Abstract:
A method and apparatus is provided for hand-off of a mobile User Equipment (UE) across a femto cellular network. The method includes dynamically clustering a plurality of neighboring femto cells in a vicinity of a first femto cell to which a mobile UE is currently attached. The dynamic cluster of neighboring femto cells is moved across the femto cellular network in accordance with movement of the mobile UE across the femto cellular network.

Description:
FIELD OF THE INVENTION 
     The invention relates to a system and method for supporting mobile communications over femto-cells as an alternative to conventional macro cells and in particular, to a system and method for supporting such communications in a communication system having an Internet Protocol (IP) multimedia subsystem (IMS). 
     BACKGROUND OF THE INVENTION 
     Due to users&#39; increasing demand for perpetual and ubiquitous access to the Internet, the end-to-end wireless/wireline communications network is gradually migrating towards a flexible wireless and wireline IP infrastructure that supports heterogeneous multimedia (e.g., voice, video, data and the like) services in an economical manner. Thus, the Third Generation Partnership Project (3GPP) has been developing the specifications and architecture of an IP multimedia subsystem (IMS) that augments the existing circuit switched &amp; 2G/3G wireless systems and expedites their gradual migration to an all IP infrastructure. IMS is built upon the open standard IP protocols defined by the IETF (Internet Engineering Task Force). It aims to serve as a ubiquitous IP service control and delivery platform for supporting all current services that existing circuit switched networks and the Internet offer as well as providing a vehicle for development and deployment of new services and applications in future. 
     The IMS technology defined by the 3GPP to provide IP Multimedia services over 3G mobile communication networks is set forth in 3GPP TS 23.228, Release 7 and TS 24.229 Release 8, which are hereby incorporated by reference in their entirety. IMS provides key features to enrich the end-user person-to-person communication experience through the integration and interaction of services. IMS allows new rich person-to-person (client-to-client) as well as person-to-content (client-to-server) communications over an IP-based network. The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between user terminals (or user terminals and web servers). SIP is an example of an application level signaling protocol used by an initiating device to locate another device in order to establish a communication session. The Session Description Protocol (SDP), carried by SIP signaling messages, is used to describe and negotiate the media components of the session. Other protocols are used for media transmission and control, such as Real-time Transport Protocol and Real-time Transport Control Protocol (RTP/RTCP), Message Session Relay Protocol (MSRP), Hyper Text Transfer Protocol (HTTP). 
       FIG. 1(   a ) depicts the simplified layered architecture of a standard IMS, showing the transport, control, and applications layers of the network. The transport layer transfers bits and packets of information across the end-to-end platform, which comprises a set of heterogeneous networks including, but not limited, to the existing public switched telephone network-(PSTN), the Internet, a 2G/3G cellular network, a WLAN (wireless local area networks), and so on. 
     The IMS control layer is responsible for setting up sessions and allocating necessary resources for supporting users&#39; services and applications. The IMS control layer primarily comprises the home subscriber server (HSS), call server control function (CSCF), and a set of edge controllers. The HSS, which stores and manages user subscription data, includes the IMS authentication, authorization, &amp; accounting-(AAA) server as well as its master user profile database. Essentially, the HSS provides the functions of a master AAA and its profile database, very much equivalent to those of the home location registrar (HLR) and the Authentication Center (AuC) in GSM networks. The CSCF entity, which implements the session initiation protocol (SIP), is the session management and call control engine of the IMS. A CSCF also acts as a SIP registrar receiving registration information (e.g., public identity, private identity, contact, etc.), and stores them in the HSS that serves as a master AAA and user profile server. The IMS edge controllers are entities that facilitate the interworking of the IMS subnet with the existing wireless and wireline networks. 
     The IMS application layer includes a set of application servers (ASs) where an AS hosts and executes one or more IMS services/applications. The CSCF uses SIP to interact with ASs and vice-versa. 
