Note: Descriptions are shown in the official language in which they were submitted.
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Dynamic Wavelength Management using Bi-directional Communication for the
prevention of Optical Beat Interference
BACKGROUND
Businesses and consumers are demanding high speed data (HSD), voice over IP
(VoIP) and
video services (CATV, IPTV), therefore access communications networks need to
be
designed to keep up with this demand. Telephone companies and Multisystem
Operators
(MS0s) satisfy this demand by bringing optical fiber deeper into the network.
This is
typically done by deploying a passive optical network (PON) such as a fiber-to-
the-premises
(FTTP), fiber-to-the-curb (FTTC), fiber-to-the-home (FTTH) or Radio Frequency
over Glass
(RFoG) network to deliver these services to the subscriber.
There are two general system architectures deployed in PON networks, time-
division multiple
access (TDMA) and frequency-division multiple access (FDMA). The TDMA method
is used
in EPON or UPON networks where the customer premises equipment (CPE) or
optical
network unit (OW) is assigned a time slot and transmits only within its
allotted time. The
FDMA method is typically found in RFoG networks. Figure 1 shows a typical
example of an
RFoG network. In the downstream direction, video, and data modulated as AM-VSB
and
QAM RF Carriers is optically modulated by an optical transmitter, amplified in
the optical
domain by an erbium doped fiber amplifier ( EDFA) and transported to the CPE
at the
subscriber site over fiber. The CPE converts the optical signal into RF and
the RF signals are
delivered to the set top box and cable modem over coaxial cable. In the
upstream direction,
the signals carrying set top box data and upstream data from the cable modem
are converted
from an RF signal to an optical signal and transmitted to an optical receiver
where they are
converted back to RF and distributed to the upstream ports of the cable modem
teiiuination
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system (CMTS) and the set top box controller.
The CMTS in the multiple system operator (MSO) headend, or hub and the cable
modems at
the subscriber sites form a point to multi-point communication network. In the
downstream
direction, the RF carriers from the CMTS are continuously on. In the upstream
direction,
since several cable modems communicate with the single CMTS upstream receiver,
both
Time Division Multiple Access (TDMA) and Frequency Division Multiple access
(FDMA)
are used. Multiple RF frequencies can be assigned to groups of cables modems,
and within a
group of cable modems that use a specific RF frequency, TDMA is used to avoid
data
collisions. However any two (or more) cable modems that are operating at
different RF
frequencies can transmit at the same time. When this happens, the lasers of
the CPEs that
they are connected to are also activated and there is a non-zero statistical
probability that the
laser wavelengths of those CPEs can overlap. It is critical to avoid this
event in any RFoG
system because when two (or more) optical signals of the same wavelength or
with
wavelengths that are close together are incident on an optical receiver, an
optical effect
known as Optical Beat Interference (OBI) can severely degrade of the signal-to-
noise ratio
(SNR) over a large RF bandwidth (Figure 2) resulting in a loss of data.
In RFoG networks, CPE lasers operate in a burst mode configuration. When the
RF level of
the upstream signal crosses a threshold level, the laser is turned on. When it
drops below a
certain threshold level, the laser is turned off. This burst mode operation
reduces the
probability of OBI, but does not eliminate it, as discussed above. In HFC
networks, the
upstream lasers are on continuously and therefore the probability of OBI is
significantly
higher.
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SUMMARY
There is a need for the following embodiments of the present disclosure. Of
course, the
present disclosure is not limited to these embodiments.
According to an embodiment of the present disclosure, a method comprises:
preventing
optical beat interference including dynamically managing an adjustable optical
transmitter
wavelength of each of a plurality of customer premises equipment, wherein each
of the
plurality of customer premises equipment is in bidirectional communication
with a customer
premises equipment controller. According to another embodiment of the present
disclosure,
an apparatus comprises a bidirectional communication system including: a
customer premises
equipment controller; and a plurality of customer premises equipment coupled
to the
customer premises equipment controller, each of the plurality of customer
premises
equipment having an adjustable optical transmitter wavelength, wherein each of
the plurality
of customer premises equipment is in bidirectional communication with the
customer
premises equipment controller to prevent optical beat interference by
dynamically managing
the adjus'Lable optical transmitter wavelength of each of the plurality of
customer premises
equipment.
