Self-Organization in Autonomous Sensor/Actuator Networks [SelfOrg]

Transkript

1 Self-Organization in Autonomous Sensor/Actuator Networks [SelfOrg] Dr.-Ing. Falko Dressler Computer Networks and Communication Systems Department of Computer Sciences University of Erlangen-Nürnberg [SelfOrg], SS

2 Overview Self-Organization Basic methodologies of self-organization; comparison of central and hierarchical control, distributed systems, and autonomous behavior; examples of self-organization Mobile Sensor/Actuator Networks Ad hoc routing; reliable communication and congestion control; sensor assistance for mobile robots; applications Coordination of Autonomous Systems Coordination and synchronization; communication aspects; clustering Bio-inspired Mechanisms Swarm intelligence; artificial immune system; intra/inter cellular information exchange [SelfOrg], SS

3 MAC Protocols for Ad Hoc Wireless Networks Design issues Design goals Classification of MAC protocols Contention-based protocols Contention-based protocols with reservation mechanisms Contention-based protocols with scheduling mechanisms MAC protocols for sensor networks Case study: S-MAC [SelfOrg], SS

4 Design Issues Bandwidth efficiency Limited radio spectrum efficient bandwidth sharing Definition bandwidth efficiency: ratio of the bandwidth used for actual data transmissions to the total available bandwidth optimization problem for MAC protocols Quality of Service support Usually complex or no bandwidth reservation resource reservation must consider the nature of wireless channels and node mobility Synchronization Inter-node synchronization required for bandwidth (time slot) reservations Time synchronization using exchanged control packets control packets must not consume too much of network bandwidth [SelfOrg], SS

5 Design Issues Hidden and exposed terminals Unique problem in wireless networks Hidden terminal problem collision of packets due to the simultaneous transmission of those nodes that are not within the direct transmission range of the sender but are within the transmission range of the receiver Exposed terminal problem inability of a node, which is blocked due to transmission by a nearby transmitting node, to transmit to another node [SelfOrg], SS

6 Hidden and exposed terminals [SelfOrg], SS

7 Hidden and exposed terminals [SelfOrg], SS

8 Design Issues [SelfOrg], SS

9 Design Issues Error-prone shared broadcast channel Broadcast nature of radio channel A transmission made by a node is received by all nodes within its direct transmission range when a node is receiving data, no other node in its neighborhood, apart from the sender, should transmit Many nodes may contend for the channel simultaneously packet collision probability is quite high in wireless networks A MAC protocol should ensure that all nodes are treated fairly with respect to bandwidth allocation [SelfOrg], SS

10 Design Issues Distributed nature/lack of central coordination Nodes must be scheduled in a distributed fashion Exchange of control information control packets must not consume too much of network bandwidth Mobility of nodes Very important factor affecting the performance (throughput) of the protocol Bandwidth reservations or control information exchanged may end up being of no use if the node mobility is very high Protocol design must take this mobility factor into consideration system performance should not significantly affected due to node mobility [SelfOrg], SS

11 Design Goals of a MAC Protocol for Ad Hoc Networks The operation of the protocol should be distributed The protocol should provide QoS support for real-time traffic The access delay, which refers to the average delay experienced by any packet to get transmitted, must be kept low The available bandwidth must be utilized efficiently The protocol should ensure fail allocation of bandwidth to nodes Control overhead must be kept as low as possible [SelfOrg], SS

12 Design Goals of a MAC Protocol for Ad Hoc Networks The protocol should minimize the effects of hidden and exposed terminal problems The protocol must be scalable to large networks It should have power control mechanisms in order to efficiently manage energy consumption of the nodes The protocol should have mechanisms for adaptive data rate control It should try to use directed antennas (reduced interference, increased spectrum reuse, reduced power consumption) The protocol should provide time synchronization among nodes [SelfOrg], SS

13 Classification of MAC Protocols [SelfOrg], SS

14 Classification of MAC Protocols Contention-based protocols No a priori resource reservation Whenever a packet should be transmitted, the node contends with its neighbors for access to the shared channel Cannot provide QoS guarantees Sender-initiated protocols packet transmissions are initiated by the sender node Single-channel sender-initiated protocols the total bandwidth is used as it is, without being divided Multi-channel sender-initiated protocols available bandwidth is divided into multiple channels; this enabled several nodes to simultaneously transmit data Receiver-initiated protocols the receiver node initiates the contention resolution protocol [SelfOrg], SS

15 Classification of MAC Protocols Contention-based protocols with reservation mechanisms Support for real-time traffic using QoS guarantees Using mechanisms for reserving bandwidth a priori Synchronous protocols require time synchronization among all nodes in the network global time synchronization is generally difficult to achieve Asynchronous protocols do not require any global time synchronization, usually rely on relative time information for effecting reservations Contention-based protocols with scheduling mechanisms Focus on packet scheduling at nodes and also scheduling nodes for access to the channel requirement for fair treatment and no starvation Used to enforce priorities among flows Sometimes battery characteristics, such as remaining battery power, are considered while scheduling nodes for access to the channel [SelfOrg], SS

16 Contention-Based Protocols Carrier sense multiple access (CSMA) General behavior 1. Sender senses the channel for the carrier signal 2. If carrier is present, it times out a random period of time before retrying CSMA does not overcome the hidden terminal problem as well as the exposed terminal problem Other approaches / solutions Multiple access collision avoidance Busy tone multiple access [SelfOrg], SS

