Wireless Networking & Mobile Computing

Transcription

1 Wireless Networking & Mobile Computing CS 752/852 - Spring 2012 Lec #4: Medium Access Control - II Tamer Nadeem Dept. of Computer Science

2 IEEE Standards Page 2 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

3 IEEE MAC Very popular wireless MAC protocol Two Architectures IEEE Medium Access Control (PCF+DCF) FHSS DSSS Infrared OFDM MAC PHY SSID BSSID Page 3 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

4 PHY Sublayers Physical layer convergence protocol (PLCP) Provides common interface for MAC Offers carrier sense status & CCA (Clear channel assesment) Performs channel synchronization / training Physical medium dependent sublayer (PMD) Functions based on underlying channel quality and characteristics E.g., Takes care of the wireless encoding Page 4 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 4

5 PLCP PLCP has two structures. All b systems have to support Long preamble. Short preamble option is provided to improve efficiency when transmitting voice, VoIP, streaming video. PLCP Frame format PLCP preamble SFD: start frame delimiter PLCP header 8-bit signal or data rate (DR) indicates how fast data will be transmitted 8-bit service field reserved for future 16-bit length field indicating the length of the ensuing MAC PDU (MAC sublayer s Protocol Data Unit) 16-bit Cyclic Redundancy Code Page 5 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 5

6 PLCP (802.11b) long preamble 192us short preamble 96us (VoIP, video) Page 6 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 6

7 Channels The IEEE channelization scheme. The 2.4-GHz band is broken down into 11 in USA. However, at most there is 3 non-overlapping channels. Page 7 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

8 Channels Page 8 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

9 IEEE MAC Two modes: DCF (distributed coordination function) PCF (point coordination function) IEEE DCF is based on CSMA/CA Physical Carrier Sense Explicit ACK from receiver (for unicast transmission) RTS/CTS reservation frames (Virtual Carrier Sensing) Retry Counters Different Timing Intervals for priorities Page 9 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

10 IEEE DCF Basics RTS/CTS & Virtual Carrier Sense RTS-CTS used for frames longer than a Threshold RTS-CTS overhead not efficient for short frames Some environments may not find RTS-CTS useful, e.g. many infrastructure networks Threshold variable can be tuned Virtual carrier sensing Duration field in all frames, including RTS and CTS, monitored by every station Duration field used to construct a network access vector (NAV) Inhibits transmission, even if no carrier detected Page 10 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

11 IEEE DCF Basics Retry Counters Counter and timer for each frame Short or long retry counter Lifetime timer Retry counter Incremented for each transmission attempt Use of short versus long retry counter based on Threshold variable Threshold limit ShortRetryLimit for short retry counter LongRetryLimit for long retry counter If threshold exceeded, frame is discarded and upper layer is notified via MAC interface Page 11 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

12 IEEE DCF Basics Timing Intervals Timing intervals are defined that control a station s access to the medium Slot time (SlotTime) Specific value depends on PMD layer Derived from propagation delay, transmitter delay, etc. (20micro-sec for DSSS and 50 for FHSS) Basic unit of time for MAC, e.g. for backoff time is a multiple of slot time Short Inter-Frame Space (SIFS) Shortest interval -- SIF < SlotTime e.g. 10 microsec for FHSS Used for highest priority access to the medium, e.g., for ACK and CTS Allows Data-ACK and RTS-CST to be atomic transactions Page 12 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

13 IEEE DCF Basics Timing Intervals Priority (or PCF) Inter-Frame Space (PIFS) PIFS = SIFS + SlotTime Used for Point Coordination Function (PCF) access to the medium Allows priority based access to the medium after ACKs but before contention based access Distributed (or DCF) Inter-Frame Space (DIFS) DIFS = SIFS + 2 SlotTime Used for Distributed Control Function (DCF) access to the medium Results in lower priority access than using SIFS or PIFS Extended Inter-Frame Space (EIFS) EIFS = SIFS + (8 ACK) + PreambleLength + PLCPHeaderLength + DIFS Used in the event that the MAC receives a frame with an error Provides an opportunity for a fast retransmit of the error frame In summary SIFS < SlotTime < PIFS < DIFS << EIFS Page 13 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

