The present invention relates to a wireless communication
system wherein a plurality of wireless stations communicate with
one another as in wireless LAN (Local Area Network) or PAN
(Personal Area Network), a wireless communication device and
a wireless communication method, and a computer program. More
particularly, it relates to a wireless communication system,
a wireless communication device and a wireless communication
method, and a computer program wherein random access is made
based on carrier detection by CSMA/CA (Carrier Sense Multiple
Access with Collision Avoidance).
Further more particularly, the present invention relates
to a wireless communication system, a wireless communication
device and a wireless communication method, and a computer
program wherein access control is carried out based on CSMA using
the RTS/CTS method to maintain communication quality. Further
moreparticularly, it relates to a wireless communication system,
a wireless communication device and a wireless communication
method, and a computer program wherein a plurality of classes
of frames, such as RTS, CTS, DATA, and ACK, are multiplexed and
thereby flexible transmitting and receiving procedures are
provided and overheads are reduced.
3
Background Art
The wireless network has received widespread attention
as a communication system which relieves users of cable wiring
. between wire communication devices. With the wireless network,
the majority of cables can be omitted in working spaces, such
as offices; therefore, communication terminals, such as personal
computers (PCs), can be relatively easily relocated. As
wireless LAN systems become faster and inexpensive, recently
the demand for the wireless network has been significantly
increased. In particular, these days, the introduction of
personal area network (PAN) has been considered to build a
small-scale wireless network among a plurality of pieces of
electronic equipment existing in people' s surroundings to carry
out information communication. For example, different wireless
communication systems and wireless communication devices are
defined using frequency bands, such as 2.4-GHz band and 5-GHz
band, for which licenses from competent authorities are
unnecessary.
Typical standards related to wireless network include IEEE
{ (The Institute of Electrical and Electronics Engineers) 802.11
{Refer to Non-Patent Document 1, for example.) , HiperLAN/2 (Refer
to Non-Patent Document 2 or Non-Patent Document 3, for example .) ,
IEEE8 02.15.3, and Bluetooth communication. With respect to the
IEEE802.il standard, various wireless transmission methods,
such as the IEEE802.11a standard and the IEEE802.11b standard,
are present depending on differences in wireless communication
method and frequency band used.
4
To build a local area network using radio engineering,
the following method is generally used: one device, designated
as "access point" or "coordinator," that functions as control
station is provided in the area. Then the network is formed
under the centralized control of the control station.
In wireless networks provided with an access point, an
access control method based on bandwidth reservation is in wide
use . In this method, when information is transmitted from some
communication device, a band required for the information
transmission is reserved at the access point. Thus,
transmission channels are utilized so that collision with
information transmission by other communication devices will
beprevented. That is, byproviding an access point, synchronous
wireless communication, wherein the communication devices
within the wireless network synchronize them with one another,
is carried out.
However, a problem arises if asynchronous communication
is carried out between a transmitting communication device and
a receiving communication devices in a wireless communication
system provided with an access point: wireless communication
must be carried out through the access point without exception,
and thus the efficiency of transmission channel utilization is
reduced by half.
To cope with this, "ad-hoc communication" in which
terminals asynchronously communicate directly with each other
has been devised as another method for building a wireless network.
In particular, in small-scale wireless networks consisting of
5
a relatively small number of clients located nearby, the ad-hoc
communication is believed to be appropriate. In the ad-hoc
communication, any terminals can carry out wireless
communication directly, or randomly, with each other without
use of any specific access point.
In ad-hoc wireless communication systems, a central
control station does not exist; therefore, it is suitable for
constituting, for example, a home network comprising household
electric appliances. The ad-hoc network has the following
features, for example: even if one unit becomes faulty or is
turned off, routing is automatically changed, and thus the
network is less prone to fail; packets are caused to hop aplurality
of times between mobile stations, and thereby data can be
transmitted a relatively long way with a high data rate maintained.
Various cases of ad-hoc system development have been known.
(Refer to Non-Patent Document 5, for example.)
For example, the IEEE802.il wireless LAN systems are
provided with ad-hoc mode in which the system autonomously and
decentralizedly operates in a peer to peer manner without
providing a control station.
With respect to wireless LAN networks in the ad-hoc
environment, it is generally known that a problem of hidden
terminal occurs. Hidden terminal is a communication station
which can be heard by one communication station as the counterpart
of communication but cannot be heard by the other when
communication is carried out between the specific communication
stations. Hidden terminals cannot carry out negotiation
6
between them,, and thus transmit operation can collide.
As one of methodologies for solving such a problem of hidden
terminal, CSMA/CA according to the RTS/CTS procedure is known.
The CSMA/CA (Carrier Sense Multiple Access with Collision
Avoidance) is a connecting method wherein multiple access is
made.based on carrier detection. In wireless communication,
it is difficult for a communication device to receive signals
by which the device itself transmits information. Therefore,
the communication device confirms that there is no information
transmission from other communication devices by the CSMA/CA
(Collision Avoidance) system, not by the CSMA/CD (Collision
Detection) system. Then, the communication device starts its
own information transmission, and thereby collision is avoided.
The CSMA system is an access method suitable for asynchronous
data communication, such as file transfer and electronic mail.
In the RTS/CTS method, the communication station as the
origin of data transmission transmits a request to send packet
RTS (Request To Send) . In response to the reception of a response
packet CTS (Clear To Send) from the communication station as
the destination of data transmission, the transmitting
communication station starts data transmission. When hidden
terminals receive either or both of RTS and CTS, they set their
own transmission stop period. The transmission stop period is
equivalent to a period during which data transmission is expected
to be performed based on the RTS/CTS procedure. Thus, collision
can be avoided. Hidden terminals for the transmitter station
receive CTS and set a transmission stop period to avoid collision
7
with data packets. Hidden terminals for the receiver station
receive RTS and stop a transmission period to avoid collision
with ACK.
Here, the wireless network will be described with-
IEEE802.Ha, an expansion standard to IEEE802.il, taken as an
example.
FIG. 15 illustrates the frame format specified by
IEEE8 02.11a. In the figure, a preamble which indicates the
presence of packet is added to the head of each packet. For
the preamble, known symbol^patterns are defined in the standard.
Based on these known patterns, receivers can judge whether a
receive signal corresponds to a preamble or not.
Following the preamble, SIGNAL field is defined. In the
SIGNAL field, information required for decoding the information
portion of the packet concerned is placed. The information
required for decoding packets is designated as PLCP header
(Physical Layer Convergence Protocol header) . The PLCP header
includes: RATE field which indicates the transmission rate of
an information portion (This includes Service field as part of
PLCP header; however, it is hereafter simply and generically
referred to as "information portion" for the purpose of
simplifying the explanation.) ; LENGTH field which indicates the
length of information portion; parity bit; Tail bit of encoder;
and the like. Receivers decode the subsequent information
portions based on the result of decoding the RATE and LENGTH
fields in the PLCP header.
The SIGNAL portion where the PLCP header is placed is
8
V
subjected to encoding resistant to noise, and is transmitted
at 6Mbps. In case of normal packets, the information portion
is transmitted in a transmission rate mode in which the highest
bit rate is provided to the extend that the occurrence of errors
is minimized according to the SNR or the like of the receiver.
IEEE802.11a defines eight classes of transmission rate
modes: 6, 9, 12, 18, 24, 36, and54Mbps, andany of themis selected.
If a transmitter/receiver is positioned nearby, a high-bit rate
transmission rate mode is selected. A communication station
positioned far away cannot decode this information sometimes.
The information portion is transferred as PSDU (Physical
Layer Service Data Unit) to the data link layer as the higher
level layer. FIG. 16 illustrates the constitution of the PSDU
frame field. IEEE802.il defines several frame types. Here,
description will be given only to four types of frames, RTS,
CTS, ACK, and DATA, required for describing the present
invention.
For each frame, Frame Control field and Duration field
are defined in common. The Frame Control field holds information
indicating the type and use of the frame concerned, and more
specifically, it describes the information listed in Table 1.
The Duration field holds information on the use of NAV (Network
Allocation Vector) (to be described later), and describes the
time that elapses before the transaction of the packet concerned
is completed.
Table 1
Field Name j Length [Sit] ) Description
Protocol Version
Type/Subtype
ToDS, FromDS
More Fragment
Retry
Power Management
More Data
WEP
Other
2
6
2
Version information
identifier indicating the frame type
Identifier determining what each ADDR indicates
Flag indicating the last fragment
Flag indicating whether this is retransmission or not
Flag indicating power management mode
Flag indicating whether more data has been accumulated or not
Flag indicating the utilization of WEP
In addition to the foregoing, the Data frame contains:
four address fields Addrl to Addr4 for identifying the
communication stations as the origin of transmission and the
destination and others; sequence field (SEQ); Frame Body which
is the primary information to be provided to the upper level
layer; and FCS (Frame Check Sequence) which is a checksum.
In addition to the foregoing, the RTS frame contains:
Receiver Address (RA) which indicates the destination;
Transmitter Address (TA) which indicates the origin of
transmission; and FCS which is a checksum.
