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Università degli Studi di Brescia A.A. 2014/2015
Francesco Gringoli
Master of Science in Communication Technologies and Multimedia
Multimedia Communication Services
Traffic Modeling and Streaming
Medium Access Control algorithms
Introduction to IEEE 802.11
IEEE 802.11 Wireless Lan: Infrastructure Mode
Standard 802.11: “cell based”
Each cell is a “Basic Service Set” (BSS) – Controlled by an Access Point (AP), may be connected to wired network
– Stations in the same cell communicates through the AP
More cells form an “Extended Service Set” (ESS) – In each cell, AP works as a Bridge for the other cells
– ESS it’s a single wide 802 network (Broadcast domain)
– Distribution System (DS) connect all APs
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DS
IEEE 802.11 Wireless Lan standard
Many PHYs: – Infrared (never implemented)
– RF @ 2.4GHz Industrial, Scientific and Medical (ISM) • Interferences possible with Bluetooth and microwave ovens
– RF @ 5GHz Unlicensed National Information Infrastructure (UNII)
Channel bandwitdh (standards pre 2014): 20MHz, 40MHz
Several modulations: – Quaternary Phase Shift Keying (QPSK), up to 2Mb/s
– Complementary Code Keying (CCK), up to 11Mb/s
– Orthogonal Frequency Division Multiplexing (OFDM), up to 150Mb/s
Multi-streams at the same time (MIMO): up to 600Mb/s (x4)
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IEEE 802.11 Wireless Lan standard/2
Standard: evolution and supported data rates – 1997: 802.11(802.1y), RLEGACY=[1Mb/s, 2Mb/s], @ 2.4GHz; B/QPSK
– 1999: 802.11b, RCCK=[5.5,11]Mb/s, @ 2.4GHz; CCK; supports RLEGACY
– 2001: 802.11a, ROFDM=[6,9,12,18,24,36,48,54]Mb/s, @ 5GHz; OFDM
• OFDM: signal transmitted over 52 orthogonal subcarriers: 48 data, 4 pilots
• Each subcarrier transmits a symbol or N bits of the original packet
• Symbols form a macro-symbol whose duration is T = 4µs
• E.g.: macro-symbol:=216b, DataRate = 216bit/4µs = 54Mb/s
• Macro-symbol separated by guard interval
– Against fading TGI = 800ns, data takes TFFT = 3200ns
• FEC: packet bits are expanded inside symbols
– E.g., 54Mb/s⇒Coding is 3/4⇒from 216b to 288b
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IEEE 802.11 Wireless Lan standard/3
Standard: evolution and supported data rates – 2003: 802.11g, R=[RLEGACY,RCCK,ROFDM], @ 2.4GHz; QPSK, CCK, OFDM
• Same rates as 802.11b + OFDM encoding (called ERP-OFDM) @ 2.4GHz
• Similar to 802.11a: additional pause (6µs) after tail (signal extension)
– 2009: 802.11n
• Same rates as OFDM (+802.11b) with many “optionals”
– More data subcarriers, from 48 to 52: 54Mb/s 58.5Mb/s (=54/48*52)
– FEC more efficient: 5/6⇒240b instead of 216b⇒ 58.5Mb/s 65Mb/s
– Guard Interval halved: symbol takes 3.6µs ⇒ 65Mb/s 72.2Mb/s
– Channel bonding: two channel of 20MHz together ⇒ 72.2Mb/s 150Mb/s(?)
– Multi-streams: up to 4 stream ⇒ 150Mb/s 600Mb/s
– Today: 802.11ac, more bw, more “optional”, exceeds 1Gb/s!
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IEEE 802.11bg: insight 2.4GHz
In ISM band there are 79(+2) 1MHz subchannels – Each 802.11b/g channel covers 22MHz
– Standard places 13 channels spaced of 5MHz + channel 14 @ 2484MHz • E.g.,: ch1 @ 2412MHz, ch2 @ 2417MHz, …, ch13 @ 2472MHz
– Regulatory domain, defines how channels are allocated worldwide • E.g., USA [1-11], Italy [1-13], Japan only channel 14.
