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Medium access control in Wireless Ad-hoc Networks
Presented by
Jin Xu and Lan Nguyen
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Outline
Classifications of MAC protocols
Five Phase Reservation Protocol (FPRP)
Distributed Wireless Ordering Protocol (DWOP)
Conclusions
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Classifications of MAC protocols
Contention-based
A node contends with its neighbors to access the channel
No QoS guarantees
MACAW, FAMA, BTMA
Contention-based with reservation
Reserve bandwidth a priori
Can provide QoS support to real time traffic
D-PRMA, CATA, FPRP
Contention-based with scheduling
Focus on transmission scheduling of the nodes
Fair and no starvation
DPS, DWOP, DLPS
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Five-Phase Reservation Protocol (FPRP)
Contention-based with reservation
Single channel time division multiple access
Fully distributed (synchronized)
Slot reservation using a 5 phase process
Parallel
Localized process
Scalable (insensitive to network size)
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FPRP: the model
Nodes keep perfect timing
A link between 2 nodes is noiseless, symmetric
The network topology not change when FPRP is
performed
When multiple packets arrive at a node, they are
destroyed
A node can tell whether 0, 1, or multiple packets are
transmitted when in receiving mode
Every node has a unique ID
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FPRP: overview
Time is divided into frames

2. 2
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FPRP: 5 phase
Reservation request (RR)
Collision report (CR)
Reservation confirmation (RC)
Reservation acknowledgement (RA)
Packing and elimination (P/E)
A node keeps global timing, and knows when a 5-phase
cycle starts.
A node can transmit or receive, but cannot do both at
the same time.
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Phase 1: Reservation request
A node which wants to make a reservation sends a
Reservation Request packet (RR) with probability p
Other nodes listen
Requesting
Node
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Phase 2: Collision report
If a node receives multiple RR’s in phase 1, it
transmits a Collision Report packet (CR)
o.w. silent
Requesting node (RN) transmission node (TN)
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Phase 3: Reservation confirmation
TN sends Reservation Confirmation packet (RC)
Every node (1 hop away) which receives RC know
the slot has been reserved cease contention,
receiving
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Phase 4: Reservation acknowledgement
A node ack a RC just received
by sending a Reservation Ack
packet (RA)
Inform nodes 2 hops away
Not transmitting
Prevent isolated node from
transmitting
Resolve isolated deadlock
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Phase 5: packing / elimination
Every node 2 hops away from TN sends a Packing
packet (PP)
A node receiving PP learns there is a TN 3 hops away
Adjust its contention prob.
to reuse time slot efficiently
TN sends an Elimination packet (EP) with a
probability of 0.5
Attempt to resolve a non-isolated deadlock

3. 3
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An example
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How to calculate the contention probability
nc : # of nodes that contend within 2 hops
nb : # of nodes within 2 hops that have to contend
in next slot due to a nearby success (cannot contend
in current slot)
R1: a portion of 1 hop neighbors from success cease
to contend in the current slot
R2: 2 hop neighbors
R3: 3 hop neighbors
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How to calculate the contention probability
At the beginning of a reservation slot, a node resets
its nc and nb:
nc = nb nb = 0
After every reservation cycle, on hearing an:
Idle: nc = nc – 1
Collision: nc = nc + 1 / (e - 2)
Success:
0 hop: done;
1 hop: nc = nc – 1 nb = nb + nc R1 nc =nc(1 – R1)
2 hops: nc = nc – 1 nb = nb + nc R2 nc =nc(1 – R2)
3 hops: nb = nb + nc R3 nc =nc(1 – R3)
P = 1/nc
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Simulation results
The # of nodes is 100, 200, 300 and 400 from bottom to top
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Effects of nodal mobility
The observation time t is 0, 0.5, 1, 2, 4, 8, 16, 32, 64, 128
seconds from bottom to top
BM RCSt = 0
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Protocol considerations and applications
Time synchronization issue:
GPS can provide accurate global time
Some applications:
TDMA schedule produced can be used to transmit user
generated packets
Make reservations for network control traffic
Can provide good service for multimedia (voice) traffic

4. 4
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Conclusions for FPRP
Fully distributed, requiring no a prior knowledge
about the network
Generate transmission schedules with low amount of
overhead
Not affected much by the network size and nodal
mobility
Suitable for use in large, mobile ad hoc network
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Problems of IEEE 802.11
Unfair channel access
Due to random access nature of wireless 802.11
Hard to support QoS (priority routing/scheduling
e.g. diverges significantly from FIFO order
Even worse in more complex topologies, e.g.
Asymmetric information
Perceived collision
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Asymmetric information
Symmetric information sharing
All nodes are within radio range of each other,
hence can hear all the RTS and CTS
All nodes have the same probability of accessing
the channel
Asymmetric information sharing (opposite)
All nodes are not within radio range of each other
Nodes have unequal channel access probabilities
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Example
Node 1 does not know about
flow B, so it has to send RTS
randomly
Node 2 may not be able to
send CTS due to either:
Deferral for flow B transmission
Didn’t receive RTS due to
collision with flow B
Node 3 knows about flow A
via node 2, hence it knows
the “right time” to send RTS
Flow A: 5%
Flow B: 95%
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A possible solution
MACAW
Node 2 sends Request-for-RTS
(RRTS) to node 1
Upon receiving RRTS, node 1
sends RTS immediately
Works only if node 2 has
received RTS
Not included in IEEE 802.11
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Perceived collision
In the previous example, flow B has info
about flow A (not the vice versa) -> flow B
gets more channel access
But this is not always the case
As knowing more information about other
flows makes a flow defer access to more
flows

