IEEE 802.11 standards

Dr. Fazekas Péter

Balogh András

BME-HIT

2016.12.05. 2

• 802.11-1997 (802.11 legacy)

– original standard 2.4 GHz ISM band,1 or 2 Mbps bitrate

• 802.11a-1999

– 5GHz ISM band, OFDM waveform, max. 54 Mbps

• 802.11b-1999

– this became popular, extension of 802.11, 11 Mbps

• 802.11g-2003

– 2.4 GHz ISM band, waveform is like in 802.11a, 54 Mbps

• 802.11n-2009

– 5 and 2.4 Gh ISM band, OFDM, MIMO, higher bandwidth, up to 600 Mbps bitrate

• 802.11ac-2013

– 802.11n advanced version, OFDM, 5GHz band, wide channel (max 2x80MHz), MIMO, 256QAM, theoretical bitrate above 6 Gbps

802.11 standards

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• 802.11ad-2012

– 60 GHz band, originally planned for consumer electronics, high definition video, single room coverage, multiple waveforms, multi-Gbps

• 802.11af-2014

– „Super-WiFi”, in unused TV bands, cognitive radio solution, based on 802.11ac, but smaller bandwidths, up to 400 Mbps

• 802.11e -2005

– extension of original MAC protocol with QoS capabilities. It was branded as WMM (Wireless MultiMedia), WME (Wireless Multimedia Extension)

• 802.11p -2010

– for vehicle to vehicle communications; IEEE 1609 WAVE (Wireless Access in Vehicular Environment) application protocol is relying on it

• 802.11k-2008

– radio resource management, measurements, reporting, fast AP selection

• 802.11s

– mesh networking

802.11 standards

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• Basic Service Set (BSS)

– one cell

– IBSS (Independent BSS): does not connect to anywhere

• Station (STA)

– wireless station that speaks the 802.11 protocol

– STA-s communicating in an IBSS-ben: ad-hoc mode

802.11 networks

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• Access Point (AP)

– an entity enabling a STA to access a network

• Distribution System (DS)

– network connecting APs

• Extended Service Set (ESS)

– looks like a single 802.11 network for higher layers:

• BSS

• +AP

• +DS

802.11 networks

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• Portal

– a device connecting 802.11 and another 802 LAN (typically:

Ethernet, 802.3)

– in practice: the AP contains, this is the bridge to Ethernet

• this is infrastructure mode-nak hívjuk

802.11 networks

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• Distributed Coordination Function (DCF)

– STA-s use the same simple rule to get access to the shared

radio channel, witout any central coordinator

– Carrier sense mechanism: STA measures whether the

channel is empty

• Point Coordination Function (PCF)

– the AP controls which terminal is to transmit

– based on polling

• Contention Period (CP), DCF

• Contention-free Period (CFP), PCF

• CPs and CFPs follow each other

– practice: PCF is optional in the standard, its rarely

implemented

WiFi MAC protocol

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• basic method is DCF

– Carrier Sense Multiple Access mechanism

– with Collision Avoidance (CSMA/CA)

• original Ethernet MAC: CSMA/CD (Collision Detection)

• When a MAC frame is ready, the STA willing to transmit senses the channel,

– CCA: Clear Channel Assessment signal is to be generated

• If the channel is occupied, the transmission is delayed

• Its efficient if the channel is not loaded very much

• Collision may occur: several STAs start transmitting at once

WiFi MAC protocol

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• Collision should be recognized and MAC should retransmit the frame ->thus higher layers, like TCP does not have to retransmit

– how to detect collision, or lost, corrupted frame: successful frame transfer is ACKnowledged with an ACK packet from the receiver

– missing ACK indicates the need for retransmission

• backoff mechanism: waiting mechanism

– if the channel is empty, the STA has to wait DIFS, DCF Inter

Frame Space time

– After DIFS time the STA has to wait an amount of empty timelots, determined by the backoff mechanism

– if any other STA transmits meanwhile: the end of transmission should be waited for, then DIFS, then the empty timeslots can be again counted

WiFi MAC protocol

WiFi MAC protocol

• if the channel remains empty and the number of backoff slots has

elapsed, the MAC frame can be transmitted

• receiver checks if the frame is corrupted, if not, sends ACK

– after SIFS (Short Interframe Space)

– SIFS is smaller than DIFS, hence noone can start transmitting

before the ACK

– no ACK for multicast packets

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WiFi MAC protocol

• backoff

– chosen randomly by the STA

– rand between 0 and CW (Contention Window)

• this is how many timeslots the waiting will last

• this is BC, backoff counter

– CW depends on the number of trials to transmit the given frame

• in WiFi: Exponential backoff: CW grows as powers of 2, as the

number of trials grow

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WiFi MAC protocol

• Whan random backoff is initiated?

– Listen to channel and its occupied

– no ACK

• after retransmission and no ACK, CW is growing

– After successful transmission

• to be fair

• CW is CW_min-re (7) in this case

• What is waiting?

