MAC Protocols AT THE END OF THIS SECTION, YOU SHOULD HAVE AN UNDERSTANDING OF THE MAC LAYER PROTOCOLS FOR SENSOR NETWORKS AND THEIR BASIC CHARACTERISTICS References H. Karl and A. Willing. Protocols and Architectures for Wireless Sensor Networks. John Wiley & Sons, 2005. (General) C. Schurgers, V. Tsiatsis, S. Ganeriwal, and M. Srivastava. Optimizing Sensor Networks in the Energy-Latency-Density Design Space, in IEEE Trans. on Mobile Computing, Vol. 1, No. 1, pp. 70-80, January-March 2002. (STEM) W. Ye, J. Heidemann, and D. Estrin. An Energy-Efficient MAC Protocol for Wireless Sensor Networks, in the Proc. of IEEE INFOCOMM, pp. 1567-1576, June 2002.(S-MAC) E. H. Callaway,Jr. Wireless Sensor Networks: Architectures and Protocols, Auerbach Publications, Chapter 4, 2004. (The meditation device protocol) L. C. Zhong, R. Shah, C. Guo, and J. Rabaey. An Ultra-Low Power and Distributed Access Protocol for Broadband Wireless Sensor Networks, in the Proc. of IEEE Broadband d Wireless Summit, May 2001. (Wakeup k radio) ) A. Woo and D. E. Culler. A Transmission Control Scheme for Media Access in Sensor Networks, in Proc. of the 7 th Annual Int l Conf. on Mobile Computing and Networking (MobiCom), July 2001. (CSMA) S. Singh and C. S. Raghavendra. PAMAS-Power Aware Multi-Access Protocol with Signaling for Ad Hoc Networks, in ACM SIGCOMM Computer Communication Review, Vol. 28, Issue 3, pp. 5-26, July 1998. (PAMAS) W. R. Heinzelman, A. Chandrakasan, and H. Balakrishnan. Energy-Efficient Communication Protocol for Wireless Microsensor Networks, in the Proc. of the 33 rd Hawaii Int l Conf. on System Sciences, Vol 2, 1o pp, January 2000. (LEACH) K. Sohrabi, J. Gao, V. Ailawadhi, and G. J. Pottie. Protocols for Self-Organization of a Wireless Sensor Network, in IEEE Personal Communications, Vol. 7, issue 5, pp. 16-27, October 2000. (SMACS) V. Rajendran, K. Obraczka, and J. J. Garcia-Luna-Aceves. Energy-Efficient Collusion-Free Medium Access Control for Wireless Sensor Networks. In the Proc. of the 1 st Int l Conf. on Embedded Networked Sensor Systems (SenSys), pp. 181-192, November 2003. (TRAMA) LAN/MAN Standards Committee of the IEEE Computer Society. IEEE Standard for Part 15.4: Wireless Medium Access Control (MAC) and Physical (PHY) October 2003. (IEEE 802.15.4 MAC) 2 Medium Access Control (MAC) Typical of other networks as well, the transmission medium must be shared in WSN. This is known as channel allocation or multiple access problem. The objective of a MAC protocol is to regulate access to the shared wireless medium such that t the performance requirements of the application are met. MAC protocols have been extensively studied in (wireless) voice and data communications. 3 1
MAC protocols: Issues 4 Nodes need to exchange some information for the right to access the communication channel at any given time. This requires the use of the communication channel itself (recursive use?)! Spatial distribution of the nodes further complicates the problem, as any information gathered by a node is at least as old as the time required for its propagation Two intertwined factors influence the aggregate behavior of a distributed MAC: the intelligence of the decision (to allow a node to transmit) made by the protocol the overhead involved MAC protocols: Performance metrics Delay the amount of time a data packet spends in the MAC layer Throughput the rate at which packets are serviced Robustness combination of reliability, availability, and dependability requirements Scalability the ability to meet the performance requirements regardless the size of the network Stability the ability to handle the traffic load over sustained periods of time Fairness allocation of the channel capacity evenly Energy efficiency paramount issue importance in WSNs 5 MAC protocols: Classification 6 MAC protocols Fixed assignment Demand assignment Random assignment FDMA centralized ALOHA TDMA CDMA distributed CSMA SDMA 2
