Note: Descriptions are shown in the official language in which they were submitted.
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PAGING METHOD FOR A MULTI-LINE TERMINAL IN A FIXED CELLULAR SYSTEM
Techriical Field
This invention generally relates to the field of
communication systems and, more particularly, to a
communication system that provides telephone service using a
Fixed Cellular Terminal (FCT).
Background
In many parts of the world, telephone service is
not readily available because of inadequate or nonexistent
infrastructure, and people desiring the service have to wait
a long time to obtain it. In order to solve this problem,
many service providers are installing terminals that are
known as Fixed Cellular Terminals (FCT), which combine
private branch exchange (PBX) technology and cellular
technology, to provide telephone service over a cellular
network.
An FCT includes a Multi-Line Terminal (MLT) and a
PBX that together provide telephone service to a group of
subscribers through a cellular network. In the FCT, a group
of subscriber units, such as conventional wired telephones,
are multiplexed through the PBX to a number of cellular
terminals (CTs) incorporated into the MLT. This arrangement
expedites installation of telephone infrastructure in remote
locations, where conventional wired telephone service is
unavailable. For example, one known FCT provides telephone
service to 100 subscribers using an MLT that has 16 CTs. A
control unit in the MLT coordinates the selection of trunks
and the communication with the PBX. Under this arrangement,
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each MLT services a group of subscriber units that can
receive or initiate calls over the cellular network.
When a call is directed to a particular subscriber
number of a conventional FCT, the cellular network transmits
a page containing the called subscriber number over a
control channel. Each one of the CTs, which operates to
direct or originate a call to and from any one of the
subscriber units, is programmed to respond to pages
addressing the group of subscriber numbers assigned to a
corresponding MLT.
Conventional FCTs are implemented using two types
of cellular networks. The first type of cellular network is
an analog cellular network, such as AMPS network, which
modulates analog voice and control signals on physical radio
frequency (RF) channels that link the CTs and a central
controller of the cellular network to each other. Under
this implementation, the CTs of a MLT tune to a known RF
control channel for receiving and transmitting analog
control information, such as paging information and voice
channel information that carry a voice call. Once the
control information is received for a called subscriber
unit, the MLT selects an idle CT to handle the call.
FCTs are also implemented in a second type of
cellular network that uses a hybrid of analog and digital
technology. One such hybrid cellular network standard is
defined by Telecommunication Industry Associated (TIA) IS-54
standard. This hybrid cellular network uses one or more
analog physical channels for communicating control
information. Voice and data signals, however, are
communicated digitally over logical channels that are formed
by subdividing a physical RF channel into a predefined
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number of time slots. In many digital systems, the logical
channels carry digital voice and traffic data using a Time
Division Multiple Access (TDMA) technique. Similar to
analog cellular networks, the CTs of the FCT operating with
the hybrid cellular network tune to a designated physical
control channel for receiving analog control information,
including the page information.
There also exists a third type of cellular
_ __network, which is a . fully_.digitized. This -type -of -cellular
network uses logical channels for communicating digital
voice or data as well as control information. One such
digital cellular standard is defined by TIA's IS-136
standard, which is being adopted rapidly throughout the
world. A cellular network implemented using the IS-136
standard transmits control information over digital control
channels (DCCHs) that carry page information over logical
paging channels (PCHs). Under the IS-136 standard, a Mobile
Switching Center (MSC) executes a predefined PCH calculation
algorithm for allocating the PCHs. In order to allocate the
PCHs, the MSC applies a number of defined parameters,
including each mobile station's mobile identification number
(MIN, to the PCH calculation algorithm. Because of the
arbitrary nature of the MIN pair, however, the PCH
calculation algorithm produces arbitrary results.
Therefore, unlike the first two types of cellular networks
that transmit paging information over a known physical
control channel, the paging information in an IS-136 system
may be scattered on arbitrarily allocated logical PCHs.
Moreover, in addition to being scattered on multiple logical
channels of one DCCH physical channel, a cell, depending on
the cell configuration of the cellular network, may
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distribute the PCH channels over up to 8 DCCH physical
channels.
The arbitrary nature of PCH complicates allocation
aver one or more physical channels the implementation of an
FCT using a digital control channel. Unlike analog and
hybrid cellular networks, which have a fixed control channel
to which the CTs can readily tune to for receiving page
information, the page information in an existing digital
cellular network are carried on PCHs that are-not--fixed. -
Since an MLT is responsible for responding to calls directed
to a large group of subscriber numbers, e.g., up to 100
subscriber numbers, its associated CTs must calculate a PCH
for each subscriber number based on an assigned MIN pair.
