EPS Overview

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Contents 1 All IP Networks 3 2 PS based architecture in 2G/3G 5 2.1 Network elements and procedures 5 2.2 3G Packet protocols 14 3 Evolution of 3gpp Standards 16 3.1 Motivation for 3gpp Releases 16 3.2 Development of the network architecture over the Releases 18

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1 All IP Networks

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Next Generation Networks will purely be based on IP. All applications will be transported via IP (all-IP). This development is not restricted to mobile networks but is also found in fixed networks. In the far future even fixed and mobile networks will technically grow together in the core network. All user devices can be connected to all applications and can be interconnected to each other using the same IP core. Since the applications will also include real-time services, the IP core must support Quality of Service functionalities and standardized signaling. Furthermore the interworking with the access networks and the QoS agreement must be standardized to guarantee end-to-end QoS for all kind of interconnections.

IP core

DSL/GPONLAN/WLAN

2G/3G CS

3G/4G PS

Mob WiMAX

PSTN

…

Application Server

Application Server

Application Server

Fig. 1 All-IP Network Architecture overview

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2 PS based architecture in 2G/3G 2.1 Network elements and procedures The classic 2G/3G mobile network (based on Release 99) can be divided into the access and the core network. While there are two alternative access technologies (GSM/GPRS and UMTS) in the Mobile Radio the division of the core is based on real-time and not-real-time services. Up to Release 99 real-time services have to be switched via the Circuit Switched Core part to guarantee real-time capabilities. Non-real-time services can be transmitted via the Packet Domain.

GERAN

UTRAN

CS Domain

PS Domain

Access Core

PSTN/PLMN

PDN

External Networks

Fig. 2 2G/3G Network Overview (R99)

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From the 2G Radio Network side two interfaces are defined. The A-Interface towards the Circuit Core which transports voice (codec G.711) in timeslots with 64 kbit/s to the MSC. The Gb-Interface is the interconnection towards the Packet Core and connects to the SGSN. The layer 2 switching technology is based on Frame Relay which forwards IP packets via BSSGP. The bandwidth of the interface can be a multiple of 64 kbit/s up to a full E1.

BTS BSC (+PCU)

BTS

BTS

A

TCE

Gb

VoTS(64kbit/s)

IP/FR(E1 channelized)

Fig. 3 2G Radio Network

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From the 3G Radio Network side two interfaces are defined. The Iu(CS)-Interface towards the Circuit Core which transports voice over ATM (codec AMR like on the radio interface) to the MSC where the voice information is transcoded into G.711. The Iu(PS) -Interface is the interconnection towards the Packet Core and connects to the SGSN. The layer 2 switching technology is based on ATM which forwards IP packets via IP-tunneling (GTP) and AAL5. The bandwidth of the interface can be a multiple of E1 or even STM-1.

RNC

Node B

Iu(CS)

Iu(PS)

VoATM(AMR/AAL2)

IP/GTP(IP/ATM/AAL5)

Node B

Node B

Fig. 4 3G Radio Network

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Gb from the 2G Radio network and Iu(PS) from the 3G Radio network are terminated at the SGSN in the Packet core. The SGSN is responsible for the Mobility of the subscriber, the set up of data connections and the forwarding of user data. The connection towards the HLR for subscriber authentication and updating the subscriber location can be circuit switched or IP-based. The protocol used for the communication is based on the Mobile Application Part. The GGSN is the gateway to external IP-based networks like the Internet or private IP-networks. It is responsible for the set up of data connections and the forwarding of user data. The SGSN connects to the GGSN via an IP network (Gn-Interface) which tunnels the user data via GTP.

SGSN GGSN

PDN

PDN

HLR

IPGn Gn

Gi

Gi

GrGb

Iu(PS)

IP/GTP(IP/L2/L1)

Signalling SLR – HLR(MAP)

IP/L2/L1

Fig. 5 2G/3G Packet Core Network

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To transport user data (or signaling information) behind the GGSN into the external IP network, the subscriber needs to attach (authorization and performing a Routing Area Update) to the network. The attach is performed by the SGSN which interacts with the HLR. On request a PDP context can be opened by the subscriber performing a PDP context activation. The request is forwarded to the SGSN and extended to the corresponding GGSN. After a successful set up of a PDP context the mobile can exchange data with the external IP-based network. This data can be user traffic of signaling information. Both types are not distinguished by the GPRS/UMTS network. The PDP context behaves like a layer 2 remote connection which transparently transmits IP packets.

