Studying IMS Signalling in LTE-based Femtocell Network

Int. J. on Recent Trends in Engineering and Technology, Vol. 11, No. 1, July 2014 Studying IMS Signalling in LTE-based Femtocell Network Sinan Ghassan Abid Ali 1, Mohd Dani Baba 2, Mohd Asri Mansur and Labeeb Mohsin Abdullah Centre for Computer Engineering Studies Faculty of Electrical Engineering, Universiti Teknologi MARA (UiTM) 40450 Shah Alam, Selangor Darul Ehsan, Malaysia sinan79alnasir@gmail.com 1, mdani074@salam.uitm.edu.my 2, pakngah60@gmail.com, labeeb96@yahoo.com Abstract Integrating femtocells with mobile networks is a challenging issue and different approaches have been proposed to achieve this convergence since the standard is still under development. One of these proposed methods is SIP/IMS-based integration. In this paper, the main focus is to overview and highlight on the technical issues on the standardization status and technological trends related to bridging between femtocells and LTE network using the IMS signalling. Index Terms IMS, Femtocell, SIP, LTE, Convergence I. INTRODUCTION The operator of the Next Generation Network (NGN) will need to support the suitable interface over the Internet in terms of scalability and security for the femtocell environment. The femtocell which is also known as Home Base Station has been adopted recently as a suitable solution for indoor coverage due to its feature of low power, low-cost, and short-range. In fact, femtocells have been installed by consumers as data access points for better indoor data and voice coverage since studies on wireless usage shows that more than 50% of all voice calls and more than 70% of data traffic are originated from indoors [1]. There are three types of network interfaces which have been proposed; cellular based interface (lu-b over IP), a Radio Access Network (RAN), gateway based Unlicensed Mobile Access (UMA) interface and the IP Multimedia Subsystem (IMS) based interface to securely connect the femtocell with the operator network [2]. The IP Multimedia Subsystem (IMS), standardized by the 3rd Generation Partnership Project (3GPP) [3][4], is the most promising candidate to replace legacy, voice-dedicated mobile networks with an All-IP technology [5] since many fixed and mobile network operators have decided to move their telecommunication networks towards an All-IP infrastructure where voice loses is controlled and becomes just one among many available services. The available standards illustrate the IMS functionality in the regular wireless networks such as in macrocell. However, femtocell is not yet accommodated by the current standard since it is a new promising technology for wireless network environment. The key advantages of the IMS in contrast with traditional IP networks are in the IMS infrastructure which allow the rapid deployment and integration of modern IP-based services, the pledged network end-to-end Quality of Service (QoS) and very flexible billing and charging while still maintaining compatibility with the available applications [6]. Furthermore, the new IMS horizontal architecture of services will join the traditionally separated packetswitched (PS) with circuit-switched (CS) networks, thus unifying both networks into one single network for all services, and that will have a vast impact on the entire way of designing, developing and deploying the DOI: 01.IJRTET.11.1.1379 Association of Computer Electronics and Electrical Engineers, 2014

future cellular business applications [6]. The remainder of this paper is organized as follows: Section II summarizes the related work in the environment of IMS and femtocell. Section III describes the notion of femtocell technology. The IMS architecture and technology are described in section IV. Section V explains the IMS and femtocell proposed integration. Finally, the conclusion is given. II. RELATED WORK Although there are many studies and scientific research in the area of either femtocell or IMS, only few studies have been done in the area of interworking between IMS and femtocell. The work in [7] provides the Session Initiation Protocol (SIP) based IMS signalling in Long Term Evolution (LTE)-femtocell network. The author proposed a particular signalling call flows for session establishment procedure. In addition, the signalling performance was intended to be determined by means of delay properties. Authors in reference [8] proposed two mechanisms for the integration of the femtocell system with IMS. However the networks used for testing the proposed mechanisms were World interoperability for Microwave Access (WiMax) and Universal Mobile Telecommunications System (UMTS). These authors had calculated the system delay for different scenario of networks. In [9], the authors proposed a mechanism to exchange media between femtocells directly in conjunction with IMS. Their experiment was based on CDMA2000 network and they measure the end-to-end system delay for the standard and the proposed scenario. The implementation of internet protocol multimedia subsystem for femtocell network is studied in [10]. The authors proposed an application plan of femtocell network. The proposed method based on IMS network for circuit switching domain using the access list. In addition [11], the open IMS testbed for exploring wireless service evolution and network architecture evolution towards System Architecture Evolution (SAE) and LTE was carried out. The work addressed the impacts of the LTE/SAE in IMS standardization. The authors proposed to deliver some service via the PS IP core and IMS which previously delivered over the CS core network in UMTS. III. FEMTOCELL CORE TECHNOLOGY Femtocell is a data access point installed by consumer in order to get a better indoor coverage for data and voice services. Femtocells have many benefits as compared to other infrastructures (such as distributed antenna and microcells) and can be summarized as follows: 1. Femtocells have better capacity and coverage, they can reduce the transmit power, attain higher signalto-interference plus noise ratio (SINR), at the same time prolong the battery life of handset since they shorten the distance between the transmitter (femtocells) and the receiver (UE) [2]. This will lead to higher capacity and an improved reception in term of consumer requirements. 2. Improve the reliability of macrocell, since femtocell network will absorb the indoors traffic over the IP backbone. Hence, the macrocell base station can redirects its resources in order to provide improved reception for the mobile users. Cost issue is an important factor for every network operator since it affects network establishment and deployment. Femtocell will reduce the capital and operating expenditure costs for the network operator. While for the microcell and distributed antenna will increase the expenditure costs due to the installation and maintenance of towers and the backhaul connection. A. Femtocell Interface Standardization work done by 3GPP is focusing on femtocell in LTE networks as defined by (3GPP TR R3.020, Rel-8). Fig. 1 shows a femtocell base station also known as Home evolved NodeB (HeNB) connection to LTE network. The femtocell is connected directly to the Mobility Management Entity (MME) or through the Femtocell Gate Way (FGW). If there is a FGW, then the femtocell and the FGW have the same S1-MME interface. As for X2 is a point to point interface between two base stations enb within the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), however the interface X2 between femtocell and enb base station is not supported by the current standards [12]. 103

