Abstract:
A method and apparatus for analyzing current voice call quality in an IP network before allowing a new voice call to enter therein. Circuitry in each gateway between a PSTN and the IP network obtains information from the IP network regarding the quality of voice call traffic being transmitted from one gateway to another gateway. A parameter is calculated by the circuitry based on this information. This parameter is compared to predetermined thresholds to guarantee acceptable quality for a new voice call that is attempting to enter the IP network. The circuitry includes three circuits. A first circuit passes voice call data to the IP network. A second circuit polls the IP network about traffic information transmitted therein. A third circuit processes the polled information to determine whether the voice call data is to be accepted by the IP network.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to the field of communications systems and more specifically to the management and admission control of voice over IP (VoIP).  
       DESCRIPTION OF THE BACKGROUND ART  
       [0002]     Telecommunications networks and other networks are increasing in both size and complexity in order to serve the growing demand for high-speed communication networks for the transfer of voice and/or data information. As these telecommunication networks grow, alternate solutions or networks are sought to meet the demand for increasing network bandwidth and new IP-based services.  
         [0003]     Traditionally, voice calls are transported entirely over the end-to-end, circuit-based Public Switched Telephone Network (PSTN). However, considerable attention has been directed toward the implementation of real-time communication across computer data networks, and particularly the ability to route voice traffic to and from the PSTN. Interest has also been raised in using Voice over IP (VoIP) solutions to facilitate voice communication between originating and terminating PSTN end points via an IP network. Using the Internet for long haul routing substantially bypasses the PSTN.  
         [0004]     For PSTN bypassing applications, voice traffic is processed into IP (or ATM) packets, transported over an IP network (or ATM network), and then processed back to PCM voice. To facilitate such call routing, originating and terminating End Office (EO) switches can be connected to PSTN/IP (or PSTN/ATM) gateways that reside as hosts on the IP (or ATM) network. Based on the called number or other signaling indicator, the EO switches route certain calls through the IP (or ATM) gateways instead of the PSTN.  
         [0005]     Unfortunately, when a new VoIP telephone voice call is established (with the intent of it being routed between two gateways in the network), there are no means to evaluate the level of congestion of the core IP network. In other words, it is possible to have too many new voice calls being introduced to the network at the same time so that the core IP network is overloaded. Under such a condition, it is highly likely that packets of information that contain the voice data will either be lost or delayed from arriving at the destination gateway. Both of such conditions result in poor Quality of Service (QoS) of the network which is an undesirable.  
         [0006]     Two possible solutions to ascertaining the level of congestion of the core IP network is to overprovision the network such that packet loss will not occur and to reserve sufficient bandwidth between gateways and count voice calls on each path. However, the first solution requires expensive overbuilding of the network and the second solution is relatively complex to operate. Accordingly an improved means for establishing an admission policy of voice calls to a network is desirable.  
       SUMMARY OF THE INVENTION  
       [0007]     The disadvantages heretofore associated with the prior art are overcome by a novel method and apparatus for analyzing the level of voice call traffic in an IP network before allowing a new voice call to enter the IP network.  
         [0008]     In particular, in one embodiment, analysis circuitry is provided to each gateway between a PSTN and the IP network. Such circuitry obtains information from the IP network regarding the level of voice call traffic being transmitted from one gateway to another gateway. A parameter is calculated by the analysis circuitry based on obtained information. This parameter is then compared to predetermined thresholds to guarantee acceptable quality for a new voice call that is attempting to enter the IP network. If the value of the parameter is below a lower threshold, voice call quality is highly acceptable and the new voice call is allowed into the IP network. If the value of the parameter is between the lower threshold and an upper threshold, voice call quality is acceptable, but the new voice is allowed into the IP network at a reduced bandwidth in comparison to existing calls in the network whose parameter was below the lower threshold. If the value of the parameter is above the upper threshold, voice call quality will be unacceptable and the new voice is not allowed into the IP network.  
         [0009]     In one embodiment, the parameter is a packet loss ratio (PLR). The PLR considers lost, late and received packets measured at a particular gateway along a particular path and reported back to a gateway that had sent such packets. The PLR is calculated by the equation A/(A+B) where A is the sum of lost and late packets arriving at the particular gateway along the particular path and B is the total number of successfully received packets arriving at the particular gateway along the particular path.  
