Patent Application
20050138451
This invention relates in general to wireless local area networks, and more particularly to power save methods for reducing power consumption at a mobile station while engaged in a time sensitive communication activity.
Wireless LAN (WLAN) systems providing broadband wireless access have experienced a spectacular rise in popularity in recent years. While the principal application of these systems has been in providing network connectivity to portable and mobile devices running data applications such as, for example, email and web browsing, there has been a tremendous and growing interest in supporting isochronous services such as telephony service and streaming video.
One of the key issues facing wireless system designers when considering voice and other time-sensitive services over a WLAN connection, such as one described by the IEEE 802.11 specification, is the power consumption of handheld devices. For example, in order to deliver competitive talk time and standby time, as compared to digital cordless or cellular devices, power conservation during voice calls become necessary. Several organizations have proposed power-efficient operation via transmit power control and physical layer rate adaptation for systems that rely on a centrally controlled contention-free channel access scheme. However, such approaches can be complex to implement and may not provide the power savings required to justify the complexity.
The 802.11 standard defines procedures which can be used to implement power management in a handheld device during periods of inactivity. In particular, three distinct building blocks are provided to support power savings: a Wakeup Procedure, a Sleep Procedure, and a Power-save Poll (PS-Poll) Procedure. A mobile client voice station (mobile station) can combine these building blocks in various manners to support power management for different applications.
Wakeup Procedure: There are generally two reasons for the mobile station to wake up, namely to transmit pending data or to retrieve buffered data from the fixed station serving the mobile station, known as an access point. Waking up to transmit data is a straightforward operation, driven by the mobile station. The decision to wake up and receive data is also made by the mobile station after monitoring its pending data bit in a periodic beacon frame transmitted by its access point. Once the mobile station decides to transition from sleep mode to active mode, it notifies the access point by sending an uplink frame with the power-save (PS) bit set to active. Following such transmission, the mobile station remains active so the access point can send any buffered downlink frames afterward.
Sleep Procedure: Similar to the wakeup procedure, a mobile station in the active mode needs to complete a successful mobile station-initiated frame exchange sequence with PS bit set to sleep to transition into the sleep mode. Following this frame exchange sequence, the access point buffers all the downlink frames to this mobile station.
PS-Poll Procedure: Instead of waiting for the access point to transmit the buffered downlink frames, a power-save mobile station can solicit an immediate delivery from its access point by using a PS-Poll frame. Upon receiving this PS-Poll, the access point can immediately send one buffered downlink frame (immediate data response) or simply send an acknowledgement message and response with a data frame later (delayed data response). For the immediate data response case, a mobile station can stay in sleep state after finishing this frame exchange since there is no need for the mobile station to transition to active state given that the access point can only send a buffered downlink frame after receiving a PS-poll from the mobile station. On the other hand, for the delayed data response case, the mobile station has to transition to the active state until receiving a downlink frame from the access point.
The architecture of a simple enterprise WLAN system is depicted in
VoIP traffic characteristics make voice over WLAN applications uniquely suited for power save operation. In particular, VoIP applications periodically generate voice frames, where the inter-arrival time between frames depends upon the voice coder chosen for an application. The process of encapsulating voice frames into IP packets is commonly referred to as packetization, which is often assumed to occur once every 20 millisecond. A typical VoIP conversation involves a bi-directional constant bit rate flow of VoIP frames, including an uplink flow from the handset to a voice gateway and a downlink flow in the reverse direction.
Since the station generally knows in advance the frame arrival rate, delay, and bandwidth requirements of its voice application, it can reserve resources and set up power management for its voice flows in agreement with the access point. A mobile station may forgo power save mode, and remain in active mode, always ready for the downlink voice transmission. In this case, the access point may transmit downlink voice frames as they arrive. However, if power save is desired, the mobile station may employ the power save building blocks described previously to wake up, exchange the VoIP frame with its access point, and go back to sleep.
In a shared-medium network, such as the access network shown in
It is possible for a mobile station to use information such as the inter-arrival time of downlink voice frames, along with a power-save mechanism, to put itself to sleep between two consecutive voice frames. Presently there are power save procedures described in various papers and WLAN related specifications.
