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Description  |
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FIELD OF THE INVENTION
This invention relates generally to communication systems and more
particularly to a communication system infrastructure.
BACKGROUND OF THE INVENTION
The term "communication system" covers a variety of networks, devices, and
systems that provide communication between a plurality of nodes. One such
communication system is a two-way wireless communication system as shown
in FIG. 1. The two-way wireless communication system 10 includes a system
controller 12, a switch 14, a plurality of base stations 16-20, a
plurality of communication resources 22-26, and a plurality of
communication units 28-32. The plurality of base stations 16-20 are
coupled to the switch 14 via wireline links 34-38. These wireline links
34-38 are typically T1 links which have a bandwidth of approximately 1.5
megahertz and a capacity of 1.5 Mbps (mega-bits per second).
In operation, a communication unit transmits an inbound signaling (ISW) via
a wireless communication resource 22-26, which has been designated as a
control channel. One of the base stations 16-20 receives the ISW and
transmits it via the T1 link 34-38 to the switch 14. The switch provides
the information to the system controller 12 which determines the validity
of the request. For a valid request, the system controller grants the
requested service, which may be a two-way wireless communication, a group
call, a private call, or a telephone interface. Note that the
communication units 28-32 may be any type of two-way wireless
communication device, such as a mobile radio, a portable radio, a cellular
telephone, or a cellular/mobile/portable radio telephone.
Having granted the request, the system controller 12 allocates a wireless
communication resource 22-26 to the requesting communication unit. This
information is sent out via the switch 14, one of the T1 links 34-38, and
a base station 16-20. With the allocation of a wireless communication
resource, the requesting communication unit may perform its wireless, or
RF (Radio Frequency), communication.
While the two-way wireless communication system 10 of FIG. 1 provides a
variety of services to a communication unit, or subscriber unit, the
infrastructure requires high bandwidth wireline transmission links between
the switch 14 and the base stations 16-20. As shown, these high bandwidth
transmission links are typically T1 links which provides transmission
capabilites of 1.5 Mbps. Alternatively, the links could be microwave links
that provide an equivalent bandwidth as the T1 links.
Regardless of the type of link used, it must have a large bandwidth to
convey large amounts of information between a base station and the switch.
Because of the large amounts of information being conveyed, low frequency
transmission lines, such as twisted pair copper telephone lines, cannot be
used for these interconnections. Typically, a twisted pair copper wire
telephone line has a bandwidth of 4 kilohertz. In two-way wireless
communication systems, the intercoupling between the switch and base
stations requires at least a 20 kilohertz bandwidth for a single data
transfer. If multiple data transfers are occurring simultaneously, which
is the typical case, the bandwidth requirements increase accordingly.
Thus, based on these bandwidth requirements, the twisted pair copper
telephone line does not provide adequate bandwidth.
Another type of communication system is illustrated in FIG. 2 as a one-way
wireless communication system 40. The one-way wireless system 40, which
may be a paging system, includes a system controller 42, a switch 44,
wireline links 46-50, a plurality of transmitters 52-56, a plurality of
communication resources 58-62, and a plurality of communication units
64-68 or pagers. In operation, the system controller receives a request to
transmit a page to at least one of the plurality of communication units
64-68. Upon receiving this request, the system controller 42 generates a
paging message which is transmitted to the switch 44 and subsequently
routed via one of the wireline links 46-50 to the appropriate transmitter.
Upon receiving the paging message, the transmitter transmits the message
to the paging unit via one of the RF communication resources.
As with the two wire two-way wireless communication system 10 of FIG. 1,
the one way wireless system 40 requires a high bandwidth wireline link
between the switch and the transmitters. Thus, a twisted pair copper
telephone line, which has a bandwidth of approximately 4 kilohertz, will
not provide the needed transmission capacity of the one-way wireless
communication system 40.
The two-way wireless system 10 and the one-way wireless system 40 each
provide the communication unit with unique features. These unique features
center around the fact that these wireless systems can transmit one
communication to a plurality of receiving units. In other Words, such
wireless systems support one-to-many communications and/or many-to-one
communications.