         [0010]     In particular, the apparatus includes a gateway for interfacing voice call data from a PSTN to an IP network. The gateway further includes a first circuit for passing said voice call data to the IP network, a second circuit for polling the IP network about traffic information transmitted therein and a third circuit for processing the polled information to determine whether the voice call data is to be accepted by the IP network.  
         [0011]     In one embodiment, the first circuit includes one or more interface cards that are connected to the IP network and the second circuit is at least one strongarm card connected to said interface card via a host CPU circuit. The third circuit compares the parameter (PLR) based on the polled information to the upper and lower thresholds to make the appropriate decision of allowing, blocking or allowing at reduced bandwidth a new voice call. In doing so, quality of all calls on the network is maintained and new calls are not permitted into the network until such time that their quality is at minimum requirements. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:  
         [0013]      FIG. 1  depicts a general overview of a communication network that employs the subject invention;  
         [0014]      FIG. 2  depicts a schematic view of a portion of the communication network shown in  FIG. 1 ;  
         [0015]      FIG. 3  depicts a detailed schematic view of a portion of one of the gateways depicted in either of  FIG. 1  or  2  detailing the interconnection of the subject invention therewith;  
         [0016]      FIG. 4  depicts a call set flow diagram that operates in accordance with the subject invention;  
         [0017]      FIG. 5  depicts a graph of offered load in a communication system employing the subject invention versus the probability of a blocked call;  
         [0018]      FIG. 6  depicts a graph of the update interval of status reports generated in accordance with the subject invention versus the blocking ability;  
         [0019]      FIG. 7  depicts a graph of the number of consecutive packets that are lost in a communication network versus the percentage of loss of said packets;  
         [0020]      FIG. 8  depicts a flow chart of a call admission process of the subject invention; and  
         [0021]      FIG. 9  depicts a flow chart of a process for updating a rules database associated with the subject invention. 
     
    
       [0022]     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.  
       DETAILED DESCRIPTION  
       [0023]     The subject invention establishes and manages VoIP traffic in a network (for example an Internet Protocol (IP) network) by monitoring certain criteria indicative of network capability and instantaneous load. Accordingly, an exemplary telecommunications system is described as one potential environment in which a subject invention operates and exists.  
         [0024]      FIG. 1  depicts an exemplary telecommunications system  100  for routing telephone calls between a first wire line subscriber  102  and a second wire line subscriber  104  in a PSTN  110 . Such telephone calls are routed across an intermediate data network  118  implementing a network layer protocol, such as IP (or a link layer protocol such as asynchronous transfer mode (ATM) or both). The telecommunications system  100  includes a first subscriber end office unit  106  connected to the first subscriber  102  and a second end office  108  connected to the second subscriber  104 . Interconnection of these components is achieved via conventional local loop subscriber lines ( 103  and  105  respectively). For example, such first subscriber line  103  and second subscriber line  105  would typically be implemented using two-element twisted pair wires carrying analog information or basic rate ISDN digital information depending on the configuration of the wire lines subscriber units  102  and  104 . Communication between the PSTN  110  and first end office  106  and second end office  108  would typically utilize trunk groups  124  carrying PCM digital voice traffic on multiplexed channels at a primary rate of 1.544 MBPS, 2.048 MBPS or better via a plurality of switches  126 .  
         [0025]     It is also possible to bypass the PSTN  110  using the data network  118 . Such an alternate communication path is established by connecting the first end office  106  to a first gateway  114  and likewise connecting the second end office unit  108  to a second gateway  116 . First gateway  114  and second gateway  116  may be a single unit (as shown as a single structure  114  or may be represented by one or more independent structures A,B and C of second gateway  116 . First and second gateways  114  and  116  respectively reside as hosts on the network  118 . They provide VoIP services on behalf of the first wire line subscriber  102  and second wire line subscriber  104  and other users (not shown) communicating over the network  118 . During VoIP communications between the first wire line subscriber  102  and the second wire line subscriber  104 , PCM traffic is routed from the first end office  106  and second end office  108  to the respective gateways  114  and  116  for routing across the data network  118 . Call control is managed through the Softswitch  112 . When a new call is set up or a completed call is torn down, signaling messages are exchanged between the first end office  106  and the Softswitch  112  and between the Softswitch  112  and the second end office  108 . The Softswitch  112  also acts as a gateway controller and exchanges messages with each gateway. In some networks there may be different Softswitches  112  controlling each gateway( 114 ,  116  or the like) and these Softswitches exchange signaling messages with each other.  
