U.S. patent application number 11/076737 was filed with the patent office on 2005-07-14 for system and method for a robust preamble and transmission delimiting in a switched-carrier transceiver.
This patent application is currently assigned to Paradyne Corporation. Invention is credited to Chapman, Joseph, Holmquist, Kurt.
Application Number | 20050152404 11/076737 |
Document ID | / |
Family ID | 34742629 |
Filed Date | 2005-07-14 |
United States Patent
Application |
20050152404 |
Kind Code |
A1 |
Holmquist, Kurt ; et
al. |
July 14, 2005 |
System and method for a robust preamble and transmission delimiting
in a switched-carrier transceiver
Abstract
A method and system for robust delimiting of messages in
switched-carrier operation in which a preamble precedes each
message. The preamble comprises symbols transmitted at a rate lower
than that of the following data. The lower rate significantly
increases the probability that decoding of the preamble symbols
will be error-free. Communication line control information can be
included in the preamble, thereby ensuring that line control
information is reliably transferred over the channel. The
preamble's first symbol can be transmitted at the lower symbol rate
and at an increased power level, thereby reliably delimiting the
beginning of a transmission. The end of the message can be reliably
delimited by sending the first symbol which contains only bits from
a next cell of information at a lower symbol rate, and including an
extra bit in that symbol. The extra bfit can indicate to a receiver
when the last cell of information begins.
Inventors: |
Holmquist, Kurt; (Largo,
FL) ; Chapman, Joseph; (Seminole, FL) |
Correspondence
Address: |
THOMAS, KAYDEN, HORSTEMEYER & RISLEY, LLP
100 GALLERIA PARKWAY, NW
STE 1750
ATLANTA
GA
30339-5948
US
|
Assignee: |
Paradyne Corporation
|
Family ID: |
34742629 |
Appl. No.: |
11/076737 |
Filed: |
March 10, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11076737 |
Mar 10, 2005 |
|
|
|
09637185 |
Aug 11, 2000 |
|
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|
60150436 |
Aug 24, 1999 |
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Current U.S.
Class: |
370/485 |
Current CPC
Class: |
H04L 1/0025 20130101;
H04L 1/0002 20130101 |
Class at
Publication: |
370/485 |
International
Class: |
H04L 005/16 |
Claims
Therefore, having thus described the invention, at least the
following is claimed:
1. A method for adapting transmission rate in a communication
device over a communication channel by including rate information
in a first portion of a communication message, comprising the steps
of: transmitting the first portion at a first rate, the first
portion operating to frame the message and to delimit the message
from silence and including rate information specifying a rate at
which data in a second portion of the communication message is
transmitted; and transmitting the data at a second rate specified
by the rate information included in the first portion.
2. The method of claim 1, wherein all transmitted messages include
the rate information.
3. The method of claim 1, further comprising the step of encoding
the first portion into a plurality of symbol indices, each of the
symbol indices encoded at a lower bits-per-symbol rate relative to
a maximum rate supported by the device over the communication
channel.
4. The method of claim 3, further comprising the step increasing
the energy of the first symbol index.
5. The method of claim 1, wherein the rate information specifies a
bits-per-symbol rate at which the data is encoded.
6. The method of claim 1, wherein the first rate is a fixed value
known to the device and to a remote device in communication with
the device over the communication channel.
7. The method of claim 1, further comprising the step of
indicating, via the first portion, whether the second portion
includes data and, if so, the format and type of data.
8. The method of claim 1, further comprising the step of
indicating, via the first portion, whether administrative
information follows the first portion.
9. A computer-readable medium containing a program for adapting
transmission rate in a communication device over a communication
channel by including rate information in a first portion of a
communication message, the program comprising the steps of:
transmitting the first portion at a first rate, the first portion
operating to frame the message and to delimit the message from
silence and including rate information specifying a rate at which
data in a second portion of the communication message is
transmitted; and transmitting the data at a second rate specified
by the rate information included in the first portion.
10. The computer-readable medium of claim 9, wherein all
transmitted messages include the rate information.
11. The computer-readable medium of claim 9, the program further
comprising the step of encoding the first portion into a plurality
of symbol indices, each of the symbol indices encoded at a lower
bits-per-symbol rate relative to a maximum rate supported by the
device over the communication channel.
12. The computer-readable medium of claim 3, the program further
comprising the step increasing the energy of the first symbol
index.
13. The computer-readable medium of claim 9, wherein the rate
information specifies a bits-per-symbol rate at which the data is
encoded.
14. The computer-readable medium of claim 9, wherein the first rate
is a fixed value known to the device and to a remote device in
communication with the device over the communication channel.
15. The computer-readable medium of claim 9, the program further
comprising the step of indicating, via the first portion, whether
the second portion includes data and, if so, the format and type of
data.
16. The computer-readable medium of claim 9, the program further
comprising the step of indicating, via the first portion, whether
administrative information follows the first portion.
17. An apparatus for adapting transmission rate in a communication
device over a communication channel by including rate information
in a first portion of a communication message, the apparatus
comprising: first logic configured to transmit the first portion at
a first rate, the first portion operating to frame the message and
to delimit the message from silence and including rate information
specifying a rate at which data in a second portion of the
communication message is transmitted; and second logic configured
to transmit the data at a second rate specified by the rate
information included in the first portion.
18. The apparatus of claim 17, wherein all transmitted messages
include the rate information.
19. The apparatus of claim 17, wherein the rate information
specifies a bits-per-symbol rate at which the data is encoded.
20. The apparatus of claim 17, wherein the first rate is a fixed
value known to the device and to a remote device in communication
with the device over the communication channel.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of Application Ser. No.
09/637,185, filed Aug. 11, 2000, which claims the benefit of U.S.
Provisional Application No. 60/150,436 filed Aug. 24, 1999. These
applications are entirely incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates generally to communications
systems, and more particularly, to a system and method for a robust
preamble and transmission delimiting in a switched-carrier
transceiver.
BACKGROUND OF THE INVENTION
[0003] Data communication typically occurs as the transfer of
information from one communication device to another. This is
typically accomplished by the use of a modem located at each
communication endpoint. In the past, the term modem denoted a piece
of communication apparatus that performed a modulation and
demodulation function, hence the term "modem". Today, the term
modem is typically used to denote any piece of communication
apparatus that enables the transfer of data and voice information
from one location to another. For example, modern communication
systems use many different technologies to perform the transfer of
information from one location to another. Digital subscriber line
(DSL) technology is one vehicle for such transfer of information.
DSL technology uses the widely available subscriber loop, the
copper wire pair that extends from a telephone company central
office to a residential location, over which communication
services, including the exchange of voice and data, may be
provisioned. DSL devices can be referred to as modems, or, more
accurately, transceivers, which connect the telephone company
central office to the user, or remote location typically, referred
to as the customer premises. DSL communication devices utilize
different types of modulation schemes and achieve widely varying
communication rates. However, even the slowest DSL communications
devices achieve data rates far in excess of conventional
point-to-point modems.
