EE4L CCC L5 Wireless Networking _v1

7/24/2019 EE4L CCC L5 Wireless Networking _v1

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EE4L

Computer & Communication

Networks

Part IV Wireless networking

Dr Costas Constantinou


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Wireless Networking

1. Overview

2. Wireless MAC layer protocols

3. Wireless LANs

4. Mobile ad hoc networks

Acknowledgements:

Slides adapted from numerous sources. Thanks go (alphabetically) to, DrRomit Roy Choudhury, Duke University; Prof Jim Kurose, Univ ofMassachusetts; Prof Nitin Vaidia, Univ of Illinois at Urbana-Champaign


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1. Overview

Wireless networks are becoming ubiquitous

The edge of the Internet is fast becoming wireless

Single hop networks:

Wireless LANs Cellular

Multi-hop networks: Personal area networks

Military


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1. Overview

RFID and

Sensor Networks

Citywatchers, Wal-Mart

Intel, Philips, Bosch

Personal Area

Networks

Motorola, Intel,

Samsung

Mesh Networks and

Wireless Backbones

Microsoft, Intel, Cisco InternetFuture network vision:


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1. Overview

wireless hosts

laptop, PDA, IP phone run applications may be stationary (non-

mobile) or mobile wireless does not

always mean mobility

Elements of a wireless network

network

infrastructure


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1. Overview


network

infrastructure

base station

typically connected towired network

relay - responsible forsending packetsbetween wired networkand wireless host(s) inits area

e.g., cell towers802.11 access points


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1. Overview


network

infrastructure

wireless link

connects mobiles tobase station

sometimes used as abackbone link

multiple access protocolcoordinates link access

data transmission rate isa function of distance

(SNIR really)


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1. Overview

infrastructure mode

base station connectsmobiles into wirednetwork

handover/handoff:mobile changes basestation


network

infrastructure


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1. Overview

Ad hoc mode no base stations nodes can only

transmit to other nodeswithin coverage

nodes organizethemselves into anetwork: route amongthemselves


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1. Overview

Characteristics of selected wireless link standards

Indoor10-30m

Outdoor50-200m

Mid-range

outdoor

200m 4 Km

Long-range

outdoor

5Km 20 Km

.056

.384

1

4

5-11

54

IS-95, CDMA, GSM 2G

UMTS/WCDMA, CDMA2000 3G

802.15

802.11b

802.11a,g

UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO 3G cellular

enhanced

802.16 (WiMAX)

802.11a,g point-to-point

200 802.11n

Datarate(Mbps

) data


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1. Overview

Wireless networks are unlike other networks

Fundamental difference is that the concept of link asa mathematical graph edge joining a pair of nodes isnot applicable

A node broadcasts its messages, it cannot send themeach to a chosen neighbour

Broadcast domains do not have well-defined boundaries,are time-varying and are subject to interference, multipath,noise, etc.

Broadcast domains are (almost always) partiallyoverlapping

Interference happens at the receiver; interference (andcarrier sense) range is longer than communication range

Nodes may move


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1. Overview

Wired vs. wireless media access Both are on shared media

Then, whats really the problem?

Wired network: Collision Detection

Nodes can transmit and receive at the same time If (Transmitted_Signal Sensed_Signal) Collision

Channel Condition is identical at Tx and Rx

Wireless network: No collision detection possible

Nodes cannot transmit and receive simultaneously on same channel Channel condition varies from node to node and is never

identical at Tx and Rx


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1. Overview

Hidden terminal problem

A, C can not hear each other means that A, C unaware oftheir interference at B

Spatial signal variation:

