Control of optical systems

A method and control system for controlling a signal from each outstation in a multiple access passive optical network including a central node and a plurality of outstations involves monitoring, at the exchange, the value of a parameter of a signal from reach outstation. The value of the parameter is monitored relative to a predetermined discrimination value to detect the signal relative to predetermined first and second reference values defining a range of acceptable values for the parameter. If the value of the parameter for a detected signal falls outside the range, a control signal is transmitted from the node to instruct the relevant outstation to alter the value of the parameter as required to bring the value into the acceptable range for subsequent signals originating from that outstation. The method and system are particularly adapted for controlling the temporal position and the amplitude of outstation signals in a TDM passive optical network. A transceiver for use with such a system is also disclosed.

The present invention relates to a method of control of an optical system a 
signal control system and a transceiver for use in such a control system 
and in particular for use with multiple access optical networks. 
The use of a multiple access network allows for potentially highly 
efficient use of network resources. More expensive equipment for network 
management, for example, may be located at a central node, or exchange and 
its facilities shared by a large number of individual network outstations. 
In such circumstances the exchange may be connected via a single main line 
with branch feeder lines linking the network outstations to the main line. 
In an optical network the branching may conveniently be effected by 
passive optical splitters. It is therefore feasible to produce a multiple 
access optical network with no active components other than at the 
exchange and at the outstations themselves. The advantages of such a 
passive network include easier maintenance and reduced overall cost. 
In a multiple access network it is necessary to ensure that signals 
intended for, or originating from, a particular outstation are correctly 
identifiable. The technique of Wavelength Division Multiplexing (WDM) or 
Time Division Multiplexing (TDM), for example, enable this identification 
to be achieved. 
Where TDM is used, each outstation is allocated an individual time slot or 
channel specifically for reception or transmission of its own signals. 
When a composite TDM signal is broadcast from the exchange each outstation 
will receive the whole of that signal from which it selects and decodes 
its own allocated channel. However, in the reverse direction, each 
outstation contributes its own signal alone and the network itself must 
effectively assemble all the separately originating channels into a 
composite TDM signal for passing back to the exchange in the correct 
order. The outstations are generally irregularly spaced at different 
distances from the exchange. It is therefore essential to provide some 
method of ensuring that transmissions from each outstation occupy a 
designated time slot in the assembled return signal irrespective of the 
physical position of the outstation relative to the exchange, both at the 
time of connection to the network and at all subsequent times. 
To deal with this problem a ranging protocol may be employed. Ranging 
protocols are known for use in radio networks and will not be further 
discussed here. However, it has been found that provision of a 
conventional ranging protocol is not necessarily sufficient to ensure the 
accurate assembly of the return TDM signal in a passive optical network as 
described above. Localised variations can occur in the transmission path, 
for example, such that the words or bits in a signal originating from an 
outstation may not be exactly within the boundaries of the allocated time 
slot in the return TDM signal. The signal from one outstation may then 
overlap the signal from another to the detriment of the network 
efficiency. 
A further problem arises because the amplitude of the signals from each 
outstation depends on the power of the transmitter associated with the 
outstation and on the attenuation of the optical path from that 
outstation. Both these factors will vary from one outstation to another. 
Thus, unless compensating measures are taken, the composite return TDM 
signal may have signal levels which differ from one time slot to the next, 
according to whichever outstation each slot is allocated. Such a variable 
signal is difficult to demultiplex. 
It is an object of the present invention to provide a method and apparatus 
controlling the signal transmission from network outstations in a multiple 
access passive optical network such that the aforementioned problems are 
substantially overcome or at least mitigated. 
According to the present invention a method of controlling a signal from an 
outstation in a multiple access passive optical network including a 
central node and a plurality of outstations comprises, at the central 
node, monitoring the value of a parameter of the signal from the 
outstation relative to a predetermined discrimination value to detect the 
signal, further monitoring the value of the parameter of the signal from 
the outstation relative to predetermined first and second reference values 
defining a range of acceptable values for the parameter; and, if the value 
of said parameter for a detected signal falls outside the said range, 
transmitting a control signal to instruct the relevant outstation to alter 
the value of said parameter as required to bring said value into the 
acceptable range for subsequent signals originating from that outstation. 
In a time division multiplex (TDM) optical network, the parameter is 
preferably the temporal position of the signal from an outstation in the 
allocated TDM time slot for that outstation. The first and second 
reference values are preferably chosen to define a central region within 
the total duration of the time slot. 
Alternatively, or additionally in a second application of the method of the 
invention, the parameter may be the signal amplitude. In this instance, 
the discrimination value is a preset amplitude level. The first and second 
reference values are then preferably two further amplitude levels defining 
an acceptable amplitude range above the discrimination value. 
