**6. Transparent handover between metro and core segments of next-generation transport networks**

Transport networks can hierarchically be broadly classified into access, metro, and core domain. End-user connectivity is catered to by the access network and a few dozen kilometers could be covered based on the specific technology requirements and the density of user population. In between, the core and the access part, lies the metro network which on one hand aggregates traffic coming from the access networks and on the other hand transports intrametro traffic (e.g., intrametro data center interconnection) and covers often up to a few hundred kilometers. Following the level of traffic aggregation, metro networks could further be segmented into metro aggregation and core. At the end, covering a bigger geographical area (e.g., state or country) is what is called the core network interconnecting the metro networks and typically spans over several hundreds to thousands of kilometers.

In the metro transport networks, the predominant technology and topology are in the form of SDH/SONET rings. But while these SDH/SONET rings had intrinsic scalability limitations, there was always an increasing need for capacity. And this gave rise to the introduction of coarse wavelength division multiplexing (CWDM) and DWDM in these networks catering to wavelength switching support at intermediate nodes thus reducing optical-electrical-optical (OEO) conversions which was initially a cost burden to the operator. In present days, the metro network is mostly infested with 2.5G or 10G wavelengths using direct-detection modulation formats and as always the increasing demand of data will be catered to using higher data rate channels (e.g., 40G and 100G) in the coming days. Compared to these, core network has a different behavior and is severely dependent on DWDM and ROADMs exhibiting asymmetrical meshed topology. The wavelength channels have a good share of 10G, 40G, and 100G. Moreover, QPSK is now the dominant modulation format for the higher data rates of 40G and 100G with coherent-detection replacing direct-detection. The utilization of higherorder modulation formats, such as 16-QAM, enables to increase channel capacity. But, the poor reach performance of these higher modulation formats penalizes a widespread deployment of the same. To mitigate this negative effect, adoption of super channels is being looked upon as an alternative to core networks where multiple carriers are grouped together to realize these higher bit-rates (e.g., 2 × 100G QPSK to create a 200G super channel on a 100GHz spectrum) [17]. Another important aspect to look upon is the optical add-drop multiplexers (OADMs) being used in core and metro networks. Metro networks are comparatively simpler with lesser nodal degrees and thus simpler OADMs of the likes of broadcast-and-select or fixed add/drop ROADMs or to the extent of nonreconfigurable fixed OADMs (ROADMs) are in use today. On the other hand, core networks use much higher capacity and also wavelength channels and thus require expensive OADMs.

The traffic flow between the above described metro and core network at the boundary node is depicted in **Figure 14** and the same is represented by metro-to-core (M2C) in the next part of this chapter.

To be more detailed, let us presume one data channel running from one metro network to another metro network via a core network and is designated as metro-to-core-to-metro Next-Generation Transport Networks Leveraging Universal Traffic Switching and Flexible Optical Transponders http://dx.doi.org/10.5772/intechopen.68953 227

**6. Transparent handover between metro and core segments of** 

works and typically spans over several hundreds to thousands of kilometers.

Transport networks can hierarchically be broadly classified into access, metro, and core domain. End-user connectivity is catered to by the access network and a few dozen kilometers could be covered based on the specific technology requirements and the density of user population. In between, the core and the access part, lies the metro network which on one hand aggregates traffic coming from the access networks and on the other hand transports intrametro traffic (e.g., intrametro data center interconnection) and covers often up to a few hundred kilometers. Following the level of traffic aggregation, metro networks could further be segmented into metro aggregation and core. At the end, covering a bigger geographical area (e.g., state or country) is what is called the core network interconnecting the metro net-

In the metro transport networks, the predominant technology and topology are in the form of SDH/SONET rings. But while these SDH/SONET rings had intrinsic scalability limitations, there was always an increasing need for capacity. And this gave rise to the introduction of coarse wavelength division multiplexing (CWDM) and DWDM in these networks catering to wavelength switching support at intermediate nodes thus reducing optical-electrical-optical (OEO) conversions which was initially a cost burden to the operator. In present days, the metro network is mostly infested with 2.5G or 10G wavelengths using direct-detection modulation formats and as always the increasing demand of data will be catered to using higher data rate channels (e.g., 40G and 100G) in the coming days. Compared to these, core network has a different behavior and is severely dependent on DWDM and ROADMs exhibiting asymmetrical meshed topology. The wavelength channels have a good share of 10G, 40G, and 100G. Moreover, QPSK is now the dominant modulation format for the higher data rates of 40G and 100G with coherent-detection replacing direct-detection. The utilization of higherorder modulation formats, such as 16-QAM, enables to increase channel capacity. But, the poor reach performance of these higher modulation formats penalizes a widespread deployment of the same. To mitigate this negative effect, adoption of super channels is being looked upon as an alternative to core networks where multiple carriers are grouped together to realize these higher bit-rates (e.g., 2 × 100G QPSK to create a 200G super channel on a 100GHz spectrum) [17]. Another important aspect to look upon is the optical add-drop multiplexers (OADMs) being used in core and metro networks. Metro networks are comparatively simpler with lesser nodal degrees and thus simpler OADMs of the likes of broadcast-and-select or fixed add/drop ROADMs or to the extent of nonreconfigurable fixed OADMs (ROADMs) are in use today. On the other hand, core networks use much higher capacity and also wave-

The traffic flow between the above described metro and core network at the boundary node is depicted in **Figure 14** and the same is represented by metro-to-core (M2C) in the next part

To be more detailed, let us presume one data channel running from one metro network to another metro network via a core network and is designated as metro-to-core-to-metro

**next-generation transport networks**

226 Optical Fiber and Wireless Communications

length channels and thus require expensive OADMs.

of this chapter.

**Figure 14.** Architectures for interconnecting metro and core networks (TrP – transponder, MuxP – muxponder). (a) Traditional all-opaque handover; (b) enhanced transparent handover.

(M2C2M) channel. The current mode of operation forces this M2C2M channel to ride on an optical channel in the metro domain (OCh M) till the metro (R)OADMs. At this (R)OADM node, the same OCh M gets terminated at a line interface only to be transferred to a transponder or muxponder which maps the same into the optical channel (OChC) ferrying inside the core network. At the boundary node in between the core and metro node, a similar process makes sure that the channel is handed over from the core to the metro network. It is to be noted that, these handover sites could be a single site at the same physical location (housing the (R)OADMs) or could be physically separated sites within a comparatively shorter distance hosting each (R)OADM looking toward the core and metro network.

This architecture represents an all-opaque architecture depicting a clear enough demarcation between the metro and core network segments where the hardware (client interfaces) performs the handover. As it already undertakes the OEO operation, these sites are used for grooming subrate data signals into higher data-rate pipes/channels. For example, the M2C2M channel with a data rate of 2.5G can be multiplexed/groomed into 10G and 100G OCh M and OCh C, respectively. Therefore, the M2C2M channel can be multiplexed/groomed in OCh M along with other subrate data channels of equal or smaller capacity starting at the same node and destined to another metro network, while in the core network the M2C2M channel can be multiplexed/groomed with other data channels that start and end in the same metro networks pair. This process directly influences the fill ratio of the core network light paths and thus contributing to the reduction of CAPEX as a result of lesser number of expensive line interfaces and 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 fiber links, higher number of amplifiers, and advanced, larger ROADMs.

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

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



**Table 2.** Qualitative comparison of all-opaque metro-to-core handover with enhanced transparent handover.

these strategies for a M2C traffic handover. Further, the analysis targets to capture technology landscape influenced differences in today's metro and core networks [21].
