Abstract:
A radio network controller and methods for reestablishing sessions in a wireless network are described. At least a portion of session information associated with a first session is saved; and in response to detecting an unexpected degradation of the first session, reestablishment of the first session is triggered using the portion of the session information.

Description:
TECHNICAL FIELD  
       [0001]     This disclosure relates to the preservation of communication sessions with client devices in a wireless network communication system despite a failure in components of the communications systems.  
       BACKGROUND  
       [0002]     In a fixed or mobile wireless Internet Protocol (IP) network, a client-side device establishes a session with a network-side access device to communicate with other entities in that network. The session represents the client-side device to the network, and includes information about the client-side device such as its IP address, location within a mobility area, permitted services and other such attributes required to communicate with the client-side device. The session may be partitioned into separate data elements representing IP connectivity and link-layer connectivity and these data elements may be stored on separate network elements. The session is typically present as long as the client-side device is present in the network, though other resources required for direct communication between the client-side device and the network device may only be in use when active communication is in process.  
         [0003]     The state of direct communication between the client-side device and the network-side device is known as a connection. In cellular wireless systems, as the client device must initiate connection, a special procedure known as paging is used to allow the network to request the client device to communicate. For example, if a network device needs to send data to a client device, the network uses the information stored in a session associated with the client device to page the client device. Paging involves transmitting a message addressed to a particular terminal over a shared channel monitored by all terminals in communication with that part of the network. This page causes the client device to initiate a connection with the network, thus enabling an exchange of data. However, if a hardware or software failure of a network-side device causes the network to lose the session information (referred to as a “session breach”), the network cannot establish a direct communication with the client device, as it has lost all knowledge of the device&#39;s identity, communication parameters and location required to page that particular device. A session for which session information has been lost due to hardware or software failure of a network-side device is referred to as a “breached session.” Because the session breach is not visible at the IP (Internet Protocol) layer, the client&#39;s peers are unaware that the client is unreachable. Thus, network-initiated connection-oriented applications such as online video teleconferencing and Internet telephony are particularly vulnerable to network-side failures. Furthermore, the client device is not immediately aware that the session breach has occurred and its recovery mechanisms operate on a sufficiently long timescale that client-initiated creation of a new session cannot be counted upon to restore network reachability for that client before it is required.  
       SUMMARY  
       [0004]     In one aspect, reestablishing a session may be accomplished by saving at least a portion of session information associated with a first session between an access terminal (e.g., a cellular telephone, a personal data assistant, or a laptop computer) and a first wireless network device; and in response to detecting an unexpected degradation of the first session, triggering a reestablishment of the first session using the portion of the session information.  
         [0005]     Implementations may include one or more of the following features. Degradation may include cessation and detecting a degradation of the first session may include detecting a state (e.g., failure) of the first wireless device. Triggering a reestablishment of the first session may include transmitting, to the access terminal, a close session message. A first session may be replicated without being closed and restored upon receiving a request to open a new session from the access terminal. The triggering may comply with a 1× Evolution-Data Optimized protocol and/or be based on a load state of a second wireless network device.  
         [0006]     Transmitting a close session message may occur immediately upon detection of a unexpected degradation of the first session and/or after receiving a request to transmit data to the access terminal. Degraded sessions may be placed in a queue and moved up in the queue in response to receiving a request to transmit data to an access terminal associated with the degraded session. Degraded sessions may include breached sessions. The session information for the session assigned to the access terminal may be deleted if the access terminal has failed to request to open a new session and/or a second wireless network device fails to reestablish the first session after a predetermined time has elapsed. The first session may be established between the access terminal and a first wireless network device. The session information may be saved on a second wireless network device that generates and/or transmits the close session message.  
         [0007]     A second session may be established between the access terminal and the second wireless network device; and at least a portion of the second session information that sufficient to reestablish the second session may be saved to a third wireless network device. The portion of the second session information may be sufficient to generate a close session message for the access terminal for the second session. In response to receiving a close session message, a breached session may be reestablished by closing the breached session and sending a request to open a new session.  
         [0008]     In another aspect, a radio network controller includes a first radio node server module configured to establish a session with a first access terminal; a storage device (e.g., non-volatile random access memory, a flash memory, and a disk memory) configured to store at least a portion of the session information that is sufficient to reestablish the session; and a control mechanism configured to cause a second radio node server module device to reestablish the session with the access terminal after detecting a degradation of the session between the first radio node server module and the access terminal.  
         [0009]     Implementations may include one or more of the following features. The session information may be sufficient to generate a close session message and/or complies with a 1× Evolution-Data Optimized protocol. The control mechanism may be configured to transmit the close session message to the access terminal and to retrieve the portion of the session information from the storage device and send the portion to the second radio node server module without causing the session to be closed. The second radio node server module may transmit a close session message immediately after the control mechanism detects a degradation of the session between the first radio node server module and the access terminal. A degradation may include a cessation. The second radio node server module may transmit a close session message only after the control mechanism receives a request to transmit data to the access terminal. Degraded sessions may be placed in a queue such that a closed session is moved to a higher entry of the queue when a request is received to transmit data to an access terminal associated with at least one of the degraded sessions. The first radio node server module may include a first processing card and the second radio node server module may include a second processing card. The control mechanism may be implemented on a processor that is connected to the first radio node server module and to the second radio node server module through a high speed bus. The control mechanism may be implemented on the second radio node server module or on a third radio node server module.  
         [0010]     In another aspect, reestablishing breached sessions in a wireless communications network is accomplished by placing a first session that has been breached in a queue for reestablishment of the first session; placing a second session that has been breached in the queue for reestablishment of the second session, which is prioritized below the first session in the queue; and promoting the second session above the first session in the queue in response to receiving a request to transmit data to an access terminal associated with the second session.  
         [0011]     Implementations may include one or more of the following features. The wireless communications network may use a 1× Evolution-Data Optimized protocol. Reestablishment of the second session may be triggered by generating and transmitting a close session message to the access terminal associated with the second session. Reestablishment may also be triggered based on a load state of a second wireless network device. The second session may be reestablished between a wireless network device and the access terminal. Triggering reestablishment of the first session may be performed after triggering reestablishment of the second session, and in some examples, only after receiving a request to transmit data to an access terminal associated with the first session. The time that the first session has spent in the queue may be monitored and the first session may be deleted if it has occupied an entry in the queue past a predetermined time period.  
         [0012]     These general and specific aspects may be implemented using a system, a method, or a computer-readable medium, or any combination of systems, methods, and computer-readable mediums.  
         [0013]     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     DESCRIPTION OF DRAWINGS  
       [0014]      FIG. 1  shows a block diagram of a wireless network system.  
         [0015]      FIG. 2  shows internal modules of a radio network controller (RNC).  
