CPSC 689: Discrete Algorithms for Mobile and Wireless Systems

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CPSC 689 Discrete Algorithms for Mobile and
Wireless Systems

• Spring 2009
• Prof. Jennifer Welch

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Lecture 3

• Topics
• MAC Protocols part 2
• Sources
• Schiller, Ch 1-3
• Balakrishnan, Ch 11
• Vaidya, Ch 1-2, 4-5
• MIT 6.885 Fall 2008 slides

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Reservation-Based Strategy

• Used in MACA and MACAW.
• For unicast transmission only.
• Assumes data to be sent can be fairly long.
• Introduces control messages, which are short, so
  less likely to collide.
• Each data transmission is preceded by a control
  message handshake to reserve the channel for a
  period of time.
• Messages RTS (Request To Send), CTS (Clear to
  Send), Data
• Sender A sends RTS.
• Contains name of sender A and receiver B, and
  length of data to be sent (amount of time the
  data transmission will take).
• Receiver B responds with CTS.
• Contains same info.
• If sender A gets CTS, sends the actual data.

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Reservation-Based Strategy (contd)

• RTS / CTS / Data protocol can be interrupted
• If sender A hears another CTS with a planned
  transmission interval that would overlap its
  proposed interval, then A doesnt transmit.
• Because some receiver C in As range has already
  OKd another transmission, say from D.
• So As transmission might collide at C with Ds
  transmission, ruining Ds transmission.

requests to send to C
clears C to send
A waits to not spoil D's msg at C
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Reservation-Based Strategy (contd)

• RTS / CTS / Data protocol can be interrupted
• If receiver B hears another RTS, intended for
  another receiver C, with a planned transmission
  interval that overlaps the one proposed by A,
  then B doesnt respond with the CTS.
• Because the two transmissions might collide at B.
• Reservation approach does not use
  carrier-sensing---just explicit messages.

requests to send to C
requests to send to B
doesn't send CTS
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Hidden Terminal Problem

• RTS / CTS / Data protocol solves the hidden
  terminal problem, assuming conditions don't
  change after the reservation.
• A requests to send to B with RTS
• B sends CTS, which is heard by A and C
• C will not send to B during A's transmission

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Exposed Terminal Problem

• Handles exposed terminal problem somewhat
• Suppose that B sets up transmission to A first.
• Sender C sends RTS to D.
• D hasnt heard the RTS from B to A (not in
  range), so D can respond to C with CTS.
• Then C can transmit to D, knowing that D
  shouldnt be receiving any other transmission.
• However
• D sends the CTS to C
• But it seems likely that C wont hear it, because
  of interference from Bs transmission.
• ?

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Acks and Reservations

• If the data transmission is followed by
  link-layer Acks, then we must be sure that Acks
  succeed also, not just the Data packets.
• Because if Ack is lost, Data must be
  retransmitted.
• So, expand the proposed reserved transmission
  time to include enough time for the Ack too.
• Essentially, an RTS / CTS / Data / Ack protocol.
• Since this involves communication in both
  directions (sender to receiver and receiver to
  sender), we should treat the link symmetrically.
• Simplest version Let each sender or receiver
  defer if it hears either an overlapping RTS or
  CTS.

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Evaluation

• RTS / CTS / Data and RTS / CTS / Data / Ack
  protocols reduce likelihood of collision
• Now collisions can occur only during the
  handshake.
• Probability of collision is lower, since control
  messages are short.
• Ex Suppose A and C send RTS at the same time
  for the same receiver B.
• B might not receive either.
• B might receive one, respond with CTS.
• B might receive both, could respond with CTS to
  one of them.

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Evaluation

• Overhead
• High, if data packets are short or collisions are
  infrequent.
• May be acceptable if data packets are long and
  collisions frequent.
• Assumes symmetrical transmission/reception
  conditions.
• Good only for unicast.
• RTS / CTS strategy sometimes called Virtual
  Carrier Sensing, since it provides an implicit
  way to tell if the channel is busy.