In addition to the foregoing, the CTS frame and ACK frame
contain RA which indicates the destination and FCS which is a
checksum.
Here description will be given to the method for coping
with access contention defined in IEEE802.il.
IEEE802.il defines four types of inter frame spaces (IFS) :
SIFS (Short IFS), PIFS (PCF IFS), DIFS (DCF IFS), and EIFS
(extended IFS) in ascending order.
IEEE802.il adopts CSMA as a basic media access procedure
(described above). Before a transmitter transmits something,
the state of the medium is monitored and further a backoff timer
9
I
10
is actuated for a random time. The transmitter is provided with
transmission right only when there is no a transmit signal during
this period, and the transmitter can transmit packets on the
medium.
When a normal packet is transmitted according to the CSMA
procedure, the DCF (Distributed Coordination Function) operate
as follows: after the transmission of a packet of some kind is
completed, the state of the medium is monitored only for a DIFS.
If there is no transmit signal during this period, random backoff
is carried out. If there is no transmit signal during this period,
either, then transmission right is provided.
When a packet, such as ACK, whose degree of urgency is
exceptionally high is transmitted, it is permitted to transmit
the packet after a short inter frame space SIFS. Thus, packets
whose degree of urgency is high can be transmitted before packets
transmitted according to the normal CSMA procedure.
To sum up, the reason why different types of inter frame
spaces IFS are defined is as 'follows: packets competing for
transmission right are given priorities according to whether
their IFS is SIFS, PIFS, or DIFS, that is, according to the length
of inter frame space.
Next, description will be given to the RTS/CTS procedure
in IEEE802.il, referring to FIG. 17 to FIG. 19. With respect
to wireless LAN networks in the ad-hoc environment, it is
generally known that a problem of hidden terminal occurs.
CSMA/CA according to the RTS/CTS procedure is known'as one of
methodologies for alleviating most of this problem (described
11
above). IEEE802.il also adopts this methodology.
FIG. 17 schematically illustrates an example of the
operation of the RTS/CTS procedure. This figure shows an example
in which information (Data) of some kind is transmitted from
STAO to STA1.
Prior to the actual transmission of information, STAO
transmits an RTS (Request To Send) packet to STA1 as the
destination of information according to the CSMA procedure . In
,• response to the reception of the RTS packet, STA1 transmits a
CTS (Clear To Send) packet to STAO, feeding back that STA1 has
^ successfully received the RTS.
If the transmitting STAO successfully receives the CTS
packet, themediumis considered to be clear, and the transmission
of information (Data) packets is immediately started. WhenSTAl
has successfully received the information (Data) packets, STA1
returns an ACK packet. Thus, the transmission and reception
transaction equivalent to one packet is completed.
FIG. 18 illustrates action which can take place in
surrounding stations when the RTS/CTS procedure is carried out
( between a transmitter station and a receiver station. In this
figure, the following communication environment is assumed: four
communication stations, STA2, STAO, STA1, and STA3, exist, and
communication stations adjoining to each other in the figure
are located within the range of radio waves . Here, it is assumed
that STAO desires to transmit information to STA1.
The transmitting STAO confirms that the medium is clear
for a certain period (from time TO to time Tl) according to the
12
CSMA procedure. Then, STAO starts the transmission of an RTS
packet to STA1 at time Tl. In the Type/Subtype field of the
Frame Control field of the RTS packet, information which
indicates the packet concerned is. RTS is described; in the
Duration field, the time that lapses before the transmission
and reception transaction for the packet concerned is completed
(i.e. the time until time T8) is described ; in the RA field,
the address of the destination communication station (STA1) is
described; and in the TA field, the address of the transmitter
station (STAO) itself is described.
Here, attention should be paid to the following point:
for STAO to describe in the Duration field the time that lapses
before the transaction according to the RTS/CTS procedure is
completed, the time when the transaction is completed must be
determined when RTS is transmitted. More specific description
will be given. The transmitting STAO must determine all the
transmission rate mode, including CTS packet, Data packet, and
ACK packet to be subsequently transmitted and received in the
transaction in question, when it transmits RTS. The
transmission rate mode determined here relates to the entire
transaction, and it is not permitted to set an individual
transmission rate mode with respect to each packet transmission
within a transaction.
FIG. 19 illustrates the procedure according to which the
transmitting STAO determines a transmission rate mode with
respect to the entire transaction concerned when it transmits
RTS . When RTS is transmitted, it must be determined where the
13
time at which the transaction is completed is (T8 in FIG. 4) .
Therefore, the transmission rate mode is determined only due
to information held by the transmitter station when if transmits
_..RTS (based on the amount of data communicated in the. entire
transaction, the transmission rate, and the like). The
transmission rate of each of the CTS, DATA, and ACK frames
thereafter transmitted and received i's basically in accordance
with the rate applied in the RTS. That is, it is not permitted
to set an individual transmission rate mode with respect to each
packet transmission within a transaction.
Here, description will be given, referring to FIG. 18 again.
The RTS packet is received'also by STA2 which is positioned in
the vicinity of STAO. When STA2 receives the RTS signal, STA2
discovers the preamble and thus starts receive operation.
Further, STA2 decodes PSDU based on the information obtained
by decoding the PLCP header. Then, STA2 recognizes from the
Frame Control field in the PSDU that the packet in question is
RTS packet, and knows that STAO desires to transmit information
of some kind. Further, STA2 recognizes from the RA field that
STA2 itself is not the destination communication station. Then,
STA2 does not monitor the medium and recognizes that the medium
is occupied until the transaction in question is completed, and
stops its own transmission so as not to hinder STAO's desire
to send. To refer to this operation, it is said that a surrounding
station "sets NAV (Network Allocation Vector}." The NAV is
effective during the period indicated in the Duration field,
and STA2 is kept in transmission prohibited state until time
14
T8.
The RTS packet is received also by STA1 which is the intended
destination. STA1 decodes the PSDU according to the same
procedure as described above/ and thereby- recognizes that -STAO
desires to transmit packets to STAl itself. Then, STA1 waits
for a SIFS (described above) and returns a CTS packet at time
T3.
At this time, the transmission rate mode of the CTS packet
must be identical with that of RTS . Further, in the Frame Control
field of the PSDU, it is described that the packet concerned
is CTS packet; in the Duration field, the time that elapses between
the transaction concerned is completed (i.e. time until.time
T8) is described; and in the RA field, the address of the
destination communication station (STAl) is described.
The CTS packet is received also by STA3 which is positioned
in the vicinity of STAl as the destination of transmission. STAl
decodes the PSDU according to the same procedure as described
above, and recognizes that "some nearby communication station
is expected to receive packets until time T8." STA3 sets an
NAVand refrains from its own transmission until the transaction
concerned is completed so as not to hinder STAl' s desire to receive.
The NAV is effective during the period indicated in the Duration
field, and STA3 is also kept in transmission prohibited state
until time T8.
The CTS packet is received also by STAO as the destination.
STAO decodes the PSDU according to the same procedure as described
above, and recognizes that STAl has got ready to receive. Then,
15
STAO waits for a SIFS, and starts the transmission of Data packet
at time T5.
If the transmission of the Data packet is completed at
time T6 and STA1 decodes .them without any error, STA1 waits for
a SIFS and returns ACK at time T7 . When STAO receives it, the
transmission and reception transaction equivalent to one packet
is completed at time T8.
At time T8, STA2 and STA3 which are nearby communication
stations put down the NAV, and return to ordinary transmitting
and receiving state.
In summary, in the above-mentioned RTS/CTS procedure, the
following surrounding stations are prohibited from carrying out
transmission: a "surrounding station for STAO as the transmitter
station," or STA2, which could receive RTS, and a "surrounding
station for STA1 as the receiver station," or STA3,'which could
receive CTS. Thus, information transmission from STAO to STA1
and a return of ACK are carried out without being hindered by
a sudden transmit signal from surrounding stations . Therefore,
the communication quality is maintained.
FIG. 20 illustrates an example of an operation sequence
followed when the above-mentioned packet transmission and
reception transaction based on the RTS/CTS procedure in
IEEE802.il is implemented with traffic through TCP/IP
(Transmission Control Protocol/Internet Protocol) . The TCP/IP
is a typical communication protocol for transmission control
and path control.
Even if communication is of one way in terms of application,
16
ACK is returned by the TCP layer typically every two segments,
and it is brought into the form of asymmetrical two-way
communication in the MAC layer. The ACK of the TCP layer is
small in amount of data, and.thus RTS/CTS. is not used together.
However, RTS/CTS is used together with respect to data traffic.
The figure illustrates such an example.
In the example illustrated in FIG. 20, packets equivalent
to 24 times in total are transmitted and received in the MAC
layer to transmit five segments, DataO to Data4 . That is, while
the operation in the TCP layer is relatively simple, complicated
operation is performed in the MAC layer.
As described up to this point, problems of access
contention and bandwidth assurance can be solved by implementing
the transmitting and receiving procedures based on CSMA in
combination with the RTS/CTS method in accordance with IEEE802 .11.
