Introduction to Wireless Networks

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Introduction to Wireless Networks

• Michalis Faloutsos

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What is an ad hoc network

• A collection of nodes that can communicate with
  each other without the use of existing
  infrastructure
• Each node is a sender, a receiver, and a relay
• There are no special nodes (in principal)
• No specialized routers, no DNS servers
• Nodes can be static or mobile
• Can be thought of us peer-to-peer communication

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Example Ad hoc network

• Nodes have power range
• Communication happens between nodes within range

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Some Introductory Things

• The MAC layer 802.11
• Typical Simulations
• The routing protocols
• TCP and ad hoc networks

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What Is Different Here?

• Broadcasts of nodes can overlap -gt collision
• How do we handle this?
• A MAC layer protocol could be the answer
• If one node broadcasts, neighbors keeps quite
• Thus, nearby nodes compete for air time
• This is called contention

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Contention in ad hoc networks

• A major difference with wireline networks
• Air-time is the critical resource
• Fact 1 connections that cross vertically
  interfere
• Fact 2 connections that do not share nodes
  interfere
• Fact 3 a single connection with itself
  interferes!

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Example of contention

• Yellow connection bothers pink connection
• Yellow bothers itself
• When A-E is active
• E-F is silent
• F-G is silent (is it?)

F
E
G
H
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The 802.11 MAC protocol
RTS
RTS
A
B
D
CTS
C
CTS

• Introduced to reduce collisions
• Sender sends Request To Send (RTS) ask
  permission
• Case A Receiver gives permission Clear To Send
  (CTS)
• Sender sends Data
• Receiver sends ACK, if received correctly
• Case B Receiver does not respond
• Sender waits, times out, exponential back-off,
  and tries again

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Why is this necessary?

• A RTS, and B replies with a CTS
• C hears RTS and avoids sending anything
• C could have been near B (not shown here)
• D hears CTS so it does not send anything to B

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Some numbers for 802.11

• Typical radius of power-range 250m
• Interference range 500m
• At 500m one can not hear, but they are bothered!
• RTS packet 40 bytes
• CTS and ACK 39 bytes
• MAC header is 47 bytes

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Typical Simulation Environment

• A 2-dimensional rectangle
• Fixed number of nodes
• Static uniformly distributed
• Dynamic way-point model
• Pick location, move with speed v, pause
• Power range fixed or variable
• Sender-receivers uniformly distributed

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Various Communication Paradigms

• Broadcasting
• one nodes reaches everybody
• Multicasting
• One node reaches some nodes
• Anycasting
• One node reaches a subset of some target nodes
  (one)
• Application Layer protocols and overlays
• Applications like peer-to-peer

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Layered and Cross Layer Protocols

• Layering
• Modular
• Isolates details of each layer
• Cross Layer
• Information of other layers is used in decisions
• Pros efficiency
• Cons deployability and compatibility

application transport Network Link physical
application transport Network Link physical
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Example application layer multicast

• Source unicasts data to some destinations
• Destinations unicast data to others
• Pros easy to deploy, no need to change network
  layer
• Cons not as efficient

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Example application layer multicast II

• Members need to make multiple copies
• It would happened anyway
• Link A B gets two packets
• Similarly in wireline multicast
• Node B sends and receives packet 4 times

s
A
B
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Some major assumptions

• The way-point model is a good model for mobility
• Homogeneity is a good assumption
• Links are bidirectional I hear U, U hear me
• Uniform distribution of location is good
• 802.11 will be used at the MAC layer
• Space is two dimensional

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Some proven claims

• The smallest the range, the better the throughput
• Mobility increases the capacity of a network
• A node should aim for 6-7 neighbors
• We can challenge these claims

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End of Introduction

• Resources
• Google
• Citeseer http//citeseer.nj.nec.com/cs
• C. Perkins book Ad Hoc Networking

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Modeling Contention(based on Nandagopal et al
MOBICOM 200)

• Seminar 260
• Michalis Faloutsos

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Problem Find Hotspot in a graph

• Given a graph and source-destinations
• Where is the bottleneck?
• Or how much bandwidth can each connection have?

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Solution Find areas of contention

• Intuition
• Step 1 create graph range connectivity
• Step 2 create graph of flows (route flows on
  graph)
• Step 3 find which flows contend for airtime
  (find areas where only one flow can be active)

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Clarification interference

• When C-gtD
• A-B, B-C, D-E, E-F can not be active!

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Clarification Dual graph

• Each edge becomes a node in G
• An edge exists between two nodes in G iff
• the edges have a common node

edge
Interference
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In more detail

1. Find topological graph
2. Find dual graph edges -gt nodes
3. Consider interference between non adjacent
   edges
4. Find Maximal cliques

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Contention Modeling conclusion

• Elegant approaches and tools are available
• The realism of the modeling must be considered
• Do not over-generalize results when heavy
  assumptions have been made

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Considering Connections

• If we know which pairs want to communicate, we
  consider only these flows as contenders
• Routing could be independent of contention of an
  area
• If routing is contention aware, then we have a
  closed loop system
• Routing -gt contention -gt routing -gt .

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Question what is optimal routing?

• Given a graph, source-destination pairs
• How do I route the flows to minimize contention?
• What happens if I do not know the connections
  ahead of time (online version of problem)?

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Modeling the Physical Channel

• There are several ways depending on degree of
  accuracy
• Binary, simplified
• in one prange you communication
• In two prange you interfere but do not communicate

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Considering the power path loss

• P_R received power
• P_t transmission power
• d distance
• alpha constant

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The physical model
Pi

Noise SUM_k Pk

• Node Y hears node i, iff received power of i is
  above a threshold beta
• Needs to rise above noise and other transmissions

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A more optimistic channel model

• Node Y hears i, if i is the loudest
• Interference from other nodes per pair
  comparison
• Deltagt0 is a protocol specified guard zone

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Channel Modeling Conclusion

• Several different models
• You need to find and justify the model you use

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Topology Control

• We cannot always control the mobility
• We can control the network topology
• Power control
• Deciding to ignore particular neighbors
• From a given graph G of possible connections we
  keep a subset G of these connections
• What is good topology?

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Topology Control Metrics

• What is good topology?
• Energy efficiency,
• Robustness to mobility,
• Throughput - capacity

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Topics Of Interest - Wireless

• Characterizing the ad hoc topology
• A snapshot
• Its evolution
• Mobility
• Realistic mobility models
• Effect of topology/mobility in
• Routing
• Multicasting in ad hoc networks

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Topics of Interest - Wireline

• Generating a realistic directed graph
• Reducing a real (directed) graph to a small
  realistic
• Survey on graph generation models
• Measuring the Internet topology
• Router level
• AS level

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We need to model contention

• First the obvious
• Adjacent edges
• Second, one edge away, considering RTS CTS
• Third, interference (500m instead of 250m)
• Modeling issue

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Typical Errors

• Mobility
• too slow or too fast
• Mobility speed may not be the expected
• Homogeneity may hide issues
• Few nodes are responsible for most traffic
• Some spots are more popular than others
• Power range is too large for the area
• Ie radius 250m, a grid of 1Km -gt one broadcast
  covers half the area

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Whats the problem?

• There is no systematic way to model and simulate
  such networks
• No clue what are the right assumptions
• Not sure how the assumptions affect the results

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Consequences

• Simulation results are
• Meaningless
• Unrepeatable
• Incomparable between different analysis
• Prone to manipulation
• Claim give me any statement, I can create
  simulations to prove it

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What Will We Do Here?

• Identify assumptions
• Some of them are subtle
• Characterize the scenarios
• Study their effect on the performance results