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Description  |
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TECHNICAL FIELD
This invention relates generally to trunked communications systems and more
particularly to trunked communication systems that transceive both voice
and data.
BACKGROUND ART
In a basic RF trunked system there exists a high degree of flexibility to
partition voice conversations between different groups so that no one
group of users is specifically aware when another group of users makes use
of the system. Typically, these groups are subdivided into subgroups so
that calls may be made upon either a group, subgroup or individual basis
depending upon the type of communication desired by an initiating
subscriber.
To establish a voice communication between a group of units operating on a
trunked system, a subscriber unit transmits a data packet called an
"inbound signalling word" (ISW) on a control channel that is maintained
for such purposes. The ISW contains at least the requesting unit's unique
ID code, which may contain or be used to obtain the requesting
subscriber's current talk-group. The request is forwarded to a central
controller, which decodes the request, and transmits on the control
channel a data packet called an "outbound signalling word" (OSW) to all
subscriber units, which continuously monitor the control channel when not
participating in a voice conversation. The OSW is a channel grant which
contains the talk-group code of the requesting unit, and the voice channel
number assigned for the conversation. The OSW causes the requesting unit
to move to the voice channel and commence transmitting, while
simultaneously causing all other subscriber units in the same talk-group
to move to the voice channel as listening units. In this way a group call
is set up. If, however, all voice channels are in use when a subscriber
unit transmits an ISW, the central controller typically send the
requesting subscriber a "busy OSW".
In addition to voice messages, it is desirable to send data information
across a trunked radio channel. In some data systems, a subscriber unit
obtains a trunked data communication channel via the same procedure used
to obtain a voice channel. However, this practice is inefficient and
spectrally wasteful, due to the time it takes for a requesting subscriber
to transmit an ISW and receive a channel grant OSW from the central, and
the time it takes to set-up and clean-down a call on a voice channel. At
contemporary data transmission rates, it is anticipated that an entire
typical data message would take substantially less time to transmit than
the time required to obtain a channel (approximately 0.5 seconds). Thus,
assigning a data channel pursuant to the same procedure as assigning a
voice channel would be wasteful of spectrum and consume precious system
time that could be better used to transmit data messages.
Other trunked communication systems desirous to accommodate data traffic
have permanently dedicated one or more channels to handling data traffic.
While this avoids the access time problem noted above, this technique is
contrary to the basic principles of trunked communication systems, which
strive to allocate channel resources across a plurality of users as
required. Therefore, the practice of having dedicated data channels,
permanently removed from the channel allocation "pool" of frequencies, is
wasteful of spectral resources and leads to inefficient system operation.
Moreover, the dedicated data channel systems lack the capacity to
dynamically redistribute or allocate the data traffic load across the
available data channels. Such systems typically permanently assign a
subscriber unit to a data channel thereby building in future problems as
the number of data subscribers increases on a particular channel.
Accordingly, there exists a need for a trunked communication system that
can accommodate both voice and data signals, and that operates in true
trunked manner to efficiently utilize spectral resources.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an
improved trunked communication system.
It is another object of the present invention to provide a procedure to
dynamically allocate data channels on a trunked radio system.
It is a further object of the present invention to redistribute or balance
the data traffic load on the particular number of data channels currently
available.
It is another object of the present invention to provide a rapid and
convenient method to broadcast system wide data messages to all data
subscribers.
Accordingly, these and other objects are achieved by the present
invention's dynamic allocation of data channels in a trunked radio system.
Briefly, according to the invention, a method is disclosed to dynamically
allocate a number of data channels on a trunked radio system. The data
activity is monitored during a predetermined time interval. If activity is
above a predetermined maximum, an additional channel is reserved for data
use. Conversely, if data traffic is low, a data channel is reallocated for
voice message use. Moreover, should the amount of data traffic between the
available data channels be unbalanced, the present invention contemplates
reassigning subscriber units to the available data channels to balance the
data traffic load, thereby providing superior access time and system
performance.
