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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Long Term Evolution (LTE) Tutorials

Here are a few hand picked links to LTE tutorials. Click here for the complete list.

LTE video tutorial

LTE video presentations

LTE physical layer

OFDM and SC-FDMA Signal Chains

LTE link layer design

data flow through PDCP, RLC, MAC and PHY layers of LTE

This article describes the LTE link-layer protocols, which abstract the physical layer and adapt its characteristics to match the requirements of higher layer protocols.The LTE link-layer protocols are optimized for low delay and low overhead and are simpler than their counterparts in UTRAN. The state -of-the-art LTE protocol design is the result of a careful crosslayer approach where the protocols interact with each other efficiently. This article provides a thorough overview of this protocol stack, including the sub-layers and corresponding interactions in between them, in a manner that is more intuitive than in the respective 3GPP specifications.

Introduction to LTE Architecture


This article provides an overview of the LTE radio interface, together with a more in-depth description of its features such as spectrum flexibility, multi-antenna transmission, and inter-cell interference control. The performance of LTE and some of its key features is illustrated with simulation results.

This article provides a high-level overview of LTE and some of its key components: spectrum flexibility, multi-antenna transmission, and ICIC. Numerical simulations are used to show the performance of the first release of LTE, as well as assess the benefit of the key features. Indeed these contribute strongly to LTE meeting its performance targets. An outlook of the evolution of LTE toward LTE-Advanced and full IMT-Advanced capabilities complete the article. Clearly, LTE offers highly competitive performance and provides a good foundation for further evolution.

LTE Protocol Stack

Click here for a more LTE tutorials that cover the entire spectrum of LTE development.


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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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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.


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LTE Video Presentations

OFDM Time Frequency Multiplexing

Rohde & Schwarz‘s presentations provide an excellent introduction to LTE. The presentations are accompanied with an audio narration. The topics covered are covered:

  1. LTE Introduction
    • Motivations for LTE
    • LTE market and background
    • Requirements
    • Evolution path to LTE
  2. LTE Parameters and Downlink Modulation
    • LTE parameters and frequency bands
    • What is OFDMA?
    • OFDMA multiple access and downlink frame structure
    • OFDMA transmit and receive chains
  3. OFDMA and Downlink Frame Structure Details
    • Downlink OFDMA time-frequency multiplexing
    • LTE Spectrum Flexibility and Bandwidth
    • FDD downlink frame structure detailed
    • TDD frame structure
  4. SC-FDMA and LTE Uplink
    • Introduction to SC-FDMA and uplink frame structure
      • Marriage of single carrier transmission and FDMA
    • Uplink SC-FDMA tranmsit and receive chains
    • Peak to Average Power Ratio (PAPR) comparison with SC-FDMA and OFDMA
  5. Network and Protocol Architecture
    • LTE/SAE network architecture
    • EPC -Evolved Packet Core
    • Base Station control plane and user plane protocol stacks
    • EPC protocol stacks
  6. Channel Mapping and UE Categories
    • Logical and transport channel mapping in downlink and uplink
    • LTE UE Categories
  7. Initial Cell Search and Cell Selection
    • Downlink physical channels and signals
    • Cell Search and Selection in LTE
      • Primary synchronization signal
      • Secondary synchronization signal
      • Reference signals
  8. System Information
    • Downlink reference signal details
    • Master Information Block on PBCH
    • System Information on DL-SCH
  9. Random Access Procedures and EPS Bearer Setup
    • Random access preamble transmission to eNodeB
    • Random access response from eNodeB
    • Resource allocation and contention resolution
    • Signaling on PDCCH
    • Hybrid ARQ
    • RRC Connection Setup and EPS Bearer Setup
  10. Uplink Channels and Signals
    • Uplink physical channels and signals
    • PU-SCH: Physical Uplink Shared Channel
    • Uplink assignment signaling on PDCCH
    • Uplink frequency hopping
    • PUCCH
  11. LTE Mobility and MIMO Introduction
    • Intra MME Handover over the X2 interface
    • RRC States
    • MIMO Basics
      • Transmit diversity
      • Spatial multiplexing
      • Beamforming
  12. Downlink and Uplink MIMO in LTE
    • Downlink MIMO modes
      • Transmit diversity
      • Spatial multiplexing
      • Cyclic delay diversity
      • Beam forming
    • Spatial multiplexing downlink transmitter chain
      • Code book based precoding
    • Uplink MIMO
      • Uplink transmit antenna selection
      • Multi-user MIMO
  13. eNodeB and UE Performance Requirements
    • eNodeB modulation quality measurements
    • eNodeB performance requirements
    • UE performance requirements
  14. UE Certification and Field Trials
    • LTE terminal testing stages
    • LTE terminal certification
    • LTE field trial scenarios


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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