         [0016]      FIG. 3  depicts the RNC internal modules of  FIG. 2  in greater detail.  
         [0017]      FIG. 4  depicts the relationship between protected RNSMs and their session data.  
         [0018]      FIG. 5A  illustrates the order in which sessions are closed for proactive session closure.  
         [0019]      FIG. 5B  illustrates the order in which sessions are closed for reactive session closure.  
         [0020]      FIG. 5C  illustrates the order in which sessions are closed for combined proactive and reactive session closure.  
         [0021]      FIG. 6  illustrates the logic of proactive session restoration performed by the Protecting RNSM of  FIG. 4 .  
         [0022]      FIG. 7  illustrates the logic of reactive session restoration performed by the Protecting RNSM of  FIG. 4 .  
         [0023]      FIG. 8  illustrates the logic of combined proactive and reactive session restoration performed by the Protecting RNSM of  FIG. 4 .  
         [0024]      FIG. 9  depicts the Session Database of  FIG. 3  and  FIG. 4 .  
         [0025]      FIG. 10  illustrates the logic of proactive session closure performed by the Protecting RNSM of  FIG. 4 .  
         [0026]      FIG. 11  illustrates the logic of reactive session closure performed by the Protecting RNSM of  FIG. 4 .  
         [0027]      FIG. 12  illustrates the logic of combined proactive and reactive session closure performed by the Protecting RNSM of  FIG. 4 .  
         [0028]      FIG. 13  illustrates the logic of session closure functions performed by the Base I/O (BIO) of  FIG. 2 .  
     
    
     DETAILED DESCRIPTION  
       [0029]     Session breaches caused by network-side hardware and/or software failures can be remedied by saving a complete copy of each user session at an alternate location that is not likely to fail at the same time as the location hosting the primary copy of that user session. As the session data changes whenever the state of the client device changes, it must only be saved when in particular states known to be consistent between the network and the client device. These states are unique to the wireless communication protocol in use and the internal design of the RNC. For this approach to be effective, all saved states must be such that a client device may immediately open a connection upon restoration of the session to that saved state.  
         [0030]     Session breaches can also be remedied by transmitting a message to a client-side device instructing the client-side device to close the current session and reopen a new session with the network. For example, the 1×-Evolution Data Optimized (1×EV-DO) protocol currently defined in the IS856 family of standards, as defined by the 3GPP2 organization, provides for a “close session message”. A close session message is a message that can be sent by either a client-side or network-side device to close a user session. For example, a client-side device (referred to as an “access terminal” in the 1×EV-DO protocol) may send a close session message to the network when the device is powering off or when its 1×EV-DO application is shut down. A network side device, such as a radio network controller, may send a close session message when it is shutting down or attempting to perform some form of overload control due to constrained resources. When an access terminal receives a close session message, it is required under the 1×EV-DO protocol to close its current session with the network. It will reopen a new session if it anticipates further communication with the network. Client devices that participate in network-initiated applications such as telephony always reopen new sessions as long as they are powered on. Therefore, a network-side device can use the close session message to reestablish session information lost due to a network-side failure. Moreover, because the information necessary to generate and transmit a close session message is less than the total amount of session information stored by a network-side device, the network does not need to maintain a complete backup of each session established on the network at a particular time. The benefit of this is that the network can maintain a larger number of user sessions for a given amount of memory available on the network-side device.  
         [0031]     Referring to  FIG. 1 , a wireless IP network  20  includes a correspondent user device  22 , a core IP network  26 , a network resource  24 , a radio network controller (RNC)  28 , a Packet Data Serving Node (PDSN)  27 , an access IP network  29 , and multiple radio nodes (RN), three of which are shown  30   a - 30   c , that each communicate with an access terminals (AT)  34   a - 34   c  using an airlink  32   a - 32   c.    
         [0032]     The access terminals, e.g., AT  34   a , are the client-side of the wireless network and Internet protocol, and may be implemented as a mobile device such as a cellular telephone, a wireless PDA, a handheld gaming device, or a wireless laptop computer or a stationary device such as a desktop computer, a parking meter, or other fixed device with which wireless communication is desired.  
         [0033]     The correspondent user device  22  connects to a core IP network  26  through an optional network resource  24  such as a telephony gateway and represents the device with which the ATs communicate over the wireless IP network. For example, the correspondent user device  22  may consist of an Internet telephone that permits a user at the device  22  to engage in a voice over IP call with a user associated with an AT. Similarly, if the ATs include a network of wireless-enabled parking meters, a user at the device  22  may use a graphical user interface to send messages to each parking meter instructing it to provide status information (e.g., space-filled/time paid; space-filled/time expired; space-empty/time remaining, space-empty/time expired, etc.). If the correspondent user device is a traditional telephone, it may connect through an optional network resource in the form of a media gateway and a soft switch that together form an interface between an IP network and a traditional telephone network.  
         [0034]     The core IP network  26  is a network of devices that use the TCP/IP network protocols to exchange data. The core IP network  26  may, for example, comprise the public Internet or a private intranet. In a CDMA cellular data system, the core IP network  26  interfaces with the wireless network through a Packet Data Serving Node (PDSN)  27 .  
         [0035]     The RNC  28  communicates with the PDSN  27  and with the RNs  30   a - 30   c  over the access IP network  29  controls the radionodes&#39; transmitters and receivers, initiates and maintains client sessions, directs data packets received from the core IP network  26 , and performs other radio access and link maintenance functions such as soft handoff and sector selection. The access IP network  29  may be a private network that is designed and operated exclusively for communication between PSDN  27 , RNC  28 , and RNs  30   a - c.    
         [0036]     Referring to  FIG. 2 , the RNC  28  includes a System Controller (SC)  40 , one or more Base Input/Output modules (BIO)  42 , and one or more Radio Node Server Modules  46   a - 46   c  (collectively referred to as RNSMs  46 ) that are interconnected over a high-speed bus  44  such as a PCI, VMEbus, USB, ISA, or PXI bus or switched fabric such as ATM (or other cell-based fabric) or Ethernet (or other packet-based fabric). Each of the three Radio Node Server Modules  46   a - 46   c  shown communicate with an AT  34   a - 34   c  using a radio node  30   a - 30   b . In reality, each RNSM will typically use multiple radio nodes to communicate with many ATs at any particular time. For simplicity, however, only one radio node per RNSM and one AT per node are illustrated.  
         [0037]     Each RNSM  46   a ,  46   b ,  46   c  is responsible for establishing and maintaining a session with the ATs that they are assigned to handle by the System Controller (SC)  40 . As previously mentioned, a session must be established before data can be exchanged. During the lifetime of a session maintained by an RNSM, the RNSM continually sends session status information to the SC  40  or to an optional Session Server  38 . If an RNSM fails, the SC detects the failure through heartbeat messages for which it receives no response. If an RNSM reboots or otherwise recovers, it registers its presence with the SC  40 , thus alerting the SC  40  to its availability to provide service.  
