EPC Cours Notes

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(1)LTE Evolved Packet Core Network Flexicourse Course Code: LT3604F. Duration: 1 day. ... delivering knowledge, maximizing performance.... LTE courses include: n. LTE/SAE Engineering Overview. n. LTE Air Interface. n. LTE Radio Access Network. n. Cell Planning for LTE Networks. n. LTE Evolved Packet Core Network. n. 4G Air Interface Awareness. n. Understanding Next Generation LTE. Wray Castle – leading the way in LTE training. www.wraycastle.com.

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(3) LTE Evolved Packet Core Network Flexicourse. LTE EVOLVED PACKET CORE NETWORK Flexicourse. First published 2009 Last updated August 2009 WRAY CASTLE LIMITED BRIDGE MILLS STRAMONGATE KENDAL LA9 4UB UK. Yours to have and to hold but not to copy The manual you are reading is protected by copyright law. This means that Wray Castle Limited could take you and your employer to court and claim heavy legal damages. Apart from fair dealing for the purposes of research or private study, as permitted under the Copyright, Designs and Patents Act 1988, this manual may only be reproduced or transmitted in any form or by any means with the prior permission in writing of Wray Castle Limited.. © Wray Castle Limited.

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(5) EPS and IMS Overview. LTE EVOLVED PACKET CORE NETWORK Flexicourse. Contents Section 1. EPS and IMS Overview. Section 2. Evolved Packet Core. Section 3. Major Protocols. Section 4. EPC Operations. Glossary. LT3604F/v1. © Wray Castle Limited. iii.

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(7) EPS and IMS Overview. Section 1. EPS and IMS Overview. LT3604F/v1. © Wray Castle Limited. v.

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(9) EPS and IMS Overview. Contents The Evolution of UMTS ........................................................................................................................ 1.1 Release 8 and Beyond ......................................................................................................................... 1.2 EPS and IMS Architecture ................................................................................................................... 1.3 PDN Connectivity Services .................................................................................................................. 1.4 E-UTRAN ............................................................................................................................................. 1.5 EPC ...................................................................................................................................................... 1.6 IMS Functions ...................................................................................................................................... 1.7 Additional Options ................................................................................................................................ 1.8 PS Interworking .................................................................................................................................... 1.9 CS Interworking .................................................................................................................................. 1.10 Non-3GPP Access Interworking ........................................................................................................ 1.11 EPS Services ..................................................................................................................................... 1.12 Glossary ............................................................................................................................................. 1.13. LT3604F/v1. © Wray Castle Limited. vii.

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(11) EPS and IMS Overview. Objectives. At the end of this section you will be able to: . discuss the evolution of 3GPP networks and show where the EPS lies within it. . outline the relationship between the EPS and the IMS. . demonstrate an understanding of the abbreviations LTE, SAE, E-UTRAN, EPC, EPS and IMS and explain their applicability to the evolved network. . describe the basic architecture of a combined EPS and IMS network. . outline the functions of the EPS. . list the main elements and interfaces found within the E-UTRAN. . outline the basic set of network elements that comprise an EPC. . describe the functions of the IMS. . list the main elements found within an IMS. . demonstrate an understanding of the interworking options available to the EPS. . list services an EPS can support. LT3604F/v1. © Wray Castle Limited. ix.

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(13) EPS and IMS Overview. 3.9G/4G R8. LTE/EPS. R7. 3.75G. R6 3.5G. R5. HSPA+. HSPA. R4 3G. R99. UMTS R99 R98. 2.75G. R97 R96. GPRS. 2G. Phase 2+ Phase 2. EDGE. 2.5G. GSM. Phase 1. 1990. 1992. 1994. 1996. 1998. 2000. 2002. 2004. 2006. 2008. 2010. 2012. The Evolution of UMTS 3GPP’s (3rd Generation Partnership Project) design for UMTS networks began with the publication of the Release 99 (also known as Release 3) specifications in the late 1990s. R99 UMTS core networks followed the basic structural architecture of the GSM/GPRS system that had preceded them and were functionally split into CS (Circuit Switched) and PS (Packet Switched) core networks. 3GPP made it clear that support for the legacy technologies employed by the CS core network would become more limited over time and stated that their intention was to evolve UMTS (Universal Mobile Telecommunications System) networks towards an ‘all IP’ basis. Release 4 specifications introduced the concept of the MGW (Media Gateway), which allowed CS services to migrate away from the time-division bearer technologies that had traditionally been used to carry voice and other real-time services towards newer, more flexible technologies such as ATM (Asynchronous Transfer Mode) and IP (Internet Protocol). The ability to provide support for real-time services carried via the PS core network was introduced in Release 5, with the IMS (IP Multimedia Subsystem). The IMS allows, among other functions, services such as voice and video calling, which were previously provided via the CS core network, to be offered via an IP-based core instead. Release 6 and Release 7 specifications introduced support for alternative access methods to 3GPP core network environments, allowing operators to interface with Wi-Fi and other types of access system.. LT3604F/v1. © Wray Castle Limited. 1.1.

(14) LTE Evolved Packet Core Network Flexicourse. EGRPS Evolution HSPA Evolution. Beyond R8. LTE Advanced IMT Advanced 3.9G/4G R8 LTE/EPS. 2010. 2012. 2014. 2016. 2018. 2020. 2022. 2024. 2026. 2028. 2030. 2032. Release 8 and Beyond The next generation of UMTS services begins with Release 8. 3GPP R8 (Release 8) specifications outline the architecture and protocols required to support what 3GPP initially called 3.9G, but which they and the rest of the industry now call 4G, services. 3GPP’s vision of 4G is generally known as the LTE (Long Term Evolution) of the radio access network and the SAE (System Architecture Evolution) of the core, although it is officially known as the EPS (Evolved Packet System). EPS radio access is provided via the E-UTRAN (Evolved Universal Terrestrial Radio Access Network) – the LTE part of the system – and the EPC (Evolved Packet Core) provides the long promised ‘all IP’ core network environment. Development for systems beyond R8 is also underway, with evolutions planned for all current generations of 3GPP-based mobile networks. 2G GSM/EDGE networks will continue to evolve with E-GPRS2 (Enhanced GPRS 2)and EDGE (Enhanced Data rates for Global Evolution) Evolution, which allow systems to offer 1 Mbit/s or more to mobile data users. 3G UMTS networks have options to increase the data rates available to HSPA (High Speed Packet Access) users using HSPA+ and HSPA Evolution. 4G LTE systems can be augmented by LTE Advanced enhancements.. 1.2. © Wray Castle Limited. LT3604F/v1.

(15) EPS and IMS Overview 2G/3G SGSN. UMTS/ GPRS. UTRAN/ GERAN. HSS. EIR. S6a. S3. PCRF. MME S4 S12 S13. E-UTRA S1-MME. S11 SGi. IP Services. S1-U S-GW Interworking to MME. Rx+ IMS. S5 E-UTRAN. S7/Gx. PDN GW. S2. WLAN or WiMAX. EPS and IMS Architecture The R8 Evolved Packet System supersedes the access and core elements of earlier iterations of UMTS, replacing the R3–R7 UTRAN radio access network with the E-UTRAN (LTE) and the differentiated CS and PS cores with a single EPC (SAE). To allow the evolved network to continue to support real-time services and connection types that require more management, 3GPP recommends that the EPS connects to an IMS. Not all connections established via the EPS will require handling within the IMS – basic Internet connections, e-mail transmission and other ‘non-real-time’ services will continue to be routed directly to their destinations via the SGi interface. There is some discussion within the mobile industry as to whether an IMS is actually required to offer real-time or CS services over LTE. Many take the view that traditional CS services – mainly consisting of voice and SMS (Short Message Service) – can be provided by reusing the GAN (Generic Access Networks) approach employed to carry CS services over Wi-Fi and other types of IP access system. The VoLGA (Voice over LTE Generic Access) consortium has proposed just such an approach, which would allow network operators to reuse their existing CS infrastructure.. Further Reading: 3GPP TS 23.401:4.2 (figure after colon relates to the section in the document) LT3604F/v1. © Wray Castle Limited. 1.3.

