U.S. patent application number 09/943276 was filed with the patent office on 2002-04-11 for method for detecting delayed data frames in a transport function.
Invention is credited to Leung, Nikolai K.N., Rezaiifar, Ramin.
Application Number | 20020041596 09/943276 |
Document ID | / |
Family ID | 22168977 |
Filed Date | 2002-04-11 |
United States Patent
Application |
20020041596 |
Kind Code |
A1 |
Rezaiifar, Ramin ; et
al. |
April 11, 2002 |
Method for detecting delayed data frames in a transport
function
Abstract
A method for detecting delayed data frames in a transport
function includes the steps of comparing a frame sequence counter
with a predetermined threshold value, detecting a delayed frame if
the sequence counter exceeds the threshold value, and processing
the detected delayed frame accordingly. In an RLP data transport
protocol, the delayed frame may be processed as a retransmitted
frame. The threshold value may be proportional to the maximum
number of frames sent in a given bundle times the maximum delay, in
units of frame intervals, between frames sent simultaneously over
the air in the same frame interval.
Inventors: |
Rezaiifar, Ramin; (San
Diego, CA) ; Leung, Nikolai K.N.; (Arlington,
VA) |
Correspondence
Address: |
Qualcomm Incorporated
Patents Department
5775 Morehouse Drive
San Diego
CA
92121-1714
US
|
Family ID: |
22168977 |
Appl. No.: |
09/943276 |
Filed: |
August 30, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09943276 |
Aug 30, 2001 |
|
|
|
09082085 |
May 20, 1998 |
|
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Current U.S.
Class: |
370/394 |
Current CPC
Class: |
H04L 1/165 20130101;
H04W 24/00 20130101; H04L 47/34 20130101; H04L 1/1841 20130101;
H04L 1/1809 20130101; H04L 69/324 20130101; H04L 9/40 20220501 |
Class at
Publication: |
370/394 |
International
Class: |
H04L 012/56 |
Claims
We claim:
1. A method of detecting a delayed frame in a transport function
wherein a plurality of frames are sent from a transmitter to a
receiver, the method comprising the steps of: determining a
threshold value proportional to a product of a number of frames
within a bundle of frames and a maximum delay time in frame-length
time increments between frames sent in said bundle of frames;
comparing, for a received frame, a frame sequencing counter number
with said threshold value, the frame sequencing counter number
being derived from a header of the received frame; and detecting
the received frame as a delayed frame if the frame sequencing
counter number exceeds said threshold value.
2. The method as recited in claim 1 wherein the plurality of frames
is sent in bundles of frames, each bundle including an equal number
of frames, the frames within any bundle being sent
simultaneously.
3. The method of claim 1, further comprising the step of processing
a detected delayed frame as a retransmitted frame.
4. The method of claim 1, wherein the transport function is a Radio
Link Protocol interface.
5. An apparatus for detecting a delayed frame in a transport
function wherein a plurality of frames are sent from a transmitter
to a receiver, the apparatus comprising: means for determining a
threshold value proportional to a product of a number of frames
within a bundle of frames and a maximum delay time in frame-length
time increments between frames sent in said bundle of frames; means
for comparing, for a received frame, a frame sequencing counter
number with said threshold value, the frame sequencing counter
number being derived from a header of the received frame; and means
for detecting the received frame as a delayed frame if the frame
sequencing counter number exceeds the threshold value.
6. The apparatus as recited in claim 5 wherein the plurality of
frames is sent in bundles of frames, each bundle including an equal
number of frames, the frames within any bundle being sent
simultaneously.
7. The apparatus of claim 5, further comprising means for
processing a detected delayed frame as a retransmitted frame.
8. The apparatus of claim 5, wherein the transport function is a
Radio Link Protocol interface.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation application of
application Ser. No. 09/082,085, filed May 20, 1998, now allowed,
entitled "Method for Detecting Delayed Data Frames in a Transport
Function" which is a continuation-in-part of U.S. application Ser.
No. 08/877,294, now U.S. Pat. No. 6,011,796, filed Jun. 17,
1997.
BACKGROUND OF THE INVENTION
[0002] I. Field of the Invention
[0003] The present invention pertains generally to the field of
wireless communications, and more specifically to detecting delayed
data frames in a transport function.
[0004] II. Background
[0005] The field of wireless communications has many applications
including, e.g., cordless telephones, paging, wireless local loops,
and satellite communication systems. A particularly important
application is cellular telephone systems for mobile subscribers.
(As used herein, the term "cellular" systems encompasses both
cellular and PCS frequencies.) Various over-the-air interfaces have
been developed for such cellular telephone systems including, e.g.,
frequency division multiple access (FDMA), time division multiple
access (TDMA), and code division multiple access (CDMA). In
connection therewith, various domestic and international standards
have been established including, e.g., Advanced Mobile Phone
Service (AMPS), Global System for Mobile (GSM), and Interim
Standard 95 (IS-95). In particular, IS-95 and its derivatives, such
as IS-95A, IS-95B, ANSI J-STD-008, IS-99, IS-657, IS-707, etc.
(often referred to collectively herein as IS-95), are promulgated
by the Telecommunication Industry Association (TIA) and other well
known standards bodies.
