**4. Protection/restoration in transport networks**

Due to several failure issues, one of the key aspects to be considered while planning and operating telecommunication networks is resiliency. With the evolution toward 5G, everyday network operators and equipment vendors struggle to come up with innovative network resiliency mechanisms which are robust, faster, and cost-effective. Transport networks are no alien to this behavior and the resiliency schemes for the same dates back to the time when SDH/SONET-based networks were at helm. Currently, OTN is the predominant transport technology. Given the multiple layers defined within the OTN, resilience mechanisms acting at the electrical and optical domains are available, the former being enforced at the ODU layer via ODU switching network elements, whereas the latter are supported at the OCh layer through ROADMs [6].

As referred, in addition to the layer at which failure survivability mechanisms are enforced, these mechanisms can be further classified as protection or restoration. Usually, protection relies on dedicated backup resources determined in advanced, reserved, and preconfigured for a particular service and which can be quickly triggered to replace the working resources. On the other hand, restoration does not require dedicated resources, i.e., backup resources can be shared, and the backup path and its resources are only assigned upon recovery from a failure. Moreover, restoration is typically triggered by the control plane, i.e., via the generalized multiprotocol label switching (GMPLS). GMPLS-based restoration can by dynamic, designated as dynamic source rerouting (DSR) and where backup resources are determined once a failure is detected, or preplanned, named preplanned shared restoration (PSR), in which case the backup resources are known in advance of the failure. The main advantage of DSR is that it can grant (best effort) survivability against a larger number of failure scenarios, namely, when multiple failures take place. On the other hand, PSR provides faster recovery of the failed service/demand since it avoids the time required to compute the backup path and associated resources [6].

**b.** Set the cost of all links in *L*ʹ, such that their cost verifies *c*(*l*ʹ) ≪ *c*(*l*

routing and grooming solution cost be *c*(π*<sup>k</sup>*

Otherwise, repeat the steps from **(Step 3a)**.

**d.** Return the routing and grooming solution for *t*

**4. Protection/restoration in transport networks**

**(Step 3)** for the next traffic demand.

218 Optical Fiber and Wireless Communications

value *S*.

through ROADMs [6].

cost. If none is found, the traffic demand is blocked.

reusing an existing light path is always preferred over creating new light paths and creating a new 200G light path is slightly more economic than creating a 100G light path (as to give preference to more spectral efficient 200G light paths in case of a tie).

**c.** Route over the auxiliary graph to determine the least cost path/cycle and let the

**Step 4**. If all traffic demands have been considered, end the algorithm. Otherwise, repeat from

Importantly, the framework is ready to support flexible-rate line interfaces, namely, capable of operating at 100G (QPSK) and 200G (16-QAM), and it can easily incorporate more line rates (e.g., 150G and 300G via 8-QAM and 64-QAM modulation formats, respectively) or can be executed for a single line rate by disabling all others. Moreover, in the case of utilizing shared restoration mechanisms to recover from failure scenarios, the fact that network resources can be shared by the backup paths of ODUs/OChs whose working paths are link- or node-disjoint also needs to be modeled. In order to control the degree of resource sharing, the number of ODUs/OChs that can share the same resource is limited by a maximum resource sharing

Due to several failure issues, one of the key aspects to be considered while planning and operating telecommunication networks is resiliency. With the evolution toward 5G, everyday network operators and equipment vendors struggle to come up with innovative network resiliency mechanisms which are robust, faster, and cost-effective. Transport networks are no alien to this behavior and the resiliency schemes for the same dates back to the time when SDH/SONET-based networks were at helm. Currently, OTN is the predominant transport technology. Given the multiple layers defined within the OTN, resilience mechanisms acting at the electrical and optical domains are available, the former being enforced at the ODU layer via ODU switching network elements, whereas the latter are supported at the OCh layer

As referred, in addition to the layer at which failure survivability mechanisms are enforced, these mechanisms can be further classified as protection or restoration. Usually, protection relies on dedicated backup resources determined in advanced, reserved, and preconfigured for a particular service and which can be quickly triggered to replace the working resources. On the other hand, restoration does not require dedicated resources, i.e., backup resources can be shared, and the backup path and its resources are only assigned upon recovery from

*d*

<sup>200</sup>ʹ) < *c*(*l*

\*). Increment *k*. If *k* = *K*, go to **(Step 3d)**.

