Note: Descriptions are shown in the official language in which they were submitted.
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EXTENSIONS TO DISTRIBUTED MAC PROTOCOLS WITH COLLISION
AVOIDANCE USING RTS/CTS EXCHANGES
This is a division of Patent Application number 2,210,030 filed on July 8, 1997.
Field of the Invention
This invention is in the field of wireless communication and relates to methods and
arrangements for Media Access Control (MAC) and their extensions to random access
protocols with collision avoidance.
Background and PriorArt
Distributed media access protocols with collision avoidance systems have been proposed
and studied in the past (see References [1]-[3]). One class of major solutions which is used
to combat the effect of collisions due to hidden nodes is based on a solution which uses
a Request-to-Send (RTS) and Clear-to-Send (CTS) frame exchange to reserve the
medium in the beginning of each transmission. In this solution, a station (A) sends one or
more data packets to another station (B) by first sending an RTS packet destined to (B).
If (B) receives the mentioned RTS packet, it replies with a CTS packet destined to (A) and
in this way, (B) announces that a transmission from (A) to (B) is about to take place and
that all stations that can interfere with such transmission and contribute to a collision at (B)
should stay off the shared medium. In essence, an RTS/CTS exchange might be
attempted for a number of times, before the medium can be reserved. This is typical for
distributed media access control protocols. We refer to the average time that a station
spends in contention trying to reserve the medium before each data transmission as the
contention time(T9. The average time each station spends sending higher layer data
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packets after a successful reservation is referred to as the transmission time(TT). In
general, the ratio TT /( TT + TC) can be used as the efficiency factor (U) of a reservation and
as U increases the throughput increases. There are a large number of parameters that
affect U such as system load or the collision window of the reservation. One way to
increase U is to send multiple data packets after each successful reservation, this scheme
is called a burst reservation.
Summary of the Invention
The methods and arrangements described herein improve the performance (measured e.g.
10 in terms of throughput) of a RTS/CTS based distributed media access control protocol. The
medium reservation is done in a hierarchical fashion where first the shared medium is
reserved for two stations called the participants. All other stations called observers stay
quiet during the time reserved for the participants. In this fashion, a shared medium can
be reserved for a subset of the plurality of devices. During this period called the reserved
period, a master (or primary), and a slave (or secondary) station attribute can be given to
the participating devices and the medium can be shared using another medium
coordination algorithm which is not necessarily the same as the one used to reserve the
medium for a reserved period in the first place. After the medium is reserved, the
participating devices can establish a conventional point-to-point connection within the
reserved time period between participating devices for a predefined period of time.
Specifically, the primary attribute can be exchanged with a secondary attribute which in
effect gives the control of the shared medium during the reserved period to another station
different from the one that originally was assigned the primary attribute. During the
reserved time, the secondary can signal the primary station that it has data to send to the
primary station and request that the primary and secondary roles or attributes to be
exchanged. In the case that a role exchange takes place, the control of the medium is
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transferred from one station to another and data transfer in an opposite direction can take
place without requiring another reservation attempt. In essence, this reduces the additional
time in contention mode.
Furthermore, in another aspect of the invention observing stations which are notparticipating in the reservation can be invited to join the initial reservation as secondary
stations if this does not cause any interference to any other existing reservations.
In other aspects of the invention, we define a number of additional extensions in the
10 RTS/CTS exchange to improve the efficiency of the medium reservation by piggy-backing
any reservation specific signals on data packets or defining new response frames such as
Hold-to-Send (HTS) and Free-to-Send (FTS) which are used for flow control techniques
in response to a congested receiving station.
In still another aspect of the invention which is applicable in a communications network
having a plurality of stations and a shared common communications medium using acollision avoidance media access protocol for communication between the stations; in
which medium access is granted to a pair of stations a requester and a target station
obtaining a successful reservation of the medium and in which Multiple stations may
request access to the medium to send data, the invention provides a method of
establishing a reservation between stations as follows:
Requesting stations send Request-to-Send (RTS) control frames and receiving
stations respond as appropriate with Clear-to-Send (CTS) control frames.
