LTE Signaling E-Book

Ralf Kreher

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 473

Veröffentlichungsjahr: 2010

Beschreibung

A comprehensive reference on the call procedures of 4G RAN and Core networks, LTE Signaling, Troubleshooting and Optimization describes the protocols and procedures of LTE. It explains essential topics from basic performance measurement counters, radio quality and user plane quality to the standards, architecture, objectives and functions of the different interfaces. The first section gives an overview of LTE/EPC network architecture, reference points, protocol stacks, information elements and elementary procedures. The proceeding parts target more advanced topics to cover LTE/EPC signalling and radio quality analysis. This book supplements the information provided in the 3GPP standards by giving readers access to a universal LTE/EPC protocol sequence to ensure they have a clear understanding of the issues involved. It describes the normal signaling procedures as well as explaining how to identify and troubleshoot abnormal network behavior and common failure causes. * Enables the reader to understand the signaling procedures and parameters that need to be analyzed when monitoring UMTS networks * Covers the essential facts on signaling procedures by providing first hand information taken from real LTE/EPC traces * A useful reference on the topic, also providing sufficient details for test and measurement experts who need to analyze LTE/EPC signaling procedures and measurements at the most detailed level * Contains a description of LTE air interface monitoring scenarios as well as other key topics up to an advanced level LTE Signaling, Troubleshooting and Optimization is the Long Term Evolution successor to the previous Wiley books UMTS Signaling and UMTS Performance Measurement.

Leseprobe

Table of Contents

Title Page

Copyright

Dedication

Foreword

Acknowledgments

Chapter 1: Standards, Protocols, and Functions

1.1 LTE Standards and Standard Roadmap

1.2 LTE Radio Access Network Architecture

1.3 Network Elements and Functions

1.4 Area and Subscriber Identities

1.5 User Equipment

1.6 QoS Architecture

1.7 LTE Security

1.8 Radio Interface Basics

1.9 LTE Network Protocol Architecture

1.10 Protocol Functions, Encoding, Basic Messages, and Information Elements

Chapter 2: E-UTRAN/EPC Signaling

2.1 S1 Setup

2.2 Initial Attach

2.3 UE Context Release Requested by eNodeB

2.4 UE Service Request

2.5 Dedicated Bearer Setup

2.6 Inter-eNodeB Handover over X2

2.7 S1 Handover

2.8 Dedicated Bearer Release

2.9 Detach

2.10 Failure Cases in E-UTRAN and EPC

Chapter 3: Radio Interface Signaling Procedures

3.1 RRC Connection Setup, Attach, and Default Bearer Setup

3.2 Failure Cases

Chapter 4: Key Performance Indicators and Measurements for LTE Radio Network Optimization

4.1 Monitoring Solutions for LTE Interfaces

4.2 Monitoring the Scheduler Efficiency

4.3 Radio Quality Measurements

4.4 Control Plane Performance Counters and Delay Measurements

4.5 User Plane KPIs

Acronyms

Bibliography

Index

This edition first published 2011

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Library of Congress Cataloging-in-Publication Data

Kreher, Ralf.

LTE signaling, troubleshooting, and optimization / Ralf Kreher, Karsten Gaenger.

p. cm.

Includes bibliographical references and index.

ISBN 978-0-470-68900-4 (hardback)

1. Long-Term Evolution (Telecommunications) I. Gaenger, Karsten. II. Title.

TK5103.48325.K74 2011

621.384—dc22

2010036847

A catalogue record for this book is available from the British Library.

Print ISBN: 978-0-470-68900-4 (H/B)

ePDF ISBN: 978-0-470-97771-2

oBook ISBN: 978-0-470-97772-9

ePub ISBN: 978-0-470-97767-5

Karsten Gaenger

To my lovely lady for her care and patience. To my parents and my sister for being with me and for their continuous support.

As a global standard, LTE might connect more people than ever before. It is my hope that as we increase our ability to communicate we increase our ability to live peaceably together.

Ralf Kreher

I dedicate this book to my grandmother Emilie. She managed to raise four children in the aftermath of World War II after losing her husband, home and all her valuables.

