Mobile Communication Networks. (Intro) Wireless Local Area Networks

Transcription

1 (Intro) Wireless Local Area Networks Boris Bellalta Part of the material has been obtained from: Leon Garciat et al.; Communication Networks. Fundamental Concepts and Key architectures McGraw Hill, 2/e

2 Mobile Internet Applications Johan Zuidweg (4th part) IMS Miquel Oliver (3rd part) SIP Anna Sfairopoulou (2nd part) TCP/UDP IP LINK WLANs PHY Boris Bellalta (1st part)

3 Report: Important Dates January 15 (deliver by e mail: boris.bellalta@upf.edu) Selected topic (1 student, 1 topic) + short explanation about it List of 5 references and a short abstract of each one (~10 lines). January 25 (deliver by e mail: boris.bellalta@upf.edu) Short presentation (12 slides, 2 slides x reference + 2 for the summary). February 4 February 18, (deliver by e mail: boris.bellalta@upf.edu) Final report (5 pages, without references and with less than 5 figures).

4 Topic/Slides Ahmad Afouneh: IEEE e Carlos Cabrera: Wireless Mesh Networks Andy Foster: IEEE n Urmass Kuresson: IEEE e Pablo Miguel Saval: IEEE n (slides?) Jelena Skulic: Multi user MIMO in WLANs (broadcast)...

5 Guidelines for the report IEEE e / EDCA Is EDCA needed? How it works? Does it guarantee any QoS? What performance gains can be achieved vs DCF? IEEE n How the higher data rates are achieved? What are the new mechanisms at MAC and PHY layer? What performance gains can be achieved vs b/a/g? Wireless Mesh Networks Overview of the IEEE s amendment. Advantages of Wireless Mesh Networks. Considerations for Wireless Mesh Networks deployments. Mu MIMO in WLANs (Broadcast Channel) Benefits of Multi user MIMO in broadcast channels. How it can be implemented in WLANs and what benefits can be obtained in terms of throughput, frame delay, contention mitigation, etc.?

6 Starting Point Wireless Networks as an Access Technology to Internet Internet Protocol (IP) everywhere

7 Some Internet Objects

8 Wireless Access Networks Access Points = {WIFI, WIMAX, GSM/GPRS, UMTS, HSDPA, LTE...}

9 Internet Structure

10 What's the Internet: a service view The Network provides communication services to applications. The different services are characterized by their capability to provide: Data loss Timing (delay) Throughput Security What services provide the current Internet? IP is a best effort (there are no loss & delay guarantees) A reliable service (based on TCP) A non reliable service (based on UDP)

11 Types of Traffic vs User perception Elastic flows (TCP based): use all the available bandwidth, fair. Streaming/Rigid flows (UDP based): use only the required bandwidth. User Perception Rigid Traffic (UDP) Video/audio streaming Voice over IP Good Bad Minimum Bandwidth B Multimedia User Perception Elastic Traffic (TCP) Better Web (HTTP) FTP, e mail,... Worst B

12 Transport service requirements of common apps

13 Internet Protocol Stack application: supporting network applications FTP, SMTP, HTTP transport: process process data transfer TCP, UDP network: routing of datagrams from source to destination IP, routing protocols link: data transfer between neighboring network elements Ethernet, WLAN physical: bits on the wire/wireless

14 Some points (SIP/IMS from the TCP/IP stack) SIP can be seen as an application protocol. The IP mobile network elements operate in the application layer and use the TCP/IP stack to interchange messages.

15 Wireless Local Area Networks

16 Introduction Significant growth of IEEE (DCF) Wireless LAN in last years. Advantages: A broadband wireless access to Internet, mobility, low cost and fast deployment. Current limitations: Only best effort services (no QoS mechanisms). Performance problems (not efficient use of resources) with simultaneous TCP like and UDP like traffic flows.

