Abstract:
The present invention relates to a system for optical transmission of information over a multiplexed logical ring structure comprising a number of nodes, of which at least one is a master node, as stated in the independent claim  1 . Said ring structure is a combination of a number of logical optical rings on the same physical fiber ring. Possible embodiments are disclosed in the dependent claims.

Description:
TECHNICAL FIELD 
     The present invention relates to optical communication systems and a method in such a system. 
     BACKGROUND OF THE INVENTION 
     The invention is based on a technique called Wavelength-Division-Multiplexing (WDM). Two different types of WDM-systems exist. The first type is called Dense-Wavelength-Division-Multiplexing (DWDM). In a DWDM system the laser chip in the laser diode of the transmitter is cooled by a peltier-element to keep the laser light source stable at a certain wavelength. The different wavelengths of different light sources at the transmitters of the DWDM system are closely spaced to each, typically between 50 GHz to 200 GHZ. DWDM systems are usually used in combination with optical amplifiers. 
     The second WDM system existing is Coarse-Wavelength-Division-Multiplexing (CWDM). CWDM is a technique that uses uncooled laser diodes for generating the different channels. The wavelength of such diodes is allowed to drift with temperature, which is compatible with the use of cheaper broadband filters. The complexity of the mechanical construction of the laser diode is reduced in comparison to laser diodes used in DWDM-systems. 
     One possible structure for an optical transmission system is at least one fibre pair ring. Said ring includes a number of nodes. Some nodes connect different rings with each other. Said nodes are called master nodes. Other nodes connect different endpoints, comprising users or subscribers, to the optical transmission system. Said nodes are connected to access rings of the system. 
     Fibre is a rare material in the city core network and two techniques or, rather, systems for transmitting information from one node to another is used. One system is a ring structure comprising electrical Time-Division-Multiplexing (TDM) add/drop multiplexers and the other system is DWDM rings with a hubbed add/drop structure. 
     In a TDM add/drop multiplexer ring structure, all nodes in the access ring are sharing the capacity that is made available by the master node. The maximum capacity is defined by the line interfaces of the nodes and all nodes have to have the same line interface. All traffic that is transported on the ring is terminated in each node, thus each node communicates with its direct neighbour. The logical and physical traffic structure is a ring. Due to the ring structure it is possible to protect the system in the transport protocol by using a two fibre ring infrastructure. This type of structure is implemented in the SDH/SONET, DTM, DPT and RPR standards. 
     In a DWDM an add/drop multiplexer ring structure, each access node is connected via its own wavelength to the master in the ring. Thus, the traffic pattern is a star with the master as a hub. DWDM systems are used as transportation systems, but this star topology suits mostly with an Ethernet star structure. If the number of nodes is increased in the ring an amplifier has to be used to compensate the losses in the optical add/drop filters. 
     From European Patent Application EP 1 063 803 A1 is a CWDM optical ring network earlier known. A dual-ring, bi-directional optical fibre transmission system interconnects a series of add/drop nodes with a hub, such that multiple, widely spaced CWDM channels are established on each ring. At each node, an optical add/drop module (OADM) includes broadband filters, such as dielectric thin film filters. Said filters are arranged to (a) extract, for the purposes of a receiver, or (b) insert, for the purposes of a transmitter, information in one or more of the channels. The signals in the one or more channels are coupled to the OADM&#39;s by a standard optical transceiver, which performs modulation and demodulation. Even though the physical topology, or structure, of the network is a ring topology, the logical topology, even called virtual topology, is a star. This means that endpoints at each of the nodes communicate with other endpoints connected to the hub. If desired, the hub in this known system can be configured to allow for selected CWDM channel optical by-pass, thereby enabling a direct connection between a pair of add/drop nodes on the ring. This connection is characterised as a point-to-point link. This means that only two points could be directly connected to each other for each wavelength used in the network system at a time. This causes a limitation in the possibility to increase the number of nodes and the capacity of this known network. 
