Bidirectional optical communications having quick data recovery without first establishing timing and phase lock
A method of transmitting and rapidly recovering a burst of data without first having to establish a timing or phase lock. The signals are transmitted as modified Manchester coded signals having pulse transitions at a clocking pulse rate which is a multiple of the clocking pulse rate at which the signals are originally generated, and wherein the MOOSE coded signal is modified by ON-OFF keying.
This application is a continuation application of U.S. application Ser. No. 09/723,804 filed Nov. 28, 2000.
TECHNICAL FIELD OF THE INVENTIONThe present invention relates generally to methods and apparatus for providing FTTH (Fiber to the Home) bidirectional communications over a single optical fiber, and more specifically to NRZ (Non-Return to Zero) coded signals at a first frequency transmitted downstream and a Manchester coded signal at a second frequency modified by ON-OFF keying and transmitted upstream. The upstream and downstream coded signals are both used to modulate a carrier light wave having a selected wavelength of light, such as, for example, 1310 nanometers of light. The invention further relates to methods and apparatus for transmitting and recovering bursts of data with a minimal number of preamble bits and without first requiring phase lock.
BACKGROUND OF THE INVENTIONThe communications industry is using more and more optical or light fibers in lieu of copper wire. Optical fibers have an extremely high bandwidth thereby allowing significantly more information than can be carried by a copper wire transmission line such as twisted pairs or coaxial cable.
Of course, modem telephone systems require bidirectional communications where each station or user on a communication channel can both transmit and receive. This is true, of course, whether using electrical wiring or optical fibers as the transmission medium. Early telephone communication systems solved this need by simply providing separate copper wires for carrying the communications in each direction, and this approach is still exclusively used in many locations and as part of the transmission path in many others. It is used to a greater degree as the signals get closer to the home or business end users. Although twisted pairs and coaxial cables are more likely to be used in distribution terminals close to the end user and homes, some modem telecommunication systems now use microwave and end-to-end optic fibers as the transmission mediums. In addition, various techniques are often used in optical transmission so that a signal optical fiber can carry more communication in both directions.
However, because of extremely high bandwidths available for use by an optical fiber, a single fiber is quite capable of carrying a great number of communications in both directions. One technique of optical transmission is WDM (Wavelength Divisional Multiplexing) which uses different wavelengths for different types of transmissions. Typical examples are the use of 1550 nanometers of light for TV signals transmission and 1310 nanometers of light for bidirectional telephony transmission.
It is noted that the term telephony is used rather than telephone to underscore the fact that communication transmission will include vocal telephone use but is not so limited. Typical telephony systems operate at a single frequency or wavelength of light which is divided into upstream and downstream carefully synchronized time windows for transmitting bursts of data. The use of such upstream and downstream synchronized windows is referred to as TDM (Time Division Multiplexing). This type of telephony systems use a single optical fiber and often may use only a single diode, for both converting electrical signals to optical signals and converting received optical signals to electrical signals.
However, as mentioned above, optical fibers have extremely high bandwidths and use of an optical fiber as a single bidirectional telephone channel is a very ineffective use of the fiber and, in fact, the available bandwidth of an optical fiber is what makes it possible to use two different and unrelated transmission techniques such as the transmission of bidirectional TDM telephone techniques at one wavelength, and the use of another technique, such as straightforward broadcasting of TV signals downstream at a second wavelength. Typically, two wavelengths regardless of the two techniques being used are combined by the use of WDM technology.
A major problem for the bidirectional telephony signals is light reflection typically occurring at optical connections or interfaces along the optical fiber, and in a worse case situation, the reflected energy may be interpreted as an actual signal transmission in the bidirectional communication. In addition, the typical use of NRZ (Non-return To Zero) coding and the need for increasing data transmission efficiency by using a minimum number of preamble bits are at odds with each other. Furthermore, clocking pulses and initial pulses of a transmitted signal are typically recovered by establishing a PLL (phase lock loop) by evaluating the time period between high to low and low to high transitions. Thus, since a consecutive string of “1”s or “0”s, using NRZ coding may result in the absence of any transition for an excessive period of time, the effect may be a shift in the timing of a data frame or “wander.” Therefore, a simple and straightforward technique to solve these problems would be of great value.
Therefore, a technique for transmitting bidirectional telephony signal bursts having minimal energy overlap, occurring from reflection and fast clocking recovery would allow the use of readily available hardware and make efficient and effective use of an optical fiber.
