Add/Drop Multiplexing in the Data-Centric Network
A new platform for add/drop multiplexing handles data more efficiently than traditional equipment.
By: Chad Dunn, Omnia Communications Inc.
Data traffic has matched and will soon overtake voice traffic in the public network. Carriers face a key choke point in the metropolitan area network that makes the migration from voice to data services painful-synchronous optical network (SONET) add/drop multiplexers (ADMs) and ancillary devices. While suitable for circuit transport, these time-division-multiplexing (TDM)-based network elements cannot scale up to support data-dominant network traffic as effectively as packet- and cell-based technologies based on statistical multiplexing.
We propose a new ADM platform based on asynchronous transfer mode (ATM) that provides a migration path from circuit-oriented SONET networks to packet- and cell-based data networks.
Provisioning data
Traditional local area network (LAN) data connections do not map well into the SONET hierarchy because SONET provisions bandwidth on fixed timeslots. The protocol's rigid TDM structure makes it difficult to allocate bandwidth efficiently. For example, a SONET network requires an entire STS-1 (51 Mbps) to provision a 10 Mbps LAN connection. The network cannot reallocate the 41 Mbps of bandwidth to carry any other traffic in the network.
Using ATM as a layer 2 protocol leverages the statistical multiplexing benefits of packet and cell technology and provisions bandwidth more efficiently through its use of flexible switched virtual circuits an paths. ATM also brings with it support for voice traffic and the multiple quality of service (QoS) levels required to mix voice, video, and data over a common infrastructure.
Although ATM is an attractive solution and TDM hinders efficient data networking, carriers desire SONET's 50-ms protection switching, alarming, and performance monitoring-all of which integrate easily into their existing operational support systems (OSSs). To provide guidance in mapping ATM across the full bandwidth of a SONET ring while maintaining the network resiliency that carriers expect, Bellcore developed the GR-2837 standard, "ATM Virtual Path Functionality in SONET Rings." Adhering to this standard enables new data-friendly network solutions that leverage SONET features. By adding an intelligent Layer-2 technology like ATM to SONET rings, carriers have a firm foundation to continue to deploy voice services as well as Internet protocol (IP) and other data services.
IP everywhere
As data traffic dominates the network, carriers look to IP to deliver integrated services. IP is the hands-down choice for data networking and is gaining momentum as a transport for voice. As carrier customers demand relatively advanced data services such as 100Mb/s Internet access and high-speed virtual private network (VPN) connectivity, wire speed routing and application-based quality of service are now basic requirements for any IP-aware device in the network. Because the statistical multiplexing benefits of routing and quality of service are most obvious near the source, new ADMs must incorporate these functions as a basic feature.
While integrating transport and access gear is a blasphemous notion to traditional network engineers, the economics associated with an integrated SONET access platform are too compelling to ignore. Examining a local SONET ring using traditional gear yields a grim picture of a number of devices that carriers must deploy, test, spare, manage, and maintain.
Today's metro network includes a SONET ADM for connectivity to the fiber ring, a channel bank or digital loop carrier (DLC) for voice connectivity, and an ATM service access multiplexer (SAM) or router for data connectivity. In the central office, a digital crossconnect breaks out the various circuits for delivery to a class 4/5 switch or to the data networks.
This collection of multivendor point solutions enables the carrier to provide multiple services, but it also drives up the cost of management and deployment, and the incremental cost of new services. Each piece of equipment requires its own management application and integration into the OSS, as well as other support services such as software upgrades and sparing.
For carriers, the cost of maintaining these networks is spiraling out of control as they deploy add-on hardware to retrofit existing SONET TDM networks to carry data traffic. These solutions often involve several devices in both the customer premise and the central office to provide services.
The solution
In a metropolitan ring based on a next-generation ADM, the important feature is what does not appear in the picture. The new ADM platform replaces the traditional SONET ADM and eliminates the SAM, DLC, and IP router. This consolidates carriers' equipment, simplifies network maintenance and management, and reduces the cost of adding advanced and incremental services.
The new ADM obviously fits into green-field network builds, but it must also offer carriers a migration path from existing networks to the new platform. While migration strategies vary widely among vendors, Omnia integrates a two-channel wavelength division multiplexer into its ADMs that provides an independent wavelength to overlay existing rings, regardless of bit rate or protocol. Passive optics maintain the integrity of the existing ring.
Replacing IP routers, DLCs, SAMs, and Digital Cross-Connects with a multiservice platform reduces procurement and management costs and increases the efficiency of the installed fiber through the use of ATM as a transport protocol. The central-office-class platform integrates the add/drop capability and reliability of a SONET ADM and the voice transport capability of a DLC with the functionality and protocol awareness of an ATM SAM and an edge router. It also provides the interfaces and the density carriers need to offer value-added data services as well as legacy transport and voice services, eliminating the management overhead associated with most "multiple box" solutions. The new ADM platform allows carriers to extend the life of installed equipment, minimize the need to deploy overlay networks and enhance the revenue generating ability of the installed fiber infrastructure.