Patent Publication Number: US-6215796-B1

Title: Process for subchannel bandwidth allocation and extraction by an ISDN communications controller

Description:
This application claims benefit to U.S. provisional application Serial No. 60/013,174, filed Mar. 12, 1996. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to digital communications systems and, in particular, to a system having a communications controller providing access to an integrated service digital network (ISDN). 
     Devices for interfacing multiple analog telephones to an ISDN line of a telecommunications network are known. For example, International PCT application WO 95/22218 published on Aug. 17, 1995 and U.S. Pat. No. 5,305,312 issued on Apr. 19, 1994 disclose such devices. However, these known devices only support a maximum of two telephones in concurrent use. 
     It is desirable to support more concurrently active telephones over the single ISDN line. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a new and improved ISDN communications controller. 
     The invention, therefore, according to a first aspect provides in a system having a plurality of end-user devices coupled to a communications controller which is connected to an integrated services digital network (ISDN) line of a telecommunications network, the ISDN line having two bearer channels of predetermined bandwidth and a data channel, a method of subchannel bandwidth allocation comprising the steps of: establishing, as demanded for a particular end-user device, a call connection though the telecommunications network between the communications controller and another network terminator; inquiring, over the data channel, whether the network terminator is a compatible communications controller; responsive to receiving a positive compatibility indication, allocating by the two controllers to the particular end-user device a subchannel from the predetermined bandwidth of the two bearer channels, wherein the subchannel consists of bandwidth up to the predetermined bandwidth if the particular end-user device is a data processing unit or bandwidth sufficient for a controller-encoded voice signal if the particular end-user device is a telephone; responsive to not receiving the positive compatibility indication, allocating by the communications controller to the particular end-user device one of the two bearer channels; and transmitting, over the subchannel or one bearer channel, outgoing signals from and incoming signals to the particular end-user device. 
     A home access network controller, manifesting the present invention, is a device that may interface subscriber premises telephone equipment with a public switched telephone network, through an ISDN interface. It is intended for residential or small office application, where it provides certain efficiencies and conveniences not previously available in that setting. A primary advantage is the capacity to support more than two and up to five telephones in concurrent use. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood from the following description of a home access network controller (HANC) system together with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic representing a first application of the HANC system; 
     FIG. 2 is a block diagram depicting functional elements of the HANC; 
     FIG. 3 illustrates an exemplary partitioning for the bandwidth of the ISDN bearer channels; 
     FIG. 4 is a schematic representing a second application having a HANC and distributed internal line interface (DILI) to provide single line voice distribution; and 
     FIG. 5 is a block diagram depicting functional elements of the HANC and DILI. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, for illustration of the present invention an application of the home access network controller (HANC) system is diagrammed. Two HANC  10  communication controller devices are shown, labeled A and B respectively, which typically would be installed in separate subscriber residences and interconnected through a backbone network that provides many readily connectable full-duplex channels of communications, namely a public switched telephone network (PSTN)  12 . Each HANC  10  connects to the PSTN  12  via its own ISDN Basic Rate Interface (BRI) line  14 , consisting of two 64 Kbps bearer (B) channels and one 16 Kbps data (D) channel multiplexed together on the same twisted pair. Coupled to the HANC  10  at each subscriber residence are subordinate devices, such as, one or more personal computers (PC)  16  connected via an Ethernet local area network (LAN)  18 , and up to five plain ordinary telephony service (POTS) telephones  20  connected in a star configuration via analog lines  22  which support dual tone multi-frequency (DTMF) and switch-hook signaling with analog voice. Additional terminal device types (not shown) may be supported. 
