EETS 8316 Wireless Networks Fall 2013

EETS 8316

Wireless Networks

Fall 2013

Shantanu Kangude

[email protected]

Lecture: WiFi Discovery, Powersave, and Beaconing

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

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

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

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

High Level Network Join Process

• Image Source: http://www.grin.com/en/doc/247563/mobility-and-association-management-for-wireless-mesh-networks

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

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

IEEE 802.11 - 2.4 GHz Channels

Slide Source: Ivan Marsic’s publicly available slides at:

http://www.ece.rutgers.edu/~marsic/books/WN/book-WN_marsic%20slides.ppt Channels 1,6,& 11 are non-overlapping

Wifi Powersave

Mainly on the Infrastructure based BSSs

Remember: All methods require time

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

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

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

Basic Infrastructure Powersave

Method

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

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

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

Picture Source in the next few

slides

http://www.csie.nctu.edu.tw/~yctseng/Wireles

Example: Unicast With TIM

IBSS Powersave Method

Quite Inefficient, and not used much in practice

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

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

802.11 ‘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

What Basic 802.11 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

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)

2 Types of APSD

• Unscheduled 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

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.

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

Regular PS vs U-APSD (More Efficient)

What Next in WiFi Powersave?

• Powersave in 802.11n

– 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