Patent Publication Number: US-8971308-B2

Title: Call admission control in VoIP systems

Description:
This application is the U.S. national phase of International Application No. PCT/EP03/50172, filed May 16, 2003, which designated the U.S. 
     FIELD OF THE INVENTION 
     The present invention relates to call admission control in Voice over IP systems. 
     BACKGROUND TO THE INVENTION 
     There is a desire amongst the operators of communication networks to transport both signaling and user traffic using Internet Protocol (IP) systems. This is because such systems can often make more efficient use of bandwidth than conventional systems such as those using circuit switched data links (e.g. TDM) and, perhaps more importantly, because the infrastructure associated with IP systems can be cheaper than the equipment required to implement conventional systems. 
     The term “IP backbone” is often used to denote an IP system used to interconnect various different types of subscriber access networks. The backbone can be thought of as providing “trunk” links between the access networks. The access networks themselves may themselves provide packet switched connections between subscribers and the IP backbone (e.g. in the case of GPRS access networks associated with cellular telephone networks), or may provide circuit switched connections (e.g in the case of Public Switched Telephone Networks (PSTN) and GSM and 3G voice networks). 
     As already mentioned, signalling and user data may be carried over an IP system such as an IP backbone: in the case of user data this is often referred to as Voice over IP (VoIP). However, a sometimes preferred mechanism is to carry user data over the IP backbone, and to carry signalling data over an SS7 network, the reason being that transmission over the IP network may be less reliable than that over an SS7 network. In order to allow for maximum flexibility, the Internet Engineering Task Force (IETF) has provided for the decomposition of gateways at network interfaces into Media Gateways (MGw:s) and Media Gateway Controllers (MGCs). The role of the MGw is to establish bearers for user data over the prescribed bearer network (e.g. the IP backbone). The role of the MGC is to handle call setup and control with peer MGCs (MGCs within the same communication plane), and to control one or more associated MGw:s so as to establish the bearers required for a negotiated call. A protocol known as the Gateway Control Protocol (GCP) has been defined for signalling over the interface between the media gateway controller and the media gateway. 
       FIG. 1  illustrates schematically a network belonging to a mobile operator. The network comprises an IP backbone  5 , and a pair of WCDMA mobile access networks. A first of these networks  2  is located in a first geographical area (“Helsinki”) whilst a second of the access networks  4  is located within a second geographical area (“Turku”). Telephony traffic between the Helsinki mobile network and the Turku mobile network traverses an IP backbone  5 , as does traffic exchanged between other pairs of mobile networks not shown in the Figure. The Helsinki area is also served by a first fixed line network or PSTN  1 , whilst the Turku area is served by a second PSTN  3 . The PSTN networks are coupled to respective mobile networks. 
       FIG. 1  illustrates by way of example a call originating in the Turlcu PSTN  3  which terminates at a mobile located within the Helsinki area. The call enters the mobile operator&#39;s network at the site Turlca, and Call servers (GMSC  6  and MSC  7 ) route the call from Turku to Helsinki and instruct the MGw:s at those sites to set up and handle the IP media bearers between the sites (in each direction, one of the MGw:s initiates a bearer and the other terminates the bearer). In this respect the call servers act as media gateway controllers for respective media gateways  8 , 9  (according to the IETF decomposition model). The IP backbone  5  contains a number of interconnected routers, although in  FIG. 1  only the edge, or site routers  10 , 11  are shown. 
     The connectionless nature of the IP backbone  5  creates the problem of how to guarantee that there is enough capacity in the network to deliver a call between sites with a sufficient QoS. The call servers (MSC and GMSC) do not know the structure, capacity or traffic distribution within the backbone, and are typically set up to allow a certain volume of calls between the two sites. This does not guarantee however that there will always be capacity for this amount of calls between the sites. Routers and links in the IP backbone can be overloaded due to traffic between other sites sharing a common link. Routers and links in the backbone may also become non-operational due to failures, reducing the traffic carrying capacity of the backbone. 
     A protocol known as Resource Reservation Setup Protocol (RSVP) has been defined by the IETF for allowing resources to be reserved over an IP network. A MGw uses RSVP to establish a call connection having some specified Quality of Service (QoS). RSVP is generally applied on a per call basis, and results in the creation of state tables at network routers. In view of the large number of sessions which a router may be handling, the resulting increase on processing requirements is undesirable. 
     Call admission control mechanisms have been proposed which operate at the bearer level to limit the number and type of calls set up over the bearer network in an attempt to avoid overloading the bearer network. In a typical scenario, one of a pair of peer MGw:s (located in the same operating plane) is responsible for setting up a bi-directional bearer to the other MGw. MGw:s receive congestion reports from other MGw:s and make decisions on call admissibility accordingly. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided a method of controlling call admission within a system comprising a plurality of media gateways interconnected by a packet switched backbone, the method comprising the steps of:
         at at least one media gateway, monitoring the level of congestion suffered by incoming packets to that gateway from other media gateways or groups of media gateways over said backbone; and   following receipt of a request for said at least one media gateway to terminate a bearer extending over said backbone from a “peer” media gateway, making a decision on the admissibility of that request based upon the previously monitored level of congestion suffered by incoming packets from that peer media gateway or from a group of gateways comprising that peer gateway.       

