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 circuit and packet- switched systems, GTP has been proven to scale very well and to fulfil the purposes for which it has been designed.
Another significant aspect of GPRS is that it uses SS7-based signalling protocols such as MAP (Mobile Application Part) and CAP (CAMEL application part), both inherited from the circuit-switched core network. MAP is used for user data management and authentication and authorization procedures, and CAP is used for CAMEL-based on line charging purposes. Further details on CAMEL (Customized Applications for Mobile networks Enhanced Logic) are far beyond the scope of this book. For our purposes, it is enough to understand that CAMEL is a concept designed to develop non-IP-based services in mobile networks. The use of SS7-based protocols can be seen a drawback for a packet network created for delivering Internet connections and IP-based services.
3GPP packet core uses a network-based mobility scheme for handling user and terminal mobility. Another aspect that was to become a target for optimization at a later date was the fact that it has two entities (i.e. SGSN and GGSN) through which user data traffic is carried. With the increased data volumes experienced as a result of WCDMA/HSPA, an optimization became necessary and was addressed in 3GPP Rel-7, completed in early 2007 with the enhancement of the packet core architecture to support a mode of operation known as ‘Direct tunnel’ where the SGSN is not used for the user plane traffic. Instead, the radio network controller connects directly to the GGSN via Iu-user plane (based on GTP). This solution, however, only applies to non-roaming cases, and also requires packet data charging functions to reside in the GGSN instead of the SGSN.
For further details on the packet core domain prior to SAE/EPC, please refer to 3GPP Technical Specification TS 23.060, [23.060].
1.2.2 3GPP2 radio access technologies
In North America another set of radio access technology standards was developed. This was developed within the standards body called 3GPP2, under the umbrella of ANSI/TIA/EIA-41 which includes North American and Asian interests towards developing global standards for those RAN technologies supported by ANSI/TIA/EIA-41.
3GPP2 developed the radio access technologies cdma2000®, providing 1xRTT and HRPD (High Rate Packet Data) services. cdma2000 1xRTT is an evolution of the older IS-95 cdma technology, increasing the capacity and supporting higher data speeds. HRPD defines a packet-only architecture with capabilities similar to the 3GPP WCDMA technology. The set of standards for the packet core network developed within 3GPP2 followed a different track to 3GPP, namely the reuse of protocols directly from the IETF, such as the Mobile IP family of protocols as well as a simpler version of IP connectivity known as Simple IP, over a PPP link. The main packet core entities in this system are known as PDSN (Packet Data Serving Node) and HA (Home Agent), where terminal-based mobility concepts from IETF are used, in conjunction with 3GPP2 developed own mechanisms. It also uses Radius-based AAA infrastructure for its user data management, authentication and authorization and accounting.
1.2.3 Other forums involved in SAE
WiMAX forum indirectly participated in the development of SAE through companies and participants who took part in both WiMAX forum and 3GPP work. A number of key WiMAX Forum contributing members were also dominant 3GPP2 members and thus the packet core network developed within WiMAX forum bears strong similarity with 3GPP2 packet core in its usage of the same IETF protocols like MIPv4, MIPv6 and Radius for the same purposes as 3GPP2.
1.2.4 Dawn of EPC
At the beginning of the work on EPC there were therefore two main variants of core network architectures designed with several different RANs in mind. With the evolution to mobile broadband, in particular LTE, the opportunity to evolve the core network to better utilize IP technologies in order to bring significant cost savings for operators and end-users, became evident. It was also immediately apparent that the new core network would need to support legacy access networks in order to ensure that existing radio network installations would be able to connect to EPC.
At the same time, many operators within 3GPP2 became interested in the evolution of the core network ongoing in 3GPP as they wished to join the LTE ecosystem and development of common packet core work under the umbrella of SAE. As a result, work in both 3GPP and 3GPP2 was established to ensure that the EPS could support interworking towards 3GPP2 networks. EPS then needed to support the evolution of two very different types of core network and that created the framework of SAE work in 3GPP.
1.2.5 SAE – building bridges between different networks
This meant that SAE was to target both improving and building a bridge between two very distinct packet core networks.
The existing packet core networks were developed to serve certain market and operator requirements. These requirements have not changed with the evolution to EPS. Rather, with the evolution towards new radio networks and also the need to deliver new types of services across the core network, the EPS is instead required to support extra requirements on top of the old ones.
