How (not) to Simulate Wireless Networks with ns

1
How (not) to SimulateWireless Networks with ns

• Brad Karp
• bkarp_at_cs.ucl.ac.uk
• University College London

ns Workshop MSR Cambridge 9th December, 2005
2
Outline

• Overview of ns wireless functionality
• History
• Simple example
• MobileNode configuration
• Controlling mobility
• Defining traffic workloads
• MobileNode architecture
• Trace file format
• Role of simulation in wireless research
• Case study of simulations pitfalls GPSR

3
History of ns Wireless Support

• David Johnsons Monarch group at CMU
• Free-space, two-ray ground reflection channel
  model
• 802.11 MAC layer
• Random waypoint mobility model
• ARP
• Ad hoc routing Dynamic Source Routing (DSR),
  TORA,
• c.
• Other major supported protocols and links
  (outside scope of todays talk) Mobile IP,
  Directed Diffusion, satellite links, c.

4
ns Wireless Architecture

• MobileNode at core of mobility support
• MobileNodes can move in a given topology,
  receive/transmit signals to/from wireless
  channels
• Wireless network stack consists of LL, ARP, MAC,
  IFQ, c.
• Allows simulations of multi-hop ad hoc networks,
  wireless LANs, sensor networks etc

5
Wireless ExampleAd hoc Routing

• Scenario
• 2 mobile nodes
• moving within 500m x 500m flat topology
• using DSDV ad hoc routing protocol
• Random Waypoint mobility model
• TCP traffic
• Examples
• ns-2/ns-tutorial/examples/simple-wireless.tcl
• ns-2/tcl/ex/wireless-demo-csci694.tcl

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An Example Step 1
Define Global Variables create simulator set
ns new Simulator create flat topology in
670m x 670m area set topo new Topography topo
load_flatgrid 500 500
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An Example Step 2
Define wireless ns trace ns trace set tracefd
open simple.tr w ns trace-all tracefd
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GOD (General Operations Director)

• Stores all-pairs Dijkstra shortest path lengths
• Allows comparison of path length with optimal
• Automatically generated, contained in scenario
  file
• set god create-god ltno of mnodesgt
• god set-dist ltfromgt lttogt lthopsgt

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Example Step 3

• Create god
• set god create-god 2
• ns at 900.00 god setdist 1 2 1
• Create wireless channel
• set thechan new Channel/WirelessChannel

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An Example Step 4
Define how a mobile node is configured ns
node-config \ -adhocRouting DSDV \ -llType LL
\ -macType Mac/802_11 \ -ifqLen 50 \ -ifqType
Queue/DropTail/PriQueue \ -antType
Antenna/OmniAntenna \ -propType
Propagation/TwoRayGround \ -phyType
Phy/WirelessPhy \ -channel thechan
\ -topoInstance topo -agentTrace ON
\ -routerTrace OFF \ -macTrace OFF
\ -movementTrace OFF
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An Example Step 5

• Use for loop to create 3 nodes
• for set i 0 i lt 2 incr i
• set node(i) ns node
• disable random motion
• node(i) random-motion 0

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MobileNode Movement

• Node position defined in 3D
• Today, z axis not used
• node set X_ ltx1gt
• node set Y_ lty1gt
• node set Z_ ltz1gt
• node at lttimegt setdest ltnewxgt ltnewygt ltspeedgt
• Lots of these events over a simulation manual
  generation tedious

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Random Waypoint Model

• Place nodes uniformly at random
• Node moves to uniformly randomly chosen
  destination, at velocity chosen uniformly at
  random
• Between move events, node stays put for pause
  time
• Meant to mimic user behavior move, work, move

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Scenario Generator Movement

• setdest MobileNode movement generator
• setdest -n ltnum_of_nodesgt -p pausetime -M
  ltmaxspeedgt -t ltsimtimegt -x ltmaxxgt -y ltmaxygt
• Source ns-2/indep-utils/cmu-scen-gen/setdest/

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Example setdest Output

• node_(2) set Z_ 0.000000000000
• node_(2) set Y_ 199.373306816804
• node_(2) set X_ 591.256560093833
• node_(1) set Z_ 0.000000000000
• node_(1) set Y_ 345.357731779204
• node_(1) set X_ 257.046298323157
• node_(0) set Z_ 0.000000000000
• node_(0) set Y_ 239.438009831261
• node_(0) set X_ 83.364418416244
• god_ set-dist 0 1 1
• ns_ at 50.000000000000 "node_(2) setdest
  369.463244915743 170.519203111152 3.371785899154"
• ns_ at 51.000000000000 "node_(1) setdest
  221.826585497093 80.855495003839 14.909259208114"
• ns_ at 33.000000000000 "node_(0) setdest
  89.663708107313 283.494644426442 19.153832288917"