(16) LTE Evolved Packet Core Network Flexicourse. PDN Connectivity Service. Evolved Packet System. EPS Bearer PDN-GW. Packet Data Network. PDN Connectivity Services. PDN Connectivity Services The EPS is designed to provide IP connectivity between a UE and a PDN (Packet Data Network). The connection provided to a UE is referred to as a PCS (PDN Connectivity Service). This consists of an EPS bearer that connects the UE to an Access Point in a PDN-GW (PDN Gateway) and traverses both the E-UTRAN and the EPC. The PDN-GW routes traffic between the EPS bearer and the external PDN. The EPS bearer, in turn, carries one or more SDF (Service Data Flow) between the UE (User Equipment) and external data services. If a UE requires additional connectivity that is only available via a different PDN-GW Access Point, then additional PDN Connectivity Services may be established in parallel.. Further Reading: 3GPP TS 23.401:4.7.1 1.4. © Wray Castle Limited. LT3604F/v1.

(17) EPS and IMS Overview. E-UTRAN. Uu. S1. X2 S1 UE. Evolved Packet Core (EPC). eNB. E-UTRAN The E-UTRAN provides radio access for the EPS. During development this part of the system was known as LTE (for the Long Term Evolution of the 3G radio path) and although the terminology used has since been standardized as the E-UTRAN, it is likely that the term LTE will remain in common use. The E-UTRAN consists of the eNB (Evolved Node B) and some IP-based interfaces, but no controller node. The evolved access network therefore has a ‘flatter’ structure than those that preceded it where the functions formerly attributed to the 2G GERAN (GPRS/EDGE Radio Access Network) BSC (Base Station Controller) or 3G UTRAN RNC (Radio Network Controller) have been subsumed into the base station itself. The main interfaces of the E-UTRAN are: . Uu – the air interface between eNB and UE. . S1-U – user traffic between eNB and EPC. . S1-MME – control traffic between eNB and EPC. . X2 – handover traffic between neighbouring eNBs. Traffic is carried over the E-UTRAN in an E-RAB (Evolved Radio Access Bearer), which runs between a UE and the EPC.. Further Reading: 3GPP TS 36.300 and 36.2xx family LT3604F/v1. © Wray Castle Limited. 1.5.

(18) LTE Evolved Packet Core Network Flexicourse Home Subscriber Server (HSS) Mobility Management Entity (MME). Policy and Charging Resource Function (PCRF). IP Network. Internet Serving Gateway (S-GW). PDN Gateway (PDN-GW). EPC. EPC (Evolved Packet Core). EPC (Evolved Packet Core) The EPC is designed to perform a set of interconnection functions similar to those performed by previous incarnations of the 3GPP core network, including session, subscriber and mobility management. As with the E-UTRAN, the EPC has been designed to offer a ‘flatter’ architecture and is therefore populated with fewer devices than were found in legacy core networks. The four basic nodes of the EPC are: . MME (Mobility Management Entity) S-GW (Serving Gateway) PDN-GW (Packet Data Network Gateway) PCRF (Policy and Charging Resource Function). The EPC network elements are interconnected via a set of ‘S’ interfaces, which are all IP-based, and which mostly reuse existing packet core network protocols such as GTP (GPRS Tunnelling Protocol).. Further Reading: 3GPP TS 23.401 1.6. © Wray Castle Limited. LT3604F/v1.

(19) EPS and IMS Overview HSS. EPS. IMS. Key: I-CSCF. Mw. I-CSCF. IP Multimedia Networks. Cx. Mw. Interworking CSCF. IM-MGW IP Multimedia MGW MGCF. Media Gateway Control Function. MRFC. Media Resource Control Function. MRFP. Multimedia Resource Function Processor. Mm S-CSCF. P-CSCF. P-CSCF Proxy Call State Control Function S-CSCF. Serving CSCF. SGW. Serving Gateway. Mr. Mg Mi. Mj MGCF. Mk BGCF. Mn. SGi. Mw MRFC. BGCF. Other IMS Networks. Mp. SGW CS Domain PDN Gateway (PDN-GW). Mb IM-MGW MRFP Mb. IMS Functions The IMS allows IP-based access networks to take advantage of the call and session control facilities provided by a range of IP-based protocols such as the SIP (Session Initiation Protocol) and the RTP (Real Time Protocol). A simplified explanation of the IMS’s function could state that it is designed to allow real-time traffic services (voice and video telephony, multiplayer gaming, etc.) to be offered by an IP-based network with the same quality of service as would have been provided by a circuit-switched system (although the remit of the IMS is wider than this narrow explanation would allow). The main functions of the IMS are the establishment and control of data sessions between users, and the associated subscriber, mobility and security management of those users while they are connected. IMS functions and architecture are performed primarily by a 3GPP-specific SIP server known as a CSCF (Call Session Control Function). There are several different types of CSCF. Subscriber management is handled by the HSS (Home Subscriber Server), which is also common to the EPS. Additionally, a variety of traffic and signalling gateways are employed to allow interworking between IP-connected IMS users and users connected to legacy network types.. Further Reading: 3GPP TS 23.228 LT3604F/v1. © Wray Castle Limited. 1.7.

(20) LTE Evolved Packet Core Network Flexicourse. 2G/3G CS Core Network. A or Iu-CS interface GANC. MSC or MSC-Server/ MGW. EPC HeNB Gateway. eNB. UE HeNB (Home eNB). Broadband Internet. Additional Options. Additional Options The EPC has been designed to be flexible enough to handle a variety of connection and service scenarios. For example, access to CS and other real-time services for LTE users is possible without an IMS. Proposals have been put forward by bodies such as the VoLGA Forum to allow CS services to be handled by existing 2G and 3G core network such as the MSC (Mobile-services Switching Centre) or MSC-Server/MGW (Media Gateway). In this scenario a GANC (Generic Access Network Controller) would be positioned between the EPC and the legacy core networks. The GANC presents itself to the legacy core as an equivalent device to a BSC (Base Station Controller) or RNC and to the EPC as just another packet data network destination. Voice and SMS (Short Message Service) traffic is permitted to travel across the legacy networks using its standard format, which is simply encapsulated into IP packets for the journey across the EPC and E-UTRAN. GAN-capable UEs (User Equipments) employ the same upper layer protocol stacks as would be found in 2G or 3G user devices. Overall, the benefit of GAN-based CS services is that operators can continue to derive value from previous investments in CS technologies. The EPC has also been provided with a standardized method of integrating ‘home’ base stations into an operator’s mix of access options. An HeNB (Home eNode B) device connects to a residential or business user’s broadband Internet service and generates a small-scale LTE cell at the user’s premises. Traffic is carried to the EPC via an HeNB GW (Home eNB Gateway), which distributes C-plane and U-plane traffic to the required EPC elements.. Further Reading: 3GPP TS 23.401, 36.300, www.volga-forum.com 1.8. © Wray Castle Limited. LT3604F/v1.

(21) EPS and IMS Overview. 2G/3G SGSN. UMTS/ GPRS. UTRAN/ GERAN. HSS. S6a. S3. PCRF. MME S4 LTE. S7/Gx. S12 S1-MME. S11. S1-U. Rx+ IMS. S5. SGi. E-UTRAN. IP Services S-GW Interworking to MME. PDN GW. S2. WLAN or WiMAX. PS Interworking The EPS forms part of the ongoing evolution of GSM and UMTS networks and therefore has been designed to offer full backwards compatibility with legacy networks and has also been provided with the ability to interwork with non-3GPP network types. The EPC S3 and S4 interfaces enable a legacy SGSN (Serving GPRS Support Node) to interwork with the EPS MME and S-GW nodes. This in turn allows UEs (User Equipment) served by 2G GERAN or 3G UTRAN access networks to establish connections via the EPC (in this scenario, access is provided via the 2G/3G SGSN, with the EPC PDN-GW taking on the ‘connection anchor’ role of the GGSN (Gateway GPRS Support Node). The interworking capabilities provided by these interfaces also allows handover of PS connections between 2G and 3G access networks and the E-UTRAN. The S12 interface has been designed to allows UMTS UTRAN elements (specifically the RNC) to interface directly with an EPC S-GW without the need for an intermediate SGSN.. Further Reading: 3GPP TS 23.401:4.2 LT3604F/v1. © Wray Castle Limited. 1.9.