     The 3GPP has adopted a highly centralized architecture in the development of the IMS specifications. It uses centralized application servers to support the various services/applications and a centralized platform to provide control layer functions (e.g., presence, AAA, and mobility management) necessary for running applications or offering services. In addition, centralized call controllers are used for session origination/modification/termination, quality of service control, and collecting charging data records. A key strength of IMS, its network-centric centralized architecture, is also the source of its main shortcomings. On the one hand, it allows the IMS to serve as a platform for efficiently supporting many services. On the other hand, it makes the introduction of IMS into legacy networks quite complex and costly. Moreover, the scalability, resiliency, and management flexibility of IMS are suboptimal. 
     To alleviate the shortcomings of the centralized IMS approach while supporting identical or at least similar services, US Patent Publication No. US 2008-0235778 A1 and US Patent Publication No. US 2008-0235185 A1 set forth a distributed architecture comprising an interconnected set of Edge Convergence Server (ECONS) customer premises equipments (CPEs). Hereafter, the said distributed architecture is referred to as ECONS architecture, the CPEs are referred to as ECONS CPEs, and the resulting IP multimedia communication network is referred to as ECONS system or ECONS network, interchangeably. 
     The tenet of ECONS architecture is to move most of the functions of the IMS application and control layers, as appropriate, into ECONS CPEs, which are distributed across the subscribers&#39; homes and premises. Such a distributed architecture can reduce the cost of introducing IMS into legacy networks and improve the scalability, flexibility and resiliency of the overall system. 
       FIG. 1(   b ) depicts the simplified layered architecture of the ECONS approach in which many of the functions of the application layer and the control layer are moved to the ECONS CPEs at the user premises. The ECONS architecture is designed to provide an operator with a decentralized, low cost, scalable solution to deliver voice and multimedia services to its customers. Instead of relying on a costly and complex centralized core network, the ECONS architecture pushes the intelligence to the edge of the network by deploying ECONS CPEs on the subscriber&#39;s premises. The ECONS CPE provides session control functions and application servers, while the core network is limited to providing AAA services for the CPEs, and gateways to other networks. The functional ECONS CPE may be embodied in a residential gateway, set top box, media center or the like. 
     The following description sets forth some of the main characteristics of the ECONS architecture. First, the ECONS is a distributed networking system. Instead of relying on a centralized core network, the main functionality of the ECONS system is provided by the ECONS CPE. The ECONS CPEs provide user registration and session management, authentication, media gateway, application servers and call control functions for user equipments (UEs) such as cell phones and the like. The core network provides authentication for CPEs and interworking functions with other networks. Second, the ECONS architecture is scalable. As the number of subscribers increases, so does the number of deployed CPEs, thus growing as required. Third, the ECONS architecture can operate in a peer to peer as well as client server manner. Whenever possible, the communication happens at the edge of the network with minimal intervention by the network core. Signaling and media are transferred directly between CPEs, without being relayed by the core network. Fourth, the ECONS architecture provides advanced functions to its users with minimal involvement of its core network. In particular, several UEs can be associated with the same CPE. Fifth, as in IMS, the session management and control in ECONS system is performed using the SIP protocol. An ECONS system can interoperate with other SIP networks, including IMS. 
     In summary, an ECONS system is a distributed IP-centric control and signaling platform comprising a set of CPEs residing at the subscribers&#39; premises which provides voice, video, data, and multimedia services to fixed and mobile subscribers. Each ECONS CPE comprises presence, location, session management, resource management, AAA, NAT traversal, CPE management enablers as well as the standard TCP/IP stack, and possibly a DSL/Cable modem for access to the Internet. The CPE is controlled and possibly owned by the operator rather than the subscriber. 
       FIG. 2  depicts the functional architecture of an ECONS network  200 . It should be emphasized that in this implementation the circuit switched network  220  and the broadband IP network  230  both belong to and are operated by the same operator. The ECONS network comprises a set of ECONS CPEs  210  connected to the operator&#39;s core network. The ECONS core network comprises the operator&#39;s circuit switched and broadband IP networks  220  and  230 . An individual ECONS CPE  210  may be connected to the operator&#39;s broadband IP network  230  or to both its broadband IP network  230  and its circuit switched network  220 . Although the ECONS architecture envisions moving most of the control and application layers functions of IMS into ECONS CPEs, it still maintains in the core network a set of common functions that operators deem essential to proper operation and security of the network (e.g., AAA server, NAT traversal, etc.). 
