Pair gain system with an ADSL repeater unit

[0002] The invention relates generally to telecommunication systems, and more particularly to ADSL (Asymmetrical Digital Subscriber Line) transmission systems in access network.

[0003] Telephone customers are moving in an evolutionary way to higher bandwidths, from sharing POTS (Plain Old Telephone Service) and modem on one line to dedicated analog modem line at 56 kbps (kilo-bits per second) and to ISDN BA (Basic Access). Now, ADSL has been introduced and become proven technology. ADSL refers to a high-speed transmission technology using existing twisted pair of wires that permits simultaneous POTS and high-speed data communication. Generally, a higher data rate is employed downstream than upstream. Ordinary twisted pair equipped with ADSL modems can transmit movies, television, dense graphics, and very high speed data. For all its capacity, ADSL leaves POTS undisturbed. A single ADSL line therefore offers simultaneous channels for personal computers, televisions, and telephones. With ADSL, telephone companies can connect homes and businesses to exciting new interactive broadband services.

[0004] The ADSL services are different from ISDN services. ISDN provides two 64 kbps channels for voice or data, while ADSL is predominantly a data pipe providing an asymmetrical bandwidth of up to 9 Mbps downstream and 800 kbps upstream. The customers for ADSL most likely already have a dedicated modem line or ISDN.

[0005] FIG. 1 shows a conventional way of implementing ADSL over a pair gain system. A pair gain system is a system which typically includes a central office terminal (COT) located in a central office (CO) of a telephone company and a remote terminal (RT) located in a customer premises. The COT may have two or more line cards, each for connecting to a pair of wires from a telephone exchange in the CO. The COT of a pair gain system multiplexes signals received from the multiple pairs connected to its line cards, over a single pair connecting between the COT and the RT of the pair gain system. The COT also demultiplexes signals received from the RT via the single pair and provides them to the telephone exchange via the multiple pairs connected to its line cards. The RT demultiplexes signals received from the single pair and provides them to a subscriber at the customer premises over the same number of pairs. The RT also multiplexes signals received from the multiple pairs at the customer premises, over the single pair connecting to the COT. Thus, a pair gain system allows a subscriber at the customer premises to have two or more telephone lines without requiring the expense of installing additional pairs between the CO and the customer premises. An example of a pair gain system is the Miniplex® 2N1, which multiplexes two pairs over a single pair. Miniplex® 2N1 is commercially available from Raychem Corporation, Menlo Park, Calif.

[0006] As illustrated in FIG. 1, at a CO 10 of a telephone company, a telephone exchange 12 provides two analog POTS lines L1 and L2 in the form of two twisted pairs of wires to a COT 15 of a pair gain system. COT 15 multiplexes L1 and L2 over a single twisted pair 13 and converts the analog signals from L1 and L2 into DSL (Digital Subscriber Line) signals, such as ISDN signals, on pair 13. An ADSL DSLAM (DSL Access Multiplexer) 18 receives broadband data from the Internet, for example and provides high-speed data transmission in the form of ADSL signals. The DSL and ADSL signals provided respectively from COT 15 and ADSL DSLAM 18 are fed to splitter 16. Splitter 16 combines the DSL and ADSL signals for transmission to a customer premises 20 over a single twisted pair 19. Splitter 16 also receives the DSL signals and ADSL signals from splitter 32 and separates the DSL signal from the ADSL signal for transmission to COT 15 and ADSL DSLAM 18, respectively.

[0007] At customer premises 20, splitter 32 separates the DSL signal from the ADSL signal. The DSL signal is provided, via a pair 33, to a RT 34 of the pair gain system, which demultiplexes the DSL signal into analog signals on two analog POTS lines L1 and L2. Each of lines L1 and L2 at customer premises 20 is for connecting to a telecommunication device (not shown), such as a telephone or a fax machine. The ADSL signal is provided to an ADSL modem 36, which is connected to either ATM 25 (asynchronous transfer mode 25 Mbps) or Ethernet. RT 34 also multiplexes analog signals from analog lines L1 and L2 into a DSL signal over pair 33 for transmission to CO 10. Splitter 32 also receives DSL and ADSL signals from RT 34 and ADSL modem 36 respectively, and combines them for transmission to CO 10 over twisted pair 19. In the case of implementing ADSL over ISDN BA (basic access), an ISDN NT1 (network termination 1) can be used as RT 34.