(22) LTE Evolved Packet Core Network Flexicourse. MGW GERAN/ UTRAN. MSC Server IMS Traffic Connection UE. MME. E-UTRAN S-GW. PDN-GW. CS Interworking CS interworking is supported via the IMS, but only for the handover of CS calls from the E-UTRAN to GERAN/UTRAN cells; it is not possible to hand calls back from GERAN/UTRAN to E-UTRAN. The support of CS handover is incorporated into the SRVCC (Single Radio Voice Call Continuity) functions specified in 3GPP TS 23.216 and the IMS Service Continuity features of TS 23.237. When a UE with active CS-type services determines that the only handover candidates are GERAN or UTRAN cells, it initiates the SRVCC process. The MME differentiates between the UE’s current CS and PS sessions; CS sessions are subject to SRVCC, whilst PS sessions are handed over to an SGSN. The MME co-ordinates the handover with a CS domain MSC-Server using a set of ‘PS to CS’ handover messages. Once the target cell and BSS/RNS have been prepared for the handover and CS resources have been reserved, the session traffic is re-routed by the IMS away from the EPC PDN-GW and towards a CS domain MGW. The MGW forwards the traffic to the selected BSS (Base Station System)/RNS (Radio Network Subsystem) and on to the UE. SRVCC and Service Continuity only apply to calls anchored in an IMS, networks that opt to use the GAN approach to CS traffic will follow a different set of procedures. A more limited CS service is also supplied by the CS Fallback feature.. Further Reading: 3GPP TS 23.401, 23.216 (SRVCC), 23.237 (IMS Service Continuity), 23.272 (CS Fallback) 1.10. © Wray Castle Limited. LT3604F/v1.

(23) EPS and IMS Overview HSS. PCRF. SWx. S6a. Rx. Gxc Gx. 3GPP Access. S-GW. IMS. SGi. S5. IP Services Gxb. PDN-GW. S6b SWm. HPLMN. 3GPP AAA Server. SWn. Non-3GPP Networks. Trusted Non-3GPP IP access. Non-trusted, Non-3GPP IP access. Gxa. SWa S2c. S2c. STa. S2c. UE Non-3GPP Access Interworking The 3GPP R6 and R7 series of specifications included several solutions aimed at allowing access to 3GPP core network environments from non-3GPP radio access networks. These facilities have been evolved to accommodate access to the EPS from non-3GPP systems also. There are specific provisions to allow access from Wi-Fi (802.11-based systems), WiMAX (Worldwide Interoperability for Microwave Access) (802.16-based systems) and 3GPP2 systems such as cdmaOne TM and CDMA2000TM, although access from other types of system is not necessarily precluded. 3GPP makes the distinction between ‘trusted’ and ‘non-trusted’ access networks and provides a range of different interworking options for each scenario. The diagram shows options supporting UEs that can access an EPC directly (via the E-UTRAN) and also via trusted and non-trusted, non-3GPP access systems.. Further Reading: 3GPP TS 23.402 LT3604F/v1. © Wray Castle Limited. 1.11.

(24) LTE Evolved Packet Core Network Flexicourse. Intranet Access. Internet Access. Voice and Video Telephony. Locationbased Services. SMS, MMS, IM Email Online Gaming. Roaming. EPS Services The EPS is designed to handle all of the traffic types handled by legacy networks. It also provides high-bit-rate data bearers (in theory, up to and beyond 100 Mbit/s) and real-time IP-based connections. Some of the services that the EPS can supply are shown in the diagram.. 1.12. © Wray Castle Limited. LT3604F/v1.

(25) EPS and IMS Overview. Glossary 3GPP (3rd Generation Partnership Project) ............................................................................................ 1 ATM (Asynchronous Transfer Mode) ...................................................................................................... 1 BSC (Base Station Controller) ................................................................................................................ 8 BSS (Base Station System) .................................................................................................................. 10 CS (Circuit Switched) .............................................................................................................................. 1 CSCF (Call Session Control Function). .................................................................................................. 7 EDGE (Enhanced Data rates for Global Evolution) ................................................................................ 2 E-GPRS2 (Enhanced GPRS 2) .............................................................................................................. 2 eNB (Evolved Node B) ............................................................................................................................ 5 EPC (Evolved Packet Core) .................................................................................................................... 2 EPS (Evolved Packet System) ................................................................................................................ 2 E-RAB (Evolved Radio Access Bearer) .................................................................................................. 5 E-UTRAN (Evolved Universal Terrestrial Radio Access Network) ......................................................... 2 GAN (Generic Access Networks) ............................................................................................................ 3 GANC (Generic Access Network Controller) .......................................................................................... 8 GERAN (GPRS/EDGE Radio Access Network) ..................................................................................... 5 GGSN (Gateway GPRS Support Node) ................................................................................................. 9 GTP (GPRS Tunnelling Protocol) ........................................................................................................... 6 HeNB (Home eNode B)........................................................................................................................... 8 HeNB GW (Home eNB Gateway) ........................................................................................................... 8 HSPA (High Speed Packet Access) ....................................................................................................... 2 HSS (Home Subscriber Server) .............................................................................................................. 7 IMS (IP Multimedia Subsystem) .............................................................................................................. 1 IP (Internet Protocol) ............................................................................................................................... 1 LTE (Long Term Evolution) ..................................................................................................................... 2 MME (Mobility Management Entity) ........................................................................................................ 6 MSC (Mobile-services Switching Centre) ............................................................................................... 8 MSC-Server/MGW (Media Gateway). ..................................................................................................... 8 PCRF (Policy and Charging Resource Function) ................................................................................... 6 PCS (PDN Connectivity Service) ............................................................................................................ 4 PDN (Packet Data Network). .................................................................................................................. 4 PDN-GW (Packet Data Network Gateway)............................................................................................. 6 PS (Packet Switched) ............................................................................................................................. 1. LT3604F/v1. © Wray Castle Limited. 1.13.

(26) LTE Evolved Packet Core Network Flexicourse R8 (Release 8) ........................................................................................................................................ 2 RNC (Radio Network Controller) ............................................................................................................. 5 RNS (Radio Network Subsystem) ......................................................................................................... 10 RTP (Real Time Protocol). ...................................................................................................................... 7 SAE (System Architecture Evolution) ..................................................................................................... 2 SDF (Service Data Flow) ........................................................................................................................ 4 SGSN (Serving GPRS Support Node) .................................................................................................... 9 S-GW (Serving Gateway)........................................................................................................................ 6 SIP (Session Initiation Protocol) ............................................................................................................. 7 SMS (Short Message Service) ................................................................................................................ 3 SRVCC (Single Radio Voice Call Continuity) ....................................................................................... 10 UE (User Equipment) .............................................................................................................................. 8 UMTS (Universal Mobile Telecommunications System) ......................................................................... 1 VoLGA (Voice over LTE Generic Access) .............................................................................................. 3 WiMAX (Worldwide Interoperability for Microwave Access) ................................................................. 11. 1.14. © Wray Castle Limited. LT3604F/v1.

(27) LTE Evolved Packet Core Network Flexicourse. Section 2. Evolved Packet Core. © Wray Castle Limited.

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(29) Evolved Packet Core. Contents EPS Network Functions ....................................................................................................................... 2.1 Network Logical Structure .................................................................................................................... 2.2 MME (Mobility Management Entity) ..................................................................................................... 2.3 S-GW (Serving Gateway)..................................................................................................................... 2.4 PDN-GW (Packet Data Network Gateway).......................................................................................... 2.5 PCRF (Policy and Charging Rules Function)....................................................................................... 2.6 Combined Functionality........................................................................................................................ 2.7 Resilience Through Pooling ................................................................................................................. 2.8 Interface Naming Convention .............................................................................................................. 2.9 S1 to E-UTRAN Interface ................................................................................................................... 2.10 S1-U Interface .................................................................................................................................... 2.11 S1 Interfaces for Home eNBs ............................................................................................................ 2.12 GTPv1-U Traffic Interfaces ................................................................................................................ 2.13 GTPv2-C C-plane Interfaces .............................................................................................................. 2.14 Diameter-based Interfaces ................................................................................................................. 2.15 PCRF Diameter Interfaces ................................................................................................................. 2.16 Interface to CS Networks ................................................................................................................... 2.17 EPS Area Identities ............................................................................................................................ 2.18 Node Identifiers .................................................................................................................................. 2.19 Subscriber Identities ........................................................................................................................... 2.20 Connection Identifiers ........................................................................................................................ 2.21 Transport Identities ........................................................................................................................... 2..22 LT3604F/v1. © Wray Castle Limited. iii.