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Scenario Generator Traffic

• Generating traffic pattern files
• CBR/TCP traffic
• ns cbrgen.tcl -type cbrtcp -nn nodes -seed
  seed -mc connections -rate rate (pkt/s)
• CBR traffic
• ns cbrgen.tcl type cbr nn 20 seed 1 mc 8
• -rate 4
• TCP traffic
• ns cbrgen.tcl type tcp -nn 15 -seed 0 mc 6
• Default packet size 512 bytes, hardwired in
  script!
• Start time uniform in 0, 180 s, hardwired in
  script!
• To evaluate routing, which traffic type would you
  use?
• Source ns-2/indep-utils/cmu-scen-gen/

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A Traffic Scenario

• set udp_(0) new Agent/UDP
• ns_ attach-agent node_(0) udp_(0)
• set null_(0) new Agent/Null
• ns_ attach-agent node_(2) null_(0)
• set cbr_(0) new Application/Traffic/CBR
• cbr_(0) set packetSize_ 512
• cbr_(0) set interval_ 4.0
• cbr_(0) set random_ 1
• cbr_(0) set maxpkts_ 10000
• cbr_(0) attach-agent udp_(0)
• ns_ connect udp_(0) null_(0)
• ns_ at 127.93667922166023 "cbr_(0) start"
• .

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An Example Step 6
Define node movement model source
ltmovement-scenario-filesgt Define traffic
model source lttraffic-scenario-filesgt
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An Example Step 7
Tell ns the simulation stop time ns at 200.0
ns halt Start your simulation ns run
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Energy Extension

• Makes MobileNode energy-aware
• Enable by adding options
• ns_ node-config \
• energyModel EnergyModel
• -initialEnergy 100.0
• -txPower 0.6
• -rxPower 0.2

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Outline

• Overview of ns wireless functionality
• History
• Simple example
• MobileNode configuration
• Controlling mobility
• Defining traffic workloads
• MobileNode architecture
• Trace file format
• Role of simulation in wireless research
• Case study of simulations pitfalls GPSR

22
Wireless Internals

• MobileNode
• Basic entity with address and port de-muxes,
  routing agent, c.
• Stack of network components, including IFQ, LL,
  MAC, NetIF, radio propagation model, c.
• Wireless channel

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Portrait of A Mobile Node
port classifier
Node
protocol agent
255
routing agent
addr classifier
defaulttarget_
ARP
LL
LL
LL
IFQ
IFQ
MAC
MAC
Propagation and antenna models
PHY
PHY
MobileNode
CHANNEL
Radio propagation/ antenna models
Prop/ant
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Mobile Node Components

• Classifiers
• defaulttarget_ points to routing agent object
• 255 is the port id assigned for rtagent_
• Routing agent
• Ad hoc routing protocol, e.g., AODV, DSDV, DSR
  or directed diffusion

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Mobile Node Components

• Link Layer
• Same as LAN, but with a separate ARP module
• Looks up IP-to-MAC mappings using ARP
• ARP
• Resolves IP address to hardware (MAC) address
• Broadcasts ARP query
• Interface queue (IFQ)
• Gives priority to routing protocol packets
• Has packet filtering (search and remove) capacity

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Mobile Node Components

• MAC
• 802.11
• IEEE RTS/CTS/DATA/ACK for unicast
• Sends DATA directly for broadcast
• Network interface (PHY)
• Used by MobileNode to access channel
• Stamps outgoing packets with meta-data
• Interface with radio/antenna models

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Mobile Node Components

• Radio Propagation Model
• Friss-space model attenuation at near distance
• Two-ray ground reflection model attenuation at
  far distance
• Shadowing model probabilistic
• Antenna
• Omni-directional, unity-gain

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Wireless Channel

• Duplicate packets to all mobile nodes attached to
  the channel except the sender
• It is the receivers responsibility to decide if
  it will accept the packet
• Collision is handled at individual receiver
• O(N2) computation!

