Abstract:
Architectures for a synchronous transport network of a telecommunications system using transparent transport capabilities are presented. The telecommunications network comprises a pair of transparent multiplexers (TMuxs) connected over a bidirectional high speed span for transparently transporting high rate traffic. Each TMux consolidates traffic from a plurality (I) of linear systems or a plurality of bidirectional self-healing rings, each ring (K i ) having a ring rate R i  and at least two nodes (A i , B i ). In another configuration, each TMux subtends a plurality of rings, such TMuxes being adapted for connection as ring nodes in a high-speed ring. The upgrades obtained with TMuxes in both the linear and ring configurations provide for per span relief for fiber exhaust where no changes to the existing systems are desired. As well, the bandwidth of an existing system may be increased on a per-span basis or the equipment count may be reduced.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is directed to architectures for a transport network of a telecommunication system, and more particularly, to network architectures using transparent transport capabilities. 
     2. Background Art 
     The rapid evolution of the technology in recent years has made the optical fiber one of the most targeted transmission media, due mostly to the high transmission rates available and reduced error rates. 
     The Synchronous Digital Hierarchy (SDH) specifies a basic rate of 155.52 Mb/s, which is called synchronous transport module level-1 (STM-1). The smaller rate of 51.840 Mb/s is called synchronous transport signal level-1 (STS-1) and is the basic rate of the SONET (Synchronous Optical NETwork) version of SDH. Higher rates (STS-N, STS-Nc) are built from STS-1, and lower rates are subsets of this. An STS-N frame comprises an overhead (OH) field with administration, operation, maintenance and provisioning information, and a payload field with user information. The optical counterpart corresponding to an STS-N signal is called OC-N. To accommodate asynchronous signals from previous generations of transport equipment, North America (SONET) and Japan base their sub-STS-1 multiplexing hierarchies on the DS-1 rate of 1.544 Mb/s, while Europe (SDH) is based on the a 2.048 Mb/s rate. The level of synchronous multiplexing hierarchies where the schemes are common occurs at the European basic rate STM-1 and the North American rate STS-3. Thereafter, the three approaches multiplex these rates in multiple integers, all being compatible with the basic rates. While the present specification describes and illustrates signals of rate (or bandwidth) according to SONET networks, it is to be understood that the invention is applicable also to other synchronous networks. 
     It is well known that the topology of a synchronous optical network can have a linear point-to-point configuration or a ring configuration. A linear configuration protects the traffic on a working fiber (W) by using a protection fiber (P) which will carry the traffic if the working fiber is interrupted. A “1:1” system has an equal number of working and protection links, a “1:N” system has N working channels and one shared protection channel. Since the protection fiber is idle most of the time, extra-traffic (ET) of lower priority may be transmitted over the protection fiber. 
     The ring topology permits the network to also recover automatically from failures due to cable cuts and site failures. Currently, two types of SDH/SONET rings are used, namely unidirectional path switched rings (UPSR), and bidirectional line switched rings (BLSR). Both ring types support unidirectional and bidirectional connections. 
     The UPSR is typically used in the access network and therefore is built for lower rates, such as STS-3/STM-1, which are sufficient for access link demands. These rings are provided with bidirectional connections between nodes, yet the traffic flow is unidirectional. The signal is always present on both working and protection fibers, therefore, the protection fiber cannot be used to carry extra-traffic (ET). 
     The BLSR is typically used in the transport network, and therefore is built to operate at higher data rates, like STS-48/STM-16. For a four-fiber BLSR (4F-BLSR) the working and protection traffic flow on separate fibers, each for one direction. For a two-fiber BLSR (2F-BLSR), the fibers between adjacent nodes carry working traffic and also have protection capacity allocated within them. Bidirectional traffic between two adjacent nodes takes place in the working time-slots, and protection traffic is inserted in the protection time-slots. Since for a BLSR configuration the protection timeslots are only used during a protection switch, they can be used for lower priority ET. Due to the working timeslots reuse capability, a BLSR always provides the optimum use of bandwidth for a given traffic pattern. However, an automatic protection switching (APS) protocol is necessary. 