SUMMARY OF THE INVENTIONShortcomings of the above-discussed bidirectional communication system are overcome by the apparatus and methods of the present invention which comprises generating a first NRZ (Non-Return to Zero) data stream having a first clocking frequency and then transmitting the first NRZ data stream by an optical fiber from a first location to a second location. The data stream is transmitted by modulating a carrier having a selected wavelength of light such as, for example, 1310 nanometers. The selected wavelength of light from the first location is received at the second location and the NRZ data stream is recovered. A second NRZ data stream intended for travel to the first location and also having the first clocking frequency is converted to a Manchester coded data stream at the first clocking frequency. The Manchester coded data stream frequency is then further modified by ON-OFF keying at a second frequency which is a selected multiple of the first clocking frequency, such as, for example, eight times (8×) the first clocking frequency. A particular combination coding discloses as a preferred embodiment herein as MOOSE (Manchester OOK Serial Encoding). The modified combination coded data stream or MOOSE coded data stream generated at the second location is transmitted to the first location by the same optical fiber used by the first NRZ coded data stream and at the same selected wavelength of light. The modified MOOSE coded data stream is then received at the first location where it is converted back to an NRZ data stream having the first clocking frequency without having to first determine clocking signals or establish phase lock loop by reconstructing the Manchester code. The reconstruction is accomplished by delaying the combination coded data stream for a period of time substantially equal to one-half cycle of the second frequency and then combining the delayed signal with an undelayed stream of the combination coded signal.
BRIEF DESCRIPTION OF THE DRAWINGSThese and other features of the present invention will be more fully disclosed when taken in conjunction with the following Detailed Description of the Preferred Embodiment(s) in which like numerals represent like elements and in which:
Referring now to
Also shown is a typical telephone system or POTS which, of course, is two-way communication typically carried by means of a twisted pair of wires. In the example shown in
As demands increase for more and more TV channels and better and more efficient transmission techniques without disruption and interference, the long runs of coaxial cable are simply becoming inefficient and inadequate. Thus, as is shown in
In the improved television transmission system, however, the transmission is achieved by a fiber optical cable as indicated by fiber optical cables 42 and 42a. As shown in
Referring now to
Further, as is shown, in addition to the series of optical fibers 42 and 42a traveling between OIU (Optical Interface Unit) 18 at Central Office 19 and a distribution terminal 20, hereinafter referred to as the HNU (Home Network Unit), there will be other optical fibers as indicated by optical fibers 42b through 42d which extend between one-to-four Optical Splitter/Coupler 44 and other home distribution terminals or HNU's similar to HNU 20. Each of the optical fibers 42b through 42d may carry light at both 1550 nanometers and 1310 nanometers.
As shown, TV signal source location 10 provides signals from equipment 12 and, in this illustrated embodiment, the TV signals may be 55-870 MHZ signals provided to a coupler or WDM (Wave Division Multiplexer) 50. It will be appreciated that cable 16 could be either an optical fiber or a coaxial cable. A copper coaxial cable 16 would carry the TV signals having a bandwidth of 55-870 MHZ to circuitry 51 which uses the electrical TV signals to modulate light having a selected wavelength which is directed or focused onto optical fiber 52. In one preferred embodiment, a particular selected wavelength for such TV signals is 1550 nanometers. Thus, the 1550 nanometer light waves are provided to optical fiber 42 by WDM 50, and according to one embodiment, travel in a single direction from WDM 50 through optical fibers 42 and 42a to distribution terminal or HNU 20 in house or building 22. Of course, once the 1550 nanometer light carrying the TV signal arrives at HNU 20, photo-diode or PD 57 strips out the TV signals such that they can be distributed throughout home or building 22, as shown by coaxial cable 12b carrying the signals to set up top box 28 and/or television 26.
Also as shown, electrical telephony or POTS (Plain Ole Telephone) signals may be carried to Central Office 19 by copper wires, such as copper wires 48, which represent a twisted pair of normal telephone communication wires. Circuitry or OIU 18 in Central Office 19 receives these electrical telephony signals as well as other broadband data signals traveling downstream. As will be discussed in detail later with respect to
Thus, ONU 18 which is connected to fiber optic cable 42 (through SWX 50) for carrying the 1310 modulated light may also receive 55-870 MHZ TV signals from the TV signal source location 10. The 55-870 MHZ electrical signals may, as an example, be used to modulate light having a wavelength of 1550 nanometers. SWX (Splitter with Division Multiplexing) 54 then combines by WDM (Wave Division Multiplexing) the plurality of 1310 nanometer wavelength signals along with the 1550 nanometer wavelength signals such that cables 42 and 42a carrying the TV signals in a downstream direction on 1550 nanometer light and carries bidirectional telephony signals in both directions at 1310 nanometers of light. Of course, fiber optical cables 42b through 42d and connected to coupler/splitter 44 carry the 1550 nanometer light and the 1310 nanometer light in a similar manner.