Meanwhile, several problems described below are left unsolved.
(1) Imperfection of Transmission Rate
When data is transmitted and received according to the
RTS/CTS procedure, the transmission rate of data packets must
be determined before an RTS packet is transmitted.
This is because, for a transmitter station to describe
in the Duration field the time that lapses before a transaction
based on the RTS/CTS procedure is completed, the time at which.
the transaction is completed must be determined when it transmits
anRTS. This means that all the transmission rate mode, including
CTSpacket, Data packet, and ACK packet subsequently transmitted
and received in that transaction must be determined when RTS
17
is transmitted. The transmission rate mode determined when RTS
is transmitted relates to the entire transaction; it is not
permitted to set an individual transmission rate mode with
respect to each packet transmission within a transaction.
Therefore/ the transmission rate is imperfect.
Further, transmitter stations cannot grasp the state of
reception in receiver stations in real time. Therefore,
determining the transmission rate of the entire transaction when
/'" . RTS is transmitted results in a lower probability that data
packets are transmitted at the optimal transmission rate
corresponding to the state of reception in the receiver station.
An example will be taken. Patent Document 1 discloses
a method for packet transfer communication. The method is such
that, when the receiving end receives an RTS packet, it measures
the environment and state of communication at that point of time;
then, the receiving end adds the result of the measurement to
the CTS packet and returns it to the transmitting end; thereby
the transmitting end optimizes the communication rate and the
like. However, this case is also nothing but another case where
the transmitting end determines the transmission rate of the
entire transaction before it transmits an RTS packet. Therefore,.
the imperfection of transmission rate is not eliminated.
The RTS transmitter station is required to: determine the
transmission rate of the entire transaction includingCTS packet,
Data packet, and ACK packet transmitted and received as the
outcome of RTS transmission; determine a value to be written
as the transaction finish time, that is, Duration based on a
18
value obtained by dividing the amount of transmitted data by
the transmission rate; and describethe determined value in RTS .
Patent Document 1 describes that the CTS transmitter station
determines a transmission rate through-the reception of RTS;
however it does not refer to how to obtain the value in the Duration
field described in RTS by the RTS transmitter station prior
thereto.
If the CTS transmitting station sets a high transmission
rate, the transaction is completed ahead of the Duration
determinedwhenRTS is transmitted. As a result, the surrounding
stations which heard the RTS still keep a NAV set and refrain
from their own transmission after the transaction is completed,
and this is a waste of bandwidth. If the CTS transmitting station
sets a low transmission rate, the transaction is not completed
even after the Duration determined when RTS is transmitted has
passed. Nevertheless, surrounding stations which heard RTS put
down the NAV and start transmit operation, and this induces
collision.
(2) Difficulty of Data Unit Multiplexing with RTS/CTS Used
Together
In the conventionally known RTS/CTS procedure, the data
transmitting end determines a data rate before it transmits an
RTS. It does not give consideration to the following: the data
receiving end determines a data rate and further transmits a
plurality of data units as one data packet. To enhance the
efficiency of the MAC layer (Refer to FIG. 20.), it is preferably
that the receiving end should be capable of determining a data
19
rate and transmitting a plurality of data units as one data packet.
However/ the conventional RTS/CTS format cannot implement this .
(3) Concomitant Use of Delay Acknowledgement and RTS/CTS
Procedure
The IEEE802.il standard is premised on the immediate ACK
wherein an acknowledgement (ACK) is returned as soon as data
is received. To reduce the overhead of ACK packets, delay ACK
can be used.
(" However, some problems arise. For example, if delay ACK
is used together with the RTS/CTS procedure, there is a
possibility that an RTS is transmitted for the purpose of
retransmission' before an ACK cannot be returned to the
transmitting end. Thus, with the existing format, the
concomitant use of delay acknowledgement and the RTS/CTS
procedure has its limit in the enhancement of efficiency.
(4) Methodology For Selective Acknowledgement
It is known that carrying out selective acknowledgement
enhances the efficiency of the MAC layer. The IEEE802.il
standard does not give consideration to selective
acknowledgement itself. The selective acknowledgement
referred to here is a retransmission control method. In this
method, ACK or NACK information is fed back to the transmitting
end with respect to an arbitrary packet which was or was not
successfully received. Thereby, retransmission is carried out
only with respect to packets which the receiving end could not
receive.
Implementation of selective acknowledgement requires
20
massive memory and its processing places a significant burden;
therefore, it has not be actually applied so much. However,
necessity for selective acknowledgement will be more and more
increased in the future. With this being the situation,, the
following case is expected: communication stations in the initial
stage do not have selective acknowledgement implemented but
communication stations developed later have selective
acknowledgement implemented. In this case, it is unfavorable
/' • that both cannot communicate with each other, and a format wherein
downward compatibility is maintained is desired.
/ , Further, up to now consideration has not given to
concomitant use of selective acknowledgement, a technique for
the receiving end-to determine a data rate, and the RTS/CTS
procedure. Therefore, this cannot be implemented by the
existing format.
(5) Influence of Overhead Produced in MAC Layer
The IEEE802.il standards independently defines RTS frame,
CTS frame, ACK frame, and the like. (Refer to FIG. 16.) For.
this reason, if individual information is to be transmitted,
they must be respectively transmitted by separate frames.
However, each time a frame is transmitted, overheads, such as
preamble, are produced. In particular, if the transmission rate
is high, the amount of overheads becomes too large to neglect.
Needless to add, increase in overheads in the MAC layer
is undesirable because it contributes in such a direction as
to limit the bandwidth which can be provided for the upper level
layers, such as TCP/IP.
21
fPatent Document 1] Japanese Unexamined Patent
Publication No. 2000-151639
[Non-Patent Document 1] Inter-national Standard ISO/IEC
8802-11: 1999(E) ANSlZlEEEStd802.il, 1999 Edition, Partll:
Wireless LANMediumAccess Control (MAC) and Physical Layer (PHY)
Specifications
[Non-Patent Document 2] ETSI Standard ETSI TS 101 761-1
VI.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type
2; Data Link Control (DLC) Layer; Parti: Basic Data Transport
Functions
[Non-Patent Document 3] ETSI TS 101 761-2 VI.3.1
Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data
Link Control (DLC) Layer; Part2: Radio Link Control (RLC)
sublayer
[Non-Patent Document 4] C. K. Tho, "AdHocMobile Wireless
Network" (Prentice Hall PTR)
Disclosure of Invention
An object of the present invention is to provide an
excellent wireless communication system, wireless
communication device and wireless communication method, and
computer program wherein the RTS/CTS method is used together
and yet access control based on CSMA can be favorably carried
out.
Another object of the present invention to provide an
excellent wireless communication system, wireless
22
communication device and wireless communication method, and
computer program wherein a plurality of types of frames, such
as RTS, CTS, DATA, and ACK, are multiplexed and thereby flexile
.-transmitting- and ..receiving procedures can. be provided -and
overheads can be reduced.
The present invention has been made with the above problems
taken into account. A first aspect of the present invention
is a wireless communication system wherein random access is
i" carried out using together .the RTS/CTS method in which a
communication station as the origin of data transmission
transmits a request to send packet RTS, and starts data
transmission in response to the reception of a response packet
CTS from a communication station as the destination of data
transmission.
The RTS receiving communication station measures the
quality of a receive signal, determines a data rate based on
the result of the measurement, describes the data rate in CTS,
and transmits the CTS.
The CTS receiving communication station carries out data
transmission as the outcome of the reception of the CTS
information based on the data rate described in the CTS.
The "system" described here refers to what is obtained
by logically aggregating a plurality of devices (or functional
modules which implement specific functions) . It does not matter
whether the devices or functional modules are placed in a single
enclosure or not.
The RTS transmitting communication station describes in
23
RTS the time that lapses before the reception of a packet/
including CTS information, transmitted as the outcome of the
reception of RTS information is completed, as medium reservation
time .information Duration. , The CTS.transmitting communication
station describes in CTS the time that lapses before the reception
of a data packet transmitted as the outcome of the reception
of CTS information is completed, as medium reservation time
information Duration. Other communication stations that
receive RTS or CTS set a NAV (Network Allocation Vector), and
keep it in effect for the medium reservation time information
Duration and bring themselves in transmission prohibited state.
In a wireless communication system according to the first
aspect of the present invention, a transmission rate can be
selected based on the quality of receive signal. Therefore,
the imperfection of transmission rate can be wiped off.
Here, the following constitution may be adopted: the RTS
sending communication station describes in RTS factor
information referred to by the RTS receiving communication
station when it determines a data rate. The RTS receiving
communication station determines a data rate with consideration
given to the factor information described in the RTS, in addition
to the result of measurement of the quality of receive signal.
As the factor information described here, information
related to whether a high data rate should be aggressively
determined or a low data rate should be passively determined
is described. Further, the following constitution may be
adopted: the RTS transmitting communication station measures
24
the transmission error rate for the RTS receiving communication
station. Then, based on the result of the measurement, factor
information (Rate Strategy) is determined. Or, factor
information may be determined based on the difference in
transmissionpower between RTS packet and data packet, the causal
relationship between data rate and transmission power, or the
like.
