HK1016372B - Time division duplexed high speed data transmission system and method - Google Patents

Description

Time division multiplexing high speed data transmission system and method

Background

The invention relates to a very high speed bi-directional digital data transmission system. In particular, a time division multiplexed data transmission scheme suitable for transmission over bundled transmission lines, such as subscriber lines, is disclosed.

The telecommunication solution Alliance (ATIS), an organization approved by the ANSI (american national standards institute) standards group, has recently finally established a discrete multi-tone based standard for digital data transmission over Asymmetric Digital Subscriber Lines (ADSL). The standard is primarily used for transmitting video data over ordinary telephone lines, although it may be used in other applications as well. The north american standard is referred to as the ANSI ti.413 ADSL standard and is incorporated herein by reference. The transmission rate according to the ADSL standard is intended to be such that the information transmission rate on twisted-pair telephone lines amounts to at least 6 million bits per second (i.e. 6+ Mbit/S). The standardized system specifies the use of a discrete multi-tone (DMT) system that uses 256 "tones" or "subchannels" each having a 4.3125KHZ bandwidth in the forward (downstream) direction. According to the telephone system, the downstream direction is defined as the transmission from the telephone central office (typically owned by the telephone company) to a remote location that may be an end user (i.e., a residential or commercial user). In other systems, the amount of audio used may vary greatly. However, typical values for the number of available subchannels (audio) are an integer power of 2, such as 128, 256, 512, 1024 or 2048 subchannels, if the modulation is efficiently performed using an Inverse Fast Fourier Transform (IFFT).

The asymmetric digital subscriber line standard also specifies the use of reverse signals transmitted at rates in the range of 16 to 800 Kbit/S. The reverse signal corresponds to transmission in the uplink direction. E.g. from a remote location to a central office. The term asymmetric digital subscriber line therefore comes from the fact that the data transmission rate is much higher in the downstream direction than in the upstream direction. This standard is of particular practical significance in systems intended to transmit video programs or teleconferencing information to a remote location over a telephone line. For example, one potential use of the system is to enable residential users to obtain video information, such as movies, over telephone lines without having to rent video tapes. Another potential use is visual conferencing.

Because the downstream and upstream signals are transmitted on the same wire pair (i.e., duplexed), they must be separated from each other in some way. The multiplexing method used in the ADSL standard is frequency division multiplexing (FDD), in which the upstream and downstream signals occupy different frequency bands and are separated by filters at the transmitter and receiver.

ANSI, written herein, has just begun to work with the next generation of subscriber line based transmission systems, which are known as the VDSL (very high digital subscriber line) standard. The VDSL standard intends to be: the transmission rate in the downstream direction is at least 25.96Mbit/S and preferably at least 51.92 Mbit/S. To achieve such rates, the transmission distance over twisted telephone pairs must generally be shorter than the length allowed by ADSL. At the same time, the digital, audio and video association (DAVIC) is also conducting research on similar systems. This system is known as "fiber to the curb" (FTTC). The transmission medium from the "curb" to the customer premises is standard Unshielded Twisted Pair (UTP) telephone line.

Many modulation schemes have been proposed for use in the VDSL and FTTC standards (hereinafter VDSL/FTTC). While written herein, all proposed VDSL/FTTC modulation schemes employ frequency division multiplexing of the upstream and downstream signals. For example, one proposed multiple carrier frequency scheme employs frequency division multiplexed discrete multi-tone signals, with upstream communications being accommodated at a low frequency band and downstream communications being accommodated at a high frequency band. The method is generally described as shown in fig. 2 (a). Another proposed approach contemplates the use of frequency division multiplexed carrierless amplitude and phase modulated (CAP) signals, with upstream communications being accommodated in the high frequency band and downstream communications being accommodated in the low frequency band. This method is generally described as shown in fig. 2 (b).

However, both of the above approaches have potential drawbacks. Most notably, in applications with relatively long loops, high frequency signals are significantly attenuated, which makes the transmission more sensitive to noise and prevents the allowed transmission rate, indeed in systems transmitting upstream signals at higher frequencies, there is a risk of losing the upstream signal altogether, which is not allowed. In asymmetric applications, there is also a greater risk that narrow-band noise significantly degrades system performance. Accordingly, an improved method of coordinating very high frequency data transmissions (i.e., having a transmission rate of at least 10 megabits per second on each transmission line) is desired.