PDN

Node B RNC SGSN GGSN

(1) Attach (Mobility Management)

(2) PDP Context activation (Session Management)

-IP based

-No standardized QoS

(3) Signalling and User data

Fig. 6 2G/3G Packet Core Network

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A full view of a 2G/3G packet architecture is shown below. The radio networks are connected to the SGSNs, the external IP networks are connected to the GGSNs. SGSNs and GGSNs are interconnected via an IP core (Gn-Interface) which also connects the IP-based HLR.

Gb

Iu(PS)BSC

RNC

Gb

Iu(PS)BSC

RNC

SGSN

SGSN

Gn

GGSN

GGSN

HLR

PDNGi

PDN

PDN

Fig. 7 2G/3G Packet Core Network

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The GPRS attach is a procedure from the GMM (GPRS Mobility Management) responsible for keeping the subscriber mobile. Other procedures from GMM are Routing Area Update (RAU), detach or paging. Also handover is performed via the GMM. The attach involves the interaction of the SGSN with the HLR to update the location of the subscriber (which SGSN) and to copy the subscriber data to the SGSN. The initial message is send from the mobile to the SGSN which performs the authentication.

Gb

Iu(PS)BSC

RNC

Gb

Iu(PS)BSC

RNC

SGSN

SGSN

Gn

GGSN

GGSN

HLR

PDNGi

PDN

PDN

UE

(1a) Attach

(1b) Update LocationInsert Subscriber Data

(1c) Attach accept

Fig. 8 3G GPRS Mobility Management - Attach

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The activation of a PDP context is initiated by the mobile. Normally it is connected to the activation of a certain application on the mobile, e.g. to transmit a MMS, to connect to the internet or to use a WAP service. The SGSN checks via the user profile in the SLR if the user is allowed to enter the requested network. It is also able to connect the corresponding QoS profile to the service. The PDN is addressed via the Access Point Name (APN). To resolve the GGSN, the APN is translated into the IP address of the GGSN at the Gn-Interface via DNS server. This server holds the entries of all possible APNs within this network. After the resolution of the GGSN IP address the activation of the PDP context can be forwarded to the corresponding GGSN. At the outgoing side of the GGSN exists the possibility of an external authentication server (e.g. RADIUS server). After successfull setup of the PDP context the procedure is completed via an accept message.

Gb

Iu(PS)BSC

RNC

Gb

Iu(PS)BSC

RNC

SGSN

SGSN

Gn

GGSN

GGSN

HLR

PDNGi

PDN

PDN

UE

(2a) activate PDP context

(2b) DNS query

(2d) activate PDP context accept

DNS

(2c) create PDP context

Fig. 9 3G Session Management - PDP Context Activation

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The user data can now be tunneled from the RNC to the GGSN via the SGSN. In later releases even a direct tunnel between RNC and GGSN is possible to avoid the extra step of switching via the SGSN. The mobile network does not distinguish user or signaling data generated by the user equipment and transmitted via IP packets. Signaling information for the PDN is transparently transmitted.

Gb

Iu(PS)BSC

RNC

Gb

Iu(PS)BSC

RNC

SGSN

SGSN

Gn

GGSN

GGSN

HLR

PDNGi

PDN

PDN

UE

(3) Signalling and user traffic transfer via GTP tunnel

Fig. 10 3G Packet User Traffic

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2.2 3G Packet protocols The picture below shows the complete end-to-end layer representation of a data transmission via a R99 UMTS network. IP packets are transmitted via PDCP on the radio side. PDCP uses a Radio Link to connect to the physical transmission channel on the radio interface. The radio resources are handled in lower layers between UE and RNC. At the RNC all radio specific protocols are removed and the IP packets are encapsulated in GTP packets to tunnel them via the SGSN to the GGSN over an existing PDP context. At the GGSN all GPRS specific headers are removed. The IP packets are extracted and encapsulated in the corresponding L2/L1 protocols at the outgoing side at the Gi interface.