In these cases, a data rate is determined according to
( the quality of RTS receive signal at the receiving end. A
transmission rate is determined with consideration additionally
given to information, such as error rate monitored by the
transmitting end. Therefore, the correspondence between
reception quality and error rate arising from fluctuation in
channel state or fluctuation in the amount of interference can
be corrected in two steps.
Further, the following constitution may be adopted: the
RTS transmitting communication station describes in RTS
information related to one or more data units the station tries
to transmit. The RTS receiving communication station
determines medium reservation time information Duration in which
the reception of all the data packets is completed. The medium
reservation time information Duration is determined based on
the information related to the data units, described in the RTS
and the determined data rate. Then, the station describes it
in CTS . In this case the data transmitting communication station
which receives the CTS carries out data transmission as the
outcome of the reception of CTS information. The data
25
transmission is carried out based on the data rate described
in the CTS so that the data transmission will be completed in
a time specified by the medium reservation time information.
Examples of the information related to data units, referred
to here, include: the data length of each data unit for which
transmission is tried; the sum of data lengths of one or more
data units for which transmission is tried; the length of time
required for transmitting one or more data units for which
transmission is tried; information on the number of data units
for which transmission is tried (in this case, however, data
units are of fixed length); and the like.
In these cases, the RTS/CTS procedure can be used together
and further data units can be effectively mult iplexed. Further,
the RTS/CTS procedure and delay ACK can be implemented. As a
result, the amount of overheads produced in the MAC layer can
be reduced.
In this case, when the RTS receiving communication station
determines medium reservation time information after the
reception of RTS, the station takes into account the time at
which the station itself completes data reception. Or, when
the RTS receiving communication station determines medium
reservation time information after the reception of RTS, it gives
consideration so that the period information described in the
RTS is not exceeded.
For example, the following constitution may be adopted:
the RTS transmitting communication station assigns sequence
numbers to transmit data units, and describes in RTS the first
26
sequence number of data the station tries to transmit. Thereby,
the station notifies of information related to the data units
it tries to transmit. Meanwhile, the RTS receiving
communication station refers to the first sequence number of
data unit, described in the RTS, and extracts and excludes data
units the station has already received from data units for which
transmission is tried. Then, the station determines medium
reservation time information.
Further, if selective acknowledgement is applied, the
following constitution may be adopted: the RTS transmitting
communication station describes reception acknowledgement
information in RTS. The reception acknowledgement information
includes the first sequence number of data units for which data
transmission is tried and bit map information obtainedby mapping
the reception confirmation information on the subsequent data
units into bits corresponding to the relative positions to the
first sequence number. Further, the station generates
information related to the data units with only data units for
which reception confirmation has not been obtained taken as
objects to be transmitted. 'Meanwhile, the RTS receiving
communication station refers to the bit map information described
in the RTS. The station extracts data units the station has
already received from data units for which transmission is tried,
and excludes them from the objects to be transmitted. Then,
the station determines medium reservation time information.
Further, the following constitution may be adopted: if
data units the RTS receiving communication station has already
27
received are included in data units as objects to be transmitted,
described in RTS, the station describes ACK information in CTS .
In this case, the CTS receiving communication station excludes
data units the RTS receiving communication station has already
received from objects to be transmitted based on the ACK
information added to the CTS. Then, the station carries out
data transmission based on the data rate described in the CTS.
Further, the following constitution may be adopted: the
RTS receiving communication station stores one or more pieces
of data length information of data units taken as objects to
be transmitted, of data for which transmission is tried as the
outcome of the RTS. Then, the station describes in CTS the
sequence number of the last data unit stored. In this case,
the CTS receiving communication station stores the sequence
number of the last'data unit described in the CTS. When an RTS
is transmitted next time, the station generates information
related to data units for which transmission is tried in such
form that the data unit indicated by the sequence number of the
last data unit is not included in objects to be transmitted.
A second aspect of the present invention is a wireless
communication system wherein a plurality of data units are
transmitted and received. Sequence numbers are assigned to
transmit data units to identify the data units. At the same
time, if selective acknowledgement is applied, the following
is caused to take place:
The receiving communication station describes the
following in reception acknowledgement information ACK: the
28
sequence numbers of data units completely received in the order
of sequence number and the bit map information represented bymapping
the state of reception of data of the subsequent sequence
numbers into relative bit positions from the sequence numbers.
The station transmits the reception acknowledgement information
ACK.
The transmitting and receiving communication stations add
information indicating a causal data flow to transmit data and
f acknowledgement. .
In a wireless communication system according to the second
, aspect of the present invention, information required for
selective acknowledgement is managed on a data flow-by-data flow
basis. Such information includes the sequence numbers of
completely received data units, bit map information wherein the
state of reception of the subsequent data units are described,
and the like. Thus, apluralityof service classes canbe handled
-by different sequence numbers. Therefore, communication can
be maintained regardless of whether selective acknowledgement
is implemented or not.
Some receiver stations do not support selective
acknowledgement. Such receiver stations transmit the bit map
information with zero set for all the bits of the bit map field.
Thus, the receiver stations can ensure communication with
transmitter stations which support the selective
acknowledgement function.
Further, some transmitter stations do not support
selective acknowledgement. Such transmitter stations
29
disregards thebitmap information transmitted from the receiving
end, and transmits the bit map information to be transmitted
with zero set for all the bits of the bit map field. Thus, the
transmitter stations can ensure communication with receiverstations
which support the selective acknowledgement function.
Further, with respect to at least one data flow, the ARQ
system wherein sequence numbers are not assigned is adopted.
When Link session is established, data is transmitted and
received through the data flow, and thereby the sequence numbers '
of the transmitter station and the receiver station are
synchronized with each other.
-A third aspect of the present invention is a wireless
communication system wherein random access is carried out using
the RTS/CTS method together. The RTS/CTS method is such that:
the communication station as the origin of data transmission
transmits a request to send packet RTS. In response to the
reception of a response packet CTS from the communication station
as the destination of data transmission, the communication
station as the origin of data transmission starts data
transmission.
The transmission and reception of packets wherein any two
or more of request, to send RTS, confirmative advice CTS, data,
and acknowledgement ACK are multiplexed are permitted.
The amount of overheads produced in the MAC layer can be
significantly reduced by including pieces of information, such
as RTS, CTS, DATA, and ACK, having different purposes in one
packet.
30
In this case, the communication station which receives
apacket containing RTS information transmits apacket containing
CTS information according to the contents of the RTS. The
communication station which receives a-packet containing-CTS
information transmits a packet containing data according to the
contents of the CTS.
Here, the following constitution may be adopted: the RTS
transmitting communication station can specify whether the
receiving communication station may add RTS information to CTS
or not. In this case, in RTS, first information which indicates
whether RTS information may be added to CTS transmitted as the
outcome of the RTS or not is described. Then, the RTS receiving
communication station judges whether it may add RTS information
to CTS based on the first information in the RTS. If there is
data the station desires to transmit to the RTS transmitting
communication station, it adds RTS information for the purpose
of transmitting the data to CTS, and transmits the CTS.
In this case, the RTS transmitting communication station
determines the first information, for example, based on
information elements contained in the packets received in the
past from the station to which the RTS is directed. The
information element described here refers to More bit which
indicates the presence of subsequent transmit data in the packet,
and the like . Alternatively, it may be judged from other means
than information elements that subsequent transmit data is
present in the station to which the RTS is directed to determine
the first information. Examples include the following case:
31
a station receives an RTS from a station to which RTS is directed.
However, the receiving station itself received a packet from
another surrounding station and thus has set a NAV. Therefore,
the station cannot return a CTS, and the data transmissionsequence
is stopped. There are also cases where it can be judged
based on any other communicationhistory that subsequent transmit
data is present in the station to which RTS is directed.
Further, the following constitution may be adopted: the
RTS receiving communication station recognizes from the
information in the RTS that the RTS transmitting communication
station is trying to transmit data the receiving communication
station itself has already received. In this case, the RTS
receiving communication station adds ACK information to CTS to
notify of the state of data reception.
Further, the following constitution may be adopted: in
the data transmitting and receiving procedure, the data
transmitting communication station specifies whether the data
receiving communication station should return an ACK. The ACK
includes immediate ACK and del ay ACK. In this case, the following
constitution may be adopted: the data receiving communication
station receives data requesting ACK, from the data transmitting
communication station. If the data receiving communication
station judges that it has not transmitted an ACK of the data,
it adds ACK information when it transmits a packet of some kind
to the data transmitting communication station. In some cases,
ACK information may be added at the transmitter station' s request.
At this time, if the transmitter station judges that it has not
32
receivedACK information related to previously transmitted data,
it sends a request to add ACKinformation. However, with respect
to packets in which RTS information is solely transmitted, ACK
information need not be added by way of exception..