     BRIEF DESCRIPTION OF THE INVENTION 
     A general problem of broadband access systems is to increase the number of access nodes. The nodes of the TDM ring share the available bandwidth. If a minimum bandwidth per node has to be guaranteed, the maximum number of nodes per ring is limited. Point-to-point WDM systems are expensive and hubbed WDM system are not optimised for TDM protocols. They are instead optimized to packet switched networks. In other words, a problem faced by many operators today is to upgrade or migrate their existing infrastructure to modern networks. 
     The basic idea is to increase the transmission capacity per fibre by combining light channels of different wavelengths on one fibre. A coarse WDM optical add/drop network structure optimised for logical TDM ring topologys is suggested. 
     In more detail, the present invention relates to a system for optical transmission of information over a multiplexed logical ring structure comprising a number of nodes, as stated in the independent claim  1 . Said ring structure is a combination of a number of logical optical rings, each at a specific wavelength, on the same physical fibre ring. Possible embodiments are disclosed in the dependent claims. For an example, one node or a number of nodes may be master nodes. 
     One advantage of the present invention is that it provides a protocol transparent solution and therefore is easy to apply to an existing system. Wavelength channels, like virtual fibre pairs, can be added one after the other without interrupting the existing traffic. 
     Further one advantage of the present invention is that different wavelength channels do not interfere with adjacent channels and, thus, do not exchange information. 
     Furthermore, one advantage is that the proposed system offers a major cost reduction compared to DWDM systems. 
     Yet another advantage is that the wavelengths are no longer dedicated to specific access node. The same wavelength is added and dropped several times throughout the network generating logical wavelength rings. 
     Another advantage is that logical rings need fewer wavelengths to connect a higher number of access nodes. For instance, only two wavelengths are necessary to connect five access co-locations. 
     Another advantage is that a number of expensive router interfaces at the Master node is reduced and by this the total cost of the solution. 
     Finally, by deploying logical wavelength rings the multicast functionality is fully exploited. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a system for optical transmission of information. 
         FIG. 2  shows a spectrum diagram for a CWDM-system. 
         FIG. 3  is a schematic illustration of a multiplexed ring structure according to the present invention. 
         FIG. 4  is an illustration of a logical ring structure according to the present invention. 
         FIG. 5  is a block diagram illustrating an optical add/drop multiplexer (OADM) in a node of the present invention. 
         FIG. 6  illustrates the data flow in the electrical domain of a CWDM-ring element with Network Management Channel. 
         FIG. 7  is an illustration of a logical ring structure of a Hybrid CWDM-DWDM system, which is further one embodiment of the present invention. 
         FIG. 8  is a spectrum diagram of a transmission system based on the Hybrid CWDM-DWDM system technology according to another embodiment of the present invention. 
         FIG. 9  shows a broadcast drop node design according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic illustration of a system for optical transmission of information, wherein said system comprises an optical fibre network  10 . Said network  10  is arranged between two geographic sites  12 ,  14 , e.g. Gothenburg and Stockholm. This long distance part of the system is called a core network  16 , sometimes even called a backbone. The core network  16  includes a trunk of optical fibres for the transmission of information. From the core network  16  is the information conducted into a metropolitan access network (MAN) ring  18 . At least one master node  20  is connected to said MAN. The master node is a common node for the MAN and an access ring  22 . The access ring comprises an optical fibre pair (not shown). Connected to said fibre pair is a series of OADM nodes  24 . Subscriber/client devices for receiving and/or transmitting information are connected to each OADM-node via subscriber/client connections  26 . 
