1. Rami Melhem Sameh Gobriel & Daniel Mosse Modeling an Energy-Efficient MAC Layer Protocol

2. Modeling An Energy-Efficient MAC Layer ProtocolDecember, 2004 ICENCO 04 2 Wireless Adhoc Networks Introduction A collection of mobile nodes forming a temporary network. Each host is an independent router. Significant impact on military and civil applications:  Conferences & meetings.  Combat field surveillance.  Target tracking.  Sensor networks.  Search & rescue operations.

3. Modeling An Energy-Efficient MAC Layer ProtocolDecember, 2004 ICENCO 04 3 Energy Consumption Challenge Nodes are low power, low cost devices. Very limited supply energy. It may be hard (or undesirable) to retrieve the nodes to change or recharge the batteries. Considerable challenge on the “Energy Consumption”. In our recent previous work [Infocom 04]:  Wasted energy in collisions & collision resolution is a significant portion of the energy consumption. BLAM goal is to minimize the energy wasted in collisions. Motivation

4. Modeling An Energy-Efficient MAC Layer ProtocolDecember, 2004 ICENCO 04 4 802.11 MAC Protocol & Collisions Collision can only occur during transmission of control frames. When a collision occurs the station defer for a random time uniformly distributed [0..CW]. The Value of the CW depends on the number of failed transmissions. 802.11 MAC Protocol A BC RTS CTS Data ACK

5. Modeling An Energy-Efficient MAC Layer ProtocolDecember, 2004 ICENCO 04 5 Why Collision Energy is Significant ?? In adhoc network a message is forwarded in more than one hop  collision is faced at each hop. A Power-aware adhoc network  decrease energy by increasing number of intermediate hops. (tendency for collision at each relay node) Collision Energy Reason 1: Number of Hops

6. Modeling An Energy-Efficient MAC Layer ProtocolDecember, 2004 ICENCO 04 6 Why Collision Energy is Significant ?? We can ’ t use low power for both data and control frames Previous work, [e.g. Vaidya, 02] proposed sending RTS-CTS with max power and Data-ACK with lower power However, control frames are the ones that face collision and retransmission Collision Energy Reason 2: Control Frames Tx Power

7. Modeling An Energy-Efficient MAC Layer ProtocolDecember, 2004 ICENCO 04 7 Why Collision Energy is Significant ?? Energy-poor nodes are the most critical ones because:  They have a lot of data to send.  They are located in the confluence of many routes.  Leaving these nodes to die may cause a “network partition”. In 802.11, all colliding nodes are equally treated  all nodes try retransmission at subsequent times. Energy-poor nodes can drain their energy colliding with high-energy nodes. Collision Energy Reason 3: Critical Nodes

8. Modeling An Energy-Efficient MAC Layer ProtocolDecember, 2004 ICENCO 04 8 BLAM: An Energy Efficient MAC BLAM BLAM conserves the channel bandwidth and energy consumption  by decreasing the total number of collisions. New partitioning philosophy  nodes are split into virtual groups based on their residual energy (no contention between low-energy and high-energy nodes). In IEEE 802.11:  When fresh data packet arrives at a node, it senses the channel. If the channel is idle  the node sends with probability 1.  When a collision is detected, the node defers transmission for a random period of time chosen uniformly in the interval [0..CW]. In BLAM:  when a fresh data packet arrives at a node or when a collision is detected  the node waits a random period of time before trying to transmit.  The random period of time is normally distributed with mean and variance depend on the residual energy.

9. Modeling An Energy-Efficient MAC Layer ProtocolDecember, 2004 ICENCO 04 9 CW Size Distribution Capacity = 1 Capacity = 0.75 Capacity = 0.5 Capacity = 0.25Capacity = 0 Full capacity  Mean =0  most probable to pick smaller window. As capacity decreases  Mean moves to left  picking larger window (i.e. waiting longer). Priority to high-energy nodes for channel access over low-energy nodes. BLAM

10. Modeling An Energy-Efficient MAC Layer ProtocolDecember, 2004 ICENCO 04 10 Transmission Priorities BLAM distributes the channel access based on the node energy. The network nodes are divided among a continuous set of transmission priorities based on the remaining energies. Probability of contention among members of the different priorities will be low  energy-poor nodes will not waste their energy colliding with high-energy nodes. The deferring time distribution is more selective (variance low) at the two extremes (C = 0 & C = 1) while less selective at C = 0.5 (close to uniform)  separate as much as possible between high and low energy nodes while not be very selective when C=0.5 (node majority). However: Probability of contention among members of the same priority will be high (pick comparable value for CW). BLAM

11. Modeling An Energy-Efficient MAC Layer ProtocolDecember, 2004 ICENCO 04 11 BLAM Effectiveness BLAM modifications are only based on the local host information:  No communication with central node is needed and no “Request- Status” messages are transmitted to neighbors.  No additional fields in the frame and no changes in the frame handling technique.  BLAM can be implemented as an open-loop control circuit  the node energy (input) and generates a random number (output) used to determine the transmission probability and random deferring time.  BLAM is transparent to the upper layers  no specific support nor changes are required.  BLAM is backward compatible can be deployed in a network that uses IEEE 802.11. BLAM

12. Modeling An Energy-Efficient MAC Layer ProtocolDecember, 2004 ICENCO 04 12 Collision Model The wireless channel around node X can be: Idle Transmit: when node X correctly transmits a data frame to R RTS-Col: when two or more nodes in the coverage area of X transmits an RTS CTS-Col: When a hidden node from X transmits an RTS to collide with the CTS sent by R. We compare analytically BLAM and IEEE 802.11 MAC protocol. We developed a collision model for the wireless network through which we can derive the total network throughput and the steady state probability of collision.

