Note: Descriptions are shown in the official language in which they were submitted.
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1NCREAS1NG TRAFFIC CAPACITY IN A CELLULAR COiVIiVIUNICATIONS
SYSTEM BY CHANGE OF TRAFFIC CHANNEL RATE
FIELD OF THE INVENTION
'The present invention relates to increasing traffic capacity in a cellular
radio
communications system, and more particularly, to increasing traffic capacity
by
changing a requested or initially selected traffic channel rate to a lower bit
rate when
extra traffic capacity is needed.
BACKGROUND AND SUMMARY OF THE INVENTION
In the Global System for Mobile communications (GSM), two bit rates for
to speech coders are defined: full rate and half rate. Full rate corresponds
to a bit rate of
13 kbits/s. and half rate corresponds to a bit rate of 6.5 kbits/s. A traffic
channel in the
GSM system can support one full rate speech call or two half rate speech
calls. Speech
ceding significantly lowers the bit rate over the radio interface (higher bit
rates take up
too much of the frequency spectrum) while still providing acceptable speech
quality. In
general, speech coding sends information about the speech (rather than the
speech
itsely from which the speech signal can be reconstructed at the receiver.
The GSM also employs time division multiple access (TDMA) with each radio
fi-cquency carrier being divided into eight time slots (TS). One time slot in
a TDMA
ii-ame is called a physical channel, and each duplex pair of frequency
carriers includes
Zo eight physical channels. Speech and other information are sent on logical
traffic
channels ('fCI-I) mapped onto the physical time slot channels. In full rate
traffic
channels. a user is assigned a single physical channel/time slot. For half
rate traffic
channels. two mobile stations share the same physical channel/time slot with
each
mobile station alternating use of the allocated time slot.
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Mobile telephone system operators live in a highly competitive world. To be
successful. a mobile telephone system operator must deliver:
~ Coverage -- a call can be set up anywhere
~ Capacity -- a call can be set up anytime
s ~ Quality -- the call is clear, undisturbed, and uninterrupted.
The present invention addresses capacity.
'l~he traditional approach to increasing capacity is to equip base stations
with as
many transceivers as possible and to invest in high capacity base station
controllers.
While this is a sound strategy in densely populated areas with evenly
distributed traffic
~u demands over time, in areas where the traffic demand is high only during
short periods.
e.g., certain streets during morning and evening rush hour, that strategy is
less sound.
'I~he more difficult issue is how to satisfy temporary high traffic demands
effectively
and ccunomically.
'Thus, it is a primary object of the invention to dynamically increase system
is capacity to meet temporary high traffic demands in an effective and
economic fashion.
It is a further object of the invention to provide enhanced ability to provide
service for connections requiring a full rate traffic channel.
Some percentage of mobile stations currently (and with increasing numbers in
the future) are capable of transceiving at different bit rates. For purposes
of explaining
2o the present invention, (and not in any way limiting the present invention),
some mobile
stations are assumed to be capable of transceiving on both full rate and half
rate
channels. Such mobile stations are referred to as having "dual rate"
capability. Dual
rate mobiles indicate their dual rate capability to the cellular network via
an initial
channel request message or possibly in periodic registration messages. Higher
rate
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-,
channels may be used in favorable traffic conditions but half rate channels
are
employed during temporary high traffic conditions.
'1-he present invention increases the capacity in a cellular radio
communications
system in accordance with the following method. Initially, a higher rate
traffic channel
s is established for communication with a mobile station located in a
particular cell area.
~'he current traffic load at that cell area is determined. If the determined
tral~lic load
exceeds a threshold, the higher rate traffic channel over which a dual rate
mobile station
is communicating is handed over to another lower rate traffic channel
available in that
cell area. For currently active traffic channels, a list is maintained for
those dual rate
ie~ mobile stations currently assigned to higher rate traffic channels.
Before making the handover from the higher rate traffic channel to the lower
rate
traffic channel, it is determined whether such a handover is permitted. If so,
the intra-
cell handover is preferably made to a traffic channel which is currently
already
Supporting another lower rate call. Otherwise, any available lower rate
traffic channel
t5 is assigned.
