       NOTE: This disposition is nonprecedential.


  United States Court of Appeals
      for the Federal Circuit
                 ______________________

     UNITED ACCESS TECHNOLOGIES, LLC,
              Plaintiff-Appellant

                            v.

    AT&T CORP., AT&T SERVICES, INC., SBC
   INTERNET SERVICES, INC., CENTURYTEL
     BROADBAND SERVICES LLC, QWEST
 CORPORATION, FRONTIER COMMUNICATIONS
               CORPORATION,
          Defendants-Cross-Appellants
            ______________________

2017-2614, 2017-2615, 2017-2616, 2018-1030, 2018-1031,
                      2018-1032
               ______________________

    Appeals from the United States District Court for the
District of Delaware in Nos. 1:11-cv-00338-LPS, 1:11-cv-
00339-LPS, 1:11-cv-00341-LPS, Chief Judge Leonard P.
Stark.
                 ______________________

               Decided: January 24, 2019
                ______________________

    ANTHONY MATTHEW GARZA, Charhon Callahan Robson
& Garza, P.C., Dallas, TX, argued for plaintiff-appellant.
Also represented by STEVEN CHASE CALLAHAN.
2            UNITED ACCESS TECHNOLOGIES LLC v. AT&T CORP.




    MICHAEL HAWES, Baker Botts, LLP, Houston, TX,
argued for all defendants-cross-appellants. Defendants-
cross-appellants AT&T Corp., AT&T Services, Inc., SBC
Internet Services, Inc. also represented by ELIZABETH
KATHLEEN BOGGS, BRYANT C. BOREN, JR., JASON R.
GERMAN, JON SWENSON, Palo Alto, CA.

    MATTHEW CHRISTOPHER GAUDET, Duane Morris LLP,
Atlanta, GA, for defendants-cross-appellants CenturyTel
Broadband Services LLC, Qwest Corporation. Also repre-
sented by ALISON HADDOCK HUTTON.

    TIMOTHY R. SHANNON, Verrill Dana LLP, Portland,
ME, for defendant-cross-appellant Frontier Communica-
tions Corporation. Also represented by SETH COBURN,
TAYLOR ROSE NEFF.
                ______________________

    Before LOURIE, BRYSON, and DYK, Circuit Judges.
BRYSON, Circuit Judge.
    Appellant United Access Technologies, LLC (“UAT”)
brought this patent infringement case against a number
of telecommunications companies.        UAT accused the
defendants of infringing U.S. Patent Nos. 5,844,596 (“the
’596 patent”), 6,243,446 (“the ’446 patent”), and 6,542,585
(“the ’585 patent”). In particular, UAT asserted claim 61
of the ’596 patent, claims 1-5 of the ’446 patent, and
claims 1, 2, 4, 8, and 9 of the ’585 patent. 1 The defend-
ants filed a joint motion for summary judgment of nonin-
fringement, which the district court granted. For the
reasons discussed below, we reverse the grant of sum-



    1   The patents-in-suit share a common specification.
References to the common specification will be to the ’596
patent.
UNITED ACCESS TECHNOLOGIES LLC v. AT&T CORP.                  3



mary judgment in part, based on an error in the district
court’s claim construction.
                               I
                     A. The Patents-in-Suit
   The technology at issue in this case involves a way of
simultaneously transmitting data and voice signals over a
single telephone line without causing interference. At the
time of the invention, providing data signals such as cable
TV transmissions to residences typically required running
a coaxial cable from the main cable trunk to each sub-
scriber. In addition, a separate segment of coaxial cable
had to be installed for every extra TV hookup within the
subscriber’s residence. Those installation requirements
were costly, as coaxial cabling was typically not included
in homes and apartment buildings at that time. See ’596
patent, col. 2, ll. 15-23; id. at col. 3, ll. 46-47.
    The inventor’s solution was to use pre-existing tele-
phone lines as the means for transmitting data from the
main cable trunk to individual subscribers. In order to
transfer data signals from the main cable trunk line to
the pre-existing telephone lines, the invention employed a
“transceiver/switch,” which the patents refer to as a
“signal interface.” 2 Id. at col. 8, ll. 9-10. The specification
explains that the signal interface “receives the [data]
signal from the source, and transmits the received [data]
signal onto at least one of the telephone lines in a selected
frequency range that is different from frequencies at
which the voice signals are carried on that telephone
line.” Id. at col. 3, line 65, through col. 4, line 2. Those


