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7: Wireless Ad Hoc Networks 7-1 Chapter 7 Wireless Ad Hoc Networks

7: Wireless Ad Hoc Networks7-1 Chapter 7 Wireless Ad Hoc Networks

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Page 1: 7: Wireless Ad Hoc Networks7-1 Chapter 7 Wireless Ad Hoc Networks

7: Wireless Ad Hoc Networks 7-1

Chapter 7

Wireless Ad Hoc Networks

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7: Wireless Ad Hoc Networks 7-2

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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7: Wireless Ad Hoc Networks 7-3

Mobile Ad Hoc Networks

May need to traverse multiple links to reach a destination

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7: Wireless Ad Hoc Networks 7-4

Mobile Ad Hoc Networks (MANET) Mobility causes route changes

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7: Wireless Ad Hoc Networks 7-5

Why Ad Hoc Networks ?

Ease of deployment

Speed of deployment

Decreased dependence on infrastructure

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7: Wireless Ad Hoc Networks 7-6

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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7: Wireless Ad Hoc Networks 7-8

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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7: Wireless Ad Hoc Networks 7-10

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.