Engineering order wire

An engineering order wire arrangement is provided for a synchronous telecommunications network comprising a plurality of nodes interconnected via transmission paths, each path accommodating a plurality of transmission channels. Engineering voice traffic is carried on a first overhead channel having a defined network route topology. A second overhead channel is used to define a model network having a a route topology identical to that of the first overhead channel. The model network is tested to determine its integrity, i.e. the absence of loops. If the model network is found to be defective, both networks are reconfigured.

This invention relates to synchronous telecommunication networks and in 
particular to an arrangement and method for providing engineering order 
wires in such networks. 
BACKGROUND OF THE INVENTION 
A typical telecommunications network comprises a number of nodes 
interconnected by communications paths. The nodes incorporate equipment 
that requires commissioning on installation and periodic servicing either 
to effect repairs or to install new software. To facilitate this exercise, 
it is conventional to provide a voice communications channel between nodes 
to allow service engineers to communicate with each other. This voice 
channel, which is independent of the network and may thus be used during 
installation and testing, is generally referred to as an engineering order 
wire (EOW). Such a facility is of particular importance in synchronous 
(SDH or SONET) networks where it is necessary for engineers located at 
various system nodes to ensure synchronisation of those nodes when the 
system is set up. 
Traditionally, when a long distance link comprising end stations and 
intermediate regenerator stations is installed, the engineering order wire 
is provided in the form of a "party line" which is private to the 
associated long distance link. Telephone terminals are provided at the end 
stations and at the regenerator station, and an engineer lifting the 
handset of any one of those telephones Is able to hear any speech on the 
line. To support the operation, means are provided for attracting the 
attention of personnel at the different sites along the line, e.g. by use 
of a simple code of one ring for site one, two rings for site two and so 
on. 
The introduction of synchronous technology has greatly increased both the 
distances and the transmission rates that can be achieved by carrying the 
traffic over optical fibre links. Rather than go to the expense of 
providing a physically separate engineering order wire connection, it has 
been proposed to provide this facility in the form of overhead channels. 
Typically, two 64 kbit/s channels are allocated for this purpose. These 
channels are extracted/inserted at all sites where an order wire facility 
is to be provided. This arrangement however suffers from two 
disadvantages. Firstly, the engineering order wire traffic is carried on 
the main fibre transmission path. If that path is cut, then the 
engineering order wire communication is also lost at a time when it is 
most needed. Secondly, network topologies have evolved to such an extent 
that engineering communication along a single unbranched path is no longer 
adequate. For example, present day networks can now comprise meshes or 
hierarchies of ring systems. These complex topologies permit the formation 
of loops when an engineering order wire is connected as a party line. This 
can lead to positive feedback or "howl" preventing effective 
communication. 
SUMMARY OF THE INVENTION 
An object of the invention is to minimise or to overcome these 
disadvantages. 
A further object of the invention is to provided an improved engineering 
order wire arrangement for a telecommunications network. 
According to the invention there is provided an engineering order wire 
arrangement for a synchronous telecommunications network comprising a 
plurality of nodes interconnected via transmission paths each 
accommodating a plurality of transmission channels, the arrangement 
including means for allocating a first overhead channel for carrying said 
voice traffic, said first channel having a defined route, means for 
allocating a second overhead channel having a route identical to that of 
said first overhead channel, means for determining from said second 
channel the integrity of the route of said first channel, and means for 
reconfiguring the common route of said first and second channels when said 
first channel route is determined to be defective. 
According to another aspect of the invention there is provided a method of 
carrying engineering order wire voice traffic in a synchronous 
telecommunications network comprising a plurality of nodes interconnected 
via transmission paths each accommodating a plurality of transmission 
channels, the method comprising allocating a first overhead channel for 
carrying said voice traffic, said first channel having a defined route, 
allocating a second overhead channel having a route identical to that of 
said first overhead channel, determining from said second channel the 
integrity of the route of said first channel, and reconfiguring the common 
route of first and second channels when said first channel route is 
determined to be defective. 
According to a further aspect of the invention there is provided a method 
of providing a communications channel for engineering order wire voice 
traffic in a synchronous telecommunications network comprising a plurality 
of nodes interconnected via transmission paths each accommodating a 
plurality of transmission channels, the method comprising allocating a 
first overhead channel for carrying said voice traffic, said first channel 
having a defined route, allocating a second overhead channel having a 
route identical to that of said first overhead channel, breaking said 
second overhead channel at selected points in the network and injecting 
test signals at said points whereby to detect the presence of loops in the 
route of the second channel, breaking the first channel route at a said 
point responsive to the detection of a loop at that point, and 
reconfiguring the routing of said first and second channels by allocating 
a new common route thereto so as to provide a loop-free common route. 
