GB1560457A - Optical fibre communication systems - Google Patents

Description

(54) OPTICAL FIBRE COMMUNICATION SYSTEMS (7il) We, STANDARD TELEPHONES AND CABLES LIMITED, a British Company of 190 Strand, London, W.C.2, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to improvements in or relating to optical fibre telecommunication systems.

For a wide range of applications a time division multiplexing (TDM) multi-terminal system has features of being highly immune to problems of optical loss and multipath effects in optical highway networks having redundant paths, and avoiding the need for any master terminal. Such systems provide electrical isolation, wide band width and immunity to cross talk and radio frequency interference. However the terminals must be specially designed to work with redundant path configurations to make the system more tolerant to limited fault conditions both in the optical fibre harness and in one or some of the terminals. In a redundant path network each terminal receives the same optical signals (assuming both paths to be in satisfactory working condition) at substantially the same time. Only when one path becomes faulty is it necessary to choose between the paths.

According to the present invention there is provided a terminal for use with an optical fibre telecommunication system having an optical highway network with at least two paths over which the same signals are transmitted to the terminal which includes a separate optical receiver means for each path, comparator means having separate inputs each coupled to the output of a respective receiver means, and switching means for connecting one or other of the receiver means outputs to a signal processing means, the switching means being operated in response to a control signal generated by the comparator means which is arranged to compare the received signals according to a predetermined characteristic.

In one embodiment of the invention the the paths and the switching means is opercomparator means is arranged to compare the relative power of the received signals on ated to connect the receiver output with the highest power to the signal processing means.

Preferably the terminal includes means for inhibiting the operation of the switching means during the reception of a burst of signals or a frame of signals in a time division multiplexed system.

Another feature of the invention relates to the control of transmission times of a terminal in a multiterminal time division multiplexed ring-main system, such as that described in Specification No. 1 534786. In such a system each terminal is allocated a separate transmission slot in a frame. Each burst of signals includes the address of the, terminal for which those signals are destined and the address of the transmitting terminal.

The terminal compares the incoming signals with its own address held in a store. When the incoming signals- contain the terminal's address the data following the address is accepted. The arrival time of the transmitting terminal's address is used to synchronise a divider chain which provides various clocks required in the terminal, one of which controls transmission from the terminal. Normally the circuitry is arranged so that the terminal anticipates the time of arrival of the received address and if this time varies a resynchronisation operation is performed, typically by resetting of an internal counter which counts a predetermined number. of time intervals between transmission slots. If all the terminals have this arrangement it is apparent that one faulty terminal could cause all the other terminals to start resynchronising.Since this could lead to a chain reaction in a ring main system it is desirable to include some means of preventing the occurrence of a single faulty address timing from affecting the other terminals.

According to the invention therefore the terminal may also include an up-down counter, means for incrementing the counter when the terminal's address is received during a predetermined period of time, means for decrementing the counter when the address is received outside the predetermined period of time, means for inhibiting the transmission of signals from the terminal during the transmission period for the terminai il .t.e counter contains a negative count, means for resetting the counter to zero when the transmission period of the terminal is finished, and means for resynchronising the terminal to the received address if that address is received when the transmitter is inhibited.

The above and other features of the invention will nonv be described in detail with reference to the accompanying drawings, in which : - 'Fig. 1 illustrates an arrangement for selectively oonnecting one of a number of incoming optical fibre paths to a signal processing means, and Fig. 2 illustrates an arrangement for negat ing he effect of a single faulty address timing in a multi-terminal time division multiplexed ring main system.

In the arrangement shown in Fig. 1 two optical fibres la, lb are terminated in con- ventional receiver units 2a, 2b respectively.

The two fibres are assumed to provide redundant paths in a network and both carry the same optical signals from a remote transmitter (not shown). The receiver outputs are led via coupling units 3a, 3b to a selector switch 4 where one or other of the outputs is connected to subsequent signal processing circuitry. The coupling units 3a, 3b each extract the same proportion of the respective receiver output power and feed this to a comparator 5. in the simplest case the comparator 5 compares the powers of the received signals and produces an output signal to operate switch 4 to connect the most powerful receiver output ;to the signal processing circuitry.To ensure that this switch ing operation is not carried out in the middle of a signal burst the comparator is also fed with a frame synchronising signal and its output is only enabled at a suitable time in the frame cycle. A memory circuit can be used to preselect one receiver based on the decision made some period b the previous frame.

As an alternative to comparing the power of the received signals they can be compared for quality. If the signals are encoded in an error detecting code, or if they include parity checking information, .then the signal rom one receiver only is normally passed on to the signal processing circuitry where error detection procedures are carried out In the event that errors are detected a changeover signal is sent aback to the switch 4 at a suitable time, as indicated by the dashed line 6, to effect changeover to the other receiver output. A combination of both power comparison and error detection procedures can be used provided one, i.e.

the error signal, is normally dominant and is only capable of being over-ridden in the vent d serious degradation in one of the receiver outputs.

To maintain synclzronisalion in the system the arrangement shown in Fig. 2 can he used. As described in Specification No.