     A traffic node is defined as the transmission equipment deployed at a site. In practical configurations, a site may comprise equipment belonging to different networks co-located in the same operation center. Such scenarios are common in big cities. There are many benefits to supporting large bandwidths on a single piece of equipment. Reducing the amount of equipment at a site simplifies the network management and also means fewer trips to a location for equipment repairs and replacement. The key benefit is lower equipment cost. 
     Telecommunications network providers are feeling the pressure of upgrading the equipment to the level of the latest technologies, as users demand ever more capacity. That factor, along with the reality of fiber congestion in the network, is causing providers to search for a solution that will increase capacity without forcing them to deploy additional fibers. 
     For an existing linear system that is experiencing fiber exhaust on a given span, the traditional solution is to replace the relevant equipment to obtain a higher line rate system. However, for a ring configuration, the line rate of the entire ring must be upgraded even if only one span is short of fiber. It is thus easy to understand why some network providers are asking for other options. 
     The add/drop multiplexer combines various STS-N input streams onto an optical fiber channel. Transparent transport is defined herein as the ability to provide continuity of all payloads and associated overhead bytes necessary to maintain a lower bit rate linear or ring system through a higher bit rate midsection, while reducing the required number of fibers interconnecting the sites. The lower bit rate linear or ring system operates as if it were directly connected without the higher bit rate midsection. Description of a transparent multiplexer, referenced as “TMux”, is provided in the U.S. patent application Ser. No. 08/847526, filed on Apr. 24, 1997 by Martin et al., assigned to Northern Telecom Limited and entitled “Transparent Multiplexer/Demultiplexer”. A method for transparently transporting higher rates signals over a mid-span is disclosed in the U.S. patent application Ser. No. 08/847529, filed on Apr. 24, 1997 by Martin et al., assigned to Northern Telecom Limited and entitled “Transparent transport”. 
     In summary, transparency in this specification implies that the bytes of the trib overhead are manipulated by the TMuxs such as to not require altering the provisioning of the existing systems, to maintain their protection switching, maintenance signalling, section/line/path performance monitoring, and to provide sufficient information for fault isolation. For example, if the trib rate is OC-48 and the midspan rate is OC-192, one solution possible is to carry the working (W) channels for all OC-48 trib systems on the OC-192 (W) channel, and the trib protection (P) channels over the OC-192 P-channel, without OC-192 protection switching enabled (defined in the above patents as the “nailed up” OC-192 option). In this arrangement, a failure of the OC-192 W-channel would trigger a span switch of all trib systems. 
     Eight OC-48 lines, or thirty OC-12/OC-3 lines can be consolidated over the high rate midspan, as detailed in the above mentioned patent applications. Bidirectional couplers may be used to further reduce the fiber count on the high rate span, i.e. from four to two fibers. It is to be noted that the bandwidth efficiency provided, 20 Gb/s bidirectional over two fibers, is accomplished without the transponders and tight tolerance transmitters and dense WDM couplers necessary in the equivalent WDM solution. 
     The invention is not limited to OC-3/OC-12/OC-48 trib signals carried by an OC-192 supercarrier, but it is also adaptable to other bit rates, in accordance with the hardware and software evolution of transport networks. Also, the invention is not limited to equipping of only identical trib rates, it is possible to carry transparently trib signals of different trib rates over the high rate span. The input tribs described in this invention have the same rate for an easier understanding of the general concept. In addition, the invention is not limited to SONET signals, and it can be applied to other synchronous transport technologies. 
     SUMMARY OF INVENTION 
     It is an object of the present invention to provide various architectures for upgrading telecommunication networks, which address fiber exhaust on a per span basis, without having to replace the equipment of all existing tributary (trib) systems. With this invention, an entire ring system does not have to be upgraded to a higher line rate due to fiber exhaust on a single span. 
     The invention is applicable to linear configurations and to ring configurations, such as OC-48 rings, although lower rate systems, such as OC-12 and OC-3 may also be upgraded. As well, the invention is applicable to higher rate rings, such as OC-192 2F-BLSR (two-fiber bidirectional line switched ring), and 4F-BLSR, where the high rate midsection is OC-768, for example. 
     It is another object of the present invention to provide a network architecture for a telecommunication system that permits tributary channels to be carried transparently over a high rate line, with no change in provisioning of tributary systems. 
