PASSIVE OPTICAL NETWORK STRUCTURE WITH HIGH FAULT TOLERANCE

Abstract: Fault tolerant topology for passive optical networks essentially based on a ''tree configuration'' with branch points or nodes (16, 17) made up of passive optical splitters, in which each node (16, 17) of a specific level is connected to the same number of nodes (16, 17) of the level immediately below by two or more branches (Rd, Rs) and a supplementary connection (S1, S2...) is provided between each pair of nodes of the same level.

Description: Description PASSIVE OPTICAL NETWORK STRUCTURE WITH HIGH FAULT TOLERANCE Technical Field This invention concerns transmission systems using optical fibres and, in particular, optical networks of passive type structured in such a way as to be highly tolerant to faults affecting the transmission mean, that is, the fibre. The passive optical fibre networks, that is, those containing no active inner components such as switches, bypasses, modems, and so on, provide very high information exchange speeds - up to a rate of around one terabit per second, using, for example, the WDM (Wavelength Division Multiplexing) technique. The present application makes particular reference to passive optical networks for use in the so-called Metropolitan Area Networks (MAN) . However, the invention is not limited to this .type of application. Background Art As we know, if we have to obtain total interconnectivity between stations linked by an optical network, the most frequently used mappings are based on star, bus and tree configurations or a suitable combination of these. Star mapping is inconvenient from the point of view of the fibre installation costs, while the bus configuration is extremely limited with regard to the power budget that is the quantity of optical power that can be carried. Tree mapping, whose limitations with regard to fibre installation costs and power budget are much less serious, does present the problem of being extremely fault-prone. If we take a metropolitan network as an example, we can see that a cut in one si-ngle fibre can completely disconnect an extensive part of a network with tree structure. To overcome this problem, the use of redundant architectur and the insertion of suitable active elements in th network nodes have been suggested, to handle for exampl the signal routing or its amplification by alternativ multiple routes contained in the network architecture. I this way, however, it is no longer possible to obtain the very high communication Bit rate (terabit/sec approx.) which can be obtained only by using optical networks realized with a completely passive physical mapping. The aim of this invention ■ is the creation of a passive optical network, designed in particular for the applications mentioned above, with high fault tolerance and complete user transparency, in the sense that the same network can be used for different types of service (requiring, for example, different transmission bit rates), rather than having to overlap a different network for each service. Another aim of this invention is to obtain the characteristics described above without using active elements in the nodes, at least in the most frequent cases, where the network is constructed with a "monotype" tree structure - a completely binary structure, for example, or completely ternary - and so on. The invention is also aimed at creating a passive optical network structure with low fibre installation costs, good signal losses and " power budget. Disclosure of Invention These objectives have been obtained by means of a tree configuration passive optical network formed by multiple nodes at different levels, linked by connector branches, where a node of a specific level is connected by m branches, with m > ^ .2, to the same number of nodes at the level immediately below, characterized by the fact to foresee a supplementary connection between each pair of nodes of the same level. In line with this invention, the passive optical fibre network should preferably be structured in such a way that each node is connected to the same number of nodes at the level immediately below by two or possibly three branches. We can see that, in this way, if we indicate the type of tree with n (n=2 for binary tree, n=3 for ternary tree, and so on) , we can reach any station by n number of mutually independent routes. Similarly, the network as conceived by this invention is absolutely insensitive to a simultaneous number of faults equal to or less than n-1. In particular, for n=2, the network is protected against a single interruption at any point, and also against multiple interruptions in the same sub-route of the two sub-routes in which we can consider the network as being divided, as we will explain below. In addition, in the network as conceived by the invention it is possible to identify the position of the fault (or faults) quickly and automatically. This is due to the fact that the route of each single branch is known and, as no further signals are received on a given branch, it is possible to identify the indication of the fault position