B, A hear each other B, C hear each other A, C can not hear each other

interfering at B

AB

C

A B C

As signalstrength

space

Cs signalstrength


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Hidden terminal problem

X is transmitting to Y

Z cannot sense X

Z transmits to Y

Collision at Y; high collision

rate; wastes bandwidth

Absence of carrier does not

always mean it is safe to

transmit

Exposed terminal problem

W is transmitting to X

Y wants to transmit to Z but

senses transmission of W and

defers

W does not exploit possiblesimultaneous transmission to Z;

high idle rate; wastes bandwidth

Presence of carrier does not

always mean it is not safe to

transmit

X Y Z X Y ZW

1. Overview


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2. Wireless MAC

Assume that you have some basicknowledge of wired MAC such as Aloha,CSMA, CSMA/CD (a.k.a. IEEE802.3)

Wireless MAC proved to be non-trivial research by [Karn90] (MACA)

research by [Bhargavan94] (MACAW)

Led to IEEE 802.11 committee

The standard was ratified in 1999

The predominant wireless MAC protocol isIEEE802.11 and its variants


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2. Wireless MAC

IEEE802.11 basic operation/handshake

CTS = Clear

To Send

RTS = Request

To Send

D

Y

S

M

K

RTS

CTS

X


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2. Wireless MAC

IEEE802.11 basic operation/handshake

D

Y

S

X

M

K

silenced

silenced

silenced

silencedData

ACK


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2. Wireless MAC


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Two modes CSMA/CAA

contention basedprotocol. In 802.11 thismode is known as

Distributed CoordinationFunction (DCF)

Priority-basedaccessA contentionfree access protocol

usable on theinfrastructure mode.Known as PointCoordination Function(PCF)

2. Wireless MAC


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2. Wireless MAC

802.11 StepsAll backlogged nodes choose a random number

R = rand (0, CW_min)

Each node counts down R Continue carrier sensing while counting down Once carrier busy, freeze countdown

Whoever reaches ZERO transmits RTS Neighbours freeze countdown, decode RTS

RTS contains (CTS + DATA + ACK) duration =T_comm

Neighbours set NAV = T_comm Remains silent for NAV time


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2. Wireless MAC

802.11 Steps (cont.)

Receiver replies with CTS Also contains (DATA + ACK) duration.

Neighbours update NAV again

Tx sends DATA, Rx acknowledges with ACK After ACK, everyone initiates remaining countdown

Tx chooses new R = rand (0, CW_min)

If RTS or DATA collides (i.e., no CTS/ACK returns)

Indicates collision RTS chooses new random no. R1 = rand (0, 2*CW_min)

Note Exponential Backoff Ri = rand (0, 2^i * CW_min)

Once successful transmission, reset to rand(0, CW_min)


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2. Wireless MAC

802.11 basic flow control

Sender sends RTS with NAV (Network Allocation Vector, i.e.

reservation parameter that determines amount of time the data packet

needs the medium) after waiting for DIFS

Receiver acknowledges via CTS after SIFS (if ready to receive)

CTS reserves channel for sender, notifying possibly hidden stations;

any station hearing CTS should be silent for NAV

Sender can now send data at once

t

DIFS

data

defer access

other

stations

receiver

senderdata

DIFS

new contention

RTS

CTSSIFS SIFS

NAV (RTS)

NAV (CTS)


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t

SIFS

DIFS

data

ACK

defer access

other

stations

receiver

senderdata

DIFS

new contention

RTS

CTSSIFS SIFS

NAV (RTS)

NAV (CTS)

2. Wireless MAC

802.11: RTS/CTS + ACK, the Final Version

802.11 adds ACK in the signaling to improve reliability

implication: to avoid conflict with ACK, any station hearing RTS should not

send for NAV

thus a station should not send for NAV if it hears either RTS and CTS

Note: RTS/CTS is optional in 802.11, and thus may not bealways turned on---some network interface cards turn it on only

when the length of a frame exceeds a given threshold


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2. Wireless MAC

802.11: PCF for Polling

tNAV

polled

wireless

stations

point

coordinator

NAV

PIFSD

U

SIFS

SIFSD

contention

period

contention free periodmedium

busy

D: downstream poll, or data from point coordinator

U: data from polled wireless station


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2. Wireless MAC

How to integrate PCF and DCF?

