CFig. 7.16 a Under no
7.8 Shared Protection Based on Pre-deployed Subconnections
The paradigm of pre-deployed subconnections can be used to avoid issues with power-level transients. The notion of a subconnection was introduced in Chap. 4, where regenerations along a path effectively break the end-to-end connection into smaller subconnections. Both ends of a subconnection are terminated in the electri-cal domain, with optielectri-cal bypass at the intermediate nodes. In Chap. 4, the design process started with a connection and broke it into subconnections for regeneration and wavelength assignment purposes. With subconnection-based protection, the process is reversed; subconnections are pre-deployed in a network and concatenated as needed to form end-to-end backup paths.
A pre-deployed subconnection refers to a lit wavelength that is routed between two transponders, where the capacity is not currently being used to carry traffic.
Thus, the transponders have been pre-deployed in the network and turned on for purposes of future traffic. Using pre-deployed subconnections as a building block for rapidly accommodating dynamic traffic or rapidly recovering from a failure was proposed in Simmons et al. [SiSB01]. Shared mesh protection based on pre-deployed subconnections is described below; further details can be found in Sim-mons [Simm07].
Consider the network shown in Fig. 7.18a, where it is assumed that the nodes are equipped with ROADM/ROADM-MDs and edge switches. Two working paths are established as indicated by the dotted lines, i.e., along A-B-C-D and A-J-K-I. There are three pre-deployed protection subconnections as indicated by the dashed lines:
A-F-G-H, H-D, and H-I. The transponders at the endpoints of the working paths as well as the transponders at the endpoints of the protection subconnections are fed into the edge switch at the respective nodes. The details of Nodes A and H are shown in Fig. 7.18b, c respectively.
The protection subconnection along A-F-G-H is shared by both working paths. If there is a failure along A-B-C-D, the edge switch at Node H concatenates the G-H subconnection with the H-D subconnection to form a backup path along A-F-G-H-D. In addition, the edge switches at Nodes A and D are reconfigured such that the client (in this case, an IP router) is connected to the backup path. Alternatively, if the failure occurs along A-J-K-I, then Node H concatenates A-F-G-H to H-I to form a protect path along A-F-G-H-I, and Nodes A and I reconfigure their edge switches to direct the client to the protect path. The scheme provides fault-independent path-based protection, such that fault localization is not needed to initiate recovery.
The most salient features of this scheme are that the transponders at either end of the protection subconnections are always on and at the desired wavelength, and that any switching occurs in the edge switch as opposed to in the ROADM-MD (i.e., client-side signals are switched, not network-side signals). Thus, the power levels on the fibers do not change as the protect path is formed, thereby avoiding issues with optical amplifier transients. After a destination detects that a connection has failed, the speed of the protection mechanism depends on the time it takes to notify
the source and the intermediate switching locations (e.g., Node H) of the failure, and the time required to reconfigure the edge switches. No turning on, retuning, or switching of WDM-compatible signals is required. In a continental-scale network, recovery on the order of 100 ms should be possible.
In addition, the scheme is compatible with restoration signaling architectures that already have been developed by carriers. For example, the Robust Optical-Lay-er End-to-End X-Connection (ROLEX) signaling mechanism [DSST99, CDLS09]
can be used to progress from one end of the restoration path to the other, selecting the subconnection to use and requesting the desired cross-connection in the edge switch. If both directions of a bidirectional connection fail, then two-ended ROLEX can be utilized, where recovery is initiated at both ends. Eventually, the processes meet at some intermediate node, such that the end-to-end bidirectional restoration path is established.
Furthermore, this protection scheme is well suited for the hierarchical protection paradigm [Simm99], such that it takes good advantage of optical bypass even for the protect wavelengths. In hierarchical protection, a subset of the nodes are chosen as “high-level” nodes, where the bulk of the protection capacity extends between these nodes, optically bypassing the “low-level” nodes. Nodes that generate a lot of
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Fig. 7.18 a Shared protection based on pre-deployed subconnections. The three protection sub-connections are indicated by the dashed lines. b Details of Node A. c Details of Node H. In both nodes, the edge switch as shown is photonic
protected traffic, nodes with a high degree, and nodes that are located strategically in the network (e.g., for regeneration) are generally favored as high-level nodes.
Applying this paradigm to the subconnection scheme described above, the majority of the protection subconnections are pre-deployed with high-level nodes as end-points (a small number of subconnections need to terminate on low-level nodes in order to provide protection for the demands that originate at these nodes). This al-lows a significant amount of optical bypass to be realized as most of the protection capacity transits the low-level nodes.