[0006] Cellular telephone systems configured in accordance with the
use of the IS-95 standard employ CDMA signal processing techniques
to provide highly efficient and robust cellular telephone service.
An exemplary cellular telephone system configured substantially in
accordance with the use of the IS-95 standard is described in U.S.
Pat. No. 5,103,459, which is assigned to the assignee of the
present invention and fully incorporated herein by reference. The
aforesaid patent illustrates transmit, or forward-link, signal
processing in a CDMA base station. Exemplary receive, or
reverse-link, signal processing in a CDMA base station is described
in U.S. application Ser. No. 08/987,172, now abandoned, filed Dec.
9, 1997, entitled MULTICHANNEL DEMODULATOR, which is assigned to
the assignee of the present invention and fully incorporated herein
by reference. In CDMA systems, over-the-air power control is a
vital issue. An exemplary method of power control in a CDMA system
is described in U.S. Pat. No. 5,056,109, which is assigned to the
assignee of the present invention and fully incorporated herein by
reference.
[0007] A primary benefit of using a CDMA over-the-air interface is
that communications are conducted over the same RF band. For
example, each mobile subscriber unit (typically a cellular
telephone) in a given cellular telephone system can communicate
with the same base station by transmitting a reverse-link signal
over the same 1.25 MHz of RF spectrum. Similarly, each base station
in such a system can communicate with mobile units by transmitting
a forward-link signal over another 1.25 MHz of RF spectrum.
[0008] Transmitting signals over the same RF spectrum provides
various benefits including, e.g., an increase in the frequency
reuse of a cellular telephone system and the ability to conduct
soft handoff between two or more base stations. Increased frequency
reuse allows a greater number of calls to be conducted over a given
amount of spectrum. Soft handoff is a robust method of
transitioning a mobile unit from the coverage area of two or more
base stations that involves simultaneously interfacing with two
base stations. (In contrast, hard handoff involves terminating the
interface with a first base station before establishing the
interface with a second base station.) An exemplary method of
performing soft handoff is described in U.S. Pat. No. 5,267,261,
which is assigned to the assignee of the present invention and
fully incorporated herein by reference.
[0009] Under the IS-99 and IS-707 standards (referred to
hereinafter collectively as IS-707), an IS-95-compliant
communications system can provide both voice and data
communications services. Data communications services allow digital
data to be exchanged using a receiver and an RF interface to one or
more transmitters. Examples of the type of digital data typically
transmitted using the IS-707 standard include computer files and
electronic mail.
[0010] In accordance with both the IS-95 and IS-707 standards, the
data exchanged between a wireless terminal and a base station is
processed in frames. To increase the likelihood that a frame will
be successfully transmitted during a data transmission, IS-707
employs a radio link protocol (RLP) to track the frames transmitted
successfully, and to perform frame retransmission when a frame is
not transmitted successfully. Retransmission is performed up to
three times in IS-707, and it is the responsibility of the higher
layer protocols to take additional steps to ensure that the frame
is successfully transmitted.
[0011] In order to track which frames have been transmitted
successfully, IS-707 calls for an eight-bit sequence number to be
included as a frame header in each frame transmitted. The sequence
number is incremented for each frame from 0 to 256 and then reset
back to zero. An unsuccessfully transmitted frame is detected when
a frame with an out-of-order sequence number is received, or an
error is detected using CRC checksum information or other error
detection methods. Once an unsuccessfully transmitted frame is
detected, the receiver transmits a negative-acknowledgment message
(NAK) to the transmit system that includes the sequence number of
the frame that was not received. The transmit system then
retransmits the frame including the sequence number as originally
transmitted. If the retransmitted frame is not received
successfully, a second negative-acknowledgment message is sent to
the transmit system. The transmit system typically responds by
notifying the controlling application or network layer of the
failed transmission.
[0012] Under IS-95A and IS-707, frames are transmitted once every
twenty milliseconds (ms). Thus, an eight-bit sequence number can
track 256 frames transmitted over a five-second interval. Five
seconds is typically sufficient to allow a failed frame
transmission to be detected, and a retransmission to be performed,
and therefore an eight-bit sequence number provides sufficient time
for frame retransmission. Thus, retransmitted frames can be
uniquely identified without ambiguity caused by a sequence
"wrap-around" whereby the eight-bit sequence number repeats.
[0013] Since the original development of IS-95A and IS-707,
however, additional protocols and standards have been proposed and
developed that allow data to be transmitted at greater rates.
Typically, these new protocols and standards use the same frame
structure as IS-95A and IS-707 in order to maintain as much
compatibility as possible with pre-existing systems and standards.
Nevertheless, while maintaining compatibility with pre-existing
standards and systems is desirable, the use of the same type of
frame within these higher rate protocols and standards
substantially increases the number of frames that are transmitted
during a given period of time. For example, if the transmission
rate is increased by a factor of four, the time required to
transmit 256 frames is reduced to 1.25 seconds, rather than the
five seconds required previously. A time period of 1.25 seconds is
typically insufficient to allow a failed frame transmission to be
detected, and a retransmission attempted, before the eight-bit
sequence number repeats. Thus, the use of an eight-bit sequence
number is insufficient to allow unique identification of frames for
the time period necessary to perform the desired retransmission
sequence.
