Abstract:
A local networking system, offers integrated communication services to multiple local voice stations and data terminals via a common transmission medium. This system, utilizing Frequency Division Multiplexing (FDM) signals as carrier, transports information with CSMA/CD protocol. Voice transmissions are organized in traditional TDMA format with fixed frame length and dynamically assigned time slots to avoid interference caused by other devices in the system. Data services operate in traditional CSMA/CD format, but restricted to specified blocks of time intervals. The voice service quality is guaranteed because the required channels and bandwidths, up to the designed maximum, are always available when needed. The grade of data service may be impacted by voice traffic volume which is automatically managed by data terminal&#39;s inherent design with CSMA/CD protocol. This unified digital multiplexing system is applicable to transmission media with restricted channels. For example, even with very limited transmission bandwidth, a Dual Area Network (DAN) for integrated voice and data transmissions still can be set up on power or telephone line. On the other hand, this technique can maximize channel utilization of any given bandwidth in a wide band transmission facility, such as radio wave, coaxial cable or infrared system.

Description:
TECHNICAL FIELD 
     This invention relates to on-premise voice and data communication systems. 
     BACKGROUND OF THE INVENTION 
     The most natural form of human communication is by voice. Besides face-to-face conversations, telephone has been providing the inter-connections between parties at different locations and times. The original direct connections quickly gave way to the switching systems provided by telephone companies, which have evolved to be the most flexible routing facilities for modern communication. 
     As the capabilities of the station instruments diversified and the subscriber&#39;s applications grew more sophisticated, additional routing locally became very desirable. Private Automatic Branch exchange (PABX) switching machines satisfied this need in business environment by assigning each station an “extension number”. Connections are then set up with direct wirings from the PABX&#39;s central switching unit, very much like the telephone company configuration. 
     A major hurdle in deploying PABX capabilities to residential or small office settings is that the wiring in these premises is very limited. For example, the existing wiring in many residential settings is limited to one pair. Adding new wiring for the purpose of service expansion discourages most of the would-be customers. 
     U.S. Pat. No. 5,596,631 resolved this limitation by introducing a station controller module that performs PABX functions with distributed architecture on single telephone line. It provides the routing capabilities of a PABX without requiring any addition or modification to the existing wiring. The basic architecture could be replicated for multi-line configurations. However, for a premise that was originally set up with a single pair of telephone wires, expanding to multiple line distributed PABX service would encounter the wiring limitation again. 
     Multiplexing voice traffic by digital technology to reduce facility requirement has been practiced for quite some time. Normally, it is set up for long distance transmission over dedicated facility that is well maintained by trained professionals. Such technology is not suited for customer premise environment whereby distance is fairly limited, while the wiring is characterized by random topology with wide range of medium quality. And, “plug and play”, “no maintenance” are key product requirements. 
     In recent years, the fast proliferation of Personal Computers (PCs) in workplaces, has created a similar need among the data equipments. Local Area Network (LAN) has been deployed to meet such demand. However, since LAN technology was developed primarily for business applications, its operation principles does not suit very well for residential situations, either. 
     The lately available Home Phoneline Networking (HPN—U.S. Pat. No. 5,696,790) technology capable of transmitting high speed digital signal over a single pair of traditional on-premise telephone wiring has made residential level LAN service feasible. By utilizing existing on-premise telephone wiring as transmission medium, a digital data bus can be established among communication nodes equipped with HPN technology based interface modules. 
     Although HPN provides data communication among multiple PCs, it does not address the voice communication need among PC&#39;s human operators. This is due to the fundamental differences between voice and data communications: 
     Connections for human voice conversation are only established on demand. Dedicated facility is used and consistent service quality has to be maintained throughout a session. For example, it would not tolerate any degradation such as noise caused by random time delay in transmission. 
