In ACM Mobile Computing and Communication Review,

1
A Real-Time Medium Access Control Protocol for Ad
Hoc Wireless Local Area Networks

• In ACM Mobile Computing and Communication Review,
  
• 1999, Vol. 3, No. 2, pp. 20-27,
• by R. O. Baldwin, N. Davis, and S. Midkiff.

2
Goal

• An enhancement of IEEE 802.11 for real-time
  communication.
• less mean delay
• less misses of deadline
• less packet collisions

3
Real-Time Communication

• Each packet has a deadline.
• After the deadline, sending this packet is
  useless.
• Ex Military personnel in the field communicate
  with their weapons remotely and wirelessly.
• Other applications
• Voice, video transmission.
• Automatic control

4
Review of IEEE 802.11

• The CW (contention window) is initially CWmin,
  and is doubled after each failure, until CWmax is
  reached.
• BV (backoff value) randomly in 0..CW-1.
• The BV is decreased after each idle slot.

5
Drawback of IEEE 802.11

• Can not meet the requirements of real-time
  communication.
• When a packet has missed its deadline, the packet
  will still be buffered and sent.
• Thus, this causes more contention, collisions,
  ...
• more packets may miss their deadlines.

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Basic Idea of RT-MAC (Real-Time MAC)

• Each packet is associated with a deadline when
  passed to the MAC layer.
• Note The deadline value does not need to be sent
  along with the packet.
• After the deadline, the packet will not be sent.

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Rule 1Enhanced Collision Avoidance

• Announcing the next BV
• When a packet is transmitted, the next BV to be
  used is placed in a field of the packet.
• Stations who hear this packet will avoid
  selecting this BV as their next backoff timer.
• BV is a random number in 0..CW-1.

8

• Details
• Prior to transmitting a packet, a station will
  select its next BV from the range of 0..CW-1,
  excluding those BVs already chosen by other
  stations.
• A station will indicate in its data packet the
  next BV value to be used.
• A station should keep a table of BV values used
  by other stations.
• After an idle slot, a station should decrease its
  own BV, as well as others BVs in its table.

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• Example
• A 3 ? 1 ? 8
• B 1 ? 6 ? ...
• C 5 ? 2 (collides with Bs, changed to 3)

B(6)
A(1)
A(8)
C(3)
B(...)
C(...)
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Rule 2Transmission Control

• A station must send when its BV value has
  expired.
• If the packet experiences transmission failure,
  it will be reexamined to see if its deadline has
  been missed.
• Note another backoff still has to be taken.
• If the packet succeeds, the next packet is
  scheduled in the same manner.

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Rule 3Contention Window Size

• CW is set to 8N, where N is the estimated number
  of real-time stations.
• N can be estimated by counting the number of
  unique addresses for a period of time.
• alternative N a function of current channel
  load.
• 8 is chosen by instinct.
• Note CW is thus not doubled after a transmission
  failure
• (compared the original IEEE 802.11 of doubling
  each time).

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Rule 4 Collision of BV

• Due to mobility, transmission error, and
  collisions, a station may receive a packet
  indicating a BV equal to its own BV.
• The station must select another BV value
  otherwise, collision will occur.
• To avoid the station being unduly penalized, the
  new BV should be selected from 0..CBV-1.
• CBV its current BV.
• I.e., the station is given higher priority.
• If all values in 0..CBV-1 are chosen, then we
  double it (i.e., 0..2CBV-1).

13
Simulation Results

• Traffic Models from the following benchmarks
• telemetry (??) traffic
• avionics (????) traffic on Boeing 777

14
Mean Delay

• Since late packets are discarded, delay decreases
  as the load increases.
• In IEEE 802.11, packets missing deadline are
  still transmitted.

15
Collision Ratio

• RT-MAC is quite stable in collision prob. with
  respect to the number of stations.

16
Missed Deadline Ratio
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Conclusions

• A new RT-MAC protocol.
• broadcasting the next BV value
• BV depends on the current number of stations
• Results
• The network behavior is quite stable in terms of
  mean delay, missed deadline ratio, and collision
  ratio.
• The mean delay is quite independent of the number
  of stations.

18
Discussion

• The RT-MAC assumes that N is already known
  (backoff window 8N).
• In a mobile/dynamic environment, how is N
  computed/estimated?