Abstract:
An apparatus and method is disclosed for protecting access to a shared medium in an optical communications system which has plural remote terminals coupled to a central terminal over the shared medium. According to one aspect, if an optical transmitter for an individual remote terminal is active for a first prescribed time interval when it has not been enabled for such transmission, the optical transmitter is disabled. According to another aspect, the continuous time that an individual terminal is enabled to transmit onto the shared medium is monitored. If the enabled transmission for the individual terminal exceeds a second prescribed time interval, the optical transmitter for the individual terminal is disabled.

Description:
BACKGROUND OF THE INVENTION 
     In Ethernet networks that operate over shared copper media, e.g., IEEE 802.3 standards for 10Base5, 10Base2 and 10BaseT, apparatus in the network access device for a connected terminal typically includes a so-called “jabber” circuit that monitors the length of transmission into the copper media for that individual terminal. If the continuous transmission of the individual terminal exceeds an accepted maximum length of time many times a maximum size packet, that terminal is disabled for a quiet period of, e.g., 500 milliseconds and then usually is allowed to resume transmission. 
     In a shared transmission medium such as a passive optical network which is shared among multiple terminals, it is important for proper operation that each terminal transmit onto the shared medium at assigned times and only for the assigned time interval. If such rules of operation are violated, the integrity of the system transmission can be severely compromised. 
     SUMMARY OF THE INVENTION 
     There is a need for a mechanism for protecting access to a shared optical medium in order to ensure that a faulty terminal does not affect system operation and integrity. 
     The present invention provides an apparatus and method for protecting access to a shared transmission medium. According to one aspect, if an optical transmitter for an individual terminal is active for a first prescribed time interval when it has not been enabled for such transmission, the optical transmitter is disabled. According to another aspect, the continuous time that an individual terminal is enabled to transmit onto the shared medium is monitored. If the enabled transmission for the individual terminal exceeds a second prescribed time interval, the optical transmitter for the individual terminal is disabled. 
     Accordingly, in an optical communications system which has plural remote terminals coupled to a central terminal or hub over a shared optical transmission medium, wherein the plural remote terminals are configured to transmit data on the shared optical transmission medium at prescribed transmission time intervals and in a prescribed sequence, a remote terminal apparatus comprises an optical transmitter for transmitting data onto the shared optical transmission medium, a fault monitoring circuit and a deactivation circuit. The fault monitoring circuit is coupled to the optical transmitter for monitoring activity of the optical transmitter and providing a fault indication signal when optical transmitter activity occurs for a duration that exceeds a fault time interval. The deactivation circuit is responsive to the fault indication signal to deactivate the optical transmitter. 
     In a first embodiment, the optical transmitter includes an optical detector for detecting the presence of optical transmitter energy into the shared medium to provide an activity signal to the fault monitoring circuit which operates to provide the fault indication signal upon the activity signal being present for a duration that exceeds the fault time interval. 
     In another embodiment, the remote terminal apparatus includes a transmitter controller that provides a transmit enable signal for enabling the optical transmitter wherein under normal operation of the remote terminal the transmit enable signal is asserted during the prescribed transmission time interval. The optical transmitter includes an optical detector for detecting the presence of optical transmitter energy into the shared medium to provide an activity signal to the fault monitoring circuit which operates to provide the fault indication signal upon the transmit enable signal being unasserted and the activity signal being present for a duration that exceeds the fault time interval. 
     According to an aspect, the fault monitoring circuit includes a timer circuit comprising a resistance-capacitance time constant circuit having an input coupled to the activity signal for establishing the fault time interval which can be less than the prescribed transmission time interval. 
     According to another aspect, the remote terminal apparatus further includes a second fault monitoring circuit for monitoring the transmit enable signal to provide a second fault indication signal upon the transmit enable signal being asserted for a duration exceeding a second fault time interval. The deactivation circuit includes a fault latch for latching the first and second fault indication signals for deactivating the optical transmitter. 
     According to yet another aspect, the second fault monitoring circuit includes a second timer circuit comprising a voltage ramp generator that has an input coupled to the transmit enable signal to provide a ramp voltage and a voltage comparator that has a first input coupled to the ramp voltage and a second input coupled to a reference voltage. The voltage comparator is operable to provide the second fault indication signal upon the ramp voltage exceeding the reference voltage over the second fault time interval. The second fault time interval is greater than the prescribed transmission time interval. 