     Physical connections among data equipments, on the other hand, are continuously maintained even if there is no information to transmit. All terminals are constantly monitoring the communication medium. Information is broadcasted whenever any terminal has message to sent. Since there is no overall system coordination, collisions among data terminals are expected as a norm. Consequently, system throughput degrades proportional to traffic. However, such degradation due to collision can be tolerated by users because there is no definitive expectation of data transmission speed. Since data equipment is designed to transmit information in packets, it can easily deal with the need of re-transmission when a packet is not received by the destination party. It is clear that even HPN technology is still not compatible with voice communication requirements. 
     It is therefore the objectives of this invention to present a methodology of extending the single line distributed PABX concept to a multi-line service, to combine with data LAN operation so that a unified local area network is formed, while staying within the restrictions of limited physical interconnect facility. 
     Another goal of this invention is to maximize system performance by dynamic resource allocation between voice and data communications according to the usage. 
     Yet another objective of this disclosure is to maintain an interconnect system that is physically modular, so that it does not require special knowledge and skill to set up and to operate. 
     SUMMARY OF THE INVENTION 
     This document discloses a novel local communication system that is based on Frequency Division Multiplexing (FDM) technology to share the same physical medium with the external wide area communication service. The local system itself utilizes digital Time Division Multiplexing (TDM) technique to integrate two types of signals, voice conversations transported by Time Division Multiple Access (TDMA) protocol and data messages transported by Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol, into one unified local network. 
     The TDM technology transports voice traffic in an orderly designated time slot fashion. However, all time slots in a traditional TDM system are continuously “used” even when there is no traffic to carry. If such “empty” time slots are consolidated into one block of contiguous time interval, it is then possible to allocate it to data terminals which operate in on-demand random-access, and collision-avoidance mode. This operation architecture can therefore merge both voice and data applications into one uniform system with dynamic resource allocation. In other words, this is a communication system that its primary goal is to provide voice communication. Under such a precondition, the system would constantly allocate as much resources for data terminals as available. 
     One basic application for this methodology is to establish simultaneous voice PABX and data LAN services in residential and SOHO (Small Office Home Office) environments where very limited transmission facility, such as one single pair of telephone wires, has been hindering the deployment of modern communication services. 
     HPN technology is used as an example vehicle to demonstrate how to extend multi-line telephone company services to individual stations, to establish intercom among the stations, and to provide data networking among computing terminals, all on a single pair of telephone wires. Thus, the connection medium needed in a multi-line PABX based on U.S. Pat. No. 5,596,631 can be reduced back to only one physical cable. Simultaneously, the data terminals perform their services by utilizing the remaining resources not occupied by voice traffic. 
     One of the advantages of the present invention is that the size of the system would be rather flexible. When all telephone services are in standby state, the entire system resources can be used for data networking. At the other extreme, the system could be fully loaded with voice traffic, while the data traffic is slowed down for the corresponding duration of time. By setting a limit on the maximum number of simultaneous voice channels allowed, minimum data networking speed is guaranteed, while the voice traffic may experience congestions during peak usage. So, this system will be suitable for a wide range of situations, from isolated island of local intercom with or without data networking, single line residential home, through SOHO, to well established small business. 
     Conventional digitized voice requires 64 Kb/s (Kilo-bits per second) digital signal to represent. The HPN technology is currently offering 1 Mb/s (Mega-bits per second) data throughput. Therefore, it is capable of carrying traffic of about 15 talkers (or, over 7 pairs of conversations). With the projected 10 Mb/s and even faster transmission speed in the next generation HPN devices, several tens of voice conversations and a large number of data terminals can share one single pair of telephone line. 
     On the other hand, if compression techniques are applied to speech signals, the portion of the system time required by voice traffic can be reduced correspondingly. Therefore, higher percentage of system resources will be available for data transmission. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing the concept of a Master Controller interfacing external communication service lines LA, LB, . . . LZ to Station Controllers (SCs), and to data Terminal Controllers (TCs) via a common System Bus  101 ′ such that a Dual (voice and data) local Area Network (DAN) with unified system architecture is formed. 
     FIG. 2 is a timing diagram showing the system concept of the DAN that transports individualized voice connection in respectively assigned time slots, and broadcasts data messages within a shared block of time interval among all data terminals. 
