SAE and the Evolved Packet Core

Driving the Mobile Broadband Revolution

Magnus Olsson, Catherine Mulligan, Stefan Rommer, Shabnam Sultana, Lars Frid

• Publisher: Academic Press
• Year: 2009
• ISBN:
  9780080888705
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

This book provides a clear, concise, complete and authoritative introduction to System Architecture Evolution (SAE) standardization work and its main outcome: the Evolved Packet Core (EPC), including potential services and operational scenarios. After providing an insightful overview of SAE's historical development, the book gives detailed explanations of the EPC architecture and key concepts as an introduction. In-depth technical descriptions of EPC follow, including thorough functional accounts of the different components of EPC, protocols, network entities and procedures. Case studies of deployment scenarios show how the functions described within EPC are placed within a live network context, while a description of the services that are predicted to be used shows what EPC as a core network can enable.

This book is an essential resource for professionals and students who need to understand the latest developments in SAE and EPC, the 'engine' that connects broadband access to the internet.

All of the authors have from their positions with Ericsson been actively involved in GPRS, SAE and 3GPP from a business and technical perspective for many years. Several of the authors have also been actively driving the standardization efforts within 3GPP.

"There is no doubt that this book, which appears just when the mobile industry starts its transition away from legacy GSM/GPRS and UMTS networks into the future will become the reference work on SAE/LTE. There are no better qualified persons than the authors of this book to provide both communication professionals and an interested general public with insights into the inner workings of SAE/LTE. Not only are they associated with one of the largest mobile network equipment vendors in the world, they have all actively contributed to and, in some cases, been the driving forces behind the development of SAE/LTE within 3GPP." - from the foreword by Dr. Ulf Nilsson, TeliaSonera R&D, Mobility Core and Connectivity

"The authors have done an excellent job in writing this book. Their familiarity with the requirements, concepts and solution alternatives, as well as the standardization work allows them to present the material in a way that provides easy communication between Architecture and Standards groups and Planning/ Operational groups within service provider organizations." - from the foreword by Dr. Kalyani Bogineni, Principal Architect, Verizon

• Up-to-date coverage of SAE including the latest standards development
• Easily accessible overview of the architecture and concepts defined by SAE
• Thorough description of the Evolved Packet Core for LTE, fixed and other wireless accesses
• Comprehensive explanation of SAE key concepts, security and Quality-of-Service
• Covers potential service and operator scenarios including interworking with existing 3GPP and 3GPP2 systems
• Detailed walkthrough of network entities, protocols and procedures
• Written by established experts in the SAE standardization process, all of whom have extensive experience and understanding of its goals, history and vision

Part I

Introduction – Background and Vision of EPC

Chapter 1 Mobile broadband and the core network evolution

The telecommunications industry is in a period of radical change with the advent of mobile broadband radio access and the convergence of Internet and mobile services. Part of this radical change is enabled by a fundamental shift in the underlying technologies; mobile telephony is moving towards an all Internet Protocol (IP) network architecture after several decades of circuit switched technology. The evolution of the core network to support the new high-bandwidth services promised by mobile broadband is a monumental breakthrough. This book covers that evolution.

The phenomenal success of GSM (Global System for Mobile Communications) was built on the foundation of circuit switching. Services, meanwhile, were built by developers specialized in telecommunication applications. During the early 1990s, usage of the Internet also took off, in later years leading to a demand for ‘Mobile Internet’; Internet services that could be accessed from an end-user’s mobile device. The first such services had limitations due to the processing capacity of terminals and also a very limited bandwidth on the radio interface. This has now changed with the evolution of radio access networks (RANs) with high data rates delivered by High Speed Packet Access (HSPA) and Long Term Evolution (LTE). The speed of this change is set to increase dramatically as a number of other developments emerge in addition to the new high-speed radio accesses; the rapid advances in the processing capacity of semiconductors for mobile terminals and also in the software that developers can use to create services. IP and packet-switched technology are soon expected to be the base for data and voice services on both the Internet and mobile communications networks.

The core network is the part that links these worlds together, combining the power of high-speed radio access technologies with the power of the innovative application development enabled by the Internet. The evolution of the core network, or Evolved Packet Core (EPC), is a fundamental cornerstone of the mobile broadband revolution; without it neither the RANs nor mobile Internet services would realize their full potential. The new core network was developed with high-bandwidth services in mind from the outset, combining the best of IP infrastructure with mobility. It is designed to truly enable mobile broadband services and applications and to ensure a smooth experience for both operators and end-users as it also connects multiple radio access technologies.

This chapter introduces the reasoning behind the evolution of the core network and a brief introduction to the technologies related to EPC.