A requesting node aborts the reservation if the medium becomes active between
its RTS transmission and the corresponding CTS response of its target station.
Arbitration is performed by stations receiving the RTS transmission as follows:
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if a receiving station receives multiple Request-to-Send (RTS) control frames
directed to the receiving station during a contention period, the receiving station responds
to the last RTS control frame received by transmitting Clear-to-Send (CTS) control frames;
if the receiving station receives multiple RTS control frames directed to different
receiving stations during a contention period, the receiving station will respect the first RTS
control frame received.
In still another aspect of the invention for use in a communications network having a
plurality of nodes and using a collision avoidance media access protocol; using a shared
o common channel medium, in which channel access results from a successful reservation.
The reservation requester is the primary station and subject target is the secondary
station. The primary station coordinates the channel data communication to the
secondary station, the secondary station acknowledges the data with Acknowledgment
(ACK) control frame. The primary station controls the reservation termination based on
transmission of an End-Of-Burst (EOB) control frame. The secondary station responds
with an End-Of-Burst-Confirm (EOBC) control signal. The method of this aspect of the
invention includes combining the with the EOB control frame and transmitting them
together as well as combining the ACK control frame and the EOBC frame and
transmitting them together.
In yet another aspect of the invention for use in a communications network having a
plurality of stations and a shared common communications medium using a collision
avoidance media access protocol for communication between the stations; in whichmedium access is granted to a pair of stations obtaining a successful reservation of the
medium; the pair of stations comprising a requester station and a target station; the
requester station is a primary station, and the target station is a secondary station, in which
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the primary station coordinates communication to the secondary station; the method of the
invention maintains the reservation in the event a transmission of a frame of data is
received from a third station not participating in the reservation as follows:
Each station participating in the reservation examines all received frames to
determine the transmitter address thereof; and
ignores frames transmitted from a non-participating station, continuing with thereservation.
Description of the Drawings
Figure 1 depicts transmissions between two stations.
Detailed Description of Preferred Embodiments of the Invention
Underlying Transmission and Media Access Scheme
In this section we consider a wireless system with the following characteristics in order to
provide a detailed description of the preferred embodiment. A random access scheme with
collision avoidance (CA) based on a RTS/CTS exchange is used to access a shared
wireless medium. Media reservations are made by exchanging RTS/CTS frames.
Data transmission from a source station (A) to a destination station (B) are followed by
sending an ACK frame from (B) to (A). The medium can be reserved by a reservation
exchange between (A) and (B) (e.g. an RTS-CTS exchange) and then one or more
packets can be exchanged between (A) and (B).
The length of the time that the media is reserved can be announced by (A) and (B) in their
reservation handshake or the termination of a reservation period can be announced by
exchanging disconnect messages at the end of the transmission period. Here (A) sends
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a End-of-Burst (EOB) frame and (B) replies with a End-of-Burst-Confirm (EOBC) frame.
Primary/Secondary Role Exchange Within Reservation
We define a primary and secondary attribute within a reservation. Initially, a primary station
(A) is the one that initiates the reservation by sending a RTS packet in a successful
reservation attempt where the secondary station (B) is the recipient of the RTS which
replies with a CTS. After the reservation is made successfully, the primary station (A) is the
owner of the reserved medium and sends data or control frames to the secondary station
(B). All other stations, such as C or D, are defined to be observers of this reservation. Here
the primary acts as a master and the secondary acts as a slave and this defines the media
coordination among active participants in the reservation. Now, a primary station (A) can
send a message called Primary Secondary Role Exchange (PSRE) to a secondary station
(B) initiating the exchange of roles between stations. As a result, the control of the medium
will be passed from one station (A) to another station (B) within a reservation and the flow
of data can be changed in the opposite direction where data frames are sent from (B) to
(A) and acknowledged by (A). As long as this is done within the reserved time, all other
stations that observe this reservation stay quiet and there should be no need for an
RTS/CTS exchange and hence the throughput of the system can be increased since the
data transmission after a PSRE happens without going through a reservation cycle or
contention period.
