EXTENDED RANGE DDS TRANSPORT VIA LOADED OR UNLOADED FACILITIES

Abstract: A method and apparatus for establishing DDS communication services between a first and second location connected by standard wiring by converting standard DDS signals into two data streams for transmission between two locations using two full-duplex communication channels is provided. The channels are established between two full-duplex communication transceivers at the first location and two full-duplex communication transceivers at the second location. At each location, the received signals are recombined into a single data stream representing DDS communications services that are made available to the channel bank at the service provider location (e.g., a D4 bank) and to the customer's terminal equipment. The transceivers establish communication via modulation formats that utilize the voice frequency band and are therefore capable of operation over longer cable lengths and over loaded cables. The devices at the central office have a network interface, a framer, and two transceivers. The network interface is for connecting the devices to the network. The network interface communicates with the framer. The framer acts as a multiplexer/demultiplexer because it separates the transmit data stream into two separate data streams, one for each transceiver and it receives two data streams from the transceivers and combines them into a single receive data stream that is forwarded to the network interface. The devices at the remote location are similarly configured, except that its interface is adapted for connection to the user's data equipment.

Description: EXTENDED RANGE DDS TRANSPORT VIA LOADED OR UNLOADED FACILITIES BACKGROUND OF THE INVENTION A. Field of the Invention This invention relates generally to the field of telecommunication and the processes by which digital data are transmitted over telephone networks to and from a customer's premises. More particularly, the invention is directed to a system for transmitting DDS (Digital Data Service, or sometimes referred to as Dataphone Digital Service) signals between the telephone network and a customer's premises. DDS services provide data communication between two points in the network, such as between customer equipment at different locations, or between equipment at a customer location and equipment at a data service provider. B. Description of the Related Art DDS signals are typically routed from a central office (CO) to customer premise equipment (CPE) by a 4-wire full-duplex circuit 8 as shown in Figure 1. The full- duplex circuit 8 is made up of two two-wire simplex circuits 10, 12. The typical DDS circuit includes an office channel unit 14 (OCU) at the CO and a data service unit 16 (DSU) at the customer premises. Those skilled in the art recognize that OCU 14 may be configured with one of a number of standard interfaces to the network. For example, the OCU may have DSX-0 interface, which is a bipolar, non-return to zero (NRZ) signal format, or a DSX-1 (Digital Service Cross-connect Level 1) interface, which is a baseband signal formatted to occupy a single time slot of a 1.544 Mbps multiplexed signal. An OCU having the latter interface is often referred to as an OCU-DP, for office channel unit - data port. An OCU-DP is located in an equipment rack commonly known as a channel bank. The channel bank generally serves as a multiplexer to combine numerous channels into a single electrical signal for transmission to another central office. The OCU-DP provides the interconnection between the four- wire digital data loop 8 and the common equipment of, for example, a D4, SLC5, SLC96, or digital loop carrier (DLC) channel bank. For purposes of the invention described herein, the distinction between an OCU and an OCU-DP is not significant. The differences between the two are primarily related to the manner in which the devices connect to the network which, in turn, vary depending upon the particular implementation of the DDS OCU and others in terms of an OCU-DP, one of ordinary skill in the art will recognize the interchangeability of the interfaces. The DSU 16 is located at the customer premises and provides the interconnection between the four-wire digital data loop 8 and the customer premise terminal equipment 18. The DSU 16 connects to the terminal equipment 18 over communication interface 20. Communication interface 20 is typically an RS-232 connection. The OCU 14 and the DSU 16 communicate using bipolar signaling over the two simplex circuits 10, 12. The data rates accommodated by present day OCU/DSU equipment are typically 2.4 Kbps, 4.8 Kbps, 9.6 Kbps, 19.2 Kbps, 38.4 Kbps, 56 Kbps, or 64 Kbps. The DDS network may be configured to provide a number of different types of services. For example, it may provide a dedicated point-to-point link between two customer locations. In such a situation, the topology of the circuit would be that shown in Fig. 2, having two DDS digital data loops 30, 32, connecting the OCU-DPs 34, 36 to the DSUs 38, 40. The OCU-DPs 34, 36 are located in channel banks 42, 44 located in two separate COs connected by a high rate multiplexed signal such as a DS-1, DS-3, etc. The DSUs 38, 40 connect the terminal equipment 46, 48. Alternatively, both customers