MAC protocols: Classification2 Schedule based regulating participants: Who may use which resource at which time (TDMA) Which frequency band can be used in a given physical space (with a given code, CDMA) Schedule can be fixed or computed on demand Contention based Risk of colliding packets is deliberately taken Coordination overhead can be saved, resulting in overall improved efficiency Mechanisms to handle/reduce probability/impact of collisions required Usually randomization used 7 MAC protocols: Fixed assignment The available resources are allocated between the nodes; no competition FDMA used by radio systems to share the (radio) spectrum; requires frequency synchronization TDMA a digital technology that uses a single frequency channel CDMA spread spectrum based scheme that allows multiple nodes to transmit simultaneously SDMA spatial separation of the nodes is used to separate their transmissions 8 MAC protocols: Demand assignment To improve channel utilization by allocating the channel to contending nodes (near-)optimally Need a control mechanism to arbitrate access Centralized polling a master control devices queries each slave node in a predetermined order Distributed reservation and token Set a time slot for reservation messages Token holder can transmit 9 3
MAC protocols: Random assignment 10 Address the short comings of fixed (static) assignment schemes Originally developed for long radio links and satellite communications ALOHA pure, slotted CSMA CD, CA Carrier Sensing Contention Non-persistent CSMA, persistent CSMA, backoff Busy-tone, RTS/CTS Hidden/exposed terminal problem The hidden-terminal: Consider the following three nodes: A and B are in mutual range, and B and C are in mutual range. But A and C cannot hear each other, thus their packets may collide. The exposed-terminal: B transmits a packet to A, and C wants to transmit a packet to D, but it cannot, because it hears B s activity. 11 A B C D RTS/CTS handshake 12 A B C D RTS CTS Data Channel busy Channel busy Ack 4
RTS/CTS failure 13 A B C D RTS CTS RTS Data RTS CTS RTS/CTS failure2 14 A B C D RTS CTS RTS Data CTS Data MAC Protocols for WSNs The balance of requirements is different in WSNs. Transmission is costly; receiving often has the same cost as transmission; idling can be cheaper but also as expensive as receiving; sleeping is almost free, but Energy problems on the MAC layer: Collusion Useless receive costs at destination, i useless transmission i costs at sender Overhearing Receiving a packet addressed for someone else Sometimes good, though, when collecting information about the WSN Protocol overhead MAC related control packets, Idle listening Costly; useless in low network loads 15 5
Energy conserving 16 A number of different MAC protocols are proposed for wireless sensor networks based on their ability to conserve energy: Protocols that explicitly attack the idle listening problem Contention-based protocols are probabilistic as the nodes risk collusions. Thus, these protocols have collusion avoidance mechanisms Schedule-based protocols, where only one node gets an opportunity based on an allocated slot Idle listening: Low duty cycle protocols 17 Try to avoid spending much time in the idle state and to reduce the communication activities to a minimum. Periodic wakeup schemes Cycled receiver; sleep periodically to receive packets (needs knowledge of the listeners) Observations: Small duty cycle transmitter is in sleep mode mostly Small duty cycle traffic to a given node is concentrated in a small time window; significant competition under heavy loads Long sleep period significant per-hop latency Short sleep periods high startup costs Idle listening: STEM 18 Sparse topology and energy management (STEM) Does not cover all aspects of a MAC protocol but provides a solution for idle listening problem and to provide fast transition into the transfer state, if necessary. Targets networks with wait-and-report behavior topology in the name: as nodes enter and leave sleep mode, network topology changes. Key requirement: the network stays connected even if some nodes are sleeping Network states Monitor state Transfer state 6
Idle listening: STEM2 Two channels Wakeup time slots (wakeup periods of length T) Data for underlying MAC protocol Listen period of length T RX << T and a sleep period 19 Wakeup period Listen period Wakeup channel Sleep period Data channel Idle listening: STEM-B The transmitter issues beacons on the wakeup channel periodically without carrier sensing Beacon contains MACs for the sender and the receiver Receiver sends an ACK on the wakeup channel on the receipt of the beacon Both nodes switch transmitters and execute regular MAC protocol Other nodes receiving the beacon not destined to them, will continue sleeping Beacon is sent continuously for at least one full wakeup period 20 Idle listening: STEM-T Transmitter sends a busy tone (no address information) on the control channel for a long time to hit on the receivers listen period All neighbor nodes switch on their data channel Packet exchange will start; all not-involved nodes will go back to sleep 21 7
