Intro to _evolved_packet_core_network

UNIVERSITY OF ALBERTA

Overview of the Evolved
packet core network
Project report submitted to the Faculty of graduate studies and research

University of Alberta
In partial fulfillment of the requirements of the degree of
Masters of Engineering (Specialization: Communications)

Amandeep Singh, ECE, Student ID: 1275809
Department of Electrical and Computer Engineering, University of Alberta
.

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Abstract

Since the advent of Mobile internet technologies, the users and their demand for the data
access with high rate has been growing exponentially. This study explores the evolution
of all IP core network named Evolution Packet Core (EPC). EPC is developed by 3GPP
under work item System Architecture Evolution (SAE). Various aspects of the EPC
which includes its architecture, interworking with other radio access technologies e.g.
GSM/ WCDMA or CDMA, major services and functions are included, in a brief manner,
are included in this study project.

Keywords- System Architecture Evolution (SAE), Evolution Packet Core (EPC), Long
Term Evolution (LTE), Mobility Management Equipment (MME), Serving Gateway
(SGW), Packet Data Network Gateway (PDN-GW), Home Subscriber Server (HSS),
eNODEB

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Table of contents

1. Introduction to Evolved packet core networks (EPC) .....................................................1
1.1 Overall cellular system architecture....................................................................1
1.2 Background of development of EPC .................................................................2
1.3 Objectives set by 3GPP for EPC .........................................................................3
2. EPC architecture ..............................................................................................................3
2.1 MME ...................................................................................................................5
2.2 Serving gateway (SGW) .....................................................................................6
2.3 Packet data network gateway (PDN-GW) ..........................................................6
2.4 Home subscriber server (HSS)............................................................................7
3. Interworking with 2G and 3G technologies .....................................................................7
3.1 Interworking between LTE and GSM or WCDMA networks ............................7
3.2 Interworking with LTE and CDMA networks ....................................................10
4. Major services of EPC ....................................................................................................11
4.1 Data services .......................................................................................................12
4.2 Voice services .....................................................................................................12
4.3 Message services .................................................................................................13
5. Major Functions of EPC ..................................................................................................14
5.1 Authentication and security ................................................................................14
5.2 Policy and charging control and QoS .................................................................17
5.3 Packet routing .....................................................................................................19
5.4 Mobility management .........................................................................................19
5.5 IP address allocation ...........................................................................................20
Conclusion .............................................................................................................................22
References ..............................................................................................................................23

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List of Figures

Figure 1- Basic cellular architecture ................................................................................... 1
Figure 2- Architecture Domains by 3GPP .......................................................................... 3
Figure 3- Basic EPC architecture for LTE .......................................................................... 4
Figure 4- Interworking of LTE with GSM or WCDMA networks ..................................... 8
Figure 5- Interworking of LTE with GSM or WCDMA networks by GTPv2 ................... 9
Figure 6- Interworking of LTE with CDMA networks .................................................... 10
Figure 7- Application and services on mobile broadband ................................................ 12
Figure 8- Flow of message services via circuit and IP domain ........................................ 14
Figure 9- Different security domains ................................................................................ 15
Figure 10- Flow of Authentication process messages ...................................................... 16
Figure 11- Example of two security domains by employing NDS/IP .............................. 17
Figure 12- Policy architecture ........................................................................................... 18
Figure 13- EPS bearer model ............................................................................................ 19

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1. Introduction to Evolved Packet Core network (EPC)

1.1. Overall cellular system architecture
In 1897, when Guglielmo Marconi first showed the world the ability to communicate
on radio with ships sailing the English Channel since then the evolution in the field
of wireless has been growing by leaps and bounds.
The first ever wireless system operated commercially in late 1970’s was AMPS
(Advanced mobile phone system) which was developed by Bell Labs. Since then
other various other standards e.g. global system for mobile communication (GSM),
GPRS, CDMA etc. have been developed and even at present the process of
development is on progress.
The basic cellular architecture of different wireless standards consists of three parts as
shown in Figure 1 below. These are:
 Mobile station.
 Base station subsystem.
 Network subsystem.

BTS
HLR VLR
Mobile
Station
BSC MSC PSTN

BTS
BSC

EIR AuC
Base Station Subsystem

Figure 1- Basic cellular architecture Network Subsystem

Mobile Station: Mobile station is equipment in the cellular system which is intended
for use while in motion. It may be hand held device or installed in vehicles. It contains
an integrated chip called subscriber identity module (SIM) which contains
International mobile subscriber identity (IMSI) and encryption keys for authorization.
Base station subsystem: Base station subsystem mainly consists of two entities Base
transceiver station (BTS) and base station controller (BSC). BTS is a fixed station in a
cellular network and used for communication with mobile stations over air interface. It

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consists of radio channels and antennas (transmitting and receiving simultaneously)
mounted on a tower. BSC provides the functions like handover, control of RF power
levels and cell configuration data in BTS and physical connectivity between BTS and
Mobile switching center (MSC). One BSC can handle various BTS simultaneously.
Network Subsystem: Network subsystem consists of Mobile Switching Center (MSC)
which provides the functions of call routing and mobile management. It is connected
to Public Switched Telephone Network (PSTN) to provide access to external networks
to the end users. Home Location Register (HLR) which stores the data related to each
and every subscriber registered in a network and provide the current location of each
user. Visitor Location Register (VLR) is database which temporarily stores the
information of a subscriber who is visiting the coverage area of MSC other than its
home MSC. The Authentication Center (AuC) is a database which is strongly
protected and handles the authentication and encryption keys for every single
subscriber in the HLR and VLR. The Authentication Center contains a register called
the Equipment Identity Register (EIR) which identifies stolen phones that transmit
identity data that does not match with information contained in either the HLR or
VLR.

