FIELD OF INVENTION:
The present invention relates to a system and method for achieving contention free Beacon Transmission .
PRIOR ART:
IEEE 802.11 ad-hoc Wireless LAN (WLAN) protocol defines mechanisms for Beacon Transmission. A randomly chosen station transmits a special Broadcast frame, called Beacon frame, announcing a Beacon Interval period. The stations that are configured to be in Power Save (PS) mode would normally be sleeping. The senders do not send frames destined to these receivers, but buffer these frames. A predefined period after a Beacon is called ATIM window. The PS receiver wakes up for the duration of this ATIM window. The sender sends an ATIM frame announcing a buffered data frame. The receiver acknowledges ATIM, and the sender sends the data frame after the ATIM window. The active stations also exchange Data frames among each other in this period after the ATIM window.
The mechanisms sometimes result in multiple Beacon Transmitters in the WLAN in multiple ways. If a STA does not hear a Beacon transmitted from another STA due to collision at the receiver, the STA also transmits a Beacon and becomes another Beacon Transmitter. Similarly, if a STA wakes up earlier than start of ATIM window and transmits a Beacon, other STAs
that wake up later do not hear this Beacon. These STAs also transmit Beacons.
Hence, different STAs in the WLAN can hear Beacon from different sources at different times. As per the time of hearing the Beacon, the perceived time for end of ATIM window varies for different STAs. Hence, some STAs may start transmitting Data frames even while the ATIMs are still being exchanged among other STAs. Thus, collision probability in Data transmission is higher leading to lower throughputs.
Us Patent No. 2003058886 [2] optimises ad-hoc WLANs performance. The patent defines methods to establish a number of communication channels between communicating nodes based on message duration. The mechanisms result in improved channel availability and quality of service (QoS) in ad-hoc Wireless Networks. However, since the method uses additional radio channels, it is not suitable for a shared-channel network. Our patent applies to throughput improvement in a shared-channel ad-hoc WLAN.
DESCRIPTION OF THE INVENTION:
The present invention relates to a system and method for achieving contention free Beacon transmission
The present method embodies new mechanisms to select a single Beacon Transmitter as winner among multiple contenders, to synchronise the end of ATIM-window time period for all STAs.
In this method the ATIM frame has been extended by adding the following additional fields -
- "Beacon_MAC": MAC address of the Beacon Transmitter heard by the sender STA
- "Beacon_Time": the time of hearing the Beacon from the Beacon Transmitter
End of ATIM window time is calculated by the receiving stations as (BeaconTime + ATIM Window time).
Each sender sends this extended ATIM when it needs to announce a buffered Data frame for a PS receiver. In the standard IEEE 802.11 protocol mechanisms, all STAs hear all frames on the channel and read the NAV field of frames, even if the frames are not destined to them. We have extended the protocol operation by making these STAs additionally read the above two additional fields. If the BeaconTime in the received ATIM is less than the time of hearing of Beacon by a STA, the STA adopts this new BeaconMAC as the selected Beacon Transmitter (winner) for the fijture. In all the future ATIM frames sent by this STA, the frame would contain these new Beacon_MAC and BeaconTime values.
As more STAs transmit ATIMs in the ATIM window, all STAs read the values in the Beacon_MAC and BeaconTime fields and become knowledgeable about the earliest Beacon Transmitter address. Hence all STAs slowly converge to select a single Beacon Transmitter as the winner,
Since, the end of ATIM window time becomes similar for all STAs, their Data transmissions start at similar times. Since, other STAs do not send ATIMs at this time, the collisions between Data frames and ATIMs are avoided. Hence, the Data throughput is better than in the standard 802.11 protocol that suffers from such collisions.
The said mechanisms also take care of margins of errors occurring due to time of hearing of a Beacon frame being different for distant STAs in a WLAN. If the time difference between BeaconTime field in an ATIM heard by a STA and the current BeaconTime value stored in the STA memory is < Delta (where, Delta is a system-wide small time interval configured for the network), both the Beacon transmitters are considered equally capable of selection. In this case, the STA makes a numerical comparison of the two MAC addresses and selects the Beacon Transmitter with lower MAC address as the winner. The details are shown in the Algorithm.
There is a possibility of STAs converging to a wrong Beacon_MAC as the winner, due to loss of some ATIM frames. If a Beacon Transmitter hears ATIMs that have selected another BeaconMAC as a winner but the time instance of its own Beacon transmission is earlier than the BeaconTime heard in the ATIMs, the Beacon Transmitter transmits a dummy announcement self-addressed ATIM. This ATIM contains its own MAC address in the BeaconMAC field, and the time of its transmitting the Beacon in the BeaconTime field. Other STAs hear this ATIM and adopt the
Algorithm for Beacon Transmitter Winner
Time Stored_Beacon_Time = 0; /* local variable */ MAC_Address Stored_Beacon_MAC = 0; /* local variable */ MAC_Address My_MAC = 0x123456; /* station MAC address */ Time Delta = .0005; /* seconds */
BeaconSentO
{
/* station transmits a Beacon */
StoredBeaconTime = get_current_system_time(); StoredBeaconMAC = MyMAC;
}
Received_ATIM()/Received_Beacon()
{
/* station receives an ATIM or Beacon */
Extract BeaconMAC address and Beacon_Time from the received frame.
if(Beacon_Time < StoredBeaconTime)
if ((StoredBeaconTime - BeaconTime) < Delta))
{
if(Beacon_MAC < StoredBeaconMAC)
Stored_Beacon_MAC = Beacon_MAC; StoredBeaconTime = Beacontime;
}
} else
{
StoredBeaconMAC = BeaconMAC; Stored Beacon Time = Beacon time:

WE CLAIM
1. A method for achieving contention free Beacon transmission, said method comprising the steps of
extending the ATIM frame for announcing a buffered data frame for a Power-Save (PS) receiver.
2. A method as claimed in Claim 1 wherein the said extended frame comprises of the address of the Beacon transmitter heard by the sender STA (BeaconMAC), the time of hearing the Beacon from the Beacon transmitter (Beacon Time).
3. A method as claimed in Claim 1 wherein the STA receiving the Beacon tramission adopts the extended ATIM frame if the BeaconTime in the received ATIM is less than the time of hearing of Beacon by the STA thereby transmitting this new BeaconMAC and BeaconTime values in all future transmission by this STA.
4. A method as claimed in Claim 1 where all STAs steadily converge to adopt a single Beacon fransmitter as more and more STAs transmit ATIMs in the extended ATIM frame.
5. A method as claimed in Claim 1 wherein the Beacon transmitter transmits a dummy announcement self addressed ATIM if a wrong BeaconMAC is heard by a Beacon transmitter.
A method as claimed in Claim 5 wherein the said dummy ATIM contains its own MAC address in the BeaconMAC field and the time of its transmitting the Beacon in the Beacon Time field.
7. A method as claimed in Claims 5 & 6 wherein other STAs hear the said ATIM and adopt the correct Beacon fransmitter and Beacon Time from this frame.
8. A system for achieving contention free Beacon transmission comprising of Transmitter, Receiver, Protocol processor means for extending ATIM frame, 'Beacon Time' addition to ATIM Logic, Beacon Time comparators. Central processing facility, Memory
9. A method for achieving contention free Beacon transmission substantially as herein described with reference to the accompanying figures.
10. A system for achieving contention free Beacon transmission substantially as herein described with reference to the accompanying figures.