     Accordingly, the invention is directed to a telecommunications network operating according to a synchronous transfer mode standard, comprising a pair of transparent multiplexers (TMuxs) connected over a bidirectional high speed span for transparently transporting high rate traffic, and a plurality (I) of bidirectional self-healing rings, each ring (K i ) having a ring rate R i , and including at least two nodes (A i , B i ) connected to each other and to the transparent multiplexers over a i-th W/P line for transporting working and protection traffic in a forward direction, and a i-th P/W line for transporting protection and working traffic in a reverse direction, wherein I is an integer, i is the index of a respective bidirectional self-healing ring, and iε[1, I], and the high rate is the sum of all the ring rates R i . 
     The invention is further directed to a telecommunications network operating in accordance with a synchronous transfer mode standard, comprising a transparent multiplexer (TMux) for connection into a high speed sub-network, a plurality (I) of bidirectional self-healing rings, each ring (K i ) including a subtended node connected to the transparent multiplexer over a i-th W/P line for transporting working and protection traffic in a forward direction, and a i-th P/W line for transporting protection and working traffic in a reverse direction at a ring rate R i , wherein I, N are integers, i is the index of a respective bidirectional self-healing ring, and iε[1, I], and the high rate is the sum of all the ring rates R i . 
     Further, a transparent ADM for a telecommunications network operating according to a synchronous transfer mode standard, at a high traffic rate comprises a trib input port and a trib output port for respectively receiving K input tribs and transmitting K output tribs, each trib of a bandwidth R i , an add/drop port for adding and dropping L local tribs, a transparent multiplexer for transparently multiplexing the K input tribs and the add local traffic into an output high rate signal, and a transparent demultiplexer for receiving an input high rate signal and demultiplexing same into the K output trib signals and the L drop tribs. 
     The invention also comprises a telecommunications network operating according to a synchronous transfer mode standard, comprising, a plurality (J) of transparent add-drop multiplexers (ADM-T) connected in a high rate bidirectional self-healing ring configuration over a high speed span, at each ADM-T j  site, a plurality (L) of nodes subtended by the ADM-T j  and connected to the ADM-T j  over a l-th W/P line for transporting working and protection traffic in a forward direction, and a l-th P/W line for transporting protection and working traffic in a reverse direction at a ring rate R l , a plurality (M) of bidirectional self-healing rings including the ADM-T j , each ring (K m ) including at least two nodes connected to each other and to the ADM-T j  over a m-th W/P line for transporting working and protection traffic in a forward direction, and a m-th P/W line for transporting protection and working traffic in a reverse direction at a ring rate R m , wherein J, L, and M are integers, j is the index of a respective ADM-T in the high rate bidirectional self-healing ring configuration, l is the index of a respective subtended node, m is the index of a respective bidirectional self-healing ring, and the high rate is L×R l +M×R m . 
     Further there is provided a telecommunications network operating according to a synchronous transfer mode standard, comprising, a first ADM and a second ADM connected in a main network over a high speed span for transmitting a high rate signal including a main signal and a subsidiary signal, a first traffic node (A) at the site of the first ADM and a second traffic node (B) at the site of the second ADM for communicating to each other over the subsidiary signal, a first additional input/output port at the first ADM for transferring the subsidiary signal to and from the first traffic node, and a second additional input/output port at the second ADM for transferring the subsidiary signal to and from the second traffic node. 
     A basic advantage of this invention is per span relief for fiber exhaust where no changes to existing systems are desired. 
     Another advantage is that a pair of TMuxs at the sites connected by the high line rate span may be a less expensive solution than the WDM (wavelength division multiplexing) approach for some network applications. For example, only one OC-192 electrical repeater is needed on the high rate span according to the invention, while four electrical repeaters are necessary in the OC-48 WDM approach. The cost of four OC-48 repeaters is about 1.6 times the cost of one OC-192 repeater. In addition, the WDM approach to accommodate higher rates on an existing network requires replacing the initially installed transmitters with a set of tight tolerance wavelength-specific (e.g. 1533 nm, 1541 nm, 1549 and 1557 nm) transmitters, adding to the overall cost of the upgrade. 
     Another advantage of the transparency is that there are no potential mid-span meet problems with the TMux-to-trib system interface regarding protection or data communication protocols, which may be the case for conventional Mux/trib system interfaces. 