1.2 Background of development of EPC
In 1990’s the various standards of cellular system e.g. GSM, CDMA etc. were based
on circuit switching and the services developed were specifically concentrated on the
typical applications of telecommunications. But the introduction of mobile internet in
early 1990’s brought a huge change or we can say the revolution in
telecommunication world. But at that time the mobile equipment were not designed
enough to support the services. Another reason was the bandwidth; the BW of radio
was not enough to support the services.
Now the trend has been changed with the evolution of new mobile broadband access
technologies and developments in semiconductor chips made it possible to support he
mobile internet services.
In November 2004, 3GPP(Third generation partnership project) started its work on
4G technologies that was like a successor of Universal mobile telecommunication
system(UMTS), particularly a work item named system architecture evolution(SAE)
along with LTE which is responsible for evolution of packet core network(EPC),
which will support the high bandwidth services at high data rate.
3GPP wanted to create a global standard for 4G technologies. Because, firstly, to give
an operator a full freedom to choose a vendor. It means whatever vendor the operator
will use, its end users would not have any disruption in services in moving from one
vendor equipment to another. It will also increase the competition between vendors.
Secondly, the creation of global standard will be helping in removing the separation
between various players like operators and vendors involved in providing services to
the end users. As an example, in no separation case, the semiconductor chip maker
company will have one larger market. So the larger the market is then larger its users.
It would help in reducing overall cost of the production and the company can achieve
high profits at lowest price levels. So the main target behind the evolution of core
networks is to provide affordable and reliable communications networks to the users.

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In the standardization process of the EPC, various bodies like 3GPP2 (Third
generation partnership project 2), Internet engineering task force (IETF), WiMAX
forum and open mobile alliance (OMA) took part very actively.”3GPP ‘owns’ the
EPS specifications and refers to IETF and occasionally OMA specifications where
necessary, while 3GPP2 complements these EPS specifications with their own
documents that cover the impact on EPS and GPP2-based systems. WiMAX forum
also refers to 3GPP documentation where appropriate for their specification work”1.

1.3 Objectives set by3GPP for EPC:
The three main promises made by 3GPP for development of SAE or EPC were to deliver:
 New core network architecture to support high data rate and reduced latency in a
time frame of next 10 years to ensure the competiveness of the 3GPP systems
 To support mobility between multiple heterogeneous access systems for e.g. like
between 3GPP and 3GPP2 systems or between 3GPP and WiMAX
 All IP architecture, to enhance the capability of 3GPP systems to cope with rapid
growths in IP data traffic

2. EPC architecture

Before we will go into the details of architecture of the EPC, we will briefly see the high-
level perspective of the complete system as defined in the SAE work item. It is called
EPS architecture. EPS stands for Evolved Packet system, which represents all IP network
and contains both EPC and LTE. It consists of different domains and each domain again
consists of logical nodes. These nodes are interworked with each other to perform any
specific set of functions. The basic network which implements the 3GPP specification is
shown below in the figure 2.
CS
networks

Circuit core domain
GSM/GPRS

WCDMA/HSPA User IMS domain
Domain
LTE

Non-3GPP Packet core domain

RAN Domains Core network domains

Figure 2- Architecture Domains by 3GPP IP
networks
1
Olsson,M., Sultana, S., Frid, L. &Mulligan,C.(2009). SAE and Evolved packet core: Driving the mobile
broadband revolution. Oxford, UK: Elsevier Ltd.

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As shown in the figure 2, there are four domains. First, GSM/GPRS represents 2G
technology domain whereas second, WCDMA/HSPA (Wide CDMA/ High speed packet
Access) represents 3G or 3.5G RAN (Radio access network). Third, LTE (Long term
evolution) is the latest domain specified by 3GPP and the fourth, Non-3GPP domain
consists of access networks, e.g. WiMAX and WLAN, Which are not specified by 3GPP
but actually provided by other standardization bodies like 3GPP2, IEEE. All four
domains are connected to packet core domain (EPC). The core domain also consists of
four basic domains. These are Circuit core domain, User domain, IMS (IP multimedia
subsystem) and Packet core domain. The circuit core domain is linked to GSM/GPRS and
WCDMA/HSPA. It supports and provides the circuit switch services in 2G and 3G
technologies. The packet core domain provides IP services over GSM, WCDMA/HSPA,
LTE and Non-3GPP technologies while the user domain provides the complete updated
information of users on request. It maintains the database to support roaming mobility of
the subscriber whether they are moving in a single network or in between different
network. The IMS provides support to services based on Session initiation protocol (SIP).
Since IMS supports IP services so it uses the IP connectivity with packet core domain to
use its function provided by its node.
Now we will turn our attention to the EPC architecture. The EPC architecture consists of
packet core domain and user domain. The following figure 3 is showing the basic
architecture of the EPC for LTE.

SGi
PDN-GW Internet

S5
CP UP

HSS S6 MME S11 SGW

S1
CP UP

eNODEB eNODEB

Mobile Device

Figure 3- Basic EPC architecture for LTE

In packet domain, it consists of:

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 Mobility management equipment(MME)
 Serving Gateway(SGW)
 Packet data network gateway(PDN-GW)

In user domain, it has only one node named Home subscriber server (HSS).