    2    The written description nowhere uses the term
“signal interface”; the term first appears in the claim
language. However, both parties agree that the “trans-
ceiver/switch” that is discussed in the written description
is the “signal interface” recited in the claims.
4            UNITED ACCESS TECHNOLOGIES LLC v. AT&T CORP.




telephone lines, which are referred to throughout the
patents-in-suit as “extended pairs,” carry both data and
voice signals from the signal interface to local networks.
See id. at Fig. 1a (referring to the telephone wiring be-
tween the signal interface and the local network interfac-
es as “extended pairs”). The patents define local networks
as the telephone wiring internal to houses, apartment
units, or rooms in commercial buildings. Id. at col. 1, ll.
32-34.
    Independent claim 61 of the ’596 patent is generally
representative of the asserted claims. It provides as
follows:
        A system for communicating information be-
    tween an external source of information and a
    plurality of destinations of information over a tel-
    ephone wiring network used for passing telephone
    signals in a telephone voice band between a plu-
    rality of telephone devices and a telephone ex-
    change, comprising:
       a plurality of transceivers coupled between
        the telephone wiring network and corre-
        sponding destinations of information, each
        including
             circuitry for accepting signals in a high
               frequency band of frequencies above
               the highest frequency of the telephone
               voice band and rejecting signals in the
               telephone voice band; and
       a signal interface coupled between the exter-
         nal source of information and the telephone
         wiring network, including
             circuitry for receiving a plurality of ex-
               ternal signals encoding a plurality of
               information streams from the external
               source of information, and
UNITED ACCESS TECHNOLOGIES LLC v. AT&T CORP.               5



             circuitry for transmitting to selected sets
               of one or more of the plurality of trans-
               ceivers a corresponding plurality of in-
               ternal signals in the high frequency
               band each encoding one of the plurality
               of information streams over the tele-
               phone wiring network;
        wherein the telephone wiring network in-
         cludes a branch network which couples one
         of the plurality of telephone devices to the
         telephone exchange telephone exchange
         [sic], and the branch network includes cir-
         cuitry for preventing transmission of signals
         in the high frequency band to the one of the
         telephone devices on the branch network.
    Independent claim 1 of the ’446 patent and independ-
ent claim 1 of the ’585 patent are generally similar, alt-
hough they use slightly different language to describe the
location of the signal interface. Claim 1 of the ’446 patent
recites “a signal interface coupled between the external
source of information and said conductive path,” and
claim 1 of the ’585 patent recites “a signal interface locat-
ed on the telephone wiring network between the tele-
phone exchange and each of the residences.”
                 B. The Accused Products
    UAT alleges that the defendants have infringed the
patents-in-suit by providing ADSL (asymmetric digital
subscriber line) service to their customers. UAT asserts
that each ADSL system meets the “signal interface” claim
limitation by the use of a type of DSLAM (digital sub-
scriber line access multiplexer).
    The defendants implement ADSL in one of two ways:
through a central-office embodiment and a remote-
terminal embodiment.    Both embodiments feature a
DSLAM that transmits data signals onto telephone lines
6               UNITED ACCESS TECHNOLOGIES LLC v. AT&T CORP.