     In this specification, the term ‘nested ring node’ is used for a traffic node which transports tributary traffic transparently over the high speed line to another nested ring node, where each nested ring node, although physically located in the higher rate system, behaves as a stand-alone tributary rate ring node. 
     In this specification, the term ‘subtended ring node’ is used for a traffic node which terminates tributary system traffic at that node, where the subtended ring node, although physically located in the higher rate system, behaves as a stand-alone tributary rate ring node. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiments, as illustrated in the appended drawings, where: 
     FIG. 1A illustrates a basic “span-by-span” application of the transparent multiplexer (TMux); 
     FIG. 1B illustrates how the eight OC-48 2F-BLSR tribs of FIG. 1A are carried transparently over a linear 4F OC-192 span using a “nailed-up” configuration (without protection for the OC-192 span); 
     FIG. 2A illustrates a “ring” application of TMux as an OC-192 TMux node; 
     FIG. 2B illustrates how the four OC-48 2F rings of FIG. 2A are consolidated transparently for transmission over a OC-192 TMux ring; 
     FIG. 3A illustrates an OC-192 ring/TMux node (ADM-T); 
     FIG. 3B illustrates a ring configuration with TMuxs, where the bandwidth over two spans needs to be increased; 
     FIG. 3C illustrates an upgrade for the configuration of FIG. 3B using OC-192 ring/TMux nodes (ADM-T); 
     FIG. 4A illustrates a configuration with TMuxs before upgrade; 
     FIG. 4B illustrates an upgrade for the configuration of FIG. 4A, where the bandwidth between all nodes has been increased using OC-192 ADM-T nodes in conjunction with bidirectional couplers; 
     FIG. 4C illustrates an upgrade for the configuration of FIG. 4A using OC-192 ring TMux nodes (ADM-T), where the equipment count has been reduced; 
     FIG. 5A illustrates another configuration with TMuxs before upgrade; 
     FIG. 5B illustrates an upgrade for the configuration of FIG. 5A where the bandwidth between all nodes has been increased using OC-192 ring nodes (ADM) in conjunction with bidirectional couplers; 
     FIG. 5C illustrates an upgrade for the configuration of FIG. 5A using OC-192 ADM ring nodes, where the equipment count has been reduced; 
     FIG. 6A illustrates a TMux-ring configuration before upgrade; 
     FIG. 6B illustrates the configuration of FIG. 6A upgraded to an OC-192 2F ring with subtended nodes; 
     FIG. 6C illustrates a further upgrade for the configuration of FIG. 6B using OC-192 ring nodes where the equipment count has been reduced; 
     FIGS. 7A,  7 B and  7 C illustrate upgrade stages for a typical backbone/spur system, showing another application of the transparent transport according to the invention. 
     FIGS. 8A,  8 B and  8 C illustrate use of TMux configurations as interim steps in upgrading of a ring to a higher bandwidth; 
     FIGS. 9A,  9 B and  9 C illustrate use of TMux configurations as interim steps in upgrading of a ring; 
     FIGS. 10A and 10B illustrates how traffic is switched between the principal and secondary nodes of FIG. 9C; and 
     FIG. 10C shows how traffic is carried between the principal and secondary nodes of FIG.  9 C. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the example illustrated in FIG. 1A, eight OC-48 2F-BLSRs have traffic nodes  1 ,  2  . . .  8  and  1 ′,  2 ′ . . .  8 ′, respectively, in two adjacent sites A and B, which could be two metropolitan areas with heavy traffic. Without TMuxs, each ring would need a fiber span between sites A and B, resulting in sixteen fibers between sites A and B. In order to reduce the fiber count, each site A, B was equipped with a respective transparent multiplexer (TMux)  100 ,  101 , which results in all traffic for the OC-48 rings being carried over a high rate  4 F midspan comprising fibers  9  and  10 , each supporting bidirectional traffic at OC-192 rate. The OC-192 protection is disabled in this configuration, but any protection switching information on a respective trib system is transmitted from the input span  11 ,  12 , . . .  18  to the output span  11 ′,  12 ′, . . .  18 ′ on the midspan  9 ,  10 . 