The role and function of each component of EPC is as follows:

2.1 Mobility Management Equipment

It is the node which is responsible for the signal exchanges between base stations and
core networks and between the subscriber and core network. Basically MME does not
involve in air interface matters so it is the non- access stratum (NAS) signalling
which is exchanged between MME and radio network. In brief following are the
basics tasks which MME performs.
 Authentication: When for the first time subscriber attached with LTE network in
particular we can say when it comes under the coverage of eNODEB for first time
then eNODEB helps in exchanging the information between the subscriber and
MME through its S1-CP (S1 control plane) interface with MME. Then MME which
is connected to HSS through S6 interface requests the authentication information
from HSS and authenticate the subscriber. After the authentication, it forwards the
encryption keys to the eNODEB so that the data and signalling exchanges between
the eNODEB and subscriber over the air interface can be ciphered or calculated
numerically.
 Establishment of Bearers: MME actually deals with the control data instead of the
user data. For the establishment of bearer it actually communicates with other
entities of the core network (SGW and PDN-GW) to establish a user IP tunnel
between a mobile subscriber and internet. It also helps in selecting a gateway router
if more than one gateway router is there in network.
 NAS mobility management: In case when there is no communication happening
between a mobile and radio network for a decided amount of time then any
connection and resources between subscriber and radio network are released by the
network. In a same tracking area (TA) the subscriber can move freely between
different base stations without notifying the MME. It saves the battery power of the
mobile device and helps in reducing the signal traffic in the network. If there is any
data arrive from the internet for this device then MME send a paging message to
every eNODEB in same tracking area then mobile device responds to the paging
message and connection re-establishes.
 Interworking support: Whenever a mobile device is reaching the boundary of LTE
then the eNODEB decides for the suitable cell, for the device or for the network
(GSM or UMTS). MME continuously makes communication with other core
network components of GSM, UMTS and CDMA to support the traffic.
 Handover support: There are some cases in which there is no X2 interface
available between two eNODEBs and mobile device is going from one eNODEB to
other eNODEB then in that case two eNODEBs transfer messages between each
other through MME.

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 Supporting traditional services like voice and messages: As LTE is pure IP
network and it should be compatible to GSM and UMTS to support the voice and
other services. MME plays the role of mapping the services from GSM or UMTS to
LTE. Details of how it supports the services are provided under major services
section of EPC.

2.2 Serving gateway (SGW)
The basic function of serving gateway is to manage the user IP tunnels between
eNODEB and packet data network gateway. Serving gateway is connected to
eNODEB through S1-UP (S1- user interface) and to PDN gateway through S5-UP
interface. S1 and S5 tunnels for an individual user are independent of each other and
it can be modified as required. It is connected to MME through S11 interface which
provides the function of creation and modification the tunnels. The S11 interface
uses GTP-C (GPRS tunnelling protocol-control) to transfer the messages sent by
MME to SGW. Generally in the standard MME and SGW are defined independently
but these entities can be defined on a same or different network node depends on the
operator choice. This allows the wireless standardization bodies to work on the
signalling traffic and user traffic independently. This was done because the
additional signalling increases the load of the processors which processes the
signalling traffic and on the other hand rising user traffic demands the evolution of
more network interfaces and routing capacity.

2.3 Packet data network gateway(PDN-GW)
The functions of PDN-GW are as follows:

 This is the gateway to Internet. It connects to the SGW through S5-UP interface and
to Internet through SGi interface. In forward direction, it takes user data packets
from SGW and transfer to internet through SGi interface. In back ward direction,
data packets are encapsulated into S5 GTP tunnel and forwarded it to SGW which is
responsible for that intended user.
 PDN gateway is also responsible for assigning IP addresses to the mobile devices.
This happens when a subscriber switched ON his/her mobile device. Mobile device
sends its request to eNODEB which uses the S1-CP and forwards to MME. MME,
after authentication, request the PDN gateway on a control plane protocol for IP
address. If PDN gateway approves the request then it sends back an assigned IP
address to MME. MME forwards it to eNODEB and eNODEB further forwards it to
the subscriber. Multiple IP addresses can be assigned to a single mobile device. This
is the case which happens when a subscriber is using a multiple services provided by
its network operator’s network such as IP multimedia subsystem.
 It plays an important role in case of international roaming scenarios. A roaming
interface is used to connect the GSM/GPRS, UMTS/HSPA, or LTE networks of
different network operators of different countries. For example, if a subscriber has
moved to another country and wants to connect to an internet then a foreign network
will query the user data base in the home network for authentication purposes. After

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authentication a bearer is established and GTP user tunnel is created between SGW of
visitor’s network and PDN-GW of subscriber’s home network over an interface
called S8.

2.4 Home subscriber server (HSS)

HSS is a data base that stores the information of each and every user in the network.
It also does the authentication and authorization of the users and services provided to
them. In UMTS and GSM, the database is referred to as Home location register
(HLR). In LTE, a protocol named DIAMETER is used to exchange the information
between MME and HSS on S6a interface. In practise, HSS and HLR are combined
physically so that the seamless roaming can be made possible between different radio
access networks. HSS stores the user parameters like IMSI, authentication
information to authenticate the subscriber, circuit switch properties e.g. user
telephone number and the services a user is allowed to use e.g. SMS, call forwarding
etc., Identity of current MSC so that incoming circuit switch calls can be routed
correctly, ID of MME or SGSN which is used in case user’s HSS profile is updated
and the changes could be notified to these nodes(MME or SGSN) and packet
switched properties such as Access point name(APN) the subscriber is allowed to use
which in turn references the properties of a connection to the Internet or other
external packet data network.