at a frequency range higher than the frequencies at which
voice signals are carried on those lines. The embodiments
differ from one another based on the location of the
DSLAM.
    In the central-office embodiment, the DSLAM is lo-
cated inside one of the defendants’ switching centers. At
the switching center, voice and data signals for multiple
subscribers are fed into the DSLAM, which directs the
signals onto multiple sets of “extended pairs” of telephone
wires that serve individual subscribers. In the remote-
terminal embodiment, the DSLAM is located at a point on
the telephone wiring network downstream of, rather than
inside, the defendants’ switching center. 3
   Downstream of the DSLAM, the defendants’ remote-
terminal embodiments are identical to the central-office
embodiments. From that point, telephone wires carrying
both voice and data signals, known as “extended pairs,”
are bundled into a common cable and distributed to
serving terminals. Those serving terminals separate the
extended pairs of wires into lines running to individual
residences.
        C. Prior Litigation Involving the Patents-in-Suit
    Prior to this litigation, Inline Connection Corporation,
a previous owner of the patents, accused Earthlink, Inc.,
and America Online, Inc., of infringement. See Inline
Connection Corp. v. AOL Time Warner Inc., 364 F. Supp.
2d 417 (D. Del. 2005). Like the defendants in this case,
Earthlink and AOL provided ADSL service to their cus-
tomers. Id. at 422. As here, Earthlink and AOL imple-
mented ADSL through both a central-office embodiment
and a remote-terminal embodiment. The Inline court


    3   As used here, downstream means closer to the lo-
cal networks; upstream means closer to the switching
centers.
UNITED ACCESS TECHNOLOGIES LLC v. AT&T CORP.                  7



construed the term “signal interface” to mean “[a] device
interposed on the opposite end (i.e., the local side) of the
public trunk line (as defined by the inventor in the pa-
tent) from the telephone exchange that performs the
recited functions of the incorporated circuitry.” Id. at 427.
Subsequently, in considering cross-motions for summary
judgment, the court granted summary judgment of non-
infringement with regard to the central-office embodiment
because that embodiment did not satisfy the “signal
interface” claim limitation. See id. at 436. However, the
court denied summary judgment with regard to the
remote-terminal embodiment. See id. at 436–48. Follow-
ing a trial, the jury found no infringement with respect to
the remote-terminal embodiments.
     On appeal, UAT challenged the Inline court’s con-
struction of “signal interface” and sought reversal of the
Inline court’s summary judgment order as to the central-
office embodiment. Earthlink argued for affirmance on
two independent grounds: (1) that the district court’s
claim construction of “signal interface” was correct, and
(2) that the jury’s verdict on the remote- terminal embod-
iment showed that the court’s construction of signal
interface “had no bearing on the jury’s decision that
Plaintiff failed to present any credible evidence of in-
fringement.” This court summarily affirmed the district
court’s judgment. United Access Techs., LLC. v. Earth-
link, Inc., 432 F. App’x 976, 977 (Fed. Cir. 2011).
           D. Proceedings in the District Court
     In the claim construction order in the present case,
the district court construed two terms relevant to this
appeal. United Access Techs., LLC v. Centurytel Broad-
band Servs., LLC, No. CV 11-338-LPS, 2016 WL 6562059
(D. Del. Nov. 4, 2016). First, the district court construed
“signal interface,” as it had been construed in the Inline
case, to mean “a device interposed on the opposite end
(i.e., the local side) of the public trunk line (i.e., the tele-
8             UNITED ACCESS TECHNOLOGIES LLC v. AT&T CORP.




phone lines comprising the public telephone network)
from the telephone exchange that performs the recited
functions of the incorporated circuitry.” Second, the court
construed the term “high frequency band,” as used in each
of the asserted claims, to refer to “frequencies above the
telephone voice band between 0.25 MHz [megahertz] and
an undetermined upper limit.”
    In light of its claim constructions, the district court al-
lowed the defendants to file an early motion for summary
judgment of non-infringement. The court then granted
summary judgment on all of UAT’s claims. United Access
Techs., LLC v. AT&T Corp., 265 F. Supp. 3d 446 (D. Del.
2017). The court determined that the defendants’ systems
do not infringe under the court’s claim construction be-
cause in both embodiments the defendants’ DSLAMs are
located on the public telephone network, not on the local
wiring downstream from the public telephone network.
The court explained that the telephone company owns the
extended pairs that run between the remote-terminal
signal interface and the network interface device that sits
between the extended pairs and the local networks. See
’596 patent, col. 9, ll. 12-15. The fact that telephone
companies own the extended pairs, the court concluded,
shows that those wires, like the wires upstream of the
signal interface, are part of the public telephone network.
Because the telephone wires both upstream and down-
stream of the DSLAMs are part of the public telephone
network, as the court applied that term, the court held
that neither the central-office nor the remote-terminal
embodiments infringe the asserted patents.
                              II
    On appeal, UAT challenges the district court’s con-
struction of the term “signal interface.” UAT advocates a
broad definition of “signal interface.” It contends that the
district court should not have defined the term with a
positional restriction—requiring the signal interface to be
UNITED ACCESS TECHNOLOGIES LLC v. AT&T CORP.               9