System Architecture Evolution (SAE) is the name of the Third Generation Partnership Project (3GPP) standardization work item which is responsible for the evolution of the packet core network, more commonly referred to as EPC. This work item is closely related to the LTE work item, covering the evolution of the radio network. Evolved Packet System (EPS) covers the radio access, the core network and the terminals that comprise the overall mobile system. Also provides support for other high-speed RANs that are not based on 3GPP standards, for example, WLAN, WiMAX or fixed access. This book is all about EPC and EPS – the evolution of the core network in order to support the mobile broadband vision and an evolution to IP-based core networks for all services.

The broad aims of the SAE work item were to evolve the packet core networks defined by 3GPP in order to create a simplified all-IP architecture, providing support for multiple radio accesses, including mobility between the different radio standards. So, what drove the requirement for evolving the core network and why did it need to be a globally agreed standard? We will start with looking into this.

1.1 The Need for Global Standards

There are many discussions today regarding the evolution of standards for the communications industries, in particular when it comes to convergence between IT and telecommunications services. A common question that pops up occasionally is why is a global standard needed at all? Why does the cellular industry follow a rigorous standards process, rather than, say, the de-facto standardization process that the computer industry often uses? There is a lot of interest in the standardization process for work items like LTE and SAE, so there is obviously a commercial reason for this, or very few companies would see value in participating in the work.

The necessity for a global standard is driven by many factors, but there are two main points. First of all, the creation of a standard is important for interoperability in a truly global, multi-vendor operating environment. Operators wish to ensure that they are able to purchase network equipment from several vendors, ensuring competition. For this to be possible nodes from different vendors must inter-work with one another; this is achieved by specifying a set of ‘interface descriptions’, through which the different nodes on a network are able to communicate with one another. A global standard therefore ensures that an operator can select whichever network equipment vendor they like and that end-users are able to select whichever handset that they like; a handset from vendor A is able to connect to a base station from vendor B and vice versa. This ensures competition which in itself attracts operators and drive deployments by ensuring a sound financial case through avoiding dependencies on specific vendors.

Secondly, the creation of a global standard is about reducing fragmentation in the market for all the actors involved in delivering network services to end-users; operators, chip manufacturers, equipment vendors, etc. A global standard ensures that there will be a certain market for the products that, for example,. an equipment vendor develops. The larger the volume of production for a product, the greater the volume there is to spread the cost of production across the end-users that will use the products. Essentially, with increased volumes a vendor should be able to produce each node at a cheaper per unit cost. Vendors can then achieve profitability at lower price levels, which ultimately leads to a more cost-effective solution for both operators and end-users. Global standards are therefore a foundation stone of the ability to provide inexpensive, reliable communications networks and the aims behind the development of EPC were no different.

There are several different standards bodies that have been directly involved in the standardization processes for the SAE work. These standards bodies include the 3GPP, the lead organization initiating the work, the Third Generation Partnership Project 2 (3GPP2), the Internet Engineering Task Force (IETF), WiMAX Forum and Open Mobile Alliance (OMA). 3GPP ‘owns’ the EPS specifications and refers to IETF and occasionally OMA specifications where necessary, while 3GPP2 complements these EPS specifications with their own documents that cover the impact on EPS and 3GPP2-based systems. WiMAX forum also refers to 3GPP documentation where appropriate for their specification work.

The readers who are not familiar with the standardization process are referred to, Appendix 1, where we provide a brief description of the different bodies involved and the processes that are followed during the development of these specifications.

1.2 Origins of the EPC

Over the years, many different radio standards that have been created worldwide, the most commonly recognized ones are GSM, CDMA and WCDMA/HSPA. The GSM/WCDMA/HSPA and CDMA radio access technologies were defined in different standards bodies and also had different core networks associated with each one as we describe below.

In order to understand why evolution was needed for 3GPP’s existing packet core, we therefore also need to consider where and how the various existing core network technologies fit together in the currently deployed systems. Chapter 2 provides more details on the background and history of the evolution towards EPC from the perspective of the standardization bodies. The following section presents a discussion around why the evolution was necessary. While the number of acronyms may appear daunting in this section for anyone new to 3GPP standards, the rest of the book explains the technology in great detail. This section highlights only some of the main technical reasons for the evolution.

1.2.1 3GPP radio access technologies

GSM was originally developed within the European Telecommunication Standards Institute (ETSI), which covered both the RAN and the core network supplying Circuit Switched telephony. The main components of the core network for GSM were the Mobile Switching Centre (MSC) and the Home Location Register (HLR). The interface between the GSM BSC (Base Station Controller) and the MSC was referred to as the ‘A’ interface. It is common practice for interfaces in 3GPP to be given a letter as a name, in later releases of the standards there are often two letters, for example, ‘Gb’ interface. Using letters is just an easy shorthand method of referring to a particular functional connection between two nodes.

