Abstract:
A method is illustrated for reducing the call setup time of high priority calls such as voice or high speed user generated data without unduly wasting RF frequency resources on non priority calls such as short message calls. This is accomplished by ascertaining the call priority at the base station controller (BSC) level in a GSM system soon after call initiation and immediately changing the channel assignment for use by the remaining signalling type messages to a different operational speed where appropriate. The concept is further extended by supplying the call priority data to a called party&#39;s BSC such that all signalling messages after initial contact can be at the operational speed appropriate to the priority of the call.

Description:
FIELD OF THE INVENTION 
     The present invention relates in general to cellular communication systems and, in particular, to methods and systems for reducing the average time required for call setup in such systems. 
     BACKGROUND OF THE INVENTION 
     According to GSM standard operating procedures, a mobile terminal or station must first request a signaling channel from the network before the call setup can proceed. This signaling channel is used for the exchange of call setup related messages between the mobile terminal and the network. Since a large number of signaling messages must be exchanged between the mobile station and the network in order to setup a call, the speed of the signaling channel directly affects the connection setup delay. 
     In the prior art, to conserve wireless telephony bandwidth, the signaling channel assigned to the mobile station at the beginning of the call is a normally low speed signalling channel designated as a “SDCCH” channel. Once the call setup related signaling is complete, the network will assign a traffic channel to the mobile station. In general the traffic channel is a higher speed channel than the signaling channel which was originally assigned to that mobile station. 
     Depending upon the type of signaling channel used for setting up the call and the time when the traffic channel is assigned to the mobile station, there are two major call setup options presently being utilized in the industry. A first option is designated in the GSM standard as early assignment (EA) while a second GSM standard option is designated as very early assignment (VEA). 
     Under EA, the mobile station is initially assigned a standalone dedicated control channel (SDCCH). The SDCCH channel is a relatively slow channel which is intended for carrying signaling traffic only. As soon as the call “setup” message from the mobile station to the mobile switching center and the return “call proceeding” message are completed, the network assigns a full traffic channel, (TCH/F), to the mobile station if a voice or user generated data call has been detected by the network. This traffic channel remains idle until the user communication starts. If no voice call is detected by the network, the call is completed at the slow channel rate. 
     Under VEA, the mobile station is assigned a TCH/F channel at the very beginning of the call setup. This full traffic channel is initially configured or used in a mode called a signaling channel and it is used for the exchange of all call setup related signaling messages. After the setup message is received from the mobile station, the network modifies the transmission mode of the TCH/F channel to a speech channel for voice or a data channel for data if the communication is voice or user generated data respectively. This TCH/F channel stays idle until user communication starts. 
     It has been found that, depending upon many variables, the average call setup delay for mobile to mobile calls using EA is in the range of 7 to 15 seconds. This call setup delay is too long in the opinion of most network providers. When the VEA setup approach is used, the call setup delay, for a given network, can be typically reduced to almost half the delay that would have occurred using the EA setup approach. However, since a TCH/F channel is always assigned to the mobile station regardless of the type of service requested by the mobile station, the amount of user traffic that can be carried by the TCH/F channel is reduced, thus the call blocking probability for the network is increased and accordingly the network capacity is decreased. 
     SUMMARY OF THE INVENTION 
     The present invention reduces the long call setup time of the EA prior art approach while preventing the increase of call blocking probability of the VEA prior art approach by detecting the type or priority of a call early in the call setup protocol and reassigning signalling channels as necessary to obtain the same speed for signalling messages as is used for communication after the call setup is completed and a connection is finalized. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a cellular system constructed in accordance with GSM standards; 
     FIG. 2 is a message flow diagram of a GSM standard EA (Early Assignment) call setup option; 
     FIG. 3 is a message flow diagram of a GSM standard VEA (Very Early Assignment) call setup option; 
     FIG. 4 is a message flow diagram of call setup in accordance with the present invention; and 
     FIG. 5 is a message flow diagram of a mobile terminating call in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The principles of the present invention and their advantages are best understood by referring to the illustrated embodiment depicted in FIGS. 1-5, in which like reference numbers describe like parts. 
     As explained briefly previously, the present invention detects the type of call being made early in the call setup series of messages and adjusts the speed of the signalling channel to match the speed of the channel to be used for the final communication after setup is complete. This is accomplished in a GSM (Global System for Mobile communications) system by checking the service type requested by the mobile station as given in the CM_service_Req message and altering the channel assignment as appropriate, before completing the remainder of the call setup process, to the type of channel, SDCCH or TCH/F, that will be used by the mobile station for the remainder of the communication. In a GSM system the SDCCH channel is a low speed signalling channel normally used for call setup and low priority messages while the TCH/F channel is higher speed and is normally used for voice traffic. 
