MAC Protocols In Sensor Networks

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MAC Protocols In Sensor Networks
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Multiple Access Control (MAC) Protocols

• MAC allows multiple users to share a common
  channel.
• Conflict-free protocols ensure successful
  transmission. Channel can be allocated to users
  statically or dynamically.
• Only static conflict-free protocols are used in
  cellular mobile communications- Frequency
  Division Multiple Access (FDMA) provides a
  fraction of the frequency range to each user for
  all the time- Time Division Multiple Access
  (TDMA) The entire frequency band is allocated
  to a single user for a fraction of time- Code
  Division Multiple Access (CDMA) provides every
  user a portion of bandwidth for a fraction of
  time
• Contention based protocols must prescribe ways to
  resolve conflicts- Static Conflict Resolution
  Carrier Sense Multiple Access (CSMA) - Dynamic
  Conflict Resolution the Ethernet, which keeps
  track of various system parameters, ordering the
  users accordingly

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Frequency Division Multiple Access (FDMA)

• Channels are assigned to the user for the
  duration of a call. No other user can access the
  channel during that time. When call terminates,
  the same channel can be re-assigned to another
  user
• FDMA is used in nearly all first generation
  mobile communication systems, like AMPS (30 KHz
  channels)
• Number of channels required to support a user
  population depends on the average number of calls
  generated, average duration of a call and the
  required quality of service (e.g. percentage of
  blocked calls)

Channel 1
Channel 2
Bandwidth
Channel 3
Channel 4
Time
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Time Division Multiple Access (TDMA)

• The whole channel is assigned to each user, but
  the users are multiplexed in time during
  communication. Each communicating user is
  assigned a particular time slot, during which it
  communicates using the entire frequency spectrum
• The data rate of the channel is the sum of the
  data rates of all the multiplexed transmissions
• There is always channel interference between
  transmission in two adjacent slots because
  transmissions tend to overlap in time. This
  interference limits the number of users that can
  share the channel

Channel 3
Channel 1
Channel 2
Channel 3
Channel 4
Channel 1
Channel 2
Bandwidth
Time
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Code Division Multiple Access (CDMA)

• CDMA, a type of a spread-spectrum technique,
  allows multiple users to share the same channel
  by multiplexing their transmissions in code
  space. Different signals from different users are
  encoded by different codes (keys) and coexist
  both in time and frequency domains
• A code is represented by a wideband pseudo noise
  (PN) signal
• When decoding a transmitted signal at the
  receiver, because of low cross-correlation of
  different codes, other transmissions appear as
  noise. This property enables the multiplexing of
  a number of transmissions on the same channel
  with minimal interference
• The maximum allowable interference (from other
  transmissions) limits the number of simultaneous
  transmissions on the same channel

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Code Division Multiple Access (CDMA)

• Spreading of the signal bandwidth can be
  performed using- Direct Sequence (DS) the
  narrow band signal representing digital data is
  multiplied by a wideband pseudo noise (PN) signal
  representing the code. Multiplication in the time
  domain translates to convolution in the spectral
  domain. Thus the resulting signal is wideband-
  Frequency Hopping (FH) carrier frequency rapidly
  hops among a large set of possible frequencies
  according to some pseudo random sequence (the
  code). The set of frequencies spans a large
  bandwidth. Thus the bandwidth of the transmitted
  signal appears as largely spread

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An Energy-Efficient MAC Protocol for Wireless
Sensor Networks (S-MAC) Ye 2002

• S- MAC protocol designed specifically for sensor
  networks to reduce energy consumption while
  achieving good scalability and collision
  avoidance by utilizing a combined scheduling and
  contention scheme
• 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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(S-MAC) Ye 2002

• S- MAC protocol consist of three major
  components
• periodic listen and sleep
• collision and overhearing avoidance
• Message passing
• 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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A Transmission Control Scheme for Media Access
in Sensor Networks Woo, 2003

• Why STUDY MAC protocols in sensor networks?
• Application behavior in sensor networks leads to
  very different traffic characteristics from that
  found in conventional computer networks
• Highly constrained resources and functionality
• Small packet size
• Deep multi-hop dynamic topologies
• The network tends to operate as a collective
  structure, rather than supporting many
  independent point-to-point flows
• Traffic tends to be variable and highly
  correlated
• Little or no activity/traffic for longer periods
  and intense traffic over shorter periods

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A Transmission Control Scheme for Media Access
in Sensor Networks Woo, 2003

• Major factors to be considered in the design of
  MAC
• Communication efficiency in terms of energy
  consumed per each packet
• Communication by radio channel consumes the
  highest energy
• Transmit , receive and idle consume roughly the
  same amount of energy
• 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
• Hidden nodes
• Contention at the upstream node may not be
  detected and results in significant loss rate
• Efficient channel utilization

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A Transmission Control Scheme for Media Access
in Sensor Networks Woo, 2003

• Major factors to be considered in the design of
  MAC
• The routing distance and degree of intermediate
  competition varies widely across the network
• The cost of dropping a packet varies with place
  and the packet
• Contribution of this paper are as follows
• Listening mechanism
• Listening is effective when there are no hidden
  nodes
• It comes at an expense of energy cost as the
  radio must be on to listen
• Many protocols such as IEEE 802.11 require
  sensing the channel even during backoff
• Shorten the length of carrier sensing and power
  off the node during backoff
• Highly synchronized nature of the traffic causes
  no packet transfer at all in the absence of
  collision detection hardware
• Introduce random delay for transmission to
  unsynchronized the nodes

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A Transmission Control Scheme for Media Access
in Sensor Networks Woo, 2003