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Wireless Trace Support

• CMU trace format differs from standard ns trace
  format!
• Configurable tracing
• MAC (very voluminous!)
• Router forwarding
• Agent events
• Movement events
• Format described in detail in ns manual
• When in doubt, no substitute for reading the
  code!
• Shortest path lengths logged per pkt rx (cost
  in-core?)
• New wireless trace format
• appears not to be in wide use yet
• same information, formatted differently

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Outline

• Overview of ns wireless functionality
• History
• Simple example
• MobileNode configuration
• Controlling mobility
• Defining traffic workloads
• MobileNode architecture
• Trace file format
• Role of simulation in wireless research
• Case study of simulations pitfalls GPSR

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Simulation as Way to Enable Area

• Ad hoc routing huge system deployment/debugging
  cost
• 1998 Broch et al. paper a watershed for ad hoc
  routing first even comparison of wide field of
  protocols
• CMU ns wireless extensions the basis of most ad
  hoc networking research for next 5 years

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Simulation as Way to Kill Area

• Simulators dont evaluate motivation!
• Why are 500 laptops in a gymnasium not simply
  connecting to base stations?
• The founding of MobiHoc
• cf. the founding of SIGMETRICS
• The founding of MobiSys
• Secret tip building a real system dramatically
  increases chances of acceptance! (because its
  HARD)

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Things to Investigate Using ns

• Coarse notion of capacity
• Coarse notion of effect of mobility
• Fine-grained behavior of your routing (or other
  hop-by-hop) protocol
• Only place you can easily have centralized view!

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Things Not to Investigate Using ns

• Realistic loss behavior (interference, multi-path
  fading, )

Roofnet measurements SIGCOMM 2004
Broadcast Packet Delivery Probability
Node Pair
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Not in ns (contd)

• Realistic topology information
• Roofnet finding RTS/CTS useless in outdoor
  network!
• Hidden terminal problem still mainstay of
  undergraduate networking courses everywhere
• GPSR free space ns simulation debacle (more
  later)
• Fine-grained mobility behavior
• Random Waypoint corresponds little to reality
• GPSR density anomaly (more later)

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When to Roll Your Own Simulator

• GPSR greedy-only simulator
• properties of node density, not of fine-grained
  radio propagation
• 250 lines of C, a mornings work
• far faster than ns will ever be
• Simple visualization
• GPSR topology visualizer
• find connected components
• show planar subgraphs
• animate forwarding

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Outline

• Overview of ns wireless functionality
• History
• Simple example
• MobileNode configuration
• Controlling mobility
• Defining traffic workloads
• MobileNode architecture
• Trace file format
• Role of simulation in wireless research
• Case study of simulations pitfalls GPSR

38
Greedy Perimeter Stateless Routing (GPSR)

• Central idea Machines can know their geographic
  locations.
• Route using geography. MobiCom 2000
• Packet destination field location of destination
• Nodes all know own positions, e.g.,
• by GPS (outdoors)
• by surveyed position (for non-mobile nodes)
• by short-range localization (indoors, ATT Camb,
  1997, Priyantha et al., 2000)
• c.
• Two forwarding algorithms
• Greedy mode simply chooses closest neighbor to
  destination
• Perimeter mode recovers when no closer next hop
  available, by routing on planar subgraph of full
  network graph

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Greedy Forwarding

• Nodes learn immediate neighbors positions from
  beaconing/piggybacking on data packets
• Locally optimal, greedy next hop choice
• Neighbor geographically nearest destination

D
x
y
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Greedy Forwarding Failure

• Greedy forwarding not always possible! Consider

D
z
v
void
y
w
x
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Void Traversal The Right-hand Rule

• Well-known graph traversal right-hand rule
• Requires only neighbors positions

z
y
x
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Planarized Graphs Example

• 200 nodes, placed uniformly at random on
  2000-by-2000-meter region 250-meter radio range

Full Graph
GG Subgraph
RNG Subgraph
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Packet Delivery Success Rate(50, 200 Dense)
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GPSR Making it Real

• We implemented full GPSR for Berkeley mote
  sensors NSDI 2005
• 3750 lines of nesC code
• also includes simple link thresholding, ARQ
• Deployed on Mica 2 dot mote testbeds
• 23-node, 50-node subsets of 100-node network in
  office building (Soda Hall office walls 433
  MHz)
• 40-node network in office building (Intel
  Research Berkeley cubicles 900 MHz)
• Delivery success workload 50 packets between all
  node pairs, serially

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50-Node Testbed, Soda Hall
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Planar, but Partitioned
Output of GPSRs Distributed GG (arrows denote
unidirectional links)
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Key Lessons and Conclusion

• Understand your algorithms correctness
  assumptions
• Does the simulator model these properties
  accurately, or at least pessimistically?
• Fading, interference, loss rate particular
  pitfalls
• No substitute for buildingeven moreso than for
  Internet congestion control, as propagation too
  complex to model accurately

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UCL A Tradition of Great Networking Research in
London

• Hiring junior and senior networking faculty
• Application deadline
• 5th January, 2006
• Details at http//www.cs.ucl.ac.uk/
• Why UCL?
• Research sdr, rat, Landmark Routing, NAT, XORP,
  TFRC, XCP, GPSR, CLDP, GHT, Autograph, Polygraph,
  
• Students (Paul Francis, Mark Handley, Jon
  Crowcroft, )
• LONDON