     FIG. 3 depicts the architecture of a Master Controller interfacing external service lines LA, LB, . . . LZ via Direct Access Arrangement (DAA) to an internal data bus pair which transmits to and receives from System Bus via transceiver T/R. The operation timing is controlled by Combinatorial Logic. 
     FIG. 4 depicts the architecture of a Station Controller interfacing station service line via Subscriber Line Interface Circuit (SLIC) to an internal data bus pair which transmits to and receives from System Bus via transceiver T/R. The operation timing is controller by Combinatorial Logic. 
     FIG. 5 depicts the architecture of a Terminal Controller interfacing data equipment signals via TIMED GATE to an internal data bus pair, which transmits to and receives from System Bus via transceiver T/R. The operation timing is controlled by Combinatorial Logic. 
     FIG. 6 shows that the architecture of FIG. 4 can be expanded by multiple SLICs for extending multiple line service to multi-line telephone instrument. 
     FIG. 7 is a combination of FIGS. 4 and 5 showing the structure of interfacing the System Bus to one voice instrument and one data terminal by sharing the same transceiver T/R. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is an illustrative system configuration of my invention. LA ( 101 ), LB ( 102 ), . . . LZ ( 108 ) represent the traditional service lines from telephone company (not shown). The services available on them are monitored by Master Controller (MC)  100 , which digitizes these signals (DC to 4 KHz of Plain Old Telephone Service—POTS), multiplexes and puts them onto System Bus  101 ′ (described below) at a different frequency (5 to 10 MHz of Home Phoneline Networking—HPN). Note that telephone company line  101  and System Bus  101 ′ are physically continuous. Yet, based on the principle of Frequency Division Multiplexing (FDM), these two signals can co-exist on the same line and operate totally independent of each other, as if the other does not exist. Due to the high frequency characteristics, the HPN signals do not propagate too far onto line  101 . Thus, a “prime” symbol is used to signify the fact that the System Bus  101 ′ is carrying signals locally in addition to those on telephone company line LA  101 . 
     The information transmission among all nodes (MC, SCs and TCs) is handled by HPN technology. All nodes listen to the signal on the System Bus  101 ′ continuously for the portions that are relevant to itself. On the other hand, each node puts signal on System bus  101 ′ at the proper time slot assigned to it (described below). 
     The Station Controllers SC# 1  ( 121 ), SC# 2  ( 122 ), . . . SC#N ( 128 ), connect to System Bus  101 ′ via signal lines  141 ,  142 , . . .  148 , respectively, and convert signals in assigned time slots to telephone station instruments STN# 1   131 , STN# 2   132 , . . . STN#N  138 , respectively. The physical connections among signal lines  141 ,  142 , . . .  148  and System Bus  101 ′ could be different lengths of wires and random topology as the dotted lines imply. 
     Based on traditional Pulse Code Modulation (PCM) technology, a 64 Kb/s digital signal stream is needed to transmit one speech. A telephone conversation normally consists of two talkers which requires 128 Kb/s to represent. For three conversations, it would need a 384 Kb/s digital stream to carry. If the System Bus  101 ′ is capable of 1 Mb/s, such as the current HPN technology can provide, there will be 616 Kb/s information carrying capability left over for communication needs among data equipments. 
     This opens up the opportunity of utilizing Terminal Controllers TC# 1   151 , TC# 2   152 , . . . TC#M  158  to interface data terminal equipments  171 ,  172 , . . .  178  to System Bus  101 ′ via line  161 ,  162 , . . .  168 , respectively. 
     Essentially, the SCs  121 ,  122 , . . .  128  work with MC  100  through System Bus  101 ′ to provide full telephone service available from the telephone company via lines  101 ,  102 , . . .  108 , respectively, while the TCs  151 ,  152 , . . .  158  perform data networking service by utilizing the remaining transmission capacity of the System Bus  101 ′. 
     For simplicity, a single “Telephone” symbol is used for all voice type station instruments at  131 ,  132 , . . .  138  which could be a telephone set, Telephone Answering Devices (TAD), or facsimile (FAX), etc. 
