Abstract: A signal transmission system has a number of wave division multiplex optical carriers linking a number of switching units arranged in a closed ring network. The carriers are amplified to compensate for loss and attenuation, but the ring gain of each carrier must be less than unity to prevent unwanted signal oscillation. An automatic gain control arrangement is operative for each optical carrier which is detected as being present at a switching unit, and the gain control is disabled for each carrier which is detected as being absent.

Abstract: A receiver in a digital communications system, in which the digital data occupy a number of levels, equalises the erroneous-count rates for the various data levels by deriving the count rates for the levels, comparing these count rates with each other and using the comparison result to adjust the threshold level which is used to detect the received data train. The error-counts are preferably derived as a byproduct of a Forward Error Correction system already available for performing normal error correction on the received data. The receiver arrangement is envisaged to find predominant application in a two-level system involving logical “1”'s and “0”'s, though it is applicable also to systems with three or more levels.

Abstract: A method for upgrading an-optical communications system in which the system comprises a plurality of nodes linked by one or more optical paths. Each path for the communication of traffic includes a single optical channel. Each node comprises add/drop multiplexers (ADM) for adding and dropping signals in electrical form, and conversion circuitry for converting between electrical and optical form signals output by the ADM for transmission via a first one of the optical paths, and for converting signals received via a second one of the optical paths for input to the ADM.

Abstract: Following a fault in a bidirectional optical communication system, the lasers which generate traffic-carrying channels and a supervisory channel are shut down to permit personnel to effect repairs safely. The low power supervisory channel is pulsed on at intervals to determine if the fault is still present, but if a receiver is able correctly to restore frame alignment from such a pulse, the reverse direction supervisory channel is enabled, and following frame alignment in both directions, the main lasers are safely turned on.

Abstract: In an Asynchronous Transfer Mode telecommunications network having a plurality of virtual paths or circuits, an encryption key used for data transmitted between a source and a receiver may be updated, the data being transmitted initially using a first encryption key by a first of the plurality of virtual paths or circuits, by a connection being estabished by a second of the plurality of virtual paths or circuits and sending a second encryption key by that connection from the source to the receiver and subsequently transmitting the data using the second encryption key by the second of the plurality of virtual paths or circuits.

Abstract: In a digital telecommunications system having data packages carrying identification data, the identification data is characterized in that it is unique to the package within a station and does not depend on the identity of the originating station.

Abstract: The present invention provides a wavelength selective optical filter device (240) for receiving input radiation and outputting corresponding filtered output radiation, characterised in that the filter device (240) includes a plurality of at least partially mutually coupled Fabry-Perot optical resonators (330, 340, 360; 360, 400, 430) for filtering the input radiation to generate the output radiation, the filter device (240) being tunable from a first radiation wavelength to a second radiation wavelength by mutually detuning the resonators in a period where the resonators are being returned from the first wavelength (&lgr;1) to the second wavelength (&lgr;2) so that the filter device (240) is substantially in a non-responsive state during the period. The resonators incorporate freely suspended mirrors (360, 430) which are electrostatically actuated to affect tuning of the resonators (330, 340, 360; 360, 400, 430).

Abstract: An improved electromagnetic seal is provided between body parts of an enclosure by providing mating surfaces of the body parts with projections to improve electrical contact between the body parts. The invention may be applied to various forms of mating surfaces, including complementary tongue and groove or stepped surfaces. Similar projections may also be introduced into slots for mounting of panels in order to improve electrical contact between the body parts and the panels.

Abstract: A depacketizer is used in terminating an ATM multiplexed data stream at a functional unit, virtual channel data being carried in cells in the data stream. The depacketizer has a plurality of cell buffers each storing the data contained in a single cell and being loaded in the sequence of a message sequence number carried by each cell and emptied in accordance with a depacketizer algorithm to form a data stream. A fill-in cell format generator causes, on the failure of a valid cell to arrive, the replacement of the missing cell by a fill-in cell in the data stream. A time-out function generator is started after each cell has been depacketized and sometimes after fill-in replacement cells have been generated. A cell frame aligner has the depacketizer with a frame alignment signal detector which detects a frame alignment signal in the non-continuous data stream and generates an associated frame start signal and a loss of frame alignment signal as appropriate.

Abstract: A higher data rate signal such as ATM (Asinchronous Transfer Mode) fixed size data packets and lower data rate signals such as ISDN are transmitted along the same link by accommodating the ISDN in unused time slots in the overall frame structure. The cells also contain a fiel containing incremented numbers of modulo-n and, in the reverse direction along the link, frames are transmitted which acknowledge receipt of those sequence numbers. If the next expected number is not received, the sequence transmitted is restarted at the number.

