7: Wireless Ad Hoc Networks 7-1
Chapter 7
Wireless Ad Hoc Networks
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Definitions: An ad-hoc network is one that comes together as
needed, not necessarily with any assistance from the existing Internet infrastructure
Instant infrastructure
A MANET is a collection of mobile platforms or nodes where each node is free to move about arbitrarily
A MANET: distributed, possibly mobile, wireless, multihop network that operates without the benefit of any existing infrastructure (infrastructure-less), except the nodes themselves
What is an Ad Hoc Network?
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Mobile Ad Hoc Networks
May need to traverse multiple links to reach a destination
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Mobile Ad Hoc Networks (MANET) Mobility causes route changes
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Why Ad Hoc Networks ?
Ease of deployment
Speed of deployment
Decreased dependence on infrastructure
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Fundamental Challenges
It is better to know some of the questions than all of the answers. — James Thurber (1835-1910)
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1. Energy Efficiency
No infrastructure means must rely on batteries (or, in general, limited energy resources)
Possible solutions Selectively sending nodes into a sleep mode Using transmitters with variable power (the
Power Control problem) Using energy-efficient paths Using cooperative techniques (still relatively
new)
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2. Mobility
Mobility-induced route changes Mobility-induced packet losses
Mobility patterns may be different Controlled e.g. robots
• Offers opportunities for improving the network functions e.g. connectivity, coverage
Uncontrolled e.g. nomadic users• Offers challenges to network design
• But also offers opportunities for improvement, e.g.
– Users “carry” delay-tolerant data closer to destination
– Delay Tolerant Network (Challenge Networks)
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3. QoS
Providing QoS even in wired networks (e.g. the Internet) is a challenging problem
Wireless RF channels further complicate the problem Unpredictability Medium access: broadcast medium with hidden
terminal problem
Possible solutions: New MAC design Cross-layer integration: allow different layers to
adapt depending on available information at other layers
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4. Scalability
Limited wireless transmission range Whether the network is able to maintain an
acceptable level of service even as the number of nodes is increased How fast the network protocol control overhead
increases as N increases
Possible solutions: Introducing hierarchy Utilizing location information Limiting reactions to changes Fixing things (e.g. paths) locally
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5. Utilizing New Technologies
What are the gains that could be achieved by using newly available technologies such as Smart directional (beamforming) antennas
• Increases the spatial reuse in cellular, but how about ad-hoc?
• Can several nodes together act as an antenna array? Practical issues?
Software Radio• The ability to quickly switch the operating frequency
may provide opportunities, but also challenging
GPS• Location information may help
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6. Security
Ease of snooping on wireless transmissions From crypto point of view, lack of a trusted
authority is one of the main challenges How to generate/share keys reliably
Harder to track or even detect attackers in a wireless environment, given that: Network relies on in-situ connections to other nodes
which may be malicious
Malicious nodes may be especially harmful by injecting bogus control packets
DoS attacks that deplete a node’s battery
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7. Lack of Reference
Lack of sufficient experimental data to confirm models What does a multi-hop path really mean? What is a link?
Simplistic models that do not capture the complexities, or complex models that do not lead to insights?
Are the protocols good enough, have they reached closed to the best possible?
