Title: Modeling Media Access in Embedded TwoFlow Topologies of Multihop Wireless Networks
1Modeling Media Access in Embedded Two-Flow
Topologies of Multi-hopWireless Networks
- Jingpu Shi
- (joint work with Michele Garetto and Edward
Knightly) - Department of Electrical and Computer Engineering
- Rice University
- April 27, 2005
2Background Introduction Wireless Medium Access
Control
b
A
- In a wireless network, how stations access
channel to accomplish transmissions ? - The MAC (Medium Access Control) is a set of rules
to determine how to access the medium (channel). - Performance Metrics
- Throughput
- Fairness etc.
a
B
(a) Ad Hoc network
(b) Hot Spot
3Background Introduction Hidden Terminal Problem
b
A
- A simple MAC solution a station starts
transmitting only after it senses the channel is
idle. - Hidden terminal can not be sensed, therefore
packets could collide, and be destroyed.
a
B
(a)
a
b
B
A
(b)
4Background Introduction IEEE 802.11 MAC
- A distributed algorithm.
- 4-way or 2-way exchange for every data packet
transmission. - 4-way exchange Control packet transmissions
precede data packet transmissions to avoid
collision. - Maintain a value CW (contention window) using a
exponential increase linear decrease algorithm.
5Background Introduction IEEE 802.11 MAC
(continued)
Sender
RTS
DATA
CTS
ACK
Receiver
Others
RTS
CTS
6Motivation of This Work
- In Multi-hop wireless networks, scenarios similar
to the hidden terminal problem lead to fairness
problems. - Those problems have not been very well addressed
and understood. - In this work, we view a network as a set of
sub-graphs consisting two flows and characterize
its media access.
Source
Destination
7Outline
- Background and Motivation
- Scenario identifications and their likelihood to
occur - Fairness simulations
- Media access modeling
8Twelve topologies
- Identical transmission range and interference
range. - We only consider one-way flows.
- A link is established when two stations are in
radio range.
9All Possible Topologies
10Scenario Classification
- Senders Connected (SC) scenarios 2-7, where
senders of each flow are in radio range. - Asymmetric Incomplete State (AIS), scenarios 11
and 12, where senders are disconnected,
asymmetric connections between the two flows. - Symmetric Incomplete State (SIS), scenario 8, 9
and 10, where senders are disconnected, symmetric
connections between the two flows.
11Scenario LikelihoodAssumptions and illustration
- Whats the probability of each scenario occurring
? - Spatial analysis, assuming the two flows are
uniformly distributed in a region and border
effect is negligible.
12Scenario LikelihoodResults for each scenario
- Scenario 11 dominates when distance becomes large
13Scenario LikelihoodResults for each group
AIS and SIS class are highly likely to occur when
distance between two hops becomes large.
14Outline
- Motivation
- Scenario identifications and their likelihood
- Fairness simulations
- Media access modeling
15Performance Simulations With CSMA/CA protocol
- Observations
- SC-No fairness problem.
- AIS-Both short-term and long-term fairness
problems. - SIS-Long-term fair, short-term unfair.
- Root cause different information about the
channel.
16Outline
- Motivation
- Scenario identifications and their likelihood
- Fairness simulations
- Modeling media access
17Modeling Framework
- Identify 4 different state
- idle channel
- channel occupied by successful transmissions
- channel occupied by a collision
- busy channel due to activity of other stations
- Define probabilities
- Probability of the four stats and throughput of
the station
18Model AIS ClassSample topology and modeling
strategy
- Use decoupling technique.
- Independently study the behavior of each
transmitting node. - Consider flow B first, and then flow A.
19Model AIS Class Strategies and steps
- For the first flow, the only unknown parameter is
collision probability, which can be computed from
the plot above. - For the second flow, the only unknown parameter
is b, which can be computed easily given
throughput of flow A is known.
20Model AIS ClassResults
- With RTS/CTS
Without RTS/CTS
21Model SIS ClassSample topology and modeling
strategy
- We analyze short-term unfairness.
- Main difficulty the two transmitting nodes are
tightly correlated. - A Markov chain model using bi-dimensional state
description.
22Model SIS Class Strategies and steps
- We represent the system state as pair (SA, SB),
where SA and SB denote the backoff stage of
Sender A and B respectively. - Transition probability of the Markov chain.
- ri is the probability that a station transmits
after one slot in backoff stage i. f is the
duration of the first packets (RTS or DATA)
transmitted. - After solving the Markov chain, we can compute
the transition time from state (m, 0) to (0,m),
where m is the maximum backoff stage.
23Model SIS ClassResults (cont.)
(C1) RTS/CTS access, m 6, CWmax 1024. (C2)
RTS/CTS access, m 8, CWmax infinity. (C3)
Basic access, m 3, CWmax 1024. (C4) Basic
access, m 6, CWmax 1024.
24Outline
- Background and Motivation
- Scenario identifications and their likelihood to
occur - Fairness simulations
- Media access modeling
25- More Questions or Comments ?
- Email jingpu_at_rice.edu