, over all *K* solutions, with smallest

<sup>100</sup>ʹ), that is,

The economics and qualitative behavior of PSR in transport networks are evaluated in the following, with particular emphasis on comparing the use of such scheme at the ODU and OCh layers and identifying in which network and traffic scenario is each one of them expected to be a more suitable option for a transport network operator.

Based on the resiliency schemes and the quality-of-service (QoS) they cater to, the working and protection/restoration paths are either link disjoint or node disjoint (the later forces the links to be disjoint though). Resources on the protection/restoration path (backup path) can be shared among multiple demands contrary to the same being dedicated to a single demand and this is enabled by the shared restoration scheme as illustrated in **Figure 5**. The sharing of resources is a novelty when compared to 1:1 dedicated protection schemes, which increase CAPEX due to a higher number of line interfaces required. Solid red and yellow highlights the working path of both the demands, respectively, while both these demands are using the same backup path. This is only possible when the working paths of both the demands are link disjoint and both the demands simultaneously stay unaffected by a single link failure and thus the same resource can be used for restoring either of the demands.

When resource sharing is enforced, savings are expected with respect to the amount of required additional resources, when compared to dedicated schemes (e.g., 1 + 1 protection), although restoration schemes usually provide slower recovery to failures. Furthermore, a

**Figure 5.** Illustration of resource sharing for shared restoration schemes.

qualitative and quantitative comparison of both schemes (ODU and OCh based restoration) can be performed, addressing key aspects such as restoration time, network element complexity, switching granularity, planning complexity, and line interface count. **Table 1** intends to summarize the qualitative comparison of both schemes.

In order to gain further insight on how PSR at the ODU and OCh level compare in terms of their resource requirements, above all in number of line interfaces, a planning case study is presented in this section. The study is realized over the 31-node backbone network covering Italy that is already used in previous studies [10] and illustrated in **Figure 6**. It is assumed that each network link supports up to 96 wavelength channels and each wavelength is operated at 100 Gb/s (i.e., carries one ODU4). A very simplified performance model is used to determine the transmission reach between regeneration sites: it consists of a maximum transmission reach of 1500 km and a reach penalty of 60 km per node traversed. With respect to the traffic pattern, 30% of the node pairs were randomly selected to exchange traffic demands. Each client traffic demand is mapped into an appropriate ODU *k*, with *k* = {0, 1, 2, 3}, which correspond to data rates of {1.25, 2.5, 10, 40} Gb/s, respectively. The traffic load offered to the network is evenly distributed over the different ODU rates, meaning that each ODU *k* accounts for around 25% of all offered traffic load [6]. As to understand the impact of the network traffic load on the effectiveness of both restoration schemes, the total traffic load is varied from 2 up to 20 Tb/s, with load increments of 2 Tb/s.

A comprehensive comparison requires not only to compute the resource requirements of enforcing ODU or OCh restoration for every traffic demand, but also to benchmark these results against the resources needed when the traffic demands are either unprotected or require 1 + 1 protection. The routing and grooming framework of Section 3 is used to plan the network. An upper bound on the number of demands sharing a common restoration resource is set to *S* = 10. Importantly, in the case of PSR at the OCh layer, the traffic is first groomed into ODU 4s and afterward the resulting ODU 4 are routed considering also the protection/ restoration mechanism being enforced.