The arrangement and method employ a first overhead channel for the 
engineering order wire and a second overhead channel for the loop 
breaking/healing functions. The second overhead channel follows the same 
route as the first, but has no telephone connections and provides no 
access. The second overhead channel can thus function as a model without 
interference with any of the telephony functions. This second channel is 
then used as a test channel to determine the integrity of the route 
currently allocated to both channels. 
The technique incorporates breaking and healing of the engineering ordrer 
wire network, where appropriate, to eliminate loops that could cause 
unwanted positive feedback. 
Testing of the model network is performed by probes which are distributed 
around the network and which are adapted to open the model network and to 
introduce test signals thereto. 
The use of a model network for test purposes eliminates the risk of 
interference with voice traffic when the testing is performed. 
In one embodiment, the second or model overhead channel is tested by the 
application of a test signal thereto in the form of a continuous 
pseudo-random sequence, e.g. a Gold code. By detecting a return of this 
sequence via a correlation technique, the system can detect the presence 
of breaks and/or loops in the transmission path. If such faults are 
detected, an alternative route can be allocated. 
In another embodiment, the test signal may comprise a burst of pseudorandom 
noise emitted by the probe.

DESCRIPTION OF PREFERRED EMBODIMENT 
Referring first to FIG. 1, this shows in highly schematic form the 
effective topology of an engineering order wire in a synchronous network. 
The topology shown is an idealised end to end route 11 having interfaces 
or bridges 12 to respective telephone sets or terminals 13 at various 
locations along the route. The engineering order wire functions as a party 
line to which each of the telephone sets has access. Thus, a conference 
can be set up between a number of terminals. It will be appreciated that 
although the path is depicted in FIG. 1 as a physical connection or 
circuit, it will in reality comprise an overhead channel allocated for 
that purpose. 
FIG. 2 shows the way in which a telephone set 13 accesses the engineering 
order wire channel. Access is provided via a node multiplexer which has 
aggregate ports 21 and tributary ports (not shown). The figure illustrates 
the audio bridge function which for each agregate port provides the sum of 
all the inputs except its own so as to provide the party line function. 
FIG. 3 shows in schematic form a synchronous, e.g. SDH or SONET network 
provided with an engineering order wire facility. The network comprises a 
number of rings 30 and access arcs 31 each having a plurality of nodes 32 
where traffic can enter or leave the respective ring. The network may also 
incorporate for example a line system 33 having regenerators 34. Each 
system node 32 incorporates an engineering order wire bridge (FIG. 2) 
which bridges an engineering order wire channel between aggregates and 
tributaries. This arrangement can lead to the formation of a number of 
engineering order wire primary loops L41, L42 . . . L46 as illustrated in 
FIG. 4. Potentially, any of the loops shown in FIG. 4 could be detrimental 
to the operation of the system. Either an initial disturbance could 
accumulate around the loop causing DC saturation, or the propagation delay 
around the loop may be such as to permit oscillation or "howl" as a result 
of positive feedback. 
To overcome these problems resulting from loop formation the engineering 
order wire arrangement comprises a first or primary voice network created 
by use of an overhead byte and bridges and to which voice access is 
provided, and a secondary or model network having a topology identical to 
that of the first and to which no voice access is provided. This may be 
effected via the following steps. 
1. Use a second overhead byte and bridges to build the model engineering 
order wire network having a topology identical to that of the primary 
network. 
2. Configure node or bridge software such that a fibre break in one 
direction results in a break in transmission in the opposite direction for 
both eow & model networks. 
3. Probe the model network for loops and break the secondary network at 
appropriate points where loops are detected so as to remove those loops. 
4. Break the primary network at points identical to those where the model 
network is broken so as to remove corresponding loops from the primary 
network. 
FIG. 5 illustrates the way in which probes 54 may be distributed around a 
communications network to effect testing of the model network associated 
with the engineering order wire primary network. The probes are arranged 
such that there is one in each potential loop of the network. The model 
network is thus used, by operation of the probes, as a test model to 
determine the integrity of the primary network and to adjust the topology 
of the primary network. 
