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Lecture on MAC

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Specific ('cross-layer') solutions, e.g. Dozer for data gathering ... like in Dozer. Slotted Aloha. We assume that the stations. are perfectly synchronous ... – PowerPoint PPT presentation

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Title: Lecture on MAC


1
Lecture on MAC
  • Anish Arora
  • CIS788.11J
  • Introduction to Wireless Sensor Networks

2
References
  • S-MAC An Energy-Efficient MAC Protocol for
    Wireless Sensor Networks, Wei Ye, John Heidemann,
    Deborah Estrin, Infocom 2002
  • T-MAC An Adaptive Energy-Efficient MAC Protocol
    for Wireless Sensor Networks, Tijs van Dam, Koen
    Langendoen, 2003
  • B-MAC Versatile Low Power Media Access for
    Wireless Sensor Networks, Joseph Polastre, Jason
    Hill, David Culler, Sensys 2004
  • Z-MAC a Hybrid MAC for Wireless Sensor Networks,
    Injong Rhee, Ajit Warrier, Mahesh Aia and Jeongki
    Min, Sensys 2005
  • O-MAC A Receiver Centric Power Management
    Protocol, Hui Cao, Kenneth W. Parker, Anish
    Arora, ICNP 2006

3
Motivation Hidden Terminal Problem
  • A sends to B, C cannot receive A
  • C wants to send to B, C senses a free medium
    (CS fails)
  • collision at B, A cannot receive the collision
    (CD fails)
  • A is hidden for C

B
A
C
4
Motivation Exposed Terminal Problem
  • B sends to A, C wants to send to D
  • C has to wait, CS signals a medium in use
  • since A is outside the radio range of C waiting
    is not necessary
  • C is exposed to B

B
A
C
D
5
Motivation - Near and Far Terminals
  • Terminals A and B send, C receives
  • the signal of terminal B hides As signal
  • C cannot receive A
  • This is also a severe problem for CDMA networks
  • precise power control required

A
B
C
6
Access Methods
  • SDMA (Space Division Multiple Access)
  • segment space into sectors, use directed antennas
  • Use cells to reuse frequencies
  • FDMA (Frequency Division Multiple Access)
  • assign a certain frequency to a transmission
    channel
  • permanent (radio broadcast), slow hopping (GSM),
    fast hopping (FHSS, Frequency Hopping Spread
    Spectrum)
  • TDMA (Time Division Multiple Access)
  • assign a fixed sending frequency for a certain
    amount of time
  • CDMA (Code Division Multiple Access)
  • Combinations!

7
Traditional MAC Protocol Classification
  • Centralized/Single-Hop Protocols
  • A base station coordinates all traffic
  • Contention Protocols (CSMA)
  • Transmit when you feel like transmitting
  • Retry if collision, try to minimize collisions,
    additional reservation modes
  • Problem Receiver must be awake as well
  • Scheduling Protocols (TDMA)
  • Use a pre-computed schedule to transmit
    messages
  • Distributed, adaptive solutions are difficult
  • Hybrid protocols
  • E.g. contention with reservation ? scheduling
  • Specific (cross-layer) solutions, e.g. Dozer
    for data gathering

8
Energy Efficient MAC Protocols
  • In sensor networks energy is often more critical
    than throughput.
  • The radio component should be turned off as much
    as possible.
  • Energy management considerations have a big
    impact on MAC protocols.
  • Idle listening costs about as much energy as
    transmitting
  • In the following we present a few ideas, stolen
    from some known protocols that try to balance
    throughput and energy consumption.
  • S-MAC, T-MAC, B-MAC, or WiseMAC
  • Many of the hundreds of MAC protocols that were
    proposed have similar ideas

9
Sensor MAC (S-MAC)
  • Coarse-grained TDMA-like sleep/awake cycles.
  • All nodes choose and announce awake schedules.
  • synchronize to awake schedules of neighboring
    nodes.
  • Uses RTS/CTS to resolve contention during listen
    intervals.
  • And allows interfering nodes to go to sleep
    during data exchange.

increased latency
10
Sensor MAC (S-MAC)
  • Problem Nodes may have to follow multiple
    schedules to avoid network partition.