     The HANC  10  represents the subscriber telephone equipment to the PSTN  12  and functions as both a router and a dynamic bandwidth controller. It accepts calls announced on the BRI D-channel and routes them to the PC  16  or the appropriate telephone  20 , each of which may be identified externally at the PSTN  12  by a unique directory number (DN). Conversely it delivers calls originating among its local subordinate devices, for example, from a particular telephone  20  to a remote telephone  24  or from the PC  16  to an Internet Access Provider  26 . The HANC  10  converts signals to and from the lines  14 ,  22  between analog voice and either 64 Kbps pulse code modulation (PCM) or 24 Kbps modified adaptive differential PCM (ADPCM) which supports an eight-to-three bit compression, depending on whether the remote device to which it is connected through the PSTN  12  is another HANC. It also acts as a network gateway for PC protocols, encapsulating LAN packets for transmission on the ISDN B-channels. 
     Referring to FIG. 2, the HANC  10  is characterized by various functional elements which comprise an ISDN BRI interface  30 , D-channels protocols  32 , bandwidth control  34 , selectable coders/decoders (CODECs)  36 , POTS interfaces  38 , Ethernet interface  40  and base logic  42 . 
     The ISDN BRI interface  30  provides the physical bi-directional interface to the ISDN line  14  of the PSTN, and separates or combines the D-channel and two B-channels while deriving the line clock. It informs the base logic  42  of line status. 
     The D-channel protocols  32  supports the physical and link layers for the Q.931 protocol with a central office of the PSTN and an X.25-based protocol with a remote HANC. Command and response packets are passed to and from the base logic  42 . 
     The bandwidth control  34  connects digital voice to or from the CODECs  36  with subchannels assigned among the 128 Kbps total bandwidth of the B-channels combined, as directed by the base logic  42 . In other words, each B-channel supports a 64 Kbps transmission rate in the form of 8,000 8-bit octets per second, and the bandwidth control  34  utilizes the combined bandwidth of 128 Kbps to multiplex the digital voice by time division thereby creating communication subchannels in the B-channel bandwidth. Data derived from or destined to an attached PC can also be directed to or from such a subchannel. 
     The selectable CODECs  36  encode analog voice signals to digital PCM or ADPCM and decode PCM or ADPCM to analog, as selected by the base logic  42 . A CODEC  36  segregates the transmitted and received analog signals. It is the actual source (on its digital side) of the various signaling tones said to be issued on the POTS interface  38  below. There is one CODEC  36 , logically at least, for each subordinate telephone. 
     The POTS interface  38  supports the subordinate analog telephones, one each, providing the physical interface including DC current. It detects switch-hook transfer and DTMF tones, reporting both to the base logic  42 . As directed by the base logic  42 , it issues dial tone, busy tone, remote ringing tone and local ringing AC and passes the bi-directional voice signal. 
     The Ethernet interface  40  connects the residential personal computer via IEEE 802.3 physical and link protocols, providing packetized data to and from the base logic  42  at the rate of 10 Mbps. 
     The base logic  42  is the processing manager of the HANC. At power-on it operates verification diagnostics, boots the operational software and seeks to establish connection with the PSTN central office on the BRI D-Channel and with the PC. It communicates with the central office or a remote HANC via packets generated or examined on the D-channel. It makes the bandwidth assignments executed by the bandwidth control  34  and may exchange PC packets for transmission in or reception from a B-Channel. It understands call control sequences and directs the selectable CODECs  36  and POTS interfaces  38  to execute them. It monitors the state of the HANC  10  device and attempts to recover from device failure by performing reset and restart. It keeps a status record accessible by the PC. 
     The HANC  10  and specifically the base logic  42  supports two categories of function in communication with an attached PC on the Ethernet interface  40 . It maintains a TCP socket by which PC software may exert control and obtain status. It also passes PC packets through to the bandwidth control  34  for inclusion in the B-channels, and vice-versa, when the PC is connected on the public network. 