could be connected to the same CO, in which case the OCUs may be configured with DSX-0 interfaces that may be interconnected without undergoing a mux/demux operation. Another typical topology is a bridged configuration as shown in Figure 3. A master DSU 50 at a customer premise communicates with an OCU 52 that feeds into a multi-junction unit 54 (MJU) located in a central office or provider's location. The MJU 54 connects in a star configuration to a number of other OCU units 56, 58, 60, 62. The OCU devices connect to DSU devices 68, 70, 76, and 78. As shown in Figure 3, one or more links may be routed through D4 channel banks 64, 66 between COs. At the channel bank 66 the signal is routed to the DSUs 68, 70 via OCU-DPs 72, 74. That is, full duplex communications is established between the MJU 54 and DSU 68 via OCU-DP 58 in channel bank 64 and OCU-DP 72 in channel bank 66. Similarly, full duplex communications is established between the MJU 54 and DSU 70 via OCU-DP 60 in channel bank 64 and OCU-DP 74 in channel bank 66. Other legs may be routed through other COs or end offices. The OCU-DPs at the end office feed to a cable facility and terminate at the customer premises with a DSU. The DSUs are connected configured to transmit messages including an address parameter that uniquely identifies a particular DSU. The MJU broadcasts the message to all of the remote DSUs, and only the addressed DSU responds. The DSUs are polled in this manner to provide communication services throughout the network of multiple remote DSUs. The DDS services may also be configured to provide switched 56 Kbps services. The network transmission rate is 64 K bps, but the least significant bit (lsb) is used to convey signaling information associated with switched services, so the lsb is not used, resulting in a data throughput of 56 Kbps. In this topology the OCU-DP provides a DS-0 interface on the network side for connection to a switch's DS-0 line card or another OCU. Furthermore, the OCU/DSU equipment must be of the type that supports standard switched 56 K bps functionality, e.g., supervisory signaling, on-hook, off-hook detection, frame detection, etc. The OCU and DSU equipment uses bipolar violations to indicate control information on the DDS link, such as on-hook and off- hook conditions. The DDS equipment converts the bipolar violations to in-band signaling that is sent using the least significant bits on the DS-0 channel. A significant limitation of the DDS network is related to the signal degradation that occurs over the subscriber loop between the OCU and DSU. The degradation is caused by impedance in the wiring that connects the OCU to the DSU. The wiring is typically 26-gauge, and exhibits approximately 42-45 dB of loss at 128 KHz over an 18 Kft length of cable. Typical devices used for DDS services can tolerate up to approximately 42 dB of loss. For this reason, 56 K bps DDS services are generally limited to subscribers that are connected by 18 Kft or less of 26-gauge wiring. The range may be extended if the wiring is a lower gauge (i.e., thicker, less lossy cable), or if lower data rates are used. Another significant limitation of the DDS network is related to its bandwidth requirements. OCUs and DSUs typically communicate over the two-simplex circuits with bipolar return to zero (RTZ) signaling shown in Figure 4a. Logical ones are represented by alternating high and low voltage pulses where the signal returns to an intermediate reference voltage at the end of a bit period. Logic zeros are represented by the absence of a pulse. Figure 4b shows a sequence of logic values (1101) in the RTZ format. Figure 4c shows the spectral properties of a full rate 64 K bps DDS RTZ signal. As seen in Figure 4c, the first spectral null occurs at 64 KHz. While the network channel capacity is 64 Kbps, the DDS subscriber loop may be a sub-rate sub-rate channel having a data rate of 9.6 Kbps, the first spectral null occurs at 9.6 KHz. As mentioned above, the impedance characteristic of the wiring is an important consideration. The impedance characteristic of the cable is not uniform across the entire bandwidth that is occupied by the signal: the cable exhibits greater loss at higher frequencies. This contributes to the distance limitation between the OCU and the DSU. Additionally, the propagation delay of signals traveling over the wiring varies according to the signal's frequency. This causes greater distortion in high bandwidth signals. Furthermore, many existing cable runs have been specifically conditioned to pass voice frequencies, which typically extend to about 4 kHz, by the addition of loading coils. While this type of line conditioning decreases the distortion in the voice frequency band, it makes higher frequencies virtually unusable for broadband data communication. Consequently, the relatively high bandwidth signals of a typical DDS circuit may not be transmitted over lines that have been conditioned by the insertion of loading coils. SUMMARY OF THE INVENTION The apparatus described herein is designed to extend the range over which DDS services may be transmitted. In particular, described herein is a method of providing DDS communication services between a first and second location connected by standard wiring by converting