[0014] A well-known protocol, the Radio Link Protocol (RLP), uses
an eight-bit sequence counter included in frames sent over the air.
The eight bits represent the least significant bits of a twelve-bit
counter kept internally at both the receiver and the transmitter.
The twelve-bit counter is updated based on the eight-bit numbers
sent over the air. It stands to reason that delayed frames present
a problem. If multiple frames are simultaneously sent from the
transmitter but are delayed with respect to each other at the
receiver, the twelve-bit counters will be incorrectly updated and
the RLP will abort.
[0015] While the number of bits in the sequence number could be
increased, such an increase would substantially alter the frame
format and therefore violate the goal of maintaining substantial
compatibility with previously existing systems and standards.
Additionally, increasing the number of bits in the sequence number
would waste available bandwidth. A conventional solution such as
increasing the number of bits used to represent the sequence
counter is therefore inadequate because it would introduce
additional overhead per transmission and decrease of the net
throughput of the transport service. Hence, it would be desirable
to provide a method for extending the sequence number range without
modifying the number of bits used for the sequence number. Such a
method would advantageously be capable of interpreting an
impossibly large number of missing data frames derived from the
sequence number as a delayed frame, thereby increasing the
throughput of the transport function. Thus, there is a need for an
efficient method of detecting delayed frames in a transport
function using a minimum number of bits.
SUMMARY OF THE INVENTION
[0016] The present invention is directed to an efficient method of
detecting delayed frames in a transport function using a minimum
number of bits. Accordingly, a method for detecting delayed data
frames in a transport function in which frames are sent from a
transmitter to a receiver includes the steps of comparing, for a
received frame, a frame sequencing counter number with a
predetermined threshold value, the frame sequencing counter number
being derived from a header of the received frame, and detecting
the received frame as a delayed frame if the frame sequencing
counter number exceeds the threshold value. In one aspect of the
invention, a data transmission system advantageously includes a
transmitter, a receiver coupled to the transmitter via an interface
for receiving data frames from the transmitter, and a protocol
processing component housed in the receiver for comparing a frame
sequencing counter number with a predetermined threshold value, the
frame sequencing counter number being derived from headers of the
data frames, the protocol processing component detecting a delayed
data frame if the frame sequencing counter number exceeds the
threshold value.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a block diagram of a cellular telephone
system.
[0018] FIG. 2 is a schematic diagram of a transmitter and
receiver.
[0019] FIG. 3 is a diagram of a frame buffer and resequencing
buffer.
[0020] FIG. 4 is a flow chart illustrating the operation of a
transmitter and a receiver during a communication.
[0021] FIG. 5 is a flow chart illustrating the operation of the
receiver during the reception of a newly transmitted frame.
[0022] FIG. 6 is a flow chart illustrating the operation of the
receiver during the reception of a retransmitted frame.
[0023] FIG. 7 is a message diagram illustrating the operation of
the transmitter and the receiver during an exemplary
communication.
[0024] FIG. 8 is a message diagram illustrating the operation of
the transmitter and the receiver during an exemplary
communication.
[0025] FIG. 9 is a flow chart illustrating the operation of the
receiver in recognizing and processing delayed frames.
[0026] FIG. 10 is a functional diagram of a shift register used in
the receiver to update a bit value specifying the next frame to be
received.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] The embodiments described hereinbelow reside in a personal
communication system operating in accordance with the use of CDMA
signal processing techniques of the IS-707 and IS-95 standards.
While the present invention is especially suited for use within
such a communications system, it should be understood that the
present invention may be employed in various other types of
communications systems that transmit data via frames or packets,
including both wireless and wireline communication systems, and
satellite-based communication systems. Additionally, throughout the
description, various well-known systems are set forth in block
form. This is done in order to avoid unnecessarily obscuring the
disclosure.
[0028] Various cellular systems for wireless telephone
communication employ fixed base stations that communicate with
mobile units via an over-the-air interface. Such cellular systems
include, e.g., AMPS (analog), IS-54 (North American TDMA), GSM
(Global System for Mobile communications TDMA), and IS-95 (CDMA).
In a preferred embodiment, the cellular system is a CDMA
system.
[0029] As illustrated in FIG. 1, a CDMA wireless telephone system
generally includes a plurality of mobile subscriber units 10, a
plurality of base stations 12, a base station controller (BSC) 14,
and a mobile switching center (MSC) 16. The MSC 16 is configured to
interface with a conventional public switch telephone network
(PSTN) 18. The MSC 16 is also configured to interface with the BSC
14. The BSC 14 is coupled to each base station 12 via backhaul
lines. The backhaul lines may be configured in accordance with any
of several known interfaces including, e.g., E1/T1, ATM, or IP. It
is to be understood that there can be more than one BSC 14 in the
system. Each base station 12 advantageously includes at least one
sector (not shown), each sector comprising an antenna pointed in a
particular direction radially away from the base station 12.
Alternatively, each sector may comprise two antennas for diversity
reception. Each base station 12 may advantageously be designed to
support a plurality of frequency assignments (each frequency
assignment comprising 1.25 MHz of spectrum). The intersection of a
sector and a frequency assignment may be referred to as a CDMA
channel. The base stations 12 may also be known as base station
transceiver subsystems (BTSs) 12. Alternatively, "base station" may
be used in the industry to refer collectively to a BSC 14 an BTSs
12, which BTSs 12 may also be denoted "cell sites" 12.