     More specifically, the functional entities in the ECONS network are the ECONS CPE  210  and the operator&#39;s control platform  240  in its core network. The operator&#39;s control platform of the ECONS core network includes a RENDEZVOUS server  242 , a NAT traversal server  270 , a DNS server  246 , a SIP proxy  248 , a VoIP gateway  250  and an authentication, authorization, and an accounting (AAA) server  252 . 
     The ECONS CPE  210  is the central entity of the system  200 . It comprises a SIP-based session manager, a policy repository, and application servers such as a presence server, web server, etc. The session manager of the ECONS CPE may control every communication flow initiated or received by the UE  260 . It may add or drop call/session legs, route sessions to any endpoint, and seamlessly move a session to any end points. The ECONS CPE  210  provides control over all audio and video sessions of the UEs  260 . It is responsible for routing SIP signaling to other ECONS CPEs as well as to other SIP devices. It also provides a local proxy/registrar for SIP based UEs, and the gateway functionality for POTS (plain old telephone service) equipments. It may also include some application servers, such as a presence server. It is important to note that the ECONS CPE  210  is installed in the user&#39;s home network. It may include a broadband modem, in which case it is connected directly to the broadband network. It can also be connected through an Ethernet link to an external broadband modem, which may implement a network address translator (NAT)  270 . In this case the ECONS CPE  210  provides the necessary mechanisms for NAT traversal for SIP signaling and media. 
     The rendezvous server  242  of the ECONS core network provides a simple way for a CPE to reach any other CPE in the system. Every ECONS CPE  210  maintains an active connection with the RENDEZVOUS server  242 . This connection can be used to send a short message to any other ECONS CPE. It is mainly used for NAT traversal. When two ECONS CPEs  210  want to establish a SIP session, they determine their public IP addresses using STUN (Simple Traversal of UDP thru NATs) protocol, and then they send their public addresses to each other through the RENDEZVOUS server  242 . After these addresses have been exchanged the rest of the communication is done directly between the CPEs without the intervention of any other node. 
     The STUN server  244  is the NAT Traversal that supports the STUN protocol and the relay usage of that protocol The STUN protocol is used by a node behind a NAT (which in the case of ECONS is an ECONS CPE) to find its own public address, determine the NAT type and the Internet side port associated by the NAT with a particular local port. The STUN server  244  also supports the relay usage. In the relay usage the server allocates a public IP address and port that the client can use for communications through the NAT. Note that in the relay usage data has to be relayed through the core of the network, which is, of course, less efficient. The detailed specifications of STUN are presented in RFC 3489. 
     The DNS server  246  of the provider&#39;s control platform  240  includes entries identifying the SIP servers for the provider&#39;s domain, an ENUM database, and the list of available STUN servers. Note that “ENUM” is a protocol that resolves fully qualified telephone numbers to fully qualified domain name addresses using a DNS-based architecture. The DNS specifications are set forth in RFC 1034 and RFC 1035. 
     The SIP proxy  248  is used to route SIP signaling between external networks and the ECONS CPEs. The SIP proxy  248  can receive SIP requests from non-ECONS SIP devices and route them to the appropriate ECONS CPE. The SIP specifications are set forth in RFC 3261. 
     The VoIP gateway  250  provides signaling and media gateway functionalities to interwork the circuit switched network with the ECONS network. 
     The AAA server  252  provides AAA services for the ECONS CPEs. It comprises an ECONS profile server as well as AAA engine that authenticates the CPEs. It also stores the billing records resulting from calls to the circuit switched domain. In principle, the AAA server  252  functionality is equivalent to those of a HSS in a centralized IMS. 
