Abstract:
A system for extending remote digital telephone set functionality over a public switched telephone network from a local PBX, comprising a local peripheral connected to the PBX for bidirectionally communicating combined digital voice and data signals with the PBX, and a remote peripheral connected to the remote digital telephone set. Each of the local and remote peripherals include circuitry for separating and recombining the combined digital voice and data signals into respective voice and data components for bidirectional communication between the peripherals over individual voice and data channels, such that the remote digital telephone set functions as if it were connected directly to the PBX.

Description:
FIELD OF THE INVENTION 
     This invention relates in general to digital telephony, and more particularly to a method and apparatus for extending digital telephone set functionality at a remote office under remote control of a central PBX (Private Branch Exchange). 
     BACKGROUND OF THE INVENTION 
     The ability to extend digital telephone set capabilities to remote offices at a reasonable cost, has been recognized as being highly desirable in the telecommunications industry. Normally, in order to provide such capabilities, a PBX networking solution is required which involves the purchasing of multiple expensive PBXs. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a method and apparatus is provided for extending digital set functionality to a remote peripheral under local control of a centralized PBX, using analog or digital trunks, thereby obviating the need to purchase additional PBXs for the remote office. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A detailed description of the preferred embodiment of the present invention is provided herein below, with reference to the drawings in which: 
     FIG. 1 is a block diagram of a telecommunications system for extending digital set functionality to a remote location, in accordance with the principles of the present invention; 
     FIG. 2 is a block diagram showing a first configuration for routing a call through a local PBX to a remote digital set, according to the present invention; and 
     FIG. 3 is a block diagram showing a second configuration for direct access of the PSTN (Public Switched Telephone System) by a remote digital set, according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1, a communication system is shown for exchanging voice and data in accordance with the well known DNIC digital telephony standard (Digital Network Interface Circuit). A local peripheral  100  is connected to a PBX  102 , and a remote peripheral  104  is connected to the local peripheral  100  via the PSTN  106 . A call originating with or terminating on telephone set  101  may be routed through switch matrix  103  of PBX  102  for communication with local peripheral  100 , in a well known manner. The PBX  102  is provided with a plurality of trunk circuits  105  on which Central Office (CO) lines are terminated, and a DNIC circuit  107  is provided for communication with a DNIC circuit  108  in the local peripheral  100  over a serial link (e.g. Mitel® ST-BUS). 
     DNIC B (voice) and D (signaling) channel data are separated in local peripheral  100  via DNIC circuit  108  for transmission over trunks  110  and  112 , respectively. Thus, the local peripheral  100  presents a DNIC set interface to the PBX  102 . The B channels are circuit switched to trunk interface circuit  110 . The D channel (e.g. Minet®) messages are extracted from the DNIC link layer and converted to modem tones via modem  114  prior to transmission over dedicated LS (Loop Start) trunk  112 . 
     The remote peripheral  104  receives the B and D channel data from the PSTN  106  via separate trunks  116  and  118 , respectively. The D channel data is demodulated via modem  120 . DNIC circuit  122  recombines the B and D channel data so as to provide connectivity to digital DNIC set  124 . 
     Each DNIC peripheral  100  and  104  is constructed using Spine technology (see LHDE0027 System Design Specification ISS Tier 1—Set Handler, by George Jeffrey and Kevin Johnson), such that up to 23 DNIC lines and 23 trunks may be supported in conjunction with a switching matrix (i.e. matrix  103 ). The LS trunks  112  and  118  are reserved for the modems  114  and  120  contained within a peripheral controller (shown in greater detail with reference to FIG.  2 ). 
     According to the present invention, two configuration options are provided for implementing the DNIC peripheral extension described generally above. According to a first option, trunk calls are rerouted from the DNIC peripherals  104  and  100  through the PBX  102  in order to obtain Least Call Routing (LCR), SMDR and/or digital trunk access. According to the second option, the trunks are configured from the DNIC peripherals to place calls directly to the Central Office (i.e. PSTN  106 ). Configuration of either of the aforementioned options is performed by means of a configuration mode on the DNIC display set  124  or via a dedicated dataset (not shown) providing RS232 connectivity to a laptop (not shown). The method by which such configuration takes place is not germane to the invention. 
     Turning now to FIG. 2, the first of the foregoing configuration options is illustrated. An example will now be provided of each of the two primary call origination scenarios, in order to better describe the invention. 
     In the event of a call originating from a subscriber  200  to a remote DNIC set  202  at, for example, a telephone number 592-5555, the call is routed through PSTN  206  in the usual manner for answering by PBX  204 . In the illustrated embodiment, which is not intended to limit the scope of the invention, the call is switched digitally through the PSTN  206  and carried over a digital trunk  208  for reception by a network gateway  210  and thence transmitted to the PBX  204  over a fiber channel  212  (e.g. FIM). The PBX  204  switches the call via an internal switch matrix  214  to a predetermined DNIC circuit  216 , within a peripheral shelf  215  associated with the called extension (e.g. ext 5555 in the example shown). 
     Upon detecting the incoming DNIC call, the local DNIC peripheral  217  places an outgoing call to a trunk  218  within remote DNIC peripheral  219 , which is associated with the  30  remote DNIC set  202  (i.e. the digits 592-5555 are dialed for ringing trunk  218 ). The local DNIC peripheral  217  connects a local DNIC voice circuit  220  to its associated trunk  222  via a circuit switch connection  224  (e.g. though the use of the aforementioned Spine architecture), under control of a peripheral controller  221 . The PBX  204  receives the digits dialed by the DNIC peripheral trunk  222  through an ONS port  226  across fiber link  228 , in a well known manner. The PBX then invokes an Automatic Route Selection (ARS) algorithm to obtain the least cost routing of the call to the remote DNIC peripheral trunk  218 . 
     When the incoming call is detected by the remote DNIC peripheral  219 , the trunk  218  answers the call and is connected to the associated set  202  in response to the subscriber at ext. 5555 going off-hook, via a circuit switch connection  230 , between the trunk  218  and associated DNIC circuit  232 , all under control of a peripheral controller  234 . 
     Any Minet® display/control messages are transported via a dedicated modem connection between the local and remote peripherals  217  and  219  by means of the additional trunks  236  (via ONS circuit  237 ) and  238 , and directed to the associated DNIC circuit  232  for set  202 . Minet information transported over the modem must be addressed in order to correlate the data with the appropriate DNIC set. Addressing of the data is accomplished by providing two one-byte fields which contain the card slot and card circuit information from the transmitting card and circuit. The data format is as follows: 
     
