Most business data traffic starts and ends on packet-switched networks, particularly Ethernet LANs. Since this business traffic is a primary target of most network operators, key services to be supported and marketed are Ethernet-based. But startup operators, with no legacy network implications, can consider establishing all packet-switched networks.

Two of the most popular packet-switched transport technologies are:

Market uptake to date of both of the above transport technologies is still relatively small.

As stated, incumbent network operators are looking to evolve their legacy Sonet/SDH networks to deliver enhanced data services at lower costs. In these cases, packet-switched transport, such as Carrier Class Ethernet and RPR, are likely to be used only as access technologies carrying data to the edge of the Sonet/SDH-based metro network. So-called Next Generation Sonet/SDH equipment being deployed already differs significantly from its legacy counterparts.

In particular, Next Generation Sonet equipment now offers three new data-plane technologies building on top of standard Sonet/SDH operation to more efficiently and effectively transport data traffic. These technologies are:

VCat allows the Sonet/SDH containers to be virtually grouped; only the terminal equipment at each end of the transport path must recognize and process the concatenated containers. In particular, any number of containers can be grouped together in Vcat, and bandwidths that closely match the required data traffic throughput can be established. It also results in lower operating expenses, thanks to higher data bandwidth efficiency as well as lower capital expenditures to replace or upgrade the path terminating equipments.

Thus, the introduction of GFP, VCat and LCAS data plane technologies has enabled legacy Sonet/SDH networks to offer enhanced data services and lower costs. Another key driver in lowering both capital and operating expenses has been the evolution in transport equipment architectures and network topologies:

The past few years have seen an increase in the integration levels associated with optical transport equipment. The functionality previously available on a fully loaded printed-circuit assembly now fits on a single application-specific IC. As a result, a rack-wide single product is now a single-slot module, and up to three or four individual tools have become a modular, reconfigurable, single-rack device that offers all the previous functionalities and more. The payoff is a significant cost savings, reduced footprint, lower power requirements and less inter-equipment cables.

Integration, however, makes sense only if separate devices are used together in the same location. Indeed, this is the case with next-generation Sonet/SDH and new equipment categories that include:

Also, changing equipment architecture has resulted in an evolution of Sonet/SDH network topologies. For example, the simple point-to-point topology has since given way to the Sonet/SDH ring topology, with its renowned Unidirectional Path Switched Ring and two- and four-fiber Bi-directional Line Switched Ring circuit-protection schemes. Such protection schemes are highly effective, but also waste bandwidth, a price that network operators are no longer prepared to pay.

The evolving data-traffic network. Source: Agilent Technologies

Enter the Sonet/SDH mesh network, which replaces circuit protection with circuit restoration, a more efficient way to offer five-nines bandwidth reliability. Mesh networks generally consist of multiple sub-tended rings. The only downside is complex Automatic Protection Switching control schemes that have less predictable performance and boundary cases.

The provisioning of appropriate paths through the transport network has until very recently been a manually intensive task. In receiving notification of a new or adjusted path requirement, a network technician would have to manually check for bandwidth availability across appropriately located equipment and links. The technician would then use multiple equipment-management systems to send instructions to each tool to configure the network, which could take hours or even days.

Obviously, this situation has been helped by the integration of equipment and the resulting drop in the number of products involved in any network. A far bigger improvement is the introduction of an effective control plane to transport networks. A transport network-control plane automatically finds an appropriate end-to-end path and configures equipment by receiving a similar path requirement notification used by a technician. It does this by using automatic procedures for network topology discovery, equipment and link status updates.

Today, transport control-plane implementations are proprietary and do not provide multivendor interoperability. However, R&D is already under way to produce a standards-based transport control plane, with GMPLS the most likely technology base.

Next-generation Sonet/SDH multiservice provisioning platform. Source: Agilent Technologies

In summary, optical transport networks are in the process of major changes, driven by the need to offer better services to data traffic and an associated requirement to significantly lower costs. Though startups with no legacy network can consider moving to all-packet-switched ones based on technologies such as Carrier Class Ethernet and RPR, the incumbent operators are evolving their huge installed base of Sonet/SDH networks to meet these new requirements while, in some cases, establishing small all-packet-switched networks. This Sonet/SDH evolution affects not only the data and control plane technologies, but also equipment architectures and network topologies.

Finally, for example, functional test equipment for Sonet/SDH networks is also evolving with:

Ronnie Neil is the optical transport marketing programs manager for Agilent Technologies (Palo Alto, Calif.).

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