The possible disadvantage of this scheme is the requirement for an edge switch at some, or all, of the nodes. However, as has been emphasized previously, an edge switch can improve the flexibility of a node, such that it may be desirable to deploy such switches anyway. Another pre-deployed-subconnection-based shared mesh protection scheme, which does not require an edge switch, was proposed in Li et al. [LiCS05]. The scheme uses the combination of tunable regenerator cards and directionless ROADM-MDs to concatenate the protection subconnections; the scheme is not compatible with a non-directionless ROADM-MD. Failure recovery requires retuning some transponders and regenerators, turning off some regen-erators, and reconfiguring the ROADM-MDs at the connection endpoints. Thus, while the requirement for the edge switch is eliminated, the restoration process is somewhat slower and does not avoid the transient issue. As systems evolve to bet-ter manage optical amplifier transients, e.g., Zhou et al. [ZhFB07], such schemes will be more viable. Furthermore, if optical amplifier transients are managed to the point that they are a nonfactor, then the subconnection paradigm can remain in place but the requirement that they be pre-lit can be removed. This allows tran-sponders to be shared among the subconnections at a node (i.e., a transponder is not assigned to a subconnection until it is actually needed), thereby reducing the number of transponders that must be pre-deployed. It also provides the op-portunity to all-optically connect two subconnections, assuming that they both support the same wavelength and the concatenated subconnection does not violate the optical reach [CCCD12].
7.8.1 Cost Versus Spare Capacity Trade-off
The pre-deployed-subconnection protection scheme inherently poses a trade-off of cost versus capacity. To achieve better sharing of the protection capacity, short-er subconnections are pre-deployed (i.e., subconnections with fewshort-er hops). This translates into a greater number of required protection subconnections, where each subconnection incurs the cost of two transponders and two edge-switch ports. Fig-ure 7.19 illustrates this trade-off. The dotted lines represent the working paths, and the dashed lines represent the protection subconnections. The same three working paths are shown in Fig. 7.19a, 7.19b: A-E, A-G-D, and C-G. In Fig. 7.19a, there are three protection subconnections, whereas in Fig. 7.19b there are four. Either configuration is sufficient to provide protection from a single link or node failure.
Figure 7.19a requires six protect transponders and seven wavelength-links of pro-tection capacity, whereas Fig. 7.19b requires eight protect transponders but requires only six wavelength-links of protection capacity. Thus, Fig. 7.19b is more capacity efficient, but more costly. By dividing the A-B-C-D protection subconnection into A-B-C and C-D, as in Fig. 7.19b, the C–D protection subconnection can be shared by all three working paths.
A study was performed to investigate the cost versus capacity trade-off further, using Reference Network 2. The network was assumed to be optical-bypass en-abled, with an optical reach of 2,500 km. Several shared mesh protection designs were performed, where in each design an increasing number of network nodes were selected as protection hubs. The protection hubs are akin to the “high-level” nodes in the hierarchical protection scheme described earlier, where protection capacity is
“chopped” into subconnections at the hubs. Thus, the greater the number of hubs, the shorter the resulting protection subconnections, yielding more opportunities for sharing, but resulting in higher cost. (The working paths can optically bypass the hubs, however.)
Demands requiring shared protection were added one by one to the network, with no knowledge of future demands. Enough demands were added such that the resulting capacity requirement on the most heavily loaded link was on the order of 100 wavelengths. All demands were at the line rate (i.e., no traffic grooming was needed). The paths of the demands were selected with an emphasis on sharing the existing protection capacity.
Varying the number of protection hubs produces the “cost” versus capacity curve plotted in Fig. 7.20. (Each point in the curve represents an average of sev-eral runs; the variance among the runs was very small.) The primary y-axis is the normalized total number of transponders required for the working and protect paths; this is used as a rough measure of network cost. (Any regeneration was tallied as two transponders.) The x-axis is the normalized total required capacity for the working and protect paths, measured in wavelength-km. The percentages
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Fig. 7.19 The working paths are indicated by the dotted lines, and the pre-deployed subconnec-tions are indicated by the dashed lines. a Three pre-deployed subconnecsubconnec-tions, requiring seven wavelength-links of capacity and six transponders. b Four pre-deployed subconnections, requiring six wavelength-links of capacity and eight transponders
next to the data points indicate the percentage of nodes that were selected as hub nodes. As expected, as the number of hubs decreases, the required total capacity increases but the cost decreases. From this graph, selecting roughly 15–20 % of the nodes as protection hubs represents a good trade-off point, where the number of transponders and the required capacity are both within ~ 15 % of their minimums.
A study was performed for several other networks in Simmons [Simm07], produc-ing similar results. Note that, while not shown on the graph, selectproduc-ing 100 % of the nodes to be protection hubs reduces the total required capacity by less than 1 % and increases the total number of transponders by almost 10 %, as compared to the scenario where 55 % of the nodes are protection hubs; thus, this is not an attractive option.
Given that the protection subconnections require transponders at the endpoints, and hence O-E-O conversion, it is interesting to investigate the amount of optical bypass attainable in the network. The top curve in Fig. 7.20 plots the average optical bypass in the network (this is the percentage of working and protect wavelengths that enter a node that optically bypass the node). As the number of hubs decreases, the average optical bypass increases because the protection capacity is being elec-tronically terminated less frequently. With 15–20 % of the nodes as hubs, the aver-age optical bypass is about 65 %, indicating that this shared protection scheme is able to take good advantage of optical bypass.
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Fig. 7.20 The lower curve represents “cost” versus capacity for shared mesh protection based on pre-deployed subconnections, in Reference Network 2. The total number of transponders ( TxRxs) required for the working and protect paths is used as a rough measure of cost. The percentages next to the data points indicate the percentage of nodes selected as protection hubs. The upper curve is the average optical-bypass percentage achieved in the network, assuming 2,500-km optical reach