Good balance between mathematical and experimental work
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Multiple-Layer Problem
PHY Adapt to rapid changes in link characteristics
MAC Minimize collision, allow fair access, and semi-reliably
transport under rapid change and hidden/exposed terminals
Network Determine efficient transmission paths when links change
often and bandwidth is at a premium Transport
Handle delay and packet loss statistics that are very different than wired networks
Application Handle frequent disconnection and reconnection as well as
varying delay and packet loss characteristics
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Several Major Issues
MAC protocols for ad hoc networks
Routing in ad hoc networks
Transport protocols for ad hoc networks
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Design Goals for MAC Protocols Allow fair access to the shared radio
medium Distributed protocol Available bandwidth must be utilized efficiently Control overhead should be minimized Ensure fair bandwidth allocation to competing
nodes Reduce the effect of hidden/exposed terminals Effectively manage the power consumption Provide QoS support for real-time traffic Protocol should be scalable
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Overall Picture
MAC Protocols for Ad Hoc
Contention-basedContention-based
with reservationContention-based
with schedulingOther
Protocols
Sender initiated Receiver initiated
Single channel Multiple channel
synchronous asynchronous
• DPS• DWOP• DLPS
• MMAC• MCSMA• PCM• RBAR
• D-PRMA• CATA• HRMA• SRMA/PA• FPRP
• MACA/PR• RTMAC
• BTMA• DBTMA• ICSMA
• MACAW• FAMA
• RI-BTMA• MACA-BI• MARCH
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Contention-based Protocols with Reservations Use a bandwidth reservation technique
Contention occurs only at resource reservation phase Node gets an exclusive access to the media once
bandwidth is reserved
D-PRMA Distributed packet reservation multiple access protocol
SRMA/PA Soft reservation multiple access with priority assignment
RTMAC Real-time medium access control protocol
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Contention-based Protocols with Scheduling Focus on packet scheduling at the
nodes and transmission scheduling of the nodes
DPS Distributed priority scheduling
DWOP Distributed wireless ordering protocol
DLPS Distributed laxity-based priority scheduling
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Contention-based Protocols w/o Reservation/Scheduling MACA
Multiple access collision avoidance protocol
MACAW Media Access Protocol for Wireless LAN
BTMA Busy tone multiple access protocol
MARCH Media access with reduced handshake
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MACA: Multiple Access Collision Avoidance Proposed as an alternative to CSMA/CA Handle hidden and exposed terminal issues using RTS-
CTS RTS and CTS packets carry the expected duration of the
data transmission A node near the sender that hearing RTS do not transmit for a time
to receive CTS A node near the receiver after hearing CTS differs its transmission If the neighbor hears the RTS only, it is free to transmit once the
waiting interval is overneighbor sender neighborreceiver
RTSRTS
CTS CTS
DataData
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MACAW: Enhancement of MACA
Issue 1: potential flow starvation due to BEB Both S1 and S2 have the high volume of traffic, S1 seizes the
channel first Packets transmitted by S2 get collided and it doubles CW The probability that S2 seizes the channel decreasing
Solution in MACAW Packet header contains the field set to the current back-off
value of the transmitting node Node receiving this packet copies this value to its back-off
counter If all the nodes can hear each other, eventually they will have
the same back-off counter (fairness)
S1
AP
S2×
BEB BEB copy
S1-AP 48.5 23.82
S2-AP 0 23.82
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MACAW (Cont.)
Issue 2: backoff calculation adjusts too rapidly After every successful transmission, return to the
case where all stations have a minimal backoff counter, and then must repeat a period of contention to increase the backoffs
Solution in MACAW Gentler adjustment
• Upon a collision, the backoff interval is increased by a multiplicative factor (1.5)
Finc(x) = MIN[l.5x, CWmax]
• Upon success it is decreased by 1
Fdec(x) = MAX[x-1, CWmin]
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MACAW (Cont.)
Issue 3: Neighbor receivers problem When node A sends an RTS to B, while node C is
receiving from D, node B cannot reply with a CTS, since B knows that D is sending to C
When the transfer from C to D is complete, node B can send a Request-to-send-RTS (RRTS) to node A Node A may then immediately send RTS to node B
A B C D
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MACAW (Cont.)
This approach, however, does not work in the scenario below Node B may not receive the RTS from A at
all, due to interference with transmission from C
A B C D
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BTMA: Busy Tone Multiple Access One of the earliest solutions for hidden
terminal problem
Multi-channel protocol Control channel: used for busy tone transmission Data channel: used for data transmission
Three variants: BTMA (Busy Tone Multiple Access) DBTMA (Dual Busy Tone Multiple Access) RI-BTMA (Receiver-Initiated BTMA)
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BTMA (Cont.)
