Wireless MAC Protocols

1
Wireless MAC Protocols

• Presenter George Nychis Xi Liu

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Outline

• Carrier Sense
• MACA
• MACAW
• MACA-BI
• Idle Sense
• OAR
• ECHOS

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
3
Carrier Sense

• Basic Idea listen before you send to avoid
  collisions
• Why?
• Avoid wasted transmissions on collision
• Avoid corruption of other transmission

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
4
Carrier Sense Design

• General Components
• Automatic Gain Control (AGC) system
• Baseband processor
• ? Instantaneous signal strength

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
5
How to Detect Transmissions

1. Preamble Detection
2. AGC unlock indicator
3. Energy Detect
4. De-correlation amplitude

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
6
Can CS be improved?

• Exposed Terminal
• Hidden Terminal

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
7
Carrier Sense Experiments

• 60-node wireless sensor network testbed
• Crossbow Mica2
• Radio _at_ 433MHz 48.4 Kbps
• What should we expected?

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
8
Carrier Sense Experiment Results
Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
9
Do we always need it?

• Oblivious to capture effect
• Performs poorly with high load
• Nodes that sent above the diagonal ? s achieved
  higher throughput

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
10
MACA

• Addresses hidden terminal and exposed terminal
  problems
• How?
• Throw away carrier sense!
• Use RTS / CTS

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
11
Collisions in MACA

• Preferably, small RTS packet collides
• Still chance of data collision
• Can we reduce this?
• Automatic Power Control
• Include S-meter in CTS
• Reduce RTS power
• Dont want to reduce CTS power
• Need extra hardware

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
12
MACAW

• 4 design details
• Contention is at the receiver
• Congestion is location dependent
• Fairness through learning of congestion levels
• Propagate synchronization information about
  contention periods

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
13
Fairness in MACAW

• Channel capture in MACA
• Backoff doubled every collision
• Reduce backoff on success
• Solution Copy backoffs

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
14
MACAW Copying Backoffs

• This does not always work as wanted

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
15
MACAW Additional Design

• Multiple Stream Model
• ACK

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
16
MACAW Additional Design

• DS
• Because carrier sense disabled

RTS
CTS
DS
DATA
Hears RTS
Doesnt hear CTS
Hears DS
Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
17
RRTS

• Problem

RTS
ACK
RTS
DS
DATA
RRTS
RTS
Backoff Increases
RRTS prevents P2 from responding
CTS
Cannot send CTS
X
DS
RTS
RTS
DATA
CTS
RTS lost
Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
18
MACAW Conclusions

• 8 extra overhead for DS and ACK
• 37 improvement in congestion
• Future work
• Multicast support
• Copying backoff

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
19
MACA-BI

• Simplified version of MACA
• MACAW Increased MACAs overhead
• Increased 3-way to 5-way handshake
• Turnaround time
• Preamble Bits
• Control Bits
• Turnaround time has more effect with higher
  channel speeds

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
20
MACA-BI Design

• Eliminate RTS
• Replace CTS with RTR (Ready to Receive)

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
21
MACA-BI Benefits

• Reduced turnaround time
• Keeps MACA functionality
• Data collision free just like MACA
• Less vulnerable to control packet corruption
  (half as many control packets)
• Receiver driven allows traffic regulation

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
22
MACA-BI Data Collision Free
2. C is transmitting to D, and B transmits an RTR
to A
1. C transmits a data packet to B
3. C is receiving an RTR from D when B sends an
RTR to A
Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
23
MACA-BI Results
Minimal hidden terminal, CSMA has less overhead
Hidden terminal problems, which is contention at
the receiver
Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
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Idle Sense - Problems of 802.11 DCF

• Short-term unfairness
• colliding hosts will be penalized
• No ACK ! collision
• Slow hosts limits the throughput of fast hosts
• capture leads to long-term unfairness

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
25
Idle Sense Basic Idea

• Make hosts use similar Contention Window (CW)
• instead of exponential backoff
• Better fairness
• Adapting CW to varying traffic conditions
• Hosts observe the number of idle slots between
  two transmission attempts
• Intuitively, this is an indicator of how many
  hosts are currently contending
• Adjust CW to an optimal value accordingly
• All hosts converge to a similar CW

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
26
Idle Sense - Analysis Result
Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
27
Idle Sense - Principles

• If we know N, things are much easier
• estimating N is undesired
• Pick a fixed target nitarget for all cases
• Hosts observe ni
• If ni lt nitarget , then CW lt- CW e
• If ni gt nitarget , then CW lt- CW / a
• AIMD has the converging property

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
28
Idle Sense - Properties

• Decouple contention control with frame loss
• Solves the capture effect problem
• No exponential backoff due to bad channel quality
• Rate adaptation protocol
• Similar CW
• short-term fairness
• Time fairness
• Scaling CW according to transmission rate
• Fully distributed
• and no information exchange