[0004] DSL transceivers can be used to provision a variety of
communication services using, for example, asynchronous transfer
mode (ATM). ATM defines a communication protocol in which 53 octet
(byte) cells are used to carry information over the DSL
communication channel. The first five octets of the ATM cell are
typically used for overhead and the remaining 48 octets are used to
carry payload data. When using a switched-carrier transmission
methodology, a control transceiver may be connected via the DSL to
one or more remote transceivers. In such a communication scheme,
the transmission is commonly referred to as "half-duplex," which is
defined as two way electronic communication that takes place in
only one direction at a time. With only a single remote transceiver
on a line, switched-carrier transmission may instead be employed in
full-duplex mode (i. e., allowing transmission in both directions
simultaneously). In this case, full-duplex operation is typically
enabled by employing either echo cancellation or frequency division
multiplexing. Hybrid techniques are possible such as one in which
there are multiple remote transceivers and communication takes
place between the control transceiver and only one remote
transceiver in full-duplex fashion. As it relates to the present
invention, the common characteristic of these communication
techniques is the use of a switched-carrier modulation in which
transmitters are deliberately silent for some interval between
signal transmissions. For simplicity, the following discussions
assume the simplest case of using switch carrier modulation with a
half-duplex (also sometimes referred to as "time domain duplex")
line usage discipline.
[0005] Before the transmission of ATM cells, a preamble containing
channel, transmission, address and administrative information may
be transmitted by the transceiver. The application of this preamble
is sometimes referred to as "framing" the data to be transmitted.
Due to the switched-carrier nature of the transmission, silence
precedes this preamble and it is of course important for all
symbols in this preamble to be received error free. It is also
desirable to have the ability to precisely delimit the beginning
and end of a transmission to within one transmitted symbol
interval. Robustly delimiting the beginning of a message enables a
receiving transceiver to reliably begin immediately decoding the
message at the correct symbol. Likewise, robustly delimiting the
end of a message enables a receiving transceiver to reliably decode
the entire message through the final symbol and then stopping so as
to prevent data loss and to prevent the inclusion of any false
data. Furthermore, by communicating the end of message indicator to
a receiving transceiver prior to the actual end of the message,
line turnaround time (i.e., idle time on the line between
transmissions) can be reduced, thereby increasing the effective use
of the available line bandwidth.
[0006] Because the most efficient signal constellation encoding
cannot allocate signal space to silence, it is impractical to
reliably discriminate silence from a signal when analyzing only a
single symbol encoding an arbitrary data value.
[0007] To improve message delimiting, existing techniques use
special marker symbols whose symbol indices are greater than those
used to encode data. At N bits per symbol (bps) data is encoded
using symbol indices 0 through 2.sup.N-1. The special symbols use
indices 2.sup.N and above. While these special marker symbols are
useful for marking the beginning and end of a transmission, their
placement at the outer edges of a constellation raises the peak
signal, thus increasing the peak to average ratio (PAR) across all
data rates by as much as 4 dB. Unfortunately, discrimination of
special symbols has the same error threshold as does decoding of
data.
[0008] Thus, it would be desirable to have a robust manner in which
to detect the beginning and end of a transmission so that line
bandwidth can be most efficiently allocated. Furthermore, it would
be desirable to robustly transmit a message preamble including
control information thereby greatly improving the probability that
the preamble is received error free.
SUMMARY OF THE INVENTION
[0009] The present invention provides an improved system and method
for robustly delimiting a message transmission in switched-carrier
communication systems. The invention provides a method and system
for transmission of a message preamble in which transmission of the
preamble is more robust than the data. In this manner, the
beginning and end of a transmission can be robustly delimited and
channel control information can be reliably conveyed to a receiving
transceiver.
[0010] The system of the present invention uses a novel header
application, which enables the transport of ATM, or any other data,
efficiently and economically over a communications channel, such as
a DSL communications channel.
[0011] Briefly described, in architecture, the system for robust
transmission delimiting comprises a communication message including
a preamble including a plurality of bits representing communication
link control information, and an encoder configured to encode the
preamble bits into a plurality of symbol indices. The symbol
indices are encoded at a lower bit per symbol rate relative to the
maximum rate capable of being supported over a communication
channel.
[0012] In another aspect, the invention is a system for delimiting
the end of a transmission. The system takes a communication message
segmented into a plurality of fixed size units, each fixed size
unit including a plurality of bits, and includes an encoder
configured to encode the plurality of bits into a plurality of
symbol indices at a first data rate. The encoder is also configured
to encode the first symbol index containing only bits from each
fixed size unit at a data rate lower than that of the first data
rate.
[0013] The present invention can also be viewed as a method for
robust transmission delimiting comprising the steps of applying a
preamble to a communication message, the preamble including a
plurality of bits representing communication link control
information, and encoding the preamble bits into a plurality of
symbol indices. The symbol indices are encoded at a lower bit per
symbol rate relative to the maximum rate capable of being
transmitted over a communication channel.
[0014] In another aspect, the invention is a method for delimiting
the end of a transmission comprising the steps of segmenting a
communication message into a plurality of fixed size units, each
unit including a plurality of bits, encoding a plurality of the
bits in the cells into a plurality of symbol indices, the symbol
indices being encoded at a first rate, and encoding the first
symbol index containing only bits from each fixed size unit at a
rate lower than that of the first rate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The invention can be better understood with reference to the
following drawings. The components in the drawings are not
necessarily to scale, emphasis instead being placed upon clearly
illustrating the principles of the present invention. Moreover, in
the drawings, like reference numerals designate corresponding parts
throughout the several views.
[0016] FIG. 1 is a schematic view illustrating a switched-carrier
half-duplex communication environment, in which DSL transceivers
containing the present invention reside;
[0017] FIB 2A is an illustration of the time-domain duplex
communication methodology employed by the DSL transceivers of FIG.
1;
[0018] FIG. 2B is a schematic view illustrating, in further detail,
a communication message of FIG. 2A;
[0019] FIG. 3A is a schematic view illustrating the bit to symbol
relationship of the communication message of FIG. 2B;
[0020] FIG. 3B is a schematic view illustrating, in further detail,
the preamble of FIG. 3A;
[0021] FIG. 4A is a graphical illustration representing a two (2)
bit per symbol signal space constellation and the increased energy
symbol of FIG. 3B;
[0022] FIG. 4B is a graphical illustration showing an exemplar
grouping of constellation points representing different bit per
symbol rates in accordance with an aspect of the invention;
[0023] FIG. 5 is a schematic view illustrating the communication
message of FIG. 3A and a technique for scrambling that further
improves reliable transmission of the message preamble;
[0024] FIG. 6 is a schematic view illustrating the communication
message of FIG. 3A and the reduced line turn around delay made
possible by an aspect of the invention;
[0025] FIG. 7 is a block diagram illustrating the control DSL
transceiver of FIG. 1;
[0026] FIG. 8 is a block diagram illustrating the encoder of FIG.
7; and
[0027] FIG. 9 is a block diagram illustrating the decoder of FIG.
7.
DETAILED DESCRIPTION OF THE INVENTION
[0028] Although, described with particular reference to the
transmission of ATM cells over a DSL communication channel, the
system and method for a robust preamble and transmission delimiting
can be implemented to transmit all forms of data in any
switched-carrier transmission system in which it is desirable to
send a robust preamble and to robustly delimit the beginning and
end of each communication message.
[0029] Furthermore, the system and method for a robust preamble and
transmission delimiting can be implemented in software, hardware,
or a combination thereof. In a preferred embodiment(s), selected
portions of the system and method for a robust preamble and
transmission delimiting are implemented in hardware and software.