Idle listening: STEM: Discussion 22 In STEM-B beacon collusions cause the scheme to fall back to STEM-T; the transmitter transmits the beacons for the maximum time, then switches to the data channel and tries the conversation with the receiver A node entering into listen period remains silent Low load situations STEM-T is preferable over STEM-B (why?) Wakeup latency is related to wakeup time T STEM-B can achieve half the wakeup latency of STEM-T if no collusions occur on the wakeup channel (how?) STEM-T can have energy consumption advantages Idle listening: S-MAC Developed at UCLA, sensor-mac (S-MAC) protocol provides mechanisms to circumvent the key energy problems Adopts a similar wakeup scheme as STEM, but requires only a single channel Listen period can be used to receive and transmit Wakeup period 23 Listen period Sleep period For SYNCH For RTS For CTS Idle listening: S-MAC: Listen phases 24 SYNCH phase Node x accepts SYNCH packets (describing their schedule) from its neighbors Divided into time slots RTS phase x listens for RTS packets from neighboring nodes (RTS/CTS procedure is used to reduce collisions of data packets due to hidden node problem CTS phase Node x transmits a CTS packet if an RTS packet is received in the previous phase. After this, packet exchange continues (into x s sleep period) 8
Idle listening: S-MAC: the protocol 25 When competing for the medium, the nodes use RTS/CTS handshake (including the virtual carrier sense mechanism via NAV); NAV can also be used to avoid overhearing When broadcasting (e.g., SYNCH packets), the nodes use CSMA with backoff Neighbors agree on the same schedule (of time slots) and create virtual clusters Idle listening: S-MAC: Virtual clusters Node x listens for at least the globally known synchronization period. x receives a SYNCH packet from a neighbor Adopts the announced schedule Broadcasts is in one of the neighbors next listen period x picks a schedule and broadcasts it x receives another node s schedule during the broadcast packet s contention period; drops its own and follows the other x receives a new schedule after its own Some neighbors use his schedule transmits its SYNCH and data packets according to both schedules No neighbor share its schedule drops own and adopts the other x periodically listens for a whole synchronization period to relearn its neighbors 26 Idle listening: S-MAC: Virtual clusters2 A large multihop network is partitioned into islands of schedule synchronization Border nodes have to follow two or more schedules (more energy consumption!) Nodes spend much time in the sleep mode Pay the price in latency; per-hop latency equals to the sleep period! Adaptive-listening reduces the latency by half 27 9
Idle listening: S-MAC: Synchronized islands 28 A A A A A A C B E B B B B E E E E E E C C C C D D D D Time 28 Idle listening: S-MAC: Message passing 29 A message larger than a packet is meaningful to an application In-network processing requires aggregating node to receive a message completely But, on wireless media, it is advisable to break a long packet into smaller ones Solution: Fragmentation: A series of fragments is transmitted with a single RTS/CTS exchange between nodes A (sender) and B (receiver) B Acks each; Duration field (in all the packets) indicates the remaining time for the whole transaction Idle listening: S-MAC: Message passing2 30 The fragmentation scheme is similar to the one used in 802.11 In 802.11, CTS and RTS reserves the medium for only the time of the first fragment, and any other frames does it for the next fragment S-MAC can be unfair at times, but fairness has a lesser weight T-MAC is a variation of S-Mac, where it adaptively shortens the listen period 10
