Alternate path FDDI standard LAN

A local area network uses standard ANSI FDDI LAN concentrator stations and a first set of mutually reverse-direction data buses or loops, in conjunction with an additional set of mutually reverse-direction data paths. The standard FDDI protocols when applied in the context of the particular connections of the concentrator station operate in the event of a break in the data path terminating at an input or output port, to reconfigure the station to interconnect the buses in a manner which routes data around even multiple breaks in the buses, to thereby maintain communication integrity.

BACKGROUND OF THE INVENTION 
This invention relates to local area communication networks, and more 
particularly to local area networks (LAN) according to ANSI Fiber 
Distributed Data Interface (FDDI) standards, which LAN are arranged for 
enhanced reliability in adverse environments. 
Modern vehicles such as aircraft, ships and the like include sophisticated 
computer systems for sensing the environment, evaluating the sensed 
information, and for controlling the vehicle and/or its payload in 
response to the evaluations. Such vehicles, and military equipment whether 
fixed or mobile, require highly reliable communications among the sensors, 
evaluation equipment, and the controlled devices. The reliability is often 
provided, at least in part, by redundancy. Thus, in addition to redundant 
computers for performing the evaluations, and redundant sensors and 
controlled devices, if possible, the interconnections should also be 
redundant. In warships, for example, there may be a relatively large 
number of sensors, including a plurality of radar, sonar, infrared and 
other sensors, all of which must be interconnected with command centers 
and with evaluation computers, and with guns, missiles and their 
controllers. 
In the past, such warships have provided redundant point-to-point 
communication or data transmission paths or links between each 
mission-critical pair of devices, with the redundant paths differently 
routed within the ship to avoid the possibility that single-point damage 
can disrupt the communication. With this arrangement, all equipments 
remain interconnected, and any one device or sensor only becomes isolated 
from the system in the event of breaks on both of its separately routed, 
mutually alternate paths. Thus, extensive damage to the ship is likely to 
break several data paths, but these data paths are not likely to include 
both data paths to a particular device, so all devices are likely to 
remain interconnected. With the advent of computerized control, each 
computer (including the redundant computer) is required to provide a 
sufficient number of input-output (IO) ports to allow connection by at 
least first (primary) and second (alternate) paths to each of the devices 
which it serves. Specialized military computers with large numbers of 
input-output ports are available for such interconnections. The rapid 
expansion of control requirements tends to quickly render obsolete any 
computer with a fixed number of IO ports, and specialized computers with 
yet larger numbers of IO ports then become necessary. 
With a view toward improved reliability and reduced cost, attention has 
been directed toward the use of a standardized local area network (LAN) in 
conjunction with a plurality of microprocessors, such as are in common 
commercial use, for performing the same functions as the special-purpose 
computers together with their point-to point communication paths. Such a 
LAN arrangement allows the distributed microprocessors to perform multiple 
functions, thereby providing another level of, and increasing the 
redundancy of the computing portion of the system, and also allows ready 
expansion of the system to include sensors or controlled devices as their 
need is discovered. The use of a standardized or commercial-type LAN 
further reduces cost, without, it is hoped, sacrificing usability. 
The American National Standards Institute (ANSI) has generated standards 
and protocols for token-passing fiber-optic LAN systems, which are known 
by the acronym FDDI, which stands for Fiber Distributed Data Interface. It 
would be desirable, if possible, to use the FDDI standards in relation to 
high-reliability systems such as those described above. At this time, 
FDDI-based systems are being introduced into shipboard applications, as 
described, for example, in an article entitled "All Aboard FDDI", by 
Adams, published at pp 13-17 of the December, 1991 issue of Military and 
Aerospace Electronics magazine. However, the FDDI standards provide only 
limited redundancy, and so cannot be used for mission-critical 
communications. 