• Backoff Mechanism
• Used to reduce the contention among the nodes
• In the sensor networks, traffic is a
  superposition of different periodic streams
• 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
• Contention-based Mechanism
• Explicit control packets like RTS and CTS are
  used to avoid contention
• ACKS indicate lack of collision
• Use of lot of control packets reduces bandwidth
  efficiency
• ACKS can be eliminated by hearing the packet
  transmission from its parent to its upstream
  which serves as an ACK for the downstream node

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A Transmission Control Scheme for Media Access
in Sensor Networks Woo, 2003

• Rate Control Mechanism
• The competition between originating traffic and
  route-thru traffic has a direct impact in
  achieving the fairness goal.
• MAC should control the rate of originating data
  of a node in order to allow route-thru traffic to
  access the channel and reach the base station and
  some kind of progressive signaling for route-thru
  traffic such the rate is controlled at the
  origin.
• A passive implicit mechanism is used to control
  the rate of transmission of both traffics

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A Transmission Control Scheme for Media Access
in Sensor Networks Woo, 2003

• Multi-hop Hidden Node problem
• It avoid the hidden node problem by constantly
  tuning the transmission rate and performing phase
  changes so that the aggregate traffic will not
  repeatedly collide with each other.
• A child can reduce a potential hidden node
  problem with its grand parent by not sending
  packets for t x packet time at the end of
  packet transmission t by its parent

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A Transmission Control Scheme for Media Access
in Sensor Networks Woo, 2003

• Advantages
• The amount of computation for this scheme is
  small and within networked sensors computation
  capability
• The scheme is totally computational which is much
  cheaper in energy cost than on the radio
• The control packet overhead is reduced

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A Transmission Control Scheme for Media Access
in Sensor Networks Woo, 2003

• Disadvantages
• The MAC protocol developed here takes into
  consideration the periodicity of the originating
  traffic which doesnt help for non periodic
  traffic

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A Transmission Control Scheme for Media Access
in Sensor Networks Woo, 2003
Suggestions/Improvements/Future Work
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An Adaptive Energy-Efficient MAC Protocol for
Wireless Sensor Networks Van dam, 2003

• T-MAC is a contention based Medium Access Control
  Protocol
• Energy consumption is reduced by introducing an
  active/sleep duty cycle
• Handles the load variations in time and location
  by introducing an adaptive duty cycle
• It reduces the amount of energy wasted on idle
  listening by dynamically ending the active part
  of it
• In T-MAC, nodes communicate using RTS, CTS, Data
  and ACK pkts which provides collision avoidance
  and reliable transmission
• When a node senses the medium idle for TA amount
  of time it immediately switches to sleep
• TA determines the minimal amount of idle
  listening time per frame
• The incoming messages between two active states
  are buffered

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An Adaptive Energy-Efficient MAC Protocol for
Wireless Sensor Networks Van dam, 2003

• The buffer capacity determines an upper bound on
  the maximum frame time
• Frame synchronization in T-MAC follows the scheme
  of virtual clustering as in S-MAC
• The RTS transmission in T-MAC starts by waiting
  and listening for a random time within a fixed
  contention interval at the beginning of the each
  active state
• The TA time is obtained using TA gt C R T
• T-MAC suffers from early sleeping problem
• Its overcome by sending Future request to send or
  taking priority on full buffers

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An Adaptive Energy-Efficient MAC Protocol for
Wireless Sensor Networks Van dam, 2003

• Advantages
• The T-MAC protocol is designed particularly for
  wireless sensor networks and hence energy
  consumption constraints are taken into account
• The T-MAC protocol tries to reduce idle listening
  by transmitting all messages in bursts of
  variable lengths and sleeping between burst
• T-MAC facilitates collision avoidance and
  overhearing -- nodes transmit their data in a
  single burst and thus do not require additional
  RTS/CTS control packets.
• By stressing on RTS retries, T-MAC gives the
  receiving nodes enough chance to listen and reply
  before it actually goes to sleep -- this
  increases the throughput in the long run

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An Adaptive Energy-Efficient MAC Protocol for
Wireless Sensor Networks Van dam, 2003

• Disadvantages
• The authors do not outline how a sender node
  would sense a FRTS packet and enable it to send a
  DS packet
• Also sending a DS packet increases the overhead.
• The network topology in the simulation considers
  that the locations of the nodes are known
• T-MAC has been observed to have a high message
  loss phenomenon
• T-MAC suffers from early sleeping problem for
  event based local unicast

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An Adaptive Energy-Efficient MAC Protocol for
Wireless Sensor Networks Van dam, 2003

• Suggestions/Improvements/Future Work
• If a buffer is full there would be a lot of
  dropped packets decreasing the throughput. A
  method to overcome this drawback is that we could
  have the node with its buffer 75 full broadcast
  a special packet Buffer Full Packet
• MAC Virtual Clustering technique needs to be
  further investigated
• An adaptive election algorithm can be
  incorporated where the schedule and neighborhood
  information is used to select the transmitter and
  receivers for the current time slot, hence
  avoiding collision and increasing energy
  conservation

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References

• Ye 2002 W. Yei, J. Heidemann and D. Estrin,
  Energy-Efficient MAC Protocol for Wireless Sensor
  Networks, Proceedings of the Twenty First
  International Annual Joint Conference of the IEEE
  Computer and Communications Societies (INFOCOM
  2002), New York, NY, USA, June 23-27 2002.
• Woo 2003 A. Woo and D. Culler, A Transmission
  Control Scheme for Media Access in Sensor
  Networks, Proceedings of the ACM/IEEE
  International Conference on Mobile Computing and
  Networking, Rome, Italy, July 2001, pp. 221-235.
  
• Van Dam 2003 T. V. Dam and K. Langendoen, An
  Adaptive Energy-Efficient MAC Protocol for
  Wireless Sensor Networks, ACM SenSys, Los
  Angeles, CA, November, 2003.