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 PM 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 PM 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 PM bit set to sleep). In the following context, the PM-bit based mechanism is referred to as LGCY6 in the art.
A second power management mechanism uses a PM-Poll frame to solicit downlink frames. Instead of waiting indefinitely for the access point to deliver downlink transmission, the PM-Poll based mechanism utilizes the PM-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 PM-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 PM-Poll frame to retrieve the buffered downlink frame. Unlike the PM-bit based mechanism, a mobile station can stay in the sleep state since the access point responds to the PM-Poll with an immediate data frame. In the following context, the PM-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 PM-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 PM 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 PM-Poll based mechanism, since a mobile station is not aware of the priority of the buffered downlink frame, the PM-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. One method of providing a particular quality of service is to use scheduled service periods at regular intervals for a given mobile station. This scheduled mode of power save deliver is referred to as automatic power save delivery (APSD). The mobile station wakes up at regular intervals and listens to the channel. The access point is synchronized to the service period, and transmits data at the scheduled time. Thus, the mobile station can put the WLAN subsystem to sleep during the periods between scheduled service intervals. However, this method limits the flexibility of the WLAN channel since there is no ability for the mobile station to deviate from the schedule. 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 method of scheduled operation by allowing a more flexible use of scheduled and unscheduled transactions. The mobile station first establishes a scheduled stream to be used in association with a high priority access category flow, such as a real time voice call or a video stream, for example. Accordingly, the mobile station enters a low power mode, and waits for a scheduled service period to begin. The scheduled service periods occur at regular intervals and have a predetermined duration. Occasionally the access point may have to terminate the service period before all buffered data can be delivered. At the end of the scheduled service period, the mobile station may receive notice from the access point that the access point still has data buffered for the mobile station, and may indicate the type or access category of data that is buffered at the access point. At the end of the scheduled service period, the mobile station may place its WLAN componentry in a low power mode. The mobile station may then initiate an unscheduled service period before the next scheduled service period to retrieve the remaining data, if conditions allow. For example, before deciding to initiate an unscheduled service period, the mobile station may check its battery status to see if there is sufficient power budget, or it may determine, based on information provided by the access point, that the data remaining at the access point is of an access category that requires immediate attention. The mobile station may also use the unscheduled transaction to service low priority data flows.
Referring now to
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Although illustrated here as two separate physical buffers, one skilled in the art will understand that a variety of buffering techniques may be used to keep reserved and unreserved data separate, without necessarily requiring separate physical buffers. Furthermore, given that the access point will respond to the polling frame with an aggregate response, the unreserved data buffer and reserved buffer may be treated as an aggregate buffer 309. In one embodiment of the invention when the access point is polled by the mobile station during an unscheduled service period the access point empties the aggregate buffer by transmitting all aggregate buffered data to the mobile station. In other power save methods, the access point will typically enforce an aging policy so as to prevent too much reserved data from being buffered at the access point. However, using the present hybrid method, the access point may rely on the mobile station initiating unscheduled transaction to retrieve remaining reserved data rather than discarding reserved data as in other methods.
Supervising the operation of the buffer manager 308, gateway 306, and transceiver 302 is a controller 314. The controller also administers resource management and controls resources so that quality of service may be assured as needed for reserved traffic streams. The controller is operably coupled to a memory 315, which it uses to track the status of call, mobile station power save states, and other parameters.
Referring now to
Referring now to
The mobile station transmissions appear on the bottom flow line 502, while the access point transmissions appear on the top flow line 504. 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 usually 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 scheduled power save mechanism can be established by mobile station implicitly by the use of a traffic specification reservation. In frames containing data for the reserved traffic stream, the unique traffic stream identifier (TSID) will be included. The mobile station can choose no power save operation, legacy power save operation, scheduled power save operation only, or the present hybrid 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 maintains a service interval timer to maintain real time operation of the flow during scheduled service periods. However, if data remains at the access point after a scheduled service period, the mobile station may choose to initiate an unscheduled service period. At the beginning of an unscheduled service period, the mobile station activates the WLAN subsystem at time 506. After which, during the time period 507, the mobile station begins contending for the WLAN channel. The mobile station initiates the unscheduled transaction by transmitting a polling frame 508. 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. The polling frame may also include signaling to indicate a desire for the access point to use an aggregate response method so that both reserved and unreserved data may be received from the access point. Alternatively, the aggregate response may be the default response mode.
In the preferred embodiment, after the access point receives the polling frame, it transmits an acknowledgement 510 within a short interframe space time period 512, which is a scheduled event, in accordance with the IEEE 802.11 specification. In response to receiving the polling frame, the access point transmits at least one response frame 516 to the mobile station, assuming the access point has aggregate buffered data for the mobile station. Assuming there is both unreserved data and reserved data in the aggregate buffer, at least a second response frame 518 will be transmitted. The access point will continue to transmit response frames until the aggregate buffer is empty, or, alternatively, if the access point must perform other scheduled tasks. Each response frame includes an end of uplink service period (EUSP) bit, such as a MORE_DATA bit to indicate whether there is more data coming from the access point, or whether the present response frame is the last response frame for the service period. It is contemplated that the access point may not completely empty the aggregate buffer of unreserved data if the access point is presently servicing a high number of reserved traffic streams for other mobile station, and the delivering the unreserved data may interfere with the delivery of reserved traffic.