In FIG. 1, a standard FDDI local area network includes first, second and 
third standard FDDI stations or connection nodes 1, 2 and 3. Each standard 
station includes first input and output ports 1i and 1o, respectively, and 
second input and output ports 2i and 2o, respectively. Input-output port 
1i, 2o is also known as an A port, and input-output port 2i, 1o is also 
known as a B port. As illustrated in FIG. 1, the first output ports lo of 
each of the three standard stations 1, 2 and 3 are connected by 
fiber-optic transmission paths to the first input port 1i of the next 
standard station. For example, a transmission path 11 connects from output 
port 1o of station 1 to input port 1i of station 2. Similarly, a 
transmission path 12 connects from output port 1o of station 2 to input 
port 1i of station 3, and a transmission path 13 connects from output port 
1o of station 3 to input port 1i of station 1, thereby forming a first 
loop, designated generally as 20, by which data can flow in the direction 
of arrow 24 among stations 1, 2 and 3. As also illustrated in FIG. 1, the 
second output ports 2o of each of the three standard stations 1, 2 and 3 
are connected by fiber-optic transmission paths to the second input port 
2i of the next standard station. For example, a transmission path 14 
connects from output port 2o of station 1 to input port 2i of station 3. 
Similarly, a transmission path 15 connects from output port 2o of station 
3 to input port 2i of station 2, and a transmission path 16 connects from 
output port 1o of station 2 to input port 2i of station 1, thereby forming 
a second loop designated generally as 22 by which data can flow in the 
direction arrow 26 among stations 1, 2 and 3. In this context, a 
transmission path is a path by which signal travels essentially unchanged 
from one location to another. A path may include cable, connectors, 
repeaters, switches, attenuators, and the like, the presence of which does 
not affect the signal. 
Each station includes a Media Access Control unit which is associated with 
one, but not both, input-output ports, so that one of the two loops is 
designated as the main or primary loop. 
The system as illustrated in FIG. 1 includes only three stations for 
simplicity of explanation, but could include a large number of such 
stations, connected in a similar manner. As illustrated, each station is 
connected to the next station by means of two fiber-optic transmission 
paths, as for example station 1 is connected to station 2 by transmission 
paths 11 and 16, in which data flows in mutually opposite directions. An 
envelope designated 28 represents the practical fact that the two fiber 
optic transmission paths 11 and 16 are often combined into a single 
multifiber cable. Similarly, envelopes 30 and 32 represent the combining 
of fibers 12 and 15 into a cable 30, and the combining of fibers 13 and 14 
into cable 32. The FDDI network as so far described in conjunction with 
FIG. 1 is used by connecting sensors, computers (and their associated 
operator inputs) or controlled devices to externally accessible data paths 
associated with each station, such as path 58 associated with station 1, 
and paths 59 and 60 associated with stations 2 and 3, respectively. In 
operation, a token is produced at the primary output port of a station 
when it has transmitted all its current information, and the next station 
on the primary loop either seizes the token to make time available for its 
own transmission, or retransmits the token on the primary loop or ring to 
the next station when it has no data to transmit. Thus, the ability to 
transmit unimpeded on the common bus steps from station to station with 
the token. Reception is not dependent on the token, and all stations may, 
essentially simultaneously, receive data which is relevant to the 
stations. 
On a vehicle such as a ship, it may be advantageous to route cable 
continuously from one end of the vehicle to the other end, connecting to 
various equipments along the way. This corresponds to that portion of FIG. 
1 including stations 1, 2 and 3, and data paths included in cables 28 and 
30, but without the data paths corresponding to those of cable 32 of FIG. 
1. As described below, the two end stations "wrap" so that communications 
occur back and forth among the stations. This linear arrangement is termed 
a "root" topology The root topology has the advantage of reduced cabling 
requirement, but introduces the distinct disadvantage that a single break 
anywhere in the system results in undesirable isolation of portions of the 
system. This disadvantage could be remedied by a complete additional set 
of station nodes and interconnection cabling. However, this more expensive 
and complex than simply running the additional cable for the return link 
corresponding to cable 32 of FIG. 1. 