[0008] One of the main limitations of ADSL is the loop length which is the distance between the CO or DSLAM and the customer premises. For service with a downstream speed of 5 to 6 Mbps (mega-bits per second), the loop length is limited to less than 12 kft, in order not to sacrifice the transmission quality. At the present time, telecom operators use very conservative installation rules to ensure that the service will always be properly delivered. However, they will soon receive requests for service from people living farther away from the central office where a DSLAM is located. At present, it is estimated that about 60% of the subscribers can be potentially reached and receive high-speed data services. This leaves the other 40% with only low-speed data services.

[0009] Of course, there are several solutions for delivering high bandwidth services over longer distances (e.g. fiber optics). One solution is the use of a repeater for the ADSL signal. However, ADSL repeaters face a specific issue with power feeding as the ADSL signal is usually transmitted simultaneously with POTS on a copper pair. In order that a repeater be practical in the field, it will need to receive remote power feeding. This is very difficult and may be impossible if ADSL is implemented with POTS on the same copper pair.

[0010] An easy solution is to run POTS and ADSL on different pairs. Several POTS lines can be multiplexed over one copper pair using subscriber line multiplexers. The copper pair used for ADSL can carry a DC current for remote power feeding an ADSL repeater. However in this case telecom operators must install two copper pairs in order to provide one or more POTS lines and an ADSL. Therefore, substantial additional expenses will be incurred by telecom operators for installing a second copper pair whenever ADSL service is requested to be delivered over longer distances.

[0011] Accordingly, there is a need for implementing ADSL over POTS over longer distances without requiring an additional copper pair for transmitting an ADSL signal along with a DC current to a customer premises.

[0012] The present invention provides an efficient way to transmit ADSL and multiple POTS signals over long distances by combining a repeater for ADSL with a pair gain system, whereby the pair gain system remotely provides DC power to the repeater over the same pair as that for transmission of DSL and ADSL signals.

[0013] According to one embodiment of the invention, a telecommunication system is provided. The telecommunication system comprises a first transmission/receiving system, located at a first location, that provides and receives a first type of signal in a first frequency spectrum and a second type of signal in a second frequency spectrum over a first pair of wires; a second transmission/receiving system, located at a second location, that receives and sends the first and second types of signals over a second pair of wires; and a repeater unit, coupled between the first and second pairs of wires, for receiving a second type of signal from the first transmission/receiving system via the first pair and providing a repeated second type of signal to the second transmission/receiving system via the second pair, the repeater unit also receiving a second type of signal from the second transmission/receiving system via the second pair and providing a repeated second type of signal to the first transmission/receiving system via the first pair; wherein the first transmission/receiving system provides DC power to the repeater unit via the first pair. In one example, the first type of signal is a digital subscriber line (DSL) signal and the second type of signal is an asymmetrical DSL (ADSL) signal. The first transmission/receiving system includes a central office system having a pair gain central office terminal (COT) that converts analog signals into a DSL signal. Also, the second transmission/receiving system includes a customer premises system having a pair gain remote terminal (RT) that converts a DSL signal into analog signals. Furthermore, the COT provides the DC power to the repeater unit via the first pair of wires. The repeater unit may also regenerate the DC power to a higher voltage level and provide regenerated (voltage-boosted) power to the RT over the second pair of wires.

[0014] In this embodiment of the invention, the repeater unit comprises an ADSL repeater including a power supply circuit, the ADSL repeater receiving an ADSL signal and providing a corresponding repeated ADSL signal; a first low pass filter, coupled between the first and second pairs of wires, for allowing only DSL and DC signals to pass through; a second low pass filter, coupled between the first pair of wires and the ADSL repeater, for allowing only a DC signal to pass through to the power supply circuit of the ADSL repeater; a first high pass filter, coupled between the first pair of wires and the ADSL repeater, for allowing only the ADSL signal to pass through; and a second high pass filter, coupled between the ADSL repeater and the second pair of wires, for allowing only the ADSL signal to pass through.

[0015] According to an alternative embodiment of the invention, the repeater unit comprises an ADSL repeater including a power supply and regeneration circuit that regenerates DC signals to predetermined higher voltage values, the ADSL repeater receiving an ADSL signal and providing a corresponding repeated ADSL signal; a band pass filter, coupled between the first and second pairs of wires, for allowing only a DSL signal to pass through; a first low pass filter, coupled between the first pair of wires and the ADSL repeater, for allowing only a DC signal to pass through to the power supply and regeneration circuit of the ADSL repeater; a second low pass filter coupled between the power supply and regeneration circuit of the ADSL repeater and the second pair of wires for allowing only a DC signal to pass through;

[0016] a first high pass filter, coupled between the first pair of wires and the ADSL repeater, for allowing only the ADSL signal to pass through; and a second high pass filter, coupled between the ADSL repeater and the second pair of wires, for allowing only the ADSL signal to pass through.

