3com Corporation

3com Technologies

Dipak M Soni

Anne G O'Connell

Peter A Saunderson

Vincent Gavin

Martin C Adams

Donal Casey

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.

Method for selecting virtual channels based on address priority in an

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.

Method of supporting unknown addresses in an interface for data

A ring data network (such as a token ring network) is divided by a security unit (1) into first and second segments (6 and 8). The second segment (8) typically corresponds to one security group in the network. Logic in the security unit reads appropriate parts of each frame (typically the two address segments) to determine whether it is appropriate to forward the frame to the next segment of the ring. If the frame does not need to go to the next segment, or is not authorized to do so, then the security unit forwards instead a modified form of the frame in which its data cannot be read (except possibly by the security unit itself). In most cases the data content of the frame will be stored in the security unit. When the modified frame returns to the security unit at its second input port (4), the original frame can (if appropriate) be reconstructed, typically by reading its data content from a store in the security unit. In this way, the frame is returned to the secure group at the same time and in the same form as if it had travelled around the first segment of the ring, but there is no opportunity for stations in the first segment to eavesdrop upon the frame.

Security device for ring network

Repeaters for digital data networks

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London, United Kingdom

Dipak M Soni