17 MACA Protocol Multiple Access Collision Avoidance Use of additional signaling packets RTS (ready-to-send) CTS (clear-to-send) General behavior If a packet is to be sent, a RTS packet is transmitted If the receiver is ready to receive the packet, it answers with a CTS packet Once the sender successfully receives the CTS without an error, it transmits the data packet If a packet is lost (collision), the node uses the binary exponential back-off (BEB) algorithm to back off for a random interval of time before retrying Each time a collision is detected, the node doubles its maximum backoff window RTS and CTS packets carry the expected duration of the data packet transmission to overcome the hidden terminal problem [SelfOrg], SS

18 MACA Protocol [SelfOrg], SS

19 MACAW Protocol The binary back-off mechanism can lead to starvation of flows Example S1 and S2 are generating a high volume of traffic If one node (S1) starts sending, the packets transmitted by S2 get collided S2 backs off and increases its back-off window the probability of node S2 acquiring the channel keeps decreasing Solution Each packet carries the current back-off window of the sender A node receiving this packet copies this value into its back-off counter [SelfOrg], SS

20 MACAW Protocol Large variations in the back-off values the back-off window increases very rapidly and is reset after each successful transmission Solution multiplicative increase and linear decrease (MILD) back-off mechanism (increase by factor 1.5) Fairness MACA: per node fairness MACAW: per flow fairness (one back-off value per flow) Error detection Originally moved to the transport layer Slow and introducing much overhead Solution New control packet type: acknowledgement (ACK) [SelfOrg], SS

21 MACAW Protocol Exposed terminal problem RTS/CTS mechanism does not solves the exposed terminal problem Solution New control packet type: data-sending (DS), a small packet (30 Byte) containing information such as the duration of the forthcoming data transmission [SelfOrg], SS

22 MACAW Protocol [SelfOrg], SS

23 BTMA Protocol Busy Tone Multiple Access The transmission channel is split into data and control channel General behavior When a node wants to transmit a packet, it senses the channel to check whether the busy tone is active If not, it turns on the busy tone signal and starts transmission Problem: very poor bandwidth utilization [SelfOrg], SS

24 DBTMA Protocol Dual Busy Tone Multiple Access Basic idea RTS/CTS on the control channel Two busy tones: BT t and BT r Performance gain Better network utilization than MACA/MACAW (about twice than most RTS/CTS schemes) [SelfOrg], SS

25 Contention-Based Protocols with Reservation MACA/PR MACA with Piggy-Backed Reservation Multi-hop routing protocol based on MACAW Main components MAC protocol Reservation protocol QoS routing protocol Differentiation of real-time and best-effort packets General behavior Slotted mechanisms Maintenance of a reservation table (RT) at each node that records all the reserved transmit and receive slots / windows of all nodes within its transmission range Network allocation vectors (NAV) for cycles Destination sequenced distance vector (DSDV) used for routing TDM-like system for real-time traffic Best-effort traffic using MACAW in free slots [SelfOrg], SS

26 MACA/PR Protocol [SelfOrg], SS

27 MAC Protocol Using Directed Antennas Properties One receiver per node, which can transmit and receive only one packet at any given time Each transceiver is equipped with M directional antennas Each antenna has a conical radiation pattern spanning an angle of 2π/M radians Basic RTS/CTS scheme (as used in MACA) [SelfOrg], SS

28 MAC Protocol Using Directed Antennas [SelfOrg], SS

29 Power Control MAC Protocol Properties RTS/CTS are transmitted with maximum power p max RTS-CTS handshake is used to determine the required transmission power p desired RTS is received at the receiver with a signal level p r Calculation of p desired Rx thresh minimum necessary received signal strength c constant pmax pdesired = Rxthresh* c pr [SelfOrg], SS

30 Power Control MAC Protocol [SelfOrg], SS

31 MAC protocols for sensor networks Main criteria (+) Collision avoidance (++) Energy efficiency (++) Scalability and adaptivity (+) Channel utilization (-) Latency (-) Throughput Goodput (+) Fairness [SelfOrg], SS

32 Case Study: S-MAC W. Ye, J. Heidemann, and D. Estrin, "An Energy-Efficient MAC Protocol for Wireless Sensor Networks," Proceedings of 21st International Annual Joint Conference of the IEEE Computer and Communications Societies (INFOCOM), vol. 3, New York, NY, USA, June 2002, pp Primary goal To retain flexibility of contention-based protocols While improving energy efficiency in multi-hop networks Approaches to reduce energy consumption from all major sources of energy waste Idle listening Collision Overhearing Control overhead [SelfOrg], SS

33 S-MAC Design Approaches Coarse-grained sleep/wakeup cycle Scheduling Low-duty-cycle operation (1-10%) All nodes choose their own listen/sleep schedules These schedules are shared with their neighbors to make communication possible between all nodes Each node periodically broadcasts its schedule in a SYNC packet, which provides simple time synchronization To reduce overhead, S-MAC encourages neighboring nodes to adopt identical schedules [SelfOrg], SS

34 S-MAC Design Approaches Data transmission RTS-CTS-DATA-ACK exchange Duration field in each packet to indicate the time needed in the current transmission Adaptive listening allows additional energy savings (nodes wake up immediately after the exchange completes for immediate contention for the channel) [SelfOrg], SS

35 S-MAC Energy Savings [SelfOrg], SS

36 S-MAC Performance Energy consumption [SelfOrg], SS

37 S-MAC Performance Latency [SelfOrg], SS

38 S-MAC Performance Energy vs. Latency and Throughput [SelfOrg], SS