Figure 1. Femtocell interface in LTE network B. Femtocell Network Infrastructure The operator is required to provide a secure and scalable interface over the Internet to support the femtocells environment. Three network interfaces have been proposed in order to connect the femtocell to the operator network namely the Unlicensed Mobile Access (UMA), the IMS/(SIP), and the lu-b over IP. 1. UMA: is a radio access network getaway that sits between the operator network and the IP and UMA task is to aggregate the traffic from femtocell. The gateway uses a standard lu-ps/cs interface in order to connect to the operator network. The UMA protocols will enable a secure IP tunnelling between the RAN gateway and femtocell for carrying the signals of femtocell over the Internet. 2. IMS/SIP: the IMS/SIP interface provides a core network reside between both the operator and femtocell. The interface of the IMS use a voice over IP (VOIP) implementation based on a SIP 3GPP standardized implementation, and runs over the standard IP [13]. 3. lu-b over IP: this technique is to connect femtocell to Radio Network Controllers (RNCs) through standard lu-cs interface present in macrocell networks. The advantages and disadvantages of the above methods are out of the scope of this study. IV. IMS CORE TECHNOLOGY IMS is a system designed to support a rugged multimedia services over diverse access technologies and across roaming boundaries. The IMS will provide ubiquitous cellular access to all service that the Internet provides since it is the key component in the 3G architecture. A. The IMS Layered Architecture The architecture of IMS which is based on horizontal service architecture will bridge the traditionally separated packet switching with circuit switching networks, merging both technologies into a single network for the whole services. This will affect the design and development of mobile application and pose huge impact on both mobile operator and consumer. The protocol architecture of IMS contains three layers [14] as shown in Fig. 2. 1. Access/Transport layer makes the IMS independent from the transport protocols by providing an access interface to all users in order to connect to the IMS network. 2. Session control layer supports functionalities that register the user equipments in the network and initiate the sessions. IMS have three types of session known as Call Session Control Function (CSCF): a. The Proxy Call Session Control Function (P-CSCF): is a SIP proxy server that supports the user equipment with the first contact point. The main task of the P-CSCF is to route signalling messages to and from the S-CSCF in the home network. A SIP proxy server is a network component used for forwarding SIP request sent to it. b. The Interrogating Call Session Control Function (I-CSCF): is also a SIP proxy server which is used as a gateway to the home network from the visited network, its hide the network topology and capacity from outside. The main function of the I-CSCF is to find a suitable S-CSCF during communication set-up and query the Home Subscriber Server (HSS). c. The HSS is a central database which saves all user related data, such as security, user profile information and location. 104

