Full duplex optical modem for broadband access network

An optical modem link for full duplex transmission of multiple, independent RF carrier signals between network elements of a Broadband Access Network, includes a first optical modem, e.g., at a CATV headend broadcast facility, linked by a single optical fiber with a second optical modem, e.g., at a distributed hub location, to form an optical modem link. Each optical modem end of a respective link is equipped with an optical transmitter, which transmits light signal having a first wavelength, a receiver which receives a light signal having a second wavelength, and a wavelength division multiplexer which directs the outgoing light signal onto, and the incoming light signal off of, respectively, the fiber link. Simultaneous transmission of digital baseband signals and independent RF signals in each direction over the optical modem link is accomplished by first forming an aggregate digital data signal from the digital baseband data signals to be transmitted, modulating an RF carrier with the aggregate digital data signal, and modulating an optical signal with both the digitally modulated RF carrier signal and the independent RF signals, respectively, so that all communication signals are transmitted in the RF domain over the optical link.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention pertains to the field of communication networks. More 
particularly, the present invention pertains to an optical modem link 
which provides full duplex transport of multiple RF signals over a single 
optical medium between, for example, a headend broadcast facility and one 
or more distributed hubs. 
2. Prior Art Systems and Methods 
In modern video broadcast networks, e.g., a cable television ("CATV") 
broadcast network, a broadcast communication signal is transmitted 
"downstream" from a headend broadcast facility to a community of 
subscribers over a broadcast distribution network. A broadcast 
distribution network may include different transmission facilities, e.g., 
optical and/or electrical, and may utilize differing transmission 
methodologies, e.g., analog RF and/or digital baseband. By way of example, 
an analog RF CATV broadcast signal may be transmitted optically from a 
headend facility to a series of distributed hub locations, each of which 
splits (and amplifies) the broadcast signal for further downstream optical 
transmission over a number of "branch" facilities to a series of remotely 
located "broadband optical network units" ("BONUs"). Each BONU serves 
subscribers of the CATV network for a defined local area, e.g., a 
residential neighborhood or office complex. At the BONU, the broadcast 
signal is converted from optical to electrical transmission and then 
delivered via an electrical carrier facility, such as a coaxial cable 
distribution network, to respective subscriber locations served by the 
BONU. 
In addition to the one-way, downstream distribution of the video broadcast 
signal, a broadcast distribution network must also be able to transmit and 
receive, (i.e., in both the downstream and "upstream" directions), 
assorted types of system-level information, referred to generally herein 
as "network management" data, between various administrative and 
monitoring systems located at the headend facility, distributed hubs and 
BONUs, respectively. As used herein, "network management" data is intended 
to encompass, but not be limited to, information transmitted to and from 
the headend facility for the purpose of carrying out system level 
functions such as network operations, administration, maintenance and 
provisioning functions, sometimes referred to in the industry as "O,A,M & 
P" data. 
For example, network management data of an "operations" nature includes 
timing signals sent downstream from the head end facility to hub and/or 
BONU locations to maintain network synchronization between the distributed 
network elements. By way of another example, network management data of an 
"administration" nature includes network configuration instructions, such 
as an "ON" or "OFF" instruction sent downstream from an administrative 
module at the headend facility to a particular optical laser or amplifier 
in the downstream network. By way of yet another example, network 
management information of a "maintenance" nature includes test signals 
comprising a series of bit patterns sent from a testing unit located in 
the headend facility to a circuit or component located elsewhere in the 
network, which are then "looped-back" upstream to the testing unit and 
examined for changes, if any, which may indicate that the circuit or 
component is faulty or malfunctioning. Yet another example of network 
management data includes alarm signals sent from a hub or BONU location to 
the headend facility indicating a problem, e.g., an equipment failure. 
The foregoing examples are made to demonstrate but a few, non-limiting 
types of both upstream and downstream network management data, typically 
required to be sent between components of a video broadcast distribution 
network in order to ensure proper network operation and service integrity. 
In known CATV broadcast distribution networks, network management data is 
typically transmitted as a digital baseband signal over a digital 
interface, e.g., a RS-232, RS-485, Ethernet or parallel TTL interface, 
between the headend facility and various hub and/or BONU locations. The 
transmission link is typically provided by separate communication channels 
"outside" the broadcast distribution network. For example, network 
management data signals may be transmitted over a series of 
"point-to-point" transport links located between the headend facility and 
each respective distributed hub, and then over additional point-to-point 
links between a hub and the respective BONU locations it is connected to. 
The transport links often require a combination of modems and copper 
twisted wire pairs or coaxial cables and may be limited by both 
transmission bandwidth and distance limitations. 