     Similarly, “Computer” is a representative symbol for various data processing equipments  171 ,  172 , . . .  178 , such as a Personal Computer (PC), printer, scanner, modem, or digital camera, etc. 
     Each one of the SCs  121 ,  122 , . . .  128  and STNs  131 ,  132 , . . .  138  of the present invention is equivalent to those SCs  121 ,  122 , . . .  128 , and STNs  131 ,  132 , . . .  138 , respectively as disclosed in U.S. Pat. No. 5,596,631. The current System Bus  101 ′ is equivalent to line  101  of the prior art with the added function of carrying multiplexed voice signals. This puts multiple telephone services on the same line offering each SC the added capability to access more than one telephone service line. For conciseness, each pair of wires in FIG. 1 of U.S. Pat. No. 5,596,631 is depicted as one line in the present FIG.  1 . 
     FIG. 2 is an operation timing diagram of an unified distributed local area network showing one System Frame  200  of a Time Division Multiplex (TDM) operation. Each System Frame  200  is then sub-divided into two Sections which are dynamically adjusted according to usage: 
     A. The first Section  203  is for voice transmission, which operates according to conventional Time Division Multiple Access (TDMA) techniques (For example, ALOHA, U.S. Pat. No. 5,515,379). 
     B. The second Section  205  is for data transmission, which follows the operation method of Carrier Sense Multiple Access with Collision Detection (CSMA/CD) technology. (For example, LAN, ISO 8802-3) 
     The Voice Section  203  is further divided into Primary  211  and Secondary  212  time segments, and each node of a party in a conversation is assigned a specific time slot for transmitting its information. That is, the MC  100  utilizes time slots P 1   221 , P 2   222 , . . . PK  228  to transmit information from the active lines among telephone company lines LA  101 , LB  102 , . . . LZ  108 . And, the active Station Controllers among SCs  121 ,  122 , . . .  128  are each assigned a corresponding time slot among S 1   231 , S 2   232 , . . . SK  238  to transmit information from corresponding STNs  131 ,  132 , . . .  138 . To coordinate the assignment of time slots, setup and take-down of a link for conversation, time slots CMD-P  202  and CMD-S  206  are used for the MC  100  and SCs  121 ,  122 , . . .  128  to exchange status information, to make request and response for service from the others, respectively. These processes are commonly known in the art (for example, digital carrier systems such as North America T 1 , and and European E 1 ), and therefore not detailed here. 
     The Data Section  205  is open to whoever among all TCs  151 ,  152 , . . .  158  with message to transmit. The data terminals operate in an on-demand, random-access and collision-avoidance mode ( 213 ). That is, TCs  151 ,  152 , . . .  158  compete for a position to transmit messages from their corresponding data terminals  171 ,  172 , . . .  178 , according to the well known LAN protocols. 
     The parameter “K” in time slot “PK” and “SK” is determined by the number of conversation pairs among external lines and internal SCs that are actively engaged in communication. For each value of “K”, the duration for Voice Section  203  is determined. Subtracting Voice Section  203  from the fixed length of a System Frame  200 , the Data Section  205  is derived. In other words, the effective capacity available to data communication is controlled by the number of active conversations. Since all T/Rs of MC, SCs and TCs are constantly listening to signals on System Bus  101 ′, this information is received by all to coordinate the individual&#39;s activity relative to the others through System Bus  101 ′. 
     A Frame flag  201  is used to set the reference for one system time cycle. A Voice/Data separation flag V/D  204  is used at the end of the Secondary voice segment  212 . That is, the TCs  151 ,  152 , . . .  158  are pre-programmed to recognize that the system resources allocated to data communication within each frame  200  are restricted to the duration after the V/D flag  204 . Since the system timing is known to all node controllers, V/D could be as simple as a length of guard time. 