         [0026]      FIG. 2  depicts a detailed schematic of the first and second gateways ( 114  and  116  respectively) and their interconnection within the IP network  118 . Specifically, first gateway  114  includes a plurality of ports  206   x  that provide access to and from the network  118 . Similarly, the second gateway  116  also has a plurality of ports  208   x  for interfacing with the network  118 . Between the first gateway  114  and network  118  and the second gateway  116  and the network  118  there may be one or more edge routers  202   x  that exist to manage the traffic flow between the respective gateways and the network  118 . Specifically, data paths are established between the first gateway  114  and for example first edge router  2021  (denoted as E1 and E2). Similar pathways (such as path E3 is established between first gateway  114  and second edge router  2022 . Likewise similar pathways are established between the second gateway  116  and a third edge router  2023  (denoted as E4) and a fourth edge router  2024  (denoted as E5 and E6).  
         [0027]     Within each gateway ( 114  and  116 ) is a corresponding admission control module (first admission control module  204   1  is in first gateway  114  and second admission control module  2042  is in second gateway  116 ). These admission control modules  204   x  monitor VoIP traffic and generate the necessary reports to decide which calls will be granted access through the network  118  to maintain overall quality of service for all subscribers. A Measurement-Based Call Admission Control (MBCAC) algorithm contained within said admission control modules  204   x  is described in greater detail below.  
         [0028]     The MBCAC algorithm operates in each gateway ( 114 , 116 ) independent of the other gateways in the network. Call quality statistics for a Real Time Transport Protocol (RTP) stream reflect the congestion status of the path followed by that stream. Thus, by observing these statistics, one can decide on the congestion status of the network paths.  FIG. 2  depicts exemplary RTP flows  210  and  212  between two gateways over the IP network  118 . Each gateway, ( 114 , 116 ) has a number of DSP chips (described in greater detail below), which convert voice streams in TDM format into IP packets. These packets are sent to destination gateways using the necessary protocols and in one embodiment is a RTP/UDP/IP protocol stack over a link protocol such as PPP.  
         [0029]     Packets traveling to a destination gateway can follow different paths based on the port  206   x  chosen for the specific RTP flow. The MBCAC algorithm assumes that the selection of a port  206   x  for an incoming call request is under the control of a call controller in the gateway. Hence, the MBCAC algorithm keeps separate admission policies for the paths from different ports to the same destination gateway. It is also assumed that multiple calls going from a particular port to the same destination gateway follows the same path, i.e., there is no load balancing within the network other than provided by the gateways through the selection of an egress port. This assumption can be satisfied if the gateways use the system IP address of the destination gateway as opposed to the IP addresses of its ports. In this framework, load balancing is supported by controlling the egress port at the source gateway (i.e., first gateway  114 ). Since each egress port would map into a unique path in the IP network  118 , the load from source gateway  114  to a destination gateway (i.e., second gateway  116 ) can be partitioned into different paths, resulting in load sharing in the network.  
         [0030]     The destination gateway  116  receives the RTP packets generated by the source gateway  114  (e.g., at port E2) and addressed to itself. For each RTP stream, the receiver measures call quality statistics like packet loss ratio, delay and interarrival jitter for the stream. The measured statistics are sent back to the source gateway  114  periodically in a special field within the RTP packets or in RTCP packets. In one example, these statistics reflect the network conditions for the path following (E2-ER1-Network-ER3-E4). Thus, the MBCAC algorithm utilizes the call quality statistics of this flow to derive the congestion status of the directed path, uniquely defined by the source gateway E2, destination gateway pair.  