At call setup, if the current traffic load in the cell area exceeds the
threshold, a
call request involving a dual rate mobile is assigned a lower rate traff is
channel. 1 I~ tim
traffic load in the cell area decreases, a call initially setup or
subsequently handed over
to a lower rate channel because of high traffic load may be handed over to a
higher rate
channel.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, Features, and advantages of the invention
will
be apparent from the following description of preferred embodiments as
illustrated in
the accompanying drawings in which reference characters refer to the same
parts
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4
throughout the various views. The drawings are net necessarily to scale with
emphasis
bcin' placed upon illustrating the principles of the invention.
Hig. 1 is a flowchart illustrating a general "change channel rate" procedure
in
accordance with a first example embodiment of the present invention:
s Fig. 2 is a function block diagram of a GSM-type mobile radio communications
system in which the present invention may be incorporated in accordance with a
second
example embodiment of the present invention;
Fig. 3 is a diagram conceptually depicting a traffic channel supporting a
single
full rate channel or two half rate channels;
io Fig. 4 are more detailed function block diagrams of specific blocks shown
in
Fig. 2: and
Figs. ~A and ~B are t7owcharts illustrating example procedures in accordance
with the second example embodiment of the present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
~s In the following description, for purposes of explanation and not
limitation.
specitic details are set forth, such as particular embodiments, hardware,
techniques, etc.
in order to provide a thorough understanding of the invention. However, it
will be
apparent to one skilled in the art that the present invention may be practiced
in other
embodiments that depart from these specific details. For example, while a
specific
~n embodiment of the present invention is described in the context of a GSM
cellular
telephone network, those skilled in the art will appreciate that the present
invention can
be implemented in any cellular telephone system. In other instances. detailed
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J
descriptions of well-known methods, interfaces, devices, and signaling
techniques are
omitted so as not to obscure the description of the present invention with
unnecessary
detai 1.
Because the application is applicable to any cellular telephone communications
a systems, a first example embodiment of the present invention is now
described in
general terms. The flowchart in Fig. 1 outlines the steps of a Change Channel
Rate
routine (block 10). Assuming a light or moderate traffic load initially.
traffic channels
are established for mobile calls in a particular cell, sector, or other
location area at a first
traffic rate (block 12) in accordance with normal channel request and
assignment
to procedures. The traffic load in the cell or location area is determined
(block 14). A
decision is made in block 16 whether the traffic load for that cell or
location area
exceeds a threshold. i.e., whether there has been temporary increase in
traffic demand.
If not, the mobile communications continue as set up (block 17). However, if
the traffic
load exceeds a threshold, a dual rate mobile communication being conducted
over a
is traffic channel at a first rate is handed over to another traffic channel
where the
communication is conducted a second lower rate (block 18). Moreover, new
incoming
calls for dual rate mobiles are set up as lower rate calls rather than higher
rate calls until
the traffic load decreases below the threshold. If the traffic load decreases
significantly,
calls may optionally be handed over from lower rate traffic channels to
available higher
2o rate traff is channels (block 17).
'1'Irus, the present invention provides a temporary increase of the traff is
capacity
of a cell by utilizing lower rate capabilities of some dual rate mobile
stations operating
in the cell. Because the communication connections are moved within the
serving cell,
i.c., infra-cell handof~(s. there is no increase in interference Icvcl that
might he
experienced if inter-cell handoffs to other cells were employed. Another
advantage of
the: present invention is that no additional hardware is required. Reserve but
often
underutilized base station transceivers are not needed to ensure sufficient
capacity for
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6
temporary high traffic demands. Yet another advantage of this invention is
that a
mobile communications network operator can control how the traffic demands of
each
cell are handled by changing the traffic load threshold to appropriate values.
An
appropriately-valued threshold ensures that only a minimum number oi~ mohile
stations
s having dual rate capability are affected by intra-cell handovers to lower
rate traffic
channels during high traffic conditions. The present invention also selects
the channel
rate for a particular traffic channel at call setup based on a current cell
traffic load level
so that during periods of high traffic, communications involving dual rate
mobile
stations are established using lower rates to further increase traffic
capacity.
