OTN offers transparent service delivery

The global migration to IP/Ethernet applications is forcing enterprises to address two major WAN issues - converging TDM and packet networks to reduce expenses, and improving network and service management to support bandwidth-intensive, delay-sensitive applications. A new global standard is addressing these challenges - ITU G.709, commonly called Optical Transport Network or digital wrapper technology.

The global migration to IP/Ethernet applications is forcing enterprises to address two major WAN issues - converging TDM and packet networks to reduce expenses, and improving network and service management to support bandwidth-intensive, delay-sensitive applications.

A new global standard is addressing these challenges - ITU G.709, commonly called Optical Transport Network (OTN) or digital wrapper technology. While digital wrapper technology has been around for a few years, standards have been in various states of completion, and only in the past year has true OTN equipment become available to make network deployments in the United States and Europe a reality.

Today, OTN is primarily offered at two rates, OTU1 and OTU2. OTU1 is defined as a 2.7Gbps signal designed to transparently carry a SONET OC-48 or synchronous digital hierarchy (SDH) STM-16 signal. OTU2 is defined to carry a 10.7Gbps signal to transparently transport an OC-192, STM-64 or 10Gbps WAN physical layer entity (PHY) as well as 10Gbps Fibre Channel.

Unlike SONET/SDH, OTU2 can be modified for special bit rates so it also can carry a 10G LAN PHY from IP/Ethernet switches and routers at full line rate. This is important, because LAN PHY typically includes proprietary overhead that increases the data rate beyond 10Gbps, making it too large for a 9.953Gbps SONET/SDH payload. The OTU3 (40Gbps) standard easily extends these networks to support future higher-bandwidth requirements.

In order to support lower-speed services, extensions to the OTN standard allow multiple subwavelength (that is, below 2.5Gbps) services to be aggregated onto a single OTU wavelength with service-level add/drop capability and bit-stream transparency. This approach simplifies and dramatically lowers the costs of access and metropolitan networks.

The emergence of OTN deployments has been driven largely by the dramatic growth of Ethernet and other packet-based services, as well as deployment of next-generation networks. One of the key benefits of OTN technology is that multiple networks and services, such as legacy SONET/SDH, can be combined seamlessly onto a common infrastructure with new Ethernet, storage and video applications.

OTN does this through transparency, which means that each service is completely preserved as it is carried through the multitude of optical network devices in the network. To do this, OTN "wraps" the client payload and overhead data into its own payload and uses its own management overhead to transport the signal across the network. This is critical for preserving a client signal's original structure to ensure performance monitoring across a network.

For example, without OTN, the media access-control overhead of an Ethernet client signal would need to be terminated at every network hop and then reinserted, resulting in the loss of the original client overhead bytes. This drastically impairs the ability of enterprises to manage individual services across a WAN.

OTN then takes that management a step further. Like SONET/SDH, OTN offers a mechanism for alarming, signaling, remote management and software downloads through its general communications channel. Unlike SONET/SDH, however, OTN extends these capabilities to asynchronous data services, such as Gigabit Ethernet, 10G Ethernet, Fibre Channel, Enterprise Systems Connection and Fibre Connection, that are unable to provide their own.

Unlike IP routers or switches, OTN is an optical technology that introduces virtually no latency or jitter into the network. This increases application throughput, eliminates the need for constant network tuning and improves overall network performance.

With the rapid migration toward IP/Ethernet-based infrastructure, OTN is the ideal technology for building a converged WAN, either directly or through OTN carrier services, based on its transparent support of legacy and packet services, ability to extend SONET/SDH-like operations and fault-management capabilities to data protocols, and seamless 10Gbps Ethernet LAN PHY support.

Alexander is CTO for Ciena. He can be reached at steve.alexander@ciena.com.

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OTN (Optical Transport Network)

03/06/03

 
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