The first prior art power management mechanism utilizes a bit in the packet header. The bit is designated as a power management (PM) bit to signal the change of the power state of the mobile station to the access point. First, a mobile station transitions from sleep mode to active mode upon having an uplink data frame to transmit by setting the PS bit to active in an uplink voice frame to notify the change of its power state. Knowing that there will be one corresponding downlink frame buffered at the access point, because uplink and downlink vocoder share the same voice frame duration, the mobile station stays in active mode for the downlink transmission. After receiving the uplink transmission, the access point then sends buffered downlink frames to the mobile station. In the last downlink frame, the access point sets the “more data” bit to FALSE to communicate the end of the downlink transmission. Finally, the mobile station needs to complete a successful station-initiated frame exchange sequence with PS bit set to sleep to transition into the sleep mode. (e.g. an uplink frame, or a Null frame if there is no uplink data frame to transmit, with the PS bit set to sleep). In the following context, the PS-bit based mechanism is referred to as LGCY6 in the art.
A second power management mechanism uses a PS-Poll frame to solicit downlink frames. Instead of waiting indefinitely for the access point to deliver downlink transmission, the PS-Poll based mechanism utilizes the PS-Poll frame to retrieve the buffered downlink frame from the access point. First, a mobile station transitions to active mode upon having an uplink data frame to transmit. The mobile station then sends out the uplink transmission. Similar to the PS-bit based mechanism, the access point sets the more data field to indicate the presence of any buffered downlink transmission. If the more data bit is TRUE, the mobile station will continue to send a PS-Poll frame to retrieve the buffered downlink frame. Unlike the PS-bit based mechanism, a mobile station can stay in the sleep state since the access point responds to the PS-Poll with an immediate data frame. In the following context, the PS-Poll based mechanism is referred to as LGCY5 in the art.
There are a couple of issues in supporting power-efficient VoIP operation using the current WLAN power save mechanisms. First, the PS-bit based mechanism is somewhat inefficient because, for example, the 802.11 standard currently only offers one way for the mobile station to transition to sleep mode, which is by initiating a frame exchange sequence with PS bit set to sleep. As a result, an extra mobile station initiated frame exchange is needed per bi-directional voice transfer in order for the mobile station to signal power state transition. Since the payload of a voice frame is small (e.g. 20 bytes for voice application with 20 ms framing and 8 Kbps vocoder), the overhead incurred by the extra frame exchange could be as high as one third of the traffic between the mobile station and access point. The significant overhead results in the inefficiency on both power consumption and system capacity.
A second issue is related to quality of service. Under the PS-Poll based mechanism, since a mobile station is not aware of the priority of the buffered downlink frame, the PS-Poll frame is sent as a the best effort access attempt, which is a data traffic mode instead of a voice traffic mode. As a result, the downlink voice transmissions essentially use the best-effort priority instead of the higher voice priority. When a system is loaded with both data traffic using best-effort priority with voice traffic, and a mobile station retrieves downlink voice traffic using a power save poll frame transmitted at the same priority as data traffic, the system will be unable to protect the voice traffic from the delays associated with a congested best-effort delivery system. Legacy power save methods may also require an uplink or poll frame to retrieve each buffered frame for the down link, or require immediate response from the access point for a given uplink frame. Therefore, given these shortcomings of the prior art, there is a need for a reliable power management protocol in a WLAN system that permits mobile station with active voice sessions to efficiently enter and exit power save mode without excessive overhead and maintain quality of service in the presence of lower priority traffic.
While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawing figures, in which like reference numerals are carried forward.