Because of the arbitrarily allocated logical PCHs,
theoretically, each mobile station must monitor every PCH on
all of the DCCHs. Although the CT has the capability of
monitoring every PCH, performing the PCH allocation
algorithm for each MIN pair of a large group of subscriber
numbers, e.g., 100, within the duration of a logical
channel, which under IS-136 standard~is about 640 ms, is
beyond the processing power of the existing CTs.
Furthermore, if the allocated PCHs are scattered over two or
more physical channels, the existing CTs, without hardware
and software modifications, are unable to concurrently
monitor PCHs over multiple physical channels.
Because of the wide acceptance of digital cellular ,
networks, it is desired to implement the FCT under a
standard that defines a digital control channel, without
substantially modifying an existing infrastructure.
Therefore, there exists a need for a communication system
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that allows for the operation of an FTC in an existing
cellular network that uses digital control channels.
Summary
5 The present invention that addresses this need is
exemplified in a communication system that includes a
cellular network that transmits page information over one or
more physical channels that are subdivided into digital
control channels (DCCHs) that include one or more logical
paging channels (PCHs). A fixed cellular terminal (FCT)
with a number of subscriber units, each having a
corresponding subscriber number, and a number of cellular
terminals (CTs) communicates with the cellular network over
the DCCHs. At least one multi-line terminal (MLT) that is
connected to the subscriber units and to the CTs that
receives the page information on at least one PCH via at
least one of the CTs.
According to some of the more detailed features of
the invention the page information directed to all of the
subscriber units that are connected to the MLT is
transmitted over a common PCH. Preferably this is done by
executing a PCH calculation algorithm for allocating the
PCH. Under this arrangement, PCH is calculated based on an
MLT identification (MLT ID), which is assigned to the MLT.
When transmitting the paging information, the subscriber
number of a called subscriber unit is transmitted over an
allocated PCH.
According to another embodiment of the invention
one CT is reserved for receiving page information over all
PCHs. Under this embodiment, other CTs ignore the page
information. The reserved CT delivers the paging information
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to the MLT, which assigns one of the other CTs to respond to
a call.
Other features and advantages of the present
invention will become apparent from the following
description of the preferred embodiment, taken in
conjunction with the accompanying drawings.
Detailed Description
Referring to FIG. 1, a communication system 10
according to the present invention provides telephone
service to subscribers using a digital cellular network 12.
In a preferred embodiment, the cellular network 12 is
implemented substantially according to the TIA~s IS-136
standard, which is hereby incorporated by reference.
Therefore, the operation of the cellular network 12 is
described to the extent necessary for understanding of the
present invention. Although, the present invention is
described as embodied using the IS-136 standard, those
skilled in the art would appreciate that the present
invention could be advantageously used in a wide variety of
other digital communication systems, such as those based on
PDC or GSM standards.
The communication system 10 provides telephone
service to fixed subscriber units 18 that communicate with
the cellular network 12 via a fixed cellular terminal (FCT)
that connects them to a number of Multi-Line Terminals ,
(MLTs} 22 via a PBX 24. In a well known manner, the
cellular network 12 communicates digital voice, data and
control information over physical channels that are
subdivided into a number of logical control channels. In
addition, the communication system provides cellular service
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to subscribers that carry mobile stations 14 while traveling
within communication cells 16 of the cellular network 12.
The cellular network 12 is a hierarchal network
with multiple levels for managing calls. Using an allocated
set of uplink and downlink RF channels, which are sub-
divided into logical control channels, mobile stations 14
and the MLTs 22 participate in calls using the allocated
logical channels. A Mobile-service Switching Center (MSC)
26 is responsible for routing calls from an originator to a
destination. In particular, the MSC 26 is responsible for
setup, control and termination of the calls. The MSC 26
also communicates with a Public Switched Telephone Network
(PSTN) 28, or other public and private network, which is
connected to conventional telephones 30. A subscriber
database 32, which is accessible by the MSC 26, maintains
records corresponding to each subscriber of the
communication system 10. By comparing a called subscriber
number, for example, to one originated by a telephone 30
against the subscriber database 32, the MSC 26 determines
whether to handle a call or not.
At a lower hierarchal level, the MSC 26 is
connected to a group of base station controllers (BSCs) 34,
which are primarily responsible for mobility management.
For example, based on reported received signal strength, a
BSC 34 determines whether to initiate a hand over, the
process by which the calls are maintained without a
noticeable break. At a still lower hierarchal level, each
one of the BSCs 34 controls a group of base transceiver
stations (BTSs) 38. Via antennas 37 and 35, the BTSs 38
primarily provide wireless links for transmission and
reception of data bursts over the logical channels. Each
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BTS 38 includes one or more transceivers (not shown) that
use the physical channels to serve a particular common
geographical area, such as one or more of the communication '
cells 16.
