The evolution of synchronous optical networks

March 1, 1995

The evolution of synchronous optical networks

Sonet`s definition has expanded to include an electrical interface for transporting an STS-1

mark r. wilson

AT&T bell laboratories

Telecommunications networks throughout North America have been evolving to a new set of common standards that collectively define a synchronous optical network. The first phase of the Sonet standard, issued in 1988 by the American National Standards Institute, was intended to standardize the industry on a common set of rates and formats based upon integer multiples of the 51.84-megabit-per-second synchronous transport signal, level 1.

Prior to Sonet, all digital equipment used the digital signal-n hierarchy for electrical signals and facilities, which defined rates and electrical interfaces to 274 Mbits/sec (the DS-4), but addressed neither higher rates nor optical interfaces. Consequently, vendors developed multiplexers and lightwave systems that multiplexed DS-1s (1.544-Mbit/sec) and DS-3s (44.736-Mbit/sec) into proprietary optical formats, making optical connectivity between vendors impossible. The Sonet standard, whose main objective was to specify a universal set of optical interface specifications, was to make such a mid-span meet a reality.

Sonet rates and formats

The Sonet standard defines the signal rates and formats that enable vendors to develop a family of Sonet products that offer end-to-end Sonet connectivity. All Sonet signal rates are integer multiples of the 51.84-Mbit/sec STS-1. Therefore, an STS-n consists of n synchronous transport signal-1s and has a rate of n䂧.84 Mbits/sec. In addition, sub-STS-1 rates are defined in the standard as virtual tributary signals. Of particular interest is the VT1.5 signal, which carries the 1.544-Mbit/sec DS-1s (at 1.728 Mbits/sec, including signal overhead). Analogous to the multiplicity of 28 DS-1s per DS-3, the STS-1 consists of 28 VT1.5 signals.

When the standard was originally released, only the optical interfaces were defined--known as optical carrier level n , or OC-n , which consisted of the optical facility transporting an STS-n . More recently, however, because network providers have demanded a Sonet electrical interface for STS-1s, the electrical carrier level 1, or EC-1, definition was added to denote the electrical facility transporting the STS-1. No external interfaces have been defined for the virtual terminal signals; they are strictly for internal use by the Sonet elements.

The Sonet element most associated with the benefits of the Sonet standard is the Sonet add/drop multiplexer, which is available at the OC-3, OC-12 and OC-48 rates. With the add/drop multiplexer, instead of using back-to-back multiplexers to add and drop traffic in an office, one terminal can add, drop or interchange constituent signals within the OC-n optical line. These add/drop multiplexers have been developed for point-to point and ring configuration. The ring systems can be applied to support point-to-point (two-node ring) and linear add/drop chain (multinode folded ring) topologies. Consequently, the ring platform, to which nodes (and other rings) can be added or removed on an in-service basis, provides complete flexibility for supporting all topologies without the need for equipment/software up grades. Because of this flexibility, most vendors have leapfrogged the development of linear add/drop multiplexer platforms in favor of the add/drop multiplexer ring.

Other major network elements that support Sonet networking are the wideband and digital crossconnect systems. Prior to the availability of Sonet interfaces, the wideband digital crossconnect systems had been used for DS-1 add/drop, grooming and performance monitoring of DS-3 facilities, and thus replaced manual DSX-1 crossconnect frames and back-to-back multiplexers. The broadband digital crossconnect systems, which consisted only of DS-3 interfaces, were used for the crossconnection of DS-3 facilities and thus replaced manual DSX-3 crossconnect frames.

Although these digital crossconnect systems initially consisted only of asynchronous interfaces, some vendors anticipated the migrations to Sonet and thus built their digital crossconnect system fabrics to switch at the Sonet rates--VT1.5 and STS-1 for the wideband and broadband digital crossconnect systems, respectively. Therefore, when the Sonet interfaces became available, these same systems became Sonet digital crossconnect systems with the addition of these Sonet interfaces.

