Making Optical Networks Flexible

FRAMINGHAM (04/27/2000) - As a result of the demand for more bandwidth, service providers are faced with the challenge of delivering bandwidth when and where customers need it.

Consider what happens when a best-selling author publishes a new book and makes it available only via the Internet, as with Stephen King's recently released Riding the Bullet. Network overloads are likely if a deluge of consumers tries to download the book simultaneously. While additional bandwidth would help, adding it takes months, and there is no way to precisely forecast when and where it will be needed.

In recognition of this problem, a new industry coalition, Optical Domain Service Interconnect (ODSI), was formed to drive innovation in service creation and bandwidth delivery for high-speed optical networks.

ODSI has identified two distinct but complementary functional elements of the optical network infrastructure: the transport network and the service network.

Transport networks create abundant raw capacity, while service networks translate this capacity into business and consumer services such as Internet access, LAN-to-LAN connectivity and extranets.

Optical transport networks create capacity by using dense wave division multiplexing (DWDM) technology to carry a large number of high-speed connections over a single pair of fiber optics. DWDM equipment available today can carry more than 100 connections at OC-48 (approximately 2.5G bit/sec each) or 40 connections at OC-192 (approximately 10G bit/sec each). However, optical transport networks are unable to look inside the contents of each connection to make switching or routing decisions at the packet or cell level. Instead, optical transport networks perform a function analogous to what airplanes provide overnight delivery companies: fast and efficient delivery between two fixed endpoints without having to know the contents of each package.

Optical service networks are typically composed of IP routers, ATM or frame relay switches, or newer products called Optical Service Switches that combine these functions into a single platform. These devices use the optical transport network to connect to each other at OC-48 or OC-192 speeds. But unlike optical transport networks, they inspect the packets or cells carried on each connection. Continuing the overnight delivery analogy, optical service networks perform the function of overnight carriers' regional service centers, making intelligent switching or routing decisions of each package, plus applying queuing and measurement policies to deliver the service level expected by customers.

The goal of ODSI is to enable service networks to rapidly dial up additional bandwidth from transport networks to meet the needs of applications. ODSI is working to create a standardized signaling interface between optical service networks and optical transport networks by applying extensions to the existing IP and Multi-protocol Label Switching protocols.

This signaling interface allows a switch or router to request a new connection (at OC-192 speeds, for example) to another switch or router within the network.

The request may also have additional constraints, such as low latency or a fast recovery if a fiber cut occurs in an optical transport network. The optical transport network attempts to find a path between the source and destination that meets the constraints. If the request succeeds, additional bandwidth can be created within seconds.

To date, the ODSI coalition has created a baseline document describing its goals, and has identified the IP and MPLS protocols that need to be extended.

The coalition intends to create a complete protocol specification by midsummer.

It will then conduct interoperability testing and provide technical recommendations to industry standards organizations such as the Internet Engineering Task Force (IETF) and Optical Internetworking Forum. Interoperable implementations of ODSI are expected for commercial deployment by year-end.

When ODSI is deployed, service providers can rapidly respond to changing network conditions. If the service network is overloaded, such as when the next bestseller is available only online, ODSI allows additional bandwidth to be instantly created upon demand, leading to more satisfied consumers and increased sales.

Kavi is director of product management at Tenor Networks. He can be reached at

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