B-MAC: Versatile Low Power Media Access for Wireless

Sensor Networks

SenSys ’04

Reminder: Proposal Presentation on Monday, March

5• Prepare powerpoint slides to motivate your

project and show the overall approach to tackling the problem

• Come at least 10 minutes before the class begins – Give your slides to the tech staff– Lean how to use the presentation tool etc.

• Teams & presentation order:– Mike & Vic (+ Brian)– Chris & Yan– Dahee & Chao– Surabh– Mehmet

Design goals of B-MAC

• Low power operation• Effective collision• Simple implementation, small code & RAM

size• Efficient channel utilization at low & high

data rates• Reconfigurable by network protocols• Tolerant to changing RF/Networking

conditions• Scalable to large numbers of nodes

Small, configurable MAC

• Export control to higher services to support wide variety of WSN workloads– WSNs are supposed to support various

applications

• S-MAC (discussed in the previous class) is more than a link layer protocol– Drawbacks of S-MAC

• Scalability: A node may have to remember many schedules and wake up accordingly

• MAC layer may not be the best place for sleep scheduling & synchronization

Adaptive, reconfigurable MAC

• Adaptive bidirectional interface for WSN applications– Reconfigure the MAC protocol based on

the current workload– Identify the best parameters for an

arbitrary low power WSN applications at compile or run time & estimate the application’s lifetime

B-MAC Design

• Really simple– CSMA via CCA (Clear Channel

Assessment) & backoff– Low power listening via Preamble– Acknowledgment

• Enable/disable anything above and allow to build anything on top of the configured B-MAC!

B-MAC Interfaces

Clear Channel Assessment

• Effective collision avoidance• Find out whether the channel is idle

– If too pessimistic: waste bandwidth– If too optimistic: more collisions

• Key observation– Ambient noise may change significantly

depending on the environment– Packet reception has fairly constant channel

energy• Software approach to estimating the noise

floor

• Take a signal sample when the channel is assumed to be free– Right after a packet is transmitted or when no valid data

is received– Take exponential moving average (EMA) of the median

signal strength• Works as a low pass filter• Smoothed idle signal level Sm(t) = a * S(t) + (1 - a) * Sm(t-1)

– Sm(t): EMA at time t – S(t-1): Signal strength of ambient noise at t – Sm(t-1): EMA at time t-1

– It contrasts to common threshold-based methods in which only a single sample is taken

– Resilient to time-varying ambient noise

CCA vs. Threshold techniques

Idle

• Threshold: waste channel utilization• CCA: Fully utilize the channel since a valid packet could have no outlier significantly below the noise floor

CCA dynamically adjusts threshold

CCA finds channelbusy/idle status withhigh accuracy

• CCA can be turned on/off– If turned off, a schedule-based protocol, e.g., S-

MAC, can be implemented atop B-MAC• If turned on, initial channel backoff when

sending a message• B-MAC does not set the backoff time, but

signals an event to the higher service that sent the packet

• The higer level service may return an initial backoff time or ignore the event– If ignored, use a short random delay

Low Power Listening: Preamble Sampling

Sender

Receiver

Preamble Send data

Preamble sampling Active to receive a message

S

R

|Preamble| ≥ Sampling period|Preamble| ≥ Sampling period

• Preamble is not a packet but a physical layer RF pulse– Minimize overhead

Optional link layer ACK

• If enabled, ACK is sent immediately after receiving a unicast packet

• Overall, B-MAC is easier to implement than S-MAC– No RTS/CTS

• Is this always good? or ?• We know RTS/CTS can reduce hidden/exposed node

problem • You may have to implement RTS/CTS on your own... • Simple but not very friendly

– No synchronization• No need for a schedule table in S-MAC• But periodic sleep & wake-up is a good approach to energy

saving

Modeling Lifetime• Monitoring applications

• E = Esleep + Elisten + Ed + Erx + Etx

• Given #nodes in the neighborhood, BMAC can estimate the network lifetime

• Lifetime tl = 1/E * Cbatt * V * 60 * 60

Derivation of Lifetime

• Ed = td * cdata * V where td = tdata * r

• Etx = ttx * ctxb * V where ttx = r * (Lpreamble + Lpacket) * ttxb

• Erx = trx * crxb * V where trx ≤ n * r * (Lpreamble + Lpacket) * trxb

– r * sum_i=1^n ( children (i) + 1 )

– Lpreamble ≥ ti / trxb

Derivation of Lifetime (Cont’d)

