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Description  |
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CROSS-REFERENCE TO RELATED APPLICATIONS
This patent application is related to U.S. patent application Guarneri, et
al., Ser. No. 08/572,286, and U.S. patent application Guarneri, et al.,
Ser. No. 08/572,297 now U.S. Pat. No. 5,631,907, filed concurrently herein
concurrently herewith.
TECHNICAL FIELD
This invention relates to the field of electronic software distribution,
and, more specifically, to distribution of executable code (i.e., object
code), which requires very highly reliable (zero fault tolerance) data
transmission.
DEFINITIONS
Telephone, computer, and data communications technologies all use common
terms that sometimes imply different meanings. A brief definition of terms
relating to current application are listed here.
1.) Synchronous Satellite: A satellite for which the mean sidereal period
of revolution is equal to the sidereal period of rotation of the primary
body about which the satellite is revolving.
2.) Geosynchronous Satellite: A synchronous satellite with the Earth as its
primary body.
3.) Satellite earth terminal: That portion of a satellite link which
receives, processes, and transmits communications between a ground station
on the Earth and a satellite.
4.) Satellite Uplink: Communications (usually microwave) link from a ground
station to a satellite.
5.) Satellite Downlink: Communications link from a satellite to a ground
station.
6.) Point-to-point connection: An arrangement whereby a communication link
is established which exchanges messages between two (and only two)
designated stations, such as station A and station B, as illustrated in
FIG. 1. The message may include data relating to the application (referred
to as a "pay load"), and data relating to the network (such as addressing,
message identifier, etc., referred to collectively as a "header"). In
addition, messages may require confirmation, i.e., an acknowledgement is
expected from the receiver (station B). Alternately, the transmitter,
Station A, may send a message "unconfirmed," in which case the
transmitting process does not wait for any acknowledgements. In the normal
or usual case (i.e., when station A sends a message to station B), a send
process 101 in station A receives data from a software process 103,
translates and/or formats it according to a previously agreed-to protocol,
and sends the message across communications link 105. Receive process 107
in station B performs any translation and/or format function, again
according to the protocol, and delivers the message to software process
109.
Additionally, send process 111 in station B sends an acknowledgement of
receipt or non-receipt (as determined by software process 109) across
communications link 105 to receive process 113 in station A. Receive
process 113 delivers the acknowledgement message to software process in
station A, and determines whether to send the next portion of the data or
to retransmit the last data sent. Thus, a reliable one-to-one transmission
protocol may be established. However, the tradeoff is that the speed of
data delivery is slow, due to the one-to-one connection and the wait for
each message to be confirmed being time consuming.
7.) Broadcasting and Multicasting: These are broad terms used in the
industry to refer to point-to-multipoint and multipoint-to-multipoint
communications, as illustrated in FIGS. 2 and 3, respectively. In a
point-to-multipoint arrangement (FIG. 2), communication is established
similarly to that described in FIG. 1 between one station, designated as
the sender, and multiple stations, designated as receivers 1-N. This type
of arrangement is typically used in transferring information from one
location, e.g., news editors, to many locations that need the information,
e.g, printing presses. In this configuration, each receive process 201-203
must acknowledge proper receipt of data through its respective send
process 204-206. All of these responses must be received by receive
process 207 in sender and delivered to software process 208 for
determination whether all receivers received the data correctly. If not,
then send process 209 in sender must retransmit the data to one or more
receive processes 201-203.
As can be readily seen in the above scenario, this type of broadcast is
very slow and expensive, for the same reasons cited above, with even more
acknowledge messages to be accounted for. There is also an upper limit to
the number of receives that can be attached to sender. With current
technology, only 30-40 receivers can be attached successfully to a single
sender before the sender exhausts both its memory and computational power.
In FIG. 3, the multipoint-to-multipoint arrangement, referred to herein as
multicast, establishes communication among many designated stations. This
type of arrangement is typically found in such applications as local area
networks and conferencing. At any given time, one of the stations is
designated as the sender by means of a token and other stations are
designated as receivers. The token passing arrangement may be pre-defined,
sequential, cyclic, or passed from station to station on demand (as is
well known in the art). However, for the purpose of the current
application, the term "broadcast" is used to refer to point-to-multipoint
arrangement, similar to FIG. 2.