     Cellular operators deploy low power base stations at subscribers&#39; premises to improve their indoor cellular coverage. These base stations are often referred to as femto cells in the cellular industry generally and as Home NodeBs in the 3GPP community in particular. These femto cells are attached to the cellular operator&#39;s core network and its centralized IMS services. Three options are currently considered for femto cell connectivity to the core network: Iu-b over IP, RAN Gateway, and SIP/IMS. First, the Iu-b over IP option uses the existing 3GPP Iu-b interface to leverage the cellular operators&#39; 3G Radio Node Controllers (RNCs) to support these femto cells, through a remote gateway along with the 3G NodeBs (i.e., 3G BTSs). Note that the Iu-b interface is primarily proposed for connection of 3G NodeBs with 3G RNCs. Second, the RAN Gateway approach uses a network controller residing between the cellular operator&#39;s core network and its IP access network to connect the femto cells to the cellular core network. Third, the SIP/IMS solution proposes to use SIP between femto cells and the IMS core of the cellular operator. 
     As the number of femto cells increases the cellular operator will eventually have in place two parallel wireless infrastructures that overlap in coverage: a series of conventional cells that each, for example, cover a distance of about 3 to 5 kms and a series of femto cells that each, for example, cover a distance of about 30-100 m. The conventional cells will be referred to herein as macrocells to distinguish them from the femto cells. Of course, the coverage offered by the femto cell infrastructure will be more complete and more fully overlap with the macrocellular infrastructure in dense metropolitan areas than in less densely populated areas. In such densely populated areas, or in an enterprise environment such as a corporate or university facility or campus, the cellular operator may be able to use the femto cell infrastructure as an alternative wireless platform to the macrocellular infrastructure. 
     One problem with using the femto cell infrastructure in this manner is ensuring seamless mobility (i.e., seamless hand-off of the UE) of the user across the femto cell infrastructure. To ensure such a seamless mobility or hand-off the UE will need to be authenticated (or re-authenticated) each time it moves from one femto cell to another. In addition, the packets destined for the UE should be forwarded to the UE as it changes its point of attachment to the network from one femto cell to another femto cell. 
     These processes, particularly the authentication (re-authentication) of the UE, are problematic because the amount of time it may take a user equipped with a UE to move (e.g., by walking) from femto cell to femto cell may be about the same or less than the amount of time needed by conventional hand-off techniques to transition a UE from one femto cell to another. Clearly, conventional hand-off techniques generally will not be suitable for use with a femto cell infrastructure. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method is provided for hand-off of a mobile User Equipment (UE) across a femto cellular network. The method includes dynamically clustering a plurality of neighboring femto cells in a vicinity of a first femto cell to which a mobile UE is currently attached. The dynamic cluster of neighboring femto cells is moved across the femto cellular network in accordance with movement of the mobile UE across the femto cellular network. 
     In accordance with another aspect of the invention, A Customer Premises Equipment (CPE) is provided that includes a wireless transceiver for establishing communication with a mobile User Equipment (UE) over a first femto cell to which the femto cell is currently attached. The CPE also includes a network interface for establishing communication with a cellular network and a session manager configured to forward an authentication message over a broadband network. The authentication message includes information needed to authenticate and attach the UE to a plurality of femto cells neighboring the first femto cell so that the UE is authenticated by the neighboring femto cells without physically entering a geographic area served by the neighboring femto cell. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the layered architecture of an IMS and an ECONS system. 
         FIG. 2  illustrates the functional architecture of an ECONS system where both broadband IP and circuit switched networks are owned and operated by the same operator. 
         FIG. 3  illustrates the architecture of an ECONS system that includes subscriber femto cells. 
         FIG. 4  illustrates one example of an ECONS CPE. 
         FIG. 5  illustrates a user-centric cell that comprises a series a neighboring femto cells. 
         FIG. 6  is a flow chart illustrating one example of the hand-off process across individual femto cells in a femto cell overlay of an end-to-end ECONS network. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 3  depicts a simplified architecture of an end-to-end ECON mobile communication network comprising the ECONS mobile communication infrastructures and control platforms of two operators, operators A and B, including their subscribers&#39; femto cells. In this example each operator has deployed an ECONS distributed platform instead of a centralized IMS for providing IMS services. 
