Merit's New Topology Brings Higher Capacity, Better Service

By Dave Katz
NSFNET Internet Engineering

Merit and its partners, MCI and IBM, announce a major redesign project to improve connectivity and move NSFNET toward the higher speeds required in the future. The new topology, scheduled for implementation starting second quarter, 1989, will increase the number of T-1 circuits in the backbone to provide multiple connections for all nodes and take advantage of MCI's Digital Reconfiguration Service (DRS) to improve network management capabilities. This redesign will significantly increase the overall capacity of the NSFNET backbone.

Initial NSFNET Configuration

This new configuration is a change from the initial NSFNET topology made operational in July 1988. This original physical topology is based on two interconnected rings linking seven nodes, along with backbone spurs that provide connectivity to six additional sites. The initial Merit plan provided network redundancy through the use of intelligent multiplexing equipment capable of imposing a logical topology over the physical circuits. The T-1 bandwidth on individual point-to-point circuits was split to accommodate multiple logical links to each site, including the spur sites which currently have single T-1 connections to the backbone.

The initial planning efforts for the new expansion were the direct result of a desire to improve connectivity for the spur nodes, providing redundant connectivity and insuring increased robustness for NSFNET as a whole. As our experience with NSFNET has grown, we have recognized the need for greater physical diversity as well as improved overall capacity to stay ahead of ever increasing demands for bandwidth and transmission speeds.

Starting last November, a planning team made up of staff from Merit, IBM, and MCI began work to develop an improved backbone design. The goal was to provide improved connectivity for spur sites within the current funding framework while providing increased capacity for the network as a whole.

The New Design

The planning team, in cooperation with the National Science Foundation, has developed a proposal which takes advantage of newly implemented technologies in both the packet- and circuit- switching arenas to make this expansion possible. This redesign will be implemented as part of a broader program for improved network functionality scheduled for implementation during 1989.

In the new design, additional T-1 circuits provide interconnectivity rather than splitting the bandwidth on individual T-1s. Each node will have multiple T-1 circuits that feed via the local telephone company into an MCI Digital Cross Connect (DXC) site. Although initially based on point-to point links between nodes, Digital Reconfiguration will make it possible to alter the paths of those links or designate alternate links between nodes at will to meet changing resource demands.

An analogy for this operation is a node with multiple circuits feeding into a "cloud" which is the MCI network. Dynamic circuit reconfiguration provides NSFNET with enhanced capabilities at the circuit level that can be implemented on demand yet remain relatively invisible to the operations at a nodal level. Merit's original proposal for NSFNET re-engineering included plans to use DRS, but this current plan includes increased functionality because of the topology reconfiguration.

Goals and Objectives for the New Design

A primary goal in the redesign is maximizing network robustness and minimizing bit traversal delay. Secondary objectives are limiting the maximum diameter of the network (the greatest distance between any two linked sites) and increasing overall bandwidth. The new plan exceeds Merit's objective for having redundant connectivity to each site, while also demonstrating our commitment to providing a dynamic network environment that takes advantage of new technologies to provide NSFNET users with ever-improving, reliable and high-speed networking services.

A PostScript version of the new topology is available by FTP from the NSFNET Information Services machine, ( With an anonymous login and "guest" as the password, the file TOPOLOGY.NEW-PS is found in the NSFNET directory.

A detailed schedule for implementation of these changes is nearing completion and will be finalized in cooperation with the NSFNET regional networks. As this work continues, future Link Letter articles will contain more details on the new network configuration. Questions should be sent to:


Taken from The Link Letter, Vol. 1 No. 13, 19 February 1989.