Understanding Optical Transport Networks: Architecture, Components and Technologies

Optical Transport Networks
An optical transport network (OTN) is a standardized network technology designed to transport multiple digital client signals with different protocols, bit rates and modulation formats over optical fiber using multiplexing and other transfer techniques common to telecommunication networks. OTN provides standardized means of transporting client signals using optical networking techniques where infrastructure providers and network operators can efficiently utilize available bandwidth.

Network Architecture and Components

An OTN consists of multiple interconnected network elements that include optical line terminals (OLTs), optical cross connects (OXCs), optical add-drop multiplexers (OADMs), wave division multiplexers (WDMs) and transponders.

OLTs are the endpoints of the Optical Transport Network where client signals interfacing with core transport network happens. OLTs terminate and multiplex client signals received from various access points into higher bit rate signals suitable for long-haul transport over optic fiber links.

OXCs are the core switching elements that provide wavelength/fiber switching capabilities within the network. OXCs perform time-division multiplexing and allow individual wavelengths to be switched between different fibers without having to be converted back into electrical signals.

OADMs allow wavelengths/client signals to be dropped from the network or optically added to the network at specific points without converting the entire traffic into electrical signals.

WDM technology multiplexes multiple optical carrier signals onto a single optical fiber by using different wavelengths of laser light. WDM systems increase bandwidth over fiber by allowing different wavelengths to carry different information simultaneously over the same fiber.

Transponders are used at both ends of core network links to convert client signals to OTN frames for transport and then extract/convert client signals back from the OTN frames. Transponders handle modulation/coding suitable for long-haul transport.

Optical Transport Hierarchy and Frame Structure

To efficiently map and transport client signals with different line rates and structures, OTN uses a multi-hierarchy encapsulation method called optical transport hierarchy or OTH.

At the lowest order, ODU0 encapsulates client signals up to 2.5Gbps. Higher orders like ODU1, ODU2, ODU3 and ODU4 can map higher rate client signals up to 100Gbps.

OTN frames have standard structures defined in ITU-T G.709 specifications. The basic frame structure in OTN is the optical data unit (ODU) frame which consist of an overhead and a payload area.

The overhead contains management, monitoring and synchronization information for transport. The payload area carries lower order ODUs, client data or filler bytes. ODU frames are then mapped into optical channel data units (ODUCs) for line transmission.

Forward Error Correction in OTN

To ensure high reliability of long-haul transmission, OTN uses forward error correction (FEC) on the ODUk frames during transport.

Two common FEC schemes used are Reed-Solomon coding and Fire code. Reed-Solomon coding provides around 7-15% error correction capability in OTN frames depending on configuration during over 10,000km transmission for bit error rates less than 1e-15. Fire code provides higher 30% FEC gain with low computational complexity.

OTN Protocol Stack and Interfaces

OTN uses a layered protocol stack with management, client adaptation and physical medium interfacing functions defined in G.709 standards.

At management layer, OTN Network Management functions enable element management, connection provisioning and fault management across the network.

At client adaptation layer, ODUk frames multiplex client signals with or without adaptation depending on client type using various client adaptation functions like GFP, SDH/PDH/Ethernet mapping.

At physical medium interface layer, OTUk frames generated after multiplexing ODUk are transmitted onto physical medium like DWDM links using forward error correction schemes. Eightb/tenth encoding, FEC codes are applied at this layer.

OTN supports various client interfaces like 10Gbit/s Ethernet, 40/100GigE, SDH, ODUFlex etc to match any required native client bit-rate.

Advantages of Optical Transport Networks

Some key advantages provided by OTN technology in core and metro networks include:

- Technology independence: OTN allows transport of virtually any client protocol as well as bit-rate flexible interfaces without client-technology dependence.

- Survivability: With intelligent ODU multiplexing and high-performance FEC, OTN ensures reliable transport of client signals over thousands of kilometers.

- Dynamic bandwidth provisioning: OTN supports bandwidth variable interfaces like ODUFlex that help optimize resources and enable quick service provisioning.

- Scalability: OTN architecture with multi-hierarchy encapsulation allows scaling of bandwidth from sub-2.5Gbps to over 100Gbps in steps which matches network growth needs.

- Manageability: Standardization of OTN operations, administration, maintenance and provisioning functions aids in automated network management across multiple vendors.

- Capacity optimization: Through optical grooming of client signals without O-E-O conversion, OTN conserves bandwidth of core optical links more efficiently.

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