The added common requirements towards the system are as follows in no particular order of importance or priority:
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Support for non-3GPP access networks
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Support handovers between 3GPP and non-3GPP accesses
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Network-based mobility mechanisms were preferred
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Common Security framework
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Common User management and Authentication and Authorization framework
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Common Policy and Charging support
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Common framework for On and Off line Charging and Accounting
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Provide Optimized handover to/from existing deployed Radio access and Packet Core networks: 3GPP’s GERAN, UTRAN and HSPA and 3GPP2’s HRPD networks
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Common Evolved Packet Core for access to Common IMS and Applications and Service framework
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Common operations and management of Terminals.
Additional requirements identified from the implementation and use of 3GPP packet core were related to:
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Improving the performance of the existing system
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Securing high performance packet handovers between 3GPP accesses
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Clear separation of control and user plane operation
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The option to have a single user plane entity during non-roaming mode of operation
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Easy migration to EPS
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Manageable impacts on roaming infrastructure upgrades, implying the ability to gradually migrate from the existing GRX (roaming) networks and the ability to support inbound/outbound roamers from LTE and to/from existing 3GPP networks belonging to operator partners that have not upgraded to EPS.
In order to support the key components described above, EPS developed multiple options of the architecture where different protocol suites were tailored to fulfil the majority of the requirements.
It was clear from early on that IETF-based protocols would play a key role in EPS. 3GPP had been very deeply involved in continuing the development of the IMS and PCC (Policy and Charging Control) Systems, where all the protocols are built on IETF-developed base protocols and then enhanced within the IETF as per 3GPP’s requirements. This was not new or unchartered territory for 3GPP member companies since 3GPP already had contributed extensively to the development within IETF of SIP, AAA, Diameter and various security related protocols.
As such, once the EPS architecture and protocol choices were settled, the work progressed quite smoothly. The most contentious area of protocol selection was related to mobility management, where there were a few competing proposals in the IETF and progress was slow. Once the IETF settled on developing PMIP (Proxy Mobile IP) as the network-based mobility protocol, however, 3GPP EPS development followed suit.
Additional standard bodies that 3GPP collaborated with in order to develop the necessary components for EPS are 3GPP2 and OMA. 3GPP2 helped develop the aspects related to CDMA technologies in relation to supporting optimized handover between HRPD and LTE as well as Single Radio Voice Call Continuity and CS Fallback solutions. OMA cooperation involved the areas of device management; focusing on aspects of network discovery and selection procedures for non-3GPP accesses.
Even though 3GPP collaborates with various other standardization fora in order to develop the final system, the actual details are worked out within 3GPP protocol working groups according to the architectural requirements set by architecture (SA) and RAN working groups. As necessary, various IETF drafts and other specifications are then prepared and progressed as per 3GPP specifications to finalize the 3GPP specification development process.
1.2.6 Introducing EPC – an operator’s and end-user’s perspective
What, then, are some of the key changes that a network operator and an end-user will experience due to the evolution of the packet core? This section will focus on the EPS, which is composed of the EPC, End-User Equipment (more commonly known as the UE), Access Network (including 3GPP access such as GERAN, UTRAN, E-UTRAN and non 3GPP accesses such as HRPD, WiMax, WLAN, etc). The combination of these enables access to an operator’s services and also to the IMS, which provides voice services.
Before getting into the technical aspects of EPS, let us look at the system from the end-user’s and network operator’s perspective. What do they need to do before their subscribers are able to connect to the new network? This depends on whether an operator is migrating to the new radio access technology or utilizing the existing 3GPP radio or other non-3GPP access technologies with EPC. For the sake of simplicity we will for the moment assume that LTE is the operator’s chosen radio access technology.
From an end-user’s perspective, the actions are quite simple; if an end-user wishes to use mobile broadband, then they need to select an operator which supports it and purchase a device capable of supporting the highest bandwidth that is provided through using EPS via E-UTRAN access. While the UICC card that the user currently owns may be reused, the enhanced security functions and data-only functions may encourage the purchase of SIM cards designed specifically for EPS.
From an operator’s perspective, in order to provide a data-only mobile broadband service, the infrastructure must be upgraded to EPS. EPS provides components that allow existing 2G/3G access networks to utilize EPC components. For those incumbent 2G/3G operators the existing CS network can provide access to voice calls in the short term, but the deployment of IMS in conjunction with EPS would provide an All-IP network with access to speech services.
In the following chapters, we will focus initially on a mobile broadband deployment scenario where access to data services is the predominant mode of operation. We will later also describe how EPS is supporting voice services.