These, and other, embodiments of the present disclosure will be better
appreciated and
understood when considered in conjunction with the following description and
the
accompanying drawings. It should be understood, however, that the following
description,
while indicating various embodiments of the present disclosure and numerous
specific details
thereof, is given for the purpose of illustration and does not imply
limitation. Many
substitutions, modifications, additions and/or rearrangements may be made
within the scope
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of embodiments of the present disclosure, and embodiments of the present
disclosure include
all such substitutions, modifications, additions and/or rearrangements.
BRIEF DESCRIPTION OF THE DRAWINGS
The drawings accompanying and forming part of this specification are included
to depict
certain embodiments of the present disclosure. A clearer concept of the
embodiments
described in this application will be readily apparent by referring to the
exemplary, and
therefore nonlimiting, embodiments illustrated in the drawings. The described
embodiments
may be better understood by reference to one or more of these drawings in
combination with
the following description presented herein. It should be noted that the
features illustrated in
the drawings are not necessarily drawn to scale.
FIG. 1 is a block schematic view of an RFoG network.
FIG. 2 shows spectrum analyzer traces of two 1550nm lasers transmitters each
modulated
with a 256-QAM signal: a) SNR ratio without OBI is ¨40 dB and b) SNR ratio
with OBI ¨10
dB.
FIG. 3 is a block schematic view of an RFoG Network with a customer premises
equipment
and a customer premises equipment controller utilizing unique RF frequencies
for
downstream and upstream communication with the customer premises equipment.
FIG. 4 is a block schematic view of an RFoG Network with the customer premises
equipment
controller downstream signal modulated on a separate optical transmitter.
FIG. 5 is a block schematic view of an RFoG customer premises equipment.
FIG. 6 is a view of a wavelength assignment process.
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DETAILED DESCRIPTION
Embodiments presented in the present disclosure and the various features and
advantageous
details thereof are explained more fully with reference to the nonlimiting
embodiments that
are illustrated in the accompanying drawings and detailed in the following
description.
Descriptions of well-known techniques, components and equipment are omitted so
as not to
unnecessarily obscure the embodiments of the present disclosure in detail. It
should be
understood, however, that the detailed description and the specific examples
are given by way
of illustration only and not by way of limitation. Various substitutions,
modifications,
additions and/or rearrangements within the scope of the underlying inventive
concept will
become apparent to those skilled in the art from this disclosure.
Embodiments of this disclosure include methods, system architectures and
apparatus to
prevent OBI in RFoG networks or in hybrid fiber coax (HFC) networks, or in any
other
networks, where multiple laser transmitters can simultaneously operate over a
common
optical fiber connected to a shared optical receiver. The system includes a
controller that
resides in the headend or the hub, and a CPE that resides at the customer
premises, as shown
in Figure 3.
Referring to figure 3, the controller 310 and the CPEs 320 form a bi-
directional
communication system. Downstream data messages from the controller are
modulated on a
RF carrier fl, optically modulated by the optical transmitter 330, combined
onto a single fiber
and delivered to the CPE through the optical splitter 340. Upstream data
messages from the
CPEs on f2 are delivered to the controller 310 through an optical receiver
350.
Referring to figure 5, at the CPE 500 the optical signal is received by a
photo diode 510,
amplified by an RF amplifier 520, and delivered to a RF demodulator 530
through an RF
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splitter 540. The RF demodulator extracts the data stream and delivers it to
the
microcontroller 550. In the upstream direction, control messages from the
microcontroller
are modulated by an RF modulator 560 on an RF frequency f2. This RF carrier is
modulated
by the laser 570 and transmitted to the optical receiver 350 (shown in figure
3) through the
optical diplexer 580, the optical fiber and the wavelength division
multiplexer (not shown in
figure 5).
In an alternate embodiment shown in figure 4, two downstream optical
transmitters 401, 402
are used. One transmitter 402 is used to modulate the downstream RF signal
from the
controller. Another optical transmitter 401 is used to modulate the downstream
video, CMTS
and set top box channels. These two transmitters operate at different optical
wavelengths
which are combined using an optical multiplexer 410 or an optical coupler. The
combined
signals are amplified, and delivered to the CPE 420. It should be noted that
it is not necessary
for the controller 430, the optical transmitter 402 that modulates the
downstream RF signal
from the controller, and the optical receiver 440 to be located at the same
location as the
transmitter 401 that modulates the other downstream channels. They can be
located at a
remote location such as a node or a pedestal.