     The operator A has a cellular network A comprising a core network A and its companion wireless access network. The cellular core network A of operator A includes a circuit switched network A and a packet switched broadband IP network A, as well as a signaling and control platform  350 . Its wireless access network includes a set of base transceiver stations (BTSs), each covering a macro cell area. In  FIG. 3 , one such macro cell of operator A is depicted, which is denoted macro cell A and which is being served by BTS  301 . The BTSs of wireless access network of operator A are typically attached by wired links to the cellular core network A of operator A. Similarly, operator B has a cellular network B comprising a cellular core network B and its companion wireless access network. The core network B of operator B includes circuit switched network B and a packet switched broadband IP network B, as well as a signaling and control platform  370 . Its wireless access network includes a set of base transceiver stations (BTSs), each covering a macro cell area. In  FIG. 3 , one such macro cell of operator B is depicted, which is denoted macro cell B and which is being served by BTS  302 . The BTSs of wireless access network of operator B are typically attached by wired links to the cellular core network B of operator B. Also shown in  FIG. 3  is a broadband network such as the Internet  360 , which is connected to the core network A of operator A as well as the core network B of operator B. 
     A femto cell is an indoor low power cellular base station with a relatively small footprint, which typically resides within the subscriber&#39;s premises (e.g., residence). A femto cell usually supports about half dozen mobile devices, and may cover an entire premises or a part of it.  FIG. 3  shows a series of femto cells  310 A associated with the subscribers of operator A and a series of femto cells  310 B associated with operator B whose areas of coverage overlap in part with the macro cell areas served by BTS  301  of cellular network A and by BTS  302  of cellular network B. 
     A detailed view of one of the femto cells  310  (e.g., one of the femto cells  310 B), denoted femto cell  312 , is also shown in  FIG. 3 . The femto cell base station  325  communicates with the subscriber&#39;s ECONS CPE  327 , which in turn communicates with the Internet  360  or other broadband network thru the subscriber&#39;s broadband modem  329  such as a DSL or cable modem. When the femto cell is located in the subscriber&#39;s premises, the subscriber&#39;s wireless devices or UEs such as PC  331  and PDA  322 , and cell phone  328  will generally attach to the femto cell base station  325  of femto cell  312  because its indoor coverage and performance is most likely better than those of the macro cellular network. Thus, all messages from (or to) the subscriber&#39;s wireless devices at her/his premises traverse through the femto cell  325 . It should be noted that the femto cell base station and the broadband modem may be integrated with the ECONS CPE (as indicated in  FIG. 2 ) or, alternatively, they may remain as separate devices (as indicated in  FIG. 3 ). 
     The ECONS CPE  327  serves as the femto cell controller and, during communication sessions, is situated in the route or path to and from the femto cells. When the ECONS CPE is attached to the broadband modem of the broadband service provider, the ECONS CPE can acquire for itself from the broadband network&#39;s DHCP server (not shown in  FIG. 3 ) the default gateway, SIP out bound proxy, DNS addresses and an IP address. 
       FIG. 4  shows one example of an ECONS CPE  500 . The CPE  500  includes a transceiver  501 , session manager  503 , and a broadband network interface  505 . The transceiver  501  is capable of communicating with the user&#39;s UE over the air interface of the femtocell. The transceiver  501  is coupled to a session manager  503 , which may include a SIP server which interfaces a SIP client to the UE to set up new communication sessions, terminate existing sessions, etc. The session manager  503  is furthermore capable of exchanging data of the communication session with the UE using a suitable protocol. The ECONS CPE communicates with the broadband IP network of one of the operators A or B via its network interface  505 . In some cases, the network interface  505  of the ECONS CPE  500  may be a broadband modem. Furthermore, the ECONS CPE includes a topology database  507 , and a guest registrar  509 . The topology database  507  contains a list of neighboring femto cells as well as overlapping macro cells in the vicinity of the subscriber&#39;s premises/residence. The guest registrar  509  is a database containing the identities and the security credentials of the UEs that are already authenticated and granted access to the femto cell attached to the ECONS CPE  500 . 