The method of the invention provides a convenient technique for automatic 
signal control in a multiple access system. The problems noted above are 
thereby avoided without recourse to time-consuming individual calibration 
of each outstation at connection. 
The method is particularly appropriate for use with TDM systems, but is 
also adaptable for use with wavelength division multiplex systems, for 
example, where the monitored parameter may conveniently be the individual 
signal wavelengths (or frequencies). 
In a multiple access passive optical network including a central node and a 
plurality of outstations, a signal control system for implementing the 
method of the invention comprises, in the central node, decision means for 
detecting the signal from an outstation and monitoring the value of a 
parameter of the signal relative to a predetermined discrimination value; 
first and second monitoring means for comparing the value of a parameter 
relative to predetermined first and second respective reference values 
defining a range of acceptable values for the parameter; control means for 
identifying the outstation sending the signal and, if the value of the 
parameter for a detected signal falls outside the said range, for 
transmitting a control signal specific to that outstation, the control 
signal carrying instructions to the outstation to alter the value of the 
parameter as required to bring the value into the acceptable range for 
subsequent signals originating from that outstation; and, in an 
outstation, decoding means for identifying a control signal specific to 
the outstation and signal control means responsive to the control signal 
for altering the value of the monitored parameter as instructed. 
Where the network is a TDM optical network, the parameter is preferably the 
temporal position of the signal as for the method. Preferably the 
monitoring means are adapted to monitor the signal position with respect 
to the time slot allocated to each particular signal. 
Alternatively or additionally, the control system may be adapted to monitor 
the signal amplitude. 
A control system according to the invention allows the problem of inter 
symbol interference (ISI) from spill-over of signals in adjacent time 
slots to be conveniently avoided. 
A control system according to the invention also provides a means for 
introducing a network-wide automatic gain control facility into the 
network. Amplitude variations within the received multiplex at the central 
node may thereby be minimised. 
In these circumstances, DC coupling in the central node receiver, which 
would otherwise be needed to maintain a given amplitude decision threshold 
(binary quantisation level) if the amplitudes were widely variable between 
signals, is not required. The performance of the exchange receiver may 
thereby be improved and the design simplified. 
A standard transmitting laser has been found to detect signals in the above 
described receive mode at an error free 2.048 Mbit/s over 10 km of 
single-mode optical fibre. A better performance is to be expected if the 
laser/diode combination were optimised for use as a transceiver. 
A control system according to the invention further permits the optical 
transmitter, usually a laser, in an outstation, to be operated at power 
levels which are effectively the minimum required for efficient operation 
of the network. Consequently, the reliability of the lasers will be 
increased to substantially the optimum possible within the network 
constraints. 
Additionally, the control system allows the condition of the outstation 
transmitter to be easily assessed. For example, the control system may be 
adapted to recognise the occurrence of a consistent trend in the 
alteration of a monitored parameter and give an early warning of a fault 
state before actual failure. 
According to another aspect of the present invention a transmitter/receiver 
(transceiver) for use with a signal control system according to the 
present invention includes an optical transmitter whose peak power output 
is controllable by the signal control means, the transmitter being also 
arranged to form at least part of an optical detector for detecting the 
control signals from the central node. 
Preferably the transceiver includes a laser diode having a back-facet 
photodiode, and a laser bias supply whereby the laser diode is forward 
biased to act as laser amplifier when not in transmitting mode. 
This has several advantages over systems using separate transmitters and 
receivers. The monitor diode supplied with commercially available laser 
transmitters which is redundant as a controller with the control system of 
the present invention when controlling the laser output is utilised in a 
manner which means the receiver package can be dispensed with. The need 
for a coupler of significant cost at the customer's end which was 
previously necessary to connect the receiver to the network is also 
removed. This reduces the costs of the customer service and reduces 
maintenance requirements.

In FIG. 1, a multiple access passive optical network 1, in this case part 
of a telephone system, consists of a central exchange 2 connected to 
outstations 5 (shown as square boxes). From the exchange 2 a main optical 
fibre link 3 is successively branched at passive optical splitters 6 
(shown circled) into branch optical fibre links 4 which ultimately link in 
to the individual outstations 5. 
For TDM operation of the network 1 it is necessary to provide a method for 
synchronising the signals originating from the outstations such that these 
signals can be passively assembled by the network into the correct 
sequence for return to the exchange 2 via the main link 3. Coarse 
synchronism is provided by use of a suitable ranging protocol (not further 
described here) such as is known in radio networks. 