automatically. Finally, a particularly useful aspect of the invention is the possibility of obtaining a "mixed" structure, that is, one made up of a first ternary sub-network near the root and a second binary .sub-network near the users, with simple switches locally controlled at the contact points between the two sub-networks. The structural and functional characteristics, as well as the further advantages offered by the mapping possible with this invention, can be made clearer by the non-exhaustive examples set out in the attached diagrams, as follows: Fig. 1 shows an example of a network in accordance with this invention, based on a three-level, or three-depth, binary tree; Fig. 2 shows a first independent route into which th network in Fig. 1 can be directed; Fig. 3 shows a second independent route into which th network in Fig. 1 can be directed; Fig. 4 shows the construction of a leaf station i greater detail; Fig. 5 sets out the general form of a network structur as conceived by this invention; and Figs. 6A and 6B are diagrams that show the probabilit of disconnection and resiliency respectively, in a networ mapped in accordance with this invention, in line with the depth of the tree at which two simultaneous failures ma occur. For the sake of simplicity, we will first give an example of a network configuration in accordance with the invention with a binary tree, that is, with two branches routed out from a node towards another at the level immediately below. In Fig. 1, 11 refers to a tree structure network equipped with a root station 12 and eight leaf (final) stations 13. Two connection lines, designated 14 and 15 respectively, leave from station 12. In fig. 1, line 15 is indicated by a dotted line. Line 14 branches out near the branch points or nodes 16, while line 15 branches out near the branch points or nodes 17. The number of branch points 16 is the same as that of branch points 17. We can see that each node is connected to two others of the level immediately following by two branches, Rs and Rd respectively, where Rs belongs to line 14 and Rd to line 15. In each node 16, the left branch between the two branches in which line 14 is divided is directed towards node 16 of the level immediately below, while the right branch is directed towards node 17 of line 15, which is at the same level. The procedure is absolutely symmetrical for each node 17. The right branch between the two branches in which line 15 is divided is directed towards node 17 of the level immediately below, while the left branch is directed towards node 16 of line 14, which is at the same level. In this invention, two adjacent nodes on the same level, or two adjacent sheet stations, are in this way linked to a transverse branch as shown in Fig. 1 with SI, S2... Branch points or nodes 16 and 17, which represent in every sense the nodes of a binary tree with transverse connections, are created by means of simple optical power splitters. Eϊxtremely advantageous is the fact that, in this way, the only additional cost with respect to a non-redundant tree configuration network is that required for the transverse connections between the nodes of the same level. As shown in greater detail in fig. 4, each leaf station 13 contains a pair of inputs 18 and 19 which can be connected alternately to unit U from a two-position switch SW. The inputs 18 are connected to line 14, and the inputs 19 are connected to line 15. A power sensor (PS.) is connected to unit U. This sensor activates its own output in. response to the detection of a break in the signal received from unit U. In the case of breakage of the optical fibre, for example the one connected to output 18, the activation of the output of the sensor PS causes the switching of the element SW in such a way as to connect a receiver/transmitter (not shown) present in unit U at input 19. If we examine figs. 2 and 3, we can see more clearly that all the stations 13 are connected with each other and station 12 by means of two completely independent routes, represented by lines 14 and 15. It is therefore clear that any single fault that interrupts the signal flow at any point of network 11 has no effect on the interconnections that link all the stations 13 and station 12. The number of nodes, and consequently the number o couplers, that a signal transmitted by the root station ha to cross to reach a leaf station is the same for bot routes represented by lines 14 and 15. This means that th difference between the signals that arrive at one or th other of inputs 18 and 19 is due only to the differen attenuation caused by the variations in length of the tw routes. The loss of power caused by the redundancy of the networ as conceived by the invention by comparison with the original tree configuration is however limited and has a very low impact on the overall quantity of power that can be carried and, consequently, the maximum number of stations that can be connected. A further advantage is that the stations 13 are fitted with instruments of a generally familiar type, indicated by CAL, for the re calibration and/or