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SONET/SDH framers gird for multiservice

Posted: 16 Dec 2003 ?? ?Print Version ?Bookmark and Share

Keywords:sonnet? sdh? mutiservice? data service? gfp?

The days of "build it and they will come" have abruptly ceased in the communications world. Most public and private communications networks continue to shift from voice traffic on their transport links toward data traffic, including Ethernet private line. Time-proven and widely installed networks for voice need to become more efficient data services without adding operational expenses.

This is not an insignificant challenge, since SONET/SDH was designed from the beginning to support TDM traffic and not the less predictable traffic flows typical of data communications.

A key component for a SONET/SDH network is the framer. Framing is the technology that designates or marks channels within a bit stream, providing the basic time slot structure, management, fault isolation and sectionalization protocol of choice for telecommunications.

New framer ICs bring two important innovations to the SONET/SDH market that distinguish them from their predecessors: VC and GFP. They integrate capabilities of traditional framers with companion devices to enable single devices, and ultimately single OEM designs, to support multirate, multiprotocol and multichannel solutions over SONET/SDH, which are key capabilities for today's multiservice providers.

Concatenation is a method used to bind a number of individual signals together to create a super-rate logical channel that provides more bandwidth than is available in one of the constituent signals. Contiguous concatenation requires that all intermediate network nodes support similar concatenation functions. Many installed network elements in SONET/SDH networks cannot support all contiguous-concatenation variations and would be prohibitively costly to upgrade or replace.

The VC specification, developed by the International Telecommunication Union, allows carriers to provision OC-48 (SONET) or STM-16 (SDH) pipes on an STS-1 or STS-3/VC-4 basis. Therefore, designers can band together the appropriate number of STS-1s or VC-4s to support a GbE stream while provisioning a different set of STS-1s/VC-4s to handle a Fibre Channel stream. Thus, VC enables the equipment, and ultimately the service provider, to provide multiple, right-sized channels for data applications.

Transporting Gigabit Ethernet over SONET (GEoS) using older technology, for example, would result in an inflexible 84 percent efficiency when using an STS-21c concatenated channel. With the traditional concatenation approach, the network provider could divide capacity into units of STS-1 (51Mbps), STS-3/STM-1 (155Mbps) or STS-12/STM-4 (622Mbps). It would be difficult, however, to mix and match the various pipes, because there would be no guarantee that the same pipes would be available across the network.

The new approach solves that problem by using VC to support pipes of different sizes. With VC, the GEoS network provider can flexibly achieve 95 percent utilization of the transport for data applications and the remaining STS-1/VC-4 channels for voice applications.

In addition, VC enables new service levels, opening the possibility for enhanced revenues. It is also possible to assign "protection bandwidth" to a signal in a virtually concatenated group that requires service-level assurance, allowing a service provider to offer network availability for the service and charge a premium.

The GFP is a key ingredient in the adoption of EoS technology. GFP is a protocol-agnostic frame delineation and encapsulation mechanism for transporting arbitrary datagrams or packets. It is a framing procedure for mapping variable-length payloads into SONET/SDH synchronous-payload envelopes. The value of GFP is that it provides simple encapsulation for native data protocols such as Ethernet or Fibre Channel. GFP comes in two flavors: frame-mapped and transparent.

GFP allows flexible and efficient transport of multiple protocols over SONET/SDH networks. GFP has extremely low overhead requirements and robust frame delineation.

With an increasing number of data services interfacing to SONET/SDH, service providers need an efficient way to support multiple protocols with simplified hardware and software. By providing GFP/VC, equipment and network efficiency can be enhanced and costs reduced.

- Chris Hamilton

Segment Marketing Director

- Nathaniel Grier

Director of Marketing, Optical Networking ICs

Agere Systems Inc.

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