Low cost fiber optic network node

The node includes plural optical receivers and transmitters. Each receiver and each transmitter of the node is connected to a user terminal by fiber optic cables. The output of each of the node receivers is electrically combined. Splitting logic is also included for splitting the electrical output of the receivers to serve as an input to the plural optical transmitters. The node may be used as an expansion mode or as a head-end unit. The optical receivers and transmitters have a low cost so that the overall node is approximately one fifth the cost of comparable nodes utilizing passive optical stars.

BACKGROUND OF THE INVENTION 
This invention relates to optical local area networks, and more 
particularly to a low cost active node design. 
Over the last several years, a number of local area network architectures 
have evolved to support data communications for a large user population. 
With the recent emergence of the personal computer, network control cards 
have been developed to permit these computers to be used as intelligent 
work stations for local area network applications. Most of these local 
area network cards employ coaxial cable as the transmission medium. This 
transmission medium has been used in both the baseband and broadband 
modes. With the rapid emergence of fiber optic technology, optical fiber 
has been examined as an alternative to coaxial cable in a number of local 
area network designs. Recently, several fiber optic components have been 
developed that can dramatically reduce the cost of integrating this 
technology into local area network designs. 
In U.S. Ser. No. 777,934, filed Sept. 19, 1985, there is disclosed and 
claimed an active star network node design employing passive optical 
stars. The teachings of U.S. Ser. No. 777,934 are incorporated herein by 
reference. Although active star nodes employing optical stars can 
accommodate high data rate protocols such as Ethernet, these nodes are 
quite expensive. In addition, the nodes employing passive optical stars 
are heavy and relatively bulky. Furthermore, the optical star elements 
suffer splitter losses which degrade system performance. 
It is therefore an object of the present invention to provide an optical 
node which is a fraction of the weight and size of known nodes and having 
a component cost about one-fifth the known design. 
Yet another object of the invention is a low cost node that eliminates the 
splitter losses associated with nodes employing optical star elements. 
SUMMARY OF THE INVENTION 
These and other objects of the present invention are achieved by an optical 
node including a plurality of optical receivers, each receiver connected 
by fiber optic cables to one of a plurality of user terminals. The 
electrical outputs of each of the optical receivers are electrically 
combined. The node also includes a plurality of optical transmitters each 
of which is connected by fiber optic cables to one of the user terminals. 
Apparatus is provided for splitting the electrical output of the receivers 
for driving each of the transmitters. The node may be used both as a 
head-end or as a repeater.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Active optical nodes can be configured to serve both as head-end nodes and 
repeater nodes. When configured as a head-end, the node receives 
information from user terminals and rebroadcasts this information to all 
of the user terminals on the network. When the node is utilized in the 
repeater or concentrator mode, the information is transmitted to a higher 
order node. In late 1985, fiber optic components suitable for integration 
into a local area network implementation were introduced. See, D. W. Tsui, 
"New Family of Miniature Fiber Optic Components Designed to Save You 
Money", the 9th International Fiber Optic Communications and Local Area 
Network Exposition (FOC/LAN 85), pp. 171, September 1985. With the 
availability of these low cost components, it became evident that a very 
affordable network node could be produced. In particular, low cost optical 
receivers and transmitters became available. An active star node 10 using 
low cost components is shown in FIG. 1. The node 10 includes a plurality 
of optical receivers 12 and a plurality of optical transmitters 14. In 
FIG. 1, a single user terminal 16 is connected to the node 10. In 
particular, a transmitter 18 of the user terminal 16 is connected by a 
fiber optic cable 20 to a receiver 22. Similarly, a receiver 24 of the 
user terminal 16 is connected by a fiber optic cable 26 to a transmitter 
28. It is to be understood that other user terminals (not shown) would be 
connected to the remaining receivers 12 and transmitters 14 of the active 
node 10. Thus, it is seen that in the present implementation, a fiber 
optic receiver 12 is dedicated to each of the inbound user lines. The 
outputs of the receivers 12 are electrical signals which are electrically 
combined in a combiner logic element 30. As shown in FIG. 1, the node 10 
is in the head-end configuration so that the combined electrical signal 
from the combiner logic element 30 is connected to a splitter logic 
element 32 which distributes the composite electrical signal to the array 
of optical transmitters 14. The array of optical transmitters 14, such as 
the optical transmitter 28 broadcasts the signal to each of the attached 
receivers such as the receiver 24 associated with the user terminal 16. If 
the node 10 is to be used in a repeater or concentrator mode, the combined 
electrical signal from the combiner logic element 30 is passed to a 
dedicated transmitter 34 forming an expansion port by means of a switch 36 
so that transmission may proceed toward a higher order node (not shown). 