BRIEF DESCRIPTION OF THE DRAWINGS
The features of the present invention which are believed to be novel are
set forth with particularity in the appended claims. The invention,
together with further objects and advantages thereof, may be understood by
reference to the following description, taken in conjunction with the
accompanying drawings, and the several figures of which like referenced
numerals identify like elements, and in which:
FIG. 1 is a block diagram of a trunked radio system that may employ the
present invention;
FIG. 2 is an illustration of the preferred signalling format for a master
data channel;
FIG. 3 is an illustration of the preferred signalling format for other data
channels;
FIG. 4 is a flow diagram illustrating the steps executed by the fixed-end
equipment of FIG. 1 in accordance with the present invention;
FIG. 5a is a flow diagram illustrating the steps executed by the fixed-end
equipment of FIG. 1 to perform the load leveling of subscriber units to
available data channels in accordance with the present invention;
FIG. 5b, is a flow diagram of the steps executed by the fixed-end equipment
of FIG. 1 to transmit a system message to the subscriber units in
accordance with the present invention; and
FIGS. 6a and 6b are flow diagrams illustrating the steps executed by the
data subscribers of FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, and in particular to FIG. 1, there is shown
a block diagram of a trunked voice/data communication system (100) that
may employ the present invention. The centralized or fixed-end equipment
comprises a central controller 102, which is responsible for allocating
the channel resources (represented here by repeaters 104a-104N) amongst
the many subscriber units. Of the available communication channels, one
(repeater 104a) is selected to be a voice control channel, which
communicates with any trunked subscriber capable of transmitting voice
traffic.
Preferably, each of the repeaters 104a-N are capable of operating as a
voice channel, control channel, or data channel. To accommodate data
traffic, such repeaters are equipped with a data interface 122. The data
interfaces 122 are responsible for encoding outbound data, decoding and
error correcting inbound data, repeater control, and providing an
interface between the repeater and the network controller 108.
Alternately, a predetermined subset of the total number of repeaters may
be equipped for data or used as a control channel. Typically, the
particular repeater selected to be the control channel (104a) is
periodically changed as a control measure.
The data network comprises at least one host computer 106, which is coupled
(i.e. wireline) to a network controller 108. The network controller 108 is
responsible for data traffic routing and communicating with the central
controller 102 to request the allocation of a data channel. Those skilled
in the art will appreciate that if the host computer 106, the network
controller 108 and the central controller 102 are co-located, the
intercouplings (124-128) may be direct. However, should remote locations
be desired, such communications may be maintained through the use of data
modems as is known in the art. Optionally, or additionally, the trunked
voice/data communication system 100 may employ one or more radio frequency
(RF) coupled host computers 118. The RF host 118 communicates as a trunked
control station via any suitable data transceiver 120. The primary
difference between the wireline host 106 and the RF host 118 is that the
data subscribers communicate directly (i.e., via both the inbound and
outbound frequencies of a data equipped repeater) with the RF host,
whereas the wireline host 106 transceives all information via the inbound
or outbound frequency of a data equipped repeater. Accordingly, the data
network of the present invention may employ several computers in either a
centralized or distributed processing arrangement.
Generally, the fixed-end equipment also includes a system manager console
110 that enables the supervisor of a communication service provider to set
a number of operational parameters that control the operation of the
trunked communication system. Typical examples of such parameters include
the maximum number of assignable data channels (if any), whether voice or
data will be the priority traffic, and various threshold values that
control when data channels may be added or reallocated to voice traffic.
Thus, at any particular time, the trunked communication system of the
present invention need not have any channels allocated for data traffic.
Conversely, if voice traffic is low, or if data traffic enjoys a priority
status or is particularly heavy, one or more channels may be allocated for
data communication.
