lecture7 infrastructure and ad hoc networks

8/2/2019 Lecture7 Infrastructure and Ad Hoc Networks

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802.11a/b/g Networks

Herbert Rubens

[email protected]

Some slides taken from UIUC Wireless Networking Group


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802.11a/b/g

Operates in 2 different modes:

Infrastructure mode

Associates with an access point

All communication goes through the access point

Used for wireless access at a company or campus

Peer-to-Peer Ad Hoc Mode

If two nodes are within range of each other theycan communicate directly with no access point

A few users in a room could quickly exchange fileswith no access point required


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Infrastructure Access

Access Points:

Provide infrastructure access to mobile users

Cover a fixed area

Wired into LAN


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Peer to Peer Ad Hoc Mode


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Infrastructure Access


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1 Mbps

2 Mbps

5.5 Mbps

11 Mbps

802.11a/b/g are multi-rate devices


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MAC Layer Fairness Models

Per Packet Fairness: If two adjacent senderscontinuously are attempting to send packets,they should each send the same number of

packets. Temporal Fairness: If two adjacent senders are

continuously attempting to send packets, theyshould each be able to send for the same

amount of medium time.

In single rate networks these are the SAME!


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Temporal Fairness Example

802.11

Packet

Fairness

OAR

Temporal

Fairness

11 MbpsLink

0.896 3.533

1 Mbps Link 0.713 0.450

TotalThroughput

1.609 3.983

1 Mbps

11 Mbps

1 Mbps

11 Mbps

Per Packet Fairness

Temporal Fairness


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802.11b Channels

11 available channels (in US)

Only 3 are non-overlapping!


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Channel 1 Channel 6

Channel 11


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Problems

Access Point placement depends on wirednetwork availability

Obstructions make it difficult to provide

total coverage of an area Site surveys are performed to determine

coverage areas

Security Concerns: rogue access points incompanies etc..

Each Access Point has limited range


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X


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Problems

Communication is only possible betweennodes which are directly in range of eachother


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What if ??

OR

Multi-hop Infrastructure AccessMulti-hop Ad Hoc Network


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Multi-hop Infrastructure Access

Nodes might be out of range of the accesspoint, BUT in range of other nodes.

The nodes in range of the access pointcould relay packets to allow out of rangenodes to communicate.

NOT part of 802.11


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Multi-hop Ad Hoc Network

If communication is required between twonodes which are out of range of eachother, intermediary nodes can forward the

packets.

NOT part of 802.11

Source Destination


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How can this be done?

ROUTING!!

Wired Networks:

Hierarchical Routing

Network is divided into subnets

Nodes look at netmask and determine if the address isdirectly reachable. If not, just forward to the defaultgateway.

Different protocols for different levels of the hierarchy

RIP, OSPF, BGP


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Wireless Routing

Flat routing

You cant assume that since a node is in your

subnet that it is directly accessible

Node must maintain or discover routes to thedestination

All nodes are routers


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Mobile Ad Hoc Networks

Formed by wireless hosts which may bemobile

Without (necessarily) using a pre-existing

infrastructure

Routes between nodes may potentially

contain multiple hops


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May need to traverse multiple links to reach adestination


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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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Challenges

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)


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Unicast Routingin



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Why is Routing in MANETdifferent ?

Host mobility

link failure/repair due to mobility may have differentcharacteristics 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


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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 wirednetworks

No single protocol works well in all environments

some attempts made to develop adaptive protocols


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Routing Protocols

Proactive protocols

Determine routes independent of traffic pattern

Traditional link-state and distance-vector routingprotocols are proactive

Reactive protocols

Maintain routes only if needed

Hybrid protocols


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Trade-Off

Latency of route discovery

Proactive protocols may have lower latency since routes aremaintained 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 aredetermined only if needed

Proactive protocols can (but not necessarily) result in higheroverhead due to continuous route updating

Which approach achieves a better trade-off depends on the trafficand mobility patterns


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Overview of Unicast Routing

Protocols


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Flooding for Data Delivery

Sender S broadcasts data packet P to allits neighbors

Each node receiving P forwards P to itsneighbors

Sequence numbers used to avoid thepossibility of forwarding the same packetmore than once


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B

A

S E

F

H

J

D

C

G

IK

Represents that connected nodes are within each

others transmission range

Z

Y

Represents a node that has received packet P

M

N

L


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B

A

S E

F

H

J

D

C

G

IK

Represents transmission of packet P

Represents a node that receives packet P forthe first time

Z

Y

Broadcast transmission

M

N

L


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B

A

S E

F

H

J

D

C

G

IK

Node H receives packet P from two neighbors:potential for collision

Z

Y

M

N

L


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B

A

S E

F

H

J

D

C

G

IK

Node C receives packet P from G and H, but does not forwardit again, because node C has already forwarded packet P once

Z

Y

M

N

L


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B

A

S E

F

H

J

D

C

G

IK

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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B

A

S E

F

H

J

D

C

G

IK

Z

Y

Node D does not forward packet P, because node Dis the intended destination of packet P

M

N

L


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B

A

S E

F

H

J

D

C

G

IK

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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B

A

S E

F

H

J

D

C

G

IK

Flooding may deliver packets to too many nodes(in the worst case, all nodes reachable from sendermay receive the packet)

Z

Y

M

N

L

Fl di f D t D li


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Flooding for Data Delivery:Advantages

Simplicity

May be more efficient than other protocols when rate of informationtransmission is low enough that the overhead of explicit routediscovery/maintenance incurred by other protocols is relativelyhigher

this scenario may occur, for instance, when nodes transmit smalldata packets relatively infrequently, and many topology changesoccur between consecutive packet transmissions

Potentially higher reliability of data delivery

Because packets may be delivered to the destination on multiplepaths

Fl di f D t D li


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Flooding for Data Delivery:Disadvantages

Potentially, very high overhead

Data packets may be delivered to too many nodes who do notneed to receive them

Potentially lower reliability of data delivery

Flooding uses broadcasting -- hard to implement reliablebroadcast delivery without significantly increasing 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 atall

lecture7 infrastructure and ad hoc networks

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