Similarly, an expansion port receiver 38 receives signals from a higher 
order node (not shown) and is converted to electrical format and split by 
the splitter logic element 32. The array of transmitters 14 then sends an 
optical signal to each of the users, such as the terminal 16, supported by 
the node. The low cost node 10 is a fraction of the weight and size of a 
corresponding node utilizing optical stars, and the component cost is 
about one-fifth as much. The major advantage of the present design is that 
the links between the user and the node are now point-point; thus, the 
system does not suffer the splitter losses attributed to optical star 
elements. Without these splitter losses, low cost modem to low cost node 
power margins match almost exactly the power margins that can be obtained 
from the higher performing modem and node components disclosed in U.S. 
Ser. No. 777,934. However, the low cost node 10 will support data rates 
only up to 5 Mb/s. For many applications, this data rate is more than 
adequate. 
A hardware mechanization of the node 10 will now be described in 
conjunction with FIG. 2. At the left hand side of the logic diagram of 
FIG. 2 are shown eight receiver units 12. Suitable receivers 12 are 
manufactured by Hewlett-Packard under the designation HFBR 2402. The only 
additional components necessary to support each of the receivers are a 
2200-ohm pull-up resister 40 and a decoupling capacitor 42. The outputs 
from each of the receivers 12 are combined in two 7408 quad AND gates 44 
and 46. When the head-end/repeater switch 48 is in the head-end position 
as shown in FIG. 2, the combined receiver signal is sent to a splitter 
array consisting of two 7404 inverter devices 50 and 52. The inverter 
devices 50 and 52 invert the signal (compensating for the inversion in the 
receivers 12) and acts as a driver for the optical transmitters 14. The 
optical transmitters 14 are driven in a shunt configuration. Suitable 
transmitters 14 are manufactured by Hewlett-Packard under the designation 
HFBR 1404. Each of the transmitters 14 is driven by a pair of 74128 line 
drivers 54 and a resistor/capacitor network 56. 
To operate the low cost active star node 10 in the repeater mode, the 
head-end/repeater switch 48 is set to the EX position. In that position, 
the receiver information from a ninth receiver 58 associated with an 
expansion port is used as the drive signal to the eight optical 
transmitters associated with users' terminal equipment on the node. In a 
similar manner, the composite information from the eight optical receivers 
associated with users' equipment on the node is routed to a ninth optical 
transmitter 60 dedicated to operation on the expansion port. To service 
this expansion port receiver 58, an additional 74128 logic device 62, and 
a resistor/capacitor network 64 are required. 
A prototype of the node disclosed herein has been constructed at The Mitre 
Corporation. The logic devices on this prototype model were all mounted on 
a single wire wrap board and interconnected with standard wire wrap 
technology. The nine optical transmitters and nine optical receivers were 
mounted on one wall of the enclosure in transmit/receiver pairs. Only 5 
volts were required to power this logic, and this voltage was supplied 
from a small external supply. The prototype unit was constructed in a cast 
metal box 71/4 inches long by 43/4 inches wide by 2-inches high. 
At present, the bandwidth of the low cost components is restricted to 5 
MHz. The fiber optic transmitter described above can perform to 50 MHz, 
but the receiver design is the limitation. A low cost complementary 
component is available with just an optical detector and preamplifier that 
can function at data rates up to 50 MHz. With the availability of such a 
receiver unit, the technology described in this patent application could 
be extended to encompass other higher rate protocols such as Ethernet. 
It is thus seen that the objects of this invention have been achieved in 
that there has been disclosed a low cost active node for interconnecting a 
plurality of user terminals in a fiber optic network. The node uses low 
cost optical receivers and transmitters and has the advantage that the 
links between the user and the node are point-point so that the system 
does not suffer the splitter losses attributed to optical star elements in 
other node designs. With low cost modems and the active star nodes 
disclosed herein, the total network cost of fiber optic local area 
networks becomes attractive when compared with coaxial installations, even 
at modest data rates. It is recognized that modifications and variations 
of the present invention will occur to those skilled in the art and it is 
intended that all such modifications and variations be included within the 
scope of the appended claims.