Optical transport network (OTN) primer

This optical transport network (OTN) primer tells you about the ITU standards designed to build more network functionality into optical networks and how traditional carriers and content providers are using this architecture.

Understanding optical transport network

Optical transport network (OTN) is a series of standards created to combine the benefits of SONET/SDH with the bandwidth-expanding capabilities of Dense Wave Division Multiplexing (DWDM) in order to build more network functionality into optical networks. As defined by the International Telecommunication Union (ITU), OTN is a set of optical network elements connected by optical fiber links capable of providing optical channel transport, multiplexing, routing, management, supervision and survivability.

Relevant ITU standards include G.878, which specifies OTN architecture, and G.709, which defines the frame format and payload mapping for carrying Ethernet, storage and SONET/SDH signals. With OTN, also called digital wrapper, these disparate traffic types are multiplexed onto and carried over a single optical transport unit (OTU) frame, either at 2.7 Gb/s (OTU-1), 10.7 Gb/s (OTU-2) or 43 Gb/s (OTU-3).

Why you need to know about optical transport network

With the ITU standards available since the early 2000s, carriers have widely deployed optical transport technology in their long-haul and metro-area networks. 

The tendency among industry watchers of late has been to dismiss the optical networking market as all-physics and low-margin, but in actuality, it continues to be a lucrative market, with more than a dozen vendors jockeying for position in the long haul and metro area, according to optical networking analyst Eve Griliches, managing partner at ACG Research. The market analysis firm pegs the worldwide optical networking market at an estimated $14 to $15 billion a year, a substantial market that is comparable in size to that of Ethernet switches.

Service providers continue to deploy OTN with the goal of boosting bandwidth and increasing network functionality. OTN has provided them a way to support different traffic types in a more cost-effective manner than by using SONET/SDH networks. Carriers are placing particular emphasis on OTN in the metro area, where they are shifting rapidly from SONET/SDH to wavelength-division multiplexing (WDM), Griliches said.

Griliches attributes the uptake to the increased spectral efficiency of the latest WDM equipment, which can handle two or more times the number of wavelengths -- 80, or in some cases, 96 -- than earlier systems. “This means it can handle bandwidth far better and at higher bit rates,” she said.

What you need to know about optical transport network

With mounting data traffic on their networks, service providers today face a decision about how to use OTN. Griliches said carriers fall into three groups:

1. Some are looking at putting IP traffic into OTN and then multiplex the OTN into DWDM;

2. Others want to use IP over DWDM, because most of their traffic is IP;

3. Some major carriers want to use OTN only for legacy time-division multiplexing (TDM) traffic, but will use packet interfaces for their IP traffic.

The level of carriers’ investment in OTN, will depend on the nature of their traffic loads, Griliches said. For those with heavy and increasing amounts of IP traffic, OTN may not be the best or quickest option for handling the higher rates. On the other hand, many providers are considering using OTN for packet traffic so they have transparency as it traverses the DWDM layers and can maintain TDM-like functionality at much higher bit rates.

As service providers strategize on how to converge their packet and optical transport departments, the discussions can be as much organizational as technical in nature. Many want to break down the traditional transport and data silos so they can effectively apply expertise from each discipline to new and rapidly evolving requirements. “Service providers need full agreement between the transport and data guys on how the network is designed and how they’ll deal with the rising amount of packet traffic,” Griliches said.

Component advances that would enable tunable signal processing are promising and will enable optical networks to be far less static and more flexible for data networking. These advances could mean greater control over lighting up, initiating and bringing down wavelengths.

“That’s been doable on a wavelength-by-wavelength basis, but not on a multiple wavelength basis,” Griliches said. “That in and of itself will solve the next biggest barrier to flexibility in the transport network. The two sides will be able to collaborate better and think a little bit more outside of the box.”

Content providers are influencing the optical transport market

Outside of what traditional carriers require from equipment vendors, the big Internet content providers -- like Google, Amazon, Facebook and LinkedIn -- are forcing equipment providers to rethink the features and functionality they bring to the market.

Content providers assume that there will be network failures and plan their architectures around that premise, so they don’t require the traditional Five 9s (99.999%), availability carrier-class guarantees typically demanded in specific optical networking gear.

“Content providers are causing a huge disruption in the optical networking space,” Griliches said. “The thinking and the approach [content providers] are bringing to the marketplace has been refreshing. I think service providers will now realize that a lot of the requirements they’ve been putting on vendors aren’t that necessary and that what content providers are saying and doing might just be something to emulate and follow, not make fun of anymore."

Optical transport network vendors: A sampling of optical networking vendors includes Alcatel-Lucent, Ciena, Cisco Systems, Ericsson, Fujitsu, Huawei, Infinera, NEC and Tellabs.

About the author: Beth Schultz is an IT writer and editor based in Chicago.

This was first published in January 2011

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