G.709 Optical Transport Network

G.709 Amendment 3 was approved by the ITU in April of 2009. Amendment 3 will have important implications for network operators all over the world. It defines new multiplexing capabilities that support both higher and lower speed interfaces. Cyan’s range of packet optical transport network platforms are designed to provide network operators solutions that support every advantage of G.709 based networking.

G.709 Optical Transport Network (OTN)

G.709 deployment has been growing steadily since its original approval in 2003. Its low profile belies its importance to the network. G.709 was initially known as ‘Digital Wrapper,’ a term that accurately describes G.709 as a utilitarian workhorse technology that operates behind the scenes as part of the network infrastructure. G.709 has come of age. Deployment can be expected to increase rapidly in the coming years.

Applications

G.709 defines interfaces for the Optical Transport Network. It addresses two different technologies: the digital part addresses mapping and mulitplexing in the electronic domain; and the optical part addresses wavelength multiplexing in the photonic domain. G.709 defines OAM capabilities for both parts. In its simplest form OTN simply wraps a single payload in a digital wrapper. The digital wrapper supports OAM and forward error correction, capabilities that go beyond 3R (regeneration, retiming and re-shaping) used prior to G.709.

FEC

G.709 adds Forward Error Correction (FEC). FEC detects and corrects bit errors. Standard Reed-Solomon FEC provides the equivalent of 5.4dB of optical coding gain. This optical coding gain translates directly to link budget improvement, extending transmission distance by kilometers. Enhanced and Ultra FEC add more FEC overhead and in return provide greater optical coding gain.  

Errors are corrected and counted. An increase in the number of corrected errors is an indicator of link degradation. This is used to proactively trigger maintenance steps, before service is actually affected.

FEC alone is sufficient reason to deploy G.709. Digital wrapper has been used to wrap OC-192 line services. In this case, G.709 supplements SONET’s section layer overhead, but plays no role in service multiplexing. This was a good solution when the total link capacity was 10G or less. G.709's multiplexing capabilities kick in at greater speeds.

Transparency

The goal of any transport technology is transparency – the transport network should affect client services as little as possible. The standard metrics for transparency are delay and loss. These and some other transparency parameters are listed below.

G.709’s role is to provide a common transport technology for a wide range of services. Adaptation is carried out at the end-points. Once the service is mapped into OTN, the network is operated independent of the different payload types. There is no requirement for payload specific monitoring at transit locations. OTN provides transparent, high capacity interconnections at the lowest total cost of ownership. Because transport is independent of the payload, the services can evolve independently of the transport infrastructure, and investment in the infrastructure benefits both new and established services.

Conclusion

The demand for high bandwidth, low latency networking is increasing. SONET/SDH is too granular and too expensive for high-bandwidth packet services. Routers and MPLS switches have greater capital cost and associated operating cost than SONET/SDH. Complex packet processing is unnecessary in transport applications, and degrades the transport service more than SONET. G.709 complements legacy SONET and MPLS network technologies, and specifically targets high bandwidth, low latency applications.
G.709 compliance is central to the architecture of Cyan’s family of Packet Optical Transport platforms. All trunk interfaces support G.709. DWDM hardware is designed in accordance with G.709 recommendations. Cyan’s DWDM multiplexer and ROADM solutions integrate the Optical Service Channel for network management. Cyan’s CyMS multilayer management system is compliant with G.709.