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Use of Time in Reservation Protocol

• The descriptions are in terms of time, so it
  sounds like the nodes should be using
  (approximately) synchronized clocks.
• But it seems that this could be implemented with
  un-synchronized local clocks.
• The reservation period is short, and everyone
  could keep track of proposed intervals in a
  relative way, in terms of their own clocks.
• Details?

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Reducing Collision Probability

• p-persistence
• At each valid transmission opportunity (when
  sensing mechanisms dont detect any activity on
  the channel), allow a node to transmit a packet
  only with probability p.
• How to choose p?
• Too small Wasted slots, hurts throughput.
• Too large High chance of collision.
• Ideal would be p 1/n, where n is the number of
  nodes that would like to transmit.
• However, the devices dont know n.
• E.g., could estimate it as some multiple of the
  number of known neighbors.
• Again, slots must be synchronized with clock
  skew, the slots must be correspondingly larger.

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p-Persistence

• p-persistence is memoryless
• Probability a node transmits in a slot doesnt
  depend on time since it last attempted to
  transmit.
• An arbitrary number of slots could pass without
  some particular node being able to transmit.
• Unfair!
• Might consider letting unsuccessful nodes
  increase their p.
• Next strategy adds some memory, for better
  fairness.

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Backoff Interval Strategy

• Ensures that a node attempts to transmit after a
  bounded number of valid transmission
  opportunities (when channel appears to be free).
• Node wanting to transmit sets a counter to a
  number b, and transmits at the bth valid
  opportunity.
• Thus, never lets more than b valid opportunities
  pass before trying to transmit.
• How to choose b?
• Too large Wastes slots
• Too small High chance of collision
• Ideal n, number of contenders, but hard to
  estimate this.
• Choose b uniformly at random in an appropriate
  range 0,bmax.
• Randomness avoids synchronized retries and the
  resulting repeated collisions.

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Backoff Interval Strategy

• To transmit
• Pick a number in 0,bmax, uniformly at random.
• Use this to initialize a counter b.
• Try periodically to sense each time
• Decrement the counter if the channel appears
  free.
• Leave the counter alone if the channel appears
  busy.
• When b 0, transmit the data.
• Can do this without synchronized clocks.

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Responding to Packet Loss

• Assume we are using the backoff interval strategy
  above, plus link-level Acks to tell if
  transmission succeeds.
• If a node A transmits and no Ack arrives, A must
  retransmit.
• The failure is evidence that there is too much
  contention, so instead of using the same bmax to
  choose b, increase bmax.
• When transmission succeeds, reset bmax to a small
  number bmin.

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Responding to Packet Loss

• How to increase bmax?
• Common strategy Binary exponential backoff
  (BEB).
• Double bmax.
• Used in Ethernet, works well there (theoretical
  results prove its optimal).
• In wireless setting, may be too pessimistic,
  since failure could be caused by noise as well as
  contention.
• Strategy is somewhat unfair
• A device that is unsuccessful keeps increasing
  its bmax, which gives it fewer and fewer chances.
• Tends to allow devices who are transmitting
  successfully to keep transmitting successfully,
  and to lock out unsuccessful devices.

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Combining Backoff with RTS/CTS

• Use backoff to tell a node when to start an
  entire RTS / CTS / Data dialog, not just when to
  transmit actual data.
• Use backoff only for initial entry to the dialog,
  i.e., for sending the RTS thereafter, just keep
  going as before.
• Of course, look out for collisions.
• If dialog is interrupted, increase bmax and start
  the entire dialog again.

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Eliminating collisions?

• In many wireless networks, no one starts out
  knowing what devices are participating.
• In an ad hoc network, we can reduce collisions to
  short intervals---time to exchange RTS/CTS
  control packets.
• In a cellular network, a base station can
  schedule data transmission.
• But there is still a very small probability that
  a device will be blocked from announcing its
  presence to the base station.
• It seems that, no matter what we do, there
  remains some possibility of collisions all we
  can do is reduce the probability.
• Q Prove a theorem?

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Comparison

• Carrier-Sense Multiple Access (CSMA) strategies,
  with Collision Avoidance (backoff), can work well
  when contention level is low.
• Little overhead, low likelihood of colliding.
• Reservation-based protocols work well when
  contention level is high.
• But they work only for unicast.
• Neither is completely adequate for ad hoc
  networks---still an active area of research.
• Can be combined, as in 802.11.

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Specific MAC Protocols

• 802.11
• MACA / MACAW

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802.11 Overview

• IEEE standard for wireless MAC layer.
• For both cellular systems and ad hoc local
  networks.
• Centralized Point Coordination Function (PCF)
• Needs an AP (Access Point base station) to
  coordinate.
• Both asynchronous and time-bounded service.
• Time-bounded service is good for supporting
  voice/video.
• AP uses a centralized polling protocol, polling
  all the devices it is servicing to see if they
  have something to send.
• Distributed Coordination Function (DCF)
• Works for both cellular and ad hoc settings.
• Just asynchronous service (no time bound
  guarantees).
• Uses CSMA/CA (with backoff), plus RTS/CTS
  reservations.
• RTS/CTS often turned off, because of overhead.

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802.11 Time Synchronization

• Cellular systems
• Access Point transmits Beacon Frames
  periodically, containing the value of the APs
  clock at the time of transmission receiving
  stations reset their clocks accordingly.
• Beacon Frames should be sent at particular
  target beacon transmission times.
• Doesn't always succeed, because of
  congestion-induced delays.
• AP does its best.

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802.11 Time Synchronization

• Ad hoc systems
• All devices try to send beacons at target times.
• Most of the time, one of the broadcasts wins over
  the others (because of characteristics of the
  broadcast medium).
• Everyone sets their clocks according to the
  received beacon.
• Sometimes no beacon will succeed at some target
  time.
• Then beacon for that time is lost algorithm
  recovers at the next target time.

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802.11 Time Synchronization

• Ad hoc solution is not completely satisfactory
• Can lead to discrepancies in large network, even
  between nearby neighbors.
• Still a research topic.
• Basic 802.11 contention management doesnt
  require time synchronization.
• Balakrishnans notes assume slots, but that is
  for performance recall that collisions are less
  likely when transmissions occur on synchronized
  slot boundaries (throughput is twice as great).

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802.11 Protocol

• Physical carrier sensing
• Compares level of energy on the channel with
  usual noise level.
• No busy-tones, just basic carrier sensing.
• Link-layer Acks
• Uses absence of Acks as a way of detecting
  collisions.
• Reservations (optional)
• RTS / CTS / Data / Ack
• Reduces collisions due to hidden terminals, by
  reducing window of vulnerability to short RTS/CTS
  handshake interval.
• Overhead
• Reservations consume channel resources.
• Often turned off because of overhead.
• But may be worth it, if data is very large.

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802.11 CSMA/CA

• Main strategy used in 802.11 DCF.
• Slots (but need not be synchronized).
• Slot size large enough to complete message
  transmission (e.g., sum of carrier-sensing delay,
  propagation delay, transmit/receive delay,).
• Each node maintains CW, the contention window
  size (like b earlier), initially CWmin (bmin).
• To transmit
• Pick a random number in 0,CW, to initialize a
  local delay timer.
• Sense at the beginning of each slot, decrementing
  the delay timer each time the channel appears
  idle.
• When delay 0, transmit data.
• If transmission seems to fail, double CW and try
  again (but just a bounded number of times, not
  forever).

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802.11 CSMA/CA

• How does the sender decide that the transmission
  has failed?
• For unicast, uses link-layer Acks.
• If no Ack received in a reasonable time,
• For broadcast ???
• Old method just guess based on physical carrier
  sensing.
• If channel is busy too many times while counting
  down delay timer,
• Not reliable.
• No good solution yet.