Basic Solution: Using Inter Frame Spacing to Prioritize Access

Different inter frame spacing (IFS): if the required IFS of a type of

message is short, the type of message has higher priority

SIFS (Short Inter Frame Spacing) highest priority, for ACK, CTS, polling response

PIFS (Point Coordination Function Spacing)

medium priority, for time-bounded service using PCF

DIFS (Distributed Coordination Function Spacing)

lowest priority, for asynchronous data service

random direct access ifmedium is free DIFS

t

medium busySIFS

PIFS

DIFS DIFS

next framecontention

Access point access ifmedium is free DIFS


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2. Wireless MAC

RTS/CTS: Does it solve hidden terminal problem?

Assuming carrier sensing zone = communication zone

C

F

A B

E

D

CTS

RTS

E does not receive CTS successfully Can later initiate transmission to D.

Hidden terminal problem remains.


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2. Wireless MAC

HT: How about increasing carrier sense range?

E will defer on sensing carrier no collision !!!

CB D

Data

A

E

CTS

RTSF


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2. Wireless MAC

HT: But what if barriers/obstructions exist?

E doesnt hear C Carrier sensing does not help

CB D

Data

A

EF

CTS

RTS


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2. Wireless MAC

ET: B should be able to transmit to A

RTS prevents this

CA B

E

D

CTS

RTS


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2. Wireless MAC

ET: B should be able to transmit to A

Carrier sensing makes the situation worse

CA B

E

D

CTS

RTS


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2. Wireless MAC

802.11 does not solve HT/ET completely Only alleviates the problem through RTS/CTS

and recommends larger CS zone

Large CS zone aggravates exposedterminals Spatial reuse reducesA tradeoff

RTS/CTS packets also consume bandwidth

Moreover, backing off mechanism is also wasteful 802.11 is still being optimized

Thus, wireless MAC research still alive


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3. Wireless LANs

Wireless LAN architecture wireless host

communicates with basestation

base station = access point

(AP) Basic Service Set (BSS)

(aka cell) in infrastructuremode contains:

wireless hosts

access point (AP): base

station ad hoc mode: hosts only

BSS 1

BSS 2

Internet

hub, switch

or routerAP

AP


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802.11b: 2.4GHz-2.485GHz spectrum divided into 11channels at different frequencies AP admin chooses frequency for AP

interference possible: channel can be same as that chosenby neighbouring AP!

host: must associate with an AP scans channels, listening for beacon frames containing

APs name (service set identifier SSID) and MACaddress

selects AP to associate with

may perform authentication will typically run DHCP to get IP address in APs subnet

3. Wireless LANs


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802.11 frame: addressing

frame

control duration

address

1

address

2

address

4

address

3 payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seq

control

Address 2: MAC address

of wireless host or AP

transmitting this frame

Address 1: MAC address

of wireless host or AP

to receive this frame Address 3: MAC addressof router interface to which

AP is attached

Address 4: used only in

ad hoc mode

3. Wireless LANs


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Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3 802.11 frame

R1 MAC addr AP MAC addr

dest. address source address

802.3 frame

3. Wireless LANs

802.11 frame: addressing


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frame

controlduration

address

1

address

2

address

4

address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seq

control

TypeFrom

APSubtype

To

AP

More

fragWEP

More

data

Power

mgtRetry Rsvd

Protocol

version

2 2 4 1 1 1 1 1 11 1

duration of reserved

transmission time (RTS/CTS)

frame seq #

(for reliable ARQ)

frame type(RTS, CTS, ACK, data)

3. Wireless LANs

802.11 frame: more


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3. Wireless LANs

Mobility within the

same subnet H1 remains in same IP

subnet: IP address canremain same

switch: which AP is

associated with H1?

self-learning: switch will see

frame from H1 and

remember which switch portcan be used to reach H1

hub or

switch

AP 2

AP 1

H1 BBS 2

BBS 1

router


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4. MANETs

Mobile ad hoc Network (MANET)