5. 5
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Perceived collision (cont.)
Flows A and C can access the channel simultaneously
Flow B contains information about both flows A & C,
i.e. it has to wait for both flows A & C
Flow A: 36%
Flow B: 28%
Flow C: 36%
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DWOP protocol
Goals
FIFO-like behavior
Piggy-backing arrival times
Modifying IEEE 802.11
Receiver participation
Stale entry detection
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Goals of DWOP
Providing fair channel access
Scheduling packets in the order that
approximates a reference scheduler
(e.g. FIFO)
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FIFO-like behavior
Wireless contention for the channel access requires a
scheduler for all the contending nodes
For FIFO, packet priority is packet’s arrival time
Contending nodes share the arrival times
A scheduling table is needed
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Piggy-backing the arrival times
A sender (receiver) piggy-
backs the arrival time of its
current (highest priority)
packet into RTS (CTS)
A sender (receiver) also
piggy-backs the arrival time
of its next highest priority
packet into the DATA (ACK)
packet’s transmission
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Piggy-backing (cont.)
Nodes overhearing RTS,
CTS, DATA, ACK packets add
the attached arrival time into
their sorted scheduling
tables along with their own
packets’ arrival times
Also, nodes remove the
arrival time entry when
overhearing the completion
of DATA and ACK

6. 6
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Modifying IEEE 802.11
If the channel is busy,
behave like IEEE 802.11
If the channel is idle,
check the scheduling
table
If its packet has the
highest priority, send
RTS
Else defer as in IEEE
802.11
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Does it work all the time ?
Only if all nodes are within radio range of each other
If not -> asymmetric information
Node 1 is not aware of flow B -> behaves like IEEE 802.11 -
> try to contend the access continuously and randomly
Flow B knows the arrival time of flow A -> defers if it has
lower priority -> less aggressive than flow A -> Flow B gets
less bandwidth
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Receiver participation
Receiver of A (node 2)
know the arrival time of
flow B
“warn” node 1
include an out-of-order
notification in CTS/ACK
Upon receiving the out-
of-order notification
Node 1 finishes it current
transmission
Node 1 goes into a
backoff
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Receiver participation (cont.)
Node 1 is allowed to complete its current out-of-order
transmission
Only approximate FIFO is achieved
DWOP can be modified to achieve perfect FIFO
But tradeoff between perfect FIFO scheduling and
network utilization
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Stale entry elimination
Stale arrival time entry occurs in a node if:
Does not receive ACK (due to collision)
Stale entry detection
If there is a deletion below its packet’s position in the
scheduling table
Reaction
Remove the first entry in the scheduling table
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Simulation experiments
Experiment setup
ns-2 simulator with cmu-wireless extension
Data packet size: 1000 bytes
Channel bandwidth: 2Mbps
CBR flows
Three topologies:
Asymmetric information topology
Perceived collision topology
More complex topology

7. 7
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Simulation experiments (cont.)
Performance metrics
Fair channel access
FIFO-like behavior
Ideal FIFO
Switch to another flow after one packet
transmission
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Asymmetric information
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Perceived collision topology
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More complex topology
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Summary of results
DWOP has less deviation from FIFO
DWOP: deviation bounded up to 4 packets
IEEE 802.11: unbounded deviation
DWOP achieves better fairness of
accessing the channel than IEEE 802.11
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Limitations of DWOP
Flows of one single hop
Fixed packet size
FIFO scheduler
Total number of received packets is about 1/3
less than IEEE 802.11
Does not consider
Mobility
Channel errors (ECF [3])
Variable packet size
Other reference schedulers

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DWOP conclusions
IEEE 802.11 does not take care of fair
channel access well
IEEE 802.11 diverges significantly from FIFO
DWOP provides fairness of channel access
DWOP approximates the FIFO order better
DWOP can be applied to other schedulers
QoS can be integrated in DWOP
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Reference
[1]. A Five-Phase Reservation Protocol (FPRP) for Mobile Ad Hoc
Networks, C Zhu, M S Corson, Proceedings of IEEE INFOCOM,
1998.
[2]. Ordered Packet Scheduling in Wireless Ad Hoc Networks:
Mechanisms and Performance Analysis, V. Kanodia, C. Li, A.
Sabharwal, B. Sadeghi, Proceedings of ACM MOBIHOC, 2002.
[3]. Fair scheduling in wireless ad-hoc networks of location
dependent channel errors, Chen J., Somani A. K., Proceedings
of the 2003 IEEE International, 2003.
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Question ?