Abstract:
A communications system ( 36 ) and method ( 60, 80 ) of serving subscribers having an originating network ( 14 ) in communication with a terminating network ( 32 ) via a core network ( 24 ). The core network ( 24 ) includes a plurality of external gateways ( 40, 42, 44 ) each adapted to route calls therebetween, with each gateway ( 40, 42, 44 ) having at least one codec ( 50, 52, 54 ). The communications system ( 36 ) includes a network node ( 37 ) for processing call requests from the subscribers having a particular codec type, wherein available gateways ( 40, 42, 44 ) are ranked based on gateway codec availability, the core network ( 24 ) being adapted to route a call as a function of the ranking. The method includes the steps of the subscriber sending a call request and the subscriber&#39;s codec type to a first node ( 37 ) within the core network ( 24 ), the first node ( 37 ) selecting one of several external gateways ( 40, 42, 44 ) in the core network as a function of the codecs ( 50, 52, 54 ) determined to be available at the various gateways. The first node ( 37 ) may consult a second node ( 43 ) within the core network ( 24 ) for information indicative of available gateways in the core network ( 24 ).

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is generally related to communication networks including wireless telephony communication networks, communicating voice and data calls between an originating network and a terminating network, and more particularly to a communication network having dissimilar compression and decompression equipment, such as codecs, in the communication networks. 
     BACKGROUND OF THE INVENTION 
     Communication networks, including wireless communication networks, typically include an originating network, a terminating network, and a communication link exchanging voice and data between these networks. In the case of telephony networks, analog speech signals are typically digitized through digital sampling prior to transmission over the communication link and then converted back to analog at the terminating network. To increase the capacity of the communication network, these digitized voice calls routed over the communication link are typically compressed through the use of compression and decompression equipment, commonly referred to as codecs, vocoders or transcoders. Typically, a codec resides at both the originating end and at the terminating end of a call, whereby the digitized voice is compressed by an encoding algorithm in a forward direction, and decompressed by a decoding algorithm at the receiving end. Other codecs may be utilized at other nodes of the communication network for a single call or data transmission. The decompressed voice signal is ultimately converted back to voice through the use of a digital to analog (D/A) converter. 
     Decompressed digitized voice signals are typically routed over a communication link, such as a public switched telephone network (PSTN) in a pulse code modulated (PCM) format, typically at 64 kbps. A compressed signal may have a rate of between 6 and 20 kbps, for example, in a GSM (Global System for Mobile Communications.) The rate of the air interface is usually lower than 64 kbps. In cellular networks utilizing “voice over IP (Internet Protocol),” codecs may be positioned in gateways at the edge of the network, resulting in transmission at lower rates as far as possible 
     SUMMARY OF THE INVENTION 
     A number of different codec types exist today. For example, in GSM there are eight types available, with more being developed. Considering the wide range of codec variety, it is likely that a gateway may not support all codec types, and therefore there may be a mismatch between the codec in the mobile station and the gateway. A mismatch may result in a downgraded codec type in a gateway being selected, as compared to the mobile station codec type, causing poor speech quality. There is desired a communication system and method for effectively selecting a gateway when mismatches in codec types exist, improving signal quality. 
     The present invention achieves technical advantages as a system and method of selecting a gateway having a codec of the same type as the subscriber codec type or best adapted for the subscriber codec type, preventing unnecessary degradation of the voice signal. A communications system serving subscribers having an originating network in communication with a terminating network via a core network is disclosed. The core network includes a plurality of gateways, with each gateway adapted to route calls therebetween. Each gateway has at least one codec, and the communications system includes a network node for processing call requests from the subscribers having a particular codec type. The gateways are ranked based on the gateway codec availability, and the core network is adapted to route a voice call as a function of the gateway ranking. 
     Also disclosed is a method of communicating an encoded signal representative of speech across a core network between a subscriber served by an originating network including an encoder having an encoding algorithm and a terminating network. The core network includes a plurality of gateways, each having at least one resident codec. The method includes the steps of the subscriber sending a call request and the subscriber&#39;s codec type to a first node within the core network, and the first node selecting one of several gateways in the core network as a function of the codecs available at the various gateways of the network. 
     Further disclosed is a method of communicating an encoded signal representative of speech across a core network between a subscriber served by an originating network including an encoder having an encoding algorithm and a terminating network. The core network includes a plurality of gateways, each gateway having at least one resident codec. The method includes the steps of the subscriber sending a call request and the subscriber&#39;s codec type to a Mobile Switching Center (MSC) server within the core network. The MSC server consults a node within the core network for information indicative of available gateways in the core network. The MSC server selects one of several gateways in the core network as a function of the codecs available at the various gateways and the subscriber profile. 