     FIG. 1B shows how the eight 2F OC-48 trib systems of FIG. 1A are carried transparently over the linear 4F OC-192 span using a ‘nailed-up’ configuration. For a 2F-BLSR trib system protection type, the traffic can be carried over either the OC-192 W-channel or the OC-192 P-channel without OC-192 protection switching enabled (hereinafter called the “nailed up” OC-192 option). As illustrated in FIG. 1B, the forward channels for four OC-48 trib systems are carried in the forward direction on the forward fiber of working (W) span  9 , and the reverse traffic is carried on reverse fiber of (W) span  9 . Similarly, the forward channels for four more OC-48 trib systems are carried in the forward direction over the forward fiber of protection (P) span  10  and over the reverse fiber of P span  10  in the reverse direction. Each fiber of the high-speed span carries a bandwidth of OC-192, resulting in a total bandwidth over span  9 ,  10  of 20 Gb/s. In this arrangement, a failure of either the OC-192 W-channel or P-channel would trigger a ring switch for the trib systems. 
     FIG. 2A shows a “ring” application of the TMux according to the invention where four OC-48 2F-BLSR rings  1 - 4  are connected to a TMux node  200  over spans  11 ,  11 ′;  12 ,  12 ′;  13 ,  13 ′; and  14 ,  14 ′, respectively. Node  200  is in turn connected in an OC-192 ring over spans  9  and  10 . 
     FIG. 2B illustrates how traffic from the four OC-48 2F-BLSR rings of FIG. 2A are consolidated transparently for transmission over the OC-192 TMux ring. Since the trib rings are 2F-BLSRs, each bidirectional span  11  and  11 ′ carries both working (W) and protection (P) traffic in the respective timeslots. For the forward direction (W-E), TMux  200  consolidates the OC-48 working traffic received over fibers  11 - 14  and transmits it over (W) fiber  9 F. Similarly, protection traffic received from fibers  11 ′ to  14 ′ is transmitted over (P) fiber  10 F. In the opposite direction (E-W), traffic received from fibers  9  and  10  is demultiplexed onto fibers  11 - 14  and  11 ′- 14 ′, respectively. OC-192 protection is again disabled in this configuration. 
     FIG. 3A shows the block diagram of an OC-192 ring/TMux (ADM-T) node. A transparent add-drop multiplexer  64  receives input tribs I 1  to I K  from K ports  54  to  56 , each connected to a tributary network. TMux  58  also receives L local add signals A 1  to A L  from add/drop port  59 . These signals are transparently multiplexed into a supercarrier S which is output from port  57  into a high rate network, in this case an OC-192 ring. Similarly, TMux  58  receives high rate signal S′ from the high rate network and demultiplexes same into K output trib signals O 1  to O K , which are then inserted in the respective trib network through ports  54  to  56 , each connected to a tributary network. TMux also provides L local drop signals D 1  to D L  to port  59 . Such a node may be used for upgrading networks to higher rates, or for saving on equipment, as shown next. 
     FIG. 3B illustrates a configuration with eight OC-48 rings using TMuxs  100  to  103 . In this configuration, OC-48 nodes  1 - 5  are co-located with TMux  100  in central office  29 , nodes  1 ′- 5 ′ are co-located with TMux  101  in central office  29 ′, nodes  6 ′- 10 ′ are co-located with TMux  103  in central office  30 ′, and nodes  6 - 10  are co-located with TMux  102  at central office  30 . A first OC-48 ring  21  includes TMuxs  100  and  101 , nodes  4 ′,  6 ′, TMuxs  103  and  102 , and nodes  6  and  4 . Similarly, OC-48 ring  22  comprises nodes  100 ,  101 ,  5 ′,  7 ′,  103 ,  102 ,  7  and  5 . TMuxs  100  and  101  are also connected in three OC-48 2F rings, a ring  23  also including nodes  1 ,  1 ′; ring  24 , including nodes  2 ,  2 ′; and ring  25  including nodes  3 ,  3 ′. Similarly, TMuxs  102  and  103  are connected over ring  23 ′ including nodes  8 ,  8 ′, ring  24 ′ including nodes  9 ,  9 ′, and ring  25 ′ including nodes  10 ,  10 ′. 
     Each TMux consolidates the traffic from its five tribs as in the configuration of FIG. 1A, therefore spans  9 ,  10 , and  9 ′,  10 ′ each carry a bandwidth of 5×OC-48, while spans  27 ,  28  and  27 ′,  28 ′ carry 2×OC-48. 