To enhance the transmit capabilities of a twisted pair copper telephone
line, several techniques have been developed. For example Integrated
Services Digital Network (ISDN) extends the bandwidth of a twisted pair
copper telephone line from 4 kilohertz up to 200 kilohertz, which provides
a transmission capability of 160 Kbps. Motorola part numbers MC145474, and
MC145472 provide ISDN services. While these devices increase the bandwidth
of a twisted pair telephone line, they are intended for one-to-one
telephone communications. For example, the ISDN chips may be used for
video telephone conferencing, facsimile transmissions, and pair gain
transmissions, where pair gain transmission is a multiplexing technique of
several telephone calls on a single line.
Another technique which increases the transmission capabilities of a
twisted pair copper telephone line is Asymmetrical Digital Subscriber
Loop, or Link, (ADSL). An ADSL device increases the bandwidth of the
twisted pair telephone line up to 1.1 megahertz, which provides
transmission capabilites up to 9 Mbps. Similar to the ISDN technique, the
ADSL technique is used for one-to-one communications and provides
additional bandwidth over the ISDN devices.
FIG. 3 illustrates a typical ADSL two wire system. The ADSL two wire system
70 includes a video server 72, which may be a camera, an asynchronous
transfer mode (ATM) switch 74, an ADSL transceiver 76, a splitter 78, a
plain old telephone service (POTS) 80, a twisted pair copper wire
telephone line 82, a second splitter 84, a telephone 86, a second ADSL
transceiver 88, and a television monitor 90. In general, the first and
second ADSL transceivers 76 and 88 communicate to establish a spectral
response of the telephone line 82. Having exchanged this information, a
transmission can begin. In operation the video camera 72 will receive an
image for teleconferencing and convert that image into digital
information. The camera 72 routes the digital information to the ATM
switch 74, which, in turn, routes the digital information to the ADSL
transceiver 76. The ADSL transceiver converts the digital information into
a Discrete Multi-Tone (DMT) symbol and conveys the DMT symbol to the other
ADSL transceiver via the splitters 78, 84 and the telephone line 82. Upon
receiving the DMT symbol, the second ADSL transceiver 88 recaptures the
digital information and routes the digital information to the TV monitor
90.
In addition to transmitting high bandwidth digital information, the ADSL
two wire system 70 can also support regular telephone communications or
POTS. This is done via the splitters 78 and 84 which route low frequency
or POTS signals to the telephone 86 or the POTS switch 80 while routing
the higher frequency signals to the ADSL transceivers.
FIG. 4 illustrates a frequency spectrum of carrier channels used in an ADSL
system. The low frequency range, 0-4 kilohertz, is reserved for plain old
telephone system (POTS) transmissions. The high frequency range, from 25
kilohertz to 1.1 megahertz, is used for ADSL transmissions. With this
defined separation, the splitters 78, 84 of FIG. 3 can easily separate the
low frequency signals from the high frequency signals.
The high frequency range of FIG. 4 is divided into 256 carrier channels
separated by approximately 4 kilohertz. The first 32 carder channels in
the range from 0 kilohertz to 138 kilohertz are full duplex channels while
the 224 channels in the frequency range from 138 kilohertz to 1.1
megahertz are half duplex channels. For the 32 carrier channels in the
full duplex range, echo cancellation must be incorporated in the splitters
to ensure proper reception and transmission of the signals.
Each carrier channels can support up to 15 bits of QAM information. The
actual amount of bits a carder channel supports varies, however, due to
the spectral response of the telephone line. For example, one carrier
channel may be able to accommodate 15 bits while another may be only able
to accommodate 4 bits. Due to ADSL requirements the minimum amount of bits
a carrier channel can support, referred to as bit loading, is 2 bits and
still carry data.
To determine the spectral response of the telephone line, the ADSL
transceivers use an algorithm as shown in FIG. 5. As shown, a first ADSL
transceivers will transmit a wide band test signal to a second ADSL
transceiver. Upon receipt, the second ADSL transceiver evaluates the
received signal to determine the spectral response of the telephone line.
Having the spectral response, the second ADSL generates a bit loading
table and sends the bit loading table to the first ADSL transceiver. The
bit loading table includes, for each carrier channel, a number of bits
that the carrier channel can support.
FIG. 6 illustrates the transmit portion of the ADSL transceiver. As shown,
the ADSL transmitter includes a multiplexer which receives a plurality of
inputs via T1 links, ADSL control, an ISDN connection, and an HO link.