     In a typical call scenario, a bi-directional link must be established between peer media gateways. Said step of making a decision on the admissibility of a request is therefore carried out at both media gateways. 
     The step of monitoring the level of congestion suffered by incoming packets to a gateway may comprise examining packets received at that gateway to determine whether or not they contain a congestion notification flag. All or only a proportion of incoming packets may be examined for this purpose. Congestion notification flags may be incorporated into a packet header by a router of the backbone forwarding that packet, when the router experiences congestion. 
     The step of monitoring the level of congestion suffered by incoming packets to a gateway may comprise monitoring the rate at which packets are dropped. This may be achieved by determining the order at which incoming packets are received with respect to packet sequence numbers, and identifying missing or out of sequence packets. 
     The step of monitoring the level of congestion suffered by incoming packets to a gateway may comprise a combination of the method described in the two preceding paragraphs. 
     Other methods of monitoring congestion may be employed. For example the “jitter” or fluctuations in an incoming packet stream may be monitored and used to monitor congestion. 
     The step of monitoring the level of congestion suffered by incoming packets to a gateway may comprise associating incoming packets or packet sequences with an originating gateway, preferably based upon source addresses or parts of source addresses. The congestion level for a given peer to peer media gateway transfer may be determined based upon congestion data collected for all packets and/or packet sequences transported between the peer gateways. 
     Preferably, said packet switched backbone is an Internet Protocol (IP) backbone. 
     In certain embodiments, said step of making a decision on the admissibility of a request for a media gateway to terminate a bearer, comprises making that decision at the media gateway. In alternative embodiments, the decision may be made at the media gateway controller controlling said at least one media gateway. Monitored congestion levels are signalled to the media gateway controller by the media gateway. 
     According to a second aspect of the present invention there is provided a media gateway arranged to control call admission within a system comprising a plurality of media gateways interconnected by a packet switched backbone, the media gateway comprising:
         means for monitoring the level of congestion suffered by incoming packets to that gateway from other media gateways or groups of media gateways over said backbone;   means for receiving a request for that media gateway to terminate a bearer extending over said backbone from a “peer” media gateway; and   means coupled to the monitoring means and receiving means for making a decision on the admissibility of that request based upon the previously monitored level of congestion suffered by incoming packets from that peer media gateway or a group of media gateways containing that peer gateway.       

     According to a third aspect of the present invention there is provided a media gateway controller arranged to control call admission within a system comprising a plurality of media gateways interconnected by a packet switched backbone, the media gateway controller comprising:
         an interface towards at least one media gateway;   means for receiving monitored congestion levels from the or each media gateway to which it has an interface, the monitored congestion levels being indicative of the congestion suffered by incoming packets to the or respective gateways from other media gateways or groups of media gateways over said backbone;   means for receiving a call request requiring that a media gateway terminate a bearer extending over said backbone from a “peer” media gateway; and   means coupled to both of the receiving means for making a decision on the admissibility of that request based upon the congestion level suffered by incoming packets from that peer media gateway or a group of media gateways containing that peer gateway.       