Foreword

As I watch my children use the latest in video conferencing technology over a mobile handset, I can't help but think of how far we've come. One might recall what the mobile communications industry was like only two decades ago, in the early 1990s. At the time, a considerable portion of the mobile addressable population simply could not comprehend why they would ever need a portable device to originate and receive phone calls. Even more difficult to believe was how this technology would spread through the business world and into the consumer population. After all, the price of a mobile minute was formidable, especially with the general lack of quality of experience and the not-to-be-forgotten 2 kg battery bag we were forced to carry.

In the mid 1990s, 2nd Generation technology was well into its general deployment. As business professionals, we gained acceptance and expectations of a world in which all of our colleagues, customers and suppliers could be reached through ever shrinking mobile devices. The service provider competitive landscape began introducing concepts such as “monthly minutes” while handset subsidization drove mobile communications into the hands of the everyday consumer. Even stranger was seeing the younger population use their thumbs (and not their mouths) to communicate with their friends. Yes, the world of the Short Message Service was taking European and Asian youth by storm.

As we headed towards the end of the century, we began to speak of the concept of Personal Communication Services. The mobile device would become our personal station for all communications, inclusive of data acquisition for business as well as rich entertainment for all users. The mobile device was quickly deemed a portable desktop and portable entertainment system. Well, perhaps a bit overstated at first! Turning the corner of the year 2000, we embraced the introduction of 2.5 Generation technology. Signs of a true portable desktop were emerging. The continuous introduction of additional spectrum for mobile communications, coupled with a now growing consumer thirst for higher bandwidth, accelerated the industry. After massive investments by highly competitive service providers, the uptake of 3rd Generation technology emerged. 3G is now carrying us to where we are today. From my perspective, the age of Personal Communication Service and its much promised devices are now upon us.

Business users can now access and upload office databases through secure wireless connections. Personal users can download music and videos, play online games and, of course, hold discussions over highly sophisticated smartphone devices. All of this has resulted in unprecedented traffic growth over the wireless mobile network and within the mobile core network. Not only is higher bandwidth per device required, but the ability of the network to essentially eliminate latency is also critical in this environment. Hence the birth of the LTE, an end-to-end IP-based architecture fully prepared to deliver these aforementioned requirements. But with LTE comes aspects of increased complexity: a highly intelligent radio interface, now responsible for mobility management, and an enhanced packet core destined to deliver speeds that rival fixed broadband service.

The ability to monitor and troubleshoot the control plane and user plane throughout the radio and core network will be essential to the success of LTE. Tektronix Communications' leadership in testing both the radio interface and the wired interface of the E-UTRAN and Enhanced Packet Core network will allow readers of this book to get a deeper insight into real world call procedures and message examples that are unmatched by other publications. Based on Tektronix Communications' experiences and best practices in troubleshooting and optimizing 3G radio access networks, we have already outlined a clear path to what needs to be done to troubleshoot and optimize 4G networks. This book serves as reference for daily work both in the lab and in the field. We also have covered the basics of network architecture and interfaces, protocols and principles of radio interface transmission in a comprehensive way that enables a smooth technology upgrade for all those who already have experience in the 2G and 3G wireless world.

The emphasis of this book is describing the signaling procedures and call flows on the radio interface, in E-UTRAN and within Enhanced Packet Core networks. We discuss the most common failure scenarios for live network troubleshooting. We also highlight essential performance measurement counters and define industry-adopted KPIs for accessibility, retainability and mobility. A large section covers in detail how to measure radio and user plane quality, especially over the air interface. All of this includes practical examples from Tektronix Communications measuring equipment, the radio interface tester K2Air and the Network and Service Analyzer platform NSA.

We are confident that this book will provide an exceptional reference for a broad range of individuals interested in next-generation mobile communications. Whether you are a highly advanced mobile network engineer or a student seeking an introduction to 3G LTE networks, you will benefit from continued accessibility to this reference. Please enjoy.

Richard Kenedi

Vice President and General Manager

Tektronix Communications

Test and Optimization Product Line

Acknowledgments

We would like to take the chance to acknowledge the efforts of all who participated directly or indirectly in creating and publishing this book.