       
         
               
               
               
             
           
               
                   
               
             
             
               
                 Card Slot 
                 Circuit 
                 Minet Message 
               
               
                   
               
             
          
         
       
     
     In the event of a call originating from remote DNIC set  202 , the remote DNIC peripheral  219  initially provides dialtone to the set (in response to the set going off-hook), by means of an internal DSP circuit (not shown). Once the subscriber at set  202  begins dialing, the digits are forwarded via the aforementioned modem connection to the local peripheral  217  which has the appearance to the set  202  of being another DNIC set. The local peripheral  217  then forwards the dialed digits to the PBX  204  which in response places an outgoing call through the PSTN  206 . 
     The local DNIC peripheral  217  establishes a voice path by initiating a LS trunk call to the PBX  204  in order to connect the LS trunk  218 . This can be accomplished in one of two ways. Firstly, the PBX  204  can set up a “hotline” via ONS circuit  226  to the remote trunk  218  such that the local peripheral  217  is merely required to cause its trunk  222  to go off-hook, in response to which the ONS circuit dials the desired number (e.g. 592-5555 for ringing the set  200 ). Alternatively, the local DNIC peripheral  217  can be preconfigured such that each trunk knows the number of the remote trunk it has to dial. 
     The remote DNIC peripheral  219  answers the incoming trunk call from local peripheral  217  (via the PBX  204 ) and provides a voice path to the associated DNIC set  202  via a circuit switch connection between local trunk  222  and the DNIC circuit  216  associated with the DNIC set  202  via circuit  220 . 
     Once the voice path has been established through the DNIC peripherals  217  and  219 , all call progression tones are fed to the remote DNIC set  202  originating the call as well as to the established voice path. 
     According to the second configuration option, as illustrated in FIG. 3, DNIC peripherals  300  and  302  access the PSTN  304  directly via trunk calls, rather than via the PBX  306 . This option reduces the number of resources required by the PBX  306  as compared to the first option illustrated in FIG. 2, and in particular eliminates the requirement for ONS lines. However, no SMDR records are recorded for trunk access, which is a feature of the first disclosed option. Further, there is no provision for digital trunk access. 
     The functional operations of the configuration of FIG. 3 remain essentially the same as for the configuration of FIG.  2 . 
     In response to a telephone set  308  originating a call to a remote DNIC set  310 , the call is first answered by PBX  306 . The call is digitally switched through the PSTN  304  and carried over a digital trunk  312  for reception by a network getaway  314  and thence transmitted to the PBX  306  over a fiber channel  316  (e.g. FIM). The PBX  306  switches the call via an internal switch matrix  318  to a predetermined DNIC circuit  320  within peripheral shelf  322  associated with the called extension. 
     Upon detecting the incoming DNIC call, the local DNIC peripheral  302  places an outgoing call to the trunk  324  (within remote DNIC peripheral  300 ) which is associated with the remote DNIC set  310 , and connects a local DNIC voice circuit  326  to its associated trunk  328  via a circuit switch connection  330 , under control of a peripheral controller  332 . 
     When the incoming call is detected by the remote DNIC peripheral  300 , the trunk  324  answers the call and is connected to the associated set  310  in response to the remote subscriber at set  310  going off-hook, via a circuit switch connection  334 , between the trunk  324  and associated DNIC circuit  336 , all under control of a peripheral controller  338 . 
     Any Minet® display/control messages are transported via a dedicated modem connection between the local and remote peripherals  302  and  300  by means of the additional trunks  340  and  342 , and directed to the associated DNIC circuit  336  for set  310 . 
     In the event of remote DNIC set  310  originating a call, the remote DNIC peripheral  300  initially provides dialtone to the set (once it has gone off-hook), by means of an internal DSP circuit (not shown). Once the subscriber at set  310  begins dialing, the digits are forwarded via the aforementioned modem connection to the local peripheral  302  which has the appearance to the set  310  of being another DNIC set. The local peripheral  302  then forwards the dialed digits to the PBX  306  which in response places an outgoing call through the PSTN  304 . 
     The local DNIC peripheral  302  establishes a voice path by initiating a LS trunk call to the LS trunk  324 . The remote DNIC peripheral  300  answers the incoming trunk call from local peripheral  302  and provides a voice path to the associated DNIC set  310 . 
     Once the voice path has been established through the DNIC peripherals  300  and  302 , all call progression tones are fed to the remote DNIC set  310  originating the call as well as to the established voice path. 
     It will be appreciated that, although a particular embodiment of the invention has been described and illustrated in detail, various changes and modifications may be made. All such changes and modifications may be made without departing from the sphere and scope of the invention as defined by the claims appended hereto.