FIELD OF THE INVENTION

The invention refers to a localisation method for bidirectional interruptions of communication links between nodes in a communication network.

BACKGROUND OF THE INVENTION

Communication networks comprise a plurality of nodes connected by links, e.g. optical fibers. The links are often bidirectional operated, whereas signals are transmitted in both directions between adjacent nodes over one single or over two fibers in the same cable.

Several protection methods like e.g. 1:1, 1+1 protection and rerouting methods are known to maintain connectivity if a link is interrupted.

Nevertheless the disrupted link has to be repaired, the exact position of the failure has to be located.

Nowadays, repair teams need to visit the adjacent nodes of the interrupted link. Special measurement devices, e.g. Opti- cal Time Domain Reflectometers (OTDR) for fiber networks, have to be employed at least at one node adjacent to the incident .

OBJECTS AND SUMMARY OF THE INVENTION

Therefore it is an object of the present invention to provide a method for quick failure localization.

According to the present invention, there is provided a lo- causation method for bidirectional interruptions of communi- cation links between nodes in a communication network comprising the steps of inserting time-stamps into the transmitted data, storing the last received time-stamp, supervising the connection and transmitting the last received time-stamps to a management system, and calculating the distance from a node to an interruption point according to the last received time-stamps transmitted from the nodes (Nl, N2) arranged on different sides of the inter- ruption point (P _F ) .

This method allows a simple calculation of an interruption point and reduces the service expenses.

Signalling and calculation can be executed by methods according to the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS Examples of the invention including a presently preferred embodiment are described below with reference to accompanying drawings, where

FIG 1 shows a simplified bloc diagram of a network link and FIG 2 shows a route-time diagram.

DETAILED DESCRIPTION OF THE INVENTION

FIG 1 shows in a bloc diagram a section of a network with two adjacent nodes Nl, N2 connected by a bidirectional link FI. Both nodes use the same exact timescale and the transmitters of both nodes inserts time-stamps TSl and TS2 respectively in the transmitted data signals, e.g. as digital information or any other defined metric. In addition e.g. byte boundaries can be used for fragmentation of the received time marks. The time synchronisation between the nodes could be realized by any synchronisation method exchanging information between the nodes or by special receivers for time signals, e.g. of the Global Positioning System.

The propagation time tg of transmitted signals between two nodes is measured or calculated. It can also be helpful to know the distance between the nodes.

The location method is now explained with reference to FIG 2. Both nodes Nl, N2 - adjacent nodes or any two nodes of a network, e.g. end nodes, exchanging data signals - are transmit- ting information comprising said time-stamps. If the data between the nodes is transmitted in data frames the time-stamps may be inserted at a certain data frame position. If the information is e.g. transmitted byte-by-byte the time-stamps are inserted periodically. The time-stamps can even be in- serted in an alarm indication signal here also regarded as data signal.

The transmission of time-stamps is illustrated by continuous lines. The transmission of a new time-stamp is represented by a new line. The last received time stamps (or the time stamps of the last received complete data frames) are always stored. The time-stamps TSl and TS2 shown in FIG 2 are the last received time-stamps before an interruption.

Now, the link is bidirectional interrupted at a "failure time" tf (interruption time) . The receivers of both nodes Nl, N2 notice the interruption and signal it along with the last received time-stamps TSl, TS2 to a management system for interruption localisation (each entity designed for failure lo- cation is here referred to as management system) . The last received time-stamps (or an even exacter calculated last successful transmission time) represent the emission times tl and t2. The distance X _F to the failure point P _F can be calculated according

X _F = Vg (tf " tl) (1)

with Vg = signal (group) velocity (calculated or measured) , tf - failure time.

The unknown failure time can be derived from

(tf - tl) + (tf - t2) = tg (2)

with tf = (tg + tl + t2) /2 (3)

inserted in (1)

X _F = Vg (t2 - tl + tg) /2 (4)

or avoiding v _g

X _F = L(t2 - tl + tg) /2tg (5) with L - distance between the nodes Nl, N2 ;

Of course, discrete components causing an additional time delay can be considered by correction the time transmitted in the time-stamp and a more exact calculation of the interruption time is possible e.g. considering a time span between the last received time-stamp and the detected loss of signal, e.g. by counting the received bytes and transmitting a time span value together with the time-stamp to the management system.

For supervising reasons it may be advantageously to forward the time-stamps continuously to the management system or to transmit (additionally) the time-stamp of the last complete received data frame. These may be the time-stamps TSl, TS2 shown in FIG 2 when the interruption occurred after the complete frames have passed the interruption point otherwise foregoing time stamps.

The present invention is not limited to the details of the above described principle. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalents of the scope of the claims are therefore to be embraced by the invention.

REFERENCE SIGNS

Nl, N2 nodes

FI link, fiber MSY management system

TSl, TS2 time stamps

P _F interruption point

X _F distance to the interruption point t _F failure time tl emission time of TSl t2 emission time of TS2

L distance between adjacent nodes