The invention solves the problems associated with the prior art by providing an uplink poll-based power save delivery (UPPSD) mode of operation in a wireless local area network (WLAN) system which permits a mobile station in power save mode to retrieve frames from an access point without requiring the access point to respond immediately to a polling frame, without requiring the mobile station to poll the access point for each downlink frame, and without requiring the mobile station to transmit a frame to inform the access point of a transition to a low power mode. When the mobile station uses the present UPPSD power save mode, it first establishes a resource reservation with an access point signaling its intention to use the UPPSD mode to retrieve data from the access point during power save operation. The mobile station indicates to the access point the intent to use the UPPSD mode by information in the a traffic specification admissions control frame sent to the access point from the mobile station. During call set-up negotiation the access point reserves sufficient resources to ensure a voice quality session, and identifies the stream with a unique traffic stream identifier which is later used by the mobile station to trigger a state transition into uplink poll-based power save delivery mode. Alternatively, the mobile station and access point may negotiate a resource reservation for an entire access category, representing an aggregate traffic stream comprised of one or more individual traffic streams. By access category it is meant the priority of the traffic associated with the reserved stream compared to other reserved stream access categories. Priority is determined in part by the minimum back off period to be used in contention for the access category. Whether the admissions control module makes a decision based on traffic streams or access categories, reserving the resources is said to be admitting the traffic stream, and all data associated with the reserved traffic stream is identified as such. Once the intent to use power save mode is communicated to the access point, the mobile station commences putting the WLAN subsystem circuitry into a low power mode, such as by turning off the WLAN chip set and associated circuits. In the preferred embodiment the mobile station operates the WLAN subsystem according to a service interval time period while engaged in a WLAN voice session. The service interval is defined as the real time duration of the data contained in a frame of data. Typically, for example, the service interval for voice traffic is on the order of 20 milliseconds. In practice, however, the actual time between service periods varies slightly from defined service interval due to factors such as the inter-arrival time between frames and other small, fluctuating delays inherent in large networks. The mobile station initiates a frame exchange with the access point by waking up the WLAN subsystem. That is, the WLAN subsystem transitions from low power mode to fully active mode, referring here to the actual power level state of the WLAN subsystem, and not the state of power save signaling bits in the frames sent to the access point. If the mobile station voice processor has produced a data packet to be transmitted, the WLAN subsystem begins acquiring the WLAN channel to transmit the data to the access point with a polling frame, identifying the polling frame as belonging to the reserved traffic stream, or identifying it as one of the reserved traffic streams if more than one has been admitted for the mobile station by the access point. If there is no data available, the WLAN subsystem preferably waits until the expiration of a polling window timer, at which time if no data has yet to be delivered by the voice processor, the WLAN subsystem acquires the WLAN channel and transmits a null frame as the polling frame. In the preferred mode the access point will transmit an acknowledgement in response to receiving the polling frame. Thereafter, the access point transmits a response frame to the mobile station. If the access point has data buffered in a buffer reserved for the reserved traffic stream, the response frame will include the data, otherwise a null frame is transmitted to the mobile station. If the access point has more than one frame of data, then the access point indicates such in the header information of the response frame. Alternatively, the access point may send data of any type it has buffered for the mobile station, regardless of admission status of the data. The mobile station maintains the WLAN subsystem in active mode until the buffered data is received from the access point. In the preferred mode the mobile station acknowledges each response frame by transmitting an acknowledgement. Once all buffered data is received from the access point for the present service period, the mobile station puts the WLAN subsystem back into low power mode.
Referring now to
Referring now to
Referring now to
The mobile station transmissions appear on the bottom flow line 402, while the access point transmissions appear on the top flow line 404. As mentioned, prior to the transaction illustrated here, the mobile station and access point will have established a reserved traffic stream, meaning the access point has reserved certain resources to maintain voice quality of the traffic stream. That is, the access point will be able to service the flow in a timely manner so that the real time effect of the flow is maintained. To prevent an overloaded scenario in a WLAN voice system, where an excessive number of high priority users might make it difficult for a system to satisfy quality of service requirements, admission control should be required for certain services, such as real time voice and video streaming. For example, in an infrastructure based voice WLAN system, a mobile station (e.g. voice user) should set up a bi-directional traffic flow for voice using a known traffic specification, and the access point should acknowledge the admission of the flow to the mobile station. By admitting the flow, it is meant that the data flow will be a reserved traffic stream having a unique traffic stream identifier. The reserved traffic stream will have a priority classification and will be apportioned a minimum amount of channel access time. During the connection setup period, the UPPSD power save mechanism can be established by mobile station. The mobile station can choose no power save operation, legacy power save operation, or the present UPPSD power save operation. After the traffic flow is admitted by the access point, the mobile station puts the WLAN subsystem in a low power state.