– Problem: each channel (22MHz) wider than channel spacing (5MHz) • Only 3 to 4 channels (22MHz) may be used at the same time!
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IEEE 802.11n: Channel Bonding
Two adjacent channels may be “bonded”, bw raises to 40MHz
– Rate double (more than 2x, signaling overhead does not double!)
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From
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IEEE 802.11n: MIMO
Multiple Input Multiple Output – Both tx’er and rx’er have multiple RF sections – Multipath with a single radio is a problem, with MIMO is useful!! – Each tx’er antenna transmits a part of the overall signal (a stream)
• Streams arrive at different rx’er antennas with different phases • Streams can be separated!
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From
[2]
IEEE 802.11b: frame format/1
Frame includes:
– PLCP: for receiver synchronization
• PLCP:= Physical Layer Convergence Procedure
– PSDU: PLCP Service Data Unit, payload with data
– PPDU:=PLCP Protocol Data Unit = PLCP + PSDU
E.g.: 802.11b
– Long preamble
– 192bit @1Mb/s
• DBPSK modulation
• ΔTPLCP = 192µs!!
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SYNC 128 bit
SFD 16 bit
SIGNAL 8 bit
SERVICE 8 bit
LENGTH 16 bit
CRC 16 bit
PLCP Preamble 144 bit
PLCP Header 48 bit
PSDU @ 1, 2, 5.5 o 11Mb/s
PPDU
From
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IEEE 802.11b: frame format/2
E.g.: Acknowledgement, PSDU:=10byte + 4byte CRC32 = 112bit – @1Mb/s, PLCP:=192µs, PSDU:= 112µs – @2Mb/s, PLCP:=192µs, PSDU:= 56µs – @5.5Mb/s, PLCP:=192µs, PSDU:= 21µs – @11Mb/s, PLCP:=192µs, PSDU:= 11µs
Problem with overhead: PLCP too long!! – 802.11b introduces Short PLCP:=72bit(@1Mb/s)+48bit(@2Mb/s) = 96µs
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Short SYNC 56 bit
Short SFD 16 bit
SIGNAL 8 bit
SERVICE 8 bit
LENGTH 16 bit
CRC 16 bit
Short PLCP Preamble 72 bit @ 1Mb/s
Short PLCP Header 48 bit @ 2Mb/s
PSDU @2, 5.5, or 11Mb/s
PPDU
DQPSK @ 2Mb/s DBPSK @ 1Mb/s
From
[1]
IEEE 802.11g: frame format/3
802.11g: new PLCP, very short – PLCP Preamble: rx’er synchronization (sig detect, diversity selection etc)
– SIGNAL contains PSDU rate and length in byte
– SERVICE: initialization scrambling sequence
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From
[1]
IEEE 802.11g: frame format/4
802.11g: frame includes – PLCP Preamble made of 10 short symbols (8µs) + 2 long ones (8µs) – PLCP Header made of
• SIGNAL field in its own symbol (4µs, PSDU length+rate)
• SERVICE field, first 16 bits of first data symbol
– PSDU made of symbols, each one carrying N bits, N depends on Rate
– PSDU terminate with CRC32 and at least 6 padding bits
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From
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IEEE 802.11g: frame format/5
802.11g: data payload – Bit expanded by convolutional encoder for FEC, R = [1/2, 2/3, 3/4] – Groups of NCBPS (Coded Bit Per Symbol) or NDBPS (Data Bit Per Symbol)
– Each subcarrier transport NBPSC bit (Bit Per SubCarrier)
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Modulation R NBPSC NCBPS NDBPS Data rate
BPSK 1/2 1 48 24 6
BPSK 3/4 1 48 36 9
QPSK 1/2 2 96 48 12
QPSK 3/4 2 96 72 18
16-QAM 1/2 4 192 96 24
16-QAM 3/4 4 192 144 36
64-QAM 2/3 6 288 192 48
64-QAM 3/4 6 288 216 54
IEEE 802.11g: frame format/6
E.g.: Acknowledgement, PSDU:=10byte + 4byte CRC32 = 112bit
PLCP: 20µs
DATAPSDU: 16b(SERVICE)+112b(PSDU)+6b(tailmin)=134b
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Data rate PLCP bit symbol ΔT Extension Total
6 20µs 134 6 24µs 6µs 50µs
9 20µs 134 4 16µs 6µs 42µs
12 20µs 134 3 12µs 6µs 38µs
18 20µs 134 2 8µs 6µs 34µs
24 20µs 134 2 8µs 6µs 34µs
36 20µs 134 1 4µs 6µs 30µs
48 20µs 134 1 4µs 6µs 30µs
54 20µs 134 1 4µs 6µs 30µs
IEEE 802.11: PSDU types
Three types:
– Management, e.g. Association Request/Response, Beacon, Auth/Deauth
• Network Advertisement, BSS Join, Authentication etc
– Control, e.g. ACK, RTS, CTS, Poll, etc
• For channel access (RTS, CTS), positive frame acknowledgment
– Data: Plain data + QoS Data, etc
• Frames with user data
– PSDU fields: depend on frame type!