Figure 2. IMS architecture d. The Serving Call Session Control Function (S-CSCF): is a major node for SIP signalling. It is the S-CSCF that plays the roles of SIP Register server and SIP proxy server, registering user information with the HSS, and routing SIP messages to other user equipment or appropriate application server. 3. Application/Service layer (AS): its purpose is to execute various IMS services and to provide the end user service logic by using distinct application servers. B. The IMS Signalling The signalling in IMS is based on SIP protocol. SIP is a textual request/response protocol which used to control everything that a subscriber does (such as voice, messaging, and data transfer). There are three kinds of SIP messaging in IMS, i.e., session based messaging, immediate messaging and deferred delivery messaging. Session establishment is an element of session based messaging. The process involves an end-to-end signalling message exchange [7]. Session established by (INVITE OK) request/response and terminated by (BYE-OK) request/response. SIP defines three types of transaction: regular, INVITE-ACK, and CANCEL. The regular transactions are initiated by any request except (CANCEL, INVITE, and ACK). The INVITE-ACK transaction is a pair of two request generated by the client. The CANCEL transactions are to cancel the previous transaction (such as INVITE transaction). The UE need to register to the IMS network, the registration procedure initiated when the UE turn on and sends a SIP Register request to the P-CSCF since it is the first entry point in the IMS. The P-CSCF will forward the request to the I-CSCF which in turn contacts the HSS to find which S-CSCF it should forward the request. The exchange of request/response messages (User-Authorization-Request/Answer (UAR/UUA)) between I-CSCF and HSS will enable the I-CSCF to select a specific S-CSCF and the Register request is forwarded to this specific S-CSCF. Due to receiving this request, S-CSCF will query the HSS to exchange authentication information that support the user of UE. The request/response messages exchanges (Multimedia Auth-Request/Answer (MAR/MAA)) between S-CSCF and HSS will enable the S-CSCF to challenge the UE by sending unauthorized response. The UE will authenticate itself by sending another Register request containing the challenge answer, hence the S-CSCF will successfully register UE which mean creating a binding between the public identity and the contact address of UE. Additionally, the S-CSCF will update the information in HSS to indicate that the user of UE has now been registered. The S-CSCF will send back a 200 (OK) response to UE to indicate that the registration procedure has succeeded. V. THE IMS FEMTOCELL INTEGRATION There are two methods for the integration of IMS network with the Femtocell Access Point (FAP). First, the direct integration between FAP and IMS network by using all IP connectivity (IMS-based Femtocell). Secondly, the integration of the IMS network to FAP through the mobile core network (cellular based femtocell) [7][8]. Femtocells will be integrated into the mobile operator s network to ensure optimal performance and to allow seamless service. This integration will be accomplished by IMS as shown in Fig. 3. 105

Figure 3. IMS/SIP model Fig. 4 shows the connectivity of FAP to the IMS network with the IMS-based Femtocell and the Cellular based Femtocell. In the latter case, the broadband connection that connects the FAP to mobile core network can be a DSL or WiMax for example. Figure 4. Femtocell - IMS model. (Red box (on the right side): IMS-based Femtocell, Blue box (on the left side): Cellular based Femtocell) 106

Using the IMS based femtocell integration is better than using the cellular based femtocell integration since the former has many advantages compared to the latter. These advantages can be summarised as follows: firstly, there is no need to upgrade the mobile operator network since the FAP has the functionality of RAN. Secondly, by using All-IP network will reduce the capital and operating expenditure of the network operator. Thirdly, it will reduce the load on the mobile operator network since the RAN is offloaded. There are two scenarios for the UE when it connect to the IMS in spite of the integration method, either the UE is trying to get services through the IMS or the UE is try to connect to another UE through the IMS. In the first scenario the signalling exchanges between UE and the AS depend on the type of AS, therefore the AS will consider as a presence service. The UE will do a registration procedure as shown in Fig. 5. Then the UE will send a Subscribe request to the FAP which forward it to the P-CSCF, I-CSCF, and S-CSCF, the request gets routed to IMS sessions like any other request and finally the S-CSCF forward the request to the AS. The AS (presence server) accept the subscription (by send 200 OK response to the UE) and deliver an initial state of the subscription (by sending notify response to UE). The UE will inform the AS about the acceptance of initial state by sending a 200 OK. Nevertheless, different types of service could be implemented in AS which follow similar request/response procedure. In the second scenario, when UE1 want to contact UE2, there are two sub-scenarios: either both the UEs are belong to the same IMS network (home), or the UEs are in different IMS networks (home and visited). Both Fig.6 and Fig.7 show the SIP signalling for the two sub-scenarios. In the first sub-scenario, both UEs are registered to the S-CSCF of the home network (or originated network). UE1 will send Register request to FAP1 after it establishment a wireless link with FAP1. FAP1 forward the request to P-CSCF, I-CSCF, and S-CSCF. The registration procedure implicit message exchange between I- CSCF and HSS (to select the specific S-CSCF) and message exchange between S-CSCF and HSS (to exchange information that support the authentication between the UE and the home IMS network). The UE2 follows the same procedure. When the UE1 and UE2 successfully register to the home IMS network then each UE contact address and public identity are stored in HSS. UE1 will send a call request (INVITE) to UE2 through IMS network. Since Both UEs belong to the home network (originated network), then they are sharing the same P-CSCF, I-CSCF, and S-CSCF. The INVITE request will forward to FAP1, P-CSCF, S-CSCF, and FAP2 (I-CSCF not includes since the S-CSCF address is known during the registration process). FAP2 forward the request to UE2 and at the same time FAP2 send 180 ringing responses (which indicate that a successful resource reservation on the wireless link) to UE1 through P-CSCF, S-CSCF, P-CSCF, and FAP1 which forward this response to UE1. During the ringing, the UE2 generate the 200 OK responses and forwarded to UE1 through the same path (i.e. FAP2, P-CSCF, S- CSCF, P-CSCF, and FAP1), the FAP1 response to this signalling and send ACK to FAP2 through the IMS session components. A media plane will initiated after the signalling plane has been completed the preparation for connecting the two UEs and the two UEs are able now to communicate and share a video or audio sessions. The connection between the two UEs is ended when one of the UE send a BYE request to the second UE, and the second UE response with OK response signal. In the second sub-scenario, each UE are registered to different IMS network, UE1 is belong to the originated network (home) while the UE2 is belong to Terminated network (visited). Each UE will register to its IMS network by following the registration procedure, UE1 register to the originated network and UE2 register to the terminated network. After the registration process complete, the UE1 send a call request (INVITE) to FAP1 which forward the request to P-CSCF and S-CSCF (I-CSCF not include since the S-CSCF address is known during the registration process) of the originating network. The S-CSCF need to forward the request to the S-CSCF of the terminated network and since S-CSCF of originated network don t know the address of S-CSCF of the terminated network, therefore its forward the request to the I-CSCF of the terminated network which will contact the HSS (in terminating network) and find which specific S-CSCF of the terminated network that serve the UE2. The request will forward to S-CSCF of the terminated network by the I-CSCF after finding its address, and the S-CSCF of the terminated network will forward the request to P-CSCF (in terminating network), which it forward further to the FAP2. FAP2 send the request to UE2 and in the same time generate a ringing signal and send it to UE1 through the same path (i.e. P-CSCF, S-CSCF of terminated network then S-CSCF, P-CSCF of originated network then to FAP1). During the ringing, the UE2 generate the 200 OK responses and forwarded to UE1 through the same path (i.e. FAP2, P-CSCF, S-CSCF of terminated network then S-CSCF, P-CSCF of originated network then to FAP1). The FAP1 response to this signalling and send ACK to FAP2 through the IMS session components. The signalling plane has prepared the connection between both UEs and therefore a media plane is established between them. Both UEs are able to communicate and share an audio or video session using the media plane. The connection is terminated 107