14 DCF Mode Principles When a sender has a data to transmit, it picks a random wait period. The wait period is decremented if the channel is idle When this period expires, the node tries to acquire the channel by sending a RTS packet The Receiving node (destination) responds with a CTS packet indicating that its ready to receive the data The sender then completes the packet transmission If the packet is received without errors, the destination node responds with an ACK If an ACK is not received, the packet is assumed to be lost and the packet is retransmitted Page 14 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

15 DCF Mode Principles If RTS fails, the node attempts to resolve the collision by doubling the wait period. (This is known as binary exponential back-off (BEB)). Station trying to send an ACK is given preference over a station that is acquiring a channel (Different waiting intervals are specified) A node needs to sense channel for Distributed Inter- Frame Space (DIFS) interval before making an RTS attempt and a Short Inter Frame Space (SIFS) interval before sending an ACK packet Page 15 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

16 DCF Mode C A B D Deferred CW DIFS C D DIFS Contention Window NAV (RTS) NAV (CTS) RTS A RTS DATA SIFS SIFS SIFS CTS ACK B Page 16 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

17 DCF Mode Notes Because SIFS is shorter than the DIFS interval, the station sending an ACK attempts transmission before a station sending a data packet In addition to physical channel sensing, virtual carrier sensing is achieved due to NAV (Network allocation vector) field in the packet NAV indicates the duration of current transmission Nodes listening to RTS, or CTS messages back off NAV amount of time before sensing the channel again Several papers describe this protocol and even suggest enhancements. Page 17 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

18 Frames Type Control Frame: RTS, CTS, ACK Data Frame Management Frame: Beacon Probe Req, Probe Resp Assoc Req, Assoc Resp Reassoc Req, Reassoc Resp Disassociation Authentication Deauthentication Page 18 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

19 Data Frame Format Ver - The Protocol Version number is always 0 Type - indicates whether the frame is a Management, Control or Data frame. Subtype - describe the detail of the frame type. To DS - set if the frame is to be sent by the AP to the Distribution System From DS - set if the frame is from the Distribution System More Frag - set if this frame is a fragment of a bigger frame and there are more fragments to follow. Retry - set if this frame is a retransmission, maybe through the loss of an ACK Page 19 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

20 Data Frame Format Power Mgmt - indicates what power mode ('save' or 'active') the station is to be in once the frame has been sent More Data - set by the AP to indicate that more frames are destined to a particular station that may be in power save mode. These frames will be buffered at the AP ready for the station should it decide to become 'active'. WEP - set if WEP is being used to encrypt the body of the frame Duration & ID - In Power save poll messages this is the station ID, whereas in all other frames this is the duration used when calculating the NAV Address 1 - The recipient station address on the BSS. If To DS is set, this is the AP address; if From DS is set then this is the station address Address 2 - The transmitter station address on the BSS. If From DS is set, this is the AP address; if To DS is set then this is the station address Address 3 - If Address 1 contains the destination address then Address 3 will contain the source address. Similarly, if Address 2 contains the source address then Address 3 will contain the destination address. Page 20 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

21 Data Frame Format Address 4 - If a Wireless Distribution System (WDS) is being used (with AP to AP communication), then Address 1 will contain the receiving AP address; Address 2 will contain the transmitting AP address; Address 3 will contain the destination station address and Address 4 the source station address. Sequence Control - contains the Fragment Number and Sequence Number that define the main frame and the number of fragments in the frame Frame Body - contains the actual data e.g. IP datagrams and can be up to 2312 octets in size CRC - 32-bit Cyclic Redundancy Check on the whole frame. Page 21 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