     FIG. 3 is a block diagram of MC  100 . It consists of the well known telephone line interface subsystem, Direct Access Arrangements (DAA)  301 ,  302 , . . .  308 , each interfaces one of the analog external service lines LA  101 , LB  102 , . . . LZ  108  to the internal digital data bus  332  via lines  351 ,  352 , . . .  358 , respectively. The HPN technology transceiver (T/R)  301 , under the control of Combinatorial Logic (CL)  310  via command line  321 , transmits information on data bus  332  to line  362  which is connected to System Bus  101 ′ via LA  101 . T/R  301  also monitors line  362  for desirable signals as determined by CL  310 , and puts them on line  331  destined for one of the DAAs  301 ,  302 , . . .  308  via lines  341 ,  342 , . . .  348 , respectively, which will be transmitted toward one of the corresponding external service lines LA  101 , LB  102 , . . . LZ  108 . Command line  361  is used for exchange status and command related messages such as ringing detect reports from DAAs to CL  310 , On/Off-hook commands from CL  310  to DAAs. These are common activities in telephone connection setup process known in the art, and therefore not detailed here. 
     CL  310  also sends out Frame signal  201 , via signal line  332  through T/R  301  to System Bus  101 ′, to synchronize the operation of the entire system. This signal also prevents the data Terminal Controllers (described below) from occupying the entire system time. 
     Furthermore, CL  310  receives service requests from Station Controllers  121 ,  122 , . . .  128  (described below) in time slot CMD-S  206  via line  331 , and assign time slots for voice traffic by CMD-P  202  via line  332 . 
     FIG. 4 is a block diagram of one of the voice Station Controllers, SC# 1   121 . The construction of its transceiver T/R module  401  is identical to that of T/R  301  in MC  100 . T/R  401 , under the control of Combinatorial Logic (CL)  410  via command line  461 , interfaces signals on line  141  which is connected with System Bus  101 ′, to internal digital data bus  421  and  422 . Such signals are connected to and from conventional Subscriber Line Interface Circuit (SLIC)  402  via lines  431  and  432 , respectively. SLIC  402  converts digital signals to analog signals with the same characteristics as Plain Old Telephone Service (POTS) to station instrument STN# 1   131  connected on line  403 . Command line  462  is used by SLIC  402  to report STN # 1   131  status such as station set ON/OFF-hook, and by CL  410  to send “ringing enable” command to SLIC  402 , etc. These are common telephony operations that are well known in the art, and therefore, not detailed here. 
     To operate the station instruments  131 ,  132 , . . .  138  as “extensions” in a PABX, an unique address has to be assigned to each SC. This is achieved through the use of dial switch ID  411 . The setting on it is fed into CL  410  via signal line  412 . When a call setup request is received on line  421 , CL  410  compares the destination address message against that on ID  411 . If a match is found, CL  410  responds by sending an answer message via line  422  to receive the call. 
     FIG. 5 is a block diagram of one of the data Terminal Controllers, TC# 1   151 . Again, transceiver T/R  501  has identical construction as  301  of MC  100 , or  401  of SC# 1   121 . T/R  501 , under the control of Combinatorial Logic (CL)  510 , via command line  551 , interfaces signals on line  161  which is connected with System Bus  101 ′, to internal data bus  521  and  522 . Such signals are connected to Timed Gate  502  via lines  531  and  532 , respectively. The Timed Gate  502 , under the control of CL  510  via command line  552  buffers signals to and from data terminal equipment  171  connected on line  541 . There could be various industrial standard interface formats on line  541 . These range from the traditional serial (RS232), parallel (Centronics), to USB (Universal Serial Bus), etc. Although they possess different characteristics and data speed, their functions are the same and are commonly known in the art. Thus, they are not discussed here. 
     Although transceivers T/R  301 ,  401 , and  501  of MC  100 , SC  121  and TC  151 , respectively, are constructed from HPN technology which operates in CSMA/CD mode, T/R  301  and  401  operate only during respectively assigned time slots when no other T/R unit is allowed to transmit according to the system timing plan shown in FIG.  2 . Thus, the CS (Carrier Sensing) and CD (Collision Detection) capabilities in T/R  301  and  401  are not utilized. Under normal operation conditions, information sent out from these modules are expected to be single transmission without any interruptions. There is no need to consider re-transmission. On the other hand, although T/R  501  handles data traffic in its inherent CSMA/CD mode, it is turned on by CL  510  only during Data Section  205  of each System Frame  200 . This layer of operation logic makes the present invention realistic. 