     As  FIG. 3  indicates, the cellular operators A and B who have dispensed the femto cells  310 A and  310 B to their mobile subscribers each ends up with a femto cell network that parallels and complements its macro cellular network. These operators may use these femto cell networks as alternatives to the wireless access networks of their macro cellular infrastructures. In this way each operator can unload traffic onto its femto cells and away from the scarce spectrum of its macro cellular network. 
     As previously mentioned, conventional hand-off techniques generally will not be suitable for use with a femto cell infrastructure because the time it takes to a UE to detach from one femto cell and attach to the next may be comparable to (or more than) the amount of time it takes the UE to traverse a femto cell and enter into an adjacent or neighboring femto cell, thus causing unacceptable disruptions in service since the user may have exited a femto cell before the attachment process is complete. To overcome this problem a user-centric cell is created which comprises a group of adjacent femto cells. The femto cell in which the user is located at any given time defines the center of the user-centric cell. As the user moves from one femto cell to another, the user-centric cell moves along with her so that the user-centric cell is always centered about the user. The concept of a user-centric cell will be further illustrated with reference to  FIG. 5 . 
       FIG. 5  shows a plan view of a group of femto cells  410 . The user is initially located in and attached to femto cell  410   1 . The user-centric cell  420  is centered about the femto cell  410   1  and has a radius of N femtocells, where in this example N=3. As the user moves m femto cells from femto cell  410   1  to femto cell  410   2 , the user-centric cell  420  shifts with her so that the new user-centric cell  420 ′ is centered about femto cell  410   2 . In the example of  FIG. 5 , m=2. 
     Each femto cell  410  within the user-centric cell  420  effectively pre-authenticates the user so that when the user enters any of those femto cells  410  the user will immediately attach to it. In order to accomplish this, each femto cell  410  in the user-centric cell  420  must be supplied with sufficient information to authenticate the user&#39;s UE before the user actually enters any of them. Such authenticating information may vary from system to system, but generally includes all or a subset of the following: the user&#39;s Network Access Identifier (NAI), security credentials (e.g., authentication vector), a mobile MAC address, (or alternatively, an International Mobile Subscriber Identity (IMSI) or an International Mobile Equipment Identity (IMEI)), and the like. Note that an authentication vector comprises a set of security parameters that allows a network operator to authenticate (or re-authenticate) a UE, when and if necessary. For instance, the authentication vector in a UMTS cellular network includes a random number, an expected response, a cipher key, an integrity key, and an authentication token. Additional information that may be provided to each femto cell  410  in user-centric cell  420  includes information (e.g., the session context) relating to an ongoing communication session in the event that a communication session is in progress as the user moves from one femto cell to another. This information is provided to each femto cell  410  in the user-centric cell  420  by the ECONS CPE that is in the originating femto cell  410  in which the user is currently located. This ECONS CPE broadcasts the necessary information over an N hop wide neighborhood of femto cells  410 . When an ECONS CPEs receives the authenticating information, it stores this data in its guest registrar (e.g., guest registrar  509  in  FIG. 4 ) so that the data can be accessed when a user enters its femto cell. The information may be broadcast in the form of an authentication message. 
     The broadcast of the authenticating information takes place over the Internet or other broadband network through which all the ECONS CPEs are connected. For instance, in  FIG. 3 , the ECONS CPE  327  sends the authentication message to each of the other ECONS CPEs in its neighborhood through the core network  330  and the Internet  360  via broadband modem  329 . This message serves to effectively establish or create the user-centric cell. In some implementations the authentication message may be sent to the neighboring ECONS CPEs using a flooding algorithm. When a flooding algorithm is employed, each ECONs CPE in the neighborhood acts as both a transmitter and receiver of the message. Each ECONS CPE tries to forward the message to each of its neighbors except the source ECONS CPE from which the message was received. In order to limit the message to N neighbors so that the user-centric cell has a radius of N femto cells, the message will generally also include a time-to-live (TTL) of N hops. As each neighbor&#39;s ECONS CPE receives the message it decrements the TTL by 1 before sending it on to its neighbors. In this way the process of forwarding the message terminates after it has been communicated to neighboring ECONS CPEs N hops away. 