A second example embodiment of the present invention will now be described as
applied to a GSM-based cellular communications system. However, those skilled
in the
art will appreciate that the present invention is in no way limited to GSi~t-
based cellular
communications systems.
Reference is made to Fig. ? which illustrates in function block format a
typical
~s GSM-based cellular communications system 30. A Gateway Mobile Switching
Center
(GMSC) 36 interfaces the cellular communications network with other
telecommunications networks including the Public Switched Telephone Network
(PSTN) 32, the Internet 34, other cellular networks, and other types of
communications
networks such as the Integrated Services Digital Network (ISDN). The Gateway
zo Mobile Switching Center 36 connects to one or more Mobile Switching Centers
(MSCs) 38. Each mobile switching center 38 is connected to one or more base
station
controllers (BSCs) 42. The BSC handles all radio related functions including
administration and remote control of the base stations 44 as well as the
handling of
connections to mobile stations 46 including handovers. Each BSC 42 is
connected to
?a plural base stations (BSs) 44 that communicate with mobile radio stations
(MS) 46.
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The aatewav MSC 36 is the interface point in the mobile radio network For
calls
to mobile subscribers. Although the GMSC 36 is shown as a separate node for
clarity
of illustration, it may be located with an MSC 38. Each mobile switching
center 38
pcriorms telephony switching functions associated with calls involving a
mobile station
a (MS) 46 including interfacing with other telecommunications networks 3? and
34 and
rc>utin'~ illObilc-originated calls.
Each MSC 38 is associated with a Visiting Location Register (VLR) 40 which
includes a database containing temporary subscriber information needed by its
associated MSC 3Ii to provide services to mobile stations in the MSC's service
area.
io 'l~ypically. when a mobile station enters a visiting network or service
area. it registers
with the VLR 40 which then requests and receives data about the roaming mobile
station from the mobile's Home Location Register (HLR) 41 and stores it. As a
result.
when the visiting mobile station is involved in a call, the VLR 40 already has
the
information needed for call setup. While the VLR 40 may be a stand-alone node,
it is
is preferably integrated with its associated MSC to eliminate signaling
between the two
nuclcs.
The home location register (HLR) 41 is a database node that stores and manages
subscriptions. For each "home" mobile subscriber, the HLR 41 contains
permanent
subscriber data such as the Mobile Station ISDN Number (MSISDN) which uniquely
2o identifies the mobile telephone subscription in a PSTN numbering plan, and
an
international mobile subscriber identity (IMSI), which is a unique identity
allocated to
cacti mobile subscriber and used for signaling for in the mobile network. All
network-
reiated subscriber information is connected to the IMSI. The HLR 41 contains a
list of
services which a mobile subscriber is authorized to use along with the current
subscriber location number cocTesponding to the address of the VLR 40
currently
servine the mobile subscriber.
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Once a serving MSCiVLR is identified (in the home or in a visiting network). a
call intended for the mobile station 46 is routed by that serving MSC to a
base
Stliti()Il =~4 aSSOClatcd with the cell in which the called mobile station is
currently
located. Using well-known. established protocols and procedures documented in
s various GSM standards, a call connection is established over the radio
interface
hctwecn the base station 44 and the mobile station 46.
During a call setup, a logical traffic channel is allocated to a radio
connection
between a base station and mobile station based on information about the
available
channel's characteristics. and in the present invention in particular, based
on the mobile
io stations capabilities. When the connection has been established over an
assigned
traffic channel, e.g., a TDMA time slot, signal strength and speech quality
are
monitored and forwarded to the BSC which may initiate a handover of the
connection
based on those reports.