     If due to customer demand more bandwidth is needed over the spans  27 ,  28  and  27 ′,  28 ′, TMuxs  100  to  103  can be upgraded to ADM-T nodes  200 - 203 , as shown in FIG. 3C, and connected in an OC-192 4F ring  31 , resulting in the configuration of FIG.  3 C. No additional fiber needs to be deployed between any sites. 
     In this way, the OC-48 traffic (both working and protection) on rings  23 - 25  and  23 ′- 25 ′ is still carried transparently over the OC-192W channel. OC-48 nodes  4 - 7  and  4 ′- 7 ′ of rings  21  and  22 , respectively, become subtended rings (multiple two-node rings), namely  4  and  5  are subtended by ADM-T  200 ,  4 ′ and  5 ′ are subtended by ADM-T  201 ,  6 ′ and  7 ′ are subtended by ADM-T  203 , and  6  and  7  are subtended by ADM-T  202 . This results in a used capacity of 4×OC-48 on spans  9 ,  10  and  9 ′,  10 ′, since nodes  4 - 7  and  4 ′- 7 ′ only add/drop STS-24 of working traffic each. One STS-48 only is used on spans  27 ,  28  and  27 ′,  28 ′. As such, the configuration of FIG. 3C results in three additional STS-48s available on each of spans  27 ,  28  and  27 ′,  28 ′. 
     FIG. 4A illustrates sixteen OC-48 2F rings. The configuration uses TMuxs  100  and  101  provided at sites  29  and  29 ′ respectively, for transparently transporting the traffic on four OC-48 rings  23 - 26  within an OC-192 supercarrier over span  9 ,  10 . Similarly, TMuxs  102  and  103  deployed at sites  29 ′ and  30 ′, respectively, transport the traffic on four OC-48 rings  32 ′- 35 ′ within an OC-192 supercarrier over span  27 ′,  28 ′, TMuxs  104  and  105  deployed at sites  30 ′ and  30 , respectively, transparently transport the traffic on four OC-48 rings  23 ′- 26 ′ within an OC-192 supercarrier over span  9 ′,  10 ′, and TMuxs  106  and  107  deployed at sites  30  and  29 , respectively, consolidate the traffic on four OC-48 rings  32 - 35  within an OC-192 supercarrier over span  27 ,  28 . The OC-192 spans have protection disabled. 
     The customers&#39; requests for more bandwidth between all sites can be addressed as shown in FIG. 4B, where the TMuxs were upgraded to OC-192 ADM-T nodes, which are connected in two OC-192 4F rings  41 ,  42 , which use the same fiber spans  9 ,  10 ;  27 ′,  28 ′;  9 ′,  10 ′ and  27 ,  28 . Reference numeral  37  illustrates a group of four 2:1 couplers. Eight such groups are necessary for directing the traffic from the two ADM-Ts at a respective site over the high-rate spans, for both forward and reverse directions. The OC-48 ring segments between the sites involved are still carried transparently by the respective supercarriers. It is apparent that no additional fibers were deployed between any sites, and that four additional OC-48 tribs may be carried over ring  41 , and  42  as shown by the thicker lines. 
     On the other hand, if reduction of equipment is desired, the TMuxs at each site could be replaced by one ADM-T node connected in an OC-192 4F ring configuration  51 , as shown in FIG.  4 C. OC-48 ring segments are still carried transparently. No additional fiber span needs to be deployed in the configuration of FIG. 4C, while four OC-192 TMux nodes are freed-up. 
     FIG. 5A illustrates a first upgrade stage configuration with TMuxs. In the initial stage (not shown) nodes  1 ,  1 ′,  9 ,  9 ′;  2 ,  2 ′,  10 ,  10 ′;  3 ,  3 ′,  11 ,  11 ′;  4 ,  4 ′,  12 ,  12 ′;  5 - 5 ′,  13 ,  13 ′;  6 ,  6 ′,  14 ,  14 ′;  7 ,  7 ′,  15 ,  15 ′; and  8 ,  8 ′,  16 ,  16 ′; were connected in eight respective 2F OC-48 rings. As in the previous examples, nodes 1-8 are located at site  29 , nodes  1 ′- 8 ′ are located at site  29 ′, nodes  9 ′- 16 ′ are at site  30 ′ and nodes  9 - 16 , at site  30 . 