(Alternatively, individual sectors of a given BTS 12 may be
referred to as cell sites.) The mobile subscriber units 10 are
typically cellular telephones 10, and the cellular telephone system
is advantageously a CDMA system configured for use in accordance
with the IS-95 standard.
[0030] During typical operation of the cellular telephone system,
the base stations 12 receive sets of reverse-link signals from sets
of mobile units 10. The mobile units 10 are conducting telephone
calls or other communications. Each reverse-link signal received by
a given base station 12 is processed within that base station 12.
The resulting data is forwarded to the BSC 14. The BSC 14 provides
call resource allocation and mobility management functionality
including the orchestration of soft handoffs between base stations
12. The BSC 14 also routes the received data to the MSC 16, which
provides additional routing services for interface with the PSTN
18. Similarly, the PSTN 18 interfaces with the MSC 16, and the MSC
16 interfaces with the BSC 14, which in turn controls the base
stations 12 to transmit sets of forward-link signals to sets of
mobile units 10.
[0031] In the embodiments described below, an algorithm serves to
map an eight-bit sequencing number for counting frames sent over
the air into a twelve-bit sequencing number in accordance with the
Radio Link Protocol (RLP), a protocol that is known in the art. The
algorithm is advantageously carried out with RLP software
instructions and a microprocessor. In one embodiment, an RLP
component may reside in a base station 12. Alternatively, the RLP
component may reside in a BSC 14. Those of skill in the art would
appreciate that the RLP algorithm may be used not only in a BSC 14
or a base station 12, but could be used in any transport layer in
which multiple data frames are received in a particular processing
period.
[0032] In FIG. 2, two communication systems configured in
accordance with an exemplary embodiment are illustrated in block
form. The higher rate communication is being conducted from
transmitter 50 to receiver 52. In an exemplary configuration,
transmitter 50 is located in a base station 12 and receiver 52 is
in a wireless terminal 10; however, the locations may be reversed.
Within transmitter 50, control system 54 receives data frames from
input/output (I/O) 56 and provides that data to encoder 58. Encoder
58 performs convolutional encoding, generating code symbols that
are received by digital modulator 60. Digital modulator 60 performs
direct sequence modulation on the code symbols with one or more
binary channel codes and one or more binary spreading codes,
yielding chipped symbols that are received by radio frequency (RF)
transmitter 62. The chipped symbols are upconverted to the carrier
frequency band by RF transmitter 62 and transmitted from antenna
system 64 via diplexer 66.
[0033] Various methods and apparatuses for performing the digital
modulation and RF upconversion can be employed. A set of
particularly useful methods and apparatus are described in U.S.
Pat. No. 6,005,855, entitled METHOD AND APPARATUS FOR PROVIDING
VARIABLE RATE DATA IN A COMMUNICATIONS SYSTEM USING STATISTICAL
MULTIPLEXING, filed Apr. 28, 1995; U.S. Pat. No. 5,777,990,
entitled METHOD AND APPARATUS FOR PROVIDING VARIABLE RATE DATA IN A
COMMUNICATIONS SYSTEMS USING NON-ORTHOGONAL OVERFLOW CHANNELS,
filed Feb. 28, 1995; and U.S. Pat. No. 6,173,005, entitled HIGH
DATA RATE SUPPLEMENTAL CHANNEL FOR CDMA TELECOMMUNICATIONS SYSTEM,
filed Jan. 15, 1997; each of which is assigned to the assignee of
the present invention and fully incorporated herein by reference.
It should be understood that some of the above-referenced patent
applications are directed to the forward link, and are therefore
more suited for use with the transmitter 50, while others are
directed to the reverse link, and are therefore more suited for use
with the receiver 52.
[0034] In an exemplary embodiment, the data transmitted from
antenna system 64 is formatted in accordance with frames 70 that
include an eight-bit sequence field (SEQ number) 72, a retransmit
flag 74, and a data field 76. A frame 70 may include other fields
that are not shown because they are not particularly relevant to
the present invention. In a preferred embodiment, the frames are
formatted substantially in accordance with the frame structures
defined in the IS-707 standard, with the addition of retransmit
flag 74.
[0035] To provide data frames to encoder 58 in an orderly manner,
control system 54 stores the frames within frame buffer 55 and
updates an index value L_V(S). Frame buffer 55 and index value
L_V(S) are preferably stored within a memory system. In a preferred
embodiment, index value L_V(S) is a twelve-bit sequence number that
is incremented after the transmission of each frame as described in
greater detail below. The least significant eight bits of index
value L_V(S) are placed in the sequence field of a frame 72.
[0036] Within receiver 52, RF receiver 80 downconverts and
digitizes the RF signals on which frame 70 is transmitted using
antenna system 82 and diplexer 84. Digital demodulator 86
demodulates the downconverted, or "baseband," signals using the
necessary binary codes, generating soft decision data that is
received by decoder 88. Decoder 88 performs maximum likelihood
trellis, or Viterbi, decoding, yielding hard decision data 90 that
is provided to controller 91.