8.) Reliable: This term refers to procedures that guarantee delivery of
information without errors. On point-to-point connections, protocols are
generally implemented to recover lost or unacknowledged messages through
retransmission. In broadcast and multicast connections, different
techniques are used to improve the efficiency of a protocol for reliable
transfer of messages. As a general practice, messages are retransmitted at
the data-link level, which includes header and payload information, or at
a "frame" level, wherein the frame encapsulates several messages involving
header and payload data into a larger message. As used herein, the term
"reliable" is used from an application perspective, not a message
perspective.
9.) Scalable: This term refers to a network architecture where the number
of receivers may be variable and may increase by several orders of
magnitude. As known in the prior art, an increase in the number of
receivers demands corresponding increase in performance requirements on
the sender (as described above in FIGS. 2 and 3, and accompanying text). A
typical server in a local area network supports broadcast service for
approximately 10 to 15 receivers. If the number of receivers were to
increase to a larger number such as 100 or 150 (a ten-fold increase),
current approaches to broadcast and multicast communications would become
ineffective as the requirements on the server grow beyond its system
capacity.
BACKGROUND OF THE INVENTION
Distribution of software, and specifically object code, for use in
processing systems has been a problem since the beginning of stored
program controlled systems. For example, in the area of telephone
switching systems, stored program control has been used since the middle
1960's. In order to distribute a new operational program (software) that
operates these systems, initially a technician had to go to each switching
office and physically remove magnetically encoded cards and install new
magnetically encoded cards. As technology improved, magnetic tapes were
used to transport programs from the point at which they are made to their
point of utilization; in fact, magnetic tapes are still used for generic
updates which currently involve large quantities (70-100 mega-bytes (MB))
of object code. All such systems required manual steps and high
transportation costs for delivery of such software, especially as the size
of the software loads grew over time.
Some recent systems rely on telephone data links for distribution of
software. For example, the prior art system of FIG. 4 illustrates a
typical software distribution system for various switching systems in a
telephone network. Such switching systems could be local central office
switches supported by a particular manufacturer, such as 5ESS.RTM.
switches manufactured and supported by AT&T, or, alternatively, may be
long distance-type switches such as the 4ESS.TM. switch, also manufactured
and supported by AT&T. Other types of program-controlled systems may
benefit from this invention without departing from the scope of the
appended claims.
Each switch is connected to a software change and notification system
(SCANS) 102. SCANS, as known in the art, provides software updates for
switching systems 104-118 by way of data transmissions over lines 120-134
using dedicated point-to-point communication links typically operate at
9600 bits per second with an X.25 protocol.
FIG. 5 illustrates such a prior art SCANS-to-switching system connection.
In the system of FIG. 5, SCANS 100 includes an application program 500,
which processes the data to be sent (in the example of switching offices,
the object code required). Application program 500 delivers the processed
object code to a plurality of communications terminal processes 502-5NN,
which communicate with the switching offices. In each communications
terminal process 502-5NN, there is a send module 504 and a receive module
506. Send module sends the object code (again, for purposes of this
example) over line 120 to switching system 104. Receive module 506 in
terminal process 502 of SCANS 100 receives acknowledgement requests for
re-tries if needed, etc., as is known in the art, from switching module
104, via line 120.
At the switching system side, switching systems (in this example 102 and
104), also include a terminal process 508-508' which contain a send module
504 and a receive module 506 which are the same, or substantially similar,
to the send 504 and receive processes 506 in the communications terminal
process 502 of SCANS 100. Terminal process 508 in switching system 104
receives data in receive process 506 and delivers the received data to
terminal process 508. Terminal process 508 determines whether the data is
received in tact, and if so, sends acknowledgements of good reception
through send process 504 or re-try requests for data if the data appeared
to be corrupted. Switching systems 104 and 102 are shown as having several
layers that communicate with communications terminal process 508. First
there is a SCANS interface 510 which performs protocol verification, etc.
and other functions, as known in the art, with SCANS 100. If the data
received appears correct, then SCANS interface 510 passes the received
data to input/output process 512, which causes administrative module 514
to further distribute the received software to where the other processes
reside. This hierarchy is very much like the system of FIG. 2.
