Power Save Multi-Poll (PSMP)

Power Save Multi-Poll (PSMP)

By CWNP On 09/14/2007 - 11 Comments

- Introduction -

In the legacy power save polling method, the station awakes from dozing and sends a PS-Poll frame to the AP that has been queueing its data traffic while it dozed.  For each PS-Poll frame the station sends to the AP, the AP responds with one data frame.  Additionally, stations have to wait for the DTIM Beacon to know whether or not it has queued traffic.  It's a high-overhead, high-latency protocol that only allows stations to doze a moderate amount - not good enough for wVoIP.


The next stage was U-APSD (adopted by the Wi-Fi Alliance as WMM Power Save (WMM-PS). WMM-PS improves the efficiency of legacy power save by increasing the amount of time the client is allowed to doze and by decreasing the number of frames that a client needs to send and receive in order to download the same number of frames buffered by the access point as before.  It consists of a signaling mechanism added to WMM that enables the access point to buffer data frames and send them to the client upon its request.  The station requests its data through use of a trigger frame (which could be a data frame).  After the client has sent a trigger frame, the access point acknowledges it is ready to send the data.  Data frames are sent during an EDCA Transmit Opportunity (TXOP) burst, with each data frame interleaved with an acknowledgement frame from the client.  On the last data frame, the access point indicates that no more data frames are available and the client can revert to its dozing state.

The 802.11n amendment defines a new power save protocol called Power Save Multi-Poll (PSMP), which has both an scheduled and unshceduled part.  Think of Scheduled PSMP (S-PSMP) as a way that APs can, during their own TXOP, send a schedule to one or more stations in the BSS letting the stations know when to be awake to receive downlink data frames and when they (individually) are allowed to begin transmitting.  By using a schedule, stations can doze for the maximum amount of time possible while missing no frames. 

Unscheduled PSMP (U-PSMP) functionality does not replace U-APSD (WMM), but rather extends it to add further functionality.  PSMP capable stations can still use WMM-PS with trigger frames when appropriate.

- Primer -

S-PSMP provides a time schedule that is used by an AP and its PSMP-capable stations (typically devices such as handhelds, where battery life is at a premium).  S-PSMP operation is controlled using the PSMP Action Frame.  The amounts of time used for downlink and uplink transmission are defined as part of the PSMP frame.  PSMP-UTT (uplink transmission time) is the time scheduled for PSMP-capable stations in the BSS to transmit frames to the AP.  PSMP-DTT (downlink transmission time) is the time scheduled for the AP to transmit frames to the stations within the BSS.  A PSMP Sequence is a series of frames that starts with a PSMP Action Frame and is followed by either PSMP uplink or downlink frames.  

The AP configures the "PSMP STAs Only" field within the HT Information Element to 1 in Beacons when it wants stations that only support PSMP operation to associate.  There are two modes of PSMP: Scheduled and Unscheduled.  It's likely that if PSMP operation is implemented by vendors (which is yet to be seen, but hopefully soon), it will be unscheduled mode that gets the call because it simply extends U-APSD, which was given to us by IEEE 802.11e and adopted by the Wi-Fi Alliance as part of WMM-PS.

- PSMP Frame Format -

A PSMP frame is an HT management action frame, and it's format is represented below.  The Destination Address (DA) field of this frame is the broadcast address or a multicast address.  The reason for using a broadcast or multicast DA is so that all stations will process the frame upon receipt.  After the frame is received, part of the frame contents (the STA_INFO Type subfield of the PSMP STA Info field) specifies whether or not the frame is actually destined to one or more individual stations, all stations, or a multicast group.

The PSMP Parameter Set is used to describe the downlink transmission time schedule (PSMP-DTT) and uplink transmission time schedule (PSMP-UTT) that immediately follows the PSMP frame.  The "PSMP Parameter Set" field is followed by zero or more "STA Info" fields that are used to specify transmission schedules for individual stations in the BSS.

The Category field is set to 7 (representing HT)
The Action field is set to 2 (representing PSMP)
PSMP Parameter Set field

In a PSMP frame, the "STA_ID" fields of all its "STA Info" fields with STA_INFO Type set to 2 (individually addressed) are unique.  For example, each station identified in the PSMP frame is identified exactly once.  The PSMP STA Info fields within a PSMP frame are ordered by STA_INFO Type as follows: broadcast (STA_INFO Type=0), multicast (STA_INFO Type=1), and then individually addressed (STA_INFO Type=2) - discussed more below.