Basic idea Node senses the control channel to check whether the
busy tone is active• If not, turns on busy tone signal and starts data
transmission• If yes, waits for a random period of time and repeats
Any node that senses the carrier on the incoming data channel also transmits a busy tone
Pros and Cons Simple with extremely low collision probability Bandwidth utilization is low (blocked in two-hop neighbor) Multiple channels
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Several Major Issues
MAC protocols for ad hoc networks
Routing in ad hoc networks
Transport protocols for ad hoc networks
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Why is Routing in MANET Different?
No specific nodes dedicated for control Host mobility
Link failure/repair due to mobility may have different characteristics than those due to other causes
Rate of link failure/repair may be high when nodes move fast
Different node characteristics E.g. power constraints, multiple access issues
New performance criteria may be used Route stability despite mobility Energy consumption
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Unicast Routing Protocols
Many protocols have been proposed Some have been invented specifically for MANET Others are adapted from previously proposed
protocols for wired networks
No single protocol works well in all environments Some attempts made to develop adaptive
protocols
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MANET Protocol Zoo
Topology based routing Proactive approach, e.g., DSDV. Reactive approach, e.g., DSR, AODV, TORA. Hybrid approach, e.g., Cluster, ZRP.
Position based routing Location Services:
• DREAM, Quorum-based, GLS, Home zone etc. Forwarding Strategy:
• Greedy, GPSR, RDF, Hierarchical, etc.
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Routing Protocols
Proactive protocols Determine routes independent of traffic pattern Traditional link-state and distance-vector routing
protocols are proactive
Reactive (on-demand) protocols Discover/maintain routes only when needed Source-initiated route discovery
Hybrid protocols
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Trade-Off
Latency of route discovery Proactive protocols may have lower latency since
routes are maintained at all times Reactive protocols may have higher latency because
a route from X to Y will be found only when X attempts to send to Y
Overhead of route discovery/maintenance Reactive protocols may have lower overhead since
routes are determined only if needed Proactive protocols can (but not necessarily) result in
higher overhead due to continuous route updating
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Tradeoff (Cont.)
Which approach achieves a better trade-off depends on the traffic and mobility patterns Reactive protocols may yield lower routing
overhead than proactive protocols when communication density is low
Reactive protocols tend to loose more packets (assuming that network layer drops packets if a route is not known)
Proactive protocols perform better with high mobility and dense communication graph
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Single Path vs. Multipath
Single path Use one path from
source to destination Similar to wired
routes Advantages:
• Simple to implement
Disadvantages:• Source must find a
new route to destination if old one fails
Multipath Use more than one
path from source to destination
Advantages:• Load balancing can
occur
• Higher tolerance to link failures
Disadvantages:• Adds complexity to
receiver and sender
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Short Hops vs. Long Hops
Research to date suggests short-hop Provides lower energy consumption
• Lower transmission power needed due to shorter distance between nodes
Provides higher link capacity• Higher received signal strength due to shorter
distance between nodes
Long-hop intuitively should have less total delay due to Less total hops Smaller total processing delay
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Some Existing Wireless Routing Protocols
DSDV WRP CGSR STAR OLSR FSR HSR GSR DSR
AODV ABR SSA FORP PLBR CEDAR ZRP ZHLS RABR
LBR COSR PAR LAR OLSB
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Dynamic Source Routing (DSR) Reactive, source-based
When node S wants to send a packet to node D, but does not know a route to D, node S initiates a route discovery
Source node S floods Route Request (RREQ)