     According to a particular implementation, the base logic  42  may be effected by a microprocessor operating under appropriate software, and the bandwidth control  34  may be a set of shift registers. Alternatively, a digital signal processor (DSP) might be used in which case it could integrate several other functional elements. Low cost integrated circuits are commercially available to support the CODEC  36  function and the POTS interface  38 , but for supporting only five telephone ports much of that logic could be provided by the DSP. The Ethernet interface  40  is conventional functionality that may be implemented in separate hardware. However, with an aggregate signal bandwidth remaining of only 144 Kbps, or 6.9 microseconds per bit, or 116 DSP instructions per bit (60 nanoseconds each), and no algorithmic challenge worse than a PCM/ADPCM lookup table, a single DSP may perform all HANC logic above the physical layer. 
     Turning back to FIG. 1, in operation each HANC  10  may support at least two concurrently active calls, for instance, with two of its subordinate telephones  20  or one telephone  20  and the PC  16  connected externally, using the two B-channels on the BRI line  14  independently with each separately routed and connected. Alternately, if the only current user is the PC  16 , both B-channels may be connected to the Internet Access Provider  26  thereby delivering maximum bandwidth of 128 Kbps. For a telephone connection, the HANC  10  may convert between analog voice and the PCM-encoded voice required on a B-channel, using 64 Kbps. 
     If the remote connection is another HANC  10 , however, additional connectivity is available. In this case analog voice from the telephones  20  is converted to 24 Kbps ADPCM before application to a B-channel. When both B-channels are available between the two compatible HANCs  10 , up to five telephones  20 , each using a three-bit allocation as a voice subchannel, and the PC  16  using a one-bit allocation as a data subchannel, may converse concurrently with their counterparts on the 16 bits derived from the 8-bit octets of both B-channels combined. 
     In FIG. 3, illustrated is an exemplary partitioning of the B-channel 8-bit octets to provide multiple communication subchannels whereby the up to five telephones and PC may communicate simultaneously over the ISDN line. In the 8-bit octets of the first B-channel, bandwidth defined by bits  1 - 3  may be allocated as a voice subchannel which is labeled VS 1  and bits  4 - 6  as another voice subchannel VS 2 , in support of two separate telephone calls. For a third active telephone call, the voice subchannel VS 3  bandwidth may occupy bits  7 - 8  of the first B-channel and bit  1  of the second B-channel. A fourth and fifth telephone call may be allocated voice subchannels VS 4  and VS 5  which occupy bits  2 - 4  and  5 - 7 , respectively, of the second B-channel bandwidth of which the remaining bit  8  may be employed as a data subchannel DS for PC communications. 
     If only one B-channel is available for HANC-to-HANC communications, for example, because the other is busy with the remote telephone  24  shown in FIG. 1, two telephones  20  and the PCs  16  may converse concurrently between HANCs. In this case, the 8 bit frame of the one B-channel for HANC-to-HANC communications may be partitioned into two three bit voice subchannels and a two bit data channel. If the call is only between PCs  16 , all available bandwidth is allocated to that connection, else three bits for each ADPCM voice subchannel. The HANC  10 , or HANCs  10 -A and  10 -B in consultation, manage this bandwidth automatically in all cases. 
     The HANC  10  deallocates bandwidth when a call disconnects. It may also deallocate (i.e., deallocate and reallocate) when a new call originates or arrives. In the case of two HANCs  10 -A and  10 -B originally connecting only their PCs  16 , a subsequent call between subordinate telephones  20  requires deallocating three bits from the PC data subchannel to construct a new voice subchannel, but requiring consultation only between HANCs. The more drastic case is a new call between a subordinate and an independent remote telephone, namely telephone  24 . An entire B-channel must be deallocated from the HANCs  10  in this case and diverted to serve the new call. This is accomplished by communication on the D-channel between HANCs as well as the PSTN  12  central office. In any case, if the necessary bandwidth is not available because too much is already allocated to relatively inflexible voice traffic, the new call is rejected on the D-channel or indicated busy to the subordinate telephone  20 . 
     When a first connection is established between a HANC (e.g., the HANC  10 -A) and a remote device through the PSTN  12 , the HANC  10 -A asks the remote if it is also a HANC using the X.25 protocol permitted end-to-end on the D-channel. If there is no valid response the HANC  10 -A treats the new call as POTS-to-POTS, but if a HANC identification is received (e.g., from the HANC  10 -B), subsequent signaling between the two HANCs,  10 -A and  10 -B, discloses the subsystem controlled by each as well as its current state. At the end of this exchange, requiring at most a few hundred milliseconds, each HANC  10 -A and  10 -B knows the devices connected to the other, their DNs, whether each is busy and how much bandwidth is available for allocation. 
     The following describes in more detail particular processes for subchannel bandwidth allocation and for external bandwidth extraction that may be adapted to the HANC system of FIG.  1 . 
     Once the connection exists via a channel through the PSTN  12  between the two HANCs  10 -A and  10 -B, the effect of subchannel bandwidth allocation is to permit complex data flow between the individual PCs, referenced as  16 -A and  16 -B, simultaneously with multiple separate conversations between telephone sets  20  on the two HANCs  10 , without calling upon the PSTN  12  for assistance. The HANCs  10 -A and  10 -B accomplish this by creating subchannels on the common network channel using time division multiplexing. Subchannels exist only as demanded by the traffic and function by appropriating a part of the full bandwidth as negotiated between the HANCs  10 -A and  10 -B. In the absence of demand for multiple subchannels most of the bandwidth is allocated to the single user. If that is a PC, its data are free to flow at the maximum rate. 
     Negotiation between HANCs  10  requires a subchannel reserved to such special communication. This is assigned that minimum of the channel bandwidth appropriate to timely negotiation and is called the control subchannel. 
     The process of allocating and removing subchannels is initiated by end user demand. An end user of one of the HANCs, such as the PC  16 -A at HANC  10 -A, first causes the initial connection through the PSTN  12  to another terminator, such as HANC  10 -B. HANC  10 -A inquires via the control subchannel whether that terminator is a compatible HANC. In the absence of a positive response it permits the PC  16 -A to communicate by its default protocol, if one exists, or breaks the connection with suitable notification. If HANC  10 -B responds, demonstrating compatibility, the two controllers proceed to open the full channel bandwidth, less that of the minimal control subchannel, to communication between the two PCs  16 -A and  16 -B. If the initial connection is requested by an analog telephone set  20 , such as  20 -A 1 , the HANC  10 -A still elicits the compatibility indication. In its absence the “natural bandwidth” required of ordinary voice communication, 64 Kbps PCM, is allocated and no suballocation on that B-channel is possible for the remainder of the call. If telephone set  20 -A 1  called a compatible telephone, such as  20 -B 2 , however, the HANCs  10 -A and  10 -B allocate the 24 Kbps of bandwidth as required by the ADPCM voice encoding, with the rest held in reserve for further demand. 
     Subchannel allocation employs the following steps, given the connection already established between PC  16 -A and PC  16 -B. As part of the initial compatibility exchange, each HANC  10 -A and  10 -B informed the other of the identifiers, i.e., telephone numbers, and types of all attached end-user devices. 
     1) User at telephone  20 -B 3  dials the DN of telephone  20 -A 2 . 
     2) HANC  10 -B notifies  10 -A of this demand and proposes the extraction of a specified part of the current PC bandwidth to support the telephone conversation, using the control subchannel. 
     3) HANC  10 -A investigates the state of telephone  20 -A 2 , which may be idle, busy or non-working, and reports accordingly. If idle it instigates ringing at that telephone while HANC  10 -B reflects a ringing tone to telephone  20 -B 3 . 
     4) If no user answers telephone  20 -A 2 , the user at telephone  20 -B 3  eventually abandons the call and HANC  10 -B so notifies HANC  10 -A. 
     5) If telephone  20 -A 2  answers, HANC  10 -A ceases to ring and accedes to the suballocation request. Coincident with an accession message transmitted on the control subchannel, HANC  10 -A extracts sufficient bandwidth from the PC subchannel to form the specified voice subchannel. After a specified period upon or after the close of the accession message, HANC  10 -A effects the bandwidth changeover in its full channel transmission toward HANC  10 -B. 
     6) HANC  10 -B transmits an acknowledgment on the control subchannel. After a specified period upon or after the close of the acknowledgment message, HANC  10 -B effects the bandwidth changeover in its transmissions toward HANC  10 -A. Note that by measuring from the close of the accession and acknowledgment messages, synchronization of bandwidth changeover is obtained with bit-level timing accuracy. Also, the point in time when the bandwidth changeover is effected upon or after the close of the accession and acknowledgment messages may be predetermined, whether immediate or after a fixed time period. 
     7) Voice communication proceeds on the new subchannel, using the ADPCM encoding method, simultaneously with data transfer on the PC subchannel. Additional voice subchannels may also be opened by the same process. The only effect apparent upon interPC communications is a reduction in throughput, which may or may not be of concern. 
     8) The user at telephone  20 -A 2  hangs up. 
     9) HANC  10 -A sends a deallocate message to HANC  10 -B on the control subchannel, identifying the voice subchannel to be deallocated. After a specified period upon or after the close of the deallocation message, HANC  10 -A restores that amount of bandwidth to the PC subchannel and effects the bandwidth changeover in its transmissions toward HANC  10 -B. 
     10) HANC  10 -B sends an acknowledgment message to HANC  10 -A and likewise changes over to the larger PC subchannel bandwidth after the acknowledgment is complete. 
     Part of the bandwidth used between two HANCs  10  may be extracted to permit communications with outside devices, such as, the independent analog telephone  24 . The process of extracting bandwidth is initiated by end user demand, either from outside the context of two connected HANCs  10 -A and  10 -B, or from a dependent user of a particular HANC  10  who wishes to communicate with an outsider. 
     Bandwidth extraction can be performed even when the HANCs  10  are used only by attached telephones  20 . The process is best illustrated, however, by its operation when the principle communication is between PCs  16 . 
     As part of the initial compatibility exchange, each HANC  10  informed the other of the identifiers, i.e., telephone numbers, and types of all attached end-user devices. 
     Outsider initiated extraction involves the following steps: 
     1) User at telephone  24  dials the DN of the telephone  20 -A 2  connected to HANC  10 -A. 
     2) The PSTN  12  notifies the HANC  10 -A of the incoming call, using processes equivalent to “Call Waiting” and “Caller-ID” as conventionally practiced. 
     3) HANC  10 -A examines its outstanding bandwidth allocation. If bandwidth insufficient to support standard network voice remains unassigned or such bandwidth cannot be removed from PC support without disconnecting any conversation or exchange already in progress, it ignores or rejects the incoming call, according to the appropriate backbone network protocol. 
     4) If the necessary bandwidth can be found, however, HANC  10 -A notifies HANC  10 -B of the demand, using the control subchannel, and proposes the extraction of a sufficient part of the current PC bandwidth to support ordinary voice. 
     5) HANC  10 -B accedes to the extraction request. Coincident with an accession message transmitted on the control subchannel, B extracts sufficient bandwidth from the PC subchannel to form the specified voice subchannel. After a specified period upon or after the close of the accession message, HANC  10 -B effects the bandwidth changeover in its full channel transmission toward HANC  10 -A. 
     6) HANC  10 -A transmits an acknowledgment on the control subchannel. After a specified period upon or after the close of the acknowledgment message, HANC  10 -A effects the bandwidth changeover in its transmissions toward HANC  10 -B. Note that by measuring from the close of the accession and acknowledgment messages, synchronization of bandwidth changeover is obtained with bit-level timing accuracy. 
     7) HANC  10 -A accepts the incoming call by notifying the PSTN  12  and creates an internal path for communication between telephones  20 -A 2  and  24 . 
     8) Voice communication proceeds on the extracted channel, using the network standard encoding method, simultaneously with data transfer between HANCs  10  on the PC subchannel. Additional voice subchannels may also be opened between the HANCs  10 , using a different process. The only effect apparent upon interPC communications is a reduction in throughput, which may or may not be of concern. 
     Insider initiated extraction involves the following steps: 
     1) The user at telephone  20 -A 2  dials a DN not administered by HANC  10 -B, such as the number corresponding to telephone  24 . 
     2) HANC  10 -A examines its outstanding bandwidth allocation. If bandwidth insufficient to support standard network voice remains unassigned or such bandwidth cannot be removed from PC support without disconnecting any conversation or exchange already in progress, it informs the telephone  20 -A 2  user that local circuits are busy. 
     3) If sufficient bandwidth is found, however, HANC  10 -A notifies HANC  10 -B of the demand, using the control subchannel, and proposes the extraction of a sufficient part of the current PC bandwidth to support ordinary voice. 
     4) HANC  10 -B accedes to the extraction request. Coincident with the accession message transmitted on the control subchannel, B extracts sufficient bandwidth from the PC subchannel to form the specified voice subchannel. After a specified period upon or after the close of the accession message, HANC  10 -B effects the bandwidth changeover in its full channel transmission toward HANC  10 -A. 
     5) HANC  10 -A transmits an acknowledgment on the control subchannel. After a specified period upon or after the close of the acknowledgment message, HANC  10 -A effects the bandwidth changeover in its transmissions toward HANC  10 -B. 
     6) HANC  10 -A notifies the PSTN  12  of its call to the DN of telephone  24  using an appropriate part of its full channel bandwidth and assigns that portion to network control, permitting the user at telephone  20 -A 2  to hear the network call progress reports. 
     7) If the telephone  24  answers, voice communication between telephones  20 -A 2  and  24  proceeds on the extracted channel, using the network standard encoding method, simultaneously with data transfer between HANCs  10  on the PC subchannel. 
     8) The user at telephone  20 -A 2  hangs up. 
     9) HANC  10 -A notifies the PSTN  12  that the connection between telephones  20 -A 2  and  24  no longer exists. 
     10) HANC  10 -A sends a restore bandwidth message to HANC  10 -B on the control subchannel, identifying the voice subchannel to be restored for use by the PCs  16 . After a specified period upon or after the close of the restore message, HANC  10 -A restores that amount of bandwidth to the PC subchannel and effects the bandwidth changeover in its transmissions toward HANC  10 -B. 
     11) HANC  10 -B sends an acknowledgment message to HANC  10 -A and likewise changes over to the larger PC subchannel bandwidth after the acknowledgment is complete. 
     In application of the above processes for subchannel bandwidth allocation and bandwidth extraction to the HANC system, the ISDN D-channel may be utilized for the control subchannel. If the initial connection through the PSTN  12  is established between PCs  16 , the called HANC furnishes the DN of its second B-channel as part of the compatibility exchange and the calling HANC immediately calls through the PSTN  12  on the second B-channel, thus obtaining the maximum 128 Kbps for use in passing PC data. Subsequent demands by telephones  20  to share the bandwidth are negotiated on the D-channel as previously described. 
     Connections through the PSTN  12  are obtained using the ISDN standard Q.931 protocols on the D-channel. The calling HANC sends a compatibility inquiry via the end-user protocols on the D-channel, which are specified to agree with X.25 standards. If it receives the compatibility indicator as an X.25 response, the call proceeds as described above. 
     Notifications by the PSTN switch of incoming calls, such as the one described from telephone  24 , arrive on the D-channel in the switch supported protocol. Subsequent negotiations between HANCs  10  use X.25 protocols. The extraction of bandwidth to support the voice demand proceeds as previously described. Here an entire 64 Kbps B-channel must be allocated to the external PCM voice flow. Only one such external connection can be supported simultaneously with the interHANC connection, which uses the other B-channel. That B-channel can still be subdivided, however, among PCs  16  and telephones  20  attached to the two HANCs  10 -A and  10 -B. 
     A further feature supported by the HANC  10  is a method for convenient call acceptance. To obtain the convenience of answering a telephone call in any part of a residence, a traditional method would place extension telephones in every part where such convenience is desired. This method bears the disadvantage that so long as an extension is busy, all the others are unavailable for separate use. 
     When the convenient call feature is active on the HANC  10 , it rings all its idle telephones  20  simultaneously with a distinctive ringing pattern, preassigned according to DN, upon receipt of a call intended for any one of the DNs. That is, at the switch in the PSTN  12 , the ISDN line  14  of the HANC  10  is represented by six DNs, one for the PC  16  and one for each analog telephone  20 . Each DN is identified by its own pattern of long rings, short rings or variably spaced longs and shorts. The HANC  10  maintains the association between the DNs and corresponding distinctive ring patterns, and it selects the appropriate pattern for an incoming call according to the dialed DN received over the data channel from the switch. 
     The first idle telephone seen by the HANC  10  to go off-hook is then connected to the incoming call. The other idle telephones remain available for another call, either outgoing or incoming, in which case the same procedure is repeated. Furthermore, the same procedure of course may be applied to incoming calls from another HANC. Described above is a process for connecting the incoming call to the telephone gone off-hook. 
     This method for convenient call acceptance is applicable to telephone sets  20  wired to the HANC  10  either in the star configuration shown in FIG. 1 or by a local voice distribution method, described in the following. 
     The typical residential subscriber of telephone services uses a single telephone line for distribution of the service within his residence. Though several telephone sets may be attached to that line, they all operate interdependently as extensions and are represented by one DN at the public switch. 
     FIG. 4 illustrates a variation in the HANC system that supports the attachment of up to five POTS telephones by a single telephone line, yet permits each to converse independently of the others and to be represented by its own DN at a central office of the PSTN. A HANC  50  connects to the PSTN  52 , via a national standard ISDN BRI line  54  consisting of two 64 Kbps B-channels and a 16 Kbps (control) D-channel, multiplexed together on the external line  54 . It supports the attachment of a (i.e., one or more) PC  56  via 802.3 LAN  57 . Up to five local analog telephones  58  may be attached via respective distributed internal line interfaces (DILIs)  60  devices to the HANC  50  over a dual twisted pair distribution line  62  located internal to a subscriber&#39;s residence. Additional device types may also be supported (not shown). 
     The HANC  50  interfaces with the public switch on the BRI D-channel and thus arranges connectibility through the PSTN  52 . It is responsible for transferring encoded voice between the ISDN B-channels and the distribution line  62  to the DILIs  60  and for signaling the DILIs  60  to inform them of call progress. Each DILI  60  interfaces an analog POTS line  64  to the distribution line  62  from the HANC  30 . It receives its operating power down that same line  62 . 
     Referring to FIG. 5, illustrated are the functional elements of the HANC  50  and the DILIs  60 . The HANC  50  is similar to the HANC  10  embodiment in FIG. 2, except that the CODECs  36  and the POTS interfaces  38  are removed from the HANC  30  and now consigned to the DILIs  60 , identified by references  36 ′ and  38 ′ respectively. In their place, the HANC  50  receives a PCM converter  66 . The digitally encoded voice signals provided by the bandwidth control  34 , either in PCM or ADPCM form, are made only PCM by the PCM converter  66  for transmission on the distribution line  62  to the DILIs  60 . 
     In each DILI  60 , the CODEC  36 ′ converts analog voice to PCM voice and vice-versa passed between the POTS interface  38 ′ and a HANC interface  68 . The POTS interface  38 ′ provides an analog line connector, line termination and impedance matching, pulse dial and DTMF detection, dial tone generator, busy tone generator, remote-ringing tone generator, and a ringing voltage generator. Functionality of the HANC interface  68  includes a distribution line connector, DC power separation and propagation on the distribution line  62 , protocol support for call progress communication with the HANC  50 , distribution physical protocol support for frame detection and channel (i.e. time slot) identification, a configurator to assign distribution channels, and a bi-directional frame redriver with drop-and-insert channel support. Each DILI  60  may be optionally equipped with caller identification (ID) display capabilities. 
     The distribution line  62  from the HANC  50  is a double twisted pair, typical residence wiring, one pair for each direction. All DILIs  60  are powered by DC on the combined pair. Signals are driven in a standard digital manner, such as Manchester or NRZI encoding, capable of AC coupling, at a bit rate relatively low but high enough to include five digital voice channels and five control channels in individual time frames. Each frame consists of five time slots corresponding to respective DILIs  60 , wherein each time slot includes one of the voice channels and one of the control channels. The frame start is identified by recurrence of a fixed bit pattern. A control channel is used to inform the HANC  50  of the switch-hook status of a particular telephone  58  and a dialed telephone numbers or DTMF tones detected. In the reverse direction it conveys instructions to issue ringing voltage, to issue dial tone, busy tone or ringing tone and to display a delivered caller ID. In the inbound direction to the HANC  50 , each DILI  60  uses a drop-and-insert technique, well known from T1 multiplexers, to insert its signals at the appropriate time slot. 
     Turning back to FIG. 4, the following illustrates the process to place a call from a DILI-connected telephone  58 . 
     1) A user lifts receiver at a particular telephone  58 . 
     2) The DILI  60  connected to that telephone  58  detects off-hook state and signals the HANC  50  on that DILI&#39;s control channel. 
     3) The HANC  50  responds with an instruction for the DILI  60  to issue dial tone. 
     4) The DILI  60 , responsive to the received instruction, generates dial tone to the receiver of the attached telephone  58 . 
     5) The user keys a called DN in DTMF tones. 
     6) The DILI  60  detects these tones, and transmits numeric codes to the HANC  50  on the control channel. 
     7) On receipt of the first numeric code, the HANC  50  instructs the DILI  60  to cease generation of the dial tone. 
     8) The HANC  50  requests connection to the called DN via its communications  54  with the central office of the PSTN  52 . If the remote telephone (i.e., called DN) is busy, the HANC  50  instructs the DILI  60  to issue a busy tone. If the remote is being rung, the HANC  50  may either feed the remote ringing tones through to the DILI  60 , if the switch provides any, or instruct the DILI  60  to generate its own. 
     9) The DILI voice channels, inbound and outbound, are activated through to the PSTN  52 , with the DILI  60  performing basic analog to digital coding-decoding and the HANC  50  assuring compatibility with PSTN switch or remote HANC (if the called DN corresponds to such) requirements. 
     When a remote telephone calls the DN of a DILI-connected telephone, the following process is effected. 
     1) The central office switch of the PSTN  52  notifies the HANC  50  of the called DN. 
     2) The HANC  50  determines the particular DILI  60  that owns this DN. 
     3) If the DILI  60  and the dependent telephone  58  are busy, the HANC  50  so notifies the PSTN switch. Otherwise, via that DILI&#39;s control channel, the HANC  50  instructs the DILI  60  to generate ringing voltage and to display the caller ID. 
     4) A user lifts receiver at that telephone  58 . 
     5) The DILI  60  informs the HANC  50  on the control channel that the telephone  58  is off-hook. 
     6) The HANC  50  instructs the DILI  60  to end ringing voltage and to activate its voice channels in both directions. 
     It is noted that the above processes are representative but not exhaustive. 
     If the local telephones  58  are connected to remote telephones not associated with a remote HANC (not shown), only two of the telephones  58  may be active simultaneously, one on each B-channel. If they are mediated by a remote HANC, however, in support of interfamily communications or small offices, up to five telephones  58  may be active concurrently, along with the PC  56 . The method of interconnecting the up to five telephones through multiple HANCs was described above. 
     Those skilled in the art will recognize that various modifications and changes could be made to the invention without departing from the spirit and scope thereof. It should therefore be understood that the claims are not to be considered as being limited to the precise embodiment of the system set forth above, in the absence of specific limitations directed to each embodiment.