3. Interworking with 2G and 3G technologies

The deployment of LTE networks are still in very early stage so it is very imperative that
LTE should be connected to 2G and 3G technologies to provide the complete services
like voice. Take a case when a user makes a call in LTE coverage and moving out of the
LTE coverage then the call should not be disconnected. So for LTE deployment
interworking with existing access networks, supporting IP connectivity becomes very
crucial. The EPS architecture provides two kinds of distinct solutions to address this
problem. The first one is LTE interworking with GSM or WCDMA access technologies
and second one describes interworking with CDMA access technologies. In the following
we will discuss these interworking in a brief manner.

3.1 Interworking between LTE and GSM or WCDMA networks

3GPP has defined two different solutions about how to do interworking between LTE
and GSM or WCDMA access networks. Before we will go further to discuss those
two solutions we just need to recall that if a terminal connects to the LTE then it will
be served by MME and in case if terminal connects to GSM or WCDMA then it will
be served by SGSN (Serving GPRS Supporting Node).
In the first solution, SGSN connects to the GSM or WCDMA networks over Gb
interfaces. The MME and PDN-GW nodes of LTE networks acts as an SGSN and
GGSN respectively. The SGSN takes MME and PDN-GW just likes as another

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SGSN and GGSN and connects to these over Gn interface. The following diagram
represents the clear picture of how LTE network is connected to GSM or WCDMA
networks.

External
HSS Networks

GGSN HLR

Gn SGi
Gr
Gn S6a PDN
SGSN

Gn
Gn Gb Iu S5/S8
MME
S11 SGW

S10
S1-MME
GSM WCDMA
LTE

eNODEB

Signalling

Voice/Data
Figure 4- Interworking of LTE with GSM or WCDMA networks

The EPC architecture supports the IP session which is established over any access
network. It is also referred as session continuity. “This is done by retaining a stable IP
anchor point in the network which allows for not having to change the IP address of
the device at all”2.
To make this solution work, it is very necessary for SGSN that it should distinguish
between a terminal that can attach to GSM or WCDMA access network only i.e.it
cannot move to LTE from a terminal that can connect to LTE but is currently
attaching to GSM or WCDMA networks due to lack of LTE coverage. The latter
terminal must always be using PDN-GW as the anchor point. It cannot use GGSN for
that because there is no logical connection between LTE and GGSN. SGSN uses
APN (Access Point Name) to choose either GGSN or PDN-GW as an IP anchor point
for a terminal. APN is a part of configuration data related to a user subscription so for
the terminals which can support LTE radio access network should be configured with
APN that is associated to PDN-GW. This actually helps the SGSN in making correct

2

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decision and ensuring that terminals that support LTE radio access network uses the
PDN-GW as an IP anchor point not the GGSN.
Another very critical part of the solution is to provide single set of user and subscriber
data. When a terminal moves between different radio access networks then there
should not be any inconsistent information in the network about to what access
network a specific terminal is attached. In GSM or WCDMA network, SGSN is
connected to HLR through Gr interface and in LTE network, MME is connected to
HSS over S6 interface. So according to the solution, HLR and HSS needs either to
share a single set of data or to make sure the consistency through other means such as
close interaction between these two entities. The 3GPP specification avoids the
problem through defining HLR as a subset of HSS in later versions of the LTE
standards.
In second solution, SGSN introduces four new interfaces. These are S3, S4, S16 and
S6d. The S3, S4 and S16 rely on updated version of GTP (Gateway Tunnel
Protocol).It is referred as GTPv2. The following figure 5 shows the details of the new
solution

External
Networks

SGi
HSS
PDN

S6d S6a S5/S8
SGSN S4
SGW
Gb Iu S3 MME S11
S16
S1-U
S10
GSM WCDMA S1-MME
LTE
eNODEB

Signalling

Voice/data
Figure 5- Interworking of LTE with GSM or WCDMA networks by GTPv2

The S3 interface is signalling only interface which is used to support inter-system
mobility between MME and SGSN. S16 is a SGSN - SGSN interface. S4 interface is
used to connect the SGW and SGSN. The fourth interface S6d is alike a MME S6a

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interface towards HSS to retrieve the subscriber data. The protocol used for S6d
interface to exchange messages is IETF’s DIAMETER protocol.
In this provided solution, the connection between the SGSN and SGW creates a
common anchor point for LTE, GSM or WCDMA in the SGW. Now, regardless the
access network to be used, all the traffic related to a particular roaming subscriber
will pass through a common point in the network. It allows the visited network’s
operator to control and monitor the traffic in a consistent way. In this solution, by a
careful look, the user traffic needs to pass through a one additional network node on
its way to PDN-GW which can be consider as a drawback of this solution. But for
the WCDMA networks the solution is available to address this problem. The RNC
(Radio network Controller) of WCDMA can be directly connected to SGW through
S12 interface. By doing this, SGSN will only considers the control signalling for
WCDMA networks not its user traffic.

