Optical network and arrangement and method in such network

An optical network which is arranged to ensure communication between nodes in a lower-order loop and a higher-order loop when there is an interruption in the lower-order loop or in the event of hub failure. Each lower-order loop consists of a bus network with hubs and one or a plurality of nodes. Two optical fibers connect the nodes in each bus network and are used for communication in opposite directions between the nodes. Each bus network comprises precisely two hubs of which the first closes the bus network end at the first end thereof and the second closes the bus network at the other end. The hubs connect the bus networks in a lower-order loop and join this loop to a higher-order loop. Each node in the bus network is arranged to communicate with each hub. Channel allocation can be carried out so that channels received in one node are re-used for transmission on the same fiber from the same node.

TECHNICAL FIELD 
The present invention concerns an optical network which is arranged to 
ensure communication between nodes in a lower-order loop and a 
higher-order loop when there is an interruption in the lower-order loop. 
The invention also concerns a process for allocating channels in the 
aforementioned optical network. 
PRIOR ART 
Within the field of telecommunications there is frequently a need for very 
high transmission capacity. Data can be transmitted very rapidly by means 
of optical transmission via modulated light signals. 
Large optical networks are frequently constructed as layered or 
hierarchical networks comprising lower-order or local loops and 
higher-order or central loops. The lower-order loops are formed by nodes 
to which a plurality of network subscribers are connected. The nodes are 
preferably connected to one another via two optical fibres in which 
messages are sent in opposite directions. Communication between nodes in 
different lower-order loops is performed in that the messages from and to 
the lower-order loops are transmitted via one or a plurality of 
higher-order loops. A hub disposed in the lower-order loop concentrates 
the traffic from the lower-order loop and transmits it to the higher-order 
loop. In a corresponding manner, the hub converts the traffic from the 
higher-order loop and transmits it to the lower-order loop in a suitable 
form. 
A large proportion of the teletrac occurs between different lower-order 
loops and it is therefore important that the possibilities for 
communication between a lower-order loop and a higher-order loop are good. 
In order to ensure this communication it is already known that a plurality 
of hubs can be arranged in a given lower-order loop. 
It is already known from U.S. Pat. No. 5,218,604 to arrange two hubs 
between a first ring network and a structurally similar higher-order ring 
network which can be compared with a local loop and a central loop. Both 
the first and the second ring networks comprise add/drop multiplexers 
(ADM) by means of which channels can be fed to or tapped from the ring 
networks. These consist of two lines which transmit data from and to the 
aforementioned ADM in two different directions, clockwise and 
anticlockwise in the ring networks. 
Each ADM in the ring networks can communicate with both hubs in that the 
channels are sent in both clockwise and anticlockwise directions in the 
networks, i.e. the same message is sent in opposite directions on the 
different lines. All the channels are sent in each line to both hubs which 
are connected to the two lines in that a given channel received in a first 
hub is tapped only partially from the line, such that residual remains of 
this channel can continue on the line to the following hub. A first hub is 
arranged to transmit the channels received from the one network to a first 
line in the second ring network and a second hub is arranged to transmit 
the same channel to a second line in the second network. 
A disadvantage with this known solution is that it is only intended for 
communication between two structurally similar ring networks which 
communicate only with each other. If the known solution is applied to an 
optical network, the ring structure causes optical noise to circulate in 
the ring network, which impairs signal quality. Furthermore the known 
solution cannot be adapted to layered networks having a plurality of 
different levels and a plurality of loops in each level. 
DESCRIPTION OF THE INVENTION 
The object of the present invention is to overcome the problem of ensuring 
communication between a lower-order loop and a higher-order loop when 
there is a cable breakdown in the lower-order loop. 
This object is achieved by an optical network having one or a plurality of 
lower-order loops connected to a higher-order loop. The lower-order loop 
comprises at least one bus network with hubs and one or a plurality of 
nodes which are connected to one another via two optical fibres. The 
optical fibres in the bus network are used for transmission in different 
directions. Each bus network comprises precisely two hubs which close each 
end of the bus network. The hubs are arranged to switch over and 
concentrate the traffic from the lower-order loop into a form which is 
suitable for transmission on the higher-order or lower-order loop. Each 
node in the bus network is arranged for transmission to one of the two 
hubs via one of the two optical fibres and for transmission to the second 
of the two hubs via the second of the two optical fibres. 