(30) LTE Evolved Packet Core Network Flexicourse. Default and Dedicated EPS Bearers.................................................................................................. 2.23 EPS Quality of Service ....................................................................................................................... 2.24 QoS Levels ......................................................................................................................................... 2.25 Providing CS Services via LTE/EPS .................................................................................................. 2.26 CS Fallback ........................................................................................................................................ 2.27 VCC (Voice Call Continuity) ............................................................................................................... 2.28 CS Service Provision via a GANC ..................................................................................................... 2.29 EPC Security Functions ..................................................................................................................... 2.30 AKA (Authentication and Key Agreement) ......................................................................................... 2.31 User Confidentiality ............................................................................................................................ 2.32 Glossary ............................................................................................................................................. 2.33. iv. © Wray Castle Limited. LT3604F/v1.

(31) Evolved Packet Core. Objectives. At the end of this section you will be able to: . outline the functions performed by EPC elements. . discuss options for interworking the EPC with legacy packet core networks. . describe the main points of interest related to EPC topics such as pooling. . list the set of S interfaces described for the EPC and outline their basic functions and protocols. . discuss options for User Plane connectivity between a UE and a PDN-GW. . outline how combinations of redundant S interfaces can provide for EPC resilience. . list the basic set of identifiers used to describe EPC areas. . outline the set of node identifiers that have been defined for the EPC. . discuss the impact of the evolved device/subscriber identifiers employed by the EPC. . outline the fundamental properties of an EPS Bearer and describe the structure of an EPS Bearer ID. . describe the relationship that exists between an EPS Bearer and an E-RAB (E-UTRAN Radio Access Bearer). . outline the role of the APN (Access Point Name) in the handling of a PCS (PDN Connectivity Service). . describe the interaction between the EPC and the GTP. . outline the interaction between the EPC, GTP and IP. . discuss the concept of the PCS and its relevance within the EPC. . outline the functions of the default EPS Bearer. . describe the differences between the default and dedicated bearer types and outline their relationship with the Service Data Flow. . describe the EPC connection hierarchy and list the set of parameter types that define them. . outline the QoS concepts employed by the EPC and define the roles of the QCI and the ARP. . outline the methods that are available for providing CS services to EPS attached UEs, including Generic Access Network functions, CS Fallback and Voice Call Continuity. . outline the security functions employed by the EPC. LT3604F/v1. © Wray Castle Limited. v.

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(33) Evolved Packet Core 2G/3G SGSN. UMTS/ GPRS. HSS. Network Access IP Functions. UTRAN/ GERAN. S6a. S3. PCRF. MME S4 S12 S7/Gx. E-UTRA S1-MME. S11. IMS S5. E-UTRAN. SGi IP Services. S1-U S-GW Interworking to MME. Rx+. PDN GW. S2. Mobility Management Anchoring. WLAN or WiMAX. Network Management. EPS Network Functions Network Access functions include providing information to assist terminals with network selection and performing admission control, authentication and authorization, charging and policy control. EPC gateway nodes are essentially IP routers with an extended capability set, and as such are primarily dedicated to performing IP packet routing functions for user traffic, signalling and network management data flows. The EPC (via the PDN-GW) is also responsible for allocating valid IP addresses to each new EPS Bearer. Regarding mobility management, the EPC has responsibility for idle mode mobility management of attached UEs and for managing the relocation of user traffic connections when a UE roams from one network area to another or to another network. The EPC is responsible for selecting the PDN-GW node that will anchor each user traffic connection (or EPS Bearer); this is achieved by selecting the appropriate PDN-GW access point for the type of service being requested by a UE. Basic network management functions performed by the EPC include load balancing and rebalancing between MMEs. The objective of these balancing functions is to prevent an MME or pool of MMEs from becoming overloaded.. Further Reading: 3GPP TS 23.401:4.3 LT3604F/v1. © Wray Castle Limited. 2.1.

(34) LTE Evolved Packet Core Network Flexicourse. User Equipment. Uu. eNode B. Non-Access Stratum (NAS) Access Stratum (AS). S1. Evolved Packet Core. Non-Access Stratum (NAS) Access Stratum (AS). Network Logical Structure As with UMTS R99, the services provided to UEs by the EPS are divided into those handled by the AS (Access Stratum) and those provided by the NAS (Non-Access Stratum). The AS comprises all of the functions performed by the E-UTRAN. The NAS consists of the Bearers and bearer control signalling functions that support them. The S1AP (S1 Application Protocol) includes provision for the direct transfer of NAS signalling between UE and MME via the eNB. Compared to the core network architecture of previous generations of mobile system such as GSM or R99 UMTS, the EPC has been provided with a much ‘flatter’ network design, which limits the number of node types deployed.. Further Reading: 3GPP TS 36.300 2.2. © Wray Castle Limited. LT3604F/v1.

(35) Evolved Packet Core. Mobility Management Entity (MME). . NAS signalling and signalling security Inter CN node signalling for mobility between 3GPP access networks UE reachability in idle mode Tracking Area list management PDN GW and serving GW selection MME selection for handovers with MME change SGSN selection Roaming connection towards home HSS Authentication Bearer management and establishment. MME (Mobility Management Entity) The MME (Mobility Management Entity) assumes many of the functions that would previously have been performed by the VLR (Visitor Location Register) or SGSN and which in the evolved network are termed EMM (EPS Mobility Management) functions. The MME’s main responsibility is to terminate the Control Plane NAS signalling flows from individual UEs and to manage the authentication and security functions for each attached UE. Unlike the legacy VLR, however, the MME is also responsible for bearer establishment. It receives Service Requests from UEs and issues appropriate instructions to the S-GW (Serving Gateway) that will handle each user plane connection. The EMM functions also include responsibility tracking UEs that are in idle mode and the MME ensures ‘UE Reachability’ by receiving TAU (Tracking Area Update) messages, maintaining the tracking area lists and performing paging of individual UEs when required. To assist with service resilience, MMEs can be grouped into ‘pools’. eNBs (Evolved Node Bs) are able to contact any MME within the pool(s) with which they are associated when passing on UE Attach requests. The MME then has flexibility as to the S-GW chosen to establish the user plane connection for each UE. The MME is also in charge of roaming and handover functions to 2G/3G SGSNs.. Further Reading: 3GPP TS 23.401:4.4.2 LT3604F/v1. © Wray Castle Limited. 2.3.

(36) LTE Evolved Packet Core Network Flexicourse. Serving Gateway (S-GW). . Local mobility anchor point for inter-eNB handover Mobility anchoring for inter-3GPP mobility Idle mode downlink packet buffering Lawful interception Packet routing and forwarding Transport level DiffServ packet marking Charging. S-GW (Serving Gateway) The S-GW handles user plane connectivity between UEs and the EPC and acts as the EPC mobility anchor for UEs roaming within part of a PLMN. This entails performing IP packet routing and buffering functions and also managing QoS by inserting DSCP (DiffServ Code Point) data into IP packet headers. The S-GW also provides mobility anchoring for connections that roam onto legacy 3GPP GERAN (2G) and UTRAN (3G) access networks. As all EPS user traffic must pass through an S-GW it is a logical node within which, in concert with the PDN-GW, to base the EPS Lawful Interception interface and also the charging functions. The standard S5 and S8 interfaces that link the S-GW and PDN-GW are based on the 3GPP GTP; many non-3GPP systems obtain similar IP mobility functionality by employing the MIPv4 (Mobile IPv4) or PMIPv6 (Proxy Mobile IPv6) protocols developed by the IETF (Internet Engineering Task Force). Adapted versions of the S5 and S8 interfaces are available that support the PMIP protocol for IP mobility. In such cases, the S-GW will also act as the FA (Foreign Agent) to anchor mobile IP tunnels. To provide some legacy perspective, taken together the MME and S-GW provide the EPC with functionality similar to that previously provided by the SGSN, with the MME handling the signalling and session control aspects and the S-GW dealing with the user traffic. Early in its development, the S-GW was also known as the UPE (User Plane Entity), although this terminology has now been dropped.. Further Reading: 3GPP TS 23.401:4.4.3.2 2.4. © Wray Castle Limited. LT3604F/v1.

