Abstract:
The present invention provides a system with a redirect functionality that removes the potential of a bottleneck inherent with a central redirect server. In a communications system according to the present invention, the central redirect server is eliminated in favor of a distributed redirect server. This distributed redirect server is distributed to, and hosted on, telephony access nodes (TANs) in the system. This configuration resolves both the problem of a potential bottleneck and provisioning of redundant redirect server capability.

Description:
FIELD OF INVENTION  
       [0001]     The present invention relates, in general, to communications systems and relates, more specifically, to the provision of services on such systems within a network.  
       BACKGROUND OF INVENTION  
       [0002]     In a communications system, a multinode messaging system may appear as a single system to the network while internally, it must distribute calls between its internal nodes. Such a system may employ a property called redirect. The redirect property is available in such protocols as the Session Initiation Protocol (SIP). Using redirect, a call is sent to a common externally known address of the messaging system. Those calls are then redirected to the node that will actually terminate the call. Since the redirection is a central-type function, it is normally placed on a centralized element of back-end hardware.  
         [0003]      FIG. 1  illustrates such a prior art system  100  that makes use of a redirect server to terminate a call. Redirect server  140  receives a message signal from switch  120 . The switch  120  may be a telephony switch such as a private branch exchange (PBX) switch. Depending on the availability of requested resources, the redirect server  140  will pass the message signal to one of telephony access nodes (TANs)  160  via an appropriate path. The TANs  160  terminate message signaling. The redirect server  140  distributes messages for the TANs  160 .  
         [0004]     There are two main drawbacks with a system such as that illustrated in  FIG. 1 . The first drawback is that the redirect server, due to its centralized nature, can cause a bottleneck in cases of increased traffic. Second, if a malfunction should occur in the redirect server, the entire system could cease to function properly, so a second redundant redirect server must be provisioned for high availability systems.  
         [0005]     Therefore, it would be desirable to have a method and system that would address the above shortcomings of the centralized redirect server.  
       SUMMARY OF INVENTION  
       [0006]     It is an object of the present invention to provide a system that removes the potential limitation of a bottleneck inherent with a central redirect server plus provides an inherent redundancy strategy.  
         [0007]     The above object is achieved by eliminating the central redirect server. The redirection functionality is instead distributed across all telephony access nodes (TANs). This configuration resolves the problem of scalability since, as TANs are added for increased capacity, the redirect server capacity is augmented as well.  
         [0008]     According to a first aspect of the invention, there is provided a communications system comprising: a switch for accepting incoming calls; a plurality of telephony access nodes for terminating said incoming calls; a distributed redirect server hosted on each of said plurality of telephony access nodes; and a load balancing unit for directing said incoming calls from said switch to one of the plurality of telephony access nodes, said plurality of telephony access nodes being connected to a back end cluster.  
         [0009]     According to another aspect of the invention, there is provided a method of terminating a call in a communications system, said communications system comprising a switch, a plurality of telephony access nodes, a distributed redirect server, and a load balancing unit the method comprising the steps of generating an incoming call at said switch; directing said incoming call from said switch via said load balancing unit to said distributed redirect server for termination at one of said telephony access nodes; verifying, at said distributed redirect server, whether said one telephony access node has sufficient resources to answer said incoming call; and terminating said incoming call at said one telephony access node.  
         [0010]     According to a further aspect of the invention, there is provided a method of terminating a call in a communications system, said communications system comprising a switch, a plurality of telephony access nodes, a distributed redirect server, and a load balancing unit, the method, comprising the steps of: accepting an incoming call at said switch; directing said incoming call from said load balancing unit to said distributed redirect server for termination at a first one of said telephony access nodes; verifying, at said distributed redirect server, whether said first telephony access node has sufficient resources to answer said incoming call; determining whether a second one of said telephony access nodes has sufficient resources to answer said incoming call when said step of verifying has concluded that said first telephony access node does not have sufficient resources to answer said incoming call, wherein any of said redirect servers would have knowledge of available resources of any other TAN, and that said TAN could send a redirect message back to the switch and would direct it to send the Invite message to a specific TAN with available resources; and terminating said incoming call at said second telephony access node. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     Embodiments of the present invention will be further described with respect to the accompanying drawings in which:  
         [0012]      FIG. 1  illustrates a prior art system that makes use of a redirect server to terminate a call;  
         [0013]      FIG. 2  illustrates a system with a distributed redirect functionality according to an embodiment of the present invention;  
         [0014]      FIG. 3A  illustrates steps in a call termination procedure in accordance with the system of  FIG. 2 ; and  
         [0015]      FIG. 3B  illustrates steps in another call termination procedure in accordance with the system of  FIG. 2 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     With reference to  FIG. 2 , a system  200  is shown which employs a distributed redirect server  240  wherein the redirect server functionality of prior art systems is distributed at each location of a TAN  260 . A load balancing unit  230  is inserted before the distributed redirect server  240  in order to accept message calls from switch  120 . The load balancing unit  230  is an unintelligent hardware device that directs incoming calls to one of the redirect servers  240  for subsequent termination at one of the TANs  260 .  