- PSMP Parameter Set field -

The PSMP Parameter Set field is used in a PSMP frame to define the number of PSMP STA Info records held in the PSMP frame, to indicate whether the PSMP sequence is to be
followed by another, and to indicate the duration of the PSMP sequence.  The PSMP Parameter Set field is 2 octets in length.  The structure of the PSMP Parameter Set field is:

N_STA (5)
More PSMP (1)
PSMP Sequence Duration/ID (10)

The N_STA field indicates the number of STA Info fields present.  The More PSMP field when set to 1 indicates whether this PSMP sequence is followed by another PSMP sequence and vice versa.  The PSMP Sequence Duration/ID field indicates the duration of the current PSMP sequence that is described by the PSMP frame, in units of 8 µs, relative to the end of the PSMP frame.  Therefore, this field can describe a PSMP sequence with a duration of up to 8.184 ms.

- PSMP STA Info field -

The PSMP STA Info field is used by the PSMP frame.  The PSMP STA Info field defines the allocation of time to the downlink (PSMP-DTT) and/or uplink (PSMP-UTT) associated
with a single receiver address.  There are three variants of the structure for individually addressed, multicast, and broadcast cases.  The length of the PSMP STA Info field is 8 octets.

The Individually-Addressed (IA) variant is structured as follows:

STA_INFO Type (2)
PSMP-DTT Start Offset (11)
PSMP-DTT Duration (8)
STA_ID (16)
PSMP-UTT Start Offset (11)
PSMP-UTT Duration (10)
reserved (6)

...where all numbers are in bits.  The STA_INFO Type field is used to define whether the frame is IA (2), multicast (1), or broadcast (0) and therefore defines the rest of the structure of the frame (which differs depending on the type).  The STA_ID field is the association identifier (AID) of the station.

The Broadcast variant is structured as follows:

STA_INFO Type (2)
PSMP-DTT Start Offset (11)
PSMP-DTT Duration (8)
reserved (43)

The Multicast variant is structured as follows:

STA_INFO Type (2)
PSMP-DTT Start Offset (11)
PSMP-DTT Duration (8)
PSMP Multicast ID (43)

The PSMP-DTT Start Offset field indicates the start of the PPDU that has the downlink data of the station.  The offset is specified relative to the end of the PSMP frame.  It is given as an integer number of 4 µs.  

The PSMP-DTT Duration field indicates the end of downlink data of a station relative to the start of the PPDU that contains the first frame destined to the station.  It contains a duration in units of 16 µs.

The PSMP-UTT Start Offset field indicates the start of the PSMP-UTT.  The first PSMP-UTT is scheduled to begin after a SIFS interval from the end of the last PSMP-DTT described in the PSMP.  The offset is specified relative to the end of the PSMP frame.  It is specified in units of 4 µs.

The PSMP-UTT Duration field indicates the maximum length of a PSMP-UTT for a station.  PSMP-UTT Duration is specified in units of 4 µs.  All transmissions by the station within the current PSMP sequence lie within the indicated PSMP-UTT.

- PSMP Operation -

Think of PSMP operation as a way that the AP can send notifications (action frames) to one or more PSMP capable stations during its own TXOP allowing them to: 1) transmit frames to the AP at specific point in time, or 2) understand that they should be awake and ready to receive one or more transmissions at a specific point in time.  If a station has frames to send, it starts transmission without performing CCA and regardless of its NAV value at the start of its PSMP-UTT Offset.  Use of the PSMP-UTT doesn't imply that a station is a TXOP holder.  Instead, it's the AP that holds the TXOP and controls downlink and uplink transmissions using the PSMP mechanism.

The AP first configures the Beacons to show whether or not only PSMP stations are allowed in the BSS.  To start a PSMP Sequence, the AP transmits a PSMP Action frame (IA, broadcast, or multicast) giving instructions to one or more stations.  If the action frame is IA, then the instructions contain information on when any downlink data will start and how long it will last, when each station (there can be a list of stations in an IA variant) is allowed to start transmitting and for how long.  If the variant is multicast or broadcast, it contains data on exactly when the downlink transmission will start and how long it will last.  With multicast variants, it also contains the multicast ID.

Either RIFS or SIFS can be used to separate uplink or downlink transmissions.  If a station runs out of uplink data to transmit during its PSMP-UTT, the station isn't allowed to truncate the TXOP with a CF-End frame because it's not the TXOP holder (the AP is the TXOP holder during a PSMP Sequence).

PSMP recovery frames, which are modified versions of the original PSMP action frame, are used when a station scheduled to transmit during a PSMP-UTT does not transmit within a PIFS of its scheduled transmission time.  Perhaps a station didn't hear the PSMP frame and thus didn't know to begin transmitting at that specific time.  The PSMP recovery frame is sent by the AP after a PIFS from the station's scheduled uplink transmission start time.  The PSMP recovery frame only modifies the schedule of the station that missed its scheduled appointment, and the schedules of other stations remain unchanged.  The station that missed its original transmission time simply has a shorter period of time in which to transmit uplink frames.

After transmission of an initial PSMP sequence, additional PSMP sequences can be transmitted by the AP in order to support resource allocation and error recovery.  An initial PSMP sequence followed by one or more PSMP sequences is termed a PSMP burst.  A station is not allowed to transmit a +HTC MPDU in which the RD/More PPDU field is set to 1 during a PSMP burst.  The RD protocol and PSMP are similar in many ways and would conflict in this case.  An AP is allowed to transmit a CF-End to end the PSMP burst.  All data frames transmitted during a PSMP burst must be QoS Data frames.

- PSMP Resource Allocation within a PSMP Burst -

If the allocated PSMP-UTT Duration is not long enough for a station to send ALL of its queued data, the station transmits only the part of the queued data that fits within the allocated PSMP-UTT Duration and may transmit a resource request to the AP within that PSMP-UTT.  The resource request is communicated by setting either the Queue Size field or the TXOP Duration Request field of the QoS Control Field that is carried in a QoS Data frame.  If a station receives an PSMP-UTT that is not long enough to transmit ANY data from its queues, it may transmit (within the PSMP-UTT) a QoS Null frame containing information about the state of its transmit queues.  The HT AP may then use this information to schedule a PSMP-UTT either in the current PSMP burst or a later PSMP burst.  The AP may allocate a PSMP-UTT Duration in the next PSMP sequence based on the resource request from the station sufficient to allow transmission of the remaining queued data.  

- PSMP Retransmission -

Any failed transmissions during PSMP-DTT and PSMP-UTT may be respectively retransmitted in PSMP-DTT and PSMP-UTT period of the next PSMP sequence.  An AP transmits BlockAck responses, if any, to a station's PSMP-UTT data transmissions in the PSMP-DTT of a subsequent PSMP sequence.  The AP may reserve a PSMP-UTT in a subsequent PSMP sequence to allow the station to retransmit failed frames.  The station may retransmit failed frames in a PSMP sequence of the current PSMP burst if a PSMP-UTT reservation is present or in a subsequent service period.  A station that cannot complete its retransmissions in the last PSMP sequence of the PSMP burst because not enough time is allocated in its PSMP-UTT, may transmit the data outside any PSMP sequence.  Frames transmitted outside the scheduled service period under EDCA are subject to the Access Category prioritization.  In the case of uplink frames transmitted outside the scheduled service period, the MTBA that acknowledges these frames is delivered in the PSMP-DTT within the next service period.

11 Responses to Power Save Multi-Poll (PSMP)

Subscribe by Email

Kevinogi jhome Says:
04/19/2018 at 21:06pm
Manfred said he didn't know that the Marlins' new owners planned to tear it washington nationals jersers down
In the early moments of atlanta braves jersers the interview, Le Batard asked Manfred whether he knew prior to the recent sale to the group oakland athletics jersers fronted by Bruce Sherman and Jeter whether the new owners planned to slash payroll. After some prodding (and Le Batard's saying that the commissioner was lying), Manfred answered, "We do not get involved in operating-level decisions in the ownership approval process."

"We did not have player-specific plans from the Miami Marlins or any other team that has been in the ownership miami marlins jersers process. Those are decisions that the individual owners make, and they do not have to be cleared by us or approved by us. ... Those are local decisions that really are not part of the approval process. Those are decisions that the individual owners make, and they do not have to be cleared with us or approved by us."

Manfred went on to say that he didn't receive a payroll plan from the Marlins until two days prior to his interview with Le Batard. More: "We don't get into, are you going to trade 'Player X' or 'Player Y' at a particular point in time, nor do we ask them to make a commitment to people before they even got in and made an evaluation of their talent level, their ability to win with the people that st. louis cardinals jersers they have. That's just not how the ownership process works."
wholesale baseball jersers

But some of that may not be true
Here's a key excerpt from a los angeles angels of anaheim jersers must-read Barry Jackson piece in the Miami Herald:

A source directly involved in the Marlins sales chicago white sox jersers process, after hearing the Le Batard cincinnati reds jersers interview, said, via text: "Commissioner said was not aware of [Jeter] plan to slash payroll. Absolutely not true. They request and receive the operating plan from all bidders.

"Project Wolverine [the name for Jeter's plan] called chicago white sox jersers on his group to reduce payroll to $85 million. This was vetted and approved by MLB prior to approval by MLB. Every [Jeter] investor and non investor has the Wolverine financial plan of slashing payroll to $85 million. Widely circulated."

First off, "Project Wolverine" is ludicrously self-important and sinister-sounding, as budget strategies go. That's the name of a secret NSA laboratory deep under the Caballo Mountains in New Mexico, not a financial schematic. Do better, Jeets. Anyhow, there's enough careful phrasing in Manfred's comments ("operating-level decisions," "'Player X' or 'Player Y'") to give him some plausible deniability. However, the idea that he didn't know about plans to engage in yet another demo job by Marlins owners strains credulity.

wiatmppgryar wiatmppgryar Says:
04/18/2018 at 03:47am
Downs 24+18 Minnesota Timberwolves constructed your home victory about Grizzlies

Philadelphia Eagles Jerseys Regarding June 10th, Beijing point in time, any NBA usual couple of years went on. Any Minnesota Timberwolves portrayed with Memphis Grizzlies. Any Timberwolves constructed a good 113-94 victory across the Grizzlies together with have scored several consecutive victories. Nelson Agholor Jerseys The end conflict could coincide when using the Nuggets. Downs have 26 tips together with 15 rebounds, Teague 26 tips together with 8 supports, Butler 15 tips; Grizzlies, McLemore 15 tips together with 7 rebounds, Brooks 15 tips, Rabba 16 tips together with 8 rebounds Nick Foles Jerseys.

Philadelphia Eagles During the earliest coint within the match, utilizing 5 seconds together with 34 minutes, Wiggins fouled McCremo's hit together with brought any Grizzlies a pair of zero cost brings. For 7 seconds 54 minutes, Gibson slipped any shot together with was initially Chalmers steals. For 9 seconds 22 minutes, Brooks passes any shot together with Brooks bites a good three-pointer. Any Timberwolves contain a significant appeal during the indoors. Rasul Douglas Jerseys The coint, these picked off nineteen rebounds, together with 10 a problem rebounds, of which Geon driven 3 rebounds. Any Grizzlies portrayed fluently. During this department, any squad finalized 8 supports as a whole, of which Chalmers provided a pair of tickets meant for teammates. Any Timberwolves crafted a good structure for iron bars over the sheltering last part. The coint provided five golf shots together with stuffed any opponent's niche end goal number that will thirty. 00%. Sidney Jones Jerseys During the earliest coint, any Grizzlies accessed the other department by using a 6-point contribute.

NFL Jerseys Cost-free coint returned, any finally minimal forty four minutes, Rab's completing slips, was initially Butler steals. For 7 seconds 59 minutes, Gyan was initially stuffed by just a break down regarding shield together with brought any Grizzlies a pair of zero cost brings. For 8 seconds together with 41 minutes, receiving a circulate with Brooks, Myke Henry have scored a good three-pointer. Any Timberwolves contain a significant appeal during the indoors. Zach Ertz Jerseys The coint, these picked off 18 rebounds, together with 5 a problem rebounds. Most notably, Downs driven 7 rebounds, together with she procured benefit for any package that will fico score 3 tips. Any Timberwolves portrayed fluently. During this department, any squad finalized 8 supports as a whole. Dorial Greenbeckham Jerseys Amongst Jones provided several tickets meant for this teammates. Any Grizzlies over and over come to any package together with attained 12 zero cost brings, 11 zero cost brings together with 91. 67% zero cost brings. Cheap Jerseys For halftime, any Timberwolves delivered any fico score that will 56-53.

Zach Ertz Jerseys During the following circle, Kobe Simmons fouled Teague's hit together with brought any Timberwolves a zero cost dispose of. 8 seconds together with 5 minutes, Raised sprang out completing slips together with was initially Brooks steals. Wes Hopkins Jerseys The main 10 seconds together with 18 minutes, attained a good circulate with Myke Henry, Rab jumper prevailed. Any Timberwolves are typically in negative circumstance regarding who the winner and now have constructed five slips. Any Timberwolves over and over come to any package together with attained 11 zero cost brings, 11 zero cost brings, in addition to a zero cost dispose of number for 100. 00%. Wendell Smallwood Jerseys Once a couple of quarters, any Grizzlies on holiday was thrown off associated with utilizing 75-83.

Wilbert Montgomery Jerseys During the latest coint, 1 minimal together with 53 minutes, Rab fouled Downs's hit together with brought any Timberwolves 1 zero cost dispose of. During the following minimal together with 40 minutes, Crawford attained a good circulate together with Communities improved for that jumper. For 9 seconds together with 43 minutes, Brooks forgotten any circulate together with was initially Wiggins steals. Any Timberwolves portrayed fluently. Vinny Curry Jerseys During this department, any squad finalized 9 supports as a whole. Amongst Raised provided several tickets meant for this teammates. Any Timberwolves contain a superior truly feel during this department. Any team's blasting number attains 61. 90%, which unfortunately improves on thirty. 00% within the Grizzlies. Trey Burton Jerseys Communities have scored mainly 13 tips during this department. When they get home within the match, any Timberwolves combat any Grizzlies 113-94 that will seal off any victory.

Ionso Ionso Says:
02/15/2018 at 03:29am
I have read a lot about the concepts to save multi poll psmp power. That we can save with best essays reviews and assignments. All the resources much important to all of our news editors and review writers.

Vishal Tyagi Says:
01/03/2018 at 05:03am
From this windows clock website you can grab the information which are suitable for you and can easily set your clock in windows 10.

05/06/2008 at 22:02pm
Thank you for your answer.
I know U-PSMP is extension of U-APSD,they are all for VoIP(WMM),and suport QoS data、power save.U-PSMP and U-APSD work in Unscheduled SP for STAs accessing the channel using EDCA. The Unscheduled SP begins when AP receives a trigger frame from STAs.
However there are some conception confusing me:Do you know what is the relationship between SP(service period)、PSMP sequence、PSMP burst、PSMP session?

04/28/2008 at 11:48am

Here's an article giving some details on U-PSMP.

Both S-PSMP and U-PSMP can operate in a contention environment. CFP isn't required.

Yes, in S-PSMP the stations can sleep. The know the schedule (including when it's their turn to transmit).

U-PSMP and S-PSMP are distinct power save modes. There are legacy power save, U-APSD, S-APSD (not used), U-PSMP, and S-PSMP (not used that I know of). Each is different.

04/28/2008 at 03:57am
1.Can you talk about the difference between S-PSMP and U-PSMP in detail;
2.I think PSMP take place in CFP,what do you think?
3.in S-PSMP,the STAs can sleep? and PSMP(include S-PSMP and U-PSMP) must be in PS mode?
Thank you!

10/27/2007 at 11:43am
While what you assert is correct, it seems that you've taken my statements completely out of context (by skipping multiple sentences). Please read my statements more carefully. thanks!

10/26/2007 at 21:32pm
You state "In the legacy power save polling method ... stations have to wait for the DTIM Beacon to know whether or not it has queued traffic."

Correction: Stations only have to wait for the next beacon (of any kind) to know if they have any individually addressed traffic queued at the AP. Stations must wait for the next DTIM beacon to determine if there is any group addressed traffic queued at the AP.

09/17/2007 at 08:39am
What is PSMP-UTT? The first paragraph under "Primer" says it is the time scheduled for AP to transmit frames to the stations.
Last paragraph in the article says AP may reserve a PSMP-UTT to let the station re-transmit failed frames (station trasmitting data).

<< prev - comments page 1 of 1 - next >>

Leave a Reply

Please login or sign-up to add your comment.
Success Stories

I literally just came out of the testing centre having taken the CWDP exam. The certification process opened my mind to different techniques and solutions. This knowledge can only broaden your perspective. Great job, CWNP, you have a great thing going on here.

Read More

Working through the CWNP coursework and certifications helped not only to deepen my technical knowledge and understanding, but also it boosted my confidence. The hard work it took to earn my CWNE has been rewarding in so many ways.

Read More

I want to commend you and all at CWNP for having a great organization. You really 'raise the bar' on knowing Wi-Fi well. I have learned a ton of information that is helping my job experience and personal career goals, because of my CWAP/CWDP/CWSP studies. Kudos to all at CWNP.

Read More