Jack of all trades: A hybrid DCS can help smooth the transition from Sonet to ATM networks

Although today's public networks still transport primarily voice traffic, data and multimedia services are becoming an increasingly important part of the application mix. By necessity, carriers are reconsidering their transport network architectures.

Synchronous transfer mode (STM) techniques-such as Sonet-have been the traditional choice for voice-dominated carrier networks. STM networks statically allocate the switching and transport resources needed to support any given connection for its duration. Once a connection has been assigned, other applications cannot use the bandwidth, even if the connection is idle.

While voice applications behave predictably in terms of static traffic characterization, data applications act as the antithesis to voice in almost every regard. Data traffic is bursty in nature, with long intervals of idle time between traffic bursts.

This places an ever-increasing demand on traditional STM-based switched services and their underlying STM transport networks. Until now, the response has been to apply more switching and transport resources to the network.

Recent thinking has focused on incorporating asynchronous transfer mode (ATM) technology into transport elements, such as digital cross-connect systems. Such an approach can provide more effective bandwidth management of traditional transport networks and services while reducing the number of costly switching resources.

ATM gains momentum ATM brings the promise of increased profits to carriers through new services that enhance revenue, and reduced costs for new and existing services. The ability of ATM networks to support some measure of bandwidth on demand has compelled carriers to consider it the basis for transporting all services, including today's circuit-switched voice and private-line traffic.

During the introductory phase of new broadband services, the primary mechanism for increasing profits is through increased revenue. Carriers are testing service acceptance and gaining operational experience through a limited, low-risk deployment of ATM network elements and transmission facilities overlaid on existing transport networks. Existing STM transport networks are suitable for these initial deployments of broadband services, but they have shortcomings that make them inefficient for widespread deployment.

The STM multiplexing hierarchy constrains the bandwidth of the paths used as access or trunk connections to ATM network elements to a small number of widely spaced values (Figure 1). This reduces service offering flexibility and inefficiently uses transport network bandwidth. In addition, the bandwidth of these STM paths is fixed with no opportunity for dynamic bandwidth allocation.

As demand for ATM grows, competition will drive the need to make transport networks more efficient to cost-effectively support wider deployment. Virtual paths can be employed in transport networks to address the additional demands of broadband services and to potentially reduce the cost of providing existing services.

Because virtual paths have arbitrary bandwidth and can be non-hierarchically multiplexed, they can allow for access and trunk connections of any size with efficient traffic aggregation toward the network backbone. Infrastructure benefits such as these are driving ATM into the network from the inside out.

To realize incremental cost benefits, carriers may leverage the fact that STM provides ATM's physical layer. This allows the use of existing STM facilities to support emerging broadband services rather than constructing separate, overlay transport networks that segregate STM and ATM traffic onto separate facilities. High-speed STM facilities will carry conventional bit streams mapped into an STM payload, as well as services that have been converted to an ATM format. These are referred to as STM-adapted services and ATM-adapted services, respectively.

A longer-term view holds that existing and emerging data, voice and video services will be transported across a ubiquitous ATM transport network. Network transitions are not instantaneous, however. The next generation of transport elements should be able to handle both STM and ATM circuits to facilitate the transition from STM to ATM transport.

A hybrid DCS is one such element. Its groom-and-fill capability can help cost-effectively transport non-switched services and route switched traffic through the smallest number of transport facilities and switch ports. In either case, the result is a net increase in the use of high-speed interoffice transport facilities and switching resources.

Architecture choices Three different network infrastructures can be used to support ATM traffic over STM facilities. One method, switching system implementation, relies heavily on ATM switching systems in central offices. The other two methods-broadband cross-connect and ATM virtual path cross-connect-rely more heavily on a hybrid DCS.

The switching system implementation of ATM transport, typically adopted in early ATM applications, relies on central office-resident ATM switching to provide transport bandwidth management. The ATM switching systems are connected by dedicated transmission facilities across distribution and interoffice subnetworks (Figure 2).

The other two network architectures rely more on CO-resident hybrid DCSs (Figure 3).

In one scenario, the hybrid DCS is used as a broadband cross-connect. STM transport is used to carry traffic between distribution and interoffice transmission facilities. The STM payload-intact cross-connect capability of the hybrid DCS is used to efficiently groom and fill STM paths. Bandwidth is allocated on an STM payload basis, offering transport at conventional rates such as STS-1 (51.84 Mb/s) and STS-3c (155 Mb/s).

The other scenario uses ATM transport-a logical transport network composed of semi-permanent virtual paths used as transmission circuits between customer premises, service access and service routing locations. Here, the hybrid DCS functions as an ATM virtual path cross-connect, enabling carriers to leverage the ability of virtual path techniques to allocate bandwidth in a more granular fashion in order to more closely approximate actual service needs.

The virtual path cross-connect approach offers an appropriate long-term solution. It allocates both the ATM transport and STM transport bandwidth management functions to the hybrid DCS, while the broadband service routing function is allocated to the ATM switching systems.

Relieving ATM switching systems of their bandwidth management responsibility facilitates their placement wherever the service routing function is deemed most appropriate. This may mean using the ATM switch as an edge switch rather than in a hub configuration. Some COs may not need a resident ATM switching system.

Given the hybrid DCS's incorporation of add/drop multiplexer (ADM) capabilities, either of the DCS-based network architectures can eliminate the need for CO-resident ADMs. The switching system-based network architecture, however, requires a CO-resident ADM for each ring to which the CO connects.

Another disadvantage of a switching system-based network architecture is that any given ATM-adapted service connection between two customers requires twice as many switching system ports as in a DCS-based implementation. This is because the switching system must provide a physical port for each of the CO-resident ADMs.

Of the two DCS-based network architectures, the virtual path cross-connect application provides greater efficiencies because ATM transport consumes fewer STM payloads per port than STM transport.

The DCS-based broadband cross-connect implementation of STM transport allocates two STM payloads between any given pair of broadband service elements-such as the ATM switching system and the hybrid DCS. This process consumes twice as many STM payloads as an equivalent DCS-based virtual path cross-connect implementation of ATM transport, where only a single STM payload needs to be allocated between the broadband service router and the hybrid DCS (Figure 4).

Another advantage of a hybrid DCS is that it supports Telecommunications Management Network-defined configuration, fault, performance and security management functions. This means a hybrid DCS can be centrally positioned within STM transport networks as a platform for circuit provisioning, performance monitoring, fault sectionalization, test access and security administration. These management functions will remain applicable as ATM transport networks evolve.

Other hybrid DCS applications As an end system, the hybrid DCS is managed as a network element within the bottom three layers of the TMN functional architecture (Figure 5). The hybrid DCS's capabilities include functions residing in the network element layer, which are controlled via a standard interface, to the management functions residing in the element management and network management layers of the TMN functional hierarchy.

In a typical implementation, element management layer functions would be allocated to the element management system, while the network management layer functions would be allocated to the network management system. Network element layer functions would be allocated to the transport nodes, including hybrid DCSs and service access nodes.

The hybrid DCS would be centrally located in transport networks, enabling it to be a gateway network element through which management and controltraffic could pass between management systems and other transport nodes. The hyb rid DCS's central location also would allow its application as an intermediate network element by virtue of its ability to provide connectivity between STM facilities and rings, thus providing the flow-through of management and control traffic between transport nodes.

DCS implementations of ATM transport yield many benefits over switching system implementations in terms of applications, scalability, flexibility and support for network evolution. Collectively, these benefits combine to form a favorable cost comparison between a hybrid DCS used in bandwidth management applications and ATM switching systems used in the same capacity.

Application support. ATM switching systems contain virtual circuit switching, statistical multiplexing, traffic management, routing and signaling features to support broadband service routing applications-none of which are needed for ATM transport bandwidth management applications. These features add unnecessary cost to an ATM switching system-based network architecture.

ATM switching systems also lack many of the capabilities associated with the maintenance and reliability of transport networks. Features such as test access, performance monitoring, facility protection and equipment redundancy often are missing in ATM switching systems because of differing maintenance and reliability criteria that transport applications impose. A hybrid DCS typically includes these features and also provides support for STM bandwidth management and ring node applications, resulting in additional cost and operational efficiencies.

Scalability. When called upon to support ATM transport bandwidth management in addition to their traditional service routing functions, some ATM switching system architectures don't scale well to the additional port and switching capacities needed. Additional ATM switching systems may be needed in the same CO, or replacements may be required in order to scale up to the larger capacities needed.

Flexibility. When relieved of all transport bandwidth management responsibilities, switching systems are freed up to be placed only where the service routing function is appropriate. This affords more flexibility in the placement of the service routing function into centralized, hub switched or distributed, edge switched topologies.

Network evolution. By encapsulating virtual path cross-connect and broadband cross-connect capabilities, the hybrid DCS provides a common platform to support network and bandwidth management of ATM-adapted service overlay and STM-adapted service overlay networks as they evolve toward integrated networks for ubiquitous ATM-adapted service transport.