         [0038]     Referring to  FIG. 3 , the System Controller  40 , which generally functions to perform protocol route computations, system configuration, network management, and centralized signaling, also maintains a session lookup database  70  of the current sessions established between the RNSMs  46   a - 46   c  and the ATs. This database  70  is normally not a backup of all session information maintained by the RNSMs but includes information sufficient to identify the RNSM on which a particular session is hosted. The session may be located by multiple identifiers, such as International Mobile Subscriber Identity (IMSI), Unicast Access Terminal Identifier (UATI), or hardware ID. This database  70  may also be implemented on a separate Session Server  38 . The use of a Session Server  38  enables location of sessions across multiple RNC chassis. The session lookup database  70  is accessed and updated by a session lookup database application  71 .  
         [0039]     The Base I/O Module  42  functions to connect the RNC  28  with the access IP network  29  and the PDSN  27 . It receives packets destined for the PDSN  27  and the RNs  30  from the RNSMs  46  and SC  40  and transmits them over its network interfaces to the access IP network  29 . It also receives packets from the PDSN  36  and the RNs  30  from the access IP network  29  and routes them to the RNSMs  46  and SC  40 . For packets received from the PDSN  36 , the BIO  42  extracts the PDSN/PCF-specific identifier (PSI) field from the packet and looks it up in its PSI forwarding table  62  to find the identity of the RNSM (e.g., RNSM  46   a ) on which the corresponding session is located. For packets received from the RNs  30  over the airlink access channel, the BIO  42  extracts the Unicast Access Terminal Identifier (UATI) from the packet and looks it up in its UATI forwarding table  63  to find the identity of the RNSM (e.g., RNSM  46   a ) on which the corresponding session is located.  
         [0040]     The RNSM (e.g., RNSM  46   a ) functions to perform the wireless-specific protocol functions necessary to implement the RNC&#39;s call processing and data handling capabilities. The call processing functions are implemented by the Call Control  72  component. The Call Control  72  component interacts with the Packet Control Function (PCF) Signaling  64  component that implements the portions of call processing that pertain to the interface with the PDSN  27 . The Call Control  72  component and the PCF Signaling  64  component maintain the state of the sessions that they set up as part of call processing in the Active Session Database  66   a.    
         [0041]     To preserve user reachability after session breach, a Session Backup Manager  68   a  on RNSM  46   a  interacts with a Session Backup Manager  68   b  on RNSM  46   b  to maintain a Backup Session Database  67   a  on RSNM  46   a  that is a backup of the Active Session Database  66   b  on RNSM  46   b  and a Backup Session Database  67   b  on RNSM  46   b  that is a backup of the Active Session Database  66   a  on RNSM  46   a . This diagram shows a specific case of two RNSMs  46   a  and  46   b  protecting each other.  
         [0000]     Protection Models  
         [0042]     Referring to  FIG. 4 , the RNSMs  46  are configured such that RNSM  46   a  maintains a Backup Session Database  67   a  to protect the Active Session Database  66   b  on RNSM  46   b  and RNSM  46   b  maintains Backup Session Database  67   b  to protect the Active Session Database  66   a  on RNSM  46   a . RNSM  46   c  and RNSM  46   d  have a similar configuration where each RNSM maintains a backup of the other&#39;s session datatbase. In some embodiments, SC  40  maintains a Backup Session Database for one or more of the RNSMs  46 .  
         [0043]     In the event of failure of RNSM  46   a , RNSM  46   b  distributes its Backup Session Database  67   b  to itself and the other RNSMs  46   c  and  46   d  as shown by the gray shaded extensions to the Active Session Databases  66   b ,  66   c  and  66   d . The options for processing these sessions are described more below. Note that, in this protection strategy, an even number of RNSMs  46  are populated into the RNC  28  to guarantee that every RNSM&#39;s session database is protected. If a new RNSM is added to the RNC  28 , it may be paired up with an odd remainder RNSM that was existing in the RNC or can remain unprotected until an additional RNSM is added to the RNC.  
         [0044]     In some implementations, each RNSM in the RNC protects one other RNSM, for example, the RNSM located to its right in the RNC&#39;s chassis with the rightmost RNSM in the chassis is responsible for protecting the leftmost RNSM in the chassis. In this manner, an RNSM protection ring is established. Thus, in this protection scheme any number of RNSMs (greater than or equal to 2) can be populated in an RNC with protection. As new RNSMs are added to the RNC, they are inserted into the protection ring, such that two other RNSMs break their existing protection relationship and establish a protection relationship with the newly added RNSM.  
         [0045]     In some other implementations, each RNSM in the RNC may partially protect multiple other RNSMs in the RNC such that each RNSM is fully protected by a set of other RNSMs.  
         [0046]     The RNSM population of the RNC chassis can be determined at power-on of the RNC, even though the RNSMs may complete their software initialization in a random order. However, because the RSNM population can be determined at power-on, in some implementations, a protection arrangement is selected at power-on but the protection does not begin until after a configurable hold-down time to allow all the RNSMs to initialize. In the event of a late appearance of an RNSM, e.g., due to failure at the RNC initialization time, it is treated as an insertion of a new RNSM module into the RNC chassis.  
         [0000]     Recovery Models  
         [0047]     The traditional approach to implementing any form of redundancy-based reliability consists of maintaining a backup of the data to be protected and restoring that data upon detection of failure of the primary copy of the data.  FIG. 5A  shows the traditional service order for restoring this data, referred to as Proactive Service Order. In this approach, the backup data is ordered in some fashion and is restored according to its existing order. It is prior art in wireline networking and I believe in wireless networking as well. Restoration by sorting UATI may be unique so perhaps we should take that sentence out.  
         [0048]     If the restoration of a session takes a significant amount of time, the proactive approach may result in longer outages of service than tolerable as the user sessions are restored in an order independent of which users are actually active and would have benefited from session restoration. In a CDMA system, network-side reachability is required only when the network, in the form of the packet data servicing node (PDSN), has a packet to send to the AT. Thus, it is possible to defer restoration of any given session until a packet destined to the AT represented by that session is received from the PDSN  27 . This is referred to as Reactive Session Restoration and its service order is depicted in  FIG. 5B . This approach minimizes the outage perceived by an average session but has a constant value for the outage perceived by a session that was required to be restored. It also has the property that session state has to be preserved for a long time after failure of an RNSM while waiting for packets to arrive from the PDSN  27  prior to the expiration of the session lifetime.  