Based on the ADSL control, the multiplexer provides one of the inputs to a
constellation encoder via a fast path or an interleave path. The fast path
includes a scramble cyclic redundancy check (CRC) block which is coupled
to a forward error correction (FEC) block. The interleave path includes a
scramble CRC, a forward error correction block, and an interleave block..
The path selected depends on the level of burst error correction needed.
If less error correction is needed, the fast path is selected, otherwise
the interleave path is selected.
The constellation encoder encodes the received signals based on the bit
loading table and an encoding sequence to produce an encoded data stream.
The encoded data stream is then provided to the Discrete Multi-Tone (DMT)
modulator which produces a DMT symbol from the encoded data stream. The
DMT symbol is then transmitted to the receiver of the other ADSL
transceiver via the telephone line.
FIG. 7 illustrates the receiver portion of the ADSL transceiver. As shown,
the ADSL receiver includes a DMT demodulator which demodulates the DMT
symbol to produce a demodulated signal. The demodulated signal is then
provided to the constellation decoder which decodes the signal based on
the bit loading information and a decoding sequence to recapture the
transmitted data stream. The recaptured data stream is then provided to
the demultiplexer via a fast path or deinterleave path. The de-multiplexer
then provides recaptured data to the appropriate output line.
While the ADSL system increases the bandwidth of a telephone line, up to
1.1 megahertz, it is designed for one-to-one communications and not
one-to-many or many-to-one communications. Therefore, a need exists for a
one-to-many and/or many-to-one communication system infrastructure that
utilizes existing telephone lines while providing the highly reliable
service subscribers of wireless communication systems expect.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a prior art two-way communication system;
FIG. 2 illustrates a prior art one-way wireless communication system;
FIG. 3 illustrates a prior art ADSL two wire communication system;
FIG. 4 illustrates a prior art representation of an ADSL frequency spectrum
allocation;
FIG. 5 illustrates a prior art flow diagram for determination of bit
loading information of a telephone line;
FIG. 6 illustrates a prior art schematic block of ADSL transmitter;
FIG. 7 illustrates a prior art schematic block diagram of ADSL receiver;
FIG. 8 illustrates a communication system in accordance with the present
invention;
FIG. 9 illustrates a two-wire communication system in accordance with the
present invention;
FIG. 10 illustrates a schematic block diagram of a primary site in
accordance with the present invention;
FIG. 11 illustrates a secondary site in accordance with the present
invention;
FIG. 12 illustrates a logic diagram for the overall operation of a primary
site in accordance with the present invention;
FIG. 13 illustrates a flow diagram for an overall operation of a secondary
site in accordance with the present invention;
FIG. 14 illustrates a logic diagram for obtaining bit loading information
in accordance with the present invention;
FIG. 15 illustrates an exemplary process for obtaining bit loading
information in accordance with the present invention;
FIG. 16 illustrates a logic diagram for determining an outbound control
channel for the system of FIG. 8 in accordance with the present invention;
FIG. 17 illustrates an exemplary process for determining an outbound
control channel in accordance with the present invention;
FIG. 18 illustrates a logic diagram for updating the outbound control
channel in accordance with the present invention;
FIG. 19 illustrates a logic diagram for determining an inbound control
channel for the system of FIG. 8 in accordance with the present invention;
FIG. 20 illustrates an exemplary allocation and utilization of an inbound
control channel in accordance with the present invention;
FIG. 21 illustrates an alternative exemplary allocation and utilization of
an inbound control channel in accordance with the present invention;
FIG. 22 illustrates a logic diagram for updating the inbound control
channel in accordance with the present invention;
FIG. 23 illustrates a logic diagram for establishing an infrastructure for
a call in accordance with the present invention;
FIG. 24 illustrates an alternative logic diagram for establishing a call in
the system of FIG. 8 or FIG. 9 in accordance with the present invention;
FIG. 25 illustrates another alternative embodiment for establishing a call
within the system of FIG. 8 or FIG. 9 in accordance with the present
invention;
FIG. 26 illustrates a logic diagram that may be used to implement carrier
channel allocations in accordance with the present invention;
FIG. 27 illustrates an alternate logic diagram that may be used to
implement carrier channel allocations in accordance with the present
invention;
FIG. 28 illustrates a logic diagram that may be used to generate an ordered
bit loading table in accordance with the present invention;
