Introduction to Wireless Networks - PowerPoint PPT Presentation

Loading...

PPT – Introduction to Wireless Networks PowerPoint presentation | free to download - id: 672c5f-NjNlY



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

Introduction to Wireless Networks

Description:

Introduction to Wireless Networks Michalis Faloutsos What is an ad hoc network A collection of nodes that can communicate with each other without the use of existing ... – PowerPoint PPT presentation

Number of Views:8
Avg rating:3.0/5.0
Slides: 43
Provided by: MichalisF9
Learn more at: http://www.cs.unm.edu
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: Introduction to Wireless Networks


1
Introduction to Wireless Networks
  • Michalis Faloutsos

2
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

3
Example Ad hoc network
  • Nodes have power range
  • Communication happens between nodes within range

4
Some Introductory Things
  • The MAC layer 802.11
  • Typical Simulations
  • The routing protocols
  • TCP and ad hoc networks

5
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

6
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!

7
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
8
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

9
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

10
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

11
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

12
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

13
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
14
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

15
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
16
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

17
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

18
End of Introduction
  • Resources
  • Google
  • Citeseer http//citeseer.nj.nec.com/cs
  • C. Perkins book Ad Hoc Networking

19
Modeling Contention (based on Nandagopal et al
MOBICOM 200)
  • Seminar 260
  • Michalis Faloutsos

20
Problem Find Hotspot in a graph
  • Given a graph and source-destinations
  • Where is the bottleneck?
  • Or how much bandwidth can each connection have?

21
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)

22
Clarification interference
  • When C-gtD
  • A-B, B-C, D-E, E-F can not be active!

23
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
24
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

25
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

26
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 .

27
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)?

28
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

29
Considering the power path loss
  • P_R received power
  • P_t transmission power
  • d distance
  • alpha constant

30
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

31
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

32
Channel Modeling Conclusion
  • Several different models
  • You need to find and justify the model you use

33
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?

34
Topology Control Metrics
  • What is good topology?
  • Energy efficiency,
  • Robustness to mobility,
  • Throughput - capacity

35
(No Transcript)
36
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

37
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

38
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

39
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

40
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

41
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

42
What Will We Do Here?
  • Identify assumptions
  • Some of them are subtle
  • Characterize the scenarios
  • Study their effect on the performance results
About PowerShow.com