synchronisation of the signal when it is necessary to switch from one route to the other, such as in a fault situation, for example. The diagram in fig..6A clearly shows that the probability of the disconnection of a station from a network with this mapping, in the case of two simultaneous faults, increases in line with the increase in the level where the faults occur. For example, if the fault occurs at level 2, that is, almost the maximum level, the probability of network disconnection in the case of the invention is only 60% instead of 100% as in the present passive optical network structures. We can therefore understand that even if the faults occur at significant depth the probability of disconnection is considerably limited and rapidly drops going toward the lower levels of the network. Fig. 6B is a diagram that shows the progress in normalised form of the so-called resiliency of a network with mapping as conceived by this invention, defined as the total number of pairs of users disconnected due to two simultaneous •p breaks in the network at random positions. As faults at higher.levels cause more serious problems, in • certain cases it is convenient to use the mapping of the 5 invention with a configuration based on a ternary tree at higher levels and a binary tree at lower levels, that is, two sub-networks as conceived by the invention which are interlinked by means of locally controlled switches. The term "locally controlled switches" means control devices 10 which require only local power supply and are substantially made up of a sensor used to detect the presence or absence of a signal in transit and automatically activate the switches. An example of a device of this type is shown in fig. 4. We should therefore explicitly eliminate switches 15 controlled remotely by a control network parallel to the optical network. In this way, we obtain an overall network with extreme fault tolerance and- low installation costs, as a ternary network (n=3) is in any case insensitive to two 20 simultaneous interruptions, and the interruptions that have serious effects are those that occur towards the root part of the tree, where on the other hand the connection branches require relatively reduced fibre lengths. Bearing the above in mind, fig. 5, which shows the general 25 structure of the network as conceived by the invention in schematic form, becomes clear. This network includes a root station 120 and branches Rl,l, Rl,2, Rl,j ... Rl,m, and leaf stations 130, with a supplementary connection (Si.j) between a generic pair of nodes (Ni+l,j, Ni+1, j+1) of the ι 30 same level. It will be understood by those skilled in the art that ■ r various modifications and changes may be made to the present invention without departing from the spirit and scope thereof. For example, a combination of modifications 35 may be made to each of blocks illustrated in Fig. 1 for implementing the present invention

Claims: Claims 1. Passive optical network structure with "tre configuration" made up of a number of nodes (Nl,l, ... Nn,p) at different levels linked by connecting branche (Rl,l, ... Rn-l,q), in which a node at level (Ni,j) is connected by m branches (Ri,j, Ri,j+1 ... Ri,m), with m 2, to the same number of nodes (Ni+1,1 ... m) of the leve immediately below, characterized in that it foresees a supplementary connection (Si,j) between each pair of nodes (*Ni+l,j, Ni+l,j+l) of the same level. 2. Passive optical network structure as described in claim 1, characterized in that each node is formed by optical power splitters. 3. Passive optical network structure as described in claims 1 and 2, characterized in that each node (16, 17) of a specific level is connected by two branches (Rd, Rs) to the same number of nodes (16, 17) of the level immediately below. 4. Passive optical network structure as described in claims 1 and 2, characterized in that each node of a specific level is connected by three branches to the same number of branches of the level immediately below. 5. Passive optical network structure as described in claims 3 and 4, characterized in that each leaf station (13) contains a unit (U), which incorporates a receiver/transmitter, connected to a signal absence detection sensor (PS) used to command a switch (SW) for the selective connection of one of the inputs (18, 19) of the leaf station (13) with said unit (U) containing a receiver/transmitter. 6. Passive optical network structure as described in claim 5, characterized in that each leaf station (13) includes means (CAL) for the recalibration and/or synchronisation of the signal. 7. Optical network consisting of a first network structure as described in claim 4 located near the root, and a second network structure as described in claim 3 located near the leaf stations, with locally controlled switches situated at the nodes common to the two structures.

Inventors: CHIARETTI GUIDO, GERLA MARIO, CHIARETTI, GUIDO, GERLA, MARIO
Applicants: ITALTEL SPA, ITALTEL SOCIETA' ITALIANA TELECOMUNICAZIONI S.P.A.
Application year: 1992
Main classification: H04L12
IPC codes: H04B10, H04J14, H04L12