     According to other aspects of the present invention there are provided computer storage media storing computer programs for causing media gateways and media gateway controllers to implement the methods described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a telecommunications system in which various access networks are interconnected via an IP backbone; 
         FIG. 2  illustrates a media gateway controller—media gateway pair for implementing an embodiment of the invention; 
         FIG. 3  illustrates signalling, associated with a call setup request, exchanged within the system of  FIG. 1 ; and 
         FIG. 4  is a flow diagram illustrating a method of operation of a media gateway of the system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     For the purpose of illustration, the general architecture of a telecommunications system shown in  FIG. 1  and described above will now be referred to. As compared to the conventional architecture, in order to implement the invention, modifications to the functionality of the media gateways and, optionally, the media gateway controllers (i.e. MSC and GMSC) is required. Modifications to other network nodes are not required. 
     It is assumed that a call is initiated for example by a PSTN subscriber in Turku to a mobile subscriber in Helsinld. A call setup message (e.g. SS7 IAM) is sent from the appropriate local exchange within the Turku PSTN  3  to the GMSC  6  in Turku owned by the mobile operator. The setup message is relayed by the GMSC  6  to the MSC  7  in Helsinki. The GMSC and the MSC act as media gateway controllers for respective media gateways in order to establish bi-directional bearers over the IP backbone  5 . 
       FIG. 2  illustrates a media gateway controller  12  and a media gateway  13  which is controlled by the media gateway controller via an interface  14 . The media gateway controller has input/output means  15  which in use is coupled to other or “peer” media gateway controllers via an appropriate network (e.g. the IP backbone  5 ). The input/output means  15  is coupled to a main processor  16  of the media gateway controller. The main processor  15  controls the transfer of data over the interface  14 . The media gateway  13  has input/output means  17  coupled in use to peer media gateways via the IP backbone  5 . This input/output means  17  is coupled to a main processor  18  which is coupled to the interface  14 . 
     Implementation of the present invention requires the use of a mechanism for detecting congestion on incoming bearers at media gateways. The following existing and well known mechanisms to detect congestion or near-congestion can be used, although other mechanisms could alternatively be used.
         Explicit Congestion Notification (ECN)
           ECN (Explicit Congestion Notification) is an Internet technology defined in RFC 3168. The basic principle is to provide a “near-congestion” notification or flag in each IP packet sent over a link that is approaching congestion. The ECN is provided by the router that forwards a packet over a nearly congested link. The ECN indicator, once set, then travels with the packet to the destination MGw, providing the receiver with information that some part of the path from source to destination is approaching congestion.   The ECN indicator is part of the IP header and uses two spare bits in the field that is also carrying the Differentiated Services CodePoints (DSCP) associated with differentiated services. The ECN is therefore transported transparently by routers not supporting ECN.   
           (Early) packet drop
           Implementation of Early packet drop results in a router starting to drop packets prior to the queue associated with a certain link and QoS becoming full, i.e. when the queue fill exceeds some predefined threshold value. (A router which has not implemented early packet drop will only drop packets when a queue is full.) The MGw receiving the packets associated with a certain stream will detect that some packets have been dropped. This detection is based on the sequence numbers in the Realtime Transport Protocol (RTP) packets or on some other mechanisms. Dropped packets will provide the receiver with information that some part of the path from source to destination media gateway is approaching congestion (early packet drop) or has reached congestion.   
               

     The packet loss rate estimate is calculated per remote site. It is assumed that the sites are organized as subnets, and thus it will be possible to identify the remote site by applying a subnet mask to the source IP address contained in a received packet. Since several IP address numbering plans can be used within one PLMN, the network id(s) and the related subnet mask(s) will be configurable in the media gateway so that the media gateway can apply an appropriate subnet mask to the remote IP address. 
     As the method of deciding upon the admissibility of a call described here treats all the IP addresses in one subnet in the same way, it will be concluded from dropped packets from certain of those IP addresses that the MGw terminating the bearer is not reachable from that subnet. Usually this will be the case. However, if load balancing or multipath routing is used between the sites, dropped packets from some IP addresses may indicate congestion for only a part of the site. In such routing scenarios the proposed admission control method may block traffic more than necessary. This problem can be overcome by using a more granular subnet mask allocation. 
     Packet loss is measured over successive fixed time periods, e.g. as a fraction of the total number of packets expected during each period, for each remote site. A moving average is then determined according to:
 
new estimate=(1 −w )*old estimate+ w *measured packet loss
 
where w (weight) is a number in the range [0 . . . 1]. The exponential weight w should be chosen carefully. With a large w the estimate follows the measurement truly, but does not suppress peaks, whereas with a small w the peaks are suppressed but the estimate follows real changes only slowly. Therefore the weight is configurable by an operator via a user interface.
 
