telecom • networking • design

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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 Random Access Procedure and Contention Resolution

LTE random access procedure is used by the UEs to initiate a data transfer. The UEs also obtain uplink timing information from the initial handshake.

This sequence diagram describes the tale of three UEs (UE-A, UE-B and UE-C) that are powered on at the same time:

  1. UEs synchronize with the downlink channel by decoding the PSS and SSS signal. The UEs are synchronized to the downlink frames after completing this procedure.
  2. The three UEs initiate the random access procedure at exactly the same time. Two of them (UE-A and UE-B) happen to pick the same preamble. This results in a resulting in a collision. UE-C picks a distinct preamble so it succeeds in the random access procedure.
  3. Contention between UE-A and UE-B is resolved in UE-A’S favor. UE-A proceeds with the RRC connection.
  4. UE-C times out and retries the random access procedure.

LTE random access procedure


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3G UMTS Mobile Terminated Call Flow

Get a detailed look at a UMTS mobile terminating call. RANAP and RRC signaling in a terminating call is describe in detail.

3G UMTS Mobile Terminating Call Flow

3G UMTS Terminating Call Sequence Diagram

The RANAP message flow presented here was generated with VisualEther from a PCAP file.  The field level details have been preserved for the RANAP messages. Click on a RANAP messages in the sequence diagram to see full field level details. For example clicking on the Paging message reveals field level detail in the message.

ranap Radio Access Network Application Part

  • per.extension_bit 0… …. Extension Bit: False
  • per.choice_index Choice Index: 0
  • ranap.RANAP_PDU RANAP-PDU: initiatingMessage (0)
    • ranap.initiatingMessage initiatingMessage
      • ranap.procedureCode procedureCode: id-Paging (14)
      • per.enum_index Enumerated Index: 1
      • ranap.criticality criticality: ignore (1)
      • per.open_type_length Open Type Length: 21
      • ranap.value value
        • ranap.Paging Paging
          • per.extension_bit 0… …. Extension Bit: False
          • per.optional_field_bit .0.. …. Optional Field Bit: False (protocolExtensions is NOT present)
          • per.sequence_of_length Sequence-Of Length: 2
          • ranap.protocolIEs protocolIEs: 2 items
            • ranap Item 0: id-CN-DomainIndicator
              • ranap.ProtocolIE_Field ProtocolIE-Field
                • ranap.id id: id-CN-DomainIndicator (3)
                • per.enum_index Enumerated Index: 1
                • ranap.criticality criticality: ignore (1)
                • per.open_type_length Open Type Length: 1
                • ranap.value value
                  • per.enum_index Enumerated Index: 0
                  • ranap.CN_DomainIndicator CN-DomainIndicator: cs-domain (0)
            • ranap Item 1: id-PermanentNAS-UE-ID
              • ranap.ProtocolIE_Field ProtocolIE-Field
                • ranap.id id: id-PermanentNAS-UE-ID (23)
                • per.enum_index Enumerated Index: 1
                • ranap.criticality criticality: ignore (1)
                • per.open_type_length Open Type Length: 9
                • ranap.value value
                  • per.extension_bit 0… …. Extension Bit: False
                  • ranap.PermanentNAS_UE_ID PermanentNAS-UE-ID: iMSI (0)
                    • per.octet_string_length Octet String Length: 8
                    • ranap.iMSI iMSI: 21436587000200f0
                    • ranap.imsi_digits IMSI digits: 123456780020000


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3G UMTS Originating Call Flow

3G UMTS Originating Call Flows

A 3G UMTS originating voice call call setup involves complex signaling to setup and release the call.

  • RRC (Radio Resource Control) signaling between the UE and RAN sets up the radio link.
  • RANAP (Radio Access Network Application Part) signaling sets up the session between the RAN and the Core Network (MSC).

Click on the image to see the full call flow. You can click on most RANAP messages in the call flow to complete field level details of the RANAP messages.

3G UMTS Originating Call with RRC and RANAP signaling

Click here for the 3G UMTS originating voice call flow 


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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.