     In the configuration shown in FIG. 5A, each site is provided with two TMuxs, a TMux for transparently transporting the traffic for all eight OC-48 rings to/from a neighbouring site. For example, TMux  100  and  101  consolidate the traffic between nodes  1 - 8  at site  29  and nodes  1 ′- 8 ′ at site  29 ′. Each span  9 ,  10 , carries transparently traffic at OC-192 rate in both directions, with no protection enabled on the OC-192 span. Similar connections are provided between sites  29 ′ and  30 ′,  30 ′ and  30 , and  30  and  29 . 
     The next upgrade stage involves replacing the TMuxs with OC-192 ring nodes  250 - 257  and connecting them into two OC-192 4F rings  51 ,  52 . While two sets of four 2:1 couplers  37  are necessary at each site, resulting in a total of 32×2:1 couplers for accommodating the bidirectional nature of the traffic and for consolidating the traffic on four fibers, no additional fiber needs to be deployed between the sites. The OC-48 nodes  1 - 8 ;  1 ′- 8 ′;  9 - 16 ; and  9 ′- 16 ′ are connected as subtended rings (multiple 2-node rings). As each OC-48 trib system uses at most a bandwidth of STS-24 of working traffic on the OC-192 node, each span  9 ,  10 , carries only a bidirectional STS-96 of working traffic. This leaves a bidirectional STS-96 available over each ring and results in a bandwidth of four STS-48s available around the two rings  51 ,  52 . 
     FIG. 5C illustrates another upgrade for the configuration of FIG. 5A for savings on equipment. The eight TMuxs  100  to  107  are here replaced with four OC-192 ring nodes  250 ,  252 ,  254  and  256 , to obtain an 4F OC-192 ring  53 . Each OC-192 ADM subtends eight OC-48 nodes, resulting in four OC-192 nodes being freed-up. No additional fiber and equipment were necessary. 
     FIG. 6A illustrates how traffic on four 2F OC-48 rings is carried transparently by an OC-192 configuration with four TMuxs  100 - 103 . Each TMux carries four 2F OC-48 rings, as shown in FIG. 2A, and each span  10 ,  28 ′,  10 ′, and  28  carries an STS-192 between adjacent sites. An upgrade is shown in FIG. 6B where the TMuxs were replaced with OC-192 ring nodes  250 ,  252 ,  254  and  256  connected into a  2 F ring  61 . The OC-48 ring nodes are now subtended (multiple two-node rings). No additional fiber and equipment were necessary. This is an interim step to the upgrade of FIG.  6 C. 
     The next upgrade stage is shown in FIG. 6C, where the outboard OC-48 NEs were eliminated, so that 16 OC-48 ring nodes (4×4) were freed-up. The OC-192 ring nodes  260 ,  262 ,  264  and  266 , and the resulting ring  62  is a 2F OC-192 which supports the same trib rates and quantities as the original subtended OC-48 ring nodes, as ring  61 . 
     FIGS. 7A,  7 B and  7 C illustrate upgrade options for a typical backbone/spur system, showing a variation of the TMux referred to as nested trib rings. The system to be upgraded, shown in FIG. 7A comprises an OC-192 backbone network  42  deployed between ADM #1 and ADM #2. Terminal TM #1 is connected to the backbone over a lower rate spur including a regenerator  43 , ADM #3 and a trib port in ADM #1, while terminal TM #2 is connected to the backbone through a separate lower rate spur system through ADM #4 and a trib port in ADM #2. 
     To improve the survivability of the spur networks, the network provider would like to close the spurs into a ring configuration. A subtended ring configuration is one option available without the TMux of the present invention, as shown in FIG.  7 B. Two new routes are provided, R #1 between the sites of ADM #1 and ADM #4, and R #2 between terminals TM#1 and TM #2. As well, an additional tributary, which acts in conjunction with the existing tributary as an embedded ADM #5, must be added to ADM #1, and terminals TM #1 and TM #2 have to be upgraded to ADM #6 and ADM #7, respectively. The changes are shown in bold on FIG.  7 B. 
