EETS 8316 Wireless Networks Fall 2013

Transcription

1 EETS 8316 Wireless Networks Fall 2013 Lecture: WiFi Discovery, Powersave, and Beaconing Shantanu Kangude

2 Discovery and Beaconing

3 Discovery? Who else is there? Any pre-existing network? Common sense methods for Discovery Hear an advertisement Hear beacons, Broadcast channels, in networks Ask someone, may be a store or a friend Probe Requests and Responses in networks Found a few options, how to decide on one Do we have common parameters we can agree with? E.g. capabilities Am I allowed? E.g. Authentication

4 Beaconing for Advertisement Network owner / AP broadcasts beacons every beacon interval with network info Name of the network (SSID) Network operation parameters PHY and MAC protocol parameters like rates supported, EDCA parameters etc. Powersave related info (more later) & more Before connecting, potential participant STAs, listen for beacons, and copy name & parameters of all networks heard

5 Probe Requests & Responses Potential participant STAs may Be impatient, & don t wait for a beacon Need to take initiative if certain beacon info suppressed (why suppressed out of scope) Probe Request Broadcast from a STA Requesting network info from APs / designated owners Probe Response Unicast from various APs to the STA With info to the STA sending request

6 Probes and Backoff After a Probe Request, all APs hearing the request are looking to respond How are collisions avoided on Probe Responses? Backoff like for all other packets Usual CSMA/CA helps Correspondence in Bluetooth Probe Request Inquiry? Probe Response Inquiry Response?

7 High Level Network Join Process Image Source:

8 Scanning in infrastructure Wifi (1/2) Look for networks in the neighborhood They can be on various different channels 2.4 Ghz => ~80 MHz available 3 non-overlapping networks possible 5 Ghz => more than 150Mhz available Many more possible Where to scan? Depends on STA s capability 11b or 11g station may scan only in the 2.4Ghz band

9 Scanning in infrastructure Wifi (2/2) Scan in different center frequencies 1 by 1 Stay in 1 frequency for certain time Collect beacon information from those heard Potentially send probe requests & collect information from responses 2 types of scanning Active scanning = Probe Requests sent Passive scanning = only wait for beacons Scan results are presented to the user as available networks Reading Assigned:

10 IEEE GHz Channels Channels 1,6,& 11 are non-overlapping 5 MHz 22 MHz 2.4 GHz GHz 1: : : : : : : : : : : Slide Source: Ivan Marsic s publicly available slides at:

11 Wifi Powersave Mainly on the Infrastructure based BSSs Remember: All methods require time synchronization, as they involve rendezvous

12 Shut Circuits (Sleep) to Powersave TX and RX RF-chains (circuits) burn significant power (TX > RX) Shut down circuits to save power, but No reception possible while shut down Others may send packets to you expecting you to receive and ACK Not responding properly to protocol activities can Cause mis-interpretation & excessive retransmissions & capacity wastage Cause eventual loss of association, authentication etc. => redo those

13 Shutting Circuits, But Responsibly When to shut circuits? No uplink or downlink traffic or flows But don t shutdown for too long Wake and do basic activities regularly to maintain connection & synch Wake up regularly to check if any downlink traffic And tell the AP, your Sleep-Wake schedule Generally wake up for specific beacons E.g. every Nth beacon Stay awake to make sure no data transfer needed in both directions

14 Another Perspective on Sleep & Wake Periods One of the nodes, typically AP, is always awake Other STA alternates between 2 states Doze /Sleep Awake Both stations should agree on When the AP should expect the STA to be awake When, after being awake, can a STA go back to sleep

15 Basic Infrastructure Powersave Method

16 Basic Concepts AP buffers packets for power-saving stations until they are awake Traffic Indication Map (TIM): A bitmap transmitted in beacons Tells which STAs have pending DL traffic Once a STA sees its TIM bit set It stays awake and gets its DL traffic Goes back to sleep after somehow informing AP For UL traffic arrival STA can wake up ANYTIME (even in scheduled sleep times) and transmit (since AP always awake) STA can change mode to active (always awake mode) for an extended period when it has traffic