[0037] Controller 91 reforms frame 70 using hard decision data 90
and determines whether the frame has been received in sequence
relative to the frames that have already been received using the
SEQ number, index variable L_V(N), and L_V(R), as well as
resequencing buffer 92 and NAK list 94 as described in further
detail below.
[0038] If controller 91 determines that the frame has been received
out of sequence relative to the frames that have already been
received, or if the frame is received in error, it generates a
negative-acknowledgment (NAK) message that is received by encoder
95. Encoder 95 performs convolutional encoding to generate code
symbols that are direct sequence spread spectrum modulated by
digital modulator 97, preferably in accordance with the IS-95
reverse link, and the chipped symbols are upconverted by RF
transmit system 98 and transmitted as NAK 83 from antenna system 82
via diplexer 84. The L_SEQ for the NAKed frame is stored within NAK
list 94.
[0039] Referring again to transmitter 50, RF receiver 67 receives
the RF signal via antenna system 64 and diplexer 66. RF receiver 67
downconverts and digitizes the RF signal, yielding samples that are
demodulated using digital demodulator 68. Decoder 69 decodes the
soft decision data from digital demodulator 68, and control system
54 receives the hard decision data from decoder 69, thereby
detecting the NAK 83 from receiver 52 contained in the hard
decision data.
[0040] Control system 54 receives NAK 83 and retrieves the NAKed
frame from transmit buffer 55. The retrieved frames are
retransmitted in accordance with the original transmission as
described above (including the original sequence number).
[0041] The configuration of frame buffer 55, resequencing buffer
92, and indexes L_V(S), L_V(N), and L_V(R), when used in accordance
with one embodiment, are illustrated in FIG. 3. Within the transmit
frame buffer 55, frames already transmitted once are shaded, and
frames to be transmitted are clear. In a preferred embodiment,
indexes L_V(S), L_V(N), and L_V(R) are twelve-bit numbers. Index
L_V(S) is set to the sequence number of the next frame to be
transmitted. When the frame is actually transmitted, the eight-bit
SEQ number of the frame is set to the eight least significant bits
of index L_V(S).
[0042] Within resequencing buffer 92, index L_V(R) is set to the
twelve-bit sequence of the next new frame expected. Index L_V(N) is
set to the twelve-bit sequence of the next frame needed for
sequential delivery, or for which processing is still pending. When
a predetermined number of NAKs 83 have been sent without receipt of
the corresponding frame, attempted processing of the frame is
terminated and the data with the missing frame is passed to the
higher layer protocols such as, e.g., the transport layer. As
shown, NAKed frames 96a-c can be received with sequence numbers
between L.sub.=13 V(N) and (L_V(R)-1) MOD 4096, inclusively.
[0043] In FIG. 4, a flow diagram illustrates the operation of the
transmitter 50 and receiver 52 during a communication performed in
accordance with one embodiment. The transmission begins at the
transmitter at step 100, with reception at the receiver occurring
at step 101. At step 102, initialization is performed, during which
index L_V(S) is set to zero within transmitter 50 and L_V(R) is set
to zero within receiver 52.
[0044] At step 108, the transmitter 50 transmits a frame (indicated
by the dashed line) when data is available for transmission, with
the SEQ number of the frame being set to the eight least
significant bits of index L_V(S), and being referred to as V(S).
Additionally, the retransmit flag is set to zero to indicate that
the frame is a newly transmitted frame. At step 112, index L_V(S)
is incremented MOD 4096, and at step 113 the transmitter performs
receive processing for any NAK message transmitted from receiver
52. In one embodiment, when no data is available, "idle" frames
having the current SEQ number may be sent repeatedly until data
becomes available (idle transmissions not shown).
[0045] At step 130 the transmitter 50 determines if a NAK 83 has
been received or is pending, and if so the NAKed frames are
retrieved from the transmit buffer using the long sequence number
contained in the NAK message and retransmitted at step 132 with the
original SEQ number and the retransmit field set to one. Once the
frame is retransmitted the pending or received NAK 83 is cleared
and processing then continues at step 113.
[0046] If a NAK message has not been received or is not pending,
the transmitter returns to step 108 and the processing
continues.
[0047] Within receiver 52, the processing begins at step 101, and
at step 106 L_V(S) is received from transmitter 50. At step 110,
receiver 52 receives any frames transmitted from transmitter 50 at
either step 108 (new transmission), or at step 132
(retransmission), and at step 114 receiver 52 examines the status
of the retransmit flag of the frame to determine if the received
frame is a retransmitted frame or a new frame. If the frame is a
retransmitted frame, retransmit processing is performed at step
116, and then the receiver returns to step 110. If the frame is not
a retransmitted frame, first transmit processing of the frame is
performed at step 120, and then step 110 is performed again.
[0048] In FIG. 5, a flow chart illustrates the operation of
receiver 52 when processing the first transmission of a frame
during step 120 of FIG. 4 in accordance with one embodiment. The
first transmission processing begins at step 150, and at step 152
L_SEQ is set according to the following equation:
L_SEQ={L.sub.--V(R)+[256+SEQ-V(R)] MOD 256} MOD 4096, (1)
[0049] where V(R) is the eight least significant bits of L_V(R) and
SEQ is the sequence number contained in the SEQ field of the frame
being processed. At step 154 it is determined whether L_SEQ is less
than L_V(N), or that the frame has been stored in the resequencing
buffer 92. If so, the frame is discarded at step 156 and the
receive system returns from first transmission processing at step
157. As noted above, L_V(N) is set to the next frame needed for
sequential delivery of the data.