         [0031]     The call quality statistics sent by the destination gateway  116  are collected by an RTP termination point in this gateway and formed into a call quality report. To support the MBCAC function, RTP flows are grouped into sets represented by (Egress port, Remote Gateway)-pair, i.e., there is a list of RTP flows for each (Egress port, Remote Gateway)-pair. Using the example introduced in  FIG. 2 , there is a set of RTP flows that are uniquely specified by the (source gateway interface E2, destination gateway)-pair. When a call quality report for a particular RTP stream arrives at the source gateway  114 , this information is processed based on the (Egress port, Destinations Gateway)-pair. For each such pair, the maximum observed packet loss ratio is reported to the call control logic (as seen in  FIG. 3  and explained in greater detail below) periodically. The period for the reporting is referred to as “CAC Update Interval”. At the end of each such interval, the maximum packet loss ratio over the set of RTP flows related to each (Egress port, Destination Gateway)-pair is determined using the most recent measurements for each flow. The result is reported to the call control logic, where the admission control decisions are made. An alternative to periodic reporting of the RTP performance information is to set the thresholds and policy so that only exceptions are reported to the call control logic. This has the advantage of reducing the messaging within the gateway and speeding up the responsiveness of the algorithm to congestion. When there are many flows associated with the same path, it would be computationally expensive to determine the maximum packet loss over all associated flows. To address this problem, the maximum packet loss can be determined for a subset of these flows. Moreover, the CAC Update windows can be made independent for each path.  
         [0032]      FIG. 3  depicts a detailed schematic of the internal arrangement and connection of components in one of the gateways associated with the subject invention. Specifically,  FIG. 3  depicts the inner connections of elements in first gateway  114 ; however, this arrangement can also be duplicated in second gateway  116  or any number of other gateways extending from the network  118 . The first gateway  114  consists of, among other things, a plurality of circuit cards interconnected in a manner so as to facilitate the passing of information packets to and from the network  118  as well as make determinations on the level of congestion on pathways in which said information packets are passed. The plurality of cards includes a shelf control card  302 , one or more MADD cards  304   x  and one or more port cards  306   x . In one embodiment of the invention, the port cards  306   x  are interface cards operating in accordance with known Ethernet protocols for interfacing with the IP network  118 . On each of said MADD cards  304   x  there is a plurality of strong arm (SARM) cards  310   x  connected to a host CPU card  312 . The shelf control card  302  contains three basic circuit components: the MBCAC algorithm circuitry or processor  204 , a rules database  314  and a call control circuit  308 . These three components interact with each other as explained in greater detail below to process VoIP traffic and new call requests into the network.  
         [0033]      FIG. 4  depicts a call set up signaling scenario between a first gateway  114  and second gateway  116  via soft switch  112  in accordance with the subject invention. While the discussed example of call flow is based on H.248 protocol, it will be understood by those skilled in the art that other types of protocols can be used in conjunction with the subject invention and achieve the desired results. Examples of such additional protocols are selected from the group consisting of SIP and H.323.  
         [0034]     The flow diagram begins at step  402  with the soft switch  112  receiving a call set of requests from the PSTN network  110  (as per  FIG. 1 ) resulting in a message being sent to first gateway  114 . Said message contains incoming call information that includes which voice trunk of first gateway  114  the voice call will arrive on. At step  404 , first gateway  114  creates a RTP port (one of the egress ports  206   x ) and maps it to the TDM trunk based upon the incoming message from the soft switch  112 . First gateway  114  then prepares a response message which contains information including for example context, RTP termination ID, IP address and ports and list of supported codecs chosen from the list presented by soft switch  112 . This message is sent back to soft switch  112  acknowledging that the TDM trunk to RTP port mapping has been accomplished.  
         [0035]     At step  406 , the soft switch  112  sends a message to second gateway  116  that includes the IP address and RTP port of first gateway  114  upon which the call is being set up as well as information about the destination PSTN switch. Since second gateway  116  now has information about first gateway  114 , second gateway  116  checks the admission control policy of the path to first gateway  114 . If the path is congested, an error message is generated at step  408  indicating that there is insufficient bandwidth to establish the desired path. If the call is to be accepted (i.e., there is sufficient bandwidth available to set up the call), second gateway  116  creates an RTP port (one of the egress ports  206   x ) and maps this into a voice trunk to the destination PSTN switch. This information is sent back to soft switch  112  as an “add response” message at step  410 . This message is forwarded by the soft switch  112  to first gateway  114  so that the RTP port in first gateway  114  can be modified to include a transmit direction. At step  412  the necessary modifications are made to the TDM trunk based on the response from second gateway  116 .  