Formed by wireless hosts which may be

mobile

Without (necessarily) using a pre-existinginfrastructure

Routes between nodes may potentially

contain multiple hops


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4. MANETs

May need to traverse multiple links to

reach a destination


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4. MANETs

Mobility causes route changes


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4. MANETs

Why ad hocNetworks ? Ease of deployment

Speed of deployment

Decreased dependence on infrastructure

Applications

Personal area networking cell phone, laptop, ear phone, wrist watch Military environments

soldiers, tanks, planes

Civilian environments taxi cab network

meeting rooms

sports stadiums boats, small aircraft

Emergency operations search-and-rescue

policing and fire fighting


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4. MANETs

Many variants Fully Symmetric Environment

all nodes have identical capabilities and responsibilities

Asymmetric Capabilities transmission ranges and radios may differ

battery life at different nodes may differ

processing capacity may be different at different nodes speed of movement

Asymmetric Responsibilities only some nodes may route packets

some nodes may act as leaders of nearby nodes (e.g., cluster head)

Traffic characteristics may differ in different ad hoc networks bit rate

timeliness constraints reliability requirements

unicast / multicast / geocast

host-based addressing / content-based addressing / capability-based addressing


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4. MANETs

Many variants (cont.) May co-exist (and co-operate) with an infrastructure-based

network

Mobility patterns may be different people sitting at an airport lounge

taxis

kids playing

military movements

personal area network

Mobility characteristics speed

predictability direction of movement

pattern of movement

uniformity (or lack thereof) of mobility characteristics among differentnodes


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4. MANETs

MANET challenges Must address all of these issues

Limited wireless transmission range

Broadcast nature of the wireless medium

Packet losses due to transmission errors

Mobility-induced route changes

Mobility-induced packet losses

Battery constraints

Potentially frequent network partitions

Ease of snooping on wireless transmissions (security hazard)

No protocol solution fits all MANET scenarios Protocol performance metrics do not scale well with

increasing mobility and/or number of nodes


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4. MANETs

Unicast routing in MANETs Whats special about MANET routing?

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

New performance criteria may be used route stability despite mobility

energy consumption

Unicast MANET routing protocol classes Proactive protocols

Determine routes independent of traffic pattern

Traditional link-state and distance-vector routing protocols are proactive Reactive protocols

Maintain routes only if needed

Hybrid protocols


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4. MANETs

Routing protocol trade-offs Delay in route discovery

Proactive protocols may have lower delay since routes are

maintained at all times

Reactive protocols may have higher delay because a route from Xto Y will be found only when X attempts to transmit to Y

Overhead of route discovery/maintenance

Reactive protocols 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

Which approach achieves a better trade-off depends on traffic &

mobility patterns


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4. MANETs

We shall examine briefly1. Flooding

2. Dynamic Source Routing (DSR) protocol

3. Ad hoc On-Demand Distance Vector (AODV) routingprotocol

4.1 Flooding for Data Delivery Sender S broadcasts data packet P to all its neighbours

Each node receiving P forwards P to its neighbours

Sequence numbers used to avoid the possibility offorwarding the same packet more than once

Packet P reaches destination D provided that D isreachable from sender S

Node D does not forward the packet


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4.1 Flooding

B

A

S E

F

H

J

D

C

G

I

K

Represents connected nodes that are within each

others transmission range

Z

Y

Represents a node that has received packet P

M

N

L


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4.1 Flooding

B

A

S E

F

H

J

D

C

G

I

K

Represents transmission of packet P

Represents a node that receives packet P for

the first time

Z

Y

Broadcast transmission

M

N

L


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4.1 Flooding

B

A

S E

F

H

J

D

C

G

I

K

Node H receives packet P from two neighbours:

Potential for collision

Z

Y

M

N

L


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4.1 Flooding

B

A

S E

F

H

J

D

C

G

I

K

Node C receives packet P from G and H, but does not forward it again,

because node C has already forwarded packet P once

Z

Y

M

N

L


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4.1 Flooding

B

A

S E

F

H

J

D

C

G

I

K

Z

Y

M

Nodes J and K both broadcast packet P to node D

Since nodes J and K are hidden from each other, their

transmissions may collide

=> Packet P may not be delivered to node D at all,

despite the use of flooding

N

L


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4.1 Flooding

B

A

S E

F

H

J

D

C

G

I

K

Z

Y

Node D does not forward packet P, because node D

is the intended destination of packet P

M

N

L


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4.1 Flooding

B

A

S E

F

H

J

D

C

G

I

K

Flooding completed

Nodes unreachable from S do not receive packet P (e.g., node Z)

Nodes for which all paths from S go through the destination D also do not

receive packet P (example: node N)

Z

Y

M

N

L


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4.1 Flooding

B

A

S E

F

H

J

D

C

G

I

K

Flooding may deliver packets to too many nodes (in the worst case, all

nodes reachable from sender may receive the packet)

Z

Y

M

N

L


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4.1 Flooding

Advantages of flooding as a data deliverymechanism

Simplicity

May be more efficient than other protocols when rate

of information transmission is low enough that theoverhead of explicit route discovery/maintenanceincurred by other protocols is relatively higher

this scenario may occur, for instance, when nodes transmitsmall data packets relatively infrequently, and many topology

changes occur between consecutive packet transmissions Potentially higher reliability of data delivery

Because packets may be delivered to the destination onmultiple paths


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4.1 Flooding

Disadvantages of flooding as a data deliverymechanism Potentially very high overhead

Data packets may be delivered to too many nodes who

do not need to receive them Potentially lower reliability of data delivery

Flooding uses broadcasting hard to implementreliable broadcast delivery without significantlyincreasing overhead

Broadcasting in IEEE 802.11 MAC is unreliable In our example, nodes J and K may transmit to node Dsimultaneously, resulting in loss of the packet

in this case, destination would not receive the packet at all


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4.1 Flooding

Flooding of control packets

Many protocols perform (potentially limited)

flooding of control packets, instead of data

packets The control packets are used to discover routes

Discovered routes are subsequently used to send

data packet(s)

Overhead of control packet flooding is amortizedover data packets transmitted between

consecutive control packet floods


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B

D

C

A

4.2 Broadcast Storm

Broadcast Storm Problem [Ni99] When node A broadcasts a route query, nodes B and C both

receive it

B and C both forward to their neighbours

B and C transmit at about the same time since they are reactingto receipt of the same message from A

This results in a high probability of collisions


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4.2 Broadcast Storm

Redundancy:A given node may receive the same route

request from too many nodes, when one copy would

have sufficed

Node D may receive from both nodes B and C

B

D

C

A


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4.2 Broadcast Storm

Possible solutions Probabilistic scheme: On receiving a route request

for the first time, a node will re-broadcast (forward)

the request with probabilityp

Also, re-broadcasts by different nodes should bestaggered by using a collision avoidance technique

(wait a random delay when channel is idle)

this will reduce the probability that nodes B and C forward a

packet simultaneously in the previous example


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B

D

C

A

F

E

4.2 Broadcast Storm

Possible solutions (cont.) Counter-Based Scheme: If node E hears more than k

neighbours broadcasting a given RREQ, before it can

forward it, then node E will not forward the request

Intuition: kneighbours together have probably alreadyforwarded the request to all of Es neighbours


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E

Z


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4.2 Broadcast Storm

Summary of Broadcast Storm Problem

Flooding is used in many protocols, such as Dynamic

Source Routing (DSR, next)

Problems associated with flooding

Collisions

Redundancy

Collisions may be reduced by jittering (waiting for a

random interval before propagating the flood)

Redundancy may be reduced by selectively re-

broadcasting packets from only a subset of the nodes


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4.2 DSR

Dynamic Source Routing (DSR) [Johnson96]