[0050] If L_SEQ is not less than L_V(N) and the frame has not been
stored in the resequencing buffer 92, it is further determined at
step 158 whether L_SEQ is greater than or equal to L_V(N) and less
than L_V(R), and whether the frame has been not stored in the
resequencing buffer 92. If so, the frame is discarded at step 156
and the receive system returns from first transmission processing
at step 157. Otherwise, it is further determined at step 160
whether L_SEQ equals L_V(R) and therefore is the next frame needed
for sequential delivery L_V(R).
[0051] If L_SEQ does not equal L_V(R), an out-of-order frame has
been received, and the frame is stored in the resequencing buffer
92 at step 162, and L_V(R) is set to L_SEQ at step 164. At step
166, the receive system transmits one or more NAK messages
requesting retransmission of all unreceived frames from L_V(N) to
(L_V(R)-1) MOD 4096, inclusive. The receive system then returns
from the first transmission processing at step 176.
[0052] If, at step 160, it is determined that L_SEQ equals L_V(R),
the frame has been received in order, causing it to further be
determined at step 170 whether L_V(N) equals L_V(R), which
indicates that no NAKed frames are outstanding. If L_V(N) equals
L_V(R), L_N(N) and L_V(R) are incremented MOD 4096 at step 172. The
data frame is delivered to the higher layer protocol at step 174,
and the receiver 52 returns from first transmit processing at step
176.
[0053] If it is determined at step 160 that L_V(N) does not equals
L_V(R), and therefore that NAKed frames remain outstanding, L_V(R)
is incremented MOD 4096 at step 178, and at step 180 the frame is
stored in the resequencing buffer 92. The receiver 52 then returns
from the first frame transmit processing at step 176.
[0054] In FIG. 6, a flow diagram illustrates the operation of the
receiver 52 during step 116 when a retransmitted frame is received
in accordance with one embodiment. The processing of the
retransmitted frame begins at step 200, and at step 202 the SEQ
field in the received frame is used as the key to look up an L_SEQ
associated with the SEQ in the NAK list 94 (see FIG. 2). At step
204 it is determined whether the L_SEQ is less than L_V(N), or
whether the frame has already been stored in the resequencing
buffer 92. If so, the frame is discarded at step 206, and the
receiver 52 returns from retransmit processing at step 208.
[0055] If L_SEQ is not less than L_V(N), and the frame has not been
stored in the resequencing buffer 92, it is further determined at
step 210 whether L_SEQ is greater than or equal to L_V(N) and less
than L_V(R), and if the frame has not been stored in the
resequencing buffer 92. If so, the frame is stored in the
resequencing buffer 92 at step 212 before step 214 is performed.
Otherwise, step 214 is performed.
[0056] At step 214, it is determined whether L_SEQ is equal to
L_V(N), and if not, the frame is discarded at step 216 because the
retransmitted frame has a sequence number that is higher than the
next new frame expected, and therefore an error has occurred. Once
the frame has been discarded, receiver 52 returns from
retransmitted frame processing at step 208.
[0057] If L_SEQ equals L_V(N), the data in all the contiguous
frames formed by the addition of the retransmitted frame being
processed from L_V(N) upward are delivered to the next higher
processing layer at step 218, and the delivered frames are removed
from the resequencing buffer 92 at step 220. At step 222 L_V(N) is
set to LAST+1, where LAST is the long sequence number (L_SEQ) of
the last frame delivered to the higher layer at step 218. At step
224 the frame is removed from the NAK list and the receiver 52
returns from processing the retransmitted frame at step 226.
[0058] In FIG. 7, a message diagram illustrates the messages
transmitted during an exemplary communication performed in
accordance with one embodiment. Transmitter 50 is shown on the
left, and receiver 52 is shown on the right. Transmitter 50
maintains index L_V(S), and frames are transmitted with value V(S)
in the sequence field, where V(S) is the eight least significant
bits of L_V(S). At the receiver 52 the NAK list after each
transmission is shown. All numbers are shown in hexadecimal.
[0059] The first frame 230 is transmitted when index L_V(S) is
equal to 0x2FE, and therefore with a SEQ number of OxFE. After the
transmission of frame 230, index L_V(S) is incremented to 0x2FF and
frame 232 is transmitted with a SEQ number of 0xFF. Both frames 230
and 232 are received successfully by receiver 52, causing index
L_V(R) to increment twice from 0x2FE to 0x300.
[0060] Frame 234 is transmitted with a SEQ number of 0x00 and is
not successfully received by receiver 52. L_V(S) is then
incremented to 0x301, and frame 236 is transmitted with a SEQ
number of 0x01 and is received successfully by receiver 52.
[0061] Upon receipt of frame 236, receiver 52 detects the
out-of-order sequence number because frame 234 was not received. In
response, receiver 52 generates NAK message 240 containing the full
twelve-bit index L_V(R) for the unreceived frame 0x300.