standard DDS signals into two data streams for transmission between two locations using two full-duplex communication channels. The channels are established between two full-duplex communication transceivers at the first location and two full-duplex communication transceivers at the second location. At each location, the received signals are recombined into a single data stream representing DDS communications services that are made available to the channel bank at the service provider location (e.g., a D4 bank) and to the customer's terminal equipment. The transceivers establish communication via modulation formats that utilize the voice frequency band and are therefore capable of operation over longer cable lengths and over loaded cables. The devices at the central office have a network interface, a framer, and two transceivers. The network interface is for connecting the devices to the network. The network interface communicates with the framer. The framer acts as a multiplexer/demultiplexer because it separates the transmit data stream into two transceivers and combines them into a single receive data stream that is forwarded to the network interface. The devices at the remote location are similarly configured, except that its interface is adapted for connection to the user's data equipment. Thus, when DDS services can not be established because the DSU at a particular customer premise is beyond the reach of an OCU/OCU-DP at the service provider's facility due to the length of the subscriber loop wiring (18Kft at 26 gauge), or if the wiring has load coils making it unsuitable for DDS transmission, then an extended range office channel unit (ER-OCU) and an extended range data service unit (ER-DSU) of the present invention is particularly useful in providing DDS services to that customer premise. The ER-OCU also has circuitry to provide power over the line to the ER-DSU. BRIEF DESCRIPTION OF THE DRAWINGS Presently preferred embodiments of the invention are depicted in the drawings, wherein like reference numerals refer to like elements in the various views, and wherein: FIG. 1 is a block diagram depicting a typical prior art DDS communication link; FIG. 2 is a block diagram depicting a prior art point-to-point DDS topology; FIG. 3 is a block diagram depicting a prior art bridged DDS topology; FIG. 4a depicts RTZ signals; FIG. 4b depicts an RTZ signal sequence; FIG. 4c depicts the frequency spectrum of a standard RTZ signal; FIG. 5 is a block diagram of an extended range DDS communication link using extended range devices; FIG. 6 is a block diagram of a DDS communication link using extended range adapter devices; FIG. 7 is a block diagram of an extended range office channel unit; FIG. 8 is a block diagram of an extended range data service unit. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The ER-DP interfaces to the network in the same manner as an OCU-DP ~ by plugging into a D4, SLC5, DLC, or SLC 96 channel bank and providing a DS-0 type serial data stream to interface with the backplane of the channel bank. The ER-DP may also be configured with alternative network interfaces such as DSX-0, if appropriate for the particular DDS implementation. Regardless of the type of interface, the central ER-DSU provides an interface to the customer's terminal equipment such as an RS-232, V.35, or other appropriate standardized interface. The terminal equipment may be a simple terminal, a router, or any other data communication device. This configuration is shown in Figure 5. The ER-OCU 100 is located at the CO. It communicates with the ER-DSU 102 over two full-duplex communication channels 104, 106. Each of the communication channels 104, 106 preferably operates over a twisted-pair two-wire line. The ER-DSU 102 communicates with the terminal equipment 108 by way of communication link 110, which is preferably an RS-232 full-duplex serial connection. An alternative embodiment of the present invention provides extended range adapters that convert the bipolar RTZ signaling of a standard OCU and DSU to the extended-range signaling format (using two full-duplex communication channels over four wires). The devices are referred to herein as an extended range adapter for an office channel unit (ERA-OCU) and an extended range adapter for a DSU (ERA-DSU). The block diagram of Figure 6 depicts the interconnection of the adapter devices in a DDS circuit. The ERA-OCU 120 provides an interface between an existing OCU 122 and an ERA-DSU 124 (or an ER-DSU), thereby providing extended range capability without the need to replace the OCU 122. The ERA-OCU 120 is located at the central office for communication with the OCU 122 via a standard bipolar RTZ signaling format over simplex lines 126, 128. The ERA-OCU 120 communicates with the ERA- DSU 124 (or an ER-DSU) via dual channel full-duplex transceivers over lines 130, 132. The ERA-DSU 124 provides an interface between the ERA-OCU 120 (or an ER-OCU) and a DSU 134, obviating the need to replace the DSU 134. The DSU 134 communicates with data equipment 140 over communication link 142. The ERA-DSU 124 is located at the customer premise (or at an intermediate remote location) for communication with the ERA-OCU 120 (or an ER-OCU) via dual channel transceivers and with the