         [0049]     A hybrid scheme is proposed, referred to as Proactive plus Reactive. In this approach, sessions are restored according to their order in the Backup Session Database as in the Proactive Approach. If, however, a packet arrives from the PDSN  27  for a session that has not already been closed, that session is promoted to the head of the list for restoration.  FIG. 5C  shows this service order.  
         [0000]     Session Backup and Restoration  
         [0050]     One general approach to preserving user reachability in the event of session breach is to save all information associated with user sessions to an alternate location and restore that information in the event of an RNSM failure. This general approach can be implemented in many different ways, some of which are described below.  
         [0051]     For example, in a first approach, all sessions hosted on a particular RNSM, known as the primary RNSM, may be stored on another RNSM, known as the backup RNSM, that is dedicated to the function of storing the session data for the primary RNSM. In this approach, each RNSM in the RNC chassis has a designated backup RNSM that is idle until it takes over the function of a failed primary RNSM. The backup RNSM constantly monitors the correct functioning of the primary RNSM and, in the event of failure of the primary RNSM, assumes the function of that primary RNSM and activates its copy of the user sessions that existed on the primary RNSM. The processing capacity of the RNC is, therefore, constrained to that of half the number of RNSMs in that RNC. This approach is known as 1:1 RNSM Redundancy.  
         [0052]     In a second approach, a group of primary RNSMs is protected by a single backup RNSM that stores a copy of the session state for all of those primary RNSMs simultaneously. This backup RNSM constantly monitors the correct functioning of all its primary RNSMs and, in the event of failure of any primary RNSM, assumes the function of that primary RNSM and activates its copy of the user sessions that existed on that primary RNSM. The processing capacity of the RNC is, therefore, reduced by one RNSM for every group of protected primary RNSMs. This approach is known as 1:N RNSM Redundancy, where N is the number of primary RNSMs protected by a single backup RNSM.  
         [0053]     A third approach involves storing a copy of the session state for any given RNSM on a single RNSM that is itself a primary RNSM. In this model, pairs of RNSMs are associated so that each is the backup RNSM for the other. This model is known as the “buddy system” and is depicted in  FIG. 4  and described previously. RNSMs may be associated symmetrically, where one RNSM protects another RNSM and that RNSM protects the first RNSM, or asymmetrically, where the RNSM protecting a particular RNSM may be protected by a different RNSM.  
         [0054]     A fourth approach involves storing a copy of the session state for any given RNSM across all other RNSMs in the RNC. Thus, every RNSM is maintaining its own sessions and a copy of some of the sessions from every other RNSM in the RNC. In the event of an RNSM failure, each of the other RNSMs in the RNC activates its copy of the user sessions associated with the faulty RNSM.  
         [0055]     A fifth approach involves storing a copy of the session state for all RNSMs in the RNC on a centralized resource within the RNC, for example the system controller.  
         [0056]     A sixth approach involves storing a copy of the session state for all RNSMs in the RNC on a dedicated session server  38  external to the RNC.  
         [0057]     Regardless of the particular backup mode, session state may be stored in several ways. First, it may be stored to Random Access Memory (RAM). Second, it may be stored to non-volatile Random Access Memory in the form of battery-backed up RAM or FLASH memory. Finally, it may be stored to a hard disk drive.  
         [0058]      FIG. 6  shows a process for proactive session restoration performed by a backup RNSM (e.g., RNSM  46   a ). The backup RNSM continually issues ( 304 ) heartbeat messages to the active RNSM (e.g., RNSM  46   b ) while performing ( 302 ) normal call processing functions. If the active RNSM is working properly, it sends an acknowledgement of the heartbeat messages to the backup RNSM. If the backup RNSM receives ( 306 ) an acknowledgement from the active RNSM, it continues to perform ( 302 ) normal call processing functions. If the backup RNSM does not receive ( 306 ) an acknowledgement from the active RNSM, the active RNSM is faulty and its sessions are breached. Thus, the backup RNSM begins the session recovery process. During this process, the backup RNSM triggers ( 308 ) the rehoming of all RNs that were served by the failed active RNSM to working RNSMs and starts a timer. The timer may be used to terminate the session restoration process in situations where session restoration may not terminate normally. The backup RNSM then retrieves the sessions from its backup session database and restores ( 314 ) them evenly to each of the working RNSMs in the RNC, including itself. In some embodiments, the backup RNSM randomly allocates individual sessions or groups of sessions to itself and the other RNSMs. For example, if there were 8 RNSMs in an RNC and 20000 sessions per RNSM, failure of one RNSM may result in the restoration of 2857 sessions to each RNSM on average. If a particular working RNSM indicates that it is overloaded, the backup RNSM may not assign additional sessions to that RNSM, in order to avoid exacerbating the overload condition. As each session or group of sessions is restored, the backup RNSM updates ( 316 ) the UATI and PSI forwarding tables of the BIOs so that subsequent traffic destined for that session will be routed properly. Upon determining ( 310 ) that either the timer has expired or that all of the breached sessions have been restored, the backup RNSM terminates ( 312 ) the restoration process, which includes deleting all unrecovered sessions. The backup RNSM also enters a non-protecting mode, in which it no longer sends heartbeat messages to other RNSMs.  
         [0059]      FIG. 7  shows a process for reactive session restoration performed by a backup RNSM, as shown in  FIG. 4 . The protecting RNSM  46   a  executes the performing ( 302 ), issuing ( 304 ), determining ( 306 ), and restoring ( 308 ) processes described for the proactive restoration process  300  illustrated in  FIG. 6 . The backup RNSM then updates ( 332 ) the BIO&#39;s UATI and PSI forwarding tables so that messages intended for the faulty RNSM are routed to the backup RNSM. In some embodiments, the BIO&#39;s UATI and PSI forwarding tables are updated using a single message by previously configuring the BIO to know the backup RNSM for every active RNSM. The backup RNSM then waits for a packet with a UATI or PSI for a breached session to be received while continually checking ( 310 ) for timer expiration or completion of the session recovery process. Upon receipt ( 334 ) of a packet with such a UATI or PSI, the backup RNSM retrieves the corresponding session from its backup session database and restores ( 314 ) it to one of the working RNSMs in the RNC, including itself. In some embodiments, the backup RNSM randomly allocates individual sessions or groups of sessions to itself and the other RNSMs. If a particular working RNSM indicates that it is overloaded, the backup RNSM may not assign additional sessions to that RNSM, in order to avoid exacerbating the overload condition. As each session or group of sessions is restored, the backup RNSM updates ( 316 ) the UATI and PSI forwarding tables of the BIOs so that subsequent traffic destined for that session will be routed to the correct RNSM for that session. Upon determining ( 310 ) that either the timer has expired or that all of the breached sessions have been restored, the backup RNSM terminates ( 312 ) the restoration process, which includes deleting all unrecovered sessions. The backup RNSM also enters a non-protecting mode, where it is no longer sending heartbeat messages to other RNSMs.  