After the WLAN subsystem is placed in low power mode, the mobile station preferably maintains a service interval timer to maintain real time operation of the flow. Preferably at the beginning of a service interval, the mobile station activates the WLAN subsystem, such as at time 406. After which, during the time period 407, the mobile station begins contending for the WLAN channel. The mobile station initiates the exchange by transmitting a polling frame 408. The polling frame may be a voice frame, which in the preferred embodiment contains a unique traffic stream identifier, and a frame of voice data if the user of the mobile station is presently speaking, or if there is no voice data to transmit presently, the polling frame will be a null frame. The polling frame will identify the reserved traffic stream and indicate UPPSD power save mode. In the preferred embodiment, after the access point receives the polling frame, it transmits an acknowledgement 410 within a short interframe space time period 412, which is a scheduled event, in accordance with the IEEE 802.11 specification. In response to receiving the polling frame, the access point transmits a response frame 416 to the mobile station. The time period between receiving the polling frame and transmitting the response frame can vary as the access point may have to finish attending to another flow for another mobile station. In the preferred embodiment, there will typically be a turnaround interframe space time period 414 between the acknowledgement and the response frame. As soon as possible, the access point will acquire the WLAN channel and transmit the response frame. However, the response frame is not sent with regard to any predetermined schedule. That is, mobile station stays active to receive the response window for an indeterminate period of time. Of course, a reasonable maximum period of time could be observed to prevent the mobile station waiting too long for a response frame or remaining active too long. In the event the maximum period occurs, the mobile station can take appropriate action, such as polling the access point a second time during the service period to check the status of the power save buffers and retrieve any frames waiting to be transmitted. The response frame will identify the reserved traffic stream. If the access point has data in the reserved buffer associated with the reserved traffic stream, the access point will transmit a frame of data from the buffer. If there is no data in the reserved buffer, the access point will transmit a null frame. In the response frame there will be signaling information, such as an end of uplink service period (EUSP) bit designated to indicate the end of the present service period, which may occur because there is no more data to transmit or because a maximum service period time has been reached. In the preferred embodiment a MORE_DATA bit may be used as the EUSP bit. If the MORE_DATA bit is cleared in the response frame, it indicates the end of the UPPSD service period due to successful transmission of all buffered frame for the mobile station in the Reserved buffer. If the access point transmits a null frame in the response frame, access point may also use the MORE_DATA bit to indicate there is no more data and to signal that the present service period is over. If the reserved buffer has only one frame of data buffered, it will transmit that frame of data, and likewise set the MORE_DATA bit to indicate there is no more data. Occasionally, however, there may be more than one data frame of data buffered at the reserved buffer, in which case the first response frame will set the MORE_DATA bit to indicate there is more data to come, so the mobile station will remain active and not go to a low power state after receiving the first response frame. In response to receiving the response frame, in the preferred embodiment, the mobile station transmits an acknowledgement 420 within a short interframe space time period 418. If the response frame indicated the end of the present service period, the mobile station then places the WLAN subsystem into a low power state after receiving the response frame at time 422.
When the mobile station and access point are not exchanging frames as part of a reserved traffic stream unreserved data flows may be serviced, such as by using other power save protocols. Unreserved data flows may be serviced before, after and even during UPPSD service periods, thereby allowing both reserved and unreserved data flows to be service using power save techniques. The time periods when unreserved flows may be serviced are between UPPSD service periods and even during UPPSD service periods, and indicated by the time periods marked 424. A first such unreserved power save service period exchange is shown in
A second method of acquiring non-reserved data from the access point while using a power save mode is shown in
Referring now to
The controller of the mobile station powers up the WLAN subsystem at the beginning of a service interval 503. At the same time a window timer is initiated to time a polling window time period 504. If, upon powering up the WLAN subsystem, there is data associated with the present reserved traffic stream to be transmitted, the WLAN subsystem will immediately begin contending for the WLAN channel to transmit a polling frame that includes the data. However, if there is no data presently available upon powering up the WLAN subsystem, the WLAN subsystem waits as the window timer proceeds. If before expiration of the window time period (506), the voice processor delivers a data frame to the WLAN subsystem that is associated with the reserved traffic stream, the WLAN subsystem immediately begins contending for the WLAN channel to transmit the data in a polling frame. If, however, at the expiration of the window time period at 506 no data has arrived, the WLAN subsystem contends for the WLAN channel and transmits a null frame as the polling frame. It will be appreciated that the window timer will have a duration that is significantly shorter than the service interval time period.
To assure priority of admitted or reserved traffic, the contention scheme used by mobile stations is modified based on the priority of data being sent. Typically contention in WLAN systems is performed by determining if the WLAN channel medium is idle or busy. If the medium is idle, then there is presently no traffic on the channel. If the medium is busy, a station is presently transmitting. There are a variety of ways a station may determine whether the medium is idle or busy, such as, for example, channel carrier sensing, or energy sensing. For carrier sensing the WLAN device tunes its receiver to the channel carrier frequency and “listens” for a carrier. The presence of a carrier indicates the channel is presently in use. Similarly, if the energy in the channel exceeds a preselected threshold, then the medium is considered to be in use by another station. When the channel is busy, the WLAN device waits for a pseudo-random time period within a range of time, and tries again. This is referred to as “back off.” At the end of the back off time period, the WLAN device again senses the channel carrier frequency until the WLAN device finds the channel to be carrier free for a brief, preselected time period. Upon finding the channel to be available the WLAN device may commence transmitting data.
Various schemes exist where, as the WLAN device repeatedly finds the channel occupied, it reduces the range of time to back off and wait. In the preferred embodiment, where priority is given to real time applications, the back off time period range used in contention is initially shorter than that used in non-reserved data traffic stream contention. By using shorter back off periods for reserved traffic streams, these streams will generally acquire the channel before non-priority traffic.
In the UPPSD power save mode of the invention after the mobile station transmits the polling frame, the mobile station stays awake until the access point transmits a response frame. The response frame is not transmitted according to any particular schedule. Instead the access point finishes whatever other transactions it is presently engaged in, if any, and then transmits the response frame or frames to the mobile station. The access point services the mobile station as soon as possible after receiving the polling frame, but not as a scheduled response, or at a predetermined time interval. One benefit of this unscheduled power save mode of operation is that the mobile station does not have to transmit a frame to indicate to the access point that the mobile station is transitioning to low power mode—it is assumed. In prior art power save mode, such as LGCY5 and LGCY6, the mobile station would often have to make three transmissions to complete a transaction or service period with the access point before placing the WLAN subsystem back into the low power state. Using the present unscheduled power save mode of the invention, the mobile station transmits a polling frame, and preferably an acknowledge frame after receiving the response frame from the access point. In transmitting the polling frame the mobile station provides a TSID to indicate the use of the UPPSD mode of operation. The access point will always respond to a polling frame when the TSID is used, and will treat the mobile station as being in low power mode during the time when the access point is not responding to the polling frame. Therefore the access point will not treat the mobile station as being in a fully active state unless the mobile station explicitly requests to exit the UPPSD power save mode, either by transitioning to the active mode or exiting the UPPSD mode entirely by modifying its resource reservation to disable UPPSD or terminate a reserved traffic stream.
Referring now to
Referring now to
Once the WLAN subsystem is in low power mode, the mobile station waits until either the arrival of a frame of data associated with the reserved traffic stream from the voice processor, or other real time media processor, or the occurrence of a service interval event, such as an interrupt (710). When new data associated with the reserved traffic stream arrives, or the service interval event occurs, the mobile station switches power back on to the WLAN subsystem (712). Next, the mobile station commences a frame exchange with the access point by initiating a frame exchange process (714) by, for example, calling a software subroutine to complete a service period. The frame exchange process is performed in accordance with the process described in reference to
Referring now to
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Referring now to
Therefore the invention provides a method of performing power save operation in a wireless local area network (WLAN) by a mobile station while performing voice or other real time communications. The method begins by admitting a reserved traffic stream at the access point, which includes establishing a reserved buffer at the access point for buffering data corresponding to the reserved traffic stream which is to be transmitted to the mobile station during the course of the call. Once a call is established, the WLAN subsystem of the mobile station is placed into a low power state. Subsequently, the method commences by waking up the WLAN subsystem of the mobile station from the low power state to transmit data to the access point, if there is any data to transmit. Once the WLAN subsystem is powered up, the method commences by acquiring the WLAN channel between the mobile station and the access point, and transmitting a polling frame to the access point over the WLAN channel, the polling frame identifies the reserved traffic stream. Acquiring the WLAN channel is preferably performed through known contention protocol, including carrier sensing. The polling frame may be a null frame if no data has arrived at the WLAN subsystem of the mobile station, but otherwise contains data from the call. In response to transmitting the polling frame, the mobile station commences receiving a response frame at the mobile station over the WLAN channel. The response frame is transmitted by the access point and identifies the reserved traffic stream. Once the response frame has been received, the mobile station commences setting the WLAN subsystem into the low power state. It should be noted that while the response frame is sent in response the polling frame, the response frame is not transmitted immediately, necessarily. The access point may have other transactions that require servicing before the response frame may be transmitted, hence the response frame is transmitted in an unscheduled fashion. In the preferred mode the polling frame and response frame are both acknowledges by the respective receiver with an acknowledgment within a specified time, such as, for example, a short interframe space as specified by IEEE 802.11. Receiving the response frame may include receiving a header of the response frame having a EUSP bit clear, or alternatively a MORE_DATA bit set to indicate a second response frame will be transmitted subsequently, and wherein the method further includes receiving a second response frame at the mobile station. The mobile station may wake up in response to the presence of data received from a voice or other real time media process of the mobile station, or in response to a service interval interrupt. Upon the occurrence of a service interval event, at the beginning of a service interval, for example, the mobile station begins running window timer having a duration shorter than the service interval. If the window timer times out and there is still no data, then the mobile station commences transmitting a null frame. The service interval is selected as the real time duration represented by a frame of data.
While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.
