1. 6 November 2002 MAC protocol 1 Optimization of the Efficiency of MAC Protocols for WLANs Raffaele Bruno MobileMAN kickoff meeting

2. 6 November 2002 MAC protocol 2 Outline  MAC protocols for WLANs: the IEEE 802.11 solution  Inefficiencies of the IEEE 802.11 MAC protocol  Definition of the optimal backoff scheme to maximize the resource utilization  Performance evaluation  Conclusions

3. 6 November 2002 MAC protocol 3 Why the MAC protocol is important?  The wireless medium is an intrinsic shared medium  The channel access becomes a fundamental factor in determining the capacity of the network and has a great impact on system complexity and/or cost  The contention MAC protocols, or random access protocols, use a direct contention to determine channel access rights. Needs for collisions avoidance and collision resolution mechanisms: 4 randomization of retransmissions 4 mechanisms for channel reservation 4 fairness in the randomization of access

4. 6 November 2002 MAC protocol 4 Power Consumption  The finite battery power of portable devices represents a severe limitation to the utility of WLANs. The data transmission and reception are one of the most power-consuming activities that these devices have to perform  About the 20% of the battery life is determined by the network interface. For the handheld devices the impact of the network interface can be up to the 50% 3 h 12 m 2 h 50 m 22 m WaveLAN ON WaveLAN OFF WaveLAN Card Consumtpion

5. 6 November 2002 MAC protocol 5 The IEEE 802.11 Protocol Stack  Each node executes locally and independently from the other nodes a Coordination Function that defines the mechanisms for sharing the bandwidth.  This technology defines a Distributed Coordination Function (DCF) for the delivery of asynchronous traffic, and a Point Coordination Function (PCF) for the delivery of synchronous traffic Point Coordination Function Distributed Coordination Function Physical Layer MAC sublayer contention free services contention services Fundamental Access Method

6. 6 November 2002 MAC protocol 6 The Distibuted Coordination Function  The Distributed Coordination Function adopts the Carrier Sensing Multiple Access (CSMA) scheme: a station is allowed to transmit only when it senses no other transmissions on the channel  The CSMA is both “physical” and “virtual”: 4 physical carrier sensing: the receiver assumes the channel busy when a radio signal power above the sensitivity threshold is detected on the air 4 virtual carrier sensing: the channel is considered busy for all the expected remaining time required to complete the actual exchange of data

7. 6 November 2002 MAC protocol 7 DCF Basic Access: overview 4 Successful transmission 4 Collision DATA ACK NAV CW DIFS SIFS Source Destination Other There is no collision detection: each collided packet is completely transmitted LALA Collision Length = collided packet maximum length (L A ) DIFSEIFS Source A Source B Source C LBLB LCLC COLLISION LENGTH

8. 6 November 2002 MAC protocol 8 DCF RTS/CTS Access: overview 4 Successful transmission 4 Collision Collision Length = length (RTS) Trade-off between the increased control overheads and the reduced collision costs RTS A DIFS Source A Source B Source C RTS B RTS C COLLISION LENGTH EIFS Source Destination Other DATA ACK NAV RTS CW DIFS SIFS RTS SIFS CTS NAV CTS NAV Data

9. 6 November 2002 MAC protocol 9 Collision Avoidance & Resolution scheme  Collision Avoidance scheme to reduce the probability to collide before transmitting a new packet: 4 it is not enough to listen the channel idle to transmit, the station set a random “backoff counter” to defer the transmission  Collision Resolution scheme to reduce the probability to collide again on the retransmissions of the same packet: 4 the “backoff counter” is increased after each retransmission of the same packet  The “backoff counter” is selected through the Binary Exponential Backoff (BEB)

10. 6 November 2002 MAC protocol 10 The BEB algorithm  The backof timer is uniformly sampled within the range [0…CW-1], where CW is the Contention Window  The Contention Window is doubled after each consecutive retransmission of the same packet  The Contention Window shall be reset to a CWmin value after each successful transmission 31 63 127 255 511 1023 # of transmissions 1234567 CWmax CWmin

11. 6 November 2002 MAC protocol 11 The BEB inefficiencies  The BEB procedure implemented is reactive and not proactive: it is triggered by a collision event, hence it does not try to predict collisions  The status information exploited by the BEB is very limited, only the number of consecutive collisions. Other feedback information as the collisions’ length can be utilized?  The BEB algorithm is known to have unpredictable fairness properties in the presence of heavy contention, since it tends to favor the station that experienced the last successful transmission

12. 6 November 2002 MAC protocol 12 Increasing the MAC efficiency  A good indication of the bandwidth efficiency is the protocol capacity, i.e., the maximum channel utilization achievable by the MAC protocol, whereas the minimum energy consumption is a good indication of the energy efficiency  Through a model of the MAC protocol operation it is feasible to derive the optimal protocol operating state that guarantees the optimal performances CHANNEL UTILIZATION ( ) = fraction of channel bandwidth used by successfully transmitted messages ENERGY CONSUMTPION = energy used by the network interface to successfully transmit a message

13. 6 November 2002 MAC protocol 13 How to model MAC protocols?  The asymptotic behavior of the IEEE 802.11 MAC protocol can be modeled via a p-persistent MAC protocol, where each station decides to transmit at the beginning of an empty slot according to a probability of transmission p * 1-p begin of an empty slot p the station transmits in the slot the station waits next empty slot * F. Calì, M. Conti, E. Gregori, "Dynamic Tuning of the IEEE 802.11Protocol to Achieve a Theoretical Throughput Limit", IEEE/ACM Transactions on Networking, December 2000