Concentrators are allowed within the FDDI standard for expanded system 
connectivity. As described in an article entitled "Using Redundancy In 
FDDI Networks", by Ocheltree, published at pages 261-267 of the 
proceedings of the 15th Conference on Local Computer Networks, 
Minneapolis, Minn. Sep. 30-Oct. 3, 1990, and in an article entitled "The 
DECconcentrator 500 Product", by Tiffany et al., published at pp 64-75 of 
the Spring, 1991 issue of Digital Technical Journal, concentrators include 
a plurality of master ports in addition to the A and B ports. FIG. 2 
illustrates a concentrator station in accordance with FDDI standards. In 
FIG. 2, an FDDI concentrator station 4 includes an A port with 1i and 1o 
ports, a B port including 2i and 2o ports, and a path 61 for connection to 
a local sensor or other device, just as in the standard FDDI stations of 
FIG. 1. In addition, concentrator station 4 of FIG. 2 includes a plurality 
of "master" (M) ports, which are available for connection of additional 
single stations 62a, 62b, 62c, . . . 62n. Such an arrangement might have 
utility, for example, in a LAN in which a multi-story building is to be 
interconnected. For this purpose, each floor might have a concentrator 
station, with the A and B ports connected with other concentrators on 
other floors in a loop as described in conjunction with FIG. 1, and at 
each floor, the M ports of the concentrator for that floor are connected 
by individual two-fiber cables to each separate single station For 
example, if concentrator 4 of FIG. 2 were on the fourth floor of such a 
building, port M1 would be connected to fourth-floor equipment station 62a 
by way of a cable 63a, port M2 would be connected to fourth-floor 
equipment station 62b by way of a cable 63b, etc. The A and B ports of the 
concentrator would, of course, be connected to other stations on the third 
and fifth floors. In the terminology of FDDI, the additional single 
stations 62 of FIG. 4 are part of a "tree", which in the case of single 
stations as in FIG. 4 cannot be expanded to further levels. If, in FIG. 2, 
each single station 62 were to be replaced by a concentrator station which 
was connected to its higher-level concentrator station (station 4 in FIG. 
1) at one of its A or B ports, additional stations at a third level could 
be connected to the various M ports of the second-level station, to 
thereby create a tree with many levels. In such an arrangement, the 
lower-level stations all couple back to loop through concentrator 4, and 
are termed "singly homed". On the other hand, if the other of the A and B 
ports of the second-level concentrator is connected to another 
concentrator station which is also in the loop, each low-level station can 
couple to the loop through two (or possibly more) paths, and they are said 
to be "dual homed", all as described, for example, in the above mentioned 
Ocheltree article. In operation of the concentrator as illustrated in FIG. 
2 when connected in a ring, as described, the token is received at the A1i 
port as in FIG. 1, and is routed in succession to each of the active M 
ports and to the stations connected thereto, i.e. from port B to port M1, 
then down through all the stations in the M1 path, then to M2, and through 
all the stations in the M2 string,... When the token has progressed 
through all the stations associated with all the M ports of the 
concentrator, it is made available at the B1o output port on the ring. 
Thus, the use of concentrators in a loop gives rise to delays which do not 
occur in standard stations, and their use is contraindicated unless 
necessary It should be noted that the paths between M ports of a 
concentrator station and the next following station are under continuing 
self-test during those intervals in which it is not being used for data. 
FIG. 3a illustrates one possible configuration of bus connections within a 
standard or concentrator station, taken as standard station 1 for 
definiteness. In FIG. 3a, first and second directional couplers 40 and 42, 
respectively, have their main through lines coupled to each other and to 
input port 1i and output port 1o. The branch lines 44 and 46 of 
directional couplers 40 and 42, respectively, are coupled to active 
circuits illustrated as a block 56, which may include light detectors and 
modulators. Similarly, third and fourth directional couplers 50 and 48 
have their through paths coupled to each other and to input and output 
ports 2i and 2o, respectively, and their coupled branch lines coupled to 
active circuit block 56. Suitable directional couplers include the 
well-known star couplers. Those skilled in the art know that this 
arrangement couples signals directly from input port 1i to output port 1o, 
and from input port 2i directly to output port 2o, and also couples the 
active circuit portion 56 to receive signals from the input ports 1i and 
2i of standard station 1, and to couple signals therefrom to output ports 
1o and 2o of station 1, but has the disadvantage of splitting the 
transmission power and thereby reducing the signal-to-noise ratio. In many 
applications, this will be of no consequence, but when large numbers of 
stations are to be interconnected, the cumulative loss of the couplers may 
exceed the regenerative power of the various stations. 