Commonly, an overlapping telecommunication network is used to provide the 
transmission facilities for network management data, e.g., over leased or 
"private" lines. For example, each hub and BONU location in a broadcast 
distribution network may be provided with several telecommunication 
network access ports, which provide a link for two-way transfer of digital 
baseband network management signals to and from the headend facility, 
respectively, i.e., between digital interface facilities at each location. 
A number of one-way upstream communication links are typically utilized 
for alarm channels, often including a separate "back-up" facility for each 
alarm channel. Because the volume of ongoing network management 
transmissions can be substantial, especially in larger CATV broadcast 
distribution networks which may serve tens of thousands of subscribers, a 
considerable number of telecommunication links may be required between a 
headend facility and each respective hub and BONU location, respectively, 
to ensure all network management messages are properly transmitted and 
received and that service integrity of the distribution network is 
maintained. These additional communication links can be quite costly and 
can be subject to undesirable distance and bandwidth limitations. 
In addition to network management data, it is also desirable to transmit 
other types of communication signals--in both the upstream and downstream 
direction--within a CATV broadcast distribution network, which would 
greatly expand the types of services supported by the network and, 
therefor, enhance its value. It is presently anticipated that several new 
services will arise requiring both point-to-point and point-to-multipoint 
transmission of independent communication signals, including a full range 
of both digital baseband and analog or digitally modulated RF carrier 
signals. Such a network is referred to generally herein as a Broadband 
Access Network. 
For example, one identified service requiring transport outside of the 
tradition broadcast distribution network includes the transmission of 
digital information between a network supervisory system at the headend 
facility and "set-top" control circuitry located at each subscriber's 
premises. As used herein, subscriber "set-top" circuitry refers generally 
to CATV control circuitry traditionally located in a box-like unit placed 
on top of a subscriber's television set--hence the "set-top" 
designation--although more recently the control circuitry is incorporated 
within the television set itself. Downstream set-top data may include, for 
example, an instruction sent from the CATV service provider to activate or 
deactivate the CATV service, or to authorize additional channel reception 
within the RF spectrum of the broadcast signal, respectively. Another 
example of downstream set-top data may include information "polling" to 
collect data on usage, e.g., for ratings or billing purposes. 
Upstream data sent from the subscriber set-top circuitry to the service 
provider, sometimes referred to as "set-top telemetry" data, may include a 
response to downstream polling, as well as other types of information, 
such as, e.g., video-on-demand subscriber service requests. As with 
network management data, upstream set-top data is typically carried over 
an "outside" network on a digital baseband interface facility. For 
example, upstream set-top data may be transmitted on a dial-in basis by 
the subscriber, i.e., via a digital interface modem link over the 
subscriber's telephone line. Usage data may also be automatically provided 
over the subscriber's telephone line at scheduled intervals, e.g., during 
off-hours such as early morning, via an installed set-top modem. 
Another identified service includes downstream, delivery of digitally 
encoded video signals, e.g., "compressed video." Known formats for 
compressed video include several Motion Picture Expert Group, ("MPEG") 
encoding formats. Yet another identified group of services requiring both 
upstream and downstream transmission of multiple, independent analog RF 
and/or digital data signals is "subscriber generated video" services. For 
example, subscriber generated video signals may comprise compressed 
modulated digital baseband or analog RF signals to be transmitted upstream 
from a subscriber location to a respective BONU, hub or headend facility. 
These RF signals may then be added to the downstream broadcast signal 
within an available RF channel spectrum for broadcast distribution. Of 
significant interest is the ability to offer point-to-point or 
point-to-multipoint transmission of subscriber generated video signals 
outside of the broadcast signal transmission on a cost effective basis. 
Thus, given the wide variety of potential communication services to be 
provided over a CATV broadcast distribution network, it is desirable to 
provide both upstream and downstream communication paths, which do not 
require use of the broadcast signal transmission bandwidth or the use of 
expensive and limited digital baseband signalling over individual private 
communication links. 
U.S. Pat. No. 5,311,344, issued to Bohn et al., discloses a bi-directional 
lightwave transmission system in which a plurality of digital baseband 
signals are time-division-multiplexed into a composite digital signal. The 
composite digital signal modulates a laser operating at a first wavelength 
for optical transmission of the signal from a headend facility, over a 
single transport fiber, to a plurality of respective individual subscriber 
terminals. Upstream digital data signals from an individual subscriber 
terminal are modulated onto an RF "subcarrier," i.e., at a frequency 
designated solely for the particular subscriber, which, in turn, modulates 
a laser operating at a second wavelength for optical transmission headend 
facility over the same transport fiber. For upstream transmission, light 
signals from the plurality of subscribers are combined onto the single 
fiber by the star coupler. At the head end facility, the combined upstream 
light signal is separated from the downstream light signal by a optical 
coupler and converted to an electrical signal. Each respective 
"subcarrier" frequency is then extracted by a corresponding band-pass 
filter and the respective subscriber signal is then demodulated back to a 
digital baseband signal. 