(37) Evolved Packet Core. PDN Gateway (PDN-GW). . Per-user-based packet filtering Lawful interception UE IP address allocation DiffServ packet marking SDF level charging SDF gating control and data rate enforcement Contains APN (Access Point Name) DHCPv4 (server and client) and DHCPv6 (client, relay and server) functions. PDN-GW (Packet Data Network Gateway). PDN-GW (Packet Data Network Gateway) If the functionality of the MME/S-GW can be thought of as analogous to that of the legacy SGSN, then the PDN-GW can be thought of as similar in function to the legacy GGSN. The PDN-GW (Packet Data Network Gateway) (also known in some versions of the specifications as the P-GW) routes traffic between EPS Bearers and the SGi interface, which leads to external data networks such as the IMS and the Internet. As all inbound and outbound EPS traffic must pass through a PDN-GW it is the logical node in which the network’s packet filtering and classification functions are based. These include the ‘deep packet inspection’ techniques that are used to classify packets into particular SDFs (Service Data Flows) before routing them over an EPS Bearer or the SGi interface, which in turn allows the PDN-GW to act as the network’s PCEF (Policy and Charging Enforcement Function) Under direction from the PCRF (Policy and Charging Rules Function) the PDN-GW will apply ‘per SDF’ charging, service level and rate enforcement and QoS-related traffic shaping functions that control the ‘gating’ of user traffic flows. Each PDN-GW contains a number of logical access points (each identified by an APN (Access Point Name)) which act as the GTP tunnel endpoints and mobility anchors of the EPS Bearers that extend service out to mobile UEs. As in the legacy GGSN, the APNs are responsible for the allocation of IP addresses to UEs during the establishment of each EPS Bearer and for routing traffic between the Bearers and particular external networks.. Further Reading: 3GPP TS 23.401:4.4.3.3 LT3604F/v1. © Wray Castle Limited. 2.5.

(38) LTE Evolved Packet Core Network Flexicourse. Policy and Charging Rules Function (PCRF). . . Decides whether and when to create additional EPS nearers Provides PCC data such as service data flow detection, gating, QoS, ARP and flow-based charging information to traffic handling entities Terminates the S7/Gx and Rx interfaces for home network service and the S9 interface point for roaming with local breakout. PCRF (Policy and Charging Rules Function) The PCRF (Policy and Charging Rules Function) is responsible for propagating the network’s connection policies and charging rules to the PDN-GW via the S7/Gx interface and to traffic gateway elements within the IMS via the Rx interface. It is the element that decides if new connections are to be allowed and, if so, whether they will be carried by an existing EPS Bearer or whether a new one is required. The PCRF is responsible for providing service data flow detection, gating, QoS and flow-based charging information to traffic handling entities within the network. This includes rules that allow the PDN-GW to provide the correct level of service to user data flows once the type of traffic being carried has been determined. For example, if the PDN-GW determines that the SDF to a user is carrying realtime traffic it may ‘gate’ up to the data rate and QoS level indicated by the PCRF and the user’s subscription profile. The PCRF’s charging rules allow the operator to apply the appropriate rating to CDRs (Call Data Records) generated for each SDF so that, for instance, real-time connections can be differentiated from an Internet browsing session. In the case of EPS roaming, when users use their terminals abroad, 3GPP has developed an extended PCRF architecture, based on the S9 interface, that defines Home Policy and Charging Rules Function (H-PCRF) and Visited Policy and Charging Rules Function (V-PCRF) logical functions.. Further Reading: 3GPP TS 23.401:4.4.7 2.6. © Wray Castle Limited. LT3604F/v1.

(39) Evolved Packet Core. S1 1. Mobility Management Entity (MME). Functions could be combined within same device S5. PDN Gateway (PDN-GW). Serving Gateway (S-GW). Combined Functionality 3GPP has deliberately designed the EPC network elements and interfaces to give vendors the greatest possible flexibility when developing their solutions. Although the MMW, S-GW, PDN-GW and PCRF all have a set of rigidly defined functions and open interfaces, the specifications make it explicit that equipment vendors are free to deploy these logical functions to physical devices in whatever way suits them best. For example, the MME and SGW functions can both be located in one device, such as an upgrade to an existing 3G SGSN platform. The S11 interface would then be internal to that combined device. In the same way it is conceivable that a vendor may decide to combine the functions of the S-GW and PDN-GW into one combined EPS gateway, rendering the S5 an internal interface.. Further Reading: 3GPP TS 23.401:4.4 LT3604F/v1. © Wray Castle Limited. 2.7.

(40) LTE Evolved Packet Core Network Flexicourse PDN-GW. Co-ordinated MME Pool and S-GW Service Area. E-UTRAN Tracking Areas served by Pools and Areas. Resilience Through Pooling In common with ongoing developments within many existing 3G core networks, the EPC is designed to take advantage of the concept of ‘pooling’, specifically of MME and S-GW nodes. The ‘S1-flex’ facility that allows each eNB in the E-UTRAN to be associated with multiple MMEs in the EPC allows those MMEs to be grouped into ‘pools’. Each pool will be responsible for the eNBs in one or more complete tracking areas. This means that when an eNB selects the MME that will handle the Attach process for a UE, that MME can continue to serve that UE as long as it remains within the tracking areas associated with the MME’s pool. This reduces the requirement for MME relocation and consequently reduces the network’s signalling load. Pooling also provides a measure of resilience for network services to the extent that, if one MME falls over, eNBs have a number of alternative devices to select. As with current implementations of the pooling concept, however, MME pooling does not protect the connections to UEs being served by a failed MME – when the MME fails all ongoing services supported by it fail too. In the same way as an MME pool area comprises a set of cells within which a UE does not need to change the serving MME, an S-GW service area is a set of cells within which a UE does not need to change S-GW. MME pools may overlap, and each MME pool area is identified by an MMEGI (MME Group Identifier). S-GW Areas are also permitted to overlap.. Further Reading: 3GPP TS 23.401:3.1 2.8. © Wray Castle Limited. LT3604F/v1.

(41) Evolved Packet Core 2G/3G SGSN. UMTS/ GPRS. HSS Roaming PCRF. UTRAN/ GERAN. S6a. S3. EIR. EIR PCRF. S9. MME S12. S4 S13. E-UTRA S1-MME. S7/Gx. S11. IMS S5. E-UTRAN. SGi IP Services. S1-U S-GW Interworking to MME. Rx+. PDN GW S8. S2 EPS Roaming. WLAN or WiMAX. Interface Naming Convention There are numerous interfaces defined for the EPC, most of which share the reference letter ‘S’. They are functionally separated into those that carry control (C-plane) and those that carry user (U-plane) traffic. Support of most S interfaces in the EPC is mandatory, although some are optional. An overview of the interfaces is given in the diagram.. Further Reading: 3GPP TS 23.401:4.2 LT3604F/v1. © Wray Castle Limited. 2.9.

(42) LTE Evolved Packet Core Network Flexicourse MME. S1-AP SCTP IP S1-MME. L2 L1. User PDU E-NB. GTPv1-U. S1-U. SCTP IP L2 S-GW. L1. S1 to E-UTRAN Interface The S1 interface can be seen as the evolved equivalent of the 3G Iu interfaces and interconnects the E-UTRAN with the EPC. Individual S1 interfaces run logically between each eNB and the set of MMEs and S-GWs to which it is associated. Messages and other control plane traffic and S1-U flows carry user plane and call control traffic. Message structures for the S1-MME interface, which operates between the eNB and the MME, are defined by the S1AP (S1 Application Protocol). S1AP performs duties that combine those performed by the legacy RANAP (Radio Access Network Application Part) and GTP-C (GPRS Tunnelling Protocol on the C plane) protocols with additional elements to support traffic flows in an all-IP environment. Data flow over the S1-MME is protected from loss and network failure by the use of SCTP (Stream Control Transmission Protocol) at the transport layer (layer 4). SCTP was specifically designed by the IETF to handle the flow of signalling and control traffic over an IP network. Retransmission of failed or missing data packets, and therefore guaranteed delivery of signalling data, is one of the facilities provided by SCTP.. Further Reading: 3GPP TS 23.401:5.1, 36.413 (S1AP) 2.10. © Wray Castle Limited. LT3604F/v1.

