IMS  

 

 

 

Overview

IMS (IP Multimedia Subsystem) is one of the terms that I heard most often since I started studying LTE. But this is one of the terms which is still not clear to me -:).

This post is to share my understanding as of now (?) with readers and I will try to put information based on my hands-on experience rather than directly coming from the specification. There may be some part which may not be 100% accurate from the real expert's point of view. I think I keep updating this page very often and very long.

As usual, the first thing I did to know something about IMS when I first heard of it, I put "IMS" in Google and as you may experienced I got a lot of different kind IMS -:). Then I put "LTE IMS" and got pretty much of the result being more relevent to my interest. But I saw from the searched link a lot of material about SIP. Reading through these links, I start asking myself.. "Hmm... Is IMS same as SIP ?".

And then I started search "SIP" in Google. Of course, a lot of result about SIP popped up but I also got almost same amout of the searched result was about "VoIP". Then I asked myself "Hmm... Is SIP as same as VoIP ?".

With the application of transition rule, I thought "Then... Is IMS same as VoIP ?". It took long time for me to get out of this confusion.

The latest version of my understanding about the relationship among IMS, SIP, VoIP is as follows ( I think there should be many other arrows in/out of IMS block). As you see, IMS is sitting on top of everything and it control/use SIP for the various media transfer.

Now let's think about SIP (Session Initiation Protocol). As the name implies, SIP is a kind of signaling protocol which mainly involved in "Initiation" and "Closing" of a media transfer. Once a session is initiated, some other protocols kick in the process to transfer the data part of the media. The specific type of prtocol is determined by the type of media data.

This image shown above illustrates a simplified overview of the IMS (IP Multimedia Subsystem) and its associated protocols, which are used for delivering multimedia and voice communication services over IP networks.

  • Diameter: A protocol used for authentication, authorization, and accounting (AAA). It's an evolution of the RADIUS protocol and provides more robust, reliable, and secure transport for signaling in IMS.
  • SIP (Session Initiation Protocol): A core protocol used for initiating, managing, and terminating multimedia sessions in IMS. It's used for voice calls (VoIP), video calls, and other types of multimedia communications.
  • Megaco (H.248): A protocol used for controlling media gateways on the network which connect traditional circuit-switched networks to the IP-based network of IMS. It allows for the management and conversion of media streams between the two different network types.

Below the high layer signaling is the types of data that can be transmitted within an IMS using the above signaling protocols:

  • Voice (VoIP): SIP is used to set up and manage voice over IP calls, which are typically carried over RTP (Real-time Transport Protocol) using codecs like AMR (Adaptive Multi-Rate).
  • Video: SIP also sets up video calls, which can use codecs such as H.263 or H.323, again typically transported via RTP, which ensures real-time delivery of media.
  • SMS: Short Message Service, a text messaging service component of most telephone, internet, and mobile device systems. It uses standardized protocols to enable messaging services in IMS.

Now let's think about how these IMS chain get involved in LTE SAE structure and I think just one diagram as shown below is good enough for a big picture.

This illustration shown above depicts the architecture of a mobile network that integrates 4G technologies, as well as the IP Multimedia Subsystem (IMS) for delivering multimedia services.

It illustrates how these components work together to manage signaling, data routing, and service delivery in a modern telecommunications network.

  • The User Equipment (UE) connects to the network via the eNodeB (the base station for LTE). The eNodeB communicates with the Mobility Management Entity (MME) , which is responsible for the signaling procedures that control the UE's access to the network.
  • The Serving Gateway (SGW) routes and forwards user data packets, while the Packet Data Network Gateway (PGW) connects the mobile network to the external IP networks, assigning IP addresses to the UE.
  • The Home Subscriber Server (HSS) is a database that contains user-related and subscription-related information. The Policy and Charging Rules Function (PCRF) makes decisions about policy control and flow-based charging according to subscriber data provided by the HSS.
  • In parallel, the IMS core , which includes the Call Session Control Function (CSCF) , handles the signaling for session establishment, management, and termination. It interfaces with the Session Initiation Protocol (SIP) Application Servers, which provide various multimedia services.
  • The SGW, PGW, HSS, MME, and PCRF are components of the Evolved Packet Core (EPC) for 4G LTE networks, which supports the data traffic. The SGSN is a similar element but for 2G/3G networks, interfacing with the older generation of base stations.

Some Video Tutorials

First, if you are a real beginner for IMS and SIP. I recommend you to see the following video a couple of times and it will give you pretty good big pictures.