Wireless LAN MAC protocols

1
Wireless LAN MAC protocols

• Murat Demirbas
• SUNY Buffalo
• CSE Dept.

2
MAC protocol categories

• Fixed assignment
• TDMA (Time Division), CDMA (Code division), FDMA
  (Frequency division)
• Unsuitable for dynamic, bursty traffic in
  wireless networks
• Random assignment
• ALOHA, CSMA (Carrier Sense)
• Predominantly used in wireless networks 802.11,
  802.15, etc.
• On-demand assignment
• Token ring
• Hard to implement requires static topology or
  neighbor discovery
• E.g., cellular networks use ALOHA for
  registration and CDMA for communication

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Goal of MAC layer

• The goal is to provide access control to manage
  multiple access
• Multiple nodes share a common channel to
  communicate (in contrast to point-to-point)
• Maximization of throughput (channel utilization)
• Minimization of latency
• Fairness
• Stability

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Challenges for MAC layer

• Transmitter collision detection is impossible
• The transmit power at the node swamps its
  receiver
• Pausing while transmission does not help since
  collisions happen on the receiver side and not
  necessarily at the sender!
• Mechanisms to cope with it
• CSMA/CD (Collision Detection) as in Ethernet is
  not viable
• CSMA/CA (Collision Avoidance) is used Random
  backoff upon detecting channel busy
• Also receiver-side CD may be used to inform any
  senders about a collision

5
Challenges for MAC layer

• Hidden terminal problem
• Two senders not in range of each other (Carrier
  Sensing fails), but in range of a common
  receiver
• Mechanisms to cope with it
• RTS/CTS handshake alleviates the problem for
  unicast traffic
• A sending node wishing to send data sends a
  Request to Send frame. The destination node
  replies with a Clear To Send frame. Any other
  node receiving either the RTS or the CTS frame
  should refrain from sending data for a given
  time.

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Challenges for MAC layer

• Exposed terminal problem
• Sensing the medium as busy and not sending, even
  though no collision will occur at the receiver
• Mechanisms to cope with it
• RTS/CTS
• Not as serious a problem as hidden terminal
• Also this is the right behavior for protocols
  that require an ACK

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Challenges for MAC layer

• Power saving
• Listening idly costs almost as much power as
  transmitting
• Scheduling sleep cycles is hard since sender and
  receiver should be wake up at the same time
• Mechanisms to cope with it
• Smart scheduling of sleep cycles

8
Challenges for MAC layer

• No support for reliable broadcast
• ACKs are useful only for unicast traffic, for
  multicast/bcast ACK implosion occurs
• Mechanisms to cope with it
• Use a dedicated slot to report collisions only
• May not address fading effects

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Wireless LAN MAC protocols

• ALOHA
• CSMA
• BTMA
• MACA
• GAMA
• EY-NPMA
• WSN MAC implementations

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ALOHA

• Hawaii 1970
• Node sends a data when it has data
• If no ACK received, data is re-send after random
  backoff
• No carrier sensing
• Works for low network contention, peak
  performance 18

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CSMA

• Carrier sensing before sending the node monitors
  the channel, if channel is busy, the node
  backoffs for a random time
• Used in 802.11, 802.15, WSN MAC layers, etc.

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BTMA

• Busy-tone multiple access
• Each node has two freqs data and control
• Solves the hidden exposed terminal problem as
  follows
• While a node is receiving on the data channel, it
  places a busy-tone on the control channel
• A sender sends iff it does not hear a busy-tone
• Downsides
• Having two frequencies sufficiently apart for
  each node is impractical
• Can be emulated (though expensive) via special
  busy-tone time-slot pays off for applications
  with long data transfers
• Links are asymmetric not hearing busy does not
  imply collision freedom
• Amplitude busy-tone

13
MACA

• Multiple access Collision Avoidance
• First-time RTS/CTS used
• All nodes (except the original sender) hearing
  CTS will defer transmission
• Solves hidden and exposed terminal problems

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GAMA

• Group Allocation Multiple Access
• Contention period and Data period (CSMA TDMA)
• In the contention period, nodes that have data to
  send contend via CSMA
• In the data period nodes in the transmission
  group transmit data respectively
• When network is lightly loaded GAMA behaves as
  CSMA, when it is crowded GAMA behaves as TDMA

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EY-NPMA

• Efficient leader election idea
• An elimination round where each node bcast a
  random priority-based length burst determines
  which node will have access to the channel in the
  communication round.
• The leader node will know it won because when it
  stops transmission of its burst the channel will
  be idle.
• Does not solve hidden terminal problem
• Might be useful for WSN MAC where best-effort
  light-weight solutions are preferred

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Remaining big challenge Multihop

• Guarantees or fairness over multihop
  communication is challenging due to contention at
  every hop

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WSN MAC implementations

• Best-effort light-weight solutions
• CSMA is implemented
• Later MACs implement RTS/CTS
• Some MACs implement ACK
• Popular TinyOS MACs
• CC1000 MAC (default with TinyOS 1.1.x)
• SMAC
• BMAC