Now that the Sonet interfaces have become available on both add/drop multiplexers and digital crossconnect systems, new applications have surfaced for digital crossconnect systems. One major application is the Sonet-to-asynchronous gateway. This gateway feature has become essential for facilitating the migration of the network to Sonet.

Other applications of Sonet digital crossconnect systems include en hanced performance monitoring of Sonet and asynchronous facilities, test access and restoration.

Network applications

The Sonet add/drop multiplexer ring has been a particularly important architecture because it has provided customers and service providers with perhaps the most beneficial ingredient for these new networks based on Sonet--self-healing networks. With the Sonet add/drop multiplexer ring, if the ring suffers any facility failure (for instance, fiber cut), service is restored in less than 60 milliseconds. Furthermore, if the ring suffers a node failure, all traffic on the ring (except that involving the failed node) is restored in less than 60 milliseconds.

Two main ring types have been developed by vendors--path-switched rings, which were offered initially, and, more recently, bidirectional line-switched rings. Both ring types are required to restore service traffic in less than 60 milliseconds (including failure detection); the choice of which type to deploy depends upon the application.

Several network engineering guidelines have arisen with the deployment of these ring systems. For example, economic studies have shown that the high capacity (OC-48) bidirectional line-switched ring system is economical for most interoffice applications. In access networks, where the traffic patterns tend to be hubbed, the lower capacity (OC-3 and OC-12) path-switched rings are preferable.

Ring bandwidth optimization

One of the most economically beneficial Sonet applications is based on the inherent grooming capability of the wide digital crossconnect systems--ring bandwidth optimization. Because access and interoffice rings use mostly EC-1 interfaces to maintain Sonet connectivity between rings, placement of the wide digital crossconnect systems between these rings to groom the interring traffic at the VT1.5 rate significantly reduces the bandwidth requirements of each ring, thereby conserving ring capacity. In a typical network, the proper placement of several wide digital crossconnect systems at interring nodes reduces the total network equipment costs by up to 25%, including the costs of the wide digital crossconnect systems deployed.

Another network issue that has received increased attention is end-to-end survivability from customer premises to an interexchange carrier point of presence. In this configuration, the customer is served by a node on an access ring, while the point of presence is a node on an interoffice ring. Because the ring topology completely restores fiber cuts and node failures (not involving the nodes of interest) in less than 60 milliseconds, the only failure scenario to be addressed is a failure in the node(s) where two rings intersect.

The recommended architecture for total survivability is illustrated with AT&T Network Systems` series 2000 products, where the access ring (DDM-2000/SLC-2000) is connected to the interoffice ring (FT-2000) via two offices, generically known as dual-ring interworking. In this dual-ring interworking architecture, the access ring employs a feature known as VT1.5 drop and continue, in which the access ring drops the customer traffic (VT1.5s) in both offices at all times. Within each office, the dual-ring interworking VT1.5s are groomed (along with other non-dual-ring interworking traffic) with the wideband digital crossconnect systems (DACS IV-2000) and sent onto the interoffice ring to the point-of-presence office. If the primary serving office (for a given set of customer DS-1s) would fail, because two copies of each dual-ring interworking VT1.5 are always present in the point-of-presence office, a VT1.5 path switch at the point of presence would restore the dual-ring interworking traffic in less than 60 milliseconds.

Although the goal of attaining end-to-end Sonet connectivity is being pursued by network service providers, most of the migration to Sonet in their networks has been based only on new growth and churn. Therefore, the existing asynchronous traffic will remain in these networks until all traffic is churned onto the Sonet elements and the old equipment is retired. Until then, the Sonet digital crossconnect systems will need to provide the Sonet-to-asynchronous gateway capability to help with the migration toward the all-Sonet network. u

Mark R. Wilson is a member of the technical staff in the transport applications planning group at AT&T Bell Laboratories, Holmdel, NJ.