• Esample = 17.3 uJ

• Elisten ≤ Esample * 1/ti

• tlisten = (tr_init + tr_on + trx/tx + tsr) * 1/ti

• tsleep = 1 – trx – ttx – td – tlisten

• Esleep = tsleep * csleep * V

• Lifetime tl = 1/E * Cbatt * V * 60 * 60

Network Parameters• Scientists may determine the physical location of the nodes &

ideal sampling rate– Compute the parameters to get the best lifetime that B-

MAC can achieve

– Best LPL check interval is the lowest line at a given

network density in the following graph

If n = 20, 50ms checkinterval is optimal

If n=60, 25ms is best

Experiments

• Compare BMAC to S-MAC & T-MAC– T-MAC is similar to S-MAC, but a receiver

goes to sleep if it does not receive any message

– B-MAC & S-MAC: implemented in TinyOS– TMAC: simulated in Matlab

TinyOS Implementation

• B-MAC does not need timestamp

Packet transmission time vs. Checking frequency

• More frequent checking of the radio– Shorter transmission time– More energy consumption

LPL check interval

Channel utilization

• Place n nodes equidistant from a receiver• Increase n to increase load• BMAC relies on higher level services to send data according to the traffic pattern

• Consider hidden node problem too• For example, after a packet is sent to the parent, nodes in the same cell wait for a certain amount of time for the parent to forward the packet up the tree Always work? More difficulty for developing sensing applications?

Energy per byte

End-to-end latency

Contention-Free Protocols

Classic Protocols

• TDMA (Time Division Multiple Access)– A node can sleep when it is not its turn to

send or receive• FDMA (Frequency Division Multiple

Access)• CDMA (Code Division Multiple Access)• No node within two hops can use the

same slot to avoid the hidden node problem

Optimal channel assignment

• Achieve contention-free communication using the minimum number of channels

• The problem of assigning a minimum number of channels for an arbitrary graph is NP-hard– Develop efficient heuristics– Centralized approaches do not scale

Stationary MAC and Startup

• Local synchronization only• Starting phase

– Handshaking on a common control channel– Each link utilizes a unique random

frequency or CDMA frequency hopping code

– Assume there are sufficiently many frequencies or codes

– Periodically use the slot

BFS/DFS-based scheduling

• Breadth-first or depth-first traversals of a data gathering tree

• Every single node is given a slot• BFS might provide more chances for

aggregation• DFS may transmit individual data

more quickly• Global synchronization required

Reservation-based synchornized MAC (ReSync)

• TDMA is not flexible enough to allow the traffic from each node to change over time

• ReSync provides more flexibility• Each node maintains an epoch based on its local

time (or can be synchronized with nearby neighbors)– Select a regular time in each epoch to send a short

intent message• Probability of collisions is low since the intent is very short

– Listen long enough to learn when the neighbor is sending the intent

– The intended receiver wakes up at the corresponding time to receive the message

– No RTS/CTS• Data transmissions are scheduled randomly

Traffic-adaptive medium access (TRAMA)

• Distributed TDMA for flexible & dynamic scheduling of time slots

• Divide time epochs into a set of short signaling slots followed by a set of longer transmission slots

• Key components– Neighbor protocol (NP)– Schedule exchange protocol (SEP)– Adaptive election algorithm (AEP)

Neighbor Protocol

• Nodes exchange one-hop neighbor info during the random access signaling slots– Ensure the slots are long enough to

allow all nodes to get consistent two-hop neighbor info

Schedule exchange protocol

• Each node publishes its schedule during the last winning slot in each epoch

• Use bitmaps to indicate the intended unicast or multicast recipients

• Sleep when not required to transmit or receive

Adaptive election algorithm

• Hash function based on node IDs and time– Ensure there’s a unique ordering of

node priorities within any two-hop region at each time

– A node transmits iff it has the highest priority among the two hop neighbors at the moment

– Sophisticated slot reuse protocol

Summary• MAC protocols in WSNs

– Arbitration for access to the wireless channel– Energy conservation

• B-MAC is a nice building block for diverse applications and workloads– TDMA, for example, can be built on top of it

• Energy savings– Preamble– S-MAC like periodic sleep & wake-up– Adaptive listening in S-MAC & T-MAC– D-MAC & DESS try to minimize the E2E delay, while using sleep

modes• TDMA

– No idle listening– Higher complexity distributed algorithms or tight

synchronization required

Questions?