Idle listening: Mediation device protocol Compatible with the P2P communication mode of the 802.15.4 WPAN standard Each node go into sleep periodically and wakeup for short periods to receive packets from neighbors No global time, each node has its own schedule At wakeup, a node transmits a short query beacon with its node address; no packets? Go back to sleep Dynamic synchronization is used A meditation device 31 Idle listening: Wakeup radio 32 Ideally, a node is in receiving state when a packet is transmitted for it, and a node is in transmitting state when it has a packet to transmit. All other times, it should sleep no idle time! One proposed wakeup MAC protocol assumes the presence of several data channels (it basically extends CSMA into multi-channel CSMA) A separate, ultra low power radio is used Idle listening: Wakeup radio The algorithm 33 A node wishing to transmit will randomly pick up a channel. If busy, will continue selecting a channel until found one, else set a timer for each and backoff The node then transmits a wakeup signal over its wakeup radio channel to the receiver asking it to turn on its main radio During idle times, only the wakeup radio is turned on 11
Idle listening: Wakeup radio The algorithm2 Problems: No such radio exists Ranges of both radios much be identical 34 Shorter range for wakeup radio not all potential nodes can be woken up Wider range for wakeup radio potential hazard in addressing The wakeup radio has to be able to carry information higher complexity Contention-based: CSMA protocols CSMA protocols are contention-based, where neighbors try their luck to transmit their packets Woo and Culler consider a multihop network with a single or a few sinks with the same traffic pattern as STEM 35 Contention-based: CSMA The protocol 36 Idle C: A: numtrials = 0 C: busy & numtrials = Max A: failure Random Delay C: non or foreign CTS & numtrials = Max A: failure Backoff C: timeout A: Listen C: idle A: send RTS C: busy & numtrials < Max A: numtrials++; set timer Await CTS C: non or foreign CTS & numtrials < Max A: numtrials++; set timer C: no Ack & numtrials < Max A: numtrials++; set timer C: no Ack & numtrials = Max A: failure C: got CTS A: send data Await Ack C: got Ack A: success Idle 12
Contention-based: CSMA The protocol2 Energy saving measures the node s transceiver can sleep during the random delay in the backoff mode Have been investigated: No random delay 37 Random listening time vs constant listening time Fixed window backoff vs exponentially increasing backoff vs exponentially decreasing backoff vs no backoff Protocols with random delay, fixed listen time, and a backoff algorithm with sleeping radio transceiver give the best throughput and lowest aggregate energy consumption Contention-based: PAMAS 38 The Power Aware Multi-access with Signaling (PAMAS) is originally designed for ad hoc networks, detailed overhearing avoid mechanism no idle listening solution Combines busy tone with RTS/CTS handshaking Uses two channels: data and signaling Contention based: PAMAS The protocol 39 New RTS; Send CTS * No packet or noise Await packet 1 time unit Receive RTS; Send CTS * Idle Receive RTS; Send CTS * Packet to send; send RTS to destination if not transmitting End of transmission Await CTS Receive RTS; Send CTS * No CTS or Busy tone or unrelated RTS BEB Time expired and destination is not transmitting; Send RTS Packet is arriving; Transmit busy tone * if data channel is idle and No noise over signaling channel Receive packet Packet received Receive CTS Transmit packet Busy tone > 2*CTS length Receive other RTS; Transmit busy Tone. Ignore all CTSs received Ignore RTS/CTS transmissions 13
Contention-based: PAMAS To power off or... Conditions to power off: No packet to transmit and a neighbor is begins transmitting One neighbor is transmitting and another is receiving Two questions to answer: How long to power off? When a packet transmission begins in the neighborhood, sleep l When wakeup, if the data channel is busy, go back to sleep, but how long? Probe protocol Send a probe(l) message l is max packet length Does a binary search to determine when the last transmission ends What happens if a neighbor wishes to transmit when sleeping? Nothing important Good power savings Sparse network, light load 20-30% (high load 10%; longer contention) Dense network, light load 60-70% (high load 30-40%) 40 Schedule-based: LEACH The Low-energy Adaptive Clustering Hierarch (LEACH) assumes dense, homogeneous sensor network with energy constraint nodes and the base station is far away from the sensors themselves Nodes are partitioned into clusters, with a dedicated di d clusterhead node in each cluster (other nodes are member nodes) 41 Member node Clusterhead Schedule-based: LEACH2 Randomized (and dynamic) rotation of clusterheads Self election with a certain probability Self elected clusterheads broadcasts their status to others Nodes determine their clusterheads Once the cluster is formed, the clusterhead creates a schedule for its nodes (nodes turn off their radios during other slots) Local data fusion (data aggregation) to reduce energy dissipation and enhanced system lifetime Compress data is sent to the base station High energy operation, but only a few nodes operate 42 14