While the Bohn et al. system provides duplex transmission of downstream and 
upstream signals between a headend facility and multiple subscriber 
terminals, respectively, several limitations to the system prevent its use 
as a full duplex transport link in a Broadband Access Network. For 
example, the input signals at both ends are strictly digital baseband, 
with no ability to support multiple, independent RF pass-through signals. 
Downstream transmission from the headend is limited to synchronous digital 
baseband signals, i.e., the transmitting laser is baseband digitally 
modulated. Therefore, no independent analog RF pass-through carrier 
signals can be accommodated. Upstream transmission requires an 
independently modulated subcarrier light signal from each subscriber 
terminal, wherein the headend facility can only receive the specific 
frequencies according to the selected RF bandwidth filters. Further, there 
is no disclosed method or capability in Bohn et al. to dynamically 
configure the digital baseband I/O ports. Moreover, the use of optical 
couplers to route the incoming and outgoing signals, respectively, at each 
end of the fiber link results in significant optical loss limitations on 
the link length. 
Thus, it remains an objective of the present invention to provide a 
communication system which performs bi-directional transport of multiple 
digital baseband data signals, such as network management data, and at the 
same time providing transparent, bi-directional transport of multiple, 
independent RF signals over the same optical medium. 
SUMMARY OF THE INVENTION 
The present invention provides an optical modem link for full duplex 
transmission of multiple, independent RF carrier signals between elements 
of a Broadband Access Network. In a preferred embodiment, a first optical 
modem, e.g., at a CATV headend broadcast facility, is linked by a single 
optical fiber with a second optical modem, e.g., at a distributed hub 
location, to form an optical modem link. Both ends of the link are 
equipped with a optical transmitter which transmits an outgoing light 
signal having a first wavelength, an optical receiver which receives an 
incoming light signal having a second wavelength, and a wavelength 
division multiplexer which directs the outgoing light signal onto, and the 
incoming light signal off of, respectively, the fiber link. 
In accordance with one aspect of the present invention, simultaneous 
transmission of digital baseband data signals and independent RF signals 
in each direction over the optical modem link is accomplished by first 
forming an aggregate digital data signal from the digital baseband data 
signals to be transmitted and modulating an RF carrier with the aggregate 
digital data signal. An outgoing optical signal is then modulated with 
both the digitally modulated RF carrier signal and the independent RF 
signals, respectively, so that all communication signals are transmitted 
in the RF domain over the optical link. A significant advantage of the 
present invention is that all RF input signals simultaneously modulate a 
single optical signal in each direction over the link. A further advantage 
of the present invention is that the actual message protocol of the 
various RF signals is not important since the optical modem link is 
transparent to the respective RF transmitting and receiving devices or 
interfaces. 
By way of example, in a preferred embodiment employed in a CATV broadcast 
distribution network, a separate optical modem link is provided from a 
headend CATV broadcast facility to each of a plurality of distributed 
network hubs. Multiple digital baseband data signals originating from a 
plurality of digital interface ports at the headend facility, (e.g., from 
separate RS 232 or 485 ports carrying network management data), are 
combined to form a single, aggregate digital data signal, which is then 
encoded with a synchronous digital clock signal, e.g., by a manchester 
encoding scheme. The encoded digital signal is used to modulate an 
(electrical) RF carrier signal, which is then combined with one or more 
independent RF signals to directly modulate the transmitting laser of a 
respective headend optical modem for transmission to a destination hub 
over a respective optical modem link. In forming the aggregate digital 
data signal, time division multiplexing (TDM) or packet switching 
techniques are preferably used so that downstream separation of the data 
signals is easily handled by known digital data handling techniques. The 
aggregate digital data signal may preferably be formed with either 
synchronous or asynchronous digital data signals. Preferably, a wide 
spectrum of RF bandwidth is provided for use for one or more digitally 
modulated RF carrier signals, as well as any other RF signals, to be 
transported over the optical modem link. 