22 Control Frame Format Page 22 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

23 Contention Window Random number selected from [0,cw] If transmission was successful, set CW = CW min If transmission fails (i.e., no ACK), CW = min{2(cw+1)-1, CW max } Small value for cw Less wasted idle slots time Large number of collisions with multiple senders (two or more stations reach zero at once) Optimal CW for known number of contenders & know packet size Computed by minimizing expected time wastage (by both collisions and empty slots) Tricky to implement because number of contenders is difficult to estimate and can be VERY dynamic Project Idea: Evaluate literature for CW calculation schemes under different scenarios Enhance/New adaptive CW scheme Page 23 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

24 Fragmentation sender receiver DIFS RTS SIFS CTS SIFS frag 1 SIFS ACK SIFS 1 frag 2 SIFS ACK2 other stations NAV (RTS) NAV (CTS) NAV (frag 1 ) NAV (ACK 1 ) DIFS contention data t Page 24 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

25 Physical Carrier Sense Mechanisms Energy detection threshold Monitors channel during idle times between packets to measure floor noise Energy levels above this floor noise by a threshold trigger carrier sense DSSS correlation threshold Monitors the channel for Direct Sequence Spread Spectrum (DSSS) coded signal Triggers carrier sense if the correlation peak is above a threshold More sensitive than energy detection (but only works for transmissions) High BER disrupts transmission but not detection Carrier can be sensed at lower levels than packets can be received Receive Range Results in larger carrier sense range than transmission range More than double the range in NS simulations Carrier Sense Range Page 25 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

26 On Issues RTS/CTS & Carrier Sense When RTS/CTS is useful? Should Carrier Sensing replace RTS/CTS? Interference Range vs. Carrier Sense Range How effective CSMA carrier sense? BER & Date rate and Transmission Range (data rate affect the SNR threshold and hence the transmission range but not the physical CS) Contention Window Size Is ACK necessary? MACA said no ACKs. Let TCP recover from losses The search for the best MAC protocol is still on. However, is being optimized too. Thus, wireless MAC research still alive Page 26 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

27 On RTS/CTS & Carrier Sense Does RTS/CTS (Virtual CS) solve hidden terminals? Assuming carrier sensing zone = communication zone A B C D CTS E RTS F E does not receive CTS successfully Can later initiate transmission that interferes with D Hidden terminal problem remains Page 27 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 27

28 On RTS/CTS & Carrier Sense Hidden Terminal: How about increasing Physical Carrier Sense range?? E will defer on sensing carrier no collision!!! CTS E RTS F A B C Data D Page 28 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

29 On RTS/CTS & Carrier Sense Exposed Terminal: B should be able to transmit to A Carrier sensing makes the situation worse RTS CTS E A B C D Page 29 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

30 On RTS/CTS & Carrier Sense does not solve HT/ET completely Only alleviates the problem through RTS/CTS and recommends larger CS zone Large CS zone aggravates exposed terminals Spatial reuse reduces A tradeoff RTS/CTS packets also consume bandwidth Moreover, backing off mechanism is also wasteful Carrier sense relies on channel measurements at the sender to infer the probability of reception at the receiver! Project Idea: Evaluation of the benefits and drawbacks of carrier sense Scheme to intelligently choose a Carrier sensing threshold Evaluate tracking correlation between channel conditions at the sender and at the receiver. Page 30 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

31 On Contention Window Size Optimal CW for known number of contenders & know packet size Computed by minimizing expected time wastage (by both collisions and empty slots) Tricky to implement because number of contenders is difficult to estimate and can be VERY dynamic adaptive scheme is unfair Under contention, unlucky nodes will use larger cw than lucky nodes (due to straight reset after a success) Lucky nodes may be able to transmit several packets while unlucky nodes are counting down for access Fair schemes should use same cw for all contending nodes Project Idea: Evaluate literature for CW calculation schemes under different scenarios Enhance/New adaptive CW scheme Page 31 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