In this manner, changes to the programs which run switching systems 102-118
may be made through a central location, for example, at a SCANS facility
100 outside of Chicago, and then sent to each switching system which
requires the change. Furthermore, software updates, where entire sections
of programs change, may also be sent to each switch 102-118 in this
manner. Finally, an entire generic update (changing the entire operating
code) may be sent from SCANS 100, via lines 120-134, to all switching
systems 102-118 which subscribe to or purchase the new generic. Therefore,
the size of the data load being transmitted to each switch may vary from a
few hundred bytes for a minor software correction to several hundred
megabytes for an entire generic.
Turning now to FIG. 6, a prior art system is shown, wherein a switching
office is connected to SCANS 100 by way of data line 120. Switching office
104 is, for example, a 5ESS switch, as manufactured by AT&T. As is known
in the art, a 5ESS switch (local switch 104) may be a distributed control
ISDN electronic telephone switching system such as the system disclosed in
U.S. Pat. No. 4,592,048, issued to M. W. Beckner, et al. on May 27, 1986,
and assigned to the assignee of this application. Alternatively, local
switch 104 may be a digital switch such as a 5ESS switch manufactured by
AT&T and described in the AT&T Technical Journal, Vol. 64, Number 6,
July/August, 1995, pages 1303-1564.
The architecture of switch 104 includes a communication module 602 as a
hub, with switching modules 604, 606, and 608 illustrated (there may be
other switching modules but these are not shown for clarity) and an
administrative module (AM) 610, emanating from communication module 602.
Communication module 602 includes a time-shared, space division switch or
time-multiplexed (TM) switch as a fabric for communications among switch
modules 604, 606, 608, and between switch modules 604, 606 and 608 the AM
610. AM 610 provides coordination of the functional components of switch
104 and human-machine interface. Switch modules 604, 606, and 608
terminate analog and/or digital subscriber lines through line units (not
shown but well-known in the art) and analog or digital trunk units (again,
not shown but well known in the art) and communicate with CM 602 over
control timeslots 611 (for sending control data) and other timeslots 613
(used for call processing). AM 610 also provides connections to other
switching systems through, for example, a signaling system 612 (such as a
common channel signaling network) by which the switching systems in a
network communicate, and to SCANS 100 via connection 120.
In the current art, SCANS 100 sends data on line 120 at typically 9600
baud. This data rate is adequate when SCANS 100 is sending small changes
(or "patches") for code to switching office 104. However, when SCANS 100
is sending major updates or a generic update over line 120, this
transmission may take many hours, depending on the size of the load or
generic which is being sent to the administrative module 610.
The burden of distributing large software loads, particularly object code,
at 9600 bps to AM 610 may interfere with other maintenance tasks of AM
610. For example, receiving an entire generic causes AM 610 to respond
more slowly to signaling messages from signaling network 612 and for
routing and administrative function requests from SMs 604-608 and CM 602.
Therefore, it has been proposed that AM 610 be assisted by a work station,
such as 614 (shown in phantom). Work station 614 is connected to SCANS 100
(instead of AM 610) and then communicates with AM 610 to build loads and
otherwise direct AM 610 with the information delivered from SCANS 100.
However, there is still a great deal of time involved delivering data from
SCANS 100 to work station 614; work station 614 merely eases some of the
processing burden on AM 610.
Furthermore, SCANS 100 can only deal with one or a small number of
switching systems at a time, due to the processing effort required to
interface with (i.e., the physical number of ports) and support
transmission on (i.e., memory and processing requirements) multiple
systems (see FIG. 2 and associated text). Returning briefly to FIG. 5,
there is a communications terminal process 508 associated with each
switching system. Each communications terminal process requires a portion
of main memory and a time-slice of the processor of SCANS 100. Therefore,
as the number of terminal processes increases, the processing memory
demands on SCANS 100 increase; thus only a limited number of switching
systems can be served at a given time.
