Tag Archives: signaling
LTE X2 Handover Sequence Diagrams
Let’s examine the X2 Handover in detail. We look at the X2 handover signaling procedure through sequence diagrams that focus on different aspects of the procedure.
The sequence diagrams presented here were generated with EventStudio System Designer.
LTE X2 Handover
Full signaling details are presented here.
Overview
Now we examine the same flow at a higher level of abstraction. The diagram focuses on the interactions between the mobile, eNodeBs and the MME/SGW.
UE Interactions
We now explore the signaling procedures that involve the UE.
Source eNodeB Role
Examine the interactions that involve the eNodeB that initiated the handover.
Target eNodeB Role
We now look at the interactions involving the eNodeB that will be serving the UE after the handover.
RRC Signaling the X2 Handover
The Radio Resource Control (RRC) signaling between the UE and the eNodeBs is covered here.
X2AP Signaling Between eNodeBs
X2AP is used for signaling between the eNodeBs. Here we examine the X2AP interactions.
Data Path Changes During an X2 Handover
The data path switching goes through several steps to accomplish a seamless handover.
LTE X2 Handover Between eNodeBs Served by the same MME
LTE eNodeBs can directly communicate with other eNodeBs on the X2 interface. The X2 interface is used to perform a handover between eNodeB.
The messaging in X2 handover is detailed in LTE X2 Handover Presentation. A few excerpts from the presentation as shown below.
X2 Handover Sequence Diagram
The X2 handover flow is shown in the following sequence diagram:
Figure 1 X2 Handover Sequence Diagram
Handover Preparation
The handover procedure is triggered by the X2AP Handover Request message. The RABs to be handover over are sent from the source eNodeB to the target eNodeB.
Figure 2X2AP Handover Request
The target eNodeB then admits the user and responds with X2AP Handover Request Acknowledge message. This message contains a transparent container that carries the Handover Command message that needs to be sent to the UE.
Figure 3 X2AP Handover Request Acknowledge
The source eNodeB sends the handover command to the UE. It then sends sequence number information to the target eNodeB.
The target eNodeB then requests the MME to switch the path from the source eNodeB to target eNodeB.
Handover Execution
Figure 4 S1AP Path Switch Request
This was an overview of the messaging involved in the X2 handover. For details refer to the LTE X2 Handover Presentation.
IMS subscriber to PSTN subscriber call flow
IP Multimedia Subsystem is an IP based signaling system for setting up and tearing down multimedia sessions. SIP based signaling is used to setup these sessions.
We have covered call flows for an IMS to IMS and PSTN to IMS calls. We now look at the call flow for a IMS to PSTN subscriber call.
The call is routed via the BGCF (Border Gateway Control Function) to the MGCF (Media Gateway Control Function). The MGCF uses one context with two terminations in IM-MGW (Media Gateway). The termination RTP1 is used towards IMS Core network subsystem entity and the bearer termination TDM1 is used for bearer towards PSTN CS network element.
The call flow is complex and it is analyzed with multiple diagrams. Some of the diagrams are presented here:

A high level view that abstracts out individual component details and just presents the flow between the UE, IMS Core and the PSTN.

Here we examine the call flow from the UE point of view. The call flow is represented as a collaboration diagram.
Link: IMS subscriber to PSTN subscriber call flow
All documents:
- IMS to PSTN Call Flow
- IMS to PSTN Call Flow Poster (11×17)
- IMS to PSTN Call Overview
- IMS to PSTN Call Context Diagram
- High Level Flow (Calling UE-IMS-PSTN)
- High Level Flow (With CSCF and H.248 Details)
- IM-MGW Call Flow
- IM-MGW Context Diagram
- MGCF Call Flow
- MGCF Context Diagram
- BGCF Call Flow
- BGCF Collaboration Diagram
- Originating UE Call Flow
- Originating UE Collaboration Diagram
PSTN subscriber to IMS subscriber call flow
IP Multimedia Subsystem is the new IP based signaling system for setting up multimedia sessions. We have already covered the call flow for an IMS subscriber calling another IMS subscriber. Here we will look at the call flow of a regular PSTN subscriber calling an IMS user.
PSTN to IMS call flow
This call flow covers the handling of a CS network originated call with ISUP. In the diagram the MGCF requests seizure of the IM CN subsystem side termination and CS network side bearer termination. When the MGCF receives an answer indication, it requests the IM-MGW to both-way through-connect the terminations. Originating and terminating end initiated call releases are also covered.
The following sequence is covered:
- ISUP IAM Handling and Initial IM-MGW and MGCF (Mn) Interactions
- Initial Handshake between MGCF and IMS CSCF Servers
- Mn Interactions for Codec selection
- ISUP ACM related interactions on Mn interface
- IMS Answer to ISUP ANM Handling
- Conversation Mode
- Call Release:
- Calling PSTN Subscriber Initiated Call Release
- Called Subsciber Initiates Call Release
Link: PSTN subscriber to IMS call flows
Focus on different aspects of the call flow
PSTN to IMS call is a very complex flow. The main sequence flow is supplemented with flows that focus on a particular aspect of the flow. A snapshot of one such diagrams is shown here:
LTE Attach Sequence Diagram
The UE initiates the Attach procedure by sending the Attach Request to the eNodeB. The message contains the old GUTI – Globally Unique Temporary Identifier…