According to the present invention, a predetermined channel ordinarily
comprises the first channel to be allocated for data. Preferably, this
first assigned data channel (hereinafter "the master data channel") has
the same frequency as any single frequency data-only subscribers (116) to
provide maximum compatibility with existing data equipment. Alternately a
random channel may be the first assigned data channel; however, the
data-only subscribers must scan the available channels to find it. Thus,
the present invention prefers to first assign a selected channel, and,
thereafter, assign any other data equipped (122) channel as an additional
data channel
The present invention allocates data channels for a time interval
determined by either the system manager or a default parameter. The length
of the allocation period may vary with the time of day, system loading or
other such parameters. By reserving a channel for data use over a time
period, data channel requests are minimized and spectral efficiency is
maximized since a data subscriber need not request a data channel for each
separate data transmission.
As a general principle, the goal of any trunked communication system is to
effectively allocate the limited channel resources amongst a plurality of
subscriber units. The present invention contemplates three classes of
subscriber units: voice-only subscribers 112, voice/data subscribers 114,
and data-only subscribers 116. The voice-only subscribers 112 are
contemplated to be any existing trunked subscriber unit having a
compatible signalling protocol to allow interaction with the system (100).
The data-only subscribers (116) are contemplated to be any multiple or
single channel data transceivers such as the KDT 800.TM., manufactured by
Motorola, Inc., or functional equivalent. Of course, receive-only data
devices, such as any of the Motorola family of display pagers, may also
operate to receive paging data over an assigned data channel. In this way,
the trunked system of the present invention accommodates existing
equipment, while providing enhanced communication capabilities. Subscriber
units are typically comprised of either mobile, portable or control
stations. Typically, mobile units are understood to be transceivers
designed to operate in a vehicle. A portable unit is typically understood
to be a transceiving or receive-only device designed primarily to be
carried on or about the person. Control stations are usually permanent or
semi-permanent installations in buildings or other such fixed locations.
These are collectively referred to herein as subscriber units, which
communicate with one another through the fixed-end equipment.
As previously mentioned, the first data channel allocated is defined as the
master data channel, the identity of which is periodically transmitted to
all subscribers over the control channel. The data subscribers (114 and
116) each determine their assigned data channel from information
transmitted over the master data channel, in addition to some locally
calculated information stored in a data channel table, which is calculated
when a subscriber first "powers up". The unit's ID code is used as a
parameter in an algorithm that generates all possible data channel
assignments (i.e., up to and including the case of all of the data
equipped channels being allocated for data traffic). Each of these
assignments is based upon the deterministic output of the algorithm, that
is selected to assure that all subscribers in a particular group (or
subgroup) are assigned to the same data channel. Those skilled in the art
will appreciate that some criteria is required to decide how to distribute
data users. The group (or subgroup) basis provides a convenient decision
criteria by which to assign data users. Of course, other bases are
available and the particular basis used in any particular implementation
is not critical to the practice of the present invention. Moreover, the
particular algorithm used is not critical to the practice or understanding
of the present inventive method. Such deterministic algorithms are widely
known and used in the art and any particular algorithm (including, for
example, simple division or addition operations) that produce a
deterministic result such that all members of the same group are assigned
the same data channel is suitable for use with the present inventive
method. In this way, data subscribers may select a data channel and
"up-load" or "down-load" information to or from the host computer 106 (or
118). As used herein, "up-load" means the transference of data or
executable code from a subscriber unit to a host computer. The term
"down-load" means the transference of data or executable code from a host
computer to a subscriber unit.
Of course, the particular data channel assignment will depend upon the
actual number of data channels currently available. For example, a
particular subscriber unit may select data channel one if the current
number of data channels is two. If the number of channels is three,
channel two may be selected, and so on. Of course, if there is only one
data channel available, all data subscribers will use that channel.
When a particular number of data channels is assigned, the central 102
monitors the voice activity, while the network controller 108 monitors the
activity on the data channels. This activity monitoring is preferably
performed for a predetermined period of time. For example, the monitoring
activity may be done by the hour (or half hour) so that during peak
loading times the trunked system (100) can quickly adapt to reallocate the
channel resources. If the network controller determines that the data
activity on the assigned data channels has exceeded a predetermined
supervisor selected threshold, the network controller 108 requests the
central to allocate another channel for data traffic. Conversely, if the
central determines that voice activity has exceeded a predetermined
threshold, the central 102 requests the network controller to relinquish a
data channel. In this way, the trunked system 100 adapts to reallocate the
channel resources.