The invention also concerns a process for allocating channels in a bus 
network in an optical network of the above-mentioned type. During channel 
allocation, at least one wavelength channel is allocated to each node for 
transmission to and reception from the hubs disposed at each end of the 
bus network. Channel allocation can be carried out such that channels 
received in one node are re-used for transmission on the same fibre from 
the same node.

PREFERRED EMBODIMENT 
In the following the invention will be explained with reference to the 
Figures and in particular to FIGS. 2, 4 and 5 which show preferred 
embodiments of a lower-order loop disposed in an optical network. 
FIG. 1 shows schematically a known construction for an optical network 
which is constructed as a layered network. In the example shown in the 
Figure, the network comprises three lower-order loops 4a-4c which 
communicate via a higher-order loop 3. Each lower-order loop comprises one 
or a plurality of nodes, shown as circles in the Figures. The optical 
nodes are connected to one another via two oppositely-directed optical 
fibres and communicate with one another via two hubs which are disposed in 
the loop and which are shown as rhombi in the Figures. Hubs are also used 
for communication between nodes in different lower-order loops, the 
higher-order loop being used for transmitting information between two hubs 
in inter-communicating loops. The so called hubs are arranged to convert 
and concentrate received signals into a form adapted for further 
transmission within the loop or to the next level. An even more extensive 
network can evidently comprise more than two levels such that each 
lower-order loop is arranged to communicate with a higher-order loop via 
one or a plurality of intermediate loops. The construction of the 
intermediate loops can be identical to that of the lower-order loops 
described here. A large part of all the teletraffic in an optical network 
occurs between different optical lower-order loops and it is therefore 
important that the possibilities for communication between a lower-order 
loop and a higher-order loop are good. 
In order to ensure operation in the optical network shown in FIG. 1, a 
plurality of geographically separate hubs can be arranged in each of the 
lower-order loops 4a-4c. According to the invention each loop 4a-4c 
consists of one or a plurality of bus networks each of which is closed by 
precisely two hubs. Via the hubs the bus networks can be connected in a 
lower-order loop 4a-4c comprising a plurality of bus networks. In the 
example shown in FIG. 1, each lower-order loop 4a-4c comprises precisely 
two bus networks which are coupled in parallel such that a lower-order 
loop is formed. 
FIG. 2 shows a first embodiment of a lower-order loop in an optical network 
according to the invention. This loop consists of a bus network 5 
comprising four different nodes A-D which are connected to one another via 
two optical fibres 1, 2 which are used for transmission in opposite 
directions. The bus network 5 comprises a first and a second hub H1, H2 
which are arranged at each end of the bus network. Each node is arranged 
to communicate with each hub via a wavelength channel such that the node 
sends one wavelength channel to the hub H2 along a fibre 1 going towards 
the right-hand side in the Figure and one wavelength channel to the hub H1 
along a fibre 2 going towards the left-hand side in the Figure. 
In the case of the embodiment shown in the Figure, the hub H1 sends four 
channels on the fibre going towards the right-hand side. A first channel 
is tapped completely from the fibre by a demultiplexer in node A and is 
prevented from continuing further on the fibre. This wavelength channel 
can therefore be re-used on the same fibre for further communication from 
node A to hub H2. The other channels continue unaffected through node A. A 
second channel is then tapped in node B and the channel can be re-used for 
transmission on the same fibre from node B to hub H2. The last two 
channels are tapped and re-used correspondingly in nodes C and D. The 
traffic going towards the left-hand side operates in the same way. Hub H2 
sends the same four channels which are tapped in nodes A, B, C and D, new 
messages being fed to the wavelength channels for transmission to hub H1. 
The order in which channels are tapped or input into the bus network can 
naturally be varied. 
The node construction shown in FIG. 3 is especially suited to the optical 
network according to the invention. By virtue of this node construction 
the same transmitter Tx can be used for transmission on the two separate 
optical fibres 1, 2 since the same channels in a node are used for 
transmission to the respective hub. In a corresponding manner, the same 
receiver Rx is used for reception on the respective fibre since each hub 
sends the same wavelength channel to an optical node via the respective 
fibre. A multiplexer 6a, 6b disposed on each optical fibre 1, 2 is 
arranged to input wavelength channels from a given transmitter Tx to both 
optical fibres. Owing to the fact that the same transmitter Tx can be used 
for transmission on the two separate optical fibres 1, 2, the costs on 
equipment are reduced. The same message is sent from one of the nodes A-D 
on both fibres 1, 2 in different directions to the two hubs H1, H2. In the 
same way the same message is received in one of the nodes A-D from both 
hubs H1, H2 via the two optical fibres 1, 2. Each node A-D comprises two 
demultiplexers 7a, 7b, of which one 7a is connected to the fibre 1 going 
towards the right-hand side and the other 7b is connected to the fibre 2 
going towards the left-hand side. These demultiplexers 7a, 7b are arranged 
to tap a given wavelength channel completely from the respective fibre to 
a receiver Rx in the respective node. In the case of reception an optical 
coupler 8 is used with the embodiment shown in the Figure for determining 
which of the signals is to be allowed to pass through to the receiver Rx. 