– Meanwhile waiting: another transmission occurs

– wait until its end, wait DIFS

– then in everzy timeslot BC--

– when BC=0 send in next slot

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WiFi MAC protocol

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Virtual carrier sense

• Virtual carrier sense

– no need to alwazs listen to the channel, the STAs can go to

sleep mode

• MAC frame header has a field containing the duration of the

transmission: STAs should ot listen to te channel until this

end

• if duration is read NAV (Network Allocation Value) is set

– hidden terminal problem

• A sends to B, C does not hear, sends to B -> collision at B

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Virtual carrier sense

• Virtual carrier sense

– before transmission RTS (Request to Send)

• with recipient address

• with duration

– after SIFS the recipient answers: Clear to Send (CTS)

• both contain the duration of the forthcoming transmission

• all who heard CTS sets their NAV and don’t trz until end

• RTS Threshold

– RTS CTS handshake is not worth doing, if the frame to be sent

is itself short

• Trend: large bitrates, short packets: the frame is itself a single slot,

usuallz no need for RTS CTS

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Virtual carrier sense

• Virtual carrier sense

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MAC performance

• usually all CSMA based protocols

– ALOHA based

– I transmit, if no success I wait for random (this was ALOHA)

– intuitively: big traffic load -> lot of frames to be transmitted ->

channel often occupied -> big backoff windows -> effective

system throughput may decrease, a lot of backoff, nobody is

transmitting, everybody is backing off

• load vs. throughput curve

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Közeghozzáférési eljárás

• random access channels all have similar characteristics

• Consequence?

– If load is at a level that the curve is on the rise -> collision -> retransmission -

>higher load ->higher throughput -> succesfull transmission -> load sets back

– If load is high, such that the curve is on the decreasing end: collision ->

retransmission -> increases load -> decreased througput -> no successful

transmission -> retransmission agein -> the throughput decreases to zero

– above a given load the system overloads

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PCF mode operation

• PCF mode

– AP polls terminals, they answer

– polls, answers, ACKs follow each other by SIFS

– the beginning of CFP is signalled by AP, those who not

participate, they set their NAV

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Power save mode • Power save mode:

– Sleeping mode, on’t listen to channel

– Still it has to be available, if the AP sends something to it, how? it sleeps and does not read the address fields of the MAC frames

– AP has a list of sleeping mobiles and stores pakets arriving to them

– periodically sends so called beacon frames, in it is tells to sleeping mobiles that they have a packet

– sleeping mobiles periodically wake up to listen to the beacon frames

– then mobile wakes up and send a power-save poll message to the AP

• mobile explicitly signals to the AP that it is now awake

– the AP may send ACK (and send the packet later, the mobile has to remain awake) or sends the packet immediately

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Frame types

• Data Frame

• Control Frame

– control functions, for processes

• Management Frame

– format is like data, but the data payload is some management

information regarding some network process

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Frame types

• Data Frame

– Data + [ CF-ACK + [CF-Poll] ]

– Null Function

– CF-ACK, CF-Poll (nodata)

• Control Frame:

– RTS, CTS, ACK

– PS-Poll, CF-End, CF-End ACK

• Management Frame

– Beacon, Probe Request & Response

– Authentication, Deauthentication

– Association Request & Response

– Reassociation Request & Response

– Disassociation

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802.11 MAC frames

• MAC Frame Control:

– Protocol Version, Type and Subtype:

• e.g. RTS, CTS, ACK, poll, authentication, stb.

– ToDS, FromDS

• to or from the network (0 both for e.g. RTS, CTS),

– More Fragments

• MAC is a piece of a bigger packets and more to come

– Retry

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802.11 MAC frames

• MAC Frame Control:

– Power Management

• STA is going to sleep after this frame

– More Data

• more frames will be sent after this

– WEP

• WEP encryption

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802.11 MAC frames

• further fields

– Duration/ID:

• in Power-Save Poll Station ID, otherwise duration

– Address Fields max. 4 addresses :

• Address-1: always recipient

• Address-2: always transmitter

• Address-3:

– packet is going/coming To/From DS : intended recipient in the

network/sender in the network

• Address-4: Wireless Distribution System is used

– AP <-> AP links used as DS

– a frame is sent between AP -> AP

– In this special case ToDS=1 és FromDS=1, Address-3 és

Address-4 is the addressed and sender

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802.11 MAC frames

• Further fields

– Sequence Control

• Fragment number and sequence number: to ensure ordered

delivery and reception, even if multiple fragments are transmitted in

a frame

– FCS (Frame Check Sum) = CRC

• for error detections

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802.11e-2005 – Quality of Service

• To provide priority to delay sensitive traffic

– this is still not a guarantee

• A 802.11e modifies the operation if the MAX

– QoS capable terminal and AP

– QoS frame format

– mechanism to determine the priority of a given traffic

– how to provide priority

• wait less before counting the backoff

• use smaller CWs in case of higher priority

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802.11e-2005 – Quality of Service

• Enhanced Distributed Channel Access (EDCA)

• Providing priority

– AIFS Arbitration inter frame space: shorter for higher priority

– CW smaller for higher priority

– transmission opportunity: a duration when the STA may transmit, it it

seized the channel

• Access Categories:

– Voice (highest priority)

– Video

– Best-Effort

– Background (lowest priority)

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