUA mode.

In the IMS architecture, Application Servers can be located in the home network or in an external third-party network. This flexibility allows for a wide range of services to be hosted and executed, depending on the specific requirements of the service.

A key feature of Application Servers is their ability to interface with other components of the IMS network, such as the S-CSCF and HSS. They can also store transaction state and session state, depending on the specific service being executed.

Check this out: Peer-to-peer SIP

Here are some key types of Application Servers:

Application Servers use Public Service Identities (PSI) to identify services hosted by them. PSI can take the form of either a SIP or Tel URI, and is stored in the HSS either as a distinct PSI or as a wildcarded PSI.

Media servers play a crucial role in Application Servers, and understanding their functions is essential for effective communication.

The Media Resource Function (MRF) is responsible for media-related functions, including voice stream mixing and playing tones and announcements. It's further divided into two components: the media resource function controller (MRFC) and the media resource function processor (MRFP).

The MRFC is a signalling plane node that interprets information from the AS and S-CSCF to control the MRFP.

The MRFP, on the other hand, is a media plane node used to mix, source, or process media streams, and manage access rights to shared resources.

Worth a look: Radio Resource Control

The Media Resource Broker (MRB) acts as a functional entity that collects and supplies published MRF information to consuming entities like the AS. It operates in two modes: Query mode, where the AS queries the MRB for media, and In-Line Mode, where the AS sends a SIP INVITE to the MRB.

In Query mode, the AS queries the MRB for media and sets up the call using the response from the MRB.

In In-Line Mode, the AS sends a SIP INVITE to the MRB, which sets up the call.

Media servers are components that operate on the media plane and are under the control of IMS core functions. Specifically, they include Media Servers (MS) and Media gateways (MGW).

Here's a breakdown of the key components:

IMS relies on a set of protocols and standards to ensure secure and efficient communication. The primary authentication mechanism is the SIM or eSIM, which verifies the legitimacy of the subscriber using the IMSI number and authentication key.

For your interest: Node B

The IMS system supports SIP, a signaling protocol used for initiating, maintaining, and terminating real-time sessions. SIP operates at the application layer of the OSI model and is widely used for VoIP and other multimedia communication applications.

The IMS system also supports Diameter, a protocol used for authentication, authorization, and accounting. The IMS system uses Diameter to authenticate users and authorize access to services.

Here are some of the key protocols and standards used in IMS:

These protocols and standards work together to provide a secure and efficient communication system.

GL's MAPS SIP IMS emulator supports a range of protocols and standards, including SIP, Diameter, and SDP.

SIP is a widely used protocol for initiating, maintaining, and terminating real-time sessions over the internet. It operates at the application layer of the OSI model and is used for VoIP and other multimedia communication applications.

GL's MAPS SIP IMS emulator specifically supports SIP extensions, such as reliability of provisional responses, the UPDATE method, private headers, and the Refer method. It also supports Diameter, a protocol used for authentication, authorization, and accounting in IP networks.

Suggestion: List of SIP Software

The emulator supports the following Diameter applications:

SDP is used for session description and negotiation, providing a structured text-based format for conveying information about session parameters.

The following table summarizes the supported protocols and standards:

Cloud-Native IMS is an advanced version of the IP Multimedia Subsystem designed to operate in cloud environments. It enables MNOs to deliver innovative voice and multimedia services efficiently.

Cloud-Native IMS supports various voice services, including VoLTE, VoNR, VoWiFi, and VoBB. These services ensure seamless communication over different networks.

VoLTE bridges voice services over LTE networks, while VoNR supports 5G by ensuring voice services over the new radio interface. VoWiFi facilitates calls over WiFi networks, enhancing coverage and reliability.

Ribbon's IMS is built using a microservices-based architecture that enhances scalability, efficiency, and operational flexibility. This design enables MNOs to deploy solutions rapidly, responding to market demands with agility.

Worth a look: Adobe Creative Cloud

IMS security mechanisms, such as those defined in TS 33.203, may not be available for early IMS and non-3GPP systems due to the lack of USIM/ISIM interfaces and prevalence of devices that support IPv4.

In these situations, 3GPP defines "early IMS security" mechanisms in TR33.978, which rely on authentication performed during network attachment procedures to bind user profiles to IP addresses. This mechanism is weak, however, because signaling is not protected on the user-network interface.

CableLabs and TISPAN have also developed similar security mechanisms for their respective systems, including the use of Digest-MD5 as a valid authentication option.

IMS systems use a SIM or eSIM for authentication, which is not just a simple key or password storage but functions more like a small computer. The phone itself does not validate the subscriber based on credentials; instead, it forwards the authentication data to the SIM, which handles the verification process.

The primary role of the SIM card is to verify the legitimacy of the subscriber, relying on the IMSI (International Mobile Subscriber Identity) number and an authentication key that verifies the IMSI.

The authentication process involves the network generating a random number, referred to as “X,” and signing it with the subscriber's authentication key to create a new number, “Y.” This process ensures that both the device and the network can confirm the identity of the subscriber securely.

