telecom • networking • design

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S1AP View of Inter-MME S1 Handover

Here we look at the S1 handover that involves an MME change. The S1 handover is analyzed at the S1AP interface. S1AP messages involved in S1 handover are shown just before the message. The messages show a high level structure of the complex messages involved in the interaction.

Inter-MME-S1-handover

Additionally, you can click on any S1AP message to see the complete field level details like the one shown below.

PDCP-transfer

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Tracking Area Updates in an LTE Network

LTE Tracking Area Update signaling flow describes how mobiles keep the network updated about their location.

A Tracking Area Update takes place if:

  • UE detects it has entered a new Tracking Area that is not in the list of TAIs that the UE registered with the network;
  • the periodic Tracking Area update timer has expired;
  • UE was in UTRAN PMM_Connected state (e.g. URA_PCH) when it reselects to E UTRAN;
  • UE was in GPRS READY state when it reselects to E UTRAN;
  • the TIN indicates “P-TMSI” when the UE reselects to E-UTRAN (e.g. due to bearer configuration modifications performed on
    GERAN/UTRAN);
  • the RRC connection was released with release cause “load re-balancing TAU required”;
  • a change of the UE Core Network Capability and/or UE Specific DRX Parameters information of the UE.

Tracking area update in LTE


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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.


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SCTP connection setup, reconfiguration and release

Stream Control Transmission Protocol (SCTP) connection setup, reconfiguration and release

Stream Control Transmission Protocol (SCTP) connection setup, reconfiguration and release

Stream Control Transmission Protocol (SCTP) is a relatively new transport layer in the IP Protocol Stack. In an LTE network, SCTP is used to provide guaranteed message delivery between the MME and eNodeB.

SCTP belongs to the SIGTRAN protocol family and has been used as the transport layer for carrying telecom signaling over IP. SCTP provides a reliable transport service that operates at message level (unlike TCP that provides a byte stream
interface with no message boundaries).

SCTP allows multiple message streams to be exchanged on a single SCTP connection. Data from multiple streams can be sent in a single SCTP message as chunks. Selective acknowledgements are supported at individual chunk level.

SCTP protocol allows dynamic configuration of the IP addresses. Similar to SS7, SCTP can be switched over from one link to another. SCTP supports a make-before-break changeover, i.e. the packet stream is moved before removing the link that needs to be taken out of service.

In this sequence diagram we will be examining some of the features of SCTP.

  1. SCTP Connection establishment.
  2. SCTP data exchange and selective acknowledgement.
  3. Addition of a new IP address to an SCTP connection.
  4. Switching over to the new IP address.
  5. Removing the old IP address.
  6. SCTP connection release

SCTP Connection Setup, Reconfiguration and Release


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LTE Attach and Default Bearer Setup

LTE reduces the latency in attach and PDP context activation. Attach and default bearer context setup take place almost in parallel.

The parallel procedures result in more complicated signaling messages. The messages involved in these procedures are explained here.

 

LTE Attach and Default Bearer Setup