1-1 Medium-Access Control

1-2 Medium Access r Radio communication: shared medium. m Throughput, delay, and fairness. r MAC for sensor networks: m Must also be energy efficient. r Contention- versus scheduled-based access. r Energy efficiency through switching nodes to low-power, sleep mode.

1-3 Hidden- and Exposed Terminals A B C Hidden Terminal Problem: a transmits to B; C does not sense A, and also transmits to B: collision! A B C D Exposed Terminal Problem: B is supposed to transmit to A and C to D; but C senses B’s transmission and defers: waste of bandwidth.

1-4 Contention- versus Scheduled-Based Access r Contention-based access performs well in low load scenarios. r Examples: m Packet radio (1970’s): Aloha, Slotted Aloha. m CSMA, CSMA-CD (Ethernet). r Collisions? r Scheduled-based access are collision free. r Example: TDMA. m No nodes within 2 hops of each other may use the same slot. r Lends itself to energy efficient approaches. m Turn nodes off when they are not sending/receiving.

1-5 MAC in Sensor Networks r Sensor networks are a special class of multihop wireless networks. Ad-hoc deployment. Self-configuring. Unattended. Battery powered.

1-6 Network Architecture Sink Thousands of nodes. Short radio range (~10m). Event-driven. Hierarchical deployment. Fault tolerance. Information Processing

1-7 MAC Protocols r Regulate channel access in a shared medium. A C B Collision at B X No coordination (ALOHA)

1-8 CSMA Listen before transmitting Stations sense the channel before transmitting data packets. SRH X Collision at R Hidden Terminal Problem

1-9 CSMA with Collision Avoidance r Stations carry out a handshake to determine which one can send a data packet (e.g., MACA, FAMA, IEEE802.11, RIMA). SRH RTS CTS Data ACK Backoff due to CTS

1-10 Idle Listening Tx Rx Idle Listen Tx Rx Sleep Sleep Scheduling Medium Access r Energy-efficient channel access is important to prolong the life-time of sensor nodes. r Conventional media-access control protocols waste energy by collisions, and idle listening. r Radios have special sleep mode for energy conservation.

1-11 Achieving energy efficiency r When a node is neither transmitting nor receiving, switch to low-power sleep mode. r Prevent collisions and retransmissions. r Need to know Tx, Rx and when transmission event occurs. Scheduled-based(time-slotted) MAC Protocols

1-12 Contention-based Channel Access Protocols: S-MAC & T-MAC

1-13 S-MAC [Ye etal] Features r Collision Avoidance m Similar to (RTS/CTS handshake). r Overhearing Avoidance m All the immediate neighbors of the sender and receiver goes to sleep. r Message Passing m Long messages are broken down in to smaller packets and sent continuously once the channel is acquired by RTS/CTS handshake. m Increases the sleep time, but leads to fairness problems.

1-14 S-MAC Overview r Time is divided in to cycles of listen and sleep intervals. r Schedules are established such that neighboring nodes have synchronous sleep and listen periods. r SYNC packets are exchanged periodically to maintain schedule synchronization.

1-15 S-MAC Operation

1-16 Schedule Establishment r Node listens for certain amount of time. r If it does not hear a schedule, it chooses a time to sleep and broadcast this information immediately. r This node is called the ‘Synchornizer’. r If a node receives a schedule before establishing its schedule, it just follows the received schedule. r If a node receives a different schedule, after it has established its schedule, it listens for both the schedules.

1-17 S-MAC Illustration

1-18 T-MAC [Dam etal] : S-MAC Adaptive Listen

1-19 T-MAC: Early Sleeping Problem

1-20 T-MAC/S-MAC Summary r Simple contention-based channel access with duty cycle-based sleeping. r Restricting channel contention to a smaller window  negative effect on energy savings due to collisions. r Requires schedule co-ordination with one hop neighbors.

1-21 Scheduling-based Channel Access Protocols: TRAMA and FLAMA

1-22 TRAMA: TRaffic-Adaptive Medium Access r Establish transmission schedules in a way that: m is self adaptive to changes in traffic, node state, or connectivity. m prolongs the battery life of each node. m is robust to wireless losses.