Discover more: Subscriber Location Function

The HSS (Home Subscriber Server) plays a key role in managing subscriber data and authentication, including verifying subscriber identities, allocating resources and services, and updating subscriber profiles.

Here are the key roles of the HSS in subscriber data and authentication:

The HSS also uses a Challenge-Response Mechanism to verify subscriber credentials and generates security tokens to secure communication sessions between the subscriber's device and the network.

See what others are reading: International Mobile Subscriber Identity

The IMS (IP Multimedia Subsystem) network relies on gateways to connect with other networks, such as the Public Switched Telephone Network (PSTN). A PSTN/CS gateway is a crucial interface that allows IMS to communicate with circuit-switched networks.

A signalling gateway (SGW) is responsible for transforming lower layer protocols, converting Stream Control Transmission Protocol (SCTP) into Message Transfer Part (MTP) to pass ISDN User Part (ISUP) from the MGCF to the CS network.

The MGCF (Media Gateway Control Function) is a SIP endpoint that interfaces with the SGW over SCTP, controlling the resources in a Media Gateway (MGW) across an H.248 interface. A media gateway (MGW) converts between RTP and PCM, and can also transcode when the codecs don't match.

Here's a summary of the IMS gateways:

IMS allows users to connect in various ways, most of which use standard IP.

IMS terminals like mobile phones and computers can register directly on IMS, even when they're roaming in another network or country.

The only requirement for registration is that the terminal can use IP and run SIP user agents.

Fixed access options like DSL, cable modems, Ethernet, and FTTx are supported.

Mobile access options like 5G NR, LTE, W-CDMA, CDMA2000, GSM, and GPRS are also supported.

Wireless access options like WLAN and WiMAX are supported as well.

IMS can even connect with other phone systems like plain old telephone service (POTS) through gateways.

H.323 and non IMS-compatible systems can also be connected through gateways.

You might like: Adobe Connect

A Breakout Gateway Control Function, or BGCF, is a SIP proxy that processes requests for routing from an S-CSCF when the session cannot be routed using DNS or ENUM/DNS.

It includes routing functionality based on telephone numbers. This makes it a crucial component in the process of routing calls.

Check this out: Dns over Https vs Tls

The BGCF is specifically designed to handle situations where traditional routing methods fail, such as when a number is not found in the DNS or ENUM/DNS database. It kicks in as a last resort to ensure the call gets through.

In these cases, the BGCF takes over and uses its routing functionality to connect the call.

See what others are reading: Azure Dns Server Ip

PSTN gateways are a crucial part of the IMS architecture, allowing for communication between the IMS network and circuit-switched networks like the Public Switched Telephone Network (PSTN).

A PSTN/CS gateway interfaces with PSTN circuit-switched (CS) networks, using different protocols for signalling and media. For signalling, CS networks use ISDN User Part (ISUP) or BICC over Message Transfer Part (MTP), while IMS uses SIP over IP.

ISUP is used for call control, and it's transformed into SIP by the MGCF, which controls the resources in a Media Gateway (MGW). The MGCF is a SIP endpoint that interfaces with the SGW over SCTP.

A fresh viewpoint: Universal Integrated Circuit Card

A signalling gateway (SGW) transforms lower layer protocols like SCTP into MTP, to pass ISUP from the MGCF to the CS network. It does call control protocol conversion between SIP and ISUP/BICC under the control of the MGCF.

Here's a breakdown of the PSTN gateway components:

The IP Multimedia Subsystem (IMS) is a standardized architectural framework that enables multimedia services to be delivered over IP networks. It's built on Session Initiation Protocol (SIP) to support a range of services, including voice, video, and data.

IMS provides a unique convergence platform for different types of networks, including mobile, satellite, broadband, cable, and fixed networks. This allows for efficient interoperability between networks.

The IMS architecture consists of three layers: the service/application layer, the control layer, and the transport layer. These layers work together to manage signals and traffic for multimedia applications.

The service layer hosts and executes services provided to users, such as 4G/5G mobile networks, WiFi, or other network infrastructure. The control layer is responsible for session controlling and management, while the transport layer supports the architecture's core network by acting as a gateway linking access layers and IP networks.

Take a look at this: Network Architecture

IMS is designed to be scalable, flexible, and resilient. It allows for dynamic, per-subscriber configuration of services, and the Home Subscriber Server (HSS) centrally manages service rules, which are downloaded to the IMS core for each subscriber.

IMS was built to be standardized, open, and interoperable. It's widely used in mobile networks and is defined by the 3GPP (3rd Generation Partnership Project).

Call Session Control Function (CSCF) plays a crucial role in handling call/session control and routing within the IMS network. CSCFs are responsible for managing session control services for User Equipment (UE).

There are three types of CSCFs: Proxy-CSCF (P-CSCF), Interrogating-CSCF (I-CSCF), and Serving-CSCF (S-CSCF). Each type of CSCF serves specific roles in the IMS framework.

Here's a brief overview of each CSCF type:

P-CSCF may perform Registrar functions, accepting registration requests from the UE and making information available through the location server (e.g., HSS). The S-CSCF manages authentication and authorization for the UE, ensuring secure access to network services.