(43) Evolved Packet Core MME. S1-AP SCTP IP S1-MME. L2 L1. User PDU E-NB. GTPv1-U. S1-U. SCTP IP L2 S-GW. L1. S1-U Interface The S1-U interface employs GTP-U (GPRS Tunnelling Protocol on the User plane) to create and manage tunnels carrying user-plane data contexts between the eNB and the S-GW. 3GPP has developed a new version of GTP (GTPv2) for use within the EPS. GTPv2 only changes the C-plane aspects of GTP, however, and is referred to as GTPv2-C. U-plane traffic continues to be carried by GTPv1. Current versions of the relevant 3GPP specifications refer to this version of GTP and GTPv1-U. The S1-U user plane carries all traffic destined to travel over the air interface to the UE, which includes all user data plus application control data such as SIP and RTP messages. It also handles the delivery of NAS signalling messages carried between the UE and the MME using the DTAP (Direct Transfer Application Part) facility. The S1-U interface employs UDP (User Datagram Protocol) at layer 4 and therefore has no data retransmission capabilities available at the transport layer. Although the S1-Flex functionality allows each eNB to connect to multiple MMEs and S-GWs, an individual UE will, unless a relocation is taking place, only ever be served by one MME and one S-GW at any one time. All signalling and traffic connections for a UE will therefore be concentrated through one pair of devices.. Further Reading: 3GPP TS 23.401:5.1, 29.274 (GTPv2-C), 23.281 (GTPv1-U) LT3604F/v1. © Wray Castle Limited. 2.11.

(44) LTE Evolved Packet Core Network Flexicourse MME. S1-AP SCTP IP L2 L1. Home eNB Gateway (HeNB GW). User PDU GTPv1-U. S1-MME Home eNB (HeNB). SCTP. Broadband. IP L2. S1-U S-GW. L1. S1 Interfaces for Home eNBs The HeNB (Home eNode B) concept provides a standardized method for creating and connecting LTE ‘femtocells’. Similar methods have been developed for the 3G HNB (Home Node B). A femtocell provides limited-area radio coverage to residential or business premises; connections are passed back to the operator’s core network via a broadband Internet connection. Indeed, femtocell devices are often incorporated into broadband routers along with the broadband modem and Wi-Fi access point. The HeNB provides the same set of services as a ‘full’ eNB and is logically connected to the EPC via the same S1-MME and S1-U interfaces. Operators may optionally deploy an HeNB GW (Home eNB Gateway) to concentrate S1-MME traffic towards the MMEs, although the HeNBs will work even without a Gateway. The HeNB presents itself to the HeNB GW as an eNB; the Gateway presents itself to the HeNB as an MME. The HeNB GW presents itself to the MME as an eNB. An X2 interface between neighbouring HeNBs is not supported, although mobility between HeNB cells and other cells via the MME/S-GW is possible.. Further Reading: 3GPP TS 36.300:4.6, TR 25.820 2.12. © Wray Castle Limited. LT3604F/v1.

(45) Evolved Packet Core RNC SGSN. S16 SGSN. S12. GTPv1-U S4. UDP Roaming EPS. IP L2. S5 S8 S-GW. L1. PDN-GW. GTPv1-U Traffic Interfaces Most EPC interfaces are based on a combination of GTPv1-U and GTPv2-C. The S4 interface carries U-plane traffic between an S-GW and an SGSN for EPC-attached UEs that have roamed onto GERAN (GPRS EDGE Radio Access Network)/UTRAN access. SGSNs that support the S4 can also be upgraded to use the S16 interface, which allows the evolved combination of GTPv1-U and GTPv2-C to be used between SGSNs. The S5 interface interconnects an S-GW to PDN-GWa within the same PLMN (Public Land Mobile Network). The S8 Interface provides roaming connectivity between a visited S-GW and a home PDNGW. The S5 interface is based on the 2G/3G Gn interface, whilst the S8 is analogous to the Gp interface. The S12 interface is used to provide a U-plane only ‘direct tunnel’ between an S-GW and a 3G RNC, which allows the user plane to bypass the SGSN and thus avoids any traffic bottlenecks that may occur.. Further Reading: 3GPP TS 23.401:5.1, 23.281 (GTPv1-U), 23.060 (GPRS) LT3604F/v1. © Wray Castle Limited. 2.13.

(46) LTE Evolved Packet Core Network Flexicourse SGSN. S16 SGSN. S10 S3 MME. MME. GTPv2-C UDP IP. S11 Roaming EPS. L2 L1. S5 S8. S-GW. PDN-GW. GTPv2-C C-plane Interfaces The S3 interface provides control plane connectivity between an MME and an SGSN and is used to carry handover and other control signalling between EPS and GERAN/UTRAN PS environments. The S16 interface carries control messaging between evolved SGSNs. The S16 interface carries control messaging between evolved SGSNs. If an S16 interface exists it can be used to handle the relocation of bearers between SGSNs without requiring the operation to be controlled by an S-GW. In addition to carrying user traffic, the S5 and S8 interfaces also carry GTPv2-C based control messaging. Networks based on non-3GPP protocols may elect to use variants of the S5 and S8 interfaces based on IETF ‘mobile IP’ protocols instead. The S10 interface carries inter-MME signalling traffic and is employed during functions such as MME relocation. This may occur, for example, when a Connected Mode UE roams out of one MME pool area into another, or when MME load balancing or rebalancing is taking place. The S10 is analogous to the Gn interface and is based on GTPv2-C running over UDP/IP.. Further Reading: 3GPP TS 23.401:5.1, 29.274 (GTPv2-C), 23.060 (GPRS) 2.14. © Wray Castle Limited. LT3604F/v1.

(47) Evolved Packet Core. HSS SGSN. S6d. Diameter. S6a. TCP/SCTP. IP L2 L1 MME S13. EIR. Diameter-based Interfaces The Diameter protocol was designed by the IETF as a more standardized successor to the venerable RADIUS (Remote Access Dial-In User Service) protocol, which provides a method of transporting AAA (Authentication, Authorization and Accounting) data over an IP network. Various proprietary adaptations of RADIUS have been developed, which were largely non-interoperable, making it a de facto closed standard. The S6a interface connects the MME to the HSS and allows the secure transfer of subscriber and other data between those nodes. The Diameter Base Protocol and the applications that enable communication between the MME and HSS run over an IP link and can be protected at the transport layer by either TCP (Transmission Control Protocol) or SCTP. The S13 interface optionally interconnects the MME and the Equipment Identity Register (EIR) and is therefore analogous to the GPRS Gf interface. Unlike the Gf, however, the S13 interface is based on the Diameter protocol. The S6d interface allows 2G/3G SGSNs that also support the S4 interface to the S-GW to connect directly to the EPS HSS (Home Subscriber Server) for mobility management and subscriber data access purposes.. LT3604F/v1. © Wray Castle Limited. 2.15.

(48) LTE Evolved Packet Core Network Flexicourse. Visited PCRF. S9. Home PCRF. Diameter. S7/Gx. Rx. TCP/SCTP. IP L2 IMS. L1. PDN-GW. PCRF Diameter Interfaces The S7 interface connects the PDN-GW to the PCRF. It carries policy lookups sent by the PDN-GW in response to connection requests and the replies generated by the PCRF that determine how or if those requests will be fulfilled. The S7 interface is based on the existing Gx interface and 3GPP specifications and diagrams use the reference names interchangeably. The Rx interface connects the PCRF to the IMS and carries a similar range of message types as the Gx. The S9 interface carries policy and charging rules data between home and visited PCRFs to allow home network policies to be applied to roaming UE connections. Visited PCRFs may have the facility to request PCC (Policy and Charging Control) details from a user’s home network but they are under no obligation to enforce them if they contradict local policies.. Further Reading: 3GPP TS 23.401:4.7.4; 23.203 2.16. © Wray Castle Limited. LT3604F/v1.

(49) Evolved Packet Core. MSC/VLR or MSC Server. SGsAP SCTP SGs/SV. IP L2 L1. MME. Interface to CS Networks. Interface to CS Networks The EPC was designed as an ‘all IP’ environment and as such carries all traffic, even voice, in IP streams but interfaces have been developed that allow for backwards compatibility with and handover of CS traffic to legacy networks, if required. The SGs interface is based on the GERAN/UTRAN Gs interface and carries mobility management and handover signalling between an MME and a legacy MSC or MSC Server. It was created to serve the interfacing requirements of the CS Fallback service, which allows EPC-Attached UEs to drop back to 2G/3G networks to handle CS calls. The SGsAP (SGs Application Part) message format employed on the interface is an adaptation of the BSSAP+ (Base Station System Application Part +) protocol employed on the legacy Gs interface, and provides much the same set of services. Other interfaces have been developed to support other forms of EPC-CS Core interaction; the SGs interface, for example, carries MME-MSC/MSC-S signalling to support the SRVCC, which allows IMSanchored real time sessions to be seamlessly handed over between EPS Bearers and GERAN/UTRAN CS Bearers.. Further Reading: 3GPP TS 23.216 (SRVCC), 23.272 (CS Fallback) LT3604F/v1. © Wray Castle Limited. 2.17.