FIG. 29 illustrates another alternate logic diagram that may be used to
implement carder channel allocations in accordance with the present
invention;
FIG. 30 illustrates a logic diagram that may be used to update carrier
channel assignments in accordance with the present invention;
FIG. 31 illustrates a logic diagram that may be used to update bit loading
tables in accordance with the present invention;
FIG. 32 illustrates an alternate logic diagram that may be used to update
carrier channel assignments in accordance with the present invention;
FIG. 33 illustrates a logic diagram that may be used to update carrier
channel assignments for a control channel in accordance with the present
invention;
FIG. 34 illustrates a functional block diagram of a DMT transmitter in
accordance with the present invention;
FIG. 35 illustrates a schematic block diagram of a data formatter of the
DMT transmitter in accordance with the present invention;
FIG. 36 illustrates a schematic block diagram of an ADSL transmitter
coupled to a data formatter in accordance with the present invention;
FIG. 37 illustrates a schematic block diagram of an ADSL data interface
coupled to an ADSL transmitter in accordance with the present invention;
FIG. 38 illustrates a logic diagram that may be used to implement a DMT
transmitter in accordance with the present invention;
FIG. 39 illustrates an alternative logic diagram that may be used to
implement a DMT transmitter in accordance with the present invention;
FIG. 40 illustrates a functional block diagram of a DMT receiver in
accordance with the present invention;
FIG. 41 illustrates a schematic block diagram of a data de-formatter of the
DMT receiver in accordance with the present invention;
FIG. 42 illustrates a schematic block diagram of an ADSL receiver coupled
to a data de-formatter in accordance with the present invention;
FIG. 43 illustrates a schematic block diagram of an ADSL data interface
coupled to an ADSL receiver in accordance with the present invention;
FIG. 44 illustrates a logic diagram that may be used to implement a DMT
receiver in accordance with the present invention;
FIG. 45 illustrates an alternative logic diagram that may be used to
implement a DMT receiver in accordance with the present invention;
FIG. 46 illustrates a logic diagram that may be used to generate addresses
for the data formatter and/or the data de-formatter in accordance with the
present invention;
FIG. 47 illustrates an alternative logic diagram that may be used to
generate addresses for the data formatter and/or the data de-formatter in
accordance with the present invention;
FIG. 48 illustrates a logic diagram that may be used to generate address
pointer information for the data formatter and/or the data de-formatter in
accordance with the present invention;
FIG. 49 illustrates another alternative logic diagram that may be used to
generate addresses for the data formatter and/or the data de-formatter in
accordance with the present invention;
FIG. 50 illustrates an alternative logic diagram that may be used to
generate address pointer information for the data formatter and/or the
data de-formatter in accordance with the present invention;
FIG. 51 illustrates an example of a DMT symbol formation in accordance with
the present invention;
FIG. 52 illustrates database information stored in memory in accordance
with the present invention;
FIGS. 53-61 illustrate an example of carrier channel allocation in
accordance with the present invention; and
FIG. 62 illustrates a data flow diagram in accordance with the present
invention.
DETAILED DESCRIPTION OF THE DRAWINGS
Generally, the present invention provides a method and apparatus for
establishing a communication system infrastructure utilizing low pass
transmission path, i.e. twisted pair telephone line. This may be
accomplished by providing a primary site which includes a controller, a
data multiplexing switch, a DMT transmitter and a DMT receiver. The
primary site is coupled to a plurality of secondary sites via low pass
transmission paths. Each of the secondary site includes a secondary DMT
receiver, a secondary DMT transmitter, a secondary DMT data multiplexing
switch, a site controller and at least one subscriber interface.
The primary site communicates with the plurality of secondary sites via
inbound and outbound low pass transmission paths. This is done via the
respective DMT receivers and DMT transmitters, wherein the DMT transmitter
transmits information over a plurality of carrier channels to a targeted
DMT receiver. The information transmitted may be control information,
i.e., the co-ordination information used by the primary site and the
secondary sites to establish data transfers, or user information, i.e.,
the information intended for a subscriber of the communication system. To
convey this information, the DMT transmitter formats the user information
and then converts it into DMT symbols, which is the actual information
transmitted over the low pass transmission path. The DMT receiver receives
the DMT symbols, deformats it to recapture the original information.