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Frame Control
Duration ID
Address 1 Address 2 Address 3 Sequence Control
Address 4 QoS Control
Frame Body
FCS
2 2 6 6 6 2 6 2 0-2312 4
IEEE 802.11: PSDU fields/1
Frame Control: – Protocol version, only 0 today
– Type and Subtype encode frame type + subtype
– ToDS: frame is for Distribution System; FromDS frame is from DS • If both set to 1, frame is transported by a Wireless DS
– More: announce other fragments are coming (PSDU is fragmented)
– Retry: help rx’er understanding this is a retransmission
– {Pwr Mgt, More Data} deal with power management, save
– Protected: announce Frame Body is encrypted
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From
[1]
IEEE 802.11: PSDU fields/2
Duration/ID – Meaning depends on PSDU type
– Data: number of µs after frame end during which medium is reserved
• Used by Virtual Carrier Sense
Address fields: they depends on ToDS/FromDS fields: – BSSID: Basic Service Set IDentification
• Address of the joined AP
– DA: Destination Address, “final destination”
– RA: Receiver Address, immediate frame destination
– SA: Source Address, who has generated this frame
– TA: Transmitter Address, who has forwarded this frame
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IEEE 802.11: PSDU fields/3
Example:
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From
[1]
IEEE 802.11: PSDU fields/4
Sequence Control:
– Fragment number, 4 bits
• For fragmented PSDU, it’s the number of this fragment
– Sequence Number, 12 bits, unique for PSDU
• Identify the PSDU (used by rx’er to avoid accepting same frame > once)
QoS Control: identify Traffic Category (check the standard)
FCS: CRC/32 Frame Check Sequence protecting the PSDU
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IEEE 802.11: PSDU types E.g., Data Frame
IP packet, no QoS, from STA to AP, N byte
– Encapsulated into a data frame
– Logical Link Control (LLC) encapsulation is used
• 8 bytes before IP: 0xAA, 0xAA, 0x03, 0x00, 0x00, 0x00, 0x08, 0x00
– Type: Data frame (0x02) & Subtype: 0 ⇒ Byte#0 FC := 0x08
– ToDS ⇒ Byte#1 FC := 0x01
– Duration: time to tx and ACK + SIFS
– Address: it’s a ToDS frame
– SeqCTRL: frag. no:=0, seq. no:=33 ⇒ SeqCTRL:=0x0210
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ethertype
08 01 3A 01 BSSID SA DA 10 02 IP FCS
2 2 6 6 6 2 N 4
AA AA 03 00 00 00 08 00
8
IEEE 802.11: Fragmentation
Ethernet: packets up to 1518byte (No jumbo frames!)