3.2 Interworking with LTE and CDMA networks
As the EPC was being developed by 3GPP under the framework of SAE, strong
efforts were made to design a solution for interworking between LTE and CDMA
technologies developed by 3GPP2 to allow smooth handover between these different
technologies. The following figure shows the interworking of LTE and 1x/1x EVDO
(eHRPD which stands for enhanced high rate packet data) networks. This figure 6
includes only details of CDMA network relevant to SAE framework.

External
Networks

SWx STa
HSS AAA
PCRF
SGi Gx
S6b
PDN-GW
S6a S10
S5/S8 S2a Gxa
Gxc

MME SGW
S103 HSGW
S1-C S1-U

eNODEB
S102
S101
eHRPD

Figure 6- Interworking of LTE with CDMA networks
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To provide the interworking between LTE and CDMA, 3GPP defined number of
additional interface in EPC architecture. The interfaces S101, S102, S103 are unique
for CDMA networks and used to provide optimal performance during handover. The
interfaces S2a, Gxa and STa are generic and may be used for any non-3GPP access
networking.
For efficient interworking between LTE and CDMA, there should be common set of
subscriber data to be used for authentication and to locate the user to know which
network is currently user attached to. For this purpose, HSS should be allowed to
common to act as a common database for all subscription data. In 3GPP2, if a
terminal is attaching over an eHRPD network then its access authentication are
handled by mechanisms which are based on IETF’s AAA (Authentication
Authorization and Accounting) functionality. For this purpose, eHRPD network is
connected to 3GPP AAA server over STa interface. In real life implementations AAA
can be a software feature inside the HSS or a different entity connected to HSS over
SWx interface. The PDN-GW is also connected to AAA server over S6b interface to
retrieve certain subscription data and also use the interface to store information
regarding the PDN-GW, the user is connected to, so that in case when a user moves
and attaches over LTE then the MME would be able to select the same PDN-GW as
was used in eHRPD network and IP session can be maintained. The user data
between eHRPD serving gateway (HSGW) and PDN-GW, which also act as a
common anchor point for eHRPD network, are transported over S2a interface via
PMIPV6 protocol. To apply common policies in eHRPD network, EPC architecture
also allows for a common policy controller (PCRF) over a Gxa interface to the
HSGW.
In addition to the core interfaces, there were three interfaces S101, S102, S103
defined to support LTE - eHRPD interworking. The S101 interface, between MME
and eHRPD, is used when a packet data handover between LTE and eHRPD network
is to take place. Before the handover, the terminal pre-register itself in the visited
network to reduce the perceived interruption time. This pre-registration and the actual
handover signalling are carried over S101 interface. The S102 interface, between
MME and eHRPD, is used to support the voice services in CDMA 1xRTT networks.
“The S103 interface, between SGW and HSGW, is used to forward any IP packets
destined to the terminal that happened to end up in SGW while the user terminal was
executing the handover to eHRPD”3. This interface is used to further optimize the
packet data handover performance. These packets can then be forwarded to the
HSGW in the eHRPD network

4. Major Services of EPC

The three major services provided by EPC are following:

3

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4.1 Data Services
As we know that EPC has flat IP architecture. It is designed to support any
application which depends on IP communications. Radio access network (LTE) and
packet core network (EPC) in 4G communications has role to provide complete IP
communication between two end users. The IP based application which a mobile
subscriber can access can either be provided by mobile operator or accessible over
internet or residing in corporate IP network. A following figure 7 shows as an
example how an end user on a lower level accesses the IP applications by using the IP
services provided by EPC.

Application level communication
Application Application

IP in point to point link Routing of IP packets
Gateway IP
IP

Mobile
Radio Network

Mobile Equipment Application server

Figure 7- Application and services on mobile broadband

In figure 7, all the communications between the two end users are point to point (by
passing first through a gateway then to application server). EPC architecture makes
assure to the subscriber that he/she can move with same IP address with same or
different radio access network.

4.2 Voice services
As EPC has flat IP architecture, there is no dedicated channel to support the voice
services like in other radio access technologies have e.g. GSM. But for the network
operator voice services have been the largest revenue generator. So in EPC two
approaches have been used to support the voice services. Either we can use the
existing circuit switched structure or the IMS technology. IMS uses MMTel
(Multimedia Telephony) developed by 3GPP to support the voice services in IMS.
 Voice services supported by IMS technology: IMS uses MMTel service for
voice calls. As IMS has IP architecture, so it offers additional media components
like video including voice component. In this way, it adds value to the end user
and is the best option for offering voice services under LTE coverage. 3GPP also

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defined single radio voice call continuity (SRVCC) to support the voice service.
This comes into a picture when a caller who has made call in LTE network and
going out to GSM or WCDMA.
 Voice services supported by circuit switched technology: 3GPP has defined a
function named circuit switched fall back (CSFB) for combining EPC supporting
LTE and circuit switched services like 3G services. CSFB is an alternative
solution to IMS and SRVCC to provide voice services to LTE users. CSFB based
on the fact that LTE users are registered in circuit switched domain when
powered ON and attaching to LTE. This is done through interaction between
MME and MSC server in circuit switched domain. There are two cases we can
consider here. In first case, when a subscriber initiated a call in LTE network and
moving out of LTE to GSM, UMTS or CDMA network. In this case, packet
services can either hand over to GSM, UMTS or CDMA network but on lower
data rate or suspended until voice call is completed. In second case, if an
incoming call is coming to a subscriber’s device which is currently attached to
LTE. In this case, MSC will request the paging in LTE through the interface
between MSC and MME. The mobile after receiving page, on temporary basis,
switches from LTE to circuit switched domain. Once the call terminates, the
mobile device attaches back to LTE.

