IES83249Y1 - ISDN interface apparatus - Google Patents

Description

This invention relates to an ISDN interface apparatus and a method of using an ISDN line.

The ISDN BRI Basic Rate Interface has two voice channels (each referred to as a “B- channel”) and one signalling channel (referred to as the “D—channel”). The interface can be provided at one of two reference points, known as S and T. At the T reference point, Figure la, the ISDN equipment NT2 has a private and direct connection to the remote terminal NT1. However at the S reference point, as shown in Figure lb, the ISDN equipment must share the basic rate ISDN line with other ISDN equipment using the S interface.

The T reference point configuration is normally only used for connecting high capacity switching equipment, such as PABX systems. It provides a four—wire, point-to-point connection. The S reference point is used to provide a connection for up to eight ISDN systems and it is normal practice to share the idle capacity equally with no priority.

However, the S reference point is not ideal for providing a secure communication link as is required, for example, for security systems, and other systems that require non-disruptive and non-blocking access to the B-channel to transmit their data. If both B-channels are in use then the system must wait until at least one is free before it can make a call. Providing a dedicated T configuration is one way to achieve this, but it is an expensive solution.

Reserving one of the two available B-charmels is also possible but is also restrictive and costly. Therefore, common practice is to use a privacy relay and a special installation wiring topology.

The current practice, illustrated in Figure 2, is to provide a privacy relay in the highest priority system 20 that is connected to an ISDN line. The relay can be operated to provide galvanic isolation between a master S interface and a slave S interface, the latter being provided by the highest priority system 20. The ISDN S wiring is first routed from a terminal NT1 to the master S interface on the higher priority device. Lower priority ISDN equipment 14 is then connected to the slave S interface on the highest priority system 20, rather than directly to the terminal NT1. When the highest priority system needs to access the line and make a call it simply physically disconnects the entire slave interface side equipment simply by operating the relay. As a result, all calls in progress are cleared down through the normal ISDN “lost call” procedure. The priority system has access to the master S interface and so can now access the ISDN line as if it were at the T reference point. This arrangement also has the disadvantage that the disadvantage is that it requires additional hardware and cost of relay and connectors on the priority system. In addition it needs special routing of the S interface wiring. As result it is disruptive and not plug and play in existing networks.

An aim of this invention is to overcome or at least ameliorate the disadvantages of the known S reference point access systems.

From a first aspect, this invention provides a controller suitable for connection to an ISDN line, the control unit being operative to monitor activity on an ISDN line to which it is connected and, when exclusive access to the line is required by a device with which it is associated, apply signals to the line that force a cleardown on any B-channel being used by one or more other device.

A particular advantage of this approach is that it is relatively simple to implement in control software for the ISDN system.

Typically, a controller embodying the invention is intended for incorporation into ISDN equipment, such as a security device, (the device with which is associated) to act as an interface between operative components of the equipment and the ISDN line.

The controller may include a processor, such as a microcontroller, and a memory device that contains program code executable by the processor. The control unit typically further comprises a line interface through which it can exchange signals with an ISDN line to which it is connected.

The controller most typically can track the state of any device attached to the S interface of a line to which it is attached. This may be done by implementing a state machine in the software program.

From a second aspect, this invention provides an ISDN installation comprising an ISDN line and a controller connected to the ISDN line, in which the controller is operative to monitor activity on the ISDN line and, when exclusive access to the line is required by a device, apply signals to the line that force a cleardown on any B-channel being used by one or more other device.

An installation embodying this aspect of the invention may include two such control units, each operable in relation to a respective one B-channel.

In an installation embodying this aspect of the invention, the or each controller is typically constituted within an interface incorporated into piece of ISDN equipment.

From a third aspect, this invention provides a method of accessing an ISDN line in T mode, by applying a signal to the network to force a cleardown of any device already connected to the line, to ensure that no other call is present which would otherwise block the placement of a new call.

An embodiment of the invention will now be described, by way of example, and with reference to the accompanying drawings, in which: Figures 1a and lb are block diagrams of a typical standard ISDN installations, and have already been discussed; Figure 2 a typical known ISDN S interface “privacy relay”, and has already been discussed; and Figure 3 is a block diagram of an ISDN system embodying the invention.