     By communicating the user&#39;s authentication information to all the ECONS CPEs within N hops of the originating femto cell in which the user is located, the corresponding femto cells in which these ECONS CPEs are located will be able to accept packets to or from the user as soon as the user enters any of these femto cells. That is, the user&#39;s UE quickly attaches to any of the femto cells as she moves from femto cell to femto cell within the user-centric cell. Once the user is attached a particular femto cell, she will receive packets addressed for her through the ECONS CPE of that femto cell in which she is located. As the user moves m femto cells away from her originating femto cell, the ECONS CPE located in the femto cell in which the user is now located will rebroadcast the user&#39;s authentication information to its N hop neighbors. In this way the user-centric cell shifts so that in  FIG. 5 , for instance, user-centric cell  420  shifts to user-centric cell  420 ′. Accordingly, this technique ensures a smooth and rapid hand-off as the user moves from one femto-cell to another. 
     To ensure seamless mobility of the UE across the end-to-end infrastructure, the packets destined for the UE should be forwarded to the UE as it moves across the femto cells of a user-centric cell, changing its point of attachment to the network from one femto cell to another. Thus in addition to forwarding the authentication message to neighboring femto cells in the user-centric cell, the originating femto cell will broadcast to its neighboring femto cells any incoming packets addressed to the user as soon as the UE leaves the originating femto cell. This ensures that the user receives packets transmitted during a user-terminated communication session. 
     The authentication message that originated from a femto cell m hops away from the receiving femto cell will have a TTL of N-m. Once the user physically enters this receiving femto cell the femto cell (which now becomes a new originating femto cell) will reset the TTL back to N before forwarding it on to its neighboring femto cells. In this way the user-centric cell will always have a radius of N hops centered about the current location of the user. The new originating femto cell will also send a redirect message to the previous originating femto cell. The redirect message, which includes the IP address of the new originating femto cell, instructs the previous originating femto cell to stop broadcasting all incoming packets or messages destined for the user to its neighboring femto cells, and instead instructs the previous originating femto cell to simply route them to the new originating femto cell. Once the redirect message is received the previous originating femto cell will no longer broadcast any packets or messages destined for the user to all the femto cells in the user-centric cell, but will only forward such packets or messages to the new originating femto cell. In the context of  FIG. 5 , femto cell  410   1  could be the previous originating femto cell and femto cell  410   2  could be the new originating femto cell that the user subsequently enters. 
     In some cases the transmission of the redirect message may be initiated by the user&#39;s UE instead of by the ECONS CPE in the new originating femto cell. For instance, if the communication system is SIP-based and the session manager (e.g., session manager  503  in  FIG. 4 ) is a SIP back to back user agent (B2BUA), the UE can send a SIP UPDATE message which “via” field contains the URL of the B2BUA of the ECONS CPE in the subsequent originating femto cell. 
     Since the user-centric cell is necessarily limited in coverage, at some point it will need to hand off the UE back to the cellular provider&#39;s macro cell. In order to accomplish this handoff, the ECON CPEs need to have sufficient knowledge about the topology of the femto-cells in order to instruct the UE in a timely manner to initiate the hand-off process to the macro cell. This information may be stored, for instance, in a database such as femto cell topology database  540  in the ECONS CPE of  FIG. 4 . For example, when a user attached to a user-centric cell of width N and within range of a macro cell is less than N hops away from an area in which there are no femto-cells to join the user-centric cell in the user&#39;s direction of movement, the ECONS CPE will instruct the user&#39;s UE to initiate a hand-off to the macro cell. In the case of a UMTS cellular network, the UE initiates the hand-off process to the macro cell by sending an attach message to the serving GPRS support node (SGSN). If this process is successful it is followed by initiation of a context activation process through the gateway GPRS support node (GGSN). The hand-off process to the macro cell will generally occur quickly since the user has already been authenticated by the AAA server of the cellular operator. 