The Rx interface plays a crucial role in controlling the setup of various call types. It operates between the P-CSCF and the PCRF, enabling the request of an appropriate policy for session establishment.

The Rx Interface operates on the Diameter protocol. This protocol is used for authentication, authorization, and accounting in the network. It's a key part of the Rx Interface's functionality.

The Rx Interface is responsible for establishing a suitable policy for incoming or outgoing calls on the network through PCRF. This includes managing resource usage in the access networks.

Here's a breakdown of the Rx Interface's key functions:

LTE SMS calls involve a complex procedure to transmit SMS messages over LTE networks.

The IP Short Messaging Gateway (IP-SM-GW) network element provides the interconnection between GSM-UMTS and LTE-EPC networks for sending and receiving SMS.

SMS messages are encapsulated in a SIP message and carried over the IMS core network to the SMSC.

Intriguing read: Do Not Call Registry Text Messages

The call flow for transmitting encapsulated SMS messages over LTE via IMS elements involves multiple nodes, including the SSCSF node and the IP-SM-GW node.

A call flow diagram depicts the transmission process, illustrating the interaction between these nodes.

The IP Multimedia Subsystem (IMS) plays a crucial role in facilitating SMS calls over LTE networks.

A fresh viewpoint: SMS

Call Emulation is a powerful feature that allows for end-to-end call emulation over LTE-IMS or 5G New Radio – IMS network.

With MAPS IMS, you can emulate all nodes to achieve an end-to-end call emulation, making voice calls happen within the same network or between two different service providers.

The following call scenarios are supported using MAPS IMS emulator:

A single Remote MAPS Server can configure and test each node in the LTE-IMS network, making it possible to simulate multiple nodes situated at various locations, which can be controlled using a single Remote Client GUI.

The Remote MAPS Server feature allows for multi-node, multi-interface simulation, and can be configured to simulate multiple nodes situated at various locations.

Here's an interesting read: Remote Radio Head

The PES architecture is a crucial part of the IP Multimedia Subsystem (IMS) that enables IP networks to provide services to analog devices. It's like a translator, allowing non-IMS devices to communicate with IMS as if they were normal SIP users.

Analog terminals can connect to the PES in two ways: via A-MGW (Access Media Gateway) or VGW (VoIP-Gateway) on customer premises. A-MGW is linked and controlled by AGCF, which is placed within the Operator's network and controls multiple A-MGW.

The A-MGW and AGCF communicate using H.248.1 (Megaco) over the P1 reference point. POTS phones connect to A-MGW over the z interface, and the signalling is converted to H.248 in the A-MGW and passed to AGCF.

AGCF has certain service-independent logic, for example, on receipt of off-hook event from A-MGW, the AGCF requests the A-MGW to play dial tone. The AGCF presents itself as P-CSCF to the S-CSCF and passes generated SIP messages to S-CSCF or to IP border via IBCF.

Suggestion: International Gateway

Here's a breakdown of how PES architecture works:

The PES architecture allows non-IMS devices to appear as normal SIP users to IMS, enabling IP networks to provide services to analog devices. This is made possible by the A-MGW and VGW, which relay call control signalling to and from the PSTN terminal.

IP Multimedia Subsystem (IMS) is a game-changer in the world of telecommunications, offering a wide range of benefits and advantages that make it an essential component of modern communication networks.

IMS provides greater flexibility and extensibility compared to traditional monolithic communications network architectures, allowing for a layered network architecture with open interfaces.

This means that applications are decoupled from the transport layer, enabling service providers to deliver common applications and services across diverse devices in 4G and 5G mobile networks, Wi-Fi networks, fixed networks, and MSO networks.

IMS also enables reusable components, reducing CAPEX and OPEX by leveraging common IMS functional elements to support multiple services.

Broaden your view: Common Gateway Interface

Standards-based solutions are another key benefit of IMS, allowing service providers to eliminate vendor lock-in and contain costs.

IMS supports policy-based QoS mechanisms for SLA assurance and to ensure satisfactory user experiences.

IMS is a key enabler in the shift from traditional circuit-switched networks to all-IP networks, which is essential for supporting advanced multimedia services.

IMS is a critical component for the deployment and operation of 4G LTE and 5G networks, providing the infrastructure for delivering high-speed data services and advanced multimedia applications.

IMS enables the delivery of high-quality multimedia services, significantly enhancing the user experience, particularly for applications like video calling, streaming, and real-time gaming.

IMS is foundational for emerging technologies like IoT (Internet of Things) and Industry 4.0, enabling a wide range of applications and services across various industries.

IMS's standardized architecture encourages innovation by allowing developers to create new services and applications that can run on IMS-enabled networks.

IMS allows the convergence of various services like voice, video, and data onto a single IP network, making it possible for users to access multiple services through a unified interface.

See what others are reading: LTE Advanced

IMS enables network operators to prioritize and manage different types of traffic, ensuring that services like voice and video receive the necessary bandwidth and low latency for a high-quality user experience.

IMS supports real-time charging and billing capabilities, allowing operators to accurately charge users for the services they consume.

Here are some of the key advantages of IMS: