FIELD OF INVENTION:
The present invention relates to a system and method for achieving a high throughput in Ad-hoc Wireless LANs.
PRIOR ART:
IEEE 802.11 ad-hoc Wireless LAN (WLAN) protocol consists of a number of stations without any central controlling equipment. All stations transmit independently after contending to acquire the shared wireless radio channel.
A station in an IEEE 802.11 ad-hoc WLAN protocol senses the channel to be free. If the channel is free it waits for a random delay and senses the channel again and then transmits the frame. However, if two stations sense the channel free exactly at the same time and transmit, collisions occur, leading to data loss. In the case of collision, the station doubles the random delay and retransmits. This contention and collisions lead to low throughput for Data transfers.
The protocol also defines power saving mechanism. 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 receiver
keeps awake after the ATIM window and receives its data frame. All subsequent Data transmissions to PS receiver follow normal contention and collisions. Multiple collisions can make the sender to postpone the Data transmission to PS receiver till the next Beacon period or even discard the Data frame. Hence, throughput is low even for transmissions to PS stations.
US Patent application no. 2003058886 optimizes 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.
DESCRIPTION OF THE INVENTION:
The present invention relates to a System and method for achieving high throughput in ad-hoc WLANs. The present method eliminates contention among transmitting stations for transmissions to PS stations, and lead to high system throughput.
In this method we have defined a reservation period in which the sending stations reserve the channel by announcing the Data frames to be transmitted to the Power Save (PS) stations. These frames are transmitted collision-free in the following transmission time period.
In the present method a station transmits a Beacon announcing two consecutive time periods, "Announcement" and "Transmission" periods. The senders first reserve the channel for transmissions to PS receivers through modified ATIM and ACK frames in the announcement period. These frames carry the Duration of Data frames to be transmitted (i.e.. Data frame size/Bit Rate). The Beacon transmitter observes these ACKs and creates an "Announce List" (AL) of stations, with information about sender and receiver stations, start time and duration of each Data frame transfer. It sends a new "Announce" frame announcing this list, at the end of the Announcement period. The senders then transmit in the following Transmission period, strictly following the announced order thereby avoiding collisions . These collision-free transfers result in increased throughput of the present protocol compared to collision-prone 802.11 ad-hoc WLAN protocol discussed in the prior art.
The present protocol also ensures that no "hidden" stations can collide with the collision-free transfers. The modified ATIM and ACK frames are also made to carry an Extended Duration (E_Duration) field announcing the time instance of the end of maximum possible Contention Free period. Both the neighbours of sender and receiver hear these frames and set their NAV so as not to transmit till that period. Thus, even if these stations are hidden from the Beacon transmitting station, they do not collide with transmissions in the Contention Free period. Henc,e the Data transmissions are always collision-free since neither the neighbours of sender nor the neighbours of receiver collide.
If these neighbours are not hidden from the Beacon transmitter and can hear the Announce frame, they would reduce their NAV to the current total contention-free period, which can be less than maximum contention-free period. Hence, this procedure ensures that no station is prevented from transmissions more than the actual contention-free period. After the end of Contention Free period (CFP), a Contention Period (CP) starts in which all the active stations can exchange their Data frames.

WE CLAIM
1. A method for contention free transmission between two nodes comprising the steps of
reserving the transmission channel by the sender by means of an announcement signal sent to the Power-Save (PS) stations before actually sending the data frames over the said channel.
2. A method as claimed in Claim 1 wherein the said reservation period has a slot for 'Announcement' and a slot for 'Transmission'.
A method as claimed in Claim 1, wherein
a beacon transmitter observes the contents of the said 'announcement' slot and creates an 'Announce List (AL) and sends a new 'Announce' frame announcing this List the sender transmits in the following 'transmission' period strictly following the order announced by it.
A method as claimed in Claim 1 wherein the information in the 'Announcement' period comprises of information regarding the size of the data frame to be transmitted, an extension duration field (E Duration) announcing the time instance of the end of maximum possible contention free period
A method as claimed in Claim 1 wherein the information contents of the 'Announce List' comprises of list of stations, sender and receiver stations, start-time, duration of each data frame to be transferred.
A method as claimed in Claim 1, wherein the neighbours of both sender and the receiver hearing the said announcement frames set their own NAVs so as not to transmit during the said reservation period.
7. A system for achieving contention free transmission between two nodes comprising of Transmitter, Receiver, Protocol processor means for creating an 'Announce List', Priority Creation Logic, memory, central processing facility (Fig.l)
8. A method for contention free transmission between two nodes substantially as herein described with reference to the accompanying figures.
9. A system for contention free transmission between two nodes substantially as herein described with reference to the accompanying figures.