Lower through loss at each station is permitted by the use of a switched 
"Trunk Coupling Unit" (TCU), illustrated in FIG. 3b. In FIG. 3b, station 1 
receives signal through input port 1i, which is applied to a moving-mirror 
fiber-optic switch 98. Switch 98 allows signal to be routed directly from 
input port 1i to output port 1o when that station or node is shut down or 
deprived of power, which is a fail-safe bypass mode of operation. In 
normal operation, when the station is powered up, switch 98 couples signal 
from input port 1i to the active portions 56 of the node, and from the 
active portion to output port 1o. The operation of the TCU associated with 
ports 2i and 2o is similar. 
The FDDI standard provides for error recognition, and for reconfiguring the 
station in response to errors for improving reliability. FIG. 4a is 
similar to FIG. 1, but includes an assumed break in a transmission line. 
While standard stations are described, the following discussion is equally 
applicable to concentrator stations. As mentioned, the transmission lines 
are actually coupled into cables including at least two paths, as for 
example cable 28 includes the two transmission paths 11 and 16 extending 
between stations 1 and 2. As a result, a break in one cable almost 
invariably breaks both optical fibers therein. The FDDI standard takes 
cognizance of this likelihood, and arranges its error detection circuits 
within each station to recognize the existence of faults, which include 
(a) a fault in the transmission path terminating at a 1i input port, and 
(b) a fault in the transmission path which starts at a 1o output port. The 
(a) type of fault may be termed an "upstream" fault, and the (b) fault may 
be termed "downstream", relative to the "main" data path or loop. In FIG. 
4, a large "X" is placed over multifiber cable 28, thereby indicating that 
it is broken, possibly as a result of environmental or hostile action. As 
to station 1, the break is a downstream "b" fault, and as to station 2, it 
is an "a" type upstream fault. The FDDI standard requires the station 
subject to an upstream fault (in this example, station 2) to disconnect 
its 2i input from its 2o output and reconnect or "wrap" it to its 1o 
output, as illustrated by dashed path 70 in station 2. The disconnection 
of input port 2i from output port 2o is of no consequence, because any 
data applied to output port 2o cannot traverse data path 16 of cable 28 
anyway. The FDDI standard also prescribes that a station subject to a 
downstream fault (station 1 in the example) must disconnect its 1i input 
port from the 1o output port, and reconnect or wrap it to the 2o output 
port, as illustrated by dash line 72 in station 1. As in the case of 
station 2, disconnecting input port 1i from output port 1o makes no 
difference, as data applied to data path 11 of cable 28 cannot traverse 
the break. The FDDI wrap protocol results in formation of a third loop to 
replace the broken first loop 20 and second loop 22. The third loop 
extends from output port 1o of station 2, through path 12, ports 1i and 1o 
of station 3 to path 13, then through input port 1i, path 72, and output 
port 2o of station 1, through path 14 to input port 2i of station 3, and 
out of output port 2o of station 3, through data path 15 to input port 2i 
of station 2, and finally through path 70 back to output port 1o of 
station 2. Thus, a single break has no significant effect on the 
communications among stations 1, 2 and 3 (although the delay may increase 
somewhat). 
FIG. 4b is similar to FIG. 4a, differing only in that a second break in the 
cabling is shown, illustrated by a large "X" over cable 30. This break 
obviously isolates station 2 from stations 1 and 3. Station 1 still 
experiences a downstream fault, and responds by producing internal 
reconfiguration path 72. Station 3 experiences an upstream fault, and 
responds by reconfiguring itself by wrapping its 2i input port to its 1o 
output port, just as station 2 did in FIG. 4a. The wrap path in station 3 
is designated 74. The wrappings provided by reconfiguration paths 72 and 
74 of stations 1 and 3, respectively, form yet another loop which includes 
stations 1 and 3, and paths 13 and 14. In addition to the isolation of 
station 2 from stations 1 and 3 by the breaks of cables 28 and 30, station 
2 is further isolated by the operation of the wrap specified by the FDDI 
protocol, which requires disconnection of paths 11 and 15 from station 2. 