17 Introduction Advantages Easy, low cost deployment Mobility & roaming: Access information anywhere Supports personal devices PDAs, laptops, data cell phones Supports communicating devices Cameras, location devices, wireless identification

18 WLAN standards

19 WLAN cards Radio Hardware PC Card Hardware frame format WMAC controller with Station Firmware (WNIC STA) Radio Hardware PC Card Hardware frame format WMAC controller with Access Point Firmware (WNIC AP) frame format frame format Driver Software (STADr) Platform Computer Ethernet V2.0 / frame format Driver Software (APDr) Bridge Software Ethernet V2.0 / frame format Kernel Software (APK) Protocol Stack Ethernet Interface STA Bridge Hardware AP

20 BSS (Basic Service Set) BSS

21 BSS (Basic Service Set) Bridge learn table STA 1 STA Association table STA 1 BSS A STA 2 Inter BSS Relay Associate ACK STA 1 Packet for STA 2 ACK Associate Packet for STA 2 STA 2

22 IBSS (Independent Basic Service Set) IBSS

23 EBSS (Extended BSS) BSS Di st r Sy ibu st tio em n BSS

24 EBSS e Backbon Bridge learn table STA 2 1 STA 1 2 Bridge learn table STA 2 2 STA 1 1 Wireless PC Card Association table Wireless PC Card STA 2 Association table STA 1 Packet for STA 2 Packet for STA 2 ACK ACK BSS B STA 1 BSS A STA 2

25 Wireless Distribution System (WDS) BSS Di st r Sy ibu st tio em n BSS

26 WDS (Wireless Distribution Subsystem) Bridge learn table Bridge learn table STA 2 2 STA 1 2 STA 2 2 STA 1 2 Wireless PC Card Association table Wireless PC Card Association table STA 1 WDS Relay STA 2 Wireless Backbone Packet for STA 2 WDS Relay ACK Packet for STA 2 Packet for STA 2 ACK ACK BSS B STA 1 BSS A STA 2

27 Frame Structure

28 Frames LLC PDU LLC MAC header PLCP PLCP preamble header MAC SDU CRC MAC layer Physical layer convergence procedure PLCP PDU designed to Support LLC Operate over many physical layers Physical medium dependent Physical layer

29 PLCPs

30 Frames Control frames Handshaking (RTS and CTS) ACKs during data transfer Data frames Data transfer

31 Frame MAC Header: 30 bytes Frame Body: bytes CRC: CCITT 32 4 bytes CRC over MAC header & frame body 2 2 Frame Control Duration/ ID MAC header (bytes) 6 6 Address 1 Address Address 3 Sequence control Address 4 Frame body CRC

32 PHY layer

33 PHY layer Specifications IEEE IEEE b IEEE a IEEE g IEEE n What's new? ac ready at 2013 (~1 Gbps) ad?? (60 GHz) Network LLC/MAC PHY

34 PHY standards Frequency Band Bit Rate Modulation Scheme GHz 1 2 Mbps Frequency Hopping Spread Spectrum, Direct Sequence Spread Spectrum b 2.4 GHz 11 Mbps Complementary Code Keying & QPSK g 2.4 GHz 54 Mbps a 5 6 GHz 54 Mbps Orthogonal Frequency Division Multiplexing & CCK for backward compatibility with b Orthogonal Frequency Division Multiplexing

35 IEEE n Release Date: June 2009 OFDM. Op. Frequency: 2.4 GHz and/or 5 GHz Data Rate (Typ): 74 Mbit/s Data Rate (Max): 300 Mbits/s (2 streams) Range (Indoor): ~70 m Two new ideas (PHY layer): 20 / 40 MHz operation to the physical (PHY) layer. MIMO (Multiple Input Multiple Output). antenna diversity and spatial multiplexing

36 OFDM BASIC IDEA : Channel bandwidth is divided into multiple sub channels to reduce frequency selective fading.

37 SISO, SIMO, MISO and MIMO

38 Spatial Diversity Spatial Multiplexing The M antennas are used: to transmit the same information (coded properly) Spatial Diversity > Lower BER to transmit different information (coded properly) Spatial Multiplexing Higher Throughput Bits Only Single user MIMO (point to point communications). Bit Split TX DSP Radio Radio DSP DSP Radio Radio DSP Bit Merge RX

39 Multiple 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. Data Rate: 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)

40 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

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

42 Auto Rate Protocols Selects the rate to be used for a packet (PHY/MAC cross layer) 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

43 IEEE a Rates

44 IEEE a Ideal rate, acceptable BER

45 DCF = MAC / LLC

46 Distributed Coordination Function Random Access based on a CSMA protocol. The MAC protocol is independent of the station and / or the traffic to transmit. For the MAC, is the same to transmit an HTTP request than a VoIP packet or a TCP ACK. Non linear performance = f(number of nodes, bandwidth req. (bps) of each node). An ARQ Stop & Wait protocol is implemented.