1 534786 every terminal would reset its internal counter timer .to agree ;with every transmission address received. The di sad- vantage of this scheme has Ibeen outlined above. In the arrangement shown in Fig. 2 a signal, Fig. 2a, is generated Qby circuitry not shown) which has pulses coinciding with withe exepected receipt of the transmission address. This pulsed signal is conveniently obtained by gating of the timing counter.

The pulses in this signal must be long enough to allow for slight variations due to propagation delays in the time of the received address, i.e. ithey must provide a "window". These pulses are fed to an up 1 down counter 20. Meanwhile a second pulse train, Fig. 2b, is generated to indicate when an address is actually received. These pulses are narrower than those of the pulses in Fig. 2a and are also fed to counter 20. If an address is received during the pulse window of Fig. 2a counter 20 is incremented, and a small change made to the terminal divider chain to during it into step. If an address is received outside the window period then the counter is decremented.

When the terminal determines that its next transmission period is due the count in counter 20 is taken into consideration. If the counter 20 contains a positive or zero count then it is reset to zero and transmission from the terminal takes place. If it contains a negative count, showing that the majority iof the previously received addresses were incorreotly timed, the terminal's .transmitter is disabled for one frame, a diag is set, and the counter reset to zero. If the next address from the next transmitting terminal is received within the window period then the counter 20 is incremented. If however the next address is also incotectly timed there is another negative count in the counter 20, and coupled with the previously set flag this results in the terminals internal timing being reset. This continues until the next period when transmission is due. The flag is cancelled and the counter 20 is reset at the same time. Tije assumption is that an isolated incorrectly received address is not necessarily indicative of the need for the terminal to retime itself, and so major timing corrections are only made if the ring is set-i.e. timing has been found to disagree with more terminals than it agrees with during one frame.

WHAT WE CLAIM LS:- 1. A terminal for use with an optical fibre telecommunication system having an optical highway network with at least two paths over which the same signals are transmitted to the terminal which includes a separate optical receiver means for each path, comparator means having separate inputs each coupled Ito the output of a respective receiver means, and switching means for connecting one or other of the receiver means outputs to a signal pro.

cessing means, the switching means being orated in response to a control signal generated by the comparator means which is arranged to compare the received signals according to a predetermined characteristic, 2. A terminal according to claim 1 wherein the comparator means is arranged to compare ;the relative power of the received signals on the paths and the switch- ing means is operated to connect the receiver output with the highest power to the signal processing means.

3. A terminal according to claim 1 or 2 which further includes means for inhibiting the operation of the switching means during the reception of a burst of signals or a frame of signals in a time division multiplexed system.

4. A terminal according to claim 1, 2 or 3 which includes means for detecting errors in the received signals in each path and means for generating control signals to operate the switching means to effect change.

over from one receiver output to another receiver output when the one receiver out.

put signals, if already connected, contain errors which are detected by the error detecting means.

5. A terminal according to claim 1 which also includes an uptdown counter, means for incrementing the counter when the address is received outside the predetermined period of time, means for inhibiting the transmission of signals from the terminal during the transmission period for the terminal if the counter contains a negative coun.t, means for resetting the counter to zero when the transmission period of the terminal is finished, and means for resynchronising the terminal to the received address if that ad- dress is received when the transmitter is inhibited.

6. A terminal for use with an optical fibre substantially as described with refer- ence to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. is set-i.e. timing has been found to disagree with more terminals than it agrees with during one frame. WHAT WE CLAIM LS:-

1. A terminal for use with an optical fibre telecommunication system having an optical highway network with at least two paths over which the same signals are transmitted to the terminal which includes a separate optical receiver means for each path, comparator means having separate inputs each coupled Ito the output of a respective receiver means, and switching means for connecting one or other of the receiver means outputs to a signal pro.

cessing means, the switching means being orated in response to a control signal generated by the comparator means which is arranged to compare the received signals according to a predetermined characteristic,

2. A terminal according to claim 1 wherein the comparator means is arranged to compare ;the relative power of the received signals on the paths and the switch- ing means is operated to connect the receiver output with the highest power to the signal processing means.

3. A terminal according to claim 1 or 2 which further includes means for inhibiting the operation of the switching means during the reception of a burst of signals or a frame of signals in a time division multiplexed system.

4. A terminal according to claim 1, 2 or 3 which includes means for detecting errors in the received signals in each path and means for generating control signals to operate the switching means to effect change.

over from one receiver output to another receiver output when the one receiver out.

put signals, if already connected, contain errors which are detected by the error detecting means.

5. A terminal according to claim 1 which also includes an uptdown counter, means for incrementing the counter when the address is received outside the predetermined period of time, means for inhibiting the transmission of signals from the terminal during the transmission period for the terminal if the counter contains a negative coun.t, means for resetting the counter to zero when the transmission period of the terminal is finished, and means for resynchronising the terminal to the received address if that ad- dress is received when the transmitter is inhibited.

6. A terminal for use with an optical fibre substantially as described with refer- ence to the accompanying drawings.