B. Bellalta Mobile Communication Networks

Transcription

1 IEEE e : EDCA B. Bellalta Mobile Communication Networks

2 Scenario STA AP STA Server Server Fixed Network STA Server Upwnlink TCP flows Downlink TCP flows STA AP STA What is the WLAN cell performance with bidirectional TCP traffic? STA

3 DCF (Distributed Coordination Function) Without the RTS/CTS handshake. EQ1:Queueing delay packet arrival (HOL) 1/λ MN1 packet arrival (queued) packet departure X1 1 DATA 1/λ MN2 BUSY 2 packet departure BUSY t backoff suspension (coll.) Queue empty X1 ACK empty slot MN3 DATA ACK BUSY COLLISION t ACK COLLISION t X2 BUSY DATA backoff suspension (succ.) The time (backoff) is slotted in empty slots of duration σ [seconds]

4 Exercise 1 Markov Chain (K=4) Traffic load: A = 4.5 * 0.2 = 0.9 Erlangs Balance Equations: Steady state probabilities: p(0)=2.4419e 01 p(1)=2.1977e 01 p(2)=1.9780e 01 p(3)=1.7802e 01 p(4)=1.6022e 01 EQ = 1.79 frames

5 Exercise 1

6 Exercise 1

7 Exercise 2 Assumptions: Poisson arrivals / Exponentially distributed service times. Real case: Statistical characterization of the service time in saturated IEEE networks Zanella, A. De Pellegrini, F. Communications Letters, IEEE Publication Date: March 2005

8 Exercise 3 (a)

9 Exercise 3 (ρ=1)

10 Exercise 3 (ρ=1)

11 Exercise 3 (ρ=0.4)

12 Exercise 3 (ρ=0.4)

13 Exercise 4 & 5 4) Difficult to answer (non-linear). n : p, N, X, alfa, rho, tau, p, N,... B : rho, tau, p, N, X, alfa, rho, p=1-(1-tau)^n, tau=f(b) Impact of n > tau In terms of performance, it is better to have few nodes. 5) For example, a fixed-point approach. Others?

14 EDCA

15 15 VoIP over WLANs Voice over IP Economical alternative to traditional telephony. Wireless LANs August 2003 Broadband access network to internet. 178,000 new Skype users per day! VoIP over WLANs. A market opportunity. Complement/Substitute of current cellular networks. Technical limitations.

16 16 VoIP over WLANs Voice over IP Economical alternative to traditional telephony. Wireless LANs Broadband access network to internet. VoIP over WLANs. A market opportunity. Complement/Substitute of current cellular networks. Technical limitations. August ,000 new Skype users per day!

17 VoIP over WLANs Limitations for VoIPoWLAN (and other real time services) Protocol overheads. Capacity varying channels (multi rate). Random Access MAC. Bandwidth penalty due to the distributed access. Unfairnes Downlink/Uplink. Difficult coexistence between heterogeneous (rigid and elastic) flows. Hand off / Roaming between WLANs. 17

18 18 VoIP over WLANs Example: Rdata = 2 Mbps, Rbasic = 1 Mbps, VoIP codec: G.729 (8 Kbps) Downlink/Uplink TCP flows simultaneous VoIP calls TCP packets ACK packets ACK packets TCP packets VoIP call ACK packets TCP packets VoIP call IEEE Solution: EDCA (Enhanced Distributed Channel Access): Traffic differentiation at MAC layer + CAC optional. EDCA is too conservative. Static Traffic Differentiation without Admission Control.

19 19 Multimedia Traffic Flows General Classification (queue occupation). Elastic flows (TCP based): use all available bandwidth, fair with other elastic flows. saturated source/queue with : B L/X, ρ =1 Streaming/Rigid flows (UDP based): use only the bandwidth required. finite load (un saturated) source/queue with: B ρ L/X, ρ 1 User Perception Rigid Traffic (UDP) Good Bad Minimum Bandwidth Video/audio streaming Voice over IP B Multimedia User Perception Elastic Traffic (TCP) Better Web (HTTP) FTP, e mail,... Worst B

20 The DCF/EDCA model

21 21 The scenario MN B=200 Kbps MN B=100 Kbps MN MN1 MN B=120 Kbps AP B=450 Kbps Be=max. av. MN B=max. av. MN B=max. av. MN MN2 B=max. av. MN B=80 Kbps MN and AP share the channel using a random access MAC protocol.