         [0060]      FIG. 8  shows a process for combined reactive and proactive session restoration performed by a backup RNSM. The protecting RNSM  46   a  executes the performing ( 302 ), issuing ( 304 ), determining ( 306 ), and restoring ( 308 ) processes described for the proactive and reactive closure processes  300  and  330  illustrated in  FIGS. 6 and 7 . The backup RNSM then updates ( 332 ) the BIO&#39;s UATI and PSI forwarding tables so that messages intended for the faulty RNSM are routed to the backup RNSM. The backup RNSM determines if a UATI or PSI for a breached session has been received while continually checking ( 310 ) for timer expiration or completion of the session recovery process. Upon receipt ( 334 ) of a packet with such a UATI or PSI, the backup RNSM retrieves the sessions from its backup session database and restores them evenly to each of the working RNSMs in the RNC, including itself. If no such packet has been received, the backup RNSM retrieves ( 314 ) the next breached session or group of sessions from its backup session database and restores them evenly to each of the working RNSMs in the RNC, including itself. In some embodiments, the backup RNSM randomly allocates individual sessions or groups of sessions to itself and the other RNSMs. If a particular working RNSM indicates that it is overloaded, the backup RNSM may not assign additional sessions to that RNSM, in order to avoid exacerbating the overload condition. As each session or group of sessions is restored, the backup RNSM updates ( 316 ) the UATI and PSI forwarding tables of the BIOs so that subsequent traffic destined for that session will be routed properly. Upon determining ( 310 ) that either the timer has expired or that all of the breached sessions have been restored, the backup RNSM terminates ( 312 ) the restoration process, which includes deleting all unrecovered sessions. The backup RNSM also enters a non-protecting mode, where it is no longer sending heartbeat messages to other RNSMs. In this manner, session restoration proceeds in an orderly fashion, with an acceleration to the head of the processing queue of any sessions for which immediate service is requested by the PDSN  27  or an RN (e.g., RN  30   a ).  
         [0000]     Session Closure  
         [0061]     As described earlier, the 1×EV-DO protocol also permits repair of session breach by the closure of a breached session from the network side, resulting in an automatic re-establishment of the session by the AT.  
         [0062]     As will be explained in more detail below, for session closure, the session database is preferably not a backup of all session information maintained by the respective RNSMs, but only includes information sufficient to generate and transmit close session messages to the ATs in the event of a RNSM failure. With reference to  FIG. 2 , if a protecting RNSM  46   a  receives an indication that a protected RNSM  46   b  may have failed, the protecting RNSM  46   a  will direct other RNSMs (e.g., RNSM  46   c ) to transmit close session messages to the effected ATs. For example, if the protecting RNSM  46   a  receives an indication that RNSM  46   b  has failed, it will use the information stored in the backup session database  67   a  to generate a close session message for AT  34   b  and assigns another RNSM (e.g., RNSM  46   c ) to transmit the close session message to AT  34   b . When AT  34   b  receives the close session message, it will terminate its session with RNSM  46   b  and open a new session with one of the operational RNSMs.  
         [0063]     The Base I/O (BIO)  42  receives data packets from the PDSN  27  and routes them to the appropriate RNSM for delivery to an AT. Upon notification of an RNSM failure, the protecting RNSM  46   a  directs the BIO  42  to flag the faulty RNSM to prevent the BIO  42  from sending any further data packets to the faulty RNSM when an application tries to contact a breached session. If the BIO  42  receives a data packet from the PDSN  27  that is addressed to a faulty RNSM  46   b , the BIO  42  forwards the data packet to the RNSM  46   a  that was protecting the faulty RNSM  46   b . The protecting RNSM  46   a  preferably saves the data packet or a subset of information contained within the data packet until a new session for the AT is established on another RNSM or until the new-session establishment procedure times out. Normally the protecting RNSM  46   a  will attempt to reestablish lost sessions according to their order in a queue; however if the protecting RNSM  46   a  receives an indication from the BIO  42  that an application is trying to contact a particular session in the queue, the protecting RNSM  46   a  will promote that session to the top of the queue and service that session immediately, as described for the combined proactive and reactive service order  170  shown in  FIG. 5C . In this manner, the protecting RNSM  46   a  uses notification sent from the BIO  42  to prioritize the close session messages.  
         [0064]     Referring to  FIG. 3 , the protecting RNSM  46   a  maintains a session database  66   a  that contains information for all sessions served by the faulty RNSM  46   b . In this particular implementation, the session database  66   a  is accessed by protection-related RNSM processes via the Session Backup Manager  68   a  software module.  
         [0065]     As shown in  FIG. 9 , the session database  66   a  stores the following information for each session  252 :  
         [0066]     (1) a Unicast Access Terminal Identifier (UATI)  254 , which is an identification code that uniquely identifies an AT  34  when it is registered with the access network;  
         [0067]     (2) a Hardware Identifier (HwID)  256 , which is a unique and permanent identification code of the physical hardware of the AT  34 ;  
         [0068]     (3) a PDSN/PCF Specific Identifier (PSI)  258 , which is a unique identification assigned to each AT  34  that has established a session with the network to allow the PCF subsystem of the RNC and the PDSN to identify to each other the particular session referenced by a data transmission;  
         [0069]     (4) a control channel cycle  260  identifying the periodic interval at which the AT monitors broadcast transmissions from the network when it does not have an active radio resource uniquely assigned to it;  
         [0070]     (5) a set of AT sectors  262  that maps each AT  34  to the particular sectors that the AT was monitoring when it last sent a route update to the network  46 ; and  
         [0071]     (6) Flags  264  that indicate the occurrence of an RNSM failure.  
         [0072]     To generate and transmit a close session message to an AT under the current 1×EV-DO protocol, the RNC maintains at least the AT&#39;s UATI, PSI, control channel cycle and last sectors received as part of a route update. The UATI is used to identify a particular AT when sending the close session message over the control channel. The Hardware Identifier is used to identify an AT in the session database so that the RNC can detect if a session no longer requires closure as it has already been closed and replaced with a new session by an AT-initiated connection attempt. The PSI identifies the session to close when a packet is received from the PDSN  27  destined to an AT whose session has been breached. The control channel cycle is used to transmit a close session message to the AT. The AT sectors are required to optimize the session closure process. If a large number of sessions are to be closed but the location of each AT is not known, the close session message for each AT must be sent over all sectors controlled by the RNC that contains the faulty RNSM. This consumes significant amounts of processing capacity and control channel bandwidth. Knowledge of which sectors from which the AT last reported pilots allows targeting of the close session message to a smaller number of sectors for each AT. The information stored in the session database  66  is sufficient to reestablish lost sessions resulting from an RNSM failure.  
