Prevention of collision for time division multiplexing optical network, apparatus and method thereof

An apparatus for preventing collision of upstream signals is provided. The apparatus is suitable for a time division multiplexing (TDM) passive optical network (PON). The apparatus includes an optical coupler device, an optic-electron converter (O/E), a control system, and an optical signal switch module. The O/E is coupled to the optical coupler device, the control system is coupled to the O/E, and the optical signal switch module is coupled to the optical coupler device and the control system. The optical coupler device receives a first optical signal and splits the first optical signal into a second optical signal and a third optical signal. The O/E converts the second optical signal into a first electrical signal. The control system generates a control signal according to the first electrical signal. The optical signal switch module determines whether to stop the third optical signal from passing the apparatus according to the control signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan application serial no. 96141055, filed on Oct. 31, 2007. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a time division multiplexing (TDM) passive optical network (PON) system for preventing signal collision, an apparatus thereof, and a method thereof.

2. Description of Related Art

The quantity of data transmitted over networks increases along with the increase of network users. In the conventional communication technique wherein data is transmitted as electric signals, network congestion may be caused due to the bandwidth limitation of the electric signals. Thus, optical fiber communication is adopted by many network service providers for providing various network services to network users.

Optical fiber communication offers greater bandwidth than wireless or cable communication using electric signals therefore can transmit data of larger quantity and provide better network services. Presently, passive optical network (PON) is usually adopted by optical fiber communication systems. A PON is easy to maintain and consumes less power because it uses passive devices and requires less switching equipment. Nowadays, many countries are dedicated to the development of PONs such as fiber to the home (FFTH), fiber to the curb (FFTC), and fiber to the building (FFTB) by using optical fiber communication techniques in order to allow network users to transmit and receive data in high speed and large quantity. Accordingly, PON is playing as one of today's major communication techniques.

In a PON, each optical network unit (ONU) has different distance to an optical line termination (OLT) in the central office. Accordingly, the transmission of upstream signals (usually optical signals of 1310 nm) can be controlled in a time division multiplexing (TDM) manner in order to prevent signal collision. As a result, the optical transceiver in each ONU has to be a laser source in burst mode in order to meet the requirement of the TDM transmission manner. The entire PON will collapse if any ONU in the PON turns from the burst mode into a continuous wave (CW) mode due to some module problems. It is impossible to transmit all the upstream signals due to the problem of signal collision.

FIG. 1Ais a schematic diagram of a conventional TDM PON100in normal operation. Referring toFIG. 1A, the TDM PON100includes an OLT101, a plurality of optical fibers102, an optical coupler device103, and a plurality of ONUs1041˜1044. The OLT101is coupled to the optical coupler device103via the optical fiber102, and the optical coupler device103is coupled to the ONUs1041˜1044via the optical fibers102. Regarding a downstream signal, the optical coupler device103splits the downstream signal so that each ONU can receive a downstream signal; and regarding upstream signals, the optical coupler device103couples an upstream signal of each ONU so that the upstream signals of the ONUs1041˜1044can be transmitted to the OLT101successfully.

The ONUs1041˜1044are respectively assigned time slots ts_1˜ts_4. If the ONU1041has upstream data, the ONU1041has to transmit the upstream data to the OLT101in time slot ts_1. Similarly, the other ONUs1042˜1044respectively transmit their upstream data to the OLT101in their own time slots ts_2˜ts_4. Accordingly, the laser sources in the ONUs1041˜1044have to operate in burst mode.

FIG. 1Bis a schematic diagram of the conventional TDM PON100operating in the condition of signal collision. When the laser source in the ONU1043turns from burst mode into CW mode due to some external or other factors such as earthquake, damaged optical fibber module, or other environmental problems, the upstream signals of the ONUs1041,1042, and1044may collide with the upstream signal of the ONU1043so that the TDM transmission manner is destroyed and accordingly the entire TDM PON100collapses.

In order to prevent foregoing situation, a method and an apparatus for transmitting a control signal through an OLT for controlling the on or off of each ONU are disclosed in U.S. Patent No. 2005/0244160 A1. However, the OLT circuit has to be re-designed in this disclosure.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to an apparatus for preventing collision of upstream signals. The apparatus is suitable for a time division multiplexing (TDM) passive optical network (PON). The apparatus has simple structure and low manufacturing cost and can be used for preventing collapse of the TDM PON caused by signal collision.

The present invention is directed to a method for preventing collision of upstream signals. The method is suitable for a TDM PON, and an apparatus executing this method can be used for preventing collapse of the TDM PON caused by signal collision.