The hardware portion of the invention can be implemented using
specialized hardware logic. The software portion can be stored in a
memory and be executed by a suitable instruction execution system
(microprocessor). The hardware implementation of the system and
method for a robust preamble and transmission delimiting can
include any or a combination of the following technologies, which
are all well known in the art: an discrete logic circuit(s) having
logic gates for implementing logic functions upon data signals, an
application specific integrated circuit having appropriate logic
gates, a programmable gate array(s) (PGA), a field programmable
gate array (FPGA), etc.
[0030] Furthermore, the robust preamble and transmission delimiting
software, which comprises an ordered listing of executable
instructions for implementing logical functions, can be embodied in
any computer-readable medium. Moreover, use by or in connection
with an instruction execution system, apparatus, or device, such as
a computer-based system, processor-containing system, or other
system that can fetch the instructions from the instruction
execution system, apparatus, or device and execute the
instructions.
[0031] In the context of this document, a "computer-readable
medium" can be any means that can contain, store, communicate,
propagate, or transport the program for use by or in connection
with the instruction execution system, apparatus, or device. The
computer readable medium can be, for example but not limited to, an
electronic, magnetic, optical, electromagnetic, infrared, or
semiconductor system, apparatus, device, or propagation medium.
More specific examples (a non-exhaustive list) of the
computer-readable medium would include the following: a electrical
connection (electronic) having one or more wires, a portable
computer diskette (magnetic), a random access memory (RAM), a
read-only memory (ROM), an erasable programmable read-only memory
(EPROM or Flash memory) (magnetic), an optical fiber (optical), and
a portable compact disc read-only memory (CDROM) (optical). Note
that the computer-readable medium could even be paper or another
suitable medium upon which the program is printed. As the program
can be electronically captured, via for instance optical scanning
of the paper or other medium, then compiled, interpreted or
otherwise processed in a suitable manner if necessary, and then
stored in a computer memory.
[0032] Turning now to the drawings, FIG. 1 is a schematic view
illustrating a switched-carrier half-duplex communication
environment 11, in which DSL transceivers containing the present
invention reside. Although the invention will be described below in
a half-duplex communication environment, the DSL transceivers
containing the invention may be used in a switched-carrier
full-duplex environment as well. In such a case, full-duplex
operation may be enabled using technologies such as echo
cancellation or frequency division multiplexing. Communication
environment 11, includes central office 12 connected via
communication channel 16 to customer premises 21. Communication
channel 16 can be any physical medium over which communications
signals can be exchanged, and in the preferred embodiment, is the
copper wire pair that extends from a telephone company central
office to an end-user location, such as a home or office. Central
office 12 includes DSL transceiver 100 connected to communication
channel 16. DSL transceiver 100 processes data via connection 14.
DSL transceiver 100 exchanges data via connection 14 with any data
terminal equipment (DTE), such as a computer or data terminal.
[0033] Customer premises 21 includes one or more DSL transceivers
150 connected via internal infrastructure wiring 18 to
communication channel 16. The infrastructure wiring 18 can be, for
example but not limited to, the telephone wiring within a private
residence or within an office. DSL transceivers 150 can be
connected to a variety of telecommunication devices located at
customer premises 21. For example, DSL transceiver 150 connects via
connection 22 to a personal computer 26. Although additional DSL
transceivers can be located at customer premises 21, an exemplar
one of which is indicated using reference numeral 155, the aspects
of the invention to be discussed below are also applicable if only
one DSL transceiver 150 is located at customer premises 21. In the
example given in FIG. 1, DSL transceiver 155 connects to computer
28 via connection 29.
[0034] The DSL transceiver 100 located at central office 12 is
considered a "control device" and the DSL transceiver 150 located
at customer premises 21 is considered a "remote device." This is so
because the control DSL transceiver 100 controls the communication
sessions by periodically polling each remote DSL transceiver 150 to
determine whether the remote device has information to transmit.
Regardless of the number of DSL transceivers located at customer
premises 21, the method of communication between DSL transceiver
100 located at central office 12 and DSL transceiver 150 located at
customer premises 21 is half-duplex in nature, sometimes referred
to as adaptive time-domain duplex, or data driven half-duplex,
unless the above-mentioned technologies such as echo cancellation
or frequency division multiplexing allow full-duplex operation
between the control transceiver 100 and one remote transceiver 150.
This means that during any time period only one DSL transceiver may
transmit at any time. In the situation in which there are multiple
DSL transceivers located at customer premises 21, the DSL
transceiver 100 located at central office 12 periodically polls
each DSL transceiver located at customer premises 21 at an
appropriate time to determine whether any of the remotely located
DSL transceivers have any information to transmit to central office
12. If only one DSL transceiver 150 is located at customer premises
21, the communication method may be half-duplex in nature or
conventional full-duplex techniques may be used (e. g., using
either frequency division multiplexing or echo cancellation).
[0035] FIG. 2A is a schematic view illustrating the time-domain
duplex communication methodology between a control DSL transceiver
100 and a remote DSL transceiver 150. When a control DSL
transceiver 100 desires to send a message to a remote DSL
transceiver 150 the control DSL transceiver 100 sends a
communication message 31 including a preamble and any information
that is to be transmitted. There are times when the communication
message may include only a preamble. After the transmission of
communication message 31, the remote DSL transceiver to which
communication message 31 is addressed (in this example remote DSL
transceiver 150) responds with communication message 32. After the
remote DSL transceiver 150 completes the transmission of
communication message 32, the control DSL transceiver 100 is now
free to send another communication message 34 to either the same
remote DSL transceiver 150 or, if present, a different remote DSL
transceiver, such as DSL transceiver 155 (remote "n") of FIG. 1. As
illustrated in FIG. 2A, remote DSL transceiver "n" responds with
communication message 36. In this manner, the communication
methodology between control DSL transceiver 100 and all remote DSL
transceivers 150, 155 . . . n, is switched-carrier and time-domain
duplexed.
[0036] FIG. 2B is a schematic view illustrating, in further detail,
the communication message 31 of FIG. 2A. Communication message 31
begins with preamble 40 followed by optional administrative header
42. In accordance with an aspect of the invention, all
communication messages, regardless of the content, begin with
preamble 40. Administrative header 42 is optional and can be used
to send information that is neither part of the preamble 40 or of
any data to follow. For example, the administrative header 42 could
convey a description of noise level conditions at one end so the
other end may opt to increase or reduce the power level of its
transmission as necessary. Likewise, the administrative header 42
sent by a remote transceiver could contain information regarding
the amount of payload information that the remote transceiver is
ready to transmit and its relative priorities so that the control
transceiver could alter the amount of time that this remote
transceiver is given to transmit its data (relative to any other
transceivers connected to the line). When the payload data
comprises ATM cells, the control transceiver could use messages
conveyed by the administrative header 42 to direct remote devices
to activate or deactivate various ATM virtual circuits.
