IEEE 802.11 standards Dr. Fazekas Péter Balogh András BME-HIT
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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