The present invention is also directed to a TDM PON system for preventing collision of upstream signals. This TDM PON system is achieved by slightly revising the structure of a conventional TDM PON, and unlike the conventional TDM PON, this TDM PON system will not collapse due to signal collision.

The present invention provides an apparatus for preventing collision of upstream signals. The apparatus is suitable for a TDM PON. The apparatus includes an optical coupler device, an optic-electron converter (O/E), a control system, and an optical signal switch module. The O/E is coupled to the optical coupler device, the control system is coupled to the O/E, and the optical signal switch module is coupled to the optical coupler device and the control system. The optical coupler device receives a first optical signal and splits the first optical signal into a second optical signal and a third optical signal. The O/E converts the second optical signal into a first electrical signal, and the control system generates a control signal according to the first electrical signal. The optical signal switch module determines whether to stop the third optical signal from passing the apparatus according to the control signal.

The present invention provides a method for preventing collision of upstream signals. The method is suitable for a TDM PON. First, a first optical signal is split into a second optical signal and a third optical signal. Next, the second optical signal is converted into a first electrical signal. After that, a control signal is generated according to the first electrical signal. Finally, whether the third optical signal is output is determined according to the control signal.

The present invention provides a TDM PON system for preventing collision of upstream signals. The TDM PON system includes an optical line termination (OLT), a first optical coupler device, a plurality of end-user devices, and a plurality of optical fibers. The optical fibers connect the OLT and the first optical coupler device and also connect the first optical coupler device and the end-user devices. Each of the end-user devices includes an optical network unit (ONU) and a signal collision prevention apparatus. The signal collision prevention apparatus is coupled to the ONU and receives a first optical signal from the ONU. The signal collision prevention apparatus splits the first optical signal into a second optical signal and a third optical signal and determines whether to output the third optical signal according to the second optical signal.

In the present invention, an apparatus for preventing collision of upstream signals is used for controlling an ONU so that a TDM PON will not collapse due to collision of upstream signals. In addition, the apparatus for preventing collision of upstream signals has simple structure, low manufacturing cost, and low hardware complexity and is easily integrated with the ONU.

DESCRIPTION OF THE EMBODIMENTS

According to embodiments of the present invention, a time division multiplexing (TDM) passive optical network (PON) system for preventing collision of upstream signals, an apparatus thereof, and a method thereof are provided.

FIG. 2Ais a schematic diagram of a TDM PON system200for preventing collision of upstream signals according to an embodiment of the present invention. Referring toFIG. 2A, the TDM PON system200includes an optical line termination (OLT)201, an optical coupler device202, a plurality of end-user devices203˜206, and a plurality of optical fibers207. The optical fibers207connect the OLT201and the optical coupler device202and also connect the optical coupler device202and the end-user devices203˜206. The optical coupler device202may be an optical coupler (OCP) or an optical splitter (OS). In short, the optical coupler device can be any device which can split or couple optical signals, and foregoing embodiment of the optical coupler device as an OCP or an optical splitter is not intended for limiting the scope of the present invention.

Regarding a downstream signal, the optical coupler device202splits the downstream signal so that each of the end-user devices203˜206can receive a downstream signal, and regarding upstream signals, the optical coupler device202couples an upstream signal of each of the end-user devices203˜206so that the upstream signal of each of the end-user devices203˜206can be successfully transmitted to the OLT201. In the TDM PON system200, the end-user devices203˜206are respectively assigned corresponding time slots ts_1˜ts_4, and the end-user devices203˜206can respectively transmit data in their own time slots ts_1˜ts_4.

The end-user device203includes an optical network unit (ONU)2031and a signal collision prevention apparatus2030. The signal collision prevention apparatus2030is coupled to the ONU2031and receives an optical signal from the ONU2031. The signal collision prevention apparatus2030splits the optical signal received from the ONU2031into a detection optical signal and a data optical signal and determines whether to output the data optical signal according to the detection optical signal. The structures of the end-user devices204˜206are similar to that of the end-user device203therefore will not be described herein.

If an error occurs to the ONU2051in the end-user device205and accordingly the transceiver in the ONU2051turns into a continuous wave (CW) mode, the TDM PON200will not collapse. This is because when the error occurs to the ONU2051, a signal collision prevention apparatus2050stops the ONU2051from constantly transmitting optical signals so that the ONUs2031,2041, and2061can transmit data in the corresponding time slots ts_1, ts_2, and ts_4.