Closer tolerances, as required in high bit-rate transmission, for example, 
may be obtained by using the method of the invention. A suitable 
adaptation of the technique is illustrated in FIG. 2. A signal received at 
the exchange is shown in the form of a conventional eye diagram. The 
central position for this signal which may be set by the ranging protocol, 
is indicated by T. For the normal purposes of information retrieval the 
signal is sampled at this instant. The signal is also monitored at two 
further sampling instants C and D at times t.sub.1 and t.sub.2 before and 
after T respectively. 
A binary code word is then constructed according to the signal level 
monitored at each of these instants. The code word is then used to 
determine what timing adjustment, if any, is required. An appropriate 
logic table is given as Table 1. 
TABLE 1 
______________________________________ 
Binary levels for time slot (outstation)N 
C T D Diagnosis Action 
______________________________________ 
0 0 0 Timing correct 
None required 
1 1 1 Timing correct 
None required 
0 1 1 Timing late Reduce outstation N delay 
1 0 0 Timing late Reduce outstation N-1 delay 
1 1 0 Timing early Increase outstation N delay 
0 0 1 Timing early Increase outstation N+1 delay 
0 1 0 Invalid Code 
1 0 1 Invalid Code 
______________________________________ 
The exchange control system then transmits an addressed control signal 
instructing the relevant outstation to increment or decrement the signal 
delay as may be required. Addressing techniques for control signals are 
well known and will not be treated in detail here. 
The method of the invention may be similarly adapted for regulating signal 
amplitude as shown in FIG. 3. 
In this case, the normal receiver decision threshold level, designated R, 
is used to distinguish between signal "ones" and "zeros". Two additional 
levels defining an acceptable amplitude range are also monitored. These 
levels' designated A and B, are monitored to detect when the signal level 
falls below an acceptable minimum or exceeds an acceptable maximum 
respectively. These levels are conveniently set according to the 
signal-to-noise ratio of the weakest outstation signal expected. Both 
levels are set about the discrimination threshold R to allow a safety 
margin between that threshold and the lower monitoring level A in order to 
reduce the risk that a signal will ever fall below the threshold and be 
entirely undetected. As previously, the signal level monitored in terms of 
the R, A, B levels is translated into a 3 bit binary code word which can 
be used to determine the required amplitude adjustment according to Table 
2. 
TABLE 2 
______________________________________ 
Binary levels 
R A B Diagnosis Action 
______________________________________ 
0 0 0 "Zero received None 
1 1 0 "One" received None 
Amplitude acceptable 
1 1 1 "One" received Instruct outstation N 
Amplitude excessive 
to decrease 
transmitter power 
1 0 0 "One" received - 
Instruct outstation N 
Amplitude too low 
to increase 
transmitter power 
0 0 1 Invalid code 
0 1 0 Invalid code 
0 1 1 Invalid code 
1 0 1 Invalid code 
______________________________________ 
As described above the method for position control (FIG. 2) and for 
amplitude regulation (FIG. 3) may be implemented separately. However, the 
signal detection at T in the time slot and the amplitude measurement can 
be viewed as interdependent. Detection of a signal at T, for example 
depends on the signal amplitude exceeding the threshold R at that time. 
Furthermore, whereas it is possible to monitor the amplitude throughout 
the duration of the time slot and irrespective of whether or not the 
threshold R is exceeded at T, as a desirable alternative the amplitude 
measurement may itself be made dependent on the detection of a signal at 
time T. This latter strategy can avoid mistaken measurement of the 
amplitude of a signal overlapping from another time slot, for example for 
the purposes of analysis using Tables 1 and 2 the logical states for R and 
T are then identical for any given signal and may be determined together 
from a single measurement. The remaining variables A, B, C, D are 
determined individually as before. 
A control system for a TDM multiple access passive optical network and 
combining position and amplitude control by the method of the invention is 
shown schematically in FIG. 4. For ease of illustration only the relevant 
components only of the exchange 2 and one outstation 5, connected by the 
passive network 3, 4, 6, are indicated in the Figure. 
In the outstation 5, date from a data source 56 passes for return signal 
transmission to the exchange 2 by a laser 51 under control of a modulator 
52. The return signal transmission is timed by a clock control which is is 
initially set by a ranging protocol as mentioned previously. A variable 
delay 55 allows fine-tuning of the timing according to the method of the 
present invention. 
The return signal amplitude is adjusted using an amplitude control register 
58 and digitial-to-analog converter 59 to control the drive current 
supplied by the modulator 52 to the laser 51. 
In the exchange 2 the return signal arrives at the TDM receiver 21, which 
passes the signal, in its appropriate time slot, via five D-type 
flip-flops 22, 23, 24, 26, 27 which perform the signal position and 
amplitude monitoring functions as detailed below. 