Wireless Lan: good reasons for using shorter packets – Very high Bit Error Rate (wrt to 802.3): Pe ∝ packet length
– If error, retransmitting shorter packets means less overhead
To maintain compatibility with Ethernet at LLC layer – Use a fragmentation mechanism at the MAC layer (below LLC)
Send-and-Wait: station transmits the same fragment – Until ack for the last (re)sent is received
– If a single fragment is retransmitted too many times, drop the packet
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IEEE 802.11: Point Coordination Function
PCF used instead of DCF for “time bounded” traffic
– E.g., voice or video traffic
AP gain access before other stations because of shorter wait
– Do not obey DIFS, wait for Point coordination IFS (PIFS)
For handling unicast traffic in a Master-Slave fashion
– AP is the Master and polls Slaves, i.e., Stations
– MAC is managed with deterministic multiplexing of traffic
• E.g., stations transmit in turn
Between adjacent PCF period, use DCF
– E.g., to transmit Best Effort traffic
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IEEE 802.11: PCF/2
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time
IEEE 802.11: PCF/3 and new techniques
Attention:
– Actually they never implemented it
– Outdated by 802.11e
• New MAC based on polling
Hybrid Coordination Function Controlled Channel Access (HCCA)
• Each Beacon interval divided into two parts
– Contention based access (DCF)
– Contention free access (HCCA)
– Outdated by 802.11aa
• New MAC for delivering multicast (groupcast) frames, e.g., video
– Deeply based on 802.11n Block Ack (frames acked in group)
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IEEE 802.11: Synchronization
Stations in a BSS requires synchronization because tx slotted
Station clock refreshed at every beacon (e.g., every 100ms)
– Beacon contains copy of AP clock when it was transmitted
– Stations simply copy this time info into their internal clock register
– Without this mechanism clocks will skew out in a while (cheap devices!)
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IEEE 802.11: Ad-hoc Networks
Standard defines AP less networks
– E.g., for exchanging documents between a couple of laptops
The first station that starts the cell will act as the AP
– Beacon generation
– Clock synchronization
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IEEE 802.11: rate choice/1
How to choose the rate is not specified by the standard – Rate Controller algorithm: RC
RCs use feedback based techniques
E.g., Minstrel algorithm, it’s the default today in Linux kernel – Count total frames transmitted PER every rate, assess success probability
– Rate that has best success delivery ratio is the winner
– Periodically (every N frames) send a frame at a test rate
• Constantly scan the entire rate set
– Rely on frames that require ACK, by counting:
• Number of attempts per packet
• Failed rate, success rate
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IEEE 802.11: rate choice/2
Example: UDP packet – RC set up these 7 attempts: [54Mb/s{1,2},48Mb/s{3,4},12Mb/s{5},1Mb/s{6,7}]
– At the end of this packet, RC refreshes its table…
• Don’t change decision (not now )
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tempo TX
RX KO KO OK OK OK KO
back
off
back
off
back
off
back
off
Try 1 Try 2 Try 3 Try 4
54 54 48 48
Rate Success Failure 54 2812/3004 (93%) 192/3004 (7%)
48 408/507 (80%) 99/507 (20%)
36 102/402 (25%) 300/402 (75%)
Rate Success Failure 54 2812/3006 (93%) 194/3006 (7%)
48 409/509 (80%) 100/509 (20%)
36 102/402 (25%) 300/402 (75%)
IEEE 802.11n: rate choice
802.11b/g: RC chooses from 4+8 possible, from 1 to 54Mb/s
802.11n: RC may choose from 4+8+64 (two streams only!) – Modulation Coding Scheme add 8 rates with 8 different configurations
• They are 8 basic encoding (52 subcarriers instead of 48) combined with {1/2 stream, channel bonding, long/short GI} that is 8 possibilities
• Actually this lead to only 55 different Data Rates over 76!
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Bibliography
[1] IEEE 802.11-2007, Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, June 2007.
[2] Tutorial on 802.11n from Cisco: http://www.wireshark.ch/download/Cisco_PSE_Day_2009.pdf
[3] G. Bianchi, “Performance analysis of the IEEE 802.11 distributed coordination function”. IEEE Journal on Selected Areas in Communications, 18(3), pp. 535-547, 2000.
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