multimedia communication...

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

Traffic Modeling and Streaming 2

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)


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!


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!


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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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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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!!


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

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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


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

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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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 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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 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)


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


 E.g.: Acknowledgement, PSDU:=10byte + 4byte CRC32 = 112bit

 PLCP: 20µs

 DATAPSDU: 16b(SERVICE)+112b(PSDU)+6b(tailmin)=134b


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!


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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 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



 Example:


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 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


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


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


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


IEEE 802.11: PCF/2


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)


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!)


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


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


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 )


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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