First of all, a special “thank you” goes to Ralf Kreher's sister Brit who created and formatted all the figures you will find in this book. Another one goes to our family members and all who supported and encouraged us to get this work done.

Eiko Seidel and his team at Nomor Research have not just created some excellent primers about LTE radio interface procedures and set up the 3GPP LTE Standards Group at www.linkedin.com, but also given us deep insight into their scheduling simulator, a tool used to design scheduling algorithms for eNodeB vendors.

Antonio Bovo working as System Architect for Tektronix Padova contributed some very detailed research on E-UTRAN protocols and functions. From his work we have derived the major part of the S1AP section of this book.

Karsten Gienskey and Marcus Garin working for Tektronix Berlin shared with us their earliest prototypes and design specifications for RLC reassembly and radio interface tracing. Without their great job we would have been “blind” on the radio interface.

Ulrich Jeczawitz, freelancer and ex-colleague of Tektronix Berlin, and the development team of the Tektronix G35 protocol simulator led by Dirk-Holger Lenz, generated traces of E-UTRAN and Enhanced Packet Core signaling procedures long before they occurred in any live network field trial.

Lars Chudzinsky, working on LTE call trace for Tektronix Berlin, contributed design specifications of protocol failure events that became the raw material for Section 2.10.

This book would not exist without the ideas, questions, and requirements contributed by customers, colleagues, and subcontractors. Besides all the others who cannot be named personally, we would like to express our thanks especially to the following people: Jürgen Forsbach, Andre Huge, Steffen Hülpüsch, Armin Klopfer, and Jens Plogsties.

In addition, our thanks go to the Management of the Tektronix Communications Test & Optimization Product Line, in particular the Human Resources Department represented by Marion Kaehlke and R&D Berlin Director Jens Dittrich who supported the idea of writing this book and approved the usage of Tektronix Communications material in the contents.

Maïssa Bahsoun and Jeanne Lancry-Gulino have been our prime contacts in 3GPP in obtaining copyright permissions and, last but not least, we also would like to express our thanks to the team at John Wiley & Sons, Ltd, especially Mark Hammond, Sarah Tilley, and Sophia Travis, for their strong support.

Ralf Kreher and Karsten Gaenger

Berlin, 1 July 2010

1

Standards, Protocols, and Functions

LTE (Long-Term Evolution) of UMTS (Universal Mobile Telecommunications Service) is one of the latest steps in an advancing series of mobile telecommunication systems. The standards body behind the paperwork is the 3rd Generation Partnership Project (3GPP).

Along with the term LTE, the acronyms EPS (Evolved Packet System), EPC (Evolved Packet Core), and SAE (System Architecture Evolution) are often heard. Figure 1.1 shows how these terms are related to each other: EPS is the umbrella that covers both the LTE of the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) and the SAE of the EPC network.

Figure 1.1 EPC and LTE under the umbrella of EPS.

LTE was and is standardized in parallel to other radio access network technologies like EDGE (Enhanced Data Rates for GSM evolution) and HSPA (High-Speed Packet Access). This means that LTE is not a simple replacement of existing technologies. Rather it is expected that different kinds of radio access will coexist in operator networks.

From this background it emerges that understanding LTE also requires understanding alternative and coexisting technologies. Indeed, one of the major challenges of LTE signaling analysis will concern the analysis of handover procedures. Especially, the options for possible inter-RAT (Radio Access Technology) handovers have multiplied compared to what was possible in UMTS Release 99. However, also intra-system handover and dynamic allocation of radio resources to particular subscribers will play an important role.

The main drivers for LTE development are:

reduced delay for connection establishment;reduced transmission latency for user plane data;increased bandwidth and bit rate per cell, also at the cell edge;reduced costs per bit for radio transmission;greater flexibility of spectrum usage;simplified network architecture;seamless mobility, including between different radio access technologies;reasonable power consumption for the mobile terminal.

