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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


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LTE Visualization Tool

The LTE Visualization Tool shows the organization of LTE channels in a subcarrier-symbol grid.

You can display:

Signals

LTE Signals

REF, S-SYNC and P-SYNC signals are shown on a subcarrier-symbol grid.

Channels

LTE Channels

PHICH, PDCCH, PBCH and PHICH channels are shown on the subcarrier symbol grid.

Throughput

LTE Throughput and Resource Element Usage

Total count of PDSCH, PBCH, PCFICH, PDCCH channels is shown. The throughout with QPSK, 16QAM and 64QAM is also calculated.

You can configure the:

  • Channel bandwidth
  • Number of antennas
  • Cell identity
  • PHICH duration
  • PHICH group scaling
  • PDCCH Quadruplets

The LTE Visualization Tool may be downloaded for free from:

LTE Visualization Tool


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LTE RRC Connection Setup

lte-rrc-connection

Establishing an RRC connection in LTE typically involves the following steps:

  1. UE initiates a session to the eNodeB using a random RA-RNTI.
  2. eNodeB assigns a C-RNTI and sends timing adjustment to the UE.
  3. UE adjusts the the timing and requests an RRC Connection Setup via the UL-SCH channel.
  4. The eNodeB sends the RRC Connection Setup to the UE on the DL-SCH channel. This message sets up the SRB. DRB may also be established at this point.
  5. Finally, the UE acknowledges the message with RRC Connection Complete message.

The messages in the RRC Connection Setup are described in the following presentation.

LTE RRC Connection Setup Messaging


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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


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LTE security: encryption and integrity protection call flow

LTE key hierarchy

LTE security is based on a shared secret key K between the USIM and the HSS. The UE, the eNodeB and the MME derive keys for encryption and integrity protection from K.

The derived keys are used for:

  • NAS encryption and integrity protection
  • RRC encryption and integrity protection
  • User plane encryption

The key derivation and the key exchange call flow is described in the following link:

LTE security procedure: authentication, encryption and integrity protection


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ROHC – Robust Header Compression

ROHC improves the throughput on the link by compressing the TCP, UDP, IP and RTP headers to a a few bytes. This is particularly important for voice over IP as in absence of ROHC, the headers would have consumed more bandwidth than the voice channel being carried.

ROHC is envisioned as an extensible framework for robust and efficient header compression over highly error–prone links with long round–trip times. This design is motivated by the large bit error rates (typically on the order of 10−4 – 10−2) and long round trip times (typically 100–200 msec) of cellular networks. The design of ROHC is based on the experiences from the header compression schemes reviewed above. In particular, ROHC incorporates elements from ROCCO and Adaptive Header Compression (ACE) which may be viewed as a preliminary form of ROHC.

ROHC

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