Before any information can be transmitted, an inbound control channel and
an outbound control channel must be established over the low pass
transmission paths. The inbound and outbound control channels established
on the low pass transmission path are different than the control channels
established in an RF communication system. The control channels of the RF
communication system are used to convey communication system operational
information to the subscribers of the system. For example, a request for a
call, RF communication resource allocation, request deny, status inquiries
and responses are all forms of communication system operational
information that are transmitted over the RF control channels. The control
channels established on the low pass transmission paths are used to convey
system infrastructure information between the primary site and the
secondary sites and to route the communication system operational
information to the sites such that the operational information may be
subsequently transmitted to the subscribers.
The outbound control channel for the system infrastructure is generally
established when the primary site transmits a training signal to each of
the plurality of secondary sites. Upon receiving the training signal, each
of the secondary sites performs a spectral response analysis of the
outbound low pass transmission path based on the training signal and then
creates bit loading information. In a sequential order, the primary site
requests and stores the bit loading information from each of the secondary
sites, wherein the bit loading information is stored in a site bit loading
table.
Having stored the bit loading information for all of the secondary sites,
the primary sites generates a lowest common denominator (LCD) bit loading
table as a compilation of all of the site bit loading tables. From the LCD
bit loading table and bandwidth requirements for the outbound control
channel, the primary site selects at least one outbound carrier channel of
the plurality of carrier channels to function as the outbound control
channel.
The inbound control channel is established in a similar way as the outbound
control channel except the primary site requests the training signal from
the secondary sites. Upon receiving the request, each of the secondary
sites responds in a sequential order. When the primary site receives the
training signal from a secondary site, the primary site performs a
spectral response on the inbound low pass transmission path based on the
training signal. From the spectral response, the primary site generates a
bit loading table for the site. Once a bit loading table for all of the
secondary sites has been generated, the primary site generates an inbound
control channel LCD bit loading table. From this table the primary site
selects, based on bandwidth requirements, inbound carrier channels to
function as the inbound control channel.
Once the control channels are established, the system may provide call
services, where a call service includes the transmission of user
information. When the primary site receives the call request, it
determines the target sites involved in the requested call. Having
identified the target sites, the primary site retrieves the site bit
loading table for each target site and generates a lowest common
denominator (LCD) call bit loading table for this particular call. From
the call bit loading table and bandwidth requirements for this particular
call, the primary site allocates carrier channels to provide the
infrastructure for this particular call. In other words, the primary site
is selecting which outbound carrier channels will convey the user
information from the primary site to the targeted secondary sites and the
inbound carrier channels that will convey the user data from the targeted
secondary sites to the primary site.
FIG. 8 illustrates a communications system 100 having a four wire
infrastructure. The communication system 100 includes a primary site 102,
a plurality of secondary sites 104-108 interoperably coupled to the
primary site via at least one inbound low pass transmission pass 148 and
at least one outbound low pass transmission path 150. The primary site 102
includes a controller 110, a DMT (Discrete Multi Tone) receiver 112, a DMT
transmitter 114, a data multiplexing switch 116, and an input/output port
118. Note that, if the secondary sites are separated by more than a given
geographic distance (12 Kft to 18 Kft) from the primary site, a secondary
site repeating the transmissions, repeaters 144-146 may be required.
Each of the secondary sites 104-108 include a secondary site controller
120, a secondary DMT receiver 122, a secondary DMT transmitter 124, a
secondary data multiplexing switch 126 and at least one subscriber
interface 128. The subscriber interface may be coupled to a radio
frequency (RF) transceiver 140, an RF transmitter 136, a facsimile device
130, a telephone 132, or a television monitor 134. A detailed discussion
of the secondary site will be provided below with reference to FIG. 11.
The general operation of communication system 100 begins by establishing an
inbound control channel on the inbound low pass transmission path 148 and
an outbound control channel on the outbound low pass transmission path
150. Establishing the inbound and outbound control channels will be
discussed in greater detail with reference to FIGS. 16-22.
Once the infrastructure control channels (i.e., the inbound and outbound
control channels) are established, user information may be transmitted to
a subscriber. This is accomplished when the primary site receives a call
request that identifies a targeted subscriber. The targeted subscriber may
be a communication unit 142, a pager unit 138, a facsimile unit 130, a
telephone 132, a television 134, or any other type of device that can
receive digital information. Upon receipt of the call request, the primary
site 102 determines the location of the target subscriber units. Having
determined the targets' locations, the primary site allocates carder
channels of the inbound and outbound low pass transmission paths 148, 150
depending on the type of call requested and the bandwidth requirements for
the call. For example, for a one way data transmission, the primary site
only needs to allocate carder channels on the outbound low pass
transmission path 150, while, for a two way data transmission, the primary
site would need to allocate carder channels in both the inbound and
outbound low pass transmission paths 148, 150.