Further, data requesting an ACK may be transmitted as a
packet directed to two or more communication stations.
Further, the following constitution may be adopted: the
RTS transmitting communication station requests the RTS
/ receiving communication station to return an ACK of data which
has been transmitted so far. In this case, the RTS transmitting
communication station multiplexes an ACK request for requesting
a return of ACK into an RTS for starting the next data transmission
transaction, and transmits it.
Even if the RTS transmitting communication station
requests an ACK, the ACK is not returned sometimes. Possible
causes include that transmit data was not properly received,
that data was property received but the reception of ACK failed,
and the like. In these cases, the RTS transmitting communication
i
i
station must verify whether to retransmit data. In these cases,
the RTS transmitting communication station may multiplex an ACK
( request requesting a return of ACK into an RTS for starting data
retransmission transaction, and transmit it.
The communication station which received an RTS with an
ACK request multiplexed returns an ACK which indicates whether
it has properly completed the reception of data transmitted from
the RTS transmitting communication station. If the station has
failed to-receive data transmitted from the RTS transmitting
33
communication station, it may multiplex an ACK indicating that
it failed to properly receive the data. Then, it may return
a CTS requesting to retransmit the data.
. . A fourth aspect of the present invention is a wireless
communication system wherein the data transmitting
communication station acquires transmission right on the medium
and carries out data communication between it and the data
receiving communication station.
I , After data transmission from the data transmitting
communication station is stopped, the presence or absence of
subsequent data at the data transmitting communication station
is detected. If the subsequent data is present, it is made easier
for the data transmitting communication station to acquire
transmission right for transmitting the subsequent data.
The state in which, though the subsequent data is present
at the data transmitting communication station, data
transmission is stopped corresponds to the following cases, for
example: cases where an RTS from the data transmitting
communication station reaches the data receiving communication
station and the data receiving communication station returns
a CTS, but the data transmitting communication station does not
start data transmission; and cases where transmit data does not
reach the data receiving communication station.
The data receiving communication station can detect the
presence/absence of the subsequent data at the data transmitting
communication station based on the communication history of
transactions based on RTS/CTS. Or, the data transmitting
34
communication station can include in a packet a specific
information element indicating the presence/absence of the
subsequent data and transmit the packet. In this case, the data
receiving communication station checks the information element,
and can thereby detect the presence/absence of the subsequent
data at the data transmitting communication station.
According to CSMA, it is confirmed only for a predetermined
period that the medium is clear, and then the operation waits
only for an arbitrary backoff time before transmission right
is acquired. In the communication environment wherein medium
access control is carried out based on CSMA, the data receiving
communication station tries to acquire transmission right on
the medium, and transmits a signal onto themedium for eliminating
other communication stations' transmission right. Thereby, it
can be made easier for the data transmitting communication
station to acquire transmission right for transmitting the
subsequent data.
In medium access control with the RTS/CTS method used
together, the communication station as the origin of data
transmission transmits a request to sendpacket RTS . In response
to the reception of a response packet CTS from the communication
station as the destination of data transmission, it starts data
transmission. In case this medium access control is carried
out, the data receiving communication station transmits a dummy
RTS onto themedium for eliminating other communication stat ions'
transmission right. Thereby, it can be made easier for the data
transmitting communication station to transmit an RTS for
35
transmitting the subsequent data. For example/ when the data
transmitting communication station receives a dummy RTS, it can
transmit an RTS with a shorter frame space for transmitting the
subsequent data. .........
A fifth aspect of the present invention is a computer
program written in a computer-readable format for performing
on a computer system processing for controlling communicating
operation in the communication environment wherein random access
is carried out with the RTS/CTS method used together. In the
RTS/CTS method, the communication station as the origin of data
transmission transmits a request to send packet RTS; in response
to the reception of a response packet CTS from the communication
station as the destination of data transmission, it starts data
transmission. The program comprises:
a communication quality measuring step in which, when an
RTS is received, the quality of receive signal is measured;
a data rate determining step in which a data rate is
determined based on the measured quality of receive signal;
a CTS transmitting step in which the data rate is described
in CTS as the outcome of the received RTS, and the CTS is
transmitted; and
a data transmitting step in which, when CTS information
is received, data transmission as the outcome of the CTS
information is carried out based on the data rate described in
the CTS.
A sixth aspect of the present invention is a computer
program written in a computer-readable format for carrying out
36
on a computer system control of communicating operation for
transmitting and receiving a plurality of data units . ' Sequence
numbers are assigned to transmit data units to identify the data
units. If selective acknowledgement is applied, the following
steps are provided:
a step in which, in response to data reception, the sequence
numbers of completely received data units in the order of sequence
number, and bit map information represented by mapping the state
of reception of data of the subsequent sequence numbers into
relative bit positions from the sequence numbers, are described
in reception acknowledgement information ACK and are
transmitted; and
a step in which information indicating a causal data flow
is added, and transmit data and an acknowledgement are
transmitted.
A seventh aspect of the present invention is a computer
program written in a computer-readable format for performing
on a computer system processing for controlling communicating
operation in the communication environment wherein random access
is carried out with the RTS/CTS method used together. In the
RTS/CTS method, the communication station as the origin of data
transmission transmits a request to send packet RTS; in response
to the reception of a response packet CTS from the communication
station as the destination of data transmission,, it starts data
transmission. The program comprises:
an information generating step in which varied information,
including request to send RTS, confirmative advice CTS, data,
37
and acknowledgement ACK, is generated; and
a packet transfer communication step in which a packet
wherein any two or more pieces of information of request to send
RTS, confirmative.advice CTS, data, and acknowledgement ACK are
multiplexed is transmitted and received.
An eighth aspect of the present invention is a computer
program written in a computer-readable format for performing
on a computer systemprocessing for controlling the communicating
operation in the following communication environment: in the
communication environment, the communication station acquires
transmission right on the medium before it carries out data
communication. The program comprises:
a step in which the presence/absence of transmit data from
other communication stations is detected; and
a medium access control step in which, in response to the
detection of the presence of transmit data from other
communication stations, a signal for eliminating the other
communication stations' transmission right is transmitted onto
the medium so as to make easier for the relevant communication
station to acquire transmission right for transmitting data.
The computer programs according to the fifth, to eighth
aspects of the present invention are computer programs written
in a computer-readable format for performing predetermined
processing on a computer system by definition. In other words,
when installed on a computer system, the computer programs
according to the fifth to eighth aspects of the present invention
deliver synergistic action on the computer system and operate
38
as wireless communication devices. When a plurality of such
wireless communication devices are started to build a wireless
network, the same action and effect as with the wireless
communication systems according to the first to fourth aspects
of the present invention can be obtained.
According to the present invention, an excellent wireless
communication system, wireless communication device and
wireless communication method, and computer program wherein the
RTS/CTS method is used together and further access control can
be favorably carried out based on CSMA can be provided.
Further, according to the present invention, an excellent
wireless communication system, wireless communication device
and wireless communication method, and computer program wherein
a plurality of types of frames, such as RTS, CTS, DATA, and ACK,
are multiplexed, and thereby flexible transmitting and receiving
procedures can be provided and overheads can be reduced can be
provided.
According to the present invention, a transmission rate
can be selected based on the quality of receive signals.
Therefore, the imperfection of transmission rate can be
eliminated. In addition, when a transmission rate is determined,
information, such as the error rat e monitored by the transmitting
end, is also taken into account. Therefore, the correspondence
between reception quality and error rate arising from fluctuation
in channel state or fluctuation in the amount of interference
can be corrected in two steps.
Further, according to the present invention, data units
39
can be effectively multiplexed while the RTS/CTS procedure is
used together. Further, the RTS/CTS procedure and delay ACK
can be implemented. As a result, the amount of overheads produced
•in the-MAC -layer can be reduced. • • -
Further, according to the present invention,
communication can be maintained regardless of whether selective
acknowledgement is implemented or not.
Further, according to the present invention, the amount
i of overheads produced in the MAC layer can be significantly
reduced by including pieces of information, such as RTS, CTS,
DATA, and ACK, having different purposes in one packet.
Other objects, features, and advantages of the present
invention will be apparent from the embodiments of the present
inventiondescribed later and themore detaileddescription taken
in connection with the accompanying drawings.
Brief Description of Drawings
FIG. 1 is a drawing illustrating an example of the
disposition of communication devices constituting a wireless
communication system in an embodiment of the present invention.
FIG. 2 is a drawing schematically illustrating the
functional constitution of a wireless communication device which
operates as a communication station in a wireless network in
an embodiment of the present invention.
FIG. 3 is a drawing illustrating an example of a frame
format used in a wireless communication system according to the
present invention.
40
FIG. 4 is a drawing illustrating an example of the
constitution of PSDU.
FIG. 5 is a drawing illustrating several examples of PSDUs
which can be defined. - - ~- -
FIG. 6 is a drawing explaining the process for determining
a transmission rate in a wireless communication system according
to the present invention.
FIG. 7 is a drawing illustrating an example of the operation
sequence of the RTS/CTS procedure according to the present
invention.