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 identifierwhen forwarding

RREQ


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4.3 DSR

B

A

S E

F

H

J

D

C

G

I

K

Z

Y

Represents a node that has received RREQ for D from S

M

N

L

Route Discovery


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4.3 DSR

B

A

S E

F

H

J

D

C

G

I

K

Represents transmission of RREQ

Z

Y

Broadcast transmission

M

N

L

[S]

[X,Y] Represents list of identifiers appended to RREQ

Route Discovery


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4.3 DSR

B

A

S E

F

H

J

D

C

G

I

K

Node H receives packet RREQ from two neighbours:

potential for collision

Z

Y

M

N

L

[S,E]

[S,C]

Route Discovery


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4.3 DSR

B

A

S E

F

H

J

D

C

G

I

K

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]

Route Discovery


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4.3 DSR

B

A

S E

F

H

J

D

C

G

I

K

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]

Route Discovery


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4.3 DSR

B

A

S E

F

H

J

D

C

G

I

K

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]

Route Discovery


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4.3 DSR

Route Discovery in DSR

Destination D on receiving the first RREQ,

sends a Route Reply (RREP)

RREP is sent on a route obtained byreversing 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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4.3 DSR

B

A

S E

F

H

J

D

C

G

I

K

Z

Y

M

N

L

RREP [S,E,F,J,D]

Represents RREP control message

Route Reply


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4.3 DSR

Route Reply can be sent by reversing the route inRoute Request (RREQ) only if links are guaranteed tobe bi-directional To ensure this, RREQ should be forwarded only if it

received on a link that is known to be bi-directional

If unidirectional (asymmetric) links are allowed, thenRREP may need a route discovery for S from node D Unless node D already knows a route to node S

If a route discovery is initiated by D for a route to S, thenthe Route Reply is piggybacked on the Route Request

from D. If IEEE 802.11 MAC is used to send data, then links

have to be bi-directional (since Ack is used)


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4.3 DSR

Node S on receiving RREP, caches the routeincluded 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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4.3 DSR

B

A

S E

F

H

J

D

C

G

I

K

Z

Y

M

N

L

DATA [S,E,F,J,D]

Packet header size grows with route length

Data Delivery


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S


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4.3 DSR

Use of Route Caching When node S learns that a route to node D is broken, it 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

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

4 3 DSR


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4.3 DSR

B

A

S E

F

H

J

D

C

G

I

K

[P,Q,R] Represents cached route at a node

(DSR maintains the cached routes in a tree format)

M

N

L

[S,E,F,J,D][E,F,J,D]

[C,S]

[G,C,S]

[F,J,D],[F,E,S]

[J,F,E,S]

Z

Use of Route Caching


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4 3 DSR


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4.3 DSR

B

A

S E

F

H

J

D

C

G

I

K

Z

Y

M

N

L

[S,E,F,J,D][E,F,J,D]

[C,S]

[G,C,S]

[F,J,D],[F,E,S]

[J,F,E,S]

RREQ

Assume that there is no link between D and Z.

Route Reply (RREP) from node K limits flooding of RREQ.

In general, the reduction may be less dramatic.

[K,G,C,S]

RREP

Use of Route Caching

Can Reduce

Propagation ofRoute

Requests

4 3 DSR


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4.3 DSR

B

A

S E

F

H

J

D

C

G

I

K

Z

Y

M

N

L

RERR [J-D]

J sends a route error to S along route J-F-E-S when its attempt to forward the

data packet S (with route SEFJD) on J-D fails

Nodes hearing RERR update their route cache to remove link J-D

Route Error (RERR)

4 3 DSR


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4.3 DSR

Disadvantages of Route Caching: Stale caches can adversely affect performance

With passage of time and host mobility, cached

routes may become invalid

A sender host may try several stale routes (obtained

from local cache, or replied from cache by other

nodes), before finding a good route

Adverse impact on TCP

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4.3 DSR

Advantages of Dynamic Source Routing: Routes maintained only between nodes who need to

communicate

Reduces overhead of route maintenance

Route caching can further reduce route discovery

overhead

A single route discovery may yield many routes to the

destination, due to intermediate nodes replying from

local caches

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4.3 DSR

Disadvantages of Dynamic Source Routing: Packet header size grows with route length