Schedule 12
Schedule 2
Schedule 1
  • A fixed sleep/awake ratio is not always optimal.
  • Variable load in the network.
  • Idea Adapt listen interval dependent on the
    current network load.
  • T-MAC

11
Low Power Listening (B-MAC)
  • Nodes wake up for a short period and check for
    channel activity.
  • Return to sleep if no activity detected.
  • If a sender wants to transmit a message, it sends
    a long preamble to make sure that the receiver
    is listening for the packet.
  • preamble has the size of a sleep interval
  • Very robust
  • No synchronization required
  • Instant recovery after channel disruption

preamble
data
listen
channel sniff
12
Low Power Listening (B-MAC)
overhearing problem
  • Problem All nodes in the vicinity of a sender
    wake-up and wait for the packet.
  • Solution 1 Send wake-up packets instead of
    preamble, wake-up packets tell when data is
    starting so that receiver can go back to sleep as
    soon as it received one wake-up packet.
  • Solution 2 Just send data several times such
    that receiver can tune in at any time and get
    tail of data first, then head.
  • Communication costs are mostly paid by the
    sender.
  • The preamble length can be much longer than the
    actual data length.
  • Idea Learn wake-up schedules from neighboring
    nodes.
  • Start sending preamble just before intended
    receiver wakes up.
  • WiseMAC

encode wake-up pattern in ACK message
13
Hybrid Protocols
  • Protocols may use information from upper layers
    to further improve their performance.
  • Information about neighborhood
  • Routing policies
  • Minimize costly overhearing of neighboring nodes
  • Inform them to change their channel sniff
    patterns
  • Use randomization to resolve schedule collisions

optimization for WiseMAC
schedule collision
like in Dozer
14
Slotted Aloha
  • We assume that the stations are perfectly
    synchronous
  • In each time slot each station transmits with
    probability p.
  • In Slotted Aloha, a station can transmit
    successfully with probability at least 1/e, or
    about 36 of the time.

15
Unslotted (Pure) Aloha
  • Unslotted Aloha simpler, no (potentially
    costly!) synchronization
  • However, collision probability increases. Why?
  • There is a factor-2-handicap of unslotted vs.
    Slotted

16
Aloha Robustness
  • We have seen that round robin has a problem when
    a new station joins. In contrast, Aloha is quite
    robust.
  • Example If the actual number of stations is
    twice as high as expected,there is still a
    successful transmission with probability 30.
    If it is onlyhalf, 27 of the slots are used
    successfully. So nodesjust need a good
    estimateof the number of nodes intheir
    neighborhood.

17
Adaptive Slotted Aloha
  • Idea Change the access probability with the
    number of stations
  • How can we estimate the current number of
    stations in the system?
  • Assume that stations can distinguish whether 0,
    1, or more than 1 stations transmit in a time
    slot.
  • Idea
  • If you see that nobody transmits, increase p.
  • If you see that more than one transmits, decrease
    p.
  • Model
  • Number of stations that want to transmit n.
  • Estimate of n
  • Transmission probability p 1/
  • Arrival rate (new stations that want to
    transmit) ? (with ? lt 1/e).

18
Adaptive Slotted Aloha QA
  • Q What if we do not know ?, or ? is changing?
  • A Use ? 1/e, and the algorithm still works.
  • Q How do newly arriving stations know ?
  • A We send with each transmission new
    stations do not send before successfully
    receiving the first transmission.
  • Q What if stations are not synchronized?
  • A Aloha (non-slotted) is twice as bad.
  • Q Can stations really listen to all time slots
    (save energy by turning off)? Can stations really
    distinguish between 0, 1, and 2 sender?
  • A Maybe. One can use systems that only rely on
    acknowledgements.

19
Backoff Protocols
  • Backoff protocols rely on acknowledgements only.
  • Binary exponential backoff
  • If a packet has collided k times, we set p 2-k
  • Or alternatively wait from random number of
    slots in 1..2k
  • It has been shown that binary exponential backoff
    is not stable for any arrival rate ? gt 0 (if
    there are infinitely many potential stations)
  • Interestingly when there are only finite
    stations, binary exponential backoff becomes
    unstable with ? gt 0.568 Polynomial backoff
    however, remains stable for any ? lt 1.
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