A Tutorial of NS2 and its Wireless Simulation

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A Tutorial of NS-2 and its Wireless Simulation

• Yue Wang
• July 21, 2007

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Agenda

• Introduction to NS-2
• What is NS-2
• NS-2 vs GloMoSim OPNET
• NS-2 Basics
• Wireless Simulation
• PHY Layer
• MAC Layer
• Example My Fading Extension
• Bugs

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What is NS-2

• NS-2 (Network Simulator 2) is
• a TCL interpreter ?
• a C project ?
• a protocol stack ?
• NS-2 is
• a network simulator using TCL scripts as
  configuration interface and using C
  objects/functions to simulate network
  components/protocols

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What is NS-2 (cont)

• NS-2
• can do packet level simulation
• supports a variety of wired/wireless protocols
• is essentially a centralized discrete event
  scheduler
• For more information
• NS-2 http//www.isi.edu/nsnam/ns/
• Baisi Forum http//www.baisi.net/forum-440-1.html
  
• My wireless tutorial http//www.cse.cuhk.edu.hk/
  ywang/ns2.html

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NS-2 vs GloMoSim OPNET
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NS-2 Basics

• NS-2 directory structure

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NS-2 Basics (cont)

• Network Components
• Simple components
• correspond to a single C object
• Compound components
• a combination of multiple simple components
• Example

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NS-2 Basics (cont)

• Node component
• Link component

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NS-2 Basics (cont)

• class Event
• defined by time, uid, next, handler
• class Packet is a subclass of Event
• Timer is a subclass of Event
• class Handler
• All network components are its subclasses
• TimerHander which handles expired timers
• Scheduler
• void Schedulerschedule(Handler h, Event e,
  double delay)

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NS-2 Basics (cont)

• Scheduling procedure

Note There are no real time, timer and packet
tx/rx in ns-2 as in UNIX network programming.
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NS-2 Basics (cont)

• Example 1 A and B are two mobile nodes within tx
  range. A wants to send packet p to B.

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NS-2 Basics (cont)

• Example 2 TimerHandler

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NS-2 Basics (cont)

• recv function is often the entry to each network
  component / protocol
• e.g. in 802.11

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Wireless Simulation

• The inner structure of a wireless node

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Limitations in Simulating Mobile Wireless Networks

• Nodes do not move significantly over the length
  of time they transmit or receive a packet. This
  assumption holds only for mobile nodes of
  high-rate and low-speed.
• Consider a node with the sending rate of 10Kbps
  and moving speed of 10m/s, during its receiving a
  packet of 1500B, the node moves 12m. Thus, the
  surrounding can change significantly and cause
  reception failure.
• Node velocity is insignificant compared to the
  speed of light. In particular, none of the
  provided propagation models include Doppler
  effects, although they could.

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PHY layer class Channel

• Its function is to deliver packets from a
  wireless node to its neighbors within sensing
  range
• I. Stamp txinfo in packets before sending, for
  receivers to calculate receiving power
• II. Send packets to the nodes within the sensing
  range distCST_ for sensing or receiving

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PHY layer class WirelessPhy (NetIF)

• Its function is to transmit/receive packets
  to/from Channel
• Transmitting
• Receiving
• Calculate receiving power
• can not sense the packet
• can sense but can not decode the packet
• can decode the packet (Note that SNR will be
  checked in MAC layer for finally receiving the
  packet)

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MAC layer class Mac802_11 (cont)

• Its function is to send/receive packets to/from
  NetIF
• Sending CSMA/CA
• Receiving SNR threshold based capture model
• Need to understand its state transition diagram

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MAC layer class Mac802_11 (cont)

• States

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MAC layer class Mac802_11 (cont)

• Channel idle state

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MAC layer class Mac802_11 (cont)

• Timers

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MAC layer class Mac802_11 (cont)

• Example 1 controlling backoff timer

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MAC layer class Mac802_11 (cont)

• Sending packets (CSMA/CA)

Note Packets will be transmitted in
backoffHandler() after mhBackoff_ expires
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MAC layer class Mac802_11 (cont)

• Receiving packets (capture)

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My Fading Extension

• Standard NS-2 does not consider fading effect

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Example 1 of fading effect

• How fading affects TCP throughput

(a) Without fading
(b) With fading
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Example 2 of fading effect

• How fading affects flow fairness

(a) Without fading
(b) With fading
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Bugs in NS-2

• Oversimplifications
• E.g. there is no scanning for WLAN
  (Discovery/Select/Authentication/Association)
• Standard Misinterpretation
• E.g. abuse of EIFS

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• Thanks!