Note: The following is the text version of a paper originally prepared with the word processor RUNOFF. Thomas Van Vleck kindly rendered the RUNOFF format into HTML. The figures are currently not available in any of the on-line versions. The full citation of the paper is

A Star-Shaped Ring Network with High Maintainability

J. H. Saltzer
K. T. Pogran

Key Words: Networks; Local Networks; Ring Networks; Star Networks; Data Communications; Network Maintainance; Network Serviceability.


Ring networks exhibit a number of desirable properties: they are simple in concept and in implementation; one-way point-to-point signal transmission minimizes analog circuitry and design problems; the cost of a small net is small; and transmission speed is not limited by propagation time. However, there are several potential reliability problems: all repeaters must be powered and operating reliably at all times; cutting of any transmission line in the ring will disrupt the entire network; and trouble-shooting may require visiting each node with test equipment.

The advantages of ring networks are sufficiently attractive to merit careful attack on the reliability and maintainability problems. This paper describes a physical organization of a ring network that we term a "star-shaped ring."(1) This organization addresses the reliability issues mentioned above by looping all internode links back through a central location so that broken lines or repeaters can be bypassed. This approach allows automatic recovery if the ring is accidentally broken. In addition, it simplifies trouble-shooting and, because the lengths of transmission links can be normalized, permits the use of still simpler transmission circuitry. Finally, the "star-shaped ring" appears to be a satisfactory approach to installing a network that can grow to a large number of nodes in a typical office building.


The M.I.T. Laboratory for Computer Science is engaged in the design and implementation of a local area network with the following principal goals, desiderata, and constraints:

This set of goals led us to conclude that the best technology choice has little to do with the usual analyses of performance, collision rate, or bit error rates, but rather with more mundane issues such as which technology is easiest to install, reconfigure, and maintain in a typical office building. We were first attracted to the technology of a repeater ring because it appeared to have one advantage of each of the alternative broadcast (Ethernet) and star configurations, without the corresponding disadvantage of each:

The repeater ring network uses point-to-point transmission and also has a hardware complement--a repeater at each node--that is a constant percentage of the system cost as the network grows. Thus it captures both advantages. In turn, a ring of repeaters appears to introduce a reliability/availability disadvantage since every component must work properly all the time. Our analysis of these considerations suggested that the advantages of the ring configuration were interesting enough to warrant devising a strategy that directly attacks the reliability and maintainability problems. The remainder of this paper describes what promises to be a successful such strategy, based on the observation that the cited advantages of a ring do not depend on the actual routing of the wire cables that link the repeaters. An earlier paper describes in detail the design of a small-delay ring repeater and the hardware and software protocols used to achieve reliable local packet transport, and those details are not repeated here.(2)

The basic ring

Proposers of ring networks usually draw pictures as in figure one, with the implication that the cables interconnecting individual repeaters follow any convenient, reasonably direct route from one repeater to the next. Installing a ring network with that approach could be expected to expose the following problems:

Bypass relays

The ring network repeater can be designed to provide a partial solution to the problem of repeater failure, since that is almost certainly the most significant of the four problems. The repeater can provide an "I-am-healthy" signal that operates a mechanical relay that would, if not energized, bypass that repeater, as in figure two. Thus turning off the primary power at a node would kill the "I-am-healthy" signal, the relay would de-energize, and its contacts would cut the repeater out of the ring. This approach replaces the repeater failure problem with four new problems, that are (one hopes) easier to cope with: The software recovery protocols of the ring network are assumed to be designed to take occasional lost packets, messages, or tokens in stride, so the bypass disruption problem is not an important concern, so long as it happens only occasionally. The relay contact failure problem may be addressed by connecting normally closed relay contacts in parallel as shown in figure two, thus requiring that two relay contacts must fail simultaneously before a de-energized relay can disrupt the ring. Reed relays that can reliably transmit high data rate digital signals seem to be readily available.