(50) LTE Evolved Packet Core Network Flexicourse PLMN = MCC+MNC. MME Group ID (MMEGI). Tracking Area ID (TAI). Evolved Cell ID (ECGI) = ID + Cell ID. eNB. EPS Area Identities The EPS continues to use the PLMN identifier employed by legacy 3GPP systems, which consists of the MCC (Mobile Country Code) and the MNC (Mobile Network Code). The MMEGI (MME Group Identifier) is a 16-bit identifier assigned to an individual MME Pool. The MMEGI only has to be unique within a PLMN. The TAI (Tracking Area Identifier) is analogous to the LA (Location Area) or RA (Routing Area) identifiers employed by the GERAN/UTRAN in that it is used to identify a group of cells in the access network. In the E-UTRAN the TA (Tracking Area) is the granularity with which each UE’s location is tracked. It is also the area within which a UE will be paged. The TAI consists of the network’s MCC and MNC followed by a TAC (Tracking Area Code). As in legacy systems it is necessary to be able to identify uniquely each cell in the network for call establishment, paging, handover and billing purposes. 3GPP has devised an updated Cell ID known as an ECGI (E-UTRAN Cell Global Identifier). The ECGI incorporates a unique eNB Identifier, which allows the S1 and X2 interface protocols to discover and identify the target nodes for functions such as EPS Bearer handover.. Further Reading: 3GPP TS 29.803, 23.401:5.2, 36.300:8.2 2.18. © Wray Castle Limited. LT3604F/v1.

(51) Evolved Packet Core Gateway Names/IP addresses Access Point Name (APN). MCC. MNC. MMEI. GUMMEI. 24 bits. MMEGI. 16 bits. MCC. MNC. eNB ID. 20 bits. MMEC. MMEI. 8 bits. Cell ID. ECGI. 8 bits. Node Identifiers. Node Identifiers The MME is primarily a signalling node and each MME has to be accessible to and exchange control data with MMEs and other devices within its own network and in other networks elsewhere in the world. For this reason, each MME is assigned a unique and globally significant identifier known as a GUMMEI (Globally Unique MME Identity). The GUMMEI consists of the network’s MCC and MNC followed by a MMEI (MME Identifier), which in turn consists of the MMEGI and the MMEC (MME Code). The MMEGI identifies the Pool to which the MME belongs and the MMEC is its index within that pool. The addressing of S-GW and PDN-GW nodes follows the model for addressing legacy PS core network nodes – ultimately, each node will be identified by an IP address, which may or may not be backed up with a DNS-resolvable device name. The termination and anchor point for an EPS Bearer is an access point in a PDN-GW, which is analogous to a PDP Context terminating on GGSN APN in 2G/3G networks. Each PDN-GW AP is assigned an IP address associated with a DNS-resolvable name – the APN (Access Point Name). The EPS ECGI is globally unique and allows individual cells to be separately identified. The ECGI is a 28-bit identifier which consists of the PLMN ID (MCC + MNC), a 20-bit eNB ID (which will be unique within a PLMN) and an 8-bit Cell ID (which will be unique within one eNB). This gives each PLMN scope to identify up to 1 million eNBs and for each eNB to control up to 256 cells.. Further Reading: 3GPP TS 23.401:5.2, 36.300 LT3604F/v1. © Wray Castle Limited. 2.19.

(52) LTE Evolved Packet Core Network Flexicourse. MCC. MNC. MMEI. M-TMSI. 24 bits. 32 bits. M-TMSI. GUTI. M-TMSI. 32 bits. MMEC. M-TMSI. 8 bits. 32 bits. S-TMSI. Subscriber Identities The main means of identifying EPS subscribers remains the IMSI (International Mobile Subscriber Identity), which is permanently assigned to a subscriber account. Temporary and anonymous identification of subscribers is provided by the GUTI (Globally Unique Temporary Identity), which is assigned by the serving MME when a UE has successfully attached and is reassigned if the UE moves to the control of a new MME. The GUTI is analogous to the legacy TMSI (Temporary Mobile Subscriber Identity), but with the additional feature that its structure uniquely identifies not only the subscriber within the MME but also the MME that assigned it. The GUTI is constructed from the GUMMEI, which identifies the MME, and the M-TMSI (MME Temporary Mobile Subscriber Identity). The M-TMSI is used to provide anonymous identification of a subscriber within an MME once that subscriber has been authenticated and attached. As with legacy TMSI use, the MME may elect to reissue the M-TMSI at periodic intervals and it will be reissued in any case if the UE passes to the control of a different MME. The M-TMSI allows a subscriber to be uniquely identified within an individual MME, whereas the S-TMSI (SAE TMSI) allows subscribers to be identified within an MME group or pool. To achieve this, the S-TMSI simply adds the one-octet MMEC (MME Code) to the M-TMSI. The MMEC is the MME’s index within its pool.. Further Reading: 3GPP TS 23.203, 23.401:5.2 2.20. © Wray Castle Limited. LT3604F/v1.

(53) Evolved Packet Core. MME. S-GW UE Radio Bearer. PDN-GW. EPS Bearer. Connection Identifiers The EPS Bearer ID is assigned by the MME upon bearer establishment. It uniquely identifies an EPS Bearer for one UE accessing via the E-UTRAN. The EPS Bearer ID is a one-octet string, which in theory means that each UE can have up to 256 EPS Bearers associated with it per MME. However, the relevant specifications currently indicate that the most significant 4 bits of the ID should be set to 0, which limits the number of EPS Bearers per UE to 16. The EPS Bearer travels between the UE and the PDN-GW; during handovers it may also extend over the X2 interface between source and target eNBs. When travelling over the S1 and X2 interfaces, there is a one-to-one mapping between the EPS Bearer and the E-RAB (E-UTRAN Radio Access Bearer) and between the identities assigned to each of those entities.. Further Reading: 3GPP TS 23.401:5.2.1 LT3604F/v1. © Wray Castle Limited. 2.21.

(54) LTE Evolved Packet Core Network Flexicourse MME UE S1-AP ID. MME S1-AP ID. S1-MME S1-AP Context. UE. X2-U (GTP TE-IDs) X2-C (UE X2-AP IDs). S1-U GTP Tunnel. S-GW. Tunnel Endpoint IDs (TE-ID). Transport Identities To allow the S1 and X2 protocols to identify the UEs that form the endpoint of each transport tunnel, terminals are assigned identities that are unique within the eNBs or MMEs that support those endpoints. The UE S1-AP ID and MME S1-AP ID are unique within the eNB and MME respectively that are handling the E-RAB/EPS Bearer to an Attached UE. The IDs are simple numerical identifiers (24-bits in the eNB and 32-bits in the MME) and are not associated with a specific instance of the S1 interface in each device. An eNB can therefore support a maximum of 224 (16.7 million) UE S1 connections and an MME 232 (4.3 billion). The UE X2-AP ID performs the same basic function as the S1-related identities, but for the X2 interface. The X2 is optional and is only used to pass handover-related traffic between source and target eNBs, so the X2-AP ID will only be created as required when a handover is initiated. The ID is 12 bits long and provides a maximum of 4096 UE X2 handover identities per eNB. The 4-byte GTP TEID (Tunnel Endpoint ID) is used in the EPS the same way as it is in legacy networks. Each device that supports a GTP tunnel refers to it in terms of the TEID assigned to the tunnel plus the IP address and UDP port number of the interface that handles it. TEIDs are assigned by the receiving side of each connection and are exchanged using S1-AP during tunnel establishment.. Further Reading: 3GPP TS 23.401:5.2; 36.413:9.2.3; 29.274 (GTPv2-C); 36.41x (S1); 36.42x (X2) 2.22. © Wray Castle Limited. LT3604F/v1.