4.3 Message services
Like voice services, EPC either uses IP based solution (SMS over IP based on
IMS) or circuit switch technology which is normally used to deliver SMS over
GSM and CDMA.
In case of IMS, sending a message from server to client is very transparent and
the message is just treated like as an IP packet. There are no specific features
required in EPC for that.
In case of circuit switching, the MME interacts with MSC which further
connected to messaging center via control channels in GSM or CDMA and by
interaction with MME, this solution can be used for LTE. Then these messages
are included in NAS signalling messages (which is between MME and mobile
device) and delivered to the destination subscriber. Note that this solution
supports only SMS text services because multimedia messages are based on IP.
The following figure 8 shows the message service flow in both above mentioned
solutions. The dotted lines express SMS transmission using signalling interfaces
whereas solid lines refer to message over IP.

Page | 13

SMSC
Messaging over
IP application

MSC SGSN
SAE Gateways

Mobile MME
GSM/CDMA device

LTE

Figure 8- Flow of message services via circuit and IP domain

5. Major functions of EPC

5.1 Authentication and security
The 3GPP TS 33.401 divides the EPS security architecture into different groups and
domains. Each domain has its own threat and security solutions. These domains are as
follows and shown in following diagram 9:
a. Network access security
b. Network domain security
c. User domain security
d. Application domain security
e. Visibility and configurability of security

Page | 14

d

a
Mobile a E-UTRAN
Terminal b Services
EPC Home
Network

USIM a

Figure 9- Different security domains

The security domains related to EPC are Network access security and Network
domain security. We will discuss these in a brief manner.
Network access security: Network access security means providing a user a secure
access to EPS. In UMTS, a new concept named mutual authentication was
introduced, which was later developed in LTE, in which UE (User Equipment) and
network authenticate each other. In addition to mutual authentication, it includes
protection of signalling traffic and user traffic. Now here we will try to figure out the
authentication and security process in E-UTRAN (evolved universal terrestrial radio
access network which is a work item under which 4G access network was developed)
only and role of EPC in that. Mutual authentication which is between UE and MME
is based on the fact that both USIM card (universal subscriber identity module) and
network have access to same security key K. This key K is permanently stored in
USIM and HSS/AuC. In LTE networks, terminals have provision to use same SIM
card which was in use in UMTS (i.e. USIM). This key is not visible to end user.
During authentication procedure, many keys are derived from key K and these keys
are used for ciphering and integrity protection of user plane and control plane traffic.
The mechanism for authentication as well as key generation in E-UTRAN is called
EPS authentication and key agreement (EPSAKA).
When a user attaches with EPS via E-UTRAN access then the MME sends the IMSI
to HSS. HSS looks up key K and a sequence number (SQN) associated with that
IMSI. HSS/AuC then uses crypto functions and key derivation functions and
generates EPS AV (EPS authentication vector). EPS AV includes KASME, XRES

Page | 15

(Expected Result), a network authentication token (AUTN) , RAND and ciphering
and integrity keys (CK and IK). HSS/AuC sends EPS AV to MME. Mutual
Authentication in E-UTRAN is performed using the parameter RAND, AUTN and
XRES. MME then forwards the AUTN and RAND to the terminal via eNODEB. The
USIM in terminal calculates its own version of AUTN using its own key K and SQN
and then compare it with AUTN received from MME. If these are equal to each other
in values then it means USIM has authenticated the network. Now USIM generates a
response key (RES) by using cryptographic functions with key K and RAND as input
parameters. It sends RES back to MME. The MME authenticate the terminal by
verifying that RES is equal to XRES. This completes the process of mutual
authentication. The following diagram 10, in brief manner, shows the flow of these
messages.

Attach request IMSI

KASME,
AUTN, XRES,
KASME, RAND
Terminal E-UTRAN MME HSS/AuC
AUTN, RAND

RES

Figure 10- Flow of Authentication process messages

Network domain security: When GSM was developed, as it was controlled by small
number of larger institutions, the threat to user traffic was not perceived at all.
Because as GSM is circuit switched network, the interfaces and the protocols it is
using are specifically for circuit switched network only and only the big telecom
operators have access to those interfaces and protocols. But with the introduction of
GPRS, IP architecture was introduced. Now user and control traffic run over more
open and accessible protocols. So there, a need came up which required the security
of the traffic. 3GPP developed some specifications about how the IP based traffic is
to be secured in core network or between different core networks. These
specifications are referred as Network domain security for IP based control planes
(NDS/IP). In this specification, a new concept was introduced named as security
domain that would be managed by single administrative authority. It makes sure that
the level of security and available security services will remain same within a security
domain. An example of the security domain could be the network of the single
operator. Security gateways (SEGs) are placed on border of the security domains to
protect the control plane traffic that passes in and out of the domain. All IP traffic
from network entities is routed via SEGs before entering in and existing out of
network. The traffic between SEGs is protected via IPsec protocol (IP security

Page | 16

protocol). To set up the IPsec security sessions, Internet key exchange (IKE)
protocols are used. This is shown in the following figure 11

Security Domain A Security Domain B

Network Entity A Network Entity A

SEG A SEG B

Network Entity B Network Entity B

Intra-domain IPsec SA

Intra-domain IKE connection

Inter-domain IPsec SA

Inter-domain IKE connection

Figure 11- Example of two security domains by employing NDS/IP

The end to end path between two network entities in two security domains is
protected in hop by hop form. Because the operator may choose the IPsec to protect
the traffic between two network entities or network entity and SEG in a single
security domain.