Schedule-based: LEACH3 The number of clusters is critical Fewer larger energy consumption (distance to the base is far) More more transmissions to the sink; larger energy consumption 5% of the nodes being clusterheads is optimal 7x-8x energy reduction compared to direct communication 43 Schedule-based: LEACH The algorithm Broken up into rounds; each round has two phases: Setup phase (Clusterhead) advertisement phase CSMA MAC protocol: cluster heads advertise, nodes select Schedule creation Based on the number of members, the clusterhead creates and broadcasts a TDMA schedule Steady-state phase Nodes transmit during their allocated transmission time (uses minimal energy, because of the clusterhead selection) Nodes can sleep until their time (and has data to transmit) Clusterhead receives data, aggregates them, and send to base station To reduce interference clusters use different CDMA codes (and informs the members) 44 Schedule-based: LEACH Rounds 45 Fixed-length round Setup phase Steady-state phase Self-election of clusterheads Time slot 1 Time slot 2 Time slot n Time slot 1 Advertisement phase Cluster setup phase Clusterheads compete with CSMA Members compete with CSMA Broadcast schedule 15
Schedule-based: SMACS Self-organizing Medium Access Control for Sensor Networks (SMACS) It is a protocol in a suite for organization, routing, management for MANETs to optimize for QoS It is an infrastructure-building protocol that forms a flat topology for sensor networks SMACS is a distributed protocol which enables a collection of nodes to discover neighbors and establish schedules for communicating with them without the need of a master node Neighbor discovery and channel assignment phases are combined 46 Schedule-based: SMACS Assumptions The available spectrum is subdivided into many channels (and many CDMA codes are available) Each node can tune to an arbitrary channel Most of the nodes are stationary and remain as such for a long time Each node divides its time locally into fixed-length superframes of T frame length Superframes are subdivided into timeslots A 47 F B D C Schedule-based: SMACS Self-organization Non Synchronous scheduled communications 48 Trans. SLOT D and A find each other T frame Recv. SLOT fx fx Node D T d fx fx Node A T a fy Node B B and C find each other T b fy Node C T c 16
Schedule-based: SMACS Node discovery 49 fy Node B B and C find each other T b fy Node C T c Node B Initial listening time Type 1 Type 2 Type 3 Type 4 Node C Type 1 Type 2 Type 3 Node G (not shown) Schedule-based: SMACS Nodes finding 50 Nodes wake up at random times, and listen to the channel for a random amount of time A node (C) will transmit an invitation (TYPE 1) by the end of its initial listen time if not heard the same from others Nodes hearing the invitation (B and G, not shown), broadcast a response (TYPE 2) during the interval following the reception of TYPE 1 at a random time If responses don t collide and heard by C, C must chose only one respondent (first) Node C sends TYPE 3 immediately after the interval following TYPE 1 to notify all respondents of the chosen one Node G was not chose, it turns off its transmitter for a while and starts the search again If C is already attached, it ll transmit its schedule info along with the time its next superframe will start in the body of TYPE 3 Node B compares the schedules and time offsets and arrives at a set of two free time intervals as the slots assigned to the link between B and C B sends this information with the randomly selected frequency band to node C in the body of TYPE 4 After a pair of short test messages, the link is added to the nodes schedules permanently Schedule-based: SMACS Startup messages TYPE 1 A short invitation containing a node s ID and number of attached neighbors (send by inviter) TYPE 2 Response to TYPE 1 by an invitee; gives the inviter and invitee s IDs and invitee s attached state TYPE 3 Response to TYPE 2; indicating which invitee is chosen. Depending on the node s attached state contains: Inviter not attached: none Both attached; inviter s schedule and frame epoch Invitee not attached, inviter attached: proposed channel for the link TYPE 4 Response to TYPE 3; contains None attached: channel determined by the invitee Invitee not attached, inviter attached: none Invitee attached, inviter not attached: channel determined by the invitee Both attached: channel determined from own and inviter s schedule information 51 17