At the downstream destination, the bidirectional telephony signals traveling on 1310 nanometer light waves are routed to equipment in HNU 20 in Home 22 which recovers the electrical telephony signals by a photo detector—(PD) 56 from the 1310 nanometer light waves traveling downstream and uses the electrical telephony signals traveling upstream to modulate light waves having a wavelength of 1310 nanometers by laser diode—(LD) 58. The electrical telephony signals are then distributed from HNU box 20 by wire pair 32b to the telephone 30b or other telephony equipment such as the 56K telephone modem 34 at home or building 22.
As was discussed above, the extremely broad bandwidth available with the use of optical fibers as a transmission medium offers many advantages and vastly increases subscriber density on a single fiber. Unfortunately, new uses and demands continue to grow at ever-increasing rates. Consequently, what may have appeared to be an overabundance of bandwidth for years to come a couple of years ago is already or threatens to become crowded in the near future. In addition, every technology has its own special set of problems and the use of optical fiber as a transmission medium for telephony communication is no exception. More particularly, using present techniques, a light wave traveling through a fiber is particularly vulnerable to reflection if the connector joining the two fibers is dirty or improperly fitted. The present invention, however, discloses transmission techniques, coding, or protocols to minimize the effects of reflective overlap of the transmitted energy spectrum, and rapid clocking recovery.
Referring now to
According to the described embodiment, a first data stream of electrical pulses in NRZ (Non-Return to Zero) format and clocking signal having a selected frequency are received through the back plane by FPGA (Field Programmable Gate Array) 60 for conditioning at inputs 62 and 64, respectively. Control and synchronizing information along with addresses and alarm data is added to the data stream by FPGA 60. The stream of conditioned NRZ coded signals are then transmitted by line 65 to a low pass filter 66 located in OIU 18 at Central Office 19. In a preferred embodiment, the NRZ electrical pulses have a frequency of around 25 MHZ. More specifically, using standard and readily-available components and parts, this frequency will actually be about 25.92 MHZ. Low pass filter 66 will typically be chosen so as to readily pass frequencies less than about 25 MHZ, while substantially blocking all frequencies above about 30 MHZ. Referring briefly to
The 1310 nanometer light carrying the NRZ data stream on fiber 42 is then provided through a splitter such as 1×4 splitter 44 to optical cable 42a and then to the distribution panel 20 in home or building 22. Splitter 44 also provides individual fiber optical cables 42b, 42c and 42d to other homes or buildings which also receive the data stream. Cable 42a is then provided to a Quplexer 82 located in distribution panel 20, which separates out the 1550 nanometer light carrying the TV signals, if any, as shown at 84, and passes the 1310 nanometer light modulated by the NRZ coded data stream to photo diode 56. Photo diode 56 in Quplexer 82 along with the very low noise amplifier recovery circuit 86 recovers the 25 MHZ NRZ coded data stream adds gain and converts this data stream to a differential voltage output on a pair of electrical conductors 88a and 88b. The two different voltage outputs are provided to low pass filter 90 and then onto Comparator or Quantizer 92. Comparator 92 uses the two signals on line 88a and 88b to regenerate the 25 MHZ NRZ data stream by improving the signal and increasing the signal-to-noise ratio such that the output of Comparator or Quantizer 92 is suitable for use by TLL Logic. This output of Quantizer 92 is then provided to PLL (Phase Lock Loop Circuit) 94 to recover the 25.92 NRZ data as well as the 25.92 clocking pulses. The 25 MHZ NRZ data is then provided by line 96 to receiving equipment.
The transmission path from the HNU 20 in the second location or Home 22 to OIU 18 at the Central Office 19 in earlier optical transmission systems might well have been a mirror image of the transmission sequence from OIU 18 to HNU 20 as discussed above. However, to assure efficiency and transmission integrity, the present invention uses a first NRZ coded transmission protocol to carry information in one direction at a first frequency of 25.92 MHZ as just discussed, and a modified Manchester coded transmission protocol at a second frequency to carry information in the opposite direction. The modified MOOSE coded data is referred to herein as Manchester/OOK coded data and provides transitions which are eight times (8×) the first frequency.