located on the local network side of the public trunk line—
or an ownership restriction—defining the boundary of the
local network and the public telephone network by the
telephone companies’ ownership of the lines. In their
cross-appeal, the defendants argue that the term “high
frequency,” which is used in all of the asserted claims, is
indefinite and that all of the claims are therefore invalid.
                    A. Signal Interface
    UAT argues that the term “signal interface” means “a
device interposed at a point of convergence [such as the
point of convergence of the extended pairs from local
networks] that is downstream from the telephone ex-
change and performs the recited functions of the incorpo-
rated circuitry.” The defendants argue that the district
court properly construed the term to mean “a device
interposed on the opposite end (i.e., the local side) of the
public trunk line (i.e., the telephone lines comprising the
public telephone network) from the telephone exchange
that performs the recited functions of the incorporated
circuitry.”
    In support of its construction of “signal interface,”
UAT first argues that the term has a plain and ordinary
meaning, which is the meaning given to the term “inter-
face” in computer science: a common boundary between
two systems, devices, or programs. But neither the speci-
fication nor any computer science dictionary to which
UAT has directed us contains a definition of “signal
interface.” And, as noted by the district court, UAT has
not explained why a person of ordinary skill in the art
would interpret “signal interface,” as that term is used in
the patents, to mean simply an interface.
    UAT next points out that the asserted claims contain
language identifying the location of the signal interface.
See, e.g., ’596 patent, claim 61 (“a signal interface coupled
between the external source of information and the tele-
phone wiring network”); ’446 patent, claim 1 (“a signal
10             UNITED ACCESS TECHNOLOGIES LLC v. AT&T CORP.




interface coupled between the external source of infor-
mation and said conductive path”); ’585 patent, claim 1 (“a
signal interface located on the telephone wiring network
between the telephone exchange and each of the residenc-
es”).
     That language, according to UAT, provides a complete
description of the location of the signal interface. But the
specification refines the claim language, making it clear
that the signal interface must be located where the public
trunk line and the lines from the local networks converge.
See ’596 patent, col. 1, line 62, through col. 2, line 3 (“The
present invention [provides] distribution [of telephonic
and non-telephonic signals] from a distribution device
[i.e., the signal interface] that connects to the trunk line of
a public or private telephone network. That device is
located where the telephone lines for multiple local net-
works converge to meet the public network trunk (or PBX,
in the case of office buildings) . . . .”); id. at col. 4, ll. 53-55
(“The interface is coupled between the telephone lines and
corresponding public telephone lines (which carry voice
signals at voiceband frequencies) that serve the residenc-
es.”). 4 Those references in the specification clarify where
the signal interface must be located. See GPNE Corp. v.
Apple Inc., 830 F.3d 1365, 1370 (Fed. Cir. 2016) (“[W]hen
a patent ‘repeatedly and consistently’ characterizes a
claim term in a particular way, it is proper to construe the
claim term in accordance with that characterization.”).