According to the present invention, reallocation begins by transmitting a
"revert to master" command over all currently assigned data channels. When
the subscriber units receive this command, they all revert to the master
data channel to listen for new data channel assignments. The allocated
data channels may be incremented or decremented by a predetermined number
(one of the preferred embodiment) and the new allocation or number of data
channels may be broadcast along with the identity of each data channel.
The subscriber units receiving this information peruse the available data
channels assignments in the data channel table, and select the appropriate
assignment. In this way, the number of data channels can be conveniently
incremented or decremented depending upon data traffic.
In addition to overall data traffic monitoring, the network controller 108
may determine that the data traffic load on a particular data channel is
excessive. Accordingly, the present invention contemplates a method to
balance the data traffic over the available data channels. This is
accomplished by transmitting a load leveling command that may include an
offset "seed" for use with the ID code and the data channel assignment
algorithm. To "load level", the subscriber units recalculate the data
channel table using the unit's ID code and the offset seed. In this way,
there is a statistical probability that the load will be spread over the
available data channels as opposed to being congested onto a particular
data channel. The data traffic may then be monitored over the next
operating period, and if the load is still unbalanced a different offset
seed may be transmitted to again redistribute the data load.
Those skilled in the art will appreciate that it is often desirable to
communicate with all subscriber units at one time in response to either an
emergency, or to distribute a message of general concern. For example, a
message announcing some emergency condition or that the host computer 106
(or 118) will be down for repair are examples of messages that would be
convenient to transmit to all subscribers at one time. Accordingly, the
present invention achieves this operation by transmitting the revert to
master command over all allocated data channels. All subscribers respond
to this command by listening to the master data channel. Just prior to
retransmitting the current number of data channels and their identities
(which may be unchanged), a system broadcast message is transmitted so
that all subscribers may receive the message prior to receiving the data
channel information. In this way rapid system wide communication is
provided by the present inventive method. After receiving the system
message, the data subscribers may return to their assigned data channels
Referring now to FIG. 2, an illustration of the preferred signalling format
for the master data channel is shown. The signalling format 200 begins
with a preamble portion 202, which may include synchronization or framing
information for the data subscriber units Following the preamble 202 is an
optional block 204 wherein either a system message or an offset seed may
be transmitted to effectuate either the system message operation or the
load leveling procedure previously mentioned. Of course, during normal
operations format block 204 would not be used and the preamble 202 would
directly precede block 206.
Basically, block 206 transmits the total number of currently available data
channels (be it 1, 2, 5, etc.) in any suitable form. Following block 206
are a plurality of blocks (208a through 208n) which transmit the identity
of the data channels. In the preferred embodiment, the transmitted
identity of the data channels is the actual frequency of the channels.
Alternately, the channels could be assigned a designated number and the
selected ones available for data use transmitted. For example, if a
particular system has five channels, it may be convenient to label them
1-5. Then, assuming the subscribers knew the frequencies, the numbers "4"
and "5" may be transmitted to indicate that channels four and five are the
data channels. The preferred method, however, is to transmit the actual
frequencies, since this allows for simple expansion of the system, and
limits the amount of information required to reside in the subscriber
units. Therefore, if there is one data channel (i.e., the master data
channel), that frequency will be transmitted in block 208a. If there are
five data channels currently available (the master data channel plus four
other data channels), those frequencies may be transmitted (for example in
blocks 208 a through 208e), and so on.
After the transmission of the identity of the last available data channel,
the master data channel may be used by the subscriber units as a data
channel as is illustrated by block 210.