     FIG. 6 is a block diagram of a SC that is made of multiple SLICs from FIG.  4 . Like elements to those in FIG. 4 have the same reference numbers. Each of the SLICs  601 ,  602 , . . .  608  is operated under the control of Combinatorial Logic (CL)  610  via command line  662  at an assigned time slot to provide one channel of standard POTS service. T/R  401  controlled by CL  610  via command line  461  interfaces System Bus  101 ′ via line  141  to internal bus  421  and  422  which provides demultiplexing and multiplexing functions to signal lines  641 ,  642 , . . .  648  and  651 ,  652 , . . .  658  that are connected to SLICs  601 ,  602 , . . .  608 , respectively. Compared to CL  410 , CL  610  possesses multiple copies of CL  410  logic and is capable to execute them simultaneously, when needed. Thus, a conventional multi-line telephone set, or a Key Telephone Set (KTS) may be used with lines  621 ,  622 , . . .  628 . For example, if only two SLICs are used, FIG. 6 would represent a station controller for the popular “2-Line phone”. 
     A closer study of FIG. 6 reveals that SLICs  601 ,  602 , . . .  608  are likely be used for somewhat related voice conversations of the same subscriber on station lines  621 ,  622 , . . .  628 . Thus, the physical devices SLICs  601 ,  602 , . . .  608  could be merged into one and used at multiple time slots within each System Frame  200 . That is, common station instrument functions such as “hold”, “conference” could be implemented by enhancing the combinatorial logic in CL  610 . Consequently, single line station instrument can be used. Essentially, the distributed SCs of my invention can extend multi-line services that were implemented in the central control unit of traditional PABX. 
     FIG. 7 is a block diagram consists of combined Station and Terminal Controllers of FIGS. 4 and 5, respectively. Like elements in those figures have the same reference numbers. Since the T/R subsystems  401  and  501  are identical but operated at different times and manners, it would be natural to share the use of it, similar as the way multiple SLICs  601 ,  602 , . . .  608  in FIG. 6 sharing one T/R subsystem  401 . Consequently, Combinatorial Logic (CL)  710  should possess combined logic of CL  410  and CL  510  to control SLIC  402  and Timed Gate  502  via command line  762 . Thus, FIG. 7 depicts an interface controller that manages a node with Dual (voice and data) local Area Networking (DAN) functionality. 
     A logical extension of this derivation would lead to the integration of Timed Gate  502  into FIG. 6, resulting in a DAN node capable of supporting multi-line service. 
     To complete the basic ingredients of a traditional PABX, several operational features are needed. These include Auto-Attendant, Call Transfer, Conference and Intercom. By utilizing the command time slots CMD-P  202  and CMD-S  206  in FIG.  2  and special purpose station instruments, these can be easily implemented: 
     Let&#39;s assume that STN # 1   131  is a TAD. Its station controller SC # 1   121  would be set up with a special station ID  411 , such as “0”. And, CL  310  in MC  100  would be programmed to direct all incoming ringing from any one of the telephone lines LA  101 , LB  102 , . . . LZ  108  by requesting STN “ 0 ” to respond which would be SC # 1   121 . This allows STN # 1   131  to greet the callers. The station ID number dialed by the caller is then broadcasted in the command channel CMD-P  202 . Any one of the SCs  122 , . . .  128  that finds a match to its own ID setting  411  would then respond through command channel CMD-S to receive the call. 
     Similarly, a call can be transferred between any SCs  121 ,  122 , . . .  128  when a station instrument  131 ,  132 , . . . or  138  dials the desired SC&#39;s ID number. 
     Conferencing is achieved by performing a call transfer but without disconnecting the station instrument after the alerted station answers. Because the speech signals from each talker in a conference are properly spaced in time, each SC participating in a conference would just monitor for all other conference parties&#39; transmission time slots to collect all speech samples, combine them and then send to its telephone instrument. To use the time slots efficiently, the first available Primary time slot is assigned to the external line. The corresponding Secondary time slot is assigned to the first internal party. The second internal party would be assigned the next available Primary time slot, while the third internal party would be using the corresponding Secondary time slot, and so on. 
     Intercom is established by a station requesting a time slot from MC  100  to be used internally without accessing one of the outside telephone lines LA, LB, . . . LZ. Thus, no DAA module is needed. The initiating SC would be assigned the next available Primary time slot. The first station responding to this intercom call would be assigned the corresponding Secondary time slot. The second station that joins this intercom would be using the next available Primary time slot while the third station that joins this intrcom would be using the corresponding Secondary time slot, and so forth. 
     For CSMA/CD technology to work efficiently, it needs a continuous block of time as long as possible, to minimize the effects of packet headers and collision delays. On the other hand, using TDMA to represent analog voice introduces a minimum time delay which is in the order of the duration of a frame. Traditional 8 K Frames/s digital multiplex system introduces 125 us delay to the voice which has no discernable effect to human listeners. However, even a full 125 us frame has only a 125 bits block for data transmission at 1 Mb/s of the current HPN line rate which is too short to send a minimum length data packet (512 bits) according to conventional data LAN utilizing CSMA/CD protocol. 
     It is known that most people can not notice speech echoes shorter than 20 ms or so (Much longer transmission delay can be tolerated because each party does not have instant knowledge about what the remote party is doing.). The current DAN is set up with 80 frames per second repetition rate which results in a System Frame 200 length of 12.5 ms. At 1 Mb/s line rate, this is capable of sending data at 12.5 Kb/Frame. With 3 conversations (384 Kb/s combined digital rate), each frame would be occupied by 4.8 Kb (384 Kb/s divided by 80) of voice and leaving a 7.7 Kb (616 Kb/s divided by 80) block per frame for data traffic. This division of system resources (time) is therefore a realistic starting point for a practical SOHO DAN. 
     For a given transmission technology (e. g. 1 Mb/s of the current HPN), line rate on System Bus  101 ′ is fixed. Maximum voice pairs “K” in FIG. 2 that is allowed in a DAN system can be preset in Master Controller&#39;s CL  310  to set a compromise between voice and data traffic levels. This avoids burdening the installer to make such a decision. A field re-programable option of this parameter would then keep the flexibility open for configurations that a special application may require. On the other hand, with higher line rate, it is possible to speed up the frame rate so that transmission delay for voice can be reduced toward that of the traditional 8 KF/s. 
     The capabilities of the SCs and TCs, on the other hand, are dtermined by the technology implemented in the transceiver T/R. No parameter need be adjusted by the user. 
     For simplicity in presentation, telephony voice encoded in 64 Kb/s conventional PCM and carried by currently available 1 Mb/s HPN transmission are employed as an example in the preferred embodiment. It should be apparent to those skilled in the art that the basic system performance can be significantly improved by substituting certain parameters in this system with technologies that are either already available or forthcoming. For example, voice compression techniques, such as ADaptive PCM (ADPCM), have proven to be very effective in sending regular speech at 32 Kb/s and 16 Kb/s. Some systems have even successfully demonstrated ADPCM at 8 Kb/s. Since the digital bit rate of my system is set by the transmission facility utilized, the fewer bits needed for voice transmission, the more bits can be allocated for data application. 
     On the other hand, if the transmission facility, such as the projected 10 Mb/s or higher HPN technology becomes available, proportionally shorter time would be needed to send the same number of digital bits for each voice channel, there will be more time in each System Frame  200  available for data equipment to send messages, as well. 
     One handicap of electronic circuitry compared to POTS is that the latter is directly supported from the telephone company&#39;s Central Office (CO) which is powered by battery bank. Upon commercial power failure, POTS is expected to continue without interruption. This is essential during emergencies such as natural disaster. Without local power, however, communication through electronic equipments, such as my invention, would become non-operational. Thus, a power fail-safe arrangement with relay  440  and switch  441  is added to the block diagram of SC in FIG.  4 . It is energized during normal operation. Upon local power failure, the relay is released to bridge line  403  from STN# 1   131  through the normally closed contacts of switch  441  to System Bus  101 ′ via line  141 . Since System Bus  101 ′ is already connected to external service line  101  from the telephone company, direct POTS operation is automatically activated. 
     Because my system could be configured with multiple external service lines  101 ,  102 , . . .  108 , it is reasonable to expect that some of them may be accessible at certain SC locations due to existing wiring. This additional telephone service access possibility is shown as line  442  in SC# 1  as detailed in FIG. 4 (This is a configuration of telephone station wiring commonly referred to as 2-Line modular jack - RJ14.). When telephone service is available on this additional line, switch  441  should be set at the normally open position. In doing so, the additional telephone line  442  will be utilized by SC # 1  for POTS under power failure. This convention would then more evenly distribute traffic load among the external service lines during local power failure. 
     My invention is a totally modular system consisting of only three types of main modules, MC, SC and TC with industrial standard interface connections. Since SCs and TCs are deployed in multiple units, it is straightforward to replace a malfunctioning module by a spare, or temporarily substitute it by a module from a location of lesser importance. The impact to system operation would be minimal. If DAA module in MC fails, it can be easily replaced by plugging in a spare card of the same type. If MC&#39;s main logic  310  fails, the SCs can be powered down to allow the station instruments to operate in POTS mode. The TCs would then run at the full speed of the System Bus capability since there is no Framing signal  201  to restrict their operation intervals. 
     As can be seen from FIG. 7, the basic function of CL  710  is to manage voice traffic from a station instrument  131  and message flow from a terminal equipment  171 . It would be natural to expand the logic in CL  710  and utilize lines  421 ,  422  to exchange advanced messages with other DAN nodes via T/R  401  to enhance the operation features available to the station instrument  131 . For example, one of the data terminals  171 ,  172 , . . .  178  could be a information server that contains databases such as telephone number directory. CL  710  could behave as if it is one of the data terminals to access such information, and then apply the retrieved telephone number to provide “speed dialing” service to its associated station instrument. Similarly, one of the station instruments  131 ,  132 , . . .  138  could be a messaging system that stores messages for all users of the telephone stations. It can be accessed by either making an intercom call to it for retrieving voice message, or contacting it as a data terminal for E-Mails or video clips, etc. This illustrates my invention&#39;s architecture, being modular, is very flexible. It can easily support communication enhancements such as the Integration of Computer and Telephony technologies commonly referred to as CTI. 
     The system architecture of DAN disclosed is also compatible with digital subscriber line products, such as various forms of Digital Subscriber Loop (XDSL) services available from local telephone companies. XDSL utilizes frequencies between POTS and HPN for signal transmission. Therefore, it is compatible, and can easily use a “frequency translation” module to integrate with the DAN system presented here to provide wide area data transmission. This will complete a unified voice and data network that works together for both local and wide area communications, starting from as few as one single pair of telephone wires connecting the telephone CO and a subscriber. 
     Alternatively, my invention may be employed to establish a stand-alone DAN service. In such a case, a MC with only intercom control logic (without DAAs) is needed to be installed at a convenient location along a single pair of wiring. Then, one station controller is connected to the wiring pair at each location that has the need for voice intercom. Terminal Controllers would still be used for each data equipment as usual. 
     The forgoing illustrates the principles of the invention and it will thus be appreciated that those skilled in the art will be able to devise alternative arrangements which, although not explicitly described herin, embody the principles of the invention and are within the spirit and scope. 
     For example, the capacity of this unified system is dependent on the System Bus  101 ′ transmission rate available from the digital multiplexing technology. Currently, the most promising candidate is HPN on telephone line. Its speed of 1 Mb/s is well suited for SOHO environment. With the projected throughput of 10 Mb/s or more, service to an established small business will be possible. Similarly, by dynamically allocating the same communication channel between voice and data applications to share common resources, instead of separate dedicated systems, even high bandwidth facilities such as radio frequency, infrared or coaxial cable based systems can benefit from this system architecture to operate more efficiently.