[0017] Accordingly, the present invention allows telecom operators to save huge costs in implementing ADSL over long distances, without sacrificing the quality of signal transmission.

[0018] Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.

[0019] In the drawings wherein like reference symbols refer to like parts:

[0020] FIG. 1 shows a conventional way of implementing ADSL over a pair gain system;

[0021] FIG. 2 illustrates a functional block diagram of one embodiment of the present invention;

[0022] FIG. 3 illustrates a functional block diagram of an ADSL repeater of the embodiment in FIG. 2;

[0023] FIG. 4 shows a functional block diagram of an alternative embodiment of the present invention; and

[0024] FIG. 5 illustrates a functional block diagram of an ADSL repeater of the alternative embodiment in FIG. 4.

[0025] FIG. 2 shows a functional block diagram of one embodiment of the present invention. In this embodiment, ADSL signals are transmitted together with basic rate ISDN (ISDN-BA) signals, which is equivalent to the technology of 2B1Q at 160 kbps, such as used by 2N1 Miniplex®, commercially available from Raychem Corporation, Menlo Park, Calif. Typically, in the this embodiment the bit rate of ADSL signal transmission is reduced by 10% compared to an embodiment in which ADSL signals are transmitted over POTS. Implementation of ADSL over ISDN-BA is described in more detail in European Telecommunications Standards Institute (ESTI) specification TS 101 388 V1.1.1 (1998-11), the disclosure of which is hereby incorporated by reference.

[0026] As illustrated in FIG. 2, a central office 10 provides ISDN and ADSL services to a customer premises 20 via a repeater unit 50, which includes low pass filters 52 and 54, high pass filters 56 and 58, and an ADSL repeater 60. Except for the pair gain system which includes a COT 15 and a RT 35, the functional blocks shown in central office 10 and customer premises 20 represent the same conventional components as those described above in connection with FIG. 1. Therefore, for simplicity the description of these components is omitted. COT 15 and RT 35 are also conventional ones, except that COT 15 uses a higher powering voltage and delivers more current and RT 35 receives a fraction of the current delivered from COT 15. The power calculations for COT 15 and RT 35 will be described later.

[0027] In ADSL repeater unit 50 shown in FIG. 2, low pass filter 52 preferably has a cutoff frequency of 100 kHz and is bi-directional. Low pass filter 52 is connected between central office 10 and customer premises 20 for filtering signals received over segments 21 and 23 of the twisted pair. There are three signals on the twisted pair: the DC power feeding current with a frequency spectrum of 0 to 10 Hz, the pair gain DSL 2B1Q signal with a frequency spectrum of 1 to 80 kHz, and the ADSL signal with a frequency spectrum of 120 to 1000 kHz. Low pass filter 52 allows the DC current and the DSL signal to pass through, but blocks the ADSL signal. Low pass filter 52 needs to be bi-directional because the DSL signal is full duplex and the repeated downstream ADSL signal should not go back towards CO exchange 12.

[0028] Low pass filter 54 of repeater unit 50 preferably has a cutoff frequency of 100 Hz. It is connected between central office 10 and ADSL repeater 60 for filtering the signals received from segment 21 of the twisted pair and provides a high impedance for the DSL and ADSL signals. Low pass filter 54 allows only the DC current to pass through to the power supply circuit of ADSL repeater 60. This allows central office 10 to remotely provide DC power to repeater unit 50. In FIG. 2, low pass filter 54 is preferably a uni-directional filter.

[0029] High pass filter 56 of repeater unit 50 preferably has a cutoff frequency of 100 kHz and is connected between central office 10 and ADSL repeater 60. High pass filter 56 blocks the DC current and the DSL signal and allows the ADSL signal to pass through to ADSL repeater 60. High pass filter 58 of repeater unit 50 preferably has a cutoff frequency of 100 kHz and is connected between ADSL repeater 60 and customer premises 20. High pass filter 58 allows the repeated ADSL signal from ADSL repeater 60 to pass through to customer premises 20. High pass filter 58 also filters signals received from customer premises 20 and allows only the ADSL signal to pass through to ADSL repeater 60. In FIG. 2, high pass filters 56 and 58 are preferably bi-directional filters.