A characteristic of a laser is that its wavelength can be adjusted by changing
its DC bias point
or its temperature. Thus any given laser with intrinsic wavelength Aio at
ambient temperature
can access a range of wavelengths from Xi man to Xa _max over temperature and
bias current:
di"
jmin or max) = + * AT - * Ai ¨diaT
where
AT = (Tinft, ¨ TO and Ai = Urn. ¨
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or
AT =(Tmax¨ To) and AI = (tram /0(T))
T0= ambient temperature, T is temperature
Rio= laser wavelength at ambient temperature
10(T) is laser bias current at given temperature for a particular output power
¨ttl. is the change in laser wavelength as a function of temperature
ar
is the change in laser wavelength as a function of laser bias current
di
The laser in the CPE is connected to a Thermo-Electric Cooler (TEC) that is
used to change
the temperature of the laser and thus its wavelength. It should be noted that
besides a TEC,
there are other ways to change the wavelength of the laser, such as a heater,
or any other
device or method that changes the wavelength of the laser. A tunable laser
could also be used.
It should be noted that the TEC can be mounted inside the laser.
The system works by defining a wavelength grid. The grid could include
wavelength ranges,
for example, known as the C-band, 0-band or any CWDM wavelength band or
combination
of bands. It can also be defined to span the entire optical communication
window from 1250
nm to 1650 nm. The range is specified by 2stQrt, Astop, and spacing between
the wavelengths,
dilbm, is defined to be large enough to eliminate the possibility of OBI.
Typically this is
greater than 3 to 5 times the adiabatic chirp of the laser plus an additional
amount to account
for dynamic wavelength shift due to burst mode transmission and a margin for
lifetime
wavelength aging of a laser device. The available wavelengths are discreet
positions given by
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= Astart * Atirt
with i =0,1,2,3...M and AM < Astop.
During the time of manufacture, the TEC is used to change the temperature of
the laser and
determine which wavelengths on the grid this particular laser can be tuned to.
It should be
noted that any one CPE may only map onto a very small subset of the M
wavelengths on the
grid. A table is created that contains the list of these wavelengths, and the
corresponding
temperature and TEC voltage that needs to be set to tune the laser to those
wavelengths. The
CPE then selects its default wavelength, which is the ideal wavelength it
should be operated
at. This wavelength would typically be in the middle of the range of
wavelengths that a CPE
laser can be tuned to. Having the CPE laser operate at the middle of the rage
is preferable
because it is easier for the TEC to maintain this wavelength over the entire
operating
temperature range of the CPE. This information about the CPE default
wavelength (and the
wavelengths above and below the default wavelength that the CPE can be tuned
to) is then
stored into the memory of the CPE. This memory can be an EEPROM, Flash,
internal
microcontroller memory, FPGA or any other method or device that can store
information.
When the CPE is first deployed in a network, it reports its default wavelength
along with the
number of wavelengths above and below the default wavelength that the CPE
laser can be
tuned to. Along with this information, the CPE also reports its unique
identifier to the
controller. This identifier can be a serial number, a MAC address, an IP
address, or any other
set of characters unique to the CPE. The controller compares this information
to its database
of information containing all the CPEs connected to that optical receiver. If
the default
wavelength of the new CPE has not been assigned to any other CPE on the same
receiver,
then the controller sends a downstream control message assigning the default
wavelength to
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the CPE. The CPE continues to operate on this default wavelength unless told
by the
controller to move to a different wavelength. If the default wavelength of the
CPE has been
assigned to another CPE connected to the same receiver, then the controller
assigns to the
CPE the closest available wavelength to its default wavelength. This ensures
that as far as
possible, the CPEs will operate at or close to their default wavelengths.
