Value of a Distinct OTN Layer
Dying Breed or Alive and Kicking?
The Optical Transport Network (OTN) architecture is now almost two decades old, and some pundits are proclaiming that its role as a distinct transport layer will soon give way to an IP-over-DWDM architecture. While IP-over-DWDM is undoubtedly gaining ground, particularly for point-to-point applications, a distinct OTN layer is very much alive and evolving to serve future optical networking needs.
It starts with the OTN standard
The OTN architecture is based on the OTN standard. This standard – also called a “digital wrapper” – frames service interfaces into a Layer 1 electrical format as the final step prior to conversion and transport as optical signals over a DWDM network. The OTN standard brings numerous benefits:
- It provides transparent transport for multiple service interfaces, including Ethernet, storage networking like Fibre Channel (FC), and SDH/SONET.
- It optimizes transmission using forward error correction (FEC) to ensure error-free transport.
- It enables smooth scaling of optical networks using schemes for multiplexing, switching, and survivability of transported optical signals.
- It facilitates service provisioning and troubleshooting of the service interfaces via operations and monitoring features.
Not only is OTN a proven standard, it is also continuously evolving to keep pace with technology advances and new service transport needs. Recently it has added service interfaces for 400GE and Flexible Ethernet (FlexE), and network interfaces for transmission above 100G.
OTN as an architecture is situated beneath layer 2 packet and other service interfaces, and above layer 0 photonics such as ROADMs and amplifiers. This layer 1 consists of “OTN transport” delivered by transponders and muxponders that implement the OTN standard, and in many configurations is supplemented by “OTN switching”.
Advantages of a distinct OTN transport layer
Implementing OTN as a distinct transport layer brings multiple advantages:
Multiservice Transport. An obvious advantage is the use of muxponders to consolidate multiple service interfaces onto a single wavelength, saving costs. Muxponders consolidate multiple lower speed service interfaces like 1GE, 10GE, and FC interfaces, onto more expensive high-speed links like 100G/200G. Going forward, muxponders will consolidate multiple 100GE interfaces onto even higher speed line interfaces, such as 600G.
Planning and Operational Flexibility. A distinct OTN transport layer enables “separation of concerns”. Service layer interfaces are easily added, upgraded, or modified, without needing to be concerned with any of the route planning, design, and implementation concerns on how they are transported. The OTN transport layer, which is managed independently to ensure optimal transport is being provided, takes care of this.
High Service Availability. An OTN transport layer implements dedicated or shared protection arrangements to deal with possible link failures, to support Service Level Agreements (SLAs) at the services layer. In addition, an OTN transport layer (working in conjunction with layer 0 ROADMs) can implement the Wavelength Switched Optical Network (WSON) architecture, which used GMPLS signaling for dynamic rerouting and restoration of wavelengths in the event of failures, adding to service survivability.
Optical Encryption. Layer 1 optical encryption (L1OE) is added at the OTN layer, and it is the only way to protect against snooping and data interception via fiber tapping. A distinct OTN transport layer facilitates adding L1OE selectively as a value-added service. It can be applied to encrypt individual service interfaces or all the interfaces being transported on a particular wavelength.
Smooth Technology Evolution. The OTN layer can evolve its client and line side interfaces independently. This provides multiple options for mapping one onto the other, to optimize transport as needs and technologies evolve. For example, OTN is now extending to support new service interfaces like Flexible Ethernet and 400GE, while on the line it is gearing up for advanced 100G+ transport mechanisms like OTUCn framing and Flex-O. In this manner, a distinct OTN layer is ideally suited to accommodate the evolving needs and speeds of service interfaces.
Additional advantages of adding OTN switching
OTN switching manipulates OTN encoded interfaces in the electrical domain prior to mapping onto a DWDM wavelength. Inputs to an OTN switch are distinct service interfaces, as well as wavelengths already containing one or more service interfaces. The switch can rearrange all the services interfaces onto different sets of wavelengths that exit the switch. Additional benefits from using OTN switching include:
Maximum Wavelength Use. Going well beyond the capabilities of muxponders, an OTN switch can groom with high density all the service interface inputs onto a reduced set of exiting wavelengths. This delivers a substantial savings in wavelength related costs by reducing transponders and transmission equipment.
Even Higher Service Availability. In the event of network failures, OTN switching provides tremendous flexibility to rearrange service assignments to wavelengths, providing an ability even beyond redundant links to assure service availability. This is implemented with the Automated Switched Optical Network (ASON) architecture, using either GMPLS signaling or centralized SDN control.
Router Bypass. Routers often connect with each other via intermediate routers. This is tremendously wasteful of router interfaces and capacity. A solution to this problem is to bypass intermediate routers at the OTN layer. This is accomplished easily using OTN switching where router-to-router service connections are switched onto express wavelengths that directly connect these routers, bypassing any intermediate routers.
Virtual Networks. OTN switching enables partitioning an optical network into virtualized domains that can be operated independently. For example, domains can be dedicated to different enterprise customers for private optical networks, or different carriers in a carrier-of-carriers business. They can also be dedicated to different applications, like creating hard 5G network slices for transporting different classes of 5G service.
In summary, OTN as a standard as and as an architecture has proven its value over the last twenty years. Even though it is seeing pressure from alternative approaches like IP-over-DWDM, we expect that implementing OTN as distinct layer of transport and switching technologies will continue to be the predominant optical architecture for years to come.
To learn more about ECI’s OTN solutions click here.