     4  For the private branch exchange (PBX) embodi-
ment, the PBX and the signal interface are both located
between the public trunk line and the local networks.
Because the signal interface is located in the same wiring
closet as the PBX, the signal interface is effectively locat-
ed where the local networks converge to meet the public
trunk line. See ’596 patent, Fig. 1b.
UNITED ACCESS TECHNOLOGIES LLC v. AT&T CORP.             11



    UAT contends that the term “signal interface” does
not provide a textual “hook” in the claim language for the
district court’s positional limitations. See NTP, Inc. v.
Research In Motion, Ltd., 418 F.3d 1282, 1310 (Fed. Cir.
2005). But as the NTP court recognized, a textual hook
arises when a “claim term [] is susceptible of clarification
by the written description,” as is true in this case. 5 Id.
We therefore reject UAT’s broad construction of the term
“signal interface” in favor of the defendants’ construction
of the term as limited to a device located on the local side
of the public trunk line.
    While we reject UAT’s principal claim construction
argument, we agree with UAT in one respect—that the
district court improperly imported an ownership limita-
tion in the course of its construction of the term “signal
interface,” and in particular in determining how the
patents define the required location of the signal inter-



   5    In its reply brief, UAT proposes a different con-
struction, arguing that the signal interface need only be
located where the wiring leading to multiple local net-
works converges, without reference to its position vis-à-vis
the public trunk line. For support, UAT cites Figure 16 of
the ’596 patent, arguing that the figure depicts a “signal
interface” sitting entirely off the telephone network. That
argument, however, has been waived because UAT failed
to include the argument in its opening brief. See, e.g.,
Aventis Pharma S.A. v. Hospira, Inc., 675 F.3d 1324,
1332–33 (Fed. Cir. 2012). In any event, all the claims in
the asserted patents that use the term “signal interface”
are addressed to a system on a telephone wiring network.
Therefore, as noted by the defendants at oral argument
and acknowledged by UAT in argument before the district
court, Figure 16 does not depict a telephone network and
is not an embodiment covered by any of the asserted
claims.
12           UNITED ACCESS TECHNOLOGIES LLC v. AT&T CORP.




face. The district court’s construction of “signal interface”
included the construction of a second term, “public trunk
line.” The district court construed the term “public trunk
line” to mean “the telephone lines comprising the public
telephone network.” Then, in the course of its summary
judgment opinion, the court further construed the term
“public telephone network,” as used in the patents, to
mean “the telephone lines and other equipment owned by
the telephone company.”
    Based on its iterative claim construction, the court
found that the defendants’ DSLAMs are located within,
rather than downstream of, the public telephone network.
The court reached that conclusion because the evidence
showed that in all of the defendants’ systems, the tele-
phone companies own telephone lines both upstream and
downstream of the DSLAMs, including the extended pairs
that connect the DSLAMs with the local network interfac-
es.
    UAT contends that the district court’s adoption of an
ownership requirement as part of its claim construction
was error. That error, according to UAT, requires that we
reverse the judgment against UAT with respect to the
remote-terminal embodiments. 6
     The problem is that the patents-in-suit do not use
ownership as the basis for determining the limits of the
public telephone network. The patents are silent with
regard to who owns the wires that make up the public
telephone network and the wires that lead from the signal
interface to the individual residences (i.e., the extended
pairs). The extended pairs can be quite lengthy and can
extend well beyond the limits of the individual subscrib-
er’s property, a fact that strongly suggests the patents do



     6 UAT does not contest the court’s summary judg-
ment as to the central-office embodiments on this ground.
UNITED ACCESS TECHNOLOGIES LLC v. AT&T CORP.             13



not contemplate that the extended pairs are privately
owned. For example, the specification states:
       There can be a large variation in the lengths
   of extended pairs 405. In an apartment building,
   the telephone wires serving different units may
   converge at a point 100 feet or less from each
   apartment unit. An example of the other extreme
   occurs when distributing signals to separate
   houses in a neighborhood. In this case, connecting
   ten houses to the [sic] a single transceiver/switch
   400 may mean that some of extended pairs 405
   will be longer than, perhaps, 1000 feet.
’596 patent, col. 14, ll. 43-50. In characterizing the pa-
tents’ description of the wiring on the local side of the
signal interfaces, the defendants repeatedly insert the
word “private” along with the word “local.” The patents,
however, provide no support for that characterization. 7 In
sum, the defendants have not convinced us that the
patents contemplate that the extended pairs, which are
located on the local side of the public trunk line and can
extend more than 1000 feet from individual houses, are
invariably owned by residents rather than by the tele-
phone company.
    The defendants’ challenge to literal infringement is
based on evidence that in their systems the telephone
companies own the extended pairs located between the
side of a house or building and the signal interface. In the
absence of any intrinsic evidence to the contrary, there is
no reason to construe the asserted claims in the opposite
fashion, to read only on systems—if any such exist—in