As mentioned, the illustrated system includes only three stations, for 
simplicity of explanation. It should be understood that station 2 of FIG. 
4b may actually represent a plurality of stations, and that the wrap 
specification will result in those additional stations forming their own 
interconnected loop, which, however, is totally isolated (by the multiple 
breaks) from the loop including stations 1 and 3. For mission-critical 
data communications, isolation of whole sections by damage to two 
unrelated data paths may not be acceptable. It should be noted that the 
looping data paths in an FDDI system are often illustrated side-by-side, 
as is shown by the alternate placement of data path 14 in the form of a 
dot-dash line 14b adjacent to data path 13 in FIG. 4b. This configuration 
results in the use of the terminology of formation of a "U" or "horseshoe" 
path instead of an alternative "loop" path when a wrap occurs. Thus, 
reconfiguration forms a horseshoe or a loop, depending upon the form or 
topology of the circuit which one has in mind, but the horseshoe and the 
loop are equivalent terms in this context. 
Thus, while it would be desirable to use the FDDI system for all shipboard 
communications, including mission-critical data, the possibility of total 
isolation of portions of the communications system due to multiple breaks 
in the transmission cable makes it unusable for mission-critical 
applications. It might be possible to use an FDDI system for auxiliary 
communications, while using point-to-point communications for 
mission-critical applications, but this would not help the key problems of 
expandability of an existing system, cost, and the need for 
special-purpose computers. If the FDDI system standards are modified to 
achieve the reliability goal, the cost benefits of commonality with 
commercial systems are lost. 
A SAFENET study group established by the Navy has conducted a review of LAN 
for mission-critical applications, and is in the process of generating 
SAFENET I standards based on IEEE 802.5 "free-token" protocol, which 
standard specifies that the two fiber transmission lines be separately 
routed, and adopts "ring hop", which amounts to abandoning a defective 
loop or ring and using the operational loop. The free-token protocol is 
not considered advantageous, because the hold time at any station is not 
controlled, and thus priority messages may be delayed excessively. The 
SAFENET group has also at least partially generated SAFENET II LAN 
standards. The SAFENET II standards define a timed token ring pursuant to 
FDDI standards, but in which the dual counter-rotating rings are 
separately routed. It supports a single failure, but degrades in the 
presence of additional failures, and therefore is no better than FDDI. 
An improved communication system is desired. 
SUMMARY OF THE INVENTION 
A LAN according to the invention uses FDDI standards, but interconnects the 
FDDI standard concentrator stations elements in a manner which, together 
with intraconnection protocols associated with each concentrator station 
which are in conformance with the FDDI standards, provide redundancy and 
reliability tending to equal that of the point-to-point system. This is 
accomplished, in short, by using, primary (or first) and alternate (or 
second) loops or data buses, each including first and second transmission 
paths operating in mutually opposite directions, with concentrator 
stations instead of FDDI standard stations. Each concentrator station 
includes arrangements which sense a break in that portion of the primary 
loop terminating at the station, and which, in that station, couples input 
and output ports originally coupled to the broken cable to an alternate 
cable segment.

DESCRIPTION OF THE INVENTION 
In FIG. 5, a communication system 500 includes a plurality of concentrator 
stations 501, 502, 503, and 504, which are coupled with a loop. Unlike the 
arrangements of FIGS. 1 and 2, the A and B ports of adjacent stations are 
not directly connected together, but instead each A or B port is connected 
to an M port of the adjacent station. More particularly, the M1 output (o) 
port of station 504 is connected by a path 513 to the B input (i) port of 
station 503. The M1 o port of station 503 is connected by path 512 to the 
B i port of station 502, and the M1 o port of station 502 is connected by 
path 511 to the B i port of station 501. As mentioned, the use of 
concentrator stations may result in a net increase in token transit time 
by comparison with standard stations, but the increased transit time is 
considered minor by comparison with the advantages of the system, as 
described below. 
As suggested by the additional letters "M" associated with station 501, 
additional M ports may be available in each concentrator station, and 
those additional M ports may be used to access additional tree-connected 
stations, if desired. 