FIG. 8 is a drawing illustrating an example of the
application of transmitting and receiving procedures according
to the present invention.
FIG. 9 is a drawing illustrating another example of the
application of transmitting and receiving procedures according
to the present invention.
FIG. 10 is a drawing illustrating a further example of
the application of transmitting and receiving procedures
according to the present invention.
FIG. 11 is a drawing illustrating the acknowledgement
procedure related to data other than data flow 0 assumed in the
present invention.
FIG. 12 is a drawing illustrating a concrete example of
the interaction of individual fields which occurs when selective
acknowledgement is carried out with RTS/CTS used together.
FIG. 13 is a drawing illustrating an example of data
transmitting and receiving procedures taken when delay ACK and
41
the RTS/CTS procedure are applied.
FIG. 14 is a drawing illustrating an example of the sequence
in which traffic is transmitted and received through TCP/IP in
• accordance with the MAC procedure according to the present
invention.
FIG. 15 is a drawing illustrating the frame format
according to IEEE8 02.11a.
FIG. 16 is a drawing illustrating the const! tut ion of frame
/ . fields in PSDU.
/ FIG. 17 is a drawing schematically illustrating an example
of the operation of the RTS/CTS procedure.
FIG. 18 is a drawing explaining the action which can occur
at surrounding stations when the RTS/CTS procedure is carried
out between the transmitter station and the receiver station.
FIG. 19 is a drawing explaining the procedure for
determining the transmission rate mode of the entire transaction
when STAO as the origin of transmission transmits an RTS.
/ FIG. 20 is a drawing illustrating an example of the
operation sequence in which packet transmission and reception
transaction based on the RTS/CTS procedure according to
IEEE802.il is implemented with traffic through TCP/IP.
FIG. 21 is a drawing explaining an example of the
application of transmitting and receiving procedures according
to the present invention.
FIG. 22 is a drawing explaining another example of the
application of transmitting and receiving procedures according
to the present invention.
42
FIG. 23 is a drawing explaining a further example of
transmitting and receiving procedures according to the present
invention.
- FIG. 2 4 is a drawing explaining the mechanism forcont
rolling the j udgment of whether "inf ormation indicating that
an RTS may be multiplexed into a CTS" should be set or not.
FIG. 25 is another drawing explaining the mechanism for
controlling the j udgment of whether winformation indicating that
/ an RTS may be multiplexed into a CTS" should be set or not.
FIG. 2 6 is a further drawing explaining the mechanism for
controlling the judgment of whether "information indicating that
an RTS may be multiplexed into a CTS" should be set or not.
Best Mode for Carrying Out the Invention
Referring to the drawings, the embodiments of the present
invention will be described in detail below.
A. System Configuration
f The propagation path for communication the present
invention assumes is wireless, and a network is built among a
plurality of communication stations. The communication the
present invention assumes is store and forward traffic, in which
information is transferred on a packe.t-by~packet basis. With
respect to each communication station, the following description
assumes single-channel station; however, the present invention
can be expanded to cases where a transmission medium comprising
a plurality of frequency channels, that is, multiple channels
is used.
43
In a wireless network according to the present invention,
each communication station can directly (randomly) transmit
information in accordance with an access procedure based on CSMA
(Carrier Sense Multiple Access). Thus, the communication
stations can build an autonomous and decentralized wireless
network.
The autonomous and decentralized wireless communication
system does not have the relation of control station and
controlled station. In such a wireless communication system,
for example, each communication station sends beacon information.
The station thereby informs other communication station located
nearby (i.e. within the range of communication) of the presence
of itself, and informs them of the constitution of the network.
A communication station which newly emerges in some communication
station's range of communication receives a beacon signal, and
thereby detects that it has entered the range of communication.
Further, the station decodes the information described in the
beacon, and can thereby recognize the network and participate
in the network.
In the wireless network according to thepresent invention,
communication stations exchange beacon signals, and modestly
and temporally synchronize with each other. Thus, transmission
control is carried out wherein channel resources are effectively
utilized by transmission (MAC) frames having the time division
multiple access structure. Therefore, each communication
station can implement an access method based on temporal
synchronization, such as reserving a bandwidth and setting a
44
preferential period of use.
The processing at each coirtmunication station, described
below, is basically carried out at every communication station
. - that participates in the network. In some cases, however, the
processing described below is not necessarily performed by all
the communication stations constituting the network.
FIG. 1 illustrates an example of the disposition of
communication devices which ' constitute the wireless
/ communication system in an embodiment of the present invention,
This wireless communication system does not have the relation
of control station and controlled station. In this
communication system, each communication device autonomously
and decentralizedly operates, and an ad-hoc network is formed.
The figure shows how the communication device #0 to the
communication device #6 are distributed in the same space.
In the figure, each communication device's range of
communication is indicated by broken line . Each communication
device can communicate with other communication devices located
within its range, and the range of communication is defined as
a range in which a signal transmitted by each station interferes
with other communication devices. More specific description
will be given. The communication device #0 is located in a range
in which it can communicate with the communication devices #1
and #4 located nearby; the communication device #1 is located
in a range in which it can communicate with the communication
devices #0, #2, and #4 located nearby; the communication device
#2 is located in a range in which it can communicate with the
45
communication devices #1, #3, and #6 located nearby; the
communication device #3 is located in a range in which it can
communicate with the communication device #2 located nearby;
the communication device #4 is located in a range in.which it
can- communicate with the communication devices #0, #1, and #5.
located nearby; the communication device #5 is located in a range
in which it can communicate with the communication device #4
located nearby; and the communication device #6 is located in
a range in which it can communicate with the communication device
#2 located nearby.
When communication is carried out between certain
communication devices, a "hidden terminal" exists. Hidden
terminal is defined as a communication device which can be heard
by either of the communication devices communicating with each
other but cannot by the other.
The scope of the present invention is not limited to the
above ad-hoc environment. It can be widely applied to other
modes of communication wherein each communication station can
directly and asynchronously transmit information in accordance
with the access procedure based on CSMA with the RTS/CTS procedure
used together.
FIG. 2 schematically illustrates the functional
constitution of .a wireless communication device which operates
as a communication station in a wireless network in an embodiment
of the'present invention. The wireless communication device
in the figure can carry out access control based on CSMA using
the RTS/CTS procedure together and form a network in the
46
autonomous and decent ral i zed communi cation environment in which
a control station is not provided.
As illustrated in the figure, the wireless communication
.device 100 comprises an interface 101, adatabuf f er 102,. a central
control unit 103, a beacon generating unit 104, a radio
transmitter unit 106, a timing control unit 107, an antenna 109,
a radio receiver unit 110, a beacon analyzing unit 112, and an
information storing unit 113.
The interface 101 exchanges varied information between
it and external equipment (e.g. personal computer (not shown))
connected to this wireless communication device 100.
The data buffer 102 is used to temporarily store data
transmitted from connected equipment through the interface 101
or data received through a wireless transmission channel before
the data is sent out through the interface 101.
The central control unit 103 manages a series of
information transmission and reception processing in the
wireless communication device 100 and controls accesses to the
transmission channel in a centralized manner. In the central
control unit 103, for example, access control is carried out
based on CSMA with the RTS/CTS procedure used together. In this
embodiment, transmitting and receiving procedures wherein a
plurality of types of frames, such as RTS, CTS, DATA, and ACK,
are multiplexed are implemented. These procedures will be
described in detail below.
The beacon generating unit 104 generates beacon signals
periodically exchanged between the wireless communication
47
device 100 and nearby wireless communication devices.
The radio transmitter unit 106 includes: a modulator which
modulates transmit signals by a predeterminedmodulat ion method;
a D-A converter which, converts digital transmit signals, into
analog signals; an up converter which up converts analog transmit
signals by frequency conversion; a power amplifier (PA) which
amplifies the power of up'converted transmit signals; and the
like. (None of these items is shown in the figure.) . The radio
transmitter unit 106 wirelessly transmits data temporarily
stored in thedatabuf f er 102 andbeacon signals at a predetermined
transmission rate.
The radio receiver unit 110 comprises: a low noise
amplifier (LNA) which voltage amplifies signals received from
other stations through the antenna 109; a down converter which
down converts voltage amplified receive signals by frequency
conversion; an automatic gain controller (AGC) ; an A-D converter
which converts analog receive signals into digital signals; a
synchronization circuit for obtaining synchronism, a channel
estimation circuit, and a demodulator which carries out
demodulation by a predetermined demodulation method; and the
like. (None of these items is shown in the figure. ) The radio
receiver unit 110 receives signals, such as information and
beacons, transmitted from other wireless communication devices
during predetermined periods.
As radio transmission and reception methods for the radio
transmitter unit 106 and the radio receiver unit 110, for example,
various transmission methods which are applicable to wireless
48
LANs and suitable for relatively short distance communication
can be applied. Specifically, the UWB (Ultra Wide Band) method,
OFDM (Orthogonal Frequency Division Multiplexing) method, CDMA
(Code Division Multiple Access) method,, and.the. like-can be
adopted.
