MACAW: A media Access Protocol for Wireless LANs

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MACAW A media Access Protocol for Wireless LANs

• Lin yu han

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• P. Karn. MACA - a new channel access method for
  packet radio. In ARRL/CRRL Amateur Radio 9th
  Computer Networking Conference, pages 134140,
  September 1990.
• V. Bharghavan, A. Demers, S. Shenker, and L.
  Zhang. MACAW A media Access Protocol for
  Wireless LANs. In Proc. ACM SIGCOMM94, pages
  212215, London,UK, August 1994.

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Purpose

• Analyze existing Media Access protocols for
  Wireless LAN (CSMA/CA and MACA)
• To design a Media Access Protocol for Wireless
  LAN (MACAW)

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Existing Protocols for Wireless LAN

• CSMA/CA
• The station senses the carrier before
  transmission.
• Avoid collisions by testing the signal strength
  in the vicinity of transmitter
• MACA
• Use Basic Dialogue (RTS-CTS-DATA) for each DATA
  packet sent
• Any station overhearing RTS and CTS has to wait

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Problems of existing protocols

• CSMA/CA
• Hidden Terminal C attempts to transmit while A
  is transmitting to B. C is unable to hear the
  transmission from A.
• Exposed Terminal B is transmitting to A and C
  attempts to transmit to other party. C is unable
  to transmit because C did not know that B is
  transmitting to another faraway party.

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Problems of existing protocols

• The design of MACA
• Fix Hidden Terminal and Exposed Terminal problem
  of CSMA/CA by using a basic dialogue
  (RTS-CTS-DATA) before sending each DATA packet.
• Using power control technique, MACA can solve the
  problem of Exposed Terminal

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Problems of existing protocols

• MACA
• The contention is likely to occur at the
  receiver, not the sender
• The protocol depends heavily on the symmetry of
  the communication
• Use BEB (Binary Exponential Backoff) with fix low
  Backoff (BO) adjustment
• The protocol is unfair due to the BO adjustment
  algorithm

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The design of MACAW

• Setting BO to minimum after successful CTS or
  DATA transmission makes Binary Exponential
  Backoff algorithm perform poorly because the
  station who has much lower BO is more likely to
  win the contention and capture the media all the
  time.
• Solution BO cannot be calculated by the station
  itself. Its value has to be calculated from
  other adjacent parties. This will help deliver
  fairness to the protocol as shown in Table 1.

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The design of MACAW

• Setting BO back to minimum also creates wild
  oscillation within the cell. This results in low
  throughput because there will be a lot of time
  that the station are not using the channel
  efficiently
• Solution Use MILD algorithm to adjust the value
  of BO as follows BO is increased by 1.5 times
  when unsuccessful TIMEOUT is received. BO is
  decreased by 1 when the request is successful.
  This multiplicative increase and linear decrease
  (MILD) helps increase the throughput.

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The design of MACAW

• MACAW now can only share the traffic IN and OUT
  of a station equally. This results in an
  unfairness of the transmission in figure 4.
• Solution Single BO should not be used here.
  Instead multiple BO should be used in order to
  adjust the BO fairly. The paper propose randomly
  choosing the value of BO for each stream and
  select the queue that has the minimum BO to
  transmit.

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The design of MACAW

• The basic MACA does not support reliable data
  transfer very well. TCP on higher layer protocol
  reduces the throughput substantially when error
  rate is higher.
• Solution MACAW implements ACK into the link
  layer in order to compensate the loss of
  throughput from the higher layer.

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The design of MACAW

• In Exposed Terminal problem, C cannot know
  whether B receives CTS from A successfully or not
  because C cannot hear CTS from A. C has to wait
  for a long time because C is assuming that B is
  transmitting.
• Solution B will send out Data-Send packet (DS)
  in order to tell C that B will transmit data.
  Without DS, the other stations cannot know the
  next contention period. Synchronization of
  contention time is important in capturing the
  media.

A
B
C
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The design of MACAW

• B1 -gt P1 P2 lt- B2
• B1 cannot send data to P1 and do not know exactly
  when to send RTS to P1 and get a successful CTS
  reply from P1 because the transmission from B2 to
  P2 is likely to block the reception of P1
• Solution Since P1 knows exactly when to content,
  P1 should inform B1 when B1 should content for
  the media. P1 will inform B1 by sending Request
  for RTS (RRTS) packet. Synchronization is
  required here.

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Known problems of MACAW

• The protocol does not allow P1 to receive while
  P2 is transmitting in Figure 7. B1 -gt P1 P2 -gt B2
  . P1 has to wait only
• This is because most of the time when B1
  initiates a data transfer by sending an RTS, P1
  cannot hear it due to P2s transmission.
• The key is the lack of synchronization
  information B1 has no way of knowing when the
  contention periods are. The RRTS packet is
  irrelevant here since P1 cannot hear the incoming
  RTS.

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• We must differentiate between the backoffs used
  to send to different pads. Each station should
  maintain a separate backoff counter for each
  stream.

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• The addition of the DS and ACK packets decrease
  the throughput by roughly 8.
• Using MACAW over MACA has yielded an improvement
  of over 37 in throughput.