**Table 1.** Qualitative comparison of ODU and OCh restoration.

Next-Generation Transport Networks Leveraging Universal Traffic Switching and Flexible Optical Transponders http://dx.doi.org/10.5772/intechopen.68953 221

**Figure 6.** Network topology – Telecom Italia National Backbone 31 Node network.

qualitative and quantitative comparison of both schemes (ODU and OCh based restoration) can be performed, addressing key aspects such as restoration time, network element complexity, switching granularity, planning complexity, and line interface count. **Table 1** intends to

In order to gain further insight on how PSR at the ODU and OCh level compare in terms of their resource requirements, above all in number of line interfaces, a planning case study is presented in this section. The study is realized over the 31-node backbone network covering Italy that is already used in previous studies [10] and illustrated in **Figure 6**. It is assumed that each network link supports up to 96 wavelength channels and each wavelength is operated at 100 Gb/s (i.e., carries one ODU4). A very simplified performance model is used to determine the transmission reach between regeneration sites: it consists of a maximum transmission reach of 1500 km and a reach penalty of 60 km per node traversed. With respect to the traffic pattern, 30% of the node pairs were randomly selected to exchange traffic demands. Each client traffic demand is mapped into an appropriate ODU *k*, with *k* = {0, 1, 2, 3}, which correspond to data rates of {1.25, 2.5, 10, 40} Gb/s, respectively. The traffic load offered to the network is evenly distributed over the different ODU rates, meaning that each ODU *k* accounts for around 25% of all offered traffic load [6]. As to understand the impact of the network traffic load on the effectiveness of both restoration schemes, the total traffic load is varied from 2

A comprehensive comparison requires not only to compute the resource requirements of enforcing ODU or OCh restoration for every traffic demand, but also to benchmark these results against the resources needed when the traffic demands are either unprotected or require 1 + 1 protection. The routing and grooming framework of Section 3 is used to plan the network. An upper bound on the number of demands sharing a common restoration resource is set to *S* = 10. Importantly, in the case of PSR at the OCh layer, the traffic is first groomed into ODU 4s and afterward the resulting ODU 4 are routed considering also the protection/

Restoration time • Faster: few hundreds of milliseconds • Slower: hundreds of milliseconds up to

Planning complexity • Multilayer: with intermediate grooming • Single-layer: without intermediate grooming

seconds

summarize the qualitative comparison of both schemes.

220 Optical Fiber and Wireless Communications

up to 20 Tb/s, with load increments of 2 Tb/s.

restoration mechanism being enforced.

**Restoration layer**

**Table 1.** Qualitative comparison of ODU and OCh restoration.

**ODU OCh**

ODU switching • Mandatory • Not required

ROADM complexity • Low: can use simple ROADMs • High: colorless and directionless

Switching granularity • Finer: starting from 1.25 Gb/s • Coarser: e.g., 40 or 100 Gb/s

Line IF count • Expected to be higher • Expected to be lower

For both ODU and OCh protection/restoration, disjointness is applied at the link level and the number of candidate routing paths/cycles *K* is set to 5 working and backup paths. **Figures 7** and **8** present the line interface count and wavelength channel utilization as a function of the offered traffic load when using resilience mechanisms acting at the ODU layer. The wavelength channel utilization is defined as the fraction of wavelengths being used overall network links.

**Figure 7.** Line interface count for ODU restoration and protection.

**Figure 8.** Wavelength channel utilization for ODU restoration and protection.

The equivalent pair of plots when supporting resilience mechanisms that operate in the OCh layer is depicted in **Figures 9** and **10**.

The main outcome of this comparison is that OCh PSR is a more cost-effective scheme than ODU PSR in a network of this size and supporting the traffic pattern defined as input [6]. In addition, it is also clear that PSR enables the network to withstand a single link failure with less resource overprovisioning than that of 1 + 1 protection.

**Figure 9.** Line interface count for OCh restoration and protection.

Next-Generation Transport Networks Leveraging Universal Traffic Switching and Flexible Optical Transponders http://dx.doi.org/10.5772/intechopen.68953 223

**Figure 10.** Wavelength channel utilization for OCh restoration and protection.

The equivalent pair of plots when supporting resilience mechanisms that operate in the OCh

The main outcome of this comparison is that OCh PSR is a more cost-effective scheme than ODU PSR in a network of this size and supporting the traffic pattern defined as input [6]. In addition, it is also clear that PSR enables the network to withstand a single link failure with

layer is depicted in **Figures 9** and **10**.

222 Optical Fiber and Wireless Communications

less resource overprovisioning than that of 1 + 1 protection.

**Figure 8.** Wavelength channel utilization for ODU restoration and protection.

**Figure 9.** Line interface count for OCh restoration and protection.