To monitor the signal position relative to the time slot the signal from 
the receiver 21 gated through the three D-types 22, 23, 24 is at the 
relevant times (cf. FIG. 2). Thus D-type 22 monitors the centre of the 
acceptable period within the time slot as defined by the gating of D-types 
23 (for C) and 24 (for D). According to the states of these position 
D-types 22, 23, 24 the time slot combinatorial logic 25 then provides the 
appropriate code work (cf. Table 1) to the control bus 31. 
The D-type 22 which monitors the central position of the signal within the 
time slot is set to trigger at the minimum signal discrimination threshold 
R. The state of this D-type therefore is also used to provide the base 
detection reference for the amplitude measurement. 
The acceptable amplitude range (cf. FIG. 3) is determined by the two 
further D-types 26, (for A) and 27 (for B). The signal is proportionately 
attenuated by the level adjusters 28 and 29 before being applied to the 
D-types 26 and 27 respectively. Consequently these D-types will only be 
triggered by a signal whose amplitude exceeds the present higher levels A, 
B above the basic discrimination threshold R. The amplitude combinatorial 
logic 30 monitors the state of the relevant D-types 22, 26, 27 and 
provides the appropriate amplitude code word (cf. Table 2) to the control 
bus 31. 
The code words for both position and amplitude are passed by the bus to the 
control processor 32 with associated memory 33. The processor 32 analyses 
the code words to determine the required action and generates a 
corresponding control signal which is then specifically addressed to the 
relevant outstations and transmitted accordingly. 
In the outstation 5, a local controller 53 is provided with a telemetry 
decoder to check the addressing of any control signals arriving from the 
network and to identify and decode those intended for the particular 
outstation. 
If the instructions in the control signal are to alter the timing of signal 
transmission, for example, the controller 53, via the local bus 60, 
increments or decrements the delay latch register 54 to vary the variable 
delay 55 as appropriate. 
If the instructions are to alter the signal amplitude, the controller 53, 
again via the local bus 60, increments or decrements the amplitude latch 
register 58. The count in the amplitude register 58 is then converted via 
the D/A converter 59 in order to control the drive current supplied to the 
laser 51 by the modulator 52. 
Although the method and control system of the present invention have been 
specifically described with reference to a TDM passive optical network, it 
will be appreciated that the application of the invention is not 
restricted. The principles of the invention may equally be applied, for 
example, to a WDM network. 
Referring now to FIG. 5, there is shown the optical link and subscriber 
sections of the network of FIG. 4 in which the laser 51 is arranged to 
operate as both a transmitter and a receiver. 
A bias current supply 62 supplies a bias current to the laser 51 sufficient 
to forward bias it below the lasing threshold to act as a laser amplifier 
when no data signal is supplied by the modulator 52. Optical signals 
arriving from the network 3,4, 6 are amplified as they pass through the 
laser diode 51 and fall on the laser diode's back facet photodiode 63 
which acts as an optical detector. The output from the photodiode 63 is 
applied to the amplifier 64. A switch 65 is arranged so that the amplifier 
64 is connected to the controller-telemetry decoder 53 only when no signal 
is being output from the variable delay 55, i.e. when the subscriber is in 
receive mode. The decoder 53 acts on received signals as it does in the 
arrangement of FIG. 4. 
The laser 51 operates in transmit mode as described above with reference to 
FIG. 4. The current from the modulator 52 will be less than before because 
of the effect of the bias current supply but will be controlled in the 
same manner to fix the laser output. 
The laser 51 in the transceiver of FIG. 5 is permanently forward biased in 
receive mode and so requires only a change in bias current amplitude to 
take it above threshold to cause it to lase and enter the transmit mode. 
Because the laser 51 operates in a low duty cycle mode and is always 
forward biased, recovery time problems are minimised when changing between 
the receive and transmit modes of laser operation. The laser transmit mode 
is only required for one time slot out of many and one can afford to lose 
a timeslot each side of that time slot as the laser transfers to receive 
mode without affecting the received signal. 
If it is found not necessary to gate out the receiver 72 during the 
transmit bursts in a particular network the switch 82 may be omitted and 
the receiver 64 permanently connected to the decoder 53. 
Other arrangements in which the transmitter is also used as a receiver are 
possible. Referring to FIG. 6 the laser diode 51 as unbiased in receive 
mode with the back-facet photodiode being used as in FIG. 5 to detect 
unamplified signals from the network. The laser diode 51 is considered a 
passive part of the optical detector guiding light to the photodiode. 
In yet another arrangement the laser diode 51 itself may be connected to 
the amplifier 64 so it acts as detector itself. In such an arrangement 
other opto-electronic devices may be used to serve as both transmitters 
and receivers, for example light emitting diodes.