When received at the respective hub optical modem, the incoming lightwave 
signal is directed by the receiving wavelength division multiplexer into a 
fiber receiver where the optical signal is converted back to the 
(electrical) multiple RF signals. The RF modulated encoded digital signal 
is filtered off from the rest of the RF signals and demodulated and 
decoded, respectively, back into an aggregate digital data signal. If the 
data was transmitted synchronously over the link, the digital clock signal 
is also preferably recovered from the encoded digital signal. The digital 
data is then separated back into multiple digital baseband data signals 
and routed to the appropriate digital interface ports at the respective 
hub end of the optical modem link. 
In accordance with another aspect of the invention, the recovered digitally 
modulated RF carrier signal serves as a "pilot tone" for an automatic gain 
attenuation control circuit at the receiver. The remaining RF signals, 
such as, e.g., RF modulated MPEG encoded digital video, analog RF 
subscriber generated video, or RF modulated set-top information, 
respectively, are passed through the modem link directly to their routing 
destination, i.e., the link being "transparent" to the RF signals. 
Thus, a general object of the present invention is to provide a cost 
effective, point-to-point, full duplex communication transport link 
between elements of a Broadband Access Network, such as a CATV broadcast 
distribution network. As will be apparent to those skilled in the art, 
other and further objects and advantages will appear hereinafter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a CATV broadcast distribution network includes a 
headend video broadcast facility 12, which utilizes one or more optical 
lasers 14 to transmit a broadband CATV RF broadcast signal. An analog RF 
broadcast signal is supplied by a broadcast feed 16 to a series of RF 
controller circuits 18. The frequency bandwidth of the broadcast signal 
may vary significantly depending on the particular selected broadcast 
channel plan, e.g., "77 channel NTSC" or "56 channel ," as well as the 
system bandwidth available for allocation. For example, in a preferred 
CATV broadcast distribution system employed by the assignee of the present 
invention, a video broadcast signal is transmitted within an RF spectrum 
bandwidth of approximately 45 MHz to 860 MHz. The respective RF controller 
circuits 18 feed the broadcast RF signal into the respective optical 
lasers 14, where it modulates an optical transport signal. The resulting 
optical signals are amplified, e.g., by an erbium-doped-fiber-amplifier 
("EDFA") 20, and delivered over respective fiber distribution networks 22 
to a plurality of respective distributed hub locations 24. 
At each hub 24, the optical broadcast signal is amplified as necessary, 
e.g., by one or more EDFAs 26, and delivered over an optical branch 
network 28 to a plurality of broadband optical network units (BONUs) 30. 
At each BONU 30, the optical broadcast signal is converted to electrical 
RF transmission by an optical receiver circuit 32, amplified 34 and 
delivered over a plurality of coaxial distribution cables 36 for 
transmission to subscriber locations (not shown) which are served by the 
respective BONU 30. A diplexer circuit 38 is preferably used to insert the 
downstream electrical broadcast signal onto each coaxial distribution 
cable 36, so that upstream electrical RF signals, such as, e.g., analog RF 
subscriber generated video signals, or RF modulated set-top telemetry data 
signals, can be received by the BONU 30 over the same coaxial cable 36. 
Operation of the headend facility 12 is overseen by a central processing 
module ("headend CPM") 40. The headend CPM 40 transmits and receives 
digital baseband network management data signals to and from, 
respectively, various headend network elements over a plurality of data 
buses 42. The CPM also performs as an interface between the headend 
network elements and a system administration module ("SAM") 44. The SAM 
44, which controls the O,A,M&P functions for the CATV broadcast network, 
transmits and receives the headend network management data signals to and 
from, respectively, the headend CPM 40 over one or more data buses 43. In 
addition, the SAM 44 transmits and receives various digital baseband 
network management data signals to and from, respectively, remotely 
located supervisory units located at the distributed hub locations 24. In 
accordance with a general aspect of the present invention, transmission of 
the downstream and upstream digital baseband network management data is 
provided by a plurality of "duplex" (i.e., simultaneous, two-way) 
transport links, each comprising a headend optical modem 46 connected by 
an optical fiber 48 to a corresponding hub optical modem 50. In this 
manner, a point-to-point "optical modem link,"--"46/48/50"--is formed 
which not only transports the two-way, (i.e., upstream and downstream), 
digital network management data signals, but also simultaneously 
transports any number of independent RF signals, including other types of 
digital signals modulated on RF carriers, between the headend 12 and each 
respective hub 24 of the CATV broadcast network. 