   7    Although the defendants refer to the extended
pairs in the patents as part of the “private, local net-
works,” the specification never uses the term “private”
except in connection with private branch exchanges.
14            UNITED ACCESS TECHNOLOGIES LLC v. AT&T CORP.




which the telephone lines leading from individual resi-
dences to the telephone company’s remote terminal are
owned by individual subscribers.
     Instead, the patents make clear that the terms “public
telephone network” and “public trunk line” refer to a
segment of the telephone wiring network located between
the local exchange and a point of convergence, where the
pre-existing interface was located. See ’596 patent, col. 8,
ll. 10-13 (“This device [i.e., the signal interface] replaces
the existing interface between the public telephone net-
work, i.e., an ordinary telephone trunk line) and the
telephone lines that lead to the individual residences.”);
id. at col. 8, ll. 15-18 (“Typically, the existing interface
will be a simple ‘punch-down’ panel that provides elec-
tronic connections between the extended pairs and the
pairs that are part of the trunk line.”); id. at col. 9, ll. 23-
25 (“Normal telephone communication on all local net-
works and between the local networks and the public
network (trunk) is preserved”); id. at col. 11, ll. 34-35
(“When transceiver/switch 400 is installed, extended pairs
405 are broken near the point of convergence”); see also
Inline Connection Corp. v. AOL Time Warner Inc., 364 F.
Supp. 2d 417, 428–36 (D. Del. 2005). The proper con-
struction does not turn on whether the telephone compa-
ny or the individual residents own the particular
equipment.
     In sum, we agree with the district court’s construction
of “signal interface” in the court’s claim construction
opinion, which defined “signal interface” as “a device
interposed on the opposite end (i.e., the local side) of the
public trunk line (i.e., the telephone lines comprising the
public telephone network) from the telephone exchange
that performs the recited functions of the incorporated
circuitry.” But we disagree with the district court’s fur-
ther construction of the term “public telephone network,”
in its summary judgment opinion, to mean “the telephone
UNITED ACCESS TECHNOLOGIES LLC v. AT&T CORP.            15



lines and other equipment owned by the telephone com-
pany.”
    There is no support for the district court’s conclusion
that the telephone company’s ownership of the extended
pairs between the network interface device (i.e., the local
network interface) and the signal interface makes those
lines part of the public telephone network, as that term is
used in the patents. We therefore set aside the district
court’s summary judgment of non-infringement as to the
remote-terminal embodiments.
                   B. High Frequency
    In their cross-appeal, the defendants argue that the
term “high frequency” is indefinite and that the asserted
claims are therefore invalid for indefiniteness. Claims are
invalid for indefiniteness if, when viewed in light of the
specification and the prosecution history, they “fail to
inform, with reasonably certainty, those skilled in the art
about the scope of the invention.” Nautilus, Inc. v. Biosig
Instruments, Inc., 572 U.S. 898, 901 (2014). “‘Reasonable
certainty’ does not require ‘absolute or mathematical
precision.’” BASF Corp. v. Johnson Matthey Inc., 875
F.3d 1360, 1365 (Fed. Cir. 2017) (quoting Biosig Instru-
ments, Inc. v. Nautilus, Inc., 783 F.3d 1374, 1381 (Fed.
Cir. 2015)). The defendants had the burden of proving
indefiniteness by clear and convincing evidence. BASF
Corp., 875 F.3d at 1365.
    The parties first argue about the standard of review.
We review “factual findings about extrinsic evidence
relevant to the [indefiniteness] question, such as evidence
about knowledge of those skilled in the art . . . for clear
error.” BASF Corp., 875 F.3d at 1365. To trigger clear
error review, however, “it is not enough that the district
court may have heard extrinsic evidence during a claim
construction proceeding—rather, the district court must
have actually made a factual finding.” Sonix Tech. Co. v.
Publications Int'l, Ltd., 844 F.3d 1370, 1376 (Fed. Cir.
16           UNITED ACCESS TECHNOLOGIES LLC v. AT&T CORP.