17 Different TIM Beacons & Sleep Intervals TIM bitmap can be there in every beacon Delivery TIM (DTIM) beacon is a beacon that indicates any broadcast/multicast traffic pending in DL ALL STAs must be awake for DTIM beacons If broadcast/multicast DL traffic is indicated Stay awake to receive it Since all STAs wake up on DTIM Typical sleep intervals (called LISTEN INTERVAL) = Submultiples of DTIM period E.g. if DTIM period is 16 Beacon Intervals, STA A, B, & C can have Listen Intervals of 1,2,4,8, or 16 beacon intervals

18 PS-Poll Frames to Retrieve DL Buffered Traffic Once TIM/DTIM indicates traffic That STA keeps awake and retrieves packets using PS-Poll frames STAs send PS-Poll frames to AP AP responds with data packets STA keeps awake generally until More Data bit in header is RESET to 0, or TIM bitmap indicates no more traffic During data transfer More Data bit in Frame- Control field in header tells about more buffered packets or not before TIM bit can in a beacon

19 Basic Powersave Operation

20 Picture Source in the next few slides snet06-02/ieee power-savingmode.ppt

21 Example: Unicast With TIM STA 1 has a listen interval of 2, and STA 2 of 3

22 Example: Broad/Multicast with DTIM

23 IBSS Powersave Method Quite Inefficient, and not used much in practice Wifi Direct Powersave much more efficient

24 Basics of IBSS Powersave No AP, so no one is always awake Traffic goes to any neighbor, plus may have Broadcast/Multicast Ensure RENDEZVOUS times when ALL are awake => ATIM Window Time Announce in ATIM Window Who all shall stay awake to receive packets Multiple announcements from multiple senders possible Random backoff, and large enough window time

25 ATIM Announcement TIM Like DTIM in infrastructure mode, all IBSS STAs should be awake for a periodic ATIM beacon ATIM window of time after every ATIM Beacon All STAs are awake during this time period All senders send announcements about receivers who should stay awake to receive packets A potential receiver who sees its name in announcement has to ACK to the potential sender All those who have successful Announcement-ACKs stay awake during beacon interval and go through transfers Others not in announcements, go to sleep No actual data in ATIM window No announcements outside ATIM window Those who stay awake, do so until next ATIM window end

26 Example: ATIM Window

27 Example IBSS Powersave Operation

28 E QoS Powersave Enhancements Scheduled and Unscheduled Automatic Power Save Delivery (APSD) Remember: 11e has 4 access categories in EDCA, and in general traffic classes

29 What Basic Powersave Lacks Quantum of time over which sleep/wake done = Beacon Intervals or Multiples Not fine grained enough For VOIP traffic, may have to be awake all the time if Beacon Interval >= 100ms PS-Poll method is a slow and uncontrolled method in delivering packets in the DL All we need to know is if the STA is awake, and any UL transmission can indicate that Synchronizing UL and DL transmissions can help save time to be awake

30 Service Period (SP) in APSD One contiguous burst of time for which a STA is awake for data transfers in APSD Service Periods end with End Of Service Period (EOSP) bit set to 1 typically by AP Like More Data bit Or Maximum SP duration is up (in Scheduled APSD)

31 Unscheduled APSD 2 Types of APSD Service Period begins with Trigger and ends with a packet with EOSP bit set Triggers frame is a QoS+Data or QoS+Null frame Scheduled APSD No triggers needed to start Service Periods Pre-negotiated PERIODIC schedules set for Doze- Wake cycles Scheduled periods are shorter than beacon intervals typically => fine grained Powersave cycles

32 Scheduled APSD Parameters APSD periodicity Beginning time Maximum Service Period duration Service Period may be ended earlier by the AP Used for periodic traffic like VOIP, Video conferencing etc.

33 Unscheduled APSD No pre-set schedules of when to wake up No inherent pre-set periodicities Service Periods (SPs) start with an UL frame SP started is of the same Access Category (AC) as the UL frame transmitted Access Categories are U-APSD enabled or disabled as per needs SPs end when a EOSP bit set DL packet received => AP says no more data

34 Regular PS vs U-APSD (More Efficient) Baseline PS U-APSD Src: wp

35 What Next in WiFi Powersave? Powersave in n Spatial Multiplexing Powersave Powersave Multi Poll (PSMP) Powersave in Wifi Direct (when one p2p STA becomes soft AP) In p2p, both nodes may sleep, so the assumptions of the OTHER NODE ALWAYS AWAKE is not valid All mechanisms are modified to fit the scenario TWO great project topics First come first serve