[0037] If data is included in communication message 31, one or more
ATM cells follow the optional administrative header 42. Although
illustrated using three ATM cells, 44, 45 and 46, there are
situations in which no ATM cells, or for that matter, no
information of any kind, follows preamble 40. In the case in which
information does follow preamble 40, and for purposes of
illustration only, ATM cells 44, 45 and 46 are each standard 53
octet ATM cells. For example, ATM cell 44 includes 5 octet ATM
header 47 and 48 octets of ATM data 48. ATM cells 45 and 46 are
identical in structure to ATM cell 44. ATM cells 44, 45 and 46
adhere to the conventional ATM cell structure as defined in
standardized ATM literature. It should be noted that optional
administrative header 42 does not follow the standard ATM cell
format and that administrative header 42 can be any number of
octets in length. As known to those having ordinary skill in the
art, an octet comprises 8 bits of information. Although described
with particular reference to the transportation of ATM cells over a
DSL communication channel, the principles of the invention are
applicable to all fixed length communication messages.
[0038] FIG. 3A is a schematic view illustrating the bit to symbol
relationship of the communication message 31 of FIG. 2B. In
accordance with an aspect of the invention, preamble 40 is placed
at the beginning of every transmission (i.e., each communication
message 31). Preamble 40 is followed by optional administrative
header 42, which is then followed, if there is data to transmit, by
one or more 53 octet ATM cell 44 and 45. Although illustrated using
only two ATM cells, any number of ATM cells may follow preamble 40
and, if included, optional administrative header 42. The ATM cells
are a stream of data information represented as a series of bits
that are placed into each ATM cell.
[0039] The preamble 40 is also a series of bits, which are encoded
into a number of communication symbols. Symbols are the
representation of the bits to be transmitted, and are represented
as signal points in a signal space constellation (to be described
below with respect to FIGS. 4A and 4B). In accordance with one
aspect of the invention, each of the bits in preamble 40 are
encoded into symbols, an exemplar one of which is illustrated using
reference numeral 55, at the lowest available bit rate that can be
transmitted over the communication channel 16. For purposes of
illustration only, the symbols that encode the bits in the preamble
40 shown in FIG. 3A are encoded at a rate of two (2) bits per
symbol. However, any number of bits per symbol lower than that of
the normally transmitted data rate can be used so long as the
symbol rate allows a receiving device to more reliably decode those
symbols. For example, if the normal data rate is five (5) bits per
symbol, then a symbol rate of two (2) bits per symbol has a
significantly (approximately 9 dB) higher noise margin than the
five (5) bit per symbol data rate, thereby allowing the symbols
that are encoded at the lower rate of two (2) bits per symbol to be
very robustly and reliably decoded by a receiving device. In this
manner, the preamble 40, which is sent at the beginning of every
communication message 31, can be made sufficiently robust so that
the chance that it will always be received error free is greatly
increased. Although very robust, there are still situations in
which the symbols into which the preamble bits are encoded can be
corrupted. However, in accordance with another aspect of the
invention, because the preamble 40 is sent at the beginning of
every communication message 31, even if the preamble 40 is
corrupted, only data following that preamble may be affected, i.e.,
lost due to corruption, if certain bits of the preamble are
corrupted.
[0040] In accordance with another aspect of the invention, the
first symbol 55 representing the first bits in the preamble 40 can
be sent using an increased power level, thereby clearly and
robustly delimiting the beginning of the communication message 31.
The effect of this increased power level symbol 55 will be
explained in greater detail below with respect to FIGS. 4A and
4B.
[0041] Still referring to FIG. 3A, if an administrative header 42
is present in communication message 31, then the bits that are
contained in administrative header 42 will be encoded at a symbol
rate of "N" bits per symbol, where N is the normal data rate. The
normal data rate can be any data rate, for example, but not limited
to, a value between 2 and 12 (inclusive) bits per symbol. For
purposes of illustration, and for simplicity of explanation, the
normal symbol rate can be five bits per symbol. This is represented
by the group of symbols 56 into which all the bits of
administrative header 42 and a portion of the bits of header 47 of
ATM cell 44 are encoded.
[0042] In accordance with another aspect of the invention, the
first symbol used to encode bits from a particular cell that
contains bits only from that cell will be encoded at a data rate
lower than that of the standard data rate used for all other bits
of each cell. For example, symbol 57 is the first symbol that
contains bits only from ATM cell 44. The last symbol 65 of symbol
group 56 contains bits from both administrative header 42 and ATM
cell 44. Likewise, symbol 60 is the first symbol containing only
bits from ATM cell 45. In accordance with this aspect of the
invention, the symbols 57 and 60 will be encoded at a data rate
that is two (2) bits per symbol lower than that of the preceding
symbol (represented by N-2 where N is the number of bits per symbol
used for encoding all other bits of the administrative header and
ATM cells.) In this manner, because of the fixed length 53 octet
ATM cells, by simple bit counting, the receiver will always know
the first symbol encoding bits from a cell that contains only bits
from this cell, and therefore has the special encoding described
herein. These N-2 bits of the cell data are grouped for
transmission and an additional bit (bit 54 for cell 44 or bit 61
for cell 45) is added for a total of N-1 bits encoded into symbol
57 or 60, respectively. This group of N-1 bits, represented by
symbol 57 or 60, is encoded into a symbol and scaled for
transmission with the scaling normally applied when encoding at N-1
bits per symbol. The extra bit 54 or 61 indicates whether or not
the cell just started (ATM cell 44 or 45, respectively) is the last
cell of the transmission. The extra bit 61 in symbol 60 is set to
logic one to indicate that ATM cell 45 is the last cell of the
transmission so that the receiver will know at the beginning of the
receipt of ATM cell 45 that ATM cell 45 is the last cell in the
transmission. For the same reason, bit 54 in symbol 57 set to zero
so that the receiver will know that at least one more cell follows
cell 44.
[0043] If N=2, then no bits are taken from the cell to encode the
next symbol (since N-2=0). Since N-1=1, the next symbol contains
just one bit, which is the last cell indicator. This effectively
inserts an entire extra symbol in each cell. Nevertheless, the same
encoding/decoding logic for this special symbol applies for any
value of N>2.
[0044] Once the receiver knows that a particular cell is the last
cell in the message, by simple counting it can readily identify the
symbol that contains the last bits of the last cell. This is
represented in FIG. 3A as symbol 51 or optionally symbol 53. Since
the number of bits remaining to be transmitted in the last symbol
(M) can be less than N, a modified encoding technique is preferable
for this symbol. One option is to add one or more padding bits (P)
52 so that M+P=N. Another option is to encode the last group of
bits at M bits per symbol as represented by symbol 53. This has the
advantage of increased robustness for the transmission of these
bits.
[0045] For simplicity, the following discussion does not address
this second technique. Having recognized the last symbol of the
transmission, the receiver does not attempt to demodulate and
decode the signal on the line following this symbol since the
transmitting station must now be sending silence.
[0046] It should be noted that although described as being encoded
at N-1 bits per symbol, the symbols 57 and 60 containing the
additional last cell indicator bit can be encoded at any symbol
rate lower than that of the standard transmission rate (N bits per
symbol). For example, if N is five (5), the specially encoded
symbols could also be encoded at N-2 or three (3) bits per symbol
so that they contain two (2) bits of cell data plus the last cell
indicator bit. In this manner, the receiver can clearly and
reliably decode the symbol 60, thereby providing a robust and
reliable end of message delimiter.