Brief description of the invention

To achieve the foregoing and other objects, and in accordance with the purpose of the invention, a method for coordinating very high speed bidirectional data transmission between a central unit and a plurality of remote units over distinct twisted pair transmission lines sharing a link is described. More specifically, uplink and downlink communication periods that are periodically synchronized are provided so as not to overlap each other. That is, the upstream and downstream communication periods of all wires sharing one connector are synchronized. With this scheme, all very high speed transmissions within the same connection are synchronized and time multiplexed so that downstream communications do not occur at times that overlap with upstream communications.

In one embodiment, a quiet period is provided to separate the upstream and downstream communication periods. During the silent period, neither uplink nor downlink communications may be transmitted. In another embodiment, the communication and silence periods are divided into symbol periods. In this embodiment, each downlink communication period comprises a number of symbol periods. Each uplink communication period includes at least one symbol period. And each quiet period includes at least one symbol period. In a particular embodiment of designing a multicarrier modulation scheme, the downlink communication period comprises 8 to 60 symbol periods, the uplink communication period comprises 1 to 30 symbol periods, and each quiet period comprises 1 to 4 symbol periods.

The described invention may be used in conjunction with a wide variety of modulation schemes, including multi-carrier transmission schemes, such as discrete multi-tone modulation (DMT), as well as single carrier transmission schemes, such as quadrature amplitude modulation; carrierless amplitude and phase modulation (CAP); quadrature Phase Shift Keying (QPSK); or vestigial sideband modulation. It may be used to include connections used to transmit low speed signals, independent of whether the low speed signals are time division multiplexed and/or synchronized with the high speed signals. This is because standardized low-speed signaling systems tend to operate at lower frequencies and are not as susceptible to near-end crosstalk as high-frequency signals.

The invention is particularly advantageous in very high performance systems, such as those designed to carry signal frequencies above about 1.0MHZ, and those capable of transmitting downstream signals over different transmission lines at transmission rates of at least 10 mbit/yarn.

In another aspect, the present invention provides for the proportional allocation of upstream data bandwidth to a plurality of set-top units sharing a single transmission line. For example, in one embodiment, each set top unit may be assigned a different segment of the upstream communication period. In another embodiment, each set top unit may be allocated a segment of the upstream communications band.

The method works regardless of whether the central unit is a central office that originates the communication or a distribution unit (e.g., an optical network unit) that receives the downstream source signal via one or more trunks or the like and transmits the information contained in the downstream source signal as the downstream communication signal. The distribution unit also transmits information contained in the upstream communication signal as an upstream source signal over the optical fiber.

Brief description of the drawings

The invention, together with its objects and advantages, may best be understood by reference to the following description taken in conjunction with the accompanying drawings. In the drawings:

fig. 1a is a block diagram of a subscriber line based communication system having a plurality of twisted-pair telephone lines extending from a central unit to remote units.

Fig. 1b is a specific example of fig. 1a, wherein the central unit is in the form of an optical network unit as a junction between a fiber trunk and a plurality of twisted wire pairs.

Fig. 2a and 2b are diagrams depicting a conventional frequency domain multiplexed transmission scheme for asymmetric subscriber line transmission;

FIGS. 3a and 3b are graphs depicting time domain multiplexed transmission schemes for a single line, with FIG. 3a representing downstream communication and FIG. 3b representing upstream communication;

FIGS. 4a-4d are diagrams depicting an asynchronous time domain multiplexed transmission scheme of a pair of transmission lines sharing the same connector, the solid lines representing transmission and the dashed lines representing reception;

FIGS. 5a-5d are diagrams depicting a synchronous time domain multiplexed transmission scheme sharing a pair of transmission lines with a connector, the solid lines representing transmission and the dashed lines representing reception;

fig. 6 is a block diagram depicting a general office and remote modem timing architecture suitable for performing synchronization operations in accordance with the present invention.