     According to one embodiment of the present invention, the network initially assigns either a slow speed channel such as a SDCCH channel or a higher speed channel such as a TCH/F channel to a mobile station requesting a channel, in accordance with the setup procedures desired by a network provider. As soon as the CM (communication management) service request message is received from a mobile station, the BSC (Base Station Controller) checks the message, unlike prior art approaches, to determine the type of service requested by the mobile station. If the initial channel assignment is determined to be inappropriate to the type of service requested, channel switching is performed at this time. The remainder of the call setup process is then completed at the speed to be used by the mobile station for the actual communication. Thus, a TCH/F (Full Traffic CHannel) is always used for the remainder of the setup messages for voice calls since the setup time of voice and user generated data calls are delay sensitive and the setup time should be a short as possible. On the other hand, a SDCCH or similar type slow signalling channel is used to complete the remainder of the setup process for other types of calls, such as short message services and location updates, which are not as delay sensitive as perceived by the mobile station user. 
     The approach used in this invention allows the reduction of the call setup delay by up to 50 percent while keeping the call blocking probability rate at a level similar to that provided by the initial early assignment (EA) approach where there is no change in system configuration of channel types available. 
     It has been determined that if the system configuration is changed such that a plurality of SDCCH or similar type slow signalling channels are combined to form one or more additional traffic channels, the call blocking probability can, in many instances of network size and configuration, be reduced. This is as compared to the blocking probability that would have occurred in the prior art early assignment approach while still reducing the call setup delay by as much as 50 percent as compared to the prior art EA approach. Thus a higher radio channel capacity is provided at each cell of the network. 
     FIG. 1 is a block diagram of a standard cellular system in accordance with GSM standards. A block  10  labeled PSTN represents the public switched telephone network and is shown connected to a block  12  designated as a mobile switching center and is often referred to by the acronym of MSC. The MSC  12  is shown connected to three BSC (Base Station Controller) blocks  14 ,  16  and  18 . Although each BSC would be connected to and control a plurality of BTSs (Base Station Transceiver)s, only three BTSs are shown connected to BSC  16  and as illustrated where the cells of the BTSs are designated as  20 ,  22  and  24 . Likewise, BSC  18  is connected to BTS cells  26 ,  28  and  30 . The BTS cell blocks are shown in hexagonal format which is the industry standard representation of a cell within which a mobile station may reliably receive communications from that particular transceiver. Within BTS cell  20  are shown two cars  32  and  34  representing mobile stations able to use the cellular network via the BTS for cell  20 . These mobile stations may leave BTS cell  20  and enter BTS cell  22  and continue communications via the BTS of cell  22  as is known to those skilled in the art. For completeness of description, a mobile station  36  is shown within BTS cell  26 . 
     In FIG. 2 a series of messages are shown as taking place between the MS (Mobile Station), the BTS that defines a cell within which the MS is located, the BSC for that BTS and the MSC when a system is configured to operate in an EA (Early Assignment) mode. When the MS, such as  32  in FIG. 1 wishes to initiate a call, a channel request  50  is sent. After several messages are exchanged between the BTS  16  and the BSC  16 , an immediate assignment message  52  is returned to the MS  32 . The channel assigned is a SDCCH channel, a slow speed control channel defined in GSM standards for use in an EA configuration. The MS responds with a CM (Communication Management) Service Request message  54 . Although message  54  includes data as to whether the request for service is voice or non-voice (i.e. short message service), the prior art GSM standard was such that the BSC  16  does not read this information. Rather the service request is merely passed on to the MSC as message  56 . It should be noted that, as used in this document, the term “voice” includes relatively high speed user supplied data that might be generated from a computer or fax as opposed to low speed short message service data. The MSC confirms the connection to the BSC with message  58  and returns an acceptance message  60  to the MS  32 . After identity, authentication and cipher mode messages are exchanged between MS  32  and MSC  12 , a setup complete message  62  is transmitted from the MS  32  to MSC  12 . After a call proceeding message  64  is transmitted, the MSC  12  issues an assignment request message to the BSC  16  to activate a TCH/F channel, a full rate traffic or voice quality channel, in those instances where the original request  54  indicated a voice quality transmission request. As known to those skilled in the art, the SDCCH channel does not have a traffic mode that needs to be altered. When the assignment of a voice channel is completed as indicated by message  68  the RF (radio frequency) channel used for the previous signalling is released for use by another MS wishing to establish communication or for other network messages such as location updates of MSs. 