32 On Interference Range vs. Carrier Sense Range physical layer (e.g., Direct Sequence Spread Spectrum (DSSS) used in b) Capture effect: two transmissions received by the same receiver, the signals of the stronger transmission will capture the receiver radio, and signals of the weaker transmission will be rejected as noise. Simple and widely accepted model: Capturing stronger signal Capturing stronger frame Received Frame Received Frame Frame 1 Frame 2 Frame 1 Frame 2 Page 32 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

33 On Interference Range vs. Carrier Sense Range Power path loss model: Capture model: Given: R=250m, R C=550, l =2, α=5 d Interference Range: I 1 2 I C Inefficiency: Page 33 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

34 On Interference Range vs. Carrier Sense Range Project Idea: How to estimate interference range (distance) Propagation Delay? Interference Aware MAC Scheme Page 34 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

35 On Transmission Date rate Bit error (p) for BPSK and QPSK : Received Power Channel Bandwidth SNR Floor Noise Data Rate Page 35 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

36 On Transmission Date rate Page 36 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

37 On ACKnowledgment APs typically backlogged with traffic Persistent traffic possibility of optimization Use implicit ACK optimization Piggyback the CTS with ACK for previous dialog Gain Implicit ACK Page 37 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

38 Backoff Backoff Backoff Backoff Backoff Backoff On ACKnowledgment The optimization timeline Implicit ACK Hybrid Channel Access T R T R T R RTS RTS RTS CTS CTS CTS Data Data Data ACK RTS Poll +ACK CTS RTS CTS +ACK Data Data Data Poll +ACK ACK RTS Data CTS +ACK Page 38 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

39 Performance Analysis of the IEEE Distributed Coordination Function (Giuseppe Bianchi) Page 39 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

40 DCF Throughput Analysis (Bianchi) Objective: Analytical Evaluation of Saturation Throughput Assumptions: Fixed number of stations having packet for transmission Each packet collide with constant and independent probability Model bi-dimensional process {s(t), b(t)} with discretetime Markov chain Analysis divided into two parts: Study the behavior of single station with a Markov model Study the events that occur within a generic slot time & expressed throughput for both Basic & RTS/CTS access method Page 40 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

41 Markov Chain Model Page 41 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

42 Markov Chain Model Closed form solution for Markov chain Stationary Probability Page 42 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

43 Markov Chain Model Probability τ that a station transmits in randomly chosen slot time When m =0 no exponential backoff is considered probability τ results independent of p In general τ depends on conditional collision probability p Page 43 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

44 Throughput Analysis Normalized system throughput S Probability of transmission P tr Probability of successful transmission P s Page 44 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

45 Throughput Analysis Normalized system throughput Specify T s and T c to compute throughput for DCF access mechanism Page 45 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

46 Throughput Analysis Considering System via Basic Access mechanism Packet header H = PHY hrd +MAC hrd Propagation delay δ Page 46 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

47 Throughput Analysis Packet transmission via RTS/CTS Access mechanism Page 47 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

48 Model Validation Compared analytical results with that obtained by means of simulation Analytical model extremely accurate Analytical results (lines) coincide with simulation results (symbols) in both Basic Access & RTS/CTS cases Saturation throughput analysis vs. simulation Page 48 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

49 Performance Evaluation Greater the network size lower is the throughput for basic access Saturation throughput vs. initial window size for Basic Access mechanism Page 49 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

50 Performance Evaluation Throughput of Basic Access mechanism depends on W W depends on number of terminals High value of W gives excellent throughput performance Saturation throughput vs. initial window size for Basic Access mechanism Page 50 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

51 Performance Evaluation Throughput obtained with RTS/CTS mechanism Independent of value of W Saturation throughput vs. initial window size for RTS/CTS mechanism Page 51 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

52 Performance Evaluation Number of transmissions per packet increases as W reduces & network size n increases. Average number of transmissions per packet Page 52 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

53 Questions Page 53 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing