T-MAC (timeout)

1
T-MAC (timeout)
2
Introduction (1/4)
3
Introduction (2/4)

• The problem is that, while latency requirements
  and buffer space are generally fixed, the message
  rate will usually vary
• The nodes must be deployed with an active time
  that can handle the highest expected load.
  Whenever the load is lower than that, the active
  time is not optimally used and energy will be
  wasted on idle listening

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Introduction (3/4)
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Introduction (4/4)

• An active period ends when no activation event
  has occurred for a time TA. An activation event
  is
• the firing of a periodic frame timer
• the reception of any data on the radio
• the sensing of communication on the radio, e.g.
  during a collision
• the end-of-transmission of a nodes own packet
• the knowledge, through overhearing prior RTS and
  CTS packets, that a data exchange of a neighbor
  has ended

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RTS operation and choosing TA (1/5)

• Fixed contention interval
• During this burst, the medium is saturated
  messages are transmitted at maximum rate
• An increasing contention interval is not useful,
  since the load is mostly high and does not change

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RTS operation and choosing TA (2/5)

• RTS retries
• When a node sends an RTS, but does not receive a
  CTS back, one of three things has happened
• the receiving node has not heard the RTS due to
  collision or
• the receiving node is prohibited from replying
  due to an overheard RTS or CTS or
• the receiving node is asleep

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RTS operation and choosing TA (3/5)

• When the sending node receives no answer within
  the interval TA, it might go to sleep. However,
  that would be wrong in cases 1 and 2 we would
  then have a situation where the sending node goes
  to sleep, while the receiving node is still awake
• Therefore, a node should retry by re-sending the
  RTS if it receives no answer. If there is still
  no reply after two retries, it should give up and
  go to sleep

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RTS operation and choosing TA (4/5)

• Determining TA
• A node should not go to sleep while its neighbors
  are still communicating, since it may be the
  receiver of a subsequent message
• TA must be long enough to receive at least the
  start of the CTS packet (Fig)
• lower limit on the length of the interval TA
• TA gt C R T

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RTS operation and choosing TA (5/5)
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Overhearing avoidance (1/2)

• The S-MAC protocol introduced the idea of
  sleeping after overhearing an RTS or CTS destined
  for another node
• In general, overhearing avoidance is a good idea,
  and it is an option in the T-MAC protocol
• A side effect collision overhead becomes higher
  a node may miss other RTS and CTS packets while
  sleeping and disturb some communication when it
  wakes up

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Overhearing avoidance (2/2)

• Consequently, the maximum throughput decreases
  for short packets by as much as 25
• Thus, although overhearing avoidance saves
  energy, it must not be used when maximum
  throughput is (at times) required