UE1 FAP1 P-CSCF I-CSCF HSS S-CSCF AS (1) Register (2) Register (3) Register (4) UAR (5) UAA (6) Register (7) MAR (8) MAA (12) 401 Unauthorized (11) 401 Unauthorized (10) 401 Unauthorized (9) 401 Unauthorized (13) Register (14) Register (15) Register (16) UAR (17) UAA (18) Register (19) SAR (20) SAA (24) 200 OK (23) 200 OK (22) 200 OK (21) 200 OK Registration Process (25) Subscribe (26) Subscribe (27) Subscribe (28) Subscribe (29) Subscribe (34) 200 OK (33) 200 OK (32) 200 OK (31) 200 OK (30) 200 OK (39) NOTIFY (41) 200 OK (42) 200 OK (43) 200 OK (44) 200 OK Figure 5. Signalling between UE and AS when one of the UE send a BYE request which is a signal to end the connection, the other UE will response with OK signal. VI. CONCLUSIONS Femtocell is the new network technology that can support today s increased in demand for indoor communication services since the data and voice traffics are mostly originated indoors. This new technology will enable the network operator to reduce their expenditure cost and improve their consumer s satisfaction. IMS is a platform where all mobile networks can be merged into this platform and hence enable the convergence of mobile telecommunication services. Connecting femtocell to LTE network using the IMS connectivity platform is the positive step towards enabling seamless connectivity between the cellular world with Internet world and making all the applications and services available at anytime, anywhere and any access platform. 108

Figure 6. Signalling between UE1 and UE2 in same home IMS network ACKNOWLEDGMENT The authors would like to thanks the Ministry of Science, Technology and Innovation (MOSTI), Malaysia for providing the Science Fund grant under the Project No: 01-01-01-SF0551. 109

Figure 7. Signalling between UE1 (originating network) and UE2 (terminating network) REFERENCES [1] Presentations by ABI Research, Picochip, Airvana, IP.access, Gartner, Telefonica Espana. Home Access Points and Femtocells. Second International Conference 2007. (http://www.avrenevents.com/dallasfemto2007/presentations.htm ) [2] Chandrasekhar, V., J. Andrews, and A. Gatherer. "Femtocell Networks: A Survey." IEEE Communication Magazine, September 2008. [3] Overview of 3GPP Release 5 V0.1. (2010). Available: (http://www.3gpp.org/ftp/information/work_plan/description_releases/ ) 110

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