Figure 6 shows how
this process would work. Since CPE1 is the first CPE to come online, it is
assigned its default
wavelength (25). CPEs 2, 3, and 4 also come online and are assigned their
default
wavelengths because those wavelengths have not been assigned to any other
CPEs. CPE 5
comes online and its default wavelength is taken by CPE3, so CPE5 is assigned
the next
closest wavelength (Ad 7) to right of the default wavelength. CPE7 also has
the same default
wavelength and it is assigned the wavelength to the left of its default
wavelength (X15).
Although this method of using a default wavelength that is in the middle of
the range of laser
wavelengths has certain advantages as stated above, a system where the default
wavelength is
not in the middle of the range can also be implemented.
In another embodiment of the method, the wavelength of the CPE laser is
measured at two
temperatures, T1 and T2. The temperatures and the corresponding laser
wavelengths and
X2 are used to create a linear equation in the form
= m* Tk +b
Where b is the constant and m is the slope defined by:
m = (k2- ki)/ (T2- TO
The equation defines the wavelength of the laser as a function of temperature.
The two data
points, the slope m and the constant b are stored in the CPEs memory. When the
CPE is
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deployed for the first time, the CPE reports this information, along with its
present operating
wavelength and its unique identifier to the controller. The initial wavelength
used by the CPE
to report this information can be any wavelength that it can tune to. The
controller compares
this information with its database of information on the wavelengths of all
other CPEs
connected to this optical receiver. It then sends control messages to the CPE
to either stay at
the CPEs present wavelength, if it is not close enough to the wavelengths of
other CPEs to
cause OBI, or to move to a different wavelength that is far enough apart from
the wavelengths
of the other CPEs so as not to cause OBI. In this method, a wavelength grid
does not have to
be defined. It is also not necessary to limit the characterization of
temperature and
wavelength to two data points. An implementation with more data points will
enable the
wavelengths to be set more precisely.
One factor that limits the operating temperature range of the CPE is the
heating and cooling
capacity of the TEC. Given this constraint, the following approach can be used
to extend the
operating temperature range of the CPE. One parameter the CPE could report to
the
controller is its temperature. The controller can collect the temperature
readings from every
CPE belonging to a single N sized cluster of CPEs connected to the optical
receiver and can
determine the distribution of temperatures for that specific cluster. There
are daily and/or
seasonal variations in ambient temperature and the controller can respond to
the moving
envelope of temperature variations. A cluster of CPEs connected to an optical
receiver is
likely confined to a geographic region where the CPEs would not experience
both extremes of
an industrial temperature range simultaneously. When the ambient temperature
moves lower
than the specified low operating temperature, the controller can move every
CPE, in concert
to a lower wavelength. The amount of wavelength shift would depend on the
amount of
change in the ambient temperature. The same approach can be utilized to extend
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operating temperature range on the high temperature side. This approach can
also minimize
the power consumption of the unit because at a lower ambient temperature, the
TEC
consumes less power to maintain wavelengths that are on the shorter side of
the optical
spectrum. Similarly, at higher ambient temperatures, the TEC consumes less
power to
maintain wavelengths that are on the longer side of the optical spectrum. The
wavelength
movement happens in a way that the CPEs will not overlap when moved. For
example when
cold ambient temperatures are experienced the wavelengths will be moved to the
shorter
values starting from the shortest wavelength one-by-one to the longest
wavelength. If the
ambient temperatures move toward the hot extreme the wavelengths will be moved
toward
the higher values starting from the longest wavelength to the shortest.
In a lxN network configuration, where N CPEs are connected to a single optical
receiver, it is
desirable to have N as large as possible. One factor that defines how large N
can be is the
variation in the intrinsic wavelengths of the lasers. If a group of CPEs use
lasers
manufactured from a single semiconductor wafer then the total available
operating
wavelength range for the group of lasers will be larger than the operating
wavelength range of
one laser from the wafer. This is because the laser chips on a wafer have a
non-zero
wavelength distribution. Typically the laser wavelengths across a single wafer
follow a
Gaussian distribution with a mean central wavelength of A and standard
deviation of ¨1nm.
