WiseMAC_ An Ultra Low Power MAC for WiseNET IP11

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                               WiseMAC, An Ultra Low Power MAC for WiseNET
                                                  Amre El-Hoiydi, J.-D. Decotignie / CSEM

                      Abstract                                Synchronized Preamble Sampling                                                                       Simulation Results
WiseMAC is a medium access control protocol            WiseMAC uses the preamble sampling technique                                                Poisson traffic injected in the left nodes, and
designed for wireless sensor networks. It is based     to minimize idle listening [2-3]. The wake-up                                               forwarded in multi-hop towards the right.
on CSMA and uses the preamble sampling                 preamble introduces a power consumption over-                                               Statistics collected on central node, in function
technique to minimize the power consumed when          head at the transmitter, the intended receiver and                                          of the injection rate.
listening to an idle medium. A unique feature of       all overhearers. In order to minimize this over-
this protocol is to exploit the knowledge of the       head, a node learns the sampling schedule of its
sampling schedule of its direct neighbors in order     neighbors. Using this knowledge, a transmission is
to use a wake-up preamble of minimized size.           started just at the right time using a preamble of
This scheme allows not only to reduce the              minimum size, as illustrated in Fig 1. The sampling
transmit and the receive power consumption, but        schedule information is gained through its
also brings a drastic reduction of the energy          inclusion in every acknowledgement packet. The
wasted due to overhearing. Backoff and medium          purpose of the wake-up preamble is to
reservation schemes have been selected to              compensate for the drift between the clock at the
provide fairness and collision avoidance. WiseMAC      source and the destination. As illustrated in Fig. 2,
requires no set-up signaling, no network-wide          this drift is proportional to the time elapsed since
time synchronization and is adaptive to the traffic    the last re-synchronization (i.e. the last time an
load. It provides an ultra-low average power           acknowledgement was received). If both quartzes
consumption in low traffic conditions and a high       have an accuracy of ±θ, the required duration for
energy efficiency in high traffic conditions. It can   the wake-up preamble is
transport sporadic, periodic as well as bursty                          TCDC = min(4θ L, TW )
traffic. Lifetimes of years can be expected using a
single AA alkaline battery under traffic loads up to   where, L is the time between communications and
one message every 10 seconds.                          TW the sampling period.
                                                                                          Arrival, w ait for      If medium idle,
                  Requirements                                                             right moment
                                                                                                            WAIT W DATA
The WiseMAC protocol has been designed to                SOURCE

operate on the WiseNET system-on-a-chip deve-                                                                         TP
                                                                                                     T                                      DOZE
loped at CSEM, which includes a low power FSK            DESTINATION
                                                                                                     W                                ACK
radio transceiver (dual-band 434 and 868 MHz),            Wake up,                Wake up,
                                                                                                         Wake up,                           TX
                                                                                                       medium busy,         T      TA TC
the CoolRISC 8 bits low power microcontroller            medium idle             medium idle         receive message

core, a random access memory as well as digital                   Fig. 1: Synchronized Preamble Sampling
and analog interfaces [1]. The WiseNET chip is
designed on the 0.18 µm standard digital CMOS                                  t =0          tx time =L-2θ L           t =L
process. It is operating from 1.5 V to 0.9 V, such                                                                               DATA
that a single inexpensive alkaline battery can be             SOURCE

used as the energy source. The WiseNET radio                                                                   θ L θL θ L θ L

transceiver targets an ultra-low power consump-                          ACK                                        TCDC
tion of 2 mA in receive mode, as well as a short
power-on latency of 800 µs. In transmit mode, the                                L
power consumption will be of about 30 mA to
reach 10 dBm output power (before the antenna).                         Fig. 2: Clock Drift Compensation
These characteristics permit the usage of the
preamble sampling technique at a low average           In addition to the clock drift compensation
power cost.                                            preamble, WiseMAC uses a medium reservation
                                                       preamble (MRP) of randomized duration to
        Main Characteristics of WiseMAC                prevent collisions between two or more nodes that                                                        Comparison with S-MAC [4]
• Based on CSMA with preamble sampling;                want to send a data frame to the same relay and
                                                       at the same target sampling time. After the wake-
• Uses link-level acknowledgements to allow for
                                                       up preamble, the WiseMAC data frame includes a
reliable communication and local synchronization;
                                                       bit synchronization preamble (SYNC) and a start
• The length of the preamble is minimized through      frame delimiter (SFD), as shown in Fig. 3.
the exploitation of the local synchronization;
                                                                                                                                                                                                                  listen sleep listen
• The length of the preamble is randomized to                          WUP (Wake- Up Preamble)

provide a collision avoidance reservation scheme;                         MRP         CDCP     SYNC SFD        MPDU

• A backoff mechanism provides fairness and col-                                                                   MAC Protocol Data Unit

lision avoidance;
                                                                                                           Start Frame Delimiter
                                                                                                    Bit Synchronization Preamble
                                                                                       Clock Drift Compensation Preamble
• The 'more' bit in data headers permits the                                 Medium Reservation Preamble

transport of traffic bursts;
                                                                       Fig. 3: WiseMAC Frame Format
• Uses a receive threshold above the sensitivity
threshold to minimize useless wake-ups;
                                                                       Latency / Energy Trade-Off                                                                                References
• The carrier sensing range is chosen to be larger                                                                                                 [1] F. Giroud et al., "Wisenet: Design of a low-power RF CMOS receiver chip for
than the interference range to mitigate the hidden                                                       D ≈ TW / 2
                                                                                                                                                   wireless communication", in CSEM Scientific and Technical Report, Page 22, 2001.

node effect;                                                                                                ( PRX − PDOZE )(TSe + TSymbol )
                                                                                                                                                   [2] A. El-Hoiydi, "Aloha with Preamble Sampling for Sporadic Traffic in Ad Hoc
                                                                                                                                                   Wireless Sensor Networks", in Proc. IEEE ICC , New York, pp. 3418-3423, 2002.
                                                                                      PIDLE = PDOZE +
• Data frame repeated in long wake-up preambles                                                                                  TW
                                                                                                                                                   [3] A. El-Hoiydi, J.-D. Decotignie, C. Enz, E. Le Roux, "Poster Abstract: WiseMAC,
                                                                                                                                                   An Ultra Low Power MAC Protocol for the WiseNET Wireless Sensor Network", in
to reduce the frame error rate, mitigate over-                                                                                                     Proc. ACM SenSys, Los Angeles, 2003.

hearing and detect interferences.                                                                              TW = 200 ms                         [4] W. Ye, J. Heidemann, D. Estrin, “An Energy-Efficient MAC protocol for Wireless
                                                                                                                                                   Sensor Networks”, in Proc. INFOCOM, New York, 2002.

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