2017) (quoting CardSoft, LLC v. VeriFone, Inc., 807 F.3d
1346, 1350 (Fed Cir. 2015)). “A party cannot transform
into a factual matter the internal coherence and context
assessment of the patent simply by having an expert offer
an opinion on it. The internal coherence and context
assessment of the patent, and whether it conveys claim
meaning with reasonable certainty, are questions of law.”
Teva Pharm. USA, Inc. v. Sandoz, Inc., 789 F.3d 1335,
1342 (Fed. Cir. 2015); see also Sonix, 844 F.3d at 1376.
    According to the defendants, the district court relied
exclusively on intrinsic evidence to reach its conclusion
that the claims were not indefinite, and the court’s con-
clusion is therefore legal in nature. UAT disagrees,
arguing that the district court made a specific factual
finding (supported by intrinsic and extrinsic evidence) as
to how one of skill in the art would consider the defend-
ants’ cited portions of the specification.
    The district court did not explicitly state that it based
any of its findings on extrinsic evidence. However, the
court’s indefiniteness ruling substantially aligns with the
testimony of UAT’s expert, Dr. Krista Jacobsen, regarding
the interpretation that one of ordinary skill would give to
the term “high frequency.” The court’s determination
thus may fairly be characterized as a factual finding
invoking clear error review. Even under de novo review,
however, we find that the defendants have failed to show
that the term “high frequency,” as used in the patents, is
indefinite.
     Based on the claim language and the specification, the
district court concluded that the bottom end of the range
of frequencies comprising the “high frequency band” must
be higher than the top end of the “voice band,” i.e., the
range of frequencies customarily used to carry telephone
signals. Looking to the specification, the district court
concluded that the minimum frequency at which the high-
frequency signal is transmitted is 0.25 MHz.
UNITED ACCESS TECHNOLOGIES LLC v. AT&T CORP.            17



    The specification makes clear that the lower limit of
the term “high frequency” is above the voice band; the
specification identifies the frequency level for the voice
band as going up to .02 MHz and the frequency level for
the transmission of data, including control signals, as
being centered between 0.5 MHz and 20 MHz. See ’596
patent, Figs. 3a, 3b, and 3c. Dr. Jacobsen explained that
the frequency band for a control signal centered at 0.5
MHz could range from 0.25 MHz to 0.75 MHz. The speci-
fication and the extrinsic evidence thus support the
district court’s choice of 0.25 MHz as a reasonable approx-
imation of the lower range of the high frequency band as
that term is used in the patents.
    The defendants argue that 0.25 MHz cannot be the
lower limit of the high frequency range because the speci-
fication contains references to high frequency that are
inconsistent with that level. The district court rejected
that argument on the ground that the specification’s
references to particular frequencies as “high” or “low”
were used “in a relative sense, characterizing a frequency
as ‘low’ or ‘high’ in the context of the preferred embodi-
ment being described, in the context of the technology at
issue, or in comparison to other preferred embodiments.”
Accordingly, the court found that a person of ordinary
skill in the art would understand that the references to
“low” or “high” were “not meant to impose numerical
limitations on the claims, but rather to distinguish and
explain different preferred embodiments and describe the
underlying science.” As such, the court concluded, the
defendants failed to show by clear and convincing evi-
dence that the references to “high” and “low” frequencies
rendered the meaning of the “high frequency” claim terms
uncertain to a person of ordinary skill in the art.
    More generally, the defendants argue that “high fre-
quency” is not synonymous with “above the voice band.”
For support, the defendants point to the difference among
three claims of the ’596 patent, each of which includes a
18            UNITED ACCESS TECHNOLOGIES LLC v. AT&T CORP.