When a call is initiated, it is the BSC that allocates an available traffic
channel
is (~~Ch1) to the mobile station. In current GSM systems, a traffic channel
(TCH) can
either support a single full rate communication such as shown at ~0 in Fig. 3
or two half
rate communications as indicated at 22 and 24 in Fig. 3. An example full rate
communication corresponds to a bit rate of l 3 kbits/s, and an example hal f
rate
communication corresponds to a bit rate of 6.5 kbits/s. It is understood that
reference to
2o a full rate traffic channel means a traffic channel currently supporting a
single lull rate
call. and reference to a half rate traffic channel means a traffic channel
currently
supporting one or two half rate calls.
More detailed block diagrams of the BSC 42, base station 44, and a dual rate
mobile station 46 are now described in conjunction with Fig. 4. For purposes
of~
illustration and description only, function blocks arc shown. However. those
Functions
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may be performed using any suitable electronic circuitry including DSP. AGIC,
suitably
programmed main processor, etc.
The traffic load in each base station's cell, sector, or location area is
monitored
by the BSC 42. However, this operation or function may be performed by other
entities
such as the MSC 38, each bases station. or other radio network control entity.
The
BSC 42 includes a traffic load detector 50 which, in this simplified example.
monitors
the traffic load in area X serviced by a base station 4=1. The BSC 42 performs
similar
tasks for each such area for each base station under its control. Traffic load
detector 50
includes a comparator 52 and a busy full rate traffic channel (TCH} counter
5~.
io Counter 54 is incremented and decremented to track a current total number
of busv
traffic channels for that call area "X". A traffic channel is considered busv
if it is
unavailable to support a full rate call, i.e.. the channel is partly or
completely allocated
for circuit-switched or packet-switched traffic. The output of counter 5~ is
input to
comparator 52 alon;= with a threshold value ~ T~ which may correspond to a
certain
~s number of traffic channels set by the mobile network operator. When the
output of the
counter exceeds the threshold, the comparator generates a traffic load signal
which may
be used to set a high load flag in controller 56 for cell area X. Preferably,
the threshold
value ~ T~ incorporates hysteresis to minimize undesired channel rate
switching. Thus,
controller 56 receives channel requests from various base stations 4=I and
issues channel
zo assignments for various mobile communications to the base stations.
Controller 56
accesses a memory 58 to store a list of dual rate capable mobile stations
currently
assigned to a full rate traffic channel. In executing various channel
assignments and
channel drops, controller 56 increments and decrements, respectively, the busy
full rate
TCH counter 54 for each base station area.
Each base station 4~1, such as the base station shown for cell area X,
includes
numerous transceivers used to establish and maintain various channels over
the; radio
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interface with mobile stations =~6 being serviced in area X. A simplified
example of a
base station transceiver shown in block 44 includes baseband processing
circuitry 60
switchablv connected via switch 62 to one of a full rate encoder/decoder 64
and a half
rate encoder/decoder 66 also switchablv connected at an opposite terminal via
switch 70
a to transceiving circuitry 72 and antenna 76. A base station controller 74
performs a
number of base station control operations including setting the position of
swiichcs 6?
and 70 depending on whether a particular communication is to be conducted over
a
traffic channel at full rate or half rate.
The dual rate mobile station 46 shown in Fig. 4 includes. among other
elements.
io baseband processing circuitry 80 switchably connected via switch 82 to one
of a full
rate encoder/decoder 84 and a half rate encoder/decoder 86 also switchably
connected at
their respective output terminals via switch 88 to transceiving circuitry 90
and
antenna 94. A dual rate mobile station controller 92 performs a number of
control
functions including controlling the position of switches 82 and 88 depending
upon
is whether a full rate or a half rate channel assignment (initially or as a
result of a
handover) is received by the mobile station.
The operation of the second e:cample embodiment of the present invention is
now described in conjunction with the flowchart illustrated in Fig. 5A. It is
understood
that change of channel rate is supported and permitted by the base station as
well as by
2o at least some of the mobile stations currently being served by that base
station. Change
of channel rate may be blocked if it is not supported or permitted. For
example, certain
types of calls such as data calls which are initially assigned to a full rate
call may not in
some instances be permitted to be changed to a half rate call after the call
is initially set
up as a full rate connection.