[0047] In accordance with this aspect of the invention, and to be
described in further detail with respect to FIG. 3B, it is also
desirable to have the ability to indicate that a message contains
only an administrative header 42. In order to accomplish this, the
first symbol containing data from the administrative header 42 can
also be encoded using the higher noise margin N-1 bit per symbol
encoding technique described above. For example, the first N-2 bits
of the administrative header 42 can be combined with a last cell
bit (such as bit 61 of symbol 60) and be encoded at the N-1 bit per
symbol rate. This can provide the extra bit to indicate whether or
not one or more ATM cells follow administrative header 42. An
alternative technique is to simply include a bit in the preamble 40
that indicates whether an administrative header 40 follows the
preamble. For simplicity, it has been assumed that this alternative
technique is used with respect to FIG. 3A and in the following
discussion.
[0048] Because each ATM cell is the smallest unit of a payload of
ATM cells, and because all ATM cells have the same length, the
first symbol of each cell that carries only bits of that cell can
readily be identified. Because these bits are transmitted using the
specially encoded symbol carrying two fewer bits than normal (as
described above), the length of each cell is effectively increased
by two bits. In some cases this can result in one extra symbol
being needed to transmit the cell. In other cases an additional
cell is not needed because the spare bits are available anyway (and
would have ended up as the padding bits (P) 52 in FIG. 3A). Because
the cells may be transmitted contiguously as a bit stream, the
addition of one extra symbol may provide sufficient extra bits to
cover the opening symbol of multiple following cells. For example,
at eight (8) bits per symbol, one (1) extra symbol is needed to
cover the end of frame signaling overhead to transmit up to four
(4) cells.
[0049] FIG. 3B is a schematic view illustrating, in further detail,
the exemplar preamble 40 of FIG. 3A. The bit stream of preamble 40
comprises four (4) bits 62 that include information regarding the
transmit rate (in bits per symbol) used to encode data following
preamble 40 (the data comprising the optional administrative header
and optional ATM cells), four (4) bits 63 that include information
regarding the rate (also in bits per symbol) that the receiver is
capable of receiving, two (2) bits 64 that identify the address of
a remote DSL transceiver if the control DSL transceiver is
transmitting (if a remote DSL transceiver is transmitting, then
these two (2) bits 64 can represent the address of that remote DSL
transceiver) and two (2) bits 66, which can be used to communicate
the format of the message to follow. For example, the two (2) bits
66 can be used to advise a receiving device whether an
administrative header 42 follows the preamble 40, whether ATM cells
follow the preamble, whether both follow or whether only the
preamble is being transmitted. The four (4) bits provided by
symbols 55 and 67 and by symbols 68 and 69 can each encode as many
as sixteen data encoding rates.
[0050] As mentioned above, the preamble 40 is sent at the beginning
of each transmission. The twelve (12) bits that comprise the
preamble 40 are encoded into symbols 55, 67, 68, 69, 70 and 71 in
accordance with that described above. In accordance with an aspect
of the invention, all of the symbols in preamble 40 are encoded at
a low bit per symbol rate. In this example, all of the symbols are
encoded at a rate of two (2) bits per symbol, however, any other
low bit per symbol rate can be used with similar results. The low
bit per symbol rate ensures a high signal-to-noise ratio for these
symbols, thereby significantly decreasing the probability that
these preamble symbols will be corrupted by noise on the
communication channel. The payload data (administrative header and
ATM cells) would typically be encoded at N bits per symbol only if
transmission at this N bit per symbol rate has an acceptably low
rate of errors (based on line length, signal strength, noise,
distortion and other impairments that may be present). Otherwise,
data transmission efficiency would suffer. Therefore, encoding the
preamble at less than N bits per symbol allows a corresponding
improvement in the reliability of transmitting this information
such that it is highly unlikely to be corrupted. Since very few
bits are needed to convey the information carried in the preamble,
a very low rate can be used without seriously reducing the overall
transmission efficiency.
[0051] In accordance with another aspect of the invention, the
first symbol 55 is encoded at a rate of two (2) bits per symbol and
has its energy increased to a point at which noise on the
communication channel is unlikely to cause a receiver to
erroneously interpret the first symbol 55 as silence. Likewise the
increased energy makes it unlikely that noise on the communication
channel will cause the receiver to erroneously interpret an
interval of the silence that precedes each message as the starting
symbol of a message. It has been found that an energy increase of 3
dB is sufficient. This aspect of the invention will be described in
greater detail below with respect to FIGS. 4A and 4B. In this
manner, the beginning of each transmission can be clearly and
robustly delimited. The remainder of the symbols 67, 68, 69, 70 and
71 that represent the bits in preamble 40 are all encoded at two
(2) bits per symbol, but do not have their energy increased.
[0052] The four (4) transmit rate bits 62 inform a receiving DSL
transceiver of the transmit rate of the information to follow the
preamble 40. Sending this information in every message has
significant benefits. It provides the transmitting transceiver the
option of changing the encoding rate for the payload from one
message to the next. Messages containing information that has been
determined to be of high priority can be transmitted using a lower
number of bits per symbol to improve the chances of its being
received without errors. If the communications system
intermittently has a reduced throughput demand, the transceivers
may instantly reduce their data rates to improve robustness without
adversely affecting real throughput. Finally, if a severe noise
condition (such as an impulse caused by plain old telephone service
(POTS) ringing signals on a subscriber line 16) happens to corrupt
one or both of the symbols 55 and 67 that encode the transmit rate,
only the payload data in this message will be improperly decoded.
The receiver's memory of a corrupted rate value lasts only until
the next transmission begins. This allows the transmit rate to
potentially be changed for every message while at the same time
avoiding the complexities of providing fail-safe communication of
the rate, such as through use of an automatic repeat request (ARQ)
protocol, that would be needed if the rate is sent only when it is
changed.
[0053] The receive rate bits 63 allow the transmitting device to
communicate to the receiving device the maximum receive rate at
which the transmitting device can receive. Inherently included in
these receive rate bits 63 are commands that instruct the opposite
device to either increase or decrease its transmit rate. This
allows the responding transceiver to instantly modify the rate it
uses for its next transmission to accommodate changes in the signal
quality that have been detected at the opposite end of the
line.
[0054] In accordance with an aspect of the invention, the address
bits 64 need only be used when the control DSL transceiver 100 is
communicating with a plurality of remote DSL transceivers in what
is commonly referred to as "multi-point" mode. When communicating
in "multi-point," mode the address bits 64 include either the
address of the remote DSL transceiver 150 that is to transmit next
(if the transmission is sent by the control DSL transceiver 100) or
the address of the responding remote DSL transceiver 150 (if the
transmission is sent by the remote DSL transceiver 150). Sending
these bits 64 at the lower bit rate of the preamble reduces the
likelihood of a remote transceiver 150 not responding or of the
incorrect remote transceiver 150 responding to a message from the
control transceiver 100. Frequent occurrence of either of these two
types of errors could adversely affect the overall data
transmission efficiency of the line.
[0055] The format bits 66 indicate whether the optional
administrative header 42 is being sent, whether one or more ATM
cells are being sent, or whether both or neither are being sent. As
described previously, the receiver uses this information in
conjunction with the transmit rate from bits 62 to identify the
special symbols at the start of each ATM cell and to determine the
symbol that is the last in the message. Robust transmission of this
information at the start of each message allows the transmitter to
dynamically modify the message format as needed from one message to
the next. Should one of the format bits be corrupted by an
abnormally severe noise event, the "damage" is restricted to the
current message only. To operate reliably, the receiver could have
a "back up" method of recognizing the end of a message such as
through detecting loss of signal energy for an extended
duration.