         [0036]     Next, first gateway  114  consults with the admission control algorithm to see if there is a path to the second gateway  116  that is not congested. If first gateway  114  is unable to find an uncongested path to second gateway  116  it sends an error message at step  414  to the soft switch  112 . In such a scenario where the call attempt has been denied the call set up process is terminated by “subtract command” messages sent by the soft switch  112  to the first and second gateways,  114  and  116  respectively at step  416 . In response to the subtract command messages of step  416 , first gateway  114  and second gateway  116  provide subtraction command responses to the soft switch  112  at step  422  thereby completing the denied call set up attempt. If there is sufficient bandwidth available to set up the incoming call based on the first gateway  114  admission control algorithm results, a “modify response” message is sent back to the soft switch  112  at step  418 . This signals the soft switch  112  that the data path for the voice call is ready for data transmission in both directions. Resultantly, an RTP session is established at step  420  and the voice call begins.  
         [0037]      FIG. 8  depicts a flow chart of the decision process executed by the admission control module  204  when practicing the admission policy (MBCAC algorithm in accordance with the subject invention). There are two separate asynchronous processes that operate. One is the updating of admission control policy based on RTP performance reports that are received. The other is the application of the admission control policy when a new call request arrives. Specifically,  FIG. 8  depicts the first process whereby the admission control policy is updated. This is shown through a series of method steps  800  that begins at step  802 . The method then proceeds to step  804  where quality of service (QoS) information is obtained for further evaluation. In one embodiment of the invention, the host CPU  312  of one of the MADD cards  304   x  will poll one of the strong arm cards  310  to receive the quality of service information from the network  118 . Quality of service information is for example packet loss information (i.e., packets that were known to be transmitted from a first point, for example first gateway  114  but not received at its destination point for example second gateway  116 ). Once the quality of service information is obtained, the method moves to step  806  where a quality statistic is computed based upon the quality of service information. In one embodiment of the subject invention, the quality statistic is a Packet Loss Ratio which is defined as  
       PLR   =       (       lost   ⁢           ⁢   packets     +     late   ⁢           ⁢   packets       )       (       received   ⁢           ⁢   packets     +     lost   ⁢           ⁢   packets     +     late   ⁢           ⁢   packets       )             
         [0038]     For the purposes of the subject invention, late packets are defined as packets that are discarded at the destination gateway (for example second gateway  116 ) since they are too late to be played or otherwise incorporated into the active voice call. Additionally, it should be noted that lost, late and received packets (collectively “sent” packets) are defined for the outgoing direction of a voice call, measured by the destination gateway (second gateway  116 ) and reported back to a source gateway (for example first gateway  114 ) in the opposite direction. Since each direction of the call takes a separate path through the IP network, there is a separate admission control decision for each direction of the call. All packet counts are defined per RTP connection. Furthermore, packet counts used in the packet loss ratio computation are counts measured over the most recent reporting period. In one embodiment of the invention, the reporting period is approximately two seconds; however, one skilled in the art will realize that various other reporting periods are possible dependent upon hardware and software being used in the overall system and network as long as the desired results are achieved. Other quality information could involve delay or delay variation.  
         [0039]     The method continues at step  808  where a first admission policy is established. In one embodiment of the invention, the admission policy consists of two threshold values. In the first decision step  808 , the first threshold value (a lower threshold P_low is introduced. The computed PLR is compared to the lower threshold P_low. If the PLR is less than P_low, the method proceeds to step  810  where the policy is set to accept new calls without any limitations. The method then awaits the next reporting period and loops back to step  804  to obtain the new quality of service information to continue evaluation. A reporting period is in the range of approximately 5-60 seconds and in one embodiment is 5 seconds. The exception reporting option will make this updating faster during congestion.  
         [0040]     Should the PLR be higher than the lower threshold, the method proceeds to step  812  where the PLR is compared to a second threshold (a high threshold P_high). If the PLR is larger than the lower threshold P_low, but lower than the higher threshold P_high, the method proceeds to step  814  where the policy is set to admit new calls at reduced bandwidth. Such action reduces the bandwidth of new incoming calls to an extent that still allows quality of service. Similar to the accepted call scenario, accepted-bandwidth limited call step  814  loops back to step  804  to await the next reporting period to attain quality of service information.  