         [0017]     As can be seen in  FIG. 2 , the TANs  260  not only terminate media and signaling from the PBX switch, but they also host each element of the distributed redirect server  240 .  
         [0018]     As mentioned previously, the distributed redirect server  240  is configured to present the messaging system as a single entity to switch  120  by means of a single external address. However, incoming message calls must still be terminated at the appropriate TAN  260 . Therefore, the distributed redirect server  240  requires that each of the TANs  260  broadcast their resource availability status to every other TAN in the system. If a particular TAN does not have the resources to answer an incoming call, it needs to understand that another TAN in the system does have the necessary resources available.  
         [0019]     The communication of resource availability status will be accomplished via an Internet Protocol (IP) multicast. Each TAN will report via IP multicast to every TAN in the system whether it has a status of Free or Busy. A TAN may transition from Free to Busy when its number of free channels drops to a value of i. This value of i may be equal to 2 so as to avoid conditions where a TAN showing a status of Free is actually busy as other calls are terminating on it while updating its status. A TAN may transition from Busy to Free as the number of free channels increases to a value j. This value j may be 4, in order to provide some hysterisis and prevent a TAN from oscillating from Free to Busy on a single call, causing excessive status multicast traffic. The actual values of i and j may be optimized on heuristics which may be based on traffic patterns as well as on the number on TANs. Each TAN will create a “map” of the status of the other TANs in the system on the basis of the information from the received multicast messages.  
         [0020]     An example of a call termination will now be described with respect to  FIG. 3A . In this figure, step  302  represents a step whereby switch  120  issues an Invite message to the external virtual IP address of the system. Load balancing unit  230  receives the Invite message and forwards it at random to one of the TANs (TAN  1  in this case). At this point, TAN  1  verifies whether it has the necessary resources to answer the call. The result of this determination is sent, as shown in step  306 , back to the switch  120 . In this case, TAN  1  has the necessary resources and answers the call.  
         [0021]     Redundancy is provided in the following manner. First, there are multiple instances of the redirect server  240 . Second, the load balancing unit  230  maintains a view of the ‘sanity’ of each of the redirect servers. If a redirect server was to fail, the load balancing server would note it and stop distributing calls to that node. Therefore, failure of any single redirect server will not impact the overall system.  
         [0022]      FIG. 3B  illustrates a situation wherein the TAN to which a call is forwarded does not have the necessary resources. Similar to the previous figure, step  312  represents a step whereby switch  120  issues an Invite message to the external virtual IP address of the system. Load balancing unit  230  receives the Invite message and forwards it at random to one of the TANs (in this case, TAN  3 ). However, in this case TAN  3  has no available resources. Prior to the arrival of this Invite message, in step  316 , TAN  3  would have multicast its transition to a Busy state, so that every other TAN in the system could update its map of TAN status. In this particular TAN 3 &#39;s map indicates that TAN  1  is available. Consequently, in step  318 , TAN  3  informs the switch  120  that it should send an invitation to the specific IP address of TAN  1  for this particular call, not to the virtual IP address of the entire system. At this point, step  320  illustrates the switch  120  issuing that invitation directly to the redirect server  240  associated with TAN  1 , thereby bypassing load balancing unit  230 . Since it is already known that TAN  1  has the necessary resources to answer the call, step  322  shows the confirmation of the ability of TAN  1  to terminate the call.  
         [0023]     In a case where a particular TAN discovers another TAN with idle resources, the call is usually relayed to the respective TAN via the redirect server. If none of the TANs in the system has available resources, the invited TAN may return a busy signal. Alternatively, the call may be placed in a queue employing the first-in-first-out methodology. The call will then be terminated on any subsequently available TAN by routing the call to the TAN with the first available free channel.