     The present invention provides several advantages. First, the quality of a voice signal is improved by selecting a gateway having the best possible codec available for the mobile station codec. Second, a cost savings may be realized by transmitting a call for as long as possible over an IP network rather than over a traditional land-based network. Furthermore, subscribers who are willing to pay a premium for superior quality calls in accordance with the present invention are given the ability to do so. Also, the communications system efficiency is improved by having codec types at gateways matched, when possible, with the codec type of the subscribers. Some signaling in the communications system may be eliminated in accordance with the present invention, because gateway negotiation is decreased. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, which form an integral part of the specification and are to be read in conjunction therewith: 
     FIG. 1 is a block diagram of a communication network according to the present invention which may include a wireless communication network; 
     FIG. 2 is a block diagram of a preferred embodiment of the present invention whereby a subscriber with high priority requests the type of codec, and the location server within the core network ranks the possible gateways according to the codec requested and optimal routing to improve signal quality therebetween; 
     FIG. 3 is a flow chart indicating the location server process; 
     FIG. 4 is a flow chart outlining the MSC server process; 
     FIG. 5 is a preferred format of a codec request from a mobile station initiating a voice call; and 
     FIG. 6 is a preferred format of a gateway selection list provided by the location server in accordance with the present invention. 
     Like numerals and symbols are employed in different figures to designate similar components in various views unless otherwise indicated. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a block diagram of a communication network  10  according to the preferred embodiment of the present invention, simplified for purposes of clarity. Communication network  10  preferably comprises a wireless telephony network that includes an originating terminal or mobile station (MS)  12  coupled by an RF link or air interface  13  to a servicing originating network (ON)  14 . The originating terminal  12  is preferably a wireless communication device such as a wireless mobile station, but may also be a fixed station. The mobile station  12  may operate based on any number of communication standards including AMPS, TDMA, D-AMPS, GSM, and IS-95 protocols. 
     The originating network  14  comprises an access network  16  in communication with a core network  24 . The access network  16  may include a plurality of base transceiver station (BTS) serviced by a plurality of base switching center (BSC) (not shown), although the access network  16  may vary as different infrastructures are available from different manufacturers and as a function of the communications standards and architecture implemented. The access network  16  is coupled to the core network  24  by an access gateway  18 , as shown. The core network  24  includes a control plane  20  where the control signaling for voice calls takes place, and a payload plane  22  over which the voice calls are transmitted. An external gateway  26  having a transcoder  28  provides the link to a transit network (XN)  30 . The transit network  30  typically comprises a public switched telephone network (PSTN), but may also comprise an optical network or Internet Protocol (IP) network. The transit network  30  interfaces and communicates electrical signals including digitized voice calls between originating network  14  and a terminating network  32 . Terminating network  32  may be the same as, or different than, the originating network  14 , and may comprise PSTN, AMPS, D-AMPS, TDMA, GSM and IS-95 networks. Terminating network (TN)  32  is coupled to and services via an RF link a terminating terminal (TT)  34 , which may comprise a fixed or mobile station such as a wireless cellular or PCS subscriber. 
     The mobile station  12  and other nodes in the access network  16  may have a codec  38  for compressing and decompressing voice and data signals into a format capable of transmission across the network. Typically, a voice signal is compressed at the mobile station  12  by a codec  38  resident at the MS  12 , to conserve bit rates on the air interface  13 . The voice signal is transmitted through and over the originating network  14  at a lower compressed bit rate. The voice signal is decompressed by the transcoder  28  of the external gateway  26  of the core network  24  before being transmitted at a higher rate through the transit network  30  and terminating network  32  to the terminating terminal  34 . 
     The codec  38  of the mobile station  12  compresses the voice and/or data signal with an encoding algorithm. Various categories of codecs are available in the art, such as half-rate (HR), full-rate (FR) or enhanced full-rate (EFR), with more advanced codecs designs currently in development. Within each category of codecs, there are several different types. Some examples of supported codecs include GSM EFR, GSM FR, GSM HR, and G.723. Full rate codecs may operate at 13 kbps, whereas half rate codecs may operate at 6.5 kbps, for example. Other codecs or vocoders are located at the BSC and other nodes in the communications system, as is known in the art. 