     FIG. 7C shows a second option possible with TMux used in a nested trib ring. R #1 is not necessary in this configuration, resulting in fiber savings. The dotted line illustrates the channel carried by R #1 of FIG. 7B, which is now nested in part of the OC-192 line. The embedded ADMs #5 and 8 are not subtended ring nodes, but nested ring nodes, where their interconnecting span is nested in the OC-192 line. Thus, by upgrading a normal linear ADM chain to include a nested trib ring (FIG.  7 C), the network operator achieves a more survivable collector network with only the addition of a single new route (R#2) rather than two as in the case shown in FIG. 7B (R#1 and R#2). 
     FIGS. 8A-8C illustrate the use of TMuxs in an interim configuration for upgrading an OC-48 2F ring  81  to an OC-192 2F ring  82 . The OC-48 2F ring of FIG. 8A includes initially nodes  240 ,  242 ,  244 , and  246 , which are OC-48 ring nodes. During the interim stage shown in FIG. 8B, the traffic is rerouted onto three TMuxs  100 ,  101  and  102  to increase bandwidth on a per span basis. Thus, OC-48 traffic is carried between nodes  240  and  242  over a high speed span  9 , while OC-48 traffic is carried between nodes  240  and  246  over high speed span  27 . No additional fiber has been deployed between nodes  240 ,  242 , and  246 , as is the case when TMuxs are used. In the final stage shown in FIG. 8C, the entire ring has been upgraded to an OC-192 2F by reconfiguring TMux&#39;s to ring ADMs. OC-48 ring nodes are now subtended, i.e. connected to the respective OC-192 ring node as an OC-48 2F ring. Each span  9 ,  27 ′,  9 ′ and  27  carries an OC-192 of traffic. 
     Still another example for illustrating use of TMux configurations as interim steps for upgrading an existing ring is shown in FIGS. 9A-9C. OC-48 and OC-192 2F rings were used for these examples, but rings of lower or higher rates may be upgraded in a similar way. 
     The example of FIG. 9 provides for two OC-48 2F rings  91  and  92 , connected by a tributary span  95  between nodes  246  and  241 , co-located at site  29 , and further connected by a tributary span  95 ′ between nodes  244  and  243  co-located at site  29 ′. 
     One option, without using transparency, is to upgrade ring  91  to a OC-192 ring by replacing the OC-48 ring nodes  240 ,  242 ,  244 , and  246  with OC-192 ring nodes  250 ,  252 ,  254 , and  256 , as shown in FIG.  9 B. 
     Another option, shown in FIG. 9C, is to upgrade ring  91  to an OC-192 2F ring  93 , with a portion of the OC-48 ring  92  nested within it. This option frees up low speed ADMs and the interconnect, and one fiber route,  10 . Traffic between nested ADMs  240  and  242  is carried over span  9 ′, as shown by dotted line. 
     FIGS. 10A and 10B illustrates how traffic is routed between ADMs  257  and  258 , while FIG. 10C expands on how the inter-ring traffic is carried over the OC-192 span between these nodes, as in the example of FIG.  9 C. OC-192 transmitter/receiver T/R#1 of primary node  257  exchanges traffic with ADM  250  (arrow A 1 ), and with ADM  258  (arrow A 3 ) of OC-192 ring  93  through OC-192 T/R#2. 
     T/R#3 of primary node  257  is connected to ADM  246  (arrow B 1 ), and to ADM  258  (arrow B 3 ) over OC-48 ring  92  through OC-192 T/R#2. T/R#2 handles both OC-192 and OC-48 traffic for the respective ring  93  (arrows A 3  and A 2 ) or  92  (arrows B 3  and B 2 ), while T/R#4 handles both OC-192 traffic as shown by arrows A 1  and A 2 , and OC-48 traffic, as shown by arrows B 1  and B 2 . Switch SW directs traffic on the respective ring. Secondary node  258  operates in a similar way. 
     The OC-48 traffic is carried over the working timeslots of span  9 ′, using half of the working bandwidth, as shown in FIG.  10 C. This maintains independence of OC-48 and OC-192 protection. For inter-ring traffic, the service selector is at the sink node, not at the principal node  257  as usual. The relationship between the principal node  257  and secondary node  258  is flipped between the rings, similar to BLSR opposite side routing. 
     While the invention has been described with reference to particular example embodiments, further modifications and improvements which will occur to those skilled in the art, may be made within the purview of the appended claims, without departing from the scope of the invention in its broader aspect.