GB2280826A

GB2280826A - Token ring fault detection

Info

Publication number: GB2280826A
Application number: GB9316254A
Authority: GB
Inventors: Francis Yun-Tai Hung; Andrew Paul Lefevre; Peter John Lee
Original assignee: Northern Telecom Ltd
Current assignee: Nortel Networks Ltd
Priority date: 1993-08-05
Filing date: 1993-08-05
Publication date: 1995-02-08
Anticipated expiration: 2013-08-05
Also published as: GB9316254D0; GB2280826B

Abstract

A user terminal (12) is coupled to a token ring (11) via a cross-point circuit (15) and a trunk coupling unit (14). The signal paths (13a, 13b) between the terminal and the cross-point circuit and between the cross-point circuit and the coupling unit are supplied each with a phantom current, there being means for isolating the cross-point circuit from the voltages driving the phantom currents. The phantom currents are used for fault detection, cessation of a phantom current causing the user link to be decoupled from the token ring. Circuit (15) is a high speed silicon cross-point integrated circuit. <IMAGE>

Description

TOKEN RING This invention relates to token rings and in particular to an improved user link between a user terminal and a token ring hub.

A typical token ring comprises a number of terminals each coupled to the ring or hub via a user link. Messages are circulated around the ring via every terminal. Each terminal extracts its own messages and relays those destined for other terminals back to the ring. It will be appreciated that if one user link in such a system should fail then the ring is broken and a complete system failure occurs. To prevent such a catastrophic failure it is necessary to provide some means of detecting faults on a user link and for isolating that link from the system in the event of a fault.

Conventionally this fault detection is accomplished by monitoring phantom currents flowing in the transmission and reception portions of the link. If an imbalance in the phantom currents is detected1 this is taken as an indication of a fault, e.g. an open circuit, and the link is isolated from the token ring by bridging its connection to the ring hub. This is necessary to maintain the integrity of the ring.

Conveniently this bridging of the connection is effected by a relay drive via the phantom current detector.

Network systems are now being proposed in which a number of terminal stations, e.g. in an office building, are coupled to or can access a number of local area networks at least some of which may comprise token rings. It is proposed that this coupling be effected via an interface comprising a high speed silicon cross-point integrated circuit.

The current token ring standards require that each terminal having access to the ring incorporates a phantom current generator having an output of 3.5 to 7 volts whereby to maintain a phantom current of 1 to 2 ma in the user link to the ring. This upper voltage limit is considerably above the typical voltage maximum of a high speed silicon integrated circuit. For many integrated circuits this maximum voltage is about 5 volts. This incompatibility of working voltage has restricted the introduction of high speed silicon circuits to such applications.

The object of the invention is to minimise or to overcome this disadvantage.

According to the invention there is provided a user link for coupling a user terminal to a token ring via a cross-point circuit, the arrangement including a first signal path between the terminal and the cross-point, a second signal path between the cross-point circuit and the token ring, means for maintaining first and second phantom currents in the first and second signal paths respectively, and means responsive to the cessation of a said phantom current for decoupling the user link from the token ring.

According to the invention there is further provided a user link for coupling a user terminal to a token ring, the arrangement including a trunk coupling associated with the ring, a cross-point circuit, a first signal path disposed between the terminal and the cross-point circuit, a second signal path disposed between the cross-point circuit and the trunk coupling unit, means for maintaining first and second phantom currents in said first and second signal paths respectively, means for isolating the cross-point circuit from ther phantom currents, and means responsive to cessation of a said phantom current for decoupling the user link from the token ring.

According to another aspect of the invention there is provided a method of coupling a user terminal to a token ring via a cross-point circuit, the method including providing a first signal path between the terminal and the cross-point circuit, providing a second signal path between the cross-point circuit and the ring, generating first and second phantom currents in the first and second signal paths respectively, and monitoring said phantom currents whereby to decouple the terminal from the ring in the event of cessation of a said phantom current.

An embodiment of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a schematic diagram of a token ring system; Fig. 2 shows in schematic form a user link for the system of Fig. 1; and Figs. 3 and 4 together shows the construction of the user link of Fig. 2.

Referring to Fig. 1, the token ring system includes a ring or hub 11 to which a plurality of user terminals 12 may be coupled via user links 13 and trunk coupling units 14. Messages are circulated around the ring via each user terminal 12. Each terminal extracts its own messages and relays back to the ring those other messages whose detinations are other system terminals. Each terminal may also transmit its own messages on to the ring system so as to communicate with other system terminals.

Referring now to Fig. 2, the user terminal 12 is coupled to the token ring hub 11 via a trunk coupling unit (TCU) 14 and a cross-point circuit 15 which effectively divides the user link into two portions 13a and 13b. The trunk coupling unit incorporates means for disconnecting the user link from the ring in the event of a fault in the user link so as to prevent a complete system failure. Although, for clarity, only one user terminal and only one token ring are shown in Fig. 2, it will be appreciated that in practice the cross-point circuit 15 provides an interface between a number of terminals and a number of local area networks. Such an arrangement may be used e.g. in an office building to provide each of a number of users with access to a selection of token ring/ local area network service.

