Abstract: A telephone instrument comprises a mechanism for providing signals for transmission, a mechanism for converting signals received by the instrument into output signals, a switching circuit for selectively establishing and terminating a connection between the instrument and a circuit-switched telephone network and for selectively establishing and terminating a connection between the instrument and a packet-switched communication network. When calling, the instrument signals over the circuit-switched network for a network address for the called instrument and the packet-switched network and establishes a dual connection over the packet-switched network if such a network address is received. When receiving a call, the instrument can terminate the call connection over the circuit-switched network and continue the call by way of the packet-switched network.

Abstract: A decision feedback encoder comprising an analog-to-digital converter for converting an input signal into digital signals representing decision levels, digital processing circuits responsive to signals from the converter for providing a succession of symbol values and a set of coefficients, digital-to-analog converters for converting the coefficients into corresponding analog values and analog circuits for forming a sum of the products of each of the symbol values and a respective one of said analog values to provide a feedback signal for the reduction of inter-symbol interference in the input signal.

Abstract: A plurality of circuit cards include at least a first card having a first internal power rail, an earth connection, respective operating circuits powered from the first power rail, external terminals coupled to its operating circuit, and a first external connection. A second of the plural cards has a second internal power rail, an earth connection, respective operating circuits powered from the second power rail, external terminals coupled to its operating circuit, and a second external connector. The first card includes a bipolar transistor with an emitter-base junction forwardly biased in response to normal voltage between the first power rail and the earth connection and a collector connected to the first external connector. The second card includes a resistive connection between its second internal power rail and the second external connector and circuits responsive to changes in voltage level at the second connector.

Abstract: A data network hub unit is stackable with other such units to constitute a hub in which each of the units can supply data packets to network data destinations and receive data packets from network data sources, and to form a closed data transmission ring which enables packets received at any unit to be transmitted from any other unit. The hub unit includes arbitration control means responsive to packet headers circulating on said ring.

Abstract: A method of processing data contained in data packets comprises storing a set of microcode instructions of which some are test instructions prescribing a respective test between a data pattern in a packet and a test pattern; defining by means of bit masks a respective multiplicity of programs each consisting a group of instructions selected from the said set of instructions; providing a particular one of said bit masks; reading out from storage the selected group of instructions in the program defined by said particular one of said bit masks, said selected group including at least one of said test instructions; and executing the instructions in the selected group on data packets. Matches between data patterns in said packets and a test pattern defined by said at least one test instruction may be detected.

Abstract: A communication system comprises a plurality of interfaces disposed for sending data packets by way of virtual circuit connections and for receiving data packets by way of virtual circuit connections in an asynchronous transfer mode. The interfaces by means of the virtual circuit connections emulate at least one local area network including unit for establishing a virtual circuit connection functioning as a BUS (broadcast and unknown server) for the broadcast of data packets throughout the emulated local area network. An asynchronous transfer mode switch receives packets by way of virtual circuit connections from each of said the interfaces, replicates the packets and sends them to all the other interfaces, thus to establish a mesh between said interfaces. At least one interface comprises a unit for mapping virtual circuit connections between one interface and at least one terminal which is not directly connectable in the mesh connection into virtual circuit connections in the mesh.

Abstract: A plurality of serial data streams are transmitted on a corresponding plurality of lines at a common frequency in equal groups of symbols. A framing signal composed of groups of symbols corresponding in number to groups of data symbols is transmitted on an additional control line. Each group of symbols in the framing signal includes a majority of symbols capable of representing a first plurality of code words and a second plurality, substantially less than the first plurality, of valid code words, and a minority of symbols which constitute parity check symbols. Each of the valid code words consists of a first sub-group of similar symbols and a second plurality of similar symbols. For some of the valid code words the symbols in the first sub-group are similar to the symbols in the second sub-group. For other valid code words the symbols in the first sub-group are different from the symbols in the second sub-group.

Abstract: A computer network operating in accordance with a management protocol such as SNMP modifies the protocol such that a request communication from a management agent can be a type specifying multiplicity of data items and has a first part which identifies the type of the request.

Abstract: In order to enable utilisation of half-bridges in networks in which a spanning tree algorithm is implemented, the priorities of the half-bridges are set such that generally the half-bridges select which of their ports should be blocked to break any communication loops which arise in preference to full bridges making the selection which may result in communication links via the backbone ports of the half-bridges being disabled. Further, a half-bridge is proposed in which more than one of its ports are available for configuring as the backbone port.

Abstract: A system is provided that protects a network from storms of multicast/broadcast data. The system includes a switch or bridge which monitors traffic through it in order to detect the onset of a storm condition from previous receptions of requests for multicasts or broadcasts. Each port or bridge or switch has associated with it one data bit which can be controlled to prevent a storm of multicasts/broadcast data being forwarded to all parts of the switch and jamming the system.