         [0073]     Referring again to  FIG. 3 , the RNSM, e.g., RNSM  46   a , includes a call control process  72  that monitors the status of each AT being served by the RNSM. As the status of ATs change, the call control  72  updates the Active Session Database  66   a . The updates trigger the Session Backup Manager  68  to replicate the whole or part of the recently updated session onto the backup RNSM if the change to the Active Session Database results in the session entering a “syncable” state that is different from the last state that was updated to the backup RNSM. For example, if a new session is created or deleted by an AT, the call control process  72  creates or deletes a corresponding session in the session database  66   a . The RNC  28  exchanges data with the core IP network  26  through an Radio-Packet (R-P) interface that carries user traffic. A single user traffic connection within this interface is an A10 as defined in the 1×EV-DO Standard. If an A10 interface is created or deleted for a session, the call control  72  updates the session database  66   a  which triggers the Session Backup Manager  68  to update the Backup Session Database  67  on the backup RNSM. If an AT  34  sends a route update including radio sectors that are different from the set of sectors which it included in its last route update, or changes the control channel cycle, the call control  72  updates the session database  66   a . As the Session Backup Manager  68  detects update messages from the call control processes, it updates the backup session database  67  accordingly. The Session Backup Manager  68  also maintains the timers required for managing the session close and retry processes.  
         [0074]     An RNSM communicates with the PDSN  27  through the Packet Control Function (PCF)  64 . The PCF  64  controls the transmission of packets between the RNC  28  and the PDSN  27 . The PCF  64  interfaces to the PDSN  27  through an A10 interface that carries user traffic between the PCF  64  and the PDSN  27 . The RNC  28  opens an A10 for each session created for an AT  34 . Upon detection of an RNSM failure, the PCF is configured to instruct the PDSN  27  to stop accounting for the activity of the A10s belonging to the faulty RNSM  46  but leave the effected A10s intact. This instruction can be accomplished by transmitting an ActiveStop message  76  to the PDSN  27 . This step functions to maintain accurate user billing information in the core IP network as the ActiveStop command is a notification that billable user activity has ceased. Because the PCF  64  is not closing the A10s, they will be kept alive indefinitely if no other action is taken. Therefore, the A10s are closed only after a session is closed or when the entire session closure process has timed out. They are reopened when a new session is created by an AT-initiated connection attempt. The Session Backup Manager  68  is also configured to notify the BIO  42  of the affected A10s by, for example, sending the BIO  42  a flag notification to label the A10 entries belonging to the faulty RNSM (e.g., RNSM  46   a ).  
         [0075]     The BIO  42  maintains a PSI table  62  that maps ATs to RNSMs and thus allows the BIO to route data packets bound for an AT to the appropriate RNSM. A PSI table  62  is a lookup structure indexed by PSI that returns the identity of the RNSM where the session addressed by that PSI is located.  
         [0076]     The BIO also includes a Fast Path component  60  that facilitates generation of close session messaging upon detection of an RNSM failure. The fast path component receives notification of faulty RNSMs, and, in response, updates the PSI forwarding table  62 , where the affected sessions are flagged. The fast path component  60  also sends received data packets to the Session Backup Manager  68  of the protecting RNSM  46   a  when it receives a packet from the core network labeled with a PSI whose corresponding RNSM is flagged for session closure in the PSI table. The Session Backup Manager  68  issues the GenerateSessionClose message, updated with the information from the Session Database  80  to a chosen RNSM  46  which then signals the AT  34  to close the session. When the BIO  42  receives a subsequent data packet  78  destined to a faulty RNSM  46 , the BIO will discard the packet  78  if it has already initiated session closure.  
         [0077]     When an RNSM fails, the sessions supported by the faulty RNSM are entered into a queue. The RNC can process close session messages for the sessions waiting in the queue in several ways. For example, as shown in  FIG. 5A , the RNC  28  executes close session messaging using a proactive process  10 , in which breached sessions are serviced immediately upon awareness of an RNSM failure according to their order in the queue  200 . If a user&#39;s session is closed and reestablished before any applications attempt to reach the session, the user will never notice that his session was breached. However, if an application attempts to reach a breached session, that application must wait until all preceding queued sessions have been serviced before that particular session is reestablished. For example, suppose an application attempts to contact the fifth session in the queue  200 . The application must wait for the preceding sessions to be processed before the fifth session can be reestablished, regardless of whether or not the preceding sessions require immediate service. If the fifth session is not reestablished before the application times out, the application will give up and terminate. Thus if many sessions are waiting to be closed, some users may find that an application tried to contact their sessions but could not because their sessions were at the end of a queue of sessions to be closed.  
         [0078]     In a reactive session closure process, the RNC  28  reduces the frequency of application timeouts by transmitting close session messages to only those ATs that an application tries to reach. An example of a reactive session closure process  150  is seen in  FIG. 5B . The fifth session in the queue  200  is highlighted as urgent  204  to indicate that an application is attempting to reach that session. All other queued sessions are marked as non-urgent  202 . The fifth session is serviced immediately while each of the remaining queued sessions must wait until an applications attempts contact. If after a predetermined time, a queued session is not contacted by an application, the RNC times that session out and deletes it from the queue.  
         [0079]     Because the reactive closure process prioritizes urgent sessions above non-urgent sessions, application timeouts are less likely to occur during reactive session closure than during proactive session closure. Furthermore, reactive session closure conserves processing resources that would have been required to close non-urgent sessions. On the other hand, reactive session closure causes all users to experience a brief period of unreachability between the time at which network-initiated traffic arrived for them and the time at which it could be delivered.  
         [0080]     A third method of session closure integrates the proactive and reactive processes to form a combined proactive and reactive process. For example, as shown in  FIG. 5C , a combined proactive and reactive process  170  services non-urgent sessions  202  according their order in the queue  200 . If a session becomes urgent  204 , the process  170  promotes  208  that session to the top of the queue  200  for immediate servicing. The advantage of the combined scheme  170  is that the closure process can be paced so that it does not significantly disrupt normal system operation yet can be facilitated immediately if session closure becomes urgent.  