The antenna 109 wirelessly transmits signals directed to
other wireless communication devices on a predetermined
frequency channel. Or, the antenna 109 gathers signals sent
from other wireless communication devices . In this embodiment,
the wireless communication device 100 is provided with a single
antenna and is incapable of carrying" out transmission and
reception in parallel.
The timing control unit 107 controls timing with which
radio signals are transmitted and received. For example, it
controls the following timing: transmission timing and reception
timing for various packets of RTS, CTS, DATA/ ACK, and the like
and packets in which these are multiplexed; timing with which
the wireless communication device 100 transmits beacons; timing
with which beacons are received from other stations; and the
like.
The beacon analyzing unit 112 analyzes beacon signals which
could be received .from neighbor stations, and analyzes the
presence of nearby wireless communication devices and the like.
For example, such information as reception timing for beacons
of neighbor stations and nearby beacon reception timing, is
stored as nearby device information in the information storing
unit 113.
49
The information storing unit 113 stores the following:
execution procedure instructions (programs wherein the
collision avoidance procedure and the like are described) for
a series of access control operation and the like, performed
in the central control unit 103; nearby device information
obtained from the result of analyses of received beacons; and
the like.
B. Frame Format (Packet Format)
FIG. 3 illustrates an example of the constitution of frame
format used in a wireless communication system according to the
present invention. However, in this figure, the preamble, PLCP
header, and the like illustrated in FIG. 15 are omitted, and
only the portion represented as PSDU (PHY Service Data Unit)
is selected and shown.
As illustrated in the figure, the PSDU comprises MAC header
portion and MSDU (MAC Service Data Unit: one data unit passed
from the upper level layer) portion. In this embodiment, a
plurality of MSDUs can be placed in one PSDU. The MAC header
portion comprises common MAC header portion, sub MAC header
portion,- and HCS (Header Check Sequence) portion. As sub MAC
header, a plurality of. sub MAC headers can be placed.
FIG. 4 illustrates an example of the constitution of PSDU
in this embodiment. These frame examples correspond to
conventional RTS frame, CTS frame, ACK frame, and data frame,
respectively. (Refer to FIG. 16.)
The header length (HLen) , destination address (RA) ,
50
transmission origin address (TA), and Duration defined in each
frame in common correspond to the common MAC header portion.
Table 2 indicates the fields in the common MAC header portion
in detail. .........
( •
(
(
51
Table 2
Field Name | Length [Bit]
H LENGTH
RA
TA
8
Description
MAC header length. Up to 255 bytes.
Refer to this value to determine to what extent the CRC
computation for HCS should be carried out.
Duration
The destination address of the relevant packet. If this is not
matched with the relevant packet's ID or broadcast ID, decoding
is immediately stopped,
The transmission origin address of the relevant packet.
Field indicating for what period of time in [fisec] reception is
expected to last after the completion of transmission of the
relevant PHY burst.
This is used for the receiver station to indicate a period for which
it can transmit and for surrounding other stations to set a NAV.
The sub MAC header portion is defined in different formats
depending on the purpose of each frame . For example, in a packet
corresponding to RTS frame, RTS SMH (Sub MAC Header) is placed
as sub MAC header; in a packet corresponding to CTS frame, CTS
SMH is placed as sub MAC header; in a packet corresponding to
ACK frame, ACK SMH is placed as sub MAC header; and in a packet
corresponding to data frame, DATA SMH is placed as sub MAC header.
With respect to the RTS, CTS, and ACK frames, PSDU is
constituted only of the MAC header portion, and the MSDU does
not exist. To the data frame, data specified by the DATA SMH
is added as frame body.
FIG. 5 illustrates several examples of PSDUs which can
be defined in the present invention. In the uppermost tier in
FIG. 5, a packet in which two MSDUs are multiplexed is shown.
In the second tier, a packet in which one MSDU and RTS information
are multiplexed is shown. In the third tier, a packet in which
CTS information and ACK information are multiplexed is shown.
In the lowermost tier, a packet inwhichoneMSDU, RTS information,
52
CTS information, and ACK information are multiplexed is shown.
If an RTS is to be transmitted, an RTS SMH is added as
SMH; if a CTS is to be transmitted, a CTS SMH is added as SMH;
-if -an-ACK-Is transmitted/ an ACK SMH is added .as . SMH;-ancL if
data units are to be transmitted, DATA SMHs are added by a number
equivalent to the number of data units to be transmitted. Thus,
pieces of information, such as RTS, CTS, DATA, and ACK, having
different purposes can be multiplexed into one packet.
Next, the details of the components of each SMH will be
described referring to Table 3 to Table 6. The leading field
of each SMH is Type field, in which an identifier indicating
what information is contained in the relevant SMH is described.
The length of SMH differs depending on the type of SMH. Since
the SMHhas an intrinsic length with respect to each Type, however,
the receiving end can learn the length of the relevant SMH by
referring to the Type field.
Fields in DATA SMH:
Table 3 illustrates the details of the fields in DATA SMH
used when general-purpose data, other than RTS, CTS, and ACK,
is transmitted. As illustrated below, the DATA SMH comprises
Attribute, Sequence, and Length fields.
53
Table 3
Field Name
Type ....
Attribute
Sequence
Length
Length [Bit]
i ._ . .
1
1
2
Description
Identifier indicating that this is an SMH for general-purpose data
transmission, " ' "*— -
Identifier indicating the attribute of the data specified by the SMH.
[2] Data Flow
[2]ACKType
[1] More Bit
[31 Fragment
The sequence number of the data specified by the SMH.
Rotation of 0 to 255 is repeated.
Indicates the length in byte of the data specified by the SMH.
In the Attribute field, the identifier indicating the
( .•
attribute of the data specified by the relevant DATA SMH is
described.
In the Data Flow portion, the data flow of the relevant
data is placed. Here, it is assumed that data flows having a
plurality of attributes on a link-by-link basis can be defined.
Data flows having different attributes are used to define
different ARQ (Automatic Repeat reQuest) methods on a data
flow-by-data flow basis, to handle traffic different in priority
t from data flow to data flow, and to attain the like intended
i purposes.
In the ACK Type portion, information which indicates what
ACK the receiver that received the data should be requested to
return is placed. Specifically, three types of information,
immediate ACK (Im-ACK) request, delay ACK (Del-ACK) request,
and ACK not required, are notified. If the immediate ACK request
is specified, the receiving end returns a packet containing ACK
information immediately after it receives data. If the delay
54
ACK request is specified, preparations for returning ACK are
made but the transmission of ACK information is not carried out
until a packet of some kind is transmitted to the origin of
• transmission of data. (When a packet of some kind-other-than
RTS is transmitted to the origin of transmission of data, an
ACK SMH is added to the packet and it is transmitted.) If ACK
not required is notified, an ACK is not returned.
In the More Bit portion, information indicating whether,
after the relevant packet is transmitted, more data to be
transmitted has been accumulated or not is placed. The receiver
stores the address of the packet transmitting station at which
the More Bit portion has been most recently set, and, in some
cases, polls that station. The polling procedure is not related
directly to the subject matter of the present invention, and
thus further description will not be given.
If the relevant data is fragmented, the Fragment portion
is used to indicate borders in the data.
In the Sequence field, the sequence number assigned to
the relevant data is placed. It is assumed that sequence numbers
0, 1, 2, 3, ... are assigned to data units in ascending order as
is performed in ordinary data communication. The sequence
numbers of 0 to 255 are repeatedly used by rotation.
In the Length field, the length of the relevant data is
placed.
Fields in ACK SMH:
Table 4 illustrates the components of ACK SMH fields used
55
when ACK information is t r a n s m i t t e d . As described below, the
ACKSMH comprises ACK A t t r i b u t e , ACK Sequence/Type, and Received
MAP f i e l d s.
Table 4
Field Name Length [Bit] Description
Type 1 Identifier indicating that this is an ACK SMH.
ACK Attribute
ACK
Sequence/Type
Received MAP
Identifier indicating the attribute of ACK.
[2] Data Flow
[6] Reserved
Transcribes DATA Type of the object of ACK if DATA Flow is 0.
Describes sequence number in the other cases.
Used'when Data Flow is not 0.
ACK/NACK information for Selective ACK.
In the ACK Attribute field, the identifier indicating the
attribute of ACK is described. The Data Flow portion indicates
in which data flow the data as the object of ACK information
is transmitted. Here it is assumed that, if data flow 0 is
selected, immediate ACK (Im-ACK) is used, and if any other data
flow is selected, selective ACK (Sel-ACK) is used.
The ACK Sequence/Type field is a field for indicating up
to which sequence number of the relevant data flow has been
sequentially received if the data flow is not 0. For example,
the value of 6 is placed, that indicates data of up to sequence
number 6 has been all successfully received. If 0 is selected
for data flow, immediate ACK is used, and the data type of that
data is transcribed to indicate which data the ACK is for.