47 CSMA basic (without RTS/CTS) Data Source SIFS ack Destination DIFS Other NAV Defer Access Wait for Reattempt Time

48 DCF (Distributed Coordination Function) Collision Avoidance (Back Off Mechanism, RTS/CTS) When station senses the channel busy, it waits until channel becomes idle for DIFS period & then begins random backoff time (in units of idle slots) Station transmits frame when backoff timer expires If collision occurs, recompute backoff over interval that is twice as long Receiving stations of error free frames send ACK Sending station interprets non arrival of ACK as loss Executes backoff and then retransmits Receiving stations use sequence numbers to identify duplicate frames

49 DCF Without the RTS/CTS handshake 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.) The time (backoff) is slotted in empty slots of duration σ [seconds]

50 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

51 CSMA with 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

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

53 Carrier Sensing (Real and Virtual) Channel busy if either sensing is busy Signal > threshold is detected. Virtual Carrier Sensing at MAC sublayer Source stations informs other stations of transmission time (in µ sec) for an MPDU Carried in Duration field of RTS & CTS Stations adjust Network Allocation Vector (NAV) to indicate when channel will become idle

54 BEB (Binary Exponential Backoff) k retry CWmin=32 CWmax=1024 B(k)=min(CW(k),CWmax) CW(k)=2k CWmin A value from 0 to CW(k) is randomly chosen before to transmit the packet (using a uniform distribution)

55 Typical DCF parameters Rdata Rbasic

56 Basic guidelines of the model Each node is modeled as an M/M/1/Q queue with packet arrival rate λ k and service time Xk. Assumption: Poisson arrivals and service time exponentially distributed. The model accounts for: queuing delays, packet losses. E Qk 1 k ( n ) k ( n )Q E D k ( n ) k X k ( n ) k Pb, k ( n ) Q 1 k (1 Pbk) 1 k ( n ) 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

57 M/M/1/K (K = Q from previous slide)

58 M/M/1/K

59 Basic guidelines of the model Reciprocal influence between nodes. Service time is function of how behave the other nodes. X k ( n ) NT, k ( n ) E Wk ( n ) k ( n ) NT, k ( n ) 1 Tc ( Lk ) T ( Lk ) Av. number of transmissions Av. collision duration Av. number of back off slots Av. succesful duration Av. duration of a slot 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

60 Basic guidelines of the model Transmission Probability k ( n) Probability to transmit in a given slot conditioned to have a packet ready in the queue. Pr(Qk 0) k ( n) E Wk ( n ) 1 E Wk ( n ) 1 k ( n ) k 1 Pb, k ( n ) X k ( n ) Conditional Collision probability Assumption: independent from the back off stage, i.e., geometrically distributed. 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

61 Basic guidelines of the model Backoff suspension When channel is sensed busy the backoff count down is suspended. Channel states k ( n ) pe, k ( n ) ps, k ( n ) T * ( Lk ) pc, k ( n ) Tc* ( Lk ) Successful transmission Collision Empty 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 backoff suspension (succ.) DATA

62 Model validation: some results Saturation points for streaming traffic (conditioned to the number of elastic flows active). DCF is a node based scheduler. Low capacity for streaming flows in presence of elastic (TCP) traffic.

63 Model validation: some results Queue utilization and collision probability.

64 Transmission Probability

65 Frame length computation

66 Throughput with different types of traffic TCP packets ACK packets ACK packets TCP packets VoIP call ACK packets TCP packets VoIP call DCF does not work well with multimedia traffic