Abstract: A large FIFO memory device has its total available memory capacity partitioned into memory sections. The partitions are in the form of programmable delimiters in order to determine flexibly the size of the memory sections.

Abstract: A method of testing data packets uses a program of selected microcode instructions of which at least one prescribes a comparison between a series of data patterns and a series of test patterns. Data patterns and test patterns are stored in dynamic random access memory. A first group of one of said series of patterns is read out and stored in temporary storage locations. A first group of the other of said series of patterns is read out and stored in temporary storage locations. Each of the patterns in the first group of the other series of patterns is compared, as it is stored, with a respective one of the stored patterns of the first series. The invention renders the testing of data packets more efficient.

Abstract: A method of transmitting over virtual channels in an asynchronous transfer mode data received in the form of data packets which are assembled in addressable buffers. A single distribution and recovery queue is maintained in conjunction with a start distribution pointer, a finish distribution pointer and a recovery pointer which is initially set to the same place as the finish distribution pointer, free buffers being entered in the queue between the start and finish distribution pointers.

Abstract: A method for operating a multi-segment repeater hub wherein at least some of various signal ports connecting signaling stations to the hub can be connected selectively to different segments of the hub, comprising: monitoring signal traffic between the stations and the hub and thereby establishing a database which indicates for each of a plurality of ports the volume of signal traffic between the respective port and each other port in said plurality and any segment through which any of said signal traffic flows and to which a port other than a port in said plurality is connected, computing the traffic to and from the ports in said plurality and said segments and the traffic between the segments for each of a plurality of possible configurations in each of which a selected port is notionally moved from connection to one segment to connection with another segment; computing according to selected criteria a preferred set of connections to the ports to the segments, and switching the connections of the ports and s

Abstract: A repeater for a computer network, and a network incorporating such a repeater are described in which repeater communication addressed to a subset of the devices connected to the repeater are transmitted in a corrupt or scrambled form to other network devices. The processing load is thus reduced for such other network devices, in particular in the context of high volume multicast traffic such as in video conferencing.

Abstract: An alarm or monitoring system for a computer network in which network devices are able to be “locked” onto the network, in which condition an alarm is raised if the device is removed. This occurs even when the device is switched off, as the monitoring of the presence of the device is performed by the network. Alternatively, the device may be “unlocked” from the network, in which condition no alarm is raised even if the device is removed. Control of whether a particular network device is subject to the alarm system is therefore in the hands of the user of the device and this is particularly useful for items such as lap-top computers which may quite legitimately be regularly connected to and disconnected from the network.

Abstract: A system for converting data in one clock domain to a second clock domain comprises a multiplexer which has a select control which is synchronous with a first frequency and is coupled to two bistable registers of which a first is clock controlled in accordance with the first frequency and a second is clock controlled by the second frequency. The data output of the first register is looped back to the second data input of the multiplexer. The select signal operates to couple a data input to the first register whereupon the multiplexer then serves to couple the data output of the first register back to the data input thereof. The arrangement ensures availability of data at the first clock frequency beyond a transition of the second clock frequency. Thus data, preferably multi-bit address data, can be transferred from one clock domain to another with less delay than in customary systems.

Abstract: An address look-up data-base is used to store medial access control addresses for resolution into channel numbers in a system for transmitting data in an asynchronous transfer mode. Static random access memory is used to contain pointer tables, there being a pointer table for each possible priority level in the data packet. Thus a search for the media access control address of a data packet in the content addressable memory yields, among other data, a priority value which when combined with other data points to a location in one of the pointer tables, which accesses the appropriate channel number.

Abstract: Data received in the form of data packets which include destination addresses is transmitted over virtual channels in an asynchronous transfer mode to emulate the operation of a plurality of emulated local area networks. A plurality of bus channels are provided, each specified for the broadcast transmission of data packets to members of a respective one of the emulated local area networks. An address look-up data-base is used to receive the destination address of a data packet and data defining the respective bus channel for the broadcast transmission of that packet in the respective emulated local area network. The data packet is then transmitted over the respective bus channel. The content addressable memory provides a data field which indicates whether a request for the resolution of the destination address into a specific virtual channel number has been made.

Abstract: The invention concerns in-line monitoring of a communication link between two stations in a frame-based communication network wherein the said two stations employ a compression algorithm for the transmission of frames and a corresponding decompression algorithm for the reception and decompression of the frames, the said algorithms requiring the maintenance at a transmitting station of a compression history in terms of the number of frames transmitted since a datum point and the maintenance of a corresponding compression history at a receiving station in terms of the number of frames received, the frames each including an identification of the compression sequence so that the receiving station can detect mismatch between the compression sequence and the receiving sequence.