5.2 Policy and charging control and QoS

On the top of EPS bearer, LTE can make use of extensive policy management
architecture. This architecture provides a very fine control over user and services it
provides. The policy architecture is shown below in figure 12.

Page | 17

Application
SPR function
Sp
Rx
PCRF

Gx Online charging
SGi Gy system
External PCEF
Network
s Gz
PGW Offline charging
system

Figure 12- Policy architecture

The Subscription profile repository (SPR) contains information such as user specific
policies and data. Online charging system is credit management system for prepaid
charging. Network operators can offer prepaid billing and usage tracking in near real
time. The policy enforcement function (PCEF) interacts with offline charging system
(which receives events from the PCEF and generates charging data records (CDRs)
for the billing system) on Gy interface to check out credit and report credit status. The
PCEF is located in the PDN-GW which makes PDN-GW a logical element to
perform traffic management functions such as deep packet inspection. PCEF enforces
gating and QoS for individual IP flows on the behalf of the PCRF. It also provides
usage measurement to support charging. The PCRF (Policy and rule function)
provides policy control and flow based charging control decisions. It receives session
information from Application function (AF) over Rx interface, subscription
information from SPR over Sp interface as well as information from the access network
via the Gx. It takes all the information and configured operator policies then creates a
service session level policy decisions which are being enforced by PCEF. The
Application function here represents the network element that supports applications
that require dynamic policy or charging control.

3GPP has defined an extensive ‘bearer model’ for EPS. Whenever user equipment
attaches to a LTE network at each time LTE assigned a bearer to the UE for
communication. “An EPS bearer is the level of granularity for bearer level QoS
control in the EPC/E-UTRAN. The decision to establish or modify a dedicated bearer
can only be taken by the EPC, and the bearer level QoS parameter values are always
assigned by the EPC. The bearer levels per QoS parameters are QCI (Qos class
identifier), ARP (Allocation and Retention Priority), GBR (Guaranteed Bit Rate),

Page | 18

MBR (Maximum Bit Rate), and AMBR (Aggregate Maximum Bit Rate)” 4 .
According to this model, the services can be allocated a particular bearer and each
EPS bearer has assigned one of the QCI. QCI defines parameters like bit rate, packet
loss and delay. The following figure 13 depicts the EPS bearer model:

Corporate
Default QCI9 APN 3 network
PDN-
GW
UE E-NODEB SGW
Dedicated QCI3 PDN- APN 2 Internet
Dedicated QCI2 GW
Dedicated QCI1
APN1

IMS
operator
services

Figure 13- EPS bearer model

In the above figure 13, EPS bearer assigned for voice has assigned QCI 1 which
means a dedicated bit rate, 100ms delay, 10-2 packet loss and priority 2 in overall
model. In total there are three different QCI classes specified in EPS and in most of
the cases operators prefer first class i.e. signalling, voice and data.

5.3 Packet routing
On the IP transport layer SGW act as a packet router. User plane packets are
forwarded transparently in upper link and downlink direction and their underlying
transport units are marked by SGW with parameters like DiffservCode point based on
QOS indicator of the associated EPS bearer.

5.4 Mobility management
In LTE, mobility management can be divided based on mobility state of the user
equipment. These are LTE_detached, LTE_IDLE, LTE_ACTIVE. If UE is in
LTE_ACTIVE state, it is registered with the MME and has RRC (Radio resource
control) connection with eNODEB. The HSS has very clear information about to
which cell the UE belongs and MME can transmit/ receive data from UE after getting
location information from home subscriber server via eNODEB. In second state,
when UE is in LTE_IDLE state, UE has no air-interface connection with eNODEB to
4
Farooq Bari, SAE and Evolved Packet core, Seattle communications (COM-19) society chapter, 2009,
http://www.ee.washington.edu/research/ieee-comm/event_nov_13_2008_files/IEEE%20-
%20SAE%20and%20Enhanced%20Packet%20Core.pdf.

Page | 19

save power consumption of the battery and reducing signalling traffic to MME. It can
change its cell in same tracking area without informing the EPC. From logical point
of view, the connection is still established and all logical bearers’ remains in place. It
means that the IP address allocated to UE by PDN-GW remain in place, in case a
mobile device wants to send IP packet. When there is IP packet arrives for UE in
IDLE state, it can be routed through core network up to the SGW. But as SGW has no
S1- user data tunnel then it requests MME to re-establish the tunnel. On the other
hand MME knows only about the TA. It send paging request to every cell of TA. The
eNODEB forwards that message to mobile device over air interface and when mobile
device responds to the paging message then S1 tunnel re-establishes. MME contacts
the SGW via S11 interface which then forwards the waiting IP packets to the mobile
device.

