Congestion Control and Fairness for Many-to-One Routing in Sensor Networks Cheng Tien Ee Ruzena Bajcsy Motivation Congestion Control Background Simulation + Implementation Results current protocols do not scale to more than 10s of nodes! motes generate more data than can be sent to base station (problem 1: congestion) packets get dropped, waste of precious energy base station receives more packets from motes (problem 2: unfair) need to tell nodes to send at particular rate, and consider fairness easy to extend solution to multiple-base stations scenarios also extends to scenarios where a subset of motes generates data first, differentiate between data generation rate and effective transmission rate data generation rate: rate at which application generates data effective transmission rate: effective rate at which mote transmits data, which may include data from downstream motes next, require 2 mechanisms at each node: should know the number of downstream motes, in other words, children in its subtree should know the local effective transmission rate Mechanism 2: Determining Effective Data Transmission Rate sensor motes send information to 1 central mote (or base station) since motes may not be within radio range of one another, intermediate motes need to route packets, resulting in multi- hoping networks focus on applications that require gathering of data that cannot be aggregated within network base station A each node sends # children + 1 to its parent done recursively towards the base station can adapt to topology changes easily eg: node C sends “1”, node B sends “3”, node A sends “4” A B C Mechanism 1: Subtree Children # basically monitor and calculate average time taken to send 1 data packet (from first attempt to actual transmission) why not use known channel rate? need to take into account interference when neighbors transmit simultaneously when interference occurs, effective rate is less rate is appended to data packet, children nodes eavesdrop on packet and updates need simulations to check performance for huge network (hundreds to thousands of nodes) simulated: radio transmission rate (19.2kbps) MAC protocol (Multiple Access Collision Avoidance, MACA) RTS/CTS packets of size 2 bytes each data packets of size 30 bytes packet loss due to interference and queue overflow implemented: in 10 Mica2dot motes MAC protocol is MACA, with hop-by-hop ARQ if parent node has lower generation rate than this node, use parent’s rate, else rate of all children in subtree = e.g.: if node A’s transmission rate is 70 pkts/sec, then any node in its subtree can generate at most 10 pkts/sec disseminate control information piggy-backed on data packets Fairness B’s queueC’s queue B F F B CA A’s queue AD E D C within a period (or epoch), each mote transmits a multiple # of packets from each subtree equal to the size of that subtree e.g. A transmits 2 packets from B, and 3 packets from C, 1 packet from itself, within 1 period requires per child queue FIFO queues subtree size (obtained as before) correctness proof: by induction A B C D E F Conclusion network adapts itself automatically exact same, simple code runs in each mote very scalable queue size can be small and constant state grows linearly with number of neighbors congestion control and fairness can be implemented in transport layer, little or no modifications need to be done to MAC layer