To effectuate a recovery process, in case any particular subscriber unit
should temporarily lose its data channel assignment, the central 102 and
the network controller 108 may periodically transmit the signalling format
200 over the voice control channel and the master data channel,
respectively. If through some error the data channel assignment is lost,
the present invention provides for all data mode subscriber units to
automatically revert to the master data channel. In this way, a subscriber
unit may receive the periodic transmissions of the channel assignments
from the master data channel and return to the proper data channel
assignment. In the event a subscriber unit loses the identity of the
master data channel, the present invention further provides for the
subscriber units to revert to the voice control channel to receive the
data channel allocation information.
Referring now to FIG. 3, the preferred format for other (i.e., not the
master) data channels is shown. Basically, the format of a data channel
300 begins with a preamble 302, which may include synchronization and
framing information. The preamble 302 precedes a plurality of variable
length data messages 304-308.
As previously mentioned, the request for assignment of a data channel is
prohibitively long compared to the typical data message transmission time.
Therefore, the present invention contemplates a subscriber unit going to
its assigned data channel and transmitting the data information without
re-requesting the channel. Operating in this manner conserves spectrum and
speeds transmission by eliminating the requirement to request a data
channel. Of course, there exists the possibility that there will be data
collisions on the data channels. However, data collision avoidance
mechanisms and methods are widely known in the art and any suitable data
collision avoidance and recovery method will be suitable for use in the
present invention.
As illustrated in FIG. 3, the lengths of data messages 1, 2 and 3 (304, 306
and 308), are all of a variable duration depending upon the amount of
information to be transmitted. Thus, once a subscriber unit gains access
to a data channel, the subscriber may transmit data for as long as
required to complete a data message. Of course, a second subscriber unit
attempting to transmit data may be required to wait until the first
subscriber has completed transmitting. Thus, a data channel may be in
constant or near constant use. During periods of high data channel use,
the preamble portion 302 need not be transmitted since the subscribers may
still be synchronized t the incoming data. However, if the data channel
use is low, the network controller 108 or a data subscriber may transmit
the preamble portion 302 prior to transmitting.
Referring now to FIG. 4, there is shown a flow diagram illustrating the
steps executed by the fixed-end equipment to implement the present
invention The routine begins with initializing step 400, wherein the
central controller 102 and the network controller 108 may set aside memory
space or perform other such functions as any particular system may
require. The routine next proceeds to step 402, which starts the period
timer over which the central controller 102 monitors the voice activity
and the network controller monitors the data activity. In step 404, these
measurements are taken such as by calculating the air-time billing
information or other such suitable means. Following step 404, decision 406
determines whether or not the time has elapsed. If the timer has not
elapsed, a loop is formed with step 404 until decision 406 determines that
the timer has expired.
Decision 408 determines whether the voice activity is high when compared to
a selected threshold that may be specified by the system supervisor. If
the determination of decision 408 is that the voice activity is high,
decision 410 determines whether the current number of data channels minus
a predetermined offset (one in the preferred embodiment) would be less
than the minimum number (if any) of data channels specified by the system
supervisor. If so, decision 410 returns control to reference letter A,
which resets the timer and the routine begins again. If, however, decision
410 determines that removing a channel would not be below the minimum
allowed data channels, or there is no minimum, the routine proceeds to
step 412, which de-allocates a channel from data traffic and returns it to
voice traffic. The routine then proceeds to reference letter A of FIG. 4.
If the determination of decision 408 is that the voice activity is not
high, the routine proceeds to decision 414, which determines whether the
data activity is high compared to a predetermined threshold selected by
the system supervisor. If the determination of decision 414 is that the
data activity is high, the routine proceeds to decision 416, which
determines whether the current number of channels plus one (in the
preferred embodiment) is greater than the maximum number (if any)
specified by the system supervisor. If the determination of decision 416
is that the additional channel would exceed the maximum, the routine
returns control to reference letter A. If, however, decision 416
determines that the addition of a data channel will not exceed the
maximum, the routine proceeds to step 418, which allocates an additional
channel from voice traffic to data traffic. The routine then returns
control to reference letter A of FIG. 4.