The various data paths of FIG. 5 are grouped into cables, as suggested by 
the encompassing circles. Each cable is referred to by the same number as 
that of the lower- numbered one of the associated data path which it 
contains. 
In operation of the system of FIG. 5, a primary loop is designated by a 
media access control unit in the stations, and includes FDDI standard B 
and M ports (the M ports are designated with numerals for convenience). As 
mentioned above, when a concentrator station includes active M ports, the 
token passes from the B port to the first active M port, which in this 
case is the M1 port. In the context of the connections illustrated in FIG. 
5, the progression from a B port to an M1 port results in a further 
connection to a B port, which corresponds to progress around the primary 
loop. End stations 501 and 504 wrap according to the FDDI standard, with 
station 501 wrapping the B i port to the B o port. Also, station 504 wraps 
the M1 i port to the M1 o port. This wrapping forms the closed primary 
loop. The closed loop so formed in the root topology by the wrapping 
includes paths 511, 512, 513, 514, 515, and 516. The other paths, in 
accordance with FDDI concentrator specifications, are in continuous 
self-testing of the individual inter-station data paths. For example, 
while the above mentioned primary path is available, the stations test 
data paths 521 and 526, which may be conceived of as a small loop, and 
also test data path pairs 522, 515; and 523, 524. Thus, alternate cables 
521, 522 and 523, while they are connected in the same manner as the 
cables 511, 512, and 513, may be considered not to form a loop, because 
the individual segments are undergoing individual testing when not being 
used for carrying data. 
In accordance with FDDI standards, each station switches from a particular 
failed segment of the primary loop to the associated alternate segment. 
For example, if an "upstream" break occurs in cable 511 of FIG. 5, as 
suggested by the letter x, or in either of the data paths 511 or 516 
contained therein, which are part of the primary loop, station 501 routes 
data, which it would otherwise have been applied to its B o port, to its A 
o port instead, so that the data can proceed over data path 521 to the M2 
i port of station 502. Station 502, according to FDDI standards, would 
ordinarily route data entering its B port to the first active M port, 
which is M1. In the presence of a "downstream" fault relative to port M1, 
station 502, pursuant to the FDDI standard, routes data incoming to its B 
i port to its M2 o port instead of to the M1 port, so that it may be 
transmitted over data path 526 to station 501. Station 502, also in 
accordance with FDDI standards, routes data incoming to its M2 i port to 
its B o port. Thus, a break on cable 511, downstream from station 502, 
results in that station rerouting its B inputs and outputs to its M2 
inputs and outputs, thereby maintaining communication with station 501 by 
way of unbroken cable 521. In effect, the FDDI reconfiguration, when used 
in a system structured as described, switches data from a segment of the 
primary loop to an alternate segment. 
More generally, if the primary one of the two cables interconnecting one 
station with its adjacent station breaks, the system as described switches 
the data flow to the corresponding alternate segment. For example, if an 
additional break occurs in the system of FIG. 5, as for example at the "x" 
in cable 513, the system switches over to alternate cable 523. It should 
be noted that a break in an alternate path while the primary path is 
operational will not cause any alteration in system operation, it merely 
results in loss of an alternative for the particular primary path for 
which it is the alternate. 
As so far described, the loop arrangement 500 of FIG. 5, according to the 
invention, includes a station 501 which includes M1 and M2 ports, which 
can be connected to the A and B ports of additional concentrator stations 
to extend the loop. In addition, each concentrator station can include 
additional M port pairs, which might be designated M3 and M4, for example, 
another kind of connection can be made. FIG. 6 illustrates a portion of 
the arrangement of FIG. 5, including stations 502, 503, and 504, and also 
illustrating additional M ports M3 and M4. As illustrated in FIG. 6, the 
M3 port of station 503 is connected to the B port of an additional 
concentrator station 506, and the M4 port of station 503 is connected to 
the A port of station 506. These connections start another loop portion 
which is effectively connected in series with the loop as described in 
conjunction with FIG. 5. A token originating at station 504 of FIG. 6 
enters the B port of station 503, and enters the active or primary one of 
the M3, M4 port pair of station 506, which in normal operation will be the 
B port. The token will proceed to the M1 output port of station 506, and 
through any stations downstream therefrom, finally returning to station 
503, and becoming available for transmission to station 502 and stations, 
such as 501, which are downstream therefrom. 