22 22 IEEE MAC Model (DCF) Each node is modelled by a M/M/1/K queue with packet arrival rate λ k (α = λ) and service time X. Assumption: Poisson arrivals and service time exponentially distributed. The model accounts for: queuing delays, packet losses. λ k X K EQ1:Queueing delay packet arrival (HOL) 1/λ MN1 packet arrival (queued) packet departure X1 1 DATA Queue empty X1 ACK 1/λ MN2 packet departure BUSY 2 DATA ACK BUSY COLLISION t ACK COLLISION t X2 BUSY backoff suspension DATA

23 23 IEEE MAC Model (DCF) Each node is modelled by a M/M/1/K queue with packet arrival rate λ k and service time X. Assumption: Poisson arrivals and service time exponentially distributed. The model accounts for: queuing delays, packet losses. λ k X K EQ1:Queueing delay packet arrival (HOL) 1/λ MN1 packet arrival (queued) packet departure X1 1 DATA Queue empty X1 ACK 1/λ MN2 packet departure BUSY 2 DATA ACK BUSY COLLISION t ACK COLLISION t X2 BUSY backoff suspension DATA

24 24 DCF EDCA EQ1:Queueing delay packet arrival (HOL) 1/λ MN1 packet arrival (queued) packet departure X1 1 DATA ACK BUSY 2 empty slot packet departure BUSY t backoff suspension (coll.) Queue empty X1 1/λ MN2 MN3 DATA ACK BUSY COLLISION t ACK COLLISION t X2 BUSY DATA backoff suspension (succ.) BEB AIFS TXOP

25 25 IEEE e MAC Model (EDCA) : TXOP Multiple packet transmission each time a MN/AP gets the channel The overall network throughput increases (S=Data/Time [bps]) Less wasted time in random access. Packet arrival MN 1 Packet arrival (Queued) DATA Packet arrival Packet arrival busy DATA ACK t DATA ACK busy DATA Packet arrival (Queued) DATA Packet arrival MN 2 Packet arrival (Queued) busy MN 2 MN 1 ACK No TXOP Service Time (X) ACK GAIN Packet arrival (Queued) busy ACK busy ACK DATA ACK TXOP t GAIN Service Time (X) DATA DATA t ACK t

26 26 IEEE e MAC Model (EDCA) : TXOP Each user is modeled as an M/G[1,B]/1 queue (bulk service time queue) TXOP (Burst length): maximum consecutive number of frames transmitted at each successful attempt i=b Renewal Analysis j K Departure distribution. Average TXOP length Average TXOP duration Stationary (equil.) distribution. Blocking Probability. Queue occuppation. Queuing delay.

27 27 IEEE e MAC Model (EDCA): AIFS AIFS MN1 BUSY BUSY BUSY BUSY BUSY t Extra blocked slots due to AIFS Aggregate load from other nodes MN1 BUSY Service Time BUSY BUSY BUSY BUSY Transmission Prob. t

28 IEEE e MAC Model (EDCA): EB, p Expected BackOFF slots Cond. Collision Probability Prioritizing a flow: TXOP AIFS CWmin This model is able to capture the impact of the other flows with different MAC parameters. 28

29 29 Wireless Scenario Single Hop Ad Hoc network. No hidden terminals and ideal channel conditions. Channel access based on the DCF (BA and RTS/CTS). Two types of flows: streaming (UPD like) and elastic (TCP like). 2 1 i n 3 n 1

30 30 Results Single cell WLAN (ad hoc) Two types of flows: S1 (streaming ~ unsaturated) Bs = 100 Kbps L = 400 bytes E1 (elastic ~ saturated) Be (network dependant) L = 1500 bytes Default WLAN MAC parameters 0 S1 flows. Solution (1): AIFS (elastic flows, A=3) 8 S1 flows The TCP throughput is reduced. S1 saturation point

31 Model Validation Single cell WLAN (ad hoc) Two types of flows: S1 (streaming ~ unsaturated) E1 (elastic ~ saturated) Bs = 100 Kbps L = 400 bytes Be (network dependant) L = 1500 bytes Solution (2): TXOP S1 (=4) 7 S1 flows. CWmin,E1 (=64) 6 S1 flows. 31