Abstract: In a Synchronous Digital Hierarchy (SDH) based communications network comprising a plurality of Intermediate Systems (IS), the IS being divided between at least one IS—IS Area and at least one non-IS—IS Area, an IS—IS Area being an area with which a routeing protocol forming part of the Network Layer (Layer 3) of the Open Systems Interconnection including routeing (OSI), is provided for routeing messages between areas, whereby where a message is routed from an IS—IS Area to a destination IS within a non-IS—IS Area and the connection to the destination IS is broken, and as a result a message is returned from the non-IS—IS Area to the originating IS—IS Area connection to the destination IS is made by a second choice connection.

Abstract: An optical network operating with burst mode data traffic signal comprises a recirculating optical loop memory which includes taking a sample of a burst mode data traffic signal and storing the sample within the recirculating optical loop memory. The network may further comprise a scanning based wavelength measurement system wherein the wavelength of the sample may be determined.

Abstract: A system for individually controlling the power levels of a plurality of wavelength division multiplexed (WDM) optical signals in a single optical guide comprising a first plurality of individually controllable optical attenuators. An example of a suitable optical attenuator is a tunable fiber Bragg grating filter whose reflectivity characteristic varies with wavelength over an active region thereof. By tuning the filter, it is possible to control the reflectivity at a particular wavelength in the active region. The system may, in addition, comprise a second plurality of optical attenuators arranged so as to compensate for any power level distortion introduced by the first plurality. Optical circulators may be used to route signals into and between the pluralities of optical attenuators.

Abstract: Optical switching arrangement for switching a number of optical signals, consists of a thermo-optic switch having a number of heaters which are heated by passing electrical current through them. The mark-space ratio of current supplied from a constant voltage source is varied to provide constant power to the heaters to compensate for resistance changes in the heaters.

Abstract: In a Synchronous Digital Hierarchy (SDH) based communications network comprising a plurality of Intermediate Systems (IS), the IS being divided between at least one IS—IS Area and at least one non-IS—IS Area, an IS—IS Area being an area within which a routeing protocol forming part of the Network Layer (Layer 3) of the Open Systems Interconnection including routeing (OSI), is provided for routeing messages between areas, a method is provided wherein static routes (Manual Adjacencies (MA) are created at IS within the IS—IS Area to point to routes to a group of one or more Network Equipments (NEs) within the non-IS—IS Area and where a failure occurs in a link to or within a group and messages from the IS—IS Area to the non-IS—IS Area are looped to the originating IS—IS Area, identification of the NEs from which messages have been looped are removed from the respective MAs allowing routeing of messages via alternative MAs.

Abstract: An optical communications network comprising a Head End Unit connected to a plurality of groups of Network Termination Equipments (NTE) and/or Optical Network Units (ONU), wherein downstream communication is by Time Division Multiplex, wherein a group is multiplexed onto a stream and a plurality of streams are combined into a higher speed multiplex and upstream communication is by Time Division Multiplex Access is characterized by enabling the NTE's or ONU's to negotiate bandwidth in respect of low level upstream traffic and by pre-allocating bandwidth to individual NTE's or ONU's in respect of high level upstream traffic.

Abstract: A retiming arrangement for use in a demultiplexer in an SDH data transmission system uses Bit Justification data, and not Pointer data, to modify a recovered clock signal and generate a clock signal for retiming purposes. The invention is especially for use in enabling third party users to carry primary rate timing data across an SDH network.

Abstract: Where the same data is stored for an extended period of time, the possibility exists of unwanted burning in of the data which would allow undesired recovery of erased data even after it has been over-written or the memory has been powered down. The memory contents are therefore periodically rewritten in the inverse of their former state, either to the same location in which they were previously stored, or elsewhere. Inversion may be performed in an invert cycle during which address decoders driven by a high speed oscillator read each cell and write back its contents inverted. On completion of a cycle, a latch may indicate whether the stored data is in a true or complement form. The latch may control programmable invertors so that cells are always read or written correctly, regardless of whether data happens to be in true or complement form.

Abstract: A multiplexing/demultiplexing method comprises, during multiplexing, taking one bit from a time slot of each of a plurality of information data signals to form an information data sequence. The information data sequence is combined with a fixed binary code and, during demultiplexing the combined signal is analyzed in fixed bit length blocks to derive the information data. The code is selected such that the combined data signal has, for each possible information data sequence, a set of unique values irrespective of the starting point of the combined signal within the block. In this way there is no need to incorporate any alignment procedures during demultiplexing.