       FIG. 6  is a flow chart illustrating one example of the hand-off process across individual femto cells in a femto cell overlay of an end-to-end ECONS network. The method begins in step  602  and continues to step  604  when an ECONS CPE receives a request from a UE to attach to the femto cell associated with the CPE. At decision step  606 , the ECONS CPE determines if the AAA record of the UE is located in its guest registrar (e.g., guest registrar  509  shown in  FIG. 4 ). If the AAA record is in the guest registrar, then the CPE grants access to the UE in step  608 . Next, in step  610 , the CPE checks its topology database (e.g., topology database  507  shown in  FIG. 4 ) to determine, at decision step  612 , whether or not the UE should undergo a hand-off to the macro cell. If yes, then the UE is instructed in step  614  to begin the handoff process to the macro cell, at which point the process terminates. On the other hand, if at decision step  612  the CPE determines that the UE should not be handed-off to the macro cell (based on the information in its topology database), the process continues to step  616 , where the CPE determines if this is the UE&#39;s first entry into the femto cell overlay or if the TTL included with the authentication message is equal to N, which is the number of hops in the user-centric femto cell. If yes, then the process proceeds to step  618 , where the CPE broadcasts the AAA record of the UE to the N hop neighborhood of femto cells. In addition, when the CPE receives packets destined for the UE at step  620 , the CPE determines at step  622  if in fact the UE is still within the femto cell associated with the CPE. If so, the CPE forwards the packet to the UE at step  624 . If the UE is not longer in the femtocell, then the CPE broadcasts the packet over the N hop neighborhood of femto cells in step  626 . Returning to decision step  616 , if the TTL of the authentication message is not equal to N, then in step  628  the CPE once again checks its topology database to determine at decision step  630  if the TTL in the authentication message is less than m, which is the number of hops the UE has moved from the original femto cell with which it attached. If the TTL is not less than m, the process returns to step  626  and any packets received for the UE are broadcast over the N hop neighborhood of femto cells. If, on the other hand, the TTL is less than m, the CPE broadcasts the AAA record of the UE over the N hop neighborhood of femto cells in step  632 . In addition, the CPE creates a tunnel to the previous originating CPE and becomes the subsequent, new originating CPE in step  634  so that it will receive packets destined for the UE directly from the previous originating CPE in step  636 . These packets received from the previous originating CPE in step  636  are then forwarded to the UE in step  624 . 
     Returning to decision step  606 , if the ECONS CPE determines that the AAA record of the UE is not located in its guest registrar, the CPE sends the AAA record to its network operator in step  638 . Next, if it is determined in step  640  that the network operator is the UE&#39;s home operator, then in step  642  it is determined if the UE should be granted access to the femto cell, and, if so, the process continues to step  610  as described above. If in step  640  it is determined that the network operator is not the UE&#39;s home operator, the UE&#39;s home operator is queried in step  644  to determine, in step  646 , if the UE should be given access to the femto cell. If so, the process once again continues to step  610  as described above. If not, the process once again terminates. 
     The steps of the processes described above, including but not limited to those shown in  FIG. 6 , may be implemented in a general, multi-purpose or single purpose processor. Such a processor will execute instructions, either at the assembly, compiled or machine-level, to perform that process. Those instructions can be written by one of ordinary skill in the art following the description provided herein and stored or transmitted on a computer readable medium. The instructions may also be created using source code or any other known computer-aided design tool. A computer readable medium may be any medium capable of carrying those instructions and include a CD-ROM, DVD, magnetic or other optical disc, tape, silicon memory (e.g., removable, non-removable, volatile or non-volatile), and/or packetized or non-packetized wireline or wireless transmission signals.