     A method of the present invention includes providing an optical transmitter for transmitting data onto a shared optical transmission medium; monitoring activity of the optical transmitter output and providing a fault indication signal upon optical transmitter activity occurring for a duration that exceeds a fault time interval; and deactivating the optical transmitter responsive to the fault indication signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     FIG. 1 is a block diagram of an optical access system. 
     FIG. 2 is a diagram that shows a upstream TDMA signal received at a central terminal of the optical access system of FIG.  1 . 
     FIG. 3 is a block diagram of a remote terminal for use in the optical access system of FIG.  1 . 
     FIG. 4 is a schematic block diagram of an optical access protection circuit of the present invention. 
     FIGS. 5A and 5B are timing diagrams illustrating operation of the optical access protection circuit of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a block diagram of an optical access system which includes a central terminal or hub  10 , remote terminals  12  designated RT 1 , RT 2 , RT 3 , . . . , RT N  and a passive optical network (PON)  14 . The system provides a downstream data signal  122  over the PON  14  from the central terminal  10  to the remote terminals  12  using time division multiplexing (TDM) transmission over the media. An upstream data signal  116  from the remote terminals  12  to the central terminal  10  over the PON  14  is provided in burst transmissions using time division multiple access (TDMA). 
     Note that the terms downstream and upstream are used herein to refer to the direction of transmission signal flow. The downstream direction refers to signals from the central terminal  10  toward the remote terminals  12 . The upstream direction refers to signals from the remote terminals  12  toward the central terminal  10 . 
     Each remote terminal  12  includes an optical transmitter described further herein which in normal operation is enabled only during a time period assigned to that particular remote terminal for transmission of data onto PON  14 . FIG. 2 shows the upstream data signal received at the central terminal  10  on the PON  14  from the remote terminals RT 1 , RT 2 , RT 3 , . . . , RT N  (FIG. 1) using TDMA in multiple timeslots designated slot  1 , slot  2 , . . . , slot N. 
     Using conventional ranging techniques, the proper ranging delay is calculated for each remote terminal to account for the corresponding propagation delay and the burst transmissions are timed to occur in accordance with assigned timeslots, e.g., remote terminal RT 1  transmits in slot  1 , RT 2  transmits in slot  2 , and RT N  transmits in slot N. It should be noted that any of the remote terminals can also be assigned to transmit in multiple timeslots depending on the services provisioned for that terminal. Between each timeslot there is a guard time period G before the next TDMA burst during which no useful data is transmitted so as to avoid overlapping of bursts in adjacent timeslots. The guard period G plus the timeslot period are shown as duration D 0 . In an embodiment, D 0  is about 12 microseconds. 
     FIG. 3 is a block diagram of remote terminal  12  of the system of FIG.  1 . The remote terminal includes an optical receiver  240 , data recovery circuit  242 , source data block  244 , transmitter controller  246 , protection circuit  248  and optical transmitter  250 . In the downstream direction, a downstream TDM data signal  122  is received in optical receiver  240  from the central terminal  10  (FIG. 1) and data is recovered in conventional data recovery circuit  242 . The data recovery circuit  242  also provides to the source data block  244  a timing synchronization signal  243  derived from the received signal  122 . In the upstream direction, synchronized source data from block  244  is provided to optical transmitter  250  through transmitter controller  246 . The optical transmitter  250  transmits a burst transmission  116 - 1  containing upstream source data in an assigned timeslot. Each of the remote terminals is similarly configured, except that each has its own ranging delay to guarantee proper slot burst alignment. 
     FIG. 4 is a schematic block diagram of the optical media access protection circuit  248  of the present invention shown connected to optical transmitter  250 . The optical transmitter  250  includes laser driver  280 , laser diode  300 , back-faced diode detector  302  and deactivation element  304 . The protection circuit  248  includes fault monitoring sections  306 ,  308  and transmitter deactivation section  310 . 