reference to frequency range. Claim 1 refers to “a fre-
quency range that is different from frequencies at which
said voice signals are carried on.” Claim 11 refers to a
frequency range that “exceeds said voiceband frequen-
cies.” And claim 61 refers to “a high frequency band of
frequencies above the highest frequency of the telephone
voice band.” The defendants argue that because the
claims employ different language to describe the frequen-
cy range used in the invention, the term “high frequency
band” must mean something different than simply “ex-
ceeds said voiceband frequency.”
    That argument is not persuasive. The characteriza-
tions of the frequency range in claims 1, 11, and 61 all
contemplate frequency ranges above the voice band,
either explicitly or by context. Moreover, the specification
confirms that understanding of the term. See ’596 patent,
col. 53, line 61 (“high-frequency (i.e., non-voiceband)
signals”); id. at col. 48, ll. 31-32 (“low pass filter 474b
passes all voiceband energy”); id. at col. 48, ll. 37-39 (“Low
pass filters 474 block transmission of the high frequency
signals transmitting through signal separators 413 be-
tween processor 418 and the local network interfaces
411.”).
    As for the upper limit of “high frequency,” the specifi-
cation explains that “attenuation, radiation, crosstalk
interference and reception of external interference all
increase as frequency increases.” Id. at col. 19, ll. 14-16.
Therefore, the specification recommends that the first
channel be “placed as close to the voiceband as feasible.”
Id. at col. 19, ll. 20-22. Yet, depending on the particular
embodiment of the invention, the upper limit of the high
frequency signals remains variable.
    The fact that the upper limit of the term “high fre-
quency” is not defined, however, does not render the term
indefinite, at least where the purpose of the limitation is
to distinguish between frequencies in a lower range and
UNITED ACCESS TECHNOLOGIES LLC v. AT&T CORP.              19



those in a higher range. See Halliburton Energy Servs.,
Inc. v. M–1 LLC, 514 F.3d 1244, 1253 n.5 (Fed. Cir. 2008)
(“Of course, a claim may contain a limitation that includes
no explicit upper bound at all (e.g., a claim limitation that
requires ‘at least 5%’ of an element). Since such a limita-
tion does not contemplate an upper bound beyond what is
practically required (e.g., the total percentage must be
less than 100%), the limitation may not present definite-
ness concerns.”). The intrinsic evidence defines the lower
limit of the high frequency band and thus makes the
meaning of the term reasonably clear; the fact that the
claim language, at least in theory, covers any frequency
higher than that does not render the claim language
indefinite, even though the use of very high frequencies
would be impractical. We therefore uphold the district
court’s conclusion that the defendants failed to show by
clear and convincing evidence that the asserted claims are
indefinite.
    Finally, based on the prosecution history, the defend-
ants argue that “high frequency band” must have a lower
limit above 0.5 MHz, rather than above the 0.25 MHz
level chosen by the district court. The defendants point to
certain prior art references that the applicant distin-
guished during prosecution. The effect of the applicant’s
discussion of those references, according to the defend-
ants, was to disclaim frequencies below 0.5 MHz. That
argument, however, does not go to indefiniteness; it goes
to a claim construction issue that is not on appeal. If a
disclaimer is ultimately found, the consequence may be to
restrict the lower limit of “high frequency” covered by the
patents-in-suit, but it would not affect the district court’s
analysis of the indefiniteness issue.
                             III
    We affirm the district court’s determination that the
asserted claims are not indefinite, and we affirm the
court’s grant of summary judgment of noninfringement as
20            UNITED ACCESS TECHNOLOGIES LLC v. AT&T CORP.




to the central-office embodiments. We reverse the court’s
grant of summary judgment of noninfringement as to the
remote-terminal embodiments and remand for further
proceedings consistent with this opinion.
     Each party shall bear its own costs for these appeals.
     AFFIRMED IN PART, REVERSED IN PART,
               AND REMANDED