Therefore, a problem in the art is that there is no method for delivering
data at a high rate of speed to multiple units simultaneously, while still
maintaining reliability of point-to-point communications. One of the
objectives of the current application is to maintain reliability of data
through simple recovery procedures even when individual messages are lost
or corrupted during data transmission. Therefore, an object of this
invention is to provide a communication means which does not have preset
limits on the scalability of the network architecture while, at the same
time, meeting other constraints on reliability, message structure,
integrity, and transmission speed.
SUMMARY OF THE INVENTION
This problem is solved and a technical advance is achieved in the art by a
system and method which can deliver data at very high data transmission
speeds to many locations simultaneously. According to an apparatus aspect
of this invention, a SCANS is supplied with a satellite uplink
communication module which transmits data to an earth orbiting satellite.
The satellite then transmits the data to a wide geographical area. Each
receiving location is equipped with a small satellite dish aimed such that
it may receive any data beamed from the satellite. Advantageously, the
satellite dish is connected to a work station in the switching office
which then processes the received data and delivers all information in a
form that is ready for use by the modules' switching office.
According to a method of this invention, the SCANS processes data to be
transmitted into blocks, such blocks including error correction
information. It then sends a transmission of the blocks to a satellite
from first block to last block without pausing for acknowledgements from
any of the receiving stations. Such data is retransmitted down from the
satellite to all of those offices identified by a broadcast identifier,
mail alias, software package identification, and/or other relevant address
information. Thus, it is possible to reach a very large number of
receiving stations. Traditional broadcast and multicast protocols with
acknowledgements require a predetermined increase in size of the sender to
support an increase in number of receivers. In contrast, the proposed
method uses an unreliable (i.e., no confirmation of data receipt)
connectionless delivery service, (e.g., User Datagram Protocol (UDP)).
Thus, there is no feedback channel from the switching offices to the SCANS
to provide acknowledgements for received messages, order and sequence of
the messages, and to provide feedback to control the rate at which
information is transmitted to switching offices. As a result, the data
transmission may result in bit-errors, burst-errors due to environmental
conditions, out-of-sequence blocks, and some blocks may be lost due to
overflow conditions. In the present invention, the responsibility for
error detection, error correction, recovery, and maintenance of data
integrity is left entirely to the receiving stations.
In this invention, it is recognized that attempts to correct errors at a
block level are inefficient when dealing with a large number of receiving
stations. Instead, errors during data transmission are noted at the
receiving station for further processing. After the blocks are broadcast
from first to last, the SCANS pauses for a predetermined interval during
which time each receiving station performs error detection, error
correction, and other recovery procedures on the blocks it just received.
The SCANS, using satellite transmission, then broadcasts the same blocks
once again from the beginning to the end. The system may be programmed to
retransmit any predetermined number of times.
Satellite broadcasts of data in this fashion without acknowledgements do
not increase performance requirements on the transmitter even if the
number of receivers increases several orders of magnitude, thus this
system is "scalable." This data transmission, however, is considered
"unreliable" as it is subject to environmental conditions and due to the
use of an unreliable broadcast protocol. This problem is solved by
addition of new design features to the broadcast protocol.
Advantageously, after the SCANS has completed the preset number of
transmissions, if a work station has yet to complete recovery of one, or a
few blocks, then it may dial up the SCANS or a maintenance center to
receive the necessary block using a point-to-point serial link
communications or other means comprising of data communication.
Advantageously, each transmitted block is encoded using forward error
correction in order to further enhance the probability of proper reception
of the data. Thus, a very high-speed broadcast of data/software updates
can be sent to many switching offices simultaneously with a guarantee of
high accuracy of reception.