To allocate the carrier channels, the primary site generates an inbound
call bit loading table from the inbound site bit loading tables of the
target sites and an outbound call bit loading table from the outbound site
bit loading tables of the target sites. From the call bit loading tables
and the bandwidth requirements of the call, the primary site selects the
carder channels to support the call. Once the carrier channels have been
selected, identity of the carrier channels are sent to the targeted
secondary sites via the outbound carrier channels supporting the outbound
control channel. Note that establishing carrier channels to support a
particular call provides a temporarily designated communication path
between the primary sites and the targeted secondary sites for this call.
Thus, the infrastructure links of the present communication system are
trunked to support a variety of calls which is in contrast to the
dedicated links of prior art communication systems.
In addition to allocating carder channels of the low pass transmission
paths 148 150, the primary site may need to allocate transmission
resources within the secondary sites to provide a communication path
between the subscriber and the secondary site. For example, in a two-way
wireless communication system, the primary site will allocate an RF
communication resource in each secondary site supporting a two-way
wireless communication. The control information identifying which RF
communication resources have been allocated will be transmitted to the
secondary sites via the outbound carrier channels functioning as the
outbound control channel.
Once the secondary sites have received and stored the carder channel
allocation information, and any RF communication resource allocation
information, the primary site 102 may transmit user information to the
targeted secondary sites via the allocated carrier channels. Upon
receiving the user information, the secondary sites can route the data to
the targeted subscriber via the allocated RF communication resource.
Additionally, the primary site, for a two-way communication, may receive
user information via the allocated inbound carder channels. Upon receipt
of this information, the primary site may retransmit this information to
the targeted secondary sites via the allocated outbound carrier channels.
FIG. 9 illustrates a communication system 160 having a two-wire
infrastructure. As shown, the primary site 102 is operably coupled to a
plurality of secondary sites 104-108 via a twisted pair low pass
transmission path 162. The low pass transmission path is a twisted pair
copper wire, which may be a telephone line. The basic interconnections of
communication system 160 are very similar to the interconnections of
communication system 100. A difference arises in that communication system
160 includes only one low pass transmission path, while communication
system 100 includes at least inbound and outbound low pass transmission
paths. Because of this difference, the primary site 102 and each secondary
site include a two-wire to four-wire conversion hybrid 164-170 to split
inbound and outbound transmissions. In addition, the conversion hybrid
provides echo cancellation such that data being transmitted via the DMT
transmitter is removed from the data being received by the DMT receiver.
The two-wire to four-wire conversion hybrids 164-170 may comprise a
transformer having a primary winding and at least one secondary winding
and echo cancellation circuitry.
FIG. 10 illustrates a schematic block diagram of the primary site 102. The
primary site 102 includes a controller 110, data multiplexing switch 116,
the DMT transmitter 114, the DMT receiver 112, memory 182, input/output
port 118, input/output port 183, a data bus 176, a control bus 178, and an
address bus 180. The controller 110 includes a processor 172 and memory
174. In practice, the controller 110 may be a micro-computer, a personal
computer, a work station, or main frame computer. Whichever type of
processing device is used as the controller 110, it should have sufficient
processing power and memory to execute the primary site functions
described throughout this specification.
In operation, the primary site determines, for data transmissions (calls),
inbound and outbound carder channel allocations for the inbound and
outbound control channels; determines inbound site bit loading information
for the plurality of secondary sites, i.e., the bit loading table for
sites based on the spectral response of the inbound low pass transmission
path; determines call bit loading tables, i.e., the bit loading tables
that are based on a set of targeted secondary sites; determines carrier
channel updates; and determines data processing for the system. Each of
these function will be discussed in greater detail below.
The primary site may receive a request for data transmission from a variety
of sources within the communication system and/or a variety of sources
external to the system. Requests from external sources may be for a page,
a facsimile, a telephone interconnect, or any type of controlling data
transfer such as a data distributor's request to send data to a particular
subscriber. To receive requests from the external sources, the primary
site would include additional input/output ports. | | |
