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802.11a/b/g Networks
Herbert Rubens
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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Peer to Peer Ad Hoc Mode
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Peer to Peer Ad Hoc Mode
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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Mobile Ad Hoc Networks
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
Mobile Ad Hoc Networks
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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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Flooding for Data Delivery
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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Flooding for Data Delivery
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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Flooding for Data Delivery
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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Flooding for Data Delivery
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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Flooding for Data Delivery
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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Flooding for Data Delivery
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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Flooding for Data Delivery
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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Flooding for Data Delivery
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