5.5 IP address allocation
In LTE-EPC networks, on basic level, one of the following ways are used to allocate
the IP addresses to user equipment
 If UE is in its home network then its local HPLMN (Home public land mobile
network)allocates IP address when the default bearer is established
 If UE is in visitor network, then VPLMN (visitor public land mobile network)
allocates IP address when the default bearer is established
 The PDN operator allocates IP address to UE when default bearer is activated

In LTE-EPC network, packet data network (PDN) types IPv4, IPv6 and IPv4v6 are
supported. EPS bearer of PDN type IPv4v6 may be associated with one IPv6 prefix
only or both IPv4 address and one EPS bearer of PDN type IPv4and IPv6 is
associated with IPv4 addresses and IPv6 prefix respectively. During a PDN
connection establishment, UE sets the requested PDN type that may be pre-
configured in the device per APN or otherwise it sets the PDN types based on its IP
stack configuration i.e. if UE supports both IPv6 and IPv4 then it can request for PDN
type IPV4 and IPv6, if UE supports only IPv4 or IPv6 then it can request for IPv4 or
IPv6 respectively and in case if UE’s TP version capability is unknown then UE can
request for IPv4v6.
In EPC, HSS stores the one or more PDN types per APN in the subscription data.
During the PDN connection establishment procedure, MME compares the requested
PDN type to the stored PDN type in HSS and set the PDN type as follows
 If the requested PDN type is allowed by the HSS then MME sets the PDN type as
requested
 If UE is requesting PDN type IPv4v6 and subscription allows only IPv4 only
then MME sets the PDN type IPv4 and send the reason back to UE. The
procedure is same in case when only IPv6 is allowed
 If in the subscriber data of UE, It is not allowed any PDN type then the request
send by the UE will be rejected by MME
 If the UE requests PDN type IPv4v6 and both IPv4 and IPv6 PDN types are
allowed but not IPv4v6 then MME shall set the PDN type to IPv4 or IPv6

Page | 20

PDN-GW also plays a role during allocation. It may restrict the usage of PDN type
IPv4v6. This is discussed in the following:
 If UE send on request of PDN type of IP4v6 but the PDN-GW operator
preferences dictate the use of IPv4 addressing only or IPv6 prefix only for this
APN then PDN type will change to single address i.e. either IPv4 or IPv6 and
reason cause shall be returned to UE
 In case when MME does not set the dual address bearer flag to support
interworking with nodes and UE requests PDN type IPv4v6 from PDN-GW then
PDN type will be changed to single version and reason shall be returned to UE

Page | 21

Conclusion
It is very much clear from the study of EPC, which is developed under a work item
named SAE, is a major achievement carried out by 3GPP and its partners. 3GPP achieves
the three main objectives set by it before the start of this SAE project in December 2004.
SAE work successfully delivered an evolved packet only core for the next generation of
mobile broadband access. Interworking with other access technologies like GSM or
UMTS and CDMA is another major breakthrough. By interworking the EPC network can
be shared across a wide community. This also opens a path of global roaming. Now a
user can access and use the services everywhere with his/her mobile equipment. The
global uptake of single technology assures more competition among different equipment
vendors and results in cost efficient network equipment and solutions.

Page | 22

References
[1] Olsson, M., Sultana, S., Frid, L. & Mulligan,C. (2009). SAE and Evolved
packet core: Driving the mobile broadband revolution. Oxford, UK:
Elsevier Ltd.

[2] Sauter, Martin. (2011). From GSM to LTE: An Introduction to mobile
networks and mobile broadband (pp. 205-274). West Sussex, UK: John
Wiley & sons.

[3] Faroor, Bari. (2009). SAE and Evolved Packet core, Seattle
communications (COM-19) society chapter. Retrieved from
http://www.ee.washington.edu/research/ieee-
comm/event_nov_13_2008_files/IEEE%20-
20SAE%20and20Enhanced%20Packet%20core.pdf.

[4] 3GPP, Technical Specification Group Services and System Aspects;
Network Architecture (Release 9), TS 23.002.

[5] 3GPP, Technical Specification Group Services and System Aspects;
System Architecture Evolution; Security Architecture (Release 11), TS
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[6] Brown, Gabriel (n.d). Heaving Reading on behalf of Cisco: Evolved
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_Policy_Management_WP.pdf

[7] Alcatel-Lucent(2009): Introduction to Evolved Packet core: White paper,
http://lte.alcatel-
lucent.com/locale/en_us/downloads/wp_evolved_packet_core.pdf

[8] Fritze, Gerhard. (2008). SAE- The Core Network for LTE, Ericsson.
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[9] Motorola (2007): Long Term Evolution (LTE): A Technical overview:
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Service%20Providers/Wireless%20Operators/LTE/_Document/Static%20
Files/6834_MotDoc_New.pdf

[10] IP Address Allocation. (2012, 07 26). Retrieved from
http://lte-epc.blogspot.com/2011/07/ip-address-allocation.html

[11] Jain, Raj. (2008). Wireless cellular architecture: 1G and 2G. Retrieved
from http://www.cse.wustl.edu/~jain/cse574-08/ftp/j_fwan.pdf

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[12] LTE SAE System Architecture Evolution (n.d). Retrieved from
http://www.radio-electronics.com/info/cellulartelecomms/lte-long-term-
evolution/sae-system-architecture-evolution-network.php

[13] Rappaport, Theodore. (2002). Wireless Communication Principle and
Practise. Upper Saddle River, NJ 07458: Prentice-Hall Inc.

[14] Kurniawan, Yousuf. The development of cellular mobile communication
system. Retrieved from http://www.slideshare.net/yusuf_k/the-
development-of-cellular-mobile-communication-system

[15] GSM Glossary. Retrieved from
http://www.argospress.com/Resources/gsm/gsmbstatiocontro.htm

Page | 24

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