1-23 TRAMA - Overview r Single, time-slotted channel access. r Transmission scheduling based on two-hop neighborhood information and one-hop traffic information. r Random access period m Used for signaling: synchronization and updating two-hop neighbor information. r Scheduled access period: m Used for contention free data exchange between nodes. m Supports unicast, multicast and broadcast communication.

1-24 TRAMA Features r Distributed TDMA-based channel access. r Collision freedom by distributed election based on Neighborhood-Aware Contention Resolution (NCR). r Traffic-adaptive scheduling to increase the channel utilization. r Radio-mode control for energy efficiency.

1-25 Time slot organization

1-26 r Each node maintains two-hop neighbor information. r Based on the time slot ID and node ID, node priorities are calculated using a random hash function. r A node with the highest two-hop priority is selected as the transmitter for the particular time slot. Neighborhood-aware contention resolution (NCR)[Bao et al., Mobicom00]

1-27 A C D B E F C Winner G H I NCR does not elect receivers and hence, no support for radio-mode control. NCR - Example

1-28 TRAMA Components r Neighbor Protocol (NP). m Gather 2-hop neighborhood information. r Schedule Exchange Protocol (SEP). m Gather 1-hop traffic information. r Adaptive Election Algorithm (AEA). m Elect transmitter, receiver and stand-by nodes for each transmission slot. m Remove nodes without traffic from election.

1-29 Neighbor Protocol r Main Function: Gather two-hop neighborhood information by using signaling packets. r Incremental neighbor updates to keep the size of the signaling packet small. r Periodically operates during random access period.

1-30 Packet Formats

1-31 Schedule Exchange Protocol (SEP) r Schedule consists of list of intended receivers for future transmission slots. r Schedules are established based on the current traffic information at the node. r Propagated to the neighbors periodically. r SEP maintains consistent schedules for the one-hop neighbors.

1-32 Schedule Packet Format

1-33 Adaptive Election Algorithm (AEA) r Decides the node state as either Transmit, Receive or Sleep. r Uses the schedule information obtained by SEP and a modified NCR to do the election. r Nodes without any data to send are removed from the election process, thereby improving the channel utilization.

1-34 Simulation Results Delivery Ratio Synthetic broadcast traffic using Poisson arrivals. 50 nodes, 500x500 area. 512 byte data. Average node density: 6

1-35 Energy Savings Percentage Sleep TimeAverage Length of sleep interval

1-36 TRAMA Limitations r Complex election algorithm and data structure. r Overhead due to explicit schedule propagation. r Higher queueing delay. Flow-aware, energy-efficient framework.

1-37 TRAMA Summary r Significant improvement in delivery ratio in all scenarios when compared to contention-based protocols. r Significant energy savings compared to S-MAC (which incurs more switching). r Acceptable latency and traffic adaptive.

1-38 Flow-aware Medium Access Framework r Avoid explicit schedule propagation. m Take advantage of application. r Simple election algorithm to suit systems with low memory and processing power (e.g., 4KB ROM in Motes). r Incorporate time-synchronization, flow discovery and neighbor discovery during random-access period.

1-39 Flow Information r Flow information characterizes application- specific traffic patterns. r Flows can be unicast, multicast or broadcast. r Characterized by source, destination, duration and rate.

1-40 Example: Data Gathering Application A C E B D Sink Fb Fc Fd Fe

1-41 Flow Discovery Mechanism r Combined with neighbor discovery during random-access period. r Adapted based on the application. m for data gathering application, flow discovery is essentially establishing the data forwarding tree.

1-42 Example S Sink initiates neighbor discovery, flow discovery and time synchronization. Broadcasts periodic SYNC packets. Potential children reply with SYNC_REQ. Source reinforces with another SYNC packet. Once associated with a parent, nodes start sending periodic SYNC broadcasts. A B C Sink SYNC SYNC_REQ

1-43 Election Process r Weighted election to incorporate traffic adaptivity. r Nodes are assigned weights based on their incoming and outgoing flows. r Highest priority 2-hop node is elected as the transmitter. r A node listens if any of its children has the highest 1-hop priority. m Can switch to sleep mode if no transmission is started. S A B C Sink ws=0 wc=1 wa=3

1-44 Simulation Results (16 nodes, 500x500 area, CC1000 radio, grid topology, edge sink) Delivery Ratio Queueing Delay

1-45 FLAMA Summary r Simple algorithm that can be implemented on a sensor platform. r Significant performance improvement by application awareness.