This coupler 8 is arranged to change between two different states. In the 
first state a signal from the demultiplexer 7a on the fibre 1 going 
towards the right-hand side is coupled to the receiver Rx whilst the 
signal from the demultiplexer 7b on the fibre 2 going towards the 
left-hand side is not taken into consideration; in contrast, in the other 
state, the signal from the demultiplexer 7b on the fibre 2 going towards 
the left-hand side is coupled to the receiver Rx, and in this case the 
signal from the demultiplexer 7a on the fibre going towards the right-hand 
side is not taken into consideration. An alternative solution which is not 
shown in the Figure is also to use two receivers. The choice of signal is 
then made in an electrical switching device before the message is further 
processed. 
FIG. 4 shows a lower-order loop with two parallel bi-directional bus 
networks 5a, 5b of the type shown in FIG. 2, which are both connected to 
the first and second hubs H1, H2. Each node in the lower-order loop with 
two bus networks 5a, 5b can, in the manner indicated in conjunction with 
FIG. 2, communicate with each of the two hubs H1, H2. Considered 
optically, the two bus networks 5a, 5b in the lower-order loop 4 are not 
connected and all communication between them occurs via the hubs H1, M2. 
Tric between two nodes in the same bus network also occurs via one of the 
hubs. Communication within the lower-order loop or with a higher-order 
loop (not shown) can thereby be maintained even if there is a cable 
breakdown in one of the bus networks 5a, 5b in the lower-order loop or if 
a hub ceases to function. 
In the node configuration shown in FIG. 4 the traffic for example from node 
A to node B passes via fibre 2 to hub H1 and from there continues to node 
B via fibre 1, or via fibre 1 to hub H2 and from there continues to node B 
via fibre 2. In a corresponding manner traffic from node B to node A 
passes via fibre 1 to hub H2 and from there continues to node A via fibre 
2 or via fibre 2 to hub H1 and from there continues to node A via fibre 1. 
In the case of traffic between two separate bus networks, for example from 
node B to node E, the tc passes in a corresponding manner via hub H1 and 
hub H2. The traffic from node B passes to hub H2 via fibre 1 and continues 
to node E, via fibre 2 from hub H2 or via fibre 2 to hub H1 and continues 
to node E via fibre 1. 
If a cable breakdown occurs for example between node A and node B in the 
example shown in FIG. 4, a wavelength channel on each fibre 1, 2 is used 
for communication between node A and hub H1. For the communication with 
node B a wavelength channel on the fibre sections which are connected to 
hub H2 are used instead. The two hubs H1, H2 are connected to a 
higher-order loop (not shown). This means that communication between the 
higher-order loop and all the nodes in the two separate bus networks 5a, 
5b is also ensured after a cable breakdown. 
It has proved advantageous to allocate an extra channel to the two bus 
networks 5a, 5b for communication between the hubs H1, H2. Without access 
to this extra channel, all communication between the hubs would have to 
pass via the higher-order loop 3 in the event of an interruption. This 
loads the higher-order loop and can therefore be a disadvantage. If an 
extra wavelength is used for handling traffic between the two hubs H1, H2, 
a wavelength channel from node A to node B in the aforementioned 
interruption situation can first be sent to hub H1 via fibre 2 where it is 
converted for transmission via hub-to-hub wavelength transmitters which 
transmit traffic to hub H2. This hub converts the received traffic and 
sends it further to node B via the actual wavelength channel on fibre 2. 
In an interruption situation traffic is also passed between two bus 
networks in the lower-order loop. Traffic from node E to node B for 
example passes via fibre 1 to hub H2 and from there continues to node B 
via fibre 2. Traffic from node B to node E passes via fibre 1 to hub H2 
and from there continues to node E via fibre 2. 
The concept according to the invention can also be extended to connect a 
plurality of bus networks 5c-5k in a grid-like lower-order loop as shown 
in FIG. 5. Each of the bus networks 5c-5k is closed at each end to a hub H 
which is common to one or a plurality of the other bus networks such that 
closed grids are formed and the loop is closed.