IP

GTP

UDP

IP

L2

L1

L2

L1

IP

L2

L1

Appl.

GTP

UDP

IP

L2

L1

GTP

UDP

IP

ATM

STM-1

GTP

UDP

IP

ATM

STM-1

PDCP

RLC

MAC

L1

PDCP

RLC

MAC

ATM

IMA E1

ATM

IMA E1L1

IP

Appl.

Iu

Radio

Node B RNC SGSN GGSN Server

ATMIP over ATM IP IP

Uu Iub Gn Gi

UE

Fig. 11 Layer representation of 3G Packet User Traffic

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Since the development of the mobile core was strongly driven by SS7 network concepts the signaling protocols from the radio side are not purely packet oriented. Between the UE and the RNC the Radio Resource Control protocol is used, towards the SGSN the Radio Access Network Application Protocol is transmitted via MTP3b over ATM, since the Iu-Interface is ATM based. At the core side the signaling between SGSN and GGSN is based on GTP control.

L1

Iu

Radio

Node B RNC SGSN

ATM ATM

Uu Iub

UE

RANAP

SCCP

MTP 3B

ATM

STM-1

RANAP

SCCP

MTP 3B

ATM

STM-1

RRC

RLC

MAC

L1

RRC

RLC

MAC

ATM

IMA E1

ATM

IMA E1

GMM / SM

GMM / SM

GGSN

IP

Gn

UDP

IP

L2

L1

GTP-C

UDP

IP

L2

L1

GTP-C

Fig. 12 Layer representation of 3G Packet Signalling

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3 Evolution of 3gpp Standards 3.1 Motivation for 3gpp Releases The development of the 3gpp releases is essentially the story how to migrate smoothly from the pure circuit based mobile network architecture of GSM to finally a pure packet based mobile network architecture. This final step might be achieved beyond Release 8 with the introduction of the fourth generation of mobile radio access. In the meanwhile real-time and non-real-time services are physically or at least virtually separated to guarantee the needs of the applications. Therefore the steps in-between are a mix of CS and PS transmission. The picture below shows the reference of the 3gpp release steps regarding the generation and the mobile technology.

Generation 2 GSM pure CS

Generation 2.5 GSM/GPRS Release 97 MixRelease 98

ofGeneration 3 UMTS Release 99

Release 4 CSGeneration 3.5 HSDPA Release 5

HSUPA Release 6 andHSPA+ Release 7

Generation 3.9 LTE/SAE Release 8 PSRelease 9

Generation 4 LTE Advanced Release 10... pure PS

Fig. 13 Development of 3gpp Releases

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The smooth transission is necessary for the operators to smoothly migrate their networks. An interworking between the existing network architectures and the new ones need to be guaranteed with every step of the evolution. I.e. a backward compatibility must be given for a very long time. Finally when a pure PS end-to-end network is achieved all benefits from such a all-IP network architecture can be used.

2G 4G

Pure circuit switching for voice and dataLittle flexibility adjusting the bandwidth (fixed time slots)Complex network architecture to multiplex from lower to higher data rates

Pure packet switching for realtimeand non-realtimeHigh flexibility adjusting the bandwidthTraffic flows can be identified using headers

smooth transmission must be given

Fig. 14 Transition from pure CS to pure PS

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3.2 Development of the network architecture over the Releases

Starting from the classic R99 2G/3G mobile network the following pictures show the evolution toward an all-IP mobile network that offers full interoperability with existing networks.

GERAN

UTRAN

CS Domain

PS Domain

Access Core

PSTN/PLMN

PDN

External Networks

Fig. 15 2G/3G Network Overview (R99)

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With the introduction of Release 4 the subscriber does not feel any changes to the services offered. The end user equipment, the mobile radio networks and also the packet switched core part do not change. The major change with Release 4 lies in the Circuit Domain. It is changed from time slot based voice transmission to Voice over IP but only within its boundaries of the core network. Therefore the other networks do not have to be changed and also the subscribers do not have to change their devices. Introducing Release 4 is the first step towards real-time services over IP in mobile networks. The IP core networks for real-time and non-real-time do not have to be separated any more. The quite complex multiplexing structure of the circuit switched core is removed with this step. As a result the WAN and also the infrastructure in widely reduced in term of cost. Also this network provides a good basis for further developments towards all IP.