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 514 between the acknowledgement and the response frame. As soon as possible, the access point will acquire the WLAN channel and transmit the response frame or frames. However, the response frame is not sent with regard to any predetermined schedule. That is, mobile station maintains the WLAN subsystem powered up 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 when it contains reserved data. 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 aggregate buffer, the access point will transmit a null frame. Alternatively, if the aggregate buffer is empty, then the acknowledgement 510 may indicate such. In the response frame there will be signaling information, such as an 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 the access point must perform other scheduled tasks. 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 unscheduled service period due to successful transmission of all buffered frame for the mobile station in the aggregate buffer, or the end of the unscheduled service period due to time considerations. 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 unscheduled 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 if the aggregate buffer is empty, otherwise the unreserved data in the aggregate buffer will also be transmitted to the mobile station. In response to receiving the response frame, in the preferred embodiment, the mobile station transmits an acknowledgement 520 within a short interframe space time period 518. If the response frame indicated the end of the present unscheduled service period, the mobile station then places the WLAN subsystem into a low power state after receiving the response frame at time 522.
Referring now to
Referring now to
In the preferred mode the access point transmits and acknowledgment which is received by the mobile station (703). If the acknowledgement is not received (704), the mobile station may back off by waiting, then retransmit the polling frame. After transmitting the polling frame, and, in the preferred mode, receiving the acknowledgment, the mobile station then waits for the access point to respond. Since the response is not scheduled, the time of the wait is variable, although the mobile station may have a preselected maximum time period to wait before undertaking an error procedure, assuming a failure of access point to respond. However, assuming normal operation, the access point will transmit an aggregation of response frames which will be received by the mobile station (706). In transmitting data from the aggregate buffer, data belonging to the traffic stream identified by the TSID used by the mobile station in the polling frame may be transmitted first, before unreserved data, in the aggregate response. Again, in the preferred mode, the mobile station will transmit an acknowledgement to assure the access point of a successful delivery. Upon receiving the response frame, the mobile station checks the EUSP bit to see if the UPSD service period is over. In the preferred embodiment, the MORE_DATA bit may be used to signal when more date is coming from the access point (708), and when it is set it indicates that the service period is continuing until at least one more response frame is received. If the MORE_DATA bit indicates subsequent frames are coming, then the mobile station remains active to receive them as it did for the first response frame. It is contemplated that subsequent response frames may contain data for a different reserved traffic stream also in use by the mobile station, or for the present reserved traffic stream. Once a response frame is received indicating no more data is coming from the access point, the process ends (710) and the mobile station places the WLAN subsystem in low power mode.
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 in which a recurring service period schedule set up between the mobile station and an access point. The scheduled service periods occur at periodic intervals and are for maintaining a reserved traffic stream. The reserved traffic stream is identified by a reserved traffic stream identifier, and the mobile station has its WLAN subsystem initially in a low power mode. The method commences by powering up the WLAN subsystem of the mobile station and commencing a scheduled service period. At the end of the scheduled service period the mobile stations receives from the access point an indication that the access point has more data in a buffer of the access point for the mobile station. After receiving the last frame of the scheduled service period, the mobile station places the WLAN subsystem into low power mode. If the mobile station decides it is appropriate, the mobile station then commences initiating an unscheduled service period to retrieve the remaining data buffered at the access point for the mobile station. The unscheduled service period begins by powering up the WLAN subsystem and transmitting a polling frame to the access point. The polling frame includes the reserved traffic stream identifier. In response, the mobile station receives at least one response frame from the access point. At the conclusion of the unscheduled service period, the mobile station places the WLAN subsystem into low power mode. In one embodiment receiving the response frame includes receiving an aggregate response in which both reserved and unreserved data is received. The aggregate mode may be a default mode, or it may be triggered by transmitting the polling frame with an aggregation bit set.
The present method also prescribes a method of retrieving data from an access point by a mobile station in a wireless local area network (WLAN), where the reserved data corresponds to a reserved traffic stream and is identified by a reserved traffic stream identifier. The method includes performing a scheduled transaction between the mobile station and access point during a scheduled service period. The mobile station transitions from a low power WLAN mode to an active WLAN mode to commence the scheduled transaction, and then transitions from the active WLAN mode to a low power WLAN mode upon completion the scheduled transaction. After the scheduled transaction is complete. The mobile station commences performing an unscheduled transaction between the mobile station and access point during an unscheduled service period. The mobile station transitions from a low power WLAN mode to an active WLAN mode to initiate the unscheduled transaction, and then transitions from the active WLAN mode to a low power WLAN mode upon completing the unscheduled transaction. It is contemplated that the unscheduled transaction may be performed in response to the access point indicating at the end of the scheduled service period that the access point still has data for the mobile station, or, alternatively, the mobile station may have data to transmit to the access point. If the access point indicates at the end of the scheduled transaction that there is still data buffered at the access point, the access point may indicate the type of data, such as the access category of the data and whether the data is part of a reserved traffic stream. Data that is part of a reserved traffic stream may be part of a live voice call. The mobile station may decide whether or not to initiate an unscheduled service period by checking various parameters, such as, for example, battery power status, signal quality level, the priority of the data buffered at the access point, and so on.
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.