The technique is illustrated in more detail in FIGS. 6 and 6a which 
illustrate a mesh arrangement showing a construction of three networks, 
i.e. the transmission network, generally indicated as 51, carrying the 
revenue earning traffic, the engineering order wire primary (voice) 
network 52 (FIG. 6a) using e.g. the En channel and which is bridged at all 
junctions or nodes 32, and the model engineering order wire network 53 
which provides no voice access and has a topology identical to that of the 
primary network 52. 
The model network has a plurality of probes 54 disposed at various points 
around the network. The probes are distributed such that one probe is 
provided in each primary loop of the network. Each of these probes 
repeatedly tests the model network by effectively breaking that network at 
the probe location and transmitting test signals in both directions 
through the network. In one embodiment, these test signals comprise bursts 
of noise. Each probe, having transmitted its signal burst, then detects 
any corresponding received signal burst which may have travelled around a 
loop. The use of a noise burst for the transmitted signal overcomes the 
potential problem of corruption of the return signal by interference 
between returns from multiple loops. In another embodiment, each probe 
transmits a continuous pseudorandom signal, e.g. comprising a Gold code. 
Each probe is allocated its own unique code sequence. Detection of the 
signal as a result of circulation around a loop is effected by a 
correlation process thus recovering the signal from the background noise. 
If a return signal indicative of a loop is detected, the model network 
remains broken at the probe site and the primary engineering order wire 
network is also broken at this point. If no return signal is detected, the 
model network is recoupled at the probe site and the primary network is 
allowed to remain unbroken. 
Where probes emitting continuous sequences are employed, they can operate 
simultaneously as each responds only to its own unique sequence. Where 
probes emitting pulsed signals are employed, these are operated 
selectively in a predetermined sequence to prevent interference of the 
signals from one probe with any other system probe. This is effected by 
allocating a unique serial number to each probe. This serial number 
determines a delay between initiation of a probing sequence and activation 
of a particular probe such that each probe operates in turn. The sequence 
commences with a short silent period, e.g. about one second, during which 
no probe transmits. The probes then transmit individually at regular 
intervals determined by their sequence number until the last probe in the 
sequence has performed its test function. This process detects the 
presence of loops so as to reconfigure the topology of the engineering 
order wire network appropriately, and may be repeated as necessary until 
all loops have been eliminated. 
The probing sequence of the model network may be undertaken at setup or 
following a network modification. The probe sequence may also be initiated 
in response to positive feedback or "howl" in the primary engineering 
order wire network indicative of the establishment of a loop. This is a 
condition that requires immediate attention to break the primary network 
at appropriate points to eliminate the feedback. Alternatively, probing 
may be a continuous process to take account of minor network changes 
arising e.g. from local fault conditions and consequent rerouting of 
traffic. 
Referring now to FIG. 7, this illustrates an arrangement for coupling a 
probe to the model (F1) network and for controlling reconfiguration of the 
engineering order wire (En) network. The probe 54 incorporates one or more 
pairs of test signal generators 71, one member of each pair for each 
direction of the model network, and corresponding detectors 72 which 
respond to returned signals e.g. from a loop. Coupling of the signal 
generators or the detectors to the network is provided via a switch 73 
disposed in the probe and a further switch 74 in the model network. The 
latter switch is operated via a control circuit 75 and is used to open or 
break the model network to effect the test procedure. A third switch 76 
activated via the control circuit 75 is provided in the engineering order 
wire network to permit controlled breaking of that network. A four wire 
analogue interface 77 between the probe and the network provides direct 
current blocking. 
The operation of the arrangement of FIG. 7 is as follows: 
1. The model (F1) network is broken via the switch 74. 
2. The probe injects signals in both directions into the broken network via 
the signal generators 71. 
3. If no return signal is detected by the detectors 72, i.e. no loops are 
found, the model nertwork is closed via the switch 74. 
4. If a loop is detected, the model network remains broken and the 
engineering order wire network is broken via the control circuit 75 and 
the switch 76. 
It will be appreciated that, under fault conditions in some network 
topologies, breaking of the model network at a particular point may 
separate that network into two disjoint parts. This is illustrated in FIG. 
8 which depicts a network comprising a number of rings. It is assumed that 
this network is initially stable, i.e. there is no positive feedback, and 
has a fibre break or fault at XX. When the probe 54a disposed in one fibre 
of the link between the right and left halves of the network initiates its 
test procedure, it breaks the secondary network at that point. Assume now 
that maintenance action to repair the network at XX results in a new fault 
at YY in the other fibre of the link between the right and left halves of 
the network. The right and left halves of the model network are now 
separated by the break and by the operation of the probe 54a. Where such a 
situation can occur it will of course be necessary to co-ordinate the 
operation of the probes in the two parts of the network. 