Referring to Figure 1 which depicts transmissions between two stations, the PSRE frame
can be sent based on a number of conditions which are described below:
After a primary station (A) finishes sending all data packets, available from higher layer
protocols such as the logical link layer (LLC), to the secondary station (B). (A) will send a
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PSRE frame to (B) if according to the rules of the MAC protocol, there is still time left for
the reservation made by (A). In this case, (B) becomes the primary of the reservation
period and starts sending data frames to (A) if (B) has any such data frames destined to
(A) and there is reservation time left to continue the transmission. If (B) does not have any
data frames to send to (A) or when the reservation time expires, (B) signals to (A) to end
the burst and the reservation is terminated implicitly by observing stations keeping track
of the reservation time or by exchanging EOB/EOBC frames. A priority transmission queue
can be set up by each station where acknowledgment packets from higher layers (e.g.
LLC) are entered in such a priority queue (QP) and after a PSRE such frames are
transmitted back to (A). This can improve the efficiency (U) and the throughput of the MAC
considerably since higher layer acknowledgments at a receiving station release
transmission windows on the transmitting station and this can be done in a rapid and
effective manner as discussed in above.
As an alternative, the PSRE can take place when (B) sends data ACK packets to (A)
where such ACK packets can indicate a request for PSRE which notify (A) that (B) had
data to send to (A). In that case (A) can initiate a PSRE after it has sent its data and if
there is still time reserved on the channel. This requires an ACK frame with PSRE
information to be defined. It is important to note that within a reservation, multiple PSRE
frames can take place.
A primary station (A) to identify the final frame that it has to send to (B) by piggy-backing
such information in the data frame sent to (B) and giving (B) the opportunity for requesting
a PSRE.
Joining a Reservation
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Another method to improve the performance for the RTS/CTS based distributed
reservation scheme is to invite observing stations to join the participating stations after a
reservation is made. Again, we denote the two stations that initially set up a reservation
with the RTS/CTS exchange as the participating stations with a primary station (A) and a
secondary station (B). All other stations that are based on observing such a reservation
stay QUIET during the reservation period as observing stations. Now, we describe an
aspect of the invention whereby an observing station can join a reservation. A primary
station can decide to "invite" an observing station (C) to become a secondary station in
addition to all other secondary stations defined during that specific reservation time by
sending an RTS to the observing station (C). Depending on the status of the set of devices
with which (C) can interfere, there exist two cases:
If (C) does not have any information about any other reservation attempt that overlaps with
the reservation originated by (A), then (C) can respond with a CTS and as a result (C)
would announces that it is about to receive data from (A) and that all stations that can
possibly interfere with (C), with the exception of (A), should stay quiet for some time T.
This time T can be announced in the RTS packet sent to (C) or it can be the remaining
reservation time based on the reservation that was originally made by (A) with respect to
(B).
If (C) is aware of any other reservation attempt beside the one made by (A) (e.g., by
observing whole or partial RTS/CTS exchanges, or frame transmissions originated by any
station other than A or B), then (C) would not respond to (A)'s RTS transmission destined
to (C) and as a result (A) would realize that (C) cannot join the reservation after it does not
receive a CTS from (C) destined to (A) within a predefined time-out period.
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Flow control mechanism with Hold-to Send (HTS) and Free-to-Send (FTS)
The flow control mechanism is designed to provide a means for a receiving station to tell
other stations it is congested and cannot handle new requests. This will prevent other
stations from making incorrect assumptions for the state of the congested station. A flow
control (HTS/FTS) type mechanism increases the efficiency of the RTS/CTS based MAC
protocols since it prevents a transmitting station from starting to send data frames to a
receiving station that is congested and will be forced to discard the frames.
The HTS flow control mechanism can be initiated as a response frame to a primary station
request control frame or piggybacked on an existing response frame.
1. HTS Indicators
The Hold-to-Send (HTS) is a control frame architected for responding to a Request-to-
Send (RTS) when the destination is not ready to accept any data. In this scenario a HTS
is sent in reply to an RTS, the transmitting station and all other stations are free to attempt
for another reservation with other non-congested stations. The HTS can also be
piggybacked on an existing response frame by using a special control bit in the control
frame. Piggybacking can be done on any control frame during the life of the reservation.