FIG. 2Bis a schematic diagram of the signal collision prevention apparatus2030. Herein only the signal collision prevention apparatus2030is described; however, the structures of the other signal collision prevention apparatuses2040˜2060are the same as that of the signal collision prevention apparatus2030. Referring toFIG. 2B, the signal collision prevention apparatus2030includes an optical coupler device2035, an optic-electron converter (O/E)2036, a control system2037, and an optical signal switch module2038. The O/E2036is coupled to the optical coupler device2035, the control system2037is coupled to the O/E2036, and the optical signal switch module2038is coupled to the optical coupler device2035and the control system2037.

The optical coupler device2035receives an optical signal from the ONU2031and splits the optical signal into a detection optical signal and a data optical signal. The O/E2036converts the detection optical signal into a detection electrical signal, and the control system2037generates a control signal according to the detection electrical signal. The optical signal switch module2038determines whether to stop the data optical signal from passing the signal collision prevention apparatus2030according to the control signal.

As shown inFIG. 2B, the optical signal switch module2038includes an optical switch (OS)203A and a photoresist device203B. The optical switch203A has an input terminal0, a first output terminal1, and a second output terminal2, wherein the input terminal0is coupled to the optical coupler device2035, the second output terminal2is coupled to the photoresist device203B, and the first output terminal1is coupled to the optical fiber207. The optical switch203A outputs the data optical signal from the first output terminal1or the second output terminal2according to the control signal, and the photoresist device203B stops the data optical signal. Additionally, in the present embodiment, the data optical signal is an upstream signal of 1310 nm; however, the wavelength of the data optical signal is not limited in the present invention.

Normally, the ONU2031inFIG. 2Boperates in a burst mode and the data optical signal is successfully output to the optical fiber207. Thus, the optical switch203A transmits the data optical signal from the input terminal0to the first output terminal1. Additionally, the optical coupler device2035may be an optical coupler (OCP) or an optical splitter embodied with a planar lightwave circuit (PLC) or a waveguide duct. The O/E2036can be embodied with a photodiode. The photodiode may be a positive-intrinsic-negative (PIN) photodiode, an avalanche photodiode (APD), or a metal-semiconductor-metal (MSM) photodiode.

Foregoing embodiments of the O/E2036, the optical coupler device2035, and the optical signal switch module2038are not intended for limiting the scope of the present invention; instead, the O/E2036, the optical coupler device2035, and the optical signal switch module2038can also be embodied differently to achieve the same functions as described above. In short,FIG. 2Billustrates only an embodiment of the present invention but not for limiting the present invention, and various changes in form and details may be made therein without departing from the spirit and scope of the present invention.

FIG. 2Cis a diagram of an optical signal when the ONU2031operates in a burst mode. When the ONU2031operates in the burst mode, the time τ for the optical signal to occupy the channel is the total of a turn-on time C00, a turn-off time C02, and a data transmission time C01of the laser source in the ONU2031. With existing PON standard, the time τ is usually no more than several microseconds, and herein the optical switch203A transmits the data optical signal to the optical fiber207via the first output terminal1so as to allow the TDM PON200to connect correctly.

FIG. 2Dis a diagram illustrating the operation of the optical switch203A when the ONU2031operates in a continuous wave (CW) mode.FIG. 2Eis a diagram of an optical signal when the ONU2031operates in the CW mode. When the ONU2031operates in the CW mode, the laser source thereof will not be turned off once it is turned on. Thus, the time τ′ of the optical signal to occupy the channel is the total of a turn-on time E00and a turned-on time E01of the laser source. Here the time τ′ is much longer than several microseconds. The signal collision prevention apparatus2030stops the data optical signal from passing so that the TDM PON200will not collapse. Accordingly, the optical switch203A transmits the data optical signal to the second output terminal2thereof so that the photoresist device203B can block the data optical signal. The principle of preventing the TDM PON200from collapsing is that the control system2037determines whether the time of the received detection electrical signal being greater than a threshold value is longer than a predetermined time. When the ONU2031is in the CW mode, the laser source emits upstream signals constantly so that the time of the detection electrical signal being greater than the threshold value is longer than the predetermined time. Accordingly, the optical signal switch module2038prevents the data optical signal from being output to the optical fiber207according to the control signal, wherein the predetermined time is longer than the time τ. When the ONU2031is in the burst mode, the laser source emits upstream signals only within the time τ. Thus, the time of the detection electrical signal being greater than the threshold value is shorter than the predetermined time, and accordingly the optical signal switch module2038allows the data optical signal to be output to the optical fiber207.