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WSN MAC challenges

• The network tends to operate as a collective
  structure, rather than supporting many
  independent point-to-point flows
• Deep multi-hop dynamic topologies, route-through
  traffic exceeds originating traffic
• Traffic tends to be variable and highly
  correlated
• Little or no activity/traffic for longer periods
  and intense traffic over shorter periods
• Highly constrained resources and functionality
• Radio should be turned off most of the time

A Transmission Control Scheme for Media Access in
Sensor Networks 2003
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WSN MAC design considerations

• Fairness of the bandwidth allocated to each node
  for end to end data delivery to sink
• Each node acts as a router as well as data
  originator resulting in two kinds of traffic
• The traffics compete for the same upstream
  bandwidth
• RATE CONTROL!
• Hidden node problem
• Solution without RTS/CTS
• Energy efficiency
• Transmit, receive and idle consume roughly the
  same amount of energy
• The cost of dropping a packet varies with place
  and the packet

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Contributions of Woo-Culler03

• Reduce idle listening
• Turn off radio during backoff
• Initial MAC delay to avoid event synchronization
• Highly synchronized nature of the traffic causes
  collisions
• Phase shift to reduce synchrony-livelock and
  achieve fairness
• Apply back off as a phase shift to the
  periodicity of the application so that the
  synchronization among periodic streams of traffic
  can be broken
• Implicit acknowledgements
• Overhearing forwarding counts as an
  acknowledgement

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Contributions of Woo-Culler01

• Heuristic for alleviating hidden-node problem
• Child reduces a potential hidden node problem
  with its grand parent by not sending packets
  between t and txpackettime after
  overhearing packet transmission at t by its
  parent
• Rate control
• Control the rate of originating data of a node to
  allow route-through traffic to reach the base
  station
• Configure a, b accordingly
• a is the linear increase to allowable traffic
  rate add a to p (probability to send)
• b is the multiplicative decrease to allowable
  traffic rate multiply p by b
• Originating traffic should have less increase
  than route-thru a_origa_route/(n1)
• Penalize route-thru traffic less than originating
  traffic so b_route1.5b_orig

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Overall

• Advantages
• Lightweight, control packet overhead is reduced
• Disadvantages
• Assumes periodicity of the originating traffic

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SMAC 2002

• Designed for energy efficiency and collision
  avoidance
• The major sources of energy waste are
• collision
• overhearing
• control packet overhead
• idle listening
• S-MAC reduce the waste of energy from all the
  sources mentioned in exchange of some reduction
  in both per-hop fairness and latency

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SMAC

• Protocol consist of three major components
• periodic listen and sleep
• collision and overhearing avoidance
• Contributions of S-MAC are
• The scheme of periodic listen and sleep helps in
  reducing energy consumption by avoiding idle
  listening. The use of synchronization to form
  virtual clusters of nodes on the same sleep
  schedule
• In-channel signaling puts each node to sleep when
  its neighbor is transmitting to another node
  (solves the overhearing problem and does not
  require additional channel)
• Message passing technique to reduce
  application-perceived latency and control
  overhead (per-node fragment level fairness is
  reduced)
• Evaluating an implementation of S-MAC over
  sensor-net specific hardware

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BMAC versatile low power MAC

• Flexible and tunable
• small core and factored functionality
• bidirectional (set and get) interfaces to MAC
  functionalities
• applications can turn them on and off for
  adapting to radio environment
• RTS/CTS, ACKs may be implemented above BMAC
• Low power operation
• Clear Channel Assessment (reducing idle
  listening)
• Low Power Listening

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CCA

• Automatic gain control
• Signal strength samples taken when channel is
  assumed to be free
• Samples go in a FIFO queue (sliding window)
• Median added to an EWMA filter
• Noise floor is established
• Comparing one signal strength reading with noise
  floor causes false negatives (noise amplitude
  fluctuates)
• Instead, detect outliers
• Samples whose energy is significantly below noise
  floor.
• This cant happen if packet is being sent.

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CCA

• Packet arrives between 22 and 54 ms

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LPL

• Sleep cycles
• Wake up, do carrier sensing
• Use CCA reduce idle listening
• If idle go back to sleep
• Else, synchronize using preamble
• Preamble length matches channel checking period
• No explicit synchronization required (unlike
  S-MAC)
• Packet checking period and Preamble length -
  configurable

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LPL

• 1-hop periodic data sampling
• Sampling rate (traffic pattern) defines optimal
  check interval
• Check interval
• Too small energy wasted on idle listening
• Too large energy wasted on transmissions (long
  preambles)
• Better to have large preamble than to check more
  often

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Implementing RTS/CTS

• RTS-CTS is implemented over BMAC
• Send RTS using LPL
• Listen for CTS using LPL
• Once CTS is heard, disable LPL, CCA
• Send data as burst
• Send link layer ACK
• Re-enable LPL, CCA
• RTS CTS/ ACK used depending on the situation

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Throughput
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Throughput vs power consumption
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Reliable Broadcasting via Collision Detection

• Murat Demirbas
• SUNY Buffalo

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Why single-hop reliable broadcast?