DSU 134 via a standard bipolar RTZ signaling format on simplex lines 136, 138. The ERA-DSU 124 may be used as an adapter at the customer premise to facilitate the use of standard DSUs with an ER-OCU. The ERA-DSU can also act as a repeater at an intermediate location to transmit an additional 18 Kft (using RTZ signaling over non-loaded cable) to the DSU at the customer premise. Figure 7 shows a DDS extended range office channel unit (ER-OCU) 200. The ER-OCU has an interface circuit 202, a framer 204, control circuitry 206, and two transceivers 208, 210 connected to two two-wire lines 212, 214. The interface circuit D4, SLC5, DLC, or SLC 96 channel bank. Alternatively, the interface circuit may provide a DSX-0 connection using bipolar NRZ signaling. The interface circuit 202 provides the necessary timing and buffering operation to pass data between the channel bank and the framer 204. The framer 204 acts as a multiplexer/demultiplexer between the single data channel of the interface circuit 202 and the two data channels corresponding to the two transceivers 208, 210 and two two-wire lines 212, 214. The two transceivers 208, 210 communicate with compatible transceivers located in the ER- DSU located at the customer premises. The ER-OCU 200 also includes control circuitry 206 to coordinate the operations of the ER-OCU 200. Preferably, the ER- OCU transceivers, framer, and control circuitry are implemented on a programmed microprocessor in conjunction with analog-to-digital and digital-to-analog converters. The preferred interface circuit 202 is implemented in a gate array and programmed microprocessor, and uses the microprocessor's speed buffer. The speed buffer is a 3-byte elastic register that phase synchronizes the data bytes to the framed PCM DS0 channel. The synchronized bytes are read out of the ER-OCU to the channel bank. The interface circuit contains the necessary logic to generate the transmit time slot and the common control channel unit identification. In the N to C path, the DS0 bytes are read into the ER-OCU and converted to customer data bytes and are provided to the framer. Figure 8 shows a DDS extended range data service unit 300 (ER-DSU). The ER-DSU provides the interface between equipment at the customer premises and the four-wire digital data circuit. Like the ER-OCU, the ER-DSU has an interface circuit 302, a framer 304, a controller 306, and two transceivers 308, 310 connected to the two two-wire lines 212, 214. The interface circuit 302 of the ER-DSU connects the ER- DSU to the customer equipment. The interface 302 is typically an RS-232 or V.35-type interface. However, the ER-DSU of the present invention may also be configured to provide an ethernet port or other appropriate standard data interface for the customer's use. The interface circuit 302 provides the necessary timing and buffering operation to pass data between the customer premise equipment and the framer 304. The framer 304 acts as a multiplexer/demultiplexer between the single data channel of the interface circuit 302 and the two data channels corresponding to the two transceivers 308, 310. The two transceivers 308, 310 communicate with compatible transceivers 208, 210 circuitry 306 to coordinate the operations of the ER-DSU. The ERA-OCU is similar to the ER-OCU shown in Fig. 7, except the interface 202 is adapted to accept a bipolar RTZ input from a standard OCU. The ERA-OCU then communicates using the extended range dual channel format with a remote extended range unit. The ERA-OCU also includes circuitry to perform the necessary supervisory signal conversion, i.e., it converts bipolar control signals coming from the network into appropriate signaling such as in-band signaling or control signals sent over the embedded operations channel. It also converts control signals from the ER- DSU into bipolar control signals that are recognized by standard OCUs. The ERA- DSU interface similarly provides a bipolar interface for a standard DSU, and performs similar control signal conversion to and from bipolar violations. The embodiments discussed above differ mainly in the types of interfaces provided, and are similar in that they all include framers and transceivers. For simplicity, the preferred embodiments of the extended range data port equipment (the ER-OCU and ERA-OCU) are collectively referred to as ER/A-OCU devices. The preferred embodiments of the extended range customer premise equipment (the ER- DSU and ERA-DSU) are collectively referred to ER/A-DSU. Additionally, all embodiments of the extended range DDS equipment of the present invention are collectively referred to as ER A-DDS devices. The framer separates the DS-0 data into two data streams, each of which can be up to 32 Kbps for a combined data throughput of 64 Kbps. Standard DS-0 channels consist of 8-bit bytes ("octets"), occurring at a rate of 8 K bytes/s, thus providing a total of 64 Kbps. Preferably, the octets are divided between the channels in an alternating fashion, resulting in every-other octet being sent over a particular transceiver. An alternative embodiment of the framer divides the 8-bit bytes into two 4-bit bytes and forwards one to each transceiver. The four least significant bits from each octet are referred to as the low-order byte, while