**2. The role of OTN switching in transport networks**

Technological advancements have seen different node architectures being proposed, each having their pros and cons. Transponders and muxponders still provide the simplest approach to getting traffic on and off a 100G transmission interface, by multiplexing one or more client interface to a single high-speed line interface. On one hand, this approach incentivizes simplicity and relatively low CAPEX, while on the other hand, penalizes the operator because of its limited ability to groom traffic, inability to perform remote configuration, and being inefficient to combine add/drop traffic with pass-through traffic from other line interfaces.

#### **2.1. First-generation OTN switch**

The present day traffic represents a blend of packet traffic and extensively installed legacy TDM traffic. To address this varied traffic pattern and mix, the modern day network architectures requires an OTN switch to cater service-agnostic switching where the multiple client service types can be mapped into ODU frames and the same can be switched at the ODU level. This will not only allow subwavelength services to be aggregated at their source and destination nodes but also allow them to be groomed at intermediated nodes and thus finally contribute to a reduction in the number of expensive WDM line interfaces in use. **Figure 1** depicts the transponder/muxponder and OTN switch architectures. The later architecture introduces a digital OTN switch, which is separated from the WDM box using short-reach optics or integrated with the WDM box to reduce footprint and power consumption [7]. According to Infonetics, the majority of service providers (86%) are choosing OTN switching as the technology best suited to fill 100G optical channels, because it enables efficient aggregation of diverse services and protocols over a single optical link. Noteworthy, it is being embraced and deployed by network providers throughout Asia, Europe, and North America [8].

Another key feature enabled by OTN switching is fast shared protection and restoration schemes with fine granularity, which cannot be achieved with a transponder or muxponder solution. Other than contributing to a reduction of CAPEX, OTN switching does include several other benefits. Among them, high scalability, fast end-to-end service provisioning, multiple traffic type support, subwavelength switching, router port offloading, and client service level mapping are a few to name [7]. But then, certain specific traffic patterns and network topology can result in the switchless architecture to have a CAPEX advantage over

**Figure 1.** Transponder/muxponder-based approach vs. OTN-based approach.

the switched architecture. The same is witnessed when client services data rates match with the WDM channel data rates or even when the traffic type is not a mix and only comprises of packet data, and in these cases, the overall node architecture can be simplified to a switchless one with simplified equipment spanning L3-L0. Another perspective also challenges network operators while including OTN switch and that is contributed by the additional power and space consumption increasing the OPEX.

The traditional or first-generation OTN switch focuses only on switching ODUs and involves dedicated cards for circuit-based switching and dedicated cards for packet-switching, as illustrated in **Figure 2**. Consequently, switching is restricted within the same switch-type domain (packet/circuit). Furthermore, within the same switching domain, interworking between different technologies, such as the OTN and the synchronous digital hierarchy (SDH)/synchronous optical network (SONET), may be also blocked. Essentially, traditional OTN switches fail to deliver "universal client port" functionalities, which would allow to support any mix of client interfaces in a single switching domain, further improving the ability to efficiently utilize the transport network resources.

#### **2.2. The universal OTN switch**

**2.1. First-generation OTN switch**

214 Optical Fiber and Wireless Communications

America [8].

The present day traffic represents a blend of packet traffic and extensively installed legacy TDM traffic. To address this varied traffic pattern and mix, the modern day network architectures requires an OTN switch to cater service-agnostic switching where the multiple client service types can be mapped into ODU frames and the same can be switched at the ODU level. This will not only allow subwavelength services to be aggregated at their source and destination nodes but also allow them to be groomed at intermediated nodes and thus finally contribute to a reduction in the number of expensive WDM line interfaces in use. **Figure 1** depicts the transponder/muxponder and OTN switch architectures. The later architecture introduces a digital OTN switch, which is separated from the WDM box using short-reach optics or integrated with the WDM box to reduce footprint and power consumption [7]. According to Infonetics, the majority of service providers (86%) are choosing OTN switching as the technology best suited to fill 100G optical channels, because it enables efficient aggregation of diverse services and protocols over a single optical link. Noteworthy, it is being embraced and deployed by network providers throughout Asia, Europe, and North

Another key feature enabled by OTN switching is fast shared protection and restoration schemes with fine granularity, which cannot be achieved with a transponder or muxponder solution. Other than contributing to a reduction of CAPEX, OTN switching does include several other benefits. Among them, high scalability, fast end-to-end service provisioning, multiple traffic type support, subwavelength switching, router port offloading, and client service level mapping are a few to name [7]. But then, certain specific traffic patterns and network topology can result in the switchless architecture to have a CAPEX advantage over

**Figure 1.** Transponder/muxponder-based approach vs. OTN-based approach.

Conversely to the traditional or first-generation OTN switch, next-generation universal OTN switch as shown in **Figure 3** can aggregate different protocols on the client side and enable transparent multiplexing of packet and TDM traffic, allowing a single device to be used in multiple applications efficiently.

The universal OTN switch is backed by a universal transport platform which is capable of switching traffic flows based on any L1-L2.5 protocol and on every port, including multiple protocols on the same port simultaneously. Hence, it can offer network operators the best of both worlds by dynamically controlling every flow on every port as a circuit or packet and providing the most efficient, future-proof solution for virtually all applications [9].

These next-generation OTN switches employ universal cards to handle TDM, OTN, carrier Ethernet (CE), and multiprotocol label switching-transport protocol (MPLS-TP) traffic and

**Figure 2.** Traditional vs. universal OTN switch.

**Figure 3.** Universal OTN switch.

provide grooming efficiency, granularity, and service classification of a packet switch along with scalability, operational efficiency, and performance of an OTN switch, as illustrated in **Figure 3**. Furthermore, the universal OTN switch can also assist in reducing router ports by distributing services from aggregated router hand-offs with virtual local area network (VLAN) to ODU mapping, as shown in **Figure 4**.

Naturally, several other benefits, common to first-generation OTN switches, are also present and include fast end-to-end service provisioning, rapid restoration, high scalability, subwavelength level switching, and easy support of new/multiple traffic types. Likewise, if the service data rates are the same as the data rates of the WDM wavelength channels or when only packet traffic is present, then the universal switch might not have a CAPEX advantage over traditional OTN switch or even the conventional transponder/muxponder with OTN encapsulation.

Depending on the location in the network and the traffic matrix, switching at the packet or STS-1/VC-4 level can have efficiency benefits over switching OTN at the ODU0 (1.25G) level or above, as long as one of two conditions are met. Either there must be a significant number of client interfaces below 1G or there must be the potential for large statistical gains from

**Figure 4.** Universal switching fabric [1].

multiplexing uncorrelated bursty traffic flows. These conditions are more likely to occur in a metro network versus the long haul network where statistical gains have often already been achieved at the IP/MPLS layer and client interface speeds are likely to be above 1G.

Importantly, universal switching platforms enable OTN, SONET/SDH, and packet switched traffic to share the same high-speed interface, with SONET/SDH mapped to ODU2, packet traffic mapped to ODU flex, and the remaining capacity available for OTN switched traffic, thus making the most efficient use of each high-speed interface and the optical spectrum it consumes. In addition, universal switching provides investment protection against changes in traffic patterns and client types. With universal fabrics, and the ability to define in software interfaces and virtual interfaces for OTN, CE (Bridging, VLAN cross-connect), or MPLS-TP/ virtual private LAN service (VPLS), and without impacting the capacity of the switch, operators can easily evolve from first-generation to universal OTN switching.