GERAN

UTRAN

CS Domain

PS Domain

Access Core

PSTN/PLMN

PDN

External Networks

Release 4: CS Domain Packet Switched

Extended Core

Fig. 16 2G/3G Network Overview - Evolution towards R4

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With Release 5 the IP Multimedia Subsystem is introduced. This signaling architecture behind the GGSN enables the PDN to interact with the user equipment based on a standardized signaling protocol - the session initiation protocol (SIP). The IMS is interconnected to all external networks and works as a bridge between all these different “worlds”. Any kind of access can be integrated into an IMS; IP based access directly and non-IP-based access (like the PSTN) via gateways for signaling and user traffic. A drawback of the IMS with Release 5 is the relatively high latency of the packet switched radio interfaces of 3G and especially 2G. They limit the ability to support real-time services in higher load situations. So, although the core network is able to establish QoS for packet services end-to-end, the mobile radio side is not able to support all requested quality parameters.

GERAN

UTRAN

CS Domain

PS Domain

Access Core

PSTN/PLMN

PDN

External Networks

Release 5: Introduction of IMS

IMS

Extended Core

IMS

Fig. 17 Network Overview - Introduction of IMS with R5

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This situation results in the question if real-time services can or cannot be supported by 3G networks. The answer is not completely straight forward. With the introduction of HSPA and HSPA+ from R5 to R7 the real-time capability of the radio interface is increased step by step. Furthermore not all real-time services are as demanding as voice. Video streaming or also video conferencing might work with 3G as long as the load in the radio network is not too high. Voice services in a cell with very high load will result in customers complains regarding the quality (delay and loss). Therefore a reduction of the CS Domain with the introduction of IMS within UMTS is not seen.

GERAN

UTRAN PS Domain

Access Core

PSTN/PLMN

PDN

External Networks

Real-time Services via IMS

IMS

Extended Core

IMS

?

Fig. 18 Reduction of CS Domain with R5

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The solution of this dilemma lies in the introduction of the next generation of mobile networks. While the 2G radio interface was fully circuit switched and also the 3G radio interfaces contained strong concepts from circuit switching (static allocation of resources) the next generation of mobile access is completely packet switched and therefore does not suffer from compromises toward circuit switching. As a result the latency is much reduced even in high load situations. With LTE access technology like the eUTRAN it is possible to establish end-to-end QoS over a packet switched bearer for all services. The new radio network is widely simplified in architecture and need a corresponding core network to support all capabilities. The evolved packet core is this step towards the system architecture evolution. eUTRAN and EPC form the evolved packet system (EPS) which - together with IMS - supports full end-to-end QoS for all applications over IP. This approach is the first full mobile all-IP solution within the 3gpp. Via the IMS all other access networks are integrated and even handover functionality between these networks is introduced.

Access Core

PSTN/PLMN

PDN

External Networks

Introduction of evolved Radio Network (LTE)

IMS

Extended Core

IMSeUTRAN EPC

Introduction of evolved Core Network (SAE)

Fig. 19 Introduction of LTE/SAE with R8

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The full picture shows all access and core networks parts with all interconnections to external networks. It can be that neither the eUTRAN nor the ECP is connected to the CS Domain.

GERAN

UTRAN

CS Domain

PS Domain

Access Core

PSTN/PLMN

PDN

External Networks

IMS

Extended Core

IMSeUTRAN EPC

No connection to the CS Domain!

Fig. 20 GSM/GPRS/UMTS/LTE/SAE Network Overview

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The EPC is also capable to interwork with the 2G/3G packet core offering an interface towards the SGSN. The PDP context can then be extended from the RNC to the SGSN and further into the EPC. Even other access types like wireless LAN can be integrated into the EPC. The standard offers solutions for trusted and untrusted network access.

GERAN

UTRAN

CS Domain

PS Domain

Access Core

PSTN/PLMN

PDN

External Networks

Integration of other access technologies

IMS

Extended Core

IMSeUTRAN EPC

WLAN

Fig. 21 Integration of other networks