The first step is for the traffic load detector to determine the traffic load
corresponding in this second embodiment to the number of currently-assigned.
lull rate
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traffic channels (block l00) using the output from the busy full rate counter
>=I, the
threshold input. and the comparator ~2 in the traffic load detector ~0. A
decision is
made in block 102 whether the current traffic load exceeds the threshold
(which
preferably incorporates a hvstcresis value}. If not, control loops back to
block ! ()0 to
s monitor the traffic load with no change of channel rate being necessary at
this time.
I f the traffic load in this cell area exceeds the threshold, the BSC
controller s6
generates a list 58 of dual rate mobile stations in this cell area X that are
currently
assigned to a full rate traffic channel that are permitted to use a half rate
traffic channel
if necessary (block 106). A decision is made in block 108 by controller s6
whether
io there are any dual rate mobile stations included in the list. If not,
control returns back to
block 100 to repeat the above procedures.
However, if there are dual rate mobile stations currently assigned to a full
rate
traffic channel entered on the list, another decision is made in block I 10
whether any
traffic channel associated with the base station for area X has one hat f~
rate connection
n idle. As described above in conjunction with Fig. 3, a traffic channel can
support two
half rate traffic channel connections. It is more efficient to fully occupy
that traffic
channel with two half rate calls. A handover of the current call on the foil
rate traffic
channel for the dual rate mobile station at the top of the list is handed over
to the
remaining idle half rate connection (block 112). The handover frees up a full
rate traffic
?o channel for other call requests some of which may require a full rate
traffic channel
during this high traffic demand period. Alternatively, the freed up full rate
channel may
he used to support two new hat l~ rate call connections.
The BSC controller 56 generates the necessary handover commands for the base
station controller 74 and mobile station controller 92. When the mobile
station -~6
initially sends a ''channel request" message to its serving base station, the
base station
includes the channel request message with a "channel required" message sent to
the
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BSC. A portion of the channel request indicates the types of channels over
which the
mobile station can communicate, e.g., full rate, half rate, etc. A ''channel
assignment"
command is then sent to the mobile station from the BSC via the serving base
station
containing a channel description of an initial or a new half~rate traffic
channel for which
the gall is to be established or handed over. The base station activates the
new hal(~rate
traffic channel. For handovers. when the new traffic channel is acknowledged.
the BSC
controller 56 sends a message to the mobile station via the old full rate
traffic channel
with information about the new half rate traffic channel frequency, time slot.
and output
power. The mobile station tunes to the new frequency and sends handover access
bursts
io on the appropriate time slot. Once those bursts are detected and
acknowledged. a
handover complete message is transmitted by the mobile, and the old full rate
traffic
channel is deactivated making it available for assignment to other
communications. .
On the other hand, if there are no traffic channels supporting a current half
rate
connection with the other half rate connection idle, a decision is made in
block 1 I ~.
is ~.vhether there is any traffic channel with both half rate channel
connections idle, i.e., a
traflic channel that is available to support one lull rate connection or two
hall~ratc
connections. If not, then the list of dual rate mobile stations ~8 is cleared
(block l22).
and the process repeats at block 100. On the other hand, if there is an
available half rate
channel connection, a handover is executed for the mobile station at the top
of list 58
'o tcom a current full rate connection to a new half rate connection on the
idle traffic
channel identified in block I 14 (block 116). When the handover is performed
in either
blocks 1 12 or 1 16. the mobile station entry at the top of the list 58 is
removed
(block 1 18). A decision is then made in block 120 whether the traffic load is
less than
the threshold by a hysteresis amount. If not, if the cell is still
experiencing high traffic
conditions and may add additional capacity by changing the channel tale from
lull rate
to half rate connections. On the other hand, if the traffic load has decreased
sufficiently
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13
beneath the threshold, the list is cleared in block 'Z2, and the process
repeated starting at
block 100.
ll~thc traffic load exceeds the threshold in decision hl~cl: 102. the: 13~C'
controller ~6 also attempts to increase capacity by assigning half rate
traffic channels
where permittedipossibIe for new call requests, see flags A in Fio. ~A. and
described in
the flowchart illustrated in Fig. ~B. For a high traffic load condition. each
new channel
request is received (block I 30), and the rate capability of the mobile
station associated
with the channel request is determined (block 132) based on the indication of
what
kinds of channels the mobile station can handle contained in the channel
request
io message. If a full rate traffic channel is required. (for whatever reason).
the channel
setup is continued to assign a full rate traffic channel assuming that one is
available.