A more detailed description of the architecture and operation of the 
aforedescribed optical modem link 46/48/50 is as follows: 
As seen in FIG. 2, each headend optical modem 46 and hub optical modem 50 
are equipped with a fiber transmitter module ("FTM") 62, which transmits 
an outgoing RF modulated optical signal having a first optical wavelength, 
(described below in conjunction with FIG. 3), a fiber receiver module 
("FRM") 64, which receives an incoming RF modulated optical signal having 
a second optical wavelength, (described below in conjunction with FIG. 4), 
and a digital channel interface circuit ("DCI") 72, which controls the 
transmission and reception of digital baseband network management data 
signals (described below in conjunction with FIG. 5), respectively. 
From the perspective of a headend optical modem 46, a plurality of 
independent outgoing, (i.e., downstream), RF carrier signals 54 are input 
into the FTM 62 and a plurality of independent incoming, (i.e., upstream), 
RF carrier signals 56 are received from the FRM 64, respectively. 
Conversely, from the perspective of the respective hub optical modem 50, 
the plurality of independent downstream RF carrier signals 54 are 
received, (i.e., incoming), from the FRM 64 and the plurality upstream RF 
carrier signals 56 are input, (i.e., outgoing), into the FTM 62, 
respectively. The frequency bandwidth allocated for the RF carrier signals 
may vary significantly depending on the particular network applications 
and/or bandwidth available for allocation. For example, in a preferred 
CATV broadcast distribution system employed by the assignee of the present 
invention, an RF spectrum of approximately 5 MHz to 42 MHz is reserved for 
the independent RF signal transmission. The signalling protocol of RF 
signals 54 and 56 are advantageously transparent to the optical modem link 
46/48/50. 
At a transmitting end of the optical modem link 46/48/50, and in a manner 
described below in greater detail, the DCI 72 combines and encodes, 
respectively, multiple outgoing digital data signals to form a single 
encoded digital signal 76, which is fed into the FTM 62. At a receiving 
end, the DCI 72 receives an incoming encoded digital signal 73 from the 
FRM 64, which it then decodes and separates, respectively, into respective 
multiple incoming digital data signals. 
Outgoing optical signals from the FTM 62 is transmitted over an optical 
feeder line 66 and into a wavelength division multiplexer (WDM) 70. The 
WDM 70 is configured to direct outgoing optical signals received from 
feeder line 66, (and having the first optical wavelength), onto the fiber 
link 48. The WDM 70 is also configured to direct incoming optical signals 
received from the fiber link 48, (having the second optical wavelength), 
onto a second optical feeder line 68, and into the FRM 64, respectively. 
In this manner, the WDM 70 provides a wavelength discrimination mechanism 
to direct the outgoing and incoming optical signals onto, and off of, 
respectively, the fiber link 48. Utilizing a WDM 70 to perform this 
function advantageously consumes significantly less optical signal power 
as compared to employing an optical coupler. This, in turn, allows for 
substantially greater fiber link 48 distances, i.e., with otherwise 
constant optical transmission power. 
The FTM 62 architecture is depicted in FIG. 3, wherein the outgoing encoded 
digital signal 76 is used to digitally modulate an RF carrier signal by an 
FSK modulation circuit 75, e.g., with an FSK modulation frequency of 
approximately 2.5 MHz (digital "zero") and 3.6 MHz (digital "one"), 
respectively. The resulting digitally modulated RF carrier signal 85 is 
then combined in a diplexer circuit 86 with the plurality of independent 
analog RF signals 54 or 56, respectively, to form a plurality of outgoing 
RF signals 87. The combined outgoing RF signals 87 are passed through a 
predistortion circuit 88 and amplified 90, respectively, preferably in a 
manner disclosed in U.S. Pat. No. 5,321,710, entitled "Predistortion 
Method And Apparatus For Laser Linearization," issued Jun. 14, 1994 to 
Cornish et al. and assigned to the assignees of the present invention, the 
disclosure of which is fully incorporated herein by reference. The 
combined outgoing RF signals 87 are then used to modulate an optical laser 
92, resulting in an single, RF modulated optical signal, which is 
transmitted, via optical feeder line 66 and WDM 70, respectively, over the 
fiber link 48. 
It is contemplated that optical lasers 92 utilized by the present invention 
may be selected which transmit any number of differing first and second 
respective wavelengths. In a preferred embodiment employed by the 
assignees of the present invention, the respective transmitting laser 92 
of a headend optical modem 46 transmits a 1550 nm lightwave signal and the 
transmitting laser of a respective hub optical modem 50 transmits a 1310 
nm lightwave signal, respectively. 1550 nm and 1310 nm lasers are 
preferable in that they are generally readily available in the marketplace 
and operate at desirable wavelengths for standard industry optical network 
equipment, e.g., fibers, amplifiers, etc. It should be readily apparent to 
one skilled in the art that the deployment of 1550 nm and 1310 nm lasers 
in this preferred embodiment could be reversed, i.e., with a 1310 nm 
transmitting laser 92 employed in the respective headend optical modems 46 
and a 1550 nm transmitting laser 92 employed in the respective hub optical 
modems 50. 