With reference to Figure 3, an ISDN installation comprises a remote terminal NT1 connected to a local installation by an ISDN line 22. The local installation includes high— priority equipment 20, such as a security system and low-priority equipment 14, such as a voice telephone, computer terminal adapter, and so forth. The high priority—equipment includes a controller 30 (being an embodiment of the second aspect of the invention) constituted by a module connected to the ISDN line 22.

The controller, in this typical embodiment, is embodied within an interface integrated circuit incorporated into the ISDN device during its manufacture. It is normal for manufacturers to implement an interface between the operative components of their equipment and ISDN by way of an interface constructed in such an integrated circuit.

The controller 30 includes a memory device 36, a microcontroller 38 that can execute software stored in the memory device 36, and a line interface 40 through which the microcontroller 38 can exchange signals with the ISDN line 22.

In operation, under the control of the software, the microcontroller 38 monitors the status of the ISDN line 22 through the line interface 40. Specifically, the software program is configured to recognise the particular state of all devices connected to the S interface in accordance with the definitions of the Q.93l ISDN state machine specification. (All ISDN-compatible devices operate in accordance with this state machine.) Through knowledge of the state, the software program can deduce the sequence of D-channel signalling that it must transmit on then D channel to mimic a call clear down sequence.

When high—priority access is required, the first step taken is, if no free B—channel is available, to force a cleardown of one other device already accessing the ISDN line.

Therefore the microcontroller 38 can obtain high priority access the line at any time simply by forcing the cleardown of any other device that is using any one of the B-channels, repeating this as required, and then proceeding with its own call on this free channel.

The controller 40 can be programmed for master or slave priority. If slave priority is set then the control unit operates in the normal mode, not interfering with any other connected device. This mode is typically used in cases where a device is fitted with a controller embodying the invention to enable exclusive access capability, but the user decides that exclusive access to the line is not required. However, if master priority is set then the slave operates to clear down any other devices when it requires sole access to the S-interface, as described.

In this way, the control unit can provide a service equivalent to the high priority device being connected on its own private T interface while requiring no special modifications to existing S interface wiring nor additional components.

The high-priority equipment 20 (and therefore, the controller 40) can be plugged into any one of the eight standard positions on the S interface. Two such pieces of equipment can share one S interface with other slave or standard ISDN equipment without conflict. Each item of high-priority equipment 20 can have exclusive access to one of the B-channels at any time.

An external interface can be provided on the control unit to which suitable equipment can be connected to enable a person to monitor and hence debug problems associated with control system and its S interface.

Claims (2)

CLAIMS: A controller suitable for connection to an ISDN line, the control unit being operative to monitor activity on an ISDN line to which it is connected and, when exclusive access to the line is required by a device with which it is associated, apply signals to the line that force a cleardown on any B-channel being used by one or more other device; optionally being intended for incorporation into ISDN equipment to act as an interface between operative components of the equipment and the ISDN line; optionally operative to force cleardowns repeatedly until it achieves the access that it requires; optionally comprising a processor, such as a microcontroller, and a memory device that contains program code executable by the processor; and optionally further comprising a line interface through which it can exchange signals with an ISDN line to which it is connected. A controller according to Claim 1 operative to track the state of any device attached to the S interface of a line to which it is attached and optionally in which the state of the attached ISDN devices is monitored by way of implementation of a state machine in the software program. An ISDN installation comprising an ISDN line and a controller connected to the ISDN line, in which the controller is operative to monitor activity on the ISDN line and, when exclusive access to the line is required by a device, apply signals to the line that force a cleardown on any B-channel being used by one or more other device; optionally including two such control units, each operable in relation to a respective one B-channel; and optionally in which the or each controller is typically constituted within an interface incorporated into piece of ISDN equipment. A method of accessing an ISDN line in T mode, comprising: a. applying a signal to force a cleardown of any device already connected to the line. A controller according to
1. Claim 1 or
2. Claim 2, substantially as herein described with reference to or as shown in