     A problem in communications systems of the prior art is that the external gateway  26  of the originating network  14  may not have a transcoder  28  available that is of the same type as the codec  38  of the mobile station  12  initiating the call. The system  10  negotiates a gateway  26  having a transcoder  28  that may be of a type downgraded from the mobile station  12 , resulting in decreased signal quality. Furthermore, each codec routing a call in the communications network causes signal degradation. Thus, decreasing the number of codecs used will maintain the integrity of a speech signal. When the transit network  30  comprises an IP network, more gateways are added at the transition points between IP and non-IP networks. These additional gateways are needed for media mapping and transcoding functions between dissimilar network, for example, from G.711 to a low bit rate code like G.723.1 or back to GSM. Each transcoding further deteriorates speech quality. 
     Referring to FIG. 2, therein is shown an expanded block diagram of the communication network  36  of the present invention. The originating network  14  is seen to include a base transceiver station (BTS)  40  serving via a radio frequency (RF) link or air interface  13  the originating mobile station (MS)  12  having a codec  38 . Each BTS  40  services multiple mobile stations  12 , although only one is shown for purposes of illustration and clarity. The terminating or external network  30  may operate according to the same operating protocol as originating network  14 , i.e., both may be GSM networks, or, the terminating network  30  may be different from the originating network  14 , i.e., the originating network  14  may be GSM and the terminating network  30  may be a PSTN or based on AMPS, D-AMPS, TDMA or IS-95 protocols. The transit or core network  24  in the preferred embodiment is preferably an Internet Protocol (IP) network, or “Voice over IP,” but may also be an ATM, for example. The system of the present invention provides a cost savings by transmitting a telephony signal over the Internet for as long as possible, before the signal is transmitted out to external networks  30 . Standards such as IETF propose an architecture similar to the one shown in FIG. 2, with an access gateway  18  facilitating connection of the radio access network  16  to the core network  16 . 
     In the EP core network  24 , there are typically thousands of gateways, represented for purposes of clarity by gateways  40 ,  42  and  44 , that serve to interconnect to non-IP networks such as the PSTN. According to the present invention, a Location Server (LS) node  43  advantageously maintains a list of external gateways  40 ,  42 ,  44  in the core network  24  available for routing calls from access gateway  18  to external network  30 . The LS node  43  is referenced by the control MSC server  37  to intelligently select one of the gateways  40 ,  42 ,  44  for routing a call. The access network  16  exchanges digitized voice data with nodes in the external networks  30  over the core network  24  through access gateway  18  and the best available gateway  40 ,  42  or  44 . The voice data or payload is transferred through the core IP network  24  at a lower rate, preferably the same rate as the one used over the air interface  13 . 
     The core network  24  includes a Home Location Register (HLR)  46  having subscriber information stored therein, HLR  46  being in communication with other nodes within the core network  24 . The core network  24  also includes a visitor location register (VLR)  41 , which includes and stores various information of the mobile stations  12  currently being served by the originating network  14 . The MSC server  37  serves to control the routing of calls, while a GMSC/Transit server  48  actually performs the routing. Each external gateway  40 ,  42  and  44  includes one or more associated transcoders (TRAs)  50 ,  52 , and  54 , respectively, for decoding the encoded speech signal provided thereto. The payload, or voice call, is transferred between the access gateway  18  and external gateway ( 40 ,  42 ,  44 ) selected. 
     The present invention derives technical advantages by introducing a Routing Preference Indicator (RPI) in a call from MS  12  that allows the MSC server  37  or other node in the core network  16  to intelligently choose a gateway  40 ,  42 , or  44  based on subscription preferences. The RPI may include a codec type (codec  38 ) request and optimal routing criteria, for example. The interrogation message  120  sent from the MSC server  37  to the LS  43  includes the codec type requested by MS  12  indicated in the RPI and the B number associated with the call, for example. The LS  43  responsively processes the codec request, using resident optimal routing criteria to scan a list of compatible gateways and provide a list  130  of suitable gateways back to the MSC server  37 . A function or module  39  in the MSC server  37  uses this list  130  to determine which gateway to route the call to. For example, the MS  12  may have a FR codec  38 , and the subscriber preference in the RPI may indicate to search for a gateway,  40  having a FR transcoder  50  so that the voice signal is not downgraded. 