Each node appends own identifier when forwarding RREQ
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Route Discovery in DSR
B
A
S E
F
H
J
D
C
G
IK
Z
Y
Represents a node that has received RREQ for D from S
M
N
L
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Route Discovery in DSR
B
A
S E
F
H
J
D
C
G
IK
Represents transmission of RREQ
Z
Y
Broadcast transmission
M
N
L
[S]
[X,Y] Represents list of identifiers appended to RREQ
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Route Discovery in DSR
B
A
S E
F
H
J
D
C
G
IK
• Node H receives packet RREQ from two neighbors: potential for collision
Z
Y
M
N
L
[S,E]
[S,C]
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Route Discovery in DSR
B
A
S E
F
H
J
D
C
G
IK
• Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once
Z
Y
M
N
L
[S,C,G]
[S,E,F]
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Route Discovery in DSR
B
A
S E
F
H
J
D
C
G
IK
Z
Y
M
• Nodes J and K both broadcast RREQ to node D• Since nodes J and K are hidden from each other, their transmissions may collide
N
L
[S,C,G,K]
[S,E,F,J]
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Route Discovery in DSR
B
A
S E
F
H
J
D
C
G
IK
Z
Y
• Node D does not forward RREQ, because node D is the intended target of the route discovery
M
N
L
[S,E,F,J,M]
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Route Discovery in DSR
Destination D on receiving the first RREQ, sends a Route Reply (RREP)
RREP is sent on a route obtained by reversing the route appended to received RREQ
RREP includes the route from S to D on which RREQ was received by node D
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Route Reply in DSR
B
A
S E
F
H
J
D
C
G
IK
Z
Y
M
N
L
RREP [S,E,F,J,D]
Represents RREP control message
RREP [S,C,G,K,D]
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Dynamic Source Routing (DSR) Node S on receiving RREP, caches the
route included in the RREP
When node S sends a data packet to D, the entire route is included in the packet header Hence the name source routing
Intermediate nodes use the source route included in a packet to determine to whom a packet should be forwarded
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DSR Optimization: Route Caching Each node caches a new route it learns by any
means When node S learns that a route to node D is broken
Uses another route from its local cache, if such a route to D exists in its cache
Otherwise, node S initiates route discovery by sending a route request
Intermediate node X on receiving a Route Request for some node D can send a Route Reply If node X knows a route to node D
Use of route cache Can speed up route discovery Can reduce propagation of route requests
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DSR Pros and Cons
Advantages: Less memory storage
needed at each node since full routing table is not needed
Lower overhead needed because no periodic update message are necessary
Nodes do not need to continually inform neighbors they are still operational
Disadvantages: Possible transmission
latency due to reactive approach
Stale routes can occur if links change frequently
Message size increases as path length increases
Collisions between route requests propagated by neighboring nodes
Route Reply Storm due to nodes replying using their local cache
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Several Major Issues
MAC protocols for ad hoc networks
Routing in ad hoc networks
Transport protocols for ad hoc networks
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Transmission Control Protocol (TCP) Reliable ordered delivery
Implements congestion avoidance and control
Reliability achieved by means of retransmissions if necessary
End-to-end semantics Acknowledgements sent to TCP sender confirm
delivery of data received by TCP receiver Ack for data sent only after data has reached
receiver
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Challenges
Throughput unfairness Unfairness at MAC layer Transport layer should take this into account
Resource constraints Power and bandwidth constraints
Separation of congestion control and reliability control
Completely decoupled transport layer Wired network: transport protocol completely
separated from underlying layer Ad hoc network: interaction with network and MAC
layer is expected for adaptability
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Challenges (Cont.)