Over time, the need to support IP traffic was identified within the mobile industry and the General Packet Radio Services (GPRS) system was created as an add-on to the existing GSM system. With the development of GPRS, the concept of a packet-switched core network was needed within the specifications. The existing GSM radio network was evolved, while two new logical network entities or nodes were introduced into the core network – the SGSN (Serving GPRS Support Node) and the GGSN (Gateway GPRS Support Node).

GPRS was developed during the period of time when PPP, X.25 and Frame Relay were state-of-the-art technologies (mid to late 1990s) for packet data transmission on data communications networks. This naturally had some influence on the standardization of certain interfaces, for example, the Gb interface, which connects the BSC in the GSM radio network with GPRS packet core.

During the move from GSM EDGE Radio Access Network (GERAN) to UMTS Terrestrial Radio Access Network (UTRAN), an industry initiative was launched to handle the standardization of radio and core network technologies in a global forum, rather than ETSI, which was solely for European standards. This initiative became known as the 3GPP and took the lead for the standardization of the core network for UTRAN/WCDMA, in addition to UTRAN radio access itself. 3GPP later also took the lead for the creation of the Common IMS specifications. IMS is short for IP Multimedia Subsystem, and targets network support for IP-based multimedia services. We discuss the IMS more in Chapter 5.

The core network for UTRAN reused much of the core network from GERAN, with a few updates. The main difference between was the addition of the interface between the UTRAN Radio Network Controller (RNC), the MSC and the SGSN, the Iu-CS and the Iu-PS, respectively. Both of these interfaces were based on the A interface, but the Iu-CS was for circuit-switched access, while the Iu-PS was for the packet-switched connections. This represented a fundamental change in thinking for the interface between the mobile terminal and the core network. For GSM, the interface handling the circuit-switched calls and the interface handling the packet-switched access were very different. For UTRAN, it was decided to have one common way to access the core network, with only small differences for the circuit-switched and packet-switched connections. A high-level view of the architecture of this date, around 1999, is shown in Figure 1.2.1.1 (to be completely accurate, the Iu-CS interface was split into two parts, but we will disregard that for now in order not to make this description too complex).

Figure 1.2.1.1 High-level architecture WCDMA and GSM radio networks.

The packet core network for GSM/GPRS and WCDMA/HSPA forms the basis for the evolution towards EPC. As a result, it is worthwhile taking the time for a brief review of the technology. Again, do not be put off by the number of acronyms, Parts II and III provide more details.

The packet core architecture was designed around a tunnelling protocol named GTP (GPRS Tunnelling Protocol) developed within ETSI and then continued within 3GPP after its creation. GTP is a fundamental part of 3GPP packet core, running between the two core network entities the SGSN and the GGSN. GTP runs over IP and provides mobility, Quality of Service (QoS) and policy control within the protocol itself. As GTP was created for use by the mobile community use, it has inherent properties that make it suitable for the robust and time-critical systems such as mobile networks. Since GTP is developed and maintained within 3GPP, it also readily facilitates the addition of the special requirements of a 3GPP network such as the use of the Protocol Configuration Option (PCO) field between the terminal and the core network. PCO carries special information between the terminal and the core network, allowing for flexible, efficient running and management of the mobile networks.

GTP has from time to time faced criticism, however, from parts of the communication industry outside 3GPP. This has mainly been due to the fact that it was not developed in the IETF community, the traditional forum for Internet and IP. GTP is instead a unique solution for 3GPP packet data and is therefore not automatically a good choice for other access technologies. GTP was instead tailor-made to suit the needs of 3GPP mobile networks. Whether the criticism is justified or not, is largely dependent on the viewpoint of each individual person.

Regardless, GTP is today a globally deployed protocol for 3GPP packet access technologies such as HSPA, which has emerged as the leading mobile broadband access technologies deployed prior to LTE. Due to the number of subscribers using GSM and WCDMA packet data networks, now counting in billions in total for both ...

Table of contents

• Cover
• Title Page
• Copyright
• Foreword by Dr. Ulf Nilsson
• Foreword by Dr. Kalyani Bogineni
• Preface
• Acknowledgements
• Part I: Introduction – Background and Vision of EPC
• Part II: Overview of EPS
• Part III: Key Concepts
• Part IV: The Nuts and Bolts of EPC
• Part V: Conclusion and Future of EPS
• References
• Appendix A: Standards Bodies Associated with EPS
• Appendix B: SAE/EPC Specifications
• Appendix C: Mobile Broadband Application Development
• Appendix D: Abbreviations
• Index