The Received MAP field is a field for indicating whether
any data that has been received after the data indicated by the
ACK Sequence exists or not. Relative to the sequence number
indicated by the ACK Sequence, the subsequent sequence numbers
56
are brought into correspondence by bit map, and. bits
corresponding to data (sequence number) which has been received
are marked. The MSB corresponds to the number next to the
• sequence number indicated by the ACK Sequence. As an example,
it is assumed that: the Received MAP field is constituted of
eight bits; the sequence number in the ACK Sequence field is
23; and the Received MAP is 00100000. This means the following:
data of up to #23 has been all received; (the Received MAP
represents the success or failure in receiving the eight pieces
of data of #24 and the following numbers by bit map) ; the reception
of data of #24 and #25 failed; data of #26 has been received;
and the subsequent data has not been received.
Themethodof transmission and reception wherein data units
received at intervals are held at the receiving end is designated
as selective acknowledgement (Selective ACK) . If the receiver
supports selective acknowledgement, a ReceivedMAP is generated
in accordance with the above-mentioned rule. If the receiver
does not support selective acknowledgement, data which could
not sequentially received' is discarded, and zero is placed in
all the bits of the Received MAP field.
Fields in RTS SMH:
Table S illustrates the components of RTS SMH fields used
when RTS information is transmitted. As described below, the
RTS SMH comprises RTS Attribute, RTS Sequence, RTS ReceivedMAP,
Max Duration, and Length (or Number Of Data Unit) fields.
57
Table 5
Field Name
Type
RTS Attribute
RTS.
Sequence
RTS
Received
MAP
Max Duration
Length
(Number of
Data Unit)
(Directed
Address)
Length [Bit]
1
1
1
2
1
1xN
1
8
Description
identifier indicating that this is a (Directed) RTS SMH.
identifier indicating the attribute of RTS.
[2] Data Row .
[2] Rate Strategy
[1] Rate Set
[1] RTS/CTS Mux
[1]ACK Request
[1] Reserved
The sequence number of the leading MSDU to be transmitted.
With respect to MSDUs subsequent to RTS Sequence, the bit
corresponding to data recognized to have been transmitted is marked
wllh'1."
The maximum data transmission time which can be allowed by the
transmitting end.
Information related to data unit to be transmitted (value indicating the
data length information).
(For example, if the data Units are of fixed length) the number of data
units to be transmitted.
(Only in case of Directed RTS) the MAC address of the node as the
object of RTS.
In the RTS Attribute field, the identifier indicating the
attribute of the RTS information is described.
In the Data Flow portion, the data flow of data for which
transmission is about to be tried is placed.
In the Rate Strategy portion, auxiliary information is
placed which is referred to when the transmission rate of data
is determined. This auxiliary information is used to bias
criteria for determining: whether to aggressively select a high
rate for transmission.rate; whether to slightly aggressively
select a slightly high rate; whether to slightly passively select
a slightly low rate; or whether to passively select a low rate.
58
The usage of the Rate Strategy portion will be described in detail
later.
In the Rate Set portion, indication is placed which
indicates whether to fix the -transmission rate at the value
applied in RTS for transmission or not. If transmission is
carried out with the transmission rate fixed, the entry in the
Rate Strategy portion is disregarded.
In the RTS/CTS Mux portion, information is placed which
indicates whether to permit adding RTS information to a CTS packet
transmitted in response to the relevant RTS or not. If the
addition of RTS is permitted, the station which receives the
relevant RTS and returns a CTS operates as follows : if the station
holds data directed to the station as the origin of transmission
of the RTS, it is permitted to add RTS information to the CTS
and transmit it. Thus, two-way communication is carried out
by one transaction.
In the ACK Request portion, information is placed which
indicates whether to request to add ACK information to a CTS
packet transmitted in response to the relevant RTS or not. If
the addition of ACK information is requested, the station which
receives the RTS and returns a CTS operates as follow: the station
adds the ACK information of data which it has received from the
station of the origin of transmission of the RTS to the CTS and
transmits it.
The RTS Sequence field is a field in which the sequence
number of the leading data of pieces of data for which the station
as the origin of transmission tries to transmit is described.
59
The station as the destination of RTS refers to this field.
Thereby, the station can learn whether the station as the origin
of transmission is trying to transmit data which is a duplicate
of the data already received or not.
The RTS Received MAP field produces an effect when
selective acknowledgement is used together. Relative to the
sequence number indicated in the RTS Sequence field, pieces of
data of the subsequent sequence numbers are brought into
correspondence by bit map. Then, the bits corresponding to data
(sequence numbers) the reception of which the receiver station
recognizes to have been completed are marked. The MSB
corresponds to the sequence number indicated by RTS Sequence.
As an example, it is assumed that: the RTS Received MAP field
is constituted of eight bits; the sequence number in the RTS
Sequence field is 23; and the Received MAP is 00100000. This
means the following: pieces of data of #23, #24, and #25 are
skipped, and for pieces of data of #2 6 and the following numbers,
transmission is continuously tried. The station as the
destination of RTS refers to this field, and can thereby learn
whether the station as the origin of transmission is trying to
transmit data which is a duplicate of the data already received.
The Max Duration field is used for the transmitting end
to set the maximum period of data transmission in the present
data transmission transaction. As an example, it is assumed
that, with respect to the present transaction, data transmission
is to be completed within 2 00 microseconds on the grounds of
the transmitter station. In this case, a value indicating 200
60
microseconds (or a value obtained by subtracting the time
required for the RTS/CTS procedure from 200 microseconds) is
placed in this field. The station as the destination of RTS
has right to determine a period of data reception. At this time,
the station determines a period of data reception so that the
period indicated in the Max Duration field will not be exceeded.
In the Length field, the length of data for which
transmission is tried is described. In case of a first example
of constitution, a plurality of pieces of Length information
(e.g. number of bits and number of bytes) are defined if it is
permitted to transmit one or more data units in one packet. Here,
it is assumed that, for example, N pieces of Length information
are defined. (N is typicallya fixed value but maybe a non-fixed
value.) Data units the reception of which the receiving end
recognizes to have been completed are excluded from data for
which transmission is tried. As an example, it is assumed that:
the sequence number in the RTS Sequence field is 23; and the
Received MAP is 00100000 . In this case, the length of data unit
of #23 is placed in LengthO, and the length of data unit of #24
is placed in Lengthl. In Length2, the length of data unit of
#26, not that of data unit of #25 which has been already received,
is placed.
As a second example of constitution, the sum of the lengths
of a plurality of data units for which transmission is tried
can be described as Length information. In this case, in the
RTS Sequence field, the sequence number of the leading data of
a plurality of data units is described.
61
As a third example of constitution, the length of data
for which transmission is tried can be expressed in time and
described in the Length field. In this case, in the Length field;
.a plurality of lengths of time for which the channel is occupied
when data or a plurality of data groups for which transmission
is tried are transmitted are described with respect to cases
where they are transmitted at several data rates. Here, it is
assumed that, for example, N pieces of Length information are
defined. (On the assumption that transmission is carried out
at N different data rates, the length of time required for data
transmission is described.)
As a fourth example of constitution, the Number Of Data
Unit field can be defined in place of the Length field if the
length of a data unit mainly transmitted is a fixed value. In
this case, in the field, the number of data units for which
transmission is tried is described.
In the RTS SMH, the Directed Address field can be defined
in addition to the foregoing. This is used in the following
case: broadcast or multicast is set in the destination (RA) field
of the common MAC header but RTS information is to be transmitted
to some specific station. In the Directed Address field, -the
address.of the destination of RTS information is described.
Fields in CTS SMH:
Table 6 illustrates the components of CTS SMH fields used
when CTS information is transmitted. As described below, the
CTS SMH comprises CTS Attribute, RATE, and Stored Sequence

Claim:
1. An information processing apparatus used in a wireless communication system,
comprising,
a processor configured to
obtain a plurality of data units,
add plurality of sub MAC headers, each of which includes length information
specifying a length of each data unit,
add a single common header including length information, and
output an aggregated data unit including the single common header, the plurality
of sub MAC headers and the plurality of data units.
2. An information processing apparatus used in a wireless communication system,
comprising,
a processor configured to
obtain an aggregated data unit including a single common header, a plurality of
sub MAC headers and a plurality of data units,
analyze the single common header including length information,
analyze one of the plurality of sub MAC headers including length information of
one of the plurality of data units, identify the one of the plurality of data units.
3. An information processing apparatus according to the claim 1 or 2, and wherein
the wireless communication system is a wireless LAN system using OFDM.
4. An information processing apparatus according to the claim 3, further comprising
an antenna configured to wirelessly transmit the aggregated data.
5. An information processing apparatus according to the claim 4, and
wherein the antenna wirelessly receives data from a transmitter transmitting the
aggregated data.
6. An information processing apparatus according to the claim 5, and
wherein the processing circuitry includes a central processing unit coupled to a
storage unit, the central processing unit processes data read from the storage unit.
7. An information processing apparatus according to the claim 6, further comprising
an interface coupled to the storage unit and wherein the interface exchanges
information with a device coupled to the information processing apparatus.