To summarize, use of multiple satellite retransmission with error
correction using the retransmission provides wide geographic coverage; use
of an unreliable broadcast protocol without acknowledgements improves
speed and number of receive transmissions, wherein the recovery procedures
during retransmissions improves the reliability; and the use of
application-level forward error correction improves overall reliability of
the system above and beyond the transmission system reliability offered by
the satellite broadcasts.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete understanding of the invention may be obtained from a
consideration of the following description in conjunction with the
drawings, in which:
FIG. 1 is a block diagram of a prior art point-to-point communication link;
FIG. 2 is a block diagram of a prior art point-to-multipoint configuration
of data transmission;
FIG. 3 is a block diagram of a prior art multipoint-to-multipoint data
distribution system;
FIG. 4 is a prior art block diagram of the current approaches to software
update system as used in telecommunication systems;
FIG. 5 is an example of a block diagram of a prior art system of FIG. 4
showing the numerous processes required in order to distribute software
through the system of FIG. 4;
FIG. 6 is a block diagram of a prior art switching office illustrating how
software received from the system as shown in FIGS. 4 and 5 is distributed
through the switching office;
FIG. 7 is a block diagram of a satellite transmission system according to
an exemplary embodiment of this invention;
FIG. 8 is a block diagram of data or code packaging as used in the
exemplary embodiment of FIG. 7;
FIG. 9 is an example of a fully packaged data transmission load of FIG. 8;
FIG. 10 is a diagram illustrating encoding forward error correction
information into the data as packaged in FIG. 9;
FIG. 11 illustrates transmission of the package blocks as shown in FIG. 10;
FIG. 12 illustrates individual cell transmissions according to the
structure of FIG. 11;
FIG. 13 shows the interaction between the satellite and the switching
office for receiving data according to the exemplary embodiment of this
invention;
FIG. 14 illustrates the iterative nature of the transmission of data blocks
according to the exemplary embodiment of this invention;
FIG. 15 illustrates those portions of a work station as illustrated in FIG.
13 which tracks the blocks that were not received properly;
FIG. 16 illustrates the processing of work station of FIG. 15 illustrating
how lost blocks are determined;
FIG. 17 is a flow chart illustrating the processing according to FIG. 16;
FIG. 18 is a flow chart illustrating the general operation of the work
station according to FIG. 15;
FIG. 19 illustrates the use of this invention in a context other than
telephone switching systems; and
FIG. 20 illustrates using the current invention in a point-to-point system
for absolute reliability.
DETAILED DESCRIPTION
FIG. 7 illustrates an exemplary embodiment of this invention distributing
data to a plurality of destinations simultaneously. In this exemplary
embodiment, telephone switching systems 102-118 will again be used to
illustrate the invention; however, this invention is applicable whenever
large amounts of data, software--particularly executable or object
code--needs to be transferred to many places at the same time.
In this exemplary embodiment, SCANS 100 receives the software or data to be
transmitted, as in the prior art. This data, for a typical switching
system such as the AT&T 5ESS Switch, is approximately 70 MB of executable
code in compressed form. The data is processed into blocks, as will be
described below, and sent from SCANS 100 to satellite uplink 200.
Satellite uplink optionally processes the data further, according to its
own format and error correction system, and transmits the data (from first
block to last block without pause) to satellite 202. Satellite 202
retransmits the data to a dish antenna at each switch office 102-118. As
will be described below, a system at each switching office 102-118
translates the data back into usable form, processes and delivers it as
required. SCANS 100 resends the data from first block to last through
satellite uplink 200 via satellite 202. Any data blocks not received in
the previous transmission(s) may thus be received. Thus, a new system for
delivering large amounts of data is shown in FIG. 7.
Turning to FIG. 8, according to one aspect of this invention, the data is
divided into memory pages of 9400 bytes, each as shown in FIG. 8. Each
page is arranged in a matrix form of 40 rows and 235 columns. Each memory
page is called an Information matrix (I).
Turning now to FIG. 9, each page of FIG. 8, matrix I, is augmented with
1880 bytes of information arranged in a matrix form of 8 rows and 235
columns which contain operational information. The operational information
includes such information as file numbers, software package
identification, sequence numbers for memory pages, ATM transport cell
identification, methods for encryption/decryption, information regarding
decompression of the user data, and broadcast addressing scheme to
activate preset receiving stations. The resulting data is called the
Operations matrix (O). Together, data from I and O arranged in 48 rows and
235 columns comprises the user data.