• Reliable broadcast is important
• Safety (consistency) reasons Sensor/actuator
  devices coordinating regulator valves should take
  consistent decisions to prevent a malfunction
• Performance (goodput) reasons Hidden terminal
  problem wastes a lot of the bandwidth
• Reliable broadcast is hard
• RTS/CTS solutions are not directly or efficiently
  generalizable to broadcast
• TDMA solutions require topology information and
  impose overhead via static scheduling of slots

35
Collision detection

• Collision detection enables reliable broadcasting
  efficiently
• Use tiny control messages to test for
  clear-to-send send data later
• Use control messages to convey unary information
  even when messages collide
• Transmitter cannot detect collisions
• Collisions occur at the receiver end
• Collisions should be detected at the receiver end
• Optionally communicate CD back to the transmitter

36
MAC layer

• MAC is implemented as a state machine
  (CC1000RadioIntM.nc)
• idle, synchronizing, receiving,
  prepare-to-transmit, and transmit states
• In the idle state when a node detects a preamble
  byte
• preamble (a predefined byte signalling that a
  message is about to be transmitted)
• synchronizing state (receiving the rest of the
  preamble bytes)
• receive state
• finally returns to idle state

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Receiver side CD

• Sample the channel in the idle state
• When the node detects intense activity in the
  medium CD is signaled
• Good indication of a collision Had this been a
  clear message, the node would be able to detect a
  preamble and be in the receive state
• Genuine activity is distinct from idle noise
• Noise has significant variance in channel energy
• Genuine activity has fairly constant channel
  energy
• Our carrier sensing at the idle state searches
  for the pits
• If for a long period no pit is found, this is a
  good indication of genuine activity

For CD we use the same carrier sensing method as
the CCA in prepare-to-transmit !
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CRC based CD

• CRC for filtering the messages received with
  errors
• The receiver calculates a running CRC for the
  message it receives
• compares this calculated CRC with the CRC
  appended to the transmitted message
• The messages that fail the test are thrown away
• Raise a collision detection at the MAC layer when
  CRC fails
• since it indicates that the receiver dropped a
  message

39
Preamble based CD

• Shadowing effect
• While a node j is receiving a message, if the
  preambles of a stronger message arrives in the
  middle of the first message, the stronger message
  dominates the first message and renders it
  undeliverable
• j synchronizes to this latter message and ignores
  the first message
• CRC for the first message does not even get
  computed so a collision detection would not be
  triggered
• To detect this case we use a preamble based
  collision detection
• In the absence of any collision, the preamble
  bytes are only heard in the synchronizing state,
  and no preamble is heard in the receive state
• When j receives a preamble byte in receive state,
  this is a good indication of a collision

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Why did CD receive no attention?

• CD is incompatible with unicast model
• When a node receives a collision, the node can
  not decide whether it should complain or not
• It can never be certain that the communication
  was addressed to itself
• When all communication is broadcast (addressed to
  all nodes) a node is justified in complaining
  about any collision it detects
• There is a need for communicating receiver side
  CD information to the transmitter efficiently and
  reliably
• Our protocols address this issue effectively by
  dedicating a slot for CD detected feedback
• CD detected feedback uses at-least-one semantics
• Collision of feedback also conveys information

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Robcast A reliable broadcast protocol

• Receiver
• Listen
• Received(Col) ? Send NCTS
• Received(RTS) ? Listen

• Transmitter
• Transmit RTS
• Receive(NCTS) ? backoff
• Transmit DATA

j
RTS
NCTS
DATA
k
l
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BEMA Busy elimination multiple access

• Control phase serves two purposes
• Deferring new senders in the presence of an
  ongoing data transmission
• Locked nodes transmit for the entire duration of
  ?
• Arbitration between multiple senders
• Each potential sender would transmit for random
  period of time bounded by fj(?)
• Transmitter of signal with the longest duration
  wins each contender listens for a busy signal or
  collision AFTER it completes its busy signal
  transmission

j
DATA
Control
k
l
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Simulations

• PROWLER wireless sensor simulation tool
• 5x5 grid of motes varying the number of motes
  contending to transmit data
• BSMA
• RTS to all neighbors start data transmission
  upon receiving at least one CTS
• Upon NAK retransmit data
• BMMM
• RTS/CTS handshake with all neighbors data
  transmission

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Number of collisions

• Collisions in BEMA and BMMM remain largely
  constant with increase in traffic load

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Goodput

• BMMM suffers heavily due to high control
  overhead
• BSMA goodput decreases almost linearly as the
  number of collisions increase
• CSMA goodput is high and constant because the
  data loss due to collisions is compensated by the
  speed gain due to NO overhead in transmission

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Round synchronization in BEMA

• Always-on solution
• FTSP time synchronization protocol
• BEMA starts after FTSP completes its initial
  synchronization round
• Periodic time synchronization messages of FTSP
  sent over BEMA to prevent interference with BEMA
  protocol
• On-demand ad-hoc solution
• Exploit collision detection info reliable
  broadcast protocol structure
• In BEMA collisions occur only in the control
  phase
• Upon hearing a collision, set phase to control
  reset the round timer
• Scheme should converge quickly for small number
  of hops