Optical Outlook: The ROADM to Bandwidth

As cable operators begin to offer more services, they need to rethink their strategy for connecting their regional headends with each other and with the Internet backbone. Many are turning to Reconfigurable Add Drop Multiplexors (ROADMs) as a key technology for adding bandwidth while keeping total costs lower by reducing the need for traditional SONET networking gear.

Verizon undertook the largest ROADM deployment to date when it launched its FiOS network, said Andrew Schmitt, directing analyst at Infonetics Research. As Verizon began to plan the massive new broadband network to the home, it realized it needed to reassess its strategy for wiring the central offices in order to feed video and data to each neighborhood.

According to Glenn Wellbrock, director of Optical Transport Network Architecture and Design at Verizon, “We had to rethink our strategy when we realized how much it would cost to provide one or more 10 Gbps links to each central office. The ROADM architecture is more economical, which is what drove us to deployment.”

ROADM Basics

The capacity of a ROADM is measured in degrees and wavelengths (a wavelength refers to a single color of light used to carry data). A single wavelength generally is used to carry between 10 and 100 Gbps of traffic. At the high end, network equipment providers are offering ROADMs capable of carrying 80 wavelengths, with a theoretical maximum capacity of 8 Tbps.

A degree is the number of separate physical ports that exit and enter a single device. Each degree can carry anywhere between zero and 80 wavelengths. Each degree generally is connected to fibers running to different cities in a network operator’s network. At the low end, a ROADM might only have four degrees, while at the high end, new ROADM components from JDS Uniphase Corp. (JDSU) are promising 23 degrees.

ROADM technology also allows network operators to install a mesh architecture that can provide the same reliability as do traditional SONET rings, using far less redundant cable. With ROADMs, network operators can deploy a mesh design with links between many cities. This allows a single circuit to provide a protection for two or more routes, which can help reduce the amount of fiber required to provide a reliable network.

SONET Out

Verizon built its initial network of about 2,000 nodes using ROADMs that supported 4 degrees and 44 wavelengths. In most cases, Verizon only is carrying 10 Gbps of traffic on each wavelength; it recently decided to standardize on ROADMs that support 8 degrees and 88 wavelengths. Both technologies are fully compatible with each other, and they can interoperate with the management infrastructure Verizon is putting into place.

Traditionally, Verizon has used SONET rings between each central office but, when it did the math, it realized a ROADM-based architecture would save between 50 percent and 60 percent of the overall capital costs by eliminating back-to-back network connections. Without ROADMs, Verizon would have had to interconnect SONET rings that converted optical signals into electrical ones, and then back again, at each central office.

Verizon chose ROADM equipment specifically from Tellabs and Fujitsu to integrate wavelength and SONET transport and switching capabilities into a single element. These hybrid devices bridge the world of optical networks with electrical switching using a common management system. The devices are distinct from routers that direct traffic at the level of TCP/IP packets, as they aggregate the traffic underneath the router so that larger pipes can be directed to the TCP/IP routers. Only 1 Gbps signals are used, while lower bit-rate streams are aggregated before they are fed into these hybrid ROADMs.

The latest generation of ROADMs uses wavelength selective switches (WSS) from vendors like JDSU. These raw optical components can be linked together to create colorless and directionless ROADMs that simplify the networking architecture and configuration. These devices then are integrated into networking equipment from vendors like Tellabs, Fujitsu, Ciena and ADVA Optical.

Using the right equipment, a network manager can rearrange the network backbone from a computer terminal without the need for a technician to touch any of the installed cable. However, this management technology still is in its infancy. One approach being cultivated by the International Telecommunication Union is the G.ASN set of protocols. Another approach being developed by the Internet Engineering Task Force is GMPLS, which Verizon is using in its deployment. Wellbrock said Verizon’s use of GMPLS eventually will allow the carrier to add switching fabric that acts like a cross connect for SONET or LAN traffic at the optical layer.

ROADM technology would have made sense, even if Verizon had not decided to roll out FiOS, but it would have a much more difficult to get management buy-in. “It is always cheaper to add one more SONET node than to install a ROADM but, by the time you install three more SONET nodes, a ROADM would have been cheaper,” Wellbrock said. “It has to be a dedicated effort where you are determined to build them out rather than just putting a few in. You really have to build a network to get the full benefit.”

–George Lawton