     The fault monitoring section  306  monitors the amount of time that an individual terminal is enabled to transmit continuously onto the shared medium. If the enabled transmission for the individual terminal exceeds a particular time interval, the laser diode  300  for the individual terminal is disabled. This fault condition can occur, for example, if the laser driver  280  operates outside its normal specification due to an internal defect of the device. The fault monitoring section  306  includes a ramp reset transistor  312  and a timer circuit which includes voltage ramp generator  314  and voltage comparator  316 . 
     The transmit enable signal  247  from transmitter controller  246  (FIG. 3) is input to the ramp reset transistor  312 . When the transmit enable signal  247  is not asserted, the reset transistor  312  conducts which prevents voltage ramp generator  314  from building up a voltage charge. When the transmit enable is asserted, the reset transistor  312  stops conducting and a slowly increasing ramp waveform signal  321  is generated by the voltage ramp generator  314 . As long as the transmit enable signal  247  is asserted, the ramp continues to increase. If the transmit duration continues long enough, the ramp waveform signal  321  exceeds the set threshold reference V REF1    323  into comparator  316 . Once the threshold is exceeded, the output of the comparator  316  provides a logic high output signal  325  which is passed to the fault latch  330  of deactivation section  310 . 
     The fault monitoring section  308  detects whether the laser diode  300  is active for some period of time when it has not been enabled for such transmission. The fault monitoring section  308  includes conditioning circuit  318 , peak detector/rectifier  320 , sample gate.  322 , a timer circuit comprising RC time constant integrator  324  and voltage comparator  326 , and AND gate  328 . 
     The output signal  303  of the back-faced sensing diode detector  302  provides a detection current of between 0.3 to 0.6 ma when the laser diode  300  is active and zero current when the laser diode  300  is off. Light entering the fiber transmission medium when the laser diode  300  is off has no effect on the output response of the back-faced sensing diode detector  302 . The mechanism of the present invention provides the closest detection point to the fiber transmission medium as physically possible. In contrast, sensing the transmission duration from any point other than the closest detection point to the laser diode itself is less desirable from the standpoint that the optical transmitter can be vulnerable to control mechanism failures which can take down the entire network due to a malfunction. 
     The detected current signal  303  is fed into conditioning circuit  318  which conditions the signal. The conditioning circuit  318  includes a transimpedance amplifier which converts the detected current into a proportional voltage that is then divided and buffered to provide a signal  305  having a more desirable operating range. The conditioned signal  305  is passed through half-wave rectifier  320  which provides a peak value of the incoming AC signal. The peak signal  307  is presented to the input of sample gate  322  which comprises a field effect transistor (FET) that conducts when transmit enable signal  247  is unasserted and does not conduct when the transmit enable signal is asserted. 
     When the sample gate transistor  322  conducts, the peak signal  307  is passed through as gate output signal  309 . The gate output signal  309  charges up RC time constant integrator  324  to remove any AC signal content and produce a DC peak signal voltage  311 . When the gate transistor  322  is disabled, after having been enabled, the integrator  324  is discharged and reset. While the gate transistor  322  is enabled, the laser diode  300  should always be off and not transmitting. However, if the laser diode  300  is active while the transmit enable signal  247  is unasserted, it is likely that there has been a fault in the laser driver  280  that controls the laser diode  300 . In that case, the laser diode  300  is taken off line as described below. 
     In the case of a fault condition with respect to the laser driver  280 , the filtering RC time constant of integrator  324  charges up to a predetermined threshold set at V REF2  input  313  of voltage comparator  326 . When the threshold is exceeded by a fixed amount (e.g., 20 mv), the output  315  of the voltage comparator  326  switches to a logic high state. This logic high state passes through AND gate  328  to the transmitter deactivation section  310  when the transmit enable signal  247  is not asserted. 
     The foregoing describes particular timer circuit embodiments for the fault monitoring sections  306 ,  308  that are readily implemented, component efficient and highly reliable. It should be noted, however, that many equivalent embodiments can be selected to provide the respective timer functions described. 
     The transmitter deactivation section  310  deactivates the laser diode  300 . The deactivation section  310  includes fault latch  330  and deactivation control block  332 . The fault latch  330  is a flip flop that is operated by either of two inputs  317 ,  325  corresponding to logic outputs from fault monitoring sections  308 ,  306  respectively. 