The FRM 64 architecture is depicted in FIG. 4, wherein the incoming optical 
signal, (i.e., having the respective "second optical wavelength"), is 
received from optical feeder line 68 and converted back to the combined RF 
signals 87 by a photo-optic detector circuit 94. The converted electrical 
RF signals 87 are initially amplified 96, attenuated by a variable level 
attentuator 98, such as, e.g., a voltage programmable (or "auto-gain") 
attenuator, and then again amplified 100, respectively. In this manner, 
the incoming RF signals 87 are maintained at a substantially constant or 
"nominal" power level. The RF signals 87 are then split by an RF coupler 
102, with a first portion 104 of the combined RF signals 87 transmitted 
into an RF output/filter circuit 106 to recover the independent RF signals 
56 or 54, respectively. A second portion 108 of the combined RF signals 87 
is used to recover the encoded incoming digital signal 73. In the 
illustrated preferred embodiment, the second portion 108 of the combined 
RF signals 87 is also used as a "pilot tone" for a gain-level control 
circuit 134, which sets the attenuation level of the variable attenuator 
98. 
In the RF output/filter circuit 106, the RF signals 87 are again amplified 
110, and attenuated 112, respectively, preferably to a preset power level. 
The signals are then passed through one or more high pass filters 114 to 
strip off the FSK modulated encoded digital signal 73, as well as to 
reduce any noise into the downstream circuitry. For example, in the 
aforementioned preferred CATV broadcast distribution system employed by 
the assignee of the present invention, a 5 MHz high pass filter is used to 
strip off the relatively low frequency FSK modulated encoded digital 
signal 73, as well as any low frequency noise that may be present in the 
signal. By the use of any number of selected further filtering 
combinations (not shown), the RF signals 56 or 54, respectively, may be 
completely separated, left combined, or some combination in between, 
depending on the particular destination and/or further routing required 
for each respective individual RF signal 56/54. 
The second portion 108 of the incoming RF signals 87 is passed through an 
FSK filter circuit 116, which includes another attentuator 118, a 5 MHz 
low pass filter 120, an amplifier 122 and an FSK bandpass filter 124, 
respectively. The 5 MHz low pass filter strips the 5 MHz to 42 MHz 
independent RF signals 56/54 from the combined RF signals 87 prior to 
further amplification 122 of the signal, in order to isolate the FSK 
modulated digital signal 85 from the rest of the combined RF signals 87. 
The FSK bandpass filter 124 frequency filters the remaining RF bandwidth 
to the FSK frequency boundaries, i.e., between approximately 3.6 MHZ and 
approximately 2.5 MHz, in order to reduce noise in the downstream 
circuitry. 
The resulting "FSK signal" 129 is then passed through an FSK demodulation 
circuit 126, which includes an adjustable receiver chip 128. The receiver 
chip 128 frequency demodulates signal 125, i.e., wherein the receiver chip 
128 outputs a digital "one" when the frequency of the input signal is 
approximately 3.6 MHz and a digital "zero" when the frequency is of the 
input signal is approximately 2.5 MHz, respectively, thereby generating 
the "incoming" encoded digital signal 73. The (demodulated) encoded 
digital signal 73 is "cleaned up" by a comparator circuit 130, which 
preferably ensures that the respective digital ones and zeros will only be 
transmitted in response to actual respective data signal frequencies and 
not to noise frequencies. A second comparator 132 is used in conjunction 
with a squelch control circuit 133 to allow transmission of the encoded 
digital signal 73, i.e., to "break squelch," only when the voltage level 
of the input signal indicates that data, and not noise, is being 
transmitted. 
The FSK signal 125 is also input into a (gain-feedback) level control 
circuit ("LCC") 134, wherein a mixer circuit 136 creates a DC voltage 135 
directly proportional to the power level of signal 125. The DC voltage 135 
is offset by a predetermined voltage level 137 and amplified, as 
necessary, by an op-amp circuit 138, resulting in a "feedback voltage" 146 
directly proportional to the power level of the FSK signal 125. A switch 
140 is provided in the LCC 134, which may be set in either "automatic" or 
"manual" gain position. When set in "automatic" gain position, the switch 
140 connects the feedback voltage 146 to the variable attenuator 98, i.e., 
wherein the level of attenuation applied to the RF signals 87 is based on 
feedback voltage 146. In this manner, the attenuation level of the 
incoming RF signals 87 is "automatically" adjusted upward or downward, 
respectively, by the feedback voltage 146, so that the incoming power 
level of signals 87 is maintained at the desired nominal level. 