Referring to FIG. 2, a block diagram of an MLT 22
is shown to include a terminal unit 39 having a number of
cellular terminal (CTs) 40 and a control unit 42. In an
exemplary embodiment, the MLT 22 includes sixteen CTs 40 for
serving 100 subscriber units 18. The control unit 42
includes a processor 44 that controls the overall operation
of the MLT 22 and interfaces with the CTs 40 via a CT
interface 46, which in an exemplary embodiment of the
invention uses a serial communication protocol for providing
communication between the control unit 42 and the CTs 40.
The MLT 22 interfaces with the PBX 24 via a PBX interface
48. The PBX 24 routes calls to and from the subscriber
units 18 via the CTs 40 of the MLT 22. The CTs 40 interface
with the cellular network 12 in a similar manner as that of
the mobile stations 14 of FIG. 1. An RF unit 41, which
includes a passive device that combines the outputs of the
CTs to a single antenna 35, receives and transmits RF
signals to and from the cellular network according to the
IS-136 standard. In this exemplary embodiment, using the
sixteen CTs 40, the MLT 22 is responsible for routing calls
to and from up to 100 subscriber units 18 through the PBX
24. An operation and maintenance (0&M) controller 50
controls the operation and maintenance of one or more MLTs
22 in a well known manner.
As described above, each physical channel is
subdivided into a number logical channels consisting of time
slots during which digital voice or data and control
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information are communicated. Referring to FTG. 3a, a
physical channel subdivided into six time slots is shown to
form a 40-ms TDMA frame. Digital voice or data are
communicated over logical digital traffic channels (DTC),
and the control information are transmitted over digital
control channels (DCCH). The DCCHs used for transmitting
control information to the CTs 40 (and mobile stations 14)
are known as Forward DCCH (FDDCH). Conversely, the DCCHs
used for receiving control information from the CTs 40 are
known as Reverse DCCH (RDDCH). Each FDDCH includes a number
of TDMA Superframes. FIG. 3b shows a TDMA Superframe that
consists of sixteen 40-ms TDMA frames for a total length of
640 ms. Each FDDCH superframe is comprised of an of an
ordered sequence of logical channels, including F-BCCH, E-
BCCH, S-BCCH, reserved slots, and SPACH. The F-BCCH
contains data elements that may be relevant to paging, such
as page continuance and PCH calculation information. The
SPACH time slots serve as the paging slots on the FDCCH.
All mobile station pages are received on these slots. A
predefined minimum and maximum number of these logical
channels may be included in each FDDCH superframe. For
example, a minimum of 3 and a maximum of 10 F-BCCH slots may
be included in a FDCCH superframe. Similarly, each FDCCH
superframe may include a minimum of 2 and a maximum of 28
SPACH time slots.
According to one embodiment of the present
invention, a method for paging a subscriber unit causes the
MSC to transmit all pages directed to an MLT on a common
PCH, which is used by all of the CTs 40 within the MLT for
monitoring the pages. More particularly, the paging method
of the invention assigns a MLT ID to each one of the MLTs,
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which are connected to a corresponding group of subscriber
units 18 via the PBX 24. In an exemplary embodiment, the
MLT IDs are assigned to the MLTs by the O & M controller 50
(shown in FIG. 2). Instead of the MIN/ESN pair, the CTs 40
and the MSC 26 all use a corresponding MLT ID, which is
associated with a corresponding group of subscriber numbers,
to calculate the common PCH. Based on a called subscriber
number, the MSC 26 applies an MLT ID associated with the
called subscriber number to the PCH calculation algorithm to
10 calculate a suitable PCH.
In the MLT, the CTs 40 also use their assigned MLT
ID to calculate their allocated PCH. Because the MSC 26 and
the CTs 40 of a particular MLT 22 use the same MLT ID, the
PCH calculation algorithm produces the same PCH.
Thereafter, the CTs 40 tune to the calculated PCH and
monitor for pages directed at the subscriber units 18 that
are connected to their corresponding MLTs 22. Once paged by
the MSC 26, idle CTs 40 compare the transmitted subscriber
number against a list of subscriber numbers assigned to
their corresponding MLT 22. If a match is found, the idle
CTs 40 direct the subscriber number to the control unit 42.
The control unit 42 routes the subscriber number to the PBX
24, which alerts the called subscriber unit 18 by causing it
to ring. If more than one idle CTs 40 respond to the page,
using an arbiter block 52 (shown in FIG. 2), the control
unit 42 selects one of the idle CTs 40 for handling the ,
call.
Under an exemplary embodiment of the invention, a
single field called "MLT ID" is added to the subscriber
database 32. For each FCT subscriber number, the MLT ID
field contains a value corresponding to an MLT ID of an MLT
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22 to which the subscriber units 18 are assigned. In other
words, the subscriber database 32 associates the FCT
subscriber numbers with an MLT ID. Preferably, the MLT ID
field is added to every subscriber number regardless of
whether they are FCT or non-FCT subscriber numbers.