         [0041]     Should the PLR be higher than the high threshold, the method proceeds to step  816  where the policy is set to block all or some percentage of new call requests from entering the network. In other words, path congestion has reached such a limit that an unacceptable number of packets are either being lost or received too late to be part of a call. As such, it is realized that no new calls can enter the network and maintain an acceptable quality of service level; therefore, such calls are not allowed into the network until path congestion is sufficiently reduced and quality of service can be maintained for all subscribers. The method ends at step  818 .  
         [0042]     Bandwidth reduction (as discussed above in step  814  of method  800 ) is achieved in a few different methods. One method is to physically change the encoder that is being used for the particular voice call. That is, there may be two or more encoders in a gateway ( 114  or  116 ) that carry out encoding tasks (one encoder having high bit rate characteristics and another having lower bit rate characteristics). If a bandwidth-reduced call is accepted, the encoder with the lower bit rate characteristics is used. When conditions allow for non-bandwidth limited channels, the system can switch back to the higher bit rate encoder. Another way in which bandwidth can be reduced is to use the same codec but increase the packet size. This will reduce the relative packet overhead. Another way of reducing bandwidth as per step  814  is to reduce the bitrate by activating silence suppression for the voice call. Briefly, silence suppression results in reducing bandwidth requirements since no packets are sent during silence periods. In most conversations only one person is talking so that, on average, in any one direction there is speech to send at most half the time. Thus suppressing packets representing silence can save considerable bandwidth. Note that silence suppression does not apply to fax calls, where picking a very large packet size would be more useful.  
         [0043]     The above calculations were given in terms of packet loss ratios. The computation of packet loss ratios involve a division operation, which can be avoided if a loss and late packet count is used. In this case, P_low and P_high are converted to low and high packet count thresholds using the packet generation rate of the flow. It is assumed that the sum of received, lost, and late packets will be fixed, and equal to the number of packets transmitted by the local gateway in RTPQOS reporting period. For example, if a codec for a RTP flow is to generate 50 packets per second, there will be 100 packets transmitted every 2-second interval. Using this assumption, it is possible to use the number of lost and late packets instead of packet loss ratio. Thus, it is possible to define packet loss ratio thresholds in terms of packet count thresholds. Continuing in the example, the lower threshold would be (lost+late)_low=100*p_low, and the higher threshold would be (lost+late) high=100*p_high. This way, the SARM  310   x  can decide if an exception report (a block-call message explained in greater detail below) should be generated or not without performing any division operation. A variation to this would be to use a computed value for the sum of lost and late packets such that this sum is equal to the “Number of packets sent by the local gateway in the RTPQOS reporting interval-local.received”. This way, the inaccuracies related to loss packet estimation in the remote gateway are avoided.  
         [0044]     Different RTP flows would have different packet rates; hence, the SARM  310   x  should take the packet rate of each flow into account. For example, with a 2-second measurement interval 1% packet loss ratio corresponds to only 1 lost packet if the packet rate is 50, while it corresponds to 2 lost packets if the packet rate is 100. The threshold values in terms of number of packets per flow will be provided by the call control during the call set-up. Moreover, if a flow is using silence suppression, the number of packets sent by the local gateway ( 114  in the above example) should be adjusted to reflect the silence suppression. Quantization of the packets may cause inaccuracies. As such, SARM  310   x  compares the value of local.received with the expected value, and if there is a large difference, it sets a flag or uses fraction and computes the packet loss ratio. Another technique omits the first set of measurement values for a newly set-up call. This way, the effect of network delay on the expected local.received is avoided.  