The voltage 146 is also input into a comparator 144, which compares voltage 
146 with a predetermined reference voltage (not shown), based on an 
expected minimum voltage level when the FSK signal 125 is present. In the 
event the FSK modulated RF signal 85 signal is not present, (i.e., when no 
transmission of the encoded digital signal 76 is taking place over the 
optical modem link 46/48/50), voltage 146 will drop below the 
predetermined reference voltage and the comparator 144 will direct a 
switch control circuit 148 to change the position of switch 140 to 
"manual" gain. In "manual" gain position, the switch 140 supplies a manual 
gain control voltage 142 to set the attenuation level of the variable 
attenuator 98. In this manner, the attenuation level of the incoming RF 
signals 87 is maintained constant until the FSK signal 125 is again 
received, wherein the comparator 144 directs the switch control 148 to 
return the position of switch 140 to "automatic" gain. 
Referring to FIG. 5, the DCI circuit 72 includes a microprocessor 150, 
which controls the transmission of multiple digital baseband data signals 
between a respective digital data interface ports locate at each end of 
the respective optical modem links 46/48/50.1 In the illustrated preferred 
embodiment, each DCI 72 is equipped with two RS 232 or 485 type 
synchronous digital data ports 78 and 80, an 8-bit parallel I/O port 82 
and an IEEE 802.3 standard Ethernet port 84, respectively. In the transmit 
direction, the microprocessor 150 preferably employs time division 
multiplexing (TDM) or packet switching techniques to form a single, 
outgoing aggregate digital data signal 149 from the outbound digital data 
streams received from digital interface ports 78, 80, 82 and 84, 
respectively. The outgoing aggregate digital data signal 149 is encoded 
with a synchronous digital clock signal 151, e.g., by a bi-phase space 
(manchester) encoder 152, to form the outgoing encoded digital signal 76. 
In the receive direction, the incoming encoded digital data signal 73 is 
decoded 154, e.g., by bi-phase space (manchester) decoding, into an 
incoming aggregate digital data signal 153 and a recovered digital clock 
signal 155, respectively. The receiving end microprocessor 150 separates 
the incoming aggregate digital signal into individual digital baseband 
data signals and transmits them to the appropriate respective ports 78, 
80, 82 and 84, respectively, preferably synchronized by the recovered 
clock signal 155. 
By employing well known digital data handling techniques, corresponding 
ports at both ends of an optical modem link 46/48/50 may be directly 
"linked" together by the respective microprocessors 150 at each 
end,--i.e., wherein the digital data is transmitted and received 
"transparently" between linked ports on each respective end. In 
particular, referring briefly to FIG. 1 along with FIG. 5, duplex digital 
baseband network management data signals are sent between the SAM 44 at 
the headend 12 and a plurality of broadband supervisory units ("BSUs") 52 
at each respective hub location 24, via the respective interface port 78 
at each end of each respective optical modem link 46/48/50. Duplex digital 
baseband network management data signals are also sent between the SAM 44 
and a plurality of hub central processing modules ("hub CPMs") 58, via the 
respective interface port 80 at each end of each respective optical modem 
link 46/48/50. Data buses 77 and 79 transport the respective data signals 
between the SAM 44 and ports 78 and 80, respectively, of each headend 
optical modem 46. Likewise, data buses 81 and 83 transport the respective 
data signals between the BSUs 52 and hub CPMs 58, respectively, and the 
respective ports 78 and 80 of each hub optical modem 50. In the 
illustrated preferred embodiment, I/O port 82 is used to transmit laser 
safety shutdown ("LSS") signals 60 sent from a respective hub broadcast 
signal amplifier 26 to a respective headend broadcast signal amplifier 20 
if a break in the broadcast fiber is detected between the headend 12 and 
respective hub 24. The ethernet port 84 may be used to link broadcast 
distribution network elements with a LAN server (not shown), e.g., in 
order to supplement, or replace, many of the functions of the SAM 44. 
The microprocessor 150 is preferably provided with a service port 156 to 
allow for direct configuration of the DCI circuit configuration, including 
"time slot" allocation of data transmitted to and received from, 
respectively, the respective digital interface ports, as well as general 
"O,A,M&P" access to the microprocessor. It is contemplated that any number 
of variations in the quantity, type and application of digital interface 
ports can be accommodated by the DCI 72, the illustrated configuration 
being merely an exemplary preferred implementation. In a preferred 
embodiment, the microprocessor 150 may also serve as the main monitor and, 
where applicable, controller of the elements of the optical modem 46 or 
50, respectively. For such purposes, a data bus 45 is preferably provided 
to provide network management data, e.g., such as alarm and status 
information relating to the operation of both ends of the optical modem 
link 46/48/50, from the respective headend microprocessor 150 to the 
headend CPM 40. 