the four most significant bits are referred to herein as the high-order byte. In either embodiment, the resultant data rate is 32 Kbps to each transceiver. An embedded operations channel (EOC) is used to provide byte synchronization so the framer at the receiving ER A-DDS device is able to reassemble the bytes in the correct sequence. The EOC uses a sequencer or framing pattern on each channel to indicate the beginning of a data frame. The framer at the receiving ER A-DDS device the ER-DDS system, the communication channels are designated as channel 1 and channel 2. Preferably, the ER/A-OCU operates as the master, while the ER A-DSU operates as the slave, and the master unit informs the slave unit as to which channel is channel 1. The odd bytes (or low-order byte, for the alternative embodiment) are then preferably sent over channel 1 and the even bytes (high-order bytes) are sent over channel 2. In this manner, once the framer has detected the frame demarcation on each channel, it merely reassembles the data bytes accordingly. In the preferred embodiment, the two transceivers are capable of transmitting at a rate of 33.6 K bps on each of the two communication channels. Thus, the EOC can occupy up to 1.6 K bps on each channel and still accommodate a 64 K bps DDS throughput. In particular, the preferred embodiment of the framer inserts four bits of overhead for every 80 data bits, thereby delimiting a frame of 80 data bits. The overhead (per channel) consists of a framing bit and three additional bits used for a cyclical redundancy check (CRC) to further improve error correction. The framing pattern used is preferably the same pattern as that used in a D-4 data stream. The EOC may also used to perform certain maintenance and performance monitoring. Each of the transceivers has a transmitter including a modulator and a receiver including a demodulator. The transmitter receives the data from the framer and modulates it for transmission over the two-wire loop. As stated previously, the transmitter is implemented on a microprocessor using digital signal processing techniques that are well known in the art. The microprocessor therefore provides digital signals to a digital-to-analog converter to generate the analog modulation signal. The receiver is also implemented on a microprocessor and accepts data samples from and analog-to-digital converter. Because each channel uses the same two wires for transmit and receive operations, the transceivers also include adaptive echo cancellers, as is well known in the art. The transceivers preferably use the modulation technique known as V.34, as described in International Telecommunications Union Recommendation V.34, and thus include adaptive equalizers, trellis encoders and precoders, which are also well known in the art. The ER/A-DSU is preferably line powered from the ER/A-OCU. The ER/A- OCU includes a line-power circuit for providing remote simplex power to the ER/A- DSU. The DDS devices are coupled to the two-wire lines via transformers having ports of the two transformers, as is common in standard DDS communication devices for providing remote power to a repeater unit. It is understood that many equivalent circuit configurations may be used to implement the functionality of the above-described elements without departing from the scope and spirit of the invention. For example, as described above, the preferred embodiment of the transceiver includes a programmed microprocessor. The microprocessor is preferably of the type known as a digital signal processor (DSP). Preferably, the DSP also performs the buffering operations of the framer, including the EOC signaling, and the necessary signal processing of a modem. Alternatively, the elements may be implemented with discrete components. For example, discrete shift registers and buffers may be used for performing the framing operation, and the modem function may be implemented via dedicated chipsets designed for transceiver operation. Similarly, the functionality of one or more components may be combined in a single circuit. For example, the interface circuit may be combined with that of the framer by providing an interface that simultaneously separates and combines the data from one data stream to two and vice-versa. Additionally, the preferred embodiment describes the use of V.34 modem technology within the transceivers. Alternative modulation techniques may be used that do not conform to the V.34 standard. All that is necessary to the present invention is that the transceiver use modulation that is limited in bandwidth, and preferably capable of full-duplex operation over a two-wire line. Thus, the detailed description of the preferred embodiment is intended as an illustration, and not as a limitation, of the present invention. While variations and modifications of the invention will occur to those skilled in the art, it is to be understood that such modifications are within the scope of the invention as defined by the following claims:

Claims: We claim: 1. A method of providing DDS communication services between a first and second location connected by standard wiring comprising the steps of: - providing two full-duplex communication transceivers at the first location and two full-duplex communication transceivers at the second location for establishing two full-duplex communication channels over the standard wiring connected between the first and second locations; dividing a data stream representing DDS signals into two complementary data streams at the first location; transmitting said two complementary data streams from the first location to the second location using said two full-duplex communication channels; and recombining said two complementary data streams into a single data stream representing DDS communications services. 2. The method of claim 1 wherein the full-duplex communication transceivers utilize a voice-frequency modulation format. 3. The method of claim 2 wherein the modulation format is V.34. 4. The method of claim 1 further comprising the step of converting bipolar DDS signals into a data stream representing DDS signals. 5. The method of claim 1 further comprising the step of converting channel bank signals into a data stream representing DDS signals. 6. The method of claim 1 further comprising the step of converting the recombined single data stream into channel bank signals. 7. The method of claim 1 further comprising the step of converting the recombined single data stream into bipolar DDS signals. 8. A method of modifying an existing DDS link having an OCU at a first location and a DSU at a remote location, where the OCU and DSU utilize bipolar signaling, comprising the steps of: connecting an OCU adapter to the OCU for sending and receiving bipolar signals to and from the OCU; and connecting a DSU adapter in series between the OCU adapter and the DSU to send and receive voice-band-modulated signals to and from the OCU adapter, and to send and receive bipolar signals to and from the DSU. location other than the remote location. 10. A method of modifying an existing DDS link having an OCU at a first location and a DSU at a remote location, where the OCU and DSU utilize bipolar signaling, comprising the following steps: replacing the OCU with an extended range OCU for sending and receiving channel bank signals; and connecting a DSU adapter in series between the extended range OCU and the DSU to send and receive voice-band-modulated signals to and from the extended range OCU, and to send and receive bipolar signals to and from the DSU. 11. The method of claim 8 wherein the DSU adapter is placed at an intermediate location other than the remote location. 12. An apparatus for transmitting DDS signals from a network to a remote location comprising: a network interface connected to the network for transmitting and receiving network-side data signals to and from the network; a framer connected to said network interface for converting said network-side data signals into first and second duplex data streams, wherein said framer includes first and second duplex ports and provides said first and second duplex data streams at said first and second duplex ports; and first and second transceivers connected to said first and second duplex ports, wherein said first and second transceivers are also adapted for connection to two two- wire lines. 13. The apparatus of claim 12 wherein said network interface utilizes a channel bank signaling format. 14. The apparatus of claim 12 wherein said network interface utilizes a bipolar signaling format. 15. The apparatus of claim 14 wherein said network interface is adapted for communicating with an OCU. 16. The apparatus of claim 12 wherein said first and second transceivers utilize a voice-frequency-band modulation format. 17. The apparatus of claim 16 wherein said voice- frequency-band modulation format is V.34. duplex transceivers. 19. An apparatus for transmitting DDS signals from a remote location to a network comprising: a remote equipment interface adapted for connection to remote equipment for transmitting and receiving customer-side data signals to and from the network; a framer connected to said terminal equipment interface for converting said customer-side data signals into first and second duplex data streams, wherein said framer includes first and second duplex ports and provides said first and second duplex data streams at said first and second duplex ports; and first and second transceivers connected to said first and second duplex ports, wherein said first and second transceivers are also adapted for connection to two two- wire lines. 20. The apparatus of claim 19 wherein said remote equipment interface utilizes a RS-232 signaling format. 21. The apparatus of claim 19 wherein said remote equipment interface utilizes a bipolar signaling format. 22. The apparatus of claim 21 wherein said remote equipment interface is adapted for communicating with a DSU. 23. The apparatus of claim 19 wherein said first and second transceivers utilize a voice-frequency-band modulation format. 24. The apparatus of claim 23 wherein said voice-frequency-band modulation format is V.34. 25. The apparatus of claim 19 wherein said first and second transceivers are full- duplex transceivers.

Inventors: GARCIA FRANCIS X, SHEETS LAURENCE L, GARCIA, FRANCIS, X., SHEETS, LAURENCE, L.
Applicants: GARCIA FRANCIS X, SHEETS LAURENCE L, TELTREND INC, GARCIA, FRANCIS, X., SHEETS, LAURENCE, L., TELTREND, INC.
Application year: 1998
Main classification: H04L5
IPC codes: H04L25