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802.11 CSMA/CA

• To handle hidden or exposed terminals
• For unicast, can replace transmission of data
  with entire RTS/CTS/Data/Ack exchange.
• For broadcast ???
• Can rely on characteristics of the medium
• If the carrier-sense range is gtgt the reception
  range, then no hidden terminals will occur.

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802.11 Pseudocode

• succeeded false
• CWmax CWmin
• while not succeeded do
• CW choose uniform0,CWmax
• while CW ? 0 do
• while channel busy do no-op
• CW CW -1
• transmit
• if successful then succeeded true
• else CWmax 2 CWmax
• Notes Do the main loop bounded number of times.
• transmit is Data/Ack or RTS/CTS/Data/Ack
• successful test based on Acks, or physical
  carrier sensing

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802.11 for ad hoc networks

• Not completely satisfactory.
• Success test not adequate.
• RTS/CTS strategy good only for unicast.
• Broadcast? Who should respond?
• Not clear whether the backoff strategy is
  optimal.
• Time synchronization strategy messy, not clear
  how well it works.
• Needs work!

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MACAW Overview

• Bharghavan, Demers, Shenker, Zhang, 1994
• MAC protocol for a single channel wireless LAN
  developed at Xerox PARC
• Based on earlier MACA strategy
• RTS / CTS / Data
• Binary exponential backoff
• Elaborates on the reservation strategy
• Modifies the backoff strategy

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MACAW Reservation Strategy

• In original RTS / CTS / Data / Ack strategy,
  nodes defer when they hear RTS or CTS with
  overlapping interval.
• Add a Data Sending (DS) packet from sender to
  receiver.
• RTS / CTS / DS / Data / Ack
• DS means CTS has arrived and data transmission is
  about to occur.
• Helps with exposed terminals
• If C hears RTS from B to A, then C should defer
  to avoid
• Having its data transmission collide (at B) with
  Bs receipt of an Ack from A.
• Having the CTS from D collide (at C) with Bs
  data transmission.
• But these are important only if B actually
  receives the CTS from A.
• DS message confirms this C defers only if it
  hears DS (or CTS).

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Exposed Terminal Scenario

• B sends RTS for A, heard by C also
• A sends CTS for B
• B sends data for A
• A sends ACK for B

• C should not interfere with B's receipt of CTS
  and ACK from A
• So C defers for a long time
• But what if A does not send to B?

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Exposed Terminal Scenario

• B sends RTS for A, heard by C also
• A sends CTS for B
• B sends (short) DS, heard by C
• B sends data for A
• A sends ACK for B

• If C does not get DS from A in a timely fashion
  after getting RTS from A, then it assumes
  transmission didnt happen, so it need not defer

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MACAW Backoff Strategy

• More gradual adjustments
• Backoff multiplies counter by 1.5 instead of 2
  (recognizing that not all failures are due to
  collisions).
• Counter decremented by 1 after success, rather
  than getting reset to bmin (recognizing the
  contention is most likely ongoing).
• Noted that backoff strategies use too little
  information---suggest sharing and remembering
  congestion information.
• Authors say that their solutions are good for the
  unicast case, but recognize that they still dont
  have good solutions for the broadcast case.

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Remaining Research on MAC Layers

• Practical
• Better strategies for handling local broadcast,
  rather than unicast.
• Better ways to estimate contention, better
  backoff protocols.
• Managing the overall network for good throughput
  (capacity).

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Remaining Research on MAC Layer

• Theoretical
• Physical layer models Capturing signal
  propagation, noise, device failure, mobility.
• Better backoff protocols, realistic analysis.
• Prove inherent limitations.
• Sort out dependence on time synchronization.
• MAC layer guarantees What are good abstract
  models for what the MAC layer should guarantee to
  higher layers?
• High-probability successful local message
  delivery, conditioned on some factors about the
  environment, like amount of contention?
• Also export other information, e.g., about amount
  of contention, or about occurrence of collisions?