         [0081]      FIG. 10  shows a process  370  for performing proactive session closure. The protecting RNSM  46   a  continually issues ( 304 ) heartbeat messages to the active RNSM  46   b  while performing normal call processing functions. If the active RNSM  46   b  is working properly, it sends an acknowledgement of the heartbeat messages to the protecting RNSM  46   a . If the protecting RNSM  46   a  receives ( 306 ) an acknowledgement from the active RNSM  46   b , it continues to perform ( 302 ) normal call processing functions. If the protecting RNSM  46   a  does not receive an acknowledgement from the active RNSM  46   b , the active RNSM  46   b  is the active RNSM is faulty and its sessions are breached. Thus, the protecting RNSM  46   a  begins the session closure process. Immediately after an RNSM failure, the RNSM  46   b  reboots. Upon reboot, the RNSM  46   b  notifies the SC  40  that it has booted, allowing the SC  40  to begin using it for new sessions.  
         [0082]     Upon the indication of an RNSM failure, the protecting RNSM  46   a  triggers the rehoming  308  of all RNs that were served by the faulty RNSM  46   b . The RNSM  46   a  sends ( 372 ) a notification of failure of RNSM  46   b  to the BIO  42  to initiate a session closure process described in  FIG. 13 . Upon receiving the notification of failure, the BIO flags all sessions being served by the faulty RNSM  46   b . The protecting RNSM  46   a  also notifies ( 372 ) the SC  40  that the RNSM  46   b  has failed. Upon notification, the SC  40  updates its Session Lookup Database  70  to reflect that the session&#39;s existence is now known on the protecting RNSM  46   a . Prior to the failure of RNSM  46   b , the protecting RNSM  46   a  had been continually storing a subset of the session information for all sessions in the RNC as given to it by the Session Backup Manager  68   a . The session information is later used to reconstitute the breached session. The PCF  64  sends an ActiveStop command  76  to the PDSN  27  ( 374 ) on behalf of each breached session and starts a timer ( 374 ). The timer may be used to terminate the session closure process in situations where session closure may not terminate normally. The ActiveStop command  76  instructs the PDSN  27  to stop billing the A10s for the sessions that were active but to leave those same A10s intact. When a session is to be closed, the protecting RNSM  46   a  assigns a flagged session to a working RNSM (e.g., RNSM  46   c ) and sends the session information to the working RNSM. The protecting RNSM  46   a  sends the session information to a selected working RNSM ( 314 ) instructing the working RNSM to issue a session closure command (i.e., a close session message) to the AT  34 . If the AT  34  requests a new session before the timeout occurs, the RNC  28  assigns the AT  34  to a working RNSM thus reestablishing the breached session. When the working RNSM registers the new session with the SessionDB Lookup App  71  on the SC  40 , the Session DB Lookup App  71  determines that the session is a duplicate because it has the same HardwareID as an existing session. The Session DB Lookup App  71  responds to the working RNSM with an acknowledgement of the registered session and notification of the RNSM on which the duplicate resides—in this case, the protecting RNSM  46   a . At this time, the working RNSM sends a message to the protecting RNSM  46   a  indicating that the session has been closed and identifying the session by its hardware ID. The protecting RNSM  46   a  then deletes that session from its list of sessions awaiting closure. However, if the AT  34  does not request a new session before the timeout, the protecting RNSM  46   a  deletes the timer and all saved session information for sessions that have not already been closed. The protecting RNSM  46   a  determines ( 378 ) whether all breached sessions, served by the faulty RNSM  46   b , have been reassigned or whether the entire session closure timeout has expired. If the determination ( 378 ) is negative, the protecting RNSM  46   a  retrieves ( 314 ) the next session from its backup session database. In some embodiments, the backup RNSM randomly allocates individual sessions or groups of sessions to itself and the other RNSMs. If a particular working RNSM indicates that it is overloaded, the backup RNSM may not assign additional sessions to that RNSM, in order to avoid exacerbating the overload condition. As each session or group of sessions is closed, the backup RNSM updates ( 316 ) the UATI and PSI forwarding tables of the BIOs so that subsequent traffic destined for the session will be routed to the working RNSM to which the session is assigned for closure. Upon determining ( 378 ) that either the timer has expired or that all of the breached sessions have been closed, the protecting RNSM  46   a  terminates ( 376 ) the closure process, which includes deleting the timer and all saved session information for sessions that have not already been closed. The protecting RNSM  46   a  enters a non-protecting mode, in which it no longer sends heartbeat messages to other RNSMs.  
         [0083]     To reduce the impact of the recovery process on normal service delivery, a reactive session closure process  390  is implemented.  FIG. 11  illustrates a reactive session closure process  390  performed by the protecting RNSM  46   a . The protecting RNSM executes the performing ( 302 ), issuing ( 304 ), determining ( 306 ), notifying ( 372 ), and sending ( 374 ) processes described for the proactive closure process  370  illustrated in  FIG. 10 .  
         [0084]     The protecting RNSM  46   a  sends the flagged session information to the BIO  42  to initiate a session closure process described in  FIG. 13 . Instead of initiating session closure immediately for every flagged session as was done in the proactive scheme  370 , the protecting RNSM  46   a  waits for an application to attempt contact with the session from the core network side before initiating a session closure. The protecting RNSM  46   a  first starts a timer to limit the duration of time during which it will attempt to close sessions reactively ( 374 ). The BIO  42  notifies the protecting RNSM  46   a  that an application is attempting contact with a particular breached session by sending the protecting RNSM  46   a  a RecoverSession message that identifies the breached session. The RNSM  46   a  determines ( 392 ) whether a RecoverSession message has been received. If no RecoverSession message is received, the protecting RNSM  46   a  resumes its process of waiting ( 392 ) for RecoverSession messages from the BIO  42  until a timeout occurs After receiving the RecoverSession message, the protecting RNSM  46   a  assigns the session identified by the BIO  42  to a working RNSM. The protecting RNSM  46   a  retrieves the session identified by the BIO  42  from the Backup Session DB  67   a  and sends ( 336 ) the saved session information to the working RNSM indicating that the session is to be closed. The working RNSM sends a close session message to the AT  34 . If the AT  34  requests a new session before the timeout occurs, the RNC  28  assigns the AT  34  to a working RNSM thus reestablishing the breached session. When the working RNSM registers the new session with the SessionDB Lookup App  71  on the SC  40 , the Session DB Lookup App  71  determines that the session is a duplicate because it has the same HardwareID as an existing session. The Session DB Lookup App  71  responds to the working RNSM with an acknowledgement of the registered session and notification of the RNSM on which the duplicate resides—in this case, the protecting RNSM  46   a . At this time, the working RNSM sends a message to the protecting RNSM  46   a  indicating that the session has been closed and identifying the session by its hardware ID. The protecting RNSM then deletes that session from its list of sessions awaiting closure. However, if the AT  34  does not request a new session before the timeout, the protecting RNSM  46   a  deletes the timer and the saved session information. The protecting RNSM  46   a  determines ( 378 ) whether all breached sessions, served by the faulty RNSM  46   b , have been reassigned or whether the entire session closure timeout has expired. If the determination ( 378 ) is negative, the protecting RNSM  46   a  waits for another RecoverSession message  80  from the BIO  42  ( 392 ) before initiating another session closure. The protecting RNSM  46   a  retrieves the session identified by the BIO  42  from the Backup Session DB  67   a  and sends ( 336 ) the saved session information to the working RNSM indicating that it is to be closed and starts a timer. The working RNSM sends a close session message to the AT  34 . As each session or group of sessions is closed, the protecting RNSM  46   a  updates ( 316 ) the UATI and PSI forwarding tables of the BIOs so that subsequent traffic destined for the session will be routed to the working RNSM to which the session is assigned for closure. Upon determining ( 378 ) that either the timer has expired or that all of the breached sessions have been closed, the protecting RNSM  46   a  terminates ( 376 ) the closure process, which includes deleting the timer and all saved session information for sessions that have not already been closed. The protecting RNSM  46   a  enters a non-protecting mode, in which it no longer sends heartbeat messages to other RNSMs.  
