WO2008012789A1

WO2008012789A1 - Method for reduced latency wireless communication having reduced latency and increased range and handoff performance between different transmitting stations

Info

Publication number: WO2008012789A1
Application number: PCT/IL2006/000866
Authority: WO
Inventors: Ilan Stark
Original assignee: Inspiair Ltd.
Priority date: 2006-07-26
Filing date: 2006-07-26
Publication date: 2008-01-31

Abstract

In a method for wireless communication between stations (CPEs) and an Access Point all operating according to a communication protocol that assigns waiting times between data packets wherein no data packets are transmitted, the waiting times are reduced and cumulative time saved during transmission of successive data packets is used to reduce latency and increase range and handoff performance between different transmitting stations. Preferably, from each station requesting to transmit to the Access Point, data is received indicating a quantity of data in a data packet to be transmitted; and a station is authorized to transmit if the quantity of data to be transmitted thereby is uniquely a minimum of all the respective quantities of data to be transmitted by the stations.

Description

Method for reduced latency wireless communication having reduced latency and increased range and handoff performance between different transmitting stations

FIELD OF THE INVENTION

This invention relates to wireless telecommunications.

BACKGROUND OF THE INVENTION

Wireless communication between an AP to AP (Access Point) and AP to CPE (Client Premises Equipment) must be carried out according to an agreed-upon set of rules that define, among other things, the format in which date packets are sent so that the receiver will be able to make sense of incoming data. Such rules are enshrined in a communications protocol, such as, for example, the IEEE 802.11/b/g/a protocols used for wireless communication. Within the context of the present description and the appended claims, the term CPE will also be referred to as "station".

Communications protocols are not entirely rigid but generally define for each parameter governed by the protocol's "rules" a range within which the parameter may validly be set. This allows some flexibility and even fine-tuning so that advantageous communication can be achieved within predefined constraints, while still conforming to the protocol.

For example, US 2004/141489 (Benveniste) published Mar. 3, 2005 and entitled "Power-saving mechanisms for 802.11 clients" discloses a technique for dealing with transmissions between telecommunications stations on a shared-communications channel that avoids some of the costs and disadvantages for doing so generally associated with the IEEE 802.11 protocol. To this extent, transmission rules are set that alleviate timing constraints, whereby stations exchanging data with each other can prepare frames for transmission far enough in advance to overcome timing constraints. US 2005/259650 (Bronner) published Nov. 24, 2005 discloses a system and method, associated with a receiver, for increasing the range or bandwidth of a wireless digital communication network. A service class detector is configured to determine a service class of a PDU received by the receiver from the wireless digital communication network and a frame check sequence checker is coupled to the service class detector and configured to disregard error-checking information in the PDU when the service class indicates that the PDU is a streaming media PDU.

US 2005/226239 (Yoshifumi et al.) published Oct. 13, 2005 and entitled "Optimizing IEEE 802.11 for TCP/IP data transfer" discloses systems and methods for increasing data transfer efficiency between networks particularly when establishing connectivity between wireless networks, such as based on IEEE 802 standards, and traditionally wired network protocols (often referred to as internet protocols), such as TCP/IP and UDP. US 2005/226239 provides formatting of network packets and then processing of network packets according to one or more optimization processes. One optimization process comprises performing partial packet retransmissions to increase network efficiency, especially in high bit error rate networks such as wireless networks. Another optimization process comprises suppressing unnecessary packet acknowledgements, therein reducing burst traffic and saving substantial overhead in lossy networks. US 2005/0201314 assigned to Matsushita Electric Industrial Co. Ltd and published as WO04006508 on Jan. 15, 2004 discloses a radio communication method and the like capable of improving the data transmission efficiency in a radio communication by the TDMA method (especially radio communication based on the IEEE802.11 standard) by arranging an empty time between data transmitted/received by a radio communication terminal on a radio interval. In a radio communication between radio communication terminals, by reducing the transmission time of the header added to data and an empty time, it is possible to improve the data transmission efficiency. More specifically, for example, a header is added to data for each predetermined number of data transmissions and the other data are transmitted without adding any header, thereby reducing the header transmission time. Moreover, it is possible to improve the data transmission efficiency by a method for acquiring a header of data on a radio interval in advance, a method for utilizing the identification information as a header, and a method for reducing the IFS (InterFrame Space) after Ack (reception acknowledgement information), thereby enabling continuous data transmission.

The IEEE802.il standard imposes a short interframe space, referred to as SIFS, following an acknowledgement message, ACK. It is apparent from Fig. 5 of US 2005/0201314 that this short time interval between frames is reduced or even omitted altogether in those cases where the communication mode allows ACK messages to be omitted. It is further apparent that a principal object of US 2005/0201314 is to reduce power consumption and this is achieved by reducing the amount of data to be conveyed between terminals. However, this is done without affecting the overall frame transmission time and so does not allow a higher rate of data transmission.

The IEEE802.il protocol employs a Network Allocation Vector (NAV) that serves to define when the broadcast medium is occupied based on expected time durations for data transmission as conveyed by transmitting and receiving devices. The IEEE802.il protocol dictates maximum time durations to be reserved for the trans- mission and reception different types of frames and the NAV uses these durations to lock out the medium for the specified duration so that only the transmitting device is able to transmit, all other devices within broadcast range being silent. If, in fact, the durations dictated by the different frames are unduly conservative and longer durations are allocated than are actually required, the result will be that the NAV will provide extended periods wherein the medium is locked out, even though during at least part of these time periods there medium is available.

It would therefore be desirable to provide improved utilization of a wireless protocol, particularly the IEEE802.i l protocol, so as to allow transmission durations to be allocated that are more closely correlated to the actual durations of data transmis- sions, so as to reduce gaps between successive transmissions and improve transmission efficiency.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide improved utilization of a wireless protocol, particularly the IEEE802.il protocol, so as to allow transmission durations to be allocated that are more closely correlated to the actual durations of data - A -

transmissions, so as to reduce gaps between successive transmissions and improve transmission efficiency.

This object is realized in accordance with an aspect of the invention by a method for wireless communication between stations and an Access Point all operating according to a communication protocol that assigns waiting times between packets wherein no packets are transmitted, said method comprising:

(a) reducing said waiting times; and

(b) using cumulative time saved during transmission of successive packets to reduce latency and increase range and handoff performance between different transmitting nodes .

According to another aspect of the invention there is provided a method for wireless communication between an Access Point and a station both operating according to a communication protocol that assigns waiting times between data packets wherein no packets are transmitted, said method comprising: (a) said Access Point receiving from the station information relating to an amount of data in a data packet to be transmitted from the station to the Access Point; (b) the Access Point instructing the station an initial permitted duration relating to a waiting time associated with said data packet; and (c) if the Access Point does not receive said data packet from the station within a specified time interval, the Access Point conveying to the station successively longer permitted durations relating to the waiting time associated with said data packet until the data packet is received

According to yet another aspect of the invention there is provided a method for allowing an Access Point to handle contention between stations in a wireless communication network, said Access Point and said stations all operating according to a communication protocol, said method comprising:

(a) from each station requesting to transmit to the Access Point, receiving data indicating a quantity of data in a data packet to be transmitted; and (b) authorizing a station to transmit if the quantity of data to be transmitted by said station is uniquely a minimum of all the respective quantities of data to be transmitted by the stations. By reducing the waiting time many unnecessary fields are removed from the 802.11 standard, and by doing so, the main processor of each Access Point has more DSP resources. The direct result is that the processor is able to apply more resources to support the weaker signals: if there is station that can be supplied with 0.9Mbps, the Access Point will have DSP reserves to make it IMbps, and if there is a remote station whose signal is weak and noisy, the Access Point will have enough DSP resources to maintain a connection therewith.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, an embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Fig. 1 is a pictorial representation showing duration of an RTS frame sent by a station to an Access Point;

Fig. 2 is a pictorial representation showing duration of a CTS frame sent by the Access Point to the station;

Fig. 3 shows pictorially the relationship between the three interframe spaces used in the 802.11 protocol to determine medium access;

Fig. 4 is a flow chart showing the principal operations carried out by the Access Point in a method according to an embodiment of the invention for reducing waiting times in a wireless protocol; and

Fig. 5 is a flow chart showing the principal operations carried out by the Access Point in a method according to an embodiment of the invention for avoiding contention in a wireless protocol.

DETAILED DESCRIPTION OF EMBODIMENTS In order to understand the invention, it is first necessary to understand some basic principles of wireless communication in general and of the IEEE 802.11 protocol as used for Wi-Fi wireless communication, in particular. The IEEE 802.i l protocol defines various frame types relating to information that is contained within a data packet transmitted from a transmitting station to a receiving station. Before a station even starts to transmit data, it must first ensure that the medium is available. It does this using carrier sensing provided by the Network Allocation Vector (NAV). Most 802.11 frames carry a duration field, which can be used to reserve the medium for a fixed time period. The NAV is a timer that indicates the amount of time the medium will be reserved. Stations set the NAV to the time for which they expect to use the medium, including any frames necessary to complete the current operation. Other stations count down from the NAV to zero. When the NAV is nonzero, the virtual carrier-sensing function indicates that the medium is busy; when the NAV reaches zero, the virtual carrier- sensing function indicates that the medium is available.

By way of analogy, the NAV acts as a stream on which the data frames are transported. The transmitting and receiving devices constantly update the NAV with time durations that cause the NAV to reserve the access medium so as to prevent other devices from transmitting at the same time and thereby avoid collisions. The NAV also serves as a common channel that is constantly accessible to all devices within broadcast range, allowing them constantly to monitor the NAV and determine when the medium is busy or available for transmission. However, as noted above, the durations that are fed to the NAV derive inter alia from the frames such as RTS, ACK, CTS etc. that specify fixed durations as part of the IEEE 802.11 protocol. In other words, the NAV reserves time for carrying each frame as dictated by the transmitting and receiving devices themselves. If, in fact, these frames are unduly conservative and allocate longer durations than are actually required, the result will be that the NAV will provide extended periods wherein the medium is locked out, even though during at least part of these time periods there medium is available. Thus, while on the one hand the NAV forms a common path that is accessible to all participating devices and prevent collisions, it also imposes an unnecessary overhead that creates gaps between communications, wherein no data is transmitted. The invention reduces these gaps, thereby ensuring that the wasted time between data transmission is reduced and more efficiently utilizing the broadcast medium. In order to understand how the invention achieves this, it is necessary to review the duration fields in different frames of the IEEE 802.11 protocol and to see from where they originate and how they impact on the time reserved by the NAV for the resulting data transmission. Before doing so, it will be instructive to review in general how data is conveyed in a wireless system, with particular reference to the IEEE 802.11 protocol. The Duration field carries the value of the NAV. Access to the medium is restricted for the time specified by the NAV.

When a transmitting device wishes to transmit to a receiving device, it must first check that the broadcast medium is available and, if so, it must ensure that time is reserved for the ensuing transmission so that during the reserved time all other stations in broadcast range are silent. This ensures collision avoidance between two transmissions that might otherwise be transmitted simultaneously by two or more stations and avoids the need for re-transmission. However, it is to be noted that wireless protocols are known which do not prevent collisions (although they usually take steps to reduce their likelihood) and make provision for one station to retransmit in the event of a collision being detected. The invention is suitable for use also with such protocols.

A station that wishes to transmit data checks the availability of the broadcast medium by transmitting a Request To Send (RTS) frame. A station is informed that the medium is available for the intended transmission by means of a Clear To Send (CTS) frame.

Fig. 1 is a pictorial representation showing duration of an RTS frame sent by a station to an Access Point. An RTS frame attempts to reserve the medium for an entire frame exchange, so the sender of an RTS frame calculates the time needed for the frame exchange sequence after the RTS frame ends. As shown in Fig. 1, the entire exchange requires three SIFS periods, the duration of one CTS, the final ACK, plus the time needed to transmit the frame or first fragment. The cumulative time required to accommodate all of these periods is time during which the medium is occupied and unable to transmit. In order to ensure that sufficient time will be allocated for the complete transmission of the frame, it is normal to estimate these periods conservatively by allocating somewhat more time for the SIFS periods, the CTS, and the ACK than minimally required. These periods constitute "waiting times" during which the medium is occupied but unable to convey actual data.

Fig. 2 is a pictorial representation showing duration of a CTS frame sent by the Access Point to the station. In accordance with conventional use of the IEEE 802.11 protocol, the access point uses the duration from the RTS field as the basis for its duration calculation. RTS frames reserve the medium for the entire RTS-CTS-frame- ACK exchange. By the time the CTS frame is transmitted, only the pending frame of 66

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fragment and its acknowledgement remain. The access point subtracts the time required for the CTS and the short interframe space that preceded the CTS from the duration in the RTS frame, and placed the result in the Duration field.

As shown in Fig. 2, the access point sending the CTS frame uses the duration from the RTS frame as the basis for its duration calculation. RTS frames reserve the medium for the entire RTS-CTS-frame-ACK exchange. By the time the CTS frame is transmitted, though, only the pending frame or fragment and its acknowledgement remain. The access point subtracts the time required for the CTS frame and the short interframe space that preceded the CTS from the duration in the RTS frame, and places the result in the Duration field.

ACK frames are used to send the positive acknowledgements required by the MAC and are used with any data transmission.

Fig. 3 shows pictorially the relationship between the three interframe spaces, SIFS, PIFS and DIFS, used in the 802.11 protocol to determine medium access and avoid collisions between competing stations. Stations delay transmission until the medium becomes idle for varying waiting times whose durations are determined by respective interframe spacings that create different priority levels for different types of traffic, so as to reduce the amount of time high priority traffic has to wait after the medium becomes idle. Therefore, if there is any high priority traffic waiting, it grabs the network before low-priority frames have a chance to try. Each interframe spacing is a fixed amount of time that is independent of transmission speed. So the use of interframe spacing as a mechanism for avoiding contention is wasteful of time, particularly when the higher-priority stations are idle and control is thus passed to stations having lower priority. Moreover, for high transmission speeds the time wasted is proportionally higher.

The short interframe space (SIFS) is used for the highest-priority transmissions, such as RTS/CTS frames and positive acknowledgements. High-priority transmissions can begin once the SIFS has elapsed. Once these high-priority transmissions begin, the medium becomes busy, so frames transmitted after the SIFS has elapsed have priority over frames that can be transmitted only after longer intervals. The PCF interframe space (PIFS) is used by the PCF during contention-free operation. Stations with data to transmit in the contention-free period can transmit after the PIFS has elapsed and preempt any contention-based traffic.

When a transmitting station wishes to transmit, it first must send a Request To Send (RTS) frame to the receiving station. The RTS comprises a Media Access Control (MAC) header, which contains separate fields defining Frame Control, Duration, Address (RA) of the receiving station and Address (TA) of the transmitting station. In addition to this header, the RTS frame also contains a Frame Check Sequence (FCS) field that serves to verify that the RTS frame is properly conveyed. The duration value is the time, in microseconds, required to transmit the pending data or management frame, plus one CTS frame, plus one ACK frame, plus three SIFS intervals and informs the receiving station how long to remain silent since it cannot receive and transmit data at the same time.

A receiving station acknowledges receipt of a frame from a transmitting station by transmitting an ACK frame to the transmitting station. The ACK frame comprises a Media Access Control (MAC) header, which contains separate fields defining Frame Control, Duration, and Address (RA) of the receiving station. The RA of the ACK frame is copied from the corresponding address field of the immediately previously directed data, management, or P S -Poll control frame. In addition to this header, the ACK frame also contains a Frame Check Sequence (FCS) field that serves to verify that the ACK frame is properly conveyed. The duration value is the time, in microseconds, required to transmit the pending data or management frame, plus one CTS frame, plus one ACK frame, plus three SIFS intervals and informs the receiving station how long to remain silent since it cannot receive and transmit data at the same time.

When a station receives a RTS frame and is willing and able to receive the data transmitted by the transmitting station, it must first inform the transmitting station that it is in order for the transmitting station to transmit. It does this by sending a Clear To Send (CTS) frame to the transmitting station. The CTS comprises a MAC header, which contains separate fields defining Frame Control, Duration and Address (RA) of the receiving station. In addition to this header, the CTS frame also contains a Frame Check Sequence (FCS) field that serves to verify that the CTS frame is properly conveyed. The duration value obtained from the Duration field of the immediately previous RTS frame minus the time, in microseconds, required to transmit the CTS frame and its SIFS interval. This in effect informs the transmitting station how long the receiving station will remain silent after the transmitting station receives the CTS frame and thus sets an upper limit to the duration of the transmission from the transmitting station.

Having explained the salient features of the 802.11 protocol, it will be under- stood that transmission of each data frame requires a transmitting station to reserve durations that effectively lock out the NAV and thereby inform all other stations within broadcast range that the medium is busy. In its broadest aspect, the invention thus provides a method for wireless communication between stations and an Access Point all operating according to a communication protocol that assigns waiting times between data packets wherein no data packets are transmitted, the method comprising:

(a) reducing said waiting times; and

(b) using cumulative time saved during transmission of successive data packets to reduce latency and increase range and handoff performance between different transmitting stations. Fig. 4 is a flow chart showing the principal operations carried out by the Access

Point in a method according to an embodiment of the invention for reducing waiting times in a wireless protocol. The Access Point receives an RTS from the station. The Access Point establishes whether the network is operating at full capacity, in which case, no action is taken since communication with the requesting station is not possible. If the network is available, the Access Point sets a default duration to the station, typically based on the measured RSSI defining the signal strength received from the station. The Access Point then sends a CTS to the station including information relating to an amount of data in a data packet to be transmitted from the station to the Access Point and conveying the previously established duration to the station. Upon receiving the CTS from the Access Point, the station constructs the next packet using the duration field received from the Access Point and transmits to the Access Point. If the Access Point receives it, this implies that the duration field previously set is adequate, although it might of course be too large. But if after a predetermined period of time, no packet is received from the station, this implies that the RSSI is too low using the specified duration field to allow propagation. In this case, the Access Point sets a higher duration and repeats the process until the station succeeds in conveying the next packet to the Access Point. Once the Access Point receives a packet from the station, it determines - li ¬

the RSSI and computes the minimum duration field required for subsequent packets to maintain this RSSI. By such means, the Access Point fixes the minimum duration field for all subsequent packets and in doing so reduces waiting time to a minimum.

It should be noted that the above-described method for reducing waiting times in a wireless protocol assumes that the durations associated with each data packet transmitted by the station to the Access Point can be dictated by the Access Point. For example, in the IEEE 802.11 protocol, durations can be assigned in one of two ways. The conventional approach that is almost universally adopted in hitherto-proposed implementations is for each station to assign the duration pertaining to the next data packet to be transmitted. Typically, it does this by means of a look-up table that stores the requisite durations as a function of RSSI. Alternatively, it is possible for the Access Point to override the duration field set by the station, in which case the Access Point must operate differently so as to convey a duration field to the station with the CTS. In such implementations, if the station receives a CTS having a duration field defined therein, it knows not to access the look-up table but to use the duration field as received from the Access Point. It is also conceivable that the station will always access the LUT but if the CTS has a duration field, it will use the CTS 's lower duration instead. It is to be understood that this mechanism is provided for by the IEEE 802.11 protocol and thus may be implemented without requiring any change to the station. It does, however, require that the Access Point be programmed so as to set the duration and convey it in the respective CTS pertaining to an RTS conveyed thereto by a station wishing to transmit data. In order that the program in the Access Point can be changed, it is necessary either to have access to the source code of the transmitter in the Access Point so as to allow suitable modification; or to be able to download a suitably modified program or possibly patch to the transmitter in the Access Point.

Fig. 5 is a flow chart showing the principal operations carried out by the Access Point in a method according to an embodiment of the invention for avoiding contention in a wireless protocol. The Access Point receives from each station wishing to transmit thereto data indicating a quantity of data in a data packet to be transmitted. The respective quantities are buffered and sorted in decreasing rank to identify one or more stations wishing to transmit the lowest quantity of data. If there is only a single unique station wishing to transmit this lowest quantity, the Access Point authorizes this station to transmit. If more than a single station competes for the lowest quantity, all such competing stations are ignored. This may be done by moving them to the end of the queue. The next lowest quantity is processed in like manner until a unique station is identified that wishes to transmit this quantity. The Access Point then sends a CTS to this station so as to authorize it to transmit.

Applying this "contention window" based sequencing establishes rules of priority over the data transfer. The system is "cleared" from smaller transmission thus allowing effective response to all transmissions, as well as, appropriate "resources" planning for larger ones. Total system resources (time, latency, range) are better utilized and as a direct result provide for enhanced system performance.

It will be appreciated that the invention also contemplates an Access Point that is programmed to operate according to the invention. Likewise, the invention contemplates a computer program being readable by an Access Point for executing the method of the invention. The invention further contemplates a machine-readable memory tangibly embodying a program of instructions executable by the machine for executing the method of the invention.

Claims

CLAIMS:

1. A method for wireless communication between stations and an Access Point all operating according to a communication protocol that assigns waiting times between data packets wherein no data packets are transmitted, said method comprising: (a) reducing said waiting times; and

(b) using cumulative time saved during transmission of successive data packets to reduce latency and increase range and handoff performance between different transmitting stations.

2. A method for wireless communication between an Access Point and a station both operating according to a communication protocol that assigns waiting times between data packets wherein no packets are transmitted, said method comprising:

(a) said Access Point receiving from the station information relating to an amount of data in a data packet to be transmitted from the station to the Access Point; (b) the Access Point instructing the station an initial permitted duration relating to a waiting time associated with said data packet; and

(c) if the Access Point does not receive said data packet from the station within a specified time interval, the Access Point conveying to the station successively longer permitted durations relating to the waiting time associated with said data packet until the data packet is received.

3. The method according to claim 2, wherein the Access Point calculates received signal strength (RSSI) from the station and calculates the permitted duration to maintain said RSSI.

4. A method for allowing an Access Point to handle contention between stations in a wireless communication network, said Access Point and said stations all operating according to a communication protocol, said method comprising:

(a) from each station requesting to transmit to the Access Point, receiving data indicating a quantity of data in a data packet to be transmitted; and (b) authorizing a station to transmit if the quantity of data to be transmitted by said station is uniquely a minimum of all the respective quantities of data to be transmitted by the stations.

5. The method according to claim 4, wherein authorizing a station to transmit includes:

(a) sorting respective quantities of each station;

(b) identifying one or more stations wishing to transmit the lowest quantity of data;

(c) if there is only a single unique station wishing to transmit this lowest quantity, authorizing said single unique station to transmit; and

(d) if more than a single station competes for the lowest quantity, ignoring all such competing stations and processing the next lowest quantity in like manner until a unique station is identified that wishes to transmit this quantity.

6. The method according to claim 5, wherein ignoring all such competing stations includes moving them to the end of the queue.

7. The method according to any one of claims 1 to 3, further including:

(e) from each station requesting to transmit to the Access Point, receiving data indicating a quantity of data in a data packet to be transmitted; and (f) authorizing a station to transmit if the quantity of data to be transmitted by said station is uniquely a minimum of all the respective quantities of data to be transmitted by the stations.

8. The method according to claim 7, wherein authorizing a station to transmit includes: (g) sorting respective quantities of each station;

(h) identifying one or more stations wishing to transmit the lowest quantity of data; (i) if there is only a single unique station wishing to transmit this lowest quantity, authorizing said single unique station to transmit; and (j) if more than a single station competes for the lowest quantity, ignoring all such competing stations and processing the next lowest quantity in like manner until a unique station is identified that wishes to transmit this quantity.

9. The method according to claim 8, wherein ignoring all such competing stations includes moving them to the end of the queue.

10. An Access Point programmed to carry out the method according to any one of claims 1 to 9.

11. A computer program comprising computer program code means for performing the method according to any one of claims 1 to 9 when said program is run on an Access Point.

12. A computer program as claimed in claim 11 embodied on a computer readable medium.