     The message diagram of FIG. 3 for the prior art GSM standard VEA option is very similar to that of FIG.  2 . However, as explained previously, all the messages shown after the immediate assignment message  52  are transmitted at the voice message rate and thus there is no release of the signalling channel as occurred in connection with message  70  of FIG.  2 . Further, rather than activate a voice or data channel, as occurred in connection with message  67  in FIG. 2 after the occurrence of message  66 , the transmission mode of the channel is merely modified as indicated in message  80  to indicate a speech or data communication mode rather than a signalling mode. As previously indicated, for a system using the VEA option of FIG. 3, all messages, including those with a low priority, use the same speed channels. In other words, with the VEA option, the SDCCH channels are not used by the MS. The term “priority” as used herein refers to how the system responds to service requests. Voice and higher speed user generated data will be considered to have a higher priority that short message service and location update type data. 
     If MS  32  is calling another MS, such as  36 , the call setup for MS  36  will use substantially identical messages and identical message speeds as originally used by MS  32 . Thus with the EA option, all the signalling messages after initial contact, with regard to MS  36  setup, would also be at the SDCCH speed. 
     A comparison of the message diagram of FIG. 4 with FIG. 3 will show that the mode of operation and the messages are identical until after CM service request message  54 . In the present invention the software of the BTS, the BSC and MSC is modified such that the BSC reads the data in the service request message  54 . The network provider may optionally setup and use the present invention to initially assign and use either slow speed SDCCH or high speed TCH/F signalling channels and then soon thereafter switch to the speed appropriate to the type or priority of service requested. 
     It will first be assumed that the network provider sets up the system to initially use the SDCCH signalling channels. If the BSC ascertains that the service requested is low priority such as for “short message service”, the BSC skips all the messages in dash line block  90  and continues with message  56  as would occur in the prior art VEA option configuration. However, the messages would be in the slow message format of SDCCH to conserve telephony bandwidth. Further the channel mode would not need to be modified from signalling to communication was illustrated by message  80  in FIG.  3 . 
     On the other hand, if BSC  16  ascertains from forwarded message  54  that the service requested is for a voice priority communication, a channel activate message  92  is issued by BSC  16 . This message  92  operates in a manner substantially identical to the message set initiated by MSC  12  with messages  66  and  67 . After the MS is reassigned to a high speed channel, the original slow speed channel is released for further use by the release message  94  in a manner substantially identical to that occurring with message  70  in connection with the EA option of FIG.  2 . Thus the service request signalling message  56  is transmitted to MSC  12  in a high speed mode as are the remainder of the signalling messages. 
     It will now be assumed that the network provider sets up the system to initially use the TCH/F signalling channels. If the BSC ascertains that the service requested is high priority such as for voice, the BSC skips all the messages in dash line block  90  and continues with message  56  as would occur in the prior art VEA option configuration. The channel mode would be modified from signalling to communication as illustrated by message  80  in FIG. 3 but would remain in the fast format. 
     On the other hand, if BSC  16  ascertains from forwarded message  54  that the service requested is for non-voice uses, a channel activate message  92  is issued by BSC  16 . This message  92  operates in a manner substantially identical to the message set initiated by MSC  12  with messages  66  and  67 . After the MS is reassigned to a low speed channel, the original TCH/F channel is released for further use by the release message  94  in a manner substantially identical to that occurring with message  70  in connection with the EA option of FIG.  2 . Thus the service request signalling message  56  is transmitted to MSC  12  in a low speed mode as are the remainder of the signalling messages to conserve telephony bandwidth. 
     The message diagram of FIG. 5 shows the manner in which a MS such as  36  receives a call in accordance with the present invention. A paging message  100  is sent from MSC  12  to BSC  18  and included therein is information as to the priority of the communication. The paging request message  102  is eventually received by MS  36  and it requests a channel as shown by message  104  in a manner similar to what would occur if it were initiating a call instead of receiving a call. The BSC  18  instructs the BTS to assign a channel as previously indicated by the paging message from the MSC. All the remaining messages follow the format of the GSM standard for the VEA option of FIG.  3 . 
     Although the invention has been described with reference to specific embodiments, these descriptions are not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore, contemplated that the claims will cover any such modifications or embodiments that fall within the true scope of the invention.