Returning to FIG. 1, further two-way communication of network management 
data and non-broadcast RF signals between the respective bud 24 and BONU 
30 locations is accomplished as follows: 
Downstream digital data transmitted from one or more BSUs 52 in a hub 24 is 
FSK modulated 55 and fed into an RF controller circuit 57. The RF 
controller circuit 55 feeds the FSK (RF) modulated signal, along with one 
or more "narrowcast" RF signals 59, into a respective transmitting optical 
laser 61, where the combined RF signals modulate an optical signal, which 
is combined with the broadcast optical signal for downstream transmission 
over the respective optical branch network 28. As used herein, a 
"narrowcast" signal 59 refers to a signal inserted into the CATV broadcast 
network at a point downstream of the headend facility 12, e.g., at hub 24 
or BONU 30 location. Narrowcast transmission is generally disclosed and 
described in U.S. Pat. No. 5,457,562, entitled "Narrowcast Optical 
Communication Networks and Methods," issued Oct. 10, 1995 to Tremblay and 
assigned to the assignees of the present invention, the disclosure of 
which is fully incorporated herein by reference. 
The source of such narrowcast signals may comprise, for example, one or 
more of the RF signals 54 transmitted from the headend 12 to a respective 
hub 24, via an optical modem link 46/48/50. Narrowcast signals may also 
comprise, by way of further example, one or more "upstream" RF signals 
sent from a BONU 30 to a respective hub, such as subscriber originated 
video signals. It should be noted that such RF signals might alternatively 
be transmitted over the branch network 28 on a more limited basis, e.g., 
point-to-point or point-to-multipoint transmission. 
The "narrowcast" laser 61 preferably outputs a different optical wavelength 
than respective transmitting broadcast laser 14 at the headend facility 
12, so that no interference is caused by the "mid-stream" insertion of the 
downstream RF signals with the broadcast signals. As such, the output 
optical signal from laser 61 is preferably not combined with the broadcast 
signal until after the final EDFA amplifier 26, as the signal may 
otherwise be filtered during the amplification process, as is described in 
the above-referenced U.S. Pat. No. 5,457,562, issued to Tremblay et al. 
At the optical receiver 32 of each respective BONU 30, the downstream FSK 
modulated network management signals are separated and demodulated, e.g., 
in a manner similar to the operation of the aforedescribed FRM 64, and 
delivered to a remote supervisory unit ("RSU") 63, within the BONU 30. In 
a similar manner, upstream digital baseband network management signals 
from the RSU 63 are RF modulated, combined with any other upstream RF 
signals received from subscribers over the coaxial cable network 36 and 
optically transmitted by a reverse fiber transmitter ("RFT") 65, (i.e., an 
optical laser), respectively, over an upstream fiber network 67 back to 
the respective hub 24. At the hub 24, optical RF signals from a plurality 
of BONUs are received by a reverse fiber receiver ("RFR") 69, which 
converts the RF signals from optical to electrical transmission and, by 
way of one or more frequency filters (not shown), separates the RF 
modulated (upstream) network management signals from the other RF signals. 
The upstream network management signals are demodulated and delivered to 
the respective BSUs 52 at the hub 24. 
The remaining upstream RF signals may be separated out by frequency, e.g., 
for insertion as a narrowcast signal, or for other downstream transmission 
from the hub 24, or may be combined by an RF combiner 71 to form part of 
the plurality of independent RF signals 56 to be transmitted upstream over 
the respective optical modem link 50/48/46. 
Thus, a new and useful architecture providing a full duplex optical link 
for connecting elements of a broadband network, such as a headend video 
broadcast facility with a network hub location, has been disclosed. While 
embodiments and applications of this invention have been shown and 
described, it would be apparent to those skilled in the art that many more 
modifications and applications are possible without departing from the 
inventive concepts herein. For example, the duplex optical modem link 
could be utilized for any type of analog or digital data transport 
application, and in any type of network, regardless of the signalling 
protocol,--the aforedescribed CATV broadcast network, including 
transmission of asynchronous and synchronous digital network management 
data, RF modulated set-top telemetry signals and analog RF subscriber 
generated video signals, respectively, being merely exemplary of 
applications for the present invention. 
The scope of the invention, therefore, is not to be restricted except in 
the spirit of the appended claims.