Overview of the Evolved packet core network

Transcription

1 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: Department of Electrical and Computer Engineering, University of Alberta. Page

2 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 Page i

3 Table of contents 1. Introduction to Evolved packet core networks (EPC) Overall cellular system architecture Background of development of EPC Objectives set by 3GPP for EPC EPC architecture MME Serving gateway (SGW) Packet data network gateway (PDN-GW) Home subscriber server (HSS) Interworking with 2G and 3G technologies Interworking between LTE and GSM or WCDMA networks Interworking with LTE and CDMA networks Major services of EPC Data services Voice services Message services Major Functions of EPC Authentication and security Policy and charging control and QoS Packet routing Mobility management IP address allocation...20 Conclusion...22 References...23 Page ii

4 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 GTPv Figure 6- Interworking of LTE with CDMA networks Figure 7- Application and services on mobile broadband Figure 8- Flow of message services via circuit and IP domain Figure 9- Different security domains Figure 10- Flow of Authentication process messages Figure 11- Example of two security domains by employing NDS/IP Figure 12- Policy architecture Figure 13- EPS bearer model Page iii

5 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. Mobile Station BTS BSC HLR MSC VLR 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 Page 1

6 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. Page 2

7 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 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 highlevel 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 GSM/GPRS Circuit core domain WCDMA/HSPA LTE User Domain IMS domain Non-3GPP Packet core domain RAN Domains Figure 2- Architecture Domains by 3GPP Core network domains 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. Page 3

8 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. PDN-GW SGi Internet CP S5 UP HSS S6 MME S11 SGW CP S1 UP enodeb enodeb Mobile Device Figure 3- Basic EPC architecture for LTE In packet domain, it consists of: Page 4

9 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. Page 5

10 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 Page 6

11 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 S 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 Page 7

12 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. HSS External Networks GGSN HLR Gn SGSN Gr Gn S6a SGi PDN Gn Gn Gb Iu S5/S8 MME S11 SGW GSM WCDMA S10 S1-MME 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 Olsson,M., Sultana, S., Frid, L. &Mulligan,C.(2009). SAE and Evolved packet core: Driving the mobile broadband revolution. Oxford, UK: Elsevier Ltd. Page 8

13 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 S16 GSM SGSN S6d S6a S5/S8 S4 Gb Iu S3 MME S11 WCDMA S10 S1-MME SGW S1-U 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 Page 9

14 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 HSS S6a SWx S10 AAA S6b STa SGi PDN-GW Gx PCRF S5/S8 S2a Gxa Gxc MME SGW S1-C S1-U S103 HSGW S101 S102 enodeb ehrpd Figure 6- Interworking of LTE with CDMA networks Page 10

15 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 Olsson,M., Sultana, S., Frid, L. &Mulligan,C.(2009). SAE and Evolved packet core: Driving the mobile broadband revolution. Oxford, UK: Elsevier Ltd. Page 11

16 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 Application level communication Application IP IP in point to point link Gateway Routing of IP packets IP Radio Mobile 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 Page 12

17 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

18 SMSC Messaging over IP application MSC SGSN SAE Gateways GSM/CDMA Mobile device MME 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 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

19 d Mobile Terminal a E-UTRAN a b EPC Home Network Services 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 K ASME, XRES Page 15

20 (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 K ASME, Terminal E-UTRAN MME AUTN, XRES, K ASME, RAND 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

21 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

22 SPR Sp Application function Rx PCRF External Network s SGi PCEF PGW Gx Gy Gz Online charging system 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

23 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: Default QCI9 PDN- APN 3 GW UE E-NODEB SGW Dedicated QCI3 PDN- APN 2 Dedicated QCI2 GW Dedicated QCI1 APN1 Corporate network Internet 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, %20SAE%20and%20Enhanced%20Packet%20Core.pdf. Page 19

24 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 preconfigured 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

25 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

26 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 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

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