Detailed description of the invention

The usual bi-directional transmission method envisages the use of a data transmission scheme based on time division multiplexing (i.e. "ping-pong"). That is, the downlink signal is first transmitted using the entire bandwidth. Thereafter, the uplink signal is transmitted using the entire bandwidth, and so on. The applicant's research has shown that this method is feasible at low frequencies in subscriber line applications. However, when higher carrier frequencies are used, e.g., carrier frequencies above 1MHz, the line-to-line near-end crosstalk sharing the same connector 205 begins to significantly degrade system performance. Thus, while time division multiplexing transmission has not been proposed for VDSL/FTTC or other subscriber line based very high speed data transmission applications as written herein, most of such proposed modulation schemes contemplate transmission over a carrier frequency band significantly higher than 1 MHz. The invention overcomes the near-end crosstalk problem at the roadside by synchronizing the time division multiplexing transmission of very high speed data transmission that shares a link.

A description of a typical subscriber line communication local loop is shown in figure 1 a. As can be seen, the central unit 201 is connected to a plurality of remote units R by separate transmission lines₁-R_NThe communication transmission line may be in the form of a conventional twisted telephone pair 206. The remote unit may be an end-user unit at home, office, or the like. Typically a large number of remote units are served by a particular central office. In recently installed systems, the remote units are often telephones, however, they may also be facsimile lines, computer terminals, televisions or various other devices capable of connecting to a "telephone line". The central unit 201 may comprise for each line a transmitter receiver 208, which is functionally divided into a transmitter 209 and a receiver 210.

In some embodiments, the central unit is a main server located at the originating central office. In other embodiments, a "central unit" may be a lower level distribution component of the system architecture that receives and retransmits signals. One of such distributed componentsOne embodiment is shown in figure 1 b. As shown, trunk 202 terminates at distribution unit 204. In the illustrated embodiment, the main trunk line employs optical cables and the distribution units employ Optical Network Units (ONUs). The distribution unit 204 is connected to a plurality of remote units R through discrete lines₁-R_NCommunication, discrete lines are also possible using conventional twisted telephone pairs 206. In the foregoing embodiments, the remote unit may be an end-user unit at a home, office, or other similar location. Typically a large number of remote units are serviced by a particular ONU. For example, in north america, a typical ONU may serve 4 to 96 remote units. In this embodiment, the ONU receives downstream source signals over one or more trunks and transmits the information contained therein as downstream communication signals to the appropriate remote units. Similarly, an ONU receives an upstream communication signal from a remote location and transmits the information contained therein as an upstream source signal. The source signal may be communicated to a central office, another distribution unit, or any other suitable location.

The distance between the central units 201, 204 and the furthest ground may vary considerably. For example, the VDSL/FTTC standard requires that twisted pair loops be allowed to have a length of up to 1000 feet (300 meters) for 51.92MHz downstream communications. Similarly, for 25.96MHZ downlink communications, the allowed loop length can be up to 3000 feet (900 meters); for 12.97MHZ downlink communications, the allowed loop length may be 5000 feet (1500 meters). Those skilled in the art will appreciate that a shorter maximum loop length generally corresponds to a higher achievable data rate.

The present invention can be applied to various data transmission schemes. This is particularly useful in transmission schemes that design efficient transmission at carrier frequencies above 1 MHZ. For example, in subscriber line applications, the concept of synchronous time division multiplexing can be applied in both multi-carrier methods, such as discrete multi-tone modulation (DMT), and single carrier methods, such as conventional Quadrature Amplitude Modulation (QAM), carrierless amplitude and phase modulation (CAP); quadrature Phase Shift Keying (QPSK); and/or vestigial sideband modulation. The system may also be used to include connections used to transmit low speed signals, independent of whether the low speed signals are time division multiplexed and/or synchronized with the high speed signals. This is because standardized low-speed signaling systems tend to operate at lower frequencies and are not as susceptible to near-end crosstalk as high-frequency signals. When low frequency noise or crosstalk causes a problem, the frequency band that becomes the problem can be entirely cancelled.

A common time division multiplexed data transmission scheme is shown in fig. 3(a) and 3(b), and as can be seen, downlink communications (shown in fig. 3(a)) are transmitted in periodic downlink communications periods 111. Upstream communications (shown in fig. 3(b)) are transmitted in periodic upstream communication periods 113 that alternate with associated downstream communication periods. The period from the start of the first downstream communication period to the start of the next downstream communication period is referred to herein as a "superframe". The actual durations of the up, down and quiet periods, as well as the superframe, may vary widely in the field of the present invention.

Most very high speed data transmission schemes are frame based systems with discrete symbols. In such systems, the number of symbols constituting a "downlink communication period" and an "uplink communication period" is typically an integer number of symbols. It is easy to make the quiet time an integer number of symbols (most typically 1), although this is not strictly required. In one such multicarrier embodiment using discrete multitone transmission, each superframe has at least a 12DMT symbol period. In such a system, the downlink communication period may be in the range of 8 to 60DMT symbol periods, and the uplink communication period may be in the range of 1 to 30 symbol periods. The quiet time is 1 to 4 symbol periods.

For example, in the embodiment shown in fig. 3(a) and 3(b), the superframe has 38 symbol periods, and each quiet period is one symbol period. Thus, there are 36 symbol periods to be divided between upstream and downstream communications. The typical relationship between downstream and upstream bandwidth in asymmetric applications is 8: 1. Such a system is depicted in the figure, where the downlink communication period is 32 symbol periods and the uplink communication period is 4 symbol periods. Referred to herein as the 32: 1: 4: 1 example. In a symmetric system, 18 symbol periods may be assigned to each of the upstream and downstream communication periods. That is, a 18: 1: 18: 1 split ratio may be used. Of course, if more bandwidth is required for upstream communication than for downstream communication, the number of symbol periods allocated for upstream communication may vary anywhere from 1 to 18 or more. Another specific discrete multi-tone instance design uses a 20 symbol superframe. In such systems, a 16: 1: 2: 1 or 8: 1: 8: 1 symbol distribution may be used. Of course, the number of symbol periods in each superframe and their respective allocations may vary greatly.

When the symbol rate is 32KHZ, the symbol period is 31.25 microseconds. In a 38-symbol superframe with a symbol rate of 32KHz, a system with a 32: 1: 4: 1 symbol distribution, the maximum transmission time for the remote is 38 symbol periods or about 1.2 milliseconds. If a shorter access time is required, a shorter superframe time may be appropriate. If an uplink bandwidth wider than a downlink bandwidth is required, it is necessary to reduce the number of symbols allocated to downlink communication and increase the number of uplink symbols. If a wider system bandwidth is required, it may be advantageous to increase the superframe length and decrease the silence period length. Indeed, in some applications, it may be desirable to remove the silence period entirely, although it will be appreciated that removing the silence period increases the likelihood of interference. It should be understood that all of these factors can vary widely depending on the requirements of a particular system.

It should be appreciated that single-carrier transmission schemes typically have a fairly short symbol period (e.g., perhaps on the order of microseconds). Thus, in such a system, a significant number of symbols may be provided in each of the downlink, uplink, and quiet periods. For example, for a downlink communication period, the period is on the order of about 1000 to 2000 symbols; 100 to 500 symbols for a quiet period; it is preferable for the uplink communication period to be 400 to 10,000 symbols.

With reference to fig. 4(a) -4(d), the deficiency of the asynchronous system is described below. In the illustrated example, twisted pair transmission lines 206(a) and 206(b) both transmit time division multiplexed discrete multi-tone signals. Each transmission line provides 16-symbol downstream communication periods 111, 2-symbol upstream communication periods 113, and 1-symbol quiet periods therebetween. In this embodiment, the communications are not synchronous. Thus, the simultaneous presence of downstream communication transmissions on lines 206(a) and 206(b) and upstream transmissions on lines 206(b) and 206(a), respectively, will result in near-end crosstalk into the associated receivers of both distribution units. Similarly, the upstream transmitters rt (a) and rt (b) will cause (even if somewhat reduced) near-end crosstalk into each other receiver. I.e. to the receivers at rt (a) and rt (b), respectively. Thus, as depicted by arrows 217 and 219, the system suffers from near end crosstalk, which will greatly reduce system performance.

To overcome this problem, all very high speed transmissions sharing a link are synchronized as shown in fig. 5(a) -5 (d). In a synchronous system, the downlink communication periods 111 all start and end simultaneously, and the uplink communication periods 113 all start and end simultaneously. Such synchronization of the upstream and downstream communication cycles effectively eliminates problems caused by near-end crosstalk.

A modem timing and synchronization scheme suitable for use in constructing such a structure is shown in fig. 6. In the illustrated embodiment, synchronization is generated by the master clock of the central unit (or ONU) which provides the superframe clock to all central unit transmitters 209. More specifically, the central unit 201 includes a master oscillator (master clock) 220 that provides a sampling clock 222, a symbol clock 224, and a superframe clock 226. Each of the three clocks 222, 224, 226 provides a single clock that is provided to each transceiver 208. The transceivers synchronize their respective symbols and superframes based on the input clock signal and transmit the data downstream to the remote unit 204. Each remote unit 204 includes a receiver 231, a transmitter 233, and a controller 235 (which obtains the superframe, symbol, and sample clocks from the downstream signal and synchronizes the upstream signal using any of a variety of clock recovery methods well known in the art). Typically, the controller 235 is in the form of a phase locked loop. Of course, the actual construction of the receiver, transmitter and controller may vary widely depending on the coding, error correction and modulation schemes, etc., used.

It should be understood that the present invention may be applied to many communication schemes. Applicants experience that modulation schemes transmitting in the carrier band well above 1MHZ are particularly susceptible to near-end crosstalk and can derive the greatest benefit from synchronization. Most of the modulation techniques considered for VDSL/FTTC and other applications requiring bit rates above 10Mbit/S envisage carrier frequencies above 1.5MHz and a great gain is obtained from the present invention. Note that in many applications, some lines that share a connection will be used for very high speed transmission (with a synchronization scheme that benefits from the synchronization scheme described above), while other lines are used to transmit conventional low speed signals. Since near-end crosstalk is not a generally serious problem at carrier frequencies below about 1MHz, such communications do not significantly interfere with high speed, or high carrier frequency time division multiplexed signals employing the synchronization scheme of the present invention.

One highly desirable feature of VDSL/FTTC systems is the ability to provide multiplexed communications to multiple set-top units (called STUs) in a subscriber room using twisted pair pairs. Each set-top unit receives, demodulates and decodes the entire downlink signal and selects the information to address it. Each STU may allow communication over successive time periods; this is Time Division Multiple Access (TDMA). Alternatively, each STU may be allocated a separate frequency band; this is known as Frequency Division Multiple Access (FDMA). Both TDMA and FDMA are well known in the art, but they are often coordinated with frequency division multiplexing to separate the upstream and downstream signals. However, in such multipoint-to-point upstream communications, the present invention is well suited to handle the allocation of upstream bandwidth. The coordination of the upstream signals from multiple STUs is the Medium Access Control (MAC). For example, media communication control information may be included as a header in a downstream file header using methods well known in the art.

As an example of TDMA, consider an embodiment in which 4 STUs, as shown in fig. 1b remote 204(e), share a transmission line. In embodiments using a 32: 1: 4: 1 symbol pattern, each STU may be assigned one of the symbols designated in the upstream symbols. In embodiments having more uplink symbol periods than STUs, a particular STU may be allocated multiple uplink symbol periods. In embodiments with more STUs than uplink symbol periods, a slightly more complex media communication control procedure will make symbol allocations less frequent than one per superframe. As an example of FDMA, consider an embodiment where a single line is shared by many STUs as shown in FIG. 1b, remote 204 (e). A selected embodiment uses a 16: 1: 2: 1 symbol pattern and, during the upstream communication period, each STU will be assigned either a large number of sub-carriers (when using discrete multi-tone modulation) or a frequency sub-band (when using single-carrier modulation) depending on its data transmission needs. In general, it is desirable to stagger the sub-carriers assigned to each STU to reduce the probability that low-band noise will significantly affect the ability of one of the STUs to communicate with the central unit. Of course, interleaving the allocated sub-carriers is not required.

Although the present invention has been described in terms of its application to a few modulation schemes, it should be understood that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the present invention. For example, although the present specification has described its use in VDSL FTTC and other subscriber line based very high frequency data transmission systems, it may also be used in other systems that suffer from near end crosstalk. In a main embodiment, the application of the invention in systems using discrete multi-tone modulation schemes has been described. However, it is also used in systems using other modulation techniques. For example, Quadrature Amplitude Modulation (QAM); carrierless amplitude and phase modulation (CAP); quadrature Phase Shift Keying (QPSK); and/or vestigial sideband modulation may be used. Importantly, the present invention can be used even when different modulation techniques are used on lines that share the same connector. When transmitting high carrier frequency signals using different modulation techniques, it is important to synchronize their time division multiplexing uplink and downlink communication periods. When transmitting low carrier frequency signals over adjacent wires that share a connection, the time division multiplexed synchronization signal will be transmitted reliably regardless of whether the low carrier frequency signals are time division multiplexed and/or synchronized with the high speed signals. This is because standardized low-speed signal transmission systems can operate in low frequency bands that are not as susceptible to near-end crosstalk as high frequency signals.

Further, it should be understood that the present invention may be practiced at both central and remote sites using a variety of modems. Accordingly, the present examples are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.

Claims (24)

1. In a subscriber communication system for bidirectional data transmission between a central unit and a plurality of remote units over different transmission lines sharing a connection, a method of coordinating data transmission comprising the steps of:

providing a periodic downlink communication period in which the central unit can transmit information to a plurality of remote units;

providing a periodic upstream communication period in which a plurality of remote units can transmit information to the central unit, the upstream communication period being designed to be non-overlapping with the downstream communication period, whereby data transmissions sharing the same link can be time division multiplexed and synchronized such that transmissions within the same link are transmitted in only one direction at a time, the central unit providing a clock signal to facilitate synchronization between the upstream and downstream communication periods.

2. The method of claim 1, further comprising the step of providing quiet periods separating upstream and downstream communication periods, wherein the quiet periods do not communicate either upstream or downstream, wherein each downstream communication period comprises a plurality of symbol periods, and each upstream communication period comprises at least one symbol period.

3. The method of claim 2, wherein the downlink communication period includes 8 to 60 symbol periods, the uplink communication period includes 1 to 30 symbol periods, and the quiet period includes 1 to 4 symbol periods.

4. The method of claim 3, wherein a preferred one of the remote units comprises a plurality of set-top units, wherein each set-top unit is assigned at least one different symbol period of the upstream communication period.

5. A method as claimed in any preceding claim, wherein the upstream and downstream communications employ discrete multi-tone modulated signals.

6. The method according to any of claims 1-4, wherein upstream and downstream communicate signals selected from the group consisting of: a carrier-free amplitude phase modulation signal; quadrature phase shift chain control modulation signals; vestigial sideband modulated signals, and quadrature amplitude modulated signals.

7. The method of claim 1, wherein upstream and downstream communications associated with a first preferred transmission line employ different modulation schemes.

8. The method of claim 1 wherein downstream communications associated with a preferred first and second transmission line employ different modulation schemes.

9. The method of claim 1, wherein:

providing an auxiliary transmission line sharing the same connector with a different transmission line for transmitting non-time division multiplexed signals at a carrier frequency below 1MHz, and

all transmission lines within a connection are time division multiplexed and synchronized at frequencies above 1 MHz.

10. The method of claim 1, wherein time division multiplexing data transmission:

including signals transmitted at carrier frequencies above about 1.0 MHz; and

downstream signals can be transmitted at a bit rate of at least 10 megabits per second on respective different transmission lines that are twisted pair transmission lines.

11. The method of claim 1, wherein the central unit is an optical network unit that:

receiving a downlink source signal through at least one optical fiber and transmitting information in the downlink source signal as a downlink communication signal, wherein the downlink communication signal is transmitted only in a downlink communication period;

information contained in an upstream communication signal, which is received in an upstream communication period through an optical fiber, is transmitted as an upstream source signal.

12. A central modem adapted for use in a subscriber communication system that facilitates communications between the central modem and a plurality of remote units over different transmission lines that share a connection, the central modem comprising:

a plurality of receiving transmitters, each receiving transmitter for communicating with an associated remote unit over an associated different transmission line sharing a connection body by using a time division multiplexing transmission scheme;

a synchronization unit for coordinating signals transmitted by the plurality of transceivers so that the transceivers transmit information only to their associated remote units during a synchronous downlink communication period; and is

Wherein the transceiver is for receiving signals from remote units that are self-correlated in a synchronous upstream communication period, the upstream communication periods being arranged such that they do not overlap with the downstream communication periods, whereby data transmissions within the connection body are time division multiplexed and synchronized such that data transmissions within the connection body are transmitted in only one direction at a time, and the synchronization unit outputs at least one clock signal to each of the plurality of transceivers to facilitate synchronization of the upstream and downstream communication periods.

13. The central modem of claim 12, wherein the synchronization unit transmits a superframe clock signal to each transceiver to facilitate coordination of upstream and downstream communication cycles.