32 32 Call Admission Control

33 Problem statement: Providing QoS Flow level Admission Control. Adaptive selection of MAC parameters. 33

34 34 Problem statement: Providing QoS Flow level Admission Control. Adaptive selection of MAC parameters. maximize performance (under the QoS) random access MAC (Performance)=f(MAC parameters, number of nodes, traffic profile each node) non-linear relation

35 35 The available bandwidth estimation problem Ideal Channel Sharing Random Access Channel Sharing Bandwidth Bandwidth Flow rejected Available bandwidth Bandwidth penalty Flow rejected Available bandwidth n... the bandwidth penalty due to the distributed access depends on the number of flows active and the traffic profile of each flow.... how to decide if a flow can be admited? We have to compute or to test if the new flow is possible. n

36 Multi rate Problems on VoIP capacity 36

37 Transmission Rates PHY Rate: The PHY Preamble and Header are transmitted at this rate (lowest) Basic Rate: Maximum rate common to all STAs Control frames are transmitted at BASIC rate. MAC Headers are transmitted at BASIC rate. Multiple Data Rates: Rate at which data is transmitted Based on the channel conditions of the STA / AP. Good: Higher rates (less redundancy bits, higher modulations) Bad: Lower rates (more redundancy bits, lower modulations)

38 Multi rate transmission (Multiple data rates) 1 Mbps 2 Mbps 5.5 Mbps 11 Mbps Direct relationship between communication rate and the channel quality required for that rate As distance increases, channel quality decreases Therefore: tradeoff between communication range and link speed Multi rate provides flexibility to meet both consumer demands Coverage Speed (high data rates)

39 Rate (Mbps) Efficiency MAC Overhead 11.0 Data Medium Time (milliseconds)

40 Auto Rate Protocols Selects the rate to use for a packet ARF Adaptive based on success/failure of previous packets Simple to implement Doesn t require the use of RTS CTS or changes to spec Receiver Based Auto Rate (RBAR) Uses SNR measurement of RTS to select rate Faster & more accurate in changing channel Requires some tweaks to the header fields Opportunistic Auto Rate (OAR) Adds packet bursting to RBAR Allows nodes to send more when channel conditions are good Implements temporal fairness instead of packet fairness

41 IEEE a Rates

42 IEEE a Ideal rate, acceptable BER

43 43 Multirate wireless 11Mbps 11Mbps AP 11Mbps 11Mbps etc

44 44 Multirate wireless 1Mbps 11Mbps AP 11Mbps 11Mbps etc

45 45 Problem Statement While total cell capacity is around 12 calls when all of them transmit at 11Mbps this capacity falls with any change from fast to slow flows Distribution of VoIP flows in a cell (G.711 codec)

46 Multihop (Mesh) Networks VoIP capacity 46

47 Hidden Terminal Problem (a) A C Data Frame A transmits data frame B (b) Data Frame A B C senses medium, station A is hidden from C Data Frame C C transmits data frame & collides with A at B

48 CSMA with Collision Avoidance (RTS/CTS) (a) B RTS C A requests to send (b) CTS B A CTS C B announces A ok to send (c) Data Frame A sends B C remains quiet

49 CSMA/CA (optional) RTS Data Source SIFS CTS SIFS SIFS Ack Destination DIFS NAV (RTS) Other NAV (CTS) NAV (Data) Defer access

50 50 VoIP Capacity in a mesh network VoIP calls (G.279) MP MP MP MP Single channel 2 Mbps Channel Contention (MAC) hidden nodes D. Niculescu. S. Ganguly, K. Kim, R. Izmailov; Performance of VoIP in a Wireless Mesh Network. IEEE Infocom 2006, Barcelona, Spain.

51 51 VoIP Capacity in a mesh network VoIP calls A MP MP MP MP Multiple channels (Two channels) Channel Contention (MAC) hidden nodes VoIP calls B MP MP MP MP NO Channel Contention (MAC) NO hidden nodes Multiple channels (Three channels)

52 VoIP Capacity in a mesh network D. Niculescu. S. Ganguly, K. Kim, R. Izmailov; Performance of VoIP in a Wireless Mesh Network. IEEE Infocom 2006, Barcelona, Spain. 52