Misinterpretation of congestion Traditional mechanism: packet loss, timeout Ad hoc loss/delay due to
• High bit error rate due to varying link condition• Packet collisions due to contention and hidden
terminal• Path breaks due to node mobility
Dynamically changing topology Frequent path breaks Partitioning and merging of networks High delay in reestablishment of path
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Performance of TCP
Several factors affect TCP performance in MANET Wireless transmission errors
Multi-hop routes on shared wireless medium• For instance, adjacent hops typically cannot transmit
simultaneously
Route failures/changes due to mobility
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Throughput over Multi-Hop Wireless Paths Connections over multiple hops are at a
disadvantage compared to shorter connections, because they have to contend for wireless access at each hop
0
200
400
600
800
1000
1200
1400
1600
1 2 3 4 5 6 7 8 9 10
Number of hops
TCPThroughtput(Kbps)TCP Throughput using
2 Mbps 802.11 MAC
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Impact of Caching
Route caching has been suggested as a mechanism to reduce route discovery overhead
Each node may cache one or more routes to a given destination
When a route from S to D is detected as broken, node S may: Use another cached route from local cache, or Obtain a new route using cached route at
another node
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Why Performance Degrades With Caching When a route is broken, route discovery returns a
cached route from local cache or from a nearby node
After a time-out, TCP sender transmits a packet on the new routeHowever, the cached route has also broken after it was cached
Another route discovery, and TCP time-out interval Process repeats until a good route is found
timeout dueto route failure
timeout, cachedroute is broken
timeout, second cachedroute also broken
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To Cache or Not to Cache
Caching can result in faster route “repair”
Faster does not necessarily mean correct
If incorrect repairs occur often enough, caching performs poorly
Need mechanisms for determining when cached routes are stale
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Caching and TCP performance Caching can reduce overhead of route
discovery even if cache accuracy is not very high
But if cache accuracy is not high enough, gains in routing overhead may be offset by loss of TCP performance due to multiple time-outs
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How to Improve Throughput(Bring Closer to Ideal) Network feedback
Inform TCP of route failure by explicit message
Let TCP know when route is repaired Probing Explicit notification
Reduces repeated TCP timeouts and backoff
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TCP with ELFN
Explicit Link Failure Notification Not totally new, e.g., ECN bits in TCP To provide the TCP sender with information about
link and route failures, so that it can avoid responding to the failures as if congestion has occurred
How does it work? When a TCP sender receives an ELFN: disables its
retransmission timers and enters a “stand-by” mode While on standby: A packet is sent at periodic intervals
to probe the network to see if a route has been established If an acknowledgment is received: leaves stand-by
mode and restores the retransmission timers
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Performance with Explicit Notification
0
0.2
0.4
0.6
0.8
1
2 10 20 30
mean speed (m/s)
thro
ug
hp
ut
as a
fra
ctio
n o
f id
eal
Base TCP
With explicitnotification
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Issues: Network Feedback
Network knows best (why packets are lost) Network feedback beneficial
Need to modify transport & network layer to receive/send feedback
Need mechanisms for information exchange between layers
[Holland99] discusses alternatives for providing feedback (when routes break and repair)
[Chandran98] also presents a feedback scheme
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TCP Performance
Two factors result in degraded throughput in presence of mobility
Loss of throughput that occurs while waiting for TCP sender to timeout This factor can be mitigated by using explicit
notifications and better route caching mechanisms
Poor choice of congestion window and RTO values after a new route has been found How to choose cwnd and RTO after a route
change?
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Issues: Window Size After Route Repair Same as before route break: may be too
optimistic
Same as startup: may be too conservative
Better be conservative than overly optimistic Reset window to small value after route repair Let TCP figure out the suitable window size Impact low on paths with small delay-bw
product
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Issues: RTO After Route Repair Same as before route break
If new route is long, this RTO may be too small, leading to timeouts
Same as TCP start-up (6 second) May be too large May result in slow response to next packet loss
Another plausible approach RTOnew = f(RTOold, route-lengthold, route-lengthnew)
E.g.: RTOnew = RTOold * route-lengthnew/route-lengthold
Not evaluated yet Pitfall: RTT is not just a function of route length
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Summary
Still many remained topics related to wireless ad hoc networks
More research opportunities in Wireless mesh networks
• With fixed infrastructure as wireless infrastructure
• Multi-radio multi-channel architecture
Wireless sensor networks• Energy consumption is one of the key challenges
• Application specific demands, including localization, coverage, event detection/collection, etc.