     FIGS. 3 and 4 illustrate flow charts for preferred processes  60  and  90  for the location server  43  and MSC server  37 , respectively. When a call is initiated by MS  12 , the MSC server  37  accesses a function or module  39  that sends an interrogation message  120  to the location server  43  (step  92  of FIG.  4 ). The interrogation message  120  contains the MS  12  RPI, which includes the codec type and subscriber profile. The LS  43  receives the request from the MSC server  37  (step  62  of FIG. 3) and scans a list of available external gateways (step  64 ), the list being located at and maintained by the LS  43 . In accordance with the present invention, the LS  43  queries whether there is an optimal gateway, available that supports the requested codec type (step  66 ). If there is, the LS  43  places the optimal gateway 0  having the requested codec type in the first position on the list  130  (step  68 ), and returns the reply  130  to the MSC server  37  (step  70 ). If the optimal gateway, does not support the requested codec, the LS  43  places the gateways 0 , although not having the requested codec type but otherwise optimal, in the first position of the list  130  (step  74 ). Then, the LS  43  finds the next gateway that supports the requested codec type ( 76 ) and places that gateway in the next position of the list  130  (step  78 ). Steps  76  and  78  are repeated until there are no more gateways available (step  80 ). The LS  43  then returns the reply in the form of a list  130 , for example, to the MSC server  37  (step  82 ). 
     Referring next to FIG. 4, the MSC server  37  receives the list  130  from the LS  43  (step  94 ). A module  39  in the MSC server  37  processes the information by querying whether the subscriber has a codec preference (step  96 ), and if not, the MSC server  37  uses prior art method of selecting an external gateway ( 40 ,  42  and  44 ) (step  98 ). If the subscriber does have a codec preference, the MSC server  37  examines the list  130  to determine whether there is an optimal gateway 0  that supports the requested codec (step  100 ). If there is, the call is set up through the optimal gateway 0  (step  102 ). If gateway 0  does not support the requested codec, the MSC server  37  determines if there is an alternative gateway available that supports the codec request (step  104 ). If not, the call is set up through gateway 0  and the voice signal is downgraded to accommodate the codec at gateway 0 . If there is an alternative gateway available that supports the codec request, the MSC server  37  checks that the alternative gateway is acceptable (step  108 ). The result of this check is typically subscription and operator determined. If the selected alternative gateway is acceptable (step  110 , the call is set up through the alternative gateway (step  112 ) such as gateway  42 . If unacceptable, steps  104 ,  108  and  110  are repeated until an acceptable alternative gateway is found. 
     It is seen that with the present invention, subscribers willing to pay an additional amount to ensure better speech quality, or conversely, incur a cost savings on their mobile service to receive lower speech quality, are provided the option to be deemed a high or low priority subscriber. If the subscriber is not determined to be high priority subscriber, the MSC server  37  selects one of gateways  40 ,  42  or  44  in accordance with methods of the prior art. 
     FIG. 5 illustrates a possible format for the interrogation message  120  sent from the MSC server  37  to the LS  43 , with at least the codec type requested and B number provided to the LS  43 . FIG. 5 shows a preferred format for the list  130  generated by the LS  43  and returned to the MSC server  37 . The ranking of the possible pathways may include not only the gateway(s) or paths chosen, but also the nodes between the end destination and the chosen gateway(s). Furthermore, the physical geographical area of the available gateways may be divided into zones, and further into subzones, to allow the MSC server  37  to select a gateway in closest proximity to the end destination. If the function or module  56  finds two or more gateways within the same zone, the best codec can be chosen within the same zone. This feature allows a call to be placed over the core IP network  16  for as long as possible, to decrease transmission costs. The list also contains the codec type supported at each gateway. 
     The novel communications system and method of communicating disclosed herein provides several advantages. First, the quality of a voice signal is improved by the use of the present invention because a gateway having the best possible codec available, compared to the mobile station codec, is selected. Second, a cost savings is realized by transmitting a call for as long as possible over an IP network rather than over a traditional land-based network. Furthermore, subscribers who are willing to pay a premium for superior quality calls in accordance with the present invention are given the ability to do so. Also, the communications system efficiency is improved by having codec types at gateways matched, when possible, with the codec type of the subscribers. Some signaling in the system may be eliminated in accordance with the present invention, because gateway negotiation is decreased. 
     Though the invention has been described with respect to a specific preferred embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present application. For example, while the function or module  39  is shown to reside in the MSC server  37  of the core network  16 , this function or module  39  may reside in another node of the network  16 , such as access gateway  18 . It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.