I-Iowever, if a full rate traffic channel is not required for the current call
request, a half
rate traffic channel is assigned (block 136) and control returns to block 100
in Fig. 5.
When the traffic load is detected to have decreased sufficiently, an optional
procedure is
to permit ongoing half rate calls to be selectively handed over to available
full rate
~i~,mncls.
Consider the following simple example. Assume that the base station ~4 has a
total of four transceivers where each transceiver includes eight time slot
channels.
accordingly, the base station includes a total of thirty-two time slots of
which three arc
~o used for control channel signaling leaving twenty-nine time slots for
traffic channels.
Again. all base station transceivers are assumed to be capable of carrying
full rate and
half rate channels. Half of the mobile stations arc assumed to be capable of
dual rate
communications with the other half being capable of full rate communication
only. The
traffic load threshold is set to seventy-five percent with a hysteresis of
five percent.
Translated to a number of traffic channels for this example, the threshold is
twenty-two
busy traffic channels with a hysteresis of one traffic channel. Thus. a high
load flay is
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1=t
set when twenty-three or more traffic channels are currently busy. and is
reset when-less
than nventv-one traffic channels are currently busy.
Assume that the traffic channels are currently used as Follows: two traffic
channels are allocated as packet data channels, ten traff is channels are
occupied by
n~obiEc stations capable of full rate only. one traffic channel is occupied by
two haifrutc
calls. two traffic channels are occupied by one half rate call per each
traffic channel,
seven traffic channels are occupied by dual rate mobile stations currently
using a full
rate connection. In total, riventy-two traffic channels are busy, and
therefore, the high
traffic load flaa is not set.
~o Suppose another full rate call is established for a dual rate mobile
station. The
high traff c load flag is then set. The base station controller 56 generates a
list of dual
rate mobile stations that are candidates for handovers. The list includes
eight entries
corresponding to the eight dual rate mobile stations currently using full rate
connections. These full rate connections are handed over one at a time to a
hal f rate
~s channel until the high load situation is alleviated. For etample, a first
full rate
connection is moved to an idle half rate connection on one of the partially-
allocated
traftic channels already carrying one half rate call connection. After this
first handover,
the number of busy traffic channels equals twenty-two, assuming that no new
calls have
been established, which is not low enough to reset the high load flag. A
second
?o connection is moved to the idle part of the second of the partially-
allocated traffic
channels. The number of busy traffic channels is reduced to twenty-one which
again is
not low enough to reset the high load flag. A third full rate connection is
moved to a
halfrate channel on one of the idle full rate channels. A fourth connection is
moved to
the other available half rate connection on that same traffic channel.
I3v transforming the four full rate connections to four new half rate
connections,
the number of busy traffic channels is reduced from twenty-three to twenty. As
a result,
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four additional traffic channels are made available to service other calls at
a relatively
minimal cost of performing for intra-cell handoffs. Surges in traffic demands
are
efficiently met in dynamic fashion without additional base station
transceivers.
While the present invention has been described with respect to particular
c~nhc~diments, those skilled in the art will recognise that the present
invention is n«t
limited to the specific embodiments described and illustrated herein.
Different formats,
embodiments, and adaptations besides those shown and described, as well as
many
variations, modifications, and equivalent arrangements may also be used to
implement
the invention. Therefore, while the present invention has been described in
relation to
io its preferred embodiments, it is to be understood that this disclosure is
only illustrative
and e~cemplary of the present invention and is merely for the purposes of
providing a
full and enabling disclosure of the invention. Accordingly, it is intended
that the
invention be limited only by the spirit and scope of the claims appended
hereto.
