Medium Access Control in Wireless Sensor Networks USC/ISI Technical Report ISI-TR-580, October 2003 Wei Ye and John Heidemann

Outline  Introduction  Scheduled Protocols  Contention-based Protocols  S-MAC  Performance  Summary

Introduction  The role of medium access control (MAC) Controls when and how each node can transmit in the wireless channel  Why do we need MAC? Wireless channel is a shared medium Radios transmitting in the same frequency band interfere with each other – collisions

MAC Attributes and Trade-offs  Collision avoidance basic task of all MAC protocols  Energy efficiency important for sensor network  Scalability and adaptivity a good MAC protocol should accommodate the changes in size, density, and topology

MAC Attributes and Trade-offs  Channel utilization how well the entire bandwidth of the channel is utilized in communications  Latency Refers to the delay from when a sender has a packet to send until the packet is successfully received by the receiver  Throughput often measured in bits or bytes per second  Fairness

Energy Efficiency in MAC Protocols  Collision major problem in contention protocols, but is generally not a problem in scheduled protocols  Overhearing when a node receives packets that are destined to other nodes  Control packet overhead

Energy Efficiency in MAC Protocols  Idle listening Is a dominant factor of radio energy consumption often 50~100% of energy required for receiving Idle:Receiving:Transmission  Stemm and Katz : 1:1.05:1.4  Wavelan card : 1:2:2.5  Mica2 mote : 1:1:1.41

Classification of MAC Protocols  According to the underlying mechanism for collision avoidance, MAC protocols can be broadly divided into two groups Scheduled-based Protocols  Scheduled nodes onto different sub-channel  Ex: TDMA FDMA CDMA Contention-based Protocols  Nodes compete in probabilistic coordination  Ex: ALOHA CSMA

Scheduled Protocols: TDMA  TDMA divides the channel into N time slots  N slots comprises a frame, which repeats cyclically  Typically, mobile nodes communicate only with the base station (low-duty-cycle)

Scheduled Protocols: TDMA  Disadvantages Requires nodes to form clusters Inter-cluster communications and interference need to be handled by other approaches, such as FDMA or CDMA limited scalability and adaptivity

Example : LEACH  TDMA  organizes nodes into cluster hierarchies  Cluster head is rotated among nodes within a cluster depending on their remaining energy levels  Nodes in cluster only talks to head  Cluster heads talk to base station over a long-range levels

Example : Bluetooth  Designed for personal area networks (PAN) with target nodes as battery- powered PDAs, cell phones, and laptops  Organizes nodes into clusters, called piconets  Inter-cluster communication uses Frequency-hopping CDMA

Example : Bluetooth  Master use polling to decide which slave to transmit a special TDMA without pre-assigned slots  Each piconet has a master and up to 7 active slaves lack of scalability  Multiple connected piconets form a scatternet difficult to handle inter-cluster communications

Contention-based Protocols  A common channel is shared by all nodes and it is allocated on-demand  Advantages 1. scale more easily across changes in node density or traffic load 2. more flexible as topologies change (no need to form cluster) 3. do not require fine-grained time synchronizations as in TDMA protocols

Contention-based Protocols  Disadvantages inefficient usage of energy node listen at all times and collisions and contention for the media can waste energy  Overcoming this disadvantage is required if contention-based protocols are to be applied to long-lived sensor networks

Example : CSMA (Carrier Sense Multiple Access)  Listening (carrier sense) before transmitting  Send immediately if channel is idle  Backoff if channel is busy  non-persistent, 1-persistent and p- persistent In p-persistent CSMA, a node transmits with probability p if the medium is idle, and with probability (1-p) to back off and restart carrier sense

Hidden Terminal Problem

Example : CSMA / CA (collision avoidance)  Establish a brief handshake between sender and receiver before transmits bac RTS CTS Request to send Clear to send Still have problem, but greatly reduced (short)

Contention Protocols: MACA and MACAW  MACA Based on CSMA/CA Add duration field in RTS/CTS informing other node about their back-off time  MACAW Improved over MACA RTS/CTS/DATA/ACK Fast error recovery at link layer

Power save (PS) mode in IEEE DCF  Assumption: all nodes are synchronized and can hear each other (single hop)  Nodes in PS mode periodically listen for beacons & ATIMs (ad hoc traffic indication messages)

Power save (PS) mode in IEEE DCF  Beacon: timing and physical layer parameters All nodes participate in periodic beacon generation One node periodically broadcasts a beacon  ATIM: tell nodes in PS mode to stay awake for Rx ATIM follows a beacon sent/received Unicast ATIM needs acknowledgement Broadcast ATIM wakes up all nodes — no ACK

Power save (PS) mode in IEEE DCF

 Problems for multi-hop Clock synchronization Neighbor discovery  Tseng et al. proposed a resolution do not synchronize Listen intervals of two nodes periodically overlap Disadvantages  1.control overhead  2.longer delay

Contention Protocols: Piconet  Develop by Bennett et al. not the same piconet in Bluetooth use 1-persistent CSMA protocol each node sleeps autonomously beacon their ID when wake up Sending node needs to listen for receiver ’ s beacon first

Contention Protocols: PAMAS (Power Aware Multi-Access with Signalling )  Proposed by Singh and Raghavendra  Try to reduce the overhearing problem but not idle listening  Improve energy efficiency from MACA  Use two channels, one for data and one for control  Node who try to transmit probe in the control channel  If any neighbor answers the probe, the node will go back to sleep  Disadvantage 1. need two radio system 2. does not reduce idle listening

Case Study : S-MAC  Basic Scheme is similar to PS mode without assuming single-hop  Major components in S-MAC Periodic listen and sleep Collision avoidance Overhearing avoidance Massage passing

Scheduling  Establish low-duty-cycle operation About 1-10%  Every node are free to choose their own listen/sleep schedules  Periodically broadcast each schedule for few listen/sleep frame in a SYNC packet (synchronization)

Scheduling  Encourages neighboring nodes to adopt identical schedules When first configures, it listen for a sync period and adopts the first schedule it hears  Periodically perform neighbor discovery, listening for an entire frame, to discovery node on different schedules  the listen period is significantly longer than clock error or drift (loose sync)

Data Transmission  Collision avoidance is simmilar to IEEE DCF  Contention only happens at a receiver ’ s listen interval  Unicast packet: CSMA + RTS-CTS- DATA-ACK (MACAW)  Broadcast packet: CSMA only  Put duration field in each packet ease the overhearing problem

Message Passing  Problem: Sensor networks in-network processing requires entire message  Solution: Long message is fragmented & sent in burst  Only one RTS and CTS packet are used to reserve medium for entire message (by duration field)  Several fragments and ACKs (with duration field)  Fragment-level error recovery ACK for each fragment extend Tx time and re-transmit immediately  Other nodes sleep for whole message time

Adaptive listening  Problema -> b -> c  solution listen b->c listen a->b listen b->c a->b Overall multi-hop latency can be reduced by at least half

Performance Measure with Mica motes

Performance Only one message in the network at a time

Summary  This paper reviews MAC protocols for wireless sensor networks  It described both scheduled and contention-based MAC protocols  Finally, it presented S-MAC as an example