It must be said that LTE as a radio access technology is flanked by a couple of significant improvements in the core network known as the EPS. Simplifying things a little, it is not wrong to state that EPS is an all-IP (Internet Protocol) transport network for mobile operators. IP will also become the physical transport layer on the wired interfaces of the E-UTRAN. This all-IP architecture is also one of the facts behind the bullet point on simplified network architecture. However, to assume that to be familiar with the TCP/IP world is enough to understand and measure LTE would be a fatal error. While the network architecture and even the basic signaling procedures (except the handovers) become simpler, the understanding and tracking of radio parameters require more knowledge and deeper investigation than they did before. Conditions on the radio interface will change rapidly and with a time granularity of 1 ms the radio resources assigned to a particular connection can be adjusted accordingly.

For instance, the radio quality that is impacted by the distance between the User Equipment (UE) and base station can determine the modulation scheme and, hence, the maximum bandwidth of a particular connection. Simultaneously, the cell load and neighbor cell interference—mostly depending on the number of active subscribers in that cell—will trigger fast handover procedures due to changing the best serving cell in city center areas, while in rural areas macro cells will ensure the best possibley coverage.

The typical footprint of a LTE cell is expected by 3GPP experts to be in the range from approximately 700 m up to 100 km. Surely, due to the wave propagation laws such macro cells cannot cover all services over their entire footprint. Rather, the service coverage within a single cell will vary, for example, from the inner to the outer areas and the maximum possible bit rates will decline. Thus, service optimization will be another challenge, too.

1.1 LTE Standards and Standard Roadmap

To understand LTE it is necessary to look back at its predecessors and follow its path of evolution for packet switched services in mobile networks.

The first stage of the General Packet Radio Service (GPRS), that is often referred to as the 2.5G network, was deployed in live networks starting after the year 2000. It was basically a system that offered a model of how radio resources (in this case, GSM time slots) that had not been used by Circuit Switched (CS) voice calls could be used for data transmission and, hence, profitability of the network could be enhanced. At the beginning there was no pre-emption for PS (Packet Switched) services, which meant that the packet data needed to wait to be transmitted until CS calls had been finished.

In contrast to the GSM CS calls that had a Dedicated Traffic Channel (DTCH) assigned on the radio interface, the PS data had no access to dedicated radio resources and PS signaling, and the payload was transmitted in unidirectional Temporary Block Flows (TBFs) as shown in Figure 1.2.

Figure 1.2 Packet data transfer in 2.5G GPRS across Radio and Abis interfaces.

These TBFs were short and the size of data blocks was small due to the fact that the blocks must fit the transported data into the frame structure of a 52-multiframe, which is the GSM radio transmission format on the physical layer. Larger Logical Link Control (LLC) frames that contain already segmented IP packets needed to be segmented into smaller Radio Link Control (RLC) blocks.

The following tasks are handled by the RLC protocol in 2.5G:

Segmentation and reassembly of LLC packets → segmentation results in RLC blocks.Provision of reliable links on the air interface → control information is added to each RLC block to allow Backward Error Correction (BEC).Performing sub-multiplexing to support more than one MS (Mobile Station) by one physical channel.

The Medium Access Control (MAC) protocol is responsible for:

point-to-point transfer of signaling and user data within a cell;channel combining to provide up to eight physical channels to one MS;mapping RLC blocks onto physical channels (time slots).

As several subscribers can be multiplexed on one physical channel, each connection has to be (temporarily) uniquely identified. Each TBF is identified by a Temporary Flow Identifier (TFI). The TBF is unidirectional (uplink (UL) and downlink (DL)) and is maintained only for the duration of the data transfer.

Toward the core network in 2.5G GPRS the Gb interface is used to transport the IP payload as well as GPRS Mobility Management/Session Management (GMM/SM) signaling messages and short messages (Short Message Service, SMS) between SGSN and the PCU (Packet Control Unit)—see . The LLC protocol is used for peer-to-peer communication between SGSN and the MS and provides acknowledged and unacknowledged transport services. Due to different transmission conditions on physical layers (E1/T1 on the Gb and Abis interfaces, 52-multiframe on the Air interface), the size of IP packets needs to be adapted. The maximum size of the LLC payload field is 1540 octets (bytes) while IP packets can have up to 65 535 octets (bytes). So the IP frame is segmented on SGSN before transmission via LLC and reassembled on the receiver side.