Additionally, the fixed-end equipment may take traffic priorities into
account before the allocation step 418 and the de-allocation step 412. If,
for example, a particular system favored voice traffic, an additional data
channel may not be allocated if voice traffic were above a predetermined
minimum. Alternately, for a system having a data traffic preference, a
data channel may not be re-allocated for voice traffic if data traffic was
above a predetermined threshold. In the absence of either a voice traffic
or data traffic preference, the absolute allocation and de-allocation
procedure of FIG. 4 is preferred.
Referring still to FIG. 4, if the determination of 414 is that the data
activity as a whole is not high, the routine proceeds to decision 420 to
determine whether the data traffic across all available data channels is
balanced or unbalanced. If the determination of decision 420 is that the
traffic is essentially balanced, the routine returns control to reference
letter A, which resets the timer of step 402. If, however, decision 420
determines that the data traffic is unbalanced the routine proceeds to the
load leveling routine of FIG. 5a.
Referring now to FIG. 5a, the steps executed by the network controller 108
to execute a load leveling of data traffic across the available data
channels is described. The routine begins in step 500, which transmits a
revert to master command across all data channels. Upon receipt of the
command, the data subscribers listen to the master data channel and may
receive an offset seed parameter or a load leveling command transmitted as
block 204 of FIG. 2 (block 502 in FIG. 5a).
Preferably, each subscriber completely recalculates the data channel table
based upon the unit's ID code, together with the offset seed, and stores
the table in a suitable memory means (such as random access memory) within
the subscriber unit. Alternately, if the total number of seeds were
suitably small, all of the seeds may be either permanently stored in, or
transmitted to, the subscriber, and all possible assignments using all the
possible seeds may be calculated. This method saves later calculation time
at the expense of memory in the subscriber units.
The load leveling routine of FIG. 5 next proceeds to step 504, where the
number of available data channels and their identities are transmitted
over the master data channel. Of course, the total number may not have
changed, but simply the offset seed parameter may have been added to
statistically redistribute the groups (or subgroups) across the available
data channels. As previously mentioned, if after monitoring the next
transmission period, the data traffic load remains unbalanced, a different
offset seed may be transmitted until an acceptable balance is obtained
between the data traffic and the available data channel resources.
Referring now to FIG. 5b, the steps executed by the network controller 108
to broadcast a system wide data message is shown. The routine begins in
step 506, where the revert to master command is transmitted to all data
subscribers. Next, in step 508, the system message is transmitted (see
FIG. 2, block 204) to the subscriber units over the master data channel.
Following step 508, the current number of data channels and their
identities (which may be unchanged) are transmitted in step 510. In this
way, a message of general concern may be rapidly and efficiently
transmitted to all data subscriber units. Lastly, after step 510, the
routine returns to reference letter A of FIG. 4. After receiving the
system message, the subscribers may return to their assigned data
channels.
Referring now to FIGS. 6a and 6b, there are shown flow diagrams
illustrating the steps executed by a data subscriber unit (114 or 116) in
accordance with the present invention. In FIG. 6a, the routine begins in
step 600, where the data subscriber performs any initialization steps
required in any particular implementation. In step 602 the data subscriber
receives the data channel allocation information from either the voice
control channel or the master data channel. Additionally, those
subscribers monitoring the master data channel may receive a system
message (see FIG. 2).
Step 604 generates the data channel assignment table using the subscriber
unit talk-group ID code and any suitable algorithm, which provides a
determinable assignment such that all subscriber units having the same
talk-group ID are assigned the same data channel. These channel
assignments are calculated for all possibilities of data channels (i.e.,
from 1 through the maximum number of data equipped channels currently
available on a particular system) and stored in any convenient storage
means such as a random access memory (RAM). Alternately, steps 602 and 604
may be reversed.
Once the subscribers have calculated the data channel table and received
the data channel allocation information, each subscriber sets a pointer
(in step 606), which assigns the subscriber unit one of the available data
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