The star-shaped ring: step one

The perambulation and installation headache problems can be attacked simultaneously simply by rearranging the inter-repeater cables so that they always loop back through a single room, called the wire center, as in figure three.(3) With this star-shaped arrangement, it is not necessary for a trouble-shooter to have keys to every office containing a repeater or to carry test equipment from point to point. With access to the signal on every cable as it passes through the wire center, one can launch a message into the ring, observe the signals on successive cables to see how far it gets, and then reconfigure the ring to disconnect temporarily the repeater or the cable that seems not to be working. Then access is needed only to the area containing the troubled repeater or cable. Installation of new repeaters is also regularized. Two cables are installed from the location of a new repeater to the wire center. Then the new repeater is spliced into the ring entirely by rearranging wires at the wire center. At no point is one tracing old cables through walls or ceilings or abandoning them there when making new installations. Unanticipated installations can be handled without creating a hodge podge of wires crisscrossing through the walls and ceilings; physical planning of a network is thus simplified. The order of repeaters on the ring is determined at the wire center and can be rearranged if for some reason reordering seems necessary. (E.g., when trouble-shooting a repeater-repeater transmission failure, quick rearrangement might be a useful technique to isolate the problem to the transmitter or receiver side of the link).

The star-shaped ring: step two

A further refinement of the wire center concept considerably reduces the wire vulnerability problem, simplifies maintenance and installation further, and normalizes transmission line lengths. This refinement involves simply moving the bypass relays from the repeaters to the wire center. The primary impact of centralizing the relays is that it provides for automatic bypass of the vulnerable data transmission lines as well as of the repeaters. To control the relay, an extra pair of wires runs from the repeater to the wire center. In practice, installation of a repeater would be accomplished by pulling a single cable containing two data transmission lines and the relay control pair from the area of the repeater to the wire center. A single plug connects all of these lines to the repeater. Almost any accident, ranging from chopping the cable to kicking out the power supply plug of the repeater has the effect of bypassing both the repeater and also the data transmission lines to and from that repeater. An important additional effect of bypassing the data transmission lines that lead to bypassed repeaters is that the path from one working repeater to the next working repeater always consists of the same cable run to the wire room, some number of bypass relays, and exactly one cable run to a receiver. This path varies mostly in the length of the final cable run; thus the range of signal levels to which the transmission system must automatically adapt is much smaller. Further, at the wire room an appropriate level-setting attenuator can be placed in series with short cables. Then a transmitter will always "see" essentially the same cable length no matter what the configuration of the ring. Thus the line length variation problem can be eliminated by this arrangement, allowing more flexibility in the design choices for the transmission system.

Physical realization of the star-shaped ring

Serviceability of the ring can be further enhanced by a suitable physical realization of the interconnections at the wire center. The design of figure four illustrates. A printed circuit board is constructed with, say, eight bypass relays in a row, and eight connectors into which cables to repeaters can be plugged. Each relay is connected to the next when its coil is de-energized. When energized, the relay cuts into the ring a pair of transmission paths that lead through a socket that can contain an attenuating network (to normalize cable lengths) to the repeater connector at the edge of the board. Current for the relay coil comes from the repeater. A light-emitting-diode is connected across the relay coil for visual observation of the "I-am-healthy" signal. The sequence of normally closed relay contacts leads to connectors at the top and bottom edges of the board. A "ring continuity cable" runs from the top connector around to the bottom one, completing the ring. In a sense, this ring of normally closed relay contacts on a single board in a controlled environment is the data communication ring, from the point of view of identifying what must be working to allow the ring to operate. If any two repeaters can energize their bypass relays, they can communicate, even though all other potential ring participants may have failed, powered down, or tripped over and disconnected their cables. Installation of a new ring participant is accomplished by installing a cable from the repeater to the wire center, attaching a connector, and plugging it into an unused bypass relay. No disruption of ring operation occurs. If all of the bypass relays have repeaters attached to them, another printed circuit board containing eight more relays can be installed next to this one, and then in a matter of seconds cabled into the ring by interchanging top ring continuity cable connectors. (This kind of ring expansion may be scheduled at times when a few-second disruption is tolerable. Then, addition of repeaters can be accomplished at any time.) Note that trouble-shooting with this physical configuration is especially straightforward. If the ring stops working, it is almost certainly because some repeater's "I-am-healthy" line is incorrectly energized. One starts by unplugging all the cable connectors to all the repeaters, checking for ring continuity, and then plugging in the cables to the repeaters one at a time to see which one (or ones) seem to disrupt continuity. Any cable whose reattachment causes trouble can be left unattached for the moment until it can be checked out more carefully. In this way the ring can be brought back into operation for the correctly behaving participants quite rapidly. Further, by rearranging the ring continuity cables that interconnect one printed circuit board to the next it is possible quickly to isolate problems to a group of eight relay/cable combinations. Finally, if a relay or printed circuit board component fails, that board can quickly be replaced or, at worst, bypassed.