14. 6 November 2002 MAC protocol 14 Protocol Capacity analysis end of j-th transmission attempt end of (j+1)-th transmission attempt Idle Period Collision/Success Protocol Capacity :  The closed formulas can be derived for a general packet length distribution, by assuming: 4 a finite network population with M stations 4 a heavily loaded network

15. 6 November 2002 MAC protocol 15 Energy Consumption analysis NOTE: PTX > PRX Tagged Success Tagged Collision Not-Tagged Success/Collision virtual transmission time  From the energy consumption standpoint, the network interface alternates between transmitting phases, where it consumes PTX power per second, and receiving phases, where it consumes PRX power per second Energy Efficiency :

16. 6 November 2002 MAC protocol 16 Capacity vs. Energy Consumption analysis  In a network with M stations, when PTX=PRX, it holds that: To attain either the Protocol Capacity or the Minimum Energy Consumption are orthogonal goals?  The Protocol Capacity analysis is a special case of the more general mathematical framework of the Energy Consumption analysis

17. 6 November 2002 MAC protocol 17 The Mp opt product l = 2 time slotsl = 100 time slots The average number of transmitting station (Mp product) that guarantees the optimal energy state (optimal capacity state) is almost independent of the M value, at least for PTX/PRX<2

18. 6 November 2002 MAC protocol 18 An approximate formula for the p opt value REMARK l=2 l=100  The derived closed formula cannot be used to tune the tune the protocol to the optimal state because it is necessary to know the number of contending stations in the network. This information is difficult to be retrieved in a mobile environment

19. 6 November 2002 MAC protocol 19 Dynamic Tuning of the MAC protocol Each station at runtime must computes the p opt value by observing the status of the channel p opt = f(M,l,PTX,PRX)  Any feedback-based strategy to achieve Power-Saving/Efficiency Optimization should be EFFECTIVE To be based only on simple channel status estimates, without the need of information about the number of active stations To require negligible computational complexity To approach as much as possible the theoretical bounds SIMPLE EASY

20. 6 November 2002 MAC protocol 20 Approximated analysis  To minimize the Energy Consumption we must minimize only the terms depending on the p value: virtual transmission time Tagged Success 1 Not-Tagged Success M-1 Collisions E[N ta ]-M Idle Periods E[N ta ] 4 4 4 4 

21. 6 November 2002 MAC protocol 21 Approximation validation The E[Energy Idle_p ] is a degreasing function of the p value, whereas the E[Energy Coll ] is an increasing function of the p value The Optimal State is the balance between these two conflicting costs

22. 6 November 2002 MAC protocol 22 Dinamic Tuning strategy: overview  stations that adopt a p value > p opt have a too much aggressive behavior  stations that adopt a p value < p opt have a too much conservative behavior virtual transmission time Collisions Idle Periods p < p opt p = p opt p > p opt

23. 6 November 2002 MAC protocol 23 Dinamic Tuning strategy: implementation n-th idle periodn-th transmission end of (n-1)-th transmission attempt updating point n-1 end of n-th transmission attempt updating point n updating

24. 6 November 2002 MAC protocol 24 Dinamic Tuning strategy: implementation  At the end of (n-1)-th transmission attempt, the transmission control strategy calculates a new p value, say p new, as a function of previous adopted p value  By exploiting the same approximation adopted to derive the p opt closed formulas, we obtain p new = p n (1+x)

25. 6 November 2002 MAC protocol 25 Power Saving-Efficient IEEE 802.11 protocol  The Power Saving (PS)-Efficient IEEE 802.11 protocol is a p-persistent IEEE 802.11 protocol where the p value is dynamically computed according to the transmission control strategy previously described  The numerical analysis performed takes in consideration all the physical and MAC overheads success_overhead = 2  +SIFS+ACK+DIFS collision_overhead =  +EIFS MAC_HDR PHY_HDRFCS DATA

26. 6 November 2002 MAC protocol 26 PS-Efficient 802.11: steady state analysis PS-Efficient IEEE 802.11 approaches theoretical lower bound for the Energy Consumption in every network configurations and traffic characteristics analyzed l=2,PTX/PRX=2l=2,PTX/PRX=10

27. 6 November 2002 MAC protocol 27 PS-Efficient 802.11: transient analysis (M=10) 201.11 (M=100) 2015.50 PS-Efficient IEEE 802.11 reaches promptly a new optimal stationary state after both sharp and frequent variation of the number of active stations l=2,PTX/PRX=2

28. 6 November 2002 MAC protocol 28 PS-Efficient 802.11: MAC delay analysis PS-Efficient IEEE 802.11 significantly improves both average and worst-case MAC Delay M=10, l=2, PTX/PRX=2 Average value 99th percentile 99.9th percentile M=10, l=100, PTX/PRX=2 Average value 99th percentile 99.9th percentile

29. 6 November 2002 MAC protocol 29 Conclusions & Future work  Conclusions : – With the current technology of network interfaces (PTX/PRX<2) the Energy Consumption Minimization and the Channel Utilization Maximization can be jointly achieved – The energy and bandwidth Efficiency in IEEE 802.11 networks can be significantly improved by modifying the backoff procedure – The intrinsic characteristics of p-persistent CSMA protocols allow us to define adaptive feedback-based backoff-tuning policies that are completely distributed and independent of the network population  Future Work: – Extension to the multi-hop case – Introduction of traffic priotization