Technology motivates multiservice and multimedia network architectures

Assorted voice, video and data services are expected to travel over robust synchronous optical and switching networks with intelligent control and automatic provisioning

BEN WAGNER AND DOUG SAYLOR

Dsc COMMUNICATIONS CORP.

The availability of interactive multimedia access/switching technologies and asynchronous transfer mode technologies with intelligent control and automatic provisioning has instigated a race to deploy voice, video and data services over fiber during the next two decades, with synchronous optical network technology as the driver.

Traditional loop, trunk and central office boundaries will begin to fade as service provisioning elements advance their way into locations distributed throughout the network. Such elements as add/drop synchronous transfer mode and ATM multiplexers, and Sonet crossconnect transport switches will replace the traditional demarcation points between the loop and interoffice networks. Moreover, they will offer rapid, flexible assignment and connection of bandwidth for a variety of switched and user-provisioned services.

Poised to emerge are products that incorporate such traditional non-intelligent functions as optical termination and multiplexers within the framework of such software-controlled and managed elements as multi-technology loop-delivery units and small, cost-effective distributed switching delivery units. This functional integration will re duce capital and operational costs, and improve reliability. In addition, it will provide service flexibility previously un available to network operators.

Integrated vision

The vision of an integrated multiservice/multimedia network architecture emerges from interconnected ring topologies found within the loop plant and interoffice facilities. Serving as nodes along a loop-plant ring, loop-delivery units provide reliable and flexible bandwidth to the network end-points. The delivery-unit concept is structured so that a network element is capable of delivering a myriad of services--video, telephony, business and personal communications--over a variety of distribution technologies, including copper, fiber, coaxial cable and radio.

Sonet will form the physical layer of baseband digital transport and offer automatic protection switching capabilities to standby lasers and fibers. Both STM and ATM transport multiplexing are employed; both use fibers for ATM carried as an STM payload and as dedicated all-ATM fibers. For video services delivered over hybrid fiber/coaxial-cable networks, analog supertrunking of broadcast channels can also be deployed.

Transport interconnections with service providers and other networks can be done at both the wire center and the hub level of the network. The choice of interconnection point will be based on traffic volumes and associated economics. This multiservice network can be implemented as a dedicated overlay network for a new service (for example, personal communications services or networks) that can be expanded for additional services or integrated into a multiservice backbone network.

Minimally, the network hub represents a concentration of the control and management segments. Hubs may also represent a switching and distribution transport hub for wire center traffic. Some overlay networks, such as PCS, may not initially require a transport hub; instead, they may rely on drop and insert capability dropping off interoffice facility rings. However, as traffic grows or other services are added, a transport hub can be added to the network--with no impact on the architecture.

Wire centers provide the bandwidth interconnection and switching between the loop plant and the interoffice plant. Switched service delivery units can be located here to provide such services as personal communications services. The loop plant can be realized through the use of various topologies and technologies, depending on the services offered and the economics of subscriber usage. Services can share loop-transport facilities or employ dedicated facilities. Residential video may be delivered via a fiber-to-the-curb/home-switched digital baseband Sonet network or a double-star hybrid fiber/coaxial network; business services will run over dual Sonet rings.

Service delivery

The network delivery of service terminates at the customer premises. The variety of delivery technologies will dictate the required customer premises network terminations. Fiber, coaxial, copper and radio technologies will all be employed by the network.

In the integrated multiservice/multimedia architecture, the control and management segment (service/management segment) is connected to the delivery segment by a high-performance ATM subnetwork. Information network architecture and telecommunications management network principles are used to define the functions and interfaces for the delivery units. These definitions represent the heart of the network architecture and provide an open, high-performance and modular service delivery network that is flexible and programmable.

Integrated multiservice/multimedia architecture supports the network blueprint for new network elements, service control elements and management elements that separate service delivery from service control and management. In this blueprint, such network elements as switches, crossconnects and add/drop multiplexers provide basic transport and management functions. These elements provide interfaces based on managed objects to invoke a transport or management function.

Service and management logic then invokes these functions to provide a specific service. Service and management logic can be layered by defining managed support objects that can be used by other service or management logic. The interfaces provided by managed objects or managed support objects are defined in contracts.

The ultimate in open network architecture is achieved by making these interfaces and the interface contracts open to all network users. The integrated multiservice/multimedia network architecture can provide the high-power control and management connectivity needed to implement open, layered, high-performance services.

Network elements and applications

Sonet, through strictly defined interfaces, will integrate optical transport functionality, regardless of the equipment supplier at either end of an optical span. In the past, all optical interfaces have been vendor proprietary, resulting in the fragmentation of the function into numerous elements. Economic savings that result from the elimination of the cost of redundant common functions that support the interconnection of these elements will be significant. When the operations and maintenance savings are also considered, it becomes apparent why network service providers were the main drivers behind the formation of the Sonet standard.

Sonet is also defined as the physical layer for network ATM and will support the introduction of ATM services into an STM infrastructure, as well as the eventual migration of this infrastructure. ATM technology embedded in Sonet transport will provide bandwidth flexibility for creating advanced local networks and services. With the explosive adoption by business customers of ATM multimedia local area network and private-network equipment, local carriers providing ATM network services can provide seamless, high-performance, cost-effective wide-area networking for these customers.

For the first time, wide area and local area network technologies are converging. The winners will be business users. For residential customers, video-on-demand and related interactive video services loom on the horizon. ATM and Sonet technologies provide local carriers with a cost-effective infrastructure for switching and multiplexing multiple rates of asymmetrical video streams. This same infrastructure, while initially justified by entertainment video revenues, can also be used for such new services as electronic publishing and software distribution.

Sonet network elements need to address the trends of network and operations simplification. These trends suggest that future network elements might be multifabric types that can access and manipulate narrowband, wideband and broadband bandwidths. These network elements configured for use in ring topologies will enable bandwidth management distribution in the network and ensure that future networks will be robust, survivable and dynamic.

From an infrastructure point of view, Sonet will form the foundation upon which the remainder of the network is built. It will serve as the medium of choice, as evidenced from the simple point-to-point networks deployed in the late 1980s to the complex ring networks of today. During this time, standards have stabilized, and the technology has matured. Consequently, point-to-point terminal configurations have been replaced by add/drop multiplexers, clearly the common choice of deployment from a service provider`s perspective.

With add/drop multiplexers, functionality is a more efficient and less expensive alternative to the back-to-back terminals available through Sonet`s inherent multiplexing scheme. In addition, requirements for outage protection and disaster recovery make it mandatory that these Sonet systems automatically re-route signals in the event of a failure. Two common protection schemes are the path-protection switch ring and the bidirectional line-switched ring.

Several regional Bell operating companies have devised a new tariffed service based on these protection schemes. This service offers to end users a guaranteed performance level at a negotiated price and presents new revenue opportunities through the distributed intelligence placed among Sonet elements.

But with these opportunities come the increasing traffic demands spurred by the continued escalation of optical transport rates. In turn, add/drop multiplexer systems will not be able to economically manage all the traffic, especially at the larger hub sites within the Sonet infrastructure. Instead, Sonet-based digital crossconnects will be deployed where large bandwidth management capabilities are required. Initially, they will serve as bridges between asynchronous and synchronous traffic. Later, next-generation digital crossconnect equipment will become available. It will be "multifabric" in the sense that the broadband and wideband matrices are combined to groom STS-1 level signals for bandwidth efficiency and fiber capacity optimization. Meanwhile, the virtual-tributary-based traffic within the STS-1 payload is manipulated for inter- or intraoffice convenience.

The digital crossconnects will also perform such functions as gateway access between the operating systems and the subtending network elements, test access for problem segregation, ring management for network simplification, and maximum fiber capacity through traffic grooming. Cell-based traffic is also emerging as LAN-based traffic at business levels and eventually will emerge as home interactive video. This traffic again places requirements on the infrastructure as the digital crossconnect system continues to evolve, this time with an ATM-based fabric.u

Ben Wagner is senior director, strategic planning, network vision and access; Doug Saylor is director, strategic planning, transmission systems at Plano, TX-based DSC Communications Corp.