Flood of route requests may potentially reach all

nodes in the network

Care must be taken to avoid collisions between route

requests propagated by neighbouring nodes

Insertion of random delays before forwarding RREQ

Increased contention if too many route replies come

back due to nodes replying using their local cache

Route Reply Storm problem

Reply storm may be eased by preventing a node from

sending RREP if it hears another RREP with a shorter route

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4.3 DSR

Disadvantages of Dynamic Source Routing(cont.):

An intermediate node may send Route Reply using a

stale cached route, thus polluting other caches

This problem can be eased if some mechanism to

purge (potentially) invalid cached routes is

incorporated.

Cache invalidation can be caused by:

Static timeouts

Adaptive timeouts based on link stability

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4.4 AODV

Ad Hoc On-Demand Distance Vector Routing (AODV)[Perkins99]

DSR includes source routes in packet headers

Resulting large headers can sometimes degrade performance

Particularly when data contents of a packet are small AODV attempts to improve on DSR by maintaining routing tables

at the nodes, so that data packets do not have to contain routes

AODV retains the desirable feature of DSR that routes are

maintained only between nodes which need to communicate

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4.4 AODV

Route Requests (RREQ) are forwarded in a mannersimilar to DSR

When a node re-broadcasts a Route Request, it sets up

a reverse path pointing towards the source

AODV assumes symmetric (bi-directional) links

When the intended destination receives a Route

Request, it replies by sending a Route Reply

Route Reply travels along the reverse path set-up when

Route Request is forwarded

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4.4 AODV

B

A

S E

F

H

J

D

C

G

I

K

Z

Y

Represents a node that has received RREQ for D from S

M

N

L

Route Request

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4.4 AODV

B

A

S E

F

H

J

D

C

G

I

K

Represents transmission of RREQ

Z

YBroadcast transmission

M

N

L

Route Request

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4.4 AODV

B

A

S E

F

H

J

D

C

G

I

K

Represents links on Reverse Path

Z

Y

M

N

L

Route Request

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4.4 AODV

B

A

S E

F

H

J

D

C

G

I

K

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

Reverse Path Setup

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4.4 AODV

B

A

S E

F

H

J

D

C

G

I

K

Z

Y

M

N

L

Reverse Path Setup

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4.4 AODV

B

A

S E

F

H

J

D

C

G

I

K

Z

Y

Node D does not forward RREQ, because node D

is the intended target of the RREQ

M

N

L

Reverse Path Setup

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4.4 AODV

B

A

S E

F

H

J

D

C

G

I

K

Z

Y

Represents links on path taken by RREP

M

N

L

Route Reply

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4.4 AODV

Route reply in AODV An intermediate node (not the destination) may also

send a Route Reply (RREP) provided that it knows amore recent path than the one previously known tosender S

To determine whether the path known to anintermediate node is more recent, destinationsequence numbers are used

The likelihood that an intermediate node will send aRoute Reply when using AODV not as high as DSR

A new Route Request by node S for a destination is assigneda higher destination sequence number. An intermediate nodewhich knows a route, but with a smaller sequence number,cannot send Route Reply

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4.4 AODV

B

A

S E

F

H

J

D

C

G

I

K

Z

Y

M

N

L

Forward links are setup when RREP travels along

the reverse path

Represents a link on the forward path

Forward Path Setup

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4.4 AODV

B

A

S E

F

H

J

D

C

G

I

K

Z

Y

M

N

L

Routing table entries used to forward data packet.

Route is not included in packet header.