       FIG. 2  shows a diagram wherein the abscissa is the optical wavelength, λ, and the ordinate is the optical effect, P opt . In a transmission system based on the CWDM (coarse-wavelength-division-multiplexing) technology a number of optical transmission bands are spread in a band of the optical spectrum.  FIG. 2  shows four optical transmission bands, each one including one channel, λ n  (n=1, 2, 3, 4, . . . ). Different wavelength channels are separated for not interfering with each other. A typical channel spacing is 20 nm (corresponding to 2400 GHz in the frequency band). CWDM is a technique that uses uncooled laser diodes for generating the different channels. The channel wavelength can change with the temperature of the laser diode, but as the original wavelength is centred in the band and the spacing is enough, the drifting of the laser diode will not cause any problem. This will reduce the price of the system, but also reduce the number of possible channels per wavelength interval. 
       FIG. 3  is a schematic illustration of a multiplexed ring structure embodiment of the invention. A physical access ring  30  comprises two optical fibres  32 ,  34  constituting a fibre pair. A number of nodes  38 , of which one is a master node  36 , are connected to said ring and fibre pair. The master node  36  connects the access ring to a metropolitan area network, MAN. However, it is not necessary that the ring has to comprise a master node. As shown in  FIG. 1 , all nodes are physically connected to the fibre pair, but logically the nodes  38  are connected to different logical wavelength rings/channels λ n  (n=1, 2, 3, 4, . . . ). This means that physically adjacent OADM-nodes, in other words neighbour nodes  38 , do not need to be logical neighbours  38   1 ;  38   2 ;  38   3 ;  38   4 . Nodes  38   n  is logical nodes and belongs to the same logical ring λ n . A master node  36  is characterised as a common point for all logical rings and it therefore allows transfer of information from one logical ring to another. The master node comprises a number of master node elements  35   n  (n=1, 2, 3, 4, . . . ), each one corresponding to a logical ring. 
       FIG. 4  is an illustration of a logical ring structure  40  according to the present invention. The invention provides a multiplexed ring structure  40  combining a number of logical optical rings  42   n  (n=1, 2, 3, 4, . . ) on the same physical fibre ring comprising a fibre pair ( 32 ,  34  in  FIG. 3 ). Each logical ring  42   n  operates on a different wavelength band λ n . The spacing between each band is such as there is no cross talk between the logical rings  42   n . Each ring is constituted by a series of OADM nodes  44 , logical neighbours, such as one wavelength is dropped and/or added, while the other wavelengths go through with minimum cross talk. Each node  44  retrieves all traffic at the wavelength λ n  defining the logical ring  42   n  it belongs to. Depending on the situation, the traffic then can be either terminated or fully regenerated and/or processed and then sent back into the logical ring. A Master node  43  intersects all logical rings and allows to transfer traffic from one ring to the other, by converting the wavelength. It acts as well as a gateway between the multiplexed logical rings and a larger core system, for instance a Wide Area Network (WAN) or a Metropolitan Area Network (MAN). One Master node is created by cascading a number of nodes  43   n  (n=1, 2, 3, 4, . . . ), each belonging to one of the rings intersecting the Master node. Each Master node element feeds one wavelength in the next Master node element, which add a new wavelength, until all the desired wavelengths are multiplexed. 
     The difference between this and other network structure is following. Compared to only TDM rings the maximum number of access nodes is now increased by a multiple with the number of wavelength used in the network. Each wavelength access node is communicating with the neighbour with the same wavelength, not with the physical/geographical neighbour. Compared to WDM hubbed rings the logical traffic pattern flow is still existing. Ring protocol like SDH/SONET, DTM, DPT and RPR are based on the assumption that the logical ring infrastructure is available. Even Gigabit Ethernet networks can be configured as rings with the help of modem switches and routers. “Hubbed” wavelength systems do not comply with this assumption as they represent a logical star topology. 
     The concept of logical rings gives the freesom to deploy new technologies alongside the existing ones, with minimal changes to the existing ones, with minimal changes to the infrastructure, examples:
         Add a DTM ring (DTM is a trademark of Cisco System INC) to an existing SDH/SONET metro network.   Add a second DPT ring (DPT is a trademark of Dynarc INC) to an existing DPT ring.   Divide a Gigabite Ethernet ring into multiple rings.   Feed distributed HFC coax islands.       