     It is possible to use a combination of the ECN and Early packet drop mechanisms to monitor congestion in the IP backbone. For example, a packet arriving at a media gateway with the ECN indicator set can be regarded as a dropped packet, with the measured packet loss for a given remote site being the total number of dropped packets as a fraction of the number of expected packets (within a given period). 
     The monitored congestion levels are used by a media gateway to determine the admissibility of a request to establish an incoming bearer received from the associated media gateway controller. A preferred approach is as follows: 
     In order to avoid overloading the IP backbone, a media gateway receiving information that a “link” is congested or is about to be congested (using either the ECN or packet drop mechanisms) performs the following actions:
         It starts a congestion control timer associated with the site from which the congested traffic is received. The site can be identified by the (sub)network number of the source IP addresses.   New call attempts that originate from the site from which the congested traffic was received are rejected for as long as the timer is running. This can be done as soon as the media gateway has received the IP address of the other media gateway. Exactly how and when the IP address is received and how calls are rejected depends on the detailed call setup scenario in use. As an example, the media gateway can obtain the remote IP address from the IPBCP Request or Accept message, depending on the bearer establishment direction.   New call attempts with a destination towards the site from which the congested traffic was received are rejected for as long as the timer is running. This can be done as soon as the media gateway has received the IP address of the other media gateway. Exactly how and when the IP address is received and how calls are rejected depends on the detailed call setup scenario in use.   The congestion control timer is restarted each time a new indication about congestion is received (ECN or packed drop) from the associated site.   If the congestion control timer expires, a gradual return to normal operation for that particular destination is carried out.       

     The result of these procedures will be that the traffic load between a remote site and the media gateway will automatically be reduced until the congestion state disappears. The traffic reduction depends entirely on calls being terminated by normal subscriber initiated release. 
     Note that all new traffic between the media gateway and the remote site will be rejected during the time that the congestion control timer is running. This is because telephony traffic is bi-directional and both media gateways are always involved in the call setup. So even if only one of the media gateways detects congestion and only in the receiving direction, all traffic between that media gateway and the remote site is affected, independent of call set up direction. 
     An alternative approach is to reject all call setup requests if the packet loss estimate is above a configurable Call Admission Control threshold value (i.e. no congestion timer is required). This requires that the media gateway send a GCP notification indicating ‘unavailable resources’. The media gateway controller shall then take the appropriate action (e.g. play a congestion tone from TDM inter-working point, if possible, and tear down the call). 
     A requirement for the processes described here is that each media gateway is aware from where the media associated with a certain call will be coming, and that the media gateway is able to reject the call setup if so required. These conditions are fulfilled through the use of the BICC protocol and the IP Bearer Control Protocol (IPBCP) bearer setup associated with the call setup. These procedures are not described here in detail as they will be well known to the person of skill in the art. 
       FIG. 3  illustrates signalling exchanged between the media gateways (MGw:s) and media gateway controllers (MGCs), where the prefixes “0” and “T” identify call originating and call terminating nodes respectively. The signalling is associated with a call setup request, and assumes that the IP Bearer Control Protocol (IPBCP) is used between media gateways to establish and control IP bearers over the IP backbone. In the example shown, as the call requires bi-directional bearers, a congestion level “check” is performed at both media gateways. Whilst the check at the terminating media gateway results in admission of the bearer setup request, the check at the originating media gateway results in rejection of the request. The call is thus released. 
       FIG. 4  is a flow diagram illustrating the call admission procedure carried out at a media gateway controller/media gateway receiving a call setup request. 
     It will be appreciated by the person of skill in that art that various modifications may be made to the above described embodiments without departing from the scope of the present invention. For example, whilst congestion monitoring is performed at the media gateway level, the decision on the admissibility of calls may be made at the media gateway controller level rather than at the media gateway level. This would require that the media gateway report congestion levels to the associated media gateway controller.