2.1 Impact of Standardization Processes on SAE

Chapter 1 outlined the process of standardization including the organization of the standards bodies and the approval processes involved within those organizations. This is only one part of the standards process; the other aspect of the process not captured in any such description is the human aspect and its impact on the standardization process. This section attempts to outline the types of discussions that affected the final set of standards known as SAE today. Some concepts and terms used in this chapter have been described more in detail in later chapters of this book. Note that any such analysis usually is subjective in nature, but we have made every attempt to provide an unbiased and objective overview of the process and events.

As with most standardization efforts within 3GPP, the SAE work item builds upon existing technologies; in the case of SAE/EPC, the base was the existing 3GPP packet core system used for GSM/GPRS and WCDMA/HSPA. As outlined in Chapter 1 the progression of the LTE is a closely related work item to SAE. As the development of LTE progressed within the RAN groups of 3GPP, the opportunities to develop improvements to Packet Core were identified by several different interested parties.

One of the key aspects to understand about any standardization activity in any standards forum worldwide is the role of conflicting and diverging goals among the participating companies/stakeholders. These are often commercially driven goals, but are also the result of technical visions and the desire for their realization into ‘real systems‘. The purpose of the standards bodies is to help resolve difficult technical and political issues leading ultimately to achieving the common goal of a strong architecture designed to serve the needs of the entire community.

The standardization process is dynamic and exciting; an analysis of these aspects is a good basis for understanding the development of the work items involved. Firstly, with the introduction of a work item that is designed to create an ‘all-IP’ network naturally meant that there was a tremendous amount of interest in 3GPP’s work in this area from new participants. In fact, the number of participants within 3GPP SA WG2 dealing with Architecture and Overall System aspects rose dramatically from around 100 people to about 180–225 participants during the peak period; approximately 75% of these participants were actively involved in attempting to shape the SAE work and EPS architecture. Naturally, with such a large increase in the number of participants, there was a period of adjustment for everyone involved. While the new participants learnt the 3GPP working procedures and also adjusted to having to handle the implications of defining an architecture that included providing a clear migration path from an existing system to a new one. Whereas the incumbents needed to adapt to a more dynamic exchange of ideas from various new sources and find a common ground to move forward.

What were the key driving forces behind some of these groups and companies just on the 3GPP accesses and its associated core network evolution? One major factor was the mix of incumbent vendors and newcomers to the 3GPP system. The enormous interest in LTE/SAE work drew a large number of companies who had previously not participated in the 3GPP standardization process, many had indeed spent time working on other systems in various other standardization fora. These new entrants to 3GPP forged alliances and joined the SAE work. This created quite a contradictory vision for the future which was initially hard to reconcile since it is the vision between ‘continuity’ and a completely ‘new beginning of sorts’.

Some vendors and operators, experienced with the existing 3GPP systems, considered it important that continuity would be maintained with the 3GPP packet core when evolving the system, whereas others initially wanted to explore new avenues based on technology used by other standardization fora rather than 3GPP. There were also a few existing 3GPP operators and vendors who solely focused on creating an architecture that was not based on the existing 3GPP architecture. All these different inflection angles and viewpoints were fed into the work on SAE, and resulting in a very dynamic and diverse working environment. Initial investigations as documented in the 3GPP Technical report 23.882, [23.882] reflect various options with one common theme; that it makes sense to separate control and user plane entities. Some of the key architecture options that were discussed during the initial stages of the standardization process may be viewed as follows:

1. Evolution of the existing 3GPP packet core architecture, i.e. evolving the GTP protocol, but not necessarily reusing the architecture, with a single User Plane Gateway (GW) for the non-roaming cases, a local anchor and a GW for roaming and using the network-based mobility protocol developed in 3GPP.

2. In principle, follow a very similar architecture as outlined in the point 1 above, but where the two GW entities; thus the roaming and non-roaming architecture is exactly the same which is slightly different than 1, the protocols between these GWs would be developed in IETF.

3. An overlay model where a control plane entity and a GW/Home Agent are used in the architecture with client-based mobility protocols.

As will be seen during the rest of the book, a suitable compromise was reached through the standardization development process, essentially an architecture that is a combination of the above proposals.

Another key element to this discussion related to the architecture of the RAN for LTE. Though not discussed in detail in this book, it played a crucial role in creating additional ambiguity and delays since there were two very distinct views. One followed the principle of a need for a central radio network controller such as an RNC as is part of the architecture of WCDMA/HSPA, while the other view argued that a flat radio network architecture without RNC is better for the future and that the drawbacks of such an RNC-like entity were higher than the advantages. A decision on this matter was crucial since the functional division between the core and radio network for the 3GPP system depended on that point. Here the progress of SAE and LTE were closely interlinked for a period of time.

