Slotted FAMA: A MAC Protocol for Underwater Acoustic Networks

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Slotted FAMA A MAC Protocol for Underwater
Acoustic Networks

• Marcal Molins and Milica Stojanovic

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MAC Protocols for UWA

• Characteristics of an underwater acoustic network
• Low bit rate
• Low Propagation Speed
• Goals of MAC protocols
• To prevent collisions
• To save energy

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Problems for Medium Access

• Hidden terminal problem
• Exposed terminal problem

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Existing Approaches

• MACA
• RTS/CTS/DATA
• MACAW
• RTS/CTS/DATA with ARQ
• Backoff algorithm
• FAMA
• RTS/CTS/DATA with ARQ
• Backoff algorithm
• Carrier sensing

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Problem with RTS/CTS
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Conditions for Collision Avoidance

• RTS length is greater than the maximum
  propagation delay
• CTS length should be greater than RTS length plus
  twice the maximum propagation delay plus the
  hardware transmit-to-receive transmission time
• .Too expensive in UWA

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Slotted FAMA

• RTS/CTS/CSMA/DATA
• Time is divided into slots
• Each packet is transmitted at the beginning of
  one slot
• The length of a slot is ??
• ? maximum transmission delay
• ? transmission time of CTS

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Slotted FAMA (contd)
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Slotted FAMA (contd)

• Receiving state
• If a node detects carrier on channel, it goes
  into receiving state
• RTS 2 slots
• CTS wait until end of the transmission
• Data wait until ACK
• ACK 1 slot
• Interference same as CTS
• Trade off between ARQ and exposed terminal
  problem
• Backoff algorithm
• No CTS for RTS, node is in backoff algorithm
• Do not reset backoff timer

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Slotted FAMA (contd)

• Transmission priority for node that just received
  a packet
• Trains of packets

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Throughput

• Throughput per node
• U time while useful data is being sent
• I idle time
• B busy time

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Simulation

• Simulation settings
• Packet size 3000 bits
• Control packet size 100 bits
• Bitrate1000bps
• Packet rate 1 packet/3000 seconds
• AUVs 2.5m/s
• Networks layout

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Simulation networks
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Simulation Results
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Simulation Results
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Simulation results
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Simulation Results
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Simulation Results
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Discussions

• Cost of synchronization ?
• Energy efficiency?
• Is it worth avoiding collision?