The construction of the user link is shown in Figs. 3 and 4, the former showing the link between the user terminal and the crosspoint circuit, and the latter showing the link between the cross-point circuit and the trunk-coupling unit.

Referring first to Fig. 3, the user terminal station is coupled to a first conductor pair TX21, TX22 via transformer T21, whereby to transmit signals to the token ring. Similarly, a further conductor pair RX21, RX22 for received signals is coupled to the user station via transformer T21,.

The user station incorporates phantom voltage generators V21, V22 whereby corresponding phantom currents are generated in the conductor pairs. Insulation between the voltage generators is provided by capacitor C21.

At the cross-point interface, blocking capacitors C22, C23, C24 and C25 provide isolation of the silicon cross-point circuit 15 from the phantom voltages. Resistors R1 to R3 in the transmit path and R4 and R5 in the receive path provides a dummy path from the station's phantom current and mimicks the presence of a trunk coupling unit (TCU) thus allowing the current station voltages to develop. When a phantom current is present in the dummy path, a corresponding DC voltage is developed across resistor R3, this voltage being detected by the cross-point circuit 15 at a phantom current detector (PCDET) input. The presence of this phantom current is signaled to the trunk coupling unit e.g. by means of a small common mode offset voltage which is superimposed on the transmitted (TX) signal. Transistor TR21 and diode D21 provide a means for setting up a phantom current imbalance in the link portion 1 3a under fault conditions.

Referring now to Fig. 4, a further phantom current for the link portion 13b is provided from voltage source V23 via transistors TR22 and TR23 driven from a phantom current control signal output (PC) of the cross-point circuit. Electrical isolation of the cross-point circuit from the voltage driving the further phantom current is provided by capacitors C32, C33, C34 and C35. The further phantom current from the transistor TR23 is split into two substantially equal portions each of which is fed via a corresponding resistor R31, R32 to the user link conductors. One portion of this current flows through the coil of relay RL21 whereby the user terminal is coupled to the token ring. A resistor R33 provides an equivalent path for the other portion of the phantom current. Transformers T31 and T32 provide coupling of the transmit and receive signals to the token ring (not shown).

If the cable link and the trunk coupling unit are properly connected and fully functional, a small DC offset voltage is developed across resistor R34, this voltage being detected at the PCDET input of the cross-point circuit.

The presence of the phantom current at the TCU interface with the ring may be signalled back to the user station via a small common mode offset voltage on the received signal. If this indication is absent from the received signal, e.g. as the result of a fault, a corresponding phantom current condition is set up in the user link portion 13a via the phantom current unbalance (PCUNB) input to transistor TR21. The user terminal then removes the phantom current to ensure that the relay RL21 is set in its by-pass mode whereby to isolate the user link from the ring.

When no phantom current is detected on the terminal side of the user link, PCUNB output is inactive. When a phantom current is first detected the PCUNB output is held inactive for a delay period to allow stabilisation of the user link on insertion into the token ring.

The arrangement is such that the phantom current in the two parts of the user link can be regarded as a single phantom current from which the cross-point circuit is effectively isolated. Thus, the user terminal 'sees' the phantom current passing through the TCU, and the TCU 'sees' the phantom current passing through the user terminal. Furthermore, disabling of the user terminal phantom current automatically disables the TCU phantom current.

Claims

CLAIMS:

1. A user link for coupling a user terminal to a token ring via a cross-point circuit, the arrangement including a first signal path between the terminal and the cross-point, a second signal path between the cross-point circuit and the token ring, means for maintaining first and second phantom currents in the first and second signal paths respectively, and means responsive to the cessation of a said phantom current for decoupling the user link from the token ring.

2. A user link for coupling a user terminal to a token ring, the arrangement including a trunk coupling associated with the ring, a cross-point circuit, a first signal path disposed between the terminal and the cross-point circuit, a second signal path disposed between the cross-point circuit and the trunk coupling unit, means for maintaining first and second phantom currents in said first and second signal paths respectively, means for isolating the crosspoint circuit from ther phantom currents1 and means responsive to cessation of a said phantom current for decoupling the user link from the token ring.

3. A user link as claimed in claim 1 or 2, and including means for maintaining said phantom currents substantially identical whereby cessation of one phantom current causes cessation of the other phantom current.

4. A user link as claimed in claim 2 or 3, wherein the first signal path incorporates an impedance equivalent to the impedance of the trunk coupling unit.

5. A user link for coupling a user terminal to a token ring substantially as described herein with reference to and as shown in the accompanying drawings.

6. A method of coupling a user terminal to a token ring via a cross-point circuit, the method including providing a first signal path between the terminal and the cross-point circuit, providing a second signal path between the cross-point circuit and the ring, generating first and second phantom currents in the first and second signal paths respectively, and monitoring said phantom currents whereby to decouple the terminal from the ring in the event of cessation of a said phantom current.

7. A method of coupling a user terminal to a token ring, which method is substantially as described herein with reference to and as shown in the accompanying drawings.