Advantageously, user data being transmitted via satellite is encoded using
forward error correction. The forward error correction of this exemplary
embodiment is known in the art as the "block-interleaved Reed-Solomon
system." This system allows for receiving stations to recover from
bit-errors and burst errors that otherwise may render an information page
to be discarded. The encoding of data is performed on a row-by-row basis
so that, for each memory page of user data, 48.times.235 bytes, the
resulting data is arranged in 48 rows and 255 columns as shown in FIG. 10.
The resulting data is referred to as a "data block," designated by matrix
B. The encoding of information is well known in the prior art, for example
see "The Theory of Error Correcting Codes," by F. J. Macwilliams and N. J.
A. Sloane, and thus, will not be discussed here. Those familiar with the
art will recognize that the resulting matrix satisfies that, for a symbol
size of one byte (or 8 bits), the number of symbols in the field is 255
(2.sup.8 -1=255) and that a loss of up to 10 symbols (1/2 the redundancy,
wherein 255-235=20) can be corrected when error positions are unknown and
up to 20 symbols can be corrected when the knowledge of the exact
positions of errored symbols is known.
Turning now to FIG. 11, after encoding, SCANS 100 has the software arranged
in blocks of 12,240 bytes, each in 48 rows and 255 columns as described
above. The original user data is coded to form blocks 1-N respectively.
One column of each block (48 bytes) is then loaded into the payload of an
ATM cell.
Turning now to FIG. 12, in this exemplary embodiment, SCANS 100 is
connected to a satellite uplink station 200. Satellite uplink stations
such as 200, are well known in the art of, for example, audio, video, and
data transmissions, and thus will not be described further. Satellite
uplink transmitter transmits data to satellite 202. Satellite 202 may be
in geosynchronous, low earth, or medium earth orbit depending on the
nature of application and geographic area to be covered. Satellite 202
retransmits the data signal to multiple locations, in this example, to a
plurality of switching offices, such as 102-118 (FIG. 7).
In this exemplary embodiment, SCANS 100 starts transmitting data from the
beginning to the end, i.e., block-1 through block-N (FIG. 11). Within each
block, SCANS transmits a single column of 48 rows as payloads of the ATM
cells as shown in FIG. 11. In this arrangement, one block of information
is transmitted as 255 ATM cells, whose beginning and end are identified by
the information encoded in the Operations matrix (O) FIG. 10.
In this embodiment, SCANS 100 sends each block of data without waiting for
any acknowledgements of receipt of previous blocks back from switching
systems 102-118 (as in for example, User Datagram Protocol (UDP)). UDP is
a well-known protocol used in computer and data communications, and, more
particularly, in the Internet connected systems and, thus, will not be
described further.
SCANS 100 broadcasts the entire program (comprising blocks 1-N) via
satellite uplink-downlink multiple times, with a waiting period between
each broadcast. Currently, broadcast speeds of up to 30 and 40 Mbps (mega
bits per second) are available. It is estimated that an entire switching
system generic, originally 70 MB of executable code, can be transmitted in
approximately 45 seconds (7447 blocks, each 12240 bytes transmitted at 30
Mbps). Therefore, even with a 5-10 minute wait period between broadcasts,
it is possible to transmit and retransmit the entire switching system
generic more than five times in one hour of satellite usage. It is thus
obvious that the relative cost of usage is minimal compared to other
alternatives used in the prior art.
Turning now to FIG. 13, a specific switching office is illustrated,
operating to receive data from satellite 202. The program is transmitted
from the SCANS 100 through satellite uplink 200, to switching office 104,
which is fitted with an outside satellite antenna 1302. In an exemplary
embodiment, this may be similar to, or the same as, the satellite receiver
dishes commercially available for satellite television reception.