Wire center interconnection

One might envision equipping a wire center with up to eight boards, each with eight relays, producing a ring of sixty-four nodes, probably enough to handle a typical building floor or wing. Another floor or wing would have its own wire center. The simplest way to interconnect the wire centers, if they are not too far apart, is to interconnect the ring continuity cables of the two wire centers. However, if the wire centers are very far apart, the center-to-center cables will add to the data transmission path between some repeaters but not between others, and the cable length variability problem will reappear. A better approach would be to build a special wire-center-to-wire-center repeater that would plug into both wire centers in the same way as a node repeater. If traffic is light enough to allow 128 nodes to be arranged in a single ring, this repeater would be a simple non-addressable full duplex repeater. If traffic grows to the point that two 64-node rings would be a better arrangement, the repeater could be replaced by a filtering bridge that forwards to the other ring only messages that are addressed to nodes not on this ring segment. (This same basic filtering bridge design could also be used to divide a 64-node ring at a single wire center into two or more smaller rings, if necessary.)

Automatic ring recovery

A further possibility for automating maintenance is to connect a microprocessor controlled repeater to one of the repeater connectors, and program the microprocessor to occasionally (say, once per second) launch a test packet around the ring. The microprocessor would also control, perhaps through another set of relay coils, the continuity between each repeater's "I-am-healthy" line and its bypass relay. If a test packet fails to make it around the ring, the microprocessor would force de-energization of all of the bypass relays (except its own), check for ring continuity, and then reconnect the "I-am-healthy" lines one at a time, testing ring continuity after each reconnection. When it finds an "I-am-healthy" line that disrupts ring continuity it might broadcast a trouble report around the ring as a way to call for a repairman. Since no other peripheral devices are needed, and the program is fixed, the extra cost of this automatic test system could be very small.

It is not clear how useful this notion of a microprocessor controlled automatic ring maintainer may be in a network consisting only of a single, small wire center. It may be overkill to automate the function because the ring may be very reliable anyway, and there is a danger that the microprocessor will fail in such a way as to disrupt the network, or that its automatic operation will be so effective that it reduces the incentive to repair misbehaving nodes. On the other hand in a campus-wide or industrial site-wide network with 100 interconnected wire centers, the automatic maintenance feature may be very helpful in getting service attention quickly and in rapidly restoring operation.

Conclusion and progress report

The star-shaped ring strategy has the decentralized control advantage of a ring network, and at the same time the centralized maintenance advantage of a star network. It thus captures some of the better properties of each while avoiding the key disadvantages of both. At the M.I.T. Laboratory for Computer Science we have been since January, 1979, operating a ring network that runs at 1 Mb/sec. and that now has five nodes consisting of Digital Equipment Corporation PDP-11 and VAX-11/780 computers. This ring uses the traditional shortest convenient path from one node to the next for wire runs. In anticipation of delivery of as many as 100 desktop computers during 1980, the ring network hardware interface, which includes the repeater, is being redesigned, simplified, and increased in speed. The star-shaped ring strategy is planned for this expanded ring network.