[0056] FIG. 4A is a graphical illustration representing a two (2)
bit per symbol signal space constellation and the increased energy
symbol of FIG. 3B. The constellation points labeled "c" represent
the points in a standard 2 bit per symbol constellation. For each
constellation point "c" transmitted, the effect of noise can make
the point appear to a receiver to have been moved with respect to
where it was when it was transmitted. The dashed circle 76
surrounding constellation point 79 represents the space within
which noise may move the point and still have the point reliably
decoded by the receiver. The point 79 appears in a different place
at the decoder due to noise induced in the communications channel
16. Each of the points "c" have a space about which they can move
and still be reliably decoded by the receiver.
[0057] The circle 77 encloses the area surrounding the origin of
the in-phase (horizontal) and quadrature (vertical) axes of FIG. 4A
about which an interval of silence (no constellation point) can be
moved by the same additive noise that can affect signal points.
This additive noise could cause the silence to be interpreted by
the decoder as one of the constellation points in a two (2) bit per
symbol constellation due to the overlap of the decoding
discrimination threshold circles 76 and 77. As shown, the circle 76
and the circle 77 have sufficient overlap in region 73 so that
silence can easily be interpreted as one of the signal points "c".
Conversely, one of the signal points "c" could also be interpreted
by the decoder as silence.
[0058] For efficient operation, it is desirable that the beginning
and end of each transmission be robustly and precisely identified
(to within one (1) symbol interval). The beginning and end of each
transmission are preceded and followed by silence on the line.
Because the most efficient constellation encoding cannot allocate
signal space to silence, it is impractical to reliably discriminate
silence from signal when analyzing only a single symbol. In other
words, it would be undesirable for silence that occurs before a
message or after a message to be interpreted as a constellation
point "c", and it would be undesirable for a constellation point
"c" to be interpreted as silence. As mentioned above, this is
possible due to the effect of noise altering the position of the
constellation signal points "c" or the position of silence.
[0059] In accordance with an aspect of the invention, the first
symbol (symbol 55 of FIG. 3B) in the preamble 40 is transmitted
with increased energy, thereby increasing the probability that it
will be reliably detected by the decoder of the receiving device.
In this manner, the beginning of each transmission is clearly and
robustly delimited. The signal point "b" in FIG. 4A is an exemplar
one of four (4) two (2) bit per symbol constellation points that
are transmitted at an increased energy level. While other increases
may provide useful, a 3 dB increase is typically sufficient and
does not increase the ratio of peak power to average power (PAR) of
the transmitted signal. As illustrated, the signal point "b" is
enclosed by dotted circle 78, within which the point "b" may move
due to noise on the communication channel 16 and still be reliably
decoded by the receiver. As shown, there is no overlap between
circle 78 and circle 77. Accordingly, by boosting the energy of the
first symbol (symbol 55 of FIG. 3B) transmitted in a communication
message (31 of FIG. 3A), there is a significantly higher
probability that the boosted symbol will be reliably decoded and
not be mistaken for silence. Nor will silence be mistaken for this
boosted energy first symbol. Preferably, the receiver places the
threshold to discriminate signal from noise at one unit from the
origin as shown by circle 77 in FIG. 4A.
[0060] FIG. 4B is a graphical illustration showing an exemplar
grouping of constellation points representing different bit per
symbol rates in accordance with an aspect of the invention. For
example purposes only, assuming that normal data is encoded at five
(5) bits per symbol, the black constellation points, an exemplar of
one of which is illustrated using reference numeral 81, represent
data encoded at five (5) bits per symbol. In accordance with an
aspect of the invention, all the symbols in the preamble 40 are
encoded at a rate of two (2) bits per symbol and are illustrated by
the four (4) constellation points labeled "c" in FIG. 4B. These two
(2) bit per symbol constellation points provide a higher
signal-to-noise ratio (high margin) than do the normal data encoded
at five (5) bits per symbol. This increased margin increases the
probability that the receiver will reliably decode all the symbols
in the preamble.
[0061] In accordance with another aspect of the invention, the four
constellation points labeled "b" in FIG. 4B represent the first
symbol (symbol 55 of FIGS. 3A and 3B), which energy is boosted by 3
dB. In this manner, the constellation points "b" representing the
boosted symbol 55 of FIGS. 3A and 3B will robustly and reliably
communicate the beginning of a transmission. Circle 82 represents
the maximum signal level of any symbols as the number of bits per
symbol becomes arbitrarily large, but the average power of the
transmitted signal is the same as it is for either the five (5)
bits per symbol (81) or the two (2) bits per symbol (points "c")
constellations shown. Therefore, as illustrated by circle 82, the
instantaneous power required by the boosted symbol points "b" is
not any higher than that used to send the normal data at any bits
per symbol value. In this manner, the boosted symbol represented by
constellation points "b" can be used to reliably indicate the start
of a message without requiring a higher transmit level capability
than that needed for normal data transmission. The non-boosted two
(2) bit per symbol constellation points indicated as "c" (having a
significantly higher signal-to-noise ratio than that of the normal
five (5) bit per symbol data) are used to transmit all symbols of
the preamble after the first symbol.
[0062] FIG. 5 is a schematic view illustrating the communication
message 31 of FIG. 3A and another aspect of the invention.
Typically, it is desirable to scramble all the data bits in a
communications message using a self-synchronizing scrambler so that
all points in the signal constellation can be used. Unfortunately,
the self-synchronizing capability of the scrambler carries the
inherent disadvantage of error propagation and extension. A single
bit in error in the received data stream is typically transformed
by the self-synchronizing descrambling process into at least 3
erroneous bits that are separated by several bits that are not in
error.
[0063] Typically, in switched-carrier operation, the scrambler
setting (state) at the end of one transmission is preserved and
used to begin scrambling the next message. (This enables full
randomization of the encoding process so as to make full use of the
available channel bandwidth.) Similarly, in a receiving device,
when descrambling, the state of the descrambler that exists at the
end of the previously received message is used to begin the
descrambling process for the next received message. This means that
the last state of the scrambler saved after scrambling the data
portion of the message would then be used to begin scrambling the
preamble bits of the next message.
[0064] Unfortunately, using this technique with the robust preamble
40 of the invention can lead to error propagation from the data
portion of the communication message to the preamble 40. Allowing
errors, which are more likely due to the larger number of bits per
symbol, in the payload data to corrupt the data in the preamble due
to the inherent error extension of the descrambling process
significantly reduces the robustness of the preamble 40. In
accordance with another aspect of the invention, a first scrambler
can be used to scramble the information contained in the preamble
40 and a second scrambler can be used to scramble the data (i.e.,
the information in the ATM cells 44, 45, etc.)
[0065] As shown in FIG. 5, line 87 indicates that a first scrambler
is used to scramble the preamble 40 of communication message 31 and
also used to scramble the preamble of communication message 86.
Similarly, line 88 indicates that a second scrambler is used to
scramble the data portion of communication message 31 and the data
portion of communication message 86. The message to message
randomizing desirable for full usage of the available channel
bandwidth can be maintained if the setting of the preamble
scrambler (to be described with respect to FIG. 8) at the end of
one preamble is used to begin the scrambling of the preamble of the
next communication message 86. Because errors in the preamble are
considered unlikely to occur, and because the bits received at the
end of a previous preamble define the descrambler state used to
descramble the next preamble, error extension from one message
preamble into the preamble is also much less likely than in the
single scrambler case.
[0066] An alternative to this that avoids the use of two scramblers
is to save the state of the preamble scrambler after scrambling the
preamble as the state to use to begin scrambling of the next
preamble. This cab be done instead of the conventional approach of
using the state of the scrambler at the end of the message. This
technique can also prevent errors at the end of one message from
corrupting the preamble of the next transmission.
[0067] FIG. 6 is a schematic view illustrating the communication
message 31 and the reduced line turn around delay made possible by
an aspect of the invention. In time-domain duplex operation any
periods during which no transceiver is transmitting represent loss
of available bandwidth. To make most efficient usage of a
communication line, it is desirable to minimize these periods. Some
intervals of silence necessarily occur between transmissions
because the transition from silence to the first symbol of the
preamble is the manner in which the beginning of the next
transmission is delimited. The process by which a transceiver makes
the transition from receiving to transmitting is referred to as
"line turn-around" and the time required may determine the minimum
amount of silence that can occur between messages. Various aspects
of the design and implementation of a time-domain duplex
transceiver may result in increased delays in the line turn-around
process. For example, transmitter filters and receiver equalizers
have inherent delays. The analog-to-digital and digital-to-analog
conversion process as well as the process of transferring digital
samples between the signal processor and converters may have some
inherent delays. If the signal processing is implemented in
firmware there may be delays between the arrival of received signal
samples and the time the processing can be performed. All of these
factors may extend the line turn-around time to the point that
transmission efficiency is significantly reduced.
[0068] As described above with respect to FIG. 3A, communication
message 31 includes a specially encoded symbol 60 transmitted at a
lower bit per symbol rate than that of the normal data encoding
rate. The symbol encodes an additional bit 61 that indicates
whether or not the ATM cell is the last cell in the communication
message 31. If it is indicated to the receiver at the beginning of
the last ATM cell 46 that the ATM cell 46 is the last cell in the
communication message, (instead of waiting to the end of the ATM
cell 46) line turn around delay can be reduced. As illustrated, if
a receiving device must wait until the end of the last message to
learn that the message is complete, there will be a delay "d"
between the time that the communication message 31 is received and
the time at which the transmission of communication message 91a can
begin. By having advance notification that the communication
message is about to be complete, a remote DSL transceiver 150 can
begin transmission of the next message before reception of the
current message has been completed. By knowing the delay
contributed by the factors such as those mentioned previously, the
transceiver can begin the transmission process, indicated by
communication message 91b, so as to reduce delay "d" as much as
possible, potentially reducing it to the minimum value needed for
the receiver to reliably detect the transition from silence to
signal at the beginning of the next message.
[0069] FIG. 7 is a block diagram illustrating the control DSL
transceiver 100 of FIG. 1. Although, described with respect to
control DSL transceiver 100, the following description is equally
applicable to a remote DSL transceiver 150. Control DSL transceiver
100 includes microprocessor 101, memory 102, transmitter 115 and
receiver 120 in communication via logical interface 108. A
bi-directional stream of ATM cells from a DTE is communicated via
line 14 to the control DSL transceiver 100. Memory 102 includes end
of transmission delimiting software 106 and robust preamble
software 104. This software resides in memory 102 and executes in
microprocessor 101 in order to achieve and perform the benefits of
the present invention. Transmitter 115 communicates with line
interface 109 via connection 112 in order to gain access to
communication channel 16. Information received on communication
channel 16 is processed by line interface 109 and sent via
connection 111 to receiver 120.
[0070] Transmitter 115 includes, among other elements that are
known to those having ordinary skill in the art, encoder 200 and
modulator 117. Similarly, receiver 120 includes, among other
elements that have been omitted for clarity, decoder 300 and
demodulator 118.
[0071] FIG. 8 is a block diagram illustrating the encoder 200 of
FIG. 7. The transmit sequencer 236 commands the multiplexer 214 via
connection 242 to select the first two (2) bits of the four (4)
bits (62 of FIG. 3B) that define the current transmit rate from
transmit rate element 206, via connection 212. This symbol is then
forwarded to preamble scrambler 217, via connection 216 for
scrambling, and is then forwarded via connection 218 to two (2) bit
per symbol preamble encoder 219. This encoded symbol is then
forwarded via connection 226 to gain increase element 227 where its
energy is increased by approximately 3 dB and is then sent via
connection 228 to multiplexer 224 and over connection 254 to
modulator 117.
[0072] The next two (2) bits of the transmit rate (62 of FIG. 3B)
are then scrambled and encoded in the same way. Next, the transmit
sequencer 236 commands the multiplexer 214 via connection 242 to
select the four (4) bits representing the requested received rate
from receive rate element 204, which bits are forwarded to
multiplexer 214 via connection 211. These four (4) bits are then
forwarded to preamble scrambler 217 where they are scrambled, and
then forwarded via connection 218 to two (2) bit per symbol
preamble encoder 219 where they are encoded into a pair of symbols.
These encoded symbols, are forwarded directly via connection 226 to
multiplexer 224 and then forwarded via connection 254 to modulator
117.
[0073] If there are multiple remote DSL transceivers 150 and 155,
then the transmit sequencer 236 commands the multiplexer 214 via
connection 242 to select the two (2) bits representing the remote
address from remote address element 202, which bits are then
forwarded via connection 209 to multiplexer 214. These two (2) bits
are then forwarded via connection 216 to preamble scrambler 217,
which scrambles the bits and forwards them via connection 218 to
the two (2) bit per symbol preamble encoder 219. The two (2) bit
per symbol preamble encoder 219 encodes the bits and transfers the
encoded symbol via connection 226 through multiplexer 224 and then
via connection 254 to modulator 117.
[0074] Transmit sequencer 236 senses if an administrative header 42
and/or ATM cells 44, 45, 46 are available for transmission via
connections 232 and 234, respectively, and uses this information to
prepare the message format indicator which is loaded by the
transmit sequencer 236 via connection 207. The transmit sequencer
236 commands the multiplexer 214 via connection 242 to select the
two (2) bits representing the message format from element 201,
which bits are then forwarded via connection 208 to multiplexer
214. These two (2) bits are then forwarded via connection 216 to
preamble scrambler 217, which scrambles the bits and forwards them
via connection 218 to the two (2) bit per symbol preamble encoder
219. The two (2) bit per symbol preamble encoder 219 encodes the
bits and transfers the encoded symbol via connection 226 through
multiplexer 224 and then via connection 254 to modulator 117.
[0075] Next, transmission of either the administrative header 42 or
the ATM cell payload begins by transmit sequencer 236 sending a
command via connection 235 to multiplexer 241 to select either the
administrative header 42 via element 229 or payload data via
element 231. These bits are supplied through multiplexer 241 via
connections 239 and 238 and are then forwarded via connection 244
to payload scrambler 246. Payload scrambler 246 scrambles the bits
and forwards them via connection 248 to N bit per symbol data
encoder 249 and N-1 bit per symbol data encoder 251. As mentioned
above with respect to FIG. 5, payload scrambler 246 may use as its
initial state either the state that exists at the end of scrambling
the preamble (supplied via connection 247) or the state that exists
after completion of scrambling the payload portion of the previous
message. As mentioned above with respect to FIG. 3A, all the data
bits are encoded at an N bit per symbol data rate by data encoder
249 and forwarded via connection 257 to multiplexer 224 until the
first symbol containing only bits from a new ATM cell is detected.
This symbol is encoded at a rate of N-1 bits per symbol by N-1 bit
per symbol data encoder 251 and forwarded via connection 256 to
multiplexer 224. The index for this symbol as delivered to payload
scrambler 246 is formed by selecting the first N-2 bits of the
first octet of the cell and adding an additional bit (i.e., bit 54
or bit 61 of FIG. 3A) representing the state of the last cell
signal 237 as selected via multiplexer 241. When instructed by
transmit sequencer 236 via connection 252, the multiplexer 224
selects the symbols from either N bit per symbol data encoder 249
or from N-1 bit per symbol data encoder 251 and forwards these
symbols via connection 254 to modulator 117.
[0076] Transmit sequencer 236 uses the payload bits per symbol
value N received via connection 212 to determine the number of
symbols to encode for each cell and to determine which symbol is to
be encoded at the N-1 bits per symbol rate and contain the last
cell indicator bit. After completing transmission of the message,
transmit sequencer 236 commands multiplexer 224 via connection 252
to select silence 221 via connection 222 as the input to the
modulator 117.
[0077] FIG. 9 is a block diagram illustrating the decoder 300 of
FIG. 7. A received transmission stream is received in demodulator
118, where it is demodulated in accordance with techniques known
those having ordinary skill in the art. The first symbol is
forwarded via connection 301 to gain reduction element 302. Gain
reduction element 302 reduces the gain of the first symbol and
supplies that reduced energy symbol via connection 304 to
multiplexer 306. Receive sequencer 328 sends a signal to
multiplexer 306 via connection 354 instructing multiplexer 306 to
select that reduced gain symbol and transfer it via connection 307
to two (2) bit per symbol preamble decoder 308. The decoded bits
from the first symbol are then sent via connection 309 to preamble
descrambler 311. Preamble descrambler 311 descrambles the first
bits in the transmission and forwards them via connection 312 to
the multiplexer 314. When instructed by receive sequencer 328 via
connection 332, the multiplexer 314 forwards these bits via
connection 324 to transmit rate element 236.
[0078] The following preamble symbols are all forwarded via
connection 301 directly to multiplexer 306, which forwards these
symbols via connection 307 for decoding by two (2) bit per symbol
preamble decoder 308. The decoded bits are forwarded via connection
309 to preamble descrambler 311 as mentioned above. These bits are
then forwarded in order via connections 324, 321, 318 and 316 to
transmit rate element 326, receive rate element 322, remote address
element 319 and message format element 317, respectively.
[0079] Next, the administrative header symbols and ATM cell data
symbols that have been encoded at N bits per symbol are forwarded
via connection 301 to N bit per symbol data decoder 337 and the ATM
cell data symbols that have been encoded at N-1 bits per symbol are
forwarded via connection 301 to N-1 bit per symbol data decoder
339. These symbols are decoded and the decoded bits are transferred
via connections 338 and 341 to multiplexer 342. Similarly, as
mentioned above with respect to FIG. 8, receive sequencer 328
insures that the symbols encoded at the rate of N-1 bits per symbol
are forwarded via connection 301 to N-1 bit per symbol data decoder
339, which forwards the decoded bits via connection 341 to
multiplexer 342. As shown, the value of N, which is the bits per
symbol value used for the N bits per symbol, or N-1 bits per symbol
decoding is controlled by the just received transmit rate bits that
have been stored in transmit rate element 326.
[0080] At the appropriate time, receive sequencer 328 commands the
multiplexer 342 via connection 347 to forward the bits via
connection 344 to payload descrambler 336. In accordance with an
aspect of the invention, the preamble descrambler 311 operates only
on the preamble bits and the payload descrambler 336 operates only
on the payload bits. As mentioned above with respect to FIG. 5, the
payload descrambler may use as its initial state either the state
of the preamble descrambler at the end of descrambling the preamble
as supplied via connection 334 or the state of the payload
descrambler at the end of descrambling the payload bits of the
previous message. The descrambled payload bits are then forwarded
via connection 346 to multiplexer 349. When ordered by receive
sequencer 328 via connection 331, the multiplexer 349 forwards the
administrative header bits via connection 351 and the payload data
bits via connection 352. These bits are then forwarded via logical
interface 108 to microprocessor 101 for processing (FIG. 7).
Receive sequencer 328 determines the presence or absence of the
administrative header and ATM cells via the just received message
format bits that have been stored in element 317 and provided to
receive sequencer 328 via connection 327. When the bits for each
symbol containing the last message bit are available at multiplexer
349, receive sequencer 328 directs the N-2 bits of payload data to
the payload data element 356 via connection 352 and receives the
last cell bit via connection 329. Receive sequencer 328 uses the
current bits per symbol value for payload data received via
connection 324 to determine the beginning and end of each cell.
Based on the message format and the value of the last cell
indicator bit, receive sequencer 328 determines when the last
symbol of the message has been decoded and instructs demodulator
118 (FIG. 7) to stop delivering demodulated symbols.
[0081] In an alternative embodiment, the special encoding of the
last cell as described above in FIG. 3A can be omitted and an "eye
pattern closure test" can be used to detect the end of the message.
In such a situation where it is acceptable to lose the advanced
notification of the end of the transmission, beneficial alternative
uses for the special encoding of the first bits of each cell are
possible. For example, this special encoding as described above
with respect to FIG. 3A wherein N-2 bits are encoded for the first
full bytes of each cell, can be used to indicate whether or not the
ATM cell header (e.g., ATM header 47 of FIG. 2B) is present. This
can be useful in the situation in which a string of ATM cells have
exactly the same header. This can happen, for example, for ATM
adaptation layer 5 (AAL5) cells that carry data from a single
protocol data unit (PDU) if no other cells have been interleaved.
The single extra bit (bit 61 of FIG. 3A) provided by the encoding
described above with respect to FIG. 3A, can be used to indicate
whether or not the following cell contains a header. If the bit 61
indicates that there is no header, the receiver copies the last
header received ahead of the payload octets of this next cell
before forwarding it to the ATM layer. Advantageously, this reduces
the approximate 10 percent overhead imposed by the five (5) octet
header (47 of FIG. 2B).
[0082] It should be emphasized that the above-described embodiments
of the present invention, particularly any "preferred" embodiments,
are merely possible examples of implementations, merely set forth
for a clear understanding of the principles of the invention. Many
variations and modifications may be made to the above-described
embodiment(s) of the invention without departing substantially from
the spirit and principles of the invention. For example, the robust
preamble and transmission delimiting system and method are
applicable to all switched-carrier transmission methodologies in
which it is desirable to reliably convey channel establishment
information and reliably delimit the beginning and end of each
communication message. All such modifications and variations are
intended to be included herein within the scope of the present
invention.
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