DATAData Delivery

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4.4 AODV

Timeouts A routing table entry maintaining a reverse path is

purged after a timeout interval

Timeout should be long enough to allow RREP to come back

A routing table entry maintaining a forward path ispurged if not usedfor a active_route_timeout interval

If no is data being sent using a particular routing table entry,

that entry will be deleted from the routing table (even if the

route may actually still be valid)

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4.4 AODV

Link failure reporting A neighbour of node X is considered active for a

routing table entry if the neighbour sent a packet

within active_route_timeout interval, which was

forwarded using that entry When the next hop link in a routing table entry breaks,

all active neighbours are informed

Link failures are propagated by means of Route Error

messages, which also update destination sequencenumbers

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4.4 AODV

Route error handling When node X is unable to forward packet P (from node S

to node D) on link (X,Y), it generates a RERR message

Node X increments the destination sequence number for Dcached at node X

The incremented sequence number Nis included in theRERR

When node S receives the RERR, it initiates a new routediscovery for D using destination sequence number atleast as large as N

Destination sequence number When node D receives the route request with destinationsequence number N, node D will set its sequence numberto N, unless it is already larger than N


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4 4 AODV


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4.4 AODV

Why use sequence numbers in AODV

Loop C-E-A-B-C

A B C D

E

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4.4 AODV

AODV algorithm optimisation: Expanding RingSearch

Route Requests are initially sent with small Time-to-

Live (TTL) field, to limit their propagation

DSR also includes a similar optimisation

If no Route Reply is received, then larger TTL tried

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4.4 AODV

Summary of AODV Routes need not be included in packet headers

Nodes maintain routing tables containing entries only

for routes that are in active use

At most one next-hop per destination maintained at

each node

DSR may maintain several routes for a single destination

Unused routes expire even if topology does not

change

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5. Open Problems

Wireless MAC Power control increases spatial reuse

Rate control based on channel quality

Exploit channel diversity

Exploit spatial diversity using directional antennas

Controlling unwanted interactions between complementary techniques

Wireless networks Concept of link between two nodes does not capture physics of

broadcast radio transmission need new mathematical

abstraction/formalism

Wireless peer-to-peer network maximum throughput scales badly with

increasing network node numbers [Gupta00] Network coding [Ahlswede00] is a promising idea for overcoming this

fundamental limitation, but is riddled with practical difficulties

Finding economical alternatives to flooding for initial location

discovery, perhaps by summarising/clustering node locations


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7 References


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7. References

Wireless MAC1. Karn, P. 1990. MACAA new channel access method for packet radio.ARRL/CRRL Amateur Radio 9th Computer Networking Conference, 134-140.

2. Bhargavan, V., Demers, A., Shenker S. and Zhang L. 1994. MACAW: A MediaAccess Protocol for Wireless LANs. Proceedings of ACM SIGCOMM 94.

3. IEEE Std 802.11Part 11: Wireless LAN Medium Access Control (MAC) andPhysical Layer (PHY) Specifications, IEEE, 1999.

MANET

1. Ni, S.-Y., Tseng, Y.-C., Chen, Y.-S. and Sheu, J.-P. 1999. The broadcast stormproblem in a mobile ad hoc network. Proc. 5thACM/IEEE MOBICOM99, 151-162.

2. Johnson, D.B. and Maltz, D.A. 1996. Dynamic source routing in ad hoc wirelessnetworks. Mobile Computing, lmielinski, T. and Korth, H. (Eds.), Kluwer, 153-81.

3. Perkins, C.E. and Royer, E.M. 1999. Ad-hoc On-Demand Distance Vector Routing.Proc. 2ndIEEE Wksp. Mobile Comp. Sys. and Apps., 90-100.

4. Haas, Z.J. and Pearlman, M.R. 1998. The performance of a new routing protocolfor the reconfigurable wireless networks. Proc. IEEE ICC98, 156-160.

Wireless networks theory1. Gupta, P. and Kumar, P.R. 2000. The capacity of wireless networks. IEEE Trans.

Inform. Theory, 46, 388-404.

2. Ahlswede, R., Cai, N., Li, S. R. and Yeung, R. 2000. Network information flow.IEEE Trans. Inform. Theory, 46, 1204-1216.

Documents

EE4L CCC L5 Wireless Networking _v1