If the laser transmitters are sourced from multiple wafers than the operating
wavelength range
could be even larger since the wafer-to-wafer mean wavelengths vary from run-
to-run when
comparing wafers manufactured from a single laser vendor. With multi-sourced
laser vendors
one could ensure a wide variation in intrinsic laser wavelengths. One could
also exploit the
natural wafer-to-wafer wavelength variation by building an array of 2 (or
more) lasers in a
single package and choosing laser chips from different wafers that have
different mean central
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wavelengths. In another implementation one could fabricate wafers with custom
wavelengths
and build a laser array to include standard laser chips and custom laser
chips. This allows an
even larger operating wavelength range to be available to the CPE allowing the
controller to
decide which laser transmitter should be turned on based on the accessible
wavelengths
available in the network at the time the CPE comes online. The decision of
which laser in the
array is turned on can be done at initial startup and could be changed later
as more CPEs
come online and more usage statistics are gathered about the network of CPEs.
In one implementation, as the controller gains more information about the
usage statistics of
each subscriber the CPE wavelengths can be moved into groups or clusters of
high data
bandwidth users and low data bandwidth users. In the case of low data
bandwidth users the
adjacent wavelength spacing criteria described above can be relaxed to allow
more spectral
bandwidth to be opened up for new CPEs as they are brought online. The relaxed
criteria may
only be needed when a new CPE that comes online cannot be assigned any of its
accessible
wavelengths because those wavelengths have been assigned to other CPEs.
The two way communication channel between the controller and the CPE can also
be used to
monitor wavelengths and periodic spectral position adjustments can be added to
improve
system performance. The improvements could be in the form of relaxing the
channel spacing
for low bandwidth users or making corrections for aging of the laser. This
could be done with
an optical spectrum analyzer, an optical channel monitor or other wavelength
monitoring
device. The monitoring device could be shared within one group of N
transmitters in a cluster
or it could be shared among multiple groups of N sized clusters.
Embodiments of the present disclosure can include method and system
architecture, and
apparatus to prevent optical beat interference in optical networks that allow
multiple laser
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transmitters to simultaneously transmit to a shared optical receiver.
Embodiments of the
present disclosure can include a system that allows bi-directional
communication path
between the CPE and the controller over a downstream and an upstream RF
frequency.
Embodiments of the present disclosure can include a system where the
downstream RF
frequency of the controller is modulated by a separate transmitter.
Embodiments of the
present disclosure can include a CPE that stores laser wavelength information
and determines
its default wavelength from this information. Embodiments of the present
disclosure can
include a CPE that transmits this information to the controller. Embodiments
of the present
disclosure can include a controller that compares this information to the
wavelength
information from other CPEs and assigns to the CPE a wavelength such that OBI
will not
occur. Embodiments of the present disclosure can include a CPE that receives
downstream
control messages from the controller carrying the wavelength assignment
information,
demodulates them, and based on this information uses a TEC to appropriately
position its
optical wavelength on the wavelength grid. Embodiments of the present
disclosure can
include a method that uses information stored on the CPE about its laser
transmitter
wavelength characteristics and communicates it to the controller. Embodiments
of the present
disclosure can include a method that uses the full spectral bandwidth of a
laser transmitter to
move it away from its default wavelength if needed. Embodiments of the present
disclosure
can include a method that places CPE wavelengths into spectrally separated
wavelength grid
to prevent signal degradation from wavelength collisions of simultaneously
transmitting
CPEs. Embodiments of the present disclosure can include a system that monitors
CPE case
temperatures and responds to daily and seasonal variations in temperature to
extend the
operational temperature range of the unit. Embodiments of the present
disclosure can include
a method to widen the spectral bandwidth available to a CPE by employing an
array of laser
transmitters. Embodiments of the present disclosure can include a method to
employ custom
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semiconductor wafer fabrication to widen the wavelength distribution of laser
transmitters
available to a CPE. Embodiments of the present disclosure can include a method
that collects
network data bandwidth usage statistics and moves CPE wavelengths into like-
user clusters.
Embodiments of the present disclosure can include a system that employs
wavelength
monitoring to improve system performance.
