**7. Conclusions**

also the network occupied bandwidth. It is noteworthy that the bandwidth usage in the core network is much more a valuable asset when compared to its metro counterpart due to longer

In this approach, the M2C2M channel crosses one line interface pair in the first metro network where they originate and client interface pair in the first M2C site, line interface pair in the following core network, client interface pair in the second M2C site, and finally a line interface pair in the terminating metro network, thus spending in total 6 and 4 line and client interfaces, respectively. But still these are being shared with other smaller data rate M2C2M channels. But this is not the entire scenario because soon we start encountering higher data rate M2C2M channels (e.g., 10G or even higher) which cannot be multiplexed/groomed into the already existing OCh C due to nonterminating OChs in the same metro networks or capacity nonavailability in the existing channels. Such use-cases foresee savings by the transparent handling of M2C2M channel between metro and core networks, which is possible if an additional fiber link exists in the M2C site in between the (R)OADMs and this higher data rate M2C2M channel is mapped on it. This transparent M2C interconnection can only be deployed by augmenting nodal degree of each (R)OADMs and additional booster/pre-amplifier deployment for each direction while employing one extra port of splitter/combiner and wavelength selective switches (WSSs) [18, 19]. Adding an optical switch between add/drop ports subset and

In order to gain insight on expected design, implementation, and operational differences between a metro core network with traditional all-opaque M2C handover versus the same network with enhanced transparent handover, a subset of these differences is highlighted in **Table 2**, to highlight the different aspects to be assessed with care before opting for either of

**All-opaque M2C handover Enhanced transparent M2C handover**

• Joint (global) optimization of line IFs used at

• Both (R)OADMs have to be augmented with

• Format used in OCh M2C2M impacts spectral

• Additional spectral inefficiencies associated to coexistence of direct detect and coherent formats in the core network: extra spacing be-

• Higher: requires integrated planning of metro

metro and core networks

one additional degree

and core networks

efficiency of both networks

tween neighboring channels [20]

• Savings in client IF count at M2C sites

fiber links, higher number of amplifiers, and advanced, larger ROADMs.

228 Optical Fiber and Wireless Communications

using them directly for the optical bypass will be a good alternative [20].

• Separate (local) optimization of line IFs required in the metro and core

• Intensive usage of client IFs at M2C

• All traffic handover done via the add/ drop ports of both (R)OADMs

metro networks first and core network

at the metro and core networks • Best suited (cost-wise, performancewise) format can be selected in each network: e.g., direct-detect 10 Gb/s in metro, more spectral efficient but ex-

pensive 100 Gb/s in core

networks

Spectral efficiency • Independent channel format selection

Planning complexity • Lower: due to sequential planning of

afterward

sites

Interface requirements

sites

(R)OADMs at M2C

The overriding purpose of this chapter is to depict the relative importance of the universal OTN switch and flexible-rate line interfaces in the context of different network scenarios and traffic conditions. To accomplish the same, results from different network studies were presented. It is evident that, when the traffic pattern involves multiple subrates coming from varied data sources (TDM, Packet), universal OTN switch yields the maximum savings from CAPEX point of view. This is contributed by the reduced number of light paths and router port savings. Needless to mention here that, the more is the mismatch between the client traffic rates and the line-side rate(s), more savings could be achieved by exploiting a multilayer optimization approach. Furthermore, exploiting the higher capacity light paths enabled by flexible-rate line interfaces and combining the same with universal OTN switching, the better of both worlds can be achieved. But it is important to mention that flexible-rate line interfaces supporting 16-QAM, for example, will be less important when the network links are very long due to the limited transparent reach with this modulation format. On the other hand, the role of these state-of-the-art node architectures in the context of protection and restoration was also visited and different alternatives were presented. It could be inferred that OCh restoration would be preferable from a CAPEX point of view, with lesser resources being required, while opting for a better QoS (e.g., in terms of recovery time), ODU restoration can be a better choice. At the end, this chapter overviews the present day metro-to-core network scenario which enforces all-opaque traffic engineering and also prospects the economic and technological feasibility of transparent handovers. Recent technological developments, for example, metro core spectral efficiency gap narrowing, alien wavelength/black link standardization aided by state-of-the-art multivendor and multilayer resource provisioning platforms will overcome the hurdles of transparent handovers, and thus exploiting the big saving potentials in terms of interfaces at these boundary nodes.