Additionally, receiver 52 updates the NAK list 94 to indicate that
a NAK 83 has been transmitted for a frame with SEQ number 0x00 and
L_SEQ number 0x300. Also, receiver 52 starts a NAK timer, which
tracks the time that has expired since the transmission of NAK
message 240.
[0062] During the transmission of NAK message 240, transmitter 50
transmits another frame 238 with a SEQ number of 0x02, which is
received successfully by receiver 52. Upon receipt of NAK message
240, transmitter 50 generates retransmitted frame 242 having SEQ
number 0x00 and the retransmit flag 74 (see FIG. 2) is set to one.
Upon receipt of retransmitted frame 242, receiver 52 detects the
retransmission bit and matches the SEQ number with the SEQ number
in NAK list 94. Once the match is made, retransmitted frame 242 is
placed within the resequencing buffer 92 (see FIG. 2) and the entry
within NAK list 94 is removed. Frames 244 and 246 are then
transmitted and received in normal fashion.
[0063] In FIG. 8, a message diagram further illustrates the
operation of transmitter 50 and receiver 52 during a transmission
in which the sequence number "wraps-around," when performed in
accordance with one embodiment. Frames 240a and 240b are
transmitted with SEQ numbers 0xFE (all numbers are in hexadecimal)
and 0xFF, respectively, which correspond to values of 0x2FE and
0x2FF for index L_V(S), and are successfully received by receiver
52, causing L_V(R) to be incremented from 0x2FE to 0x300.
[0064] Frame 240c includes SEQ number 0x00 but is not successfully
received by receiver 52. Frame 240d includes SEQ number 0x01 and is
received properly by receiver 52. Upon receipt of frame 240d,
receiver 52 detects that the SEQ number is greater than the eight
least significant bits of L_V(R), and therefore that a frame has
been received out of order. In response, receiver 52 updates L_V(R)
to 0x302, which corresponds to the next expected frame, and places
the SEQ number of the unreceived frame into NAK list 94.
Additionally, receiver 52 transmits NAK 241 containing the complete
L_SEQ number 0x300 of the frame that was not received, and
initiates a timer that tracks the amount of time that has expired
since the transmission of the NAK 241. As shown in FIG. 8, however,
NAK 241 is not received successfully by transmitter 50.
[0065] Transmitter 50 continues to transmit frames as shown,
including frames 240e-240j, all of which are successfully received
by receiver 52. During the transmission of frames 240e-240j, index
L_V(S) changes from 0x302 to 0x400, causing a wrap-around in eight
least significant bits, and therefore in the SEQ number contained
in the frames.
[0066] Frame 240k is transmitted with SEQ number 0x01 and is not
received successfully by receiver 52. Frame 240l is transmitted
with SEQ number 0x02 and is received successfully by receiver 52.
Upon receipt of frame 240l, receiver 52 detects an out-of-order
transmission, and responds by transmitting NAK 243 containing
sequence value 0x401 and by adding sequence number 0x401 to NAK
list 94. Additionally, at this time the timer for NAK 241 expires,
causing a second NAK 245 containing sequence value 0x300 to be
transmitted to transmitter 50. Thus, a second NAK is transmitted
for frame 240c. Additionally, receiver 52 sets L_V(R) to the next
expected sequence number 0x403. It should be noted that the
sequence numbers transmitted in NAKs 243 and 245 could be
transmitted in a single NAK message.
[0067] Transmitter 50 responds to NAKs 243 and 245 by transmitting
retransmitted frame 242a containing the data from frame 240k, and
retransmitted frame 242b containing the data from frame 240c. Upon
receipt of retransmission frame 242a, receiver 52 identifies the
frame as a retransmitted frame based on the status of retransmit
flag 74 (see FIG. 2). Once the frame is identified as a
retransmitted frame, receiver 52 performs a lookup within NAK list
94, using the SEQ number, and determines which frame has been
retransmitted. Retransmitted frame 242a is then placed in the
appropriate location within resequencing buffer 92 (see FIG. 2),
and the corresponding entry is removed from NAK list 94.
[0068] Upon receipt of retransmission frame 242b, receiver 52 also
identifies the type of frame and performs a lookup within NAK list
94. When the identity of the frame is determined, it is placed
within the resequencing buffer 92 (see FIG. 2), and the
corresponding entry is removed from NAK list 94. Transmitter 50
then transmits frame 240m having sequence number 0x03, which is
successfully received by receiver 52. At this point, NAK list 94 is
empty.
[0069] As should be evident from the transmission shown in FIG. 8,
marking frames as either "new" or "retransmitted" allows the
receiver 52 to properly process both new and retransmitted frames
that have the same SEQ numbers even when wrap-around of the
sequence number occurs during a retransmission. This is possible
because a retransmitted frame with the same SEQ number as a newly
transmitted frame can be distinguished by the retransmit flag.
Thus, a greater number of frames may be processed using an
eight-bit sequence number, which supports significantly higher data
rates while maintaining substantial computability with pre-existing
standards.