     The logic high state on either inputs  317 ,  325  from respective fault monitoring sections  308 ,  306  sets the fault latch  330  to a logic high output state. The output  327  of the fault latch  330  is connected to deactivation control block  332  which includes an enhancement mode FET transistor that conducts when a logic high input from the fault latch. The current conduction of the transistor draws current through the deactivation element  304  (e.g., a fuse) of optical transmitter  250  which operates (opens) in several milliseconds to disable the laser diode  300  and thereby prevent it from transmitting on the shared medium. 
     While the foregoing describes an embodiment of the deactivation mechanism which disables power to the laser diode  300  through a fuse element, it should be understood that in alternate embodiments the laser diode  300  can be deactivated by disabling transmit enable control to the device. 
     There can be several mechanisms for recovery from the aforementioned deactivation of a faulty optical transmitter. In one approach, recovery can be done by requiring manual intervention of a craftsperson to replace the faulty optical transmitter. In another approach, the deactivation element can be a thermally sensed element rather than a fused element as described above. In this case, a gross time out (on the order of seconds) of the thermally sensed element can be required to occur before the optical transmitter is operational again. 
     It should be apparent to those skilled in the art that the principles of the present protection mechanism can be applied to a shared multi-drop network in which the interconnect medium is copper based, for example, in a local area interconnect or backplane application. 
     The operation of the protection circuit  248  (FIG. 4) can be further understood with reference to the timing diagrams of FIGS. 5A and 5B. 
     FIG. 5A shows the timing diagram for operation of monitoring section  306  (FIG.  4 ). As noted above, fault monitoring section  306  monitors the amount of time that an individual terminal is enabled to transmit onto the shared medium. If the enabled transmission exceeds a particular time interval, the laser diode  300  for the individual terminal is disabled. 
     The transmit enable signal  247  normally is asserted for a duration D 1  that begins and ends in succeeding guard periods G and thus approximately equals the duration D 0  (FIG.  2 ). This waveform is designated  402  in FIG.  5 A. When the transmit enable is asserted, the reset transistor  312  (FIG. 4) stops conducting and a slowly increasing ramp waveform signal  321  is generated by the voltage ramp generator  314 . As long as the transmit enable signal  247  is asserted, the ramp continues to increase. At the end of the normal duration D 1 , when the transmit enable is no longer asserted, the ramp reset conducts which resets the ramp waveform designated  403 . However, if the transmit enable duration continues long enough, as shown for transmit enable waveform designated  404 , the ramp waveform signal designated  405  exceeds the set threshold reference V REF1  into comparator  316 . This latter duration is designated D 2 . In an embodiment, the parameters of the voltage ramp generator  314  and voltage reference V REF1  are selected such that the duration D 2  is 2.5 times normal duration D 1  or about 30 microseconds. Once the threshold is exceeded, the logic high output signal  325  sets the fault latch output  327  to a logic high state to cause the deactivation control signal  329  to deactivate the laser diode. 
     FIG. 5B shows the timing diagram for operation of monitoring section  308  (FIG.  4 ). As noted above, the fault monitoring section  308  detects whether the laser diode  300  is active for some period of time when it has not been enabled for such transmission. FIG. 5B shows the following signals: transmit enable signal  247 , detected current signal  303 , sample gate output signal  309  and integrator signal  311 . 
     While the gate transistor  322  is enabled, the laser diode  300  should always be off and not transmitting. This is shown in FIG. 5B wherein the sample gate output signal  309  tracks the normal duration D, of the transmit enable signal  247 . However, if the laser diode  300  is active while the transmit enable signal  247  is unasserted, there has been a fault in the laser driver  280  that controls the laser diode  300 . This is shown in FIG. 5B as detected current signal  303  rising at a time t 3  when the transmit enable signal  247  is unasserted. 
     In this case of a fault condition with respect to the laser driver  280 , the filtering RC time constant of integrator  324  charges up to a predetermined threshold set at voltage reference V REF2 . When the threshold is exceeded by a fixed amount (e.g., 20 mv), a logic high state is passed to set the fault latch. Fault latch output  327  at a logic high state causes the deactivation control signal  329  to deactivate the laser diode. The RC time constant is selected to have a duration D 3  that is 0.5 times normal duration D 1  or about 6 microseconds. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.