Multimedia Communication Services Traffic Modeling and Streaming

Transcription

1 Multimedia Communication Services Traffic Modeling and Streaming Medium Access Control algorithms Introduction to IEEE Università degli Studi di Brescia A.A. 2014/2015 Francesco Gringoli Master of Science in Communication Technologies and Multimedia

2 IEEE Wireless Lan: Infrastructure Mode Standard : 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 DS Traffic Modeling and Streaming 2

3 IEEE Wireless Lan standard Many PHYs: Infrared (never implemented) 2.4GHz Industrial, Scientific and Medical (ISM) Interferences possible with Bluetooth and microwave ovens 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) Traffic Modeling and Streaming 3

4 IEEE Wireless Lan standard/2 Standard: evolution and supported data rates 1997: (802.1y), R LEGACY =[1Mb/s, 2.4GHz; B/QPSK 1999: b, R CCK 2.4GHz; CCK; supports R LEGACY 2001: a, R OFDM 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 T GI = 800ns, data takes T FFT = 3200ns FEC: packet bits are expanded inside symbols Tratto da [1] E.g., 54Mb/s Coding is 3/4 from 216b to 288b Traffic Modeling and Streaming 4

5 IEEE Wireless Lan standard/3 Standard: evolution and supported data rates 2003: g, R=[R LEGACY,R CCK,R OFDM 2.4GHz; QPSK, CCK, OFDM Same rates as b + OFDM encoding (called 2.4GHz Similar to a: additional pause (6µs) after tail (signal extension) 2009: n Same rates as OFDM ( b) 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: ac, more bw, more optional, exceeds 1Gb/s! Traffic Modeling and Streaming 5

6 IEEE bg: insight 2.4GHz In ISM band there are 79(+2) 1MHz subchannels Each b/g channel covers 22MHz Standard places 13 channels spaced of 5MHz + channel 2484MHz E.g.,: 2412MHz, 2417MHz,, 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! Traffic Modeling and Streaming 6

7 IEEE n: Channel Bonding Two adjacent channels may be bonded, bw raises to 40MHz Rate double (more than 2x, signaling overhead does not double!) From [2] Traffic Modeling and Streaming 7

8 IEEE n: 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! From [2] Traffic Modeling and Streaming 8

9 IEEE b: 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.: b Long preamble SYNC 128 bit SFD 16 bit SIGNAL 8 bit SERVICE 8 bit LENGTH 16 bit CRC 16 bit DBPSK modulation PLCP Preamble 144 bit PLCP Header 48 bit 1, 2, 5.5 o 11Mb/s From [1] ΔT PLCP = 192µs!! PPDU Traffic Modeling and Streaming 9

10 IEEE b: frame format/2 E.g.: Acknowledgement, PSDU:=10byte + 4byte CRC32 = PLCP:=192µs, PSDU:= PLCP:=192µs, PSDU:= PLCP:=192µs, PSDU:= PLCP:=192µs, PSDU:= 11µs Problem with overhead: PLCP too long!! b introduces Short PLCP:=72bit(@1Mb/s)+48bit(@2Mb/s) = 96µs Short SYNC Short SFD 56 bit 16 bit 1Mb/s SIGNAL 8 bit SERVICE LENGTH 8 bit 16 bit 2Mb/s CRC 16 bit Short PLCP Preamble 72 1Mb/s Short PLCP Header 48 2Mb/s 5.5, or 11Mb/s From [1] PPDU Traffic Modeling and Streaming 10

11 IEEE g: frame format/ g: 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 From [1] Traffic Modeling and Streaming 11

12 IEEE g: frame format/ g: 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 From [1] Traffic Modeling and Streaming 12

13 IEEE g: frame format/ g: data payload Bit expanded by convolutional encoder for FEC, R = [1/2, 2/3, 3/4] Groups of N CBPS (Coded Bit Per Symbol) or N DBPS (Data Bit Per Symbol) Each subcarrier transport N BPSC bit (Bit Per SubCarrier) Modulation R N BPSC N CBPS N DBPS Data rate BPSK 1/ BPSK 3/ QPSK 1/ QPSK 3/ QAM 1/ QAM 3/ QAM 2/ QAM 3/ Traffic Modeling and Streaming 13

14 IEEE g: frame format/6 E.g.: Acknowledgement, PSDU:=10byte + 4byte CRC32 = 112bit PLCP: 20µs DATA PSDU : 16b(SERVICE)+112b(PSDU)+6b(tail min )=134b Data rate PLCP bit symbol ΔT Extension Total 6 20µs µs 6µs 50µs 9 20µs µs 6µs 42µs 12 20µs µs 6µs 38µs 18 20µs µs 6µs 34µs 24 20µs µs 6µs 34µs 36 20µs µs 6µs 30µs 48 20µs µs 6µs 30µs 54 20µs µs 6µs 30µs Traffic Modeling and Streaming 14

15 IEEE : 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 Traffic Modeling and Streaming 15

16 IEEE : 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 From [1] Traffic Modeling and Streaming 16

17 IEEE : 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 Traffic Modeling and Streaming 17

18 IEEE : PSDU fields/3 Example: From [1] Traffic Modeling and Streaming 18

19 IEEE : 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 Traffic Modeling and Streaming 19

20 IEEE : 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 N A 01 BSSID SA DA AA AA IP FCS Traffic Modeling and Streaming 20

21 IEEE : 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): P e 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 Traffic Modeling and Streaming 21

22 IEEE : 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 Traffic Modeling and Streaming 22

23 IEEE : PCF/2 time Traffic Modeling and Streaming 23

24 IEEE : PCF/3 and new techniques Attention: Actually they never implemented it Outdated by e 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 aa New MAC for delivering multicast (groupcast) frames, e.g., video Deeply based on n Block Ack (frames acked in group) Traffic Modeling and Streaming 24

25 IEEE : 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!) Traffic Modeling and Streaming 25

26 IEEE : 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 Traffic Modeling and Streaming 26

27 IEEE : 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 Traffic Modeling and Streaming 27

28 IEEE : rate choice/2 Example: UDP packet TX RX backoff RC set up these 7 attempts: [54Mb/s {1,2},48Mb/s {3,4},12Mb/s {5},1Mb/s {6,7} ] tempo backoff backoff KO KO OK KO OK OK Try 1 Try 2 Try 3 Try 4 backoff At the end of this packet, RC refreshes its table Rate Success Failure /3004 (93%) 192/3004 (7%) /507 (80%) 99/507 (20%) /402 (25%) 300/402 (75%) Rate Success Failure /3006 (93%) 194/3006 (7%) /509 (80%) 100/509 (20%) /402 (25%) 300/402 (75%) Don t change decision (not now ) Traffic Modeling and Streaming 28

29 IEEE n: rate choice b/g: RC chooses from 4+8 possible, from 1 to 54Mb/s n: RC may choose from (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! Tratto da [2] Traffic Modeling and Streaming 29

30 Bibliography [1] IEEE , Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, June [2] Tutorial on n from Cisco: [3] G. Bianchi, Performance analysis of the IEEE distributed coordination function. IEEE Journal on Selected Areas in Communications, 18(3), pp , Traffic Modeling and Streaming 30