Referring once again to FIG. 5, it will be noted that the right-most 
concentrator 504 includes unused A and B ports. In that respect, 
communication system 500 of FIG. 5 is similar to an unused standard FDDI 
station, such as any one of the stations of FIG. 1 by itself. 
Consequently, a plurality of the communications systems 500 of FIG. 5 can 
be interconnected in the manner illustrated in FIG. 1. More particularly, 
a plurality of systems 500 can be arrayed in a "pitchfork" configuration, 
such as that illustrated in FIG. 7. In FIG. 7, elements corresponding to 
those of FIG. 5 are designated by like reference numerals In FIG. 7, the 
bottom-most system 500 corresponds in principle to station 3 of FIG. 1, 
and it therefore carries the additional parenthetical designation (3). The 
B port of system (3) of FIG. 7 is connected by a two-conductor or two-path 
cable designated 832 (corresponding to cable 32 of FIG. 1) to the A port 
of the right-most station 504 of system 500 (1), which corresponds in 
principle to station 1 of FIG. 1. The B port of station 504 of system 500 
(1) is connected over a cable 728, corresponding to cable 28 of FIG. 1, to 
the A port of station 504 of system 500 (2) of FIG. 7. Lastly, the B port 
of station 504 of system 500 (2) of FIG. 7 is connected by a cable 730, 
corresponding to cable 30 of FIG. 1, to the A port of station 504 of 
system 500 (3) of FIG. 7. Thus, the concentrator structure according to 
the invention can be connected into supraloops pursuant to the FDDI 
standard. Unfortunately, the supraloops formed in this fashion do not 
enjoy the same survivability in the presence of multiple breaks as that of 
the systems according to the invention. 
FIG. 8 is similar to FIG. 5, and corresponding elements are similarly 
numbered. FIG. 8 differs from FIG. 5 in that additional cables 818, 828 
(defining the cable by the lowest transmission path or fiber designator) 
are connected from the M1 port of station 501 to the B port of station 
504, and from the M2 port of station 501 to the A port of station 504. 
These cables, together with the other structure of FIG. 8, forms loops. 
For example, starting at path 820, the data or token enters the B i port 
of station 504, and proceeds to its M1 o port, and through path 513 to the 
B i port of station 503. The data proceeds through station 503 to its M1 o 
port, through path 512 to the B i port of station 502 and thence to its M1 
o port. From the M1 o port of station 502, the data proceeds through data 
path 511 and the B i port of station 501, and through the M1 o port of 
station 501 and back onto path 820. It is believed that this configuration 
may fail to recognize the fact that the M1 port of station 501 is 
connected to the B port of station 504, and that under normal conditions 
of unbroken paths, that the station will not use the additional path 
provided by cable 818. When a break occurs in one of cables 511, 512 or 
513, however, it is believed that the system may then recognize the 
presence of cable 818, and retain a fully operational primary path, 
without recourse to the alternate paths. This would advantageously provide 
another level of redundancy or reconfiguration. Similar comments apply to 
the presence of additional cable 828 of FIG. 8 in the alternate paths. 
Other embodiments of the invention will be apparent to those skilled in the 
art. For example, while fiber-optic digital data paths are contemplated, 
the invention may be used with any form of transmission path, such as 
coaxial cable or telephone wire. Also, while digital data is described, 
that data may originate from a voice, video or other analog source, and 
may even be transmitted in analog form. As mentioned, the LAN according to 
the invention may be used in vehicles for adverse environments, as for 
example commercial and military aircraft and spacecraft, ships and 
submarines, land vehicles such as trains and buses, military land vehicles 
such as tanks and APCs, and in fixed stations where operability in the 
presence of damage is important, as in power stations, nuclear plants, 
radar and other defense systems, airport and air traffic control systems, 
and the like, and also other systems where outages are important, such as 
medical and financial systems.