All traffic directed at the congested station will be held until the Free-to-Send (FTS) frame
is heard by the other stations.
2. FTS Indicators
The Free-to-Send (FTS) indicator must be sent to inform listening stations that congested
condition has been resolved and it is now permissible to resume reservation attempts. The
FTS indicator can be advertised in response to a received RTS frame if one is available
or it can be sent via a control bit in the next available frame queued for transmission.
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The flow control HTS/FTS mechanism can be honored by every station contending and
within range or it may be honored by only participating stations. Either approach may be
selected with different advantages for each.
Atomic RTS/CTS Exchanges
The atomic exchange is used to reduce the collision window in a medium which must
contend with hidden nodes. This approach will minimize scenarios which create
overlapping or ambiguous reservation scenarios.
The rules can be broken down into 2 multiple points:
Station (B) who receives multiple RTS frames, during the contention period, all directed to
station (B) will use the LAST IN WIN rule. For example, if a station (B) receives multiple
RTS packets destined to (B) which are originated from different stations before (B) replies
with a CTS, it replies to the last RTS that it has received.
Station (B) who receives multiple RTS frames, during the contention period, directed at
DIFFERENT stations will obey the FIRST IN RULE. For example, if a station (B) received
an RTS, directed at station (A) followed by an RTS directed at station (B) it would respect
the first RTS frame, with the destination address not equal to station (B) and accept the
loss of a reservation.
The second RTS frame directed at station (B) would indicate that the originator of the
second RTS could not hear the originator of the first RTS and would eventually collide due
to the hidden node problem. The recommended action in this situation is to ignore both
RTS frames and return to the random BACKOFF state to prepare for the next contention
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period. This is considered as the conservative backoff approach to accommodate the
hidden node problem.
The atomic RTS/CTS exchange is used to try and resolve asymmetric reservation
scenarios by forcing both sides to participate in the reservation before accepting it as
successful. An extension to the atomic RTS/CTS is to also force the requirement of an
RTS/CTS/DATA atomic exchange. This would be used to cover the situation where a
station could here the RTS and not a CTS. The DATA would confirm the success of the
attempted reservation.
The atomic exchange is an additional rule to the RTS/CTS protocol as defined in [1]-[3].
This feature can decrease the collision probability in the medium and hence increase the
performance of the protocol.
Combined DATA-EOB/ACK-EOBC Frames
In a burst reservation where end of burst transmission period is announced by sending a
End-of- Burst (EOB) / End-of-Burst-Confirm (EOBC) pair of packets, the EOB information
can be piggy-backed by the last Data frame and the EOBC can be piggy-backed by the
ACK frame. This in turn increases the efficiency of the protocol.
Forgiving Channel
The completion of the atomic reservation transaction enables participants of thereservation to receive and process unauthorized packets, from non participating nodes,
during the life span of the reservation. The action of permitting unauthorized packets from
stations which have not recognized the state of the reservation cycle is considered as
FORGIVING.
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The aspect of a forgiving channel makes reservation participants within a reservation
persistent with respect to channel interference, during DATA / ACK frame exchanges, and
enables all participating stations, both the primary and secondary, suffering from
interference to be persistent with the transmission inside of a reservation. During the
reservation, if either the primary or secondary stations receive a control or data frame sent
by a non-participating station (ie. the source address is not equal to either the primary or
secondary station), the frame will be ignored. The status of the reservation will be
unaffected.
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References
[1] V. Bhargavan, A. Demers, S. Shenker, L. Zhang, "MACAW: A Media Access Protocol
for Wireless LANs" Proceeding of SIGCOMM 94, London, England, 8/94.
[2] K.C. Chen, "Medium Access Control of Wireless LANs for Mobile Computing", IEEE
Network, Vol 8, No. 5, 1994.
[3] Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY)
specifications, Draft Standard IEEE 802.11, May 1995
[4] MAC Protocol for Wireless Communications, Patent CA9-93-019, 1993