Schedule-based: TRAMA 52 The Traffic-Adaptive Medium Access Protocol (TRAMA) reduces energy consumption by providing collision-free transmissions and low-power idle state Assumes single time-slotted channel and uses a distributed election scheme to determine which node can transmit at a particular slot Divides time into: Random access: signaling slots Scheduled access: transmission slots Consists of: Neighbor protocol (NP) Schedule exchange protocol (SEP) Adaptive election algorithm (AEA) NP Schedule-based: TRAMA2 53 Propagates one-hop neighbor information among neighboring nodes during random access period (contention based channel acquisition and signaling) SEP Exchange traffic-based information, or schedules (information on traffic originating from a node), with neighbors AEA Selects transmitters and receivers to achieve collision-free transmission using the information from NP and SEP Random transmission collisions Transmitters without receivers energy waste Schedule-based: TRAMA Packet contents 54 Type Source address Destination address DelNum AddNum Deleted Node IDs Added Node IDs Signal header Source Type address Data header Destination address Timeout NumSlots Bitmap Source address Schedule packet Timeout NumSlots Bitmaps Width Bitmaps N=4; 4 one-hop neighbors Reserved Changeover Slot Giveup Schedule announcement 18
Schedule-based: TRAMA NP 55 TRAMA starts in random access mode where each node selects a slot randomly Nodes can only join the network during the random access periods (occur more often in dynamic networks) NP gathers neighborhood information by exchanging small signaling packets, carrying incremental neighborhood updates If no updates, the signaling packets serve as keep-alive beacons A node times out its neighbor if it does not hear from it for a certain period of time The updates are transmitted to ensure 99% probability of success Schedule-based: TRAMA SEP 56 Establishes and maintains traffic-based schedule information required by the transmitter (e.g. slot re-use) and the receiver (i.e. sleep state switching) A node s schedule captures a window of traffic to be transmitted by the node; schedules have timeouts Nodes announce their schedule via schedule packets The intended receiver information is conveyed using a bitmap A schedule summary is also send during data transmission to minimize effects of packet loss in schedule dissemination Nodes maintain schedule information for their one-hop neighbors, which is consulted when needed An unused slot is called Changeover slot; all nodes listen during the Changeover slot of the transmitter to synchronize their schedule Schedule-based: TRAMA AEA At any given time slot t during the scheduled access period, the state of a node u is determined based on its two-hop neighborhood information and the schedules of it s one-hop neighbors; possible states are: transmit (TX), receive (RX), or sleep (SL) Node u is in TX state if (1) u has the highest priority among its contending set and (2) u has data to send Node u is in RX state when it is the intended receiver of the current transmitter Otherwise the node can be turned off to SL state 57 19
Schedule-based: TRAMA Winners 58 The state of a node u depends on the Absolute Winner and the schedules of its one-hop neighbors From node u s perspective, the Absolute Winner at a time slot t can be: Node u itself Node v that lies in the two-hop neighborhood of node u in which case the Alternate Winner atx(u) is to be considered if hidden from node v Node w that lies in node u s one-hop neighborhood The Absolute Winner is the assumed transmitter unless the Alternative Winner is hidden from the Absolute Winner and it belongs to the Possible Transmitter Set The IEEE 802.11.4 MAC protocol 59 The standard covers both the physical and the MAC layers of a low-rate Wireless Personal Area Network (WPAN) The (asymmetric) MAC protocol combines both contention-based and schedule-based schemes Two types of nodes: Full Function Device (FFD); it can be a PAN coordinator, a simple coordinator, and a device A Reduced Function Device (RFD); can operate as only a device A device must be associated with a FFD to form a star network. The coordinator Manages a collection of associated devices Deals with device addressing Assigns short addresses to its devices Regularly transmits frame beacon packets Announces the PAN identifier, list of outstanding frames, etc. Exchanges data packets with devices and with peer coordinators 60 20
