Abstract:
A system that packetizes telephone calls using loss-less codecs for transmission over a communication link. The system includes a call admission control mechanism that admits calls for transmission over the communication link. An adaptive system which monitors bandwidth usage and which dynamically provides a feedback loop to the CAC system. The bandwidth allocation for transmission of the calls and the admission of calls is based upon actual bandwidth usage conditions in the system.

Description:
FIELD OF THE INVENTION 
     This invention relates to electronic communications networks and more particularly to packet networks. 
     BACKGROUND OF THE INVENTION 
     Abbreviations used herein:
         CBR Constant Bit Rate   VBR Variable Bit Rate   TDM Time Division Multiplex   LLC loss-less compression codec   VBR variable bit rate   RT Real Time   CAC Call Admission Control   BS Base Station   BTS Base Transceiver Station   BSC Base Station Controller   GSM Global System for Mobile communications   VAS Voice Activity Status       

     Traditional telephone networks transmit multiple telephone calls over a single channel using time division multiplex (TDM) technology. When TDM is used to transmit telephone calls, the communication channels transmits the data at a constant bit rate (CBR). 
     Telephone calls can also be transmitted over packet networks. Loss-less compression codecs (LLC) can be used to packetize telephone calls for transmission over a packet network While traditional TDM transmits data at a CBR, LLCs produce a variable bit rate (VBR) data stream. 
     Typically when multiple streams of VBR traffic are transmitted over a particular link, the amount of bandwidth allocated for each channel is based on the expected average bit rate of the various channels. In many situations the bit rate of the VBR sources are not correlated and hence averages and expectations are a reasonable approach to allocating bandwidth for such links. In many situations, the bandwidth allocated to a VBR connection is established at call set up time. The bandwidth allocation remains constant for all VBR calls which have the same characteristics. 
     The VBR data streams produced by LLCs operating on voice calls have some special characteristics. Such channels in effect emulate Constant Bit Rate (CBR) channels. Furthermore, they must have a very low error bit rate since they are a replacement for TDM links which have a very low bit error rate. In a typical VBR link, if bandwidth usage exceeds that which can be accommodated over a particular link, a policing mechanism kicks in, and in general this causes higher error rates. The VBR data streams produced by LLCs operating on voice calls generally can not tolerate high error rates. 
     The present invention also addresses the problem that occurs when a VBR communication link is oversubscribed. That is, the present invention addresses the problem that occurs when there is not enough bandwidth to handle all of the calls destined for a particular link. 
     In many communication links, a Call Admission Control (CAC) system is used to allocate calls to various links. Typically CAC systems allocate bandwidth for each call when the call is set up. Bandwidth is allocated based on a formula that takes into account the nature of the call in a conventional system, the bandwidth allocation formula does not take into account the amount of bandwidth actually being used by other calls that are being handled by the system at that particular time. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention provides an adaptive system which monitors bandwidth usage and which dynamically provides a feedback loop to the CAC system. Thus with the present invention bandwidth allocation and call admission is based upon actual conditions in the system. The present invention monitors actual bandwidth usage, and if the amount of bandwidth required by a new call exceeds the bandwidth actually available, the new call is not admitted into the system. 
     The present invention also provides a policing mechanism that insures that if over subscription occurs, the most important data channels are not significantly affected. That is, with the present invention, if over-subscription occurs, low priority channels are suppressed and hence, only the low priority channels are affected. If there are no low priority channels, a small number of channels is suppressed so that only the selected small number of channels are affected by the over-subscription. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is an overall system diagram of a preferred embodiment. 
         FIG. 2  is block diagram showing the feedback path to the Call Admission Control. 
         FIG. 3  is program flow diagram showing how the real time bandwidth allocation operates. 
         FIG. 4  is a program flow diagram showing the bandwidth policing operation. 
     
    
    
     DETAILED DESCRIPTION 
     Cellular phone networks typically include multiple Base Transceiver Stations(BTS) and at least one Base Station Controller (SC). The BTS maintain communications with mobile radio-telephone sets within a given range. The BTS includes (or is connected to) antennas and radio equipment necessary to provide wireless service in a particular area. Base Transceiver Stations are sometimes referred to as Base Stations (BS). 
     A Base Station Controller (BSC) performs radio signal management functions for one or more base stations. The BSC provides functions such as frequency assignment and handoff. 
     Cellular systems in Europe generally operate using the “Global System for Mobile Communication” (GSM) system. GSM is a globally accepted standard for digital cellular communication systems that operate at 900 MHz. 
     In the preferred embodiment shown and described herein, the invention is applied to link between a BTS and a BSC in a GSM system.  FIG. 1  is an overall diagram of the preferred embodiment. As shown in  FIG. 1 , a packet communication channel  120  connects a GSM BTS  100  to a BSC  130 . The BTS has an associated Call Admission Control (CAC) System  105 . The CAC  105  may be located at the BTS or it may be remote and it may be part of a larger CAC that handles several BTS locations. Channel  120  also carries control data associated with call set up. In some alternate embodiments, channel  120  may also carry other unrelated data from a unrelated data source  110 . Channel  120  may be a conventional internet communication link. Data from the BTS  100  is packetized by loss-less codecs  115  prior to transmission through channel  120 . The data is de-compressed and de-packetized by unit  125  at the output of the transmission channel  120 . The call admission control  105  is a standard device; however, with the present system it is provided with additional feedback data that helps it determine the number of calls that can be admitted to the transmission link  120  at any particular time. 
     The communication channel  120  has a limited bandwidth. For example it may be a conventional T 1  line which has a number of DS0s carrying GSM cellular calls. In a typical GSM system, each DS0 carries four GSM sub-channels. That is, four calls can be handled by one DS0. One LLC handles the data for one DS0. The system will include as many LLCs as there are DS0s being utilized. 
     With prior art techniques, a particular amount of bandwidth is allocated for each DS0 at call set up time. In practice, the actual amount of bandwidth used by each DS0 varies over time for two primary reasons. First, the amount of bandwidth used depends upon the number of calls being transmitted on the particular DS0. Since each DS0, that is, each loss-less codec, can accommodate from zero to four calls. The bandwidth actually used by any particular DS0 depends on the number of calls being handled by the particular DS0 and the particular loss-less codec. Second, for each particular call there is a difference between the bandwidth used during “spurts of talk” and periods of silence. 
     Using some prior art techniques, the assignment of bandwidth is based on the assumption that the bandwidth required by the various channels is not coordinated. That is, one can get an average bandwidth assuming that when one channel is over the “average” there are another channels that would be under average. In a telephone transmission link, the use of channels is often found to be coordinated and thus, use of averages has certain disadvantages. 
     In the embodiment shown there is no pre-assignment of bandwidth to each of the channels. Instead, the actual bandwidth used is monitored. As shown in  FIG. 2 , there is a feedback path from the DSPs  203  that execute the loss-less codec  115 , to the call access control  101 . The actual bandwidth used is monitored and when bandwidth usage approaches the limit of available bandwidth, the number of calls admitted is decreased. The DSPs and the loss-less codecs are not shown and described in detail in that they are conventional. Obtaining data from the DSPs concerning bandwidth used is also conventional technology. 
       FIG. 3  is a block diagram showing the operation of the system. It should be understood that the monitoring and policing  202  and the feedback to the call admission control  101  are operations performed by computer programs. These programs would generally be in an operating system that controls the unit. 
     The system continually collects data from the DSPs that implement to loss-less codecs  115  as indicated by block  301 . The data collected includes data which shows the amount of compression and the amount of bandwidth actually being used. The collected data is used to make a determination concerning the amount of bandwidth being used and the amount of bandwidth that is available at that particular time as indicated by block  302 . 
     The rest of the operations shown in  FIG. 3  occur at the time when a new call is being set up as indicated by block  303 . As indicated by block  304 , the total amount of bandwidth actually being used is calculated. If for example a particular channel (i.e. a particular DS0) is only carrying one call, it will be using less bandwidth than if it were carrying four calls. The bandwidth being used by all the DS0s on the transmission network is calculated. 
     A determination is then made concerning whether or not the system is approaching a pre-established bandwidth limit as indicated by block  303 . If there is available bandwidth, the call admission control  105  is informed that it can continue admitting calls as indicated by block  304 . As indicated by block  305 , if the bandwidth usage has reached the pre-established limit, the call admission control is informed that no further calls should be admitted. 
     Thus, the number of call admitted by the call admission control is based upon the actual bandwidth usage situation in the system. There is not pre-assigned bandwidth for the various channels. 
     The operation of the policing operation is shown in  FIG. 4 . First a determination is made concerning the bandwidth actually being used as indicated by block  401 . A determination is made as indicated by block  402  as to whether or not the system is approaching its bandwidth limit. If it is not approaching the limit no action is needed. If the system is approaching the bandwidth limit, the low priority calls are selected as indicated by block  404 . The bandwidth assigned to these calls is then reduced and transmission of packets related to these calls is suppressed. That is, the number of packets allowed for these calls is reduced to zero. As a result, the caller will hear no sound and eventually hang up. When a party hangs up, the call is terminated using the normal mechanism for call termination. The calculations indicated in  FIG. 4  can be made as frequently as computer resources allow or for example each time a new call is admitted. 
     As indicated by the dotted block  110  in  FIG. 1 , the packet transmission network  120  may also be carrying other unrelated data. This may be low priority data, which is only transmitted over the network when the monitoring system  202  indicates that the network as unused network capacity. On the other hand, it may be high priority data which would in fact limit the amount of bandwidth available for the telephone calls. In such a case the total available bandwidth calculated in step  303  would be reduced as appropriate. 
     While the invention has been shown and described with respect to preferred embodiments thereof, it should be understood that various changes in form and detail may be made without departing from the spirit and scope of the invention. The scope of the invention is limited defined only by the appended claims.