Referring now to
Referring again to
Referring again to
The unique Manchester ON-OFF Keying or MOOSE coded data signal is then provided from ON-OFF keying device 118 to BPF 134, TX driver circuit 136 and then to diode 58 in Quplexer 82 where it is used to modulate light directed onto optical fiber 80 for transmission to distribution terminal 18. Referring to
The output 158 is split and applied to delay line 160 and multiplier 162. Similarly, the reference output 164 is split and applied to delay line 166 and multiplier 168. According to the present embodiment, delay lines 160 and 166 provide a delay of approximately 4.8 nanoseconds as shown in
Referring now to
Referring now to
In a similar manner, the data transmitted and received upstream at terminal 18 is passed through a band pass filter 156 which readily passes frequencies of between about 50 MHz and 150 MHz while substantially attenuating frequencies having a frequency of less than about 50 MHz and greater than about 150 MHz. The threshold (pass-no pass) lines 140 and 142 thus clearly illustrate how the upstream transmitted data is limited to the center burst of data 178 between 50 MHz and 150 MHz.
Referring now to
Similarly,
Referring now to
The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed.
Claims
1. A distribution terminal, comprising:
- a coding unit operable to receive a first coded data stream, the coding unit operable to convert the first coded data stream into a second coded data stream;
- a keying device operable to modify the second coded data stream into a modified second coded data stream; and
- a laser source operable to modulate an optical signal with the modified second coded data stream.
2. The distribution terminal of claim 1, wherein the first coded data stream is a NRZ coded data stream.
3. The distribution terminal of claim 1, wherein the second coded data stream is a Manchester coded data stream.
4. The distribution terminal of claim 1, further comprising:
- a clock multiplier operable to provide a clock signal to the coding unit and the keying device.
5. The distribution terminal of claim 4, wherein the clock signal provided to the coding unit is twice a clock rate associated with the first coded data stream.
6. The distribution terminal of claim 4, wherein the clock signal provided to the keying device to modify the second coded data stream is a multiple of a clock rate associated with the first coded data stream.
7. The distribution terminal of claim 1, wherein the keying device provides a reference voltage having a first level for the modified second coded data stream corresponding to a first voltage level of the second coded data stream.
8. The distribution terminal of claim 6, wherein the keying device provides reference voltages alternating between a second level and the first level for the modified second coded data stream corresponding to a second voltage level of the second coded data stream.
9. The distribution terminal of claim 1, further comprising:
- a bandpass filter operable to pass only a portion of the modified second coded data stream within a predetermined threshold region.
10. The distribution terminal of claim 1, wherein modulation of the optical signal is performed without establishing a timing or phase lock.
11. A method for generating an optical signal, comprising:
- receiving a first coded data stream;
- converting the first coded data stream into a second coded data stream;
- modifying the second coded data stream into a modified second coded data stream;
- modulating an optical signal with the modified second coded data stream.
12. The method of claim 11, further comprising:
- passing a portion of the modified second coded data stream falling within a predetermined threshold region.
13. The method of claim 11, wherein modifying the second coded data stream includes:
- providing a reference voltage having a first level corresponding to a first voltage level of the second coded data stream.
14. The method of claim 13, wherein modifying the second coded data stream includes:
- providing alternating reference voltages between a second level and the first level corresponding to a second voltage level of the second coded stream.
15. The method of claim 11, wherein the first coded data stream is a NRZ coded data stream and the second coded data stream is a Manchester coded data stream.
16. A system for generating an optical signal, comprising:
- means for receiving a first coded data stream;
- means for converting the first coded data stream into a second coded data stream;
- means for modifying the second coded data stream into a modified second coded data stream;
- means for modulating an optical signal with the modified second coded data stream.
17. The system of claim 16, further comprising:
- means for passing a portion of the modified second coded data stream falling within a predetermined threshold region.
18. The system of claim 16, wherein the means for modifying the second coded data stream includes:
- means for providing a reference voltage having a first level corresponding to a first voltage level of the second coded data stream.
19. The system of claim 18, wherein the means for modifying the second coded data stream includes:
- means for providing alternating reference voltages between a second level and the first level corresponding to a second voltage level of the second coded stream.
20. The system of claim 16, wherein the first coded data stream is a NRZ coded data stream and the second coded data stream is a Manchester coded data stream.
Type: Application
Filed: Aug 29, 2005
Publication Date: Jan 5, 2006
Inventors: Muneer Zuhdi (Lewisville, TX), George Buabbud (South Lake, TX)
Application Number: 11/215,613
International Classification: H04L 27/10 (20060101);