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
29
Idle Sense - Throughput
Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
30
Idle Sense - Fairness
Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
31
Idle Sense - Convergence Speed
Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
32
Idle Sense - Time Fairness
Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
33
Rate Adaptation Protocols

• Auto Rate Fallback (ARF)
• senders attempt to use higher transmission rate
  after consecutive successes
• revert to lower rate after failures
• Receiver Based Auto Rate (RBAR)
• receiver measures channel quality
• piggyback in CTS
• sender decide transmission rate according to this
  information
• Idle Sense

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
34
Opportunistic Auto Rate (OAR)

• Observation
• Coherence time (duration where hosts have better
  channel quality) is at least several packet time
• Idea
• If the channel is of high quality, user can
  transmit multiple packets
• Temporal fairness vs. throughput fairness

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
35
OAR - Implementation Issues

• How to estimate channel condition
• Use ARF, RBAR, Idle Sense
• How to transmit several packet
• Utilize 802.11 fragmentation
• set more fragments bit
• clear fragment number subfield

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
36
OAR - Benefits

• Channel is better utilized, then better
  throughput
• No RTS/CTS for subsequent packets
• Reduce contention time per packet
• Time fairness

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
37
ECHOS (Enhance Capacity 802.11 Hotspots)

• AP-CST Access Point (modifies) Carrier Sense
  Threshold
• multiple flows can co-exist by dynamically
  modifying the carrier sense threshold
• RNC-SC Radio Network Controller (uses) Secondary
  Channels
• centralized algorithm that assigns hosts to
  cells/channels and adjust transmission power

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
38
ECHOS - Three Ranges in Wireless

• Obtained from QualNet
• Rate 2Mbps
• CST -93dBm
• Power 15dBm
• Pr kPt / d4
• transmission range
• interference range
• carrier sense range

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
39
ECHOS - Example

• Node 5 7 can hear only each other
• Node 3 9 can hear only each other
• Node 1 hear all transmissions

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
40
ECHOS - Observations 1

• The use of physical carrier sensing with default
  fixed carrier sensing threshold (CST) can
  unnecessarily couple together several flows
  reducing per-flow and aggregate throughput

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
41
ECHOS - Observation 2

• The optimum value of CST is that which the
  carrier sense range of the transmitter just
  covers the interference range of the receiver

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
42
ECHOS - AP-CST

• Basic idea
• choose the appropriate CST to be used by each AP
  and stations
• Reducing CST may allow several flows to operate
  together without interference
• Stations
• Given the signal strength of a station at AP, CST
  of a station is set to maximum signal strength
  that can be ignored while attempting to transmit,
  
• Then it is adjusted so that it can hear other
  stations from inside its own cell avoid hidden
  terminal
• AP
• Minimum SNR of the AP at its clients decides the
  CST of AP

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
43
ECHOS - RNC-SC

• Measure and determine if a cell is overloaded
• MAC service time time between frame is submitted
  and ACK is acknowledged
• average over all users
• Choose client and secondary channel
• Only when secondary AP and client can operate at
  low transmit power

Carrier Sense
MACA
MACAW
MACA-BI
Idle Sense
OAR
Echos
44
References

• E.1 Phil Karn, "MACA - A New Channel Access
  Method for Packet Radio", 1990.
• E.2 V. Bharghavan, A. Demers, S. Shenker , and
  L. Zhang. MACAW A Media Access Protocol for
  Wireless LANs , In Proc. ACM SIGCOMM , London,
  U.K., September 1994, pp. 212-225.
• E.3 B. Sadeghi, V. Kanodia, A. Sabharwal, and
  E. Knightly, Opportunistic Media Access for
  Multirate Ad Hoc Networks, in Proceedings of ACM
  MOBICOM 2002.
• E.5 Martin Heusse, Franck Rousseau, Romaric
  Guillier, and Andrzej Duda. Idle Sense An
  Optimal Access Method for High Throughput and
  Fairness in Rate Diverse Wireless LANs In Proc.
  of ACM SIGCOMM, Aug. 2005
• E.6 A. Vasan, R. Ramjee, and T. Woo. Echos -
  Enhanced Capacity 802.11 Hotspots  In Proc. of
  IEEE Infocom'05, Mar 2005
• E.7 F. Talucci, M. Gerla, and L. Fratta.
  MACA-BI (MACA by invitation) a receiver oriented
  access protocol for wireless multihop networks.
  In Proceedings of IEEE PIMRC, 1997.630970
• E.8 Kyle Jamieson, Bret Hull, Allen K. Miu,
  Hari Balakrishnan. Understanding the Real-World
  Performance of Carrier Sense. ACM SIGCOMM
  Workshop on Experimental Approaches to Wireless
  Network Design and Analysis (E-WIND)

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Q A

• Thanks!

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Questions

• Why hasnt MAC changed much?