Abstract: A modular test instrument comprises a base unit and one of a selection of application modules, neither being capable of performing end-user functions without the other. When physically assembled to one another, the base unit and application module comprise a structurally unitary device specialized for performance of application-specific end-user functions.

Abstract: A modular test instrument comprises a base unit and one of a selection of application modules, neither being capable of performing end-user functions without the other. When physically assembled to one another, the base unit and application module comprise a structurally unitary device specialized for performance of application-specific end-user functions.

Abstract: A modular test instrument comprises a base unit and one of a selection of application modules, neither being capable of performing end-user functions without the other. When physically assembled to one another, the base unit and application module comprise a structurally unitary device specialized for performance of application-specific end-user functions.The base unit is generic to all types of test to be provided, and comprises a display, a power supply, a user interface, and generic software to operate the display and user interface. Each application module is connected to the base unit in the same manner, so that the same base unit can be employed with a wide variety of application modules. The application modules can be readily and conveniently removed from and assembled to the base unit.

Abstract: Round trip travel time of cells in an asynchronous transfer mode (ATM) communication network is measured by injecting a time stamp into test cells in a first test instrument, transmitting these test cells to a remote node in the network, looping the test cells back from the remote node to an originating node, and detecting their arrival. Preferably the time stamps are injected into test cells configured as OAM-PM (operation, administration, and maintenance performance management) cells. A time-of-departure stamp is injected into the payload of the test cell on transmission, and a time-of-arrival time stamp is appended upon its receipt. The time-of-arrival and time of departure time stamps are compared to determine the actual round trip travel time of the cell within the network. This information can then be used to optimize network traffic flow, especially for available bit rate virtual connections.

Abstract: A test instrument for monitoring operation of ATM communication networks is capable of identifying individual cells passing through a node to which the instrument is connected as belonging to a particular virtual connection, and transmitting test cells replacing the identified cells, transmitting modified identified cells, or augmenting cells transmitted over the identified virtual connection with operations, administration, and maintenance cells. Specific test cells can be detected by a second test instrument connected to a node elsewhere in the network for accurately monitoring specific network operating characteristics. Cell delay, cell delay variation, cell error rates, and the like may all be monitored by the test instrument according to the invention.

Abstract: An instrument for identifying the types of individual cells in a stream of ATM cells by individually examining the cell header and cell payload information of each cell. A microprogram within a microsequencer is vectored responsive to each cell type for updating appropriate statistical counters. Identification of individual cell types by examination of their cell headers on networks operating at speeds of up to 622 Mbps allows numerous network utilization statistics of interest to be gathered.

Abstract: A test instrument for measuring the rate and size of bursts of cells received at a given node of an ATM network with respect to a particular virtual connection includes a cell filter for identifying cells belonging to the virtual connection. The "leaky bucket" algorithm is implemented for determining conformity with contractual performance standards, and for delineating the bursts.

Abstract: An improvement to T1 digital communications systems uses the alternate mark inversion (AMI) encoding format, typically used in T1 systems, and the D.C. system that powers the repeaters for enabling quick and accurate in-service detection and identification of malfunctioning or failed repeaters.