13. Modeling An Energy-Efficient MAC Layer ProtocolDecember, 2004 ICENCO 04 13 Probability of Transmission 1. Assume that the probability of transmission for a node in a given time slot is p. 2. Using p we compute the probabilities of transition from each state to the other assuming a uniform distributed network. 3. Using the equilibrium equations of the state transition diagram  we get the steady state collision probability and also the percentage of total time the wireless channel around node x is in each state. Collision Model  The difference between BLAM and IEEE 802.11 is in p  the probability of transmission per time slot.

14. Modeling An Energy-Efficient MAC Layer ProtocolDecember, 2004 ICENCO 04 14 Probability of Transmission 1. As an approximation, if we assume that the CW value is held constant (at mid range) then:  In IEEE 802.11 the probability of transmission per time slot [ As proved in Bianchi,00 ] :  In BLAM the probability of node X to transmit in slot “i” is a function of the node’s energy level Rx. 2. For any neighborhood with a given distribution of energies among M nodes the probability of transmission in a given time slot i is given by: Collision Model

15. Modeling An Energy-Efficient MAC Layer ProtocolDecember, 2004 ICENCO 04 15 Comparing BLAM & 802.11 Using the steady state equations of the collision model we compare BLAM and IEEE 802.11:  Average collision probability  Total network throughput The comparison is done in two cases:  BLAM Worst case  nodes in the neighborhood are having equal residual energy.  BLAM Best case  nodes in the neighborhood are having a uniform distribution of remaining energy. Analytical Results Parameter Value RTS Duration13 slot time CTS Duration12 slot time Data Packet Duration287 slot time ACK Duration12 slot time CW size256 slot time Nodes per neighborhood16 nodes Probability of TxP 802.11 (i) & P blam (i)

16. Modeling An Energy-Efficient MAC Layer ProtocolDecember, 2004 ICENCO 04 16 Comparing BLAM & 802.11 To verify the correctness of the collision model we simulated a single hop network using NS2. Two sets of scenarios are simulated:  All the nodes have full energy  The nodes have uniform distribution of the remaining battery energy. The energy distribution is forced to be fixed from the start to the end of the simulation. Analytical Results Worst case: BLAM only increased the probability of collision by 13% Best case: BLAM decreased the probability of collision by almost 4 folds.

17. Modeling An Energy-Efficient MAC Layer ProtocolDecember, 2004 ICENCO 04 17 Simulation Environment Simulation Parameter Value Number of simulation runs10 Network size375 X 375 m 2 Node range250 m Number of nodes32 Number of connections60 Flow TypeCBR Packet Size512 bytes Transmission rate per source6 pkts/sec The previous simulation analysis is presented to verify the correctness of the model (single- hop network, fixed-energy, fully-saturated & uniform-distributed nodes). Another simulation analysis is presented for a real network scenario. In our simulations, we compared 3 MAC layer protocols:  Basic 802.11  Modified 802.11  BLAM

18. Modeling An Energy-Efficient MAC Layer ProtocolDecember, 2004 ICENCO 04 18 Total Number of Collisions BLAM decreased Collisions by 40% over basic protocol. Initially the collision should be higher. Once a node starts transmission, it moves towards another priority (no contention). Simulation Results

19. Modeling An Energy-Efficient MAC Layer ProtocolDecember, 2004 ICENCO 04 19 Network Lifetime Simulation Results BLAM saved the energy wasted in Collisions, resolution & retransmissions. BLAM conserved the energy of the critical nodes by avoiding colliding with high-energy nodes. BLAM increased the network lifetime by 15%.

20. Modeling An Energy-Efficient MAC Layer ProtocolDecember, 2004 ICENCO 04 20 Total Number of Rx Packets Simulation Results Energy savings can be reached trivially by making nodes send less frequently (decrease throughput). BLAM was able to deliver more data packets to its final destination. BLAM increased the number of data packets received by 39% over the basic protocol.

21. Modeling An Energy-Efficient MAC Layer ProtocolDecember, 2004 ICENCO 04 21 Conclusions Conclusion We showed that the IEEE 802.11 protocol is not optimal for adhoc networks. We introduce BLAM, a new energy-efficient MAC layer enhancement. We used a collision model to analytically compare the behavior of BLAM and the IEEE 802.11 DCF protocols. We showed that the worst case probability of collision in BLAM is 13% higher than that of the IEEE 802.11, while in the best case a 4 folds improvement in the collision probability is achievable. We validated the correctness of the proposed model through simulation analysis for a single hop adhoc network. We also simulated a real network scenario and showed that BLAM decreased the number of collisions & increased the network lifetime  This indicates that the worst case of BLAM is not frequent.

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