14. The central modem of claim 13, wherein the synchronization unit further outputs a symbol clock signal and a sampling clock signal to each transceiver to further facilitate coordination of upstream and downstream communication cycles.

15. The central modem of claim 12, wherein the synchronization unit comprises:

a superframe clock for outputting a superframe clock signal to each transceiver;

a symbol clock for outputting a symbol clock signal to each transceiver; and

a sampling clock for outputting a sampling clock signal to each transceiver; and is

Wherein the transceiver uses a clock signal to coordinate upstream and downstream communication cycles.

16. The central modem of claim 12 wherein the central modem is configured to ensure that all transmissions on the transmission lines within the connector are time division multiplexed and synchronized in the frequency band above about 1MHz, and each transceiver is capable of transmitting downstream signals at a bit rate of at least 10 megabits per second.

17. The central modem of claim 12, wherein the central modem is an optical network unit that:

receiving a downlink source signal through at least one optical fiber, and sending out information contained in the downlink source signal as a downlink communication signal, wherein the downlink communication signal is only sent in a downlink communication period; and

information included in an upstream signal is transmitted as an upstream source signal, and the upstream signal is received through an optical fiber in an upstream communication period.

18. The central modem of any of claims 12-15, 17 and 18, wherein the transmitter receiver is configured to receive and transmit signals taken from the group consisting of: quadrature phase shift keying modulation signals; a carrier-free amplitude phase modulation signal; a vestigial sideband modulated signal; a modulated signal modulated by quadrature amplitude and a discrete multi-tone modulated signal.

19. A modem adapted for use in a subscriber communication system that facilitates communications between a central modem and a plurality of remote units over different transmission lines that share a connection, the modem comprising:

a transmitter adapted to transmit upstream communication signals from the modem to the central unit over the designated transmission line;

a receiver adapted to receive a downlink communication signal from the central unit via the designated transmission line; and

a controller for detecting the sampling clock signal, the symbol clock signal and the superframe clock signal in the downlink communication signal and synchronizing the uplink communication signal transmitted by the transmitter with the downlink communication signal so that the uplink and downlink communication signals are time division multiplexed, whereby the uplink communication signal of the modem is synchronized with the uplink communication signal transmitted by the other remote units, whereby data transmission within the link is time division multiplexed and synchronized so that data transmission within the link is performed in only one direction at a time, wherein the remote units communicate with the central unit through different transmission lines of the link in common with a designated transmission line.

20. The modem of claim 19 wherein the transmitter is configured to transmit a signal selected from the group consisting of: a carrier-free amplitude and phase modulated signal; quadrature phase shift keying modulation signals; a vestigial sideband modulated signal; a signal modulated by a quadrature amplitude system.

21. The modem of claim 19 wherein the transmitter is configured to transmit discrete multi-tone modulated signals and the receiver is configured to receive discrete multi-tone modulated signals.

22. A designated remote unit for use in a subscriber communication system that facilitates communication between a central modem and a plurality of remote units over different transmission lines that share a connection, the designated remote unit comprising:

a plurality of set-top units, each set-top unit including a modem, wherein each set-top unit is configured to share the designated transmission line, the modem comprising:

a transmitter adapted to transmit upstream communication signals from the modem to the central unit over the designated transmission line;

a receiver adapted to receive a downlink communication signal from the central unit via the designated transmission line; and

a controller for detecting the sampling clock signal, the symbol clock signal and the superframe clock signal in the downlink communication signal, and synchronizing the uplink communication signal transmitted by the transmitter with the downlink communication signal so that the uplink and downlink communication signals are time division multiplexed, whereby the uplink communication signal of the modem is synchronized with the uplink communication signal transmitted by the other remote units, whereby data transmission within the link is time division multiplexed and synchronized so that data transmission within the link is performed in only one direction at a time, wherein the remote units communicate with the central unit through different transmission lines of the link in common with a designated transmission line.

23. The designated remote unit of claim 22, wherein the transmitter of the modem is configured to transmit a signal selected from the group consisting of: a carrier-free amplitude and phase modulated signal; quadrature phase shift keying modulation signals; a vestigial sideband modulated signal; a signal modulated by a quadrature amplitude system.

24. The designated remote unit of claim 22, wherein the transmitter of the modem is configured to transmit a discrete multi-tone modulated signal and the receiver is configured to receive the discrete multi-tone modulated signal.