Multi-ring topologies forge Sonet fiber backbone

Multi-ring topologies forge Sonet fiber backbone


A high-speed, fiber-optic backbone multiplexer supports multi-ring topologies that do away with network design distance constraints. The Premnet backbone multi-ring infrastructure, by Racal-Datacom Inc., Sunrise, FL, links multiple fiber backbone rings to form a seamless, centrally managed multi-ring network.

Multi-ring configurations increase design flexibility in configuring a network that meets the needs of a building, campus or metropolitan area. They can take the form of a chain with rings linked end-on-end for maximum distance coverage; a daisy configuration in which multiple rings are linked around a central ring; or a tree topology in which multiple rings branch out from a single parent ring.

Charles M. Dillon, vice president of the Fiber Systems Group at Racal-Datacom, explains that with multi-ring capability, networks can be extended to 2000 kilometers, or about 1240 miles. In these networks, five rings can be linked together using 74 nodes. Each ring would use 15 to 16 nodes, on average, each up to 50 km apart.

Dillon says multi-ring capability allows synchronous optical network/synchronous digital hierarchy, or Sonet/SDH, rings to be connected to each other or interconnected with standard 100-megabit-per-second Premnet rings for handling such broadband requirements as multimedia, interactive applications, graphics file-sharing and full-motion video.

Ronald O. Brown, a communications consultant in Melrose, MA, says Racal-Datacom`s ring architecture offers the best availability for the networking dollar. "Ring architecture was used years ago in the military and then adopted by such competitive access providers as MFS and Teleport," he says. "It is now clear that network designers are rapidly moving toward using the ring as the fundamental network building block. In the future, all large networks will be made of interconnected rings."

Traffic integration

The high-speed fiber backbone system is also capable of integrating data, voice, video and asynchronous transfer mode, or ATM, traffic over the same fiber-optic cable. With this capability, individual rings transporting disparate types of network traffic can be seamlessly integrated into a single, centrally managed network (see Lightwave, October 1995, page 3).

Another industry analyst raises questions about Racal-Datacom`s multi-ring configurations. Skip MacAskill, senior research analyst for the Gartner Group in Stamford, CT, asks, "Does this solution rely on old technology, since Premnet is multiplexer-based?"

The big push now is to craft these types of backbones using ATM switching technology, MacAskill notes. "Is multiplexing the way to go if the industry as a whole is heading to ATM? I would like to see more detail and expansion in the network management area. If a user is going to maintain his own fiber backbone, then he`s going to require some robust network management tools," he observes.

Despite MacAskill`s reservations, Racal-Datacom`s Charles Dillon maintains that his company`s solution is viable: "This multi-ring capability eliminates network design roadblocks both in terms of configuration and distance. Network designers no longer need to think of single-ring architectures. The Premnet fiber backbone networks can form metropolitan-area networks incorporating two cities that are miles apart, or even link to a Sonet/SDH cloud to span an entire continent. Designers can consider linking fiber backbones to transport data across large distances or to remote locations previously unable to be included on a single backbone network. The multi-ring capability applies to many industries."

Expanding upon this point, Dillon says universities with isolated campus backbones in multiple locations can create a seamless network to deliver shared learning resources. Furthermore, the multi-ring capability allows the Premnet backbone`s real-time video capability to transmit video images simultaneously to classes at multiple locations without the need for special video equipment or expensive codecs. But it is necessary to install fiber links to all locations.

Applications in education

In the academic environment, Premnet is used to broadcast lectures, laboratory sessions or company messages to remote sites. In this application, the video/audio signal is broadcast from a central site to one or more locations. While video in this application is unidirectional only, audio is bidirectional.

Videoconferencing applications for a full-duplex Racal-Datacom video interface module can be either within a campus or in a wide area network. According to Joe Mocerino, Racal-Datacom product manager, his company has 256 videoconferencing modules in the field. "The Premnet multiplexer supports complex backbone networks with high ring and node counts," says Mocerino. He notes the multi-ring capability does away with most distance limitations by enabling multiple Premnet rings to be linked together to span longer distances.

From an engineering perspective, multiple Premnet rings can now be linked to a Sonet/SDH wide area network using a standard Sonet/SDH add/drop multiplexer. Premnet Sonet/SDH optical links have been tested and are fully compliant with applicable sections of Bellcore GR-253-Core, "Synchronous Optical Network: Sonet Transport System Common Generic Criteria; ITU-T G.707, "Synchronous Digital Hierarchy Bit Rates;" G.708, "Network Node Interface for Synchronous Digital Hierarchy," and G.709, "Synchronous Multiplexing Structure."

Network management

The company claims the Premnet multiplexer`s network management capability using simple network management protocol, or SNMP, makes a complex multi-ring backbone network easier to manage. The multiplexer`s master node manages the entire backbone network, allowing all network operations to be monitored from a single location. This single-point-of-control enables built-in network management for the entire backbone network.

Additional management console terminals may be connected to interconnect nodes (nodes that are members of two rings) that have a clear communication path to the master node, allowing full management of all nodes on the network.

Fault tolerance

Mocerino notes that reliability and redundancy are part of the Premnet fault-tolerant system architecture. A dual counter-rotating ring architecture prevents data transmission from being interrupted in the event of a fiber break or other network failure and makes the system suitable for mission-critical applications such as air traffic control. Multi-ring monitoring, alarm and fault-recovery features survey the backbone and provide early warning of potential network problems.

Premnet users include regional Bell operating companies and competitive access providers, which are using the system to offer local area network services to local and regional subscribers.

Commenting on these applications, Michael H. Smith, a communications analyst with Datapro Information Services Group, Delran, NJ, says, "The regional Bell operating companies and many competitive access providers have recognized the need to provide data networking services at native local area network speeds within a metropolitan area.

"With Ethernet LANs capable of operating at speeds at 10 Mbits/sec, token ring LANs operating at 4 or 16 Mbits/sec, and [fiber distributed data interface] LANs operating at speeds of 100 Mbits/sec, the [wide area network] link becomes a real network bottleneck. For example, a DS-1 circuit can only support transmission speeds of 1.544 Mbits/sec. However, with native LAN interconnection services, the [regional Bell operating companies] and [competitive access providers] can support transmission speeds well beyond those offered by traditional dedicated services," Smith says.

"By constructing Sonet/SDH rings in metropolitan areas and installing products such as Racal-Datacom`s Premnet backbone multiplexer, local service providers can leverage the high performance and enhanced reliability offered by fiber-optic transmission facilities. Clearly, the ability to interconnect multiple Sonet/SDH rings is an attractive option for the service providers. The [regional Bell operating companies] and [competitive access providers] can serve as a stable and potentially profitable distribution channel for Racal-Datacom," Smith observes. q

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