Satellite receiver dish 1302 is connected to work station 614 by a
receiver 1310 and modem 1312. Work station 614 includes an interface for
receiving data from modem 1312, and, as known in the art, performs any
usual modem-performed translations. Furthermore, data may be encrypted
and/or compressed in order to prevent others from intercepting the data
transmission and to cut down on transmission time. Work station 614 also
performs such decryption and decompression functions in order to process
the received data and make available the original object code for the
switching system 104. Further, work station 614 receives information from
AM 610 regarding office configuration and compiles such data into a usable
generic. Work station 614 then downloads the generic to AM 610, which in
turn, propagates CM data to CM 602 and SM data through CM 602 to the SMs
represented by 604-608. The physical link connecting CM 202 to SMs 604-608
supports 512 timeslots, and in one instance of the exemplary embodiment,
two timeslots are used as control timeslots and the remainder are used for
telephone calls.
Turning now to FIG. 14, a time chart showing the transmissions of data
blocks is shown. It is recognized that not every switching office will
necessarily receive every ATM cell, data frame, or data block correctly.
Furthernmore, each switching office may have problems with reception of a
different data block. However, since SCANS 100 broadcasts the data
multiple times, each individual office has a high probability of receiving
all of the data blocks after all iterations. In the example of FIG. 14,
the first transmission of, for example, a generic object code, begins at
time X and ends at time Y. There is a wait time of interval W during which
each work station processes the data received and determines which data
blocks were incorrectly received and could not be recovered through means
of error correction. A second transmission then begins at time A and
proceeds through to end at time B. The data transmitted in Transmission
A-B is identical to the data transmitted in Transmission X-Y. This
mechanism of data transmission and wait time of interval W continues
through to the last transmission, which again broadcasts the exact same
data as Transmission X-Y and Transmission A-B. The number of transmissions
is a parameter which may be varied according to field of experience,
environment and weather conditions, and the nature and criticality of an
application.
Turning now to FIG. 15, the operation of work station 614 is illustrated in
block diagram form. Work station 614 comprises, as is generally known in
the art, a CPU 1502, memory 1504, an interface to the switch 1506
(specifically AM 610), and a bus 1508. Additionally, work station has a
SCANS interface 1510, as known in the art. Finally, work station 614 also
includes a satellite dish interface 1512. The satellite dish interface
includes a receiver and a modem as used in data communications. In one
implementation, interface 1512 may process all data transmissions received
from the satellite interface and pass the received ATM cells to work
station 614 for further processing. In this arrangement, satellite
transmitter-receiver units can be supplied by a variety of service
providers and maintain an open (non-proprietary) interface between the
work station bus 1508 and interface 1512. Alternatively, the receiver and
modem unit interface 1512 may be enhanced with software provided by the
SCANS 100, i.e., combine the functions of satellite receiver 1512 and the
SCANS interface 1510 into one integrated system which allows SCANS error
detection, correction, and recovery procedures to work directly with the
satellite receiver for efficient processing. It is known in the prior art
that such integration of functions can be efficiently implemented in
hardware but be proprietary to the manufacturer whereas the software
structure, described above, may be inefficient but have an open
architecture.
In operation, data is received from the satellite dish 1302, and is sent to
interface 1512. Interface 1512 processes the data received based on data
link layer checks, such as frame check sequence and/or cyclical redundancy
checks, to determine bit-errors during data transmission. Some errors may
be recovered based on procedures built into the transmitter and the
receiver. For example, when using ATM transport, the 5 bytes of ATM header
information may correct 1 bit errors during data transmission. Turning to
FIG. 10, advantageously, additional layers of Forward Error Correction are
generally built into the commercially available transmitter-receiver
systems. The receiver interface 1512 processes the received data as
necessary and sends the data via bus 1508 to memory 1504, under control of
CPU 1502. SCANS interface 1510 assimilates all of the received data in the
block structure, block-1 through block-N, as arranged at the transmitting
end.
SCANS interface 1510, under the control of CPU 1502, performs the error
detection, correction, and recovery procedures to determine if any of the
blocks are unusable due to bit-errors, corruption, or lost cells. This
procedure is performed on each of the received blocks, (as illustrated in
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