         [0085]      FIG. 12  illustrates a combined proactive and reactive session closure scheme  400  that integrates the proactive  370  and the reactive  390  session closure schemes described in  FIG. 10  and  FIG. 11 , respectively. Both the reactive session closure process  390  of  FIG. 11  and the combined closure process  400  of  FIG. 12  use the RecoverSession message sent from the BIO  42  to prioritize close session messaging. The protecting RNSM  46   a  executes the performing ( 302 ), issuing ( 304 ), determining ( 306 ), notifying ( 372 ), and sending ( 374 ) processes described for the proactive and reactive closure process  370  and  390  illustrated in  FIGS. 10 and 11 .  
         [0086]     In the combined process  400 , the protecting RNSM  46   a  determines ( 378 ) whether all breached sessions in queue  200  have been reassigned or whether the entire session closure timeout has expired. If the determination ( 378 ) is negative, the protecting RNSM  46   a  determines ( 392 ) whether a RecoverSession message  80  from the BIO  42  has been received.  
         [0087]     If the protecting RNSM  46   a  receives a RecoverSession message from the BIO  42 , the protecting RNSM  46   a  will immediately process ( 336 ) the session identified by the BIO  42 , thus effectively promoting that session to the front of the queue  200 . If the protecting RNSM  46   a  does not receive a RecoverSession message from the BIO  42 , the protecting RNSM  46   a  will process ( 314 ) the sessions according to their order in the queue  200  at a user-configurable rate. The rate may be selected to optimize a trade-off between the speed at which sessions are closed and the processing power required to close the sessions.  
         [0088]     As each session or group of sessions is closed, the protecting RNSM  46   a  updates ( 316 ) the UATI and PSI forwarding tables of the BIOs so that subsequent traffic destined for the session will be routed to the working RNSM to which the session is assigned for closure. Upon determining ( 378 ) that either the timer has expired or that all of the breached sessions have been closed, the protecting RNSM  46   a  terminates ( 376 ) the closure process, which includes deleting the timer and all saved session information for sessions that have not already been closed. The protecting RNSM  46   a  enters a non-protecting mode, in which it no longer sends heartbeat messages to other RNSMs.  
         [0089]     Referring to  FIG. 13 , the close session messaging process  420  performed by the BIO  42  is shown. While performing ( 422 ) normal procedures, the BIO  42  continually determines ( 424 ) whether a RNSM failure notification has been received from a protecting RNSM  46   a . The BIO  42  flags ( 426 ) all session entries in its PSI table  62  and its UATI table  63  for the faulty RNSM  46   b  and updates the location of the sessions in the session entries to point to the protecting RNSM  46   a . The BIO  42  receives data packets  78  from the PDSN  27  and determines ( 428 ) whether the packets are directed toward a faulty RNSM. If the packets are directed toward a faulty RNSM, the BIO  42  determines ( 434 ) whether the session, to which the packets belong, are flagged as “pending recovery.” If the session is not flagged, the BIO  42  flags ( 440 ) the session as “pending recovery,” starts a timer, and sends a RecoverSession message to the protecting RNSM  46   a . The RecoverSession message notifies the protecting RNSM  46   a  that an application has attempted to contact a breached session. The BIO  42  determines ( 436 ) whether the timer has expired. Subsequent packets received for the flagged sessions while the timer is not yet expired are discarded ( 438 ). If the timer has expired, a new RecoverSession is sent ( 440 ) to the protecting RNSM  46   a  and the timer is restarted. If the BIO  42  determines ( 428 ) that a received packet is not directed toward a faulty RNSM, the BIO  42  determines ( 430 ) if the timers for any other breached sessions that are pending recovery have expired. If the timers have not expired, the BIO  42  continues receiving packets and determining ( 428 ) whether they are directed toward faulty RNSMs. If a timer for a session has expired, the BIO  42  restarts the timer and generates ( 432 ) a new RecoverSession message for the session.  
         [0090]     The RNC  28  could be implemented by a number of different hardware configurations. One possible configuration is a dedicated chassis containing multiple processing cards, where each card performs the functions of either the SC  40 , the BIO  42 , or the RNSM (e.g., RNSM  46   a ). In a preferred embodiment, the processing card is a Compact-PCI or Advanced TeleCommunications Architecture (A-TCA) Card containing non-volatile RAM, flash memory, and a flash disk. A second configuration is a blade server. A blade server consists of a chassis containing multiple cards where each card is equivalent to a complete single-board computer, except that common resources such as hard disk drives and power supplies are shared between all the cards. The RNC  28  could also be implemented on standalone servers arranged as a collection of independent computers in which each computer plays the role of an RNC  28 . In this standalone-server configuration, each computer consolidates the functions of the SC  40 , the BIO  42 , and the RNSM (e.g., RNSM  46   a ) into a single processor. An alternate embodiment of the standalone server model utilizes each computer as either an SC, RNSM or BIO. Another configuration for the RNC  28  could be an integrated system containing embedded processors that perform the functions of the SC  40 , the BIO  42 , or the RNSM (e.g., RNSM  46   a ). A further configuration is an application-specific integrated circuit (ASIC) in which all functions of the RNC  28  are performed by a single chip. The PCF  64  could also be implemented as a virtual PCF and reside on all RNSMs  46  simultaneously or on a given RNSM (e.g., RNSM  46   a ) with the ability to be relocated to another RNSM (e.g., RNSM  46   b ).  
         [0091]     Other embodiments are within the scope of the following claims. For example, the steps described in  FIG. 6 ,  FIG. 7 ,  FIG. 8 ,  FIG. 10 ,  FIG. 11 ,  FIG. 12  and  FIG. 13  could be performed in an order that is different than the ordering shown in the figures.