However, it would be appreciated that the MLT ID field could
be added only for FCT subscribers. In an exemplary
embodiment, the corresponding MLT-ID fields of non-FCT
subscriber numbers may contain no value. When a call is
detected, the MSC 26 looks up a received subscriber number
in its subscriber database 32. If found, the MSC 26 checks
the corresponding MLT_ID field. If this field was loaded
with an MLT ID value, the MSC 26 calculates a PCH based on
the MLT ID associated with the subscriber number of the
called subscriber unit 18. Using the calculated PCH
channel, the MSC 26 transmits a page that includes the
subscriber number of the called subscriber unit 18.
Therefore, the MSC 26 only uses the MLT ID for selecting a
PCH. Once selected, a corresponding subscriber number is
used for paging a called subscriber unit.
In this way, the MSC 26 pages every subscriber
unit 18, which is assigned to an MLT, using a single PCH
that is calculated based on the MLT ID. The advantage of
this solution over using a specific MIN from the list of 100
already assigned to the MLT is that it is independent of the
non-FCT subscribers and does not require the MSC to
distinguish between the FCT and non-FCT subscribers. This
allows the MSC 26 to page all types of subscribers in the
network 12 without requiring significant configuration
modification in the existing MSC 26 or the mobile stations
14 .
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In another embodiment of the invention, the MLTs
operate without being assigned an MLT ID. Rather, the
processing of the received pages over PCHs is off loaded to
the processor of the control unit 42, which has a higher
processing power than that of the CTs 40. While the CTs 40
are capable of monitoring every SPACH slot on the DCCH, they
are not capable of processing the pages. The CTs 40 must
read every message from every slot and compare it against a
large list of stored subscriber numbers, for example 100
subscriber numbers. Since each SPACH could contain 5 pages,
under this scenario, a CT 40 must perform up to 500
comparisons for each 20-ms SPACH slot. With maximum of 28
SPACH slots, theoretically, the CT would need to complete up
to 14000 comparisons in 560 ms. As described before, the
processors of existing CTs do not have such a high
processing power. Further more, if an ~~Additional DCCH
Information~~ element is communicated over the F-DCCH, the
CTs may be required to monitor for pages on up to 8
different physical DCCHs simultaneously, or multiple logical
DCCHs on the same frequency. The current CTs are also
incapable of monitoring different physical channels
simultaneously.
Therefore, according to this embodiment of the
invention, one of the CTs 40 of the MLT 22 is reserved for
PCH monitoring. The reserved CT is only responsible for
monitoring pages on every PCH in the superframe. The
reserved CT does not perform any comparison or other
processing on the page information. All other non-reserved
CTs in the MLT would ignore any pages encountered in a
superframe. The reserved CT passes all page information
received on every PCH to the control unit 42 for processing.
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Under this embodiment, the reserved CT acts as a
data gathering unit in the MLT 22. Upon receiving the page
information from the reserved CT, the control unit 42
compares the MIN transmitted in the page information with a
list of corresponding subscriber numbers contained in the
control unit 42. If a match is found, the control units 42
selects an idle CT as the trunk to handle the received page
and sends the page to the selected CT 40. The selected CT
40 would then respond to the page as if it had received it
from the MSC 26 under a normal cellular environment.
Preferably, if a reserved CT became inoperable, the control
unit 42 reserves a different CT for monitoring the PCH. If
a cell uses multiple DCCHs, up to 8 CTs 40 may be designated
as reserved CTs. Alternatively, multiple receivers may be
incorporated into the RF unit 41 (shown in FIG. 2) for
monitoring multiple DCCHs.
From the foregoing description, it will be
appreciated that the two embodiments of the invention allow
for implementation of an FCT that uses an existing digital
cellular network for providing telephone service for
subscriber units via a PBX. By using separate MLT IDs in
the PCH calculation algorithms of the CTs and the MSC, one
embodiment of the invention forces paging of the subscriber
units on a common PCH. This way, the overhead associated
with handling arbitrary PCH allocation is avoided. In the
other embodiment, the present invention reduces the burden
of monitoring all PCHs by CTs by designating a reserved CT,
which monitors and reports page information received over
all of the PCH to the control unit. This embodiment of the
invention, therefore, off-loads the processing requirement
of the monitoring pages transmitted on arbitrary channels
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from the CTs to the more powerful processor of the control
unit. Accordingly, both embodiments of the invention allow
for implementation of an FCT that operates with a cellular
network that uses a digital control channel.
Although the invention has been described in
detail with reference only to a preferred embodiment, those
skilled in the art will appreciate that various
modifications can be made without departing from the
invention. Accordingly, the invention is defined only by
the following claims which are intended to embrace all
equivalents thereof.