         [0045]     The SARMs  310   x  send blocking rules to the admission control module  204   1  in the shelf controller  302  as a result of the analysis conducted by method  800 . To reduce the amount of transmitted data, the blocking rules may be reported as exceptions. If the determined admission rule is Accept, nothing is reported to admission control module  204   1 . However, if the rule is Reduce or Block, the SARM  310   x  reports the value to the admission control module  2041  through the host CPU  312  as an exception report. Once an exception report (Reduce or Block) is sent to the admission control module  204   1  for a flow, there should be no reporting for the same RTP flow for a time interval of length T_u, which is called “exception update interval”. This update interval could be different than the periodic update interval if periodic updates are used instead of exception reports. One exception to this rule is if the last reported rule is Reduce and the newly computed rule is Block, the new value should be passed to the admission control module  204   1  immediately. (Note that this is not applicable when Reduce rule is disabled by setting P_low to zero.) In this case, there should be no more reporting for the same RTP flow for a time interval equal to the update interval. A new exception report should be generated if the Reduce or Block rule is determined using a QoS report that was received after the update interval is over. This type of periodic exception reporting should be continued until an Accept rule is detected for the RTP flow. There is no need to report an Accept rule.  
         [0046]     An alternative to the exception reporting per RTP is to perform exception reporting per (local interface, remote IP address). This way, the number of update messages can be greatly reduced. This approach results in a maximum of two reports generated within an update interval per (local interface, remote IP address)-pair as opposed to being per RTP flow.  
         [0047]     The exception report, delivered to the admission control module  2041 , includes the routeID of the flow that the measurement belongs to and the blocking rule. The admission control module  204   1  uses routeID to determine the local interface and remote IP address of the flow. Note that the information regarding the mapping between the routeID and the (local interface, remote IP address)-pair should be located in the shelf controller  302 . If this is not possible, the host CPU  312  should provide the explicit information as local interface and remote RTP address when submitting an exception report to the admission control module  2041 .  
         [0048]     When the admission control module  204 , is initialized, the rules database  306  is empty. With time, blocking rules will be reported by the SARMs  310   x . This blocking information is kept in the rules database  306  which is used by the admission policy function. An entry in this database is indexed by the Remote IP address. Moreover, each entry consists of subentries. Each subentry contains a blocking rule, an index to a local Ethernet interface, and a timestamp for the subentry. This way, each subentry shows the admission rule for the path defined by (local interface, Remote IP address)—pair. The number of subentries for Remote IP address is variable, where the maximum number is equal to the number of local interface cards, configured in the system. Note that the rules database  306  reflects the congestion status of the network paths from the local gateway to remote gateways. The opposite direction is handled similarly in the remote gateway. When the admission control module  204   1  is initialized, information about the existing interfaces is determined.  
         [0049]     The admission control module  204   1  continuously listens to the exception reports from the host CPUs  312 . When an exception report is received for an RTP flow, the blocking rule for the endpoints of the flow is updated. A method of updating the blocking rules is shown in  FIG. 9  as a series of method steps  900 . Specifically, the method starts at step  902  and proceeds to step  904  where a search of the rules database entries is performed to search for the remote gateway IP address reported in the exception report. The method proceeds to step  906  where a first decision is invoked to determine whether the reported IP address is found. If the remote IP address is not found, the method proceeds to step  908  where a database entry is created for the remote gateway IP address in the exception report. Further in that step, a subentry is created. The subentry includes the index to the local egress ports (e.g., 208 x ), blocking rule and the timestamp (set equal to the current time). At this step, it is also possible to create other subentries with indices to other local egress ports with a blocking rule of “Accept”. After such entries are created, the method proceeds to step  916 .  
         [0050]     Should the IP address of the remote gateway be found at step  906 , the method proceeds to step  910  where a second decision step is invoked. The second decision step  910  determines if a subentry for the local egress port  208   x  is found. If such subentry is found, the method proceeds to step  914  where the admission policy is updated and the timestamp is reset to the current time. If the subentry is not found, the method proceeds to step  912  where an appropriate subentry is created and the timestamp is set to the current time. At the conclusion of either of steps  912  or  914 , the method proceeds to its final step at  916 .  