       FIG. 5  is a block diagram illustrating an optical add/drop multiplexer (OADM)  50  in a node of the present invention. The multiplexer is connected to the fibre pair,  52  and  53 , of the access ring via the contact interfaces, east  54  and west  55 . Information is transported in both directions on the pair. The function of the OADM is following. The present node drops λ 2 -channel information by use of a drop filter  51 . Said information is received by use of a CWDM receiver  56  that forwards the information to a low cost transceiver  57 . The transceiver is an optical connection or an electric interface to a processing unit  58  that comprises an information processor. The processed information is returned via the low cost transceiver to a CWDM transmitter  59  that transmits the processed information on to the λ 2  channel. The transmitter is connected to an add-filter  60  that adds the information onto the same fibre. 
       FIG. 6  illustrates the data flow in the electrical domain of a CWDM-ring element with Network Management Channel, which is an embodiment of the present invention. A Network Management Channel can be modulated as an overtone in the electrical frequency domain on the transmission signal channel. By doing this, information from one node can be distributed in the system. Said CWDM-ring element comprises a device for adding/removing the pilot tone/overtone. Distributed information can be for example link losses between nodes or information collected at each node from other equipment. 
       FIG. 7  is an illustration of a logical ring structure of a Hybrid CWDM-DWDM system  70 , which is further one embodiment of the present invention. This structure is similar to the logical ring structure in  FIG. 4 , and therefore are corresponding reference numbers for equivalent details used. Due the fact that CWDM channels use a wavelength band with a bandwidth of around 13 nm, it is possible to build a hybrid system. One of the channel bands is used for a multi-channel DWDM system, in this case  16  (λ 5 -λ 20 ). The add/drop configuration of the DWDM system then would be a hubbed configuration, also called star topology/structure, and thus some nodes  73  with an extraordinary need of broadband access could be supplied by this system. The DWDM system has no logical ring structure and acts as point-to-point structure from the master to each node  73 . This will constitute a hybrid system that can have a spectrum diagram illustrated in  FIG. 8 . 
       FIG. 8  is a similar spectrum diagram as earlier illustrated in  FIG. 1 , wherein the abscissa is the optical wavelength, λ, and the ordinate is the optical effect, P opt . The transmission system based on the Hybrid CWDM-DWDM system technology has a number of optical transmission bands that are spread in a band of the optical spectrum. The third CWDM-channel λ 3  replaced by a number of DWDM-channels λ 5 -λ 20 . 
       FIG. 9  shows a broadcast drop node design  90 . Such a design could be implemented in the CWDM ring. For example, it is of interest to broadcast cable-TV via optical signals on channels λ 1 -λ 4 . In this case, 5% of the optical effect of all the signals on the different channels λ 1 -λ 4  is divided by use of an optical coupler  92  connected to a network fibre  91 . The rest of the 95% of the optical effect of the signals continues through the network fibre  91 . A filter  94  is connected to the optical coupler  92  by use of a fibre  93  and the dropped signals is conducted to said filter. The filter extracts a predetermined CATV-signal (in this case λ 3 ) and stops the remaining signals (λ 1 -λ 2  and λ 4 ) to pass through the filter. The predetermined CATV-signal is conducted via a fibre  96  to a connected receiver R x    95 , in which the optical signal can be converted to the electric domain. In the next broadcast nodes a similar procedure is performed. The only difference is how much of the optical effect is divided and which channel signal that is allowed to pass said filter. 
     The advantage by using this type of system is that traffic (e.g. cable-TV) is on one wavelength and bi-directional traffic (e.g. voice, data, . . . ) is on another wavelength. Different end-equipment could be used together with different wavelength. 
     The present invention is not limited to the above-described preferred embodiments. Various alternatives, modifications and equivalents may be used. Therefore, the above embodiments should not be taken as limiting the scope of the invention, which is defined by the appended claims.