[0030] In FIG. 2, ADSL repeater 60 receives the downstream signal from high pass filter 56 and remodulates it for transmission towards the subscriber at customer premises 20. The subscriber at customer premises 20 would not recognize any difference from a standard ADSL over pair gain system. ADSL repeater 60 also receives the upstream signal from high pass filter 58 and remodulates it for transmission towards central office 10.

[0031] FIG. 3 shows a functional block diagram of ADSL repeater 60, which includes ADSL transceivers 62 and 64, a repeater power supply 66 and a controller 68. ADSL transceivers 62 and 64 are conventional devices. An example of such transceiver is CopperGold MC 145650 commercially available from Motorola, or TNETD 3000C commercially available from Texas Instruments. ADSL transceiver 62 receives ADSL signals from high pass filter 56, amplifies the received ADSL signals, and then sends the amplified ADSL signals to ADSL transceiver 64 for forwarding to high pass filter 58. Similarly, ADSL transceiver 64 receives ADSL signals from high pass filter 58, amplifies the ADSL signals, and then sends the amplified ADSL signals to ADSL transceiver 62 for forwarding to high pass filter 56. Low pass filter 54 provides DC signals to repeater power supply 66 for powering ADSL repeater 60. Controller 68 controls the operations of ADSL transceivers 62 and 64.

[0032] FIG. 4 shows an alternative embodiment of the present invention. For simplicity, description of the elements with the same reference symbols as those previously described is omitted. In this embodiment, the DSL signals received by repeater unit 70 pass through a bi-directional band pass filter 72 having a frequency spectrum preferably between 100 Hz and 100 kHz. Band pass filter 72 blocks the DC signals. The DC power feeding current supplied from central office 10 passes through low pass filter 54 to power ADSL repeater 80. A power supply and regeneration circuit 86 in ADSL repeater 80 will regenerate the feeding voltage to a predetermined higher value for sending, via a low pass filter 74, to customer premises to power RT 35. Like low pass filter 54, low pass filter 74 has a cutoff frequency of approximately 100 Hz. This embodiment allows signals to be transmitted over longer distances.

[0033] FIG. 5 shows a functional block diagram of ADSL repeater 80, which, except for power supply and regeneration circuit 86, is similar to ADSL repeater 60 shown in FIG. 3. Therefore, detailed description of FIG. 5 is omitted.

[0034] The systems in FIGS. 2 and 4 each are shown for ADSL combined with a 2:1 pair gain system, i.e., 2 lines multiplexed over one pair. For example, one can use a Miniplex® 2N1 pair gain system commercially available from Raychem Corporation, Menlo Park, Calif. It is also possible to use a pair gain system with more than 2:1, e.g., a 4:1 pair gain system—4 lines multiplexed over one pair, such as a Miniplex® 4N1 pair gain system, which is also commercially available from Raychem Corporation. A 4:1 pair gain system uses more bandwidth, but the same principles apply. For an ADSL over a 4:1 pair gain system, the bit rate may be about 20% lower than for the full-rate of an ADSL over POTS. This is not so critical when the ADSL repeater unit of the present invention is used. If the full rate can achieve 6 Mbps over a loop length of 10,000 ft, then an ADSL over 4:1 pair gain system should be able to reach a speed of 5 Mbps. With the aid of an ADSL repeater unit of the present invention, a speed of 5 Mbps over a loop length of 20,000 ft can be achieved.

[0035] The power calculations for the pair gain system in FIG. 2 are now described. The known and unknown values in the calculations are listed below:

[0036] The known values are:

[0037] Prt: power consumption of the Miniplex® RT,

[0038] Prep: power consumption of the ADSL repeater,

[0039] Rf: resistance of the feeder part of the loop,

[0040] Rd: resistance of the distribution part of the loop, and

[0041] Vlc: powering voltage at the line card of the pair gain COT.

[0042] The unknown values are:

[0043] Vrt: voltage at the RT,

[0044] Irt: current drawn by the RT,

[0045] Vrep: voltage at the repeater,

[0046] Irep: current drawn by the repeater, and

[0047] IIc: current to be delivered by the Miniplex® COT.

[0048] The following equations are used for power calculations for the pair gain system in FIG. 2.