Definitions
The terms program and software and/or the phrases program elements, computer
program and
computer software are intended to mean a sequence of instructions designed for
execution on
a computer system (e.g., a program and/or computer program, may include a
subroutine, a
function, a procedure, an object method, an object implementation, an
executable application,
an applet, a servlet, a source code, an object code, a shared library/dynamic
load library
and/or other sequence of instructions designed for execution on a computer or
computer
system). The phrase radio frequency (RF) is intended to mean frequencies less
than or equal
to approximately 300 GHz. The term light is intended to mean frequencies
greater than or
equal to approximately 300 GHz.
The term uniformly is intended to mean unvarying or deviate very little from a
given and/or
expected value (e.g., within 10% of). The term substantially is intended to
mean largely but
not necessarily wholly that which is specified. The term approximately is
intended to mean at
least close to a given value (e.g., within 10% of). The term generally is
intended to mean at
least approaching a given state. The term coupled is intended to mean
connected, although
not necessarily directly, and not necessarily mechanically. The term
proximate, as used
herein, is intended to mean close, near adjacent and/or coincident; and
includes spatial
situations where specified functions and/or results (if any) can be carried
out and/or achieved.
The term distal, as used herein, is intended to mean far, away, spaced apart
from and/or non-
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coincident, and includes spatial situation where specified functions and/or
results (if any) can
be carried out and/or achieved. The term deploying is intended to mean
designing, building,
shipping, installing and/or operating.
The terms first or one, and the phrases at least a first or at least one, are
intended to mean the
singular or the plural unless it is clear from the intrinsic text of this
document that it is meant
otherwise. The temis second or another, and the phrases at least a second or
at least another,
are intended to mean the singular or the plural unless it is clear from the
intrinsic text of this
document that it is meant otherwise. Unless expressly stated to the contrary
in the intrinsic
text of this document, the term or is intended to mean an inclusive or and not
an exclusive or.
Specifically, a condition A or B is satisfied by any one of the following: A
is true (or present)
and B is false (or not present), A is false (or not present) and B is true (or
present), and both A
and B are true (or present). The terms a and/or an are employed for
grammatical style and
merely for convenience.
The term plurality is intended to mean two or more than two. The term any is
intended to
mean all applicable members of a set or at least a subset of all applicable
members of the set.
The phrase any integer derivable therein is intended to mean an integer
between the
corresponding numbers recited in the specification. The phrase any range
derivable therein is
intended to mean any range within such corresponding numbers. The term means,
when
followed by the term "for" is intended to mean hardware, firmware and/or
software for
achieving a result. The term step, when followed by the term "for" is intended
to mean a
(sub)method, (sub)process and/or (sub)routine for achieving the recited
result. Unless
otherwise defined, all technical and scientific terms used herein have the
same meaning as
commonly understood by one of ordinary skill in the art to which this present
disclosure
belongs. In case of conflict, the present specification, including
definitions, will control.
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The described embodiments and examples are illustrative only and not intended
to be
limiting. Although embodiments of the present disclosure can be implemented
separately,
embodiments of the present disclosure may be integrated into the system(s)
with which they
are associated. All the embodiments of the present disclosure disclosed herein
can be made
and used without undue experimentation in light of the disclosure. Embodiments
of the
present disclosure are not limited by theoretical statements (if any) recited
herein. The
individual steps of embodiments of the present disclosure need not be
performed in the
disclosed manner, or combined in the disclosed sequences, but may be performed
in any and
all manner and/or combined in any and all sequences. The individual components
of
embodiments of the present disclosure need not be combined in the disclosed
configurations,
but could be combined in any and all configurations.
Various substitutions, modifications, additions and/or rearrangements of the
features of
embodiments of the present disclosure may be made without deviating from the
scope of the
underlying inventive concept. All the disclosed elements and features of each
disclosed
embodiment can be combined with, or substituted for, the disclosed elements
and features of
every other disclosed embodiment except where such elements or features are
mutually
exclusive. The scope of the underlying inventive concept as defined by the
appended claims
and their equivalents cover all such substitutions, modifications, additions
and/or
rearrangements.
The appended claims are not to be interpreted as including means-plus-function
limitations,
unless such a limitation is explicitly recited in a given claim using the
phrase(s) "means for"
or "mechanism for" or "step for". Sub-generic embodiments of this disclosure
are delineated
by the appended independent claims and their equivalents. Specific embodiments
of this
disclosure are differentiated by the appended dependent claims and their
equivalents.
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