Sonet/SDH framers gird for multiservice

Sonet/SDH framers gird for multiservice
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 time-division-multiplexed traffic and not the less predictable traffic flows typical of data communications. The wealth of investment in equipment, software, knowledge and network management experience underscores the need to leverage the existing infrastructure.

For the service provider, and ultimately for the OEM equipment vendor, this environment leads to the following technical and business challenges:

• Carriers and equipment manufacturers must derive more revenue from existing network infrastructure by using it more efficiently, without the need for complete network upgrades and capital expenditures.

• Service providers must be able to generate additional revenue streams through new and differentiated services. As new access technologies-like Ethernet in the local loop or metropolitan Ethernet-come to market, the ability to interface them with the existing network infrastructure at minimum expense, rework or disruption to services becomes crucial.

• The price points of how services are built, upgraded and maintained will need to reflect the new economic realities. Customers need flexibility with scalability to preserve their infrastructure investments. And they need technologies that enable incremental capital investment for each service activation, thus improving return on investment.

• Equipment will need to provide higher port densities with reduced footprint, power and cost. OEMs require design reuse that enables them to reuse hardware (board) and software development and intellectual property. This should be combined with modular expandable hardware elements like multichannel, multirate framers and pluggable optics. The goal is to cut development time and the costs associated with development, deployment and operation.

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: virtual concatenation (VC) and Generic Framing Protocol (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 spec, developed by the International Telecommunication Union (ITU), 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 Gigabit Ethernet 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 (1.244-Gbit/second) concatenated channel. With the traditional concatenation approach, the network provider could divide capacity into units of STS-1 (51 Mbits/s), STS-3/STM-1 (155 Mbits/s) or STS-12/STM-4 (622 Mbits/s). 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 (STS-1-21v for each Gigabit Ethernet stream for Sonet or VC-4-7v for SDH) and the remaining STS-1/VC-4 channels for voice applications. In some multiprotocol applications, VC may reduce wasted capacity by as much as 75 percent.

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

The GFP is a key ingredient in the adoption of EoS technology. Defined under the ITU G.7041 specification, GFP is a protocol-agnostic frame delineation and encapsulation mechanism for transporting arbitrary datagrams or packets.

GFP 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. In the case of Gigabit Ethernet and Internet Protocol, the frame-mapped method is the most efficient and common.

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 designs. By providing GFP/VC, equipment and network efficiency can be enhanced and costs reduced.

While GFP and VC are common implementations, the link-capacity adjustment scheme (LCAS) and the link-access procedure for SDH are currently optional additions for manufacturers. LCAS can be viewed as a supplement to VC, allowing designers to adjust the capacity of the VC group in real-time. Therefore, carriers and equipment manufacturers can adjust the amount of STS-1s/VC-4s provided in a group to meet the changing needs of end users and adjust to changing conditions in a network. LCAS allows the number of STS-1/VC-4 containers to be adjusted dynamically within the network.

Real-time adjustments

For service providers, the payoff with LCAS is being able to provide real-time scalable data services profitably and maintain service-level-agreement delivery. VC on its own cannot deliver real-time, error-free adjustment, because both ends of the resized pipe need to be synchronized. Although this synchronization can be achieved with software over several seconds, service providers face challenges when crossing over operational-support-system domains, since they do not support this seamless handoff today. Without LCAS in such circumstances, real-time bandwidth adjustment is difficult.

LCAS' structure adds a level of design complexity and risk for any OEM trying to decide how and when to build in support. Ultimately, framer silicon that supports LCAS offers a lower-cost and lower-risk solution to the problem.

Advances in semiconductor technology are giving chip designers higher performance and space to add functionality. For example, GFP, VC and LCAS are integrated in next-generation Sonet/SDH framers. The expanded capabilities and modularity of these new ICs are changing OEM design constraints and promise service providers new levels of flexibility. Other Sonet/SDH framer ICs address the multiprotocol, multirate issues with GEoS. These devices combine Gigabit Ethernet support with add/drop multiplexer, VC and other functions on an IC, reducing an OEM's time-to-market while improving a system's cost, power and size.

Although Ethernet is a simpler, less expensive solution, it is not optimized for the ring topologies of Sonet/SDH networks. Thus, Ethernet typically does not take advantage of installed-ring-topology functions to improve availability. New GEoS solutions can transport Gigabit Ethernet frames over existing Sonet/SDH rings by taking advantage of GFP/VC to use arbitrary groups of STS-1/VC-4 signals. The devices build higher-capacity payloads whereby a stream of data is divided among a set of STS-1/VC-4 containers as it is transmitted, then realigned and multiplexed as it is received. This capability enables data clients to utilize Sonet/SDH networks more quickly, reducing overall system power and cost for EoS and packet-over-Sonet/SDH applications.

See related chart

See related chart