The superframe The coordinator operating in beacon mode organizes channel access and data transmission with the help of superframe structure The lengths of the active and inactive periods as well as the length of a single time slot and the usage of GTS slots are configurable 61 Ati Active period id Inactive period Beacon Contention access period (CAP) Guaranteed time slots (GTS) GTS management The coordinator allocates GTS to devices when receive requests packets (for a transmit or a receive slot) during the CAP 62 Answers the request packets in two steps: An immediate acknowledgement After receiving the acknowledgement, the device must track the coordinator s beacons for a while to see when the required time slots are allocated. The device can use the slots as long as they are announced in the GTS descriptor Allocates GTS to a node if has sufficient resources and, until resources become scarce and the GTS is explicitly deallocated Devices can renegotiate if a GTS allocation request fails Data transfer 63 If a device has an allocated transmit GTS, it wakes up just before the slot and sends its packet immediately This is only possible if the allocated slots are large enough to hold the data, as well as the coordinator acknowledgment and appropriate p InterFrame Spaces (IFSs) Otherwise, the data is sent during the CAP using a slotted CSMA protocol The coordinator always sends an acknowledgement packet When the coordinator is unable to use a receive GTS (of the device), a simple handshake protocol is executed and the coordinator transmits the data 21
Slotted CSMA-CA protocol 64 NB=0; CW=2; BE=macMinBE N Await next backoff period boundary CW--; CW=0? Y Success Transmit data Random delay Failure Y Y CCA on backoff period boundary Channel idle? N NB++; CW=2; BE=min(BE+1,a MaxBE) NB>Max CSMA Backoffs? N Nonbeaconed mode 65 The coordinator does not send beacon frames, nor is there any GTS mechanism (time synchronization is disabled) All packets are transmitted using a unslotted CSMA-CA protocol Coordinators must always be on, but devices can follow their own sleep schedule and wake up when To send a data/control packet to the coordinator To fetch a packet destined to self from the coordinator using the data request/acknowledgement/data/acknowledgement handshake Data request packet is sent through unslotted CSMA-CA mechanism (device must stay awake for a certain period of time) IEEE 802.11 and Bluetooth Given a number of wireless MAC protocols particularly IEEE 802.11 and Bluetooth), why not use them? Bluetooth is designed as a WPAN with one major application Connection of devices to a PC It is already been tried for a wireless sensor network application Drawbacks: Constantly need a master polling its slaves Limited number of active slaves (8) per piconet IEEE 802.11 family of protocols have several physical layers to share a single MAC protocol Drawbacks: A node x must constantly be in listen mode since another node y may attempt transmitting to x at the same time Nodes need to overhear RTS/CTS to adjust their NAVs properly Tailored for higher bit rates It is a single-hop protocol for both infrastructure and ad hoc scenarios 66 22
Summary 67 Idle Over Collusion Flat/ # of listening hearing Protocol Overhead Clustered Channels Avoidance STEM Both 2 Periodic sleep STEM-B Depends on MAC wakeup beacons S-MAC Flat 1 Periodic sleep NAV RTS/CTS RTS/CTS; SYNC; Mediation Flat 1 Periodic sleep implicit No Mediator device service; Wakeup radio Flat 2 Wakeup signal Wakeup signal Multichannel Wakeup radio CSMA CSMA Flat 1 --- Sleep RTS/CTS RTS/CTS PAMAS Flat 2 --- Yes RTS/CTS; busy tone Signaling channel LEACH Rotating clusters 1 TDMA TDMA TDMA Cluster formation; SMACS Flat Many TDMA TDMA TDMA Channel setup; TRAMA Flat 1 Scheduling Scheduling Scheduling NP; SEP 23