[0070] In FIG. 9, a flow chart illustrates the operation of the
receiver 52 in recognizing and processing delayed frames in
accordance with one embodiment. A delayed frame may be defined as
an RLP frame that is transmitted at the same time on the
over-the-air interface with a group, or bundle, of other RLP
frames, but has experienced a significantly different delay (e.g.,
due to a different path length) on its way to the receiver 52. In
accordance with the IS-707-A standard and RLP, a known protocol for
retransmission of data frames, frames are sent over the air in
twenty-millisecond (ms) intervals. If the difference in the delay
is more than twenty ms, the delayed frame will be received in one
of the following twenty-ms processing intervals identified in
IS-707-A. If not detected as a delayed frame, a delayed RLP frame
may cause an RLP reset.
[0071] Equation (1), described above in connection with FIGS. 4-6,
presents a method for mapping the eight-bit SEQ number (which is
transmitted as a frame header over the air) into a twelve-bit L_SEQ
number at the receiver 52 to keep track of frame sequence. If, by
way of example, frames 1, 2, and 4 of a four-frame bundle are
received within the same twenty-ms time interval, but frame 3 is
delayed and is received in the following twenty-ms time interval,
equation (1) yields the following value for L_SEQ: 1 L_SEQ = { L_V
( R ) + [ 256 + SEQ - V ( R ) ] MOD 256 } MOD 4096 = { 5 + [ 256 +
3 - 5 ] MOD 256 } MOD 4096 = 5 + 254 ,
[0072] which indicates that 254 frames are missing. Clearly, this
is not the correct interpretation because it is impossible to miss
254 frames within a twenty-ms time interval. In the embodiment
depicted in FIG. 9, an RLP algorithm advantageously categorizes
such a frame as a delayed frame at the receiver 52.
[0073] In the embodiment of FIG. 9, the value D denotes the maximum
difference in the arrival time for RLP frames that are transmitted
in the same twenty-ms time interval on the over-the-air interface.
D is expressed in units of twenty-ms time intervals and is
typically zero, one, or two. The number V(R).sub.T-D denotes the
value of V(R) at a time of D.times.20 ms ago. The value N.sub.max
denotes the maximum number of frames that can be sent in one
twenty-ms time interval. N.sub.max may be eight in a particular
embodiment. In another embodiment, N.sub.max may be four.
[0074] In step 300, a frame is received at the receiver 52. The
algorithm then proceeds to step 302 and determines whether the
frame is a new frame. If the frame is a new frame, the algorithm
proceeds to step 304. If the frame is not a new frame, the
algorithm proceeds to step 306 to process the frame as a
retransmitted frame. In step 306, the algorithm processes the frame
as a retransmitted frame via a table, as described above. The
algorithm then returns to step 300 and receives the next frame.
[0075] In step 304, the algorithm determines whether L_V(R) has
been updated within the past D.times.20 ms. L_V(R) is the
twelve-bit value of V(R), which points to the next frame the RLP
algorithm expects to receive in the receive buffer. If L_V(R) has
not been updated in the past D.times.20 ms, equation (1) will not
yield an impossibly large number of missing frames, so the
algorithm proceeds to step 306, processing the frame as a
retransmitted frame. If L_V(R) has been updated within the past
D.times.20 ms, there is a possibility that the new frame is a
delayed frame, and the algorithm proceeds to step 308.
[0076] In step 308, the algorithm commences new frame processing by
computing the value H=(256+SEQ-V(R).sub.T-D) MOD 256. The algorithm
then proceeds to step 310. In step 310, the algorithm determines
whether H is greater than N.sub.max.times.D. If H is greater than
N.sub.max.times.D, the algorithm detects the frame as a delayed
frame and proceeds to step 306, advantageously processing the
detected delayed frame as a retransmitted frame. Those of skill in
the art would understand that because L_SEQ equals [L_V(R)+H] MOD
4096 (see equation (1)), checking if H is greater than the
threshold value N.sub.max.times.D (which represents the maximum
number of frames that could be missing) simply amounts to comparing
L_SEQ with a threshold value. If L_SEQ is found to exceed the
threshold value, a delayed frame is detected and processed
accordingly. Those of skill in the art would likewise appreciate
that in an alternate embodiment not employing RLP frames, the
delayed frame need not necessarily be processed as a retransmitted
frame, but might instead be processed in some other manner. If H is
not greater than N.sub.max.times.D, the algorithm proceeds to step
312 and processes the frame as a new frame via equation (1), as
described above. The algorithm then returns to step 300 and
receives the next frame.
[0077] In one embodiment, illustrated in FIG. 10, a shift register
400 in the receiver 52 (see FIG. 2) may advantageously be used to
keep track of the value of V(R).sub.T-D. The shift register 400
must have D+1 stages (i.e., the shift register 400 must have a
number of bits equal to (D+1) multiplied by the bit length of
V(R)). The bit value V(R) is put into the shift register 400 and,
as shown, the shift register 400 is shifted every twenty ms to
update V(R).sub.T-D. If L_V(R) (or, equivalently, V(R)) was not
updated within the past twenty-ms time interval, a special value
predetermined to denote a "null" symbol is advantageously placed in
the shift register 400 to represent that no change took place.
[0078] Preferred embodiments of the present invention have thus
been shown and described. It would be apparent to one of ordinary
skill in the art, however, that numerous alterations may be made to
the embodiments herein disclosed without departing from the spirit
or scope of the invention. Therefore, the present invention is not
to be limited except in accordance with the following claims.
* * * * *