[0049] Prt=Vrt*Irt  (1)

[0050] Vrep=Vrt+Rd*Irt  (2)

[0051] Prep=Vrep*Irep  (3)

[0052] Vlc=Vrep+Rf*IIc  (4)

[0053] IIc=Irep+Irt  (5)

[0054] From (1) and (3):

[0055] Irt=Prt/Vrt  (6)

[0056] Irep=Prep/Vrep  (7)

[0057] By substituting equations (6) and (7) in equations (2) an (3), the following are derived:

[0058] Vrt=Vrep−Rd*(Prt/Vrt)  (8)

[0059] Vrep=Vic−Rf*((Prep/Vrep)+(Prt/Vrt))  (9)

[0060] It is easy to solve (8) and (9) together iteratively by using, e.g., a spreadsheet, such as Microsoft Excel®.

[0061] The following examples illustrate the power calculations for two different systems: ADSL over 2:1 pair gain system and ADSL over 4:1 pair gain system. In the calculations of the these examples, it is assumed that Miniplex® 2N1 and Miniplex® 4N1 pair gain systems are used. Furthermore, the following assumptions are made:

[0062] power consumption of an ADSL repeater unit=6 W,

[0063] power consumption of Miniplex® 2N1 RT=4 W,

[0064] power consumption of Miniplex® 4N1 RT=6 W.

[0065] Also, all the powering voltages and currents remain within the GR-1089 A3 limits, as provided by Bellcore, “Electromagnetic Compatibility and Electrical Safety—Generic Criteria for Network Telecommunications Equipment,” Issue 2, December 1997. The results of the calculations are shown in Table 1.

[0066] The power calculations for the pair gain system in FIG. 4 are next described.

[0067] The known values are:

[0068] Prt: power consumption of the Miniplex® RT,

[0069] Prep: power consumption of the ADSL repeater,

[0070] Rf: resistance of the feeder part of the loop,

[0071] Rd: resistance of the distribution part of the loop, and

[0072] Vlc: powering voltage at the line card of the pair gain COT.

[0073] Vrepout powering voltage at the output of the repeater

[0074] Eff_rep efficiency of the voltage boosting circuit in the repeater

[0075] The unknown values are:

[0076] Vrt: voltage at the RT,

[0077] Irt: current drawn by the RT,

[0078] Prepout power delivered from the repeater to the Miniplex® RT

[0079] Vrep: input voltage at the repeater,

[0080] IIc: current to be delivered by the Miniplex COT.

[0081] The following equations are used for power calculations for the pair gain system in FIG. 4.

[0082] Prt=Vrt*Irt  (10)

[0083] Vrepout=Vrt+Rd*Irt  (11)

[0084] Prepout=Vrepout*Irt  (12)

[0085] Prep+(Prepout/Eff_—rep)=Vrep*IIc  (13)

[0086] Vlc=Vrep+Rf*IIc  (14)

[0087] From (10) and (11):

[0088] Irt=Prt/(Vrepout−Rd*Irt)  (15)

[0089] This can be readily solved for Irt. Substituting the value of Irt in (12) provides the value for Prepout.

[0090] Combining (13) and (14) gives:

[0091] IIc*(Vlc−Rf*IIc)=Prep+(Prepout/Eff_—rep)  (16)

[0092] This can be readily solved for IIc. Substituting the value of IIc in (13) or (14) allows one to calculate the value of Vrep.

[0093] The following examples illustrate the power calculations for two different systems: ADSL over 2:1 pair gain system and ADSL over 4:1 pair gain system. In the calculations of the these examples, it is assumed that Miniplex® 2N1 and Miniplex® 4N1 pair gain systems are used. Furthermore, the following assumptions are made:

[0094] power consumption of an ADSL repeater unit=6 W,

[0095] power consumption of Miniplex® 2N1 RT=4 W,

[0096] power consumption of Miniplex® 4N1 RT=6 W.

[0097] Also, all the powering voltages and currents remain within the GR-1089 A3 limits, as provided by Bellcore, “Electromagnetic Compatibility and Electrical Safety—Generic Criteria for Network Telecommunications Equipment,” Issue 2, December 1997. The results of the calculations are shown in Table 2.

[0098] While the invention has been described in conjunction with several specific embodiments, it is evident to those skilled in the art that many further alternatives, modifications and variations will be apparent in light of the foregoing description. For example, the invention could also be used in VDSL (Very-high-bit rate DSL) applications. Thus, the invention described herein is intended to embrace all such alternatives, modifications, applications and variations as may fall within the spirit and scope of the appended claims.