         [0051]     Admission control module  204   1  periodically revises its database to remove subentries that were not updated in the last T_u+Delta seconds. The interval T_u is the time window where the SARMs  310   x  suppress reporting packet loss values for a connection following a report for the same connection. Here, Delta should be slightly larger than the QoS reporting period of the gateways, so that SARM  310  will have a chance of sending a second exception report before the admission control module  204   1  removes the blocking rule. Delta can be 3 seconds since the RTP QoS reporting is done every 2 seconds. Based on this scheme, the admission control module  204   1  assumes that a path is not congested if there is no exception report within the most recent time interval of length last T_u+Delta seconds. This action is beneficial because the host CPU  312  does not report when the packet loss ratio goes below the threshold value. Reporting of the below threshold value crossing is not needed since there might be more than one flow responsible for the blocking rule, which should be relaxed only if there is no update for the rule in the last T_u+Delta time interval by none of the related flows. Since the mapping of a rule for a path to all the flows, which reported exceptions for the path, would be computationally inefficient, an indirect method is used to remove the blocking condition. The period to revise the database to detect aged blocking rules should be chosen as small as possible to keep the database size small so that search operations will be efficient during a call to function as explained below.  
         [0052]      FIG. 5  depicts a graph of the probability of blocking calls and packet loss ratios versus the offered load of the network employing the method and apparatus of the subject invention. That is, graph  500  depicts four plots showing results when using the algorithm of the subject invention. Specifically, the first plot  502  plots the packet loss ratio versus the offered load using the subject invention. It can be seen that as the offered load exceeds up to 20% of network capacity, very few packets are lost (much less than 1% of packets are lost). First plot  502  can be compared to second plot  504  for direct comparison of results of using the subject invention versus not using the subject invention. That is, the second plot  504  plots packet loss ratio without using the admission control protocols or algorithm. As can be seen from the graph  500 , as the system capacity approaches 100%, almost 2% of packets are lost. This number grows to more than 16% as offer load increases to 20% over capacity. The third plot  506  plots the call blocking probability when using the admission control algorithm. This plot is compared to fourth plot  508  which plots blocking probability for an ideal algorithm in which the exact configuration of calls that is known. As can be seen, third plot  506  and fourth plot  508  are nearly identical. As such, the algorithm is sufficiently characterized and developed so as to effectively block calls to maintain a quality of service based on current network conditions.  
         [0053]      FIG. 6  depicts the corresponding results for the case where the update interval of the reports is varied while the offered load is kept constant at approximately 10% over capacity. Graph  600  depicts four plots that show the results of the packet loss ratio with and without employing the subject invention as well as the blocking probabilities using the subject algorithm. Specifically, first plot  602  shows packet loss ratio using the algorithm in accordance with the subject invention as the update intervals are increased from approximately 5 to 60 seconds. As can be seen, there is very little increase in the packet loss ratio as this parameter is varied. Second plot  604  depicts packet loss ratio without using the algorithm in accordance with the subject invention. As can be seen packet loss ratio is significantly higher when not using the algorithm. Third plot  606  depicts the blocking probability when using the algorithm in accordance with the subject invention while fourth plot  608  depicts the same blocking probability when the exact number of calls moving the network is known. As can be seen by inspection of the third and fourth plots  606  and  608  respectively, the blocking probability tends to increase as the report interval increases. This occurs because once a packet loss ratio of 1% is detected, the admission policy is set to block new call arrivals. All new calls arriving during the update interval thus are blocked (resulting in the increase in blocked call numbers) thus, the admission policy update interval is an important performance variable when examining a blocked call probability more than packet loss ratio.  
         [0054]      FIG. 7  depicts a graph  700  of packet loss probability in relation to burst losses. In other words, the graph shows the percentage or likelihood of losing a consecutive number of packets when using and not using the algorithm in accordance with the subject invention. Specifically, along the X axis of the graph is an increasing number of consecutive packets lost. The percentage of losing a number of consecutive packets is shown by vertical bars extending upward from the X axis. Lightly colored bars  702  denote percentage of loss of consecutive packets when not using the admission control protocols in accordance with the subject invention. Darker colored bars  704  denote probabilities or percentage of consecutive lost packets when using the admission control protocols. As can be seen, there is a significant number of consecutive packets that are lost when not using the admission control protocols in accordance with the subject invention as opposed to when these protocols are in use. For example, the likelihood of losing three or more consecutive packets when not using the admission control protocols is approximately 1%. However, the likelihood of losing the same number of consecutive packets when using the admission control protocol is nearly zero. Therefore, the admission control protocol or algorithm presents a significant advantage when considering of consecutive lost packets that may occur. This is important as the number of consecutive lost packets can seriously degrade voice call quality.  
         [0055]     Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.