(55) Evolved Packet Core. Both Bearers share same IP address. Initial or Default EPS Bearer. eNB. IMS. PDN-GW. S-GW Subsequent or Dedicated EPS Bearer. UE. Both Bearers routed via same APN. Internet. Default and Dedicated Bearers. Default and Dedicated EPS Bearers Each UE will establish an initial or default EPS Bearer as part of the attach process. This will provide the required ‘always on’ IP connectivity to the UE and may be to a ‘default APN’, if one is stored in the user’s subscriber profile, or to an APN selected by the network. In networks that interconnect to an IMS, the default bearer allows the UE to perform SIP registration and thereafter to provide a path for session initiation messaging. In these circumstances, the data rate and QoS assigned initially to the default bearer is commensurate with the expected low level of SIPbased traffic flow, but these parameters can be modified to accommodate the requirements of application traffic flows when a connection is established. If a UE has a requirement to establish an application connection whose QoS or data rate demands are incompatible with those currently assigned to the default bearer (but which can still be routed through the current APN), the PDN-GW or PCRF may initiate the establishment of an additional EPS Bearer to carry the new traffic flow. Any additional bearers assigned to a UE in addition to the default bearer are termed dedicated bearers and will be identified by different EPS Bearer/E-RAB and radio bearer IDs. A UE may have more than one PDN Connectivity Service running if it has connections established through more than one APN/PDN-GW. In that case, there will be one Default Bearer and an optional number of Dedicated Bearers created for each PCS. The 4-bit EPS Bearer ID limits the total number of bearers established for one UE to sixteen (numbered 0 to 15).. Further Reading: 3GPP TS 23.401:4.7.2 LT3604F/v1. © Wray Castle Limited. 2.23.

(56) LTE Evolved Packet Core Network Flexicourse. EPS QoS Characteristics. PCEF (Policy and Charging Enforcement Function) in PDN-GW. EPS. QCI (QoS Class Identifier) ARP (Allocation and Retention Priority) GBR (Guaranteed Bit Rate) MBR (Maximum Bit Rate). EPS Quality of Service QoS in the EPS is defined by a combination of four parameters: . QCI (QoS Class Identifier). . ARP (Allocation and Retention Priority). . GBR (Guaranteed Bit Rate). . MBR (Maximum Bit Rate). EPS QoS is applied between the UE and the PDN-GW.. Further Reading: 3GPP TS 23 2.24. © Wray Castle Limited. LT3604F/v1.

(57) Evolved Packet Core. EPS bearer with GBR QoS. APN-AMBR for non-GBR EPS bearers to PDN-GW 2. S-GW. PDN-GW 1. UE. PDN-GW 2. UE-AMBR for all non-GBR EPS Bearers from UE APN-AMBR for non-GBR EPS bearers to PDN-GW 2. QoS Levels QoS in the EPC is currently defined by three levels: GBR (Guaranteed Bit Rate), MBR (Maximum Bit Rate) and AMBR (Aggregate Maximum Bit Rate). GBR connections are assigned a guaranteed data rate and are therefore useful for carrying certain types of real-time and delay-sensitive traffic. MBR connections are non-guaranteed, variable-bit-rate services with a defined maximum data rate. If a connection’s data rate goes beyond the set maximum the network may decide to begin discarding the excess traffic. GBR and MBR parameters are applied on a ‘per bearer’ basis, whereas AMBR is applied to a group of bearers; specifically, a group of non-GBR bearers that terminate on the same UE. AMBR allows the EPS to set a maximum aggregate bit rate for the whole group of bearers that can then be shared between them. The APN-AMBR parameter sets the shared bit rate available to a group of non-GBR bearers that terminate on the same APN and can therefore be seen to be applied on a ‘per PCS’ basis; the UE-AMBR parameter aggregates all non-GBR bearers associated with one UE. Dedicated bearers can be established as GBR or non-GBR (i.e. MBR) as required. Default bearers, due to the probable need to adjust their bandwidth after the initial Attach has taken place, must be non-GBR.. Further Reading: 3GPP TS 23.401:4.7.3 LT3604F/v1. © Wray Castle Limited. 2.25.

(58) LTE Evolved Packet Core Network Flexicourse. MGW. A/Iu. GERAN/UTRAN Access. Gb/Iu. SGSN. Mc. S4. GERAN/UTRAN. CS traffic. MSC-S. S3. IMS. Sv. EPS. PS traffic MME UE using E-UTRAN access. SGi. S11. E-UTRAN Access. S5. S1 S-GW. PDN-GW. Providing CS Services via LTE/EPS The EPC was designed to handle non-real-time IP-based PS applications such as Internet access and messaging by providing an EPS Bearer between a UE and an external network or AF. 3GPP’s intention was that real-time and more traditional services, especially those that were handled by CS networks – voice, fax, SMS, dial-up data, supplementary services, emergency calls, etc – would be handled in conjunction with an IMS. It was always accepted that some network operators may wish to continue to make use of their legacy CS core networks, either in place of an IMS or alongside one, and 3GPP and a number of industry bodies have proposed methods of achieving this.. 2.26. © Wray Castle Limited. LT3604F/v1.

(59) Evolved Packet Core. MGW. GERAN/UTRAN Access. Gb/Iu. EPS Attached UE. A/Iu. Paged via E-UTRAN SGSN. Falls back to GERAN/UTRAN for connection. IMS not required. Mc. CS traffic. Returns to E-UTRAN when Idle. S4. GERAN/UTRAN. MSC-S. S3 Sv. EPS. CS signalling MME SGi. S11. E-UTRAN Access. S5. S1 S-GW. PDN-GW. CS Fallback Arguably, the simplest solution to the problem of providing CS services without necessarily deploying an IMS is to use 3GPP’s CS Fallback service. CS Fallback allows an EPS UE to perform combined Attach/Location Update functions with the EPS and the legacy CS core. Mobile-Terminated CS transactions, such as inbound calls or SMS, are directed to the legacy CS core as usual. The MSC or MSC Server that receives the inbound transaction alerts the UE’s serving MME via the SGs interface and the MME pages the UE. When it responds, the UE is directed to drop down to a ‘CS capable cell’ in the GERAN/UTRAN to receive the inbound service. Mobile-Originated CS services are handled in the same way, with the UE requesting the service via the EPS but being directed to GERAN/UTRAN access resources to complete the transaction. Once the CS transaction is over, the UE will return to idle mode and will camp onto an E-UTRAN cell. Any EPS Bearers carrying PS traffic will be handed over to the GERAN/UTRAN via an SGSN, if possible, when the CS Fallback is initiated. CS Fallback can operate in conjunction with IMS-based services or could be used as an interim measure by an operator that is not yet ready to deploy one.. Further Reading: 3GPP TS 23.272 (CS Fallback) LT3604F/v1. © Wray Castle Limited. 2.27.

(60) LTE Evolved Packet Core Network Flexicourse. MGW. GERAN/UTRAN Access UE HO to GERAN/UTRAN access. Gb/Iu. A/Iu. SGSN. CS call employs SRVCC PS uses standard HO techniques. Mc. IMS S4. GERAN/UTRAN. CS traffic. MSC-S. S3 Sv. EPS. PS traffic MME SGi. S11. E-UTRAN Access. S5. S1 S-GW. PDN-GW. VCC (Voice Call Continuity) VCC (Voice Call Continuity) is designed to make use of the combined resources of the IMS and legacy CS core network by allowing IMS-anchored real-time or CS calls to be handed over from the E-UTRAN and the GERAN/UTRAN. The specific variant of this concept outlined in the diagram is SRVCC (Single Radio VCC), which supports UEs that only contain one radio and can therefore only connect to one air interface method at a time; in this scenario, the UE is capable of connecting to E-UTRAN, UTRAN or GERAN cells but only one at a time. Call- and handover-related signalling is passed between the MME and MSC–MSC Server via the Sv interface. Handover or hand back of calls from UTRAN/GERAN to E-UTRAN is not supported; once a call drops down to 2G/3G it stays there. Any active PS sessions will be split from the CS sessions and handed over to a 2G/3G SGSN at the same time as the CS sessions are transferred. The SRVCC specification also provides options for handing over IMS-anchored real-time sessions from UTRAN (HSPA) and 3GPP2 1xRTT CDMA2000 access networks to GERAN/UTRAN resources.. Further Reading: 3GPP TS 23.216 (SRVCC) 2.28. © Wray Castle Limited. LT3604F/v1.

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