A significant amount of time and effort were placed in discussing the functional division between core and radio networks and the pros and cons of the different approaches. In the end, the 3GPP community made the decision to go with the architecture option without RNC-like entity for the Radio Access Network and the community then focused on settling the functions belonging to Radio and Core aspects and move forward with the investigations of developing the new architecture. Great efforts were put into the work on arriving at the preferred functional split between the LTE RAN (now only consisting of base stations) and the packet core network as defined by EPC. 3GPP finally arrived at a decision where the functional split between RAN and CN for LTE are similar in nature as for WCDMA with few exceptions.

Another key aspect that influenced the work on EPC was what role WiMAX as a technology would have. WiMAX was at the time promoted by some parties as a potentially important and preferred next generation mobile radio access technology, partly in competition with LTE. A key question here was the ambition level for supporting connection of WiMAX access networks (e.g. interworking and handover) to the 3GPP family of access technologies. The 3GPP community here took decisive action to postpone this tight coupling for the time being, clearing the path for completion of the SAE work. The main rationale behind this decision was that the most important interworking cases with rigid performance demands are between LTE and legacy access networks which are already widely deployed, meaning GSM, WCDMA/HSPA and CDMA/HRPD. Interworking with all other access technologies including WiMAX were considered to have less stringent performance demands.

Another difficult architecture decision was the selection of Policy Control and Charging mechanism. In the 3GPP systems so far, the GTP protocol carries not only mobility information but also QoS and Charging and policy control information. This way of transferring information that supports the PCC infrastructure is also known as the ‘On-Path model’. Since two options for handling mobility within EPC was decided to be supported using GTP and PMIP respectively, and the IETF PMIP protocol is unable to carry QoS and charging information itself the On-Path model used for GTP could not be supported for PMIP. The two options then considered in case of PMIP PCC was a form of On-Path model where a protocol like for example Diameter may be used directly between the two involved entities (see subsequent section for PCC for more details on the two models) to carry the necessary data and the other model also known as Off-Path model where the data is then carried via the PCC infrastructure and thus taking some extra hops on its way. There were some additional subtleties between the two models where QoS enforcement and management are not handled by the same entities for GTP vs. the PMIP variant of the architecture. In the end, the community selected to use Off-Path model for the PMIP variant, and thus the major hurdles for the architecture work were considered removed.

As usual, any new area of work brings a lot of research ideas to the forefront and the same happened for the SAE work. This has special significance when the issues are crucial aspects for operators, e.g. migration, interworking with existing infrastructure, and the overall cost of implementation and deployment. These issues were not necessarily considered as a priority until the end of the architecture development phase.

With such an approach the direction of work risked becoming in some sense decoupled from the reality of the every day business of running the networks. The positive aspect of this was that the development progressed with a completely new vision for the future though still considering that 3GPP needed to maintain some, if not most, of the key functions of the current system.

As investigations of various alternatives progressed, some new enthusiasm in the form of a few incumbent CDMA operators came into the 3GPP community about mid way through the evaluation/development process. These CDMA operators made strong commitments to help develop SAE/LTE and took steps to commit to the SAE/LTE track as an alternative next step for the migration of existing HRPD network.

As work progressed, it became clear that the SAE work would not emerge with one set of protocols and design choices for the EPS. With somewhat divergent operator requirements and migration/evolution strategies, 3GPP needed to take a hard decision; either following one architecture alternative (which was rather impossible to achieve), or allowing for multiple alternatives. At the end of the day, 3GPP emerged with multiple protocol variants used within one overall architecture framework satisfying these requirements. An aspect worth noting is that not only did 3GPP select two protocol options with slightly different architectural variants for network-based mobility, including the GTP evolution and PMIP-track based on the IETF-developed PMIP protocol suite and 3GPP specific extensions to this mobility mechanism, 3GPP also continued to develop a terminal-based mobility option though with rather limited interest from operator community.

A final twist came in the conclusion phase when it was decided that the existing packet core architecture should be maintained in parallel with EPC, while the original assumption and understanding of the work was that EPC would replace the existing packet core architecture. The consequence of this decision was the creation of two variants of how to interconnect to GERAN and WCDMA/HSPA access networks towards packet data networks (e.g. GPRS and EPC).