The optical switch203A may be a micro-electro-mechanical system (MEMS) latch type optical switch or a semiconductor optical amplifier (SOA) switch.FIG. 3Ais a schematic diagram of the optical switch203A according to an embodiment of the present invention.FIG. 3Bis a diagram illustrating an optical signal output to the second output terminal2of the optical switch203A.FIG. 3Cis a diagram illustrating an optical signal output to the first output terminal1of the optical switch203A. The optical switch203A inFIGS. 3A˜3Bis embodied as a MEMS latch type optical switch300. Referring toFIG. 3A, the optical switch300includes a reflector301which can moved up and down, wherein the movement of the reflector301is controlled by the control signal. Referring toFIG. 3B, when the control signal controls the reflector301to move upwards, the reflector301reflects the data optical signal to the second output terminal2. Referring toFIG. 3C, when the control signal controls the reflector301to move downwards, the data optical signal is directly transmitted to the first output terminal1.

Even though foregoing optical switch203A is embodied with a MEMS latch type optical switch300, the present invention is not limited thereto. In other words, the optical switch203A may also be embodied differently to achieve the function described above without departing from the scope of the present invention.

FIG. 4is a flowchart of a method for preventing collision of upstream signals according to an embodiment of the present invention. The method is suitable for a TDM PON. In this method, first, an optical signal sent by an ONU is split into a data optical signal and a detection optical signal (step S400). Next, the detection optical signal is converted into a detection electrical signal (step S401). After that, a control signal is generated according to the detection electrical signal (step S402). Finally, whether to output the data optical signal is determined according to the control signal (step S403). If the time for the intensity of the detection electrical signal to be higher than a threshold value is longer than a predetermined time, the data optical signal is blocked and not output to the TDM PON, so that collision of upstream signals can be prevented. In addition, the predetermined time and the threshold value can be adjusted according to the actual system requirement.

FIG. 5Ais an eye diagram of an optical signal when there is no CW mode ONU in a PON according to an embodiment of the present invention.FIG. 5Bis an eye diagram of an optical signal when any ONU in a PON is in CW mode and which causes the PON to collapse according to the present embodiment of the present invention.FIG. 5Cis an eye diagram of an optical signal when any ONU in a PON is in CW mode and the method and apparatus provided by the present invention are adopted according to an embodiment of the present invention. In foregoing PON, the distance from the OLT to an ONU is about 20 km, and all the upstream signals and downstream signals in the PON pass through a 1×8 optical splitter. Laser sources of 1490 nm and 1310 nm are respectively used as the laser sources of downstream signals and upstream signals. In foregoing PON, an optical signal is modulated through 1.25 Gbps direct modulation, and 231-1 bits are generated in a non-return-to-zero (NRZ) pseudo random binary sequence (PRBS) mode for measuring the bit error rate (BER) of the entire PON.

As shown inFIG. 5A, when there is no ONU in the CW mode, the eye diagram of the optical signal is very complete and the extinction ratio (ED) thereof is greater than 10 dB. If there is one ONU in CW mode, the upstream signal received by the OLT is seriously distorted due to the problem of signal collision. As shown inFIG. 5B, the eye diagram of the optical signal is very bad because of signal collision, and the ED thereof is nearly undetectable. InFIG. 5B, the received power of the ONU in the CW mode is about −30 dBm, which is very low. The eye diagram of the optical signal will become worse if the received power is under ˜30 dBm. If the apparatus and method for preventing collision of upstream signals provided by the present invention are adopted in the PON, the optical signals of an ONU in CW mode are not be transmitted to the PON so that the PON will not collapse. As shown in5C, since the optical signal of the ONU in CW mode cannot be transmitted to the PON, the eye diagram of the upstream signal received by the OLT is nearly perfect.

FIG. 6is a diagram illustrating the on/off time of an optical switch according to an embodiment of the present invention.FIG. 7is a graph of BER vs. the received power of a downstream signal according to an embodiment of the present invention. As shown inFIG. 6, the response time of the optical switch in the TDM PON system illustrated inFIGS. 5A˜5Cis about 7 ms. As shown inFIG. 7, the curve denoted with circles represents the BER of back to back transmission; the curve denoted with diamonds represents the BER when there is collision of upstream signals and no collision prevention mechanism is adopted; and the curve denoted with squares represents the BER when there is collision of upstream signals and the apparatus and method for preventing collision of upstream signals provided by the present invention are adopted. As shown inFIG. 7, regardless of whether the collision prevention mechanism is started, the power penalty produced by a downstream signal of 1490 nm is not obviously increased (lower than 0.2 dB) when BER is equal to 10−9. Accordingly, the apparatus, network system, and method provided by the present invention do not reduce the performance of downstream signals obviously.

In summary, the present invention provides an apparatus, a PON system, and a method for preventing collision of upstream signals such that system collapse caused by collision of upstream signals can be prevented. Moreover, the apparatus provided by the present invention has simple structure, Low manufacturing cost, and low hardware complexity and is easily integrated with an ONU.