A practical implementation of the engineering order wire arrangement 
requires three elements: 
Bridges and overhead access 
Probe function 
Phone function 
These will be discussed below with reference to FIGS. 9 to 11. 
Bridge and overhead access 
This function may be implemented digitally inside an equipment and is 
described in this context. A typical SDH multiplexer has aggregate and 
tributary ports providing access to 64 kbit/s overhead channels and fibre 
interfaces. The provision of the required functionality is illustrated in 
FIG. 9. In this arrangement, conference bridges 91a, 91b are used to 
individually assign aggregate or tributary ports to the conference, there 
being one bridge (91a) for the engineering order wire network and one 
(91b) for the model network. First and second controlled bi-directional 
break switches are used, one (93) to permit attachment of a probe, and the 
other (94) to permit the breaking of the engineering order wire under the 
control of the probe. Allocation of the audio interface is illustrated by 
way of example in FIG. 10 which illustrates the use of two codecs to 
support probe connection to the model network. An analogue interface codec 
(not shown) is assigned to the voice conference bridge. Two codecs 96a, 
96b are assigned either side of the model (F1) network break. Switches 97a 
and 97b are opened only when a probe is present at that site and can be 
operated by a control signal from the probe. In addition, the probe can 
control voice break via a control signal as described above. A practical 
implementation of this arrangement is illustrated in FIG. 11 which shows a 
chip configuration supporting the functions of FIG. 9. 
FIG. 12 shows a typical burst mode probe test cycle. Each probe is arranged 
to signal on one interface and receive on the other. It then receives on 
the one interface and signals on the other. The two signal receive phases 
are separated by a compute/action phase in which the test data is analysed 
and the En and F1 networks are opened or closed as appropriate. A first 
guard period is provided at the start of each probe cycle and a second 
guard period is provided at the end of the cycle to allow for any timing 
inaccuracies between probes. The first guard period makes provision for 
the situation where the network has been divided into two portions by a 
fault. 
As discussed above, the probes operate selectively in a sequence defined by 
the allocated sequence number of each probe. The sequence illustrated in 
FIG. 13 is preceded by a quiet interval after which each probe performs 
its test cycle in the sequence, each cycle starting with the first guard 
period and concluding with the second guard period as described above. 
FIG. 14 shows the telephone (phone) function which provides an interface 
between a network element 121 and a conventional DTMF two-wire telephone 
set 131. The arrangement of FIG. 13 comprises a subscriber line interface 
which provides the standard telephone functions e.g. of battery feed, 
signalling and hybrid. A DTMF timer detects dial codes and rings the bell 
of the terminal if the corresponding dial code is recognised. The 
telephone function interfaces to the network element via a four wire 
analogue interface and a control channel, the latter carrying signals 
indicative of the on-hook/off-hook status of the attached telephone set. 
The network element responds to the on-hook status signal by muting the 
telephone input to a conference bridge to prevent noise build-up from 
on-hook telephones. 
The network element 121 and the underlying transmission network provide a 
party line connection to all telephone functions in the engineering order 
wire network. All calling/signalling behaviour is controlled by the 
combination of the telephone functions and the respective DTMF telephones. 
When a telephone set goes from its on-hook to its off-hook condition, it is 
immediately connected to the party line. Thus, if a telephone conversation 
is already taking place, the newcomer can join in that conversation from 
the time of connection. The calling/signalling behaviour of the 
telephone/telephone function have no influence on connectivity. 
The calling/signalling behaviour of the telephone function is such as to 
recognise when its particular number is being called and to ring the bell 
or tone sounder of the telephone once for a pre-set period. The telephone 
function also provides for such facilities as Selective Call, Group Call 
and PSTN/PABX Call from a recognition of appropriate calling tones. 
Further telephone functions are envisaged which permit interfacing the 
engineering order wire network with a public network or a PABX. In such 
circumstances, the telephone functions could be exposed to calling tones 
for PABX numbers and functions in addition to the calling tones which 
alert the telephone function at the PABX interface. This problem may be 
addressed by ensuring, e.g. by way of a timer, that each telephone 
function ignores all tones in a sequence after the first three tones which 
relate exclusively to engineering order wire functions. 
It will be appreciated that, although the engineering order wire 
arrangement and method have been described with particular reference to 
SDH and SONET synchronous networks, it is not limited to those particular 
protocols but is of more general application to digital communications 
networks.