One milestone accomplishment during the architecture work is that regardless of choice of network-based mobility model (GTP or PMIP), the user device acts in the same way. There is no dependency between the mobility protocol used by the network entities and how the terminal connects to the network. This transparency should definitely help future development of EPS as a whole.

Figure 2.1.1 provides a timeline for some of key decisions taken during the SAE development phases primarily in 3GPP Architecture Working Group also known as SA WG2. Though other working groups were working in parallel in areas such as Security, Charging, Lawful Intercept, the main emphasis in this section has been centred around SA WG2 and its relation with other WGs including RAN and other fora.

2.2 Terminologies Used in this Book

As you progress through the chapters in this book, you will notice that there are several different acronyms used to describe different aspects of the core network evolution. You will notice that these acronyms are being used extensively in the industry as well, so here we have included a brief description of the meanings of the different acronyms and how we have selected to use these terms in this book.

The common terminology used in the industry is not necessarily the same as the terms that have been used in standardization. On the contrary there is somewhat of a mismatch between the mostly used terms in the mobile industry and the terms actually used in 3GPP specification work. SAE is a prime example of a term that has one meaning inside the 3GPP standardization community and a different meaning outside. Formally, in 3GPP the term SAE is the name of the work item developing the overall architecture for the EPS. The term SAE does not appear as a name on any part of the system in the 3GPP specifications but in the industry and the general public it is probably more familiar than the term EPS, which would be the formally correct way to refer to the overall system that implements the LTE radio.

Below follows a list of terms describing some of the most common acronyms in this book.

EPC: The new Packet Core architecture itself as defined in 3GPP Rel-8.

SAE: The work item, or standardization activity, within 3GPP that was responsible for defining the EPC specifications.

EPS: A 3GPP term which refers to a complete end-to-end system, that is, the User Equipment, E-UTRAN (and UTRAN and GERAN connected via EPC) and Core Network.

SAE/LTE: A term often used to refer to the complete NW; it is more commonly used outside of 3GPP instead of EPS. In this book we have used the terms EPS instead of the term SAE/LTE.

E-UTRAN: Evolved UTRAN; the 3GPP term denoting the RAN that implements the LTE radio interface technology.

UTRAN: The RAN for WCDMA/HSPA.

GERAN: The GSM RAN.

LTE: Formally the name of the 3GPP work item (Long Term Evolution) that developed the radio access technology and E-UTRAN, but in daily talk, it is used more commonly instead of E-UTRAN itself. In the book we use LTE for the radio interface technology. In the overall descriptions we have allowed ourselves to use LTE for both the RAN and the radio interface technology. In the more technical detailed chapters (Parts III and IV) of the book we strictly use the terms E-UTRAN for the RAN and LTE for the radio interface technology.

2G/3G: A common term for both the GSM and WCDMA/HSPA radio access and the core networks. In a 3GPP2-based network 2G/3G refers to the complete network supporting CDMA/HRPD.

GSM: 2G RAN. In this book, the term does not include the core network.

GSM/GPRS: 2G RAN and the GPRS core network for packet data.

WCDMA: The air interface technology selected for the 3G UMTS standard. Commonly also used to refer to the RAN formally known as UTRAN.

WCDMA/HSPA: 3G RAN and the enhancements of the 3G RAN to high-speed packet services. Commonly also used to refer to a UTRAN that is upgraded to support HSPA.

GSM/WCDMA: Both the second and third-generation radio access technologies and RAN.

HSPA: A term which covers both HSDPA (High Speed Downlink Packet Access) and Enhanced Uplink together. HSPA introduces several concepts into WCDMA allowing for the provision of downlink and uplink data rates up to 42 and 12 Mb/s respectively (February 2009).

CDMA: For the purposes of this book, CDMA refers to the system and standards defined by 3GPP2; in the context of this book, it is used as a short form for cdma2000®, referring to the access and core networks for both circuit switched services and packet data.

HRPD: High Rate Packet Data; the high-speed CDMA-based wireless data technology. For EPC, HRPD has been enhanced further to connect to EPS and support handover to and from LTE. Thus we also refer to as eHRPD; evolved HRPD network which supports interworking with EPS.

We also want to bring the attention to the use of UE, Terminal, and Mobile Device in this book. These terms are used interchangeably in the book and all refer to the device that an end-user accesses the network with.

Also, we use the word ‘interface’ to refer to both the reference points and the actual interfaces. cdma2000® is the trademark for the technical nomenclature for certain specifications and standards of the Organizational Partners (OPs) of 3GPP2. Geographically (and as of the date of publication), cdma2000® is a registered trademark of the Telecommunications Industry Association (TIA-USA) in the United States.